The hypothalamic-pituitary axis; co-development of two organs

833

The hypothalamic-pituitary axis: co-development of two organs Mathias Treier* and Michael G Rosenfeldt Development of the anterior pituitary gland ultimately leads to the appearance of five distinct cell types that are defined by the trophic hormones which they produce, providing an instructive model system for elucidating the molecular mechanisms that underlie the determination of distinct cell phenotypes within an organ from a common precursor lineage. The recent identification of several homeodomain transcription factors expressed specifically in the anterior pituitary gland has revealed a transcriptional cascade orchestrating a developmental program that leads to the determination of the five mature cell types. Recent data from gene-targeting experiments in mice further imply that the execution of this program is dependent on inductive signals originating in the floor of the diencephalon. Addresses Howard Hughes Medical Institute, University of California at San Diego, Department and School of Medicine, 9500 Gilman Drive, La Jolla, CA 92093-0648, USA *e-mail: [email protected] re-mail: [email protected] Current Opinion in Cell Biology 1996, 8:833-843 © Current Biology Ltd ISSN 0955-0674 Abbreviations ACTH adrenocorticotropin AP-1 activator protein-1 AVP arginine vasopressin E embryonic day of development c~GSU ~-glycoprotein subunit FSH follicle-stimulating hormone GH growth hormone GHF GH factor GHRH GH-releasing hormone LH luteinizing hormone MAPK mitogen-activated protein kinase OT oxytocin POMC pro-opiomelanocortin P-OTX pituitary OTX-related factor PRL prolactin Ptxl pituitary homeobox 1 PVH paraventricular hypothalamic Rpx Rathke's pouch homeobox SF-1 steroidogenic factor-1 SO supraoptic T/EBP thyroid-specific enhancer-binding protein TSH thyroid-stimulating hormone TTF-1 thyroid-specific transcription factor-1 XANF-2 Xenopusanterior neural fold-2

Introduction T h e anterior pituitary gland exerts critical functions in the homeostatic control of vertebrates. The gland integrates complex feedback mechanisms, receiving information from the brain via the hypothalamus, and signaling to peripheral endocrine organs such as the thyroid, adrenal gland and gonads, thereby regulating

such vital processes as metabolism, growth, reproduction and behavior. T h e five cell types that compose the anterior pituitary gland are defined by the hormones that they produce and secrete. Corticotropes secrete adrenocorticotropin (ACTH) which regulates adrenal function by proteolytic processing of pro-opiomelanocortin (POMC); thyrotropes secrete thyroid-stimulating hormone (TSH) which controls thyroid gland growth and hormone production; and gonadotropes secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH) which influence gonadal function. FSH, L H and T S H are heterodimeric polypeptide hormones consisting of a common ~x-glycoprotein subunit (~GSU) and a distinct 13 subunit (FSH13, LH13 or TSH13). Somatotropes secrete growth hormone (GH) and lactotropes secrete prolactin (PRL), which regulate linear growth and milk production, respectively. T h e description of the temporal and spatial expression patterns of the above-mentioned peptide hormones has revealed a precise order in which these five cell types appear during anterior pituitary development [1,2], making it an excellent model system in which to investigate the transcriptional mechanisms that regulate cell-specific gene expression during mammalian organogenesis. Characterization of the regulatory sequences that govern the cell-specific expression of individual hormones has led to the identification of the anterior pituitary specific transcription factor Pit-1 (also known as GHF-1) [3,4], the subject of several previous reviews [5-7]. Classical embryological experiments have suggested that the development of the anterior pituitary is dependent on inductive signals from the diencephalon [8,9], which will later give rise to the hypothalamus. How these two organs, each of different embryological origin, co-develop to produce the hypothalamic-pituitary axis introduces a new level of developmental complexity to study. Results obtained during the past two years have begun to shed light on this interplay. In this review, we will focus on the recent identification and characterization of tissue-restricted transcription factors expressed in the hypothalamic-pituitary axis, with the aim of highlighting them in the context of a signal-dependent transcriptional cascade that regulates anterior pituitary determination and differentiation.

Origin a n d early c o m m i t m e n t of the a n t e r i o r pituitary g l a n d Rathke's pouch, the primordium of the anterior pituitary, first appears as an invagination of an ectodermal layer of cells beneath the diencephalon at around embryonic day of development (E)8.5 in the mouse [10]. Fate-map

834

Cell differentiation

analyses in frog, chicken and mouse have shown that the most anterior part of the neural plate [11-13], the so-called anterior neural ridge, will later give rise to non-neural structures, including the anterior pituitary, the nasal cavity ectoderm and the olfactory placode. In contrast, the adjacent region of the neural plate will become the most anterior neural structures, encompassing hypothalamus, posterior pituitary, optic vesicles and ventral forebrain. These results indicate that, before the first visible appearance of Rathke's pouch, certain cell compartments are already precommitted and possess the competence to develop into an anterior pituitary. The homeodomain factors Six3 and Rpx T h e cloning of the homeobox gene Six3 [14"], the mouse homolog of the Drosophila sine oculis gene, has provided, together with Pax6, a molecular marker which exhibits an expression pattern that is consistent with the above mentioned fate-map studies [15,16]. Six3 expression is first detected at approximately E6.5 in mouse in the most anterior border of the neural plate, and later Six3 is highly expressed in Rathke's pouch and in the hypothalamus and other anterior structures. Six1, another member of this gene family, exhibits a pattern of expression that overlaps with that of Six3 in Rathke's pouch [17]. Identification of the mouse homeobox gene Rpx [18"] (Rathke's pouch homeobox; also known as Hesxl [19]), which is related to the paired gene of Drosophila, has provided an additional marker. Rpx, like Six3, is expressed early in development in the anterior neural plate of the mouse embryo. In contrast to Six3, however, Rpx expression later becomes restricted solely to Rathke's pouch. Therefore, it is likely that Rpx is involved in the determination of the anterior pituitary and subdivides the Six3 + compartment into smaller developmental units during organogenesis. Loss-of-function studies for Six3 and Rpx will be required to prove that both genes are indeed essential for the early commitment of the anterior pituitary. XANF-2, the Xenopus homolog of Rpx T h e analysis of the Xenopus homolog of Rpx, XANF-2 (for Xenopus anterior neural fold-2) [20], has revealed some of the early inductive signals that lead to the expression of this homeobox gene. Members of the Hedgehog family in Xenopus are able to directly induce XANF-2 expression in animal cap explants [21]. T h e expression of XANF-2 by itself seems not to be sufficient to drive terminal differentiation of the anterior pituitary, however, as there is a lack of expression of the late pituitary specific marker POMC. POMC expression can be induced, however, in ectodermal explants by coculture with mesoderm [22], strongly suggesting that full anterior pituitary differentiation requires additional signaling from the underlying mesoderm. Consistent with this hypothesis, the activation of XANF-2 by Hedgehog is highly

elevated in activin-treated animal caps which therefore contain mesoderm [23"]. S p e c i f i c a t i o n o f a n t e r i o r p i t u i t a r y cell l i n e a g e s Determination of the anterior pituitary occurs during the formation of Rathke's pouch, when the ectodermal primordium of the anterior and intermediate lobes of the pituitary comes into contact with the neuroectoderm of the ventral diencephalon. Inductive interactions at this stage are required for the proper development of Rathke's pouch into the mature pituitary gland (see Fig. 1). The LIM homeodomain factor Lhx3/P-Lim/mLim-3 Concurrent with the organ determination, the mouse LIM homeobox gene Lhx3 [24] (also known as P-Lira [25] and mLim-3 [26]) is expressed in Rathke's pouch. Lhx3, like its Xenopus counterpart Xlim-3 [27], continues to be expressed throughout the course of anterior pituitary development and in the mature gland. Targeted gene disruption in mice has shown that Lhx3 is essential for differentiation and proliferation of anterior pituitary lineages [28"]. Although Rathke's pouch initially forms in Lhx3 mutant embryos and Rpx expression is detected, the pituitary fails to grow and differentiate. In addition, analyses with pituitary-specific markers reveal that, with the exception of the corticotropes, none of the pituitary cell lineages have formed. Therefore, either Lhx3 is not expressed in the precursor of the corticotropes, or it is not required for the determination of this cell lineage. These results suggest that the time between E10.5-E12.5 in mouse embryonic development is the critical period for Lhx3 gene function in anterior pituitary ontogenesis. Expression of Lhx3 in the mature gland also points to a role of Lhx3 in maintenance of one or more of the cell types [25]. Indeed, it has been shown that Lhx3 can bind to, and activate, the cxGSU promoter and can synergize with Pit-1 in transcriptional activation of genes encoding terminal differentiation markers such as TSH[3 and prolactin [25]. T h e etGSU transcript appears at approximately the same time as does Lhx3 in the presumptive Rathke's pouch, and subsequently becomes restricted to thyrotropes and gonadotropes [1]. Although Rathke's pouch is forming, c~GSU expression is absent in Lhx3 mutant animals, strongly suggesting that c~GSU expression may be directly regulated by Lhx3. Nevertheless, immunohistochemistry will be required to finally establish which cell lineage(s) in the mature gland express Lhx3. Recently, it was shown that LH-2, another LIM homeobox gene, is also expressed in pituitary cell lines and that its gene product is able to bind to an enhancer element in the &GSU gene [29]. A requirement for LH-2 during pituitary development has yet to be established, however. Interestingly, the analysis of expression patterns for LIM homeobox genes during the development of the rainbow trout has revealed that more than one LIM homeodomain factor is expressed in the pituitary [30]. This points to the possibility that, as in spinal cord development of the mouse where

The hypothalamic-pituitary axis Treier and Rosenfeld

several different LIM homeobox genes are expressed in a combinatorial fashion [31], cell-type specification in the pituitary may be also determined by a combinatorial code of LIM homeodomain factors.

The h o m e o d o m a i n factor Ptxl/P-OTX

It has been suggested that a factor other than Lhx3 may influence corticotrope gene activation. This factor was independently cloned by two laboratories and named Ptxl (pituitary homeobox 1) and P - O T X (pituitary O T X related factor) [32°,33°], respectively. Ptxl/P-OTX, a homeobox-containing transcription factor, is, early in development, expressed in most cells of Rathke's pouch and later becomes restricted to corticotropes in the mature gland. Additional domains of expression include the first branchial arch and the nasal and oral epithelium in the head region, in addition to the hindlimb. Its homeobox is most closely related to those of the anterior-specific genes Otx-1 and Otx-2 in mammals and bicoid and orthodenticle in

Drosophila. Ptxl was cloned by expression screening of an ART-20 library with a defined corticotrope-specific regulatory element of the POMC promoter. Ptx was further shown to have only POMC promoter specific activity in cotransfection studies in heterologous cells. In the adult gland, Ptxl transcripts colocalize with A C T H [32°]. In contrast, P - O T X was identified by a two-hybrid screen for factors that interact with the amino terminus of Pit-1 (see below). In transient transfection studies, P-OTX activates the promoters of o~GSU, POMC and growth hormone, and exhibits synergistic interactions with Pit-1 on the prolactin gene [33°]. Clearly, the full developmental role of this factor remains to be clarified.

835

differentiation of these three cell lineages. Allelic mouse mutants known as Snell or Jackson (dw) animals, which have been identified by their dwarf phenotypes, have been shown to harbor mutations in the Pit-1 gene [36]. T h e hypoplastic pituitary glands of these dwarf animals lack thyrotropes, somatotropes and lactotropes, indicating that functional Pit-1 is required for the proliferation and/or survival of these three cell types, and for the transcriptional activation of their trophic hormone genes. T h e Ames (dr) mouse transmits a recessive mutation mapped to mouse chromosome 11 [37], distinct from the Pit-1 gene locus on mouse chromosome 16. These Ames mice exhibit a phenotype similar to that of the Snell and Jackson animals, displaying hypocellularity of their anterior pituitaries due to a general lack of thyrotropes, somatotropes and lactotropes [38,39]. T h e cloning of the gene that harbors the mutation that produces the dwarf phenotype of the Ames mouse may shed light on the early activation of the Pit-1 gene. Recent studies of the Ames mice have shown that the initial activation of the Pit-1 gene is not detectable by in situ hybridization [40°]. Only rarely are cells observed which immunostain for TSH, G H or P R L in Ames animals. T h e same cells that are positive for TSH, G H or PRL also immunostain for Pit-l, showing that they are Pit-l-dependent and following the normal developmental pathway [41°]. This contrasts with pituitaries from Snell and Jack~on animals in which initial transactivation of the Pit-1 gene occurs, but in which G H and P R L are never detected because the Pit-1 transcript encodes an nonfunctional Pit-1 protein [36].

The POU h o m e o d o m a i n factor Pit-1

T h e difference between the Ames and Snell/Jackson pituitaries is consistent with a study indicating that G H and P R L may exert neurotrophic effects on dopaminergic neurons in the arcuate nucleus of the hypothalamus [42]. In Snell animals nearly all dopaminergic neurons in the arcuate nucleus are missing, whereas their number is only reduced by half in the Ames animals. An additional study has further shown, by assaying the behavior of df/df cells in chimeric mice consisting of df/df and normal (wt/wt) cells, that the d f g e n e product must be a cell-autonomous factor [41°]. T h e detailed analysis of the Ames mouse, and its near identical phenotype to Snell animals, has provided compelling evidence that the df gene product and Pit-1 function in a common developmental pathway during anterior pituitary ontogenesis.

Pit-1 is a transcription factor that was initially identified (and its gene cloned) as a transcriptional activator of the G H and PRL genes [3,4]. Its expression is restricted to the thyrotropes, somatotropes and lactotropes of the anterior pituitary gland. Pit-1 gene activation precedes, both temporally and spatially, the activation of transcription of its distal target genes, which include the GH, PRL, GH-releasing hormone (GHRH) receptor [35] and TSH[3 [36] genes. Mouse genetics has provided direct evidence for an essential role of Pit-1 in the determination and

If the df mutation results in a complete loss-of-function mutation, the appearance of rare Pit-l-positive cells would argue that the df gene product may work in parallel to Pit-1. Alternatively, the df mutation may produce only a partial loss-of-function of the encoded protein, and in this case the gene product may function upstream of the Pit-1 gene. T h e resolution of this question awaits the cloning of the df gene and characterization of the underlying mutation in the Ames mouse.

As mentioned above, Lhx3 is required for the proliferation and determination of the gonadotrope, thyrotrope, somatotrope and lactotrope lineages. T h e further differentiation of the Lhx3/Ptxl/P-OTX + cells into the four lineages can be monitored by the appearance of two transcription factors: the POU domain transcription factor Pit-1 and the orphan nuclear receptor SF-1 (steroidogenic factor-l) [34]. Both factors are first detectable at E13.5 in the anterior pituitary gland (see below).

836

Cell differentiation

Figure 1

Embryonic day

Factor ontogeny

Organogenesis Endocrine hypothalamus

7.0

iE "7

x I-0

Organ competence Anterior pituitary

9.0

Organ determination

1 1.0

Cell-type determination

14.5

Cell-type differentiation

Birth

Organ maturation

\

IMP-4

t

GF

;,,._-,, "" " POMC

TSH r

GH FSH LH

('~

PRL © 1996 Current Opinion in Cell Biology

The hypothalamic-pituitary axis Treier and Rosenfeld

837

Figure 1 Development of the anterior pituitary gland in the mouse. At ET.0 in mouse, the most anterior midline region of the neural plate, the so-called anterior neural ridge, is precommitted to an anterior pituitary fate. The adjacent rnidline region of the neural plate gives rise to the endocrine hypothalamus. The black crescent-shaped region symbolizing the future head ectoderm also possesses the competence to become anterior pituitary if brought into contact experimentally with the floor of the diencephalon. At this developmental stage, three homeodomain factors, Pax6, Rpx, and Six3, are expressed in the anterior region in an overlapping pattern. Two days later at E9.0, Rathke's pouch, the primordium of the anterior pituitary gland, appears as an invagination of an ectodermal layer of cells that contacts the neuroectoderm of the primordium of the ventral hypothalamus. Concurrently, expression of two homeodomain factors, Lhx3/P-Lim and Ptxl/P-OTX, is seen in Rathke's pouch. Another two days later, before the appearance of the trophic hormones, cell-type determination has occurred. Secreted factors such as WNT and bone morphogenetic protein-4 (BMP-4), which originate in the diencephalon, in addition to fibroblast growth factors (FGFs) and Hedgehog (HH), may be involved in this patterning process. At approximately E13.5, expression of the POU homeodomain factor Pit-1 and the orphan nuclear factor SF-1 can be detected, starting the final cell-type differentiation. At birth, the five different cell types are present: corticotropes which produce ACTH by proteolytic processing of POMC; gonadotropes which produce FSH and LH; and the three Pit-l-dependent cell types, which are thyrotropes which produce TSHI3, somatotropes which produce GH and lactotropes which produce prolactin (PRL). The rostral tip thyrotropes (TSHr), which are a Pit-l-independent cell population, disappear near the time of birth [82].

The orphan nuclear receptor SF-1

SF-1 is expressed in the adrenal gland, the gonads, the ventral diencephalon and the gonadotropes of the anterior pituitary gland. Targeted disruption of the Ftz-F1 gene, which encodes SF-1, has shown that this nuclear receptor is essential for adrenal and gonadal development [43]. A follow-up study of the Ftz-Fl-disrupted mice has focused on the potential role of SF-1 in the gonadotropes, showing that three gonadotropic-specific markers, LHI3, FSHI3 and the receptor for gonadotropin-releasing hormone (GnRH), are missing in the mutant mice [44]. As the appearance of SF-I temporarily precedes the expression of the gonadotropic hormones, it was suggested that SF-1 is involved in the establishment of the gonadotropic lineage and/or directly regulates the expression of multiple genes required for gonadotropin expression. Surprisingly, it was found later that treatment of the Ftz-Fl-disrupted mice with G n R H fully restores the expression of L H and FSH in the anterior pituitary [45], demonstrating that the gonadotropic lineage is present in these animals, making it unlikely that SF-1 is solely required for the expression of these hormones. Regulation

of the trophic

hormones

Several new findings in the past two years have extended our understanding of the regulation of expression of the trophic hormone genes in the anterior pituitary gland, and will help in deciphering the molecular mechanisms that dictate their cell type specific expression.

TSHI~, growth hormone and prolactin Although Pit-1 is expressed in the thyrotropes, somatotropes and lactotropes and is required for the activation of expression of TSHI3, G H and prolactin, expression of each trophic hormone gene is restricted to a single cell type. Therefore, additional mechanisms must exist that dictate gene-specific action by Pit-1. Interactions of Pit-1 with other transcription factors are well established for the different distal target genes. For example, the estrogen receptor has been shown to produce synergistic transactivation with Pit-1 on the

prolactin distal enhancer [46], while on the G H gene, the zinc-finger protein Zn-15 and the thyroid hormone receptor may potentially be required [47,48], together with other factors, to interact with Pit-1 for full gene activation. Finally, synergy between Pit-1 and an AP-l-like factor has been suggested in activation of the TSHI] gene [49]. Evidence from transgenic experiments suggests that, in addition to synergistic gene activation, there may be mechanisms operating to exclude expression of prolactin in thyrotropes [50]. Although these putative restricting factors await identification, possible candidates, which belong to the KriJppel family of zinc-finger-containing transcription factors [51], have been identified recently and may work as such repressors. Interestingly, a recent study has shown that Pit-1 itself can display specificity in its action [52"], providing a second potential mechanism for cell type specific gene activation events. It was found that, depending on the DNA site to which it binds, Pit-1 will bind as either a monomer or a dimer, and that this difference allows Pit-1 to make specific use of a tyrosine-dependent synergy domain in its amino terminus. Thus, the critical DNA site in the prolactin enhancer, for which this domain is required, binds Pit-1 as a monomer, whereas the Pit-l-binding sites in the G H gene, which do not require this synergy domain, bind Pit-1 as a dimer. Prolactin gene transcription is hormonally regulated and its promoter is responsive to the Ras/MAPK (mitogenactivated protein kinase) signaling pathway in vitro [53]. Although Pit-1 is phosphorylated in response to several signaling pathways, recent data suggest that these phosphorylations are not required for hormonal regulation of the prolactin promoter. Instead, it has been shown that the prolactin enhancer encompasses several consensus DNA binding sites for members of the Ets transcription factor family [54,55], which are well known to integrate Ras signaling. These studies show that Pit-1 can functionally synergize with Ets-domain factors and that this interaction underlies the transcriptional responsiveness of the prolactin gene to the Ras/MAPK

838

Cell differentiation

pathway. The identities of the endogenous Ets-domain factors and their roles in viv'o remain to be determined. The signals that are required to let a Pit-1 + precursor cell differentiate into one of the three potential cell lines (thyrotropes, somatotropes and lactotropes) have remained elusive. Studies with the established cell line GH-3, which is derived from a rat anterior pituitary tumor, have provided some hints about how this cell-fate decision may be accomplished [56]. GH-3 cells can produce both PRL and GH; therefore, they can be considered as bipotential precursor cells [56]. GHRH induces the conversion of GH-3 cells into somatotrope-like cells, whereas exposure to nerve growth factor (NGF) leads to a differentiation into the lactotropc lineage, as monitored by an increase in the secretion of PRL and the expression of the lactotrope-specific dopamine D-2 receptor. Consistent with their response to NGE GH-3 cells express the high- and low-affinity N G F receptors, TrkA and gp75. In addition, it has been shown that cultured primary pituitary cells and somatolactotrope and lactotrope cell lines contain N G E raising the possibility of autocrine and/or paracrine actions in the lactotrope lineage [57]. &GSU

ctGSU, the common subunit in the pituitary hormones LH, FSH, and TSH, is expressed in Rathke's pouch and later becomes restricted to the Pit-l-dependent thyrotrope lineage and the Pit-l-independent gonadotrope lineage [1]. To understand better this promiscuous behavior, a lot of effort has gone into understanding the regulation of the ctGSU gene and its biological function. Transgenic mice have been used to determine the ds-regulatory elements that are required for regulation of the temporal and spatial expression patterns of the mouse c~-glycoprotein subunit [58]. Whereas 0.48kb of promoter sequence are sufficient to correctly target c~-GSU expression to gonadotropes and thyrotropes, an additional enhancer element has been discovered between -2.7 kb and -4.6 kb that is required in e,i~'o to give high level expression of a /acZ reporter gene. The regulatory sequences that restrict &GSU expression from the other lineages should then also be included in the 0.48kb fragment and await identification. The responsiveness of the o~-subunit gene to gonadotropin-releasing hormone is mediated via the MAPK pathway and bv a consensus DNA binding site in the 0.48kb fragment for factors belonging to the Ets transcription factor family [59]. It has been also shown that this element harbors E-box-binding sites for basic helix-loop-helix transcriptional activators which ate required for basal promoter activity [60]. To further elucidate the roles of c~GSU and fetal TSH, I,H and FSH during development, mice lacking a functional o~-glycoprotein subunit have been generated by gene targeting [61"]. The homozygous mutant animals are hvpogonadal and exhibit profound hypothyroidism resulting in dwarf animals. In addition, there is a severe alteration

in the ratio of the pituitary cell types. The cells producing the TSH~ subunit exhibit dramatic hypertrophy and hyperplasia as a result of the lack of thyroid function, which normally exhibits negative feedback regulation on the TSH-producing cells through thyroid hormone. The proliferative response of the thyrotropes was suggested to occur at the expense of somatotropes and lactotropes. Normally each of these two cell types constitutes 30~10% of the adult pituitary gland, but lactotropes are almost completely absent and somatotrope numbers are strongly reduced in the pituitaries of mutant mice. FSH

It is well known that activins, members of the transforming growth factor-J3 superfamily, stimulate pituitary synthesis of FSH [62]. Activins can bind to several different type I and type II serine/threonine kinase receptors in vitro [63]. To confirm the significance of this interaction m vi~,o, a null mutation in the type II activin receptor (ActRclI) has been generated in mice by gene targeting [64"]. The anterior pituitary- gland, which highly expresses ActRclI, was examined for the expression of LH and FSH. LH levels are normal in gonadotropes of mutant animals, whereas FSH is strongly suppressed in both male and female mutant mice. This result is also reflected in decreased FSH serum levels and female sterility. These results therefore confirm the proposed role of activins in stimulation of FSH synthesis through signaling via the ActRclI in gonadotropes in vie, o.

A n t e r i o r pituitary induction by t h e floor of t h e diencephalon As already pointed out, a critical time for anterior pituitary organ determination is the formation of Rathke's pouch. During that time, the ectodermal primordium of the anterior and intermediate lobes of the pituitary makes contact with the neuroectoderm of the floor of the diencephalon. Several studies have suggested that inductive interactions between these tissues are required for their interdependent development [9,65,66]. Results obtained from three gene-targeting experiments, each resulting in the inactivation of a homeodomain transcription factor, are providing clues to the molecular mechanisms underlying these inductive interactions.

The homeodomain factor T/EBP/TTF-1/Nkx-2.1 The suggestion that a signal from the ventral diencephalon is indeed essential for the formation of the anterior pituitary gland is confirmed by the targeted disruption of the gene encoding the thyroid-specific enhancerbinding protein (T/EBP) [67°°1, also named thyroidspecific transcription factor-1 (TTF-1) or Nkx-2.1. T/EBP was originally described as governing thyroid-specific expression of the rat thyroglobulin gene [68]. In situ hybridization later showed that T/ehp is expressed not only in the thyroid but also in the lung bronchial epithelium and ventral forebrain. In the forebrain in particular, expression is seen in the preoptic and hypothalamic

The hypothalamic-pituitary axis Treier and Rosenfeld

areas, with highest levels of expression in the ventral regions of the third ventricular neuroepithelium [69]. In contrast, Rathke's pouch does not express T/ebp at any time. Mice homozygous for the disrupted T/ebp gene are born dead and lack the pituitary gland, the thyroid gland and the lung parenchyma. In the forebrain, most nuclei of the hypothalamus, including the premamillary nucleus, the arcuate nucleus, the mamillary body and the supramamillary nucleus, are not found [67"]. T h e absence of the posterior pituitary, which is derived from the infundibulum of the diencephalon, is consistent with the T/ebp expression pattern. Surprisingly, the anterior pituitary and intermediate pituitary are also missing, clearly demonstrating that the presence of the infundibulum and/or T/ebp gene expression in the floor of the ventral diencephalon is required for full development of the anterior and intermediate pituitary. Furthermore, the T/ebp null homozygous mice develop to full term at birth, which suggests that all the hormones needed for general growth and survival of the fetus can be supplied by the mother through the placenta. Although the authors demonstrate the absence of the pituitary at mouse E 18.5 [67°°], it would be of great interest to know if, in the T/ebp-null mice, invagination of the ectoderm and subsequent formation of Rathke's pouch occurs at all, or if Rathke's pouch later degenerates because of the absence of trophic factors from the hypothalamus. Irrespective of this issue, the study clearly asks for the identification of the factor(s) and their receptors that are involved in this inductive interaction between the diencephalon and the ectoderm that will give rise to Rathke's pouch. Presently, we can correlate the expression patterns of factors such as bone morphogenetic protein-4 (BMP-4), fibroblast growth factor-8 (FGF-8) and W N T - 7 in the diencephalon with developmental events in the anterior pituitary [70-73]. If and how these factors are involved in the induction and patterning of Rathke's pouch remains to be determined. Furthermore, cloning of the gene encoding the putative transmembrane protein neuronatin, and characterization of its highly restricted expression in the brain and Rathke's pouch, opens the possibility that novel signaling pathways may operate in the induction and patterning of Rathke's pouch [74]. The h o m e o d o m a i n factor Gsh-1

Targeted mutation of the mouse homeobox gene Gsh-1 has further underscored the importance of the hypothalamus for anterior pituitary development. One site of Gsh-1 expression in the brain is in the hypothalamus directly adjacent to Rathke's pouch [75]. Homozygous Gsh-1deficient mice exhibit extreme dwarfism, sexual infantilism and significant perinatal mortality [76"]. Analysis of the pituitaries of homozygous mutant mice has revealed a hypocellularity of the gland, which results from a severe reduction in the number of the growth-hormone- and prolactin-producing cells. This reduction in cell number is most likely a direct effect of the absence of G H R H

839

in the arcuate nucleus of the hypothalamus in Gsh-1 homozygous mutant mice. T h e decrease in somatotrope number resembles the phenotype of the little mouse which is known to have a mutation in the G H R H receptor [77,78]. In contrast to the little mouse, however, there is also a severe reduction in the number of lactotropes and a decrease in L H levels in Gsh-l-mutant mice. Thus, the Gsh-1 mutant phenotype is much more severe, suggesting that, in addition to the G H R H deficiency, multiple functions of the hypothalamus are affected that still remain to be detected and analyzed. The POU h o m e o d o m a i n factor Bin-2

T h e posterior pituitary gland, which originates from the infundibulum of the diencephalon, consists of pituicytes, specialized astroglia of the posterior lobe, and axon terminals of the magnocellular neurosecretory system. T h e magnocellular system includes neurons of the paraventricular hypothalamic (PVH) and supraoptic (SO) nuclei that synthesize the peptide hormones oxytocin (OT) and arginine vasopressin (AVP). Both peptides are released in an activity-dependent manner from axon terminals in the posterior lobe of the pituitary. Several members of the class III POU-domain gene family are expressed uniquely or in overlapping patterns in the developing hypothalamus, suggesting that combinatorial codes of class III POU-domain factors are involved in the determination of specific nuclei in the hypothalamus [79]. Two groups have reported the targeted deletion of the POU-domain factor Brn-2, and the phenotype of these mice supports this hypothesis [80°°,81°°]. Homozygous Brn-2 mutant mice have lost endocrine hypothalamic nuclei and the posterior pituitary gland. In particular, examination of the ontogeny of posterior pituitary development in Brn-2-deleted mice has revealed normal cellularity until E14.5, a normal population of pituicytes at El6, but a complete lack of axonal projections that normally arrive from the magnocellular neuroendocrine system. Consistent with this observation, O T and AVP are absent in the posterior pituitary, and activation of these hormones in the magnocellular neurons of either the PVH or the SO nuclei in homozygous mutant animals cannot be detected. Concurrently, the pituicytes start to disappear, suggesting that the magnocellular axons elaborate trophic factors required for maintenance of the posterior pituitary pituicytes. T h e activation of another peptide hormone, corticotropin-releasing hormone (CRH), which is normally produced in the parvocellular cells of the PVH and governs pituitary-adrenal responses, is also never observed. However, examination of the cell types in the anterior and intermediate pituitary has revealed no abnormalities. These findings strongly suggest that signals from the undifferentiated neuroepithelium of the ventral diencephalon, but not from terminally differentiated hypothalamic cells, are required for normal anterior pituitary development.

840

Cell differentiation

T h e developmental programs of the anterior pituitary and the endocrine hypothalamus, two structures of different embryonic origin which have to develop in a coordinated fashion to produce the mature pituitary gland, seem at least in part to be directed in parallel by two POU domain transcription factors, Pit-1 and Brn-2. Brn-2, like Pit-l, is

required both for activation of terminal target genes and for cellular survival (see Fig. 2). Conclusions

T h e hypothalamic-pituitary axis, which results from the coordinated development of two organs of distinct

Figure 2

Brn-2

PVH

Gsh-1

SO

P~

l

OT

f

ARH

DA

A

Lactotropes !

I Portal vascular system

-

Pit-1

Corticotropes

Gonadotropes IThyr°tr°pes I © 1996 Current Opinion in Cell Biology

I Lhx3/P-Lim

The hypothalamic-pituitary axis. The magnocellular neurosecretory system includes neurons in the PVH and SO nuclei that synthesize the peptide hormones OT and AVP, and release them in an activity-dependent manner from axonal terminals in the posterior lobe (P) of the pituitary gland. In addition, the PVH harbors separate populations of parvocellular cells which synthesize corticotropin-releasing hormone (CRH) and thyrotropin-releasing hormone (TRH). These neuropeptides are delivered to the median eminence for conveyance via the hypophyseal-portal vascular system to modulate the synthesis and release of ACTH and TSH in the anterior pituitary gland (A). Centered in ventrally contiguous cell groups in the anterior periventricular (PVa) or the arcuate nuclei (ARH) of the hypothalamus are hypophysiotrophic neurons that provide both the dopaminergic (DA) control of PRL secretion, and somatostatin (SS) or GHRH which impart the principal inhibitory and stimulator/regulation of GH, respectively. Cells of the intermediate lobe (I) of the pituitary gland produce melanocyte-stimulating hormone (MSH) by proteolytic processing of POMC (not shown). The consequences that result from the loss of gene function for each of the five homeodomain transcription factors (Brn-2, T/EBP, Pit-l, Gsh-1 and Lhx3/P-Lim) are indicated by crosses. Brn-2-deficient mice have lost AVP, OT and CRH expression in the PVH and SO nuclei. Gsh-l-mutant mice lack GHRH expression in the ARH. Absence of Lhx3/P-Lim function results in loss of gonadotropes, thyrotropes, somatotropes and lactotropes, whereas in the case of a nonfunctional Pit-1 protein, thyrotropes, somatotropes and lactotropes are missing. The most dramatic phenotype is observed in T/EBP-deficient mice, in which the endocrine hypothalamus and the pituitary gland are completely absent.

The hypothalamic-pituitary axis Treier and Rosenfeld

embryonic origin, provides an excellent model system in which to study the control of mammalian organogenesis during sequential developmental stages which lead to the precisely regulated appearance of specific cell types. Six3 and Rpx, together with Pax6, provide molecular markers that will guide further investigation of the prepatterning code that directs the competence of ectodermal tissue to respond to the still undefined inductive signal(s) from the ventral diencephalon. T h e s e signals may be necessary to generate Rathke's pouch. As classical embryological studies have shown that only the ventral diencephalon is able to induce Rathke's pouch, a specific combination of signaling molecules, if not a unique signal, must operate. T h e identification of specific LIM-homeodomain and orthodenticle-related factors in the developing gland, and their potential subsequent spatial restriction, suggests combinatorial codes that are required for determination of the different cell types in the mature gland from a common precursor lineage. These factors will provide tools with which to investigate whether the determination of the cell types follows an intrinsic mechanism (e.g. as in the case of the sensory organ development in Drosophila) and/or whether a second round of inductive signaling is necessary, perhaps also originating from the hypothalamus (e.g. as seen in Drosophila eye and Caenorhabditis elegans vulval development). Thus, the anterior pituitary gland, with its distinct spatial and temporal order of appearance of a series of cell phenotypes, provides a particularly instructive model with which to unravel the complex genetic circuitry and machinery that control mammalian organ development.

5.

Voss JW, Rosenfeld MG: Anterior pituitary development:, short tales from dwarf mice. Cell 1992, 70:527-530.

6.

Rhodes SJ, DiMattia GE, Rosenfeld MG: Transcriptional mechanisms in anterior pituitary cell differentiation. Curt Opin Genet Day 1994, 4:709-717.

7.

Andersen B, Rosenfeld MG: Pit-1 determines cell types during development of the anterior pituitary gland. J Biol Chem 1994, 269:29335-29338.

8.

Watanabe YG: Effects of brain and mesenchyme upon the cytogenesis of rat adenohypophysis in vitro. Cell Tissue Ras 1982, 227:257-266.

9.

Fedtsova NG, Barabanov VM: The distribution of competence for adenohypophysis development in the ectoderm of chick embryos. Ontogenez 1990, 21:254-260.

10.

Jacobson AG, Miyamoto DM, Mai S-H: Rathke's pouch morphogenesis in the chick embryo. J Exp Zoo/1979, 207:351-366.

11.

Eagleson GW, Harris WA: Mapping of the presumptive brain regions in the neural plate of Xenopus laevis. J Neurobio/1990, 21:427-440.

12.

Couly G, Le Douarin NM: The fate map of the cephalic neural primordium at the presomitic to the 3-somita stage in the avian embryo. Development 1988, 103 (Suppl):101-113.

13.

Osumi-Yamashita N, Ninomiya Y, Doi H, Eto K: The contribution of both forebrain and midbrain crest cells to the mesenchyme in the frontonasal mass of mouse embryos. Dev Bio11994, 164:409-419,

14, •

Oliver G, Mallhos A, Wehr R, Copeland NG, Jenkins NA, Gruss P: Six3, a murine homologue of the sine oculis gene, demarcates the most anterior border of the developing neural plate and is expressed during eye development. Development 1995, 121:4045-4055. This paper describes the cloning of Six3, one of the most anteriorly expressed homeobox genes reported to date. Its expression pattern during mouse development supports the idea that Rathke's pouch derives from the anterior neural ridge. 15.

Walther C, Gruss P: Pax-6, a murine paired box gene, is expressed in the developing CNS. Development 1991, 113:1435-1449.

16.

Li HS, Yang JM, Jacobson RD, Pasko D, Sundin O: Pax-6 is first expressed in a region of ectoderm anterior to the early neural plate: implications for stepwise determination of the lens. Dev Bio/1994, 162:181-194

17.

Oliver G, Wehr R, Jenkins NA, Copeland NG, Cheyette BN, Hartenstein V, Zipursky SL, Gruss P: Homeobox genes and connective tissue patterning. Development 1995, 121:693-705.

Acknowledgements We thank Kathleen M Scully and Anatoli Gleiberman for critical reading of this manuscript. We gratefully acknowledge Peggy Myer for her expertise and assistance in preparation of illustrations. Mathias Treier was supported by a Boehringer Ingelheim Fonds postdoctoral fellowship. Michael G Rosenfeld is an Investigator with the Howard Hughes Medical Institute.

References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: • ••

of special interest of outstanding interest Simmons DM, Voss JW, Ingraham HA, Holloway JM, Broide RS, Rosenfeld MG, Swanson LW: Pituitary cell phenotypes involve cell-specific Pit-1 mRNA translation and synergistic interactions with other classes of transcription factors. Genes Dev 1990, 4:695-711. Japbn MA, Rubinstein M, Low MJ: In situ hybridization analysis of anterior pituitary hormone gene expression during fetal mouse development. J Histochem Cytochem 1994, 42:1117-1125. Bodner M, Castrillo JL, Theill LE, Deerinck T, Ellisman M, Karin M: The pituitary-specific transcription factor GHF-1 is a homeobox-containing protein. Cell 1988, 50:267-275.

4.

Ingraham HA, Chen R, Mangalam HJ, Elshottz HP, Flynn SE, Lin CR, Simmons DM, Swanson L, Rosenfeld MG: A tissuespecific transcription factor containing a homeodomain specifies a pituitary phenotype. Cell 1988, 50:519-529.

841

18. .

Hermesz E, Mackem S, Mahon KA: Rpx: a novel anteriorrestricted homebox gene progressively activated in the prechordal plate, anterior neural plate and Rathke's pouch of the mouse embryo. Development 1996, 122:41-52. The authors report the cloning of Rpx, a homeobox gene which becomes restricted to Rathke's pouch (the primordium of the pituitary), suggesting that it may be involved in the determination of the pituitary gland. Downregulation of Rpx gene expression in the pouch coincides with the differentiation of pituitary-specific cell types. 19.

Thomas PQ, Johnson BV, Rathjen J, Rathjen PD: Sequence, genomic organisation, and expression of the novel homeobox gene Hesxl. J Biol Chem 1995, 270:3869-3875.

20.

Mathers PH, Miller A, Doniach T, Dirksen M-L, Jamrich M: Initiation of anterior head-specific gene expression in uncommitted ectoderm of Xenopus laevis by ammonium chloride. Dev Bio/ 1995, 171:641-654.

21.

EkkerSC, McGraw LL, Lai C-J, Lee JL, Von Kessler DP, Moon RT, Beachy PA: Distinct expression and shared activities of members of the hedgehog gene family of Xenopus laevis. Development 1995, 121:2337-2347.

22.

Heideveld M, Ayoubi TAY, Van de Wial MHJ, Martens GJM, Durston AJ: Proopiomelanocortin gene expression as a neural marker during the embryonic development of Xenopus leevis. Differentiation 1993, 52:195-200.

23. •

Lai C-J, Ekker SC, Beachy PA, Moon RT: Patterning of the neural ectoderm of Xenopus laevis by the amino-terminal product of hedgehog autoproteolytic cleavage. Development 1995, 121:2349-2360.

842

Cell differentiation

The paper demonstrates that Hedgehog can directly induce and elevate the expression of anterior neural genes, suggesting a role in anterio-posterior patterning. One of the markers that is induced is XANF-2, the Xenopus homolog of Rpx. 24.

Zhadanov AB, Bertuzzi S, Taira M, Dawid IB, Westphal H: Expression pattern of the murine LIM class homeobox gene Lhx3 in subsets of neural and neuroendocrine tissues. Dev Dyn 1995, 202:354-364.

25.

Bach I, Rhodes S J, Pearse RV II, Heinzel T, Gloss B, Scully KM, Sawchenko PE, Rosenfeld MG: P-Lim, a LIM homeodomain factor, is expressed during pituitary organ and cell commitment and synergizes with Pit-1. Proc Nat/Acad Sci USA 1995, 92:2720-2724.

26.

Seidah NG, Barale J-C, Marcinkiewicz M, Mattei M-G, Day R, Chretien M: The mouse homeoprotein mLIM-3 is expressed early in cells derived from the neuroepithelium and persists in adult pituitary. DNA Ceil Biol 1994, 13:1163-1180.

27.

Taira M, Hayes WP, Otani H, Dawid IB: Expression of LIM class h o m e o b o x gene Xlim-3 in Xenopus development is limited to neural and neuroendocrine tissues. Dev Bio/1993, 159:245-256.

31.

Andersen B, Pearse II RV, Jenne K, Sornson M, Lin S-C, Bartke A, Rosenfeld MG: The ames dwarf gene is required for Pit-1 gene activation. Dev Bio/1995, 172:495-503. This paper demonstrates that the initial activation of the Pit-1 gene is deficient in Ames dwarf animals, suggesting that the df factor acts upstream of Pit-1 in pituitary development. •

41.

Gage PJ, Roller ML, Saunders TL, Scarlett LM, Camper SA: Anterior pituitary cells defective in the cell-autonomous factor, df, undergo cell lineage specification but not expansion. Deve/opment 1996, 122:151-160. Provides evidence that expression of Pit-1 and limited commitment of anterior pituitary cells occurs in Ames pituitaries. Therefore, it was suggested that df may play a crucial role in lineage-specific proliferation, rather than in cytodifferentiation. Furthermore, the use of chimeric mice demonstrates that df functions as a ceIFautonomous factor. •

43.

Luo X, Ikeda Y, Parker KL: A cell-specific nuclear receptor is essential for adrenal and gonadal development and sexual differentiation. Ceil 1994, 77:481-490.

44.

Ingraham HA, Lala DS, Ikeda Y, Luo X, Shen W-H, Nachtigal MW, Abbud R, Nilson JH, Parker KL: The nuclear receptor steroidogenic factor 1 acts at multiple levels of the reproductive axis. Genes Dev 1994, 8:2302-2312.

45.

Ikeda Y, Luo X, Abbud R, Nilson JH, Parker KL: The nuclear receptor steroidogenic factor 1 is essential for the formation of the ventromedial hypothalamic nucleus. Mol Endocrinol 1995, 9:478-486.

46.

Gong Z, Hui C-C, Hew CL: Presence of isl-l-related LIM domain homeobox genes in teleost and their similar patterns of expression in brain and spinal cord. J Bio/Chem 1995, 270:3335-3345.

Nowakowski BE, Maurer IRA: Multiple Pit-1 binding sites facilitate estrogen responsiveness of the prolactin gene. Mol Endocrinol 1994, 8:1742-1749.

47.

Tsuchida T, Ensini M, Morton SB, Baldassare M, Edlund T, Jessell TM, Pfaff SL: Topographic organization of embryonic motor neurons defined by expression of LIM homeobox genes. Ceil 1994, 79:957-970.

Lipkin SM, N~&r AM, Kalla KA, Sack RA, Rosenfeld MG: Identification of a novel zinc finger protein binding a conserved element critical for Pit-l-dependent growth hormone gene expression. Genes Dev 1993, 7:1674-1687.

48.

Schaufele F, West BL, Baxter JD: Synergistic activation of the rat growth hormone promoter by Pit-1 and the thyroid hormone receptor, Mol Endocrinol 1992, 6:656-665.

49.

Kim MK, McClaskey JH, Bodenner DL, Weintraub BD: An APl-like factor and the pituitary-specific factor Pit-1 are both necessary to mediate hormonal induction of human thyrotropin gone expression. J Biol Chem 1993, 268:23366-23375.

50.

Crenshaw EBI, Kalla K, Simmons DM, Swanson LW, Rosenfeld MG: Cell-specific expression of the prolactin gone in transgenic mice is controlled by synergistic interactions between promoter and enhancer elements. Genes Dev 1989, 3:959-972.

51.

Jiang Y, Yu VC, Buchholz F, O'Connell S, Rhodes S J, Candeloro C, Xia Y-R, Lusis AJ, Rosenfeld MG: A novel family of Cys-Cys, H i s - C y s zinc finger transcription factors expressed in developing nervous system and pituitary gland. J Bio/Chem 1996, 271:10723-10730.

Roberson MS, Schoderbek WE, Tremml G, Maurer RA: Activation of the glycoprotein hormone cesubunit promoter by a LIMhomeodomain transcription factor. Mo/Ceil Bio/1994, 14:2985-2993.

32. •

Lamonerie T, Tremblay J.I, Lanctot C, Therrien M, Gauthier Y, Drouin J: Ptx-1, a bicoid-related homeobox transcription factor involved in transcription of the pro-opiomelanocortin gene. Genes Dev 1996, 10:1284-1295. This paper, together with [33"], describes the cloning of a homeobox gene that is highly expressed in Rathke's pouch and which possesses a homeodomain that is most closely related to those of the anterior-specific genes bicoid and orthodenticle in Drosophila and Otx-1 and Otx-2 in mammals. 33. •

Szeto DP, Ryan AK, OConnell SM, Rosenfeld MG: P-OTX: a Pit-1 interacting homeodomain factor expressed during anterior pituitary gland development. Proc Nat/Acad Sci USA 1996, 93:7706-7710. See annotation [32"]. 34.

40.

Phelps C J: Pituitary hormones as neurotrophic signals: anomalous hypophysiotrophic neuron differentiation in hypopituitary dwarf mice. Proc Soc Exp Bio/ Med 1994, 206:6-23.

Sheng HZ, Zhadanov AB, Mosinger B Jr, Fulii T, Bertuzzi S, Grinberg A, Lee EJ, Huang S-P, Mahon KA, Westphal H: Specification of pituitary cell lineages by the LIM homeobox gene Lhx3. Science 1996,272:1004-1007. The authors describe the phenotype of mice homozygous for a mutation in the LIM homeobox gene Lhx3. In these animals, Rathke's pouch forms but fails to grow and differentiate; therefore, the mice lack both the anterior and the intermediate lobes of the pituitary gland. The determination of all the pituitary cell lineages, except the corticotropes, is affected.

30.

Cheng T, Beamer W, Phillips J, Bartke A, Mallonee R, Dowling C: Etiology of growth hormone deficiency in little, Ames, and Snell dwarf mice. Endocn'nology 1983, 113:1669-1678.

42.

28. ••

29.

39.

Lala DS, Rice DA, Parker KL: Steroidogenic factor 1, a key regulator of steridogenic enzyme expression, is the mouse homolog of fushi tarazu-factor 1. Mol Endocrinol 1992, 6:1249-1258.

35.

Lin C, Lin S-C, Chang C-P, Rosenfeld MG: Pit-l-dependent expression of the receptor for growth hormone releasing factor mediates pituitary cell growth. Nature 1992, 360:765-768.

36.

Li S, Crenshaw EBI, Rawson EJ, Simmons DM, Swanson LW, Rosenfeld MG: Dwarf locus mutants, which lack three pituitary cell types, result from mutations in the POU domain gene, Pit-1. Nature 1990, 347:528-533.

37.

Buckwalter M, Katz R, Camper S: Localization of the panhypopituitary dwarf mutation (df) on mouse chromosome 11 in an intersubspecific backcross. Genomics 1991, 10:515-526.

38.

Bartke A: The response of two types of dwarf mice to growth hormone, thyrotropin, and thyroxine. Gen Comp Endocrinol 1965, 5:418-426.

52.

Holloway JM, Szeto DP, Scully KM, Glass CK, Rosenfeld MG: Pit-1 binding to specific DNA sites as a monomer or dimer determines gene-specific use of a tyrosine-dependent synergy domain. Genes Dev 1995, 9:1992-2006. This paper describes the finding that the sequence of specific DNA sites in the prolactin and growth hormone genes, respectively, dictates alternative Pit-1 synergy domain utilization based on monomeric or dimeric binding, suggesting an additional regulatory strategy for differential target gene activation in distinct cell types. =

53.

Conrad KE, Oberwetter JM, Vaillancourt R, Johnson GL, GutierrezHartmann A: Identification of the functional components of the Ras signaling pathway regulating pituitary cell-specific gene expression. Mol Ceil Biol 1994, 14:1553-1565.

54.

Bradford AP, Conrad KE, Wasylyk C, Wasylyk B, GutierrezHartmann A: Functional interaction of c-Ets-1 and GHF-1/Pit-1 mediates Ras activation of pituitary-specific gene expression: mapping of the essential c-Ets-1 domain. Mol Ceil B/o/1995, 15:2849-2857.

55.

Howard PW, Maurer RA: A composite Ets/Pit-1 binding site in the prolactin gene can mediate transcriptional responses

The hypothalamic-pituitary axis Treier and Rosenfeld

to multiple signal transduction pathways. J Bio/Chem 1995, 270:20930-20935. 58.

Missale C, Boroni F, Sigala S, Zanellato A, Dal Toso R, Balsari A, Spano P: Nerve growth factor directs differentiation of the bipotential cell line GH-3 into the mammotroph phenotype. Endocrinology 1994, 135:290-298.

57.

Missale C, Boroni F, Fressine M, Caruso A, Spano P: Nerve growth factor promotes the differentiation of pituitary mammotroph cells in vitro. Endocrinology 1995, 136:1205-1213.

58.

Kendall SK, Gordon DF, Birkmeier TS, Petrey D, Sarapura VD, O'Shea KS, Wood WM, Lloyd RV, Ridgway EC, Camper SA: Enhancer-mediated high level expression of mouse pituitary glycoprotein hormone ¢¢-subunit transgene in thyrotropes, gonadotropes, and developing pituitary gland. Mol Endocrino/ 1994, 8:1420-1433.

59.

Roberson MS, Misra-Press A, Laurence ME, Stork PJS, Maurer RA: A role for mitogen-activated protein kinase in mediating activation of the glycoprotein hormone (x-subunit promoter by gonadotropin-releasing hor;'aone./viol Ceil Biol 1995, 15:3531-3539.

60.

Jackson SM, Gutierrez-Hartmann A, Hoeffler JP: Upstream stimulatory factor, a basic-helix-loop-helix-zipper protein, regulates the activity of the ec-glycoprotein hormone subunit gene in pituitary cells. Mol Endocrino11995, 9:278-291.

Kendall SK, Samuelson LC, Saunders TL, Wood RI, Camper SA: Targeted disruption of the pituitary glycoprotein hormone ¢¢-subunit produces hypogonadal and hypothyroid mice. Genes Dev 1995, 9:2007-2019. This paper reports the gene knockout for the common (x subunit of the pituitary hormones TSH, LH and FSH. The homozygous mutant mice are hypogonadal and exhibit profound hypothyroidism resulting in dwarfism. The pituitary cells that produce the TSHI3 subunit show a dramatic hypertrophy and hyperplasia as a result of the lack of thyroid function.

68.

Civitareale D, Lonigro R, Sinclair AJ, Di Lauro R: A thyroidspecific nuclear protein essential for tissue-specific expression of the thyroglobulin promoter. EMBO J 1989, 8:2537-2541.

69.

Price M, Lazzaro D, Pohl T, Mattei M-G, RLither U, Olivo J-C, Duboule D, Di Lauro R: Regional expression of the homeobox gene Nkx-2.2 in the developing mammalian forebrain. Neuron 1992, 8:241-255.

70.

JonesCM, Lyons KM, Hogan BLM: Involvement of bone morphogenetic protein-4 (BMP-4) and Vgr-1 in morphogenesis and neurogenesis in the mouse. Development 1991, 111:531-542.

71.

Crossley PH, Martin GR: The mouse Fgf8 gene encodes a family of polypeptides end is expressed in regions that direct outgrowth and patterning in the developing embryo. Development 1995, 121:439-451.

72.

Parr BA, Shea MJ, Vassileva G, McMahon AP: Mouse WNT genes exhibit discrete domains of expression in the early embryonic CNS and limb buds. Development 1993, 119:247-261.

73.

Watanabe YG: An organ culture study on the site of determination of ACTH and LH cells in the rat adenohypophysis. Cell Tissue Res 1982, 227:267-275.

74.

Wijnholds J, Chowdhury K, Wehr R, Gruss P: Segment-specific expression of the neuronatin gene during early hindbrain development. Dev Bio/1995, 171:73-84.

75.

ValeriusMT, Li H, Stock JL, Weinstein M, Kaur S, Singh G, Potter SS: Gsh-l: • novel murine homeobox gene expressed in the central nervous system. Dev Dyn 1995, 203:337-351.

61. •

62.

Carroll RS, Corrigan AZ, Gharib SD, Vale W, Chin WW: Inhibin, activin end folliststin: regulation of follicle-stimulating hormone messenger ribonucleic acid levels. Mol Endocrinol 1989, 3:1969-1976.

63.

DalkinAC, Haisenleder DJ, Yasin M, Gilrain JT: Pituitary activin receptor subtypes and follistatin gene expression in female rats: differential regulation by activin and follistatin. Endocrinology 1996, 137:548-554.

64. •

Matzuk MM, Kumar TR, Bradley A: Different phenotypes for mice deficient in either activins or activin receptor type II. Nature 1995, 374:356-360. The analysis of mice with a disruption of the gene for the type II activin receptor (ActRcll) confirms that activin is required for positve regulation of FSH expression in vivo. 65.

KawamuraK, Kikuyama S: Evidence that hypophysis and hypothalamus constitute a single entity from the primary stage of histogenesis. Development 1992, 115:1-9.

66.

KawamuraK, Kikuyama S: Induction from posterior hypothalamus is essential for the development of the pituitary proopiomelanocortin (POMC) cells of the toad (Bufo japonicus). Cell Tissue Res 1995, 279:233-239.

67 ••

KimuraS, Hara Y, Pineau T, Fernandez-Salguero P, Fox CH, Ward JM, Gonzaiez FJ: The T/ebp null mouse: thyroid-specific enhancer-binding protein is essential for the organogenesis of the thyroid, lung, ventral forebrain, and pituitary. Genes Dev 1996, 10:60-69. A good example of a gene knockout giving novel and unequivocal results. The entire pituitary is missing in t/ebp-mutant animals. This is surprising as T/ebp is not expressed in the intermediate and anterior lobe of the pituitary at any time; in contrast, expression is detected in the floor of the diencephalon which is absent in homozygous animals. This finding confirms classical embryological studies which have suggested that a signal originating in the diencephalon that is necessary to determine Rathke's pouch must exist.

843

76. •

Li H, Zeitler PS, Valerius MT, Small K, Potter SS: Gsh-1, an orphan Hox gene, is required for normal pituitary development. EMBO J 1996, 15:714-724. Description of pleiotropic effects on pituitary development and function in Gsh-1 mutant animals. Particularly, Gsh-1 is shown to be essential for GHRH gene expression in the arcuate nucleus of the hypothalamus. 77.

Lin S-C, Lin CR, Gukosvsky I, Lusis AJ, Sawchenko PE, Rosenfeld MG: Molecular basis of the little mouse phenotype and implications for cell type-specific growth. Nature 1993, 364:208-213.

78.

Godfrey P, Rahal JO, Beamer WG, Copeland NG, Jenkins NA, Mayo KE: GHRH receptor of little mice contains a missense mutation in the extracellular domain that disrupts receptor function. Nat Genet 1993, 4:227-232

79.

Alvarez-Bolado G, Rosenfeld MG, Swanson LW: Model for forebrain regionalization based on spatiotemporal patterns of POU-II! homeobox gene expression, birthdates, end morphological features. J Comp Neuro/1995, 355:237-295.

80. •-

Schonemann MD, Ryan AK, McEvilly RJ, O'Connell SM, Arias CA, Kalla KA, Li P, Sawchenko PE, Rosenfeld MG: Development and survival of the endocrine hypothalamus and posterior pituitary gland requires the neuronal POU domain factor Brn-2. Genes Dev 1995, 9:3122-3135. This paper, together with [81"'], describes the phenotypa of mice with a deletion of the Bin-2 genomic locus. Lack of Brn-2 results in loss of endocrine hypothalamic nuclei and the posterior pituitary gland. This finding demonstrates that the developmental program of the endocrine hypothalamus, like that of the anterior pituitary gland, seems, at least in part, to be directed by a POU domain transcription factor. This transcription factor is Brn-2 in the endocrine hypothalamus and Pit-1 in the anterior pituitary gland. 81. ••

NakaiS, Kawano H, Yudate T, Nishi M, Kuno J, Nagata A, Jishage K, Hamada H, Fujii H, Kawarnura K: The POU domain transcription factor Brn-2 is required for the determination of specific neuronal lineages in the hypothalamus of the mouse. Genes Dev 1995, 9:3109-3121. See annotation [80"]. 82.

Lin S-C, Li S, Drolet DW, Rosenfeld MG: Pituitary ontogeny of the Snell dwarf mouse reveals Pit-l-independent and Pit1-dependent origins of the thyrotrope. Development 1994, 120:515-522.