Methylmercury Induces Acute Oxidative Stress, Altering Nrf2 Protein Level in Primary Microglial Cells

Abstract

The neurotoxicity of methylmercury (MeHg) is well documented in both humans and animals. MeHg causes acute and chronic damage to multiple organs, most profoundly the central nervous system (CNS). Microglial cells are derived from macrophage cell lineage, making up approximately 12% of cells in the CNS, yet their role in MeHg-induced neurotoxicity is not well defined. The purpose of the present study was to characterize microglial vulnerability to MeHg and their potential adaptive response to acute MeHg exposure. We examined the effects of MeHg on microglial viability, reactive oxygen species (ROS) generation, glutathione (GSH) level, redox homeostasis, and Nrf2 protein expression. Our data showed that MeHg (1-5 microM) treatment caused a rapid (within 1 min) concentration- and time-dependent increase in ROS generation, accompanied by a statistically significant decrease in the ratio of GSH and its oxidized form glutathione disulfide (GSSG) (GSH:GSSG ratio). MeHg increased the cytosolic Nrf2 protein level within 1 min of exposure, followed by its nuclear translocation after 10 min of treatment. Consistent with the nuclear translocation of Nrf2, quantitative real-time PCR revealed a concentration-dependent increase in the messenger RNA level of Ho-1, Nqo1, and xCT 30 min post MeHg exposure, whereas Nrf2 knockdown greatly reduced the upregulation of these genes. Furthermore, we observed increased microglial death upon Nrf2 knockdown by the small hairpin RNA approach. Taken together, our study has demonstrated that microglial cells are exquisitely sensitive to MeHg and respond rapidly to MeHg by upregulating the Nrf2-mediated antioxidant response.

Effects of Centrally Administered Etanercept on Behavior, Microglia, and Astrocytes in Mice Following a Peripheral Immune Challenge

“…This may relate to the dose of LPS, as chronic administration (daily for 7 days) generated an astrocyte response in another study, whereas in our study only a single LPS administration was employed (Bian et al, 2013). Another possibility is that there is a temporal difference between the reactivity of the two cell types, with microglia responding more quickly to this insult than astrocytes (Kuhlmann and Guilarte, 2000;Ni et al, 2010). Kuhlmann and Guilarte (2000) also reported that while microglia responded within 48 h to an insult generated by administration of neurotoxicant trimethyltin, astrocytic response was delayed, seen at 14 days but not 48 h following injury.…”

Glia and Methylmercury Neurotoxicity

“…As a key protective protein against oxidative stress, Nrf-2 increases in both cell types after MeHg exposure (Ni et al 2010; Wang et al 2009). In the absence of oxidative stress, Nrf2 is bound to Kelch-like ECH-associating protein 1 (Keap1) protein in the cytoplasm and the complex is degraded by proteasomes (Kensler and Wakabayashi 2010).…”

Role of Intracellular Defense Factors against Methylmercury Toxicity

“…11) Since the revelation of these findings, Nrf2 has been found to contribute to the reduction of MeHg toxicity in astrocytes and microglial cells. 14,15) Recent research has also shown that administration of the Nrf2 activator sulforaphane to mice suppressed MeHg neurotoxicity. 16) Therefore, from cultured cells to mice, Nrf2 is thought to function as a protective factor against MeHg toxicity.…”