Autophagy-mediated selective degradation of Smoothened limits Hedgehog-driven regeneration in aged fibroblasts

Hypothesis

In aged fibroblasts, persistent autophagy activation selectively degrades Smoothened (SMO) to preserve amino acid pools for essential metabolism, thereby attenuating GLI-mediated transcription and impairing tissue regeneration.

Mechanistic Basis

Autophagy operates as a rationing system that prioritizes cargo based on metabolic need Autophagy receptors like p62/SQSTM1 and NUFIP1 earmark specific cargo. Under chronic nutrient stress, mTORC1 remains active while AMPK drives autophagy, recycling mitochondria, ribosomes and lipid droplets to sustain ATP Autophagy accounts for approximately one-third of mitochondrial protein turnover. In senescent cells, this flux fuels lysosomal amino acid pools that maintain mTORC1 activity during starvation Senescent cells exhibit persistent mTORC1 signaling with elevated basal autophagy. Recent work shows that autophagy inhibition stabilizes primary cilia, indicating that the pathway actively removes ciliary components Pharmacological inhibition of autophagy with bafilomycin A1 or chloroquine stabilizes primary cilia. Since SMO must reside within the cilium to transduce Hedgehog signals, we propose that autophagy receptors recognize SMO-associated ubiquitin ligatures or ciliary sheath proteins as expendable cargo during siege‑like conditions. Selective removal of SMO would diminish GLI activator formation, shifting the balance toward SUFU‑mediated repression and reducing transcription of regenerative targets.

Testable Predictions

1. In aged human fibroblasts subjected to serum starvation, SMO protein levels will decline faster than total cilia length, and this loss will be rescued by autophagy inhibitors (bafilomycin A1) or genetic knockdown of ATG5 or ATG7.

2. Proximity ligation assays will detect increased ubiquitin‑SMO conjugates in lysosomes of aged cells, colocalizing with LAMP2 and p62, whereas young cells show minimal conjugates.

3. Restoring SMO expression via a cilia‑targeted, autophagy‑resistant mutant (e.g., lysine‑arginine substitutions at predicted ubiquitination sites) will rescue GLI1 transcriptional activity and improve collagen‑gel contraction in aged fibroblasts, even under nutrient‑limited conditions.

4. In vivo, topical delivery of chloroquine to aged murine skin will increase SMO‑cilia occupancy and enhance epidermal thickness after wounding, whereas autophagy activation (rapamycin) will blunt this effect.

Implications

If validated, this hypothesis reframes autophagy activators not as universal longevity boosters but as context‑dependent modulators that may inadvertently sabotage regenerative signaling in stressed, aged niches. Therapeutic strategies would need to balance bulk cytoprotection with preservation of specific signaling hubs, possibly by inhibiting selective autophagy receptors (e.g., NUFIP1) rather than the core autophagic machinery.