Hypothesis

Chronic elevation of neuronal lactate in aging brains promotes lactylation of extracellular endosulfatases (SULF1/2), increasing their enzymatic activity toward chondroitin sulfate chains in perineuronal nets (PNNs). This post‑translational modification shifts the C4S:C6S ratio toward higher C4S, reinforcing ECM structural memory and inhibiting axonal plasticity, thereby contributing to age‑related cognitive decline.

Mechanistic Rationale

• Lactate accumulates in the extracellular space with age due to altered microglial metabolism and reduced neuronal uptake [https://www.aging-us.com/article/101256/text].
• Lactate can directly modify lysine residues on proteins via lactylation, a process shown to regulate enzyme activity in other contexts [https://pubmed.ncbi.nlm.nih.gov/41608227/].
• Endosulfatases remove 6‑O‑sulfate groups from chondroitin sulfate; increased activity would lower C6S levels while leaving C4S unchanged, raising the C4S:C6S ratio observed in aged PNNs [https://pmc.ncbi.nlm.nih.gov/articles/PMC8724624/].
• Elevated C4S:C6S renders PNNs more inhibitory to axon growth, stabilising ECM structural memory that resists turnover [https://pmc.ncbi.nlm.nih.gov/articles/PMC11140655/].
• Reversing lactate‑induced lactylation should reduce endosulfatase activity, restore youthful C4S:C6S ratios, and weaken ECM memory, permitting enhanced plasticity.

Testable Predictions

1. Aged brain extracellular fluid will show higher lactate concentrations and increased lactylated SULF2 compared with young tissue.
2. Pharmacological inhibition of lactate production (e.g., with LDHA blockers) or scavenging of extracellular lactate will decrease SULF2 lactylation, lower C4S:C6S ratios in PNNs, and improve axon outgrowth in vitro.
3. Genetic mutation of lactylation sites on SULF2 (lysine to arginine) will mimic the youthful sulfation pattern despite aged lactate levels.
4. Culturing aged hippocampal slices in lactate‑reduced artificial cerebrospinal fluid will rescue PNN permissiveness for neurite extension, an effect blocked by exogenous lactylated SULF2.

Experimental Approach

• Measure lactate and lactylated SULF2 in microdialysates from young (3 mo) and aged (24 mo) rats using targeted LC‑MS/MS with lactate‑specific enrichment and anti‑lysine‑lactate immunoblotting [https://www.aging-us.com/article/101256/text]; [https://pubmed.ncbi.nlm.nih.gov/41608227/].
• Use CRISPR‑knock‑in rats expressing lactylation‑deficient SULF2 (K→R) and assess PNN C4S:C6S ratios via disaccharide‑specific LC‑MS/MS (UVPD) [https://pmc.ncbi.nlm.nih.gov/articles/PMC8724624/].
• Treat organotypic hippocampal slices from aged rats with lactate dehydrogenase inhibitor (GSK‑2837808A) or lactate oxidase, then quantify axon growth from transfected neurons and PNN integrity (WFA staining).
• Perform rescue experiments by adding recombinant lactylated SULF2 to lactate‑reduced slices to confirm specificity.

Potential Impact

If validated, this hypothesis links metabolic state to ECM glycomics through a direct post‑translational mechanism, offering a druggable axis (lactate‑SULF2 lactylation) to modulate ECM structural memory in neurodegeneration and brain aging. Success would justify developing lactate‑scavenging or lactylation‑blocking strategies as novel therapeutic approaches to restore youthful PNN sulfation and enhance axonal resilience.