Hypothesis

Chronic low‑grade mitochondrial ROS triggers a retrograde signal via oxidized mtDNA that activates the cytosolic cGAS‑STING pathway. This pathway induces type‑I interferon signaling in parenchymal cells and CD38 upregulation in macrophages, which together suppress NAMPT‑mediated NAD+ biosynthesis and increase NAD+ consumption. The resulting NAD+ decline is not a passive marker of damage but an active, evolution‑conserved program that reduces energy‑intensive repair and transcriptional fidelity pathways (PARP, sirtuins) to limit the amplification of senescent‑associated secretory phenotype (SASP) and thereby protect tissue integrity in the short term.

Mechanistic Rationale

1. Mitochondrial ROS → mtDNA release – Persistent ROS oxidizes cardiolipin, promoting mitochondrial outer‑membrane permeabilization and release of oxidised mtDNA fragments into the cytosol.
2. cGAS‑STING activation – Cytosolic mtDNA binds cGAS, generating 2'3'-cGAMP that activates STING, leading to TBK1‑IRF3 phosphorylation and IFN‑β transcription.
3. IFN‑β signaling – Autocrine/paracrine IFN‑β engages JAK‑STAT1 signaling, which transcriptionally represses NAMPT (the rate‑limiting NAD+ salvage enzyme) and induces CD38 expression in tissue‑resident macrophages.
4. NAD+ pool contraction – Reduced NAMPT limits NAD+ regeneration while elevated CD38 increases NAD+ hydrolysis, driving NAD+ down.
5. Functional outcome – Lower NAD+ diminishes PARP‑mediated DNA repair (conserving NAD+ for glycolysis) and attenuates SIRT1‑dependent deacetylation of NF‑κB and PGC‑1α, shifting cells toward a glycolytic, senescence‑prone state that limits SASP amplification.

This creates a feedback loop where NAD+ decline serves as a protective brake on inflammaging, analogous to a budget cut that curtails costly investments when future returns are uncertain.

Testable Predictions

• Prediction 1: In aged mice, pharmacologic inhibition of STING (e.g., H-151) will increase NAMPT expression, raise NAD+ levels, and exacerbate SASP markers (IL‑6, CXCL10) in senescent tissue unless CD38 is simultaneously blocked.
• Prediction 2: Macrophage‑specific CD38 knockout mice will show blunted NAD+ decline with aging, but will display heightened SASP and accelerated tissue dysfunction when given NAD+ precursors (NMN/NR).
• Prediction 3: Exogenous IFN‑β treatment of young human fibroblasts will reduce NAMPT mRNA, lower intracellular NAD+, and increase resistance to SASP induction upon oncogenic stress.
• Prediction 4: Elevating mtDNA oxidation (via mito‑Paraquat) in vitro will increase cytosolic mtDNA, activate cGAS‑STING, and produce the NAD+‑lowering/SASP‑suppressive phenotype.

Experimental Design

1. In vivo – Use aged wild‑type, STING‑fl/fl crossed with Ubc‑CreERT2 (tamoxifen‑inducible STING deletion), and CD38‑fl/fl crossed with LysM‑Cre (macrophage‑specific CD38 KO) mice. Treat cohorts with NMN (400 mg/kg/day) for 8 weeks. Measure tissue NAD+ (LC‑MS), SASP cytokines (ELISA), senescence burden (p16^INK4a^ immunostaining), and functional outcomes (grip strength, treadmill endurance).
2. In vitro – Treat primary human dermal fibroblasts with mito‑Paraquat (10 µM, 24h) to induce mtDNA oxidation. Assess cytosolic mtDNA (qPCR after fractionation), cGAMP levels (ELISA), p‑STAT1, NAMPT expression (Western), NAD+ levels, and SASP after irradiation‑induced senescence.
3. Rescue – Co‑treat with STING inhibitor H‑151 or CD38 inhibitor 78c to test whether NAD+ restoration exacerbates SASP only when the upstream ROS‑cGAS‑STING axis is intact.

Potential Confounds & Mitigation

• Off‑target effects of STING/CD38 inhibitors – Use genetic knock‑outs alongside pharmacological tools and confirm target engagement via pathway read‑outs (p‑TBK1 for STING, NADase activity for CD38).
• Compensatory NAD+ salvage pathways – Monitor NRK1/2 and NAMPT-independent salvage (e.g., nicotinamide riboside kinases) to ensure observed NAD+ changes are primarily due to NAMPT regulation.
• Cell‑type heterogeneity – Employ single‑cell RNA‑seq on sorted parenchymal vs. immune compartments to disentangle cell‑specific contributions.

Implications

If validated, this hypothesis reframes NAD+ decline as an adaptive, signaling‑driven attenuation of metabolic investment rather than a mere biomarker of damage. It suggests that NAD+ boosting strategies must be coupled with modulation of the mitochondrial ROS‑cGAS‑STING‑CD38 axis to avoid inadvertently amplifying inflammaging, aligning interventions with the organism’s genuine "budgetary" state rather than overriding it.