Transient receptor potential cation channel, subfamily A, member 1, also known as transient receptor potential ankyrin 1, TRPA1, or The Mustard and Wasabi Receptor, is a protein that in humans is encoded by the TRPA1 (and in mice and rats 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 pain, cold and itch in humans and other mammals, as well as a sensor for environmental irritants giving rise to other protective responses (tears, airway resistance, and cough).
Structure In 2016, cryo-electron microscopy was employed to obtain a three-dimensional structure of TRPA1. This work revealed that the channel assembles as a homotetramer, and possesses several structural features that hint at its complex regulation by irritants, cytoplasmic second messengers (e.g., calcium), cellular co-factors (e.g., inorganic anions like polyphosphates), and lipids (e.g., PIP2). 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 two 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.
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. One of the specific functions of this protein involves a role in the detection, integration, and initiation of pain signals in the peripheral nervous system. It can be activated at sites of tissue injury or sites of inflammation directly by endogenous mediators or indirectly as a downstream target via signaling from a number of distinct G-protein coupled receptors (GPCRs), such as bradykinin. The role of TRPA1 in pain sensing was first revealed when TRPA1 was identified as the receptor for mustard oil (allyl isothiocyanate), the pungent ingredient in mustard and wasabi. Recent studies indicate that TRPA1 is activated by a number of reactive (cinnamaldehyde, farnesyl thiosalicylic acid, formalin, hydrogen peroxide, 4-hydroxynonenal, acrolein, and tear gases) and non-reactive compounds (nicotine, PF-4840154) and is thus 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 are considered important sensors of nociception in humans, and their activation will under normal conditions give rise to pain. Indeed, TRPA1 is considered 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 (complete Freund's adjuvant and formalin). Although it is not fully 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. TRPA1 also serves as an infrared detector in the antennae of beetle pollinators, enabling them to detect infrared radiation that cycad plants generate as a pollination cue.
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 (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, 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 (via reactive oxygen generation from mitochondria). This may cause a characteristic side-effect of oxaliplatin (cold-triggered acute peripheral neuropathy).
Ligands
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