In vitro investigation of caspase-3 dependent and independent apoptotic processes in cortical and hippocampal neurons triggered by S-sulfocysteine-induced cell death

Purpose: S-sulfocysteine (SSC), a toxic byproduct of sulfur-containing amino acid metabolism, accumulates in conditions such as molybdenum cofactor deficiency (MoCD) and isolated sulfite oxidase deficiency (iSOD), leading to severe neurodegeneration. Despite evidence of SSC's neurotoxicity, the apoptotic mechanisms it triggers remain unclear, particularly in different brain regions. This study aimed to investigate the region-specific apoptotic pathways induced by SSC in hippocampal HT-22 and primary cortical neurons, focusing on caspase-dependent and -independent mechanisms. Materials and methods: Neuronal cells were treated with increasing doses of SSC, and cell viability, glutathione (GSH) levels, and apoptosis-related proteins (AIF, calpain, cytochrome c, caspase-3) were assessed using CCK-8 assay, GSH enzymatic assay, and ELISA. The protective effects of specific inhibitors targeting AIF, calpain, and cytochrome c were also evaluated. Results: SSC reduced cell viability in both neuronal types with half-maximal lethal dose (LD50) values of 150 μM (HT-22) and 155 μM (cortical neurons). In both models, SSC elevated AIF and calpain levels, whereas cytochrome c and caspase-3 were significantly increased only in cortical neurons. GSH levels initially rose at 2–8 hours and declined by 16 hours. Inhibitors of AIF, calpain, and cytochrome c partially restored viability, with combined administration offering the most robust protection. Conclusion: SSC induces both caspase-independent and caspase-dependent apoptosis in a region-specific manner: HT-22 cells predominantly activate AIF and calpain, while cortical neurons engage additional cytochrome c and caspase-3 pathways. These findings suggest distinct molecular vulnerabilities and offer potential targets for therapeutic intervention in sulfite-related neurodegenerative diseases.