341-LB: Chronic ER Calcium Depletion Resulting from Sarcoendoplasmic Reticulum Ca2⁺ Atpase 2b (SERCA2b) Deficiency Leads to Impaired Proinsulin Processing in the Pancreatic ß Cell

Reductions in pancreatic β cell endoplasmic reticulum (ER) Ca²⁺ levels are associated with the development of ER stress and impaired proinsulin processing in both type 1 and type 2 diabetes. The SERCA pump transports Ca²⁺ into the ER lumen in order to maintain a steep Ca²⁺ gradient between the ER and cytosol. SERCA2 activity and expression are reduced in diabetes, however, the relationship between SERCA2 and proinsulin processing remains incompletely understood. To this end, we developed mice with β cell specific deletion of SERCA2 (βS2KO mice) and S2KO INS1 cells. βS2KO mice exhibited glucose intolerance and reduced insulin secretion in response to glucose without evidence of impaired insulin sensitivity. Islets from βS2KO mice had significantly reduced ER Ca²⁺ levels and impaired glucose-induced Ca²⁺ oscillations, while serum proinsulin/insulin (PI/I) ratios and whole pancreas PI/I content were elevated. Immunoblot analysis of βS2KO islets and S2KO INS-1 cells revealed reduced total levels of the proinsulin processing enzymes, PC1/3, PC2 and CPE, and increased levels of proPC2. Restoration of SERCA2b via adenoviral transduction in S2KO INS1 cells was sufficient to restore PC1/3 and PC2 maturation and enzyme activity. Brefeldin A treatment in INS1 cells recapitulated the impairments in PC1/3 and PC2 maturation observed in S2KO INS1 cells, suggesting a disturbance in protein trafficking between the ER and Golgi. Trafficking assays were next performed using a vesicular stomatitis virus G (VSVG) protein construct and revealed a significantly slower rate of VSVG movement from the ER to the Golgi in S2KO INS1 cells. Taken together, these data indicate that loss of SERCA2 activity and impairments in ER Ca²⁺ lead to reduced proinsulin processing via altered protein trafficking through the β cell secretory pathway.