G Protein βγ-subunit signaling mediates airway hyperresponsiveness and inflammation in allergic asthma

Asthma: The Use of Animal Models and Their Translational Utility

Asthma is characterized by chronic lower airway inflammation that results in airway remodeling, which can lead to a permanent decrease in lung function. The pathophysiology driving the development of asthma is complex and heterogenous. Animal models have been and continue to be essential for the discovery of molecular pathways driving the pathophysiology of asthma and novel therapeutic approaches. Animal models of asthma may be induced or naturally occurring. Species used to study asthma include mouse, rat, guinea pig, cat, dog, sheep, horse, and nonhuman primate. Some of the aspects to consider when evaluating any of these asthma models are cost, labor, reagent availability, regulatory burden, relevance to natural disease in humans, type of lower airway inflammation, biological samples available for testing, and ultimately whether the model can answer the research question(s). This review aims to discuss the animal models most available for asthma investigation, with an emphasis on describing the inciting antigen/allergen, inflammatory response induced, and its translation to human asthma.

Homeostatic glucocorticoid signaling in airway smooth muscle: A roadmap to asthma pathogenesis

Homeostasis is the self-regulating process by which the body maintains internal stability within a narrow physiological range (i.e., "normality") as it dynamically adjusts to disruptive influences. Thus, whereas homeostasis maintains bodily health, disrupted homeostasis at the tissue or systemic level leads to disease. Airway smooth muscle (ASM) is the pivotal site of disrupted homeostasis in asthma. While extensive research has greatly expanded our understanding of ASM behavior under pro-asthmatic conditions, the cellular signaling mechanisms that underlie ASM homeostasis under these conditions remain elusive. Based on a broad collection of published studies, a homeostasis mechanism intrinsic to ASM and exhibited under inflammatory and non-inflammatory pro-asthmatic conditions is identified herein. Central to this mechanism is the novel unifying concept that the pro-asthmatic-exposed ASM can independently generate its own active glucocorticoid (i.e., cortisol), produce its own newly activated glucocorticoid receptors for the steroid, and, accordingly, use this molecular strategy to homeostatically prevent induction of the asthmatic state. This article addresses the experimental evidence that underlies the proposed homeostatic glucocorticoid signaling mechanism in ASM, followed by a discussion and depiction of the feed-forward and feedback intrinsic ASM signaling circuitry that constitutes the homeostatic state. The proposed mechanism offers a practical roadmap for future basic and translational research aimed at identifying potential key site(s) of disrupted ASM homeostasis leading to asthma.

NADPH oxidase in the vasculature: Expression, regulation and signalling pathways; role in normal cardiovascular physiology and its dysregulation in hypertension

The last 20-25 years have seen an explosion of interest in the role of NADPH oxidase (NOX) in cardiovascular function and disease. In vascular smooth muscle and endothelium, NOX generates reactive oxygen species (ROS) that act as second messengers, contributing to the control of normal vascular function. NOX activity is altered in response to a variety of stimuli, including G-protein coupled receptor agonists, growth-factors, perfusion pressure, flow and hypoxia. NOX-derived ROS are involved in smooth muscle constriction, endothelium-dependent relaxation and smooth muscle growth, proliferation and migration, thus contributing to the fine-tuning of blood flow, arterial wall thickness and vascular resistance. Through reversible oxidative modification of target proteins, ROS regulate the activity of protein tyrosine phosphatases, kinases, G proteins, ion channels, cytoskeletal proteins and transcription factors. There is now considerable, but somewhat contradictory evidence that NOX contributes to the pathogenesis of hypertension through oxidative stress. Specific NOX isoforms have been implicated in endothelial dysfunction, hyper-contractility and vascular remodelling in various animal models of hypertension, pulmonary hypertension and pulmonary arterial hypertension, but also have potential protective effects, particularly NOX4. This review explores the multiplicity of NOX function in the healthy vasculature and the evidence for and against targeting NOX for antihypertensive therapy.

Effects of the Th2-dominant milieu on allergic responses in Der f 1-activated mouse basophils and mast cells

Although basophils and mast cells share similar phenotypic and functional properties, little is known about the difference in the initial Th2 immune responses of these cells following exposure to proteolytic allergens. Here, we investigated the mechanisms of Th2-mediated immune responses in mouse bone marrow-derived basophils (BMBs) and mast cells (BMMCs) via stimulation with the cysteine protease allergen Der f 1. Our results showed that Th2 cytokines were induced from BMBs by active recombinant Der f 1 (rDer f 1 independently with Toll-like receptor (TLR) 2 and TLR4. Although both BMBs and BMMCs expressed protease-activated receptors on their surfaces, PAR expression following exposure to rDer f 1 was altered only in basophils. G protein-coupled receptors in basophils were found to be associated with interleukin (IL)-4 and IL-13 production from BMBs upon Der f 1 treatment. Secretion of Th2 cytokines from rDer f 1-treated basophils was mediated by G protein βγ and phosphatidylinositol 3-kinase γ through the extracellular signal-regulated kinase and c-Jun N-terminal kinase pathways. These findings provide insights into the roles of cysteine proteases in Th2 immune responses, such as allergic diseases, and improve our understanding of the mechanisms of Th2 cytokine production.

Long-term Air Pollution Exposure, Genome-wide DNA Methylation and Lung Function in the LifeLines Cohort Study

BACKGROUND

Long-term air pollution exposure is negatively associated with lung function, yet the mechanisms underlying this association are not fully clear. Differential DNA methylation may explain this association.

OBJECTIVES

Our main aim was to study the association between long-term air pollution exposure and DNA methylation.

METHODS

We performed a genome-wide methylation study using robust linear regression models in 1,017 subjects from the LifeLines cohort study to analyze the association between exposure to nitrogen dioxide (NO₂) and particulate matter (PM_2.5, fine particulate matter with aerodynamic diameter ≤2.5 μm; PM₁₀, particulate matter with aerodynamic diameter ≤10 μm) and PM_{2.5absorbance}, indicator of elemental carbon content (estimated with land-use-regression models) with DNA methylation in whole blood (Illumina^® HumanMethylation450K BeadChip). Replication of the top hits was attempted in two independent samples from the population-based Cooperative Health Research in the Region of Augsburg studies (KORA).

RESULTS

Depending on the p-value threshold used, we found significant associations between NO₂ exposure and DNA methylation for seven CpG sites (Bonferroni corrected threshold p<1.19×10^-7) or for 4,980 CpG sites (False Discovery Rate<0.05). The top associated CpG site was annotated to the PSMB9 gene (i.e., cg04908668). None of the seven Bonferroni significant CpG-sites were significantly replicated in the two KORA-cohorts. No associations were found for PM exposure.

CONCLUSIONS

Long-term NO₂ exposure was genome-wide significantly associated with DNA methylation in the identification cohort but not in the replication cohort. Future studies are needed to further elucidate the potential mechanisms underlying _NO2-exposure-related respiratory disease. https://doi.org/10.1289/EHP2045.

Translocation of 5' mRNA cap analogue--peptide conjugates across the membranes of giant unilamellar vesicles

Cell-penetrating peptides (CPPs) have been extensively studied because of their ability to deliver various cargo molecules, which are often potential therapeutic agents. However, in most cases, the exact entry mechanism of CPPs is still unknown. In this study, we focused our attention on the membrane permeability sequence (MPS) peptide (AAVALLPAVLLALLAK) conjugated to analogues of a 5' mRNA cap. This unique RNA structure plays a pivotal role in eukaryotic gene expression and has a large therapeutic application potential. We validated the translocation abilities of conjugates across the membranes of giant unilamellar vesicles (GUVs) composed of POPC lipids by application of fluorescence microscopy. Translocation of the MPS peptide itself was observed in contrast to peptide conjugates containing mono- and dinucleotide cap analogues, indicating that even for such small cargos, passive translocation does not occur. However, membrane permeability was observed in the case of conjugated mononucleotides. Fluorescence lifetime microscopy (FLIM) of the C6-NBD-phospholipid revealed changes in lipid packing induced by a penetrating peptide. Our results support the usefulness of artificial membrane systems applied to elucidate membrane crossing mechanisms.

Long-Acting Beta Agonists Enhance Allergic Airway Disease

Asthma is one of the most common of medical illnesses and is treated in part by drugs that activate the beta-2-adrenoceptor (β₂-AR) to dilate obstructed airways. Such drugs include long acting beta agonists (LABAs) that are paradoxically linked to excess asthma-related mortality. Here we show that LABAs such as salmeterol and structurally related β₂-AR drugs such as formoterol and carvedilol, but not short-acting agonists (SABAs) such as albuterol, promote exaggerated asthma-like allergic airway disease and enhanced airway constriction in mice. We demonstrate that salmeterol aberrantly promotes activation of the allergic disease-related transcription factor signal transducer and activator of transcription 6 (STAT6) in multiple mouse and human cells. A novel inhibitor of STAT6, PM-242H, inhibited initiation of allergic disease induced by airway fungal challenge, reversed established allergic airway disease in mice, and blocked salmeterol-dependent enhanced allergic airway disease. Thus, structurally related β₂-AR ligands aberrantly activate STAT6 and promote allergic airway disease. This untoward pharmacological property likely explains adverse outcomes observed with LABAs, which may be overcome by agents that antagonize STAT6.

Salmeterol Efficacy and Bias in the Activation and Kinase-Mediated Desensitization of β2-Adrenergic Receptors

Salmeterol is a long-acting β2-adrenergic receptor (β2AR) agonist that is widely used as a bronchodilator for the treatment of persistent asthma and chronic obstructive pulmonary disease in conjunction with steroids. Previous studies demonstrated that salmeterol showed weak efficacy for activation of adenylyl cyclase; however, its efficacy in the complex desensitization of the β2AR remains poorly understood. In this work, we provide insights into the roles played by the G protein-coupled receptor kinase/arrestin and protein kinase A in salmeterol-mediated desensitization through bioluminescence resonance energy transfer (BRET) studies of liganded-β2AR binding to arrestin and through kinetic studies of cAMP turnover. First, BRET demonstrated a much reduced efficacy for salmeterol recruitment of arrestin to β2AR relative to isoproterenol. The ratio of BRETISO/BRETSALM after 5-minute stimulation was 20 and decreased to 5 after 35 minutes, reflecting a progressive decline in BRETISO and a stable BRETSALM. Second, to assess salmeterol efficacy for functional desensitization, we examined the kinetics of salmeterol-induced cAMP accumulation (0-30 minutes) in human airway smooth muscle cells in the presence and absence of phosphodiesterase inhibition. Analysis of shaping of cAMP turnover for both agonists demonstrated significant salmeterol desensitization, although it was reduced relative to isoproterenol. Using an isoproterenol rescue protocol after either short-term (10 minutes) or long-term (2 and 14 hours) salmeterol pretreatments, we found that salmeterol progressively depressed isoproterenol stimulation but did not prevent subsequent rescue by isoproterenol and additional isoproterenol-mediated desensitization. Our findings reveal a complex efficacy for functional desensitization, demonstrating that although salmeterol shows weak efficacy for adenylyl cyclase activation and G protein-coupled receptor kinase/arrestin-mediated desensitization, it acts as a strong agonist in highly amplified protein kinase A-mediated events.

Constitutively active signaling by the G protein βγ-subunit mediates intrinsically increased phosphodiesterase-4 activity in human asthmatic airway smooth muscle cells

Signaling by the Gβγ subunit of Gi protein, leading to downstream c-Src-induced activation of the Ras/c-Raf1/MEK-ERK1/2 signaling pathway and its upregulation of phosphodiesterase-4 (PDE4) activity, was recently shown to mediate the heightened contractility in proasthmatic sensitized isolated airway smooth muscle (ASM), as well as allergen-induced airway hyperresponsiveness and inflammation in an in vivo animal model of allergic asthma. This study investigated whether cultured human ASM (HASM) cells derived from asthmatic donor lungs exhibit constitutively increased PDE activity that is attributed to intrinsically upregulated Gβγ signaling coupled to c-Src activation of the Ras/MEK/ERK1/2 cascade. We show that, relative to normal cells, asthmatic HASM cells constitutively exhibit markedly increased intrinsic PDE4 activity coupled to heightened Gβγ-regulated phosphorylation of c-Src and ERK1/2, and direct co-localization of the latter with the PDE4D isoform. These signaling events and their induction of heightened PDE activity are acutely suppressed by treating asthmatic HASM cells with a Gβγ inhibitor. Importantly, along with increased Gβγ activation, asthmatic HASM cells also exhibit constitutively increased direct binding of the small Rap1 GTPase-activating protein, Rap1GAP, to the α-subunit of Gi protein, which serves to cooperatively facilitate Ras activation and, thereby, enable enhanced Gβγ-regulated ERK1/2-stimulated PDE activity. Collectively, these data are the first to identify that intrinsically increased signaling via the Gβγ subunit, facilitated by Rap1GAP recruitment to the α-subunit, mediates the constitutively increased PDE4 activity detected in asthmatic HASM cells. These new findings support the notion that interventions targeted at suppressing Gβγ signaling may lead to novel approaches to treat asthma.

Directional secretory response of double stranded RNA-induced thymic stromal lymphopoetin (TSLP) and CCL11/eotaxin-1 in human asthmatic airways

BACKGROUND

Thymic stromal lymphoproetin (TSLP) is a cytokine secreted by the airway epithelium in response to respiratory viruses and it is known to promote allergic Th2 responses in asthma. This study investigated whether virally-induced secretion of TSLP is directional in nature (apical vs. basolateral) and/or if there are TSLP-mediated effects occurring at both sides of the bronchial epithelial barrier in the asthmatic state.

METHODS

Primary human bronchial epithelial cells (HBEC) from control (n = 3) and asthmatic (n = 3) donors were differentiated into polarized respiratory tract epithelium under air-liquid interface (ALI) conditions and treated apically with dsRNA (viral surrogate) or TSLP. Sub-epithelial effects of TSLP were examined in human airway smooth muscle cells (HASMC) from normal (n = 3) and asthmatic (n = 3) donors. Clinical experiments examined nasal airway secretions obtained from asthmatic children during naturally occurring rhinovirus-induced exacerbations (n = 20) vs. non-asthmatic uninfected controls (n = 20). Protein levels of TSLP, CCL11/eotaxin-1, CCL17/TARC, CCL22/MDC, TNF-α and CXCL8 were determined with a multiplex magnetic bead assay.

RESULTS

Our data demonstrate that: 1) Asthmatic HBEC exhibit an exaggerated apical, but not basal, secretion of TSLP after dsRNA exposure; 2) TSLP exposure induces unidirectional (apical) secretion of CCL11/eotaxin-1 in asthmatic HBEC and enhanced CCL11/eotaxin-1 secretion in asthmatic HASMC; 3) Rhinovirus-induced asthma exacerbations in children are associated with in vivo airway secretion of TSLP and CCL11/eotaxin-1.

CONCLUSIONS

There are virally-induced TSLP-driven secretory immune responses at both sides of the bronchial epithelial barrier characterized by enhanced CCL11/eotaxin-1 secretion in asthmatic airways. These results suggest a new model of TSLP-mediated eosinophilic responses in the asthmatic airway during viral-induced exacerbations.

Inhibition of Gαi activity by Gβγ is mediated by PI 3-kinase-γ- and cSrc-dependent tyrosine phosphorylation of Gαi and recruitment of RGS12

Others and we have characterized several Gβγ-dependent effectors in smooth muscle, including G protein-coupled receptor kinase 2 (GRK2), PLCβ3, and phosphatidylinositol (PI) 3-kinase-γ, and have identified various signaling targets downstream of PI 3-kinase-γ, including cSrc, integrin-linked kinase, and Rac1-Cdc42/p21-activated kinase/p38 MAP kinase. This study identified a novel mechanism whereby Gβγ acting via PI 3-kinase-γ and cSrc exerts an inhibitory influence on Gαi activity. The Gi2-coupled δ-opioid receptor agonist d-penicillamine (2,5)-enkephalin (DPDPE) activated cSrc, stimulated tyrosine phosphorylation of Gαi2, and induced regulator of G protein signaling 12 (RGS12) association; all three events were blocked by PI 3-kinase (LY294002) and cSrc (PP2) inhibitors and by expression of the COOH-terminal sequence of GRK2-(495-689), a Gβγ-scavenging peptide. Inhibition of forskolin-stimulated cAMP and muscle relaxation by DPDPE was augmented by PP2, LY294002, and a selective PI 3-kinase-γ inhibitor, AS-605420. Expression of tyrosine-deficient (Y69F, Y231F, or Y321F) Gαi2 mutant or knockdown of RGS12 blocked Gαi2 phosphorylation and Gαi2-RGS12 association and caused greater inhibition of cAMP. Parallel studies using somatostatin, cyclopentyl adenosine, or ACh to activate, respectively, Gi1-coupled somatostatin (sstr3) receptors, and Gi3-coupled adenosine A1 or muscarinic m2 receptors elicited cSrc activation, Gαi1 or Gαi3 phosphorylation, Gαi1-RGS12 or Gαi3-RGS12 association, and inhibition of cAMP. Inhibition of cAMP and muscle relaxation was greatly increased by AS-605240 and PP2. The results demonstrate that Gβγ-dependent tyrosine phosphorylation of Gαi1/2/3 by cSrc facilitated recruitment of RGS12, a Gαi-specific RGS protein with a unique phosphotyrosine-binding domain, resulting in rapid deactivation of Gαi and facilitation of smooth muscle relaxation.

The Gβγ-Src signaling pathway regulates TNF-induced necroptosis via control of necrosome translocation

Formation of multi-component signaling complex necrosomes is essential for tumor necrosis factor α (TNF)-induced programmed necrosis (also called necroptosis). However, the mechanisms of necroptosis are still largely unknown. We isolated a TNF-resistant L929 mutant cell line generated by retrovirus insertion and identified that disruption of the guanine nucleotide-binding protein γ 10 (Gγ10) gene is responsible for this phenotype. We further show that Gγ10 is involved in TNF-induced necroptosis and Gβ2 is the partner of Gγ10. Src is the downstream effector of Gβ2γ10 in TNF-induced necroptosis because TNF-induced Src activation was impaired upon Gγ10 knockdown. Gγ10 does not affect TNF-induced activation of NF-κB and MAPKs and the formation of necrosomes, but is required for trafficking of necrosomes to their potential functioning site, an unidentified subcellular organelle that can be fractionated into heterotypic membrane fractions. The TNF-induced Gβγ-Src signaling pathway is independent of RIP1/RIP3 kinase activity and necrosome formation, but is required for the necrosome to function.

Role of periostin, FENO, IL-13, lebrikzumab, other IL-13 antagonist and dual IL-4/IL-13 antagonist in asthma

INTRODUCTION

Asthma markedly diminishes quality of life due to limited activity, absences from work or school and hospitalizations. Patients with severe asthma which are not controlled despite taking effective therapy are most in need of new treatment approaches. IL-13 was demonstrated as 'central mediator of allergic asthma'.

AREAS COVERED

IL-13 has been implicated in the pathogenesis of asthma, idiopathic pulmonary fibrosis and COPD. IL-13 levels in the sputum and bronchial biopsy samples remain elevated in severe asthma despite the use of inhaled and systemic corticosteroids. Thus, IL-13 is a mediator involved in corticosteroid resistance. Periostin enhances profibrotic TGF-β signaling in subepithelial fibrosis associated with asthma. IL-13 induces bronchial epithelial cells to secrete periostin. Periostin may be a biomarker for Th2 induced airway inflammation. Lebrikizumab is a monoclonal antibody against IL-13. Lebrikizumab improved lung function in asthmatics who were symptomatic despite treatment with long acting beta agonist and inhaled corticosteroids and provided benefit in the treatment of severe uncontrolled asthma.

EXPERT OPINION

Lebrikizumab block IL-13 signaling through the IL-13Rα1/IL-4Rα receptor. There was a larger reduction in FENO in the high periostin subgroup than in the low periostin subgroup (34.4 vs 4.3%). Serum CCL17, CCL13 and total IgE levels decreased in the lebrikizumab group.

A new perspective on muscarinic receptor antagonism in obstructive airways diseases

Acetylcholine has traditionally only been regarded as a neurotransmitter of the parasympathetic nervous system, causing bronchoconstriction and mucus secretion in asthma and COPD by muscarinic receptor activation on airway smooth muscle and mucus-producing cells. Recent studies in experimental models indicate that muscarinic receptor stimulation in the airways also induces pro-inflammatory, pro-proliferative and pro-fibrotic effects, which may involve activation of airway structural and inflammatory cells by neuronal as well as non-neuronal acetylcholine. In addition, mechanical changes caused by muscarinic agonist-induced bronchoconstriction may be involved in airway remodeling. Crosstalk between muscarinic receptors and β2-adrenoceptors on airway smooth muscle causes a reduced bronchodilator response to β2-agonists, and a similar mechanism could possibly apply to the poor inhibition of inflammatory and remodeling processes by these drugs. Collectively, these findings provide novel perspectives for muscarinic receptor antagonists in asthma and COPD, since these drugs may not only acutely affect cholinergic airways obstruction, but also have important beneficial effects on β2-agonist responsiveness, airway inflammation and remodeling. The clinical relevance of these findings is presently under investigation and starting to emerge.

Pro-asthmatic cytokines regulate unliganded and ligand-dependent glucocorticoid receptor signaling in airway smooth muscle

To elucidate the regulation of glucocorticoid receptor (GR) signaling under pro-asthmatic conditions, cultured human airway smooth muscle (HASM) cells were treated with proinflammatory cytokines or GR ligands alone and in combination, and then examined for induced changes in ligand-dependent and -independent GR activation and downstream signaling events. Ligand stimulation with either cortisone or dexamethsone (DEX) acutely elicited GR translocation to the nucleus and, comparably, ligand-independent stimulation either with the Th2 cytokine, IL-13, or the pleiotropic cytokine combination, IL-1β/TNFα, also acutely evoked GR translocation. The latter response was potentiated by combined exposure of cells to GR ligand and cytokine. Similarly, treatment with either DEX or IL-13 alone induced GR phosphorylation at its serine-211 residue (GR^Ser211), denoting its activated state, and combined treatment with DEX+IL-13 elicited heightened and sustained GR^Ser211 phosphorylation. Interestingly, the above ligand-independent GR responses to IL-13 alone were not associated with downstream GR binding to its consensus DNA sequence or GR transactivation, whereas both DEX-induced GR:DNA binding and transcriptional activity were significantly heightened in the presence of IL-13, coupled to increased recruitment of the transcriptional co-factor, MED14. The stimulated GR signaling responses to DEX were prevented in IL-13-exposed cells wherein GR^Ser211 phosphorylation was suppressed either by transfection with specific serine phosphorylation-deficient mutant GRs or treatment with inhibitors of the MAPKs, ERK1/2 and JNK. Collectively, these novel data highlight a heretofore-unidentified homeostatic mechanism in HASM cells that involves pro-asthmatic cytokine-driven, MAPK-mediated, non-ligand-dependent GR activation that confers heightened glucocorticoid ligand-stimulated GR signaling. These findings raise the consideration that perturbations in this homeostatic cytokine-driven GR signaling mechanism may be responsible, at least in part, for the insensirtivity to glucocorticoid therapy that is commonly seen in individuals with severe asthma.

The cellular and molecular basis of bitter tastant-induced bronchodilation

Bitter tastants can activate bitter taste receptors on constricted smooth muscle cells to inhibit L-type calcium channels and induce bronchodilation.

Bronchodilators are a standard medicine for treating airway obstructive diseases, and β2 adrenergic receptor agonists have been the most commonly used bronchodilators since their discovery. Strikingly, activation of G-protein-coupled bitter taste receptors (TAS2Rs) in airway smooth muscle (ASM) causes a stronger bronchodilation in vitro and in vivo than β2 agonists, implying that new and better bronchodilators could be developed. A critical step towards realizing this potential is to understand the mechanisms underlying this bronchodilation, which remain ill-defined. An influential hypothesis argues that bitter tastants generate localized Ca²⁺ signals, as revealed in cultured ASM cells, to activate large-conductance Ca²⁺-activated K⁺ channels, which in turn hyperpolarize the membrane, leading to relaxation. Here we report that in mouse primary ASM cells bitter tastants neither evoke localized Ca²⁺ events nor alter spontaneous local Ca²⁺ transients. Interestingly, they increase global intracellular [Ca²⁺]_i, although to a much lower level than bronchoconstrictors. We show that these Ca²⁺ changes in cells at rest are mediated via activation of the canonical bitter taste signaling cascade (i.e., TAS2R-gustducin-phospholipase Cβ [PLCβ]- inositol 1,4,5-triphosphate receptor [IP3R]), and are not sufficient to impact airway contractility. But activation of TAS2Rs fully reverses the increase in [Ca²⁺]_i induced by bronchoconstrictors, and this lowering of the [Ca²⁺]_i is necessary for bitter tastant-induced ASM cell relaxation. We further show that bitter tastants inhibit L-type voltage-dependent Ca²⁺ channels (VDCCs), resulting in reversal in [Ca²⁺]_i, and this inhibition can be prevented by pertussis toxin and G-protein βγ subunit inhibitors, but not by the blockers of PLCβ and IP3R. Together, we suggest that TAS2R stimulation activates two opposing Ca²⁺ signaling pathways via Gβγ to increase [Ca²⁺]_i at rest while blocking activated L-type VDCCs to induce bronchodilation of contracted ASM. We propose that the large decrease in [Ca²⁺]_i caused by effective tastant bronchodilators provides an efficient cell-based screening method for identifying potent dilators from among the many thousands of available bitter tastants.

Bitter taste receptors (TAS2Rs), a G-protein-coupled receptor family long thought to be solely expressed in taste buds on the tongue, have recently been detected in airways. Bitter substances can activate TAS2Rs in airway smooth muscle to cause greater bronchodilation than β2 adrenergic receptor agonists, the most commonly used bronchodilators. However, the mechanisms underlying this bronchodilation remain elusive. Here we show that, in resting primary airway smooth muscle cells, bitter tastants activate a TAS2R-dependent signaling pathway that results in an increase in intracellular calcium levels, albeit to a level much lower than that produced by bronchoconstrictors. In bronchoconstricted cells, however, bitter tastants reverse the bronchoconstrictor-induced increase in calcium levels, which leads to the relaxation of smooth muscle cells. We find that this reversal is due to inhibition of L-type calcium channels. Our results suggest that under normal conditions, bitter tastants can activate TAS2Rs to modestly increase calcium levels, but that when smooth muscle cells are constricted, they can block L-type calcium channels to induce bronchodilation. We postulate that this novel mechanism could operate in other extraoral cells expressing TAS2Rs.

Asthmatic airway hyperresponsiveness: the ants in the tree

Airways from asthmatics have a propensity to narrow excessively in response to spasmogens (i.e., contractile agonists), a feature called airway hyperresponsiveness (AHR). AHR is an important contributor to asthma symptoms because the degree of responsiveness dictates the amount of airway narrowing that occurs in response to inflammation-derived spasmogens produced endogenously following exposure to environmental triggers, such as allergens, viruses, or pollutants. The smooth muscle encircling the airways is responsible for responsiveness because it constricts the airway lumen when commanded to contract by spasmogens. However, whether AHR seen in asthmatics is due to stronger muscle is equivocal. In this opinion article, I propose that environmental triggers and other inflammatory molecules released during asthma attacks contribute to AHR by increasing muscle force.

Muscarinic receptors on airway mesenchymal cells: novel findings for an ancient target

Since ancient times, anticholinergics have been used as a bronchodilator therapy for obstructive lung diseases. Targets of these drugs are G-protein-coupled muscarinic M(1), M(2) and M(3) receptors in the airways, which have long been recognized to regulate vagally-induced airway smooth muscle contraction and mucus secretion. However, recent studies have revealed that acetylcholine also exerts pro-inflammatory, pro-proliferative and pro-fibrotic actions in the airways, which may involve muscarinic receptor stimulation on mesenchymal, epithelial and inflammatory cells. Moreover, acetylcholine in the airways may not only be derived from vagal nerves, but also from non-neuronal cells, including epithelial and inflammatory cells. Airway smooth muscle cells seem to play a major role in the effects of acetylcholine on airway function. It has become apparent that these cells are multipotent cells that may reversibly adopt (hyper)contractile, proliferative and synthetic phenotypes, which are all under control of muscarinic receptors and differentially involved in bronchoconstriction, airway remodeling and inflammation. Cholinergic contractile tone is increased by airway inflammation associated with asthma and COPD, resulting from exaggerated acetylcholine release as well as increased expression of contraction related proteins in airway smooth muscle. Moreover, muscarinic receptor stimulation promotes proliferation of airway smooth muscle cells as well as fibroblasts, and regulates cytokine, chemokine and extracellular matrix production by these cells, which may contribute to airway smooth muscle growth, airway fibrosis and inflammation. In line, animal models of chronic allergic asthma and COPD have recently demonstrated that tiotropium may potently inhibit airway inflammation and remodeling. These observations indicate that muscarinic receptors have a much larger role in the pathophysiology of obstructive airway diseases than previously thought, which may have important therapeutic implications.