Hypothesis

Aged satellite cells exhibit a shift in GLI2/GLI3 stoichiometry toward the repressor form (GLI3R) due to impaired primary cilium–dependent processing, which attenuates SMO agonist efficacy. Restoring ciliogenesis rebalances GLI gradients and enables SMO‑driven repair.

Mechanistic Rationale

• Primary cilium length and integrity decline with age in muscle satellite cells, altering the subcellular hub where SMO activates GLI2 and where GLI3 is processed to its repressor isoform [3].
• In young tissue, Shh ligand promotes SMO accumulation at the cilium tip, favoring GLI2 activator formation and limiting GLI3R production [1].
• With aging, reduced IFT‑B trafficking and increased HDAC6‑mediated tubulin deacetylation shorten cilia, biasing GLI3 toward proteolytic cleavage into GLI3R, which sequesters SUFU and suppresses Shh‑target genes despite SMO activation [2].
• Consequently, SMO agonists raise total pathway output but fail to shift the GLI2/GLI3R ratio, leaving repair genes silent.

Testable Predictions

1. In aged mouse muscle, satellite‑cell primary cilia are significantly shorter and show reduced acetylated tubulin versus young controls.
2. GLI3R/GLI2 protein ratio in nuclear extracts from aged satellite cells is elevated; SMO agonist (SAG) treatment alone does not decrease this ratio, whereas combined SAG + HDAC6 inhibitor (tubacin) normalizes it.
3. Forced elongation of satellite‑cell cilia (via KIF3A overexpression) restores GLI2 activator predominance and enhances SMO‑induced c‑Jun expression and neurite‑outgrowth‑like phenotypes in vitro.
4. In vivo, aged mice receiving SAG + tubacin show improved muscle regeneration (greater centrally nucleated fibers and force recovery) compared with SAG alone, correlating with restored GLI2/GLi3R balance in regenerating myofibers.

Experimental Approach

• Isolate Pax7+ satellite cells from young (3 mo) and aged (24 mo) mice; quantify cilium length via immunofluorescence (acetyl‑tubulin, ARL13B) and measure GLI2/GLI3R by western blot of nuclear fractions.
• Treat cells with SAG (5 µM) ± tubacin (1 µM) for 24 h; assess GLI2/GLI3R ratio, c‑Jun expression, and proliferation (EdU incorporation).
• Rescue experiments: transduce aged satellite cells with KIF3A‑GFP or shRNA against HDAC6; repeat SMO agonist stimulation.
• In vivo: inject cardiotoxin into tibialis anterior of aged mice; administer SAG (50 mg/kg i.p.) ± tubacin (10 mg/kg i.p.) daily for 5 days; evaluate regeneration histology, GLI2/GLI3R immunostaining, and grip strength.

If predictions hold, the hypothesis establishes that cilium‑dependent GLI processing, not merely SMO activity, gates the regenerative response to Hedgehog agonists in aged tissue.