Identification of Cells Involved in the Induction of TNF-A Expression by Methylmercury in the Mouse Brain and Its Molecular Mechanisms

摘要

Methylmercury is an environmental pollutant that causes neurotoxicity. Recently, we found that tumor necrosis factor-a (TNF-a; inflammatory cytokine) is selectively induced in the brain of mice administered methylmercury. Although TNF-a may be involved in the neurotoxicity caused by methylmercury, the mechanisms involved in the induction of TNF-a expression by methylmercury are unknown. In this study, we aimed to identify cells involved in the induction of TNF-a expression by methylmercury in mice brain, to elucidate the underlying molecular mechanisms. Seven days after subcutaneous injection of C57BL6 mice with methylmercuric chloride (25 mg/kg), TNF-a mRNA levels were determined in the brain tissues by in situ hybridization. We found that TNF-a was hardly expressed in the saline-administered group, whereas TNF-a-expressing cells were observed in the whole area of the brain in the methylmercury-treated group. It was reported that astrocytes and microglial cells are involved in the induction of TNF-a expression. Therefore, we performed immunostaining using antibodies specific for GFAP or IBA1, which are specifically expressed in astrocytes and microglial cells, respectively. The TNF-a-expressing cells did not overlap with the GFAP-positive cells, but with the IBAl-positive cells. The results suggest that microglial cells are mainly involved in the induction of TNF-a expression by methylmercury in the mouse brain. In addition, methylmercury induced TNF-a expression in mouse microglial cell line BV2. MAP kinases such as JNK, ERK, and p38 are known to be involved in the induction of TNF-a expression in microglial cells. We found that all three MAP kinases were phosphorylated by methylmercury treatment. Moreover, when the cells were pretreated with specific inhibitors, only the p38 inhibitor suppressed the induction of TNF-a expression by methylmercury. These results indicate that methylmercury may induce TNF-a expression via activation of p38 in microglial cells.