Author ORCID Identifier
Year of Publication
Doctor of Philosophy (PhD)
Molecular and Cellular Biochemistry
Dr. Matthew S. Gentry
Dr. Tianyan Gao
Glycogen is a carbohydrate molecule that is traditionally viewed as a convenient and easily accessible energy storage form of glucose. However,emerging evidence supports the role of glycogen as more than a glucose storage form. During the last 20 years, glycogen has been shown to play pivotal roles in learning and memory, signaling events, viscosity, protein glycosylation, and be acritical hallmark in devastating diseases. Not only, does glycogen play a role as an energy substrate and critical metabolite during energy deprivation, glycogen is central for neurotransmitter homeostasis, tumor initiation, and ontributes to proper protein glycosylation in the brain. In this work, the crucial role of glycogen homeostasis in the healthy and diseased brain is elucidated with a focus on our emerging understanding of glycogen as a critical metabolite. These aspects will be discussed concerning our understanding of diseases with a focus on Glycogen Storage Diseases (GSDs), utilizing Lafora disease (LD) and Glucose transporter 1 deficiency syndrome (G1D) as model systems. Although these diseases have different genetic causes, they share several clinical and biochemical characteristics, including perturbed glycogen metabolism. Utilizing LD as a primary disease, we elucidated glycogen’sinfluence on neurotransmitter metabolism, the metabolic profile as a biomarker, and brain glycosylation. LD is classified as a glycogen storage disease and progressive myoclonus epilepsy caused by mutations in the genes EPM2A and EMP2B, encoding the glycogen phosphatase, laforin, and the E3 ubiquitin ligase, malin. Although laforin and malin are structurally and functionally different proteins, they are both involved in glycogen metabolism. Importantly, mutations in either gene lead to this devasting and fatal disease clinically presenting with increasing and worsening treatment-resistant epileptic seizure, loss of muscular control, dementia, and severe cognitive decline. The first symptoms generally start in the second decade of individuals who appears to develop normally, and the disease eventually leads to a vegetative state followed by death typically around 11 years after disease onset. A key hallmark of LD is the formation of insoluble glycogen-like aggregates known as Lafora bodies (LBs). LBs form in multiple tissues and several laboratories, using various mouse models, have demonstrated that LBs drive disease progression. However, the mechanism of disease is poorly understood, and there is a lack of robust biomarkers. To that end, we aimed to determine the metabolic phenotype of LD mouse and human samples by mass spectrometry analysis. The results revealed complex disturbances of central carbon metabolism in LD brain with perturbed oxidative glucose metabolism in both brain slices and primary cultures of neurons and astrocytes. Results from LD patient CSF samples indicate perturbations linked to neurotransmitter and glycogen metabolism as well as the hexosamine pathway.The distinct metabolic profile and perturbations in the hexosamine pathway suggest a connection to other carbohydrate pathways, including glycosylation.Indeed, we recently demonstrated that brain glycogen plays a key role in brain glycosylation, a posttranslational modification, and LBs disrupt this homeostasis.
To further elucidate this aspect of glycogen metabolism, we here characterized glycogen and glycosylation in an LD mouse model, utilizing cutting-edge techniques such as Matrix-Assisted Laser Desorption Ionization Mass Spectrometry Imaging and Gas Chromatography-Mass Spectrometry.Furthermore, we determined how sustained oral administration of glucosamine, a building block for glycosylation and brain glycogen, impacts glycogen metabolism, glycosylation, and behavior in mice. Overall, oral supplementation of glucosamine positively affects the biochemical characteristics of the disease and impacts mouse behavior. Thus, we provide evidence of a crucial connection between perturbed glycogen metabolism and glycosylation in LD.Collectively, this work demonstrates divergent roles of glycogen as more than a glucose cache. We show that glycogen perturbations affect protein glycosylation and the potential value of a metabolic profile as a biomarker for GSDs. Future work will continue to elucidate our understanding of glycogen in health and disease.
Digital Object Identifier (DOI)
Research reported in this dissertation was supported by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health under Award Numbers R35 NS116824 (MSG) and P01 NS097197 (M.S.G).
Markussen, Kia H., "BRAIN GLYCOGEN – BEYOND ENERGY STORAGE IN GLYCOGEN STORAGE DISEASES" (2023). Theses and Dissertations--Molecular and Cellular Biochemistry. 65.
Available for download on Wednesday, May 14, 2025