Balancing NAD+ deficits with nicotinamide riboside: therapeutic possibilities and limitations

Alterations in cellular nicotinamide adenine dinucleotide (NAD+) levels have been observed in multiple lifestyle and age-related medical conditions. This has led to the hypothesis that dietary supplementation with NAD+ precursors, or vitamin B3s, could exert health benefits. Among the different molecules that can act as NAD+ precursors, Nicotinamide Riboside (NR) has gained most attention due to its success in alleviating and treating disease conditions at the pre-clinical level. However, the clinical outcomes for NR supplementation strategies have not yet met the expectations generated in mouse models. In this review we aim to provide a comprehensive view on NAD+ biology, what causes NAD+ deficits and the journey of NR from its discovery to its clinical development. We also discuss what are the current limitations in NR-based therapies and potential ways to overcome them. Overall, this review will not only provide tools to understand NAD+ biology and assess its changes in disease situations, but also to decide which NAD+ precursor could have the best therapeutic potential.

Crosstalk between Drp1 phosphorylation sites during mitochondrial remodeling and their impact on metabolic adaptation

Mitochondria constantly undergo fusion and fission events, referred as mitochondrial dynamics, which determine mitochondrial architecture and bioenergetics. Cultured cell studies demonstrate that mitochondrial dynamics are acutely regulated by phosphorylation of the mitochondrial fission orchestrator dynamin-related protein 1 (Drp1) at S579 or S600. However, the physiological impact and crosstalk of these phosphorylation sites is poorly understood. Here, we describe the functional interrelation between S579 and S600 phosphorylation sites in vivo and their role on mitochondrial remodeling. Mice carrying a homozygous Drp1 S600A knockin (Drp1 KI) mutation display larger mitochondria and enhanced lipid oxidation and respiratory capacities, granting improved glucose tolerance and thermogenic response upon high-fat feeding. Housing mice at thermoneutrality blunts these differences, suggesting a role for the brown adipose tissue in the protection of Drp1 KI mice against metabolic damage. Overall, we demonstrate crosstalk between Drp1 phosphorylation sites and provide evidence that their modulation could be used in the treatment and prevention of metabolic diseases.

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Drp1 phosphorylation at S600 promotes the phosphorylation at the S579 site

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Both Drp1 P-S600 and P-S579 are required for maximal mitochondrial fragmentation

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Drp1 S600A knockin mice are protected against diet-induced metabolic damage

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Drp1 phosphorylation controls brown adipose tissue thermogenic capacity in mice

A reduced form of nicotinamide riboside defines a new path for NAD+ biosynthesis and acts as an orally bioavailable NAD+ precursor

OBJECTIVE

A decay in intracellular NAD⁺ levels is one of the hallmarks of physiological decline in normal tissue functions. Accordingly, dietary supplementation with NAD⁺ precursors can prevent, alleviate, or even reverse multiple metabolic complications and age-related disorders in diverse model organisms. Within the constellation of NAD⁺ precursors, nicotinamide riboside (NR) has gained attention due to its potent NAD⁺ biosynthetic effects in vivo while lacking adverse clinical effects. Nevertheless, NR is not stable in circulation, and its utilization is rate-limited by the expression of nicotinamide riboside kinases (NRKs). Therefore, there is a strong interest in identifying new effective NAD⁺ precursors that can overcome these limitations.

METHODS

Through a combination of metabolomics and pharmacological approaches, we describe how NRH, a reduced form of NR, serves as a potent NAD⁺ precursor in mammalian cells and mice.

RESULTS

NRH acts as a more potent and faster NAD⁺ precursor than NR in mammalian cells and tissues. Despite the minor structural difference, we found that NRH uses different steps and enzymes to synthesize NAD⁺, thus revealing a new NRK1-independent pathway for NAD⁺ synthesis. Finally, we provide evidence that NRH is orally bioavailable in mice and prevents cisplatin-induced acute kidney injury.

CONCLUSIONS

Our data identify a new pathway for NAD⁺ synthesis and classify NRH as a promising new therapeutic strategy to enhance NAD⁺ levels.

Endogenous nicotinamide riboside metabolism protects against diet-induced liver damage

Supplementation with the NAD⁺ precursor nicotinamide riboside (NR) ameliorates and prevents a broad array of metabolic and aging disorders in mice. However, little is known about the physiological role of endogenous NR metabolism. We have previously shown that NR kinase 1 (NRK1) is rate-limiting and essential for NR-induced NAD⁺ synthesis in hepatic cells. To understand the relevance of hepatic NR metabolism, we generated whole body and liver-specific NRK1 knockout mice. Here, we show that NRK1 deficiency leads to decreased gluconeogenic potential and impaired mitochondrial function. Upon high-fat feeding, NRK1 deficient mice develop glucose intolerance, insulin resistance and hepatosteatosis. Furthermore, they are more susceptible to diet-induced liver DNA damage, due to compromised PARP1 activity. Our results demonstrate that endogenous NR metabolism is critical to sustain hepatic NAD⁺ levels and hinder diet-induced metabolic damage, highlighting the relevance of NRK1 as a therapeutic target for metabolic disorders.

Nicotinamide adenine dinucleotide (NAD⁺) sustains cellular energy metabolism, functions as a substrate of Sirt and PARP enzymes, and its supplementation is explored therapeutically in aging and other contexts. Here the authors provide insight into the role of endogenous NAD⁺ metabolism by studying nicotinamide riboside kinase 1 (NRK1) deficient mice.

Evaluation of the Toxicity of Intravitreally Injected PLGA Microspheres and Rods in Monkeys and Rabbits: Effects of Depot Size on Inflammatory Response

Purpose

Poly(lactic-co-glycolic) acid (PLGA) inserts have been successfully developed for the treatment of posterior eye disease as a means of reducing injection frequency of intravitreally administered therapeutics. PLGA microspheres are also of interest for the delivery of intravitreal drugs, since they offer the advantage of being easily injected without surgical procedures or large injectors.

Methods

In the current study, the toxicity of PLGA microspheres and rods was investigated in nonhuman primates (NHPs) and rabbits. An in vitro assessment of cytokine responses to PLGA in peripheral blood mononuclear cells (PBMCs) and macrophages was also performed.

Results

Intravitreal administration of 3, 10, or 12.5 mg/eye of PLGA microspheres in NHPs resulted in a severe immune response characterized by a foreign body response. Follow-up studies in the rabbit confirmed this finding for PLGA microspheres ranging in size from 20 to 100 μm. In contrast, administration of PLGA rod implants with a similar PLGA mass did not elicit a significant immune response. In vitro assays in PBMCs and macrophages confirmed proinflammatory cytokine release upon treatment with PLGA microspheres but not PLGA rods.

Conclusions

These data demonstrate a lack of tolerability of PLGA microspheres upon intravitreal injection, and suggest that the size, shape, and/or surface area of PLGA depots are critical attributes in determining ocular toxicity.

Mutagenic potential of DNA glycation: miscoding by (R)- and (S)-N2-(1-carboxyethyl)-2'-deoxyguanosine

Elevated circulating glucose resulting from complications of obesity and metabolic disease can result in the accumulation of advanced glycation end products (AGEs) of proteins, lipids, and DNA. The formation of DNA-AGEs assumes particular importance as these adducts may contribute to genetic instability and elevated cancer risk associated with metabolic disease. The principal DNA-AGE, N(2)-(1-carboxyethyl)-2'-deoxyguanosine (CEdG), is formed as a mixture of R and S isomers at both the polymer and monomer levels. In order to examine the miscoding potential of this adduct, oligonucleotides substituted with (R)- and (S)-CEdG and the corresponding triphosphates (R)- and (S)-CEdGTP were synthesized, and base-pairing preferences for each stereoisomer were examined using steady-state kinetic approaches. Purine dNTPs were preferentially incorporated opposite template CEdG when either the Klenow (Kf(-)) or Thermus aquaticus (Taq) polymerases were used. The Kf(-) polymerase preferentially incorporated dGTP, whereas Taq demonstrated a bias for dATP. Kf(-) incorporated purines opposite the R isomer with greater efficiency, but Taq favored the S isomer. Incorporation of (R)- and (S)-CEdGTP only occurred opposite dC and was catalyzed by Kf(-) with equal efficiencies. Primer extension from a 3'-terminal CEdG was observed only for the R isomer. These data suggest CEdG is the likely adduct responsible for the observed pattern of G transversions induced by exposure to elevated glucose or its alpha-oxoaldehyde decomposition product methylglyoxal. The results imply that CEdG within template DNA and the corresponding triphosphate possess different syn/anti conformations during replication which influence base-pairing preferences. The implications for CEdG-induced mutagenesis in vivo are discussed.