pix-1 Controls Early Elongation in Parallel with mel-11 and let-502 in Caenorhabditis elegans

Abstract

Cell shape changes are crucial for metazoan development. During Caenorhabditis elegans embryogenesis, epidermal cell shape changes transform ovoid embryos into vermiform larvae. This process is divided into two phases: early and late elongation. Early elongation involves the contraction of filamentous actin bundles by phosphorylated non-muscle myosin in a subset of epidermal (hypodermal) cells. The genes controlling early elongation are associated with two parallel pathways. The first one involves the rho-1/RHOA-specific effector let-502/Rho-kinase and mel-11/myosin phosphatase regulatory subunit. The second pathway involves the CDC42/RAC-specific effector pak-1. Late elongation is driven by mechanotransduction in ventral and dorsal hypodermal cells in response to body-wall muscle contractions, and involves the CDC42/RAC-specific Guanine-nucleotide Exchange Factor (GEF) pix-1, the GTPase ced-10/RAC and pak-1.In this study, pix-1 is shown to control early elongation in parallel with let-502/mel-11, as previously shown for pak-1. We show that pix-1, pak-1 and let-502 control the rate of elongation, and the antero-posterior morphology of the embryos. In particular, pix-1 and pak-1 are shown to control head, but not tail width, while let-502 controls both head and tail width. This suggests that let-502 function is required throughout the antero-posterior axis of the embryo during early elongation, while pix-1/pak-1 function may be mostly required in the anterior part of the embryo. Supporting this hypothesis we show that low pix-1 expression level in the dorsal-posterior hypodermal cells is required to ensure high elongation rate during early elongation.

The interplay of stiffness and force anisotropies drives embryo elongation

“…We observed that the head, body and tail diameter (at the levels of H1, V3 and V6, respectively) decreased at different rates over time (Figure 3c), as also observed by Martin and colleagues (Martin et al, 2014). The head diameter did not diminish between the 1.3F and 1.5F stages when the stress was nearly isotropic, but decreased significantly between the 1.5F and 1.7F stages as the stress anisotropy increased.…”

The interplay of stiffness and force anisotropies drive embryo elongation

“…To correlate the stress anisotropy with the morphological changes of the embryo, we used markers 141 labelling cortical actin (an ABD) and junctions (HMR-1/E-cadherin). We observed that the head, body 142 and tail diameter (at the level of H1, V3 and V6, respectively) decreased at different rates over time 143 (figure 3c), as also observed by Martin and colleagues (Martin, 2014). The head diameter did not 144 diminish between the 1.3F and 1.5F stages when the stress was nearly isotropic, but decreased 145 significantly between the 1.5F and 1.7F stages as the stress anisotropy increased.…”

The RhoGAP myosin 9/HUM-7 integrates membrane signals to modulate Rho/RHO-1 during embryonic morphogenesis inC. elegans

“…The role of the GTPases Rho/RHO-1 and Cdc42/CDC-42 in the ventral enclosure step of morphogenesis is only beginning to be described (Fotopoulos et al, 2013). In contrast, the role of RHO-1 in a later morphogenetic step, epidermal elongation, is exceedingly well studied (Diogon et al, 2007; Gally et al, 2009; Martin et al, 2014; Piekny et al, 2003; Piekny et al, 2000). Since hum-7 behaved genetically and molecularly like a candidate GAP for RHO-1 (and CDC-42), we investigated if hum-7 mutants have defects in Rho-dependent processes, like elongation.…”

“…During elongation the epidermis constricts circumferentially to squeeze the embryo into a tubular worm. Embryonic elongation also depends on regulators of Rho-1, and on Rho-1 targets including the Rho Kinase/LET-502 and non-muscle myosins (Chisholm and Hardin, 2005; Diogon et al, 2007; Fotopoulos et al, 2013; Gally et al, 2009; Martin et al, 2014; Piekny and Mains, 2003; Piekny et al, 2003; Quintin et al, 2008; Wissmann et al, 1997; Zhang et al, 2010). Elongation is driven by high levels of tension, mainly in the two rows of lateral seam cells.…”