Methods for Studying Glycogen Metabolism

ARTICLES DISCUSSED:
Wilson, W. A. Spectrophotometric methods for the study of eukaryotic glycogen metabolism. Journal of Visualized Experiments. (174), ...

radiochemical-assessment-glycogen-synthase-enzyme-activity-animal

Radiochemical Assessment of Glycogen Synthase Enzyme Activity in Animal Tissue

Glycogen is an important metabolite to many eukaryotic cells, integral to energy storage and quick energy release. The building of the molecule is ...

Indiana University School of Medicine-Muncie and Ball State University

Brain glycogen is proposed to function under both physiological and pathological conditions. Pharmacological elevation of this glucose polymer in brain is hypothesized to protect neurons against hypoglycemia-induced cell death. Elevation of brain glycogen levels due to prior hypoglycemia is postulated to contribute to the development of hypoglycemia-associated autonomic failure (HAAF) in insulin-treated diabetic patients. This latter mode of elevating glycogen levels is termed "supercompensation." We tested whether brain glycogen supercompensation occurs in healthy, conscious mice after recovery from insulin-induced acute or recurrent hypoglycemia. Blood glucose levels were lowered to less than 2.2 mmol/liter for 90 min by administration of insulin. Brain glucose levels decreased at least 80% and brain glycogen levels decreased approximately 50% after episodes of either acute or recurrent hypoglycemia. After these hypoglycemic episodes, mice were allowed access to food for 6 or 27 hr. After 6 hr, blood and brain glucose levels were restored but brain glycogen levels were elevated by 25% in mice that had been subjected to either acute or recurrent hypoglycemia compared with saline-treated controls. After a 27-hr recovery period, the concentration of brain glycogen had returned to baseline levels in mice previously subjected to either acute or recurrent hypoglycemia. We conclude that brain glycogen supercompensation occurs in healthy mice, but its functional significance remains to be established.

Brain glycogen supercompensation in the mouse after recovery from insulin-induced hypoglycemia.

Brain glycogen synthesis is a regulated, multi-step process that begins with glucose transport across the blood brain barrier and culminates with the actions of glycogen synthase and the glycogen branching enzyme to elongate glucose chains and introduce branch points in a growing glycogen molecule. This review focuses on the synthesis of glycogen in the brain, with an emphasis on glycogen synthase, but draws on salient studies in mammalian muscle and liver as well as baker's yeast, with the goal of providing a more comprehensive view of glycogen synthesis and highlighting potential areas for further study in the brain. In addition, deficiencies in the glycogen biosynthetic enzymes which lead to glycogen storage diseases in humans are discussed, highlighting effects on the brain and discussing findings in genetically modified animal models that recapitulate these diseases. Finally, implications of glycogen synthesis in neurodegenerative and other diseases that impact the brain are presented.

Structure and Regulation of Glycogen Synthase in the Brain.

Browning induction or transplantation of brown adipose tissue (BAT) or brown/beige adipocytes derived from progenitor or induced pluripotent stem cells (iPSCs) can represent a powerful strategy to treat metabolic diseases. However, our poor understanding of the mechanisms that govern the differentiation and activation of brown adipocytes limits the development of such therapy. Various genetic factors controlling the differentiation of brown adipocytes have been identified, although most studies have been performed using in vitro cultured pre-adipocytes. We investigate here the differentiation of brown adipocytes from adipose progenitors in the mouse embryo. We demonstrate that the formation of multiple lipid droplets (LDs) is initiated within clusters of glycogen, which is degraded through glycophagy to provide the metabolic substrates essential for de novo lipogenesis and LD formation. Therefore, this study uncovers the role of glycogen in the generation of LDs.

Glycogen Dynamics Drives Lipid Droplet Biogenesis during Brown Adipocyte Differentiation.

Pompe disease is caused by acid alpha-glucosidase (GAA) deficiency, resulting in lysosomal glycogen accumulation. This disease is characterized by progressive skeletal muscle weakness, respiratory distress, and in the infantile-onset form, cardiomyopathy. The only approved treatment is enzyme replacement therapy (ERT) with human recombinant GAA. While ERT therapy extends life span, residual symptoms remain, with poor muscle uptake and immunogenicity limiting efficacy. We examined a novel Centyrin protein-short interfering ribonucleic acid (siRNA) conjugate targeting CD71 (transferrin receptor type 1, TfR1) and GYS1, a key enzyme involved in glycogen synthesis. Unlike existing ERTs designed to degrade aberrant glycogen deposits observed in Pompe patients, the CD71 Centyrin:Gys1 siRNA is designed to restore glycogen balance by inhibiting glycogen synthesis. To this end, we administered the CD71 Centyrin:Gys1 siRNA conjugate to the 6/6 Pompe mouse model. Once bound to TfR1, siRNA-conjugated Centyrin is internalized into cells to facilitate gene knockdown. We found that treatment with this conjugate significantly reduced GYS1 protein expression, glycogen synthase enzymatic activity, and glycogen levels in muscle. In addition, impaired treadmill exercise performance of male Pompe mice was improved. These data suggest that Centyrin-mediated delivery of Gys1 siRNA may be an effective next generation therapy for late-onset Pompe disease or, in combination with ERT, for infantile-onset Pompe disease.

ArticleTotal : 2	Year
Publication title	2023
Publication title	2025

Article	Year
Brain glycogen supercompensation in the mouse after recovery from insulin-induced hypoglycemia. Journal of neuroscience research\| PubMed ID: 21259334	2011
Structure and Regulation of Glycogen Synthase in the Brain. Advances in neurobiology\| PubMed ID: 31667806	2019
Glycogen Dynamics Drives Lipid Droplet Biogenesis during Brown Adipocyte Differentiation. Cell reports\| PubMed ID: 31693883	2019
A novel CD71 Centyrin:Gys1 siRNA conjugate reduces glycogen synthesis and glycogen levels in a mouse model of Pompe disease. Molecular therapy : the journal of the American Society of Gene Therapy\| PubMed ID: 39604266	2025

Bartholomew A. Pederson

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