Overview

The synaptic pruning cascade that refines circuits during development is re‑engaged in aging, but its selectivity drifts from pure activity‑tagging to a hybrid rule that also weighs mitochondrial output. In young tissue, complement protein C1q tags synapses with low firing rates, while CD47 delivers a 'don't eat me' signal to active connections 1. With age, rising inflammatory cytokines shift the C1q/CD47 balance toward elimination, but we propose that an additional sensor—membrane potential‑dependent ATP/ADP ratio—feeds into the microglial decision loop, causing the removal of synapses that are energetically expensive despite moderate activity.

Mechanism

Microglia express the metabolic sensor AMPK, which becomes activated when the local ATP/ADP ratio falls below a threshold. Activated AMPK phosphorylates the phagocytic receptor MerTK, increasing its affinity for C1q‑opsonized synapses. Simultaneously, AMPK suppresses CD47 transcription via HIF‑1α inhibition, lowering the 'don't eat me' signal. Thus, a synapse that fires enough to avoid pure activity‑based tagging but sits in a region with high mitochondrial demand (e.g., dense axonal arbor) will experience a low ATP/ADP ratio, trigger AMPK, and be earmarked for removal. This creates a second‑order filter: expensive, moderately active connections are pruned preferentially when energy is scarce.

Testable Predictions

1. In aged mice, pharmacological inhibition of AMPK in microglia will preserve synapses that show intermediate calcium imaging activity but high mitochondrial marker (Mitotracker) signal, without affecting low‑activity synapses that are already C1q‑positive 2.
2. Synaptosomal isolates from old prefrontal cortex will exhibit a higher ratio of phosphorylated MerTK to total MerTK compared with young tissue, and this ratio will correlate with regional ATP levels measured by luciferase assay 3.
3. Transient overexpression of CD47 in hippocampal neurons will rescue spine density in aged animals only when combined with a mild metabolic stressor (e.g., 2‑deoxyglucose), indicating that the 'don't eat me' signal can overcome AMPK‑driven pruning 4.
4. Human PET‑MRI studies using a novel C1q‑binding tracer should show that regions with elevated amyloid‑negative but high FDG uptake (reflecting mitochondrial load) lose synaptic density faster than regions with low FDG uptake, independent of overall firing rates measured by MEG.

Implications

If aging brains prune synapses not just for inactivity but also for metabolic inefficiency, interventions that boost neuronal energetics—such as NAD+ boosters, mitochondrial uncouplers at low dose, or exercise‑induced lactate shuttling—could shift the AMPK/CD47 balance back toward preservation. This reframes cognitive decline as a maladaptive energy‑allocation problem rather than pure wiring loss, suggesting that metabolic rescue may complement synaptic enrichment strategies.