Loss of mitochondrial membrane integrity and release of apoptogenic factors are a key step in the signaling cascade leading to neuronal cell death in various
neurological disorders, including ischemic injury. Emerging evidence has suggested that the intramitochondrial
protein apoptosis-inducing factor (AIF) translocates to the nucleus and promotes
caspase-independent cell death induced by
glutamate toxicity, oxidative stress,
hypoxia, or
ischemia. However, the mechanism by which AIF is released from mitochondria after neuronal injury is not fully understood. In this study, we identified
calpain I as a direct activator of AIF release in neuronal cultures challenged with
oxygen-
glucose deprivation and in the rat model of transient global
ischemia. Normally residing in both neuronal cytosol and mitochondrial intermembrane space,
calpain I was found to be activated in neurons after
ischemia and to cleave intramitochondrial AIF near its N terminus. The truncation of AIF by
calpain activity appeared to be essential for its translocation from mitochondria to the nucleus, because neuronal transfection of the mutant AIF resistant to
calpain cleavage was not released after
oxygen-
glucose deprivation. Adeno-associated virus-mediated overexpression of
calpastatin, a specific
calpain-inhibitory
protein, or
small interfering RNA-mediated knockdown of
calpain I expression in neurons prevented
ischemia-induced AIF translocation. Moreover, overexpression of
calpastatin or knockdown of AIF expression conferred neuroprotection against cell death in neuronal cultures and in hippocampal CA1 neurons after transient global
ischemia. Together, these results define
calpain I-dependent AIF release as a novel signaling pathway that mediates neuronal cell death after
cerebral ischemia.