Genes involved in cysteine metabolism of Chironomus tepperi are regulated differently by copper and by cadmium

Freshwater invertebrates are often exposed to metal contamination, and changes in gene expression patterns can help understand mechanisms underlying toxicity and act as pollutant-specific biomarkers. In this study the expressions of genes involved in cysteine metabolism are characterized in the midge Chironomus tepperi during exposures to sublethal concentrations of cadmium and copper. These metals altered gene expression of the cysteine metabolism differently. Both metals decreased S-adenosylhomocysteine hydrolase expression and did not change the expression of S-adenosylmethionine synthetase. Cadmium exposure likely increased cystathionine production by up-regulating cystathionine-β-synthase (CβS) expression, while maintaining control level cysteine production via cystathionine-γ-lyase (CγL) expression. Conversely, copper down-regulated CβS expression and up-regulated CγL expression, which in turn could diminish cystathionine to favor cysteine production. Both metals up-regulated glutathione related expression (γ-glutamylcysteine synthase and glutathione synthetase). Only cadmium up-regulated metallothionein expression and glutathione S-transferase d1 expression was up-regulated only by copper exposure. These different transcription responses of genes involved in cysteine metabolism in C. tepperi point to metal-specific detoxification pathways and suggest that the transsulfuration pathway could provide biomarkers for identifying specific metals.