Contribution of inorganic polyphosphate towards regulation of mitochondrial free calcium

Background: Calcium signaling plays a key role in the regulation of multiple processes in mammalian mitochondria, from cellular bioenergetics to the induction of stress-induced cell death. While the total concentration of calcium inside the mitochondria can increase by several orders of magnitude, the concentration of bioavailable free calcium in mitochondria is maintained within the micromolar range by the mitochondrial calcium buffering system. This calcium buffering system involves the participation of inorganic phosphate. However, the mechanisms of its function are not yet understood. Specifically, it is not clear how calcium-orthophosphate interactions, which normally lead to formation of insoluble precipitates, are capable to dynamically regulate free calcium concentration. Here we test the hypothesis that inorganic polyphosphate, which is a polymerized form of orthophosphate, is capable to from soluble complexes with calcium, playing a significant role in the regulation of the mitochondrial free calcium concentration. Methods: We used confocal fluorescence microscopy to measure the relative levels of mitochondrial free calcium in cultured hepatoma cells (HepG2) with variable levels of inorganic polyphosphate (polyP). Results: The depletion of polyP leads to the significantly lower levels of mitochondrial free calcium concentration under conditions of pathological calcium overload. These results are coherent with previous observations showing that inorganic polyphosphate (polyP) can inhibit calcium-phosphate precipitation and, thus, increase the amount of free calcium. Conclusions: Inorganic polyphosphate plays an important role in the regulation of mitochondrial free calcium, leading to its significant increase. General significance: Inorganic polyphosphate is a previously unrecognized integral component of the mitochondrial calcium buffering system.