Airway Surface Dehydration by Transforming Growth Factor β (TGF-β) in Cystic Fibrosis Is Due to Decreased Function of a Voltage-dependent Potassium Channel and Can Be Rescued by the Drug Pirfenidone

Deficiency of the BKCa potassium channel displayed significant implications for the physiology of the human bronchial epithelium

Plasma membrane large-conductance calcium-activated potassium (BKCa) channels are important players in various physiological processes, including those mediated by epithelia. Like other cell types, human bronchial epithelial (HBE) cells also express BKCa in the inner mitochondrial membrane (mitoBKCa). The genetic relationships between these mitochondrial and plasma membrane channels and the precise role of mitoBKCa in epithelium physiology are still unclear. Here, we tested the hypothesis that the mitoBKCa channel is encoded by the same gene as the plasma membrane BKCa channel in HBE cells. We also examined the impact of channel loss on the basic function of HBE cells, which is to create a tight barrier. For this purpose, we used CRISPR/Cas9 technology in 16HBE14o- cells to disrupt the KCNMA1 gene, which encodes the α-subunit responsible for forming the pore of the plasma membrane BKCa channel. Electrophysiological experiments demonstrated that the disruption of the KCNMA1 gene resulted in the loss of BKCa-type channels in the plasma membrane and mitochondria. We have also shown that HBE ΔαBKCa cells exhibited a significant decrease in transepithelial electrical resistance which indicates a loss of tightness of the barrier created by these cells. We have also observed a decrease in mitochondrial respiration, which indicates a significant impairment of these organelles. In conclusion, our findings indicate that a single gene encodes both populations of the channel in HBE cells. Furthermore, this channel is critical for maintaining the proper function of epithelial cells as a cellular barrier.

The combination of propylene glycol and vegetable glycerin e-cigarette aerosols induces airway inflammation and mucus hyperconcentration

Despite concerns over their safety, e-cigarettes (e-cigs) remain a popular tobacco product. Although nicotine and flavors found in e-cig liquids (e-liquids) can cause harm in the airways, whether the delivery vehicles propylene glycol (PG) and vegetable glycerin (VG) are innocuous when inhaled remains unclear. Here, we investigated the effects of e-cig aerosols generated from e-liquid containing only PG/VG on airway inflammation and mucociliary function in primary human bronchial epithelial cells (HBEC) and sheep. Primary HBEC were cultured at the air-liquid interface (ALI) and exposed to e-cig aerosols of 50%/50% v/v PG/VG. Ion channel conductance, ciliary beat frequency, and the expression of inflammatory markers, cell type-specific markers, and the major mucins MUC5AC and MUC5B were evaluated after seven days of exposure. Sheep were exposed to e-cig aerosols of PG/VG for five days and mucus concentration and matrix metalloproteinase-9 (MMP-9) activity were measured from airway secretions. Seven-day exposure of HBEC to e-cig aerosols of PG/VG caused a significant reduction in the activities of apical ion channels important for mucus hydration, including the cystic fibrosis transmembrane conductance regulator (CFTR) and large conductance, Ca2+-activated, and voltage-dependent K+ (BK) channels. PG/VG aerosols significantly increased the mRNA expression of the inflammatory markers interleukin-6 (IL6), IL8, and MMP9, as well as MUC5AC. The increase in MUC5AC mRNA expression correlated with increased immunostaining of MUC5AC protein in PG/VG-exposed HBEC. On the other hand, PG/VG aerosols reduced MUC5B expression leading overall to higher MUC5AC/MUC5B ratios in exposed HBEC. Other cell type-specific markers, including forkhead box protein J1 (FOXJ1), keratin 5 (KRT5), and secretoglobin family 1A member 1 (SCGB1A1) mRNAs, as well as overall ciliation, were significantly reduced by PG/VG exposure. Finally, PG/VG aerosols increased MMP-9 activity and caused mucus hyperconcentration in sheep in vivo. E-cig aerosols of PG/VG induce airway inflammation, increase MUC5AC expression, and cause dysfunction of ion channels important for mucus hydration in HBEC in vitro. Furthermore, PG/VG aerosols increase MMP-9 activity and mucus concentration in sheep in vivo. Collectively, these data show that e-cig aerosols containing PG/VG are likely to be harmful in the airways.

High-resolution structures illuminate key principles underlying voltage and LRRC26 regulation of Slo1 channels

Multi-modal regulation of Slo1 channels by membrane voltage, intracellular calcium, and auxiliary subunits enables its pleiotropic physiological functions. Our understanding of how voltage impacts Slo1 conformational dynamics and the mechanisms by which auxiliary subunits, particularly of the LRRC (Leucine Rich Repeat containing) family of proteins, modulate its voltage gating remain unresolved. Here, we used single particle cryo-electron microscopy to determine structures of human Slo1 mutants which functionally stabilize the closed pore (F315A) or the activated voltage-sensor (R207A). Our structures, obtained under calcium-free conditions, reveal that a key step in voltage-sensing by Slo1 involves a rotameric flip of the voltage-sensing charges (R210 and R213) moving them by ∼6 Å across a hydrophobic gasket. Next we obtained reconstructions of a complex of human Slo1 with the human LRRC26 (γ1) subunit in absence of calcium. Together with extensive biochemical tests, we show that the extracellular domains of γ1 form a ring of interlocked dominos that stabilizes the quaternary assembly of the complex and biases Slo1:γ1 assembly towards high stoichiometric complexes. The transmembrane helix of γ1 is kinked and tightly packed against the Slo1 voltage-sensor. We hypothesize that γ1 subunits exert relatively small effects on early steps in voltage-gating but structurally stabilize non-S4 helices of Slo1 voltage-sensor which energetically facilitate conformational rearrangements that occur late in voltage stimulated transitions.

Furin as a therapeutic target in cystic fibrosis airways disease

Clinical management of cystic fibrosis (CF) has been greatly improved by the development of small molecule modulators of the CF transmembrane conductance regulator (CFTR). These drugs help to address some of the basic genetic defects of CFTR; however, no suitable CFTR modulators exist for 10% of people with CF (PWCF). An alternative, mutation-agnostic therapeutic approach is therefore still required. In CF airways, elevated levels of the proprotein convertase furin contribute to the dysregulation of key processes that drive disease pathogenesis. Furin plays a critical role in the proteolytic activation of the epithelial sodium channel; hyperactivity of which causes airways dehydration and loss of effective mucociliary clearance. Furin is also responsible for the processing of transforming growth factor-β, which is increased in bronchoalveolar lavage fluid from PWCF and is associated with neutrophilic inflammation and reduced pulmonary function. Pathogenic substrates of furin include Pseudomonas exotoxin A, a major toxic product associated with Pseudomonas aeruginosa infection and the spike glycoprotein of severe acute respiratory syndrome coronavirus 2, the causative pathogen for coronavirus disease 2019. In this review we discuss the importance of furin substrates in the progression of CF airways disease and highlight selective furin inhibition as a therapeutic strategy to provide clinical benefit to all PWCF.

E-cigarette aerosols of propylene glycol impair BK channel activity and parameters of mucociliary function

Propylene glycol (PG) is a common delivery vehicle for nicotine and flavorings in e-cigarette (e-cig) liquids and is largely considered safe for ingestion. However, little is known about its effects as an e-cig aerosol on the airway. Here, we investigated whether pure PG e-cig aerosols in realistic daily amounts impact parameters of mucociliary function and airway inflammation in a large animal model (sheep) in vivo and primary human bronchial epithelial cells (HBECs) in vitro. Five-day exposure of sheep to e-cig aerosols of 100% PG increased mucus concentrations (% mucus solids) of tracheal secretions. PG e-cig aerosols further increased the activity of matrix metalloproteinase-9 (MMP-9) in tracheal secretions. In vitro exposure of HBECs to e-cig aerosols of 100% PG decreased ciliary beating and increased mucus concentrations. PG e-cig aerosols further reduced the activity of large conductance, Ca2+-activated, and voltage-dependent K+ (BK) channels. We show here for the first time that PG can be metabolized to methylglyoxal (MGO) in airway epithelia. PG e-cig aerosols increased levels of MGO and MGO alone reduced BK activity. Patch-clamp experiments suggest that MGO can disrupt the interaction between the major pore-forming BK subunit human Slo1 (hSlo1) and the gamma regulatory subunit LRRC26. PG exposures also caused a significant increase in mRNA expression levels of MMP9 and interleukin 1 beta (IL1B). Taken together, these data show that PG e-cig aerosols cause mucus hyperconcentration in sheep in vivo and HBECs in vitro, likely by disrupting the function of BK channels important for airway hydration.

Cell signaling and regulation of CFTR expression in cystic fibrosis cells in the era of high efficiency modulator therapy

Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP- and protein kinase A (PKA)-regulated channel, expressed on the luminal surface of secretory and absorptive epithelial cells. CFTR has a complex, cell-specific regulatory network playing a major role in cAMP- and Ca2+-activated secretion of electrolytes. It secretes intracellular Cl- and bicarbonate and regulates absorption of electrolytes by differentially controlling the activity of the epithelial Na+ channel (ENaC) in colon, airways, and sweat ducts. The CFTR gene expression is regulated by cell-specific, time-dependent mechanisms reviewed elsewhere [1]. This review will focus on the transcriptional, post-transcriptional, and translational regulation of CFTR by cAMP-PKA, non-coding (nc)RNAs, and TGF-β signaling pathways in cystic fibrosis (CF) cells.

BPIFA1 is a secreted biomarker of differentiating human airway epithelium

In vitro culture and differentiation of human-derived airway basal cells under air-liquid interface (ALI) into a pseudostratified mucociliated mucosal barrier has proven to be a powerful preclinical tool to study pathophysiology of respiratory epithelium. As such, identifying differentiation stage-specific biomarkers can help investigators better characterize, standardize, and validate populations of regenerating epithelial cells prior to experimentation. Here, we applied longitudinal transcriptomic analysis and observed that the pattern and the magnitude of OMG, KRT14, STC1, BPIFA1, PLA2G7, TXNIP, S100A7 expression create a unique biosignature that robustly indicates the stage of epithelial cell differentiation. We then validated our findings by quantitative hemi-nested real-time PCR from in vitro cultures sourced from multiple donors. In addition, we demonstrated that at protein-level secretion of BPIFA1 accurately reflects the gene expression profile, with very low quantities present at the time of ALI induction but escalating levels were detectable as the epithelial cells terminally differentiated. Moreover, we observed that increase in BPIFA1 secretion closely correlates with emergence of secretory cells and an anti-inflammatory phenotype as airway epithelial cells undergo mucociliary differentiation under air-liquid interface in vitro.

Losartan ameliorates TGF-β1-induced CFTR dysfunction and improves correction by cystic fibrosis modulator therapies

Highly effective modulator therapies dramatically improve the prognosis for those with cystic fibrosis (CF). The triple combination of elexacaftor, tezacaftor, and ivacaftor (ETI) benefits many, but not all, of those with the most common F508del mutation in the CF transmembrane conductance regulator (CFTR). Here, we showed that poor sweat chloride concentration responses and lung function improvements upon initiation of ETI were associated with elevated levels of active TGF-β1 in the upper airway. Furthermore, TGF-β1 impaired the function of ETI-corrected F508del-CFTR, thereby increasing airway surface liquid (ASL) absorption rates and inducing mucus hyperconcentration in primary CF bronchial epithelial cells in vitro. TGF-β1 not only decreased CFTR mRNA, but was also associated with increases in the mRNA expression of TNFA and COX2 and TNF-α protein. Losartan improved TGF-β1-mediated inhibition of ETI-corrected F508del-CFTR function and reduced TNFA and COX2 mRNA and TNF-α protein expression. This likely occurred by improving correction of mutant CFTR rather than increasing its mRNA (without an effect on potentiation), thereby reversing the negative effects of TGF-β1 and improving ASL hydration in the CF airway epithelium in vitro. Importantly, these effects were independent of type 1 angiotensin II receptor inhibition.

The LRRC family of BK channel regulatory subunits: potential roles in health and disease

Large conductance K⁺ channels, termed BK channels, regulate a variety of cellular and physiological functions. Although universally activated by changes in voltage or [Ca²⁺ ]_i , the threshold for BK channel activation varies among loci of expression, often arising from cell-specific regulatory subunits including a family of leucine rich repeat-containing (LRRC) γ subunits (LRRC26, LRRC52, LRRC55 and LRRC38). The 'founding' member of this family, LRRC26, was originally identified as a tumour suppressor in various cancers. An LRRC26 knockout (KO) mouse model recently revealed that LRRC26 is also highly expressed in secretory epithelial cells and partners with BK channels in the salivary gland and colonic goblet cells to promote sustained K⁺ fluxes likely essential for normal secretory function. To accomplish this, LRRC26 negatively shifts the range of BK channel activation such that channels contribute to K⁺ flux near typical epithelial cell resting conditions. In colon, the absence of LRRC26 increases vulnerability to colitis. LRRC26-containing BK channels are also likely important regulators of epithelial function in other loci, including airways, female reproductive tract and mammary gland. Based on an LRRC52 KO mouse model, LRRC52 regulation of large conductance K⁺ channels plays a role both in sperm function and in cochlear inner hair cells. Although our understanding of LRRC-containing BK channels remains rudimentary, KO mouse models may help define other organs in which LRRC-containing channels support normal function. A key topic for future work concerns identification of endogenous mechanisms, whether post-translational or via gene regulation, that may impact LRRC-dependent pathologies.

Revisiting the Large-Conductance Calcium-Activated Potassium (BKCa) Channels in the Pulmonary Circulation

Potassium ion concentrations, controlled by ion pumps and potassium channels, predominantly govern a cell′s membrane potential and the tone in the vessels. Calcium-activated potassium channels respond to two different stimuli-changes in voltage and/or changes in intracellular free calcium. Large conductance calcium-activated potassium (BKCa) channels assemble from pore forming and various modulatory and auxiliary subunits. They are of vital significance due to their very high unitary conductance and hence their ability to rapidly cause extreme changes in the membrane potential. The pathophysiology of lung diseases in general and pulmonary hypertension, in particular, show the implication of either decreased expression and partial inactivation of BKCa channel and its subunits or mutations in the genes encoding different subunits of the channel. Signaling molecules, circulating humoral molecules, vasorelaxant agents, etc., have an influence on the open probability of the channel in pulmonary arterial vascular cells. BKCa channel is a possible therapeutic target, aimed to cause vasodilation in constricted or chronically stiffened vessels, as shown in various animal models. This review is a comprehensive collation of studies on BKCa channels in the pulmonary circulation under hypoxia (hypoxic pulmonary vasoconstriction; HPV), lung pathology, and fetal to neonatal transition, emphasising pharmacological interventions as viable therapeutic options.

Cystic Fibrosis Lung Disease in the Aging Population

The demographics of the population with cystic fibrosis (CF) is continuously changing, with nowadays adults outnumbering children and a median predicted survival of over 40 years. This leads to the challenge of treating an aging CF population, while previous research has largely focused on pediatric and adolescent patients. Chronic inflammation is not only a hallmark of CF lung disease, but also of the aging process. However, very little is known about the effects of an accelerated aging pathology in CF lungs. Several chronic lung disease pathologies show signs of chronic inflammation with accelerated aging, also termed “inflammaging”; the most notable being chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF). In these disease entities, accelerated aging has been implicated in the pathogenesis via interference with tissue repair mechanisms, alterations of the immune system leading to impaired defense against pulmonary infections and induction of a chronic pro-inflammatory state. In addition, CF lungs have been shown to exhibit increased expression of senescence markers. Sustained airway inflammation also leads to the degradation and increased turnover of cystic fibrosis transmembrane regulator (CFTR). This further reduces CFTR function and may prevent the novel CFTR modulator therapies from developing their full efficacy. Therefore, novel therapies targeting aging processes in CF lungs could be promising. This review summarizes the current research on CF in an aging population focusing on accelerated aging in the context of chronic airway inflammation and therapy implications.

Modulation of Ion Transport to Restore Airway Hydration in Cystic Fibrosis

Cystic fibrosis (CF) is a life-limiting genetic disorder caused by loss-of-function mutations in the gene which codes for the CF transmembrane conductance regulator (CFTR) Cl^- channel. Loss of Cl^- secretion across the apical membrane of airway lining epithelial cells results in dehydration of the airway surface liquid (ASL) layer which impairs mucociliary clearance (MCC), and as a consequence promotes bacterial infection and inflammation of the airways. Interventions that restore airway hydration are known to improve MCC. Here we review the ion channels present at the luminal surface of airway epithelial cells that may be targeted to improve airway hydration and MCC in CF airways.

Losartan reduces cigarette smoke-induced airway inflammation and mucus hypersecretion

The aim was to determine whether losartan reduces cigarette smoke (CS)-induced airway inflammation and mucus hypersecretion in an in vitro model and a small clinical trial. Primary human bronchial epithelial cells (HBECs) were differentiated at the air-liquid interface (ALI) and exposed to CS. Expression of transforming growth factor (TGF)-β1 and the mucin MUC5AC, and expression or activity of matrix metalloproteinase (MMP)-9 were measured after CS exposure. Parameters of mucociliary clearance were evaluated by measuring airway surface liquid volumes, mucus concentrations, and conductance of cystic fibrosis transmembrane conductance regulator (CFTR) and large conductance, Ca²⁺-activated and voltage-dependent potassium (BK) channels. Nasal cells were collected from study participants and expression of MUC5AC, TGF-β1, and MMP-9 mRNAs was measured before and after losartan treatment. In vitro, CS exposure of HBECs caused a significant increase in mRNA expression of MUC5AC and TGF-β1 and MMP-9 activity and decreased CFTR and BK channel activities, thereby reducing airway surface liquid volumes and increasing mucus concentrations. Treatment of HBECs with losartan rescued CS-induced CFTR and BK dysfunction and caused a significant decrease in MUC5AC expression and mucus concentrations, partially by inhibiting TGF-β signalling. In a prospective clinical study, cigarette smokers showed significantly reduced mRNA expression levels of MUC5AC, TGF-β1, and MMP-9 in the upper airways after 2 months of losartan treatment. Our findings suggest that losartan may be an effective therapy to reduce inflammation and mucus hypersecretion in CS-induced chronic airway diseases.

Hyperglycaemia in cystic fibrosis adversely affects BK channel function critical for mucus clearance

Large-conductance, Ca²⁺-activated, voltage-dependent K⁺ (BK) channel function is critical for adequate airway hydration and mucociliary function. In airway epithelia, BK function is regulated by its γ-subunit, leucine-rich repeat-containing protein 26 (LRRC26). Since patients with cystic fibrosis (CF)-related diabetes mellitus (CFRD) have worse lung function outcomes, this study determined the effects of hyperglycaemia on BK function in CF bronchial epithelial (CFBE) cells in vitro and evaluated the correlation between glycaemic excursions and mRNA expression of LRRC26 in the upper airways of CF and CFRD patients.CFBE cells were redifferentiated at the air-liquid interface (ALI) in media containing either 5.5 mM or 12.5 mM glucose. BK activity was measured in an Ussing chamber. Airway surface liquid (ASL) volume was estimated by meniscus scanning and inflammatory marker expression was measured by quantitative real-time PCR and enzyme-linked immunosorbent assay (ELISA). CF patients were assessed by 7 days of continuous glucose monitoring (CGM). LRRC26 mRNA expression was measured by quantitative real-time PCR from nasal cells obtained at the end of glucose monitoring.BK currents were significantly decreased in CFBE cells cultured under high glucose. These cells revealed significantly lower ASL volumes and increased inflammation, including the receptor for advanced glycation endproducts (RAGE), compared to cells cultured in normal glucose. In vivo, nasal cell expression of LRRC26 mRNA was inversely correlated with hyperglycaemic excursions, consistent with the in vitro results.Our findings demonstrate that hyperglycaemia induces inflammation and impairs BK channel function in CFBE cells in vitro These data suggest that declining lung function in CFRD patients may be related to BK channel dysfunction.

Significant functional differences in differentiated Conditionally Reprogrammed (CRC)- and Feeder-free Dual SMAD inhibited-expanded human nasal epithelial cells

BACKGROUND

Patient-derived airway cells differentiated at Air Liquid Interface (ALI) are valuable models for Cystic fibrosis (CF) precision therapy. Different culture expansion methods have been established to extend expansion capacity of airway basal cells, while retaining functional airway epithelium physiology. Considerable variation in response to CFTR modulators is observed in cultures even within the same CFTR genotype and despite the use of similar ALI culture techniques. We aimed to address culture expansion method impact on differentiation.

METHODS

Nasal epithelial brushings from 14 individuals (CF=9; non-CF=5) were collected, then equally divided and expanded under conditional reprogramming culture (CRC) and feeder-serum-free "dual-SMAD inhibition" (SMADi) methods. Expanded cells from each culture were differentiated with proprietary PneumaCult™-ALI media. Morphology (Immunofluorescence), global proteomics (LC-MS/MS) and function (barrier integrity, cilia motility, and ion transport) were compared in CRC^ALI and SMADi^ALI under basal and CFTR corrector treated (VX-809) conditions.

RESULTS

No significant difference in the structural morphology or baseline global proteomics profile were observed. Barrier integrity and cilia motility were significantly different, despite no difference in cell junction morphology or cilia abundance. Epithelial Sodium Channels and Calcium-activated Chloride Channel activity did not differ but CFTR mediated chloride currents were significantly reduced in SMADi^ALI compare to their CRC^ALI counterparts.

CONCLUSION

Alteration of cellular physiological function in vitro were more prominent than structural and differentiation potential in airway ALI. Since initial expansion culture conditions significantly influence CFTR activity, this could lead to false conclusions if data from different labs are compared against each other without specific reference ranges.

Targeting the Heme Oxygenase 1/Carbon Monoxide Pathway to Resolve Lung Hyper-Inflammation and Restore a Regulated Immune Response in Cystic Fibrosis

In individuals with cystic fibrosis (CF), lung hyper-inflammation starts early in life and is perpetuated by mucus obstruction and persistent bacterial infections. The continuous tissue damage and scarring caused by non-resolving inflammation leads to bronchiectasis and, ultimately, respiratory failure. Macrophages (MΦs) are key regulators of immune response and host defense. We and others have shown that, in CF, MΦs are hyper-inflammatory and exhibit reduced bactericidal activity. Thus, MΦs contribute to the inability of CF lung tissues to control the inflammatory response or restore tissue homeostasis. The non-resolving hyper-inflammation in CF lungs is attributed to an impairment of several signaling pathways associated with resolution of the inflammatory response, including the heme oxygenase-1/carbon monoxide (HO-1/CO) pathway. HO-1 is an enzyme that degrades heme groups, leading to the production of potent antioxidant, anti-inflammatory, and bactericidal mediators, such as biliverdin, bilirubin, and CO. This pathway is fundamental to re-establishing cellular homeostasis in response to various insults, such as oxidative stress and infection. Monocytes/MΦs rely on abundant induction of the HO-1/CO pathway for a controlled immune response and for potent bactericidal activity. Here, we discuss studies showing that blunted HO-1 activation in CF-affected cells contributes to hyper-inflammation and defective host defense against bacteria. We dissect potential cellular mechanisms that may lead to decreased HO-1 induction in CF cells. We review literature suggesting that induction of HO-1 may be beneficial for the treatment of CF lung disease. Finally, we discuss recent studies highlighting how endogenous HO-1 can be induced by administration of controlled doses of CO to reduce lung hyper-inflammation, oxidative stress, bacterial infection, and dysfunctional ion transport, which are all hallmarks of CF lung disease.

Single Molecule Fluorescence Imaging Reveals the Stoichiometry of BKγ1 Subunit in Living HEK293 Cell Expression System

Large conductance Ca²⁺-activated K⁺ (BK_Ca) channels are ubiquitously expressed in plasma membrane of both excitable and non-excitable cells and possess significant physiological functions. A tetrameric complex of α subunit (BKα) forms a functional pore of BK_Ca channel. The properties of BK_Ca channel, such as voltage-dependence, Ca²⁺ sensitivity and pharmacological responses, are extensively modulated by co-expressing accessory β subunits (BKβ), which can associate with BKα in one to one manner. Although the functional significance of newly identified γ subunits (BKγ) has been revealed, the stoichiometry between BKα and BKγ1 remains unclear. In the present study, we utilized a single molecule fluorescence imaging with a total internal reflection fluorescence (TIRF) microscope to directly count the number of green fluorescent protein (GFP)-tagged BKγ1 (BKγ1-GFP) within a single BK_Ca channel complex in HEK293 cell expression system. BKγ1-GFP significantly enhanced the BK channel activity even when the intracellular Ca²⁺ concentration was kept lower, i.e., 10 nM, than the physiological resting level. BKγ1-GFP stably formed molecular complexes with BKα-mCherry in the plasma membrane. Counting of GFP bleaching steps revealed that a BK_Ca channel can contain up to four BKγ1 per channel at the maximum. These results suggest that BKγ1 forms a BK_Ca channel complex with BKα in a 1 : 1 stoichiometry in a human cell line.

The Role of MicroRNA in the Airway Surface Liquid Homeostasis

Mucociliary clearance, mediated by a coordinated function of cilia bathing in the airway surface liquid (ASL) on the surface of airway epithelium, protects the host from inhaled pathogens and is an essential component of the innate immunity. ASL is composed of the superficial mucus layer and the deeper periciliary liquid. Ion channels, transporters, and pumps coordinate the transcellular and paracellular movement of ions and water to maintain the ASL volume and mucus hydration. microRNA (miRNA) is a class of non-coding, short single-stranded RNA regulating gene expression by post-transcriptional mechanisms. miRNAs have been increasingly recognized as essential regulators of ion channels and transporters responsible for ASL homeostasis. miRNAs also influence the airway host defense. We summarize the most up-to-date information on the role of miRNAs in ASL homeostasis and host-pathogen interactions in the airway and discuss concepts for miRNA-directed therapy.

Regulation of BK Channels by Beta and Gamma Subunits

Ca²⁺- and voltage-gated K⁺ channels of large conductance (BK channels) are expressed in a diverse variety of both excitable and inexcitable cells, with functional properties presumably uniquely calibrated for the cells in which they are found. Although some diversity in BK channel function, localization, and regulation apparently arises from cell-specific alternative splice variants of the single pore-forming α subunit ( KCa1.1, Kcnma1, Slo1) gene, two families of regulatory subunits, β and γ, define BK channels that span a diverse range of functional properties. We are just beginning to unravel the cell-specific, physiological roles served by BK channels of different subunit composition.

Involvement of the γ1 subunit of the large-conductance Ca2+-activated K+ channel in the proliferation of human somatostatinoma cells

Pancreatic neuroendocrine tumors (pNETs) occur due to the abnormal growth of pancreatic islet cells and predominantly develop in the duodenal-pancreatic region. Somatostatinoma is one of the pNETs associated with tumors of pancreatic δ cells, which produce and secrete somatostatin. Limited information is currently available on the pathogenic mechanisms of somatostatinoma. The large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel is expressed in several types of cancer cells and regulates cell proliferation, migration, invasion, and metastasis. In the present study, the functional expression of the BK_Ca channel was examined in a human somatostatinoma QGP-1 cell line. In QGP-1 cells, outward currents were elicited by membrane depolarization at pCa 6.5 (300 nM) in the pipette solution and inhibited by the specific BK_Ca channel blocker, paxilline. Paxilline-sensitive currents were detected, even at pCa 8.0 (10 nM) in the pipette solution, in QGP-1 cells. In addition to the α and β2-4 subunits of the BK_Ca channel, the novel regulatory γ1 subunit (BK_Caγ1) was co-localized with the α subunit in QGP-1 cells. Paxilline-sensitive currents at pCa 8.0 in the pipette solution were reduced by the siRNA knockdown of BK_Caγ1. Store-operated Ca²⁺ entry was smaller in BK_Caγ1 siRNA-treated QGP-1 cells. The proliferation of QGP-1 cells was attenuated by paxilline or the siRNA knockdown of BK_Caγ1. These results strongly suggest that BK_Caγ1 facilitates the proliferation of human somatostatinoma cells. Therefore, BK_Caγ1 may be a novel therapeutic target for somatostatinoma.

Losartan Rescues Inflammation-related Mucociliary Dysfunction in Relevant Models of Cystic Fibrosis

Rationale: Despite therapeutic progress in treating cystic fibrosis (CF) airway disease, airway inflammation with associated mucociliary dysfunction remains largely unaddressed. Inflammation reduces the activity of apically expressed large-conductance Ca2+-activated and voltage-dependent K+ (BK) channels, critical for mucociliary function in the absence of CFTR (CF transmembrane conductance regulator).Objectives: To test losartan as an antiinflammatory therapy in CF using CF human bronchial epithelial cells and an ovine model of CF-like airway disease.Methods: Losartan's antiinflammatory effectiveness to rescue BK activity and thus mucociliary function was tested in vitro using primary, fully redifferentiated human airway epithelial cells homozygous for F508del and in vivo using a previously validated, now expanded pharmacologic sheep model of CF-like, inflammation-associated mucociliary dysfunction.Measurements and Main Results: Nasal scrapings from patients with CF showed that neutrophilic inflammation correlated with reduced expression of LRRC26 (leucine rich repeat containing 26), the γ subunit mandatory for BK function in the airways. TGF-β1 (transforming growth factor β1), downstream of neutrophil elastase, decreased mucociliary parameters in vitro. These were rescued by losartan at concentrations achieved by nebulization in the airway and oral application in the bloodstream: BK dysfunction recovered acutely and over time (the latter via an increase in LRRC26 expression), ciliary beat frequency and airway surface liquid volume improved, and mucus hyperconcentration and cellular inflammation decreased. These effects did not depend on angiotensin receptor blockade. Expanding on a validated and published nongenetic, CF-like sheep model, ewes inhaled CFTRinh172 and neutrophil elastase for 3 days, which resulted in prolonged tracheal mucus velocity reduction, mucus hyperconcentration, and increased TGF-β1. Nebulized losartan rescued both mucus transport and mucus hyperconcentration and reduced TGF-β1.Conclusions: Losartan effectively reversed CF- and inflammation-associated mucociliary dysfunction, independent of its angiotensin receptor blockade.

Roles of LRRC26 as an auxiliary γ1-subunit of large-conductance Ca2+-activated K+ channels in bronchial smooth muscle cells

In visceral smooth muscle cells (SMCs), the large-conductance Ca²⁺-activated K⁺ (BK) channel is one of the key elements underlying a negative feedback mechanism that is essential for the regulation of intracellular Ca²⁺ concentration. Although leucine-rich repeat-containing (LRRC) proteins have been identified as novel auxiliary γ-subunits of the BK channel (BKγ) in several cell types, its physiological roles in SMCs are unclear. The BKγ expression patterns in selected SM tissues were examined using real-time PCR analyses and Western blotting. The functional contribution of BKγ1 to BK channel activity was examined by whole cell patch-clamp in SMCs and heterologous expression systems. BKγ1 expression in mouse bronchial SMCs (mBSMCs) was higher than in other several SMC types. Coimmunoprecipitation and total internal reflection fluorescence imaging analyses revealed molecular interaction between BKα and BKγ1 in mBSMCs. Under voltage-clamp, steady-state activation of BK channel currents at pCa 8.0 in mBSMCs occurred in a voltage range comparable to that of reconstituted BKα/BKγ1 complex. However, this range was much more negative than in mouse aortic SMCs (mASMCs) or in HEK293 cells expressing BKα alone and β-subunit (BKβ1). Mallotoxin, a selective activator of BK channel that lacks BKγ1, dose-dependently activated BK currents in mASMCs but not in mBSMCs. The abundant expression of BKγ1 in mBSMCs extensively facilitates BK channel activity to keep the resting membrane potential at negative values and prevents contraction under physiological conditions.

The Effects of the Anti-aging Protein Klotho on Mucociliary Clearance

α-klotho (KL) is an anti-aging protein and has been shown to exert anti-inflammatory and anti-oxidative effects in the lung and pulmonary diseases such as chronic obstructive pulmonary disease (COPD) and cystic fibrosis. The current study investigated the direct effect of KL on the bronchial epithelium in regards to mucociliary clearance parameters. Primary human bronchial and murine tracheal epithelial cells, cultured, and differentiated at the air liquid interface (ALI), were treated with recombinant KL or infected with a lentiviral vector expressing KL. Airway surface liquid (ASL) volume, airway ion channel activities, and expression levels were analyzed. These experiments were paired with ex vivo analyses of mucociliary clearance in murine tracheas from klotho deficient mice and their wild type littermates. Our results showed that klotho deficiency led to impaired mucociliary clearance with a reduction in ASL volume in vitro and ex vivo. Overexpression or exogenous KL increased ASL volume, which was paralleled by increased activation of the large-conductance, Ca2+-activated, voltage-dependent potassium channel (BK) without effect on the cystic fibrosis transmembrane conductance regulator (CFTR). Furthermore, KL overexpression downregulated IL-8 levels and attenuated TGF-β-mediated downregulation of LRRC26, the γ subunit of BK, necessary for its function in non-excitable cells. In summary, we show that KL regulates mucociliary function by increasing ASL volume in the airways possibly due to underlying BK activation. The KL mediated BK channel activation may be a potentially important target to design therapeutic strategies in inflammatory airway diseases when ASL volume is decreased.

Transforming Growth Factor-β1 Selectively Recruits microRNAs to the RNA-Induced Silencing Complex and Degrades CFTR mRNA under Permissive Conditions in Human Bronchial Epithelial Cells

Mutations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene lead to cystic fibrosis (CF). The most common mutation F508del inhibits folding and processing of CFTR protein. FDA-approved correctors rescue the biosynthetic processing of F508del-CFTR protein, while potentiators improve the rescued CFTR channel function. Transforming growth factor (TGF-β1), overexpressed in many CF patients, blocks corrector/potentiator rescue by inhibiting CFTR mRNA in vitro. Increased TGF-β1 signaling and acquired CFTR dysfunction are present in other lung diseases. To study the mechanism of TGF-β1 repression of CFTR, we used molecular, biochemical, and functional approaches in primary human bronchial epithelial cells from over 50 donors. TGF-β1 destabilized CFTR mRNA in cells from lungs with chronic disease, including CF, and impaired F508del-CFTR rescue by new-generation correctors. TGF-β1 increased the active pool of selected micro(mi)RNAs validated as CFTR inhibitors, recruiting them to the RNA-induced silencing complex (RISC). Expression of F508del-CFTR globally modulated TGF-β1-induced changes in the miRNA landscape, creating a permissive environment required for degradation of F508del-CFTR mRNA. In conclusion, TGF-β1 may impede the full benefit of corrector/potentiator therapy in CF patients. Studying miRNA recruitment to RISC under disease-specific conditions may help to better characterize the miRNAs utilized by TGF-β1 to destabilize CFTR mRNA.

Regulation of BK Channels by Beta and Gamma Subunits

Ca²⁺- and voltage-gated K⁺ channels of large conductance (BK channels) are expressed in a diverse variety of both excitable and inexcitable cells, with functional properties presumably uniquely calibrated for the cells in which they are found. Although some diversity in BK channel function, localization, and regulation apparently arises from cell-specific alternative splice variants of the single pore-forming α subunit ( KCa1.1, Kcnma1, Slo1) gene, two families of regulatory subunits, β and γ, define BK channels that span a diverse range of functional properties. We are just beginning to unravel the cell-specific, physiological roles served by BK channels of different subunit composition.

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.

TGFβ as a therapeutic target in cystic fibrosis

INTRODUCTION

Cystic fibrosis (CF) is a genetic disease characterized by progressive lung disease. Most CF therapies focus on treating secondary pulmonary complications rather than addressing the underlying processes inducing airway remodeling and ineffective response to infection. Transforming growth factor beta (TGFβ) is a cytokine involved in fibrosis, inflammation, and injury response as well as a genetic modifier and biomarker of CF lung disease. Targeting the TGFβ pathway has been pursued in other diseases, but the mechanism of TGFβ effects in CF is less well understood. Areas covered: In this review, we discuss CF lung disease pathogenesis with a focus on potential links to TGFβ. TGFβ signaling in lung health and disease is reviewed. Recent studies investigating TGFβ's impact in CF airway epithelial cells are highlighted. Finally, an overview of potential therapies to target TGFβ signaling relevant to CF are addressed. Expert opinion: The broad impact of TGFβ signaling on numerous cellular processes in homeostasis and disease is both a strength and a challenge to developing TGFβ dependent therapeutics in CF. We discuss the challenges inherent in developing TGFβ-targeted therapy, identifying appropriate patient populations, and questions regarding the timing of treatment. Future directions for research into TGFβ focused therapeutics are discussed.

ATP12A promotes mucus dysfunction during Type 2 airway inflammation

Allergic airway disease is known to cause significant morbidity due to impaired mucociliary clearance, however the mechanism that leads to the mucus dysfunction is not entirely understood. Interleukin 13 (IL-13), a key mediator of Type 2 (T2) inflammation, profoundly alters the ion transport properties of airway epithelium. However, these electrophysiological changes cannot explain the thick, tenacious airway mucus that characterizes the clinical phenotype. Here we report that IL-13 dramatically increases the airway surface liquid (ASL) viscosity in cultured primary human bronchial epithelial cells and thereby inhibits mucus clearance. These detrimental rheological changes require ATP12A, a non-gastric H⁺/K⁺-ATPase that secretes protons into the ASL. ATP12A knockdown or inhibition prevented the IL-13 dependent increase in ASL viscosity but did not alter the ASL pH. We propose that ATP12A promotes airway mucus dysfunction in individuals with T2 inflammatory airway diseases and that ATP12A may be a novel therapeutic target to improve mucus clearance.

Airway Hydration, Apical K(+) Secretion, and the Large-Conductance, Ca(2+)-activated and Voltage-dependent Potassium (BK) Channel

Large-conductance, calcium-activated, and voltage-gated K(+) (BK) channels are expressed in many tissues of the human body, where they play important roles in signaling not only in excitable but also in nonexcitable cells. Because BK channel properties are rendered in part by their association with four β and four γ subunits, their channel function can differ drastically, depending on in which cellular system they are expressed. Recent studies verify the importance of apically expressed BK channels for airway surface liquid homeostasis and therefore of their significant role in mucociliary clearance. Here, we review evidence that inflammatory cytokines, which contribute to airway diseases, can lead to reduced BK activity via a functional down-regulation of the γ regulatory subunit LRRC26. Therefore, manipulation of LRRC26 and pharmacological opening of BK channels represent two novel concepts of targeting epithelial dysfunction in inflammatory airway diseases.

Klotho Inhibits Interleukin-8 Secretion from Cystic Fibrosis Airway Epithelia

Chronic inflammation is a hallmark of cystic fibrosis (CF) and associated with increased production of transforming growth factor (TGF) β and interleukin (IL)-8. α-klotho (KL), a transmembrane or soluble protein, functions as a co-receptor for Fibroblast Growth Factor (FGF) 23, a known pro-inflammatory, prognostic marker in chronic kidney disease. KL is downregulated in airways from COPD patients. We hypothesized that both KL and FGF23 signaling modulate TGF β-induced IL-8 secretion in CF bronchial epithelia. Thus, FGF23 and soluble KL levels were measured in plasma from 48 CF patients and in primary CF bronchial epithelial cells (CF-HBEC). CF patients showed increased FGF23 plasma levels, but KL levels were not different. In CF-HBEC, TGF-β increased KL secretion and upregulated FGF receptor (FGFR) 1. Despite increases in KL, TGF-β also increased IL-8 secretion via activation of FGFR1 and Smad 3 signaling. However, KL excess via overexpression or supplementation decreased IL-8 secretion by inhibiting Smad 3 phosphorylation. Here, we identify a novel signaling pathway contributing to IL-8 secretion in the CF bronchial epithelium with KL functioning as an endocrine and local anti-inflammatory mediator that antagonizes pro-inflammatory actions of FGF23 and TGF-β.

Role of Smad3 and p38 Signalling in Cigarette Smoke-induced CFTR and BK dysfunction in Primary Human Bronchial Airway Epithelial Cells

Mucociliary clearance (MCC) is a major airway host defence system that is impaired in patients with smoking-associated chronic bronchitis. This dysfunction is partially related to a decrease of airway surface liquid (ASL) volume that is in part regulated by apically expressed cystic fibrosis transmembrane conductance regulator (CFTR) and large-conductance, Ca²⁺-activated, and voltage dependent K⁺ (BK) channels. Here, data from human bronchial epithelial cells (HBEC) confirm that cigarette smoke not only downregulates CFTR activity but also inhibits BK channel function, thereby causing ASL depletion. Inhibition of signalling pathways involved in cigarette smoke-induced channel dysfunction reveals that CFTR activity is downregulated via Smad3 signalling whereas BK activity is decreased via the p38 cascade. In addition, pre-treatment with pirfenidone, a drug presently used to inhibit TGF-β signalling in idiopathic pulmonary fibrosis, ameliorated BK dysfunction and ASL volume loss. Taken together, our results highlight the importance of not only CFTR but also BK channel function in maintaining ASL homeostasis and emphasize the possibility that pirfenidone could be employed as a novel therapeutic regimen to help improve MCC in smoking-related chronic bronchitis.

Large-conductance Ca2+ -activated K+ channel activation by apical P2Y receptor agonists requires hydrocortisone in differentiated airway epithelium

KEY POINTS

Hydrocortisone (HC) is required for activation of large-conductance Ca²⁺ -activated K⁺ current (BK) by purinergic receptor agonists. HC reduces insertion of the stress-regulated exon (STREX) in the KCNMA1 gene, permitting protein kinase C (PKC)-dependent channel activation. Overlapping and unique purinergic signalling regions exist at the apical border of differentiated surface cells. BK channels localize in the cilia of surface cells.

ABSTRACT

In the present study we investigated the role of hydrocortisone (HC) on uridine-5'-triphosphate (UTP)-stimulated ion transport in differentiated, pseudostratified epithelia derived from normal human bronchial basal cells. The presence of a UTP-stimulated, paxilline-sensitive large-conductance Ca²⁺ -activated K⁺ (BK) current was demonstrated in control epithelia but was not stimulated in epithelia differentiated in the absence of HC (HC0). Addition of the BK channel opener NS11021 directly activated channels in control epithelia; however, under HC0 conditions, activation only occurred when UTP was added after NS11021. The PKC inhibitors GF109203x and Gö6983 blocked BK activation by UTP in control epithelia, suggesting that PKC-mediated phosphorylation plays a permissive role in purinoceptor-stimulated BK activation. Moreover, HC0 epithelia expressed significantly more KCNMA1 containing the stress-regulated exon (STREX), a splice-variant of the α-subunit that displays altered channel regulation by phosphorylation, compared to control epithelia. Furthermore, BK channels as well as purinergic receptors were shown to localize in unique and overlapping domains at the apical membrane of ciliated surface cells. These results establish a previously unrecognized role for glucocorticoids in regulation of BK channels in airway epithelial cells.

Mechanistic insight into the heme-independent interplay between iron and carbon monoxide in CFTR and Slo1 BKCa channels

Ion channels have been extensively reported as effectors of carbon monoxide (CO). However, the mechanisms of heme-independent CO action are still not known. Because most ion channels are heterologously expressed on human embryonic kidney cells that are cultured in Fe³⁺-containing media, CO may act as a small and strong iron chelator to disrupt a putative iron bridge in ion channels and thus to tune their activity. In this review CFTR and Slo1 BK_Ca channels are employed to discuss the possible heme-independent interplay between iron and CO. Our recent studies demonstrated a high-affinity Fe³⁺ site at the interface between the regulatory domain and intracellular loop 3 of CFTR. Because the binding of Fe³⁺ to CFTR prevents channel opening, the stimulatory effect of CO on the Cl^- and HCO₃^- currents across the apical membrane of rat distal colon may be due to the release of inhibitive Fe³⁺ by CO. In contrast, CO repeatedly stimulates the human Slo1 BK_Ca channel opening, possibly by binding to an unknown iron site, because cyanide prohibits this heme-independent CO stimulation. Here, in silico research on recent structural data of the slo1 BK_Ca channels indicates two putative binuclear Fe²⁺-binding motifs in the gating ring in which CO may compete with protein residues to bind to either Fe²⁺ bowl to disrupt the Fe²⁺ bridge but not to release Fe²⁺ from the channel. Thus, these two new regulation models of CO, with iron releasing from and retaining in the ion channel, may have significant and extensive implications for other metalloproteins.

Relationship between auxiliary gamma subunits and mallotoxin on BK channel modulation

The large-conductance, calcium- and voltage-activated K⁺(BK) channel consists of the pore-forming α subunits (BKα) and auxiliary subunits. The auxiliary γ1-3 subunits potently modulate the BK channel by shifting its voltage-dependence of channel activation toward the hyperpolarizing direction by approximately 145 mV (γ1), 100 mV (γ2), and 50 mV (γ3). Mallotoxin is a potent small-molecule BK channel activator. We analyzed the relationship between mallotoxin and the γ subunits in their BK channel-activating effects in membrane patches excised from HEK-293 cells. We found that mallotoxin, when applied extracellularly, shifted the half-activation voltage (V_1/2) of BKα channels by −72 mV. The channel-activating effect of mallotoxin was greatly attenuated in the presence of the γ1, γ2, or γ3 subunit, with resultant ΔV_{1/2 (+/− mallotoxin)} values of −9, −28, or −15 mV, respectively. Most examined γ1 mutant subunits antagonized mallotoxin’s channel-activating effect in a manner that was largely dependent on its own modulatory function. However, mallotoxin caused an irreversible functional and structural disengagement of the γ1-F273S mutant from BK channels. We infer that the auxiliary γ subunit effectively interferes with mallotoxin on BK channel modulation via either a direct steric competition or an indirect allosteric influence on mallotoxin’s binding and action on BKα.

The single transmembrane segment determines the modulatory function of the BK channel auxiliary γ subunit

Researchers identify regions of tissue-specific γ subunits that are responsible for regulating the activity of calcium-activated potassium channels.

The large-conductance, calcium-activated potassium (BK) channels consist of the pore-forming, voltage- and Ca²⁺-sensing α subunits (BKα) and the tissue-specific auxiliary β and γ subunits. The BK channel γ1 subunit is a leucine-rich repeat (LRR)–containing membrane protein that potently facilitates BK channel activation in many tissues and cell types through a vast shift in the voltage dependence of channel activation by ∼140 mV in the hyperpolarizing direction. In this study, we found that the single transmembrane (TM) segment together with its flanking charged residues is sufficient to fully modulate BK channels upon its transplantation into the structurally unrelated β1 subunit. We identified Phe273 and its neighboring residues in the middle of the TM segment and a minimum of three intracellular juxtamembrane Arg residues as important for the γ1 subunit’s modulatory function and observed functional coupling between residues of these two locations. We concluded that the TM segment is a key molecular determinant for channel association and modulation and that the intracellular positively charged cluster is involved mainly in channel association, likely through its TM-anchoring effect. Our findings provide insights into the structure–function relationship of the γ1 subunit in understanding its potent modulatory effects on BK channels.

Engineered cationic antimicrobial peptide (eCAP) prevents Pseudomonas aeruginosa biofilm growth on airway epithelial cells

OBJECTIVES

Chronic infections with the opportunistic pathogen Pseudomonas aeruginosa are responsible for the majority of the morbidity and mortality in patients with cystic fibrosis (CF). While P. aeruginosa infections may initially be treated successfully with standard antibiotics, chronic infections typically arise as bacteria transition to a biofilm mode of growth and acquire remarkable antimicrobial resistance. To address the critical need for novel antimicrobial therapeutics that can effectively suppress chronic bacterial infections in challenging physiological environments, such as the CF lung, we have rationally designed a de novo engineered cationic antimicrobial peptide, the 24-residue WLBU2, with broad-spectrum antibacterial activity for pan-drug-resistant P. aeruginosa in liquid culture. In the current study, we tested the hypothesis that WLBU2 also prevents P. aeruginosa biofilm growth.

METHODS

Using abiotic and biotic biofilm assays, co-culturing P. aeruginosa with polarized human airway epithelial cells, we examined the ability of WLBU2 to prevent biofilm biogenesis alone and in combination with currently used antibiotics.

RESULTS

We observed a dose-dependent reduction in biofilm growth on an abiotic surface and in association with CF airway epithelial cells. WLBU2 prevented P. aeruginosa biofilm formation when co-cultured with mucus-producing primary human CF airway epithelial cells and using CF clinical isolates of P. aeruginosa, even at low pH and high salt conditions that mimic the CF airway. When used in combination, WLBU2 significantly increases killing by the commonly used antibiotics tobramycin, ciprofloxacin, ceftazidime and meropenem.

CONCLUSIONS

While other studies have demonstrated the ability of natural and synthetic antimicrobial peptides to prevent abiotic bacterial biofilm formation, the current studies for the first time demonstrate the effective peptide treatment of a biotic bacterial biofilm in a setting similar to the CF airway, and without negative effects on human airway epithelial cells, thus highlighting the unique potential of this engineered cationic antimicrobial peptide for treatment of human respiratory infections.

Modulation of BK Channel Function by Auxiliary Beta and Gamma Subunits

The large-conductance, Ca(2+)- and voltage-activated K(+) (BK) channel is ubiquitously expressed in mammalian tissues and displays diverse biophysical or pharmacological characteristics. This diversity is in part conferred by channel modulation with different regulatory auxiliary subunits. To date, two distinct classes of BK channel auxiliary subunits have been identified: β subunits and γ subunits. Modulation of BK channels by the four auxiliary β (β1-β4) subunits has been well established and intensively investigated over the past two decades. The auxiliary γ subunits, however, were identified only very recently, which adds a new dimension to BK channel regulation and improves our understanding of the physiological functions of BK channels in various tissues and cell types. This chapter will review the current understanding of BK channel modulation by auxiliary β and γ subunits, especially the latest findings.

Dual Oxidase 2 (Duox2) Regulates Pannexin 1-mediated ATP Release in Primary Human Airway Epithelial Cells via Changes in Intracellular pH and Not H2O2 Production

Human airway epithelial cells express pannexin 1 (Panx1) channels to release ATP, which regulates mucociliary clearance. Airway inflammation causes mucociliary dysfunction. Exposure of primary human airway epithelial cell cultures to IFN-γ for 48 h did not alter Panx1 protein expression but significantly decreased ATP release in response to hypotonic stress. The IFN-γ-induced functional down-regulation of Panx1 was due to the up-regulation of dual oxidase 2 (Duox2). Duox2 suppression by siRNA led to an increase in ATP release in control cells and restoration of ATP release in cells treated with IFN-γ. Both effects were reduced by the pannexin inhibitor probenecid. Duox2 up-regulation stoichiometrically increases H2O2 and proton production. H2O2 inhibited Panx1 function temporarily by formation of disulfide bonds at the thiol group of its terminal cysteine. Long-term exposure to H2O2, however, had no inhibitory effect. To assess the role of cellular acidification upon IFN-γ treatment, fully differentiated airway epithelial cells were exposed to ammonium chloride to alkalinize the cytosol. This led to a 2-fold increase in ATP release in cells treated with IFN-γ that was also inhibited by probenecid. Duox2 knockdown also partially corrected IFN-γ-mediated acidification. The direct correlation between intracellular pH and Panx1 open probability was shown in oocytes. Therefore, airway epithelial cells release less ATP in response to hypotonic stress in an inflammatory environment (IFN-γ exposure). Decreased Panx1 function is a response to cell acidification mediated by IFN-γ-induced up-regulation of Duox2, representing a novel mechanism for mucociliary dysfunction in inflammatory airway diseases.