Hypothesis

Chronic mTOR inhibition extends lifespan by mimicking nutrient scarcity, but continuous suppression of the S6K1→GLI1 axis impairs Hedgehog-dependent stromal repair in aged tissues. Intermittent rapamycin dosing—alternating periods of inhibition with nutrient-replete windows—will sustain sufficient GLI1 transcriptional activity to support fibroblast-mediated tissue maintenance and wound healing, while still eliciting the systemic metabolic reprogramming that underlies longevity.

Mechanistic Rationale

mTORC1 activity drives S6K1-mediated phosphorylation of GLI1 at Ser84, enhancing its transcriptional output independent of SMO [4]. In aged fibroblasts, this GLI1 activity promotes expression of extracellular‑matrix remodelers and paracrine factors that facilitate tissue repair after injury [5]. Continuous rapamycin blocks S6K1, thereby dampening GLI1‑driven repair programs and potentially contributing to fibrosis or delayed healing. However, the longevity effects of mTOR inhibition arise from reduced protein synthesis, enhanced autophagy, and activation of FOXO‑mediated stress defenses [1,2,3], which are maintained as long as the average mTOR activity remains low.

An intermittent schedule creates a pulsatile signal: during off‑drug phases, mTORC1 reactivates S6K1, allowing transient GLI1 phosphorylation and repair‑gene expression; during on‑drug phases, the catabolic, stress‑resistant state is reinstated. This mirrors natural feast‑famine cycles that evolution likely tuned to balance growth and maintenance.

Predictions & Experimental Design

1. Molecular readout – In aged mouse dermal fibroblasts, continuous rapamycin (2 mg/kg/day) will reduce p‑S6K1 and p‑GLI1(S84) levels by >70 % relative to controls, whereas intermittent rapamycin (2 mg/kg/day 5 days on/2 days off) will maintain p‑GLI1 at ≥50 % of baseline during off‑days.
2. Functional assay – Excisional wound healing models will show delayed closure (≥30 % slower) under continuous rapamycin, but comparable closure rates to controls under intermittent dosing.
3. Longevity outcome – Both regimens will extend median lifespan by ~10‑15 % versus ad libitum fed controls, confirming that the systemic longevity signal is preserved despite inter‑dose mTOR reactivation.
4. Fibrosis marker – Hydroxyproline content and collagen‑I immunostaining will be elevated in continuously treated aged skin but not in intermittently treated skin.

Experiments will use C57BL/6 mice aged 18 months, with treatment groups: vehicle, continuous rapamycin, intermittent rapamycin, and pair‑fed controls. Tissue will be harvested at defined intervals for western blot, immunofluorescence, and histological analysis.

Potential Implications

If validated, this hypothesis reframes rapamycin not as a static gerostatic but as a temporally patterned intervention that uncouples systemic longevity from local regenerative competence. It suggests that clinical regimens aiming to extend healthspan should consider dosing schedules that preserve signaling nodes like S6K1→GLI1 in stromal niches, thereby avoiding the trade‑off between slowed aging and impaired tissue repair.