Hypothesis

Chronic mTORC1 inhibition restores the cellular competence to engage Hedgehog (Hh) signaling by stabilizing GLI2 and permitting SMO ciliary localization, but it simultaneously blocks the proteolytic conversion of full-length GLI2/GLI3 into their transcriptional activator forms. Consequently, aged tissues exhibit a "signaling‑competent but transcriptionally inert" Hh state: the pathway is poised at the membrane, yet downstream GLI‑dependent genes required for proliferative regeneration remain suppressed. This mechanistic uncoupling explains why rapamycin improves Hh readouts (e.g., PTCH1 expression) without rescuing tissue repair, and predicts that pharmacologically forcing GLI activator formation will restore regeneration even under continuous mTORC1 inhibition.

Rationale

1. mTORC1 regulates GLI processing – In Drosophila and mammalian cells, mTORC1 activity promotes GSK3β inhibition, favoring GLI2/GLI3 stabilization as activators; conversely, mTORC1 loss enhances GSK3β activity, driving GLI proteolytic processing toward repressor forms (Zhang et al., 2021). Although not directly cited in the seed papers, this cross‑talk is well established and provides a mechanistic link between the observed mTORC1‑Hh paradox and regenerative failure.
2. Rapamycin rescues trafficking but not processing – Rapamycin restores GLI2 protein levels and SMO ciliary accumulation (refs [3][4]), yet it does not address the cytosolic kinase cascade that determines GLI2’s fate. Thus, cells accumulate full‑length GLI2 that is competent for ciliary entry but remains phosphorylated by GSK3β, leading to β‑TRCP‑mediated degradation or repressor generation.
3. Regenerative proliferation requires GLI‑activator‑driven transcription – Mitogenic Hh targets such as CyclinD1, Myc, and Aurka depend on GLI2/GLI3 activator isoforms; repressor isoforms antagonize these genes (ref [5]). Therefore, even with intact ciliary signaling, a shift toward GLI repressors blocks the proliferative program essential for muscle, liver, and neuron repair.

Testable Predictions

• Prediction 1: In aged human fibroblasts treated with rapamycin (100 nM, 48 h), GLI2 will show increased ciliary SMO localization (as previously reported) but a decreased ratio of GLI2‑activator (full‑length, phosphorylated at S210) to GLI2‑repressor (processed N‑terminal fragment) compared with untreated controls.
• Prediction 2: Pharmacological inhibition of GSK3β (e.g., CHIR99021, 3 µM) during rapamycin treatment will restore the GLI2‑activator/repressor ratio and rescue Hh‑dependent transcription of CyclinD1 and Myc without affecting SMO ciliary localization.
• Prediction 3: In a murine model of aged muscle injury, continuous rapamycin will improve PTCH1 expression but fail to augment satellite cell proliferation; co‑administration of a GSK3β inhibitor will restore both proliferation and fiber cross‑sectional area to levels seen with intermittent rapamycin or vehicle.
• Falsification: If rapamycin does not alter the GLI2‑activator/repressor ratio, or if GSK3β inhibition fails to rescue proliferative Hh targets despite restoring activator levels, the hypothesis is refuted.

Experimental Approach

1. Cellular assays – Treat primary human dermal fibroblasts from donors >65 years with rapamycin ± CHIR99021 for 48 h. Assess SMO ciliary localization by immunofluorescence (acetyl‑tubulin + SMO), GLI2 processing by western blot using antibodies distinguishing full‑length versus repressor fragments, and Hh transcriptional output via qPCR for PTCH1, GLI1, CyclinD1.
2. In vivo validation – Use 20‑month‑old C57BL/6 mice subjected to tibialis anterior crush injury. Groups: vehicle, rapamycin (4 mg/kg/day i.p.), rapamycin + CHIR99021 (10 mg/kg/day i.p.), and intermittent rapamycin (3 days on/4 days off). Measure Pax7⁺ satellite cell proliferation (EdU incorporation), myofiber diameter, and Hh target expression at 3 and 7 days post‑injury.
3. Readouts – Primary: GLI2‑activator/repressor ratio, EdU⁺ satellite cells. Secondary: PTCH1, GLI1, CyclinD1, Myc expression; fibrosis (collagen I staining).

Implications

If validated, this hypothesis reframes the mTORC1‑Hh interaction from a simple "competence vs. output" trade‑off to a specific node—GLI2 proteolytic processing—that can be therapeutically decoupled. It suggests that intermittent or pulsatile mTORC1 inhibition, or combined mTORC1/GSK3β targeting, may yield the longevity benefits of hormesis without compromising regenerative capacity, addressing a critical limitation of current rapamycin‑based geroprotective strategies.