Rapamycin extends lifespan by activating an integrated stress response that simulates nutrient scarcity, thereby suppressing mTORC1-driven biosynthesis and reducing pathogenic von Willebrand factor (VWF) release from senescent endothelium. However, continuous mTORC1 inhibition also dampens mTORC2‑Akt signaling, a pathway essential for endothelial survival and repair. We hypothesize that the longevity benefit of rapamycin depends on dosing schedule: intermittent administration yields a transient stress signature that lowers thrombotic risk without compromising Akt‑mediated endothelial regeneration, whereas chronic exposure produces a persistent stress state that impairs repair and accelerates endothelial senescence despite reduced coagulopathy.

Testable Predictions

1. Biomarker Dynamics – In aged mice, continuous rapamycin (2 mg/kg diet) will sustain low plasma VWF:Ag and FVIII:C (as previously reported) but will show decreased phospho‑Akt (Ser473) and increased endothelial senescence markers (p16^INK4a^, SA‑β‑gal) compared with untreated controls. Intermittent rapamycin (same total weekly dose given 3 days on/4 days off) will maintain low VWF/FVIII while preserving phospho‑Akt levels and reducing senescence markers relative to continuous treatment.
2. Functional Outcome – Mice receiving intermittent rapamycin will exhibit superior endothelial‑dependent vasodilation (acetylcholine‑induced aortic relaxation) and faster re‑endothelialization after femoral artery injury than those on continuous rapamycin, despite comparable reductions in thrombosis susceptibility (FeCl3‑induced carotid occlusion time).
3. Mechanistic Link – The protective effect of intermittent dosing will be attenuated by endothelial‑specific Akt1 knockout, indicating that Akt signaling is required for the repair advantage. Conversely, pharmacologic activation of eIF2α phosphatase (e.g., ISRIB) during continuous rapamycin will restore phospho‑Akt and rescue endothelial function without raising VWF, confirming that the stress response itself is not detrimental when its duration is limited.

Experimental Design

• Use 20‑month‑old C57BL/6 mice divided into four groups (n=12 per group): vehicle, continuous rapamycin, intermittent rapamycin, and intermittent rapamycin + ISRIB. Measure plasma VWF:Ag, FVIII:C, phospho‑Akt (Ser473) in aortic lysates, endothelial senescence (p16, SA‑β‑gal), and functional endpoints as described above. Statistical analysis via two‑way ANOVA with post‑hoc Tukey test.

Falsifiability

If intermittent rapamycin fails to preserve phospho‑Akt or endothelial function while still lowering VWF, or if continuous rapamycin does not exacerbate senescence markers relative to vehicle, the hypothesis would be refuted. Likewise, if Akt1 endothelial deletion does not abolish the functional advantage of intermittent dosing, the proposed mechanistic link would be invalid.

Broader Implication

This hypothesis reframes mTOR inhibition not as a simple mimetic of scarcity but as a tunable stress intervention whose net effect on aging hinges on balancing catabolic adaptation with anabolic repair signals. Optimizing dosing schedules could convert a compensatory response into a genuine rejuvenative strategy, extending healthspan without trading thrombotic protection for endothelial frailty.