Hypothesis

Chronic microglial JNK/AP-1 signaling does not merely increase perineuronal net (PNN) deposition; it reprograms the sulfation machinery of astrocytes and microglia to favor chondroitin‑4‑sulfate (CS‑4S) over chondroitin‑6‑sulfate (CS‑6S), producing an overly rigid extracellular matrix that suppresses surprise‑driven plasticity while preserving neuronal viability.

Mechanistic proposal

Persistent JNK activation maintains AP‑1 dimers (c‑Jun/c‑Fos) bound to enhancer regions of chondroitin sulfotransferase genes CHST3 (CS‑4S) and CHST11 (CS‑6S). In aged microglia, AP‑1 recruits histone acetyltransferase p300, increasing H3K27ac at the CHST3 promoter and concurrently engaging DNA methyltransferase DNMT3a to silence CHST11 via CpG methylation. The resulting enzymatic bias elevates CS‑4S incorporation into nascent proteoglycans, which are secreted via microglial exosomes and taken up by perineuronal astrocytes, accelerating PNN maturation. Because CS‑4S confers higher binding affinity for tenascin‑R and link protein, the lattice becomes less permissive to protease‑mediated remodeling, limiting synapse turnover around parvalbumin‑positive interneurons. Importantly, the structural scaffold remains intact, explaining why neurons resist amyloid‑β toxicity yet exhibit reduced long‑term potentiation and impaired reversal learning.

Testable predictions

1. In aged wild‑type mice, microglial nuclei will show increased H3K27ac at CHST3 and elevated DNA methylation at CHST11 compared with young controls.
2. Pharmacological inhibition of JNK (SP600125) for two weeks in aged mice will normalize CHST3/CHST11 expression ratios and shift PNN sulfation toward CS‑6S without reducing overall PNN density.
3. Astrocyte‑specific overexpression of CHST11 (via AAV9‑GFAP‑CHST11) in aged JNK‑intact mice will rescue CS‑6S levels, improve performance on the attentional set‑shifting task, and leave amyloid‑β burden unchanged.
4. Microglial‑specific JNK knockout (Cx3cr1‑CreER;Jnk1^fl/fl) will prevent the epigenetic shift and maintain youthful PNN composition, confirming cell‑autonomous control.

Experimental approach

• Epigenetic profiling: isolate microglia from young (3 mo) and aged (24 mo) mice; perform ChIP‑seq for H3K27ac and whole‑genome bisulfite sequencing targeting CHST3/CHST11 loci.
• Pharmacology: treat aged mice with JNK inhibitor or vehicle; quantify PNN sulfation using lectin staining (WFA for total PNN, VVA for CS‑6S) and immunoblot for CHST3/CHST11.
• Genetic rescue: inject AAV9‑GFAP‑CHST11 into hippocampus of aged mice; assess reversal learning and PNN composition after four weeks.
• Cell‑specific knockout: induce JNK deletion in microglia; longitudinal behavioral testing and two‑photon imaging of dendritic spine turnover.

Potential confounders and controls

• Off‑target effects of SP600125 controlled by using a second structurally distinct JNK inhibitor (AS602801) and measuring p‑JNK levels.
• Viral spread confined to astrocytes verified by co‑labeling with Aldh1l1; neuronal CHST11 overexpression excluded by using a Synapsin‑promoter control vector.
• Compensatory upregulation of other sulfotransferases monitored via qPCR panel for CHST family members.
• Ensure that amyloid‑β load remains similar across groups by ELISA to isolate plasticity effects from pathology.

If the epigenetic reprogramming of sulfotransferases is the critical link between chronic JNK/AP-1 signaling and PNN over‑consolidation, then correcting the enzyme balance should restore surprise‑driven learning without triggering neurodegeneration—directly testing the "over‑confident map" hypothesis of cognitive aging.