Scientists at the University of Kentucky and colleagues used imaging techniques for studying brain metabolism to identify the reservoir for a necessary sugar in the brain.
The team discovered that glucose was not the only sugar contained in glycogen in the brain. Brain glycogen also contained glucosamine. The study (“Brain glycogen serves as a critical glucosamine cache required for protein glycosylation”) appears in Cell Metabolism.
“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,” write the investigators.
“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.”
Some forms of glucosamine, such as glucosamine sulfate and glucosamine hydrochloride, are common supplements used to improve joint movement. However, within cells, glucosamine is an essential sugar needed for the complex carbohydrate chains that are attached to proteins in a process called glycosylation. These sugar chains decorate proteins, and the sugar decorations are critical for the appropriate function of myriad proteins.
Key insight into neurological diseases
Discovering that glucosamine is a major component of brain glycogen provides key insight into neurological diseases caused by aberrant glycogen-like cellular aggregates called polyglucosan bodies (PGBs), according to the researchers. Lafora disease is a rare, inherited childhood dementia caused by PGBs and this study demonstrates that the Lafora disease PGBs sequester glucosamine, leading to numerous cellular perturbations. PGBs also accumulate in the brain as people age and in people with other forms of dementia.
Thus, the discovery that glycogen is also a storage cache for glucosamine has broad implications for understanding neurological changes associated with aging, noted Ramon Sun, PhD, assistant professor of neuroscience, Markey Cancer Center at the University of Kentucky College of Medicine.
Using biochemical approaches, the researchers determined the sugar composition of glycogen in the muscle, liver, and brain of mice. Unlike muscle glycogen, which had only 1% glucosamine, and liver glycogen, which had less than 1% glucosamine, brain glycogen contained 25% glucosamine. “The discovery that brain glycogen is comprised of 25% glucosamine was stunning,” said Sun.
Upon making this surprising discovery, they then identified the enzymes responsible for incorporating glucosamine into glycogen and for releasing glucosamine from glycogen. Again, the discovery was unexpected as these enzymes are the same ones used to incorporate glucose into and release glucose from glycogen.
To understand the implications of their findings for Lafora disease and neurological problems arising from PGBs, the researchers used their newly developed technique called matrix-assisted laser desorption/ionization traveling-wave ion-mobility high-resolution mass spectrometry (MALDI TW IMS) to measure and visualize the amount of glycogen in different regions of the brain. They also used this technique to quantify changes in the specific patterns of the sugar decorations on proteins in multiple regions of the brain.
The team applied MALDI TW IMS to analyze the brains of healthy mice and of two different mouse models of glycogen storage diseases: a model of Lafora disease and a model of glucose storage disease (GSD) type III.
“This new technique allows us to quantify the amount of these sugars with high accuracy while also maintaining the spatial distribution within the brain regarding where the sugars are located. It is crucial that the brain has the correct sugars in the right location within the brain,” explained Sun.
These studies revealed that without the ability to properly regulate brain glycogen metabolism, not only do PGBs form, which perturbs cell metabolism, but the sugar decoration of proteins is also altered. They could restore protein sugar decoration by injecting an antibody-enzyme fusion (VAL-0417) into the brains of Lafora disease mice to degrade the PGBs.
The findings, which show a direct connection between abnormal glycogen storage and defective protein function in the brain, have implications for many other GSDs and congenital disorders of glycosylation, which cause severe neurological symptoms, including epilepsy and dementia, pointed out Matthew Gentry, PhD, professor of molecular and cellular biochemistry and director of the Lafora Epilepsy Cure Initiative at the University of Kentucky College of Medicine.
“Multiple neurological diseases have blockades in these metabolic pathways. I’m sure these pathways are going to be important in other neuro-centric diseases as well. Brain glycogen is comprised of glucose and glucosamine and brain metabolism has to balance both in order to stay healthy,” explained Gentry.