The exceptionally high reactivity of Cys 621 is critical for electrophilic activation of the sensory nerve ion channel TRPA1

Ligustilide covalently binds to Cys703 in the pre-S1 helix of TRPA1, blocking the opening of channel and relieving pain in rats with acute soft tissue injury

ETHNOPHARMACOLOGICAL RELEVANCE

The natural anodyne Ligustilide (Lig), derived from Angelica sinensis (Oliv.) Diels and Ligusticum chuanxiong Hort., has been traditionally employed for its analgesic properties in the treatment of dysmenorrhea and migraine, and rheumatoid arthritis pain. Despite the existing reports on the correlation between TRP channels and the analgesic effects of Lig, a comprehensive understanding of their underlying mechanisms of action remains elusive.

AIM OF THE STUDY

The objective of this study is to elucidate the mechanism of action of Lig on the analgesic target TRPA1 channel.

METHODS

The therapeutic effect of Lig was evaluated in a rat acute soft tissue injury model. The analgesic target was identified through competitive inhibition of TRP channel agonists at the animal level, followed by Fluo-4/Ca2+ imaging on live cells overexpressing TRP proteins. The potential target was verified through in-gel imaging, colocalization using a Lig-derived molecular probe, and a drug affinity response target stability assay. The binding site of Lig was identified through protein spectrometry and further analyzed using molecular docking, site-specific mutation, and multidisciplinary approaches.

RESULTS

The administration of Lig effectively ameliorated pain and attenuated oxidative stress and inflammatory responses in rats with soft tissue injuries. Moreover, the analgesic effects of Lig were specifically attributed to TRPA1. Mechanistic studies have revealed that Lig directly activates TRPA1 by interacting with the linker domain in the pre-S1 region of TRPA1. Through metabolic transformation, 6,7-epoxyligustilide (EM-Lig) forms a covalent bond with Cys703 of TRPA1 at high concentrations and prolonged exposure time. This irreversible binding prevents endogenous electrophilic products from entering the cysteine active center of ligand-binding pocket of TRPA1, thereby inhibiting Ca2+ influx through the channel opening and ultimately relieving pain.

CONCLUSIONS

Lig selectively modulates the TRPA1 channel in a bimodal manner via non-electrophilic/electrophilic metabolic conversion. The epoxidized metabolic intermediate EM-Lig exerts analgesic effects by irreversibly inhibiting the activation of TRPA1 on sensory neurons. These findings not only highlight the analgesic mechanism of Lig but also offer a novel nucleophilic attack site for the development of TRPA1 antagonists in the pre-S1 region.

Development of covalent chemogenetic K2P channel activators

K2P potassium channels regulate excitability by affecting cellular resting membrane potential in the brain, cardiovascular system, immune cells, and sensory organs. Despite their important roles in anesthesia, arrhythmia, pain, hypertension, sleep, and migraine, the ability to control K2P function remains limited. Here, we describe a chemogenetic strategy termed CATKLAMP (covalent activation of TREK family K+ channels to clamp membrane potential) that leverages the discovery of a K2P modulator pocket site that reacts with electrophile-bearing derivatives of a TREK subfamily small-molecule activator, ML335, to activate the channel irreversibly. We show that CATKLAMP can be used to probe fundamental aspects of K2P function, as a switch to silence neuronal firing, and is applicable to all TREK subfamily members. Together, our findings exemplify a means to alter K2P channel activity that should facilitate molecular and systems level studies of K2P function and enable the search for new K2P modulators.

TRPA1 Covalent Ligand JT010 Modifies T Lymphocyte Activation

Transient Receptor Potential Ankyrin 1 (TRPA1) is a non-selective cation channel involved in sensitivity to a plethora of irritating agents and endogenous mediators of oxidative stress. TRPA1 influences neuroinflammation and macrophage and lymphocyte functions, but its role is controversial in immune cells. We reported earlier a detectable, but orders-of-magnitude-lower level of Trpa1 mRNA in monocytes and lymphocytes than in sensory neurons by qRT-PCR analyses of cells from lymphoid organs of mice. Our present goals were to (a) further elucidate the expression of Trpa1 mRNA in immune cells by RNAscope in situ hybridization (ISH) and (b) test the role of TRPA1 in lymphocyte activation. RNAscope ISH confirmed that Trpa1 transcripts were detectable in CD14+ and CD4+ cells from the peritoneal cavity of mice. A selective TRPA1 agonist JT010 elevated Ca2+ levels in these cells only at high concentrations. However, a concentration-dependent inhibitory effect of JT010 was observed on T-cell receptor (TcR)-induced Ca2+ signals in CD4+ T lymphocytes, while JT010 neither modified B cell activation nor ionomycin-stimulated Ca2+ level. Based on our present and past findings, TRPA1 activation negatively modulates T lymphocyte activation, but it does not appear to be a key regulator of TcR-stimulated calcium signaling.

Targeting TRP channels: recent advances in structure, ligand binding, and molecular mechanisms

Transient receptor potential (TRP) channels are a large and diverse family of transmembrane ion channels that are widely expressed, have important physiological roles, and are associated with many human diseases. These proteins are actively pursued as promising drug targets, benefitting greatly from advances in structural and mechanistic studies of TRP channels. At the same time, the complex, polymodal activation and regulation of TRP channels have presented formidable challenges. In this short review, we summarize recent progresses toward understanding the structural basis of TRP channel function, as well as potential ligand binding sites that could be targeted for therapeutics. A particular focus is on the current understanding of the molecular mechanisms of TRP channel activation and regulation, where many fundamental questions remain unanswered. We believe that a deeper understanding of the functional mechanisms of TRP channels will be critical and likely transformative toward developing successful therapeutic strategies targeting these exciting proteins. This endeavor will require concerted efforts from computation, structural biology, medicinal chemistry, electrophysiology, pharmacology, drug safety and clinical studies.

Human Transient Receptor Potential Ankyrin 1 Channel: Structure, Function, and Physiology

The transient receptor potential ion channel TRPA1 is a Ca2+-permeable nonselective cation channel widely expressed in sensory neurons, but also in many nonneuronal tissues typically possessing barrier functions, such as the skin, joint synoviocytes, cornea, and the respiratory and intestinal tracts. Here, the primary role of TRPA1 is to detect potential danger stimuli that may threaten the tissue homeostasis and the health of the organism. The ability to directly recognize signals of different modalities, including chemical irritants, extreme temperatures, or osmotic changes resides in the characteristic properties of the ion channel protein complex. Recent advances in cryo-electron microscopy have provided an important framework for understanding the molecular basis of TRPA1 function and have suggested novel directions in the search for its pharmacological regulation. This chapter summarizes the current knowledge of human TRPA1 from a structural and functional perspective and discusses the complex allosteric mechanisms of activation and modulation that play important roles under physiological or pathophysiological conditions. In this context, major challenges for future research on TRPA1 are outlined.

A Novel Flp Reporter Mouse Shows That TRPA1 Expression Is Largely Limited to Sensory Neuron Subsets

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal cation channel that is activated by electrophilic irritants, oxidative stress, cold temperature, and GPCR signaling. TRPA1 expression has been primarily identified in subsets of nociceptive sensory afferents and is considered a target for future analgesics. Nevertheless, TRPA1 has been implicated in other cell types including keratinocytes, epithelium, enterochromaffin cells, endothelium, astrocytes, and CNS neurons. Here, we developed a knock-in mouse that expresses the recombinase FlpO in TRPA1-expressing cells. We crossed the TRPA1Flp mouse with the R26ai65f mouse that expresses tdTomato in a Flp-sensitive manner. We found tdTomato expression correlated well with TRPA1 mRNA expression and sensitivity to TRPA1 agonists in subsets of TRPV1 (transient receptor potential vanilloid receptor type 1)-expressing neurons in the vagal ganglia and dorsal root ganglia (DRGs), although tdTomato expression efficiency was limited in DRG. We observed tdTomato-expressing afferent fibers centrally (in the medulla and spinal cord) and peripherally in the esophagus, gut, airways, bladder, and skin. Furthermore, chemogenetic activation of TRPA1-expressing nerves in the paw evoked flinching behavior. tdTomato expression was very limited in other cell types. We found tdTomato in subepithelial cells in the gut mucosa but not in enterochromaffin cells. tdTomato was also observed in supporting cells within the cochlea, but not in hair cells. Lastly, tdTomato was occasionally observed in neurons in the somatomotor cortex and the piriform area, but not in astrocytes or vascular endothelium. Thus, this novel mouse strain may be useful for mapping and manipulating TRPA1-expressing cells and deciphering the role of TRPA1 in physiological and pathophysiological processes.

Development of covalent chemogenetic K2P channel activators

K2P potassium channels regulate excitability by affecting cellular resting membrane potential in the brain, cardiovascular system, immune cells, and sensory organs. Despite their important roles in anesthesia, arrhythmia, pain, hypertension, sleep, and migraine, the ability to control K2P function remains limited. Here, we describe a chemogenetic strategy termed CATKLAMP (Covalent Activation of TREK family K+ channels to cLAmp Membrane Potential) that leverages the discovery of a site in the K2P modulator pocket that reacts with electrophile-bearing derivatives of a TREK subfamily small molecule activator, ML335, to activate the channel irreversibly. We show that the CATKLAMP strategy can be used to probe fundamental aspects of K2P function, as a switch to silence neuronal firing, and is applicable to all TREK subfamily members. Together, our findings exemplify a new means to alter K2P channel activity that should facilitate studies both molecular and systems level studies of K2P function and enable the search for new K2P modulators.

TRPA1-dependent and -independent activation by commonly used preservatives

Background and purpose: Addition of preservatives ensures microbial stability, especially in multidose containers of parenterally administered pharmaceuticals. These compounds can cause side effects, and particularly at the site of application, might elicit or facilitate pain. TRPA1 is a cation channel expressed in peripheral neurons which contributes to pain and inflammation and is sensitive to many irritants. The most commonly used preservatives, in particular with a focus on parenteral formulations, were investigated for their potential to activate TRPA1. Experimental approach: Sixteen preservatives were screened for mediating calcium influx in human TRPA1-transfected HEK293t cells. Untransfected cells served as control, results were further validated in mouse sensory neurons. In addition, proinflammatory mediators serotonin, histamine and prostaglandin E2 were co-administered to probe a potential sensitisation of preservative-induced TRPA1 activation. Key results: Butylparaben, propylparaben, ethylparaben, bronopol, methylparaben, phenylethyl alcohol and phenol induced a TRPA1-dependent calcium influx in transfected HEK293t cells at concentrations used for preservation. Other preservatives increased calcium within the used concentration ranges, but to a similar degree in untransfected controls. Serotonin, histamine, and prostaglandin enhanced TRPA1 activation of phenylethyl alcohol, bronopol, ethylparaben, propylparaben and butylparaben. Conclusion and implications: Systematic screening of common preservatives applied for parenterally administered drugs resulted in identifying several preservatives with substantial TRPA1 channel activation. This activation was enhanced by the addition of proinflammatory meditators. This allows selecting a preservative without TRPA1 activation, particularly in case of pharmaceuticals that could act proinflammatory.

Elucidation of the binding mode of organic polysulfides on the human TRPA1 receptor

Introduction: Previous studies have established that endogenous inorganic polysulfides have significant biological actions activating the Transient Receptor Potential Ankyrin 1 (TRPA1) receptor. Organic polysulfides exert similar effects, but they are much more stable molecules, therefore these compounds are more suitable as drugs. In this study, we aimed to better understand the mechanism of action of organic polysulfides by identification of their binding site on the TRPA1 receptor. Methods: Polysulfides can readily interact with the thiol side chain of the cysteine residues of the protein. To investigate their role in the TRPA1 activation, we replaced several cysteine residues by alanine via site-directed mutagenesis. We searched for TRPA1 mutant variants with decreased or lost activating effect of the polysulfides, but with other functions remaining intact (such as the effects of non-electrophilic agonists and antagonists). The binding properties of the mutant receptors were analyzed by in silico molecular docking. Functional changes were tested by in vitro methods: calcium sensitive fluorescent flow cytometry, whole-cell patch-clamp and radioactive calcium-45 liquid scintillation counting. Results: The cysteines forming the conventional binding site of electrophilic agonists, namely C621, C641 and C665 also bind the organic polysulfides, with the key role of C621. However, only their combined mutation abolished completely the organic polysulfide-induced activation of the receptor. Discussion: Since previous papers provided evidence that organic polysulfides exert analgesic and anti-inflammatory actions in different in vivo animal models, we anticipate that the development of TRPA1-targeted, organic polysulfide-based drugs will be promoted by this identification of the binding site.

Transient Receptor Potential Ankyrin 1 (TRPA1) Channel as a Sensor of Oxidative Stress in Cancer Cells

Moderate levels of reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂), fuel tumor metastasis and invasion in a variety of cancer types. Conversely, excessive ROS levels can impair tumor growth and metastasis by triggering cancer cell death. In order to cope with the oxidative stress imposed by the tumor microenvironment, malignant cells exploit a sophisticated network of antioxidant defense mechanisms. Targeting the antioxidant capacity of cancer cells or enhancing their sensitivity to ROS-dependent cell death represent a promising strategy for alternative anticancer treatments. Transient Receptor Potential Ankyrin 1 (TRPA1) is a redox-sensitive non-selective cation channel that mediates extracellular Ca²⁺ entry upon an increase in intracellular ROS levels. The ensuing increase in intracellular Ca²⁺ concentration can in turn engage a non-canonical antioxidant defense program or induce mitochondrial Ca²⁺ dysfunction and apoptotic cell death depending on the cancer type. Herein, we sought to describe the opposing effects of ROS-dependent TRPA1 activation on cancer cell fate and propose the pharmacological manipulation of TRPA1 as an alternative therapeutic strategy to enhance cancer cell sensitivity to oxidative stress.

Bipartite Activation of Sensory Neurons by a TRPA1 Agonist Allyl Isothiocyanate Is Reflected by Complex Ca2+ Influx and CGRP Release Patterns: Enhancement by NGF and Inhibition with VAMP and SNAP-25 Cleaving Botulinum Neurotoxins

The trafficking of transient receptor potential (TRP) channels to the plasma membrane and the release of calcitonin gene-related peptide (CGRP) from trigeminal ganglion neurons (TGNs) are implicated in some aspects of chronic migraines. These exocytotic processes are inhibited by cleavage of SNAREs with botulinum neurotoxins (BoNTs); moreover, type A toxin (/A) clinically reduces the frequency and severity of migraine attacks but not in all patients for unknown reasons. Herein, neonatal rat TGNs were stimulated with allyl isothiocyanate (AITC), a TRPA1 agonist, and dose relationships were established to link the resultant exocytosis of CGRP with Ca2+ influx. The CGRP release, quantified by ELISA, was best fit by a two-site model (EC50 of 6 and 93 µM) that correlates with elevations in intracellular Ca2+ [Ca2+]i revealed by time-lapse confocal microscopy of fluo-4-acetoxymethyl ester (Fluo-4 AM) loaded cells. These signals were all blocked by two TRPA1 antagonists, HC-030031 and A967079. At low [AITC], [Ca2+]i was limited because of desensitisation to the agonist but rose for concentrations > 0.1 mM due to a deduced non-desensitising second phase of Ca2+ influx. A recombinant BoNT chimera (/DA), which cleaves VAMP1/2/3, inhibited AITC-elicited CGRP release to a greater extent than SNAP-25-cleaving BoNT/A. /DA also proved more efficacious against CGRP efflux evoked by a TRPV1 agonist, capsaicin. Nerve growth factor (NGF), a pain-inducing sensitiser of TGNs, enhanced the CGRP exocytosis induced by low [AITC] only. Both toxins blocked NGF-induced neuropeptide secretion and its enhancement of the response to AITC. In conclusion, NGF sensitisation of sensory neurons involves TRPA1, elevated Ca2+ influx, and CGRP exocytosis, mediated by VAMP1/2/3 and SNAP-25 which can be attenuated by the BoNTs.

CYP1B1-derived epoxides modulate the TRPA1 channel in chronic pain

Pain is often debilitating, and current treatments are neither universally efficacious nor without risks. Transient receptor potential (TRP) ion channels offer alternative targets for pain relief, but little is known about the regulation or identities of endogenous TRP ligands that affect inflammation and pain. Here, transcriptomic and targeted lipidomic analysis of damaged tissue from the mouse spinal nerve ligation (SNL)-induced chronic pain model revealed a time-dependent increase in Cyp1b1 mRNA and a concurrent accumulation of 8,9-epoxyeicosatrienoic acid (EET) and 19,20-EpDPA post injury. Production of 8,9-EET and 19,20-EpDPA by human/mouse CYP1B1 was confirmed in vitro, and 8,9-EET and 19,20-EpDPA selectively and dose-dependently sensitized and activated TRPA1 in overexpressing HEK-293 cells and Trpa1-expressing/AITC-responsive cultured mouse peptidergic dorsal root ganglia (DRG) neurons. TRPA1 activation by 8,9-EET and 19,20-EpDPA was attenuated by the antagonist A967079, and mouse TRPA1 was more responsive to 8,9-EET and 19,20-EpDPA than human TRPA1. This latter effect mapped to residues Y933, G939, and S921 of TRPA1. Intra-plantar injection of 19,20-EpDPA induced acute mechanical, but not thermal hypersensitivity in mice, which was also blocked by A967079. Similarly, Cyp1b1-knockout mice displayed a reduced chronic pain phenotype following SNL injury. These data suggest that manipulation of the CYP1B1-oxylipin-TRPA1 axis might have therapeutic benefit.

Isolation of human TRPA1 channel from transfected HEK293 cells and identification of alkylation sites after sulfur mustard exposure

Transient receptor potential (TRP) channels are important in the sensing of pain and other stimuli. They may be triggered by electrophilic agonists after covalent modification of certain cysteine residues. Sulfur mustard (SM) is a banned chemical warfare agent and its reactivity is also based on an electrophilic intermediate. The activation of human TRP ankyrin 1 (hTRPA1) channels by SM has already been documented, however, the mechanism of action is not known in detail. The aim of this work was to purify hTRPA1 channel from overexpressing HEK293 cells for identification of SM-induced alkylation sites. To confirm hTRPA1 isolation, Western blot analysis was performed showing a characteristic double band at 125 kDa. Immunomagnetic separation was carried out using either an anti-His-tag or an anti-hTRPA1 antibody to isolate hTRPA1 from lysates of transfected HEK293 cells. The identity of the channel was confirmed by micro liquid chromatography-electrospray ionization high-resolution tandem-mass spectrometry. Following SM exposure, hTRPA1 channel modifications were found at Cys⁴⁶² and Cys⁶⁶⁵, as well as at Asp³³⁹ and Glu³⁴¹ described herein for the first time. Since Cys⁶⁶⁵ is a well-known target of hTRPA1 agonists and is involved in hTRPA1 activation, SM-induced modifications of cysteine, as well as aspartic acid and glutamic acid residues may play a role in hTRPA1 activation. Considering hTRPA1 as a target of other SM-related chemical warfare agents, analogous adducts may be predicted and identified applying the analytical approach described herein.

Potent Activation of Human but Not Mouse TRPA1 by JT010

Transient receptor potential (TRP) ankyrin repeat 1 (TRPA1), which is involved in inflammatory pain sensation, is activated by endogenous factors, such as intracellular Zn²⁺ and hydrogen peroxide, and by irritant chemical compounds. The synthetic compound JT010 potently and selectively activates human TRPA1 (hTRPA1) among the TRPs. Therefore, JT010 is a useful tool for analyzing TRPA1 functions in biological systems. Here, we show that JT010 is a potent activator of hTRPA1, but not mouse TRPA1 (mTRPA1) in human embryonic kidney (HEK) cells expressing hTRPA1 and mTRPA1. Application of 0.3-100 nM of JT010 to HEK cells with hTRPA1 induced large Ca²⁺ responses. However, in HEK cells with mTRPA1, the response was small. In contrast, both TRPA1s were effectively activated by allyl isothiocyanate (AITC) at 10-100 μM. Similar selective activation of hTRPA1 by JT010 was observed in electrophysiological experiments. Additionally, JT010 activated TRPA1 in human fibroblast-like synoviocytes with inflammation, but not TRPA1 in mouse dorsal root ganglion cells. As cysteine at 621 (C621) of hTRPA1, a critical cysteine for interaction with JT010, is conserved in mTRPA1, we applied JT010 to HEK cells with mutations in mTRPA1, where the different residue of mTRPA1 with tyrosine at 60 (Y60), with histidine at 1023 (H1023), and with asparagine at 1027 (N1027) were substituted with cysteine in hTRPA1. However, these mutants showed low sensitivity to JT010. In contrast, the mutation of hTRPA1 at position 669 from phenylalanine to methionine (F669M), comprising methionine at 670 in mTRPA1 (M670), significantly reduced the response to JT010. Moreover, the double mutant at S669 and M670 of mTRPA1 to S669E and M670F, respectively, induced slight but substantial sensitivity to 30 and 100 nM JT010. Taken together, our findings demonstrate that JT010 potently and selectively activates hTRPA1 but not mTRPA1.

The human TRPA1 intrinsic cold and heat sensitivity involves separate channel structures beyond the N-ARD domain

TRP channels sense temperatures ranging from noxious cold to noxious heat. Whether specialized TRP thermosensor modules exist and how they control channel pore gating is unknown. We studied purified human TRPA1 (hTRPA1) truncated proteins to gain insight into the temperature gating of hTRPA1. In patch-clamp bilayer recordings, ∆1-688 hTRPA1, without the N-terminal ankyrin repeat domain (N-ARD), was more sensitive to cold and heat, whereas ∆1-854 hTRPA1, also lacking the S1-S4 voltage sensing-like domain (VSLD), gained sensitivity to cold but lost its heat sensitivity. In hTRPA1 intrinsic tryptophan fluorescence studies, cold and heat evoked rearrangement of VSLD and the C-terminus domain distal to the transmembrane pore domain S5-S6 (CTD). In whole-cell electrophysiology experiments, replacement of the CTD located cysteines 1021 and 1025 with alanine modulated hTRPA1 cold responses. It is proposed that hTRPA1 CTD harbors cold and heat sensitive domains allosterically coupled to the S5-S6 pore region and the VSLD, respectively.

Drimane Sesquiterpene Aldehydes Control Candida Yeast Isolated from Candidemia in Chilean Patients

Drimys winteri J.R. (Winteraceae) produce drimane sesquiterpenoids with activity against Candida yeast. In this work, drimenol, polygodial (1), isotadeonal (2), and a new drimane α,β-unsaturated 1,4-dialdehyde, named winterdial (4), were purified from barks of D. winteri. The oxidation of drimenol produced the monoaldehyde drimenal (3). These four aldehyde sesquiterpenoids were evaluated against six Candida species isolated from candidemia patients in Chilean hospitals. Results showed that 1 displays fungistatic activity against all yeasts (3.75 to 15.0 µg/mL), but irritant effects on eyes and skin, whereas its non-pungent epimer 2 has fungistatic and fungicide activities at 1.9 and 15.0 µg/mL, respectively. On the other hand, compounds 3 and 4 were less active. Molecular dynamics simulations suggested that compounds 1–4 are capable of binding to the catalytic pocket of lanosterol 14-alpha demethylase with similar binding free energies, thus suggesting a potential mechanism of action through the inhibition of ergosterol synthesis. According to our findings, compound 2 appears as a valuable molecular scaffold to pursue the future development of more potent drugs against candidiasis with fewer side effects than polygodial. These outcomes are significant to broaden the alternatives to treat fungal infections with increasing prevalence worldwide using natural compounds as a primary source for active compounds.

Sensory TRP Channels in Three Dimensions

Transient receptor potential (TRP) ion channels are sophisticated signaling machines that detect a wide variety of environmental and physiological signals. Every cell in the body expresses one or more members of the extended TRP channel family, which consists of over 30 subtypes, each likely possessing distinct pharmacological, biophysical, and/or structural attributes. While the function of some TRP subtypes remains enigmatic, those involved in sensory signaling are perhaps best characterized and have served as models for understanding how these excitatory ion channels serve as polymodal signal integrators. With the recent resolution revolution in cryo-electron microscopy, these and other TRP channel subtypes are now yielding their secrets to detailed atomic analysis, which is beginning to reveal structural underpinnings of stimulus detection and gating, ion permeation, and allosteric mechanisms governing signal integration. These insights are providing a framework for designing and evaluating modality-specific pharmacological agents for treating sensory and other TRP channel-associated disorders.

Anticancer Activity of Natural and Semi-Synthetic Drimane and Coloratane Sesquiterpenoids

Drimane and coloratane sesquiterpenoids are present in several plants, microorganisms, and marine life. Because of their cytotoxic activity, these sesquiterpenoids have received increasing attention as a source for new anticancer drugs and pharmacophores. Natural drimanes and coloratanes, as well as their semi-synthetic derivatives, showed promising results against cancer cell lines with in vitro activities in the low micro- and nanomolar range. Despite their high potential as novel anticancer agents, the mode of action and structure–activity relationships of drimanes and coloratanes have not been completely enlightened nor systematically reviewed. Our review aims to give an overview of known structures and derivatizations of this class of sesquiterpenoids, as well as their activity against cancer cells and potential modes-of-action. The cytotoxic activities of about 40 natural and 25 semi-synthetic drimanes and coloratanes are discussed. In addition to that, we give a summary about the clinical significance of drimane and coloratane sesquiterpenoids.

Atomistic mechanisms of human TRPA1 activation by electrophile irritants through molecular dynamics simulation and mutual information analysis

The ion channel TRPA1 is a promiscuous chemosensor, with reported response to a wide spectrum of noxious electrophilic irritants, as well as cold, heat, and mechanosensation. It is also implicated in the inception of itch and pain and has hence been investigated as a drug target for novel analgesics. The mechanism of electrophilic activation for TRPA1 is therefore of broad interest. TRPA1 structures with the pore in both open and closed states have recently been published as well as covalent binding modes for electrophile agonists. However, the detailed mechanism of coupling between electrophile binding sites and the pore remains speculative. In addition, while two different cysteine residues (C621 and C665) have been identified as critical for electrophile bonding and activation, the bound geometry has only been resolved at C621. Here, we use molecular dynamics simulations of TRPA1 in both pore-open and pore-closed states to explore the allosteric link between the electrophile binding sites and pore stability. Our simulations reveal that an open pore is structurally stable in the presence of open ‘pockets’ in the C621/C665 region, but rapidly collapses and closes when these pockets are shut. Binding of electrophiles at either C621 or C665 provides stabilisation of the pore-open state, but molecules bound at C665 are shown to be able to rotate in and out of the pocket, allowing for immediate stabilisation of transient open states. Finally, mutual information analysis of trajectories reveals an informational path linking the electrophile binding site pocket to the pore via the voltage-sensing-like domain, giving a detailed insight into the how the pore is stabilized in the open state.

Polygodial, a drimane sesquiterpenoid dialdehyde purified from Drimys winteri, inhibits voltage-gated sodium channels

Drimys winteri J.R.Forst. & G.Forst, a South American evergreen shrub that is used by the Mapuche people for treatment of several painful conditions, contains polygodial, a lipophilic drimane-type sesquiterpene dialdehyde with known activity at transient receptor potential channel family members including TRPA1 and TRPV1. We sought to assess the activity of polygodial at Na_V1.7 and Na_V1.8, two key isoforms of the voltage-gated sodium channel family involved in nociception. Polygodial was isolated from D. winteri by thin-layer chromatography and analysed structurally by 1 D and 2 D nuclear magnetic resonance (NMR) spectroscopy. Activity at heterologously expressed Na_V1.7 and Na_V1.8 was assessed using automated whole-cell patch-clamp electrophysiology. Here, we show that polygodial inhibits members of the voltage-gated sodium channel family, specifically Na_V1.7 and Na_V1.8, without changing the voltage-dependence of activation or inactivation. Activity of polygodial at voltage-gated sodium channels may contribute to the previously reported antinociceptive properties.

TRPA1: Pharmacology, natural activators and role in obesity prevention

Transient receptor potential ankyrin 1 (TRPA1) channel is a calcium permeable, non-selective cation channel, expressed in the sensory neurons and non-neuronal cells of different tissues. Initially studied for its role in pain and inflammation, TRPA1 has now functionally involved in multiple other physiological functions. TRPA1 channel has been extensively studied for modulation by pungent compounds present in the spices and herbs. In the last decade, the role of TRPA1 agonism in body weight reduction, secretion of hunger and satiety hormones, insulin secretion and thermogenesis, has unveiled the potential of the TRPA1 channel to be used as a preventive target to tackle obesity and associated comorbidities including insulin resistance in type 2 diabetes. In this review, we summarized the recent findings of TRPA1 based dietary/non-dietary modulation for its role in obesity prevention and therapeutics.

Mini-review: The nociceptive sensory functions of the polymodal receptor Transient Receptor Potential Ankyrin Type 1 (TRPA1)

Over the last 17 years since its cloning in 2003, the receptor-channel TRPA1 has received increasing attention due to its polymodal features and prominent role in pain signaling in a variety of human disease states. While evidence has been accumulating for non-neuronal TRPA1 expression, it is the presence of this channel in nociceptive nerve endings which has taken centre stage, due to its potential clinical ramifications. As a consequence, we shall focus in this review on the sensory functions of TRPA1 related to its expression in the peripheral nervous system. While substantial research has been focused on the putative role of TRPA1 in detecting irritant compounds, noxious cold and mechanical stimuli, the current overall picture is, to some extent, still cloudy. The chemosensory function of the channel is well demonstrated, as well as its involvement in the detection of oxidative and nitrosative stress; however, the other sensory features of TRPA1 have not been fully elucidated yet. The current state of the experimental evidence for these physiological roles of TRPA1 in mammals, and particularly in humans, will be discussed in this review.

Functional evidence of distinct electrophile-induced activation states of the ion channel TRPA1

Transient Receptor Potential Ankyrin 1 (TRPA1) is a tetrameric, nonselective cation channel expressed on nociceptive sensory nerves whose activation elicits nocifensive responses (e.g. pain). TRPA1 is activated by electrophiles found in foods and pollution, or produced during inflammation and oxidative stress, via covalent modification of reactive cysteines, but the mechanism underlying electrophilic activation of TRPA1 is poorly understood. Here we studied TRPA1 activation by the irreversible electrophiles iodoacetamide and N-ethylmaleimide (NEM) following transient expression in HEK293 cells. We found that in Ca2+ imaging studies C621 is critical for electrophile-induced TRPA1 activation, but the role of C665 in TRPA1 activation is dependent on the size of the electrophile. We identified slower TRPA1 activation in whole-cell recordings compared to studies with intact cells, which is rescued by pipette solution supplementation with the antioxidant glutathione. Single-channel recordings identified two distinct electrophilic-induced TRPA1 activation phases: a partial activation that, in some channels, switched to full activation with continued electrophile exposure. Full activation but not the initial activation was regulated by C665. Fitting of open time distributions suggests that full activation correlated with an additional (and long) exponential component, thus suggesting the phases are manifestations of distinct activation states. Our results suggest that distinct NEM-induced TRPA1 activation states are evoked by sequential modification of C621 then C665.

Transient Receptor Potential Channel Ankyrin 1: A Unique Regulator of Vascular Function

TRPA1 (transient receptor potential ankyrin 1), the lone member of the mammalian ankyrin TRP subfamily, is a Ca²⁺-permeable, non-selective cation channel. TRPA1 channels are localized to the plasma membranes of various cells types, including sensory neurons and vascular endothelial cells. The channel is endogenously activated by byproducts of reactive oxygen species, such as 4-hydroxy-2-noneal, as well as aromatic, dietary molecules including allyl isothiocyanate, a derivative of mustard oil. Several studies have implicated TRPA1 as a regulator of vascular tone that acts through distinct mechanisms. First, TRPA1 on adventitial sensory nerve fibers mediates neurogenic vasodilation by stimulating the release of the vasodilator, calcitonin gene-related peptide. Second, TRPA1 is expressed in the endothelium of the cerebral vasculature, but not in other vascular beds, and its activation results in localized Ca²⁺ signals that drive endothelium-dependent vasodilation. Finally, TRPA1 is functionally present on brain capillary endothelial cells, where its activation orchestrates a unique biphasic propagation mechanism that dilates upstream arterioles. This response is vital for neurovascular coupling and functional hyperemia in the brain. This review provides a brief overview of the biophysical and pharmacological properties of TRPA1 and discusses the importance of the channel in vascular control and pathophysiology.

Aberrant Complement System Activation in Neurological Disorders

The complement system is an assembly of proteins that collectively participate in the functions of the healthy and diseased brain. The complement system plays an important role in the maintenance of uninjured (healthy) brain homeostasis, contributing to the clearance of invading pathogens and apoptotic cells, and limiting the inflammatory immune response. However, overactivation or underregulation of the entire complement cascade within the brain may lead to neuronal damage and disturbances in brain function. During the last decade, there has been a growing interest in the role that this cascading pathway plays in the neuropathology of a diverse array of brain disorders (e.g., acute neurotraumatic insult, chronic neurodegenerative diseases, and psychiatric disturbances) in which interruption of neuronal homeostasis triggers complement activation. Dysfunction of the complement promotes a disease-specific response that may have either beneficial or detrimental effects. Despite recent advances, the explicit link between complement component regulation and brain disorders remains unclear. Therefore, a comprehensible understanding of such relationships at different stages of diseases could provide new insight into potential therapeutic targets to ameliorate or slow progression of currently intractable disorders in the nervous system. Hence, the aim of this review is to provide a summary of the literature on the emerging role of the complement system in certain brain disorders.

Differential sensitivity of cinnamaldehyde-evoked calcium fluxes to ruthenium red in guinea pig and mouse trigeminal sensory neurons

Objective

Transient receptor potential ankyrin 1 (TRPA1) is an excitatory ion channel expressed on a subset of sensory neurons. TRPA1 is activated by a host of noxious stimuli including pollutants, irritants, oxidative stress and inflammation, and is thought to play an important role in nociception and pain perception. TRPA1 is therefore a therapeutic target for diseases with nociceptive sensory signaling components. TRPA1 orthologs have been shown to have differential sensitivity to certain ligands. Cinnamaldehyde has previously been shown to activate sensory neurons via the selective gating of TRPA1. Here, we tested the sensitivity of cinnamaldehyde-evoked responses in mouse and guinea pig sensory neurons to the pore blocker ruthenium red (RuR).

Results

Cinnamaldehyde, the canonical TRPA1-selective agonist, caused robust calcium fluxes in trigeminal neurons dissociated from both mice and guinea pigs. RuR effectively inhibited cinnamaldehyde-evoked responses in mouse neurons at 30 nM, with complete block seen with 3 μM. In contrast, responses in guinea pig neurons were only partially inhibited by 3 μM RuR. We conclude that RuR has a decreased affinity for guinea pig TRPA1 compared to mouse TRPA1. This study provides further evidence of differences in ligand affinity for TRPA1 in animal models relevant for drug development.

Air Pollution-Induced Autonomic Modulation

Air pollutants pose a serious worldwide health hazard, causing respiratory and cardiovascular morbidity and mortality. Pollutants perturb the autonomic nervous system, whose function is critical to cardiopulmonary homeostasis. Recent studies suggest that pollutants can stimulate defensive sensory nerves within the cardiopulmonary system, thus providing a possible mechanism for pollutant-induced autonomic dysfunction. A better understanding of the mechanisms involved would likely improve the management and treatment of pollution-related disease.

Electrophile-Induced Conformational Switch of the Human TRPA1 Ion Channel Detected by Mass Spectrometry

The human Transient Receptor Potential A1 (hTRPA1) ion channel, also known as the wasabi receptor, acts as a biosensor of various potentially harmful stimuli. It is activated by a wide range of chemicals, including the electrophilic compound N-methylmaleimide (NMM), but the mechanism of activation is not fully understood. Here, we used mass spectrometry to map and quantify the covalent labeling in hTRPA1 at three different concentrations of NMM. A functional truncated version of hTRPA1 (Δ1-688 hTRPA1), lacking the large N-terminal ankyrin repeat domain (ARD), was also assessed in the same way. In the full length hTRPA1, the labeling of different cysteines ranged from nil up to 95% already at the lowest concentration of NMM, suggesting large differences in reactivity of the thiols. Most important, the labeling of some cysteine residues increased while others decreased with the concentration of NMM, both in the full length and the truncated protein. These findings indicate a conformational switch of the proteins, possibly associated with activation or desensitization of the ion channel. In addition, several lysines in the transmembrane domain and the proximal N-terminal region were labeled by NMM, raising the possibility that lysines are also key targets for electrophilic activation of hTRPA1.

Irritant-evoked activation and calcium modulation of the TRPA1 receptor

The TRPA1 ion channel is expressed by primary afferent nerve fibers, where it functions as a low threshold sensor for structurally diverse electrophilic irritants ranging from small volatile environmental toxicants to endogenous algogenic lipids¹. TRPA1 is also a ‘receptor-operated’ channel whose activation downstream of metabotropic receptors elicits inflammatory pain or itch, making it an attractive target for novel analgesic therapies². However, we lack mechanistic insight into how TRPA1 recognizes and responds to electrophiles or cytoplasmic second messengers. Here, we show that electrophiles act through a two-step process in which modification of a highly reactive cysteine (C621) promotes reorientation of a cytoplasmic loop to enhance nucleophilicity and modification of a nearby cysteine (C665), thereby stabilizing the loop in an activating configuration. These actions modulate two restrictions controlling ion permeation, including widening of the selectivity filter to enhance calcium permeability and opening of a canonical gate at the cytoplasmic end of the pore. We propose a model to explain functional coupling between electrophile action and these control points. We also characterize a calcium binding pocket that is remarkably conserved across TRP channel subtypes and accounts for all aspects of calcium-dependent TRPA1 regulation, including potentiation, desensitization, and activation by metabotropic receptors. These findings provide a structural framework for understanding how a broad-spectrum irritant receptor is controlled by endogenous and exogenous agents that elicit or exacerbate pain and itch.

Structural Insights into Electrophile Irritant Sensing by the Human TRPA1 Channel

Transient receptor potential channel subfamily A member 1 (TRPA1) is a Ca2+-permeable cation channel that serves as one of the primary sensors of environmental irritants and noxious substances. Many TRPA1 agonists are electrophiles that are recognized by TRPA1 via covalent bond modifications of specific cysteine residues located in the cytoplasmic domains. However, a mechanistic understanding of electrophile sensing by TRPA1 has been limited due to a lack of high-resolution structural information. Here, we present the cryoelectron microscopy (cryo-EM) structures of nanodisc-reconstituted ligand-free TRPA1 and TRPA1 in complex with the covalent agonists JT010 and BITC at 2.8, 2.9, and 3.1 Å, respectively. Our structural and functional studies provide the molecular basis for electrophile recognition by the extraordinarily reactive C621 in TRPA1 and mechanistic insights into electrophile-dependent conformational changes in TRPA1. This work also provides a platform for future drug development targeting TRPA1.

Regulation of vascular tone by transient receptor potential ankyrin 1 channels

The Ca²⁺-permeable, non-selective cation channel, TRPA1 (transient receptor potential ankyrin 1), is the sole member of the ankyrin TRP subfamily. TRPA1 channels are expressed on the plasma membrane of neurons as well as non-neuronal cell types, such as vascular endothelial cells. TRPA1 is activated by electrophilic compounds, including dietary molecules such as allyl isothiocyanate, a derivative of mustard. Endogenously, the channel is thought to be activated by reactive oxygen species and their metabolites, such as 4-hydroxynonenal (4-HNE). In the context of the vasculature, activation of TRPA1 channels results in a vasodilatory response mediated by two distinct mechanisms. In the first instance, TRPA1 is expressed in sensory nerves of the vasculature and, upon activation, mediates release of the potent dilator, calcitonin gene-related peptide (CGRP). In the second, work from our laboratory has demonstrated that TRPA1 is expressed in the endothelium of blood vessels exclusively in the cerebral vasculature, where its activation produces a localized Ca²⁺ signal that results in dilation of cerebral arteries. In this chapter, we provide an in-depth overview of the biophysical and pharmacological properties of TRPA1 channels and their importance in regulating vascular tone.

Semi-synthetic cinnamodial analogues: Structural insights into the insecticidal and antifeedant activities of drimane sesquiterpenes against the mosquito Aedes aegypti

The Aedes aegypti mosquito serves as a major vector for viral diseases, such as dengue, chikungunya, and Zika, which are spreading across the globe and threatening public health. In addition to increased vector transmission, the prevalence of insecticide-resistant mosquitoes is also on the rise, thus solidifying the need for new, safe and effective insecticides to control mosquito populations. We recently discovered that cinnamodial, a unique drimane sesquiterpene dialdehyde of the Malagasy medicinal plant Cinnamosma fragrans, exhibited significant larval and adult toxicity to Ae. aegypti and was more efficacious than DEET-the gold standard for insect repellents-at repelling adult female Ae. aegypti from blood feeding. In this study several semi-synthetic analogues of cinnamodial were prepared to probe the structure-activity relationship (SAR) for larvicidal, adulticidal and antifeedant activity against Ae. aegypti. Initial efforts were focused on modification of the dialdehyde functionality to produce more stable active analogues and to understand the importance of the 1,4-dialdehyde and the α,ß-unsaturated carbonyl in the observed bioactivity of cinnamodial against mosquitoes. This study represents the first investigation into the SAR of cinnamodial as an insecticide and antifeedant against the medically important Ae. aegypti mosquito.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

TRPA1 and issues relating to animal model selection for extrapolating toxicity data to humans

The transient receptor potential ankyrin 1 (TRPA1) ion channel is a sensor for irritant chemicals, has ancient lineage, and is distributed across animal species including humans, where it features in many organs. Its activation by a diverse panel of electrophilic molecules (TRPA1 agonists) through electrostatic binding and/or covalent attachment to the protein causes the sensation of pain. This article reviews the species differences between TRPA1 channels and their responses, to assess the suitability of different animals to model the effects of TRPA1-activating electrophiles in humans, referring to common TRPA1 activators (exogenous and endogenous) and possible mechanisms of action relating to their toxicology. It concludes that close matching of in vitro and in vivo models will help optimise the identification of relevant biochemical and physiological responses to benchmark the efficacy of potential therapeutic drugs, including TRPA1 antagonists, to counter the toxic effects of those electrophiles capable of harming humans. The analysis of the species issue provided should aid the development of medical treatments to counter poisoning by such chemicals.

Non-Analgesic Symptomatic or Disease-Modifying Potential of TRPA1

TRPA1, a versatile ion channel of the Transient Receptor Potential (TRP) channel family, detects a large variety of chemicals and can contribute to signal processing of other stimuli, e.g., due to its sensitivity to cytosolic calcium elevation or phosphoinositolphosphate modulation. At first, TRPA1 was found on sensory neurons, where it can act as a sensor for potential or actual tissue damage that ultimately may elicit pain or itch as warning symptoms. This review provides an update regarding the analgesic and antipruritic potential of TRPA1 modulation and the respective clinical trials. Furthermore, TRPA1 has been found in an increasing amount of other cell types. Therefore, the main focus of the review is to discuss the non-analgesic and particularly the disease-modifying potential of TRPA1. This includes diseases of the respiratory system, cancer, ischemia, allergy, diabetes, and the gastrointestinal system. The involvement of TRPA1 in the respective pathophysiological cascades is so far mainly based on pre-clinical data.

Noncanonical Ion Channel Behaviour in Pain

Ion channels contribute fundamental properties to cell membranes. Although highly diverse in conductivity, structure, location, and function, many of them can be regulated by common mechanisms, such as voltage or (de-)phosphorylation. Primarily considering ion channels involved in the nociceptive system, this review covers more novel and less known features. Accordingly, we outline noncanonical operation of voltage-gated sodium, potassium, transient receptor potential (TRP), and hyperpolarization-activated cyclic nucleotide (HCN)-gated channels. Noncanonical features discussed include properties as a memory for prior voltage and chemical exposure, alternative ion conduction pathways, cluster formation, and silent subunits. Complementary to this main focus, the intention is also to transfer knowledge between fields, which become inevitably more separate due to their size.

A Cell-Penetrating Scorpion Toxin Enables Mode-Specific Modulation of TRPA1 and Pain

TRPA1 is a chemosensory ion channel that functions as a sentinel for structurally diverse electrophilic irritants. Channel activation occurs through an unusual mechanism involving covalent modification of cysteine residues clustered within an amino-terminal cytoplasmic domain. Here, we describe a peptidergic scorpion toxin (WaTx) that activates TRPA1 by penetrating the plasma membrane to access the same intracellular site modified by reactive electrophiles. WaTx stabilizes TRPA1 in a biophysically distinct active state characterized by prolonged channel openings and low Ca²⁺ permeability. Consequently, WaTx elicits acute pain and pain hypersensitivity but fails to trigger efferent release of neuropeptides and neurogenic inflammation typically produced by noxious electrophiles. These findings provide a striking example of convergent evolution whereby chemically disparate animal- and plant-derived irritants target the same key allosteric regulatory site to differentially modulate channel activity. WaTx is a unique pharmacological probe for dissecting TRPA1 function and its contribution to acute and persistent pain.

Antimycin A-induced mitochondrial dysfunction activates vagal sensory neurons via ROS-dependent activation of TRPA1 and ROS-independent activation of TRPV1

Inflammation causes activation of nociceptive sensory nerves, resulting in debilitating sensations and reflexes. Inflammation also induces mitochondrial dysfunction through multiple mechanisms. Sensory nerve terminals are densely packed with mitochondria, suggesting that mitochondrial signaling may play a role in inflammation-induced nociception. We have previously shown that agents that induce mitochondrial dysfunction, such as antimycin A, activate a subset of nociceptive vagal sensory nerves that express transient receptor potential (TRP) channels ankyrin 1 (A1) and vanilloid 1 (V1). However, the mechanisms underlying these responses are incompletely understood. Here, we studied the contribution of TRPA1, TRPV1 and reactive oxygen species (ROS) to antimycin A-induced vagal sensory nerve activation in dissociated neurons and at the sensory terminals of bronchopulmonary C-fibers. Nociceptive neurons were defined chemically and genetically. Antimycin A-evoked activation of vagal nociceptors in a Fura2 Ca²⁺ assay correlated with TRPV1 responses compared to TRPA1 responses. Nociceptor activation was dependent on both TRP channels, with TRPV1 predominating in a majority of responding nociceptors and TRPA1 predominating only in nociceptors with the greatest responses. Surprisingly, both TRPA1 and TRPV1 were activated by H₂O₂ when expressed in HEK293. Nevertheless, targeting ROS had no effect of antimycin A-evoked TRPV1 activation in either HEK293 or vagal neurons. In contrast, targeting ROS inhibited antimycin A-evoked TRPA1 activation in HEK293, vagal neurons and bronchopulmonary C-fibers, and a ROS-insensitive TRPA1 mutant was completely insensitive to antimycin A. We therefore conclude that mitochondrial dysfunction activates vagal nociceptors by ROS-dependent (TRPA1) and ROS-independent (TRPV1) mechanisms.

A Human TRPA1-Specific Pain Model

The cation channel transient receptor potential ankyrin 1 (TRPA1) plays an important role in sensing potentially hazardous substances. However, TRPA1 species differences are substantial and limit translational research. TRPA1 agonists tested previously in humans also have other targets. Therefore, the sensation generated by isolated TRPA1 activation in humans is unknown. The availability of 2-chloro-N-(4-(4-methoxyphenyl)thiazol-2-yl)-N-(3-methoxypropyl)-acetamide (JT010), a potent and specific TRPA1 agonist, allowed us to explore this issue. To corroborate the specificity of JT010, it was investigated whether the TRPA1 antagonist (1E,3E)-1-(4-fluorophenyl)-2-methyl-1-penten-3-one oxime (A-967079) abolishes JT010-elicited pain. Sixteen healthy volunteers of both sexes rated pain due to intraepidermal injections of different concentrations and combinations of the substances. The study design was a double-blind crossover study. All subjects received all types of injections, including a placebo without substances. Injections of the TRPA1 agonist dose-dependently caused pain with a half-maximal effective concentration of 0.31 μm Coinjection of A-967079 dose-dependently reduced and at a high concentration abolished JT010-induced pain. Quantification of JT010 by HPLC showed that a substantial part is adsorbed when in contact with polypropylene surfaces, but that this was overcome by handling in glass vials and injection using glass syringes. Isolated TRPA1 activation in humans causes pain. Thus, intradermal JT010 injection can serve as a tool to validate new TRPA1 antagonists concerning target engagement. More importantly, TRPA1-specific tools allow quantification of the TRPA1-dependent component in physiology and pathophysiology.SIGNIFICANCE STATEMENT This study showed that activation of the ion channel transient receptor potential ankyrin 1 (TRPA1) alone indeed suffices to elicit pain in humans, independent of other receptors previously found to be involved in pain generation. The newly established TRPA1-specific pain model allows different applications. First, it can be tested whether diseases are associated with compromised or exaggerated TRPA1-dependent painful sensations in the skin. Second, it can be investigated whether a new, possibly systemically applied drug directed against TRPA1 engages its target in humans. Further, the general possibility of quantitative inhibition of TRPA1 allows identification of the TRPA1-dependent disease component, given that the substance reaches its target. This contributes to a better understanding of pathophysiology, can lay the basis for new therapeutic approaches, and can bridge the gap between preclinical research and clinical trials.

Identification of molecular targets for toxic action by persulfate, an industrial sulfur compound

Persulfate salts are broadly used as industrial chemicals and exposure to them causes occupational asthma, occupational rhinitis and contact dermatitis. However, the mechanisms underlying these toxic actions are not fully elucidated. Transient receptor potential (TRP) vanilloid 1 (V1), ankyrin 1 (A1) and melastatin 8 (M8) are non-selective cation channels preferentially expressing sensory neurons. These channels are known to be involved in respiratory and skin diseases. In the present study, we investigated the effects of sodium persulfate on these TRP channels. In wild-type mouse sensory neurons, persulfate evoked [Ca²⁺]_i increases that were inhibited by removal of extracellular Ca²⁺ or blockers of TRPA1 but not by those of TRPV1 and TRPM8. Persulfate failed to evoke [Ca²⁺]_i responses in neurons from TRPA1(-/-) mice, but did evoke them in neurons from TRPV1(-/-) mice. In HEK 293 cells expressing mouse TRPA1 (mTRPA1-HEK), persulfate induced [Ca²⁺]_i increases. Moreover, in HEK 293 cells expressing mouse TRPV1 (mTRPV1-HEK), a high concentration of persulfate also evoked [Ca²⁺]_i increases. Similar [Ca²⁺]_i responses were observed in HEK 293 cells expressing human TRPA1 and human TRPV1. Current responses were also elicited by persulfate in mTRPA1- and mTRPV1-HEK. Analysis using mutated channels revealed that persulfate acted on electrophilic agonist-sensitive cysteine residues of TRPA1, and it indirectly activated TRPV1 due to the external acidification, because of the disappearance of [Ca²⁺]_i responses in acid-insensitive mTRPV1 mutant. These results demonstrate that persulfate activates nociceptive TRPA1 and TRPV1 channels. It is suggested that activation of these nociceptive channels may be involved in respiratory and skin injuries caused by exposure to this industrial sulfur compound. Thus, selective TRPA1 and TRPV1 channel blockers may be effective to remedy persulfate-induced toxic actions.

TRPA1: a molecular view

The transient receptor potential ankyrin 1 (TRPA1) ion channel is expressed in pain-sensing neurons and other tissues and has become a major target in the development of novel pharmaceuticals. A remarkable feature of the channel is its long list of activators, many of which we are exposed to in daily life. Many of these agonists induce pain and inflammation, making TRPA1 a major target for anti-inflammatory and analgesic therapies. Studies in human patients and in experimental animals have confirmed an important role for TRPA1 in a number of pain conditions. Over the recent years, much progress has been made in elucidating the molecular structure of TRPA1 and in discovering binding sites and modulatory sites of the channel. Because the list of published mutations and important molecular sites is steadily growing and because it has become difficult to see the forest for the trees, this review aims at summarizing the current knowledge about TRPA1, with a special focus on the molecular structure and the known binding or gating sites of the channel.

Improving redox sensitivity of roGFP1 by incorporation of selenocysteine at position 147

Objective

Redox-sensitive green fluorescent protein (roGFP) is a genetically-encoded redox-sensitive protein used to detect cellular oxidative stress associated with reactive oxygen species production. Here we replaced the cysteine at position 147 of roGFP1 (variant of roGFP) with selenocysteine in order to increase redox sensitivity of the redox reporter.

Results

Expression of roGFP1 selenoprotein (roGFP1-Se147) in HEK293 cells required the presence of a selenocysteine insertion sequence and was augmented by co-expression with SBP2. roGFP1-Se147 demonstrated a similar excitation and emission spectra to roGFP1. Although expression of roGFP1-Se147 was limited, it was sufficient enough to perform live cell imaging to evaluate sensitivity to oxidation and reduction. roGFP1-Se147 exhibited a 100-fold increase in sensitivity to oxidation with H₂O₂ in comparison to roGFP1 as well as a 20-fold decrease in the EC₅₀ of H₂O₂. Furthermore, roGFP1-Se147, unlike roGFP1, was able to detect oxidation caused by the mitochondrial electron transport complex III inhibitor antimycin A. Unfortunately roGFP-Se147 exhibited a diminished dynamic range and photoinstability.

Electronic supplementary material

The online version of this article (10.1186/s13104-018-3929-x) contains supplementary material, which is available to authorized users.

Activation Stoichiometry and Pore Architecture of TRPA1 Probed with Channel Concatemers

The nociceptor ion channel TRPA1 detects a wide range of hazardous chemicals, including reactive electrophiles such as cinnamaldehyde, which gate the channel allowing Na⁺ and Ca²⁺ entry. TRPA1 assembles as a tetramer, with a central pore within which an aspartate residue (D918) determines Ca²⁺ permeability. Here, we report that introduction of histidine at this position, D918H, makes TRPA1 channels sensitive to block by nanomolar concentration of Zn²⁺ and can be used to functionally tag subunits in concatemers. Concatemers with increasing numbers of D918H subunits display increasing sensitivity to Zn²⁺ inhibition, indicating that the four side chains at position 918 of the tetramer directly coordinate Zn²⁺ and other permeating divalent cations. In the published structure of TRPA1, this requires a rearrangement of the pore region which may represent the true open state of the channel. Concatemeric channels containing subunits mutated to be insensitive to reactive electrophiles (C622S) could be activated by cinnamaldehyde when as few as two subunits contained intact ligand binding sites. Activation upon liganding of just two of the four possible subunits may represent an optimal strategy to rapidly and reliably detect noxious chemicals.

Molecular basis of TRPA1 regulation in nociceptive neurons. A review

Transient receptor potential A1 (TRPA1) is an excitatory ion channel that functions as a cellular sensor, detecting a wide range of proalgesic agents such as environmental irritants and endogenous products of inflammation and oxidative stress. Topical application of TRPA1 agonists produces an acute nociceptive response through peripheral release of neuropeptides, purines and other transmitters from activated sensory nerve endings. This, in turn, further regulates TRPA1 activity downstream of G-protein and phospholipase C-coupled signaling cascades. Despite the important physiological relevance of such regulation leading to nociceptor sensitization and consequent pain hypersensitivity, the specific domains through which TRPA1 undergoes post-translational modifications that affect its activation properties are yet to be determined at a molecular level. This review aims at providing an account of our current knowledge on molecular basis of regulation by neuronal inflammatory signaling pathways that converge on the TRPA1 channel protein and through modification of its specific residues influence the extent to which this channel may contribute to pain.

An environmental pollutant, 9,10-phenanthrenequinone, activates human TRPA1 via critical cysteines 621 and 665

Transient receptor potential ankyrin 1 (TRPA1) is activated by noxious cold, mechanical stimulation, and irritant chemicals. In our recent study, 9, 10‐phenanthrenequinone (9,10‐PQ) is the most potent irritant for activation of NRF2 among 1395 cigarette smoke components and it may be, therefore, important to find its additional targets. Here, we show that 9,10‐PQ functions as an activator of TRPA1 in human embryonic kidney (HEK) cells expressing human wild‐type TRPA1 (HEK‐wTRPA1) and human alveolar A549 (A549) cells. Application of 9,10‐PQ at 0.1–10 μmol/L induced a concentration‐dependent Ca²⁺ response as well as inward currents at −50 mV in HEK‐wTRPA1 cells. The current response was blocked by TRPA1 antagonists, HC‐030031 (HC) and A‐967079. To test whether 9,10‐PQ affects the cysteine residues of TRPA1, we expressed mutant TRPA1 channels in HEK cells (HEK‐muTRPA1) in which six different cysteine residues were replaced with serine. Among them, a mutation of cysteine 621 (C621S) abolished the 9,10‐PQ‐induced Ca²⁺ and current responses. The channel activity induced by 9,10‐PQ was also abolished in excised inside‐out patches isolated from HEK‐muTRPA1 cells with the C621S substitution. Although a mutation of cysteine 665 (C665S) reduced the 9,10‐PQ‐induced response, channel sensitization by pretreatment with Cu²⁺ plus 1,10‐phenanthroline and by internal dialysis of 3 μmol/L Ca²⁺ restored the response. However, a double mutant with C621S and C665S substitutions had little response to 9,10‐PQ, even when sensitized by Ca²⁺ dialysis. In A549 cells, 9,10‐PQ induced an HC‐sensitive Ca²⁺ response. Our findings demonstrate that 9,10‐PQ activation of human TRA1 is dependent on cysteine residues 621 and 665.

Structure-Pungency Relationships and TRP Channel Activation of Drimane Sesquiterpenes in Tasmanian Pepper (Tasmannia lanceolata)

Sensory-guided fractionation of extracts of Tasmanian pepper berries revealed 20 drimane sesquiterpens, among which polygodial, warburganal, and 1β-acetoxy-9-deoxy-isomuzigadial exhibited the lowest pungency threshold concentrations on the tongue surface (0.6-2.8 nmol/cm²) and elicited a dose-dependent calcium influx into mTRPA1 expressing CHO cells with the lowest EC₅₀ values (4.5 ± 1.0 to 16.7 ± 7.5 μmol/L) and a good correlation to oral pungency thresholds (R² = 0.986, linear regression). Calcium imaging assays demonstrated these chemosensates to induce a calcium influx into cultured trigeminal neurons prepared from wildtype (TRPA1^+/+) mice, whereas no calcium influx was observed in neurons from TRPA1 knockout mice (TRPA1^-/-), thus confirming the α,β-unsaturated 1,4-dialdehyde structure to be the required structural motif for a low oral puncency thresholds and activation of the Transient Receptor Potential Channel A1 (TRPA1). Time-resolved NMR experiments confirmed the pungency mediating mechanism for electrophilic drimane sesquiterpene dialdehydes to be different from that found for other electrophilic pungent agents like isothiocyanates, which have been shown to undergo a covalent binding with cysteine residues in TRPA1. Instead, the high-impact chemosensates polygodial, warburganal, and 1β-acetoxy-9-deoxy-isomuzigadial showed immediate reactivity with the ε-amino group of lysine side chains to give pyrrole-type conjugates, thus showing evidence for TRPA1 activation by covalent lysine modification.