Comparative analysis of embryonic gene expression defines potential interaction sites for Xenopus EphB4 receptors with ephrin-B ligands.

The Eph family of receptor tyrosine kinases and their ligands, the ephrins, act as signaling molecules regulating the migratory behavior of neurons and neural crest cells, and are implicated in tissue patterning, blood vessel formation, and tumorigenesis. On the basis of structural similarities and overlapping binding specificities, Eph receptors as well as their ligands can be divided into A and B subfamilies with orthologues found in all vertebrates. We describe here the isolation of cDNAs encoding Xenopus EphB4 receptors and show that embryonic expression is prominently associated with the developing vasculature, newly forming somites, the visceral arches, and non-neuronal tissues of the embryonic head. In a screen to identify potential ligands for EphB4 in Xenopus embryos, we isolated cDNAs for the Xenopus ephrin-B2 and -B3, which demonstrates that the Xenopus genome harbors genes encoding orthologues to all three currently known mammalian ephrin-B genes. We next performed in situ hybridizations to identify tissues and organs where EphB4 receptors may encounter ephrin-B ligands during embryonic development. Our analysis revealed distinct, but overlapping patterns of ephrin-B gene expression. Interestingly, each ephrin-B ligand displayed expression domains either adjacent to or within EphB4-expressing tissues. These findings indicate that EphB4 receptors may interact in vivo with multiple B-class ephrins. The expression patterns also suggest that EphB4 receptors and their ligands may be involved in visceral arch formation, somitogenesis, and blood vessel development.