Altered distribution of mitochondria impairs calcium homeostasis in rat hippocampal neurons in culture

The specificity of Ca²⁺ signals is conferred in part by limiting changes in cytosolic Ca²⁺ to subcellular domains. Mitochondria play a major role in regulating Ca²⁺ in neurons and may participate in its spatial localization. We examined the effects of changes in the distribution of mitochondria on NMDA-induced Ca²⁺ increases. Hippocampal cultures were treated with the microtubule-destabilizing agent vinblastine, which caused the mitochondria to aggregate and migrate towards one side of the neuron. This treatment did not appear to decrease the energy status of mitochondria, as indicated by a normal membrane potential and pH gradient across the inner membrane. Moreover, electron microscopy showed that vinblastine treatment altered the distribution but not the ultrastructure of mitochondria. NMDA (200 μM, 1 min) evoked a greater increase in cytosolic Ca²⁺ in vinblastine-treated cells than in untreated cells. This increase did not result from impaired Ca²⁺ efflux, enhanced Ca ²⁺ influx, opening of the mitochondrial permeability transition pore or altered function of endoplasmic reticulum Ca²⁺ stores. Ca ²⁺ uptake into mitochondria was reduced by 53% in vinblastine-treated cells, as reported by mitochondrially targeted aequorin. Thus, the distribution of mitochondria maintained by microtubules is critical for buffering Ca²⁺ influx. A subset of mitochondria close to a Ca²⁺ source may preferentially regulate Ca²⁺ microdomains, set the threshold for Ca²⁺-induced toxicity and participate in local ATP production.