The receptor tyrosine kinase EphB4 and ephrin-B ligands restrict angiogenic growth of embryonic veins in Xenopus laevis

The cues and signaling systems that guide the formation of embryonic blood vessels in tissues and organs are poorly understood, Members of the Eph fam ily of receptor tyrosine kinases and their cell membrane-anchored ligands, the ephrins, have been assigned important roles in the control of cell migr ation during embryogenesis, particularly in axon guidance and neural crest migration, Here we investigated the role of EphB receptors and their ligand s during embryonic blood vessel development in Xenopus laevis. In a survey of tadpole-stage Xenopus embryos for EphB receptor expression, we detected expression of EphB4 receptors in the posterior cardinal veins and their der ivatives, the intersomitic veins, Vascular expression of other EphB recepto rs, including EphB1, EphB2 or EphB3, could however not be observed, suggest ing that EphB3 is the principal EphB receptor of the early embryonic vascul ature of Xenopus. Furthermore, we found that ephrin-B ligands are expressed complementary to EphB4 in the somites adjacent to the migratory pathways t aken by intersomitic veins during angiogenic growth. We performed RNA injec tion experiments to study the function of EphB4 and its ligands in intersom itic vein development, Disruption of EphB4 signaling by dominant negative E phB4 receptors or misexpression of ephrin-B ligands in Xenopus embryos resu lted in intersomitic veins growing abnormally into the adjacent semitic tis sue, Our findings demonstrate that EphB4 and B-class ephrins act as regulat ors of angiogenesis possibly by mediating repulsive guidance cues to migrat ing endothelial cells.