Hypothesis

Aging cells actively suppress autophagy through a NAD+-sirtuin axis that becomes hypoactive, leading to hyperacetylation of key autophagy initiators (ULK1, VPS34, BECN1) and thereby reinforcing mTORC1‑driven inhibition. Transient partial reprogramming restores NAD+ levels and sirtuin activity, deacetylates these proteins, and re‑enables autophagic flux without needing to overcome mTORC1 hyperactivity directly.

Mechanistic Basis

• NAD+ declines with age, reducing SIRT1 and SIRT2 activity.
• Hyperacetylated ULK1 loses its ability to be phosphorylated by AMPK and becomes more susceptible to mTORC1‑mediated inhibitory phosphorylation.
• Acetylated VPS34 shows reduced lipid kinase activity, impairing PI3P production needed for phagophore nucleation.
• Acetylated BECN1 binds less effectively to VPS34 and more to inhibitory partners such as RUBCN, further dampening the complex.
• These acetylation changes synergize with the mTORC1‑ULK1 block described in , creating a layered suppression that is not merely due to upstream kinase activity but to loss of deacetylase‑mediated priming. Evidence: SIRT1 activation by NAD+ precursors rescues autophagy in aged models ([3]; [4]), and acetyl‑mimic mutants of ULK1 and VPS34 phenocopy the autophagy block seen in senescent cells.

Testable Predictions

1. In old fibroblasts, NAD+ supplementation will increase SIRT1/2 activity, decrease acetyl‑ULK1 (K->R) and acetyl‑VPS34 (K->R) levels, and restore LC3‑II turnover even when mTORC1 remains pharmacologically active.
2. Expressing a non‑acetylatable ULK1 mutant (K->R) in aged cells will bypass the NAD+‑sirtuin requirement and restore autophagosome formation despite low NAD+.
3. Transient expression of Yamanaka factors for 48 h will raise intracellular NAD+ via upregulation of NAMPT, increase SIRT1 deacetylase activity, and lead to rapid deacetylation of ULK1/VPS34 preceding any detectable reduction in mTORC1 signaling.
4. Preventing the NAD+ rise during partial reprogramming (by FK866 inhibition) will block the autophagy‑restoring effect, confirming that the NAD+‑sirtuin axis is necessary for the observed flux rescue.

Experimental Approach

• Cell model: Human dermal fibroblasts cultured to replicative senescence (passage 20‑25).
• Interventions: (a) NAD+ precursor (NR or NMN) 1 mM for 48 h; (b) CRISPR knock‑in of ULK1 K->R and VPS34 K->R; (c) 4‑factor OSKM inducible system pulsed for 48 h; (d) combined treatments with rapamycin or FK866 as controls.
• Readouts: Western blot for acetyl‑ULK1 (Ac‑KXXX), acetyl‑VPS34, phospho‑ULK1 (Ser757), LC3‑II/I ratio with and without bafilomycin A1, mTORC1 activity (p‑S6K), NAD+ levels (enzymatic assay), SIRT1/2 activity (fluorometric deacetylase assay).
• Functional assay: Mitochondrial ROS, p16^INK4a expression, and senescence‑associated β‑galactosidase to link autophagy restoration to phenotypic rejuvenation.
• Statistical plan: n = 3 biological replicates, ANOVA with Tukey post‑hoc, significance set at p < 0.05.

If the data show that NAD+‑dependent deacetylation precedes and is sufficient for autophagy recovery, the hypothesis that active suppression operates via an acetyl‑sensitive checkpoint will be supported. Conversely, if autophagy fails to recover despite NAD+ elevation and sirtuin activation, the model would be falsified, pointing to alternative dominant suppressors.