Senescent Keratocytes as Stromal Homeostasis Rheostats in Healthy Aging Cornea

Hypothesis

In the naturally aging cornea, low‑level senescence of keratocytes functions not as a passive by‑product of damage but as an active rheostat that tempers stromal fibroblast activation and preserves extracellular‑matrix (ECM) order. Senescent keratocytes secrete a constrained SASP enriched in TGF‑β1, PGE2 and specific matricellular proteins (e.g., decorin, lumican) that engage SMAD2/3 and EP4 receptors on neighboring keratocytes, promoting a quiescent, anti‑fibrotic phenotype. When this senescent brake is removed, keratocytes exhibit hyper‑proliferation, dysregulated collagen fibrillogenesis, and a shift toward a pro‑fibrotic, haze‑prone state.

Mechanistic Basis

Recent data show that senescent corneal cells in injury or transplant models release CXCL1/8, MMP2 and collagen fragments that drive inflammation and fibrosis [3]. However, the SASP is context‑dependent; in homeostatic aging, proteostatic stress triggers a p16^INK4a^‑dependent senescent program that selectively up‑regulates TGF‑β1 secretion while keeping NF‑κB‑driven chemokines at basal levels. TGF‑β1 signaling through TβRII induces SMAD2/3‑mediated transcription of ECM‑organizing genes (lumican, keratocan) and suppresses α‑SMA expression, thereby maintaining stromal transparency. PGE2, acting via EP4 receptors, elevates cAMP and further inhibits keratocyte proliferation and myofibroblast differentiation. This creates a paracrine milieu that opposes the run‑away fibroblast activation seen in keratoconus or post‑injury fibrosis.

Testable Predictions

1. Ablation of senescent keratocytes in healthy, aged mouse corneas will increase Ki67^+^ keratocyte density within 2 weeks.
2. Loss of senescence will correlate with disrupted collagen fibril spacing (measured by second‑harmonic generation) and elevated light scattering (higher haze score).
3. Treated eyes will show up‑regulation of α‑SMA and collagen‑I, and down‑regulation of lumican/decorin mRNA relative to controls.
4. Exogenous low‑dose TGF‑β1 or PGE2 supplementation will rescue the proliferative and fibrotic phenotypes after senolytic treatment.

Experimental Approach

• Use p16‑3MR mice to label and selectively eliminate senescent keratocytes with ganciclovir in 12‑month‑old animals (natural aging) vs. young controls.
• Quantify keratocyte proliferation (Ki67 immunofluorescence), apoptosis (cleaved caspase‑3), and SASP composition (ELISA for TGF‑β1, PGE2, CXCL8).
• Assess stromal architecture via confocal SHG imaging and corneal haze using a slit‑lamp‑based scoring system.
• Measure corneal epithelial barrier function (fluorescein staining) and transcriptomic shifts (RNA‑seq) for fibrosis‑related pathways.
• Conduct rescue experiments with topical TGF‑β1 (0.5 ng/µL) or PGE2 (1 µL of 10 µM) administered post‑senolytic.

Potential Implications

If validated, this hypothesis reframes corneal senescence as a dynamic, protective checkpoint that preserves stromal homeostasis during aging. It cautions against indiscriminate senolytic use in the healthy aging eye and suggests that therapeutic strategies should aim to modulate, rather than abolish, the senescent SASP—perhaps by biasing it toward TGF‑β1/PGE2‑rich profiles. Such an approach could maintain transparency while still mitigating pathological senescence in disease settings.