Formation of Polyphosphate-Poly-Beta-Hydroxybutyrate Granule-Like Complexes in Heart Failure Myocytes

Abstract

Inorganic polyphosphate (polyP) is a naturally occurring polyanion made of ten to several hundred orthophosphates linked together by phosphoanhydride bonds. Specific physiological roles of polyP vary dramatically depending on its size, concentration, tissue and subcellular localization. Recently we reported that mitochondria of rabbit ventricular myocytes contain significant amounts (280 ± 60 pmol/mg of protein) of polyP with an average length of 25 orthophosphates, and that polyP is involved in Ca2+-dependent activation of the mitochondrial permeability transition pore (mPTP). Using DAPI as a probe for polyP, we showed that polyP levels depend on the activity of the mitochondrial respiratory chain. Here, we visualized intracellular polyP distribution in cardiomyocytes using the affinity of the Xpress epitope-tagged recombinant polyphosphate-binding domain (PPBD) of E. coli exopolyphosphatase and immunocytochemical approach. Primarily mitochondrial location of polyP was demonstrated in both control and heart failure (HF) myocytes from two different HF models: (1), non-ischemic rabbit HF model induced by combined aortic insufficiency and stenosis and (2), targeted microbead embolization of the first diagonal branch of left anterior descending coronary artery in Yucatan mini-pigs. However, enhanced formation of polyP dense spots was observed in HF conditions. Furthermore, polyP dense spots overlapped with poly-beta-hydroxybutyrate (PHB) accumulation areas in HF myocytes as detected with specific PHB antibodies (anti-PHB IgG) raised against a synthetic 8-mer of PHB. It is known that polyP can form channels with Ca2+ and PHB, a polymerized form of the ketone body beta-hydroxybutyrate, with characteristics similar to mPTP. These data suggest that polyP-PHB complex formation in HF myocytes could contribute to the enhanced propensity of mPTP opening despite decreased mitochondrial Ca2+ uptake.