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

<div><p>Cell shape changes are crucial for metazoan development. During <i>Caenorhabditis elegans</i> 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 <i>rho-1/</i>RHOA-specific effector <i>let-502/</i>Rho-kinase and <i>mel-11</i>/myosin phosphatase regulatory subunit. The second pathway involves the CDC42/RAC-specific effector <i>pak-1</i>. 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) <i>pix-1</i>, the GTPase <i>ced-10/</i>RAC and <i>pak-1</i>.</p><p>In this study, <i>pix-1</i> is shown to control early elongation in parallel with <i>let-502/mel-11</i>, as previously shown for <i>pak-1</i>. We show that <i>pix-1</i>, <i>pak-1</i> and <i>let-502</i> control the rate of elongation, and the antero-posterior morphology of the embryos. In particular, <i>pix-1</i> and <i>pak-1</i> are shown to control head, but not tail width, while <i>let-502</i> controls both head and tail width. This suggests that <i>let-502</i> function is required throughout the antero-posterior axis of the embryo during early elongation, while <i>pix-1/pak-1</i> function may be mostly required in the anterior part of the embryo. Supporting this hypothesis we show that low <i>pix-1</i> expression level in the dorsal-posterior hypodermal cells is required to ensure high elongation rate during early elongation.</p>