Ephrin-A5 induces rounding, blebbing and de-adhesion of EphA3-expressing 293T and melanoma cells by CrkII and Rho-mediated signalling

TRPM7 regulates cell adhesion by controlling the calcium-dependent protease calpain

m-Calpain is a protease implicated in the control of cell adhesion through focal adhesion disassembly. The mechanism by which the enzyme is spatially and temporally controlled is not well understood, particularly because the dependence of calpain on calcium exceeds the submicromolar concentrations normally observed in cells. Here we show that the channel kinase TRPM7 localizes to peripheral adhesion complexes with m-calpain, where it regulates cell adhesion by controlling the activity of the protease. Our research revealed that overexpression of TRPM7 in cells caused cell rounding with a concomitant loss of cell adhesion that is dependent upon the channel of the protein but not its kinase activities. Knockdown of m-calpain blocked TRPM7-induced cell rounding and cell detachment. Silencing of TRPM7 by RNA interference, however, strengthened cell adhesion and increased the number of peripheral adhesion complexes in the cells. Together, our results suggest that the ion channel TRPM7 regulates cell adhesion through m-calpain by mediating the local influx of calcium into peripheral adhesion complexes.

EphA Receptors Inhibit Anti-CD3-Induced Apoptosis in Thymocytes

The EphA receptor tyrosine kinases interact with membrane-bound ligands of the ephrin-A subfamily. Interaction induces EphA receptor oligomerization, tyrosine phosphorylation, and, as a result, EphA receptor signaling. EphA receptors have been shown to regulate cell survival, migration, and cell-cell and cell-matrix interactions. However, their functions in lymphoid cells are only beginning to be described. We show in this study that functional EphA receptors are expressed by murine thymocytes, including CD4(+)CD8(+), CD4(+)CD8(-), and CD4(-)CD8(+) subpopulations. We demonstrate that activation of EphA receptors by the ephrin-A1 ligand inhibits the anti-CD3-induced apoptosis of CD4(+)CD8(+) double-positive thymocytes. Furthermore, ephrin-A1 costimulation suppresses up-regulation of both the IL-2R alpha-chain (CD25) and early activation Ag CD69 and can block IL-2 production by CD4(+) single-positive cells. In agreement, EphA receptor activation in thymocytes also inhibits TCR-induced activation of the Ras-MAPK pathway. Our findings suggest that EphA receptor activation is antithetical to TCR signaling in thymocytes, and that the level of engagement by ephrin-A proteins on thymic APCs regulates thymocyte selection.

Eph receptors inactivate R-Ras through different mechanisms to achieve cell repulsion

Eph receptor tyrosine kinases regulate the spatial organization of cells within tissues. Central to this function is their ability to modulate cell shape and movement in response to stimulation by the ephrin ligands. The EphB2 receptor was reported to inhibit cell-matrix adhesion by phosphorylating tyrosine 66 in the effector domain of R-Ras, a Ras family protein known to regulate cell adhesion and motility. Here, we further characterize the role of R-Ras downstream of both EphA and EphB receptors. Our data show that besides inhibiting R-Ras function through phosphorylation, Eph receptors can reduce R-Ras activity through the GTPase-activating protein, p120RasGAP. By using R-Ras mutants that cannot be inactivated by p120RasGAP and/or cannot be phosphorylated at tyrosine 66, we show that the two forms of R-Ras negative regulation - through increased GTP hydrolysis and phosphorylation - differentially contribute to various ephrin-mediated responses. Retraction of the COS cell periphery depends only on R-Ras inactivation through p120RasGAP. By contrast, both reduced R-Ras GTP levels and tyrosine 66 phosphorylation contribute to the ephrin inhibitory effects on COS cell migration and to ephrin-dependent growth cone collapse in primary neurons. Therefore, Eph receptors can regulate R-Ras in two different ways to achieve cell repulsion.

Ephrin-B2 controls cell motility and adhesion during blood-vessel-wall assembly

New blood vessels are initially formed through the assembly or sprouting of endothelial cells, but the recruitment of supporting pericytes and vascular smooth muscle cells (mural cells) ensures the formation of a mature and stable vascular network. Defective mural-cell coverage is associated with the poorly organized and leaky vasculature seen in tumors or other human diseases. Here we report that mural cells require ephrin-B2, a ligand for Eph receptor tyrosine kinases, for normal association with small-diameter blood vessels (microvessels). Tissue-specific mutant mice display perinatal lethality; vascular defects in skin, lung, gastrointestinal tract, and kidney glomeruli; and abnormal migration of smooth muscle cells to lymphatic capillaries. Cultured ephrin-B2-deficient smooth muscle cells are defective in spreading, focal-adhesion formation, and polarized migration and show increased motility. Our results indicate that the role of ephrin-B2 and EphB receptors in these processes involves Crk-p130(CAS) signaling and suggest that ephrin-B2 has some cell-cell-contact-independent functions.

Down-regulation of Rap1 activity is involved in ephrinB1-induced cell contraction

Ephrins are cell surface ligands that activate Eph receptor tyrosine kinases. This ligand-receptor interaction plays a central role in the sorting of cells. We have previously shown that the ephrinB-EphB signalling pathway is also involved in the migration of intestinal precursor cells along the crypts. Using the colon cell line DLD1 expressing the EphB2 receptor, we showed that stimulation of these cells with soluble ephrinB1 results in a rapid retraction of cell extensions and a detachment of cells. On ephrinB1 stimulation, the small GTPases Rho and Ras are activated and Rap1 is inactivated. Importantly, when a constitutively active Rap1 mutant was introduced into these cells, ephrinB1-induced retraction was inhibited. From these results, we conclude that down-regulation of Rap1 is a prerequisite for ephrin-induced cell retraction in colon cells.

PTHrP induces changes in cell cytoskeleton and E-cadherin and regulates Eph/Ephrin kinases and RhoGTPases in murine secondary trophoblast cells

The differentiation of murine trophoblast giant cells (TGCs) is well characterised at the molecular level and, to some extent, the cellular level. Currently, there is a rudimentary understanding about factors regulating the cellular differentiation of secondary TGCs. Using day 8.5 p.c.-ectoplacental cone (EPC) explant in serum-free culture, we have found parathyroid hormone-related protein (PTHrP) to regulate cellular changes during TGC differentiation. PTHrP greatly stimulated the formation and organisation of actin stress fibres and actin expression in trophoblast outgrowth. This coincided with changing cell shape into a flattened/fibroblastic morphology, suppression of E-cadherin expression, and increased cell spreading in culture. PTHrP also increased the nuclear staining of beta-catenin and, similar to activator protein-2gamma (AP-2gamma), showed microtubule-dependent nuclear localisation in vitro. These cellular and behavioural changes correlated with changes in the expression of RhoGTPases and in both expression and phosphorylation of Eph/Ephrin kinases. The effects of PTHrP on trophoblast cellular differentiation were abolished after blocking its action. In conclusion, PTHrP provides an excellent example of the extrinsic factors that, through their network of activities, plays an important role in cellular differentiation of secondary TGCs.

Adam meets Eph: an ADAM substrate recognition module acts as a molecular switch for ephrin cleavage in trans

The Eph family of receptor tyrosine kinases and their ephrin ligands are mediators of cell-cell communication. Cleavage of ephrin-A2 by the ADAM10 membrane metalloprotease enables contact repulsion between Eph- and ephrin-expressing cells. How ADAM10 interacts with ephrins in a regulated manner to cleave only Eph bound ephrin molecules remains unclear. The structure of ADAM10 disintegrin and cysteine-rich domains and the functional studies presented here define an essential substrate-recognition module for functional interaction of ADAM10 with the ephrin-A5/EphA3 complex. While ADAM10 constitutively associates with EphA3, the formation of a functional EphA3/ephrin-A5 complex creates a new molecular recognition motif for the ADAM10 cysteine-rich domain that positions the proteinase domain for effective ephrin-A5 cleavage. Surprisingly, the cleavage occurs in trans, with ADAM10 and its substrate being on the membranes of opposing cells. Our data suggest a simple mechanism for regulating ADAM10-mediated ephrin proteolysis, which ensures that only Eph bound ephrins are recognized and cleaved.

Rho GTPase expression in tumourigenesis: evidence for a significant link

Rho proteins belong to the small GTPases superfamily. They function as molecular switches that, in response to diverse stimuli, control key signaling and structural aspects of the cell. Although early studies proposed a role for Rho GTPases in cellular transformation, this effect was underestimated due to the fact that no genetic mutations affecting Rho-encoding genes were found in tumors. Recently, it has become evident that Rho GTPases participate in the carcinogenic process by either overexpression of some of the members of the family with oncogenic activity, downmodulation of other members with suggested tumor suppressor activity, or by alteration of upstream modulators or downstream effectors. Thus, alteration of the levels of expression of different members of the family of Rho GTPases has been detected in many types of human tumors leading to a great interest in the cellular effects elicited by these oncoproteins. This essay reviews the current evidence of dysregulation of Rho signaling by overexpression in human tumors.

A key role for Abl family kinases in EphA receptor-mediated growth cone collapse

The ephrin-As, and their EphA receptor tyrosine kinases, guide retinal axons by contact-mediated repulsion to their correct target in the midbrain. We have developed a co-culture assay to observe the dynamic cytoskeletal rearrangements comprising retinal growth cone collapse stimulated by contact with an ephrin-A-expressing fibroblast. We show that EphA-ephrin-A interaction at membrane contact sites triggers rapid loss of growth cone lamellipodia followed by axon retraction and cell-cell separation. Using this assay, in combination with soluble ephrin-A5-induced growth cone collapse, we show that inhibiting the Rho effector, ROCK, prevents only ephrin-A-induced retinal axon retraction, but not loss of growth cone lamellipodia. This suggests that actin/myosin driven cell contraction alone does not mediate ephrin-A-induced repulsive responses. We provide evidence that Abl family kinases are a major effector of ephrin-A-induced retinal ganglion cell repulsion since the Abl inhibitor, STI571, prevents both loss of growth cone lamellipodia and axon retraction. These results comprise the first evidence that Abl family kinases play a role in EphA receptor-mediated axon guidance.

Growth factor-sensitive molecular targets identified in primary and metastatic head and neck squamous cell carcinoma using microarray analysis

Polypeptide growth factors play key roles in the processes of cell migration and invasion. In this study, we have used cDNA microarrays to identify target genes whose expression is differentially modulated by the growth factors TGFbeta and EGF. HN4 and HN12 cell lines, established from primary tumor and a lymph node metastasis arising in one patient with head and neck squamous cell carcinoma, were treated with 2nM EGF or 50pM TGFbeta for 24h and extracted RNA was used to prepare labeled cDNAs which were hybridized to NCI UniGem 2.0 cDNA microarrays containing 9128 features. Results revealed constitutive overexpression of 41 genes and underexpression of 109 genes in metastatic HN12 compared to HN4 under conditions of serum withdrawal. Furthermore, TGFbeta treatment resulted in relative upregulation of 53 genes and downregulation of 91 genes in HN12 compared with HN4, whereas cells treated with EGF showed relative upregulation of 67 genes and downregulation of 113 genes. Partial overlap was found between TGFbeta and EGF-modulated gene sets. Results were verified for a subset of each category using quantitative PCR, western blotting and zymography. The data indicate that TGFbeta and EGF differentially affect gene expression in primary and metastatic HNSCC cells, and likely contribute to the invasive properties of metastatic cells through regulation of both common and specific mediators for each growth factor.

Concurrent binding of anti-EphA3 antibody and ephrin-A5 amplifies EphA3 signaling and downstream responses: potential as EphA3-specific tumor-targeting reagents

The Eph receptor tyrosine kinases and their membrane-bound ephrin ligands form a unique cell-cell contact-mediated system for controlling cell localization and organization. Their high expression in a wide variety of human tumors indicates a role in tumor progression, and relatively low Eph and ephrin levels in normal tissues make these proteins potential targets for anticancer therapies. The monoclonal antibody IIIA4, previously used to isolate EphA3, binds with subnanomolar affinity to a conformation-specific epitope within the ephrin-binding domain that is closely adjacent to the "low-affinity" ephrin-A5 heterotetramerization site. We show that similar to ephrin-A5, preclustered IIIA4 effectively triggers EphA3 activation, contraction of the cytoskeleton, and cell rounding. BIAcore analysis, immunoblot, and confocal microscopy of wild-type and mutant EphA3 with compromised ephrin-A5 or IIIA4-binding capacities indicate that IIIA4 binding triggers an EphA3 conformation which is permissive for the assembly of EphA3/ephrin-A5-type signaling clusters. Furthermore, unclustered IIIA4 and ephrin-A5 Fc applied in combination initiate greatly enhanced EphA3 signaling. Radiometal conjugates of ephrin-A5 and IIIA4 retain their affinity, and in mouse xenografts localize to, and are internalized rapidly into EphA3-positive, human tumors. These findings show the biological importance of EphA3/ephrin-A5 interactions and that ephrin-A5 and IIIA4 have great potential as tumor targeting reagents.

Eph-modulated cell morphology, adhesion and motility in carcinogenesis

Eph receptor tyrosine kinases (Ephs) and their membrane anchored ephrin ligands (ephrins) form an essential cell-cell communication system that directs the positioning, adhesion and migration of cells and cell layers during development. While less prominent in normal adult tissues, there is evidence that up-regulated expression and de-regulated function of Ephs and ephrins in a large variety of human cancers may promote a more aggressive and metastatic tumour phenotype. However, in contrast to other RTKs, Ephs do not act as classical proto-oncogenes and do not effect cell proliferation or differentiation. Mounting evidence suggests that Eph receptors, through de-regulated re-emergence of their mode of action in the embryo may direct cell movements and positioning during metastasis, invasion and tumour angiogenesis. This review discusses these and other emerging roles of Eph receptors during oncogenesis.

Ephrin-B2 is differentially expressed in the intestinal epithelium in Crohn’s disease and contributes to accelerated epithelial wound healing in vitro

AIM: Eph receptor tyrosine kinases and their membrane bound receptor-like ligands, the ephrins, represent a bi-directional cell-cell contact signaling system that directs epithelial movements in development. The meaning of this system in the adult human gut is unknown. We investigated the Eph/ephrin mRNA expression in the intestinal epithelium of healthy controls and patients with inflammatory bowel disease (IBD).

METHODS: mRNA expression profiles of all Eph/ephrin family members in normal small intestine and colon were established by real-time RT-PCR. In addition, differential expression in IBD was investigated by cDNA array technology, and validated by both real-time RT-PCR and immunohistochemistry. Potential effects of enhanced EphB/ephrin-B signaling were analyzed in an in vitro IEC-6 cell scratch wound model.

RESULTS: Human adult intestinal mucosa exhibits a complex pattern of Eph receptors and ephrins. Beside the known prominent co-expression of EphA2 and ephrinA1, we found abundantly co-expressed EphB2 and ephrin-B1/2. Interestingly, cDNA array data, validated by real-time PCR and immunohistochemistry, showed upregulation of ephrin-B2 in both perilesional and lesional intestinal epithelial cells of IBD patients, suggesting a role in epithelial homeostasis. Stimulation of ephrin-B signaling in ephrin-B1/2 expressing rat IEC-6-cells with recombinant EphB1-Fc resulted in a significant dose-dependent acceleration of wound closure. Furthermore, fluorescence microscopy showed that EphB1-Fc induced coordinated migration of wound edge cells is associated with enhanced formation of lamellipodial protrusions into the wound, increased actin stress fiber assembly and production of laminin at the wound edge.

CONCLUSION: EphB/ephrin-B signaling might represent a novel protective mechanism that promotes intestinal epithelial wound healing, with potential impact on epithelial restitution in IBD.

Three Distinct Molecular Surfaces in Ephrin-A5 Are Essential for a Functional Interaction with EphA3

Eph receptor tyrosine kinases (Ephs) function as molecular relays that interact with cell surface-bound ephrin ligands to direct the position of migrating cells. Structural studies revealed that, through two distinct contact surfaces on opposite sites of each protein, Eph and ephrin binding domains assemble into symmetric, circular heterotetramers. However, Eph signal initiation requires the assembly of higher order oligomers, suggesting additional points of contact. By screening a random library of EphA3 binding-compromised ephrin-A5 mutants, we have now determined ephrin-A5 residues that are essential for the assembly of high affinity EphA3 signaling complexes. In addition to the two interfaces predicted from the crystal structure of the homologous EphB2.ephrin-B2 complex, we identified a cluster of 10 residues on the ephrin-A5 E alpha-helix, the E-F loop, the underlying H beta-strand, as well as the nearby B-C loop, which define a distinct third surface required for oligomerization and activation of EphA3 signaling. Together with a corresponding third surface region identified recently outside of the minimal ephrin binding domain of EphA3, our findings provide experimental evidence for the essential contribution of three distinct protein-interaction interfaces to assemble functional EphA3 signaling complexes.

The motility of glioblastoma tumour cells is modulated by intracellular cofilin expression in a concentration-dependent manner

The invasive behaviour of tumour cells has been attributed in part to dysregulated cell motility. Members of the ADF/Cofilin family of actin-binding proteins are known to increase microfilament dynamics by increasing the rate at which actin monomers leave the pointed end of the filament and by a filament-severing activity. As depolymerisation is a rate-limiting step in actin dynamics, ADF/Cofilins are suspected to facilitate the motility of cells. To test this, we investigated the influence of cofilin on tumour motility by transient and stably overexpressing cofilin in the human glioblastoma cell line, U373 MG. Several different methods were used to ascertain the level of cofilin in overexpressing clones and this was correlated with their rate of random locomotion. A biphasic relationship between cofilin level and locomotory rate was found. Clones that displayed a moderate amount of overproduction of cofilin were found to have increased rates of locomotion approximately linear to the overproduction of cofilin up to an optimal cofilin level of about 4.5 times that of wild type cells at which the cells were almost twice as fast. However, clones producing more than this optimal amount were found to locomote at progressively reduced speeds. Cells that overexpress cofilin have reduced stress fibres compared to control cells showing that the excess cofilin affects the actin cytoskeleton. We conclude that overexpression of cofilin enhances the motility of glioblastoma tumour cells in a concentration-dependent fashion, which is likely to contribute to their invasiveness.

Ephrin-A1 binding to CD4+ T lymphocytes stimulates migration and induces tyrosine phosphorylation of PYK2

Eph receptors, the largest subfamily of receptor tyrosine kinases, and their ephrin ligands are important mediators of cell-cell communication regulating cell attachment, shape, and mobility. Here we demonstrate that CD4+ T lymphocytes express the EphA1 and EphA4 receptors and that these cells bind the ligand ephrin-A1. Further we show ephrin-A1 expression in vivo on high endothelial venule (HEV) endothelial cells. Ephrin-A1 binding to CD4+ T cells stimulates both stromal cell-derived factor 1α (SDF-1α)- and macrophage inflammatory protein 3β (MIP3β)-mediated chemotaxis. In line with the increased chemotactic response, increased actin polymerization is observed in particular with the combination of ephrin-A1 and SDF-1α. Signaling through EphA receptors induces intracellular tyrosine phosphorylation. In particular, proline-rich tyrosine kinase 2 (PYK2) is phosphorylated on tyrosine residues 402 and 580. Ephrin-A1-induced chemotaxis and intracellular tyrosine phosphorylation, including EphA1 and Pyk2, was inhibited by Tyrphostin-A9. In conclusion, ligand engagement of EphA receptors on CD4+ T cells stimulates chemotaxis, induces intracellular tyrosine phosphorylation, and affects actin polymerization. This, together with our finding that ephrin-A1 is expressed by HEV endothelial cells, suggests a role for Eph receptors in transendothelial migration.

The role of ephrins and Eph receptors in cancer

Eph receptors are the largest receptor tyrosine kinase family of transmembrane proteins with an extracellular domain capable of recognizing signals from the cells' environment and influencing cell-cell interaction and cell migration. Ephrins are the ligands to Eph receptors and stimulate bi-directional signaling of the Eph/ephrin axis. Eph receptor and ephrin overexpression can result in tumorigenesis as related to tumor growth and survival and is associated with angiogenesis and metastasis in many types of human cancer. Recent data suggest that Eph/ephrin signaling could play an important role in the development of novel inhibition strategies and cancer treatments to potentially target this receptor tyrosine kinase and/or its ligand. A deeper understanding of the molecular basis for normal versus defective cell-cell interaction through the Eph/ephrin axis will enable the potential development of novel cancer treatments. This review emphasizes the biology of Eph/ephrin as well as the potential for novel targeted therapy through this pathway.

Diverse roles of eph receptors and ephrins in the regulation of cell migration and tissue assembly

Eph receptor tyrosine kinases and ephrins have key roles in regulation of the migration and adhesion of cells required to form and stabilize patterns of cell organization during development. Activation of Eph receptors or ephrins can lead either to cell repulsion or to cell adhesion and invasion, and recent work has found that cells can switch between these distinct responses. This review will discuss biochemical mechanisms and developmental roles of the diverse cell responses controlled by Eph receptors and ephrins.

Inhibition of integrin-mediated cell adhesion but not directional cell migration requires catalytic activity of EphB3 receptor tyrosine kinase. Role of Rho family small GTPases

Genetic studies have shown that Eph receptor tyrosine kinases have both kinase-dependent and kinase-independent functions through incompletely understood mechanisms. We report here that ephrin-B1 stimulation of endogenous EphB kinases in LS174T colorectal epithelial cells inhibited integrin-mediated adhesion and HGF/SF-induced directional cell migration. Using 293 cells stably transfected with wild type (WT)- or kinase-deficient (KD-EphB3), we found that inhibition of integrin-mediated cell adhesion and induction of cell rounding was kinase-dependent. Unexpectedly, in two independent assays, both KD- and WT-EphB3 significantly inhibited directional cell migration. Upon ephrin-B1 stimulation, the activities of Rac1 and Cdc42 were reduced in both WT- and KD-EphB3-expressing cells that were induced to migrate. Pharmacological evidence demonstrates that a relative increase in RhoA signaling as a result of decreased Rac1/Cdc42 activities contributes to the inhibitory effects. Furthermore, EphB3-mediated inhibitory effect on cell adhesion but not migration was abolished by the integrin activating antibodies, suggesting that the inhibition of cell migration is not because of down-regulation of integrin function. These results uncover a differential requirement for EphB3 catalytic activity in the regulation of cell adhesion and migration, and suggest that while catalytic activity of EphB3 is required for inhibition of integrin-mediated cell adhesion, a distinct signaling pathway to Rho GTPases shared by WT- and KD-EphB3 receptor mediates inhibition of directional cell migration.

Recruitment of Eph receptors into signaling clusters does not require ephrin contact

Eph receptors and their cell membrane-bound ephrin ligands regulate cell positioning and thereby establish or stabilize patterns of cellular organization. Although it is recognized that ephrin clustering is essential for Eph function, mechanisms that relay information of ephrin density into cell biological responses are poorly understood. We demonstrate by confocal time-lapse and fluorescence resonance energy transfer microscopy that within minutes of binding ephrin-A5-coated beads, EphA3 receptors assemble into large clusters. While remaining positioned around the site of ephrin contact, Eph clusters exceed the size of the interacting ephrin surface severalfold. EphA3 mutants with compromised ephrin-binding capacity, which alone are incapable of cluster formation or phosphorylation, are recruited effectively and become phosphorylated when coexpressed with a functional receptor. Our findings reveal consecutive initiation of ephrin-facilitated Eph clustering and cluster propagation, the latter of which is independent of ephrin contacts and cytosolic Eph signaling functions but involves direct Eph-Eph interactions.

Eph receptor–ephrin bidirectional signals that target Ras and Rho proteins

The ability of cells to respond to their surrounding environment and relay signals to the cell interior is essential for numerous processes during the development and maintenance of tissues. Eph receptors and their membrane-bound ligands, the ephrins, are unique in the receptor tyrosine kinase family in that their signaling is bidirectional, through both the receptor and the ligand. Eph receptors and ephrins are essential for a variety of biological processes, and play a particularly important role in regulating cell shape and cell movement. Recent data have linked Eph receptor-ephrin signaling complexes to the Ras and Rho families of small molecular weight GTPases and also to heterotrimeric G proteins. Understanding the signaling networks involved is an important step to understand the molecular basis for normal and defective cell-cell communication through Eph receptors and ephrins.

EphA/ephrin-A interactions during optic nerve regeneration: restoration of topography and regulation of ephrin-A2 expression

During visual system development, interactions between Eph tyrosine kinase receptors and their ligands, the ephrins, guide retinal ganglion cell (RGC) axons to their topographic targets in the optic tectum. Here we show that Eph/ephrin interactions are also involved in restoring topography during RGC axon regeneration in goldfish. Following optic nerve crush, EphA/ephrin-A interactions were blocked by intracranial injections of recombinant Eph receptor (EphA3-AP) or phospho-inositol phospholipase-C. Topographic errors with multiple inputs to some tectal loci were detected electrophysiologically and increased projections to caudal tectum demonstrated by RT-97 immunohistochemistry. In EphA3-AP-injected fish, ephrin-A2-expressing cells in the retino-recipient tectal layers were reduced in number compared to controls and their distribution was no longer graded. The findings, supported by in vitro studies, implicate EphA/ephrin-A interactions in restoring precise topography and in regulating ephrin-A2 expression during regeneration.

EphA2 receptor tyrosine kinase regulates endothelial cell migration and vascular assembly through phosphoinositide 3-kinase-mediated Rac1 GTPase activation

Angiogenesis is critical for vascular remodeling during development and contributes to the pathogenesis of diseases such as cancer. Targeted disruption of several EphB class receptor tyrosine kinases results in vascular remodeling defects during embryogenesis. The role of EphA class receptors in vascular remodeling, however, is not well-characterized. We recently demonstrated that global inhibition of EphA receptors disrupts endothelial migration induced by ephrin, VEGF or tumor-derived signals, though the specific target remained undefined. Here, we report that EphA2 regulates endothelial cell assembly and migration through phosphoinositide (PI) 3-kinase-mediated activation of Rac1 GTPase in two model systems: primary bovine and murine pulmonary microvascular endothelial cells. EphA2-deficient endothelial cells fail to undergo vascular assembly and migration in response to ephrin-A1 in vitro. Ephrin-A1 stimulation induces PI3-kinase-dependent activation of Rac1 in wild-type endothelial cells, whereas EphA2-deficient cells fail to activate Rac1 upon stimulation. Expression of dominant negative PI3-kinase or Rac1 inhibits ephrin-A1-induced endothelial cell migration. Consistent with in vitro data, EphA2-deficient mice show a diminished angiogenic response to ephrin-A1 in vivo. Moreover, EphA2-deficient endothelial cells fail to assemble in vivo when transplanted into recipient mice. These data suggest that EphA2 is an essential regulator of post-natal angiogenesis.

EphA3 is induced by CD28 and IGF-1 and regulates cell adhesion

Stimulation of CD28 alone has been shown to regulate cytokine gene transcription and expression of the type 1 insulin-like growth factor receptor (IGF-1R) in lymphocytes. In this study, the ephrin receptor tyrosine kinase ephA3, was identified as a new CD28-responsive gene in Jurkat cells by using a human cytokine/receptor array. EphA3 was not detected in normal peripheral T cells, in any subset of thymus-derived developing T cells, or in Hodgkin's lymphoma. However, contrary to previous findings, EphA3 was detected in a panel of T-cell lymphomas. Stimulation of Jurkat cells with ephrin-A5 resulted in loss of cell adhesion to fibronectin and recruitment of the adapter protein CrkII to EphA3. Interestingly, EphA3 expression in CD28-stimulated Jurkat cells was enhanced by IGF-1 or by overexpression of the IGF-1R, and was suppressed by anti-IGF-1R blocking antibodies. The data suggest that CD28- and IGF-1-regulated expression of EphA3 is associated with adherence and that it may be involved in the motility of malignant T cells.

Differential gene expression of Eph receptors and ephrins in benign human tissues and cancers

BACKGROUND

Eph receptors and their ligands, the ephrins, represent a large class of cell-cell communication molecules with well-defined developmental functions. Their role in healthy adult tissues and in human disease is still largely unknown, although diverse roles in carcinogenesis have been postulated.

METHODS

We established a set of fluorescent PCR probes and primers for the definition of individual gene expression profiles of 12 different Eph receptors and 8 ephrins in 13 different healthy tissues. The mRNA expression profiles were studied in human lung, colorectal, kidney, liver, and brain cancers.

RESULTS

The family of Eph receptors/ephrins was widely expressed in adult tissues with organ-site-specific patterns: EphB6 was highest in the thymus, compatible with an involvement in T-cell maturation. Brain and testis shared a unique pattern with EphA6, EphA8, and EphB1 being the most prominent. EphA7 had a high abundance in the kidney vasculature. Ephrin-A3 was up-regulated 26-fold in lung cancer, and EphB2 was up-regulated 9-fold in hepatocellular carcinoma. EphA8 was down-regulated in colon cancer, and EphA1/EphA8 was down-regulated in glioblastomas.

CONCLUSION

Eph/Ephrin genes are widely expressed in all adult organs with certain organ-site-specific patterns. Because their function in adult tissues remains unknown, further analysis of their role in disease may disclose new insights beyond their well-defined meaning in development.

Dissecting the EphA3/Ephrin-A5 interactions using a novel functional mutagenesis screen

The EphA3 receptor tyrosine kinase preferentially binds ephrin-A5, a member of the corresponding subfamily of membrane-associated ligands. Their interaction regulates critical cell communication functions in normal development and may play a role in neoplasia. Here we describe a random mutagenesis approach, which we employed to study the molecular determinants of the EphA3/ephrin-A5 recognition. Selection and functional characterization of EphA3 point mutants with impaired ephrin-A5 binding from a yeast expression library defined three EphA3 surface areas that are essential for the EphA3/ephrin-A5 interaction. Two of these map to regions identified previously in the crystal structure of the homologous EphB2-ephrin-B2 complex as potential ligand/receptor interfaces. In addition, we identify a third EphA3/ephrin-A5 interface that falls outside the structurally characterized interaction domains. Functional analysis of EphA3 mutants reveals that all three Eph/ephrin contact areas are essential for the assembly of signaling-competent, oligomeric receptor-ligand complexes.

Ephs and ephrins during early stages of chick embryogenesis

The Eph family of receptor tyrosine kinases and their ligands, the ephrins, are membrane-bound proteins that mediate bidirectional signals between adjacent cells. By modulating cytoskeleton dynamics affecting cell motility and adhesion, Ephs and ephrins orchestrate cell movements during multiple morphogenetic processes, including gastrulation, segmentation, angiogenesis, axonal pathfinding, and neural crest cell migration. The full repertoire of developmental Eph/ephrin functions remains uncertain, however, because coexpression of multiple receptor and ligand family members, and promiscuous interactions between them, can result in functional redundancy. A complete understanding of expression patterns, therefore, is a necessary prerequisite to understanding function. Here, we present a comprehensive expression overview for 10 Eph and ephrin genes during the first 48 hr of chick embryo development. First, dynamic expression domains are described for each gene between Hamburger and Hamilton stages 4 and 12; second, comparative analyses are presented of Eph/ephrin expression patterns in the primitive streak, the somites, the vasculature, and the brain. Complex spatially and temporally dynamic expression patterns are revealed that suggest novel functions for Eph and ephrin family members in both known and previously unrecognized processes. This study will provide a valuable resource for further experimental investigations of Eph and ephrin functions during early embryonic development.

'Eph'ective signaling: forward, reverse and crosstalk

The Eph receptors comprise the largest group of receptor tyrosine kinases and are found in a wide variety of cell types in developing and mature tissues. Their ligands are the ephrins, a family of membrane-bound proteins found in lipid rafts. In the past decade, Eph receptors and ephrins have been implicated in a vast array of cellular processes. Unlike other receptor tyrosine kinases, however, the Eph receptors seem to be geared towards regulating cell shape and movement rather than proliferation. Studies have uncovered intricate signaling networks that center around the ligand-receptor complex, and this may account for the broad repertoire of functions of Eph proteins. Deciphering the bi-directional pathways emanating from an Eph receptor-ephrin complex will not only help us to understand basic biological processes, but may also provide important insight into disease.

Ephrin-B1 reverse signaling activates JNK through a novel mechanism that is independent of tyrosine phosphorylation

Eph receptors and their cognate ligand ephrins play important roles in various biological processes such as cell migration, axon guidance, and synaptic plasticity. One characteristic feature of the Eph-ephrin signal transduction is that, upon interaction with the receptor, the transmembrane B-class ephrins become tyrosine-phosphorylated and transduce intracellular signals that lead to reorganization of the cytoskeleton. Although in vitro and genetic studies have demonstrated unequivocally the significance of this reverse signaling, the underlying mechanism remains unclear. We report here that transfection of ephrin-B1 into 293 cells resulted in robust increase in JNK activity, whereas expression of truncated ephrin-B1 lacking the cytoplasmic domain had a negligible effect, indicating that the induction of JNK activity was attributed mainly to the reverse signaling. The ephrin-B1-mediated JNK activation was reduced significantly by dominant-negative TAK1, MKK4, or MKK7. Ephrin-B1 over-expressing 293 cells became rounded in morphology. Surprisingly, ephrin-B1 that lacked all six intracellular tyrosine residues still triggered JNK activation and rounding morphology of the transfected cells. Consistent with these observations, activation of JNK and the resulting morphological changes mediated by ephrin-B1 could be abolished by the JNK inhibitor SP600125 but not the Src inhibitor PP2. Taken together, our findings have identified a novel reverse signaling pathway transduced by ephrin-B1, which is independent of tyrosine phosphorylation but involves the activation of JNK through TAK1 and MKK4/MKK7 and leads to changes in cell morphology.

Contact-dependent signaling during the late events of platelet activation

Signaling events downstream from collagen receptors and G protein-coupled receptors are responsible for the initiation and extension of platelet plug formation. This creates the platelet plug and hopefully results in the cessation of bleeding. It is not, however, all that is required for hemostasis, and growing evidence is emerging that the perpetuation of a stable hemostatic plug requires additional intracellular signaling. At least part of this process is made possible by the persistent close contacts between platelets that can only occur after the onset of aggregation. This review discusses several examples of such signaling mechanisms that help to perpetuate the platelet plug in a contact-dependent manner, including outside-in signaling through integrins, signaling though Eph kinases and ephrins, and the role of CD40L.

Ephrin-A1 induces c-Cbl phosphorylation and EphA receptor down-regulation in T cells

Eph receptor tyrosine kinases are expressed by T lineage cells, and stimulation with their ligands, the ephrins, has recently been shown to modulate T cell behavior. We show that ephrin-A1 stimulation of Jurkat T cells induces tyrosine phosphorylation of EphA3 receptors and cytoplasmic proteins, including the c-cbl proto-oncogene. Cbl phosphorylation was also observed in peripheral blood T cells. In contrast, stimulation of Jurkat cells with the EphB receptor ligand ephrin-B1 does not cause Cbl phosphorylation. EphA activation also induced Cbl association with Crk-L and Crk-II adapters, but not the related Grb2 protein. Induction of Cbl phosphorylation upon EphA activation appeared to be dependent upon Src family kinase activity, as Cbl phosphorylation was selectively abrogated by the Src family inhibitor 4-amino-5(4-chlorophenyl-7-(tert-butyl)pyrazolo[3,4-d]pyrimidine, while EphA phosphorylation was unimpaired. Ephrin-A1 stimulation of Jurkat cells was also found to cause down-regulation of endogenous EphA3 receptors from the cell surface and their degradation. In accordance with the role of Cbl as a negative regulator of receptor tyrosine kinases, overexpression of wild-type Cbl, but not its 70-Z mutant, was found to down-regulate EphA receptor expression. Receptor down-regulation could also be inhibited by blockage of Src family kinase activity. Our findings show that EphA receptors can actively signal in T cells, and that Cbl performs multiple roles in this signaling pathway, functioning to transduce signals from the receptors as well as regulating activated EphA receptor expression.

EphrinA1 inactivates integrin-mediated vascular smooth muscle cell spreading via the Rac/PAK pathway

Interactions between the Eph receptor tyrosine kinase and ephrin ligands transduce short-range signals regulating axon pathfinding, development of the cardiovascular system, as well as migration and spreading of neuronal and non-neuronal cells. Some of these effects are believed to be mediated by alterations in actin dynamics. The members of the small Rho GTPase family elicit various effects on actin structures and are probably involved in Eph receptor-induced actin modulation. EphrinA1 is proposed to contribute to angiogenesis as it is strongly expressed at sites of neovascularization. Moreover, angiogenic factors induce the expression of ephrinA1 in endothelial cells. In this study, using rat vascular smooth muscle cells (VSMCs), we investigated the contribution of the small Rho GTPases in ephrinA1-induced integrin inactivation. EphrinA1 did not significantly affect early adhesion of VSMCs on purified laminin or fibronectin, but strongly impaired cell spreading. The Rho kinase inhibitor Y-27632 partly reversed the ephrinA1 effect, suggesting involvement of Rho in this model. However, inhibition of RhoA synthesis with short interfering (si)RNA had a modest effect, suggesting that RhoA plays a limited role in ephrinA1-mediated inhibition of spreading in VSMCs. The ephrinA1-mediated morphological alterations correlated with inhibition of Rac1 and p21-activated kinase 1 (PAK1) activity, and were antagonized by the expression of a constitutively active Rac mutant. Moreover, repression of Rac1 synthesis with siRNA amplifies the ephrinA1-induced inhibition of spreading. Finally, sphingosine-1-phosphate (S1P), a lipid mediator known to inhibit Rac activation in VSMCs amplifies the ephrinA1 effect. In conclusion, our results emphasize the role of the Rac/PAK pathway in ephrinA1-mediated inhibition of spreading. In this way, ephrinA1, alone or in synergy with S1P, can participate in blood vessel destabilization, a prerequisite for angiogenesis.

EphB1 associates with Grb7 and regulates cell migration

EphB1 is a member of the Eph family of receptor tyrosine kinases that play important roles in diverse biological processes including nervous system development, angiogenesis, and neural synapsis formation and maturation. Grb7 is an adaptor molecule implicated in the regulation of cell migration. Here we report identification of an interaction between Grb7 and the cytoplasmic domain of EphB1 by using Grb7 as a "bait" in a yeast two-hybrid screening. Co-immunoprecipitation was used to confirm the interaction of Grb7 with the cytoplasmic domain of EphB1 as well as the full-length receptor in intact cells. This interaction is mediated by the SH2 domain of Grb7 and requires tyrosine autophosphorylation of EphB1. Furthermore, Tyr-928 of EphB1 was identified as the primary binding site for Grb7. Stimulation of endogenous EphB1 in embryonal carcinoma P19 cells with its ligand ephrinB1 increased its association with Grb7, which is consistent with a role for the autophosphorylation of EphB1. We also found that EphB1 could phosphorylate Grb7 and mutation of either Tyr-928 or Tyr-594 to Phe decreased this activity. Finally, we show that EphB1 could stimulate fibroblast motility on extracellular matrix in a kinase-dependent manner, which also correlated with its association with Grb7. Consistent with this, co-expression of Grb7 with EphB1 further enhanced cell motility, whereas co-expression of the Grb7 SH2 domain abolished EphB1-stimulated cell migration. Together, our results identified a novel interaction between EphB1 with the adaptor molecule Grb7 and suggested that this interaction may play a role in the regulation of cell migration by EphB1.