The C. elegans Hypertonic Stress Response: Big Insights from Shrinking Worms

Abstract

Cell volume is one of the most aggressively defended physiological set points in biology. Changes in intracellular ion and water concentrations, which are induced by changes in metabolism or environmental exposures, disrupt protein folding, enzymatic activity, and macromolecular assemblies. To counter these challenges, cells and organisms have evolved multifaceted, evolutionarily conserved molecular mechanisms to restore cell volume and repair stress induced damage. However, many unanswered questions remain regarding the nature of cell volume 'sensing' as well as the molecular signaling pathways involved in activating physiological response mechanisms. Unbiased genetic screening in the model organism C. elegans is providing new and unexpected insights into these questions, particularly questions relating to the hypertonic stress response (HTSR) pathway. One surprising characteristic of the HTSR pathway in C. elegans is that it is under strong negative regulation by proteins involved in protein homeostasis and the extracellular matrix (ECM). The role of the ECM in particular highlights the importance of studying the HTSR in the context of a live organism where native ECM-tissue associations are preserved. A second novel and recently discovered characteristic is that the HTSR is regulated at the post-transcriptional level. The goal of this review is to describe these discoveries, to provide context for their implications, and to raise outstanding questions to guide future research.

Keywords: Hypertonic stress; Osmoregulation; Stress response; Compatible solute.

Figure 1.. Hypertonic stress response pathways in C. elegans.

The induction of osmotically regulated genes, such as gpdh-1, during hypertonic stress (HTS) is regulated through both transcriptional (blue) and post-transcriptional (red) mechanisms. At least three pathways regulate the transcriptional induction of gpdh-1. First, inhibition of proteins that maintain protein folding and new protein synthesis activate gpdh-1 expression. HTS-induced decreases in protein translation lead to increased gpdh-1 transcription through a gcn-1/2 and wnk-1/gck-3 dependent pathway. Additionally, while significant work shows that HTS causes unique types of protein damage and inhibition of protein homeostasis genes activates the HTSR, the specific signaling mechanisms linking HTS-induced protein damage to gpdh-1 mRNA upregulation are not known. Second, the HTSR transcriptional response is negatively regulated by extracellular proteins that function upstream of the transmembrane protein PTR-23/patched-related protein 23, and the GATA erythroid-like transcription factors ELT-2 and ELT-3. However, ptr-23 is not required for all osmotically regulated gene expression and as such at least one ptr-23 independent pathway must exist. It is unknown if this ptr-23 independent pathway functions through the GATA transcription factors. It also remains unknown if HTS itself can activate ELT-2/-3 through an extracellular protein and PTR-23 independent pathway. Finally, the O-GlcNAc transferase OGT-1 regulates GPDH-1 protein translation through a post-transcriptional pathway. ogt-1 is required for osm-8 and osm-11 phenotypes, suggesting there is some crosstalk between the extracellular protein transcriptional pathway and the ogt-1 post-transcriptional pathway. The precise mechanism by which ogt-1 induces GPDH-1 protein expression is unknown, but it could include regulation of mRNA cleavage, 3’ UTR usage, mRNA export, initiation factor interactions or ribosomal elongation. The output of both the transcriptional and post-transcriptional pathways are two-fold. First, they mediate acclimation that enable animals to survive and reproduce in hypertonic environments. Second, these pathways may also modulate the function of osmosensory neurons such that once acclimation has occurred, behavioral avoidance to hypertonic stimuli is no longer necessary.