Hypothesis

In aged regenerating tissue, the Hedgehog (Hh) pathway deficit arises not from SMO dysfunction but from an age‑induced increase in HDAC3 activity that selectively removes H3K27 acetylation at GLI3‑bound repressor enhancers. This creates a chromatin environment that favors GLI3 repressor occupancy and suppresses GLI1 activator function, thereby biasing the GLI gradient toward repression. Supraphysiological SMO stimulation (e.g., high‑dose SAG) can overcome this blockade by two coupled mechanisms: (1) heightened GLI1 phosphorylation that reduces its affinity for HDAC complexes, and (2) enhanced adenosine‑to‑inosine (A‑to‑I) RNA editing of GLI1 transcripts, which increases GLI1 transcriptional potency and competitively displaces GLI3 from shared DNA sites.

Mechanistic Rationale

• Epigenetic gatekeeper: HDAC3 is known to deacetylate H3K27 at enhancers, converting active chromatin to a repressed state. Aging elevates HDAC3 expression in fibroblasts and satellite cells (see HDAC activity shifts in aged muscle)【https://pmc.ncbi.nlm.nih.gov/articles/PMC10111369/】. We propose that HDAC3 is recruited to GLI3‑bound HH‑dependent enhancers via a protein‑protein interface that strengthens with age, leading to focal loss of H3K27ac specifically at these sites.
• GLI gradient shift: GLI3 repressor competes with GLI1 activator for overlapping binding motifs. Loss of H3K27ac diminishes GLI1 binding affinity while preserving or increasing GLI3 binding (GLI3 prefers hypoacetylated chromatin). Consequently, the ratio of GLI1‑to‑GLI3 occupancy at HH‑responsive loci drops, attenuating transcription of regenerative programs (c‑Jun, Myc, Pax7).
• SMO bypass: High‑dose SMO agonists increase GLI1 phosphorylation via PKA‑independent kinases (e.g., CK1ε), which reduces GLI1’s interaction with HDACs and promotes its nuclear retention. Simultaneously, elevated GLI1 levels drive ADAR2‑mediated A‑to‑I editing at specific GLI1 codons, altering its transactivation domain and boosting co‑activator recruitment (CBP/p300). Edited GLI1 exhibits higher transcriptional output even when chromatin is partially repressed.
• Predicted outcome: In young tissue, baseline SMO activity yields a balanced GLI1/GLI3 ratio and robust enhancer acetylation. In aged tissue, HDAC3‑mediated deacetylation tips the balance toward GLI3 repression; only suprathreshold SMO activation restores the activator‑dominant state via combined GLI1 phosphorylation and RNA editing.

Experimental Plan

1. Chromatin profiling: Perform ChIP‑seq for GLI1, GLI3, H3K27ac, and HDAC3 in young (3 mo) and aged (24 mo) mouse skeletal muscle 3 days after cardiotoxin injury. Compare occupancy and acetylation at annotated HH‑dependent enhancers (the ~6 % identified in Gli enhancer study).
2. Pharmacologic intervention: Treat aged injured mice with vehicle, low‑dose SAG (10 mg/kg), or high‑dose SAG (50 mg/kg) for 48 h. Assess GLI1 phosphorylation (phospho‑specific Western), GLI1 RNA editing levels (RNA‑seq + REDItools), and downstream target expression (qPCR for Ptch1, Gli1, c‑Jun).
3. Functional readouts: Measure regenerative outcomes—myofiber cross‑sectional area, central nucleation, and force production. Predict that only high‑dose SAG rescues the regenerative deficit in aged mice, accompanied by increased GLI1 editing and restored H3K27ac at GLI1‑bound enhancers.
4. Genetic validation: Use satellite‑cell‑specific HDAC3 knockout mice (Pax7‑CreERT2;Hdac3^fl/fl) to test whether HDAC3 loss phenocopies young chromatin states and eliminates the age‑dependent shift in GLI gradient. Conversely, overexpress a deacetylase‑dead HDAC3 mutant to see if it sustains repression despite high‑dose SAG.

Falsifiability

If aged tissue shows no increase in HDAC3 occupancy or H3K27ac loss at GLI3 sites, or if high‑dose SAG fails to elevate GLI1 phosphorylation/RNA editing and regenerative output, the hypothesis would be refuted. Conversely, confirmation of the predicted chromatin and molecular changes would support a model where aging imposes a reversible epigenetic brake on Hh signaling that can be lifted by suprathreshold SMO activation coupled to GLI1 post‑transcriptional remodeling.