A family of cysteine proteases (caspases) form the core machinery of programmed cell death (PCD) in mammals. However, in certain contexts, cell death proceeds normally in the absence of caspase function, suggesting that there exist caspase-independent pathways of PCD. Recently, a novel death effector called apoptosis-inducing factor (AIF) was cloned and shown in cell lines to induce hallmarks of PCD independent of caspases. To explore the physiological role of AIF, I disrupted the mouse aif gene in embryonic stem cells using gene targeting. I show that AIF is required for cell death in response to serum withdrawal, but not to DNA damage or inhibition of kinases. Moreover, in response to oxidative stress, AIF and caspases function redundantly to induce cell death. These results provide the first genetic evidence for a caspaseindependent pathway of PCD. Furthermore, they support a model whereby distinct death stimuli trigger distinct effector pathways of cell death.
Cardiac and skeletal muscle critically depend on mitochondrial energy metabolism for their normal function. Recently, we showed that apoptosis-inducing factor (AIF), a mitochondrial protein implicated in programmed cell death, plays a role in mitochondrial respiration. However, the in vivo consequences of AIF-regulated mitochondrial respiration resulting from a loss-of-function mutation in Aif are not known. Here, we report tissue-specific deletion of Aif in the mouse. Mice in which Aif has been inactivated specifically in cardiac and skeletal muscle exhibit impaired activity and protein expression of respiratory chain complex I. Mutant animals develop severe dilated cardiomyopathy, heart failure, and skeletal muscle atrophy accompanied by lactic acidemia consistent with defects in the mitochondrial respiratory chain. Isolated hearts from mutant animals exhibit poor contractile performance in response to a respiratory chain-dependent energy substrate, but not in response to glucose, supporting the notion that impaired heart function in mutant animals results from defective mitochondrial energy metabolism. These data provide genetic proof that the previously defined cell death promoter AIF has a second essential function in mitochondrial respiration and aerobic energy metabolism required for normal heart function and skeletal muscle homeostasis.
