Inhibition of airway liquid secretion and its effect on the physical properties of airway mucus

PAR-2-activated secretion by airway gland serous cells: role for CFTR and inhibition by Pseudomonas aeruginosa

Airway submucosal gland serous cells are important sites of fluid secretion in conducting airways. Serous cells also express the cystic fibrosis (CF) transmembrane conductance regulator (CFTR). Protease-activated receptor 2 (PAR-2) is a G protein-coupled receptor that activates secretion from intact airway glands. We tested if and how human nasal serous cells secrete fluid in response to PAR-2 stimulation using Ca²⁺ imaging and simultaneous differential interference contrast imaging to track isosmotic cell shrinking and swelling reflecting activation of solute efflux and influx pathways, respectively. During stimulation of PAR-2, serous cells exhibited dose-dependent increases in intracellular Ca²⁺. At stimulation levels >EC₅₀ for Ca²⁺, serous cells simultaneously shrank ∼20% over ∼90 s due to KCl efflux reflecting Ca²⁺-activated Cl^- channel (CaCC, likely TMEM16A)-dependent secretion. At lower levels of PAR-2 stimulation (<EC₅₀ for Ca²⁺), shrinkage was not evident due to failure to activate CaCC. Low levels of cAMP-elevating VIP receptor (VIPR) stimulation, also insufficient to activate secretion alone, synergized with low-level PAR-2 stimulation to elicit fluid secretion dependent on both cAMP and Ca²⁺ to activate CFTR and K⁺ channels, respectively. Polarized cultures of primary serous cells also exhibited synergistic fluid secretion. Pre-exposure to Pseudomonas aeruginosa conditioned media inhibited PAR-2 activation by proteases but not peptide agonists in primary nasal serous cells, Calu-3 bronchial cells, and primary nasal ciliated cells. Disruption of synergistic CFTR-dependent PAR-2/VIPR secretion may contribute to reduced airway surface liquid in CF. Further disruption of the CFTR-independent component of PAR-2-activated secretion by P. aeruginosa may also be important to CF pathophysiology.

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

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

TMEM16A in Cystic Fibrosis: Activating or Inhibiting?

The inflammatory airway disease cystic fibrosis (CF) is characterized by airway obstruction due to mucus hypersecretion, airway plugging, and bronchoconstriction. The cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel is dysfunctional in CF, leading to defects in epithelial transport. Although CF pathogenesis is still disputed, activation of alternative Cl- channels is assumed to improve lung function in CF. Two suitable non-CFTR Cl- channels are present in the airway epithelium, the Ca2+ activated channel TMEM16A and SLC26A9. Activation of these channels is thought to be feasible to improve hydration of the airway mucus and to increase mucociliary clearance. Interestingly, both channels are upregulated during inflammatory lung disease. They are assumed to support fluid secretion, necessary to hydrate excess mucus and to maintain mucus clearance. During inflammation, however, TMEM16A is upregulated particularly in mucus producing cells, with only little expression in ciliated cells. Recently it was shown that knockout of TMEM16A in ciliated cells strongly compromises Cl- conductance and attenuated mucus secretion, but does not lead to a CF-like lung disease and airway plugging. Along this line, activation of TMEM16A by denufosol, a stable purinergic ligand, failed to demonstrate any benefit to CF patients in earlier studies. It rather induced adverse effects such as cough. A number of studies suggest that TMEM16A is essential for mucus secretion and possibly also for mucus production. Evidence is now provided for a crucial role of TMEM16A in fusion of mucus-filled granules with the apical plasma membrane and cellular exocytosis. This is probably due to local Ca2+ signals facilitated by TMEM16A. Taken together, TMEM16A supports fluid secretion by ciliated airway epithelial cells, but also maintains excessive mucus secretion during inflammatory airway disease. Because TMEM16A also supports airway smooth muscle contraction, inhibition rather than activation of TMEM16A might be the appropriate treatment for CF lung disease, asthma and COPD. As a number of FDA-approved and well-tolerated drugs have been shown to inhibit TMEM16A, evaluation in clinical trials appears timely.

Development of an airway mucus defect in the cystic fibrosis rat

The mechanisms underlying the development and natural progression of the airway mucus defect in cystic fibrosis (CF) remain largely unclear. New animal models of CF, coupled with imaging using micro-optical coherence tomography, can lead to insights regarding these questions. The Cftr-/- (KO) rat allows for longitudinal examination of the development and progression of airway mucus abnormalities. The KO rat exhibits decreased periciliary depth, hyperacidic pH, and increased mucus solid content percentage; however, the transport rates and viscoelastic properties of the mucus are unaffected until the KO rat ages. Airway submucosal gland hypertrophy develops in the KO rat by 6 months of age. Only then does it induce increased mucus viscosity, collapse of the periciliary layer, and delayed mucociliary transport; stimulation of gland secretion potentiates this evolution. These findings could be reversed by bicarbonate repletion but not pH correction without counterion donation. These studies demonstrate that abnormal surface epithelium in CF does not cause delayed mucus transport in the absence of functional gland secretions. Furthermore, abnormal bicarbonate transport represents a specific target for restoring mucus clearance, independent of effects on periciliary collapse. Thus, mature airway secretions are required to manifest the CF defect primed by airway dehydration and bicarbonate deficiency.

Both Ways at Once: Keeping Small Airways Clean

The small airways of the lungs are under constant assault from the pathogens and debris in the air that they must conduct to alveoli. Although hygiene is of paramount importance for respiratory health, the underlying principles of airway clearance have not been well integrated or established. Newly emerging concepts of simultaneous absorption and secretion of airway surface liquid (ASL) and the role of [Formula: see text] in the maturation of mucins have advanced from experimental evidence as well as observations from the congenital disease cystic fibrosis (CF) to present a novel model that integrates microanatomy with organ physiology to meet the constant challenge of cleaning small airways.

Defective postsecretory maturation of MUC5B mucin in cystic fibrosis airways

In cystic fibrosis (CF), airway mucus becomes thick and viscous, and its clearance from the airways is impaired. The gel-forming mucins undergo an ordered "unpacking/maturation" process after granular release that requires an optimum postsecretory environment, including hydration and pH. We hypothesized that this unpacking process is compromised in the CF lung due to abnormal transepithelial fluid transport that reduces airway surface hydration and alters ionic composition. Using human tracheobronchial epithelial cells derived from non-CF and CF donors and mucus samples from human subjects and domestic pigs, we investigated the process of postsecretory mucin unfolding/maturation, how these processes are defective in CF airways, and the probable mechanism underlying defective unfolding. First, we found that mucins released into a normal lung environment transform from a compact granular form to a linear form. Second, we demonstrated that this maturation process is defective in the CF airway environment. Finally, we demonstrated that independent of HCO₃^- and pH levels, airway surface dehydration was the major determinant of this abnormal unfolding process. This defective unfolding/maturation process after granular release suggests that the CF extracellular environment is ion/water depleted and likely contributes to abnormal mucus properties in CF airways prior to infection and inflammation.

Acidic pH increases airway surface liquid viscosity in cystic fibrosis

Cystic fibrosis (CF) disrupts respiratory host defenses, allowing bacterial infection, inflammation, and mucus accumulation to progressively destroy the lungs. Our previous studies revealed that mucus with abnormal behavior impaired mucociliary transport in newborn CF piglets prior to the onset of secondary manifestations. To further investigate mucus abnormalities, here we studied airway surface liquid (ASL) collected from newborn piglets and ASL on cultured airway epithelia. Fluorescence recovery after photobleaching revealed that the viscosity of CF ASL was increased relative to that of non-CF ASL. CF ASL had a reduced pH, which was necessary and sufficient for genotype-dependent viscosity differences. The increased viscosity of CF ASL was not explained by pH-independent changes in HCO3- concentration, altered glycosylation, additional pH-induced disulfide bond formation, increased percentage of nonvolatile material, or increased sulfation. Treating acidic ASL with hypertonic saline or heparin largely reversed the increased viscosity, suggesting that acidic pH influences mucin electrostatic interactions. These findings link loss of cystic fibrosis transmembrane conductance regulator-dependent alkalinization to abnormal CF ASL. In addition, we found that increasing Ca2+ concentrations elevated ASL viscosity, in part, independently of pH. The results suggest that increasing pH, reducing Ca2+ concentration, and/or altering electrostatic interactions in ASL might benefit early CF.

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.

Resveratrol ameliorates abnormalities of fluid and electrolyte secretion in a hypoxia-Induced model of acquired CFTR deficiency

OBJECTIVE/HYPOTHESIS

Ineffective mucociliary clearance (MCC) is a common pathophysiologic process that underlies airway inflammation and infection. A dominant fluid and electrolyte secretory pathway in the nasal airways is governed by the cystic fibrosis transmembrane conductance regulator (CFTR). Decreased transepithelial Cl(-) transport secondary to an acquired CFTR deficiency may exacerbate respiratory epithelial dysfunction by diminishing MCC and increasing mucus viscosity. The objectives of the present study are to 1) develop a model of acquired CFTR deficiency in sinonasal epithelium using hypoxia, 2) investigate whether the polyphenol resveratrol promotes CFTR-mediated anion transport, 3) explore resveratrol mechanism of action and determine therapeutic suitability for overcoming acquired CFTR defects, and 4) test the drug in the hypoxic model of acquired CFTR deficiency in preparation for a clinical trial in human sinus disease. We hypothesize that hypoxia will induce depletion of airway surface liquid (ASL) secondary to acquired CFTR deficiency and that resveratrol will restore transepithelial Cl(-) secretion and recover ASL hydration.

STUDY DESIGN

Basic science.

METHODS

Murine nasal septal (MNSE) and human sinonasal epithelial (HSNE) cultures were incubated under hypoxic conditions (1% O2 , 5% CO2 ) and transepithelial ion transport (change in short-circuit current = ΔISC ) evaluated in Ussing chambers. Resveratrol was tested using primary cells and HEK293 cells expressing human CFTR by Ussing chamber and patch clamp techniques under both phosphorylating and nonphosphorylating conditions. CFTR activation was evaluated in human explants and by murine in vivo (nasal potential difference) assessment. Cellular cyclic adenosine monophosphate (cAMP) (ELISA) and subsequent CFTR regulatory domain (R-D) phosphorylation (gel-shift assay) were also evaluated. Effects of hypoxia and resveratrol on ASL were tested using confocal laser scanning microscopy (CLSM) and micro-optical coherence tomography (µOCT).

RESULTS

Hypoxia significantly decreased ΔISC (in µA/cm(2) ) attributable to CFTR at 12 and 24 hours of exposure in both MNSE (13.55 ± 0.46 [12 hours]; 12.75 ± 0.07 [24 hours] vs. 19.23 ± 0.18 [control]; P < 0.05) and HSNE (19.55 ± 0.56 [12 hours]; 17.67 ± 1.13 [24 hours] vs. 25.49 ± 1.48 [control]; P < 0.05). We have shown that resveratrol (100 μM) enhanced CFTR-dependent Cl(-) secretion in HSNE to an extent comparable to the recently Food and Drug Administration-approved CFTR potentiator, ivacaftor. Cl(-) transport across human sinonasal explants (78.42 ± 1.75 vs. 1.75 ± 1.5 [control]; P < 0.05) and in vivo murine nasal epithelium (-4 ± 1.8 vs. -0.8 ± 1.7 mV [control]; P < 0.05) were also significantly increased by the drug. No increase in cAMP or CFTR R-D phosphorylation was detected. Inside-out patches showed increased CFTR open probability (NPo/N (N = channel number]) compared to controls in both MNSE (0.329 ± 0.116 vs. 0.119 ± 0.059 [control]; P < 0.05) and HEK293 cells (0.22 ± 0.048 vs. 0.125 ± 0.07 [control]; P < 0.05). ASL thickness was decreased under hypoxic conditions when measured by CLSM (4.19 ± 0.44 vs. 6.88 ± 0.67 [control]; P < 0.05). A 30-minute apical application of resveratrol increased ASL depth in normal epithelium (8.08 ± 1.68 vs. 6.11 ± 0.47 [control]; P < 0.05). Furthermore, hypoxia-induced abnormalities of fluid and electrolyte secretion in sinonasal epithelium were restored with resveratrol treatment (5.55 ± 0.74 vs. 3.13 ± 0.17 [control]; P < 0.05).

CONCLUSIONS

CFTR activation with a leading edge Cl(-) secretagogue such as resveratrol represents an innovative approach to overcoming acquired CFTR defects in sinus and nasal airway disease. This exciting new strategy bears further testing in non-CF individuals with chronic rhinosinusitis.

LEVEL OF EVIDENCE

N/A. Laryngoscope, 125:S1-S13, 2015.

Model demonstrates functional purpose of the nasal cycle

Background

Despite the occurrence of the nasal cycle being well documented, the functional purpose of this phenomenon is not well understood. This investigation seeks to better understand the physiological objective of the nasal cycle in terms of airway health through the use of a computational nasal air-conditioning model.

Method

A new state-variable heat and water mass transfer model is developed to predict airway surface liquid (ASL) hydration status within each nasal airway. Nasal geometry, based on in-vivo magnetic resonance imaging (MRI) data is used to apportion inter-nasal air flow.

Results

The results demonstrate that the airway conducting the majority of the airflow also experiences a degree of ASL dehydration, as a consequence of undertaking the bulk of the heat and water mass transfer duties. In contrast, the reduced air conditioning demand within the other airway allows its ASL layer to remain sufficiently hydrated so as to support continuous mucociliary clearance.

Conclusions

It is quantitatively demonstrated in this work how the nasal cycle enables the upper airway to accommodate the contrasting roles of air conditioning and the removal of entrapped contaminants through fluctuation in airflow partitioning between each airway.

Mucociliary clearance and submucosal gland secretion in the ex vivo ferret trachea

In many species submucosal glands are an important source of tracheal mucus, but the extent to which mucociliary clearance (MCC) depends on gland secretion is unknown. To explore this relationship, we measured basal and agonist-stimulated MCC velocities in ex vivo tracheas from adult ferrets and compared the velocities with previously measured rates of ferret glandular mucus secretion (Cho HJ, Joo NS, Wine JJ. Am J Physiol Lung Cell Mol Physiol 299: L124-L136, 2010). Stimulated MCC velocities (mm/min, means ± SE for 10- to 35-min period poststimulation) were as follows: 1 μM carbachol: 19.1 ± 3.3 > 10 μM phenylephrine: 15.3 ± 2.4 ≈ 10 μM isoproterenol: 15.0 ± 1.9 ≈ 10 μM forskolin: 14.6 ± 3.1 > 1 μM vasoactive intestinal peptide (VIP): 10.2 ± 2.2 >> basal (t15): 1.8 ± 0.3; n = 5-10 for each condition. Synergistic stimulation of MCC was observed between low concentrations of carbachol (100 nM) and isoproterenol (300 nM). Bumetanide inhibited carbachol-stimulated MCC by ~70% and abolished the increase in MCC stimulated by forskolin + VIP, whereas HCO3 (-)-free solutions did not significantly inhibit MCC to either intracellular Ca(2+) concentration or intracellular cAMP concentration ([cAMP]i)-elevating agonists. Stimulation and inhibition of MCC and gland secretion differed in several respects: most importantly, elevating [cAMP]i increased MCC much more effectively than expected from its effects on gland secretion, and bumetanide almost completely inhibited [cAMP]i-stimulated MCC while it had a smaller effect on gland secretion. We conclude that changes in glandular fluid secretion are complexly related to MCC and discuss possible reasons for this.

A pilot study of an in-vitro bovine trachea model of the effect of continuous positive airway pressure breathing on airway surface liquid

Background

Continuous positive air pressure (CPAP) users frequently report troublesome symptoms of airway dryness and nasal congestion. Clinical investigations have demonstrated that supplementary humidification reduces these symptoms but the reason for their occurrence remains unexplained. Investigations using human computational air-conditioning models are unable to reproduce or quantify the apparent airway drying experienced during CPAP therapy. The purpose of this study was to determine whether augmented air pressures change overall mucosal airway surface liquid (ASL) water supply and, if so, the extent of this effect.

Method

In an original in vitro experimental set up, maximal ASL supply was determined in whole bovine trachea when exposed to simulated tidal breathing stresses over a range of air pressures.

Results

At ambient pressure, the maximal supply of ASL was found to compare well to previously published data (31.2 μl/cm².hr). CPAP pressures from 5 cm H₂O above ambient were found to reduce ASL supply by 22%. Statistical analysis (n = 8) showed a significant difference existed between the ambient and CPAP results (p < 0.0001), and that there was no significant variation between all pressurized results (p = 0.716).

Conclusions

These findings provide preliminary data that ASL supply is reduced by CPAP therapy which may explain the airway drying symptoms associated with this therapy.

Comparison of cystic fibrosis transmembrane conductance regulator (CFTR) and ciliary beat frequency activation by the CFTR Modulators Genistein, VRT-532, and UCCF-152 in primary sinonasal epithelial cultures

IMPORTANCE

Pharmacologic activation of mucociliary clearance (MCC) represents an emerging therapeutic strategy for patients with chronic rhinosinusitis, even in the absence of congenital mutations of the CFTR gene. Drug discovery efforts have identified small molecules that activate the cystic fibrosis transmembrane conductance regulator (CFTR), including potentiators under development for treatment of cystic fibrosis.

OBJECTIVE

To evaluate the properties of CFTR modulators and their effects on ciliary beat frequency (CBF) in human sinonasal epithelium (HSNE).

DESIGN

Primary HSNE cultures (wild type and F508del/F508del) were used to compare stimulation of CFTR-mediated Cl- conductance and CBF by the CFTR modulators genistein, VRT-532, and UCCF-152.

MAIN OUTCOMES AND MEASURES

Increase in CFTR-dependent anion transport and CBF.

RESULTS

HSNE cultures were analyzed using pharmacologic manipulation of ion transport (change in short-circuit current [∆ISC]) and high-speed digital imaging (CBF). Activation of CFTR-dependent anion transport was significantly different among agonists (P < .001), with genistein exerting the greatest effect (mean [SD] ∆ISC, genistein, 23.1 [1.8] μA/cm2² > VRT-532, 8.1 [1.0] μA/cm² > UCCF-152, 3.4 [1.4] μA/cm² > control, 0.7 [0.2] μA/cm²; Tukey-Kramer P < .05) in the absence of forskolin. Genistein and UCCF-152 augmented CBF (under submerged conditions) significantly better (Tukey-Kramer P < .05) than cells treated with VRT-532 or dimethyl sulfoxide vehicle control (mean [SD] fold change over baseline, genistein, 1.63 [0.06]; UCCF-152, 1.56 [0.06]; VRT-532, 1.38 [0.08]; control, 1.27 [0.02]). Activation of CBF was blunted in F508del/F508del HSNE cultures.

CONCLUSIONS AND RELEVANCE

The degree of CBF stimulation was not dependent on the magnitude of Cl- secretion, suggesting that different mechanisms of action may underlie MCC activation by these small molecule potentiators. Agents that activate both CFTR-dependent ISC and CBF are particularly attractive as therapeutics because they may address 2 independent pathways that contribute to deficient MCC in chronic rhinosinusitis.

The Novel Dry Extract BNO 1011 Stimulates Chloride Transport and Ciliary Beat Frequency in Human Respiratory Epithelial Cultures

Background

Herbal remedies predate written history and continue to be used more frequently than conventional pharmaceutical medications. The novel dry extract BNO 1011 is based on a combination of five herbs that is used to treat acute and chronic rhinosinusitis. We evaluated the pharmacologic effects of the novel dry extract BNO 1011 on human respiratory epithelial cultures specifically addressing electrolyte transport and cilia beat frequency (CBF).

Methods

Well-differentiated human bronchial epithelial cultures grown at an air–liquid interface were treated on the apical or basolateral surface with varying concentrations of dry extract BNO 1011. Changes in transepithelial sodium and chloride transport were determined in Ussing chambers under voltage-clamped conditions. Changes in CBF were determined using the Sissons-Ammons Video Analysis system (Ammons Engineering, Mt. Morris, MI).

Results

When applied to the apical surface, dry extract BNO 1011 activated forskolin-stimulated chloride secretion and ciliary beat in a dose-dependent fashion. Basolateral application of dry extract BNO 1011 did not alter the measured physiological properties.

Conclusion

Apical application of dry extract BNO 1011 stimulates both chloride secretion and CBF and therefore may augment mucociliary clearance.

Mechanisms of bicarbonate secretion: lessons from the airways

Early studies showed that airway cells secrete HCO(3)(-) in response to cAMP-mediated agonists and HCO(3)(-) secretion was impaired in cystic fibrosis (CF). Studies with Calu-3 cells, an airway serous model with high expression of CFTR, also show the secretion of HCO(3)(-) when cells are stimulated with cAMP-mediated agonists. Activation of basolateral membrane hIK-1 K(+) channels inhibits HCO(3)(-) secretion and stimulates Cl(-) secretion. CFTR mediates the exit of both HCO(3)(-) and Cl(-) across the apical membrane. Entry of HCO(3)(-) on a basolateral membrane NBC or Cl(-) on the NKCC determines which anion is secreted. Switching between these two secreted anions is determined by the activity of hIK-1 K(+) channels.

Cystic fibrosis transmembrane conductance regulator modulation by the tobacco smoke toxin acrolein

OBJECTIVES/HYPOTHESIS

Evidence indicates that decreased mucociliary clearance (MCC) is a major contributing feature to chronic rhinosinusitis. Tobacco-smoke exposure is thought to inhibit transepithelial Cl(-) secretion, a major determinant of airway surface liquid hydration and MCC. The objective of the current study was to evaluate the effects of acrolein exposure (a prominent tobacco smoke toxin) on vectorial Cl(-) transport through the major apical anion channel cystic fibrosis transmembrane conductance regulator (CFTR) in sinonasal epithelium.

STUDY DESIGN

In vitro investigation.

METHODS

Primary murine nasal septal epithelia (MNSE; wild-type and transgenic CFTR(-/-)) cultures were exposed to acrolein in Ussing chambers and the effects on Cl(-) secretion investigated using pharmacologic manipulation. Cellular cyclic adenosine monophosphate (cAMP) signaling and cytotoxicity were also investigated.

RESULTS

Acrolein stimulated Cl(-) secretion (ΔI(SC) - change in short-circuit current in μA/cm(2)) at concentrations similar to smoker's airways (100 μM, 15.8 ± 2.2 vs. 2.4 ± 0.8 [control]; P < .0001), suppressed forskolin-stimulated C- transport at 300 μM (13.3 ± 1.2 vs. 19.9 ± 1.0; P < .01), and completely abolished all transport at 500 μM (-1.1 ± 1.6). Stimulated Cl(-) secretion was solely reliant upon the presence of CFTR (confirmed in transgenic CFTR(-/-) MNSE), but independent of cAMP signaling. Inhibition at higher concentrations was not secondary to cellular cytotoxicity.

CONCLUSIONS

The present study demonstrates that acrolein has complex but pronounced interaction with the major apical Cl(-) transport mechanism that uses CFTR. Further investigations are required to determine acrolein's impact as a tobacco smoke constituent on mucociliary transport.

Transepithelial ion transport is suppressed in hypoxic sinonasal epithelium

OBJECTIVES/HYPOTHESIS

Sinonasal respiratory epithelial mucociliary clearance is dependent on the transepithelial transport of ions such as Cl(-) . The objectives of the present study were to investigate the role of oxygen restriction in 1) Cl(-) transport across primary sinonasal epithelial monolayers, 2) expression of the apical Cl(-) channels cystic fibrosis transmembrane conductance regulator (CFTR) and transmembrane protein 16A (TMEM16A), and 3) the pathogenesis of chronic rhinosinusitis.

STUDY DESIGN

In vitro investigation.

METHODS

Murine nasal septal epithelial (MNSE), wild type, and human sinonasal epithelial (HSNE) cultures were incubated under hypoxic conditions (1% O(2) , 5% CO(2) ). Cultures were mounted in Ussing chambers for ion transport measurements. CFTR and TMEM16A expression were measured using quantitative reverse-transcription polymerase chain reaction (RT-PCR).

RESULTS

The change in short-circuit current (ΔI(SC) in microamperes per square centimeter) attributable to CFTR (forskolin-stimulated) was significantly decreased due to a 12-hour hypoxia exposure in both MNSE (13.55 ± 0.46 vs. 19.23 ± 0.18) and HSNE (19.55 ± 0.56 vs. 25.49 ± 1.48 [control]; P < .05). TMEM16A (uridine triphosphate-stimulated transport) was inhibited by 48 hours of hypoxic exposure in MNSE (15.92 ± 2.87 vs. 51.44 ± 3.71 [control]; P < .05) and by 12 hours of hypoxic exposure in HSNE (16.75 ± 0.68 vs. 24.15 ± 1.35 [control]). Quantitative RT-PCR (reported as relative mRNA levels ± standard deviation) demonstrated significant reductions in both CFTR and TMEM16A mRNA expression in MNSE and HSNE owing to airway epithelial hypoxia.

CONCLUSIONS

Sinonasal epithelial CFTR and TMEM16A-mediated Cl(-) transport and mRNA expression were robustly decreased in an oxygen-restricted environment. These findings indicate that persistent hypoxia may lead to acquired defects in sinonasal Cl(-) transport in a fashion likely to confer mucociliary dysfunction in chronic rhinosinusitis.

Hesperidin stimulates cystic fibrosis transmembrane conductance regulator-mediated chloride secretion and ciliary beat frequency in sinonasal epithelium

OBJECTIVE

Pharmacologic agents designed to promote mucociliary clearance (MCC) in chronic rhinosinusitis (CRS) represent a novel therapeutic strategy. The objectives of the present study were to investigate whether the natural bioflavonoid hesperidin 1) increases transepithelial chloride (Cl(-)) secretion in vitro and in vivo, 2) enhances ciliary beat frequency (CBF), and 3) exerts its mechanistic effects through cAMP/PKA-dependent pathways.

STUDY DESIGN

In vitro and in vivo study.

SETTING

Laboratory.

SUBJECTS AND METHODS

Transepithelial Cl(-) transport (Ussing chamber) and CBF were investigated in primary murine nasal septal (MNSE) and human sinonasal epithelial (HSNE) cultures. In vivo activity was measured using the murine nasal potential difference (NPD) assay, cystic fibrosis transmembrane conductance regulator (CFTR) R-domain phosphorylation, and cAMP levels were investigated to rule out a cAMP/PKA-dependent mechanism of activation.

RESULTS

Hesperidin significantly increased CFTR-mediated Cl(-) transport (change in short-circuit current, DeltaI(SC)) in both MNSE (13.51 +/- 0.77 vs 4.4 +/- 0.66 [control]; P < 0.05) and HSNE (12.28 +/- 1.08 vs 0.69 +/- 0.32 [control]; P < 0.05). Cl(-) transport across in vivo murine nasal epithelium was also significantly enhanced with hesperidin (-2.3 +/- 1.0 vs -0.8 +/- 0.8 mV [control], P < 0.05). There was no increase in cellular cAMP or phosphorylation of the CFTR R-domain. Hesperidin significantly increased CBF (ratio of pretreatment to post-treatment) with both basal (1.31 +/- 0.07 vs 0.93 +/- 0.06 [control]; P < 0.05), apical (1.72 +/- 0.09 vs 1.40 +/- 0.07 [control]; P < 0.05), and basal + apical delivery (2.26 +/- 0.18 vs 1.60 +/- 0.21, respectively; P < 0.05).

CONCLUSION

Our in vitro and in vivo investigations provide strong support for future testing of this robust Cl(-) secretagogue and CBF activator in human clinical trials for CRS.

Exposure to cigarette smoke condensate reduces calcium activated chloride channel transport in primary sinonasal epithelial cultures

OBJECTIVES/HYPOTHESIS

The cystic fibrosis transmembrane conductance regulator (CFTR) serves as a predominant Cl(-) transport conduit in airway epithelium and is inhibited by cigarette smoke in vitro and in vivo. Activation of secondary Cl(-) transport pathways through calcium-activated Cl(-) channels (CaCC) has been postulated as a mechanism to bypass defects in CFTR-mediated transport. Because it is not known whether CaCCs are also inhibited by tobacco exposure, the current study was designed to investigate the effect of cigarette smoke condensate (CSC) on CaCC transport.

STUDY DESIGN

In vitro study.

METHODS

Well-characterized primary murine nasal septal epithelial (MNSE) and human sinonasal epithelial (HSNE) cultures were exposed to CSC in Ussing chambers. We monitored CaCC short-circuit current through stimulation of P2Y purinergic receptors with uridine triphosphate or adenosine triphosphate and selective inhibition of the CFTR-dependent secretory pathway. Characterization of CaCC current was also accomplished in primary airway cells derived from transgenic CFTR(-/-) (knockout) murine models.

RESULTS

Change in CaCC-mediated current (DeltaI(SC) representing transepithelial Ca-mediated Cl(-) secretion in muA/cm(2)) was significantly decreased in CSC-exposed wild type MNSE when compared to controls (32.8 +/- 4.6 vs. 47.5 +/- 2.3; respectively; P < .02). A similar effect was demonstrated in CFTR(-/-) MNSE cultures (33.4 +/- 2.8 vs. 38.6 +/- 2.0; P < .05>. HSNE cultures also had a significant reduction in I(SC) (16.1 +/- 0.6 vs. 22.7 +/- 0; P = .008).

CONCLUSIONS

CSC affects multiple pathways fundamental to airway ion transport, including both cyclic adenosine monophosphate and calcium activated Cl(-) transport. Inhibition of Cl(-) transport may contribute to common diseases of the airways, such as chronic rhinosinusitis and chronic obstructive pulmonary disease.

Comparison of vectorial ion transport in primary murine airway and human sinonasal air-liquid interface cultures, models for studies of cystic fibrosis, and other airway diseases

BACKGROUND

The purpose of this study was to compare vectorial ion transport within murine trachea, murine nasal septa, and human sinonasal cultured epithelium. Our hypothesis is that murine septal epithelium, rather than trachea, will more closely mimic the electrophysiology properties of human sinonasal epithelium.

METHODS

Epithelium from murine trachea, murine septa, and human sinonasal tissue were cultured at an air-liquid interface to confluence and full differentiation. A limited number of homozygous dF508 epithelia were also cultured. Monolayers were mounted in modified Ussing chambers to investigate pharmacologic manipulation of ion transport.

RESULTS

The change in forskolin-stimulated current (delta-I(SC), expressed as micro-A/cm(2)) in murine septal (n = 19; 16.84 +/- 2.09) and human sinonasal (n = 18; 12.15 +/- 1.93) cultures was significantly increased over murine tracheal cultures (n = 15; 6.75 +/- 1.35; p = 0.035 and 0.0005, respectively). Forskolin-stimulated I(SC) was inhibited by the specific cystic fibrosis transmembrane regulator (CFTR) inhibitor INH-172 (5 microM). No forskolin-stimulated I(SC) was shown in cultures of dF508 homozygous murine septal epithelium (n = 3). Murine septal I(SC) was largely inhibited by amiloride (12.03 +/- 0.66), whereas human sinonasal cultures had a very limited response (0.70 +/- 0.47; p < 0.0001). The contribution of CFTR to stimulated chloride current as measured by INH-172 was highly significantly different between all groups (murine septa, 19.51 +/- 1.28; human sinonasal, 11.12 +/- 1.58; murine trachea, 4.85 +/- 0.49; p < 0.0001).

CONCLUSION

Human sinonasal and murine septal epithelial cultures represent a useful model for studying CFTR activity and may provide significant advantages over lower airway tissues for investigating upper and lower respiratory pathophysiology.

Substance P stimulates human airway submucosal gland secretion mainly via a CFTR-dependent process

Chronic bacterial airway infections are the major cause of mortality in cystic fibrosis (CF). Normal airway defenses include reflex stimulation of submucosal gland mucus secretion by sensory neurons that release substance P (SubP). CFTR is an anion channel involved in fluid secretion and mutated in CF; the role of CFTR in secretions stimulated by SubP is unknown. We used optical methods to measure SubP-mediated secretion from human submucosal glands in lung transplant tissue. Glands from control but not CF subjects responded to mucosal chili oil. Similarly, serosal SubP stimulated secretion in more than 60% of control glands but only 4% of CF glands. Secretion triggered by SubP was synergistic with vasoactive intestinal peptide and/or forskolin but not with carbachol; synergy was absent in CF glands. Pig glands demonstrated a nearly 10-fold greater response to SubP. In 10 of 11 control glands isolated by fine dissection, SubP caused cell volume loss, lumen expansion, and mucus flow, but in 3 of 4 CF glands, it induced lumen narrowing. Thus, in CF, the reduced ability of mucosal irritants to stimulate airway gland secretion via SubP may be another factor that predisposes the airways to infections.

HCO3(-) secretion by murine nasal submucosal gland serous acinar cells during Ca2+-stimulated fluid secretion

Airway submucosal glands contribute to airway surface liquid (ASL) composition and volume, both important for lung mucociliary clearance. Serous acini generate most of the fluid secreted by glands, but the molecular mechanisms remain poorly characterized. We previously described cholinergic-regulated fluid secretion driven by Ca²⁺-activated Cl⁻ secretion in primary murine serous acinar cells revealed by simultaneous differential interference contrast (DIC) and fluorescence microscopy. Here, we evaluated whether Ca²⁺-activated Cl⁻ secretion was accompanied by secretion of HCO₃⁻, possibly a critical ASL component, by simultaneous measurements of intracellular pH (pH_i) and cell volume. Resting pH_i was 7.17 ± 0.01 in physiological medium (5% CO₂–25 mM HCO₃⁻). During carbachol (CCh) stimulation, pH_i fell transiently by 0.08 ± 0.01 U concomitantly with a fall in Cl⁻ content revealed by cell shrinkage, reflecting Cl⁻ secretion. A subsequent alkalinization elevated pH_i to above resting levels until agonist removal, whereupon it returned to prestimulation values. In nominally CO₂–HCO₃⁻-free media, the CCh-induced acidification was reduced, whereas the alkalinization remained intact. Elimination of driving forces for conductive HCO₃⁻ efflux by ion substitution or exposure to the Cl⁻ channel inhibitor niflumic acid (100 μM) strongly inhibited agonist-induced acidification by >80% and >70%, respectively. The Na⁺/H⁺ exchanger (NHE) inhibitor dimethylamiloride (DMA) increased the magnitude (greater than twofold) and duration of the CCh-induced acidification. Gene expression profiling suggested that serous cells express NHE isoforms 1–4 and 6–9, but pharmacological sensitivities demonstrated that alkalinization observed during both CCh stimulation and pH_i recovery from agonist-induced acidification was primarily due to NHE1, localized to the basolateral membrane. These results suggest that serous acinar cells secrete HCO₃⁻ during Ca²⁺-evoked fluid secretion by a mechanism that involves the apical membrane secretory Cl⁻ channel, with HCO₃⁻ secretion sustained by activation of NHE1 in the basolateral membrane. In addition, other Na⁺-dependent pH_i regulatory mechanisms exist, as evidenced by stronger inhibition of alkalinization in Na⁺-free media.

Effects of purinergic stimulation on ciliary beat frequency and chloride secretion in sinusitis

OBJECTIVES

We aimed to identify the functional abnormality of sinusitis-affected mucosa by observing the responsiveness of the mucosa to purinergic stimulation after the onset of sinusitis and during the recovery period. We also aimed to identify possible beneficial effects of purinergic agonists on sinusitis.

METHODS

A rabbit sinusitis model was developed by blocking maxillary ostia. Sinus mucosae were harvested immediately and 1 and 4 weeks after reopening the ostia. We measured chloride secretion and ciliary beat frequencies responding to purinergic stimulation.

RESULTS

The increases of ciliary beat frequency by adenosine triphosphate (100 micromol/L) were 3.2%+/-8.5%, 7.9%+/-2.3%, and 12.2%+/-1.9% immediately after establishment of sinusitis and 1 week and 4 weeks after reopening of ostia, respectively. Chloride secretion stimulated by adenosine triphosphate also showed gradual increase during the recovery period. Grossly, the mucosae appeared to have normalized in 80% (4 of 5) after 4 weeks; however, functional and microscopic improvements were still incomplete.

CONCLUSIONS

Mucosal functions, assessed by increase of ciliary activity and ion secretion by purinergic stimulation, and microscopic findings showed gradual but incomplete recovery after 4 weeks of recovery, in contrast to the gross normalization. Purinergic agonists may have beneficial effects on sinusitis by stimulating decreased ciliary motility and chloride secretion in sinusitis.

Synergistic airway gland mucus secretion in response to vasoactive intestinal peptide and carbachol is lost in cystic fibrosis

Cystic fibrosis (CF) is caused by dysfunction of the CF transmembrane conductance regulator (CFTR), an anion channel whose dysfunction leads to chronic bacterial and fungal airway infections via a pathophysiological cascade that is incompletely understood. Airway glands, which produce most airway mucus, do so in response to both acetylcholine (ACh) and vasoactive intestinal peptide (VIP). CF glands fail to secrete mucus in response to VIP, but do so in response to ACh. Because vagal cholinergic pathways still elicit strong gland mucus secretion in CF subjects, it is unclear whether VIP-stimulated, CFTR-dependent gland secretion participates in innate defense. It was recently hypothesized that airway intrinsic neurons, which express abundant VIP and ACh, are normally active and stimulate low-level gland mucus secretion that is a component of innate mucosal defenses. Here we show that low levels of VIP and ACh produced significant mucus secretion in human glands via strong synergistic interactions; synergy was lost in glands of CF patients. VIP/ACh synergy also existed in pig glands, where it was CFTR dependent, mediated by both Cl(-) and HCO(3) (-), and clotrimazole sensitive. Loss of "housekeeping" gland mucus secretion in CF, in combination with demonstrated defects in surface epithelia, may play a role in the vulnerability of CF airways to bacterial infections.

Fluid secretion by submucosal glands of the tracheobronchial airways

Submucosal glands of the tracheobronchial airways provide the important functions of secreting mucins, antimicrobial substances, and fluid. This review focuses on the ionic mechanism and regulation of gland fluid secretion and examines the possible role of gland dysfunction in the lethal disease cystic fibrosis (CF). The fluid component of gland secretion is driven by the active transepithelial secretion of both Cl(-) and HCO(3)(-) by serous cells. Gland fluid secretion is neurally regulated with acetylcholine, substance P, and vasoactive intestinal peptide (VIP) playing prominent roles. The cystic fibrosis transmembrane conductance regulator (CFTR) is present in the apical membrane of gland serous cells and mediates the VIP-induced component of liquid secretion whereas the muscarinic component of liquid secretion appears to be at least partially CFTR-independent. Loss of CFTR function, which occurs in CF disease, reduces the capacity of glands to secrete fluid but not mucins. The possible links between the loss of fluid secretion capability and the complex airway pathology of CF are discussed.

Parasympathetic control of airway submucosal glands: central reflexes and the airway intrinsic nervous system

Airway submucosal glands produce the mucus that lines the upper airways to protect them against insults. This review summarizes evidence for two forms of gland secretion, and hypothesizes that each is mediated by different but partially overlapping neural pathways. Airway innate defense comprises low level gland secretion, mucociliary clearance and surveillance by airway-resident phagocytes to keep the airways sterile in spite of nearly continuous inhalation of low levels of pathogens. Gland secretion serving innate defense is hypothesized to be under the control of intrinsic (peripheral) airway neurons and local reflexes, and these may depend disproportionately on non-cholinergic mechanisms, with most secretion being produced by VIP and tachykinins. In the genetic disease cystic fibrosis, airway glands no longer secrete in response to VIP alone and fail to show the synergy between VIP, tachykinins and ACh that is observed in normal glands. The consequent crippling of the submucosal gland contribution to innate defense may be one reason that cystic fibrosis airways are infected by mucus-resident bacteria and fungi that are routinely cleared from normal airways. By contrast, the acute (emergency) airway defense reflex is centrally mediated by vagal pathways, is primarily cholinergic, and stimulates copious volumes of gland mucus in response to acute, intense challenges to the airways, such as those produced by very vigorous exercise or aspiration of foreign material. In cystic fibrosis, the acute airway defense reflex can still stimulate the glands to secrete large amounts of mucus, although its properties are altered. Importantly, treatments that recruit components of the acute reflex, such as inhalation of hypertonic saline, are beneficial in treating cystic fibrosis airway disease. The situation for recipients of lung transplants is the reverse; transplanted airways retain the airway intrinsic nervous system but lose centrally mediated reflexes. The consequences of this for gland secretion and airway defense are poorly understood, but it is possible that interventions to modify submucosal gland secretion in transplanted lungs might have therapeutic consequences.

A physical linkage between cystic fibrosis airway surface dehydration and Pseudomonas aeruginosa biofilms

A vexing problem in cystic fibrosis (CF) pathogenesis has been to explain the high prevalence of Pseudomonas aeruginosa biofilms in CF airways. We speculated that airway surface liquid (ASL) hyperabsorption generates a concentrated airway mucus that interacts with P. aeruginosa to promote biofilms. To model CF vs. normal airway infections, normal (2.5% solids) and CF-like concentrated (8% solids) mucus were prepared, placed in flat chambers, and infected with an approximately 5 x 10(3) strain PAO1 P. aeruginosa. Although bacteria grew to 10(10) cfu/ml in both mucus concentrations, macrocolony formation was detected only in the CF-like (8% solids) mucus. Biophysical and functional measurements revealed that concentrated mucus exhibited properties that restrict bacterial motility and small molecule diffusion, resulting in high local bacterial densities with high autoinducer concentrations. These properties also rendered secondary forms of antimicrobial defense, e.g., lactoferrin, ineffective in preventing biofilm formation in a CF-like mucus environment. These data link airway surface liquid hyperabsorption to the high incidence of P. aeruginosa biofilms in CF via changes in the hydration-dependent physical-chemical properties of mucus and suggest that the thickened mucus gel model will be useful to develop therapies of P. aeruginosa biofilms in CF airways.

Innate immune response in CF airway epithelia: hyperinflammatory?

The lack of functional cystic fibrosis (CF) transmembrane conductance regulator (CFTR) in the apical membranes of CF airway epithelial cells abolishes cAMP-stimulated anion transport, and bacteria, eventually including Pseudomonas aeruginosa, bind to and accumulate in the mucus. Flagellin released from P. aeruginosa triggers airway epithelial Toll-like receptor 5 and subsequent NF-kappaB signaling and production and release of proinflammatory cytokines that recruit neutrophils to the infected region. This response has been termed hyperinflammatory because so many neutrophils accumulate; a response that damages CF lung tissue. We first review the contradictory data both for and against the idea that epithelial cells exhibit larger-than-normal proinflammatory signaling in CF compared with non-CF cells and then review proposals that might explain how reduced CFTR function could activate such proinflammatory signaling. It is concluded that apparent exaggerated innate immune response of CF airway epithelial cells may have resulted not from direct effects of CFTR on cellular signaling or inflammatory mediator production but from indirect effects resulting from the absence of CFTRs apical membrane channel function. Thus, loss of Cl-, HCO3-, and glutathione secretion may lead to reduced volume and increased acidification and oxidation of the airway surface liquid. These changes concentrate proinflammatory mediators, reduce mucociliary clearance of bacteria and subsequently activate cellular signaling. Loss of apical CFTR will also hyperpolarize basolateral membrane potentials, potentially leading to increases in cytosolic [Ca2+], intracellular Ca2+, and NF-kappaB signaling. This hyperinflammatory effect of CF on intracellular Ca2+ and NF-kappaB signaling would be most prominently expressed during exposure to both P. aeruginosa and also endocrine, paracrine, or nervous agonists that activate Ca2+ signaling in the airway epithelia.

Ionic mechanism of forskolin-induced liquid secretion by porcine bronchi

cAMP-elevating agents such as forskolin and vasoactive intestinal peptide induce liquid secretion by tracheobronchial submucosal glands. This pathway is thought to be CFTR dependent and thus defective in cystic fibrosis; however, the ionic mechanism that drives this secretion process is incompletely understood. To better define this mechanism, we studied the effects of ion transport inhibitors on the forskolin-induced liquid secretion response (Jv) of porcine distal bronchi. The forskolin-induced Jv was driven by a combination of bumetanide-sensitive Cl− secretion and DIDS-sensitive HCO3− secretion. When Cl− secretion was inhibited with bumetanide, Na+/H+ exchange-dependent HCO3− secretion was apparently induced to compensate for the loss of Cl− secretion. The forskolin-induced Jv was significantly inhibited by the anion channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid, diphenylamine-2-carboxylate, and glibenclamide. We conclude that the forskolin-induced Jv shares many characteristics of cholinergically induced secretion except for the presence of a DIDS-sensitive component. Although the identity of the DIDS-sensitive component is unclear, we speculate that it represents a basolateral membrane Na+-HCO3− cotransporter or an Na+-dependent anion exchanger, which could account for transepithelial HCO3− secretion.

Restoration of mucociliary transport in the fluid-depleted trachea by surface-active instillates

Severe impairment of mucociliary transport (MCT) is a hallmark of cystic fibrosis (CF) lung disease. Recent studies demonstrate that pharmacologic inhibition of anion and liquid secretion in pig tracheas models the MCT defect in CF through depletion of the periciliary fluid component of airway surface liquid. In the present study, the effectiveness of various aqueous instillates on rehydration of the airway surface and restoration of MCT was assessed in this model. Excised porcine tracheas were mounted in a chamber that permitted simultaneous measurement of MCT and adventitial exposure of the airways to Krebs solution. When anion and liquid secretion were inhibited by treatment with bumetanide and dimethylamiloride, MCT was greatly reduced. Luminal instillation of aqueous solutions containing surface-active substances (1% Tween80 or calfactant) transiently restored MCT to high rates in nearly all tissues. Mucosal treatment with only Krebs solution or hypertonic saline restored MCT in only one half of the tracheas. We conclude that aqueous salt solutions alone can hydrate airway surfaces and restore MCT in some tissues, but surface-active substances may provide additional benefit in restoring MCT in this model of mucociliary stasis. We speculate that administration of surface-active substances, by aerosol or lavage, might help to restore MCT in the airways of patients with CF.

Inhibition of chloride secretion in human bronchial epithelial cells by cigarette smoke extract

Chronic bronchitis, a disease mainly of cigarette smokers, shares many clinical features with cystic fibrosis, a disease of altered ion transport, suggesting that the negative effects of cigarette smoke on mucociliary clearance may be mediated through alterations in ion transport. We tested the hypothesis that cigarette smoke extract would inhibit chloride secretion in human bronchial epithelial cells. In agreement with studies in canine trachea, cigarette smoke extract inhibited net chloride secretion without affecting sodium transport. We performed microelectrode impalements and impedance analysis studies to investigate the physiological mechanisms of this inhibition. These data demonstrated that cigarette smoke extract caused an acute increase in membrane resistances in conjunction with apical membrane hyperpolarization, an effect consistent with inhibition of an apical membrane anion conductance. After this acute phase, both membrane resistances decreased while membrane potentials continued to hyperpolarize, indicating that cigarette smoke extract also inhibited the basolateral entry of chloride into the cell. Furthermore, cigarette smoke extract caused an increase in mucin secretion. Therefore, the ion transport phenotype of human bronchial epithelial cells exposed to cigarette smoke extract is similar to that of cystic fibrosis epithelia in which there is sodium absorption out of proportion to chloride secretion in the setting of increased mucus secretion.

Submucosal gland dysfunction as a primary defect in cystic fibrosis

It has been proposed that defective submucosal gland function in CF airways is a major determinant of CF airway disease. We tested the hypothesis that submucosal gland function is defective early in CF subjects with minimal clinical disease. Functional assays of gland fluid secretion rate and viscosity were performed on freshly obtained nasal biopsies from 6 CF subjects and 5 non-CF controls (age range 2-22 years). Secretions from individual submucosal glands were visualized by light/fluorescence microscopy after orienting and immobilizing biopsy specimens in a custom chamber. The viscosity of freshly secreted gland fluid after pilocarpine, measured by fluorescence recovery after photobleaching of microinjected FITC-dextran, was 4.9 +/- 0.2- vs. 2.2 +/- 0.2-fold greater than water viscosity in CF vs. non-CF specimens, respectively (SE, P<10(-4)). Gland fluid secretion rate in CF specimens, measured by video imaging (4.5+/-0.5 nL/min/gland, n=6), was 2.7-fold reduced compared to non-CF specimens (n=3, P<0.05). Quantitative histology revealed similar size and morphology of submucosal glands in CF and non-CF specimens. Our results suggest that defective airway submucosal gland function is an early, primary defect in CF. Therapies directed at normalizing gland fluid secretion early in CF may thus reduce lung disease.

A “virtual gland” method for quantifying epithelial fluid secretion

We developed a new apparatus, the virtual gland (VG), for measuring the rate of fluid secretion ( Jv), its composition, and the transepithelial potential (TEP) in cultured epithelial cells under open circuit. The VG creates a 10-μl chamber above the apical surface of epithelial cells on a Costar filter with a small hole leading to an oil-filled reservoir. After the chamber is primed with a fluid of choice, secreted fluid is forced through the hole into the oil, where it forms a bubble that is monitored optically to determine Jv and collected for analysis. Calu-3 cells were mounted in the VG with a basolateral bath consisting of Krebs-Ringer bicarbonate buffer at 37°C. Basal Jv was 2.7 ± 0.1 μl·cm−2·h−1 ( n = 42), and TEP was −9.2 ± 0.6 mV ( n = 33); both measures were reduced to zero by ouabain ( n = 6). Jv and TEP were stimulated 64 and 59%, respectively, by 5 μM forskolin ( n = 10), 173 and 101% by 1 mM 1-ethyl-2-benzimidazolinone ( n = 5), 213 and 122% by 333 nM thapsigargin ( n = 5), and 520 and 240% by forskolin + thapsigargin ( n = 6). Basal Jv and TEP were inhibited to 82 and 63%, respectively, with 10 μM bumetanide ( n = 5), 71 and 82% with 100 μM acetazolamide ( n = 5), and 47 and 56% with 600 μM glibenclamide ( n = 4). Basal Jv and TEP were 52 and 89% of control values, respectively, after HCO3− replacement with HEPES ( n = 16). The net HCO3− concentration of the secreted fluid was close to that of the bath (25 mM), except when stimulated with forskolin or VIP, when it increased (∼80 mM). These results validate the use of the VG apparatus and provide the first direct measures of Jv in Calu-3 cells.

Liquid secretion properties of airway submucosal glands

The tracheobronchial submucosal glands secrete liquid that is important for hydrating airway surfaces, supporting mucociliary transport, and serving as a fluid matrix for numerous secreted macromolecules including the gel-forming mucins. This review details the essential structural elements of airway glands and summarizes what is currently known regarding the ion transport processes responsible for producing the liquid component of gland secretion. Liquid secretion most likely arises from serous cells and is principally under neural control with muscarinic agonists, substance P, and vasoactive intestinal peptide (VIP) functioning as effective secretogogues. Liquid secretion is driven by the active transepithelial secretion of both Cl(-) and HCO(3)(-) and at least a portion of this process is mediated by the cystic fibrosis transmembrane conductance regulator (CFTR), which is highly expressed in glands. The potential role of submucosal glands in cystic fibrosis lung disease is discussed.

Update on pathogenesis of cystic fibrosis lung disease

PURPOSE OF REVIEW

It has been an ongoing challenge to translate knowledge pertaining to the molecular basis of cystic fibrosis (CF) into a clear understanding of the development of CF lung disease. Various hypotheses have attempted to explain the apparent breach of innate defenses in CF, although a definitive explanation has been elusive.

RECENT FINDINGS

Recent data suggest that altered ion transport functions--namely sodium hyperabsorption and reduced chloride secretion--lead to a depletion of airway surface liquid. As a result, the overlying mucus layer may encroach upon cell surfaces and become adherent, thus interfering with cilia-dependent and cough clearance. These static, and ultimately anaerobic, niches provide a favorable environment for the development of bacterial biofilms and persistent infection with Pseudomonas aeruginosa.

SUMMARY

With a better understanding of pathogenic steps leading to CF lung disease, we may now be able to direct the development of therapies that will substantially improve disease outcomes.

A small molecule CFTR inhibitor produces cystic fibrosis-like submucosal gland fluid secretions in normal airways

Airway submucosal glands have been proposed as a primary site for initiating and sustaining airway disease in cystic fibrosis (CF). However, it has been difficult to define the role of CFTR in gland fluid secretion because of concerns in interpreting experiments on diseased CF human airways subjected to chronic infection and inflammation. Here, we test the role of CFTR in gland fluid secretion by using a selective CFTR inhibitor (CFTRinh-172) in pig and human airways. Measurements of single-gland fluid secretion rates showed inhibition of both cholinergic and cAMP-stimulated fluid secretion by CFTRinh-172. Secreted fluid [Na+] and [Cl-] measured by fluorescence ratio imaging were 101 and 116 mM, respectively, and not significantly altered by secretory agonists or CFTR inhibition. Gland fluid pH was 7.1 and reduced by 0.4 units after CFTR inhibition. Gland fluid viscosity, determined by photobleaching of FITC-dextran, was threefold increased in pilocarpine-stimulated gland fluid after CFTR inhibition, and protein concentration was increased from 12 to 20 mg/ml. Our data provide strong evidence that gland fluid secretion is CFTR-dependent. The relatively hyper-viscous and acidic fluid secretions produced by acute CFTR inhibition support a role for defective gland function in CF lung disease and provide a rational basis for pharmacological creation of a large animal model of CF.

Adenosine receptors and phosphodiesterase inhibitors stimulate Cl- secretion in Calu-3 cells

We investigated cystic fibrosis transmembrane conductance regulator (CFTR) activation by clinically used phosphodiesterase inhibitors (PDEis) in Calu-3 cell monolayers alone and in combination with A2B adenosine receptor stimulation. This receptor pathway has previously been shown to activate wild-type and mutant CFTR molecules. Several PDEis, including milrinone, cilostazol (Pletal), papaverine, rolipram, and sildenafil (Viagra), produced a short circuit current (Isc) that was glibenclamide-sensitive, achieving 20-85% of forskolin-stimulated Isc. Papaverine, cilostazol, and rolipram also augmented both the magnitude and the duration of Isc following low dose stimulation of adenosine receptors with Ado (0.1-1.0 microM, P < 0.01). Subsequent studies demonstrated that very low concentrations of cilostazol or papaverine (approximately 1/2 peak serum concentrations) were sufficient to activate Isc, and both agents markedly augmented Ado-stimulated Isc (1 microM, P < 0.01). Our results provide evidence that select PDEis, at concentrations achieved as part of systemic therapies, can activate CFTR-dependent Isc in Calu-3 cell monolayers. These studies also indicate that PDEis have the capacity to augment an endogenous CFTR-activating pathway in an "in vivo"-like model system, and supports future investigations of these agents relevant to cystic fibrosis.

Disruptive effects of anion secretion inhibitors on airway mucus morphology in isolated perfused pig lung

Since anion secretion inhibitors reproduce important aspects of cystic fibrosis (CF) lung disease, the effects of these antagonists on airway mucus morphology were assessed in isolated perfused pig lungs. Maximal inhibitory concentrations of bumetanide and dimethylamiloride, which respectively block Cl- and HCO3- secretion in porcine airways, induced the formation of dense 'plastered' mucus on the airway surface, depletion of periciliary fluid and collapse of cilia. This effect was more pronounced when lungs were also exposed to bethanechol to stimulate submucosal gland secretion, when plastered mucus covered > 98 % of the airway surface. Bethanechol also reduced gland duct mucin content in the absence, but not presence, of the anion secretion inhibitors. Anion secretion inhibitors did not induce measurable increases in goblet cell degranulation. We conclude that inhibition of anion and liquid secretion in porcine lungs disrupts the normal morphology of airway surface mucus, providing further evidence that impaired anion secretion alone could account for critical aspects of CF lung disease.

Role of airway surface liquid and submucosal glands in cystic fibrosis lung disease

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) protein, an epithelial chloride channel expressed in the airways, pancreas, testis, and other tissues. A central question is how defective CFTR function in CF leads to chronic lung infection and deterioration of lung function. Several mechanisms have been proposed to explain lung disease in CF, including abnormal airway surface liquid (ASL) properties, defective airway submucosal gland function, altered inflammatory response, defective organellar acidification, loss of CFTR regulation of plasma membrane ion transporters, and others. This review focuses on the physiology of the ASL and submucosal glands with regard to their proposed role in CF lung disease. Experimental evidence for defective ASL properties and gland function in CF is reviewed, and deficiencies in understanding ASL/gland physiology are identified as areas for further investigation. New model systems and measurement technologies are being developed to make progress in establishing lung disease mechanisms in CF, which should facilitate mechanism-based design of therapies for CF.

Liquid secretion inhibitors reduce mucociliary transport in glandular airways

Because of its possible importance in cystic fibrosis (CF) pulmonary pathogenesis, the effect of anion and liquid secretion inhibitors on airway mucociliary transport was examined. When excised porcine tracheas were treated with ACh to induce gland liquid secretion, the rate of mucociliary transport was increased nearly threefold from 2.5 +/- 0.5 to 6.8 +/- 0.8 mm/min. Pretreatment with both bumetanide and dimethylamiloride (DMA), to respectively inhibit Cl(-) and HCO secretion, significantly reduced mucociliary transport in the presence of ACh by 92%. Pretreatment with the anion channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid similarly reduced mucociliary transport in ACh-treated airways by 97%. These agents did not, however, reduce ciliary beat frequency. Luminal application of benzamil to block liquid absorption significantly attenuated the inhibitory effects of bumetanide and DMA on mucociliary transport. We conclude that anion and liquid secretion is essential for normal mucociliary transport in glandular airways. Because the CF transmembrane conductance regulator protein likely mediates Cl(-), HCO, and liquid secretion in normal glands, we speculate that impairment of gland liquid secretion significantly contributes to defective mucociliary transport in CF.

Clara cell impact in air-side activation of CFTR in small pulmonary airways

The Clara cells are nonciliated, nonmucous, secretory cells containing characteristic peptidergic granules; they constitute up to 80% of the epithelial cell population of the distal airways. Despite this exposed histotopology and abundance within the terminal airways where fluid secretion is of pivotal importance, the functional role of the Clara cells remained poorly understood. At the transcriptional, translational, and cellular levels, we provide evidence that the Clara cells are well equipped with the bioactive peptide guanylin and proteins of the cGMP-signaling system including guanylate cyclase C, cGMP-dependent protein kinase II, and cystic fibrosis transmembrane conductance regulator (CFTR) together with the two CFTR scaffolding proteins EBP50/NHERF and E3KARP/NHERF-2 that are essential for proper function of CFTR. Guanylin was localized to secretory granules underneath the apical membrane of Clara cells and was, in addition, detected in high concentrations in bronchoalveolar lavage fluid, predicting release of the peptide luminally into the bronchiolar airways. On the other hand, the guanylin-receptor guanylate cyclase C, CFTR, and proteins linked to CFTR activation and function were all confined to the adluminal membrane of Clara cells, implicating an intriguing air-side route of action of guanylin. Whole-cell patch-clamp recordings in the Clara cell line H441 revealed that guanylin activates CFTR Cl(-) conductance via the cGMP but not the cAMP-signaling pathway. Hence, in the critical location of distal airways in situ, the Clara cells may play the outstanding role of CFTR-dependent regulation of epithelial electrolyte/water secretion through a sophisticated paracrine/luminocrine mode of guanylin-induced CFTR activation.

Effects of intravenous furosemide on mucociliary transport and rheological properties of patients under mechanical ventilation

The use of intravenous (IV) furosemide is common practice in patients under mechanical ventilation (MV), but its effects on respiratory mucus are largely unknown. Furosemide can affect respiratory mucus either directly through inhibition of the NaK(Cl)2 co-transporter on the basolateral surface of airway epithelium or indirectly through increased diuresis and dehydration. We investigated the physical properties and transportability of respiratory mucus obtained from 26 patients under MV distributed in two groups, furosemide (n = 12) and control (n = 14). Mucus collection was done at 0, 1, 2, 3 and 4 hours. The rheological properties of mucus were studied with a microrheometer, and in vitro mucociliary transport (MCT) (frog palate), contact angle (CA) and cough clearance (CC) (simulated cough machine) were measured. After the administration of furosemide, MCT decreased by 17 +/- 19%, 24 +/- 11%, 18 +/- 16% and 18 +/- 13% at 1, 2, 3 and 4 hours respectively, P < 0.001 compared with control. In contrast, no significant changes were observed in the control group. The remaining parameters did not change significantly in either group. Our results support the hypothesis that IV furosemide might acutely impair MCT in patients under MV.

Mechanism of substance P-induced liquid secretion across bronchial epithelium

The present study was undertaken to identify and determine the mechanism of noncholinergic pathways for the induction of liquid secretion across airway epithelium. Excised porcine bronchi secreted substantial and significant quantities of liquid when exposed to acetylcholine, substance P, or forskolin but not to isoproterenol, norepinephrine, or phenylephrine. Bumetanide, an inhibitor of Na(+)-K(+)-2Cl(-) cotransport, reduced the liquid secretion response to substance P by 69%. Approximately two-thirds of bumetanide-insensitive liquid secretion was blocked by dimethylamiloride (DMA), a Na(+)/H(+) exchange inhibitor. Substance P responses were preserved in airways after surface epithelium removal, suggesting that secreted liquid originated from submucosal glands. The anion channel blockers diphenylamine-2-carboxylate (DPC) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) inhibited >90% of substance P-induced liquid secretion, whereas DIDS had no effect. DMA, DPC, and NPPB had greater inhibitory effects on net HCO(3)(-) secretion than on liquid secretion. Although preserved relative to liquid secretion, net HCO(3)(-) secretion was reduced by 39% in the presence of bumetanide. We conclude that substance P induces liquid secretion from bronchial submucosal glands of pigs through active transport of Cl(-) and HCO(3)(-). The pattern of responses to secretion agonists and antagonists suggests that the cystic fibrosis transmembrane conductance regulator mediates this process.

Optical method for quantifying rates of mucus secretion from single submucosal glands

We describe an optical method to quantify single- gland secretion. Isolated tracheal mucosa were mounted at the air-Krebs interface and coated with oil. Gland secretions formed spherical bubbles that were digitally imaged at intervals, allowing rates of secretion to be calculated. We monitored 340 glands in 54 experiments with 12 sheep. Glands secreted basally at low rates (0.57 +/- 0.04 nl x min(-1) x gland(-1), 123 glands) in tissues up to 9 h postharvest and at lower rates for up to 3 days. Carbachol (10 microM) stimulated secretion with an early transient and a sustained or oscillating phase. Peak secretion was 15.7 +/- 1.2 nl x min(-1) x gland(-1) (60 glands); sustained secretion was 4.5 +/- 0.5 nl x min(-1) x gland(-1) (10 glands). Isoproterenol and phenylephrine (10 microM each) stimulated only small, transient responses. We confirmed that cats have a large secretory response to phenylephrine (11.6 +/- 3.7 nl x min(-1) x gland(-1), 12 glands), but pigs, sheep, and humans all have small responses (<2 nl x min(-1)m x gland(-1)). Carbachol-stimulated peak secretion was inhibited 56% by bumetanide, 67% by HCO replacement with HEPES, and 92% by both. The distribution of secretion rates was nonnormal, suggesting the existence of subpopulations of glands.

ENaC- and CFTR-dependent ion and fluid transport in mammary epithelia

Mammary epithelial 31EG4 cells (MEC) were grown as monolayers on filters to analyze the apical membrane mechanisms that help mediate ion and fluid transport across the epithelium. RT-PCR showed the presence of cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial Na(+) channel (ENaC) message, and immunomicroscopy showed apical membrane staining for both proteins. CFTR was also localized to the apical membrane of native human mammary duct epithelium. In control conditions, mean values of transepithelial potential (apical-side negative) and resistance (R(T)) are -5.9 mV and 829 Omega x cm(2), respectively. The apical membrane potential (V(A)) is -40.7 mV, and the mean ratio of apical to basolateral membrane resistance (R(A)/R(B)) is 2.8. Apical amiloride hyperpolarized V(A) by 19.7 mV and tripled R(A)/R(B). A cAMP-elevating cocktail depolarized V(A) by 17.6 mV, decreased R(A)/R(B) by 60%, increased short-circuit current by 6 microA/cm(2), decreased R(T) by 155 Omega x cm(2), and largely eliminated responses to amiloride. Whole cell patch-clamp measurements demonstrated amiloride-inhibited Na(+) currents [linear current-voltage (I-V) relation] and forskolin-stimulated Cl(-) currents (linear I-V relation). A capacitance probe method showed that in the control state, MEC monolayers either absorbed or secreted fluid (2--4 microl x cm(-2) x h(-1)). Fluid secretion was stimulated either by activating CFTR (cAMP) or blocking ENaC (amiloride). These data plus equivalent circuit analysis showed that 1) fluid absorption across MEC is mediated by Na(+) transport via apical membrane ENaC, and fluid secretion is mediated, in part, by Cl(-) transport via apical CFTR; 2) in both cases, appropriate counterions move through tight junctions to maintain electroneutrality; and 3) interactions among CFTR, ENaC, and tight junctions allow MEC to either absorb or secrete fluid and, in situ, may help control luminal [Na(+)] and [Cl(-)].

Submucosal gland secretions in airways from cystic fibrosis patients have normal [Na(+)] and pH but elevated viscosity

Fluid and macromolecule secretion by submucosal glands in mammalian airways is believed to be important in normal airway physiology and in the pathophysiology of cystic fibrosis (CF). An in situ fluorescence method was applied to measure the ionic composition and viscosity of freshly secreted fluid from airway glands. Fragments of human large airways obtained at the time of lung transplantation were mounted in a humidified perfusion chamber and the mucosal surface was covered by a thin layer of oil. Individual droplets of secreted fluid were microinjected with fluorescent indicators for measurement of [Na(+)], [Cl(-)], and pH by ratio imaging fluorescence microscopy and viscosity by fluorescence recovery after photobleaching. After carbachol stimulation, 0.1--0.5 microl of fluid accumulated in spherical droplets at gland orifices in approximately 3--5 min. In gland fluid from normal human airways, [Na(+)] was 94 +/- 8 mM, [Cl(-)] was 92 +/- 12 mM, and pH was 6.97 +/- 0.06 (SE, n = 7 humans, more than five glands studied per sample). Apparent fluid viscosity was 2.7 +/- 0.3-fold greater than that of saline. Neither [Na(+)] nor pH differed in gland fluid from CF airways, but viscosity was significantly elevated by approximately 2-fold compared to normal airways. These results represent the first direct measurements of ionic composition and viscosity in uncontaminated human gland secretions and indicate similar [Na(+)], [Cl(-)], and pH to that in the airway surface liquid. The elevated gland fluid viscosity in CF may be an important factor promoting bacterial colonization and airway disease.