Hypothesis

Rapamycin extends lifespan by amplifying a nutrient‑scarcity signal that boosts serotonin release from enterochromaffin (EC) cells, thereby shifting physiological priority from growth and reproduction to somatic maintenance. This effect does not reverse age‑related EC‑cell loss or mitochondrial damage; instead it phenocopies the adaptive response to famine that evolved to survive periods of low energy intake.

Mechanistic Rationale

• mTORC2 inhibition in placental trophoblasts increases serotonin secretion by 49% without altering TPH1 activity [PMC8807024]; chronic rapamycin elevates brain serotonin and produces antidepressant‑like effects across ages [PMC3454865][PubMed 18534253].
• Chronic stress suppresses mTORC1 signaling and reduces hippocampal synapses, mirroring the serotonergic outcome of rapamycin [Frontiers Pharmacol 2021].
• Gut EC cells generate >90% of bodily serotonin and upregulate TPH1 2‑3‑fold in response to microbiome‑derived short‑chain fatty acids (SCFAs), a pathway activated during caloric restriction [PMC5839326][PMC4396604].
• No evidence shows that rapamycin restores aged EC‑cell numbers or repairs mitochondrial dysfunction; the serotonergic rise is a downstream read‑out of pathway inhibition that reproduces a scarcity state.

We propose that rapamycin does not repair cellular injury but instead hijacks the SCFA‑TPH1 axis, tricking EC cells into secreting more serotonin as if the organism were experiencing famine. This signal then activates downstream longevity programs (e.g., FOXO, autophagy) that were selected to preserve energy during shortage, not to rejuvenate damaged tissues.

Testable Predictions

1. EC‑cell serotonin output will rise rapidly after rapamycin treatment in young, well‑fed animals, but the magnitude of increase will be blunted in germ‑free or SCFA‑deficient mice.
2. Chronic rapamycin will not increase EC‑cell density or mitochondrial membrane potential in aged intestine; histological counts will remain unchanged relative to untreated controls.
3. Blocking TPH1 with p‑chlorophenylalanine will abolish the lifespan‑extending effect of rapamycin without affecting its inhibition of mTORC1 signaling (measured by p‑S6K levels).
4. Transcriptomic profiling of EC cells after rapamycin will show enrichment of famine‑response genes (e.g., PPARα, FGF21) and not of DNA‑repair or senescence‑associated secretory phenotype markers.

Experimental Approach

• Model: Use C57BL/6 mice aged 6 months (young) and 24 months (old). Treat subgroups with rapamycin (14 ppm diet) or vehicle for 8 weeks. Include germ‑free and antibiotic‑treated cohorts to manipulate SCFA availability.
• Readouts:
  • Serotonin levels in portal blood and intestinal lumen (ELISA).
  • EC‑cell density via chromogranin A immunostaining and TPH1 quantification.
  • Mitochondrial health assessed by JC‑1 staining and Seahorse OCR in isolated intestinal epithelial cells.
  • Lifespan monitoring for a subset to correlate serotonergic shift with survival.
  • RNA‑seq of FACS‑sorted EC cells to pathway‑enrichment analysis.
• Intervention: Administer TPH1 inhibitor (p‑chlorophenylalanine, 100 mg/kg i.p.) concurrently with rapamycin in a separate arm.

Potential Implications

If validated, this hypothesis reframes rapamycin‑induced longevity as a hormetic mimetic of ancestral famine rather than a gerotherapeutic repair mechanism. It suggests that combining rapamycin with interventions that genuinely restore EC‑cell number or mitochondrial function (e.g., senolytics, NAD⁺ boosters) could yield additive benefits. Conversely, reliance solely on mTOR inhibition may produce a life extended under a persistent “low‑energy” signal, raising questions about the quality of such prolonged states.