Hypothesis

When base excision repair (BER) capacity falls below a critical threshold in aged neurons, the resulting NAD+ depletion from PARP1 hyperactivation creates a bioenergetic checkpoint that couples persistent oxidative DNA damage to activity‑dependent calcium signaling. This combined stress activates the AMPK‑P53 axis, which upregulates complement C3 tagging on vulnerable neurons. Microglia expressing CR3 then phagocytose these tagged cells, implementing a selective eviction program that favors metabolically expensive, weakly active circuits. In essence, the brain does not simply lose neurons to damage; it actively removes those that fail an energy‑damage‑activity triage.

Mechanistic Reasoning

1. BER decline → NAD+ sink – Reduced OGG1 and APE1 activity increases 8‑oxoguanine lesions, causing prolonged PARP1 binding and NAD+ consumption [5576892].
2. NAD+ drop → AMPK activation – Low NAD+/high AMP ratios trigger AMPK, which phosphorylates and stabilizes P53 independent of canonical DNA‑damage kinases [5576894].
3. Activity‑dependent Ca2+ influx – Neurons with low firing rates experience insufficient calcium‑dependent BDNF signaling, reducing survival pathways; however, sporadic synaptic events still generate micro‑domains of Ca2+ that, when NAD+ is low, favor activation of the calcium‑sensitive phosphatase calcineurin, which dephosphorylates and further activates P53.
4. P53‑driven complement expression – P53 transcribes C3 and other complement components, tagging the neuron for microglial recognition [2474780].
5. Microglial phagocytosis – Complement‑tagged neurons are engulfed via microglial CR3, completing an eviction loop that mirrors developmental pruning but is fueled by metabolic stress rather than trophic competition.

This model extends the observed regional selectivity (CA3, striatum, hippocampus, cerebellum) by predicting that neurons with high basal metabolic rates but low average firing—such as CA3 pyramidal cells—will reach the NAD+‑AMPK‑P53 threshold earliest when BER wanes.

Testable Predictions

• Prediction 1: Pharmacological NAD+ replenishment (e.g., nicotinamide riboside) in aged mice will reduce neuronal loss in BER‑deficient models without significantly lowering 8‑oxoguanine levels, indicating that cell death is mediated by the energy checkpoint, not lesion load alone [5576886].
• Prediction 2: Genetic deletion of microglial C3ra (the complement receptor) will block neurodegeneration in the same BER‑deficient mice, despite persistent DNA damage and PARP1 activation.
• Prediction 3: Chemogenetic suppression of activity in specific vulnerable populations (e.g., CA3) will accelerate their eviction, whereas optogenetic enhancement of firing will delay loss, even when NAD+ remains low.
• Prediction 4: Pharmacological inhibition of AMPK (Compound C) or P53 (pifithrin‑α) will attenuate complement C3 upregulation and microglial phagocytosis in cultured neurons subjected to PARP1‑induced NAD+ depletion.

Falsifiability

If NAD+ restoration fails to rescue neuronal survival despite reducing PARP1 activity, or if microglial C3 blockade does not prevent loss, the energy‑checkpoint coupling hypothesis would be refuted. Conversely, observing that boosting neuronal activity alone rescues cells independent of NAD+ levels would also challenge the model.

Implications

Reframing age‑related neuronal decline as an active, metabolism‑sensitive quality‑control process shifts therapeutic focus from merely preventing DNA damage to modulating the NAD+‑AMPK‑P53‑complement axis. Interventions that sustain bioenergetic homeostasis or adjust microglial complement signaling could preserve useful circuitry while allowing the brain to continue its adaptive pruning under energetic constraints.

Key sources: BER decline and apoptosis [5576886], NER deficiency and apoptotic death [2474780], checkpoint activation in post‑mitotic neurons [5576894], regional vulnerability patterns [852002], PARP1‑NAD+ depletion [5576892]