Calcium Extrusion in the Context of Pancreatic Islet Function and Dysfunction

Vanderbilt University, Nashville TN

Investigators

Abstract

Project Summary Glucose control of islet hormone secretion is perturbed in patients with type-2 diabetes (T2D), which contributes to hyperglycemia. It is generally accepted that islet insulin and glucagon secretion occur in response to elevated intracellular Ca2+. However, how islet Ca2+ extrusion modulates hormone secretion and is altered in T2D remain largely unknown. Data from our lab finds that the plasma membrane calcium-transporting ATPase 1 (PMCA1) serves to set basal - and -cell cytoplasmic Ca2+ levels and reduce secretagogue-stimulated Ca2+ influx. Whereas the sodium/calcium exchanger (NCX1) limits - and-cell Ca2+ oscillation amplitude and is activated during arginine stimulation. Further data show that -cell specific ablation of PMCA1 drastically improves glucose tolerance. Finally, our preliminary data provide the first evidence that diabetic conditions inhibit NCX1 activation in -cells, which likely alters Ca2+ handling. Based on these exciting data, the objective of this proposal is to elucidate how PMCA1 and NCX1 modulate islet hormone secretion and health under physiological and diabetic conditions. This project will test the central hypothesis that islet PMCA1 and NCX1 mediated Ca2+ extrusion lowers basal and secretagogue-stimulated cytoplasmic Ca2+, which limits hormone secretion. We further propose that stress-induced inhibition of PMCA1 and inactivation of NCX1 increase islet cell Ca2+ entry and hormone secretion in diabetes. The rationale that underlies this project is that understanding how PMCA1 and NCX1 control islet Ca2+ handling and hormone secretion will expose novel therapeutic strategies for restoring islet hormone secretion in T2D. This project will be accomplished with the following two specific aims: 1) Elucidate the mechanisms governing PMCA1 and NCX1 control of β-cell Ca2+ extrusion and insulin secretion; and 2) Determine how PMCA1 and NCX1 regulate -cell Ca2+ handling and glucagon secretion. Under the first aim, transgenic mice with -cell ablation of PMCA1 and NCX1 as well as human pseudoislets with ShRNA knockdown of -cell PMCA1 and NCX1 will be utilized to assess their roles during secretagogue and inhibitor control of Ca2+ handling and insulin secretion. Aim1 will also determine how -cell PMCA1 and NCX1 function are altered under diabetic conditions and the resulting impacts on insulin secretion, -cell health, and glucose homeostasis. Under the second aim, the roles of PMCA1 and NCX1 on -cell function will be determined in mice with -cell specific ablation of PMCA1 and NCX1 and in human pseudoislets containing -cells with knockdown of PMCA1 and NCX1. Furthermore, Aim2 will determine how alterations in-cell Ca2+ extrusion due to reduced PMCA1 and NCX1 activity under diabetogenic conditions contribute to -cell dysfunction. This project is significant because it is expected to illuminate mechanisms that control -cell and -cell Ca2+ extrusion and how alterations of their function contribute to disrupted islet hormone secretion in T2D. Moreover, this project plans to identify potential signals that can be used to modulate Ca2+ extrusion to help normalize hormone secretion, reduce islet dysfunction, improve islet health, and restore euglycemia in T2D.