Genome wide association (GWA) studies have linked the G6PC2 gene to variations in fasting blood glucose (FBG) and hemoglobin A1C levels in humans, parameters that are associated with both the risk of type 2 diabetes and cardiovascular-associated mortality. The overall objective of this application is to build on the results of these GWA studies by determining the function of G6PC2. Our preliminary data show that the human G6PC2 gene is selectively expressed in pancreatic islet beta cells and that G6PC2 hydrolyzes glucose-6-phosphate (G6P). Based on these data our first hypothesis is that the glucose-6-phosphatase activity of G6PC2 opposes the action of glucokinase (GCK), which catalyses the conversion of glucose to G6P. Glycolytic flux has been shown to determine the S {0.5} of glucose-stimulated insulin secretion (GSIS) and the existing paradigm in the islet field proposes that GCK alone is the beta cell glucose sensor. The significance of our observations is that they challenge this paradigm and suggest that G6PC2 is a fundamental inhibitory component of that sensor. Instead we propose that a GCK/G6PC2 futile cycle acts as the beta cell glucose sensor determining glycolytic flux and the S{0.5} of GSIS. Additional preliminary data show that the mouse G6pc2 gene is also selectively expressed in pancreatic islet beta cells and that G6pc2 also hydrolyzes G6P. This suggests that the use of G6pc2 knockout (KO) mice represents an innovative and appropriate tool to study the function of G6PC2. Deletion of the mouse G6pc2 gene results in reduced FBG levels, consistent with the human GW A study data. But in addition we have found that deletion of the G6pc2 gene also results in exercise intolerance, characterized by hypoglycemia and inappropriately high GSIS. Based on these data our second hypothesis is that the GCK/G6PC2 futile cycle is physiologically important for the attenuation of insulin secretion during exercise. Neural inputs to the islet are activated during exercise and the existing paradigm in the islet field proposes that these inputs inhibit insulin secretion by hyperpolarizing the beta cell and also directly inhibiting the exocytotic machinery. The significance of our observations is that they challenge this paradigm and suggest that G6PC2 is a fundamental component of the machinery through which GSIS is inhibited during exercise. As with the control of FBG and hemoglobin AIC, this topic is also clinically important because exercise induced hypoglycemia is a major problem in individuals with diabetes that limits the duration and hence the beneficial effects of exercise. The goal of this proposal is to test our two hypotheses. The application is divided into two matching Specific Aims.
Aim I explores the function of G6pc2 at a molecular level whereas Aim 2 explores the physiological importance of the Gck/G6pc2 futile cycle for the attenuation of insulin secretion during exercise.
Impaired insulin secretion that results in elevated fasting blood glucose and hemoglobin A (1C) levels in humans is associated with increased risk for the development of type 2 diabetes and cardiovascular-associated mortality. In contrast, an inability to suppress insulin secretion results in exercise induced hypoglycemia, which is a major problem in individuals with diabetes. The experiments proposed in this application aim to elucidate the function of a protein called G6PC2 that we hypothesize plays a critical role in the control of fasting blood glucose and hemoglobin A (1C) levels as well as the termination of insulin secretion during exercise.
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