Not just sweet: the sugar branches that shape the brain

Gifu University scientists have uncovered how a brain-specific enzyme reshapes protein-linked sugar chains to facilitate the formation of complex glycans essential for normal brain function. These insights could inform future research into glycan-related brain disorders and open new avenues for therapeutic investigation.

The study was published in the Journal of Biological Chemistry on Jan. 7.

Proteins in the brain are modified by O-mannose glycans, specialized sugar chains that play key roles in neural structure and signaling. Rather than growing only as one long string, these glycans can also form branches, created when a sugar side chain is added to an existing chain. Disruptions in this branching process have previously been linked to neurological conditions such as demyelination (in which nerve insulation is damaged) 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.”

Seeking an answer, the team compared a structural model of GnT-IX bound to its O-mannose substrate with the crystal structure of a closely related enzyme. The results revealed the arginine amino acid at position 304 (R304) in the GnT-IX protein as being critical for substrate recognition. When R304 was altered, GnT-IX largely lost its ability to selectively act on O-mannose glycans. This discovery pinpointed a key molecular determinant of brain-specific glycan branching.

The researchers then turned to the question of why branching matters. Using mouse brains that lack GnT-IX, they found that levels of keratan sulfate, a complex glycan vital for brain structure and function, were significantly reduced. This indicated that branching of O-mannose glycans is required for efficient keratan sulfate formation.

Further enzymatic tests explained why. Enzymes involved in keratan sulfate biosynthesis were far more active on branched O-mannose glycans than on linear, unbranched ones. In effect, branching by GnT-IX is not just a structural modification; it creates a molecular scaffold that allows other enzymes to extend the glycan 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 clarifying how brain-specific O-mannose glycans are built step by step, the study advances fundamental knowledge of glycan biosynthesis and facilitates future research into neurological disorders associated with disrupted glycosylation.

The team plans to investigate whether this principle applies more broadly across glycan biosynthesis. Many enzymes involved in glycan extension remain poorly understood, particularly in terms of whether they prefer branched structures or function independently of them.

“Our ultimate goal is to fully understand and manipulate the biosynthesis of complex and diverse glycan structures on proteins,” Kizuka said.

Funding 

• FOREST program no. JPMJFR215Z from the Japan Science and Technology Agency (JST),
• Grant-in-Aid for Scientific Research (B) no. 24K02222 from the Japan Society for the Promotion of Science (JSPS),
• AMED-CREST grant no. JP23gm1410011 from the Japan Agency for Medical Research and Development (AMED),
• Human Glycome Atlas Project (HGA) from the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT).