Hypothesis

Continuous mTOR inhibition mimics a permanent scarcity signal, eliciting stress‑adaptation pathways without triggering the anabolic phases needed for macromolecular repair. We propose that delivering rapamycin in intermittent bursts aligned with the endogenous circadian trough of hepatic mTOR activity will restore rhythmic cycles of catabolism and anabolism, thereby enabling true rejuvenation—measured by epigenetic age reversal, improved proteostasis, and restored proliferative capacity—while retaining the lifespan‑extending benefits of autophagy induction.

Mechanistic Rationale

• mTORC1 activity naturally oscillates over the 24‑h cycle, peaking during the active phase and declining during rest [[https://pmc.ncbi.nlm.nih.gov/articles/PMC6075448/]]. This rhythmic suppression coincides with heightened autophagy and reduced protein synthesis [[https://www.jci.org/articles/view/64099]].
• Constant rapamycin exposure flattens this rhythm, creating a chronic “hardship” state that upregulates stress defenses (e.g., inflammation reduction, stem‑cell niche support) but does not activate repair programs that require transient mTORC1 re‑engagement, such as translation‑fidelity checkpoint activation or NAD^+‑dependent sirtuin activity.
• Brief periods of mTORC1 reactivation (e.g., during the circadian peak) can restore ribosome biogenesis and chaperone‑mediated folding, allowing cells to clear accumulated misfolded proteins and reset epigenetic marks via increased acetyl‑CoA production.
• Importantly, intermittent inhibition preserves autophagy induction because each rapamycin pulse still triggers LC3‑II conversion and autophagosome formation [[https://pmc.ncbi.nlm.nih.gov/articles/PMC4141208/]], while the off‑windows permit proteasomal degradation and DNA‑repair pathways that are suppressed under sustained mTORC1 inhibition.

Testable Predictions

1. Mice receiving rapamycin every 48 h at zeitgeber time 18 (the hepatic mTOR trough) will show a significant reduction in epigenetic age (Horvath clock) compared to continuously dosed and untreated controls after 6 months.
2. The intermittent group will exhibit increased proteasome activity and reduced ubiquitinated protein aggregates in liver and muscle, without a decline in autophagic flux (LC3‑II/I ratio unchanged).
3. Immune competence, measured by survival after West Nile virus challenge, will be preserved in the intermittent regimen, unlike the continuous group which shows heightened susceptibility [[https://pmc.ncbi.nlm.nih.gov/articles/PMC4326902/]].
4. Translational fidelity, assessed by misincorporation rates of fluorescent reporter proteins, will be improved only in the intermittent condition, indicating periodic mTORC1‑dependent quality‑control activation.

Experimental Design

• Use male C57BL/6J mice, 4 months old, randomized into three groups (n = 15 per group): vehicle control, continuous rapamycin (14 mg/kg chow), intermittent rapamycin (same total weekly dose administered via intraperitoneal injection every 48 h at ZT18).
• Monitor food intake, body weight, and glucose tolerance monthly.
• At 6 months, collect tissues for: epigenetic sequencing (automated CpG analysis), proteasome activity assays, ubiquitin‑Western blot, LC3‑II/I immunofluorescence, and spleen‑derived T‑cell function assays.
• Challenge a subset with subcutaneous West Nile virus and record survival over 14 days.
• Perform puromycin‑based SUnSUCH assay to quantify nascent chain fidelity in hepatocytes.

If intermittent, circadian‑timed mTOR inhibition delivers the predicted improvements in aging biomarkers without sacrificing immune resilience, the hypothesis will be supported, reframing longevity pharmacology from static hardship imitation to dynamic metabolic reprogramming.