Heterogeneous expression of CFTR in insulin-secreting β-cells of the normal human islet

Abstract

Cystic fibrosis (CF) is due to mutations in the CF-transmembrane conductance regulator (CFTR) and CF-related diabetes (CFRD) is its most common co-morbidity, affecting ~50% of all CF patients, significantly influencing pulmonary function and longevity. Yet, the complex pathogenesis of CFRD remains unclear. Two non-mutually exclusive underlying mechanisms have been proposed in CFRD: i) damage of the endocrine cells secondary to the severe exocrine pancreatic pathology and ii) intrinsic β-cell impairment of the secretory response in combination with other factors. The later has proven difficult to determine due to low expression of CFTR in β-cells, which results in the general perception that this Cl−channel does not participate in the modulation of insulin secretion or the development of CFRD. The objective of the present work is to demonstrate CFTR expression at the molecular and functional levels in insulin-secreting β-cells in normal human islets, where it seems to play a role. Towards this end, we have used immunofluorescence confocal and immunofluorescence microscopy, immunohistochemistry, RT-qPCR, Western blotting, pharmacology, electrophysiology and insulin secretory studies in normal human, rat and mouse islets. Our results demonstrate heterogeneous CFTR expression in human, mouse and rat β-cells and provide evidence that pharmacological inhibition of CFTR influences basal and stimulated insulin secretion in normal mouse islets but not in islets lacking this channel, despite being detected by electrophysiological means in ~30% of β-cells. Therefore, our results demonstrate a potential role for CFTR in the pancreatic β-cell secretory response suggesting that intrinsic β-cell dysfunction may also participate in the pathogenesis of CFRD.

Islet Function in the Pathogenesis of Cystic Fibrosis-Related Diabetes Mellitus

“…Another factor that might play a role is if the tissue material is fresh or has been stored embedded in paraffin for a long time. In one of the latter studies, 38 Di Fulvio and colleagues made a large effort testing several different antibodies and they came to the conclusion that some but not all CFTR antibodies detect CFTR in pancreatic islets. Their conclusion, which can be agreed on, was that CFTR is expressed in a subpopulation of β-cells.…”

“…Moreover, we and others have reported the presence of CFTR currents in primary human β-cells by patch clamp. 11 , 12 , 38 In recent years, much attention has been given to islet cell heterogeneity, and that subtypes of β-cells and α-cells have different gene expression and physiological roles. 42 , 43 Expression studies are snapshots, and it has not yet been described if a cell stays within a certain expression pattern forever.…”

“… 12 In a study by Di Fulvio and co-workers performed in rat β-cells a CFTR Cl − current was measured in 33% of the cells. 38 Finally, we have measured the presence of a CFTR Cl-current in primary human β-cells. 11 Thus, it is likely that CFTR has a role in the depolarization of β-cells.…”

“…CFTR has been detected by immunohistochemistry in mouse, rat, and human α-cells. 38 , 40 , 62 Like in β-cells there are also some studies against the presence of CFTR in α-cells. 10 , 39 In our hands, inhibition of CFTR increased cAMP potentiated glucagon secretion in human islets.…”

“…The altered islet hormone secretion in CFRD patients, in particular after meal intake, 17 shows that normal islet cell physiology is disrupted. A subpopulation of β-cells express CFTR, 38 and several in vivo and in vitro studies have shown that pharmacological manipulation of CFTR in both β-cell and α-cells disrupt hormone secretion. 11 , 12 , 62 , 63 Some results available so far suggest effects on α- and β-cell membrane potential, other data like the differential secretion of proinsulin is indicative of that CFTR has a role in insulin granule priming.…”