Hypothesis

Rapamycin extends lifespan not by systemic damage repair but by selectively suppressing enterochromaffin (EC) cell‑derived serotonin, thereby generating a localized gut‑derived famine signal that triggers intestinal epithelial autophagy and downstream metabolic re‑programming.

Mechanistic Rationale

• mTORC1 inhibition in EC cells reduces TPH1 activity and serotonin release [2][3].
• Loss of luminal 5‑HT relieves its inhibitory tone on AKT/mTOR in neighboring epithelial cells [4], paradoxically allowing a low‑level AMPK activation that stimulates ULK1‑dependent autophagy.
• Autophagic flux in enterocytes enhances barrier integrity, reduces inflammaging, and shifts microbiota toward SCFA producers [5], which further feeds back to sustain low serotonin via SCFA‑mediated TPH1 repression [3].
• The resulting gut‑centric signal mimics nutrient scarcity without systemic caloric restriction, explaining why rapamycin preserves some metabolic benefits of CR while diverging on immune outcomes [6].

Testable Predictions

1. EC‑specific TPH1 overexpression in mice treated with rapamycin will abolish the lifespan extension seen with rapamycin alone.
2. Intestinal epithelial autophagy reporters (e.g., mCherry‑GFP‑LC3) will show increased flux within 24 h of rapamycin administration, an effect lost in EC‑cell‑specific mTOR knockout mice.
3. Fecal serotonin levels will drop significantly after rapamycin treatment, correlating with increased circulating SCFA concentrations and improved gut barrier markers (e.g., FITC‑dextran permeability).
4. Germ‑free mice receiving rapamycin will show attenuated lifespan extension, indicating that the microbiome‑SCFA feedback loop is required for the full effect.

Falsification Criteria

If any of the following are observed, the hypothesis is weakened:

• Rapamycin still extends lifespan in EC‑TPH1‑overexpressing mice to the same degree as controls.
• Autophagic flux in intestinal epithelium does not change after rapamycin, or changes are independent of EC‑cell serotonin levels.
• Fecal serotonin remains unchanged or increases despite rapamycin treatment.

Implications

This reframes rapamycin’s action as a targeted "gut-deception" strategy: pharmacologically silencing a gut‑derived satiety/hardship signal to engage evolutionarily conserved autophagy programs, thereby separating the benefits of longevity from the systemic costs of true famine.

Experimental Approach

To test these predictions, we propose using villin‑CreERT2;Tph1fl/fl mice for inducible EC‑specific TPH1 knockout, combined with rapamycin dosing. Lifespan cohorts will be monitored alongside intestinal autophagy reporters (Rosa26‑mTmG;LC3). Metabolomics of portal blood will quantify SCFAs, while 16S rRNA sequencing will assess microbiota shifts. Statistical comparisons will use Cox proportional hazards models for survival and ANOVA for autophagy flux, with significance set at p<0.05.