Differential activation of the volume-sensitive cation channel TRP12 (OTRPC4) and volume-regulated anion currents in HEK-293 cells

The detection of changes in volume and osmolality is an essential function in vertebrate cells. A novel member of the transient receptor potential (trp) family of ion channels, which is sensitive to changes in cell volume, has been described recently. Heterologous expression of TRP12 in HEK cells resulted in the appearance of a swelling-activated cation current. The permeability sequence of this cation current for various monovalent cations, as determined from shifts in reversal potential upon extracellular cation substitution, was PK>PCs>PNa>PLi, corresponding to an Eisenman-IV sequence characteristic for a weak-field-strength site. Surprisingly, over-expression of this channel in HEK cells was accompanied by a dramatic down-regulation of the volume-regulated anion channel (VRAC), which is activated by cell swelling in non-transfected cells. In contrast to VRAC, TRP12 could not be activated at constant volume by a reduction of intracellular ionic strength or by intracellular perfusion with guanosine 5'-O-(3-thiotriphosphate (GTPgammaS). The kinetic and pharmacological profile of VRAC and TRP12 currents were also different.     

A brief history of trp: commentary and personal perspective

The history of the discovery of the transient receptor potential (TRP) cation channel superfamily began in 1969 with Cosens and Manning's isolation of the Drosophila transient receptor potential mutant, in which the photoreceptor response decays during continuous illumination. Early studies from Minke found that the elementary light response was unaffected in trp mutants, and he attributed the defect to an intermediate stage of phototransduction. Montell and Rubin cloned the trp gene in 1989: they recognised it as a transmembrane protein, but also concluded that it did not encode the light-sensitive channels. In 1991, Minke and Selinger proposed that TRP represented a Ca2+ transporter required for refilling intracellular InsP3-sensitive Ca2+ stores, in turn required for activation of the light-sensitive channels. Also in 1991, after developing a photoreceptor patch clamp preparation, I showed that the light-sensitive channels themselves were highly permeable to Ca2+, questioning the need for such a dedicated Ca2+ transporter. In 1992, in collaboration with Minke, I resolved this paradox by showing there were two classes of light-sensitive channels, one highly Ca2+ permeable and eliminated in trp mutants. This represented the first and compelling evidence that TRP represented a light-sensitive channel and was supported by the cloning of the second light-sensitive channel, TRPL, by Kelly's lab. Three years later, in 1995, the labs of Montell and Birnbaumer independently cloned TRPC1, the first of 29 vertebrate TRP isoforms distributed amongst seven subfamilies.     

The history of the Drosophila TRP channel: the birth of a new channel superfamily

Transient receptor potential (TRP) channels are polymodal cellular sensors involved in a wide variety of cellular processes, mainly by changing membrane voltage and increasing cellular Ca(2+). This review outlines in detail the history of the founding member of the TRP family, the Drosophila TRP channel. The field began with a spontaneous mutation in the trp gene that led to a blind mutant during prolonged intense light. It was this mutant that allowed for the discovery of the first TRP channels. A combination of electrophysiological, biochemical, Ca(2+) measurements, and genetic studies in flies and in other invertebrates pointed to TRP as a novel phosphoinositide-regulated and Ca(2+)-permeable channel. The cloning and sequencing of the trp gene provided its molecular identity. These seminal findings led to the isolation of the first mammalian homologues of the Drosophila TRP channels. We now know that TRP channel proteins are conserved through evolution and are found in most organisms, tissues, and cell-types. The TRP channel superfamily is classified into seven related subfamilies: TRPC, TRPM, TRPV, TRPA, TRPP, TRPML, and TRPN. A great deal is known today about participation of TRP channels in many biological processes, including initiation of pain, thermoregulation, salivary fluid secretion, inflammation, cardiovascular regulation, smooth muscle tone, pressure regulation, Ca(2+) and Mg(2+) homeostasis, and lysosomal function. The native Drosophila photoreceptor cells, where the founding member of the TRP channels superfamily was found, is still a useful preparation to study basic features of this remarkable channel.     

TRP channels as sensors of oxygen availability

An ability to adapt to changes in oxygen availability is essential for survival in both prokaryotic and eukaryotic organisms. Recently, cation channels encoded by the transient receptor potential (trp) gene superfamily have been recognized as multimodal sensors of a wide variety of factors inside the cells and in the extracellular environment and also as transducers of electrical and chemical signals mediated by ions such as Ca²⁺. The functional features of TRP channels enable the body to react and adapt to different forms of environmental changes, including oxygen levels. A subclass of TRP channels regulates various cellular processes in response to fluctuations in oxygen. In this article, we describe the physiological and pathological significance of the oxygen-sensitive TRP channels, which are heterogeneous in the cellular responses to acute changes in oxygen, by contrasting their oxygen monitoring function with that of other ion channels, transporters, and enzymes. We also discuss the physiological relevance of oxygen-sensitive TRP channels as a novel class of target proteins for pharmaceutical therapeutics.     

Modulation of TRPs by PIPs

The TRP superfamily of cation channels encompasses 28 mammalian members related to the product of the Drosophila trp (transient receptor potential) gene. TRP channels have a widespread distribution in many cell types and organs and gate in response to a broad variety of physical and chemical stimuli; as such, they can be considered as ubiquitous cellular sensors. Several recent studies reported modulation of different TRP channels by phosphoinositides, in particular by phosphatidylinositol 4,5-bisphosphate (PIP₂). In most cases, PIP₂ promotes TRP channel activation. Here we provide a brief overview of current insights and controversies about the mechanisms and structural determinants of PIP₂–TRP channel interactions, and zoom in on the regulation of the Ca²⁺- and voltage-gated TRPM4 by phosphoinositides.     

Transient receptor potential channels as molecular substrates of receptor-mediated cation entry

Calcium is a versatile multitarget intracellular second messenger in eukaryotic cells. In addition to calcium release from intracellular stores and influx via voltage- or ligand-operated channels, agonist-induced calcium entry constitutes one of the main pathways by which cytosolic calcium is elevated. Receptor-stimulated currents are initiated in response to agonist binding to G-protein-coupled receptors and to receptor tyrosine kinases. Within the past few years our knowledge about the molecular identity of receptor-stimulated channels has expanded substantially. Drosophila melanogaster visual transduction channels associated with the transient receptor potential (trp) and the trp-like (trpl) mutant visual phenotypes were the first members of this category of channels to be identified at the molecular level. Since then an entire mammalian gene family of TRP homologues has been discovered by homology cloning. Only now are we beginning to fully understand the functional roles of TRP channels in mammalian cells. We review recent findings in TRP channel research and discuss the role of these proteins for receptor-activated cation entry.     

Drosophila TRP channels

The transient receptor potential (TRP) superfamily comprises a large group of related cation channels that display surprising diversity in the specific modes of activation and cation selectivities. However, a unifying theme is that many TRP channels play important roles in sensory physiology. The superfamily includes 28 mammalian members, which are subdivided into multiple subfamilies. Each of these subfamilies is represented by at least one of the 13 members in Drosophila, suggesting common evolutionary relationships. In recent years it has become clear that TRP channels in flies and mammals participate in similar sensory modalities. These include, but are not limited to, hearing, thermosensation, and certain specialized types of vision. With the recent flurry of new studies, 9 out of the 13 TRPs have been addressed in various contexts. As a result, the repertoire of biological roles attributed to Drosophila TRPs has increased considerably and is likely to lead to many additional surprises over the next few years.     

TRP channels and their implications in metabolic diseases

The transient receptor potential (TRP) channel superfamily is composed of 28 nonselective cation channels that are ubiquitously expressed in many cell types and have considerable functional diversity. Although changes in TRP channel expression and function have been reported in cardiovascular disease and renal disorders, the pathogenic roles of TRP channels in metabolic diseases have not been systemically reviewed. In this review, we summarised the distribution of TRP channels in several metabolic tissues and discussed their roles in mediating and regulating various physiological and pathophysiological metabolic processes and diseases including diabetes, obesity, dyslipidaemia, metabolic syndrome, atherosclerosis, metabolic bone diseases and electrolyte disturbances. This review provides new insight into the involvement of TRP channels in the pathogenesis of metabolic disorders and implicates these channels as potential therapeutic targets for the management of metabolic diseases.     

Olfactory adaptation in Drosophila larvae

To accommodate both high sensitivity as well as the ability to respond to a broad range of stimulus concentrations, an organism must possess some means of modulating the gain of its sensory systems. This phenomenon is known as adaptation. Here, we demonstrate that Drosophila larvae can adapt to three odorants in a behavioral paradigm. Larval olfactory adaptation is concentration- and dose-dependent. Olfactory and visual adaptation in Drosophila melanogaster adults is dependent on the transient receptor potential (trp) calcium channel. Recovery from olfactory adaptation, which is TRP-dependent in adults, is shown to be unaffected by a loss-of-function trp mutation in larvae. Moreover, the TRP gene product is not expressed in the larval olfactory organs. These observations suggest a role for trp in mediating sensory function that is conserved between sensory modalities in adults but is not conserved between developmental stages.     

Homo- and heteromeric assembly of TRP channel subunits

Mammalian homologues of the Drosophila melanogaster transient receptor potential (TRP) channels are the second largest cation channel family within the superfamily of hexahelical cation channels. Most mammalian TRP channels function as homooligomers and mediate mono- or divalent cation entry upon activation by a variety of stimuli. Because native TRP channels may be multimeric proteins of possibly complex composition, it is difficult to compare cation conductances in native tissues to those of clearly defined homomeric TRP channel complexes in living cells. Therefore, the possibility of heteromeric TRP channel assembly has been investigated in recent years by several groups. As a major conclusion of these studies, most heteromeric TRP channel complexes appear to consist of subunit combinations only within relatively narrow confines of phylogenetic subfamilies. Although the general capability of heteromer formation between closely related TRP channel subunits is now clearly established, we are only beginning to understand whether these heteromeric complexes are of physiological significance. This review summarizes the current knowledge on the promiscuity and specificity of the assembly of channel complexes composed of TRPC-, TRPV- and TRPM-subunits of mammalian TRP channels.     

Substance P evokes cation currents through TRP channels in HEK293 cells

Activation of any of the three known tachykinin receptors (NK1R, -2R, or -3R) can cause a rise in [Ca2+]i via a pertussis toxin-insensitive heterotrimeric G protein, Gq/G11, activation of phospholipase C (PLC), and a membrane depolarization. Tachykinins can depolarize neurons by two distinct mechanisms: 1) they reduce a resting K+ current in many neurons or 2) in parasympathetic and vagal primary sensory neurons, they activate a nonspecific cation current (Icat). Transient receptor potential channels (TRPC) are nonspecific cation channels that can be activated by a rise in [Ca2+]i in a PLC-dependent manner. The present work tests whether NK2R can signal TRPC. We applied standard whole cell patch-clamp recordings to HEK293 cells stably transfected with the human TRP3 channels (TRP3C), and transiently transfected with a functional NK2R-EGFP. Bath applied Substance P (SP, 1 microM) induced an Icat in the cells expressing both TRP3C and NK2R. Icat reached its peak value in approximately 3 min (195 +/- 120.0 s, mean +/- SE, n = 20), had a peak density of 11.3 +/- 3.48 pA/pF (n = 24), and was blocked by an NK2R-specific antagonist (SR48968, 100 nM). The Erev value for the SP current was 6.8 +/- 7.66 mV (n = 6), suggestive of a nonspecific cation channel. Icat was not measurable in TRP3C-expressing HEK293 cells without NK2R expression (n = 6) or in wild-type HEK293 cells with NK2R expression (n = 12). These data indicate that NK2R can be functionally coupled to TRP channels in HEK293 cells and suggest that SP-induced cation currents in vagal primary sensory neurons might be mediated by TRPC.     

TRP channels and mechanosensory transduction: insights into the arterial myogenic response

Mechano-gated ion channels are implicated in a variety of key physiological functions ranging from touch sensitivity to arterial pressure regulation. Seminal work in prokaryotes and invertebrates provided strong evidence for the role of specific ion channels in volume regulation, touch sensitivity, or hearing, specifically the mechanosensitive channel subunits of large and small conductances (MscL and MscS), the mechanosensory channel subunits (MEC) and the transient receptor potential channel subunits (TRP). In mammals, recent studies further indicate that members of the TRP channel family may also be considered as possible candidate mechanosensors responding to either tension, flow, or changes in cell volume. However, contradictory results have challenged whether these TRP channels, including TRPC1 and TRPC6, are directly activated by mechanical stimulation. In the present review, we will focus on the mechanosensory function of TRP channels, discuss whether a direct or indirect mechanism is at play, and focus on the proposed role for these channels in the arterial myogenic response to changes in intraluminal pressure.     

TRPV1通道蛋白介导的生理病理机制

瞬时感受器电位离子通道蛋白(transient receptor potentialion dannel protein,TRP)是一类组织分布非常广泛的非选择性阳离子通道蛋白。到目前为止,有超过30个TRP通道家族成员在哺乳动物中先后被克隆。TRPV1是近年研究较多、机制较为清楚的TRPV亚家族成员之一,主要分布于外周感觉神经。研究表明,TRPV1通道蛋白可被多种外源或内源性介质敏化或激活,其主要生理功能是感受热、痛等伤害性刺激,且与炎症、咳嗽等多种病理过程密切相关。     

Transient receptor potential cation channels in disease

The transient receptor potential (TRP) superfamily consists of a large number of cation channels that are mostly permeable to both monovalent and divalent cations. The 28 mammalian TRP channels can be subdivided into six main subfamilies: the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and the TRPA (ankyrin) groups. TRP channels are expressed in almost every tissue and cell type and play an important role in the regulation of various cell functions. Currently, significant scientific effort is being devoted to understanding the physiology of TRP channels and their relationship to human diseases. At this point, only a few channelopathies in which defects in TRP genes are the direct cause of cellular dysfunction have been identified. In addition, mapping of TRP genes to susceptible chromosome regions (e.g., translocations, breakpoint intervals, increased frequency of polymorphisms) has been considered suggestive of the involvement of these channels in hereditary diseases. Moreover, strong indications of the involvement of TRP channels in several diseases come from correlations between levels of channel expression and disease symptoms. Finally, TRP channels are involved in some systemic diseases due to their role as targets for irritants, inflammation products, and xenobiotic toxins. The analysis of transgenic models allows further extrapolations of TRP channel deficiency to human physiology and disease. In this review, we provide an overview of the impact of TRP channels on the pathogenesis of several diseases and identify several TRPs for which a causal pathogenic role might be anticipated.     

TRP channels in Drosophila photoreceptor cells

TRP cation channels are conserved throughout animal phylogeny and include many members that function in sensory physiology. The founding TRP is required for Drosophila phototransduction and has served as a paradigm for unravelling the roles and macromolecular organizations of TRP channels in native tissues. Two other TRPC channels, TRPL and TRPγ, are expressed in photoreceptor cells and form heteromultimers with TRP and with each other. TRP is a member of a supramolecular signalling complex, the signalplex, which includes the PDZ scaffold protein, INAD, and two other core members that remain bound and depend on INAD for localization. Other INAD binding proteins are proposed to interact dynamically with INAD, one of which, TRPL, undergoes light-dependent translocation in photoreceptor cells. Surprisingly, TRP has non-channel functions, including an anchoring role necessary for retaining INAD in the rhabdomeres. Loss of TRP function or constitutive TRP activity results in retinal degeneration, which can be suppressed by disruption or overexpression of the Na⁺/Ca²⁺ exchanger, CalX, respectively. Given that hypoxia-induced constitutive activity of some mammalian TRPs leads to neuronal cell death, interventions that increase Na⁺/Ca²⁺ exchanger or decrease TRP function have the potential to reduce the severity of cell death due to ischaemia.     

A network perspective on unraveling the role of TRP channels in biology and disease

Transient receptor potential (TRP) channels are a large family of non-selective cation channels that mediate numerous physiological and pathophysiological processes; however, still largely unknown are the underlying molecular mechanisms. With data generated on an unprecedented scale, network-based approaches have been revolutionizing the way in which we understand biology and disease, discover disease genes, and develop therapeutic strategies. These circumstances have created opportunities to encounter TRP channel research to data-intensive science. In this review, we provide an introduction of network-based approaches in biomedical science, describe the current state of TRP channel network biology, and discuss the future direction of TRP channel research. Network perspective will facilitate the discovery of latent roles and underlying mechanisms of TRP channels in biology and disease.     

Expression of transient receptor potential (TRP) channel mRNAs in the mouse olfactory bulb

Transient receptor potential (TRP) channels are a large family of cation channels. The 28 TRP channel subtypes in rodent are divided into 6 subfamilies: TRPC1-7, TRPV1-6, TRPM1-8, TRPP2/3/5, TRPML1-3 and TRPA1. TRP channels are involved in peripheral olfactory transduction. Several TRPC channels are expressed in unidentified neurons in the main olfactory bulb (OB), but the expression of most TRP channels in the OB has not been investigated. The present study employed RT-PCR as an initial survey of the expression of TRP channel mRNAs in the mouse OB and in 3 cell types: external tufted, mitral and granule cells. All TRP channel mRNAs except TRPV5 were detected in OB tissue. Single cell RT-PCR revealed that external tufted, mitral and granule cell populations expressed in aggregate 14 TRP channel mRNAs encompassing members of all 6 subfamilies. These different OB neuron populations expressed 7–12 channel mRNAs. Common channel expression was more similar among external tufted and mitral cells than among these cells and granule cells. These results indicate that a large number of TRP channel subtypes are expressed in OB neurons, providing the molecular bases for these channels to regulate OB neuron activity and central olfactory processing.     

Inhibition of N and PQ calcium channels by calcium entry through L channels in chromaffin cells

Transient receptor potential (TRP) polycystin 3 (TRPP3) is a member of the TRP superfamily of cation channels. Murine TRPP3 has been reported to form an acid-activated cation channel on the plasma membrane when coexpressed with the polycystin 1-like protein 3 (PKD1L3); however, the function and biophysical properties of TRPP3-dependent channels have not yet been characterized in detail. Here we show that overexpression of murine TRPP3 channel in HEK293 cells, without coexpression of PDK1-like proteins, leads to robust channel activity. These channels exhibit a high single-channel conductance of 184 pS at negative potentials, are Ca²⁺-permeable, and relatively nonselective between cations. Whole-cell experiments showed a characteristic form of voltage-dependent gating of TRPP3 channels, whereby repolarization after depolarization caused large transient inward TRPP3 tail currents. Moreover, we found that TRPP3 activity was increased upon cell swelling and by alkalization. Taken together, our results demonstrate that TRPP3, on its own, can act as a voltage-dependent, pH- and volume-sensitive plasma membrane cation channel. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Defining features of the hair cell mechanoelectrical transducer channel

This review summarizes current knowledge of the hair cell mechanotransducer channel, the ion channel responsible for detecting mechanical stimuli in the inner ear and one of the few channels whose molecular structure is still unknown. Several candidate proteins have been proposed, especially members of the transient receptor potential (TRP) channel family, but all have so far failed in one test or another. Furthermore, none has biophysical properties exactly matching the native channel. The defining features of the native mechanotransducer channel are documented, including ionic permeability, channel structure inferred from blocking agents, diversity in channel conductance, and regulation by Ca²⁺, which are compared with a potential candidate, TRP channels of the polycystin family. The strengths and weaknesses of a TRP channel contender are discussed.