CFTR: a hub for kinases and crosstalk of cAMP and Ca2+

Elevated Prostaglandin E2 Synthesis Is Associated with Clinical and Radiological Disease Severity in Cystic Fibrosis

Background: Previous studies found high but very variable levels of tetranor-PGEM and PGDM (urine metabolites of prostaglandin (PG) E2 and PGD2, respectively) in persons with cystic fibrosis (pwCF). This study aims to assess the role of cyclooxygenase COX-1 and COX-2 genetic polymorphisms in PG production and of PG metabolites as potential markers of symptoms' severity and imaging findings. Methods: A total of 30 healthy subjects and 103 pwCF were included in this study. Clinical and radiological CF severity was evaluated using clinical scoring methods and chest computed tomography (CT), respectively. Urine metabolites were measured using liquid chromatography/tandem mass spectrometry. Variants in the COX-1 gene (PTGS1 639 C>A, PTGS1 762+14delA and COX-2 gene: PTGS2-899G>C (-765G>C) and PTGS2 (8473T>C) were also analyzed. Results: PGE-M and PGD-M urine concentrations were significantly higher in pwCF than in controls. There were also statistically significant differences between clinically mild and moderate disease and severe disease. Patients with bronchiectasis and/or air trapping had higher PGE-M levels than patients without these complications. The four polymorphisms did not associate with clinical severity, air trapping, bronchiectasis, or urinary PG levels. Conclusions: These results suggest that urinary PG level testing can be used as a biomarker of CF severity. COX genetic polymorphisms are not involved in the variability of PG production.

The role of CFTR channel in female infertility

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-activated trans-membrane ATP gated anion channel present in most epithelia, which transports chloride and bicarbonate ions across the apical membrane. Mutations in the CFTR protein are known to result in defective expression or function, notably the inhibition of chloride and bicarbonate transport. This can result in cystic fibrosis (CF), a disorder characterised by thickness of the mucus lining of the epithelial cells of the alimentary and respiratory tracts, sweat ducts and reproductive organs. As a consequence, there is a reduction in fluid transport at the apical surface. While the most devastating effect of CF is mortality, about 98% of men with CF are infertile, consequent of early blockage of or failure to develop the mesonephrotic ducts as well as the vas deferens. The effect of CF of female fertility is less well-understood. This review highlights the genetics and pathophysiology as well as the mechanism of action of CF on female infertility.

Pathogenic Relationships in Cystic Fibrosis and Renal Diseases: CFTR, SLC26A9 and Anoctamins

The Cl--transporting proteins CFTR, SLC26A9, and anoctamin (ANO1; ANO6) appear to have more in common than initially suspected, as they all participate in the pathogenic process and clinical outcomes of airway and renal diseases. In the present review, we will therefore concentrate on recent findings concerning electrolyte transport in the airways and kidneys, and the role of CFTR, SLC26A9, and the anoctamins ANO1 and ANO6. Special emphasis will be placed on cystic fibrosis and asthma, as well as renal alkalosis and polycystic kidney disease. In essence, we will summarize recent evidence indicating that CFTR is the only relevant secretory Cl- channel in airways under basal (nonstimulated) conditions and after stimulation by secretagogues. Information is provided on the expressions of ANO1 and ANO6, which are important for the correct expression and function of CFTR. In addition, there is evidence that the Cl- transporter SLC26A9 expressed in the airways may have a reabsorptive rather than a Cl--secretory function. In the renal collecting ducts, bicarbonate secretion occurs through a synergistic action of CFTR and the Cl-/HCO3- transporter SLC26A4 (pendrin), which is probably supported by ANO1. Finally, in autosomal dominant polycystic kidney disease (ADPKD), the secretory function of CFTR in renal cyst formation may have been overestimated, whereas ANO1 and ANO6 have now been shown to be crucial in ADPKD and therefore represent new pharmacological targets for the treatment of polycystic kidney disease.

Dietary Na+ depletion up-regulates NKCC1 expression and enhances electrogenic Cl- secretion in rat proximal colon

The corticosteroid hormone, aldosterone, markedly enhances K+ secretion throughout the colon, a mechanism critical to its role in maintaining overall K+ balance. Previous studies demonstrated that basolateral NKCC1 was up-regulated by aldosterone in the distal colon specifically to support K+ secretion-which is distinct from the more well-established role of NKCC1 in supporting luminal Cl- secretion. However, considerable segmental variability exists between proximal and distal colonic ion transport processes, especially concerning their regulation by aldosterone. Furthermore, delineating such region-specific effects has important implications for the management of various gastrointestinal pathologies. Experiments were therefore designed to determine whether aldosterone similarly up-regulates NKCC1 in the proximal colon to support K+ secretion. Using dietary Na+ depletion as a model of secondary hyperaldosteronism in rats, we found that proximal colon NKCC1 expression was indeed enhanced in Na+-depleted (i.e., hyperaldosteronemic) rats. Surprisingly, electrogenic K+ secretion was not detectable by short-circuit current (ISC) measurements in response to either basolateral bumetanide (NKCC1 inhibitor) or luminal Ba2+ (non-selective K+ channel blocker), despite enhanced K+ secretion in Na+-depleted rats, as measured by 86Rb+ fluxes. Expression of BK and IK channels was also found to be unaltered by dietary Na+ depletion. However, bumetanide-sensitive basal and agonist-stimulated Cl- secretion (ISC) were significantly enhanced by Na+ depletion, as was CFTR Cl- channel expression. These data suggest that NKCC1-dependent secretory pathways are differentially regulated by aldosterone in proximal and distal colon. Development of therapeutic strategies in treating pathologies related to aberrant colonic K+/Cl- transport-such as pseudo-obstruction or ulcerative colitis-may benefit from these findings.

Thiopurines impair the apical plasma membrane expression of CFTR in pancreatic ductal cells via RAC1 inhibition

BACKGROUND AND AIMS

Thiopurine-induced acute pancreatitis (TIP) is one of the most common adverse events among inflammatory bowel disease patients treated with azathioprine (AZA), representing a significant clinical burden. Previous studies focused on immune-mediated processes, however, the exact pathomechanism of TIP is essentially unclear.

METHODS

To model TIP in vivo, we triggered cerulein-induced experimental pancreatitis in mice receiving a daily oral dose of 1.5 mg/kg AZA. Also, freshly isolated mouse pancreatic cells were exposed to AZA ex vivo, and acinar cell viability, ductal and acinar Ca²⁺ signaling, ductal Cl^- and HCO₃^- secretion, as well as cystic fibrosis transmembrane conductance regulator (CFTR) expression were assessed using microscopy techniques. Ras-related C3 botulinum toxin substrate (RAC1) activity was measured with a G-LISA assay. Super-resolution microscopy was used to determine protein colocalization.

RESULTS

We demonstrated that AZA treatment increases tissue damage in the early phase of cerulein-induced pancreatitis in vivo. Also, both per os and ex vivo AZA exposure impaired pancreatic fluid and ductal HCO₃^- and Cl^- secretion, but did not affect acinar cells. Furthermore, ex vivo AZA exposure also inhibited RAC1 activity in ductal cells leading to decreased co-localization of CFTR and the anchor protein ezrin, resulting in impaired plasma membrane localization of CFTR.

CONCLUSIONS

AZA impaired the ductal HCO₃^- and Cl^- secretion through the inhibition of RAC1 activity leading to diminished ezrin-CFTR interaction and disturbed apical plasma membrane expression of CFTR. We report a novel direct toxic effect of AZA on pancreatic ductal cells and suggest that the restoration of ductal function might help to prevent TIP in the future.

Loss of CFTR function is associated with reduced bitter taste receptor-stimulated nitric oxide innate immune responses in nasal epithelial cells and macrophages

Introduction

Bitter taste receptors (T2Rs) are G protein-coupled receptors identified on the tongue but expressed all over the body, including in airway cilia and macrophages, where T2Rs serve an immune role. T2R isoforms detect bitter metabolites (quinolones and acyl-homoserine lactones) secreted by gram negative bacteria, including Pseudomonas aeruginosa, a major pathogen in cystic fibrosis (CF). T2R activation by bitter bacterial products triggers calcium-dependent nitric oxide (NO) production. In airway cells, the NO increases mucociliary clearance and has direct antibacterial properties. In macrophages, the same pathway enhances phagocytosis. Because prior studies linked CF with reduced NO, we hypothesized that CF cells may have reduced T2R/NO responses, possibly contributing to reduced innate immunity in CF.

Methods

Immunofluorescence, qPCR, and live cell imaging were used to measure T2R localization, calcium and NO signaling, ciliary beating, and antimicrobial responses in air-liquid interface cultures of primary human nasal epithelial cells and immortalized bronchial cell lines. Immunofluorescence and live cell imaging was used to measure T2R signaling and phagocytosis in primary human monocyte-derived macrophages.

Results

Primary nasal epithelial cells from both CF and non-CF patients exhibited similar T2R expression, localization, and calcium signals. However, CF cells exhibited reduced NO production also observed in immortalized CFBE41o- CF cells and non-CF 16HBE cells CRISPR modified with CF-causing mutations in the CF transmembrane conductance regulator (CFTR). NO was restored by VX-770/VX-809 corrector/potentiator pre-treatment, suggesting reduced NO in CF cells is due to loss of CFTR function. In nasal cells, reduced NO correlated with reduced ciliary and antibacterial responses. In primary human macrophages, inhibition of CFTR reduced NO production and phagocytosis during T2R stimulation.

Conclusions

Together, these data suggest an intrinsic deficiency in T2R/NO signaling caused by loss of CFTR function that may contribute to intrinsic susceptibilities of CF patients to P. aeruginosa and other gram-negative bacteria that activate T2Rs.

The Impact of Highly Effective Modulator Therapy on Cystic Fibrosis Microbiology and Inflammation

Highly effective cystic fibrosis (CF) transmembrane conductance regulator (CFTR) modulator therapy (HEMT) corrects the underlying molecular defect causing CF disease. HEMT decreases symptom burden and improves clinical metrics and quality of life for most people with CF (PwCF) and eligible cftr mutations. Improvements in measures of pulmonary health suggest that restoration of function of defective CFTR anion channels by HEMT not only enhances airway mucociliary clearance, but also reduces chronic pulmonary infection and inflammation. This article reviews the evidence for how HEMT influences the dynamic and interdependent processes of infection and inflammation in the CF airway, and what questions remain unanswered.

CFTR, Cell Junctions and the Cytoskeleton

The multi-organ disease cystic fibrosis (CF) is caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR) protein, a cAMP regulated chloride (Cl⁻) and bicarbonate (HCO₃⁻) ion channel expressed at the apical plasma membrane (PM) of epithelial cells. Reduced CFTR protein results in decreased Cl⁻ secretion and excessive sodium reabsorption in epithelial cells, which consequently leads to epithelial dehydration and the accumulation of thick mucus within the affected organs, such as the lungs, pancreas, gastrointestinal (GI) tract, reproductive system and sweat glands. However, CFTR has been implicated in other functions besides transporting ions across epithelia. The rising number of references concerning its association to actin cytoskeleton organization, epithelial cell junctions and extracellular matrix (ECM) proteins suggests a role in the formation and maintenance of epithelial apical basolateral polarity. This review will focus on recent literature (the last 10 years) substantiating the role of CFTR in cell junction formation and actin cytoskeleton organization with its connection to the ECM.

Dysfunction in the Cystic Fibrosis Transmembrane Regulator in Chronic Obstructive Pulmonary Disease as a Potential Target for Personalised Medicine

In recent years, numerous pathways were explored in the pathogenesis of COPD in the quest for new potential therapeutic targets for more personalised medical care. In this context, the study of the cystic fibrosis transmembrane conductance regulator (CFTR) began to gain importance, especially since the advent of the new CFTR modulators which had the potential to correct this protein's dysfunction in COPD. The CFTR is an ion transporter that regulates the hydration and viscosity of mucous secretions in the airway. Therefore, its abnormal function favours the accumulation of thicker and more viscous secretions, reduces the periciliary layer and mucociliary clearance, and produces inflammation in the airway, as a consequence of a bronchial infection by both bacteria and viruses. Identifying CFTR dysfunction in the context of COPD pathogenesis is key to fully understanding its role in the complex pathophysiology of COPD and the potential of the different therapeutic approaches proposed to overcome this dysfunction. In particular, the potential of the rehydration of mucus and the role of antioxidants and phosphodiesterase inhibitors should be discussed. Additionally, the modulatory drugs which enhance or restore decreased levels of the protein CFTR were recently described. In particular, two CFTR potentiators, ivacaftor and icenticaftor, were explored in COPD. The present review updated the pathophysiology of the complex role of CFTR in COPD and the therapeutic options which could be explored.

Caco-2/HT29-MTX co-cultured cells as a model for studying physiological properties and toxin-induced effects on intestinal cells

Infectious gastrointestinal diseases are frequently caused by toxins secreted by pathogens which may impair physiological functions of the intestines, for instance by cholera toxin or by heat-labile enterotoxin. To obtain a functional model of the human intestinal epithelium for studying toxin-induced disease mechanisms, differentiated enterocyte-like Caco-2 cells were co-cultured with goblet cell-like HT29-MTX cells. These co-cultures formed a functional epithelial barrier, as characterized by a high electrical resistance and the presence of physiological intestinal properties such as glucose transport and chloride secretion which could be demonstrated electrophysiologically and by measuring protein expression. When the tissues were exposed to cholera toxin or heat-labile enterotoxin in the Ussing chamber, cholera toxin incubation resulted in an increase in short-circuit currents, indicating an increase in apical chloride secretion. This is in line with typical cholera toxin-induced secretory diarrhea in humans, while heat-labile enterotoxin only showed an increase in short-circuit-current in Caco-2 cells. This study characterizes for the first time the simultaneous measurement of physiological properties on a functional and structural level combined with the epithelial responses to bacterial toxins. In conclusion, using this model, physiological responses of the intestine to bacterial toxins can be investigated and characterized. Therefore, this model can serve as an alternative to the use of laboratory animals for characterizing pathophysiological mechanisms of enterotoxins at the intestinal level.

Adenylate Cyclase 1 Links Calcium Signaling to CFTR-Dependent Cytosolic Chloride Elevations in Chick Amacrine Cells

The strength and sign of synapses involving ionotropic GABA and glycine receptors are dependent upon the Cl^- gradient. We have shown that nitric oxide (NO) elicits the release of Cl^- from internal acidic stores in retinal amacrine cells (ACs); temporarily altering the Cl^- gradient and the strength or even sign of incoming GABAergic or glycinergic synapses. The underlying mechanism for this effect of NO requires the cystic fibrosis transmembrane regulator (CFTR) but the link between NO and CFTR activation has not been determined. Here, we test the hypothesis that NO-dependent Ca²⁺ elevations activate the Ca²⁺-dependent adenylate cyclase 1 (AdC1) leading to activation of protein kinase A (PKA) whose activity is known to open the CFTR channel. Using the reversal potential of GABA-gated currents to monitor cytosolic Cl^-, we established the requirement for Ca²⁺ elevations. Inhibitors of AdC1 suppressed the NO-dependent increases in cytosolic Cl^- whereas inhibitors of other AdC subtypes were ineffective suggesting that AdC1 is involved. Inhibition of PKA also suppressed the action of NO. To address the sufficiency of this pathway in linking NO to elevations in cytosolic Cl^-, GABA-gated currents were measured under internal and external zero Cl^- conditions to isolate the internal Cl^- store. Activators of the cAMP pathway were less effective than NO in producing GABA-gated currents. However, coupling the cAMP pathway activators with the release of Ca²⁺ from stores produced GABA-gated currents indistinguishable from those stimulated with NO. Together, these results demonstrate that cytosolic Ca²⁺ links NO to the activation of CFTR and the elevation of cytosolic Cl^-.

Intestinal organoid-based 2D monolayers mimic physiological and pathophysiological properties of the pig intestine

Gastrointestinal infectious diseases remain an important issue for human and animal health. Investigations on gastrointestinal infectious diseases are classically performed in laboratory animals leading to the problem that species-specific models are scarcely available, especially when it comes to farm animals. The 3R principles of Russel and Burch were achieved using intestinal organoids of porcine jejunum. These organoids seem to be a promising tool to generate species-specific in vitro models of intestinal epithelium. 3D Organoids were grown in an extracellular matrix and characterized by qPCR. Organoids were also seeded on permeable filter supports in order to generate 2D epithelial monolayers. The organoid-based 2D monolayers were characterized morphologically and were investigated regarding their potential to study physiological transport properties and pathophysiological processes. They showed a monolayer structure containing different cell types. Moreover, their functional activity was demonstrated by their increasing transepithelial electrical resistance over 18 days and by an active glucose transport and chloride secretion. Furthermore, the organoid-based 2D monolayers were also confronted with cholera toxin derived from Vibrio cholerae as a proof of concept. Incubation with cholera toxin led to an increase of short-circuit current indicating an enhanced epithelial chloride secretion, which is a typical characteristic of cholera infections. Taken this together, our model allows the investigation of physiological and pathophysiological mechanisms focusing on the small intestine of pigs. This is in line with the 3R principle and allows the reduction of classical animal experiments.

Transcriptomic analysis of CFTR-impaired endothelial cells reveals a pro-inflammatory phenotype

Cystic fibrosis (CF) is a life-threatening disorder characterised by decreased pulmonary mucociliary and pathogen clearance, and an exaggerated inflammatory response leading to progressive lung damage. CF is caused by bi-allelic pathogenic variants of the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which encodes a chloride channel. CFTR is expressed in endothelial cells (ECs) and EC dysfunction has been reported in CF patients, but a role for this ion channel in ECs regarding CF disease progression is poorly described.We used an unbiased RNA sequencing approach in complementary models of CFTR silencing and blockade (by the CFTR inhibitor CFTRinh-172) in human ECs to characterise the changes upon CFTR impairment. Key findings were further validated in vitro and in vivo in CFTR-knockout mice and ex vivo in CF patient-derived ECs.Both models of CFTR impairment revealed that EC proliferation, migration and autophagy were downregulated. Remarkably though, defective CFTR function led to EC activation and a persisting pro-inflammatory state of the endothelium with increased leukocyte adhesion. Further validation in CFTR-knockout mice revealed enhanced leukocyte extravasation in lung and liver parenchyma associated with increased levels of EC activation markers. In addition, CF patient-derived ECs displayed increased EC activation markers and leukocyte adhesion, which was partially rescued by the CFTR modulators VX-770 and VX-809.Our integrated analysis thus suggests that ECs are no innocent bystanders in CF pathology, but rather may contribute to the exaggerated inflammatory phenotype, raising the question of whether normalisation of vascular inflammation might be a novel therapeutic strategy to ameliorate the disease severity of CF.

Prostaglandin E2 stimulates anion and fluid secretion triggered by lipopolysaccharide in rat vaginal epithelium

Prostaglandin E2 (PGE2) is a principal lipid mediator mediating various biological processes including immune responses and fluid secretion. As the first line of host defense against infection, vaginal epithelium plays orchestrated roles in vaginal innate immunity. However, the effect of PGE2 triggered by pro-inflammatory stimuli on vaginal epithelium remains elusive. This study aimed to investigate the regulatory role of PGE2 on vaginal epithelium after lipopolysaccharide (LPS) stimulation. RT-PCR and western blot analysis revealed that E-prostanoid (EP) receptors EP2 and EP4 were expressed in rat vagina. Basolateral application of PGE2 induced anion secretion mediated by cystic fibrosis transmembrane conductance regulator (CFTR) via EP-adenylate cyclase-cAMP signaling pathway in rat vaginal epithelial cells. The in vivo study showed that PGE2 promoted fluid secretion in rat vagina. Moreover, LPS stimulation facilitated cyclooxygenase-dependent PGE2 synthesis and vaginal fluid secretion in vivo. Conclusively, LPS stimulation triggered epithelium-derived PGE2 production in vaginal epithelium, leading to CFTR-mediated anion secretion and luminal flushing. This study provides valuable insights into the physiological role of PGE2 during vaginal bacterial infection.

Cystic fibrosis: Physiopathology and the latest pharmacological treatments

Cystic fibrosis (CF) is a lethal autosomal recessive genetic disease, caused by a mutation in the cystic fibrosis transmembrane conductance regulator gene (CFTR), which primarily affects the lungs and digestive system. This gene encodes the CFTR protein, a distinctive membrane transporter of the ATP-binding cassette (ABC) superfamily. It functions as a chloride channel, allowing the balance and transport of chloride through the apical membrane of epithelial cells. Due to its ubiquitous location, mutations in the CFTR gene trigger multiple changes in ion transport and metabolic pathways, affecting various organs, as it will be herein explained. Pulmonary impairment is the most characteristic comorbidity of CF and respiratory failure is the main cause of death. This review presents the importance of an early diagnosis of CF to establish, as soon as possible, a primary therapy for symptomatic prevention and relief. It also mentions new therapeutic approaches that include CFTR modulators. They are correctors and/or potentiators of the deficient CFTR channel. In an attempt to overcome the disadvantages of CFTR modulators, the application of biotechnology techniques is addressed, such as gene therapy, gene editing, RNA therapy and therapeutic microRNAs. The potential of the intranasal administration route is another presented aspect.

Targeting the phosphoinositide-3-kinase/protein kinase B pathway in airway innate immunity

The airway innate immune system maintains the first line of defense against respiratory infections. The airway epithelium and associated immune cells protect the respiratory system from inhaled foreign organisms. These cells sense pathogens via activation of receptors like toll-like receptors and taste family 2 receptors (T2Rs) and respond by producing antimicrobials, inflammatory cytokines, and chemokines. Coordinated regulation of fluid secretion and ciliary beating facilitates clearance of pathogens via mucociliary transport. Airway cells also secrete antimicrobial peptides and radicals to directly kill microorganisms and inactivate viruses. The phosphoinositide-3-kinase/protein kinase B (Akt) kinase pathway regulates multiple cellular targets that modulate cell survival and proliferation. Akt also regulates proteins involved in innate immune pathways. Akt phosphorylates endothelial nitric oxide synthase (eNOS) enzymes expressed in airway epithelial cells. Activation of eNOS can have anti-inflammatory, anti-bacterial, and anti-viral roles. Moreover, Akt can increase the activity of the transcription factor nuclear factor erythroid 2 related factor-2 that protects cells from oxidative stress and may limit inflammation. In this review, we summarize the recent findings of non-cancerous functions of Akt signaling in airway innate host defense mechanisms, including an overview of several known downstream targets of Akt involved in innate immunity.

NME/NM23/NDPK and Histidine Phosphorylation

The NME (Non-metastatic) family members, also known as NDPKs (nucleoside diphosphate kinases), were originally identified and studied for their nucleoside diphosphate kinase activities. This family of kinases is extremely well conserved through evolution, being found in prokaryotes and eukaryotes, but also diverges enough to create a range of complexity, with homologous members having distinct functions in cells. In addition to nucleoside diphosphate kinase activity, some family members are reported to possess protein-histidine kinase activity, which, because of the lability of phosphohistidine, has been difficult to study due to the experimental challenges and lack of molecular tools. However, over the past few years, new methods to investigate this unstable modification and histidine kinase activity have been reported and scientific interest in this area is growing rapidly. This review presents a global overview of our current knowledge of the NME family and histidine phosphorylation, highlighting the underappreciated protein-histidine kinase activity of NME family members, specifically in human cells. In parallel, information about the structural and functional aspects of the NME family, and the knowns and unknowns of histidine kinase involvement in cell signaling are summarized.

Segment-specific effects of resveratrol on porcine small intestinal dipeptide absorption depend on the mucosal pH and are due to different mechanisms: potential roles of different transport proteins and protein kinases

Numerous beneficial features of the polyphenol resveratrol (RSV) have been demonstrated in several tissues and cell culture models. There is also evidence, that RSV impairs intestinal nutrient transport but the underlying mechanisms are not understood. The aim of the present study was to evaluate whether RSV has also an impact on the H⁺-coupled transport of peptides via the peptide transporter 1 (PepT1) and to characterize RSV mediated changes in the apical abundance of nutrients transport proteins and protein kinases that may be involved. RSV decreased the H⁺-coupled transport of peptides in the porcine small intestines in a pH and location specific manner (jejunum vs ileum) as measured in Ussing chamber experiments. The comparison of the effects of RSV with the effects of the cAMP/PKA-activating agent forskolin indicates that different mechanisms may be responsible in the intestinal segments. Additionally, it seems that the transport of peptides and glucose in the jejunum are inhibited via the same mechanism while there might be two mechanisms involved in the ileum. Functional data and protein expression data indicate, that, besides PepT1, the activity of the Na⁺/H⁺-exchanger 3 (NHE3) may be involved. Protein kinase A (PKA) and AMP-activated kinase (AMPK) are both activated by RSV while the extracellular regulated kinase (ERK) and the serum and glucocorticoid induced kinase (SGK) are widely unaffected. Although PKA and AMPK are activated, AMPK seems not to be related to the effects of RSV. Additionally, both the functional data and the protein expression data reveal some interesting pH- and segment-specific differences.

Macrophage dysfunction in cystic fibrosis: Nature or nurture?

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) affect the homeostasis of chloride flux by epithelial cells. This has deleterious consequences, especially in respiratory epithelia, where the defect results in mucus accumulation distinctive of cystic fibrosis. CFTR is, however, also expressed in phagocytic cells, like macrophages. Immune cells are highly sensitive to conditioning by their environment; thus, CFTR dysfunction in epithelia influences macrophages by affecting the lung milieu, but the mutations also appear to be directly consequential for intrinsic macrophage functions. Particular mutations can alter CFTR's folding, traffic of the protein to the membrane and function. As such, understanding the intrinsic effects of CFTR mutation requires distinguishing the secondary effects of misfolded CFTR on cell stress pathways from the primary defect of CFTR dysfunction/absence. Investigations into CFTR's role in macrophages have exploited various models, each with their own advantages and limitations. This review summarizes these methodologic approaches, discussing their physiological correspondence and highlighting key findings. The controversy surrounding CFTR-dependent acidification is used as a case study to highlight difficulties in commensurability across model systems. Recent work in macrophage biology, including polarization and host-pathogen interaction studies, brought into the context of CFTR research, offers potential explanations for observed discrepancies between studies. Moreover, the rapid advancement of novel gene editing technologies and new macrophage model systems makes this assessment of the field's models and methodologies timely.

Regulation of TMEM16A by CK2 and Its Role in Cellular Proliferation

Casein kinase 2 (CK2) is a highly ubiquitous and conserved serine/threonine kinase that forms a tetramer consisting of a catalytic subunit (CK2α) and a regulatory subunit (CK2β). Despite being ubiquitous, CK2 is commonly found at higher expression levels in cancer cells, where it inhibits apoptosis, and supports cell migration and proliferation. The Ca²⁺-activated chloride channel TMEM16A shows similar effects in cancer cells: TMEM16A increases cell proliferation and migration and is highly expressed in squamous cell carcinoma of the head and neck (HNSCC) as well as other malignant tumors. A microscopy-based high-throughput screening was performed to identify proteins that regulate TMEM16A. Within this screen, CK2 was found to be required for proper membrane expression of TMEM16A. small interfering (si) RNA-knockdown of CK2 reduced plasma membrane expression of TMEM16A and inhibited TMEM16A whole cell currents in (cystic fibrosis bronchial epithelial) CFBE airway epithelial cells and in the head and neck cancer cell lines Cal33 and BHY. Inhibitors of CK2, such as TBB and the preclinical compound CX4549 (silmitasertib), also blocked membrane expression of TMEM16A and Ca²⁺-activated whole cell currents. siRNA-knockout of CK2 and its pharmacological inhibition, as well as knockdown or inhibition of TMEM16A by either niclosamide or Ani9, attenuated cell proliferation. Simultaneous inhibition of CK2 and TMEM16A strongly potentiated inhibition of cell proliferation. Although membrane expression of TMEM16A is reduced by inhibition of CK2, our data suggest that the antiproliferative effects by inhibition of CK2 are mostly independent of TMEM16A. Simultaneous inhibition of TMEM16A by niclosamide and inhibition of CK2 by silmitasertib was additive with respect to blocking cell proliferation, while cytotoxicity was reduced when compared to solely blockade of CK2. Therefore, parallel blockade TMEM16A by niclosamide may assist with anticancer therapy by silmitasertib.

Neuropeptide regulation of secretion and inflammation in human airway gland serous cells

Airway submucosal gland serous cells are sites of expression of the cystic fibrosis transmembrane conductance regulator (CFTR) and are important for fluid secretion in conducting airways. To elucidate how neuropeptides regulate serous cells, we tested if human nasal turbinate serous cells secrete bicarbonate (HCO₃ ^-), important for mucus polymerisation and antimicrobial peptide function, during stimulation with cAMP-elevating vasoactive intestinal peptide (VIP) and if this requires CFTR. Serous cells stimulated with VIP exhibited a ∼15-20% cAMP-dependent decrease in cell volume and a ∼0.15 unit decrease in intracellular pH (pH_i), reflecting activation of Cl^- and HCO₃ ^- secretion, respectively. HCO₃ ^- secretion was directly dependent on CFTR and was absent in cells from CF patients. In contrast, neuropeptide Y (NPY) reduced VIP-evoked cAMP increases, CFTR activation, and Cl^-/HCO₃ ^- secretion. Culture of primary serous cells in a model that maintained a serous phenotype confirmed the activating and inhibiting effects of VIP and NPY, respectively, on fluid and HCO₃ ^- secretion. Moreover, VIP enhanced antimicrobial peptide secretion and antimicrobial efficacy of secretions while NPY reduced antimicrobial efficacy. In contrast, NPY enhanced cytokine release while VIP reduced cytokine release through a mechanism requiring CFTR. As levels of VIP and NPY are up-regulated in diseases like allergy, asthma, and chronic rhinosinusitis, the balance of these two peptides in the airway may control mucus rheology and inflammatory responses in serous cells. Furthermore, the loss of CFTR conductance in serous cells may contribute to CF pathophysiology by increasing serous cells inflammatory responses in addition to directly impairing Cl^- and HCO₃ ^- secretion.

CFTR deletion affects mouse osteoblasts in a gender-specific manner

Advancements in research and care have contributed to increase life expectancy of individuals with cystic fibrosis (CF). With increasing age comes a greater likelihood of developing CF bone disease, a comorbidity characterized by a low bone mass and impaired bone quality, which displays gender differences in severity. However, pathophysiological mechanisms underlying this gender difference have never been thoroughly investigated. We used bone marrow-derived osteoblasts and osteoclasts from Cftr^+/+ and Cftr^-/- mice to examine whether the impact of CF transmembrane conductance regulator (CFTR) deletion on cellular differentiation and functions differed between genders. To determine whether in vitro findings translated into in vivo observations, we used imaging techniques and three-point bending testing. In vitro studies revealed no osteoclast-autonomous defect but impairment of osteoblast differentiation and functions and aberrant responses to various stimuli in cells isolated from Cftr^-/- females only. Compared with wild-type controls, knockout mice exhibited a trabecular osteopenic phenotype that was more pronounced in Cftr^-/- males than Cftr^-/- females. Bone strength was reduced to a similar extent in knockout mice of both genders. In conclusion, we find a trabecular bone phenotype in Cftr^-/- mice that was slightly more pronounced in males than females, which is reminiscent of the situation found in patients. However, at the osteoblast level, the pathophysiological mechanisms underlying this phenotype differ between males and females, which may underlie gender differences in the way bone marrow-derived osteoblasts behave in absence of CFTR.

The Function of NM23-H1/NME1 and Its Homologs in Major Processes Linked to Metastasis

Metastasis suppressor genes (MSGs) inhibit different biological processes during metastatic progression without globally influencing development of the primary tumor. The first MSG, NM23 (non-metastatic clone 23, isoform H1) or now called NME1 (stands for non-metastatic) was identified some decades ago. Since then, ten human NM23 paralogs forming two groups have been discovered. Group I NM23 genes encode enzymes with evolutionarily highly conserved nucleoside diphosphate kinase (NDPK) activity. In this review we summarize how results from NDPKs in model organisms converged on human NM23 studies. Next, we examine the role of NM23-H1 and its homologs within the metastatic cascade, e.g. cell migration and invasion, proliferation and apoptosis. NM23-H1 homologs are well known inhibitors of cell migration. Drosophila studies revealed that AWD, the fly counterpart of NM23-H1 is a negative regulator of cell motility by modulating endocytosis of chemotactic receptors on the surface of migrating cells in cooperation with Shibire/Dynamin; this mechanism has been recently confirmed by human studies. NM23-H1 inhibits proliferation of tumor cells by phosphorylating the MAPK scaffold, kinase suppressor of Ras (KSR), resulting in suppression of MAPK signalling. This mechanism was also observed with the C. elegans homolog, NDK-1, albeit with an inverse effect on MAPK activation. Both NM23-H1 and NDK-1 promote apoptotic cell death. In addition, NDK-1, NM23-H1 and their mouse counterpart NM23-M1 were shown to promote phagocytosis in an evolutionarily conserved manner. In summary, inhibition of cell migration and proliferation, alongside actions in apoptosis and phagocytosis are all mechanisms through which NM23-H1 acts against metastatic progression.

Increased Levels of cAMP by the Calcium-Dependent Activation of Soluble Adenylyl Cyclase in Parkin-Mutant Fibroblasts

Almost half of autosomal recessive early-onset parkinsonism has been associated with mutations in PARK2, coding for parkin, which plays an important role in mitochondria function and calcium homeostasis. Cyclic adenosine monophosphate (cAMP) is a major second messenger regulating mitochondrial metabolism, and it is strictly interlocked with calcium homeostasis. Parkin-mutant (Pt) fibroblasts, exhibiting defective mitochondrial respiratory/OxPhos activity, showed a significant higher value of basal intracellular level of cAMP, as compared with normal fibroblasts (CTRL). Specific pharmacological inhibition/activation of members of the adenylyl cyclase- and of the phosphodiesterase-families, respectively, as well as quantitative reverse transcription polymerase chain reaction (RT-qPCR) analysis, indicate that the higher level of cAMP observed in Pt fibroblasts can contribute to a higher level of activity/expression by soluble adenylyl cyclase (sAC) and to low activity/expression of the phosphodiesterase isoform 4 (PDE4). As Ca²⁺ regulates sAC, we performed quantitative calcium-fluorimetric analysis, showing a higher level of Ca²⁺ in the both cytosol and mitochondria of Pt fibroblasts as compared with CTRL. Most notably, inhibition of the mitochondrial Ca²⁺ uniporter decreased, specifically the cAMP level in PD fibroblasts. All together, these findings support the occurrence of an altered mitochondrial Ca²⁺-mediated cAMP homeostasis in fibroblasts with the parkin mutation.

A 30-year journey from volume-regulated anion currents to molecular structure of the LRRC8 channel

Strange et al. review recent advances in our understanding of the molecular and structural basis of volume-regulated anion channel function within the framework of classical biophysical and physiological studies.

The swelling-activated anion channel VRAC has fascinated and frustrated physiologists since it was first described in 1988. Multiple laboratories have defined VRAC’s biophysical properties and have shown that it plays a central role in cell volume regulation and possibly other fundamental physiological processes. However, confusion and intense controversy surrounding the channel’s molecular identity greatly hindered progress in the field for >15 yr. A major breakthrough came in 2014 with the demonstration that VRAC is a heteromeric channel encoded by five members of the Lrrc8 gene family, Lrrc8A–E. A mere 4 yr later, four laboratories described cryo-EM structures of LRRC8A homomeric channels. As the melee of structure/function and physiology studies begins, it is critical that this work be framed by a clear understanding of VRAC biophysics, regulation, and cellular physiology as well as by the field’s past confusion and controversies. That understanding is essential for the design and interpretation of structure/function studies, studies of VRAC physiology, and studies aimed at addressing the vexing problem of how the channel detects cell volume changes. In this review we discuss key aspects of VRAC biophysics, regulation, and function and integrate these into our emerging understanding of LRRC8 protein structure/function.

Calcium-induced chloride secretion is decreased by Resveratrol in ileal porcine tissue

Objective

Chloride (Cl⁻) secretion is crucial for intestinal fluid secretion. Therefore, effects of the polyphenol Resveratrol (RSV) on Cl⁻ secretion have been investigated. In a previous study, we observed effects of RSV on forskolin-induced Cl⁻ secretion in the porcine jejunum but not the ileum although RSV itself induced a transepithelial ion current that may represent Cl⁻ secretion in the ileum. The aim of this study was to gain further insights regarding the effects of RSV on characteristics of Cl⁻ secretion in the porcine ileum using the Ussing chamber technique (recording of short circuit currents (I_sc) as a measure for epithelial net ion transfer).

Results

RSV increased the I_sc in the porcine ileum but not in the porcine jejunum as is already known. This increase was absent in a Cl⁻-free buffer system, indicating that RSV indeed induces Cl⁻ secretion. However, the carbachol-induced I_sc was significantly inhibited by RSV indicating an inhibition of Ca²⁺-induced Cl⁻ secretion. The cellular basis for these contradictory, segment specific results of RSV on Cl⁻ secretion has to be subjected to further studies. The results also underline, that is difficult to generalize effects of RSV between different intestinal locations, organs, cell culture models or species.

Electronic supplementary material

The online version of this article (10.1186/s13104-018-3825-4) contains supplementary material, which is available to authorized users.

Diet-Induced Dysbiosis and Genetic Background Synergize With Cystic Fibrosis Transmembrane Conductance Regulator Deficiency to Promote Cholangiopathy in Mice

The most typical expression of cystic fibrosis (CF)-related liver disease is a cholangiopathy that can progress to cirrhosis. We aimed to determine the potential impact of environmental and genetic factors on the development of CF-related cholangiopathy in mice. Cystic fibrosis transmembrane conductance regulator (Cftr)^-/- mice and Cftr ^+/+ littermates in a congenic C57BL/6J background were fed a high medium-chain triglyceride (MCT) diet. Liver histopathology, fecal microbiota, intestinal inflammation and barrier function, bile acid homeostasis, and liver transcriptome were analyzed in 3-month-old males. Subsequently, MCT diet was changed for chow with polyethylene glycol (PEG) and the genetic background for a mixed C57BL/6J;129/Ola background (resulting from three backcrosses), to test their effect on phenotype. C57BL/6J Cftr ^-/- mice on an MCT diet developed cholangiopathy features that were associated with dysbiosis, primarily Escherichia coli enrichment, and low-grade intestinal inflammation. Compared with Cftr ^+/+ littermates, they displayed increased intestinal permeability and a lack of secondary bile acids together with a low expression of ileal bile acid transporters. Dietary-induced (chow with PEG) changes in gut microbiota composition largely prevented the development of cholangiopathy in Cftr ^-/- mice. Regardless of Cftr status, mice in a mixed C57BL/6J;129/Ola background developed fatty liver under an MCT diet. The Cftr ^-/- mice in the mixed background showed no cholangiopathy, which was not explained by a difference in gut microbiota or intestinal permeability, compared with congenic mice. Transcriptomic analysis of the liver revealed differential expression, notably of immune-related genes, in mice of the congenic versus mixed background. In conclusion, our findings suggest that CFTR deficiency causes abnormal intestinal permeability, which, combined with diet-induced dysbiosis and immune-related genetic susceptibility, promotes CF-related cholangiopathy.

Functional roles of glutamic acid E143 and E705 residues in the N-terminus and transmembrane domain 7 of Anoctamin 1 in calcium and noxious heat sensing

Anoctamin 1 (ANO1) is an anion channel that is activated by changes in cytosolic Ca²⁺ concentration and noxious heat. Although the critical roles of ANO1 have been elucidated in various cell types, the control of its gating mechanisms by Ca²⁺ and heat remain more elusive. To investigate critical amino acid residues for modulation of Ca²⁺ and heat sensing, we constructed a randomized mutant library for ANO1. Among 695 random mutants, reduced Ca²⁺ sensitivity was observed in two mutants (mutant 84 and 87). Consequently, the E143A mutant showed reduced sensitivity to Ca²⁺ but not to high temperatures, whereas the E705V mutant exhibited reduced sensitivity to both Ca²⁺ and noxious heat. These results suggest that the glutamic acids (E) at 143 and 705 residues in ANO1 are critical for modulation of Ca²⁺ and/or heat responses. Furthermore, these findings help to provide a better understanding of the Ca²⁺-mediated activation and heat-sensing mechanism of ANO1.

NM23 proteins: innocent bystanders or local energy boosters for CFTR?

NM23 proteins NDPK-A and -B bind to the cystic fibrosis (CF) protein CFTR in different ways from kinases such as PKA, CK2 and AMPK or linkers to cell calcium such as calmodulin and annexins. NDPK-A (not -B) interacts with CFTR through reciprocal AMPK binding/control, whereas NDPK-B (not -A) binds directly to CFTR. NDPK-B can activate G proteins without ligand-receptor coupling, so perhaps NDPK-B's binding influences energy supply local to a nucleotide-binding site (NBD1) needed for CFTR to function. Curiously, CFTR (ABC-C7) is a member of the ATP-binding cassette (ABC) protein family that does not obey 'clan rules'; CFTR channels anions and is not a pump, regulates disparate processes, is itself regulated by multiple means and is so pleiotropic that it acts as a hub that orchestrates calcium signaling through its consorts such as calmodulin/annexins. Furthermore, its multiple partners make CFTR dance to different tunes in different cellular and subcellular locations as it recycles from the plasma membrane to endosomes. CFTR function in airway apical membranes is inhibited by smoking which has been dubbed 'acquired CF'. CFTR alone among family members possesses a trap for other proteins that it unfurls as a 'fish-net' and which bears consensus phosphorylation sites for many protein kinases, with PKA being the most canonical. Recently, the site of CFTR's commonest mutation has been proposed as a knock-in mutant that alters allosteric control of kinase CK2 by log orders of activity towards calmodulin and other substrates after CFTR fragmentation. This link from CK2 to calmodulin that binds the R region invokes molecular paths that control lumen formation, which is incomplete in the tracheas of some CF-affected babies. Thus, we are poised to understand the many roles of NDPK-A and -B in CFTR function and, especially lumen formation, which is defective in the gut and lungs of many CF babies.

CFTR mutation enhances Dishevelled degradation and results in impairment of Wnt-dependent hematopoiesis

Mutations of cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis (CF) with a multitude of clinical manifestations. Some CF patients develop clinically significant anemia, suggesting that CFTR may regulate hematopoiesis. Here, we report that cftr mutant zebrafish model exhibits primitive and definitive hematopoietic defects with impaired Wnt signaling. Cftr is found to interact, via its PDZ-binding domain (PDZBD), with Dishevelled (Dvl), a key component of Wnt signaling required for hematopoietic progenitor specification, thus protecting Dvl from Dapper1 (Dpr1)-induced lysosomal degradation. Defective hematopoiesis and impaired Wnt signaling in cftr mutant can be rescued by overexpression of wild-type or channel function-defective G551D mutant CFTR with an intact PDZBD, but not Cftr with mutations in the PDZBD. Analysis of human database (http://r2.amc.nl) shows that CFTR is positively correlated with DVL2 and Wnt-related hematopoietic factors in human blood system. The results reveal a previously unrecognized role of CFTR, which is independent of its channel function, in regulating DVL degradation and thus Wnt signaling required for hematopoiesis in both zebrafish and humans, providing an explanation for the anemic phenotype of CF patients.

Airway surface liquid pH is not acidic in children with cystic fibrosis

Modulation of airway surface liquid (ASL) pH has been proposed as a therapy for cystic fibrosis (CF). However, evidence that ASL pH is reduced in CF is limited and conflicting. The technical challenges associated with measuring ASL pH in vivo have precluded accurate measurements in humans. In order to address this deficiency, ASL pH was measured in vivo in children using a novel luminescent technology integrated with fibre-optic probes. Here we show that ASL pH in children with CF is similar to that of children without CF. Findings were supported by highly controlled direct pH measurements in primary human airway epithelial cell culture models, which also suggest that the potential ASL pH gradient produced by defective apical ion transport is balanced out by paracellular shunting of acid/base. Thus, reduced baseline ASL pH is unlikely to be an important pathobiological factor in early CF lung disease.

Modulation of airway surface liquid pH has been proposed as a therapy for cystic fibrosis, but whether pH is indeed altered in cystic fibrosis is controversial. Here, the authors develop a novel fibre-optic based pH measurement technology, and show that pH is not altered in children with cystic fibrosis.

Mechanisms of endothelial cell dysfunction in cystic fibrosis

Although cystic fibrosis (CF) patients exhibit signs of endothelial perturbation, the functions of the cystic fibrosis conductance regulator (CFTR) in vascular endothelial cells (EC) are poorly defined. We sought to uncover biological activities of endothelial CFTR, relevant for vascular homeostasis and inflammation. We examined cells from human umbilical cords (HUVEC) and pulmonary artery isolated from non-cystic fibrosis (PAEC) and CF human lungs (CF-PAEC), under static conditions or physiological shear. CFTR activity, clearly detected in HUVEC and PAEC, was markedly reduced in CF-PAEC. CFTR blockade increased endothelial permeability to macromolecules and reduced trans‑endothelial electrical resistance (TEER). Consistent with this, CF-PAEC displayed lower TEER compared to PAEC. Under shear, CFTR blockade reduced VE-cadherin and p120 catenin membrane expression and triggered the formation of paxillin- and vinculin-enriched membrane blebs that evolved in shrinking of the cell body and disruption of cell-cell contacts. These changes were accompanied by enhanced release of microvesicles, which displayed reduced capability to stimulate proliferation in recipient EC. CFTR blockade also suppressed insulin-induced NO generation by EC, likely by inhibiting eNOS and AKT phosphorylation, whereas it enhanced IL-8 release. Remarkably, phosphodiesterase inhibitors in combination with a β₂ adrenergic receptor agonist corrected functional and morphological changes triggered by CFTR dysfunction in EC. Our results uncover regulatory functions of CFTR in EC, suggesting a physiological role of CFTR in the maintenance EC homeostasis and its involvement in pathogenetic aspects of CF. Moreover, our findings open avenues for novel pharmacology to control endothelial dysfunction and its consequences in CF.

A role for the cystic fibrosis transmembrane conductance regulator in the nitric oxide-dependent release of Cl- from acidic organelles in amacrine cells

γ-Amino butyric acid (GABA) and glycine typically mediate synaptic inhibition because their ligand-gated ion channels support the influx of Cl^- However, the electrochemical gradient for Cl^- across the postsynaptic plasma membrane determines the voltage response of the postsynaptic cell. Typically, low cytosolic Cl^- levels support inhibition, whereas higher levels of cytosolic Cl^- can suppress inhibition or promote depolarization. We previously reported that nitric oxide (NO) releases Cl^- from acidic organelles and transiently elevates cytosolic Cl^-, making the response to GABA and glycine excitatory. In this study, we test the hypothesis that the cystic fibrosis transmembrane conductance regulator (CFTR) is involved in the NO-dependent efflux of organellar Cl^- We first establish the mRNA and protein expression of CFTR in our model system, cultured chick retinal amacrine cells. Using whole cell voltage-clamp recordings of currents through GABA-gated Cl^- channels, we examine the effects of pharmacological inhibition of CFTR on the NO-dependent release of internal Cl^- To interfere with the expression of CFTR, we used clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 genome editing. We find that both pharmacological inhibition and CRISPR/Cas9-mediated knockdown of CFTR block the ability of NO to release Cl^- from internal stores. These results demonstrate that CFTR is required for the NO-dependent efflux of Cl^- from acidic organelles.NEW & NOTEWORTHY Although CFTR function has been studied extensively in the context of epithelia, relatively little is known about its function in neurons. We show that CFTR is involved in an NO-dependent release of Cl^- from acidic organelles. This internal function of CFTR is particularly relevant to neuronal physiology because postsynaptic cytosolic Cl^- levels determine the outcome of GABA- and glycinergic synaptic signaling. Thus the CFTR may play a role in regulating synaptic transmission.

Hydrogen sulfide stimulates CFTR in Xenopus oocytes by activation of the cAMP/PKA signalling axis

Hydrogen sulfide (H₂S) has been recognized as a signalling molecule which affects the activity of ion channels and transporters in epithelial cells. The cystic fibrosis transmembrane conductance regulator (CFTR) is an epithelial anion channel and a key regulator of electrolyte and fluid homeostasis. In this study, we investigated the regulation of CFTR by H₂S. Human CFTR was heterologously expressed in Xenopus oocytes and its activity was electrophysiologically measured by microelectrode recordings. The H₂S-forming sulphur salt Na₂S as well as the slow-releasing H₂S-liberating compound GYY4137 increased transmembrane currents of CFTR-expressing oocytes. Na₂S had no effect on native, non-injected oocytes. The effect of Na₂S was blocked by the CFTR inhibitor CFTR_inh172, the adenylyl cyclase inhibitor MDL 12330A, and the protein kinase A antagonist cAMPS-Rp. Na₂S potentiated CFTR stimulation by forskolin, but not that by IBMX. Na₂S enhanced CFTR stimulation by membrane-permeable 8Br-cAMP under inhibition of adenylyl cyclase-mediated cAMP production by MDL 12330A. These data indicate that H₂S activates CFTR in Xenopus oocytes by inhibiting phosphodiesterase activity and subsequent stimulation of CFTR by cAMP-dependent protein kinase A. In epithelia, an increased CFTR activity may correspond to a pro-secretory response to H₂S which may be endogenously produced by the epithelium or H₂S-generating microflora.

Cellular distribution and function of ion channels involved in transport processes in rat tracheal epithelium

Transport of water and electrolytes in airway epithelia involves chloride-selective ion channels, which are controlled either by cytosolic Ca2+ or by cAMP The contributions of the two pathways to chloride transport differ among vertebrate species. Because rats are becoming more important as animal model for cystic fibrosis, we have examined how Ca2+- dependent and cAMP- dependent Cl- secretion is organized in the rat tracheal epithelium. We examined the expression of the Ca2+-gated Cl- channel anoctamin 1 (ANO1), the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel, the epithelial Na+ channel ENaC, and the water channel aquaporin 5 (AQP5) in rat tracheal epithelium. The contribution of ANO1 channels to nucleotide-stimulated Cl- secretion was determined using the channel blocker Ani9 in short-circuit current recordings obtained from primary cultures of rat tracheal epithelial cells in Ussing chambers. We found that ANO1, CFTR and AQP5 proteins were expressed in nonciliated cells of the tracheal epithelium, whereas ENaC was expressed in ciliated cells. Among nonciliated cells, ANO1 occurred together with CFTR and Muc5b and, in addition, in a different cell type without CFTR and Muc5b. Bioelectrical studies with the ANO1-blocker Ani9 indicated that ANO1 mediated the secretory response to the nucleotide uridine-5'-triphosphate. Our data demonstrate that, in rat tracheal epithelium, Cl- secretion and Na+ absorption are routed through different cell types, and that ANO1 channels form the molecular basis of Ca2+-dependent Cl- secretion in this tissue. These characteristic features of Cl--dependent secretion reveal similarities and distinct differences to secretory processes in human airways.

Mutations of Cystic Fibrosis Transmembrane Conductance Regulator Gene Cause a Monocyte-Selective Adhesion Deficiency

RATIONALE

Cystic fibrosis (CF) is a common genetic disease caused by mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Persistent lung inflammation, characterized by increasing polymorphonuclear leukocyte recruitment, is a major cause of the decline in respiratory function in patients with CF and is a leading cause of morbidity and mortality. CFTR is expressed in various cell types, including leukocytes, but its involvement in the regulation of leukocyte recruitment is unknown.

OBJECTIVES

We evaluated whether CF leukocytes might present with alterations in cell adhesion and migration, a key process governing innate and acquired immune responses.

METHODS

We used ex vivo adhesion and chemotaxis assays, flow cytometry, immunofluorescence, and GTPase activity assays in this study.

MEASUREMENTS AND MAIN RESULTS

We found that chemoattractant-induced activation of β1 and β2 integrins and of chemotaxis is defective in mononuclear cells isolated from patients with CF. In contrast, polymorphonuclear leukocyte adhesion and chemotaxis were normal. The functionality of β1 and β2 integrins was restored by treatment of CF monocytes with the CFTR-correcting drugs VRT325 and VX809. Moreover, treatment of healthy monocytes with the CFTR inhibitor CFTR(inh)-172 blocked integrin activation by chemoattractants. In a murine model of lung inflammation, we found that integrin-independent migration of CF monocytes into the lung parenchyma was normal, whereas, in contrast, integrin-dependent transmigration into the alveolar space was impaired. Finally, signal transduction analysis showed that, in CF monocytes, chemoattractant-triggered activation of RhoA and CDC42 Rho small GTPases (controlling integrin activation and chemotaxis, respectively) was strongly deficient.

CONCLUSIONS

Altogether, these data highlight the critical regulatory role of CFTR in integrin activation by chemoattractants in monocytes and identify CF as a new, cell type-selective leukocyte adhesion deficiency disease, providing new insights into CF pathogenesis.

Insulin signaling via the PI3-kinase/Akt pathway regulates airway glucose uptake and barrier function in a CFTR-dependent manner

Cystic fibrosis-related diabetes is the most common comorbidity associated with cystic fibrosis (CF) and correlates with increased rates of lung function decline. Because glucose is a nutrient present in the airways of patients with bacterial airway infections and because insulin controls glucose metabolism, the effect of insulin on CF airway epithelia was investigated to determine the role of insulin receptors and glucose transport in regulating glucose availability in the airway. The response to insulin by human airway epithelial cells was characterized by quantitative PCR, immunoblot, immunofluorescence, and glucose uptake assays. Phosphatidylinositol 3-kinase/protein kinase B (Akt) signaling and cystic fibrosis transmembrane conductance regulator (CFTR) activity were analyzed by pharmacological and immunoblot assays. We found that normal human primary airway epithelial cells expressed glucose transporter 4 and that application of insulin stimulated cytochalasin B-inhibitable glucose uptake, consistent with a requirement for glucose transporter translocation. Application of insulin to normal primary human airway epithelial cells promoted airway barrier function as demonstrated by increased transepithelial electrical resistance and decreased paracellular flux of small molecules. This provides the first demonstration that airway cells express insulin-regulated glucose transporters that act in concert with tight junctions to form an airway glucose barrier. However, insulin failed to increase glucose uptake or decrease paracellular flux of small molecules in human airway epithelia expressing F508del-CFTR. Insulin stimulation of Akt1 and Akt2 signaling in CF airway cells was diminished compared with that observed in airway cells expressing wild-type CFTR. These results indicate that the airway glucose barrier is regulated by insulin and is dysfunctional in CF.

Synergy of cAMP and calcium signaling pathways in CFTR regulation

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that encodes a chloride channel located in the apical membrane of epithelia cells. The cAMP signaling pathway and protein phosphorylation are known to be primary controlling mechanisms for channel function. In this study, we present an alternative activation pathway that involves calcium-activated calmodulin binding of the intrinsically disordered regulatory (R) region of CFTR. Beyond their potential therapeutic value, these data provide insights into the intersection of calcium signaling with control of ion homeostasis and the ways in which the local CFTR microdomain organizes itself.

Cystic fibrosis results from mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, leading to defective apical chloride transport. Patients also experience overactivation of inflammatory processes, including increased calcium signaling. Many investigations have described indirect effects of calcium signaling on CFTR or other calcium-activated chloride channels; here, we investigate the direct response of CFTR to calmodulin-mediated calcium signaling. We characterize an interaction between the regulatory region of CFTR and calmodulin, the major calcium signaling molecule, and report protein kinase A (PKA)-independent CFTR activation by calmodulin. We describe the competition between calmodulin binding and PKA phosphorylation and the differential effects of this competition for wild-type CFTR and the major F508del mutant, hinting at potential therapeutic strategies. Evidence of CFTR binding to isolated calmodulin domains/lobes suggests a mechanism for the role of CFTR as a molecular hub. Together, these data provide insights into how loss of active CFTR at the membrane can have additional consequences besides impaired chloride transport.

Mitochondrial cAMP signaling

Cyclic adenosine 3, 5'-monophosphate (cAMP) is a ubiquitous second messenger regulating many biological processes, such as cell migration, differentiation, proliferation and apoptosis. cAMP signaling functions not only on the plasma membrane, but also in the nucleus and in organelles such as mitochondria. Mitochondrial cAMP signaling is an indispensable part of the cytoplasm-mitochondrion crosstalk that maintains mitochondrial homeostasis, regulates mitochondrial dynamics, and modulates cellular stress responses and other signaling pathways. Recently, the compartmentalization of mitochondrial cAMP signaling has attracted great attentions. This new input should be carefully taken into account when we interpret the findings of mitochondrial cAMP signaling. In this review, we summarize previous and recent progress in our understanding of mitochondrial cAMP signaling, including the components of the signaling cascade, and the function and regulation of this signaling pathway in different mitochondrial compartments.

New Insights into the Molecular Mechanisms Targeting Tubular Channels/Transporters in PKD Development

BACKGROUND

Autosomal dominant polycystic kidney disease (PKD) or autosomal recessive PKD is caused by a mutation in the PKD1, PKD2 or PKHD1 gene, which encodes polycystin-1, polycystin-2 or fibrocystin, respectively. Embryonic and postnatal mutation studies show that transport or channel function is dysregulated before the initiation of cystogenesis, suggesting that the abnormality of transport or channel function plays a critical role in the pathology of PKD.

SUMMARY

Polycystin-2 by itself is a calcium-permeable cation channel, and its channel function can be regulated by polycystin-1 or fibrocystin. In this paper, we reviewed the current knowledge about calcium transports and cyclic adenosine monophosphate (cAMP)-driven chloride transports in PKD. In addition, the function and the underlining mechanism of glucose transporters, phosphate transporters and water channels in PKD are also discussed.

KEY MESSAGES

Abnormalities in calcium handling and exuberant cAMP-dependent cystic fibrosis transmembrane conductance regulator-mediated fluid secretion in the collecting duct are the most important issues in the pathogenesis of PKD.

Using Drugs to Probe the Variability of Trans-Epithelial Airway Resistance

Background

Precision medicine aims to combat the variability of the therapeutic response to a given medicine by delivering the right medicine to the right patient. However, the application of precision medicine is predicated on a prior quantitation of the variance of the reference range of normality. Airway pathophysiology provides a good example due to a very variable first line of defence against airborne assault. Humans differ in their susceptibility to inhaled pollutants and pathogens in part due to the magnitude of trans-epithelial resistance that determines the degree of epithelial penetration to the submucosal space. This initial ‘set-point’ may drive a sentinel event in airway disease pathogenesis. Epithelia differentiated in vitro from airway biopsies are commonly used to model trans-epithelial resistance but the ‘reference range of normality’ remains problematic. We investigated the range of electrophysiological characteristics of human airway epithelia grown at air-liquid interface in vitro from healthy volunteers focusing on the inter- and intra-subject variability both at baseline and after sequential exposure to drugs modulating ion transport.

Methodology/Principal Findings

Brushed nasal airway epithelial cells were differentiated at air-liquid interface generating 137 pseudostratified ciliated epithelia from 18 donors. A positively-skewed baseline range exists for trans-epithelial resistance (Min/Max: 309/2963 Ω·cm²), trans-epithelial voltage (-62.3/-1.8 mV) and calculated equivalent current (-125.0/-3.2 μA/cm²; all non-normal, P<0.001). A minority of healthy humans manifest a dramatic amiloride sensitivity to voltage and trans-epithelial resistance that is further discriminated by prior modulation of cAMP-stimulated chloride transport.

Conclusions/Significance

Healthy epithelia show log-order differences in their ion transport characteristics, likely reflective of their initial set-points of basal trans-epithelial resistance and sodium transport. Our data may guide the choice of the background set point in subjects with airway diseases and frame the reference range for the future delivery of precision airway medicine.

Calcium-ATPases: Gene disorders and dysregulation in cancer

Ca(2+)-ATPases belonging to the superfamily of P-type pumps play an important role in maintaining low, nanomolar cytoplasmic Ca(2+) levels at rest and priming organellar stores, including the endoplasmic reticulum, Golgi, and secretory vesicles with high levels of Ca(2+) for a wide range of signaling functions. In this review, we introduce the distinct subtypes of Ca(2+)-ATPases and their isoforms and splice variants and provide an overview of their specific cellular roles as they relate to genetic disorders and cancer, with a particular emphasis on recent findings on the secretory pathway Ca(2+)-ATPases (SPCA). Mutations in human ATP2A2, ATP2C1 genes, encoding housekeeping isoforms of the endoplasmic reticulum (SERCA2) and secretory pathway (SPCA1) pumps, respectively, confer autosomal dominant disorders of the skin, whereas mutations in other isoforms underlie various muscular, neurological, or developmental disorders. Emerging evidence points to an important function of dysregulated Ca(2+)-ATPase expression in cancers of the colon, lung, and breast where they may serve as markers of differentiation or novel targets for therapeutic intervention. We review the mechanisms underlying the link between calcium homeostasis and cancer and discuss the potential clinical relevance of these observations. This article is part of a Special Issue entitled: Calcium and Cell Fate. Guest Editors: Jacques Haiech, Claus Heizmann, Joachim Krebs, Thierry Capiod and Olivier Mignen.

Anoctamin 1 (Ano1) is required for glucose-induced membrane potential oscillations and insulin secretion by murine β-cells

Anions such as Cl(-) and HCO3 (-) are well known to play an important role in glucose-stimulated insulin secretion (GSIS). In this study, we demonstrate that glucose-induced Cl(-) efflux from β-cells is mediated by the Ca(2+)-activated Cl(-) channel anoctamin 1 (Ano1). Ano1 expression in rat β-cells is demonstrated by reverse transcriptase-polymerase chain reaction, western blotting, and immunohistochemistry. Typical Ano1 currents are observed in whole-cell and inside-out patches in the presence of intracellular Ca(++): at 1 μM, the Cl(-) current is outwardly rectifying, and at 2 μM, it becomes almost linear. The relative permeabilities of monovalent anions are NO3 (-) (1.83 ± 0.10) > Br(-) (1.42 ± 0.07) > Cl(-) (1.0). A linear single-channel current-voltage relationship shows a conductance of 8.37 pS. These currents are nearly abolished by blocking Ano1 antibodies or by the inhibitors 2-(5-ethyl-4-hydroxy-6-methylpyrimidin-2-ylthio)-N-(4-(4-methoxyphenyl)thiazol-2-yl)acetamide (T-AO1) and tannic acid (TA). These inhibitors induce a strong decrease of 16.7-mM glucose-stimulated action potential rate (at least 87 % on dispersed cells) and a partial membrane repolarization with T-AO1. They abolish or strongly inhibit the GSIS increment at 8.3 mM and at 16.7 mM glucose. Blocking Ano1 antibodies also abolish the 16.7-mM GSIS increment. Combined treatment with bumetanide and acetazolamide in low Cl(-) and HCO3 (-) media provokes a 65 % reduction in action potential (AP) amplitude and a 15-mV AP peak repolarization. Although the mechanism triggering Ano1 opening remains to be established, the present data demonstrate that Ano1 is required to sustain glucose-stimulated membrane potential oscillations and insulin secretion.

Anoctamin 1 (Ano1) is required for glucose-induced membrane potential oscillations and insulin secretion by murine β-cells

Anions such as Cl⁻ and HCO₃⁻ are well known to play an important role in glucose-stimulated insulin secretion (GSIS). In this study, we demonstrate that glucose-induced Cl⁻ efflux from β-cells is mediated by the Ca²⁺-activated Cl⁻ channel anoctamin 1 (Ano1). Ano1 expression in rat β-cells is demonstrated by reverse transcriptase–polymerase chain reaction, western blotting, and immunohistochemistry. Typical Ano1 currents are observed in whole-cell and inside-out patches in the presence of intracellular Ca⁺⁺: at 1 μM, the Cl⁻ current is outwardly rectifying, and at 2 μM, it becomes almost linear. The relative permeabilities of monovalent anions are NO₃⁻ (1.83 ± 0.10) > Br⁻ (1.42 ± 0.07) > Cl⁻ (1.0). A linear single-channel current–voltage relationship shows a conductance of 8.37 pS. These currents are nearly abolished by blocking Ano1 antibodies or by the inhibitors 2-(5-ethyl-4-hydroxy-6-methylpyrimidin-2-ylthio)-N-(4-(4-methoxyphenyl)thiazol-2-yl)acetamide (T-AO1) and tannic acid (TA). These inhibitors induce a strong decrease of 16.7-mM glucose-stimulated action potential rate (at least 87 % on dispersed cells) and a partial membrane repolarization with T-AO1. They abolish or strongly inhibit the GSIS increment at 8.3 mM and at 16.7 mM glucose. Blocking Ano1 antibodies also abolish the 16.7-mM GSIS increment. Combined treatment with bumetanide and acetazolamide in low Cl⁻ and HCO₃⁻ media provokes a 65 % reduction in action potential (AP) amplitude and a 15-mV AP peak repolarization. Although the mechanism triggering Ano1 opening remains to be established, the present data demonstrate that Ano1 is required to sustain glucose-stimulated membrane potential oscillations and insulin secretion.

CFTR Knockdown induces proinflammatory changes in intestinal epithelial cells

Background

Hyperinflammation is a hallmark feature of cystic fibrosis (CF) airways. However, inflammation has also been documented systemically and, more recently, in extrapulmonary CF-affected tissues such as the pancreas and intestine. The pathogenesis of CF-related inflammation and more specifically the role of the cystic fibrosis transmembrane conductance regulator (CFTR) in that respect are not entirely understood. We have tested the hypothesis that genetic depletion of CFTR will affect the inflammatory status of human intestinal epithelial cell lines.

Methods

CFTR expression was genetically depleted from Caco-2/15 and HT-29 cells using short hairpin RNA interference (shRNAi). Inflammatory conditions were induced by the addition of human recombinant tumor necrosis factor (TNF) or Interleukin-1β (IL-1β) for various periods of time. Gene expression, mRNA stability and secreted levels of interleukin (IL)-6, −8 and 10 were assessed. Analysis of pro- and anti-inflammatory signaling pathways including mitogen-activated protein kinases (p38, ERK 1/2 and JNK), nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor alpha (IκBα), and nuclear factor-kappa B (NF-κB) was also performed. Eosinophils were counted in the jejunal mucosa of Cftr−/− and Cftr+/+ mice.

Results

CFTR gene and protein knockdown caused a significant increase in basal secretion of IL-8 as well as in IL-1β-induced secretion of IL-6 and −8. Release of the anti-inflammatory cytokine, IL-10, remained unaffected by CFTR depletion. The enhanced secretion of IL-8 stems in part from increased IL8 mRNA levels and greater activation of ERK1/2 MAPK, IκBα and NF-κB in the CFTR knockdown cells. By contrast, phosphorylation levels of p38 and JNK MAPK did not differ between control and knockdown cells. We also found a higher number of infiltrating eosinophils in the jejunal mucosa of Cftr −/− females, but not males, compared to Cftr +/+ mice, thus providing in vivo support to our in vitro findings.

Conclusion

Collectively, these data underscore the role played by CFTR in regulating the intestinal inflammatory responses. Such findings lend support to the theory that CFTR exerts functions that may go beyond its role as a chloride channel whereby its disruption may prevent cells to optimally respond to exogenous or endogenous challenges. These observations are of particular interest to CF patients who were found to display alterations in their intestinal microbiota, thus predisposing them to pathogens that may elicit exaggerated inflammatory responses.

Electronic supplementary material

The online version of this article (doi:10.1186/s12950-015-0107-y) contains supplementary material, which is available to authorized users.

Repairing the basic defect in cystic fibrosis - one approach is not enough

Cystic fibrosis has attracted much attention in recent years due to significant advances in the pharmacological targeting of the basic defect underlying this recessive disorder: the deficient functional expression of mutant cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels at the apical membrane of epithelial cells. However, increasing evidence points to the reduced efficacy of single treatments, thus reinforcing the need to combine several therapeutic strategies to effectively target the multiple basic defect(s). Protein-repair therapies that use potentiators (activating membrane-located CFTR) or correctors (promoting the relocation of intracellular-retained trafficking mutants of CFTR) in frequent mutations such as F508del and G551D have been put forward and made their way to the clinic with moderate to good efficiency. However, alternative (or additional) approaches targeting the membrane stability of mutant proteins, or correcting the cellular phenotype through a direct effect upon other ion channels (affecting the overall electrolyte transport or simply promoting alternative chloride transport) or targeting less frequent mutations (splicing variants, for example), have been proposed and tested in the field of cystic fibrosis (CF). Here, we cover the different strategies that rely on novel findings concerning the CFTR interactome and signalosome through which it might be possible to further influence the cellular trafficking and post-translational modification machinery (to increase rescued CFTR abundance and membrane stability). We also highlight the new data on strategies aiming at the regulation of sodium absorption or to increase chloride transport through alternative channels. The development and implementation of these complementary approaches will pave the way to combinatorial therapeutic strategies with increased benefit to CF patients.

"Store-operated" cAMP signaling contributes to Ca2+-activated Cl- secretion in T84 colonic cells

Apical cAMP-dependent CFTR Cl(-) channels are essential for efficient vectorial movement of ions and fluid into the lumen of the colon. It is well known that Ca(2+)-mobilizing agonists also stimulate colonic anion secretion. However, CFTR is apparently not activated directly by Ca(2+), and the existence of apical Ca(2+)-dependent Cl(-) channels in the native colonic epithelium is controversial, leaving the identity of the Ca(2+)-activated component unresolved. We recently showed that decreasing free Ca(2+) concentration ([Ca(2+)]) within the endoplasmic reticulum (ER) lumen elicits a rise in intracellular cAMP. This process, which we termed "store-operated cAMP signaling" (SOcAMPS), requires the luminal ER Ca(2+) sensor STIM1 and does not depend on changes in cytosolic Ca(2+). Here we assessed the degree to which SOcAMPS participates in Ca(2+)-activated Cl(-) transport as measured by transepithelial short-circuit current (Isc) in polarized T84 monolayers in parallel with imaging of cAMP and PKA activity using fluorescence resonance energy transfer (FRET)-based reporters in single cells. In Ca(2+)-free conditions, the Ca(2+)-releasing agonist carbachol and Ca(2+) ionophore increased Isc, cAMP, and PKA activity. These responses persisted in cells loaded with the Ca(2+) chelator BAPTA-AM. The effect on Isc was enhanced in the presence of the phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX), inhibited by the CFTR inhibitor CFTRinh-172 and the PKA inhibitor H-89, and unaffected by Ba(2+) or flufenamic acid. We propose that a discrete component of the "Ca(2+)-dependent" secretory activity in the colon derives from cAMP generated through SOcAMPS. This alternative mode of cAMP production could contribute to the actions of diverse xenobiotic agents that disrupt ER Ca(2+) homeostasis, leading to diarrhea.

Characterizing diverse orthologues of the cystic fibrosis transmembrane conductance regulator protein for structural studies

As an ion channel, the cystic fibrosis transmembrane conductance regulator (CFTR) protein occupies a unique niche within the ABC family. Orthologues of CFTR are extant throughout the animal kingdom from sharks to platypods to sheep, where the osmoregulatory function of the protein has been applied to differing lifestyles and diverse organ systems. In humans, loss-of-function mutations to CFTR cause the disease cystic fibrosis, which is a significant health burden in populations of white European descent. Orthologue screening has proved fruitful in the pursuit of high-resolution structural data for several membrane proteins, and we have applied some of the princples developed in previous studies to the expression and purification of CFTR. We have overexpressed this protein, along with evolutionarily diverse orthologues, in Saccharomyces cerevisiae and developed a purification to isolate it in quantities sufficient for structural and functional studies.