Hypothesis

Age‑dependent rise in the C4S/C6S ratio within brain perineuronal nets (PNNs) is not caused by altered sulfotransferase expression but by myeloperoxidase (MPO)–mediated oxidation of existing 6‑O‑sulfonated chondroitin sulfate chains. This oxidative conversion generates 4‑O‑sulfonated disaccharides, increasing the C4S/C6S ratio and enhancing the inhibitory capacity of PNNs on axon growth.

Mechanistic Reasoning

MPO, secreted by activated microglia and infiltrating neutrophils in the aging brain, produces hypochlorous acid (HOCl). HOCl can electrophilically attack sulfate esters, leading to transient desulfation and subsequent spontaneous resulfation at the adjacent 4‑O position via a sulfoxide intermediate. This post‑synthetic shift would occur without changes in the transcriptional levels of CHST or sulfatase genes, consistent with the observed dissociation between sulfation pattern and sulfotransferase expression in aged PNNs [Brain ageing changes proteoglycan sulfation]. The resulting enrichment of C4S motifs strengthens electrostatic interactions with cationic axon‑guidance molecules, thereby amplifying the growth‑inhibitory signal noted in aged perineuronal nets [Brain ageing changes proteoglycan sulfation].

Testable Predictions

1. MPO activity correlates with C4S/C6S ratio – In rodent brain tissue across ages, MPO activity (measured by Amplex Red assay) will positively correlate with the C4S/C6S ratio determined by LC‑MS disaccharide analysis.
2. MPO inhibition prevents sulfation shift – Chronic treatment with the MPO inhibitor 4‑amino‑benzoic acid hydrazide (ABAH) or a neutrophil‑specific MPO knockout will attenuate the age‑related increase in C4S/C6S ratio compared with vehicle‑treated controls.
3. Rescue of axonal outgrowth – Ex vivo organotypic slice cultures from aged mice treated with ABAH will show reduced inhibition of neurite extension from dorsal root ganglion explants, quantified by increased average neurite length relative to untreated aged slices.
4. Behavioral relevance – Mice receiving long‑term MPO inhibition will perform better in hippocampal‑dependent learning tasks (e.g., Morris water maze) than age‑matched controls, linking the biochemical change to functional outcome.

Experimental Approach

• Subjects: Male C57BL/6 mice at 3, 12, and 18 months of age; include MPO‑KO and wild‑type cohorts.
• Assays:
  • MPO activity via fluorometric substrate.
  • PNN isolation followed by chondroitin sulfate extraction; disaccharide profiling by LC‑MS/MS to quantify C4S and C6S.
  • Immunohistochemistry for WFA‑positive PNNs and neurofilament‑positive axons.
  • Neurite outgrowth assay using organotypic hippocampal slices and fluorescently labeled DRG neurons.
  • Cognitive testing in Morris water maze.
• Controls: Vehicle-treated wild‑type, IgG‑treated MPO‑KO, and a rescue group where exogenous C6S‑rich chondroitin sulfate is supplied to test specificity.

Falsifiability

If MPO inhibition fails to alter the C4S/C6S ratio in aged PNNs, or if axonal growth inhibition persists despite normalization of sulfation patterns, the hypothesis would be refuted. Conversely, a consistent correlation between MPO activity, C4S/C6S elevation, and neurite inhibition—reversible by MPO blockade—would support the proposed oxidative remodeling mechanism.

Broader Implications

Demonstrating an enzyme‑driven, post‑synthetic mechanism for ECM sulfation shifts would expand the concept of "structural memory" beyond transcriptional regulation, offering a druggable target to modulate pericellular matrix properties in neurodegeneration and possibly other tissues where similar sulfation inversions occur (e.g., intervertebral disc, cartilage) [Ageing affects chondroitin sulfates and their synthetic enzymes].