Several cellular mechanisms are known to be involved in methylmercury (MeHg) induced central nervous system (CNS) toxicity, including the dysregulation of intracellular Ca2+, redox, and glutamate homeostasis. However, the factors that make particular neurons susceptible to MeHg toxicity, and the latency period of neurological signs and symptoms, have not yet been clearly delineated. For example, the spinal dorsal root ganglia (DRG) is the primary target of MeHg. Mercury (Hg) granules are first detected in spinal cord motor neurons (SMNs) in the non-symptomatic phase, whereas Hg granules are detected in glia later, following with neurological symptoms (Møller-Madsen, 1991). This finding suggested that the latent period (non-symptomatic phase) is associated with Hg accumulation in neurons, while the symptomatic phase occurs following Hg accumulation in glia, and the susceptibility is not associated with Hg granule accumulation in cells (Møller-Madsen, 1991). Astrocytes generally provide glutathione (GSH) for neurons to detoxify toxic insult. In the spinal cord, MeHg might perturb the antioxidant pathway, Keap1-Nrf2-ARE pathway in the spinal cord astrocytes (SCAs) consequently contribute to DRG or SMN susceptibility to MeHg toxicity. In this study, the comparative responses of different SCAs maturity to a non-toxic MeHg concentration (0.1 μM) suggested that the fully mature SCAs (Day in vitro 30; DIV30), were more susceptible to MeHg than SCAs on DIV14. The perturbation of the Keap1-Nrf2-ARE pathway in SCAs (DIV 30) during exposure to sub-toxic MeHg concentration (0.50 μM) caused a biphasic increase in antioxidant genes such as Keap1, Nrf2, Gclc, Abcc1 mRNAs expression. The concomitant increase of glutamate transporter Slc7a11 encoded for the system Xc-, and Slc1a3 encoded for EAAT1, and Slc1a2 encoded for EAAT2 expression during MeHg exposure might suggest the cooperative expression or function of these glutamate transporters. This concomitant expression was further demonstrated in studies using Nrf2-knockout (Nrf2-KO) derived SCAs. The increase of basal Slc7a11 mRNA, was concurrent to the increase of basal Slc1a3 and Slc1a2 mRNA expressions in Nrf2-KO derived SCA. The function of time of MeHg exposure indicated that Nrf2-KO derived SCAs were more susceptible to MeHg than the wild-type (WT)-derived SCAs. The pronounced susceptibility of Nrf2-KO derived SCAs was mainly due to the loss of GSH) metabolism and transport genes Gclc, GPx1, GPx4, and Abcc1 mRNAs in this genotype. MeHg significantly reduced these mRNA expressions in both genotypes. However, not all Nrf2-ARE regulated genes were affected by MeHg in similar ways in these genotypes. For example, MeHg induced the increase of Slc7a11 mRNA expression in WT-derived SCAs, but it appears to cause the reduction of this mRNA expression in Nrf2 KO-derived SCAs. Administration of antioxidant N-acetyl-L-cystine (NAC) in pretreatment (NP), co-treatment (CO), and post-treatment of MeHg (MP) prevented the reduction of SCAs metabolic functions for over 160h. The mechanism of NAC action in preventing MeHg induced SCAs degeneration is primarily due to its thiol antioxidant property.In conclusion, this study suggests that age and genetic predisposition contribute to SCAs susceptibility to MeHg toxicity. The dysregulation of the antioxidant pathways and glutamate homeostasis in SCAs potentially contributes to SMNs or DRG susceptible to MeHg.
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