Hypothesis

Circadian‑timed intermittent rapamycin dosing restores the proteostatic advantage of caloric restriction by rescuing BMAL1‑gated mTOR oscillation and reducing maladaptive protein half‑life extension.

Mechanistic rationale

Caloric restriction (CR) suppresses mTOR in a circadian‑dependent manner, peaking during the active phase when BMAL1 drives expression of downstream catabolic programs [2]. This temporal gating creates oscillatory windows of mTOR activity that allow transient bursts of protein synthesis necessary for quality control, while extended low‑mTOR periods engage autophagy and reduce oxidative damage [1]. Rapamycin, when given continuously, flattens this oscillation, leading to constitutively low mTOR signaling. Although this reduces translation initiation, it also abolishes the synthetic bursts that replenish short‑lived, damage‑prone proteins, causing a net increase in protein half‑life without a corresponding rise in degradation [1]. The result is a proteome that is longer‑lived but enriched in aged, oxidized species, explaining why CR females retain superior muscle quality despite lower mass [5].

By delivering rapamycin in short pulses aligned with the BMAL1‑driven trough of mTOR activity (e.g., early night in mice), we predict that the drug will mimic CR’s natural oscillation: each pulse suppresses mTOR just enough to activate autophagy and lower oxidative stress, while the intervening trough permits a physiological surge in translation that restores protein turnover. This should lower the average protein half‑life toward CR levels, improve the synthesis‑degradation balance, and preserve functional tissue outcomes.

Testable predictions

1. Mice receiving circadian‑timed intermittent rapamycin (e.g., 5 mg/kg every 24 h at ZT14) will show a significant reduction in polysome loading compared to continuously dosed rapamycin, approximating the decrease seen under CR [1].
2. Proteomic pulse‑chase assays will reveal a decrease in median protein half‑life from the ~15 % extension seen with constant rapamycin to the ~35‑60 % extension characteristic of CR, indicating restored turnover [1].
3. Muscle‑specific readouts (grip strength, NMJ integrity) will be superior to those of constant‑dose rapamycin and non‑inferior to CR, despite similar overall mTOR inhibition measured by p‑S6 levels [4][5].
4. Disruption of BMAL1 (muscle‑specific Bmal1 knockout) will abolish the advantage of timed rapamycin, bringing outcomes back to those of continuous dosing, confirming the circadian gate as mechanistic linchpin.

Experimental approach

• Use male and female C57BL/6J mice aged 12 months. Four groups: ad libitum control, CR (40 % reduction), continuous rapamycin (encapsulated pellets delivering ~14 mg/kg/day), and intermittent rapamycin (same total weekly dose split into daily pulses at ZT14).
• Monitor food intake, body weight, and circadian locomotor activity to confirm timing.
• After 3 months, collect quadriceps and gastrocnemius for polysome profiling, SUnSET puromycin labeling, and oxidative carbonylation assays.
• Perform dynamic SILAC pulse‑chase to measure protein‑specific half‑lives.
• Assess grip strength, rotarod performance, and NMJ staining (α‑bungaroxin).
• In a parallel cohort, employ muscle‑specific Bmal1‑KO mice to test dependence on the circadian clock.

If timed rapamycin recapitulates CR’s proteostatic profile and functional benefits while preserving lifespan extension, the hypothesis will be supported; failure to improve protein turnover or tissue function despite matched mTOR inhibition will falsify the claim that circadian gating is essential for genuine rejuvenation beyond stress mimicry.