Eph-Ephrin Bidirectional Signaling in Physiology and Disease

Receptor tyrosine kinases of the Eph family bind to cell surface-associated ephrin ligands on neighboring cells. The ensuing bidirectional signals have emerged as a major form of contact-dependent communication between cells. New findings reveal that Eph receptors and ephrins coordinate not only developmental processes but also the normal physiology and homeostasis of many adult organs. Imbalance of Eph/ephrin function may therefore contribute to a variety of diseases. The challenge now is to better understand the complex and seemingly paradoxical signaling mechanisms of Eph receptors and ephrins, which will enable effective strategies to target these proteins in the treatment of diseases such as diabetes and cancer. Receptor tyrosine kinases of the Eph family bind to cell surface-associated ephrin ligands on neighboring cells. The ensuing bidirectional signals have emerged as a major form of contact-dependent communication between cells. New findings reveal that Eph receptors and ephrins coordinate not only developmental processes but also the normal physiology and homeostasis of many adult organs. Imbalance of Eph/ephrin function may therefore contribute to a variety of diseases. The challenge now is to better understand the complex and seemingly paradoxical signaling mechanisms of Eph receptors and ephrins, which will enable effective strategies to target these proteins in the treatment of diseases such as diabetes and cancer. Since its discovery two decades ago, the Eph family of receptor tyrosine kinases has been implicated in an increasing number of physiological and pathological processes in many cell types and different organs. Therefore, elucidating the mechanism of action of the Eph receptors and their signaling networks is important for understanding developmental processes, the physiology of adult organs and, as is becoming increasingly evident, the pathogenesis of many diseases. Eph receptors have diverse activities, including widespread effects on the actin cytoskeleton, cell-substrate adhesion, intercellular junctions, cell shape, and cell movement (Egea and Klein, 2007Egea J. Klein R. Bidirectional Eph-ephrin signaling during axon guidance.Trends Cell Biol. 2007; 17: 230-238Abstract Full Text Full Text PDF PubMed Scopus (294) Google Scholar, Himanen et al., 2007Himanen J.P. Saha N. Nikolov D.B. Cell-cell signaling via Eph receptors and ephrins.Curr. Opin. Cell Biol. 2007; 19: 534-542Crossref PubMed Scopus (191) Google Scholar, Pasquale, 2005Pasquale E.B. Eph receptor signalling casts a wide net on cell behaviour.Nat. Rev. Mol. Cell Biol. 2005; 6: 462-475Crossref PubMed Scopus (817) Google Scholar). In addition, effects on cell proliferation, survival, differentiation, and secretion have also been described. These activities depend on the interaction of the Eph receptors with the ephrins (Eph receptor interacting proteins). In the human genome, there are nine EphA receptors that bind to five GPI-linked ephrin-A ligands and five EphB receptors that bind to three transmembrane ephrin-B ligands. Interactions are promiscuous within each class, and some Eph receptors can also bind to ephrins of the other class. Several of the domains in the Eph receptor extracellular region can bind to the ephrins. The amino-terminal “ephrin-binding” domain contains a high-affinity binding site that mediates receptor-ephrin interaction between cells (Figure 1) (Himanen et al., 2007Himanen J.P. Saha N. Nikolov D.B. Cell-cell signaling via Eph receptors and ephrins.Curr. Opin. Cell Biol. 2007; 19: 534-542Crossref PubMed Scopus (191) Google Scholar, Wimmer-Kleikamp and Lackmann, 2005Wimmer-Kleikamp S.H. Lackmann M. Eph-modulated cell morphology, adhesion and motility in carcinogenesis.IUBMB Life. 2005; 57: 421-431Crossref PubMed Scopus (59) Google Scholar). Two additional lower-affinity ephrin-binding sites have also been identified in the ephrin-binding domain and the cysteine-rich region, which are thought to facilitate clustering of multiple Eph-ephrin complexes. The Eph fibronectin type III domain closer to the membrane can also bind to ephrins, if they are located on the same cell surface. A distinctive feature of Eph-ephrin complexes is their ability to generate bidirectional signals that affect both the receptor-expressing and ephrin-expressing cells (Pasquale, 2005Pasquale E.B. Eph receptor signalling casts a wide net on cell behaviour.Nat. Rev. Mol. Cell Biol. 2005; 6: 462-475Crossref PubMed Scopus (817) Google Scholar). Eph receptor “forward” signaling depends on the tyrosine kinase domain, which mediates autophosphorylation as well as phosphorylation of other proteins, and on the associations of the receptor with various effector proteins. Ephrin-B “reverse” signaling also depends in part on tyrosine phosphorylation of the ephrin cytoplasmic region (mediated by Src family kinases and some receptor tyrosine kinases) and on associated proteins. Most Eph receptors and the B-type ephrins also have a carboxy-terminal PDZ domain-binding site, which is particularly important for the physiological functions of ephrin-B (Egea and Klein, 2007Egea J. Klein R. Bidirectional Eph-ephrin signaling during axon guidance.Trends Cell Biol. 2007; 17: 230-238Abstract Full Text Full Text PDF PubMed Scopus (294) Google Scholar). The mechanisms of reverse signaling for ephrin-A are less understood, but these GPI-linked ephrins probably use associated transmembrane proteins to fulfill their signaling function. Several candidates have been reported at meetings, including the p75 low-affinity nerve growth factor receptor (T.R. McLaughlin et al., 2007, Soc. Neurosci., abstract). Eph receptors and ephrins use some common signaling effectors, such as Src family kinases and Ras/Rho family GTPases, which are particularly important for the organization of the actin cytoskeleton and cell adhesion (Figure 1). Some signaling connections may apply only to a particular Eph class, including those between EphA receptors and the Rho exchange factor Ephexin or between EphB receptors and the exchange factors Intersectin and Kalirin. Others are more selective. For example, the lipid phosphatase Ship2 was found to interact only with EphA2, and the GTPase-activating proteins SPAR/E6TP1 interacted only with EphA4 and EphA6 among several EphA and EphB receptors examined (Richter et al., 2007Richter M. Murai K.K. Bourgin C. Pak D. Pasquale E.B. The EphA4 receptor regulates neuronal morphology through SPAR-mediated inactivation of Rap GTPases.J. Neurosci. 2007; 27: 14205-14215Crossref PubMed Scopus (68) Google Scholar, Zhuang et al., 2007Zhuang G. Hunter S. Hwang Y. Chen J. Regulation of EphA2 receptor endocytosis by SHIP2 lipid phosphatase via phosphatidylinositol 3-Kinase-dependent Rac1 activation.J. Biol. Chem. 2007; 282: 2683-2694Crossref PubMed Scopus (106) Google Scholar). An emerging theme is that Eph receptors and ephrins activate complex bidirectional signaling networks that often include signaling pathways with opposite effects (Figure 1). This may explain why differences in cellular context can dramatically alter the outcome of Eph/ephrin stimulation. Furthermore, the degree of Eph/ephrin clustering may not only affect signal strength but may also differentially regulate downstream pathways thus leading to variable outcomes (Pasquale, 2005Pasquale E.B. Eph receptor signalling casts a wide net on cell behaviour.Nat. Rev. Mol. Cell Biol. 2005; 6: 462-475Crossref PubMed Scopus (817) Google Scholar, Poliakov et al., 2004Poliakov A. Cotrina M. Wilkinson D.G. Diverse roles of eph receptors and ephrins in the regulation of cell migration and tissue assembly.Dev. Cell. 2004; 7: 465-480Abstract Full Text Full Text PDF PubMed Scopus (347) Google Scholar). Further increasing versatility, forward and reverse signaling can also be independently regulated, for example through Eph receptor dephosphorylation (Konstantinova et al., 2007Konstantinova I. Nikolova G. Ohara-Imaizumi M. Meda P. Kucera T. Zarbalis K. Wurst W. Nagamatsu S. Lammert E. EphA-Ephrin-A-mediated beta cell communication regulates insulin secretion from pancreatic islets.Cell. 2007; 129: 359-370Abstract Full Text Full Text PDF PubMed Scopus (231) Google Scholar). In addition, interactions between Eph receptors and ephrins located on the same cell surface appear to represent a mechanism for silencing bidirectional signaling, although it is unclear under what circumstances Eph receptors and ephrins intermingle rather than segregate in different microdomains of the plasma membrane (Egea and Klein, 2007Egea J. Klein R. Bidirectional Eph-ephrin signaling during axon guidance.Trends Cell Biol. 2007; 17: 230-238Abstract Full Text Full Text PDF PubMed Scopus (294) Google Scholar). A well-characterized effect of Eph forward signaling is retraction of the cell periphery following contact with ephrin-expressing cells (Pasquale, 2005Pasquale E.B. Eph receptor signalling casts a wide net on cell behaviour.Nat. Rev. Mol. Cell Biol. 2005; 6: 462-475Crossref PubMed Scopus (817) Google Scholar). This repulsive response is particularly important for axon guidance and sorting of Eph-expressing cells from ephrin-expressing cells during development. Several mechanisms can explain how the initial adhesive contact evolves into cell separation. One is removal of the adhesive Eph-ephrin complexes from the cell surface by endocytosis of vesicles containing plasma membrane fragments derived from both cells (Egea and Klein, 2007Egea J. Klein R. Bidirectional Eph-ephrin signaling during axon guidance.Trends Cell Biol. 2007; 17: 230-238Abstract Full Text Full Text PDF PubMed Scopus (294) Google Scholar). An implication of this unusual mechanism is that the two cells exchange Eph receptors or ephrins and possibly their associated proteins, which may continue to signal from intracellular compartments. Another way to convert cell adhesion into repulsion is proteolytic cleavage (Egea and Klein, 2007Egea J. Klein R. Bidirectional Eph-ephrin signaling during axon guidance.Trends Cell Biol. 2007; 17: 230-238Abstract Full Text Full Text PDF PubMed Scopus (294) Google Scholar, Himanen et al., 2007Himanen J.P. Saha N. Nikolov D.B. Cell-cell signaling via Eph receptors and ephrins.Curr. Opin. Cell Biol. 2007; 19: 534-542Crossref PubMed Scopus (191) Google Scholar). Studies have shown that metalloproteases and other proteases can cleave the extracellular portions of EphB receptors and ephrins. The remaining membrane-anchored fragments are further cleaved by γ-secretase, followed by proteasomal degradation. Proteolytic cleavage not only terminates the adhesive Eph-ephrin interaction and causes downregulation of the proteins, but it can also generate Eph/ephrin fragments with new activities. For example, the ephrin-B cytoplasmic peptide released by γ-secretase activates the tyrosine kinase Src, which in turn phosphorylates the cytoplasmic domain of intact B-type ephrins and perhaps other substrates (Egea and Klein, 2007Egea J. Klein R. Bidirectional Eph-ephrin signaling during axon guidance.Trends Cell Biol. 2007; 17: 230-238Abstract Full Text Full Text PDF PubMed Scopus (294) Google Scholar). Furthermore, the soluble Eph and ephrin extracellular portions released by metalloproteases might reach distant cells and trigger effects that are independent of cell-cell contact. They could, for example, function as monomeric inhibitors of bidirectional signaling. Alternatively, soluble A-type ephrins oligomerized by transglutamination may serve to activate EphA receptors at a distance (Alford et al., 2007Alford S.C. Bazowski J. Lorimer H. Elowe S. Howard P.L. Tissue transglutaminase clusters soluble A-type ephrins into functionally active high molecular weight oligomers.Exp. Cell Res. 2007; 313: 4170-4179Crossref PubMed Scopus (23) Google Scholar). Although bidirectional signaling is their best characterized modus operandi, Eph receptor and ephrins may also function independently of each other and/or in concert with other cell-surface communication systems (Figure 2). For example, recent studies have proposed that members of the epidermal growth factor (EGF) receptor family can coopt EphA2 as an effector to promote cell motility and proliferation, perhaps independently of ephrin stimulation (Brantley-Sieders et al., 2008Brantley-Sieders D.M. Zhuang G. Hicks D. Fang W.B. Hwang Y. Cates J.M. Coffman K. Jackson D. Bruckheimer E. Muraoka-Cook R.S. et al.The receptor tyrosine kinase EphA2 promotes mammary adenocarcinoma tumorigenesis and metastatic progression in mice by amplifying ErbB2 signaling.J. Clin. Invest. 2008; 118: 64-78Crossref PubMed Scopus (198) Google Scholar, Larsen et al., 2007Larsen A.B. Pedersen M.W. Stockhausen M.T. Grandal M.V. van Deurs B. Poulsen H.S. Activation of the EGFR gene target EphA2 inhibits epidermal growth factor-induced cancer cell motility.Mol. Cancer Res. 2007; 5: 283-293Crossref PubMed Scopus (98) Google Scholar). Other studies have shown association and synergistic responses of fibroblast growth factor (FGF) receptors and EphA4, and that phosphorylation by FGF receptors inhibits ephrin-B1 activities (Arvanitis and Davy, 2008Arvanitis D. Davy A. Eph/ephrin signaling: networks.Genes Dev. 2008; 22: 416-429Crossref PubMed Scopus (209) Google Scholar). Intricate links between EphB/ephrin-B and Wnt signaling have also been revealed in different model systems. EphB receptors and Ryk, a Wnt receptor containing an inactive tyrosine kinase domain, can physically associate and likely function together in craniofacial development and axon guidance (Arvanitis and Davy, 2008Arvanitis D. Davy A. Eph/ephrin signaling: networks.Genes Dev. 2008; 22: 416-429Crossref PubMed Scopus (209) Google Scholar, Schmitt et al., 2006Schmitt A.M. Shi J. Wolf A.M. Lu C.C. King L.A. Zou Y. Wnt-Ryk signalling mediates medial-lateral retinotectal topographic mapping.Nature. 2006; 439: 31-37Crossref PubMed Scopus (226) Google Scholar). Furthermore, both EphB receptors and B-type ephrins can signal through components of the noncanonical Wnt pathway (Figure 1B) (Kida et al., 2007Kida Y.S. Sato T. Miyasaka K.Y. Suto A. Ogura T. Daam1 regulates the endocytosis of EphB during the convergent extension of the zebrafish notochord.Proc. Natl. Acad. Sci. USA. 2007; 104: 6708-6713Crossref PubMed Scopus (49) Google Scholar, Lee et al., 2006Lee H.S. Bong Y.S. Moore K.B. Soria K. Moody S.A. Daar I.O. Dishevelled mediates ephrinB1 signalling in the eye field through the planar cell polarity pathway.Nat. Cell Biol. 2006; 8: 55-63Crossref PubMed Scopus (88) Google Scholar). This pathway in turn causes endocytic removal of EphB receptors from the cell surface, whereas canonical Wnt signaling upregulates EphB transcripts and downregulates ephrin-B transcripts (Clevers and Batlle, 2006Clevers H. Batlle E. EphB/EphrinB receptors and Wnt signaling in colorectal cancer.Cancer Res. 2006; 66: 2-5Crossref PubMed Scopus (122) Google Scholar, Kida et al., 2007Kida Y.S. Sato T. Miyasaka K.Y. Suto A. Ogura T. Daam1 regulates the endocytosis of EphB during the convergent extension of the zebrafish notochord.Proc. Natl. Acad. Sci. USA. 2007; 104: 6708-6713Crossref PubMed Scopus (49) Google Scholar). Some forms of crosstalk occur at epithelial cell junctions, others have been reported in neurons and other cell types. RTK, receptor tyrosine kinase; yellow circles, tyrosine phosphorylation; the scissors indicate proteolytic cleavage. E-cadherin-dependent intercellular adhesion can also regulate Eph receptor expression, cell-surface localization, and ephrin-dependent activation (Arvanitis and Davy, 2008Arvanitis D. Davy A. Eph/ephrin signaling: networks.Genes Dev. 2008; 22: 416-429Crossref PubMed Scopus (209) Google Scholar, Ireton and Chen, 2005Ireton R.C. Chen J. EphA2 receptor tyrosine kinase as a promising target for cancer therapeutics.Curr. Cancer Drug Targets. 2005; 5: 149-157Crossref PubMed Scopus (140) Google Scholar). The regulation is reciprocal, and EphB signaling drives E-cadherin to the cell surface thus promoting the formation of epithelial adherens junctions and enabling EphB/ephrin-B-dependent cell sorting. Conversely, inhibiting EphB-ephrin-B binding was found to disturb adherens junctions (Cortina et al., 2007Cortina C. Palomo-Ponce S. Iglesias M. Fernandez-Masip J.L. Vivancos A. Whissell G. Huma M. Peiro N. Gallego L. Jonkheer S. et al.EphB-ephrin-B interactions suppress colorectal cancer progression by compartmentalizing tumor cells.Nat. Genet. 2007; 39: 1376-1383Crossref PubMed Scopus (205) Google Scholar, Noren and Pasquale, 2007Noren N.K. Pasquale E.B. Paradoxes of the EphB4 receptor in cancer.Cancer Res. 2007; 67: 3994-3997Crossref PubMed Scopus (116) Google Scholar). EphA2 overexpression, on the other hand, has been shown to destabilize adherens junctions through a pathway involving Src, the low-molecular-weight phosphotyrosine phosphatase, and p190RhoGAP, resulting in increased RhoA activity (Figure 1B) (Fang et al., 2008Fang W.B. Ireton R.C. Zhuang G. Takahashi T. Reynolds A. Chen J. Overexpression of EPHA2 receptor destabilizes adherens junctions via a RhoA-dependent mechanism.J. Cell Sci. 2008; 121: 358-368Crossref PubMed Scopus (95) Google Scholar). The Eph system also affects integrin-mediated cell communication with the extracellular environment (Bourgin et al., 2007Bourgin C. Murai K.K. Richter M. Pasquale E.B. The EphA4 receptor regulates dendritic spine remodeling by affecting beta1-integrin signaling pathways.J. Cell Biol. 2007; 178: 1295-1307Crossref PubMed Scopus (115) Google Scholar, Pasquale, 2005Pasquale E.B. Eph receptor signalling casts a wide net on cell behaviour.Nat. Rev. Mol. Cell Biol. 2005; 6: 462-475Crossref PubMed Scopus (817) Google Scholar, Wimmer-Kleikamp and Lackmann, 2005Wimmer-Kleikamp S.H. Lackmann M. Eph-modulated cell morphology, adhesion and motility in carcinogenesis.IUBMB Life. 2005; 57: 421-431Crossref PubMed Scopus (59) Google Scholar). Crosstalk of EphA2 or ephrin-B1 with claudins, which are components of epithelial tight junctions, has been implicated in the regulation of cell adhesion and intercellular permeability (Arvanitis and Davy, 2008Arvanitis D. Davy A. Eph/ephrin signaling: networks.Genes Dev. 2008; 22: 416-429Crossref PubMed Scopus (209) Google Scholar). Some claudins can also cause ephrin-B1 tyrosine phosphorylation independently of EphB receptors. Gap junction proteins are also critical for Eph/ephrin function in cell sorting, insulin secretion, and osteogenic differentiation (Davy et al., 2006Davy A. Bush J.O. Soriano P. Inhibition of gap junction communication at ectopic Eph/ephrin boundaries underlies craniofrontonasal syndrome.PLoS Biol. 2006; 4: e315https://doi.org/10.1371/journal.pbio.0040315Crossref PubMed Scopus (123) Google Scholar, Konstantinova et al., 2007Konstantinova I. Nikolova G. Ohara-Imaizumi M. Meda P. Kucera T. Zarbalis K. Wurst W. Nagamatsu S. Lammert E. EphA-Ephrin-A-mediated beta cell communication regulates insulin secretion from pancreatic islets.Cell. 2007; 129: 359-370Abstract Full Text Full Text PDF PubMed Scopus (231) Google Scholar, Poliakov et al., 2004Poliakov A. Cotrina M. Wilkinson D.G. Diverse roles of eph receptors and ephrins in the regulation of cell migration and tissue assembly.Dev. Cell. 2004; 7: 465-480Abstract Full Text Full Text PDF PubMed Scopus (347) Google Scholar). Reciprocal communication also occurs between EphB receptors and calcium channels (Figure 2). Following ephrin binding, EphB2 associates with the NMDA receptors, which are calcium channels, and promotes clustering of these neurotransmitter receptors at synapses (Yamaguchi and Pasquale, 2004Yamaguchi Y. Pasquale E.B. Eph receptors in the adult brain.Curr. Opin. Neurobiol. 2004; 14: 288-296Crossref PubMed Scopus (125) Google Scholar). Moreover, activation of Src family kinases downstream of EphB2 leads to NMDA receptor phosphorylation, which increases NMDA-dependent calcium influx. Interestingly, increased intracellular calcium in turn promotes proteolytic degradation of EphB2, demonstrating that Eph levels can be regulated by intracellular calcium independently of ephrin binding (Litterst et al., 2007Litterst C. Georgakopoulos A. Shioi J. Ghersi E. Wisniewski T. Wang R. Ludwig A. Robakis N.K. Ligand binding and calcium influx induce distinct ectodomain/gamma-secretase-processing pathways of EphB2 receptor.J. Biol. Chem. 2007; 282: 16155-16163Crossref PubMed Scopus (102) Google Scholar). More information on Eph signaling mechanisms and crosstalk with other signaling systems can be found in recent reviews (Arvanitis and Davy, 2008Arvanitis D. Davy A. Eph/ephrin signaling: networks.Genes Dev. 2008; 22: 416-429Crossref PubMed Scopus (209) Google Scholar, Egea and Klein, 2007Egea J. Klein R. Bidirectional Eph-ephrin signaling during axon guidance.Trends Cell Biol. 2007; 17: 230-238Abstract Full Text Full Text PDF PubMed Scopus (294) Google Scholar, Himanen et al., 2007Himanen J.P. Saha N. Nikolov D.B. Cell-cell signaling via Eph receptors and ephrins.Curr. Opin. Cell Biol. 2007; 19: 534-542Crossref PubMed Scopus (191) Google Scholar, Noren and Pasquale, 2004Noren N.K. Pasquale E.B. Eph receptor-ephrin bidirectional signals that target Ras and Rho proteins.Cell. Signal. 2004; 16: 655-666Crossref PubMed Scopus (144) Google Scholar, Pasquale, 2005Pasquale E.B. Eph receptor signalling casts a wide net on cell behaviour.Nat. Rev. Mol. Cell Biol. 2005; 6: 462-475Crossref PubMed Scopus (817) Google Scholar, Poliakov et al., 2004Poliakov A. Cotrina M. Wilkinson D.G. Diverse roles of eph receptors and ephrins in the regulation of cell migration and tissue assembly.Dev. Cell. 2004; 7: 465-480Abstract Full Text Full Text PDF PubMed Scopus (347) Google Scholar). The activities of Eph receptors and ephrins in the nervous system have been extensively studied. Neurons form complex networks where electrical signals travel from axonal to dendritic processes through specialized junctions called synapses. Here, neurotransmitters released from the presynaptic terminal in response to electrical signals activate postsynaptic ion channel receptors that initiate new electrical and chemical signals in the postsynaptic neuron. The network of neuronal processes is embedded among surrounding glial cells, which regulate many properties of the neurons including their ability to form synapses. Eph-ephrin bidirectional signaling is important not only for the communication between neurons but also for that between neurons and glial cells (Yamaguchi and Pasquale, 2004Yamaguchi Y. Pasquale E.B. Eph receptors in the adult brain.Curr. Opin. Neurobiol. 2004; 14: 288-296Crossref PubMed Scopus (125) Google Scholar). Eph receptors and ephrins are highly expressed in the developing nervous system, where they have well-known roles in the establishment of neuronal connectivity by guiding axons to the appropriate targets and regulating the formation of synaptic connections. The trajectories of many axonal projections depend on Eph receptors and ephrins distributed in gradients or forming boundaries (Luo and Flanagan, 2007Luo L. Flanagan J.G. Development of continuous and discrete neural maps.Neuron. 2007; 56: 284-300Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar, Pasquale, 2005Pasquale E.B. Eph receptor signalling casts a wide net on cell behaviour.Nat. Rev. Mol. Cell Biol. 2005; 6: 462-475Crossref PubMed Scopus (817) Google Scholar, Poliakov et al., 2004Poliakov A. Cotrina M. Wilkinson D.G. Diverse roles of eph receptors and ephrins in the regulation of cell migration and tissue assembly.Dev. Cell. 2004; 7: 465-480Abstract Full Text Full Text PDF PubMed Scopus (347) Google Scholar). A number of Ras/Rho regulatory proteins have been implicated over the years in axon guidance by the Eph receptors, including several guanine nucleotide exchange factors for Rho GTPases (Figure 1B). Only recently four simultaneous studies have also implicated a GTPase-activating protein for Rac1, α2-chimaerin, as a critical EphA4 effector (Beg et al., 2007Beg A.A. Sommer J.E. Martin J.H. Scheiffele P. alpha2-chimaerin is an essential EphA4 effector in the assembly of neuronal locomotor circuits.Neuron. 2007; 55: 768-778Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar, Iwasato et al., 2007Iwasato T. Katoh H. Nishimaru H. Ishikawa Y. Inoue H. Saito Y.M. Ando R. Iwama M. Takahashi R. Negishi M. et al.Rac-GAP alpha-chimerin regulates motor-circuit Formation as a key mediator of ephrinB3/EphA4 forward signaling.Cell. 2007; 130: 742-753Abstract Full Text Full Text PDF PubMed Scopus (128) Google Scholar, Shi et al., 2007Shi L. Fu W.Y. Hung K.W. Porchetta C. Hall C. Fu A.K. Ip N.Y. Alpha2-chimaerin interacts with EphA4 and regulates EphA4-dependent growth cone collapse.Proc. Natl. Acad. Sci. USA. 2007; 104: 16347-16352Crossref PubMed Scopus (78) Google Scholar, Wegmeyer et al., 2007Wegmeyer H. Egea J. Rabe N. Gezelius H. Filosa A. Enjin A. Varoqueaux F. Deininger K. Schnutgen F. Brose N. et al.EphA4-dependent axon guidance is mediated by the RacGAP alpha2-chimaerin.Neuron. 2007; 55: 756-767Abstract Full Text Full Text PDF PubMed Scopus (111) Google Scholar). Remarkably, α2-chimaerin mutant mice have defects in the formation of cortical and spinal motor circuits that phenocopy those in the EphA4 knockout mice, indicating that α2-chimaerin is essential for certain axon guidance decisions that depend on EphA4. Mice lacking the adaptor proteins Nck1 and Nck2 in the nervous system also exhibited similar defects, suggesting that Nck adaptors, which can bind both EphA4 and α2-chimaerin, may also play a role in the pathway (Fawcett et al., 2007Fawcett J.P. Georgiou J. Ruston J. Bladt F. Sherman A. Warner N. Saab B.J. Scott R. Roder J.C. Pawson T. Nck adaptor proteins control the organization of neuronal circuits important for walking.Proc. Natl. Acad. Sci. USA. 2007; 104: 20973-20978Crossref PubMed Scopus (71) Google Scholar, Wegmeyer et al., 2007Wegmeyer H. Egea J. Rabe N. Gezelius H. Filosa A. Enjin A. Varoqueaux F. Deininger K. Schnutgen F. Brose N. et al.EphA4-dependent axon guidance is mediated by the RacGAP alpha2-chimaerin.Neuron. 2007; 55: 756-767Abstract Full Text Full Text PDF PubMed Scopus (111) Google Scholar). In vitro and in vivo analyses of hippocampal and cortical neurons have revealed that the EphB receptors and B-type ephrins regulate multiple steps in the assembly and maturation of the pre- and postsynaptic sides of excitatory synapses. Interestingly, different Eph receptor domains can control different aspects of synaptogenesis. The EphB2 extracellular region, for example, is sufficient to promote the assembly of presynaptic structures even when expressed in non-neuronal cells (Kayser et al., 2006Kayser M.S. McClelland A.C. Hughes E.G. Dalva M.B. Intracellular and trans-synaptic regulation of glutamatergic synaptogenesis by EphB receptors.J. Neurosci. 2006; 26: 12152-12164Crossref PubMed Scopus (166) Google Scholar). This activity requires the ephrin-binding domain, suggesting a trans-synaptic interaction with axonal ephrins. This ability of EphB2 to promote presynaptic specializations, however, may vary in different brain regions because it was detected in cortical but not hippocampal neurons. Activation of ephrin-B reverse signaling by postsynaptic EphB2 has also been recently implicated in the morphological and functional maturation of developing retinotectal synapses in the Xenopus optic tectum (Lim et al., 2008Lim B.K. Matsuda N. Poo M.M. Ephrin-B reverse signaling promotes structural and functional synaptic maturation in vivo.Nat. Neurosci. 2008; 11: 160-169Crossref PubMed Scopus (93) Google Scholar). The EphB2 extracellular portion also associates with NMDA neurotransmitter receptors and promotes their clustering at synapses following ephrin-B stimulation (Dalva et al., 2007Dalva M.B. McClelland A.C. Kayser M.S. Cell adhesion molecules: signalling functions at the synapse.Nat. Rev. Neurosci. 2007; 8: 206-220Crossref PubMed Scopus (409) Google Scholar). Furthermore, EphB2 promotes AMPA neurotransmitter receptor clustering and endocytosis, and these activities respectively depend on the PDZ domain-binding site of EphB2 and its kinase activity. Most excitatory synapses are located on small dendritic protrusions called dendritic spines, which compartmentalize the postsynaptic space from the dendritic shaft, but some are also located on the dendritic shaft (Dalva et al., 2007Dalva M.B. McClelland A.C. Kayser M.S. Cell adhesion molecules: signalling functions at the synapse.Nat. Rev. Neurosci. 2007; 8: 206-220Crossref PubMed Scopus (409) Google Scholar, Yamaguchi and Pasquale, 2004Yamaguchi Y. Pasquale E.B. Eph receptors in the adult brain.Curr. Opin. Neurobiol. 2004; 14: 288-296Crossref PubMed Scopus (125) Google Scholar). EphB receptors selectively promote the formation of the synapses located on spines and also play a critical role in spine maturation, which results in the characteristic mushroom shape determined by the actin cytoskeleton. Studies with cultured neurons have implicated several nucleotide exchange factors for Rho GTPases in EphB-dependent spine elaboration, including Kalirin, Intersectin, and Tiam1 (Figure 1B) (Tolias et al., 2007Tolias K.F. Bikoff J.B. Kane C.G. Tolias C.S. Hu L. Greenberg M.E. The Rac1 guanine nucleotide exchange factor Tiam1 mediates EphB receptor-dependent dendritic spine development.Proc. Natl. Acad. Sci. USA. 2007; 104: 7265-7270Crossref PubMed Scopus (158) Google Scholar, Yamaguchi and Pasquale, 2004Yamaguchi Y. Pasquale E.B. Eph receptors in the adult brain.Curr. Opin. Neurobiol. 2004; 14: 288-296Crossref PubMed Scopus (125) Google Scholar). It is not known whether these exchange factors function in different subsets of dendritic spines and whether there are differences in their effects on the spines. Ephrin-B ligands are also found postsynaptically, and ephrin-B3 expressed in non-neuronal ce