Pivotal role of mitochondrial calcium uptake in neural cell apoptosis and necrosis

Perturbed cellular calcium homeostasis has been implicated in both apoptosi s and necrosis, but the role of altered mitochondrial calcium handling in t he cell death process is unclear. The temporal ordering of changes in cytop lasmic ([Ca2+]C) and intramitochondrial ([Ca2+]M) calcium levels in relatio n to mitochondrial reactive oxygen species (ROS) accumulation and membrane depolarization (MD) was examined in cultured neural cells exposed to either an apoptotic (staurosporine; STS) or a necrotic (the toxic aldehyde 4-hydr oxynonenal; HNE) insult. STS and HNE each induced an early increase of [Ca2 +]C followed by delayed increase of [Ca2+]M. Overexpression of Bcl-2 blocke d the elevation of [Ca2+]M and the MD in cells exposed to STS but not in ce lls exposed to HNE. The cytoplasmic calcium chelator BAPTA-AM and the inhib itor of mitochondrial calcium uptake ruthenium red prevented both apoptosis and necrosis. STS and HNE each induced mitochondrial ROS accumulation and MD, which followed the increase of [Ca2+]M. Cyclosporin A prevented both ap optosis and necrosis, indicating critical roles for MD in both forms of cel l death. Caspase activation occurred only in cells undergoing apoptosis and preceded increased [Ca2+]M. Collectively, these findings suggest that mito chondrial calcium overload is a critical event in both apoptotic and necrot ic cell death.