The EphA4 and EphB1 receptor tyrosine kinases and ephrin-B2 ligand regulate targeted migration of branchial neural crest cells

BACKGROUND During vertebrate head development, neural crest cells migrate from hindbrain segments to specific branchial arches, where they differentiate into distinct patterns of skeletal structures. The rostrocaudal identity of branchial neural crest cells appears to be specified prior to migration, so it is important that they are targeted to the correct destination. In Xenopus embryos, branchial neural crest cells segregate into four streams that are adjacent during early stages of migration. It is not known what restricts the intermingling of these migrating cell populations and targets them to specific branchial arches. Here, we investigated the role of Eph receptors and ephrins-mediators of cell-contact-dependent interactions that have been implicated in neuronal pathfinding-in this targeted migration. RESULTS Xenopus EphA4 and EphB1 are expressed in migrating neural crest cells and mesoderm of the third arch, and third plus fourth arches, respectively. The ephrin-B2 ligand, which interacts with these receptors, is expressed in the adjacent second arch neural crest and mesoderm. Using truncated receptors, we show that the inhibition of EphA4/EphB1 function leads to abnormal migration of third arch neural crest cells into second and fourth arch territories. Furthermore, ectopic activation of these receptors by overexpression of ephrin-B2 leads to scattering of third arch neural crest cells into adjacent regions. Similar disruptions occur when the expression of ephrin-B2 or truncated receptors is targeted to the neural crest. CONCLUSIONS These data indicate that the complementary expression of EphA4/EphB1 receptors and ephrin-B2 is involved in restricting the intermingling of third and second arch neural crest and in targeting third arch neural crest to the correct destination. Together with previous work showing that Eph receptors and ligands mediate neuronal growth cone repulsion, our findings suggest that similar mechanisms are used for neural crest and axon pathfinding.