THE MITOCHONDRIAL PERMEABILITY TRANSITION IN CELL-DEATH - A COMMON MECHANISM IN NECROSIS, APOPTOSIS AND AUTOPHAGY

Using confocal microscopy, onset of the mitochondrial permeability tra nsition (MPT) in individual mitochondria within living cells can be vi sualized by the redistribution of the cytosolic fluorophore, calcein, into mitochondria. Simultaneously, mitochondria release membrane poten tial-indicating fluorophores like tetramethylrhodamine methylester. Th e MPT occurs in several forms of necrotic cell death, including oxidat ive stress, pH-dependent ischemia/reperfusion injury and Ca2+ ionophor e toxicity. Cyclosporin A (CsA) and trifluoperazine block the MPT in t hese models and prevent cell killing, showing that the MPT is a causat ive factor in necrotic cell death. During oxidative injury induced by t-butylhydroperoxide, onset of the MPT is preceded by pyridine nucleot ide oxidation, mitochondrial generation of reactive oxygen species, an d an increase of mitochondrial free Ca2+, all changes that promote the MPT. During tissue ischemia, acidosis develops. Because of acidotic p H, anoxic cell death is substantially delayed. However, when pH is res tored to normal after reperfusion (reoxygenation at pH 7.4), cell deat h occurs rapidly (pH paradox). This killing is caused by pH-dependent onset of the MPT, which is blocked by reperfusion at acidotic pH or wi th CsA. Zn isolated mitochondria, toxicants causing Reye's syndrome, s uch as salicylate and valproate, induce the MPT. Similarly, salicylate induces a CsA-sensitive MPT and killing of cultured hepatocytes. Thes e in vitro findings suggest that the MPT is the pathophysiological mec hanism underlying Reye's syndrome in vivo. Kroemer and coworkers propo sed that the MPT is a critical event in the progression of apoptotic c ell death. Using confocal microscopy, the MPT can be directly document ed during tumor necrosis factor-alpha induced apoptosis in hepatocytes . CsA blocks this MPT and prevents apoptosis. The MPT does not occur u niformly during apoptosis. Initially, a small proportion of mitochondr ia undergo the MPT, which increases to nearly 100% over 1-3 h. A techn ique based on fluorescence resonance energy transfer can selectively r eveal mitochondrial depolarization. After nutrient deprivation, a smal l fraction of mitochondria spontaneously depolarize and enter an acidi c lysosomal compartment, suggesting that the MPT precedes the normal p rocess of mitochondrial autophagy. A model is proposed in which onset of the MPT to increasing numbers of mitochondria within a cell leads p rogressively to autophagy, apoptosis and necrotic cell death. (C) 1998 Elsevier Science B.V. All rights reserved.