Transient Receptor Potential Cation Channel Subfamily A Member 1 (TRPA1)
Historical Background and Structure
The transient receptor potential ankyrin-1 (TRPA1) is the only member of TRPA channel subfamily in mammals. Initially identified in human lung fibroblasts (Jaquemar et al. 1999), as an ankyrin-like protein with transmembrane domains protein 1 (ANKTM1), it was successively recognized as a TRP channel for its homology with several members of the same superfamily (Story et al. 2003). TRPA1 that has been cloned in invertebrates, such as Caenorhabditis elegans and fruit fly, and in a variety of species, including zebrafish, chicken, mouse, rat, dog, and humans (Nilius and Szallasi 2014), is a protein of about 1100 amino acids (120–130 kDa), although shorter splice variants have also been reported. The human trpa1 gene is in chromosome 8q13 and consists of 27 exons and spans 55,701 base pairs. Like other TRPs, TRPA1 is composed of six transmembrane domains with a pore region between domains 5 and 6 and two intracellular cytoplasmic regions located to the N- and C-termini. Its name derives from an abundant number (14–19) of ankyrin repeat regions within the N-terminal, which appear to play a major role in protein–protein interactions, as they connect transmembrane proteins to the cytoskeleton and are involved in channel trafficking to the plasma membrane. Another important functional site required for channel activation by electrophilic molecules (Macpherson et al. 2007) and for its desensitization is represented by at least 11 reactive cysteine and lysine amino acid residues in the N-terminal domain.
TRPA1 ion channel is composed by six transmembrane helices and intracellular NH3+ and COO− termini. The transmembrane S1–S6 with S5–S6 helices form the central pore and selectivity filter. Lysine and cysteine amino acid residues, critical for channel activation by electrophilic and reactive agonists, are located within the N-terminal domain before the several ankyrin repeat (14–19) regions. The calcium (Ca2+)-binding helix-loop-helix structural domain (EF-hand domain) in N-terminal region represents the most common mechanism for many Ca2+-interacting proteins. The TRPA1 channel is gated by a host of exogenously or endogenously produced agents, thereby evoking influx of cations. A number of drugs and their metabolites have been also reported to activate TRPA1.
TRPA1 protein is abundantly expressed in a subpopulation of primary sensory neurons of the dorsal root (DRG), trigeminal (TG), and vagal (VG) ganglia. TRPA1-expressing neurons have unmyelinated C- and thinly myelinated Aδ-fibers, and only occasionally large myelinated fibers (Story et al. 2003). However, the original proposal that TRPA1 expression is confined to peptidergic nociceptors (Story et al. 2003) has been challenged by recent evidence of colocalization of TRPA1 with markers of nonpeptidergic neurons, including the purinergic P2X3 receptor, isolectin B4 (IB4), or the Na(V)1.8 channel. The neuropeptides expressed by TRPA1-positive nociceptors are the tachykinins, substance P (SP) and neurokinin A (NKA), and calcitonin gene-related peptide (CGRP) that mediate neurogenic inflammatory responses, when released from peripheral nerve terminals.
In the central nervous system, TRPA1 is expressed in hippocampal neurons, where it seems to be linked with the cannabinoid receptor CB1, in astrocytes, where it appears to contribute to resting intracellular Ca2+ levels and regulating inhibitory synapses modulating the extracellular concentration of γ-aminobutirric acid, and in oligodendrocytes, with possible detrimental roles in ischemia and neurodegeneration (Hamilton et al. 2016). TRPA1 has also been identified in a variety of extraneuronal cells or tissues. These include the inner ear and the organ of Corti, where it may contribute to mechanical transduction, vascular endothelial cells, where it modulates vessel tone, keratinocytes and skin fibroblasts, where it mediates secretion of eicosanoids, such as prostaglandin E2 (PGE2) and leukotriene B4 (LTB4), dental pulp fibroblasts, where it contributes to the perception of noxious cold and to cold hypersensitivity, fibroblasts and epithelial and smooth muscle cells of airway and lung, where it modulates interleukin-8 release, rat pancreatic islets, where it facilitates insulin release, and enterochromaffin cells, where, by serotonin release, it modulates gastrointestinal motility. TRPA1 is also expressed in taste cells, where it associates with the bitter taste receptor TAS2R60.
Channel Modulation and Intracellular Pathways
G protein-coupled receptors (GPCRs) through their second-messenger signaling cascades affect TRPA1 function. Thus, stimulation of the bradykinin receptor 2 (B2), the prostaglandins receptors (EP), the protease-activated receptor 2 (PAR2), the MAS-related GPCRs (MrgprA3 and MrgprC11), and the bile acid receptor (TGR5) has been found to activate/sensitize the TRPA1 channel. TRPA1 sensitivity was potentiated through phospholipase C (PLC)-coupled B2 receptor activation. The pathway that ultimately resulted in TRPA1 signaling required the removal of the inhibitory effect of phosphatidyl inositol-4,5-bisphosphate (PIP2) and the inositol triphosphate (IP3)-dependent Ca2+ release from intracellular stores. GPCR-dependent TRPA1 potentiation was inhibited by protein kinase A (PKA) blockers, indicating an additive potentiating effect mediated by the PKA pathway.
PAR2 receptor expressed in sensory nerves is functionally coupled to TRPA1 by PLC-dependent pathways, similar to that described for the B2 receptor. PAR2, cleaved and activated by the proinflammatory protease, trypsin, and mast cell tryptase, induces PLC activation and by this mechanism increases TRPA1 sensitivity to agonists. Pruritogens that induce histamine-independent itch activate the MrgprA3 and MrgprC11 receptors, which via Gβγ and PLC signaling, respectively, sensitize TRPA1 and exacerbate scratching behavior in mice models of itch. Protein kinase C (PKC) activated by TGR5 and coexpressed with TRPA1 by a subpopulation of cutaneous afferents, seems necessary for TRPA1 sensitization mediated by bile acids.
EP, B2 and PAR2 receptors can activate and sensitize the TRPA1 channel through phospholipase C (PLC) and protein kinase A (PKA) signaling. TRPA1 activation pathway mediated by PLC requires the removal of the inhibitory effect of phosphatidyl inositol-4,5-bisphosphate (PIP2) and the inositol triphosphate (IP3)-dependent Ca2+ release from intracellular stores, whereas PKA activation, mediated by cyclic adenosine monophosphate (cAMP) increase, directly phosphorylates and sensitizes TRPA1. The pruritogens, chloroquine, and bovine adrenal medulla peptide (BAM8–22) targeting MAS-related (Mrgpr) receptor sensitize TRPA1 through phospholipase C (PLC) and the G protein βγ signaling. TGR5 receptor sensitizes TRPA1 through protein kinase C (PKC) activation. NGF by activating TrkA receptor, enhances the phosphorylated p38 MAPK, thus regulating TRPA1 expression.
Modes of Activation and Functions
TRPA1, initially described as a sensor of noxious cold (< 15°C) (Story et al. 2003), in some species appears to mediate acute cold responses, being directly activated by cold either via Ca2+-dependent or Ca2+-independent mechanisms. Exposure to cold temperature sensitizes the channel via an allosteric and Ca2+-independent process. In vivo findings confirmed that cold-like response by TRPA1 agonists detected in wild-type animals was absent in TRPA1-deficient mice. However, the TRPA1 role in cold sensation remains controversial with remarkable variability across species, as indicated by the fact that a single residue in S5 transmembrane domain accounts for the different response to cold stimuli between rodent and primate TRPA1. In contrast, the contribution of TRPA1 to cold hypersensitivity has been established in a variety of rodent models of inflammatory and neuropathic pain.
The role of TRPA1 has also been advocated in mechanosensation and, particularly, in mechanical hypersensitivity. The TRPA1 worm ortholog is sensitive to mechanical pressure, and several amphipathic molecules, such as trinitrophenol and chlorpromazine, produce a membrane curvature that leads to TRPA1 activity modulation. While channel ability to mediate the acute painful consequences of mechanical pressure is still matter of debate, increasing evidence indicates a key role for TRPA1 in mechanical allodynia and hyperalgesia in rodent models of both inflammatory and neuropathic pain. The remarkable TRPA1 sensitivity for an unprecedented series of chemical compounds, which has led to define TRPA1 as an ideal chemosensor, is undisputable. In the last 10 years, a plethora of chemical mediators that, produced by tissue or nerve injury and by targeting TRPA1 promote inflammatory and neuropathic pain have been identified. These can be divided into two main groups, according to the diverse mechanisms of channel activation. The first group comprises the molecules which gate the channel by modifying or interacting with nucleophilic cysteine and lysine residues of the channel N-terminus (Macpherson et al. 2007), by several chemical modes of covalent modification of the amino acidic residues, including Michael addition, formation of a thiocarbamate intermediate and of cysteine-disulfide products, or alkylation. The second group encompasses nonelectrophilic compounds which activate the channel through a noncovalent protein modification. Among different TRP channels, including TRPV1, TRPV4, TRPC5, TRPV3, and TRPV2, TRPA1 exhibits the highest sensitivity to oxidation.
TRPA1 agonists may derive from exogenous sources or are produced endogenously under circumstances of inflammation or tissue injury. Exogenous agonists include a large variety of chemical species, such as molecules from natural or alimentary origin, drugs or drug metabolites, and environmental irritants. Among natural compounds, the best-known activators, with moderate to severe irritant properties, are cinnamaldehyde, extracted from the Cinnamomum, and several isothiocyanate compounds, such as allyl isothiocyanate (mustard oil) obtained from the Brassica seeds, contained in mustard or wasabi, and allicin and diallyl disulfide, contained in garlic (Allium sativum). Less potent or selective TRPA1 activators are gingerol, contained in ginger, which also gates TRPV1, thymol, a major component of thyme (Thymus vulgari) and oregano (Origanum vulgare), and carvacrol, contained in oregano. All these reactive compounds covalently modify specific cysteine residues within the cytoplasmic N-terminal region of the channel, resulting in TRPA1 activation. The nonelectrophilic compound, delta-9-tetrahydrocannabinol (THC), contained in Cannabis sativa, or other phytocannabinoids, activate TRPA1 without any requirement of covalent modification. TRPA1 is highly sensitive to intracellular divalent biological metals, such as Zn2+ and Cu2+, as low nanomolar concentrations activate TRPA1 and modulate its sensitivity. Environmental irritants described as TRPA1 channel stimulants include among others acrolein (Bautista et al. 2006) tear gases, toluene diisocyanate, formaldehyde (McNamara et al. 2007), acetaldehyde, and crotonaldehyde, contained in cigarette smoke.
Drugs or their metabolites may gate TRPA1. General anesthetics, including isoflurane, desflurane, sevuflurane, and propofol, directly activate TRPA1, thereby producing irritation and neurogenic inflammatory responses in the respiratory tract. Local anesthetics, including high concentrations of lidocaine, can activate TRPA1 by both covalent modification of intracellular cysteine residues and by interacting with the S5 transmembrane domain. Nicotine activates TRPA1 in nociceptors in a membrane-delimited manner, stabilizing the open state and destabilizing the closed state of the channel. Elevated concentrations of nonsteroidal anti-inflammatory drugs (NSAIDs), including flufenamic and niflumic acid, diclofenac, indomethacin, and ketoprofen, have been shown to activate the rodent and human TRPA1, in vitro. Other currently used drugs such as the antimycotic clotrimazole, the antidiabetic glibenclamide, the nonnarcotic morphine derivative, apomorphine, the antirheumatic medicine, auranophin, and the antihypertensive dihydropyridines have also been shown to activate TRPA1, albeit with low potency.
More importantly, TRPA1 has been identified as a sensor (Bessac et al. 2008) of a wide array of byproducts of oxidative stress, including reactive oxygen (ROS) and nitrogen (RNS) species. Among ROS, hydrogen peroxide (H2O2), hypochlorite (OCl-), and superoxide (O2−) activate TRPA1. Among RNS, NO and ONOO− are TRPA1 stimulants. The peroxidation or nitrosylation of plasma membrane phospholipids generate reactive metabolites, including 4-hydroxy-2-nonenal (4-HNE), 4-hydroxy-hexenal (4-HHE), 4-oxo-nonenal (4-ONE), and nitrooleic acid (9-OA-NO2), all identified as remarkable TRPA1 activators. ROS gates TRPA1 through a cysteine oxidation or disulfide formation, whereas RNS uses S-nitrosylation reaction. The reactive aldehydes, 4-HNE and acrolein, gate the channel by a Michael-addition reaction between their electrophilic C = C double bond and the sulfhydryl group of cysteine, the ε-amino group of lysine, or the imidazole group of histidine residues (Macpherson et al. 2007). Cyclopentenone prostaglandins (PGs) and isoprostanes (iso-PGs), including 15-deoxy-Δ12,14-PGJ2, PGA2, and PGA1, and 8-isoprostane-PGA2 generated via a nonenzymatic dehydration from proinflammatory and proalgesic prostaglandins and isoprostanes directly target TRPA1. Finally, hydrogen sulfide, produced by cysteine metabolism and endowed with vasodilatatory and other properties, has also been shown to stimulate TRPA1 (Fig. 1).
Antagonists and Desensitizing Agents
The first selective TRPA1 antagonist was HC-030031 (Hydra Biosciences Inc., Cambridge, MA, USA) (McNamara et al. 2007). Chemically, HC-030031 contains a xanthine alkaloid core similar to caffeine, a low potency TRPA1 activator, and covalently binds to the receptor (McNamara et al. 2007). The low potency of HC-030031 is compensated by a good selectivity, which allows its use for specific TRPA1 blockade. Additional antagonists successfully used in models of inflammatory and neuropathic pain are the oxime derivative, AP-18 (Novartis Research Foundation, San Diego, CA, USA), A-967079 (Abbott Laboratories, Abbott Park, IL, USA), and the HC-030031 derivative, Chembridge-5,861,528 (ChemBridge Corporation, San Diego, CA, USA). A new series of compounds based on 7-substituted-1,3-dimethyl-1,5-dihydro-pyrrolo[3,2-d]pyrimidine-2,4-dione derivatives (Department of Pharmaceutical Sciences, University of Ferrara, Ferrara, Italy) has been reported. More recently, AMG0902 (Amgen Inc., Thousand Oaks, CA, USA) while blocked AITC-evoked responses, surprisingly, did not exhibit any antihyperalgesic effect in models of inflammatory and neuropathic pain.
A series of herbal extracts used for the treatment of pain or migraine have been identified as TRPA1 agonists. Albeit this action would conflict with the beneficial effects of these compounds, they have been also found to act as partial agonists and agents capable of desensitizing the channel and nociceptors. Parthenolide, the active constituent of Tanacetum parthenium, recommended for migraine treatment, is a partial TRPA1 agonist and produces a prolonged desensitization of the TRPA1 channel and of the dual function of primary sensory neurons, thus reducing both neurogenic inflammatory and nociceptive responses. Ligustilide, used to treat pain and headaches, is contained in elevated concentrations in herbal medicines, and like parthenolide, by activating TRPA1, promotes nociceptor desensitization. The mechanism of action of the universally used analgesic and antipyretic medicine, acetaminophen (paracetamol), is unknown. The highly reactive acetaminophen metabolite, N-acetyl-p-benzoquinone imine (NAPQI), responsible for the major toxic effects if acetaminophen is overdosed, targets TRPA1, thereby, causing channel desensitization and inhibiting pain transmission at relay structures of the dorsal spinal cord. More importantly, antipyrine, propyphenazone, and dipyrone (metamizole), old analgesics still successfully used by hundreds of millions of patients for treating pain and migraine, are very weak cyclooxygenase inhibitors, but potently and selectively antagonized TRPA1, at concentrations fully compatible to the clinical doses (Nassini et al. 2015).
The pain of migraine and cluster headache attacks cannot be considered as a mere example of inflammatory pain because these conditions respond to triptans and CGRP inhibitors, which are unable to attenuate any other type of inflammatory and neuropathic pain. However, the unquestionable antimigraine efficacy of NSAIDs indicates the role of prostaglandins. The role of TRPA1 in migraine and cluster headache originated from the observation that certain migraine or cluster headache triggers, including umbellulone and acrolein, are TRPA1 stimulants. Furthermore, herbal extracts used for migraine prophylaxis or acetaminophen via its reactive metabolite, NAPQI, partially antagonize TRPA1 and desensitize peptidergic trigeminal nociceptors. Finally, dipyrone (metamizole), a potent and selective TRPA1 antagonist (Nassini et al. 2015), is an effective and largely prescribed antimigraine drug.
Estrogen receptor positive breast cancer patients undergo a 5-year daily therapy with aromatase inhibitors to reduce the hormone-dependent proliferative burden. Unfortunately, these otherwise effective drugs produce in about 40% of patients a constellation of pain and inflammatory symptoms defined as aromatase inhibitor musculoskeletal symptoms (AIMSS). All clinically used aromatase inhibitors (exemestane, letrozole, and anastrozole) are direct TRPA1 agonists. Pharmacological inhibition or genetic deletion of TRPA1 reversed or prevented AIMSS-like responses in mice. Exemestane, the false aromatase substrate, that blocks estrone/estradiol formation is an analog of the naturally occurring estrogen precursor, androstenedione. Notably, androstenedione engages TRPA1 in peptidergic primary sensory neurons and dramatically lowers the concentration of letrozole required for TRPA1 activation. This synergistic action results in AIMSS-like responses in mice by doses of letrozole fully compatible with the plasma concentration found in patients (De Logu et al. 2016).
TRPA1 contribution to neuropathic pain was first shown in a model of lumbar spinal nerve ligation (SNL) where prolonged mechanical hypersensitivity was reversed by HC-030031, and was further confirmed by the use of A-967079. In models of radicular pain induced by chronic compression of the dorsal root ganglia, intrathecal injection of TRPA1 antagonists reduced mechanical allodynia. TRPA1 inhibition also induced an antiallodynic effect in a model of chronic postischemic pain in rats. Recent data propose TRPA1 as a major contributor to trigeminal neuropathic pain. Trigeminal mechanical and cold allodynia evoked in mice by constriction of the infraorbital nerve was attenuated in TRPA1-deficient mice and by TRPA1 antagonists (Trevisan et al. 2016) (Fig. 3). As a macrophage-depleting agent and local antioxidants also abated neuropathic pain, the hypothesis was proposed that oxidative stress byproducts generated by macrophages recruited at the site of nerve injury target TRPA1, thus maintaining trigeminal neuropathic pain (Trevisan et al. 2016).
Diabetic Peripheral Neuropathy
Spontaneous pain and hypersensitivity to mechanical and cold stimuli are common painful symptoms described by diabetic patients. TRPA1 appears to contribute to pain conditions, not generated by a mechanical insult, such as that associated to chronic hyperglycemia. Systemic administration of TRPA1 antagonists reduced mechanical allodynia in rodent models of streptozotocin-induced diabetes. Similar antiallodynic effects were observed when the antagonist was injected intrathecally. The reactive compounds, 4-hydroxynonenal and methylglyoxal, overproduced in hyperglycemia are known to activate the TRPA1 channel and may sustain TRPA1-mediated nociceptor activation, causing both the early pain hypersensitivity and the subsequent loss of cutaneous nerve fiber function.
Chemotherapy-induced peripheral neuropathy (CIPN), which affects a substantial proportion of patients treated with anticancer drugs, is characterized by pain symptoms localized to the extremities that are exacerbated by exposure to mechanical and cold stimuli. The underlying mechanism of CIPN is unknown and consequently patients are undertreated. The prolonged mechanical and cold hypersensitivity induced by a single dose of oxaliplatin or cisplatin, in rodents, was completely abolished by TRPA1 gene deletion or antagonism. As antioxidants prevent oxaliplatin-evoked mechanical hyperalgesia and oxaliplatin or cisplatin did not target directly TRPA1, it is possible that oxidative stress, generated by the anticancer drugs, gates the channel to maintain the allodynic condition. Paclitaxel, unable to gate TRPA1, induced in mice mechanical and cold hypersensitivities that were attenuated by channel antagonism or gene deletion. A single administration of bortezomib produced prolonged mechanical and cold hypersensitivities that were absent in TRPA1-deleted mice. The observation that bortezomib-evoked hypersensitivities were transiently attenuated by antioxidants or TRPA1 antagonists (Trevisan et al. 2013) strengthened the general hypothesis that the known ability of anticancer drugs to generate oxidative stress causes TRPA1 activation in nociceptors, thus generating prolonged neuropathic pain.
TRPA1 has been proposed to contribute to different airway inflammatory diseases, including chronic cough, asthma, and chronic obstructive pulmonary diseases (COPD), by activating neurogenic and nonneurogenic inflammatory responses. TRPA1 has been detected in nonneuronal airway cells (human small cell lung cancer cells, fibroblasts, epithelial cells, and smooth muscle cells) whereby increasing intracellular Ca2+ releases proinflammatory cytokines, such as interleukin-8, prevents apoptosis and promotes cell survival, via an extracellular-signal regulated kinases (ERK1/2)-dependent pathway, and favors detrimental processes in epithelial airway cells taken from cystic fibrosis patients. Neurogenic and nonneurogenic inflammatory responses to cigarette smoke, allergen (Caceres et al. 2009), or NAPQI were found to be attenuated by TRPA1 inhibition. However, the clinical relevance of these findings obtained in rodents remains to be determined.
TRPA1-expressing primary sensory neurons terminals are found in many tissues, including the skin. Scratch behavior that is evoked by toxic agents and irritants characterizes some inflammatory skin circumstances, such as atopic dermatitis and psoriasis. Itch is largely histamine-dependent although acute and chronic itch conditions may be insensitive to antihistaminic drugs. The TRPA1 agonist trans-cinnamaldehyde caused a burning sensation and skin irritation and itch-related dysesthetic states of allokinesis and hyperkinesis. Itch produced by the antimalaric drug chloroquine and the bovine adrenal medulla peptide (BAM8–22) caused histamine-independent itch by acting on the MrgprA3 and MrgprC11 receptors, respectively, via increases in intracellular Ca2+ mediated by TRPA1 (Wilson et al. 2013). TRPA1 mediates itch produced by endothelin, hydrogen peroxide, serotonin, and cytokines overexpressed in atopic dermatitis, such as thymic stromal lymphopoietin (TLPS) and interleukin-13 (Wilson et al. 2013). Pruritus evoked by bile acids through TRG5 receptors is also mediated by TRPA1 activation (Alemi et al. 2013). Finally, both TRPA1 and TRPV1 cooperate to promote itch evoked by leukotriene B4, inteleukin-31, and lysophosphatidic acid.
Both intrinsic and extrinsic neurons in the gastrointestinal tract express TRPA1 which may contain SP/NKA and CGRP. In 4,6-trinitrobenzene sulfonic acid (TNBS) or dextran sodium sulfate (DSS) models of visceral inflammatory disease, a TRPA1-dependent increase in the abdominal hypersensitivity, which seems to be related to the local release of CGRP and SP, has been reported (Engel et al. 2011). Similar results were obtained in a caerulein-induced model of pancreatitis in which pharmacological inhibition or genetic deletion of TRPA1 attenuated both inflammation and hyperalgesia. TRPA1 is highly expressed in serotonin (5-HT)-containing enterochromaffin (EC) cells. Stimulation of EC by TRPA1 agonists induces Ca2+ influx and 5-HT release, which via 5-HT3 receptor activation contracts rodents intestine wall. Thus, gastrointestinal functions can be regulated by TRPA1-dependent neuronal and nonneuronal pathways.
Although this issue remains highly controversial, expression of TRPA1 in thermoregulatory sensory nerves suggests that TRPA1 can influence body temperature and contribute to temperature sensation. TRPA1 mediates sensation of the rate of temperature change in Drosophila larvae, determining a protective rolling behavior in this species. However, remarkable species differences have been found in the phylogeny, as cold activates rat and mouse TRPA1, but is ineffective in human or rhesus monkey TRPA1 orthologs. Further complexity derives from the observation that fruit fly, mosquitos, snake, frog, lizard, and chicken TRPA1 is a heat, and not a cold, sensor. Of clinical relevance is the observation that hypothermia produced by systemic cinnamaldehyde administration was greatly reduced in TRPA1-deleted mice and that a TRPA1 antagonist inhibited acetaminophen-induced hypothermia and acetaminophen did not lower body temperature in TRPA1-deleted mice. Protection of cutaneous tissues against cold injury is obtained by a cold-induced vascular response, consisting of vasoconstriction followed by vasodilatation. TRPA1, producing superoxide generation and the ensuing vasoconstriction, mediates both the initial cold-dependent vasoconstriction and the subsequent restorative increase in blood flow via CGRP and SP release (Aubdool et al. 2014).
Few studies have explored whether pain conditions are associated with genetic TRPA1 variants. In 2010, in a Colombian family, a point mutation in the S4 domain of TRPA1 that induced an autosomal-dominant Mendelian heritable episodic pain syndrome was identified (Kremeyer et al. 2010). Patients show severe episodes of pain triggered by conditions of fatigue, fasting, and cold, mainly affecting the thorax and arms and occasionally radiating to the abdomen and legs. Affected patients displayed hypersensitivity to mustard oil and this effect correlated with a gain of function of the mutant TRPA1 channel inducing channel hypersensitivity to agonists and cold in vitro (Kremeyer et al. 2010). Another study has identified a single-nucleotide mutation in N-terminal of TRPA1 channel (E179K) linked to paradoxical heat sensation in patient suffering from neuropathic pain (Binder et al. 2011).
Localization of TRPA1 in a large variety of cells and tissues suggests pleiotropic roles. TRPA1, while maintaining a conserved function in chemosensation, showed remarkable variability across species regarding cold and mechanical sensitivity. The elevated levels of expression in a subset of peptidergic nociceptors indicate a primary role in pain and neurogenic inflammation. With the caveat that the majority of pain models are reproduced in rodents, TRPA1 contribution to inflammatory and neuropathic pain is robustly emerging. The pronounced sensitivity of TRPA1 to oxidative stress and its byproducts emphasizes the channel contribution in pain signaling at sites of tissues injury and inflammation. Initial promising results with TRPA1 antagonists which are under current clinical scrutiny require, however, further confirmation before TRPA1 can be validated as a major target for pain therapy.