Transient Receptor Potential A1 Activation Research Articles

Identification of a new class of non-electrophilic TRPA1 agonists by a structure-based virtual screening approach

Recent work has gradually been clarifying the binding site of non-electrophilic agonists on the transient receptor potential A1 (TRPA1). This study searched for non-electrophilic TRPA1 agonists by means of in silico drug discovery techniques based on three-dimensional (3-D) protein structure. First, agonist-bound pocket structures were explored using an advanced molecular dynamics simulation starting from the cryo-electron microscopic structure of TRPA1, and several pocket structures suitable for virtual screening were extracted by structure evaluation using known non-electrophilic TRPA1 agonists. Next, 49 compounds were selected as new non-electrophilic agonist candidates from a library of natural products comprising 10,555 compounds by molecular docking toward these pocket structures. Measurement of the TRPA1 agonist activity of these compounds showed notable TRPA1 activation with three compounds (decanol, 2-ethyl-1-hexanol, phenethyl butanoate). Decanol and 2-ethyl-1-hexanol, which are categorized as fatty alcohols, in particular have a novel chemical scaffold for TRPA1 activation. The results of this study are expected to be of considerable use in understanding the molecular mechanism of TRPA1 recognition by non-electrophilic agonists.

Catechol estrogens stimulate insulin secretion in pancreatic β-cells via activation of the transient receptor potential A1 (TRPA1) channel

Estrogen hormones play an important role in controlling glucose homeostasis and pancreatic β-cell function. Despite the significance of estrogen hormones for regulation of glucose metabolism, little is known about the roles of endogenous estrogen metabolites in modulating pancreatic β-cell function. In this study, we evaluated the effects of major natural estrogen metabolites, catechol estrogens, on insulin secretion in pancreatic β-cells. We show that catechol estrogens, hydroxylated at positions C2 and C4 of the steroid A ring, rapidly potentiated glucose-induced insulin secretion via a nongenomic mechanism. 2-Hydroxyestrone, the most abundant endogenous estrogen metabolite, was more efficacious in stimulating insulin secretion than any other tested catechol estrogens. In insulin-secreting cells, catechol estrogens produced rapid activation of calcium influx and elevation in cytosolic free calcium. Catechol estrogens also generated sustained elevations in cytosolic free calcium and evoked inward ion current in HEK293 cells expressing the transient receptor potential A1 (TRPA1) cation channel. Calcium influx and insulin secretion stimulated by estrogen metabolites were dependent on the TRPA1 activity and inhibited with the channel-specific pharmacological antagonists or the siRNA. Our results suggest the role of estrogen metabolism in a direct regulation of TRPA1 activity with potential implications for metabolic diseases.

Expression of proteinase-activated receptor-2 and transient receptor potential A1 in vagal afferent nerve of rat after lung schemia-reperfusion injury.

Lung ischemia-reperfusion injury (LIRI) is a common and severe clinical complication. As the injury occurs, the pulmonary afferent nerves play an important role in regulating respiratory functions under pathophysiological conditions. The purpose of this study was to examine expression of proteinaseactivated receptor-2 (PAR2) and transient receptor potential A1 (TRPA1) in pulmonary vagal afferent nerves of LIRI and further to determine molecular mediators linking activation of PAR2 and TRPA1. A rat model of LIRI was used. Enzyme-linked immunosorbent assay (ELISA) and Western blot analysis were employed to examine pro-inflammatory cytokines (PICs, i.e., IL-1β, IL-6 and TNF-α), and the protein levels of PIC receptors, PAR2, TRPA1, and intracellular signals. In the results, IL-1β, IL-6 and TNF-α along with their receptors were amplified in afferent nerves of LIRI rats as compared with control rats. Sensory PAR2 and TRPA1 were also upregulated by LIRI. Blocking PAR2 by infusion of FSLLRY-NH2 attenuated upregulation of TRPA1 via intracellular signals, namely p38-MAPK and JNK. Moreover, blocking individual PIC receptor attenuated PAR2 and TRPA1 in pulmonary vagal afferent nerves. Our data showed specific signaling pathways leading LIRI to activation of PIC signal and activation of PAR2 and TRPA1 in pulmonary vagal afferent nerves via intracellular mediators. Targeting one or more of these signaling molecules may present opportunities to improve the abnormalities in vagal afferent nerve-mediated respiratory functions observed as LIRI occurs.

Molecular basis of TRPA1 regulation in nociceptive neurons. A review.

Transient receptor potential A1 (TRPA1) is an excitatory ion channel that functions as a cellular sensor, detecting a wide range of proalgesic agents such as environmental irritants and endogenous products of inflammation and oxidative stress. Topical application of TRPA1 agonists produces an acute nociceptive response through peripheral release of neuropeptides, purines and other transmitters from activated sensory nerve endings. This, in turn, further regulates TRPA1 activity downstream of G-protein and phospholipase C-coupled signaling cascades. Despite the important physiological relevance of such regulation leading to nociceptor sensitization and consequent pain hypersensitivity, the specific domains through which TRPA1 undergoes post-translational modifications that affect its activation properties are yet to be determined at a molecular level. This review aims at providing an account of our current knowledge on molecular basis of regulation by neuronal inflammatory signaling pathways that converge on the TRPA1 channel protein and through modification of its specific residues influence the extent to which this channel may contribute to pain.

Intracellular TRPA1 mediates Ca2+ release from lysosomes in dorsal root ganglion neurons.

Transient receptor potential A1 (TRPA1) is a nonselective cation channel implicated in thermosensation and inflammatory pain. In this study, we show that TRPA1 (activated by allyl isothiocyanate, acrolein, and 4-hydroxynonenal) elevates the intracellular Ca2+ concentration ([Ca2+]i) in dorsal root ganglion (DRG) neurons in the presence and absence of extracellular Ca2+ Pharmacological and immunocytochemical analyses revealed the presence of TRPA1 channels both on the plasma membrane and in endolysosomes. Confocal line-scan imaging demonstrated Ca2+ signals elicited from individual endolysosomes ("lysosome Ca2+ sparks") by TRPA1 activation. In physiological solutions, the TRPA1-mediated endolysosomal Ca2+ release contributed to ∼40% of the overall [Ca2+]i rise and directly triggered vesicle exocytosis and calcitonin gene-related peptide release, which greatly enhanced the excitability of DRG neurons. Thus, in addition to working via Ca2+ influx, TRPA1 channels trigger vesicle release in sensory neurons by releasing Ca2+ from lysosome-like organelles.

Blocking PAR2 Alleviates Bladder Pain and Hyperactivity via TRPA1 Signal.

Bladder disorders associated with interstitial cystitis are frequently characterized by increased contractility and pain. The goals of this study were to examine 1) the effects of blocking proteinase-activated receptor-2 (PAR2) on the exaggerated bladder activity and pain evoked by cystitis and 2) the underlying mechanisms responsible for the role of PAR2 in regulating cystic sensory activity. The protein expression of PAR2 was amplified in rats with cystitis by inducing it with systemic administration of cyclophosphamide (CYP) as compared with control rats. Blocking PAR2 by intrathecal infusion of PAR2 antagonist FSLLRY-NH2 attenuated bladder hyperactivity and pain. In addition, blocking PAR2 attenuated the transient receptor potential A1 (TRPA1) signal pathway, whereas inhibition of the TRPA1 decreased bladder hyperactivity and pain. The data revealed specific signaling pathways leading to CYP-induced bladder hyperactivity and pain, including the activation of PAR2 and TRPA1. Inhibition of these pathways alleviates cystic pain. Targeting one or more of these signaling molecules may present new opportunities for treatment and management of overactive bladder and pain often observed in cystitis.

Hydrogen Sulfide Plays a Key Role in the Inhibitory Neurotransmission to the Pig Intravesical Ureter

According to previous observations nitric oxide (NO), as well as an unknown nature mediator are involved in the inhibitory neurotransmission to the intravesical ureter. This study investigates the hydrogen sulfide (H2S) role in the neurogenic relaxation of the pig intravesical ureter. We have performed western blot and immunohistochemistry to study the expression of the H2S synthesis enzymes cystathionine γ-lyase (CSE) and cystathionine β-synthase (CBS), measurement of enzymatic production of H2S and myographic studies for isometric force recording. Immunohistochemical assays showed a high CSE expression in the intravesical ureter muscular layer, as well as a strong CSE-immunoreactivity within nerve fibres distributed along smooth muscle bundles. CBS expression, however, was not consistently observed. On ureteral strips precontracted with thromboxane A2 analogue U46619, electrical field stimulation (EFS) and the H2S donor P-(4-methoxyphenyl)-P-4-morpholinylphosphinodithioic acid (GYY4137) evoked frequency- and concentration-dependent relaxations. CSE inhibition with DL-propargylglycine (PPG) reduced EFS-elicited responses and a combined blockade of both CSE and NO synthase (NOS) with, respectively, PPG and NG-nitro-L-arginine (L-NOARG), greatly reduced such relaxations. Endogenous H2S production rate was reduced by PPG, rescued by addition of GYY4137 and was not changed by L-NOARG. EFS and GYY4137 relaxations were also reduced by capsaicin-sensitive primary afferents (CSPA) desensitization with capsaicin and blockade of ATP-dependent K+ (KATP) channels, transient receptor potential A1 (TRPA1), transient receptor potential vanilloid 1 (TRPV1), vasoactive intestinal peptide/pituitary adenylyl cyclase-activating polypeptide (VIP/PACAP) and calcitonin gene-related peptide (CGRP) receptors with glibenclamide, HC030031, AMG9810, PACAP6–38 and CGRP8–37, respectively. These results suggest that H2S, synthesized by CSE, is involved in the inhibitory neurotransmission to the pig intravesical ureter, through an NO-independent pathway, producing smooth muscle relaxation via KATP channel activation. H2S also promotes the release of inhibitory neuropeptides, as PACAP 38 and/or CGRP from CSPA through TRPA1, TRPV1 and related ion channel activation.

Annexin A2 regulates TRPA1-dependent nociception.

The transient receptor potential A1 (TRPA1) channel is essential for vertebrate pain. Even though TRPA1 activation by ligands has been studied extensively, the molecular machinery regulating TRPA1 is only poorly understood. Using an unbiased proteomics-based approach we uncovered the physical association of Annexin A2 (AnxA2) with native TRPA1 in mouse sensory neurons. AnxA2 is enriched in a subpopulation of sensory neurons and coexpressed with TRPA1. Furthermore, we observe an increase of TRPA1 membrane levels in cultured sensory neurons from AnxA2-deficient mice. This is reflected by our calcium imaging experiments revealing higher responsiveness upon TRPA1 activation in AnxA2-deficient neurons. In vivo these findings are associated with enhanced nocifensive behaviors specifically in TRPA1-dependent paradigms of acute and inflammatory pain, while heat and mechanical sensitivity as well as TRPV1-mediated pain are preserved in AnxA2-deficient mice. Our results support a model whereby AnxA2 limits the availability of TRPA1 channels to regulate nociceptive signaling in vertebrates.

Bimodal voltage dependence of TRPA1: mutations of a key pore helix residue reveal strong intrinsic voltage-dependent inactivation

Transient receptor potential A1 (TRPA1) is implicated in somatosensory processing and pathological pain sensation. Although not strictly voltage-gated, ionic currents of TRPA1 typically rectify outwardly, indicating channel activation at depolarized membrane potentials. However, some reports also showed TRPA1 inactivation at high positive potentials, implicating voltage-dependent inactivation. Here we report a conserved leucine residue, L906, in the putative pore helix, which strongly impacts the voltage dependency of TRPA1. Mutation of the leucine to cysteine (L906C) converted the channel from outward to inward rectification independent of divalent cations and irrespective to stimulation by allyl isothiocyanate. The mutant, but not the wild-type channel, displayed exclusively voltage-dependent inactivation at positive potentials. The L906C mutation also exhibited reduced sensitivity to inhibition by TRPA1 blockers, HC030031 and ruthenium red. Further mutagenesis of the leucine to all natural amino acids individually revealed that most substitutions at L906 (15/19) resulted in inward rectification, with exceptions of three amino acids that dramatically reduced channel activity and one, methionine, which mimicked the wild-type channel. Our data are plausibly explained by a bimodal gating model involving both voltage-dependent activation and inactivation of TRPA1. We propose that the key pore helix residue, L906, plays an essential role in responding to the voltage-dependent gating.

Roles of TRPA1 in Pain Pathophysiology and Implications for the Development of a New Class of Analgesic Drugs

The Transient Receptor Potential A1 (TRPA1) ion channel has evolved in animals to respond to signals from a variety of sensory stimuli. Many structural determinants of its multimodal activation have been identified to date. TRPA1 activities include responses to exogenous chemical irritants, responses to endogenous inflammatory mediators, zinc, voltage, temperature or stretch and subtle yet critical modulation by calcium ions. TRPA1 has emerged as an important target for several types of pain and inflammatory conditions because of its limited expression profile and its demonstrated roles in mediating different types of pain and sensitization of peripheral sensory afferents. Despite observed species differences in channel pharmacology, recent genetic evidence in human brings some hope that preclinical efficacy in disease models will translate to patient condition. During the past decade, various groups have investigated the development of a new class of analgesic drugs or anti-tussive agents aimed at blocking TRPA1 activity in primary sensory afferents. Several companies are advancing toward clinical proof of concept studies. This review aims to summarize key advances in the understanding of TRPA1 with regard to its roles and implications for patient conditions.

UV light phototransduction activates transient receptor potential A1 ion channels in human melanocytes

Human skin is constantly exposed to solar ultraviolet radiation (UVR), the most prevalent environmental carcinogen. Humans have the unique ability among mammals to respond to UVR by increasing their skin pigmentation, a protective process driven by melanin synthesis in epidermal melanocytes. The molecular mechanisms used by melanocytes to detect and respond to long-wavelength UVR (UVA) are not well understood. We recently identified a UVA phototransduction pathway in melanocytes that is mediated by G protein-coupled receptors and leads to rapid calcium mobilization. Here we report that in human epidermal melanocytes physiological doses of UVR activate a retinal-dependent current mediated by transient receptor potential A1 (TRPA1) ion channels. The TRPA1 photocurrent is UVA-specific and requires G protein and phospholipase C signaling, thus contributing to UVA-induced calcium responses to mediate downstream cellular effects and providing evidence for TRPA1 function in mammalian phototransduction. Remarkably, TRPA1 activation is required for the UVR-induced and retinal-dependent early increase in cellular melanin. Our results show that TRPA1 is essential for a unique extraocular phototransduction pathway in human melanocytes that is activated by physiological doses of UVR and results in early melanin synthesis.

Ultraviolet Light Phototransduction Activates TRPA1 to Mediate Melanin Synthesis in Human Melanocytes

Human skin is constantly exposed to solar ultraviolet radiation (UVR), the most prevalent environmental carcinogen. Humans have the unique ability among mammals to respond to UVR by increasing their skin pigmentation, a protective process driven by melanin synthesis in epidermal melanocytes. However, the mechanism used by melanocytes to detect and respond to UVR is not well understood. Here we report that transient receptor potential A1 (TRPA1) ion channels expressed in human melanocytes are activated by UVR and mediate early melanin synthesis. We show that in human epidermal melanocytes (HEMs) physiological doses of UVR activate a retinal-dependent current mediated by TRPA1. The UVR photocurrent density was reduced by TRPA1 antagonists and abolished in HEMs expressing TRPA1-targeted miRNA. The TRPA1 photocurrent is UVA specific and requires G protein signaling, providing the first evidence for TRPA1 function in mammalian phototransduction. In HEMs, TRPA1 activation contributes to UVR-induced calcium responses to mediate downstream cellular effects. Remarkably, the UVR-induced and retinal-dependent early increase in cellular melanin content was significantly reduced in HEMs treated with TRPA1 antagonists and abolished in HEMs expressing TRPA1-targeted miRNA, suggesting that TRPA1 is required for early melanin synthesis. Our results show that TRPA1 is essential for a novel extra-ocular phototransduction pathway in human melanocytes that is activated by physiological doses of UVR and results in early melanin synthesis.

TRPA1‐activated anion secretion is linked with PGE2 receptor EP4 in human and rat colon

The transient receptor potential A1 (TRPA1) channel is considered a chemosensor in several sensory tissues. In the present study, we investigated the secretory effect of potential TRPA1 agonist allyl isothiocyanate (AITC) in rat and human colons using the Ussing flux chamber. The mucosal application of AITC (10−6–10−3 M) induced Cl− and HCO3 − secretion in a concentration‐dependent manner, while the serosal application induced weaker effect. AITC‐evoked anion secretion was attenuated by the pretreatment with cyclooxygenase inhibitor piroxicam and prostaglandin (PG) E2, but the secretion was not affected by tetrodotoxin, atropine, or extracellular Ca2+ depletion. These results indicated that TRPA1 activation induces anion secretion through PG synthesis independent of neural pathway in the colon. Further analysis also indicated that AITC‐evoked anion secretion is mainly mediated by EP4. In addition, RT‐PCR using isolated colonic crypts, in situ hybridization and immunohistochemistry supported the TRPA1 expression in epithelial cells. Therefore, we conclude that the activation of TRPA1 activity on the colonic epithelial cells is likely involved in host defense mechanism through rapid anion secretion.

Warmth Suppresses and Desensitizes Damage-Sensing Ion Channel TRPA1

BackgroundAcute or chronic tissue damage induces an inflammatory response accompanied by pain and alterations in local tissue temperature. Recent studies revealed that the transient receptor potential A1 (TRPA1) channel is activated by a wide variety of substances that are released following tissue damage to evoke nociception and neurogenic inflammation. Although the effects of a noxious range of cold temperatures on TRPA1 have been rigorously studied, it is not known how agonist-induced activation of TRPA1 is regulated by temperature over an innocuous range centred on the normal skin surface temperature. This study investigated the effect of temperature on agonist-induced currents in human embryonic kidney (HEK) 293 cells transfected with rat or human TRPA1 and in rat sensory neurons.ResultsAgonist-induced TRPA1 currents in HEK293 cells were strongly suppressed by warm temperatures, and almost abolished at 39°C. Such inhibition occurred when TRPA1 was activated by either electrophilic or non-electrophilic agonists. Warming not only decreased the apparent affinity of TRPA1 for mustard oil (MO), but also greatly enhanced the desensitization and tachyphylaxis of TRPA1. Warming also attenuated MO-induced ionic currents in sensory neurons. These results suggest that the extent of agonist-induced activity of TRPA1 may depend on surrounding tissue temperature, and local hyperthermia during acute inflammation could be an endogenous negative regulatory mechanism to attenuate persistent pain at the site of injury.ConclusionThese results indicate that warmth suppresses and desensitizes damage-sensing ion channel TRPA1. Such warmth-induced suppression of TRPA1 may also explain, at least in part, the mechanistic basis of heat therapy that has been widely used as a supplemental anti-nociceptive approach.

Activation of TRPA1 by luminal stimuli induces EP4-mediated anion secretion in human and rat colon

In gastrointestinal (GI) physiology, anion and fluid secretion is an important function for host defense and is induced by changes in the luminal environment. The transient receptor potential A1 (TRPA1) channel is considered to be a chemosensor in several sensory tissues. Although the function of TRPA1 has been studied in GI motility, its contribution to the transepithelial ion transport system has rarely been discussed. In the present study, we investigated the secretory effect of the potential TRPA1 agonist allyl isothiocyanate (AITC) in rat and human colon using an Ussing chamber. The mucosal application of AITC (10(-6)-10(-3) M) induced Cl(-) and HCO(3)(-) secretion in a concentration-dependent manner, whereas the serosal application induced a significantly weaker effect. AITC-evoked anion secretion was attenuated by tissue pretreatment with piroxicam and prostaglandin (PG) E(2); however, this secretion was not affected by TTX, atropine, or extracellular Ca(2+) depletion. These experiments indicate that TRPA1 activation induces anion secretion through PG synthesis, independent of neural pathways in the colon. Further analysis also indicates that AITC-evoked anion secretion is mediated mainly by the EP(4) receptor subtype. The magnitude of the secretory response exhibited segmental heterogeneity in rat colon. Real-time PCR analysis showed the segmental difference was corresponding to the differential expression of EP(4) receptor and cyclooxygenase-1 and -2. In addition, RT-PCR, in situ hybridization, and immunohistochemical studies showed TRPA1 expression in the colonic epithelia. Therefore, we conclude that the activation of TRPA1 in colonic epithelial cells is likely involved in the host defense mechanism through rapid anion secretion.

Sustained TRPA1 activation in vivo

Transient receptor potential A1 (TRPA1) is a calcium permeable non-selective cation channel that is selectively localized to peptidergic C-fibres in the pain pathway. TRPA1 is highly conserved across the animal kingdom and it is able to detect a wide range of potentially toxic environmental chemicals. An unusual mechanism of TRPA1 activation was recently elucidated in which reactive agonists bind covalently to cysteines and lysine in the intracellular N-terminus. Despite a covalent activation mechanism, only transient TRPA1 activation is seen in the maintained presence of reactive agonists in whole-cell patch clamp experiments. I suggest that previous patch clamp studies are performed under conditions that do not fully mimic all aspects of TRPA1 activation. Here, I argue that compelling evidence exists for sustained TRPA1 activation in several chronic (neuropathic) pain-related pathophysiological conditions in vivo. I discuss briefly putative mechanisms that are likely to contribute to and maintain sustained TRPA1 agonist levels through increased production and/or decreased metabolism and inactivation. Chronic pain can be understood as a false alarm evoked by sustained and increased levels of endogenous TRPA1 agonists in various pathophysiological conditions.

08 Increased risk of cardiac arrhythmia in rats exposed to diesel exhaust air pollution is mediated by the nociceptive transient receptor potential A1 (TRPA1)

Epidemiological studies demonstrate a significant association between arrhythmias and air pollution exposure. Sensitivity to aconitine-induced arrhythmia has been employed to examine the factors that increase the risk of such dysfunction....

Overexpression of Human Transient Receptor Potential M5 Upregulates Endogenous Human Transient Receptor Potential A1 in a Stable HEK Cell Line

Transient receptor potential M5 (TRPM5), a monovalent cation channel, is primarily activated by increases in intracellular calcium. However, we found unexpectedly that allyl isothiocyanate (AITC) and structural analogs triggered a membrane potential and calcium dye responses in TRPM5-HEK cells (AITC EC₅₀ = 9.0 ± 2.4 μM, n = 5). Although AITC and its analogs were more potent on transient receptor potential A1 (TRPA1)-HEK cells (AITC EC₅₀ = 0.23 ± 0.03 μM, n = 4), the rank order potency of these compounds were similar for TRPM5- and TRPA1-HEK cells. No response to these compounds was seen in parental HEK cells, TRPM5-CHO cells, and TRPM4b-, TRPM8-, or TRPV1-transfected HEK cells. An AITC-evoked current in TRPM5-HEK cells was confirmed in whole-cell voltage clamp recording. AITC elicited an intracellular calcium increase that was not dependent on phorpholipase C(β)₂ (PLC(β)₂) activation but was dependent on extracellular calcium concentration. TRPA1 mRNA was upregulated fourfold in TRPM5-HEK cells compared with parental cells. In contrast, TRPA1 was not upregulated in HEK cells transfected in a similar manner with TRPV1 or TRPM8 genes. The AITC response was blocked by a TRPA1 inhibitor and reduced by a TRPM5 inhibitor and by targeted TRPA1 siRNA. These results suggest that TRPM5 may play a role in upregulating endogenous expression of TRPA1, that TRPA1 activation may be an additional trigger for co-expressed calcium-dependent ion channels such as TRPM5, and that TRPM5 may amplify responses to TRPA1 ligands.

Transient receptor potential V1 regulates activation and modulation of transient receptor potential A1 by Ca 2+

The transient receptor potential A1 (TRPA1) channel contributes to nociceptive signaling in certain pain models. It has been suggested that Ca 2+, which activates and modulates TRPA1, could play a critical regulatory role in this process. Since TRPA1 and transient receptor potential V1 (TRPV1) channels are co-expressed and interact in neurons, we investigated whether activation and modulation of TRPA1 by Ca 2+ is regulated by TRPV1. Cell-attached recordings showed that TRPA1 is activated by extracellular Ca 2+ ([Ca 2+] e) in concentration-response fashion. This activation, especially by 2 mM [Ca 2+] e was substantially suppressed by co-expression with TRPV1. Inside-out recordings demonstrated that intracellular Ca 2+ ([Ca 2+] i)-triggered activation of TRPA1 was attenuated by the presence of TRPV1 only at 2 mM [Ca 2+] e, but not in Ca 2+-free conditions. Further, depletion of internal Ca 2+ stores by thapsigargin generated TRPA1-mediated currents, which is affected by TRPV1 in both Chinese hamster ovary cells and sensory neurons. Since mustard oil current (I MO) is modulated by [Ca 2+] e, we next examined whether alterations in the Ca 2+-permeability of TRPV1 by mutating Y671 effect I MO properties. First it was demonstrated that the mutations in TRPV1 did not affect association of the TRPA1 and TRPV1 channels. However, these TRPV1 mutations, particularly Y671K, altered the following characteristics of TRPA1: magnitude of I MO in presence and absence of [Ca 2+] e; the influence of [Ca 2+] e on the voltage-dependency of I MO, and open probability of single-channel I MO. In summary, activation of TRPA1 by [Ca 2+] e and [Ca 2+] i is controlled by the TRPV1 channel, and characteristics of I MO depend on Ca 2+ permeability of the TRPV1 channel.

Heavy metals zinc, cadmium and copper stimulate pulmonary C‐fibers via direct activation of TRPA1

Airway exposure to zinc dust and zinc‐containing ambient particulates can cause symptoms of airway irritation and inflammation. However, the underlying molecular and cellular mechanisms are largely unknown. Transient receptor potential A1 (TRPA1) is selectively expressed in a subpopulation of pulmonary C‐fiber afferents, and has been considered as a major irritant sensor in the lung and airways. Using whole‐cell patch‐clamp recording and Ca2+ imaging, we have demonstrated that application of ZnCl2 concentration‐dependently evoked inward current and Ca2+ transient in isolated vagal pulmonary sensory neurons; both responses were almost completely prevented after pretreatment with AP18, a selective TRPA1 antagonist. In anesthetized spontaneously breathing animals, intratracheal instillation of ZnCl2 (2 mM, 25 μl) evoked pronounced respiratory sensory irritation in wild‐type mice, and the effect was essentially absent in TRPA1‐deficient mice. In addition, our study showed that two other heavy metals cadmium and copper also stimulated pulmonary sensory neurons via direct activation of TRPA1. In summary, our results suggest that activation of TRPA1 in pulmonary C‐fiber sensory nerves may contribute to the respiratory toxicity induced by airway exposure to these three heavy metals. Funded in part by grants from NIH AI076714 and AHA 0835320N.