Characterization of basolateral K+ channels underlying anion secretion in the human airway cell line Calu-3

Convergent gene losses and pseudogenizations in multiple lineages of stomachless fishes

The regressive evolution of independent lineages often results in convergent phenotypes. Several teleost groups display secondary loss of the stomach, and four gastric genes, atp4a, atp4b, pgc, and pga2 have been co-deleted in agastric (stomachless) fish. Analyses of genotypic convergence among agastric fishes showed that four genes, slc26a9, kcne2, cldn18a, and vsig1, were co-deleted or pseudogenized in most agastric fishes of the four major groups. kcne2 and vsig1 were also deleted or pseudogenized in the agastric monotreme echidna and platypus, respectively. In the stomachs of sticklebacks, these genes are expressed in gastric gland cells or surface epithelial cells. An ohnolog of cldn18 was retained in some agastric teleosts but exhibited an increased non-synonymous substitution when compared with gastric species. These results revealed novel convergent gene losses at multiple loci among the four major groups of agastric fish, as well as a single gene loss in the echidna and platypus.

Reduction of TRPC1/TRPC3 mediated Ca2+-signaling protects oxidative stress-induced COPD

Oxidative stress is a predisposing factor in Chronic Obstructive Pulmonary Disease (COPD). Specifically, pulmonary epithelial (PE) cells reduce antioxidant capacity during COPD because of the continuous production of reactive oxygen species (ROS). However, the molecular pathogenesis that governs such ROS activity is unclear. Here we show that the dysregulation of intracellular calcium concentration ([Ca2+]i) in PE cells from COPD patients, compared to the healthy PE cells, is associated with the robust functional expressions of Transient Receptor Potential Canonical (TRPC)1 and TRPC3 channels, and Ca2+ entry (SOCE) components, Stromal Interaction Molecule 1 (STIM1) and ORAI1 channels. Additionally, the elevated expression levels of fibrotic, inflammatory, oxidative, and apoptotic markers in cells from COPD patients suggest detrimental pathway activation, thereby reducing the ability of lung remodeling. To further delineate the mechanism, we used human lung epithelial cell line, A549, since the behavior of SOCE and the expression patterns of TRPC1/C3, STIM1, and ORAI1 were much like PE cells. Notably, the knockdown of TRPC1/C3 in A549 cells substantially reduced the SOCE-induced [Ca2+]i rise, and reversed the ROS-mediated oxidative, fibrotic, inflammatory, and apoptotic responses, thus confirming the role of TRPC1/C3 in SOCE driven COPD-like condition. Higher TRPC1/C3, STIM1, and ORAI1 expressions, along with a greater Ca2+ entry, via SOCE in ROS-induced A549 cells, led to the rise in oxidative, fibrotic, inflammatory, and apoptotic gene expression, specifically through the extracellular signal-regulated kinase (ERK) pathway. Abatement of TRPC1 and/or TRPC3 reduced the mobilization of [Ca2+]i and reversed apoptotic gene expression and ERK activation, signifying the involvement of TRPC1/C3. Together these data suggest that TRPC1/C3 and SOCE facilitate the COPD condition through ROS-mediated cell death, thus implicating their likely roles as potential therapeutic targets for COPD. SUMMARY: Alterations in Ca2+ signaling modalities in normal pulmonary epithelial cells exhibit COPD through oxidative stress and cellular injury, compromising repair, which was alleviated through inhibition of store-operated calcium entry. SUBJECT AREA: Calcium, ROS, Cellular signaling, lung disease.

Hormonal regulation of non-cystic fibrosis transmembrane conductance regulator ion channels in the endocervix

OBJECTIVE

To characterize ion channel expression and localization in the endocervix under different hormonal conditions using a nonhuman primate primary endocervical epithelial cell model.

DESIGN

Experimental.

SETTING

University-based, translational science laboratory.

INTERVENTIONS

We cultured and treated conditionally reprogrammed primary rhesus macaque endocervix cells with estradiol and progesterone and measured gene expression changes for several known ion channel and ion channel regulators of mucus secreting epithelia. Using both rhesus macaque endocervical samples and human samples, we localized channels in the endocervix using immunohistochemistry.

MAIN OUTCOME MEASURES

The relative abundance of transcripts was evaluated using real-time polymerase chain reaction. Immunostaining results were evaluated qualitatively.

RESULTS

Compared with controls, we found that estradiol increased gene expression for ANO6, NKCC1, CLCA1, and PDE4D. Progesterone down-regulated gene expression for ANO6, SCNN1A, SCNN1B, NKCC1, and PDE4D (P≤.05). Immunohistochemistry confirmed endocervical cell membrane localization of ANO1, ANO6, KCNN4, LRR8CA, and NKCC1.

CONCLUSIONS

We found several ion channels and ion channel regulators that are hormonally sensitive in the endocervix. These channels, therefore, may play a role in the cyclic fertility changes in the endocervix and could be further investigated as targets for future fertility and contraceptive studies.

Impact of KvLQT1 potassium channel modulation on alveolar fluid homeostasis in an animal model of thiourea-induced lung edema

Alveolar ion and fluid absorption is essential for lung homeostasis in healthy conditions as well as for the resorption of lung edema, a key feature of acute respiratory distress syndrome. Liquid absorption is driven by active transepithelial sodium transport, through apical ENaC Na⁺ channels and basolateral Na⁺/K⁺-ATPase. Our previous work unveiled that KvLQT1 K⁺ channels also participate in the control of Na⁺/liquid absorption in alveolar epithelial cells. Our aim was to further investigate the function of KvLQT1 channels and their interplay with other channels/transporters involved in ion/liquid transport in vivo using adult wild-type (WT) and KvLQT1 knock-out (KO) mice under physiological conditions and after thiourea-induced lung edema. A slight but significant increase in water lung content (WLC) was observed in naïve KvLQT1-KO mice, relative to WT littermates, whereas lung function was generally preserved and histological structure unaltered. Following thiourea-induced lung edema, KvLQT1-KO did not worsen WLC or lung function. Similarly, lung edema was not aggravated by the administration of a KvLQT1 inhibitor (chromanol). However, KvLQT1 activation (R-L3) significantly reduced WLC in thiourea-challenged WT mice. The benefits of R-L3 were prevented in KO or chromanol-treated WT mice. Furthermore, R-L3 treatment had no effect on thiourea-induced endothelial barrier alteration but restored or enhanced the levels of epithelial alveolar AQP5, Na⁺/K⁺-ATPase, and ENaC expressions. Altogether, the results indicate the benefits of KvLQT1 activation in the resolution of lung edema, probably through the observed up-regulation of epithelial alveolar channels/transporters involved in ion/water transport.

A Novel Role of Arrhythmia-Related Gene KCNQ1 Revealed by Multi-Omic Analysis: Theragnostic Value and Potential Mechanisms in Lung Adenocarcinoma

The early diagnosis, prognostic prediction, and personalized therapy of lung adenocarcinoma (LUAD) remains a challenging issue. KCNQ1 (potassium voltage-gated channel subfamily Q Member 1) is implicated in long QT syndrome (LQTS) and cardiac arrhythmia, while its significance in LUAD remains unclear. In this study, we aimed to explore the significance of KCNQ1 in terms of clinical value, tumor immunity, underlying mechanisms, and a precision medicine approach by means of multi-omics analysis. The association of KCNQ1 with LUAD was first explored. Both altered variants and high expression of KCNQ1 in a TCGA-LUAD cohort indicated a favorable outcome. KCNQ1 levels had a negative correlation with tumor proliferation index Ki67 levels. siRNA-knockdown of KCNQ1 promoted the migration ability of lung cancer cells. KCNQ1 levels were decreased in LUAD tissue compared to normal tissue. A receiver operating characteristic (ROC) curve indicated good diagnostic efficiency of KCNQ1. High KCNQ1 is associated with an immunoactive profile of immune infiltration and immunomodulators and is involved in the inhibition of the cell cycle and DNA replication. Lapatinib was identified as a potent drug for LUAD in the context of low KCNQ1. This study unveiled the significance of KCNQ1 in diagnosis and prognosis and provided a corresponding precision medicine strategy for LUAD.

Cyclic nucleotide phosphodiesterase inhibitors as therapeutic interventions for cystic fibrosis

Cystic Fibrosis (CF) lung disease results from mutations in the CFTR anion channel that reduce anion and fluid secretion by airway epithelia. Impaired secretion compromises airway innate defence mechanisms and leads to bacterial colonization, excessive inflammation and tissue damage; thus, restoration of CFTR function is the goal of many CF therapies. CFTR channels are activated by cyclic nucleotide-dependent protein kinases. The second messengers 3'5'-cAMP and 3'5'-cGMP are hydrolysed by a large family of cyclic nucleotide phosphodiesterases that provide subcellular spatial and temporal control of cyclic nucleotide-dependent signalling. Selective inhibition of these enzymes elevates cyclic nucleotide levels, leading to activation of CFTR and other downstream effectors. Here we examine members of the PDE family that are likely to regulate CFTR-dependent ion and fluid secretion in the airways and discuss other actions of PDE inhibitors that can influence cyclic nucleotide-regulated mucociliary transport, inflammation and bronchodilation. Finally, we review PDE inhibitors and the potential benefits they could provide as CF therapeutics.

Bitter taste receptor agonists regulate epithelial two-pore potassium channels via cAMP signaling

Background

Epithelial solitary chemosensory cell (tuft cell) bitter taste signal transduction occurs through G protein coupled receptors and calcium-dependent signaling pathways. Type II taste cells, which utilize the same bitter taste signal transduction pathways, may also utilize cyclic adenosine monophosphate (cAMP) as an independent signaling messenger in addition to calcium.

Methods

In this work we utilized specific pharmacologic inhibitors to interrogate the short circuit current (Isc) of polarized nasal epithelial cells mounted in Ussing chambers to assess the electrophysiologic changes associated with bitter agonist (denatonium) treatment. We also assessed release of human β-defensin-2 from polarized nasal epithelial cultures following treatment with denatonium benzoate and/or potassium channel inhibitors.

Results

We demonstrate that the bitter taste receptor agonist, denatonium, decreases human respiratory epithelial two-pore potassium (K2P) current in polarized nasal epithelial cells mounted in Ussing chambers. Our data further suggest that this occurs via a cAMP-dependent signaling pathway. We also demonstrate that this decrease in potassium current lowers the threshold for denatonium to stimulate human β-defensin-2 release.

Conclusions

These data thus demonstrate that, in addition to taste transducing calcium-dependent signaling, bitter taste receptor agonists can also activate cAMP-dependent respiratory epithelial signaling pathways to modulate K2P currents. Bitter-agonist regulation of potassium currents may therefore serve as a means of rapid regional epithelial signaling, and further study of these pathways may provide new insights into regulation of mucosal ionic composition and innate mechanisms of epithelial defense.

The ubiquitous flavonoid quercetin is an atypical KCNQ potassium channel activator

Many commonly consumed plants are used as folk medicines, often with unclear molecular mechanisms. Recent studies uncovered the ubiquitous and influential KCNQ family of voltage-gated potassium (Kv) channels as a therapeutic target for several medicinal plant compounds. Capers - immature flower buds of Capparis spinosa - have been consumed for food and medicinal purposes for millennia. Here, we show that caper extract hyperpolarizes cells expressing KCNQ1 or KCNQ2/3 Kv channels. Capers are the richest known natural source of quercetin, the most consumed dietary flavonoid. Quercetin potentiated KCNQ1/KCNE1, KCNQ2/3 and KCNQ4 currents but, unusually, not KCNQ5. Strikingly, quercetin augmented both activation and inactivation of KCNQ1, via a unique KCNQ activation mechanism involving sites atop the voltage sensor and in the pore. The findings uncover a novel potential molecular basis for therapeutic effects of quercetin-rich foods and a new chemical space for atypical modes of KCNQ channel modulation.

Kaitlyn E. Redford and Geoffrey W. Abbott show that quercetin, a flavonoid highly expressed in capers, potentiates KCNQ currents to varying degrees depending on the subunit composition of the channel complex. By combining in silico docking, mutagenesis, and electrophysiology they show that this flavonoid can bind KCNQ channels atop the voltage sensor and within the pore module, highlighting an atypical mode of channel modulation.

Cyclic AMP-Dependent Regulation of Kv7 Voltage-Gated Potassium Channels

Voltage-gated Kv7 potassium channels, encoded by KCNQ genes, have major physiological impacts cardiac myocytes, neurons, epithelial cells, and smooth muscle cells. Cyclic adenosine monophosphate (cAMP), a well-known intracellular secondary messenger, can activate numerous downstream effector proteins, generating downstream signaling pathways that regulate many functions in cells. A role for cAMP in ion channel regulation has been established, and recent findings show that cAMP signaling plays a role in Kv7 channel regulation. Although cAMP signaling is recognized to regulate Kv7 channels, the precise molecular mechanism behind the cAMP-dependent regulation of Kv7 channels is complex. This review will summarize recent research findings that support the mechanisms of cAMP-dependent regulation of Kv7 channels.

Kv7 Channels in Lung Diseases

Lung diseases constitute a global health concern causing disability. According to WHO in 2016, respiratory diseases accounted for 24% of world population mortality, the second cause of death after cardiovascular diseases. The Kv7 channels family is a group of voltage-dependent K⁺ channels (Kv) encoded by KCNQ genes that are involved in various physiological functions in numerous cell types, especially, cardiac myocytes, smooth muscle cells, neurons, and epithelial cells. Kv7 channel α-subunits are regulated by KCNE1–5 ancillary β-subunits, which modulate several characteristics of Kv7 channels such as biophysical properties, cell-location, channel trafficking, and pharmacological sensitivity. Kv7 channels are mainly expressed in two large groups of lung tissues: pulmonary arteries (PAs) and bronchial tubes. In PA, Kv7 channels are expressed in pulmonary artery smooth muscle cells (PASMCs); while in the airway (trachea, bronchus, and bronchioles), Kv7 channels are expressed in airway smooth muscle cells (ASMCs), airway epithelial cells (AEPs), and vagal airway C-fibers (VACFs). The functional role of Kv7 channels may vary depending on the cell type. Several studies have demonstrated that the impairment of Kv7 channel has a strong impact on pulmonary physiology contributing to the pathophysiology of different respiratory diseases such as cystic fibrosis, asthma, chronic obstructive pulmonary disease, chronic coughing, lung cancer, and pulmonary hypertension. Kv7 channels are now recognized as playing relevant physiological roles in many tissues, which have encouraged the search for Kv7 channel modulators with potential therapeutic use in many diseases including those affecting the lung. Modulation of Kv7 channels has been proposed to provide beneficial effects in a number of lung conditions. Therefore, Kv7 channel openers/enhancers or drugs acting partly through these channels have been proposed as bronchodilators, expectorants, antitussives, chemotherapeutics and pulmonary vasodilators.

Dexamethasone reduces airway epithelial Cl- secretion by rapid non-genomic inhibition of KCNQ1, KCNN4 and KATP K+ channels

Basolateral membrane K⁺ channels play a key role in basal and agonist stimulated Cl^- transport across airway epithelial cells by generating a favourable electrical driving force for Cl^- efflux. The K⁺ channel sub-types and molecular mechanisms of regulation by hormones and secretagoues are still poorly understood. Here we have identified the type of K⁺ channels involved in cAMP and Ca²⁺ stimulated Cl^- secretion and uncovered a novel anti-secretory effect of dexamethasone mediated by inhibition of basolateral membrane K⁺ channels in a human airway cell model of 16HBE14o^- cells commonly used for ion transport studies. Dexamethasone produced a rapid inhibition of transepithelial chloride ion secretion under steady state conditions and after stimulation with cAMP agonist (forskolin) or a Ca²⁺ mobilizing agonist (ATP). Our results show three different types of K⁺ channels are targeted by dexamethasone to reduce airway secretion, namely Ca²⁺-activated secretion via KCNN4 (KCa3.1) channels and cAMP-activated secretion via KCNQ1 (Kv7.1) and KATP (Kir6.1,6.2) channels. The down-regulation of KCNN4 and KCNQ1 channel activities by dexamethasone involves rapid non-genomic activation of PKCα and PKA signalling pathways, respectively. Dexamethasone signal transduction for PKC and PKA activation was demonstrated to occur through a rapid non-genomic pathway that did not implicate the classical nuclear receptors for glucocorticoids or mineralocorticoids but occurred via a novel signalling cascade involving sequentially a G_i-protein coupled receptor, PKC, adenylyl cyclase Type IV, cAMP, PKA and ERK1/2 activation. The rapid, non-genomic, effects of dexamethasone on airway epithelial ion transport and cell signalling introduces a new paradigm for glucocorticoid actions in lung epithelia which may serve to augment the anti-inflammatory activity of the steroid and enhance its therapeutic potential in treating airway hypersecretion in asthma and COPD.

The KCNE2 potassium channel β subunit is required for normal lung function and resilience to ischemia and reperfusion injury

The KCNE2 single transmembrane-spanning voltage-gated potassium (K_v) channel β subunit is ubiquitously expressed and essential for normal function of a variety of cell types, often via regulation of the KCNQ1 K_v channel. A polymorphism upstream of KCNE2 is associated with reduced lung function in human populations, but the pulmonary consequences of KCNE2 gene disruption are unknown. Here, germline deletion of mouse Kcne2 reduced pulmonary expression of potassium channel α subunits Kcnq1 and Kcnb1 but did not alter expression of other Kcne genes. Kcne2 colocalized and coimmunoprecipitated with Kcnq1 in mouse lungs, suggesting the formation of pulmonary Kcnq1-Kcne2 potassium channel complexes. Kcne2 deletion reduced blood O₂, increased CO₂, increased pulmonary apoptosis, and increased inflammatory mediators TNF-α, IL-6, and leukocytes in bronchoalveolar lavage (BAL) fluids. Consistent with increased pulmonary vascular leakage, Kcne2 deletion increased plasma, BAL albumin, and the BAL:plasma albumin concentration ratio. Kcne2^-/- mouse lungs exhibited baseline induction of the reperfusion injury salvage kinase pathway but were less able to respond via this pathway to imposed pulmonary ischemia/reperfusion injury (IRI). We conclude that KCNE2 regulates KCNQ1 in the lungs and is required for normal lung function and resistance to pulmonary IRI. Our data support a causal relationship between KCNE2 gene disruption and lung dysfunction.-Zhou, L., Köhncke, C., Hu, Z., Roepke, T. K., Abbott, G. W. The KCNE2 potassium channel β subunit is required for normal lung function and resilience to ischemia and reperfusion injury.

Involvement of the TREK-1 channel in human alveolar cell membrane potential and its regulation by inhibitors of the chloride current

K⁺ channels of the alveolar epithelium control the driving force acting on the ionic and solvent flow through the cell membrane contributing to the maintenance of cell volume and the constitution of epithelial lining fluid. In the present work, we analyze the effect of the Cl^- channel inhibitors: (4-[(2-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-inden-5-yl)oxy] butanoic acid (DCPIB) and 9-anthracenecarboxylic acid (9-AC) on the total current in a type II pneumocytes (A549 cell line) model by patch clamp, immunocytochemical, and gene knockdown techniques. We noted that DCPIB and 9-AC promote the activation of K conductance. In fact, they significantly increase the intensity of the current and shift its reversal potential to values more negative than the control. By silencing outward rectifier channel in its anoctamin 6 portion, we excluded a direct involvement of Cl^- ions in modulation of I_K and, by means of functional tests with its specific inhibitor spadin, we identified the TREK-1 channel as the presumable target of both drugs. As the activity of TREK-1 has a key role for the correct functioning of the alveolar epithelium, the identification of DCPIB and 9-AC molecules as its activators suggests their possible use to build new pharmacological tools for the modulation of this channel.

Protective and antifungal properties of Nanodisk-Amphotericin B over commercially available Amphotericin B

Objective

Amphotericin B (AMB), a potent antifungal agent, has been employed as topical and systemic therapy for sinonasal fungal infections. A novel formulation of nanodisc (ND) containing super aggregated AMB (ND-AMB) for the treatment of fungal infections has been recently developed to provide greater protection from AMB toxicity than current, clinically approved lipid-based formulations. The objective of the current study was to evaluate the safety and potency of ND-AMB for sinonasal delivery using an in vitro model.

Methods

Human sinonasal tissue was harvested during endoscopic sinus surgery and grown at air–liquid interface until well-differentiated. Cultures were exposed to ND-AMB vs AMB and changes in K⁺ permeability and resistance were measured and recorded via Ussing chamber assay. Ciliary beat frequency (CBF) was analyzed in parallel as well as cytotoxic assay. Potency was assessed using real-time PCR measurement of the Aspergillus fumigatus 18S rRNA.

Results

Ussing chamber studies revealed K⁺ currents that increased rapidly within 30 s of adding AMB (10 μg/mL) to the apical side, indicating apical membranes had become permeable to K⁺ ions. In contrast, negligible induction of K⁺ current was obtained following addition of ND-AMB [AMB = (107.7 ± 15.9) μA/cm² AMB vs ND-AMB = (2.3 ± 0.7) μA/cm² ND-AMB; P = 0.005]. ND-AMB also protected nasal epithelial cells from cytotoxicity of AMB (P < 0.05). There was no difference in ciliary beat frequency between the two groups (P = 0.96). The expression of A. fumigatus 18S rRNA with exposure of lower dose of ND-AMB was significantly lower compared to that with AMB (P < 0.05).

Conclusions

Data from the present study suggests ND-AMB protects human nasal epithelia membranes from AMB toxicity by protecting against apical cell K⁺ permeability while maintaining uncompromised antifungal property compared to AMB. ND-AMB could provide a novel topical therapy for sinonasal fungal diseases.

Airway Gland Structure and Function

Submucosal glands contribute to airway surface liquid (ASL), a film that protects all airway surfaces. Glandular mucus comprises electrolytes, water, the gel-forming mucin MUC5B, and hundreds of different proteins with diverse protective functions. Gland volume per unit area of mucosal surface correlates positively with impaction rate of inhaled particles. In human main bronchi, the volume of the glands is ∼ 50 times that of surface goblet cells, but the glands diminish in size and frequency distally. ASL and its trapped particles are removed from the airways by mucociliary transport. Airway glands have a tubuloacinar structure, with a single terminal duct, a nonciliated collecting duct, then branching secretory tubules lined with mucous cells and ending in serous acini. They allow for a massive increase in numbers of mucus-producing cells without replacing surface ciliated cells. Active secretion of Cl(-) and HCO3 (-) by serous cells produces most of the fluid of gland secretions. Glands are densely innervated by tonically active, mutually excitatory airway intrinsic neurons. Most gland mucus is secreted constitutively in vivo, with large, transient increases produced by emergency reflex drive from the vagus. Elevations of [cAMP]i and [Ca(2+)]i coordinate electrolyte and macromolecular secretion and probably occur together for baseline activity in vivo, with cholinergic elevation of [Ca(2+)]i being mainly responsive for transient increases in secretion. Altered submucosal gland function contributes to the pathology of all obstructive diseases, but is an early stage of pathogenesis only in cystic fibrosis.

Complementary roles of KCa3.1 channels and β1-integrin during alveolar epithelial repair

BACKGROUND

Extensive alveolar epithelial injury and remodelling is a common feature of acute lung injury and acute respiratory distress syndrome (ARDS) and it has been established that epithelial regeneration, and secondary lung oedema resorption, is crucial for ARDS resolution. Much evidence indicates that K(+) channels are regulating epithelial repair processes; however, involvement of the KCa3.1 channels in alveolar repair has never been investigated before.

RESULTS

Wound-healing assays demonstrated that the repair rates were increased in primary rat alveolar cell monolayers grown on a fibronectin matrix compared to non-coated supports, whereas an anti-β1-integrin antibody reduced it. KCa3.1 inhibition/silencing impaired the fibronectin-stimulated wound-healing rates, as well as cell migration and proliferation, but had no effect in the absence of coating. We then evaluated a putative relationship between KCa3.1 channel and the migratory machinery protein β1-integrin, which is activated by fibronectin. Co-immunoprecipitation and immunofluorescence experiments indicated a link between the two proteins and revealed their cellular co-distribution. In addition, we demonstrated that KCa3.1 channel and β1-integrin membrane expressions were increased on a fibronectin matrix. We also showed increased intracellular calcium concentrations as well as enhanced expression of TRPC4, a voltage-independent calcium channel belonging to the large TRP channel family, on a fibronectin matrix. Finally, wound-healing assays showed additive effects of KCa3.1 and TRPC4 inhibitors on alveolar epithelial repair.

CONCLUSION

Taken together, our data demonstrate for the first time complementary roles of KCa3.1 and TRPC4 channels with extracellular matrix and β1-integrin in the regulation of alveolar repair processes.

Hydrogen peroxide stimulation of CFTR reveals an Epac-mediated, soluble AC-dependent cAMP amplification pathway common to GPCR signalling

BACKGROUND AND PURPOSE

H2 O2 is widely understood to regulate intracellular signalling. In airway epithelia, H2 O2 stimulates anion secretion primarily by activating an autocrine PGE2 signalling pathway via EP4 and EP1 receptors to initiate cytic fibrosis transmembrane regulator (CFTR)-mediated Cl(-) secretion. This study investigated signalling downstream of the receptors activated by H2 O2 .

EXPERIMENTAL APPROACH

Anion secretion by differentiated bronchial epithelial cells was measured in Ussing chambers during stimulation with H2 O2 , an EP4 receptor agonist or β2 -adrenoceptor agonist in the presence and absence of inhibitors of ACs and downstream effectors. Intracellular calcium ([Ca(2+) ]I ) changes were followed by microscopy using fura-2-loaded cells and PKA activation followed by FRET microscopy.

KEY RESULTS

Transmembrane adenylyl cyclase (tmAC) and soluble AC (sAC) were both necessary for H2 O2 and EP4 receptor-mediated CFTR activation in bronchial epithelia. H2 O2 and EP4 receptor agonist stimulated tmAC to increase exchange protein activated by cAMP (Epac) activity that drives PLC activation to raise [Ca(2+) ]i via Ca(2+) store release (and not entry). Increased [Ca(2+) ]i led to sAC activation and further increases in CFTR activity. Stimulation of sAC did not depend on changes in [HCO3 (-) ]. Ca(2+) -activated apical KCa 1.1 channels and cAMP-activated basolateral KV 7.1 channels contributed to H2 O2 -stimulated anion currents. A similar Epac-mediated pathway was seen following β2 -adrenoceptor or forskolin stimulation.

CONCLUSIONS AND IMPLICATIONS

H2 O2 initiated a complex signalling cascade that used direct stimulation of tmACs by Gαs followed by Epac-mediated Ca(2+) crosstalk to activate sAC. The Epac-mediated Ca(2+) signal constituted a positive feedback loop that amplified CFTR anion secretion following stimulation of tmAC by a variety of stimuli.

H2O2 stimulates cystic fibrosis transmembrane conductance regulator through an autocrine prostaglandin pathway, using multidrug-resistant protein-4

Cystic fibrosis transmembrane conductance regulator (CFTR) activity is essential for the maintenance of airway surface liquid depth, and therefore mucociliary clearance. Reactive oxygen species, increased during inflammatory airway diseases, alter CFTR activity. Here, H2O2 levels in the surface liquid of normal human bronchial epithelial cultures differentiated at the air-liquid interface were estimated, and H2O2-mediated changes in CFTR activity were examined. In Ussing chambers, H2O2-induced anion currents were sensitive to the CFTR inhibitors CFTRinh172 and GlyH-101. These currents were absent in cells from patients with cystic fibrosis. Responses to greater than 500 μM H2O2 were transient. Cyclooxygenase inhibitors blocked the H2O2 response, as did EP1 and EP4 receptor antagonists. A multidrug-resistant protein (MRP) inhibitor and short hairpin RNA directed against MRP4 blocked H2O2 responses. EP1 and EP4 agonists mimicked H2O2 in both control and MRP4 knockdown cells. Thus, H2O2 activates the synthesis, export, and binding of prostanoids via EP4 and, interestingly, EP1 receptors in normal, differentiated human airway epithelial cells to activate cyclic adenosine monophosphate pathways that in turn activate CFTR channels in the apical membrane.

Genetic variation in genes encoding airway epithelial potassium channels is associated with chronic rhinosinusitis in a pediatric population

Background

Apical potassium channels regulate ion transport in airway epithelial cells and influence air surface liquid (ASL) hydration and mucociliary clearance (MCC). We sought to identify whether genetic variation within genes encoding airway potassium channels is associated with chronic rhinosinusitis (CRS).

Methods

Single nucleotide polymorphism (SNP) genotypes for selected potassium channels were derived from data generated on the Illumnia HumanHap550 BeadChip or Illumina Human610-Quad BeadChip for 828 unrelated individuals diagnosed with CRS and 5,083 unrelated healthy controls from the Children's Hospital of Philadelphia (CHOP). Statistical analysis was performed with set-based tests using PLINK, and corrected for multiple testing.

Results

Set-based case control analysis revealed the gene KCNMA1 was associated with CRS in our Caucasian subset of the cohort (598 CRS cases and 3,489 controls; p = 0.022, based on 10,000 permutations). In addition there was borderline evidence that the gene KCNQ5 (p = 0.0704) was associated with the trait in our African American subset of the cohort (230 CRS cases and 1,594 controls). In addition to the top significant SNPs rs2917454 and rs6907229, imputation analysis uncovered additional genetic variants in KCNMA1 and in KCNQ5 that were associated with CRS.

Conclusions

We have implicated two airway epithelial potassium channels as novel susceptibility loci in contributing to the pathogenesis of CRS.

Biology of the KCNQ1 Potassium Channel

Ion channels are essential for basic cellular function and for processes including sensory perception and intercellular communication in multicellular organisms. Voltage-gated potassium (Kv) channels facilitate dynamic cellular repolarization during an action potential, opening in response to membrane depolarization to facilitate K+ efflux. In both excitable and nonexcitable cells other, constitutively active, K+ channels provide a relatively constant repolarizing force to control membrane potential, ion homeostasis, and secretory processes. Of the forty known human Kv channel pore-forming α subunits that coassemble in various combinations to form the fundamental tetrameric channel pore and voltage sensor module, KCNQ1 is unique. KCNQ1 stands alone in having the capacity to form either channels that are voltage-dependent and require membrane depolarization for activation, or constitutively active channels. In mammals, KCNQ1 regulates processes including gastric acid secretion, thyroid hormone biosynthesis, salt and glucose homeostasis, and cell volume and in some species is required for rhythmic beating of the heart. In this review, the author discusses the unique functional properties, regulation, cell biology, diverse physiological roles, and involvement in human disease states of this chameleonic K+ channel.

Identification of KvLQT1 K+ channels as new regulators of non-small cell lung cancer cell proliferation and migration

K+ channels, which are overexpressed in several cancers, have been identified as regulators of cell proliferation and migration, key processes in cancer development/propagation. Their role in lung cancer has not been studied extensively; but we showed previously that KvLQT1 channels are involved in cell migration, proliferation and repair of normal lung epithelial cells. We now investigated the role of these channels in lung cancer cell lines and their expression levels in human lung adenocarcinoma (AD) tissues. First, we observed that the wound-healing rates of A549 lung adenocarcinoma cell monolayers were reduced by clofilium and chromanol or after silencing with KvLQT1-specific siRNA. Dose-dependent decrease of A549 cell growth and cell accumulation in G0/G1 phase were seen after KvLQT1 inhibition. Clofilium also affected 2D and 3D migration of A549 cells. Similarly, H460 cell growth, migration and wound healing were diminished by this drug. Because K+ channel overexpression has been encountered in some cancers, we assessed KvLQT1 expression levels in tumor tissues from patients with lung AD. KvLQT1 protein expression in tumor samples was increased by 1.5- to 7-fold, compared to paired non-neoplastic tissues, in 17 of 26 patients. In summary, our data reveal that KvLQT1 channel blockade efficiently reduces A549 and H460 cell proliferation and migration. Moreover, KvLQT1 overexpression in AD samples suggests that it could be a potential therapeutic target in lung cancer.

Evidence of K+ channel function in epithelial cell migration, proliferation, and repair

Efficient repair of epithelial tissue, which is frequently exposed to insults, is necessary to maintain its functional integrity. It is therefore necessary to better understand the biological and molecular determinants of tissue regeneration and to develop new strategies to promote epithelial repair. Interestingly, a growing body of evidence indicates that many members of the large and widely expressed family of K(+) channels are involved in regulation of cell migration and proliferation, key processes of epithelial repair. First, we briefly summarize the complex mechanisms, including cell migration, proliferation, and differentiation, engaged after epithelial injury. We then present evidence implicating K(+) channels in the regulation of these key repair processes. We also describe the mechanisms whereby K(+) channels may control epithelial repair processes. In particular, changes in membrane potential, K(+) concentration, cell volume, intracellular Ca(2+), and signaling pathways following modulation of K(+) channel activity, as well as physical interaction of K(+) channels with the cytoskeleton or integrins are presented. Finally, we discuss the challenges to efficient, specific, and safe targeting of K(+) channels for therapeutic applications to improve epithelial repair in vivo.

Influenza A virus (H1N1) increases airway epithelial cell secretion by up-regulation of potassium channel KCNN4

Influenza infects the epithelial cells lining the airways. Normally epithelial cells move solutes through ion channels to create the osmotic drive to hydrate the airways. Viral alteration of this process could explain, in part, the fluid imbalance in the lungs and the resulting pulmonary edema that occurs during severe influenza infections. Using western blot and RT-qPCR, we measured ion channel and cytokine expression in the Calu3 airway cell line after infection with influenza virus (H1N1) for 48 h. We simultaneously measured chloride and potassium channel function by means of a short-circuit current (I(sc)) produced in an Ussing chamber. At a multiplicity of infection (MOI) of 10, viral M1 protein and pro-inflammatory cytokine expression was observed 24h post-infection, despite a lack of measurable change in Isc. However, we observed a decreased secretory response in cAMP- and calcium-induced Isc 48 h post-infection. This correlated with a decrease in CFTR and KCNN4 protein levels. Interestingly, a viral dose of an MOI 0.6 revealed an increased secretory response that correlated with pro-inflammatory cytokine expression. This increased secretory response seemed to be primarily driven through KCNN4. We detected an increase in KCNN4 mRNA and protein, while CFTR function and expression remained unchanged. Furthermore, inhibition of the KCNN4-stimulated I(sc) with TRAM-34, a specific inhibitor, ameliorated the response, implicating KCNN4 as the main driving force behind the secretory phenotype.

Pseudohalide anions reveal a novel extracellular site for potentiators to increase CFTR function

BACKGROUND AND PURPOSE

There is great interest in the development of potentiator drugs to increase the activity of the cystic fibrosis transmembrane conductance regulator (CFTR) in cystic fibrosis. We tested the ability of several anions to potentiate CFTR activity by a novel mechanism.

EXPERIMENTAL APPROACH

Patch clamp recordings were used to investigate the ability of extracellular pseudohalide anions (Co(CN)(6) (3-) , Co(NO(2) )(6) (3-) , Fe(CN)(6) (3-) , IrCl(6) (3-) , Fe(CN)(6) (4-) ) to increase the macroscopic conductance of mutant CFTR in intact cells via interactions with cytoplasmic blocking anions. Mutagenesis of CFTR was used to identify a possible molecular mechanism of action. Transepithelial short-circuit current recordings from human airway epithelial cells were used to determine effects on net anion secretion.

KEY RESULTS

Extracellular pseudohalide anions were able to increase CFTR conductance in intact cells, as well as increase anion secretion in airway epithelial cells. This effect appears to reflect the interaction of these substances with a site on the extracellular face of the CFTR protein.

CONCLUSIONS AND IMPLICATIONS

Our results identify pseudohalide anions as increasing CFTR function by a previously undescribed molecular mechanism that involves an interaction with an extracellular site on the CFTR protein. Future drugs could utilize this mechanism to increase CFTR activity in cystic fibrosis, possibly in conjunction with known intracellularly-active potentiators.

Emerging genetics of COPD

Since the discovery of alpha-1 antitrypsin in the early 1960s, several new genes have been suggested to play a role in chronic obstructive pulmonary disease (COPD) pathogenesis. Yet, in spite of those advances, much about the genetic basis of COPD still remains to be discovered. Unbiased approaches, such as genome-wide association (GWA) studies, are critical to identify genes and pathways and to verify suggested genetic variants. Indeed, most of our current understanding about COPD candidate genes originates from GWA studies. Experiments in form of cross-study replications and advanced meta-analyses have propelled the field towards unravelling details about COPD's pathogenesis. Here, we review the discovery of genetic variants in association with COPD phenotypes by discussing the available approaches and current findings. Limitations of current studies are considered and future directions provided.

Luminal cholinergic signalling in airway lining fluid: a novel mechanism for activating chloride secretion via Ca²⁺-dependent Cl⁻ and K⁺ channels

BACKGROUND AND PURPOSE

Recent studies detected the expression of proteins involved in cholinergic metabolism in airway epithelial cells, although the function of this non-neuronal cholinergic system is not known in detail. Thus, this study focused on the effect of luminal ACh as a regulator of transepithelial ion transport in epithelial cells.

EXPERIMENTAL APPROACH

RT-PCR experiments were performed using mouse tracheal epithelial cells for ChAT and organic cation transporter (OCT) transcripts. Components of tracheal airway lining fluid were analysed with desorption electrospray ionization (DESI) MS. Effects of nicotine on mouse tracheal epithelial ion transport were examined with Ussing-chamber experiments.

KEY RESULTS

Transcripts encoding ChAT and OCT1-3 were detected in mouse tracheal epithelial cells. The DESI experiments identified ACh in the airway lining fluid. Luminal ACh induced an immediate, dose-dependent increase in the transepithelial ion current (EC₅₀: 23.3 µM), characterized by a transient peak and sustained plateau current. This response was not affected by the Na⁺-channel inhibitor amiloride. The Cl⁻-channel inhibitor niflumic acid or the K⁺-channel blocker Ba²⁺ attenuated the ACh effect. The calcium ionophore A23187 mimicked the ACh effect. Luminal nicotine or muscarine increased the ion current. Experiments with receptor gene-deficient animals revealed the participation of muscarinic receptor subtypes M₁ and M₃.

CONCLUSIONS AND IMPLICATIONS

The presence of luminal ACh and activation of transepithelial ion currents by luminal ACh receptors identifies a novel non-neuronal cholinergic pathway in the airway lining fluid. This pathway could represent a novel drug target in the airways.

Nongenomic effects of fluticasone propionate and budesonide on human airway anion secretion

This study investigated the physiological effects of inhaled corticosteroids, which are used widely to treat asthma. The application of fluticasone propionate (FP, 100 μM) induced sustained increases in the short-circuit current (I(SC)) in human airway Calu-3 epithelial cells. The FP-induced I(SC) was prevented by the presence of H89 (10 μM, a protein kinase A inhibitor) and SQ22536 (100 μM, an adenylate cyclase inhibitor). The FP-induced responses involved bumetanide (a Na(+)-K(+)-2Cl(-) cotransporter inhibitor)-sensitive and 4,4'-dinitrostilbene-2,2'-disulfonic acid (an inhibitor of HCO(3)(-)-dependent anion transporters)-sensitive components, both of which reflect basolateral anion transport. Further, FP augmented apical membrane Cl(-) current (I(Cl)), reflecting cystic fibrosis transmembrane conductance regulator (CFTR)-mediated conductance, in the nystatin-permeabilized monolayer. In I(SC) and I(Cl) responses, FP failed to enhance the responses to forskolin (10 μM, an adenylate cyclase activator). Nevertheless, we found that FP synergistically increased cytosolic cAMP concentrations in combination with forskolin. All these effects of FP were reproduced with the use of budesonide. Collectively, inhaled corticosteroids such as FP and budesonide stimulate CFTR-mediated anion transport through adenylate cyclase-mediated mechanisms in a nongenomic fashion, thus sharing elements of a common pathway with forskolin. However, the corticosteroids cooperate with forskolin for synergistic cAMP production, suggesting that the corticosteroids and forskolin do not compete with each other to exert their effects on adenylate cyclase. Considering that such synergism was also observed in the FP/salmeterol combination, these nongenomic aspects may play therapeutic roles in mucus congestive airway diseases, in addition to genomic aspects that are generally recognized.

Regulation of normal and cystic fibrosis airway epithelial repair processes by TNF-α after injury

Chronic infection and inflammation have been associated with progressive airway epithelial damage in patients with cystic fibrosis (CF). However, the effect of inflammatory products on the repair capacity of respiratory epithelia is unclear. Our objective was to study the regulation of repair mechanisms by tumor necrosis factor-α (TNF-α), a major component of inflammation in CF, in a model of mechanical wounding, in two bronchial cell lines, non-CF NuLi and CF CuFi. We observed that TNF-α enhanced the NuLi and CuFi repair rates. Chronic exposure (24–48 h) to TNF-α augmented this stimulation as well as the migration rate during repair. The cellular mechanisms involved in this stimulation were then evaluated. First, we discerned that TNF-α induced metalloproteinase-9 release, epidermal growth factor (EGF) shedding, and subsequent EGF receptor transactivation. Second, TNF-α-induced stimulation of the NuLi and CuFi wound-closure rates was prevented by GM6001 (metalloproteinase inhibitor), EGF antibody (to titrate secreted EGF), and EGF receptor tyrosine kinase inhibitors. Furthermore, we recently reported a relationship between the EGF response and K+channel function, both controlling bronchial repair. We now show that TNF-α enhances KvLQT1 and KATPcurrents, while their inhibition abolishes TNF-α-induced repair stimulation. These results indicate that the effect of TNF-α is mediated, at least in part, through EGF receptor transactivation and K+channel stimulation. In contrast, cell proliferation during repair was slowed by TNF-α, suggesting that TNF-α could exert contrasting actions on repair mechanisms of CF airway epithelia. Finally, the stimulatory effect of TNF-α on airway wound repair was confirmed on primary airway epithelial cells, from non-CF and CF patients.

A role for two-pore K⁺ channels in modulating Na⁺ absorption and Cl⁻ secretion in normal human bronchial epithelial cells

Mucociliary clearance is the primary innate physical defense mechanism against inhaled pathogens and toxins. Vectorial ion transport, primarily sodium absorption and anion secretion, by airway epithelial cells supports mucociliary clearance. This is evidenced by diseases of abnormal ion transport such as cystic fibrosis and pseudohypoaldosteronism that are characterized by changes in mucociliary clearance. Sodium absorption and chloride secretion in human bronchial epithelial cells depend on potassium channel activity, which creates a favorable electrochemical gradient for both by hyperpolarizing the apical plasma membrane. Although the role of basolateral membrane potassium channels is firmly established and extensively studied, a role for apical membrane potassium channels has also been described. Here, we demonstrate that bupivacaine and quinidine, blockers of four-transmembrane domain, two-pore potassium (K2P) channels, inhibit both amiloride-sensitive sodium absorption and forskolin-stimulated anion secretion in polarized, normal human bronchial epithelial cells at lower concentrations when applied to the mucosal surface than when applied to the serosal surface. Transcripts from four genes, KCNK1 (TWIK-1), KCNK2 (TREK-1), KCNK5 (TASK-2), and KCNK6 (TWIK-2), encoding K2P channels were identified by RT-PCR. Protein expression at the apical membrane was confirmed by immunofluorescence. Our data provide further evidence that potassium channels, in particular K2P channels, are expressed and functional in the apical membrane of airway epithelial cells where they may be targets for therapeutic manipulation.

Genome-wide association and large-scale follow up identifies 16 new loci influencing lung function

Pulmonary function measures reflect respiratory health and are used in the diagnosis of chronic obstructive pulmonary disease. We tested genome-wide association with forced expiratory volume in 1 second and the ratio of forced expiratory volume in 1 second to forced vital capacity in 48,201 individuals of European ancestry with follow up of the top associations in up to an additional 46,411 individuals. We identified new regions showing association (combined P < 5 × 10(-8)) with pulmonary function in or near MFAP2, TGFB2, HDAC4, RARB, MECOM (also known as EVI1), SPATA9, ARMC2, NCR3, ZKSCAN3, CDC123, C10orf11, LRP1, CCDC38, MMP15, CFDP1 and KCNE2. Identification of these 16 new loci may provide insight into the molecular mechanisms regulating pulmonary function and into molecular targets for future therapy to alleviate reduced lung function.

Polystyrene nanoparticles activate ion transport in human airway epithelial cells

BACKGROUND

Over the last decade, nanotechnology has provided researchers with new nanometer materials, such as nanoparticles, which have the potential to provide new therapies for many lung diseases. In this study, we investigated the acute effects of polystyrene nanoparticles on epithelial ion channel function.

METHODS

Human submucosal Calu-3 cells that express cystic fibrosis transmembrane conductance regulator (CFTR) and baby hamster kidney cells engineered to express the wild-type CFTR gene were used to investigate the actions of negatively charged 20 nm polystyrene nanoparticles on short-circuit current in Calu-3 cells by Ussing chamber and single CFTR Clchannels alone and in the presence of known CFTR channel activators by using baby hamster kidney cell patches.

RESULTS

Polystyrene nanoparticles caused sustained, repeatable, and concentration-dependent increases in short-circuit current. In turn, these short-circuit current responses were found to be biphasic in nature, ie, an initial peak followed by a plateau. EC(50) values for peak and plateau short-circuit current responses were 1457 and 315.5 ng/mL, respectively. Short-circuit current was inhibited by diphenylamine-2-carboxylate, a CFTR Cl(-) channel blocker. Polystyrene nanoparticles activated basolateral K(+) channels and affected Cl(-) and HCO(3) (-) secretion. The mechanism of short-circuit current activation by polystyrene nanoparticles was found to be largely dependent on calcium-dependent and cyclic nucleotide-dependent phosphorylation of CFTR Cl(-) channels. Recordings from isolated inside-out patches using baby hamster kidney cells confirmed the direct activation of CFTR Cl(-) channels by the nanoparticles.

CONCLUSION

This is the first study to identify the activation of ion channels in airway cells after exposure to polystyrene-based nanomaterials. Thus, polystyrene nanoparticles cannot be considered as a simple neutral vehicle for drug delivery for the treatment of lung diseases, due to the fact that they may have the ability to affect epithelial cell function and physiological processes on their own.

A phospholipid-apolipoprotein A-I nanoparticle containing amphotericin B as a drug delivery platform with cell membrane protective properties

Amphotericin B (AMB), a potent antifungal agent, has been employed as an inhalable therapy for pulmonary fungal infections. We recently described a novel nano-sized delivery vehicle composed of phospholipid (PL) and apolipoprotein A-I, NanoDisk (ND), to which we added AMB as a payload (ND-AMB). The goal of the present study was to evaluate whether ND-AMB, compared to other formulations, preserves lung cell integrity in vitro, as AMB can be toxic to mammalian cells and reduce lung function when inhaled. Epithelial integrity was assessed by measuring K(+) ion flux across a model airway epithelium, Calu-3 cells. In this assay ND-AMB was at least 8-fold less disruptive than AMB/deoxycholate (DOC). Cell viability studies confirmed this observation. Unexpectedly, the ND vehicle restored the integrity of a membrane compromised by prior exposure to AMB. An alternative formulation of ND-AMB containing a high load of AMB per ND was not protective, suggesting that ND with a low ratio of AMB to PL can sequester additional AMB from membranes. ND-AMB also protected HepG2 cells from the cytotoxicity of AMB, as determined by cellular viability and lactate dehydrogenase (LDH) levels. This study suggests that ND-AMB may be safe for administration via inhalation and reveals a unique activity whereby ND-AMB protects lung epithelial membranes from AMB toxicity.

A biophysical model for integration of electrical, osmotic, and pH regulation in the human bronchial epithelium

A dynamical biophysical model for the functioning of an epithelium is presented. This model integrates the electrical and osmotic behaviors of the epithelium, taking into account intracellular conditions. The specific tissue modeled is the human bronchial epithelium, which is of particular interest, as it is the location of the most common lethal symptoms of cystic fibrosis. The model is implemented in a modular form to facilitate future application of the code to other epithelial tissue by inputting different transporters, channels, and geometric parameters. The model includes pH regulation as an integral component of overall regulation of epithelial function, through the interdependence of pH, bicarbonate concentration, and current. The procedures for specification, the validation of the model, and parametric studies are presented using available experimental data of cultured human bronchial epithelium. Parametric studies are performed to elucidate a), the contribution of basolateral chloride channels to the short-circuit current functional form, and b), the role that regulation of basolateral potassium conductance plays in epithelial function.

Electrophysiological characterization of electrolyte and nutrient transport across the small intestine in horses

The aim of this study was to characterize the transport mechanisms of electrolytes and nutrients across the jejunum of nine healthy horses electrophysiologically. The stripped mucosa was mounted in Ussing chambers and tissue conductances (G(t)) and short circuit currents (I(sc)) were continuously monitored. After blocking the sodium and potassium channels with amiloride, tetraethylammonium chloride (TEA) and barium, chloride secretion was stimulated by carbachol and forskolin. Subsequently, chloride channels were inhibited by 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid, 5-nitro-2-(3-phenylpropylamino)benzoic acid, CFTR(inh)-172, N-(2-naphtalenyl)-(3.5-dibromo-2.4-dihydroxyphenyl)methylene glycine hydrazide (GlyH-101) and glibenclamide and their dose-response effect was investigated. The response to glucose, l-alanine and glycyl-l-glutamine was determined at two different mucosal pH values (pH 7.4 and 5.4 respectively). Mean basal I(sc) was -0.47 +/- 0.31 microEq/cm(2)h and mean G(t) was 22.17 +/- 1.78 mS/cm(2). Amiloride and TEA did not alter the baseline I(sc). Barium, carbachol and forskolin significantly increased I(sc). Irrespective of the dose, none of the chloride inhibitors changed I(sc). All nutrients induced a significant increase in I(sc) with the increase being significantly higher at pH 7.4 than at pH 5.4. In conclusion, there is evidence that chloride secretion in horses may be different from respective transport mechanisms in other species. The glucose absorption is suggestive of a sodium-dependent glucose cotransporter 1. However, a decrease in luminal pH did not stimulate current response to peptides as shown for other mammals.

[K+ channels and lung epithelial physiology]

Transcripts of more than 30 different K(+) channels have been detected in the respiratory epithelium lining airways and alveoli. These channels belong to the 3 main classes of K(+) channels, i.e. i) voltage-dependent or calcium-activated, 6 transmembrane segments (TM), ii) 2-pores 4-TM and iii) inward-rectified 2-TM channels. The physiological and functional significance of this high molecular diversity of lung epithelial K(+) channels is not well understood. Surprisingly, relatively few studies are focused on K(+) channel function in lung epithelial physiology. Nevertheless, several studies have shown that KvLQT1, KCa and K(ATP) K(+) channels play a crucial role in ion and fluid transport, contributing to the control of airway and alveolar surface liquid composition and volume. K(+) channels are involved in other key functions, such as O(2) sensing or the capacity of the respiratory epithelia to repair after injury. This mini-review aims to discuss potential functions of lung K(+) channels.

Volume regulation in the human airway epithelial cell line Calu-3

Cells regulate their volume in response to changes in the osmolarity of both their extracellular and their intracellular environments. We investigated the ability of the human airway epithelial cell line Calu-3 to respond to changes in extracellular osmolarity. Although switching Calu-3 cells from an isosmotic to a hyperosmotic environment resulted in cell shrinkage, there was no compensatory mechanism for the cells to return to their original volume. In contrast, switching to a hyposmotic environment resulted in an initial cell swelling response, followed by a regulatory volume decrease (RVD). Pharmacologic studies demonstrate that the voltage-activated K+ channels Kv4.1 and (or) Kv4.3 play a crucial role in mediating this RVD response, and we demonstrated expression of these channel types at the mRNA and protein levels. Furthermore, inhibition of the large- and intermediate-conductance Ca2+-activated K+ channels KCa1.1 (maxi-K) and KCa3.1 (hIK) also implicated these channels as playing a role in volume recovery in Calu-3 cells. This report describes the nature of volume regulation in the widely used model cell line Calu-3.

In situ measurement of airway surface liquid [K+] using a ratioable K+-sensitive fluorescent dye

The airway surface liquid (ASL) is the thin fluid layer lining airway surface epithelial cells, whose volume and composition are tightly regulated and may be abnormal in cystic fibrosis (CF). We synthesized a two-color fluorescent dextran to measure ASL [K(+)], TAC-Lime-dextran-TMR, consisting of a green-fluorescing triazacryptand K(+) ionophore-Bodipy conjugate, coupled to dextran, together with a red fluorescing tetramethylrhodamine reference chromophore. TAC-Lime-dextran-TMR fluorescence was K(+)-selective, increasing >4-fold with increasing [K(+)] from 0 to 40 mm. In well differentiated human airway epithelial cells, ASL [K(+)] was 20.8 +/- 0.3 mm and decreased by inhibition of the Na(+)/K(+) pump (ouabain), ENaC (amiloride), CF transmembrane conductance regulator (CFTR(inh)-172), or K(+) channels (TEA or XE991). ASL [K(+)] was increased by forskolin but not affected by Na(+)/K(+)/2Cl(-) cotransporter inhibition (bumetanide). Functional and expression studies indicated the involvement of [K(+)] channels KCNQ1, KCNQ3, and KCNQ5 as determinants of ASL [K(+)]. [K(+)] in CF cultures was similar to that in non-CF cultures, suggesting that abnormal ASL [K(+)] is not a factor in CF lung disease. In intact airways, ASL [K(+)] was also well above extracellular [K(+)]: 22 +/- 1 mm in pig trachea ex vivo and 16 +/- 1 mm in mouse trachea in vivo. Our results provide the first noninvasive measurements of [K(+)] in the ASL and indicate the involvement of apical and basolateral membrane ion transporters in maintaining a high ASL [K(+)].

Inhibition of the KCa3.1 channels by AMP-activated protein kinase in human airway epithelial cells

The vectorial transport of ions and water across epithelial cells depends to a large extent on the coordination of the apical and basolateral ion fluxes with energy supply. In this work we provide the first evidence for a regulation by the 5'-AMP-activated protein kinase (AMPK) of the calcium-activated potassium channel KCa3.1 expressed at the basolateral membrane of a large variety of epithelial cells. Inside-out patch-clamp experiments performed on human embryonic kidney (HEK) cells stably transfected with KCa3.1 first revealed a decrease in KCa3.1 activity following the internal addition of AMP at a fixed ATP concentration. This effect was dose dependent with half inhibition at 140 muM AMP in 1 mM ATP. Evidence for an interaction between the COOH-terminal region of KCa3.1 and the gamma1-subunit of AMPK was next obtained by two-hybrid screening and pull-down experiments. Our two-hybrid analysis confirmed in addition that the amino acids extending from Asp(380) to Ala(400) in COOH-terminal were essential for the interaction AMPK-gamma1/KCa3.1. Inside-out experiments on cells coexpressing KCa3.1 with the dominant negative AMPK-gamma1-R299G mutant showed a reduced sensitivity of KCa3.1 to AMP, arguing for a functional link between KCa3.1 and the gamma1-subunit of AMPK. More importantly, coimmunoprecipitation experiments carried out on bronchial epithelial NuLi cells provided direct evidence for the formation of a KCa3.1/AMPK-gamma1 complex at endogenous AMPK and KCa3.1 expression levels. Finally, treating NuLi monolayers with the membrane permeant AMPK activator 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) caused a significant decrease of the KCa3.1-mediated short-circuit currents, an effect reversible by coincubation with the AMPK inhibitor Compound C. These observations argue for a regulation of KCa3.1 by AMPK in a functional epithelium through protein/protein interactions involving the gamma1-subunit of AMPK.

Molecular diversity and function of K+ channels in airway and alveolar epithelial cells

Multiple K(+) channels are expressed in the respiratory epithelium lining airways and alveoli. Of the three main classes [1) voltage-dependent or Ca(2+)-activated, 6-transmembrane domains (TMD), 2) 2-pores 4-TMD, and 3) inward-rectified 2-TMD K(+) channels], almost 40 different transcripts have already been detected in the lung. The physiological and functional significance of this high molecular diversity of lung epithelial K(+) channels is intriguing. As detailed in the present review, K(+) channels are located at both the apical and basolateral membranes in the respiratory epithelium, where they mediate K(+) currents of diverse electrophysiological and regulatory properties. The main recognized function of K(+) channels is to control membrane potential and to maintain the driving force for transepithelial ion and liquid transport. In this manner, KvLQT1, KCa and K(ATP) channels, for example, contribute to the control of airway and alveolar surface liquid composition and volume. Thus, K(+) channel activation has been identified as a potential therapeutic strategy for the resolution of pathologies characterized by ion transport dysfunction. K(+) channels are also involved in other key functions in lung physiology, such as oxygen-sensing, inflammatory responses and respiratory epithelia repair after injury. The purpose of this review is to summarize and discuss what is presently known about the molecular identity of lung K(+) channels with emphasis on their role in lung epithelial physiology.

EGF and K+ channel activity control normal and cystic fibrosis bronchial epithelia repair

Severe lesions of airway epithelia are observed in cystic fibrosis (CF) patients. The regulatory mechanisms of cell migration and proliferation processes, involved in the repair of injured epithelia, then need to be better understood. A model of mechanical wounding of non-CF (NuLi) and CF (CuFi) bronchial monolayers was employed to study the repair mechanisms. We first observed that wound repair, under paracrine and autocrine EGF control, was slower (up to 33%) in CuFi than in NuLi. Furthermore, EGF receptor (EGFR) activation, following wounding, was lower in CuFi than in NuLi monolayers. Cell proliferation and migration assays indicated a similar rate of proliferation in both cell lines but with reduced (by 25%) CuFi cell migration. In addition, cell migration experiments performed in the presence of conditioned medium, collected from NuLi and CuFi wounded bronchial monolayers, suggested a defect in EGF/EGFR signaling in CF cells. We (49) recently demonstrated coupling between the EGF response and K(+) channel function, which is crucial for EGF-stimulated alveolar repair. In CuFi cells, lower EGF/EGFR signaling was accompanied by a 40-70% reduction in K(+) currents and KvLQT1, ATP-sensitive potassium (K(ATP)), and Ca(2+)-activated K(+) (KCa3.1) channel expression. In addition, EGF-stimulated bronchial wound healing, cell migration, and proliferation were severely decreased by K(+) channel inhibitors. Finally, acute CFTR inhibition failed to reduce wound healing, EGF secretion, and K(+) channel expression in NuLi. In summary, the delay in CuFi wound healing could be due to diminished EGFR signaling coupled with lower K(+) channel function, which play a crucial role in bronchial repair.

KCNQ-encoded channels regulate Na+ transport across H441 lung epithelial cells

H441 cells are a model of absorptive airway epithelia that are characterised by a pronounced apical Na+ flux through amiloride-sensitive Na+ channels. The flux of Na+ is intimately linked to Na+ handling by the cell as well as the membrane potential across the apical membrane. As KCNQ-encoded K+ channels influence chloride secretion in gastrointestinal epithelia, the goal of the present study was to ascertain the expression of KCNQ genes in H441 cells and determine the functional role of the expression products. Message for KCNQ3 and KCNQ5 was detected by RT-polymerase chain reaction and the translated proteins were observed by immunocytochemistry. Ussing experiments showed that the pan-KCNQ channel blocker XE991, but not KCNQ1 selective blockers, reduced the short circuit current and the amiloride-sensitive component. These data show for the first time that potassium channels encoded by KCNQ3 or KCNQ5 are crucial determinants of epithelial Na+ flux.

Dual pathway activated by tert-butyl hydroperoxide in human airway anion secretion

We analyzed the mechanisms underlying the ion transport induced by tert-butyl hydroperoxide (t-BOOH), a membrane-permeant oxidant that has been widely used as a model of oxidative stress, in human airway epithelial cells (Calu-3). We found that t-BOOH induced a short-circuit current that was composed of two distinct components, a peaked component (PC) and a sustained component (SC). Both components were reduced by the presence of H-89 (N-[2-(4-bromocinnamylamino)ethyl]-5-isoquinoline) [10 microM, a protein kinase A (PKA) inhibitor] and clofilium (100 microM, a cAMP-dependent K+ channel inhibitor) but not by charybdotoxin (50 nM, a human intermediate conductance Ca2+-activated K+ channel inhibitor), suggesting that both PC and SC were generated through a common PKA-dependent/Ca2+-independent pathway. Notwithstanding, analyses of the physiological properties revealed that PC and SC were attributable to different pathways. PC, but not SC, was correlated with apical membrane Cl- conductance and was inhibited by the cyclooxygenase (COX)-2 inhibitor NS-398 (N-[2-(cyclohexyloxyl)-4-nitrophenyl]-methane sulfonamide; 10 microM). In contrast, SC, but not PC, was composed of a component sensitive to bumetanide (50 microM), an inhibitor of the basolateral Na+-K+-2Cl- cotransporter (NKCC1), and was abolished by the cytoskeleton dysfunction induced by cytochalasin D (10 microM) and (R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexane carboxamide (Y-27632; 20 microM). Collectively, t-BOOH induces PKA-related anion secretion through two independent pathways: rapid activation of apical anion efflux through a COX-2-dependent/cytoskeleton-independent pathway and relatively delayed activation of NKCC1 for basolateral anion uptake through a COX-2-independent/cytoskeleton-dependent pathway.

Apical adenosine regulates basolateral Ca2+-activated potassium channels in human airway Calu-3 epithelial cells

In airway epithelial cells, apical adenosine regulates transepithelial anion secretion by activation of apical cystic fibrosis transmembrane conductance regulator (CFTR) via adenosine receptors and cAMP/PKA signaling. However, the potent stimulation of anion secretion by adenosine is not correlated with its modest intracellular cAMP elevation, and these uncorrelated efficacies have led to the speculation that additional signaling pathways may be involved. Here, we showed that mucosal adenosine-induced anion secretion, measured by short-circuit current (Isc), was inhibited by the PLC-specific inhibitor U-73122 in the human airway submucosal cell line Calu-3. In addition, the Isc was suppressed by BAPTA-AM (a Ca2+ chelator) and 2-aminoethoxydiphenyl borate (2-APB; an inositol 1,4,5-trisphosphate receptor blocker), but not by PKC inhibitors, suggesting the involvement of PKC-independent PLC/Ca2+ signaling. Ussing chamber and patch-clamp studies indicated that the adenosine-induced PLC/Ca2+ signaling stimulated basolateral Ca2+-activated potassium (KCa) channels predominantly via A2B adenosine receptors and contributed substantially to the anion secretion. Thus, our data suggest that apical adenosine activates contralateral K+ channels via PLC/Ca2+ and thereby increases the driving force for transepithelial anion secretion, synergizing with its modulation of ipsilateral CFTR via cAMP/PKA. Furthermore, the dual activation of CFTR and KCa channels by apical adenosine resulted in a mixed secretion of chloride and bicarbonate, which may alter the anion composition in the secretion induced by secretagogues that elicit extracellular ATP/adenosine release. Our findings provide novel mechanistic insights into the regulation of anion section by adenosine, a key player in the airway surface liquid homeostasis and mucociliary clearance.

Enhancement of P2Y6-induced Cl- secretion by IL-13 and modulation of SK4 channels activity in human bronchial cells

Expression of functional P2Y(6) receptors was demonstrated in primary cultures of human bronchial cells (NHBE cells). P2Y(6) receptors were located only on the apical membranes of NHBE cells. Their stimulation by UDP induced a chloride secretion (short-circuit current) reflected by the development of two I(sc) components (I(fast) and I(late)). A pharmacological characterization of those two I(sc) components showed the involvement of CaCC and CFTR channel activity in I(fast) and I(late) respectively. I(fast) was also found to be under control of basolateral SK4 channels. Indeed, inhibition of SK4 channels opening by clotrimazole dramatically reduced I(fast) amplitude. The epithelial ion transporting phenotype depends on the cellular state of differentiation. As previously reported, we observed that Ultroser G increased the epithelial tightness and Na(+)-transport capacity while IL-13 switch the epithelial ion transport phenotype from a Na(+)-absorbing to a Cl(-)-secreting one. In our study, we report for the first time a change in the K(+) cell permeability associated to IL-13-induced cell differentiation. IL-13 treatment increased the-resting K(+) permeability as well as the Ca(2+)-dependent K(+) permeability stimulated by UDP or ionomycin. SK4 channels activity, underlying the Ca(2+)-dependent K(+) permeability was in particular increased by IL-13. The on/off effect of IL-13 on P2Y(6)-induced Cl-secretion may help to identify the molecular determinants responsible for the CaCC channel activity.

Contribution of KCNQ1 to the regulatory volume decrease in the human mammary epithelial cell line MCF-7

Using the human mammary epithelial cell line MCF-7, we have investigated volume-activated changes in response to hyposmotic stress. Switching MCF-7 cells from an isosmotic to a hyposmotic solution resulted in an initial cell swelling response, followed by a regulatory volume decrease (RVD). This RVD response was inhibited by the nonselective K+ channel inhibitors Ba2+, quinine, and tetraethylammonium chloride, implicating K+ channel activity in this volume-regulatory mechanism. Additional studies using chromonol 293B and XE991 as inhibitors of the KCNQ1 K+ channel, and also a dominant-negative NH2-terminal truncated KCNQ1 isoform, showed complete abolition of the RVD response, suggesting that KCNQ1 plays an important role in regulation of cell volume in MCF-7 cells. We additionally confirmed that KCNQ1 mRNA and protein is expressed in MCF-7 cells, and that, when these cells are cultured as a polarized monolayer, KCNQ1 is located exclusively at the apical membrane. Whole cell patch-clamp recordings from MCF-7 cells revealed a small 293B-sensitive current under hyposmotic, but not isosmotic conditions, while recordings from mammalian cells heterologously expressing KCNQ1 alone or KCNQ1 with the accessory subunit KCNE3 reveal a volume-sensitive K+ current, inhibited by 293B. These data suggest that KCNQ1 may play important physiological roles in the mammary epithelium, regulating cell volume and potentially mediating transepithelial K+ secretion.

Involvement of KATP and KvLQT1 K+ channels in EGF-stimulated alveolar epithelial cell repair processes

Several respiratory diseases are associated with extensive damage of lung epithelia, and the regulatory mechanisms involved in their regeneration are not clearly defined. Growth factors released by epithelial cells or fibroblasts from injured lungs are important regulators of alveolar repair by stimulating cell motility, proliferation, and differentiation. In addition, K(+) channels regulate cell proliferation/migration and are coupled with growth factor signaling in several tissues. We decided to explore the hypothesis, never investigated before, that K(+) could play a prominent role in alveolar repair. We employed a model of mechanical wounding of rat alveolar type II epithelia, in primary culture, to study their response to injury. Wound healing was suppressed by one-half upon epidermal growth factor (EGF) titration with EGF-antibody (Ab) or erbB1/erbB2 tyrosine-kinase inhibition with AG-1478/AG-825. The addition of exogenous EGF slightly stimulated the alveolar wound healing and enhanced, by up to five times, alveolar cell migration measured in a Boyden-type chamber. Conditioned medium collected from injured alveolar monolayers also stimulated cell migration; this effect was abolished in the presence of EGF-Ab. The impact of K(+) channel modulators was examined in basal and EGF-stimulated conditions. Wound healing was stimulated by pinacidil, an ATP-dependent K(+) channel (K(ATP)) activator, which also increased cell migration, by twofold, in basal conditions and potentiated the stimulatory effect of EGF. K(ATP) or KvLQT1 inhibitors (glibenclamide, clofilium) reduced EGF-stimulated wound healing, cell migration, and proliferation. Finally, EGF stimulated K(ATP) and KvLQT1 currents and channel expression. In summary, stimulation of K(+) channels through autocrine activation of EGF receptors could play a crucial role in lung epithelia repair processes.

Prostaglandin E2 enhances acetylcholine-induced, Ca2+-dependent ionic currents in swine tracheal mucous gland cells

Airway submucosal gland cell (SMGC) secretions are under the control of various neurotransmitters and hormones. Interactions between different pathways, such as those mediated by cAMP and Ca(2+), in controlling mucus or electrolyte secretions are not well understood. Prostaglandin E(2) (PGE(2)) or forskolin has been shown to enhance acetylcholine (ACh)-induced short circuit current (I(SC)) in SMGC mucous cell monolayers. We show that PGE(2), by activating cAMP-dependent protein kinase A (PKA), enhanced ACh-induced, Ca(2+)-mediated current and changes in [Ca(2+)](i) in mucous cells. PGE(2) pretreatment sensitized ACh-induced I(SC) (DeltaI(SC)) by activating endoprostanoid (EP(2)) receptors. PKA inhibitors 14-22 amide PKI (PKI) and Rp-diastereomer (Rp) of cAMPs prevented the effect of PGE(2). Removing external Ca(2+) or pretreatment with the Ca(2+) entry blocker, SKF96365 [1-[beta-(3-(4-methoxyphenyl) propoxy)-4-methoxyphenethyl]-1H-imidazole hydrochloride1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl) propoxy] ethyl] imidazole], shifted the concentration-response relationships for ACh to the right but did not abolish PGE(2)-induced sensitization of the ACh response. An inositol 1,4,5-trisphosphate (IP(3)) receptor antagonist and Ca(2+) entry blocker, 2-aminoethoxydiphenyl borate, abolished the ACh-induced response. Charybdotoxin, but not iberiotoxin (IbTX), inhibited the ACh-induced DeltaI(SC). Clotrimazole, but not IbTX, inhibited the ACh-induced serosal K(+) current. Under whole-cell patch clamp, ACh-induced K(+) and Cl(-) currents were coincident with increases in [Ca(2+)](i) in single mucous cells. PGE(2) or forskolin pretreatment did not induce current or [Ca(2+)](i) changes but enhanced ACh-induced currents, membrane hyperpolarization, and [Ca(2+)](i) changes. Intra-cellular dialysis with the PKA-catalytic subunit enhanced ACh-induced whole-cell current as well. These findings demonstrate that PGE(2), via EP(2) receptors and the cAMP/PKA pathway, activates Ca(2+) entry-independent mechanisms, possibly by increasing IP(3)-mediated Ca(2+) release, resulting in the sensitization of ACh-induced currents.