CFTR and Anoctamin 1 (ANO1) contribute to cAMP amplified exocytosis and insulin secretion in human and murine pancreatic beta-cells
- Anna Edlund1,
- Jonathan LS Esguerra1,
- Anna Wendt1,
- Malin Flodström-Tullberg†2 and
- Lena Eliasson†1Email author
© Edlund et al.; licensee BioMed Central Ltd. 2014
Received: 9 December 2013
Accepted: 10 April 2014
Published: 28 May 2014
Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene lead to the disease cystic fibrosis (CF). Although patients with CF often have disturbances in glucose metabolism including impaired insulin release, no previous studies have tested the hypothesis that CFTR has a biological function in pancreatic beta-cells.
Experiments were performed on islets and single beta-cells from human donors and NMRI-mice. Detection of CFTR was investigated using PCR and confocal microscopy. Effects on insulin secretion were measured with radioimmunoassay (RIA). The patch-clamp technique was used to measure ion channel currents and calcium-dependent exocytosis (as changes in membrane capacitance) on single cells with high temporal resolution. Analysis of ultrastructure was done on transmission electron microscopy (TEM) images.
We detected the presence of CFTR and measured a small CFTR conductance in both human and mouse beta-cells. The augmentation of insulin secretion at 16.7 mM glucose by activation of CFTR by cAMP (forskolin (FSK) or GLP-1) was significantly inhibited when CFTR antagonists (GlyH-101 and/or CFTRinh-172) were added. Likewise, capacitance measurements demonstrated reduced cAMP-dependent exocytosis upon CFTR-inhibition, concomitant with a decreased number of docked insulin granules. Finally, our studies demonstrate that CFTR act upstream of the chloride channel Anoctamin 1 (ANO1; TMEM16A) in the regulation of cAMP- and glucose-stimulated insulin secretion.
Our work demonstrates a novel function for CFTR as a regulator of pancreatic beta-cell insulin secretion and exocytosis, and put forward a role for CFTR as regulator of ANO1 and downstream priming of insulin granules prior to fusion and release of insulin. The pronounced regulatory effect of CFTR on insulin secretion is consistent with impaired insulin secretion in patients with CF.
KeywordsCFTR Cystic Fibrosis Diabetes Insulin secretion Islet of Langerhans Beta-cell Exocytosis
The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel that belongs to the family of ATP-binding cassette (ABC)-transporters . As many other ABC-transporters, CFTR contains two membrane spanning domains (MSDs) and two nucleotide binding domains (NBDs) that interact with ATP. In addition, CFTR has a regulatory domain (R) that contains several phosphorylation sites. The chloride selective pore of the channel is formed by the MSDs, whereas the other domains control channel gating . Apart from being an anion channel, CFTR has been suggested to act as a regulator of other proteins and ion channels  in similarity with other members of the family of ABC-transporters, such as the sulfonylurea receptor (SUR1).
CFTR is primarily present in epithelial cells in airways, intestine and in cells with exocrine functions. Mutations in the gene encoding the channel protein complex (cftr) cause the autosomal recessive disease cystic fibrosis (CF). In patients with CF the defective chloride transport through CFTR leads to production of thick viscous mucus caused by a disturbed ion and water transport across epithelial membranes. The aberrant secretory functions cause obstruction of the distal airways and intestine, pancreatitis and malabsorption .
With increasing lifespan many CF patients develop cystic fibrosis-related diabetes (CFRD). The mean age of CFRD onset is around 20 years and the prevalence varies between 30 to 50% depending on the population studied [4–7]. Despite an increased awareness of the clinical impact of CFRD surprisingly little is known about its etiopathogenesis. CFRD is associated with exocrine pancreatic insufficiency caused by duct obstruction and fibrosis, but not all patients with pancreatic insufficiency develop diabetes [8, 9]. It has been suggested that the endocrine problem is, at least in part, mechanical [4, 9]. Abnormalities in the islet architecture and beta-cell loss have been observed while alpha- and delta-cell mass have remained intact or even increased [10–13]. However, observations showing that CFRD often correlates with insulin deficiency and/or a significant lower first-phase insulin secretion (for example, in [8, 9, 14, 15]), clearly indicate that other, beta-cell intrinsic factors, may play a causative role in CFRD. Moreover, many CF patients, also those without CFRD, have normal fasting plasma glucose levels, but suffer from postprandial hyperglycemia indicating that the beta-cells fail to respond upon increased insulin demand. A recent pilot study  performed on five CFRD patients with the G551D mutation having reduced/absent acute insulin response is in support of an insulin secretion defect in CFRD. One-month treatment with Ivacaftor, a newly developed CFTR potentiator, increased the insulin response to oral glucose in four out of the five patients. On a cellular level, data from insulin secreting HIT-cells support a role for an ATP-sensitive and cAMP-activated current in insulin secreting cells . Boom et al. revealed that CFTR mRNA and protein are expressed by both alpha- and beta-cells in rats , but did not investigate the physiological importance of CFTR for the functions of these cells. In the present study, we have used murine and human pancreatic islets and single beta-cells to test the hypothesis that active CFTR is present in beta-cells and essential in regulating insulin secretion.
Animal procedures were approved by the local ethics committee for use of laboratory animals in Malmö, Sweden. Human islet isolation from deceased donors and experimental protocols were approved by the ethics committee in Uppsala and Malmö, Sweden.
Islet isolation and cell culture
Islets from 43 non-diabetic human donors (F/M 18/25, age 59 ± 1.3, body mass index (BMI) 25.5 ± 0.5 kg/m2, HbA1c 5.7 ± 0.04, days in culture 4.1 ± 0.3) were used for secretion measurements, patch-clamp experiments, qPCR and immunohistochemistry. For these experiments islets were hand-picked to ensure high purity. Human islets were provided by the Nordic Network for Clinical Islet Transplantation (Uppsala, Sweden) through the LUDC Human Tissue Laboratory. Female NMRI mice (Bolmholtgaard, Ry, Denmark) were sacrificed by cervical dislocation and islets were isolated by collagenase digestion and hand-picked prior to experiments. For patch-clamp measurements, human and mouse islets were dispersed into single cells as previously described .
mRNA expression measured with RT-qPCR
Total RNA from human and mouse islets were prepared as described . CFTR mRNA expression was measured by RT-qPCR using primers and probes from Taqman mRNA assays (Life Technologies, California, USA). Human CFTR: Hs00357011_m1 expression was normalized against human HPRT1: 4333768 F, while mouse CFTR: Mm00445197_m1 expression was normalized against mouse HPRT1: Mm00446968. RT-qPCR runs were performed for individual batches of human (N = 5) and mouse (N = 5) islets in triplicate wells of 384-well plate on a 7900 HT RT-PCR system (Applied Biosystems, California, USA).
Human or mouse single cells were fixed and stained as described elsewhere . Primary antibodies; Mouse monoclonal anti-CFTR (MATG-1061, RD-Biotech, France), Guinea pig monoclonal anti-insulin (Linco, Billerica, MA, USA), Guinea pig monoclonal anti-insulin (EuroDiagnostica, Malmö, Sweden) and rabbit polyclonal anti-Stx1A (Synaptic Systems, Goettingen, Germany). Secondary antibodies; guinea pig conjugated to Cy2, mouse conjugated to Cy3, guinea pig and rabbit conjugated to Cy5 (all from Jackson, UK). Immunofluorescence was detected with a confocal microscope (META 510, Zeiss, Germany) and unspecific binding of the secondary antibodies was excluded by parallel experiments in the absence of the primary antibodies. Localization of CFTR was analyzed as described elsewhere . In short, the ratio between the mean fluorescent intensity in the plasma membrane region (P1) and the cytosolic region (P2) was measured using ZEN software (Zeiss, Jena, Germany). Pixel by pixel co-localization analysis was also performed using Zen software. The same laser setting was retained between experiments enabling comparison of experiments performed at different occasions.
EPC10 amplifier and 8.80 pulse software (HEKA, Lambrecht/Pfalz, Germany) was used to evoke and record whole-cell currents and changes in membrane capacitance on single mouse and human islet cells as described . Current and capacitance measurements were performed at RT and 32 to 33°C, respectively. Extracellular solution contained: 118 mM NaCl, 20 mM TEACl, 5.6 mM KCl, 2.6 mM CaCl2, 1.2 mM MgCl2, 5 mM HEPES, 3 (current measurements)/5 (capacitance measurements) mM D-glucose pH 7.4 (NaOH), supplemented with 10 μM FSK, 200 μM 4,4'-Diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS), 10 μM CFTRinh-172 (Sigma Aldrich, Sweden), 25 μM (current measurements) or 40 μM (capacitance measurements) GlyH-101 (Calbiochem, USA) [23, 24] and 50 μM T16Ainh-AO1 (gift from A.S. Verkman, Department of Medicine, UCSF School of Medicine) as indicated. The intracellular solution contained: 125 mM CsOH, 125 mM Glutamate, 10 mM CsCl, 10 mM NaCl, 1 mM MgCl2, 3 mM Mg-ATP, 4 mM EGTA (current measurement)/0.05 mM EGTA (capacitance measurement), 5 mM HEPES and 0.1 mM cAMP (capacitance measurements) (pH 7.15 with CsOH).
Insulin secretion measurements
Insulin secretion was measured in static batch incubations described previously . Briefly islets were pre-incubated in 1 mM glucose for 30 minutes followed by 1 h incubation in extracellular solution with variable glucose concentration. Incubation was 15 min when 50 mM KCl was used as stimulator. The extracellular solution was supplemented with 10 μM FSK, 0.1 μM GLP-1 (Bachem, Weil am Rhein, Switzerland), 100 μM Tolbutamide (Sigma Aldrich, Stockholm, Sweden), 200 μM DIDS, 50 μM TMinh-AO1, 40 μM CFTRinh-172 and/or 50 μM GlyH-101 as indicated. Insulin secretion was measured using radioimmunoassay kit (mouse: Linco Research Inc.,; human: Millipore, Billerica, MA, USA).
Transmission electron microscopy
Islets were collected after insulin secretion assay and prepared for electron microscopy, examined and analyzed as previously described . The granule volume density (Nv) and surface density (Ns) were calculated using in-house software programmed in MatLab.
Data are presented as mean ± SEM of N number of individuals or independent experiments and number of cells or biological replicates. Statistical significance was calculated using ANOVA or Student’s t test and P <0.05 was considered significant.
CFTR-antagonists inhibit insulin secretion and an ATP-sensitive and cAMP-dependent current in mouse and human beta-cells
In addition to the ion channel function, CFTR has been attributed a role as regulator of other ion channels and proteins, such as other chloride channels [2, 33]. To investigate the possibility that CFTR regulates the function of other chloride channels we used the non-specific chloride channel blocker DIDS that blocks a wide variety of chloride channels, while CFTR is insensitive to this antagonist [34, 35]. The cAMP-stimulated current, in human and mouse beta-cells, was significantly reduced by DIDS (Figure 2E, F). The presence of active CFTR channels was proven by a significant reduction in current conductance in the simultaneous presence of GlyH-101 and DIDS as compared with DIDS alone (Figure 2E, F). The DIDS-sensitive component constituted 38 ± 10% and CFTR 37 ± 15% (n = 5) of the FSK-activated current at negative potentials (−100 to −50 mV) in human beta-cells. In the mouse, the DIDS-sensitive current was 41 ± 12% and CFTR 29 ± 9% (n = 8) of the FSK-activated current. From these measurements the CFTR conductance was estimated to be 2.8 ± 0.6 pS/pF in human (n = 5; N = 3) and 12 ± 4.0 pS/pF in mouse (n = 8; N = 5) beta-cells at negative potentials (−100 mV to −50 mV). Taken together these data indicate that 1) the cAMP-induced chloride current has one DIDS-sensitive component and one part sensitive to CFTR inhibition, 2) the current through CFTR in beta-cells is small, and 3) CFTR likely acts as a regulator of DIDS-sensitive chloride channel(s).
CFTR likely regulates the chloride channel Anoctamin 1 (ANO1)
Recently, it was suggested that channels from the family of Anoctamins (ANO1-10) are controlled by CFTR [40, 41]. The first member in this family, ANO1, is a voltage- and calcium-dependent chloride channel localized in the plasma membrane that has been suggested to interact with CFTR to control chloride conductance in epithelial cells . Interestingly, ANO1 gene expression has been measured in rat and human islets [42, 43]. To investigate the possible combined involvement of ANO1 and CFTR in the regulation of insulin secretion we performed insulin secretion measurements in mouse and human islets in the presence or absence of specific blockers of the respective channel. Indeed, insulin secretion enhanced by GLP-1 was reduced by TM16Ainh-AO1 (AO1), a specific blocker of ANO1 , and in the combined presence of GlyH-101 and AO1 the reduction in GLP-1 enhanced glucose-stimulated insulin secretion was not significantly different from the reduction observed after application of either of them alone (Figure 3C, D). In human islets, the inhibitory effect of AO1 was significantly more potent than GlyH-101 alone, but, as in the mouse, inclusion of GlyH-101 in the simultaneous presence of AO1 did not cause an additive decrease in GLP-1 amplified insulin secretion at 16.7 mM glucose (Figure 3D). It was confirmed that AO1 did not have any inhibitory effect at 1 mM glucose (Figure 3E).
The presence of ANO1 chloride current in beta-cells was measured by patch-clamp recordings on single mouse beta-cells using the standard whole-cell mode and the ramp-protocol as used above. It was obvious that the FSK-activated current was reduced by AO1, and the conductance was decreased (Figure 3F). A small, but significant, further reduction in conductance was obtained in the simultaneous presence of AO1 and the CFTR-inhibitor GlyH-101 (Figure 3F). The AO1-sensitive current was calculated to represent 50 ± 10% and CFTR 30 ± 5% of the FSK-activated current at negative potentials (n = 5). The CFTR conductance was estimated to 14 ± 3 pS/pA (n = 5). Interestingly, these are similar to values obtained from the DIDS-experiments in Figure 2 (12 pS/PA and 29%). This experiment demonstrates that the blockers AO1 and GlyH-101 most likely act on separate channels (Figure 3E). Hence, the lack of an additive effect on secretion (Figure 3C, D) would suggest that CFTR interacts with ANO1 in regulating insulin secretion.
Inhibition of CFTR results in decreased exocytosis in mouse and human beta-cells
CFTR has been shown to act through additional mechanisms, one being interaction with the SNARE-protein syntaxin 1A [50, 51], a protein crucial for beta-cell granule docking and exocytosis . To investigate the possibility that CFTR interacts with this SNARE-protein, we investigated the distribution of CFTR and syntaxin 1A, and found that CFTR was co-localized with syntaxin 1A in the plasma membrane of human beta-cells (86 ± 2% co-localization, n = 39; N = 2; Figure 5E).
Chloride channels play key roles in pancreatic beta-cells [17, 52–54], but the exact identity of all chloride channels involved remains to be established. A previous study indicated that endocrine cells in the rat pancreas express the mRNA encoding CFTR . Here we provide novel data adding CFTR to the list of chloride channels having important functions in human and mouse beta-cells [17, 52–54], and suggest that CFTR acts upstream of ANO1 [2, 33, 41] to control insulin secretion.
In a previous study, Kinard and Satin  measured an ATP- and cAMP-dependent chloride current in insulin secreting cells that could be activated under hypotonic conditions. The current was termed ICl,islet and had similar properties in terms of size and reversal potential as the chloride current obtained here in the presence of glucose and FSK/GLP-1 (Figure 2). It was suggested  that the ICl,islet contributes to cAMP-dependent depolarization at negative membrane potentials although the exact identity of the channel was not described. It has been suggested that the channel is a volume-regulated anion channel (VRAC; ), due to similarities with ICl,islet in terms of activation and electrophysiological properties. Our focus was CFTR and we did not investigate the presence of a current sensitive to cell swelling. However, our data do not rule out the presence of VRAC. Whereas VRAC is suggested to enhance electrical activity, we hypothesize that the main function of CFTR/ANO1 is in the control of exocytosis. We confirm that activation of the ATP- and cAMP-dependent chloride current contributes to a small depolarization at potentials below the equilibrium potential for chloride, when the flux of chloride ions is outward from the cell comparable to the depolarization obtained by cAMP on electrical activity measured on whole islets [56, 57].
The measured current conductance in the presence of FSK varies among different batches of cells investigated and amounts between approximately 80 and 120 pS/pF and approximately 20 and 40 pS/pF in mouse beta-cells (Figures 2B, F and 3F) and human beta-cells (Figure 2B, E), respectively. Biological variation is common in studies on primary tissue and human primary tissue in particular. However, more importantly, the estimated CFTR conductance after blockade of other chloride-currents using DIDS or by blocking ANO1 becomes the same (13 pS/pF and 12 pS/pF, respectively), strongly supporting the presence of a CFTR conductance. Although small, the impact on insulin secretion is large, suggesting that CFTR has a function upstream of many other cAMP-activated processes involved in insulin secretion [19, 27, 49, 56, 58].
We provide evidence suggesting that the main consequence of the cAMP-activated chloride current is unrelated to membrane depolarization (Figure 4A). More specifically our studies point to a role of CFTR and ANO1 in cAMP augmented calcium-dependent exocytosis. Indeed, both FSK- and GLP-1-enhanced, glucose-stimulated insulin secretion is reduced by CFTR-inhibitors. We observed that the inhibitors are more potent on GLP-1 than on FSK-stimulated secretion. This we mainly attribute to the fact that FSK increases cAMP to a higher level than GLP-1 in islets (see, for example, ). The participation of CFTR in the exocytotic process is supported by the capacitance measurements showing that exocytosis is blocked by CFTR-inhibitors in both human and mouse beta-cells (Figure 4). Exocytosis is a calcium-dependent process and the reduced exocytotic response observed in the presence of the CFTR antagonists might be explained by a reduced calcium current , but this was proven not to be the case (Figure 4F, G). Based upon our observations from insulin secretion and electrophysiological ramp-protocol measurements we instead hypothesize that CFTR through ANO1 act directly on exocytosis. Indeed, we found CFTR to co-localize with the SNARE-protein syntaxin 1A (Figure 5E) as demonstrated in other tissues [50, 51]. This suggests that CFTR, as syntaxin 1A, is part of the exocytotic machinery.
From our novel results we suggest that CFTR plays a key role in priming of the insulin granules; this is evident from the capacitance measurements and the electron micrographic data. The increase in membrane capacitance evoked by a train of membrane depolarizations initiates exocytosis of release-ready (primed) granules by the first two depolarizations, whereas the latter depolarizations enable granules within a larger reserve pool to be released . Our data demonstrate a pronounced reduction in exocytosis evoked during the two first depolarizations (Figure 4B-E). Hence, CFTR can be suggested to have its main function in the priming of insulin granules. Moreover, the ultrastructural analysis revealed that the number of granules in close vicinity to the plasma membrane (<300 nm) was reduced after CFTR inhibition (Figure 5D). The exact molecular mechanism by which CFTR contributes to granular priming is hitherto unknown. Interestingly, DIDS has earlier been demonstrated to reduce insulin exocytosis, an effect that has been coupled with intragranular ClC3 chloride channels and priming of the insulin granules . A role for ClC3 in insulin granular priming and exocytosis has also been proven by knock-out animals . Here we demonstrate that CFTR inhibition reduces exocytosis to the same extent as has earlier been demonstrated for DIDS  and removal of ClC3 . As DIDS is a chloride channel blocker inhibiting the current through most chloride channels except for CFTR, the ANO1 current is likely reduced by this treatment (compare Figures 2F and 3F). Our insulin secretion measurements and chloride current measurements suggest that CFTR acts on ANO1. The mechanisms by which ANO1 is regulated by CFTR is yet to be investigated, but the small influx of chloride through CFTR seems to have some function since the ANO1 current was inhibited by the CFTR-specific open-channel blocker Gly-H 101 (Figure 3F). The fact that ANO1 is calcium activated  is consistent with a role of this channel in exocytosis. It can be argued that the increase in cAMP will enhance the voltage-dependent calcium-influx  and thereby increase the ANO1 current and insulin secretion. This process can most likely occur in parallel with the cAMP-dependent regulation of ANO1 via CFTR as demonstrated here. Our data, however, demonstrate a role for CFTR in regulating insulin secretion through a direct effect on exocytosis.
Depolarization-evoked exocytosis of primed granules is hypothesized to correspond to the first phase insulin secretion , suggesting an important role for CFTR at this stage. Indeed, many CF patients suffer from postprandial hyperglycemia although they have normal fasting plasma glucose levels indicating that the beta-cells fail to respond upon increased insulin demand. For these patients, impaired insulin secretion is mostly apparent during the first phase, strongly indicating a defect at the level of the pancreatic beta-cell [9, 14–16]. The impaired insulin secretion has been suggested to be due to severe exocrine tissue damage destroying the beta-cells [4, 10, 11, 13, 15], but others have indicated direct effects on beta-cell function [9, 15, 16, 60, 61]. Indeed, reduced or absent acute insulin response to glucose in patients with CFRD was recently shown to be compensated by pharmacological CFTR potentiation . Here we provide a likely explanation to the clinical observations supporting an effect on beta-cell function. We demonstrate for the first time that CFTR has a role at the cellular level, in both human and mouse beta-cells, by regulating insulin secretion. Specifically, we provide evidence that interference with CFTR affects cAMP-dependent rapid exocytosis of primed granules important for first phase insulin release.
- ABC transporters:
Anoctamin 1 chloride channel
Body Mass Index
Cystic fibrosis-related diabetes
Cystic fibrosis transmembrane conductance regulator
Exchange protein directly activated by cAMP 2
granular Sulfonylura receptor
Membrane spanning domain
Nucleotide binding domain
surface density of granules
volume density of granules
Soluble NSF attachment protein receptor
Transmission electron microscopy
Volume regulated anion channel.
We thank Professors C. Wollheim (Lund University, Sweden and University of Geneva, Switzerland) and S. Cushman (Lund University, Sweden and NIH, USA) for critical reading of the manuscript; Professor A.S. Verkman (UCSF, USA) for kindly providing TM16Ainh-AO1; B.M. Nilsson, A.M.V. Ramsay and R. Wallén (Lund University, Sweden) for technical assistance; and the LUDC Human Tissue Laboratory (Lund University, Sweden) and The Nordic Network for Islet Transplantation for providing human islets (Uppsala University, Sweden).
This work was supported by the Swedish Research Council through project grant (L.E), Linneus grant (LUDC) and Strategic Research grant (Exodiab), the Knut and Alice Wallenberg Foundation, the European Union (EU-FP-7, BetaBat), The Diabetes Wellness Foundation (LE), The Swedish Diabetes Foundation (L.E.), Albert Påhlsson Foundation (L.E.) and Region Skåne (ALF; L.E.). MFT. is supported by Karolinska Institutet, Stockholm, Sweden and a VINNMER fellowship from VINNOVA, Sweden. LE is a senior researcher at the Swedish Research Council.
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