Differential dysregulation of glutamate transporter expression contributes to relative susceptibility to methylmercury of wild-type, and Nrf2-knockout derived spinal astrocytes.

Dysregulation of redox and glutamate homeostasis is involved in methylmercury (MeHg)- induced neuronal and astrocytic degeneration. MeHg disrupts not only the expression of Nrf2, a master regulator of the antioxidant pathway but also glutamate transporter expression and its concentration. To characterize the role of Nrf2 in MeHg- induced spinal astrocyte degeneration, spinal cord astrocytes (SCAs) viability and antioxidant genes from Nrf2 knockout derived SCAs (Nrf2-KO SCA) were compared to the wild-type derived SCA (WT-SCAs). The results showed that Nrf2-KO SCAs were more susceptible to MeHg compared to WT-SCAs.  A one-hour exposure to 5 µM MeHg significantly reduced the viability of Nrf2-KO SCAs by 60% compared to WT-SCAs. The viability of Nrf2-KO SCAs further decreased and was considerably lower than the WT-SCA with prolonged exposure to MeHg. While antioxidant genes such as Gclc, GPx1, and GPx4 involved in glutathione synthesis were significantly reduced in both WT- and Nrf2-KO SCAs during MeHg exposure, the cystine/glutamate antiporter (or system Xc-) gene (Slc7a11) increased in WT-SCA (5-fold induction) and appeared to reduce in Nrf2-KO SCAs. The excitatory amino acid transporter 2 (EAAT2) gene, Slc1a2 was significantly reduced following MeHg exposure for 18h in both WT- and Nrf2-KO SCAs compared to their same genotype treated with the vehicle. The EAAT1 gene, Slc1a3 was significantly reduced only in Nrf2-KO derived SCAs, but only slightly affected in the WT SCAs. There was a differential expression of Slc1a3 and Slca1a2 in Nrf2 KO SCAs in which the Slc1a3 expression was significantly higher than WT-SCAs (4-fold higher), and Slc1a2 expression was about 2-fold higher than WT SCAs. The perturbation of Nrf2 in MeHg-induced SCA toxicity involved the reduction of GSH synthesis genes. Differential susceptibility between Nrf2-KO SCA and WT-SCA through the differential glutamate transporter expressions suggests a genetic background also plays a role in differential susceptibility to MeHg.