Aging-induced limbal stem cell (LSC) dysfunction correlates with reduced PAX6 and dysregulation of the lncRNA SDHAP2_miR-17-5p/miR-20b-5p axis Age-related alterations in this lncRNA/miRNA network, but the mechanistic link to cellular senescence remains unexplored. Here, I hypothesize that senescent stromal cells—marked by p16INK4a—drive lncRNA SDHAP2 downregulation, which releases miR-17-5p and miR-20b-5p to directly target and suppress PAX6 mRNA in limbal epithelial cells, precipitating LSC exhaustion and aberrant differentiation.

Core Mechanistic Proposal

PAX6 is strictly epithelial-specific and essential for maintaining LSC fate PAX6 expression in the limbal niche; its loss triggers a switch to epidermal-like keratins Loss of PAX6 fundamentally disrupts LSC fate commitment. While the lncRNA SDHAP2_miR-17-5p/miR-20b-5p axis is implicated in aging, its direct regulation of PAX6 isn’t established Age-related alterations. I propose that senescent stromal cells in the aged niche create a permissive environment for this axis:

• Step 1: p16INK4a+ senescent stromal cells secrete SASP factors (e.g., IL-6, TGF-β), which downregulate lncRNA SDHAP2 in adjacent epithelial cells via epigenetic silencing (e.g., promoter methylation).
• Step 2: Reduced SDHAP2 fails to sponge miR-17-5p and miR-20b-5p, increasing their free cytoplasmic levels.
• Step 3: These miRNAs directly target the 3’ UTR of PAX6 mRNA, suppressing its translation and destabilizing the transcript. This is novel—current research hasn’t linked these miRNAs to PAX6 directly.
• Step 4: PAX6 reduction leads to quiescent stem-like cell accumulation, reduced proliferation PAX6 deficiency increases quiescent stem-like cells, and a fate switch toward K1/K10 expression, exhausting the LSC pool.

Testable Predictions

This model is falsifiable through targeted experiments:

1. Co-localization Studies: Use immunofluorescence to detect p16INK4a+ stromal cells adjacent to epithelial cells with low SDHAP2 and PAX6 in aged human limbal biopsies. Prediction: Inverse correlation between p16INK4a and SDHAP2/PAX6.
2. Functional Assays: In vitro, treat young limbal epithelial cells with conditioned media from senescent stromal cells (induced by irradiation). Measure SDHAP2, miR-17-5p/miR-20b-5p, and PAX6 levels via qPCR/Western blot. Prediction: Senescent media will suppress SDHAP2 and PAX6 while elevating miRNAs.
3. Direct miRNA Targeting: Perform luciferase reporter assays with PAX6 3’ UTR fragments containing predicted miR-17-5p/miR-20b-5p binding sites. Prediction: Mutation of binding sites will rescue PAX6 expression under senescent conditions.
4. In Vivo Intervention: In aged mice, use senolytics (e.g., dasatinib/quercetin) to clear p16INK4a+ cells. Prediction: This will restore SDHAP2, reduce miRNAs, increase PAX6, and improve corneal regeneration capacity.

Implications and Extensions

If validated, this links the niche’s aging stroma directly to epithelial fate via a lncRNA-miRNA-mRNA axis, challenging the view that LSC exhaustion is primarily cell-autonomous PAX6 deficiency increases quiescent stem-like cells. It also suggests that targeting senescent cells or miRNAs could reactivate PAX6, offering a pathway for therapy—critical given no FDA-approved treatments for LSCD no FDA-approved therapies exist. Future work should explore if other PAX isoforms (e.g., PAX2) are co-regulated or compensatory presence and functional roles of other PAX isoforms, and whether ABCB5+ LSC subpopulations ABCB5+ limbal epithelial cells are differentially affected by this axis.