Methylmercury induces neuronal cell death by inducing TNF-α expression through the ASK1/p38 signaling pathway in microglia

Abstract

We recently found that tumor necrosis factor-α (TNF-α) may be involved in neuronal cell death induced by methylmercury in the mouse brain. Here, we examined the cells involved in the induction of TNF-α expression by methylmercury in the mouse brain by in situ hybridization. TNF-α-expressing cells were found throughout the brain and were identified as microglia by immunostaining for ionized calcium binding adaptor molecule 1 (Iba1). Methylmercury induced TNF-α expression in mouse primary microglia and mouse microglial cell line BV2. Knockdown of apoptosis signal-regulating kinase 1 (ASK1), an inflammatory cytokine up-regulator that is responsible for reactive oxygen species (ROS), decreased methylmercury-induced TNF-α expression through decreased phosphorylation of p38 MAP kinase in BV2 cells. Suppression of methylmercury-induced reactive oxygen species (ROS) by antioxidant treatment largely abolished the induction of TNF-α expression and phosphorylation of p38 by methylmercury in BV2 cells. Finally, in mouse brain slices, the TNF-α antagonist (WP9QY) inhibited neuronal cell death induced by methylmercury, as did the p38 inhibitor SB203580 and liposomal clodronate (a microglia-depleting agent). These results indicate that methylmercury induces mitochondrial ROS that are involved in activation of the ASK1/p38 pathway in microglia and that this is associated with induction of TNF-α expression and neuronal cell death.

Necrotic-like BV-2 microglial cell death due to methylmercury exposure

“…BV-2 microglial cells were exposed to MeHg followed, or not, by stimulation with LPS, for 6 h. In the absence of LPS, microglial cells did not exhibit changes in the production and secretion of both pro-inflammatory cytokines when exposed to different MeHg concentrations. However, upon LPS stimulation, we observed a significant increase in IL-6 and TNF-α content, according to the previously observed in several studies ( Pinheiro et al, 2007 ; Toyama et al, 2021 ; Yuan et al, 2021 ). Moreover, treatment with 10 µM-MeHg significantly decreased the LPS-stimulated cytokines production and release.…”

“…Several reports have highlighted discrepancies concerning IL-6 and TNF-α production and secretion during MeHg exposure in animal studies ( Toyama et al, 2021 ) and microglial cell lines ( Sass et al, 2001 ; Thameem Dheen et al, 2007 ; Ge et al, 2019 ). Indeed, in N9 microglial cells treated with several concentrations of MeHg for 8h, the results showed that 10 µM-MeHg induces a significant increase in IL-6 production ( Garg and Chang, 2006 ).…”

Deubiquitinase USP54 attenuates methylmercury toxicity in human embryonic kidney 293 cells

“…The Human Protein Atlas database indicates that USP54 is highly expressed in oligodendrocytes and microglia, glial cells of the central nervous system, and is mainly distributed in mitochondria. We have recently found that excessive activation of microglia, immune cells in the brain, is involved in methylmercury-induced central nervous system damage (Hoshi et al, 2019;Toyama et al, 2021a). Because mitochondria are a main target organelle of methylmercury (Lee et al, 2016;Mori et al, 2011;Sato et al, 2020), USP54 might attenuate its toxicity by suppressing the inflammatory response in the brain induced by methylmercury through an unknown function in mitochondria.…”

Potential Association between Methylmercury Neurotoxicity and Inflammation

“…The differences between these results may arise from differences in experimental conditions; however, the details are unknown. Other cytokines, such as TNF-α 52,54,66,69) and IL-1β, 52,66) are also increased following MeHg exposure, as revealed by mRNA and protein assays. However, reduced expression of TNF-α mRNA following low-dose MeHg exposure 52) and no change of TNF-α following MeHg exposure 53) have also been reported.…”