Endoplasmic reticulum Ca2+ release through ryanodine and IP3 receptors contributes to neuronal excitotoxicity

Overactivation of ionotropic glutamate receptors induces a Ca2+ overload into the cytoplasm that leads neurons to excitotoxic death, a process that has been linked to several neurodegenerative disorders. While the role of mitochondria and its involvement in excitotoxicity have been widely studied, the contribution of endoplasmic reticulum (ER), another crucial intracellular store in maintaining Ca2+ homeostasis, is not fully understood. In this study, we analyzed the contribution of ER-Ca2+ release through ryanodine (RyR) and IP3 (IP3R) receptors to a neuronal in vitro model of excitotoxicity. NMDA induced a dose-dependent neuronal death, which was significantly decreased by ER-Ca2+ release inhibitors in cortical neurons as well as in organotypic slices. Furthermore, ryanodine and 2APB, RyR and IP3R inhibitors respectively, attenuated NMDA-triggered intracellular Ca2+ increase and oxidative stress, whereas 2APB reduced mitochondrial membrane depolarization and caspase-3 cleavage. Consistent with ER-Ca2+ homeostasis disruption, we observed that NMDA-induced ER stress, characterized here by eIF2α phosphorylation and over-expression of GRP chaperones which were regulated by ER-Ca2+ release inhibitors. These results demonstrate that Ca2+ release from ER contributes to neuronal death by both promoting mitochondrial dysfunction and inducing specific stress and apoptosis pathways during excitotoxicity.