Hypothesis

Neuronal eviction in aging is gated by a mitochondrial NAD+‑dependent checkpoint that directs microglial complement tagging.

Mechanistic Rationale

• Aging neurons show ribosome stalling and proteostatic stress that lowers NAD+ levels, impairing sirtuin activity and mitochondrial membrane potential [[https://news.stanford.edu/stories/2025/07/brain-aging-mechanism-proteostasis-neurodegenerative-diseases-als-parkinsons-alzheimers]]
• Low NAD+ reduces SIRT3‑mediated deacetylation of mitochondrial enzymes, increasing ROS and decreasing ATP output, a signal interpreted as “inefficient”
• Microglia in a primed state express complement components C1q and C3 that are upregulated by neuronal mitochondrial stress signals (e.g., extracellular NAD+ metabolites, ATP release) [[https://pmc.ncbi.nlm.nih.gov/articles/PMC5479435/]]
• In females, sustained estrogen signaling maintains microglial sensitivity to these metabolic cues, leading to prolonged complement‑mediated pruning throughout adulthood [[https://pubmed.ncbi.nlm.nih.gov/41324815/]]
• Conversely, activation of the REST pathway suppresses neuronal hyperactivity, preserving NAD+ pools and reducing the inefficiency signal [[https://news.harvard.edu/gazette/story/2019/10/nervous-system-activity-might-influence-human-longevity-neural-activity/]]

Predictions & Experimental Design

1. Prediction: Elevating neuronal NAD+ via nicotinamide mononucleotide (NMN) supplementation will decrease microglial C1q‑positive contacts with neurons in the aged hippocampus.
2. Design: 24‑month‑old C57BL/6J mice receive NMN (400 mg/kg/day) or vehicle for 8 weeks. Immunohistochemistry quantifies C1q‑microglia apposition to NeuN+ cells, mitochondrial membrane potential (TMRM), and NAD+ levels (enzyme‑based assay). Stereological neuron counts assess eviction.
3. Expected outcome: NMN‑treated mice show ↑ neuronal NAD+, ↑ mitochondrial Δψm, ↓ C1q‑microglia contacts, and preserved neuron numbers without loss of baseline synaptic density (measured by PSD‑95 puncta).
4. Sex‑specific test: Parallel cohorts of males and females evaluate whether estrogen receptor antagonism abolishes the female‑specific persistence of complement tagging despite NMN treatment.

Potential Confounds & Alternative Outcomes

• If NMN fails to alter complement tagging despite raising NAD+, the checkpoint may lie downstream (e.g., sirtuin‑independent ROS sensing) or involve microglial NAD+ metabolism directly.
• A reduction in C1q contacts without neuron survival improvement would suggest that tagging is separable from execution of phagocytosis, implicating downstream effectors like CR3 or lysosomal enzymes.
• Conversely, an unexpected increase in synaptic pruning markers could indicate that NAD+ boost restores plasticity‑related activity, inadvertently reactivating developmental pruning pathways.

Implications

Confirming a metabolic checkpoint would reframe age‑related neuronal loss as a regulatable quality‑control mechanism rather than inevitable damage. It would raise the possibility of modulating neuronal metabolism to preserve circuitry in neurodegenerative contexts while preserving the brain’s adaptive efficiency.