Abstract 203: Differences Between Mammalian and Drosophila Mitochondrial Calcium Handling Help Identify Mechanisms Regulating the Permeability Transition.

Studies of cardiomyocyte death during calcium overload induced by ischemia-reperfusion injury or heart failure have implicated the mitochondrial permeability transition as a key pathway. During the permeability transition, an opening of a channel in the inner membrane leads to mitochondrial depolarization and swelling. Despite extensive studies in mammalian systems, the machinery responsible for this phenomenon remains only partially identified. If present in non-mammalian species, the components of the permeability transition may be further elucidated, given potential advantages within these systems for high-throughput screens. However, the existence of a permeability transition remains controversial in non-mammalian organisms. In Drosophila , prior studies have documented calcium-induced mitochondrial depolarization, but no obvious swelling. Here we show that Drosophila S2R+ cells do possess the machinery for permeability transition, but that the threshold for a calcium trigger is significantly higher than in mammalian systems. Using a calcein-loading method, we show that Drosophila permeability transition can be triggered by calcium overload, using ionomycin, and by cysteine oxidation, using phenylarsine oxide. As in mammalian systems, blockade of mitochondrial cyclophilin or the ATP/ADP transporter appears to inhibit the Drosophila permeability transition. Finally, we examine three alternative hypotheses that may explain these differences in permeability transition. First, we test if perturbing the pathways for calcium influx into S2R+ mitochondria can trigger this phenomenon. Second, we test if the discrepancy in the calcium threshold is due to structural differences in the key regulators, particularly the mitochondrial cyclophilin. Third, we compare Drosophila and human genomes to see if any novel molecules may be responsible for setting the lower threshold for calcium-induced permeability transition in mammalian cells. Since the Drosophila cells possess such significant resistance to permeability transition, the results of our investigations suggest potential new strategies for the development of therapeutics inhibiting mitochondrial permeability transition in cardiac calcium-induced injury.