1 Reversing sodium-driven metabolic reprogramming in heart failure using a novel optogenetic approach

<h3></h3> One of the hallmarks of heart failure is a rise in intracellular Na concentration which directly and causally contributes to contractile dysfunction and arrhythmias. Recently, Na elevation has also been shown to drive metabolic reprogramming, mediating a substrate preference switch away from fatty acid oxidation towards glucose metabolism. The aim of this project is to use isolated hearts and cell culture models to establish whether lowering Na is therapeutically beneficial in heart failure. A model of acute Na elevation was established by treating isolated Langendorff-perfused rat hearts with the Na/K ATPase inhibitor ouabain. Na elevation in this model resulted in reversible energetic dysfunction and metabolic derangement. However, this could be secondary to Na/Ca exchange-mediated cytoplasmic Ca elevation. In separate studies, this possibility was excluded by comparison with experiments where intracellular Ca alone was elevated. To investigate the therapeutic potential of lowering Na in the failing myocardium, the light driven outward Na pump eKR2 was used as an optogenetic tool. This pump was transiently expressed in isolated neonatal rat cardiomyocytes treated with 100 μM phenylephrine for 48hr to induce hypertrophy. At baseline, intracellular Na measured using ²³Na-NMR in cardiomyocytes was 9.9±1.6 mM and this increased by approximately 54% after phenylephrine treatment. Glycolytic excretion of lactate was elevated in this model of hypertrophy where Na is elevated. Expression and activation of eKR2 (illumination with 0.5mW/mm2 for 30 mins) lowered Na by approximately 44% and significantly reduced lactate production (measured using ¹H-NMR) in the hypertrophic cells, suggesting a reversal of the Na-driven metabolic reprogramming. These findings show that Na plays a direct role in metabolic reprogramming and cardiac energetics in heart failure, and that these are reversible in a cell-based model of hypertrophy using the light-driven outward Na pump eKR2.