Abstract Body (Do not enter title and authors here): Introduction: Diabetic cardiomyopathy (DC) is characterized by metabolic inflexibility, inductions of NAD loss and hypoxic signaling, and lipotoxicity.
Methods: To identify mechanisms linking these features we performed a multi-level transcriptomic analyses on diabetic mouse hearts.
Results: Bulk RNA-seq revealed dehydrogenase/reductase-related transcripts were widely dysregulated (51 downregulated, 32 upregulated) in diabetic hearts. Many of the downregulated genes were in the electron transport chain (ETC) while some of the upregulated genes regulate lipid metabolism (beta-oxidation, GPD1). These transcriptomic changes coincided with a decrease in NAD levels and upregulated HIF1a. To resolve cell-type specific changes, we performed single-nuclear RNA-seq. Downregulated ETC genes were specifically found in diabetic cardiomyocytes (CM), while changes of lipid metabolic genes were stochastically distributed in cardiac cells. LDHA/LDHB expression ratio was proposed as a cell marker of glycolytic (high) vs oxidative metabolism (low). CM had much lower LDHA/LDHB ratio than endothelial cells (EC), indicating the oxidative nature of CM. A subpopulation of diabetic CM had increased LDHA/LDHB ratio and glycolytic gene expression, suggesting hypoxic adaptation in CM. These LDHA-hi CMs had suppressed lipid metabolism genes, signifying metabolic heterogeneity in the diabetic cardiac cells. Diabetic ECs showed angiogenic gene upregulation, another indicator of hypoxic stress.
We performed MERFISH-based spatial transcriptomics and found upregulation of HIF-responsive PDK4 in diabetic CM, a metabolic switch for increased lipid metabolism and lactate production. The stochastic changes of expression of lipid metabolic genes in diabetic cardiac cells was also observed in MERFISH, suggesting unknown spatial mechanisms in cardiac cells. MERFISH showed reduced vasculature (EC) in proximity to PDK4-hi CMs, indicating potential islands of hypoxia resulting in metabolic derangement in diabetic CM. We tested if SARM1 NAD hydrolase mediates NAD loss and found that diabetic KO mice had restored cardiac NAD levels and reversed cardiac dysfunction, along with normalized lipid- and hypoxia-related markers (ceramides, lipid-related genes, PDK4, HIF1a).
Summary: Our data indicate SARM1 regulates metabolic signaling involving NAD and hypoxia in DC. Observations from multi-level transcriptomic analyses suggest local cross-talks of cardiac cells to regulate lipid metabolism in DC.