Hypothesis

NAD+ depletion in aging neurons is not a passive consequence of DNA damage but actively drives a bistable metabolic switch that locks cells into a low‑NAD+, high‑PARP1 state, suppressing BER and promoting a senescent‑like phenotype.

Mechanistic Rationale

1. Persistent oxidative lesions (e.g., 8‑oxoG) activate PARP1, which consumes NAD+ to synthesize PAR chains.[2]
2. Falling NAD+ reduces SIRT1 activity, decreasing deacetylation of PGC‑1α and FOXO factors, thereby lowering mitochondrial oxidative capacity and increasing ROS production.[3]
3. Low NAD+ also limits the activity of NAMPT‑dependent salvage, creating a positive feedback loop that further depresses NAD+ levels.
4. In parallel, NAD+ shortage impairs the acetyl‑CoA synthetase ACSS2, reducing acetyl‑CoA for histone acetylation and shifting gene expression toward a repressed, SASP‑like profile.
5. The combined effect is a self‑reinforcing low‑NAD+ attractor state where BER enzymes (OGG1, APE1) remain under‑resourced, DNA damage accumulates, and the neuron adopts a hypometabolic, repair‑deficient phenotype.

Predictions

• P1: In aged neurons, artificially raising NAD+ (e.g., via NR) will break the bistable loop, decreasing PARP1 activity and restoring SIRT1‑mediated deacetylation, even without altering DNA lesion load.
• P2: Inhibiting PARP1 in young neurons subjected to acute oxidative stress will prevent the transition to the low‑NAD+ state, preserving BER capacity.
• P3: Forced overexpression of NAMPT in aged neurons will shift the system back to the high‑NAD+ attractor, reducing markers of senescence (p16^INK4a^, SASP cytokines) and improving cognitive readouts in animal models.

Experimental Approach

• In vitro: Primary cortical neurons from young (3 mo) and aged (24 mo) mice treated with H2O2 to induce 8‑oxoG. Measure NAD+, PAR, SIRT1 activity, OGG1/APE1 levels, and senescence markers over time. Apply NR, PARP inhibitor (Olaparib), or NAMPT overexpression via AAV.
• In vivo: Aged APP/PS1 mice receive chronic NR supplementation or brain‑specific NAMPT AAV. Assess 8‑oxoG accumulation, tau phosphorylation, inflammatory cytokines, and performance in Morris water maze.
• Readouts: LC‑MS NAD+/NADH ratios, immunoblot for PAR, SIRT1, acetyl‑p53, immunofluorescence for 8‑oxoG, qPCR for SASP genes, behavioral testing.

If NR or NAMPT rescue fails to lower PARP1 activity or improve BER despite raising NAD+, the hypothesis would be falsified, indicating that NAD+ loss is merely a downstream epiphenomenon rather than a driver of the bistable switch.