Methylmercury-induced hair cell loss requires hydrogen peroxide production and leukocytes in zebrafish embryos

Hearing impairment and deafness is frequently observed as one of the neurological signs in patients with Minamata disease caused by methylmercury (MeHg) poisoning. Loss of hair cells in humans and animals is a consequence of MeHg poisoning. However, it is still not clear how MeHg causes hearing deficits. We employed the hair cells of the lateral line system of zebrafish embryos as a model to explore this question. We exposed transgenic zebrafish embryos to MeHg (30 - 360 μg/L) at the different stages, and scored the numbers of hair cells. We find that MeHg-induced reduction of hair cells is in a concentration dependent manner. By employing antisense morpholino against to pu.1, we confirm that loss of hair cells involves the action of leukocytes. Moreover, hair cell loss is attenuated by co-treating MeHg-exposed embryos with pharmacological inhibitors of NADPH oxidases named diphenyleneiodonium (DPI) and VAS2870. In situ gene expression analysis showed that genes encoding the SQSTM1-Keap1-Nrf2 systems involved in combating oxidative stress and immune responses are highly expressed in the lateral line organs of embryos exposed to MeHg. This suggests that induction of hydrogen peroxide (H2O2) is the primary effect of MeHg on the hair cells. Genes induced by MeHg are also involved in regeneration of the hair cells. These features are likely related to the capacity of the zebrafish to regenerate the lost hair cells.