Circadian‑Timed mTOR Pulsatility Determines Whether Senolytic‑Fasting Synergy Clears Senescent Cells or Triggers Regenerative Risk

Hypothesis

Timed suppression of mTORC1 during the active phase of the circadian cycle creates a metabolic window in which healthy cells boost NAD+-dependent SIRT1 activity, priming senescent cells for senolytic-induced apoptosis, whereas mistimed mTOR inhibition blunts this priming and may allow surviving senescent cells to provoke proliferative signaling that increases tumorigenesis risk.

Mechanistic Rationale

• mTORC1 activity suppresses AMPK, lowering NAD+ synthesis and reducing SIRT1 deacetylase function.
• During fasting, mTORC1 drops, AMPK rises, NAD+ levels climb, and SIRT1 activates FOXO and PGC‑1α pathways that enhance expression of pro‑apoptotic BCL‑2 family members (e.g., BIM) and downregulate anti‑apoptotic BCL‑XL in senescent cells.
• Senolytic drugs such as dasatinib‑quercetin rely on BCL‑2 family inhibition to trigger apoptosis; thus, cells with heightened SIRT1‑FOXO signaling are more susceptible.
• The circadian clock gates NAD+ salvage enzymes (NAMPT) and mTORC1 sensitivity, producing peaks of NAD+ availability at specific times (early active phase). Aligning TRF‑induced mTOR trough with this NAD+ peak maximizes SIRT1‑mediated priming.
• If mTOR suppression occurs outside this window, NAD+ remains low, SIRT1 stays inactive, senescent cells retain BCL‑XL expression, and senolytics are less effective; residual SASP may stimulate neighboring stem cells, raising oncogenic transformation odds.

Experimental Design

Model: 20‑month‑old C57BL/6 mice (n=10 per group). Groups:

1. Ad libitum feeding + vehicle (control).
2. Ad libitum + intermittent dasatinib‑quercetin (DQ) 5 mg/kg dasatinib + 50 mg/kg quercetin, weekly.
3. Time‑restricted feeding (TRF) 8‑hour window aligned to early active phase (ZT0‑ZT8) + vehicle.
4. TRF (early window) + DQ given at ZT2 (mid‑fast).
5. TRF (early window) + DQ given at ZT14 (early rest phase, opposite circadian timing).
6. TRF with inverted window (ZT10‑ZT18) + DQ at ZT12 (to test window dependence). Duration: 3 months. Readouts:

• Senescent cell burden (p16^Ink4a^+ immunostaining) in liver, kidney, adipose.
• SASP cytokine panel (IL‑6, CXCL1).
• Hepatic NAD+ levels, SIRT1 activity, FOXO1 acetylation.
• Stem cell proliferation (Ki67^+ Lgr5^+ crypt cells).
• Tumor incidence (histopathology).
• Median and maximal lifespan.

Predictions & Falsifiability

• Prediction 1: Group 4 (TRF early + DQ at ZT2) will show the greatest reduction in senescent cells (>50% vs control) and lowest SASP, accompanied by increased NAD+/SIRT1 activity and enhanced stem cell function without rise in tumorigenesis.
• Prediction 2: Group 5 (TRF early + DQ at ZT14) will have senescent cell clearance comparable to DQ alone (no added benefit) and may exhibit elevated IL‑6 and increased tumor incidence relative to Group 4.
• Prediction 3: Group 6 (inverted TRF window) will fail to improve clearance despite DQ timing, confirming that the metabolic state, not just drug timing, drives synergy. Falsification: If groups 4 and 5 show no difference in senescent cell burden or tumor rates, or if NAD+/SIRT1 readouts do not correlate with clearance outcomes, the hypothesis is refuted.

Potential Implications

Aligning mTOR-modulating interventions with circadian NAD+ peaks could refine senolytic protocols, maximizing healthspan while minimizing cancer risk. It also suggests that circadian-disrupted lifestyles (shift work, irregular eating) may blunt the benefits of fasting‑senolytic combos, offering a mechanistic explanation for variable human responses to time‑restricted eating trials.