Circadian rhythms are disrupted in ageing, which could contribute to ageing-associated pathologies, but the molecular mechanisms linking circadian rhythms and ageing remain elusive. Bass and colleagues now uncover a role for nicotinamide adenine dinucleotide (NAD+) in preventing age-related decline in circadian function in mice.
Cellular levels of NAD+ decline with ageing, and boosting NAD+ production by administration of its precursors promotes youthful behavioural and physiological functions in mice. To investigate the role of NAD+ in circadian gene expression in mice, the authors gave mice drinking water that was supplemented with NAD+ precursors for 4 months and analysed circadian gene expression in the liver. This revealed that the expression pattern of approximately half of circadian-regulated hepatic genes changed upon NAD+ increase.
NAD+ is a cofactor for sirtuin deacetylases, which are known to promote healthspan and lifespan. Sirtuin 1 (SIRT1) regulates circadian rhythms by binding to the core clock complex — comprising heterodimeric circadian transcription activator CLOCK–BMAL1 and its repressor PER2 — and driving PER2 deacetylation and subsequent degradation. Liver-specific Sirt1 knockout abrogated the changes in hepatic circadian gene expression that were observed when NAD+ levels were increased, while NAD+ precursor administration enhanced chromatin occupancy of BMAL1 and promoted BMAL1-mediated chromatin opening in mouse livers. Moreover, PER2 nuclear localization was increased in SIRT1-deficient cultured cells. Thus, NAD+ promotes BMAL1 transcriptional activity via the SIRT1–PER2 axis.
NAD+ depletion led to PER2 hyperacetylation on Lys680; this reduced phosphorylation of nearby Ser659, which promotes PER2 nuclear retention and when mutated causes circadian disruption manifesting as a sleep disorder in humans. NAD+ depletion also increased PER2 phosphorylation on other sites, including Ser693, which has been linked to PER2 stability and activity. Altogether, NAD+ — through SIRT1-mediated deacetylation of PER2 and reduction of regulatory site phosphorylation — curbs PER2 activity as a transcription repressor, thereby increasing circadian gene expression via BMAL1.
Next, the authors found that in the liver of old mice, chromatin occupancy of BMAL1 was decreased, which coincided with increased PER2 levels and decreased amplitude of circadian gene oscillations. Administration of NAD+ precursor to the old mice for 6 months restored BMAL1 chromatin binding and function to levels observed in young animals. In addition, late-night locomotor activity, normally reduced in old mice, was restored to youthful levels with NAD+ precursor administration.
“NAD+ promotes BMAL1 transcriptional activity via the SIRT1–PER2 axis”
In summary, elevating NAD+ has the capacity to reverse ageing-associated dysfunction of circadian rhythms. Correction of circadian rhythmicity with NAD+ could also be tested for the management of circadian disorders linked to PER2 deregulation, such as those caused by PER2 mutations or shift work.
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