Hypothesis Rapamycin extends lifespan only while mTORC1 remains inhibited; stopping the drug causes a rapid collapse of the NAD+-dependent sirtuin‑autophagy axis that underlies its health benefits, revealing that the intervention mimics a transient famine signal rather than enacting lasting repair.

Mechanistic reasoning mTORC1 suppression by rapamycin reduces protein synthesis and activates autophagy via ULK1 and TFEB. This state also raises the NAD+/NADH ratio, boosting SIRT1 and SIRT3 activity, which deacetylate autophagy proteins and sustain mitochondrial quality control. When rapamycin is withdrawn, mTORC1 reactivation suppresses ULK1, lowers NAD+ levels through increased glycolytic flux, and diminishes sirtuin deacetylation, causing autophagy flux to drop despite any residual lysosomal capacity. Thus the longevity phenotype depends on continuous pathway suppression to keep the stress‑responsive metabolic loop alive.

Prediction If the NAD+-sirtuin loop is experimentally maintained after rapamycin cessation, the survival benefit will persist; if it is disrupted, benefits will be lost even with ongoing rapamycin.

Experimental design

1. Treatment groups (n=30 per group, male C57BL/6J mice, start at 12 months):
   • Vehicle control
   • Continuous rapamycin (14 mg/kg diet)
   • Intermittent rapamycin (8 weeks on/off)
   • Intermittent rapamycin + NAD+ booster (nicotinamide riboside 300 mg/kg diet)
   • Intermittent rapamycin + SIRT1 inhibitor (EX527 10 mg/kg)
2. Readouts (collected every 4 weeks):
   • Survival curves
   • Frailty index
   • Liver and muscle autophagy flux (LC3‑II turnover with bafilomycin)
   • NAD+ levels (enzymatic assay)
   • SIRT1 activity (fluorometric deacetylation assay)
   • Phospho‑S6K as mTORC1 readout
3. Analysis
   • Compare median lifespan and healthspan across groups using log‑rank test.
   • Test interaction between rapamycin withdrawal and NAD+ boosting on autophagy flux with two‑way ANOVA.
   • Correlate SIRT1 activity with LC3‑II levels to validate the proposed loop.

Potential outcomes

• If intermittent rapamycin alone shows a lifespan gain that disappears after withdrawal, while the NAD+ booster cohort retains extended survival and high autophagy flux, the hypothesis is supported.
• If SIRT1 inhibition abolishes the booster’s effect, confirming sirtuin dependence.
• If neither NAD+ nor SIRT1 manipulation alters the post‑withdrawal decline, the hypothesis is falsified, suggesting other mechanisms sustain rapamycin’s benefits.

References [1] https://doi.org/10.1101/2023.10.26.564115 [2] https://pubmed.ncbi.nlm.nih.gov/28329151/ [3] https://doi.org/10.1111/acel.13086 [4] https://www.nad.com/news/rapamycin-dietary-restriction-brain-mice [5] https://pubmed.ncbi.nlm.nih.gov/39127848 [6] https://pmc.ncbi.nlm.nih.gov/articles/PMC12766144/