CFTR in pancreatic islets

Abstract: AbstractCystic fibrosis (CF) is caused by mutations in the anion channel and protein regulator CFTR. The most common co-morbidity in CF is CF-related diabetes (CFRD) affecting ~50% of adult patients. The etiopathology of CFRD is largely unknown but the destruction of the exocrine pancreas is thought to contribute. However, the hypothesis that CFTR has a direct role in the endocrine pancreas has not been explored. In this thesis I have investigated if CFTR have a direct function in insulin, glucagon and somatostatin secretion. Experiments were performed on islets and single cells from NMRI mice and the CFTRTM1Eur mouse model (F508del) that carries the most common human mutation in CFTR, the deletion of a phenylalanine at position 508. We found CFTR to be present in human and mouse alpha and beta cells but not in delta cells. A CFTR-dependent Cl- current was recorded in alpha- and beta-cells. In beta cells, a large part of the CFTR-dependent current was mediated by the Ca2+-activated chloride channel Anoctamin 1 (ANO1), and we suggested that CFTR most likely regulates ANO1 in beta cells. In human and mouse islets inhibition of CFTR or ANO1 reduced cAMP-enhanced insulin secretion by direct effects on exocytosis. In addition, ANO1 transcripts were found to be upregulated in islets from type 2 diabetic (T2D) donors. Analysis of transmission electron microscopy micrographs revealed that beta cells from the F508del CF mouse model or in NMRI mice after pharmacological inhibition of CFTR have reduced number of docked granules. CFTR also co-localizes to the SNARE protein Syntaxin 1A and places CFTR in the exocytotic machinery. Moreover, isolated islets from F508del mice have an increased proinsulin secretion and correspondingly decrease in release of c-peptide. This was especially evident during cAMP stimulation when CFTR is activated. Based on these findings we propose a model where CFTR is involved in granular priming and maturation of insulin. Cl- ions are believed to be necessary to lower the pH to levels needed for cleavage of proinsulin to insulin and c-peptide. Glucagon secretion is dysregulated in diabetes. We found that inhibition of CFTR increased glucagon secretion in isolated islets from human and mouse. Mathematical modelling of alpha cell physiology revealed that the CFTR-dependent current was involved in the regulation of alpha cell electrical activity. Moreover, the F508del mice had increased serum glucagon and isolated F508del islets had increased glucagon secretion. Inhibition of ANO1 in islets from T2D donors enhanced glucagon secretion while having no effect on islets from normal glucose tolerant donors, further supporting the regulatory role of Cl- in glucagon secretion.The novel data presented in this thesis suggests that CFTR regulates human and mouse beta cells by direct effects on exocytosis of the insulin granule. A defective CFTR increases secretion of immature proinsulin. Furthermore, CFTR negatively regulates glucagon secretion by direct effects on electrical activity and thereby CFTR works as a break in alpha cells.