Cell-cell adhesion mediated by binding of membrane-anchored ligand LERK-2 to the EPH-related receptor human embryonal kinase 2 promotes tyrosine kinase activity

Overexpression of EphB6 and EphrinB2 controls soma spacing of cortical neurons in a mutual inhibitory way

To establish functional circuitry, neurons settle down in a particular spatial domain by spacing their cell bodies, which requires proper positioning of the soma and establishing of a zone with unique connections. Deficits in this process are implicated in neurodevelopmental diseases. In this study, we examined the function of EphB6 in the development of cerebral cortex. Overexpression of EphB6 via in utero electroporation results in clumping of cortical neurons, while reducing its expression has no effect. In addition, overexpression of EphrinB2, a ligand of EphB6, also induces soma clumping in the cortex. Unexpectedly, the soma clumping phenotypes disappear when both of them are overexpressed in cortical neurons. The mutual inhibitory effect of EphB6/ EphrinB2 on preventing soma clumping is likely to be achieved via interaction of their specific domains. Thus, our results reveal a combinational role of EphrinB2/EphB6 overexpression in controlling soma spacing in cortical development.

Identification of a novel agrin-dependent pathway in cell signaling and adhesion within the erythroid niche

Establishment of cell–cell adhesion is crucial in embryonic development as well as within the stem cell niches of an adult. Adhesion between macrophages and erythroblasts is required for the formation of erythroblastic islands, specialized niches where erythroblasts proliferate and differentiate to produce red blood cells throughout life. The Eph family is the largest known family of receptor tyrosine kinases (RTKs) and controls cell adhesion, migration, invasion and morphology by modulating integrin and adhesion molecule activity and by modifying the actin cytoskeleton. Here, we identify the proteoglycan agrin as a novel regulator of Eph receptor signaling and characterize a novel mechanism controlling cell–cell adhesion and red cell development within the erythroid niche. We demonstrate that agrin induces clustering and activation of EphB1 receptors on developing erythroblasts, leading to the activation of α5β1 integrins. In agreement, agrin knockout mice display severe anemia owing to defective adhesion to macrophages and impaired maturation of erythroid cells. These results position agrin-EphB1 as a novel key signaling couple regulating cell adhesion and erythropoiesis.

Eph receptor signaling and ephrins

The Eph receptors are the largest of the RTK families. Like other RTKs, they transduce signals from the cell exterior to the interior through ligand-induced activation of their kinase domain. However, the Eph receptors also have distinctive features. Instead of binding soluble ligands, they generally mediate contact-dependent cell-cell communication by interacting with surface-associated ligands-the ephrins-on neighboring cells. Eph receptor-ephrin complexes emanate bidirectional signals that affect both receptor- and ephrin-expressing cells. Intriguingly, ephrins can also attenuate signaling by Eph receptors coexpressed in the same cell. Additionally, Eph receptors can modulate cell behavior independently of ephrin binding and kinase activity. The Eph/ephrin system regulates many developmental processes and adult tissue homeostasis. Its abnormal function has been implicated in various diseases, including cancer. Thus, Eph receptors represent promising therapeutic targets. However, more research is needed to better understand the many aspects of their complex biology that remain mysterious.

Eph family receptors and ligands in vascular cell targeting and assembly

Members of the Eph family of receptor tyrosine kinases determine neural cell aggregation and targeting behavior, functions that are also critical in vascular assembly and remodeling. Among this class of diverse receptors, EphA2 (Eck) and EphB1 (ELK) represent prototypes for two receptor subfamilies distinguished by high-affinity interaction with either glycerophosphatidylinositol (GPI)-linked or transmembrane ligands, respectively. EphA2 participates in angiogenic responses to tumor necrosis factor (TNF) through an autocrine loop affecting endothelial cell migration. EphB1 and its ligand Ephrin-B1 (LERK-2) are important determinants of assembly of endothelial cells from the microvasculature of the kidney, where both are expressed in endothelial progenitors and in glomerular microvascular endothelial cells. Ephrin-B1 activation of EphB1 promotes assembly of these cells into capillary-like structures. Interaction trap approaches have identified downstream signaling proteins that complex with ligand-activated EphA2 or EphB1, including nonreceptor tyrosine kinases and SH2 domain-containing adapter proteins. The Grb 10 adapter is one of a subset that binds activated EphB1, but not EphA2, defining distinct signaling mechanisms for these related endothelial receptors. On the basis of observations in vascular endothelial cells and recent results defining Eph receptor and ligand roles in neural cell targeting, we propose that these receptors direct cell-cell recognition events that are critical in vasculogenesis and angiogenesis. (Trends Cardiovasc Med 1997;7:329-334). © 1997, Elsevier Science Inc.

EphB3 suppresses non-small-cell lung cancer metastasis via a PP2A/RACK1/Akt signalling complex

Eph receptors are implicated in regulating the malignant progression of cancer. Here we find that despite overexpression of EphB3 in human non-small-cell lung cancer, as reported previously, the expression of its cognate ligands, either ephrin-B1 or ephrin-B2, is significantly downregulated, leading to reduced tyrosine phosphorylation of EphB3. Forced activation of EphB3 kinase in EphB3-overexpressing non-small-cell lung cancer cells inhibits cell migratory capability in vitro as well as metastatic seeding in vivo. Furthermore, we identify a novel EphB3-binding protein, the receptor for activated C-kinase 1, which mediates the assembly of a ternary signal complex comprising protein phosphatase 2A, Akt and itself in response to EphB3 activation, leading to reduced Akt phosphorylation and subsequent inhibition of cell migration. Our study reveals a novel tumour-suppressive signalling pathway associated with kinase-activated EphB3 in non-small-cell lung cancer, and provides a potential therapeutic strategy by activating EphB3 signalling, thus inhibiting tumour metastasis.

The cis side of juxtacrine signaling: a new role in the development of the nervous system

Cell-cell communication by juxtacrine signaling plays a key role in the development of the nervous system, from cell fate determination through axonal guidance to synaptogenesis. Interestingly, several juxtacrine signaling systems exhibit an inhibitory interaction between receptors and ligands in the same cell, termed cis inhibition. These include the Notch, semaphorin and ephrin signaling systems. Here we review the role of cis inhibition in these signaling systems in the development of the nervous system. We compare and contrast cis inhibition mechanisms and discuss their potential cellular function as a threshold-generating mechanism. The prevalence of cis inhibition suggests that these interactions and their functional regulatory roles may serve as a general design principle for juxtacrine signaling-mediated processes during and beyond neurodevelopment.

Multiple effects of ephrin-A5 on cortical neurons are mediated by SRC family kinases

The Eph receptor tyrosine kinases and their membrane-bound ligands, the ephrins, are involved in a variety of developmental processes such as axonal guidance, cell migration, cell adhesion, proliferation, and differentiation. In addition to repulsive effects, ephrins can also induce attractive responses. Up to now, little was known about the underlying signaling mechanisms that regulate attractive versus repulsive effects. In this study, we show that ephrin-A5 enhances the motility of cortical neurons that is dependent on the activity of Src-family kinases (SFKs). Ephrin-A5 further changes the adhesive properties of neurons by inducing the formation of cell aggregates. Using the stripe assay, we found that the motogenic effect of ephrin-A5 is the result of repulsive ephrin-A interactions. Blocking SFK function leads to a conversion of repulsion into adhesion, suggesting that SFKs can act as a biological switch for the response of EphA receptors. Finally, we discovered a ligand-induced release of membrane particles containing EphA receptors, suggesting membrane ripping as a novel mechanism to overcome the "ephrin paradox" of repulsion after high-affinity receptor-ligand binding.

Tumor inhibition by genomically integrated inducible RNAi-cassettes

RNA interference (RNAi) has emerged as a powerful tool to induce loss-of-function phenotypes by post-transcriptional silencing of gene expression. In this study we wondered whether inducible RNAi-cassettes integrated into cellular DNA possess the power to trigger neoplastic growth. For this purpose inducible RNAi vectors containing tetracycline (Tet)-responsive derivatives of the H1 promoter for the conditional expression of short hairpin RNA (shRNA) were used to target human polo-like kinase 1 (Plk1), which is overexpressed in a broad spectrum of human tumors. In the absence of doxycycline (Dox) HeLa clones expressing TetR, that carry the RNAi-cassette stably integrated, exhibited no significant alteration in Plk1 expression levels. In contrast, exposure to Dox led to marked downregulation of Plk1 mRNA to 3% and Plk1 protein to 14% in cell culture compared to mismatch shRNA/Plk1-expressing cells. As a result of Plk1 depletion cell proliferation decreased to 17%. Furthermore, for harnessing RNAi for silencing disease-related genes in vivo we transplanted inducible RNAi-HeLa cells onto nude mice. After administration of Dox knockdown of Plk1 expression was observed correlating to a significant inhibition of tumor growth. Taken together, our data revealed that genomically integrated RNAi-elements are suitable to hamper tumor growth by conditional expression of shRNA.

Eph receptors and their ephrin ligands are expressed in developing mouse pancreas

Pancreas development involves branching morphogenesis concomitantly to differentiation of endocrine, exocrine and ductal cell types from a single population of pancreatic precursors. These processes depend on many signals and factors that also control development of the central nervous system. In the latter, Eph receptors and their class-A (GPI-anchored) and class-B (transmembrane) ephrin ligands control cell migration and axon-pathfinding, help establish regional patterns and act as labels for cell positioning. This raised the question as to whether and where Ephs and ephrins are expressed during pancreas development. Here we have identified the Eph and ephrin genes that are expressed in mouse embryonic pancreas, as detected by RT-PCR analysis. In situ hybridization experiments showed that Ephs and ephrins are mainly expressed in the burgeoning structures of the epithelium which differentiate into exocrine acini. Binding experiments on whole pancreas demonstrated the presence of functional Eph receptors. They showed that EphBs are expressed by the pancreatic epithelium at embryonic day (e) 12.5 and that, from e14.5 on, Ephs of both classes are expressed by the pancreatic epithelium and then become restricted to developing acini. We conclude that specific members of the Eph/ephrin family are expressed in embryonic pancreas according to a dynamic temporal and regional pattern.

Silencing of EphA3 through a cis interaction with ephrinA5

EphAs and ephrinAs are expressed in multiple areas of the developing brain in overlapping countergradients, notably in the retina and tectum. Here they are involved in targeting retinal axons to their correct topographic position in the tectum. We have used truncated versions of EphA3, single-amino acid point mutants of ephrinA5 and fluorescence resonance energy transfer technology to uncover a cis interaction between EphA3 and ephrinA5 that is independent of the established ligand-binding domain of EphA3. This cis interaction abolishes the induction of tyrosine phosphorylation of EphA3 and results in a loss of sensitivity of retinal axons to ephrinAs in trans. Our data suggest that formation of this complex transforms the uniform expression of EphAs in the nasal part of the retina into a gradient of functional EphAs and has a key role in controlling retinotectal mapping.

Investigations on the inducible and endothelial nitric oxide synthases in human breast cancer cell line MCF-7 - estrogen has an influence on e-NOS, but not on i-NOS

As a model for hormone-dependent breast cancer, we studied the MCF-7 cell line to examine differences in the stimulation of the inducible (i) and endothelial (e) nitric oxide synthase (NOS) and the role of 17beta-estradiol (E(2)). MCF-7 cells were stimulated with (a) E(2) (10(-8)M) and (b) a combination of different cytokines such as interleukin-1 beta (Il-1beta), tumor necrosis factor alpha (TNF-alpha) and interferon gamma (INF-gamma), and lipopolysaccharide (LPS). e-NOS and i-NOS proteins were measured using Western blot analysis. Using the Griess method nitric oxide (NO) was estimated by assessing the stable product nitrite (NO(2-)) in the culture medium, and a direct method, employing EPR spin trapping also was used. Western blot analysis revealed the presence of e-NOS and i-NOS in MCF-7 cells. In Western blot analysis, e-NOS, but not i-NOS, expression could be stimulated by E(2). An increase in NO(2-) was noted after stimulation of MCF-7 using different combinations of cytokines Il-1beta, TNFalpha and INFgamma, and LPS, but not after E(2). In conclusion, e-NOS and i-NOS are weakly expressed in the MCF-7 cell line, but are stimulated differently. The MCF-7 cell may contain both a constitutive NOS and an inducible NOS.

Biphasic functions of the kinase-defective Ephb6 receptor in cell adhesion and migration

EphB6 is a unique member in the Eph family of receptor tyrosine kinases in that its kinase domain contains several alterations in conserved amino acids and is catalytically inactive. Although EphB6 is expressed both in a variety of embryonic and adult tissues, biological functions of this receptor are largely unknown. In the present study, we examined the function of EphB6 in cell adhesion and migration. We demonstrated that EphB6 exerted biphasic effects in response to different concentrations of the ephrin-B2 ligand; EphB6 promoted cell adhesion and migration when stimulated with low concentrations of ephrin-B2, whereas it induced repulsion and inhibited migration upon stimulation with high concentrations of ephrin-B2. A truncated EphB6 receptor lacking the cytoplasmic domain showed monophasic-positive effects on cell adhesion and migration, indicating that the cytoplasmic domain is essential for the negative effects. EphB6 is constitutively associated with the Src family kinase Fyn. High concentrations of ephrin-B2 induced tyrosine phosphorylation of EphB6 through an Src family kinase activity. These results indicate that EphB6 can both positively and negatively regulate cell adhesion and migration, and suggest that tyrosine phosphorylation of the receptor by an Src family kinase acts as the molecular switch for the functional transition.

Conditional inhibition of cancer cell proliferation by tetracycline-responsive, H1 promoter-driven silencing of PLK1

RNA interference (RNAi) is a powerful tool for studying gene function. We developed an inducible genetic element for short interfering RNA-mediated gene silencing. This system uses a tetracycline (Tet)-responsive derivative of the H1 promoter and the Tet repressor (TetR) for conditional expression of short hairpin RNA (shRNA) in HeLa cells. Promoter constructs were generated, which contain the Tet operator (TetO) derived from a prokaryotic Tet resistance transposon upstream and/or downstream of the TATA box. To quantify the response of controllable transcription units for shRNA expression, we examined the functional activity of polo-like kinase 1 (PLK1), a key component of mitotic progression, that is overexpressed in many human tumors. Cotransfection of plasmids for the expression of TetR and shRNA/PLK1 under the control of an H1 promoter-variant carrying TetO upstream of the TATA box did not alter PLK1 expression and proliferation properties of HeLa cells in the absence of doxycycline. Addition of the antibiotic led to marked downregulation of endogenous PLK1 accompanied by strong inhibition of cellular proliferation. Our data indicate that an inducible transcription system for shRNAs based on the human H1 promoter could be a versatile tool for controlled gene silencing in vitro.

Functions of ephrin/Eph interactions in the development of the nervous system: emphasis on the hippocampal system

Ephrins and their Eph receptors are membrane-anchored proteins that have key roles in the development of the Central Nervous System. The main characteristics of ephrin/Eph interactions are that their effect is mediated by cell-to-cell contacts and that they can propagate bidirectional signals downstream of the ligand-receptor complex. These characteristics make ephrins and Eph receptors critical cues in the regulation of migrating cells or axons, and in the establishment of tissue patterns and topographic maps in distinct regions of the developing brain. In addition, ephrins and Eph receptors regulate synapse formation and plasticity. These roles would be promoted by complementary gradual expression of receptors and ligands in the neurons involved. Although, historically, ephrins and Eph receptors have been considered as repulsion signals through barriers or gradients, new evidence indicates that they may be both inhibitory and permissive/active cues depending on expression levels. The expression of distinct ligands and receptors in the developing and mature hippocampus suggests that these proteins are involved in distinct processes during the development and maturation of the hippocampal region. In fact, recent studies have shown that ephrin/Eph signaling participates in the formation of the layer-specific patterns of hippocampal afferents, in synaptogenesis and in plasticity. Therefore, ephrin/Eph interactions should be considered a crucial system in the development and maturation of the brain regions, including the hippocampus.

EPHA2/EFNA1 expression in human gastric cancer

The erythropoietin-producing hepatocellular (EPH)A2 receptor, tyrosine kinase, is overexpressed and phosphorylated in several types of human tumors and has been associated with malignant transformation. A recent report, however, indicated that stimulation of the EPHA2 receptor ligand, ephrinA1 (EFNA1), inhibits the growth of EPHA2-expressing breast cancer. The authors examined the expression of EPHA2 and EFNA1 using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) in four gastric cancer cell lines and 49 primary gastric cancer samples, as well as in normal gastric tissue. EPHA2 was more highly expressed in tumor tissue than in normal tissue in 27 cases (55%). EFNA1 was overexpressed in tumor tissue in 28 cases (57%). No significant correlation was detected between the expression levels and histologic features such as tumor size, age, vessel invasion, or lymph node involvement. However, EPHA2 overexpression was more prominent in macroscopic type 3 and 4 tumors than in type 1 or 2 advanced gastric cancer. The authors observed EPHA2 expression in three of the four gastric cancer cell lines (AGS, KATO3, and MKN74) that were examined. In one cell line, TMK1, EPHA2 expression was barely detectable using northern blotting, RT-PCR, and western blotting. In contrast, EFNA1 was detected in all cell lines. In the gastric cancer cell lines that endogenously expressed EPHA2, stimulation with ephrinA1-Fc led to decreased EPHA2 protein expression and increased EPHA2 phosphorylation. Finally, the growth of EPHA2-expressing cells was inhibited by repetitive stimulation with soluble ephrinA1-Fc. Taken together, these findings suggest that EPHA2 and EFNA1 expression may influence the behavior of human gastric cancer.

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.

Silencing of the HER2/neu gene by siRNA inhibits proliferation and induces apoptosis in HER2/neu-overexpressing breast cancer cells

In eukaryotes, double-stranded (ds) RNA induces sequence-specific inhibition of gene expression referred to as RNA interference (RNAi). We exploited RNAi to define the role of HER2/neu in the neoplastic proliferation of human breast cancer cells. We transfected SK-BR-3, BT-474, MCF-7, and MDA-MB-468 breast cancer cells with short interfering RNA (siRNA) targeted against human HER2/neu and analyzed the specific inhibition of HER2/neu expression by Northern and Western blots. Transfection with HER2/neu-specific siRNA resulted in a sequence-specific decrease in HER2/neu mRNA and protein levels. Moreover, transfection with HER2/neu siRNA caused cell cycle arrest at G0/G1 in the breast cancer cell lines SK-BR-3 and BT-474, consistent with a powerful RNA silencing effect. siRNA treatment resulted in an antiproliferative and apoptotic response in cells overexpressing HER2/neu, but had no influence in cells with almost no expression of HER2/neu proteins like MDA-MB-468 cells. These data indicate that HER2/neu function is essential for the proliferation of HER2/neu-overexpressing breast cancer cells. Our observations suggest that siRNA targeted against human HER2/neu may be valuable tools as antiproliferative agents that display activity against neoplastic cells at very low doses.

Cell adhesion to ephrinb2 is induced by EphB4 independently of its kinase activity

Cell to cell interaction in bone marrow is crucial for differentiation of hematopoietic cells. We have shown that EphB4 receptor is expressed in erythroid progenitor and its activation accelerates erythroid differentiation. To elucidate the role of EphB4 activation in erythropoiesis, we analyzed effects of EphB4 on cell adhesive pathways. Cell adhesion with the extension of filopodial pseudopod was observed by EphB4 activation. EphB4 activation also enhanced an effect of fibronectin-mediated adhesive pathway along with formation of the c-Cbl/CrkL complex. The tyrosine kinase activity of EphB4 was dispensable for those phenomena. These results suggest that activation of EphB4 participates in adhesive but not repulsive signals independently of its tyrosine kinase activity in hematopoietic cells.

EphA receptor tyrosine kinases interact with co-expressed ephrin-A ligands in cis

Eph receptor tyrosine kinases have been implicated in various developmental processes, including axonal guidance, angiogenesis and morphogenesis. In general, Eph receptors and their ligands, ephrins, are reciprocally compartmentalized during embryogenesis. However, they are expressed in an overlapping fashion in some developing neural and non-neural tissues. Results from the overexpression or mutant mice of ephrin ligands in the retino-tectal system suggest that ephrin-As regulate co-expressed EphA receptor activity, but little is known about the molecular mechanisms. Here we show that EphA receptors and co-expressed ephrin-A ligands interact directly in cis via their functional binding domains, and that this interaction does not seem to mediate intracellular signals, but has an inhibitory effect on the trans interaction.

Abnormal hippocampal axon bundling in EphB receptor mutant mice

Axons travel frequently in bundles to reach their target. After arriving at the target, axon terminals defasciculate, migrate to topographically defined positions, and form synapses with appropriate target neurons. Here we present evidence that the B-type receptors of the erythropoietin-producing hepatocellular (Eph) family and a ligand, ephrin-B3, influence hippocampal axon defasciculation. The EphB receptors are expressed in the hippocampus, and the ligand, ephrin-B3, is transcribed in the lateral septum, the major subcortical target of hippocampal neurons. Ephrin-B3 promotes adhesion of hippocampal neurons to the ligand-expressing substrates in vitro, and the loss of the receptor EphB2 abrogates the effects of ephrin-B3. In mice deficient in EphB2 and EphB3, many hippocampal axons remain in bundles. This phenotype was also observed in mice that were specifically deleted for the cytoplasmic domain of EphB2. These observations indicate that the EphB receptors and their ligand regulate hippocampal axon defasciculation at the septal target, possibly through a receptor-mediated forward signaling mechanism.

Cell-cell signaling during synapse formation in the CNS

Synapses join individual nerve cells into a functional network. Specific cell-cell signaling events regulate synapse formation during development and thereby generate a highly reproducible connectivity pattern. The accuracy of this process is fundamental for normal brain function, and aberrant connectivity leads to nervous system disorders. However, despite the overall precision with which neuronal circuits are formed, individual synapses and synaptic networks are also plastic and can readily adapt to external stimuli or perturbations. In recent studies, several trans-synaptic signaling systems have been identified that can mediate various aspects of synaptic differentiation in the central nervous system. It appears that these individual pathways functionally cooperate, thereby generating robustness and flexibility, which ensure normal nervous system function.

Effect of RNA silencing of polo-like kinase-1 (PLK1) on apoptosis and spindle formation in human cancer cells

BACKGROUND

Expression of polo-like kinase-1 (PLK1), which has several functions in mitotic progression, is elevated in a broad range of human tumors. To investigate the role of PLK1 in neoplastic proliferation, we used the technique of RNA interference.

METHODS

Cells from several different cancer cell lines (MCF-7 breast cancer cells, HeLa S3 cervical cancer cells, SW-480 colon cancer cells, and A549 lung cancer cells) were transfected with small interfering (si) RNAs targeted against the human PLK1 or lamin genes. Northern and western blot analyses were used to examine PLK1 gene expression in transfected cancer cells and normal cells (human mammary epithelial cells [HMECs]). The phenotype, proliferation, and cell cycle distribution of cells transfected with siRNAs were also monitored by fluorescence microscopy and fluorescence-activated cell sorting analysis.

RESULTS

All cancer cell lines transfected with low doses of siRNAs targeted to PLK1 had greatly decreased levels of PLK1 mRNA and protein. siRNA4, which had the strongest inhibitory effect, reduced PLK1 mRNA in MCF-7 cells by 70% and PLK1 protein in MCF-7 cells by 95% 24 hours after transfection. Cell proliferation was reduced by between 66% and 99% 48 hours after transfection, and apoptosis was increased from 1%-5% to 13%-50% in transfected cells. Transfected SW-480 cells were mitotically arrested, and their centrosomes had lost the ability to nucleate microtubules. HMECs took up siRNAs less efficiently than cancer cells, and transfection with siRNAs targeted to PLK1 did not inhibit their proliferation.

CONCLUSIONS

PLK1 function appears to be essential for centrosome-mediated microtubule events and, consequently, for spindle assembly. siRNAs targeted against human PLK1 may be valuable tools as antiproliferative agents that display activity against a broad spectrum of neoplastic cells at very low doses.

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.

Ephrin-B ligands play a dual role in the control of neural crest cell migration

Little is known about the mechanisms that direct neural crest cells to the appropriate migratory pathways. Our aim was to determine how neural crest cells that are specified as neurons and glial cells only migrate ventrally and are prevented from migrating dorsolaterally into the skin, whereas neural crest cells specified as melanoblasts are directed into the dorsolateral pathway. Eph receptors and their ephrin ligands have been shown to be essential for migration of many cell types during embryonic development. Consequently, we asked if ephrin-B proteins participate in the guidance of melanoblasts along the dorsolateral pathway, and prevent early migratory neural crest cells from invading the dorsolateral pathway. Using Fc fusion proteins, we detected the expression of ephrin-B ligands in the dorsolateral pathway at the stage when neural crest cells are migrating ventrally. Furthermore, we show that ephrins block dorsolateral migration of early-migrating neural crest cells because when we disrupt the Eph-ephrin interactions by addition of soluble ephrin-B ligand to trunk explants, early neural crest cells migrate inappropriately into the dorsolateral pathway. Surprisingly, we discovered the ephrin-B ligands continue to be expressed along the dorsolateral pathway during melanoblast migration. RT-PCR analysis, in situ hybridisation, and cell surface-labelling of neural crest cell cultures demonstrate that melanoblasts express several EphB receptors. In adhesion assays, engagement of ephrin-B ligands to EphB receptors increases melanoblast attachment to fibronectin. Cell migration assays demonstrate that ephrin-B ligands stimulate the migration of melanoblasts. Furthermore, when Eph signalling is disrupted in vivo, melanoblasts are prevented from migrating dorsolaterally, suggesting ephrin-B ligands promote the dorsolateral migration of melanoblasts. Thus, transmembrane ephrins act as bifunctional guidance cues: they first repel early migratory neural crest cells from the dorsolateral path, and then later stimulate the migration of melanoblasts into this pathway. The mechanisms by which ephrins regulate repulsion or attraction in neural crest cells are unknown. One possibility is that the cellular response involves signalling to the actin cytoskeleton, potentially involving the activation of Cdc42/Rac family of GTPases. In support of this hypothesis, we show that adhesion of early migratory cells to an ephrin-B-derivatized substratum results in cell rounding and disruption of the actin cytoskeleton, whereas plating of melanoblasts on an ephrin-B substratum induces the formation of microspikes filled with F-actin.

Topographic mapping in dorsoventral axis of the Xenopus retinotectal system depends on signaling through ephrin-B ligands

Ephrin-B and EphB are distributed in matching dorsoventral gradients in the embryonic Xenopus visual system with retinal axons bearing high levels of ligand (dorsal) projecting to tectal regions with high receptor expression (ventral). In vitro stripe assays show that dorsal retinal axons prefer to grow on EphB receptor stripes supporting an attractive guidance mechanism. In vivo disruption of EphB/ephrin-B function by application of exogenous EphB or expression of dominant-negative ephrin-B ligand in dorsal retinal axons causes these axons to shift dorsally in the tectum, while misexpression of wild-type ephrin-B in ventral axons causes them to shift ventrally. These dorsoventral targeting errors are consistent with the hypothesis that an attractive mechanism that requires ephrin-B cytoplasmic domain is critical for retinotectal mapping in this axis.

Downregulation of human polo-like kinase activity by antisense oligonucleotides induces growth inhibition in cancer cells

A central role for polo-like kinases (PLK) in regulating several stages of mitotic progression has been born out in several species. Overexpression of PLK1 is observed in the majority of hitherto analysed human tumors. PLK1 overexpression is a negative prognostic factor in patients suffering from non-small cell lung cancer, head and neck tumors, esophageal carcinomas and melanomas. In order to define the role of PLK1 for mitotic progression of human cells and for neoplastic cell growth, phosphorothioate antisense oligonucleotides (ASOs) were tested to selectively downregulate PLK1 expression in MDA-MB-435 (breast cancer), HeLa S3 (cervical carcinoma) and A549 (non-small cell lung cancer) cells. ASOs were identified which suppress PLK1 mRNA and protein in a dose-dependent and sequence-specific manner. This approach also led to reduced PLK1 serine/threonine kinase activity. Downregulation of cellular PLK1 levels in cancer cells altered cell cycle progression moderately with an elevated percentage (20-30%) of cells in G(2)/M. Furthermore, cells with reduced PLK1 protein gained a rounded phenotype with multiple centrosomes. Moreover, ASO treatment resulted in potent antiproliferative effects in cell culture. Considerable antitumor activity was observed in vivo against A549 cells. This study suggests that antisense inhibitors targeted against PLK1 at well tolerated doses may be considered as a cancer therapeutic agent.

The kinase-null EphB6 receptor undergoes transphosphorylation in a complex with EphB1

Uniquely for the Eph family of receptor tyrosine kinases, the EphB6 receptor is catalytically inactive due to the alteration of several critical residues in its kinase domain. This has cast doubt upon its ability to participate in cytoplasmic signaling events. We show here that despite its lack of kinase activity, EphB6 undergoes inducible tyrosine phosphorylation upon stimulation with the Eph-B receptor subfamily ligand ephrin-B1. We also demonstrate, for the first time, evidence of cross-talk between Eph receptors. Overexpression of a catalytically active member of the Eph-B subfamily, EphB1, resulted in increased EphB6 phosphorylation. EphB1-induced EphB6 phosphorylation was ligand-dependent and required the functional catalytic activity of EphB1. EphB1 not only transphosphorylated EphB6, but together they also formed a stable hetero-complex. In addition, we identify the proto-oncogene c-Cbl as an EphB6-binding protein. Although EphB6-Cbl association appeared to be constitutive, Cbl required a functional phosphotyrosine binding domain in order to bind the receptor, whereas its RING finger motif ubiquitin-transfer domain was not necessary. Our findings demonstrate that EphB6 is an actively signaling receptor that undergoes transphosphorylation upon ligand binding and that can initiate specific cytoplasmic signaling events.

Signals from Eph and ephrin proteins: a developmental tool kit

Interactions between Eph receptors and their ligands the ephrin proteins are critically important in many key developmental processes. Emerging evidence also supports a role for these molecules in postembryonic tissues, particularly in pathological processes, including tissue injury and tumor metastasis. We review the signaling mechanisms that allow the 14 Eph and nine ephrin proteins to deliver intracellular signals that regulate cell shape and movement. What emerges is that the initiation of these signals is critically dependent on which Eph and ephrin proteins are expressed, the level of their expression, and, in some cases, which splice variants are expressed. Diversity at the level of initial interaction and in the downstream signaling processes regulated by Eph-ephrin signaling provides a subtle, versatile system of regulation of intercellular adhesion, cell shape, and cell motility.

Downregulation of the Ras-mitogen-activated protein kinase pathway by the EphB2 receptor tyrosine kinase is required for ephrin-induced neurite retraction

Activation of the EphB2 receptor tyrosine kinase by clustered ephrin-B1 induces growth cone collapse and neurite retraction in differentiated NG108 neuronal cells. We have investigated the cytoplasmic signaling events associated with EphB2-induced cytoskeletal reorganization in these neuronal cells. We find that unlike other receptor tyrosine kinases, EphB2 induces a pronounced downregulation of GTP-bound Ras and consequently of the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) pathway. A similar inhibition of the Ras-MAPK pathway was observed on stimulation of endogenous EphB2 in COS-1 cells. Inactivation of Ras, induced by ephrin B1 stimulation of NG108 neuronal cells, requires EphB2 tyrosine kinase activity and is blocked by a truncated form of p120-Ras GTPase-activating protein (p120-RasGAP), suggesting that EphB2 signals through the SH2 domain protein p120-RasGAP to inhibit the Ras-MAPK pathway. Suppression of Ras activity appears functionally important, since expression of a constitutively active variant of Ras impaired the ability of EphB2 to induce neurite retraction. In addition, EphB2 attenuated the elevation in ERK activation induced by attachment of NG108 cells to fibronectin, indicating that the EphB2 receptor can modulate integrin signaling to the Ras GTPase. These results suggest that a primary function of EphB2, a member of the most populous family of receptor tyrosine kinases, is to inactivate the Ras-MAPK pathway in a fashion that contributes to cytoskeletal reorganization and adhesion responses in neuronal growth cones.

Eph signalling functions downstream of Val to regulate cell sorting and boundary formation in the caudal hindbrain

Rhombomeres are segmental units of the developing vertebrate hindbrain that underlie the reiterated organisation of cranial neural crest migration and neuronal differentiation. valentino (val), a zebrafish homologue of the mouse bzip transcription factor-encoding gene, kreisler, is required for segment boundary formation caudal to rhombomere 4 (r4). val is normally expressed in r5/6 and is required for cells to contribute to this region. In val(−) mutants, rX, a region one rhombomere in length and of mixed identity, lies between r4 and r7. While a number of genes involved in establishing rhombomeric identity are known, it is still largely unclear how segmental integrity is established and boundaries are formed. Members of the Eph family of receptor tyrosine kinases and their ligands, the ephrins, are candidates for functioning in rhombomere boundary formation. Indeed, expression of the receptor ephB4a coincides with val in r5/6, whilst ephrin-B2a, which encodes a ligand for EphB4a, is expressed in r4 and r7, complementary to the domain of val expression. Here we show that in val(−) embryos, ephB4a expression is downregulated and ephrin-B2a expression is upregulated between r4 and r7, indicating that Val is normally required to establish the mutually exclusive expression domains of these two genes. We show that juxtaposition of ephB4a-expressing cells and ephrin-B2a-expressing cells in the hindbrain leads to boundary formation. Loss of the normal spatial regulation of eph/ephrin expression in val mutants correlates not only with absence of boundaries but also with the inability of mutant cells to contribute to wild-type r5/6. Using a genetic mosaic approach, we show that spatially inappropriate Eph signalling underlies the repulsion of val(−) cells from r5/6. We propose that Val controls eph expression and that interactions between EphB4a and Ephrin-B2a mediate cell sorting and boundary formation in the segmenting caudal hindbrain.

Ephrin-A6, a new ligand for EphA receptors in the developing visual system

In the embryonic visual system, EphA receptors are expressed on both temporal and nasal retinal ganglion cell axons. Only the temporal axons, however, are sensitive to the low concentrations of ephrin-A ligands found in the anterior optic tectum. The poor responsiveness of nasal axons to ephrin-A ligands, which allows them to traverse the anterior tectum and reach their targets in the posterior tectum, has been attributed to constitutive activation of the EphA4 receptor expressed in these axons. EphA4 is highly expressed throughout the retina, but is preferentially phosphorylated on tyrosine (activated) in nasal retina. In a screen for EphA4 ligands expressed in chicken embryonic retina, we have identified a novel ephrin, ephrin-A6. Like ephrin-A5, ephrin-A6 has high affinity for EphA4 and activates this receptor in cultured retinal cells. In the embryonic day 8 (E8) chicken visual system, ephrin-A6 is predominantly expressed in the nasal retina and ephrin-A5 in the posterior tectum. Thus, ephrin-A6 has the properties of a ligand that activates the EphA4 receptor in nasal retinal cells. Ephrin-A6 binds with high affinity to several other EphA receptors as well and causes growth cone collapse in retinal explants, demonstrating that it can elicit biological responses in retinal neurons. Ephrin-A6 expression is high at E6 and E8, when retinal axons grow to their tectal targets, and gradually declines at later developmental stages. The asymmetric distribution of ephrin-A6 in retinal cells, and the time course of its expression, suggest that this new ephrin plays a role in the establishment of visual system topography.

Profiling and verification of gene expression patterns in normal and malignant human prostate tissues by cDNA microarray analysis

cDNA microarray technology allows the "profiling" of gene expression patterns for virtually any cellular material. In this study, we applied cDNA microarray technology to profile changes in gene expression associated with human prostate tumorigenesis. RNA prepared from normal and malignant prostate tissue was examined for the expression levels of 588 human genes. Four different methods for data normalization were utilized. Of these, normalization to ACTB expression proved to be the most rigorous technique with the least probability of producing spurious results. After normalization to ACTB expression, 15 of 588 (2.6%) genes examined by array analysis were differentially expressed by a factory of 2x or more in malignant compared to normal prostate tissues. The expression patterns for 8 of 15 genes have been reported previously in prostate tissues (TGFbeta3, TGFBR3, IGFII, IGFBP2, VEGF, FGF7, ERBB3, MYC), but those of seven genes are reported here for the first time (MLH1, CYP1B1, RFC4, EPHB3, MGST1, BTEB2, MLP). These genes describe at least four metabolic and signaling pathways likely disrupted in human prostate tumorigenesis. Reverse transcriptase polymerase chain reaction (RT-PCR) and Northern blot analyses quantitated with reference to ACTB expression levels verified the trends in gene expression levels observed by array analysis for 14/15 and 8/8 genes, respectively. However, RT-PCR and Northern blot analyses accurately verified the "fold" differences in expression levels for only 6/15 (40%) and 7/8 (88%) of genes examined, respectively, demonstrating the need to better validate quantitative differences in gene expression revealed by array-based techniques.

Neurobiology. Turning attraction into repulsion

A series of molecular cues guide axons as they extend in the embryonic environment. In her Perspective, Pasquale discusses new findings that reveal how signals originating from adhesive contacts between axons and the cells they encounter can repel the axon, causing it to break the adhesive contacts and move away (Hattori et al. and Galko and Tessier-Lavigne).

Phosphorylation of tyrosine residues in the kinase domain and juxtamembrane region regulates the biological and catalytic activities of Eph receptors

Members of the Eph family of receptor tyrosine kinases exhibit a striking degree of amino acid homology, particularly notable in the kinase and membrane-proximal regions. A mutagenesis approach was taken to address the functions of specific conserved tyrosine residues within these catalytic and juxtamembrane domains. Ligand stimulation of wild-type EphB2 in neuronal NG108-15 cells resulted in an upregulation of catalytic activity and an increase in cellular tyrosine phosphorylation, accompanied by a retraction of neuritic processes. Tyrosine-to-phenylalanine substitutions within the conserved juxtamembrane motif abolished these responses. The mechanistic basis for these observations was examined using the highly related EphA4 receptor in a continuous coupled kinase assay. Tandem mass spectrometry experiments confirmed autophosphorylation of the two juxtamembrane tyrosine residues and also identified a tyrosine within the kinase domain activation segment as a phosphorylation site. Kinetic analysis revealed a decreased affinity for peptide substrate upon substitution of activation segment or juxtamembrane tyrosines. Together, our data suggest that the catalytic and therefore biological activities of Eph receptors are controlled by a two-component inhibitory mechanism, which is released by phosphorylation of the juxtamembrane and activation segment tyrosine residues.

Expression of Eph receptors and ephrins is differentially regulated by E-cadherin

E-cadherin is the main cell adhesion molecule of early embryonic and adult epithelial cells. Downregulation of E-cadherin is associated with epithelial-mesenchymal transition during embryonic mesoderm formation and tumor progression. To identify genes whose expression is affected by the loss of E-cadherin, we compared mRNA expression patterns between wild-type and E-cadherin null mutant embryonic stem (ES) cells. We found that expression of several Eph receptors and ephrins is dependent on E-cadherin. Rescue of E-cadherin null ES cells with E-cadherin cDNA restores the wild-type expression pattern of Eph family members. Rescue of E-cadherin null ES cells with N-cadherin cDNA does not restore the wild-type expression pattern, indicating that the regulation of differential expression of Eph family members is specific to E-cadherin. Constitutive ectopic expression of E-cadherin in non-epithelial NIH3T3 cells results in the production of the EphA2 receptor. In epithelial cells, E-cadherin is required for EphA2 receptor localization at cell-cell contacts; in the absence of functional E-cadherin, EphA2 localizes to the perinuclear region. Our results indicate that E-cadherin may be directly or indirectly required for the membrane localization of Eph receptors and their membrane-bound ligands.

Eph receptors and ephrins: regulators of guidance and assembly

Recent advances have started to elucidate the developmental functions and biochemistry of Eph receptor tyrosine kinases and their membrane-bound ligands, ephrins. Interactions between these molecules are promiscuous, but they largely fall into two groups: EphA receptors bind to GPI-anchored ephrin-A ligands, while EphB receptors bind to ephrin-B proteins that have a transmembrane and cytoplasmic domain. Remarkably, ephrin-B proteins transduce signals, such that bidirectional signaling can occur upon interaction with Eph receptor. In many tissues, specific Eph receptors and ephrins have complementary domains, whereas other family members may overlap in their expression. An important role of Eph receptors and ephrins is to mediate cell-contact-dependent repulsion. Complementary and overlapping gradients of expression underlie establishment of a topographic map of neuronal projections in the retinotectal system. Eph receptors and ephrins also act at boundaries to channel neuronal growth cones along specific pathways, restrict the migration of neural crest cells, and via bidirectional signaling prevent intermingling between hindbrain segments. Intriguingly, Eph receptors and ephrins can also trigger an adhesive response of endothelial cells and are required for the remodeling of blood vessels. Biochemical studies suggest that the extent of multimerization of Eph receptors modulates the cellular response and that the actin cytoskeleton is one major target of the intracellular pathways activated by Eph receptors. Eph receptors and ephrins have thus emerged as key regulators of the repulsion and adhesion of cells that underlie the establishment, maintenance, and remodeling of patterns of cellular organization.

Nck-interacting Ste20 kinase couples Eph receptors to c-Jun N-terminal kinase and integrin activation

The mammalian Ste20 kinase Nck-interacting kinase (NIK) specifically activates the c-Jun amino-terminal kinase (JNK) mitogen-activated protein kinase module. NIK also binds the SH3 domains of the SH2/SH3 adapter protein Nck. To determine whether Nck functions as an adapter to couple NIK to a receptor tyrosine kinase signaling pathway, we determined whether NIK is activated by Eph receptors (EphR). EphRs constitute the largest family of receptor tyrosine kinases (RTK), and members of this family play important roles in patterning of the nervous and vascular systems. In this report, we show that NIK kinase activity is specifically increased in cells stimulated by two EphRs, EphB1 and EphB2. EphB1 kinase activity and phosphorylation of a juxtamembrane tyrosine (Y594), conserved in all Eph receptors, are both critical for NIK activation by EphB1. Although pY594 in the EphB1R has previously been shown to bind the SH2 domain of Nck, we found that stimulation of EphB1 and EphB2 led predominantly to a complex between NIK/Nck, p62(dok), RasGAP, and an unidentified 145-kDa tyrosine-phosphorylated protein. Tyrosine-phosphorylated p62(dok) most probably binds directly to the SH2 domain of Nck and RasGAP and indirectly to NIK bound to the SH3 domain of Nck. We found that NIK activation is also critical for coupling EphB1R to biological responses that include the activation of integrins and JNK by EphB1. Taken together, these findings support a model in which the recruitment of the Ste20 kinase NIK to phosphotyrosine-containing proteins by Nck is an important proximal step in the signaling cascade downstream of EphRs.

Growth factor signaling pathways in vascular development

Recent research on the formation and maintenance of the vasculature in the embryo and in the adult has provided a greater understanding of the cellular signals involved in these processes. With this understanding comes the potential means of controlling vascularization in pathological situations such as tumorigenesis and wounding. For the purpose of this review, we will discuss the key receptor tyrosine kinases involved in vascular function and the molecules which relay signals downstream of receptor activation. The receptor tyrosine kinases discussed include the vascular endothelial cell growth factor receptors, Eph receptors, Tie1, and Tie2, all of which are expressed on vascular endothelial cells. We also discuss the roles of the platelet derived growth factor receptors which are expressed on vascular smooth muscle cells. While all of these receptor tyrosine kinases activate many similar effector molecules, some of the signals initiated appear to be distinct. This may explain, at least in part, how different receptor tyrosine kinases expressed in overlapping patterns on the developing vasculature, direct unique biological functions.

Topographic targeting and pathfinding errors of retinal axons following overexpression of ephrinA ligands on retinal ganglion cell axons

In the retinotectal projection, the Eph receptor tyrosine kinase ligands ephrinA2 and ephrinA5 are differentially expressed not only in the tectum, but also in a high-nasal-to-low-temporal pattern in the retina. Recently, we have shown that retrovirally driven overexpression of ephrinA2 on retinal axons leads to topographic targeting errors of temporal axons in that they overshoot their normal termination zones in the rostral tectum and project onto the mid- and caudal tectum. The behavior of nasal axons, however, was only marginally affected. Here, we show that overexpression of ephrinA5 affects the topographic targeting behavior of both temporal and nasal axons. These data reinforce the idea that differential ligand expression on retinal axons contributes to topographic targeting in the retinotectal projection. Additionally, we found that ectopic expression of ephrinA2 and ephrinA5 frequently leads to pathfinding errors at the chiasm, resulting in an increased stable ipsilateral projection.

Endothelial adhesion molecules in the development of the vascular tree: the garden of forking paths

In the past, year targeted null mutation studies have further supported the concept that endothelial cell-matrix and cell-cell adhesion is involved in the formation and maintenance of the network of branched tubes within the vascular tree. In addition, recent results derived from the closely related experimental system of branching tubulogenesis in epithelial cells may provide an appealing model for endothelial biology.

Surface densities of ephrin-B1 determine EphB1-coupled activation of cell attachment through alphavbeta3 and alpha5beta1 integrins

Receptors of the Eph family and their ligands (ephrins) mediate developmental vascular assembly and direct axonal guidance. Migrating cell processes identify appropriate targets within migratory fields based on topographically displayed ephrin gradients. Here, EphB1 regulated cell attachment by discriminating the density at which ephrin-B1 was displayed on a reconstituted surface. EphB1-ephrin-B1 engagement did not promote cell attachment through mechanical tethering, but did activate integrin-mediated attachment. In endothelial cells, attachment to RGD peptides or fibrinogen was mediated through alphavbeta3 integrin. EphB1 transfection conferred ephrin-B1-responsive activation of alpha5beta1 integrin-mediated cell attachment in human embryonic kidney cells. Activation-competent but signaling-defective EphB1 point mutants failed to stimulate ephrin-B1 dependent attachment. These findings lead us to propose that EphB1 functions as a 'ligand density sensor' to signal integrin-mediated cell-matrix attachment.

Requirement for EphA receptor signaling in the segregation of Xenopus third and fourth arch neural crest cells

We describe here the isolation of a full-length cDNA encoding a Xenopus orthologue of the mammalian EphA2 receptor tyrosine kinase and investigate its role in cranial neural crest migration. We show that the primary sites of Xenopus EphA2 expression are rhombomere 4 of the developing hindbrain, migratory cranial neural crest cells and mesoderm of the visceral arches. To interfere with EphA2 and related receptors during cranial neural crest migration, we took a dominant negative approach. Overexpression of kinase-deficient EphA2 receptor variants led to abnormal migration of cranial neural crest cells. Neural crest cells of the third arch were found to mismigrate posteriorly, resulting in the failure of third and fourth arch neural crest to separate into distinct streams. These defects could be rescued by expression of full-length EphA2 receptors. A comparison of the expression domains of EphA2-binding proteins mapped by receptor affinity probe (RAP) in situ staining with those for EphA2 receptors revealed co-expression of ligands and receptors in the visceral arch mesenchyme. Taken together, these results suggest that EphA receptors may mediate attractive or adhesive signals during migration of cranial neural crest cells.

PDZ-domain-mediated interaction of the Eph-related receptor tyrosine kinase EphB3 and the ras-binding protein AF6 depends on the kinase activity of the receptor

Eph-related receptor tyrosine kinases (RTKs) have been implicated in intercellular communication during embryonic development. To elucidate their signal transduction pathways, we applied the yeast two-hybrid system. We could demonstrate that the carboxyl termini of the Eph-related RTKs EphA7, EphB2, EphB3, EphB5, and EphB6 interact with the PDZ domain of the ras-binding protein AF6. A mutational analysis revealed that six C-terminal residues of the receptors are involved in binding to the PDZ domain of AF6 in a sequence-specific fashion. Moreover, this PDZ domain also interacts with C-terminal sequences derived from other transmembrane receptors such as neurexins and the Notch ligand Jagged. In contrast to the association of EphB3 to the PDZ domain of AF6, the interaction with full-length AF6 clearly depends on the kinase activity of EphB3, suggesting a regulated mechanism for the PDZ-domain-mediated interaction. These data gave rise to the idea that the binding of AF6 to EphB3 occurs in a cooperative fashion because of synergistic effects involving different epitopes of both proteins. Moreover, in NIH 3T3 and NG108 cells endogenous AF6 is phosphorylated specifically by EphB3 and EphB2 in a ligand-dependent fashion. Our observations add the PDZ domain to the group of conserved protein modules such as Src-homology-2 (SH2) and phosphotyrosine-binding (PTB) domains that regulate signal transduction through their ability to mediate the interaction with RTKs.

Signaling by Eph receptors and their ephrin ligands

What are the mechanisms by which Eph receptor tyrosine kinases guide axons and migrating cells? Recent results show that Eph receptors bind some of the well-known effector molecules that are implicated in the organization of the actin cytoskeleton; moreover, biochemical and genetic evidence suggests a signaling role for transmembrane ephrin ligands in axon pathfinding.

The Eph family receptors and ligands

The Eph family is the largest of all known tyrosine kinase receptor-ligand systems. They are expressed in distinct, but overlapping, spatial and temporal patterns during embryonic development and postnatal life, and function in a variety of morphogenic events. The best known function is their role in the guidance of migration of axons and cells in the nervous system through repulsive interactions. They may also play a role in angiogenesis, tissue patterning, and tumor formation.

Eph receptors discriminate specific ligand oligomers to determine alternative signaling complexes, attachment, and assembly responses

Eph family receptor tyrosine kinases (including EphA3, EphB4) direct pathfinding of neurons within migratory fields of cells expressing gradients of their membrane-bound ligands. Others (EphB1 and EphA2) direct vascular network assembly, affecting endothelial migration, capillary morphogenesis, and angiogenesis. To explore how ephrins could provide positional labels for cell targeting, we tested whether endogenous endothelial and P19 cell EphB1 (ELK) and EphB2 (Nuk) receptors discriminate between different oligomeric forms of an ephrin-B1/Fc fusion ligand. Receptor tyrosine phosphorylation was stimulated by both dimeric and clustered multimeric ephrin-B1, yet only ephrin-B1 multimers (tetramers) promoted endothelial capillary-like assembly, cell attachment, and the recruitment of low-molecular-weight phosphotyrosine phosphatase (LMW-PTP) to receptor complexes. Cell-cell contact among cells expressing both EphB1 and ephrin-B1 was required for EphB1 activation and recruitment of LMW-PTP to EphB1 complexes. The EphB1-binding site for LMW-PTP was mapped and shown to be required for tetrameric ephrin-B1 to recruit LMW-PTP and to promote attachment. Thus, distinct EphB1-signaling complexes are assembled and different cellular attachment responses are determined by a receptor switch mechanism responsive to distinct ephrin-B1 oligomers.