The ability to maintain osmotic homeostasis between the cytoplasm and the environment is essential for cellular life. Recently, we developed C. elegans as a model system in which to define animal osmoregulatory signaling pathways. In response to hypertonicity, C. elegans accumulates the osmolyte glycerol by upregulating the glycerol biosynthetic enzyme
gpdh-1 >350 fold.
gpdh-1 is not detectably expressed under isotonic conditions. Using forward and reverse genetic screens, we recently identified >100 gene inhibitions that cause mis-expression of
gpdh-1, elevated glycerol levels, and resistance to hypertonic dehydration. Whole genome-microarray analyses suggest that these mutants cause constitutive activation of osmoregulatory signaling pathways. We are using one such mutant,
osm-11, in genetic suppressor screens to identify genes that function to transduce osmoregulatory signals. Wild type animals rapidly dehydrate and stop moving following exposure to 500 mM NaCl. However, >90% of
osm-11 mutants remain motile. Inhibition of genes that function downstream from
osm-11 should cause
osm-11 mutants to dehydrate and cease motility under hypertonic conditions. Since kinases are frequently involved in signal transduction pathways, we used RNAi to inhibit the function of 395/431 kinase genes in
osm-11 mutants and tested the motility of these animals following exposure to 500 mM NaCl. From the 395 screened genes, a total of 7 reproducible ‘hits were identified. Interestingly, one of these genes,
lit-1, encodes a MAP kinase with 28% identity to S. cereviase HOG1 and 52% identity to mammalian Nemo-like Kinases. In yeast, HOG1 plays an essential role in osmoregulatory signal transduction. Consistent with our RNAi data,
lit-1(
t1512) mutants exhibit acute and chronic sensitivity to hypertonic stress. During C. elegans embryogenesis,
lit-1 regulates Wnt/<font face=symbol>b</font>-catenin signaling to promote intestinal development, so we are currently testing the role of Wnt/<font face=symbol>b</font>-catenin signaling genes in osmotic stress responses. Additionally, we are crossing
lit-1(
t1512) to a variety of osmosensitive GFP reporter strains to test the requirement of
lit-1 in osmosensitive gene expression. Overall, our data suggest that
lit-1 functions as an essential transducer of osmotic stress signals in C. elegans.