Effects of CaMKII-Mediated Phosphorylation of Ryanodine Receptor Type 2 on Islet Calcium Handling, Insulin Secretion, and Glucose Tolerance

Sayali S Dixit,Jeongkyung Lee,Tiannan Wang,David Y Chiang,Nicole Ryan,Eiffel John Q Manzano,Vijay K Yechoor,Xander H T Wehrens,Jonathan L Respress,Shin Yoo,Thierry Alquier

doi:10.1371/journal.pone.0058655

Sayali S Dixit, Jeongkyung Lee + Show 9 more

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https://doi.org/10.1371/journal.pone.0058655

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Journal: PloS one	Publication Date: Mar 13, 2013
Citations: 44	License type: CC BY 4.0

Affiliation: Baylor College of Medicine

Abstract

Altered insulin secretion contributes to the pathogenesis of type 2 diabetes. This alteration is correlated with altered intracellular Ca2+-handling in pancreatic β cells. Insulin secretion is triggered by elevation in cytoplasmic Ca2+ concentration ([Ca2+]cyt) of β cells. This elevation in [Ca2+]cyt leads to activation of Ca2+/calmodulin-dependent protein kinase II (CAMKII), which, in turn, controls multiple aspects of insulin secretion. CaMKII is known to phosphorylate ryanodine receptor 2 (RyR2), an intracellular Ca2+-release channel implicated in Ca2+-dependent steps of insulin secretion. Our data show that RyR2 is CaMKII phosphorylated in a pancreatic β-cell line in a glucose-sensitive manner. However, it is not clear whether any change in CaMKII-mediated phosphorylation underlies abnormal RyR2 function in β cells and whether such a change contributes to alterations in insulin secretion. Therefore, knock-in mice with a mutation in RyR2 that mimics its constitutive CaMKII phosphorylation, RyR2-S2814D, were studied. This mutation led to a gain-of-function defect in RyR2 indicated by increased basal RyR2-mediated Ca2+ leak in islets of these mice. This chronic in vivo defect in RyR2 resulted in basal hyperinsulinemia. In addition, S2814D mice also developed glucose intolerance, impaired glucose-stimulated insulin secretion and lowered [Ca2+]cyt transients, which are hallmarks of pre-diabetes. The glucose-sensitive Ca2+ pool in islets from S2814D mice was also reduced. These observations were supported by immunohistochemical analyses of islets in diabetic human and mouse pancreata that revealed significantly enhanced CaMKII phosphorylation of RyR2 in type 2 diabetes. Together, these studies implicate that the chronic gain-of-function defect in RyR2 due to CaMKII hyperphosphorylation is a novel mechanism that contributes to pathogenesis of type 2 diabetes.

Highlights

Diabetes mellitus is a metabolic disease characterized by high blood glucose levels
ryanodine receptor 2 (RyR2) is the Predominant Isoform in Mouse Islets To determine the predominant ryanodine receptor (RyR) isoform found in pancreatic islets, mRNA levels of RyR1, RyR2, and RyR3 were determined using quantitative RT-PCR
Our findings suggest that the RyR2mediated Ca2+ leak in islets from S2814D knock-in mice (S2814D) mice lowers the amount of Ca2+ storage in b cells, resulting in defective glucose-stimulated Ca2+ handling and insulin release from b cells (Figure 5C)

Summary

Introduction

High blood glucose levels result from either the impaired pancreatic production or secretion of, or the cellular response to insulin [1]. Glucose is transported into b cells and metabolized, which increases the concentration of ATP ([ATP]) This increase in [ATP] leads to the closure of ATP-sensitive K+ channels and depolarization of the cellular membrane. This depolarization activates voltage-gated Ca2+ channels, allowing entry of extracellular Ca2+ into b cells, which in turn triggers a greater release of Ca2+ from intracellular pools [1]. Under type 2 diabetic conditions, defects in insulin secretion are found to be associated with alterations in intracellular Ca2+ handling of both rodent and human pancreatic b cells [2]

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