Circadian NAD+‑SIRT1 gating of JNK‑AP-1 explains inflammaging and predicts timed NAD+ rescue

Core Hypothesis

Loss of rhythmic BMAL1 diminishes NAMPT‑driven NAD+ synthesis, lowering SIRT1 deacetylase activity. This permits persistent acetylation and activation of the JNK‑AP-1 module, converting transient stress signals into a chronic proinflammatory state. Restoring NAD+ rhythmicity reinstates SIRT1‑mediated JNK suppression, thereby blocking the acute‑to‑chronic inflammatory transition.

Mechanistic Rationale

• BMAL1 directly activates NAMPT transcription, linking the clock to the NAD+ salvage pathway (1).
• SIRT1 deacetylates JNK and its upstream kinases (ASK1, MKK4/7), reducing their catalytic activity in a NAD+-dependent manner.
• Circadian decline of NAD+ therefore leads to hyperacetylated, active JNK, driving sustained AP-1‑dependent transcription of SASP components (5).
• REV-ERBα, which represses BMAL1/CLOCK, is itself a SIRT1 target; reduced SIRT1 activity amplifies REV-ERBα acetylation, weakening its repression and further destabilizing clock output (3).
• Mitochondrial ROS in senescent cells activates JNK (4), but without SIRT1 restraint this signal becomes uncoupled from circadian gating, feeding a feed‑forward loop of inflammation.

Testable Predictions

1. In BMAL1‑deficient fibroblasts, JNK phosphorylation will show abolished circadian oscillation and elevated basal levels compared with wild‑type controls.
2. Pharmacological NAD+ augmentation (e.g., NMN) administered at the subjective dawn will restore SIRT1 deacetylase activity, reduce acetylated JNK, and reinstate rhythmic AP-1 target gene expression.
3. Aged mice receiving timed NMN (ZT0) will exhibit lower serum IL‑6 and TNF‑α, reduced hepatic SASP markers, and improved circadian JNK activity rhythms relative to vehicle or mistimed NMN.
4. SIRT1 knockdown will abolish the protective effect of NMN on JNK acetylation, confirming SIRT1 as the mediator.

Falsifiability

If NAD+ supplementation fails to modify JNK acetylation or AP-1 rhythmicity in BMAL1‑compromised cells, or if JNK activity remains constitutively high despite restored NAD+ levels, the hypothesis would be refuted. Conversely, observing rescued JNK rhythmicity and attenuated inflammaging would support the model.

Broader Implications

This positions the NAD+/SIRT1 axis as a molecular rheostat that translates clock output into precise control of stress‑kinase signaling. It suggests that chronotherapeutic NAD+ boosting—not merely clock gene overexpression—could serve as a potent geroprotective strategy by reinstating the firewall that gates JNK‑AP-1 driven inflammaging.