The human brain is composed of over 100 billion highly specialized cells that regulate cognition, emotion, and behavior through intricate networks of molecular signaling. Within this sophisticated system, a biological modification known as glycosylation is increasingly emerging as a central focus of neuroscience research. Although glycans constitute less than 1% of the brain's total mass, their biological impact far exceeds their quantitative proportion.

In October 2025, a comprehensive review article published in Experimental & Molecular Medicine by the team of Hyun Joo An from Chungnam National University systematically summarized the latest advancements in the study of brain glycosylation. This article will guide you through the key findings of this burgeoning field.

What is Glycosylation? Glycan Modifications in the Brain

Glycosylation is a critical biosynthetic modification process catalyzed by hundreds of glycosidases and glycosyltransferases, resulting in the generation of a vast array of glycan molecules attached to proteins or lipids. In the brain, this process is essential for maintaining homeostasis; its specific functions include:

• Regulating protein folding and stability
• Mediating receptor trafficking
• Facilitating intercellular communication
• Guiding neural development and synaptic plasticity

In terms of molecular classification, brain glycosylation is broadly categorized into two main types: protein-linked glycosylation and lipid-linked glycosylation.

Fig. 1 Overview of glycosylation pathways in the brain. (Seo, et al. 2025)

Glycan Tags on Proteins: N-Glycans and O-Glycans

N-Glycosylation: A Complex Modification Initiated in the Endoplasmic Reticulum

N-glycosylation begins within the endoplasmic reticulum (ER). A precursor oligosaccharide is assembled onto a lipid carrier and subsequently transferred en bloc to specific asparagine residues on nascent proteins. This is followed by a series of trimming and monosaccharide-addition reactions within the Golgi apparatus, ultimately yielding a diverse array of N-glycan structures.

Key Functional Examples

• PSA-NCAM (Polysialylated Neural Cell Adhesion Molecule): Promotes the migration and differentiation of neural stem cells; highly expressed in the developing central nervous system, where it influences axon growth and the fasciculation (bundling) of neuronal fibers. In the adult brain, it is primarily expressed in the olfactory bulb, hippocampus, and hypothalamus, retaining neurogenic potential.
• Glutamate Receptor GluN1 Subunit: Removal of N-glycans alters glutamate sensitivity, indicating that N-glycosylation participates in the regulation of receptor trafficking and excitatory neurotransmission.
• TREM2 (Triggering Receptor Expressed on Myeloid Cells 2): As a transmembrane glycoprotein found in microglia, its N-glycosylation is critical for cell surface localization and intracellular signaling, and it plays a role in maintaining neuroimmune homeostasis.

O-Glycosylation: A Diverse Array of Modifications

O-glycosylation encompasses several distinct types:

• O-GlcNAc Modification: Occurs within the cell nucleus, where O-GlcNAc transferase attaches a single GlcNAc residue to serine or threonine residues. This modification is dynamically reversible, analogous to the role of phosphorylation in cellular signaling.
• Mucin-type O-Glycosylation (O-GalNAcylation): Initiated in the Golgi apparatus and catalyzed by members of the family of 20 polypeptide N-acetylgalactosaminyltransferases; these structures can be further extended to form complex, tissue-specific O-glycans.
• O-Mannosylation and O-Fucosylation: Commonly found on brain-associated proteins—such as components of the extracellular matrix, alpha-dystroglycan, and thrombospondins—and involved in neuronal development and signal transduction.

Alpha-dystroglycan (α-DG) serves as a quintessential example of O-glycosylation; the O-mannosyl glycans it bears mediate its binding to the extracellular matrix, thereby guiding neuronal migration and axon guidance during development. The binding of laminin to O-mannosylated α-DG plays a pivotal role in the formation of the basement membrane.

Glycans on Lipids: The Mysteries of Gangliosides

Gangliosides are the most abundant glycosphingolipids in the brain, composed of a ceramide core linked to a sialylated glycan chain. Their synthesis begins in the endoplasmic reticulum with the formation of ceramide, followed by the stepwise addition of sugar residues in the Golgi apparatus—catalyzed by specific glycosyltransferases—to generate a structurally diverse array of molecules, including numerous isomers.

Dynamic Transitions During Development

During brain development, the expression patterns of gangliosides undergo significant shifts:

• Embryonic Stage: Dominated by simple forms, such as GM3 and GD3.
• Adulthood: Transitions to complex forms, including GM1a, GD1a, GD1b, and GT1b.

In the adult brain, four major gangliosides—GM1, GD1a, GD1b, and GT1b—account for over 90% of the total ganglioside content.

Functions of Major Gangliosides

GM1: Neuroprotection and Synaptic Plasticity

• Highly enriched in neuronal membranes; interacts with various cell-surface receptors, including neurotrophin receptors, neurotransmitter receptors, and ion channels.
• Enhances neuroprotective signaling—thereby promoting neuronal survival and synaptic plasticity—by stabilizing the interaction between Trk receptors and neurotrophic factors.
• Regulates calcium ion dynamics through interactions with ion channels.

GD1a and GT1b: Myelin Stability

• Highly enriched in myelinated fibers within the adult mouse brain (e.g., the corpus callosum and corticospinal tract).
• Colocalize with myelin-associated glycoproteins and act as axonal receptors involved in stabilizing the myelin sheath.

GD3: Neurogenesis and Developmental Regulation

• Particularly critical during brain development, supporting the maintenance of neural stem cells, neurogenesis, myelination, and neuronal differentiation.
• Regulates neuronal apoptosis during developmental pruning, ensuring appropriate cellular turnover and survival.

Abnormal Glycan Chains in Disease

Neurodegenerative Diseases: Alzheimer's Disease and Parkinson's Disease

Fig. 2 Alterations in brain glycosylation in Alzheimer’s and Parkinson’s diseases. (Seo, et al. 2025)

Alzheimer's Disease (AD)

• Increased N-glycan sialylation in microglia, particularly in the vicinity of amyloid plaques; approximately 65% of microglia within plaque-containing regions exhibit α-2,6-linked sialylation.
• Global glycoprotein sialylation levels are reduced, particularly regarding highly branched and extended N-glycan chains.
• Site-specific hypersialylation occurs at select locations—such as the localized compensatory regulation observed in Clusterin (CLU).
• PSA-NCAM levels are typically reduced in the perirhinal cortex and hippocampus; however, expression increases in the dentate gyrus and CA1 region during moderate-to-severe AD, demonstrating dynamic changes that are specific to both region and disease stage.
• In cerebrospinal fluid (CSF), levels of biantennary and core-fucosylated N-glycans are elevated, while sialylation is reduced; this phenomenon may be associated with the overexpression of N-acetylglucosaminyltransferase III (MGAT3).
• Ganglioside composition undergoes a shift from GM1 toward GM3, with GM3 exhibiting significant spatial colocalization with amyloid deposits.

Parkinson's Disease (PD)

• Sialylation of striatal O-glycans is significantly increased, while sulfation is reduced; this may disrupt ligand recognition by lectins (glycan-binding proteins)—such as Sialic acid-binding immunoglobulin-type lectin 3 (Siglec-3) and Galectin-3—as well as impair the functions of complement components C1q and Factor H.
• Analysis of urinary N-glycans reveals a reduction in biantennary galactosylated and sialylated structures.
• Serum N-glycans exhibit increased core fucosylation, biantennary GlcNAc structures, and α2,6-linked sialylation.
• Aberrant ganglioside regulation is associated with mutations in the GBA gene, particularly in patients exhibiting pathology within the middle temporal gyrus, cingulate gyrus, and striatum. Multiple brain regions, among others, exhibit elevated levels of GM1, GM2, GM3, GD2, and GD3.

Psychiatric Disorders: Depression, Post-Traumatic Stress Disorder (PTSD), and Schizophrenia

Depression

• In models of both acute and chronic mild stress, hippocampal N-glycosylation undergoes alterations, characterized by a reduction in biantennary glycan chains.
• Chronic mild stress preferentially reduces multiantennary glycan chains, whereas acute stress primarily reduces high-mannose-type glycan chains; this suggests that distinct stress paradigms elicit different glycosylation responses.
• Increased O-GlcNAc modification in astrocytes within the medial prefrontal cortex can modulate glutamatergic transmission, thereby influencing stress susceptibility and depression-like behaviors.
• Levels of α2,6-sialylated glycan chains are reduced in both plasma and extracellular vesicles, accompanied by a decrease in signals corresponding to wheat germ agglutinin (WGA)-bound GlcNAc and sialic acid.

Post-Traumatic Stress Disorder (PTSD)

In stress-susceptible animals, core Fucosylation of biantennary glycan chains in the prefrontal cortex is increased, accompanied by a shift toward simpler, less sialylated glycan structures.

Schizophrenia

• In male patients, serum levels of polylactosaminylated glycan chains and SLex-containing glycan chains are elevated.
• Cerebrospinal fluid (CSF) exhibits reduced levels of biantennary and sialylated glycan chains, suggesting potential gender-specific glycosylation patterns.

Neurodevelopmental Disorders: Muscular Dystrophy and Attention Deficit Hyperactivity Disorder (ADHD)

Muscular Dystrophy-Dystroglycanopathy (MDDG)

• Defects in the O-mannosylation of α-dystroglycan (α-DG) lead to impaired laminin binding, thereby disrupting neuron–glia–extracellular matrix interactions.
• Mutations in the B3GALNT2 gene impair the formation of the core M3 structure, preventing the synthesis of functional matrix glycans (GlcA-Xyl repeats) and resulting in congenital muscular dystrophy with variable cerebral and ocular involvement.

Attention-Deficit/Hyperactivity Disorder (ADHD)

• Alterations in serum glycosylation include: increased antennary fucosylation, reduced bi-branching of bi-/tri-antennary N-glycans (specifically a reduction in bisecting GlcNAc), and decreased α2,3-sialylation.
• Studies using St3gal5-deficient mouse models demonstrate that impaired GM3 ganglioside synthesis leads to hyperactivity and anxiety-like behaviors, accompanied by compromised insulin receptor signaling and electroencephalogram (EEG) abnormalities.

Autism Spectrum Disorder (ASD)

• Reduced plasma Sialic Aacid Levels suggest potential defects in ganglioside biosynthesis.
• Elevated levels of anti-GM1 antibodies suggest an immune-mediated disruption of neuronal ganglioside function.

Technical Breakthrough: Deciphering the Brain's Glyco-Code

Traditional studies of brain glycosylation have relied on techniques such as gene knockout/knockdown models, Western blotting, and lectin arrays; while these methods have provided important insights, they struggle to capture the full complexity of the brain's glycome. In recent years, methodological breakthroughs have revolutionized this field:

• Mass Spectrometry: High-sensitivity, high-resolution mass spectrometric analysis—combined with liquid chromatography (LC-MS/MS)—enables the detailed characterization of glycan types, monosaccharide composition, linkage patterns, and isomeric variants.
• Glycan-Specific Enrichment Strategies: Improved sample preparation protocols facilitate the efficient extraction and enrichment of glycans.
• Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging (MALDI-MSI): This technique enables the spatial mapping of glycosylation patterns within brain tissue, integrating molecular information with histological context.
• Advanced Tandem Mass Spectrometry: Techniques such as stepped collision-induced dissociation (sCID) and electron transfer/high-energy collision dissociation (ETD/HCD) support the comprehensive structural characterization of N-glycans, O-glycans, and glycosphingolipids.

Future Outlook: From Basic Research to Clinical Application

Brain glycosylation research currently stands at the threshold of translational medicine. Future directions include:

Multi-omics Integration Strategies

Integrating Glycomics with genomics, transcriptomics, proteomics, and metabolomics to construct large-scale, glycomics-centric multi-omics platforms. This will support population-level studies—inspired by genome-wide association studies (GWAS)—to explore the biological significance of glycosylation heterogeneity and its associations with genetic background, physiological traits, and disease susceptibility.

AI-Driven Bioinformatics

The immense structural diversity and biosynthetic complexity of glycans remain major bottlenecks in data interpretation. Artificial intelligence (AI) and machine learning approaches are becoming indispensable tools for facilitating the analysis of high-dimensional glycomics datasets, enabling the integration of glycan profiles with transcriptomic, proteomic, and clinical data to unlock novel biological insights.

Construction of Brain Glycome Atlases and Databases

Establishing brain region-specific and cell-type-specific glycome atlases and robust databases—analogous to the Human Protein Atlas—will provide critical resources for both basic and translational research, while complementing global brain research initiatives such as the HUMAN Brain Project and the BRAIN Initiative Cell Census Network.

Therapeutic Prospects

• Diagnostic Biomarkers: Specific alterations in cerebrospinal fluid (CSF) N-glycans—such as bi-antennary and core fucosylation structures—have demonstrated potential as biomarkers for Alzheimer's disease; furthermore, specific GM3 and GQ3 ganglioside species possess high diagnostic value.
• Therapeutic Targets: The ganglioside GM1 has exhibited neuroprotective effects in models of ischemic stroke, reducing infarct volume and improving neurological functional recovery; moreover, regulatory strategies targeting glycosylation enzymes may offer novel therapeutic avenues for a wide range of brain disorders.
• Precision Medicine: Individualized therapeutic stratification based on glycosylation profiles—particularly in the field of psychiatric disorders—is gaining traction; notably, the largest-scale meta-analysis of genome-wide association studies (GWAS) for major depressive disorder has identified several glycosylation-related genetic loci associated with antidepressant response.

Conclusion

Research into brain glycosylation is transitioning from the exploration of molecular complexity toward the realization of therapeutic potential. Spanning the spectrum from neurodevelopment to aging, and from neurodegenerative diseases to psychiatric disorders, abnormalities in glycan modifications are intimately linked to a diverse array of brain diseases. Driven by advancements in analytical technologies, the accumulation of large-scale datasets, and the integration of artificial intelligence methodologies, we are progressively deciphering the "glyco-code" of the brain.

The evolution of this field not only deepens our understanding of brain function but also paves the way for the development of next-generation diagnostic tools and precision therapeutic strategies. In this era of integrated multi-omics and AI-driven biomedical research, brain glycosylation holds immense promise as a transformative frontier in neuroscience, ultimately offering new hope to millions of patients worldwide suffering from brain disorders.

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Reference

1. Seo, Y., et al. (2025). The emerging landscape of brain glycosylation: from molecular complexity to therapeutic potential. Experimental & Molecular Medicine, 1-12. DOI: 1038/s12276-025-01560-8.