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

Neural Mechanisms Underlying the Coughing Reflex

Breathing is an intrinsic natural behavior and physiological process that maintains life. The rhythmic exchange of gases regulates the delicate balance of chemical constituents within an organism throughout its lifespan. However, chronic airway diseases, including asthma and chronic obstructive pulmonary disease, affect millions of people worldwide. Pathological airway conditions can disrupt respiration, causing asphyxia, cardiac arrest, and potential death. The innervation of the respiratory tract and the action of the immune system confer robust airway surveillance and protection against environmental irritants and pathogens. However, aberrant activation of the immune system or sensitization of the nervous system can contribute to the development of autoimmune airway disorders. Transient receptor potential ion channels and voltage-gated Na+ channels play critical roles in sensing noxious stimuli within the respiratory tract and interacting with the immune system to generate neurogenic inflammation and airway hypersensitivity. Although recent studies have revealed the involvement of nociceptor neurons in airway diseases, the further neural circuitry underlying airway protection remains elusive. Unraveling the mechanism underpinning neural circuit regulation in the airway may provide precise therapeutic strategies and valuable insights into the management of airway diseases.

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.

Regulatory Peptides in Asthma

Numerous regulatory peptides play a critical role in the pathogenesis of airway inflammation, airflow obstruction and hyperresponsiveness, which are hallmarks of asthma. Some of them exacerbate asthma symptoms, such as neuropeptide Y and tachykinins, while others have ameliorating properties, such as nociception, neurotensin or β-defensin 2. Interacting with peptide receptors located in the lungs or on immune cells opens up new therapeutic possibilities for the treatment of asthma, especially when it is resistant to available therapies. This article provides a concise review of the most important and current findings regarding the involvement of regulatory peptides in asthma pathology.

Combined agonists act synergistically to increase mucociliary clearance in a cystic fibrosis airway model

Mucus clearance, a primary innate defense mechanism of airways, is defective in patients with cystic fibrosis (CF) and CF animals. In previous work, the combination of a low dose of the cholinergic agonist, carbachol with forskolin or a β adrenergic agonist, isoproterenol synergistically increased mucociliary clearance velocity (MCCV) in ferret tracheas. Importantly, the present study shows that synergistic MCCV can also be produced in CF ferrets, with increases ~ 55% of WT. Synergistic MCCV was also produced in pigs. The combined agonists increased MCCV by increasing surface fluid via multiple mechanisms: increased fluid secretion from submucosal glands, increased anion secretion across surface epithelia and decreased Na⁺ absorption. To avoid bronchoconstriction, the cAMP agonist was applied 30 min before carbachol. This approach to increasing mucus clearance warrants testing for safety and efficacy in humans as a potential therapeutic for muco-obstructive diseases.

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

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

[Endothelium-related and neuro-mediated mechanisms of emphysema development in chronic obstructive pulmonary disease]

Emphysema is one of the main manifestations of chronic obstructive pulmonary disease (COPD), and smoking is one of the most significant risk factors. The results of studies in humans and animals show the vascular endothelium initiates and modulates the main pathological processes in COPD and smoking is an important factor initiating, developing and persisting inflammation and remodeling of blood vessels and tissues, including the destruction of small respiratory tracts with the development of lung tissue destruction and emphysema. The latest studies describe mechanisms not just associated with the endothelium, but specific neuro-mediated mechanisms. There is reason to believe that neuro-mediated and neuro-similar mechanisms associated and not related to endothelial dysfunction may play the significant role in the pathogenesis of COPD and emphysema formation. Information about components and mechanisms of neurogenic inflammation in emphysema development is fragmentary and not systematized in the literature. It is described that long-term tobacco smoking can initiate processes not only of cells and tissues damage, but also become a trigger for excessive release of neurotransmitters, which entails whole cascades of adverse reactions that have an effect on emphysema formation. With prolonged and/or intensive stimulation of sensor fibers, excessive release of neuropeptides is accompanied by a number of plastic and destructive processes due to a cascade of pathological reactions of neurogenic inflammation, the main participants of which are classical neuropeptides and their receptors. The most important consequences can be the maintenance and stagnation of chronic inflammation, activation of the mechanisms of destruction and remodeling, inadequate repair processes in response to damage, resulting in irreversible loss of lung tissue. For future research, there is interest to evaluate the possibilities of therapeutic and prophylactic effects on neuro-mediated mechanisms of endothelial dysfunction and damage emphysema in COPD and smoking development.

Mycoplasma hyopneumoniae surface-associated proteases cleave bradykinin, substance P, neurokinin A and neuropeptide Y

Mycoplasma hyopneumoniae is an economically-devastating and geographically-widespread pathogen that colonises ciliated epithelium, and destroys mucociliary function. M. hyopneumoniae devotes ~5% of its reduced genome to encode members of the P97 and P102 adhesin families that are critical for colonising epithelial cilia, but mechanisms to impair mucociliary clearance and manipulate host immune response to induce a chronic infectious state have remained elusive. Here we identified two surface exposed M. hyopneumoniae proteases, a putative Xaa-Pro aminopeptidase (MHJ_0659; PepP) and a putative oligoendopeptidase F (MHJ_0522; PepF), using immunofluorescence microscopy and two orthogonal proteomic methodologies. MHJ_0659 and MHJ_0522 were purified as polyhistidine fusion proteins and shown, using a novel MALDI-TOF MS assay, to degrade four pro-inflammatory peptides that regulate lung homeostasis; bradykinin (BK), substance P (SP), neurokinin A (NKA) and neuropeptide Y (NPY). These findings provide insight into the mechanisms used by M. hyopneumoniae to influence ciliary beat frequency, impair mucociliary clearance, and initiate a chronic infectious disease state in swine, features that are a hallmark of disease caused by this pathogen.

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.

Mucus strands from submucosal glands initiate mucociliary transport of large particles

Mucus produced by submucosal glands is a key component of respiratory mucociliary transport (MCT). When it emerges from submucosal gland ducts, mucus forms long strands on the airway surface. However, the function of those strands is uncertain. To test the hypothesis that mucus strands facilitate transport of large particles, we studied newborn pigs. In ex vivo experiments, interconnected mucus strands moved over the airway surface, attached to immobile spheres, and initiated their movement by pulling them. Stimulating submucosal gland secretion with methacholine increased the percentage of spheres that moved and shortened the delay until mucus strands began moving spheres. To disrupt mucus strands, we applied reducing agents tris-(2-carboxyethyl)phosphine and dithiothreitol. They decreased the fraction of moving spheres and delayed initiation of movement for spheres that did move. We obtained similar in vivo results with CT-based tracking of microdisks in spontaneously breathing pigs. Methacholine increased the percentage of microdisks moving and reduced the delay until they were propelled up airways. Aerosolized tris-(2-carboxyethyl)phosphine prevented those effects. Once particles started moving, reducing agents did not alter their speed either ex vivo or in vivo. These findings indicate that submucosal glands produce mucus in the form of strands and that the strands initiate movement of large particles, facilitating their removal from airways.

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.

Synergistic mucus secretion by histamine and IL-4 through TMEM16A in airway epithelium

Histamine is an important mediator of allergic reactions, and mucus hypersecretion is a major allergic symptom. However, the direct effect of histamine on mucus secretion from airway mucosal epithelia has not been clearly demonstrated. TMEM16A is a Ca²⁺-activated chloride channel, and it is closely related to fluid secretion in airway mucosal epithelia. We investigated whether histamine directly induces fluid secretion from epithelial cells or submucosal glands (SMG) and mechanisms related, therewith, in allergic airway diseases. In pig airway tissues from the nose or trachea, histamine was a potent secretagogue that directly induced strong responses. However, gland secretion from human nasal tissue was not induced by histamine, even in allergic rhinitis patients. Histamine type 1 receptor (H1R) and histamine type 2 receptor (H2R) were not noted in SMG by in situ hybridization. Cultured primary human nasal epithelial (NHE) cells were used for the measurement of short-circuit current changes with the Ussing chamber. Histamine-induced slight responses of anion secretions under normal conditions. The response was enhanced by IL-4 stimulation through TMEM16A, which might be related to fluid hypersecretion in allergic rhinitis. Pretreatment with IL-4 augmented the histamine response that was suppressed by a TMEM16A inhibitor. TMEM16A expression was enhanced by 24-h treatment of IL-4 in human nasal epithelial cells. The expression of TMEM16A was significantly elevated in an allergic rhinitis group, compared with a control group. We elucidated histamine-induced fluid secretions in synergy with IL-4 through TMEM16A in the human airway epithelium. In addition, we observed species differences between pigs and humans in terms of gland secretion of histamine.

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.

The effect of nasal irrigation formulation on the antimicrobial activity of nasal secretions

BACKGROUND

Saline-based irrigation solutions are evidence-based rhinological treatments; however, the formulation of these solutions could theoretically alter the function of innate antimicrobial peptides. The aim of this study was to determine if the antimicrobial activity of normal human nasal secretions in vivo is altered by commercially available large volume irrigation solutions.

METHODS

Minimally manipulated sinonasal secretions were collected from patients with chronic rhinosinusitis (CRS; n = 10) and normal healthy volunteers (n = 20). In a subset of control patients (n = 10) secretions were collected prior to, and at 1 hour, 6 hours, and 24 hours after nasal irrigation with 4 commercial irrigation solutions. Lysozyme and lactoferrin levels were analyzed and the antimicrobial activity of secretions determined using a radial diffusion assay.

RESULTS

The antimicrobial activity of nasal secretions was reduced in CRS patients compared to healthy volunteers (p < 0.01), but there was no significant difference in antimicrobial peptide concentrations. Isotonic nasal irrigation reduced lysozyme and lactoferrin levels, which returned to baseline levels by 6 hours; in addition to a sustained decrease in antimicrobial activity before returning to baseline at 24 hours. Low-salt solution stimulated peptide secretion by approximately 40% at 6 hours and 24 hours, but produced a transient decrease in antimicrobial activity, returning to baseline levels by 6 hours. Hypertonic solution initially decreased lysozyme and lactoferrin levels but maintained baseline levels of antimicrobial activity and increased peptide secretion by approximately 30% at 24 hours.

CONCLUSION

The formulation of nasal irrigation solutions significantly affects the measured levels and functionality of sinonasal antimicrobial peptides.

A spatial model of fluid recycling in the airways of the lung

The genetic disease cystic fibrosis (CF) is a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, and results in viscous mucus and impaired mucociliary clearance leading to chronic recurring pulmonary infections. Although extensive experimental research has been conducted over the last few decades, CF lung pathophysiology remains controversial. There are two competing explanations for the observed depletion of periciliary liquid (PCL) in CF lungs. The low volume hypothesis assumes fluid hyperabsorption through surface epithelia due to an over-active epithelial Na(+) channel (ENaC), and the low secretion hypothesis assumes inspissated mucins secreted from glands due to lack of serous fluid secreted from gland acini. We present a spatial mathematical model that reflects in vivo fluid recycling via submucosal gland (SMG) secretion, and absorption through surface epithelia. We then test the model in CF conditions by increasing ENaC open probability and decreasing SMG flux while simultaneously reducing CFTR open probability. Increasing ENaC activity only results in increased fluid absorption across surface epithelia, as seen in in vitro experiments. However, combining potential CF mechanisms results in markedly less fluid absorbed while providing the largest reduction in PCL volume, suggesting that a compromise in gland fluid secretion dominates over increased ENaC activity to decrease the amount of fluid transported transcellularly in CF lungs in vivo. Model results also indicate that a spatial model is necessary for an accurate calculation of total fluid transport, as the effects of spatial gradients can be severe, particularly in close proximity to the SMGs.

Upregulation of TMEM16A Protein in Bronchial Epithelial Cells by Bacterial Pyocyanin

Induction of mucus hypersecretion in the airway epithelium by Th2 cytokines is associated with the expression of TMEM16A, a Ca²⁺-activated Cl^- channel. We asked whether exposure of airway epithelial cells to bacterial components, a condition that mimics the highly infected environment occurring in cystic fibrosis (CF), also results in a similar response. In cultured human bronchial epithelial cells, treatment with pyocyanin or with a P. aeruginosa culture supernatant caused a significant increase in TMEM16A function. The Ca²⁺-dependent Cl^- secretion, triggered by stimulation with UTP, was particularly enhanced by pyocyanin in cells from CF patients. Increased expression of TMEM16A protein and of MUC5AC mucin by bacterial components was demonstrated by immunofluorescence in CF and non-CF cells. We also investigated TMEM16A expression in human bronchi by immunocytochemistry. We found increased TMEM16A staining in the airways of CF patients. The strongest signal was observed in CF submucosal glands. Our results suggest that TMEM16A expression/function is upregulated in CF lung disease, possibly as a response towards the presence of bacteria in the airways.

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.

Recent advances in cystic fibrosis

PURPOSE OF REVIEW

The field of cystic fibrosis (CF) continues to evolve at a fast pace thanks to novel observations that have enabled deeper understanding of the disease pathophysiology. Parallel groundbreaking developments in innovative therapies permit, for the first time, distinct disease modification.

RECENT FINDINGS

This review highlights important discoveries in fluid homeostasis and mucus secretion in CF that further informs the pathophysiology of the airway disease that characterizes CF. In addition, current concepts and novel paradigms, such as 'theratypes' and 'CF transmembrane conductance regulator chaperome', which will be important for the continued development of disease modifying therapies, are reviewed.

SUMMARY

The rate of progress in the field continues to accelerate with new knowledge informing the development of innovative therapies. This has already led to tangible substantial and unprecedented clinical benefit for selected subsets of the CF patient population. In the years ahead, further knowledge acquisition may motivate the extension of these benefits to the larger population of people with CF.

Proteomic analysis of pure human airway gland mucus reveals a large component of protective proteins

Airway submucosal glands contribute to innate immunity and protect the lungs by secreting mucus, which is required for mucociliary clearance and which also contains antimicrobial, anti-inflammatory, anti-proteolytic and anti-oxidant proteins. We stimulated glands in tracheal trimmings from three lung donors and collected droplets of uncontaminated mucus as they formed at the gland orifices under an oil layer. We analyzed the mucus using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Analysis identified 5486 peptides and 441 proteins from across the 3 samples (269-319 proteins per subject). We focused on 269 proteins common to at least 2 0f 3 subjects, of which 102 (38%) had protective or innate immunity functions. While many of these have long been known to play such roles, for many others their cellular protective functions have only recently been appreciated in addition to their well-studied biologic functions (e.g. annexins, apolipoproteins, gelsolin, hemoglobin, histones, keratins, and lumican). A minority of the identified proteins are known to be secreted via conventional exocytosis, suggesting that glandular secretion occurs via multiple mechanisms. Two of the observed protective proteins, major vault protein and prohibitin, have not been observed in fluid from human epithelial cultures or in fluid from nasal or bronchoalveolar lavage. Further proteomic analysis of pure gland mucus may help clarify how healthy airways maintain a sterile environment.

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.

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.

cAMP and Ca²⁺ signaling in secretory epithelia: crosstalk and synergism

The Ca(2+) and cAMP/PKA pathways are the primary signaling systems in secretory epithelia that control virtually all secretory gland functions. Interaction and crosstalk in Ca(2+) and cAMP signaling occur at multiple levels to control and tune the activity of each other. Physiologically, Ca(2+) and cAMP signaling operate at 5-10% of maximal strength, but synergize to generate the maximal response. Although synergistic action of the Ca(2+) and cAMP signaling is the common mode of signaling and has been known for many years, we know very little of the molecular mechanism and mediators of the synergism. In this review, we discuss crosstalk between the Ca(2+) and cAMP signaling and the function of IRBIT (IP3 receptors binding protein release with IP3) as a third messenger that mediates the synergistic action of the Ca(2+) and cAMP signaling.

Mechanism and synergism in epithelial fluid and electrolyte secretion

A central function of epithelia is the control of the volume and electrolyte composition of bodily fluids through vectorial transport of electrolytes and the obligatory H2O. In exocrine glands, fluid and electrolyte secretion is carried out by both acinar and duct cells, with the portion of fluid secreted by each cell type varying among glands. All acinar cells secrete isotonic, plasma-like fluid, while the duct determines the final electrolyte composition of the fluid by absorbing most of the Cl(-) and secreting HCO3 (-). The key transporters mediating acinar fluid and electrolyte secretion are the basolateral Na(+)/K(+) /2Cl(-) cotransporter, the luminal Ca(2+)-activated Cl(-) channel ANO1 and basolateral and luminal Ca(2+)-activated K(+) channels. Ductal fluid and HCO3 (-) secretion are mediated by the basolateral membrane Na(+)-HCO3 (-) cotransporter NBCe1-B and the luminal membrane Cl(-)/HCO3 (-) exchanger slc26a6 and the Cl(-) channel CFTR. The function of the transporters is regulated by multiple inputs, which in the duct include major regulation by the WNK/SPAK pathway that inhibit secretion and the IRBIT/PP1 pathway that antagonize the effects of the WNK/SPAK pathway to both stimulate and coordinate the secretion. The function of these regulatory pathways in secretory glands acinar cells is yet to be examined. An important concept in biology is synergism among signaling pathways to generate the final physiological response that ensures regulation with high fidelity and guards against cell toxicity. While synergism is observed in all epithelial functions, the molecular mechanism mediating the synergism is not known. Recent work reveals a central role for IRBIT as a third messenger that integrates and synergizes the function of the Ca(2+) and cAMP signaling pathways in activation of epithelial fluid and electrolyte secretion. These concepts are discussed in this review using secretion by the pancreatic and salivary gland ducts as model systems.

Substance P stimulates production of inflammatory cytokines in human disc cells

STUDY DESIGN

Laboratory study.

OBJECTIVE

The aims of this study were as follows: (1) to confirm that Substance P (SP) is expressed by nucleus pulposus (NP) and annulus fibrosus (AF) cells; (2) to determine the effect of SP on expression of inflammatory mediators in human disc cells and the effect of inflammatory mediators on the expression of SP; and (3) to characterize the relative expression of SP receptor isoforms in disc tissue and describe whether exposure to SP changes receptor expression.

SUMMARY OF BACKGROUND DATA

SP, classically described as a neurotransmitter, acts as an inflammatory regulator in other tissue types, but its role within the intervertebral disc has not been characterized.

METHODS

Human AF and NP cells from 7 individuals were expanded in monolayer and maintained in alginate bead culture. Cells were treated with SP or interleukin (IL)-1β/tumor necrosis factor-α (TNF-α). After treatment, the cells were recovered and then RNA was isolated and transcribed into cDNA. Quantitative reverse-transcriptase polymerase chain reaction was performed to evaluate expression of inflammatory mediators and SP and its receptors.

RESULTS

Disc cells treated with SP demonstrated significant upregulation of IL-1β, IL-6, and IL-8 in NP and AF cells whereas significant upregulation of RANTES and TNF occurred only in the AF cells. AF and NP cells expressed SP at low levels; expression did not change significantly with SP treatment but was significantly upregulated after treatment with IL-1β/TNF-α. Both SP receptor isoforms were expressed by NP and AF cells.

CONCLUSION

SP upregulates inflammatory mediators in disc cells. SP and its receptors were expressed in both NP and AF cells, and expression did not change after treatment with SP but increased after treatment with IL-1β/TNF-α. SP likely acts in an autocrine or paracrine manner in intervertebral disc cells and may be involved in "crosstalk" between disc cells and neurons, providing a potential mechanism for transmission of painful discogenic stimuli.

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.

Inspissated oral secretions and a review of their clinical, biological, and physiological significance

People with some chronic diseases may dehydrate and develop thick, viscous inspissated oronasal secretions that include cellular debris. This material can lead to ductal or airway obstructions that can prove to be life threatening. Asthma, allergy with superinfection, cystic fibrosis, intubated ventilation, burn injuries, and medication-induced complications are discussed in this paper. Many patients with chronic debilitating conditions may also be unable to communicate, and so may be unable to verbally convey that they have a compromised airway or an obstruction. Therefore, it is essential to maintain hydration and good oral hygiene that not only addresses the teeth and prostheses, but also the oral mucosal surfaces. People who are institutionalized and bed-ridden, in particular, need to be closely monitored to prevent adverse sequellae. A daily oral sweep with a 4 × 4 surgical sponge moistened with chlorhexidine may prevent aspiration pneumonia or a fatality due to an airway obstruction. Human oronasal secretions are involved with immunity, digestion, lubrication, and speech. Saliva is the most volumetrically important. These secretions moisturize inspired and expired air but can lose water, causing an increase in viscosity. The viscous secretions trap particles, food debris, and bacterial colonies, thereby increasing inspissations that may obstruct the airway.

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.

CFTR, mucins, and mucus obstruction in cystic fibrosis

Mucus pathology in cystic fibrosis (CF) has been known for as long as the disease has been recognized and is sometimes called mucoviscidosis. The disease is marked by mucus hyperproduction and plugging in many organs, which are usually most fatal in the airways of CF patients, once the problem of meconium ileus at birth is resolved. After the CF gene, CFTR, was cloned and its protein product identified as a cAMP-regulated Cl(-) channel, causal mechanisms underlying the strong mucus phenotype of the disease became obscure. Here we focus on mucin genes and polymeric mucin glycoproteins, examining their regulation and potential relationships to a dysfunctional cystic fibrosis transmembrane conductance regulator (CFTR). Detailed examination of CFTR expression in organs and different cell types indicates that changes in CFTR expression do not always correlate with the severity of CF disease or mucus accumulation. Thus, the mucus hyperproduction that typifies CF does not appear to be a direct cause of a defective CFTR but, rather, to be a downstream consequence. In organs like the lung, up-regulation of mucin gene expression by inflammation results from chronic infection; however, in other instances and organs, the inflammation may have a non-infectious origin. The mucus plugging phenotype of the β-subunit of the epithelial Na(+) channel (βENaC)-overexpressing mouse is proving to be an archetypal example of this kind of inflammation, with a dehydrated airway surface/concentrated mucus gel apparently providing the inflammatory stimulus. Data indicate that the luminal HCO(3)(-) deficiency recently described for CF epithelia may also provide such a stimulus, perhaps by causing a mal-maturation of mucins as they are released onto luminal surfaces. In any event, the path between CFTR dysfunction and mucus hyperproduction has proven tortuous, and its unraveling continues to offer its own twists and turns, along with fascinating glimpses into biology.

Protease-activated receptor 2 mediates mucus secretion in the airway submucosal gland

Protease-activated receptor 2 (PAR2), a G protein-coupled receptor expressed in airway epithelia and smooth muscle, plays an important role in airway inflammation. In this study, we demonstrated that activation of PAR2 induces mucus secretion from the human airway gland and examined the underlying mechanism using the porcine and murine airway glands. The mucosa with underlying submucosal glands were dissected from the cartilage of tissues, pinned with the mucosal side up at the gas/bath solution interface of a physiological chamber, and covered with oil so that secretions from individual glands could be visualized as spherical bubbles in the oil. Secretion rates were determined by optical monitoring of the bubble diameter. The Ca(2+)-sensitive dye Fura2-AM was used to determine intracellular Ca(2+) concentration ([Ca(2+)](i)) by means of spectrofluorometry. Stimulation of human tracheal mucosa with PAR2-activating peptide (PAR2-AP) elevated intracellular Ca(2+) and induced glandular secretion equal to approximately 30% of the carbachol response in the human airway. Porcine gland tissue was more sensitive to PAR2-AP, and this response was dependent on Ca(2+) and anion secretion. When the mouse trachea were exposed to PAR2-AP, large amounts of secretion were observed in both wild type and ΔF508 cystic fibrosis transmembrane conductance regulator mutant mice but there is no secretion from PAR-2 knock out mice. In conclusion, PAR2-AP is an agonist for mucus secretion from the airway gland that is Ca(2+)-dependent and cystic fibrosis transmembrane conductance regulator-independent.

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.

Protease-activated receptor 2-dependent fluid secretion from airway submucosal glands by house dust mite extract

BACKGROUND

The submucosal gland (SMG) is important in the control of airway surface fluid. Protease-activated receptor (PAR) 2 contributes to the pathophysiology of allergies in response to nonspecific allergens bearing proteases and anion secretion. House dust mites (HDMs) have abundant proteases that can activate PAR2, but little is known about the direct effect of HDM on SMG secretion.

OBJECTIVE

The aim of this study was to investigate the effect of HDMs on glandular secretion and its mechanism in allergic patients, patients with chronic rhinosinusitis (CRS), or both.

METHODS

Inferior nasal turbinates were harvested from 55 patients and classified into 4 groups (the control, allergic rhinitis [AR], CRS, and AR+CRS groups). A microscope attached to a digital camera was used to quantify mucus bubbles from individual SMGs while stimulated with HDM extract, PAR2-activating peptide, and carbachol. PAR2 expression in the SMG was determined by means of immunostaining with anti-PAR2 mAb.

RESULTS

HDM induced a significantly higher secretion rate and number of responding glands in the AR and AR+CRS groups than in the control group. Interestingly, patients in the CRS group, who had no HDM-specific IgE antibody, showed a higher response than the control group, and its response was suppressed by a PAR2-selective antagonist. The responses to PAR2-activating peptide were similar to those to HDM, and their secretion rates positively correlated with HDM responses. PAR2 was highly expressed in all 3 disease groups with immunostaining.

CONCLUSIONS

HDM allergens can induce glandular secretion in patients with AR, CRS, or both, and PAR2 represents a possible mechanism for nonspecific hyperreactivity in inflammatory airway diseases.

Novel role for pendrin in orchestrating bicarbonate secretion in cystic fibrosis transmembrane conductance regulator (CFTR)-expressing airway serous cells

In most HCO(3)(-)-secreting epithelial tissues, SLC26 Cl(-)/HCO(3)(-) transporters work in concert with the cystic fibrosis transmembrane conductance regulator (CFTR) to regulate the magnitude and composition of the secreted fluid, a process that is vital for normal tissue function. By contrast, CFTR is regarded as the only exit pathway for HCO(3)(-) in the airways. Here we show that Cl(-)/HCO(3)(-) anion exchange makes a major contribution to transcellular HCO(3)(-) transport in airway serous cells. Real-time measurement of intracellular pH from polarized cultures of human Calu-3 cells demonstrated cAMP/PKA-activated Cl(-)-dependent HCO(3)(-) transport across the luminal membrane via CFTR-dependent coupled Cl(-)/HCO(3)(-) anion exchange. The pharmacological and functional profile of the luminal anion exchanger was consistent with SLC26A4 (pendrin), which was shown to be expressed by quantitative RT-PCR, Western blot, and immunofluorescence. Pendrin-mediated anion exchange activity was confirmed by shRNA pendrin knockdown (KD), which markedly reduced cAMP-activated Cl(-)/HCO(3)(-) exchange. To establish the relative roles of CFTR and pendrin in net HCO(3)(-) secretion, transepithelial liquid secretion rate and liquid pH were measured in wild type, pendrin KD, and CFTR KD cells. cAMP/PKA increased the rate and pH of the secreted fluid. Inhibiting CFTR reduced the rate of liquid secretion but not the pH, whereas decreasing pendrin activity lowered pH with little effect on volume. These results establish that CFTR predominately controls the rate of liquid secretion, whereas pendrin regulates the composition of the secreted fluid and identifies a critical role for this anion exchanger in transcellular HCO(3)(-) secretion in airway serous cells.

Defective fluid secretion from submucosal glands of nasal turbinates from CFTR-/- and CFTR (ΔF508/ΔF508) pigs

Background

Cystic fibrosis (CF), caused by reduced CFTR function, includes severe sinonasal disease which may predispose to lung disease. Newly developed CF pigs provide models to study the onset of CF pathophysiology. We asked if glands from pig nasal turbinates have secretory responses similar to those of tracheal glands and if CF nasal glands show reduced fluid secretion.

Methodology/Principal Findings

Unexpectedly, we found that nasal glands differed from tracheal glands in five ways, being smaller, more numerous (density per airway surface area), more sensitive to carbachol, more sensitive to forskolin, and nonresponsive to Substance P (a potent agonist for pig tracheal glands). Nasal gland fluid secretion from newborn piglets (12 CF and 12 controls) in response to agonists was measured using digital imaging of mucus bubbles formed under oil. Secretion rates were significantly reduced in all conditions tested. Fluid secretory rates (Controls vs. CF, in pl/min/gland) were as follows: 3 µM forskolin: 9.2±2.2 vs. 0.6±0.3; 1 µM carbachol: 143.5±35.5 vs. 52.2±10.3; 3 µM forskolin + 0.1 µM carbachol: 25.8±5.8 vs. CF 4.5±0.9. We also compared CF^{ΔF508/ΔF508} with CFTR^-/- piglets and found significantly greater forskolin-stimulated secretion rates in the ΔF508 vs. the null piglets (1.4±0.8, n = 4 vs. 0.2±0.1, n = 7). An unexpected age effect was also discovered: the ratio of secretion to 3 µM forskolin vs. 1 µM carbachol was ∼4 times greater in adult than in neonatal nasal glands.

Conclusions/Significance

These findings reveal differences between nasal and tracheal glands, show defective fluid secretion in nasal glands of CF pigs, reveal some spared function in the ΔF508 vs. null piglets, and show unexpected age-dependent differences. Reduced nasal gland fluid secretion may predispose to sinonasal and lung infections.

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.

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.

Expression of neural receptors in mouse meibomian gland

PURPOSE

To investigate gene and protein expression profiles of neural receptors found in the mouse meibomian gland. RNA and protein levels were determined for neuropeptide Y (NPY) receptor, vasoactive intestinal peptide (VIP) receptor, substance P (SP) receptor, and muscarinic cholinergic receptor (mAChR) subtypes M1-M5 in the mouse meibomian gland.

METHODS

Frozen sections of Balb/c mouse eyelids were subjected to laser capture microdissection to isolate pure samples of meibomian gland ductal and acinar cells. Real-time polymerase chain reaction, immunolabeling, and Western blot analysis for SP receptor, VIP receptor, NPY receptor, and mAChR subtypes M1-M5 were performed on meibomian gland ductal and acinar cells.

RESULTS

Expression of NPY1 receptor, VIP receptor 1, SP receptor, and all 5 mAChR subtypes was found in all meibomian gland ductal and acinar cells analyzed by real-time polymerase chain reaction. Immunolabeling and Western blot analysis confirmed the presence of NPY1 receptor, VIP receptor 1, SP receptor, and all 5 mAChR subtypes in the meibomian gland. The levels were variable with the duct showing greater levels of NPY1 receptor, SP receptor, and mAChRs 1, 2, 4, and 5 than with the gland.

CONCLUSIONS

VIP receptor 1, SP receptor, NPY1 receptor, and mAChR subtypes may be involved in the regulation of meibomian gland secretion. Laser capture microdissection in conjunction with gene expression analysis provides an excellent approach for studying meibomian gland cells about which relatively little is known at the molecular level.

cAMP-activated Ca2+ signaling is required for CFTR-mediated serous cell fluid secretion in porcine and human airways

Cystic fibrosis (CF), which is caused by mutations in CFTR, affects many tissues, including the lung. Submucosal gland serous acinar cells are primary sites of fluid secretion and CFTR expression in the lung. Absence of CFTR in these cells may contribute to CF lung pathogenesis by disrupting fluid secretion. Here, we have isolated primary serous acinar cells from wild-type and CFTR-/- pigs and humans without CF to investigate the cellular mechanisms and regulation of fluid secretion by optical imaging. Porcine and human serous cells secrete fluid in response to vasoactive intestinal polypeptide (VIP) and other agents that raise intracellular cAMP levels; here, we have demonstrated that this requires CFTR and a cAMP-dependent rise in intracellular Ca2+ concentration ([Ca2+]i). Importantly, cAMP induced the release of Ca2+ from InsP3-sensitive Ca2+ stores also responsive to cAMP-independent agonists such as cholinergic, histaminergic, and purinergic agonists that stimulate CFTR-independent fluid secretion. This provides two types of synergism that strongly potentiated cAMP-mediated fluid secretion but differed in their CFTR dependencies. First, CFTR-dependent secretion was strongly potentiated by low VIP and carbachol concentrations that individually were unable to stimulate secretion. Second, higher VIP concentrations more strongly potentiated the [Ca2+]i responses, enabling ineffectual levels of cholinergic stimulation to strongly activate CFTR-independent fluid secretion. These results identify important molecular mechanisms of cAMP-dependent secretion, including a requirement for Ca2+ signaling, and suggest new therapeutic approaches to correct defective submucosal gland secretion in CF.

Transgenic animals may help resolve a sticky situation in cystic fibrosis

Cystic fibrosis (CF) is caused by defects in the CFTR, a cAMP-activated Cl- channel of epithelia. The resulting reduction in epithelial fluid transport creates abnormally viscous secretions from airway mucous glands that may be a major factor in CF pathology. Mouse airways have few mucous glands, and the mouse model of CF exhibits no significant airway disease. Pigs and ferrets, however, have approximately the same number of airway mucous glands as humans. In this issue of the JCI, three independent research groups conclude that changes in airway mucous gland function in CFTR-deficient animals of these species resemble the changes seen in human CF. It is expected, therefore, that these animals will develop lung disease similar to human CF and prove to be valuable models on which to test potential therapies.

Pseudomonas aeruginosa Homoserine lactone activates store-operated cAMP and cystic fibrosis transmembrane regulator-dependent Cl- secretion by human airway epithelia

The ubiquitous bacterium Pseudomonas aeruginosa frequently causes hospital-acquired infections. P. aeruginosa also infects the lungs of cystic fibrosis (CF) patients and secretes N-(3-oxo-dodecanoyl)-S-homoserine lactone (3O-C12) to regulate bacterial gene expression critical for P. aeruginosa persistence. In addition to its effects as a quorum-sensing gene regulator in P. aeruginosa, 3O-C12 elicits cross-kingdom effects on host cell signaling leading to both pro- or anti-inflammatory effects. We find that in addition to these slow effects mediated through changes in gene expression, 3O-C12 also rapidly increases Cl(-) and fluid secretion in the cystic fibrosis transmembrane regulator (CFTR)-expressing airway epithelia. 3O-C12 does not stimulate Cl(-) secretion in CF cells, suggesting that lactone activates the CFTR. 3O-C12 also appears to directly activate the inositol trisphosphate receptor and release Ca(2+) from the endoplasmic reticulum (ER), lowering [Ca(2+)] in the ER and thereby activating the Ca(2+)-sensitive ER signaling protein STIM1. 3O-C12 increases cytosolic [Ca(2+)] and, strikingly, also cytosolic [cAMP], the known activator of CFTR. Activation of Cl(-) current by 3O-C12 was inhibited by a cAMP antagonist and increased by a phosphodiesterase inhibitor. Finally, a Ca(2+) buffer that lowers [Ca(2+)] in the ER similar to the effect of 3O-C12 also increased cAMP and I(Cl). The results suggest that 3O-C12 stimulates CFTR-dependent Cl(-) and fluid secretion in airway epithelial cells by activating the inositol trisphosphate receptor, thus lowering [Ca(2+)] in the ER and activating STIM1 and store-operated cAMP production. In CF airways, where CFTR is absent, the adaptive ability to rapidly flush the bacteria away is compromised because the lactone cannot affect Cl(-) and fluid secretion.

Hyposecretion of fluid from tracheal submucosal glands of CFTR-deficient pigs

Cystic fibrosis (CF) results from mutations that disrupt CF transmembrane conductance regulator (CFTR), an anion channel found mainly in apical membranes of epithelial cells. CF leads to chronic infection of the airways with normally innocuous bacteria and fungi. Hypotheses to explain the pathophysiology of CF airways have been difficult to test because mouse models of CF do not develop human-like airway disease. The recent production of pigs lacking CFTR and pigs expressing the most common CF-causing CFTR mutant, DeltaF508, provide another model that might help clarify the pathophysiology of CF airway disease. Here, we studied individual submucosal glands from 1-day-old piglets in situ in explanted tracheas, using optical methods to monitor mucus secretion rates from multiple glands in parallel. Secretion rates from control piglets (WT and CFTR+/-) and piglets with CF-like disease (CFTR-/- and CFTR-/DeltaF508) were measured under 5 conditions: unstimulated (to determine basal secretion), stimulated with forskolin, stimulated with carbachol, stimulated with substance P, and, as a test for synergy, stimulated with forskolin and a low concentration of carbachol. Glands from piglets with CF-like disease responded qualitatively to all agonists like glands from human patients with CF, producing virtually no fluid in response to stimulation with forskolin and substantially less in response to all other agonists except carbachol. These data are a step toward determining whether gland secretory defects contribute to CF airway disease.

CFTR-adenylyl cyclase I association responsible for UTP activation of CFTR in well-differentiated primary human bronchial cell cultures

Chloride secretion by airway epithelial cells is defective in cystic fibrosis (CF). The conventional paradigm is that CFTR is activated through cAMP and protein kinase A (PKA), whereas the Ca(2+)-activated chloride channel (CaCC) is activated by Ca(2+) agonists like UTP. We found that most chloride current elicited by Ca(2+) agonists in primary cultures of human bronchial epithelial cells is mediated by CFTR by a mechanism involving Ca(2+) activation of adenylyl cyclase I (AC1) and cAMP/PKA signaling. Use of selective inhibitors showed that Ca(2+) agonists produced more chloride secretion from CFTR than from CaCC. CFTR-dependent chloride secretion was reduced by PKA inhibition and was absent in CF cell cultures. Ca(2+) agonists produced cAMP elevation, which was blocked by adenylyl cyclase inhibition. AC1, a Ca(2+)/calmodulin-stimulated adenylyl cyclase, colocalized with CFTR in the cell apical membrane. RNAi knockdown of AC1 selectively reduced UTP-induced cAMP elevation and chloride secretion. These results, together with correlations between cAMP and chloride current, suggest that compartmentalized AC1-CFTR association is responsible for Ca(2+)/cAMP cross-talk. We further conclude that CFTR is the principal chloride secretory pathway in non-CF airways for both cAMP and Ca(2+) agonists, providing a novel mechanism to link CFTR dysfunction to CF lung disease.

Neurotrophic and neuroimmune responses to early-life Pseudomonas aeruginosa infection in rat lungs

Early-life respiratory infection with Pseudomonas aeruginosa is common in children with cystic fibrosis or immune deficits. Although many of its clinical manifestations involve neural reflexes, little information is available on the peripheral nervous system of infected airways. This study sought to determine whether early-life infection triggers a neurogenic-mediated immunoinflammatory response, the mechanisms of this response, and its relationship with other immunoinflammatory pathways. Weanling and adult rats were inoculated with suspensions containing P. aeruginosa (PAO1) coated on alginate microspheres suspended in Tris-CaCl(2) buffer. Five days after infection, rats were injected with capsaicin to stimulate nociceptive nerves in the airway mucosa, and microvascular permeability was measured using Evans blue as a tracer. PAO1 increased neurogenic inflammation in the extra- and intrapulmonary compartments of weanlings but not in adults. The mechanism involves selective overexpression of NGF, which is critical for the local increase in microvascular permeability and for the infiltration of polymorphonuclear leukocytes into infected lung parenchyma. These effects are mediated in part by induction of downstream inflammatory cytokines and chemokines, especially IL-1beta, IL-18, and leptin. Our data suggest that neurogenic-mediated immunoinflammatory mechanisms play important roles in airway inflammation and hyperreactivity associated with P. aeruginosa when infection occurs early in life.

Mucus secretion from individual submucosal glands of the ferret trachea

Mucus secretion from individual tracheal glands in adult ferrets was studied with time-lapse optical imaging of mucus droplets under an oil layer. Density of functional glands (determined by responses to 1 muM carbachol) was 1.5 +/- 0.3 per mm(2) (n = 6). Secretion rates (in pl.min(-1).gland(-1)) were as follows: 4.1 +/- 0.7 basal (unstimulated; n = 27, 669 glands), 338 +/- 70 to 10 microM forskolin (n = 8, 90 glands), 234 +/- 13 to 1 microM VIP (n = 6, 57 glands), 183 +/- 92 to 10 microM isoproterenol (n = 3, 33 glands), 978 +/- 145 to 1 microM carbachol (n = 11, 131 glands), and 1,348 +/- 325 to 10 muM phenylephrine (n = 7, 74 glands). The potency (EC(50), in microM) and efficacy (V(max), in pl x min(-1) x gland(-1)) were 7.6 (EC(50)) and 338 +/- 16 (V(max)) to forskolin, 1.0 (EC(50)) and 479 +/- 19 (V(max)) to VIP, 0.6 (EC(50)) and 1,817 +/- 268 (V(max)) to carbachol, and 3.7 (EC(50)) and 1,801 +/- 95 (V(max)) to phenylephrine. Although carbachol and phenylephrine were equally effective secretagogues, only carbachol caused contractions of the trachealis muscle. Synergy was demonstrated between 300 nM isoproterenol and 100 nM carbachol, which, when combined, produced a secretion rate almost fourfold greater than predicted from their additive effect. The dependence of fluid secretion on Cl(-) and HCO(3)(-) varied depending on the mode of stimulation. Secretion stimulated by VIP or forskolin was reduced by approximately 60% by blocking either anion, while carbachol-stimulated secretion was blocked 68% by bumetanide and only 32% by HEPES replacement of HCO(3)(-). These results provide parametric data for comparison with fluid secretion from glands in ferrets lacking CFTR.

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.