Molecular mechanism of Ca2+-induced mitochondrial shape transition in metazoans

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PROJECT SUMMARY / ABSTRACT Ca2+ is a critical second messenger that is required for several cellular processes. Cytosolic Ca2+ (cCa2+) transients are shaped by the mitochondria due to the highly negative membrane potential and through the mitochondrial calcium uniporter (MCU). Mitochondrial Ca2+ (mCa2+) is utilized by the matrix dehydrogenases for maintaining cellular bioenergetics. Reciprocally, dysregulated elevation of cCa2+ under conditions of stroke, ischemia/reperfusion injury drives mCa2+ overload that in turn leads to mitochondrial permeability transition pore opening that triggers necrotic cell death. Hence, it was thought that preventing mCa2+ overload can be protective under conditions of elevated cCa2+. Contrary to this, mice knocked-out for MCU, which demonstrated no mCa2+ uptake and hence no mitochondrial swelling, surprisingly did not offer any protection from IR mediated cell death, suggesting that loss of MCU-mediated Ca2+ overload was not sufficient to protect cells from Ca2+-induced necrosis. To understand the molecular mechanisms of elevated Ca2+-induced cell death, we performed ultra-structural analysis of liver harvested from liver specific MCU-/- (MCUHEP) and MCUfl/fl animals. Electron microscopy studies revealed stark contrast in the shape of mitochondria: MCUfl/fl liver sections showed long and filamentous mitochondria (spaghetti-like) while MCUHEP mitochondria were short and circular (donut-like). We hypothesized this Mitochondrial Shape Transition phenomenon that we refer hereafter as MiST, to be cCa2+-induced and independent of mitochondrial swelling or Drp1-mediated mitochondrial fission. Based on our preliminary results, we hypothesize that pathophysiological elevation of cCa2+ induces MiST and that is Miro-1 driven. Because cellular mitochondrial networks allow for the sharing of metabolites, proteins, mitochondrial DNA and potential energy distribution, there is an extensive risk for local mitochondrial failures to quickly spread over the entire network and compromise cellular energy conversion. Like power networks that physically segment elements with circuit breakers, we hypothesize that MiST protects mitochondrial networks from propagating local failures. Our recently completed whole genome-wide CRISPR/Cas9 Library screen in MEFs identified a conserved protein, S100z to be the cytosolic component for MiST. We expect MiST to be a sequential step with a major determinant to be the cCa2+ transients and the molecular component to be shared by the cytosol (S100Z) and the mitochondria (Miro1). We also hypothesize that MiST is likely to be conserved in metazoans and would facilitate lysosomal removal by autophagy/mitophagy depending on the varying cCa2+ transients, thus preserving the quality of the mitochondrial network. The revelation of this Ca2+-induced phenomenon and the identification of the molecular components will resolve the spatio-temporal molecular mechanisms of MiST. Successful accomplishment of our proposed experiments using our cellular, biochemical, and imaging techniques will authentically demonstrate MiST to be key regulator in maintaining mitochondrial quality control under pathophysiological conditions.