Hexokinase Isoforms and Glucose Metabolism in Adult and Neonatal Cardiac Myocytes

Abstract

Localization of hexokinase (HK) isoforms to the cytoplasm or mitochondria controls their anabolic (glycogen synthesis) and catabolic (glycolysis) activities. In this study, we compared the effects of HKI and HKII in isolated adult (ARVM) and neonatal rat ventricular myocytes (NRVM) using a set of novel genetically-encoded optical imaging tools to track, in real-time, the subcellular distributions of HKI and HKII, as well as the functional consequences on glucose utilization. We show that HKII, the predominant isoform in ARVM, dynamically distributes between the mitochondria and cytoplasm. Removal of extracellular glucose displaces HKII from mitochondria in ARVM, but not in NRVM, whereas iodoacetate (IAA) displaces HKII in both. HKI, the predominant isoform in NRVM, always remains bound to mitochondria when overexpressed in either AVRM and NRVM, and is not displaced by the above interventions. In ARVM, overexpression of HKI, but not HKII, increased glycolytic activity. In NVRM, knock-down of HKI, but not HKII, decreased glycolytic activity. Swapping the N terminus between HKI and HKII, as well as specific mutations, revealed that the N terminus of HKI is necessary, but not sufficient, for high affinity mitochondrial binding to promote glycolysis. In conclusion, HKI and HKII play major roles in defining the different metabolic profiles of ARVM and NRVM, accounting for the markedly increased glycolytic activity of NRVM.