03-P016 Ephrin–Eph signaling mediates tissue separation between involuting mesoderm and noninvoluting ectoderm, that is critical for Xenopus gastrulaiton movement

Here we show that Ephrin–Eph signaling regulates Xenopus gastrulation via mediating tissue separation. XEph receptor is exclusively expressed in the involuting mesoderm, whereas XEphrin ligand is localized in the ectoderm region during Xenopus gastrulaiton. At the beginning of gastrulation, XEph expressing mesoderm is around blastopore lip, and XEphrin ligand expressing ecotoderm is placed right above of mesoderm tissue. As the gastrulation proceeds, mesoderm involutes and then converges and extends along the A-P axis, whereas outer ectoderm moves down by epiboly. As a result, XEph expressing mesoderm and XEphrin ligand expressing ectoderm confront each other, then generate the interface between them by tissue separation. Loss-of-XEph-function studies show gastrulation movement defect phenotypes generated by the deficiency of tissue separation. However, there are no effects on mesoderm fate or DMZ elongation. We also demonstrate that XEph coordinates cell polarity and migratory activity of the involuting mesoderm thus selectively regulates convergence of DMZ tissues. XEph functions in a cooperative manner with XPAPC and XFz7 which have been reported for functioning in tissue separation. The deficiency of Brachet’s cleft formation induced by XEph-MO is rescued only by RhoA, but not by any of XFz7 downstream components, such as Dsh and PKCa. We further show that XEph modulates RhoA activity whose activity is required for normal tissue separation. In this study, we present novel evidences that Ephrin–Eph signaling is required in the tissue separation during Xenopus gastrulation. Despite XEph and XFz7 dependent PCP signaling share the common downstream component, RhoA, they are not redundant.