Abstract 413: NAD + -dependent Pathogenic Mechanisms and Metabolic Interventions for Mitochondrial Disease and its Associated Cardiomyopathy

Abstract

Leigh syndrome (LS) is a congenital mitochondrial disease. Patients with LS develop metabolic abnormality and progressive dysfunctions in brain, skeletal and cardiac muscles, eventually leading to early death. A mouse model of LS with deficiency of a Complex I subunit, Ndufs4-null mice (KO), was used in this study. Depleted NAD + level and NAD + redox imbalance were observed in KO brain and lead to inhibitions of poly (ADP-ribose) polymerase (PARP)-dependent DNA repair and SOD2 activity via protein hyperacetylation. These inhibitions promoted oxidative stress and increased H2Ax phosphorylation, a DNA damage marker, leading to neurological impairments in KO. To counteract the NAD + depletion associated with the pathogenesis of KO, we targeted NAD + salvage pathway to increase NAD + pool size. Supplementation of nicotinamide mononucleotide (NMN), a NAD + precursor, doubled lifespan of KO mice from 55 to 110 days, but did not attenuate neurological impairments, owing to the failure to elevate NAD + levels in brain. We next determined the underlying mechanisms by which NMN extended lifespan of KO without impacting brain NAD + pool. NMN elevated NAD + levels in skeletal and cardiac muscles of KO, resulting in blunted hyperacetylation and attenuated systolic dysfunction. NMN lowered lactate levels in serum and muscles of KO, associated with suppression of hypoxic signaling (lowered HIF1α, LDHA levels). The data suggest a NAD + -sensitive mechanism that suppresses glycolysis in KO treated by NMN via hypoxic signaling. While acetylation levels of HIF1α protein were unchanged in KO, acetylation of glutamate dehydrogenase (GDH) was upregulated in KO and suppressed by NMN. Deacetylation of GDH after NMN treatment promoted upregulation of α-ketoglutarate (KG) level in KO, a co-substrate needed for HIF1α degradation. The results suggested that KG may be beneficial to KO. To determine the therapeutic potential of KG, dimethyl-ketoglutarate (DMKG), a cell permeable form of KG, was administered to KO. DMKG extended lifespan and improved systolic function in KO, associated with suppressed hypoxic signaling. Collectively, this study identified NAD + -dependent pathogenic mechanisms that may lead to translatable interventions for LS and mitochondrial cardiomyopathy.