Methyl-mercury induces apoptosis through ROS-mediated endoplasmic reticulum stress and mitochondrial apoptosis pathways activation in rat cortical neurons

Methyl-mercury (MeHg) is a dangerous environmental contamination biotransformed from mercury or inorganic mercury compounds in waterways, which induces severe toxic effects in central nervous system. Oxidative stress is involved in various ways of intracellular physiological or pathological processes including neuronal apoptosis. For understanding the ways that oxidative stress participating in MeHg-induced apoptosis, the current study attempted to explore the effects of oxidative stress on endoplasmic reticulum (ER) and mitochondria function, especially focussing on ER stress followed by unfold protein response (UPR), as well as mitochondrial apoptosis pathways activation in primary cultured cortical neurons. Cells were exposed to 0, 0.25, 0.5, or 1 mu M MeHg for 1-6 h, respectively, followed by cell viability quantification. For further experiments, 100 mu M of 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (trolox) pre-treatment for 3 h followed by 1 mu M MeHg for 6 h were performed for evaluation of oxidative stress, neuronal apoptosis, ER stress, UPR activation, and mitochondrial dysfunction. Results showed that MeHg induced neuronal oxidative stress, apoptosis, up-regulating glucose-regulated protein (GRP78, GRP94), spliced Xbp1, activating transcription factor 4 (ATF4) mRNA, with activation of UPR including PKR-like ER kinase-eIF2 alpha, inositol-requiring enzyme 1, and ATF6 pathways, as well as C/EBP homologous transcription factor protein and cleaved caspase-12 up-regulation. In addition, mitochondrial function was disrupted by MeHg, which was supported by caspase-3 and caspase-9 activation, and high levels of cytoplasm cytochrome C and apoptosis induce factor. Trolox pre-treatment significantly blocked neuronal apoptosis, ER stress, UPR activation, as well as mitochondrial dysfunction, in addition to the direct anti-oxidation. In conclusion, MeHg induces neuronal apoptosis through ER and mitochondria pathway, oxidative stress plays important roles in mediating apoptosis pathways activation.