Hypothesis: PAX6 expression determines the position of the optimal mTOR activity zone in limbal stem cells (LSCs). High PAX6 raises the threshold for mTOR‑driven exhaustion by increasing expression of the endogenous mTORC1 inhibitor DEPTOR, thereby widening the therapeutic window. Conversely, PAX6 loss narrows this window, forcing aged LSCs to rely on higher mTOR activity to escape quiescence while becoming more sensitive to mTOR‑induced stress.

Mechanistic reasoning: PAX6 is a transcription factor that directly binds the promoter of DEPTOR (DEP domain containing mTOR‑interacting protein), enhancing its transcription. DEPTOR binds mTORC1 and reduces its kinase activity, buffering fluctuations in upstream signals such as amino acids or growth factors. In young limbal epithelium, robust PAX6 sustains DEPTOR levels, keeping mTORC1 activity within a range that supports proliferation without triggering γH2AX‑mediated senescence. With age, declining PAX6 reduces DEPTOR, lifting this buffer; mTORC1 becomes hyper‑responsive to stimulatory cues, so even modest activation pushes cells toward exhaustion, while inhibition quickly drops activity below the level needed for renewal.

Testable predictions:

1. Forced PAX6 overexpression in LSCs from aged donors will increase DEPTOR protein, shift the rapamycin concentration that yields maximal colony‑forming efficiency to lower doses, and reduce γH2AX and p16^INK4a^ levels at doses that are inhibitory in controls.
2. CRISPRi‑mediated PAX6 knockdown in young LSCs will decrease DEPTOR, narrow the effective rapamycin range (requiring higher concentrations to achieve the same proliferation boost) and increase senescence markers at lower rapamycin doses.
3. Chromatin immunoprecipitation followed by qPCR will show PAX6 occupancy at the DEPTOR promoter in limbal basal cells, and this occupancy will correlate with DEPTOR mRNA across young vs. aged donor samples.

Experimental design: Obtain limbal epithelial explants from donors stratified by age (≤30 yr vs. ≥60 yr). Transduce with lentiviral vectors for PAX6 overexpression or CRISPRi guide RNAs, alongside appropriate controls. Treat cultures with a rapamycin gradient (0–20 nM) or substance P/NK1R agonist to modulate mTORC1 activity. After 72 h, assess:

• Proliferation: Ki67+ fraction and EdU incorporation.
• Stemness: colony‑forming efficiency and holoclone formation.
• Differentiation: K3/K12 expression via immunostaining.
• Stress/senescence: γH2AX foci, p16^INK4a^ and SA‑β‑gal activity.
• DEPTOR levels: western blot and immunofluorescence.

Expected outcome if hypothesis true: PAX6‑high cells will display a left‑shifted dose‑response curve, achieving peak proliferation at lower rapamycin concentrations while maintaining low senescence; PAX6‑low cells will show a right‑shifted curve and heightened senescence even at sub‑inhibitory doses. If PAX6 manipulation does not alter the mTOR optimal zone or DEPTOR levels, the hypothesis is falsified.

This framework links a transcription‑factor‑driven rheostat to the "civilization‑versus‑survival" mTOR dial, offering a concrete, falsifiable route to explain why aged limbal niches fail to regenerate despite apparent stem cell presence.