Identification of transmembrane domain 5 as a critical molecular determinant of menthol sensitivity in mammalian TRPA1 channels

TRPA1 channels: molecular sentinels of cellular stress and tissue damage

TRPA1 is a non-selective cation channel expressed in mammalian peripheral pain receptors, with a major role in chemonociception. TRPA1 has also been implicated in noxious cold and mechanical pain sensation. TRPA1 has an ancient origin and plays important functions in lower organisms, including thermotaxis, mechanotransduction and modulation of lifespan. Here we highlight the role of TRPA1 as a multipurpose sensor of harmful signals, including toxic bacterial products and UV light, and as a sensor of stress and tissue damage. Sensing roles span beyond the peripheral nervous system to include major barrier tissues: gut, skin and lung. Tissue injury, environmental irritants and microbial pathogens are danger signals that can threaten the health of organisms. These signals lead to the coordinated activation of the nociceptive and the innate immune system to provide a homeostatic response trying to re-establish physiological conditions including tissue repair. Activation of TRPA1 participates in protective neuroimmune interactions at multiple levels, sensing ROS and bacterial products and triggering the release of neuropeptides. However, an exaggerated response to danger signals is maladaptive and can lead to the development of chronic inflammatory conditions.

An environmental pollutant, 9,10-phenanthrenequinone, activates human TRPA1 via critical cysteines 621 and 665

Transient receptor potential ankyrin 1 (TRPA1) is activated by noxious cold, mechanical stimulation, and irritant chemicals. In our recent study, 9, 10‐phenanthrenequinone (9,10‐PQ) is the most potent irritant for activation of NRF2 among 1395 cigarette smoke components and it may be, therefore, important to find its additional targets. Here, we show that 9,10‐PQ functions as an activator of TRPA1 in human embryonic kidney (HEK) cells expressing human wild‐type TRPA1 (HEK‐wTRPA1) and human alveolar A549 (A549) cells. Application of 9,10‐PQ at 0.1–10 μmol/L induced a concentration‐dependent Ca²⁺ response as well as inward currents at −50 mV in HEK‐wTRPA1 cells. The current response was blocked by TRPA1 antagonists, HC‐030031 (HC) and A‐967079. To test whether 9,10‐PQ affects the cysteine residues of TRPA1, we expressed mutant TRPA1 channels in HEK cells (HEK‐muTRPA1) in which six different cysteine residues were replaced with serine. Among them, a mutation of cysteine 621 (C621S) abolished the 9,10‐PQ‐induced Ca²⁺ and current responses. The channel activity induced by 9,10‐PQ was also abolished in excised inside‐out patches isolated from HEK‐muTRPA1 cells with the C621S substitution. Although a mutation of cysteine 665 (C665S) reduced the 9,10‐PQ‐induced response, channel sensitization by pretreatment with Cu²⁺ plus 1,10‐phenanthroline and by internal dialysis of 3 μmol/L Ca²⁺ restored the response. However, a double mutant with C621S and C665S substitutions had little response to 9,10‐PQ, even when sensitized by Ca²⁺ dialysis. In A549 cells, 9,10‐PQ induced an HC‐sensitive Ca²⁺ response. Our findings demonstrate that 9,10‐PQ activation of human TRA1 is dependent on cysteine residues 621 and 665.

Hypotonicity-induced cell swelling activates TRPA1

Hypotonic solutions can cause painful sensations in nasal and ocular mucosa through molecular mechanisms that are not entirely understood. We clarified the ability of human TRPA1 (hTRPA1) to respond to physical stimulus, and evaluated the response of hTRPA1 to cell swelling under hypotonic conditions. Using a Ca²⁺-imaging method, we found that modulation of AITC-induced hTRPA1 activity occurred under hypotonic conditions. Moreover, cell swelling in hypotonic conditions evoked single-channel activation of hTRPA1 in a cell-attached mode when the patch pipette was attached after cell swelling under hypotonic conditions, but not before swelling. Single-channel currents activated by cell swelling were also inhibited by a known hTRPA1 blocker. Since pre-application of thapsigargin or pretreatment with the calcium chelator BAPTA did not affect the single-channel activation induced by cell swelling, changes in intracellular calcium concentrations are likely not related to hTRPA1 activation induced by physical stimuli.

Ethyl Vanillin Activates TRPA1

The nonselective cation channel transient receptor potential ankryn subtype family 1 (TRPA1) is expressed in neurons of dorsal root ganglia and trigeminal ganglia and also in vagal afferent neurons that innervate the lungs and gastrointestinal tract. Many TRPA1 agonists are reactive electrophilic compounds that form covalent adducts with TRPA1. Allyl isothiocyanate (AITC), the common agonist used to identify TRPA1, contains an electrophilic group that covalently binds with cysteine residues of TRPA1 and confers a structural change on the channel. There is scientific motivation to identify additional compounds that can activate TRPA1 with different mechanisms of channel gating. We provide evidence that ethyl vanillin (EVA) is a TRPA1 agonist. Using fluorescent calcium imaging and whole-cell patch-clamp electrophysiology on dissociated rat vagal afferent neurons and TRPA1-transfected COS-7 cells, we discovered that EVA activates cells also activated by AITC. Both agonists display similar current profiles and conductances. Pretreatment with A967079, a selective TRPA1 antagonist, blocks the EVA response as well as the AITC response. Furthermore, EVA does not activate vagal afferent neurons from TRPA1 knockout mice, showing selectivity for TRPA1 in this tissue. Interestingly, EVA appears to be pharmacologically different from AITC as a TRPA1 agonist. When AITC is applied before EVA, the EVA response is occluded. However, they both require intracellular oxidation to activate TRPA1. These findings suggest that EVA activates TRPA1 but via a distinct mechanism that may provide greater ease for study in native systems compared with AITC and may shed light on differential modes of TRPA1 gating by ligand types.

Discovery of a Series of Indazole TRPA1 Antagonists

A series of TRPA1 antagonists is described which has as its core structure an indazole moiety. The physical properties and in vitro DMPK profiles are discussed. Good in vivo exposure was obtained with several analogs, allowing efficacy to be assessed in rodent models of inflammatory pain. Two compounds showed significant activity in these models when administered either systemically or topically. Protein chimeras were constructed to indicate compounds from the series bound in the S5 region of the channel, and a computational docking model was used to propose a binding mode for example compounds.

Lipids as central modulators of sensory TRP channels

The transient receptor potential (TRP) ion channel family is involved in a diversity of physiological processes including sensory and homeostatic functions, as well as muscle contraction and vasomotor control. Their dysfunction contributes to the etiology of several diseases, being validated as therapeutic targets. These ion channels may be activated by physical or chemical stimuli and their function is highly influenced by signaling molecules activated by extracellular signals. Notably, as integral membrane proteins, lipid molecules also modulate their membrane location and function either by direct interaction with the channel structure or by modulating the physico-chemical properties of the cellular membrane. This lipid-based modulatory effect is being considered an alternative and promising approach to regulate TRP channel dysfunction in diseases. Here, we review the current progress in this exciting field highlighting a complex channel regulation by a large diversity of lipid molecules and suggesting some diseases that may benefit from a membrane lipid therapy. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

TRPA1 expression levels and excitability brake by KV channels influence cold sensitivity of TRPA1-expressing neurons

The molecular sensor of innocuous (painless) cold sensation is well-established to be transient receptor potential cation channel, subfamily M, member 8 (TRPM8). However, the role of transient receptor potential cation channel, subfamily A, member 1 (TRPA1) in noxious (painful) cold sensation has been controversial. We find that TRPA1 channels contribute to the noxious cold sensitivity of mouse somatosensory neurons, independent of TRPM8 channels, and that TRPA1-expressing neurons are largely non-overlapping with TRPM8-expressing neurons in mouse dorsal-root ganglia (DRG). However, relatively few TRPA1-expressing neurons (e.g., responsive to allyl isothiocyanate or AITC, a selective TRPA1 agonist) respond overtly to cold temperature in vitro, unlike TRPM8-expressing neurons, which almost all respond to cold. Using somatosensory neurons from TRPM8-/- mice and subtype-selective blockers of TRPM8 and TRPA1 channels, we demonstrate that responses to cold temperatures from TRPA1-expressing neurons are mediated by TRPA1 channels. We also identify two factors that affect the cold-sensitivity of TRPA1-expressing neurons: (1) cold-sensitive AITC-sensitive neurons express relatively more TRPA1 transcripts than cold-insensitive AITC-sensitive neurons and (2) voltage-gated potassium (K_V) channels attenuate the cold-sensitivity of some TRPA1-expressing neurons. The combination of these two factors, combined with the relatively weak agonist-like activity of cold temperature on TRPA1 channels, partially explains why few TRPA1-expressing neurons respond to cold. Blocking K_V channels also reveals another subclass of noxious cold-sensitive DRG neurons that do not express TRPM8 or TRPA1 channels. Altogether, the results of this study provide novel insights into the cold-sensitivity of different subclasses of somatosensory neurons.

Evolutionary tuning of TRPA1 and TRPV1 thermal and chemical sensitivity in vertebrates

Thermal perception is an essential sensory system for survival since temperature fluctuations affect various biologic processes. Therefore, evolutionary changes in thermosensory systems may have played important roles in adaptation processes. Comparative analyses of sensory receptors among different species can provide us with important clues to understand the molecular basis for adaptation. Several ion channels belonging to the transient receptor potential (TRP) superfamily serve as thermal sensors in a wide variety of animal species. These TRP proteins are multimodal receptors that are activated by temperature as well as other sensory stimuli. Among them TRPV1 and TRPA1 are activated by noxious ranges of thermal stimuli and irritating chemicals, and are mainly expressed in nociceptive sensory neurons. Comparative analyses of TRPV1 and TRPA1 among various vertebrate species revealed evolutionary changes that likely contributed to diversification of sensory perception. Whereas heat-induced TRPV1 responses have been conserved across many vertebrates, TRPA1 varied among species. Mutagenesis experiments using these two channels from various species also helped characterize the molecular basis for their activation and inhibition. Meanwhile, recent detailed comparative analyses using closely related species showed shifts in TRPV1 and TRPA1 thermal sensitivity that allowed adaptation to different thermal environments. Changes in TRPV1 heat responses appear to arise from just a few amino acid differences among species. These observations suggest that evolutionary changes in peripheral sensors are likely driving force for shifting thermal perception in adaptation processes.

Identification of a putative binding site critical for general anesthetic activation of TRPA1

General anesthetics suppress CNS activity by modulating the function of membrane ion channels, in particular, by enhancing activity of GABA_A receptors. In contrast, several volatile (isoflurane, desflurane) and i.v. (propofol) general anesthetics excite peripheral sensory nerves to cause pain and irritation upon administration. These noxious anesthetics activate transient receptor potential ankyrin repeat 1 (TRPA1), a major nociceptive ion channel, but the underlying mechanisms and site of action are unknown. Here we exploit the observation that pungent anesthetics activate mammalian but not Drosophila TRPA1. Analysis of chimeric Drosophila and mouse TRPA1 channels reveal a critical role for the fifth transmembrane domain (S5) in sensing anesthetics. Interestingly, we show that anesthetics share with the antagonist A-967079 a potential binding pocket lined by residues in the S5, S6, and the first pore helix; isoflurane competitively disrupts A-967079 antagonism, and introducing these mammalian TRPA1 residues into dTRPA1 recapitulates anesthetic agonism. Furthermore, molecular modeling predicts that isoflurane and propofol bind to this pocket by forming H-bond and halogen-bond interactions with Ser-876, Met-915, and Met-956. Mutagenizing Met-915 or Met-956 selectively abolishes activation by isoflurane and propofol without affecting actions of A-967079 or the agonist, menthol. Thus, our combined experimental and computational results reveal the potential binding mode of noxious general anesthetics at TRPA1. These data may provide a structural basis for designing drugs to counter the noxious and vasorelaxant properties of general anesthetics and may prove useful in understanding effects of anesthetics on related ion channels.

4-isopropylcyclohexanol has potential analgesic effects through the inhibition of anoctamin 1, TRPV1 and TRPA1 channel activities

Interactions between calcium-activated chloride channel anoctamin 1 (ANO1) and transient receptor potential vanilloid 1 (TRPV1) enhance pain sensations in mice, suggesting that ANO1 inhibition could have analgesic effects. Here we show that menthol and the menthol analogue isopropylcyclohexane (iPr-CyH) inhibited ANO1 channels in mice. The iPr-CyH derivative 4-isopropylcyclohexanol (4-iPr-CyH-OH) inhibited mouse ANO1 currents more potently than iPr-CyH. Moreover, 4-iPr-CyH-OH inhibited the activities of TRPV1, TRP ankyrin 1 (TRPA1), TRP melastatin 8 (TRPM8) and TRPV4. Single-channel analysis revealed that 4-iPr-CyH-OH reduced TRPV1 and TRPA1 current open-times without affecting unitary amplitude or closed-time, suggesting that it affected gating rather than blocking the channel pore. The ability of 4-iPr-CyH-OH to inhibit action potential generation and reduce pain-related behaviors induced by capsaicin in mice suggests that 4-iPr-CyH-OH could have analgesic applications. Thus, 4-iPr-CyH-OH is a promising base chemical to develop novel analgesics that target ANO1 and TRP channels.

Recent Progress in the Discovery and Development of TRPA1 Modulators

TRPA1 is a well-validated therapeutic target in areas of high unmet medical need that include pain and respiratory disorders. The human genetic rationale for TRPA1 as a pain target is provided by a study describing a rare gain-of-function mutation in TRPA1, causing familial episodic pain syndrome. There is a growing interest in the TRPA1 field, with many pharmaceutical companies reporting the discovery of TRPA1 chemical matter; however, GRC 17536 remains to date the only TRPA1 antagonist to have completed Phase IIa studies. A key issue in the progression of TRPA1 programmes is the identification of high-quality orally bioavailable molecules. Most published TRPA1 ligands are commonly not suitable for clinical progression due to low lipophilic efficiency and/or poor absorption, distribution, metabolism, excretion and pharmaceutical properties. The recent TRPA1 cryogenic electron microscopy structure from the Cheng and Julius labs determined the structure of full-length human TRPA1 at up to 4Å resolution in the presence of TRPA1 ligands. This ground-breaking science paves the way to enable structure-based drug design within the TRPA1 field.

Structural basis of TRPA1 inhibition by HC-030031 utilizing species-specific differences

Pain is a harmful sensation that arises from noxious stimuli. Transient receptor potential ankyrin 1 (TRPA1) is one target for studying pain mechanisms. TRPA1 is activated by various stimuli such as noxious cold, pungent natural products and environmental irritants. Since TRPA1 is an attractive target for pain therapy, a few TRPA1 antagonists have been developed and some function as analgesic agents. The responses of TRPA1 to agonists and antagonists vary among species and these species differences have been utilized to identify the structural basis of activation and inhibition mechanisms. The TRPA1 antagonist HC-030031 (HC) failed to inhibit frog TRPA1 (fTRPA1) and zebrafish TRPA1 activity induced by cinnamaldehyde (CA), but did inhibit human TRPA1 (hTRPA1) in a heterologous expression system. Chimeric studies between fTRPA1 and hTRPA1, as well as analyses using point mutants, revealed that a single amino acid residue (N855 in hTRPA1) significantly contributes to the inhibitory action of HC. Moreover, the N855 residue and the C-terminus region exhibited synergistic effects on the inhibition by HC. Molecular dynamics simulation suggested that HC stably binds to hTRPA1-N855. These findings provide novel insights into the structure-function relationship of TRPA1 and could lead to the development of more effective analgesics targeted to TRPA1.

The N-terminal Ankyrin Repeat Domain Is Not Required for Electrophile and Heat Activation of the Purified Mosquito TRPA1 Receptor

Temperature sensors are crucial for animals to optimize living conditions. The temperature response of the ion channel transient receptor potential A1 (TRPA1) is intriguing; some orthologs have been reported to be activated by cold and others by heat, but the molecular mechanisms responsible for its activation remain elusive. Single-channel electrophysiological recordings of heterologously expressed and purified Anopheles gambiae TRPA1 (AgTRPA1), with and without the N-terminal ankyrin repeat domain, demonstrate that both proteins are functional because they responded to the electrophilic compounds allyl isothiocyanate and cinnamaldehyde as well as heat. The proteins' similar intrinsic fluorescence properties and corresponding quenching when activated by allyl isothiocyanate or heat suggest lipid bilayer-independent conformational changes outside the N-terminal domain. The results show that AgTRPA1 is an inherent thermo- and chemoreceptor, and analogous to what has been reported for the human TRPA1 ortholog, the N-terminal domain may tune the response but is not required for the activation by these stimuli.

The Outer Pore and Selectivity Filter of TRPA1

TRPA1 (transient-receptor-potential-related ion channel with ankyrin domains) is a direct receptor or indirect effector for a wide variety of nociceptive signals, and thus is a compelling target for development of analgesic pharmaceuticals such as channel blockers. Recently, the structure of TRPA1 was reported, providing insights into channel assembly and pore architecture. Here we report whole-cell and single-channel current recordings of wild-type human TRPA1 as well as TRPA1 bearing point mutations of key charged residues in the outer pore. These measurements demonstrate that the glutamate at position 920 plays an important role in collecting cations into the mouth of the pore, by changing the effective surface potential by ~16 mV, while acidic residues further out have little effect on permeation. Electrophysiology experiments also confirm that the aspartate residue at position 915 represents a constriction site of the TRPA1 pore and is critical in controlling ion permeation.

TRPA1 Channels in Drosophila and Honey Bee Ectoparasitic Mites Share Heat Sensitivity and Temperature-Related Physiological Functions

The transient receptor potential cation channel, subfamily A, member 1 (TRPA1) is conserved between many arthropods, and in some has been shown to function as a chemosensor for noxious compounds. Activation of arthropod TRPA1 channels by temperature fluctuations has been tested in only a few insect species, and all of them were shown to be activated by heat. The recent identification of chemosensitive TRPA1 channels from two honey bee ectoparasitic mite species (VdTRPA1 and TmTRPA1) have provided an opportunity to study the temperature-dependent activation and the temperature-associated physiological functions of TRPA1 channels in non-insect arthropods. We found that both mite TRPA1 channels are heat sensitive and capable of rescuing the temperature-related behavioral defects of a Drosophila melanogaster trpA1 mutant. These results suggest that heat-sensitivity of TRPA1 could be conserved between many arthropods despite its amino acid sequence diversity. Nevertheless, the ankyrin repeats (ARs) 6 and 7 are well-conserved between six heat-sensitive arthropod TRPA1 channels and have critical roles for the heat activation of VdTRPA1.

Cold sensitivity of TRPA1 is unveiled by the prolyl hydroxylation blockade-induced sensitization to ROS

Mammalian transient receptor potential ankyrin 1 (TRPA1) is a polymodal nociceptor that plays an important role in pain generation, but its role as a cold nociceptor is still controversial. Here, we propose that TRPA1 can sense noxious cold via transduction of reactive oxygen species (ROS) signalling. We show that inhibiting hydroxylation of a proline residue within the N-terminal ankyrin repeat of human TRPA1 by mutation or using a prolyl hydroxylase (PHD) inhibitor potentiates the cold sensitivity of TRPA1 in the presence of hydrogen peroxide. Inhibiting PHD in mice triggers mouse TRPA1 sensitization sufficiently to sense cold-evoked ROS, which causes cold hypersensitivity. Furthermore, this phenomenon underlies the acute cold hypersensitivity induced by the chemotherapeutic agent oxaliplatin or its metabolite oxalate. Thus, our findings provide evidence that blocking prolyl hydroxylation reveals TRPA1 sensitization to ROS, which enables TRPA1 to convert ROS signalling into cold sensitivity.

The transient receptor potential ankyrin 1 (TRPA1) is a cation channel that is involved in nociceptive pain sensing. Here, the authors show that hydroxylation of a proline in the N terminus of TRPA1 renders it sensitive to reactive oxygen species resulting from noxious cold.

Enhancing effect of menthol on nicotine self-administration in rats

RATIONALE

Tobacco smoking is a leading preventable cause of premature death in the USA. Menthol is a significant flavoring additive in tobacco products. Clinical evidence suggests that menthol may promote tobacco smoking and nicotine dependence. However, it is unclear whether menthol enhances the reinforcing actions of nicotine and thus facilitates nicotine consumption. This study employed a rat model of nicotine self-administration to examine the effects of menthol on nicotine-taking behavior.

METHODS

Male Sprague-Dawley rats were trained in daily 1-h sessions to press a lever for intravenous nicotine self-administration under a fixed-ratio 5 schedule of reinforcement. In separate groups, rats self-administered nicotine at four different doses (0.0075, 0.015, 0.03, and 0.06 mg/kg/infusion). Five minutes prior to the two test sessions, menthol (5 mg/kg) or its vehicle was administered intraperitoneally in all rats in a counterbalanced design within each group. In separate rats that self-administered 0.015 mg/kg/infusion nicotine, menthol dose-response function was determined. Menthol was also tested on food self-administration.

RESULTS

An inverted U-shaped nicotine dose-response curve was observed. Menthol pretreatment shifted the nicotine dose-response curve to the left. The facilitating effect of menthol on the self-administration of 0.015 mg/kg/infusion nicotine was dose-dependent, whereas it produced similar effects at doses above the threshold of 2.5 mg/kg. Menthol tended to suppress the self-administration of food pellets.

CONCLUSIONS

These data demonstrate that menthol enhances the reinforcing effects of nicotine, and the effect of menthol was specific to nicotine. The findings suggest that menthol directly facilitates nicotine consumption, thereby contributing to tobacco smoking.

Effects of N-Glycosylation of the human cation channel TRPA1 on agonist-sensitivity

Determining the functional significance of post-translational modifications advances our understanding of many broadly-expressed proteins, and particularly ion channels. The enzymes that catalyze these modifications are often expressed in a cell-type specific manner, resulting in considerable structural diversity among post-translationally modified proteins that are expressed across a variety of cell types. TRP channels exhibit notably variable behavior between cell types in vitro and in vivo , and they are frequently modified with N-glycans that contribute to protein function. TRPA1 possesses two putative N-linked glycosylation sites at N747 and N753 that have not yet been studied in detail. Here, we show that both of these sites can be modified with an N-glycan and that the glycan at position N747 modulates agonist-sensitivity of TRPA1 in vitro Additionally, we found that N-glycosylation also modulates cooperative effects of temperature and the agonist cinnamaldehyde on TRPA1 channel activation. Collectively, these findings suggest a dynamic role played by the N-glycosylation of human TRPA1. They also provide further evidence of the versatility of N-glycans and will assist in efforts to fully understand the complex regulation of TRPA1 activity.

Quaternary Lidocaine Derivative QX-314 Activates and Permeates Human TRPV1 and TRPA1 to Produce Inhibition of Sodium Channels and Cytotoxicity

BACKGROUND

The relatively membrane-impermeable lidocaine derivative QX-314 has been reported to permeate the ion channels transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential cation channel, subfamily A, member 1 (TRPA1) to induce a selective inhibition of sensory neurons. This approach is effective in rodents, but it also seems to be associated with neurotoxicity. The authors examined whether the human isoforms of TRPV1 and TRPA1 allow intracellular entry of QX-314 to mediate sodium channel inhibition and cytotoxicity.

METHODS

Human embryonic kidney 293 (HEK-293) cells expressing wild-type or mutant human (h) TRPV1 or TRPA1 constructs as well as the sodium channel Nav1.7 were investigated by means of patch clamp and ratiometric calcium imaging. Cytotoxicity was examined by flow cytometry.

RESULTS

Activation of hTRPA1 by carvacrol and hTRPV1 by capsaicin produced a QX-314-independent reduction of sodium current amplitudes. However, permeation of QX-314 through hTRPV1 or hTRPA1 was evident by a concentration-dependent, use-dependent inhibition of Nav1.7 activated at 10 Hz. Five and 30 mM QX-314 activated hTRPV1 via mechanisms involving the intracellular vanilloid-binding domain and hTRPA1 via unknown mechanisms independent of intracellular cysteins. Expression of hTRPV1, but not hTRPA1, was associated with a QX-314-induced cytotoxicity (viable cells 48 ± 5% after 30 mM QX-314) that was ameliorated by the TRPV1 antagonist 4-(3-chloro-2-pyridinyl)-N-[4-(1,1-dimethylethyl)phenyl]-1-piperazinecarboxamide (viable cells 81 ± 5%).

CONCLUSIONS

The study data demonstrate that QX-314 directly activates and permeates the human isoforms of TRPV1 and TRPA1 to induce inhibition of sodium channels, but also a TRPV1-dependent cytotoxicity. These results warrant further validation of this approach in more intact preparations and may be valuable for the development of this concept into clinical practice.

Agonist-induced sensitisation of the irritant receptor ion channel TRPA1

KEY POINTS

The transient receptor potential ankyrin 1 (TRPA1) ion channel is expressed in nociceptive neurons and its activation causes ongoing pain and inflammation; TRPA1 is thought to play an important role in inflammation in the airways. TRPA1 is sensitised by repeated stimulation with chemical agonists in a calcium-free environment and this sensitisation is very long lasting following agonist removal. We show that agonist-induced sensitisation is independent of the agonist's binding site and is also independent of ion channel trafficking or of other typical signalling pathways. We find that sensitisation is intrinsic to the TRPA1 protein and is accompanied by a slowly developing shift in the voltage dependence of TRPA1 towards more negative membrane potentials. Agonist-induced sensitisation may provide an explanation for sensitisation following long-term exposure to harmful irritants and pollutants, particularly in the airways.

ABSTRACT

The TRPA1 ion channel is expressed in nociceptive (pain-sensitive) neurons and responds to a wide variety of chemical irritants, such as acrolein in smoke or isothiocyanates in mustard. Here we show that in the absence of extracellular calcium the current passing through TRPA1 gradually increases (sensitises) during prolonged application of agonists. Activation by an agonist is essential, because activation of TRPA1 by membrane depolarisation did not cause sensitisation. Sensitisation is independent of the site of action of the agonist, because covalent and non-covalent agonists were equally effective, and is long lasting following agonist removal. Mutating N-terminal cysteines, the target of covalent agonists, did not affect sensitisation by the non-covalent agonist carvacrol, which activates by binding to a different site. Sensitisation is unaffected by agents blocking ion channel trafficking or by block of signalling pathways involving ATP, protein kinase A or the formation of lipid rafts, and does not require ion flux through the channel. Examination of the voltage dependence of TRPA1 activation shows that sensitisation is accompanied by a slowly developing shift in the voltage dependence of TRPA1 towards more negative membrane potentials, and is therefore intrinsic to the TRPA1 channel. Sensitisation may play a role in exacerbating the pain caused by prolonged activation of TRPA1.

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.

Sensory Effects of Menthol and Nicotine in an E-Cigarette

INTRODUCTION

Despite the longstanding use and popularity of menthol as a flavorant in tobacco products, its sensory interactions with inhaled nicotine have never been measured independently of the other irritants in tobacco smoke. We therefore measured the perception of menthol in an E-cigarette with the primary goal of assessing its analgesic effect on the sensory irritation produced by inhaled nicotine.

METHODS

Adult cigarette smokers sampled aerosolized E-liquids containing five different concentrations of nicotine with 0%, 0.5%, or 3.5% l-menthol, as well as two commercial menthol flavors with and without nicotine. For each of the E-liquids participants used a labeled magnitude scale to rate the Overall Sensation intensity, Coolness/Cold, and Irritation/Harshness they experienced, and a Labeled Hedonic Scale to indicate how much they liked/disliked the overall flavor.

RESULTS

The main findings were that (1) perceived Irritation/Harshness was unaffected by a low (0.5%) menthol concentration, whereas a high menthol concentration (3.5%) led to higher perceived Irritation/Harshness at low nicotine concentrations but to lower Irritation/Harshness at the highest nicotine concentration (24mg/ml); (2) a commercial Menthol-Mint flavor produced similar results; (3) nicotine tended to enhance rather than suppress sensations of Coolness/Cold; and (4) menthol tended to slightly increase liking independently of nicotine concentration.

CONCLUSION

In addition to adding a sensation of coolness, menthol can reduce perceived airway irritation and harshness produced by inhalation when nicotine concentration is high, and contributes to the sensory impact of E-liquids when nicotine concentration is low.

IMPLICATIONS

The evidence presented here indicates that menthol can potentially improve the appeal of E-cigarettes not only via its coolness and minty flavor, but also by reducing the harshness from high concentrations of nicotine. As the first direct demonstration of an analgesic effect of menthol on inhaled nicotine in humans, these data also have implications for the role of menthol flavors in other inhaled tobacco products.

Chronic l-menthol-induced browning of white adipose tissue hypothesis: A putative therapeutic regime for combating obesity and improving metabolic health

INTRODUCTION

Obesity constitutes a serious global health concern reaching pandemic prevalence rates. The existence of functional brown adipose tissue (BAT) in adult humans has provoked intense research interest in the role of this metabolically active tissue in whole-body energy balance and body weight regulation. A number of environmental, physiological, pathological, and pharmacological stimuli have been proposed to induce BAT-mediated thermogenesis and functional thermogenic BAT-like activity in white adipose tissue (WAT), opening new avenues for therapeutic strategies based on enhancing the number of beige adipocytes in WAT.

HYPOTHESIS

Recent evidence support a role of l-menthol cooling, mediated by TRPM8 receptor, on UCP1-dependent thermogenesis and BAT-like activity in classical WAT depots along with the recruitment of BAT at specific anatomical sites. l-Menthol-induced BAT thermogenesis has been suggested to occur by a β-adrenergic-independent mechanism, avoiding potential side-effects due to extensive β-adrenergic stimulation mediated by available beta receptor agonists. l-Menthol has been also linked to the activation of the cold-gated ion channel TRPA1. However, its role in l-menthol-induced UCP1-dependent thermogenic activity in BAT and WAT remains undetermined. White adipose tissue plasticity has important clinical implications for obesity prevention and/or treatment because higher levels of UCP1-dependent thermogenesis can lead to enhanced energy expenditure at a considerable extent. We hypothesize that chronic dietary l-menthol treatment could induce TRPM8- and TRPA1-dependent WAT adaptations, resembling BAT-like activity, and overall improve whole-body metabolic health in obese and overweight individuals.

CONCLUSIONS

The putative impact of chronic l-menthol dietary treatment on the stimulation of BAT-like activity in classical WAT depots in humans remains unknown. A detailed experimental design has been proposed to investigate the hypothesized l-menthol-induced browning of WAT. If our hypothesis was to be confirmed, TRPM8/TRPA1-induced metabolic adaptations of WAT to BAT-like activity could provide a promising novel therapeutic approach for increasing energy expenditure, regulating body weight, and preventing obesity and its related co-morbidities in humans.

Activation of Transient Receptor Potential Ankyrin-1 by Insoluble Particulate Material and Association with Asthma

Inhaled irritants activate transient receptor potential ankyrin-1 (TRPA1), resulting in cough, bronchoconstriction, and inflammation/edema. TRPA1 is also implicated in the pathogenesis of asthma. Our hypothesis was that particulate materials activate TRPA1 via a mechanism distinct from chemical agonists and that, in a cohort of children with asthma living in a location prone to high levels of air pollution, expression of uniquely sensitive forms of TRPA1 may correlate with reduced asthma control. Variant forms of TRPA1 were constructed by mutating residues in known functional elements and corresponding to single-nucleotide polymorphisms in functional domains. TRPA1 activity was studied in transfected HEK-293 cells using allyl-isothiocynate, a model soluble electrophilic agonist; 3,5-ditert butylphenol, a soluble nonelectrophilic agonist and a component of diesel exhaust particles; and insoluble coal fly ash (CFA) particles. The N-terminal variants R3C and R58T exhibited greater, but not additive, activity with all three agonists. The ankyrin repeat domain-4 single nucleotide polymorphisms E179K and K186N exhibited decreased response to CFA. The predicted N-linked glycosylation site residues N747A and N753A exhibited decreased responses to CFA, which were not attributable to differences in cellular localization. The pore-loop residue R919Q was comparable to wild-type, whereas N954T was inactive to soluble agonists but not CFA. These data identify roles for ankyrin domain-4, cell surface N-linked glycans, and selected pore-loop domain residues in the activation of TRPA1 by insoluble particles. Furthermore, the R3C and R58T polymorphisms correlated with reduced asthma control for some children, which suggest that TRPA1 activity may modulate asthma, particularly among individuals living in locations prone to high levels of air pollution.

A novel 3-(4,5-diphenyl-1,3-oxazol-2-yl)propanal oxime compound is a potent Transient Receptor Potential Ankyrin 1 and Vanilloid 1 (TRPA1 and V1) receptor antagonist

Transient Receptor Potential Ankyrin 1 and Vanilloid 1 (TRPA1, TRPV1) ion channels expressed on nociceptive primary sensory neurons are important regulators of pain and inflammation. TRPA1 is activated by several inflammatory mediators including formaldehyde and methylglyoxal that are products of the semicarbazide-sensitive amine-oxidase enzyme (SSAO). SZV-1287 is a new 3-(4,5-diphenyl-1,3-oxazol-2-yl)propanal oxime SSAO inhibitor, its chemical structure is similar to other oxime derivatives described as TRPA1 antagonists. Therefore, we investigated its effects on TRPA1 and TRPV1 receptor activation on the cell bodies and peripheral terminals of primary sensory neurons and TRPA1 or TRPV1 receptor-expressing cell lines. Calcium influx in response to the TRPA1 agonist allyl-isothiocyanate (AITC) (200 μM) and the TRPV1 stimulator capsaicin (330 nM) in rat trigeminal neurons or TRPA1 and TRPV1 receptor-expressing cell lines was measured by microfluorimetry or radioactive (45)Ca(2+) uptake experiments. Calcitonin gene-related peptide (CGRP) release as the indicator of 100 μM AITC - or 100 nM capsaicin-induced peripheral sensory nerve terminal activation was measured by radioimmunoassay. SZV-1287 (100, 500 and 1000 nM) exerted a concentration-dependent significant inhibition on both AITC- and capsaicin-evoked calcium influx in trigeminal neurons and TRPA1 or TRPV1 receptor-expressing cell lines. It also significantly inhibited the TRPA1, but not the TRPV1 activation-induced CGRP release from the peripheral sensory nerve endings in a concentration-dependent manner. In contrast, the reference SSAO inhibitor LJP 1207 with a different structure had no effect on TRPA1 or TRPV1 activation in either model system. This is the first evidence that our novel oxime compound SZV-1287 originally developed as a SSAO inhibitor has a potent dual antagonistic action on TRPA1 and TRPV1 ion channels on primary sensory neurons.

When Choice Makes Sense: Menthol Influence on Mating, Oviposition and Fecundity in Drosophila melanogaster

The environment to which insects have been exposed as larvae and adults can affect subsequent behaviors, such as mating, oviposition, food preference or fitness. Experience can change female preference for oviposition, particularly in phytophagous insects. In Drosophila melanogaster, females avoid laying eggs on menthol rich-food when given the choice. Exposure to menthol during larval development reduces this aversion. However, this observation was not reproduced in the following generation. Recently, we have shown that oviposition-site preference (OSP) differs between wild-type D. melanogaster lines freely or forcibly exposed to menthol. After 12 generations, menthol "forced" lines still exhibit a persistent aversion to menthol whereas 'free-choice' lines show a decreased aversion for menthol rich-food. Here, we compare courtship behavior, mating and female fecundity in "forced" and "free-choice" lines, raised either on menthol rich-food (Menthol-lines) or on menthol-free food (Plain-lines). "Forced" males did not discriminate between decapitated virgin females of the two lines. They courted and mated with intact females of both "forced" lines in a comparable rate. However "forced" M-line males did mate significantly more rapidly with "forced" M-line females. In the "free-choice" procedure, P-line males show a similar pattern as "forced" males for discrimination ability and courtship. M-line males courted significantly more M-line females. Both 'free-choice' lines males mated significantly more with females of their own line. Female fecundity was assessed during 10 days in 'free-choice' lines. Menthol-line females laid more eggs during the first 4 days than female Plain-lines and parental control-line. The total number of eggs laid during the first 10 days of female adult life is comparable in M-line and parental control line. However, Menthol-line females laid eggs earlier than both parental control and Plain-lines. Our findings show that in D. melanogaster, as for OSP, mating and fecundity are more rapidly influenced when flies have a choice between alternative resources compared to flies permanently exposed to menthol.

Naturally Produced Defensive Alkenal Compounds Activate TRPA1

(E)-2-alkenals are aldehydes containing an unsaturated bond between the alpha and beta carbons. 2-alkenals are produced by many organisms for defense against predators and secretions containing (E)-2-alkenals cause predators to stop attacking and allow the prey to escape. Chemical ecologists have described many alkenal compounds with 3-20 carbons common, having varied positions of double bonds and substitutions. How do these defensive alkenals act to deter predators? We have tested the effects of (E)-2-alkenals with 6-12 carbons on transient receptor potential channels (TRP) commonly found in sensory neurons. We find that (E)-2-alkenals activate transient receptor potential ankyrin subtype 1 (TRPA1) at low concentrations-EC50s 10-100 µM (in 0 added Ca(2+) external solutions). Other TRP channels were either weakly activated (TRPV1, TRPV3) or insensitive (TRPV2, TRPV4, TRPM8). (E)-2-alkenals may activate TRPA1 by modifying cysteine side chains. However, target cysteines include others beyond the 3 in the amino-terminus implicated in activation, as a channel with cysteines at 621, 641, 665 mutated to serine responded robustly. Related chemicals, including the aldehydes hexanal and decanal, and (E)-2-hexen-1-ol also activated TRPA1, but with weaker potency. Rat trigeminal nerve recordings and behavioral experiments showed (E)-2-hexenal was aversive. Our results suggest that TRPA1 is likely a major target of these commonly used defensive chemicals.

Taurolidine and congeners activate hTRPA1 but not hTRPV1 channels and stimulate CGRP release from mouse tracheal sensory nerves

Taurolidine has long been in clinical use as an antimicrobial irrigation that does not impede wound healing. It can even be administered intravenously (30 g/day) to treat sepsis or to exert newly recognized antineoplastic actions. Only one irritant effect is reported, that is, to temporarily induce burning pain of unknown origin when applied to body cavities or peripheral veins. The structure of the molecule suggested the chemoreceptor channel TRPA1 as a potential target, which was verified measuring stimulated CGRP release from sensory nerves of the isolated mouse trachea and calcium influx in hTRPA1‐transfected HEK293 cells. With both methods, the concentration–response relationship of taurolidine exceeded the threshold value below 500 μmol/L and 100 μmol/L, respectively, and reached saturation at 1 mmol/L. The clinical 2% taurolidine solution did not evoke greater or longer lasting responses. The reversible tracheal response was abolished in TRPA1^−/− but retained in TRPV1^−/− mice. Consistently, hTRPV1‐HEK showed no calcium influx as a response, likewise native HEK293 cells and hTRPA1‐HEK deprived of extracellular calcium did not respond to taurolidine 1 mmol/L. The metabolite taurultam and its oxathiazine derivative, expected to cause less burning pain, showed weak tracheal irritancy only at 10 mmol/L, acting also through hTRPA1 but not hTRPV1. In conclusion, taurolidine, its metabolite, and a novel derivative showed no unspecific cellular effects but selectively, concentration‐dependently and reversibly activated the irritant receptor TRPA1 in CGRP‐expressing, thus nociceptive, neurons. The clinical solution of 2% taurolidine (~70 mmol/L) can, thus, rightly be expected to cause transient burning pain and neurogenic inflammation.

Abnormal activity of corneal cold thermoreceptors underlies the unpleasant sensations in dry eye disease

Dry eye disease (DED) affects >10% of the population worldwide, and it provokes an unpleasant sensation of ocular dryness, whose underlying neural mechanisms remain unknown. Removal of the main lachrymal gland in guinea pigs caused long-term reduction of basal tearing accompanied by changes in the architecture and density of subbasal corneal nerves and epithelial terminals. After 4 weeks, ongoing impulse activity and responses to cooling of corneal cold thermoreceptor endings were enhanced. Menthol (200 μM) first excited and then inactivated this augmented spontaneous and cold-evoked activity. Comparatively, corneal polymodal nociceptors of tear-deficient eyes remained silent and exhibited only a mild sensitization to acidic stimulation, whereas mechanonociceptors were not affected. Dryness-induced changes in peripheral cold thermoreceptor responsiveness developed in parallel with a progressive excitability enhancement of corneal cold trigeminal ganglion neurons, primarily due to an increase of sodium currents and a decrease of potassium currents. In corneal polymodal nociceptor neurons, sodium currents were enhanced whereas potassium currents remain unaltered. In healthy humans, exposure of the eye surface to menthol vapors or to cold air currents evoked unpleasant sensations accompanied by increased blinking frequency that we attributed to cold thermoreceptor stimulation. Notably, stimulation with menthol reduced the ongoing background discomfort of patients with DED, conceivably due to use-dependent inactivation of cold thermoreceptors. Together, these data indicate that cold thermoreceptors contribute importantly to the detection and signaling of ocular surface wetness, and develop under chronic eye dryness conditions an injury-evoked neuropathic firing that seems to underlie the unpleasant sensations experienced by patients with DED.

Sensitivities of Two Zebrafish TRPA1 Paralogs to Chemical and Thermal Stimuli Analyzed in Heterologous Expression Systems

Transient receptor potential A1 (TRPA1) is the only member of the mouse, chick, and frog TRPA family, whereas 2 paralogs (zTRPA1a and zTRPA1b) are present in zebrafish. We herein investigated functional differences in the 2 zebrafish TRPA1s. HEK293T cells were used as heterologous expression systems, and the sensitivities of these cells to 4 chemical irritants (allyl isothiocyanate [AITC], caffeine, auto-oxidized epigallocatechin gallate [EGCG], and hydrogen peroxide [H2O2]) were compared with Ca(2+) imaging techniques. Sensitivities to the activators for AITC, oxidized EGCG, and H2O2 were higher in cells expressing zTRPA1a than in those expressing zTRPA1b, whereas caffeine appeared to activate both cells equally. We also characterized the thermal sensitivity of Xenopus oocytes expressing each TRPA1 electrophysiologically using a 2-electrode voltage clamp. Although endogenous currents induced by a cold stimulation were observed in control oocytes in some batches, oocytes expressing zTRPA1b showed significantly stronger cold- and heat-induced responses. However, significant thermal activation was not observed in oocytes expressing zTRPA1a. The results obtained using in vitro expression systems suggest that zTRPA1a is specialized for chemical sensing, whereas zTRPA1b responds to thermal stimuli. Furthermore, characterization of the chimeric molecule of TRPA1a and 1b revealed the importance of the N-terminal region in chemical and thermal sensing by zTRPA1s.

The discovery of a potent series of carboxamide TRPA1 antagonists

Potent and selective carboxamide TRPA1 antagonists were identified by high throughput screening, with efficacy demonstrated in a topical inflammation model.

How the TRPA1 receptor transmits painful stimuli: Inner workings revealed by electron cryomicroscopy

A new high-resolution structure of a pain-sensing ion channel, TRPA1, provides a molecular scaffold to understand channel function. Unexpected structural features include a TRP-domain helix similar to TRPV1, a novel ligand-binding site, and an unusual C-terminal coiled coil stabilized by inositol hexakisphosphate (IP6). TRP-domain helices, which structurally act as a nexus for communication between the channel gates and its other domains, may thus be a feature conserved across the entire TRP family and, possibly, other allosterically-gated channels. Similarly, the TRPA1 antagonist-binding site could also represent a druggable location in other ion channels. Combined with known TRPA1 functional properties, the structural role for IP6 leads us to propose that polyphosphate unbinding could act as a molecular kill switch for TRPA1 inactivation. Finally, although packing of the TRPA1 membrane-proximal region hints at a mechanism for electrophile sensing, the details of how TRPA1 responds to noxious reactive electrophiles and temperature await future studies.

Physiological, pharmacological and behavioral evidence for a TRPA1 channel that can elicit defensive responses in the medicinal leech

Transient receptor potential ankyrin subtype 1 (TRPA1) channels are chemosensitive to compounds such as allyl isothiocyanate (AITC, the active component of mustard oil) and other reactive electrophiles and may also be thermodetectors in many animal phyla. In this study, we provide the first pharmacological evidence of a putative TRPA1-like channel in the medicinal leech. The leech's polymodal nociceptive neuron was activated by both peripheral and central application of the TRPA1 agonist AITC in a concentration-dependent manner. Responses to AITC were inhibited by the selective TRPA1 antagonist HC030031, but also by the TRPV1 antagonist SB366791. Other TRPA1 activators - N-methylmaleimide (NMM) and cinnamaldehyde (CIN) - also activated this nociceptive neuron, although HC030031 only inhibited the effects of NMM. The polymodal nociceptive neurons responded to moderately cold thermal stimuli (<17°C) and these responses were blocked by HC030031. AITC sensitivity was also found in the pressure-sensitive sensory neurons and was blocked by HC030031, but not by SB366791. AITC elicited a nocifensive withdrawal of the posterior sucker in a concentration-dependent manner that could be attenuated with HC030031. Peripheral application of AITC in vivo also produced swimming-like behavior that was attenuated by HC030031. These results suggest the presence of a TRPA1-like channel in the medicinal leech nervous system that responds to cold temperatures and may interact with the leech TRPV-like channel.

Differential Contribution of TRPA1, TRPV4 and TRPM8 to Colonic Nociception in Mice

Background

Various transient receptor potential (TRP) channels in sensory neurons contribute to the transduction of mechanical stimuli in the colon. Recently, even the cold-sensing menthol receptor TRPM(melastatin)8 was suggested to be involved in murine colonic mechano-nociception.

Methods

To analyze the roles of TRPM8, TRPA1 and TRPV4 in distension-induced colonic nociception and pain, TRP-deficient mice and selective pharmacological blockers in wild-type mice (WT) were used. Visceromotor responses (VMR) to colorectal distension (CRD) in vivo were recorded and distension/pressure-induced CGRP release from the isolated murine colon ex vivo was measured by EIA.

Results

Distension-induced colonic CGRP release was markedly reduced in TRPA1^-/- and TRPV4^-/- mice at 90/150 mmHg compared to WT. In TRPM8-deficient mice the reduction was only distinct at 150 mmHg. Exposure to selective pharmacological antagonists (HC030031, 100 μM; RN1734, 10 μM; AMTB, 10 μM) showed corresponding effects. The unselective TRP blocker ruthenium red (RR, 10 μM) was as efficient in inhibiting distension-induced CGRP release as the unselective antagonists of mechanogated DEG/ENaC (amiloride, 100 μM) and stretch-activated channels (gadolinium, 50 μM). VMR to CRD revealed prominent deficits over the whole pressure range (up to 90 mmHg) in TRPA1^-/- and TRPV4^-/- but not TRPM8^-/- mice; the drug effects of the TRP antagonists were again highly consistent with the results from mice lacking the respective TRP receptor gene.

Conclusions

TRPA1 and TRPV4 mediate colonic distension pain and CGRP release and appear to govern a wide and congruent dynamic range of distensions. The role of TRPM8 seems to be confined to signaling extreme noxious distension, at least in the healthy colon.

Perceptual mapping of chemesthetic stimuli in naïve assessors

Chemesthetic compounds, responsible for sensations such as burning, cooling, and astringency, are difficult stimuli to work with, especially when the evaluation task requires retasting. Here, we developed a protocol by which chemesthetic compounds can be assessed using sorting. We compared the performance of two cohorts of untrained assessors on this task, one with nose clips and the other without. Similarity matrices were analyzed using multidimensional scaling (MDS) to produce perceptual maps for the two cohorts. Overall, the groupings from the nose open cohort tended to follow a biological basis, consistent with previous findings that suggest compounds that activate a common receptor will elicit similar sensations. The nose-open and nose-pinched cohorts generated significantly different maps. The nose-pinched cohort had a higher variance in the MDS solution than the nose-open group. While the nose-open cohort generated seven clusters, the nose-pinched cohort generated only two clusters, seemingly based on the ready identification of chemesthetic sensations or not. There was less consensus regarding the attributes used to describe the samples in the nose-pinched cohort than in the nose-open cohort as well, as this cohort collectively generated more attributes but fewer were significant in regression.

Perceptual mapping of chemesthetic stimuli in naïve assessors

Chemesthetic compounds, responsible for sensations such as burning, cooling, and astringency, are difficult stimuli to work with, especially when the evaluation task requires retasting. Here, we developed a protocol by which chemesthetic compounds can be assessed using sorting. We compared the performance of two cohorts of untrained assessors on this task, one with nose clips and the other without. Similarity matrices were analyzed using multidimensional scaling (MDS) to produce perceptual maps for the two cohorts. Overall, the groupings from the nose open cohort tended to follow a biological basis, consistent with previous findings that suggest compounds that activate a common receptor will elicit similar sensations. The nose-open and nose-pinched cohorts generated significantly different maps. The nose-pinched cohort had a higher variance in the MDS solution than the nose-open group. While the nose-open cohort generated seven clusters, the nose-pinched cohort generated only two clusters, seemingly based on the ready identification of chemesthetic sensations or not. There was less consensus regarding the attributes used to describe the samples in the nose-pinched cohort than in the nose-open cohort as well, as this cohort collectively generated more attributes but fewer were significant in regression.

Menthol Enhances the Desensitization of Human α3β4 Nicotinic Acetylcholine Receptors

The α3β4 nicotinic acetylcholine receptor (nAChR) subtype is widely expressed in the peripheral and central nervous systems, including in airway sensory nerves. The nAChR subtype transduces the irritant effects of nicotine in tobacco smoke and, in certain brain areas, may be involved in nicotine addiction and/or withdrawal. Menthol, a widely used additive in cigarettes, is a potential analgesic and/or counterirritant at sensory nerves and may also influence nicotine's actions in the brain. We examined menthol's effects on recombinant human α3β4 nAChRs and native nAChRs in mouse sensory neurons. Menthol markedly decreased nAChR activity as assessed by Ca(2+) imaging, (86)Rb(+) efflux, and voltage-clamp measurements. Coapplication of menthol with acetylcholine or nicotine increased desensitization, demonstrated by an increase in the rate and magnitude of the current decay and a reduction of the current integral. These effects increased with agonist concentration. Pretreatment with menthol followed by its washout did not affect agonist-induced desensitization, suggesting that menthol must be present during the application of agonist to augment desensitization. Notably, menthol acted in a voltage-independent manner and reduced the mean open time of single channels without affecting their conductance, arguing against a simple channel-blocking effect. Further, menthol slowed or prevented the recovery of nAChRs from desensitization, indicating that it probably stabilizes a desensitized state. Moreover, menthol at concentrations up to 1 mM did not compete for the orthosteric nAChR binding site labeled by [(3)H]epibatidine. Taken together, these data indicate that menthol promotes desensitization of α3β4 nAChRs by an allosteric action.

Structure of the TRPA1 ion channel suggests regulatory mechanisms

The TRPA1 ion channel (a.k.a the ‘wasabi receptor’) is a detector of noxious chemical agents encountered in our environment or produced endogenously during tissue injury or drug metabolism. These include a broad class of electrophiles that activate the channel through covalent protein modification. TRPA1 antagonists hold potential for treating neurogenic inflammatory conditions provoked or exacerbated by irritant exposure. Despite compelling reasons to understand TRPA1 function, structural mechanisms underlying channel regulation remain obscure. Here, we use single-particle electron cryo-microscopy to determine the structure of full-length human TRPA1 to ~4Å resolution in the presence of pharmacophores, including a potent antagonist. A number of unexpected features are revealed, including an extensive coiled-coil assembly domain stabilized by polyphosphate co-factors and a highly integrated nexus that converges on an unpredicted TRP-like allosteric domain. These findings provide novel insights into mechanisms of TRPA1 regulation, and establish a blueprint for structure-based design of analgesic and anti-inflammatory agents.

TRPA1 as a drug target--promise and challenges

The transient receptor potential ankyrin 1 (TRPA1) channel is a nonselective cation channel belonging to the superfamily of transient receptor potential (TRP) channels. It is predominantly expressed in sensory neurons and serves as an irritant sensor for a plethora of electrophilic compounds. Recent studies suggest that TRPA1 is involved in pain, itch, and respiratory diseases, and TRPA1 antagonists have been actively pursued as therapeutic agents. Here, we review the recent progress, unsettled issues, and challenges in TRPA1 research and drug discovery.

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.

Sensory TRP channels: the key transducers of nociception and pain

Peripheral detection of nociceptive and painful stimuli by sensory neurons involves a complex repertoire of molecular detectors and/or transducers on distinct subsets of nerve fibers. The majority of such molecular detectors/transducers belong to the transient receptor potential (TRP) family of cation channels, which comprise both specific receptors for distinct nociceptive stimuli, as well as for multiple stimuli. This chapter discusses the classification, distribution, and functional properties of individual TRP channel types that have been implicated in various nociceptive and/or painful conditions.

Transient receptor potential channels and itch: how deep should we scratch?

Over the past 30 years, transient receptor potential (TRP) channels have evolved from a somewhat obscure observation on how fruit flies detect light to become the center of drug discovery efforts, triggering a heated debate about their potential as targets for therapeutic applications in humans. In this review, we describe our current understanding of the diverse mechanism of action of TRP channels in the itch pathway from the skin to the brain with focus on the peripheral detection of stimuli that elicit the desire to scratch and spinal itch processing and sensitization. We predict that the compelling basic research findings on TRP channels and pruritus will be translated into the development of novel, clinically useful itch medications.