Mechanism of glycan extension in the brain elucidated by Gifu University and collaborators

Professor Yasuhiko Kizuka, Graduate Student Tomoya Itoh, and their colleagues from the Institute for Glyco-core Research (iGCORE) at Gifu University, in collaboration with the University of Mississippi in the US, the University of Osaka, and the Tokyo Metropolitan Institute for Geriatrics and Gerontology, announced that they have elucidated the mechanism by which O-mannose (Man) glycans extend in the brain. This discovery is expected to contribute to understanding the mechanisms of glycan biosynthesis in the brain and the pathology of demyelinating diseases. The results were published in The Journal of Biological Chemistry on January 7.

Glycans are structures in which sugars such as glucose link together in a chain and branch, and they typically exist in a state bound to proteins or lipids. Disease-specific changes in glycans are used for cancer diagnosis and other applications. Elucidating the mechanisms that bring about changes in glycans is considered important.

Glycans attached to proteins are produced through the action of glycosyltransferases within cells, and humans have approximately 180 types of glycosyltransferases.

GnT-IX (MGAT5B) is a glycosyltransferase that exists specifically in the brain and acts on glycans called O-mannose glycans attached to proteins to create branched structures. Abnormalities in its formation are known to cause muscular dystrophy.

It has been reported that mice lacking GnT-IX recover more quickly from demyelinating diseases, and that glioma, a type of brain tumor, shows suppressed growth in cancer cells lacking GnT-IX. However, the mechanisms and significance of glycosyltransferases in general are not well understood.

Therefore, in this study, the researchers used the brains of GnT-IX knockout mice to investigate the glycan structures that extend from O-Man glycans. They found that keratan sulfate, a structure that exists at the terminus of extended glycans, was significantly reduced in knockout mice. It was revealed that the branching of O-Man glycans is necessary for keratan sulfate to be produced normally. Keratan sulfate is a long glycan with repeating structures that is abundant in the brain and cornea, exists only on glycans of specific proteins, and is involved in nerve axon regeneration and neurodegenerative diseases.

Therefore, the researchers purified six types of biosynthetic enzymes that form keratan sulfate and conducted reactions using linear and branched O-Man glycans as substrates to examine whether enzyme activity differs depending on the presence or absence of branching. As a result, the enzymes showed markedly higher activity toward branched chains than toward linear chains.

The reduction in keratan sulfate in the brains of GnT-IX knockout mice was thought to be due to the loss of O-Man glycan branching, which made it more difficult for keratan sulfate enzymes to function.

Kizuka commented: "The shapes of glycans attached to proteins are complex, and the brain in particular has many specially shaped glycans, with many mysteries surrounding how they are produced. This time, we discovered the mechanism by which glycans involved in demyelination and nerve damage extend. To investigate the activity of enzymes that extend glycans, experiments collecting many different forms of glycans were necessary, but through the diligent efforts of our graduate students, we obtained clear results. In the future, if we can understand the mechanisms of many enzymes that produce glycans, I expect this will lead to freely modifying glycans and elucidating the mechanisms by which glycans change in diseases."

Journal Information
Publication: The Journal of Biological Chemistry
Title: Enzymatic basis of branching and extension of O-Man glycans for keratan sulfate biosynthesis
DOI: 10.1016/j.jbc.2026.111140