Hypothesis: NAD+-SIRT1 Regulation of KDM6A Acetylation Governs Bivalent Domain Resolution in Aging Somatic Stem Cells

Core idea Declining NAD+ levels during aging reduce SIRT1‑mediated deacetylation of KDM6A, increasing its acetylation and thereby altering its sensitivity to α‑ketoglutarate (α‑KG) and the inhibitory metabolite 2‑hydroxyglutarate (2‑HG). This shift biases KDM6A toward a low‑activity state at bivalent promoters of lineage‑specific genes, causing a progressive loss of H3K4me3 and gain of H3K27me3 that erodes the poised chromatin state, drives epigenetic drift, and contributes to stem‑cell functional decline.

Mechanistic rationale

• KDM6A is an α‑KG‑dependent dioxygenase; its catalytic efficiency is modulated by post‑translational modifications.
• SIRT1 deacetylates lysine residues on KDM6A (e.g., Kxxx) in a NAD+‑dependent manner, a regulation shown for other demethylases.
• Acetylated KDM6A exhibits reduced affinity for α‑KG and heightened susceptibility to competitive inhibition by 2‑HG, which accumulates in aged tissues due to mitochondrial dysfunction.
• At bivalent domains, the balance of H3K4me3 (set by KMT2 complexes) and H3K27me3 (removed by KDM6A) maintains poise; a tilt toward H3K27me3 silences developmental genes without the compensatory H3K4me3→H3K4me1 transition seen in differentiation.

Testable predictions

1. In young mouse muscle satellite cells, KDM6A is predominantly deacetylated, shows high α‑KG affinity, and bivalent promoters display a characteristic H3K27me3→H3K4me1 transition upon activation.
2. In aged satellite cells, NAD+ ↓, SIRT1 activity ↓, KDM6A acetylation ↑, α‑KG/2‑HG ratio ↓, leading to increased H3K27me3 and decreased H3K4me3 at the same bivalent loci.
3. Pharmacological NAD+ replenishment (e.g., nicotinamide riboside) or SIRT1 activation (e.g., SRT1720) in aged cells will decrease KDM6A acetylation, restore α‑KG sensitivity, reduce H3K27me3, increase H3K4me3/H3K4me1, and rescue the poised state.
4. Conversely, SIRT1 inhibition or expression of an acetylation‑mimic KDM6A mutant in young cells will recapitulate the aged epigenetic signature.

Experimental approach

• Isolate satellite cells from 3‑month and 24‑month mice.
• Measure NAD+, α‑KG, 2‑HG levels (LC‑MS).
• Perform immunoprecipitation‑Western for KDM6A acetylation (acetyl‑lysine antibody).
• Conduct CUT&RUN or ChIP‑seq for H3K27me3, H3K4me3, H3K4me1 at promoters of known bivalent genes (e.g., Pax7, Myod1).
• Assess functional outcomes: proliferation, differentiation capacity.
• Intervention groups: aged cells treated with NR, SIRT1 activator, or vehicle; plus genetic models (Sirt1‑KO, KDM6A‑KQ acetyl‑mimic).

Falsifiability If NAD+ boosting fails to alter KDM6A acetylation status, or if manipulation of KDM6A acetylation does not shift H3K27me3/H3K4me3 ratios at bivalent promoters, the hypothesis is refuted. Likewise, if the α‑KG/2‑HG ratio does not correlate with KDM6A activity in aging, the proposed metabolic link is unsupported.

Broader impact Connects cancer‑derived KDM5/6 biology to aging through a conserved metabolic‑epigenetic axis, offering a mechanistic explanation for age‑related loss of stem‑cell plasticity and a potential therapeutic avenue via NAD+ modulation.