Timing and mechanism of the initial cue establishing handed left­right asymmetry in Caenorhabditis elegans embryos.

By the six-cell stage, embryos of Caenorhabditis elegans are morphologically L­R asymmetric with an invariant handedness that persists throughout development. We used intracellular markers to ask whether breaking of L­R symmetry could be observed at earlier stages. Observation of two- to three-cell embryos carrying intracellular markers indicated that L­R symmetry is broken concomitantly with establishment of D­V axis polarity during division of the anterior AB cell. The AB cleavage furrow initiates asymmetrically and always from the left, suggesting L­R differences in the AB cell cortex. An invariantly handed cortical rotation observed earlier during first cleavage implies that the one-cell embryo has an intrinsic chirality. We propose that L­R differences in the cortex could result from mechanical forces on asymmetric components of a chiral cortical network during the off-axis elongation of the AB-cell spindle prior to AB cleavage.

The Rho GTPase-activating proteins RGA-3 and RGA-4 are required to set the initial size of PAR domains in Caenorhabditis elegans one-cell embryos.

Caenorhabditis elegans embryos establish cortical domains of PAR proteins of reproducible size before asymmetric cell division. The ways in which the size of these domains is set remain unknown. Here we identify the GTPase-activating proteins (GAPs) RGA-3 and RGA-4, which regulate the activity of the small GTPase RHO-1. rga-3/4(RNAi) embryos have a hypercontractile cortex, and the initial relative size of their anterior and posterior PAR domains is altered. Thus, RHO-1 activity appears to control the level of cortical contractility and concomitantly the size of cortical domains. These data support the idea that in C. elegans embryos the initial size of the PAR domains is set by regulating the contractile activity of the acto-myosin cytoskeleton through the activity of RHO-1. RGA-3/4 have functions different from CYK-4, the other known GAP required for the first cell division, showing that different GAPs cooperate to control the activity of the acto-myosin cytoskeleton in the first cell division of C. elegans embryos.

CDC-42 and RHO-1 coordinate acto-myosin contractility and PAR protein localization during polarity establishment in C. elegans embryos.

In C. elegans one-cell embryos, polarity is conventionally defined along the anteroposterior axis by the segregation of partitioning-defective (PAR) proteins into anterior (PAR-3, PAR-6) and posterior (PAR-1, PAR-2) cortical domains. The establishment of PAR asymmetry is coupled with acto-myosin cytoskeleton rearrangements. The small GTPases RHO-1 and CDC-42 are key players in cytoskeletal remodeling and cell polarity in a number of different systems. We investigated the roles of these two GTPases and the RhoGEF ECT-2 in polarity establishment in C. elegans embryos. We show that CDC-42 is required to remove PAR-2 from the cortex at the end of meiosis and to localize PAR-6 to the cortex. By contrast, RHO-1 activity is required to facilitate the segregation of CDC-42 and PAR-6 to the anterior. Loss of RHO-1 activity causes defects in the early organization of the myosin cytoskeleton but does not inhibit segregation of myosin to the anterior. We therefore propose that RHO-1 couples the polarization of the acto-myosin cytoskeleton with the proper segregation of CDC-42, which, in turn, localizes PAR-6 to the anterior cortex.