Hypothesis

Circadian disruption drives hepatic aging by uncoupling the NAD+-SIRT1-FXR deacetylation cycle, leading to loss of rhythmic FXR transcriptional activity and epigenetic instability.

Mechanistic Rationale

• The circadian clock regulates NAD+ biosynthesis through NAMPT, producing daily NAD+ peaks that activate SIRT1.
• SIRT1 deacetylates FXR and its coregulators (e.g., SRC-1), enhancing FXR DNA binding and transcriptional repression of inflammatory genes.
• With age, microbiome‑derived secondary bile acid decline reduces FXR ligand availability, while circadian NAD+ oscillations flatten, diminishing SIRT1 activity.
• Hypo‑acetylated FXR fails to recruit SHP and induce Fgf15, causing chronic ERK/MAPK activation and hepatic fibrosis.
• Concurrently, loss of rhythmic FXR signaling disrupts circadian epigenome maintenance, allowing drift of cytosine methylation at metabolic promoters.

Testable Predictions

1. Timed NAD+ restoration – Oral NMN administered at zeitgeber time 6 (ZT6) in aged mice will restore diurnal NAD+ peaks, increase SIRT1‑FXR deacetylation, normalize bile acid FXR target expression (Shp, Fgf15), and reduce liver fibrosis scores compared with vehicle or mistimed dosing.
2. SIRT1 dependence – Liver‑specific SIRT1 knockout will abolish the beneficial effects of timed NMN on FXR target rhythms and hepatic aging markers, confirming SIRT1 as the required mediator.
3. Epigenetic read‑out – Whole‑genome bisulfite sequencing of livers from aged mice receiving circadian‑aligned NMN will show reduced age‑associated methylation variance at FXR‑regulated loci relative to controls.
4. Microbiome‑FXR feedback – Fecal transplantation from young donors combined with timed NMN will synergistically amplify secondary bile acid production, further enhancing circadian FXR signaling beyond either intervention alone.

Falsifiability

If timed NMN fails to restore FXR transcriptional rhythms or improve hepatic histology in aged wild‑type mice, or if SIRT1 loss does not block the effect, the hypothesis would be refuted.

Potential Impact

Demonstrating that a simple chrononutritional intervention can re‑synchronize the NAD+-SIRT1-FXR axis would position circadian‑aligned NAD+ boosting as a potent, low‑risk geroprotective strategy for liver aging.