Mitochondrial DNA Stress Reactivates Hedgehog‑GLI Signaling in Aged Stromal Fibroblasts via ATF4‑Mediated Chromatin Remodeling

The hypothesis posits that somatic mtDNA mutations activate the integrated stress response (ISR) transcription factor ATF4, which then remodels chromatin at enhancers of GLI‑target genes, leading to aberrant Hedgehog pathway activation in aged stromal fibroblasts. This reactivation drives a maladaptive repair program characterized by excessive extracellular matrix deposition and impaired stem‑cell niche function, contributing to age‑related tissue fibrosis.

Key mechanistic steps:

1. mtDNA point mutations increase mitochondrial unfolded protein response (UPRmt) and ISR signaling, elevating ATF4 protein levels (as shown in POLG mutator mice) [https://www.aging-us.com/article/100109/text].
2. ATF4 binds to specific DNA motifs upstream of GLI1 and GLI2 promoters, recruiting histone acetyltransferases (e.g., p300) that increase H3K27ac marks, thereby opening chromatin at these loci.
3. Open chromatin permits GLI transcription factors to access their target enhancers even in the absence of canonical Hedgehog ligand, resulting in ligand‑independent GLI activation.
4. Persistent GLI signaling upregulates profibrotic genes (COL1A1, ACTA2) and suppresses adipogenic differentiation, shifting fibroblast phenotype toward a myofibroblast‑like state.
5. Secreted mitokines such as GDF15 amplify this effect systemically, propagating the fibrotic response to distant tissues [https://www.frontiersin.org/journals/cell-and-developmental-biology/articles/10.3389/fcell.2025.1652353/full].

Testable predictions:

• Fibroblasts isolated from POLG mutator mice will show higher basal GLI1/GLI2 transcriptional activity and increased H3K27ac at GLI enhancers compared with wild‑type controls; ATF4 knockdown (siRNA) will normalize these marks and reduce GLI target expression.
• Pharmacological inhibition of ISR (e.g., with ISRIB) will attenuate GLI activation and reduce collagen production in mutator fibroblasts without affecting mitochondrial respiration.
• Chromatin immunoprecipitation followed by sequencing (ChIP‑seq) for ATF4 in aged human dermal fibroblasts will reveal enrichment at GLI1/GLI2 regulatory regions; CRISPR‑mediated deletion of these ATF4 binding sites will diminish age‑associated GLI signaling.
• In vivo, topical application of a GLI antagonist (GANT61) to aged mutator mouse skin will decrease fibrosis markers and improve wound‑healing kinetics, whereas ISR activation in young wild‑type mice (via low‑dose tunicamycin) will prematurely induce GLI‑dependent fibrotic phenotypes.

Falsification criteria: If ATF4 loss or ISR inhibition fails to alter GLI enhancer acetylation or GLI‑target expression in mtDNA‑mutant fibroblasts, or if GLI blockade does not ameliorate fibrosis in aged mutator models, the proposed mechanistic link between mtDNA stress and Hedgehog‑GLI reactivation would be refuted, suggesting that mtDNA‑driven aging operates through alternative pathways.

This hypothesis extends the current view of mtDNA dysfunction as a passive accumulator of damage by positioning it as an active regulator of developmental signaling networks, thereby offering a novel target for interventions aimed at reversing age‑related fibrotic degeneration.