Hypothesis

Circadian-gated mTORC1 pulsatility balances tissue regeneration with senescence clearance to extend healthspan.

Mechanistic Rationale

mTORC1 acts as a nutrient‑sensing rheostat that drives anabolic growth when amino acids and IGF‑1 are abundant (active phase) and suppresses autophagy, whereas its inhibition during the rest phase allows AMPK‑mediated catabolism, autophagy and senescent cell removal (1,2,3,4,5,6). In senescent cells, mTORC1 becomes constitutively active and nutrient‑insensitive, locking them into a pro‑inflammatory state (6). Stem and progenitor niches, however, require transient mTORC1 activity for proliferation and regeneration (7). Aligning mTORC1 peaks with the organism’s feeding‑active window and troughs with the resting window therefore permits periodic civilizational growth (tissue building) followed by survival‑mode cleanup (damage clearance) without chronic suppression that exhausts regenerative capacity.

Predictions

1. Mice with inducible, liver‑specific mTORC1 oscillation synchronized to the light‑dark cycle will exhibit higher hepatocyte proliferation (Ki67+), enhanced autophagic flux (LC3‑II/I), and lower senescence markers (p16^Ink4a+, SA‑β‑gal) compared with constant mTORC1 inhibition or activation.
2. Timed rapamycin administration restricted to the rest phase (ZT12-ZT24) will produce greater median lifespan extension than the same total dose given randomly or during the active phase.
3. Loss of core clock function (e.g., Bmal1^-/-) will abolish the benefits of intermittent mTORC1 inhibition, leading to accelerated accumulation of senescent cells and reduced healthspan despite identical dosing regimens.

Experimental Design

Genetic model: Alb‑CreERT2;Raptor^fl/fl mice allow tamoxifen‑inducible hepatocyte‑specific mTORC1 knockout. Tamoxifen will be delivered in pulsed regimes: (a) active‑phase pulses (ZT0-ZT12) every 48 h, (b) rest‑phase pulses (ZT12-ZT24) every 48 h, (c) continuous pulses (every 24 h) as control, and (d) vehicle. Parallel groups receive rapamycin either constantly, only during ZT12-ZT24, or only during ZT0-ZT12. Readouts: Serum ALT/AST, albumin production, hepatocyte Ki67 index, LC3‑II/I western blot, p16^Ink4a immunostaining, senescence‑associated secretory phenotype cytokines (IL-6, MCP-1), fibrosis scoring (Sirius Red), and survival curves. Clock test: Repeat the above in Bmal1^-/- background to determine whether circadian gating is required.

Potential Pitfalls

• Compensatory mTORC2 activation could confound interpretation; measuring phospho‑AKT(S473) will monitor this.
• Liver‑specific manipulation may not reflect whole‑body aging; complementary muscle‑ or brain‑specific lines can be added.
• Ensuring true temporal precision of tamoxifen‑induced knockout requires pharmacokinetic validation (e.g., Raptor immunoblot at multiple ZTs).