Glycogen is a branched storage polymer of glucose that serves as an energy reserve in many cell types. In absolute terms, liver and skeletal muscle house the largest glycogen deposits in mammals and are critical to whole body glucose metabolism. The overall goal of this proposal is to improve understanding of the mechanism and control of glycogen synthesis, its relevance to whole body glucose metabolism and its impairment in metabolic diseases like diabetes and certain glycogen storage diseases. An important site of regulation is glycogen synthase, a key glycogen biosynthetic enzyme, which is controlled by several hormones, including insulin and epinephrine as well as by exercise.
Aim (i) Control of glycogen synthase during exercise. Exercise has complex and poorly understood effects on the activity of glycogen synthase. Exhaustive exercise of mice led to a stable inactivation of muscle glycogen synthase by a mechanism not explained by known covalent control of the enzyme by phosphorylation.
This aim would attempt to determine the nature of this potentially novel control mechanism.
Aim (ii) Novel potential regulators of glycogen synthase activity. During the last funding period, two novel potential regulators of glycogen synthase emerged. First, a dual specificity phosphatase, BEDP, was discovered that activates glycogen synthase when co-expressed in cultured cells. The mechanism for this activation and the identity of responsible BEDP substrate(s) will be sought. Second, a protein kinase, PAS kinase, whose yeast orthologs phosphorylate yeast glycogen synthase, was found to be also a potent inactivator of the mammalian enzyme.
The aim will attempt to establish whether the kinase has a role in vivo.
Aim (iii) Glycogen in mouse models of glucose homeostasis. Mice with the muscle glycogen synthase gene (GYS1) disrupted have improved glucose tolerance and elevated serum insulin is more sustained than in wild type mice during a glucose tolerance test. Whether there is altered islet function in these animals will be one focus of the aim. It will also address the relative importance of the liver and muscle glycogen stores for whole body glucose metabolism through other mouse models, including mice lacking liver glycogen (also a model for glycogen storage disease type 0) or with tissue-specific ablation of the GYS1 gene.
Aim (iii) Glycogen branching, laforin and Lafora disease. Laforin is a dual specificity protein phosphatase that contains a glycogen binding domain. Mutations in the EPM2A gene, encoding laforin, cause Lafora disease, a form of epilepsy in which poorly branched glycogen deposits, Lafora bodies, accumulate in neurons and other tissues. The goal is to understand how defects in this glycogen associated phosphatase cause the accumulation of poorly branched glycogen.
Aim (iv) UDP-glucose pyrophosphatase (UGPPase). This recently discovered enzyme hydrolyzes the substrate for glycogen synthase, UDP-glucose, to glucose-1-P and UMP. Its action could therefore profoundly affect glycogen metabolism. The goal of this aim is to establish whether UGPPase influences UDP-glucose and glycogen levels in vivo. ? ?
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