Mitochondrial Regulation of Store-operated Calcium Signaling in T Lymphocytes

Abstract

Mitochondria act as potent buffers of intracellular Ca ²⁺ in many cells, but a more active role in modulating the generation of Ca ²⁺ signals is not well established. We have investigated the ability of mitochondria to modulate store-operated or “capacitative” Ca ²⁺ entry in Jurkat leukemic T cells and human T lymphocytes using fluorescence imaging techniques. Depletion of the ER Ca ²⁺ store with thapsigargin (TG) activates Ca ²⁺ release-activated Ca ²⁺ (CRAC) channels in T cells, and the ensuing influx of Ca ²⁺ loads a TG- insensitive intracellular store that by several criteria appears to be mitochondria. Loading of this store is prevented by carbonyl cyanide m-chlorophenylhydrazone or by antimycin A1 + oligomycin, agents that are known to inhibit mitochondrial Ca ²⁺ import by dissipating the mitochondrial membrane potential. Conversely, intracellular Na ⁺ depletion, which inhibits Na ⁺-dependent Ca ²⁺ export from mitochondria, enhances store loading. In addition, we find that rhod-2 labels mitochondria in T cells, and it reports changes in Ca ²⁺ levels that are consistent with its localization in the TG-insensitive store. Ca ²⁺ uptake by the mitochondrial store is sensitive (threshold is <400 nM cytosolic Ca ²⁺), rapid (detectable within 8 s), and does not readily saturate. The rate of mitochondrial Ca ²⁺ uptake is sensitive to extracellular [Ca ²⁺], indicating that mitochondria sense Ca ²⁺ gradients near CRAC channels. Remarkably, mitochondrial uncouplers or Na ⁺ depletion prevent the ability of T cells to maintain a high rate of capacitative Ca ²⁺ entry over prolonged periods of >10 min. Under these conditions, the rate of Ca ²⁺ influx in single cells undergoes abrupt transitions from a high influx to a low influx state. These results demonstrate that mitochondria not only buffer the Ca ²⁺ that enters T cells via store-operated Ca ²⁺ channels, but also play an active role in modulating the rate of capacitative Ca ²⁺ entry.