The antimalarial artemisinin is a non-electrophilic agonist of the transient receptor potential ankyrin type 1 receptor-channel

Artemisinin and its derivatives are the main therapeutic drugs against Plasmodium protists, the causative agents of malaria. While several putative mechanisms of action have been proposed, the precise molecular targets of these compounds have not been fully elucidated. In addition to their antimalarial properties, artemisinins have been reported to act as anti-tumour agents and certain antinociceptive effects have also been proposed. We investigated the effect of the parent compound, artemisinin, on a number of temperature-gated Transient Receptor Potential ion channels (so called thermoTRPs), given their demonstrated roles in pain-sensing and cancer. We report that artemisinin acts as an agonist of the Transient Receptor Potential Ankyrin type 1 (TRPA1) receptor channel. Artemisinin was able to evoke calcium transients in HEK293T cells expressing recombinant human TRPA1, as well as in a subpopulation of mouse dorsal root ganglion (DRG) neurons which also responded to the selective TRPA1 agonist allyl isothiocyanate (AITC) and these responses were reversibly abolished by the selective TRPA1 antagonist A967079. Artemisinin also triggered whole-cell currents in HEK293T cells transiently transfected with human TRPA1, as well as in TRPA1-expressing DRG neurons, and these currents were inhibited by A967079. Interestingly, using human TRPA1 mutants, we demonstrate that artemisinin acts as a non-electrophilic agonist of TRPA1, activating the channel in a similar manner to carvacrol and menthol. These results may provide a better understanding of the biological actions of the very important antimalarial and anti-tumour agent artemisinin.

Multiple Targets for Oxysterols in Their Regulation of the Immune System

Oxysterols, or cholesterol oxidation products, are naturally occurring lipids which regulate the physiology of cells, including those of the immune system. In contrast to effects that are mediated through nuclear receptors or by epigenetic mechanism, which take tens of minutes to occur, changes in the activities of cell-surface receptors caused by oxysterols can be extremely rapid, often taking place within subsecond timescales. Such cell-surface receptor effects of oxysterols allow for the regulation of fast cellular processes, such as motility, secretion and endocytosis. These cellular processes play critical roles in both the innate and adaptive immune systems. This review will survey the two broad classes of cell-surface receptors for oxysterols (G-protein coupled receptors (GPCRs) and ion channels), the mechanisms by which cholesterol oxidation products act on them, and their presence and functions in the different cell types of the immune system. Overall, this review will highlight the potential of oxysterols, synthetic derivatives and their receptors for physiological and therapeutic modulation of the immune system.

A diacylglycerol photoswitching protocol for studying TRPC channel functions in mammalian cells and tissue slices

Small molecular probes designed for photopharmacology and opto-chemogenetics are rapidly gaining widespread recognition for investigations of transient receptor potential canonical (TRPC) channels. This protocol describes the use of three photoswitchable diacylglycerol analogs—PhoDAG-1, PhoDAG-3, and OptoDArG—for ultrarapid activation and deactivation of native TRPC2 channels in mouse vomeronasal sensory neurons and olfactory type B cells, as well as heterologously expressed human TRPC6 channels. Photoconversion can be achieved in mammalian tissue slices and enables all-optical stimulation and shutoff of TRPC channels.

For complete details on the use and execution of this protocol, please refer to Leinders-Zufall et al. (2018).

•

DAG photoswitching enables ultrarapid activation and deactivation of TRPC channels

•

Multiple photoswitchable DAG analogs are now available

•

DAG photoconversion is sufficient for the gating of TRPC2, TRPC3, and TRPC6

•

Photoswitching combined with Ca²⁺ imaging enables all-optical stimulation and recording

In Vivo Pharmacokinetic Analysis Utilizing Non-Targeted and Targeted Mass Spectrometry and In Vitro Assay against Transient Receptor Potential Channels of Maobushisaishinto and Its Constituent Asiasari Radix

The Japanese traditional medicine maobushisaishinto (MBST) has been prescribed for treating upper respiratory tract infections, such as a common cold. However, its mode of action is poorly understood, especially concerning the MBST constituent Asiasari Radix (AR). In this study, we focused on AR, with an objective of clarifying its bioavailable active ingredients and role within MBST by performing pharmacokinetic and pharmacological studies. Firstly, we performed qualitative non-targeted analysis utilizing high-resolution mass spectrometry to explore the bioavailable ingredients of AR as well as quantitative targeted analysis to reveal plasma concentrations following oral administration of MBST in rats. Secondly, we performed in vitro pharmacological study of bioavailable AR ingredients in addition to other ingredients of MBST to confirm any agonistic activities against transient receptor potential (TRP) channels. As a result, methyl kakuol and other compounds derived from AR were detected in the rat plasma and showed agonistic activity against TRPA1. This study suggests that methyl kakuol as well as other compounds have the potential to be an active ingredient in AR and thus presumably would contribute in part to the effects exerted by MBST.

Global alignment and assessment of TRP channel transmembrane domain structures to explore functional mechanisms

The recent proliferation of published TRP channel structures provides a foundation for understanding the diverse functional properties of this important family of ion channel proteins. To facilitate mechanistic investigations, we constructed a structure-based alignment of the transmembrane domains of 120 TRP channel structures. Comparison of structures determined in the absence or presence of activating stimuli reveals similar constrictions in the central ion permeation pathway near the intracellular end of the S6 helices, pointing to a conserved cytoplasmic gate and suggesting that most available structures represent non-conducting states. Comparison of the ion selectivity filters toward the extracellular end of the pore supports existing hypotheses for mechanisms of ion selectivity. Also conserved to varying extents are hot spots for interactions with hydrophobic ligands, lipids and ions, as well as discrete alterations in helix conformations. This analysis therefore provides a framework for investigating the structural basis of TRP channel gating mechanisms and pharmacology, and, despite the large number of structures included, reveals the need for additional structural data and for more functional studies to establish the mechanistic basis of TRP channel function.

A Single Natural Variation Determines Cytosolic Ca2+-Mediated Hyperthermosensitivity of TRPA1s from Rattlesnakes and Boas

Transient receptor potential ankyrin 1 from rattlesnakes (rsTRPA1) and boas (bTRPA1) was previously proposed to underlie thermo-sensitive infrared sensing based on transcript enrichment in infrared-sensing neurons and hyper-thermosensitivity expressed in Xenopus oocytes. It is unknown how these TRPA1s show thermosensitivities that overwhelm other thermoreceptors, and why rsTRPA1 is more thermosensitive than bTRPA1. Here, we show that snake TRPA1s differentially require Ca2+ for hyper-thermosensitivity and that predisposition to cytosolic Ca2+ potentiation correlates with superior thermosensitivity. Extracellularly applied Ca2+ upshifted the temperature coefficients (Q10s) of both TRPA1s, for which rsTRPA1, but not bTRPA1, requires cytosolic Ca2+. Intracellular Ca2+ chelation and substitutive mutations of the conserved cytosolic Ca2+-binding domain lowered rsTRPA1 thermosensitivity comparable to that of bTRPA1. Thapsigargin-evoked Ca2+ or calmodulin little affected rsTRPA1 activity or thermosensitivity, implying the importance of precise spatiotemporal action of Ca2+. Remarkably, a single rattlesnake-mimicking substitution in the conserved but presumably dormant cytosolic Ca2+-binding domain of bTRPA1 substantially enhanced thermosensitivity through cytosolic Ca2+ like rsTRPA1, indicating the capability of this single site in the determination of both cytosolic Ca2+ dependence and thermosensitivity. Collectively, these data suggest that Ca2+ is essential for the hyper-thermosensitivity of these TRPA1s, and cytosolic potentiation by permeating Ca2+ may contribute to the natural variation of infrared senses between rattlesnakes and boas.

Atomistic Insights of Calmodulin Gating of Complete Ion Channels

Intracellular calcium is essential for many physiological processes, from neuronal signaling and exocytosis to muscle contraction and bone formation. Ca²⁺ signaling from the extracellular medium depends both on membrane potential, especially controlled by ion channels selective to K⁺, and direct permeation of this cation through specialized channels. Calmodulin (CaM), through direct binding to these proteins, participates in setting the membrane potential and the overall permeability to Ca²⁺. Over the past years many structures of complete channels in complex with CaM at near atomic resolution have been resolved. In combination with mutagenesis-function, structural information of individual domains and functional studies, different mechanisms employed by CaM to control channel gating are starting to be understood at atomic detail. Here, new insights regarding four types of tetrameric channels with six transmembrane (6TM) architecture, Eag1, SK2/SK4, TRPV5/TRPV6 and KCNQ1–5, and its regulation by CaM are described structurally. Different CaM regions, N-lobe, C-lobe and EF3/EF4-linker play prominent signaling roles in different complexes, emerging the realization of crucial non-canonical interactions between CaM and its target that are only evidenced in the full-channel structure. Different mechanisms to control gating are used, including direct and indirect mechanical actuation over the pore, allosteric control, indirect effect through lipid binding, as well as direct plugging of the pore. Although each CaM lobe engages through apparently similar alpha-helices, they do so using different docking strategies. We discuss how this allows selective action of drugs with great therapeutic potential.

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.

A multiscale model of mechanotransduction by the ankyrin chains of the NOMPC channel

Our senses of touch and hearing are dependent on the conversion of external mechanical forces into electrical impulses by the opening of mechanosensitive channels in sensory cells. This remarkable feat involves the conversion of a macroscopic mechanical displacement into a subnanoscopic conformational change within the ion channel. The mechanosensitive channel NOMPC, responsible for hearing and touch in flies, is a homotetramer composed of four pore-forming transmembrane domains and four helical chains of 29 ankyrin repeats that extend 150 Å into the cytoplasm. Previous work has shown that the ankyrin chains behave as biological springs under extension and that tethering them to microtubules could be involved in the transmission of external forces to the NOMPC gate. Here we combine normal mode analysis (NMA), full-atom molecular dynamics simulations, and continuum mechanics to characterize the material properties of the chains under extreme compression and extension. NMA reveals that the lowest-frequency modes of motion correspond to fourfold symmetric compression/extension along the channel, and the lowest-frequency symmetric mode for the isolated channel domain involves rotations of the TRP domain, a putative gating element. Finite element modeling reveals that the ankyrin chains behave as a soft spring with a linear, effective spring constantof 22 pN/nm for deflections ≤15 Å. Force-balance analysis shows that the entire channel undergoes rigid body rotation during compression, and more importantly, each chain exerts a positive twisting moment on its respective linker helices and TRP domain. This torque is a model-independent consequence of the bundle geometry and would cause a clockwise rotation of the TRP domain when viewed from the cytoplasm. Force transmission to the channel for compressions >15 Å depends on the nature of helix-helix contact. Our work reveals that compression of the ankyrin chains imparts a rotational torque on the TRP domain, which potentially results in channel opening.

Structural basis for PtdInsP2-mediated human TRPML1 regulation

Transient receptor potential mucolipin 1 (TRPML1), a lysosomal channel, maintains the low pH and calcium levels for lysosomal function. Several small molecules modulate TRPML1 activity. ML-SA1, a synthetic agonist, binds to the pore region and phosphatidylinositol-3,5-bisphosphate (PtdIns(3,5)P2), a natural lipid, stimulates channel activity to a lesser extent than ML-SA1; moreover, PtdIns(4,5)P2, another natural lipid, prevents TRPML1-mediated calcium release. Notably, PtdIns(3,5)P2 and ML-SA1 cooperate further increasing calcium efflux. Here we report the structures of human TRPML1 at pH 5.0 with PtdIns(3,5)P2, PtdIns(4,5)P2, or ML-SA1 and PtdIns(3,5)P2, revealing a unique lipid-binding site. PtdIns(3,5)P2 and PtdIns(4,5)P2 bind to the extended helices of S1, S2, and S3. The phosphate group of PtdIns(3,5)P2 induces Y355 to form a π-cation interaction with R403, moving the S4-S5 linker, thus allosterically activating the channel. Our structures and electrophysiological characterizations reveal an allosteric site and provide molecular insight into how lipids regulate TRP channels.

Transient Receptor Potential (TRP) Channels

Transient Receptor Potential (TRP) channels are evolutionarily conserved integral membrane proteins. The mammalian TRP superfamily of ion channels consists of 28 cation permeable channels that are grouped into six subfamilies based on sequence homology (Fig. 6.1). The canonical TRP (TRPC) subfamily is known for containing the founding member of mammalian TRP channels. The vanilloid TRP (TRPV) subfamily has been extensively studied due to the heat sensitivity of its founding member. The melastatin-related TRP (TRPM) subfamily includes some of the few known bi-functional ion channels, which contain functional enzymatic domains. The ankyrin TRP (TRPA) subfamily consists of a single chemo-nociceptor that has been proposed to be a target for analgesics. The mucolipin TRP (TRPML) subfamily channels are found primarily in intracellular compartments and were discovered based on their critical role in type IV mucolipidosis (ML-IV). The polycystic TRP (TRPP) subfamily is a diverse group of proteins implicated in autosomal dominant polycystic kidney disease (ADPKD). Overall, this superfamily of channels is involved in a vast array of physiological and pathophysiological processes making the study of these channels imperative to our understanding of subcellular biochemistry.

TRP Channels as Sensors of Bacterial Endotoxins

The cellular and systemic effects induced by bacterial lipopolysaccharides (LPS) have been solely attributed to the activation of the Toll-like receptor 4 (TLR4) signalling cascade. However, recent studies have shown that LPS activates several members of the Transient Receptor Potential (TRP) family of cation channels. Indeed, LPS induces activation of the broadly-tuned chemosensor TRPA1 in sensory neurons in a TLR4-independent manner, and genetic ablation of this channel reduced mouse pain and inflammatory responses triggered by LPS and the gustatory-mediated avoidance to LPS in fruit flies. LPS was also shown to activate TRPV4 channels in airway epithelial cells, an effect leading to an immediate production of bactericidal nitric oxide and to an increase in ciliary beat frequency. In this review, we discuss the role of TRP channels as sensors of bacterial endotoxins, and therefore, as crucial players in the timely detection of invading gram-negative bacteria.

TRPs et al.: a molecular toolkit for thermosensory adaptations

The ability to sense temperature is crucial for the survival of an organism. Temperature influences all biological operations, from rates of metabolic reactions to protein folding, and broad behavioral functions, from feeding to breeding, and other seasonal activities. The evolution of specialized thermosensory adaptations has enabled animals to inhabit extreme temperature niches and to perform specific temperature-dependent behaviors. The function of sensory neurons depends on the participation of various types of ion channels. Each of the channels involved in neuronal excitability, whether through the generation of receptor potential, action potential, or the maintenance of the resting potential have temperature-dependent properties that can tune the neuron's response to temperature stimuli. Since the function of all proteins is affected by temperature, animals need adaptations not only for detecting different temperatures, but also for maintaining sensory ability at different temperatures. A full understanding of the molecular mechanism of thermosensation requires an investigation of all channel types at each step of thermosensory transduction. A fruitful avenue of investigation into how different molecules can contribute to the fine-tuning of temperature sensitivity is to study the specialized adaptations of various species. Given the diversity of molecular participants at each stage of sensory transduction, animals have a toolkit of channels at their disposal to adapt their thermosensitivity to their particular habitats or behavioral circumstances.

The Touching Tail of a Mechanotransduction Channel

In mechanotransduction, sensory receptors convert force into electrical signals to mediate such diverse functions as touch, pain, and hearing. In this issue of Cell, Zhang et al. present evidence that the fly NompC channel senses mechanical stimuli using its N-terminal tail as a tether between the cell membrane and microtubules.

TRP channels interaction with lipids and its implications in disease

Transient receptor potential (TRP) proteins are a family of ion channels central for sensory signaling. These receptors and, in particular, those involved in thermal sensing are also involved in pain signaling. Noteworthy, thermosensory receptors are polymodal ion channels that respond to both physical and chemical stimuli, thus integrating different environmental clues. In addition, their activity is modulated by algesic agents and lipidergic substances that are primarily released in pathological states. Lipids and lipid-like molecules have been found that can directly activate some thermosensory channels or modulate their activity by either potentiating or inhibiting it. To date, more than 50 endogenous lipids that can regulate TRP channel activity in sensory neurons have been described, thus representing the majority of known endogenous TRP channel modulators. Lipid modulators of TRP channels comprise lipids from a variety of metabolic pathways, including metabolites of the cyclooxygenase, lipoxygenase and cytochrome-P450 pathways, phospholipids and lysophospholipids. Therefore, TRP-channels are able to integrate and interpret incoming signals from the different metabolic lipid pathways. Taken together, the large number of lipids that can activate, sensitize or inhibit neuronal TRP-channels highlights the pivotal role of these molecules in sensory biology as well as in pain transduction and perception. This article is part of a Special Issue entitled: Lipid-protein interactions. Guest Editors: Amitabha Chattopadhyay and Jean-Marie Ruysschaert.

Role of TRP channels in the cardiovascular system

The transient receptor potential (TRP) superfamily consists of a large number of nonselective cation channels with variable degree of Ca(2+)-permeability. The 28 mammalian TRP channel proteins can be grouped into six subfamilies: canonical, vanilloid, melastatin, ankyrin, polycystic, and mucolipin TRPs. The majority of these TRP channels are expressed in different cell types including both excitable and nonexcitable cells of the cardiovascular system. Unlike voltage-gated ion channels, TRP channels do not have a typical voltage sensor, but instead can sense a variety of other stimuli including pressure, shear stress, mechanical stretch, oxidative stress, lipid environment alterations, hypertrophic signals, and inflammation products. By integrating multiple stimuli and transducing their activity to downstream cellular signal pathways via Ca(2+) entry and/or membrane depolarization, TRP channels play an essential role in regulating fundamental cell functions such as contraction, relaxation, proliferation, differentiation, and cell death. With the use of targeted deletion and transgenic mouse models, recent studies have revealed that TRP channels are involved in numerous cellular functions and play an important role in the pathophysiology of many diseases in the cardiovascular system. Moreover, several TRP channels are involved in inherited diseases of the cardiovascular system. This review presents an overview of current knowledge concerning the physiological functions of TRP channels in the cardiovascular system and their contributions to cardiovascular diseases. Ultimately, TRP channels may become potential therapeutic targets for cardiovascular diseases.

[The progress of TRP channels in structural studies]

Transient receptor potential (TRP) channels are a super-family of nonselective cation channels that play critical roles in the responses to various environmental changes and stimuli. Over the past 10 years much progress has been made in the structural studies of TRP channels. To date, the structures of 5 full-length TRP channels and their 11 cytoplasmic domains have been determined. These structural insights enhance our understanding of TRP channel gating, assembly and regulation. In this review, the structures of TRP channels are summarized with an emphasis on the progress made during the last two years.

Transient receptor potential is essential for high temperature tolerance in invasive Bemisia tabaci Middle East Asia minor 1 cryptic species

Temperature is an important factor in affecting population dynamics and diffusion distribution of organisms. Alien species can successfully invade and colonize to various temperature environments, and one of important reasons is that alien species have a strong resistance to stress temperature. Recently, researchers have focused on the mechanisms of temperature sensing to determine the sensing and regulation mechanisms of temperature adaptation. The transient receptor potential (TRP) is one of the key components of an organism’s temperature perception system. TRP plays important roles in perceiving temperature, such as avoiding high temperature, low temperature and choosing the optimum temperature. To assess high temperature sensation and the heat resistance role of the TRP gene, we used 3′ and 5′ rapid-amplification of cDNA ends to isolate the full-length cDNA sequence of the TRP gene from Bemisia tabaci (Gennadius) MEAM1 (Middle East Asia Minor 1), examined the mRNA expression profile under various temperature conditions, and identified the heat tolerance function. This is the first study to characterize the TRP gene of invasive B. tabaci MEAM1 (MEAM1 BtTRP). The full-length cDNA of MEAM1 BtTRP was 3871 bp, and the open reading frames of BtTRP was 3501 bp, encoding 1166 amino acids. Additionally, the BtTRP mRNA expression level was significantly increased at 35°C. Furthermore, compared with control treatments, the survival rate of B. tabaci MEAM1 adults was significantly decreased under high temperature stress conditions after feeding with dsRNA BtTRP. Collectively, these results showed that MEAM1 BtTRP is a key element in sensing high temperature and plays an essential role in B. tabaci MEAM1 heat tolerance ability. Our data improved our understanding of the mechanism of temperature sensation in B. tabaci MEAM1 at the molecular level and could contribute to the understanding of the thermal biology of B. tabaci MEAM1 within the context of global climate change.

Are sensory TRP channels biological alarms for lipid peroxidation?

Oxidative stress induces numerous biological problems. Lipid oxidation and peroxidation appear to be important steps by which exposure to oxidative stress leads the body to a disease state. For its protection, the body has evolved to respond to and eliminate peroxidation products through the acquisition of binding proteins, reducing and conjugating enzymes, and excretion systems. During the past decade, researchers have identified a group of ion channel molecules that are activated by oxidized lipids: transient receptor potential (TRP) channels expressed in sensory neurons. These ion channels are fundamentally detectors and signal converters for body-damaging environments such as heat and cold temperatures, mechanical attacks, and potentially toxic substances. When messages initiated by TRP activation arrive at the brain, we perceive pain, which results in our preparing defensive responses. Excessive activation of the sensory neuronal TRP channels upon prolonged stimulations sometimes deteriorates the inflammatory state of damaged tissues by promoting neuropeptide release from expresser neurons. These same paradigms may also work for pathologic changes in the internal lipid environment upon exposure to oxidative stress. Here, we provide an overview of the role of TRP channels and oxidized lipid connections during abnormally increased oxidative signaling, and consider the sensory mechanism of TRP detection as an alert system.

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.

Transient receptor potential (TRP) channels: a clinical perspective

Transient receptor potential (TRP) channels are important mediators of sensory signals with marked effects on cellular functions and signalling pathways. Indeed, mutations in genes encoding TRP channels are the cause of several inherited diseases in humans (the so-called 'TRP channelopathies') that affect the cardiovascular, renal, skeletal and nervous systems. TRP channels are also promising targets for drug discovery. The initial focus of research was on TRP channels that are expressed on nociceptive neurons. Indeed, a number of potent, small-molecule TRPV1, TRPV3 and TRPA1 antagonists have already entered clinical trials as novel analgesic agents. There has been a recent upsurge in the amount of work that expands TRP channel drug discovery efforts into new disease areas such as asthma, cancer, anxiety, cardiac hypertrophy, as well as obesity and metabolic disorders. A better understanding of TRP channel functions in health and disease should lead to the discovery of first-in-class drugs for these intractable diseases. With this review, we hope to capture the current state of this rapidly expanding and changing field.

Structural biology of TRP channels

Membrane proteins remain challenging targets for structural biologists, despite recent technical developments regarding sample preparation and structure determination. We review recent progress towards a structural understanding of TRP channels and the techniques used to that end. We discuss available low-resolution structures from electron microscopy (EM), X-ray crystallography, and nuclear magnetic resonance (NMR) and review the resulting insights into TRP channel function for various subfamily members. The recent high-resolution structure of TRPV1 is discussed in more detail in Chapter 11. We also consider the opportunities and challenges of using the accumulating structural information on TRPs and homologous proteins for deducing full-length structures of different TRP channel subfamilies, such as building homology models. Finally, we close by summarizing the outlook of the "holy grail" of understanding in atomic detail the diverse functions of TRP channels.

TRPA1 channels: chemical and temperature sensitivity

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal excitatory ion channel found in sensory neurons of different organisms, ranging from worms to humans. Since its discovery as an uncharacterized transmembrane protein in human fibroblasts, TRPA1 has become one of the most intensively studied ion channels. Its function has been linked to regulation of heat and cold perception, mechanosensitivity, hearing, inflammation, pain, circadian rhythms, chemoreception, and other processes. Some of these proposed functions remain controversial, while others have gathered considerable experimental support. A truly polymodal ion channel, TRPA1 is activated by various stimuli, including electrophilic chemicals, oxygen, temperature, and mechanical force, yet the molecular mechanism of TRPA1 gating remains obscure. In this review, we discuss recent advances in the understanding of TRPA1 physiology, pharmacology, and molecular function.

Temperature sensing by thermal TRP channels: thermodynamic basis and molecular insights

All organisms need to sense temperature in order to survive and adapt. But how they detect and perceive temperature remains poorly understood. Recent discoveries of thermal Transient Receptor Potential (TRP) ion channels have shed light on the problem and unravel molecular entities for temperature detection and transduction in mammals. Thermal TRP channels belong to the large family of transient receptor potential channels. They are directly activated by heat or cold in physiologically relevant temperature ranges, and the activation is exquisitely sensitive to temperature changes. Thermodynamically, this strong temperature dependence of thermal channels occurs due to large enthalpy and entropy changes associated with channel opening. Thus understanding how the channel proteins obtain their exceptionally large energetics is central toward determining functional mechanisms of thermal TRP channels. The purpose of this chapter is to provide a comprehensive review on critical issues and challenges facing the problem, with emphases on underlying biophysical and molecular mechanisms.

TRPA1

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

Gating of thermally activated channels

A class of ion channels that belongs to the transient receptor potential (TRP) superfamily and is present in specialized neurons that project to the skin has evolved as temperature detectors. These channels are classified into subfamilies, namely canonical (TRPC), melastatin (TRPM), ankyrin (TRPA), and vanilloid (TRPV). Some of these channels are activated by heat (TRPM2/4/5, TRPV1-4), while others by cold (TRPA1, TRPC5, and TRPM8). The general structure of these channels is closely related to that of the voltage-dependent K(+) channels, with their subunits containing six transmembrane segments that form tetramers. Thermal TRP channels are polymodal receptors. That is, they can be activated by temperature, voltage, pH, lipids, and agonists. The high temperature sensitivity in these thermal TRP channels is due to a large enthalpy change (∼100 kcal/mol), which is about five times the enthalpy change in voltage-dependent gating. The characterization of the macroscopic currents and single-channel analysis demonstrated that gating by temperature is complex and best described by branched or allosteric models containing several closed and open states. The identification of molecular determinants of temperature sensitivity in TRPV1, TRPA1, and TRPV3 strongly suggest that thermal sensitivity arises from a specific protein domain.

TRPML2 and mucolipin evolution

The TRPML2 protein, encoded by the Mcoln2 gene, is one of the three mucolipins (TRPML1-3), a subset of the TRP superfamily of ion channels. Although there are no thorough studies on the cellular distribution of TRPML2, its mRNA appears to be largely restricted to lymphocytes and other immune cells. This contrasts with the ubiquitous expression of TRPML1 and the limited but diverse expression of TRPML3 and clearly suggests a specialized role for TRPML2 in immunity. Localization studies indicate that TRPML2 is present in lysosomes (including the specialized lysosome-related organelle that B-lymphocytes use for processing of the antigen-bound B-cell receptor), late endosomes, recycling endosomes, and, at a much lower level, the plasma membrane. Heterologously expressed TRPML2, like TRPML1 and/or TRPML3, forms ion channels that can be activated by a gain-of-function mutation (alanine to proline in the fifth transmembrane domain, close to the pore) that favors the open state, by a transient reduction of extracellular sodium followed by sodium replenishment, by small chemicals related to sulfonamides, and by PI(3,5)P2, a rare phosphoinositide that naturally accumulates in the membranes of endosomes and lysosomes and thus could act as a physiologically relevant agonist. TRPML2 channels are inwardly rectifying and permeable to Ca(2+), Na(+), and Fe(2+). When heterologously co-expressed, TRPML2 can form heteromultimers with TRPML1 and TRPML3. In B-lymphocytes, TRPML2 and TRPML1 may play redundant roles in the function of their specialized lysosome. Although the specific subcellular function of TRPML2 is unknown, distribution and channel properties suggest roles in calcium release from endolysosomes, perhaps to regulate vesicle fusion and/or subsequent scission or to release calcium from intracellular acidic stores for signaling in the cytosol. Alternatively, TRPML2 could function in the plasma membrane, and its abundance in vesicles of the endocytic pathway could simply be due to regulation by endocytosis and exocytosis. The Mcoln2 gene is closely downstream from and in the same orientation as Mcoln3 in the genomes of most jawed vertebrates (from humans to sharks) with the exception of pigs, Xenopus tropicalis, and ray-finned fishes. The close homology of TRPML2 and 3 (closer to each other than to TRPML1) suggests that Mcoln2 and Mcoln3 arose from unequal crossing over that duplicated a common ancestor and placed both gene copies in tandem. These genes would have come apart subsequently in pigs, Xenopus, and the ancestor to ray-finned fishes. All jawed vertebrates for which we have thorough genomic knowledge have distinct Mcoln1, 2, and 3 genes (except ray-finned fishes which, probably due to the whole-genome duplication in their common ancestor, have two Mcoln1-like genes and two Mcoln3-like genes, although only one Mcoln2 gene). However, the available genomes of invertebrate deuterostomes (a sea urchin, lancelet, and two tunicates) contain a single mucolipin gene that is equally distant from the three vertebrate mucolipins. Hence, vertebrate mucolipins arose through two rounds of gene duplication (the first one likely producing Mcoln1 and the ancestor to Mcoln2 and 3) at some time between the onset of craniates and that of jawed vertebrates. This is also the evolutionary period during which adaptive immunity appeared. Given the restricted expression of TRPML2 in immune cells, this evolutionary history suggests a functional role in the adaptive immunity characteristic of vertebrates.

Phosphorylation of the Drosophila transient receptor potential ion channel is regulated by the phototransduction cascade and involves several protein kinases and phosphatases

Protein phosphorylation plays a cardinal role in regulating cellular processes in eukaryotes. Phosphorylation of proteins is controlled by protein kinases and phosphatases. We previously reported the light-dependent phosphorylation of the Drosophila transient receptor potential (TRP) ion channel at multiple sites. TRP generates the receptor potential upon stimulation of the photoreceptor cell by light. An eye-enriched protein kinase C (eye-PKC) has been implicated in the phosphorylation of TRP by in vitro studies. Other kinases and phosphatases of TRP are elusive. Using phosphospecific antibodies and mass spectrometry, we here show that phosphorylation of most TRP sites depends on the phototransduction cascade and the activity of the TRP ion channel. A candidate screen to identify kinases and phosphatases provided in vivo evidence for an involvement of eye-PKC as well as other kinases and phosphatases in TRP phosphorylation.

A novel ion channel formed by interaction of TRPML3 with TRPV5

TRPML3 and TRPV5 are members of the mucolipin (TRPML) and TRPV subfamilies of transient receptor potential (TRP) cation channels. Based on sequence similarities of the pore forming regions and on structure-function evidence, we hypothesized that the pore forming domains of TRPML and TRPV5/TRPV6 channels have similarities that indicate possible functional interactions between these TRP channel subfamilies. Here we show that TRPML3 and TRPV5 associate to form a novel heteromeric ion channel. This novel conductance is detectable under conditions that do not activate either TRPML3 or TRPV5. It has pharmacological similarity with TRPML3 and requires functional TRPML3 as well as functional TRPV5. Single channel analyses revealed that TRPML3 and TRPV5 heteromers have different features than the respective homomers, and furthermore, that they occur in potentially distinct stoichiometric configurations. Based on overlapping expression of TRPML3 and TRPV5 in the kidney and the inner ear, we propose that TRPML3 and TRPV5 heteromers could have a biological function in these organs.

Identification and analysis of cation channel homologues in human pathogenic fungi

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

C-terminal acidic cluster is involved in Ca2+-induced regulation of human transient receptor potential ankyrin 1 channel

The transient receptor potential ankyrin 1 (TRPA1) channel is a Ca(2+)-permeable cation channel whose activation results from a complex synergy between distinct activation sites, one of which is especially important for determining its sensitivity to chemical, voltage and cold stimuli. From the cytoplasmic side, TRPA1 is critically regulated by Ca(2+) ions, and this mechanism represents a self-modulating feedback loop that first augments and then inhibits the initial activation. We investigated the contribution of the cluster of acidic residues in the distal C terminus of TRPA1 in these processes using mutagenesis, whole cell electrophysiology, and molecular dynamics simulations and found that the neutralization of four conserved residues, namely Glu(1077) and Asp(1080)-Asp(1082) in human TRPA1, had strong effects on the Ca(2+)- and voltage-dependent potentiation and/or inactivation of agonist-induced responses. The surprising finding was that truncation of the C terminus by only 20 residues selectively slowed down the Ca(2+)-dependent inactivation 2.9-fold without affecting other functional parameters. Our findings identify the conserved acidic motif in the C terminus that is actively involved in TRPA1 regulation by Ca(2+).

Phosphoinositide sensitivity of ion channels, a functional perspective

Phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] are required for the activity of many different ion channels. This chapter will highlight various aspects of this paradigm, by discussing current knowledge on four different ion channel families: inwardly rectifying K(+) (Kir) channels, KCNQ voltage gated K(+) channels, voltage gated Ca(2+) (VGCC) channels and Transient Receptor Potential (TRP) channels. Our main focus is to discuss functional aspects of this regulation, i.e. how changes in the concentration of PtdIns(4,5)P(2) in the plasma membrane upon phospholipase C activation may modulate the activity of ion channels, and what are the major determinants of this regulation. We also discuss how channels act as coincidence detectors sensing phosphoinositide levels and other signalling molecules. We also briefly discuss the available methods to study phosphoinositide regulation of ion channels, and structural aspects of interaction of ion channel proteins with these phospholipids. Finally, in several cases the effect of PtdIns(4,5)P(2) is more complex than a simple dependence of ion channel activity on the lipid, and we will discuss some these complexities.

Identification of intracellular and plasma membrane calcium channel homologues in pathogenic parasites

Ca²⁺ channels regulate many crucial processes within cells and their abnormal activity can be damaging to cell survival, suggesting that they might represent attractive therapeutic targets in pathogenic organisms. Parasitic diseases such as malaria, leishmaniasis, trypanosomiasis and schistosomiasis are responsible for millions of deaths each year worldwide. The genomes of many pathogenic parasites have recently been sequenced, opening the way for rational design of targeted therapies. We analyzed genomes of pathogenic protozoan parasites as well as the genome of Schistosoma mansoni, and show the existence within them of genes encoding homologues of mammalian intracellular Ca²⁺ release channels: inositol 1,4,5-trisphosphate receptors (IP₃Rs), ryanodine receptors (RyRs), two-pore Ca²⁺ channels (TPCs) and intracellular transient receptor potential (Trp) channels. The genomes of Trypanosoma, Leishmania and S. mansoni parasites encode IP₃R/RyR and Trp channel homologues, and that of S. mansoni additionally encodes a TPC homologue. In contrast, apicomplexan parasites lack genes encoding IP₃R/RyR homologues and possess only genes encoding TPC and Trp channel homologues (Toxoplasma gondii) or Trp channel homologues alone. The genomes of parasites also encode homologues of mammalian Ca²⁺influx channels, including voltage-gated Ca²⁺ channels and plasma membrane Trp channels. The genome of S. mansoni also encodes Orai Ca²⁺ channel and STIM Ca²⁺ sensor homologues, suggesting that store-operated Ca²⁺ entry may occur in this parasite. Many anti-parasitic agents alter parasite Ca²⁺ homeostasis and some are known modulators of mammalian Ca²⁺ channels, suggesting that parasite Ca²⁺ channel homologues might be the targets of some current anti-parasitic drugs. Differences between human and parasite Ca²⁺ channels suggest that pathogen-specific targeting of these channels may be an attractive therapeutic prospect.

Molecular modeling studies on 11H-dibenz[b,e]azepine and dibenz[b,f][1,4]oxazepine derivatives as potent agonists of the human TRPA1 receptor

A computational strategy based on comparative molecular fields analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) was performed on a series of the 11H-dibenz[b,e]azepine and dibenz[b,f][1,4]oxazepine derivatives as potent agonists of the human TRPA1 receptor. The CoMFA and CoMSIA models resulting from a 21 molecule training set gave r²(cv) values of 0.631 and 0.542 and r² values of 0.986 and 0.981, respectively. The statistically significant models were validated by a test set of five compounds with predictive r²(pred). values of 0.967 and 0.981 for CoMFA and CoMSIA, respectively. A systemic external validation was also performed on the established models. The information obtained from 3D counter maps could facilitate the design of more potent human TRPA1 receptor agonists.

Molecular convergence of infrared vision in snakes

It has been discovered that the transient receptor potential ankyrin 1 (TRPA1) proteins of Boidae (boas), Pythonidae (pythons), and Crotalinae (pit vipers) are used to detect infrared radiation, but the molecular mechanism for detecting the infrared radiation is unknown. Here, relating the amino acid substitutions in their TRPA1 proteins and the functional differentiations, we propose that three parallel amino acid changes (L330M, Q391H, and S434T) are responsible for the development of infrared vision in the three groups of snakes. Protein modeling shows that the three amino acid changes alter the structures of the central region of their ankyrin repeats.

Heteromultimeric TRPML channel assemblies play a crucial role in the regulation of cell viability models and starvation-induced autophagy

The mucolipin (TRPML) subfamily of transient receptor potential (TRP) cation channels consists of three members that play various roles in the regulation of membrane and protein sorting along endo-lysosomal pathways. Loss-of-function mutations in TRPML1 cause the neurodegenerative lysosomal storage disorder, mucolipidosis type IV (MLIV), whereas a gain-of-function mutation in TRPML3 is principally implicated in the hearing-impaired and abnormally pigmented varitint-waddler mouse. Currently, TRPML2 is not implicated in any pathological disorder, but we have recently shown that it is a functional cation channel that physically interacts with TRPML1 and TRPML3 to potentially regulate lysosomal integrity. Here, we show that mutant TRPMLs heteromultimerize with other mutant and wild-type TRPMLs to regulate cell viability and starvation-induced autophagy, a process that mediates macromolecular and organellar turnover under cell starvation conditions. Heteromultimerization of dominant-negative TRPMLs with constitutively active TRPMLs rescues cells from the cytotoxic effects of TRPML constitutive activity. Moreover, dominant-negative TRPML1 channels, including a mutant channel directly implicated in MLIV pathology, also inhibit starvation-induced autophagy by interacting with and affecting native TRPML channel function. Collectively, our results indicate that heteromultimerization of TRPML channels plays a role in various TRPML-regulated mechanisms.

Function and regulation of TRP family channels in C. elegans

Seventeen transient receptor potential (TRP) family proteins are encoded by the C. elegans genome, and they cover all of the seven TRP subfamilies, including TRPC, TRPV, TRPM, TRPN, TRPA, TRPP, and TRPML. Classical forward and reverse genetic screens have isolated mutant alleles in every C. elegans trp gene, and their characterizations have revealed novel functions and regulatory mechanisms of TRP channels. For example, the TRPC channels TRP-1 and TRP-2 control nicotine-dependent behavior, while TRP-3, a sperm TRPC channel, is regulated by sperm activation and required for sperm-egg interactions during fertilization. Similar to their vertebrate counterparts, C. elegans TRPs function in sensory physiology. For instance, the TRPV channels OSM-9 and OCR-2 act in chemosensation, osmosensation, and touch sensation, the TRPA member TRPA-1 regulates touch sensation, while the TRPN channel TRP-4 mediates proprioception. Some C. elegans TRPM, TRPP, and TRPML members exhibit cellular functions similar to their vertebrate homologues and have provided insights into human diseases, including polycystic kidney disease, hypomagnesemia, and mucolipidosis type IV. The availability of a complete set of trp gene mutants in conjunction with its facile genetics makes C. elegans a powerful model for studying the function and regulation of TRP family channels in vivo.

A TRP channel is expressed in Spodoptera littoralis antennae and is potentially involved in insect olfactory transduction

The molecular characterization of post-receptor actors involved in insect olfactory transduction has yet to be understood. We have investigated the presence of a Transient Receptor Potential (TRP) channel in the peripheral olfactory system of the moth Spodoptera littoralis. A cDNA encoding a Lepidopteran TRP channel (TRPgamma) was identified by analysis of a male-antennal EST database and subsequently cloned by RACE PCR. In adult males, the TRPgamma transcript was detected in antennae, at the base of olfactory sensilla. Moreover, TRPgamma was observed in antennae in both pupal and adult stages. This work is the first step in understanding the involvement of TRPgamma in signalling pathways involved in the development and function of the insect olfactory system.

[Effect of mutations imitating the phosphorylation by TRPM7 kinase on the function of the N-terminal domain of tropomodulin]

It has been shown that tropomodulin 1 is phosphorylated at serine and threonine residues by TRPM7 kinase. The phosphorylation sites for TRPM7 kinase in the N-terminal functional domain of tropomodulin 1 have been identified, which include tropomyosin-binding and actin-capping regions. It has been found that the phosphorylation-mimicking mutation T54E resulted in the loss of the capping ability of the N-terminal tropomodulin domain; however, its tropomyosin-binding ability did not change. We further hypothesize that the phosphorylation of tropomodulin by TRPM7 kinase may play a role in the regulation of the dynamics of actin filaments.

Cluster formation of transmembrane proteins due to hydrophobic mismatching

Membranes are the defining envelopes of living cells. At this boundary a multitude of transmembrane proteins mediate signal and mass transfer between cells and their environment. Clustering of these proteins is a frequent and often vital phenomenon that relies at least in part on membrane-mediated interactions. Indeed, the mismatch between proteins' hydrophobic transmembrane domains and the surrounding lipid bilayer has been predicted to facilitate clustering, yet unequivocal quantitative data in support of these predictions have been lacking. Here, we have used coarse-grained membrane simulations to thoroughly address the clustering of transmembrane proteins in detail. Our results emphasize the universal nature of membrane-mediated attraction which relaxes the need for a plethora of fine-tuned interactions between membrane proteins.

A primer on ankyrin repeat function in TRP channels and beyond

Transient receptor potential (TRP) channels are rapidly gaining attention as important receptors and transducers of diverse sensory and environmental cues. Recent progress in the field has provided new insights into the structure and function of the ankyrin repeat motifs present in the N-terminal cytosolic domain of many TRP channels. The topics addressed in this Highlight include the structural features of canonical ankyrin repeats, new clues into the functions these repeats perform in cells, and how this information can be applied to develop further experiments on TRP channels and other proteins containing ankyrin repeats.

Yeast screens show aromatic residues at the end of the sixth helix anchor transient receptor potential channel gate

Transient receptor potential (TRP) channels are first elements in sensing chemicals, heat, and force and are widespread among protists and fungi as well as animals. Despite their importance, the arrangement and roles of the amino acids that constitute the TRP channel gate are unknown. The yeast TRPY1 is activated in vivo by osmotically induced vacuolar membrane deformation and by cytoplasmic Ca(2+). After a random mutagenesis, we isolated TRPY1 mutants that responded more strongly to mild osmotic upshocks. One such gain-of-function mutant has a Y458H substitution at the C terminus of the predicted sixth transmembrane helix. Direct patch-clamp examination of vacuolar membranes showed that Y458H channels were already active with little stimulus and showed marked flickers between the open and intraburst closed states. They remained responsive to membrane stretch force and to Ca(2+), indicating primary defects in the gate region but not in the sensing of gating principles. None of the other 18 amino acid replacements engineered here showed normal channel kinetics except the two aromatic substitutions, Y458F and Y458W. The Y458 of TRPY1 has its aromatic counterpart in mammalian TRPM. Furthermore, conserved aromatics one alpha-helical turn downstream from this point are also found in animal TRPC, TRPN, TRPP, and TRPML, suggesting that gate anchoring with aromatics may be common among many TRP channels. The possible roles of aromatics at the end of the sixth transmembrane helix are discussed.

The TRP channel superfamily: insights into how structure, protein-lipid interactions and localization influence function

TRP (transient receptor potential) cationic channels are key molecules that are involved in a variety of diverse biological processes ranging from fertility to osmosensation and nociception. Increasing our knowledge of these channels will help us to understand a range of physiological and pathogenic processes, as well as highlighting potential therapeutic drug targets. The founding members of the TRP family, Drosophila TRP and TRPL (TRP-like) proteins, were identified within the last two decades and there has been a subsequent explosion in the number and type of TRP channel described. Although information is accumulating as to the function of some of the TRP channels, the activation and inactivation mechanisms, structure, and interacting proteins of many, if not most, are awaiting elucidation. The Cell and Molecular Biology of TRP Channels Meeting held at the University of Bath included speakers working on a number of the different subfamilies of TRP channels and provided a basis for highlighting both similarities and differences between these groups. As the TRP channels mediate diverse functions, this meeting also brought together an audience with wide-ranging research interests, including biochemistry, cell biology, physiology and neuroscience, and inspired lively discussion on the issues reviewed herein.

Sensing the air around us: the voltage-gated-like ion channel family

Ion channels are a complex set of proteins having many important physiologic and potentially pathologic roles. The flow of ions through these channels and the subsequent cellular depolarization can trigger complex mechanisms such as cardiac rhythm, hormone secretion, and numerous sensory experiences. The transient receptor potential (TRP) channels are an important means for multiple organ systems to interact with their environment. The various TRP channel subfamilies respond to voltage or to ligands such as G-protein coupled receptors. Their ability to sense temperature, pain, stretch, and osmolarity among others enables them to mediate responses such as smooth muscle contraction, cough, or sensation of pain.

Dissection of the components for PIP2 activation and thermosensation in TRP channels

Phosphatidylinositol 4,5-bisphosphate (PIP2) plays a central role in the activation of several transient receptor potential (TRP) channels. The role of PIP2 on temperature gating of thermoTRP channels has not been explored in detail, and the process of temperature activation is largely unexplained. In this work, we have exchanged different segments of the C-terminal region between cold-sensitive (TRPM8) and heat-sensitive (TRPV1) channels, trying to understand the role of the segment in PIP2 and temperature activation. A chimera in which the proximal part of the C-terminal of TRPV1 replaces an equivalent section of TRPM8 C-terminal is activated by PIP2 and confers the phenotype of heat activation. PIP2, but not temperature sensitivity, disappears when positively charged residues contained in the exchanged region are neutralized. Shortening the exchanged segment to a length of 11 aa produces voltage-dependent and temperature-insensitive channels. Our findings suggest the existence of different activation domains for temperature, PIP2, and voltage. We provide an interpretation for channel-PIP2 interaction using a full-atom molecular model of TRPV1 and PIP2 docking analysis.

Both translocon and a cation channel are involved in the passive Ca2+ leak from the endoplasmic reticulum: A mechanistic study on rat liver microsomes

Steady-state levels of calcium ions in endoplasmic reticulum reflect a balance between active inward transport, mediated by MgATP-dependent Ca(2+) pumps, and passive backflux of the ions, through putative "leak channels". We have investigated the efflux of Ca(2+) from rat liver microsomal vesicles, passively pre-equilibrated in the presence radiolabelled Ca(2+). Similarly, we have also evaluated the efflux of a low-Mwt uncharged compound, i.e., sucrose. The results show that two major passive Ca(2+) efflux pathways exist. One appeared to involve the translocon pore, since it was stimulated by the translocon opener puromycin, and also allowed the passage of sucrose. Putative channels likely mediated the other one, since it required counter ion influx and was inhibited by Gd(3+) and La(3+). The latter pathway did not appear to involve inactive Ca(2+) pumps, Bcl2 proteins, or known channels, such as the InsP3 and ryanodine receptors. While sucrose efflux was highly represented in a rough microsomal subfraction--enriched in the translocon component Sec61alpha--the efflux of Ca(2+) was represented both in smooth and in rough microsomes. We conclude that the passive efflux of Ca(2+) from the (liver) ER could be mediated by both the translocon pore and putative Ca(2+) leak channels. However, the relative role of these Ca(2+) efflux pathways in the intact cell as well as the molecular nature of the Ca(2+) leak channel(s) remain to be clarified.