Molecular Mechanism of the Response to Hydrogen Sulfide in C. elegans

Abstract

Hydrogen sulfide (H2S) has long been known as a noxious gas, however, we now know that H2S also acts an endogenous signaling molecule in humans. H2S has been shown to mediate a wide range of biological effects, ranging from sensing hypoxia, mitigating ischemia reperfusion injury, neuro-modulation and even increasing lifespan in C. elegans. The molecular mechanisms behind these organismal effects of H2S are not well understood. To identify and better understand the proteins involved in the response to H2S, I utilize C. elegans as a model system to study the organismal response to exogenous H2S. To ensure that I can accurately control H2S exposure in our experimental setup, I created continuous flow atmospheric chambers, which allows the maintenance of a precise concentration of H2S over long periods of time. These chambers, coupled with the use of C. elegans, which obtain gas by diffusion, allows for unprecedented control of the concentration of H2S to which each cell is exposed. Using these chambers, I identified novel functions in H2S for four genes; sqrd-1, skn-1, rhy-1 and cysl-1. Sulfide quinone oxoreductase (sqrd-1) acts to oxidize H2S, feeding electrons into the electron transport chain. I show that sqrd-1 is necessary for C. elegans to maintain protein translation upon exposure to H2S. These translational effects are unique to sqrd-1 mutants and are not seen with other genes key to the organismal response to H2S. SQRD-1 acts to maintain proteostasis in H2S, as sqrd-1 mutants show upregulation of the unfolded protein response to both ER and mitochondria upon exposure to H2S. This suggests that SQRD-1 acts not only to detoxify H2S but may play a role in H2S signaling. The hypoxia inducible factor (hif-1) is necessary for the initial transcriptional response to H2S and hif-1-null animals die upon exposure to low concentrations of H2S. I undertook a forward genetic screen for mutations that suppress hif-1 lethality in H2S. This screen identified mutations in wdr-23 and skn-1, that increase SKN-1 transcriptional activity, promote survival in H2S independent of hif-1. This suppression of hif-1 is specific to H2S, as increasing SKN-1 activity does not impact other hif-1 phenotypes. SKN-1 acts to suppress hif-1 by increasing rhy-1 expression and rhy-1 overexpression alone is sufficient to allow hif-1-null animals to survive in H2S. Increased SKN-1 activity also requires cysl-1 to suppress hif-1 in H2S. Both rhy-1 and cysl-1 have been previously shown to regulate HIF-1 activity. Our data show that rhy-1 and cysl-1 act in a novel hif-1-independent pathway to promote survival in H2S. My work highlights previously unknown signaling pathways by which C. elegans appropriately responds to H2S. This work lays the groundwork, by identifying key proteins in the response to H2S, to further our understanding of H2S signaling in humans.