Hypothesis

Chronic NAD+ depletion in aging does not merely reflect energy shortage; it actively rewires chromatin through a lactate‑histone lactylation axis that locks cells into a low‑ambition, senescence‑associated secretory phenotype (SASP). When NAD+ falls, SIRT1 activity drops, HIF‑1α stabilizes, LDHA expression rises, and intracellular lactate accumulates. Elevated lactate non‑enzymatically lactylates histone lysines (H3K18la, H4K5la), neutralizing positive charges and loosening nucleosome‑DNA interactions at promoters of pro‑inflammatory genes. This epigenetic shift sustains SASP expression even if NAD+ is later replenished, explaining why NAD+ precursors can prolong dysfunctional cells in damage‑driven phenotypes.

Mechanistic Rationale

1. NAD+ → SIRT1 → HIF‑1α – NAD+‑dependent SIRT1 deacetylates and destabilizes HIF‑1α. NAD+ loss → HIF‑1α accumulation → transcription of glycolytic enzymes (LDHA, PDK1) → increased pyruvate‑to‑lactate conversion [5].
2. Lactate → Histone Lactylation – Lactate reacts with acetyl‑CoA‑derived acetyl groups on histones via non‑enzymatic lactylation, a modification shown to correlate with gene activation in macrophages and neurons (Zhang et al., 2019). In senescent fibroblasts, H3K18la enrichment is observed at IL6 and IL8 promoters.
3. Lactylation → SASP Lock – Lactylated chromatin maintains an open configuration at SASP loci, reducing the reliance on ongoing NAD+ consumption for transcriptional activation. Thus, even after NAD+ restoration, the epigenome remains "stuck" in a pro‑inflammatory state unless lactyl marks are removed.
4. Feedback Loop – SASP factors (IL‑6, IL‑8) reinforce PARP activation via NF‑κB signaling, further draining NAD+ and perpetuating the cycle.

Testable Predictions

• Prediction 1: In cells exhibiting damage‑driven NAD+ depletion (high PARP activity, low NAMPT), pharmacological inhibition of LDHA or supplementation with lactate dehydrogenase inhibitor (e.g., oxamate) will reduce intracellular lactate, decrease H3K18la levels, and lower SASP mRNA/protein without altering NAD+ concentrations.
• Prediction 2: Overexpressing NAMPT to boost NAD+ will fail to suppress SASP if lactate production remains high; combined NAMPT overexpression + LDHA knock‑down will synergistically reduce SASP markers.
• Prediction 3: Chromatin immunoprecipitation followed by mass spectrometry (ChIP‑MS) will show increased H3K18la and H4K5la at SASP promoters in aged human fibroblasts that correlate inversely with SIRT1 activity and directly with lactate concentration.
• Prediction 4: In vivo, aged mice treated with NMN alone will exhibit improved NAD+ levels but persistent colonic SASP if lactate is not concurrently inhibited; adding oxamate will enhance tissue repair and reduce senescence markers beyond NMN alone.

Experimental Approach

• Cellular Models: Use primary human fibroblasts subjected to chronic low‑dose etoposide (DNA damage) to generate a damage‑driven NAD+ depletion phenotype; parallel NAMPT‑KO cells for biosynthetic failure.
• Interventions: NMN (500 µM), LDHA inhibitor oxamate (1 mM), SIRT1 activator SRT2104 (10 µM), and combinations.
• Readouts: NAD+/NADH ratios (enzymatic assay), PARP activity (ELISA), lactate (colorimetric), global and site‑specific histone lactylation (anti‑H3K18la/H4K5la Western blot, ChIP‑qPCR), SASP cytokines (IL‑6, IL‑8 ELISA), senescence (SA‑β‑gal, p16^INK4a^).
• Computational Layer: Integrate longitudinal metabolomics, proteomics, and epigenomics into a personal digital twin (as per ’s framework) to simulate whether lactate‑centric interventions shift the phenotype from "damage‑driven depletion" to a reparative state.

Implications

If validated, this hypothesis reframes NAD+ supplementation as insufficient without addressing the lactate‑lactyl epigenetic lock that sustains a low‑ambition, SASP‑prone state. It suggests a two‑step therapeutic strategy: first, curb lactate production (via LDHA inhibition or glycolysis modulation) to erase lactyl marks; second, replenish NAD+ to restore SIRT1‑mediated chromatin fidelity and mitochondrial communication. Such an approach could prevent the paradoxical prolongation of senescent cells observed with naïve NAD+ boosting and improve the precision of anti‑aging interventions guided by digital‑twin diagnostics.

[1]: https://pmc.ncbi.nlm.nih.gov/articles/PMC5419884/ [2]: https://www.contemporaryhealth.co.uk/digital-health/ai-powered-digital-twin-developed-to-anticipate-individual-health-trajectories/ [3]: https://pmc.ncbi.nlm.nih.gov/articles/PMC9185337/ [4]: https://pmc.ncbi.nlm.nih.gov/articles/PMC10776128/ [5]: https://doi.org/10.1016/j.cell.2013.11.037 [6]: https://doi.org/10.1038/sigtrans.2017.17