Mitochondrial Morphology Regulates Organellar Ca2+ Uptake and Changes Cellular Ca2+ Homeostasis

Changes in mitochondrial size and shape have been implicated in several physiological processes, but their role in mitochondrial Ca2+ uptake regulation and overall cellular Ca2+ homeostasis is largely unknown. Here we show that modulating mitochondrial dynamics towards increased fusion through expression of a dominant negative form of the fission protein DRP1 (DRP1-DN) markedly increased both mitochondrial Ca2+ retention capacity and Ca2+ uptake rates in permeabilized C2C12 cells. Similar results were seen using the pharmacological fusion-promoting M1 molecule. Conversely, promoting a fission phenotype through the knockdown of the fusion protein mitofusin 2 (MFN2) strongly reduced mitochondrial Ca2+ uptake speed and capacity in these cells. These changes were not dependent on modifications in inner membrane potentials or the mitochondrial permeability transition. Implications of mitochondrial morphology modulation on cellular calcium homeostasis were measured in intact cells; mitochondrial fission promoted lower basal cellular calcium levels and lower endoplasmic reticulum (ER) calcium stores, as measured by depletion with thapsigargin. Indeed, mitochondrial fission was associated with ER stress. Additionally, the calcium-replenishing process of store-operated calcium entry (SOCE) was impaired in MFN2 knockdown cells, while DRP1-DN-promoted fusion resulted in faster cytosolic Ca2+ increase rates. Overall, our results show a novel role for mitochondrial morphology in the regulation of mitochondrial Ca2+ uptake, which impacts on cellular Ca2+ homeostasis.