Hypothesis

Elevating NAD+ levels in aged myocardium suppresses the senescence‑associated secretory phenotype (SASP) of c‑Kit+ cardiac progenitor cells (CPCs) by restoring SIRT1‑dependent autophagy, thereby interrupting the paracrine senescence amplification loop and improving regenerative capacity after injury.

Rationale

• Senescent CPCs accumulate with age and secrete SASP factors that induce senescence in neighboring healthy CPCs [2].
• Autophagy suppression directly drives senescence in cardiomyocytes [4], and c‑Kit signaling is required for CPC function [3].
• NAD+ activates SIRT1, which deacetylates autophagy genes (e.g., LC3, ATG5) and promotes mitophagy, a process shown to reduce SASP in other cell types.
• Systemic NAD+ decline correlates with aged stem‑cell exhaustion [5], suggesting a nodal point for intervention.

Predictions

1. Molecular – NAD+ supplementation (e.g., nicotinamide riboside, NR) will increase SIRT1 activity and autophagic flux specifically in c‑Kit+ CPCs from aged mice.
2. Cellular – Senescent CPC frequency (β‑galactosidase+, p16^INK4a^+) will decrease by ≥30 % relative to vehicle‑treated aged controls, without altering total CPC number.
3. Secretory – SASP cytokine levels (IL‑6, IL‑8, MMP‑3) in cardiac interstitial fluid will be reduced proportionally to the decline in senescent CPCs.
4. Functional – After myocardial infarction, NR‑treated aged mice will show a higher proportion of Ki‑67^+^ cardiomyocytes and improved ejection fraction compared with aged controls.
5. Sex‑Specific – Female mice will exhibit a greater reduction in SASP due to estrogen‑mediated upregulation of NAD+ salvage enzymes (NAMPT), predicting a sex‑dependent effect size.

Experimental Design

• Animals: 24‑month‑old male and female C57BL/6J mice; groups: vehicle, NR (400 mg/kg/day oral), NR + SIRT1 inhibitor (EX527) to test pathway specificity.
• Intervention: 8‑week pretreatment before coronary artery ligation.
• Readouts (at baseline, 3 days, and 14 days post‑MI):
  • Flow cytometry for c‑Kit^+^ cells stained with senescence markers.
  • SIRT1 activity assay and LC3‑II/I ratio in isolated CPCs.
  • Multiplex ELISA of SASP factors in pericardial lavage.
  • Histology for proliferating cardiomyocytes (Ki‑67^+^/cTnT^+^) and echocardiography for functional recovery.
• Statistical: Power analysis targeting 80 % power to detect a 25 % difference in senescent CPC fraction (α = 0.05).

Potential Outcomes and Interpretation

• If predictions hold: NAD+ elevation restores autophagy, curtails SASP, and breaks the senescence amplification loop, supporting a senomorphic strategy complementary to senolytics.
• If SASP declines without autophagy increase: Suggests NAD+ acts via alternative routes (e.g., PARP inhibition or inflammasome modulation), prompting mechanistic follow‑up.
• If no effect is observed: Implies that NAD+/SIRT1 signaling is insufficient to override age‑related epigenetic locks in CPCs, redirecting focus to combinatorial epigenetic reprogramming.

This hypothesis is directly falsifiable: a failure to observe increased autophagy and reduced SASP in CPCs despite NAD+ boost would refute the proposed mechanistic link.