TRP channels and Ca2+ signaling

There is a rapidly growing interest in the family of transient receptor potential (TRP) channels because TRP channels are not only important for many sensory systems, but they are crucial components of the function of neurons, epithelial, blood and smooth muscle cells. These facts make TRP channels important targets for treatment of diseases arising from the malfunction of these channels in the above cells and for treatment of inflammatory pain. TRP channels are also important for a growing number of genetic diseases arising from mutations in various types of TRP channels. The Minerva-Gentner Symposium on TRP channels and Ca(2+) signaling, which took place in Eilat, Israel (February 24-28, 2006) has clearly demonstrated that the study of TRP channels is a newly emerging field of biomedicine with prime importance. In the Eilat symposium, investigators who have contributed seminal publications and insight into the TRP field presented their most recent, and in many cases still unpublished, studies. The excellent presentations and excitement generated by them demonstrated that much progress has been achieved. Nevertheless, it was also evident that the field of TRP channels is still in its infancy in comparison to other fields of ion channels, and even the fundamental knowledge of the gating mechanism of TRP channels is still unsolved. The beautiful location of the symposium, together with informal intensive discussions among the participants, contributed to the success of this meeting.     

Transient receptor potential, TRP channels: a new family of channels broadly expressed

Transient receptor potential, TRP channels are a new superfamily of functionally versatile non-selective cation channels present from yeast to mammals. On the basis of their structural homology, TRP channels are subdivided in 7 groups : TRPC 1-7 Canonical, TRPV 1-6 Vanilloid, TRPM 1-8 Melastatin, TRPP 1-3 Polycystin, TRPML Mucolipin, TRPA Ankyrin and TRPN (NO mechanotransducer potential C), the latter not expressed in mammals. Their cloning and heterologous expression allowed to demonstrating that these channels are generally weakly voltage-dependent. They are activated by various ligands involving a signal transduction cascade as well as directly by multiple compounds, heat and pH. TRP channels are found in a broad range of cell types. TRP channels are essential in allowing animals to sense the outside world and cells to sense their local environment. Following mutations or anomalous behaviour, these channels have a major role in several human diseases.     

Interplay between TRP channels and the cytoskeleton in health and disease

Transient receptor potential (TRP) channels are a family of cation channels that play a key role in ion homeostasis and cell volume regulation. In addition, TRP channels are considered universal integrators of sensory information required for taste, vision, hearing, touch, temperature, and the detection of mechanical force. Seminal investigations exploring the molecular mechanisms of phototransduction in Drosophila have demonstrated that TRP channels operate within macromolecular complexes closely associated with the cytoskeleton. More recent evidence shows that mammalian TRP channels similarly connect to the cytoskeleton to affect cytoskeletal organization and cell adhesion via ion-transport-dependent and -independent mechanisms. In this review, we discuss new insights into the interplay between TRP channels and the cytoskeleton and provide recent examples of such interactions in different physiological systems.     

Mechanism of Ca(2+)-dependent desensitization in TRP channels

The 30+ members of the family of TRP channels are diverse in their physiological roles, yet the mechanisms that regulate their gating may be conserved. In particular, all TRP channels show an activity-dependent inhibition which is mediated by Ca(2+). The mechanism by which Ca(2+) inhibits TRP channels is currently a matter of intense debate, with Ca(2+)-regulated kinases, phosphatases, phospholipases and calmodulin all proposed to be involved. In this review, we will discuss different mechanisms for Ca(2+)-dependent desensitization in TRP channels. We will conclude with a model that focuses on Ca(2+)-dependent activation of phospholipase C and Ca(2+) binding to calmodulin and propose that the phospholipase C and calmodulin pathways are structurally and functionally coupled.     

The role of adipose TRP channels in the pathogenesis of obesity

The prevalence of obesity is continuously increasing worldwide. Transient receptor potential (TRP) channels constitute a family of nonselective cation channels that are ubiquitously expressed in mammalian tissues, including adipose tissue. Although TRP channels might be regarded as therapeutic targets for obesity due to the inhibitory effects of their agonists on body weight and adiposity, the exact role of TRP channels in the development of obesity by modulating the function of adipose tissue has not been systemically reviewed. Multiple TRP channels are present in adipocytes and are involved in diverse aspects of cellular function, including differentiation and maturation of white adipose tissue (WAT), browning of WAT and thermogenesis of brown adipose tissue (BAT). Most of these functions are mediated by alterations in intracellular Ca²⁺ levels or subcellular Ca²⁺ signaling pathway. TRP channels influence intracellular Ca²⁺ dynamics through directly mediating Ca²⁺ entry (TRPVs and others) or store-operated mechanisms (TRPCs). Intracellular Ca²⁺ displays a biphasic effect on regulation adipocyte behaviors depending on the differentiation stage, which may account for the different roles of individual TRP channels in regulation of adiposity. This review emphasizes the contribution of TRP channels to obesity and provide an in-depth discussion on the complexity of their mechanism of actions.     

TRP gating is linked to the metabolic state and maintenance of the Drosophila photoreceptor cells

The Drosophila light-activated channel TRP is the founding member of a large and diverse family of channel proteins that is conserved throughout evolution. In spite of much progress, the gating mechanism of TRP channels is still unknown. However, recent studies have shown multi-faceted functions of the Drosophila light-sensitive TRP channel that may shed light on TRP gating. Accordingly, metabolic stress, which leads to depletion of cellular ATP, reversibly activates the Drosophila TRP and TRPL channels in the dark in a constitutive manner. In several Drosophila mutants, constitutive activity of TRP channels lead to a rapid retinal degeneration in the dark, while genetic elimination of TRP protects the cells from degeneration. Additional studies have shown that TRPL translocates in a light-dependent manner between the signaling membranes and the cell body. This light-activated translocation is accompanied by reversible morphological changes leading to partial and reversible collapse of the microvillar signaling membranes into the cytosol, which allows turnover of signaling molecules. These morphological changes are also blocked by genetic elimination of TRP channels. The link of TRP gating to the metabolic state and maintenance of cells makes cells expressing TRP extremely vulnerable to metabolic stress via a mechanism that may underlie retinal degeneration and neuronal cell death upon malfunction.     

Heteromerization of TRP channel subunits: extending functional diversity

Transient receptor potential (TRP) channels are widely found throughout the animal kingdom. By serving as cellular sensors for a wide spectrum of physical and chemical stimuli, they play crucial physiological roles ranging from sensory transduction to cell cycle modulation. TRP channels are tetrameric protein complexes. While most TRP subunits can form functional homomeric channels, heteromerization of TRP channel subunits of either the same subfamily or different subfamilies has been widely observed. Heteromeric TRP channels exhibit many novel properties compared to their homomeric counterparts, indicating that co-assembly of TRP channel subunits has an important contribution to the diversity of TRP channel functions.     

瞬时受体电位通道研究进展

瞬时受体电位通道(TRP channels)是位于细胞膜上的一类重要的阳离子通道超家族.根据氨基酸序列的同源性,将已发现的28种哺乳动物,TRP通道分为:TRPC、TRPV、TRPM、TRPA、TRPP和TRPML 6个亚家族.所有的TRP通道都具有6次跨膜结构域.不同的TRP通道对钙离子和钠离子选择性不同.TRP通道分布广泛,调节机制各异,通过感受细胞内外环境的各种刺激,参与痛温觉、机械感觉、味觉的发生和维持细胞内外环境的离子稳态等众多生命活动.     

Application of Amphipols for Structure–Functional Analysis of TRP Channels

Amphipathic polymers (amphipols), such as A8-35 and SApol, are a new tool for stabilizing integral membrane proteins in detergent-free conditions for structural and functional studies. Transient receptor potential (TRP) ion channels function as tetrameric protein complexes in a diverse range of cellular processes including sensory transduction. Mammalian TRP channels share ~20 % sequence similarity and are categorized into six subfamilies: TRPC (canonical), TRPV (vanilloid), TRPA (ankyrin), TRPM (melastatin), TRPP (polycystin), and TRPML (mucolipin). Due to the inherent difficulties in purifying eukaryotic membrane proteins, structural studies of TRP channels have been limited. Recently, A8-35 was essential in resolving the molecular architecture of the nociceptor TRPA1 and led to the determination of a high-resolution structure of the thermosensitive TRPV1 channel by cryo-EM. Newly developed maltose-neopentyl glycol (MNG) detergents have also proven to be useful in stabilizing TRP channels for structural analysis. In this review, we will discuss the impacts of amphipols and MNG detergents on structural studies of TRP channels by cryo-EM. We will compare how A8-35 and MNG detergents interact with the hydrophobic transmembrane domains of TRP channels. In addition, we will discuss what these cryo-EM studies reveal on the importance of screening different types of surfactants toward determining high-resolution structures of TRP channels.     

TRP channels in hypertension

Pulmonary and systemic arterial hypertension are associated with profound alterations in Ca(2+) homeostasis and smooth muscle cell proliferation. A novel class of non-selective cation channels, the transient receptor potential (TRP) channels, have emerged at the forefront of research into hypertensive disease states. TRP channels are identified as molecular correlates for receptor-operated and store-operated cation channels in the vasculature. Over 10 TRP isoforms are identified at the mRNA and protein expression levels in the vasculature. Current research implicates upregulation of specific TRP isoforms to be associated with increased Ca(2+) influx, characteristic of vasoconstriction and vascular smooth muscle cell proliferation. TRP channels are implicated as Ca(2+) entry pathways in pulmonary hypertension and essential hypertension. Caveolae have recently emerged as membrane microdomains in which TRP channels may be co-localized with the endoplasmic reticulum in both smooth muscle and endothelial cells. Such enhanced expression and function of TRP channels and their localization in caveolae in pathophysiological hypertensive disease states highlights their importance as potential targets for pharmacological intervention.     

Structural biology of thermoTRPV channels

Essential for physiology, transient receptor potential (TRP) channels constitute a large and diverse family of cation channels functioning as cellular sensors responding to a vast array of physical and chemical stimuli. Detailed understanding of the inner workings of TRP channels has been hampered by a lack of atomic structures, though structural biology of TRP channels has been an enthusiastic endeavor since their molecular identification two decades ago. These multi-domain integral membrane proteins, exhibiting complex polymodal gating behavior, have been a challenge for traditional X-ray crystallography, which requires formation of well-ordered protein crystals. X-ray structures remain limited to a few TRP channel proteins to date. Fortunately, recent breakthroughs in single-particle cryo-electron microscopy (cryo-EM) have enabled rapid growth of the number of TRP channel structures, providing tremendous insights into channel gating and regulation mechanisms and serving as foundations for further mechanistic investigations. This brief review focuses on recent exciting developments in structural biology of a subset of TRP channels, the calcium-permeable, non-selective and thermosensitive vanilloid subfamily of TRP channels (TRPV1-4), and the permeation and gating mechanisms revealed by structures.     

TRP gating is linked to the metabolic state and maintenance of the Drosophila photoreceptor cells

The Drosophila light-activated channel TRP is the founding member of a large and diverse family of channel proteins that is conserved throughout evolution. In spite of much progress, the gating mechanism of TRP channels is still unknown. However, recent studies have shown multi-faceted functions of the Drosophila light-sensitive TRP channel that may shed light on TRP gating. Accordingly, metabolic stress, which leads to depletion of cellular ATP, reversibly activates the Drosophila TRP and TRPL channels in the dark in a constitutive manner. In several Drosophila mutants, constitutive activity of TRP channels lead to a rapid retinal degeneration in the dark, while genetic elimination of TRP protects the cells from degeneration. Additional studies have shown that TRPL translocates in a light-dependent manner between the signaling membranes and the cell body. This light-activated translocation is accompanied by reversible morphological changes leading to partial and reversible collapse of the microvillar signaling membranes into the cytosol, which allows turnover of signaling molecules. These morphological changes are also blocked by genetic elimination of TRP channels. The link of TRP gating to the metabolic state and maintenance of cells makes cells expressing TRP extremely vulnerable to metabolic stress via a mechanism that may underlie retinal degeneration and neuronal cell death upon malfunction.     

TRP channels and pain

Since the molecular identification of the capsaicin receptor, now known as TRPV1, transient receptor potential (TRP) channels have occupied an important place in the understanding of sensory nerve function in the context of pain. Several TRP channels exhibit sensitivity to substances previously known to cause pain or pain-like sensations; these include cinnamaldehyde, menthol, gingerol, and icillin. Many TRP channels also exhibit significant sensitivity to increases or decreases in temperature. Some TRP channels are sensitized in vitro by the activation of other receptors such that these channels may be activated by processes, such as inflammation that result in pain. TRP channels are suggested to be involved in processes as diverse as sensory neuron activation events, neurotransmitter release and action in the spinal cord, and release of inflammatory mediators. These functions strongly suggest that specific and selective inhibition of TRP channel activity will be of use in alleviating pain.     

TRP channels in hypertension

Pulmonary and systemic arterial hypertension are associated with profound alterations in Ca²⁺ homeostasis and smooth muscle cell proliferation. A novel class of non-selective cation channels, the transient receptor potential (TRP) channels, have emerged at the forefront of research into hypertensive disease states. TRP channels are identified as molecular correlates for receptor-operated and store-operated cation channels in the vasculature. Over 10 TRP isoforms are identified at the mRNA and protein expression levels in the vasculature. Current research implicates upregulation of specific TRP isoforms to be associated with increased Ca²⁺ influx, characteristic of vasoconstriction and vascular smooth muscle cell proliferation. TRP channels are implicated as Ca²⁺ entry pathways in pulmonary hypertension and essential hypertension. Caveolae have recently emerged as membrane microdomains in which TRP channels may be co-localized with the endoplasmic reticulum in both smooth muscle and endothelial cells. Such enhanced expression and function of TRP channels and their localization in caveolae in pathophysiological hypertensive disease states highlights their importance as potential targets for pharmacological intervention.     

Translocation of the Drosophila transient receptor potential-like (TRPL) channel requires both the N- and C-terminal regions together with sustained Ca2+ entry

In Drosophila photoreceptors the transient receptor potential-like (TRPL), but not the TRP channels undergo light-dependent translocation between the rhabdomere and cell body. Here we studied which of the TRPL channel segments are essential for translocation and why the TRP channels are required for inducing TRPL translocation. We generated transgenic flies expressing chimeric TRP and TRPL proteins that formed functional light-activated channels. Translocation was induced only in chimera containing both the N- and C-terminal segments of TRPL. Using an inactive trp mutation and overexpressing the Na(+)/Ca(2+) exchanger revealed that the essential function of the TRP channels in TRPL translocation is to enhance Ca(2+)-influx. These results indicate that motifs present at both the N and C termini as well as sustained Ca(2+) entry are required for proper channel translocation.     

TRP channels and analgesia

Since cloning and characterizing the first nociceptive ion channel Transient Receptor Potential (TRP) Vanilloid 1 (TRPV1), other TRP channels involved in nociception have been cloned and characterized, which include TRP Vanilloid 2 (TRPV2), TRP Vanilloid 3 (TRPV3), TRP Vanilloid 4 (TRPV4), TRP Ankyrin 1 (TRPA1) and TRP Melastatin 8 (TRPM8), more recently TRP Canonical 1, 5, 6 (TRPC1, 5, 6), TRP Melastatin 2 (TRPM2) and TRP Melastatin 3 (TRPM3). These channels are predominantly expressed in C and Aδ nociceptors and transmit noxious thermal, mechanical and chemical sensitivities. TRP channels are modulated by pro-inflammatory mediators, neuropeptides and cytokines. Significant advances have been made targeting these receptors either by antagonists or agonists to treat painful conditions. In this review, we will discuss TRP channels as targets for next generation analgesics and the side effects that may ensue as a result of blocking/activating these receptors, because they are also involved in physiological functions such as release of vasoactive neuropeptides and regulation of vascular tone, maintenance of the body temperature, gastrointestinal motility, urinary bladder control, etc.     

A Transient Receptor Potential Ion Channel in Chlamydomonas Shares Key Features with Sensory Transduction-Associated TRP Channels in Mammals

Sensory modalities are essential for navigating through an ever-changing environment. From insects to mammals, transient receptor potential (TRP) channels are known mediators for cellular sensing. Chlamydomonas reinhardtii is a motile single-celled freshwater green alga that is guided by photosensory, mechanosensory, and chemosensory cues. In this type of alga, sensory input is first detected by membrane receptors located in the cell body and then transduced to the beating cilia by membrane depolarization. Although TRP channels seem to be absent in plants, C. reinhardtii possesses genomic sequences encoding TRP proteins. Here, we describe the cloning and characterization of a C. reinhardtii version of a TRP channel sharing key features present in mammalian TRP channels associated with sensory transduction. In silico sequence-structure analysis unveiled the modular design of TRP channels, and electrophysiological experiments conducted on Human Embryonic Kidney-293T cells expressing the Cr-TRP1 clone showed that many of the core functional features of metazoan TRP channels are present in Cr-TRP1, suggesting that basic TRP channel gating characteristics evolved early in the history of eukaryotes.     

TRP channels and pain

Since the molecular identification of the capsaicin receptor, now known as TRPV1, transient receptor potential (TRP) channels have occupied an important place in the understanding of sensory nerve function in the context of pain. Several TRP channels exhibit sensitivity to substances previously known to cause pain or pain-like sensations; these include cinnamaldehyde, menthol, gingerol, and icillin. Many TRP channels also exhibit significant sensitivity to increases or decreases in temperature. Some TRP channels are sensitized in vitro by the activation of other receptors such that these channels may be activated by processes, such as inflammation that result in pain. TRP channels are suggested to be involved in processes as diverse as sensory neuron activation events, neurotransmitter release and action in the spinal cord, and release of inflammatory mediators. These functions strongly suggest that specific and selective inhibition of TRP channel activity will be of use in alleviating pain.     

TRP通道与信号转导

TRP(transient receptor potential)通道是一类在外周和中枢神经系统分布很广泛的通道蛋白.到目前为止,有超过30个TRP通道家族成员在哺乳动物中被克隆.TRP通道均为六次跨膜蛋白,其N末端和C末端均在胞内,由第五和第六跨膜结构域共同构成非选择性阳离子孔道.这些通道可被许多种因素调节,包括温度、渗透压、pH值、机械力,以及一些内、外源性配体和细胞内信号分子.TRP通道家族包含七个亚族.目前,它们最公认的功能是介导感觉信号的传递,其他功能包括调节细胞钙平衡和影响发育等.