Mucociliary transport in porcine trachea: differential effects of inhibiting chloride and bicarbonate secretion

Therapeutic Targets and Precision Medicine in COPD: Inflammation, Ion Channels, Both, or Neither?

The development of a wider range of therapeutic options is a key objective in drug discovery for chronic obstructive pulmonary disease (COPD). Fundamental advances in lung biology have the potential to greatly expand the number of therapeutic targets in COPD. The recently reported successful Phase 3 clinical trial of the first biologic agent for COPD, the monoclonal antibody dupilumab, adds additional support to the importance of targeting inflammatory pathways in COPD. However, numerous other cellular mechanisms are important targets in COPD therapeutics, including airway remodeling, the CFTR ion channel, and mucociliary function. Some of these emerging targets can be exploited by the expanded use of existing COPD drugs, such as roflumilast, while targeting others will require the development of novel molecular entities. The identification of additional therapeutic targets and agents has the potential to greatly expand the value of using clinical and biomarker data to classify COPD into specific subsets, each of which can be predictive of an enhanced response to specific subset(s) of targeted therapies. The author reviews established and emerging drug targets in COPD and uses this as a framework to define a novel classification of COPD based on therapeutic targets. This novel classification has the potential to enhance precision medicine in COPD patient care and to accelerate clinical trials and pre-clinical drug discovery efforts.

The role of CFTR channel in female infertility

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-activated trans-membrane ATP gated anion channel present in most epithelia, which transports chloride and bicarbonate ions across the apical membrane. Mutations in the CFTR protein are known to result in defective expression or function, notably the inhibition of chloride and bicarbonate transport. This can result in cystic fibrosis (CF), a disorder characterised by thickness of the mucus lining of the epithelial cells of the alimentary and respiratory tracts, sweat ducts and reproductive organs. As a consequence, there is a reduction in fluid transport at the apical surface. While the most devastating effect of CF is mortality, about 98% of men with CF are infertile, consequent of early blockage of or failure to develop the mesonephrotic ducts as well as the vas deferens. The effect of CF of female fertility is less well-understood. This review highlights the genetics and pathophysiology as well as the mechanism of action of CF on female infertility.

Physiology and pathophysiology of human airway mucus

The mucus clearance system is the dominant mechanical host defense system of the human lung. Mucus is cleared from the lung by cilia and airflow, including both two-phase gas-liquid pumping and cough-dependent mechanisms, and mucus transport rates are heavily dependent on mucus concentration. Importantly, mucus transport rates are accurately predicted by the gel-on-brush model of the mucociliary apparatus from the relative osmotic moduli of the mucus and periciliary-glycocalyceal (PCL-G) layers. The fluid available to hydrate mucus is generated by transepithelial fluid transport. Feedback interactions between mucus concentrations and cilia beating, via purinergic signaling, coordinate Na+ absorptive vs Cl- secretory rates to maintain mucus hydration in health. In disease, mucus becomes hyperconcentrated (dehydrated). Multiple mechanisms derange the ion transport pathways that normally hydrate mucus in muco-obstructive lung diseases, e.g., cystic fibrosis (CF), chronic obstructive pulmonary disease (COPD), non-CF bronchiectasis (NCFB), and primary ciliary dyskinesia (PCD). A key step in muco-obstructive disease pathogenesis is the osmotic compression of the mucus layer onto the airway surface with the formation of adherent mucus plaques and plugs, particularly in distal airways. Mucus plaques create locally hypoxic conditions and produce airflow obstruction, inflammation, infection, and, ultimately, airway wall damage. Therapies to clear adherent mucus with hydrating and mucolytic agents are rational, and strategies to develop these agents are reviewed.

Tromethamine improves mucociliary clearance in cystic fibrosis pigs

In cystic fibrosis (CF), the loss of cystic fibrosis transmembrane conductance regulator (CFTR) mediated Cl-  and HCO3 -  secretion across the epithelium acidifies the airway surface liquid (ASL). Acidic ASL alters two key host defense mechanisms: Rapid ASL bacterial killing and mucociliary transport (MCT). Aerosolized tromethamine (Tham) increases ASL pH and restores the ability of ASL to rapidly kill bacteria in CF pigs. In CF pigs, clearance of insufflated microdisks is interrupted due to abnormal mucus causing microdisks to abruptly recoil. Aerosolizing a reducing agent to break disulfide bonds that link mucins improves MCT. Here, we are interested in restoring MCT in CF by aerosolizing Tham, a buffer with a pH of 8.4. Because Tham is hypertonic to serum, we use an acidified formulation as a control. We measure MCT by tracking the caudal movement of individual tantalum microdisks with serial chest computed tomography scans. Alkaline Tham improves microdisk clearance to within the range of that seen in non-CF pigs. It also partially reverses MCT defects, including reduced microdisk recoil and elapse time until they start moving after methacholine stimulation in CF pig airways. The effect is not due to hypertonicity, as it is not seen with acidified Tham or hypertonic saline. This finding indicates acidic ASL impairs CF MCT and suggests that alkalinization of ASL pH with inhaled Tham may improve CF airway disease.

Inhibition of the sodium-dependent HCO3- transporter SLC4A4, produces a cystic fibrosis-like airway disease phenotype

Bicarbonate secretion is a fundamental process involved in maintaining acid-base homeostasis. Disruption of bicarbonate entry into airway lumen, as has been observed in cystic fibrosis, produces several defects in lung function due to thick mucus accumulation. Bicarbonate is critical for correct mucin deployment and there is increasing interest in understanding its role in airway physiology, particularly in the initiation of lung disease in children affected by cystic fibrosis, in the absence of detectable bacterial infection. The current model of anion secretion in mammalian airways consists of CFTR and TMEM16A as apical anion exit channels, with limited capacity for bicarbonate transport compared to chloride. However, both channels can couple to SLC26A4 anion exchanger to maximise bicarbonate secretion. Nevertheless, current models lack any details about the identity of the basolateral protein(s) responsible for bicarbonate uptake into airway epithelial cells. We report herein that the electrogenic, sodium-dependent, bicarbonate cotransporter, SLC4A4, is expressed in the basolateral membrane of human and mouse airways, and that it's pharmacological inhibition or genetic silencing reduces bicarbonate secretion. In fully differentiated primary human airway cells cultures, SLC4A4 inhibition induced an acidification of the airways surface liquid and markedly reduced the capacity of cells to recover from an acid load. Studies in the Slc4a4-null mice revealed a previously unreported lung phenotype, characterized by mucus accumulation and reduced mucociliary clearance. Collectively, our results demonstrate that the reduction of SLC4A4 function induced a CF-like phenotype, even when chloride secretion remained intact, highlighting the important role SLC4A4 plays in bicarbonate secretion and mammalian airway function.

Effects of Different Inhalation Therapy on Ventilator-Associated Pneumonia in Ventilated COVID-19 Patients: A Randomized Controlled Trial

The effect of routine inhalation therapy on ventilator-associated pneumonia (VAP) in mechanically ventilated patients with the coronavirus disease (COVID-19) has not been well-defined. This randomized controlled trial included 175 eligible adult patients with COVID-19 who were treated with mechanical ventilation at the University Hospital of Split between October 2020 and June 2021. Patients were randomized and allocated to a control group (no routine inhalation) or one of the treatment arms (inhalation of N-acetylcysteine; 5% saline solution; or 8.4% sodium bicarbonate). The primary outcome was the incidence of VAP, while secondary outcomes included all-cause mortality. Routine inhalation therapy had no effect on the incidence of bacterial or fungal VAP nor on all-cause mortality (p > 0.05). Secondary analyses revealed a significant reduction of Gram-positive and methicillin-resistant Staphylococcus aureus (MRSA) VAP in the treatment groups. Specifically, the bicarbonate group had a statistically significantly lower incidence of Gram-positive bacterial VAP (4.8%), followed by the N-acetylcysteine group (10.3%), 5% saline group (19.0%), and control group (34.6%; p = 0.001). This difference was driven by a lower incidence of MRSA VAP in the bicarbonate group (2.4%), followed by the N-acetylcysteine group (7.7%), 5% saline group (14.3%), and control group (34.6%; p < 0.001). Longer duration of ventilator therapy was the only significant, independent predictor of any bacterial or fungal VAP in the multivariate analysis (aOR 1.14, 95% CI 1.01−1.29, p = 0.038 and aOR 1.05, 95% CI 1.01−1.10, p = 0.028, respectively). In conclusion, inhalation therapy had no effect on the overall VAP incidence or all-cause mortality. Further studies should explore the secondary findings of this study such as the reduction of Gram-positive or MRSA-caused VAP in treated patients.

Mucus concentration-dependent biophysical abnormalities unify submucosal gland and superficial airway dysfunction in cystic fibrosis

Cystic fibrosis (CF) is characterized by abnormal transepithelial ion transport. However, a description of CF lung disease pathophysiology unifying superficial epithelial and submucosal gland (SMG) dysfunctions has remained elusive. We hypothesized that biophysical abnormalities associated with CF mucus hyperconcentration provide a unifying mechanism. Studies of the anion secretion-inhibited pig airway model of CF revealed elevated SMG mucus concentrations, osmotic pressures, and SMG mucus accumulation. Human airway studies revealed hyperconcentrated CF SMG mucus with raised osmotic pressures and cohesive forces predicted to limit SMG mucus secretion/release. Using proline-rich protein 4 (PRR4) as a biomarker of SMG secretion, CF sputum proteomics analyses revealed markedly lower PRR4 levels compared to healthy and bronchiectasis controls, consistent with a failure of CF SMGs to secrete mucus onto airway surfaces. Raised mucus osmotic/cohesive forces, reflecting mucus hyperconcentration, provide a unifying mechanism that describes disease-initiating mucus accumulation on airway surfaces and in SMGs of the CF lung.

Bicarbonate transport of airway surface epithelia in luminally perfused mice bronchioles

HCO3- secretion in distal airways is critical for airway mucosal defense. HCO3-/H+ transport across the apical membrane of airway surface epithelial cells was studied by measuring intracellular pH in luminally microperfused freshly dissected mice bronchioles. Functional studies demonstrated that CFTR, ENaC, Cl--HCO3- exchange, Na+-H+ exchange, and Na+-HCO3- cotransport are involved in apical HCO3-/H+ transport. RT-PCR of isolated bronchioles detected fragments from Cftr, α, β, γ subunits of ENaC, Ae2, Ae3, NBCe1, NBCe2, NBCn1, NDCBE, NBCn2, Nhe1, Nhe2, Nhe4, Nhe5, Slc26a4, Slc26a6, and Slc26a9. We assume that continuous decline of intracellular pH following alkaline load demonstrates time course of HCO3- secretion into the lumen which is perfused with a HCO3--free solution. Forskolin-stimulated HCO3- secretion was substantially inhibited by luminal application of CFTRinh-172 (5 μM), H2DIDS (200 μM), and amiloride (1 μM). In bronchioles from a cystic fibrosis mouse model, basal and acetylcholine-stimulated HCO3- secretion was substantially impaired, but forskolin transiently accelerated HCO3- secretion of which the magnitude was comparable to wild-type bronchioles. In conclusion, we have characterized apical HCO3-/H+ transport in native bronchioles. We have demonstrated that cAMP-mediated and Ca2+-mediated pathways are involved in HCO3- secretion and that apical HCO3- secretion is largely mediated by CFTR and H2DIDS-sensitive Cl--HCO3- exchanger, most likely Slc26a9. The impairment of HCO3- secretion in bronchioles from a cystic fibrosis mouse model may be related to the pathogenesis of early lung disease in cystic fibrosis.

Hepatobiliary acid-base homeostasis: Insights from analogous secretory epithelia

Many epithelia secrete bicarbonate-rich fluid to generate flow, alter viscosity, control pH and potentially protect luminal and intracellular structures from chemical stress. Bicarbonate is a key component of human bile and impaired biliary bicarbonate secretion is associated with liver damage. Major efforts have been undertaken to gain insight into acid-base homeostasis in cholangiocytes and more can be learned from analogous secretory epithelia. Extrahepatic examples include salivary and pancreatic duct cells, duodenocytes, airway and renal epithelial cells. The cellular machinery involved in acid-base homeostasis includes carbonic anhydrase enzymes, transporters of the solute carrier family, and intra- and extracellular pH sensors. This pH-regulatory system is orchestrated by protein-protein interactions, the establishment of an electrochemical gradient across the plasma membrane and bicarbonate sensing of the intra- and extracellular compartment. In this review, we discuss conserved principles identified in analogous secretory epithelia in the light of current knowledge on cholangiocyte physiology. We present a framework for cholangiocellular acid-base homeostasis supported by expression analysis of publicly available single-cell RNA sequencing datasets from human cholangiocytes, which provide insights into the molecular basis of pH homeostasis and dysregulation in the biliary system.

Variability in tracheal mucociliary transport is not controlled by beating cilia in lambs in vivo during ventilation with humidified and nonhumidified air

Mucociliary transport in the respiratory epithelium depends on beating of cilia to move a mucus layer containing trapped inhaled particles toward the mouth. Little is known about the relationship between cilia beat frequency (CBF) and mucus transport velocity (MTV) in vivo under normal physiological conditions and when inspired air is dry or not fully humidified. This study was designed to use video-microscopy to simultaneously measure CBF and MTV in the tracheal epithelium through an implanted optical window in mechanically ventilated lambs. The inspired air in 6 animals was heated to body temperature and fully saturated with water for 4 hours as a baseline. In another series of experiments, 5 lambs were ventilated with air at different temperatures and humidities and the mucosal surface temperature was monitored with infrared macro-imaging. In the baseline experiments, during ventilation with fully humidified air at body temperature, CBF remained constant, mean 13.9 ± 1.6 Hz but MTV varied considerably between 0.1 and 26.1 mm/min with mean 11.0 ± 3.9 mm/min, resulting in a maximum mucus displacement of 34.2 µm/cilia beat. Fully humidified air at body temperature prevented fluctuations in the surface temperature during breathing indicating a thermodynamic balance in the airways. When lambs were ventilated with dryer air, the mucosal surface temperature and MTV dropped without a significant change in CBF. When inspired air was dry, mainly latent heat (92%) was transferred to air in the trachea, reducing the surface temperature by 5 °C. Reduced humidity of the inspired air lowered the surface temperature and reduced MTV in the epithelium during ventilation.

Novel Therapy of Bicarbonate, Glutathione, and Ascorbic Acid Improves Cystic Fibrosis Mucus Transport

Defective airway mucus clearance is a defining characteristic of cystic fibrosis lung disease, and improvements to current mucolytic strategies are needed. Novel approaches targeting a range of contributing mechanisms are in various stages of preclinical and clinical development. ARINA-1 is a new nebulized product comprised of ascorbic acid, glutathione, and bicarbonate. Using microoptical coherence tomography, we tested the effect of ARINA-1 on central features of mucociliary clearance in F508del/F508del primary human bronchial epithelial cells to assess its potential as a mucoactive therapy in cystic fibrosis. We found that ARINA-1 significantly augmented mucociliary transport rates, both alone and with CFTR (cystic fibrosis transmembrane conductance regulator) modulator therapy, whereas airway hydration and ciliary beating were largely unchanged compared with PBS vehicle control. Analysis of mucus reflectivity and particle-tracking microrheology indicated that ARINA-1 restores mucus clearance by principally reducing mucus layer viscosity. The combination of bicarbonate and glutathione elicited increases in mucociliary transport rate comparable to those seen with ARINA-1, indicating the importance of this interaction to the impact of ARINA-1 on mucus transport; this effect was not recapitulated with bicarbonate alone or bicarbonate combined with ascorbic acid. Assessment of CFTR chloride transport revealed an increase in CFTR-mediated chloride secretion in response to ARINA-1 in CFBE41o^- cells expressing wild-type CFTR, driven by CFTR activity stimulation by ascorbate. This response was absent in CFBE41o^- F508del cells treated with VX-809 and primary human bronchial epithelial cells, implicating CFTR-independent mechanisms for the effect of ARINA-1 on cystic fibrosis mucus. Together, these studies indicate that ARINA-1 is a novel potential therapy for the treatment of impaired mucus clearance in cystic fibrosis.

Transport properties in CFTR-/- knockout piglets suggest normal airway surface liquid pH and enhanced amiloride-sensitive Na+ absorption

Previous analysis of CFTR-knockout (CFTR-/-) in piglets has provided important insights into the pathology of cystic fibrosis. However, controversies exist as to the true contribution of CFTR to the pH balance in airways and intestine. We therefore compared ion transport properties in newborn wild-type (CFTR+/+) and CFTR-knockout (CFTR-/- piglets). Tracheas of CFTR-/- piglets demonstrated typical cartilage malformations and muscle cell bundles. CFTR-/- airway epithelial cells showed enhanced lipid peroxidation, suggesting inflammation early in life. CFTR was mainly expressed in airway submucosal glands and was absent in lungs of CFTR-/- piglets, while expression of TMEM16A was uncompromised. mRNA levels for TMEM16A, TMEM16F, and αβγENaC were unchanged in CFTR-/- airways, while mRNA for SLC26A9 appeared reduced. CFTR was undetectable in epithelial cells of CFTR-/- airways and intestine. Small intestinal epithelium of CFTR-/- piglets showed mucus accumulation. Secretion of both electrolytes and mucus was activated by stimulation with prostaglandin E2 and ATP in the intestine of CFTR+/+, but not of CFTR-/- animals. pH was measured inside small bronchi using a pH microelectrode and revealed no difference between CFTR+/+ and CFTR-/- piglets. Intracellular pH in porcine airway epithelial cells revealed only a small contribution of CFTR to bicarbonate secretion, which was absent in cells from CFTR-/- piglets. In contrast to earlier reports, our data suggest a minor impact of CFTR on ASL pH. In contrast, enhanced amiloride-sensitive Na⁺ absorption may contribute to lung pathology in CFTR-/- piglets, along with a compromised CFTR- and TMEM16A-dependent Cl^- transport.

Chemical and Microstructural Characterization of pH and [Ca2+] Dependent Sol-Gel Transitions in Mucin Biopolymer

Mucus is responsible for controlling transport and barrier function in biological systems, and its properties can be significantly affected by compositional and environmental changes. In this study, the impacts of pH and CaCl₂ were examined on the solution-to-gel transition of mucin, the primary structural component of mucus. Microscale structural changes were correlated with macroscale viscoelastic behavior as a function of pH and calcium addition using rheology, dynamic light scattering, zeta potential, surface tension, and FTIR spectroscopic characterization. Mucin solutions transitioned from solution to gel behavior between pH 4–5 and correspondingly displayed a more than ten-fold increase in viscoelastic moduli. Addition of CaCl₂ increased the sol-gel transition pH value to ca. 6, with a twofold increase in loss moduli at low frequencies and ten-fold increase in storage modulus. Changing the ionic conditions—specifically [H⁺] and [Ca²⁺] —modulated the sol-gel transition pH, isoelectric point, and viscoelastic properties due to reversible conformational changes with mucin forming a network structure via  non-covalent cross-links between mucin chains.

Mucociliary Transport in Healthy and Cystic Fibrosis Pig Airways

Cystic fibrosis (CF) lung disease is the major cause of morbidity and mortality in people with CF. Abnormal mucociliary transport has been the leading hypothesis for the underlying pathogenesis of CF airway disease. However, this has been difficult to investigate at very early time points. A porcine CF model, which recapitulates many features of CF disease in humans, enables studies to be performed in non-CF and CF pigs on the day that they are born. In newborn CF pigs, we found that under basal conditions, mucociliary transport rates in non-CF and CF pigs are similar. However, after cholinergic stimulation, which stimulates submucosal gland secretion, particles become stuck in the CF airways owing to a failure of mucus strands to release from submucosal glands. In this review, we summarize these recent discoveries and also discuss the morphology, composition, and function of mucins in the porcine lung.

Calmodulin and protein kinases A/G mediate ciliary beat response in the human nasal epithelium

BACKGROUND

Mucociliary clearance of the airway epithelium is an essential function for mucosal defense. We recently proposed a hypothetical mechanism of ciliary beat regulation, in which the pannexin-1 (Panx1)-P2X7 unit serves as an oscillator generating a periodic increase in intracellular Ca²⁺ ([Ca²⁺ ]_i ). In the present study, we examined the localization of Panx1 and P2X7 at the ultrastructural level, and investigated the regulatory pathway subsequent to [Ca²⁺ ]_i increase.

METHODS

The inferior turbinate mucosa was collected from patients with chronic hypertrophic rhinitis during endoscopic sinonasal surgery. The mucosa was examined by transmission immunoelectron microscopy for Panx1 and P2X7. Alternatively, the mucosa was cut into thin strips, and ciliary beat frequency (CBF) was measured under a phase-contrast light microscope with a high-speed digital video camera.

RESULTS

In immunoelectron microscopy, immunoreactivities for Panx1 and P2X7 were localized along the plasma membrane of the entire length of the cilia. CBF was significantly increased by stimulation with 100 µM acetylcholine (Ach). The Ach-induced CBF increase was significantly inhibited by calmidazolium (calmodulin antagonist), SQ22536 (adenylate cyclase inhibitor), ODQ (guanylate cyclase inhibitor), KT5720 (protein kinase A inhibitor), and KT5823 (protein kinase G inhibitor). Fluorodinitrobenzene (creatine kinase inhibitor) completely inhibited the ciliary beat in a time- and dose-dependent manner.

CONCLUSION

These results indicate that Panx1 and P2X7 coexist at the cilia of the human nasal epithelial cells and that the ciliary beat is regulated by calmodulin, adenylate/guanylate cyclases and protein kinases A/G, and crucially depends on creatine kinase.

Intranasal instillation of distilled water, hypertonic saline and sodium bicarbonate promotes redox imbalance and acute lung inflammation in adult mice

Bronchial obstruction, caused by retained secretions, is often treated by the administration of mucoactive agents including distilled water, saline, hypertonic saline, and sodium bicarbonate. However, the inflammatory effect of these solutions on the lungs remains unclear. This study evaluated the instillation effects of different solutions on oxidative stress and lung inflammatory response in C57BL/6 mice. Fifty C57BL/6 mice were divided into 5 groups: control (CG); distilled water (DWG), hypertonic saline (HSG), saline (SG) and sodium bicarbonate (SBG). CG was exposed to ambient air while DWG, HSG, SG and SBG had 50 μl of respective solutions administered intranasally for 5 consecutive days. Twenty-four hours after the last intranasal instillation, all animals were euthanized for subsequent analysis. All solutions promoted increased recruitment of inflammatory cells to the lung compared to controls. Superoxide dismutase activity was lower in HSG compared to all other groups; catalase activity was reduced in SG, while it increased in SBG and DWG compared to CG. Finally, there was an increase in the inflammatory markers TNF-α, CCL2 and IFN-γ in DWG compared to CG, SG and HSG. In conclusions, the intranasal instillation of different solutions promotes redox imbalance and inflammation on lungs of adult mice.

Decreased ciliary beat responsiveness to acetylcholine in the nasal polyp epithelium

OBJECTIVE

We investigated the difference in ciliary beat responsiveness to acetylcholine in ex vivo and the difference in the expressions of associated molecules (M1/M3 muscarinic receptors, pannexin-1 and P2X7 purinergic receptor) between the nasal polyp and turbinate mucosa.

STUDY DESIGN

Laboratorial study.

PARTICIPANTS

Nasal polyp and inferior turbinate were collected from patients with hypertrophic rhinitis and/or nasal polyp during endoscopic sinonasal surgery.

MAIN OUTCOME MEASURES

The mucosa was cut into thin strips, and ciliary movement was observed under a phase-contrast light microscope equipped with a high-speed digital video camera. The samples were also examined by scanning electron microscopy, fluorescence immunohistochemistry, and quantitative reverse transcription-polymerase chain reaction.

RESULTS

Cilia were well preserved in both tissues at the ultrastructural level. The baseline ciliary beat frequency (CBF) was not different between the two tissues. The CBF of the turbinate was significantly increased by stimulation with acetylcholine (P < 0.001), but that of the polyp was not. The ratio of the acetylcholine-stimulated CBF to the baseline CBF was significantly lower in the polyp than in the turbinate (P < 0.001). Immunohistochemical study revealed that immunoreactivities for M3, pannexin-1 and P2X7 were weaker in the polyp than in the turbinate. The mRNA expressions of M1, M3 and P2X7 were significantly lower and that of pannexin-1 tended to be lower in the polyp than in the turbinate.

CONCLUSIONS

These results indicate that ciliary beat responsiveness to acetylcholine is decreased in the nasal polyp. This may be explained by the decreased expressions of M3, P2X7 and probably pannexin-1 in this tissue.

Extracellular pH and lung infections in cystic fibrosis

Cystic fibrosis (CF) is an autosomal recessive disease caused by CFTR mutations. It is characterized by high NaCl concentration in sweat and the production of a thick and sticky mucus, occluding secretory ducts, intestine and airways, accompanied by chronic inflammation and infections of the lungs. This causes a progressive and lethal decline in lung function. Therefore, finding the mechanisms driving the high susceptibility to lung infections has been a key issue. For decades the prevalent hypothesis was that a reduced airway surface liquid (ASL) volume and composition, and the consequent increased mucus concentration (dehydration), create an environment favoring infections. However, a few years ago, in a pig model of CF, the Na⁺/K⁺ concentrations and the ASL volume were found intact. Immediately a different hypothesis arose, postulating a reduced ASL pH as the cause for the increased susceptibility to infections, due to a diminished bicarbonate secretion through CFTR. Noteworthy, a recent report found normal ASL pH values in CF children and in cultured primary airway cells, challenging the ASL pH hypothesis. On the other hand, recent evidences revitalized the hypothesis of a reduced ASL secretion. Thus, the role of the ASL pH in the CF is still a controversial matter. In this review we discuss the basis that sustain the role of CFTR in modulating the extracellular pH, and the recent results sustaining the different points of view. Finding the mechanisms of CFTR signaling that determine the susceptibility to infections is crucial to understand the pathophysiology of CF and related lung diseases.

Acetylcholine-induced Ciliary Beat of the Human Nasal Mucosa Is Regulated by the Pannexin-1 Channel and Purinergic P2X Receptor

Background Airway mucociliary transport is an important function for the clearance of inhaled foreign particulates in the respiratory tract. The present study aimed at investigating the regulatory mechanism of acetylcholine (Ach)-induced ciliary beat of the human nasal mucosa in ex vivo. Methods The inferior turbinate mucosa was collected from patients with chronic hypertrophic rhinitis during endoscopic surgery. The mucosa was cut into thin strips, and ciliary movement was observed under a phase-contrast light microscope with a high-speed digital video camera. The sample was alternatively subjected to scanning electron microscopic observation. Results Cilia on the turbinate epithelium were well preserved at the ultrastructural level. The baseline ciliary beat frequency (CBF) was 6.45 ± 0.32 Hz. CBF was significantly increased by stimulation with 100 µM Ach and 100 µM adenosine triphosphate. The Ach-induced CBF increase was completely inhibited by removing extracellular Ca²⁺. Significant inhibition of the Ach-induced CBF was also observed by the addition of 1 µM atropine, 40 µM 2-aminoethoxydiphenyl borate (inositol trisphosphate [IP₃] receptor antagonist), 10 µM carbenoxolone (pannexin-1 blocker), 1 mM probenecid (pannexin-1 blocker), 100 µM pyridoxalphosphate-6-azophenyl-20,40-disulfonic acid (P2X antagonist), and 300 µM flufenamic acid (connexin blocker). Meanwhile, 30 nM bafilomycin A1 (vesicular transport inhibitor) did not inhibit the Ach-induced CBF increase.

CONCLUSIONS

These results indicate that the regulatory mechanism of the Ach-induced ciliary beat is dependent on extracellular Ca²⁺ and involves the muscarinic Ach receptor, IP₃ receptor, pannexin-1 channel, purinergic P2X receptor, and connexin channel. We proposed a tentative intracellular signaling pathway of the Ach-induced ciliary beat, in which the pannexin-1-P2X unit may play a central role in ciliary beat regulation.

The role of mucus on drug transport and its potential to affect therapeutic outcomes

A layer of mucus covers the surface of all wet epithelia throughout the human body. Mucus is a hydrogel mainly composed of water, mucins (glycoproteins), DNA, proteins, lipids, and cell debris. This complex composition yields a tenacious viscoelastic hydrogel that lubricates and protects the exposed epithelia from external threats and enzymatic degradation. The natural protective role of mucus is nowadays acknowledged as a major barrier to be overcome in non-invasive drug delivery. The heterogeneity of mucus components offers a wide range of potential chemical interaction sites for macromolecules, while the mesh-like architecture given to mucus by the intermolecular cross-linking of mucin molecules results in a dense network that physically, and in a size-dependent manner, hinders the diffusion of nanoparticles through mucus. Consequently, drug diffusion, epithelial absorption, drug bioavailability, and ultimately therapeutic outcomes of mucosal drug delivery can be attenuated.

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.

Hydrator Therapies for Chronic Bronchitis. Lessons from Cystic Fibrosis

Patients with the chronic bronchitis form of chronic obstructive pulmonary disease and cystic fibrosis share similar clinical features, including mucus obstruction of airways and the development of chronic/recurrent airways infections that often manifest as disease exacerbations. There is growing evidence that these diseases may have parallels in disease pathogenesis as well, including cystic fibrosis transmembrane conductance regulator dysfunction, mucus dehydration, and defective mucociliary clearance. As progress is made in the development of therapies that target the basic defects that lead to cystic fibrosis lung disease, it is possible that similar approaches could also benefit patients with chronic bronchitis. A deeper understanding of how tobacco smoke and other triggers of chronic bronchitis actually lead to disease, and exploration of the concept that therapies that restore cystic fibrosis transmembrane conductance regulator function, mucus hydration, and/or mucociliary clearance may benefit patients with chronic bronchitis, hold the prospect of significant progress in treating this prevalent disease.

Physicochemical properties of mucus and their impact on transmucosal drug delivery

Mucus is a selective barrier to particles and molecules, preventing penetration to the epithelial surface of mucosal tissues. Significant advances in transmucosal drug delivery have recently been made and have emphasized that an understanding of the basic structure, viscoelastic properties, and interactions of mucus is of great value in the design of efficient drug delivery systems. Mucins, the primary non-aqueous component of mucus, are polymers carrying a complex and heterogeneous structure with domains that undergo a variety of molecular interactions, such as hydrophilic/hydrophobic, hydrogen bonds and electrostatic interactions. These properties are directly relevant to the numerous mucin-associated diseases, as well as delivering drugs across the mucus barrier. Therefore, in this review we discuss regional differences in mucus composition, mucus physicochemical properties, such as pore size, viscoelasticity, pH, and ionic strength. These factors are also discussed with respect to changes in mucus properties as a function of disease state. Collectively, the review seeks to provide a state of the art roadmap for researchers who must contend with this critical barrier to drug delivery.

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.

The role of mucoactive agents in the mechanically ventilated patient: a review of the literature

INTRODUCTION

The management of airway secretions in the mechanically ventilated patient is a routine task throughout all intensive care units. The current treatment strategies are primarily based on anecdotal experiences rather than statistical evidence. Areas covered: This review article evaluates the data from published trials surrounding mucoactive agents and their use in the critically ill patient population. We completed an extensive search through PUBMED and CINAHL via EBSCO, along with the Cochran library to find all trials using mucoactive agents in the critically ill patient population. Expert commentary: Overall, the role of mucoactive agents in the intensive care unit is a field within pulmonary critical care that is in need of evidence-based recommendations. We feel that there is great opportunity for investigators to evaluate different mucoactive therapies in this patient population and to determine their effect on clinical outcomes.

High-resolution mucociliary transport measurement in live excised large animal trachea using synchrotron X-ray imaging

BACKGROUND

The Australian Synchrotron Imaging and Medical Beamline (IMBL) was designed as the world's widest synchrotron X-ray beam, enabling both clinical imaging and therapeutic applications for humans as well as the imaging of large animal models. Our group is developing methods for imaging the airways of newly developed CF animal models that display human-like lung disease, such as the CF pig, and we expect that the IMBL can be utilised to image airways in animals of this size.

METHODS

This study utilised samples of excised tracheal tissue to assess the feasibility, logistics and protocols required for airway imaging in large animal models such as pigs and sheep at the IMBL. We designed an image processing algorithm to automatically track and quantify the tracheal mucociliary transport (MCT) behaviour of 103 μm diameter high refractive index (HRI) glass bead marker particles deposited onto the surface of freshly-excised normal sheep and pig tracheae, and assessed the effects of airway rehydrating aerosols.

RESULTS

We successfully accessed and used scavenged tracheal tissue, identified the minimum bead size that is visible using our chosen imaging setup, verified that MCT could be visualised, and that our automated tracking algorithm could quantify particle motion. The imaging sequences show particles propelled by cilia, against gravity, up the airway surface, within a well-defined range of clearance speeds and with examples of 'clumping' behaviour that is consistent with the in vivo capture and mucus-driven transport of particles.

CONCLUSION

This study demonstrated that the wide beam at the IMBL is suitable for imaging MCT in ex vivo tissue samples. We are now transitioning to in vivo imaging of MCT in live pigs, utilising higher X-ray energies and shorter exposures to minimise motion blur.

Cilia and Mucociliary Clearance

Mucociliary clearance (MCC) is the primary innate defense mechanism of the lung. The functional components are the protective mucous layer, the airway surface liquid layer, and the cilia on the surface of ciliated cells. The cilia are specialized organelles that beat in metachronal waves to propel pathogens and inhaled particles trapped in the mucous layer out of the airways. In health this clearance mechanism is effective, but in patients with primary cilia dyskinesia (PCD) the cilia are abnormal, resulting in deficient MCC and chronic lung disease. This demonstrates the critical importance of the cilia for human health. In this review, we summarize the current knowledge of the components of the MCC apparatus, focusing on the role of cilia in MCC.

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.

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

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

Acetylcholine-induced ex vivo ATP release from the human nasal mucosa

OBJECTIVE

The present study aimed at investigating ATP release in response to acetylcholine (Ach) and pharmacologically elucidating the intracellular signal transduction pathway of this reaction in an ex vivo experiment.

METHODS

The inferior turbinate mucosa was collected from 21 patients with chronic hypertrophic rhinitis who underwent endoscopic turbinectomy. The mucosa was shaped into a filmy round piece, and incubated with chemical(s) in Hank's balanced salt solution for 10min. After incubation, the ATP concentration was measured by a luciferin-luciferase assay.

RESULTS

The baseline release of ATP without stimulus was 57.2±10.3fM. The ATP release was significantly increased by stimulation with 100μM Ach. The Ach-induced ATP release was completely inhibited by removing extracellular Ca²⁺. Significant inhibition of the Ach-induced ATP release was also observed by the addition of 1μM atropine, 40μM 2-APB, 10μM CBX, and 100μM PPADS, whereas 30nM bafilomycin A1 did not affect the ATP release.

CONCLUSION

These results indicate that the Ach-induced ATP release from the human nasal mucosa is dependent on the pannexin-1 channel and purinergic P2X7 receptor, suggesting that these two molecules constitute a local autocrine/paracrine signaling system in the human nasal epithelium.

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.

Role of epithelial sodium channels in the regulation of lung fluid homeostasis

In utero, fetal lung epithelial cells actively secrete Cl(-) ions into the lung air spaces while Na(+) ions follow passively to maintain electroneutrality. This process, driven by an electrochemical gradient generated by the Na(+)-K(+)-ATPase, is responsible for the secretion of fetal fluid that is essential for normal lung development. Shortly before birth, a significant upregulation of amiloride-sensitive epithelial channels (ENaCs) on the apical side of the lung epithelial cells results in upregulation of active Na(+) transport. This process is critical for the reabsorption of fetal lung fluid and the establishment of optimum gas exchange. In the adult lung, active Na(+) reabsorption across distal lung epithelial cells limits the degree of alveolar edema in patients with acute lung injury and cardiogenic edema. Cl(-) ions are transported either paracellularly or transcellularly to preserve electroneutrality. An increase in Cl(-) secretion across the distal lung epithelium has been reported following an acute increase in left atrial pressure and may result in pulmonary edema. In contrast, airway epithelial cells secrete Cl(-) through apical cystic fibrosis transmembrane conductance regulator and Ca(2+)-activated Cl(-) channels and absorb Na(+). Thus the coordinated action of Cl(-) secretion and Na(+) absorption is essential for maintenance of the volume of epithelial lining fluid that, in turn, maximizes mucociliary clearance and facilitates clearance of bacteria and debris from the lungs. Any factor that interferes with Na(+) or Cl(-) transport or dramatically upregulates ENaC activity in airway epithelial cells has been associated with lung diseases such as cystic fibrosis or chronic obstructive lung disease. In this review we focus on the role of the ENaC, the mechanisms involved in ENaC regulation, and how ENaC dysregulation can lead to lung pathology.

CFTR, bicarbonate, and the pathophysiology of cystic fibrosis

The gene that encodes for the cystic fibrosis transmembrane regulator protein (CFTR) was identified in 1989, yet major pathophysiologic questions remain unanswered. There is emerging evidence that CFTR is a bicarbonate channel, a driver of chloride-bicarbonate exchange and through its action on local pH, a regulator of other ion channels and of proteins that function optimally in a neutral environment. In both the respiratory and gastrointestinal (GI) tracts, bicarbonate drives ionic content and fluid on epithelial surfaces, allows mucins to unfold and become slippery, and contributes to innate immunity. In the GI tract bicarbonate neutralizes gastric acid to support digestion and absorption. When CFTR is dysfunctional, lack of bicarbonate secretion disrupts these normal processes and thus leads directly to the clinical symptoms and signs of CF. This article synthesizes evidence from cell, animal, and human investigations that support these concepts. Bicarbonate secretion does not seem to be the same in all tissues and varies with physiologic demand. Thus, tissue type and whether conditions are baseline or stimulated needs to be taken into account when evaluating the evidence concerning the role of bicarbonate in the pathophysiology of CF as a regulator of local pH. Basic and applied research that focuses on the role of CFTR-mediated bicarbonate secretion helps explain many of the diverse clinical manifestations that are CF.

Excess Secretion of Gel-Forming Mucins and Associated Innate Defense Proteins with Defective Mucin Un-Packaging Underpin Gallbladder Mucocele Formation in Dogs

Mucosal protection of the gallbladder is vital yet we know very little about the mechanisms involved. In domestic dogs, an emergent syndrome referred to as gallbladder mucocele formation is characterized by excessive secretion of abnormal mucus that results in obstruction and rupture of the gallbladder. The cause of gallbladder mucocele formation is unknown. In these first mechanistic studies of this disease, we investigated normal and mucocele-forming dog gallbladders to determine the source, identity, biophysical properties, and protein associates of the culprit mucins with aim to identify causes for abnormal mucus behavior. We established that mucocele formation involves an adoptive excess secretion of gel forming mucins with abnormal properties by the gallbladder epithelium. The mucus is characterized by a disproportionally significant increase in Muc5ac relative to Muc5b, defective mucin un-packaging, and mucin-interacting innate defense proteins that are capable of dramatically altering the physical and functional properties of mucus. These findings provide an explanation for abnormal mucus behavior and based on similarity to mucus observed in the airways of people with cystic fibrosis, suggest that abnormal mechanisms for maintenance of gallbladder epithelial hydration may be an instigating factor for mucocele formation in dogs.

Continuous mucociliary transport by primary human airway epithelial cells in vitro

Mucociliary clearance (MCC) is an important innate defense mechanism that continuously removes inhaled pathogens and particulates from the airways. Normal MCC is essential for maintaining a healthy respiratory system, and impaired MCC is a feature of many airway diseases, including both genetic (cystic fibrosis, primary ciliary dyskinesia) and acquired (chronic obstructive pulmonary disease, bronchiectasis) disorders. Research into the fundamental processes controlling MCC, therefore, has direct clinical application, but has been limited in part due to the difficulty of studying this complex multicomponent system in vitro. In this study, we have characterized a novel method that allows human airway epithelial cells to differentiate into a mucociliary epithelium that transports mucus in a continuous circular track. The mucociliary transport device allows the measurement and manipulation of all features of mucociliary transport in a controlled in vitro system. In this initial study, the effect of ciliary beat frequency and mucus concentration on the speed of mucociliary transport was investigated.

A functional anatomic defect of the cystic fibrosis airway

RATIONALE

The mechanisms underlying cystic fibrosis (CF) lung disease pathogenesis are unknown.

OBJECTIVES

To establish mechanisms linking anion transport with the functional microanatomy, we evaluated normal and CF piglet trachea as well as adult swine trachea in the presence of selective anion inhibitors.

METHODS

We investigated airway functional microanatomy using microoptical coherence tomography, a new imaging modality that concurrently quantifies multiple functional parameters of airway epithelium in a colocalized fashion.

MEASUREMENTS AND MAIN RESULTS

Tracheal explants from wild-type swine demonstrated a direct link between periciliary liquid (PCL) hydration and mucociliary transport (MCT) rates, a relationship frequently invoked but never experimentally confirmed. However, in CF airways this relationship was completely disrupted, with greater PCL depths associated with slowest transport rates. This disrupted relationship was recapitulated by selectively inhibiting bicarbonate transport in vitro and ex vivo. CF mucus exhibited increased viscosity in situ due to the absence of bicarbonate transport, explaining defective MCT that occurs even in the presence of adequate PCL hydration.

CONCLUSIONS

An inherent defect in CF airway surface liquid contributes to delayed MCT beyond that caused by airway dehydration alone and identifies a fundamental mechanism underlying the pathogenesis of CF lung disease in the absence of antecedent infection or inflammation.

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.

Cystic fibrosis: an inherited disease affecting mucin-producing organs

Our current understanding of cystic fibrosis (CF) has revealed that the biophysical properties of mucus play a considerable role in the pathogenesis of the disease in view of the fact that most mucus-producing organs are affected in CF patients. In this review, we discuss the potential causal relationship between altered cystic fibrosis transmembrane conductance regulator (CFTR) function and the production of mucus with abnormal biophysical properties in the intestine and lungs, highlighting what has been learned from cell cultures and animal models that mimic CF pathogenesis. A similar cascade of events, including mucus obstruction, infection and inflammation, is common to all epithelia affected by impaired surface hydration. Hence, the main structural components of mucus, namely the polymeric, gel-forming mucins, are critical to the onset of the disease. Defective CFTR leads to epithelial surface dehydration, altered pH/electrolyte composition and mucin concentration. Further, it can influence mucin transition from the intracellular to extracellular environment, potentially resulting in aberrant mucus gel formation. While defective HCO3(-) production has long been identified as a feature of CF, it has only recently been considered as a key player in the transition phase of mucins. We conclude by examining the influence of mucins on the biophysical properties of CF sputum and discuss existing and novel therapies aimed at removing mucus from the lungs. This article is part of a Directed Issue entitled: Cystic Fibrosis: From o-mics to cell biology, physiology, and therapeutic advances.

Assessing mucociliary transport of single particles in vivo shows variable speed and preference for the ventral trachea in newborn pigs

Mucociliary transport (MCT) is an innate defense mechanism that removes particulates, noxious material, and microorganisms from the lung. Several airway diseases exhibit abnormal MCT, including asthma, chronic bronchitis, and cystic fibrosis. However, it remains uncertain whether MCT abnormalities contribute to the genesis of disease or whether they are secondary manifestations that may fuel disease progression. Limitations of current MCT assays and of current animal models of human disease have hindered progress in addressing these questions. Therefore, we developed an in vivo assay of MCT, and here we describe its use in newborn wild-type pigs. We studied pigs because they share many physiological, biochemical, and anatomical features with humans and can model several human diseases. We used X-ray multidetector-row-computed tomography to track movement of individual particles in the large airways of newborn pigs. Multidetector-row-computed tomography imaging provided high spatial and temporal resolution and registration of particle position to airway anatomy. We discovered that cilia orientation directs particles to the ventral tracheal surface. We also observed substantial heterogeneity in the rate of individual particle movement, and we speculate that variations in mucus properties may be responsible. The increased granularity of MCT data provided by this assay may provide an opportunity to better understand host defense mechanisms and the pathogenesis of airway disease.

Non-invasive airway health assessment: synchrotron imaging reveals effects of rehydrating treatments on mucociliary transit in-vivo

To determine the efficacy of potential cystic fibrosis (CF) therapies we have developed a novel mucociliary transit (MCT) measurement that uses synchrotron phase contrast X-ray imaging (PCXI) to non-invasively measure the transit rate of individual micron-sized particles deposited into the airways of live mice. The aim of this study was to image changes in MCT produced by a rehydrating treatment based on hypertonic saline (HS), a current CF clinical treatment. Live mice received HS containing a long acting epithelial sodium channel blocker (P308); isotonic saline; or no treatment, using a nebuliser integrated within a small-animal ventilator circuit. Marker particle motion was tracked for 20 minutes using PCXI. There were statistically significant increases in MCT in the isotonic and HS-P308 groups. The ability to quantify in vivo changes in MCT may have utility in pre-clinical research studies designed to bring new genetic and pharmaceutical treatments for respiratory diseases into clinical trials.

Protein composition of bronchoalveolar lavage fluid and airway surface liquid from newborn pigs

The airway mucosa and the alveolar surface form dynamic interfaces between the lung and the external environment. The epithelial cells lining these barriers elaborate a thin liquid layer containing secreted peptides and proteins that contribute to host defense and other functions. The goal of this study was to develop and apply methods to define the proteome of porcine lung lining liquid, in part, by leveraging the wealth of information in the Sus scrofa database of Ensembl gene, transcript, and protein model predictions. We developed an optimized workflow for detection of secreted proteins in porcine bronchoalveolar lavage (BAL) fluid and in methacholine-induced tracheal secretions [airway surface liquid (ASL)]. We detected 674 and 3,858 unique porcine-specific proteins in BAL and ASL, respectively. This proteome was composed of proteins representing a diverse range of molecular classes and biological processes, including host defense, molecular transport, cell communication, cytoskeletal, and metabolic functions. Specifically, we detected a significant number of secreted proteins with known or predicted roles in innate and adaptive immunity, microbial killing, or other aspects of host defense. In greatly expanding the known proteome of the lung lining fluid in the pig, this study provides a valuable resource for future studies using this important animal model of pulmonary physiology and disease.