Pancreatic Beta Cell Identity Regulated by the Endoplasmic Reticulum Calcium Sensor Stromal Interaction Molecule 1

Abstract

Type 2 diabetes mellitus is a chronic disorder characterized by hyperglycemia, insulin resistance, and insufficient insulin secretion from the pancreatic β cells. To maintain adequate levels of insulin secretion, β cells rely on highly coordinated control of luminal ER Ca2+. Stromal Interaction Molecule 1 (STIM1) is an ER Ca2+ sensor that serves to replenish ER Ca2+ stores in response to depletion by gating plasmalemmal Orai1 channels in a process known as store-operated calcium entry (SOCE). We developed a method for the direct measurement of SOCE in pancreatic β cells and found that deletion of STIM1 in INS-1 cells (STIM1KO) is sufficient to block Ca2+ influx in response to store-depletion. To determine the physiological importance of β cell STIM1, we created mice with pancreatic β cell specific deletion of STIM1 (STIM1Δβ) and placed them on a high fat diet (HFD) with 60% of kilocalories derived from fat. After 8 weeks of HFD, female, but not male, STIM1Δβ mice exhibited increased body weight and fat mass as well as significant glucose intolerance and impaired insulin secretion without observable differences in insulin tolerance. Immunohistochemical analysis revealed a reduction of β cell mass and an increase of α cell mass; ELISA of islet lysates revealed a similar significant reduction in insulin content and increased glucagon content. RNA-sequencing performed on STIM1Δβ islets revealed differentially expressed genes for functions related to apoptosis, lipid metabolism, and epithelial cell differentiation, as well as loss of β cell identity. Proteomics analysis of STIM1KO cells phenocopied the metabolic findings, revealing significantly increased glucagon expression. Analysis of islet RNA-sequencing results showed modulation of pathways related to 17-β estradiol (E2) signaling, with notable downregulation of G-protein coupled estrogen receptor 1 (GPER1) expression. Consistently, treatment of female wild-type islets with pharmacological SOCE inhibitors led to reduced expression GPER1, while STIM1KO cells showed lower mobilization of intracellular cAMP levels in response to GPER agonist treatment. Taken together, these findings identify a novel interaction between SOCE and E2 signaling in the female islet and suggest that loss of STIM1 and impairments in SOCE may contribute to diabetes pathophysiology through loss of β cell identity.