TRP channels as new pharmacological targets

Identification of two natural coumarin enantiomers for selective inhibition of TRPV2 channels

Thermosensitive transient receptor potential vanilloid 2 (thermoTRPV2) is a nonselective Ca²⁺ -permeable cation channel broadly expressed, and is implicated in the pathology of diseases such as diabetes and pancreatitis. However, the physiological and pharmacological functions of TRPV2 channels have not been extensively investigated because of the absence of specific modulators. In this study, we report a pair of natural coumarin derivative enantiomers (-)-murraxocin (B304-1) and (+)-murraxocin (B304-2) from Murraya exotica for their selective inhibition of TRPV2 channels expressed in HEK293 cells and native TRPV2 currents in differentiated brown adipocytes. Whole-cell patch clamp recordings confirmed the enantiomers B304-1 and B304-2 could selectively inhibit the agonist mediated activation of TRPV2 current with IC₅₀ values of 22.2 ± 7.8 μM and 3.7 ± 0.7 μM, respectively. Molecular docking and site-directed mutagenesis revealed a key residue I600 of TRPV2 critical for the binding of the enantiomers. Furthermore, B304-1 and B304-2 significantly reversed TRPV2 agonist-induced inhibition of mouse brown adipocyte differentiation. Taken together, our identification of two natural coumarin enantiomers provides valuable tools and chemical leads for further elucidation of TRPV2 channel function, and pharmacological modulation of thermoTRPV2 in brown adipocytes may represent a new therapeutic strategy for treatment of energy imbalance or metabolic disorders.

Atria-selective antiarrhythmic drugs in need of alliance partners

Atria-selective antiarrhythmic drugs in need of alliance partners. Guideline-based treatment of atrial fibrillation (AF) comprises prevention of thromboembolism and stroke, as well as antiarrhythmic therapy by drugs, electrical rhythm conversion, ablation and surgical procedures. Conventional antiarrhythmic drugs are burdened with unwanted side effects including a propensity of triggering life-threatening ventricular fibrillation. In order to solve this therapeutic dilemma, 'atria-selective' antiarrhythmic drugs have been developed for the treatment of supraventricular arrhythmias. These drugs are designed to aim at atrial targets, taking advantage of differences in atrial and ventricular ion channel expression and function. However it is not clear, whether such drugs are sufficiently antiarrhythmic or whether they are in need of an alliance partner for clinical efficacy. Atria-selective Na⁺ channel blockers display fast dissociation kinetics and high binding affinity to inactivated channels. Compounds targeting atria-selective K⁺ channels include blockers of ultra rapid delayed rectifier (Kv1.5) or acetylcholine-activated inward rectifier K⁺ channels (Kir3.x), inward rectifying K⁺ channels (Kir2.x), Ca²⁺-activated K⁺ channels of small conductance (SK), weakly rectifying two-pore domain K⁺ channels (K_2P), and transient receptor potential channels (TRP). Despite good antiarrhythmic data from in-vitro and animal model experiments, clinical efficacy of atria-selective antiarrhythmic drugs remains to be demonstrated. In the present review we will briefly summarize the novel compounds and their proposed antiarrhythmic action. In addition, we will discuss the evidence for putative improvement of antiarrhythmic efficacy and potency by addressing multiple pathophysiologically relevant targets as possible alliance partners.

Role of Chemosensory TRP Channels in Lung Cancer

Transient receptor potential (TRP) channels represent a large family of cation channels and many members of the TRP family have been shown to act as polymodal receptor molecules for irritative or potentially harmful substances. These chemosensory TRP channels have been extensively characterized in primary sensory and neuronal cells. However, in recent years the functional expression of these proteins in non-neuronal cells, e.g., in the epithelial lining of the respiratory tract has been confirmed. Notably, these proteins have also been described in a number of cancer types. As sensor molecules for noxious compounds, chemosensory TRP channels are involved in cell defense mechanisms and influence cell survival following exposure to toxic substances via the modulation of apoptotic signaling. Of note, a number of cytostatic drugs or drug metabolites can activate these TRP channels, which could affect the therapeutic efficacy of these cytostatics. Moreover, toxic inhalational substances with potential involvement in lung carcinogenesis are well established TRP activators. In this review, we present a synopsis of data on the expression of chemosensory TRP channels in lung cancer cells and describe TRP agonists and TRP-dependent signaling pathways with potential relevance to tumor biology. Furthermore, we discuss a possible role of TRP channels in the non-genomic, tumor-promoting effects of inhalational carcinogens such as cigarette smoke.

TRPV channel-mediated calcium transients in nociceptor neurons are dispensable for avoidance behaviour

Animals need to sense and react to potentially dangerous environments. TRP ion channels participate in nociception, presumably via Ca²⁺ influx, in most animal species. However, the relationship between ion permeation and animals’ nocifensive behaviour is unknown. Here we use an invertebrate animal model with relevance for mammalian pain. We analyse the putative selectivity filter of OSM-9, a TRPV channel, in osmotic avoidance behaviour of Caenorhabditis elegans. Using mutagenized OSM-9 expressed in the head nociceptor neuron, ASH, we study nocifensive behaviour and Ca²⁺ influx. Within the selectivity filter, M⁶⁰¹-F⁶⁰⁹, Y604G strongly reduces avoidance behaviour and eliminates Ca²⁺ transients. Y604F also abolishes Ca²⁺ transients in ASH, while sustaining avoidance behaviour, yet it disrupts behavioral plasticity. Homology modelling of the OSM-9 pore suggests that Y⁶⁰⁴ may assume a scaffolding role. Thus, aromatic residues in the OSM-9 selectivity filter are critical for pain behaviour and ion permeation. These findings have relevance for understanding evolutionary roots of mammalian nociception.

TRPs are calcium-permeable channels involved in the sensing of damaging stimuli but the relationship between calcium influx and pain behaviour has been elusive. Here the authors find that the TRP channel OSM-9 functions as an ion channel in vivo in C. elegans, and establish residues that are critical for worm pain-like behaviour.

State-of-the-Art Automated Patch Clamp Devices: Heat Activation, Action Potentials, and High Throughput in Ion Channel Screening

Ion channels are essential in a wide range of cellular functions and their malfunction underlies many disease states making them important targets in drug discovery. The availability of standardized cell lines expressing ion channels of interest lead to the development of diverse automated patch clamp (APC) systems with high-throughput capabilities. These systems are now available for drug screening, but there are limitations in the application range. However, further development of existing devices and introduction of new systems widen the range of possible experiments and increase throughput. The addition of well controlled and fast solution exchange, temperature control and the availability of the current clamp mode are required to analyze standard cell lines and excitable cells such as stem cell-derived cardiomyocytes in a more physiologically relevant environment. Here we describe two systems with different areas of applications that meet the needs of drug discovery researchers and basic researchers alike. The here utilized medium throughput APC device is a planar patch clamp system capable of recording up to eight cells simultaneously. Features such as temperature control and recordings in the current clamp mode are described here. Standard cell lines and excitable cells such as stem cell-derived cardiomyocytes have been used in the voltage clamp and current clamp modes with the view to finding new drug candidates and safety testing methods in a more physiologically relevant environment. The high-throughput system used here is a planar patch clamp screening platform capable of recording from 96 cells in parallel and offers a throughput of 5000 data points per day. Full dose response curves can be acquired from individual cells reducing the cost per data point. The data provided reveals the suitability and relevance of both APC platforms for drug discovery, ion channel research, and safety testing.

Increased susceptibility to cardiovascular effects of dihydrocapcaicin in resuscitated rats. Cardiovascular effects of dihydrocapsaicin

Background

Survivors of a cardiac arrest often have persistent cardiovascular derangements following cardiopulmonary resuscitation including decreased cardiac output, arrhythmias and morphological myocardial damage. These cardiovascular derangements may lead to an increased susceptibility towards the external and internal environment of the cardiovascular system as compared to the healthy situation.

Methods

Here we tested the hypothesis that the cardiovascular system in healthy rats and rats resuscitated from a cardiac arrest may be differentially affected by a transient receptor potential vanilloid type 1 agonist, by continuous intravenous infusion of dihydrocapsaicin (DHC).

Results

Compared to baseline, infusion of DHC caused an initial increase in mean arterial blood pressure in both healthy and resuscitated rats of 25% and 10%, respectively. Also, we observed an initial response of tachycardia in both healthy and resuscitated rats of 30% and 20%, respectively. Then, at high levels of DHC infusion (> 2.0 mg/kg/hr) we observed two single episodes of transient bradycardia and hypotension in 33% of the healthy rats, which was consistent with a TRPV1 agonist induced Bezold-Jarisch reflex. In contrast, in resuscitated rats we observed multiple episodes of bradycardia/hypotension in 100% of the rats and at a dose of DHC of 0.65 mg/kg/hr. Notably, this DHC effect could be completely blocked in the resuscitated rats by pre-treatment with atropine, a muscarinic acetylcholine antagonist.

Conclusions

Our results indicate that the susceptibility of the rats towards TRPV1 agonist induced Bezold-Jarisch reflex is increased in those resuscitated from cardiac arrest compared to the healthy situation.

Arterial response to shear stress critically depends on endothelial TRPV4 expression

BACKGROUND

In blood vessels, the endothelium is a crucial signal transduction interface in control of vascular tone and blood pressure to ensure energy and oxygen supply according to the organs' needs. In response to vasoactive factors and to shear stress elicited by blood flow, the endothelium secretes vasodilating or vasocontracting autacoids, which adjust the contractile state of the smooth muscle. In endothelial sensing of shear stress, the osmo- and mechanosensitive Ca(2+)-permeable TRPV4 channel has been proposed to be candidate mechanosensor. Using TRPV4(-/-) mice, we now investigated whether the absence of endothelial TRPV4 alters shear-stress-induced arterial vasodilation.

METHODOLOGY/PRINCIPAL FINDINGS

In TRPV4(-/-) mice, loss of the TRPV4 protein was confirmed by Western blot, immunohistochemistry and by in situ-patch-clamp techniques in carotid artery endothelial cells (CAEC). Endothelium-dependent vasodilation was determined by pressure myography in carotid arteries (CA) from TRPV4(-/-) mice and wild-type littermates (WT). In WT CAEC, TRPV4 currents could be elicited by TRPV4 activators 4alpha-phorbol-12,13-didecanoate (4alphaPDD), arachidonic acid (AA), and by hypotonic cell swelling (HTS). In striking contrast, in TRPV4(-/-) mice, 4alphaPDD did not produce currents and currents elicited by AA and HTS were significantly reduced. 4alphaPDD caused a robust and endothelium-dependent vasodilation in WT mice, again conspicuously absent in TRPV4(-/-) mice. Shear stress-induced vasodilation could readily be evoked in WT, but was completely eliminated in TRPV4(-/-) mice. In addition, flow/reperfusion-induced vasodilation was significantly reduced in TRPV4(-/-) vs. WT mice. Vasodilation in response to acetylcholine, vasoconstriction in response to phenylephrine, and passive mechanical compliance did not differ between genotypes, greatly underscoring the specificity of the above trpv4-dependent phenotype for physiologically relevant shear stress.

CONCLUSIONS/SIGNIFICANCE

Genetically encoded loss-of-function of trpv4 results in a loss of shear stress-induced vasodilation, a response pattern critically dependent on endothelial TRPV4 expression. Thus, Ca(2+)-influx through endothelial TRPV4 channels is a molecular mechanism contributing significantly to endothelial mechanotransduction.

TRPV channels' role in osmotransduction and mechanotransduction

In signal transduction of metazoan cells, transient receptor potential (TRP) ion channels have been identified that respond to diverse external and internal stimuli, among them osmotic and mechanical stimuli. This chapter will summarize findings on the TRPV subfamily, both its vertebrate and invertebrate members. Of the six mammalian TRPV channels, TRPV1, -V2, and -V4 were demonstrated to function in transduction of osmotic and/or mechanical stimuli. TRPV channels have been found to function in cellular as well as systemic osmotic homeostasis in vertebrates. Invertebrate TRPV channels, five in Caenorhabditis elegans and two in Drosophila, have been shown to play a role in mechanosensation, such as hearing and proprioception in Drosophila and nose touch in C. elegans, and in the response to osmotic stimuli in C. elegans. In a striking example of evolutionary conservation of function, mammalian TRPV4 has been found to rescue mechanosensory and osmosensory deficits of the TRPV mutant line osm-9 in C. elegans, despite no more than 26% orthology of the respective amino acid sequences.

Capsaicin inhibits catecholamine secretion and synthesis by blocking Na+ and Ca2+ influx through a vanilloid receptor-independent pathway in bovine adrenal medullary cells

We report here the effects of capsaicin, a flavoring ingredient in the hot pepper Capsicum family, on catecholamine secretion and synthesis in cultured bovine adrenal medullary cells. Capsaicin inhibited catecholamine secretion (IC(50)=9.5, 11.8, and 62 microM) stimulated by carbachol, an agonist of the nicotinic acetylcholine receptor, by veratridine, an activator of voltage-dependent Na(+) channels, and by high K(+), an activator of voltage-dependent Ca(2+) channels, respectively. Capsaicin also suppressed carbachol-induced (22)Na(+) influx (IC(50)=5.0 microM) and (45)Ca(2+) influx (IC(50)=24.4 muM), veratridine-induced (22)Na(+) influx (IC(50)=2.4 microM) and (45)Ca(2+) influx (IC(50)=1.1 microM), and high K(+)-induced (45)Ca(2+) influx (IC(50)=5.8 microM). The reduction in catecholamine secretion caused by capsaicin was not overcome by increasing the concentration of carbachol. Furthermore, capsazepine (10 microM), a competitive antagonist for the transient receptor potential vanilloid 1, and ruthenium red (30 microM), a nonselective cation channel antagonist, did not block the inhibition by capsaicin of catecholamine secretion. Capsaicin also suppressed both basal and carbachol-stimulated (14)C-catecholamine synthesis (IC(50)=10.6 and 26.4 microM, respectively) from [(14)C] tyrosine but not from L: -3, 4-dihydroxyphenyl [3-(14)C] alanine ([(14)C] DOPA) as well as tyrosine hydroxylase activity (IC(50)=8.4 and 39.0 microM, respectively). The present findings suggest that capsaicin inhibits catecholamine secretion and synthesis via suppression of Na(+) and Ca(2+) influx through a vanilloid receptor-independent pathway.

Small molecule vanilloid TRPV1 receptor antagonists approaching drug status: can they live up to the expectations?

The cloning of the transient receptor potential vanilloid type-1 (TRPV1) receptor initiated the discovery of potent small molecule antagonists, many of which are in preclinical phase or already undergoing clinical trials. While animal experiments imply a therapeutic value for these compounds as novel analgesic-antiphlogistic drugs, new findings with TRPV1 deficient (trpv1 -/-) mice signal troubles for TRPV1 antagonists as clinical research gains impetus. An emerging concept with important implications for drug development is that TRPV1 may be differentially regulated under physiological and pathological conditions. If so, it is conceivable that such TRPV1 ligands can be synthesized that specifically target TRPV1 in diseased (e.g. inflamed or neoplastic) tissues but spare TRPV1 that subserves its physiological functions in healthy organs. This review explores the current status of this field and seeks an answer to the question how these new discoveries could be factored into TRPV1 drug discovery and development.

TRPV1 (vanilloid receptor) in the urinary tract: expression, function and clinical applications

The transient receptor potential vanilloid subfamily 1 (TRPV1) is an ion channel activated by capsaicin, heat, protons and endogenous ligands such as anandamide. It is largely expressed in the urinary tract of mammals. Structures in which the receptor expression is firmly established include sensory fibers and urothelial cells, although the presence of TRPV1 in other cell types has been reported. As in other systems, pain perception was the first role attributed to TRPV1 in the urinary tract. However, it is now increasingly clear that TRPV1 also regulates the frequency of bladder reflex contractions, either through direct excitation of sensory fibers or through urothelial-sensory fiber cross talk involving the release of neuromediators from the epithelial cells. In addition, the recent identification of the receptor in urothelial and prostatic cancer cells raise the exciting hypothesis that TRPV1 is involved in cell differentiation. Desensitization of the receptor by capsaicin and resiniferatoxin has been investigated for therapeutic purposes. For the moment, lower urinary tract dysfunctions in which some benefit was obtained include painful bladder syndrome and overactive bladder of neurogenic and non-neurogenic origin. However, desensitization may become obsolete when non-toxic, potent TRPV1 antagonists become available.

Clinical Relevance of Ion Channels for Diagnosis and Therapy of Cancer

Ion channels have a critical role in cell proliferation and it is well documented that channel blockers can inhibit the growth of cancer cells. The concept of ion channels as therapeutic targets or prognostic biomarkers attracts increasing interest, but the lack of potent and selective channel modulators has hampered a critical verification for many years. Today, the knowledge of human ion channel genes is almost complete and molecular correlates for many native currents have already been identified. This information triggered a wave of experimental results, identifying individual ion channels with relevance for specific cancer types. The current pattern of cancer-related ion channels is not arbitrary, but can be reduced to few members from each ion channel family. This review aims to provide an overview of the molecularly identified ion channels that might be relevant for the most common human cancer types. Possible applications of these candidates for a targeted cancer therapy or for clinical diagnosis are discussed.