Mutations in the C. elegans vab-1 gene disrupt the coordinated movements of cells during two periods of embryogenesis. vab-1 mutants are defective in the movement of neuroblasts during closure of the ventral gastrulation cleft and in the movements of epidermal cells during ventral enclosure of the embryo by the epidermis. We show that vab-1 encodes a receptor tyrosine kinase of the Eph family. Disruption of the kinase domain of VAB-1 causes weak mutant phenotypes, indicating that VAB-1 may have both kinase-dependent and kinase-independent activities. VAB-1 is expressed in neurons during epidermal enclosure and is required in these cells for normal epidermal morphogenesis, demonstrating that cell-cell interactions are required between neurons and epidermal cells for epidermal morphogenesis.

Background In the last stage of the C. elegans equivalent of epiboly, an open pocket in the epidermis is closed by marginal epidermal P/pocket cells that express and require VAB-1/Eph and PLX-2/plexin receptors for migration toward and alignment with contralateral partners at the ventral midline. Cellular mechanisms affected by these signaling proteins remain unknown. Results A cellular bridge comprising four neuron cell bodies that spans the open pocket serves as a substratum for migration of contra-lateral P cell pair P9/10 to the midline which can facilitate similar migration of neighboring P cells. This bridge is formed by a stereotypical rearrangement of five sister pairs of PLX-2 and VAB-1 expressing cells, of which three pairs serve as a scaffold for bridge assembly and two pairs form the bridge. Bridge formation requires VAB-1 kinase-dependent extension of presumptive bridge cell protrusions toward the ventral midline. An unassembled mutant bridge obstructs but does not block P cell progression toward the midline, however, cell type-specific rescue experiments show that VAB-1 or a nearly complete cytoplasmic deletion of VAB-1 expressed by scaffold and bridge cells or by P9/10 can facilitate P cell progression to the midline. MAB-20/semaphorin and VAB-1 also exhibit complex redundancies to regulate adhesion and prevent gaps between sister bridge and scaffold forming cells that would otherwise completely block P cell migration. Conclusions The Eph receptor functions to mediate cell extensions required for bridge formation, independently facilitates P cell migration to the ventral midline, and acts redundantly with PLX-2/plexin to prevent gaps between sister plexin band cells that normally serve as a substratum for P9/10 cell migration.

The mechanism by which epithelial cells undergo directed rearrangement is central to morphogenesis, yet the regulation of these movements remains poorly understood. We have investigated epithelial cell rearrangement (intercalation) in the dorsal hypodermis, or embryonic epidermis, of the C. elegans embryo by analyzing the die-1(w34) mutant, which fails to undergo normal intercalation. Dorsal hypodermal cells of die-1(w34) homozygous embryos initiate but fail to complete the process of intercalation. Multiphoton microscopy reveals that intercalating cells extend monopolar, basolateral protrusions in their direction of migration; posterior dorsal hypodermal cells in die-1(w34) mutants appear to extend protrusions normally, but fail to translocate their cell bodies to complete rearrangement. Despite abnormal intercalation, the subsequent morphogenetic movements that enclose the embryo with epithelial cells and the process of dorsal cell fusion still occur. However, elongation of the embryo into a wormlike shape is disrupted in die-1(w34) embryos, suggesting that intercalation may be necessary for subsequent elongation of the embryo. Actin filaments are not properly organized within the dorsal hypodermis of die-1(w34) embryos, consistent with intercalation's being a necessary prerequisite for elongation. The die-1 gene encodes a C2H2 zinc finger protein containing four fingers, which likely acts as a transcriptional regulator. DIE-1 is present in the nuclei of hypodermal, muscle, gut, and pharyngeal cells; its distribution suggests that DIE-1 acts in each of these tissues to regulate morphogenetic movements. die-1(w34) mutants display morphogenetic defects in the pharynx, gut, and muscle quadrants, in addition to the defects in the dorsal hypodermis, consistent with the DIE-1 expression pattern. Mosaic analysis indicates that DIE-1 is autonomously required in the posterior dorsal hypodermis for intercalation. Our analysis documents for the first time the dynamics of protrusive activity during epithelial cell rearrangement. Moreover, our analysis of die-1 shows that the events of epithelial cell rearrangement are under transcriptional control, and that early and later phases of epithelial cell rearrangement are genetically distinguishable.