Structure-function analysis of TRPV channels

A bibliometrics analysis and visualization study of TRPV1 channel

Background: At the end of the 1990s, transient receptor potential vanilloid 1 (TRPV1) was first identified and cloned, serving as a key pain and heat sensor in humans. A large body of evidence have revealed its polymodal structure, complex function and wide-spread distribution, the specific mechanism of the ion channel remains unclear. Our goal here is to perform a bibliometric analysis and visualization study to present hotspots and trends in TRPV1 channel. Materials and Methods: TRPV1-related publications from inception to 2022 were retrieved from the Web of Science database. Excel, VOSviewer, and CiteSpace software were utilized for co-authorship, co-citation and co-occurrence analysis. Results: There were 9,113 publications included in the study, the number of publications increased rapidly after 1989, from 7 in 1990 to 373 in 2007, during which the number of citations per publication (CPP) also reached a peak in 2000 (CPP = 106.52). A total of 1,486 journals published TRPV1 articles, mainly belong to Q1 or Q2 divisions; The United States published the most articles (TP = 3,080), followed by Japan (TP = 1,221), China (TP = 1,217), and England (TP = 734); In recent years, the TRPV1-related research direction has been broaden to multiple fields related to inflammation, oxidative stress, and apoptosis; Keyword clustering refined the topic distributions and could be generalized as neuralgia, endogenous cannabinoid system, TRPV1 mediated airway hyperresponsiveness, involvement of apoptosis, TRPV1 antagonists as therapy targets. Conclusion: By conducting an exhaustive bibliographic search, this review refined the topic distributions and generalized as neuralgia, endogenous cannabinoid system, TRPV1 mediated airway hyperresponsiveness, involvement of apoptosis, TRPV1 antagonists as therapy targets. It is currently being clarified how exactly TRPV1 works as an ion channel, and much more in-depth basic research is needed in the future.

Role of TRPM2 in brain tumours and potential as a drug target

Ion channels are ubiquitously expressed in almost all living cells, and are the third-largest category of drug targets, following enzymes and receptors. The transient receptor potential melastatin (TRPM) subfamily of ion channels are important to cell function and survival. Studies have shown upregulation of the TRPM family of ion channels in various brain tumours. Gliomas are the most prevalent form of primary malignant brain tumours with no effective treatment; thus, drug development is eagerly needed. TRPM2 is an essential ion channel for cell function and has important roles in oxidative stress and inflammation. In response to oxidative stress, ADP-ribose (ADPR) is produced, and in turn activates TRPM2 by binding to the NUDT9-H domain on the C-terminal. TRPM2 has been implicated in various cancers and is significantly upregulated in brain tumours. This article reviews the current understanding of TRPM2 in the context of brain tumours and overviews the effects of potential drug therapies targeting TRPM2 including hydrogen peroxide (H2O2), curcumin, docetaxel and selenium, paclitaxel and resveratrol, and botulinum toxin. It is long withstanding knowledge that gliomas are difficult to treat effectively, therefore investigating TRPM2 as a potential therapeutic target for brain tumours may be of considerable interest in the fields of ion channels and pharmacology.

Transient Receptor Potential (TRP) and Thermoregulation in Animals: Structural Biology and Neurophysiological Aspects

This review presents and analyzes recent scientific findings on the structure, physiology, and neurotransmission mechanisms of transient receptor potential (TRP) and their function in the thermoregulation of mammals. The aim is to better understand the functionality of these receptors and their role in maintaining the temperature of animals, or those susceptible to thermal stress. The majority of peripheral receptors are TRP cation channels formed from transmembrane proteins that function as transductors through changes in the membrane potential. TRP are classified into seven families and two groups. The data gathered for this review include controversial aspects because we do not fully know the mechanisms that operate the opening and closing of the TRP gates. Deductions, however, suggest the intervention of mechanisms related to G protein-coupled receptors, dephosphorylation, and ligands. Several questions emerge from the review as well. For example, the future uses of these data for controlling thermoregulatory disorders and the invitation to researchers to conduct more extensive studies to broaden our understanding of these mechanisms and achieve substantial advances in controlling fever, hyperthermia, and hypothermia.

Dynamic evolution of transient receptor potential vanilloid (TRPV) ion channel family with numerous gene duplications and losses

The transient receptor potential vanilloid (TRPV) ion channel family is involved in multiple sensory and physiological functions including thermosensing and temperature-dependent neuroendocrine regulation. The objective of the present study was to investigate the number, origin and evolution of TRPV genes in metazoans, with special focus on the impact of the vertebrate whole-genome duplications (WGD). Gene searches followed by phylogenetic and synteny analyses revealed multiple previously undescribed TRPV genes. The common ancestor of Cnidaria and Bilateria had three TRPV genes that became four in the deuterostome ancestor. Two of these were lost in the vertebrate ancestor. The remaining two genes gave rise to two TRPV subfamilies in vertebrates, consisting of subtypes 1, 2, 3, 4, 9 and 5, 6, 7, 8, respectively. This gene expansion resulted from the two basal vertebrate WGD events (1R and 2R) and three local duplications before the radiation of gnathostomes. TRPV1, 4 and 5 have been retained in all gnathostomes investigated, presumably reflecting important functions. TRPV7 and 8 have been lost independently in various lineages but are still retained in cyclostomes, actinistians (coelacanth), amphibians, prototherians and basal actinopterygians (Polypteridae). TRPV3 and 9 are present in extant elasmobranchs, while TRPV9 was lost in the osteichthyan ancestor and TRPV3 in the actinopterygian ancestor. The coelacanth has retained the ancestral osteichthyan repertoire of TRPV1, 3, 4, 5, 7 and 8. TRPV2 arose in the tetrapod ancestor. Duplications of TRPV5 occurred independently in various lineages, such as cyclostomes, chondrichthyans, anuran amphibians, sauropsids, mammals (where the duplicate is called TRPV6), and actinopterygians (Polypteridae and Esocidae). After the teleost-specific WGD (3R) only TRPV1 retained its duplicate, whereas TRPV4 and 5 remained as single genes. Both 3R-paralogs of TRPV1 were kept in some teleost species, while one paralog was lost in others. The salmonid-specific WGD (4R) duplicated TRPV1, 4, and 5 leading to six TRPV genes. The largest number was found in Xenopus tropicalis with no less than 15 TRPV genes. This study provides a comprehensive evolutionary scenario for the vertebrate TRPV family, revealing additional TRPV types and proposing a phylogeny-based classification of TRPV across metazoans.

TRPV Protein Family-From Mechanosensing to Cancer Invasion

Biophysical cues from the cellular microenvironment are detected by mechanosensitive machineries that translate physical signals into biochemical signaling cascades. At the crossroads of extracellular space and cell interior are located several ion channel families, including TRP family proteins, that are triggered by mechanical stimuli and drive intracellular signaling pathways through spatio-temporally controlled Ca2+-influx. Mechanosensitive Ca2+-channels, therefore, act as critical components in the rapid transmission of physical signals into biologically compatible information to impact crucial processes during development, morphogenesis and regeneration. Given the mechanosensitive nature of many of the TRP family channels, they must also respond to the biophysical changes along the development of several pathophysiological conditions and have also been linked to cancer progression. In this review, we will focus on the TRPV, vanilloid family of TRP proteins, and their connection to cancer progression through their mechanosensitive nature.

Letrozole induced low estrogen levels affected the expressions of duodenal and renal calcium-processing gene in laying hens

Estrogen regulates the calcium homeostasis in hens, but the mechanisms involved are still unclear fully. In this study, we investigated whether letrozole (LZ) induced low estrogen levels affected the calcium absorption and transport in layers. In the duodenum, we observed a significant decrease of mRNA expressions of Calbindin-28k (CaBP-28k) and plasma membrane Ca²⁺-ATPase (PMCA 1b) while CaBP-28k protein expression was declined in birds with LZ treatment, and the mRNA levels of duodenal transient receptor potential vanilloid 6 (TRPV6) and Na⁺/Ca²⁺ exchanger 1 (NCX1) were not affected. Interestingly, we observed the different changes in the kidney. The renal mRNA expressions of TRPV6 and NCX1 were unregulated while the PMCA1b was down-regulated in low estrogen layers, however, the CaBP-28k gene and protein expressions were no changed in the kidney. Furthermore, it showed that the duodenal estradiol receptor 2 (ESR2) transcripts rather than parathyroid hormone 1 receptor (PTH1R) and calcitonin receptor (CALCR) played key roles to down-regulate calcium transport in LZ-treated birds. In conclusion, CaBP-28k, PMCA 1b and ESR2 genes in the duodenum may be primary targets for estrogen regulation in order to control calcium homeostasis in hens.

Plasma membrane stretch activates transient receptor potential vanilloid and ankyrin channels in Merkel cells from hamster buccal mucosa

Merkel cells (MCs) have been proposed to form a part of the MC-neurite complex with sensory neurons. Many transient receptor potential (TRP) channels have been identified in mammals; however, the activation properties of these channels in oral mucosal MCs remain to be clarified. We investigated the biophysical and pharmacological properties of TRP vanilloid (TRPV)-1, TRPV2, TRPV4, TRP ankyrin (TRPA)-1, and TRP melastatin (TRPM)-8 channels, which are sensitive to osmotic and mechanical stimuli by measurement of intracellular free Ca(2+) concentration ([Ca(2+)]i) using fura-2. We also analyzed their localization patterns through immunofluorescence. MCs showed immunoreaction for TRPV1, TRPV2, TRPV4, TRPA1, and TRPM8 channels. In the presence of extracellular Ca(2+), the hypotonic test solution evoked Ca(2+) influx. The [Ca(2+)]i increases were inhibited by TRPV1, TRPV2, TRPV4, or TRPA1 channel antagonists, but not by the TRPM8 channel antagonist. Application of TRPV1, TRPV2, TRPV4, TRPA1, or TRPM8 channel selective agonists elicited transient increases in [Ca(2+)]i only in the presence of extracellular Ca(2+). The results indicate that membrane stretching in MCs activates TRPV1, TRPV2, TRPV4, and TRPA1 channels, that it may be involved in synaptic transmission to sensory neurons, and that MCs could contribute to the mechanosensory transduction sequence.

The many faces of calmodulin in cell proliferation, programmed cell death, autophagy, and cancer

Calmodulin (CaM) is a ubiquitous Ca(2+) receptor protein mediating a large number of signaling processes in all eukaryotic cells. CaM plays a central role in regulating a myriad of cellular functions via interaction with multiple target proteins. This review focuses on the action of CaM and CaM-dependent signaling systems in the control of vertebrate cell proliferation, programmed cell death and autophagy. The significance of CaM and interconnected CaM-regulated systems for the physiology of cancer cells including tumor stem cells, and processes required for tumor progression such as growth, tumor-associated angiogenesis and metastasis are highlighted. Furthermore, the potential targeting of CaM-dependent signaling processes for therapeutic use is discussed.

Modulating molecular chaperones improves sensory fiber recovery and mitochondrial function in diabetic peripheral neuropathy

Quantification of intra-epidermal nerve fibers (iENFs) is an important approach to stage diabetic peripheral neuropathy (DPN) and is a promising clinical endpoint for identifying beneficial therapeutics. Mechanistically, diabetes decreases neuronal mitochondrial function and enhancing mitochondrial respiratory capacity may aid neuronal recovery from glucotoxic insults. We have proposed that modulating the activity and expression of heat shock proteins (Hsp) may be of benefit in treating DPN. KU-32 is a C-terminal Hsp90 inhibitor that improved thermal hypoalgesia in diabetic C57Bl/6 mice but it was not determined if this was associated with an increase in iENF density and mitochondrial function. After 16 weeks of diabetes, Swiss Webster mice showed decreased electrophysiological and psychosensory responses and a >30% loss of iENFs. Treatment of the mice with ten weekly doses of 20mg/kg KU-32 significantly reversed pre-existing deficits in nerve conduction velocity and responses to mechanical and thermal stimuli. KU-32 therapy significantly reversed the pre-existing loss of iENFs despite the identification of a sub-group of drug-treated diabetic mice that showed improved thermal sensitivity but no increase in iENF density. To determine if the improved clinical indices correlated with enhanced mitochondrial activity, sensory neurons were isolated and mitochondrial bioenergetics assessed ex vivo using extracellular flux technology. Diabetes decreased maximal respiratory capacity in sensory neurons and this deficit was improved following KU-32 treatment. In conclusion, KU-32 improved physiological and morphologic markers of degenerative neuropathy and drug efficacy may be related to enhanced mitochondrial bioenergetics in sensory neurons.

Introduction to TRP channels: structure, function, and regulation

Transient receptor potential or TRP families of ion channels demonstrate great diversity in activation and inhibition, and they are diverse in selectivity of ion conductance. TRP ion channels function as signal integrators through their ion conductance properties, and in some cases kinase activity. They mediate processes such as vision, taste, olfaction, hearing, touch, and thermo- and osmosensation. TRP cation channels function by mediating the flux of Na(+) and Ca(2+) across the plasma membrane and into the cytoplasm. The influx of cations into the cytoplasm depolarizes cells and is necessary for action potentials in excitable cells such as neurons. In non-excitable cells, membrane depolarization by TRP ) and-channels stimulates voltage- dependent channels (Ca(2+), K(+), Cl(-) influences many cellular events, such as transcription, translation, contraction, and migration. TRP channels are important in human physiology, and mutations in TRP genes are associated with at least four diseases. Furthermore, altered expression, function, and/or regulation of TRP channels have been implicated in diseases such as pulmonary hypertension.

Calcium transport in strongly calcifying laying birds: mechanisms and regulation

Birds that lay long clutches (series of eggs laid sequentially before a "pause day"), among them the high-producing, strongly-calcifying Gallus gallus domesticus (domestic hen) and Coturnix coturnix japonica (Japanese quail), transfer about 10% of their total body calcium daily. They appear, therefore, to be the most efficient calcium-transporters among vertebrates. Such intensive transport imposes severe demands on ionic calcium (Ca2+) homeostasis, and activates at least two extremely effective mechanisms for Ca2+ transfer from food and bone to the eggshell. This review focuses on the development, action and regulation of the mechanisms associated with paracellular and transcellular Ca2+ transport in the intestine and the eggshell gland (ESG); it also considers some of the proteins (calbindin, Ca2+ATPase, Na+/Ca2+ exchange, epithelial calcium channels (TRPVs), osteopontin and carbonic anhydrase (CA) associated with this phenomenon. Calbindins are discussed in some detail, as they appear to be a major component of the transcellular transport system, and as only they have been studied extensively in birds. The review aims to gather old and new knowledge, which could form a conceptual basis, albeit not a completely accepted one, for our understanding of the mechanisms associated with this phenomenon. In the intestine, the transcellular pathway appears to compensate for low Ca2+ intake, but in birds fed adequate calcium the major drive for calcium absorption remains the electrochemical potential difference (ECPD) that facilitates paracellular transport. However, the mechanisms involved in Ca2+ transport into the ESG lumen are not yet established. In the ESG, the presence of Ca2+-ATPase and calbindin--two components of the transcellular transport pathway--and the apparently uphill transport of Ca2+ support the idea that Ca2+ is transported via the transcellular pathway. However, the positive (plasma with respect to mucosa) electrical potential difference (EPD) in the ESG, among other findings, indicates that there may be major alternative or complementary paracellular passive transport pathways. The available evidence hints that the flow from the gut to the ESG, which occurs during a relatively short period (11 to 14 h out the 24- to 25.5-h egg cycle), is primarily driven by carbonic anhydrase (CA) activity in the ESG, which results in high HCO3(-) content that, in turn, "sucks out" Ca2+ from the intestinal lumen via the blood and ESG cells, and deposits it in the shell crystals. The increased CA activity appears to be dependent on energy input, whereas it seems most likely that the Ca2+ movement is secondary, that it utilizes passive paracellular routes that fluctuate in accordance with the appearance of the energy-dependent CA activity, and that the level of Ca2+ movement mimics that of the CA activity. The on-off signals for the overall phenomenon have not yet been identified. They appear to be associated with the circadian cycle of gonadal hormones, coupled with the egg cycle: it is most likely that progesterone acts as the "off" signal, and that the "on" signal is provided by the combined effect of an as-yet undefined endocrine factor associated with ovulation and with the mechanical strain that results from "egg white" formation and "plumping". This strain may initially trigger the formation of the mammillae and the seeding of shell calcium crystals in the isthmus, and thereafter initiate the formation of the shell in the ESG.

TRP channels entering the structural era

Transient receptor potential (TRP) channels are important in many neuronal and non-neuronal physiological processes. The past 2 years have seen much progress in the use of structural biology techniques to elucidate molecular mechanisms of TRP channel gating and regulation. Two approaches have proven fruitful: (i) a divide-and-conquer strategy has provided high-resolution structural details of TRP channel fragments although it fails to explain how these fragments are integrated in the full channel; and (ii) electron microscopy of entire TRP channels has yielded low-resolution images that provide a basis for testable models of TRP channel architecture. The results of each approach, summarized in this review, provide a preview of what the future holds in TRP channel structural biology.

Structure of TRPV1 channel revealed by electron cryomicroscopy

The transient receptor potential (TRP) family of ion channels participate in many signaling pathways. TRPV1 functions as a molecular integrator of noxious stimuli, including heat, low pH, and chemical ligands. Here, we report the 3D structure of full-length rat TRPV1 channel expressed in the yeast Saccharomyces cerevisiae and purified by immunoaffinity chromatography. We demonstrate that the recombinant purified TRPV1 channel retains its structural and functional integrity and is suitable for structural analysis. The 19-A structure of TRPV1 determined by using single-particle electron cryomicroscopy exhibits fourfold symmetry and comprises two distinct regions: a large open basket-like domain, likely corresponding to the cytoplasmic N- and C-terminal portions, and a more compact domain, corresponding to the transmembrane portion. The assignment of transmembrane and cytoplasmic regions was supported by fitting crystal structures of the structurally homologous Kv1.2 channel and isolated TRPV1 ankyrin repeats into the TRPV1 structure.

Transient receptor potential vanilloid type 2 (TRPV2) expression in normal urothelium and in urothelial carcinoma of human bladder: correlation with the pathologic stage

OBJECTIVE

To evaluate the expression of transient receptor potential vanilloid type 2 (TRPV2) in normal human bladder and urothelial carcinoma (UC) tissues.

METHODS

Bladder specimens were obtained by transurethral resection or radical cystectomy. TRPV2 mRNA expression in normal human urothelial cells (NHUCs), UC cell lines, and formalin-fixed paraffin-embedded normal (n=6) and cancer bladder tissues (n=58) was evaluated by polymerase chain reaction (PCR) and quantitative real-time PCR (RT-PCR). TRPV2 protein expression was assessed by cytofluorimetric and confocal microscopy analyses in NHUCs and UC cells and by Western blotting and immunohistochemistry in normal and UC tissues.

RESULTS

Enhanced TRPV2 mRNA and protein expression was found in high-grade and -stage UC specimens and UC cell lines. Both the full-length TRPV2 (hTRPV2) and a short splice-variant (s-TRPV2) were detected in NHUC and normal bladder specimens, whereas a progressive decline of s-TRPV2 in pTa, pT1, and pT2 stages was observed, up to a complete loss in pT3 and pT4 UC specimens.

CONCLUSIONS

Normal human urothelial cells and bladder tissue specimens express TRPV2 at both the mRNA and protein levels. A progressive loss of s-TRPV2 accompanied by a marked increase of hTRPV2 expression was found in high-grade and -stage UC tissues.

The ankyrin repeats of TRPV1 bind multiple ligands and modulate channel sensitivity

TRPV1 plays a key role in nociception, as it is activated by heat, low pH, and ligands such as capsaicin, leading to a burning pain sensation. We describe the structure of the cytosolic ankyrin repeat domain (ARD) of TRPV1 and identify a multiligand-binding site important in regulating channel sensitivity within the TRPV1-ARD. The structure reveals a binding site that accommodates triphosphate nucleotides such as ATP, and biochemical studies demonstrate that calmodulin binds the same site. Electrophysiology experiments show that either ATP or PIP2 prevent desensitization to repeated applications of capsaicin, i.e., tachyphylaxis, while calmodulin plays an opposing role and is necessary for tachyphylaxis. Mutations in the TRPV1-ARD binding site eliminate tachyphylaxis. We present a model for the calcium-dependent regulation of TRPV1 via competitive interactions of ATP and calmodulin at the TRPV1-ARD-binding site and discuss its relationship to the C-terminal region previously implicated in interactions with PIP2 and calmodulin.

Prediction of Ion Channel Activity Using Binary Kernel Discrimination

Voltage-gated ion channels are a diverse family of pharmaceutically important membrane proteins for which limited 3D information is available. A number of virtual screening tools have been used to assist with the discovery of new leads and with the analysis of screening results. One such tool, and the subject of this paper, is binary kernel discrimination (BKD), a machine-learning approach that has recently been applied to applications in chemoinformatics. It uses a training set of compounds, for which both structural and qualitative activity data are known, to produce a model that can then be used to rank another set of compounds in order of likely activity. Here, we report the use of BKD to build models for the prediction of five different ion channel targets using two types of activity data. The results obtained suggest that the approach provides an effective way of prioritizing compounds for acquisition and testing.

TRPV6

The ion channel TRPV6 is likely to function as an epithelial calcium channel in organs with high calcium transport requirements such as the intestine, kidney, and placenta. Transcriptional regulation of TRPV6 messenger RNA (mRNA) is controlled by 1,25-dihydroxyvitamin D, which is the active hormonal form of vitamin D3, and by additional calcium-dependent and vitamin D3-independent mechanisms. Under physiological conditions, the conductance of the channel itself is highly calcium-selective and underlies complex inactivation mechanisms triggered by intracellular calcium and magnesium ions. There is growing evidence that transcriptional regulation of TRPV6 in certain tissues undergoing malignant transformation, such as prostate cancer, is linked to cancer progression.

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.

Identification of two domains involved in the assembly of transient receptor potential canonical channels

Transient receptor potential canonical (TRPC) channels are associated with calcium entry activity in nonexcitable cells. TRPCs can form homo- or heterotetrameric channels, in which case they can assemble together within a subfamily groups. TRPC1, 4, and 5 represent one group, and TRPC3, 6, and 7 represent the other. The molecular determinants involved in promoting subunit tetramerization are not known. To identify them, we generated chimeras by swapping the different domains of TRPC4 with the same regions in TRPC6. We showed that TRPC4 coimmunoprecipitated with the chimeras containing the ankyrin repeats and coiled-coil domains of TRPC4 into TRPC6. However, chimeras containing only the ankyrin repeats or only the coiled-coil domain of TRPC4 did not coimmunoprecipitate with TRPC4. We also showed that a second domain of interaction composed of the pore region and the C-terminal tail is involved in the oligomerization of TRPC4. However, chimeras containing only the pore region or only the C-terminal tail of TRPC4 did not coimmunoprecipitate with TRPC4. Furthermore, we showed that the N terminus of TRPC6 coimmunoprecipitated with the C terminus of TRPC6. Overexpression in HEK293T cells of chimeras that contained an N terminus and a C terminus from different subfamily groups increased intracellular calcium entry subsequent to stimulation of G(q) protein-coupled receptors. These results suggest that two types of interactions are involved in the assembly of the four subunits of the TRPC channel. The first interaction occurs between the N termini and involves two regions. The second interaction occurs between the N terminus and the C terminus and does not appear to be necessary for the activity of TRPCs.

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.

An introduction to TRP channels

The aim of this review is to provide a basic framework for understanding the function of mammalian transient receptor potential (TRP) channels, particularly as they have been elucidated in heterologous expression systems. Mammalian TRP channel proteins form six-transmembrane (6-TM) cation-permeable channels that may be grouped into six subfamilies on the basis of amino acid sequence homology (TRPC, TRPV, TRPM, TRPA, TRPP, and TRPML). Selected functional properties of TRP channels from each subfamily are summarized in this review. Although a single defining characteristic of TRP channel function has not yet emerged, TRP channels may be generally described as calcium-permeable cation channels with polymodal activation properties. By integrating multiple concomitant stimuli and coupling their activity to downstream cellular signal amplification via calcium permeation and membrane depolarization, TRP channels appear well adapted to function in cellular sensation. Our review of recent literature implicating TRP channels in neuronal growth cone steering suggests that TRPs may function more widely in cellular guidance and chemotaxis. The TRP channel gene family and its nomenclature, the encoded proteins and alternatively spliced variants, and the rapidly expanding pharmacology of TRP channels are summarized in online supplemental material.

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.

Human TRPV4 channel splice variants revealed a key role of ankyrin domains in multimerization and trafficking

The TRPV4 cation channel exhibits a topology consisting of six predicted transmembrane domains (TM) with a putative pore loop between TM5 and TM6 and intracellular N- and C-tails, the former containing at least three ankyrin domains. Functional transient receptor potential (TRP) channels are supposed to result following the assembly of four subunits. However, the rules governing subunit assembly and protein domains implied in this process are only starting to emerge. The ankyrin, TM, and the C-tail domains have been identified as important determinants of the oligomerization process. We now describe the maturation and oligomerization of five splice variants of the TRPV4 channel. The already known TRPV4-A and TRPV4-B (delta384-444) variants and the new TRPV4-C (delta237-284), TRPV4-D (delta27-61), and TRPV4-E (delta237-284 and delta384-444) variants. All alternative spliced variants involved deletions in the cytoplasmic N-terminal region, affecting (except for TRPV4-D) the ankyrin domains. Subcellular localization, fluorescence resonance energy transfer, co-immunoprecipitation, glycosylation profile, and functional analysis of these variants permitted us to group them into two classes: group I (TRPV4-A and TRPV4-D) and group II (TRPV4-B, TRPV4-C, and TRPV4-E). Group I, unlike group II variants, were correctly processed, homo- and heteromultimerized in the endoplasmic reticulum, and were targeted to the plasma membrane where they responded to typical TRPV4 stimuli. Our results suggest that: 1) TRPV4 biogenesis involves core glycosylation and oligomerization in the endoplasmic reticulum followed by transfer to the Golgi apparatus for subsequent maturation; 2) ankyrin domains are necessary for oligomerization of TRPV4; and 3) lack of TRPV4 oligomerization determines its accumulation in the endoplasmic reticulum.

Specific detection and semi-quantitative analysis of TRPC4 protein expression by antibodies

In mouse tissues two variants of the transient receptor potential (canonical) (TRPC) 4 protein are expressed: the "full-length" TRPC4 protein and a slightly smaller variant, called TRPC4Delta(761-864), which lacks 84 amino acid residues. Although the presence of mRNA encoding the TRPC4 protein in mammalian cells and the detection of the heterologously expressed TRPC4 protein by Western blot analysis have been reported, the unequivocal detection of endogenous TRPC4 proteins has proven difficult. In the present study we compared polyclonal antibodies for the detection of TRPC4 proteins in mouse tissues and monitored their specificity and reliability by analysing corresponding tissues from TRPC4-deficient mice. In addition we introduced a procedure that allows us to estimate the amount of TRPC4 protein expressed in a single cell. Using this technique it appears that the amount of TRPC4 protein expressed stably in HEK 293 cells is at least fourfold higher than the amount of TRPC4 protein expressed endogenously in the bovine adrenocortical cell line SBAC.

Functional characterization and physiological relevance of the TRPC3/6/7 subfamily of cation channels

The mammalian transient receptor potential (TRP) superfamily of cation channels can be divided into six major families. Among them, the "classical" or "canonical" TRPC family is most closely related to Drosophila TRP, the founding member of the superfamily. All seven channels of this family designated TRPC1-7 share the common property of activation through phospholipase C (PLC)-coupled receptors, but their gating by receptor- or store-operated mechanisms is still controversial. The TRPC3, 6, and 7 channels are 75% identical and are also gated by direct exposure to diacylglycerols (DAG). TRPC3, 6, and 7 interact physically and, upon coexpression, coassemble to form functional tetrameric channels. This review will focus on the TRPC3/6/7 subfamily and describe their functional properties and regulation as homomers obtained from overexpression studies in cell lines. It will also summarize their heteromultimerization potential in vitro and in vivo and will present preliminary data concerning their physiological functions analyzed in isolated tissues with downregulated channel activity and gene-deficient mouse models.

Transcriptional regulation and processing increase the functional variability of TRPM channels

Mammalian TRP channels display heterogenous biophysical properties and are involved in a variety of signal transduction pathways. To carry out their diverse biological functions and to adapt these functions to changes of the environment, mechanisms to regulate their molecular structure are required. Transcriptional regulation and post-transcriptional RNA processing represent essential instruments to generate TRP channel variants with modified properties. TRP variants are expressed depending on the tissue and developmental state. They can show distinct biophysical properties and mechanisms of activation, and thereby determine channel function and malfunction in certain human diseases. In this review, we give an overview of the variants of a given TRP gene, with the focus on the TRPM subfamily, and discuss their relevance with respect to their function under physiological and pathological conditions.