Transient receptor potential ankyrin 1 mediates toluene diisocyanate-evoked respiratory irritation

A Novel Flp Reporter Mouse Shows That TRPA1 Expression Is Largely Limited to Sensory Neuron Subsets

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal cation channel that is activated by electrophilic irritants, oxidative stress, cold temperature, and GPCR signaling. TRPA1 expression has been primarily identified in subsets of nociceptive sensory afferents and is considered a target for future analgesics. Nevertheless, TRPA1 has been implicated in other cell types including keratinocytes, epithelium, enterochromaffin cells, endothelium, astrocytes, and CNS neurons. Here, we developed a knock-in mouse that expresses the recombinase FlpO in TRPA1-expressing cells. We crossed the TRPA1Flp mouse with the R26ai65f mouse that expresses tdTomato in a Flp-sensitive manner. We found tdTomato expression correlated well with TRPA1 mRNA expression and sensitivity to TRPA1 agonists in subsets of TRPV1 (transient receptor potential vanilloid receptor type 1)-expressing neurons in the vagal ganglia and dorsal root ganglia (DRGs), although tdTomato expression efficiency was limited in DRG. We observed tdTomato-expressing afferent fibers centrally (in the medulla and spinal cord) and peripherally in the esophagus, gut, airways, bladder, and skin. Furthermore, chemogenetic activation of TRPA1-expressing nerves in the paw evoked flinching behavior. tdTomato expression was very limited in other cell types. We found tdTomato in subepithelial cells in the gut mucosa but not in enterochromaffin cells. tdTomato was also observed in supporting cells within the cochlea, but not in hair cells. Lastly, tdTomato was occasionally observed in neurons in the somatomotor cortex and the piriform area, but not in astrocytes or vascular endothelium. Thus, this novel mouse strain may be useful for mapping and manipulating TRPA1-expressing cells and deciphering the role of TRPA1 in physiological and pathophysiological processes.

Transient receptor potential Ankyrin-1 (TRPA1) agonists suppress myelination and induce demyelination in organotypic cortical slices

Oligodendrocytes are highly specialized glial cells characterized by their production of multilayer myelin sheaths that wrap axons to speed up action potential propagation. It is due to their specific role in supporting axons that impairment of myelin structure and function leads to debilitating symptoms in a wide range of degenerative diseases, including Multiple Sclerosis and Leukodystrophies. It is known that myelin damage can be receptor-mediated and recently oligodendrocytes have been shown to express Ca2+ -permeable Transient Receptor Potential Ankyrin-1 (TRPA1) channels, whose activation can result in myelin damage in ischemia. Here, we show, using organotypic cortical slice cultures, that TRPA1 activation, by TRPA1 agonists JT010 and Carvacrol for varying lengths of time, induces myelin damage. Although TRPA1 activation does not appear to affect oligodendrocyte progenitor cell number or proliferation, it prevents myelin formation and after myelination causes internodal shrinking and significant myelin degradation. This does not occur when the TRPA1 antagonist, A967079, is also applied. Of note is that when TRPA1 agonists are applied for either 24 h, 3 days or 7 days, axon integrity appears to be preserved while mature myelinated oligodendrocytes remain but with significantly shortened internodes. These results provide further evidence that TRPA1 inhibition could be protective in demyelination diseases and a promising therapy to prevent demyelination and promote remyelination.

Antimony Compounds Associate with Atopic Dermatitis and Influence Models of Itch and Dysbiosis

Compared to the myriad of known triggers for rhinitis and asthma, environmental exposure research for atopic dermatitis (AD) is not well established. We recently reported that an untargeted search of U.S. Environmental Protection Agency (EPA) databases versus AD rates by United States (U.S.) postal codes revealed that isocyanates, such as toluene diisocyanate (TDI), are the pollutant class with the strongest spatiotemporal and epidemiologic association with AD. We further demonstrated that (di)isocyanates disrupt ceramide-family lipid production in commensal bacteria and activate the thermo-itch host receptor TRPA1. In this report, we reanalyzed regions of the U.S. with low levels of diisocyanate pollution to assess if a different chemical class may contribute. We identified antimony compounds as the top associated pollutant in such regions. Exposure to antimony compounds would be expected from brake dust in high-traffic areas, smelting plants, bottled water, and dust from aerosolized soil. Like TDI, antimony inhibited ceramide-family lipid production in Roseomonas mucosa and activated TRPA1 in human neurons. While further epidemiologic research will be needed to directly evaluate antimony exposure with surrounding AD prevalence and severity, these data suggest that compounds which are epidemiologically associated with AD, inhibit commensal lipid production, and activate TRPA1 may be causally related to AD pathogenesis.

Diisocyanates influence models of atopic dermatitis through direct activation of TRPA1

We recently used EPA databases to identify that isocyanates, most notably toluene diisocyanate (TDI), were the pollutant class with the strongest spatiotemporal and epidemiologic association with atopic dermatitis (AD). Our findings demonstrated that isocyanates like TDI disrupted lipid homeostasis and modeled benefit in commensal bacteria like Roseomonas mucosa through disrupting nitrogen fixation. However, TDI has also been established to activate transient receptor potential ankyrin 1 (TRPA1) in mice and thus could directly contribute to AD through induction of itch, rash, and psychological stress. Using cell culture and mouse models, we now demonstrate that TDI induced skin inflammation in mice as well as calcium influx in human neurons; each of these findings were dependent on TRPA1. Furthermore, TRPA1 blockade synergized with R. mucosa treatment in mice to improve TDI-independent models of AD. Finally, we show that the cellular effects of TRPA1 are related to shifting the balance of the tyrosine metabolites epinephrine and dopamine. This work provides added insight into the potential role, and therapeutic potential, or TRPA1 in the pathogenesis of AD.

Narrative Review of the Mechanisms and Treatment of Cough in Asthma, Cough Variant Asthma, and Non-asthmatic Eosinophilic Bronchitis

Chronic cough is a debilitating condition affecting 10-12% of the general population and is one of the leading causes for referral to secondary care. Many conditions have been associated with chronic cough, including asthma, gastro-esophageal reflux disease and upper airways cough syndrome. Inflammatory airway conditions including cough variant asthma (CVA) and non-asthmatic eosinophilic bronchitis (NAEB) contribute to a significant proportion of presentations with chronic cough, with differing diagnostic criteria and different responses to commonly used asthma therapy for their respective diagnoses. Mechanistic studies in both animal models and humans have identified increased neuronal sensitivity and subsequent central sensitization. These mechanisms include inflammatory-mediated nociceptor sensitization and alterations of afferent nerve terminal excitability, phenotypic changes in the vagal afferent neurons over time, and central neuroplasticity resulting from increased synaptic signalling from peripheral afferents. The aim of this review is to discuss the mechanisms, neurophysiology, and management approaches currently available for patients presenting with chronic cough with underlying asthma, CVA, and NAEB and to shed a light on areas of further research required to elucidate the mechanisms of cough in this patient population.

Exposure of Toluene Diisocyanate Induces DUSP6 and p53 through Activation of TRPA1 Receptor

Toluene diisocyanate (TDI), a major intermediate agent used in the manufacturing industry, causes respiratory symptoms when exposed to the human body. In this study, we aimed to determine the molecular mechanism of TDI toxicity. To investigate the impact of TDI exposure on global gene expression, we performed transcriptomic analysis of human bronchial epithelial cells (BEAS-2B) after TDI treatment. Differentially expressed genes (DEGs) were sorted and used for clustering and network analysis. Among DEGs, dual-specificity phosphatase 6 (DUSP6) was one of the genes significantly changed by TDI exposure. To verify the expression level of DUSP6 and its effect on lung cells, the mRNA and protein levels of DUSP6 were analyzed. Our results showed that DUSP6 was dose-dependently upregulated by TDI treatment. Thereby, the phosphorylation of ERK1/2, one of the direct inhibitory targets of DUSP6, was decreased. TDI exposure also increased the mRNA level of p53 along with its protein and activity which trans-activates DUSP6. Since TRPA1 is known as a signal integrator activated by TDI, we analyzed the relevance of TRPA1 receptor in DUSP6 regulation. Our data revealed that up-regulation of DUSP6 mediated by TDI was blocked by a specific antagonist against TRPA1. TDI exposure attenuated the apoptotic response, which suggests that it promotes the survival of cancerous cells. In conclusion, our results suggest that TDI induces DUSP6 and p53, but attenuates ERK1/2 activity through TRPA1 receptor activation, leading to cytotoxicity.

TRPA1: Pharmacology, natural activators and role in obesity prevention

Transient receptor potential ankyrin 1 (TRPA1) channel is a calcium permeable, non-selective cation channel, expressed in the sensory neurons and non-neuronal cells of different tissues. Initially studied for its role in pain and inflammation, TRPA1 has now functionally involved in multiple other physiological functions. TRPA1 channel has been extensively studied for modulation by pungent compounds present in the spices and herbs. In the last decade, the role of TRPA1 agonism in body weight reduction, secretion of hunger and satiety hormones, insulin secretion and thermogenesis, has unveiled the potential of the TRPA1 channel to be used as a preventive target to tackle obesity and associated comorbidities including insulin resistance in type 2 diabetes. In this review, we summarized the recent findings of TRPA1 based dietary/non-dietary modulation for its role in obesity prevention and therapeutics.

Advances in TRP channel drug discovery: from target validation to clinical studies

Transient receptor potential (TRP) channels are multifunctional signalling molecules with many roles in sensory perception and cellular physiology. Therefore, it is not surprising that TRP channels have been implicated in numerous diseases, including hereditary disorders caused by defects in genes encoding TRP channels (TRP channelopathies). Most TRP channels are located at the cell surface, which makes them generally accessible drug targets. Early drug discovery efforts to target TRP channels focused on pain, but as our knowledge of TRP channels and their role in health and disease has grown, these efforts have expanded into new clinical indications, ranging from respiratory disorders through neurological and psychiatric diseases to diabetes and cancer. In this Review, we discuss recent findings in TRP channel structural biology that can affect both drug development and clinical indications. We also discuss the clinical promise of novel TRP channel modulators, aimed at both established and emerging targets. Last, we address the challenges that these compounds may face in clinical practice, including the need for carefully targeted approaches to minimize potential side-effects due to the multifunctional roles of TRP channels.

Functional evidence of distinct electrophile-induced activation states of the ion channel TRPA1

Transient Receptor Potential Ankyrin 1 (TRPA1) is a tetrameric, nonselective cation channel expressed on nociceptive sensory nerves whose activation elicits nocifensive responses (e.g. pain). TRPA1 is activated by electrophiles found in foods and pollution, or produced during inflammation and oxidative stress, via covalent modification of reactive cysteines, but the mechanism underlying electrophilic activation of TRPA1 is poorly understood. Here we studied TRPA1 activation by the irreversible electrophiles iodoacetamide and N-ethylmaleimide (NEM) following transient expression in HEK293 cells. We found that in Ca2+ imaging studies C621 is critical for electrophile-induced TRPA1 activation, but the role of C665 in TRPA1 activation is dependent on the size of the electrophile. We identified slower TRPA1 activation in whole-cell recordings compared to studies with intact cells, which is rescued by pipette solution supplementation with the antioxidant glutathione. Single-channel recordings identified two distinct electrophilic-induced TRPA1 activation phases: a partial activation that, in some channels, switched to full activation with continued electrophile exposure. Full activation but not the initial activation was regulated by C665. Fitting of open time distributions suggests that full activation correlated with an additional (and long) exponential component, thus suggesting the phases are manifestations of distinct activation states. Our results suggest that distinct NEM-induced TRPA1 activation states are evoked by sequential modification of C621 then C665.

Irritant Inhalation Evokes P Wave Morphological Changes in Spontaneously Hypertensive Rats via Reflex Modulation of the Autonomic Nervous System

Irritant inhalation is associated with increased incidence of atrial fibrillation (AF) and stroke. Irritant inhalation acutely regulates cardiac function via autonomic reflexes. Increases in parasympathetic and sympathetic reflexes may increase atrial susceptibility to ectopic activity and the initiation of arrhythmia such as AF. Both age and hypertension are risk factors for AF. We have shown that irritant-evoked pulmonary–cardiac reflexes are remodeled in spontaneously hypertensive (SH) rats to include a sympathetic component in addition to the parasympathetic reflex observed in normotensive Wistar-Kyoto (WKY) rats. Here, we analyzed P wave morphology in 15-week old WKY and SH rats during inhalation of the transient receptor potential ankyrin 1 agonist allyl isothiocyanate (AITC). P Wave morphology was normal during vehicle inhalation but was variably modulated by AITC. AITC increased RR intervals (RRi), PR intervals, and the P Wave duration. In SH rats only, AITC inhalation increased the occurrence of negative P waves. The incidence of AITC-evoked negative P waves in SH rats was dependent on RRi, increasing during bradycardic and tachycardic cardiac cycles. Inhibition of both parasympathetic (using atropine) and sympathetic (using atenolol) components of the pulmonary–cardiac reflex decreased the incidence of negative P waves. Lastly, the probability of evoking a negative P Wave was increased by the occurrence of preceding negative P waves. We conclude that the remodeled irritant-evoked pulmonary–cardiac reflex in SH rats provides a substrate for altered P Wave morphologies. These are likely ectopic atrial beats that could provide a trigger for AF initiation in structurally remodeled atria.

Inhibition of TRPA1 reduces airway inflammation and hyperresponsiveness in mice with allergic rhinitis

This study was conducted to investigate whether a transient receptor potential ankyrin 1 (TRPA1) antagonist (HC-030031) can reduce airway inflammation and hyperresponsiveness in a murine allergic rhinitis (AR) model. BALB/c mice were sensitized and challenged by ovalbumin (OVA) to induce AR. HC-030031 or vehicle was administrated to mice via intraperitoneal injection prior to OVA challenges. Nose-scratching events, histopathologic alterations of the airways, and bronchial hyperresponsiveness (BHR) were assessed. Differential cells and proinflammatory cytokines in the nasal lavage (NAL) and bronchoalveolar lavage (BAL) fluid were measured. Expressions of TRPA1 in nasal mucosa were examined by immunohistochemistry. TRPA1-expressing vagal neurons were labeled by immunofluorescent staining. HC-030031-treated AR mice had markedly reduced type-2 inflammation in nasal mucosa and ameliorated-nose-scratching events than AR mice received vehicle. HC-030031 treatment also dramatically reduced leucocyte numbers and IL-8 level in the BAL fluid, inhibited lower airway remodeling and fibrosis, and nearly abolished BHR. HC-0300031 treatment significantly inhibited the upregulated number of TRPA1 expressing nasal epithelial cells and TRPA1 expressing sensory neurons, leading to downregulation of SP in both upper and lower airways. Targeting TRPA1 may represent a promising strategy for treating AR and AR-related asthma.

Differential sensitivity of cinnamaldehyde-evoked calcium fluxes to ruthenium red in guinea pig and mouse trigeminal sensory neurons

Objective

Transient receptor potential ankyrin 1 (TRPA1) is an excitatory ion channel expressed on a subset of sensory neurons. TRPA1 is activated by a host of noxious stimuli including pollutants, irritants, oxidative stress and inflammation, and is thought to play an important role in nociception and pain perception. TRPA1 is therefore a therapeutic target for diseases with nociceptive sensory signaling components. TRPA1 orthologs have been shown to have differential sensitivity to certain ligands. Cinnamaldehyde has previously been shown to activate sensory neurons via the selective gating of TRPA1. Here, we tested the sensitivity of cinnamaldehyde-evoked responses in mouse and guinea pig sensory neurons to the pore blocker ruthenium red (RuR).

Results

Cinnamaldehyde, the canonical TRPA1-selective agonist, caused robust calcium fluxes in trigeminal neurons dissociated from both mice and guinea pigs. RuR effectively inhibited cinnamaldehyde-evoked responses in mouse neurons at 30 nM, with complete block seen with 3 μM. In contrast, responses in guinea pig neurons were only partially inhibited by 3 μM RuR. We conclude that RuR has a decreased affinity for guinea pig TRPA1 compared to mouse TRPA1. This study provides further evidence of differences in ligand affinity for TRPA1 in animal models relevant for drug development.

TRP channels in airway sensory nerves

Transient Receptor Potential (TRP) channels expressed in specific subsets of airway sensory nerves function as transducers and integrators of a diverse range of sensory inputs including chemical, mechanical and thermal signals. These TRP sensors can detect inhaled irritants as well as endogenously released chemical substances. They play an important role in generating the afferent activity carried by these sensory nerves and regulating the centrally mediated pulmonary defense reflexes. Increasing evidence reported in recent investigations has revealed important involvements of several TRP channels (TRPA1, TRPV1, TRPV4 and TRPM8) in the manifestation of various symptoms and pathogenesis of certain acute and chronic airway diseases. This mini-review focuses primarily on these recent findings of the responses of these TRP sensors to the biological stresses emerging under the pathophysiological conditions of the lung and airways.

Respiratory Health Effects of Exposure to Cleaning Products

There is consistent and growing evidence of an epidemic of "asthma-like" symptoms among professional cleaners. Questions include how big is this problem worldwide, which cleaning agents are dangerous, how do they affect the lungs, and is it really asthma? This issue is important to public health because of the increasing number of professional cleaners, many from vulnerable categories. There are implications for anybody exposed to cleaning products during housekeeping, including children. This article uses available evidence to give a broad but concise overview on what we know so far and how we can prevent the cleaning-associated respiratory public health burden.

Transient receptor potential channels in pulmonary chemical injuries and as countermeasure targets

The lung is highly sensitive to chemical injuries caused by exposure to threat agents in industrial or transportation accidents, occupational exposures, or deliberate use as weapons of mass destruction (WMD). There are no antidotes for the majority of the chemical threat agents and toxic inhalation hazards despite their use as WMDs for more than a century. Among several putative targets, evidence for transient receptor potential (TRP) ion channels as mediators of injury by various inhalational chemical threat agents is emerging. TRP channels are expressed in the respiratory system and are essential for homeostasis. Among TRP channels, the body of literature supporting essential roles for TRPA1, TRPV1, and TRPV4 in pulmonary chemical injuries is abundant. TRP channels mediate their function through sensory neuronal and nonneuronal pathways. TRP channels play a crucial role in complex pulmonary pathophysiologic events including, but not limited to, increased intracellular calcium levels, signal transduction, recruitment of proinflammatory cells, neurogenic inflammatory pathways, cough reflex, hampered mucus clearance, disruption of the integrity of the epithelia, pulmonary edema, and fibrosis. In this review, we summarize the role of TRP channels in chemical threat agents-induced pulmonary injuries and how these channels may serve as medical countermeasure targets for broader indications.

Air Pollution-Induced Autonomic Modulation

Air pollutants pose a serious worldwide health hazard, causing respiratory and cardiovascular morbidity and mortality. Pollutants perturb the autonomic nervous system, whose function is critical to cardiopulmonary homeostasis. Recent studies suggest that pollutants can stimulate defensive sensory nerves within the cardiopulmonary system, thus providing a possible mechanism for pollutant-induced autonomic dysfunction. A better understanding of the mechanisms involved would likely improve the management and treatment of pollution-related disease.

Transient Receptor Potential Ion Channels Mediate Adherens Junctions Dysfunction in a Toluene Diisocyanate-Induced Murine Asthma Model

Disruption of epithelial cell-cell junctions is essential for the initiation and perpetuation of airway inflammation in asthma. We've previously reported compromised epithelial barrier integrity in a toluene diisocyanate (TDI)-induced occupational asthma model. This study is aimed to explore the role of transient receptor potential vanilloid 4 (TRPV4) and transient receptor potential ankyrin 1 (TRPA1) in the dysfunction of adherens junctions in TDI-induced asthma. Mice were sensitized and challenged with TDI for a chemical-induced asthma model. Selective blockers of TRPV4 glycogen synthase kinase (GSK)2193874, 5 and 10 mg/kg) and TRPA1 (HC030031, 10 and 20 mg/kg) were intraperitoneally given to the mice. Immunohistochemistry revealed different expression pattern of TRPV4 and TRPA1 in lung. TDI exposure increased TRPV4 expression in the airway, which can be suppressed by GSK2193874, while treatment with neither TDI alone nor TDI together with HC030031 led to changes of TRPA1 expression in the lung. Blocking either TRPV4 or TRPA1 blunted TDI-induced airway hyperreactivity, airway neutrophilia and eosinophilia, as well as Th2 responses in a dose-dependent manner. At the same time, membrane levels of E-cadherin and β-catenin were significantly decreased after TDI inhalation, which were inhibited by GSK2193874 or HC030031. Moreover, GSK2193874 and HC030031 also suppressed serine phosphorylation of glycogen synthase kinase 3β, tyrosine phosphorylation of β-catenin, as well as activation and nuclear transport of β-catenin in mice sensitized and challenged with TDI. Our study suggested that both TRPV4 and TRPA1 contribute critically to E-cadherin and β-catenin dysfunction in TDI-induced asthma, proposing novel therapeutic targets for asthma.

Urban-level environmental factors related to pediatric asthma

During the 20th century, urbanization has increasing and represented a major demographic and environmental change in developed countries. This ever-changing urban environment has an impact on disease patterns and prevalence, namely on noncommunicable diseases, such as asthma and allergy, and poses many challenges to understand the relationship between the changing urban environment and the children health. The complex interaction between human beings and urbanization is dependent not only on individual determinants such as sex, age, social or economic resources, and lifestyles and behaviors, but also on environment, including air pollution, indoors and outdoors, land use, biodiversity, and handiness of green areas. Therefore, the assessment and identification of the impact of urban environment on children's health have become a priority and many recent studies have been conducted with the goal of better understanding the impacts related to urbanization, characterizing indoor air exposure, identifying types of neighborhoods, or characteristics of neighborhoods that promote health benefits. Thus, this review focuses on the role of urban environmental factors on pediatric asthma.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

Exposure to indoor endocrine-disrupting chemicals and childhood asthma and obesity

BACKGROUND

Indoor air contaminants may act as endocrine-disrupting chemicals (EDCs). However, to what extent these contaminants affect health is poorly known. We aimed to assess the association between EDCs exposure and asthma, respiratory symptoms and obesity in schoolchildren.

METHODS

Data from a cross-sectional analysis of 815 participants from 20 schools in Porto, Portugal, were analysed. Symptoms were assessed, asthma was defined on lung function, and airway reversibility and body mass index (BMI) were calculated. The concentrations of 13 volatile organic compounds and 2 aldehydes identified as EDCs were measured in 71 classrooms throughout 1 week. Principal component analysis (PCA) was used to assess the effect of co-exposure. Associations were estimated by regression coefficients using linear and logistic regression models.

RESULTS

Increased individual and combined EDCs levels were found in classrooms having more children with asthma and obesity. Higher levels of hexane, styrene, cyclohexanone, butylated hydroxytoluene and 2-butoxyethanol were associated with obesity, and higher levels of cyclohexanone were associated with increased child BMI. Toluene, o-xylene, m/p-xylene and ethylbenzene were significantly associated with nasal obstruction. A positive association was found between PC1 and the risk of obese asthma (OR = 1.43, 95% CI 1.01, 1.98) and between PC2 and overweight (OR = 1.51, 95% CI 1.28, 1.79). PC1 and PC2 were also associated with nasal obstruction, and PC2 was associated with breathing difficulties and lean body mass, although EDCs concentrations were low.

CONCLUSIONS

Our findings further support the role of EDCs in asthma and obesity development. Moreover, even low levels of indoor exposure may influence the risk of asthma, respiratory symptoms and obesity.

Role of TRPA1 receptors in skin inflammation induced by volatile chemical irritants in mice

Contact dermatitis is a very common inflammatory reaction in the skin, causing not only aesthetic problems but also loss functionality at work. The molecular mechanisms of contact dermatitis induced by chemical irritants are still unclear. Considering that transient receptor potential channels (TRP) may induce neurogenic inflammation and the exacerbation of inflammatory responses, here we investigated the role of transient receptor potential channel ankyrin type-1 (TRPA1) in skin inflammation evoked by chemical irritants. Ear oedema and nociceptive responses elicited by the topical application of xylene and toluene were measured in Swiss mice, wild type and TRPA1 knockout (Trpa1^-/-) C57BL/6 mice. Histological analyses were performed in mice subjected to the ear oedema assay. Topical application of xylene and toluene in the mouse ear induced an edematogenic response (0.113 ± 0.008 mm and 0.067 ± 0.011 mm), compared to vehicle (0.008 ± 0.008 mm), assessed by ear thickness measurements and histological analyses. These responses were prevented by topical pretreatment with a selective TRPA1 antagonist, HC-030031 (% inhibition: xylene 36.8 ± 9.4% and toluene 50.7 ± 11.0%), and by the genetic deletion of TRPA1 ((% inhibition: xylene 66.6 ± 16.7% and toluene 75 ± 0%). In addition, the topical application of xylene and toluene to the mouse paw elicited nociceptive responses, which were significantly reduced by oral treatment with HC-030031 ((% of inhibition: 84.9 ± 1.3% and 27.1 ± 8.0%, respectively); nociceptive responses were almost completely abolished in Trpa1^-/-mice. Our data suggest that the activation of TRPA1 could be involved in some of the symptoms of irritant-mediated contact dermatitis, such as oedema, pain and neurogenic inflammation.

Nociceptive pulmonary-cardiac reflexes are altered in the spontaneously hypertensive rat

KEY POINTS

We investigated the cardiovascular and respiratory responses of the normotensive Wistar-Kyoto (WKY) rat and the spontaneously hypertensive (SH) rat to inhalation and intravenous injection of the noxious stimuli allyl isothiocyanate (AITC). AITC inhalation evoked atropine-sensitive bradycardia in conscious WKY rats, and evoked atropine-sensitive bradycardia and atenolol-sensitive tachycardia with premature ventricular contractions (PVCs) in conscious SH rats. Intravenous injection of AITC evoked bradycardia but no tachycardia/PVCs in conscious SHs, while inhalation and injection of AITC caused similar bradypnoea in conscious SH and WKY rats. Anaesthesia (inhaled isoflurane) inhibited the cardiac reflexes evoked by inhaled AITC but not injected AITC. Data indicate the presence of a de novo nociceptive pulmonary-cardiac reflex triggering sympathoexcitation in SH rats, and this reflex is dependent on vagal afferents but is not due to steady state blood pressure or due to remodelling of vagal efferent function.

ABSTRACT

Inhalation of noxious irritants/pollutants activates airway nociceptive afferents resulting in reflex bradycardia in healthy animals. Nevertheless, noxious pollutants evoke sympathoexcitation (tachycardia, hypertension) in cardiovascular disease patients. We hypothesize that cardiovascular disease alters nociceptive pulmonary-cardiac reflexes. Here, we studied reflex responses to irritants in normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive (SH) rats. Inhaled allyl isothiocyanate (AITC) evoked atropine-sensitive bradycardia with atrial-ventricular (AV) block in conscious WKY rats, thus indicating a parasympathetic reflex. Conversely, inhaled AITC in conscious SH rats evoked complex brady-tachycardia with both AV block and premature ventricular contractions (PVCs). Atropine abolished the bradycardia and AV block, but the atropine-insensitive tachycardia and PVCs were abolished by the β₁ -adrenoceptor antagonist atenolol. The aberrant AITC-evoked reflex in SH rats was not reduced by acute blood pressure reduction by captopril. Surprisingly, intravenous AITC only evoked bradycardia in conscious SH and WKY rats. Furthermore, anaesthesia reduced the cardiac reflexes evoked by inhaled but not injected AITC. Nevertheless, anaesthesia had little effect on AITC-evoked respiratory reflexes. Such data suggest distinct differences in nociceptive reflex pathways dependent on cardiovascular disease, administration route and downstream effector. AITC-evoked tachycardia in decerebrate SH rats was abolished by vagotomy. Finally, there was no difference in the cardiac responses of WKY and SH rats to vagal efferent electrical stimulation. Our data suggest that AITC inhalation in SH rats evokes de novo adrenergic reflexes following vagal afferent activation. This aberrant reflex is independent of steady state hypertension and is not evoked by intravenous AITC. We conclude that pre-existing hypertension aberrantly shifts nociceptive pulmonary-cardiac reflexes towards sympathoexcitation.

Activation of TRPA1 by volatile organic chemicals leading to sensory irritation

Many volatile organic chemicals (VOCs) have not been tested for sensory pulmonary irritation. Development of in vitro non-animal sensory irritation assay suitable for a large number of chemicals is needed to replace the mouse assay. An adverse outcome pathway (AOP) is designed to provide a clear description of the biochemical and cellular processes leading to toxicological effects or an adverse outcome. The AOP for chemical sensory pulmonary irritation was developed according to the Organization for Economic Co-operation and Development guidance including the Bradford Hill criteria for a weight of evidence to determine the confidence of the AOP. The proposed AOP is based on an in-depth review of the relevant scientific literature to identify the initial molecular event for respiratory irritation. The activation of TRPA1 receptor (transient receptor potential cation channel, subfamily A, member 1) is the molecular initial event (MIE) leading to sensory irritation. A direct measure of TRPA1 activation in vitro should identify chemical sensory irritants and provide an estimate of potency. Fibroblasts expressing TRPA1 are used to determine TRPA1 activation and irritant potency. We report a linear relationship between the in vivo RD₅₀ and the in vitro pEC₅₀ values (R=0.81) to support this hypothesis. We propose that this in vitro assay after additional analysis and validation could serve as a suitable candidate to replace the mouse sensory irritation assay.

Sympathoexcitation in response to cardiac and pulmonary afferent stimulation of TRPA1 channels is attenuated in rats with chronic heart failure

Excessive sympathoexcitation characterizes the chronic heart failure (CHF) state. An exaggerated cardiac sympathetic afferent reflex (CSAR) contributes to this sympathoexcitation. Prior studies have demonstrated that the CSAR to capsaicin [transient receptor potential (TRP) vanilloid 1 agonist] is exaggerated in CHF animal models. We recently discovered that capsaicin application to the lung visceral pleura in anesthetized, vagotomized, open-chested rats increases mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA). We named this response the pulmonary spinal afferent reflex (PSAR). Due to the similarities between TRP vanilloid 1 and TRP ankyrin 1 (TRPA1) channels as well as the excessive sympathoexcitation of CHF, we hypothesized that stimulation of the CSAR and PSAR with a specific TRPA1 agonist would result in an augmented response in CHF rats (coronary ligation model) compared with sham control rats. In response to a TRPA1 agonist, both CSAR and PSAR in sham rats resulted in biphasic changes in MAP and increases in HR and RSNA 10-12 wk postmyocardial infarction (post-MI). These effects were blunted in CHF rats. Assessment of TRPA1 expression levels in cardiopulmonary spinal afferents by immunofluorescence, quantitative RT-PCR, and Western blot analysis 10-12 wk post-MI all indicates reduced expression in CHF rats but no reduction at earlier time points. TRPA1 protein was reduced in a dorsal root ganglia cell culture model of inflammation and simulated tissue ischemia, raising the possibility that the in vivo reduction of TRPA1 expression was, in part, caused by CHF-related tissue ischemia and inflammation. These data provide evidence that reflex responses to cardiopulmonary spinal afferent TRPA1 stimulation may be attenuated in CHF rather than enhanced. NEW & NOTEWORTHY Excessive sympathoexcitation characterizes chronic heart failure (CHF). The contribution of transient receptor potential ankyrin 1 (TRPA1) channel-mediated reflexes to this sympathoexcitation is unknown. We found that application of TRPA1 agonist to the heart and lung surface resulted in increased heart rate and sympathetic output and a biphasic change in mean arterial pressure in control rats. These effects were attenuated in CHF rats, decreasing the likelihood that TRPA1 channels contribute to cardiopulmonary afferent sensitization in CHF.

Inhalation of electronic cigarette aerosol induces reflex bronchoconstriction by activation of vagal bronchopulmonary C-fibers

The electronic cigarette (e-cig) has been suggested as a safer alternative to tobacco cigarettes. However, the health effects of e-cigs on the airways have not been fully investigated. Nicotine, the primary chemical constituent of the e-cig aerosol, has been shown to stimulate vagal bronchopulmonary C-fiber sensory nerves, which upon activation can elicit vigorous pulmonary defense reflexes, including airway constriction. In this study, we investigated the bronchomotor response to e-cig inhalation challenge in anesthetized guinea pigs and the mechanisms involved in regulating these responses. Our results showed that delivery of a single puff of e-cig aerosol into the lung triggered immediately a transient bronchoconstriction that sustained for >2 min. The increase in airway resistance was almost completely abolished by a pretreatment with either intravenous injection of atropine or inhalation of aerosolized lidocaine, suggesting that the bronchoconstriction was elicited by cholinergic reflex mechanism and stimulation of airway sensory nerves was probably involved. Indeed, electrophysiological recording further confirmed that inhalation of e-cig aerosol exerted a pronounced stimulatory effect on vagal bronchopulmonary C-fibers. These effects on airway resistance and bronchopulmonary C-fiber activity were absent when the e-cig aerosol containing zero nicotine was inhaled, indicating a critical role of nicotine. Furthermore, a pretreatment with nicotinic acetylcholine receptor antagonists by inhalation completely prevented the airway constriction evoked by e-cig aerosol inhalation. In conclusion, inhalation of a single puff of e-cig aerosol caused a transient bronchoconstriction that was mediated through cholinergic reflex and triggered by a stimulatory effect of nicotine on vagal bronchopulmonary C-fiber afferents.

Mrgprs on vagal sensory neurons contribute to bronchoconstriction and airway hyper-responsiveness

Asthma, accompanied by lung inflammation, bronchoconstriction and airway hyper-responsiveness, is a significant public health burden. Here we report that Mas-related G protein-coupled receptors (Mrgprs) are expressed in a subset of vagal sensory neurons innervating the airway and mediates cholinergic bronchoconstriction and airway hyper-responsiveness. These findings provide insights into the neural mechanisms underlying the pathogenesis of asthma.

The emerging role of transient receptor potential channels in chronic lung disease

Chronic lung diseases such as asthma, chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis are a major and increasing global health burden with a high unmet need. Drug discovery efforts in this area have been largely disappointing and so new therapeutic targets are needed. Transient receptor potential ion channels are emerging as possible therapeutic targets, given their widespread expression in the lung, their role in the modulation of inflammatory and structural changes and in the production of respiratory symptoms, such as bronchospasm and cough, seen in chronic lung disease.

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

Toluene diisocyanate exposure induces airway inflammation of bronchial epithelial cells via the activation of transient receptor potential melastatin 8

Toluene diisocyanate (TDI) is the most important cause of occupational asthma (OA), and various pathogenic mechanisms have been suggested. Of these mechanisms, neurogenic inflammation is an important inducer of airway inflammation. Transient receptor potential melastatin 8 (TRPM8) is a well-established cold-sensing cation channel that is expressed in both neuronal cells and bronchial epithelial cells. A recent genome-wide association study of TDI-exposed workers found a significant association between the phenotype of TDI-induced OA and the single-nucleotide polymorphism rs10803666, which has been mapped to the TRPM8 gene. We hypothesized that TRPM8 located in airway epithelial cells may be involved in the pathogenic mechanisms of TDI-induced OA and investigated its role. Bronchial epithelial cells were treated with TDI in a dose- and time-dependent manner. The expression levels of TRPM8 mRNA and protein were determined by quantitative real-time polymerase chain reaction and western blotting. TDI-induced morphological changes in the cells were evaluated by immunocytochemistry. Alterations in the transcripts of inflammatory cytokines were examined in accordance with TRPM8 activation by TDI. TRPM8 expression at both the mRNA and protein levels was enhanced by TDI in airway epithelial cells. TRPM8 activation by TDI led to significant increases in the mRNA of interleukin (IL)-4, IL-13, IL-25 and IL-33. The increased expression of the cytokine genes by TDI was partly attenuated after treatment with a TRPM8 antagonist. TDI exposure induces increased expression of TRPM8 mRNA in airway epithelial cells coupled with enhanced expression of inflammatory cytokines, suggesting a novel role of TRPM8 in the pathogenesis of TDI-induced OA.

Blocking TRPA1 in Respiratory Disorders: Does It Hold a Promise?

Transient Receptor Potential Ankyrin 1 (TRPA1) ion channel is expressed abundantly on the C fibers that innervate almost entire respiratory tract starting from oral cavity and oropharynx, conducting airways in the trachea, bronchi, terminal bronchioles, respiratory bronchioles and upto alveolar ducts and alveoli. Functional presence of TRPA1 on non-neuronal cells got recognized recently. TRPA1 plays a well-recognized role of “chemosensor”, detecting presence of exogenous irritants and endogenous pro-inflammatory mediators that are implicated in airway inflammation and sensory symptoms like chronic cough, asthma, chronic obstructive pulmonary disease (COPD), allergic rhinitis and cystic fibrosis. TRPA1 can remain activated chronically due to elevated levels and continued presence of such endogenous ligands and pro-inflammatory mediators. Several selective TRPA1 antagonists have been tested in animal models of respiratory disease and their performance is very promising. Although there is no TRPA1 antagonist in advanced clinical trials or approved on market yet to treat respiratory diseases, however, limited but promising evidences available so far indicate likelihood that targeting TRPA1 may present a new therapy in treatment of respiratory diseases in near future. This review will focus on in vitro, animal and human evidences that strengthen the proposed role of TRPA1 in modulation of specific airway sensory responses and also on preclinical and clinical progress of selected TRPA1 antagonists.

Alternative in vitro assays to assess the potency of sensory irritants-Is one TRP channel enough?

One important function of the peripheral nervous system is the detection of noxious chemicals in the environment as well as the recognition of tissue damage throughout the body. Transient receptor potential (TRP) ion channels are able to sense a multitude of signaling factors involved in these processes. Via the sensory ganglia these sentinels convey information to the central nervous system, where perceptions of nociception or sensory irritation are generated. From the 28 members of the 6 subfamilies present in mammals, researchers in toxicology paid special attention to TRPA1 and TRPV1 channels. Various xenobiotics (e.g., acrolein, formaldehyde) can open these channels causing sensory irritations and defense mechanisms like sneezing, coughing and lacrimation. Heterologous expression of these two channels and the subsequent investigation of ion fluxes have been proposed as in vitro models for the assessment of sensory irritation. In a series of experiments using acetophenone, isophorone, and 2-ethylhexanol (2-EH) we investigated the effects of these irritants on heterologously expressed TRP channels in comparison to a primary cell culture of trigeminal ganglia neurons of mice. We confirmed acetophenone as a specific TRPA1 agonist that activates the receptor in concentrations >3mM, whereas isophorone specifically activates TRPV1 in concentrations >100μM. 2-EH can activate heterologously expressed TRPA1 concentration-dependently (1 mM-10mM). In Ca²⁺ imaging we observed 2-EH as an agonist of multiple channels (TRPA1, TRPV1, GPCRs) that activates the trigeminal neurons by application of μM 2-EH concentrations. The convergent results of our experiments further support the specificity of acetophenone and isophorone to activate only one of these investigated TRP channels and a more unspecific activation in the case of 2-EH. However, the results of the two different in vitro systems also showed that both TRPA1 and TRPV1 channel activation is important for the perception of irritants and only the combined and tiered testing might lead to precise estimates describing the potency of a xenobiotic to cause sensory irritation or pain.

Deciphering Subtype-Selective Modulations in TRPA1 Biosensor Channels

The transient receptor potential (TRP) proteins are a family of ion channels that act as cellular sensors. Several members of the TRP family are sensitive to oxidative stress mediators. Among them, TRPA1 is remarkably susceptible to various oxidants, and is known to mediate neuropathic pain and respiratory, vascular and gastrointestinal functions, making TRPA1 an attractive therapeutic target. Recent studies have revealed a number of modulators (both activators and inhibitors) that act on TRPA1. Endogenous mediators of oxidative stress and exogenous electrophiles activate TRPA1 through oxidative modification of cysteine residues. Non-electrophilic compounds also activate TRPA1. Certain non-electrophilic modulators may act on critical non-cysteine sites in TRPA1. However, a method to achieve selective modulation of TRPA1 by small molecules has not yet been established. More recently, we found that a novel N-nitrosamine compound activates TRPA1 by S-nitrosylation (the addition of a nitric oxide (NO) group to cysteine thiol), and does so with significant selectivity over other NO-sensitive TRP channels. It is proposed that this subtype selectivity is conferred through synergistic effects of electrophilic cysteine transnitrosylation and molecular recognition of the non-electrophilic moiety on the N-nitrosamine. In this review, we describe the molecular pharmacology of these TRPA1 modulators and discuss their modulatory mechanisms.

Neuro-immune interactions in chemical-induced airway hyperreactivity

Asthma may be induced by chemical sensitisers, via mechanisms that are still poorly understood. This type of asthma is characterised by airway hyperreactivity (AHR) and little airway inflammation. Since potent chemical sensitisers, such as toluene-2,4-diisocyanate (TDI), are also sensory irritants, it is suggested that chemical-induced asthma relies on neuro-immune mechanisms.We investigated the involvement of transient receptor potential channels (TRP) A1 and V1, major chemosensors in the airways, and mast cells, known for their ability to communicate with sensory nerves, in chemical-induced AHR.In vitro intracellular calcium imaging and patch-clamp recordings in TRPA1- and TRPV1-expressing Chinese hamster ovarian cells showed that TDI activates murine TRPA1, but not TRPV1. Using an in vivo model, in which an airway challenge with TDI induces AHR in TDI-sensitised C57Bl/6 mice, we demonstrated that AHR does not develop, despite successful sensitisation, in Trpa1 and Trpv1 knockout mice, and wild-type mice pretreated with a TRPA1 blocker or a substance P receptor antagonist. TDI-induced AHR was also abolished in mast cell deficient Kit(Wsh) (/Wsh) mice, and in wild-type mice pretreated with the mast cell stabiliser ketotifen, without changes in immunological parameters.These data demonstrate that TRPA1, TRPV1 and mast cells play an indispensable role in the development of TDI-elicited AHR.

The involvement of TRP channels in sensory irritation: a mechanistic approach toward a better understanding of the biological effects of local irritants

Peripheral nerves innervating the mucosae of the nose, mouth, and throat protect the organism against chemical hazards. Upon their stimulation, characteristic perceptions (e.g., stinging and burning) and various reflexes are triggered (e.g., sneezing and cough). The potency of a chemical to cause sensory irritation can be estimated by a mouse bioassay assessing the concentration-dependent decrease in the respiratory rate (50 % decrease: RD50). The involvement of the N. trigeminus and its sensory neurons in the irritant-induced decrease in respiratory rates are not well understood to date. In calcium imaging experiments, we tested which of eight different irritants (RD50 5-730 ppm) could induce responses in primary mouse trigeminal ganglion neurons. The tested irritants acetophenone, 2-ethylhexanol, hexyl isocyanate, isophorone, and trimethylcyclohexanol stimulated responses in trigeminal neurons. Most of these responses depended on functional TRPA1 or TRPV1 channels. For crotyl alcohol, 3-methyl-1-butanol, and sodium metabisulfite, no activation could be observed. 2-ethylhexanol can activate both TRPA1 and TRPV1, and at low contractions (100 µM) G protein-coupled receptors (GPCRs) seem to be involved. GPCRs might also be involved in the mediation of the responses to trimethylcyclohexanol. By using neurobiological tools, we showed that sensory irritation in vivo could be based on the direct activation of TRP channels but also on yet unknown interactions with GPCRs present in trigeminal neurons. Our results showed that the potency suggested by the RD50 values was not reflected by direct nerve-compound interaction.

Role of reactive oxygen species and TRP channels in the cough reflex

The cough reflex is evoked by noxious stimuli in the airways. Although this reflex is essential for health, it can be triggered chronically in inflammatory and infectious airway disease. Neuronal transient receptor potential (TRP) channels such as ankyrin 1 (TRPA1) and vanilloid 1 (TRPV1) are polymodal receptors expressed on airway nociceptive afferent nerves. Reactive oxygen species (ROS) and other reactive compounds are associated with inflammation, from either NADPH oxidase or mitochondria. These reactive compounds cause activation and hyperexcitability of nociceptive afferents innervating the airways, and evidence suggests key contributions of TRPA1 and TRPV1.

Characterization of cardiovascular reflexes evoked by airway stimulation with allylisothiocyanate, capsaicin, and ATP in Sprague-Dawley rats

Acute inhalation of airborne pollutants alters cardiovascular function and evidence suggests that pollutant-induced activation of airway sensory nerves via the gating of ion channels is critical to these systemic responses. Here, we have investigated the effect of capsaicin [transient receptor potential (TRP) vanilloid 1 (TRPV1) agonist], AITC [TRP ankyrin 1 (TRPA1) agonist], and ATP (P2X2/3 agonist) on bronchopulmonary sensory activity and cardiovascular responses of conscious Sprague-Dawley (SD) rats. Single fiber recordings show that allyl isothiocyanate (AITC) and capsaicin selectively activate C fibers, whereas subpopulations of both A and C fibers are activated by stimulation of P2X2/3 receptors. Inhalation of the agonists by conscious rats caused significant bradycardia, atrioventricular (AV) block, and prolonged PR intervals, although ATP-induced responses were lesser than those evoked by AITC or capsaicin. Responses to AITC were inhibited by the TRP channel blocker ruthenium red and the muscarinic antagonist atropine. AITC inhalation also caused a biphasic blood pressure response: a brief hypertensive phase followed by a hypotensive phase. Atropine accentuated the hypertensive phase, while preventing the hypotension. AITC-evoked bradycardia was not abolished by terazosin, the α1-adrenoceptor inhibitor, which prevented the hypertensive response. Anesthetics had profound effects on AITC-evoked bradycardia and AV block, which was abolished by urethane, ketamine, and isoflurane. Nevertheless, AITC inhalation caused bradycardia and AV block in paralyzed and ventilated rats following precollicular decerebration. In conclusion, we provide evidence that activation of ion channels expressed on nociceptive airway sensory nerves causes significant cardiovascular effects in conscious SD rats via reflex modulation of the autonomic nervous system.

Transient receptor potential A1 activation prolongs isoflurane induction latency and impairs respiratory function in mice

BACKGROUND

Isoflurane is a potent volatile anesthetic; however, it evokes airway irritation and neurogenic constriction through transient receptor potential (TRP) A1 channels and sensitizes TRPV1 channels, which colocalizes with TRPA1 in most of the vagal C-fibers innervating the airway. However, little is known about the precise effects of these two channels on the respiratory function during isoflurane anesthesia.

METHODS

By using a rodent behavioral model and whole-body plethysmograph, the authors examined the response of Trpa1 and Trpv1 mice to isoflurane anesthesia and monitored their respiratory functions during anesthesia.

RESULTS

This study showed that Trpa1 mice (n = 9), but not Trpv1 mice (n = 11), displayed a shortened induction latency compared with wild-type mice (n = 10) during isoflurane anesthesia (33 ± 2.0 s in wild-type and 33 ± 3.8 s in Trpv1 vs. 17 ± 1.8 in Trpa1 at 2.2 minimum alveolar concentrations). By contrast, their response to the nonpungent volatile anesthetic sevoflurane is indistinguishable from wild-type mice (24 ± 3.6 s in wild-type vs. 26 ± 1.0 s in Trpa1 at 2.4 minimum alveolar concentrations). The authors discovered that Trpa1 mice inhaled more anesthetic but maintained better respiratory function. Further respiration pattern analysis revealed that isoflurane triggered nociceptive reflexes and led to prolonged resting time between breaths during isoflurane induction as well as decreased dynamic pulmonary compliance, an indicator of airway constriction, throughout isoflurane anesthesia in wild-type and Trpv1 mice, but not in Trpa1 mice.

CONCLUSION

Activation of TRPA1 by isoflurane negatively affects anesthetic induction latency by altering respiratory patterns and impairing pulmonary compliance.

Oxidative stress as activators of sensory nerves for cough

Excessive activation of the cough reflex is a major clinical problem in respiratory diseases. The cough reflex is triggered by activation of nociceptive sensory nerve terminals innervating the airways by noxious stimuli. Oxidative stress is a noxious stimuli associated with inhalation of pollutants and inflammatory airway disease. Here, we discuss recent findings that oxidative stress, in particular downstream of mitochondrial dysfunction, evokes increased electrical activity in airway nociceptive sensory nerves. Mechanisms include activation of transient receptor potential (TRP) channels and protein kinase C. Such mechanisms may contribute to excessive cough reflexes in respiratory diseases.

Toluene diisocyanate: Induction of the autotaxin-lysophosphatidic acid axis and its association with airways symptoms

Diisocyanates are industrial chemicals which have a wide range of applications in developed and developing countries. They are notorious lung toxicants and respiratory sensitizers. However, the mechanisms behind their adverse effects are not adequately characterized. Autotaxin (ATX) is an enzyme producing lysophosphatidic acid (LPA), and the ATX-LPA axis has been implicated in lung related inflammatory conditions and diseases, including allergic asthma, but not to toxicity of environmental low-molecular-weight chemicals. We investigated effects of toluene diisocyanate (TDI) on ATX induction in human lung epithelial cell models, and we correlated LPA-levels in plasma to biomarkers of TDI exposure in urine collected from workers exposed to <5ppb (parts per billion). Information on workers' symptoms was collected through interviews. One nanomolar TDI robustly induced ATX release within 10min in vitro. A P2X7- and P2X4-dependent microvesicle formation was implicated in a rapid ATX release and a subsequent protein synthesis. Co-localization between purinergic receptors and ATX was documented by immunofluorescence and confocal microscopy. The release was modulated by monocyte chemoattractant protein-1 (MCP-1) and by extracellular ATP. In workers, we found a dose-response relationship between TDI exposure biomarkers in urine and LPA levels in plasma. Among symptomatic workers reporting "sneezing", the LPA levels were higher than among non-symptomatic workers. This is the first report indicating induction of the ATX-LPA axis by an environmental low-molecular-weight chemical, and our data suggest a role for the ATX-LPA axis in TDI toxicity.

The TRPA1 channel in inflammatory and neuropathic pain and migraine

The transient receptor potential ankyrin 1 (TRPA1), a member of the TRP superfamily of channels, is primarily localized to a subpopulation of primary sensory neurons of the trigeminal, vagal, and dorsal root ganglia. This subset of nociceptors produces and releases the neuropeptides substance P (SP) and calcitonin gene-related peptide (CGRP), which mediate neurogenic inflammatory responses. TRPA1 is activated by a number of exogenous compounds, including molecules of botanical origin, environmental irritants, and medicines. However, the most prominent feature of TRPA1 resides in its unique sensitivity for large series of reactive byproducts of oxidative and nitrative stress. Here, the role of TRPA1 in models of different types of pain, including inflammatory and neuropathic pain and migraine, is summarized. Specific attention is paid to TRPA1 as the main contributing mechanism to the transition of mechanical and cold hypersensitivity from an acute to a chronic condition and as the primary transducing pathway by which oxidative/nitrative stress produces acute nociception, allodynia, and hyperalgesia. A series of migraine triggers or medicines have been reported to modulate TRPA1 activity and the ensuing CGRP release. Thus, TRPA1 antagonists may be beneficial in the treatment of inflammatory and neuropathic pain and migraine.

The role of flavor and fragrance chemicals in TRPA1 (transient receptor potential cation channel, member A1) activity associated with allergies

TRPA1 has been proposed to be associated with diverse sensory allergic reactions, including thermal (cold) nociception, hearing and allergic inflammatory conditions. Some naturally occurring compounds are known to activate TRPA1 by forming a Michael addition product with a cysteine residue of TRPA1 through covalent protein modification and, in consequence, to cause allergic reactions. The anti-allergic property of TRPA1 agonists may be due to the activation and subsequent desensitization of TRPA1 expressed in sensory neurons. In this review, naturally occurring TRPA1 antagonists, such as camphor, 1,8-cineole, menthol, borneol, fenchyl alcohol and 2-methylisoborneol, and TRPA1 agonists, including thymol, carvacrol, 1'S-1'- acetoxychavicol acetate, cinnamaldehyde, α-n-hexyl cinnamic aldehyde and thymoquinone as well as isothiocyanates and sulfides are discussed.

Species-specific temperature sensitivity of TRPA1

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal ion channel sensitive to temperature and chemical stimuli. The importance of temperature and aversive chemical detection for survival has driven the evolutionary diversity of TRPA1 sensitivity. This diversity can be observed in the various roles of TRPA1 in different species, where it is proposed to act as a temperature-insensitive chemosensor, a heat transducer, a noxious cold transducer, or a detector of low-intensity heat for prey localization. Exploring the variation of TRPA1 functions among species provides evolutionary insight into molecular mechanisms that fine-tune thermal and chemical sensitivity, and offers an opportunity to address basic principles of temperature gating in ion channels. A decade of research has yielded a number of hypotheses describing physiological roles of TRPA1, modulators of its activity, and biophysical principles of gating. This review surveys the diversity of TRPA1 adaptations across evolutionary taxa and explores possible mechanisms of TRPA1 activation.

Transient receptor potential channels and occupational exposure

PURPOSE OF REVIEW

The discovery that a number of transient receptor potential (TRP) channels are expressed in a subpopulation of primary sensory neurons innervating the upper and lower airways as well as in nonneuronal cells in the airways and lungs has initiated a quest for the understanding of their role in the physiology and pathophysiology of the respiratory tract.

RECENT FINDINGS

Various members of the TRP vanilloid subfamily (TRPV1, TRPV4) and the TRP ankyrin 1 (TRPA1), because of their localization in peptidergic sensory neurons, promote airway neurogenic inflammation. In particular, TRPA1, which is gated by oxidative and nitrative stress byproducts, has been found to mediate inflammatory responses produced by an unprecedented series of toxic and irritant agents produced by air pollution, contained in cigarette smoke, and produced by accidental events at the workplace. The observation that reactive molecules endogenously produced in the airways/lungs of asthma, work-related asthma, and chronic obstructive pulmonary disease target TRPA1 underscores the primary role of the TRPA1 channel in these conditions.

SUMMARY

Identification of TRP channels, and especially TRPA1, as major targets of oxidative/nitrative stress and a variety of irritant environmental agents supports the hypothesis that neurogenic inflammation plays an important role in work-related inflammatory diseases and that antagonists for such channels may be novel therapeutic options for the treatment of these diseases.

Redox regulation of transient receptor potential channels

SIGNIFICANCE

Environmental and endogenous reactive species such as reactive oxygen species (ROS), reactive nitrogen species (RNS), and other electrophiles are not only known to exert toxic effects on organisms, but are also emerging as molecules that mediate cell signaling responses. However, the mechanisms underlying this cellular redox signaling by reactive species remains largely uncharacterized.

RECENT ADVANCES

Ca2+-permeable cation channels encoded by the transient receptor potential (trp) gene superfamily are characterized by a wide variety of activation triggers that act from outside and inside the cell. Recent studies have revealed that multiple TRP channels sense reactive species and induce diverse physiological and pathological responses, such as cell death, chemokine production, and pain transduction. TRP channels sense reactive species either indirectly through second messengers or directly via oxidative modification of cysteine residues. In this review, we describe the activation mechanisms and biological roles of redox-sensitive TRP channels, including TRPM2, TRPM7, TRPC5, TRPV1, and TRPA1.

CRITICAL ISSUES

The sensitivity of TRP channels to reactive species in vitro has been well characterized using molecular and pharmacological approaches. However, the precise activation mechanism(s) and in vivo function(s) of ROS/RNS-sensitive TRP channels remain elusive.

FUTURE DIRECTIONS

Redox sensitivity of TRP channels has been shown to mediate previously unexplained biological phenomena and is involved in various pathologies. Understanding the physiological significance and activation mechanisms of TRP channel regulation by reactive species may lead to TRP channels becoming viable pharmacological targets, and modulators of these channels may offer therapeutic options for previously untreatable diseases.

Intraganglionic signaling as a novel nasal-meningeal pathway for TRPA1-dependent trigeminovascular activation by inhaled environmental irritants

Headache is the most common symptom associated with air pollution, but little is understood about the underlying mechanism. Nasal administration of environmental irritants activates the trigeminovascular system by a TRPA1-dependent process. This report addresses questions about the anatomical pathway involved and the function of TRP channels in this pathway. TRPV1 and TRPA1 are frequently co-localized and interact to modulate function in sensory neurons. We demonstrate here that resiniferatoxin ablation of TRPV1 expressing neurons significantly reduces meningeal blood flow responses to nasal administration of both TRPV1 and TRPA1 agonists. Accordingly resiniferatoxin also significantly reduces TRPV1 and CGRP immunostaining and TRPV1 and TRPA1 message levels in trigeminal ganglia. Sensory neurons of the trigeminal ganglia innervate the nasal epithelium and the meninges, but the mechanism and anatomical route by which nasal administration evokes meningeal vasodilatation is unclear. Double retrograde labeling from the nose and meninges reveals no co-localization of fluorescent label, however nasal and meningeal labeled cells are located in close proximity to each other within the trigeminal ganglion. Our data demonstrate that TRPV1 expressing neurons are important for TRPA1 responses in the nasal-meningeal pathway. Our data also suggest that the nasal-meningeal pathway is not primarily by axon reflex, but may instead result from intraganglionic transmission.

Transient receptor potential ankyrin 1 receptor activation in vitro and in vivo by pro-tussive agents: GRC 17536 as a promising anti-tussive therapeutic

Cough is a protective reflex action that helps clear the respiratory tract which is continuously exposed to airborne environmental irritants. However, chronic cough presents itself as a disease in its own right and despite its global occurrence; the molecular mechanisms responsible for cough are not completely understood. Transient receptor potential ankyrin1 (TRPA1) is robustly expressed in the neuronal as well as non-neuronal cells of the respiratory tract and is a sensor of a wide range of environmental irritants. It is fast getting acceptance as a key biological sensor of a variety of pro-tussive agents often implicated in miscellaneous chronic cough conditions. In the present study, we demonstrate in vitro direct functional activation of TRPA1 receptor by citric acid which is routinely used to evoke cough in preclinical and clinical studies. We also show for the first time that a potent and selective TRPA1 antagonist GRC 17536 inhibits citric acid induced cellular Ca⁺² influx in TRPA1 expressing cells and the citric acid induced cough response in guinea pigs. Hence our data provides a mechanistic link between TRPA1 receptor activation in vitro and cough response induced in vivo by citric acid. Furthermore, we also show evidence for TRPA1 activation in vitro by the TLR4, TLR7 and TLR8 ligands which are implicated in bacterial/respiratory virus pathogenesis often resulting in chronic cough. In conclusion, this study highlights the potential utility of TRPA1 antagonist such as GRC 17536 in the treatment of miscellaneous chronic cough conditions arising due to diverse causes but commonly driven via TRPA1.

TRPA1 channels: chemical and temperature sensitivity

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal excitatory ion channel found in sensory neurons of different organisms, ranging from worms to humans. Since its discovery as an uncharacterized transmembrane protein in human fibroblasts, TRPA1 has become one of the most intensively studied ion channels. Its function has been linked to regulation of heat and cold perception, mechanosensitivity, hearing, inflammation, pain, circadian rhythms, chemoreception, and other processes. Some of these proposed functions remain controversial, while others have gathered considerable experimental support. A truly polymodal ion channel, TRPA1 is activated by various stimuli, including electrophilic chemicals, oxygen, temperature, and mechanical force, yet the molecular mechanism of TRPA1 gating remains obscure. In this review, we discuss recent advances in the understanding of TRPA1 physiology, pharmacology, and molecular function.

TRPA1

The transient receptor potential ankyrin subtype 1 protein (TRPA1) is a nonselective cation channel permeable to Ca(2+), Na(+), and K(+). TRPA1 is a promiscuous chemical nocisensor that is also involved in noxious cold and mechanical sensation. It is present in a subpopulation of Aδ- and C-fiber nociceptive sensory neurons as well as in other sensory cells including epithelial cells. In primary sensory neurons, Ca(2+) and Na(+) flowing through TRPA1 into the cell cause membrane depolarization, action potential discharge, and neurotransmitter release both at peripheral and central neural projections. In addition to being activated by cysteine and lysine reactive electrophiles and oxidants, TRPA1 is indirectly activated by pro-inflammatory agents via the phospholipase C signaling pathway, in which cytosolic Ca(2+) is an important regulator of channel gating. The finding that non-electrophilic compounds, including menthol and cannabinoids, activate TRPA1 may provide templates for the design of non-tissue damaging activators to fine-tune the activity of TRPA1 and raises the possibility that endogenous ligands sharing binding sites with such non-electrophiles exist and regulate TRPA1 channel activity. TRPA1 is promising as a drug target for novel treatments of pain, itch, and sensory hyperreactivity in visceral organs including the airways, bladder, and gastrointestinal tract.

The effects of nasal irritant induced responses on breathing and cough in anaesthetized and conscious animal models

There is little evidence to support the down-regulation of coughing from the nose. The cough response to citric acid (CA) was studied in anesthetized and conscious guinea pigs after nasal pretreatment with saline, 1% DMSO, allylisothiocyanate (TRPA1 agonist) and allylisothiocyanate +AP-18 (TRPA1 antagonist). Cough was induced by adding citric acid (CA) to the tracheal perfusion in anaesthetized animals, or by inhaling 0.4M CA in conscious animals. The cough response was counted from the dose response curves, airflow traces and cough sound analysis. In conscious animals, nasal allylisothiocyanate induced reproducible, dose dependent nasal symptoms and a significant drop in respiratory rate. Cough induced by CA was suppressed after nasal allylisothiocyanate (p<0.05), and this effect was prevented by AP-18 (1mM). In anaesthetized animals, nasal allylisothiocyanate induced a significant drop in respiratory rate. Cough induced subsequently by CA was suppressed when compared to baseline and vehicle responses (p<0.05). The reasons for the suppression of CA induced cough by TRPA1 agonist applied to the nose are not clear and remain to be elucidated.