# Hypothesis
Post‑mitotic neurons maintain exceptionally low somatic copy‑number variation (CNV) through constitutively active DNA‑repair pathways. This hyper‑stable genome physically or signaling‑wise engages the p75^NTR^ receptor, which in turn biases synaptic plasticity toward long‑term depression (LTD) and away from long‑term potentiation (LTP). The result is a cognitively rigid network that over‑consolidates existing models and resists new learning—not because neurons are damaged, but because their DNA‑maintenance machinery actively suppresses plasticity.

## Mechanistic Rationale
- **Genomic stability signal:** In non‑dividing cells, efficient base‑excision repair and homologous recombination constantly scan chromatin. Persistent repair complexes may recruit or modify p75^NTR^ at the nuclear membrane or cytoplasm, altering its downstream signaling balance toward proBDNF‑p38‑RhoA‑ROCK2‑LIMK1‑cofilin pathways that promote actin depolymerization and spine shrinkage—core LTD effectors. 
- **p75^NTR^ as a molecular hub:** Deletion of p75^NTR^ prevents age‑related declines in LTP and memory [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC7884039/), indicating that its activity is sufficient to induce plasticity loss. We propose that chronic, low‑level DNA‑repair activity in aged neurons sustains p75^NTR^ in an active conformation, thereby linking genome integrity to synaptic weakening.
- **Tissue‑specific contrast:** Mitotic tissues (blood, skin) accumulate mosaic chromosomal alterations (mCAs) rapidly because replication overwhelms repair [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6367020/), [4](https://www.science.org/doi/10.1126/sciadv.adf4163). Neurons, by contrast, show minimal mCNV but maximal functional rigidity, supporting the idea that *too much* stability, not instability, drives cognitive decline.

## Testable Predictions
1. **Pharmacological dampening of DNA‑repair activity in neurons** (e.g., transient inhibition of PARP1 or DNA‑PKcs) will reduce p75^NTR^ activation and rescue LTP in aged mice without increasing CNV burden.
2. **Neuron‑specific overexpression of a repair‑deficient mutant** (e.g., Ku70‑ΔN) will raise somatic CNV levels modestly and correlate with restored LTP and improved spatial‑learning performance.
3. **p75^NTR^‑dependent phosphoproteomics** will show enhanced cofilin phosphorylation (inactive) and reduced RhoA‑ROCK2 signaling when DNA‑repair is inhibited, indicating a shift from LTD‑favoring to LTP‑favoring states.
4. **Human biomarker correlation:** Elderly individuals with high peripheral‑blood mCA burden (reflecting lower systemic repair fidelity) will exhibit *better* cognitive performance on tasks requiring cognitive flexibility, inversely correlating with neuronal‑specific DNA‑repair activity proxies (e.g., CSF γ‑H2AX levels).

## Experimental Approach
- **In vivo:** Use AAV‑Cre to deliver a floxed‑STOP‑shRNA targeting Parp1 or Prkdc specifically to hippocampal excitatory neurons of 18‑month‑old mice. Measure LTP/LTD ex vivo, mCNV via single‑cell PCR, and behavior (Morris water maze reversal learning).
- **In vitro:** Differentiate iPSC‑derived neurons from young and old donors; treat with low‑dose PARP inhibitor; assess p75^NTR^ cleavage, cofilin activity, and electrophysiological LTP induction.
- **Human:** Analyze existing cohorts (e.g., ADNI) for plasma mCA load [6](https://www.genome.gov/27548594/2012-release-scientists-find-that-chromosomal-abnormalities-are-associated-with-aging-and-cancer) and CSF DNA‑repair markers; correlate with neuropsychological flexibility scores.

## Falsifiability
If inhibiting neuronal DNA repair fails to alter p75^NTR^ signaling, LTP/LTD balance, or cognitive flexibility—while leaving global mCNV unchanged—the hypothesis is refuted. Conversely, if enhanced repair (e.g., overexpressing Ku70) worsens rigidity, the inverse relationship would also falsify the model.

## Broader Implication
This reframes cognitive aging not as loss of capacity but as an actively maintained state of *predictive overconfidence*. Interventions that safely introduce genomic “noise” or modulate the repair‑p75^NTR^ axis could re‑engage plasticity without compromising neuronal longevity.