Hypothesis

Continuous mTOR inhibition (e.g., chronic rapamycin) fails to repair the aging gut barrier because it simultaneously blocks the regenerative intestinal stem cell (ISC) expansion required for epithelial renewal. We hypothesize that intermittent, circadian‑aligned pulses of mTOR inhibition will improve tight junction (TJ) structure and reduce systemic inflammaging by allowing two complementary phases: (1) an inhibition window that activates autophagy‑mediated clearance of damaged TJ proteins, and (2) a refeeding window that permits mTORC1‑driven ISC proliferation and differentiation to replace lost epithelium.

Mechanistic Rationale

1. Autophagic removal of compromised TJ complexes – mTORC1 suppresses autophagy; its inhibition triggers LC3‑dependent engulfment of ubiquitinated occludin, ZO‑1, and claudin‑2, facilitating their degradation and preventing the accumulation of leak‑prone pores (3).
2. ISC dependence on mTORC1 activity – Intestinal stem cells require transient mTORC1 activation for protein synthesis, metabolic reprogramming, and clonal expansion during injury or homeostatic turnover (5). Chronic rapamycin abolishes this proliferative burst, leading to epithelial atrophy despite reduced inflammation.
3. Feedback between barrier repair and inflammaging – Restored TJ integrity limits bacterial endotoxin translocation, lowering TLR4‑NF‑κB signaling and consequently decreasing miR‑191‑5p upregulation and CB₁‑driven cytokine loops (1).
4. Circadian nutrient sensing – Feeding‑fasting cycles naturally oscillate mTORC1 activity; aligning drug pulses with the fasting phase mimics physiological autophagy peaks while preserving the feeding‑phase mTORC1 surge needed for ISC activity.

Testable Predictions

• Experimental design: Use 24‑month-old C57BL/6 mice randomized to four groups (n=10 per group): (i) vehicle control, (ii) continuous rapamycin (2 mg/kg/day, i.p.), (iii) intermittent rapamycin (2 mg/kg/day, 5 days on/2 days off, synchronized with the dark‑phase fasting window), (iv) intermittent rapamycin plus autophagy inhibitor (chloroquine) to confirm autophagy dependence.
• Readouts (after 8 weeks):
  • TJ protein levels: immunoblot and immunofluorescence for ZO‑1, occludin, claudin‑2 in distal colon (2).
  • Barrier function: FITC‑dextran (4 kDa) serum appearance after oral gavage.
  • Systemic inflammation: serum IL‑6, TNF‑α, LPS‑binding protein (ELISA).
  • ISC activity: Lgr5‑GFP+ crypt counts, Ki67 proliferation index, and organoid formation efficiency.
  • Autophagy flux: LC3‑II/I ratio and p62 accumulation with/without chloroquine treatment.
• Expected outcomes:
  • Intermittent rapamycin will show ↑ ZO‑1/occludin, ↓ claudin‑2, reduced FITC‑dextran permeability, and lower serum cytokines vs. both control and continuous rapamycin groups.
  • These improvements will correlate with enhanced LC3‑II conversion during inhibition days and increased Lgr5+ Ki67+ cells during refeeding days.
  • Co‑administration of chloroquine will abolish the barrier benefits, confirming autophagy dependence.
  • Continuous rapamycin will maintain low mTORC1 signaling but fail to increase ISC markers and will not improve TJ integrity, potentially worsening atrophy.

Falsifiability

If intermittent rapamycin does not produce superior TJ repair or anti‑inflammatory effects compared to continuous rapamycin or control, or if autophagy inhibition does not negate the benefits, the hypothesis is falsified. Likewise, if ISC proliferation remains suppressed despite the refeeding pulses, the mechanistic link between mTORC1‑dependent regeneration and barrier restoration would be refuted.

Implications

Confirming this hypothesis would shift the longevity‑promoting use of mTOR inhibitors from chronic suppression to strategic, timing‑based regimens that harness the body’s innate repair cycles, offering a clinically translatable strategy to target gut‑derived inflammaging in aging.