Regulation of murine airway surface liquid volume by CFTR and Ca2+-activated Cl- conductances

Dynamic measurement of airway surface liquid volume with an ex vivo trachea-chip

The volume and composition of airway surface liquid (ASL) is regulated by liquid secretion and absorption across airway epithelia, controlling the pH, solute concentration, and biophysical properties of ASL in health and disease. Here, we developed a method integrating explanted tracheal tissue with a micro-machined device (referred to as "ex vivo trachea-chip") to study the dynamic properties of ASL volume regulation. The ex vivo trachea-chip allows real-time measurement of ASL transport (Jv) with intact airway anatomic structures, environmental control, high-resolution, and enhanced experimental throughput. Applying this technology to freshly excised tissue we observed ASL absorption under basal conditions. The apical application of amiloride, an inhibitor of airway epithelial sodium channels (ENaC), reduced airway liquid absorption. Furthermore, the basolateral addition of NPPB, a Cl- channel inhibitor, reduced the basal rate of ASL absorption, implicating a role for basolateral Cl- channels in ASL volume regulation. When tissues were treated with apical amiloride and basolateral methacholine, a cholinergic agonist that stimulates secretion from airway submucosal glands, the net airway surface liquid production shifted from absorption to secretion. This ex vivo trachea-chip provides a new tool to investigate ASL transport dynamics in pulmonary disease states and may aid the development of new therapies targeting ASL regulation.

Function and Regulation of the Calcium-Activated Chloride Channel Anoctamin 1 (TMEM16A)

Various human tissues express the calcium-activated chloride channel Anoctamin 1 (ANO1), also known as TMEM16A. ANO1 allows the passive chloride flux that controls different physiological functions ranging from muscle contraction, fluid and hormone secretion, gastrointestinal motility, and electrical excitability. Overexpression of ANO1 is associated with pathological conditions such as hypertension and cancer. The molecular cloning of ANO1 has led to a surge in structural, functional, and physiological studies of the channel in several tissues. ANO1 is a homodimer channel harboring two pores - one in each monomer - that work independently. Each pore is activated by voltage-dependent binding of two intracellular calcium ions to a high-affinity-binding site. In addition, the binding of phosphatidylinositol 4,5-bisphosphate to sites scattered throughout the cytosolic side of the protein aids the calcium activation process. Furthermore, many pharmacological studies have established ANO1 as a target of promising compounds that could treat several illnesses. This chapter describes our current understanding of the physiological roles of ANO1 and its regulation under physiological conditions as well as new pharmacological compounds with potential therapeutic applications.

KCa3.1 differentially regulates trachea and bronchi epithelial gene expression in a chronic-asthma mouse model

Ion channels are potentially exploitable as pharmacological targets to treat asthma. This study evaluated the role of KCa3.1 channels, encoded by Kcnn4, in regulating the gene expression of mouse airway epithelium and the development of asthma traits. We used the ovalbumin (OVA) challenge as an asthma model in wild type and Kcnn4^-/- mice, performed histological analysis, and measured serum IgE to evaluate asthma traits. We analyzed gene expression of isolated epithelial cells of trachea or bronchi using mRNA sequencing and gene ontology and performed Ussing chamber experiments in mouse trachea to evaluate anion secretion. Gene expression of epithelial cells from mouse airways differed between trachea and bronchi, indicating regional differences in the inflammatory and transepithelial transport properties of proximal and distal airways. We found that Kcnn4 silencing reduced mast cell numbers, mucus, and collagen in the airways, and reduced the amount of epithelial anion secretion in the OVA-challenged animals. Additionally, gene expression was differentially modified in the trachea and bronchi, with Kcnn4 genetic silencing significantly altering the expression of genes involved in the TNF pathway, supporting the potential of KCa3.1 as a therapeutic target for asthma.

Lung Microbiome in Cystic Fibrosis

The defective mucociliary clearance due to CFTR malfunctioning causes predisposition to the colonization of pathogens responsible for the recurrent inflammation and rapid deterioration of lung function in patients with cystic fibrosis (CF). This has also a profound effect on the lung microbiome composition, causing a progressive reduction in its diversity, which has become a common characteristic of patients affected by CF. Although we know that the lung microbiome plays an essential role in maintaining lung physiology, our comprehension of how the microbial components interact with each other and the lung, as well as how these interactions change during the disease's course, is still at an early stage. Many challenges exist and many questions still to be answered, but there is no doubt that manipulation of the lung microbiome could help to develop better therapies for people affected by CF.

Mechanisms of Post-Pancreatitis Diabetes Mellitus and Cystic Fibrosis-Related Diabetes: A Review of Preclinical Studies

Anatomical proximity and functional correlations between the exocrine and endocrine pancreas warrant reciprocal effects between the two parts. Inflammatory diseases of the exocrine pancreas, such as acute or chronic pancreatitis, or the presence of cystic fibrosis disrupt endocrine function, resulting in diabetes of the exocrine pancreas. Although novel mechanisms are being increasingly identified, the intra- and intercellular pathways regulating exocrine-endocrine interactions are still not fully understood, making the development of new and more effective therapies difficult. Therefore, this review sought to accumulate current knowledge regarding the pathogenesis of diabetes in acute and chronic pancreatitis, as well as cystic fibrosis.

CFTR regulation of aquaporin-mediated water transport

The cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel responsible for the direct transport of bicarbonate and chloride. CFTR-dependent ionic transport is crucial for pH regulation and fluid homeodynamics among epithelial surfaces. Particularly, CFTR performs an essential role in the male reproductive tract, which requires a tight regulation of water and electrolytes in order to produce healthy spermatozoa. The absence or malfunction of CFTR results in cystic fibrosis, the most common lethal disease among Caucasians, that is characterized by an impaired fluid and ionic homeostasis in the whole organism. Due to the wide expression and importance of CFTR, the male reproductive tract is highly affected by cystic fibrosis, resulting in male infertility. Although CFTR is not permeable to water, this protein acts as a regulator of other protein channels, such as aquaporins. In fact, CFTR acts as a molecular partner of aquaporins in epithelial cells, regulating fluid homeodynamics. Herein, up-to-date data concerning the regulation of aquaporin-mediated water transport by CFTR will be discussed, highlighting the role of both channels in the male reproductive tract.

Comparison of intestinal glucose flux and electrogenic current demonstrates two absorptive pathways in pig and one in Nile tilapia and rainbow trout

The mucosal-to-serosal flux of ¹⁴C 3-O-methyl-d-glucose was compared against the electrogenic transport of d-glucose across ex vivo intestinal segments of Nile tilapia, rainbow trout, and pig in Ussing chambers. The difference in affinities (K_m "fingerprints") between pig flux and electrogenic transport of glucose, and the absence of this difference in tilapia and trout, suggest two absorptive pathways in the pig and one in the fish species examined. More specifically, the total mucosal-to-serosal flux revealed a super high-affinity, high-capacity (sHa/Hc) total glucose transport system in tilapia; a super high-affinity, low-capacity (sHa/Lc) total glucose transport system in trout and a low-affinity, low-capacity (La/Lc) total glucose transport system in pig. Comparatively, electrogenic glucose absorption revealed similar K_m in both fish species, with a super high-affinity, high capacity (sHa/Hc) system in tilapia; a super high-affinity/super low-capacity (sHa/sLc) system in trout; but a different K_m fingerprint in the pig, with a high-affinity, low-capacity (Ha/Lc) system. This was supported by different responses to inhibitors of sodium-dependent glucose transporters (SGLTs) and glucose transporter type 2 (GLUT2) administered on the apical side between species. More specifically, tilapia flux was inhibited by SGLT inhibitors, but not the GLUT2 inhibitor, whereas trout lacked response to inhibitors. In contrast, the pig responded to inhibition by both SGLT and GLUT2 inhibitors with a higher expression of GLUT2. Altogether, it would appear that two pathways are working together in the pig, allowing it to have continued absorption at high glucose concentrations, whereas this is not present in both tilapia and trout.

Attenuated Amiloride-Sensitive Current and Augmented Calcium-Activated Chloride Current in Marsh Rice Rat (Oryzomys palustris) Airways

Prolonged heat and sea salt aerosols pose a challenge for the mammalian airway, placing the protective airway surface liquid (ASL) at risk for desiccation. Thus, mammals inhabiting salt marshes might have acquired adaptations for ASL regulation. We studied the airways of the rice rat, a rodent that inhabits salt marshes. We discovered negligible Na+ transport through the epithelial sodium channel (ENaC). In contrast, carbachol induced a large Cl- secretory current that was blocked by the calcium-activated chloride channel (CaCC) inhibitor CaCCinhi-A01. Decreased mRNA expression of α, β, and γ ENaC, and increased mRNA expression of the CaCC transmembrane member 16A, distinguished the rice rat airway. Rice rat airway cultures also secreted fluid in response to carbachol and displayed an exaggerated expansion of the ASL volume when challenged with 3.5% NaCl. These data suggest that the rice rat airway might possess unique ion transport adaptations to facilitate survival in the salt marsh environment.

Concurrent absorption and secretion of airway surface liquids and bicarbonate secretion in human bronchioles

Although small airways account for the largest fraction of the total conducting airway surfaces, the epithelial fluid and electrolyte transport in small, native airway epithelia has not been well characterized. Investigations have been limited, no doubt, by the complex tissue architecture as well as by its inaccessibility, small dimensions, and lack of applicable assays, especially in human tissues. To better understand how the critically thin layer of airway surface liquid (ASL) is maintained, we applied a "capillary"-Ussing chamber (area ≈1 mm2) to measure ion transport properties of bronchioles with diameters of ~2 mm isolated from resected specimens of excised human lungs. We found that the small human airway, constitutively and concurrently, secretes and absorbs fluid as observed in porcine small airways (50). We found that the human bronchiolar epithelium is also highly anion selective and constitutively secretes bicarbonate ( HCO 3 - ), which can be enhanced pharmacologically by cAMP as well as Ca2+-mediated agonists. Concurrent secretion and absorption of surface liquid along with HCO 3 - secretion help explain how the delicate volume of the fluid lining the human small airway is physiologically buffered and maintained in a steady state that avoids desiccating or flooding the small airway with ASL.

Sigmoidal kinetics define porcine intestinal segregation of electrogenic monosaccharide transport systems as having multiple transporter population involvement

Kinetic characterization of electrogenic sodium-dependent transport in Ussing chambers of d-glucose and d-galactose demonstrated sigmoidal/Hill kinetics in the porcine jejunum and ileum, with the absence of transport in the distal colon. In the jejunum, a high-affinity, super-low-capacity (Ha/sLc) kinetic system accounted for glucose transport, and a low-affinity, low-capacity (La/Lc) kinetic system accounted for galactose transport. In contrast, the ileum demonstrated a high-affinity, super-high-capacity (Ha/sHc) glucose transport and a low-affinity, high-capacity (La/Hc) galactose transport systems. Jejunal glucose transport was not inhibited by dapagliflozin, but galactose transport was inhibited. Comparatively, ileal glucose and galactose transport were both sensitive to dapagliflozin. Genomic and gene expression analyses identified 10 of the 12 known SLC5A family members in the porcine jejunum, ileum, and distal colon. Dominant SGLT1 (SLC5A1) and SGLT3 (SLC5A4) expression was associated with the sigmoidal Ha/sLc glucose and La/Lc galactose transport systems in the jejunum. Comparatively, the dominant expression of SGLT1 (SLC5A1) in the ileum was only associated with Ha glucose and La galactose kinetic systems. However, the sigmoidal kinetics and overall high capacity (Hc) of transport is unlikely accounted for by SGLT1 (SLC5A1) alone. Finally, the absence of transport and lack of pharmacological inhibition in the colon was associated with the poor expression of SLC5A genes. Altogether, the results demonstrated intestinal segregation of monosaccharide transport fit different sigmoidal kinetic systems. This reveals multiple transporter populations in each system, supported by gene expression profiles and pharmacological inhibition. Overall, this work demonstrates a complexity to transporter involvement in intestinal electrogenic monosaccharide absorption systems not previously defined.

Intestinal electrogenic sodium-dependent glucose absorption in tilapia and trout reveal species differences in SLC5A-associated kinetic segmental segregation

Electrogenic sodium-dependent glucose transport along the length of the intestine was compared between the omnivorous Nile tilapia ( Oreochromis niloticus) and the carnivorous rainbow trout ( Oncorhynchus mykiss) in Ussing chambers. In tilapia, a high-affinity, high-capacity kinetic system accounted for the transport throughout the proximal intestine, midintestine, and hindgut segments. Similar dapagliflozin and phloridzin dihydrate inhibition across all segments support this homogenous high-affinity, high-capacity system throughout the tilapia intestine. Genomic and gene expression analysis supported findings by identifying 10 of the known 12 SLC5A family members, with homogeneous expression throughout the segments with dominant expression of sodium-glucose cotransporter 1 (SGLT1; SLC5A1) and sodium-myoinositol cotransporter 2 (SMIT2; SLC5A11). In contrast, trout's electrogenic sodium-dependent glucose absorption was 20-35 times lower and segregated into three significantly different kinetic systems found in different anatomical segments: a high-affinity, low-capacity system in the pyloric ceca; a super-high-affinity, low-capacity system in the midgut; and a low-affinity, low-capacity system in the hindgut. Genomic and gene expression analysis found 5 of the known 12 SLC5A family members with dominant expression of SGLT1 ( SLC5A1), sodium-glucose cotransporter 2 (SGLT2; SLC5A2), and SMIT2 ( SLC5A11) in the pyloric ceca, and only SGLT1 ( SLC5A1) in the midgut, accounting for differences in kinetics between the two. The hindgut presented a low-affinity, low-capacity system partially attributed to a decrease in SGLT1 ( SLC5A1). Overall, the omnivorous tilapia had a higher electrogenic glucose absorption than the carnivorous trout, represented with different kinetic systems and a greater expression and number of SLC5A orthologs. Fish differ from mammals, having hindgut electrogenic glucose absorption and segment specific transport kinetics.

Ca2+-activated Cl- channel TMEM16A/ANO1 identified in zebrafish skeletal muscle is crucial for action potential acceleration

The Ca²⁺-activated Cl⁻ channel (CaCC) TMEM16A/Anoctamin 1 (ANO1) is expressed in gastrointestinal epithelia and smooth muscle cells where it mediates secretion and intestinal motility. However, ANO1 Cl⁻ conductance has never been reported to play a role in skeletal muscle. Here we show that ANO1 is robustly expressed in the highly evolved skeletal musculature of the euteleost species zebrafish. We characterised ANO1 as bonafide CaCC which is activated close to maximum by Ca²⁺ ions released from the SR during excitation-contraction (EC) coupling. Consequently, our study addressed the question about the physiological advantage of implementation of ANO1 into the euteleost skeletal-muscle EC coupling machinery. Our results reveal that Cl⁻ influx through ANO1 plays an essential role in restricting the width of skeletal-muscle action potentials (APs) by accelerating the repolarisation phase. Resulting slimmer APs enable higher AP-frequencies and apparently tighter controlled, faster and stronger muscle contractions, crucial for high speed movements.

The Ca²⁺-activated Cl^- channel TMEM16A/Anoctamin 1 (ANO1) mediates secretion and intestinal motility in gastrointestinal epithelia and smooth muscle cells. Here, the authors show a role for ANO1 in zebrafish skeletal muscle which is activated by SR Ca²⁺ release during excitation-contraction coupling.

Slippery When Wet: Airway Surface Liquid Homeostasis and Mucus Hydration

The ability to regulate cell volume is crucial for normal physiology; equally the regulation of extracellular fluid homeostasis is of great importance. Alteration of normal extracellular fluid homeostasis contributes to the development of several diseases including cystic fibrosis. With regard to the airway surface liquid (ASL), which lies apically on top of airway epithelia, ion content, pH, mucin and protein abundance must be tightly regulated. Furthermore, airway epithelia must be able to switch from an absorptive to a secretory state as required. A heterogeneous population of airway epithelial cells regulate ASL solute and solvent composition, and directly secrete large mucin molecules, antimicrobials, proteases and soluble mediators into the airway lumen. This review focuses on how epithelial ion transport influences ASL hydration and ASL pH, with a specific focus on the roles of anion and cation channels and exchangers. The role of ions and pH in mucin expansion is also addressed. With regard to fluid volume regulation, we discuss the roles of nucleotides, adenosine and the short palate lung and nasal epithelial clone 1 (SPLUNC1) as soluble ASL mediators. Together, these mechanisms directly influence ciliary beating and in turn mucociliary clearance to maintain sterility and to detoxify the airways. Whilst all of these components are regulated in normal airways, defective ion transport and/or mucin secretion proves detrimental to lung homeostasis as such we address how defective ion and fluid transport, and a loss of homeostatic mechanisms, contributes to the development of pathophysiologies associated with cystic fibrosis.

A revised airway epithelial hierarchy includes CFTR-expressing ionocytes

The airways of the lung are the primary sites of disease in asthma and cystic fibrosis. Here we study the cellular composition and hierarchy of the mouse tracheal epithelium by single-cell RNA-sequencing (scRNA-seq) and in vivo lineage tracing. We identify a rare cell type, the Foxi1+ pulmonary ionocyte; functional variations in club cells based on their location; a distinct cell type in high turnover squamous epithelial structures that we term 'hillocks'; and disease-relevant subsets of tuft and goblet cells. We developed 'pulse-seq', combining scRNA-seq and lineage tracing, to show that tuft, neuroendocrine and ionocyte cells are continually and directly replenished by basal progenitor cells. Ionocytes are the major source of transcripts of the cystic fibrosis transmembrane conductance regulator in both mouse (Cftr) and human (CFTR). Knockout of Foxi1 in mouse ionocytes causes loss of Cftr expression and disrupts airway fluid and mucus physiology, phenotypes that are characteristic of cystic fibrosis. By associating cell-type-specific expression programs with key disease genes, we establish a new cellular narrative for airways disease.

Mucin Production and Hydration Responses to Mucopurulent Materials in Normal versus Cystic Fibrosis Airway Epithelia

RATIONALE

Cystic fibrosis (CF) airways disease produces a mucoobstructive lung phenotype characterized by airways mucus plugging, epithelial mucous cell metaplasia/hyperplasia, chronic infection, and inflammation. Simultaneous biochemical and functional in vivo studies of mucin synthesis and secretion from CF airways are not available. In vitro translational models may quantitate differential CF versus normal mucin and fluid secretory responses to infectious/inflammatory stimuli.

OBJECTIVES

We tested the hypothesis that CF airways exhibit defective epithelial fluid, but not mucin, secretory responses to bacterial/inflammatory host products.

METHODS

Well-differentiated primary human bronchial epithelial cultures were exposed to supernatant from mucopurulent material (SMM) from human CF airways as a test of bacterial/inflammatory host product stimulus. Human bronchial epithelia (HBE) with normal CF transmembrane conductance regulator function were compared with ΔF508/ΔF508 CF HBE.

MEASUREMENTS AND MAIN RESULTS

Acute (up to 60 min) SMM exposure promoted mucin secretion, but mucins were degraded by the proteolytic enzymes present in SMM. Chronic SMM exposure induced upregulation of mucin synthesis and storage and generated absolute increases in basal and stimulated mucin release in normal and CF cultures. These responses were similar in normal and CF cultures. In contrast, SMM produced a coordinated CF transmembrane conductance regulator-mediated Cl- secretory response in normal HBE, but not in CF HBE. The absence of the fluid secretory response in CF produced quantitatively more dehydrated mucus.

CONCLUSIONS

Our study reveals the interplay between regulation of mucin and fluid secretion rates in inflamed versus noninflamed conditions and why a hyperconcentrated mucus is produced in CF airways.

Therapeutic Approaches to Acquired Cystic Fibrosis Transmembrane Conductance Regulator Dysfunction in Chronic Bronchitis

Chronic obstructive pulmonary disease is a common cause of morbidity and a rising cause of mortality worldwide. Its rising impact indicates the ongoing unmet need for novel and effective therapies. Previous work has established a pathophysiological link between the chronic bronchitis phenotype of chronic obstructive pulmonary disease and cystic fibrosis as well as phenotypic similarities between these two airways diseases. An extensive body of evidence has established that cigarette smoke and its constituents contribute to acquired dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) protein in the airways, pointing to a mechanistic link with smoking-related and chronic bronchitis. Recent interest surrounding new drugs that target both mutant and wild-type CFTR channels has paved the way for a new treatment opportunity addressing the mucus defect in chronic bronchitis. We review the clinical and pathologic evidence for modulating CFTR to address acquired CFTR dysfunction and pragmatic issues surrounding clinical trials as well as a discussion of other ion channels that may represent alternative therapeutic targets.

Enhancement of lung gene delivery after aerosol: a new strategy using non-viral complexes with antibacterial properties

The pathophysiology of obstructive pulmonary diseases, such as cystic fibrosis (CF), leads to the development of chronic infections in the respiratory tract. Thus, the symptomatic management of the disease requires, in particular, repetitive antibiotherapy. Besides these antibacterial treatments, certain pathologies, such as CF or chronic obstructive pulmonary disease (COPD), require the intake of many drugs. This simultaneous absorption may lead to undesirable drug interactions. For example, Orkambi® (lumacaftor/Ivacaftor, Vertex), a pharmacological drug employed to treat F508del patients, cannot be used with antibiotics such as rifampicin or rifabutin (rifamycin family) which are necessary to treat Mycobacteriaceae. As far as gene therapy is concerned, bacteria and/or biofilm in the airways present an additional barrier for gene transfer. Thus, aerosol administration of nanoparticles have to overcome many obstacles before allowing cellular penetration of therapeutic compounds. This review focusses on the development of aerosol formulations adapted to the respiratory tract and its multiple barriers. Then, formulations that are currently used in clinical applications are summarized depending on the active molecule delivered. Finally, we focus on new therapeutic approaches to reduce possible drug interactions by transferring the antibacterial activity to the nanocarrier while ensuring the transfection efficiency.

The therapeutic potential of CFTR modulators for COPD and other airway diseases

Airways diseases, especially chronic obstructive pulmonary disease (COPD) and asthma, are common causes of morbidity and mortality worldwide. There is an ongoing unmet need for novel and effective therapies. There is an established pathophysiological link and phenotypic similarity between the chronic bronchitis phenotype of COPD and cystic fibrosis (CF). New evidence suggests that CFTR dysfunction may play a role in other common airways diseases such as COPD, non-atopic asthma and non-CF bronchiectasis. Newly approved and investigational drugs that target both mutant and wild-type CFTR channels have provided a new treatment opportunity addressing the mucus defect in pulmonary diseases that share the same pathophysiology with CF.

Ivacaftor withdrawal syndrome in cystic fibrosis patients with the G551D mutation

Ivacaftor use can lead to dramatic health improvements in cystic fibrosis (CF) patients with gating mutations. Here, we report five instances of dramatic clinical decline following withdrawal of ivacaftor in three individuals with the G551D-CFTR mutation. In each case, the patient's lung function and symptoms rapidly deteriorated after cessation of treatment. Awareness of this phenomenon should inform both clinical practices as well as the design of future clinical trials of highly active CFTR modulators.

Bacterial Sphingomyelinase is a State-Dependent Inhibitor of the Cystic Fibrosis Transmembrane conductance Regulator (CFTR)

Sphingomyelinase C (SMase) inhibits CFTR chloride channel activity in multiple cell systems, an effect that could exacerbate disease in CF and COPD patients. The mechanism by which sphingomyelin catalysis inhibits CFTR is not known but evidence suggests that it occurs independently of CFTR's regulatory "R" domain. In this study we utilized the Xenopus oocyte expression system to shed light on how CFTR channel activity is reduced by SMase. We found that the pathway leading to inhibition is not membrane delimited and that inhibited CFTR channels remain at the cell membrane, indicative of a novel silencing mechanism. Consistent with an effect on CFTR gating behavior, we found that altering gating kinetics influenced the sensitivity to inhibition by SMase. Specifically, increasing channel activity by introducing the mutation K1250A or pretreating with the CFTR potentiator VX-770 (Ivacaftor) imparted resistance to inhibition. In primary bronchial epithelial cells, we found that basolateral, but not apical, application of SMase leads to a redistribution of sphingomyelin and a reduction in forskolin- and VX-770-stimulated currents. Taken together, these data suggest that SMase inhibits CFTR channel function by locking channels into a closed state and that endogenous CFTR in HBEs is affected by SMase activity.

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.

Tobacco Smoke Constituents Trigger Cytoplasmic Calcium Release

Cytosolic Ca²⁺ is a universal second messenger that is involved in many processes throughout the body, including the regulation of cell growth/cell division, apoptosis, and the secretion of both ions, and macromolecules. Tobacco smoke exerts multiple effects on airway epithelia and we have previously shown that Kentucky reference cigarette smoke exposure elevated the second messenger Ca²⁺, leading to dysfunctional ion secretion. In this study, we tested whether little cigar and commercial cigarette smoke exposure exerts similar effects on intracellular Ca²⁺ levels. Indeed, Swisher Sweets, Captain Black, and Cheyenne little cigars, as well as Camel, Marlboro, and Newport cigarettes, triggered a comparable increase in intracellular Ca²⁺ as seen with Kentucky reference cigarettes in human bronchial epithelia. We also found that Kentucky reference cigarette smoke exposure caused increases in Ca²⁺ in HEK293T cells and that similar increases in Ca²⁺ were seen with the tobacco smoke metabolites 1-NH₂-naphthalene, formaldehyde, nicotine, and nicotine-derived nitrosamine ketone. Given the large number of physiological processes governed by changes in cytosolic Ca²⁺, our data suggest that Ca²⁺ signaling is a useful and reproducible assay that can be used to probe the propensity of tobacco products and their constituents to cause toxicity.

Role of CFTR in epithelial physiology

Salt and fluid absorption and secretion are two processes that are fundamental to epithelial function and whole body fluid homeostasis, and as such are tightly regulated in epithelial tissues. The CFTR anion channel plays a major role in regulating both secretion and absorption in a diverse range of epithelial tissues, including the airways, the GI and reproductive tracts, sweat and salivary glands. It is not surprising then that defects in CFTR function are linked to disease, including life-threatening secretory diarrhoeas, such as cholera, as well as the inherited disease, cystic fibrosis (CF), one of the most common life-limiting genetic diseases in Caucasian populations. More recently, CFTR dysfunction has also been implicated in the pathogenesis of acute pancreatitis, chronic obstructive pulmonary disease (COPD), and the hyper-responsiveness in asthma, underscoring its fundamental role in whole body health and disease. CFTR regulates many mechanisms in epithelial physiology, such as maintaining epithelial surface hydration and regulating luminal pH. Indeed, recent studies have identified luminal pH as an important arbiter of epithelial barrier function and innate defence, particularly in the airways and GI tract. In this chapter, we will illustrate the different operational roles of CFTR in epithelial function by describing its characteristics in three different tissues: the airways, the pancreas, and the sweat gland.

Pharmacological analysis of epithelial chloride secretion mechanisms in adult murine airways

Defective epithelial chloride secretion occurs in humans with cystic fibrosis (CF), a genetic defect due to loss of function of CFTR, a cAMP-activated chloride channel. In the airways, absence of an active CFTR causes a severe lung disease. In mice, genetic ablation of CFTR function does not result in similar lung pathology. This may be due to the expression of an alternative chloride channel which is activated by calcium. The most probable protein performing this function is TMEM16A, a calcium-activated chloride channel (CaCC). Our aim was to assess the relative contribution of CFTR and TMEM16A to chloride secretion in adult mouse trachea. For this purpose we tested pharmacological inhibitors of chloride channels in normal and CF mice. The amplitude of the cAMP-activated current was similar in both types of animals and was not affected by a selective CFTR inhibitor. In contrast, a CaCC inhibitor (CaCCinh-A01) strongly blocked the cAMP-activated current as well as the calcium-activated chloride secretion triggered by apical UTP. Although control experiments revealed that CaCCinh-A01 also shows inhibitory activity on CFTR, our results indicate that transepithelial chloride secretion in adult mouse trachea is independent of CFTR and that another channel, possibly TMEM16A, performs both cAMP- and calcium-activated chloride transport. The prevalent function of a non-CFTR channel may explain the absence of a defect in chloride transport in CF mice.

Current and Emerging Therapies for the Treatment of Cystic Fibrosis or Mitigation of Its Symptoms

Clinical presentation of the chronic, heritable condition cystic fibrosis (CF) is complex, with a diverse range of symptoms often affecting multiple organs with varying severity. The primary source of morbidity and mortality is due to progressive destruction of the airways attributable to chronic inflammation arising from microbial colonisation. Antimicrobial therapy combined with practises to remove obstructive mucopurulent deposits form the cornerstone of current therapy. However, new treatment options are emerging which offer, for the first time, the opportunity to effect remission from the underlying cause of CF. Here, we discuss these therapies, their mechanisms of action, and their successes and failures in order to illustrate the shift in the nature of how CF will likely be managed into the future.

Calcium-calmodulin does not alter the anion permeability of the mouse TMEM16A calcium-activated chloride channel

Ca²⁺-calmodulin fails to affect TMEM16A anion permeability.

The transmembrane protein TMEM16A forms a Ca²⁺-activated Cl⁻ channel that is permeable to many anions, including SCN⁻, I⁻, Br⁻, Cl⁻, and HCO₃⁻, and has been implicated in various physiological functions. Indeed, controlling anion permeation through the TMEM16A channel pore may be critical in regulating the pH of exocrine fluids such as the pancreatic juice. The anion permeability of the TMEM16A channel pore has recently been reported to be modulated by Ca²⁺-calmodulin (CaCaM), such that the pore of the CaCaM-bound channel shows a reduced ability to discriminate between anions as measured by a shift of the reversal potential under bi-ionic conditions. Here, using a mouse TMEM16A clone that contains the two previously identified putative CaM-binding motifs, we were unable to demonstrate such CaCaM-dependent changes in the bi-ionic potential. We confirmed the activity of CaCaM used in our study by showing CaCaM modulation of the olfactory cyclic nucleotide–gated channel. We suspect that the different bi-ionic potentials that were obtained previously from whole-cell recordings in low and high intracellular [Ca²⁺] may result from different degrees of bi-ionic potential shift secondary to a series resistance problem, an ion accumulation effect, or both.

Protein phosphatase 1 coordinates CFTR-dependent airway epithelial HCO3- secretion by reciprocal regulation of apical and basolateral membrane Cl(-)-HCO3- exchangers

BACKGROUND AND PURPOSE

Our recent studies on human airway serous-like Calu-3 cells showed that cAMP agonists stimulated a HCO3(-) rich secretion containing up to 80 mM HCO3(-). This alkaline secretion relied on a coordinated switch in the activity of distinct Cl(-)-HCO3(-) anion exchangers (AE) located at different regions of the cell. At the apical membrane, cAMP agonists activated the electroneutral AE pendrin (SLC26A4), together with cystic fibrosis transmembrane conductance regulator (CFTR), while at the basolateral membrane the agonists inhibited AE2 (SLC4A2). However, the underlying mechanism(s) that orchestrates this cAMP-dependent switch in AE activity has not been elucidated.

EXPERIMENTAL APPROACH

Apical and basolateral Cl(-)-HCO3(-) exchange was assessed by measuring Cl(-)-dependent changes in intracellular pH (pH(i)).

KEY RESULTS

We show that protein phosphatase 1 (PP1), together with CFTR, play central roles in this reciprocal regulation of AE activity. Activation of pendrin by cAMP agonists, but not inhibition of the basolateral exchanger, was protein kinase A-dependent. Knocking down CFTR expression, or blocking its activity with GlyH-101, led to incomplete inhibition of the basolateral AE by cAMP, supporting a role for CFTR in this process. Addition of the PP1/2A inhibitor, okadaic acid, but not the PP2A specific inhibitor fostreicin, mimicked the effect of cAMP stimulation. Furthermore, okadaic acid-treated Calu-3 monolayers produced a more alkaline fluid than untreated cells, which was comparable with that produced by cAMP stimulation.

CONCLUSIONS AND IMPLICATIONS

These results identify PP1 as a novel regulator of AE activity which, in concert with CFTR, coordinates events at both apical and basolateral membranes, crucial for efficient HCO3(-) secretion from Calu-3 cells.

Potentiator ivacaftor abrogates pharmacological correction of ΔF508 CFTR in cystic fibrosis

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR). Newly developed "correctors" such as lumacaftor (VX-809) that improve CFTR maturation and trafficking and "potentiators" such as ivacaftor (VX-770) that enhance channel activity may provide important advances in CF therapy. Although VX-770 has demonstrated substantial clinical efficacy in the small subset of patients with a mutation (G551D) that affects only channel activity, a single compound is not sufficient to treat patients with the more common CFTR mutation, ΔF508. Thus, patients with ΔF508 will likely require treatment with both correctors and potentiators to achieve clinical benefit. However, whereas the effectiveness of acute treatment with this drug combination has been demonstrated in vitro, the impact of chronic therapy has not been established. In studies of human primary airway epithelial cells, we found that both acute and chronic treatment with VX-770 improved CFTR function in cells with the G551D mutation, consistent with clinical studies. In contrast, chronic VX-770 administration caused a dose-dependent reversal of VX-809-mediated CFTR correction in ΔF508 homozygous cultures. This result reflected the destabilization of corrected ΔF508 CFTR by VX-770, markedly increasing its turnover rate. Chronic VX-770 treatment also reduced mature wild-type CFTR levels and function. These findings demonstrate that chronic treatment with CFTR potentiators and correctors may have unexpected effects that cannot be predicted from short-term studies. Combining these drugs to maximize rescue of ΔF508 CFTR may require changes in dosing and/or development of new potentiator compounds that do not interfere with CFTR stability.

A comprehensive strategy to identify stoichiometric membrane protein interactomes

There are numerous experimental approaches to identify the interaction networks of soluble proteins, but strategies for the identification of membrane protein interactomes remain limited. We discuss in detail the logic of an experimental design that led us to identify the interactome of a membrane protein of complex membrane topology, the calcium activated chloride channel Anoctamin 1/Tmem16a (Ano1). We used covalent chemical stabilizers of protein-protein interactions combined with magnetic bead immuno-affinity chromatography, quantitative SILAC mass-spectrometry and in silico network construction. This strategy led us to define a putative Ano1 interactome from which we selected key components for functional testing. We propose a combination of procedures to narrow down candidate proteins interacting with a membrane protein of interest for further functional studies.

Soluble adenylyl cyclase in health and disease

The second messenger cAMP is integral for many physiological processes. Soluble adenylyl cyclase (sAC) was recently identified as a widely expressed intracellular source of cAMP in mammalian cells. sAC is evolutionary, structurally, and biochemically distinct from the G-protein-responsive transmembranous adenylyl cyclases (tmAC). The structure of the catalytic unit of sAC is similar to tmAC, but sAC does not contain transmembranous domains, allowing localizations independent of the membranous compartment. sAC activity is stimulated by HCO(3)(-), Ca²⁺ and is sensitive to physiologically relevant ATP fluctuations. sAC functions as a physiological sensor for carbon dioxide and bicarbonate, and therefore indirectly for pH. Here we review the physiological role of sAC in different human tissues with a major focus on the lung. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease, guest edited by J. Buck and L.R. Levin.

Structure and function of TMEM16 proteins (anoctamins)

TMEM16 proteins, also known as anoctamins, are involved in a variety of functions that include ion transport, phospholipid scrambling, and regulation of other membrane proteins. The first two members of the family, TMEM16A (anoctamin-1, ANO1) and TMEM16B (anoctamin-2, ANO2), function as Ca2+-activated Cl- channels (CaCCs), a type of ion channel that plays important functions such as transepithelial ion transport, smooth muscle contraction, olfaction, phototransduction, nociception, and control of neuronal excitability. Genetic ablation of TMEM16A in mice causes impairment of epithelial Cl- secretion, tracheal abnormalities, and block of gastrointestinal peristalsis. TMEM16A is directly regulated by cytosolic Ca2+ as well as indirectly by its interaction with calmodulin. Other members of the anoctamin family, such as TMEM16C, TMEM16D, TMEM16F, TMEM16G, and TMEM16J, may work as phospholipid scramblases and/or ion channels. In particular, TMEM16F (ANO6) is a major contributor to the process of phosphatidylserine translocation from the inner to the outer leaflet of the plasma membrane. Intriguingly, TMEM16F is also associated with the appearance of anion/cation channels activated by very high Ca2+ concentrations. Furthermore, a TMEM16 protein expressed in Aspergillus fumigatus displays both ion channel and lipid scramblase activity. This finding suggests that dual function is an ancestral characteristic of TMEM16 proteins and that some members, such as TMEM16A and TMEM16B, have evolved to a pure channel function. Mutations in anoctamin genes (ANO3, ANO5, ANO6, and ANO10) cause various genetic diseases. These diseases suggest the involvement of anoctamins in a variety of cell functions whose link with ion transport and/or lipid scrambling needs to be clarified.

Anoctamin 1 mediates thermal pain as a heat sensor

Vertebrates can sense and avoid noxious heat that evokes pain. Many thermoTRP channels are associated with temperature sensation. TRPV1 is a representative ion channel that is activated by noxious heat. Anoctamin 1 (ANO1) is a Cl- channel activated by calcium that is highly expressed in small sensory neurons, colocalized with markers for nociceptors, and most surprisingly, activated by noxious heat over 44oC. Although ANO1 is a Cl- channel, opening of this channel leads to depolarization of sensory neurons, suggesting a role in nociception. Indeed, the functional deletion of ANO1 in sensory neurons triggers the reduction in thermal pain sensation. Thus, it seems clear that ANO1 is a heat sensor in a nociceptive pathway. Since ANO1 modulators are developed for the purpose of treating chronic diseases such as cystic fibrosis, this finding is likely to predict unwanted effects and provide a guide for better developmental strategy

Ca2+-activated Cl- channels

Ca(2+)-activated Cl(-) channels (CaCCs) are plasma membrane proteins involved in various important physiological processes. In epithelial cells, CaCC activity mediates the secretion of Cl(-) and of other anions, such as bicarbonate and thiocyanate. In smooth muscle and excitable cells of the nervous system, CaCCs have an excitatory role coupling intracellular Ca(2+) elevation to membrane depolarization. Recent studies indicate that TMEM16A (transmembrane protein 16 A or anoctamin 1) and TMEM16B (transmembrane protein 16 B or anoctamin 2) are CaCC-forming proteins. Induced expression of TMEM16A and B in null cells by transfection causes the appearance of Ca(2+)-activated Cl(-) currents similar to those described in native tissues. Furthermore, silencing of TMEM16A by RNAi causes disappearance of CaCC activity in cells from airway epithelium, biliary ducts, salivary glands, and blood vessel smooth muscle. Mice devoid of TMEM16A expression have impaired Ca(2+)-dependent Cl(-) secretion in the epithelial cells of the airways, intestine, and salivary glands. These animals also show a loss of gastrointestinal motility, a finding consistent with an important function of TMEM16A in the electrical activity of gut pacemaker cells, that is, the interstitial cells of Cajal. Identification of TMEM16 proteins will help to elucidate the molecular basis of Cl(-) transport.

New pulmonary therapies directed at targets other than CFTR

Our current understanding of the pathogenesis of cystic fibrosis (CF) lung disease stresses the importance of the physical and chemical properties of the airway surface liquid (ASL). In particular, the loss of cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel function in CF reduces the volume and fluidity of the ASL, thus impairing mucociliary clearance and innate antimicrobial mechanisms. Besides direct approaches to restoring mutant CFTR function, alternative therapeutic strategies may also be considered to correct the basic defect of impaired salt and water transport. Such alternative strategies are focused on the restoration of mucociliary transport by (1) reducing sodium and fluid absorption by inhibiting the ENaC channel; (2) activating alternative chloride channels; and (3) increasing airway surface hydration with osmotic agents. Therapeutic approaches directed at targets other than CFTR are attractive because they are potentially useful to all patients irrespective of their genotype. Clinical trials are underway to test the efficacy of these approaches.

Fusion-activated cation entry (FACE) via P2X₄ couples surfactant secretion and alveolar fluid transport

Two fundamental mechanisms within alveoli are essential for lung function: regulated fluid transport and secretion of surfactant. Surfactant is secreted via exocytosis of lamellar bodies (LBs) in alveolar type II (ATII) cells. We recently reported that LB exocytosis results in fusion-activated cation entry (FACE) via P2X₄ receptors on LBs. We propose that FACE, in addition to facilitating surfactant secretion, modulates alveolar fluid transport. Correlative fluorescence and atomic force microscopy revealed that FACE-dependent water influx correlated with individual fusion events in rat primary ATII cells. Moreover, ATII cell monolayers grown at air-liquid interface exhibited increases in short-circuit current (Isc) on stimulation with ATP or UTP. Both are potent agonists for LB exocytosis, but only ATP activates FACE. ATP, not UTP, elicited additional fusion-dependent increases in Isc. Overexpressing dominant-negative P2X₄ abrogated this effect by ∼50%, whereas potentiating P2X4 lead to ∼80% increase in Isc. Finally, we monitored changes in alveolar surface liquid (ASL) on ATII monolayers by confocal microscopy. Only stimulation with ATP, not UTP, led to a significant, fusion-dependent, 20% decrease in ASL, indicating apical-to-basolateral fluid transport across ATII monolayers. Our data support the first direct link between LB exocytosis, regulation of surfactant secretion, and transalveolar fluid resorption via FACE.

Status of fluid and electrolyte absorption in cystic fibrosis

Salt and fluid absorption is a shared function of many of the body's epithelia, but its use is highly adapted to the varied physiological roles of epithelia-lined organs. These functions vary from control of hydration of outward-facing epithelial surfaces to conservation and regulation of total body volume. In the most general context, salt and fluid absorption is driven by active Na(+) absorption. Cl(-) is absorbed passively through various available paths in response to the electrical driving force that results from active Na(+) absorption. Absorption of salt creates a concentration gradient that causes water to be absorbed passively, provided the epithelium is water permeable. Key differences notwithstanding, the transport elements used for salt and fluid absorption are broadly similar in diverse epithelia, but the regulation of these elements enables salt absorption to be tailored to very different physiological needs. Here we focus on salt absorption by exocrine glands and airway epithelia. In cystic fibrosis, salt and fluid absorption by gland duct epithelia is effectively prevented by the loss of cystic fibrosis transmembrane conductance regulator (CFTR). In airway epithelia, salt and fluid absorption persists, in the absence of CFTR-mediated Cl(-) secretion. The contrast of these tissue-specific changes in CF tissues is illustrative of how salt and fluid absorption is differentially regulated to accomplish tissue-specific physiological objectives.

Progress in cystic fibrosis and the CF Therapeutics Development Network

Cystic fibrosis (CF), the most common life-shortening genetic disorder in Caucasians, affects approximately 70 000 individuals worldwide. In 1998, the Cystic Fibrosis Foundation (CFF) launched the CF Therapeutics Development Network (CF-TDN) as a central element of its Therapeutics Development Programme. Designed to accelerate the clinical evaluation of new therapies needed to fulfil the CFF mission to control and cure CF, the CF-TDN has conducted 75 clinical trials since its inception, and has contributed to studies as varied as initial safety and proof of concept trials to pivotal programmes required for regulatory approval. This review highlights recent and significant research efforts of the CF-TDN, including a summary of contributions to studies involving CF transmembrane conductance regulator (CFTR) modulators, airway surface liquid hydrators and mucus modifiers, anti-infectives, anti-inflammatories, and nutritional therapies. Efforts to advance CF biomarkers, necessary to accelerate the therapeutic goals of the network, are also summarised.

Association of TMEM16A chloride channel overexpression with airway goblet cell metaplasia

The TMEM16A protein has a potential role as a Ca(2+)-activated Cl(-) channel (CaCC) in airway epithelia where it may be important in the homeostasis of the airway surface fluid. We investigated the function and expression of TMEM16A in primary human bronchial epithelial cells and in a bronchial cell line (CFBE41o-). Under resting conditions, TMEM16A protein expression was relatively low. However, TMEM16A silencing with short-interfering RNAs caused a marked inhibition of CaCC activity, thus demonstrating that a low TMEM16A expression is sufficient to support Ca(2+)-dependent Cl(-) transport. Following treatment for 24-72 h with interleukin-4 (IL-4), a cytokine that induces mucous cell metaplasia, TMEM16A protein expression was strongly increased in approximately 50% of primary bronchial epithelial cells, with a specific localization in the apical membrane. IL-4 treatment also increased the percentage of cells expressing MUC5AC, a marker of goblet cells. Interestingly, MUC5AC was detected specifically in cells expressing TMEM16A. In particular, MUC5AC was found in 15 and 60% of TMEM16A-positive cells when epithelia were treated with IL-4 for 24 or 72 h, respectively. In contrast, ciliated cells showed expression of the cystic fibrosis transmembrane conductance regulator Cl(-) channel but not of TMEM16A. Our results indicate that TMEM16A protein is responsible for CaCC activity in airway epithelial cells, particularly in cells treated with IL-4, and that TMEM16A upregulation by IL-4 appears as an early event of goblet cell differentiation. These findings suggest that TMEM16A expression is particularly required under conditions of mucus hypersecretion to ensure adequate secretion of electrolytes and water.

mCLCA3 does not contribute to calcium-activated chloride conductance in murine airways

Ca(2+)-activated Cl(-) channels (CaCCs) contribute to airway Cl(-) and fluid secretion, and were implicated in the modulation of disease severity and as a therapeutic target in cystic fibrosis (CF). Previous in vitro studies suggested that members of the CLCA gene family, including the murine mCLCA3, contribute to CaCCs. However, the role of mCLCA3 in ion transport in native airway epithelia has not been studied, to the best of our knowledge. In this study, we used mCLCA3-deficient mice and determined bioelectric properties in freshly excised tracheal tissue, airway morphology, and gene expression studies, to determine the role of mCLCA3 in airway ion transport and airway structure. Bioelectric measurements did not detect any differences in basal short-circuit current, amiloride-sensitive Na(+) absorption, cyclic adenosine monophosphate-dependent Cl(-) secretion, and activation of Ca(2+)-activated (uridine-5'-triphosphate-mediated) Cl(-) secretion in mCLCA3-deficient mice compared with wild-type mice. Moreover, no histological changes were observed in the respiratory tract or any other tissues of mCLCA3-deficient mice when compared with wild-type control mice. The intratracheal instillation of IL-13 produced an approximately 30-fold up-regulation of mCLCA3 transcripts without inducing CaCC activity in wild-type airways, and induced goblet-cell hyperplasia and mucin gene expression to similar levels in both genotypes. Further, multiple specific reverse-transcriptase quantitative PCR assays for other CaCC candidates, including mCLCA1, mCLCA2, mCLCA4, mCLCA5, mCLCA6, mCLCA7, mBEST1, mBEST2, mCLC4, mTTYH3, and mTMEM16A, failed to identify the differential expression of genes in the respiratory tract that may compensate for a lack of mCLCA3 function. Together, these findings argue against a role of mCLCA3 in CaCC-mediated Cl(-) secretion in murine respiratory epithelia.

International Union of Basic and Clinical Pharmacology. LXXXV: calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) are widely expressed in various tissues and implicated in physiological processes such as sensory transduction, epithelial secretion, and smooth muscle contraction. Transmembrane proteins with unknown function 16 (TMEM16A) has recently been identified as a major component of CaCCs. Detailed molecular analysis of TMEM16A will be needed to understand its structure-function relationships. The role this channel plays in physiological systems remains to be established and is currently a subject of intense investigation.

Physiological roles and diseases of Tmem16/Anoctamin proteins: are they all chloride channels?

The Tmem16 gene family was first identified by bioinformatic analysis in 2004. In 2008, it was shown independently by 3 laboratories that the first two members (Tmem16A and Tmem16B) of this 10-gene family are Ca(2+)-activated Cl(-) channels. Because these proteins are thought to have 8 transmembrane domains and be anion-selective channels, the alternative name, Anoctamin (anion and octa=8), has been proposed. However, it remains unclear whether all members of this family are, in fact, anion channels or have the same 8-transmembrane domain topology. Since 2008, there have been nearly 100 papers published on this gene family. The excitement about Tmem16 proteins has been enhanced by the finding that Ano1 has been linked to cancer, mutations in Ano5 are linked to several forms of muscular dystrophy (LGMDL2 and MMD-3), mutations in Ano10 are linked to autosomal recessive spinocerebellar ataxia, and mutations in Ano6 are linked to Scott syndrome, a rare bleeding disorder. Here we review some of the recent developments in understanding the physiology and structure-function of the Tmem16 gene family.

Voltage- and calcium-dependent gating of TMEM16A/Ano1 chloride channels are physically coupled by the first intracellular loop

Ca(2+)-activated Cl(-) channels (CaCCs) are exceptionally well adapted to subserve diverse physiological roles, from epithelial fluid transport to sensory transduction, because their gating is cooperatively controlled by the interplay between ionotropic and metabotropic signals. A molecular understanding of the dual regulation of CaCCs by voltage and Ca(2+) has recently become possible with the discovery that Ano1 (TMEM16a) is an essential subunit of CaCCs. Ano1 can be gated by Ca(2+) or by voltage in the absence of Ca(2+), but Ca(2+)- and voltage-dependent gating are very closely coupled. Here we identify a region in the first intracellular loop that is crucial for both Ca(2+) and voltage sensing. Deleting (448)EAVK in the first intracellular loop dramatically decreases apparent Ca(2+) affinity. In contrast, mutating the adjacent amino acids (444)EEEE abolishes intrinsic voltage dependence without altering the apparent Ca(2+)affinity. Voltage-dependent gating of Ano1 measured in the presence of intracellular Ca(2+) was facilitated by anions with high permeability or by an increase in [Cl(-)](e). Our data show that the transition between closed and open states is governed by Ca(2+) in a voltage-dependent manner and suggest that anions allosterically modulate Ca(2+)-binding affinity. This mechanism provides a unified explanation of CaCC channel gating by voltage and ligand that has long been enigmatic.

Characterization of the oligomeric structure of the Ca(2+)-activated Cl- channel Ano1/TMEM16A

Members of the Anoctamin (Ano)/TMEM16A family have recently been identified as essential subunits of the Ca(2+)-activated chloride channel (CaCC). For example, Ano1 is highly expressed in multiple tissues including airway epithelia, where it acts as an apical conduit for transepithelial Cl(-) secretion and helps regulate lung liquid homeostasis and mucus clearance. However, little is known about the oligomerization of this protein in the plasma membrane. Thus, utilizing mCherry- and eGFP-tagged Ano1 constructs, we conducted biochemical and Förster resonance energy transfer (FRET)-based experiments to determine the quaternary structure of Ano1. FRET and co-immunoprecipitation studies revealed that tagged Ano1 subunits directly associated before they reached the plasma membrane. This association was not altered by changes in cytosolic Ca(2+), suggesting that this is a fixed interaction. To determine the oligomeric structure of Ano1, we performed chemical cross-linking, non-denaturing PAGE, and electromobility shift assays, which revealed that Ano1 exists as a dimer. These data are the first to probe the quaternary structure of Ano1. Understanding the oligomeric nature of Ano1 is an essential step in the development of therapeutic drugs that could be useful in the treatment of cystic fibrosis.

The effect of intracellular calcium oscillations on fluid secretion in airway epithelium

Airway epithelium has been shown to elicit fluid secretion after a rise in intracellular calcium. This rise in intracellular calcium has been shown to display complex oscillations in many species after the binding of particular agonists to extracellular receptors. Fluid secreted by the airway epithelium is used to maintain the depth of the periciliary liquid (PCL) above the apical membrane of the epithelial cells lining the bronchial airways. Previous mathematical models have been published which separately consider the electrophysiology involved in regulating periciliary liquid depth, and the transmission of intracellular calcium waves in airway epithelial tissue. In this paper we present a mathematical model that combines these previous models and allows the effect of oscillations in intracellular calcium on fluid secretion by airway epithelial cells to be investigated. We show that an oscillatory calcium response produces different fluid secretion properties to that elicited by a tonic rise in intracellular calcium. These differences are shown to be due to saturation of the Ca(2+) activated ion channels.

Primary bronchial epithelial cell culture from explanted cystic fibrosis lungs

Lung disease is responsible for more than 95% of morbidity and mortality in cystic fibrosis. The exact pathogenesis of cystic fibrosis lung disease remains poorly understood. Experimental models are therefore vital for use in research. Animal models and immortalized cell lines both have inherent limitations. Explanted lungs removed from people with cystic fibrosis at the time of transplantation represent a potentially valuable but technically and logistically challenging source of primary cystic fibrosis bronchial epithelial cells. In this study, pieces of segmental bronchus from explanted lungs were treated with patient-specific antimicrobials prior to isolation of bronchial epithelial cells. Cultured cells were characterized by their morphology under light microscopy, cytokeratin and hematoxylin-eosin staining, and electrophysiological profile. Primary bronchial epithelial cells were successfully cultured from 15 of 22 patients attempted. The cells exhibited typical epithelial morphology, staining for cytokeratin, lack of responsiveness to forskolin treatment, and remained viable after storage in liquid nitrogen. Seven unsuccessful cultures failed due to early infection with bacteria known to colonize the airways pretransplant. The results show that primary bronchial epithelial cell culture is possible from explanted cystic fibrosis lungs. This provides an important cellular model to elucidate the pathogenic mechanisms in cystic fibrosis lung disease and to investigate potential therapeutic targets.

Effects of secretagogues on net and unidirectional liquid fluxes across porcine bronchial airways

Rates of liquid secretion and absorption across the bronchopulmonary airways are important for regulating airway surface liquid volume and maintaining mucociliary transport. The current study demonstrates the feasibility of measuring not just net liquid movements but unidirectional liquid movements across isolated intact bronchi from swine. Airways were liquid filled to assess both net liquid movements, and, in the presence of NPPB to selectively inhibit secretion, unidirectional absorption. Unidirectional liquid secretion rates were determined by subtraction. For comparison, net liquid movements were assessed in air-filled airways in parallel. In the absence of secretagogues, unidirectional absorption was observed (4.63 +/- 0.53 microl.cm(-2).h(-1)) with little unidirectional secretion (1.42 +/- 0.36 microl.cm(-2).h(-1)). ACh, substance P (SP), and vasoactive intestinal peptide (VIP) all induced unidirectional secretion (10.64 +/- 1.52 microl.cm(-2).h(-1), 14.16 +/- 1.39 microl.cm(-2).h(-1), and 4.25 +/- 0.25 microl.cm(-2).h(-1), respectively) without affecting unidirectional absorption. Net liquid secretion in air-filled airways was close to that in liquid-filled airways except with VIP. VIP induced net secretion in air-filled airways (4.44 +/- 1.26 microl.cm(-2).h(-1)), but negligible net change in liquid movement occurred in liquid-filled airways. This effect was likely to have been caused by the higher solid content of the VIP-induced mucous liquid (3.98 +/- 0.26%) compared with the ACh- and SP-induced liquid (2.06 +/- 0.07% and 2.15 +/- 0.07%, respectively). We conclude that this technique allows important quantitative distinctions to be made between liquid secretion and absorption in intact bronchial airways.

Airway surface liquid depth measured in ex vivo fragments of pig and human trachea: dependence on Na+ and Cl- channel function

The airway surface liquid (ASL) is the thin fluid layer lining the airways whose depth may be reduced in cystic fibrosis. Prior measurements of ASL depth have been made in airway epithelial cell cultures. Here, we established methodology to measure ASL depth to approximately 1-microm accuracy in ex vivo fragments of freshly obtained human and pig tracheas. Airway fragments were mounted in chambers designed for perfusion of the basal surface and observation of the apical, fluorescently stained ASL by scanning confocal microscopy using a high numerical aperture lens immersed in perfluorocarbon. Measurement accuracy was verified using standards of specified fluid thickness. ASL depth in well-differentiated primary cultures of human nasal respiratory epithelium was 8.0 +/- 0.5 microm (SE 10 cultures) under basal conditions, 8.4 +/- 0.4 microm following ENaC inhibition by amiloride, and 14.5 +/- 1.2 microm following CFTR stimulation by cAMP agonists. ASL depth in human trachea was 7.0 +/- 0.7 microm under basal conditions, 11.0 +/- 1.7 microm following amiloride, 17.0 +/- 3.4 microm following cAMP agonists, and 7.1 +/- 0.5 microm after CFTR inhibition. Similar results were found in pig trachea. This study provides the first direct measurements of ASL depth in intact human airways and indicates the involvement of ENaC sodium channels and CFTR chloride channels in determining ASL depth. We suggest that CF lung disease may be caused by the inability of CFTR-deficient airways to increase their ASL depth transiently following secretory stimuli that in non-CF airways produce transient increases in ASL depth.

Regulation of TMEM16A chloride channel properties by alternative splicing

Expression of TMEM16A protein is associated with the activity of Ca(2+)-activated Cl(-) channels. TMEM16A primary transcript undergoes alternative splicing. thus resulting in the generation of multiple isoforms. We have determined the pattern of splicing and assessed the functional properties of the corresponding TMEM16A variants. We found three alternative exons, 6b, 13, and 15, coding for segments of 22, 4, and 26 amino acids, respectively, which are differently spliced in human organs. By patch clamp experiments on transfected cells, we found that skipping of exon 6b changes the Ca(2+) sensitivity by nearly 4-fold, resulting in Cl(-) currents requiring lower Ca(2+) concentrations to be activated. At the membrane potential of 80 mV, the apparent half-effective concentration decreases from 350 to 90 nm when the segment corresponding to exon 6b is excluded. Skipping of exon 13 instead strongly reduces the characteristic time-dependent activation observed for Ca(2+)-activated Cl(-) channels at positive membrane potentials. This effect was also obtained by deleting only the second pair of amino acids corresponding to exon 13. Alternative splicing appears as an important mechanism to regulate the voltage and Ca(2+) dependence of the TMEM16A-dependent Cl(-) channels in a tissue-specific manner.

A mathematical model of calcium-induced fluid secretion in airway epithelium

Regulation of periciliary liquid (PCL) depth is of central importance to mucociliary clearance by the airway epithelium. Without adequate hydration mucociliary transport would cease, leading to build up of mucus in the airways, and impairing the clearance of any trapped inhaled particulates. Airway epithelial cells are known to release ATP under a number of stress conditions. Cell surface receptors bind ATP and trigger an intracellular calcium response which regulates the gating of specific ion channels on the apical and basolateral cell membranes. This shifts the electrochemical balance, resulting in the accumulation of Na(+) and Cl(-) in the periciliary liquid, and providing an osmotic driving force for water flux. In this study, we present a mathematical model of a single airway epithelial cell which describes the fluid secretion elicited after a rise in intracellular calcium. The model provides a basis to quantitatively analyse the influence of intracellular calcium signalling on fluid movement. The model demonstrates behaviour consistent with a number of experimental data on manipulating periciliary liquid volume and tonicity, and provides a quantitative basis for analysing the role of the different membrane ion channels in determining water flux following different physiological stimuli.