Hypothesis

Restoring NAD+ levels at the circadian trough of NAD+ biosynthesis will reinstate BMAL1‑driven KLF4 expression oscillations in aged macrophages, thereby breaking the feed‑forward loop where senescent cells drive inflammaging and circadian disruption.

Rationale

• Circadian misalignment directly shortens lifespan and expands senescent immune cell pools by desynchronizing repair pathways [1].
• Core clock proteins BMAL1 and CLOCK regulate immune function; loss of BMAL1 accelerates age‑related phenotypes [2].
• In aged macrophages, despite intact BMAL1, KLF4 abundance falls and its circadian oscillation is lost, uncoupling immune responses from the clock [1].
• NAD+ is a critical cofactor for SIRT1, which deacetylates BMAL1 and enhances its transcriptional activity; NAD+ declines with age and exhibits a circadian rhythm [3].
• Senolytic clearance reduces pro‑inflammatory cytokines (TNF‑α, IL‑6) and alleviates inflammaging [4]; however, senolytics alone do not restore circadian KLF4 rhythm.

We propose that NAD+ supplementation timed to the endogenous NAD+ nadir (subjective night) will boost SIRT1 activity, increase BMAL1 transcriptional potency, and restore KLF4 oscillation, thereby normalizing macrophage cytokine output and reducing senocyte‑induced inflammation.

Predictions (Falsifiable)

1. In aged mice, timed NAD+ precursor (e.g., NR) administration will increase nuclear SIRT1 activity and BMAL1 DNA‑binding at circadian time points where NAD+ is normally low.
2. This intervention will restore ~24‑hour oscillation of KLF4 protein and its target genes (e.g., MerTK, Cd36) in peritoneal macrophages, which is absent in vehicle‑treated aged controls.
3. Restored KLF4 rhythm will correlate with decreased secretion of IL‑1β, TNF‑α, and IL‑6 upon LPS challenge, mirroring the cytokine profile of young macrophages.
4. Consequently, senolytic treatment combined with timed NAD+ will produce a synergistic reduction in tissue‑resident senocyte burden (p16^INK4a^+ cells) compared with either monotherapy.
5. Disrupting SIRT1 (using EX‑527) or BMAL1 (macrophage‑specific Bmal1 KO) will abolish the NAD+‑induced KLF4 rescue, confirming mechanistic dependence.

Experimental Approach

Animal model: C57BL/6 mice, 20‑month old (aged) vs. 3‑month old (young).

Interventions (2‑week course, n=10 per group):

• Vehicle control
• NAD+ precursor (nicotinamide riboside, 400 mg/kg/day) administered at ZT0 (lights‑on) vs. ZT12 (lights‑off) to test timing dependence
• Senolytic (fisetin, 100 mg/kg, 3×/week) alone
• Combination of timed NAD+ + senolytic
• Pharmacologic SIRT1 inhibition (EX‑527) added to timed NAD+ group to test necessity

Readouts:

• Liver and spleen NAD+ levels (LC‑MS) at ZT0, ZT6, ZT12, ZT18 to confirm rhythm
• Macrophage isolation (CD11b^+ F4/80^+) followed by western blot for SIRT1 activity (acetyl‑p53), BMAL1 binding (ChIP‑qPCR at Klf4 promoter), and KLF4 protein over 24 h
• Cytokine secretion (ELISA) from ex‑vivo LPS‑stimulated macrophages
• Flow cytometry for p16^INK4a^+ senocytes in adipose tissue and liver
• In vivo imaging of inflammaging (FDG‑PET) and survival monitoring (up to 6 months)

Statistical analysis: Two‑way ANOVA (treatment × time) with post‑hoc Tukey; survival curves compared via log‑rank test.

Potential Outcomes and Interpretation

• If timed NAD+ restores KLF4 oscillation and reduces inflammaging, it supports the hypothesis that metabolic‑clock coupling is a lever to re‑synchronize immune circadian control, positioning NAD+ replenishment as a chronotherapeutic geroprotector.
• If no KLF4 rhythm is rescued despite NAD+ increase, the hypothesis would be falsified, suggesting that age‑related loss of KLF4 oscillation requires additional epigenetic or transcriptional regulators beyond SIRT1‑BMAL1.
• If senolytic + timed NAD+ shows synergy only in reducing senocyte burden but not cytokine rhythms, it would indicate that inflammaging and senescence are partially separable outputs of the circadian‑immune axis.

Overall, this experiment tests a concrete, falsifiable mechanistic link between metabolic state, circadian transcription factors, and macrophage‑mediated inflammaging, offering a clear path toward combined chrono‑senolytic interventions for aging.