Receptor tyrosine kinases and respiratory motor plasticity

Respiratory Physiology & Neurobiology 164 (2008) 242–251

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Receptor tyrosine kinases and respiratory motor plasticity Francis J. Golder ∗ Department of Clinical Studies – Philadelphia, University of Pennsylvania, United States

a r t i c l e

i n f o

Article history: Accepted 18 June 2008 Keywords: Receptor tyrosine kinases Respiratory motor plasticity Breathing

a b s t r a c t Protein kinases are a family of enzymes that transfer a phosphate group from adenosine tri-phosphate to an amino acid residue on a protein. The receptor tyrosine kinases (RTKs) are expressed on the outer cell membrane, bind extracellular protein ligands, and phosphorylate tyrosine residues on other proteins—essentially permitting communication between cells. Such activity regulates multiple aspects of cellular physiology including cell growth and differentiation, adhesion, motility, cell death, and morphological and synaptic plasticity. This review will focus on the role of RTKs in respiratory motor plasticity, with particular emphasis on long-term changes in respiratory motoneuron function. Reflecting the predominant literature, specific attention will be devoted to the role of tropomyosin-related kinase type B (TrkB) activation on phrenic motoneuron activity. However, many RTKs share similar patterns of expression and mechanisms of ligand-induced activation and downstream signaling. Thus, a perspective based on TrkB-induced phrenic motor plasticity may provide insight into the potential roles of other RTKs in the neural control of breathing. Finally, understanding how different RTKs affect respiratory motor output in the long-term may provide future avenues for pharmacological development with the goal of increasing respiratory motor output in disorders such as obstructive sleep apnea and after spinal cord injury. This is best illustrated in recent studies where we have used small, highly diffusible molecules to transactivate TrkB receptors near phrenic motoneurons to improve breathing after cervical spinal cord injury. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Breathing, and its control, must be adaptable during an individual’s lifetime. Transient adjustments in breathing can be achieved through various feedforward (i.e., corticospinal projections) and feedback mechanisms (i.e., chemoreception and lung-volume mechanoreception). However, with repeated or chronic perturbations (i.e., during disease or following injury) the neural control of breathing can exhibit functional plasticity—a change in system performance based on prior experience [Mitchell and Johnson, 2003]. The term plasticity contrasts with short-term modulation, where a stimulus (i.e., hypoxia or a receptor ligand) elicits a change in system performance but this change returns to pre-stimulus levels once the inciting cause is removed (i.e., return of blood gases to pre-stimulus levels or the ligand vacates the receptor) [Mitchell and Johnson, 2003]. This review will primarily focus on the role of receptor tyrosine kinases (RTKs) in respiratory motor plasticity. However, given that factors that modulate respiratory motor output often also elicit respiratory motor plasticity, albeit under

∗ Correspondence address: 3900 Delancey Street, Philadelphia, PA 19104, United States. Tel.: +1 215 746 0137; fax: +1 215 573 8183. E-mail address: [email protected]. 1569-9048/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.resp.2008.06.018

different experiment conditions such a prolonged or episodic exposure, we will also briefly review studies that describe a role for RTKs in respiratory motor modulation. 2. Receptor tyrosine kinases Protein kinases are a family of enzymes that transfer a phosphate group from adenosine tri-phosphate to an amino acid residue on a protein. Such activity regulates multiple aspects of cellular physiology including cell growth and differentiation, adhesion, motility, cell death, and morphological and synaptic plasticity. Protein kinases are classified based on their preferred amino acid substrates to which they add a phosphate group (i.e., serine/threonine kinases preferentially phosphorylate serine and threonine residues, whereas tyrosine kinases phosphorylate tyrosine residues) and their cellular compartmentalization (e.g., RTKs are associated with the cell surface, whereas cytoplasmic kinases are completely intracellular). Although our focus here is on RTKs, new evidence suggests that cellular protein kinases also may be critical factors in regulating respiratory modulation and plasticity [Feldman et al., 2005]. Approximately 90 tyrosine kinases have been identified in the mammalian genome, of which 58 are classified as RTKs [Robinson et al., 2000]. The RTKs are further divided into 17 families based on their structure, biology, and function (see Table 1). General

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Table 1 The families of receptor tyrosine kinases and respiratory motor plasticity Class

Receptor tyrosine kinase family

Reviews of biology, structure, and function

I II III IV V VI VII VIII IX X XI XII XIII XIV XV XVI XVII

Epidermal growth factor receptor (EGF) Insulin receptor (Ins, IGF) Platelet-derived growth factor receptor (PDGF) Fibroblast growth factor receptor (FGF) Vascular endothelial growth factor receptor (VEGF) Hepatocyte growth factor receptor (MET) Tropomyosin-related kinase receptor (TRK) Ephrin receptor (EPH) Adhesion-related kinase receptor (AXL) Anaplastic lymphoma kinase receptor (ALK) Angiopoietin receptor (TIE) Orphan receptor (ROR) Discoidin domain receptor (DDR) Glial cell line-derived neurotrophic factor (RET) Cholecystokinase-4 receptor (KLG or CCK4) Related to receptor tyrosine kinases (RYK) Muscle specific kinase (MuSK)

Warren and Landgraf (2006) Dupont and Holzenberger (2003) Betsholtz (2003) Eswarakumar et al. (2005) Ferrara et al. (2003) Gentile et al. (2008) Huang and Reichardt (2003) Murai and Pasquale (2004) Hafizi and Dahlback (2006) Pulford et al. (2004) Gale and Yancopoulos (1999) Forrester (2002) Vogel et al. (2006) Runeberg-Roos and Saarma (2007) Kobus and Fleming (2005) Keeble and Cooper (2006) Strochlic et al. (2005)

tyrosine kinase structure and function have been reviewed elsewhere [Hubbard and Till, 2000], and will only be described here in brief. Generally, RTKs are glycoproteins that permit communication between cells via extracellular protein ligands (e.g., hormones or growth factors). However, other signaling mechanisms are possible, for example intracellular activation of immature RTKs, which does not require direct interaction with extracellular ligands [Sorkin, 2005]. Canonical signal transduction involves ligand binding to specific sites on the RTK extracellular domain, RTK dimerization, activation of the intracellular tyrosine kinase domain on the RTK (autophosphorylation), and subsequent phosphorylation of downstream effector proteins. Ligand binding stabilizes the dimeric configuration of the RTK and can induce conformation changes in protein structure. The close proximity of the two intracellular catalytic domains is believed to be sufficient to permit transautophosphorylation of tyrosine residues of each RTK monomer, whereas the protein conformational changes may be sufficient for cis-autophosphorylation. Autophosphorylation increases the intrinsic activity of the kinases and generates intracellular binding sites for proteins that recognize phosphotyrosine-containing sequences. Thus, downstream signaling molecules can closely associate with the RTK kinase domain and undergo tyrosine residue phosphorylation. In general, RTK signaling activates three major downstream effectors. These are: (1) members of the mitogen-activated protein kinase family (MAP kinases, i.e., ERK1/2) [Avruch et al., 2001], (2) the PI3-kinase/protein kinase B (Akt) family [Cohen, 1999], and (3) phospho-lipase C gamma (PLC-␥) [Kamat and Carpenter, 1997]. Schematic representations of the RTK activation and downstream effector signaling are depicted elsewhere [Huang and Reichardt, 2003]. Using the RTK family tropomyosin-related kinase (Trk receptors) as an example, Trk-dependent models of synaptic plasticity have traditionally been attributed to activation of the MAP kinases [Reichardt, 2006]. ERK1/2 may contribute to synaptic plasticity by controlling protein synthesis via translational and/or transcriptional regulation [Sweatt, 2004], modulating potassium channels [Yuan et al., 2002], and/or increasing membrane expression of glutamate receptors or altering receptor subunit composition [Malenka, 2003]. Although primarily associated with promoting cell survival, the PI3K/Akt signaling cascade was recently demonstrated to also regulate synaptic plasticity within the adult CNS [Wang et al., 2003]. Like ERK1/2, Akt may contribute to synaptic plasticity by promoting protein synthesis [Horwood et al., 2006]. The role of PLC-␥ in Trk-dependent synaptic plasticity is less clear. PLC-␥ knockout mice have deficiencies in the induction, but not

maintenance, phase of hippocampal LTP—an ex vivo model of synaptic plasticity [Minichiello et al., 2002]. 3. The tropomyosin-related kinases (Trk receptors) The family of RTKs most often associated with respiratory motor plasticity, are the Trk receptors (TrkA, TrkB, and TrkC). Trk receptors and their endogenous ligands, the neurotrophins, were first identified as neuronal survival and growth factors [Cohen, 2004]. The neurotrophins (Nerve Growth Factor (NGF), BrainDerived Neurotrophic Factor (BDNF), Neurotrophin-3 (NT-3), and Neurotrophin-4 (NT-4)) are distinct from other growth factors in that they regulate growth and differentiation (but not proliferation) and are synthesized at a distance from the cell body of the neurotrophin-sensitive neuron [Skaper, 2008]. The neurotrophins (and presumably their receptors) are synthesized throughout the CNS including tissues associated with the neural control of breathing [Dreyfus et al., 1999; Maisonpierre et al., 1990]. Indeed, normal growth and development of respiratory neurons, and their penultimate function-ventilation, requires neurotrophin expression, in particular BDNF [Katz, 2003, 2005]. Neurotrophins bind to two classes of receptors: the Trk receptors and the p75 receptor. The latter is not an RTK and will not be discussed further. The biology and function of p75 receptors has been thoroughly reviewed elsewhere [Blochl and Blochl, 2007; Reichardt, 2006]. Furthermore, this review will focus on the role of Trk receptors in respiratory motor plasticity: the general biology and function of the Trk receptors is reviewed elsewhere [Huang and Reichardt, 2003; Reichardt, 2006]. 3.1. Peripheral mechanisms of Trk-dependent respiratory motor plasticity BDNF and TrkB receptors are present in carotid body type I cells (the chemoreceptor elements) and their afferent petrosal ganglia neurons in the carotid sinus nerve, and are important factors in carotid body development and function [Wang and Bisgard, 2005]. Removal of the carotid body at birth, or creating BDNF deficient mice, decreases sensory innervation of the carotid body, an effect that is reversed by exogenous administration of BDNF [Brady et al., 1999; Hertzberg et al., 1994]. Thus, BDNF acts as a carotid body-derived survival factor for petrosal ganglion neurons during development. Resting ventilation and the response to hyperoxia were severely depressed in BDNF knockout postnatal mice [Erickson et al., 1996].

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These effects may be related to the loss of chemosensory neurons due to BDNF-deficient carotid bodies. Alternatively, BDNF may have additional roles in chemosensitive reflexes, such as at the level of the chemoafferent synapse in the NTS [Katz, 2003]. For example, electrical stimulation of petrosal ganglion neurons increases BDNF release in the NTS, and exogenously administered BDNF decreases AMPA-mediated currents in sensory relay neurons in the NTS [Balkowiec and Katz, 2000; Balkowiec et al., 2000]. These data suggest that chemosensory afferent-derived BDNF may regulate synaptic strength within the NTS. Thus, it is plausible that, in addition to direct changes in carotid body sensitivity, BDNF-TrkB interactions within the NTS may contribute to respiratory motor plasticity during chronic carotid body stimulation (i.e., ventilatory acclimatization during chronic hypoxia at altitude). 3.2. Supraspinal mechanisms of Trk-dependent respiratory motor plasticity The preBötzinger complex within the ventrolateral medulla contains the neural network associated with inspiratory rhythm generation [Feldman and Del Negro, 2006]. Based on the premise that BDNF and TrkB receptors are located within the preBötzinger complex [Thoby-Brisson et al., 2003], and that BDNF has a significant role in the maturation of respiratory rhythm [Katz, 2003, 2005], recent studies have investigated the acute effects of exogenous BDNF administration on preBötzinger neuronal function. Thoby-Brisson et al. (2003) recorded spontaneous and evoked currents from preBötzinger neurons before and after applying BDNF to brain slices from 1 to 4 day old mice. Their experiments demonstrated that exogenous BDNF increased the amplitude of spontaneous and postsynaptic glutamatergic currents, and decreased respiratory frequency. The authors suggest that their data are consistent with previous studies [Butera et al., 1999; Del Negro et al., 2001] where increasing excitatory coupling between preBötzinger neurons decreases respiratory frequency. BDNF-induced facilitation of spontaneous synaptic currents persisted beyond the 30 min BDNF washout period, suggesting that plasticity had occurred. In addition, the effects of BDNF on frequency required a lag time of 15 min. BDNF–TrkB interactions initiate cellular responses with lag times from milliseconds to minutes depending on the mechanisms downstream of TrkB activation. For example, canonical TrkB signaling via activated PLC-␥, Akt, and/or MAP kinases, in general, requires minutes to hours before a phenotypic response is observed. In contrast, activated TrkB receptors can directly gate sodium channels (within milliseconds) without the need for intracellular signaling mediators, essentially conferring excitatory transmitter-like properties to BDNF [Kovalchuk et al., 2004; Rose et al., 2004]. Given the lag time and persistence of effect observed by Thoby-Brisson et al. (2003), the effects of BDNF on PreBötzinger neurons were likely due to long-term signaling mechanisms downstream of TrkB receptor activation. The Kölliker-Fuse nucleus contributes to respiratory pattern formation and afferent feedback processing. Recently, investigators have demonstrated a role for BDNF in synaptic plasticity of respiratory neurons within the Kölliker-Fuse nucleus [Kron et al., 2007a,b, 2008]. Kron et al. revealed that exogenous administration of BDNF potentiated NMDA receptor currents in juvenile, but not neonatal rats. This effect of BDNF was blocked by inhibitors of PLC activation, suggesting that BDNF may interact with NMDA receptors via Trk-dependent activation of PLC-␥. These data also suggest that developmental age may have considerable influence on the ability of RTKs to elicit respiratory motor plasticity in animals. BDNF injected into the juvenile Kölliker-Fuse nucleus did not show significant changes in phrenic motor activity.

3.3. Spinal mechanisms of Trk-dependent respiratory motor plasticity 3.3.1. Intermittent hypoxia-induced phrenic long-term facilitation Long-term changes in respiratory motoneuron function can occur via plasticity of their synaptic inputs [Mitchell and Johnson, 2003]. The most frequently studied model of respiratory synaptic plasticity is respiratory long-term facilitation, LTF, following acute intermittent hypoxia [Feldman et al., 2003; Mitchell et al., 2001]. Respiratory LTF is expressed as a progressive increase in inspiratory motor output (often recorded from hypoglossal and phrenic motoneurons) that persists for hours after exposure to repeated bouts of hypoxia (see Fig. 2A for a representative example of phrenic LTF). LTF is primarily a facilitation of inspiratory burst amplitude, however, a small but significant increase in respiratory frequency is often also present [Baker-Herman and Mitchell, 2008]. In the experimental paradigm of anesthetized, mechanically ventilated, and vagotomized animals, LTF represents a model of activity-independent synaptic plasticity. In the unanesthetized freely behaving animal, the physiological significance of this phenomenon to breathing is remains unclear. Hypoglossal and phrenic LTF may contribute to long-term adjustments in upper airway resistance and tidal volume, respectively, in response to intermittent hypoxia, such as may occur during obstructive sleep apnea [Mahamed and Mitchell, 2007]. This hypothesis is strengthened by resent observations that experimentally-induced episodic apnea also elicits phrenic LTF in anesthetized rats [Mahamed and Mitchell, 2008]. Mahamed and Mitchell (2007) propose that LTF may promote airway stability and limit the recurrence of obstructive apneas, thereby delaying the onset and/or severity of clinical manifestations of sleep-disordered breathing. If true, then investigating the factors that contribute to, or diminish, LTF in the anesthetized animal preparation may provide important clues to the epidemiology, pathophysiology, and future therapeutic approaches to treating obstructive sleep apnea in people. A mechanistic model describing the expression of phrenic LTF has recently been proposed [Baker-Herman et al., 2004]. Phrenic LTF requires spinal serotonin receptor activation and new spinal protein synthesis near phrenic motoneurons [Baker-Herman and Mitchell, 2002]. Baker-Herman et al. (2004) revealed that new synthesis of BDNF (but not synthesis of NT-3) near phrenic motoneurons was necessary for phrenic LTF following intermittent hypoxia. BDNF may elicit phrenic LTF by activating spinal TrkB receptors, which in turn strengthen excitatory glutamatergic synapses from brainstem respiratory neurons onto phrenic motoneurons. Using ELISA analysis, Baker-Herman et al. demonstrated that intermittent hypoxia increased BDNF in the ventral C4 spinal cord, which contains the majority of phrenic motoneuron soma. Whether these changes in BDNF are restricted to motoneurons or more widely distributed throughout the spinal segment is unknown. Therefore, we recently measured the change in BDNF immunoreactivity in the C4 spinal cord collected from anesthetized, vagotomized, and mechanically ventilated rats at 1 h after exposure to intermittent hypoxia (Fig. 1). In time controls (n = 5), weak BDNF immunoreactivity was observed throughout the spinal. After intermittent hypoxia (n = 6), BDNF immunoreactivity was significantly increased throughout the spinal cord, but also within large neurons within the ventral horn (presumptive motoneurons). These data confirm that intermittent hypoxia increases BDNF near phrenic motoneurons, although the changes in BDNF were not specific to any cell type. Thus, it is plausible that there may be multiple sources of BDNF responsible for phrenic LTF–phrenic motoneurons themselves, and/or adjacent neurons and glia.

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Fig. 1. The effects of intermittent hypoxia on brain-derived neurotrophic factor expression in the C4 ventral horn from rats not exposed to intermittent hypoxia (Time Controls = TC) and rats exposed to intermittent hypoxia (IH). Rats were perfused with 4% paraformaldehyde solution at the end of each experiment (TC n = 4; IH n = 6). C4 spinal segments were collected and prepared BDNF immunohistochemistry run in triplicate. Sections were incubated with anti-BDNF antibody made in rabbit (BDNF-N20: 1/1000, Santa Cruz, CA) overnight at 4 ◦ C, washed and then incubated with a biotinylated secondary antibody made in goat (goat anti-rabbit: 1/1000, Vector BA-5000) for 1 h. Sections were re-washed and then incubated with ABC solution (PK-6100 Standard, Vector Labs), followed by DAB solution (SK-4100, Vector Labs) for 7 min. Sections were examined using a light microscope (Leica DM4000B) and photomicrographs were captured using a SPOT RT-SE camera (Model 9.4 Slider6). BDNF immunoreactivity was measured using an offline image analysis software (NIH Image J). (Upper panel) Representative photomicrographs of the C4 ventral horn. BDNF expression increases after IH throughout the spinal cord, but is particularly noticeable in presumptive motoneurons (arrows). (Lower panel) Grouped data of the change in pixel grey density in the ventral horn compared to control rats. The change in BDNF immunoreactivity was largest in rats exposed to IH. * p < 0.05 compared to time controls.

The role of full length TrkB receptor activation in phrenic LTF has not been directly investigated. K252a, a protein kinase inhibitor, blocked phrenic motor facilitation following intermittent hypoxia, which supports a role for TrkB activation in phrenic LTF [BakerHerman et al., 2004]. However, K252a also inhibits other protein kinases, such as protein kinase A (PKA) and protein kinase C (PKC). Recent studies suggest that PKA and PKC activation can modulate respiratory motoneuron output and elicit respiratory motor plasticity, respectively [Feldman et al., 2005; Neverova et al., 2007]. BDNF may also modulate phrenic motor output by binding to truncated TrkB receptors, which lack an intracellular tyrosine kinase domain and are located on spinal motoneurons [Johnson et al., 1999; Zhang and Huang, 2006]. The role of truncated TrkB receptors in synaptic plasticity is unknown, however a recent report suggest that overexpression of truncated TrkB receptors in hippocampal neurons induces morphological plasticity of dendritic [Hartmann et al., 2004]. A potential role for the truncated TrkB receptor in respiratory motor plasticity is intriguing and warrants further investigation. 3.3.2. Transactivation of spinal Trk receptors RTK activation has the potential to match respiratory motor function with repetitive changes in physiological demand and highlights the potential for RTK activators as pharmacological strategies to improve breathing in situations where respiratory motor func-

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tion is impaired (i.e., following spinal cord injury or in ALS patients). The use of hypoxia to activate RTKs on respiratory motoneurons is limited clinically because intermittent hypoxia is a non-specific stimulus that may affect many physiological functions and induce pathology [Bass et al., 2004]. Thus, pharmaceuticals that activate RTKs are more likely candidates for the therapeutic development than intermittent hypoxia. Given that BDNF is sufficient to elicit phrenic motor facilitation [Baker-Herman et al., 2004], BDNF could be administered intrathecally to increase phrenic motor output (and presumably ventilation). However, penetration of exogenous proteins to the CNS is limited by the blood brain barrier and may be associated with unintended activation of immune responses against a foreign protein. An alternative approach is the use of small, highly permeable molecules with properties that mimic the effects of BDNF, and TrkB receptor activation, on respiratory motor output. In PC12 cells, adenosine 2a (A2a) receptor activation phosphorylates Trk receptors, initiates Trk-dependent signaling in the absence of neurotrophins, and increases cell survival [Jeanneteau and Chao, 2006; Lee and Chao, 2001; Lee et al., 2002]. A2a receptors belong to the family of G-protein coupled receptors (GPCR), which traditionally transmit extracellular signals to the cell’s interior, stimulate second messenger signals, and regulate the activity of effector enzymes and ion channels [Luttrell, 2006]. GPCRs also can activate growth factor signaling pathways via transactivation of RTKs. Transactivation occurs when a ligand binds to a GPCR, elicits G-protein dependent signaling, which in turn promotes phosphorylation of an RTK. As such, the GPCR ligand does not directly associate with the RTK. GPCR-induced transactivation of RTKs has been described for epidermal growth factor receptors [Fischer et al., 2006], platelet derived growth factor ␤ receptors [Oak et al., 2001], fibroblast growth factor (FGF) receptors [Belcheva et al., 2002], and more recently in vivo for the Trk receptors [Golder et al., 2008; Wiese et al., 2007]. The physiological role of GPCR-induced transactivation of RTKs is unknown, although multiple advantages have been proposed. First, signaling from diverse sources (such as neurotrophins or unrelated GPCR ligands) can be integrated, providing more subtle control of RTKs. Second, the specific effector pathway activated by a RTK can depend on how the receptor was activated. For example, a neurotrophin binding to a Trk receptor activates multiple effector pathways (Akt, ERK1/2, and PLC-␥) whereas GPCR-mediated transactivation of Trk receptors favors only one downstream effector, namely Akt [Lee and Chao, 2001]. Finally, GPCR-mediated transactivation of Trk receptors may substitute for declining expression of neurotrophins in the aging spinal cord [Nakamura and Bregman, 2001], thereby maintaining Trk-dependent forms of motor plasticity in the adult CNS. Given that A2a receptors are located on neuron dendrites and axon terminals throughout the CNS, including phrenic motoneurons [Correia-de-Sa and Ribeiro, 1994], we speculated that spinal A2a receptor activation could mimic the effects of BDNF on phrenic motor output, in particular by eliciting phrenic motor facilitation. In our recent study [Golder et al., 2008], we report that intrathecal administration of the A2a receptor agonist, CGS 21680, activates cervical spinal A2a receptors, strengthens synaptic pathways to phrenic motoneurons, and elicits phrenic motor facilitation in rats. Intravenous administration of A2a receptor agonists also increased in hypoglossal motor output, suggesting that A2a-induced respiratory motor plasticity is not restricted to spinal motoneurons. A2a-induced phrenic motor facilitation is phenotypically similar to that observed following intermittent hypoxia and intrathecal BDNF administration. Interestingly, the magnitude of phrenic motor facilitation is greatest when induced by exogenous BDNF or A2a receptor agonist administration compared to intermittent

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Fig. 2. Phrenic motor facilitation elicited by three separate stimuli. (A) Representative traces of integrated phrenic nerve discharge before, during, and after exposure to intermittent hypoxia (upper trace) and A2a-receptor activation (lower trace). (B) Average data collected at 60-min post-stimulus depicting the percentage change in phrenic burst amplitude above baseline values. Groups include rats not exposed to any stimulus (Time Controls = TC; n = 4), rats exposed to 3 episodes of hypoxia (IH; n = 6), after intrathecal administration of brain-derived neurotrophic factor (BDNF; n = 6), and after intrathecal injections of the A2a receptor agonist, CGS 21680 (A2a; n = 8). A detailed description of the methods used to record phrenic motor output is available in Golder et al. (2008). * p < 0.05, significantly different from TC; ‡ p < 0.05, significantly different from IH; data are mean ± S.E.M.

hypoxia in the same rat substrain (Fig. 2). In this figure, data are compared at 60 min post-stimulus. In rats exposed to intermittenthypoxia, the magnitude of LTF does not change significantly beyond 60 min post-hypoxia. In rats exposed to exogenous BDNF or A2a receptor agonists, phrenic motor facilitation usually increases further and then remains constantly elevated for the duration of the recordings, up to 3 h post-stimulus. This difference in magnitude and time course supports the concept that, although the phenotype may be similar, the mechanisms contributing to phrenic motor facilitation differ depending on the inciting stimulus. To date, our studies have focused on the effects of A2a receptor agonists on respiratory motor output. However, adenosine interacts with four adenosine receptors, A1, A2a, A2b, and A3, all of which are G-protein coupled receptors, GPCRs [Fredholm et al., 2001]. To our knowledge, there are no endogenous molecules that are selective for the A2a receptor. As such, the physiological relevance of studies using selective A2a receptor ligands is unclear. Sebastiao and Ribeiro (2000) have attempted to address this issue by investigating the functional effects of adenosine on the phrenic–diaphragm neuromuscular junction. This group suggests that adenosine is capable of preferentially activating A2a receptor s even when A1 receptors are located on the same nerve terminal. Synaptic adenosine is derived from: (1) hydrolysis of ATP co-localized and released with a variety of neurotransmitters, and (2) direct release from neighboring neurons and glia. Studies sug-

gest that adenosine derived from ATP acts preferentially on A2a receptors and adenosine released directly from cells preferentially activates A1 receptors [Cunha et al., 1996]. This selectivity could result from a different localization of A1 and A2A receptors in relation to adenosine release sites and to the localization of the enzyme ecto-nucleotidase, which hydrolyses released ATP into adenosine. Alternatively, receptor selectivity could be a consequence of the burst-like formation of adenosine from released ATP. Using phrenic–diaphragm neuromuscular junctions as a model of neurotransmission, the Ribeiro laboratory suggests that the pattern of neuronal firing influences the pattern of purine release and extracellular formation, and consequently, influences the way purines modulate synaptic transmission. High-frequency stimulation favors ATP release and adenosine formation, which in turn preferentially activates A2A receptors, whereas low-frequency stimulation favors adenosine release and A1 receptor activation [Correia-de-Sa et al., 1996]. Given that premotor respiratory drive stimulates phrenic motoneurons with a high-frequency pattern, it is plausible that A2a-induced phrenic motor facilitation may indeed have a physiological correlate-modulation of synaptic strength by synaptic release of co-localized ATP. Our working model proposes that the cellular mechanisms responsible for strengthening synaptic pathways to phrenic motoneurons occur postsynaptically (Fig. 3). However, given that A2a receptors are located presynaptically and postsynaptically on many neurons [Rebola et al., 2005], it is plausible that both presynaptic and/or postsynaptic mechanisms may contribute to A2a-induced phrenic motor facilitation. Presynaptic mechanisms that may contribute to A2a-induced phrenic motor plasticity include increased quantal release of glutamate and enlargement of vesicular release sites [Krueger and Fitzsimonds, 2006]. Postsynaptic mechanisms include structural plasticity of dendritic spines [Muller et al., 2002], and insertion and post-translational modification of the ionotropic glutamate receptors (i.e., AMPA and/or NMDA). We have recently completed preliminary experiments that suggest that NMDA receptor activation is necessary for A2a-induced phrenic motor plasticity. Pretreatment with intrathecally administered MK801, an NMDA receptor antagonist, dose-dependently blocked A2a-induced phrenic motor facilitation in anesthetized, vagotomized, pump-ventilated rats (Golder, unpublished data). In general, NMDA receptors are considered to be primarily post-synaptic, which would support a postsynaptic mechanistic loci. The cellular processes connecting A2a receptor activation to NMDA-induced phrenic motor facilitation are not completely understood, although certain key factors have been identified (Fig. 3) [Golder et al., 2008]. Cervical spinal A2a receptor activation increased the synthesis and phosphorylation of an immature isoform of TrkB protein in the vicinity of phrenic motoneurons, including within presumptive motoneurons in the C4 spinal segment. New TrkB synthesis and activation was necessary for the effects of A2a receptor activation on phrenic motor output. A2a-induced TrkB activation did not require new BDNF expression, supporting a model of GPCR-induced transactivation of Trk receptors. However, the role of preformed BDNF (i.e., increased extracellular release of BDNF) and new or preformed NT-4 in A2ainduced phrenic motor facilitation remains unknown. How intracellular TrkB protein becomes phosphorylated without canonical neurotrophin-TrkB interactions remains unclear. One possible mechanism has recently been proposed by Rajagopal et al. (2004). Increasing intracellular TrkB protein can induce constitutive Trk signaling in the absence of the relevant neurotrophin [Watson et al., 1999], possibly because increased local concentration of Trk monomer enables auto-dimerization and subsequent phosphorylation. To test this theory, we administered tunicamycin

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Fig. 3. A schematic figure comparing and contrasting the proposed mechanisms contributing to intermittent hypoxia-induced phrenic long-term facilitation and A2ainduced phrenic motor facilitation. Intermittent hypoxia stimulates new BDNF synthesis near phrenic motoneurons. BDNF is secreted extracellularly and binds to mature, fully glycosylated (CHO), TrkB receptors expressed on the outer cell membrane. BDNF phosphorylates (P) mature TrkB receptors, which in turn strengthen excitatory glutamatergic synapses onto phrenic motoneurons via activated MAP kinases (ERK1/2). A2a receptor agonists activate Gs protein and increase synthesis and phosphorylation of intracellular, hypoglycosylated, immature TrkB protein without the need for BDNF (transactivation). Phosphorylated intracellular TrkB strengthens excitatory glutamatergic synapses onto phrenic motoneurons via activated Pi3K and Protein Kinase B (Akt) (with activated ERK1/2 acting as an alternative pathway. Both pathways converge on NMDA receptors to strengthen synaptic pathways to phrenic motoneurons.

(20 ␮l; 100 ␮M), a protein glycosylation inhibitor, intrathecally over the C4 spinal segment in two anesthetized rats. Proteins expressed on the outer cell membrane (i.e., RTKs) are generally glycosylated after translation. These oligosaccharides are crucial for trafficking proteins from intracellular compartments to the extracellular membrane [Takahashi et al., 2004]. Tunicamycin increases Trk expression and activation on intracellular membranes [Watson et al., 1999]. In our preliminary study, phrenic burst amplitude increased after tunicamycin injection, reaching a peak facilitation of 35% at 60 min. This data supports the concept that increasing intracellular TrkB protein near/within phrenic motoneurons is sufficient to initiate phrenic motor facilitation. Further studies using more specific mechanisms of increasing intracellular TrkB protein (i.e., viral vectoring TrkB cDNA to phrenic motoneurons) seem warranted. Consistent with in vitro models [Lee et al., 2002], cervical spinal A2a-induced TrkB activation increases phosphorylation of Akt, but not ERK1/2 or PLC-␥ [Golder et al., 2008]. The mechanism responsible for selective effector activation is unknown, but may reflect regional differences in the cellular distribution of adapter proteins/enzymes such as Ras, an adapter protein/enzyme that bridges Trk protein with MAP kinases [Rajagopal et al., 2004]. Alternatively, Raf-Akt cross-talk may account for this effect. Raf-Akt cross-talk occurs when elevated phospho-Akt levels inhibit ERK1/2 activation by inhibiting Raf, an effector kinase upstream of ERK1/2 [Zimmermann and Moelling, 1999]. Thus, under the correct preconditions (i.e., decreased Akt activation) ERK1/2 may become activated, instead of Akt, following A2a-induced TrkB signaling. Our preliminary studies support this concept that not all rats express an increase in C4 spinal Akt activation after A2a-induced TrkB phosphorylation, in some rats, ERK1/2 is preferentially activated, and a negative correlation exists between phospho-Akt and phosphoERK1/2 levels in rats exposed to cervical A2a receptor agonists.

3.4. Trk receptors and ventilatory facilitation in unanesthetized animals Inspiratory phrenic activity is positively correlated to inspiratory pressure and tidal volume in spontaneously breathing mammals [Eldridge, 1975]. As such, long-term facilitation of phrenic motor activity may be evident as persistent increases in minute ventilation in unanesthetized, freely behaving animals. Intermittent hypoxia and A2a receptor agonists have been administered to unanesthetized animals to elicit ventilatory facilitation [Golder et al., 2008; Prabhakar and Kline, 2002]. However, the role of RTKs and ventilatory facilitation has not been directly studied. A2a receptor activation elicits ventilatory facilitation in normal and C2 spinally injured unanesthetized rats [Golder et al., 2008]. Rats with a unilateral C2 spinal injury maintain normal minute ventilation albeit using a rapid shallow breathing pattern, in part because C2 injury disrupts descending synaptic pathways from the brainstem to the spinal cord [Golder et al., 2001]. A2a receptor activation increased tidal volume to normal values in C2-injured rats with a time course that closely matched the time course for A2a-induced phrenic motor facilitation in anesthetized preparations. Thus, we propose that A2a-induced spinal RTK activation contributes to A2a-induced ventilatory facilitation. Further mechanistic studies are warranted to separate likely peripheral (i.e., A2a receptors in the carotid bodies increase phrenic motor output [Kobayashi et al., 2000]) and central mechanisms acting concurrently to shape the ventilatory response to A2a receptor agonists. 3.5. Potential therapeutic roles for Trk activation in respiratory control Neurotrophins, or small molecules that transactivate Trk receptors, may have therapeutic potential to treat disorders in breathing.

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Rett syndrome, a developmental disorder, is caused by mutations in the methyl-CpG-binding protein 2 gene (Mecp2) [Francke, 2006]. This mutation often leads to severe respiratory complications including episodic hyperventilation and apneusis [Weese-Mayer et al., 2006]. In a mouse model of Rett Syndrome, central and peripheral deficits in BDNF expression exist after birth [Wang et al., 2006]. Given that BDNF is necessary for the normal development of brainstem respiratory neurons, it has been postulated that deficits in BDNF-TrkB signaling may contribute to the respiratory complications associated with Rett Syndrome [Wang et al., 2006]. Thus, it follows that increasing BDNF expression may improve breathing in this patient population. To test this hypothesis, Ogier et al. (2007) investigated the effects of increasing BDNF expression on ventilation in Mecp2 null mice. The pattern of breathing was measured before and after a 3-day treatment with the ampakine drug, CX546. Ampakines increase neuronal activity by prolonging AMPA-mediated inward currents, thereby increasing activity-dependent expression of BDNF. CX546 increased BDNF expression within brainstem respiratory neurons and decreased respiratory frequency. Ogier et al. suggest that CX546 may improve breathing in people with Rhett Syndrome by increasing BDNF expression (and presumably BDNF-TrkB interactions) near respiratory neurons. In our recent study, A2a receptor agonists increased tidal volume in spinally injured rats. Although the percent change in function was small, even modest increments in respiratory function can significantly enhance quality of life and reduce morbidity in ventilator-dependent patients, particularly if the functional gain allows even partial ventilator independence. A2a-induced TrkB transactivation also may occur in motoneurons subserving other motor functions, such as locomotion and upper airway patency. Thus, A2a receptor activation may represent a novel therapy for improving motoneuron function in general and in the treatment of obstructive sleep apnea. 4. RET RET, the protein from the Ret (rearranged-during-transfection) gene, binds extracellular ligands belonging to the glial cell linederived neurotrophic factor (GDNF) family of ligands: GDNF, neurturin, artemin, and persephin [Runeberg-Roos and Saarma, 2007]. RET is activated by a complex formed between a GDNF family ligand and one of four co-receptors belonging to the GDNF family receptor ␣ (GFR␣1–4). In general, the GDNF family ligands and RET signaling is crucial for the development of the enteric, sympathetic, parasympathetic, motor, and sensory nervous systems. Although we do not know of any role for RET in respiratory control, other than maintaining carotid chemoafferent attachments during carotid body development, we will outline studies that demonstrate a potential for RET and its ligands to elicit respiratory motor plasticity. The role of RET in carotid body development is reviewed elsewhere in this issue [see review by Katz in this issue]. Most motoneurons express abundant RET protein [Garces et al., 2000; Jongen et al., 2007], thereby providing a substrate for direct effects of RET activation on motoneuron function. Indeed, RET is expressed in rat hypoglossal motoneurons and transported anterogradally towards the neuromuscular junction [Russell et al., 2000]. Sources of GDNF family ligands that may activate motoneuron RET receptors include target muscles, Schwann cells, astrocytes, and possibly the motoneurons themselves [English, 2003; Zhao et al., 2004]. Whether RET protein is present in phrenic motoneurons remains unknown. However, given that GDNF is expressed in the rat diaphragm and is found in the C3 to C6 ventral spinal cord [Johnson et al., 2000], it is plausible that RET receptors are also present on or near phrenic motoneurons.

RET signaling may contribute to respiratory motoneuron survival in models of axonal injury, and synaptic plasticity. For example, GDNF elicits protective effects against facial nerve axotomy by decreasing programmed motoneuron death, maintaining normal choline acetyltransferase expression, and attenuating the characteristic astrogliosis in the facial nucleus [Zhao et al., 2004]. Thus, GDNF may be useful as a promoter of respiratory motor recovery after peripheral nerve injuries that axotomize respiratory motoneurons. GDNF gene therapy has been used in rat models of laryngeal paralysis to improve laryngeal function [Shiotani et al., 2007]. One month post-gene transfer, GDNF increased the number of surviving motoneurons, increased nerve conduction velocity, and elicited better vocal fold motor function compared to control rats. The GDNF family ligands and RET may promote synaptic plasticity. Chen et al. (2006) administered a neurotoxin to denervate the striatum of dopamine neurons in mice. Transgenic GDNF restored LTP and LTD in striatal slices; however, whether this effect was due to rescue of dopaminergic neurons or plasticity of remaining synapses is unknown. Supporting the latter mechanism, investigators have demonstrated that GDNF potentiates calcium channels and neurotransmitter release centrally and in muscles, possibly by upregulating frequenin, a calcium-binding protein [Wang et al., 2001]. 5. Platelet-derived growth factor receptor PDGF receptor activation is similar to that described for Trk receptors: ligand-induced receptor dimerization, autophosphorylation, and downstream effector activation (i.e., MAP kinases, Akt, and PLC-␥). PDGF receptor may contribute to the late-phase of the hypoxic ventilatory response. With sustained hypoxia, ventilation progressively declines (sometimes termed “roll-off”) relative to the acute hypoxic response [Powell et al., 1998]. Various inhibitory factors (e.g., adenosine and GABA) and reduced NMDA receptor function in the NTS have been implicated in the mechanisms contributing to roll-off. NMDA receptor activity is believed to be high during the acute hypoxic ventilatory response, and then declines as the duration of hypoxia increases. Recently, Gozal et al. propose that platelet-derived growth factor ␤ (PDGF-␤), and PDGF-␤ receptor activation, may contribute to this phenomenon. The Gozal laboratory demonstrated that sustained hypoxia increases PDGF expression in the NTS and that pharmacological inhibition of PDGF receptors abolishes ventilatory roll-off. In their model, PDGF-induced activation of the PDGF receptor activates PLC-␥, which in turn increases protein phosphatase activity. Activated protein phosphatase decreases NMDA receptor phosphorylation and diminishes NMDA receptor activity. Whether PDGF has similar effects in the vicinity of respiratory motoneurons as yet to be elucidated. 6. Fibroblast growth factor receptor The effects of fibroblast growth factor receptor activation on respiratory function after thoracic spinal cord injury has been evaluated in rats [Teng et al., 1999]. T8 contusion injury elicited a rapid shallow breathing pattern and reduced the hypercapnic ventilatory response. FGF injected intrathecally over a T8 contusion injury significantly improved motoneuron survival, and prevented the SCI-induced respiratory deficiencies. The authors propose that most, if not all, the effects of FGF on respiratory function were due to increased survival of intercostal motoneurons, in part, because a linear correlation existed between changes in tidal volume during hypercapnia and the number of neurons located in the ventral horn rostral to the injury epicenter. However, FGF could also poten-

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tially elicit synaptic plasticity of remaining motoneurons, either by inducing axon terminal sprouting [Ramirez et al., 1999] or acting as a trophic factor for serotonergic terminals [Lindholm et al., 1994].

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injury demonstrate the potential for RTK activators as therapeutic tools in patients with respiratory deficiencies.

References 7. Vascular endothelial growth factor receptor Recently, increased vascular-endothelial growth factor (VEGF), and its RTK, have been implicated in the pathophysiology of sleepdisordered breathing, such as obstructive sleep apnea (OSA) in children [Gozal et al., 2002]. However, this association remains inconclusive because plasma VEGF levels were higher or similar between patients with obstructive sleep-disordered breathing and in controls without snoring. Whether circulating VEGF concentration is a sensitive indicator of VEGF expression near respiratory motoneurons is debatable. In contrast, another group suggest that VEGF may instead be a long-term adaptive mechanism in OSA, reducing mortality in elderly OSA patients [Peled et al., 2007]. Further studies are necessary before definitive conclusions can be reached. VEGF may increase respiratory motor output when expressed in the vicinity of phrenic motoneurons. VEGF injected intrathecally over the C4 spinal segment in anesthetized rats progressively increases phrenic motor output (Mitchell, personal communication), reminiscent of phrenic facilitation following intrathecal BDNF and A2a receptor agonist administration. Thus, VEGF may be protective in some cases of sleep-disordered breathing by eliciting respiratory motor plasticity and increasing upper airway stability. 8. Insulin-like growth factor receptor Insulin-like growth factor (IGF) receptor may contribute to progestin-induced improvements in ventilation in postmenopausal women with chronic respiratory insufficiencies [Saaresranta et al., 2000]. Saaresranta et al. measured the magnitude and duration of changes in IGF expression during progestin therapy in a small group of postmenopausal women with hypercapnic or hypoxaemic respiratory failure. Serum IGF increased and followed a similar time course as the improvements seen in ventilatory performance. The mechanism responsible for this interaction between IGF and progestin remains unknown. 9. Conclusions The study of RTKs and their ligands in respiratory motor plasticity is an emerging field in respiratory neurobiology and will significantly contribute to our understanding how breathing adapts during an individual’s lifetime, either during normal physiological mechanisms or in response to injury or disease. It is clear from the studies reviewed here that RTK activation can elicit respiratory plasticity via multiple direct and indirect mechanisms. These include increasing motoneuron survival after injury or following loss of motoneurons through degenerative neurological diseases (such as ALS), increasing motoneuron axon terminal sprouting at target muscles, inducing synaptic plasticity of excitatory synaptic connections to motoneurons, and increasing neurotransmitter release pre- and postsynaptically. Furthermore, the location of RTKs and the stimulation paradigm can govern the magnitude and direction (i.e., facilitation or depression) of respiratory motor plasticity. For example, chronic hypoxia elicits TrkB-dependent respiratory motor depression at the level of the NTS, whereas intermittent hypoxia elicits TrkB-dependent phrenic LTF. Finally, preliminary studies using small molecules to transactivate TrkB receptors near phrenic motoneurons and improve tidal volume after spinal cord

Avruch, J., et al., 2001. Ras activation of the Raf kinase: tyrosine kinase recruitment of the MAP kinase cascade. Recent Prog. Horm. Res. 56, 127–155. Baker-Herman, T.L., Mitchell, G.S., 2002. Phrenic long-term facilitation requires spinal serotonin receptor activation and protein synthesis. J. Neurosci. 22, 6239–6246. Baker-Herman, T.L., Mitchell, G.S., 2008. Determinants of frequency long-term facilitation following acute intermittent hypoxia in vagotomized rats. Respir. Physiol. Neurobiol. 162, 8–17. Baker-Herman, T.L., et al., 2004. BDNF is necessary and sufficient for spinal respiratory plasticity following intermittent hypoxia. Nat. Neurosci. 7, 48–55. Balkowiec, A., Katz, D.M., 2000. Activity-dependent release of endogenous brainderived neurotrophic factor from primary sensory neurons detected by ELISA in situ. J. Neurosci. 20, 7417–7423. Balkowiec, A., et al., 2000. Brain-derived neurotrophic factor acutely inhibits AMPA-mediated currents in developing sensory relay neurons. J. Neurosci. 20, 1904–1911. Bass, J.L., et al., 2004. The effect of chronic or intermittent hypoxia on cognition in childhood: a review of the evidence. Pediatrics 114, 805–816. Belcheva, M.M., et al., 2002. The fibroblast growth factor receptor is at the site of convergence between mu-opioid receptor and growth factor signaling pathways in rat C6 glioma cells. J. Pharmacol. Exp. Ther. 303, 909–918. Betsholtz, C., 2003. Biology of platelet-derived growth factors in development. Birth Defects Res. C Embryo Today 69, 272–285. Blochl, A., Blochl, R., 2007. A cell-biological model of p75NTR signaling. J. Neurochem. 102, 289–305. Brady, R., et al., 1999. BDNF is a target-derived survival factor for arterial baroreceptor and chemoafferent primary sensory neurons. J. Neurosci. 19, 2131–2142. Butera Jr., R.J., et al., 1999. Models of respiratory rhythm generation in the pre-Botzinger complex. II. Populations Of coupled pacemaker neurons. J. Neurophysiol. 82, 398–415. Chen, L.W., Zhang, J.P., Kwok-Yan Shum, D., Chan, Y.S., 2006. Localization of nerve growth factor, neurotrophin-3, and glial cell line-derived neurotrophic factor in nestin-expressing reactive astrocytes in the caudate-putamen of 1-methyl4-phenyl-1,2,3,6-tetrahydropyridine-treated C57/Bl mice. J Comp Neurol 497, 898–909. Cohen, P., 1999. The Croonian Lecture 1998. Identification of a protein kinase cascade of major importance in insulin signal transduction. Philos. Trans. R. Soc. Lond. B. Biol. Sci. 354, 485–495. Cohen, S., 2004. Origins of growth factors: NGF and EGF. Ann. N. Y Acad. Sci. 1038, 98–102. Correia-de-Sa, P., Ribeiro, J.A., 1994. Evidence that the presynaptic A2a-adenosine receptor of the rat motor nerve endings is positively coupled to adenylate cyclase. Naunyn Schmiedebergs Arch. Pharmacol. 350, 514–522. Correia-de-Sa, P., et al., 1996. Presynaptic A1 inhibitory/A2A facilitatory adenosine receptor activation balance depends on motor nerve stimulation paradigm at the rat hemidiaphragm. J. Neurophysiol. 76, 3910–3919. Cunha, R.A., et al., 1996. Preferential activation of excitatory adenosine receptors at rat hippocampal and neuromuscular synapses by adenosine formed from released adenine nucleotides. Br. J. Pharmacol. 119, 253–260. Del Negro, C.A., et al., 2001. Models of respiratory rhythm generation in the preBotzinger complex. III. Experimental tests of model predictions. J. Neurophysiol. 86, 59–74. Dreyfus, C.F., et al., 1999. Expression of neurotrophins in the adult spinal cord in vivo. J. Neurosci. Res. 56, 1–7. Dupont, J., Holzenberger, M., 2003. Biology of insulin-like growth factors in development. Birth Defects Res. C Embryo Today 69, 257–271. Eldridge, F.L., 1975. Relationship between respiratory nerve and muscle activity and muscle force output. J. Appl. Physiol. 39, 567–574. English, A.W., 2003. Cytokines, growth factors and sprouting at the neuromuscular junction. J. Neurocytol. 32, 943–960. Erickson, J.T., et al., 1996. Mice lacking brain-derived neurotrophic factor exhibit visceral sensory neuron losses distinct from mice lacking NT4 and display a severe developmental deficit in control of breathing. J. Neurosci. 16, 5361–5371. Eswarakumar, V.P., et al., 2005. Cellular signaling by fibroblast growth factor receptors. Cytokine Growth Factor Rev. 16, 139–149. Feldman, J.L., Del Negro, C.A., 2006. Looking for inspiration: new perspectives on respiratory rhythm. Nat. Rev. Neurosci. 7, 232–242. Feldman, J.L., et al., 2003. Breathing: rhythmicity, plasticity, chemosensitivity. Annu. Rev. Neurosci. 26, 239–266. Feldman, J.L., et al., 2005. Modulation of hypoglossal motoneuron excitability by intracellular signal transduction cascades. Respir. Physiol. Neurobiol. 147, 131–143. Ferrara, N., et al., 2003. The biology of VEGF and its receptors. Nat. Med. 9, 669–676. Fischer, O.M., et al., 2006. Dissecting the epidermal growth factor receptor signal transactivation pathway. Methods Mol. Biol. 327, 85–97.

250

F.J. Golder / Respiratory Physiology & Neurobiology 164 (2008) 242–251

Forrester, W.C., 2002. The Ror receptor tyrosine kinase family. Cell Mol. Life Sci. 59, 83–96. Francke, U., 2006. Mechanisms of disease: neurogenetics of MeCP2 deficiency. Nat. Clin. Pract. Neurol. 2, 212–221. Fredholm, B.B., et al., 2001. International Union of Pharmacology. XXV. Nomenclature and classification of adenosine receptors. Pharmacol. Rev. 53, 527–552. Gale, N.W., Yancopoulos, G.D., 1999. Growth factors acting via endothelial cellspecific receptor tyrosine kinases: VEGFs, angiopoietins, and ephrins in vascular development. Genes Dev. 13, 1055–1066. Garces, A., et al., 2000. GFRalpha 1 is required for development of distinct subpopulations of motoneuron. J. Neurosci. 20, 4992–5000. Gentile, A., et al., 2008. The Met tyrosine kinase receptor in development and cancer. Cancer Metastasis Rev. 27, 85–94. Golder, F.J., et al., 2008. Spinal adenosine A2a receptor-activation elicits long-lasting phrenic motor facilitation. J. Neurosci. 28, 2033–2042. Golder, F.J., et al., 2001. Cervical spinal cord injury alters the pattern of breathing in anesthetized rats. J. Appl. Physiol. 91, 2451–2458. Gozal, D., et al., 2002. Circulating vascular endothelial growth factor levels in patients with obstructive sleep apnea. Sleep 25, 59–65. Hafizi, S., Dahlback, B., 2006. Signalling and functional diversity within the Axl subfamily of receptor tyrosine kinases. Cytokine Growth Factor Rev. 17, 295–304. Hartmann, M., et al., 2004. Truncated TrkB receptor-induced outgrowth of dendritic filopodia involves the p75 neurotrophin receptor. J. Cell Sci. 117, 5803–5814. Hertzberg, T., et al., 1994. BDNF supports mammalian chemoafferent neurons in vitro and following peripheral target removal in vivo. Dev. Biol. 166, 801–811. Horwood, J.M., et al., 2006. Signalling mechanisms mediated by the phosphoinositide 3-kinase/Akt cascade in synaptic plasticity and memory in the rat. Eur. J. Neurosci. 23, 3375–3384. Huang, E.J., Reichardt, L.F., 2003. Trk receptors: roles in neuronal signal transduction. Annu. Rev. Biochem. 72, 609–642. Hubbard, S.R., Till, J.H., 2000. Protein tyrosine kinase structure and function. Annu. Rev. Biochem. 69, 373–398. Jeanneteau, F., Chao, M.V., 2006. Promoting neurotrophic effects by GPCR ligands. Novartis Found Symp. 276, 181–189 (discussion 189–192, 233–187, 275–181). Johnson, H., et al., 1999. Expression of p75(NTR), trkB and trkC in nonmanipulated and axotomized motoneurons of aged rats. Brain. Res. Mol. Brain Res. 69, 21–34. Johnson, R.A., et al., 2000. Cervical dorsal rhizotomy increases brain-derived neurotrophic factor and neurotrophin-3 expression in the ventral spinal cord. J. Neurosci. 20, RC77. Jongen, J.L., et al., 2007. Distribution of RET immunoreactivity in the rodent spinal cord and changes after nerve injury. J. Comp. Neurol. 500, 1136–1153. Kamat, A., Carpenter, G., 1997. Phospholipase C-gamma1: regulation of enzyme function and role in growth factor-dependent signal transduction. Cytokine Growth Factor Rev. 8, 109–117. Katz, D.M., 2003. Neuronal growth factors and development of respiratory control. Respir. Physiol. Neurobiol. 135, 155–165. Katz, D.M., 2005. Regulation of respiratory neuron development by neurotrophic and transcriptional signaling mechanisms. Respir. Physiol. Neurobiol. 149, 99–109. Keeble, T.R., Cooper, H.M., 2006. Ryk: a novel Wnt receptor regulating axon pathfinding. Int. J. Biochem. Cell Biol. 38, 2011–2017. Kobayashi, S., et al., 2000. Gene expression and function of adenosine A(2A) receptor in the rat carotid body. Am. J. Physiol. Lung Cell Mol. Physiol. 279, L273–282. Kobus, F.J., Fleming, K.G., 2005. The GxxxG-containing transmembrane domain of the CCK4 oncogene does not encode preferential self-interactions. Biochemistry 44, 1464–1470. Kovalchuk, Y., et al., 2004. Neurotrophin action on a rapid timescale. Curr. Opin. Neurobiol. 14, 558–563. Kron, M., et al., 2007a. Developmental changes in brain-derived neurotrophic factormediated modulations of synaptic activities in the pontine Kolliker-Fuse nucleus of the rat. J. Physiol. 583, 315–327. Kron, M., et al., 2008. Emergence of BDNF induced postsynaptic potentiation of NMDA-currents during the postnatal maturation of the Kolliker-Fuse nucleus of the rat. J. Physiol.. Kron, M., et al., 2007b. Developmental changes in the BDNF-induced modulation of inhibitory synaptic transmission in the Kolliker-Fuse nucleus of rat. Eur. J. Neurosci. 26, 3449–3457. Krueger, S., Fitzsimonds, R.M., 2006. Remodeling the plasticity debate: the presynaptic locus revisited. Physiology (Bethesda) 21, 346–351. Lee, F.S., Chao, M.V., 2001. Activation of Trk neurotrophin receptors in the absence of neurotrophins. Proc. Natl. Acad. Sci. U.S.A. 98, 3555–3560. Lee, F.S., et al., 2002. Distinctive features of Trk neurotrophin receptor transactivation by G protein-coupled receptors. Cytokine Growth Factor Rev. 13, 11–17. Lindholm, D., et al., 1994. Fibroblast growth factor-5 promotes differentiation of cultured rat septal cholinergic and raphe serotonergic neurons: comparison with the effects of neurotrophins. Eur. J. Neurosci. 6, 244–252. Luttrell, L.M., 2006. Transmembrane signaling by G protein-coupled receptors. Methods Mol. Biol. 332, 3–49. Mahamed, S., Mitchell, G.S., 2007. Sleep apnoea & hypertension: physiological bases for a causal relation: is there a link between intermittent hypoxia-induced respiratory plasticity and obstructive sleep apnoea? Exp. Physiol. 92, 27–37. Mahamed, S., Mitchell, G.S., 2008. Simulated apnoeas induce serotonin-dependent respiratory long-term facilitation in rats, J Physiol. 586., 2171–2178. Maisonpierre, P.C., et al., 1990. NT-3, BDNF, and NGF in the developing rat nervous system: parallel as well as reciprocal patterns of expression. Neuron 5, 501–509.

Malenka, R.C., 2003. Synaptic plasticity and AMPA receptor trafficking. Ann. N.Y. Acad. Sci. 1003, 1–11. Minichiello, L., et al., 2002. Mechanism of TrkB-mediated hippocampal long-term potentiation. Neuron 36, 121–137. Mitchell, G.S., et al., 2001. Invited review: intermittent hypoxia and respiratory plasticity. J. Appl. Physiol. 90, 2466–2475. Mitchell, G.S., Johnson, S.M., 2003. Neuroplasticity in respiratory motor control. J. Appl. Physiol. 94, 358–374. Muller, D., et al., 2002. LTP, memory and structural plasticity. Curr. Mol. Med. 2, 605–611. Murai, K.K., Pasquale, E.B., 2004. Eph receptors, ephrins, and synaptic function. Neuroscientist 10, 304–314. Nakamura, M., Bregman, B.S., 2001. Differences in neurotrophic factor gene expression profiles between neonate and adult rat spinal cord after injury. Exp. Neurol. 169, 407–415. Neverova, N.V., et al., 2007. Episodic stimulation of alpha1-adrenoreceptors induces protein kinase C-dependent persistent changes in motoneuronal excitability. J. Neurosci. 27, 4435–4442. Oak, J.N., et al., 2001. Dopamine D(4) and D(2L) receptor stimulation of the mitogen-activated protein kinase pathway is dependent on transactivation of the platelet-derived growth factor receptor. Mol. Pharmacol. 60, 92–103. Ogier, M., Wang, H., Hong, E., Wang, Q., Greenberg, M.E., Katz, D.M., 2007. Brainderived neurotrophic factor expression and respiratory function improve after ampakine treatment in a mouse model of Rett syndrome. J Neurosci 27, 10912–10917. Peled, N., et al., 2007. Association of elevated levels of vascular endothelial growth factor in obstructive sleep apnea syndrome with patient age rather than with obstructive sleep apnea syndrome severity. Respiration 74, 50–55. Powell, F.L., et al., 1998. Time domains of the hypoxic ventilatory response. Respir. Physiol. 112, 123–134. Prabhakar, N.R., Kline, D.D., 2002. Ventilatory changes during intermittent hypoxia: importance of pattern and duration. High Alt. Med. Biol. 3, 195–204. Pulford, K., et al., 2004. The emerging normal and disease-related roles of anaplastic lymphoma kinase. Cell Mol. Life Sci. 61, 2939–2953. Rajagopal, R., et al., 2004. Transactivation of Trk neurotrophin receptors by Gprotein-coupled receptor ligands occurs on intracellular membranes. J. Neurosci. 24, 6650–6658. Ramirez, J.J., et al., 1999. Basic fibroblast growth factor enhances axonal sprouting after cortical injury in rats. Neuroreport 10, 1201–1204. Rebola, N., et al., 2005. Different synaptic and subsynaptic localization of adenosine A2A receptors in the hippocampus and striatum of the rat. Neuroscience 132, 893–903. Reichardt, L.F., 2006. Neurotrophin-regulated signalling pathways. Philos. Trans. R. Soc. Lond. B. Biol. Sci. 361, 1545–1564. Robinson, D.R., et al., 2000. The protein tyrosine kinase family of the human genome. Oncogene 19, 5548–5557. Rose, C.R., et al., 2004. From modulator to mediator: rapid effects of BDNF on ion channels. Bioessays 26, 1185–1194. Runeberg-Roos, P., Saarma, M., 2007. Neurotrophic factor receptor RET: structure, cell biology, and inherited diseases. Ann. Med., 1–9. Russell, F.D., et al., 2000. Anterograde axonal transport of glial cell line-derived neurotrophic factor and its receptors in rat hypoglossal nerve. Neuroscience 97, 575–580. Saaresranta, T., et al., 2000. IGF-I and ventilation after short-term progestin in postmenopausal women with chronic respiratory insufficiency. Respir. Med. 94, 909–916. Sebastiao, A.M., Ribeiro, J.A., 2000. Fine-tuning neuromodulation by adenosine. Trends Pharmacol. Sci. 21, 341–346. Shiotani, A., et al., 2007. Gene therapy for laryngeal paralysis. Ann. Otol. Rhinol. Laryngol. 116, 115–122. Skaper, S.D., 2008. The biology of neurotrophins, signalling pathways, and functional peptide mimetics of neurotrophins and their receptors. CNS Neurol. Disord. Drug Targets 7, 46–62. Sorkin, A., 2005. TRKing signals through the Golgi. Sci STKE, pe1. Strochlic, L., et al., 2005. The synaptic muscle-specific kinase (MuSK) complex: new partners, new functions. Bioessays 27, 1129–1135. Sweatt, J.D., 2004. Mitogen-activated protein kinases in synaptic plasticity and memory. Curr. Opin. Neurobiol. 14, 311–317. Takahashi, M., et al., 2004. Role of N-glycans in growth factor signaling. Glycoconj J. 20, 207–212. Teng, Y.D., et al., 1999. Basic fibroblast growth factor increases long-term survival of spinal motor neurons and improves respiratory function after experimental spinal cord injury. J. Neurosci. 19, 7037–7047. Thoby-Brisson, M., et al., 2003. Expression of functional tyrosine kinase B receptors by rhythmically active respiratory neurons in the pre-Botzinger complex of neonatal mice. J. Neurosci. 23, 7685–7689. Vogel, W.F., et al., 2006. Sensing extracellular matrix: an update on discoidin domain receptor function. Cell Signal 18, 1108–1116. Wang, C.Y., et al., 2001. Ca(2+) binding protein frequenin mediates GDNFinduced potentiation of Ca(2+) channels and transmitter release. Neuron 32, 99–112. Wang, H., et al., 2006. Dysregulation of brain-derived neurotrophic factor expression and neurosecretory function in Mecp2 null mice. J. Neurosci. 26, 10911–10915.

F.J. Golder / Respiratory Physiology & Neurobiology 164 (2008) 242–251 Wang, Q., et al., 2003. Control of synaptic strength, a novel function of Akt. Neuron 38, 915–928. Wang, Z.Y., Bisgard, G.E., 2005. Postnatal growth of the carotid body. Respir. Physiol. Neurobiol. 149, 181–190. Warren, C.M., Landgraf, R., 2006. Signaling through ERBB receptors: multiple layers of diversity and control. Cell Signal. 18, 923–933. Watson, F.L., et al., 1999. TrkA glycosylation regulates receptor localization and activity. J. Neurobiol. 39, 323–336. Weese-Mayer, D.E., et al., 2006. Autonomic nervous system dysregulation: breathing and heart rate perturbation during wakefulness in young girls with Rett syndrome. Pediatr. Res. 60, 443–449. Wiese, S., et al., 2007. Adenosine receptor A2A-R contributes to motoneuron survival by transactivating the tyrosine kinase receptor TrkB. Proc. Natl. Acad. Sci. U.S.A. 104, 17210–17215.

251

Yuan, L.L., et al., 2002. Protein kinase modulation of dendritic K+ channels in hippocampus involves a mitogen-activated protein kinase pathway. J. Neurosci. 22, 4860–4868. Zhang, J., Huang, E.J., 2006. Dynamic expression of neurotrophic factor receptors in postnatal spinal motoneurons and in mouse model of ALS. J. Neurobiol. 66, 882–895. Zhao, Z., et al., 2004. Overexpression of glial cell line-derived neurotrophic factor in the CNS rescues motoneurons from programmed cell death and promotes their long-term survival following axotomy. Exp. Neurol. 190, 356–372. Zimmermann, S., Moelling, K., 1999. Phosphorylation and regulation of Raf by Akt (protein kinase B). Science 286, 1741–1744.