Hypothesis Chronic rapamycin treatment in aged mice maintains epithelial tight‑junction (TJ) protein levels and low basal permeability but impairs the regenerative response to epithelial injury, resulting in exaggerated inflammaging after barrier challenge.

Rationale Aging reduces ZO‑1 and occludin expression, increasing basal leak and driving systemic NF‑κB activation via luminal LPS and muramyl dipeptide (source; source; source). mTORC1 inhibition by rapamycin preserves baseline TJ integrity in non‑injured epithelium (source), yet genetic mTOR loss in intestinal epithelium causes normal homeostasis but catastrophic repair after irradiation or DSS, with depleted Olfm4+ stem cells (source). ZO‑1 loss similarly uncouples basal barrier from Wnt/β‑catenin‑driven restitution (source). We propose that rapamycin mimics nutrient scarcity, tightening homeostatic TJ assembly while suppressing the anabolic burst needed for rapid TJ restitution and stem‑cell proliferation during repair. Consequently, the barrier appears 'young' at rest but behaves like a damaged, low‑capacity system when injured.

Predictions

1. In aged mice (≥20 mo), rapamycin (8 mg kg⁻¹ d⁻¹, 4 weeks) will not further decrease ZO‑1 or occludin mRNA/protein compared with vehicle, and basal TEER will be unchanged or slightly higher.
2. After 5 % DSS exposure, rapamycin‑treated aged mice will show:
   • larger increase in FITC‑dextran flux (higher permeability) than vehicle controls,
   • reduced Olfm4+ and Lgr5+ crypt cells at 48 h post‑injury,
   • diminished phospho‑S6 (mTORC1 activity) in regenerating crypts,
   • elevated serum LPS, TNF‑α, and IL‑6 relative to vehicle‑injured aged mice.
3. Young mice (3 mo) receiving rapamycin will retain baseline TJ preservation but will not exhibit the repair deficit, indicating an age‑dependent interaction.

Experimental Design

• Groups: young vehicle, young rapamycin, aged vehicle, aged rapamycin (n=10 per group).
• Rapamycin administered via chow for 4 weeks before any manipulation.
• Baseline measurements: immunofluorescence for ZO‑1/occludin, TEER on Ussing chambers, qPCR for TJ genes.
• Injury: 5 % DSS in drinking water for 7 days, followed by 48 h recovery.
• Outcomes: FITC‑dextran (4 kDa) serum quantification, crypt isolation and flow cytometry for Olfm4+/Lgr5+, Western blot for phospho‑S6 and NF‑κB p65, serum cytokine ELISA.
• Statistical analysis: two‑way ANOVA (age × treatment) with post‑hoc Tukey; significance set at P<0.05.

Potential Outcomes If rapamycin impairs injury‑induced TJ restitution, we will observe a significant age × treatment interaction for permeability and stem‑cell markers, confirming the hypothesis. If rapamycin improves both baseline and repair, the interaction will be absent, falsifying the claim that mTOR inhibition merely impersonates a harder life.

Implications This work would distinguish prophylactic TJ preservation from genuine barrier rejuvenation, guiding clinical use of rapamycin analogues in older adults: chronic dosing may protect against leak‑induced inflammaging but could compromise mucosal healing after infection, chemotherapy, or colitis.