Hypothesis

Continuous rapamycin treatment extends lifespan by mimicking a persistent scarcity signal, but this chronic mTORC1 inhibition also dampens mTORC2‑AKT signaling and blunts the NAD⁺‑dependent AMPK‑SIRT1 axis that is essential for DNA repair and senescence clearance. We hypothesize that pulsatile rapamycin administration, synchronized to the nadir of circadian NAD⁺ levels, will produce greater healthspan and lifespan benefits than continuous dosing by allowing intermittent mTORC1 inhibition to trigger hormetic stress pathways while preserving mTORC2‑AKT‑mediated tissue repair and NAD⁺‑driven SIRT1 activity.

Mechanistic Rationale

• Rapamycin’s primary longevity effect stems from activation of SKN‑1/NRF2, autophagy, and proteasome activity [1][2]
• Chronic mTORC1 inhibition reduces feedback inhibition of AKT via mTORC2, leading to attenuated insulin‑like signaling that is required for stem‑cell proliferation and tissue homeostasis [5]
• NAD⁺ levels oscillate with the circadian clock; SIRT1 activity peaks when NAD⁺ is high, promoting deacetylation of repair proteins and FOXO transcription factors [7]
• Brief mTORC1 inhibition activates AMPK, which raises NAD⁺ via NAMPT, creating a window where SIRT1 can amplify DNA‑repair and senescent‑cell clearance without the deleterious effects of sustained AKT suppression.

Testable Predictions

1. Lifespan & Healthspan: Mice receiving rapamycin in 6‑hour pulses at zeitgeber time ZT0‑ZT6 (low NAD⁺) will show a >15 % increase in median lifespan and improved frailty indices compared with matched continuous‑dose controls.
2. Molecular Signatures: Pulsatile treatment will yield:
   • Higher hepatic SIRT1 activity and increased deacetylation of p53 and Ku70.
   • Elevated autophagic flux (LC3‑II/I ratio) during pulse windows.
   • Preserved phospho‑AKT (Ser473) levels during off‑pulse periods, indicating intact mTORC2 signaling.
3. Damage Markers: Continuous rapamycin will accumulate more γH2AX foci and senescent‑cell burden (p16^Ink4a^+) in liver and muscle than the pulsatile regimen.
4. Falsifiability: If pulsatile dosing fails to improve any of the above endpoints, or if continuous dosing outperforms it, the hypothesis is refuted.

Experimental Design

• Use C57BL/6J mice, n=30 per group, start treatment at 20 months.
• Groups: (i) vehicle, (ii) continuous rapamycin (14 mg/kg chow), (iii) pulsatile rapamycin (same total weekly dose delivered in 6‑h pulses every 24 h aligned to ZT0).
• Monitor survival, monthly frailty scoring, quarterly blood NAD⁺/NADH ratios, and tissue collection at 30 months for the assays listed.

Potential Impact

Confirming that timing matters would shift rapamycin from a blunt “impersonation of hardship” to a chronotherapeutic strategy that couples scarcity signaling with the body’s intrinsic repair cycles, optimizing the trade‑off between stress resistance and anabolic maintenance.