Anterior Pituitary and Pars Intermedia Space

C H A P T E R

8 Anterior Pituitary and Pars Intermedia Space: Corticotrophs (ACTH) and Melanotrophs (a-MSH) Nicola Romano`, Michael J. Shipston Centre for Discovery Brain Sciences, University of Edinburgh, Edinburgh, UK

1. INTRODUCTION Proopiomelanocortin (POMC) is the 241-amino-acidlong peptide whose posttranslational processing gives rise to several bioactive peptide hormones controlling a large array of different physiologic functions ranging from stress, feeding, and energy balance to pigmentation and analgesia. Two separate cell lineages in the pituitary gland express POMC: the corticotrophs of the anterior pituitary, central to the hormonal stress response, and the melanotrophs of the pars intermedia (PI), important for determination of skin and fur pigmentation. Differential posttranslational processing of POMC in these two populations results in production of adrenocorticotropic hormone (ACTH) in corticotrophs and a-melanocyte-stimulating hormone (a-MSH) in melanotrophs. This chapter will focus on the physiology of ACTH and a-MSH, although references to other POMC-derived hormones will be made where necessary.

2. POMC-DERIVED PEPTIDES AND THEIR RECEPTORS: STRUCTURE AND PROCESSING 2.1 POMCdSynthesis and Processing The synthesis and processing of POMC and its related peptides have been extensively studied since the 1940s, when a peptide with the ability of “repairing” the adrenal cortex of hypophysectomized rats, later named adrenocorticotropic hormone (ACTH), was initially isolated from the sheep pituitary gland (Li et al., 1955, 1954, Hormonal Signaling in Biology and Medicine https://doi.org/10.1016/B978-0-12-813814-4.00008-0

1942). It was not until several decades later with the cloning of the POMC gene that the existence of a precursor gene for ACTH and beta lipotropin (b-LPH) was confirmed (Nakanishi et al., 1979). Expression of POMC is highest in the pituitary gland and the hypothalamus; however, lower levels of POMC and its products have been detected in other tissues such as the male reproductive tract, which, in many species including humans, produces b-endorphin (Shu-Dong et al., 1982; Sharp and Pekary, 1981), and the skin, where keratinocytes and melanocytes produce and secrete POMC as well as ACTH, a-MSH, and b-endorphin (Rousseau et al., 2007; Wakamatsu et al., 1997). The production of POMC and its products is regulated by a complex interplay of transcriptional and posttranscriptional control.

2.2 Transcriptional Control of the POMC Gene The transcriptional control of the POMC gene allows for its cell-specific expression and has been the subject of a large amount of studies. Three main genomic regions are involved in the control of pituitary POMC expression; the promoter region, a
300 bp enhancer, and a
7 kb enhancer. Hypothalamic POMC neurons use two further enhancers at
10 kb and
12 kb (Drouin, 2016; de Souza et al., 2005). Along with binding sites for some widely expressed transcription factors (TFs), these regulatory elements also contain additional binding sites for TFs expressed in a cell-specific manner, which mediate the corresponding POMC cell type-specific expression pattern in corticotrophs and melanotrophs. Tpit (T-box TF) and Pitx1 (bicoid-type homeodomain TF pituitary homeobox 1) are important for both corticotrophs and

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8. ANTERIOR PITUITARY AND PARS INTERMEDIA SPACE: CORTICOTROPHS (ACTH) AND MELANOTROPHS (a-MSH)

melanotrophs; NeuroD1 (neurogenic differentiation 1), a member of the basic helix-loop-helix (bHLH) family, is key for corticotroph POMC expression; finally, Pax7 (Paired Box 7) is central for defining the specific transcriptional profile of melanotrophs (Drouin, 2016). Tpit and Pitx1 bind to a composite Tpit/Pitx1 responsive element (Poulin et al., 1997; Lamonerie et al., 1996), while NeuroD1 binds to an E-box sequence, named EboxNeuro, upstream of the Tpit/Pitx1 binding site (Philips et al., 1997). Pitx1 gives broad pituitary specificity, due to its role in transcription of many anterior pituitary hormones (Tremblay et al., 1998). TPIT expression, on the other hand, is restricted to corticotrophs and melanotrophs (Lamolet et al., 2001), limiting POMC transcription to these cell populations. Finally, NeuroD1 expression is high in corticotrophs during development, being required for their early differentiation, but it strongly declines in the adult (Lamolet et al., 2004); it is never expressed by melanotrophs. The binding of these TFs to the enhancer allows synergistic regulation of POMC transcription (Therrien and Drouin, 1991); for instance, NeuroD1 can synergistically interact with Pitx1, as shown by an approximately 10-fold increase in POMC transcription when both of these factors are present compared to either of them alone (Poulin et al., 2000). Furthermore, studies on the binding site for NeuroD1 have revealed that it acts as a heterodimer with another TF of the bHLH family, Pan1, through a NeuroD1-specific E-box, called E-boxNeuro. Upstream of

this element is E-boxUbi, which is activated by binding of ubiquitously present bHLH TFs. The two E-box elements are equally effective in activating POMC transcription; indeed, E-boxNeuro can be substituted by an E-boxUbi (that does not bind NeuroD1) without affecting transcription levels (Poulin et al., 2000). However, mutations or removal of E-boxNeuro severely affect the levels of POMC transcription both during development and in the adult (Lavoie et al., 2008; Poulin et al., 2000). Since NeuroD1 levels are low in adult corticotrophs, this has suggested that another bHLH TF, such as Ascl1, which is expressed in adult corticotrophs and melanotrophs, may bind to E-boxNeuro in adult life (Drouin, 2016). Melanotroph identity is instead defined by the Pax7 TF; this acts as a pioneer factor able to remodel chromatin to allow access to promoters for genes that are not normally accessible in corticotrophs (Budry et al., 2012). Indeed, exogenous expression of Pax7 in the AtT-20 corticotroph cell line is sufficient to change their epigenetic asset and confer them a melanotroph-like identity (Mayran et al., 2018).

2.3 Posttranslational Control of POMC Production A second layer of control in the production of POMCderived peptides happens at the level of posttranslational modifications (Fig. 8.1). POMC is processed through the regulated secretory pathway (RSP),

FIGURE 8.1 POMC posttranscriptional processing. Schematic of the posttranscriptional processing of POMC. The precursor peptide POMC is processed through a series of proteolytic cuts by the proteases PC1 and PC2. Corticotrophs express only PC1 and can therefore process POMC into ACTH and b-lipotropin, while in melanotrophs PC2 further processes these products to form a- and b-MSH and b-endorphin. Production of a-MSH also requires further cleavage by carboxypeptidase E (CPE), and amidation and acetylation through the subsequent actions of peptidylglycine a-amidating monooxygenase (PAM) and N-acetyltransferase (NAT). Finally the lysosomal enzyme prolylcarboxypeptidase (PRCP) is involved in degradation of a-MSH. CLIP, Corticotropin-like intermediate lobe peptide; EP, endorphin; JP, junction peptide; LPH, lipotropin; Sig, signal peptide.

3. PITUITARY CELLS EXPRESSING POMC: CORTICOTROPHS AND MELANOTROPHS

whereby the precursor peptide is transported through the rough endoplasmic reticulum (RER) to the Golgi apparatus, where it is cleaved and modified, for its products to be targeted in the secretory vesicles of the RSP. In the Golgi apparatus, a complex series of posttranslational cleavage and modification reactions of the POMC precursor results in the production of several peptide hormones including the melanocortins ACTH, a-, b-, and g-melanocyte-stimulating hormone (a-, b-, and g-MSH), and the opioids b-endorphin and b- and g-lipotropin (b- and g-LPH) (Roberts and Herbert, 1977; Mains et al., 1977) (Fig. 8.1). Cell type-specific expression of the two main endoproteases responsible for the processing of POMC, the proprotein convertases 1 and 2 (PC1 also called PC3, and PC2) (Benjannet et al., 1991), allows control of the production of specific hormones. PC1 and PC2 are endoproteases that recognize the [R/K]-Xn-[R/K]Y motif (where Y indicates the cutting site) (Seidah and Chre´tien, 1999). Studies in rodents and humans (Takumi et al., 1998; Seidah et al., 1991) have shown localization of PC1 in the anterior pituitary corticotrophs; this results in cleavage of POMC to produce ACTH and b-LPH. In the melanotrophs of the PI, on the other hand, expression of both PC1 and PC2 allows almost complete cleavage of b-LPH to b-endorphin and g-LPH, and it allows selective production of a-MSH from cleavage of ACTH specifically in these cells (Day et al., 1992; Seidah et al., 1991). Processing of POMC and sorting through the RSP happens in a sequential fashion. Translocation to the RER is allowed by the presence of two disulphide bridges at the N-terminal of POMC, forming a hairpin loop structure (Cool et al., 1995; Cool and Peng, 1994; Tam et al., 1993). This structure allows binding to the sorting receptor, which has been identified as carboxypeptidase E (CPE, also known as carboxypeptidase H or enkephalin convertase), a widely expressed exopeptidase present in many endocrine tissues and involved in the biosynthesis of several hormones (Fricker, 1988; Hook et al., 1982). Specifically, it is the membrane-bound form of CPE that functions as the sorting receptor for POMC in the trans-Golgi network (Cool et al., 1997). Electron microscopy studies have shown that processing of POMC is limited to the outermost cisternae of the Golgi in corticotrophs (Schnabel et al., 1989). Here, acidic pH allows the autocatalytic activation of PC1 (Seidah et al., 2008), which first cleaves POMC into b-LPH and pro-ACTH, then further cleaves the latter to produce ACTH, a junction peptide, and NPOMC. In parallel to the activity of PC1 and PC2, there is evidence for the role of the protease cathepsin L, which is located in secretory vesicles, in the maturation of POMC to ACTH, a-MSH, and b-endorphin, as evidenced by the fact that cathepsin L knockout mice show major reduction in the levels of these hormones (Hook et al., 2009; Funkelstein et al., 2008).

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In cells expressing PC2, such as the melanotrophs of the intermediate zone, the skin melanocytes, or the hypothalamic POMC neurons, further processing of ACTH to a-MSH and of b-LPH to g-LPH and b-endorphin happens at the level of secretory vesicles (Fig. 8.1). After cleavage of the full-length ACTH(1-39) molecule by PC2 to give ACTH(1-14), the C-terminal of the latter is cleaved by a soluble form of CPE (Ji et al., 2017). Finally, through the subsequent actions of peptidylglycine a-amidating monooxygenase (PAM) and an N-acetyltransferase (NAT), the active acetylated and amidated form of a-MSH is synthesized. Finally, these posttranslational events can also be modulated by changes in the level of the enzymes involved in POMC processing: for example, in rat melanotrophs, PC1, PC2, and PAM transcription and translation are under dopaminergic control, being increased by haloperidol treatment and decreased by bromocriptine (Oyarce et al., 1996; Day et al., 1992; Birch et al., 1991). Similarly, expression of PC1 in corticotrophs has been shown to depend on activation of the cAMP/PKA pathway, for instance through the inducible cAMP early repressor (ICER), an isoform of the cAMP response element modulator (Lamas et al., 1997); this is also in line with evidence showing that PC1 is regulated through CRE elements (Jansen et al., 1995).

3. PITUITARY CELLS EXPRESSING POMC: CORTICOTROPHS AND MELANOTROPHS 3.1 Corticotrophs and ACTH 3.1.1 ACTH Release from Corticotrophs: A Key Mediator of the Neuroendocrine Response to Stress The release of ACTH from corticotrophs of the anterior pituitary gland is a key component of the hypothalamicepituitaryeadrenal (HPA) axis that coordinates the neuroendocrine response to stress (Fig. 8.2). Activation of the HPA axis results in the release of ACTH from corticotrophs into the circulation. ACTH in turn stimulates the synthesis and release of glucocorticoid hormones from the adrenal gland (primarily cortisol in man, corticosterone in rodents) that have powerful and pleiotropic roles in a range of systems. Glucocorticoids, in turn, feed back at the pituitary corticotroph, as well as other levels of the HPA axis, to limit output of the axis as part of an adaptive response to restore homeostasis. Corticotrophs and ACTH release thus provide an important communication link between the brain neural circuitry and the peripheral release of glucocorticoids from the endocrine adrenal gland. Importantly, release of ACTH is pulsatile across multiple time domains with both ultradian and circadian

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FIGURE 8.2 Control of ACTH and a-MSH production. Schematic of the control of corticotroph and melanotroph activity. PC1 in anterior pituitary corticotrophs processes POMC to ACTH. This is released in response to release of corticotropin-releasing hormone (CRH) and/or arginine vasopressin (AVP) from hypothalamic neuroendocrine neurons in the portal vasculature as part of the hormonal response to stress. ACTH stimulates the synthesis and release of glucocorticoid hormones from fasciculata cells of the adrenal cortex to control a variety of physiologic functions. Glucocorticoid negative feedback controls the HPA axis at multiple levels and over different time scales. Melanotrophs of the pars intermedia are directly innervated from hypothalamic fibers. They are under tonic inhibitory control from dopaminergic and GABAergic inputs, as well as receiving other neuropeptidergic inputs that may control their function (see text for details). In these cells the combined actions of PC1 and PC2 allow the production of a-MSH, important for the control of skin pigmentation.

rhythmsdthis pulsatility is critically important for the physiologic response of target organs to ACTH and glucocorticoids and is modified in aging and disease (Spiga et al., 2014; Lightman and Conway-Campbell, 2010). The release of ACTH from corticotrophs is thus tightly controlled to allow ultradian/circadian rhythmicity while being able to respond appropriately to stressors throughout the day. Acute stress is beneficial by priming the organism for the demands of its immediate environment, for example, by increasing alertness and behavioral and cognitive performance. Indeed, glucocorticoids mediate powerful and pleiotropic physiologic and metabolic responses including energy mobilization, cardiovascular and antiinflammatory responses, as well as suppression of digestive and reproductive function to prepare the organism for a stressful situation (Sapolsky et al., 2000). However, our inability to adapt to persistent stress is now well recognized as increasing the likelihood of developing a range of debilitating disorders including cardiovascular disease, metabolic disruption such as obesity and diabetes, as well as neurological disorders such as depression, anxiety, and cognitive dysfunction. In general, we are not well equipped to cope with

chronic activation of stress pathways, and as recognized by Hans Selye, “it is not stress that kills us, it is our reaction to it.” While “activation” of the HPA axis is commonly associated with responses to aversive stimuli, it is important to remember that output is under both ultradian and circadian control (Spiga et al., 2014; Lightman and Conway-Campbell, 2010), as well stimulated by “pleasurable” behaviors including appetite reward and sexual encounter. 3.1.2 Corticotroph Development and Anatomy in the Anterior Pituitary Gland In the adult anterior pituitary gland, corticotrophs represent approximately 5%e10% of the endocrine secretory cell population. Until relatively recently, corticotrophs been considered largely “randomly” distributed throughout the anterior pituitary as single cells/ clusters intermingled with other pituitary cells. However, using whole tissue imaging and genetic labeling of corticotrophs reveals that corticotrophs, as for other pituitary cells, form homotypic networks that extend throughout the gland (Le Tissier et al., 2012; Budry et al., 2011). In mice, corticotrophs form columns and

3. PITUITARY CELLS EXPRESSING POMC: CORTICOTROPHS AND MELANOTROPHS

sheets of cells from the ventral surface toward the center of the anterior pituitary gland, and many cells have long cytoplasmic projections (cytonemes) that make contact with corticotrophs separated by other cell lineages (Le Tissier et al., 2012; Budry et al., 2011). Moreover, corticotrophs are generally further away from capillary blood vessels, so whether ACTH release into the blood stream is from cytonemes located adjacent to the vessels and/or is first released into the interstitial space is unclear (Le Tissier et al., 2017). However, whether this anatomic network in fact results in functional corticotroph networks, as has been demonstrated for somatotrophs and lactrotrophs (Hodson et al., 2012; Lafont et al., 2010; Bonnefont et al., 2005), is not known. Functional corticotroph networks and the relationship of corticotrophs to the vascular bed are both likely to play an important role in the dynamics (including amplitude, synchronization, and robustness) of pulsatile ACTH release into the peripheral vasculature, and perhaps for a form of corticotroph “memory” following a stressor, as for other pituitary hormones (Hodson et al., 2012). Corticotrophs are the first cells of the anterior pituitary to form anatomic networks and the first cells to reach terminal differentiation that is determined by a complex network of TFs (Drouin, 2016). Corticotroph terminal differentiation is dependent upon the expression of Tpit, which binds to the POMC promoter in corticotrophs, resulting in corticotrophs being detected by about e12.5a in mice (Lamolet et al., 2001). Tpit is expressed 0.5 days before detectable expression of POMC and as well as being critical for corticotroph terminal differentiation genetic deletion of the TPIT gene in mice reveals a role for Tpit in both corticotroph expansion and maintenance (Pulichino et al., 2003). In humans, multiple TPIT mutations are associated with the inherited recessive condition of isolated ACTH deficiency that results in a lack of pituitary ACTH and secondary adrenal glucocorticoid deficiency (Couture et al., 2012; Vallette-Kasic et al., 2005) while sparing other pituitary hormones. Corticotroph specificity of POMC expression is also dependent upon the neurogenic basic helix-loop-helix factor NeuroD1 (Poulin et al., 1997) and a complex network of protein interactions. In particular, NeuroD1 binds to a bipartite regulatory element that includes a Nur response element (NurRE) important for corticotrophin-releasing hormone (CRH) stimulation of POMC transcription (Philips et al., 1997). Together the Tpit/PitX and NeuroD1/Nur response elements act synergistically to stimulate POMC transcription (Drouin, 2016). Importantly, corticotroph specification is not dependent upon CRH or AVP from the hypothalamus. For example, corticotroph development is largely normal in CRH-deficient mice (Muglia et al., 1995), and in mice lacking mature CRH-

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and AVP-producing hypothalamic neurons, maturation of anterior pituitary cells was normal (Schonemann et al., 1995). 3.1.3 CRH and Glucocorticoid Control of POMC Expression in Corticotrophs CRH, but not AVP, is however a powerful stimulator of POMC transcription and thus ACTH synthesis and storage in corticotrophs. A major target for CRHmediated stimulation of POMC transcription is through an orphan nuclear receptor related to nerve growth factor 1-B (Nurr77) that binds the NurRE and is activated by the MAPK pathway (Philips et al., 1997). The effect of CRH via Nurr77 is enhanced by a number of coactivators including members of the steroid receptor coactivator family (Maira et al., 2003). In turn, glucocorticoids antagonize CRH-stimulated POMC transcription that appears to be mediated primarily through a transrepression mechanism resulting from proteineprotein interactions of the glucocorticoid receptor (GR) complex with Nur factors rather than the GR binding directly to glucocorticoid response elements in the POMC promoter (Martens et al., 2005; Philips et al., 1997). This glucocorticoid-mediated transrepression of POMC transcription requires recruitment of histone deacetylase (HDAC2) and the ATPase component of the SW1/SNF chromatin remodeling complex, BRG1. Indeed, loss of nuclear expression of either HDAC2 or BRG1 may account for loss of glucocorticoid repression of POMC in a significant proportion of patients with Cushing syndrome (Drouin, 2016). 3.1.4 Regulation of ACTH Secretion by CRH, AVP, and Glucocorticoids The release of ACTH from corticotrophs is controlled by two major hypothalamic neuropeptides, CRH, also known as corticotrophin releasing factor (CRF), and arginine vasopressin (AVP). Perception of a stressor by the brain is ultimately relayed to neuroendocrine neurons in the hypothalamus that release CRH and/or AVP into the hypophysial portal circulation (Fig. 8.2). At the level of the corticotroph, CRH and AVP can act synergistically through the activation of distinct G proteinecoupled receptor (GPCR) signaling pathways (Fig. 8.3). 3.1.5 CRH and AVP Activate Distinct G ProteineCoupled Receptor Signaling Pathways in Corticotrophs CRH, originally isolated and characterized from ovine hypothalamus by Wyllie Vale and colleagues in 1981 (Vale et al., 1981), belongs to a family of four related neuropeptidesdCRH, urocortin 1 (UCN1), UCN2, and UCN3dthat are widely expressed in the brain (Dedic et al., 2018). CRH is most closely related to UCN1 (43% sequence identity), and its biologic active, mature form

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FIGURE 8.3 Stimulus secretion-coupling in corticotrophs and melanotrophs. In corticotrophs (left), CRH and AVP stimulate ACTH secretion through activation of distinct G proteinecoupled receptors (CRHR1 and AVPR1, respectively). Activation of CRHR1 activates adenylate cyclase (AC) with a resulting increase in the second messenger cyclic adenosine monophosphate (cAMP) that activates protein kinase A (PKA). Activation of AVP1b results in stimulation of phospholipase C (PLC), resulting in liberation of diacylglycerol (DAG) and inositol trisphosphate (IP3). DAG activates protein kinase C (PKC), whereas IP3 releases calcium from intracellular calcium stores. CRH and AVP control electrical excitability of corticotrophs through a variety of plasma membrane ions channels including voltage-gated calcium channels and potassiumselective channels. The exocytosis of ACTH is ultimately controlled by an increase in intracellular free calcium. Similar mechanisms control a-MSH secretion from melanotrophs where, as in corticotrophs, cAMP and intracellular calcium are central for hormonal secretion. In these cells the two main hypothalamic controllers are dopamine (DA) and GABA. Through its action on the D2 receptors, DA inhibits production of cAMP by AC. GABA acts through both the through the ionotropic GABAA receptor, which provides rapid depolarization, and through the metabotropic GABAB receptor, which provides sustained inhibition by blocking cAMP production.

is a 41 amino acid peptide generated from a 196 amino acid precursor by proteolytic cleavage. CRH is the major physiologic activator of the HPA axis in man and rodents, and its bioavailability is regulated by CRHbinding protein (Seasholtz et al., 2001). In mice, constitutive genetic deletion of CRH blunts basal and stress-evoked stress responses (Muglia et al., 1995). CRH mRNA and peptide is high in parvocellular neurons of the hypothalamus that project to the median eminence and release CRH into the hypophysial portal circulation. CRH signals through activation of two G proteinecoupled receptors (CRHR1 and CRHR2), which share w70% amino acid identity (Dedic et al., 2018). CRH shows a much higher affinity for CRHR1 than CRHR2; however, at the level of anterior pituitary corticotrophs, CRHR1 is the major CRH GPCR on corticotrophs, where it couples to adenylate cyclase via Gs proteins to stimulate the synthesis of cAMP. Thus, the majority, if not all, effects of CRH at the level of pituitary corticotrophs are mediated through cAMP and subsequent activation of cAMP-dependent protein kinase, although the role of other cAMP-mediated pathways has not been systematically explored. Genetic deletion of CRHR1 in mice abolishes CRH-induced ACTH secretion and cAMP accumulation in isolated pituitary cells and significantly blunts stress-induced ACTH and corticosterone secretion in vivo (Timpl et al., 1998; Smith et al., 1998).

A role for AVP in driving ACTH secretion in response to a variety of stressors has long been recognized, including from studies using the Brattleboro rat, which lacks endogenous AVP (Antoni, 1993). AVP, also known as antidiuretic hormone, is a nonapeptide, structurally related to the related neuropeptide oxytocin (OT) that is synthesized from a larger precursor pre-proAVP. AVP is expressed in both magnocellular and parvocellular neuroendocrine neurons of the paraventricular nucleus that project to the median eminence (Antoni, 1993). AVP mediates its physiologic actions through three distinct GPCRs the V1a, V1b, and V2 receptors that share 40%e55% amino acid identity (Koshimizu et al., 2012). However, genetic and pharmacologic evidence reveals that the V1b receptor, which is highly expressed in pituitary corticotrophs (Roper et al., 2011; Lolait et al., 1995; Antoni, 1993; Jard et al., 1986), mediates the actions of AVP to stimulate ACTH release from corticotrophs. Genetic deletion of V1b receptors in mice (V1bR-KO) leads to loss of AVP-stimulated but not CRH-stimulated ACTH release in isolated pituitary cells (Roper et al., 2010; Tanoue et al., 2004). Moreover, in vivo, V1bR-KO mice or wild-type animals injected with V1b receptor antagonists such as ORG52186 or SSR149415 display blunted ACTH responses to AVP stimulation (Roper et al., 2010; Spiga et al., 2009; Tanoue et al., 2004). In corticotrophs, the V1b receptor is coupled to Gq/11 proteins and stimulates phospholipase C

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(PLC) activity, resulting in the activation of protein kinase C signaling (via diacylglycerol) and release of calcium from intracellular inositol trisphosphate (IP3)dsensitive stores (Roper et al., 2011; Lolait et al., 1995; Antoni, 1993; Jard et al., 1986). 3.1.6 Synergy of CRH- and AVP-Stimulated ACTH Secretion Although CRH is the major ACTH secretagogue in most species, AVP plays an important role through its synergistic action on ACTH secretion in combination with CRH observed in vitro and in vivo (Lamberts et al., 1984; Gillies et al., 1982). Mechanistically, synergy is also observed at the level of CRH-stimulated cAMP production and is dependent upon activation of PKC by AVP in cell population assays. In isolated rat corticotrophs, CRH has been proposed to stimulate two distinct isoforms of adenylate cyclase (AC): the Ca2þ-dependent AC9 and Ca2þ-independent, but PKC-activated, AC7 (Antoni et al., 2003). AC9 is the major adenylate cyclase responsible for CRH-stimulated cAMP accumulation at low physiologic levels of CRH. As AC9 is inhibited by Ca2þ, cAMP accumulation at low CRH is subject to robust Ca2þ-dependent negative feedback. In this model, AVP, via PKC-dependent activation of AC7, is proposed to act as a switch, so CRH-dependent cAMP accumulation is now predominantly through activation of AC7, rather than AC9. This promotes cAMP synthesis that is now largely resistant to Ca2þ-feedback inhibition and likely also involves differential regulation of phosphodiesterases that break down cAMP (Antoni et al., 2003). Whether synergy occurs downstream of cAMP signaling is not well understood. As discussed subsequently, differential regulation of ion channels may lead to synergistic actions of CRH and AVP at the level of electrical excitability. However, at the population level, rat corticotrophs do not display synergy at the level of intracellular Ca2þ accumulation evoked by physiologic levels of CRH and AVP (Romano` et al., 2017). However, distinct intercellular heterogeneity in synergy is observed with approximately 40% of corticotrophs displaying synergy in response to repeated exposure to CRH, AVP, and CRH/AVP (Romano` et al., 2017). Thus, synergy at the corticotroph population level of ACTH secretion may also arise due to heterogeneity in ACTH secretion from single cells in response to CRH and AVP. Indeed, using the hemolytic plaque assay that allows secretion detection in response to several hours of secretagogue exposure has revealed corticotrophs that respond differentially to CRH and AVP. For example, ACTH secretion from single cells was dose dependently increased by CRH, but not AVP, whereas AVP recruited more cells to secrete ACTH (Canny et al., 1992). Finally, synergy may occur at the level of Ca2þ-dependent exocytosis of ACTH vesicles. In

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corticotrophs the Ca2þ dependence of fast exocytosis has a third-power relationship (Tse and Lee, 2000). Thus, as CRH and AVP together promote a stronger depolarization (Zemkova et al., 2016; Lee et al., 2015; Duncan et al., 2015), and hence increase in voltagedependent Ca2þ entry, due to the steep dependence of voltage-gated calcium channels (VGCC) at physiologic membrane potentials, this may provide a synergistic effect at the level of ACTH vesicle exocytosis. 3.1.7 Electrical Excitability, Calcium Signaling, and the Control of ACTH Secretion Metabolically intact corticotrophs are electrically excitable cells that display spontaneous (20- to 100-ms duration) action potentials, although there is considerable heterogeneity between cells (Fletcher et al., 2017; Zemkova et al., 2016; Liang et al., 2011; Kuryshev et al., 1996). Modeling studies suggest this heterogeneity can arise from small differences in functional ion channel expression rather than from distinct “subtypes” of corticotrophs, as individual cells can display multiple behaviors (Fletcher et al., 2017). The ion channel targets downstream of CRH and AVP receptor activation are still poorly understood; however, recent evidence reveals both secretagogues control distinct as well as common ion channels to ultimately control elevation of intracellular free calcium. Physiologic (subnanomolar) concentrations of CRH in murine corticotrophs result in cell depolarization and an increase in electrical excitability characterized by both an increase in action potential frequency as well a transition to a “pseudo plateau bursting” mode (Fletcher et al., 2017; Zemkova et al., 2016; Duncan et al., 2015). Modeling and experimental studies suggest that the initial membrane depolarization is dependent on activation of a background sodium conductance and inhibition of a background potassium conductance. In mouse, the background Kþ current may be mediated by the two-pore TREK-1 potassium channel that is also reported to be inhibited by AVP, providing a mechanism for additive effects of CRH and AVP on initial depolarization (Lee et al., 2015). This depolarization is important for activation of VGCC, with L-type (dihydropyridine sensitive) VGCCs, playing a critical role in both calcium entry and CRH-evoked ACTH secretion (Ritchie et al., 1996; Kuryshev et al., 1996). The transition to bursting is promoted by large conductance calcium-and voltage-activated potassium channels that play a paradoxical role in facilitating sustained membrane depolarization and voltage-gated Ca2þ entry (Fletcher et al., 2017; Zemkova et al., 2016; Duncan et al., 2015). This increase in excitability is maintained for several minutes following removal of CRH. In contrast, physiologic levels of AVP largely evoke a smaller depolarization and an increase in action

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potential frequency rather than a significant transition to bursting. Importantly, the response to AVP is maintained >10 min following AVP withdrawal. At supraphysiological (>2 nM) levels of AVP, the release of intracellular free calcium is predominant and reported to result in transient hyperpolarization of the membrane potential in both rat and mouse corticotrophs (Lee et al., 2015; Tse and Lee, 1998). Indeed, the relative importance of voltage-gated Ca2þ entry compared to release from intracellular IP3-sensitive stores for AVP-stimulated ACTH release is likely to be concentration dependent. The more potent effect of CRH, compared to AVP, in stimulating secretion does not appear to be a result of either cAMP or the CRH-induced spatial calcium gradient during VGCC Ca2þ entry, being more effective in promoting exocytosis of the readily releasable pool of ACTH vesicles in rat corticotrophs (Tse and Lee, 2000, 1998). However, whether CRH is more effective at sustaining recruitment and slower exocytosis of vesicles compared to AVP during sustained stimulation is not known. 3.1.8 Glucocorticoid Feedback Inhibition of ACTH Release Glucocorticoids feed back at multiple levels of the stress axis to suppress HPA activity in both a timeand concentration-dependent manner (Keller-Wood and Dallman, 1984). At the level of the anterior pituitary, corticotroph glucocorticoids inhibit ACTH secretion in three, mechanistically distinct, time domains: (1) fast inhibition (seconds to minutes) that is rapidly reversible and nongenomic; (2) early inhibition (10 mine2 h); and (3) slow inhibition (h/days). Fast glucocorticoid inhibition at the level of the pituitary is observed in vivo upon injection of corticosterone immediately prior to stimulation of ACTH secretion with CRH (Hinz and Hirschelmann, 2000). Recent in vitro studies, using perifused pituitary cells at physiologic CRH concentrations, have suggested that fast inhibition is mediated downstream of CRH-induced calcium signaling involving a membrane-bound form of the classical GR through a nongenomic mechanism (Deng et al., 2015). Importantly, this rapid inhibition was seen at concentrations of corticosterone that would typically be observed during stress (>1 mM), or near the peak of an ultradian pulse, in corticotrophs that had been previously exposed to nanomolar concentrations of glucocorticoid prior to stimulation. In corticotrophs exposed to glucocorticoid-free conditions for 6 h prior to stimulation, 10e100 nM corticosterone could now also evoke rapid and reversible inhibition of CRHstimulated ACTH release. Slow glucocorticoid inhibition results in both suppression of basal and evoked ACTH secretion involving multiple genomic mechanisms including suppression of

POMC transcription (see earlier) (Drouin, 2016) as well as suppression of CRH-R1 mRNA expression (Pozzoli et al., 1996). The long-term effects of glucocorticoids or stress on other parameters of corticotroph function such as Ca2þ signaling or electrical excitability have not been explored systematically. In contrast, early glucocorticoid inhibition involves multiple mechanisms that largely inhibit evoked, rather than basal ACTH secretion through control of both membrane excitability and suppression of CRH- and AVP- evoked elevations of intracellular free calcium. In perifused rat corticotrophs subjected to pulses of CRH and/or CRH þ AVP, glucocorticoid inhibition of evoked ACTH secretion is observed within 10 min of glucocorticoid exposure that lasts for w2 h following steroid withdrawal. Inhibition is blocked by classical type II glucocorticoid antagonists as well as inhibitors of transcription and translation and is thus a largely genomic effect of glucocorticoid (Shipston and Antoni, 1991; Dayanithi and Antoni, 1989). In the same time frame, glucocorticoids suppress both CRH- and AVP-evoked increases in intracellular calcium signaling at the single cell level in rat corticotrophs (Romano` et al., 2017) and suppress CRH-mediated Ca2þ signaling in mouse AtT20D16:16 cells (Antoni et al., 1992). Importantly, in native rat corticotrophs, suppression of calcium signaling involved an increase in the recruitment of individual cells to glucocorticoid inhibition, the time course also being dependent on the secretagogue combination (Romano` et al., 2017). Cells exposed to CRH and AVP in combination cells were also more resistant to feedback inhibition. Glucocorticoid feedback inhibition of Ca2þ signaling is, at least in part, due to suppression of CRH- and AVP-stimulated electrical excitability through multiple mechanisms. In native mouse corticotrophs, glucocorticoids prevent the transition to bursting in response to CRH. This inhibition of CRH-induced bursting involves control of BK channels, as corticotrophs that were pretreated with corticosterone could be induced to burst, in the presence of CRH, by reintroducing a BK-like fast current into cells using dynamic clamp (Duncan et al., 2016). Corticosterone also dampens excitability independently of BK channels as corticosterone inhibited both CRH- and AVP-induced increases in action potential frequency in the absence of BK channels. However, glucocorticoids had little effect on CRH/AVP-induced depolarization, suggesting that the mechanisms largely target channels involved in control spike frequency and bursting, rather than initial depolarization from rest (Duncan et al., 2016). Corticosterone had differential effects on basal activitydwith no effect on resting membrane potential following pretreatment for 4 or 150 min but with a significant hyperpolarization of RMP observed after 90 min of exposure. Thus, glucocorticoids

4. MELANOTROPHS AND a-MSH

control excitability through multiple ionic mechanisms in the early time domain although the molecular targets and mechanisms remain to be defined.

4. MELANOTROPHS AND a-MSH Production of a-MSH from melanotrophs is key for the control of skin and fur pigmentation in vertebrates. A large volume of studies on melanotrophs has been performed in fish, amphibians, and reptiles, where the role of a-MSH in controlling color changes and skin pigmentation is well established (Nilsson Sko¨ld et al., 2013; Vaudry et al., 2006; Tonon et al., 1993, 1988). The role of a-MSH in pigmentation in mammals has also been subject of many investigations, and although its role in human skin pigmentation was initially debated, it is now clear that a-MSH acting through its receptor MC1R plays a central role in the control of eumelanin production in humans as well (Nasti and Timares, 2015; Abdel-Malek et al., 2014, 1999).

4.1 Anatomic Considerations Melanotrophs are located in the PI of the pituitary gland, a band of cells located at the interface between the anterior and neural part of the hypophysis, where these cells receive direct innervation from hypothalamic neurons (Takeuchi, 2001). Melanotrophs are the main cell population of the PI; morphologic studies have revealed heterogeneity in this population, in terms of shape, amount, and size of secretory granules, as well as levels of POMC expression (Takeuchi, 2001; Ishii and Ishibashi, 1989; Chronwall et al., 1987; Murakami et al., 1968). Interestingly, it has been shown that some species, including humans and higher apes (Plaut, 1936), cetaceans (Geiling et al., 1940), elephants (Wislocki, 1940), and birds (Rahn and Painter, 1941), lack a well-defined PI or only present a rudimentary or vestigial one. The presence of a functional PI in humans has been a longstanding matter of debate; although a clear PI is found in the fetus (Murakami et al., 1968), this is thought to regress in the adult, leaving residual colloid-filled cysts (Larkin and Ansorge, 2000; Horvath et al., 1999; Rasmussen, 1930). Although in cetaceans there are reports of melanotrophs in the anterior pituitary (Panin et al., 2013), it is unknown whether this is the case in humans.

4.2 Control of Melanotroph Activity In most species, vascularization of the PI is fairly limited when compared to that of the anterior pituitary (Stojilkovic et al., 2010; Takeuchi, 2001), in agreement

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with the fact that cells in this region are controlled through direct innervation from dopaminergic, GABAergic, and neuropeptidergic hypothalamic neurons (Fig. 8.2), rather than receiving hormonal signals from the hypothalamus through the portal vasculature, as it is the case for hormone-producing cells in the anterior pituitary. Mammalian melanotrophs are under tonic GABAergic and dopaminergic inhibition (Stojilkovic et al., 2010); in amphibians, the role of several neuropeptides (such as TRH, CRH, and NPY) has also been investigated (Shibuya and Douglas, 1993; Leenders et al., 1993; de Rijk et al., 1992), while the role of these peptides in mammals remains less clear. When cultured (thus relieving their tonic hypothalamic inhibition), dissociated melanotrophs show a diversity of spontaneous activity; they have been reported to display low frequency (<5 Hz) spontaneous action potentials, although the proportion of active cells varies between different studies and species (Zemkova et al., 2016; Valentijn et al., 1991a; Stack and Surprenant, 1991; Trouslard et al., 1989; Louiset et al., 1988). These action potentials are TTX-sensitive (Stack and Surprenant, 1991; Trouslard et al., 1989) and appear either in the form of single spikes or of bursts of spikes seen on top of spontaneous fluctuations in membrane potential. Along with these are TTX-insensitive, Ca2þ-dependent small oscillations in membrane potential, correlated with fluctuations in intracellular Ca2þ (Lee, 1996; Stack and Surprenant, 1991). This is consistent with the expression of both TTX-dependent and -independent Naþ channels in these cells (Kehl, 1994). The inhibitory actions of dopamine (DA) on melanotrophs are mediated through binding to the D2 receptors, a 7-transmembrane GPCR, coupled to Gi (thus inhibiting the cAMP pathway, Fig. 8.3). D2 receptor is abundantly expressed in the PI, and its expression is controlled by DA itself (Autelitano et al., 1989). Tonic activation of D2 receptors controls melanotrophs growth, as demonstrated by the PI hyperplasia in D2-null mice (Saiardi et al., 1997) or mice where dopaminergic innervation was removed through 6-hydroxydopamine treatment (Gary and Chronwall, 1992). Interestingly, in D2-null animals, melanotroph proliferation is also accompanied by an increase in blood levels not only of a-MSH, but also of b-endorphin and ACTH, secondary to increase in expression of PC1 and PC2 (Saiardi et al., 1997). This is not due to modulation of corticotroph function, since these cells are not responsive to DA (Vale et al., 1983). When applied to cultured melanotrophs, DA rapidly hyperpolarize them, thus reducing or completely abolishing their spontaneous firing (Valentijn et al., 1991a; Douglas and Taraskevich, 1978). DA actions have been shown to depend on activation of a potassium current and a pertussis toxin (PTX)-sensitive G protein, that is independent from cAMP (Valentijn et al., 1991b; Stack and Surprenant,

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1991; Taraskevich and Douglas, 1990). This is also associated with a drop in intracellular Ca2þ levels (Lee, 1996; Taraskevich and Douglas, 1990) and reduced transcription of POMC (Cote et al., 1986). Similarly, in vivo treatment of rats with the dopamine agonist 2-Br-alphaergocryptine resulted in a strong decrease in POMC transcript levels, while conversely, blockade of D2 receptor with haloperidol resulted in their increase. Stimulation with GABA results in a short transient increase of a-MSH secretion, followed by a decrease in secretion both from mammalian and amphibian melanotrophs (Desrues et al., 1995; Tomiko et al., 1983). Correspondingly, application of GABA rapidly depolarizes melanotrophs, leading to a brief burst of action potentials, followed by silencing of the cells (Taraskevich and Douglas, 1982). Furthermore, GABA is able to prevent the KCl-mediated depolarization and increase in a-MSH secretion from melanotrophs (Tomiko et al., 1983; Taraskevich and Douglas, 1982). In mammals, these seemingly paradoxical results have been explained by the interaction between the rapid depolarizing actions of the GABAA and the inhibitory actions of the GABAB receptors on melanotrophs (Demeneix et al., 1986, 1984, Fig. 8.3); however, in frog melanotrophs, both of these effects are mediated through GABAA (Desrues et al., 2005). Although not as abundant as in the anterior pituitary, CRH receptor immunoreactivity and transcripts are seen in the PI of the rat (Mano-Otagiri et al., 2016; De Souza et al., 1984), and CRH is able to stimulate secretion from rat and sheep melanotrophs in vitro (Lugo and Pintar, 1996; Gagner and Drouin, 1987; Vale et al., 1983; Meunier and Labrie, 1982) and in vivo (Proulx-Ferland et al., 1982). However, neither CRH nor AVP were able to affect electrical activity of mouse melanotrophs in culture, possibly indicating species specificity of these responses (Zemkova et al., 2016). Since CRH innervation is seen in the posterior pituitary but not in the PI (Burlet et al., 1983), for these actions to happen physiologically, CRH would have to diffuse from the posterior pituitary to the PI. The effect of CRH on melanotroph’s secretion is not mirrored in transcription of POMC, which is not affected by CRH treatment (Gagner and Drouin, 1987); chronic stress, which affects POMC transcription in corticotrophs, did not alter POMC transcription in certain rat models of chronic stress (chronic electrical foot shock or chronic arthritis) (Ho¨llt et al., 1986). However, in rats exposed to a different type of chronic stress (cage filled with water), PI POMC transcription, as well as increased a-MSH plasma levels, were increased. These contrasting results may suggest that different types of stressors can activate different physiologic homeostatic mechanisms to respond to stress. Finally, treatment with glucocorticoids does not affect secretion in these cells in vitro (Vale et al., 1983), nor

in vivo (Proulx-Ferland et al., 1982), and it does not alter POMC transcription in melanotrophs (Gagner and Drouin, 1987). This is in agreement with the low level of expression of the GR in the PI, possibly due to the tonic dopaminergic inhibition in these cells (Budry et al., 2012; Takeuchi, 2001; Antakly et al., 1985). Interestingly, recent studies on the important chromatin-remodeling role of the pioneer factor Pax7 in melanotrophs show that GR immunoreactivity increases in the PI of Pax7-null animals, indicating a role of this factor in modulation of GR transcription (Budry et al., 2011).

5. THE MELANOCORTIN RECEPTORS (MCRS), ACCESSORY PROTEINS (MRAP), AND THEIR ACTIONS Five melanocortin receptors (MC1R to MC5R) are responsible for mediating the wide variety of physiologic actions of melanocortins in all tetrapods. These are 7transmembrane G proteinecoupled receptors of the rhodopsin family, showing an average level of 40% e50% sequence similarity, which are coupled to GaS, thus activating the production of cAMP upon binding of their ligands; MC2R is selective for ACTH, while all other MCRs can be bound and activated, with different affinity, by all melanocortins. All of the melanocortins are characterized by the presence of a His-Phe-Arg-Trp pharmacophore, which has been recognized necessary for their action (Sahm et al., 1994); selective binding of ACTH to the MC2R is permitted by the presence of the 15 Lys-Lys-Arg-Arg-19Pro sequence, which is required for its steroidogenic activity (Costa et al., 2004). The activity and surface expression of all MCRs can be modulated by melanocortin receptor accessory proteins (MRAP1 and 2) (Chan et al., 2009), whose role has been principally studied in regard to its effects on MC2R. Studies on the evolution of POMC and MCRs in chordates support the idea that the genes coding for these proteins have coevolved, and that the generation of five different receptors has resulted from duplication events happening during the course of the last 500 million years. The current view states that a first duplication event of an ancestral presumptive opioid precursor gene resulted in the generation of the current chordate’s POMC gene, as well as a presumptive proenkephalin/prodynorphin precursor gene (Dores and Baron, 2011; Sundstro¨m et al., 2010). In parallel to these events, three distinct duplication events resulted in the evolution of the five MCRs from a hypothetical precursor gene (Schio¨th et al., 2005, 2003). Further evolutionary events during the radiation of chordates resulted in selectivity of MC2R for ACTH in modern teleosts and tetrapods, while in cartilaginous fishes, MC2R can also be activated by a-MSH.

5. THE MELANOCORTIN RECEPTORS (MCRS), ACCESSORY PROTEINS (MRAP), AND THEIR ACTIONS

5.1 MC2R Historically, evidence for an ACTH-specific receptor in adrenal cells came from experiments showing that only ACTH but not a-MSH could activate production of steroids in adrenal cortical cells (Baumann et al., 1986; Ramachandran et al., 1976). The ACTH receptor mediating these effects remained elusive for another 15 years when MC2R was finally characterized (Mountjoy et al., 1992). The study of MC2R was further hindered by the fact that it was not possible to exogenously express it at the membrane in nonadrenal cell lines (Noon et al., 2002), suggesting that some cofactor was necessary for the membrane localization of MC2R. The identity of this cofactor was unclear until the role of the melanocortin 2 receptoreassociated protein 1 and 2 (MRAP1 and MRAP2) in melanocortin receptor function was discovered (Chan et al., 2009; Metherell et al., 2005). MRAP1 is necessary both for the translocation of MC2R at the membrane and its activation, while MRAP2 is able to drive translocation, but it prevents activation of the receptor (Sebag and Hinkle, 2010). MRAP1 and 2 are single transmembrane proteins, and they are peculiar in that they can exist both in a topology where the N-terminal is facing the external part of the membrane, and in one where the C-terminal faces outside. Two MRAP molecules with these different topologies can bind to form antiparallel homo- or heterodimers (Sebag and Hinkle, 2007); these can interact with MC2R in the RER, with a stoichiometry of one MC2R:2 MRAP. MRAP1 has not only the role of allowing MC2R to be expressed at the cell membrane, it is also important for the activity of the receptor. Specifically, amino acids 18e21 (LDYI in mouse, LDYL in human) are necessary for the increase in cAMP production following MC2R activation; mutant versions of MRAP1 lacking the 18e21 sequence allow translocation of MC2R to the membrane, but only result in minimal cAMP production (Sebag and Hinkle, 2007). Contrary to MRAP1, MRAP2 can shuttle MC2R to the membrane, but it prevents its activation by ACTH. Indeed, MRAP2 acts as a natural negative dominant form of MRAP (Sebag and Hinkle, 2010), and the interaction between MRAP, MRAP2, and MC2R could provide a means of regulation of ACTH signaling in cells expressing these two accessory proteins. Furthermore, evidence for dimerization of melanocortin receptors has shown that formation of a hetero-hexameric complex of four MRAP with two MC2R is necessary both for membrane localization (Sebag and Hinkle, 2009) and function of the receptor (Cooray et al., 2011). Although no MCR crystal structure has yet been reported, the selectivity of MC2R for ACTH has been extensively studied, and the specific interactions

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between MRAP, ACTH, and MC2R have been elucidated. It has been proposed that one of the two MRAP molecules binds to a region between the transmembrane region (TM) 3 and TM4 of MC2R, where an ER arrest signal is located, thus allowing translocation to the membrane. The other MRAP molecule binds between TM4 and TM5 and participates in forming a binding site for the ACTH-specific pharmacophore Lys-Lys-Arg-Arg-Pro. Following binding of ACTH, conformational changes lead to the formation of a second binding site for the His-Phe-Arg-Trp pharmacophore, which eventually allows for the activity of the receptor (Fridmanis et al., 2010). Upon binding of ACTH to MC2R the cAMP/PKA pathway is activated, leading to a range of physiologic effects. The main role of MC2R is to mediate the stimulating effects of ACTH on steroidogenesis in the adrenal cortex (Mountjoy et al., 1992). Activation of MC2R increases cAMP levels, resulting in activation of proteins important for steroidogenesis; this happens both through rapid effects such as the PKA-dependent phosphorylation of the steroidogenic acute regulatory protein (StAR) (Krueger and Orme-Johnson, 1983) and through slower actions on the transcription of several steroidogenic genes (Spiga et al., 2017). After ACTH binding, 20%e30% of MC2R/MRAP complexes are internalized, in a ligand concentration- and arrestindependent manner; about a third of the internalized receptors are then recycled at the membrane and mediate sustained cAMP responses (Roy et al., 2011). Aside from its role in adrenal steroidogenesis, MC2R has also been involved in other physiologic processes. MC2R is expressed by osteoblasts both in mice (Isales et al., 2010) and humans (Zhong et al., 2005), and ACTH was shown to be able to prevent glucocorticoidinduced osteonecrosis in a rabbit model, supporting the hypothesis that it may play a role in bone remodeling (Zaidi et al., 2010). In rodents, MC2R has also been identified in adipocytes, where it mediates ACTHdependent lipolysis (Møller et al., 2011); however, this is unlikely to be the case in humans, where ACTH does not stimulate lipolysis, even at supraphysiological doses (Boston, 2006).

5.2 MC1R Independently cloned by two groups in 1992 (Mountjoy et al., 1992; Chhajlani and Wikberg, 1992), the melanocortin 1 receptor MC1R is central for the control of skin, hair, and eye pigmentation in vertebrates. The physiologic ligands of MC1R are a-MSH, acting as an agonist, and the agouti peptide, which acts as an antagonist (Lu et al., 1994). Binding of a-MSH produced by

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the pituitary as well as by keratinocytes (Rousseau et al., 2007; Wakamatsu et al., 1997) results in stimulation of the cAMP/PKA pathway and activation via phosphorylation of the CREB transcription factor (Price et al., 1998), eventually resulting in increased transcription of genes involved in the control of pigmentation. Upon binding of a-MSH, MC1R undergoes agonist dose-dependent homologous desensitization, resulting in internalization and rapid recycling (Swope et al., 2012; Sa´nchez-Ma´s et al., 2005). Central for skin pigmentation are melanocytes, specialized epidermal cells containing melanin-rich organelles, called melanosomes; through extension of processes into the dermis, these cells can transfer melanin to keratinocytes, allowing protection from UV radiation. The specific color of the skin depends on the ratio of the two types of melanin present in melanocytes, the black/brown photoprotective eumelanin and the red/ yellow phaeomelanin, which does not protect from UV radiation and may even play a role in UV-dependent skin damage (Thody et al., 1991); eumelanin is related with darker skin tones, while more phaeomelanin is found in individuals with lighter skin tones (Nasti and Timares, 2015). Activation of MC1R by a-MSH promotes the switch from phaeomelanin to eumelanin, through activation of the cAMP pathway. Central to this is the microphthalmia-associated transcription factor gene (Mitf) (Vance and Goding, 2004), a transcription factor playing an important role in melanocyte survival (Opdecamp et al., 1997). Five different isoforms of Mitf have been identified, amongst which M-MIFT has been shown to be melanocytes-specific (Shibahara et al., 2001; Fuse et al., 1996). Transcription of M-MITF is CREB dependent, and thus responsive to MC1R activation by a-MSH, thanks to the presence of a CREBresponsive element (CRE) sequence in its promoter (Price et al., 1998; Bertolotto et al., 1998). Other factors such as Sox10 have also been shown to be necessary for M-MITF transcription (Huber et al., 2003; Shibahara et al., 2001) possibly being responsible for restricting its transcription to melanocytes. Transcription of M-MITF is key to melanogenesis, since this transcription factor promotes transcription of tyrosinase (TYR) and tyrosinase-related-protein 1 and 2 (TRP-1 and TRP-2), enzymes involved in the formation of eumelanin. Because of this, loss-of-function mutations of MC1R are associated with red/yellow coloring, and conversely, activating mutations produce darker coats in many species (Va˚ge et al., 1997; Marklund et al., 1996; Klungland et al., 1995; Robbins et al., 1993). In humans, several variants of the MC1R have been associated with red hair, fair skin, and poor tanning responses (Valverde et al., 1995); some of these correspond to loss-of-function mutations that are not able to stimulate cAMP production, while others are able to generate increases in cAMP

levels, but show impaired membrane translocation of MC1R (Newton et al., 2007, 2005; Beaumont et al., 2005). As for the other MCRs, the activity of MC1R has been shown to be modulated by binding to MRAP2. Coexpression of MRAP decreases the a-MSH mediated increase in cAMP, particularly in the presence of both MRAP1 and 2, without affecting membrane translocation of MC1R (Chan et al., 2009). While the specific molecular interactions between MRAP and MC1R remain to be clarified, some molecular models for the interaction between a-MSH and MC1R have been proposed. Because of the technical issues of GPCR crystallization, these predictions are based on homology modeling with the “archetypal” GPCR rhodopsin and validated through site-directed mutagenesis. One model shows that the ligand-binding pocket is located between TM1, -2, and -7, with Glu94 and Asp294 playing an important role in ligand binding (Prusis et al., 1997). In agreement with this, mutating Glu94 to a basic residue results in strongly impaired ligand binding (Prusis et al., 1997); furthermore the Asp294His is a common SNP associated with red hair and fair skin tone (Valverde et al., 1995). The importance of Glu94 has also been highlighted by a second molecular model, which also proposes an important role for Asp117 and Asp121 on TM-3 in forming a hydrophilic pocket that binds to Arg8 of a-MSH (Yang et al., 1997; Haskell-Luevano et al., 1996). The role of Asp117 in binding a-MSH has been debated (Prusis et al., 1997), although data indicating the opposite is focused on its interaction with His6 rather than Arg8 (Schio¨th et al., 1997). Aromatic residues in TM7 are also thought to form a hydrophobic pocket that is able to generate aromatic interactions with Phe7 and Trp9 of a-MSH, although these residues appear less important for ligand binding and receptor activation (Yang et al., 1997; Haskell-Luevano et al., 1996). Finally, sitedirected mutagenesis experiments have revealed a possible ligand-binding role for residues in the extracellular loops of MC1R (Chhajlani et al., 1996).

5.3 MC5R Independently cloned by several groups (Labbe´ et al., 1994; Griffon et al., 1994; Gantz et al., 1994; Barrett et al., 1994; Chhajlani and Wikberg, 1992), the fifth melanocortin receptor, MC5R, is important for exocrine gland function. Expression of MC5R is high in exocrine and endocrine peripheral tissue including sebaceous, lacrimal, and preputial glands, as well as prostate, adrenal, pancreas, uterus, and skin (Thiboutot et al., 2000; Chen et al., 1997; Labbe´ et al., 1994). This is consistent with the results of binding studies showing melanocortin binding sites on these sites (Tatro and Reichlin, 1987) and with some of the early discovered physiologic

7. DISEASE AND AGING EFFECTS

effects of a-MSH, such as the promotion of secretion from lacrimal, preputial, and sebaceous glands (Cripps et al., 1987; Jahn et al., 1982; Thody and Shuster, 1973, 1975; Ebling et al., 1975). Indeed, studies in MC5R knockout mice showed impaired sebum production, resulting in defects in water repulsion and defective thermoregulation (Chen et al., 1997). In mice, MC5R has also been implicated in production of pheromones such as a- and b-farnesene from the preputial gland in mice, resulting in decreased aggressive behavior in MC5R knockout mice (Morgan and Cone, 2006; Morgan et al., 2004). As for the other MCRs, MC5R can also interact with MRAP; opposite to their effect on MC2R, expression of either MRAP1 or MRAP2 results in reduced MC5R surface expression and diminished cAMP responses to a-MSH stimulation (Sebag and Hinkle, 2009; Chan et al., 2009).

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C-terminal is important for the stability of this hormone (Imura et al., 1967). However, studies in rats and dogs on degradation of both endogenous and exogenously administered ACTH have showed remarkably short (3e5 min) half-lives for this hormone (Matsuyama et al., 1972; Everson et al., 1969), suggesting that degradation happens somewhere else than in plasma. Indeed kidney, liver, and muscle seem to be the major locations involved in the metabolism of ACTH (Baker et al., 1976; Cats and Kassenaar, 1957). ACTH is then cleaved into smaller inactive fragments that could be then secreted in the plasma (Matsuyama et al., 1971) before getting metabolized into their constituent amino acids. Degradation of a-MSH is also rapid and thought to occur through the action of the lysosomal enzyme prolylcarboxypeptidase (PRCP). This cleaves 1-13a-MSH to the inactive 1-12a-MSH (Wallingford et al., 2009). Indeed, mice lacking PRCP show a lean phenotype and are resistant to diet-induced obesity (Wallingford et al., 2009).

5.4 MC3R and MC4R The melanocortin 3 and 4 receptors play an important role in regulation of feeding and energy homoeostasis, as demonstrated by the phenotypes of mice where these receptors have been deleted. Indeed, MC4R null mice are hyperphagic, hyperglycemic, and obese (Huszar et al., 1997), while MC3R null mice are not obese, but present with a strong metabolic phenotype, with an increase in adipose mass associated with reduced energy expenditure (Huszar et al., 1997). Expression of MC3R is mostly restricted to several hypothalamic nuclei (Roselli-Rehfuss et al., 1993; Gantz et al., 1993) where this receptor plays an important part for energy homeostasis. MC4R is expressed throughout the CNS, where it mediates its effects on food intake (Mountjoy et al., 1994; Gantz et al., 1994), and more recently, it has been shown to have a possible role in skin pigmentation (Spencer and Schallreuter, 2009). Because the effects mediated by these receptors depend for the major part on POMC and a-MSH produced in the brain rather than in the pituitary gland, we will refer the reader to some of the many extensive reviews written about their role in the hypothalamic control of feeding (Anderson et al., 2016; Mountjoy, 2015, 2010).

6. HORMONE INACTIVATION In vitro experiments showed that ACTH is stable in fresh plasma for at least 4 h at room temperature (Imura et al., 1967). Progressive removal of the C-terminal part of ACTH results in quicker loss of activity in fresh plasma, with 1-18ACTH losing most of its activity only after 1 h in fresh plasma, supporting the idea that the

7. DISEASE AND AGING EFFECTS Given the large spectrum of physiologic effects mediated by POMC and its products, it is unsurprising that its impaired or excessive production will result in important pathologic conditions. This last section will give an overview of the main conditions associated with over- or underproduction of POMC.

7.1 Addison DiseasedAdrenal Insufficiency Adrenal insufficiency (Addison disease) is the clinical manifestation of reduced glucocorticoid production (hypocortisolism), or action of glucocorticoid hormones, that may result from dysfunction at three different levels of the HPA axis (Charmandari et al., 2014; Bornstein, 2009). Primary adrenal insufficiency is a result of disease or failure intrinsic to the adrenal cortex (primary adrenal failure), in which glucocorticoid deficiency is also typically associated with loss of other adrenal steroids. In contrast, central adrenal insufficiency is a result of dysfunction at either the level of the pituitary, ultimately resulting in reduced release of ACTH (secondary adrenal insufficiency), or hypothalamus as a consequence of reduced CRH and/or AVP secretion (tertiary adrenal insufficiency). Secondary adrenal insufficiency is the most common form in adults (estimated at around 200 per million), with a higher prevalence in women than men. ACTH deficiency can result from both genetic disorders as well as a wide range of insults to the pituitary including pituitary tumors, infections such as meningitis and tuberculosis, surgery, and lymphocytic hypophysitis,

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often as a result of autoimmunity (Andrioli et al., 2006). Thus, secondary adrenal insufficiency may be associated with other pituitary hormone defects as well as being isolated. Indeed, loss of function of a range TFs that are important for normal pan pituitary development such as HESX homeobox 1 (HESX1), LIM homeobox 4 (LIM 4), orthodenticle homeobox 2 (OTX2), and SRY (sex-determining region Y)-box 3 (SOX3) are associated with early development deficits and hypopituitarism. PROP paired homeobox 1 (PROP1) is implicated in late-onset ACTH deficiency, and more than 60% of patients with PradereWilli syndrome display secondary adrenal insufficiency. Isolated ACTH deficiency can result from either loss of function of the POMC gene (Yaswen et al., 1999; Krude et al., 1998), mutations in the proprotein convertase 1 (PC1) that disrupts POMC peptide processing (Jackson et al., 1997; Nussey et al., 1993), or a variety of loss-offunction mutations in the transcription factor T-PIT (TBX19 in humans) (Couture et al., 2012; Vallette-Kasic et al., 2005) that controls corticotroph specification. In humans, several mutations in POMC that result in a loss of POMC transcription and/or translation also are associated with early onset obesity, hypopigmentation, and red hair, a phenotype also seen in mice with a genetic deletion of the POMC gene (Mendiratta et al., 2011; Farooqi et al., 2006; Krude et al., 1998, 2003; Yaswen et al., 1999). Genetic screening of early onset obesity patients has revealed several mutations associated with impaired processing of the POMC precursor. For example, mutations affecting the N-terminal region result in impaired processing and sorting of the precursor peptide, since this region contains the ER signaling sequence (Creemers et al., 2008). Mutations at POMC processing site can result in improper production of POMC-related peptides; for instance, mutations at the Arg236Gly site have been described as preventing the cleavage of b-MSH and b-endorphin, resulting in a fusion peptide that can bind with high affinity to MC4R, but only minimally activating it (Challis et al., 2002). Tertiary adrenal insufficiency is a result of reduced secretion of CRH and/or AVP as a consequence of hypothalamic dysfunction. The most common causes are in patients with long-term glucocorticoid treatment or patients recovering from Cushing syndrome, in which glucocorticoid negative feedback at the hypothalamic level suppresses CRH/AVP secretion and hence reduced pituitary corticotroph drive (Broersen et al., 2015; Krasner, 1999; Gomez et al., 1993). The majority (80%e90%) of patients with primary adrenal insufficiency result from autoimmune adrenalitis that results in destruction of the adrenal cortex (Charmandari et al., 2014; Bornstein, 2009). Indeed, autoantibodies against several enzymes required for steroid

biosynthesis have been detected in patients, with antibodies against 21-hydroxylase being most prevalent in patients with idiopathic adrenal insufficiency. Other insults to the adrenal including infections, surgery, and drug-induced adrenal insufficiency also contribute to primary adrenal insufficiency. Furthermore, a broad spectrum of genetic mutations are included in a variety of enzymes required for steroid biosynthesis. Deficiency of 21-hyroxylase (CYP21A2) is a classic and common mutation (Charmandari et al., 2014; Bornstein, 2009), especially in children. Importantly, as discussed earlier, mutations in MC2R or MRAP also lead to loss of adrenal sensitivity to ACTH, leading to familial glucocorticoid deficiency and ACTH insensitivity syndromes (Charmandari et al., 2014; Bornstein, 2009). Glucocorticoid replacement is the treatment of choice for these conditions, although dosage may vary depending on the severity of the adrenal insufficiency, and replacement may not even be required for milder cases (Guo et al., 2013; Ouleghzal et al., 2012; Andrioli et al., 2006). Mineralocorticoid production is generally not affected in these patients, so replacement is not required (Andrioli et al., 2006).

7.2 Cushing Disease Cushing syndrome is a collection of disorders manifest as hypercortisolism that can arise through multiple ACTH-dependent or ACTH-independent mechanisms. The resultant chronic exposure of tissues to glucocorticoids leads to a range of disorders from obesity and diabetes to hypertension and osteoporosis. The most common form (>70% of cases) of endogenous Cushing syndrome results from ACTH-secreting pituitary adenomas (called Cushing disease) with ectopic (nonpituitary) ACTH secretion (such as ACTH-secreting lung tumors) being less common (Sbiera et al., 2015; Raff and Carroll, 2015). In both cases, excessive ACTH stimulates adrenal MC2R receptors, leading to excessive glucocorticoid release and adrenal hyperplasia. Transphenoidal surgery to remove the adenoma is the treatment of choice with pharmacological interventions to either inhibit pituitary ACTH release, glucocorticoid synthesis, or antagonize GRs in patients with inoperable tumors or following relapse. A major hallmark of Cushing disease is a relative resistance to glucocorticoid-negative feedback inhibition of ACTH release arising from a higher than normal set point for effective glucocorticoid inhibition. Two major cellular mechanisms are thus implicated in Cushing disease. Firstly, ACTH-secreting pituitary corticotroph adenomas result predominantly from monoclonal expansion of corticotrophs with genetic mutations (Zhou et al., 2014; Gicquel et al., 1992; Biller

8. CONCLUSIONS AND FUTURE DIRECTIONS

et al., 1992; Schulte et al., 1991). These adenomas are typically associated with increased expression of CRHR1 and AVPR1b as well as cell cycle regulators (such as epidermal growth factor receptor and cyclin E) and tumor suppressors, including bone morphogenic protein-4 (BMP-4) and suppressor of cytokine signaling 1 (SOCS1) as well as nuclear TFs such as testicular orphan receptor 4 (TR4) (Sbiera et al., 2015; Zhou et al., 2014). In approximately 60% of adenomas from patients with Cushing disease, whole-exome sequencing has identified somatic mutations in the deubiquitinase gene USP8 (Reincke et al., 2015; PerezRivas et al., 2015; Ma et al., 2015). An important target for USP8-dependent deubiquitination is the EGFR receptor, and thus USP8 protects the degradation of the EGFR receptor by the lysosomal pathway. The activity of USP8 is normally kept in check by binding to the adapter protein 14-3-3. Identified gain-of-function mutations in USP8 disrupt the interaction of USP8 with 14-3-3, resulting in an increase in constitutive USP8 deubiquitination activity. This, in turn, increases plasma membrane EGFR expression in adenomas, resulting in increased activation of the MAPK signaling pathway that enhances POMC transcription and ACTH synthesis. Thus, knockdown of USP8 or pharmacological inhibition of EGFR activity results in a suppression of ACTH release and tumor size, suggesting potential new therapeutic avenues (Sbiera et al., 2015; Reincke et al., 2015; Perez-Rivas et al., 2015; Ma et al., 2015; Fukuoka et al., 2011). Secondly, the levels of ACTH in the circulating plasma of patients with Cushing syndrome are inappropriate for the elevated levels of circulating glucocorticoid. Although not abolished, glucocorticoid negative feedback is significantly attenuated in Cushing’ disease. The mechanisms of reduced glucocorticoid feedback are poorly understood; however as discussed earlier, dysregulation of glucocorticoidmediated transrepression of POMC transcription due to a loss of nuclear HDAC2 and BRG1 is also likely involved (Drouin, 2016; Bilodeau et al., 2006).

7.3 Familial Glucocorticoid Deficiency Several mutations in MC2R have been associated with familial glucocorticoid deficiency (FGD), a rare autosomal recessive disorder (Clark et al., 1993) in which the cells of the zona fasciculata of the adrenal gland fail to produce cortisol in response to ACTH. This is characterized by hypocortisolism in the presence of high ACTH, which results in hypoglycemic symptoms in early life, accompanied by hyperpigmentation of the skin due to activation of MC1R in the skin (Tsigos, 1999; Weber and Clark, 1994). Furthermore, the

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high levels of ACTH have been suggested to stimulate bone growth, probably through activation of MC3R, explaining the tall stature often associated with FGD (Imamine et al., 2005; Elias et al., 2000). FGD, however, is a heterogeneous disease, and mutations in MC2R are not the only cause of FGD, accounting for approximately 25% of cases, designated as type I FGD (Ge´nin et al., 2002; Naville et al., 1996; Weber and Clark, 1994). Genetic mapping of FGD patients not showing MC2R mutations have identified two other loci linked to this disease, on chromosome 21q22.1 (Metherell et al., 2005) and 8q12.1 to 8q21.2 (Ge´nin et al., 2002). The first have shown to depend on mutations of MRAP2 (Metherell et al., 2005) and have been designated FGD type 2. Mutations in STAR (which have usually been associated with nonclassic lipoid congenital adrenal hyperplasia) have been shown to be the underlying cause of FGD in the second group of patients, identified as type 3 FGD (Metherell et al., 2009). Overall, these account for just over 50% of FGD cases (Dias et al., 2010), and other genetic causes of FGD are presently unknown.

7.4 Changes with Aging As for most pituitary functions, the activity of corticotrophs and melanotrophs is also changed by aging (Veldhuis, 2013). ACTH secretion following CRH injection is amplified in the elderly, in a gender-dependent manner (Born et al., 1995). However, ACTH secretion was higher in young compared to old men (but not women) in response to social stress (Kudielka et al., 2004). Furthermore, a decrease in the amplitude of circadian variations in ACTH and glucocorticoid secretion has been reported in older individuals (Sherman et al., 1985). Rats display an age-dependent increase in the size of the PI; however, they do not display major changes in plasma a-MSH levels. The precise molecular mechanisms underlying these changes, and the cellular and molecular properties of corticotrophs and melanotrophs during aging, however, still remain to be determined.

8. CONCLUSIONS AND FUTURE DIRECTIONS This chapter has highlighted the main features of the two different cell populations of the pituitary gland producing POMC and its proteolytic products. Corticotrophs in the anterior pituitary gland, central to the endocrine stress response, and melanotrophs of the PI, important for the control of pigmentation.

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The control of these cell populations has been widely studied and modeled both at the cell and system levels, in a variety of cellular and animal model systems, including humans. Still, many questions remain open. Several anatomic and functional observations over the years have supported the idea that both these cell types are highly heterogeneous, but the cause and functional role of this heterogeneity still remains to be clarified. Furthermore, much work on these systems has been performed in vitro on cell line or primary dissociated pituitary cells; while these studies have been instrumental for understanding the molecular function of corticotrophs and melanotrophs, an always growing literature supports the idea of a functional role for the 3D structure of pituitary cells. Exploration of the functionality of corticotrophs and melanotrophs at the single cell level and in their native anatomic context (e.g., in slices or in vivo) has not been attempted to date. This as well as studies targeted to get a better understanding of the interactions between the different cell types of the pituitary gland are much needed. Secondly, a better understanding of how timevarying hypothalamic signals (e.g., CRH and AVP) are interpreted by corticotrophs, how these inputs vary, and how they affect glucocorticoid output during the course of different type of stressors or different physiologic situations will help developing better systemwide models of the HPA axis. This is of particular clinical relevance, since glucocorticoids are one of the most widely prescribed drugs for a variety of clinical conditions, and approximately 1% of the general population receives long-term glucocorticoid treatment (Overman et al., 2013; Fardet et al., 2011; van Staa et al., 2000). Although some treatment regimens with glucocorticoids try to mimic the physiologic circadian variations in the level of these hormones (Oksnes et al., 2014; Johannsson et al., 2012; Verma et al., 2010), they fail to recapitulate their physiologic ultradian release, which has been shown to be important for their function, resulting in important undesired side effects. Proof-of-concept studies using a subcutaneous delivery system show that it is possible to replicate near-physiologic levels of glucocorticoids, to produce pulsatile administration that could improve clinical outcomes (Russell and Lightman, 2014; Russell et al., 2014). Thus, understanding the mechanisms that control ACTH and glucocorticoid pulsatility may in the future inform clinicians, with the hope of developing better treatment for stress-related pathologies as well as improving the use of glucocorticoids in the clinic.

References Abdel-Malek, Z., Suzuki, I., Tada, A., Im, S., Akcali, C., 1999. The melanocortin-1 receptor and human pigmentation. Ann. N. Y. Acad. Sci. 885, 117e133. Abdel-Malek, Z.A., Swope, V.B., Starner, R.J., Koikov, L., Cassidy, P., Leachman, S., 2014. Melanocortins and the melanocortin 1 receptor, moving translationally towards melanoma prevention. Arch. Biochem. Biophys. 563, 4e12. https://doi.org/10.1016/ j.abb.2014.07.002. Anderson, E.J.P., C ¸ akir, I., Carrington, S.J., Cone, R.D., GhamariLangroudi, M., Gillyard, T., Gimenez, L.E., Litt, M.J., 2016. 60 YEARS OF POMC: regulation of feeding and energy homeostasis by a-MSH. J. Mol. Endocrinol. 56, T157eT174. https://doi.org/ 10.1530/JME-16-0014. Andrioli, M., Pecori Giraldi, F., Cavagnini, F., 2006. Isolated corticotrophin deficiency. Pituitary 9, 289e295. https://doi.org/10.1007/ s11102-006-0408-5. Antakly, T., Sasaki, A., Liotta, A.S., Palkovits, M., Krieger, D.T., 1985. Induced expression of the glucocorticoid receptor in the rat intermediate pituitary lobe. Science 229, 277e279. Antoni, F.A., 1993. Vasopressinergic control of pituitary adrenocorticotropin secretion comes of age. Front. Neuroendocrinol. 14, 76e122. https://doi.org/10.1006/frne.1993.1004. Antoni, F.A., Hoyland, J., Woods, M.D., Mason, W.T., 1992. Glucocorticoid inhibition of stimulus-evoked adrenocorticotrophin release caused by suppression of intracellular calcium signals. J. Endocrinol. 133, R13eR16. Antoni, F.A., Sosunov, A.A., Haunso, A., Paterson, J.M., Simpson, J., 2003. Short-term plasticity of cyclic adenosine 30 ,50 -monophosphate signaling in anterior pituitary corticotrope cells: the role of adenylyl cyclase isotypes. Mol. Endocrinol. 17, 692e703. https://doi.org/ 10.1210/me.2002-0369. Autelitano, D.J., Snyder, L., Sealfon, S.C., Roberts, J.L., 1989. Dopamine D2-receptor messenger RNA is differentially regulated by dopaminergic agents in rat anterior and neurointermediate pituitary. Mol. Cell. Endocrinol. 67, 101e105. Baker, J.R., Bennett, H.P., Hudson, A.M., McMartin, C., Purdon, G.E., 1976. On the metabolism of two adrenocorticotrophin analogues. Clin. Endocrinol. 5 (Suppl. l), 61Se72S. Barrett, P., MacDonald, A., Helliwell, R., Davidson, G., Morgan, P., 1994. Cloning and expression of a new member of the melanocytestimulating hormone receptor family. J. Mol. Endocrinol. 12, 203e213. Baumann, J.B., Eberle, A.N., Christen, E., Ruch, W., Girard, J., 1986. Steroidogenic activity of highly potent melanotropic peptides in the adrenal cortex of the rat. Acta Endocrinol. 113, 396e402. Beaumont, K.A., Newton, R.A., Smit, D.J., Leonard, J.H., Stow, J.L., Sturm, R.A., 2005. Altered cell surface expression of human MC1R variant receptor alleles associated with red hair and skin cancer risk. Hum. Mol. Genet. 14, 2145e2154. https://doi.org/ 10.1093/hmg/ddi219. Benjannet, S., Rondeau, N., Day, R., Chre´tien, M., Seidah, N.G., 1991. PC1 and PC2 are proprotein convertases capable of cleaving proopiomelanocortin at distinct pairs of basic residues. Proc. Natl. Acad. Sci. U. S. A. 88, 3564e3568. Bertolotto, C., Abbe, P., Hemesath, T.J., Bille, K., Fisher, D.E., Ortonne, J.P., Ballotti, R., 1998. Microphthalmia gene product as a signal transducer in cAMP-induced differentiation of melanocytes. J. Cell Biol. 142, 827e835. Biller, B.M., Alexander, J.M., Zervas, N.T., Hedley-Whyte, E.T., Arnold, A., Klibanski, A., 1992. Clonal origins of adrenocorticotropin-secreting pituitary tissue in Cushing’s disease. J. Clin. Endocrinol. Metab. 75, 1303e1309. https:// doi.org/10.1210/jcem.75.5.1358909.

REFERENCES

Bilodeau, S., Vallette-Kasic, S., Gauthier, Y., Figarella-Branger, D., Brue, T., Berthelet, F., Lacroix, A., Batista, D., Stratakis, C., Hanson, J., Meij, B., Drouin, J., 2006. Role of Brg1 and HDAC2 in GR trans-repression of the pituitary POMC gene and misexpression in Cushing disease. Genes Dev. 20, 2871e2886. https://doi.org/ 10.1101/gad.1444606. Birch, N.P., Tracer, H.L., Hakes, D.J., Loh, Y.P., 1991. Coordinate regulation of mRNA levels of pro-opiomelanocortin and the candidate processing enzymes PC2 and PC3, but not furin, in rat pituitary intermediate lobe. Biochem. Biophys. Res. Commun. 179, 1311e1319. https://doi.org/10.1016/0006-291X(91)91716-P. Bonnefont, X., Lacampagne, A., Sanchez-Hormigo, A., Fino, E., Creff, A., Mathieu, M.-N., Smallwood, S., Carmignac, D., Fontanaud, P., Travo, P., Alonso, G., Courtois-Coutry, N., Pincus, S.M., Robinson, I.C.A.F., Mollard, P., 2005. Revealing the large-scale network organization of growth hormone-secreting cells. Proc. Natl. Acad. Sci. U. S. A. 102, 16880e16885. https:// doi.org/10.1073/pnas.0508202102. Born, J., Ditschuneit, I., Schreiber, M., Dodt, C., Fehm, H.L., 1995. Effects of age and gender on pituitary-adrenocortical responsiveness in humans. Eur. J. Endocrinol. 132, 705e711. Bornstein, S.R., 2009. Predisposing factors for adrenal insufficiency. N. Engl. J. Med. 360, 2328e2339. https://doi.org/10.1056/ NEJMra0804635. Boston, B.A., 2006. The role of melanocortins in adipocyte function. Ann. N. Y. Acad. Sci. 885, 75e84. https://doi.org/10.1111/j.17496632.1999.tb08666.x. Broersen, L.H.A., Pereira, A.M., Jørgensen, J.O.L., Dekkers, O.M., 2015. Adrenal insufficiency in corticosteroids use: systematic review and meta-analysis. J. Clin. Endocrinol. Metab. 100, 2171e2180. https:// doi.org/10.1210/jc.2015-1218. Budry, L., Balsalobre, A., Gauthier, Y., Khetchoumian, K., L’honore´, A., Vallette, S., Brue, T., Figarella-Branger, D., Meij, B., Drouin, J., 2012. The selector gene Pax7 dictates alternate pituitary cell fates through its pioneer action on chromatin remodeling. Genes Dev. 26, 2299e2310. https://doi.org/10.1101/gad.200436.112. Budry, L., Lafont, C., El Yandouzi, T., Chauvet, N., Cone´jero, G., Drouin, J., Mollard, P., 2011. Related pituitary cell lineages develop into interdigitated 3D cell networks. Proc. Natl. Acad. Sci. U. S. A. 108, 12515e12520. https://doi.org/10.1073/pnas.1105929108. Burlet, A., Tonon, M.C., Tankosic, P., Coy, D., Vaudry, H., 1983. Comparative immunocytochemical localization of corticotropin releasing factor (CRF-41) and neurohypophysial peptides in the brain of Brattleboro and Long-Evans rats. Neuroendocrinology 37, 64e72. https://doi.org/10.1159/000123517. Canny, B.J., Jia, L.G., Leong, D.A., 1992. Corticotropin-releasing factor, but not arginine vasopressin, stimulates concentration-dependent increases in ACTH secretion from a single corticotrope. Implications for intracellular signals in stimulus-secretion coupling. J. Biol. Chem. 267, 8325e8329. Cats, A., Kassenaar, A.A., 1957. Influence of the kidney on the disappearance rate of labelled corticotrophin from the blood stream. Acta Endocrinol. 24, 43e49. Challis, B.G., Pritchard, L.E., Creemers, J.W.M., Delplanque, J., Keogh, J.M., Luan, J., Wareham, N.J., Yeo, G.S.H., Bhattacharyya, S., Froguel, P., White, A., Farooqi, I.S., O’Rahilly, S., 2002. A missense mutation disrupting a dibasic prohormone processing site in pro-opiomelanocortin (POMC) increases susceptibility to early-onset obesity through a novel molecular mechanism. Hum. Mol. Genet. 11, 1997e2004. Chan, L.F., Webb, T.R., Chung, T.-T., Meimaridou, E., Cooray, S.N., Guasti, L., Chapple, J.P., Egertova´, M., Elphick, M.R., Cheetham, M.E., Metherell, L.A., Clark, A.J.L., 2009. MRAP and MRAP2 are bidirectional regulators of the melanocortin receptor family. Proc. Natl. Acad. Sci. U. S. A. 106, 6146e6151. https:// doi.org/10.1073/pnas.0809918106.

161

Charmandari, E., Nicolaides, N.C., Chrousos, G.P., 2014. Adrenal insufficiency. Lancet 383, 2152e2167. https://doi.org/10.1016/ S0140-6736(13)61684-0. Chen, W., Kelly, M.A., Opitz-Araya, X., Thomas, R.E., Low, M.J., Cone, R.D., 1997. Exocrine gland dysfunction in MC5-R-deficient mice: evidence for coordinated regulation of exocrine gland function by melanocortin peptides. Cell 91, 789e798. Chhajlani, V., Wikberg, J.E., 1992. Molecular cloning and expression of the human melanocyte stimulating hormone receptor cDNA. FEBS Lett. 309, 417e420. Chhajlani, V., Xu, X., Blauw, J., Sudarshi, S., 1996. Identification of ligand binding residues in extracellular loops of the melanocortin 1 receptor. Biochem. Biophys. Res. Commun. 219, 521e525. https://doi.org/10.1006/bbrc.1996.0266. Chronwall, B.M., Millington, W.R., Griffin, W.S., Unnerstall, J.R., O’Donohue, T.L., 1987. Histological evaluation of the dopaminergic regulation of proopiomelanocortin gene expression in the intermediate lobe of the rat pituitary, involving in situ hybridization and [3H]thymidine uptake measurement. Endocrinology 120, 1201e1211. https://doi.org/10.1210/endo-120-3-1201. Clark, A.J.L., Grossman, A., McLoughlin, L., 1993. Familial glucocorticoid deficiency associated with point mutation in the adrenocorticotropin receptor. Lancet 341 (8843), 461e462. https://doi.org/ 10.1016/0140-6736(93)90208-X. Originally published as vol. 1. Cool, D.R., Fenger, M., Snell, C.R., Loh, Y.P., 1995. Identification of the sorting signal motif within pro-opiomelanocortin for the regulated secretory pathway. J. Biol. Chem. 270, 8723e8729. https://doi.org/ 10.1074/jbc.270.15.8723. Cool, D.R., Normant, E., Shen, F., Chen, H.-C., Pannell, L., Zhang, Y., Loh, Y.P., 1997. Carboxypeptidase E is a regulated secretory pathway sorting receptor: genetic obliteration leads to endocrine disorders in Cpefat mice. Cell 88, 73e83. https://doi.org/ 10.1016/S0092-8674(00)81860-7. Cool, D.R., Peng, L., 1994. Identification of a sorting signal for the regulated secretory pathway at the N-terminus of proopiomelanocortin. Biochimie 76, 265e270. https://doi.org/ 10.1016/0300-9084(94)90156-2. Cooray, S.N., Chung, T.-T., Mazhar, K., Szidonya, L., Clark, A.J.L., 2011. Bioluminescence resonance energy transfer reveals the adrenocorticotropin (ACTH)-Induced conformational change of the activated ACTH receptor complex in living cells. Endocrinology 152, 495e502. https://doi.org/10.1210/en.2010-1053. Costa, J.L., Bui, S., Reed, P., Dores, R.M., Brennan, M.B., Hochgeschwender, U., 2004. Mutational analysis of evolutionarily conserved ACTH residues. Gen. Comp. Endocrinol. 136, 12e16. https://doi.org/10.1016/j.ygcen.2003.11.005. Cote, T.E., Felder, R., Kebabian, J.W., Sekura, R.D., Reisine, T., Affolter, H.U., 1986. D-2 dopamine receptor-mediated inhibition of pro-opiomelanocortin synthesis in rat intermediate lobe. Abolition by pertussis toxin or activators of adenylate cyclase. J. Biol. Chem. 261, 4555e4561. Couture, C., Saveanu, A., Barlier, A., Carel, J.C., Fassnacht, M., Flu¨ck, C.E., Houang, M., Maes, M., Phan-Hug, F., Enjalbert, A., Drouin, J., Brue, T., Vallette, S., 2012. Phenotypic homogeneity and genotypic variability in a large series of congenital isolated ACTHdeficiency patients with TPIT gene mutations. J. Clin. Endocrinol. Metab. 97, E486eE495. https://doi.org/10.1210/jc.2011-1659. Creemers, J.W.M., Lee, Y.S., Oliver, R.L., Bahceci, M., Tuzcu, A., Gokalp, D., Keogh, J., Herber, S., White, A., O’Rahilly, S., Farooqi, I.S., 2008. Mutations in the amino-terminal region of proopiomelanocortin (POMC) in patients with early-onset obesity impair POMC sorting to the regulated secretory pathway. J. Clin. Endocrinol. Metab. 93, 4494e4499. https://doi.org/10.1210/jc.2008-0954. Cripps, M.M., Bromberg, B.B., Patchen-Moor, K., Welch, M.H., 1987. Adrenocorticotropic hormone stimulation of lacrimal peroxidase secretion. Exp. Eye Res. 45, 673e682.

162

8. ANTERIOR PITUITARY AND PARS INTERMEDIA SPACE: CORTICOTROPHS (ACTH) AND MELANOTROPHS (a-MSH)

Day, R., Schafer, M.K., Watson, S.J., Chre´tien, M., Seidah, N.G., 1992. Distribution and regulation of the prohormone convertases PC1 and PC2 in the rat pituitary. Mol. Endocrinol. 6, 485e497. https://doi.org/10.1210/mend.6.3.1316544. Dayanithi, G., Antoni, F.A., 1989. Rapid as well as delayed inhibitory effects of glucocorticoid hormones on pituitary adrenocorticotropic hormone release are mediated by type II glucocorticoid receptors and require newly synthesized messenger ribonucleic acid as well as protein. Endocrinology 125, 308e313. https://doi.org/ 10.1210/endo-125-1-308. de Rijk, E.P., van Strien, F.J., Roubos, E.W., 1992. Demonstration of coexisting catecholamine (dopamine), amino acid (GABA), and peptide (NPY) involved in inhibition of melanotrope cell activity in Xenopus laevis: a quantitative ultrastructural, freeze-substitution immunocytochemical study. J. Neurosci. 12, 864e871. De Souza, E.B., Perrin, M.H., Rivier, J., Vale, W.W., Kuhar, M.J., 1984. Corticotropin-releasing factor receptors in rat pituitary gland: autoradiographic localization. Brain Res. 296, 202e207. de Souza, F.S.J., Santangelo, A.M., Bumaschny, V., Avale, M.E., Smart, J.L., Low, M.J., Rubinstein, M., 2005. Identification of neuronal enhancers of the proopiomelanocortin gene by transgenic mouse analysis and phylogenetic footprinting. Mol. Cell. Biol. 25, 3076e3086. https://doi.org/10.1128/MCB.25.8.3076-3086.2005. Dedic, N., Chen, A., Deussing, J.M., 2018. The CRF family of neuropeptides and their receptorsemediators of the central stress response. Curr. Mol. Pharmacol. 11, 4e31. https://doi.org/10.2174/ 1874467210666170302104053. Demeneix, B.A., Desaulles, E., Feltz, P., Loeffler, J.P., 1984. Dual population of GABAA and GABAB receptors in rat pars intermedia demonstrated by release of alpha MSH caused by barium ions. Br. J. Pharmacol. 82, 183e190. Demeneix, B.A., Taleb, O., Loeffler, J.P., Feltz, P., 1986. GABAA and GABAB receptors on porcine pars intermedia cells in primary culture: functional role in modulating peptide release. Neuroscience 17, 1275e1285. Deng, Q., Riquelme, D., Trinh, L., Low, M.J., Tomic, M., Stojilkovic, S., Aguilera, G., 2015. Rapid glucocorticoid feedback inhibition of ACTH secretion involves ligand-dependent membrane association of glucocorticoid receptors. Endocrinology 156, 3215e3227. https://doi.org/10.1210/EN.2015-1265. Desrues, L., Castel, H., Malagon, M.M., Vaudry, H., Tonon, M.-C., 2005. The regulation of alpha-MSH release by GABA is mediated by a chloride-dependent [Ca2þ]c increase in frog melanotrope cells. Peptides 26, 1936e1943. https://doi.org/10.1016/ j.peptides.2004.11.022. Desrues, L., Vaudry, H., Lamacz, M., Tonon, M.C., 1995. Mechanism of action of gamma-aminobutyric acid on frog melanotrophs. J. Mol. Endocrinol. 14, 1e12. Dias, R.P., Chan, L.F., Metherell, L.A., Pearce, S.H.S., Clark, A.J.L., 2010. Isolated Addison’s disease is unlikely to be caused by mutations in MC2R, MRAP or STAR, three genes responsible for familial glucocorticoid deficiency. Eur. J. Endocrinol. 162, 357e359. https:// doi.org/10.1530/EJE-09-0720. Dores, R.M., Baron, A.J., 2011. Evolution of POMC: origin, phylogeny, posttranslational processing, and the melanocortins. Ann. N. Y. Acad. Sci. 1220, 34e48. https://doi.org/10.1111/j.17496632.2010.05928.x. Douglas, W.W., Taraskevich, P.S., 1978. Action potentials in gland cells of rat pituitary pars intermedia: inhibition by dopamine, an inhibitor of MSH secretion. J. Physiol. 285, 171e184. Drouin, J., 2016. 60 YEARS OF POMC: transcriptional and epigenetic regulation of POMC gene expression. J. Mol. Endocrinol. 56, T99eT112. https://doi.org/10.1530/JME-15-0289. Duncan, P.J., Sengu¨l, S., Tabak, J., Ruth, P., Bertram, R., Shipston, M.J., 2015. Large conductance Ca2þ-activated Kþ (BK) channels promote secretagogue-induced transition from spiking to bursting in murine

anterior pituitary corticotrophs. J. Physiol. 593, 1197e1211. https:// doi.org/10.1113/jphysiol.2015.284471. Duncan, P.J., Tabak, J., Ruth, P., Bertram, R., Shipston, M.J., 2016. Glucocorticoids inhibit CRH/AVP-evoked bursting activity of male murine anterior pituitary corticotrophs. Endocrinology 157, 3108e3121. https://doi.org/10.1210/en.2016-1115. Ebling, F.J., Ebling, E., Randall, V., Skinner, J., 1975. The synergistic action of alpha-melanocyte-stimulating hormone and testosterone of the sebaceous, prostate, preputial, Harderian and lachrymal glands, seminal vesicles and brown adipose tissue in the hypophysectomized-castrated rat. J. Endocrinol. 66, 407e412. Elias, L.L., Huebner, A., Metherell, L.A., Canas, A., Warne, G.L., Bitti, M.L., Cianfarani, S., Clayton, P.E., Savage, M.O., Clark, A.J., 2000. Tall stature in familial glucocorticoid deficiency. Clin. Endocrinol. 53, 423e430. Everson, R.A., Smith, P.D., Dobson, E.L., 1969. Kinetics of Adrenocorticotropic Hormone Inactivation in the Dog. Fardet, L., Petersen, I., Nazareth, I., 2011. Prevalence of long-term oral glucocorticoid prescriptions in the UK over the past 20 years. Rheumatology 50, 1982e1990. https://doi.org/10.1093/rheumatology/ ker017. Farooqi, I.S., Drop, S., Clements, A., Keogh, J.M., Biernacka, J., Lowenbein, S., Challis, B.G., O’Rahilly, S., 2006. Heterozygosity for a POMC-null mutation and increased obesity risk in humans. Diabetes 55, 2549e2553. https://doi.org/10.2337/db06-0214. Fletcher, P.A., Zemkova, H., Stojilkovic, S.S., Sherman, A., 2017. Modeling the diversity of spontaneous and agonist-induced electrical activity in anterior pituitary corticotrophs. J. Neurophysiol. https://doi.org/10.1152/jn.00948.2016 jn.00948.2016. Fricker, L.D., 1988. Carboxypeptidase E. Annu. Rev. Physiol. 50, 309e321. https://doi.org/10.1146/annurev.ph.50.030188.001521. Fridmanis, D., Petrovska, R., Kalnina, I., Slaidina, M., Peculis, R., Schio¨th, H.B., Klovins, J., 2010. Identification of domains responsible for specific membrane transport and ligand specificity of the ACTH receptor (MC2R). Mol. Cell. Endocrinol. 321, 175e183. https://doi.org/10.1016/j.mce.2010.02.032. Fukuoka, H., Cooper, O., Ben-Shlomo, A., Mamelak, A., Ren, S.-G., Bruyette, D., Melmed, S., 2011. EGFR as a therapeutic target for human, canine, and mouse ACTH-secreting pituitary adenomas. J. Clin. Investig. 121, 4712e4721. https://doi.org/10.1172/ JCI60417. Funkelstein, L., Toneff, T., Mosier, C., Hwang, S.-R., Beuschlein, F., Lichtenauer, U.D., Reinheckel, T., Peters, C., Hook, V., 2008. Major role of cathepsin L for producing the peptide hormones ACTH, beta-endorphin, and alpha-MSH, illustrated by protease gene knockout and expression. J. Biol. Chem. 283, 35652e35659. https://doi.org/10.1074/jbc.M709010200. Fuse, N., Yasumoto, K., Suzuki, H., Takahashi, K., Shibahara, S., 1996. Identification of a melanocyte-type promoter of the microphthalmia-associated transcription factor gene. Biochem. Biophys. Res. Commun. 219, 702e707. https://doi.org/10.1006/ bbrc.1996.0298. Gagner, J.P., Drouin, J., 1987. Tissue-specific regulation of pituitary proopiomelanocortin gene transcription by corticotropin-releasing hormone, 30 ,50 -cyclic adenosine monophosphate, and glucocorticoids. Mol. Endocrinol. 1, 677e682. https://doi.org/ 10.1210/mend-1-10-677. Gantz, I., Konda, Y., Tashiro, T., Shimoto, Y., Miwa, H., Munzert, G., Watson, S.J., DelValle, J., Yamada, T., 1993. Molecular cloning of a novel melanocortin receptor. J. Biol. Chem. 268, 8246e8250. Gantz, I., Shimoto, Y., Konda, Y., Miwa, H., Dickinson, C.J., Yamada, T., 1994. Molecular cloning, expression, and characterization of a fifth melanocortin receptor. Biochem. Biophys. Res. Commun. 200, 1214e1220. https://doi.org/10.1006/bbrc.1994.1580. Gary, K.A., Chronwall, B.M., 1992. The onset of dopaminergic innervation during ontogeny decreases melanotrope proliferation in the

REFERENCES

intermediate lobe of the rat pituitary. Int. J. Dev. Neurosci. 10, 131e142. Geiling, E.M.K., Vos, B.J., Oldham, F.K., 1940. The pharmacology and anatomy of the hypophysis of the porpoise. Endocrinology 27, 309e316. https://doi.org/10.1210/endo-27-2-309. Ge´nin, E., Huebner, A., Jaillard, C., Faure, A., Halaby, G., Saka, N., Clark, A.J.L., Durand, P., Be´geot, M., Naville, D., 2002. Linkage of one gene for familial glucocorticoid deficiency type 2 (FGD2) to chromosome 8q and further evidence of heterogeneity. Hum. Genet. 111, 428e434. https://doi.org/10.1007/s00439-002-0806-3. Gicquel, C., Le Bouc, Y., Luton, J.P., Girard, F., Bertagna, X., 1992. Monoclonality of corticotroph macroadenomas in Cushing’s disease. J. Clin. Endocrinol. Metab. 75, 472e475. https://doi.org/ 10.1210/jcem.75.2.1322426. Gillies, G.E., Linton, E.A., Lowry, P.J., 1982. Corticotropin releasing activity of the new CRF is potentiated several times by vasopressin. Nature 299, 355e357. https://doi.org/10.1038/299355a0. Gomez, M.T., Magiakou, M.A., Mastorakos, G., Chrousos, G.P., 1993. The pituitary corticotroph is not the rate limiting step in the postoperative recovery of the hypothalamic-pituitary-adrenal axis in patients with Cushing syndrome. J. Clin. Endocrinol. Metab. 77, 173e177. https://doi.org/10.1210/jcem.77.1.8392083. Griffon, N., Mignon, V., Facchinetti, P., Diaz, J., Schwartz, J.C., Sokoloff, P., 1994. Molecular cloning and characterization of the rat fifth melanocortin receptor. Biochem. Biophys. Res. Commun. 200, 1007e1014. Guo, Q., Lu, J., Mu, Y., Chen, K., Pan, C., 2013. Adult idiopathic isolated ACTH deficiency: a short series and literature review. Neuroendocrinol. Lett. 34, 693e700. Haskell-Luevano, C., Sawyer, T.K., Trumpp-Kallmeyer, S., Bikker, J.A., Humblet, C., Gantz, I., Hruby, V.J., 1996. Three-dimensional molecular models of the hMC1R melanocortin receptor: complexes with melanotropin peptide agonists. Drug Des. Discov. 14, 197e211. Hinz, B., Hirschelmann, R., 2000. Rapid non-genomic feedback effects of glucocorticoids on CRF-induced ACTH secretion in rats. Pharm. Res. 17, 1273e1277. Hodson, D.J., Schaeffer, M., Romano, N., Fontanaud, P., Lafont, C., Birkenstock, J., Molino, F., Christian, H., Lockey, J., Carmignac, D., Fernandez-Fuente, M., Le Tissier, P., Mollard, P., 2012. Existence of long-lasting experience-dependent plasticity in endocrine cell networks. Nat. Commun. 3, 605. https://doi.org/ 10.1038/ncomms1612. Ho¨llt, V., Przewłocki, R., Haarmann, I., Almeida, O.F., Kley, N., Millan, M.J., Herz, A., 1986. Stress-induced alterations in the levels of messenger RNA coding for proopiomelanocortin and prolactin in rat pituitary. Neuroendocrinology 43, 277e282. https:// doi.org/10.1159/000124541. Hook, V., Funkelstein, L., Toneff, T., Mosier, C., Hwang, S.-R., 2009. Human pituitary contains dual cathepsin L and prohormone convertase processing pathway components involved in converting POMC into the peptide hormones ACTH, a-MSH, and b-endorphin. Endocrine 35, 429e437. https://doi.org/10.1007/ s12020-009-9163-5. Hook, V.Y., Eiden, L.E., Brownstein, M.J., 1982. A carboxypeptidase processing enzyme for enkephalin precursors. Nature 295, 341e342. Horvath, E., Kovacs, K., Lloyd, R.V., 1999. Pars intermedia of the human pituitary revisited: morphologic aspects and frequency of hyperplasia of POMC-peptide immunoreactive cells. Endocr. Pathol. 10, 55e64. https://doi.org/10.1007/BF02738816. Huber, W.E., Price, E.R., Widlund, H.R., Du, J., Davis, I.J., Wegner, M., Fisher, D.E., 2003. A tissue-restricted cAMP transcriptional response: SOX10 modulates alpha-melanocyte-stimulating hormone-triggered expression of microphthalmia-associated transcription factor in melanocytes. J. Biol. Chem. 278, 45224e45230. https://doi.org/10.1074/jbc.M309036200.

163

Huszar, D., Lynch, C.A., Fairchild-Huntress, V., Dunmore, J.H., Fang, Q., Berkemeier, L.R., Gu, W., Kesterson, R.A., Boston, B.A., Cone, R.D., Smith, F.J., Campfield, L.A., Burn, P., Lee, F., 1997. Targeted disruption of the melanocortin-4 receptor results in obesity in mice. Cell 88, 131e141. Imamine, H., Mizuno, H., Sugiyama, Y., Ohro, Y., Sugiura, T., Togari, H., 2005. Possible relationship between elevated plasma ACTH and tall stature in familial glucocorticoid deficiency. Tohoku J. Exp. Med. 205, 123e131. Imura, H., Matsuyama, H., Matsukura, S., Miyake, T., Fukase, M., 1967. Stability of ACTH preparations in human plasma incubated in vitro. Endocrinology 80, 599e602. https://doi.org/10.1210/ endo-80-4-599. Isales, C.M., Zaidi, M., Blair, H.C., 2010. ACTH is a novel regulator of bone mass. Ann. N. Y. Acad. Sci. 1192, 110e116. https://doi.org/ 10.1111/j.1749-6632.2009.05231.x. Ishii, T., Ishibashi, T., 1989. The ultrastructure of the pars intermedia and the subdivision of its glandular cells in the young sheep pituitary. Arch. Histol. Cytol. 52, 123e133. Jackson, R.S., Creemers, J.W., Ohagi, S., Raffin-Sanson, M.L., Sanders, L., Montague, C.T., Hutton, J.C., O’Rahilly, S., 1997. Obesity and impaired prohormone processing associated with mutations in the human prohormone convertase 1 gene. Nat. Genet. 16, 303e306. https://doi.org/10.1038/ng0797-303. Jahn, R., Padel, U., Porsch, P.H., So¨ling, H.D., 1982. Adrenocorticotropic hormone and alpha-melanocyte-stimulating hormone induce secretion and protein phosphorylation in the rat lacrimal gland by activation of a cAMP-dependent pathway. Eur. J. Biochem. 126, 623e629. Jansen, E., Ayoubi, T.A., Meulemans, S.M., Van de Ven, W.J., 1995. Neuroendocrine-specific expression of the human prohormone convertase 1 gene. Hormonal regulation of transcription through distinct cAMP response elements. J. Biol. Chem. 270, 15391e15397. Jard, S., Gaillard, R.C., Guillon, G., Marie, J., Schoenenberg, P., Muller, A.F., Manning, M., Sawyer, W.H., 1986. Vasopressin antagonists allow demonstration of a novel type of vasopressin receptor in the rat adenohypophysis. Mol. Pharmacol. 30, 171e177. Ji, L., Wu, H.-T., Qin, X.-Y., Lan, R., 2017. Dissecting carboxypeptidase E: properties, functions and pathophysiological roles in disease. Endocr. Connect. 6, R18eR38. https://doi.org/10.1530/EC-17-0020. Johannsson, G., Nilsson, A.G., Bergthorsdottir, R., Burman, P., Dahlqvist, P., Ekman, B., Engstro¨m, B.E., Olsson, T., Ragnarsson, O., Ryberg, M., Wahlberg, J., Biller, B.M.K., Monson, J.P., Stewart, P.M., Lennerna¨s, H., Skrtic, S., 2012. Improved cortisol exposure-time profile and outcome in patients with adrenal insufficiency: a prospective randomized trial of a novel hydrocortisone dual-release formulation. J. Clin. Endocrinol. Metab. 97, 473e481. https://doi.org/10.1210/jc.2011-1926. Kehl, S.J., 1994. Voltage-clamp analysis of the voltage-gated sodium current of the rat pituitary melanotroph. Neurosci. Lett. 165, 67e70. Keller-Wood, M.E., Dallman, M.F., 1984. Corticosteroid inhibition of ACTH secretion. Endocr. Rev. 5, 1e24. https://doi.org/10.1210/ edrv-5-1-1. Klungland, H., Va˚ge, D.I., Gomez-Raya, L., Adalsteinsson, S., Lien, S., 1995. The role of melanocyte-stimulating hormone (MSH) receptor in bovine coat color determination. Mamm. Genome 6, 636e639. Koshimizu, T., Nakamura, K., Egashira, N., Hiroyama, M., Nonoguchi, H., Tanoue, A., 2012. Vasopressin V1a and V1b receptors: from molecules to physiological systems. Physiol. Rev. 92, 1813e1864. https://doi.org/10.1152/physrev.00035.2011. Krasner, A.S., 1999. Glucocorticoid-induced adrenal insufficiency. J. Am. Med. Assoc. 282, 671e676. Krude, H., Biebermann, H., Luck, W., Horn, R., Brabant, G., Gru¨ters, A., 1998. Severe early-onset obesity, adrenal insufficiency and red hair pigmentation caused by POMC mutations in humans. Nat. Genet. 19, 155e157. https://doi.org/10.1038/509.

164

8. ANTERIOR PITUITARY AND PARS INTERMEDIA SPACE: CORTICOTROPHS (ACTH) AND MELANOTROPHS (a-MSH)

Krude, H., Biebermann, H., Schnabel, D., Tansek, M.Z., Theunissen, P., Mullis, P.E., Gru¨ters, A., 2003. Obesity due to proopiomelanocortin deficiency: three new cases and treatment trials with thyroid hormone and ACTH4-10. J. Clin. Endocrinol. Metab. 88, 4633e4640. https://doi.org/10.1210/jc.2003-030502. Krueger, R.J., Orme-Johnson, N.R., 1983. Acute adrenocorticotropic hormone stimulation of adrenal corticosteroidogenesis. Discovery of a rapidly induced protein. J. Biol. Chem. 258, 10159e10167. Kudielka, B.M., Buske-Kirschbaum, A., Hellhammer, D.H., Kirschbaum, C., 2004. HPA axis responses to laboratory psychosocial stress in healthy elderly adults, younger adults, and children: impact of age and gender. Psychoneuroendocrinology 29, 83e98. Kuryshev, Y.A., Childs, G.V., Ritchie, A.K., 1996. Corticotropinreleasing hormone stimulates Ca2þ entry through L- and P-type Ca2þ channels in rat corticotropes. Endocrinology 137, 2269e2277. https://doi.org/10.1210/endo.137.6.8641175. Labbe´, O., Desarnaud, F., Eggerickx, D., Vassart, G., Parmentier, M., 1994. Molecular cloning of a mouse melanocortin 5 receptor gene widely expressed in peripheral tissues. Biochemistry 33, 4543e4549. Lafont, C., Desarme´nien, M.G., Cassou, M., Molino, F., Lecoq, J., Hodson, D., Lacampagne, A., Mennessier, G., El Yandouzi, T., Carmignac, D., Fontanaud, P., Christian, H., Coutry, N., Fernandez-Fuente, M., Charpak, S., Le Tissier, P., Robinson, I.C.A.F., Mollard, P., 2010. Cellular in vivo imaging reveals coordinated regulation of pituitary microcirculation and GH cell network function. Proc. Natl. Acad. Sci. U. S. A. 107, 4465e4470. https://doi.org/10.1073/pnas.0902599107. Lamas, M., Molina, C., Foulkes, N.S., Jansen, E., Sassone-Corsi, P., 1997. Ectopic ICER expression in pituitary corticotroph AtT20 cells: effects on morphology, cell cycle, and hormonal production. Mol. Endocrinol. 11, 1425e1434. https://doi.org/10.1210/ mend.11.10.9987. Lamberts, S.W., Verleun, T., Oosterom, R., de Jong, F., Hackeng, W.H., 1984. Corticotropin-releasing factor (ovine) and vasopressin exert a synergistic effect on adrenocorticotropin release in man. J. Clin. Endocrinol. Metab. 58, 298e303. https://doi.org/10.1210/jcem58-2-298. Lamolet, B., Poulin, G., Chu, K., Guillemot, F., Tsai, M.-J., Drouin, J., 2004. Tpit-independent function of NeuroD1(BETA2) in pituitary corticotroph differentiation. Mol. Endocrinol. 18, 995e1003. https://doi.org/10.1210/me.2003-0127. Lamolet, B., Pulichino, A.M., Lamonerie, T., Gauthier, Y., Brue, T., Enjalbert, A., Drouin, J., 2001. A pituitary cell-restricted T box factor, Tpit, activates POMC transcription in cooperation with Pitx homeoproteins. Cell 104, 849e859. Lamonerie, T., Tremblay, J.J., Lanctoˆt, C., Therrien, M., Gauthier, Y., Drouin, J., 1996. Ptx1, a bicoid-related homeo box transcription factor involved in transcription of the pro-opiomelanocortin gene. Genes Dev. 10, 1284e1295. Larkin, S., Ansorge, O., 2000. Development and microscopic anatomy of the pituitary gland. In: De Groot, L.J., Chrousos, G., Dungan, K., Feingold, K.R., Grossman, A., Hershman, J.M., Koch, C., Korbonits, M., McLachlan, R., New, M., Purnell, J., Rebar, R., Singer, F., Vinik, A. (Eds.), Endotext. MDText.com, Inc., South Dartmouth, MA. Lavoie, P.-L., Budry, L., Balsalobre, A., Drouin, J., 2008. Developmental dependence on NurRE and EboxNeuro for expression of pituitary proopiomelanocortin. Mol. Endocrinol. 22, 1647e1657. https:// doi.org/10.1210/me.2007-0567. Le Tissier, P., Campos, P., Lafont, C., Romano`, N., Hodson, D.J., Mollard, P., 2017. An updated view of hypothalamic-vascular-pituitary unit function and plasticity. Nat. Rev. Endocrinol. 13, 257e267. https://doi.org/10.1038/nrendo.2016.193.

Le Tissier, P.R., Hodson, D.J., Lafont, C., Fontanaud, P., Schaeffer, M., Mollard, P., 2012. Anterior pituitary cell networks. Front. Neuroendocrinol. 33, 252e266. https://doi.org/10.1016/ j.yfrne.2012.08.002. Lee, A.K., 1996. Dopamine (D2) receptor regulation of intracellular calcium and membrane capacitance changes in rat melanotrophs. J. Physiol. 495, 627e640. Lee, A.K., Tse, F.W., Tse, A., 2015. Arginine vasopressin potentiates the stimulatory action of CRH on pituitary corticotropes via a protein kinase Cedependent reduction of the background TREK-1 current. Endocrinology 156, 3661e3672. https://doi.org/10.1210/ en.2015-1293. Leenders, H.J., de Koning, H.P., Ponten, S.P., Jenks, B.G., Roubos, E.W., 1993. Differential effects of coexisting dopamine, GABA and NPY on alpha-MSH secretion from melanotrope cells of Xenopus laevis. Life Sci. 52, 1969e1975. Li, C.H., Geschwind, I.I., Cole, R.D., Raacke, I.D., Harris, J.I., Dixon, J.S., 1955. Amino-acid sequence of alpha-corticotropin. Nature 176, 687. https://doi.org/10.1038/176687a0. Li, C.H., Geschwind, I.I., Levy, A.L., Harris, J.I., Dixon, J.S., Pon, N.G., Porath, J.O., 1954. Isolation and properties of alpha-corticotrophin from sheep pituitary glands. Nature 173, 251e253. Li, C.H., Simpson, M.E., Evans, H.M., 1942. Isolation of adrenocorticotropic hormone from sheep pituitaries. Science 96, 450. https:// doi.org/10.1126/science.96.2498.450. Liang, Z., Chen, L., McClafferty, H., Lukowski, R., MacGregor, D., King, J.T., Rizzi, S., Sausbier, M., McCobb, D.P., Knaus, H.-G., Ruth, P., Shipston, M.J., 2011. Control of hypothalamic-pituitary-adrenal stress axis activity by the intermediate conductance calciumactivated potassium channel, SK4. J. Physiol. 589, 5965e5986. https://doi.org/10.1113/jphysiol.2011.219378. Lightman, S.L., Conway-Campbell, B.L., 2010. The crucial role of pulsatile activity of the HPA axis for continuous dynamic equilibration. Nat. Rev. Neurosci. 11, 710e718. https://doi.org/ 10.1038/nrn2914. Lolait, S.J., O’Carroll, A.M., Mahan, L.C., Felder, C.C., Button, D.C., Young, W.S., Mezey, E., Brownstein, M.J., 1995. Extrapituitary expression of the rat V1b vasopressin receptor gene. Proc. Natl. Acad. Sci. U. S. A. 92, 6783e6787. Louiset, E., Cazin, L., Lamacz, M., Tonon, M.C., Vaudry, H., 1988. Patch-clamp study of the ionic currents underlying action potentials in cultured frog pituitary melanotrophs. Neuroendocrinology 48, 507e515. https://doi.org/10.1159/000125057. Lu, D., Willard, D., Patel, I.R., Kadwell, S., Overton, L., Kost, T., Luther, M., Chen, W., Woychik, R.P., Wilkison, W.O., 1994. Agouti protein is an antagonist of the melanocyte-stimulating-hormone receptor. Nature 371, 799e802. https://doi.org/10.1038/ 371799a0. Lugo, D.I., Pintar, J.E., 1996. Ontogeny of basal and regulated secretion from POMC cells of the developing anterior lobe of the rat pituitary gland. Dev. Biol. 173, 95e109. https://doi.org/10.1006/ dbio.1996.0009. Ma, Z.-Y., Song, Z.-J., Chen, J.-H., Wang, Y.-F., Li, S.-Q., Zhou, L.-F., Mao, Y., Li, Y.-M., Hu, R.-G., Zhang, Z.-Y., Ye, H.-Y., Shen, M., Shou, X.-F., Li, Z.-Q., Peng, H., Wang, Q.-Z., Zhou, D.-Z., Qin, X.L., Ji, J., Zheng, J., Chen, H., Wang, Y., Geng, D.-Y., Tang, W.-J., Fu, C.-W., Shi, Z.-F., Zhang, Y.-C., Ye, Z., He, W.-Q., Zhang, Q.-L., Tang, Q.-S., Xie, R., Shen, J.-W., Wen, Z.-J., Zhou, J., Wang, T., Huang, S., Qiu, H.-J., Qiao, N.-D., Zhang, Y., Pan, L., Bao, W.-M., Liu, Y.-C., Huang, C.-X., Shi, Y.-Y., Zhao, Y., 2015. Recurrent gainof-function USP8 mutations in Cushing’s disease. Cell Res. 25, 306e317. https://doi.org/10.1038/cr.2015.20. Mains, R.E., Eipper, B.A., Ling, N., 1977. Common precursor to corticotropins and endorphins. Proc. Natl. Acad. Sci. U. S. A. 74, 3014e3018.

REFERENCES

Maira, M., Couture, C., Le Martelot, G., Pulichino, A.-M., Bilodeau, S., Drouin, J., 2003. The T-box factor Tpit recruits SRC/p160 coactivators and mediates hormone action. J. Biol. Chem. 278, 46523e46532. https://doi.org/10.1074/jbc.M305626200. Mano-Otagiri, A., Nemoto, T., Yamauchi, N., Kakinuma, Y., Shibasaki, T., 2016. Distribution of corticotrophin-releasing factor type 1 receptor-like immunoreactivity in the rat pituitary. J. Neuroendocrinol. 28 https://doi.org/10.1111/jne.12440. Marklund, L., Moller, M.J., Sandberg, K., Andersson, L., 1996. A missense mutation in the gene for melanocyte-stimulating hormone receptor (MC1R) is associated with the chestnut coat color in horses. Mamm. Genome 7, 895e899. Martens, C., Bilodeau, S., Maira, M., Gauthier, Y., Drouin, J., 2005. Protein-protein interactions and transcriptional antagonism between the subfamily of NGFI-B/Nur77 orphan nuclear receptors and glucocorticoid receptor. Mol. Endocrinol. 19, 885e897. https://doi.org/10.1210/me.2004-0333. Matsuyama, H., Mims, R.B., Ruhmann-Wennhold, A., Nelson, D.H., 1971. Bioassay and radioimmunoassay of plasma ACTH in adrenalectomized rats. Endocrinology 88, 696e701. https://doi.org/ 10.1210/endo-88-3-696. Matsuyama, H., Ruhmann-Wennhold, A., Johnson, L.R., Nelson, D.H., 1972. Disappearance rates of exogenous and endogenous ACTH from rat plasma measured by bioassay and radioimmunoassay. Metabolism 21, 30e35. Mayran, A., Khetchoumian, K., Hariri, F., Pastinen, T., Gauthier, Y., Balsalobre, A., Drouin, J., 2018. Pioneer factor Pax7 deploys a stable enhancer repertoire for specification of cell fate. Nat. Genet. 50, 259e269. https://doi.org/10.1038/s41588-017-0035-2. Mendiratta, M.S., Yang, Y., Balazs, A.E., Willis, A.S., Eng, C.M., Karaviti, L.P., Potocki, L., 2011. Early onset obesity and adrenal insufficiency associated with a homozygous POMC mutation. Int. J. Pediatr. Endocrinol. 2011, 5. https://doi.org/10.1186/1687-98562011-5. Metherell, L.A., Chapple, J.P., Cooray, S., David, A., Becker, C., Ru¨schendorf, F., Naville, D., Begeot, M., Khoo, B., Nu¨rnberg, P., Huebner, A., Cheetham, M.E., Clark, A.J.L., 2005. Mutations in MRAP, encoding a new interacting partner of the ACTH receptor, cause familial glucocorticoid deficiency type 2. Nat. Genet. 37, 166e170. https://doi.org/10.1038/ng1501. Metherell, L.A., Naville, D., Halaby, G., Begeot, M., Huebner, A., Nu¨rnberg, G., Nu¨rnberg, P., Green, J., Tomlinson, J.W., Krone, N.P., Lin, L., Racine, M., Berney, D.M., Achermann, J.C., Arlt, W., Clark, A.J.L., 2009. Nonclassic lipoid congenital adrenal hyperplasia masquerading as familial glucocorticoid deficiency. J. Clin. Endocrinol. Metab. 94, 3865e3871. https://doi.org/ 10.1210/jc.2009-0467. Meunier, H., Labrie, F., 1982. beta-Adrenergic, CRF-ergic and dopaminergic mechanisms controlling alpha-MSH secretion in rat pars intermedia cells in primary culture. Prog. NeuroPsychopharmacol. Biol. Psychiatry 6, 411e415. ˚ ., Holst, B., CondeMøller, C.L., Raun, K., Jacobsen, M.L., Pedersen, T.A Frieboes, K.W., Wulff, B.S., 2011. Characterization of murine melanocortin receptors mediating adipocyte lipolysis and examination of signalling pathways involved. Mol. Cell. Endocrinol. 341, 9e17. https://doi.org/10.1016/j.mce.2011.03.010. Morgan, C., Cone, R.D., 2006. Melanocortin-5 receptor deficiency in mice blocks a novel pathway influencing pheromone-induced aggression. Behav. Genet. 36, 291e300. https://doi.org/10.1007/ s10519-005-9024-9. Morgan, C., Thomas, R.E., Ma, W., Novotny, M.V., Cone, R.D., 2004. Melanocortin-5 receptor deficiency reduces a pheromonal signal for aggression in male mice. Chem. Senses 29, 111e115. Mountjoy, K.G., 2015. Pro-opiomelanocortin (POMC) neurones, POMC-derived peptides, melanocortin receptors and obesity: how understanding of this system has changed over the last

165

decade. J. Neuroendocrinol. 27, 406e418. https://doi.org/ 10.1111/jne.12285. Mountjoy, K.G., 2010. Functions for pro-opiomelanocortin-derived peptides in obesity and diabetes. Biochem. J. 428, 305e324. https://doi.org/10.1042/BJ20091957. Mountjoy, K.G., Mortrud, M.T., Low, M.J., Simerly, R.B., Cone, R.D., 1994. Localization of the melanocortin-4 receptor (MC4-R) in neuroendocrine and autonomic control circuits in the brain. Mol. Endocrinol. 8, 1298e1308. https://doi.org/10.1210/mend.8.10.7854347. Mountjoy, K.G., Robbins, L.S., Mortrud, M.T., Cone, R.D., 1992. The cloning of a family of genes that encode the melanocortin receptors. Science 257, 1248e1251. Muglia, L., Jacobson, L., Dikkes, P., Majzoub, J.A., 1995. Corticotropinreleasing hormone deficiency reveals major fetal but not adult glucocorticoid need. Nature 373, 427e432. https://doi.org/ 10.1038/373427a0. Murakami, M., Yoshida, T., Nakayama, Y., Hashimoto, J., Hirata, S., 1968. The fine structure of the pars intermedia of the pituitary in human fetus. Arch. Histol. Jpn. Nihon Soshikigaku Kiroku 30, 61e73. Nakanishi, S., Inoue, A., Kita, T., Inoue, A., Nakamura, M., Chang, A.C.Y., Cohen, S.N., Numa, S., 1979. Nucleotide sequence of cloned cDNA for bovine corticotropin-b-lipotropin precursor. Nature 278, 423e427. https://doi.org/10.1038/278423a0. Nasti, T.H., Timares, L., 2015. Invited review MC1R, eumelanin and pheomelanin: their role in determining the susceptibility to skin cancer. Photochem. Photobiol. 91, 188e200. https://doi.org/ 10.1111/php.12335. Naville, D., Barjhoux, L., Jaillard, C., Faury, D., Despert, F., Esteva, B., Durand, P., Saez, J.M., Begeot, M., 1996. Demonstration by transfection studies that mutations in the adrenocorticotropin receptor gene are one cause of the hereditary syndrome of glucocorticoid deficiency. J. Clin. Endocrinol. Metab. 81, 1442e1448. https:// doi.org/10.1210/jcem.81.4.8636348. Newton, R.A., Roberts, D.W., Leonard, J.H., Sturm, R.A., 2007. Human melanocytes expressing MC1R variant alleles show impaired activation of multiple signaling pathways. Peptides 28, 2387e2396. https://doi.org/10.1016/j.peptides.2007.10.003. Newton, R.A., Smit, S.E., Barnes, C.C., Pedley, J., Parsons, P.G., Sturm, R.A., 2005. Activation of the cAMP pathway by variant human MC1R alleles expressed in HEK and in melanoma cells. Peptides 26, 1818e1824. https://doi.org/10.1016/ j.peptides.2004.11.031. Nilsson Sko¨ld, H., Aspengren, S., Wallin, M., 2013. Rapid color change in fish and amphibians e function, regulation, and emerging applications. Pigm. Cell Melanoma Res. 26, 29e38. https:// doi.org/10.1111/pcmr.12040. Noon, L.A., Franklin, J.M., King, P.J., Goulding, N.J., Hunyady, L., Clark, A.J.L., 2002. Failed export of the adrenocorticotrophin receptor from the endoplasmic reticulum in non-adrenal cells: evidence in support of a requirement for a specific adrenal accessory factor. J. Endocrinol. 174, 17e25. Nussey, S.S., Soo, S.C., Gibson, S., Gout, I., White, A., Bain, M., Johnstone, A.P., 1993. Isolated congenital ACTH deficiency: a cleavage enzyme defect? Clin. Endocrinol. 39, 381e385. Oksnes, M., Bjo¨rnsdottir, S., Isaksson, M., Methlie, P., Carlsen, S., Nilsen, R.M., Broman, J.-E., Triebner, K., Ka¨mpe, O., Hulting, A.L., Bensing, S., Husebye, E.S., Løva˚s, K., 2014. Continuous subcutaneous hydrocortisone infusion versus oral hydrocortisone replacement for treatment of Addison’s disease: a randomized clinical trial. J. Clin. Endocrinol. Metab. 99, 1665e1674. https:// doi.org/10.1210/jc.2013-4253. Opdecamp, K., Nakayama, A., Nguyen, M.T., Hodgkinson, C.A., Pavan, W.J., Arnheiter, H., 1997. Melanocyte development in vivo and in neural crest cell cultures: crucial dependence on the Mitf basic-helix-loop-helix-zipper transcription factor. Development 124, 2377e2386.

166

8. ANTERIOR PITUITARY AND PARS INTERMEDIA SPACE: CORTICOTROPHS (ACTH) AND MELANOTROPHS (a-MSH)

Ouleghzal, H., Rosales, C., Raffin-Sanson, M.-L., 2012. Treatment of corticotroph deficiency. Ann. Endocrinol. 73, 12e19. https:// doi.org/10.1016/j.ando.2012.01.001. Overman, R.A., Yeh, J.-Y., Deal, C.L., 2013. Prevalence of oral glucocorticoid usage in the United States: a general population perspective. Arthritis Care Res. 65, 294e298. https://doi.org/10.1002/ acr.21796. Oyarce, A.M., Hand, T.A., Mains, R.E., Eipper, B.A., 1996. Dopaminergic regulation of secretory granule-associated proteins in rat intermediate pituitary. J. Neurochem. 67, 229e241. Panin, M., Giurisato, M., Peruffo, A., Ballarin, C., Cozzi, B., 2013. Immunofluorescence evidence of melanotrophs in the pituitary of four odontocete species. An immunohistochemical study and a critical review of the literature. Ann. Anat. Anat. Anz. 195, 512e521. https://doi.org/10.1016/j.aanat.2013.06.004. Perez-Rivas, L.G., Theodoropoulou, M., Ferrau`, F., Nusser, C., Kawaguchi, K., Stratakis, C.A., Faucz, F.R., Wildemberg, L.E., Assie´, G., Beschorner, R., Dimopoulou, C., Buchfelder, M., Popovic, V., Berr, C.M., To´th, M., Ardisasmita, A.I., Honegger, J., Bertherat, J., Gadelha, M.R., Beuschlein, F., Stalla, G., Komada, M., Korbonits, M., Reincke, M., 2015. The gene of the ubiquitin-specific protease 8 is frequently mutated in adenomas causing Cushing’s disease. J. Clin. Endocrinol. Metab. 100, E997eE1004. https://doi.org/10.1210/jc.2015-1453. Philips, A., Lesage, S., Gingras, R., Maira, M.H., Gauthier, Y., Hugo, P., Drouin, J., 1997. Novel dimeric Nur77 signaling mechanism in endocrine and lymphoid cells. Mol. Cell. Biol. 17, 5946e5951. Plaut, A., 1936. Investigations on the pars intermedia of the hypophysis in anthropoid apes and man. J. Anat. 70, 242e249. Poulin, G., Lebel, M., Chamberland, M., Paradis, F.W., Drouin, J., 2000. Specific protein-protein interaction between basic helix-loop-helix transcription factors and homeoproteins of the Pitx family. Mol. Cell. Biol. 20, 4826e4837. Poulin, G., Turgeon, B., Drouin, J., 1997. NeuroD1/beta2 contributes to cell-specific transcription of the proopiomelanocortin gene. Mol. Cell. Biol. 17, 6673e6682. Pozzoli, G., Bilezikjian, L.M., Perrin, M.H., Blount, A.L., Vale, W.W., 1996. Corticotropin-releasing factor (CRF) and glucocorticoids modulate the expression of type 1 CRF receptor messenger ribonucleic acid in rat anterior pituitary cell cultures. Endocrinology 137, 65e71. https://doi.org/10.1210/endo.137.1.8536643. Price, E.R., Horstmann, M.A., Wells, A.G., Weilbaecher, K.N., Takemoto, C.M., Landis, M.W., Fisher, D.E., 1998. a-melanocytestimulating hormone signaling regulates expression of microphthalmia, a gene deficient in Waardenburg syndrome. J. Biol. Chem. 273, 33042e33047. https://doi.org/10.1074/jbc.273.49.33042. Proulx-Ferland, L., Labrie, F., Dumont, D., Coˆte´, J., Coy, D.H., Sveiraf, J., 1982. Corticotropin-releasing factor stimulates secretion of melanocyte-stimulating hormone from the rat pituitary. Science 217, 62e63. Prusis, P., Schio¨th, H.B., Muceniece, R., Herzyk, P., Afshar, M., Hubbard, R.E., Wikberg, J.E.S., 1997. Modeling of the threedimensional structure of the human melanocortin 1 receptor, using an automated method and docking of a rigid cyclic melanocytestimulating hormone core peptide. J. Mol. Graph. Model. 15, 307e317. https://doi.org/10.1016/S1093-3263(98)00004-7. Pulichino, A.-M., Vallette-Kasic, S., Tsai, J.P.-Y., Couture, C., Gauthier, Y., Drouin, J., 2003. Tpit determines alternate fates during pituitary cell differentiation. Genes Dev. 17, 738e747. https:// doi.org/10.1101/gad.1065703. Raff, H., Carroll, T., 2015. Cushing’s syndrome: from physiological principles to diagnosis and clinical care. J. Physiol. 593, 493e506. https://doi.org/10.1113/jphysiol.2014.282871. Rahn, H., Painter, B.T., 1941. A comparative histology of the bird pituitary. Anat. Rec. 79, 297e311. https://doi.org/10.1002/ ar.1090790304.

Ramachandran, J., Farmer, S.W., Liles, S., Li, C.H., 1976. Comparison of the steroidogenic and melanotropic activities of corticotropin, alpha-melanotropin and analogs with their lipolytic activities in rat and rabbit adipocytes. Biochim. Biophys. Acta 428, 347e354. Rasmussen, A.T., 1930. Origin of the basophilic cells in the posterior lobe of the human hypophysis. Am. J. Anat. 46, 461e475. https://doi.org/10.1002/aja.1000460305. Reincke, M., Sbiera, S., Hayakawa, A., Theodoropoulou, M., Osswald, A., Beuschlein, F., Meitinger, T., Mizuno-Yamasaki, E., Kawaguchi, K., Saeki, Y., Tanaka, K., Wieland, T., Graf, E., Saeger, W., Ronchi, C.L., Allolio, B., Buchfelder, M., Strom, T.M., Fassnacht, M., Komada, M., 2015. Mutations in the deubiquitinase gene USP8 cause Cushing’s disease. Nat. Genet. 47, 31e38. https://doi.org/10.1038/ng.3166. Ritchie, A.K., Kuryshev, Y.A., Childs, G.V., 1996. Corticotropinreleasing hormone and calcium signaling in corticotropes. Trends Endocrinol. Metab. 7, 365e369. Robbins, L.S., Nadeau, J.H., Johnson, K.R., Kelly, M.A., RoselliRehfuss, L., Baack, E., Mountjoy, K.G., Cone, R.D., 1993. Pigmentation phenotypes of variant extension locus alleles result from point mutations that alter MSH receptor function. Cell 72, 827e834. Roberts, J.L., Herbert, E., 1977. Characterization of a common precursor to corticotropin and beta-lipotropin: cell-free synthesis of the precursor and identification of corticotropin peptides in the molecule. Proc. Natl. Acad. Sci. U. S. A. 74, 4826e4830. Romano`, N., McClafferty, H., Walker, J.J., Le Tissier, P., Shipston, M.J., 2017. Heterogeneity of calcium responses to secretagogues in corticotrophs from male rats. Endocrinology. https://doi.org/10.1210/ en.2017-00107. Roper, J., O’Carroll, A.-M., Young, W., Lolait, S., 2011. The vasopressin Avpr1b receptor: molecular and pharmacological studies. Stress 14, 98e115. https://doi.org/10.3109/10253890.2010.512376. Roper, J.A., Craighead, M., O’Carroll, A.-M., Lolait, S.J., 2010. Attenuated stress response to acute restraint and forced swimming stress in arginine vasopressin 1b receptor subtype (Avpr1b) receptor knockout mice and wild-type mice treated with a novel Avpr1b receptor antagonist. J. Neuroendocrinol. 22, 1173e1180. https:// doi.org/10.1111/j.1365-2826.2010.02070.x. Roselli-Rehfuss, L., Mountjoy, K.G., Robbins, L.S., Mortrud, M.T., Low, M.J., Tatro, J.B., Entwistle, M.L., Simerly, R.B., Cone, R.D., 1993. Identification of a receptor for gamma melanotropin and other proopiomelanocortin peptides in the hypothalamus and limbic system. Proc. Natl. Acad. Sci. U. S. A. 90, 8856e8860. Rousseau, K., Kauser, S., Pritchard, L.E., Warhurst, A., Oliver, R.L., Slominski, A., Wei, E.T., Thody, A.J., Tobin, D.J., White, A., 2007. Proopiomelanocortin (POMC), the ACTH/melanocortin precursor, is secreted by human epidermal keratinocytes and melanocytes and stimulates melanogenesis. FASEB J. 21, 1844e1856. https:// doi.org/10.1096/fj.06-7398com. Roy, S., Roy, S.J., Pinard, S., Taillefer, L.-D., Rached, M., Parent, J.-L., Gallo-Payet, N., 2011. Mechanisms of melanocortin-2 receptor (MC2R) internalization and recycling in human embryonic kidney (HEK) cells: identification of Key Ser/Thr (S/T) amino acids. Mol. Endocrinol. 25, 1961e1977. https://doi.org/10.1210/me.2011-0018. Russell, G.M., Durant, C., Ataya, A., Papastathi, C., Bhake, R., Woltersdorf, W., Lightman, S., 2014. Subcutaneous pulsatile glucocorticoid replacement therapy. Clin. Endocrinol. 81, 289e293. https://doi.org/10.1111/cen.12470. Russell, G.M., Lightman, S.L., 2014. Can side effects of steroid treatments be minimized by the temporal aspects of delivery method? Expert Opin. Drug Saf. 13, 1501e1513. https://doi.org/10.1517/ 14740338.2014.965141. Sahm, U.G., Olivier, G.W.J., Branch, S.K., Moss, S.H., Pouton, C.W., 1994. Synthesis and biological evaluation of a-MSH analogues substituted with alanine. Peptides 15, 1297e1302. https:// doi.org/10.1016/0196-9781(94)90157-0.

REFERENCES

Saiardi, A., Bozzi, Y., Baik, J.H., Borrelli, E., 1997. Antiproliferative role of dopamine: loss of D2 receptors causes hormonal dysfunction and pituitary hyperplasia. Neuron 19, 115e126. Sa´nchez-Ma´s, J., Guillo, L.A., Zanna, P., Jime´nez-Cervantes, C., Garcı´aBorro´n, J.C., 2005. Role of G protein-coupled receptor kinases in the homologous desensitization of the human and mouse melanocortin 1 receptors. Mol. Endocrinol. 19, 1035e1048. https://doi.org/ 10.1210/me.2004-0227. Sapolsky, R.M., Romero, L.M., Munck, A.U., 2000. How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions. Endocr. Rev. 21, 55e89. https://doi.org/10.1210/edrv.21.1.0389. Sbiera, S., Deutschbein, T., Weigand, I., Reincke, M., Fassnacht, M., Allolio, B., 2015. The new molecular landscape of Cushing’s disease. Trends Endocrinol. Metab. 26, 573e583. https://doi.org/ 10.1016/j.tem.2015.08.003. Schio¨th, H.B., Haitina, T., Ling, M.K., Ringholm, A., Fredriksson, R., Cerda´-Reverter, J.M., Klovins, J., 2005. Evolutionary conservation of the structural, pharmacological, and genomic characteristics of the melanocortin receptor subtypes. Peptides 26, 1886e1900. https://doi.org/10.1016/j.peptides.2004.11.034. Schio¨th, H.B., Muceniece, R., Szardenings, M., Prusis, P., Lindeberg, G., Sharma, S.D., Hruby, V.J., Wikberg, J.E.S., 1997. Characterisation of D117A and H260A mutations in the melanocortin 1 receptor. Mol. Cell. Endocrinol. 126, 213e219. https://doi.org/10.1016/S03037207(96)03993-7. Schio¨th, H.B., Raudsepp, T., Ringholm, A., Fredriksson, R., Takeuchi, S., Larhammar, D., Chowdhary, B.P., 2003. Remarkable synteny conservation of melanocortin receptors in chicken, human, and other vertebrates. Genomics 81, 504e509. Schnabel, E., Mains, R.E., Farquhar, M.G., 1989. Proteolytic processing of pro-ACTH/endorphin begins in the Golgi complex of pituitary corticotropes and AtT-20 cells. Mol. Endocrinol. 3, 1223e1235. https://doi.org/10.1210/mend-3-8-1223. Schonemann, M.D., Ryan, A.K., McEvilly, R.J., O’Connell, S.M., Arias, C.A., Kalla, K.A., Li, P., Sawchenko, P.E., Rosenfeld, M.G., 1995. Development and survival of the endocrine hypothalamus and posterior pituitary gland requires the neuronal POU domain factor Brn-2. Genes Dev. 9, 3122e3135. Schulte, H.M., Oldfield, E.H., Allolio, B., Katz, D.A., Berkman, R.A., Ali, I.U., 1991. Clonal composition of pituitary adenomas in patients with Cushing’s disease: determination by X-chromosome inactivation analysis. J. Clin. Endocrinol. Metab. 73, 1302e1308. https://doi.org/10.1210/jcem-73-6-1302. Seasholtz, A.F., Burrows, H.L., Karolyi, I.J., Camper, S.A., 2001. Mouse models of altered CRH-binding protein expression. Peptides 22, 743e751. Sebag, J.A., Hinkle, P.M., 2010. Regulation of G proteinecoupled receptor signaling: specific dominant-negative effects of melanocortin 2 receptor accessory protein 2. Sci. Signal. 3, ra28. https://doi.org/ 10.1126/scisignal.2000593. Sebag, J.A., Hinkle, P.M., 2009. Opposite effects of the melanocortin-2 (MC2) receptor accessory protein MRAP on MC2 and MC5 receptor dimerization and trafficking. J. Biol. Chem. 284, 22641e22648. https://doi.org/10.1074/jbc.M109.022400. Sebag, J.A., Hinkle, P.M., 2007. Melanocortin-2 receptor accessory protein MRAP forms antiparallel homodimers. Proc. Natl. Acad. Sci. U. S. A. 104, 20244e20249. https://doi.org/10.1073/pnas.0708916105. Seidah, N.G., Chre´tien, M., 1999. Proprotein and prohormone convertases: a family of subtilases generating diverse bioactive polypeptides. Brain Res. 848, 45e62. https://doi.org/10.1016/ S0006-8993(99)01909-5. Published on the World Wide Web on 17 August 1999.

167

Seidah, N.G., Marcinkiewicz, M., Benjannet, S., Gaspar, L., Beaubien, G., Mattei, M.G., Lazure, C., Mbikay, M., Chre´tien, M., 1991. Cloning and primary sequence of a mouse candidate prohormone convertase PC1 homologous to PC2, Furin, and Kex2: distinct chromosomal localization and messenger RNA distribution in brain and pituitary compared to PC2. Mol. Endocrinol. 5, 111e122. https://doi.org/10.1210/mend-5-1-111. Seidah, N.G., Mayer, G., Zaid, A., Rousselet, E., Nassoury, N., Poirier, S., Essalmani, R., Prat, A., 2008. The activation and physiological functions of the proprotein convertases. Int. J. Biochem. Cell Biol. 40, 1111e1125. https://doi.org/10.1016/j.biocel.2008.01.030. Sharp, B., Pekary, A.E., 1981. b-endorphin 61e91 and other b-endorphin-immunoreactive peptides in human semen. J. Clin. Endocrinol. Metab. 52, 586e588. https://doi.org/10.1210/jcem-52-3-586. Sherman, B., Wysham, C., Pfohl, B., 1985. Age-related changes in the circadian rhythm of plasma cortisol in man. J. Clin. Endocrinol. Metab. 61, 439e443. https://doi.org/10.1210/jcem-61-3-439. Shibahara, S., Takeda, K., Yasumoto, K., Udono, T., Watanabe, K., Saito, H., Takahashi, K., 2001. Microphthalmia-associated transcription factor (MITF): multiplicity in structure, function, and regulation. J. Investig. Dermatol. Symp. Proc. 6, 99e104. https:// doi.org/10.1046/j.0022-202x.2001.00010.x. Shibuya, I., Douglas, W.W., 1993. Spontaneous cytosolic calcium pulsing detected in Xenopus melanotrophs: modulation by secretoinhibitory and stimulant ligands. Endocrinology 132, 2166e2175. https://doi.org/10.1210/endo.132.5.8386613. Shipston, M.J., Antoni, F.A., 1991. Early glucocorticoid feedback in anterior pituitary corticotrophs: differential inhibition of hormone release induced by vasopressin and corticotrophin-releasing factor in vitro. J. Endocrinol. 129, 261e268. Shu-Dong, T., Phillips, D.M., Halmi, N., Krieger, D., Bardin, C.W., 1982. Beta-endorphin is present in the male reproductive tract of five species. Biol. Reprod. 27, 755e764. Smith, G.W., Aubry, J.M., Dellu, F., Contarino, A., Bilezikjian, L.M., Gold, L.H., Chen, R., Marchuk, Y., Hauser, C., Bentley, C.A., Sawchenko, P.E., Koob, G.F., Vale, W., Lee, K.F., 1998. Corticotropin releasing factor receptor 1-deficient mice display decreased anxiety, impaired stress response, and aberrant neuroendocrine development. Neuron 20, 1093e1102. Spencer, J.D., Schallreuter, K.U., 2009. Regulation of pigmentation in human epidermal melanocytes by functional high-affinity betamelanocyte-stimulating hormone/melanocortin-4 receptor signaling. Endocrinology 150, 1250e1258. https://doi.org/ 10.1210/en.2008-1212. Spiga, F., Harrison, L.R., Wood, S., Knight, D.M., MacSweeney, C.P., Thomson, F., Craighead, M., Lightman, S.L., 2009. Blockade of the V(1b) receptor reduces ACTH, but not corticosterone secretion induced by stress without affecting basal hypothalamic-pituitaryadrenal axis activity. J. Endocrinol. 200, 273e283. https:// doi.org/10.1677/JOE-08-0421. Spiga, F., Walker, J.J., Terry, J.R., Lightman, S.L., 2014. HPA axisrhythms. Compr. Physiol. 4, 1273e1298. https://doi.org/10.1002/ cphy.c140003. Spiga, F., Zavala, E., Walker, J.J., Zhao, Z., Terry, J.R., Lightman, S.L., 2017. Dynamic responses of the adrenal steroidogenic regulatory network. Proc. Natl. Acad. Sci. U. S. A. 114, E6466eE6474. https://doi.org/10.1073/pnas.1703779114. Stack, J., Surprenant, A., 1991. Dopamine actions on calcium currents, potassium currents and hormone release in rat melanotrophs. J. Physiol. 439, 37e58. Stojilkovic, S.S., Tabak, J., Bertram, R., 2010. Ion channels and signaling in the pituitary gland. Endocr. Rev. 31, 845e915. https://doi.org/ 10.1210/er.2010-0005.

168

8. ANTERIOR PITUITARY AND PARS INTERMEDIA SPACE: CORTICOTROPHS (ACTH) AND MELANOTROPHS (a-MSH)

Sundstro¨m, G., Dreborg, S., Larhammar, D., 2010. Concomitant duplications of opioid peptide and receptor genes before the origin of jawed vertebrates. PLoS One 5, e10512. https://doi.org/10.1371/ journal.pone.0010512. Swope, V., Jameson, J., McFarland, K., Supp, D., Miller, W., McGraw, D., Patel, M.A., Nix, M.A., Milhauser, G., Babcock, G., Abdel-Malek, Z.A., 2012. Defining MC1R regulation in human melanocytes by its agonist a-melanocortin and antagonists agouti signaling protein and b-defensin 3. J. Investig. Dermatol. 132, 2255e2262. https://doi.org/10.1038/jid.2012.135. Takeuchi, M., 2001. The mammalian pars intermedia e structure and function. Zool. Sci. 18, 133e144. Takumi, I., Steiner, D.F., Sanno, N., Teramoto, A., Osamura, R.Y., 1998. Localization of prohormone convertases 1/3 and 2 in the human pituitary gland and pituitary adenomas: analysis by immunohistochemistry, immunoelectron microscopy, and laser scanning microscopy. Mod. Pathol. 11, 232e238. Tam, W.W.H., Andreasson, K.I., Loh, Y.P., 1993. The amino-terminal sequence of pro-opiomelanocortin directs intracellular targeting to the regulated secretory pathway. Eur. J. Cell Biol. 62, 294e306. Tanoue, A., Ito, S., Honda, K., Oshikawa, S., Kitagawa, Y., Koshimizu, T.-A., Mori, T., Tsujimoto, G., 2004. The vasopressin V1b receptor critically regulates hypothalamic-pituitary-adrenal axis activity under both stress and resting conditions. J. Clin. Investig. 113, 302e309. https://doi.org/10.1172/JCI19656. Taraskevich, P.S., Douglas, W.W., 1990. Dopamine (D2) or gammaaminobutyric acid (GABAB) receptor activation hyperpolarizes rat melanotrophs and pertussis toxin blocks these responses and the accompanying fall in [Ca2þ]i. Neurosci. Lett. 112, 205e209. Taraskevich, P.S., Douglas, W.W., 1982. GABA directly affects electrophysiological properties of pituitary pars intermedia cells. Nature 299, 733e734. Tatro, J.B., Reichlin, S., 1987. Specific receptors for alpha-melanocytestimulating hormone are widely distributed in tissues of rodents. Endocrinology 121, 1900e1907. https://doi.org/10.1210/endo121-5-1900. Therrien, M., Drouin, J., 1991. Pituitary pro-opiomelanocortin gene expression requires synergistic interactions of several regulatory elements. Mol. Cell. Biol. 11, 3492e3503. Thiboutot, D., Sivarajah, A., Gilliland, K., Cong, Z., Clawson, G., 2000. The melanocortin 5 receptor is expressed in human sebaceous glands and rat preputial cells. J. Investig. Dermatol. 115, 614e619. https://doi.org/10.1046/j.1523-1747.2000.00094.x. Thody, A.J., Higgins, E.M., Wakamatsu, K., Ito, S., Burchill, S.A., Marks, J.M., 1991. Pheomelanin as well as eumelanin is present in human epidermis. J. Investig. Dermatol. 97, 340e344. Thody, A.J., Shuster, S., 1975. Control of sebaceous gland function in the rat by alpha-melanocyte-stimulating hormone. J. Endocrinol. 64, 503e510. Thody, A.J., Shuster, S., 1973. Possible role of MSH in the mammal. Nature 245, 207e209. https://doi.org/10.1038/245207a0. Timpl, P., Spanagel, R., Sillaber, I., Kresse, A., Reul, J.M., Stalla, G.K., Blanquet, V., Steckler, T., Holsboer, F., Wurst, W., 1998. Impaired stress response and reduced anxiety in mice lacking a functional corticotropin-releasing hormone receptor 1. Nat. Genet. 19, 162e166. https://doi.org/10.1038/520. Tomiko, S.A., Taraskevich, P.S., Douglas, W.W., 1983. GABA acts directly on cells of pituitary pars intermedia to alter hormone output. Nature 301, 706e707. Tonon, M.-C., Danger, J.-M., Lamacz, M., Leroux, P., Adjeroud, S., Andersen, A.C., Verburg-van, K.L., Jenks, B.G., Pelletier, G., Stoeckel, L., Burlet, A., Kupryszewski, G., Vaudry, H., 1988. Multihormonal control of melanotropin secretion in cold-blooded vertebrates. In: The Melanotropic Peptides. Taylor & Francis Group.

Tonon, M.C., Desrues, L., Lamacz, M., Chartrel, N., Jenks, B., Vaudry, H., 1993. Multihormonal regulation of pituitary melanotrophs. Ann. N. Y. Acad. Sci. 680, 175e187. Tremblay, J.J., Lanctoˆt, C., Drouin, J., 1998. The pan-pituitary activator of transcription, Ptx1 (pituitary homeobox 1), acts in synergy with SF-1 and Pit1 and is an upstream regulator of the Limhomeodomain gene Lim3/Lhx3. Mol. Endocrinol. 12, 428e441. https://doi.org/10.1210/mend.12.3.0073. Trouslard, J., Demeneix, B.A., Feltz, P., 1989. Spontaneous spiking activities of porcine pars intermedia cells: effects of thyrotropinreleasing hormone. Neuroendocrinology 50, 33e43. https:// doi.org/10.1159/000125199. Tse, A., Lee, A.K., 2000. Voltage-gated Ca2þ channels and intracellular Ca2þ release regulate exocytosis in identified rat corticotrophs. J. Physiol. 528 (Pt 1), 79e90. Tse, A., Lee, A.K., 1998. Arginine vasopressin triggers intracellular calcium release, a calcium-activated potassium current and exocytosis in identified rat corticotropes. Endocrinology 139, 2246e2252. https://doi.org/10.1210/endo.139.5.5999. Tsigos, C., 1999. Isolated glucocorticoid deficiency and ACTH receptor mutations. Arch. Med. Res. 30, 475e480. https://doi.org/10.1016/ S0188-0128(99)00057-3. Va˚ge, D.I., Lu, D., Klungland, H., Lien, S., Adalsteinsson, S., Cone, R.D., 1997. A non-epistatic interaction of agouti and extension in the fox, Vulpes vulpes. Nat. Genet. 15, 311e315. https://doi.org/10.1038/ ng0397-311. Vale, W., Spiess, J., Rivier, C., Rivier, J., 1981. Characterization of a 41residue ovine hypothalamic peptide that stimulates secretion of corticotropin and beta-endorphin. Science 213, 1394e1397. Vale, W., Vaughan, J., Smith, M., Yamamoto, G., Rivier, J., Rivier, C., 1983. Effects of synthetic ovine corticotropin-releasing factor, glucocorticoids, catecholamines, neurohypophysial peptides, and other substances on cultured corticotropic cells. Endocrinology 113, 1121e1131. https://doi.org/10.1210/endo-113-3-1121. Valentijn, J.A., Louiset, E., Vaudry, H., Cazin, L., 1991a. Involvement of non-selective cationic channels in the generation of pacemaker depolarizations and firing behaviour in cultured frog melanotrophs. Brain Res. 560, 175e180. Valentijn, J.A., Louiset, E., Vaudry, H., Cazin, L., 1991b. Dopamine regulates the electrical activity of frog melanotrophs through a G protein-mediated mechanism. Neuroscience 44, 85e95. Vallette-Kasic, S., Brue, T., Pulichino, A.-M., Gueydan, M., Barlier, A., David, M., Nicolino, M., Malpuech, G., De´chelotte, P., Deal, C., Van Vliet, G., De Vroede, M., Riepe, F.G., Partsch, C.-J., Sippell, W.G., Berberoglu, M., Atasay, B., de Zegher, F., Beckers, D., Kyllo, J., Donohoue, P., Fassnacht, M., Hahner, S., Allolio, B., Noordam, C., Dunkel, L., Hero, M., Pigeon, B., Weill, J., Yigit, S., Brauner, R., Heinrich, J.J., Cummings, E., Riddell, C., Enjalbert, A., Drouin, J., 2005. Congenital isolated adrenocorticotropin deficiency: an underestimated cause of neonatal death, explained by TPIT gene mutations. J. Clin. Endocrinol. Metab. 90, 1323e1331. https://doi.org/10.1210/jc.2004-1300. Valverde, P., Healy, E., Jackson, I., Rees, J.L., Thody, A.J., 1995. Variants of the melanocyte-stimulating hormone receptor gene are associated with red hair and fair skin in humans. Nat. Genet. 11, 328e330. https://doi.org/10.1038/ng1195-328. van Staa, T.P., Leufkens, H.G., Abenhaim, L., Begaud, B., Zhang, B., Cooper, C., 2000. Use of oral corticosteroids in the United Kingdom. QJM 93, 105e111. Vance, K.W., Goding, C.R., 2004. The transcription network regulating melanocyte development and melanoma. Pigm. Cell Res. 17, 318e325. https://doi.org/10.1111/j.1600-0749.2004.00164.x. Vaudry, H., Chartrel, N., Desrues, L., Galas, L., Kikuyama, S., Mor, A., Nicolas, P., Tonon, M.C., 2006. The pituitary-skin connection in amphibians: reciprocal regulation of melanotrope cells and dermal

REFERENCES

melanocytes. Ann. N. Y. Acad. Sci. 885, 41e56. https://doi.org/ 10.1111/j.1749-6632.1999.tb08664.x. Veldhuis, J.D., 2013. Changes in pituitary function with aging and implications for patient care. Nat. Rev. Endocrinol. 9, 205e215. https://doi.org/10.1038/nrendo.2013.38. Verma, S., Vanryzin, C., Sinaii, N., Kim, M.S., Nieman, L.K., Ravindran, S., Calis, K.A., Arlt, W., Ross, R.J., Merke, D.P., 2010. A pharmacokinetic and pharmacodynamic study of delayed- and extended-release hydrocortisone (Chronocort) vs. conventional hydrocortisone (Cortef) in the treatment of congenital adrenal hyperplasia. Clin. Endocrinol. 72, 441e447. https://doi.org/ 10.1111/j.1365-2265.2009.03636.x. Wakamatsu, K., Graham, A., Cook, D., Thody, A.J., 1997. Characterisation of ACTH peptides in human skin and their activation of the melanocortin-1 receptor. Pigm. Cell Res. 10, 288e297. Wallingford, N., Perroud, B., Gao, Q., Coppola, A., Gyengesi, E., Liu, Z.-W., Gao, X.-B., Diament, A., Haus, K.A., Shariat-Madar, Z., Mahdi, F., Wardlaw, S.L., Schmaier, A.H., Warden, C.H., Diano, S., 2009. Prolylcarboxypeptidase regulates food intake by inactivating alpha-MSH in rodents. J. Clin. Investig. 119, 2291e2303. https:// doi.org/10.1172/JCI37209. Weber, A., Clark, A.J.L., 1994. Mutations of the ACTH receptor gene are only one cause of familial glucocorticoid deficiency. Hum. Mol. Genet. 3, 585e588. https://doi.org/10.1093/hmg/3.4.585. Wislocki, G.B., 1940. The topography of the hypophysis in the elephant, manatee and hyrax. Anat. Rec. 77, 427e445.

169

Yang, Y. k, Dickinson, C., Haskell-Luevano, C., Gantz, I., 1997. Molecular basis for the interaction of [Nle4,D-Phe7]melanocyte stimulating hormone with the human melanocortin-1 receptor. J. Biol. Chem. 272, 23000e23010. Yaswen, L., Diehl, N., Brennan, M.B., Hochgeschwender, U., 1999. Obesity in the mouse model of pro-opiomelanocortin deficiency responds to peripheral melanocortin. Nat. Med. 5, 1066e1070. https://doi.org/10.1038/12506. Zaidi, M., Sun, L., Robinson, L.J., Tourkova, I.L., Liu, L., Wang, Y., Zhu, L.-L., Liu, X., Li, J., Peng, Y., Yang, G., Shi, X., Levine, A., Iqbal, J., Yaroslavskiy, B.B., Isales, C., Blair, H.C., 2010. ACTH protects against glucocorticoid-induced osteonecrosis of bone. Proc. Natl. Acad. Sci. U. S. A. 107, 8782e8787. https://doi.org/ 10.1073/pnas.0912176107. Zemkova, H., Tomic, M., Kucka, M., Aguilera, G., Stojilkovic, S.S., 2016. Spontaneous and CRH-induced excitability and calcium signaling in mice corticotrophs involves sodium, calcium, and cationconducting channels. Endocrinology 157, 1576e1589. https:// doi.org/10.1210/en.2015-1899. Zhong, Q., Sridhar, S., Ruan, L., Ding, K.-H., Xie, D., Insogna, K., Kang, B., Xu, J., Bollag, R.J., Isales, C.M., 2005. Multiple melanocortin receptors are expressed in bone cells. Bone 36, 820e831. https:// doi.org/10.1016/j.bone.2005.01.020. Zhou, Y., Zhang, X., Klibanski, A., 2014. Genetic and epigenetic mutations of tumor suppressive genes in sporadic pituitary adenoma. Mol. Cell. Endocrinol. 386, 16e33. https://doi.org/10.1016/ j.mce.2013.09.006.