Mitochondrial calcium overload triggers complement-dependent superoxide-mediated programmed cell death in Trypanosoma cruzi

The epimastigote stage of Trypanosoma cruzi undergoes PCD (programmed cell death) when exposed to FHS (fresh human serum). Although it has been known for over 30 years that complement is responsible for FHS-induced death, the link between complement activation and triggering of PCD has not been established. We have previously shown that the mitochondrion participates in the orchestration of PCD in this model. Several changes in mitochondrial function were described, and in particular it was shown that mitochondrion-derived O-2(center dot-) (superoxide radical) is necessary for PCD. In the present study, we establish mitochondrial Ca2+ overload as the link between complement deposition and the observed changes in mitochondrial physiology and the triggering of PCD. We show that complement activation ends with the assembly of the MAC (membrane attack complex), which allows influx of Ca2+ and release of respiratory substrates to the medium. Direct consequences of these events are accumulation Of Ca2+ in the mitochondrion and decrease in cell respiration. Mitochondrial Ca2+ causes partial dissipation of the inner membrane potential and consequent, mitochondrial uncoupling. Moreover, we provide evidence that mitochondrial Ca2+ overload is responsible for the increased O-2(center dot-) production, and that if cytosolic Ca2+ rise is not accompanied by the accumulation of the cation in the mitochondrion and consequent production of O-2(center dot-), epimastigotes die by necrosis instead of PCD. Thus Our results suggest a model in which MAC assembly oil the parasite surface allows Ca2+ entry and its accumulation in the mitochondrion, leading to O-2(center dot-) production, which in turn constitutes a PCD signal.