Islet-intrinsic effects of CFTR mutation

The Role of Ion-Transporting Proteins in Human Disease

This Special Issue focuses on the significance of ion-transporting proteins, such as ion channels and transporters, providing evidence for their significant contribution to bodily and cellular functions via the regulation of signal transduction and ionic environments [...].

Essential Fatty Acid Deficiency in Cystic Fibrosis Disease Progression: Role of Genotype and Sex

Adequate intake of nutrients such as essential fatty acids (EFA) are critical in cystic fibrosis (CF). The clinical course of deterioration of lung function in people with CF has been shown to relate to nutrition. Independent of the higher energy consumption and malabsorption due to pancreatic insufficiency, EFA deficiency is closely associated with the risk of pulmonary infection, the most significant pathology in CF. This review will focus on the EFA deficiency identified in people with CF, as well as the limited progress made in deciphering the exact metabolic pathways that are dysfunctional in CF. Specifically, people with CF are deficient in linoleic acid, an omega 6 fatty acid, and the ratio of arachidonic acid (omega 6 metabolite) and docosahexaenoic acid (omega 3 metabolite) is increased. Analysis of the molecular pathways in bronchial cells has identified changes in the enzymes that metabolise EFA. However, fatty acid metabolism primarily occurs in the liver, with EFA metabolism in CF liver not yet investigated, indicating that further research is required. Despite limited understanding in this area, it is well known that adequate EFA concentrations are critical to normal membrane structure and function, and thus are important to consider in disease processes. Novel insights into the relationship between CF genotype and EFA phenotype will be discussed, in addition to sex differences in EFA concentrations in people with CF. Collectively, investigating the specific effects of genotype and sex on fatty acid metabolism may provide support for the management of people with CF via personalised genotype- and sex-specific nutritional therapies.

Bidirectional causal associations between type 2 diabetes and COVID-19

Observational studies have reported high comorbidity between type 2 diabetes (T2D) and severe COVID-19. However, the causality between T2D and COVID-19 has yet to be validated. We performed genetic correlation and Mendelian randomization (MR) analyses to assess genetic relationships and potential causal associations between T2D and three COVID-19 outcomes (severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2] infection, COVID-19 hospitalization, and critical COVID-19). Molecular pathways connecting SARS-CoV-2 and COVID-19 were reconstructed to extract insights into the potential mechanisms underlying the connection. We identified a high genetic overlap between T2D and each COVID-19 outcome (genetic correlations 0.21-0.28). The MR analyses indicated that genetic liability to T2D confers a causal effect on hospitalized COVID-19 (odds ratio 1.08, 95% confidence interval [CI] 1.04-1.12) and critical COVID-19 (1.09, 1.03-1.16), while genetic liability to SARS-CoV-2 infection exerts a causal effect on T2D (1.25, 1.00-1.56). There was suggestive evidence that T2D was associated with an increased risk for SARS-CoV-2 infection (1.02, 1.00-1.03), while critical COVID-19 (1.06, 1.00-1.13) and hospitalized COVID-19 (1.09, 0.99-1.19) were associated with an increased risk for T2D. Pathway analysis identified a panel of immunity-related genes that may mediate the links between T2D and COVID-19 at the molecular level. Our study provides robust support for the bidirectional causal associations between T2D and COVID-19. T2D may contribute to amplifying the severity of COVID-19, while the liability to COVID-19 may increase the risk for T2D.

The effect of cystic fibrosis transmembrane conductance regulator modulators on impaired glucose tolerance and cystic fibrosis related diabetes

Cystic fibrosis (CF) is an autosomal recessive disorder, with a prevalence of 1 in 2,500 live births. It is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. With the significant advancement in CFTR-directed therapies, life expectancy of CF patients has steadily increased. With improved survival, CF related co-morbidities have become more apparent. The most common endocrine complication includes Cystic fibrosis related diabetes (CFRD). Impaired glucose tolerance and insulin deficiency in CFRD leads to a decline in pulmonary function in CF patients. Here we review the underlying mechanisms involved in the pathogenesis of CFRD, focusing on the role of CFTR in the regulation of insulin secretion from the β-cell. We then discuss CFTR modulators and their effect on impaired glucose tolerance and CFRD.

Cystic Fibrosis-Related Diabetes Mellitus and Pregnancy: A Retrospective Study

INTRODUCTION

Cystic fibrosis-related diabetes mellitus (CFRDM) is becoming a more common issue in pregnancy care as the life expectancy of females living with cystic fibrosis has improved, with an increasing number of pregnancies in this population. Despite the Republic of Ireland having the highest incidence of cystic fibrosis globally, there is limited Irish data on pregnancy outcomes for those with CFRDM. This study aimed to retrospectively review maternal and foetal outcomes of pregnancies affected by maternal CFRDM.

METHODS

The patient records of all women with CFRDM who attended the National Maternity Hospital Dublin for obstetric care between 2015 and 2019 were retrospectively reviewed.

RESULTS

A search of patient records identified 15 pregnancies in 12 women with CFRDM during the study period. CFRDM was diagnosed pre-conception in ten of the 15 pregnancies. Median neonatal weight at birth was lower in women with CFRDM diagnosed pre-conception compared to women diagnosed during pregnancy (2.8 vs. 3.02 kg). The median weight gain in women with CFRDM diagnosed pre-conception was 10.9 kg compared to 11.9 kg for those diagnosed during pregnancy. The majority of women (62.5%) with CFRDM diagnosed pre-conception delivered via caesarean section. Admission for CF exacerbations during pregnancy in women with CFRDM diagnosed pre-conception was very common (87.5%) compared with 75% of those diagnosed during their pregnancy.

CONCLUSION

Women diagnosed with CFRDM were likely to require caesarean section, to be treated with insulin, and to be frequently admitted to hospital for CF exacerbations. Our review highlights the importance of good glucose control, stable cystic fibrosis before pregnancy and a multidisciplinary team approach.

Angiotensin(1-7) activates MAS-1 and upregulates CFTR to promote insulin secretion in pancreatic β-cells: the association with type 2 diabetes

OBJECTIVE

The beneficial effect of angiotensin(1-7) (Ang(1-7)), via the activation of its receptor, MAS-1, has been noted in diabetes treatment; however, how Ang(1-7) or MAS-1 affects insulin secretion remains elusive and whether the endogenous level of Ang(1-7) or MAS-1 is altered in diabetic individuals remains unexplored. We recently identified an important role of cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated Cl- channel, in the regulation of insulin secretion. Here, we tested the possible involvement of CFTR in mediating Ang(1-7)'s effect on insulin secretion and measured the level of Ang(1-7), MAS-1 as well as CFTR in the blood of individuals with or without type 2 diabetes.

METHODS

Ang(1-7)/MAS-1/CFTR pathway was determined by specific inhibitors, gene manipulation, Western blotting as well as insulin ELISA in a pancreatic β-cell line, RINm5F. Human blood samples were collected from 333 individuals with (n = 197) and without (n = 136) type 2 diabetes. Ang(1-7), MAS-1 and CFTR levels in the human blood were determined by ELISA.

RESULTS

In RINm5F cells, Ang(1-7) induced intracellular cAMP increase, cAMP-response element binding protein (CREB) activation, enhanced CFTR expression and potentiated glucose-stimulated insulin secretion, which were abolished by a selective CFTR inhibitor, RNAi-knockdown of CFTR, or inhibition of MAS-1. In human subjects, the blood levels of MAS-1 and CFTR, but not Ang(1-7), were significantly higher in individuals with type 2 diabetes as compared to those in non-diabetic healthy subjects. In addition, blood levels of MAS-1 and CFTR were in significant positive correlation in type-2 diabetic but not non-diabetic subjects.

CONCLUSION

These results suggested that MAS-1 and CFTR as key players in mediating Ang(1-7)-promoted insulin secretion in pancreatic β-cells; MAS-1 and CFTR are positively correlated and both upregulated in type 2 diabetes.

Understanding the Pancreatic Islet Microenvironment in Cystic Fibrosis and the Extrinsic Pathways Leading to Cystic Fibrosis Related Diabetes

Cystic fibrosis (CF) is an autosomal recessive chronic condition effecting approximately 70 000 to 100 000 people globally and is caused by a loss-of-function mutation in the CF transmembrane conductance regulator. Through improvements in clinical care, life expectancy in CF has increased considerably associated with rising incidence of secondary complications including CF-related diabetes (CFRD). CFRD is believed to result from β-cell loss as well as insufficient insulin secretion due to β-cell dysfunction, but the underlying pathophysiology is not yet fully understood. Here we review the morphological and cellular changes in addition to the architectural remodelling of the pancreatic exocrine and endocrine compartments in CF and CFRD pancreas. We consider also potential underlying proinflammatory signalling pathways impacting on endocrine and specifically β-cell function, concluding that further research focused on these mechanisms may uncover novel therapeutic targets enabling restoration of normal insulin secretion.

The Distribution and Role of the CFTR Protein in the Intracellular Compartments

Cystic fibrosis is a hereditary disease that mainly affects secretory organs in humans. It is caused by mutations in the gene encoding CFTR with the most common phenylalanine deletion at position 508. CFTR is an anion channel mainly conducting Cl⁻ across the apical membranes of many different epithelial cells, the impairment of which causes dysregulation of epithelial fluid secretion and thickening of the mucus. This, in turn, leads to the dysfunction of organs such as the lungs, pancreas, kidney and liver. The CFTR protein is mainly localized in the plasma membrane; however, there is a growing body of evidence that it is also present in the intracellular organelles such as the endosomes, lysosomes, phagosomes and mitochondria. Dysfunction of the CFTR protein affects not only the ion transport across the epithelial tissues, but also has an impact on the proper functioning of the intracellular compartments. The review aims to provide a summary of the present state of knowledge regarding CFTR localization and function in intracellular compartments, the physiological role of this localization and the consequences of protein dysfunction at cellular, epithelial and organ levels. An in-depth understanding of intracellular processes involved in CFTR impairment may reveal novel opportunities in pharmacological agents of cystic fibrosis.

Lumacaftor/ivacaftor in cystic fibrosis: effects on glucose metabolism and insulin secretion

PURPOSE

The question whether the new cystic fibrosis transmembrane conductance regulator (CFTR) modulator drugs aimed at restoring CFTR protein function might improve glucose metabolism is gaining attention, but data on the effect of lumacaftor/ivacaftor treatment (LUMA/IVA) on glucose tolerance are limited. We evaluated the variation in glucose metabolism and insulin secretion in CF patients homozygous for Phe508del CFTR mutation after one-year treatment with LUMA/IVA in comparison to patients with the same genotype who did not receive such treatment.

METHODS

We performed a retrospective case-control study on 13 patients with a confirmed diagnosis of CF, homozygous for the Phe508del CFTR mutation, who received LUMA/IVA for one year (cases) and 13 patients with identical genotype who did not receive this treatment (controls). At the beginning and conclusion of the follow-up, all subjects received a modified 3 h OGTT, sampling at baseline, and at 30 min intervals for plasma glucose, serum insulin, and c-peptide concentrations to evaluate glucose tolerance, and quantify by modeling beta-cell insulin secretion responsiveness to glucose, insulin clearance and insulin sensitivity.

RESULTS

LUMA/IVA did not produce differences in glucose tolerance, insulin secretory parameters, clearance and sensitivity with respect to matched controls over one-year follow-up.

CONCLUSION

We found no evidence of improvements in glucose tolerance mechanisms in patients with CF after one-year treatment with LUMA/IVA.

[Cystic fibrosis being a polyendocrine disease (Review)]

The cystic fibrosis transmembrane regulator (CFTR) gene encodes the synthesis of a protein of the same name, which functions as a direct activator of anionic transport. Chloride is the most abundant anion; as an antagonist of Na+ and K+, it provides electroneutrality of cell membranes at rest; together with cations, it serves as an important osmolyte and forms water flow across cell membranes for transepithelial secretion.Glandular cells in CF trap Cl- and Na+, and the prodused secretion is excessively viscous. Subnormal CFTR activity leads to stagnation of mucociliary clearance, inhibition of intestinal transport.In addition to exocrine disorders, CFTR mutations are associated with a decrease in volume, mass, increased apoptosis of β-cells of the pancreas, a significant suppression of insulin exocytosis in response to stimulation with glucose and glucagon-like peptide-1, hyperglucagonemia against the background of a defect in the suppression of α-cell function by insulin, but a decrease in maximum capacity α-cells.Deficiency and progressive decline in bone mineral density is an expected secondary manifestation of CF due to pancreatic exocrine insufficiency with malabsorption of nutrients and fat-soluble vitamins. However, in patients with the F508del mutation, a significant decrease in the synthesis of OPG, COX-2, PGE2 in the osteoblastic formation, and an increase in the activity of the antianabolic NF-kB were found. We are talking about a defect in the canonical signaling pathway (Wnt/β-catenin), which regulates the expression of genes-activators of osteoblastogenesis, dissociation of the stages of physiological bone remodeling.In addition to congenital bilateral or unilateral aplasia of the vas deferens, an increase in the frequency of CFTR mutations is also found in non-obstructive azoospermia, oligo-, astheno- and teratospermia. CFTR is involved in the entry of HCO3- into Sertoli cells to trigger cAMP-dependent transcription and its defects lead to suppression of FSH-dependent gene expression of spermatogenesis, loss of sequence in the Wnt cascade, destruction of the PGE2-dependent transepithelial interaction and, as a consequence, the blood-testicular barrier.CF is characterized, along with classical signs, by endocrine dysfunction of the pancreas, osteoporosis with suppression of osteoblastogenesis, and a defect in spermatogenesis.

Cystic Fibrosis-Related Diabetes (CFRD): Overview of Associated Genetic Factors

Cystic fibrosis (CF) is the most common autosomal recessive disease in the Caucasian population and is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene that encodes for a chloride/bicarbonate channel expressed on the membrane of epithelial cells of the airways and of the intestine, as well as in cells with exocrine and endocrine functions. A common nonpulmonary complication of CF is cystic fibrosis-related diabetes (CFRD), a distinct form of diabetes due to insulin insufficiency or malfunction secondary to destruction/derangement of pancreatic betacells, as well as to other factors that affect their function. The prevalence of CFRD increases with age, and 40–50% of CF adults develop the disease. Several proposed hypotheses on how CFRD develops have emerged, including exocrine-driven fibrosis and destruction of the entire pancreas, as well as contrasting theories on the direct or indirect impact of CFTR mutation on islet function. Among contributors to the development of CFRD, in addition to CFTR genotype, there are other genetic factors related and not related to type 2 diabetes. This review presents an overview of the current understanding on genetic factors associated with glucose metabolism abnormalities in CF.

Lumacaftor/ivacaftor therapy fails to increase insulin secretion in F508del/F508del CF patients

BACKGROUND

Glucose tolerance abnormalities including cystic fibrosis related diabetes (CFRD) are common in patients with cystic fibrosis (CF). The underlying pathophysiology is not fully understood. Emerging evidence suggests that CFTR dysfunction may directly or indirectly impact β-cell function, offering the potential for improvement with CFTR modulator therapy. In small pilot studies, treatment with ivacaftor improved insulin secretion in patients with the G551D CFTR mutation. In the current study, we examined the impact of lumacaftor/ivacaftor therapy on glucose tolerance and insulin secretion in patients with CF who were homozygous for the F508del mutation.

METHODS

39 subjects from the PROSPECT Part B study who had been prescribed lumacaftor/ivacaftor by their CF care team at a CF Foundation's Therapeutic Development Network center were recruited. Subjects underwent 2-hour oral glucose tolerance tests (OGTTs) at baseline prior to first dose of lumacaftor/ivacaftor, and at 3, 6 and 12 months on therapy. OGTT glucose, insulin and c-peptide parameters were compared.

RESULTS

Compared to baseline, OGTT fasting and 2 hour glucose levels, glucose area under the curve, insulin area under the curve and time to peak insulin level were not significantly different at 3, 6 and 12 months on lumacaftor/ivacaftor therapy. Similarly, C-peptide levels were no different.

CONCLUSIONS

Lumacaftor/ivacaftor therapy did not improve insulin secretion or glucose tolerance in patients with CF who were homozygous for the F508del mutation.

Cystic fibrosis-related diabetes: an update on pathophysiology, diagnosis, and treatment

Cystic fibrosis (CF) is a highly prevalent autosomal recessive disorder that is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene (7q31.2), which encodes the CFTR chloride-anion channel that is expressed in several tissues. Life expectancy has increased significantly over the past few decades due to therapeutic advances and early diagnosis through neonatal screening. However, new complications have been identified, including CF-related diabetes (CFRD). The earliest detectable glycemic abnormality is postprandial hyperglycemia that progresses into fasting hyperglycemia. CFRD is associated with a decline in lung function, impairments in weight gain and growth, pubertal development, and increased morbidity and mortality. Annual screening with oral glucose tolerance test is recommended beginning at the age of 10, and screenings are recommended for any age group during the first 48 h of hospital admission. Fasting plasma glucose levels ≥126 mg/dL (7.0 mmol/L) or 2-h postprandial plasma glucose levels ≥200 mg/dL (11.1 mmol/L) that persist for more than 48 h are diagnostic criteria for CFRD. Under stable health condition, the diagnosis is made when laboratory abnormalities in accordance with the American Diabetes Association criteria are detected for the first time; however, levels of HbA1c <6.5% do not rule out the diagnosis. Treatment for CFRD includes insulin replacement and a hypercaloric and hyperproteic diet that does not restrict carbohydrates, fats or salt, and diabetes self-management education. The most important CFRD complications are nutritional and pulmonary disease deterioration, though the microvascular complications of diabetes have already been described.

Characteristics of cystic fibrosis-related diabetes: Data from two different sources the European cystic fibrosis society patient registry and German/Austrian diabetes prospective follow-up registry

BACKGROUND

Standardized patient registries provide a unique basis to get insight into cystic fibrosis (CF)-related diabetes (CFRD), the most common comorbidity in CF.

METHODS

A total of 3853 CFRD patients from the European CF Society Patient Registry (ECFSPR) and 752 from the German/Austrian diabetes prospective follow-up (diabetes patienten verlaufsdokumentation [DPV]) were studied. To adjust for age and sex, multivariable regression was used (SAS 9.4).

RESULTS

DPV subjects were younger (26.5 [20.2-32.6] vs 28.3 [21.7-36.0] years, P < 0.001) and more often female (59.6 vs 50.9%, P < 0.001). In both registries, F508del homozygotes were most frequent, with higher proportion in DPV (80.9 vs 57.8%, P = 0.003). After adjustment, lung-transplantation (LTX) was more common in ECFSPR (18.9 vs 4.9%, P < 0.001), although duration since LTX (4.8 ± 0.2 vs 5.5 ± 0.7 years, P = 0.33) did not differ. In DPV patients without LTX, a lower BMI (19.6 ± 0.1 vs 21.0 ± 0.1 kg/m² , P < 0.001), higher proportion of underweight (41.2 vs 20.2%, P < 0.001) and a tendency towards worse lung function (%FEV₁ : 42.3 ± 4.2 vs 48.3 ± 0.5%, P = 0.16) were observed.

CONCLUSIONS

Between both registries, demographic and clinical differences of CFRD were present. Besides different kind of data sources, diverse treatment structures between countries may play a role. The results may further indicate a more serious illness in patients treated in specialized diabetes clinics, documenting their data in DPV.

MARKEDLY DELAYED INSULIN SECRETION AND A HIGH RATE OF UNDETECTED OVERT DIABETES CHARACTERIZE GLUCOSE METABOLISM IN ADULT PATIENTS WITH CYSTIC FIBROSIS AFTER LUNG TRANSPLANTATION

OBJECTIVE

Cystic fibrosis-related diabetes (CFRD) is associated with adverse clinical outcomes and should be screened for by an annual oral glucose tolerance test (OGTT). Since pathophysiologic studies have mainly been performed in a pediatric/adolescent, nontransplanted collective, we aimed to assess parameters of insulin secretion and sensitivity in adult cystic fibrosis (CF) patients after lung transplantation (LT).

METHODS

Twelve adult CF patients after LT without known diabetes (33.3 ± 11.5 years; body mass index [BMI] 21.5 ± 3.3 kg/m²) and 8 control subjects matched by age (36.0 ± 6.6 years; P>.05), BMI (22.3 ± 1.5 kg/m²; P>.05), and gender (CON group) underwent a 3-hour OGTT with glucose, insulin, and C-peptide measurements. Parameters of insulin secretion and sensitivity as well as lipid profiles were assessed.

RESULTS

In the CF group, 4 patients were diagnosed with overt diabetes (CFRD) compared to CF patients without diabetes (CF-noDM), of whom 6 had indeterminate glycemia with 1-h glucose values >200 mg/dL. The insulin peak after glucose load occurred after 30 minutes in CON, after 90 minutes in CF-noDM, and was missing in CFRD. Insulin sensitivity was comparable between the groups. Beta-cell glucose sensitivity was markedly reduced in CFRD (10.7 ± 5.8 pmol/min*m²*mM), higher in CF-noDM (39.9 ± 23.4 pmol/min*m²*mM), but still significantly lower compared to CON (108.3 ± 53.9 pmol/min*m²*mM; P = .0008). CFRD patients exhibited increased triglyceride levels and decreased high-density lipoprotein levels.

CONCLUSION

Adult CF patients after LT have profound disturbances in glucose metabolism, with a high rate of undetected diabetes and markedly delayed insulin secretion. Curbed beta-cell glucose sensitivity rather than insulin resistance explains postprandial hyperglycemia and is accompanied by abnormalities in lipid metabolism.

ABBREVIATIONS

AUC = area under the curve; BMI = body mass index; CF = cystic fibrosis; CFRD = cystic fibrosis-related diabetes; CFTR = cystic fibrosis transmembrane-conductance regulator; CF-TX = cystic fibrosis patients who underwent lung transplantation; CGM = continuous glucose monitoring; HbA1c = glycated hemoglobin; HDL = high-density lipoprotein; INDET = indeterminate glycemia; LDL = low-density lipoprotein; LT = lung transplantation; OGIS = oral glucose sensitivity index; OGTT = oral glucose tolerance test; QUICKI = quantitative insulin sensitivity check index.

High glucose alters fetal rat islet transcriptome and induces progeny islet dysfunction

Offspring of diabetic mothers are susceptible to developing type 2 diabetes due to pancreatic islet dysfunction. However, the initiating molecular pathways leading to offspring pancreatic islet dysfunction are unknown. We hypothesized that maternal hyperglycemia alters offspring pancreatic islet transcriptome and negatively impacts offspring islet function. We employed an infusion model capable of inducing localized hyperglycemia in fetal rats residing in the left uterine horn, thus avoiding other factors involved in programming offspring pancreatic islet health. While maintaining euglycemia in maternal dams and right uterine horn control fetuses, hyperglycemic fetuses in the left uterine horn had higher serum insulin and pancreatic beta cell area. Upon completing infusion from GD20 to 22, RNA sequencing was performed on GD22 islets to identify the hyperglycemia-induced altered gene expression. Ingenuity pathway analysis of the altered transcriptome found that diabetes mellitus and inflammation/cell death pathways were enriched. Interestingly, the downregulated genes modulate more diverse biological processes, which includes responses to stimuli and developmental processes. Next, we performed ex and in vivo studies to evaluate islet cell viability and insulin secretory function in weanling and adult offspring. Pancreatic islets of weanlings exposed to late gestation hyperglycemia had decreased cell viability in basal state and glucose-induced insulin secretion. Lastly, adult offspring exposed to in utero hyperglycemia also exhibited glucose intolerance and insulin secretory dysfunction. Together, our results demonstrate that late gestational hyperglycemia alters the fetal pancreatic islet transcriptome and increases offspring susceptibility to developing pancreatic islet dysfunction.

Survival in a bad neighborhood: pancreatic islets in cystic fibrosis

In cystic fibrosis (CF), ductal plugging and acinar loss result in rapid decline of exocrine pancreatic function. This destructive process results in remodeled islets, with only a modest reduction in insulin producing β cells. However, β-cell function is profoundly impaired, with decreased insulin release and abnormal glucose tolerance being present even in infants with CF. Ultimately, roughly half of CF subjects develop diabetes (termed CF-related diabetes, CFRD). Importantly, CFRD increases CF morbidity and mortality via worsening catabolism and pulmonary disease. Current accepted treatment options for CFRD are aimed at insulin replacement, thereby improving glycemia as well as preventing nutritional losses and lung decline. CFRD is a unique form of diabetes with a distinct pathophysiology that is as yet incompletely understood. Recent studies highlight emerging areas of interest. First, islet inflammation and lymphocyte infiltration are common even in young children with CF and may contribute to β-cell failure. Second, controversy exists in the literature regarding the presence/importance of β-cell intrinsic functions of CFTR and its direct role in modulating insulin release. Third, loss of the CF transmembrane conductance regulator (CFTR) from pancreatic ductal epithelium, the predominant site of its synthesis, results in paracrine effects that impair insulin release. Finally, the degree of β-cell loss in CFRD does not appear sufficient to explain the deficit in insulin release. Thus, it may be possible to enhance the function of the remaining β cells using strategies such as targeting islet inflammation or ductal CFTR deficiency to effectively treat or even prevent CFRD.

Defective exocytosis and processing of insulin in a cystic fibrosis mouse model

Cystic fibrosis-related diabetes (CFRD) is a common complication for patients with cystic fibrosis (CF), a disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR). The cause of CFRD is unclear, but a commonly observed reduction in first-phase insulin secretion suggests defects at the beta cell level. Here we aimed to examine beta- and alpha-cell function in the Cftrtm1EUR/F508del mouse model (C57BL/6J), which carries the most common human mutation in CFTR, the F508del mutation. CFTR expression, beta cell mass, insulin granule distribution, hormone secretion and single cell capacitance changes were evaluated using islets (or beta cells) from F508del mice and age-matched wild-type mice aged 7-10 weeks. Granular pH was measured with DND-189 fluorescence. Serum glucose, insulin and glucagon levels were measured in vivo, and glucose tolerance was assessed using IPGTT. We show increased secretion of proinsulin and concomitant reduced secretion of C-peptide in islets from F508del mice compared to WT mice. Exocytosis and number of docked granules was reduced. We confirmed reduced granular pH by CFTR stimulation. We detected decreased pancreatic beta cell area, but unchanged beta cell number. Moreover, the F508del mutation caused failure to suppress glucagon secretion leading to hyperglucagonemia. In conclusion, F508del mice have beta cell defects resulting in 1) reduced number of docked insulin granules and reduced exocytosis, and 2) potential defective proinsulin cleavage and secretion of immature insulin. These observations provide insight into the functional role of CFTR in pancreatic islets and contribute to increased understanding of the pathogenesis of CFRD.

Induced pluripotent stem cells for treating cystic fibrosis: State of the science

Induced pluripotent stem cells (iPSCs) are a recently developed technology in which fully differentiated cells such as fibroblasts from individual CF patients can be repaired with [wildtype] CFTR, and reprogrammed to differentiate into fully differentiated cells characteristic of the proximal and distal airways. Here, we review properties of different epithelial cells in the airway, and the in vitro genetic roadmap which iPSCs follow as they are step-wise differentiated into either basal stem cells, for the proximal airway, or into Type II Alveolar cells for the distal airways. The central theme is that iPSC-derived basal stem cells, are penultimately dependent on NOTCH signaling for differentiation into club cells, goblet cells, ciliated cells, and neuroendocrine cells. Furthermore, given the proper matrix, these cellular progenies are also able to self-assemble into a fully functional pseudostratified squamous proximal airway epithelium. By contrast, club cells are reserve stem cells which are able to either differentiate into goblet or ciliated cells, but also to de-differentiate into basal stem cells. Variant club cells, located at the transition between airway and alveoli, may also be responsible for differentiation into Type II Alveolar cells, which then differentiate into Type I Alveolar cells for gas exchange in the distal airway. Using gene editing, the mutant CFTR gene in iPSCs from CF patients can be repaired, and fully functional epithelial cells can thus be generated through directed differentiation. However, there is a limitation in that the lung has other CFTR-dependent cells besides epithelial cells. Another limitation is that there are CFTR-dependent cells in other organs which would continue to contribute to CF disease. Furthermore, there are also bystander or modifier genes which affect disease outcome, not only in the lung, but specifically in other CF-affected organs. Finally, we discuss future personalized applications of the iPSC technology, many of which have already survived the "proof-of-principle" test. These include (i) patient-derived iPSCs used as a "lung-on-a-chip" tool for personalized drug discovery; (ii) replacement of mutant lung cells by wildtype lung cells in the living lung; and (iii) development of bio-artificial lungs. It is hoped that this review will give the reader a roadmap through the most complicated of the obstacles, and foster a guardedly optimistic view of how some of the remaining obstacles might one day be overcome.

The Clinical Biology of Cystic Fibrosis Transmembrane Regulator Protein: Its Role and Function in Extrapulmonary Disease

Normal cystic fibrosis (CF) transmembrane regulator (CFTR) protein has multiple functions in health and disease. Many mutations in the CFTR gene produce abnormal or absent protein. CFTR protein dysfunction underlies the classic CF phenotype of progressive pulmonary and GI pathology but may underlie diseases not usually associated with CF. This review highlights selected extrapulmonary disease that may be associated with abnormal CFTR. Increasing survival in CF is associated with increasing incidence of diseases associated with aging. CFTR dysfunction in older individuals may have novel effects on glucose metabolism, control of insulin release, regulation of circadian rhythm, and cancer cell pathophysiology. In individuals who have cancers with acquired CFTR suppression, their tumors may more likely exhibit rapid expansion, epithelial-to-mesenchymal transformation, abnormally reduced apoptosis, and increased metastatic potential. The new modulators of CFTR protein synthesis could facilitate the additional exploration needed to better understand the unfolding clinical biology of CFTR in human disease, even as they revolutionize treatment of patients with CF.

Cystic Fibrosis: Emerging Understanding and Therapies

Cystic fibrosis (CF) is the most common life-limiting genetic disease in Caucasian patients. Continued advances have led to improved survival, and adults with CF now outnumber children. As our understanding of the disease improves, new therapies have emerged that improve the basic defect, enabling patient-specific treatment and improved outcomes. However, recurrent exacerbations continue to lead to morbidity and mortality, and new pathogens have been identified that may lead to worse outcomes. In addition, new complications, such as CF-related diabetes and increased risk of gastrointestinal cancers, are creating new challenges in management. For patients with end-stage disease, lung transplantation has remained one of the few treatment options, but challenges in identifying the most appropriate patients remain.

LRRC8/VRAC anion channels enhance β-cell glucose sensing and insulin secretion

Glucose homeostasis depends critically on insulin that is secreted by pancreatic β-cells. Serum glucose, which is directly sensed by β-cells, stimulates depolarization- and Ca²⁺-dependent exocytosis of insulin granules. Here we show that pancreatic islets prominently express LRRC8A and LRRC8D, subunits of volume-regulated VRAC anion channels. Hypotonicity- or glucose-induced β-cell swelling elicits canonical LRRC8A-dependent VRAC currents that depolarize β-cells to an extent that causes electrical excitation. Glucose-induced excitation and Ca²⁺ responses are delayed in onset, but not abolished, in β-cells lacking the essential VRAC subunit LRRC8A. Whereas Lrrc8a disruption does not affect tolbutamide- or high-K⁺-induced insulin secretion from pancreatic islets, it reduces first-phase glucose-induced insulin secretion. Mice lacking VRAC in β-cells have normal resting serum glucose levels but impaired glucose tolerance. We propose that opening of LRRC8/VRAC channels increases glucose sensitivity and insulin secretion of β-cells synergistically with K_ATP closure. Neurotransmitter-permeable LRRC8D-containing VRACs might have additional roles in autocrine/paracrine signaling within islets.

LRRC8/VRAC anion channels enhance β-cell glucose sensing and insulin secretion

Glucose homeostasis depends critically on insulin that is secreted by pancreatic β-cells. Serum glucose, which is directly sensed by β-cells, stimulates depolarization- and Ca²⁺-dependent exocytosis of insulin granules. Here we show that pancreatic islets prominently express LRRC8A and LRRC8D, subunits of volume-regulated VRAC anion channels. Hypotonicity- or glucose-induced β-cell swelling elicits canonical LRRC8A-dependent VRAC currents that depolarize β-cells to an extent that causes electrical excitation. Glucose-induced excitation and Ca²⁺ responses are delayed in onset, but not abolished, in β-cells lacking the essential VRAC subunit LRRC8A. Whereas Lrrc8a disruption does not affect tolbutamide- or high-K⁺-induced insulin secretion from pancreatic islets, it reduces first-phase glucose-induced insulin secretion. Mice lacking VRAC in β-cells have normal resting serum glucose levels but impaired glucose tolerance. We propose that opening of LRRC8/VRAC channels increases glucose sensitivity and insulin secretion of β-cells synergistically with K_ATP closure. Neurotransmitter-permeable LRRC8D-containing VRACs might have additional roles in autocrine/paracrine signaling within islets.

Abnormal CFTR Affects Glucagon Production by Islet α Cells in Cystic Fibrosis and Polycystic Ovarian Syndrome

Glucagon, produced by islet α cells, functions to increase blood glucose. Abnormal glucose levels are often seen in cystic fibrosis (CF), a systematic disease caused by mutations of the CF transmembrane conductance regulator (CFTR), and in polycystic ovarian syndrome (PCOS), an endocrine disorder featured with hyperandrogenism affecting 5-10% women of reproductive age. Here, we explored the role of CFTR in glucagon production in α cells and its possible contribution to glucagon disturbance in CF and PCOS. We found elevated fasting glucagon levels in CFTR mutant (DF508) mice compared to the wildtypes. Glucagon and prohormone convertase 2 (PC2) were also upregulated in CFTR inhibitor-treated or DF508 islets, as compared to the controls or wildtypes, respectively. Dihydrotestosterone (DHT)-induced PCOS rats exhibited significantly lower fasting glucagon levels with higher CFTR expression in α cells compared to that of controls. Treatment of mouse islets or αTC1-9 cells with DHT enhanced CFTR expression and reduced the levels of glucagon and PC2. The inhibitory effect of DHT on glucagon production was blocked by CFTR inhibitors in mouse islets, and mimicked by overexpressing CFTR in αTC1-9 cells with reduced phosphorylation of the cAMP/Ca2+ response element binding protein (p-CREB), a key transcription factor for glucagon and PC2. These results revealed a previously undefined role of CFTR in suppressing glucagon production in α-cells, defects in which may contribute to glucose metabolic disorder seen in CF and PCOS.

Oral glucose tolerance test and continuous glucose monitoring to assess diabetes development in cystic fibrosis patients

INTRODUCTION

Patients with cystic fibrosis (CF) undergo a slow and progressive process toward diabetes. Oral glucose tolerance test (OGTT) is recommended to diagnose impaired glucose levels in these patients. Continuous glucose monitoring (CGM) measures glucose profiles under real-life conditions.

OBJECTIVE

To compare OGTT and CGM results in CF patients.

METHODS

Paired OGTT and 6-day CGM profiles (146.2±9.1h/patient) were performed in 30 CF patients aged 10-18 years.

RESULTS

According to OGTT, 14 patients had normal glucose tolerance (NGT), 14 abnormal glucose tolerance (AGT), and two cystic fibrosis-related diabetes (CFRD). In 27 patients (13 NGT, 13 AGT, 1 CFRD), CGM showed glucose values ranging from 140 to 200mg/dL during similar monitoring times (2%-14% with NGT, 1%-16.9% with AGT, and 3% with CFRD). Glucose peak levels ≥200mg/dL were seen in seven patients (3 NGT, 3 AGT, 1 CFRD). According to CGM, two patients had all glucose values under 140mg/dL (1 NGT, 1 AGT). Seventeen patients had glucose levels ranging from 140 to 200mg/dL (10 NGT, 6 AGT, 1 CFRD). Ten patients (3 NGT, 7 AGT) had glucose values ≥200mg/dL for ≤1% of the monitoring time and one (CFRD) for >1% of the monitoring time.

CONCLUSIONS

OGTT results did not agree with those of the CGM. CGM allows for diagnosis of glucose changes not detected by OGTT. Such changes may contribute to optimize pre-diabetes management in CF patients.

Evolving Mechanistic Views and Emerging Therapeutic Strategies for Cystic Fibrosis-Related Diabetes

Diabetes is a common and important complication of cystic fibrosis, an autosomal recessive genetic disease due to mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Cystic fibrosis-related diabetes (CFRD) is associated with profound detrimental effects on the disease course and mortality and is expected to increase in prevalence as the survival of patients with cystic fibrosis continues to improve. Despite progress in the functional characterization of CFTR molecular defects, the mechanistic basis of CFRD is not well understood, in part because of the relative inaccessibility of the pancreatic tissue and the limited availability of representative animal models. This review presents a concise overview of the current understanding of CFRD pathogenesis and provides a cutting-edge update on novel findings from human and animal studies. Potential contributions from paracrine mechanisms and β-cell compensatory mechanisms are highlighted, as well as functional β-cell and α-cell defects, incretin defects, exocrine pancreatic insufficiency, and loss of islet cell mass. State-of-the-art and emerging treatment options are explored, including advances in insulin administration, CFTR modulators, cell replacement, gene replacement, and gene editing therapies.

CFTR Influences Beta Cell Function and Insulin Secretion Through Non-Cell Autonomous Exocrine-Derived Factors

Although β-cell dysfunction in cystic fibrosis (CF) leads to diabetes, the mechanism by which the cystic fibrosis transmembrane conductance regulator (CFTR) channel influences islet insulin secretion remains debated. We investigated the CFTR-dependent islet-autonomous mechanisms affecting insulin secretion by using islets isolated from CFTR knockout ferrets. Total insulin content was lower in CF as compared with wild-type (WT) islets. Furthermore, glucose-stimulated insulin secretion (GSIS) was impaired in perifused neonatal CF islets, with reduced first, second, and amplifying phase secretion. Interestingly, CF islets compensated for reduced insulin content under static low-glucose conditions by secreting a larger fraction of islet insulin than WT islets, probably because of elevated SLC2A1 transcripts, increased basal inhibition of adenosine triphosphate-sensitive potassium channels (K-ATP), and elevated basal intracellular Ca2+. Interleukin (IL)-6 secretion by CF islets was higher relative to WT, and IL-6 treatment of WT ferret islets produced a CF-like phenotype with reduced islet insulin content and elevated percentage insulin secretion in low glucose. CF islets exhibited altered expression of INS, CELA3B, and several β-cell maturation and proliferation genes. Pharmacologic inhibition of CFTR reduced GSIS by WT ferret and human islets but similarly reduced insulin secretion and intracellular Ca2+ in CFTR knockout ferret islets, indicating that the mechanism of action is not through CFTR. Single-molecule fluorescent in situ hybridization, on isolated ferret and human islets and ferret pancreas, demonstrated that CFTR RNA colocalized within KRT7+ ductal cells but not endocrine cells. These results suggest that CFTR affects β-cell function via a paracrine mechanism involving proinflammatory factors secreted from islet-associated exocrine-derived cell types.

The Mechanistic Links between Insulin and Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Cl- Channel

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel belongs to the ATP-binding cassette (ABC) transporter superfamily and regulates Cl- secretion in epithelial cells for water secretion. Loss-of-function mutations to the CFTR gene cause dehydrated mucus on the apical side of epithelial cells and increase the susceptibility of bacterial infection, especially in the airway and pulmonary tissues. Therefore, research on the molecular properties of CFTR, such as its gating mechanism and subcellular trafficking, have been intensively pursued. Dysregulated CFTR trafficking is one of the major pathological hallmarks in cystic fibrosis (CF) patients bearing missense mutations in the CFTR gene. Hormones that activate cAMP signaling, such as catecholamine, have been found to regulate the intracellular trafficking of CFTR. Insulin is one of the hormones that regulate cAMP production and promote trafficking of transmembrane proteins to the plasma membrane. The functional interactions between insulin and CFTR have not yet been clearly defined. In this review article, I review the roles of CFTR in epithelial cells, its regulatory role in insulin secretion, and a mechanism of CFTR regulation by insulin.

Cystic Fibrosis Related Diabetes: a Unique Challenge in Diabetes Care

Cystic Fibrosis (CF) is a common autosomal recessive disease that affects multiple organs due to a defect in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR). This transporter is present in various organs and tissues, including the airway epithelium, sinuses, pancreas, intestine, biliary tree, the vas deferens, and the sweat ducts, making CF a multi-system disease1. As CF patients are living longer, pancreatic function declines and diabetes emerges, further complicating the nutritional status and care of these patients.