2197-P: SEL1L-HRD1 Endoplasmic Reticulum-Associated Degradation Is Required for Maintaining Glucagon Production in Pancreatic Alpha Cells

Endoplasmic reticulum-associated degradation (ERAD) is a key component of cellular protein quality control, an essential process for pancreatic islet endocrine cells, which synthesize peptide prohormones in high quantities. Two highly conserved ER membrane proteins comprise the core ERAD machinery: HRD1, a ubiquitin ligase that retrotranslocates and ubiquitinates misfolded proteins for proteasomal degradation, and SEL1L, an adaptor protein that identifies ERAD target proteins and is required for HRD1 stability and function. Pancreatic alpha cells, intestinal L-cells, and certain neurons synthesize and differentially cleave proglucagon into tissue-specific hormones, including glucagon and glucagon-like peptide 1 (GLP-1). To investigate the role of ERAD in these cells, we targeted SEL1L deletion with the Cre-lox system, using mice expressing Cre recombinase under the proglucagon promoter. We found that Sel1L^Gcg mice showed normal growth and glucose homeostasis in vivo. Oral glucose tolerance was equally impaired by administration of the GLP-1 receptor antagonist exendin 9-39 in both Sel1L^Gcg and control mice, suggesting intact GLP-1 signaling in Sel1L^Gcg mice. Glucagon secretion in vivo was not significantly different following a 24 hour fast or insulin-induced hypoglycemia. Interestingly, pancreatic glucagon content was reduced by half in Sel1L^Gcg mice, despite unchanged pancreatic weight. By immunofluorescence, pancreatic islet morphology in Sel1L^Gcg mice was normal, though α-cells demonstrated increased expression of the ER chaperone BiP and ERAD-associated proteins OS9 and XTP3-B. By transmission electron microscopy, a large subset of α-cells showed significant ER dilation and fewer, smaller glucagon granules. Our data demonstrate that SEL1L-HRD1 ERAD is required for maintaining normal glucagon expression and ER morphology in pancreatic α-cells but does not entirely disrupt proglucagon signaling in vivo.