Hypothesis

Elevating NAD+ levels in hematopoietic stem cells (HSCs) rejuvenates immune surveillance by reducing mitochondrial DNA release and cGAS‑STING activation, thereby breaking the inflammaging feedback loop that drives systemic aging.

Mechanistic Rationale

• NAD+ decline with age impairs sirtuin activity, leading to hyperacetylated mitochondrial proteins and increased mitochondrial ROS, which promotes mtDNA efflux into the cytosol 1.
• Cytosolic mtDNA activates the cGAS‑STING pathway in immune cells, driving NF‑κB–dependent SASP production and reinforcing senescence 2.
• Restoring NAD+ via precursors (e.g., NR or NMN) reactivates SIRT3/SIRT5, enhancing mitochondrial fidelity and limiting mtDNA leakage 3.
• Lower mtDNA release diminishes cGAS‑STING signaling, reducing SASP from senescent T cells, macrophages, and neutrophils, and curbing the feed‑forward loop that spreads senescence to parenchyma 4.
• Consequently, a youthful HSC compartment generates a balanced lymphoid‑myeloid output, improving clearance of senescent cells and decreasing tissue‑level inflammation 5.

Testable Predictions

1. In aged mice, NAD+ supplementation will decrease cytosolic mtDNA in HSCs and downstream immune cells, measurable by reduced cGAMP levels and STING phosphorylation.
2. This biochemical shift will correlate with lowered plasma IL‑6, IL‑1β, and TNF‑α, and with reduced p16^Ink4a^‑positive senescent cells in liver, kidney, and lung.
3. Functionally, treated mice will show enhanced clearance of transplanted senescent fibroblasts and improved grip strength and treadmill endurance compared with vehicle controls.
4. Genetic ablation of STING in HSCs should mimic the NAD+ effect, confirming pathway specificity.

Experimental Design

• Groups: young (3 mo), aged (20 mo) vehicle, aged + NAD+ precursor (NR 400 mg/kg/day), aged + HSC‑specific Sting knockout (using Vav‑Cre‑Sting^fl/fl).
• Interventions: 8‑week treatment; collect blood, bone marrow, and organs at endpoint.
• Assays: mtDNA quantification by qPCR after subcellular fractionation, Western blot for p‑STING, p‑TBK1, p‑IRF3; cytokine ELISA; senescence‑associated β‑gal staining; flow cytometry for HSC lineage bias; functional assays (senescent cell clearance via luciferase‑labeled fibroblasts, grip strength, rotarod).
• Analysis: Two‑way ANOVA with post‑hoc tests; significance set at p<0.05.

If NAD+ boost fails to lower mtDNA/cGAS‑STING activity or does not improve immune‑mediated senescence clearance, the hypothesis is falsified. Conversely, a rescue of youthful immune phenotypes and delayed tissue aging would support the claim that metabolic reprogramming of HSCs is a upstream lever to halt inflammaging-driven aging.