Identification and characterization of AIFsh2, a mitochondrial apoptosis-inducing factor (AIF) isoform with NADH oxidase activity

Apoptosis-inducing factor (AIF) is a bifunctional NADH oxidase involved in mitochondrial respiration and caspase-independent apoptosis. Three alternatively spliced mRNA isoforms of AIF have been identified previously: AIF, AIF-exB, and AIFsh. Here, we report the cloning and the biochemical characterization of a new isoform named AIF short 2 (AIFsh2). AIFsh2 transcript includes a previously unknown exon placed between exons 9 and 10 of AIF. The resulting AIFsh2 protein, which localizes in mitochondria, corresponds to the oxidoreductase domain of AIF. In this way, AIFsh2 exhibits similar NADH oxidase activity to AIF and generates reactive oxygen species. Like AIF, AIFsh2 is released from mitochondria to cytosol after an apoptotic insult in a calpain or cathepsin-dependent manner. However, in contrast to AIF, AIFsh2 does not induce nuclear apoptosis. Thus, it seems that the reactive oxygen species produced by the oxidoreductase domain of AIF/AIFsh2 are not important for AIF-dependent nuclear apoptosis. In addition, we demonstrate that the AIFsh2 mRNA is absent in normal brain tissue, whereas it is expressed in neuroblastoma-derived cells, suggesting a different regulation in normal and transformed cells from the brain lineage. Together, our results reveal that AIF yields an original and independent genetic regulation of the two AIF functions. This is an important issue to understand the physiological role of this protein.