Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCB221

Poster Communications

Dysfunctional insulin secretion in the CftrEURtm1/F508del CFTR mouse model due to defective processing of insulin and reduced exocytosis.

L. Eliasson1, M. Hühn2, M. Abels1, I. Mollet1, E. Svedin2, A. Wendt1, N. Wierup1, B. J. Scholte3, M. Flodström-Tullberg2

1. Dept Clinical Sciences Malmö, Lund University, Malmö, Sweden. 2. Karolinska Institute, Stockholm, Sweden. 3. Erasmus MC, Rotterdam, Netherlands.

Background and Problem statement: The most common complication in cystic fibrosis (CF) is CF-related diabetes (CFRD) yet the etiopathology of CFRD is largely unknown. CF patients with the F508del mutation have the highest risk for CFRD. The CFTREURtm1 mouse model of CF carries the human F508del CFTR mutation. The presence of CFTR in islets cells is not fully elucidated. Our previous data show presence of CFTR in both human and mouse pancreatic alpha- and beta-cells. In beta cells CFTR regulates priming and exocytosis of insulin-containing granules (Edlund, BMC Medicine, 2014), whereas in alpha cells it is involved in the control of the electrical activity (Edlund et al., Sci Rep, 2017). Here we aimed to investigate the regulation of insulin secretion in the F508del mouse model (F508del). Methods: In vivo serum basal insulin and glucagon levels were measured in F508del and corresponding controls. Pancreatic hormone secretion was measured on isolated islets analyzed with RIA or ELISA. Exocytosis was investigated using the patch-clamp technique. CFTR was detected in beta cells using confocal microscopy, and islet and beta cell morphology was investigated with immunohistochemistry and transmission electron microscopy, respectively. Results: .Pancreatic islets from F508del mice had reduced beta cell mass (n=21 pancreatic sections from N=7 mice of each genotype, p<0.01). This was not due to reduced beta cell number rather to reduced beta cell size. The F508del islets had an increased proinsulin secretion especially pronounced in the presence of cAMP (NWT=4; NF508del=4, p<0.01). Proinsulin is cleaved into insulin and c-peptide in the granule and the increase in proinsulin secretion coincided with a decrease in c-peptide secretion (NWT=4; NF508del=4, p<0.05). The conversion of proinsulin-to-insulin require an acidic pH. Priming is a process important for granular acidification and for Ca2+-dependent exocytosis. Effects on priming and exocytosis in single F508del beta cells measured as changes in membrane capacitance was reduced (nWT=16, nF508del=16 ; p<0.05), indicating reduced exocytosis and defective priming in this mouse model. Moreover, the beta cells had a reduced number of docked granules (nF508del=33, nWT=29, p<0.01). Serum levels of insulin was not different but the F508del had three-fold increase in serum glucagon (NWT=14, NF508del=13, p<0.005). In addition, glucagon secretion measured on isolated islets was increased compared to WT (n=4, p<0.05). Conclusions: We conclude that the CftrEURtm1/F508del CFTR mouse model has reduced insulin secretion mainly due to impaired priming causing defect processing of proinsulin to insulin and reduced Ca2+-dependent exocytosis. In line with previous investigations, we hypothesize that impaired insulin secretion contributes to the development of CFRD.

Where applicable, experiments conform with Society ethical requirements