EphB receptors couple dendritic filopodia motility to synapse formation.

Motile dendritic filopodial processes are thought to be precursors of spine synapses, but how motility relates to cell-surface cues required for axon-dendrite recognition and synaptogenesis remains unclear. We demonstrate with dynamic imaging that loss of EphBs results in reduced motility of filopodia in cultured cortical neurons and brain slice. EphB knockdown and rescue experiments during different developmental time windows show that EphBs are required for synaptogenesis only when filopodia are most abundant and motile. In the context of EphB knockdown and reduced filopodia motility, independent rescue of either motility with PAK or of Eph-ephrin binding with an EphB2 kinase mutant is not sufficient to restore synapse formation. Strikingly, the combination of PAK and kinase-inactive EphB2 rescues synaptogenesis. Deletion of the ephrin-binding domain from EphB2 precludes rescue, indicating that both motility and trans-cellular interactions are required. Our findings provide a mechanistic link between dendritic filopodia motility and synapse differentiation.

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.

Eph/ephrin A- and B-family expression patterns in the leopard frog (Rana utricularia).

Eph/ephrin expression was studied in Rana utricularia larvae and adults with in situ receptor and ligand affinity probes. From stages TK-II (early limb bud) to VI (early foot paddle larva), tectal EphB expression is highest in a band extending transversely across the posterior optic tectum and grades off anteriorly and posteriorly. The ephrin-A expression gradient is parallel to the EphB gradient rather than being orthogonal to it. However, its high point occupies the posterior pole, and it runs from high-posteriorly to low-anteriorly. Tectal EphA expression is high anteriorly and low posteriorly, while ephrin-Bs are expressed only in a thin line at the dorsal midline. At later stages and in adults, tectal EphB expression becomes uniform.

Characterization of a novel Eph receptor tyrosine kinase, EphA10, expressed in testis.

In mammals, 14 members of the Eph receptor tyrosine kinase family have been described so far. Here we present a not yet described member of this family denoted EphA10. We report the identification of three putative EphA10 isoforms: one soluble and two transmembrane isoforms. One of the latter isoforms lacked the sterile alpha motif commonly found in Eph receptors. The gene encoding EphA10 is located on chromosome 1p34 and expression studies show that EphA10 mRNA is mainly expressed in testis. Binding studies to ephrin ligands suggests that this receptor belongs to the EphA subclass of Eph receptors binding mainly to ephrin-A ligands.

[Ephrins, neuronal development and plasticity]. / Efrinas, desarrollo neuronal y plasticidad.

AIMS: In this work we review the main characteristics of ephrins and their Eph receptors (ER), as well as descriptions that have been published to date of the different functions the ephrin/Eph system (EES) performs in neuronal development. DEVELOPMENT: ER constitute the largest group of tyrosine kinase receptors and are found in many different types of cells during development and in mature tissues. Their ligands, the ephrins, are membrane anchored proteins that are divided into class A ephrins, with a glycosylphosphatidylinositol bond, and class B ephrins, with a hydrophobic transmembrane region and a cytoplasmic domain. The EES is the only one that involves bidirectional signalling. Thus, the ephrin Eph interaction both activates the tyrosine kinase domain of the ER, with the resulting signal transduction in the cell that expresses Eph, and produces a reverse signal in the cells that contain the ligands. Over the last decade a number of studies have been conducted that establish the involvement of the EES in neuronal development. Although the classic function of this system is that of establishing patterns of both cellular and axonal organisation, recent reports describe how the ER and their ephrin ligands regulate synaptogenesis and the maturation of terminals during development, as well as the plasticity of the adult brain. CONCLUSIONS: Recent findings open up new expectations regarding the possible functions carried out by the interaction of ephrin and Eph. They also confirm the crucial role played by this system in all the processes involved in allowing neuronal development to take place in a correct fashion.

Analysis of EphB receptors and their ligands in the developing retinocollicular system of the wallaby reveals dynamic patterns of expression in the retina.

The expression of EphB1 and B2 receptors and ephrins-B1, -B2 and -B3 in the retina and superior colliculus of the wallaby (Macropus eugenii) was examined during the development of the retinocollicular projection, using reverse transcription-polymerase chain reaction and immunohistochemistry. There was an early transient differential expression of EphB2 that was higher in ventral retina and restricted to the outer neuroblast layer, whereas a high ventral to low dorsal gradient of ephrin-B2 expression occurred there throughout the study period. However, there was no dorsoventral gradient of receptors or ligands in retinal ganglion cells or a mediolateral gradient of ephrins in the colliculus. These findings suggest a limited role for these molecules in topographic mapping across the mediolateral colliculus in the wallaby. Early in retinal development there is a complementary pattern of expression of ephrin-B1 and -B2 in the outer neuroblast layer that overlaps with expression of EphB2. Ganglion and amacrine cells also express EphB2. As development proceeds subpopulations of putative horizontal and bipolar cells, also expressing EphB2, come to reside in the inner nuclear layer and ephrin-B1 is expressed throughout the outer nuclear layer. At the same time cells expressing ephrin-B2, and subpopulations of horizontal and bipolar cells come to reside in the inner nuclear layer and there is a corresponding decrease in ephrin-B2 expression in the outer nuclear layer. This pattern of coexpression of receptors and ligands suggests a role for them in cell migration and maintenance of laminar boundaries.

mRNA expression of ephrins and Eph receptor tyrosine kinases in the neonatal and adult mouse central nervous system.

Ephrins and Eph receptors are a family of molecules that have been implicated in axonal pathfinding. A unique feature of B-class ephrins and Eph receptors is their ability to transmit bidirectional signals in both ephrin- and Eph receptor-expressing cells upon cell-cell contact. These signals can lead to cytoskeletal alterations that have been attributed to regulating neuronal growth responses. Examination of gene-target knockout mice has supported this hypothesis, revealing numerous developmental defects in the nervous systems of mice mutant for both B-class ephrins and Eph receptors. To examine the potential scope of action for these genes in the nervous system, we have used in situ hybridization to study the mRNA expression of ephrins (B1, B2, and B3) and Eph receptors (B1, B2, B3, A4) in neonatal and adult mice. We found ephrins and Eph receptors to be expressed throughout the CNS. Expression was observed in the epithelium and migratory regions of the neonate and adult tissues as well as in discrete regions of high plasticity, including the adult olfactory bulb, hippocampus, and cerebellum. These studies suggest additional potential roles for these molecules in the postnatal and adult CNS and will serve as a guide in the detailed evaluation of mutant mice.