Transient receptor potential channels in sensory neurons are targets of the antimycotic agent clotrimazole

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.

Targeting temperature-sensitive transient receptor potential channels in hypertension: far beyond the perception of hot and cold

Transient receptor potential (TRP) channels are nonselective cation channels and participate in various physiological roles. Thus, changes in TRP channel function or expression have been linked to several disorders. Among the many TRP channel subtypes, the TRP ankyrin type 1 (TRPA1), TRP melastatin type 8 (TRPM8), and TRP vanilloid type 1 (TRPV1) channels are temperature-sensitive and recognized as thermo-TRPs, which are expressed in the primary afferent nerve. Thermal stimuli are converted into neuronal activity. Several studies have described the expression of TRPA1, TRPM8, and TRPV1 in the cardiovascular system, where these channels can modulate physiological and pathological conditions, including hypertension. This review provides a complete understanding of the functional role of the opposing thermo-receptors TRPA1/TRPM8/TRPV1 in hypertension and a more comprehensive appreciation of TRPA1/TRPM8/TRPV1-dependent mechanisms involved in hypertension. These channels varied activation and inactivation have revealed a signaling pathway that may lead to innovative future treatment options for hypertension and correlated vascular diseases.

Clotrimazole-Based Modulators of the TRPM3 Ion Channel Reveal Narrow Structure-Activity Relationship

TRPM3 is an ion channel that is highly expressed in nociceptive neurons and plays a key role in pain perception. In the presence of the endogenous TRPM3 ligand, pregnenolone sulfate (PS), the antifungal compound clotrimazole (Clt) augments Ca²⁺ signaling and opens a non-canonical pore, permeable to Na⁺, which aggravates TRPM3-induced pain. To date, little is known about structural features that govern the Clt modulatory effect of TRPM3. Here, we synthesized and evaluated several Clt analogues in order to gain insights into their structure-activity relationship. Our results reveal a tight SAR with the three phenyl rings on the trityl moiety being essential for the activity, as well as the presence of fluorine or chlorine substituents on the trityl group. Imidazole as a heterocycle is also necessary for activity. Interestingly, we identified a pentafluoro-trityl analogue (29a) that is able to act as a TRPM3 agonist in the absence of PS. The compounds we report in this work will be useful tools for the further study of TRPM3 modulation and its effect on pain perception.

In Vitro and In Vivo Pharmacological Characterization of a Novel TRPM8 Inhibitor Chemotype Identified by Small-Scale Preclinical Screening

Transient receptor potential melastatin type 8 (TRPM8) is a target for the treatment of different physio-pathological processes. While TRPM8 antagonists are reported as potential drugs for pain, cancer, and inflammation, to date only a limited number of chemotypes have been investigated and thus a limited number of compounds have reached clinical trials. Hence there is high value in searching for new TRPM8 antagonistic to broaden clues to structure-activity relationships, improve pharmacological properties and explore underlying molecular mechanisms. To address this, the EDASA Scientific in-house molecular library has been screened in silico, leading to identifying twenty-one potentially antagonist compounds of TRPM8. Calcium fluorometric assays were used to validate the in-silico hypothesis and assess compound selectivity. Four compounds were identified as selective TRPM8 antagonists, of which two were dual-acting TRPM8/TRPV1 modulators. The most potent TRPM8 antagonists (BB 0322703 and BB 0322720) underwent molecular modelling studies to highlight key structural features responsible for drug–protein interaction. The two compounds were also investigated by patch-clamp assays, confirming low micromolar potencies. The most potent compound (BB 0322703, IC₅₀ 1.25 ± 0.26 μM) was then profiled in vivo in a cold allodinya model, showing pharmacological efficacy at 30 μM dose. The new chemotypes identified showed remarkable pharmacological properties paving the way to further investigations for drug discovery and pharmacological purposes.

The role of TRP ion channels in migraine and headache

Migraine afflicts more than 10% of the general population. Although its mechanism is poorly understood, recent preclinical and clinical evidence has identified calcitonin gene related peptide (CGRP) as a major mediator of migraine pain. CGRP, which is predominantly expressed in a subset of primary sensory neurons, including trigeminal afferents, when released from peripheral terminals of nociceptors, elicits arteriolar vasodilation and mechanical allodynia, a hallmark of migraine attack. Transient receptor potential (TRP) channels include several cationic channels with pleiotropic functions and ubiquitous distribution in various cells and tissues. Some members of the TRP channel family, such as the ankyrin 1 (TRPA1), vanilloid 1 and 4 (TRPV1 and TRPV4, respectively), and TRPM3, are abundantly expressed in primary sensory neurons and are recognized as sensors of chemical-, heat- and mechanical-induced pain, and play a primary role in several models of pain diseases, including inflammatory, neuropathic cancer pain, and migraine pain. In addition, TRP channel stimulation results in CGRP release, which can be activated or sensitized by various endogenous and exogenous stimuli, some of which have been proven to trigger or worsen migraine attacks. Moreover, some antimigraine medications seem to act through TRPA1 antagonism. Here we review the preclinical and clinical evidence that highlights the role of TRP channels, and mainly TRPA1, in migraine pathophysiology and may be proposed as new targets for its treatment.

Activation of Drosophila melanogaster TRPA1 Isoforms by Citronellal and Menthol

BACKGROUND

The transient receptor potential ankyrin 1 (TRPA1) cation channels function as broadly-tuned sensors of noxious chemicals in many species. Recent studies identified four functional TRPA1 isoforms in Drosophila melanogaster (dTRPA1(A) to (D)), but their responses to non-electrophilic chemicals are yet to be fully characterized.

METHODS

We determined the behavioral responses of adult flies to the mammalian TRPA1 non-electrophilic activators citronellal and menthol, and characterized the effects of these compounds on all four dTRPA1 channel isoforms using intracellular Ca²⁺ imaging and whole-cell patch-clamp recordings.

RESULTS

Wild type flies avoided citronellal and menthol in an olfactory test and this behavior was reduced in dTrpA1 mutant flies. Both compounds activate all dTRPA1 isoforms in the heterologous expression system HEK293T, with the following sensitivity series: dTRPA1(C) = dTRPA1(D) > dTRPA1(A) ≫ dTRPA1(B) for citronellal and dTRPA1(A) > dTRPA1(D) > dTRPA1(C) > dTRPA1(B) for menthol.

CONCLUSIONS

dTrpA1 was required for the normal avoidance of Drosophila melanogaster towards citronellal and menthol. All dTRPA1 isoforms are activated by both compounds, but the dTRPA1(B) is consistently the least sensitive. We discuss how these findings may guide further studies on the physiological roles and the structural bases of chemical sensitivity of TRPA1 channels.

TRPM2 Oxidation Activates Two Distinct Potassium Channels in Melanoma Cells through Intracellular Calcium Increase

Tumor microenvironments are often characterized by an increase in oxidative stress levels. We studied the response to oxidative stimulation in human primary (IGR39) or metastatic (IGR37) cell lines obtained from the same patient, performing patch-clamp recordings, intracellular calcium ([Ca²⁺]_i) imaging, and RT-qPCR gene expression analysis. In IGR39 cells, chloramine-T (Chl-T) activated large K⁺ currents (KROS) that were partially sensitive to tetraethylammonium (TEA). A large fraction of KROS was inhibited by paxilline—a specific inhibitor of large-conductance Ca²⁺-activated BK channels. The TEA-insensitive component was inhibited by senicapoc—a specific inhibitor of the Ca²⁺-activated KCa3.1 channel. Both BK and KCa3.1 activation were mediated by an increase in [Ca²⁺]_i induced by Chl-T. Both KROS and [Ca²⁺]_i increase were inhibited by ACA and clotrimazole—two different inhibitors of the calcium-permeable TRPM2 channel. Surprisingly, IGR37 cells did not exhibit current increase upon the application of Chl-T. Expression analysis confirmed that the genes encoding BK, KCa3.1, and TRPM2 are much more expressed in IGR39 than in IGR37. The potassium currents and [Ca²⁺]_i increase observed in response to the oxidizing agent strongly suggest that these three molecular entities play a major role in the progression of melanoma. Pharmacological targeting of either of these ion channels could be a new strategy to reduce the metastatic potential of melanoma cells, and could complement classical radio- or chemotherapeutic treatments.

High-throughput screening of TRPV1 ligands in the light of the Bioluminescence Resonance Energy Transfer technique

Ion channels are attractive drug targets for many therapeutic applications. However, high-throughput screening (HTS) of drug candidates is difficult and remains very expensive. We thus assessed the suitability of the Bioluminescence Resonance Energy Transfer (BRET) technique as a new HTS method for ion-channel studies by taking advantage of our recently characterized intra- and intermolecular BRET probes targeting the TRPV1 ion channel. These BRET probes monitor conformational changes during TRPV1 gating and subsequent coupling with Calmodulin, two molecular events that are intractable using reference techniques such as automated calcium assay (ACA) and automated patch-clamp (APC). We screened the small-sized Prestwick chemical library, encompassing 1200 compounds with high structural diversity, using either intra- and intermolecular BRET probes or ACA. Secondary screening of the detected hits was done using APC. Multiparametric analysis of our results shed light on the capability of calmodulin inhibitors included in the Prestwick library to inhibit TRPV1 activation by Capsaicin (CAPS). BRET was the lead technique for this identification process. Finally, we present data exemplifying the use of intramolecular BRET probes to study other TRPs and non-TRPs ion channels. Knowing the ease of use of BRET biosensors and the low cost of the BRET technique, these assays may advantageously be included for extending ion-channel drug screening. Significance Statement We screened a chemical library against TRPV1 ion channel using Bioluminescence Resonance Energy Transfer (BRET) molecular probes, and compared the results with the ones obtained using reference techniques such as automated calcium assay and automated patch-clamp. Multiparametric analysis of our results shed light on the capability of Calmodulin antagonists to inhibit chemical activation of TRPV1, and indicates that BRET probes may advantageously be included in ion channel drug screening campaigns.

The Role of TRPA1 in Skin Physiology and Pathology

The transient receptor potential ankyrin 1 (TRPA1), a member of the TRP superfamily of channels, acts as 'polymodal cellular sensor' on primary sensory neurons where it mediates the peripheral and central processing of pain, itch, and thermal sensation. However, the TRPA1 expression extends far beyond the sensory nerves. In recent years, much attention has been paid to its expression and function in non-neuronal cell types including skin cells, such as keratinocytes, melanocytes, mast cells, dendritic cells, and endothelial cells. TRPA1 seems critically involved in a series of physiological skin functions, including formation and maintenance of physico-chemical skin barriers, skin cells, and tissue growth and differentiation. TRPA1 appears to be implicated in mechanistic processes in various immunological inflammatory diseases and cancers of the skin, such as atopic and allergic contact dermatitis, psoriasis, bullous pemphigoid, cutaneous T-cell lymphoma, and melanoma. Here, we report recent findings on the implication of TRPA1 in skin physiology and pathophysiology. The potential use of TRPA1 antagonists in the treatment of inflammatory and immunological skin disorders will be also addressed.

Non-electrophilic TRPA1 agonists, menthol, carvacrol and clotrimazole, open epithelial tight junctions via TRPA1 activation

Activation of the transient receptor potential A1 channel (TRPA1) by electrophilic agonists was reported to induce the opening of tight junctions (TJs). Because compounds that increase TJ permeability can be paracellular permeability enhancers, we investigated the effect of non-electrophilic TRPA1 activators, including food ingredients (menthol and carvacrol) and medication (clotrimazole), on epithelial permeability. We show that all three compounds induced increase of the permeability of fluorescein isothiocyanate-conjugated dextran (4 kDa) and decrease of transepithelial electrical resistance, accompanied by Ca2+ influx and cofilin activation in epithelial MDCK II monolayers. These phenotypes were attenuated by pretreatment of a TRPA1 antagonist, suggesting TRPA1-mediated opening of TJs. These results suggest that non-electrophilic TRPA1 activators with established safety can be utilized to regulate epithelial barriers.

A KCa3.1 Channel Opener, ASP0819, Modulates Nociceptive Signal Processing from Peripheral Nerves in Fibromyalgia-Like Pain in Rats

Purpose

Although abnormal peripheral and central pain processing has been observed in fibromyalgia (FM) patients, the biomechanics and pathophysiology, surrounding the peripheral mechanism are not well understood. An intermediate conductance channel, K_Ca3.1, is expressed in peripheral sensory nerve fibers where it maintains the resting membrane potential and controls nerve firing, making it a plausible target for peripheral therapeutic interventions. ASP0819, a K_Ca3.1 channel opener, is an orally available molecular entity and is used in this investigation to elucidate the role of K_Ca3.1 in signal processing of pain in FM.

Methods

Human or rat K_Ca3.1 channel-expressing cells were used for evaluating the main action of the compound. Effects of the compound on withdrawal behavior by mechanical stimulation were examined in reserpine-induced myalgia (RIM) and vagotomy-induced myalgia (VIM) models of rats. In addition, in vivo electrophysiological analysis was performed to examine the peripheral mechanisms of action of the compound. Other pain models were also examined.

Results

ASP0819 increased the negative membrane potential in a concentration-dependent manner. Oral administration of ASP0819 significantly recovered the decrease in muscle pressure threshold in rat FM models of RIM and VIM. The in vivo electrophysiological experiments showed that Aδ- and C-fibers innervating the leg muscles in the RIM model demonstrated increased spontaneous and mechanically evoked firing compared with normal rats. Intravenous infusion of ASP0819 significantly reduced both the spontaneous activity and mechanically evoked responses in Aδ-fibers in the rat RIM model. ASP0819 significantly reduced the number of abdominal contractions as an indicator of abdominal pain behaviors in the rat visceral extension model and withdrawal responses in the osteoarthritis model, respectively.

Conclusion

These findings suggest that ASP0819 may be a promising analgesic agent with the ability to modulate peripheral pain signal transmission. Its use in the treatment of several pain conditions should be explored, chief amongst these being FM pain.

Multicomponent crystals of clotrimazole: a combined theoretical and experimental study

Compared with clotrimazole, some multicomponent crystals showed an improvement in solubility and dissolution rate.

(Endo)Cannabinoids and Gynaecological Cancers

Gynaecological cancers can be primary neoplasms, originating either from the reproductive tract or the products of conception, or secondary neoplasms, representative of metastatic disease. For some of these cancers, the exact causes are unknown; however, it is recognised that the precise aetiopathogeneses for most are multifactorial and include exogenous (such as diet) and endogenous factors (such as genetic predisposition), which mutually interact in a complex manner. One factor that has been recognised to be involved in the pathogenesis and progression of gynaecological cancers is the endocannabinoid system (ECS). The ECS consists of endocannabinoids (bioactive lipids), their receptors, and metabolic enzymes responsible for their synthesis and degradation. In this review, the impact of plant-derived (Cannabis species) cannabinoids and endocannabinoids on gynaecological cancers will be discussed within the context of the complexity of the proteins that bind, transport, and metabolise these compounds in reproductive and other tissues. In particular, the potential of endocannabinoids, their receptors, and metabolic enzymes as biomarkers of specific cancers, such as those of the endometrium, will be addressed. Additionally, the therapeutic potential of targeting selected elements of the ECS as new action points for the development of innovative drugs will be presented.

Pharmacological and genetic inhibition of TRPC6-induced gene transcription

Transient receptor potential canonical-6 (TRPC6) channels are non-selective cation channels that can be activated by hyperforin, a constituent of Hypericum perforatum. TRPC6 activation has been linked to a variety of biological functions and pathologies, including focal segmental glomerulosclerosis and the development of various tumor entities. Thus, TRPC6 is an interesting drug target, and a specific pharmacological inhibitor would be very valuable for both basic research and therapy of TRPC6-mediated human pathologies. Here, we assessed the biological activity of various TRP channel inhibitors on hyperforin-stimulated TRPC6 channel signaling. Hyperforin stimulates the activity of the transcription factor AP-1 via TRPC6. Expression experiments involving a TRPC6-specific small hairpin RNA confirmed that hyperforin-induced gene transcription requires TRPC6. Cellular AP-1 activity was measured to assess which compound interrupted the TRPC6-induced intracellular signaling cascade. The results show that the compounds 2-APB, clotrimazole, BCTC, TC-I 2014, SAR 7334, and larixyl acetate blocked TRPC6-mediated activation of AP-1. In contrast, the TRPM8-specific inhibitor RQ-00203078 did not inhibit TRPC6-mediated signaling. 2-APB, clotrimazole, BCTC, and TC-I 2014 are broad-spectrum Ca²⁺ channel inhibitors, while SAR 7334 and larixyl acetate have been proposed to function as rather TRPC6-specific inhibitors. In this study it is shown that both compounds, in addition to inhibiting TRPC6-induced signaling, completely abolished pregnenolone sulfate-mediated signaling via TRPM3 channels. Thus, SAR 7334 and larixyl acetate are not TRPC6-specific inhibitors.

Medicinal chemistry perspective of TRPM2 channel inhibitors: where we are and where we might be heading?

Transient receptor potential melastatin 2 (TRPM2) is a Ca²⁺- permeable nonselective cation channel that is involved in diverse biological functions as a cellular sensor for oxidative stress and temperature. It has been considered a promising therapeutic target for the treatment of ischemia/reperfusion (IR) injury, inflammation, cancer, and neurodegenerative diseases. Development of highly potent and selective TRPM2 inhibitors and validation of their use in relevant disease models will advance drug discovery. In this review, we describe the molecular structures and gating mechanism of the TRPM2 channel, and offer a comprehensive review of advances in the discovery of TRPM2 inhibitors. Furthermore, we analyze the properties of reported TRPM2 inhibitors with an emphasis on how specific inhibitors targeting this channel could be better developed.

TRPA1 as a therapeutic target for nociceptive pain

Introduction

Chronic pain affects approximatively 30–50% of the population globally. Pathologies such as migraine, diabetic neuropathy, nerve injury and treatment with chemotherapeutic agents, can induce chronic pain. Members of the transient receptor potential (TRP) channels, including the TRP ankyrin 1 (TRPA1), have a major role in pain.

Areas covered

We focus on TRPA1 as a therapeutic target for pain relief. The structure, localization, and activation of the channel and its implication in different pathways to signal pain are described. This paper underlines the role of pharmacological interventions on TRPA1 to reduce pain in numerous pain conditions. We conducted a literature search in PubMed up to and including July 2020.

Expert opinion

Our understanding of the molecular mechanisms underlying the sensitization of central and peripheral nociceptive pathways is limited. Preclinical evidence indicates that, in murine models of pain diseases, numerous mechanisms converge on the pathway that encompasses oxidative stress and Schwann cell TRPA1 to sustain chronic pain. Programs to identify and develop treatments to attenuate TRPA1-mediated chronic pain have emerged from this knowledge. Antagonists explored as a novel class of analgesics have a new and promising target in the TRPA1 expressed by peripheral glial cells.

Pharmacological inhibition of TRPM8-induced gene transcription

Transient receptor potential melastatin-8 (TRPM8) channels are activated by cold temperature, menthol and icilin, leading to cold sensation. TRPM8 activation is connected with various diseases, indicating that a specific pharmacological antagonist, allowing nongenetic channel suppression, would be a valuable tool for therapy and basic research. Here, we assessed the biological activity and specificity of various TRPM8 inhibitors following stimulation of TRPM8 channels with either icilin or menthol. Recently, we showed that icilin strikingly upregulates the transcriptional activity of AP-1. By measuring AP-1 activity, we assessed which compound interrupted the TRPM8-induced intracellular signaling cascade from the plasma membrane to the nucleus. We tested the specificity of various TRPM8 inhibitors by analyzing AP-1 activation following stimulation of TRPM3 and TRPV1 channels, L-type voltage-gated Ca²⁺ channels, and Gαq-coupled receptors, either in the presence or absence of a particular TRPM8 inhibitor. The results show that the TRPM8 inhibitors BCTC, RQ-00203078, TC-1 2014, 2-APB, and clotrimazole blocked TRPM8-mediated activation of AP-1. However, only the compound RQ-00203078 showed TRPM8-specificity, while the other compounds function as broad-spectrum Ca²⁺ channel inhibitors. In addition, we show that progesterone interfered with TRPM8-induced activation of AP-1, as previously shown for TRPM3 and TRPC6 channels. TRPM8-induced transcriptional activation of AP-1 was additionally blocked by the compound PD98059, indicating that extracellular signal-regulated protein kinase-1/2 is essential to couple TRPM8 stimulation with transcriptional activation of AP-1. Moreover, an influx of Ca²⁺-ions is essential to induce the intracellular signaling cascade leading to the activation of AP-1.

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.

TRPs in Tox: Involvement of Transient Receptor Potential-Channels in Chemical-Induced Organ Toxicity-A Structured Review

Chemicals can exhibit significant toxic properties. While for most compounds, unspecific cell damaging processes are assumed, a plethora of chemicals exhibit characteristic odors, suggesting a more specific interaction with the human body. During the last few years, G-protein-coupled receptors and especially chemosensory ion channels of the transient receptor potential family (TRP channels) were identified as defined targets for several chemicals. In some cases, TRP channels were suggested as being causal for toxicity. Therefore, these channels have moved into the spotlight of toxicological research. In this review, we screened available literature in PubMed that deals with the role of chemical-sensing TRP channels in specific organ systems. TRPA1, TRPM and TRPV channels were identified as essential chemosensors in the nervous system, the upper and lower airways, colon, pancreas, bladder, skin, the cardiovascular system, and the eyes. Regarding TRP channel subtypes, A1, M8, and V1 were found most frequently associated with toxicity. They are followed by V4, while other TRP channels (C1, C4, M5) are only less abundantly expressed in this context. Moreover, TRPA1, M8, V1 are co-expressed in most organs. This review summarizes organ-specific toxicological roles of TRP channels.

Redox regulation of transient receptor potential channels in the endothelium

ROS and RNS are important mediators of signaling pathways in the endothelium. Specific members of the TRP superfamily of cation channels act as important Ca²⁺ influx pathways in endothelial cells and are involved in endothelium-dependent vasodilation, regulation of barrier permeability, and angiogenesis. ROS and RNS can modulate the activity of certain TRP channels mainly by modifying specific cysteine residues or by stimulating the production of second messengers. In this review, we highlight the recent literature describing redox regulation of TRP channel activity in endothelial cells as well as the physiological importance of these pathways and implication for cardiovascular diseases.

Synthesis, high-throughput screening and pharmacological characterization of β-lactam derivatives as TRPM8 antagonists

The mammalian transient receptor potential melastatin channel 8 (TRPM8), highly expressed in trigeminal and dorsal root ganglia, mediates the cooling sensation and plays an important role in the cold hypersensitivity characteristic of some types of neuropathic pain, as well as in cancer. Consequently, the identification of selective and potent ligands for TRPM8 is of great interest. Here, a series of compounds, having a β-lactam central scaffold, were prepared to explore the pharmacophore requirements for TRPM8 modulation. Structure-activity studies indicate that the minimal requirements for potent β-lactam-based TRPM8 blockers are hydrophobic groups (benzyl preferentially or ^tBu) on R¹, R², R³ and R⁵ and a short N-alkyl chain (≤3 carbons). The best compounds in the focused library (41 and 45) showed IC₅₀ values of 46 nM and 83 nM, respectively, in electrophysiology assays. These compounds selectively blocked all modalities of TRPM8 activation, i.e. menthol, voltage, and temperature. Molecular modelling studies using a homology model of TRPM8 identified two putative binding sites, involving networks of hydrophobic interactions, and suggesting a negative allosteric modulation through the stabilization of the closed state. Thus, these β-lactams provide a novel pharmacophore scaffold to evolve TRPM8 allosteric modulators to treat TRPM8 channel dysfunction.

TRPA1 expression levels and excitability brake by KV channels influence cold sensitivity of TRPA1-expressing neurons

The molecular sensor of innocuous (painless) cold sensation is well-established to be transient receptor potential cation channel, subfamily M, member 8 (TRPM8). However, the role of transient receptor potential cation channel, subfamily A, member 1 (TRPA1) in noxious (painful) cold sensation has been controversial. We find that TRPA1 channels contribute to the noxious cold sensitivity of mouse somatosensory neurons, independent of TRPM8 channels, and that TRPA1-expressing neurons are largely non-overlapping with TRPM8-expressing neurons in mouse dorsal-root ganglia (DRG). However, relatively few TRPA1-expressing neurons (e.g., responsive to allyl isothiocyanate or AITC, a selective TRPA1 agonist) respond overtly to cold temperature in vitro, unlike TRPM8-expressing neurons, which almost all respond to cold. Using somatosensory neurons from TRPM8-/- mice and subtype-selective blockers of TRPM8 and TRPA1 channels, we demonstrate that responses to cold temperatures from TRPA1-expressing neurons are mediated by TRPA1 channels. We also identify two factors that affect the cold-sensitivity of TRPA1-expressing neurons: (1) cold-sensitive AITC-sensitive neurons express relatively more TRPA1 transcripts than cold-insensitive AITC-sensitive neurons and (2) voltage-gated potassium (K_V) channels attenuate the cold-sensitivity of some TRPA1-expressing neurons. The combination of these two factors, combined with the relatively weak agonist-like activity of cold temperature on TRPA1 channels, partially explains why few TRPA1-expressing neurons respond to cold. Blocking K_V channels also reveals another subclass of noxious cold-sensitive DRG neurons that do not express TRPM8 or TRPA1 channels. Altogether, the results of this study provide novel insights into the cold-sensitivity of different subclasses of somatosensory neurons.

Activation of TRPA1 Channel by Antibacterial Agent Triclosan Induces VEGF Secretion in Human Prostate Cancer Stromal Cells

Accruing evidence indicates that exposure to environmental compounds may adversely affect human health and promote carcinogenesis. Triclosan (TCS), an antimicrobial agent widely used as a preservative in personal care products, has been shown to act as an endocrine disruptor in hormone-dependent tissues. Here, we demonstrate a new molecular mechanism by which TCS stimulates the secretion by human prostate cancer stromal cells of vascular endothelial growth factor (VEGF), a factor known to promote tumor growth. This mechanism involves an increase in intracellular calcium levels due to the direct activation of a membrane ion channel. Using calcium imaging and electrophysiology techniques, we show for the first time that environmentally relevant concentrations of TCS activate a cation channel of the TRP family, TRPA1 (Transient Receptor Potential Ankirin 1), in primary cultured human prostate cancer stromal cells. The TCS-induced TRPA1 activation increased basal calcium in stromal cells and stimulated the secretion of VEGF and epithelial cells proliferation. Interestingly, immunofluorescence labeling performed on formalin-fixed paraffin-embedded prostate tissues showed an exclusive expression of the TRPA1 channel in prostate cancer stromal cells. Our data demonstrate an impact of the environmental factor TCS on the tumor microenvironment interactions, by activating a tumor stroma-specific TRPA1 ion channel. Cancer Prev Res; 10(3); 177-87. ©2017 AACR.

Agonist-induced sensitisation of the irritant receptor ion channel TRPA1

KEY POINTS

The transient receptor potential ankyrin 1 (TRPA1) ion channel is expressed in nociceptive neurons and its activation causes ongoing pain and inflammation; TRPA1 is thought to play an important role in inflammation in the airways. TRPA1 is sensitised by repeated stimulation with chemical agonists in a calcium-free environment and this sensitisation is very long lasting following agonist removal. We show that agonist-induced sensitisation is independent of the agonist's binding site and is also independent of ion channel trafficking or of other typical signalling pathways. We find that sensitisation is intrinsic to the TRPA1 protein and is accompanied by a slowly developing shift in the voltage dependence of TRPA1 towards more negative membrane potentials. Agonist-induced sensitisation may provide an explanation for sensitisation following long-term exposure to harmful irritants and pollutants, particularly in the airways.

ABSTRACT

The TRPA1 ion channel is expressed in nociceptive (pain-sensitive) neurons and responds to a wide variety of chemical irritants, such as acrolein in smoke or isothiocyanates in mustard. Here we show that in the absence of extracellular calcium the current passing through TRPA1 gradually increases (sensitises) during prolonged application of agonists. Activation by an agonist is essential, because activation of TRPA1 by membrane depolarisation did not cause sensitisation. Sensitisation is independent of the site of action of the agonist, because covalent and non-covalent agonists were equally effective, and is long lasting following agonist removal. Mutating N-terminal cysteines, the target of covalent agonists, did not affect sensitisation by the non-covalent agonist carvacrol, which activates by binding to a different site. Sensitisation is unaffected by agents blocking ion channel trafficking or by block of signalling pathways involving ATP, protein kinase A or the formation of lipid rafts, and does not require ion flux through the channel. Examination of the voltage dependence of TRPA1 activation shows that sensitisation is accompanied by a slowly developing shift in the voltage dependence of TRPA1 towards more negative membrane potentials, and is therefore intrinsic to the TRPA1 channel. Sensitisation may play a role in exacerbating the pain caused by prolonged activation of TRPA1.

Transient Receptor Potential Melastatin 8 Channel (TRPM8) Modulation: Cool Entryway for Treating Pain and Cancer

TRPM8 ion channels, the primary cold sensors in humans, are activated by innocuous cooling (<28 °C) and cooling compounds (menthol, icilin) and are implicated in sensing unpleasant cold stimuli as well as in mammalian thermoregulation. Overexpression of these thermoregulators in prostate cancer and in other life-threatening tumors, along with their contribution to an increasing number of pathological conditions, opens a plethora of medicinal chemistry opportunities to develop receptor modulators. This Perspective seeks to describe current known modulators for this ion channel because both agonists and antagonists may be useful for the treatment of most TRPM8-mediated pathologies. We primarily focus on SAR data for the different families of compounds and the pharmacological properties of the most promising ligands. Furthermore, we also address the knowledge about the channel structure, although still in its infancy, and the role of the TRPM8 protein signalplex to channel function and dysfunction. We finally outline the potential future prospects of the challenging TRPM8 drug discovery field.

TRP channels in schistosomes

Praziquantel (PZQ) is effectively the only drug currently available for treatment and control of schistosomiasis, a disease affecting hundreds of millions of people worldwide. Many anthelmintics, likely including PZQ, target ion channels, membrane protein complexes essential for normal functioning of the neuromusculature and other tissues. Despite this fact, only a few classes of parasitic helminth ion channels have been assessed for their pharmacological properties or for their roles in parasite physiology. One such overlooked group of ion channels is the transient receptor potential (TRP) channel superfamily. TRP channels share a common core structure, but are widely diverse in their activation mechanisms and ion selectivity. They are critical to transducing sensory signals, responding to a wide range of external stimuli. They are also involved in other functions, such as regulating intracellular calcium and organellar ion homeostasis and trafficking. Here, we review current literature on parasitic helminth TRP channels, focusing on those in schistosomes. We discuss the likely roles of these channels in sensory and locomotor activity, including the possible significance of a class of TRP channels (TRPV) that is absent in schistosomes. We also focus on evidence indicating that at least one schistosome TRP channel (SmTRPA) has atypical, TRPV1-like pharmacological sensitivities that could potentially be exploited for future therapeutic targeting.

•

We provide an overview of transient receptor potential (TRP) channels in schistosomes and other parasitic helminths.

•

TRP channels are important for sensory signaling, ion homeostasis, organellar trafficking, and a host of other functions.

•

Very little work has been done on TRP channels in parasitic helminths.

•

TRPV channels, found throughout the Metazoa, appear not to be present in parasitic platyhelminths.

•

TRP channels in schistosomes appear to have atypical pharmacology, perhaps an entrée for therapeutic targeting.

Deciphering Subtype-Selective Modulations in TRPA1 Biosensor Channels

The transient receptor potential (TRP) proteins are a family of ion channels that act as cellular sensors. Several members of the TRP family are sensitive to oxidative stress mediators. Among them, TRPA1 is remarkably susceptible to various oxidants, and is known to mediate neuropathic pain and respiratory, vascular and gastrointestinal functions, making TRPA1 an attractive therapeutic target. Recent studies have revealed a number of modulators (both activators and inhibitors) that act on TRPA1. Endogenous mediators of oxidative stress and exogenous electrophiles activate TRPA1 through oxidative modification of cysteine residues. Non-electrophilic compounds also activate TRPA1. Certain non-electrophilic modulators may act on critical non-cysteine sites in TRPA1. However, a method to achieve selective modulation of TRPA1 by small molecules has not yet been established. More recently, we found that a novel N-nitrosamine compound activates TRPA1 by S-nitrosylation (the addition of a nitric oxide (NO) group to cysteine thiol), and does so with significant selectivity over other NO-sensitive TRP channels. It is proposed that this subtype selectivity is conferred through synergistic effects of electrophilic cysteine transnitrosylation and molecular recognition of the non-electrophilic moiety on the N-nitrosamine. In this review, we describe the molecular pharmacology of these TRPA1 modulators and discuss their modulatory mechanisms.

Tetrahydroisoquinoline-Derived Urea and 2,5-Diketopiperazine Derivatives as Selective Antagonists of the Transient Receptor Potential Melastatin 8 (TRPM8) Channel Receptor and Antiprostate Cancer Agents

Tetrahydroisoquinoline derivatives containing embedded urea functions were identified as selective TRPM8 channel receptor antagonists. Structure-activity relationships were investigated, with the following conclusions: (a) The urea function and the tetrahydroisoquinoline system are necessary for activity. (b) Bis(1-aryl-6,7dimethoxy-1,2,3,4-tetrahydroisoquinolyl)ureas are more active than compounds containing one tetrahydroisoquinoline ring and than an open phenetylamine ureide. (c) Trans compounds are more active than their cis isomers. (d) Aryl substituents are better than alkyls at the isoquinoline C-1 position. (e) Electron-withdrawing substituents lead to higher activities. The most potent compound is the 4-F derivative, with IC50 in the 10(-8) M range and selectivities around 1000:1 for most other TRP receptors. Selected compounds were found to be active in reducing the growth of LNCaP prostate cancer cells. TRPM8 inhibition reduces proliferation in the tumor cells tested but not in nontumor prostate cells, suggesting that the activity against prostate cancer is linked to TRPM8 inhibition.

TRPM8: a potential target for cancer treatment

Transient receptor potential (TRP) cation channel superfamily plays critical roles in variety of processes, including temperature perception, pain transduction, vasorelaxation, male fertility, and tumorigenesis. One of seven families within the TRP superfamily of ion channels, the melastatin, or TRPM family comprises a group of eight structurally and functionally diverse channels. Of all the members of TRPM subfamily, TRPM8 is the most notable one. A lot of literatures have demonstrated that transient receptor potential melastatin 8 (TRPM8) could perform a myriad of functions in vertebrates and invertebrates alike. In addition to its well-known function in cold sensation, TRPM8 has an emerging role in a variety of biological systems, including thermoregulation, cancer, bladder function, and asthma. Recent studies have shown that TRPM8 is necessary to the initiation and progression of tumors, and the aberrant expression of TRPM8 was found in varieties of tumors, such as prostate tumor, melanoma, breast adenocarcinoma, bladder cancer, and colorectal cancer, making it a novel molecular target potentially useful in the diagnosis and treatment of cancer. This review outlines our current understanding on the role of TRPM8 in occurrence and development of different kinds of tumor and also includes discussion about the regulation of TRPM8 during carcinogenesis as well as therapeutic potential of targeting TRPM8 in tumor, which may be utilized for a potential pharmacological use as a target for anti-cancer therapy.

The TRPA1 channel in inflammatory and neuropathic pain and migraine

The transient receptor potential ankyrin 1 (TRPA1), a member of the TRP superfamily of channels, is primarily localized to a subpopulation of primary sensory neurons of the trigeminal, vagal, and dorsal root ganglia. This subset of nociceptors produces and releases the neuropeptides substance P (SP) and calcitonin gene-related peptide (CGRP), which mediate neurogenic inflammatory responses. TRPA1 is activated by a number of exogenous compounds, including molecules of botanical origin, environmental irritants, and medicines. However, the most prominent feature of TRPA1 resides in its unique sensitivity for large series of reactive byproducts of oxidative and nitrative stress. Here, the role of TRPA1 in models of different types of pain, including inflammatory and neuropathic pain and migraine, is summarized. Specific attention is paid to TRPA1 as the main contributing mechanism to the transition of mechanical and cold hypersensitivity from an acute to a chronic condition and as the primary transducing pathway by which oxidative/nitrative stress produces acute nociception, allodynia, and hyperalgesia. A series of migraine triggers or medicines have been reported to modulate TRPA1 activity and the ensuing CGRP release. Thus, TRPA1 antagonists may be beneficial in the treatment of inflammatory and neuropathic pain and migraine.

Transient receptor potential channels and itch: how deep should we scratch?

Over the past 30 years, transient receptor potential (TRP) channels have evolved from a somewhat obscure observation on how fruit flies detect light to become the center of drug discovery efforts, triggering a heated debate about their potential as targets for therapeutic applications in humans. In this review, we describe our current understanding of the diverse mechanism of action of TRP channels in the itch pathway from the skin to the brain with focus on the peripheral detection of stimuli that elicit the desire to scratch and spinal itch processing and sensitization. We predict that the compelling basic research findings on TRP channels and pruritus will be translated into the development of novel, clinically useful itch medications.

TRP channels in the skin

Emerging evidence suggests that transient receptor potential (TRP) ion channels not only act as 'polymodal cellular sensors' on sensory neurons but are also functionally expressed by a multitude of non-neuronal cell types. This is especially true in the skin, one of the largest organs of the body, where they appear to be critically involved in regulating various cutaneous functions both under physiological and pathophysiological conditions. In this review, we focus on introducing the roles of several cutaneous TRP channels in the regulation of the skin barrier, skin cell proliferation and differentiation, and immune functions. Moreover, we also describe the putative involvement of several TRP channels in the development of certain skin diseases and identify future TRP channel-targeted therapeutic opportunities.

A novel pan-negative-gating modulator of KCa2/3 channels, fluoro-di-benzoate, RA-2, inhibits endothelium-derived hyperpolarization-type relaxation in coronary artery and produces bradycardia in vivo

Small/intermediate conductance KCa channels (KCa2/3) are Ca(2+)/calmodulin regulated K(+) channels that produce membrane hyperpolarization and shape neurologic, epithelial, cardiovascular, and immunologic functions. Moreover, they emerged as therapeutic targets to treat cardiovascular disease, chronic inflammation, and some cancers. Here, we aimed to generate a new pharmacophore for negative-gating modulation of KCa2/3 channels. We synthesized a series of mono- and dibenzoates and identified three dibenzoates [1,3-phenylenebis(methylene) bis(3-fluoro-4-hydroxybenzoate) (RA-2), 1,2-phenylenebis(methylene) bis(3-fluoro-4-hydroxybenzoate), and 1,4-phenylenebis(methylene) bis(3-fluoro-4-hydroxybenzoate)] with inhibitory efficacy as determined by patch clamp. Among them, RA-2 was the most drug-like and inhibited human KCa3.1 with an IC50 of 17 nM and all three human KCa2 subtypes with similar potencies. RA-2 at 100 nM right-shifted the KCa3.1 concentration-response curve for Ca(2+) activation. The positive-gating modulator naphtho[1,2-d]thiazol-2-ylamine (SKA-31) reversed channel inhibition at nanomolar RA-2 concentrations. RA-2 had no considerable blocking effects on distantly related large-conductance KCa1.1, Kv1.2/1.3, Kv7.4, hERG, or inwardly rectifying K(+) channels. In isometric myography on porcine coronary arteries, RA-2 inhibited bradykinin-induced endothelium-derived hyperpolarization (EDH)-type relaxation in U46619-precontracted rings. Blood pressure telemetry in mice showed that intraperitoneal application of RA-2 (≤100 mg/kg) did not increase blood pressure or cause gross behavioral deficits. However, RA-2 decreased heart rate by ≈145 beats per minute, which was not seen in KCa3.1(-/-) mice. In conclusion, we identified the KCa2/3-negative-gating modulator, RA-2, as a new pharmacophore with nanomolar potency. RA-2 may be of use to generate structurally new types of negative-gating modulators that could help to define the physiologic and pathomechanistic roles of KCa2/3 in the vasculature, central nervous system, and during inflammation in vivo.

Transient receptor potential ankyrin 1 (TRPA1) channel activation by the thienopyridine-type drugs ticlopidine, clopidogrel, and prasugrel

Transient receptor potential A1 (TRPA1) is widely expressed throughout the human and animal organism, including the dorsal root ganglia as well as the bladder, stomach and small intestine. Here, we examined the effect of three platelet aggregation inhibitors on TRPA1: ticlopidine, clopidogrel and prasugrel. Utilising fluorometric Ca(2+) influx analysis and electrophysiological whole cell measurements in TRPA1-expressing HEK293 and in human enterochromaffin-like QGP-1 cells, we found that ticlopidine, clopidogrel and prasugrel are direct activators of TRPA1. Although this polymodal channel commonly contributes to the perception of pain, temperature and chemical irritants, recent studies provide evidence for its involvement in the release of serotonin (5-HT) from enterochromaffin cells. Therefore, we further investigated the ability of ticlopidine, clopidogrel and prasugrel to stimulate 5-HT release from QGP-1 cells. We could determine 5-HT in supernatants from cultured QGP-1 cells upon treatment with ticlopidine and clopidogrel but not with prasugrel. These findings indicate that a robust TRPA1 activation by ticlopidine and clopidogrel correlates with the stimulatory effect on the secretion of 5-HT. As recipients of ticlopidine and clopidogrel frequently complain about gastrointestinal adverse events such as nausea, vomiting and diarrhoea, an activation of TRPA1 may contribute to adverse effects of such drugs in the digestive system.

Female reproductive tract pain: targets, challenges, and outcomes

Pain from the female reproductive tract (FRT) is a significant clinical problem for which there are few effective therapies. The complex neuroanatomy of pelvic organs not only makes diagnosis of pelvic pain disorders difficult but represents a challenge to development of targeted therapies. A number of potential therapeutic targets have been identified on sensory neurons supplying the FRT but our knowledge on the basic neurophysiology of these neurons is limited compared with other viscera. Until this is addressed we can only guess if the new experimental therapies proposed for somatic, gastrointestinal, or bladder pain will translate to the FRT. Once suitable therapeutic targets become clear, the next challenge is drug delivery. The FRT represents a promising system for topical drug delivery that could be tailored to act locally or systemically depending on formulation. Development of these therapies and their delivery systems will need to be done in concert with more robust in vivo and in vitro models of FRT pain.

A tarantula-venom peptide antagonizes the TRPA1 nociceptor ion channel by binding to the S1-S4 gating domain

BACKGROUND

The venoms of predators have been an excellent source of diverse highly specific peptides targeting ion channels. Here we describe the first known peptide antagonist of the nociceptor ion channel transient receptor potential ankyrin 1 (TRPA1).

RESULTS

We constructed a recombinant cDNA library encoding ∼100 diverse GPI-anchored peptide toxins (t-toxins) derived from spider venoms and screened this library by coexpression in Xenopus oocytes with TRPA1. This screen resulted in identification of protoxin-I (ProTx-I), a 35-residue peptide from the venom of the Peruvian green-velvet tarantula, Thrixopelma pruriens, as the first known high-affinity peptide TRPA1 antagonist. ProTx-I was previously identified as an antagonist of voltage-gated sodium (NaV) channels. We constructed a t-toxin library of ProTx-I alanine-scanning mutants and screened this library against NaV1.2 and TRPA1. This revealed distinct partially overlapping surfaces of ProTx-I by which it binds to these two ion channels. Importantly, this mutagenesis yielded two novel ProTx-I variants that are only active against either TRPA1or NaV1.2. By testing its activity against chimeric channels, we identified the extracellular loops of the TRPA1 S1-S4 gating domain as the ProTx-I binding site.

CONCLUSIONS

These studies establish our approach, which we term "toxineering," as a generally applicable method for isolation of novel ion channel modifiers and design of ion channel modifiers with altered specificity. They also suggest that ProTx-I will be a valuable pharmacological reagent for addressing biophysical mechanisms of TRPA1 gating and the physiology of TRPA1 function in nociceptors, as well as for potential clinical application in the context of pain and inflammation.

TRPA1

The transient receptor potential ankyrin subtype 1 protein (TRPA1) is a nonselective cation channel permeable to Ca(2+), Na(+), and K(+). TRPA1 is a promiscuous chemical nocisensor that is also involved in noxious cold and mechanical sensation. It is present in a subpopulation of Aδ- and C-fiber nociceptive sensory neurons as well as in other sensory cells including epithelial cells. In primary sensory neurons, Ca(2+) and Na(+) flowing through TRPA1 into the cell cause membrane depolarization, action potential discharge, and neurotransmitter release both at peripheral and central neural projections. In addition to being activated by cysteine and lysine reactive electrophiles and oxidants, TRPA1 is indirectly activated by pro-inflammatory agents via the phospholipase C signaling pathway, in which cytosolic Ca(2+) is an important regulator of channel gating. The finding that non-electrophilic compounds, including menthol and cannabinoids, activate TRPA1 may provide templates for the design of non-tissue damaging activators to fine-tune the activity of TRPA1 and raises the possibility that endogenous ligands sharing binding sites with such non-electrophiles exist and regulate TRPA1 channel activity. TRPA1 is promising as a drug target for novel treatments of pain, itch, and sensory hyperreactivity in visceral organs including the airways, bladder, and gastrointestinal tract.

Intimacies and physiological role of the polymodal cold-sensitive ion channel TRPM8

The detection of environmental temperature is critical for the survival of the most diverse organisms. Thermosensitive transient receptor potential (thermoTRP) channels have evolved as a class of ion channels activated by a wide range of temperatures. These molecular thermal sensors are spread through the different TRP channel subfamilies. Among the Melastatin subfamily of TRP channels, the eighth member, TRPM8, is a calcium-permeable cationic ion channel activated by cold, by substances that evoke cold sensation such as menthol, and by voltage. This channel is considered the main molecular entity responsible for the sensitivity to cold of primary sensory neurons of the somatosensory system. Here we present to the readers a summary of some the most relevant biophysical properties, physiological role, and molecular intimacies of this polymodal thermoTRP channel.

TRPM8

Transient receptor potential melastatin 8 (TRPM8) was originally cloned from prostate tissue. Shortly thereafter, the protein was identified as a cold- and menthol-activated ion channel in peripheral sensory neurons, where it plays a critical role in cold temperature detection. In this chapter, we review our current understanding of the molecular and biophysical properties, the pharmacology, and the modulation by signaling molecules of this TRP channel. Finally, we examine the physiological role of TRPM8 and its emerging link to various human diseases, including pain, prostate cancer, dry eye disease, and metabolic disorders.

TRPC6: physiological function and pathophysiological relevance

TRPC6 is a non-selective cation channel 6 times more permeable to Ca(2+) than to Na(+). Channel homotetramers heterologously expressed have a characteristic doubly rectifying current-voltage relationship and are directly activated by the second messenger diacylglycerol (DAG). TRPC6 proteins are also regulated by specific tyrosine or serine phosphorylation and phosphoinositides. Given its specific expression pattern, TRPC6 is likely to play a number of physiological roles which are confirmed by the analysis of a Trpc6 (-/-) mouse model. In smooth muscle Na(+) influx through TRPC6 channels and activation of voltage-gated Ca(2+) channels by membrane depolarisation is the driving force for contraction. Permeability of pulmonary endothelial cells depends on TRPC6 and induces ischaemia-reperfusion oedema formation in the lungs. TRPC6 was also identified as an essential component of the slit diaphragm architecture of kidney podocytes and plays an important role in the protection of neurons after cerebral ischaemia. Other functions especially in immune and blood cells remain elusive. Recently identified TRPC6 blockers may be helpful for therapeutic approaches in diseases with highly activated TRPC6 channel activity.

The use of extemporaneously compounded 1% tetracaine to improve adherence with clotrimazole 1% topical solution in the treatment of ear infection: a case report

For most medically amenable conditions, adherence to drug therapy is a necessary condition for a successful outcome. Drug side effects, especially pain, can interfere with the desired outcome. We report a case of non-adherence due to severe pain associated with the topical use of clotrimazole 1% solution in the ear. Instillation of tetracaine 1% solution prior to the administration of the clotrimazole blocked the pain sensation allowing the patient to successfully complete the antifungal therapy.

Lack of correlation between the amplitudes of TRP channel-mediated responses to weak and strong stimuli in intracellular Ca(2+) imaging experiments

It is often observed in intracellular Ca(2+) imaging experiments that the amplitudes of the Ca(2+) signals elicited by newly characterized TRP agonists do not correlate with the amplitudes of the responses evoked subsequently by a specific potent agonist. We investigated this rather controversial phenomenon by first testing whether it is inherent to the comparison of the effects of weak and strong stimuli. Using five well-characterized TRP channel agonists in commonly used heterologous expression systems we found that the correlation between the amplitudes of the Ca(2+) signals triggered by two sequentially applied stimuli is only high when both stimuli are strong. Using mathematical simulations of intracellular Ca(2+) dynamics we illustrate that the innate heterogeneity in expression and functional properties of Ca(2+) extrusion (e.g. plasma membrane Ca(2+) ATPase) and influx (TRP channels) pathways across a cellular population is a sufficient condition for low correlation between the amplitude of Ca(2+) signals elicited by weak and strong stimuli. Taken together, our data demonstrate that this phenomenon is an expected outcome of intracellular Ca(2+) imaging experiments that cannot be taken as evidence for lack of specificity of low-efficacy stimuli, or as an indicator of the need of other cellular components for channel stimulation.

Allyl isothiocyanate sensitizes TRPV1 to heat stimulation

The powerful plant-derived irritant allyl isothiocyanate (AITC, aka mustard oil) induces hyperalgesia to heat in rodents and humans through mechanisms that are not yet fully understood. It is generally believed that AITC activates the broadly tuned chemosensory cation channel transient receptor potential cation channel subfamily A member 1 (TRPA1), triggering an inflammatory response that sensitizes the heat sensor transient receptor potential cation channel subfamily V member 1 (TRPV1). In the view of recent data demonstrating that AITC can directly activate TRPV1, we here explored the possibility that this compound sensitizes TRPV1 to heat stimulation in a TRPA1-independent manner. Patch-clamp recordings and intracellular Ca(2+) imaging experiments in HEK293T cells over-expressing mouse TRPV1 revealed that the increase in channel activation induced by heating is larger in the presence of AITC than in control conditions. The analysis of the effects of AITC and heat on the current-voltage relationship of TRPV1 indicates that the mechanism of sensitization is based on additive shifts of the voltage dependence of activation towards negative voltages. Finally, intracellular Ca(2+) imaging experiments in mouse sensory neurons isolated from Trpa1 KO mice yielded that AITC enhances the response to heat, specifically in the subpopulation expressing TRPV1. Furthermore, this effect was strongly reduced by the TRPV1 inhibitor capsazepine and virtually absent in neurons isolated from double Trpa1/Trpv1 KO mice. Taken together, these findings demonstrate that TRPV1 is a locus for cross sensitization between AITC and heat in sensory neurons and may help explaining, at least in part, the role of this channel in AITC-induced hyperalgesia to heat.

Mechanisms of transient receptor potential vanilloid 1 activation and sensitization by allyl isothiocyanate

Allyl isothiocyanate (AITC; aka, mustard oil) is a powerful irritant produced by Brassica plants as a defensive trait against herbivores and confers pungency to mustard and wasabi. AITC is widely used experimentally as an inducer of acute pain and neurogenic inflammation, which are largely mediated by the activation of nociceptive cation channels transient receptor potential ankyrin 1 and transient receptor potential vanilloid 1 (TRPV1). Although it is generally accepted that electrophilic agents activate these channels through covalent modification of cytosolic cysteine residues, the mechanism underlying TRPV1 activation by AITC remains unknown. Here we show that, surprisingly, AITC-induced activation of TRPV1 does not require interaction with cysteine residues, but is largely dependent on S513, a residue that is involved in capsaicin binding. Furthermore, AITC acts in a membrane-delimited manner and induces a shift of the voltage dependence of activation toward negative voltages, which is reminiscent of capsaicin effects. These data indicate that AITC acts through reversible interactions with the capsaicin binding site. In addition, we show that TRPV1 is a locus for cross-sensitization between AITC and acidosis in nociceptive neurons. Furthermore, we show that residue F660, which is known to determine the stimulation by low pH in human TRPV1, is also essential for the cross-sensitization of the effects of AITC and low pH. Taken together, these findings demonstrate that not all reactive electrophiles stimulate TRPV1 via cysteine modification and help understanding the molecular bases underlying the surprisingly large role of this channel as mediator of the algesic properties of AITC.

TRPM8 ion channels differentially modulate proliferation and cell cycle distribution of normal and cancer prostate cells

Overexpression of the cation-permeable channel TRPM8 in prostate cancers might represent a novel opportunity for their treatment. Inhibitors of TRPM8 reduce the growth of prostate cancer cells. We have used two recently described and highly specific blockers, AMTB and JNJ41876666, and RNAi to determine the relevance of TRPM8 expression in the proliferation of non-tumor and tumor cells. Inhibition of the expression or function of the channel reduces proliferation rates and proliferative fraction in all tumor cells tested, but not of non-tumor prostate cells. We observed no consistent acceleration of growth after stimulation of the channel with menthol or icilin, indicating that basal TRPM8 expression is enough to sustain growth of prostate cancer cells.

Identification and analysis of cation channel homologues in human pathogenic fungi

Fungi are major causes of human, animal and plant disease. Human fungal infections can be fatal, but there are limited options for therapy, and resistance to commonly used anti-fungal drugs is widespread. The genomes of many fungi have recently been sequenced, allowing identification of proteins that may become targets for novel therapies. We examined the genomes of human fungal pathogens for genes encoding homologues of cation channels, which are prominent drug targets. Many of the fungal genomes examined contain genes encoding homologues of potassium (K⁺), calcium (Ca²⁺) and transient receptor potential (Trp) channels, but not sodium (Na⁺) channels or ligand-gated channels. Some fungal genomes contain multiple genes encoding homologues of K⁺ and Trp channel subunits, and genes encoding novel homologues of voltage-gated K_v channel subunits are found in Cryptococcus spp. Only a single gene encoding a homologue of a plasma membrane Ca²⁺ channel was identified in the genome of each pathogenic fungus examined. These homologues are similar to the Cch1 Ca²⁺ channel of Saccharomyces cerevisiae. The genomes of Aspergillus spp. and Cryptococcus spp., but not those of S. cerevisiae or the other pathogenic fungi examined, also encode homologues of the mitochondrial Ca²⁺ uniporter (MCU). In contrast to humans, which express many K⁺, Ca²⁺ and Trp channels, the genomes of pathogenic fungi encode only very small numbers of K⁺, Ca²⁺ and Trp channel homologues. Furthermore, the sequences of fungal K⁺, Ca²⁺, Trp and MCU channels differ from those of human channels in regions that suggest differences in regulation and susceptibility to drugs.