Hypothesis

Age‑related decline in cellular NAD+ weakens the SIRT1‑mediated bridge between the core circadian clock and endothelial nitric oxide synthase (eNOS), causing a dual loss of temporal NO production and clock gene rhythmicity that drives arterial stiffening.

Mechanistic Basis

The circadian clock regulates eNOS activity through rhythmic phosphorylation and transcriptional control [1, 2]. Conversely, NO feeds back to enhance clock gene expression via cAMP‑CREB and E‑box pathways [1]. SIRT1, a NAD+-dependent deacetylase, sits at the intersection of these loops: it deacetylates BMAL1 and PER2 to stabilize clock function [3] and directly deacetylates eNOS at Lys496, increasing its activity and NO output [4]. With age, NAD+ levels fall, SIRT1 activity drops, and the clock‑eNOS positive feedback frays. This leads to daytime eNOS hypophosphorylation, reduced NO bioavailability, and loss of circadian BP variation [2, 5].

Testable Predictions

1. In aged mice, NAD+ supplementation (e.g., nicotinamide riboside) will restore SIRT1-dependent deacetylation of BMAL1, PER2, and eNOS, thereby reinvolving circadian NO peaks and improving endothelium‑dependent relaxation.
2. Endothelial‑specific SIRT1 knockout will abolish the beneficial effects of NAD+ boosting on clock gene rhythmicity and NO production, despite restored NAD+ levels.
3. Pharmacological inhibition of SIRT1 in young mice will mimic the aged phenotype: blunted NO circadian rhythm, elevated arterial stiffness, and loss of BP dipping, even when NAD+ levels are normal.

Experimental Approach

• Animal groups: young (3 mo) and aged (24 mo) C57BL/6 mice receiving (i) vehicle, (ii) NAD+ precursor (NR 400 mg/kg/day), (iii) NAD+ precursor + endothelial SIRT1 siRNA, (iv) young mice treated with SIRT1 inhibitor EX527. Duration 8 weeks.
• Readouts: (a) Liver and aorta tissue collected every 4 h over 24 h for qPCR of Bmal1, Per2, Cry1; (b) Western blot for acetylated BMAL1, PER2, and eNOS (Lys496); (c) NO metabolites (nitrite/nitrate) in plasma and aortic supernatants; (d) ex vivo aortic ring acetylcholine‑induced relaxation; (e) pulse wave velocity for arterial stiffness; (f) telemetry BP monitoring for circadian amplitude.
• Statistical analysis: Two‑way ANOVA (age × treatment) with post‑hoc Tukey; rhythmicity assessed by JTK_CYCLE.

Falsifiability

If NAD+ supplementation fails to improve either clock gene oscillations or eNOS activity/NO levels in aged animals, or if endothelial SIRT1 deletion does not block these improvements, the hypothesis would be refuted. Conversely, confirmation would position NAD+-SIRT1 as a actionable node to reinstate the circadian‑NO firewall against vascular aging.