Hypothesis

Age‑related NAD+ loss in senescent cells does not merely reflect passive consumption; it actively rewires cellular metabolism toward aerobic glycolysis, raising intracellular lactate that promotes pro‑inflammatory macrophage polarization and sustains the senescence‑associated secretory phenotype (SASP). This creates a feed‑forward loop that accelerates inflammaging and tissue dysfunction.

Mechanistic Rationale

NAD+ is a cofactor for sirtuins and PARPs, which modulate mitochondrial function and DNA repair. When NAD+ falls, the NAD+/NADH ratio drops, stabilizing HIF‑1α even under normoxic conditions [7]. HIF‑1α drives transcription of glycolytic enzymes (e.g., LDHA) and suppresses pyruvate dehydrogenase, shunting pyruvate to lactate. Elevated lactate exits senescent cells via MCT4 and acts as a signaling molecule that polarizes nearby macrophages toward an M1‑like phenotype, increasing TNF‑α, IL‑1β, and further SASP factor secretion [3]. SASP factors, in turn, upregulate CD38 on immune and stromal cells [2], boosting NAD+ consumption and deepening the deficit. Thus, NAD+ decline becomes an active driver of a metabolic‑immune circuit that amplifies inflammation rather than a passive side‑effect.

Testable Predictions

1. In aged tissues, senescent cells will show a higher lactate production rate and a lower NAD+/NADH ratio compared with young counterparts.
2. Pharmacological inhibition of lactate export (MCT4 blockade) will reduce M1 macrophage markers and attenuate SASP expression despite persistent NAD+ loss.
3. Genetic overexpression of LDHA in senescent cells will exacerbate inflammaging, whereas LDH knockdown will mitigate it even when NAD+ levels remain low.
4. Combining CD38 inhibition with lactate signaling blockade will produce synergistic improvements in metabolic healthspan beyond either intervention alone.

Experimental Approach

• Model: Use aged (24‑month) and young (3‑month) C57BL/6 mice; isolate senescent cells via p16‑Ink4a reporter sorting.
• Metabolomics: Measure intracellular NAD+, NADH, lactate, and pyruvate levels via LC‑MS; calculate NAD+/NADH ratio.
• Flux Analysis: Employ ^13C‑glucose tracing to quantify glycolytic flux and lactate export.
• Interventions: Treat mice with CD38 inhibitor (78c), MCT4 inhibitor (SYN‑0012), LDHA siRNA delivered via p16‑promoter‑driven AAV, or combinations.
• Readouts: Flow cytometry for macrophage CD86/CD206 ratios; ELISA for SASP cytokines (IL‑6, IL-1β, TNF‑α); insulin tolerance test and indirect calorimetry for metabolic flexibility.
• Validation: Repeat key findings in human senescent fibroblasts cultured with autologous macrophages.

Potential Implications

If validated, this hypothesis reframes NAD+ decline as a mechanistic hub linking cellular senescence to chronic inflammation through metabolic reprogramming. It suggests that targeting lactate signaling—perhaps alongside NAD+ boosters—could break the inflammaging cycle, offering a complementary strategy to senolytics or NAD+ supplementation for age‑related diseases.