Impact of dietary selenium on methylmercury toxicity in juvenile Atlantic cod: A transcriptional survey

Selenium (Se) and its derivatives are known to have protective effects against mercury (Hg) toxicity in mammals. In this study we wanted to evaluate whether Se co-exposure affect the transcription of methylmercury (MeHg) toxicity-relevant genes in early life stages of fish. Juvenile Atlantic cod were exposed to regular feed (control), Se-spiked feed (3 mg Se kg−1), MeHg-spiked feed (10 mg Hg kg−1) or to Se- and MeHg-spiked feed (3 mg Se kg−1 and 10 mg Hg kg−1, respectively) for ten weeks. Liver tissue was harvested for transcriptional analysis when the fish were weighing 11.4 ± 3.2 g. Accumulated levels of Hg in liver of the two groups of fish exposed to MeHg were 1.5 mg Hg kg−1 wet weight, or 44-fold higher than in the control group, while the Se concentrations differed with less than 2-fold between the fish groups. Selenium co-exposure had no effect on the accumulated levels of Hg in liver tissue; however, MeHg co-exposure reduced the accumulated level of Se. Dietary exposure to MeHg had no effect on fish growth. Interaction effects between Se and MeHg exposure were observed for the transcriptional levels of CAT, GPX1, GPX3, NFE2L2, UBA52, SEPP1 and DNMT1. Significant effects of MeHg exposure were seen for DNMT1 and PPARG, while effects of Se exposure were seen for GPX4B and SEPP1A, as well as for DNA methyltransferase activity. The transcriptional results suggest, by considering up-regulation as a proxy for negative impact and at the tested concentrations, a pro-oxidative effect of Se co-exposure with MeHg, rather than an antioxidative effect.