NAD+ and Mitochondrial Metabolism: What the Research Literature Reports

NAD+ is studied both as a redox carrier for mitochondrial oxidative metabolism and as a consumed substrate for signalling enzymes. A review of cell and rodent literature framed NAD+ as a cofactor that links metabolic flux to sirtuin activity and to mitochondrial quality-control pathways such as the mitochondrial unfolded protein response [1]. A separate review of NAD+ metabolic pathways described how NAD+ availability regulates sirtuin-catalysed deacylation reactions relevant to mitochondrial homeostasis and biogenesis, and catalogued nicotinamide riboside and nicotinamide mononucleotide as NAD+ precursors under investigation [10]. A central question in this literature has been how the mitochondrial matrix obtains NAD+. In work using mammalian cell lines, isolated mitochondria, and yeast mitochondria lacking endogenous NAD+ carriers, SLC25A51 was identified as a mammalian mitochondrial NAD+ transporter; loss of SLC25A51 decreased mitochondrial but not whole-cell NAD+ content, impaired mitochondrial respiration, and blocked NAD+ uptake into isolated mitochondria, while overexpression of SLC25A51 or SLC25A52 increased mitochondrial NAD+ and restored uptake in the yeast system [2]. Follow-up in vitro work combining mutational analysis with localised NAD+ biosensors reported that the mitochondrial membrane potential, together with charged residues in the carrier pore, enabled sustained import of NAD+ against its electrochemical gradient, and that dissipating the membrane potential or mutating selected residues led to equilibration of matrix NAD+ [3]. In mice and mouse hepatocytes, hepatic Slc25a51 expression was induced by fasting and showed a circadian-like pattern that was disrupted by liver-specific deletion of the clock gene Bmal1; knockdown lowered mitochondrial NAD+ levels and SIRT3 activity, as reflected by increased acetylation of the SIRT3 targets IDH2 and ACADL, reduced oxygen consumption in intact hepatocytes, and was accompanied by hepatic steatosis and hypertriglyceridaemia [4]. Methodological studies have addressed how compartmentalised NAD+ redox state is measured and perturbed. Genetically encoded SoNar-based biosensors targeted to mitochondria or cytosol responded linearly to physiological NAD+/NADH ratios in live cells, and indicated that the two pools responded rapidly but differently to acute metabolic perturbations, with communication reported through the malate–aspartate shuttle and sensitivity to NAD+ precursor levels [5]. In human cells, expression of a water-forming bacterial NADH oxidase (LbNOX) in either cytosol or mitochondria raised the NAD+/NADH ratio in a compartment-specific manner and was reported to alter metabolic fluxes, gluconeogenesis, and signalling, and to ameliorate proliferative and metabolic defects caused by an impaired electron transport chain in that cell model [6]. Several preclinical studies have linked NAD+ availability to mitochondrial oxidative function through sirtuins. In mice, the circadian transcription feedback loop generated cycles of NAD+ biosynthesis, ATP production, and mitochondrial respiration via changes in mitochondrial protein acetylation, and NAD+ supplementation restored protein deacetylation and increased oxygen consumption in circadian mutant animals [7]. In mammalian cells and mouse tissues, nicotinamide riboside raised NAD+ levels and was associated with SIRT1 and SIRT3 activation, enhanced oxidative metabolism, and protection against high-fat-diet-induced metabolic abnormalities in the mouse model [8]. In aged mice and Caenorhabditis elegans, NAD+ levels were reported to be reduced, and genetic or pharmacological restoration of NAD+ promoted longevity in worms in a manner dependent on the deacetylase sir-2.1 and involving the mitochondrial unfolded protein response and DAF-16/FOXO signalling [9]. Animal work has extended these observations to stem cells and the nervous system. In mice, the NAD+-consuming enzyme CD38 promoted haematopoietic stem cell proliferation by inducing mitochondrial Ca2+ influx and mitochondrial metabolism in young animals, whereas its upregulation with age was associated with stem cell deterioration and dysregulated NAD+ metabolism, and pharmacological CD38 inactivation altered these ageing-associated changes in aged mice [11]. In cross-species Alzheimer's disease models, nicotinamide mononucleotide restored NAD+ metabolic profiles and increased mitochondrial stress response signalling through an ATF4-dependent mitochondrial unfolded protein response, with reduced mitochondrial proteotoxicity, reduced neuronal loss, and less brain atrophy reported in the mouse model; the same report described dysregulated mitochondrial stress response profiles in plasma samples from people with Alzheimer's disease [12]. In mice genetically engineered to express Pfkfb3 in neurons, glycolysis-mediated NAD+ reduction was associated with complex I disassembly, mitochondrial redox stress, impaired sirtuin-dependent autophagy, and cognitive and metabolic phenotypes that were corrected by brain NAD+ restoration or by ablating mitochondrial redox stress in that model [13]. In cultured cells, mitochondrial ADP-ribosylation—another NAD+-consuming reaction—increased reversibly after respiratory chain inhibition and showed reciprocal behaviour with nuclear ADP-ribosylation, which the authors interpreted as NAD+-mediated mitochondrial–nuclear crosstalk [14]. Human data in this set are largely observational or intraoperative. In human ovarian tissue and immature oocytes, advanced maternal age was associated with markers of oxidative damage and with metabolite changes consistent with reduced mitochondrial function, including depletion of NAD+, purines, and pyrimidines alongside accumulation of glycolytic substrates and glutamine [15]. In more than 80 patients infused with 13C-labelled nutrients during kidney tumour resection, clear cell renal cell carcinomas showed suppressed labelling of tricarboxylic acid cycle intermediates and lower electron transport chain activity in isolated human tumour mitochondria, while paired mouse experiments indicated that stimulating respiration or NADH recycling promoted metastasis and that complex I inhibition decreased it [16]. Disease-focused reviews in this set place NAD redox state within broader metabolic remodelling. A review of cardiac energy metabolism in heart failure described alterations in NAD redox state, alongside transcriptional and post-translational changes, as part of the metabolic profile of the failing heart [17]. A review of diabetic cardiomyopathy summarised a decreased mitochondrial NAD+/NADH ratio together with an increased acetyl-CoA/CoA ratio in the diabetic heart and its relationship to protein acetylation [18]. A review of ischaemia–reperfusion research described increased activity of NAD+-consuming enzymes and inhibition of oxidative phosphorylation in injured mitochondria in preclinical models [19]. A review of tryptophan metabolism described the kynurenine pathway as the sole de novo NAD+ biosynthetic route and noted that kynurenine pathway interventions extended lifespan in fruit flies and nematodes, and that NAD+ or NAD+-precursor supplementation was reported to increase longevity in flies, nematodes, and mice [20]. Across this literature, the mechanistic transport and redox findings come from cell and isolated-mitochondria systems [2][3][5][6][14], the intervention findings come from invertebrate and rodent models [7][8][9][11][12][13], and the human entries are descriptive metabolic measurements rather than intervention outcomes [15][16].