The maintenance of osmotic homeostasis is essential for cellular survival. Disruption of osmotic homeostasis is a stress faced by all organisms and can occur under both physiological and pathophysiological conditions. Increased extracellular osmolarity (hyperosmolarity) causes a rapid decrease in cell volume leading to increased intracellular ionic strength, macromolecular damage, and cell-ECM mechanical strain. These stressors can be fatal if left unchecked. To counteract these stressors, cells activate several physiological pathways. One major pathway involves the upregulation of genes that facilitate the accumulation of solutes called organic osmolytes. Organic osmolytes restore cell volume, reduce cell-ECM mechanical strain, and prevent macromolecular damage. However, the mechanisms by which multicellular animals, including humans, detect osmotic dyshomeostasis and activate osmosensitive gene expression, including genes involved in osmolyte accumulation, remain poorly understood. Similar to humans, the nematode Caenorhabditis elegans responds to hyperosmotic stress by synthesizing organic osmolytes from glucose metabolic products. C. elegans synthesizes the osmolyte glycerol by upregulating the enzyme that catalyzes the rate-limiting step in glycerol biosynthesis, glycerol-3-phosphate dehydrogenase enzyme (GPDH-1). To understand how the osmotic stress response is coordinated in multicellular animals, we conducted an ENU-based genetic screen with a
gpdh-1::GFP reporter to identify mutants with no induction of osmolyte biosynthesis gene expression. Two of the mutants we identified have distinct nonsense SNPs in the conserved O-GlcNAC transferase OGT. OGT is the only enzyme in metazoans that O-GlcNAcylates serine and threonine residues on cytoplasmic and nuclear proteins to regulate protein activity, localization, and stability. Surprisingly, we found that
gpdh-1 mRNA is still induced in
ogt-1 mutants. However, GPDH-1 protein is not upregulated.
ogt-1 is also required for all of the osmotic stress resistant phenotypes caused by loss of the ECM protein OSM-8. Our data show that
ogt-1 is essential for both physiological and genetic activation of the osmotic stress response through a post-transcriptional mechanism. We are currently investigating the molecular requirements for
ogt-1 to regulate the hyperosmotic stress response.