A cadmium toxicity assay using stress responsive Caenorhabditis elegans mutant strains

To test the applicability of Caenorhabditis elegans mutant for toxicity screening, the sensitivity of cadmium (Cd) in C. elegans was investigated on 14 mutant strains using median lethal concentration (LC50) tests, with further analysis on growth and reproduction conducted on five selected strains. The 24 h LC50 of Cd observed on the wildtype and mutant strains of C. elegans was in the order of age-1(hx546) > mtl-2(gk125) > sod-3(gk235) > daf-21(p673) > cyp35a2(gk317) > skn-1(or13) > daf-12(rh62rh157) > hsp-16.2(gk249) > daf-18(e1375) > ctl-2(ok1137) > wildtype(N2) > sod-1(or13) > daf-16(mu86) > cep-1(gk138) > cdr-2(ok1996). Compared to the wildtype response, a decreased reproduction potential was observed in mtl-2(gk125), sod-3(gk235), cdr-2(ok1996) and cep-1(gk138) strains. To gain a mechanistic understanding of different sensitivities of the mutant strains, a time-course gene expression analysis was also performed on the five genes. A dramatic increase in the expression of the mtl-2 gene due to Cd exposure confirmed the importance of this gene in C. elegans Cd toxicity. An increased expression of the sod-3 gene at the longer exposure time period (48 h) suggests that oxidative stress may not be a direct toxic mechanism, but may rather be a consequence of Cd toxicity. Even though, LC50 values for the age-1(hx546) mutant strain were the highest among the tested strains, the response on the reproduction potential in age-1(hx546) mutant was unchanged compared to the wildtype, and the age-1 gene expression remained unaltered on exposure to Cd, which may be interpreted as the maintenance of age-1 expression level is needed for the exertion of Cd toxicity; however, the role of the age-1 gene in Cd toxicity may not be via a reproduction-related pathway. The overall results suggest that the C. elegans mutant assay seems to be a promising tool for the study of toxic mechanisms, as well as for toxicity screening in ecotoxicological research.