Hypothesis

It's becoming clear that aging impairs microglial mitochondrial fitness, which tilts the C1q/CD47 balance toward indiscriminate phagocytosis. This metabolic shift causes the brain’s neuronal‑eviction system to lose precision, removing both inefficient and functional neurons while sparing truly damaged cells.

Mechanistic Basis

• Microglia rely on oxidative phosphorylation to fuel phagocytic signaling and to sustain surface CD47, the “don’t eat me” cue 4.
• Age‑related mtDNA damage and falling NAD+ levels blunt microglial ATP output, lowering CD47 transcription and raising C1q deposition 3.
• The resulting C1q‑high/CD47‑low state flags synapses and somata of merely low‑activity neurons for removal, even when those cells are still functional 2.
• Damaged lysosomal enzymes from accumulated lipofuscin further hinder degradation of engulfed material, sparking a secondary inflammatory loop that dampens phagocytic competence.

Predictions and Tests

1. Metabolic rescue restores selectivity – Giving aged microglia NAD+ precursors (e.g., nicotinamide riboside) or mitochondrial antioxidants should lift CD47, cut C1q, and steer pruning back toward truly inactive neurons. Expected result: better memory precision without a drop in total neuron count in aged mice.
2. Cell‑type‑specific manipulation – Microglial‑specific TREM2 knockout will worsen the C1q/CD47 imbalance, accelerating premature neuronal loss and cognitive decline; conversely, microglial overexpression of PGC‑1α (a mitochondrial biogenesis driver) should safeguard synaptic density.
3. Biomarker correlation – In human CSF, the soluble C1q/CD47 ratio will positively track with PET‑measured microglial mitochondrial signal (e.g., [18F]BCPP‑EF) and inversely with hippocampal volume across adulthood.
4. Functional read‑out – Chemogenetically silencing low‑activity dentate gyrus neurons in aged mice will mimic metabolic rescue, showing that preserving these cells rescues pattern‑separation performance only when microglial metabolism is intact.

Potential Implications

If microglial metabolic fitness governs the selectivity of neuronal eviction, therapies that boost microglial energetics could re‑balance pruning, preserving useful circuits while still clearing maladaptive neurons. This view recasts age‑related cognitive decline not as inevitable neuron loss but as a correctable quality‑control failure, opening treatment routes that target microglial metabolism rather than broad neuroprotection.