Hypothesis

Intermittent low‑dose rapamycin produces lifespan extension by selectively enhancing uORF‑mediated translation of longevity factors without triggering the sustained stress‑response programs seen with continuous inhibition.

Mechanistic Basis

Recent work shows that rapamycin reshapes aging‑associated translation through upstream open reading frames (uORFs) in a manner not recapitulated by caloric restriction【5】. uORFs can repress translation of downstream main ORFs under nutrient‑rich conditions but allow selective bypass when eIF2α phosphorylation is transiently altered. We propose that short pulses of mTORC1 inhibition cause a brief dip in eIF2B activity, leading to a timed window where specific uORFs governing autophagy regulators (e.g., ATG5, BECN1) and senescent‑cell clearance factors are translated, while chronic inhibition maintains eIF2α‑P elevation, activating broad ATF4‑driven stress pathways (e.g., CHOP, GADD34) that underlie side effects.

Thus, the balance between true rejuvenation and stress mimicry hinges on the kinetic profile of uORF translation rather than the absolute degree of mTOR suppression.

Testable Predictions

1. Biomarker split – In mouse liver, intermittent rapamycin (e.g., 5 mg/kg twice weekly) will increase LC3II/Beclin‑1 and decrease p62 without raising cleaved caspase‑3 or phospho‑eIF2α, whereas continuous dosing will elevate both autophagy and apoptosis markers.
2. uORF specificity – Ribosome profiling will reveal enhanced ribosome occupancy on the main ORFs of ATG5 and BECN1 after intermittent treatment, but not after continuous treatment, despite similar overall mTORC1 inhibition (p‑S6K levels).
3. Lifespan decoupling – Mice receiving intermittent rapamycin will show median lifespan extension comparable to continuous dosing, yet exhibit significantly lower incidence of cataract, glucose intolerance, and thymic atrophy—phenotypes linked to chronic stress activation.
4. Rescue experiment – Pharmacological inhibition of eIF2α phosphatase (e.g., using guanabenz) during intermittent rapamycin will blunt the uORF‑driven translation boost and abolish the longevity benefit, confirming mechanistic reliance on the eIF2α‑uORF axis.

Experimental Design

• Animal cohorts (n=30 per group, male and female C57BL/6J): (1) control chow, (2) continuous rapamycin (14 mg/kg chow), (3) intermittent rapamycin (same total weekly dose given in two injections), (4) intermittent rapamycin + guanabenz (eIF2α‑P stabilizer).
• Readouts at 6, 12, and 18 months: liver and muscle lysates for LC3II, Beclin‑1, p62, cleaved caspase‑3, phospho‑eIF2α, ATF4 targets; ribosome footprint sequencing focused on uORF‑containing transcripts; frailty index and glucose tolerance tests.
• Survival monitoring until natural death, with cause‑of‑death classification.
• Statistical analysis – Kaplan‑Meier with log‑rank test for lifespan; two‑way ANOVA for biomarker time points; interaction terms to test whether guanabenz abolishes intermittent‑rapamycin effects.

If intermittent dosing yields improved healthspan without the molecular signatures of chronic stress, while continuous dosing does not, the hypothesis is supported. Conversely, if both regimens produce identical uORF translation patterns and stress markers, the hypothesis is falsified, indicating that lifespan benefits stem from a uniform stress‑mimicry mechanism rather than differential translational control.