Abstract

During development, epidermal morphogenesis encloses the C. elegans embryo in a layer of epidermal cells (by dorsal intercalation and ventral enclosure) and lengthens the embryo into the characteristic worm shape (elongation). These processes occur by a combination of cell shape changes, migration and adhesion. Genes that are involved in these events include Rho GTPases, nucleators of F-actin, nonmuscle myosin regulators and the catenin/cadherin complex that mediates junction formation.
Elongation occurs due to actin-myosin mediated contractions within the lateral epidermal cells. Rho kinase (LET-502), a RhoA effector that activates nonmuscle myosin, and myosin phosphatase (MEL-11), which down regulates myosin activity, are required for elongation. Biochemical analyses performed in other organisms predict that RhoA (RHO-1) functions upstream of Rho kinase (LET-502) and nonmuscle myosin in the elongation pathway, however, due to the lack of rho-1 alleles, genetic tests had not verified this. We used a zygotic null allele of rho-1 recently generated by the Knockout Consortium to show that rho-1 is required for elongation. Crosses between rho-1 and elongation mutants, including mlc-4 (nonmuscle myosin regulatory light chain), let-502 (Rho-kinase) and mel-11 (myosin phosphatase regulatory subunit), confirmed rho-1 as an upstream regulator of nonmuscle myosin.


Ventral enclosure is the process where ventral epidermal cells migrate over the ventral surface of the embryo and meet at the midline to adhere with their contralateral neighbours. Some of the genes required for ventral enclosure include actin regulators such as Rac, Arp 2/3 and Wave/Scar (which nucleate the formation of branched F-actin to form lamellipodia) and adhesion components including the cadherin/catenin complex. Other genes contribute to ventral enclosure non-autonomously, such as vab-1 and vab-2, which are expressed in specific subsets of neuroblast cells that function as a substrate for the overlying ventral epidermal cells. It is not fully understood how neuroblasts non-autonomously regulate epidermal cell migration. Here, we show that C. elegans anillin (ANI-1), a scaffold protein that binds to F-actin and active myosin, is non-autonomously required for ventral enclosure. In ani-1 RNAi embryos expressing AJM-1::GFP (a marker for epidermal cell junctions), ventral epidermal cells were mis-aligned and failed to adhere at the ventral surface of the embryo. Furthermore, imunostaining revealed that ANI-1 is primarily expressed in dividing neuroblast cells. These data suggest that neuroblasts may mechanically regulate ventral epidermal cell migration, shedding light on one of the mechanisms by which metazoan tissues may form in vivo.