1768-P: Sel1L-Hrd1 Endoplasmic Reticulum-Associated Degradation Regulates PC2 Function and Proglucagon Processing in Islet α-Cells Free

Pancreatic islets rely on cellular protein quality control to maintain glucoregulatory hormone production. Proglucagon, precursor of glucagon and glucagon-like peptide 1 (GLP1), is expressed in islet α cells, intestinal L-cells, and some neurons. Islet α cells predominantly convert proglucagon to glucagon through prohormone convertase 2 (PC2). Islet-derived GLP1 is increasingly recognized as an important contributor to islet function, but the factors controlling its production remain unclear. To understand how protein quality control affects proglucagon processing in α cells, we inactivated the highly conserved Sel1L-Hrd1 endoplasmic reticulum-associated degradation (ERAD) system using Cre-loxP recombination in mice. Compared to littermate controls, Sel1L^ΔGcg mice showed an age-related decline in glucagon secretion in vivo, associated with a 43% decrease in pancreatic glucagon content (76.1 +/- 14.1 vs. 134.4 +/- 14.9 ng/g). Pancreatic GLP1 content was unchanged in Sel1L^ΔGcg mice (779.4 +/- 60.7 vs. 773.4 +/- 128.6 pg/g), despite a 37% reduction in α cell mass (0.19 +/- 0.04 vs. 0.12 +/- 0.04 mg/pancreas). Expression of the precursor form of PC2 (proPC2) was increased in Sel1L^ΔGcg α cells, suggesting accumulation of misfolded protein and/or impaired maturation. To investigate the mechanism underlying these changes, we used CRISPR to delete Sel1L in αTC cells. ERAD-deficient αTC-ΔSel1L cells showed 1.75-fold higher expression of proPC2, in addition to appearance of a novel truncated (~56 kD) form of proPC2. Though expression of the mature (64 kD) PC2 form was unchanged in αTC-ΔSel1L cells, PC2 activity was reduced by half in a specific enzyme assay. This was accompanied by decreased expression of 7B2, a chaperone required for proPC2 transit and activation in the secretory pathway. These data show that Sel1L-Hrd1 ERAD is required to ensure proper proPC2 maturation and glucagon production in α cells, thus maintaining classic α cell identity.