Aged hippocampi accumulate perineuronal nets (PNNs) that restrict synaptic plasticity, yet retain core LTP/LTD machinery when driven strongly [1][2][3][4]. We hypothesize that brief, pulsed expression of OSKM transcription factors via mRNA lipid nanoparticles triggers a downstream cascade that remodels the extracellular matrix, lowering PNN density and restoring experience‑dependent plasticity without causing tumorigenic transformation or sustained dedifferentiation.

Mechanistically, OSKM activation is known to upregulate genes involved in matrix metalloproteinase (MMP) secretion and to repress tissue inhibitor of metalloproteinases (TIMP) expression in other tissues. In the aged hippocampus, we predict that a transient OSKM pulse will increase the MMP‑9/TIMP‑1 ratio, enhancing proteolytic cleavage of aggrecan and link proteins that stabilize PNNs. This ECM loosening should reduce the physical barrier to dendritic spine motility, permitting NMDA‑dependent LTP induction under physiological stimulation patterns that are normally ineffective in old animals.

Importantly, we propose that the plasticity‑permissive state created by OSKM must be paired with controlled uncertainty—such as intermittent exposure to novel spatial contexts or reversed reward contingencies—to bias the network toward updating existing schemas rather than indiscriminate sprouting. The combination of a permissive matrix and predictive error signals should engage metaplastic mechanisms that shift the BCM‑like threshold for synaptic change, allowing the latent LTP/LTD capacity observed in slices to manifest behaviorally.

Testable predictions

1. Aged mice (≥18 mo) receiving two weekly injections of OSKM‑mRNA LNPs for three weeks will show a significant decrease in hippocampal PNN intensity (measured by WFA staining) and an increased MMP‑9/TIMP‑1 ratio compared with saline or GFP‑mRNA controls.
2. Electrophysiologically, acute slices from OSKM‑treated aged mice will exhibit LTP induction with standard theta‑burst protocols that fail in untreated aged controls, while LTD magnitude remains unchanged.
3. Behaviorally, OSKM‑treated aged mice that also receive weekly novelty‑exposure sessions will outperform controls on reversal learning and surprise‑based discrimination tasks (e.g., probabilistic reversal learning), whereas OSKM treatment without novelty will not improve performance beyond baseline.
4. Long‑term follow‑up (6 months post‑treatment) will reveal no increase in proliferative markers (Ki‑67, Sox2) or ectopic neurogenesis, and no tumor formation, distinguishing transient from sustained OSKM expression.

Falsifiability If OSKM mRNA delivery fails to alter PNN density, MMP‑9/TIMP‑1 balance, or LTP susceptibility, or if behavioral improvements occur without novelty exposure, the core mechanistic link between transient reprogramming, ECM remodeling, and uncertainty‑driven plasticity would be refuted. Conversely, observation of any tumorigenic transformation would indicate that the safety window for transient OSKM in the CNS is narrower than predicted, challenging the hypothesis’s translational viability.

This framework directly tests whether re‑introducing controlled molecular youthfulness—via a brief, defined OSKM pulse—can unlock the brain’s latent plasticity by loosening an over‑consolidated matrix, turning the interpretation of cognitive aging from irreversible decline to a reversible state of excessive stability.