Role of Mitochondrial Morphology in Bioenergetics

Abstract

Mitochondria in cells undergo constant morphological changes mainly through fission and fusion. However, functional significance of mitochondrial fission and fusion is not fully understood. To test the importance of mitochondrial morphology in maintaining mitochondrial function, first, we used glucose-stimulated insulin secretion in pancreatic β-cells as an experimental model because insulin secretion upon elevated plasma glucose concentration requires intact mitochondrial function. Increased ATP production in mitochondria from glucose metabolism induces plasma membrane depolarization and subsequent increase of cytosolic Ca2+ triggers insulin exocytosis. We found that glucose stimulation of the β-cell line INS-1E induces transient mitochondrial shortening and recovery. Inhibiting mitochondrial fission by expressing the dominant-negative fission mutant DLP1-K38A abolished the dynamic change of mitochondrial morphology in glucose stimulation. Importantly, we discovered that abolition of the glucose-induced mitochondrial morphology change suppresses glucose-stimulated insulin secretion. Measuring respiration under fission inhibition showed an increase of mitochondrial uncoupling, and thus significantly diminished the mitochondrial ATP production in response to glucose stimulation. Further evaluation of mitochondrial membrane potential in primary hepatocytes revealed that inhibition of mitochondrial fission induces large-scale fluctuations of the potentiometric fluorescence in mitochondria within cells. Frequencies and intervals of the fluorescence oscillation were random and insensitive to inhibitors of anion channels and mitochondrial permeability, and superoxide scavenger. This suggests that the fission inhibition-induced fluctuation of the inner membrane potential is a previously unrecognized unique phenomenon. These observations demonstrate that inhibition of mitochondrial fission induces a large-scale fluctuation of the mitochondrial inner membrane potential, which is functionally reflected in mitochondrial uncoupling. Taken together, our findings indicate that mitochondrial fission plays a role in regulating the coupling efficiency of oxidative phosphorylation.