TRPA1


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 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.

Function

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.
Recent studies indicate that TRPA1 is activated by a number of reactive and non-reactive compounds and considered as a "chemosensor" in the body. TRPA1 is co-expressed with TRPV1 on nociceptive primary afferent C-fibers in humans. This sub-population of peripheral C-fibers is considered important sensors of nociception in humans and their activation will under normal conditions give rise to pain. Indeed, TRPA1 is considered as an attractive pain target. TRPA1 knockout mice showed near complete attenuation of nocifensive behaviors to formalin, tear-gas and other reactive chemicals. TRPA1 antagonists are effective in blocking pain behaviors induced by inflammation.
Although it is not firmly confirmed whether noxious cold sensation is mediated by TRPA1 in vivo, several recent studies clearly demonstrated cold activation of TRPA1 channels in vitro.
In the heat-sensitive loreal pit organs of many snakes TRPA1 is responsible for the detection of infrared radiation.

Structure

In 2016, the laboratories of David Julius and Yifan Cheng at the University of California, San Francisco used cryo-electron microscopy to solve the three-dimensional structure of TRPA1. This work revealed that the channel assembles as a homotetramer, and possesses numerous intriguing structural features that hint at its complex regulation by irritants, cytoplasmic second messengers, cellular co-factors, and lipids. Most notably, the site of covalent modification and activation for electrophilic irritants was localized to a tertiary structural feature on the membrane-proximal intracellular face of the channel, which has been termed the 'allosteric nexus', and which is composed of a cysteine-rich linker domain and the eponymous TRP domain. Breakthrough research combining cryo-electron microscopy and electrophysiology later elucidated the molecular mechanism of how the channel functions as a broad-spectrum irritant detector. With respect to electrophiles, which activate the channel by covalent modification of three cysteines in the allosteric nexus, it was shown that these reactive oxidative species act step-wise to modify two critical cysteine residues in the allosteric nexus. Upon covalent attachment, the allosteric nexus adopts a conformational change that is propagated to the channel's pore, dilating it to permit cation influx and subsequent cellular depolarization. With respect to activation by the second messenger calcium, the structure of the channel in complex with calcium localized the binding site for this ion and functional studies demonstrated that this site controls the various different effects of calcium on the channel – namely potentiation, desensitization, and receptor-operation.

Clinical significance

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 ". A gain-of-function mutation in the fourth transmembrane domain causes the channel to be overly sensitive to pharmacological activation.
Metabolites of paracetamol have been demonstrated to bind to the TRPA1 receptors, which may desensitize the receptors in the way capsaicin does 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. This may cause a characteristic side-effect of oxaliplatin.

Ligand binding

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.
Activation of the TRPA1 ion channel by the olive oil phenolic compound oleocanthal appears to be responsible for the pungent or "peppery" sensation in the back of the throat caused by olive oil.
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 CR gas|dibenzoxazepine derivative substituted by a carboxylic methylester at position 10 is the most potent TRPA1 agonist discovered to date. The pyrimidine PF-4840154 is a potent, non-covalent activator of both the human and rat 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 pathway of arachidonic acid metabolism, 12S-hydroperoxy-5Z,8Z,10E,14Z-eicosatetraenoic acid and the hepoxilins, HxA3 and HxB3 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 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 made by the metabolism of arachidonic acid by any one of several cytochrome P450 enzymes 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 EP3 G 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, 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 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.

TRPA1 inhibition

Resolvin D1 and RvD2 and maresin 1 are metabolites of the omega 3 fatty acid, docosahexaenoic acid. They are members of the specialized proresolving mediators 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 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.

Ligand examples

Agonists