Semaphorin and Eph Receptor Signaling Guide a Series of Cell Movements for Ventral Enclosure in C. elegans

BACKGROUND In the last stage of the Caenorhabditis elegans body wall closure, an open pocket in the epidermis is closed by the migration of marginal epidermal P/pocket cells to the ventral midline. The cellular and molecular mechanisms of this closure remain unknown. RESULTS Cells within the pocket align to form a bridge for migration of contralateral P cell pair P9/10 L,R (and neighboring P cells) to the midline. Bridge formation involves rearrangement of five sister pairs of PLX-2/plexin and VAB-1/Eph receptor expressing "plexin band" cells, of which three pairs form a scaffold for bridge assembly and two pairs form the bridge. Bridge formation requires VAB-1 kinase-dependent extension of presumptive bridge cells over scaffold cells toward the ventral midline. An unassembled vab-1 null mutant bridge obstructs P cell migration, which is largely overcome by plexin band expression of VAB-1 or VAB-1(delC) (a kinase deletion of VAB-1). VAB-1 also functions redundantly with MAB-20/semaphorin to prevent perdurant gaps between sister plexin band cells that block P cell migration. CONCLUSIONS The Eph receptor mediates cellular extensions required for bridge formation, independently facilitates P cell migration to the midline, and functions redundantly with PLX-2/plexin to prevent gaps in the bridge used for P9/10 cell migration in body wall closure.