Mitochondria and cell death

Mitochondria play a central role in both apoptosis and necrosis through the opening of the mitochondrial permeability transition pore (MPTP). This is thought to be formed through a Ca2+-triggered conformational change of the adenine nucleotide translocase (ANT) bound to matrix cyclophilin-D and we h ave now demonstrated this directly by reconstitution of the pure components . Opening of the MPTP causes swelling and uncoupling of mitochondria which, unrestrained, leads to necrosis. In ischaemia/reperfusion injury of the he art we have shown MPTP opening directly. Recovery of hearts correlates with subsequent closure, and agents that prevent opening or enhance closure pro tect from injury. Transient MPTP opening may also be involved in apoptosis by initially causing swelling and rupture of the outer membrane to release cytochrome c (cyt c), which then activates the caspase cascade and sets apo ptosis in motion. Subsequent MPTP closure allows ATP levels to be maintaine d, ensuring that cell death remains apoptotic rather than necrotic, Apoptos is in the hippocampus that occurs after a hypoglycaemic or ischaemic insult is triggered by this means. Other apoptotic stimuli such as cytokines or r emoval of growth factors also involve mitochondrial cyt c release, but here there is controversy over whether the MPTP is involved. In many cases cyt c release is seen without any mitochondrial depolarization, suggesting that the MPTP does not open. Recent data of our own and others have revealed a specific outer-membrane cyt c-release pathway involving porin that does not release other intermembrane proteins such as adenylate kinase. This is ope ned by pro-apototic members of the Bcl-2 family such as BAX and prevented b y anti-apoptotic members such as Bcl-x(L). Our own data suggest that this p athway may interact directly with the ANT in the inner membrane at contact sites.