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Programmed cell death is induced by many different factors and involves numerous signaling pathways, some dependent on caspase proteases and others that are caspase independent. Like cytochrome C, AIF (apoptosis-inducing factor) is a protein found in mitochondria that is released from mitochondria by apoptotic stimuli. While cytochrome C is linked to caspase-dependent apoptotic signaling, AIF release stimulates caspase-independent apoptosis, moving into the nucleus where it binds DNA. DNA binding by AIF stimulates chromatin condensation, and DNA fragmentation, perhaps through recruitment of nucleases. DNA-binding by AIF occurs through a distinct domain of the protein in a manner that does not rely on specific DNA sequences. AIF also has another domain that acts as an NADH oxidase, a redox enzyme. The NADH oxidase activity of AIF is separable from its DNA-binding activity and is not required for AIF to induce apoptosis. In this role, AIF protects against apoptosis rather than inducing apoptosis. In mice down regulation of the AIF gene created harlequin mice with oxidative damage to neurons and increased neuronal cell death. Loss of AIF also increased sensitivity to peroxides, suggesting that AIF may act as a peroxide scavenger. The mechanism by which AIF protects against peroxide damage is not clear, but may involve a more indirect mechanism than direct peroxide scavenging since the enzyme does not appear to have this activity in vitro. Elucidating further the role of AIF as a redox enzyme will shed further light on its normal function in mitochondria as well as its role in apoptosis and disease. (This definition may be outdated - see the DesignNote.)
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