Scientists at Gifu University have identified how a brain-specific enzyme, GnT-IX (also known as MGAT5B), modifies protein-linked sugar chains to enable the formation of complex glycans that are important for normal brain function. The findings, published in the Journal of Biological Chemistry on January 7, may help guide future research into brain disorders related to glycan abnormalities and support new therapeutic approaches.
O-mannose glycans are specialized sugar chains attached to proteins in the brain, playing key roles in neural structure and signaling. These glycans can form branches when a side chain is added, rather than growing only as linear strings. Disruptions in this branching process have been linked to neurological conditions such as demyelination and brain tumors.
"O-mannose glycans are uniquely branched in the brain by the enzyme GnT-IX, also known as MGAT5B," said Yasuhiko Kizuka, professor at Gifu University's Institute for Glyco-core Research (iGCORE) and lead author of the study.
"However, it remains unclear how GnT-IX recognizes O-mannose glycans or, critically, how branched O-mannose glycans are extended into more complex structures."
To address these questions, researchers compared a structural model of GnT-IX bound to its O-mannose substrate with the crystal structure of a similar enzyme. They found that an arginine amino acid at position 304 (R304) in GnT-IX is crucial for recognizing its substrate. Altering R304 caused GnT-IX to lose much of its ability to act selectively on O-mannose glycans. This highlighted a molecular feature essential for glycan branching specific to the brain.
The team then explored why branching is important by studying mouse brains lacking GnT-IX. They observed significantly reduced levels of keratan sulfate—a complex glycan vital for brain structure and function—suggesting that branching is necessary for efficient keratan sulfate production.
Further enzymatic experiments showed that enzymes involved in making keratan sulfate were much more active on branched O-mannose glycans than on unbranched ones. This suggests that branching by GnT-IX not only changes glycan structure but also creates a scaffold that helps other enzymes extend these molecules efficiently.
"Our results demonstrated that branching of O-mannose glycans promotes their extension," Kizuka said. "This is the first clear demonstration of a direct relationship between branching and extension of a particular glycan."
By detailing how these specialized sugar chains are built step by step in the brain, this research improves understanding of glycan biosynthesis and could aid studies into neurological disorders involving disrupted glycosylation.
The researchers plan further investigations into whether this principle applies more broadly across different types of glycan biosynthesis. Many enzymes responsible for extending glycans remain poorly understood regarding their preference for branched or linear structures.
"Our ultimate goal is to fully understand and manipulate the biosynthesis of complex and diverse glycan structures on proteins," Kizuka said.
