The therapeutic perspective of NAD+ precursors in age-related diseases.

Nicotinamide adenine dinucleotide (NAD+) is the fundamental molecule that performs numerous biological reactions and is crucial for maintaining cellular homeostasis. Studies have found that NAD+ decreases with age in certain tissues, and age-related NAD+ depletion affects physiological functions and contributes to various aging-related diseases. Supplementation of NAD+ precursor significantly elevates NAD+ levels in murine tissues, effectively mitigates metabolic syndrome, enhances cardiovascular health, protects against neurodegeneration, and boosts muscular strength. Despite the versatile therapeutic functions of NAD+ in animal studies, the efficacy of NAD+ precursors in clinical studies have been limited compared with that in the pre-clinical study. Clinical studies have demonstrated that NAD+ precursor treatment efficiently increases NAD+ levels in various tissues, though their clinical proficiency is insufficient to ameliorate the diseases. However, the latest studies regarding NAD+ precursors and their metabolism highlight the significant role of gut microbiota. The studies found that orally administered NAD+ intermediates interact with the gut microbiome. These findings provide compelling evidence for future trials to further explore the involvement of gut microbiota in NAD+ metabolism. Also, the reduced form of NAD+ precursor shows their potential to raise NAD+, though preclinical studies have yet to discover their efficacy. This review sheds light on NAD+ therapeutic efficiency in preclinical and clinical studies and the effect of the gut microbiota on NAD+ metabolism.

Adipocyte NMNAT1 expression is essential for nuclear NAD+ biosynthesis but dispensable for regulating thermogenesis and whole-body energy metabolism.

Nicotinamide adenine dinucleotide (NAD+) functions as an essential cofactor regulating a variety of biological processes. The purpose of the present study was to determine the role of nuclear NAD+ biosynthesis, mediated by nicotinamide mononucleotide adenylyltransferase 1 (NMNAT1), in thermogenesis and whole-body energy metabolism. We first evaluated the relationship between NMNAT1 expression and thermogenic activity in brown adipose tissue (BAT), a key organ for non-shivering thermogenesis. We found that reduced BAT NMNAT1expression was associated with inactivation of thermogenic gene program induced by obesity and thermoneutrality. Next, we generated and characterized adiponectin-Cre-driven adipocyte-specific Nmnat1 knockout (ANMT1KO) mice. Loss of NMNAT1 markedly reduced nuclear NAD+ concentration by approximately 70% in BAT. Nonetheless, adipocyte-specific Nmnat1 deletion had no impact on thermogenic (rectal temperature, BAT temperature and whole-body oxygen consumption) responses to ß-adrenergic ligand norepinephrine administration and acute cold exposure, adrenergic-mediated lipolytic activity, and metabolic responses to obesogenic high-fat diet feeding. In addition, loss of NMNAT1 did not affect nuclear lysine acetylation or thermogenic gene program in BAT. These results demonstrate that adipocyte NMNAT1 expression is required for maintaining nuclear NAD+ concentration, but not for regulating BAT thermogenesis or whole-body energy homeostasis.

Deletion of Nmnat1 in Skeletal Muscle Leads to the Reduction of NAD+ Levels but Has No Impact on Skeletal Muscle Morphology and Fiber Types.

Nicotinamide adenine dinucleotide (NAD+) is a coenzyme that mediates many redox reactions in energy metabolism. NAD+ is also a substrate for ADP-ribosylation and deacetylation by poly (ADP-ribose) polymerase and sirtuin, respectively. Nicotinamide mononucleotide adenylyltransferase 1 (Nmnat1) is a NAD+ biosynthesizing enzyme found in the nucleus. Recent research has shown that the maintaining NAD+ levels is critical for sustaining muscle functions both in physiological and pathological conditions. However, the role of Nmnat1 in skeletal muscle remains unexplored. In this study, we generated skeletal muscle-specific Nmnat1 knockout (M-Nmnat1 KO) mice and investigated its role in skeletal muscle. We found that NAD+ levels were significantly lower in the skeletal muscle of M-Nmnat1 KO mice than in control mice. M-Nmnat1 KO mice, in contrast, had similar body weight and normal muscle histology. Furthermore, the distribution of muscle fiber size and gene expressions of muscle fiber type gene expression were comparable in M-Nmnat1 KO and control mice. Finally, we investigated the role of Nmnat1 in muscle regeneration using cardiotoxin-induced muscle injury model, but muscle regeneration appeared almost normal in M-Nmnat1 KO mice. These findings imply that Nmnat1 has a redundancy in the pathophysiology of skeletal muscle.

Loss of hepatic Nmnat1 has no impact on diet-induced fatty liver disease.

Nicotinamide adenine dinucleotide (NAD+), a biological molecule integral to redox reactions involved in multiple cellular processes, has the potential to treat nonalcoholic fatty liver diseases (NAFLDs) and nonalcoholic steatohepatitis (NASH). Nicotinamide mononucleotide adenylyltransferase (Nmnat1), one of the NAD+ biosynthesizing enzymes, plays a central role in all NAD+ metabolic pathways and it is vital to embryonic development. However, the function of Nmnat1 in metabolic pathology and, specifically, in the development and progression of NAFLD and NASH remains unexplored. First, we generated hepatic Nmnat1 knockout (H-Nmnat1-/-) mice to investigate the physiological function of Nmnat1 and found that NAD+ levels were significantly lower in H-Nmnat1-/- mice than control mice. However, H-Nmnat1-/- mice appeared normal with comparable metabolic activity. Next, we used three different diet-induced NASH models to assess the pathophysiological role of Nmant1 in metabolic disorders and discovered that hepatic loos of Nmnat1 decreased 35%-40% of total NAD+ in an obese state. Nevertheless, our analysis of phenotypic variations found comparable body composition, gene expression, and liver histology in all NASH models in H-Nmnat1-/- mice. We also found that aged H-Nmnat1-/- mice exhibited comparable liver phenotypes with control mice. These findings suggest that Nmnat1 has a redundancy to the pathophysiology of obesity-induced hepatic disorders.

BST1 regulates nicotinamide riboside metabolism via its glycohydrolase and base-exchange activities.

Nicotinamide riboside (NR) is one of the orally bioavailable NAD+ precursors and has been demonstrated to exhibit beneficial effects against aging and aging-associated diseases. However, the metabolic pathway of NR in vivo is not yet fully understood. Here, we demonstrate that orally administered NR increases NAD+ level via two different pathways. In the early phase, NR was directly absorbed and contributed to NAD+ generation through the NR salvage pathway, while in the late phase, NR was hydrolyzed to nicotinamide (NAM) by bone marrow stromal cell antigen 1 (BST1), and was further metabolized by the gut microbiota to nicotinic acid, contributing to generate NAD+ through the Preiss-Handler pathway. Furthermore, we report BST1 has a base-exchange activity against both NR and nicotinic acid riboside (NAR) to generate NAR and NR, respectively, connecting amidated and deamidated pathways. Thus, we conclude that BST1 plays a dual role as glycohydrolase and base-exchange enzyme during oral NR supplementation.

Molecular characterization of Leucoanthocyanidin reductase and Flavonol synthase gene in Arachis hypogaea.

Arachis hypogaea (peanut) is a potential source of bioactive compounds including flavonols and proanthocyanidins, which have gained particular interest of metabolic engineering owing to their significance in the growth, development and defense responses in plants. To gain insight of proanthocyanidins and flavonols production in A. hypogaea, Leucoanthocyanidin reductase (AhLAR) and Flavonol synthase (AhFLS) enzymes responsible for their production, have been structurally, transcriptionally and functionally characterized. Structural and functional analysis of putative protein sequence of AhFLS indicated two functional motifs 2OG-FeII_Oxy and DIOX_N, while six functional motifs belonging to the families of NAD-dependent dehydratase, 3, ß hydroxysteroid dehydrogenase and NmrA-like family were observed in case of AhLAR. Promoter sequence analysis unraveled several promoter elements related to the development regulation, environmental stress responses and hormonal signaling. Furthermore, the expression analysis of AhFLS and AhLAR and accumulation pattern analysis of proanthocyanidins and flavonols in three selected cultivars of A. hypogaea under saline environment confirmed their role against salinity in genotype-dependent and stress level-dependent manner. Correlation studies revealed that AhFLS and AhLAR expression is not directly dependent on the antioxidant enzymes activity, biochemical and growth parameters but higher Pearson r value depicted some level of dependency. This detailed study of AhLAR and AhFLS can assist in the metabolic engineering of flavonoid biosynthetic pathway to produce stress tolerant varieties and production of proanthocyanidins and flavonols at an industrial scale.