SPLUNC1 regulates airway surface liquid volume by protecting ENaC from proteolytic cleavage

MicroRNA-200b regulates distal airway development by maintaining epithelial integrity

miR-200b plays a role in epithelial-to-mesenchymal transition (EMT) in cancer. We recently reported abnormal expression of miR-200b in the context of human pulmonary hypoplasia in congenital diaphragmatic hernia (CDH). Smaller lung size, a lower number of airway generations, and a thicker mesenchyme characterize pulmonary hypoplasia in CDH. The aim of this study was to define the role of miR-200b during lung development. Here we show that miR-200b-/- mice have abnormal lung function due to dysfunctional surfactant, increased fibroblast-like cells and thicker mesenchyme in between the alveolar walls. We profiled the lung transcriptome in miR-200b-/- mice, and, using Gene Ontology analysis, we determined that the most affected biological processes include cell cycle, apoptosis and protein transport. Our results demonstrate that miR-200b regulates distal airway development through maintaining an epithelial cell phenotype. The lung abnormalities observed in miR-200b-/- mice recapitulate lung hypoplasia in CDH.

Tubular lipid binding proteins (TULIPs) growing everywhere

Tubular lipid binding proteins (TULIPs) have become a focus of interest in the cell biology of lipid signalling, lipid traffic and membrane contact sites. Each tubular domain has an internal pocket with a hydrophobic lining that can bind a hydrophobic molecule such as a lipid. This allows TULIP proteins to carry lipids through the aqueous phase. TULIP domains were first found in a large family of extracellular proteins related to the bacterial permeability-inducing protein (BPI) and cholesterol ester transfer protein (CETP). Since then, the same fold and lipid transfer capacity have been found in SMP domains (so-called for their occurrence in synaptotagmin, mitochondrial and lipid binding proteins), which localise to intracellular membrane contact sites. Here the methods for identifying known TULIPs are described, and used to find previously unreported TULIPs, one in the silk polymer and another in prokaryotes illustrated by the E. coli protein YceB. The bacterial TULIP alters views on the likely evolution of the domain, suggesting its presence in the last universal common ancestor. The major function of TULIPs is to handle lipids, but we still do not know how they work in detail, or how many more remain to be discovered. This article is part of a Special Issue entitled: Membrane Contact Sites edited by Christian Ungermann and Benoit Kornmann.

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Proteins with the tubular lipid binding fold exist in a wider variety than is usually appreciated.

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TULIPs are found in prokaryotes, altering views on their evolution.

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It is not yet known whether TULIPs transfer lipids as tunnels or as shuttles.

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Tests have not yet been done to say if TULIPs with SMP domains (for example E-syts and ERMES components) tether contact sites.

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It is likely that more TULIPs remain to be discovered.

Little Cigars are More Toxic than Cigarettes and Uniquely Change the Airway Gene and Protein Expression

Little cigars (LCs) are regulated differently than cigarettes, allowing them to be potentially targeted at youth/young adults. We exposed human bronchial epithelial cultures (HBECs) to air or whole tobacco smoke from cigarettes vs. LCs. Chronic smoke exposure increased the number of dead cells, lactate dehydrogenase release, and interleukin-8 (IL-8) secretion and decreased apical cilia, cystic fibrosis transmembrane conductance regulator (CFTR) protein levels, and transepithelial resistance. These adverse effects were significantly greater in LC-exposed HBECs than cigarette exposed cultures. LC-exposure also elicited unique gene expression changes and altered the proteomic profiles of airway apical secretions compared to cigarette-exposed HBECs. Gas chromatography-mass spectrometry (GC-MS) analysis indicated that LCs produced more chemicals than cigarettes, suggesting that the increased chemical load of LCs may be the cause of the greater toxicity. This is the first study of the biological effects of LCs on pulmonary epithelia and our observations strongly suggest that LCs pose a more severe danger to human health than cigarettes.

Identification of BPIFA1/SPLUNC1 as an epithelium-derived smooth muscle relaxing factor

Asthma is a chronic airway disease characterized by inflammation, mucus hypersecretion and abnormal airway smooth muscle (ASM) contraction. Bacterial permeability family member A1, BPIFA1, is a secreted innate defence protein. Here we show that BPIFA1 levels are reduced in sputum samples from asthmatic patients and that BPIFA1 is secreted basolaterally from healthy, but not asthmatic human bronchial epithelial cultures (HBECs), where it suppresses ASM contractility by binding to and inhibiting the Ca²⁺ influx channel Orai1. We have localized this effect to a specific, C-terminal α-helical region of BPIFA1. Furthermore, tracheas from Bpifa1^-/- mice are hypercontractile, and this phenotype is reversed by the addition of recombinant BPIFA1. Our data suggest that BPIFA1 deficiency in asthmatic airways promotes Orai1 hyperactivity, increased ASM contraction and airway hyperresponsiveness. Strategies that target Orai1 or the BPIFA1 deficiency in asthma may lead to novel therapies to treat this disease.

Molecular Characteristics and Biological Functions of Surface-Active and Surfactant Proteins

Many critical biological processes take place at hydrophobic:hydrophilic interfaces, and a wide range of organisms produce surface-active proteins and peptides that reduce surface and interfacial tension and mediate growth and development at these boundaries. Microorganisms produce both small lipid-associated peptides and amphipathic proteins that allow growth across water:air boundaries, attachment to surfaces, predation, and improved bioavailability of hydrophobic substrates. Higher-order organisms produce surface-active proteins with a wide variety of functions, including the provision of protective foam environments for vulnerable reproductive stages, evaporative cooling, and gas exchange across airway membranes. In general, the biological functions supported by these diverse polypeptides require them to have an amphipathic nature, and this is achieved by a diverse range of molecular structures, with some proteins undergoing significant conformational change or intermolecular association to generate the structures that are surface active.

Tight junctions in pulmonary epithelia during lung inflammation

Inflammatory lung diseases like asthma bronchiale, chronic obstructive pulmonary disease and allergic airway inflammation are widespread public diseases that constitute an enormous burden to the health systems. Mainly classified as inflammatory diseases, the treatment focuses on strategies interfering with local inflammatory responses by the immune system. Inflammatory lung diseases predispose patients to severe lung failures like alveolar oedema, respiratory distress syndrome and acute lung injury. These life-threatening syndromes are caused by increased permeability of the alveolar and airway epithelium and exudate formation. However, the mechanism underlying epithelium barrier breakdown in the lung during inflammation is elusive. This review emphasises the role of the tight junction of the airway epithelium as the predominating structure conferring epithelial tightness and preventing exudate formation and the impact of inflammatory perturbations on their function.

Marked increases in mucociliary clearance produced by synergistic secretory agonists or inhibition of the epithelial sodium channel

Mucociliary clearance (MCC) is a critical host innate defense mechanism in airways, and it is impaired in cystic fibrosis (CF) and other obstructive lung diseases. Epithelial fluid secretion and absorption modify MCC velocity (MCCV). We tested the hypotheses that inhibiting fluid absorption accelerates MCCV, whereas inhibiting fluid secretion decelerates it. In airways, ENaC is mainly responsible for fluid absorption, while anion channels, including CFTR and Ca²⁺-activated chloride channels mediate anion/fluid secretion. MCCV was increased by the cAMP-elevating agonists, forskolin or isoproterenol (10 μM) and by the Ca²⁺-elevating agonist, carbachol (0.3 μM). The CFTR-selective inhibitor, CFTR_inh-172, modestly reduced MCCV-increases induced by forskolin or isoproterenol but not increases induced by carbachol. The ENaC inhibitor benzamil increased basal MCCV as well as MCCV increases produced by forskolin or carbachol. MCC velocity was most dramatically accelerated by the synergistic combination of forskolin and carbachol, which produced near-maximal clearance rates regardless of prior treatment with CFTR or ENaC inhibitors. In CF airways, where CFTR-mediated secretion (and possibly synergistic MCC) is lost, ENaC inhibition via exogenous agents may provide therapeutic benefit, as has long been proposed.

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.

Short Palate, Lung, and Nasal Epithelial Clone 1 Has Antimicrobial and Antibiofilm Activities against the Burkholderia cepacia Complex

The opportunistic bacteria of the Burkholderia cepacia complex (Bcc) are extremely pathogenic to cystic fibrosis (CF) patients, and acquisition of Bcc bacteria is associated with a significant increase in mortality. Treatment of Bcc infections is difficult because the bacteria are multidrug resistant and able to survive in biofilms. Short palate, lung, and nasal epithelial clone 1 (SPLUNC1) is an innate defense protein that is secreted by the upper airways and pharynx. While SPLUNC1 is known to have antimicrobial functions, its effects on Bcc strains are unclear. We therefore tested the hypothesis that SPLUNC1 is able to impair Bcc growth and biofilm formation. We found that SPLUNC1 exerted bacteriostatic effects against several Bcc clinical isolates, including B. cenocepacia strain J2315 (50% inhibitory concentration [IC50] = 0.28 μM), and reduced biofilm formation and attachment (IC50 = 0.11 μM). We then determined which domains of SPLUNC1 are responsible for its antimicrobial activity. Deletions of SPLUNC1's N terminus and α6 helix did not affect its function. However, deletion of the α4 helix attenuated antimicrobial activity, while the corresponding α4 peptide displayed antimicrobial activity. Chronic neutrophilia is a hallmark of CF lung disease, and neutrophil elastase (NE) cleaves SPLUNC1. However, we found that the ability of SPLUNC1 to disrupt biofilm formation was significantly potentiated by NE pretreatment. While the impact of CF on SPLUNC1-Bcc interactions is not currently known, our data suggest that understanding this interaction may have important implications for CF lung disease.

Differential short palate, lung, and nasal epithelial clone 1 suppression in eosinophilic and noneosinophilic chronic rhinosinusitis with nasal polyps: implications for pathogenesis and treatment

PURPOSE OF REVIEW

Short palate, lung, and nasal epithelial clone 1 (SPLUNC1) is an epithelium-secreted protein that is involved in innate immunity. A protective role for SPLUNC1 in lower respiratory inflammation and chronic rhinosinusitis (CRS) has recently been recognized.

RECENT FINDINGS

An impaired epithelial immune barrier has been proposed to play a critical role in the pathogenesis of CRS. Recent research has demonstrated that SPLUNC1 is profoundly reduced in polyp tissues of CRS with nasal polyps (CRSwNP) compared with control tissues. Studies investigating the differential expression of SPLUNC1 in eosinophilic and noneosinophilic CRSwNP have been published. Nasal SPLUNC1 expression was inhibited by Th2 cytokines (IL-4 and IL-13) but was stimulated by toll-like receptor (TLR) agonists and glucocorticoids. Decreased SPLUNC1 expression in the sinus mucosa is associated with positive Pseudomonas aeruginosa bacterial colonization and poor surgical outcomes in CRS patients.

SUMMARY

These studies identify the role of SPLUNC1 in sinonasal innate immunity and the pathogenesis of CRS. Defective expression of SPLUNC1 in CRSwNP patients may lead to insufficient maintenance of the epithelial barrier function and enhanced bacterial colonization. The use of SPLUNC1 as a therapeutic target for CRSwNP remains to be determined.

The function and regulation of acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC): IUPHAR Review 19

Acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC) are both members of the ENaC/degenerin family of amiloride-sensitive Na(+) channels. ASICs act as proton sensors in the nervous system where they contribute, besides other roles, to fear behaviour, learning and pain sensation. ENaC mediates Na(+) reabsorption across epithelia of the distal kidney and colon and of the airways. ENaC is a clinically used drug target in the context of hypertension and cystic fibrosis, while ASIC is an interesting potential target. Following a brief introduction, here we will review selected aspects of ASIC and ENaC function. We discuss the origin and nature of pH changes in the brain and the involvement of ASICs in synaptic signalling. We expose how in the peripheral nervous system, ASICs cover together with other ion channels a wide pH range as proton sensors. We introduce the mechanisms of aldosterone-dependent ENaC regulation and the evidence for an aldosterone-independent control of ENaC activity, such as regulation by dietary K(+) . We then provide an overview of the regulation of ENaC by proteases, a topic of increasing interest over the past few years. In spite of the profound differences in the physiological and pathological roles of ASICs and ENaC, these channels share many basic functional and structural properties. It is likely that further research will identify physiological contexts in which ASICs and ENaC have similar or overlapping roles.

Epithelial Anion Transport as Modulator of Chemokine Signaling

The pivotal role of epithelial cells is to secrete and absorb ions and water in order to allow the formation of a luminal fluid compartment that is fundamental for the epithelial function as a barrier against environmental factors. Importantly, epithelial cells also take part in the innate immune system. As a first line of defense they detect pathogens and react by secreting and responding to chemokines and cytokines, thus aggravating immune responses or resolving inflammatory states. Loss of epithelial anion transport is well documented in a variety of diseases including cystic fibrosis, chronic obstructive pulmonary disease, asthma, pancreatitis, and cholestatic liver disease. Here we review the effect of aberrant anion secretion with focus on the release of inflammatory mediators by epithelial cells and discuss putative mechanisms linking these transport defects to the augmented epithelial release of chemokines and cytokines. These mechanisms may contribute to the excessive and persistent inflammation in many respiratory and gastrointestinal diseases.

Inhibition of airway surface fluid absorption by cholinergic stimulation

In upper airways airway surface liquid (ASL) depth and clearance rates are both increased by fluid secretion. Secretion is opposed by fluid absorption, mainly via the epithelial sodium channel, ENaC. In static systems, increased fluid depth activates ENaC and decreased depth inhibits it, suggesting that secretion indirectly activates ENaC to reduce ASL depth. We propose an alternate mechanism in which cholinergic input, which causes copious airway gland secretion, also inhibits ENaC-mediated absorption. The conjoint action accelerates clearance, and the increased transport of mucus out of the airways restores ASL depth while cleansing the airways. We were intrigued by early reports of cholinergic inhibition of absorption by airways in some species. To reinvestigate this phenomenon, we studied inward short-circuit currents (Isc) in tracheal mucosa from human, sheep, pig, ferret, and rabbit and in two types of cultured cells. Basal Isc was inhibited 20–70% by the ENaC inhibitor, benzamil. Long-lasting inhibition of ENaC-dependent Isc was also produced by basolateral carbachol in all preparations except rabbit and the H441 cell line. Atropine inhibition produced a slow recovery or prevented inhibition if added before carbachol. The mechanism for inhibition was not determined and is most likely multi-factorial. However, its physiological significance is expected to be increased mucus clearance rates in cholinergically stimulated airways.

Polymorphisms associated with expression of BPIFA1/BPIFB1 and lung disease severity in cystic fibrosis

BPI fold containing family A, member 1 (BPIFA1) and BPIFB1 are putative innate immune molecules expressed in the upper airways. Because of their hypothesized roles in airway defense, these molecules may contribute to lung disease severity in cystic fibrosis (CF). We interrogated BPIFA1/BPIFB1 single-nucleotide polymorphisms in data from an association study of CF modifier genes and found an association of the G allele of rs1078761 with increased lung disease severity (P = 2.71 × 10(-4)). We hypothesized that the G allele of rs1078761 is associated with decreased expression of BPIFA1 and/or BPIFB1. Genome-wide lung gene expression and genotyping data from 1,111 individuals with lung disease, including 51 patients with CF, were tested for associations between genotype and BPIFA1 and BPIFB1 gene expression levels. Findings were validated by quantitative PCR in a subset of 77 individuals. Western blotting was used to measure BPIFA1 and BPIFB1 protein levels in 93 lung and 101 saliva samples. The G allele of rs1078761 was significantly associated with decreased mRNA levels of BPIFA1 (P = 4.08 × 10(-15)) and BPIFB1 (P = 0.0314). These findings were confirmed with quantitative PCR and Western blotting. We conclude that the G allele of rs1078761 may be detrimental to lung function in CF owing to decreased levels of BPIFA1 and BPIFB1.

Airway surface liquid homeostasis in cystic fibrosis: pathophysiology and therapeutic targets

Cystic fibrosis (CF) is a life-limiting disease characterised by recurrent respiratory infections, inflammation and lung damage. The volume and composition of the airway surface liquid (ASL) are important in maintaining ciliary function, mucociliary clearance and antimicrobial properties of the airway. In CF, these homeostatic mechanisms are impaired, leading to a dehydrated and acidic ASL. ASL volume depletion in CF is secondary to defective anion transport by the abnormal cystic fibrosis transmembrane conductance regulator protein (CFTR). Abnormal CFTR mediated bicarbonate transport creates an unfavourable, acidic environment, which impairs antimicrobial function and alters mucus properties and clearance. These disease mechanisms create a disordered airway milieu, consisting of thick mucopurulent secretions and chronic bacterial infection. In addition to CFTR, there are additional ion channels and transporters in the apical airway epithelium that play a role in maintaining ASL homeostasis. These include the epithelial sodium channel (ENaC), the solute carrier 26A (SLC26A) family of anion exchangers, and calcium-activated chloride channels. In this review we discuss how the ASL is abnormal in CF and how targeting these alternative channels and transporters could provide an attractive therapeutic strategy to correct the underlying ASL abnormalities evident in CF.

Interleukin-13 Inhibits Lipopolysaccharide-Induced BPIFA1 Expression in Nasal Epithelial Cells

Short palate, lung, and nasal epithelium clone 1 (SPLUNC1) protein is expressed in human nasopharyngeal and respiratory epithelium and has demonstrated antimicrobial activity. SPLUNC1 is now referred to as bactericidal/permeability-increasing fold containing family A, member 1 (BPIFA1). Reduced BPIFA1 expression is associated with bacterial colonization in patients with chronic rhinosinusitis with nasal polyps (CRSwNP). Interleukin 13 (IL-13), predominately secreted by T helper 2 (T_H2) cells, has been found to contribute to airway allergies and suppress BPIFA1 expression in nasal epithelial cells. However, the molecular mechanism of IL-13 perturbation of bacterial infection and BPIFA1 expression in host airways remains unclear. In this study, we found that lipopolysaccharide (LPS)-induced BPIFA1 expression in nasal epithelial cells was mediated through the JNK/c-Jun signaling pathway and AP-1 activation. We further demonstrated that IL-13 downregulated the LPS-induced activation of phosphorylated JNK and c-Jun, followed by attenuation of BPIFA1 expression. Moreover, the immunohistochemical analysis showed that IL-13 prominently suppressed BPIFA1 expression in eosinophilic CRSwNP patients with bacterial infection. Taken together, these results suggest that IL-13 plays a critical role in attenuation of bacteria-induced BPIFA1 expression that may result in eosinophilic CRSwNP.

New insights on the palate, lung, and nasal epithelium clone (PLUNC) proteins: Based on molecular and functional analysis of its homolog of YH1/SPLUNC1

The palate, lung, and nasal epithelium clone (PLUNC) proteins are intricate immune molecules and arisen questions from them are still unresolved. In order to identify the role of PLUNC family proteins, we had analyzed its homolog protein YH1/SPLUNC1, which highly expresses in nontumor nasopharyngeal epithelium while expresses weakly in nasopharyngeal carcinoma (NPC) tissues. It is found that YH1/SPLUNC1 protein expression level was higher in chronic normal nasopharynx inflammatory cells compared to NPC tissue cells. An approach to produce active YH1/SPLUNC1 protein had been established and recombinant YH1/SPLUNC1 protein could bind to all four Gram-positive and four Gram-negative bacteria we tested, and triggered the aggregation of those bacteria. Interestingly, YH1/SPLUNC1 protein has antimicrobial activity, and it can directly kill Escherichia coli and Acinetobacter haemolyticus. The microorganism cell showed morphological changes in cell wall such as cell damage and cytoplasmic leakage after exposure to YH1/SPLUNC1 protein, indicating that YH1/SPLUNC1 directly killed the microorganisms by cell wall permeabilization. All these results indicated that YH1/SPLUNC1 might be an important antimicrobial protein involved in innate immunity defense.

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.

Airway hydration and COPD

Chronic obstructive pulmonary disease (COPD) is one of the prevalent causes of worldwide mortality and encompasses two major clinical phenotypes, i.e., chronic bronchitis (CB) and emphysema. The most common cause of COPD is chronic tobacco inhalation. Research focused on the chronic bronchitic phenotype of COPD has identified several pathological processes that drive disease initiation and progression. For example, the lung's mucociliary clearance (MCC) system performs the critical task of clearing inhaled pathogens and toxic materials from the lung. MCC efficiency is dependent on: (1) the ability of apical plasma membrane ion channels such as the cystic fibrosis transmembrane conductance regulator (CFTR) and the epithelial Na(+) channel (ENaC) to maintain airway hydration; (2) ciliary beating; and (3) appropriate rates of mucin secretion. Each of these components is impaired in CB and likely contributes to the mucus stasis/accumulation seen in CB patients. This review highlights the cellular components responsible for maintaining MCC and how this process is disrupted following tobacco exposure and with CB. We shall also discuss existing therapeutic strategies for the treatment of chronic bronchitis and how components of the MCC can be used as biomarkers for the evaluation of tobacco or tobacco-like-product exposure.

Protein networks in induced sputum from smokers and COPD patients

RATIONALE

Subtypes of cigarette smoke-induced disease affect different lung structures and may have distinct pathophysiological mechanisms.

OBJECTIVE

To determine if proteomic classification of the cellular and vascular origins of sputum proteins can characterize these mechanisms and phenotypes.

SUBJECTS AND METHODS

Individual sputum specimens from lifelong nonsmokers (n=7) and smokers with normal lung function (n=13), mucous hypersecretion with normal lung function (n=11), obstructed airflow without emphysema (n=15), and obstruction plus emphysema (n=10) were assessed with mass spectrometry. Data reduction, logarithmic transformation of spectral counts, and Cytoscape network-interaction analysis were performed. The original 203 proteins were reduced to the most informative 50. Sources were secretory dimeric IgA, submucosal gland serous and mucous cells, goblet and other epithelial cells, and vascular permeability.

RESULTS

Epithelial proteins discriminated nonsmokers from smokers. Mucin 5AC was elevated in healthy smokers and chronic bronchitis, suggesting a continuum with the severity of hypersecretion determined by mechanisms of goblet-cell hyperplasia. Obstructed airflow was correlated with glandular proteins and lower levels of Ig joining chain compared to other groups. Emphysema subjects' sputum was unique, with high plasma proteins and components of neutrophil extracellular traps, such as histones and defensins. In contrast, defensins were correlated with epithelial proteins in all other groups. Protein-network interactions were unique to each group.

CONCLUSION

The proteomes were interpreted as complex "biosignatures" that suggest distinct pathophysiological mechanisms for mucin 5AC hypersecretion, airflow obstruction, and inflammatory emphysema phenotypes. Proteomic phenotyping may improve genotyping studies by selecting more homogeneous study groups. Each phenotype may require its own mechanistically based diagnostic, risk-assessment, drug- and other treatment algorithms.

Bactericidal/Permeability-increasing protein fold-containing family member A1 in airway host protection and respiratory disease

Bactericidal/permeability-increasing protein fold-containing family member A1 (BPIFA1), formerly known as SPLUNC1, is one of the most abundant proteins in respiratory secretions and has been identified with increasing frequency in studies of pulmonary disease. Its expression is largely restricted to the respiratory tract, being highly concentrated in the upper airways and proximal trachea. BPIFA1 is highly responsive to airborne pathogens, allergens, and irritants. BPIFA1 actively participates in host protection through antimicrobial, surfactant, airway surface liquid regulation, and immunomodulatory properties. Its expression is modulated in multiple lung diseases, including cystic fibrosis, chronic obstructive pulmonary disease, respiratory malignancies, and idiopathic pulmonary fibrosis. However, the role of BPIFA1 in pulmonary pathogenesis remains to be elucidated. This review highlights the versatile properties of BPIFA1 in antimicrobial protection and its roles as a sensor of environmental exposure and regulator of immune cell function. A greater understanding of the contribution of BPIFA1 to disease pathogenesis and activity may clarify if BPIFA1 is a biomarker and potential drug target in pulmonary disease.

Targeting ion channels in cystic fibrosis

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene cause a characteristic defect in epithelial ion transport that plays a central role in the pathogenesis of cystic fibrosis (CF). Hence, pharmacological correction of this ion transport defect by targeting of mutant CFTR, or alternative ion channels that may compensate for CFTR dysfunction, has long been considered as an attractive approach to a causal therapy of this life-limiting disease. The recent introduction of the CFTR potentiator ivacaftor into the therapy of a subgroup of patients with specific CFTR mutations was a major milestone and enormous stimulus for seeking effective ion transport modulators for all patients with CF. In this review, we discuss recent breakthroughs and setbacks with CFTR modulators designed to rescue mutant CFTR including the common mutation F508del. Further, we examine the alternative chloride channels TMEM16A and SLC26A9, as well as the epithelial sodium channel ENaC as alternative targets in CF lung disease, which remains the major cause of morbidity and mortality in patients with CF. Finally, we will focus on the hurdles that still need to be overcome to make effective ion transport modulation therapies available for all patients with CF irrespective of their CFTR genotype.

SPLUNC1 Is a Significant Marker in Pleural Effusion from Lung Cancer Compared to Tuberculosis

SPLUNC1 (Short palate, lung and nasal epithelium clone1) protein is an abundant secretory product of epithelia present throughout the conducting airways. Although its function is still not fully known, most studies have focused on its defensive effect in the infection of human airways and its potential to serve as a molecular marker for lung cancer. In this study, we further evaluated the SPLUNC1 expression in patients with lung disease to explore its role in cancer or tuberculosis at the protein level. We generated a panel of antibodies by using protein from a eukaryotic expression system as the immunogen to mice. It was the panel of SPLUNC1 monoclonal antibodies that allowed us to comparatively determine SPLUNC1 protein in lung cancer and tuberculosis infection by detecting sera and pleural effusion other than airway surface. The results showed that the SPLUNC1 level was not significantly changed either from sera of lung cancer or control. There was a significant increase in pleural effusion from lung cancer when compared to tuberculosis. These results indicate that SPLUNC1 may be a useful marker for tracing lung cancer cells, based on its epithelial origin property in pleural effusion.

Gammaherpesvirus infection modulates the temporal and spatial expression of SCGB1A1 (CCSP) and BPIFA1 (SPLUNC1) in the respiratory tract

Murine γ-herpesvirus 68 (MHV-68) infection of Mus musculus-derived strains of mice is an established model of γ-herpesvirus infection. We have previously developed an alternative system using a natural host, the wood mouse (Apodemus sylvaticus), and shown that the MHV-68 M3 chemokine-binding protein contributes significantly to MHV-68 pathogenesis. Here we demonstrate in A. sylvaticus using high-density micro-arrays that M3 influences the expression of genes involved in the host response including Scgb1a1 and Bpifa1 that encode potential innate defense proteins secreted into the respiratory tract. Further analysis of MHV-68-infected animals showed that the levels of both protein and RNA for SCGB1A1 and BPIFA1 were decreased at day 7 post infection (p.i.) but increased at day 14 p.i. as compared with M3-deficient and mock-infected animals. The modulation of expression was most pronounced in bronchioles but was also present in the bronchi and trachea. Double staining using RNA in situ hybridization and immunohistology demonstrated that much of the BPIFA1 expression occurs in club cells along with SCGB1A1 and that BPIFA1 is stored within granules in these cells. The increase in SCGB1A1 and BPIFA1 expression at day 14 p.i. was associated with the differentiation of club cells into mucus-secreting cells. Our data highlight the role of club cells and the potential of SCGB1A1 and BPIFA1 as innate defense mediators during respiratory virus infection.

Automated acquisition and analysis of airway surface liquid height by confocal microscopy

The airway surface liquid (ASL) is a thin-liquid layer that lines the luminal side of airway epithelia. ASL contains many molecules that are involved in primary innate defense in the lung. Measurement of ASL height on primary airway cultures by confocal microscopy is a powerful tool that has enabled researchers to study ASL physiology and pharmacology. Previously, ASL image acquisition and analysis were performed manually. However, this process is time and labor intensive. To increase the throughput, we have developed an automatic ASL measurement technique that combines a fully automated confocal microscope with novel automatic image analysis software that was written with image processing techniques derived from the computer science field. We were able to acquire XZ ASL images at the rate of ∼ 1 image/s in a reproducible fashion. Our automatic analysis software was able to analyze images at the rate of ∼ 32 ms/image. As proofs of concept, we generated a time course for ASL absorption and a dose response in the presence of SPLUNC1, a known epithelial sodium channel inhibitor, on human bronchial epithelial cultures. Using this approach, we determined the IC50 for SPLUNC1 to be 6.53 μM. Furthermore, our technique successfully detected a difference in ASL height between normal and cystic fibrosis (CF) human bronchial epithelial cultures and detected changes in ATP-stimulated Cl(-)/ASL secretion. We conclude that our automatic ASL measurement technique can be applied for repeated ASL height measurements with high accuracy and consistency and increased throughput.

Proteomic analysis of pure human airway gland mucus reveals a large component of protective proteins

Airway submucosal glands contribute to innate immunity and protect the lungs by secreting mucus, which is required for mucociliary clearance and which also contains antimicrobial, anti-inflammatory, anti-proteolytic and anti-oxidant proteins. We stimulated glands in tracheal trimmings from three lung donors and collected droplets of uncontaminated mucus as they formed at the gland orifices under an oil layer. We analyzed the mucus using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Analysis identified 5486 peptides and 441 proteins from across the 3 samples (269-319 proteins per subject). We focused on 269 proteins common to at least 2 0f 3 subjects, of which 102 (38%) had protective or innate immunity functions. While many of these have long been known to play such roles, for many others their cellular protective functions have only recently been appreciated in addition to their well-studied biologic functions (e.g. annexins, apolipoproteins, gelsolin, hemoglobin, histones, keratins, and lumican). A minority of the identified proteins are known to be secreted via conventional exocytosis, suggesting that glandular secretion occurs via multiple mechanisms. Two of the observed protective proteins, major vault protein and prohibitin, have not been observed in fluid from human epithelial cultures or in fluid from nasal or bronchoalveolar lavage. Further proteomic analysis of pure gland mucus may help clarify how healthy airways maintain a sterile environment.

Targeting LUNX inhibits non-small cell lung cancer growth and metastasis

There remains a great need for effective therapies for lung cancer, the majority of which are non-small cell lung cancers (NSCLC). Here, we report the identification of a novel candidate therapeutic target, LUNX, as a molecule overexpressed in primary NSCLC and lymph node metastases that is associated with reduced postoperative survival. Functional studies demonstrated that LUNX overexpression promoted lung cancer cell migration and proliferation by interactions with the chaperone protein 14-3-3. Conversely, LUNX silencing disrupted primary tumor growth, local invasion, and metastatic colonization. The finding that LUNX was expressed on cell membranes prompted us to generate and characterize LUNX antibodies as a candidate therapeutic. Anti-LUNX could downregulate LUNX and reduce lung cancer cell proliferation and migration in vitro. Administered in vivo to mice bearing lung cancer xenografts, anti-LUNX could slow tumor growth and metastasis and improve mouse survival. Together, our work provides a preclinical proof of concept for LUNX as a novel candidate target for immunotherapy in lung cancer.

ENaC inhibitors for the treatment of cystic fibrosis

The epithelial Na+ channel, ENaC, is a key regulator of the volume of airway surface liquid in the human airway epithelium. In cystic fibrosis (CF), Na+ hyperabsorption through ENaC in the absence of CFTR-mediated anion secretion results in the dehydration of respiratory secretions and the impairment of mucociliary clearance. The hypothesis of utilizing an ENaC-blocking molecule to facilitate restoration of the airway surface liquid volume sufficiently to allow normal mucociliary clearance is of interest in the management of lung disease in CF patients. This article summarizes the published patent applications from 2010 that claim approaches to inhibit the function of ENaC for utility in the treatment of CF. Patents were located though SciFinder®, using “ENaC” as the keyword from 2010 onwards; documents not relevant to CF were then manually removed.

Cathepsin B contributes to Na+ hyperabsorption in cystic fibrosis airway epithelial cultures

In cystic fibrosis (CF) lung disease, the absence of functional CF transmembrane conductance regulator results in Cl(-)/HCO3 (-) hyposecretion and triggers Na(+) hyperabsorption through the epithelial Na(+) channel (ENaC), which contribute to reduced airway surface liquid (ASL) pH and volume. Prostasin, a membrane-anchored serine protease with trypsin-like substrate specificity has previously been shown to activate ENaC in CF airways. However, prostasin is typically inactive below pH 7.0, suggesting that it may be less relevant in acidic CF airways. Cathepsin B (CTSB) is present in both normal and CF epithelia and is secreted into ASL, but little is known about its function in the airways. We hypothesized that the acidic ASL seen in CF airways may stimulate CTSB to activate ENaC, contributing to Na(+) hyperabsorption and depletion of CF ASL volume. In Xenopus laevis oocytes, CTSB triggered α- and γENaC cleavage and induced an increase in ENaC activity. In bronchial epithelia from both normal and CF donor lungs, CTSB localized to the apical membrane. In normal and CF human bronchial epithelial cultures, CTSB was detected at the apical plasma membrane and in the ASL. CTSB activity was significantly elevated in acidic ASL, which correlated with increased abundance of ENaC in the plasma membrane and a reduction in ASL volume. This acid/CTSB-dependent activation of ENaC was ameliorated with the cell impermeable, CTSB-selective inhibitor CA074, suggesting that CTSB inhibition may have therapeutic relevance. Taken together, our data suggest that CTSB is a pathophysiologically relevant protease that activates ENaC in CF airways.

PLUNC proteins positivity in patients with chronic rhinosinusitis: a case-control study

Introduction. Innate immunity is the first protection against microorganisms. Nowadays, there is a growing interest in innate immune molecule known as palate, lung, nasal epithelial clone (PLUNC). PLUNC is a specific product of the airways, of approximately 25 kDa, encoded by adjacent genes found within a 300 kb region of chromosome 20; these proteins must be detected predominantly in the upper respiratory tract. Materials and Methods. We performed a case-control study to investigate the presence of this protein in nasal tissue of patients affected by chronic rhinosinusitis. 59 patients were enrolled (44 cases, 15 controls). We have examined the correlation between the presence of pathology and the PLUNC proteins positivity. Results. 100% of controls have a +++ rated PLUNC proteins positivity, while cases have a lower percentage of positivity. We used χ² statistical test to analyze the results of the study and there is a difference statistically significant between cases and controls in PLUNC proteins positivity. Conclusions. These observations suggest that, in response to agents or chemical factors, nasal mucosal epithelium will react and produce PLUNC proteins. So PLUNC proteins have a protective function on upper airways mucosa, as we can see by evaluating the high positivity in control group.

Mammalian short palate lung and nasal epithelial clone 1 (SPLUNC1) in pH-dependent airway hydration

The epithelia that line the conducting airways are the lung's first point of contact with inhaled pathogens and toxicants. As such, they are known to play an important role in the lung's innate defense system, which includes (i) the production of airway surface liquid (ASL) that helps cleanse the airways through the physical removal of pathogens and toxicants on the mucociliary escalator and (ii) the secretion of anti-microbial proteins into the ASL to kill inhaled pathogens. Interestingly, the recently crystallized short palate lung and nasal epithelial clone 1 (SPLUNC1) protein appears to be a multi-functional protein. That is, it not only acts as an anti-microbial agent, but also modulates ASL homeostasis by acting as an endogenous inhibitor of the epithelial Na(+) channel (ENaC). This review will focus on the latter function of SPLUNC1, and will discuss new structural and physiological data regarding SPLUNC1's failure to function as a regulator of ASL hydration in CF airways.

CFTR: cystic fibrosis and beyond

Cystic fibrosis (CF) remains the most common fatal hereditary lung disease. The discovery of the cystic fibrosis transmembrane conductance regulator (CFTR) gene 25 years ago set the stage for: 1) unravelling the molecular and cellular basis of CF lung disease; 2) the generation of animal models to study in vivo pathogenesis; and 3) the development of mutation-specific therapies that are now becoming available for a subgroup of patients with CF. This article highlights major advances in our understanding of how CFTR dysfunction causes chronic mucus obstruction, neutrophilic inflammation and bacterial infection in CF airways. Furthermore, we focus on recent breakthroughs and remaining challenges of novel therapies targeting the basic CF defect, and discuss the next steps to be taken to make disease-modifying therapies available to a larger group of patients with CF, including those carrying the most common mutation ΔF508-CFTR. Finally, we will summarise emerging evidence indicating that acquired CFTR dysfunction may be implicated in the pathogenesis of chronic obstructive pulmonary disease, suggesting that lessons learned from CF may be applicable to common airway diseases associated with mucus plugging.

Investigation of anti-infection mechanism of lactoferricin and splunc-1

The innate immune system is the first line in the defense system and prevents the body from further bacteria, virus, or fungal infections. Most of the innate immune system is relevant to mucosa immunity. Lactotransferrin is secreted from the human mammal breast duct epithelial tissue and strengthens infant immunity to defense with regard to outward pathogens. Splunc-1 is also an innate material secreted from the soft palate, lung, nasal cavity epithelium, and mucosa. It helps with mucosa defense against bacterial, virus, and even fungus. LPS is the main etiology of Gram-negative bacilla infection source. And studies of lactoferricin and slpunc-1 both can combine with LPS and subsequently cause insults to the mucosa. Although, we know that both of them partake in an important role in innate immunity, we do not know the effects when they work together. In this study, we just overview silicon stimulation to examine the combination of Lactoferricin and Splunc-1 and the effect with regard to LPS.

Dual acute proinflammatory and antifibrotic pulmonary effects of short palate, lung, and nasal epithelium clone-1 after exposure to carbon nanotubes

Carbon nanotubes (CNTs; allotropes of carbon with a cylindrical nanostructure) have emerged as one of the most commonly used types of nanomaterials, with numerous applications in industry and biomedicine. However, the inhalation of CNTs has been shown to elicit pulmonary toxicity, accompanied by a robust inflammatory response with an early-onset fibrotic phase. Epithelial host-defense proteins represent an important component of the pulmonary innate immune response to foreign inhalants such as particles and bacteria. The short palate, lung, and nasal epithelium clone-1 (SPLUNC1) protein, a member of the bactericidal/permeability-increasing-fold (BPIF)-containing protein family, is a 25-kD secretory protein that is expressed in nasal, oropharyngeal, and lung epithelia, and has been shown to have multiple functions, including antimicrobial and chemotactic activities, as well as surfactant properties. This study sought to assess the importance of SPLUNC1-mediated pulmonary responses in airway epithelial secretions, and to explore the biological relevance of SPLUNC1 to inhaled particles in a single-walled carbon nanotube (SWCNT) model. Using Scgb1a1-hSPLUNC1 transgenic mice, we observed that SPLUNC1 significantly modified host inflammatory responses by increasing leukocyte recruitment and enhancing phagocytic activity. Furthermore, we found that transgenic mice were more susceptible to SWCNT exposure at the acute phase, but showed resistance against lung fibrogenesis through pathological changes in the long term. The binding of SPLUNC1 also attenuated SWCNT-induced TNF-α secretion by RAW 264.7 macrophages. Taken together, our data indicate that SPLUNC1 is an important component of mucosal innate immune defense against pulmonary inhaled particles.

The Concise Guide to PHARMACOLOGY 2013/14: ion channels

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Ion channels are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Identification of the SPLUNC1 ENaC-inhibitory domain yields novel strategies to treat sodium hyperabsorption in cystic fibrosis airway epithelial cultures

The epithelial sodium channel (ENaC) is responsible for Na(+) and fluid absorption across colon, kidney, and airway epithelia. Short palate lung and nasal epithelial clone 1 (SPLUNC1) is a secreted, innate defense protein and an autocrine inhibitor of ENaC that is highly expressed in airway epithelia. While SPLUNC1 has a bactericidal permeability-increasing protein (BPI)-type structure, its NH2-terminal region lacks structure. Here we found that an 18 amino acid peptide, S18, which corresponded to residues G22-A39 of the SPLUNC1 NH2 terminus inhibited ENaC activity to a similar degree as full-length SPLUNC1 (∼2.5 fold), while SPLUNC1 protein lacking this region was without effect. S18 did not inhibit the structurally related acid-sensing ion channels, indicating specificity for ENaC. However, S18 preferentially bound to the βENaC subunit in a glycosylation-dependent manner. ENaC hyperactivity is contributory to cystic fibrosis (CF) lung disease. Unlike control, CF human bronchial epithelial cultures (HBECs) where airway surface liquid (ASL) height was abnormally low (4.2 ± 0.6 μm), addition of S18 prevented ENaC-led ASL hyperabsorption and maintained CF ASL height at 7.9 ± 0.6 μm, even in the presence of neutrophil elastase, which is comparable to heights seen in normal HBECs. Our data also indicate that the ENaC inhibitory domain of SPLUNC1 may be cleaved away from the main molecule by neutrophil elastase, suggesting that it may still be active during inflammation or neutrophilia. Furthermore, the robust inhibition of ENaC by the S18 peptide suggests that this peptide may be suitable for treating CF lung disease.

Estradiol activates epithelial sodium channels in rat alveolar cells through the G protein-coupled estrogen receptor

Female sex predisposes individuals to poorer outcomes during respiratory disorders like cystic fibrosis and influenza-associated pneumonia. A common link between these disorders is dysregulation of alveolar fluid clearance via disruption of epithelial sodium channel (ENaC) activity. Recent evidence suggests that female sex hormones directly regulate expression and activity of alveolar ENaC. In our study, we identified the mechanism by which estradiol (E2) or progesterone (P4) independently regulates alveolar ENaC. Using cell-attached patch clamp, we measured ENaC single-channel activity in a rat alveolar cell line (L2) in response to overnight exposure to either E2 or P4. In contrast to P4, E2 increased ENaC channel activity (NPo) through an increase in channel open probability (Po) and an increased number of patches with observable channel activity. Apical plasma membrane abundance of the ENaC α-subunit (αENaC) more than doubled in response to E2 as determined by cell surface biotinylation. αENaC membrane abundance was approximately threefold greater in lungs from female rats in proestrus, when serum E2 is greatest, compared with diestrus, when it is lowest. Our results also revealed a significant role for the G protein-coupled estrogen receptor (Gper) to mediate E2's effects on ENaC. Overall, our results demonstrate that E2 signaling through Gper selectively activates alveolar ENaC through an effect on channel gating and channel density, the latter via greater trafficking of channels to the plasma membrane. The results presented herein implicate E2-mediated regulation of alveolar sodium channels in the sex differences observed in the pathogenesis of several pulmonary diseases.

Managing the underlying cause of cystic fibrosis: a future role for potentiators and correctors

Cystic fibrosis (CF), a severe genetic disease, is caused by mutations that alter the structure and function of CFTR, a plasma membrane channel permeable to chloride and bicarbonate. Defective anion transport in CF irreversibly damages the lungs, pancreas, liver, and other organs. CF mutations cause loss of CFTR function in multiple ways. In particular, class 3 mutations such as p.Gly551Asp strongly decrease the time spent by CFTR in the open state (gating defect). Instead, class 2 mutations impair the maturation of CFTR protein and its transport from the endoplasmic reticulum to the plasma membrane (trafficking defect). The deletion of phenylalanine 508 (p.Phe508del), the most frequent mutation among CF patients (70-90 %), destabilizes the CFTR protein, thus causing both a trafficking and a gating defect. These two defects can be overcome with drug-like molecules generically called correctors and potentiators, respectively. The potentiator Kalydeco™ (also known as Ivacaftor or VX-770), developed by Vertex Pharmaceuticals, has been recently approved by the US FDA and the European Medicines Agency (EMA) for the treatment of CF patients carrying at least one CFTR allele with the p.Gly551Asp mutation (2-5 % of all patients). In contrast, the corrector VX-809, which significantly improves p.Phe508del-CFTR trafficking in vitro, is still under study in clinical trials. Because of multiple defects caused by the p.Phe508del mutation, it is probable that rescue of the mutant protein will require combined treatment with correctors having different mechanisms of action. This review evaluates the status of experimental and clinical research in pharmacotherapy for the CF basic defect.

Reduced sodium transport with nasal administration of the prostasin inhibitor camostat in subjects with cystic fibrosis

BACKGROUND

Prostasin, a trypsin-like serine protease, is a channel-activating protease and major regulator of epithelial sodium channel-mediated sodium absorption. Its direct inhibition by camostat represents a potential approach to inhibiting sodium transport in cystic fibrosis (CF).

METHODS

To determine whether a topical formulation of camostat represents an efficacious and tolerable approach to reducing Na+ transport in the CF airway, we conducted a two-part randomized, double-blind, placebo-controlled, crossover, ascending single-dose study to evaluate the pharmacodynamics, safety, and pharmacokinetics of camostat administered through a nasal spray pump in subjects with CF. Nasal potential difference (PD) was measured before and after treatment, and safety and pharmacokinetics were assessed by a standardized approach.

RESULTS

In part 1, nine subjects were enrolled, and six completed crossover dosing at the maximally tolerated dose. The change in maximal (most polarizing) basal PD 2 h following administration of camostat was +13.1 mV (1.6-mg dose group) compared with -8.6 mV following placebo (P<.005). Intrasubject change in Ringer and amiloride-sensitive PDs exhibited similar and consistent responses. Bayesian analysis in an additional six subjects in part 2 estimated a dose of 18 μg/mL to provide 50% of the maximum effect. There was no significant change in chloride transport or total nasal symptom score, nasal examination rating, and laboratory parameters.

CONCLUSIONS

This study establishes the proof of concept that a reduction in sodium transport in the human CF airway can be achieved through inhibition of prostasin activity, identifying a potential therapeutic target in the disease.

TRIAL REGISTRATION

ClinicalTrials.gov; No.: NCT00506792; URL: www.clinicaltrials.gov.

Increased susceptibility to pulmonary Pseudomonas infection in Splunc1 knockout mice

The airway epithelium is the first line of host defense against pathogens. The short palate, lung, and nasal epithelium clone (SPLUNC)1 protein is secreted in respiratory tracts and is a member of the bacterial/permeability increasing (BPI) fold-containing protein family, which shares structural similarities with BPI-like proteins. On the basis of its homology with BPIs and restricted expression of SPLUNC1 in serous cells of submucosal glands and surface epithelial cells of the upper respiratory tract, SPLUNC1 is thought to possess antimicrobial activity in host defense. SPLUNC1 is also reported to have surfactant properties, which may contribute to anti-biofilm defenses. The objective of this study was to determine the in vivo functions of SPLUNC1 following Pseudomonas aeruginosa infection and to elucidate the underlying mechanism by using a knockout (KO) mouse model with a genetic ablation of Splunc1. Splunc1 KO mice showed accelerated mortality and increased susceptibility to P. aeruginosa infection with significantly decreased survival rates, increased bacterial burdens, exaggerated tissue injuries, and elevated proinflammatory cytokine levels as compared with those of their wild-type littermates. Increased neutrophil infiltration in Splunc1 KO mice was accompanied by elevated chemokine levels, including Cxcl1, Cxcl2, and Ccl20. Furthermore, the expression of several epithelial secretory proteins and antimicrobial molecules was considerably suppressed in the lungs of Splunc1 KO mice. The deficiency of Splunc1 in mouse airway epithelium also results in increased biofilm formation of P. aeruginosa. Taken together, our results support that the ablation of Splunc1 in mouse airways affects the mucociliary clearance, resulting in decreased innate immune response during Pseudomonas-induced respiratory infection.

Molecular basis for pH-dependent mucosal dehydration in cystic fibrosis airways

The ability to maintain proper airway surface liquid (ASL) volume homeostasis is vital for mucus hydration and clearance, which are essential aspects of the mammalian lung's innate defense system. In cystic fibrosis (CF), one of the most common life-threatening genetic disorders, ASL dehydration leads to mucus accumulation and chronic infection. In normal airways, the secreted protein short palate lung and nasal epithelial clone 1 (SPLUNC1) effectively inhibits epithelial Na(+) channel (ENaC)-dependent Na(+) absorption and preserves ASL volume. In CF airways, it has been hypothesized that increased ENaC-dependent Na(+) absorption contributes to ASL depletion, and hence increased disease. However, this theory is controversial, and the mechanism for abnormal ENaC regulation in CF airways has remained elusive. Here, we show that SPLUNC1 is a pH-sensitive regulator of ENaC and is unable to inhibit ENaC in the acidic CF airway environment. Alkalinization of CF airway cultures prevented CF ASL hyperabsorption, and this effect was abolished when SPLUNC1 was stably knocked down. Accordingly, we resolved the crystal structure of SPLUNC1 to 2.8 Å. Notably, this structure revealed two pH-sensitive salt bridges that, when removed, rendered SPLUNC1 pH-insensitive and able to regulate ASL volume in acidic ASL. Thus, we conclude that ENaC hyperactivity is secondary to reduced CF ASL pH. Together, these data provide molecular insights into the mucosal dehydration associated with a range of pulmonary diseases, including CF, and suggest that future therapy be directed toward alkalinizing the pH of CF airways.

Short palate, lung, and nasal epithelial clone-1 is a tightly regulated airway sensor in innate and adaptive immunity

Short palate, lung, and nasal epithelial clone-1 (SPLUNC1) is a protein abundantly expressed by the respiratory epithelium of the proximal lower respiratory tract, a site of great environmental exposure. Previous studies showed that SPLUNC1 exerts antimicrobial effects, regulates airway surface liquid and mucociliary clearance, and suppresses allergic airway inflammation. We studied SPLUNC1 to gain insights into its role in host defense. In the lower respiratory tract, concentrations of SPLUNC1 are high under basal conditions. In models of pneumonia caused by common respiratory pathogens, and in Th1-induced and Th2-induced airway inflammation, SPLUNC1 secretion is markedly reduced. Pathogen-associated molecular patterns and IFN-γ act directly on airway epithelial cells to inhibit SPLUNC1 mRNA expression. Thus, SPLUNC1 is quickly suppressed during infection, in response to an insult on the epithelial surface. These experiments highlight the finely tuned fluctuations of SPLUNC1 in response to exposures in the respiratory tract, and suggest that the loss of SPLUNC1 is a crucial feature of host defense across air-breathing animal species.

Does epithelial sodium channel hyperactivity contribute to cystic fibrosis lung disease?

  Airway epithelia absorb Na+ through the epithelial Na+ channel (ENaC) and secrete Cl- through the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel. This balance maintains sufficient airway surface liquid hydration to permit efficient mucus clearance, which is needed to maintain sterility of the lung. Cystic fibrosis (CF) is a common autosomal recessive inherited disease caused by mutations in the CFTR gene that lead to the reduction or elimination of the CFTR protein. CF is a multi-organ disease that affects epithelia lining the intestines, lungs, pancreas, sweat ducts and vas deferens, among others. CF lungs are characterized by viscous, dehydrated mucus, persistent neutrophilia and chronic infections. ENaC is negatively regulated by CFTR and, in patients with CF, the absence of CFTR results in a double hit of reduced Cl-/HCO3- and H2O secretion as well as ENaC hyperactivity and increased Na+ and H2O absorption. Together, these effects are hypothesized to trigger mucus dehydration, resulting in a failure to clear mucus. Rehydrating CF mucus has become a recent clinical focus and yields important end-points for clinical trials. However, while ENaC hyperactivity in CF airways has been detected in vivo and in vitro, recent data have brought the role of ENaC in CF lung disease pathogenesis into question. This review will focus on our current understanding of the contribution of ENaC to CF pathogenesis.

Mechanosensitive ATP release maintains proper mucus hydration of airways

The clearance of mucus from the airways protects the lungs from inhaled noxious and infectious materials. Proper hydration of the mucus layer enables efficient mucus clearance through beating of cilia on airway epithelial cells, and reduced clearance of excessively concentrated mucus occurs in patients with chronic obstructive pulmonary disease and cystic fibrosis. Key steps in the mucus transport process are airway epithelia sensing and responding to changes in mucus hydration. We reported that extracellular adenosine triphosphate (ATP) and adenosine were important luminal autocrine and paracrine signals that regulated the hydration of the surface of human airway epithelial cultures through their action on apical membrane purinoceptors. Mucus hydration in human airway epithelial cultures was sensed by an interaction between cilia and the overlying mucus layer: Changes in mechanical strain, proportional to mucus hydration, regulated ATP release rates, adjusting fluid secretion to optimize mucus layer hydration. This system provided a feedback mechanism by which airways maintained mucus hydration in an optimum range for cilia propulsion. Understanding how airway epithelia can sense and respond to changes in mucus properties helps us to understand how the mucus clearance system protects the airways in health and how it fails in lung diseases such as cystic fibrosis.

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.

Deuterium oxide dilution: a novel method to study apical water layers and transepithelial water transport

Lung epithelia regulate the water flux between gas filled airways and the interstitial compartment in order to maintain organ function. Current methodology to assess transepithelial water transport is limited. We present a D2O dilution method to quantify submicroliter volumes of aqueous solutions on epithelial cell layers. Evaluating D2O/H2O mixtures using mid-infrared (2-25 μm) attenuated total reflection (ATR) spectroscopy, with a resolution of 0.06% vol/vol change, corresponding to 24 nL, was achieved. Using this method, we demonstrate that water transport across NCI-H441 lung epithelial cell layers and apical surface liquid (ASL) volumes are coupled to dexamethasone dependent amiloride-sensitive ion transport. However, contrary to current dogma, electrogenic transport is not rate-limiting for water transport. This clearly indicates the need to directly assess net water rather than ion transport across epithelial cell layers. The presented D2O dilution method enables such direct and quick quantification of transepithelial water transport with high resolution.

SPLUNC1/BPIFA1 contributes to pulmonary host defense against Klebsiella pneumoniae respiratory infection

Epithelial host defense proteins comprise a critical component of the pulmonary innate immune response to infection. The short palate, lung, nasal epithelium clone (PLUNC) 1 (SPLUNC1) protein is a member of the bactericidal/permeability-increasing (BPI) fold-containing (BPIF) protein family, sharing structural similarities with BPI-like proteins. SPLUNC1 is a 25 kDa secretory protein that is expressed in nasal, oropharyngeal, and lung epithelia, and has been implicated in airway host defense against Pseudomonas aeruginosa and other organisms. SPLUNC1 is reported to have surfactant properties, which may contribute to anti-biofilm defenses. The objective of this study was to assess the importance of SPLUNC1 surfactant activity in airway epithelial secretions and to explore its biological relevance in the context of a bacterial infection model. Using cultured airway epithelia, we confirmed that SPLUNC1 is critically important for maintenance of low surface tension in airway fluids. Furthermore, we demonstrated that recombinant SPLUNC1 (rSPLUNC1) significantly inhibited Klebsiella pneumoniae biofilm formation on airway epithelia. We subsequently found that Splunc1(-/-) mice were significantly more susceptible to infection with K. pneumoniae, confirming the likely in vivo relevance of this anti-biofilm effect. Our data indicate that SPLUNC1 is a crucial component of mucosal innate immune defense against pulmonary infection by a relevant airway pathogen, and provide further support for the novel hypothesis that SPLUNC1 protein prevents bacterial biofilm formation through its ability to modulate surface tension of airway fluids.