SkillsMechanism: Rapamycin inhibits mTORC1, reducing cholesterol and oxysterol, which impairs Shh ligand lipidation and flattens GLI gradients, thereby hindering regeneration. Readout: Readout: Co-administration of Smoothened agonist or exogenous cholesterol restores GLI activity and progenitor proliferation, rescuing tissue repair while maintaining rapamycin's longevity benefits.
Hypothesis
Rapamycin‑mediated mTORC1 inhibition extends lifespan by simulating nutrient scarcity, but this catabolic state depletes intermediates required for Hedgehog (Hh) ligand lipidation, thereby weakening GLI activator gradients in aged tissues.
Mechanistic Rationale
- mTORC1 inhibition reduces SREBP‑driven transcription of HMG‑CoA reductase and downstream enzymes, lowering cellular cholesterol and oxysterol pools [[https://pmc.ncbi.nlm.nih.gov/articles/PMC10330278/]].
- Hh ligands (Shh, Ihh) require cholesterol modification and oxysterol‑mediated activation for long‑range signaling and GLI2/3 processing [[https://www.jci.org/articles/view/64099]].
- Depleted cholesterol/oxysterol limits production of active Shh, causing a shallow GLI gradient and reduced proliferation of Gli1+ progenitors during injury [[https://onlinelibrary.wiley.com/doi/10.1111/acel.13302]].
- This catabolic shift opposes the anabolic demand for protein synthesis and membrane expansion needed for morphogen gradient formation, a process not rescued by autophagy alone [[https://pmc.ncbi.nlm.nih.gov/articles/PMC9982835/]].
Testable Predictions
- In aged mice subjected to muscle crush injury, chronic rapamycin treatment will decrease Shh cholesterolylation levels and flatten GLI1 transcriptional reporters compared with vehicle [[https://pmc.ncbi.nlm.nih.gov/articles/PMC7960700/]].
- The reduction in GLI activity will correlate with diminished satellite‑cell proliferation and delayed functional recovery.
- Co‑administration of a Smoothened agonist (SAG) or exogenous cholesterol‑modified Shh peptide will restore GLI gradient steepness and progenitor expansion without abolishing rapamycin‑induced autophagy or lifespan extension.
- In progeroid DNA repair‑deficient mice, rapamycin alone will fail to improve tissue repair, whereas rapamycin + SAG will rescue both regenerative output and longevity metrics.
These predictions are falsifiable: if rapamycin does not alter Shh lipidation or GLI gradients, or if SAG fails to rescue regeneration, the hypothesis is refuted.
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