Due to the natural environment of the worm, pathways pertaining to osmotic balance must be crucial for survival. However, in C. elegans the ways in which the organism deals with hypoosmotic shock is relatively poorly understood. Here we show that a serine proteinase inhibitor (serpin) is involved in a novel phenotype, hypoosmotic shock lethal (osl). Serpins are a superfamily of serine and cysteine proteinase inhibitors. We identified one member of the C. elegans serpin family,
srp-6, that inhibits the lysosomal cysteine proteinases.
srp-6 was expressed predominantly in the excretory gland cell and the vulval muscles. The longevity and morphology of the
srp-6 knockout animals were normal. However the serpin knockout animal had an adverse response to hypoosmotic stress. Compared to N2 worms,
srp-6 (
ok319) animals showed a marked decreased in survivability when placed in water (98% vs 10% viability, respectively). The phenotype is characterized by rapid swelling, vacuolization and cell death of the intestinal cells upon exposure to water. Using genetic and pharmacological approaches, we showed that osl phenotype in
srp-6 animals was associated with prolonged elevations in intracellular calcium suggesting a role for calcium signaling in hypoosmotic shock.
srp-6 (RNAi) on mutant alleles of intracellular ER associated calcium channels (e.g.
itr-1,
ryr-1) could suppress the phenotype.
srp-6 (RNAi) on the CED mutants indicates that the cell death was independent of the classical apoptotic cell death pathways. RNAi using cDNAs of the cysteine proteinases in
srp-6 animals have identified putative proteinase interactors. These data suggest that
srp-6 plays a protective role in blocking the activation of a systemic proteolytic cascade that is activated in response to hypoosmotic shock.