GCK‐3, a Caenorhabditis elegans homologue of PASK, is essential for whole‐animal osmotic homeostasis

We recently identified a new member of the Ste20 kinase superfamily in C. elegans , GCK‐3, which is a homologue of mammalian PASK. PASK is expressed in transporting epithelia including the kidney, and was shown recently to interact with and regulate cell volume dependent Na‐K‐2Cl and K‐Cl cotransporters. In C. elegans , GCK‐3 regulates cell volume‐dependent activity of CLH‐3b, a member of the ClC superfamily of voltage‐gated Cl − channels. GCK‐3 is co‐expressed with clh‐3 in the worm oocyte and excretory cell, which functions as the worm kidney. We have proposed that GCK‐3 and PASK are orthologs that likely play evolutionarily conserved roles in cell volume sensing and regulation of systemic osmotic homeostasis. To begin testing this idea, we characterized the effect of GCK‐3 knockdown on whole animal osmoregulation. When exposed to a growth medium containing 350 mM NaCl, control worms rapidly lose 35% of their body volume and become paralyzed. However, complete recovery of body volume occurs within 6 hours and normal movement and feeding behaviors resume by 8 hours. Worms fed bacteria that express GCK‐3 double‐stranded RNA lose a similar amount of body volume (39%), but never recover this volume, never resume movement, and die within 72 h. Our studies demonstrate that GCK‐3 plays an essential role in whole animal osmotic homeostasis. Importantly, the GCK‐3 loss‐of‐function phenotype provides a powerful basis for forward and reverse genetic analysis of GCK‐3 signaling mechanisms. Elucidation of GCK‐3 signaling pathways will likely provide important insights into PASK signaling and function in mammals. Supported by NIH grant DK61168.