Hypothesis: Age‑Related Loss of 6‑Sulfated Dermatan Sulfate on Decorin Undermines Perineuronal Net Neuroprotection by Dysregulating Growth‑Factor Sequestration and Microglial Activation

Background

Recent work shows that with aging human skin shifts from versican‑rich chondroitin sulfate to decorin‑rich dermatan sulfate, and that adult decorin carries fewer 6‑sulfated disaccharides than fetal decorin[2]. Cardiac tissue analyses reveal heightened transcription of GAG synthesis and sulfation enzymes in aged tissue[3]. In the brain, perineuronal nets (PNNs) are stable ECM scaffolds that stabilize long‑term memory, promote fast‑spiking interneuron activity, and limit lipofuscin accumulation in Alzheimer’s disease[4]. However, the impact of age‑dependent sulfation changes on PNN function remains unexplored.

Mechanistic Insight

We propose that the specific loss of 6‑sulfation on decorin‑derived dermatan sulfate chains reduces the net’s capacity to bind basic fibroblast growth factor (bFGF) and heparin‑binding cytokines through electrostatic interactions. In young tissue, 6‑sulfate clusters create high‑affinity reservoirs that sequester bFGF, limiting its availability to microglia and astrocytes. With age, diminished 6‑sulfation weakens this reservoir, leading to elevated extracellular bFGF, enhanced microglial MAPK signaling, and a shift toward a pro‑inflammatory phenotype. The resulting oxidative stress overwhelms the protective capacity of PNNs, accelerating lipofuscin buildup and compromising memory‑stabilizing scaffolding.

Additionally, reduced 6‑sulfation may alter the susceptibility of dermatan sulfate to chondroitinase‑like enzymes secreted by microglia, thereby increasing net remodeling and further destabilizing PNNs.

Testable Predictions

• P1: Aged mice exhibiting lower decorin 6‑sulfation will show decreased bFGF binding in PNN fractions compared with young mice.
• P2: Restoring 6‑sulfation via neuronal‑specific overexpression of the sulfotransferase CHST3 (or CHST11) in aged mice will increase bFGF sequestration, reduce microglial activation markers (Iba1, CD68), lower lipofuscin accumulation, and improve performance on hippocampus‑dependent memory tasks.
• P3: Pharmacological blockade of bFGF signaling (using FGFR inhibitor PD173074) in aged wild‑type mice will phenocopy the protective effects of enhanced 6‑sulfation, rescuing PNN integrity and memory despite low sulfation.
• P4: In vitro, purified aged decorin dermatan sulfate will bind less bFGF than fetal decorin dermatan sulfate in surface plasmon resonance assays, and supplementation with synthetic 6‑sulfated disaccharide will restore binding affinity.

Experimental Approach

1. Biochemical quantification: Isolate PNNs from cortices of young (3 mo) and aged (24 mo) mice; perform disaccharide analysis via LC‑MS to quantify 6‑sulfated dermatan sulfate[2].
2. Binding assays: Use SPR or ELISA‑based pull‑down to measure bFGF affinity for PNN extracts.
3. Genetic manipulation: Deploy AAV‑Syn‑CHST3 vectors to overexpress the 6‑O‑sulfotransferase in excitatory neurons of aged mice; confirm overexpression and sulfation increase by immunostaining with 6‑sulfate‑specific antibody (CS‑56).
4. Cellular readouts: Quantify microglial activation (Iba1/CD68) and astrocytic reactivity (GFAP); measure lipofuscin via autofluorescence microscopy.
5. Behavioral testing: Conduct Morris water maze and fear conditioning to assess memory consolidation.
6. Pharmacological validation: Treat aged wild‑type mice with PD173074 and repeat steps 2‑5.

Potential Outcomes and Falsifiability

• If P1–P4 are supported—i.e., aged mice show reduced 6‑sulfation, diminished bFGF binding, and genetic or pharmacological restoration of 6‑sulfation/bFGF signaling rescues PNN integrity, microglial phenotype, lipofuscin load, and memory—the hypothesis gains mechanistic weight.
• Conversely, if enhancing 6‑sulfation fails to alter bFGF sequestration, microglial activation, lipofuscin, or memory despite confirmed biochemical rescue, the hypothesis would be falsified, indicating that other sulfation positions or ECM components dominate PNN aging effects.

This framework directly links a quantifiable ECM modification to a neuroimmune cascade, offering a falsifiable path to determine whether the “sulfation code” decorates perineuronal nets as a regulatory layer of brain aging.