Abstract Body (Do not enter title and authors here): Background: Cardiac NAD decline is reported in cardiometabolic diseases. SARM1 consumes NAD in axonal injury and causes mitochondrial dysfunction, but its role in heart disease is not known. We hypothesized that inhibiting SARM1 is cardioprotective via mitochondrial mechanisms.
Methods: Diabetic stress (DIA) and diet-induced obesity (DIO) were induced by streptozotocin or high-fat diet. Genetic (KO mice) or pharmacologic (DSRM-3716) inhibition of SARM1 was employed. Echocardiography, respirometry, transmission electron microscopy and immunoblotting were used to characterize cardiac, mitochondrial and molecular changes.
Results: Systolic and diastolic dysfunction in male and female DIA-WT mice was attenuated in KO mice, while DIO-induced diastolic dysfunction and hypertrophy were suppressed in male KO mice. SARM1 inhibition by DSRM reversed pre-existing cardiac dysfunction in DIA-WT mice. These lines of genetic and pharmacologic evidence show that SARM1 inhibition is protective in cardiometabolic diseases.
DIA-WT heart mitochondria had reduced OXPHOS respiration, cristae count, score and density, and OPA1 protein, all restored in DIA-KO. The cardio- and mito-protection were associated with elevated tissue NAD, but not mito-NAD. Similarly, DSRM treatment reversed the decline in OXPHOS capacity, but not mito-NAD decline, in DIA-WT. H9c2 cell line with a cytosolic NAD sensor was generated. Cytosolic NAD decline by ectopic SARM1 expression was reversed by DSRM, suggesting that SARM1 regulates extramitochondrial NAD.
We next explored the role of mitophagy in cardio- and mito-protection. DIA-WT hearts had elevated BNIP3 and p-DRP1, and more fragmented mitochondria, indicating activation of mitophagy. Upregulated mito-LC3 and reduced Rab5, Rab7, Rab9 and RILP expression in DIA-WT indicated reduced endosomal trafficking, causing compromised mitophagic flux and accumulation of damaged mitochondria, despite upregulated lysosomal protease cathepsin D. Impaired mitophagy markers (BNIP3, p-DRP1, mito-LC3, Rab5, Rab7, RILP) were restored in DIA-KO, suggesting that SARM1 may regulate multiple steps in the mitophagy pathway. Bnip3 transcription is controlled by HIF1α, and DIA-WT hearts showed upregulated HIF1α and its target genes, including Bnip3. These alterations were normalized in DIA-KO, suggesting HIF1α-dependent regulation of BNIP3-mitophagy by SARM1.
Conclusion: SARM1 is a therapeutic target to fine-tune mitophagy and protect mitochondria in cardiometabolic disease.