Substance P stimulates CFTR-dependent fluid secretion by mouse tracheal submucosal glands

Pulmonary neuroendocrine cells sense succinate to stimulate myoepithelial cell contraction

Pulmonary neuroendocrine cells (PNECs) are rare airway cells with potential sensory capacity linked to vagal neurons and immune cells. How PNECs sense and respond to external stimuli remains poorly understood. We discovered PNECs located within pig and human submucosal glands, a tissue that produces much of the mucus that defends the lung. These PNECs sense succinate, an inflammatory molecule in liquid lining the airway surface. The results indicate that succinate migrates down the submucosal gland duct to the acinus, where it triggers apical succinate receptors, causing PNECs to release ATP. The short-range ATP signal stimulates the contraction of myoepithelial cells wrapped tightly around the submucosal glands. Succinate-triggered gland contraction may complement the action of neurotransmitters that induce mucus release but not gland contraction to promote mucus ejection onto the airway surface. These findings identify a local circuit in which rare PNECs within submucosal glands sense an environmental cue to orchestrate the function of airway glands.

Lipopolysaccharides induce a RAGE-mediated sensitization of sensory neurons and fluid hypersecretion in the upper airways

Thoracic dorsal root ganglia (tDRG) contribute to fluid secretion in the upper airways. Inflammation potentiates DRG responses, but the mechanisms remain under investigation. The receptor for advanced glycation end-products (RAGE) underlies potentiation of DRG responses in pain pathologies; however, its role in other sensory modalities is less understood. We hypothesize that RAGE contributes to electrophysiological and biochemical changes in tDRGs during inflammation. We used tDRGs and tracheas from wild types (WT), RAGE knock-out (RAGE-KO), and with the RAGE antagonist FPS-ZM1, and exposed them to lipopolysaccharides (LPS). We studied: capsaicin (CAP)-evoked currents and action potentials (AP), tracheal submucosal gland secretion, RAGE expression and downstream pathways. In WT neurons, LPS increased CAP-evoked currents and AP generation, and it caused submucosal gland hypersecretion in tracheas from WT mice exposed to LPS. In contrast, LPS had no effect on tDRG excitability or gland secretion in RAGE-KO mice or mice treated with FPS-ZM1. LPS upregulated full-length RAGE (encoded by Tv1-RAGE) and downregulated a soluble (sRAGE) splice variant (encoded by MmusRAGEv4) in tDRG neurons. These data suggest that sensitization of tDRG neurons contributes to hypersecretion in the upper airways during inflammation. And at least two RAGE variants may be involved in these effects of LPS.

Nebulized hypertonic saline triggers nervous system-mediated active liquid secretion in cystic fibrosis swine trachea

Inhaled hypertonic saline (HTS) treatment is used to improve lung health in patients with cystic fibrosis (CF). The current consensus is that the treatment generates an osmotic gradient that draws water into the airways and increases airway surface liquid (ASL) volume. However, there is evidence that HTS may also stimulate active secretion of ASL by airway epithelia through the activation of sensory neurons. We tested the contribution of the nervous system and airway epithelia on HTS-stimulated ASL height increase in CF and wild-type swine airway. We used synchrotron-based imaging to investigate whether airway neurons and epithelia are involved in HTS treatment-triggered ASL secretion in CFTR^−/− and wild-type swine. We showed that blocking parasympathetic and sensory neurons in airway resulted in ~50% reduction of the effect of HTS treatment on ASL volume in vivo. Incubating tracheal preparations with inhibitors of epithelial ion transport across airway decreased secretory responses to HTS treatment. CFTR^−/− swine ex-vivo tracheal preparations showed substantially decreased secretory response to HTS treatment after blockage of neuronal activity. Our results indicated that HTS-triggered ASL secretion is partially mediated by the stimulation of airway neurons and the subsequent activation of active epithelia secretion; osmosis accounts for only ~50% of the effect.

Neuropeptides in asthma, chronic obstructive pulmonary disease and cystic fibrosis

The nervous system mediates key airway protective behaviors, including cough, mucus secretion, and airway smooth muscle contraction. Thus, its involvement and potential involvement in several airway diseases has become increasingly recognized. In the current review, we focus on the contribution of select neuropeptides in three distinct airway diseases: asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis. We present data on some well-studied neuropeptides, as well as call attention to a few that have not received much consideration. Because mucus hypersecretion and mucus obstruction are common features of many airway diseases, we place special emphasis on the contribution of neuropeptides to mucus secretion. Finally, we highlight evidence implicating involvement of neuropeptides in mucus phenotypes in asthma, COPD and cystic fibrosis, as well as bring to light knowledge that is still lacking in the field.

Cystic fibrosis swine fail to secrete airway surface liquid in response to inhalation of pathogens

Cystic fibrosis is caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) channel, which can result in chronic lung disease. The sequence of events leading to lung disease is not fully understood but recent data show that the critical pathogenic event is the loss of the ability to clear bacteria due to abnormal airway surface liquid secretion (ASL). However, whether the inhalation of bacteria triggers ASL secretion and whether this is abnormal in cystic fibrosis has never been tested. Here we show, using a novel synchrotron-based in vivo imaging technique, that wild-type pigs display both a basal and a Toll-like receptor-mediated ASL secretory response to the inhalation of cystic fibrosis relevant bacteria. Both mechanisms fail in CFTR^-/- swine, suggesting that cystic fibrosis airways do not respond to inhaled pathogens, thus favoring infection and inflammation that may eventually lead to tissue remodeling and respiratory disease.Cystic fibrosis is caused by mutations in the CFTR chloride channel, leading to reduced airway surface liquid secretion. Here the authors show that exposure to bacteria triggers secretion in wild-type but not in pig models of cystic fibrosis, suggesting an impaired response to pathogens contributes to infection.

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.

Protein phosphatase 1 coordinates CFTR-dependent airway epithelial HCO3- secretion by reciprocal regulation of apical and basolateral membrane Cl(-)-HCO3- exchangers

BACKGROUND AND PURPOSE

Our recent studies on human airway serous-like Calu-3 cells showed that cAMP agonists stimulated a HCO3(-) rich secretion containing up to 80 mM HCO3(-). This alkaline secretion relied on a coordinated switch in the activity of distinct Cl(-)-HCO3(-) anion exchangers (AE) located at different regions of the cell. At the apical membrane, cAMP agonists activated the electroneutral AE pendrin (SLC26A4), together with cystic fibrosis transmembrane conductance regulator (CFTR), while at the basolateral membrane the agonists inhibited AE2 (SLC4A2). However, the underlying mechanism(s) that orchestrates this cAMP-dependent switch in AE activity has not been elucidated.

EXPERIMENTAL APPROACH

Apical and basolateral Cl(-)-HCO3(-) exchange was assessed by measuring Cl(-)-dependent changes in intracellular pH (pH(i)).

KEY RESULTS

We show that protein phosphatase 1 (PP1), together with CFTR, play central roles in this reciprocal regulation of AE activity. Activation of pendrin by cAMP agonists, but not inhibition of the basolateral exchanger, was protein kinase A-dependent. Knocking down CFTR expression, or blocking its activity with GlyH-101, led to incomplete inhibition of the basolateral AE by cAMP, supporting a role for CFTR in this process. Addition of the PP1/2A inhibitor, okadaic acid, but not the PP2A specific inhibitor fostreicin, mimicked the effect of cAMP stimulation. Furthermore, okadaic acid-treated Calu-3 monolayers produced a more alkaline fluid than untreated cells, which was comparable with that produced by cAMP stimulation.

CONCLUSIONS AND IMPLICATIONS

These results identify PP1 as a novel regulator of AE activity which, in concert with CFTR, coordinates events at both apical and basolateral membranes, crucial for efficient HCO3(-) secretion from Calu-3 cells.

Sinonasal solitary chemosensory cells "taste" the upper respiratory environment to regulate innate immunity

BACKGROUND

It is not fully understood how sinonasal epithelial cells detect the presence of pathogens and activate innate defense responses necessary for protecting the upper airway from infection. One mechanism is through bitter taste receptors (T2Rs), which are expressed in the sinonasal cavity. One T2R isoform, T2R38, is expressed in ciliated cells and detects quorum-sensing molecules from gram-negative bacteria, activating antimicrobial nitric oxide production. More recent studies have examined the role of T2Rs expressed in a sinonasal cell type that has only recently been identified in humans, the solitary chemosensory cell (SCC). We sought to provide an overview of SCCs and taste receptor function in human sinonasal defense as well as implications for chronic rhinosinusitis (CRS).

METHODS

A literature review of the current knowledge of SCCs and taste receptors in sinonasal physiology and CRS was conducted.

RESULTS

Human sinonasal SCCs express both bitter T2R and sweet T1R2/3 receptors. Activation of SCC T2Rs activates a calcium signal that propagates to the surrounding epithelial cells and causes secretion of antimicrobial peptides. T1R2/3 sweet receptor activation by physiological airway surface liquid (ASL) glucose concentrations attenuates the T2R response, likely as a mechanism to prevent full activation of the T2R pathway except during times of infection, when pathogens may consume ASL glucose and reduce its concentration.

CONCLUSION

SCCs appear to be important mediators of upper airway innate immunity, as the SCC T2Rs regulate antimicrobial peptide secretion, but further study is needed to determine the specific T2R isoforms involved as well as whether polymorphisms in these isoforms affect susceptibility to infection or patient outcomes in CRS. The inhibitory role of T1R2/3 sweet receptor suggests that T1R2/3 blockers may have therapeutic potential in some CRS patients, particularly those with diabetes mellitus. However, further clinical study of the relationship between infection and T1R2/3 genotype is required.

Taste receptors in innate immunity

Taste receptors were first identified on the tongue, where they initiate a signaling pathway that communicates information to the brain about the nutrient content or potential toxicity of ingested foods. However, recent research has shown that taste receptors are also expressed in a myriad of other tissues, from the airway and gastrointestinal epithelia to the pancreas and brain. The functions of many of these extraoral taste receptors remain unknown, but emerging evidence suggests that bitter and sweet taste receptors in the airway are important sentinels of innate immunity. This review discusses taste receptor signaling, focusing on the G-protein-coupled receptors that detect bitter, sweet, and savory tastes, followed by an overview of extraoral taste receptors and in-depth discussion of studies demonstrating the roles of taste receptors in airway innate immunity. Future research on extraoral taste receptors has significant potential for identification of novel immune mechanisms and insights into host-pathogen interactions.

Pseudomonas aeruginosa triggers CFTR-mediated airway surface liquid secretion in swine trachea

Cystic fibrosis (CF) is an autosomal recessive genetic disorder caused by mutations in the gene encoding for the anion channel cystic fibrosis transmembrane conductance regulator (CFTR). Several organs are affected in CF, but most of the morbidity and mortality comes from lung disease. Recent data show that the initial consequence of CFTR mutation is the failure to eradicate bacteria before the development of inflammation and airway remodeling. Bacterial clearance depends on a layer of airway surface liquid (ASL) consisting of both a mucus layer that traps, kills, and inactivates bacteria and a periciliary liquid layer that keeps the mucus at an optimum distance from the underlying epithelia, to maximize ciliary motility and clearance of bacteria. The airways in CF patients and animal models of CF demonstrate abnormal ASL secretion and reduced antimicrobial properties. Thus, it has been proposed that abnormal ASL secretion in response to bacteria may facilitate the development of the infection and inflammation that characterize CF airway disease. Whether the inhalation of bacteria triggers ASL secretion, and the role of CFTR, have never been tested, however. We developed a synchrotron-based imaging technique to visualize the ASL layer and measure the effect of bacteria on ASL secretion. We show that the introduction of Pseudomonas aeruginosa and other bacteria into the lumen of intact isolated swine tracheas triggers CFTR-dependent ASL secretion by the submucosal glands. This response requires expression of the bacterial protein flagellin. In patients with CF, the inhalation of bacteria would fail to trigger ASL secretion, leading to infection and inflammation.

Tachykinins and their receptors: contributions to physiological control and the mechanisms of disease

The tachykinins, exemplified by substance P, are one of the most intensively studied neuropeptide families. They comprise a series of structurally related peptides that derive from alternate processing of three Tac genes and are expressed throughout the nervous and immune systems. Tachykinins interact with three neurokinin G protein-coupled receptors. The signaling, trafficking, and regulation of neurokinin receptors have also been topics of intense study. Tachykinins participate in important physiological processes in the nervous, immune, gastrointestinal, respiratory, urogenital, and dermal systems, including inflammation, nociception, smooth muscle contractility, epithelial secretion, and proliferation. They contribute to multiple diseases processes, including acute and chronic inflammation and pain, fibrosis, affective and addictive disorders, functional disorders of the intestine and urinary bladder, infection, and cancer. Neurokinin receptor antagonists are selective, potent, and show efficacy in models of disease. In clinical trials there is a singular success: neurokinin 1 receptor antagonists to treat nausea and vomiting. New information about the involvement of tachykinins in infection, fibrosis, and pruritus justifies further trials. A deeper understanding of disease mechanisms is required for the development of more predictive experimental models, and for the design and interpretation of clinical trials. Knowledge of neurokinin receptor structure, and the development of targeting strategies to disrupt disease-relevant subcellular signaling of neurokinin receptors, may refine the next generation of neurokinin receptor antagonists.

Ca²⁺ signaling and fluid secretion by secretory cells of the airway epithelium

Cytoplasmic Ca(2+) is a master regulator of airway physiology; it controls fluid, mucus, and antimicrobial peptide secretion, ciliary beating, and smooth muscle contraction. The focus of this review is on the role of cytoplasmic Ca(2+) in fluid secretion by airway exocrine secretory cells. Airway submucosal gland serous acinar cells are the primary fluid secreting cell type of the cartilaginous conducting airways, and this review summarizes the current state of knowledge of the molecular mechanisms of serous cell ion transport, with an emphasis on their regulation by intracellular Ca(2+). Many neurotransmitters that regulate secretion from serous acinar cells utilize Ca(2+) as a second messenger. Changes in intracellular Ca(2+) concentration regulate the activities of ion transporters and channels involved in transepithelial ion transport and fluid secretion, including Ca(2+)-activated K(+) channels and Cl(-) channels. We also review evidence of interactions of Ca(2+) signaling with other signaling pathways (cAMP, NO) that impinge upon different ion transport pathways, including the cAMP/PKA-activated cystic fibrosis (CF) transmembrane conductance regulator (CFTR) anion channel. A better understanding of Ca(2+) signaling and its targets in airway fluid secretion may identify novel strategies to intervene in airway diseases, for example to enhance fluid secretion in CF airways.

Cellular mechanism underlying formaldehyde-stimulated Cl- secretion in rat airway epithelium

BACKGROUND

Recent studies suggest that formaldehyde (FA) could be synthesized endogeneously and transient receptor potential (TRP) channel might be the sensor of FA. However, the physiological significance is still unclear.

METHODOLOGY/PRINCIPAL FINDINGS

The present study investigated the FA induced epithelial Cl(-) secretion by activation of TRPV-1 channel located in the nerve ending fiber. Exogenously applied FA induced an increase of I(SC) in intact rat trachea tissue but not in the primary cultured epithelial cells. Western blot and immunofluorescence analysis identified TRPV-1 expression in rat tracheal nerve ending. Capsazepine (CAZ), a TRPV-1 specific antagonist significantly blocked the I(SC) induced by FA. The TRPV-1 agonist capsaicin (Cap) induced an increase of I(SC), which was similar to the I(SC) induced by FA. L-703606, an NK-1 specific inhibitor and propranolol, an adrenalin β receptor inhibitor significantly abolished the I(SC) induced by FA or Cap. In the ion substitute analysis, FA could not induce I(SC) in the absence of extracelluar Cl(-). The I(SC) induced by FA could be blocked by the non-specific Cl(-) channel inhibitor DPC and the CFTR specific inhibitor CFTR(i-172), but not by the Ca(2+)-activated Cl(-) channel inhibitor DIDS. Furthermore, both forskolin, an agonist of adenylate cyclase (AC) and MDL-12330A, an antagonist of AC could block FA-induced I(SC).

CONCLUSION

Our results suggest that FA-induced epithelial I(SC) response is mediated by nerve, involving the activation of TRPV-1 and release of adrenalin as well as substance P.

Why mouse airway submucosal gland serous cells do not secrete fluid in response to cAMP stimulation

Airway submucosal glands are important sites of cystic fibrosis transmembrane conductance regulator (CFTR) chloride (Cl(-)) channel expression and fluid secretion in the airway. Whereas both mouse and human submucosal glands and their serous acinar cells express CFTR, human glands and serous cells secrete much more robustly than mouse cells/glands in response to cAMP-generating agonists such as forskolin and vasoactive intestinal peptide. In this study, we examined mouse and human serous acinar cells to explain this difference and reveal further insights into the mechanisms of serous cell secretion. We found that mouse serous cells possess a robust cAMP-activated CFTR-dependent Cl(-) permeability, but they lack cAMP-activated calcium (Ca(2+)) signaling observed in human cells. Similar to human cells, basal K(+) conductance is extremely small in mouse acinar cells. Lack of cAMP-activated Ca(2+) signaling in mouse cells results in the absence of K(+) conductances required for secretion. However, cAMP activates CFTR-dependent fluid secretion during low-level cholinergic stimulation that fails to activate secretion on its own. Robust CFTR-dependent fluid secretion was also observed when cAMP stimulation was combined with direct pharmacological activation of epithelial K(+) channels with 1-ethyl-2-benzimidazolinone (EBIO). Our data suggest that mouse serous cells lack cAMP-mediated Ca(2+) signaling to activate basolateral membrane K(+) conductance, resulting in weak cAMP-driven serous cell fluid secretion, providing the likely explanation for reduced cAMP-driven secretion observed in mouse compared with human glands.

The cytokines interleukin-1β and tumor necrosis factor-α stimulate CFTR-mediated fluid secretion by swine airway submucosal glands

The airway is kept sterile by an efficient innate defense mechanism. The cornerstone of airway defense is mucus containing diverse antimicrobial factors that kill or inactivate pathogens. Most of the mucus in the upper airways is secreted by airway submucosal glands. In patients with cystic fibrosis (CF), airway defense fails and the lungs are colonized by bacteria, usually Pseudomonas aeruginosa. Accumulating evidence suggests that airway submucosal glands contribute to CF pathogenesis by failing to respond appropriately to inhalation of bacteria. However, the regulation of submucosal glands by the innate immune system remains poorly understood. We studied the response of submucosal glands to the proinflammatory cytokines interleukin-1β and tumor necrosis factor-α. These are released into the airway submucosa in response to infection with the bacterium P. aeruginosa and are elevated in CF airways. Stimulation with IL-1β and TNF-α increased submucosal gland secretion in a concentration-dependent manner with a maximal secretion rate of 240 ± 20 and 190 ± 40 pl/min, respectively. The half maximal effective concentrations were 11 and 20 ng/ml, respectively. The cytokine effect was dependent on cAMP but was independent of cGMP, nitric oxide, Ca(2+), or p38 MAP kinase. Most importantly, IL-1β- and TNF-α-stimulated secretion was blocked by the CF transmembrane conductance regulator (CFTR) blocker, CFTRinh172 (100 μmol/l) but was not affected by the Ca(2+)-activated Cl(-) channel blocker, niflumic acid (1 μmol/l). The data suggest, that during bacterial infections and resulting release of proinflammatory cytokines, the glands are stimulated to secrete fluid, and this response is mediated by cAMP-activated CFTR, a process that would fail in patients with CF.

Functional regulation of cystic fibrosis transmembrane conductance regulator-containing macromolecular complexes: a small-molecule inhibitor approach

CFTR (cystic fibrosis transmembrane conductance regulator) has been shown to form multiple protein macromolecular complexes with its interacting partners at discrete subcellular microdomains to modulate trafficking, transport and signalling in cells. Targeting protein-protein interactions within these macromolecular complexes would affect the expression or function of the CFTR channel. We specifically targeted the PDZ domain-based LPA2 (type 2 lysophosphatidic acid receptor)-NHERF2 (Na+/H+ exchanger regulatory factor-2) interaction within the CFTR-NHERF2-LPA2-containing macromolecular complexes in airway epithelia and tested its regulatory role on CFTR channel function. We identified a cell-permeable small-molecule compound that preferentially inhibits the LPA2-NHERF2 interaction. We show that this compound can disrupt the LPA2-NHERF2 interaction in cells and thus compromises the integrity of macromolecular complexes. Functionally, it elevates cAMP levels in proximity to CFTR and upregulates its channel activity. The results of the present study demonstrate that CFTR Cl- channel function can be finely tuned by modulating PDZ domain-based protein-protein interactions within the CFTR-containing macromolecular complexes. The present study might help to identify novel therapeutic targets to treat diseases associated with dysfunctional CFTR Cl- channels.

CGRP induction in cystic fibrosis airways alters the submucosal gland progenitor cell niche in mice

In cystic fibrosis (CF), a lack of functional CF transmembrane conductance regulator (CFTR) chloride channels causes defective secretion by submucosal glands (SMGs), leading to persistent bacterial infection that damages airways and necessitates tissue repair. SMGs are also important niches for slow-cycling progenitor cells (SCPCs) in the proximal airways, which may be involved in disease-related airway repair. Here, we report that calcitonin gene-related peptide (CGRP) activates CFTR-dependent SMG secretions and that this signaling pathway is hyperactivated in CF human, pig, ferret, and mouse SMGs. Since CGRP-expressing neuroendocrine cells reside in bronchiolar SCPC niches, we hypothesized that the glandular SCPC niche may be dysfunctional in CF. Consistent with this hypothesis, CFTR-deficient mice failed to maintain glandular SCPCs following airway injury. In wild-type mice, CGRP levels increased following airway injury and functioned as an injury-induced mitogen that stimulated SMG progenitor cell proliferation in vivo and altered the proliferative potential of airway progenitors in vitro. Components of the receptor for CGRP (RAMP1 and CLR) were expressed in a very small subset of SCPCs, suggesting that CGRP indirectly stimulates SCPC proliferation in a non-cell-autonomous manner. These findings demonstrate that CGRP-dependent pathways for CFTR activation are abnormally upregulated in CF SMGs and that this sustained mitogenic signal alters properties of the SMG progenitor cell niche in CF airways. This discovery may have important implications for injury/repair mechanisms in the CF airway.

Anion secretion by a model epithelium: more lessons from Calu-3

Anion transport drives fluid into the airways and is essential for humidifying inspired air and supplying surface liquid for mucociliary transport. Despite the importance of airway secretion in diseases such as cystic fibrosis, the cellular mechanisms remain poorly understood, in part due to the small size and complicated structure of the submucosal glands that produce most of the fluid. The Calu-3 human lung adenocarcinoma cell line has become a popular model for studying airway secretion because it can be cultured as a flat sheet, expresses the cystic fibrosis transmembrane conductance regulator and several acinar cell markers, forms polarized monolayers with tight junctions, has robust cAMP-stimulated anion transport, and responds to secretagogues that regulate the glands in vivo. However, some properties of Calu-3 cells are less consistent with those of native tissue. In particular, Calu-3 monolayers do not secrete chloride when stimulated by forskolin under short-circuit conditions. Bicarbonate ions are thought to carry the short-circuit current (I(sc)) and the drive secretion of alkaline fluid, in contrast to the neutral pH secretions that are produced by submucosal glands. Calu-3 cells also have abnormal chromosomes and characteristics of both serous and mucus cells. In this article, we discuss Calu-3 as a model in light of our ongoing studies, which suggest that Calu-3 monolayers resemble submucosal glands more closely than was previously thought. For example, we find that net HCO(3)(-) flux fully accounts for I(sc) as previously suggested but Cl(-) is the main anion transported under physiological conditions. A novel, HCO(3)(-) -dependent mechanism of Cl(-) transport is emerging which may explain secretion by Calu-3 and perhaps other epithelial cells.

Properties of substance P-stimulated mucus secretion from porcine tracheal submucosal glands

Human and pig airway submucosal glands secrete mucus in response to substance P (SubP), but in pig tracheal glands the response to SubP is >10-fold greater than in humans and shares features with cholinergically produced secretion. CFTR-deficient pigs provide a model for human cystic fibrosis (CF), and in newborn CF pigs the response of tracheal glands to SubP is significantly reduced (Joo et al. J Clin Invest 120: 3161–3166, 2010). To further define features of SubP-mediated gland secretion, we optically measured secretion rates from individual adult porcine glands in isolated tracheal tissues in response to mucosal capsaicin and serosal SubP. Mucosal capsaicin (EC50 = 19 μM) stimulated low rates of secretion that were partially inhibited by tetrodotoxin and by inhibitors for muscarinic, VIP, and SubP receptors, suggesting reflex stimulation of secretion by multiple transmitters. Secretion in response to mucosal capsaicin was inhibited by CFTRinh-172, but not by niflumic acid. Serosal SubP (EC50 = 230 nM) stimulated 10-fold more secretion than mucosal capsaicin, with a Vmax similar to that of carbachol. Secretion rates peaked within 5 min and then declined to a lower sustained rate. SubP-stimulated secretion was inhibited 75% by bumetanide, 53% by removal of HCO3−, and 85% by bumetanide + removal of HCO3−; it was not inhibited by atropine but was inhibited by niflumic acid, clotrimazole, BAPTA-AM, nominally Ca2+-free bath solution, and the adenylate cyclase inhibitor MDL-12330A. Ratiometric measurements of fura 2 fluorescence in dissociated gland cells showed that SubP and carbachol increased intracellular Ca2+ concentration by similar amounts. SubP produced rapid volume loss by serous and mucous cells, expansion of gland lumina, mucus flow, and exocytosis but little or no contraction of myoepithelial cells. These and prior results suggest that SubP stimulates pig gland secretion via CFTR- and Ca2+-activated Cl− channels.

Control of enteric neuromuscular functions by purinergic A(3) receptors in normal rat distal colon and experimental bowel inflammation

BACKGROUND AND PURPOSE

Adenosine A(3) receptors mediate beneficial effects in experimental colitis, but their involvement in enteric neuromuscular functions during bowel inflammation is undetermined. This study investigated the regulatory role of A(3) receptors on colonic motility in the presence of experimental colitis.

EXPERIMENTAL APPROACH

Colitis was induced in rats by 2,4-dinitrobenzenesulfonic acid. A(3) receptors and adenosine deaminase (ADA, adenosine catabolic enzyme) mRNA were examined by RT-PCR. Tissue distribution of A(3) receptors was detected by confocal immunofluorescence. The effects of 2,3-ethyl-4,5-dipropyl-6-phenylpyridine-3-thiocarboxylate-5-carboxylate (MRS1523) (MRS, A(3) receptor antagonist), 2-chloro-N(6) -(3-iodobenzyl)-adenosine-5'-N-methyluronamide (2Cl-IB-MECA) (CIB, A(3) receptor agonist), dipyridamole (DIP, adenosine transport inhibitor) and ADA were assayed on contractile responses evoked by electrical stimulation (ES) or carbachol in colonic longitudinal muscle preparations (LMP).

KEY RESULTS

RT-PCR showed A(3) receptors and ADA mRNA in normal colon and their increased level in inflamed tissues. Immunofluorescence showed a predominant distribution of A(3) receptors in normal myenteric ganglia and an increased density during colitis. MRS enhanced ES-induced cholinergic contractions in normal LMP, but was less effective in inflamed tissues. After pretreatment with dipyridamole plus ADA, to reduce extracellular adenosine, CIB decreased cholinergic motor responses of normal LMP to ES, with enhanced efficacy in inflamed LMP. A(3) receptor ligands did not affect carbachol-induced contractions in LMP from normal or inflamed colon.

CONCLUSIONS AND IMPLICATIONS

Normally, adenosine modulated colonic cholinergic motility via activation of A(3) receptors in the myenteric plexus. A(3) receptor-mediated tonic inhibitory control by adenosine was impaired in inflamed bowel, despite increased density of functioning and pharmacologically recruitable A(3) receptors.

Functional study of mucus secretion of the eustachian tube in Guinea pigs

OBJECTIVES

Our aim was to verify neural regulation of submucous gland mucus secretions in the Eustachian tubes of guinea pigs.

STUDY DESIGN

Prospective animal study.

METHODS

Eustachian tubes harvested from 12 guinea pigs were used for this study. For real-time resolution of pure glandular secretion, we used a modified method of single-gland optical measurement. Secretory monitoring was undertaken after each preparation with phenylephrine, isoproterenol, forskolin, and substance P. To confirm the viability of each tissue, we examined glandular secretion after treatment with carbachol. Secretory effects of each agonist were evaluated by comparing with basal secretion using a Student's t test (p < 0.01).

RESULTS

The Ca-elevating agonists carbachol and substance P showed greater effects on submucous gland secretions of the Eustachian tube than the cyclic adenosine monophosphate (cAMP)-elevating agonists forskolin and isoproterenol. However, phenylephrine, although it belongs to the Ca-elevating agonist group, did not show any significant secretory effect.

CONCLUSION

The optical measurement method used in this study had the merit of real-time resolution of submucous glandular secretion. Submucous glandular secretion in the Eustachian tube was regulated by both Ca- and cAMP-elevating agonists, and Ca-elevating agonists seemed to be more potent than cAMP-elevating agonists except phenylephrine. Our results suggest that not only the autonomic nerve system but also the neuropeptides such as substance P are closely related to glandular secretion in the Eustachian tube, and beta-adrenergic receptors seem to be more related to submucous glandular secretion of the Eustachian tube in guinea pig than alpha-adrenergic receptors.

A new role for bicarbonate secretion in cervico-uterine mucus release

Cervical mucus thinning and release during the female reproductive cycle is thought to rely mainly on fluid secretion. However, we now find that mucus released from the murine reproductive tract critically depends upon concurrent bicarbonate (HCO(3)(-)) secretion. Prostaglandin E(2) (PGE(2))- and carbachol-stimulated mucus release was severely inhibited in the absence of serosal HCO(3)(-), HCO(3)(-) transport, or functional cystic fibrosis transmembrane conductance regulator (CFTR). In contrast to mucus release, PGE(2)- and carbachol-stimulated fluid secretion was not dependent on bicarbonate or on CFTR, but was completely blocked by niflumic acid. We found stimulated mucus release was severely impaired in the cystic fibrosis F508 reproductive tract, even though stimulated fluid secretion was preserved. Thus, CFTR mutations and/or poor bicarbonate secretion may be associated with reduced female fertility associated with abnormal mucus and specifically, may account for the increased viscosity and lack of cyclical changes in cervical mucus long noted in women with cystic fibrosis.

Compartmentalized cyclic adenosine 3',5'-monophosphate at the plasma membrane clusters PDE3A and cystic fibrosis transmembrane conductance regulator into microdomains

Formation of multiple-protein macromolecular complexes at specialized subcellular microdomains increases the specificity and efficiency of signaling in cells. In this study, we demonstrate that phosphodiesterase type 3A (PDE3A) physically and functionally interacts with cystic fibrosis transmembrane conductance regulator (CFTR) channel. PDE3A inhibition generates compartmentalized cyclic adenosine 3',5'-monophosphate (cAMP), which further clusters PDE3A and CFTR into microdomains at the plasma membrane and potentiates CFTR channel function. Actin skeleton disruption reduces PDE3A-CFTR interaction and segregates PDE3A from its interacting partners, thus compromising the integrity of the CFTR-PDE3A-containing macromolecular complex. Consequently, compartmentalized cAMP signaling is lost. PDE3A inhibition no longer activates CFTR channel function in a compartmentalized manner. The physiological relevance of PDE3A-CFTR interaction was investigated using pig trachea submucosal gland secretion model. Our data show that PDE3A inhibition augments CFTR-dependent submucosal gland secretion and actin skeleton disruption decreases secretion.

Mechanisms of Ca2+-stimulated fluid secretion by porcine bronchial submucosal gland serous acinar cells

The serous acini of airway submucosal glands are important for fluid secretion in the lung. Serous cells are also sites of expression of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. However, the mechanisms of serous cell fluid secretion remain poorly defined. In this study, serous acinar cells were isolated from porcine bronchi and studied using optical techniques previously used to examine fluid secretion in rat parotid and murine nasal acinar cells. When stimulated with the cholinergic agonist carbachol, porcine serous cells shrank by approximately 20% (observed via DIC microscopy) after a profound elevation of intracellular [Ca(2+)] ([Ca(2+)](i); measured by simultaneous fura 2 fluorescence imaging). Upon removal of agonist and relaxation of [Ca(2+)](i) to resting levels, cells swelled back to resting volume. Similar results were observed during stimulation with histamine and ATP, and elevation of [Ca(2+)](i) was found to be necessary and sufficient to activate shrinkage. Cell volume changes were associated with changes in [Cl(-)](i) (measured using SPQ fluorescence), suggesting that shrinkage and swelling are caused by loss and gain of intracellular solute content, respectively, likely reflecting changes in the secretory state of the cells. Shrinkage was inhibited by niflumic acid but not by GlyH-101, suggesting Ca(2+)-activated secretion is mediated by alternative non-CFTR Cl(-) channels, possibly including Ano1 (TMEM16A), expressed on the apical membrane of porcine serous cells. Optimal cell swelling/solute uptake required activity of the Na(+)K(+)2Cl(-) cotransporter and Na(+)/H(+) exchanger, both of which are expressed on the basolateral membrane of serous acini and likely contribute to sustaining transepithelial secretion.

Effects of secretagogues on net and unidirectional liquid fluxes across porcine bronchial airways

Rates of liquid secretion and absorption across the bronchopulmonary airways are important for regulating airway surface liquid volume and maintaining mucociliary transport. The current study demonstrates the feasibility of measuring not just net liquid movements but unidirectional liquid movements across isolated intact bronchi from swine. Airways were liquid filled to assess both net liquid movements, and, in the presence of NPPB to selectively inhibit secretion, unidirectional absorption. Unidirectional liquid secretion rates were determined by subtraction. For comparison, net liquid movements were assessed in air-filled airways in parallel. In the absence of secretagogues, unidirectional absorption was observed (4.63 +/- 0.53 microl.cm(-2).h(-1)) with little unidirectional secretion (1.42 +/- 0.36 microl.cm(-2).h(-1)). ACh, substance P (SP), and vasoactive intestinal peptide (VIP) all induced unidirectional secretion (10.64 +/- 1.52 microl.cm(-2).h(-1), 14.16 +/- 1.39 microl.cm(-2).h(-1), and 4.25 +/- 0.25 microl.cm(-2).h(-1), respectively) without affecting unidirectional absorption. Net liquid secretion in air-filled airways was close to that in liquid-filled airways except with VIP. VIP induced net secretion in air-filled airways (4.44 +/- 1.26 microl.cm(-2).h(-1)), but negligible net change in liquid movement occurred in liquid-filled airways. This effect was likely to have been caused by the higher solid content of the VIP-induced mucous liquid (3.98 +/- 0.26%) compared with the ACh- and SP-induced liquid (2.06 +/- 0.07% and 2.15 +/- 0.07%, respectively). We conclude that this technique allows important quantitative distinctions to be made between liquid secretion and absorption in intact bronchial airways.

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.