Reciprocal Regulation of Mitochondrial Dynamics and Calcium Signaling in Astrocyte Processes.

We recently showed that inhibition of neuronal activity, glutamate uptake, or reversed-Na⁺/Ca²⁺-exchange with TTX, TFB-TBOA, or YM-244769, respectively, increases mitochondrial mobility in astrocytic processes. In the present study, we examined the interrelationships between mitochondrial mobility and Ca²⁺ signaling in astrocyte processes in organotypic cultures of rat hippocampus. All of the treatments that increase mitochondrial mobility decreased basal Ca²⁺.As recently reported, we observed spontaneous Ca²⁺spikes with half-lives of∼1 s that spread ~6μmand are almost abolished by a TRPA1 channel antagonist. Virtually all of these Ca²⁺spikes overlap mitochondria (98%), and 62% of mitochondria are overlapped by these spikes. Although tetrodotoxin, TFB-TBOA, or YM-244769 increased Ca²⁺ signaling, the specific effects on peak, decay time, and/or frequency were different. To more specifically manipulate mitochondrial mobility, we explored the effects of Miro motor adaptor proteins. We show that Miro1 and Miro2 are both expressed in astrocytes and that exogenous expression of Ca²⁺-insensitive Miro mutants (KK) nearly doubles the percentage of mobile mitochondria. Expression of Miro1KK had a modest effect on the frequency of these Ca²⁺ spikes but nearly doubled the decay half-life. The mitochondrial proton ionophore, FCCP, caused a large, prolonged increase in cytosolic Ca²⁺ followed by an increase in the decay time and the spread of the spontaneous Ca²⁺ spikes. Photo-ablation of mitochondria in individual astrocyte processes has similar effects on Ca²⁺. Together, these studies show that Ca²⁺ regulates mitochondrial mobility, and mitochondria in turn regulate Ca²⁺ signals in astrocyte processes. [ABSTRACT FROM AUTHOR]