Differential effect of epidermal growth factor on serous and mucous cells in porcine airway submucosal gland

Effects of indacaterol on the LPS-evoked changes in fluid secretion rate and pH in swine tracheal membrane

An acquired dysregulation of airway secretion is likely involved in the pathophysiology of chronic bronchitis and chronic obstructive pulmonary disease (COPD). Nowadays, it is widely known that several kinds of long-acting bronchodilators reduce the frequency of COPD exacerbations. However, limited data are available concerning the complementary additive effects on airflow obstruction. Using an optical method and a selective pH indicator, we succeeded in evaluating the gland secretion rate and the pH in swine tracheal membrane. A physiologically relevant concentration of acetylcholine (ACh) 100 nM induced a gradual increase in the amount of gland secretion. Lipopolysaccharides (LPS) accelerated the ACh-induced secretory responses up to around threefold and lowered the pH level significantly. Long-acting β₂-agonists (LABAs) including indacaterol (IND), formoterol, and salmeterol restored the LPS-induced changes in both the hypersecretion and acidification. The subsequent addition of the long-acting muscarine antagonist, glycopyrronium, further increased the pH values. Two different inhibitors for cystic fibrosis transmembrane conductance regulator (CFTR), NPPB and CFTR_inh172, abolished the IND-mediated pH normalization in the presence of both ACh and ACh + LPS. Both immunofluorescence staining and western blotting analysis revealed that LPS downregulated the abundant expression of CFTR protein. However, IND did not restore the LPS-induced decrease in CFTR expression on Calu-3 cells. These findings suggest that the activation of cAMP-dependent HCO₃⁻ secretion through CFTR would be partly involved in the IND-mediated pH normalization in gland secretion and may be suitable for the maintenance of airway defense against exacerbating factors including LPS.

TLR7 agonist attenuates acetylcholine-induced, Ca2+ -dependent ionic currents in swine tracheal submucosal gland cells

NEW FINDINGS

What is the central question of this study? Does Toll-like receptor 7 (TLR7) have any direct effects on Ca²⁺ -dependent physiological function of tracheal submucosal gland cells? What is the main finding and its importance? TLR7 is co-localized with SERCA2 in tracheal submucosal gland cells and causes a rapid attenuation of acetylcholine (ACh)-induced, Ca²⁺ -dependent ionic currents through the activation of SERCA2-dependent Ca²⁺ clearance. TLR7 is abundantly expressed in the airways of both swine and healthy human subjects, but is significantly downregulated in chronic obstructive pulmonary disease (COPD) airways. These findings suggest that a dysfunction of TLR7 in COPD removes the brake on ACh-induced serous secretion during viral infections, resulting in prolonged airway hypersecretion, and that it is one of the triggers of COPD exacerbations.

ABSTRACT

Airway surface fluids are mainly secreted from submucosal glands (SMGs) and play important roles in the defence of airways via the activation of mucociliary transport. Toll-like receptor 7 (TLR7) recognizes and eliminates single stranded RNA (ssRNA) viruses through the induction of innate immunity. However, there is no obvious connection between TLR7 and mucus secretion, aside from TLR7 recognizing ssRNA viruses, which are often associated with airway hypersecretion in chronic obstructive pulmonary disease (COPD). Here, we investigated whether TLR7 has any direct effects on the Ca²⁺ -dependent physiological function of tracheal SMG cells. Patch-clamp analyses revealed that TLR7 ligand inhibited the acetylcholine (ACh)-induced ionic currents in isolated tracheal SMG cells. Intracellular calcium assays and pharmacological analyses revealed that TLR7 attenuated the transient rises in the intracellular calcium concentration evoked by ACh by activating sarco/endoplasmic reticulum Ca²⁺ -ATPase 2 (SERCA2). Immunofluorescence staining and immunohistochemical staining revealed that TLR7 was co-localized with SERCA2. These findings suggest that the activation of TLR7 during viral infections contributes to the rapid attenuation of ACh-induced ionic currents through an increase in SERCA2-dependent Ca²⁺ clearance in healthy airway SMG cells. Our study also revealed that TLR7 expression was significantly downregulated in COPD airways. Based on these findings, we speculate that a dysfunction of TLR7 may not only have an adverse effect on the elimination of these viruses but also remove the brake on ACh-induced serous secretion, resulting in prolonged hypersecretion and acting as one of the triggers of COPD exacerbations.

How does rhinovirus cause the common cold cough?

Cough is a protective reflex to prevent aspiration and can be triggered by a multitude of stimuli. The commonest form of cough is caused by upper respiratory tract infection and has no benefit to the host. The virus hijacks this natural defence mechanism in order to propagate itself through the population. Despite the resolution of the majority of cold symptoms within 2 weeks, cough can persist for some time thereafter. Unfortunately, the mechanism of infectious cough brought on by pathogenic viruses, such as human rhinovirus, during colds, remains elusive despite the extensive work that has been undertaken. For socioeconomic reasons, it is imperative we identify the mechanism of cough. There are several theories which have been proposed as the causative mechanism of cough in rhinovirus infection, encompassing a range of different processes. Those of which hold most promise are physical disruption of the epithelial lining, excess mucus production and an inflammatory response to rhinovirus infection which may be excessive. And finally, neuronal modulation, the most convincing hypothesis, is thought to potentiate cough long after the original stimulus has been cleared. All these hypotheses will be briefly covered in the following sections.

High-intensity focused ultrasound leads to histopathologic changes of the inferior turbinate mucosa with allergic inflammation

This study was aimed at understanding the histopathologic changes that occur in the nasal mucosa of patients with perennial allergic rhinitis after high-intensity focused ultrasound (HIFU) treatment. Biopsy specimens of the inferior turbinate mucosa were taken from 11 PAR patients before, immediately after and 1 y after HIFU treatment. Morphometric analysis revealed that the density of eosinophils and other inflammatory cells increased immediately after treatment and then were decreased significantly 1 y post-treatment. Submucosal glands were swollen and venous sinusoids were dilated, but there was no statistically significant change in their density, immediately after treatment. However, both glands and venous sinusoids significantly decreased in number 1 y after HIFU treatment. The ciliated epithelium or basement membrane of the nasal mucosa was well preserved at all stages. In conclusion, HIFU is a tolerable and effective treatment to reduce inflammation of the inferior turbinate mucosa in patients with perennial allergic rhinitis.

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

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

Flagellin/TLR5 signaling potentiates airway serous secretion from swine tracheal submucosal glands

Airway serous secretion is essential for the maintenance of mucociliary transport in airway mucosa, which is responsible for the upregulation of mucosal immunity. Although there are many articles concerning the importance of Toll-like receptors (TLRs) in airway immune systems, the direct relationship between TLRs and airway serous secretion has not been well investigated. Here, we focused on whether TLR5 ligand flagellin, which is one of the components of Pseudomonas aeruginosa, is involved in the upregulation of airway serous secretion. Freshly isolated swine tracheal submucosal gland cells were prepared, and the standard patch-clamp technique was applied for measurements of the whole cell ionic responses of these cells. Flagellin showed potentiating effects on these oscillatory currents induced by physiologically relevant low doses of acetylcholine (ACh) in a dose-dependent manner. These potentiating effects were TLR5 dependent but TLR4 independent. Both nitric oxide (NO) synthase inhibitors and cGMP-dependent protein kinase (cGK) inhibitors abolished these flagellin-induced potentiating effects. Furthermore, TLR5 was abundantly expressed on tracheal submucosal glands. Flagellin/TLR5 signaling further accelerated the intracellular NO synthesis induced by ACh. These findings suggest that TLR5 takes part in the airway mucosal defense systems as a unique endogenous potentiator of airway serous secretions and that NO/cGMP/cGK signaling is involved in this rapid potentiation by TLR5 signaling.

Vitamin A deficiency induces fluid hyposecretion from the airway submucosal glands of mice

Vitamin A deficiency (VAD) alters the phenotype of airway epithelium and attenuates the epithelial defense system, and many studies have reported the association of VAD with respiratory disease. In this study, we investigated changes in submucosal glands (SMG) in a mouse model of VAD. C57BL/6 mice were fed a vitamin A-devoid diet and the others were fed a control diet (1.2 mg retinol/kg). The areas of serous and mucous cells of SMG were measured in 4-, 8-, and 20-wk-old male mice. The volume and lysozyme concentration of glandular secretions were also measured. The 2 groups did not differ in body weight or general morbidity at 3-10 wk of age, although serum retinol concentrations were greater in the control mice than in the VAD mice after 4 wk. Upon histological evaluation, we found that the areal ratio of serous cells:total SMG cells was significantly lower after 8 wk in the VAD mice compared with the control mice, although the total area of SMG did not differ between groups throughout the 20-wk experiment. The number of secretory bubbles did not differ between the groups, but total secretion volume was reduced by 35% in 8-wk-old VAD mice compared with controls. Furthermore, the concentration of lysozyme in secretions from 8-wk-old VAD mice was also less than in controls, compounding the effect of diminished secretion volume. In this study, we found serous cell hypotrophy/hypoplasia and dysfunction in VAD mice, which may contribute to the susceptibility to airway infection linked to VAD.

Regulation of airway inflammation and remodeling by muscarinic receptors: perspectives on anticholinergic therapy in asthma and COPD

Acetylcholine is the primary parasympathetic neurotransmitter in the airways and an autocrine/paracrine secreted hormone from non-neuronal origins including inflammatory cells and airway structural cells. In addition to the well-known functions of acetylcholine in regulating bronchoconstriction and mucus secretion, it is increasingly evident that acetylcholine regulates inflammatory cell chemotaxis and activation, and also participates in signaling events leading to chronic airway wall remodeling that is associated with chronic obstructive airway diseases including asthma and COPD. As muscarinic receptors appear responsible for most of the pro-inflammatory and remodeling effects of acetylcholine, these findings have significant implications for anticholinergic therapy in asthma and COPD, which is selective for muscarinic receptors. Here, the regulatory role of acetylcholine in inflammation and remodeling in asthma and COPD will be discussed including the perspectives that these findings offer for anticholinergic therapy in these diseases.

Effect of resection of the posterior nasal nerve on functional and morphological changes in the inferior turbinate mucosa

CONCLUSIONS

The underlying pathophysiological mechanisms of the posterior nasal nerve (PNN) resection involved the suppression of the secretogogue motor and the inhibition of neurogenic inflammation induced by parasympathetic and sensory denervation.

OBJECTIVE

The study was designed to clarify the underlying mechanisms of the resection of the PNN.

PATIENTS AND METHODS

Ten patients with allergic rhinitis and non-allergic chronic rhinitis were enrolled in the study. Clinical symptoms were evaluated before and after administration of oxitropium bromide aerosol, and after the PNN resection. Biopsy specimens from the inferior turbinate mucosa obtained from five patients before and after resection of the PNN were examined.

RESULTS

The application of oxitropium bromide resulted in a significant reduction of both watery rhinorrhea and nasal obstruction, but not sneezing or postnasal drip. Resection of the PNN also significantly improved both rhinorrhea and nasal obstruction. Morphometric analysis of the density of the nasal gland showed a significant reduction, whereas no significant change was recognized in the density of the vessels. A significant reduction in the number of infiltrating neutrophils, eosinophils, and lymphocytes was recognized.

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

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

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

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

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

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

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

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

A potentiating effect of endogenous NO in the physiologic secretion from airway submucosal glands

It is known that several second messengers, such as Ca(2+) or cAMP, play important roles in the intracellular pathway of electrolyte secretion in tracheal submucosal gland. However, the participation of cGMP, and therefore nitric oxide (NO), is not well understood. To investigate the physiologic role of NO, we first examined whether tracheal glands can synthesize NO in response to acetylcholine (ACh), and then whether endogenous NO has some effects on the ACh-triggered ionic currents. From the experiments using the NO-specific fluorescent indicator 4,5-diaminofluorescein diacetate salt (DAF-2DA), we found that a physiologically relevant low dose of ACh (100 nM) stimulated the endogenous NO synthesis, and it was almost completely suppressed in the presence of the nonspecific NO synthase (NOS) inhibitor Nomega-Nitro-L-arginine Methyl Ester Hydrochloride (L-NAME) or the neuronal NOS (nNOS)-specific inhibitor 7-Nitroindazole (7-NI). Patch-clamp experiments revealed that both the NOS inhibitors (L-NAME or 7-NI) and cGK inhibitors (KT-5823 or Rp-8-Br-cGMP) partially decreased ionic currents induced by 30 nM of ACh, but not in the case of 300 nM of ACh. Our results indicate that NO can be synthesized through the activation of nNOS endogenously and has potentiating effects on the gland secretion, under a physiologically relevant ACh stimulation. When cells were stimulated by an inadequately potent dose of ACh, which caused an excess elevation in [Ca(2+)](i), the cells were desensitized. Therefore, due to NO, gland cells become more sensitive to calcium signaling and are able to maintain electrolyte secretion without desensitization.

Muscarinic receptor signaling in the pathophysiology of asthma and COPD

Anticholinergics are widely used for the treatment of COPD, and to a lesser extent for asthma. Primarily used as bronchodilators, they reverse the action of vagally derived acetylcholine on airway smooth muscle contraction. Recent novel studies suggest that the effects of anticholinergics likely extend far beyond inducing bronchodilation, as the novel anticholinergic drug tiotropium bromide can effectively inhibit accelerated decline of lung function in COPD patients. Vagal tone is increased in airway inflammation associated with asthma and COPD; this results from exaggerated acetylcholine release and enhanced expression of downstream signaling components in airway smooth muscle. Vagally derived acetylcholine also regulates mucus production in the airways. A number of recent research papers also indicate that acetylcholine, acting through muscarinic receptors, may in part regulate pathological changes associated with airway remodeling. Muscarinic receptor signalling regulates airway smooth muscle thickening and differentiation, both in vitro and in vivo. Furthermore, acetylcholine and its synthesizing enzyme, choline acetyl transferase (ChAT), are ubiquitously expressed throughout the airways. Most notably epithelial cells and inflammatory cells generate acetylcholine, and express functional muscarinic receptors. Interestingly, recent work indicates the expression and function of muscarinic receptors on neutrophils is increased in COPD. Considering the potential broad role for endogenous acetylcholine in airway biology, this review summarizes established and novel aspects of muscarinic receptor signaling in relation to the pathophysiology and treatment of asthma and COPD.

Cystic fibrosis and airway submucosal glands

The chronic pulmonary infections and inflammation associated with cystic fibrosis (CF) are responsible for almost all the morbidity and mortality of this disease. Our understanding of the mechanisms that underlie the very early stages of CF lung disease, that result directly from mutations in the CF gene, is relatively poor. However, the demonstration that the predominant sites of expression of the CF gene in normal lungs are the submucosal glands, together with the histological observations showing that hyperplasia of these glands and mucin occlusion of the gland ducts are the earliest signs of disease in the CF lung, suggest that malfunction of the submucosal glands may be an important factor contributing to the early pathophysiology of CF lung disease. This review describes the function of submucosal glands in normal lungs, and the way in which their function may be disrupted in CF and may thus contribute to the early stages of CF lung disease.