Roles of NAD Metabolism in Diabetic Cardiomyopathy
Authors: Lee Chi Fung , Light Christine , Shi Xiaojian , Tran Huyen , Tian Rong , Sadoshima Junichi , Gu Haiwei , Chiao Ying Ann 
Published: April 2020

Diabetes is a major risk factor of heart failure. Diabetes and heart failure have been linked to altered NAD metabolism, whose roles in the progression of diabetic cardiomyopathy (DC) are far from established. We induced diabetes in wild type C57BL6N mice (WT) by streptozotocin (STZ) injections. WT mice with chronic diabetic stress (fasting glucose ~600 mg/dl) for 16 weeks had gradual declines in systolic and diastolic function and lowered NAD/NADH ratio, while acute STZ treatment (1‐day) did not affect cardiac function and NAD pool. To determine whether NAD redox imbalance promotes DC, we employed cardiac‐specific Ndufs4‐KO mice (cKO), which exhibit lowered cardiac NAD/NADH ratio without overt dysfunction. While insulin depletion and hyperglycemia were similar in diabetic control and cKO mice stressed by the same diabetogenic protocol, systolic and diastolic dysfunctions were accelerated in diabetic cKO mice. We next determined how NAD metabolism is altered to accelerate DC. In addition to the NAD redox balance, NAD metabolism also involves metabolites and enzymes in the NAD consumption and synthesis pathways, coordinating the homeostasis of NAD. Targeted analyses of transcripts and metabolites involved in these pathways were performed using qPCR and quantitative LC‐MS/MS analyses. Of 22 genes measured, we identified that Nmrk mRNA levels were up‐regulated in diabetic cKO hearts, compared to diabetic control hearts. Of thirteen cardiac NAD metabolite levels surveyed, product metabolites of NMRK enzymatic reaction (i.e. NMN or NAMN) showed decreases in levels in diabetic cKO hearts. The transcript and metabolite data suggest a role of NMRK in DC. Histologically, cardiac fibrosis levels of diabetic control and cKO hearts were slightly elevated compared to non‐diabetic controls, but they were not different. The results suggest that the accelerated decline of cardiac function in diabetic cKO hearts is not be due to altered extracellular matrix environment. We next analyzed acetylation‐dependent pathways to account for the accelerated decline of function in diabetic cKO hearts. NAD redox imbalance in diabetic cKO hearts promoted protein acetylation, including SOD2 acetylation (SOD2‐K68Ac). SOD2 acetylation inhibits its antioxidant function and we observed elevated protein oxidation levels in diabetic cKO hearts. To gain further insights how NAD redox imbalance may regulate cardiomyocyte function, phosphorylation levels of myosin binding protein C (MyBPC) and troponin I (TnI) were examined. In diabetic cKO hearts, TnI phosphorylation was elevated while MyBPC phosphorylation remained unchanged, suggesting a regulation of cardiomyocyte contraction/relaxation by NAD‐dependent mechanisms. To normalize NAD redox imbalance, we elevated NAD levels in diabetic cKO mice with cardiac‐specific NAMPT overexpression. Cardiac NAMPT expression slowed the accelerated decline of systolic and diastolic functions in diabetic cKO hearts. Our results support that altered NAD metabolism is a critical determinant for the progression of DC and warrants further investigations.Support or Funding Information1. Scientist Development Grant, AHA2. Seed Funding, Presbyterian Health Foundation of Oklahoma City 1.