Hypothesis

Transient expression of OSKM factors in aged microglia reprograms their epigenetic state, restoring developmental-like selectivity in synapse pruning by re‑establishing activity‑dependent complement tagging and Crq‑mediated phagocytosis, thereby shifting elimination from indiscriminate debris clearance to efficient removal of weak or redundant synapses.

Mechanistic Rationale

1. Epigenetic reset – OSKM binds to promoters of phagocytic regulators (e.g., Crq, C1qa), removing age‑associated heterochromatin marks and re‑opening chromatin for activity‑dependent transcription [2].
2. Restored complement tagging – In young brain, inactive synapses are marked by C1q/C3 for microglial engulfment [1]; aging elevates basal C3 irrespective of activity, promoting maladaptive phagocytosis [4]. OSKM‑driven chromatin remodeling reduces constitutive C3 expression while preserving its inducibility by neuronal activity.
3. Microglial metabolic shift – Transient reprogramming lowers AMPK hyperactivation in microglia, decreasing autophagy‑mediated synaptic loss that is unrelated to synaptic weakness [4].
4. Outcome – Synapse elimination becomes correlated with low neuronal firing and weak connectivity, mirroring developmental pruning, while overall neuron number remains stable.

Testable Predictions

• In aged mice receiving transient OSKM mRNA via lipid nanoparticles, microglial Crq and inducible C1q expression will increase, whereas basal C3 levels will decline relative to untreated aged controls (measured by flow cytometry and qPCR).
• Synaptic density in hippocampal CA1 will be preserved, but the proportion of eliminated synapses will show a higher correlation with low c‑Fos activity (immunostaining for activated microglia juxtaposed with low neuronal activity markers).
• Behavioral assays (e.g., Morris water maze) will reveal improved spatial memory without a significant increase in total NeuN‑positive neuron counts.
• Blocking microglial phagocytosis with annexin V or inhibiting C3 will abolish the OSKM‑induced memory benefit, indicating that the effect depends on restored selective pruning.

Falsifiability

If OSKM treatment does not alter the selectivity of microglial synapse removal—i.e., eliminated synapses remain uncorrelated with neuronal activity markers—or if it leads to indiscriminate loss of synapses comparable to untreated aged brains, the hypothesis is refuted. Similarly, failure to rescue cognitive performance despite molecular changes would indicate that restored pruning selectivity is insufficient for functional benefit.

References

[1] https://elifesciences.org/articles/101779 [2] https://doi.org/10.1101/2024.06.24.600214 [3] https://news.harvard.edu/gazette/story/2019/10/nervous-system-activity-might-influence-human-longevity-neural-activity/ [4] https://doi.org/10.1038/s41419-019-1464-x [5] https://doi.org/10.1038/s41598-017-05927-4 [6] https://pmc.ncbi.nlm.nih.gov/articles/PMC4632662/