Abstract

Tissue morphogenesis is crucial for the development of metazoans. C. elegans, is a model system for studying tissue morphogenesis, as there are many genetic tools available, and embryos are amenable to microscopy. We study ventral enclosure, which is the process where ventral epidermal cells migrate to enclose the C. elegans embryo in a single layer of epidermal cells. This process is initiated by the migration and adhesion of two pairs of anterior leading cells, followed by the migration and adhesion of eight pairs of posterior pocket cells. The migration of the leading cells is mediated by F-actin rich filopodia-like protrusions, which are under the control of the Rac – Wave/Scar – Arp2/3 pathway. Cables of F-actin become enriched around the margins of the pocket cells to form a ring, and early studies showed that this ring is under tension. Based on this data, actomyosin contractility was predicted to mediate the closure of this ring during ventral enclosure. However, nonmuscle myosin has not been studied in ventral enclosure, where it could regulate ring closure in addition to regulating cell shape changes and/or adhesion. For example, proteins that form adhesion junction complexes are also required for ventral enclosure, as they maintain contacts between cells so they can migrate as a unit and form new junctions with contralateral neighbors.
Our studies support a role for myosin contractility in ventral enclosure. RhoA regulates nonmuscle myosin contractility for cytokinesis in the early embryo and for elongation of the lateral epidermal cells in late embryogenesis. ECT-2 is the GEF that activates RhoA during cytokinesis, and a different GEF, RHGF-2, activates RhoA during late embryogenesis. A hypomorphic, maternal ts allele of ect-2, ax751, is required for polarity in the early embryo, but displays few cytokinesis defects, especially at non-permissive temperatures. Using this allele, we found that ect-2 is required for the migration of neuroblasts during earlier embryonic stages, and ventral epidermal cells during ventral enclosure. Genetic crosses suggest that ect-2 functions in parallel to the Rac pathway and may be part of the Rho pathway to regulate actomyosin contractility, supporting a role for myosin in ventral enclosure. Imaging and quantification of embryos expressing GFP-tagged myosin showed that myosin forms into a supracellular structure around the margins of the ventral pocket cells, reminiscent of the actin ring described previously. We found that ect-2 is required for the enrichment and organization of myosin, supporting a requirement for ect-2 in regulating myosin contractility. Interestingly, ect-2 may also function in the cadherin/catenin pathway that forms adhesion junctions, suggesting ect-2 could also regulate the actomyosin filaments that contribute to adhesion.