Bax Regulates Neuronal Ca 2+ Homeostasis

Excessive Ca 2+ entry during glutamate receptor overactivation (“excitotoxicity”) induces acute or delayed neuronal death. We report here that deficiency in bax exerted broad neuroprotection against excitotoxic injury and oxygen/glucose deprivation in mouse neocortical neuron cultures and reduced infarct size, necrotic injury, and cerebral edema formation after middle cerebral artery occlusion in mice. Neuronal Ca 2+ and mitochondrial membrane potential (Δψ m ) analysis during excitotoxic injury revealed that bax -deficient neurons showed significantly reduced Ca 2+ transients during the NMDA excitation period and did not exhibit the deregulation of Δψ m that was observed in their wild-type (WT) counterparts. Reintroduction of bax or a bax mutant incapable of proapoptotic oligomerization equally restored neuronal Ca 2+ dynamics during NMDA excitation, suggesting that Bax controlled Ca 2+ signaling independently of its role in apoptosis execution. Quantitative confocal imaging of intracellular ATP or mitochondrial Ca 2+ levels using FRET-based sensors indicated that the effects of bax deficiency on Ca 2+ handling were not due to enhanced cellular bioenergetics or increased Ca 2+ uptake into mitochondria. We also observed that mitochondria isolated from WT or bax -deficient cells similarly underwent Ca 2+ -induced permeability transition. However, when Ca 2+ uptake into the sarco/endoplasmic reticulum was blocked with the Ca 2+ -ATPase inhibitor thapsigargin, bax -deficient neurons showed strongly elevated cytosolic Ca 2+ levels during NMDA excitation, suggesting that the ability of Bax to support dynamic ER Ca 2+ handling is critical for cell death signaling during periods of neuronal overexcitation.