Mechanistic link between diesel exhaust particles and respiratory reflexes.

BACKGROUND: Diesel exhaust particles (DEPs) are a major component of particulate matter in Europe's largest cities, and epidemiologic evidence links exposure with respiratory symptoms and asthma exacerbations. Respiratory reflexes are responsible for symptoms and are regulated by vagal afferent nerves, which innervate the airway. It is not known how DEP exposure activates airway afferents to elicit symptoms, such as cough and bronchospasm. OBJECTIVE: We sought to identify the mechanisms involved in activation of airway sensory afferents by DEPs. METHODS: In this study we use in vitro and in vivo electrophysiologic techniques, including a unique model that assesses depolarization (a marker of sensory nerve activation) of human vagus. RESULTS: We demonstrate a direct interaction between DEP and airway C-fiber afferents. In anesthetized guinea pigs intratracheal administration of DEPs activated airway C-fibers. The organic extract (DEP-OE) and not the cleaned particles evoked depolarization of guinea pig and human vagus, and this was inhibited by a transient receptor potential ankyrin-1 antagonist and the antioxidant N-acetyl cysteine. Polycyclic aromatic hydrocarbons, major constituents of DEPs, were implicated in this process through activation of the aryl hydrocarbon receptor and subsequent mitochondrial reactive oxygen species production, which is known to activate transient receptor potential ankyrin-1 on nociceptive C-fibers. CONCLUSIONS: This study provides the first mechanistic insights into how exposure to urban air pollution leads to activation of guinea pig and human sensory nerves, which are responsible for respiratory symptoms. Mechanistic information will enable the development of appropriate therapeutic interventions and mitigation strategies for those susceptible subjects who are most at risk.

Targeting fatty acid amide hydrolase as a therapeutic strategy for antitussive therapy.

Cough is the most common reason to visit a primary care physician, yet it remains an unmet medical need. Fatty acid amide hydrolase (FAAH) is an enzyme that breaks down endocannabinoids, and inhibition of FAAH produces analgesic and anti-inflammatory effects. Cannabinoids inhibit vagal sensory nerve activation and the cough reflex, so it was hypothesised that FAAH inhibition would produce antitussive activity via elevation of endocannabinoids.Primary vagal ganglia neurons, tissue bioassay, in vivo electrophysiology and a conscious guinea pig cough model were utilised to investigate a role for fatty acid amides in modulating sensory nerve activation in vagal afferents.FAAH inhibition produced antitussive activity in guinea pigs with concomitant plasma elevation of the fatty acid amides N-arachidonoylethanolamide (anandamide), palmitoylethanolamide, N-oleoylethanolamide and linoleoylethanolamide. Palmitoylethanolamide inhibited tussive stimulus-induced activation of guinea pig airway innervating vagal ganglia neurons, depolarisation of guinea pig and human vagus, and firing of C-fibre afferents. These effects were mediated via a cannabinoid CB2/Gi/o-coupled pathway and activation of protein phosphatase 2A, resulting in increased calcium sensitivity of calcium-activated potassium channels.These findings identify FAAH inhibition as a target for the development of novel, antitussive agents without the undesirable side-effects of direct cannabinoid receptor agonists.

Prostaglandin D2 and the role of the DP1, DP2 and TP receptors in the control of airway reflex events.

Prostaglandin D2 (PGD2) causes cough and levels are increased in asthma suggesting that it may contribute to symptoms. Although the prostaglandin D2 receptor 2 (DP2) is a target for numerous drug discovery programmes little is known about the actions of PGD2 on sensory nerves and cough. We used human and guinea pig bioassays, in vivo electrophysiology and a guinea pig conscious cough model to assess the effect of prostaglandin D2 receptor (DP1), DP2 and thromboxane receptor antagonism on PGD2 responses. PGD2 caused cough in a conscious guinea pig model and an increase in calcium in airway jugular ganglia. Using pharmacology and receptor-deficient mice we showed that the DP1 receptor mediates sensory nerve activation in mouse, guinea pig and human vagal afferents. In vivo, PGD2 and a DP1 receptor agonist, but not a DP2 receptor agonist, activated single airway C-fibres. Interestingly, activation of DP2 inhibited sensory nerve firing to capsaicin in vitro and in vivo. The DP1 receptor could be a therapeutic target for symptoms associated with asthma. Where endogenous PGD2 levels are elevated, loss of DP2 receptor-mediated inhibition of sensory nerves may lead to an increase in vagally associated symptoms and the potential for such adverse effects should be investigated in clinical studies with DP2 antagonists.

The role of adenylyl cyclase isoform 6 in ß-adrenoceptor signalling in murine airways.

BACKGROUND AND PURPOSE: Adenylyl cyclase (AC) is a key signalling enzyme for many GPCRs and catalyses the conversion of ATP to cAMP which, in turn, is a crucial determinant of many biological responses. ß-Adrenoceptor agonists are prescribed as bronchodilators for asthma and chronic obstructive pulmonary disease, and it is commonly assumed that they elicit their actions via AC-dependent production of cAMP. However, empirical evidence in support of this is lacking and the exact mechanism by which these drugs acts remains elusive. This is partly due to the existence of at least 10 different isoforms of AC and the absence of any truly selective pharmacological inhibitors. Here, we have used genetically modified mice and model systems to establish the role of AC isoforms in the airway responses to ß-adrenoceptor agonists. EXPERIMENTAL APPROACH: Receptors mediating responses to ß-adrenoceptor agonists in airway smooth muscle (ASM) and sensory nerve were identified in isolated tissue systems. Expression of mRNA for the AC isoforms in ASM and neurones was determined by qPCR. Functional responses were assessed in AC isoform KO mice and wild-type controls. KEY RESULTS: Airway and vagal tissue expressed mRNA for various isoforms of AC. AC6 was the most prominent isoform. Responses to ß-adrenoceptor agonists in tissues from AC6 KO mice were virtually abolished. CONCLUSIONS AND IMPLICATIONS: AC6 played a critical role in relaxation of ASM to ß1 -adrenoceptor agonists and in modulation of sensory nerves by ß1-3 -adrenoceptor agonists. These results further unravel the signalling pathway of this extensively prescribed class of medicine.

Role of transient receptor potential and pannexin channels in cigarette smoke-triggered ATP release in the lung.

BACKGROUND: COPD is an inflammatory disease usually associated with cigarette smoking (CS) with an increasing global prevalence and no effective medication. Extracellular ATP is increased in the COPD affected lung and may play a key role in driving CS-induced airway inflammation, but the mechanism involved in ATP release has eluded researchers. Recently, the transient receptor potential (TRP) and pannexin-1 channels have been suggested to play a role in other experimental paradigms. Thus, the aim of this work is to investigate if these channels are involved in CS-induced ATP release in the lung. METHODS: Primary human cells were exposed to CS and extracellular ATP levels measured. Mice were exposed to mainstream CS and airway inflammation assessed. TRPV1/4 mRNA expression was assessed in human lung parenchyma. RESULTS: CS exposure caused a dose-related increase in ATP from primary airway bronchial epithelial cells. This was attenuated by blockers of TRPV1, TRPV4 and pannexin-1 channels. Parallel data was obtained using murine acute CS-driven model systems. Finally, TRPV1/4 mRNA expression was increased in lung tissue samples from patients with COPD. CONCLUSIONS: Extracellular ATP is increased in the COPD affected lung and may play a key role in driving disease pathophysiology. These experiments uncover a novel mechanism which may be responsible for CS-induced ATP release. These findings highlight novel targets that could lead to the development of medicine to treat this devastating disease.

TLR4 activation induces IL-1ß release via an IPAF dependent but caspase 1/11/8 independent pathway in the lung.

BACKGROUND: The IL-1 family of cytokines is known to play an important role in inflammation therefore understanding the mechanism by which they are produced is paramount. Despite the recent plethora of publications dedicated to the study of these cytokines, the mechanism by which they are produced in the airway following endotoxin, Lipopolysaccharide (LPS), exposure is currently unclear. The aim was to determine the mechanism by which the IL-1 cytokines are produced after LPS inhaled challenge. METHODS: Mice were challenged with aerosolised LPS, and lung tissue and bronchiolar lavage fluid (BALF) collected. Targets were measured at the mRNA and protein level; caspase activity was determined using specific assays. RESULTS: BALF IL-1b/IL-18, but not IL-1a, was dependent on Ice Protease-Activating Factor (IPAF), and to a lesser extent Apoptosis-associated Speck-like protein containing a CARD (ASC). Interestingly, although we measured an increase in mRNA expression for caspase 1 and 11, we could not detect an increase in lung enzyme activity or a role for them in IL-1a/b production. Further investigations showed that whilst we could detect an increase in caspase 8 activity at later points in the time course (during resolution of inflammation), it appeared to play no role in the production of IL-1 cytokines in this model system. CONCLUSIONS: TLR4 activation increases levels of BALF IL-1b/IL-18 via an IPAF dependent and caspase 1/11/8 independent pathway. Furthermore, it would appear that the presence of IL-1a in the BALF is independent of these pathways. This novel data sheds light on innate signalling pathways in the lung that control the production of these key inflammatory cytokines.

G-protein coupled receptors regulating cough.

Cough is a protective mechanism but can occur excessively in disease. Cough can be modulated by a range of GPCRs which can be either inhibitory or excitatory. Prostaglandin E2 and bradykinin can activate airway sensory nerves via EP3 and B2 receptors receptively and have both been shown to mediate their effects though TRPV1 and TRPA1 receptors. Activation of the ß2-adrenoceptor and cannabinoid CB2 receptors can inhibit sensory nerves and prevent cough. It is currently thought that activation of the ß2-adrenoceptor causes c-AMP dependent activation of PKA; however, recent research has suggested that the pathway involves PKG-mediated opening of the BKCa channel leading to hyperpolarization.