Human Prune Regulates the Metabolism of Mammalian Inorganic Polyphosphate and Bioenergetics

Inorganic polyphosphate (polyP) is an evolutionarily conserved and ubiquitous polymer that is present in all studied organisms. PolyP consists of orthophosphates (Pi) linked together by phosphoanhydride bonds. The metabolism of polyP still remains poorly understood in higher eukaryotes. Currently, only F₀F₁-ATP synthase, Nudt3, and Prune have been proposed to be involved in this metabolism, although their exact roles and regulation in the context of polyP biology have not been fully elucidated. In the case of Prune, in vitro studies have shown that it exhibits exopolyphosphatase activity on very short-chain polyP (up to four units of Pi), in addition to its known cAMP phosphodiesterase (PDE) activity. Here, we expand upon studies regarding the effects of human Prune (h-Prune) on polyP metabolism. Our data show that recombinant h-Prune is unable to hydrolyze short (13-33 Pi) and medium (45-160 Pi) chains of polyP, which are the most common chain lengths of the polymer in mammalian cells. Moreover, we found that the knockdown of h-Prune (h-Prune KD) results in significantly decreased levels of polyP in HEK293 cells. Likewise, a reduction in the levels of polyP is also observed in Drosophila melanogaster loss-of-function mutants of the h-Prune ortholog. Furthermore, while the activity of ATP synthase, and the levels of ATP, are decreased in h-Prune KD HEK293 cells, the expression of ATP5A, which is a main component of the catalytic subunit of ATP synthase, is upregulated in the same cells, likely as a compensatory mechanism. Our results also show that the effects of h-Prune on mitochondrial bioenergetics are not a result of a loss of mitochondrial membrane potential or of significant changes in mitochondrial biomass. Overall, our work corroborates the role of polyP in mitochondrial bioenergetics. It also demonstrates a conserved effect of h-Prune on the metabolism of short- and medium-chain polyP (which are the predominant chain lengths found in mammalian cells). The effects of Prune in polyP are most likely exerted via the regulation of the activity of ATP synthase. Our findings pave the way for modifying the levels of polyP in mammalian cells, which could have pharmacological implications in many diseases where dysregulated bioenergetics has been demonstrated.