Aging brains show selective loss of metabolically expensive, weakly connected neurons that matches developmental pruning logic rather than random damage. Evidence indicates that silencing neuronal activity is sufficient to drive degeneration of entorhinal cortex layer 2 stellate cells, and that loss of noradrenergic locus coeruleus neurons correlates with severe cognitive decline. Humanin, a mitochondria‑derived peptide, activates FPRL1/2‑ERK1/2 and STAT3 pathways to promote survival, and its overexpression extends lifespan in invertebrates and mice. Yet no study has tested whether declining humanin (or related SHLPs) precedes the activity drop that predicts elimination.

We hypothesize that a gradual reduction in mitochondrial peptide export lowers local ATP production and attenuates ERK/STAT3 signaling in vulnerable neurons. This bioenergetic deficit reduces basal firing and calcium influx, weakening synaptic drive and making the cells appear "inefficient" to microglia‑mediated complement tagging. Consequently, activity‑dependent elimination pathways (e.g., C1q‑C3‑CR3) target these neurons before overt damage accumulates. In contrast, neurons that sustain sufficient humanin‑mediated signaling maintain electrophysiological activity and evade eviction despite advancing age.

Testable predictions:

1. In aged mice, single‑cell transcriptomics of entorhinal cortex layer 2 will reveal a stepwise decrease in humanin/SHLP mRNA and downstream p‑ERK/p‑STAT3 levels in neurons that later show reduced c‑Fos (activity marker) and increased C3 deposition.
2. Viral overexpression of humanin specifically in reelin+ stellate cells will rescue their activity (measured by in vivo two‑photon calcium imaging) and prevent their loss, even when global mitochondrial function is impaired.
3. Pharmacological blockade of FPRL1/2 in young mice will mimic the aged phenotype: lowered neuronal activity, increased microglial phagocytosis, and accelerated loss of weakly connected neurons.
4. Conversely, enhancing mitochondrial peptide export via overexpression of the mitochondrial inner‑membrane transporter SLIRP will elevate extracellular humanin, sustain ERK/STAT3 signaling, and delay activity‑dependent eviction without altering global oxidative stress markers.

These experiments directly link mitochondrial retrograde signaling to the activity checkpoint that governs neuronal eviction. If humanin loss precedes and causally drives activity decline, interventions that boost mitochondrial peptide signaling could preserve network efficiency by halting a programmed, quality‑control pruning process rather than merely combating passive neurodegeneration.