Unbalanced ER-mitochondrial calcium homeostasis promotes mitochondrial dysfunction and associated apoptotic pathways...

Methylmercury (MeHg) is a cumulative environmental pollutant that can easily cross the blood-brain barrier and cause damage to the brain, mainly targeting the central nervous system. The purpose of this study is to investigate the role of calcium ion (Ca²⁺ ) homeostasis between the endoplasmic reticulum (ER) and mitochondria in MeHg-induced neurotoxicity. Rat primary cortical neurons exposed to MeHg (0.25-1 μm) underwent dose-dependent cell damage, accompanied by increased Ca²⁺ release from the ER and elevated levels of free Ca²⁺ in cytoplasm and mitochondria. MeHg also increased the protein and messenger RNA expressions of the inositol 1,4,5-triphosphate receptor, ryanodine receptor 2, and mitochondrial calcium uniporter. Ca²⁺ channel inhibitors 2-aminoethyl diphenylborinate and procaine reduced the release of Ca²⁺ from ER, while RR and 4,4'-diisothiocyanatostilbene-2,2'-disulfonate inhibited Ca²⁺ uptake from mitochondria. In addition, pretreatment with Ca²⁺ chelator BAPTA-AM effectively restored mitochondrial membrane potential levels, inhibited over opening of mitochondrial permeability transition pore, and maintained mitochondrial function stability. Meanwhile, the expression of mitochondrial apoptosis-related proteins recovered to some extent, along with the reduction of the early apoptosis ratio. These results suggest that Ca²⁺ homeostasis plays an essential role in mitochondrial damage and apoptosis induced by MeHg, which may be one of the important mechanisms of MeHg-induced neurotoxicity.