NAD+‑SIRT6 chromatin brake hypothesis

Aging cells lose NAD+ not just as a fuel shortage but as a signal that engages a nuclear deacetylase, SIRT6, to remodel chromatin at metabolic‑gene promoters. Low NAD+ reduces SIRT6 activity, which in turn allows aberrant acetylation of histone H3K9 at promoters of genes governing mitochondrial biogenesis and autophagy. This aberrant acetylation recruits a repressive complex containing HDAC3 and the corepressor NCoR, shutting down transcription. The resulting drop in mitochondrial output further limits acetyl‑CoA synthesis, creating a vicious cycle that locks the cell into a low‑energy, low‑repair state. The cycle is testable: restoring NAD+ should reactivate SIRT6, normalize H3K9 acetylation, and revive mitochondrial gene expression only if the SIRT6‑HDAC3 axis is intact.

Key predictions

• In aged mouse liver, nuclear NAD+ levels will correlate inversely with H3K9ac at the promoters of Ppargc1a (PGC‑1α) and Map1lc3b (LC3B) NAD+ decline driven by CD38.
• Genetic ablation of SIRT6 in young mice will mimic the aged chromatin signature and reduce oxygen consumption rate (OCR) by ~30 % without altering total NAD+ pools SIRT1‑AMPK axis.
• Overexpressing a catalytically dead SIRT6 mutant will block the benefits of NAD+ precursor supplementation on histone acetylation and OCR, confirming that SIRT6 activity—not merely NAD+ abundance—is required for the rescue ACSS2 phosphorylation.
• Pharmacologic inhibition of HDAC3 will prevent the repressive complex formation, allowing NAD+ repletion to boost acetylation and respiration even when SIRT6 is low acetyl‑CoA pools autophagy.

Experimental outline

1. Measure nuclear NAD+, total NAD+, and SIRT6 activity in liver isolates from 3‑month vs 24‑month C57BL/6 mice CD38-mediated NAD+ consumption.
2. Perform ChIP‑qPCR for H3K9ac at Ppargc1a and Map1lc3b promoters; correlate with NAD+ and SIRT6 readouts.
3. Generate Alb‑Cre;Sirt6^fl/fl mice (liver‑specific SIRT6 KO) and young controls; assess NAD+, H3K9ac, OCR, and ATP levels.
4. Treat aged wild‑type and SIRT6 KO mice with nicotinamide riboside (NR) for 4 weeks; repeat ChIP‑qPCR and Seahorse OCR assay acetate supplementation.
5. In parallel, administer the HDAC3 inhibitor RGFP966 to a subset of NR‑treated aged mice; evaluate whether HDAC3 blockade restores H3K9ac and mitochondrial function despite low SIRT6.
6. Extend to functional readouts: grip strength, frailty index, and lifespan in a cohort of NR ± RGFP966‑treated aged mice.

Falsifiability If NAD+ repletion improves histone acetylation and mitochondrial respiration irrespective of SIRT6 status, or if SIRT6 loss does not recapitulate the aged chromatin and metabolic defects, the hypothesis is refuted. Likewise, if HDAC3 inhibition fails to rescue the NR response in SIRT6‑deficient cells, the proposed SIRT6‑HDAC3 repressive axis is incorrect.

Broader implication This frames NAD+ decline as a trigger for a chromatin‑based metabolic brake rather than a passive energy shortage, linking the nucleotide‑redox state directly to epigenetic enforcement of a low‑activity state. Interventions that uncouple NAD+ levels from SIRT6‑HDAC3 signaling—either by boosting SIRT6 activity or blocking HDAC3—could therefore restore youthful chromatin landscapes without merely flooding the cell with NAD+.