Transient receptor potential cation channel, subfamily A, member 1, also known as transient receptor potential ankyrin 1 or TRPA1, is a protein that in humans is encoded by the TRPA1 (and in other species by the Trpa1) gene.
TRPA1 is an ion channel located on the plasma membrane of many human and animal cells. This ion channel is best known as a sensor for environmental irritants giving rise to somatosensory modalities, such as pain, cold and itch, and other protective responses (tears, airway resistance, cough...).
TRPA1 is a member of the transient receptor potential channel family. TRPA1 contains 14 N-terminal ankyrin repeats and is believed to function as a mechanical and chemical stress sensor. The specific function of this protein has not yet been determined; however, studies indicate that the function may involve a role in somatosensory neuron mediated protective responses, signal transduction and growth control.
In 2008, it was observed that caffeine suppresses activity of human TRPA1, but it was found that mouse TRPA1 channels expressed in sensory neurons cause an aversion to drinking caffeine-containing water, suggesting that the TRPA1 channels mediate the perception of caffeine.
TRPA1 has also been implicated in airway irritation by cigarette smoke, cleaning supplies and in the skin irritation experienced by some smokers trying to quit by using nicotine replacement therapies such as inhalers, sprays, or patches.
A missense mutation of TRPA1 was found to be the cause of a hereditary episodic pain syndrome. A family from Colombia suffers from "debilitating upper-body pain starting in infancy" that is "usually triggered by fasting or fatigue (illness, cold temperature, and physical exertion being contributory factors)". A gain-of-function mutation in the fourth transmembrane domain causes the channel to be overly sensitive to pharmacological activation.
Metabolites of paracetamol (acetaminophen) have been demonstrated to bind to the TRPA1 receptors (probably agonism which then can lead to desensitization of TRPA1 receptors in the way that capsaicin does it in the spinal cord of mice, causing an antinociceptive effect. This is suggested as the antinociceptive mechanism for paracetamol.
Oxalate, a metabolite of an anti cancer drug oxaliplatin, has been demonstrated to inhibit prolyl hydroxylase, which endows cold-insensitive human TRPA1 with pseudo cold sensitivity (via reactive oxygen generation from mitochondria). This may cause a characteristic side-effect of oxaliplatin (cold-triggered acute peripheral neuropathy).
TRPA1 can be considered to be one of the most promiscuous TRP ion channels, as it seems to be activated by a large number of noxious chemicals found in many plants, food, cosmetics and pollutants.
Although several nonelectrophilic agents such as thymol and menthol have been reported as TRPA1 agonists, most of the known activators are electrophilic chemicals that have been shown to activate the TRPA1 receptor via the formation of a reversible covalent bond with cysteine residues present in the ion channel. For a broad range of electrophilic agents, chemical reactivity in combination with a lipophilicity enabling membrane permeation is crucial to TRPA1 agonistic effect. A dibenz[b,f][1,4]oxazepine derivative substituted by a carboxylic methylester at position 10 is the most potent TRPA1 agonist discovered to date (EC50 = 50 pM). The pyrimidine PF-4840154 is a potent, non-covalent activator of both the human (EC50 = 23 nM) and rat (EC50 = 97 nM) TrpA1 channels. This compound elicits nociception in a mouse model through TrpA1 activation. Furthermore, PF-4840154 is superior to allyl isothiocyanate, the pungent component of mustard oil, for screening purposes.
The eicosanoids formed in the ALOX12 (i.e. arachidonate-12-lipoxygnease) pathway of arachidonic acid metabolism, 12S-hydroperoxy-5Z,8Z,10E,14Z-eicosatetraenoic acid (i.e. 12S-HpETE; see 12-Hydroxyeicosatetraenoic acid) and the hepoxilins (Hx), HxA3 (i.e. 8R/S-hydroxy-11,12-oxido-5Z,9E,14Z-eicosatrienoic acid) and HxB3 (i.e. 10R/S-hydroxy-11,12-oxido-5Z,8Z,14Z-eicosatrienoic acid) (see Hepoxilin#Pain perception) directly activate TRPA1 and thereby contribute to the hyperalgesia and tactile allodynia responses of mice to skin inflammation. In this animal model of pain perception, the hepoxilins are released in spinal cord and directly activate TRPA (and also TRPV1) receptors to augment the perception of pain. 12S-HpETE, which is the direct precursor to HxA3 and HxB3 in the ALOX12 pathway, may act only after being converted to these hepoxilins. The epoxide, 4,5-epoxy-8Z,11Z,14Z-eicosatrienoic acid (4,5-EET) made by the metabolism of arachidonic acid by any one of several cytochrome P450 enzymes (see Epoxyeicosatrienoic acid) likewise directly activates TRPA1 to amplify pain perception.
Studies with mice, guinea pig, and human tissues and in guinea pigs indicate that another arachidonic acid metabolite, Prostaglandin E2, operates through its prostaglandin EP3G protein coupled receptor to trigger cough responses. Its mechanism of action does not appear to involve direct binding to TRPA1 but rather the indirect activation and/or sensitization of TRPA1 as well as TRPV1 receptors. Genetic polymorphism in the EP3 receptor (rs11209716), has been associated with ACE inhibitor-induce cough in humans.
More recently, a peptide toxin termed the wasabi receptor toxin from the Australian black rock scorpion (Urodacus manicatus) was discovered; it was shown to bind TRPA1 non-covalently in the same region as electrophiles and act as a gating modifier toxin for the receptor, stabilizing the channel in an open conformation.
Resolvin D1 (RvD1) and RvD2 (see resolvins) and maresin 1 are metabolites of the omega 3 fatty acid, docosahexaenoic acid. They are members of the specialized proresolving mediators (SPMs) class of metabolites that function to resolve diverse inflammatory reactions and diseases in animal models and, it is proposed, in humans. These SPMs also damp pain perception arising from various inflammation-based causes in animal models. The mechanism behind their pain-dampening effect involves the inhibition of TRPA1, probably (in at least certain cases) by an indirect effect wherein they activate another receptor located on neurons or nearby microglia or astrocytes. CMKLR1, GPR32, FPR2, and NMDA receptors have been proposed to be the receptors through which SPMs may operate to down-regulate TRPs and thereby pain perception.
^Jaquemar D, Schenker T, Trueb B (Mar 1999). "An ankyrin-like protein with transmembrane domains is specifically lost after oncogenic transformation of human fibroblasts". The Journal of Biological Chemistry. 274 (11): 7325–33. doi:10.1074/jbc.274.11.7325. PMID10066796.
^ abClapham DE, Julius D, Montell C, Schultz G (Dec 2005). "International Union of Pharmacology. XLIX. Nomenclature and structure-function relationships of transient receptor potential channels". Pharmacological Reviews. 57 (4): 427–50. doi:10.1124/pr.57.4.6. PMID16382100.
^ abAndersen HH, Elberling J, Arendt-Nielsen L (May 2015). "Human Surrogate Models of Histaminergic and Non-histaminergic Itch". Acta Dermato-Venereologica. 95 (7): 771–7. doi:10.2340/00015555-2146. PMID26015312.
^Andersson DA, Gentry C, Alenmyr L, Killander D, Lewis SE, Andersson A, Bucher B, Galzi JL, Sterner O, Bevan S, Högestätt ED, Zygmunt PM (2011-11-22). "TRPA1 mediates spinal antinociception induced by acetaminophen and the cannabinoid Δ(9)-tetrahydrocannabiorcol". Nature Communications. 2 (2): 551. doi:10.1038/ncomms1559. PMID22109525.
^Macpherson LJ, Dubin AE, Evans MJ, Marr F, Schultz PG, Cravatt BF, Patapoutian A (Feb 2007). "Noxious compounds activate TRPA1 ion channels through covalent modification of cysteines". Nature. 445 (7127): 541–5. doi:10.1038/nature05544. PMID17237762.
^Gijsen HJ, Berthelot D, Zaja M, Brône B, Geuens I, Mercken M (Oct 2010). "Analogues of morphanthridine and the tear gas dibenz[b,f][1,4]oxazepine (CR) as extremely potent activators of the human transient receptor potential ankyrin 1 (TRPA1) channel". Journal of Medicinal Chemistry. 53 (19): 7011–20. doi:10.1021/jm100477n. PMID20806939.
^ abcKoivisto A, Chapman H, Jalava N, Korjamo T, Saarnilehto M, Lindstedt K, Pertovaara A (January 2014). "TRPA1: a transducer and amplifier of pain and inflammation". Basic & Clinical Pharmacology & Toxicology. 114 (1): 50–5. doi:10.1111/bcpt.12138. PMID24102997.
^Grilo A, Sáez-Rosas MP, Santos-Morano J, Sánchez E, Moreno-Rey C, Real LM, Ramírez-Lorca R, Sáez ME (2011). "Identification of genetic factors associated with susceptibility to angiotensin-converting enzyme inhibitors-induced cough". Pharmacogenetics and Genomics. 21 (1): 10–7. doi:10.1097/FPC.0b013e328341041c. PMID21052031.