How Glycocalyx Dysfunction Affects the Blood-Brain Barrier in Aging and Disease
The inner lining of blood vessels in our brain is covered by a dense network of Glycan Chains called the glycocalyx. Like a sugary protective umbrella for the blood vessels, it serves as the first physical and chemical line of defense for the blood-brain barrier against harmful substances in the blood. However, what happens to this crucial defense mechanism during aging and disease? And what does this mean for brain health?
On February 26, 2025, a study published in Nature by a team from Stanford University led by Tony Wyss-Coray and Carolyn R. Bertozzi, titled "Glycocalyx dysregulation impairs blood–brain barrier in ageing and disease" provided answers. The scientists systematically mapped the compositional and structural changes of the brain endothelial glycocalyx during aging, discovering a significant downregulation of a specific type of mucin-like glycoprotein and its O-Glycosylation Modifications. Even more remarkably, the study demonstrated that restoring the synthesis of these key glycan chains through gene therapy effectively repaired the damaged blood-brain barrier in aged mice, reduced neuroinflammation, and even improved cognitive function. This finding opens up entirely new therapeutic avenues for understanding and intervening in blood-brain barrier dysfunction associated with aging and neurodegenerative diseases.
The research team first used lanthanum nitrate staining and electron microscopy to visually observe the glycocalyx layer of cortical capillaries in young (3-month-old) and aged (21-month-old) mice.
The results were surprising: compared to young mice, the thickness and area of the glycocalyx layer on the inner wall of blood vessels in aged mice were significantly reduced. This indicates that aging is accompanied by structural degradation of the protective sugar coating on brain blood vessels.
To explore the molecular basis behind this structural change, the researchers analyzed the transcriptome data of brain endothelial cells. They found that the expression of many glycosylation-related genes was disrupted during aging. Specifically, the expression of enzyme genes responsible for the synthesis of mucin-type O-glycans (especially core 1 structures), such as C1GALT1 and B3GNT3, was significantly downregulated. This suggests that a reduction in mucin-type O-glycosylation may be a key reason for glycocalyx degradation.
Fig. 1 The BBB and brain endothelial glycocalyx layer. (Shi, et al. 2025)
Mucin-type O-glycosylation primarily modifies a class of proteins with dense serine/threonine sequences, forming glycosylated mucin domains. These proteins have previously been studied mainly in mucosal epithelium, but they are also widely expressed in brain vascular endothelial cells.
How to specifically study these Glycoproteins? The research team used an ingenious tool: a modified, enzymatically inactive bacterial mucinase, StcE(E447D). Like a probe, it binds to mucin domains with high specificity, allowing for the visualization and labeling of these glycoproteins.
Using this tool, they found that the mucin domain glycoprotein signal in the brain vascular endothelium of aged mice was significantly weaker than in young mice. Furthermore, intravenous injection of active StcE enzyme to degrade these glycoproteins quickly led to blood-brain barrier leakage in young mice, even causing cerebral hemorrhage in severe cases. This directly demonstrates that these glycoproteins are crucial structural components for maintaining the integrity of the blood-brain barrier.
Fig. 2 Mucin-type O-glycan biosynthetic pathway. (Shi, et al. 2025)
Is this change universal? Researchers turned their attention to patient samples and models of Alzheimer's disease and Huntington's disease.
Single-cell nuclear RNA sequencing analysis showed that the mucin-type O-glycosylation biosynthesis pathway was also significantly downregulated in brain endothelial cells of patients with these neurodegenerative diseases, highly similar to the aging state. A reduction in C1GALT1 protein and mucin domain glycoprotein signals was also directly observed in the microvasculature of Alzheimer's disease patients' brains.
This suggests that the downregulation of mucin O-glycosylation may be a common molecular marker of blood-brain barrier dysfunction in various aging-related brain diseases.
To confirm the causal relationship, the research team conducted precise in vivo functional experiments.
They used brain endothelial cell-specific adeno-associated viral vectors to knock down the expression of the key Glycosyltransferase C1galt1 in young mice. As a result, the mucin signaling in the brain endothelial cells of these mice was weakened, and the permeability of the blood-brain barrier to small tracers significantly increased, showing leakage points similar to those in aged mice.
Conversely, they performed rescue experiments in aged mice. Overexpression of C1GALT1 or B3GNT3 in brain endothelial cells via viral vectors successfully restored the mucin glycan chain signaling in the vascular wall. Excitingly, this manipulation significantly reduced blood-brain barrier leakage in aged mice, bringing their barrier function closer to that of young mice.
Does repairing the glycocalyx of the blood-brain barrier merely repair the barrier itself? Research provides a more in-depth answer.
In aged mice overexpressing B3GNT3, researchers observed improved cognitive function. These mice showed better spatial working memory in the Y-maze test and stronger hippocampus-dependent learning and memory abilities in the contextual fear conditioning test.
To understand the underlying cellular and molecular changes, single-cell nuclear RNA sequencing was performed. Analysis revealed that B3GNT3 Overexpression induced a shift in gene expression towards a younger state in multiple brain cell types, for example, enhancing neuronal homeostasis-related pathways while suppressing disease-related activation states in microglia and astrocytes. Immunofluorescence staining also confirmed a significant reduction in CD68, a marker of activated microglia, in the brains of treated aged mice.
These results indicate that repairing the vascular glycocalyx not only strengthens the physical barrier but also creates a microenvironment more conducive to neuronal health by reducing the infiltration of harmful blood components, thereby improving overall brain homeostasis and function.
How exactly does glycocalyx damage lead to blood-brain barrier dysfunction? Research has revealed several potential pathways:
These findings collectively paint a picture where mucin glycans not only provide physical protection and an electrical charge barrier but are also essential for maintaining endothelial cell homeostasis, intercellular connections, and normal signal transduction. Its absence triggers a chain reaction that ultimately dismantles the blood-brain barrier.
This study is groundbreaking:
Of course, many questions remain unanswered, such as which specific mucin domain glycoproteins play a core role? What is their precise protective mechanism? In the future, further deciphering the functions of various Glycoconjugates in the glycocalyx will open a promising new window for our deeper understanding of brain aging and resistance to neurodegenerative diseases.
Reference
