It has long been known that exposure to certain stresses (e.g., hyperthermia or hypoxia) can induce the expression of a set of genes known as the heat shock proteins. Sodium azide, which inhibits cytochrome oxidase c and ATP synthase, creates a chemical hypoxia in cells and can induce certain heat shock proteins. It has been used as a worm anesthetic but the mechanism by which C. elegans can survive prolonged exposure to this chemical is not known. We hypothesized that chemicals affecting energy metabolism will also induce the heat shock response in C. elegans. We have found that exposure to low levels of sodium azide confers thermotolerance in the nematode (see WBG 15(1): 38). Our studies have shown that worms exposed to 10 mM sodium azide for 60 minutes will show a significantly elevated survival probability after exposure to 37 oC. The survival probability for control (no azide or heat exposure) worms taken directly to 37 oC is 0.07, while worms exposed to 10 mM sodium azide have a survival probability of 0.68 (p<0.05). This compares well with worms exposed to 33 oC and then taken to 37 oC. Their survival probability is 0.92 (p<0.05). We then hypothesized that the molecular mechanism of the response to azide is the same as the response to elevated temperatures, specifically regarding induction of the heat shock proteins. Snutch and Baillie (Can. J. Biochem. Cell Biol 61: 480-487, 1983) demonstrated that exposing C. elegans to 33 oC resulted in the induction of
hsp70 and hsp 16. Using SDS-PAGE and Coomassie Brilliant Blue staining, we observed a 250% induction of
hsp70 after exposure to 33 oC, but only a 25% induction after exposure to sodium azide. Using a more sensitive silver staining technique, this pattern was still observed. Neither staining technique was able to resolve induction of a low molecular weight hsp. Using an anti-
hsp16 antibody kindly provided by Peter Candido, we determined via Western Blot analysis that
hsp16 was induced after exposure to sodium azide. In conjunction with the SDS-PAGE results, Western blots did not demonstrate an induction of any other hsp examined (
hsp90 and
hsp60). To further elucidate the mechanism, we also tested the only known hsp mutant in C. elegans,
daf-21 (JT6130). Elizabeth Malone and Jim Thomas reported at the 1997 Worm Meeting that this constitutive dauer forming mutant strain carries a conserved glu -> lys change in
hsp90. We tested this temperature sensitive mutant and found when grown at the permissive temperature, it survived heat shock after receiving pre-treatment with either azide or 33 oC. However, these worms showed a reduction in survival probability when compared to N2. At the restricted temperature, the worms pre-treated with either azide or 33oC did not survive the heat shock. These results strongly suggest that while
hsp90 is not induced by hyperthermia or sodium azide, it is an essential part of the worm's response to stress. Our data indicate that while the mechanism of the worm!s response to sodium azide is similar to its response to elevated temperatures, there are significant differences in the degree of the response.