Mouse SERCA2b is Critical in Compensatory Response to High Fat Diet by Maintaining Pancreatic β Cell Proliferation

Type 2 diabetes (T2D) affects over 380 million individuals worldwide and is characterized by peripheral insulin resistance and inadequate insulin secretion from the pancreatic β cell. During the development of T2D and in the face of increasing peripheral insulin resistance, the β cell undergoes a compensatory response to both increase insulin production and secretion. Here, we tested the hypothesis that intact activity of the sarco‐endoplasmic reticulum calcium ATPase (SERCA) pump, which transports 2 calcium molecules into the ER lumen during each catalytic cycle and is responsible for maintaining a robust ER calcium pool, is a critical determinant of the β cell's adaptive response to diet‐induced obesity. To this end, mice with a total body heterozygous deletion of SERCA2 (S2HET) and wild‐type littermate controls (WT) were placed on high fat diet (HFD) containing 45% of kilocalories from fat for 16 weeks. Compared to WT controls, S2HET mice showed equivalent body weight gain and fat content. Interestingly, S2HET mice demonstrated fasting hyperglycemia, hypoinsulinemia, and significantly impaired glucose tolerance. Insulin sensitivity and insulin signaling in liver, skeletal muscle, and adipose tissue were not different between S2HETs and WT controls, suggesting a primary defect at the level of the β cell. Histologic analysis of pancreatic sections obtained after 16 weeks of HFD revealed significantly reduced β cell mass as well as reduced β cell proliferation in S2HET mice. To define mechanisms underlying these observations, a SERCA2 deficient rat insulinoma cell line (S2KO INS‐1) was created using CRISPR/Cas9 technology. At baseline, thymidine incorporation was reduced in S2KO INS‐1 cells. Cell cycle analysis revealed a dramatic shift towards decreased proliferation and a G1/S block, while SERCA2b restoration by adenoviral transduction was able to fully rescue these defects. In the meantime, we observed the expression of adaptive and terminal ER stress markers including Bip, Dnajc3, hsp90b1, Pdia4 and spliced Xbp‐1 were elevated in both S2KO INS‐1 cells and isolated S2HET islets. Consistent with this, further analysis by polyribosomal profiling analysis revealed a translational initiation block in both S2KO cells and islets isolated from S2HET mice. In aggregate, these data suggest that impaired SERCA2 activity and altered ER calcium homeostasis lead to increased β cell ER stress and impaired proliferation, thereby limiting the β cell compensatory response to diet‐induced obesity. Support or Funding Information NIH R01 DK 093954 and VA Merit Award 1I01BX001733