Contribution of Inorganic Polyphosphate Towards Regulation of Mitochondrial Free Calcium

Abstract

Calcium signaling plays a key role in the regulation of multiple processes in mammalian mitochondria, ranging from the physiological regulation of 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. It is established that this calcium buffering system involves participation of orthophosphate. However, the mechanisms of its function are not yet understood. Specifically, it is not clear how orthophosphate is capable to regulate free calcium concentration in the micromolar range. We hypothesized that inorganic polyphosphate, a biological polymer, made of up to 100 orthophosphates, plays a significant role in this regulation on mitochondria. We used confocal fluorescent microscopy to measure the relative levels of free calcium in this organelle, in cultured hepatoma (HepG2) cells with variable levels of inorganic polyphosphate. We found that the depletion of inorganic polyphosphate leads to the significantly lower levels of mitochondrial free calcium concentration, under conditions of pathological calcium overload. We conclude that inorganic polyphosphate is a previously unrecognized integral component of the mitochondrial calcium buffering system.