Regulation of NAD Metabolism in Diastolic Dysfunction Induced by Metabolic Stress
Authors: Lee Chi Fung 
Published: May 2022

Patients of heart failure with preserved ejection fraction (HFpEF) have diastolic dysfunction. Intervention targeting mechanisms of diastolic dysfunction is lacking, leading to a growing population of patients with HFpEF. Diabetes and obesity are key risk factors of HFpEF. NAD depletion is associated with heart failure, diabetes and obesity, and is due to NAD redox imbalance, suppressed synthesis and/or activated consumption pathways. Here, we used mouse models manipulating NAD redox balance and hydrolysis to determine their roles in metabolic cardiomyopathy. Cardiac Ndufs4‐KO mice (cKO) have lowered cardiac NAD/NADH ratio, normal baseline function, geometry and energetics. Chronic diabetic stress induced by streptozotocin (STZ) or diet‐induced obesity (DIO) in wild type (WT) mice promoted diastolic dysfunction, which was further exacerbated in cKO mice with latent cardiac NAD redox imbalance. Elevated oxidative stress by SOD2 acetylation and increased TnI‐S150 phosphorylation by energetic deficiency in diabetic cKO hearts led to exacerbated diastolic dysfunction. Elevation of cardiac NAD levels normalized NAD/NADH ratio, ameliorated these pathogenic mechanisms and diastolic dysfunction in metabolically stressed mice. These results supported the causal role of NAD redox imbalance in metabolic cardiomyopathy.

Next, we tested whether deficiency of SARM1, an intracellular NAD hydrolase, will ameliorate diastolic dysfunction induced by metabolic stresses. WT and total SARM1‐KO mice were challenged with diabetic stress or DIO. These metabolic stresses led to a decline in diastolic function, which was improved in SARM1‐KO mice. DIO also promoted cardiac hypertrophy that was ameliorated by SARM1 deficiency. To determine that the improvement of diastolic function by SARM1 deficiency was not due to a reversal of systemic metabolic state, a comprehensive plasma metabolomic analyses was performed. Hyperglycemia was similar in diabetic WT and SARM1‐KO plasma. Metabolomic analyses showed significant changes in 58 out of 298 aqueous metabolites and in 26 out of 85 lipid species measured, when comparing non‐diabetic WT to diabetic WT plasma. When comparing the 84 metabolites with diabetic changes from diabetic WT and diabetic SARM1‐KO plasma, only one of the lipid metabolites (PC 16:0/16:0) showed a mild further decrease in SARM1‐KO plasma. The metabolomic data suggest that WT and SARM1‐KO hearts experienced similar metabolic stress, and that the improvement of diastolic dysfunction by SARM1 deficiency was due to the reversal of cardiac pathogenic mechanisms.

To determine how SARM1 deficiency affected cardiac NAD metabolism, transcript levels of genes in NAD metabolic pathways were assessed. Expressions of Bst1 and Cd38 NAD hydrolases did not change in SARM1‐KO hearts, suggesting that SARM1 deficiency did not induce compensatory changes in expression of the other NAD hydrolases. SARM1‐KO hearts showed lower expressions in several other NAD consuming genes (e.g. Sirtuins) and NAD synthesis genes (e.g. Nmnats). Cardiac fibrosis, pro‐fibrotic genes (e.g. ctgf) and Nppb expression were induced in diabetic WT hearts and were suppressed by SARM1 deficiency. In summary, our data support the critical roles of NAD redox imbalance and SARM1 in diastolic dysfunction induced by metabolic stress.