Distinctive features of Trk neurotrophin receptor transactivation by G protein-coupled receptors

Cytokine & Growth Factor Reviews 13 (2002) 11–17

Mini-review

Distinctive features of Trk neurotrophin receptor transactivation by G protein-coupled receptors Francis S. Lee a , Rithwick Rajagopal b , Moses V. Chao b,∗ b

a Department of Psychiatry, Weill Medical College of Cornell University, New York, NY 10021, USA Departments of Cell Biology and Physiology and Neuroscience, New York University School of Medicine, Molecular Neurobiology Program, Skirball Institute of Biomolecular Medicine, New York, NY 10016, USA

Abstract Ligands for G protein-coupled receptors (GPCR) are capable of activating mitogenic receptor tyrosine kinases, in addition to the mitogen-activated protein (MAP) kinase signaling pathway and classic G protein-dependent signaling pathways involving adenylyl cyclase and phospholipase. For example, receptors for epidermal growth factor (EGF), insulin-like growth-1 and platelet-derived growth factor and can be transactivated through G protein-coupled receptors. Neurotrophins, such as NGF, BDNF and NT-3 also utilize receptor tyrosine kinases, namely TrkA, TrkB and TrkC. Recently, it has been shown that activation of Trk receptor tyrosine kinases can also occur via a G protein-coupled receptor mechanism, without involvement of neurotrophins. Adenosine and adenosine agonists can activate Trk receptor phosphorylation specifically through the seven transmembrane spanning adenosine 2A (A2A ) receptor. Several features of Trk receptor transactivation are noteworthy and differ significantly from other transactivation events. Trk receptor transactivation is slower and results in a selective increase in activated Akt. Unlike the biological actions of other tyrosine kinase receptors, increased Trk receptor activity by adenosine resulted in increased cell survival. This article will discuss potential mechanisms by which adenosine can activate trophic responses through Trk tyrosine kinase receptors. © 2002 Published by Elsevier Science Ltd. Keywords: GPCR; NGF; BDNF; PLC␥; Akt; Shc

1. Introduction Many examples of transactivation of mitogenic growth factor receptors in response to G protein-coupled receptor (GPCR) signaling have now been reported. In each case, increased dimerization and tyrosine phosphorylation of receptor tyrosine kinases occurs, followed by association of receptors with tyrosine phosphorylated adaptor proteins and Ras-dependent activation of MAP kinases [1,2]. A variety of diverse ligands for GPCRs including isoproterenol, thrombin, lysophosphatidic acid (LPA), endothelin, thyrotropin-releasing hormone, carbachol and angiotensin II rapidly increase epidermal growth factor (EGF) receptor autophosphorylation. The occurrence of these events in Abbreviations: GPCR, G protein-coupled receptors; A2A , adenosine 2A; MAP, mitogen-activated protein; NGF, nerve growth factor; BDNF, brain derived neurotrophic factor; EGF, epidermal growth factor; PLC␥, phospholipase C␥; PI-3 kinase, phosphatidylinositol 3-kinase; LPA, lysophosphatidic acid; CNS, central nervous system ∗ Corresponding author. Tel.: +1-212-263-0761; fax: +1-212-263-0723. E-mail address: [email protected] (M.V. Chao). 1359-6101/02/$ – see front matter © 2002 Published by Elsevier Science Ltd. PII: S 1 3 5 9 - 6 1 0 1 ( 0 1 ) 0 0 0 2 4 - 7

different cell backgrounds such as vascular smooth muscle, fibroblastic, neuronal and non-neuronal cells [3–6] suggests that this is a general mechanism of cross-talk between these two receptor systems. Through GPCR signaling, it is well established that adenylyl cyclase and cAMP levels are regulated, as well as phospholipase C (PLC) and MAP kinase activities. Therefore, GPCR and receptor tyrosine kinases both can activate MAP kinase signaling. Furthermore, these interactions imply that growth factor receptors may be viewed as substrates of GPCRs. Several issues are raised by these observations. What are the cellular mechanisms that account for these events? Second, how is downstream signaling by GPCR and receptor tyrosine kinases differentially regulated. What are the biological functions for cross-talk between GPCR signaling and receptor tyrosine kinases? Finally, growth factor receptors are normally associated with enhanced cell proliferation, but what is the significance of receptor transactivation in postmitotic cells in the nervous system? This article will focus on transactivation events involving neurotrophin receptor tyrosine kinases, which provide a number of insights into these questions.

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2. Neurotrophin receptor transactivation Neurotrophins represent a unique family of proteins required for cell survival, differentiation and plasticity during development of the nervous system. NGF, brain derived neurotrophic factor, neurotrophin-3 (NT-3) and NT-4 are produced as precursor proteins which are cleaved to mature proteins of 118–120 amino acids that associate as non-covalent homodimers [7]. The actions of neurotrophins are dictated by two classes of cell surface receptors, the Trk receptor tyrosine kinase and the p75 neurotrophin receptor, a member of the TNF receptor superfamily [8]. There are three vertebrate trk receptor genes. NGF binds exclusively to the TrkA receptor, whereas BDNF and NT-4 are the ligands for TrkB. NT-3 binds to the TrkC receptor. The principal domains that determine specificity of neurotrophin binding are the immunoglobulin-C2 like sequences found in the extracellular domain of each Trk receptor. Signal transduction through the Trk receptors involves the recruitment of the adaptor proteins Shc and FRS2, and effectors, such as phosphatidylinositol 3-kinase (PI-3 kinase) and PLC␥ [9]. The key docking sites on the Trk receptor are tyrosine residue 490 in the juxtamembrane region and tyrosine residue 790 in the tail of the cytoplasmic domain. PLC␥ binds to Tyr-790 and this interaction has been proposed to facilitate interactions with ion channels, such as the VR1 capsaisin receptor [10]. Through binding of Shc or FRS2 at Tyr-490, these adaptor proteins become tyrosine phosphorylated and provide a scaffold for other signaling proteins that lead to the activation of the Ras-MAP kinase or the PI-3 kinase/Akt pathways (Fig. 1). We have found that Trk tyrosine kinase receptors are activated as a result of adenosine treatment in PC12 cells, as well as primary cultures of hippocampal neurons [11]. This

activation was observed when cells were treated with either adenosine or adenosine agonists such as CGS 21680. Not only does adenosine activate Trk receptors, but effectors of Trk are also phosphorylated. Adenosine treatment of PC12 cells promoted the phosphorylation of the three Shc adaptor proteins, as well as PLC␥, analogous to the induction observed with NGF (Fig. 1). Similar results were seen for PI-3 kinase and Akt. A major difference between adenosine versus NGF activation is the lag in stimulating the phosphorylation of substrates of the Trk receptor, such as the Shc adaptor proteins, PLC␥ and PI-3 kinase. These phosphorylation events required over 1 h of adenosine treatment in PC12 cells. Adenosine is a neuromodulator, whose levels are increased when ATP is converted to adenosine or during injury such as hypoxia or ischemia. Its effects are mediated by specific P1 purinoceptors. Four adenosine receptors have been characterized. A1 and A3 adenosine receptors couple to Gi and Go or Gq, whereas A2A and A2B adenosine receptors couple to Gs [12]. Use of specific adenosine agonists and antagonists established that the A2A receptor functions in PC12 cells to mediate TrkA receptor activation. Transactivation of Trk receptors by adenosine can be specifically blocked by K252a, a well-established inhibitor of Trk tyrosine kinases. K252a inhibits NGF activation of Trk receptors and the subsequent biological effects of neurotrophins, without affecting other receptor tyrosine kinases, such as the EGF and FGF receptors [13]. A number of striking differences exist between transactivation of the Trk receptor tyrosine kinases and the EGF receptor. A notable feature is the relatively slow time course of activation. Phosphorylation of Trk receptors required at least 60–90 min, compared to the activation of EGF receptors by GPCR ligands angiotensin II, LPA, bradykinin or

Fig. 1. Signal transduction of the Trk NGF receptor. Left: model of the major Trk receptor signaling pathways [9]. Right: PC12 cells were treated with adenosine for various time periods and the phosphorylation of immunoprecipitated Shc proteins and PLC␥ were assessed with anti-phosphotyrosine antibodies.

F.S. Lee et al. / Cytokine & Growth Factor Reviews 13 (2002) 11–17

isoproterenol which occurs rapidly, within minutes of treatment [1,4,5]. The effects of adenosine were considerably delayed compared to treatment with NGF, which activates Trk within a minute. The phosphorylation of Trk substrates, Shc and PLC␥, induced by adenosine also followed this slower time course (Fig. 1). In contrast, EGF receptor transactivation and phosphorylation of downstream substrates followed a fast and similar time course with either EGF or LPA [4]. Another major difference is that many GPCR ligands that transactivate EGF receptor do not affect the activity of Trks. Administration of bradykinin, carbachol, ATP, apomorphine, quinpirole or angiotensin II to PC12 cells did not result in TrkA activation [11], even though receptors for these ligands are expressed on these cells. By comparison, many of these ligands are capable of stimulating EGF receptors and other mitogenic growth factor receptors [14]. Conversely, adenosine and adenosine agonists do not activate EGF receptors at either short or longer time points. As GPCRs differ in function and in their associated G protein subunits, transactivation events directed for each receptor tyrosine kinase must reflect specific signaling pathways. Thus far, an intracellular pathway that directly explains cross-talk between GPCR and receptor tyrosine kinases has not been elucidated. One of the most notable features is in downstream signaling events. Activation of MAP kinases is considered a critical event during GPCR-mediated transactivation of mitogenic growth factor receptors. An unexpected finding is a selective activation of Akt activity by adenosine-induced Trk signaling. This response had not been associated with adenosine, because many of its actions were previously evaluated at much earlier time points. One of adenosine’s early actions is the induction of MAP kinase activity, which occurs within 5 min of treatment [15–17]. However, MAP kinase activity is not sustained at later times. After 2 h, adenosine did not elevate MAP kinase activity in PC12 cells, even though Trk activity was increased during this period. Therefore, MAP kinase induction by adenosine G protein signaling is early (minutes) Trk-independent, whereas activity is delayed

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and Trk-dependent later. These measurements indicate that adenosine leads to a long lasting Akt kinase, and not MAP kinase, activity. Since both Akt and MAP kinase pathways are mediated by Shc binding to the Trk receptor at tyrosine 490, this suggests there is a preferential induction of Akt signaling over the MAP kinase pathway. How this occurs is not clear, but competition between adaptor proteins for the Trk receptor may account for this unique outcome.

3. Mechanism of transactivation A simple explanation for the effects of adenosine is that adenosine interacts directly with Trk receptors to promote dimerization and autophosphorylation of Trk receptors. However, binding experiments indicate that adenosine does not compete with NGF for binding to the TrkA receptor. This is relevant because the Trk binding site for NGF are represented by the IgG domains, which are also responsible for regulating dimerization [18]. Furthermore, adenosine displays a very short half life, which would be inconsistent with the long lag in activating Trk receptors. The long time course of transactivation suggests that adenosine treatment might lead to the production of NGF in PC12 cells that could act in an autocrine fashion to stimulate TrkA receptors. In EGF receptor transactivation by endothelin or LPA or carbachol, a metalloprotease cleaves pro-heparin-binding-EGF at the plasma membrane of COS or 293 cells [19]. The extracellularly released EGF then binds to EGF receptors on these cells, resulting in autophosphorylation of the receptors (Fig. 2). Hence, activation of EGF receptors by GPCRs occurs directly by EGF produced and released at the cell surface. Several observations argue that transactivation of Trk receptors does not involve release of neurotrophins. First, NGF blocking antibodies do not affect adenosine’s ability to activate TrkA receptors in PC12 cells at concentrations that block the effects of NGF added exogeneously to PC12 cells

Fig. 2. Transactivation of EGF receptors by GPCR ligands. Transactivation of the EGF receptor by ligands of GPCR, such as isoproterenol, thrombin, endothelin, carbachol, angiotensin II or bradykinin can occur by several different mechanisms. Several ligands, such as angiotensin and bradykinin, require Ca2+ or Src activity [2]. Alternatively, activation of GPCRs can lead to cleavage of heparin-binding EGF at the cell surface by specific matrix metalloproteases [19,29]. The release of heparin-binding EGF leads to a soluble ligand that activates EGF receptor in an autocrine or paracrine manner.

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Table 1 Dissection of TrkA receptor transactivation mechanisma Treatment Anti-NGF K-252a PD98059 LY294002 PP1 BAPTA/AM EGTA EDTA 8-bromo-cAMP H-89 GF10923X Calphostin C Cycloheximide Actinomycin D

Block Trk transactivation? NGF inhibitor Trk inhibitor MEK1 activation inhibitor PI-3 Kinase inhibitor Src family inhibitor Intracellular Ca2+ chelator Extracellular Ca2+ chelator Extracellular Ca2+ chelator PKA activator PKA inhibitor PKC inhibitor PKC inhibitor Translational inhibitor Transcriptional inhibitor

No Yes No No Yes Yes No No No No No No Yes Yes

a Activation of TrkA receptors in PC12 cells was assessed by western blot analysis after a 2 h treatment with adenosine and each of the following reagents.

(Table 1). Second, treatment of PC12 cells with adenosine or adenosine agonists does not result in the production of neurotrophic activities. Supernatants of PC12 cells subjected to adenosine treatment did not give any neurite outgrowth activity typical of NGF treatment (Fig. 3). Therefore, in this cell system, adenosine activation of Trk receptors does not result via an autocrine/paracine mechanism involving the release of neurotrophin. The mechanisms by which a GPCR activates receptor tyrosine kinases are not completely understood. Various candidate signaling intermediates were examined in the case of adenosine. Interestingly, activation of Trk receptor activity by adenosine was not affected by protein kinase C or protein kinase A inhibitors (Table 1). Another activity that precedes that transactivation of Trk receptors is increased MAP kinase induction. However, treatment of PC12 cells with PD98059, an established MEK inhibitor, did not affect Trk receptor activation (Table 1). In addition, an intermediate

that appears to be involved in many GPCR activation events of mitogenic receptor tyrosine kinases are members of the Src family kinases. Indeed, treatment of PC12 cells with the Src kinase inhibitor PP1 resulted in a marked decrease in the level of tyrosine phosphorylated TrkA receptors elicited by adenosine (Table 1). These results suggest that the regulation of TrkA activity by adenosine may be mediated by a Src tyrosine kinase activity downstream of GPCR. Another proposed intermediate for transactivation of Trk receptors by GPCR signaling is a calcium-dependent step. Treatment of PC12 cells with BAPTA/AM, an intracellular calcium chelator, led to a complete inhibition of transactivation of Trk receptors by adenosine (Table 1). In contrast, an extracellular calcium chelator, EGTA, as well as a general chelator, EDTA, had no effect upon TrkA tyrosine kinase activity. These results suggest that transactivation of Trk receptors requires mobilization of intracellular calcium, together with intracellular tyrosine phosphorylation events (Fig. 4).

4. Functional consequences of transactivated Trk receptors Phosphatidylinositol 3-kinase (PI3-kinase)/Akt is an important pathway that is directly influenced by many receptor tyrosine kinases. That GPCR ligands such as adenosine use this pathway through a receptor tyrosine kinase represents a new cross-talk mechanism. The activity of Akt is widely accepted as a signaling module for neuronal cell survival [20]. The time course of Akt activation was very similar to TrkA autophosphorylation induced by adenosine. Activated Akt was observed well after an hour of treatment, whereas NGF causes a much quicker induction of Akt activity. The increase in Akt activity was eliminated by pretreatment with 100 nM K-252a, demonstrating the reliance upon Trk receptor activity. Conversely, pretreatment of PC12 cells with LY294002, a PI-3 kinase inhibitor, eliminated Akt

Fig. 3. Adenosine treatment does not lead to NGF release. Supernatants from PC12 cells treated with 10 ␮M adenosine for 1–2 days were tested for their ability to induce neurite outgrowth. Adenosine does not lead to the production of NGF, or to an activity, that stimulates neurite outgrowth in PC12 cells.

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Fig. 4. Proposed model of transactivation of Trk receptors by adenosine.

activity, but did not affect the transactivation of Trk receptor by adenosine (Table 1). Previous studies indicated that adenosine could exert neuroprotective effects on a variety of neurons from the central nervous system [21–23], in a manner similar to the effects of neurotrophins. For example, hippocampal neuron survival is promoted by BDNF and its withdrawal from hippocampal neurons leads to rapid cell death. We found that adenosine maintains the survival of hippocampal neurons grown in the absence of BDNF [11]. The action of adenosine required A2A receptor and TrkB receptor activity, as well as Akt activity. Adenosine was able to reverse cell death in hippocampal neurons initiated by withdrawal of trophic support by BDNF. Therefore, a likely mechanism to account for adenosine’s neurotrophic effects is through the action of Trk receptors.

5. Discussion These findings indicate that small molecules acting through GPCRs may be used to promote trophic activities mediated by receptor tyrosine kinases. Adenosine can activate the neurotrophin signaling system in the absence of neurotrophins. This is significant since neurotrophins provide signals to promote neuronal survival, synaptic efficacy and plasticity in the nervous system. Depending upon the circumstances, adenosine may be neuroprotective against injury initiated by ischemia, hypoxia or vascular damage [22,24]. In contrast to other transactivation events involving receptor tyrosine kinases that lead to transient increases in MAP kinase activity, adenosine signaling through Trk neurotrophin receptors leads to selective activation of the PI-3 kinase/Akt pathway over a prolonged time course. Intracellular signaling interactions between adenosine and Trk

receptors provide a new avenue for developing approaches to address neurological and psychiatric disorders. Small molecules like adenosine may be used to target populations of neurons that express both adenosine and Trk receptors and therefore be considered as potential treatments for a wide number of nervous system disorders, including cerebral ischemia, amyotrophic lateral sclerosis and Parkinson’s diseases and other neurodegenerative conditions. Adenosine and neurotrophin signaling not only share similar signaling pathways and consequences, but their receptors have considerable overlap in their CNS and PNS distribution. Analysis of targeted mutations of neurotrophin factors in mice indicate there are substantial losses of peripheral neurons, such as sensory and sympathetic neurons for NGF. However, very few losses in neuronal populations were observed in the CNS of mice deficient in either NGF, BDNF or NT-3 [25]. Taken together, the lack of a survival phenotype in the CNS of mice lacking neurotrophins suggest that other factors may substitute for neurotrophins during brain and spinal cord development. Survival of neurons in these populations may rely upon other factors, such as adenosine with the capability of tranducing signals through receptor tyrosine kinases. The intracellular events that promote signaling between G protein-coupled receptors and receptor tyrosine kinases remain elusive. The delayed nature of Trk receptor activation may be due to several cellular requirements. First, transactivation may require new protein synthesis or gene activation events. This is consistent with the inhibition of Trk activation by adenosine after cycloheximide or actinomycin D treatment (Table 1). Elevation of intracellular calcium levels also may play a critical role in the transactivation of Trk receptors, as has been shown for the EGF receptor [6]. In addition, the transactivation of Trk receptors appears to involve Src tyrosine kinases. Src family nonreceptor tyrosine

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kinases are involved in several MAP kinase activation and transactivation events mediated by G protein-coupled receptors [26,27]. Src family members may participate in regulating the catalytic activity of Trk receptors, or internalization and trafficking of Trk receptors. Another important aspect is that post-translational processing of Trk receptors may be responsible for receptor autophosphorylation. The Trk receptor undergoes glycosylation during the time period of transactivation and the longer time course may reflect the autophosphorylation of newly processed receptors (Fig. 4). Indeed, the endocytosis and intracellular targeting of receptors have been key steps proposed for ␣2A and ␤2 adrenergic receptor signaling and transactivation of receptor tyrosine kinases [28,29]. It is likely that multi-subunit complexes exist between G protein-coupled receptors and receptor tyrosine kinases and their associated proteins and substrates [30]. Such complexes may be extremely relevant in specifying local and long range signaling in axons and cell bodies of neurons during retrograde transport of Trk receptors. It is surprising how prevalent receptor tyrosine kinases are used downstream of GPCRs. However, the mechanisms that account for these events are likely to be conserved and relevant for many physiological processes. We have shown that in addition to their established roles in cell proliferation, receptor tyrosine kinases represent downstream mediators in G protein-coupled receptor signaling for cell survival in neuronal populations. Given that there are hundreds of G protein-coupled receptors and a more limited repertoire of receptor tyrosine kinases, there must be a great deal of selectivity in coupling these systems. The link of adenosine with neurotrophin signaling raises a number of interesting issues. Mice deficient in A2A receptors display a number of behavioral effects also seen in mice lacking NGF or BDNF [31,32]. This includes decreased sensitivity to heat and aggressive behavior. Additionally, neurotrophins exert noticeable effects upon synaptic plasticity, learning and memory [33–36]. It is likely that modulation of these activities may be influenced by cross-talk between neurotrophin receptor tyrosine kinases with a variety of G protein-coupled receptors.

Acknowledgements FSL was supported by the DeWitt-Wallace Fund in the New York Community Trust and an American Psychiatric Association PMRTP Fellowship and MVC was supported by grants from the NIH.

References [1] Luttrell L, Della Rocca G, van Biesen T, Luttrell D, Lefkowitz R. G␤␥ subunits mediate Src-dependent phosphorylation of the epidermal growth factor receptor. J Biol Chem 1997;272:4637–44.

[2] Hackel P, Zwick E, Prenzel N, Ullrich A. Epidermal growth factor receptors: critical mediators of multiple receptor pathways. Curr Opin Cell Biol 1999;11:184–9. [3] Daub H, Weiss FU, Wallasch CS Ullrich F A. Role of transactivation of the EGF receptor in signalling by G-protein coupled receptors. Nature 1996;379:557–60. [4] Daub H, Wallash C, Lankenau A, Herrlich A, Ullrich A. Signal characteristics of G protein-transactivated EGF receptor. EMBO J 1997;16:7032–44. [5] Eguchi S, Numaguchi K, Iwasaki H, et al. Calcium-dependent epidermal growth factor receptor transactivation mediates the angiotensin II-induced mitogen-activated protein kinase activation in vascular smooth muscle cells. J Biol Chem 1998;273:8890–6. [6] Zwick E, Wallasch C, Daub H, Ullrich A. Distinct calcium-dependent pathways of epidermal growth factor receptor transactivation and PYK2 tyrosine phosphorylation in PC12 cells. J Biol Chem 1999;274:20989–96. [7] McDonald N, Chao MV. Structural determinants of neurotrophin action. J Biol Chem 1995;270:19669–72. [8] Chao MV, Hempstead BL. p75 and trk: a two receptor system. Trends Neurosci 1995;19:321–6. [9] Kaplan DR, Miller FD. Neurotrophin signal transduction in the nervous system. Curr Opin Neurobiol 2000;10(3):381–91. [10] Chuang H-h, Prescott E, Kong H. Bradykinin and nerve growth factor release the capsaicin receptor from PtdIns(4,5)P2-mediated inhibition. Nature 2001;411:957–62. [11] Lee F, Chao M. Activation of Trk neurotrophin receptors in the absence of neurotrophins. Proc Natl Acad Sci USA 2001;92:3555– 60. [12] Ralevic V, Burnstock G. Receptors for purines and pyrimidines. Pharmacol Rev 1998;50:413–92. [13] Berg MM, Sternberg D, Parada LF, Chao MV. K-252a inhibits NGF-induced trk proto-oncogene tyrosine phosphorylation and kinase activity. J Biol Chem 1992;267:13–6. [14] Zwick E, Hackel P, Prenzel N, Ullrich A. The EGF receptor as central transducer of heterologous signaling systems. TIPS 1999;20:408–12. [15] Gao Z, Chen T, Weber M, Linden J. A2B adenosine and P2Y2 receptors stimulate mitogen-activated protein kinase in human embryonic kidney 293 cells. J Biol Chem 1999;274:5972–80. [16] Seidel M, Klinger M, Freissmuth M, Holler C. Activation of mitogen-activated protein kinase by the A2A-adenosine receptor via a rap1-dependent and via a p21ras-dependent pathway. J Biol Chem 1999;274:25833–41. [17] Sexl V, Mancusi G, Holler C, Gloria-Maercker E, Schutz W, Freissmuth M. Stimulation of the mitogen-activated protein kinase via the A2A-adenosine receptor in primary endothelial cells. J Biol Chem 1997;272:5792–9. [18] Arevalo J, Conde B, Hempstead B, Chao M, Martin-Zanca D, Perez P. TrkA immunoglobulin-like ligand binding domains inhibit spontaneous activation of the receptor. Mol Cell Biol 2000;20:5908– 16. [19] Prenzel N, Zwick E, Daub H, et al. EGF receptor transactivation by G-protein-coupled receptors requires metalloproteinase cleavage of proHB-EGF. Nature 1999;402:884–8. [20] Brunet A, Datta S, Greenberg M. Transcription-dependent and -independent control of neuronal survival by the PI3K-Akt signaling pathway. Curr Opin Neurobiol 2001;11:297–305. [21] Rudolphi K, Schubert P, Parkinson F, Fredholm B. Neuroprotective role of adenosine in cerebral ischaemia. Trends Pharm Sci 1992;13:439–45. [22] Jacobson K, von Lubitz D, Daly J, Fredholm B. Adenosine receptor ligands: differences with acute versus chronic treatment. Trends Pharm Sci 1996;17:108–13. [23] Kobayashi S, Millhorn D. Stimulation of expression for the adenosine A2A receptor gene by hypoxia in PC12 cells. J Biol Chem 1999;274:20358–65.

F.S. Lee et al. / Cytokine & Growth Factor Reviews 13 (2002) 11–17 [24] Neary J, Rathbone M, Cattabeni F, Abbracchio M, Burnstock G. Trophic actions of extracellular nucleotides and nucleosides on glial and neuronal cells. Trends Neurosci 1996;19:1996. [25] Snider WD. Functions of the neurotrophins during nervous-system development — what the knockouts are teaching us. Cell 1994;77(5):627–38. [26] Luttrell L, Daaka Y, Lefkowitz R. Regulation of tyrosine kinase cascades by G-protein-coupled receptors. Curr Opin Cell Biol 1999;11:177–83. [27] Andreev J, Galisteo M, Kranenburg O, et al. Src and Pyk2 mediate G-protein-coupled receptor activation of epidermal growth factor receptor (EGFR) but are not required for coupling to the mitogen-activated protein (MAP) kinase signaling cascade. J Biol Chem 2001;276:20130–5. [28] Luttrell L, Roudabush F, Choy E, et al. Activation and targeting of extracellular signal-regulated kinases by b-arrestin scaffolds. Proc Natl Acad Sci USA 2001;98:2449–54. [29] Pierce K, Tohgo A, Ahn S, Field M, Luttrell L, Lefkowitz R. Epidermal growth factor (EGF) receptor-dependent ERK activation by G protein-coupled receptors. J Biol Chem 2001;276: 23155–60.

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[30] Maudsley S, Pierce K, Musa Zamah A. The ␤2-adrenergic receptor mediates extracellular signal-regulated kinase activation via assembly of a multi-receptor complex with the epidermal growth factor receptor. J Biol Chem 2000;275:9572–80. [31] Ledent C, Vaugeois J-M, Schiffmann SN, et al. Aggressiveness, hypoalgesia and high blood pressure in mice lacking the adenosine A2a receptor. Nature 1997;388:674–8. [32] Lyons WE, Mamounas LA, Ricaurte GA, et al. Brain-derived neurotrophic factor-deficient mice develop aggressiveness and hyperphagia in conjunction with brain serotonergic abnormalities. Proc Natl Acad Sci USA 1999;96(26):15239–44. [33] Cabelli RJ, Hohn A, Shatz CJ. Inhibition of ocular dominance column formation by infusion of NT-4/5 or BDNF. Science 1995;267:1662– 6. [34] Thoenen H. Neurotrophins and neuronal plasticity. Science 1995;270:593–8. [35] Minichiello L, Korte M, Wolfer D, et al. Essential role for TrkB receptors in hippocampus-mediated learning. Neuron 1999;24(2):401–14. [36] Schinder A, Poo M-M. The neurotrophin hypothesis for synaptic plasticity. Trends Neurosci 2000;23:639–45.