The main focus of the laboratory is the study of the hormonal regulation of glycogen metabolism. Dr. Brady and his lab have identified a novel PP1 regulatory subunit, termed PTG for Protein Targeting to Glycogen. This molecule binds to PP1 and glycogen, thus targeting the phosphatase to the glycogen particle. Additionally, PTG specifically binds to several PP1 substrates that are key enzymatic regulators of glycogen metabolism. Overexpression of PTG in cultured cells and intact animals causes the intracellular redistribution of PP1 and glycogen metabolizing enzymes, and a marked increase in glycogen stores. Conversely, overexpression of a dominant negative PTG molecule reduced endogenous glycogen levels by 90% in primary rat hepatocytes. hese results indicate that PTG acts as a molecular scaffold, assembling PP1 with specific substrate proteins, allowing for the efficient hormonal regulation of glycogen metabolism.

Dr. Brady is interested in the effects of modulating glycogen levels on insulin secretion from pancreatic -cells and insulin metabolic signaling. Initial efforts have been directed at adenoviral-mediated overexpression of wild type, super active and potential dominant negative PTG constructs in insulin secreting cell lines. The principal question is whether modulating the glycogen synthetic capacity of these cells will affect glucose-induced insulin secretion. Can glucose be shunted from glycolysis and ATP production to glycogen storage? If so, will PTG overexpression cause a right shift in the glucose dose response curve for insulin secretion? Conversely, what are the effects of disrupting glycogen metabolism on glucose sensing by the beta cell? Future plans include generation of transgenic mouse lines with beta cell specific expression of PTG constructs, and examination of islet differentiation and function in vivo.

A second major project in the lab is the examination of the interdependency of intracellular glycogen levels and insulin metabolic signaling in 3T3-L1 adipocytes. By using adenoviral vectors, several PTG constructs will be over expressed in these cells. The effects of elevation or depletion of cellular glycogen on insulin-regulated glucose uptake and storage will then be examined. Since lipid and glycogen are the principal forms of carbon storage in mammals, another priority is the adipose specific expression of PTG constructs in transgenic animals. Can adipocytes be made to shunt more stored energy from lipid to glycogen? What is the physiological role of glycogen metabolism in fat cells, especially with regards to secretion of a variety of factors such as leptin, resistin, TNF? and Acrp30? And most importantly, what are the ramifications of altering adipose tissue glycogen stores (up or down) on whole animal metabolism, diet induced obesity and insulin resistance?

Together, these studies may generate new insight into the hormonal regulation of glycogen synthesis and the potential role for altered glycogen metabolism in the development and progression of type I and II diabetes.