Hypothesis

Chronic low‑grade inflammation in the aging limbal niche up‑regulates DNA methyltransferases and specific microRNAs, leading to promoter hyper‑methylation and post‑transcriptional repression of PAX6 in limbal epithelial progenitor cells (LEPCs). This epigenetic silencing reduces PAX6 protein levels, weakening the PAX6‑WNK2 signaling axis, shifting LEPCs toward a stress‑prone, quasi‑quiescent state, and impairing corneal epithelial regeneration. Topical CSB‑001 (HGF/c‑MET agonist) can counteract this decline by activating downstream ERK/MAPK signaling, which phosphorylates and stabilizes PAX6 protein, thereby restoring transcriptional output and functional stem cell activity.

Mechanistic Rationale

• Inflammatory trigger: Aging limbal stroma shows elevated IL‑6 and TNF‑α (see [5]). IL‑6 signaling via JAK/STAT3 induces DNMT3B expression, promoting CpG methylation at the PAX6 promoter.
• Epigenetic layer: Bisulfite sequencing of young vs. aged human limbal epithelium predicts increased methylation at PAX6 promoter CpG islands, correlating with reduced PAX6 mRNA.
• Post‑transcriptional layer: miR‑204, known to target PAX6 3′‑UTR, is up‑regulated in aged limbal tissue (consistent with stress‑marker profiles in [3]), further suppressing PAX6 translation.
• Functional consequence: Reduced PAX6 diminishes transcription of WNK2 and downstream effectors (e.g., ERK1/2), weakening the PAX6–WNK2 axis that maintains proliferation and migration ([4]). LEPCs accumulate stress markers (Tmem176b, Apoe, Lcn2) and exhibit defective differentiation (low Krt12, Klf4, Zfp750).
• Rescue by HGF: HGF/c‑MET activation stimulates ERK/MAPK, which can phosphorylate PAX6 on serine residues, protecting it from proteasomal degradation and enhancing its transcriptional activity independent of mRNA levels.

Testable Predictions

1. Methylation & miRNA: Limbal tissue from donors >60 y will show significantly higher PAX6 promoter methylation (≥2‑fold increase) and elevated miR‑204 levels compared with donors <30 y.
2. IL‑6 blockade: Ex‑vivo aged limbal explants treated with an IL‑6 neutralizing antibody will exhibit reduced DNMT3B activity, decreased promoter methylation, and restored PAX6 mRNA/protein.
3. DNMT inhibition: Treatment with 5‑aza‑2′‑deoxycytidine will demethylate the PAX6 promoter and increase PAX6 expression, rescuing colony‑forming efficiency and differentiation marker expression.
4. HGF rescue: Adding recombinant HGF or CSB‑001 to aged limbal cultures will increase phosphorylated ERK, stabilize PAX6 protein (detected by cycloheximide chase), and improve wound‑closure rates and Krt12/Klf4 expression, even when promoter methylation remains high.
5. In vivo validation: Aged mouse models receiving topical CSB‑001 will display increased limbal PAX6 protein (immunostaining), reduced apoptosis (cleaved caspase‑3), and improved corneal transparency versus vehicle controls.

Falsifiability

If any of the following observations are made, the hypothesis is weakened or refuted:

• No age‑related difference in PAX6 promoter methylation or miR‑204 levels.
• IL‑6 blockade fails to alter PAX6 expression or functional outcomes in aged explants.
• Demethylating agents or HGF treatment do not rescue PAX6 protein levels or regenerative capacity despite pathway activation.
• CSB‑001 treatment does not improve PAX6 stability or corneal phenotype in aged animals.

By linking inflammatory epigenetics, post‑transcriptional control, and protein‑stabilizing rescue mechanisms, this hypothesis provides a concrete, experimentally tractable framework to explain age‑related limbal stem cell decline and to evaluate therapeutic strategies beyond simple PAX6 gene augmentation.