Mechanisms of insulin secretion mediated by alpha cells Pancreatic islets contain multiple endocrine cells that produce hormones intimately involved in the control of glucose homeostasis. ?-cells, which secrete insulin, are both the most abundant cell type and the most studied cell type. ?-cell dysfunction leads to insufficient insulin production and ultimately hyperglycemia, the major pathophysiological phenotype of type 2 diabetes (T2D). ?-cells are the second most abundant cell type within islets, yet substantially less is known about ?-cell function or the role of these cells in glucose homeostasis. ?- cells secrete glucagon, which is canonically described as a hyperglycemic agent and an important counter- regulatory hormone in the prevention of hypoglycemia. ?-cell dysfunction is described as inappropriate glucagon secretion and is considered to be a driving force for hyperglycemia in T2D. However, glucagon receptors (GCGRs) are expressed on ?-cells and glucagon is a potent insulin secretagogue. Furthermore, it has been recently proposed that ?-cells synthesize and secrete glucagon-like peptide 1 (GLP-1), through alternative processing of the proglucagon gene transcript. GLP-1 receptors (GLP-1R) are also expressed on ?-cells, and GLP-1R agonists are currently available for use in T2D to increase insulin production. This raises the intriguing possibility that ?- to ?-cell paracrine interactions control insulin production and ultimately glucose homeostasis. Moreover, this perspective allows for the possibility that the increased glucagon secretion seen in T2D is a compensatory mechanism by which the ?-cells are attempting to increase ?-cell insulin production. Based on this, we hypothesize that increased ?-cell activity in T2D is not pathogenic per se, but rather, it is the interruption of ?- to ?-cell communication that results in decreased insulin production and hyperglycemia. Therefore, the overall goal of this project is to understand how ?-cells regulate ?-cell function, determine the importance of this relationship for normal glucose tolerance, and identify how severing the key node is pathogenic. We will accomplish this by first understanding the importance of ?- to ?-cell communication in the context of ?-cell function and overall glucose tolerance, and then determining if alterations in this axis occur during the development of ?-cell dysfunction. We will also define the relative importance of ?-cell glucagon versus GLP-1 production to delineate why ?-cells would produce two insulin secretagogues. Similarly, we will identify the importance of GCGR versus GLP-1R in ?-cells to determine if these are redundant systems or if they have unique roles in control of ?-cell function. This work will encompass cell lines and primary tissues, utilize novel mouse models, and determine the translational potential by conducting key experiments in human subjects in order to comprehensively understand the importance of ?- to ?-cell communication in normal and diabetic environments. Completion of this project will enhance our understanding of ?-cell function in order to identify how best to therapeutically target this cell type for the treatment of T2D.
/Relevance Completion of the project will enhance our understanding of the role ?-cells and their key product, glucagon, in the pathogenesis of type 2 diabetes. Increased ?-cell activity and hyperglucagonemia is currently viewed as pathogenic, spurring the development of glucagon receptor antagonists to lower glycemia. However, our data suggest that ?-cell control of ?-cell function is critical for normal glucose homeostasis but is interrupted in diabetes, indicating that a better understanding of ?-cell function would provide more direct and efficacious therapeutic targets for the treatment of diabetes.
