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Nicotinamide-induced mitophagy: event mediated by high NAD+/NADH ratio and SIRT1 protein activation.

ABSTRACT: Active autophagy coupled with rapid mitochondrial fusion and fission constitutes an important mitochondrial quality control mechanism and is critical to cellular health. In our previous studies, we found that exposure of cells to nicotinamide causes a decrease in mitochondrial content and an increase in mitochondrial membrane potential (MMP) by activating autophagy and inducing mitochondrial fragmentation. Here, we present evidence to show that the effect of nicotinamide is mediated through an increase of the [NAD(+)]/[NADH] ratio and the activation of SIRT1, an NAD(+)-dependent deacetylase that plays a role in autophagy flux. The [NAD(+)]/[NADH] ratio was inversely correlated with the mitochondrial content, and an increase in the ratio by the mobilization of the malate-aspartate shuttle resulted in autophagy activation and mitochondrial transformation from lengthy filaments to short dots. Furthermore, treatment of cells with SIRT1 activators, fisetin or SRT1720, induced similar changes in the mitochondrial content. Importantly, the activators induced mitochondrial fragmentation only when SIRT1 expression was intact. Meanwhile, MMP did not increase when the cells were treated with the activators, suggesting that the change in MMP is not induced by the mitochondrial turnover per se and that elevation of the [NAD(+)]/[NADH] ratio may activate additional mechanisms that cause MMP augmentation. Together, our results indicate that a metabolic state resulting in an elevated [NAD(+)]/[NADH] ratio can modulate mitochondrial quantity and quality via pathways that may include SIRT1-mediated mitochondrial autophagy.

SUBMITTER: Jang SY

PROVIDER: S-EPMC3365962 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Mitochondrial dysfunction is a common feature in neurodegeneration and aging. We identify mitochondrial dysfunction in xeroderma pigmentosum group A (XPA), a nucleotide excision DNA repair disorder with severe neurodegeneration, in silico and in vivo. XPA deficient cells show defective mitophagy with excessive cleavage of PINK1 and increased mitochondrial membrane potential. The mitochondrial abnormalities appear to be caused by decreased activation of the NAD+-SIRT1-PGC-1? axis triggered by hyperactivation of the DNA damage sensor PARP1. This phenotype is rescued by PARP1 inhibition or by supplementation with NAD+ precursors that also rescue the lifespan defect in xpa-1 nematodes. Importantly, this pathogenesis appears common to ataxia-telangiectasia and Cockayne syndrome, two other DNA repair disorders with neurodegeneration, but absent in XPC, a DNA repair disorder without neurodegeneration. Our findings reveal a novel nuclear-mitochondrial cross-talk that is critical for the maintenance of mitochondrial health. Mice carrying WT, or CX (Csa-/-/Xpa-/-) alleles in a C57BL/6 background were maintained under standard laboratory conditions and allowed free access to water and control casein pelleted diet (Research Diets D12450B). At 3 months of age, 3 replicates of each of the CX and WT mice were given subcutaneous interscapular injections of 500 mg of Nicotinamide riboside/kg body weight/day or the equivalent volume of saline for 14 consecutive days at 4:00 pm. On day 15, the mice were sacrificed and half of the cerebellum was harvested for purification of mitochondria, with the left half snap-frozen, homogenized, and aliquoted for RNA isolation. Total RNA extraction was done using a TRIzol Plus RNA purification kit as per manufacturer’s protocol. Quality and quantity of the total RNA was tested using the Agilent 2100 Bio-Analyzer and RNA 6000 nano kits. The RNA was labeled using the standard Illumina protocol and hybed overnight to Mouse Ref-8 Illumina arrays. The arrays were scanned using the Beadstation 500 X from Illumina.

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Nicotinamide-induced mitophagy: event mediated by high NAD+/NADH ratio and SIRT1 protein activation.

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