Regulation of calcium homeostasis in the pancreatic beta cell

Rlr Va Medical Center, Indianapolis IN

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Abstract

PROJECT SUMMARY/ABSTRACT Diabetes mellitus affects nearly one in four Veterans and over 463 million individuals worldwide. Inadequate insulin secretion from the pancreatic β cell plays a primary role in diabetes pathogenesis; however, the mechanisms responsible for β cell failure in diabetes remain poorly understood. The long-term goal of our VA- funded research program is to identify how altered β cell calcium (Ca2+) homeostasis contributes to diabetes pathophysiology. During the last funding cycle, we identified a previously unappreciated role for impaired store- operated calcium entry (SOCE) in human and rodent models of type 2 diabetes. SOCE is a process that is activated in response to endoplasmic reticulum (ER) Ca2+ depletion and is initiated when Ca2+ dissociates from the ER Ca2+ sensor, STIM1, leading to STIM1 oligomerization and translocation to the plasma membrane. Here, STIM1 complexes with the Ca2+ selective ion channel, Orai1, leading to Orai1 opening, Ca2+ influx from the extracellular space, and subsequent transfer of Ca2+ into the ER. To study the in vivo role of SOCE, mice with β cell specific loss of STIM1 (STIM1Δβ) were generated and challenged with a high fat diet to model type 2 diabetes. Our results revealed an unusual sexually dimorphic phenotype, where female (but not male) STIM1Δβ mice displayed significantly impaired glucose tolerance as well as reduced insulin secretion and β cell mass that was associated with reduced expression of key β cell identity markers and a reciprocal increase in α cell mass. Unbiased as well as targeted transcriptional analysis revealed a reduction in G-protein coupled estrogen receptor (GPER) expression that was linked with reduced estradiol and GPER-mediated signaling. This finding was notable because loss of endogenous estradiol signaling has been linked with reduced β cell function and increased diabetes susceptibility, while estradiol has beneficial effects in both male and female β cells. Against this background, we hypothesize that impaired SOCE leads to reduced estradiol-mediated signaling through GPER, resulting in β cell de-differentiation and reduced β cell function in type 2 diabetes. To test this hypothesis, two Specific Aims are proposed in this VA Merit renewal application. In Aim 1, lineage tracing experiments will be performed to track the fate of β cells in female STIM1Δβ mice fed a high fat diet, allowing us to define whether there is bona fide β cell de-differentiation in this model. We will test whether mice with a β cell specific deletion of Orai1 (Orai1Δβ) exhibit a similar sexually dimorphic phenotype and loss of β cell identity. Finally, we will determine whether naturally occurring loss of function mutations in STIM1 and Orai1 impact directed differentiation of induced pluripotent stem cells into β cells. In Aim 2, experiments will be performed to determine how GPER and SOCE interact to regulate β cell function. Here, an ovariectomy model will be leveraged to test whether endogenous estradiol signaling is required for the maintenance of β cell identity in the setting of diet- induced obesity, and we will determine how β cell specific GPER deletion impacts β cell function and differentiation status. A specific pharmacological agonist of GPER will be used to test whether β cell de- differentiation is reversible in STIM1Δβ, Orai1Δβ, and ovariectomized mice and in human islets from donors with T2D. Using the diet-induced obesity model, we will determine if GPER agonists have beneficial and/or distinct effects in the male islet, and we will complete mechanistic experiments aimed at testing the functional interaction between SOCE and GPER signaling. Across all Aims, questions will be translated into human model systems by utilizing human islets from donors with and without diabetes, pancreatic sections from pre- and post- menopausal female organ donors, and iPSCs from humans with STIM1 and Orai1 loss of function mutations. The translational impact of this work will be the identification of pathways that can be targeted clinically to improve β cell health in diabetes with the guiding principle of personalized therapeutic approaches that consider disease specific mechanisms that may differ and overlap between males and females.