UniPR129 is a competitive small molecule Eph-ephrin antagonist blocking in vitro angiogenesis at low micromolar concentrations

BACKGROUND AND PURPOSE

The Eph receptor tyrosine kinases and their ephrin ligands are key players in tumorigenesis and many reports have correlated changes in their expression with a poor clinical prognosis in many solid tumours. Agents targeting the Eph-ephrin system might emerge as new tools useful for the inhibition of different components of cancer progression. Even if different classes of small molecules targeting Eph-ephrin interactions have been reported, their use is hampered by poor chemical stability and low potency. Stable and potent ligands are crucial to achieve robust pharmacological performance.

EXPERIMENTAL APPROACH

UniPR129 (the L-homo-Trp conjugate of lithocholic acid) was designed by means of computational methods, synthetized and tested for its ability to inhibit the interaction between the EphA2 receptor and the ephrin-A1 ligand in an elisa binding study. The ability of UniPR129 to disrupt EphA2-ephrin-A1 interaction was functionally evaluated in a prostate adenocarcinoma cell line and its anti-angiogenic effect was tested in vitro using cultures of HUVECs.

KEY RESULTS

UniPR129 disrupted EphA2-ephrin-A1 interaction with Ki = 370 nM in an elisa binding assay and with low micromolar potency in cellular functional assays, including inhibition of EphA2 activation, inhibition of PC3 cell rounding and disruption of in vitro angiogenesis, without cytotoxic effects.

CONCLUSIONS AND IMPLICATIONS

The discovery of UniPR129 represents not only a major advance in potency compared with the existing Eph-ephrin antagonists but also an improvement in terms of cytotoxicity, making this molecule a useful pharmacological tool and a promising lead compound.

EphA4 constitutes a population-specific guidance cue for motor neurons

Motor neurons in the ventral neural tube project axons specifically to their target muscles in the periphery. Although many of the transcription factors that specify motor neuron cell fates have been characterized, less is understood about the mechanisms that guide motor axons to their correct targets. We show that ectopic expression of EphA4 receptor tyrosine kinase alters the trajectories of a specific population of motor axons in the avian hindlimb. Most motor neurons in the medial portion of the lateral motor column (LMC) extend their axons aberrantly in the dorsal nerve trunk at the level of the crural plexus, in the presence of ectopic EphA4. This misrouting of motor axons is not accompanied by alterations in motor neuron identity, settling patterns in the neural tube, or the fasciculation of spinal nerves. However, ectopic EphA4 axons do make errors in pathway selection during sorting in the plexus at the base of the hindlimb. These results suggest that EphA4 in motor neurons acts as a population-specific guidance cue to control the dorsal trajectory of their axons in the hindlimb.

EphA4/ephrin-A5 interactions in muscle precursor cell migration in the avian forelimb

Limb muscles derive from muscle precursor cells that lie initially in the lateral portion of the somitic dermomyotome and subsequently migrate to their target limb regions, where muscle-specific gene transcription is initiated. Although several molecules that control the generation and delamination of muscle precursor cells have been identified, little is known about the mechanisms that guide muscle precursor cell migration in the limb. We have examined the distribution of members of the Eph family during muscle precursor cell development. The EphA4 receptor tyrosine kinase and its ligand, ephrin-A5, are expressed by muscle precursor cells and forelimb mesoderm in unique spatiotemporal patterns during the period when muscle precursors delaminate from the dermomyotome and migrate into the limb. To test the function of EphA4/ephrin-A5 interactions in muscle precursor migration, we used targeted in ovo electroporation to express ephrin-A5 ectopically specifically in the presumptive limb mesoderm. In the presence of ectopic ephrin-A5, Pax7-positive muscle precursor cells are significantly reduced in number in the proximal limb, compared with controls, and congregate abnormally near the lateral dermomyotome. In stripe assays, isolated muscle precursor cells avoid substrate-bound ephrin-A5 and this avoidance is abolished by addition of soluble ephrin-A5. These data suggest that ephrin-A5 normally restricts migrating, EphA4-positive muscle precursor cells to their appropriate territories in the forelimb, disallowing entry into abnormal embryonic regions.

In vivo tyrosine phosphorylation sites of activated ephrin-B1 and ephB2 from neural tissue

EphB2 is a receptor tyrosine kinase of the Eph family and ephrin-B1 is one of its transmembrane ligands. In the embryo, EphB2 and ephrin-B1 participate in neuronal axon guidance, neural crest cell migration, the formation of blood vessels, and the development of facial structures and the inner ear. Interestingly, EphB2 and ephrin-B1 can both signal through their cytoplasmic domains and become tyrosine-phosphorylated when bound to each other. Tyrosine phosphorylation regulates EphB2 signaling and likely also ephrin-B1 signaling. Embryonic retina is a tissue that highly expresses both ephrin-B1 and EphB2. Although the expression patterns of EphB2 and ephrin-B1 in the retina are different, they partially overlap, and both proteins are substantially tyrosine-phosphorylated. To understand the role of ephrin-B1 phosphorylation, we have identified three tyrosines of ephrin-B1 as in vivo phosphorylation sites in transfected 293 cells stimulated with soluble EphB2 by using mass spectrometry and site-directed mutagenesis. These tyrosines are also physiologically phosphorylated in the embryonic retina, although the extent of phosphorylation at each site may differ. Furthermore, many of the tyrosines of EphB2 previously identified as phosphorylation sites in 293 cells (Kalo, M. S., and Pasquale, E. B. (1999) Biochemistry 38, 14396-14408) are also phosphorylated in retinal tissue. Our data underline the complexity of ephrin-Eph bidirectional signaling by implicating many tyrosine phosphorylation sites of the ligand-receptor complex.

EphB/syndecan-2 signaling in dendritic spine morphogenesis

We previously reported that the cell surface proteoglycan syndecan-2 can induce dendritic spine formation in hippocampal neurons. We demonstrate here that the EphB2 receptor tyrosine kinase phosphorylates syndecan-2 and that this phosphorylation event is crucial for syndecan-2 clustering and spine formation. Syndecan-2 is tyrosine phosphorylated and forms a complex with EphB2 in mouse brain. Dominant-negative inhibition of endogenous EphB receptor activities blocks clustering of endogenous syndecan-2 and normal spine formation in cultured hippocampal neurons. This is the first evidence that Eph receptors play a physiological role in dendritic spine morphogenesis. Our observations suggest that spine morphogenesis is triggered by the activation of Eph receptors, which causes tyrosine phosphorylation of target molecules, such as syndecan-2, in presumptive spines.

Multiple signaling interactions of Abl and Arg kinases with the EphB2 receptor

The Eph family of receptor tyrosine kinases and the Abl family of non-receptor tyrosine kinases have both been implicated in tissue morphogenesis. They regulate the organization of the actin cytoskeleton in the developing nervous system and participate in signaling pathways involved in axon growth. Both Eph receptors and Abl are localized in the neuronal growth cone, suggesting that they play a role in axon pathfinding. Two-hybrid screens identified regions of Abl and Arg that bind to the EphB2 and EphA4 receptors, suggesting a novel signaling connection involving the two kinase families. The association of full-length Abl and Arg with EphB2 was confirmed by co-immunoprecipitation and found to involve several distinct protein interactions. The SH2 domains of Abl and Arg bind to tyrosine-phosphorylated motifs in the juxtamembrane region of EphB2. A second, phosphorylation-independent interaction with EphB2 involves non-conserved sequences in the C-terminal tails of Abl and Arg. A third interaction between Abl and EphB2 is probably mediated by an intermediary protein because it requires tyrosine phosphorylation of EphB2, but not the binding sites for the Abl SH2 domain. The connection between EphB2 and Abl/Arg appears to be reciprocal. Activated EphB2 causes tyrosine phosphorylation of Abl and Arg, and vice versa. Interestingly, treatment of COS cells and B35 neuronal-like cells with ephrin-B1 to activate endogenous EphB2 decreased the kinase activity of endogenous Abl. These data are consistent with the opposite effects that Eph receptors and Abl have on neurite ougrowth and suggest that Eph receptors and Abl family kinases have shared signaling activities.

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.

The ephrin-A1 ligand and its receptor, EphA2, are expressed during tumor neovascularization

Eph receptor tyrosine kinases and their ephrin ligands have been implicated in embryonic vascular development and in in vivo models of angiogenesis. Eph proteins may also regulate tumor neovascularization, but this role has not been previously investigated. To screen for Eph proteins expressed in tumor blood vessels, we used tumor xenografts grown in nude mice from MDA-MB-435 human breast cancer cells or KS1767 human Kaposi's sarcoma cells. By immunohistochemistry, the ephrin-A1 ligand and one of its receptors, EphA2, were detected throughout tumor vasculature. Double-labeling with anti-CD34 antibodies demonstrated that both ephrin-A1 and EphA2 were expressed in xenograft endothelial cells and also tumor cells. Furthermore, EphA2 was tyrosine-phosphorylated in the xenograft tumors, indicating that it was activated, presumably by interacting with ephrin-A1. Ephrin-A1 and EphA2 were also detected in both the vasculature and tumor cells of surgically removed human cancers. In an in vitro angiogenesis model, a dominant negative form of EphA2 inhibited capillary tube-like formation by human umbilical vein endothelial cells (HUVECs), demonstrating a requirement for EphA receptor signaling. These data suggest that ephrin-A1 and EphA2 play a role in human cancers, at least in part by influencing tumor neovascularization. Eph proteins may represent promising new targets for antiangiogenic cancer treatments.

Eph receptors and ephrin ligands: embryogenesis to tumorigenesis

Protein tyrosine kinase genes are the largest family of oncogenes. This is not surprising since tyrosine kinases are important components of signal transduction pathways that control cell shape, proliferation, differentiation, and migration. At 14 distinct members, the Eph kinases constitute the largest family of receptor tyrosine kinases. Although they have been most intensively studied for their roles in embryonic development, increasing evidence also implicates Eph family proteins in cancer. This review will address the recent progress in understanding the function of Eph receptors in normal development and how disregulation of these functions could promote tumorigenesis.

Developmental regulation of EphA4 expression in the chick auditory brainstem

The avian auditory brainstem nuclei nucleus magnocellularis (NM) and nucleus laminaris (NL) display highly precise patterns of neuronal connectivity. NM projects tonotopically to the dorsal dendrites of ipsilateral NL neurons and to the ventral dendrites of contralateral NL neurons. The precision of this binaural segregation is evident at the earliest developmental stage at which connections can be observed. We have begun to examine the possibility that Eph receptor tyrosine kinase signaling is involved in establishing these spatially segregated connections. The expression of the EphA4 tyrosine kinase was examined at several developmental stages. EphA4 is expressed in rhombomere 5, which contains progenitors for both NM and NL. In this rhombomere, the labeling becomes striped during the time that precursor cells migrate to the auditory anlage. At the precise time when NM-NL projections are forming, EphA4 expression in NL is asymmetric, with markedly higher expression in the dorsal NL neuropil than in the ventral neuropil, suggesting a possible role in guiding growing axons to the appropriate region. At later embryonic ages EphA4 expression is symmetric around NL, and is absent in NM. As auditory function matures, EphA4 expression decreases so that by 4 days after hatch no EphA4 antibody labeling is evident in the auditory brainstem nuclei.

Expression of EphA4, ephrin-A2 and ephrin-A5 during axon outgrowth to the hindlimb indicates potential roles in pathfinding

During neural development, spinal motor axons extend in a precise manner from the ventral portion of the developing spinal cord to innervate muscle targets in the limb. Although classical studies in avians have characterized the cellular interactions that influence motor axon pathfinding to the limb, less is known about the molecular mechanisms that mediate this developmental event. Here, we examine the spatiotemporal distributions of the EphA4 receptor tyrosine kinase (RTK) and its cognate ligands, ephrin-A2 and ephrin-A5, on motor neurons, their axons and their pathways to the avian hindlimb to determine whether these molecules may influence axonal projections. The expression patterns of EphA4, ephrin-A2 and ephrin-A5 mRNAs and proteins are highly complex and appear to exhibit some overlap during motor axon outgrowth and pathfinding to the hindlimb, reminiscent of the co-expression of Eph RTKs and ephrins in the retinotectal system. EphA4, similar to the carbohydrate moiety polysialic acid, strikingly marks the main dorsal, but not ventral, nerve trunk after axon sorting at the limb plexus region. Our results suggest that EphA4 RTK and its ligands may influence axon fasciculation and the sorting of axons at the limb plexus, contributing to the correct dorsoventral organization of nerve branches in the hindlimb.

Eph receptors and ephrins in the developing chick cerebellum: relationship to sagittal patterning and granule cell migration

Spatiotemporal expression patterns of six members of the Eph gene family (EphA4, EphA3, EphB2, ephrin-B1, ephrin-A2, and ephrin-A5) were characterized immunocytochemically at various stages of chick cerebellar development. EphA4 expression is observed in the cerebellar anlage as early as embryonic day 5 (E5) and continues in the posthatch cerebellum. During the early period of cerebellar development (E3-E8), complementarity is observed between EphA4 and ephrin-A5 expression within the cerebellar-isthmal region. By E8, differential expression of EphA4 in parasagittal Purkinje cell bands is evident, and the expression remains banded in the posthatch cerebellum. Banded expression of the ephrin-A5 ligand complements EphA4 expression during the middle period (E9-E15). During this period, ephrin-A2 and EphA3 are coexpressed in a banded pattern and with variable correlation to EphA4. Variability in the banding expression is observed for EphA4, EphA3, ephrin-A5, and ephrin-A2 across different lobes, and graded complementarity in the expression pattern of EphA3 and ephrin-A5 is observed in the external granular layer between the posterior and anterior lobes. Analysis of Purkinje cell birth date in correlation with Eph-ephrin expression during the middle period reveals that early-born cells express EphA4, whereas late-born cells express ephrin-A5. Finally, EphA4 expression domains are respected by migrating granule cell ribbons, which express both ephrin-B1 and EphB2. These expression patterns suggest multiple roles for the Eph-ephrin system in cerebellar development, including demarcation/enforcement of boundaries of the cerebellar anlage, formation/maintenance of Purkinje cell compartments, and restriction of the early phase of granule cell migration to ribbons.

Spinal motor axons and neural crest cells use different molecular guides for segmental migration through the rostral half-somite

The peripheral nervous system in vertebrates is composed of repeating metameric units of spinal nerves. During development, factors differentially expressed in a rostrocaudal pattern in the somites confine the movement of spinal motor axons and neural crest cells to the rostral half of the somitic sclerotome. The expression patterns of transmembrane ephrin-B ligands and interacting EphB receptors suggest that these proteins are likely candidates for coordinating the segmentation of spinal motor axons and neural crest cells. In vitro, ephrin-B1 has indeed been shown to repel axons extending from the rodent neural tube (Wang & Anderson, 1997). In avians, blocking interactions between EphB3 expressed by neural crest cells and ephrin-B1 localized to the caudal half of the somite in vivo resulted in loss of the rostrocaudal patterning of trunk neural crest migration (Krull et al., 1997). The role of ephrin-B1 in patterning spinal motor axon outgrowth in avian embryos was investigated. Ephrin-B1 protein was found to be expressed in the caudal half-sclerotome and in the dermomyotome at the appropriate time to interact with the EphB2 receptor expressed on spinal motor axons. Treatment of avian embryo explants with soluble ephrin-B1, however, did not perturb the segmental outgrowth of spinal motor axons through the rostral half-somite. In contrast, under the same treatment conditions with soluble ephrin-B1, neural crest cells migrated aberrantly through both rostral and caudal somite halves. These results indicate that the interaction between ephrin-B1 and EphB2 is not required for patterning spinal motor axon segmentation. Even though spinal motor axons traverse the same somitic pathway as neural crest cells, different molecular guidance mechanisms appear to influence their movement.

Replacing two conserved tyrosines of the EphB2 receptor with glutamic acid prevents binding of SH2 domains without abrogating kinase activity and biological responses

Eph receptor tyrosine kinases play key roles in pattern formation during embryonic development, but little is known about the mechanisms by which they elicit specific biological responses in cells. Here, we investigate the role of tyrosines 605 and 611 in the juxtamembrane region of EphB2, because they are conserved Eph receptor autophosphorylation sites and demonstrated binding sites for the SH2 domains of multiple signaling proteins. Mutation of tyrosines 605 and 611 to phenylalanine impaired EphB2 kinase activity, complicating analysis of their function as SH2 domain binding sites and their contribution to EphB2-mediated signaling. In contrast, mutation to the negatively charged glutamic acid disrupted SH2 domain binding without reducing EphB2 kinase activity. By using a panel of EphB2 mutants, we found that kinase activity is required for the changes in cell-matrix and cell - cell adhesion, cytoskeletal organization, and activation of mitogen-activated protein (MAP) kinases elicited by EphB2 in transiently transfected cells. Instead, the two juxtamembrane SH2 domain binding sites were dispensable for these effects. These results suggest that phosphorylation of tyrosines 605 and 611 is critical for EphB2-mediated cellular responses because it regulates EphB2 kinase activity.

An Eph receptor regulates integrin activity through R-Ras

The ability of integrins to mediate cell attachment to extracellular matrices and to blood proteins is regulated from inside the cell. Increased ligand-binding activity of integrins is critical for platelet aggregation upon blood clotting and for leukocyte extravasation to inflamed tissues. Decreased adhesion is thought to promote tumor cell invasion. R-Ras, a small intracellular GTPase, regulates the binding of integrins to their ligands outside the cell. Here we show that the Eph receptor tyrosine kinase, EphB2, can control integrin activity through R-Ras. Cells in which EphB2 is activated become poorly adherent to substrates coated with integrin ligands, and a tyrosine residue in the R-Ras effector domain is phosphorylated. The R-Ras phosphorylation and loss of cell adhesion are causally related, because forced expression of an R-Ras variant resistant to phosphorylation at the critical site made cells unresponsive to the anti-adhesive effect of EphB2. This is an unusual regulatory pathway among the small GTPases. Reduced adhesiveness induced through the Eph/R-Ras pathway may explain the repulsive effect of the Eph receptors in axonal pathfinding and may facilitate tumor cell invasion and angiogenesis.

A novel signaling intermediate, SHEP1, directly couples Eph receptors to R-Ras and Rap1A

The Eph family of receptor tyrosine kinases has been implicated in many developmental patterning processes, including cell segregation, cell migration, and axon guidance. The cellular components involved in the signaling pathways of the Eph receptors, however, are incompletely characterized. Using a yeast two-hybrid screen, we have identified a novel signaling intermediate, SHEP1 (SH2 domain-containing Eph receptor-binding protein 1), which is expressed in the embryonic and adult brain. SHEP1 contains an Src homology 2 domain that binds to a conserved tyrosine-phosphorylated motif in the juxtamembrane region of the EphB2 receptor and may itself be a target of EphB2 kinase activity, since it becomes heavily tyrosine-phosphorylated in cells expressing activated EphB2. SHEP1 also contains a domain similar to Ras guanine nucleotide exchange factor domains and binds to the GTPases R-Ras and Rap1A, but not Ha-Ras or RalA. Thus, SHEP1 directly links activated, tyrosine-phosphorylated Eph receptors to small Ras superfamily GTPases.

Multiple in vivo tyrosine phosphorylation sites in EphB receptors

Autophosphorylation regulates the function of receptor tyrosine kinases. To dissect the mechanism by which Eph receptors transmit signals, we have developed an approach using matrix-assisted laser desorption-ionization (MALDI) mass spectrometry to map systematically their in vivo tyrosine phosphorylation sites. With this approach, phosphorylated peptides from receptors digested with various endoproteinases were selectively isolated on immobilized anti-phosphotyrosine antibodies and analyzed directly by MALDI mass spectrometry. Multiple in vivo tyrosine phosphorylation sites were identified in the juxtamembrane region, kinase domain, and carboxy-terminal tail of EphB2 and EphB5, and found to be remarkably conserved between these EphB receptors. A number of these sites were also identified as in vitro autophosphorylation sites of EphB5 by phosphopeptide mapping using two-dimensional chromatography. Only two in vitro tyrosine phosphorylation sites had previously been directly identified for Eph receptors. Our data further indicate that in vivo EphB2 and EphB5 are also extensively phosphorylated on serine and threonine residues. Because phosphorylation at each site can affect receptor signaling properties, the multiple phosphorylation sites identified here for the EphB receptors suggest a complex regulation of their functions, presumably achieved by autophosphorylation as well as phosphorylation by other kinases. In addition, we show that MALDI mass spectrometry can be used to determine the binding sites for Src homology 2 (SH2) domains by identifying the EphB2 phosphopeptides that bind to the SH2 domain of the Src kinase.

Signal transfer by Eph receptors

The Eph receptors are a unique family of receptor tyrosine kinases that enforce cellular position in tissues through mainly repulsive signals generated upon cell-cell contact. Together, Eph receptors and their membrane-anchored ligands. the ephrins, are key molecules for establishing tissue organization through signaling pathways that control axonal projection, cell migration, and the maintenance of cellular boundaries. Through their SH2 (Src Homology 2) and PDZ (postsynaptic density protein, disks large, zona occludens) domains, several signaling molecules have been demonstrated to interact with the activated cytoplasmic domain of Eph receptors by using the yeast two-hybrid system and in vitro biochemical assays. Most proteins found to interact with Eph receptors are well-known regulators of cytoskeletal organization and cell adhesion, and also cell proliferation. Promoting growth, however, does not appear to be a primary role of Eph receptors. Explaining which signaling interactions identified for the Eph receptors have physiological significance, how Eph receptor signaling cascades are propagated, and characterizing the intrinsic signaling properties of the ephrins are all exciting questions currently being investigated.

Eph receptors and ephrins demarcate cerebellar lobules before and during their formation

The formation of the ten cerebellar lobules is an unsolved problem in brain development. We report a screen for the four subfamilies of Eph receptors and their ligands (ephrins) in developing mouse cerebellum, using soluble receptor-immunoglobulin and ligand-immunoglobulin fusion proteins, and antibodies against EphA and ephrin-B proteins. Our results identify Eph receptors and ephrins as the first molecules known to demarcate individual lobules during development. Staining for ephrin-A ligands is in lobule VIII as it forms, across the whole width of the cerebellum. Staining for three EphA receptors approximately coincides with presumptive lobules VI and/or VII before and just after birth, whereas a fourth EphA receptor (EphA4, which binds ligands of both subfamilies) has more widespread expression. Staining for EphB receptors is in lobules VII, VIII, and IX. Staining for ephrin-B ligands is much weaker, becomes detectable only after birth, and does not appear to be lobule-specific. Staining for all subfamilies spreads to at least some adjacent lobules as maturation proceeds. The lobule-specific patterns appear before the lobules form, and initially extend across the width of the cerebellum, in spite of the lesser conservation of the lateral extensions of the lobules. These expression patterns define previously unknown developmental units and suggest that Eph family proteins may contribute to cerebellar morphogenesis.

Complex formation between EphB2 and Src requires phosphorylation of tyrosine 611 in the EphB2 juxtamembrane region

The cellular components of the neuronal signaling pathways of Eph receptor tyrosine kinases are only beginning to be elucidated. Here we show that in vivo tyrosine phosphorylation sites of the Eph receptors EphA3, EphA4, and EphB2 in embryonic retina serve as binding sites for the Src-homology 2 (SH2) domain of Src kinase. Furthermore, tyrosine-phosphorylated EphB2 was detected in Src immunoprecipitates from transfected Cos cells, indicating that EphB2 and Src can physically associate. Interestingly, a form of Src with reduced electrophoretic mobility and increased tyrosine phosphorylation was detected in Cos cells expressing tyrosine-phosphorylated EphB2, suggesting a functional interaction between EphB2 and Src. Yeast two-hybrid analysis in conjunction with site-directed mutagenesis demonstrated that phosphorylated tyrosine 611 in the juxtamembrane region of EphB2 is crucial for the interaction with the SH2 domain of Src. In contrast, binding of the carboxy-terminal SH2 domain of phospholipase Cgamma was not abolished upon mutation of tyrosine 611 in EphB2. Phosphopeptide mapping of autophosphorylated full-length EphB2, and wild-type and tyrosine to phenylalanine mutants of the EphB2 cytoplasmic domain fused to LexA, showed tyrosine 611 in the sequence motif YEDP as a major site of autophosphorylation in EphB2. Our mutational analysis also indicated that tyrosines 605 and 611 are important for EphB2 kinase activity. We propose Src kinase as a downstream effector that mediates the neuron's response to Eph receptor activation.

Expression and tyrosine phosphorylation of Eph receptors suggest multiple mechanisms in patterning of the visual system

The EphA3 receptor tyrosine kinase has been implicated in guiding the axons of retinal ganglion cells as they extend in the optic tectum. A repulsive mechanism involving opposing gradients of the EphA3 receptor on retinal axons and its ligands, ephrin-A2 and ephrin-A5, in the tectum influences topographic mapping of the retinotectal projection. To investigate the overall role of the Eph family in patterning of the visual system, we have used in situ hybridization to localize nine Eph receptors in the chicken retina and optic tectum at Embryonic Day 8. Three of the receptors examined correspond to the novel chicken homologs of EphA2, EphA6, and EphA7. Unexpectedly, we found that many Eph receptors are expressed not only in retinal ganglion cells, but also in tectal cells, In particular, EphA3 mRNA is prominently expressed in the anterior tectum, with a pattern reciprocal to that of ephrin-A2 and ephrin-A5. Similarly, ephrin-A5 is expressed not only in tectal cells but also in the nasal retina, with a pattern reciprocal to that of its receptor EphA3 and partially overlapping with that of its other receptor EphA4. Consistent with the even distribution of EphA4 and the polarized distribution of EphA4 ligands in the retina, probing EphA4 immunoprecipitates from different sectors of the retina with anti-phosphotyrosine antibodies revealed spatial differences in receptor phosphorylation. These complex patterns of expression and tyrosine phosphorylation suggest that Eph receptors and ephrins contribute to establishing topography of retinal axons through multiple mechanisms, in addition to playing a role in intraretinal and intratectal organization.

The Eph family: a multitude of receptors that mediate cell recognition signals

The Eph receptor tyrosine kinases are emerging as molecules that guide the migration of cells and growth cones during embryonic development. Based on their concentration in embryonic regions containing growing neuronal processes, the Eph receptors were suspected early on to have a role in regulating aspects of axon growth. The most distinctive role of the Eph receptors appears to be their ability to mediate cell-cell repulsion through the binding of a ligand on an adjacent cell surface. The repulsive interactions are presumably mediated by transient receptor activation at the boundaries of complementary regions of high ligand or receptor expression. In contrast, overlapping expression patterns may regulate cell adhesion and cytoskeletal organization with possible consequences on the overall growth and fasciculation of neuronal processes. A notable feature of Eph receptor signaling is that, upon receptor binding, responses may also be elicited in the ligand-expressing cells. A better understanding of Eph receptor function requires the elucidation of their signaling properties. Recent evidence suggests a functional interaction between the Eph receptor EphB2 and neural cell adhesion molecules of the L1 family, which have well-recognized roles in the formation of neuronal projections. Only a few cytoplasmic signaling molecules that bind to the activated Eph receptors have been identified. Several of these molecules are known to transduce signals regulating cytoskeletal organization and neurite outgrowth. It is currently unclear why there is a need for fourteen distinct Eph receptor genes, many of which appear to encode several variant forms with distinct functional properties, but it is tempting to speculate that such diversity is necessary to refine the spatial organization of embryonic structures.

Immunolocalization of the receptor tyrosine kinase EphA4 in the adult rat central nervous system

EphA4 is a receptor tyrosine kinase of the Eph family previously designated Cek8 in chicken, Tyro1 in rat, and Sek1 in mouse, which is preferentially expressed in the embryonic and adult nervous system. We have mapped the distribution of EphA4 in the adult rat brain and spinal cord using a polyclonal antibody raised against a synthetic carboxy-terminal peptide. Immunoblotting experiments revealed that EphA4 is widely distributed in various regions of the adult rat brain. At the light microscopic level, intense immunoreactivity was apparent in the cerebral cortex, hippocampus, matrix compartment of the neostriatum, cholinergic neurons in the basal forebrain, cerebellar Purkinje cells, and substantia gelatinosa of the spinal cord. Among white matter tracts, EphA4 expression was detected in the corpus callosum, fornix, and posterior portion of the anterior commissure, but not in the lateral olfactory tract, mammillothalamic tract, or optic chiasm. Interestingly, expression in the optic chiasm is high at postnatal day 6, but decreases with the maturation of this structure. While in some regions of the neuropil neuronal cell bodies were prominently labeled, in others EphA4 immunoreactivity was detected in a punctate pattern. This punctuate staining did not coincide with synaptophysin localization. At the electron microscopic level, EphA4 immunoreactivity was observed in dendrites in the gray matter, particularly associated with dendritic spines, and in myelinated axons, but not their myelin sheaths in the white matter. The widespread distribution and diverse subcellular compartmentalization of EphA4 suggest that this receptor is important for the maintenance of multiple structures in the adult nervous system.

The Eph family of receptors

Eph receptor tyrosine kinases have recently been identified as instructive molecules that guide the topographic movement of cells and growth cones. The activation of Eph receptors by their ligands, which are membrane-anchored molecules, involves a cell-cell recognition event that often causes cell repulsion. Therefore, Eph receptors mediate signals that can override cell adhesion. Transmembrane ligands for Eph receptors also exhibit properties of signal transducing molecules, suggesting that bidirectional signaling occurs when receptor-expressing cells contact ligand-expressing cells.

Shared and distinct functions of RAGS and ELF-1 in guiding retinal axons

Two ligands for Eph-related receptor tyrosine kinases, RAGS and ELF-1, have been implicated in the control of development of the retinotectal projection. Both molecules are expressed in overlapping gradients in the tectum, the target area of retinal ganglion cell axons. In two in vitro assays ELF-1 is shown to have a repellent axon guidance function for temporal, but apparently not for nasal axons. RAGS on the other hand is repellent for both types of axons, though to different degrees. Thus, RAGS and ELF-1 share some and differ in other properties. The biological activities of these molecules correlate with the strength of interaction with their receptors expressed on RGC axons. The meaning of these findings for guidance of retinal axons in the tectum is discussed.

Tyrosine phosphorylation of L1 family adhesion molecules: implication of the Eph kinase Cek5

The L1 family comprises transmembrane cell adhesion molecules of the immunoglobulin superfamily that play an important role in neuronal migration and axon outgrowth, fasciculation, and myelination. Consistent with a crucial role in developmental processes, mutations in L1 cause severe brain malformations. Although L1 activates intracellular signaling pathways, little is known about the membrane proximal events of L1 signaling. The cytoplasmic domains of L1 family proteins contain several conserved tyrosine residues that are potential targets for receptor tyrosine kinases. Here, we report that the L1 family protein Ng-CAM is phosphorylated on tyrosine in embryonic day 13 chicken retina. This is the first demonstration of in vivo tyrosine phosphorylation of an L1-like molecule. Because chicken embryo kinase 5 (Cek5) is a receptor tyrosine kinase expressed in neuronal processes and activated in the chicken embryonic retina, we have analyzed the possible role of Cek5 in L1 phosphorylation. The rat glioblastoma cell line B28 was stably transfected with human L1. Additional transient transfection with Cek5 cDNA led to expression of Cek5 in its tyrosine-phosphorylated, activated form. Biochemical analysis revealed that L1 is phosphorylated on tyrosine in Cek5-transfected cells but not in control transfectants. Furthermore, direct phosphorylation of the L1 cytoplasmic domain by Cek5 was demonstrated in an in vitro kinase assay. Tyrosine phosphorylation may represent a novel mechanism of signal cascade initiation through L1.

Tyrosine phosphorylation of transmembrane ligands for Eph receptors

Axonal pathfinding in the nervous system is mediated in part by cell-to-cell signaling events involving members of the Eph receptor tyrosine kinase (RTK) family and their membrane-bound ligands. Genetic evidence suggests that transmembrane ligands may transduce signals in the developing embryo. The cytoplasmic domain of the transmembrane ligand Lerk2 became phosphorylated on tyrosine residues after contact with the Nuk/Cek5 receptor ectodomain, which suggests that Lerk2 has receptorlike intrinsic signaling potential. Moreover, Lerk2 is an in vivo substrate for the platelet-derived growth factor receptor, which suggests crosstalk between Lerk2 signaling and signaling cascades activated by tyrosine kinases. It is proposed that transmembrane ligands of Eph receptors act not only as conventional RTK ligands but also as receptorlike signaling molecules.

Reciprocal expression of the Eph receptor Cek5 and its ligand(s) in the early retina

Recent evidence suggests that Eph receptor tyrosine kinases and their ligands provide positional information in the developing visual system. We previously found that the Eph receptor Cek5 is more highly expressed in the ventral than dorsal chicken embryonic retina. We now report the identification of a chicken ligand for Cek5 (cCek5-L) that is 75% identical to the ligand LERK2. In situ hybridization experiments do not reveal a dorsoventral gradient of cCek5-L transcripts in the optic tectum at Embryonic Day 8, suggesting that this ligand is not involved in guiding Cek5-expressing axons in the tectum. Surprisingly, it is in the retina that high levels of cCek5-L mRNA are present. In the early retina, cCek5-L is more highly expressed in the dorsal than the ventral aspect. Similarly, a Cek5 Ig chimera labels dorsal but not ventral retina, indicating that even if several Cek5 ligands are present, their overall distribution is complementary to that of Cek5. Hence, Cek5 and cCek5-L may both contribute to define anatomical compartments within the early retina. In contrast, in the 11-day embryonic retina the distributions of Cek5 and its ligand(s) show considerable overlap, suggesting changing functions as development progresses. In dissociated cultures of dorsal or ventral retinal cells seeded on plates coated with either receptor or ligand Ig chimeras, the interaction between Cek5 and its ligand(s) or cCek5-L and its receptor(s) is sufficient to mediate cell adhesion and allows neurite outgrowth.

Developmental expression and distinctive tyrosine phosphorylation of the Eph-related receptor tyrosine kinase Cek9

Cek9 is a receptor tyrosine kinase of the Eph subfamily for which only a partial cDNA sequence was known (Sajjadi, F.G., and E.B. Pasquale. 1993. Oncogene. 8:1807-1813). We have obtained the entire cDNA sequence and identified a variant form of Cek9 that lacks a signal peptide. We subsequently examined the spatio-temporal expression and tyrosine phosphorylation of Cek9 in the chicken embryo by using specific antibodies. At embryonic day 2, Cek9 immunoreactivity is concentrated in the eye, the brain, the posterior region of the neural tube, and the most recently formed somites. Later in development, Cek9 expression is widespread but particularly prominent in neural tissues. In the developing visual system, Cek9 is highly concentrated in areas containing retinal ganglion cell axons, suggesting a role in regulating their outgrowth to the optic tectum. Unlike other Eph-related receptors, Cek9 is substantially phosphorylated on tyrosine in many tissues at various developmental stages. Since autophosphorylation of receptor protein-tyrosine kinases typically correlates with increased enzymatic activity, this suggests that Cek9 plays an active role in embryonic signal transduction pathways.

B61, a ligand for the Eck receptor protein-tyrosine kinase, exhibits neurotrophic activity in cultures of rat spinal cord neurons

Although the Eph subfamily represents the largest group of receptor protein-tyrosine kinases, the biological roles of the Eph-related receptors and their ligands are not well understood. B61 has been identified recently by receptor affinity chromatography as a ligand for the Eph-related receptor Eck (Bartley et al.: Nature 368:558-560, 1994). Here we show that Eck immunoreactivity is localized in areas of the embryonic rat spinal cord that are rich in axons, suggesting that Eck plays a role in this region of the developing nervous system. To examine the biological function of Eck, monolayer cultures of dissociated cells from embryonic rat spinal cord were treated with soluble B61. With an ED50 of approximately 10 ng/ml, B61 treatment improved the survival of the overall neuronal population. Furthermore, in the presence of B61 neurites were longer and more elaborated. B61 similarly affected survival and neurite length in cultures enriched in motor neurons. These neurotrophic effects of B61 were not observed in the presence of anti-Eck antibodies, indicating that these effects are likely to be mediated by the Eck receptor.

Genomic organization and alternatively processed forms of Cek5, a receptor protein-tyrosine kinase of the Eph subfamily

The genomic organization of Cek5, a receptor tyrosine kinase of the Eph subfamily, was elucidated utilizing a strategy involving PCR amplification of Cek5 genomic DNA. Cek5 is the first Eph-related kinase for which the exon-intron structure of the entire coding region has been determined. The Cek5 gene spans over 35 kb and comprises at least 16 exons. The exon-intron structure of Cek5 can be correlated with the proposed domain structure of the Eph subfamily, with the exception of an Ig motif in the extracellular domain. Intron positions in the Cek5 gene coincide with the locations of the deletions, substitutions, or insertions that have been described in a number of Eph-related kinases. This suggests that alternative processing plays a major role in generating the structural variability observed in the Eph subfamily. Consistent with this hypothesis, analysis of the Cek5 gene indicate that: (i) a variant form of Cek5 containing an insertion in the juxtamembrane region (Cek5 +) arises through the use of alternative 5' splice sites, and (ii) a soluble form of Cek5 comprising only the extracellular domain (Cek5s) may exists, which originates by alternative polyadenylation. RT-PCR analysis and RNase protection assays revealed the expression of both Cek5 + and Cek5s at various stages of chicken development.

Polarized expression of the receptor protein tyrosine kinase Cek5 in the developing avian visual system

Receptor protein tyrosine kinases of the Eph subfamily have been proposed to play roles in pattern formation based on their distribution during embryonic development. Cek5 (chicken embryo kinase 5) and Cek8 (chicken embryo kinase 8) are Eph-related kinases highly expressed in the chicken embryonic retina. To assess their potential roles in the development of the visual pathway, we examined their distribution by immunoperoxidase labeling. Cek8 is expressed throughout the pathway of the retinal ganglion cell axons, including the nerve fiber layer of the retina, optic nerve, optic chiasm, and stratum opticum of the tectum. Cek5 immunoreactivity is highly concentrated in only a portion of the optic nerve and optic chiasm, and in retinal cultures, Cek5 is detected in neurons. This prompted us to examine the regional distribution of Cek5 in the developing retina and led to the observation that Cek5 is most concentrated in the ventral aspect. RT-PCR established that the differential regulation of Cek5 expression in different portions of the retina occurs at the transcriptional level. Immunoblotting analysis revealed that this unusual expression pattern is distinctive for Cek5, as three other members of the Eph subfamily, Cek4, Cek8, and Cek9, are evenly expressed across the dorsal-ventral axis of the retina. Both Cek5 and Cek8 are distributed in manners which are consistent with their regulating the outgrowth of retinal ganglion cell axons to the tectum. Furthermore, Cek5 represents the first signal transduction molecule found to exhibit the polarized pattern of expression predicted for proteins that control the specificity of the retinotectal projections.

Membrane-bound LERK2 ligand can signal through three different Eph-related receptor tyrosine kinases

The Eph-related family of receptor tyrosine kinases consists of at least 13 members, several of which display distinctive expression patterns in the developing and adult nervous system. Recently, a small family of ligands, structurally related to the B61 protein, was identified. Binding of these ligands to Eph-related receptors did not, however, elicit measurable biological signals in cultured cells. In order to study functional interactions between B61-related ligands and Eph-related receptors, we constructed chimeric receptors, containing an Eph-related ectodomain and the cytoplasmic domain of the TrkB neurotrophin receptor. Expression and activation of such chimeric receptors in NIH 3T3 cells induced transformation in focus formation assays. Membrane-bound LERK2 ligand is shown to signal through three different Eph-related receptors, namely Cek5, Cek10 and Elk. LERK2, however, fails to interact functionally with the Cek9 receptor. Quantitative analysis including binding assays indicates that Cek10 is the preferred LERK2 receptor. Preliminary mutagenesis of the LERK2 protein suggests a negative regulatory role for its cytoplasmic domain in LERK2 signaling.

Characterization of the expression of the Cek8 receptor-type tyrosine kinase during development and in tumor cell lines

Cek8 is a receptor-type tyrosine kinase gene that was identified by screening a 10 day chicken embryo library with a DNA probe corresponding to the related kinase Cek4 (Sajjadi & Pasquale, 1993). Here we report the characterization of the Cek8 protein and its expression in embryonic tissues and tumor cell lines. The 120 kd Cek8 protein is detected early in embryogenesis, is developmentally regulated and preferentially, but not exclusively, expressed in neural tissues. In the stage 24 chick embryo, Cek8 immunoreactivity is prominent in the spinal cord and spinal nerves. At embryonic day 6, Cek8 expression becomes concentrated to the ventral portion of the spinal nerves, suggesting a role in axonogenesis of specific subsets of neurons. Cek8 is expressed in nearly all of the tumor cell lines examined, including cell lines derived from tumors of the central nervous system. Although the phosphorylation on tyrosine of Cek8 during development is moderate or undetectable, Cek8 is substantially phosphorylated on tyrosine (and thus presumably activated) in many of the transformed cell lines. Because of its high frequency of expression in tumor cell lines, Cek8 differs from previously investigated Eph-related kinases. However, as we show, Cek8 is not unique among the Eph-related kinases: another member of the Eph subclass, Cek5, has similar patterns of expression and phosphorylation in tumor cells. Based on its binding to a variety of lectin columns, Cek8 contains complex N-linked oligosaccharides. Cross-linking of Cek8 molecules on the cell surface with wheat germ agglutinin caused their rapid phosphorylation on tyrosine. Autophosphorylation on tyrosine is typically the first step in the activation of a receptor tyrosine kinase by a ligand.

cDNA cloning and characterization of a ligand for the Cek5 receptor protein-tyrosine kinase

We have isolated a murine cDNA encoding a ligand for the Cek5 receptor protein-tyrosine kinase (RPTK), a member of the Eph/Eck RPTK subfamily. Sequence analysis predicts an open reading frame of 345 amino acids with a predicted molecular mass of 38 kDa. Metabolic labeling and immunoprecipitation of cells transfected with a cDNA encoding the Cek5 ligand revealed the mature protein to have an apparent molecular mass of 45 kDa. The extracellular domain of the Cek5 ligand shows a 27% sequence identity at the protein level to B61, a ligand for the related Eck RPTK (Bartley, T. D., et al. (1994) Nature 368, 558-560). Consistent with the presence of a transmembrane domain, flow cytometry analysis revealed the Cek5 ligand to be expressed on the cell surface. The expressed Cek5 ligand is functionally active as it induces autophosphorylation of the Cek5 RPTK.

Altered localization and cytoplasmic domain-binding properties of tyrosine-phosphorylated beta 1 integrin

We describe a novel approach to study tyrosine-phosphorylated (PY) integrins in cells transformed by virally encoded tyrosine kinases. We have synthesized a peptide (PY beta 1 peptide) that represents a portion of the cytoplasmic domain of the beta 1 integrin subunit and is phosphorylated on the tyrosine residue known to be the target of oncogenic tyrosine kinases. Antibodies prepared against the PY beta 1 peptide, after removal of cross-reacting antibodies by absorption and affinity purification, recognized the PY beta 1 peptide and the tyrosine-phosphorylated form of the intact beta 1 subunit, but did not bind the nonphosphorylated beta 1 peptide, the nonphosphorylated beta 1 subunit or other unrelated tyrosine-phosphorylated proteins. The anti-PY beta 1 antibodies labeled the podosomes of Rous sarcoma virus-transformed fibroblasts, but did not detectably stain nontransformed fibroblasts. The localization of the tyrosine phosphorylated beta 1 subunits appeared distinct from that of the beta 1 subunit. Adhesion plaques were stained by the anti-beta 1 subunit antibodies in Rous sarcoma virus-transformed fibroblasts plated on fibronectin, whereas neither podosomes nor adhesion plaques were labeled on vitronectin or on uncoated plates. Anti-phosphotyrosine antibodies labeled podosomes, adhesion plaques and cell-cell boundaries regardless of the substratum. One of the SH2 domains of the p85 subunit of phosphatidylinositol-3-kinase bound to the PY beta 1 peptide, but not to the non-phosphorylated beta 1 cytoplasmic peptide. Other SH2 domains did not bind to the PY beta 1 peptide. These results show that the phosphorylated form of the beta 1 integrin subunit is detected in a different subcellular localization than the nonphosphorylated form and suggest that the phosphorylation on tyrosine of the beta 1 subunit cytoplasmic domain may affect cellular signaling pathways.

Cek5, a tyrosine kinase of the Eph subclass, is activated during neural retina differentiation

The expression of Cek5, a receptor-type tyrosine kinase of the Eph subclass, and its variant form Cek5+ were examined in the chick neural retina during development. Cek5 is present at high levels at all stages of retinal development examined, while Cek5+ is most abundant during differentiation. Cek5 mRNA expression and immunoreactivity are evenly distributed in the undifferentiated retina. With differentiation, Cek5 becomes concentrated in the inner and outer plexiform layers. While only moderate changes in Cek5 protein expression are observed throughout retinal development, Cek5 phosphorylation on tyrosine in vivo is dramatically increased during differentiation. This suggests that the Cek5 ligand is expressed at high levels and causes Cek5 activation. Thus, Cek5 is likely to play an active role in retinal morphogenesis, particularly during the establishment of interneuronal contacts.

The alpha v beta 3 integrin associates with a 190-kDa protein that is phosphorylated on tyrosine in response to platelet-derived growth factor

Integrins are cell surface heterodimers that mediate cell adhesion to the extracellular matrix. We show that in mouse 3T3 fibroblasts the alpha v beta 3 integrin (vitronectin receptor) coprecipitates with a tyrosine-phosphorylated 190-kDa protein, as detected by antibodies to phosphotyrosine. Three different antibodies to the vitronectin receptor, all of which precipitate the alpha/beta complex, coprecipitated a 190-kDa protein. The three antibodies were raised against the purified placental vitronectin receptor, the platelet alpha IIb beta 3 integrin, and the cytoplasmic domain of the alpha v subunit. The association was specific for the vitronectin receptor, since an antibody to the alpha 5 beta 1 integrin (fibronectin receptor) did not coprecipitate any tyrosine-phosphorylated protein. The phosphorylation of the 190-kDa protein was observed only following cell activation by platelet-derived growth factor, which is known to stimulate tyrosine kinase activity and to modulate cell adhesion. Antibodies raised against the platelet-derived growth factor alpha and beta receptors (M(r) = 170,000 and 190,000, respectively) did not recognize the 190-kDa, integrin-associated phosphorylated protein. Occupancy of the vitronectin receptor by one of its ligands, vitronectin, resulted in an increased amount of tyrosine phosphorylation of the 190-kDa protein. Our data suggest that the association of tyrosine-phosphorylated proteins with integrins may play an important role in growth factor-mediated modulation of cell adhesion.

Five novel avian Eph-related tyrosine kinases are differentially expressed

We have identified cDNA clones that encode five new avian receptor-like tyrosine kinases of the Eph subclass, by screening two chicken embryonic cDNA libraries with DNA probes. We have designated them Cek6 to Cek10. The identification of these kinases indicates that the Eph subclass comprises at least 10 members and, therefore, represents a very large family of receptor-like tyrosine kinases. Variants of Cek10 and of Cek5 (a previously identified Eph-related kinase) containing amino acid insertion sequences in the juxtamembrane domain were also isolated. The Cek5 variant is expressed in the brain, but not in other tissues of the 10-day chick embryo. Analysis of 10-day chick embryo mRNAs shows the newly identified tyrosine kinases to be all expressed in both the embryonic brain and body tissues. In adult tissues, they display distinct patterns of expression. Cek6, Cek7, Cek8, Cek9 and Cek10 are likely to play significant roles in embryonic signal transduction pathways, including those involved in neural development. Their distinct tissue distributions in the adult suggest that the different members of the Eph family may each serve specific functions.

Different members of the fibroblast growth factor receptor family are specific to distinct cell types in the developing chicken embryo

Single-stranded RNA probes for the three chicken fibroblast growth factor (FGF) receptors, cek-1, cek-2, and cek-3, in conjunction with in situ hybridization were used to characterize the distribution of the corresponding mRNAs in the developing chicken embryo. Cek-1 was expressed diffusely in most tissues examined, whereas the expression of cek-2 and cek-3 was more restricted. The highest levels of FGF receptor expression were seen in the developing bones; in skeletal, cardiac, and smooth muscle; and in some areas of the brain. Although all three receptors were expressed in a number of the same tissues, the expression of each receptor within a given tissue was generally specific for different cell types. In addition, the distribution of each of these receptors did not correlate with the previously characterized distributions of individual FGFs. These results suggest that the members of the FGF receptor family may represent cell-type-specific receptors rather than ligand-specific receptors. Thus, the interaction between a growth factor of the FGF family and a given FGF receptor is likely to be controlled to a large extent by spatial constraints, rather than exclusively by high binding affinities.

Cek5, a membrane receptor-type tyrosine kinase, is in neurons of the embryonic and postnatal avian brain

Cek5 is a recently identified receptor-type tyrosine kinase of the Eph subclass that is nearly ubiquitously expressed during embryonic development (Pasquale, 1991). Cek5 is predominantly expressed in the avian CNS throughout development, and high levels remain apparent in adult neurons. By means of immunofluorescence microscopy and high-resolution immunoelectron microscopy, Cek5 was found to be expressed in many regions of the chicken brain at various developmental stages, most notably in the hippocampus and cerebellum. The highest concentration of Cek5 was observed in the molecular layer of the cerebellum, associated within the axons of mature granule cells (parallel fibers) and with the cell bodies of immature granule cells. In the axons of parallel fibers, Cek5 was concentrated in the fasciculated nonsynaptic portions. This localization, together with the "adhesion" motifs present in the Cek5 extracellular region suggest that Cek5 may interact with other cell surface-associated molecules and be involved in the growth, guidance, and/or bundling of certain unmyelinated axonal processes. Alternatively (or in addition), Cek5 may represent the receptor for a neurotrophic substance, similar to several other neuronal transmembrane tyrosine kinases.

Identification of a new eph-related receptor tyrosine kinase gene from mouse and chicken that is developmentally regulated and encodes at least two forms of the receptor

We have identified cDNA clones that encode a new member of the Eph receptor tyrosine kinase subclass by using antibodies to phosphotyrosine to screen a chicken embryonic cDNA expression library. These cDNAs were in turn used to isolate the entire coding sequence of this receptor from both mouse and chicken (designated Mek4 and Cek4, respectively). A cDNA that encodes a putative secreted form of the murine receptor's ligand-binding domain and lacks the transmembrane and kinase domains was also isolated. Analysis of the appropriate Mek4 genomic region revealed the presence of sequences required for the production of the secreted form of the receptor. Analysis of RNAs from various tissues shows the receptor to be highly expressed in mouse and chicken embryos, with the greatest levels of expression occurring in the brain. In adult mouse, the pattern of RNA expression is altered and appears to be confined primarily to the brain. However, a shorter transcript was found to be expressed at reduced levels in the testis. This new Eph-related receptor may play an important role during development and in signal transduction pathways.

Identification of chicken embryo kinase 5, a developmentally regulated receptor-type tyrosine kinase of the Eph family

Chicken embryo kinase 5 (Cek5) is a transmembrane tyrosine kinase of the Eph family that was identified by screening a 10-d chicken embryo cDNA expression library with anti-phosphotyrosine antibodies. The extracellular region of Cek5 contains a cysteine rich N-terminal subdomain and a C-terminal subdomain mostly devoid of cysteines and comprising two repeats similar to fibronectin type III repeats. Immunoblotting experiments with anti-Cek5 polyclonal antibodies indicated that Cek5 is a membrane-associated 120-kDa protein containing intramolecular (but not intermolecular) disulfide bonds. Cek5 is already expressed in 2-d-old chicken embryos and is also expressed, at higher levels, later in development. In 10-d-old chicken embryos, Cek5 is expressed at substantial levels in nearly all the tissues examined, whereas in adult it is expressed predominantly in the brain. The expression of Cek5 in the brain gradually diminishes during embryonic development, whereas in the skeletal muscle of the thigh a sharp decrease in Cek5 expression was detected at the time of terminal muscle differentiation. Its wide tissue distribution throughout development and its sustained expression in adult brain suggest that Cek5 is an important component of signal transduction pathways, likely to interact with a widely distributed and important ligand, which is as yet unknown.

A distinctive family of embryonic protein-tyrosine kinase receptors

Two closely related protein-tyrosine kinases with the characteristics of growth factor receptors were identified by screening a chicken embryo cDNA expression library with anti-phosphotyrosine antibodies and were designated Cek2 and Cek3 (chicken embryo kinases 2 and 3). Cek2 and Cek3 are structurally related to Cek1, a chicken basic fibroblast growth factor receptor, and presumably represent receptors for basic fibroblast growth factor-related molecules. The identification of Cek2 and Cek3 establishes the existence of a family of protein-tyrosine kinases that includes Cek1 and that is likely to be implicated in the control of developmental processes. Among protein-tyrosine kinases, this family of receptors, which may include other as yet unknown members, is most closely related to the protooncogene product Ret and the platelet-derived growth factor receptor family.

Identification of a developmentally regulated protein-tyrosine kinase by using anti-phosphotyrosine antibodies to screen a cDNA expression library

To identify the protein-tyrosine kinases that are expressed during chicken embryonic development, a 10-day chicken embryo cDNA expression library was screened with anti-phosphotyrosine antibodies. Of the positive clones isolated, many encoded the same protein-tyrosine kinase, which we designate Cek1 (chicken embryo kinase 1). Its amino acid sequence suggests that the Cek1 protein is a transmembrane tyrosine kinase and presumably the receptor for an unknown ligand. Antibodies prepared to the cloned Cek1 kinase recognize, in immunoblotting experiments, two protein bands with apparent molecular weights of 100,000 and 110,000. The Cek1 protein was detected in many chicken embryonic tissues, but not in the corresponding adult tissues (with the exception of brain). The Cek1 kinase appears to be very closely related to two protein-tyrosine kinases whose partial sequences have been recently determined, human Flg and mouse Bek. Cloning using anti-phosphotyrosine antibodies has allowed us to detect, in addition to Cek1, several other protein-tyrosine kinases that are expressed during chicken embryonic development, some of which have not been previously identified.

Heat shock induces protein tyrosine phosphorylation in cultured cells

We examined the effect of heat shock on protein tyrosine phosphorylation in cultured animal cells using antiphosphotyrosine antibodies in immunoblotting and immunofluorescence microscopy experiments. Heat shock significantly elevated the level of phosphotyrosine in proteins in most of the cultured cells examined, including fibroblasts, epithelial cells, nerve cells, and muscle cells, but not in Rous sarcoma virus-transformed fibroblasts. The increase in protein tyrosine phosphorylation induced by heat shock occurred in proteins with a wide range of molecular masses and was dependent on the temperature and duration of the heat shock.

Comparative study of the tyrosine phosphorylation of proteins in Swiss 3T3 fibroblasts stimulated by a variety of mitogenic agents

We have carried out a comparative study of the protein tyrosine phosphorylation induced by a wide range of mitogenic stimuli on a single cell type, Swiss 3T3 mouse fibroblasts. For this purpose we have used high-affinity antibodies directed to phosphotyrosine residues on proteins (Wang: Mol. Cell. Biol. 5:3640-3643, 1985) in immunoblotting and immunofluorescence microscopy experiments. Immunoblotting experiments showed that all of the mitogens tested, including epidermal growth factor, platelet-derived growth factor, basic fibroblast growth factor, insulin, fetal calf serum, trypsin, and 12-O-tetradecanoylphorbol-13-acetate, increased the phosphorylation on tyrosine of a number of proteins. Most of the increase in tyrosine phosphorylation induced by each factor involved a small set of proteins with apparent molecular weights (Mr) above 50,000. Following stimulation with epidermal growth factor, platelet-derived growth factor, and basic fibroblast growth factor, increased phosphotyrosine modification of proteins with molecular weights corresponding to those of the respective receptors was observed. A protein band of apparent Mr 160,000 contained substantially increased levels of phosphotyrosine following insulin treatment, but tyrosine phosphorylation of the insulin receptor was apparently below the level of detectability. The phosphotyrosine content of proteins with apparent Mr of 220,000, 120,000, and 70,000 was increased by all the agents tested. Phosphorylation on tyrosine of most of the proteins increased within a few minutes of the mitogenic stimulation, reached a peak, and returned more slowly to basal levels. Immunofluorescence labeling with the antibodies specific for phosphotyrosine showed a substantial increase in the amount of phosphotyrosine containing proteins only in the presence of platelet-derived growth factor and fetal calf serum. This finding suggests that most of the proteins phosphorylated on tyrosine in Swiss 3T3 fibroblasts are not concentrated in specific subcellular structures, but rather are diffusely distributed throughout the cell and are therefore not detectable by immunofluorescence microscopy.