The place of mitochondria in apoptosis

Apoptosis research has recently experienced a change from a paradigm in which the nucleus determined the apoptotic process to a paradigm in which caspases and, more recently, mitochondria constitute the center of death control. Mitochondria undergo major changes in membrane integrity before classical signs of cell death become manifest. These changes concern both the inner and the outer mitochondrial membranes, leading to the dissipation of the inner transmembrane potential and/or the release of intermembrane proteins through the outer membrane. An ever increasing number of endogenous, viral, or xenogeneic effectors directly act on mitochon-dria to trigger permeabilization. At least in some cases, this is achieved by a direct action on the permeability transition pore complex (PTPC), a multi-protein ensemble containing proteins from both mitochondrial membranes which interact with pro- and anti-apoptotic members of the Bcl-2 family. At present, it is elusive whether opening of the PTPC is the only physiological mechanism leading to mitochondrial membrane permeabilization. Proteins released from mitochondria during apoptosis include caspases (mainly caspases 2, 3 and 9), caspase activators (cytochrome c, hsp 10, Smac/DIABLO), as well as a caspase-independent death effector, AIF (apoptosis inducing factor). Apoptosis inducing factor (AIF) is encoded for by one single gene located on the X chromosome. AIF is ubiquitously expressed, both in normal tissues and in a variety of cancer cell lines.

The AIF precursor is synthesized in the cytosol and is imported into mitochondria, The mature AIF protein, a flavoprotein (prosthetic group: FAD) with significant homology to plant ascorbate reductases and bacterial NADH oxidases, is normally confined to the mitochondrial intermembrane space. In a variety of different apoptosis-inducing conditions, AIF translocates through the outer mitochondrial membrane to the cytosol and to the nucleus. Ectopic (extra-mitochondrial) AIF increases the permeability of the outer mitochondrial membrane, thereby triggering the release of the caspase activator cytochrome c. Moreover, AIF induces nuclear chromatin condensation, as well as large scale (∼ 50 kbp) DNA fragmentation. Thus, similar to cytochrome c, AIF is a phylogenetically old, bifunctional protein with an electron acceptor/donor (oxidore-ductase) function and a second apoptogenic function. In contrast to cytochrome c, however, AIF acts in a caspase-independent fashion. The molecular mechanisms via which AIF induces apoptosis, as well as the phenotype of AIF knock-out cells will be discussed.