HORMONAL CONTROL OF GLYCOGEN METABOLISM

Indiana Univ-Purdue Univ At Indianapolis, Indianapolis IN

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Abstract

DESCRIPTION(adapted from applicant's abstract): The goal is to understand the control of glycogen synthesis in mammalian cells and the relative roles of liver and muscle glycogen deposits in whole body glucose metabolism. Defects in glucose metabolism are associated with metabolic diseases, including diabetes and glycogen storage diseases. An important site of regulation is glycogen synthase which is controlled by several hormones (insulin, glucagon, and epinephrine). The enzyme undergoes complex multisite phosphorylation and activation by the key regulatory metabolite glucose-6-P. Other research addresses the initiation phase, which is mediated by a specialized initiator protein, glycogenin. Aim 1: G6P and the activation of glycogen synthase. The goal is to define residues involved in G6P activation. By the study of such mutants [sic] in vitro and in cultured cells, the interplay between phosphorylation and ligand binding in the control of glycogen synthesis will be explored. 2. Analysis of novel GS kinase. Work in the last funding period suggested that novel mechanisms exist for the phosphorylation of important COOH-terminal phosphorylation sites and we will pursue identification of a new protein kinase involved in this process. 3. Analysis of glycogenin and interacting proteins. We will continue study of muscle glycogenin to define regions of the protein involved in dimerization and in interacting with a newly defined glycogenin binding protein called GNIP1. We will characterize GNIP1 and also search for proteins that interact with a newly discovered liver form of glycogenin, glycogenin-2. 4. Role of glycogenin in the ocntrol of liver glycogen biosynthesis. We will continue characterization of glycogenin-2 in order to understand the role and importance of this new self-glucosylating species. We have found that overexpression in cells causes increased glycogen accumulation. If its expression is controlled by nutritional and hormonal factors, we would seek to understand the molecular basis. A serious question has risen as to whether the protein even exists in rats and mice, common models for the study of glucose metabolism. We will seek to resolve this issue. 5. Glycogen in mouse models of glucose homeostasis. In this aimn, we will develop genetically altered mice as experimental models to test the relative roles of the muscle and liver glycogen deposits in blood glucose homeostasis. We will also attempt to assess in animal models the roles of phosphorylation and G6P activation of GS in controlling glycogen synthesis