Brain glycogen serves as a critical glucosamine cache required for protein glycosylation

Glycosylation defects are a hallmark of many nervous system diseases. However, the molecular and metabolic basis for this pathology is not fully understood. In this study, we found that N-linked protein glycosylation in the brain is metabolically channeled to glucosamine metabolism through glycogenolysis. We discovered that glucosamine is an abundant constituent of brain glycogen, which functions as a glucosamine reservoir for multiple glycoconjugates. We demonstrated the enzymatic incorporation of glucosamine into glycogen by glycogen synthase, and the release by glycogen phosphorylase by biochemical and structural methodologies, in primary astrocytes, and in vivo by isotopic tracing and mass spectrometry. Using two mouse models of glycogen storage diseases, we showed that disruption of brain glycogen metabolism causes global decreases in free pools of UDP-N-acetylglucosamine and N-linked protein glycosylation. These findings revealed fundamental biological roles of brain glycogen in protein glycosylation with direct relevance to multiple human diseases of the central nervous system. [Display omitted] •Brain glycogen contains 25% glucosamine•A novel mass spectrometry imaging method reveals brain glycogen distributions•Glucosamine flux through glycogen is mediated by the enzymes GYS, GP, and GDE•Glycogen storage disease mutations impair protein glycosylation in the brain Sun et al. demonstrate that glycogen serves as a critical reservoir of glucosamine in the nervous system and map protein N-glycosylation patterns in the brain. In models of glycogen storage diseases, impaired flux of glucosamine through glycogen by the same enzymes that mediate flux of glucose is associated with reduced protein N-glycosylation.