Hypothesis: Aging induces primary cilium shortening in stromal fibroblasts, which reduces SMO dwell time at the cilium tip and favors GLI2 proteolysis into repressor form, flattening the Shh gradient needed for stem cell activation. Senescence‑associated NF‑κB signaling further phosphorylates GLI2 at destabilizing sites, compounding repressor bias. We predict that pharmacological inhibition of GSK3β (which normally primes GLI2 for β‑TrCP‑mediated degradation) will shift the GLI2 processing balance toward the activator isoform, restoring a steep Shh gradient and regenerative output even when SMO ciliary trafficking is compromised. Crucially, this rescue should occur without increasing tumorigenic risk because GSK3β inhibition does not drive uncontrolled SMO activity; instead, it corrects downstream GLI processing that is uncoupled from oncogenic SMO mutations.

Testable predictions:

1. In young versus aged mouse muscle after cardiotoxin injury, immunofluorescence for acetylated tubulin and ARL13B will show a ~30% reduction in cilium length in aged PDGFRa+ fibroblasts (see et al., PMID:41673424).
2. FRAP of Smo‑GFP will reveal a decreased mobile fraction and faster recovery half‑time in aged fibroblasts, indicating shorter ciliary residency.
3. A Gli‑responsive luciferase reporter introduced into muscle explants will display a flattened spatial gradient (high proximal, low distal) in aged tissue; treatment with the GSK3β inhibitor CHIR99021 will steepen the gradient, whereas purmorphamine alone will not.
4. RNA‑seq of sorted fibroblasts will show increased expression of Gli3 repressor target genes and decreased Gli1/Gli2 activator targets in aged tissue, a pattern reversed by CHIR99021 but unchanged by purmorphamine.
5. Senescence markers (p16^Ink4a, SASP IL‑6) will remain elevated after CHIR99021 treatment, confirming that the effect occurs downstream of senescence.
6. Longitudinal lineage tracing of fibroblasts treated chronically with CHIR99021 will not show increased hyperplasia or tumor formation, distinguishing this approach from constitutive SMO activation that raises cancer risk.

If any of these predictions fails—for example, if GSK3β inhibition does not restore the Gli luciferase gradient or if it leads to tumorigenic overgrowth—the hypothesis is falsified. This framework shifts the focus from merely activating SMO to correcting the intracellular processing of GLI factors, offering a mechanistic route to rejuvenate Shh‑dependent regeneration in aged tissues while mitigating oncogenic concerns.