Hypothesis

Aging neurons accumulate epigenetic rigidity not because histone marks decay but because the sleep‑dependent glymphatic system fails to clear inhibitory complexes that tether KDM6 demethylases in the cytoplasm, leading to chronic H3K27me3 loss at synaptic enhancers and a locked transcriptional state.

Mechanistic basis

1. Sleep‑glymphatic flux regulates KDM6 localization – During wakefulness, neuronal activity drives calcium‑dependent phosphorylation of KDM6 (KDM6B/JMJD3) promoting its binding to cytoplasmic scaffolds (e.g., 14‑3‑3 proteins) that prevent nuclear entry. During sleep, expanded extracellular space enhances glymphatic inflow, flushing these scaffolds and releasing KDM6 for nuclear import.[1]
2. Age‑related decline in glymphatic efficiency reduces this nocturnal clearance, leaving KDM6 sequestered and thereby diminishing H3K27me3 deposition at bivalent promoters and enhancers.[2] The concurrent stability of H3K4me3 (via constant KDM5 activity) and increased MLL2‑mediated H3K4me2 creates an activating bias that favors stress‑response programs while silencing synaptic genes via loss of H3K27ac at enhancers.[3]
3. Resulting chromatin landscape shows global H3K27me3 loss, preserved bivalency, and reduced transcriptional variance – the hallmarks of over‑consolidation described in the seed idea.

Testable predictions

• Prediction 1: In aged mice (18‑24 mo), pharmacological enhancement of glymphatic flow (e.g., via intrathecal acetazolamide or chronic AQP4 over‑expression) will increase nuclear KDM6B levels by ≥30 % compared with age‑matched controls, measurable by subcellular fractionation and immunoblot.[4]
• Prediction 2: This nuclear KDM6 rise will restore H3K27me3 at synaptic enhancers (e.g., Synaptophysin, GluA1 promoters) to youthful levels without altering global H3K4me3, detectable by ChIP‑seq.
• Prediction 3: Behavioral assays of cognitive flexibility (reversal learning in a water maze) will show improved performance in treated aged mice, approximating young adult performance, whereas blockade of glymphatic inflow (using intracerebroventricular TNF‑α to suppress AQP4 polarity) will exacerbate rigidity and worsen performance.
• Prediction 4: Acute sleep deprivation in young adult mice will mimic the aged epigenetic signature: cytoplasmic KDM6 accumulation, H3K27me3 loss at synaptic loci, and increased transcriptional variance suppression, reversible by subsequent sleep recovery.

Falsifiability

If enhancing glymphatic clearance fails to increase nuclear KDM6 or does not rescue H3K27me3 levels and cognitive flexibility, the hypothesis that sleep‑dependent clearance of KDM6 regulators drives epigenetic consolidation would be refuted. Similarly, if glymphatic blockade does not worsen the aged epigenetic profile, the proposed causal link is unsupported.

Broader implication

This model reframes cognitive aging as a reversible state of epigenetic over‑locking that can be loosened by restoring the brain’s nocturnal housekeeping, shifting the therapeutic goal from restoring lost plasticity to re‑introducing controlled uncertainty via physiological clearance mechanisms.