Elevated intracellular calcium causes distinct mitochondrial remodelling and calcineurin-dependent fission in astrocytes

Disruptions of mitochondrial dynamics have been implicated in the pathogenesis of neurodegenerative diseases. The regulation mechanisms of mitochondrial dynamics have not been fully elucidated; however, calcium has been suggested to play a role. In the present study, we examined the role of intracellular calcium in regulating mitochondrial morphology and motility in cortical astrocytes employing different concentrations of a calcium ionophore. High levels of calcium caused a dramatic reduction in mitochondrial length, the result of two distinct phenomena: mitochondrial remodelling (or “rounding”) and fission. Quantitative analysis revealed that mitochondrial remodelling/rounding was the predominant process. In addition, mitochondrial motility was reduced, as reported previously in neurons. By contrast, prolonged, more modest levels of intracellular calcium resulted in a reduction in mitochondrial length without significant effects upon mitochondrial motility. This calcium-induced reduction in mitochondrial length was not affected by the presence of calcineurin inhibitors; however, when mitochondrial fission events were specifically examined, calcineurin inhibitors had a significant inhibitory effect. This suggests that changes in mitochondrial length were primarily due to mitochondrial remodelling as opposed to fission. In the present study, we have therefore dissected the effects of calcium on mitochondrial motility, remodelling and fission. Our results suggest independent mechanisms for regulating these processes.