TRPA1

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.

Streptozotocin Stimulates the Ion Channel TRPA1 Directly: INVOLVEMENT OF PEROXYNITRITE

Streptozotocin (STZ)-induced diabetes is the most commonly used animal model of diabetes. Here, we have demonstrated that intraplantar injections of low dose STZ evoked acute polymodal hypersensitivities in mice. These hypersensitivities were inhibited by a TRPA1 antagonist and were absent in TRPA1-null mice. In wild type mice, systemic STZ treatment (180 mg/kg) evoked a loss of cold and mechanical sensitivity within an hour of injection, which lasted for at least 10 days. In contrast, Trpa1(-/-) mice developed mechanical, cold, and heat hypersensitivity 24 h after STZ. The TRPA1-dependent sensory loss produced by STZ occurs before the onset of diabetes and may thus not be readily distinguished from the similar sensory abnormalities produced by the ensuing diabetic neuropathy. In vitro, STZ activated TRPA1 in isolated sensory neurons, TRPA1 cell lines, and membrane patches. Mass spectrometry studies revealed that STZ oxidizes TRPA1 cysteines to disulfides and sulfenic acids. Furthermore, incubation of tyrosine with STZ resulted in formation of dityrosine, suggesting formation of peroxynitrite. Functional analysis of TRPA1 mutants showed that cysteine residues that were oxidized by STZ were important for TRPA1 responsiveness to STZ. Our results have identified oxidation of TRPA1 cysteine residues, most likely by peroxynitrite, as a novel mechanism of action of STZ. Direct stimulation of TRPA1 complicates the interpretation of results from STZ models of diabetic sensory neuropathy and strongly argues that more refined models of diabetic neuropathy should replace the use of STZ.

Species-specific temperature sensitivity of TRPA1

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal ion channel sensitive to temperature and chemical stimuli. The importance of temperature and aversive chemical detection for survival has driven the evolutionary diversity of TRPA1 sensitivity. This diversity can be observed in the various roles of TRPA1 in different species, where it is proposed to act as a temperature-insensitive chemosensor, a heat transducer, a noxious cold transducer, or a detector of low-intensity heat for prey localization. Exploring the variation of TRPA1 functions among species provides evolutionary insight into molecular mechanisms that fine-tune thermal and chemical sensitivity, and offers an opportunity to address basic principles of temperature gating in ion channels. A decade of research has yielded a number of hypotheses describing physiological roles of TRPA1, modulators of its activity, and biophysical principles of gating. This review surveys the diversity of TRPA1 adaptations across evolutionary taxa and explores possible mechanisms of TRPA1 activation.

Urate crystal induced inflammation and joint pain are reduced in transient receptor potential ankyrin 1 deficient mice--potential role for transient receptor potential ankyrin 1 in gout

Introduction

In gout, monosodium urate (MSU) crystals deposit intra-articularly and cause painful arthritis. In the present study we tested the hypothesis that Transient Receptor Poten-tial Ankyrin 1 (TRPA1), an ion channel mediating nociceptive signals and neurogenic in-flammation, is involved in MSU crystal-induced responses in gout by utilizing three experi-mental murine models.

Methods

The effects of selective pharmacological inhibition (by HC-030031) and genetic depletion of TRPA1 were studied in MSU crystal-induced inflammation and pain by using 1) spontaneous weight-bearing test to assess MSU crystal-induced joint pain, 2) subcutaneous air-pouch model resembling joint inflammation to measure MSU crystal-induced cytokine production and inflammatory cell accumulation, and 3) MSU crystal-induced paw edema to assess acute vascular inflammatory responses and swelling.

Results

Intra-articularly injected MSU crystals provoked spontaneous weight shift off from the affected limb in wild type but not in TRPA1 knock-out mice referring alleviated joint pain in TRPA1 deficient animals. MSU crystal-induced inflammatory cell infiltration and accumulation of cytokines MCP-1, IL-6, IL-1beta, MPO, MIP-1alpha and MIP-2 into subcu-taneous air-pouch (resembling joint cavity) was attenuated in TRPA1 deficient mice and in mice treated with the selective TRPA1 inhibitor HC-030031 as compared to control animals. Further, HC-030031 treated and TRPA1 deficient mice developed tempered inflammatory edema when MSU crystals were injected into the paw.

Conclusions

TRPA1 mediates MSU crystal-induced inflammation and pain in experimental models supporting the role of TRPA1 as a potential mediator and a drug target in gout flare.

ThermoTRPs and Pain

The ability of the body to perceive noxious stimuli lies in a heterogeneous group of primary somatosensory neurons termed nociceptors. The molecular receptors of noxious mechanical, temperature, or chemical stimuli are expressed in these neurons and have drawn considerable attention as possible targets for analgesic development to improve treatment for the millions who suffer from chronic pain conditions. A number of thermoTRPs, a subset of the transient receptor potential family of ion channels, are activated by a wide range on noxious stimuli. In this review, we review the function of these channels and examine the evidence that thermoTRPs play a vital role in acute, inflammatory and neuropathic nociception.

TRP Channels in Respiratory Pathophysiology: the Role of Oxidative, Chemical Irritant and Temperature Stimuli

There is rapidly growing evidence indicating multiple and important roles of Ca(2+)- permeable cation TRP channels in the airways, both under normal and disease conditions. The aim of this review was to summarize the current knowledge of TRP channels in sensing oxidative, chemical irritant and temperature stimuli by discussing expression and function of several TRP channels in relevant cell types within the respiratory tract, ranging from sensory neurons to airway smooth muscle and epithelial cells. Several of these channels, such as TRPM2, TRPM8, TRPA1 and TRPV1, are discussed in much detail to show that they perform diverse, and often overlapping or contributory, roles in airway hyperreactivity, inflammation, asthma, chronic obstructive pulmonary disease and other respiratory disorders. These include TRPM2 involvement in the disruption of the bronchial epithelial tight junctions during oxidative stress, important roles of TRPA1 and TRPV1 channels in airway inflammation, hyperresponsiveness, chronic cough, and hyperplasia of airway smooth muscles, as well as TRPM8 role in COPD and mucus hypersecretion. Thus, there is increasing evidence that TRP channels not only function as an integral part of the important endogenous protective mechanisms of the respiratory tract capable of detecting and ensuring proper physiological responses to various oxidative, chemical irritant and temperature stimuli, but that altered expression, activation and regulation of these channels may also contribute to the pathogenesis of respiratory diseases.

Acupuncture for visceral pain: neural substrates and potential mechanisms

Visceral pain is the most common form of pain caused by varied diseases and a major reason for patients to seek medical consultation. Despite much advances, the pathophysiological mechanism is still poorly understood comparing with its somatic counterpart and, as a result, the therapeutic efficacy is usually unsatisfactory. Acupuncture has long been used for the management of numerous disorders in particular pain and visceral pain, characterized by the high therapeutic benefits and low adverse effects. Previous findings suggest that acupuncture depresses pain via activation of a number of neurotransmitters or modulators including opioid peptides, serotonin, norepinephrine, and adenosine centrally and peripherally. It endows us, by advancing the understanding of the role of ion channels and gut microbiota in pain process, with novel perspectives to probe the mechanisms underlying acupuncture analgesia. In this review, after describing the visceral innervation and the relevant afferent pathways, in particular the ion channels in visceral nociception, we propose three principal mechanisms responsible for acupuncture induced benefits on visceral pain. Finally, potential topics are highlighted regarding the future studies in this field.

Targeting TRP channels for novel migraine therapeutics

Migraine is increasingly understood to be a disorder of the brain. In susceptible individuals, a variety of “triggers” may influence altered central excitability, resulting in the activation and sensitization of trigeminal nociceptive afferents surrounding blood vessels (i.e., the trigeminovascular system), leading to migraine pain. Transient receptor potential (TRP) channels are expressed in a subset of dural afferents, including those containing calcitonin gene related peptide (CGRP). Activation of TRP channels promotes excitation of nociceptive afferent fibers and potentially lead to pain. In addition to pain, allodynia to mechanical and cold stimuli can result from sensitization of both peripheral afferents and of central pain pathways. TRP channels respond to a variety of endogenous conditions including chemical mediators and low pH. These channels can be activated by exogenous stimuli including a wide range of chemical and environmental irritants, some of which have been demonstrated to trigger migraine in humans. Activation of TRP channels can elicit CGRP release, and blocking the effects of CGRP through receptor antagonism or antibody strategies has been demonstrated to be effective in the treatment of migraine. Identification of approaches that can prevent activation of TRP channels provides an additional novel strategy for discovery of migraine therapeutics.

Human TRPA1 is intrinsically cold- and chemosensitive with and without its N-terminal ankyrin repeat domain

We have purified and reconstituted human transient receptor potential (TRP) subtype A1 (hTRPA1) into lipid bilayers and recorded single-channel currents to understand its inherent thermo- and chemosensory properties as well as the role of the ankyrin repeat domain (ARD) of the N terminus in channel behavior. We report that hTRPA1 with and without its N-terminal ARD (Δ1-688 hTRPA1) is intrinsically cold-sensitive, and thus, cold-sensing properties of hTRPA1 reside outside the N-terminal ARD. We show activation of hTRPA1 by the thiol oxidant 2-((biotinoyl)amino)ethyl methanethiosulfonate (MTSEA-biotin) and that electrophilic compounds activate hTRPA1 in the presence and absence of the N-terminal ARD. The nonelectrophilic compounds menthol and the cannabinoid Δ(9)-tetrahydrocannabiorcol (C16) directly activate hTRPA1 at different sites independent of the N-terminal ARD. The TRPA1 antagonist HC030031 inhibited cold and chemical activation of hTRPA1 and Δ1-688 hTRPA1, supporting a direct interaction with hTRPA1 outside the N-terminal ARD. These findings show that hTRPA1 is an intrinsically cold- and chemosensitive ion channel. Thus, second messengers, including Ca(2+), or accessory proteins are not needed for hTRPA1 responses to cold or chemical activators. We suggest that conformational changes outside the N-terminal ARD by cold, electrophiles, and nonelectrophiles are important in hTRPA1 channel gating and that targeting chemical interaction sites outside the N-terminal ARD provides possibilities to fine tune TRPA1-based drug therapies (e.g., for treatment of pain associated with cold hypersensitivity and cardiovascular disease).

H2S and NO cooperatively regulate vascular tone by activating a neuroendocrine HNO-TRPA1-CGRP signalling pathway

Nitroxyl (HNO) is a redox sibling of nitric oxide (NO) that targets distinct signalling pathways with pharmacological endpoints of high significance in the treatment of heart failure. Beneficial HNO effects depend, in part, on its ability to release calcitonin gene-related peptide (CGRP) through an unidentified mechanism. Here we propose that HNO is generated as a result of the reaction of the two gasotransmitters NO and H₂S. We show that H₂S and NO production colocalizes with transient receptor potential channel A1 (TRPA1), and that HNO activates the sensory chemoreceptor channel TRPA1 via formation of amino-terminal disulphide bonds, which results in sustained calcium influx. As a consequence, CGRP is released, which induces local and systemic vasodilation. H₂S-evoked vasodilatatory effects largely depend on NO production and activation of HNO–TRPA1–CGRP pathway. We propose that this neuroendocrine HNO–TRPA1–CGRP signalling pathway constitutes an essential element for the control of vascular tone throughout the cardiovascular system.

Nitric oxide (NO) and hydrogen sulphide (H₂S) are two gaseous signalling molecules produced in tissues. Here the authors propose that NO and H₂S react with each other to form nitroxyl (HNO), which activates the TRPA1 channel in nerve cells and triggers the release of the vasoactive peptide CGRP.