Structure-functional intimacies of transient receptor potential channels

The Pentameric Ligand-Gated Ion Channel Family: A New Member of the Voltage Gated Ion Channel Superfamily?

In this report we present seven lines of bioinformatic evidence supporting the conclusion that the Pentameric Ligand-gated Ion Channel (pLIC) Family is a member of the Voltage-gated Ion Channel (VIC) Superfamily. In our approach, we used the Transporter Classification Database (TCDB) as a reference and applied a series of bioinformatic methods to search for similarities between the pLIC family and members of the VIC superfamily. These include: (1) sequence similarity, (2) compatibility of topology and hydropathy profiles, (3) shared domains, (4) conserved motifs, (5) similarity of Hidden Markov Model profiles between families, (6) common 3D structural folds, and (7) clustering analysis of all families. Furthermore, sequence and structural comparisons as well as the identification of a 3-TMS repeat unit in the VIC superfamily suggests that the sixth transmembrane segment evolved into a re-entrant loop. This evidence suggests that the voltage-sensor domain and the channel domain have a common origin. The classification of the pLIC family within the VIC superfamily sheds light onto the topological origins of this family and its evolution, which will facilitate experimental verification and further research into this superfamily by the scientific community.

Signaling Roleplay between Ion Channels during Mammalian Sperm Capacitation

In order to accomplish their primary goal, mammalian spermatozoa must undergo a series of physiological, biochemical, and functional changes crucial for the acquisition of fertilization ability. Spermatozoa are highly polarized cells, which must swiftly respond to ionic changes on their passage through the female reproductive tract, and which are necessary for male gametes to acquire their functional competence. This review summarizes the current knowledge about specific ion channels and transporters located in the mammalian sperm plasma membrane, which are intricately involved in the initiation of changes within the ionic milieu of the sperm cell, leading to variations in the sperm membrane potential, membrane depolarization and hyperpolarization, changes in sperm motility and capacitation to further lead to the acrosome reaction and sperm-egg fusion. We also discuss the functionality of selected ion channels in male reproductive health and/or disease since these may become promising targets for clinical management of infertility in the future.

TRP (transient receptor potential) ion channel family: structures, biological functions and therapeutic interventions for diseases

Transient receptor potential (TRP) channels are sensors for a variety of cellular and environmental signals. Mammals express a total of 28 different TRP channel proteins, which can be divided into seven subfamilies based on amino acid sequence homology: TRPA (Ankyrin), TRPC (Canonical), TRPM (Melastatin), TRPML (Mucolipin), TRPN (NO-mechano-potential, NOMP), TRPP (Polycystin), TRPV (Vanilloid). They are a class of ion channels found in numerous tissues and cell types and are permeable to a wide range of cations such as Ca²⁺, Mg²⁺, Na⁺, K⁺, and others. TRP channels are responsible for various sensory responses including heat, cold, pain, stress, vision and taste and can be activated by a number of stimuli. Their predominantly location on the cell surface, their interaction with numerous physiological signaling pathways, and the unique crystal structure of TRP channels make TRPs attractive drug targets and implicate them in the treatment of a wide range of diseases. Here, we review the history of TRP channel discovery, summarize the structures and functions of the TRP ion channel family, and highlight the current understanding of the role of TRP channels in the pathogenesis of human disease. Most importantly, we describe TRP channel-related drug discovery, therapeutic interventions for diseases and the limitations of targeting TRP channels in potential clinical applications.

Cost-Effective Pipeline for a Rational Design and Selection of Capsaicin Analogues Targeting TRPV1 Channels

Transient receptor potential (TRP) channels constitute a large group of membrane receptors associated with sensory pathways in vertebrates. One of the most studied is TRPV1, a polymodal receptor tuned for detecting heat and pungent compounds. Specific inhibition of the nociceptive transduction at the peripheral nerve represents a convenient approach to pain relief. While acting as a chemoreceptor, TRPV1 shows high sensitivity and selectivity for capsaicin. In contrast to the drugs available on the market that target the inflammatory system, TRPV1 antagonists act as negative modulators of nociceptive transduction. Therefore, the development of compounds modulating TRPV1 activity has expanded dramatically over time. Experimental data suggest that most agonist and antagonist drugs interact at or near capsaicin's binding site. In particular, the properties of capsaicin's head play an essential role in modulating potency and affinity. Here, we explored a cost-efficient pipeline to predict the effects of introducing chemical modifications into capsaicin's head region. An extensive set of molecules was selected by first considering the geometrical properties of capsaicin's binding site and then molecular docking. Finally, the novel ligands were ranked by combining molecular and pharmacokinetic predictions.

Single-cell analysis reveals prognostic fibroblast subpopulations linked to molecular and immunological subtypes of lung cancer

Fibroblasts are poorly characterised cells that variably impact tumour progression. Here, we use single cell RNA-sequencing, multiplexed immunohistochemistry and digital cytometry (CIBERSORTx) to identify and characterise three major fibroblast subpopulations in human non-small cell lung cancer: adventitial, alveolar and myofibroblasts. Alveolar and adventitial fibroblasts (enriched in control tissue samples) localise to discrete spatial niches in histologically normal lung tissue and indicate improved overall survival rates when present in lung adenocarcinomas (LUAD). Trajectory inference identifies three phases of control tissue fibroblast activation, leading to myofibroblast enrichment in tumour samples: initial upregulation of inflammatory cytokines, followed by stress-response signalling and ultimately increased expression of fibrillar collagens. Myofibroblasts correlate with poor overall survival rates in LUAD, associated with loss of epithelial differentiation, TP53 mutations, proximal molecular subtypes and myeloid cell recruitment. In squamous carcinomas myofibroblasts were not prognostic despite being transcriptomically equivalent. These findings have important implications for developing fibroblast-targeting strategies for cancer therapy.

Involvement of TRPM7 in Alcohol-Induced Damage of the Blood-Brain Barrier in the Presence of HIV Viral Proteins

Ethanol (EtOH) exerts its effects through various protein targets, including transient receptor potential melastatin 7 (TRPM7) channels, which play an essential role in cellular homeostasis. We demonstrated that TRPM7 is expressed in rat brain microvascular endothelial cells (rBMVECs), the major cellular component of the blood-brain barrier (BBB). Heavy alcohol drinking is often associated with HIV infection, however mechanisms underlying alcohol-induced BBB damage and HIV proteins, are not fully understood. We utilized the HIV-1 transgenic (HIV-1Tg) rat to mimic HIV-1 patients on combination anti-retroviral therapy (cART) and demonstrated TRPM7 expression in rBMVECs wass lower in adolescent HIV-1Tg rats compared to control animals, however control and HIV-1Tg rats expressed similar levels at 9 weeks, indicating persistent presence of HIV-1 proteins delayed TRPM7 expression. Binge exposure to EtOH (binge EtOH) decreased TRPM7 expression in control rBMVECs in a concentration-dependent manner, and abolished TRPM7 expression in HIV-1Tg rats. In human BMVECs (hBMVECs), TRPM7 expression was downregulated after treatment with EtOH, HIV-1 proteins, and in combination. Next, we constructed in vitro BBB models using BMVECs and found TRPM7 antagonists enhanced EtOH-mediated BBB integrity changes. Our study demonstrated alcohol decreased TRPM7 expression, whereby TRPM7 could be involved in the mechanisms underlying BBB alcohol-induced damage in HIV-1 patients on cART.

Regulation of ThermoTRP Channels by PIP2 and Cholesterol

Transient receptor potential (TRP) ion channels are proteins that are expressed by diverse tissues and that play pivotal functions in physiology. These channels are polymodal and are activated by several stimuli. Among TRPs, some members of this family of channels respond to changes in ambient temperature and are known as thermoTRPs. These proteins respond to heat or cold in the noxious range and some of them to temperatures considered innocuous, as well as to mechanical, osmotic, and/or chemical stimuli. In addition to this already complex ability to respond to different signals, the activity of these ion channels can be fine-tuned by lipids. Two lipids well known to modulate ion channel activity are phosphatidylinositol 4,5-bisphosphate (PIP2) and cholesterol. These lipids can either influence the function of these proteins through direct interaction by binding to a site in the structure of the ion channel or through indirect mechanisms, which can include modifying membrane properties, such as curvature and rigidity, by regulating their expression or by modulating the actions of other molecules or signaling pathways that affect the physiology of ion channels. Here, we summarize the key aspects of the regulation of thermoTRP channels by PIP2 and cholesterol.

ThermoTRP channels in pain sexual dimorphism: new insights for drug intervention

Chronic pain is a major burden for the society and remains more prevalent and severe in females. The presence of chronic pain is linked to persistent alterations in the peripheral and the central nervous system. One of the main types of peripheral pain transducers are the transient receptor potential channels (TRP), also known as thermoTRP channels, which intervene in the perception of hot and cold external stimuli. These channels, and especially TRPV1, TRPA1 and TRPM8, have been subjected to profound investigation because of their role as thermosensors and also because of their implication in acute and chronic pain. Surprisingly, their sensitivity to endogenous signaling has been far less studied. Cumulative evidence suggests that the function of these channels may be differently modulated in males and females, in part through sexual hormones, and this could constitute a significant contributor to the sex differences in chronic pain. Here, we review the exciting advances in thermoTRP pharmacology for males and females in two paradigmatic types of chronic pain with a strong peripheral component: chronic migraine and chemotherapy-induced peripheral neuropathy (CIPN). The possibilities of peripheral druggability offered by these channels and the differential exploitation for men and women represent a development opportunity that will lead to a significant increment of the armamentarium of analgesic medicines for personalized chronic pain treatment.

Histological and immunohistochemical study of gilthead seabream tongue from the early stage of development: TRPV4 potential roles

BACKGROUND

Taste buds, the morphofunctional units for taste perception, transduce gustatory stimuli using G protein-coupled receptors, and a complex arrangement of ion channels, among which TRPV4, a member of the TRP superfamily. Studies on taste buds development on gilthead seabream are unknown, and the TRPV4 expression on fish taste cells studies were conducted only on zebrafish.

METHODS

In our study, we have investigated the histological features of the gilthead seabream tongue dorsal surface from the earliest stage of development using Masson trichrome with aniline blue staining. Additionally, TRPV4 expression pattern was studied by means of immunohistochemical labeling and quantitative RT-PCR.

RESULTS

We have recorded for the first time on gilthead seabream lingual dorsal surface the presence of, stage specific, three types of taste buds: type I, type II and type III in larvae, juvenile and adults respectively. At 40 days post hatching, taste buds were mature-looking. TRPV4 expression was detected in a subpopulation of taste cells of larvae, juveniles, and adults. Furthermore, TRPV4 was expressed in the basal epithelial cells of the tongue at the larvae and juvenile stage, while this expression pattern was more diffused within all the epithelial cell layers on the adult.

CONCLUSION

Our findings presume a taste sensory role of TRPV4 in the three stage-specific taste buds and oral epithelia of gilthead seabream. In addition to its sensory role on the epithelial cell layers, we hypothesize that TRPV4 is implicated in epithelial cells differentiation and membrane protection.

Sequence and structural conservation reveal fingerprint residues in TRP channels

Transient receptor potential (TRP) proteins are a large family of cation-selective channels, surpassed in variety only by voltage-gated potassium channels. Detailed molecular mechanisms governing how membrane voltage, ligand binding, or temperature can induce conformational changes promoting the open state in TRP channels are still a matter of debate. Aiming to unveil distinctive structural features common to the transmembrane domains within the TRP family, we performed phylogenetic reconstruction, sequence statistics, and structural analysis over a large set of TRP channel genes. Here, we report an exceptionally conserved set of residues. This fingerprint is composed of twelve residues localized at equivalent three-dimensional positions in TRP channels from the different subtypes. Moreover, these amino acids are arranged in three groups, connected by a set of aromatics located at the core of the transmembrane structure. We hypothesize that differences in the connectivity between these different groups of residues harbor the apparent differences in coupling strategies used by TRP subgroups.

Supramolecular assembly of protein building blocks: from folding to function

Several phenomena occurring throughout the life of living things start and end with proteins. Various proteins form one complex structure to control detailed reactions. In contrast, one protein forms various structures and implements other biological phenomena depending on the situation. The basic principle that forms these hierarchical structures is protein self-assembly. A single building block is sufficient to create homogeneous structures with complex shapes, such as rings, filaments, or containers. These assemblies are widely used in biology as they enable multivalent binding, ultra-sensitive regulation, and compartmentalization. Moreover, with advances in the computational design of protein folding and protein-protein interfaces, considerable progress has recently been made in the de novo design of protein assemblies. Our review presents a description of the components of supramolecular protein assembly and their application in understanding biological phenomena to therapeutics.

Structural mechanism of heat-induced opening of a temperature-sensitive TRP channel

Numerous physiological functions rely on distinguishing temperature through temperature-sensitive transient receptor potential channels (thermo-TRPs). Although the function of thermo-TRPs has been studied extensively, structural determination of their heat- and cold-activated states has remained a challenge. Here, we present cryo-EM structures of the nanodisc-reconstituted wild-type mouse TRPV3 in three distinct conformations: closed, heat-activated sensitized and open states. The heat-induced transformations of TRPV3 are accompanied by changes in the secondary structure of the S2-S3 linker and the N and C termini and represent a conformational wave that links these parts of the protein to a lipid occupying the vanilloid binding site. State-dependent differences in the behavior of bound lipids suggest their active role in thermo-TRP temperature-dependent gating. Our structural data, supported by physiological recordings and molecular dynamics simulations, provide an insight for understanding the molecular mechanism of temperature sensing.

Allyl Isothiocyanate: A TAS2R38 Receptor-Dependent Immune Modulator at the Interface Between Personalized Medicine and Nutrition

Understanding individual responses to nutrition and medicine is of growing interest and importance. There is evidence that differences in bitter taste receptor (TAS2R) genes which give rise to two frequent haplotypes, TAS2R38-PAV (functional) and TAS2R38-AVI (non-functional), may impact inter-individual differences in health status. We here analyzed the relevance of the TAS2R38 receptor in the regulation of the human immune response using the TAS2R38 agonist allyl isothiocyanate (AITC) from Brassica plants. A differential response in calcium mobilization upon AITC treatment in leucocytes from healthy humans confirmed a relevance of TAS2R38 functionality, independent from cation channel TRPV1 or TRPA1 activation. We further identified a TAS2R38-dependence of MAPK and AKT signaling activity, bactericidal (toxicity against E. coli) and anti-inflammatory activity (TNF-alpha inhibition upon cell stimulation). These in vitro results were derived at relevant human plasma levels in the low micro molar range as shown here in a human intervention trial with AITC-containing food.

Functional Expression of TRPV1 Ion Channel in the Canine Peripheral Blood Mononuclear Cells

TRPV1, known as a capsaicin receptor, is the best-described transient receptor potential (TRP) ion channel. Recently, it was shown to be expressed by non-excitable cells such as lymphocytes. However, the data regarding the functional expression of the TRPV1 channel in the immune cells are often contradictory. In the present study, we performed a phylogenetical analysis of the canine TRP ion channels, we assessed the expression of TRPV1 in the canine peripheral blood mononuclear cells (PBMC) by qPCR and Western blot, and we determined the functionality of TRPV1 by whole-cell patch-clamp recordings and calcium assay. We found high expression of TRPV2, -M2, and -M7 in the canine PBMCs, while expression of TRPV1, -V4 and, -M5 was relatively low. We confirmed that TRPV1 is expressed on the protein level in the PBMC and it localizes in the plasma membrane. The whole-cell patch-clamp recording revealed that capsaicin application caused a significant increase in the current density. Similarly, the results from the calcium assay show a dose-dependent increase in intracellular calcium level in the presence of capsaicin that was partially abolished by capsazepine. Our study confirms the expression of TRPV1 ion channel on both mRNA and protein levels in the canine PBMC and indicates that the ion channel is functional.

An overview on transient receptor potential channels superfamily

The transient receptor potential (TRP) channel superfamily is comprised of a large group of cation-permeable channels, which display an extraordinary diversity of roles in sensory signaling and are involved in plethora of animal behaviors. These channels are activated through a wide variety of mechanisms and participate in virtually every sensory modality. Modulating TRP channel activity provides an important way to regulate membrane excitability and intracellular calcium levels. This is reflected by the fact that small molecule compounds modulating different TRPs have all entered clinical trials for a variety of diseases. The role of TRPs will be further elucidated in complex diseases of the nervous, intestinal, renal, urogenital, respiratory, and cardiovascular systems in diverse therapeutic areas including pain and itch, headache, pulmonary function, oncology, neurology, visceral organs, and genetic diseases. This review focuses on recent developments in the TRP ion channel-related area and highlights evidence supporting TRP channels as promising targets for new analgesic drugs for therapeutic intervention. This review presents a variety of: (1) phylogeny aspects of TRP channels; (2) some structural and functional characteristics of TRPs; (3) a general view and short characteristics of main seven subfamilies of TRP channels; (4) the evidence for consider TRP channels as therapeutic and analgesic targets; and finally (5) further perspectives of TRP channels research.

Novel TRPV1 Channel Agonists With Faster and More Potent Analgesic Properties Than Capsaicin

The transient receptor potential vanilloid 1 (TRPV1) ion channel is a member of the family of Transient Receptor Potential (TRP) channels that acts as a molecular detector of noxious signals in primary sensory neurons. Activated by capsaicin, heat, voltage and protons, it is also well known for its desensitization, which led to the medical use of topically applied TRPV1 agonist capsaicin for its long-lasting analgesic effects. Here we report three novel small molecules, which were identified using a Structure-Based Virtual Screening for TRPV1 from the ZINC database. The three compounds were tested using electrophysiological assays, which confirmed their capsaicin-like agonist activity. von Frey filaments were used to measure the analgesic effects of the compounds applied topically on tactile allodynia induced by intra-plantar carrageenan. All compounds had anti-nociceptive activity, but two of them showed faster and longer lasting analgesic effects than capsaicin. The present results suggest that TRPV1 agonists different from capsaicin could be used to develop topical analgesics with faster onset and more potent effects.

Evolutionary Aspects of TRPMLs and TPCs

Transient receptor potential (TRP) or transient receptor potential channels are a highly diverse family of mostly non-selective cation channels. In the mammalian genome, 28 members can be identified, most of them being expressed predominantly in the plasma membrane with the exception of the mucolipins or TRPMLs which are expressed in the endo-lysosomal system. In mammalian organisms, TRPMLs have been associated with a number of critical endo-lysosomal functions such as autophagy, endo-lysosomal fusion/fission and trafficking, lysosomal exocytosis, pH regulation, or lysosomal motility and positioning. The related non-selective two-pore cation channels (TPCs), likewise expressed in endosomes and lysosomes, have also been found to be associated with endo-lysosomal trafficking, autophagy, pH regulation, or lysosomal exocytosis, raising the question why these two channel families have evolved independently. We followed TRP/TRPML channels and TPCs through evolution and describe here in which species TRP/TRPMLs and/or TPCs are found, which functions they have in different species, and how this compares to the functions of mammalian orthologs.

TRPV4: A Physio and Pathophysiologically Significant Ion Channel

Transient Receptor Potential (TRP) channels are a family of ion channels whose members are distributed among all kinds of animals, from invertebrates to vertebrates. The importance of these molecules is exemplified by the variety of physiological roles they play. Perhaps, the most extensively studied member of this family is the TRPV1 ion channel; nonetheless, the activity of TRPV4 has been associated to several physio and pathophysiological processes, and its dysfunction can lead to severe consequences. Several lines of evidence derived from animal models and even clinical trials in humans highlight TRPV4 as a therapeutic target and as a protein that will receive even more attention in the near future, as will be reviewed here.

TRPA1 regulates macrophages phenotype plasticity and atherosclerosis progression

BACKGROUND AND AIMS

TRPA1 is a calcium permeable non-selective cation channel, its expression is up-regulated in atherosclerosis plaque, yet its function in macrophages activation is unknown. We sought to establish the role of TRPA1 in inflammatory macrophages activation.

METHODS

TRPA1^-/-ApoE^-/- mice and C57BL/6 J control were treated with a high-fat diet (HFD) and the TRPA1 agonist cinnamaldehyde (CIN). Third-order branches of superior aorta of patients and mice were collected. Oil Red O staining and hematoxylin and eosin staining were performed to measure atherosclerotic lesions. The RNA-seq was performed to identify TRPA1 function in atherosclerosis. The expression of bone marrow-derived macrophages (BMDMs) markers was tested by Western blot. In addition, the levels of inflammatory factors were checked by ELISA. Chromatin immunoprecipitation (ChIP)-PCR and luciferase reporter gene assays were used to explore if TRPA1 could regulate histone modifications.

RESULTS

TRPA1^-/-ApoE^-/- mice showed a significant increase in atherosclerosis plaques compared to ApoE^-/- mice after HFD treatment. Conversely, activation of TRPA1 by CIN sharply reduced atherosclerosis progression. Atherosclerosis was associated with a significant change in macrophage polarization toward the M1 proinflammatory phenotype. We found that inhibition of TRPA1 remarkably stimulated M1 marker genes expression, while repressed M2 marker genes expression. The interaction between TRPA1 and Ezh2, a subunit of polycomb repressive complex 2, suppressed the proteasome-dependent degradation of Ezh2. Thus, TRPA1 epigenetically regulated H3K27 trimethylation level in macrophages.

CONCLUSIONS

Our results demonstrate that TRPA1, up-regulated in atherosclerosis plaque, could regulate the macrophages toward an inflammatory phenotype, thereby modulating atherosclerosis progression. Activation of TRPA1 might serve as an atherosclerosis therapeutic target.

TRPM3_miR-204: a complex locus for eye development and disease

First discovered in a light-sensitive retinal mutant of Drosophila, the transient receptor potential (TRP) superfamily of non-selective cation channels serve as polymodal cellular sensors that participate in diverse physiological processes across the animal kingdom including the perception of light, temperature, pressure, and pain. TRPM3 belongs to the melastatin sub-family of TRP channels and has been shown to function as a spontaneous calcium channel, with permeability to other cations influenced by alternative splicing and/or non-canonical channel activity. Activators of TRPM3 channels include the neurosteroid pregnenolone sulfate, calmodulin, phosphoinositides, and heat, whereas inhibitors include certain drugs, plant-derived metabolites, and G-protein subunits. Activation of TRPM3 channels at the cell membrane elicits a signal transduction cascade of mitogen-activated kinases and stimulus response transcription factors. The mammalian TRPM3 gene hosts a non-coding microRNA gene specifying miR-204 that serves as both a tumor suppressor and a negative regulator of post-transcriptional gene expression during eye development in vertebrates. Ocular co-expression of TRPM3 and miR-204 is upregulated by the paired box 6 transcription factor (PAX6) and mutations in all three corresponding genes underlie inherited forms of eye disease in humans including early-onset cataract, retinal dystrophy, and coloboma. This review outlines the genomic and functional complexity of the TRPM3_miR-204 locus in mammalian eye development and disease.

The ion selectivity filter is not an activation gate in TRPV1-3 channels

Activation of TRPV1 channels in sensory neurons results in opening of a cation permeation pathway that triggers the sensation of pain. Opening of TRPV1 has been proposed to involve two gates that appear to prevent ion permeation in the absence of activators: the ion selectivity filter on the external side of the pore and the S6 helices that line the cytosolic half of the pore. Here we measured the access of thiol-reactive ions across the selectivity filters in rodent TRPV1-3 channels. Although our results are consistent with structural evidence that the selectivity filters in these channels are dynamic, they demonstrate that cations can permeate the ion selectivity filters even when channels are closed. Our results suggest that the selectivity filters in TRPV1-3 channels do not function as activation gates but might contribute to coupling structural rearrangements in the external pore to those in the cytosolic S6 gate.

The structural changes of the mutated ankyrin repeat domain of the human TRPV4 channel alter its ATP binding ability

The transient receptor potential (TRP) channel TRPV4 is a calcium-permeable cation channel protein which plays a mechanosensory and osmosensory role in several musculoskeletal tissues. Previous studies have shown that some specific mutations in the ankyrin repeat domain (ARD) of TRPV4 can reduce channel activity and further cause the osteoarthropathy related disease. Mutations in this region probably influence the constitutive activity of the channel, which mainly regulated by the binding of a small ligand such as adenosine triphosphate (ATP). These findings suggest that it is crucial to understand the fundamental mechanisms regulated by chemical ligands such as ATP binding with the ankyrin repeat domain (ARD) of TRPV4. However, how these mutations at the molecular level resulting in the related diseases are still unclear. Here we use full atomistic simulations to investigate the mutation induced conformational changes and ATP binding ability differences of TRPV4-ARD. Conformation characteristics of different mutations of TRPV4-ARD are explored. Optimal communication paths are studied to explain how a point mutation away from aim region (Finger 3) can cause a significant alteration on the conformation. We identify two molecular mechanisms through the conformation of Finger 3 and through alter the ATP binding mechanism correspondently to explain these unknowns. Our study provides fundamental insights into the mutation induced structural changes of the TRPV4-ARD and helps to explain how the mutations alter the ATP binding ability of the TRPV4-ARD.

A Cell-Penetrating Scorpion Toxin Enables Mode-Specific Modulation of TRPA1 and Pain

TRPA1 is a chemosensory ion channel that functions as a sentinel for structurally diverse electrophilic irritants. Channel activation occurs through an unusual mechanism involving covalent modification of cysteine residues clustered within an amino-terminal cytoplasmic domain. Here, we describe a peptidergic scorpion toxin (WaTx) that activates TRPA1 by penetrating the plasma membrane to access the same intracellular site modified by reactive electrophiles. WaTx stabilizes TRPA1 in a biophysically distinct active state characterized by prolonged channel openings and low Ca²⁺ permeability. Consequently, WaTx elicits acute pain and pain hypersensitivity but fails to trigger efferent release of neuropeptides and neurogenic inflammation typically produced by noxious electrophiles. These findings provide a striking example of convergent evolution whereby chemically disparate animal- and plant-derived irritants target the same key allosteric regulatory site to differentially modulate channel activity. WaTx is a unique pharmacological probe for dissecting TRPA1 function and its contribution to acute and persistent pain.

Marine Toxins and Nociception: Potential Therapeutic Use in the Treatment of Visceral Pain Associated with Gastrointestinal Disorders

Visceral pain, of which the pathogenic basis is currently largely unknown, is a hallmark symptom of both functional disorders, such as irritable bowel syndrome, and inflammatory bowel disease. Intrinsic sensory neurons in the enteric nervous system and afferent sensory neurons of the dorsal root ganglia, connecting with the central nervous system, represent the primary neuronal pathways transducing gut visceral pain. Current pharmacological therapies have several limitations, owing to their partial efficacy and the generation of severe adverse effects. Numerous cellular targets of visceral nociception have been recognized, including, among others, channels (i.e., voltage-gated sodium channels, VGSCs, voltage-gated calcium channels, VGCCs, Transient Receptor Potential, TRP, and Acid-sensing ion channels, ASICs) and neurotransmitter pathways (i.e., GABAergic pathways), which represent attractive targets for the discovery of novel drugs. Natural biologically active compounds, such as marine toxins, able to bind with high affinity and selectivity to different visceral pain molecular mediators, may represent a useful tool (1) to improve our knowledge of the physiological and pathological relevance of each nociceptive target, and (2) to discover therapeutically valuable molecules. In this review we report the most recent literature describing the effects of marine toxin on gastrointestinal visceral pain pathways and the possible clinical implications in the treatment of chronic pain associated with gut diseases.

Nociceptor Signalling through ion Channel Regulation via GPCRs

The prime task of nociceptors is the transformation of noxious stimuli into action potentials that are propagated along the neurites of nociceptive neurons from the periphery to the spinal cord. This function of nociceptors relies on the coordinated operation of a variety of ion channels. In this review, we summarize how members of nine different families of ion channels expressed in sensory neurons contribute to nociception. Furthermore, data on 35 different types of G protein coupled receptors are presented, activation of which controls the gating of the aforementioned ion channels. These receptors are not only targeted by more than 20 separate endogenous modulators, but can also be affected by pharmacotherapeutic agents. Thereby, this review provides information on how ion channel modulation via G protein coupled receptors in nociceptors can be exploited to provide improved analgesic therapy.

Role of TRPV1 and TRPA1 Ion Channels in Inflammatory Bowel Diseases: Potential Therapeutic Targets?

Inflammatory bowel diseases (IBD) have long been recognized to be accompanied by pain resulting in high morbidity. Transient receptor potential vanilloid 1 (TRPV1) and ankyrin 1 (TRPA1) ion channels located predominantly on the capsaicin-sensitive sensory neurons play a complex role in hyperalgesia and neurogenic inflammation. This review provides an overview of their expression and role in intestinal inflammation, in particular colitis, that appears to be virtually inconsistent based on the thorough investigations of the last twenty years. However, preclinical results with pharmacological interventions, as well as scarcely available human studies, more convincingly point out the potential therapeutic value of TRPV1 and TRPA1 antagonists in colitis and visceral hypersensitivity providing future therapeutical perspectives through a complex, unique mechanism of action for drug development in IBD.

New Structures and Gating of Voltage-Dependent Potassium (Kv) Channels and Their Relatives: A Multi-Domain and Dynamic Question

Voltage-dependent potassium channels (Kv channels) are crucial regulators of cell excitability that participate in a range of physiological and pathophysiological processes. These channels are molecular machines that display a mechanism (known as gating) for opening and closing a gate located in a pore domain (PD). In Kv channels, this mechanism is triggered and controlled by changes in the magnitude of the transmembrane voltage sensed by a voltage-sensing domain (VSD). In this review, we consider several aspects of the VSD–PD coupling in Kv channels, and in some relatives, that share a common general structure characterized by a single square-shaped ion conduction pore in the center, surrounded by four VSDs located at the periphery. We compile some recent advances in the knowledge of their architecture, based in cryo-electron microscopy (cryo-EM) data for high-resolution determination of their structure, plus some new functional data obtained with channel variants in which the covalent continuity between the VSD and PD modules has been interrupted. These advances and new data bring about some reconsiderations about the use of exclusively a classical electromechanical lever model of VSD–PD coupling by some Kv channels, and open a view of the Kv-type channels as allosteric machines in which gating may be dynamically influenced by some long-range interactional/allosteric mechanisms.

Role of transient receptor potential channels in regulating spermatozoa functions: A mini-review

Flagellar navigation along the genital tract of male and female in spermatozoa is accomplished through a number of biological, physiological, biochemical, and electrophysiological alterations in spermatozoa. These alterations are highly precise, dynamic, and regulated through a number of ion channels along with their associated pathways. Beating of flagella along with intracellular metabolism of spermatozoa is associated with fluxing of Ca++ as well as release of Ca++ from different sources. Calcium fluxing through the spermatozoa is mediated through sperm-specific calcium channel and also through transient receptor potential (TRP) channels which are diversified multifamily of ion channels which are activated through a number of extracellular agents such as pH, temperature, chemicals, and pathogens. Research has shown the dynamic role of TRP channels in regulating sperm functions such as sperm chemotaxis, rheotaxis, thermotaxis, and eventually fertilization. Diversified forms of TRP and their involvement in regulation of sperm function opens new horizons of understanding of the sperm function and, in specific, issues related to infertility. This mini-review is an attempt to draw some insights into the action of TRP channels in regulating sperm fertility competence through both calcium-dependent and calcium-independent mechanisms.

TRP channels in cardiac and intestinal fibrosis

It is now widely accepted that advanced fibrosis underlies many chronic inflammatory disorders and is the main cause of morbidity and mortality of the modern world. The pathogenic mechanism of advanced fibrosis involves diverse and intricate interplays between numerous extracellular and intracellular signaling molecules, among which the non-trivial roles of a stress-responsive Ca²⁺/Na⁺-permeable cation channel superfamily, the transient receptor potential (TRP) protein, are receiving growing attention. Available evidence suggests that several TRP channels such as TRPC3, TRPC6, TRPV1, TRPV3, TRPV4, TRPA1, TRPM6 and TRPM7 may play central roles in the progression and/or prevention of fibroproliferative disorders in vital visceral organs such as lung, heart, liver, kidney, and bowel as well as brain, blood vessels and skin, and may contribute to both acute and chronic inflammatory processes involved therein. This short paper overviews the current knowledge accumulated in this rapidly growing field, with particular focus on cardiac and intestinal fibrosis, which are tightly associated with the pathogenesis of atrial fibrillation and inflammatory bowel diseases such as Crohn's disease.

TRP Channels as Potential Targets for Sex-Related Differences in Migraine Pain

Chronic pain is one of the most debilitating human diseases and represents a social and economic burden for our society. Great efforts are being made to understand the molecular and cellular mechanisms underlying the pathophysiology of pain transduction. It is particularly noteworthy that some types of chronic pain, such as migraine, display a remarkable sex dimorphism, being up to three times more prevalent in women than in men. This gender prevalence in migraine appears to be related to sex differences arising from both gonadal and genetic factors. Indeed, the functionality of the somatosensory, immune, and endothelial systems seems modulated by sex hormones, as well as by X-linked genes differentially expressed during development. Here, we review the current data on the modulation of the somatosensory system functionality by gonadal hormones. Although this is still an area that requires intense investigation, there is evidence suggesting a direct regulation of nociceptor activity by sex hormones at the transcriptional, translational, and functional levels. Data are being accumulated on the effect of sex hormones on TRP channels such as TRPV1 that make pivotal contributions to nociceptor excitability and sensitization in migraine and other chronic pain syndromes. These data suggest that modulation of TRP channels' expression and/or activity by gonadal hormones provide novel pathways for drug intervention that may be useful for targeting the sex dimorphism observed in migraine.

Transient Receptor Potential Channels and Chronic Airway Inflammatory Diseases: A Comprehensive Review

Chronic airway inflammatory diseases remain a major problem worldwide, such that there is a need for additional therapeutic targets and novel drugs. Transient receptor potential (TRP) channels are a group of non-selective cation channels expressed throughout the body that are regulated by various stimuli. TRP channels have been identified in numerous cell types in the respiratory tract, including sensory neurons, airway epithelial cells, airway smooth muscle cells, and fibroblasts. Different types of TRP channels induce cough in sensory neurons via the vagus nerve. Permeability and cytokine production are also regulated by TRP channels in airway epithelial cells, and these channels also contribute to the modulation of bronchoconstriction. TRP channels may cooperate with other TRP channels, or act in concert with calcium-dependent potassium channels and calcium-activated chloride channel. Hence, TRP channels could be the potential therapeutic targets for chronic airway inflammatory diseases. In this review, we aim to discuss the expression profiles and physiological functions of TRP channels in the airway, and the roles they play in chronic airway inflammatory diseases.

Behavioral Aversion to AITC Requires Both Painless and dTRPA1 in Drosophila

There has been disagreement over the functional roles of the painless gene product in the detection and subsequent behavioral aversion to the active ingredient in wasabi, allyl isothiocyanate (AITC). Originally, painless was reported to eliminate the behavioral aversion to AITC, although subsequent reports suggested that another trpA homolog, dTRPA1, was responsible for AITC aversion. We re-evaluated the role of the painless gene in the detection of AITC, employing several different behavioral assays. Using the proboscis extension reflex (PER) assay, we observed that AITC did not reduce PER frequencies in painless or dTRPA1 mutants but did in wild-type genotypes. Quantification of food intake showed a significant decline in food consumption in the presence of AITC in wild-type, but not painless mutants. We adapted an oviposition choice assay and found wild-type oviposit on substrates lacking AITC, in contrast to painless and dTRPA1 mutants. Lastly, tracking individual flies relative to a point source of AITC, showed a consistent clustering of wild-type animals away from the point source, which was absent in painless mutants. We evaluated expression patterns of both dTRPA1 and painless, which showed expression in distinct central and peripheral populations. We identified the transmitter phenotypes of subsets of painless and dTRPA1 neurons and found similar neuropeptides as those expressed by mammalian trpA expressing neurons. Using a calcium reporter, we observed AITC-evoked responses in both painless and dTRPA1 expressing neurons. Collectively, these results reaffirm the necessity of painless in nociceptive behaviors and suggest experiments to further resolve the molecular basis of aversion.

Down-regulation of transient receptor potential melastatin member 7 prevents migration and invasion of renal cell carcinoma cells via inactivation of the Src and Akt pathway

Purpose

Transient receptor potential melastatin member 7 (TRPM7), an ion channel and serine/threonine protein kinase, has been linked with distinct human malignancies. However, the role of TRPM7 in renal cell carcinoma (RCC) has not been investigated. The aim of this study is to determine whether TRPM7 regulates the migration and invasion of RCC cells. Its relationship with signal transduction pathways was also studied.

Materials and Methods

The human RCC cell lines ACHN and SN12C were chosen for this study. The molecular mechanisms of TRPM7 action were studied using Western blot analysis and small interfering RNA (siRNA)-based knockdown. The effect of TRPM7 knockdown on RCC cells was measured by using Transwell invasion and wound healing migration assays.

Results

siRNA-induced silencing of TRPM7 notably decreased the migration and invasion of ACHN and SN12C RCC cells. The phosphorylation levels of Src in both cells were obviously reduced after TRPM7 silencing compared with that of the control ACHN and SN12C cells. Furthermore, the phosphorylation levels of Akt were greatly decreased in ACHN cells after siRNA-induced knockdown of TRPM7. Additionally, the treatment of cells with Src and Akt inhibitors clearly limited the migration and invasion of RCC cells.

Conclusions

Our data show that TRPM7 regulated ACHN and SN12C RCC cell invasion via the Src/Akt signaling pathway. Therefore, targeting the Src/Akt signaling pathway and/or the expression or function of TRPM7 could be a potential beneficial treatment for patients with RCC.

"Immuno-Transient Receptor Potential Ion Channels": The Role in Monocyte- and Macrophage-Mediated Inflammatory Responses

Monocytes and macrophages play important roles in health and disease. They have a central role in protecting the host, as they clear pathogens and modulate other immune cell functions through the production of regulatory molecules. Their functions include immune surveillance, bacterial killing, tissue remodeling and repair, clearance of cell debris and more. Macrophages can have beneficial and detrimental effects on the outcome of several diseases depending on the microenvironment and the activation state of cells. Over the past few years, there has been an increasing interest in the expression and functions of ion channels, in particular of transient receptor potential (TRP) channel family in immune cells. The 30 members of mammalian TRP channels are subdivided into TRPC, TRPV, TRPM, TRPML, TRPP, and TRPA superfamily, and several members of TRP subfamily have been found to be functionally expressed in monocytes and macrophages. TRP are cation-selective channels that are weakly voltage-sensitive and diversely gated by temperature, mechanical force, electrophiles, ligands, and internal cues, such as membrane composition and pH, contributing to immune and inflammatory responses. The TRP channels play major roles in controlling several monocyte and macrophage functions such as phagocytosis, production of chemokines and cytokines, cell survival, polarization and so forth. In addition, they can also be potential therapeutic targets in a variety of inflammatory diseases. Thus, the goal of this review is to describe the role of TRP channels in the control of monocyte–macrophage functions in inflammatory and immune-mediated diseases.

Transient Receptor Potential Vanilloid 1 Expression Mediates Capsaicin-Induced Cell Death

The transient receptor potential (TRP) ion channel family consists of a broad variety of non-selective cation channels that integrate environmental physicochemical signals for dynamic homeostatic control. Involved in a variety of cellular physiological processes, TRP channels are fundamental to the control of the cell life cycle. TRP channels from the vanilloid (TRPV) family have been directly implicated in cell death. TRPV1 is activated by pain-inducing stimuli, including inflammatory endovanilloids and pungent exovanilloids, such as capsaicin (CAP). TRPV1 activation by high doses of CAP (>10 μM) leads to necrosis, but also exhibits apoptotic characteristics. However, CAP dose-response studies are lacking in order to determine whether CAP-induced cell death occurs preferentially via necrosis or apoptosis. In addition, it is not known whether cytosolic Ca²⁺ and mitochondrial dysfunction participates in CAP-induced TRPV1-mediated cell death. By using TRPV1-transfected HeLa cells, we investigated the underlying mechanisms involved in CAP-induced TRPV1-mediated cell death, the dependence of CAP dose, and the participation of mitochondrial dysfunction and cytosolic Ca²⁺ increase. Together, our results contribute to elucidate the pathophysiological steps that follow after TRPV1 stimulation with CAP. Low concentrations of CAP (1 μM) induce cell death by a mechanism involving a TRPV1-mediated rapid and transient intracellular Ca²⁺ increase that stimulates plasma membrane depolarization, thereby compromising plasma membrane integrity and ultimately leading to cell death. Meanwhile, higher doses of CAP induce cell death via a TRPV1-independent mechanism, involving a slow and persistent intracellular Ca²⁺ increase that induces mitochondrial dysfunction, plasma membrane depolarization, plasma membrane loss of integrity, and ultimately, cell death.

Irreversible temperature gating in trpv1 sheds light on channel activation

Temperature-activated TRP channels or thermoTRPs are among the only proteins that can directly convert temperature changes into changes in channel open probability. In spite of a wealth of functional and structural information, the mechanism of temperature activation remains unknown. We have carefully characterized the repeated activation of TRPV1 by thermal stimuli and discovered a previously unknown inactivation process, which is irreversible. We propose that this form of gating in TRPV1 channels is a consequence of the heat absorption process that leads to channel opening.

Uncovering the arrhythmogenic potential of TRPM4 activation in atrial-derived HL-1 cells using novel recording and numerical approaches

Aims

Transient receptor potential cation channel subfamily melastatin member 4 (TRPM4), a Ca2+-activated nonselective cation channel abundantly expressed in the heart, has been implicated in conduction block and other arrhythmic propensities associated with cardiac remodelling and injury. The present study aimed to quantitatively evaluate the arrhythmogenic potential of TRPM4.

Methods and results

Patch clamp and biochemical analyses were performed using expression system and an immortalized atrial cardiomyocyte cell line (HL-1), and numerical model simulation was employed. After rapid desensitization, robust reactivation of TRPM4 channels required high micromolar concentrations of Ca2+. However, upon evaluation with a newly devised, ionomycin-permeabilized cell-attached (Iono-C/A) recording technique, submicromolar concentrations of Ca2+ (apparent Kd = ∼500 nM) were enough to activate this channel. Similar submicromolar Ca2+ dependency was also observed with sharp electrode whole-cell recording and in experiments coexpressing TRPM4 and L-type voltage-dependent Ca2+ channels. Numerical simulations using a number of action potential (AP) models (HL-1, Nygren, Luo-Rudy) incorporating the Ca2+- and voltage-dependent gating parameters of TRPM4, as assessed by Iono-C/A recording, indicated that a few-fold increase in TRPM4 activity is sufficient to delay late AP repolarization and further increases (≥ six-fold) evoke early afterdepolarization. These model predictions are consistent with electrophysiological data from angiotensin II-treated HL-1 cells in which TRPM4 expression and activity were enhanced.

Conclusions

These results collectively indicate that the TRPM4 channel is activated by a physiological range of Ca2+ concentrations and its excessive activity can cause arrhythmic changes. Moreover, these results demonstrate potential utility of the first AP models incorporating TRPM4 gating for in silico assessment of arrhythmogenicity in remodelling cardiac tissue.

Regulation of Pain and Itch by TRP Channels

Nociception is an important physiological process that detects harmful signals and results in pain perception. In this review, we discuss important experimental evidence involving some TRP ion channels as molecular sensors of chemical, thermal, and mechanical noxious stimuli to evoke the pain and itch sensations. Among them are the TRPA1 channel, members of the vanilloid subfamily (TRPV1, TRPV3, and TRPV4), and finally members of the melastatin group (TRPM2, TRPM3, and TRPM8). Given that pain and itch are pro-survival, evolutionarily-honed protective mechanisms, care has to be exercised when developing inhibitory/modulatory compounds targeting specific pain/itch-TRPs so that physiological protective mechanisms are not disabled to a degree that stimulus-mediated injury can occur. Such events have impeded the development of safe and effective TRPV1-modulating compounds and have diverted substantial resources. A beneficial outcome can be readily accomplished via simple dosing strategies, and also by incorporating medicinal chemistry design features during compound design and synthesis. Beyond clinical use, where compounds that target more than one channel might have a place and possibly have advantageous features, highly specific and high-potency compounds will be helpful in mechanistic discovery at the structure-function level.

TRPV1 gain-of-function mutation impairs pain and itch sensations in mice

Transient receptor potential vanilloid 1 (TRPV1) is a non-selective cation channel, which can detect various noxious stimuli that cause pain, inflammation, hyperalgesia, and itch. TRPV1 knock-out mice show deficiency in nociception, but the in vivo effects of persistent activation of TRPV1 are not completely understood. Here, we generated TRPV1 knock-in mice with a G564S mutation. In the heterologous expression system, an electrophysiological study showed that the G564S mutation in mouse TRPV1 caused increased basal current and a leftward shift of voltage dependence. Intriguingly, using behavioral analysis, we found that knock-in mice showed a thermosensory defect, impaired inflammatory thermal pain, and capsaicin sensitivity. We also demonstrated an attenuated behavioral response to the pruritic agent histamine in the knock-in mice. Indeed, calcium imaging together with electrophysiology showed that the overactive mutant had decreased capsaicin sensitivity. Western blot analysis revealed that the G564S mutant reduced TRPV1 phosphorylation and cell membrane trafficking. Together, we have generated a mouse model with a gain-of-function mutation in Trpv1 gene and demonstrated that the pain and histamine-dependent itch sensations in these mice are impaired due to a decreased phosphorylation level and reduced membrane localization of TRPV1.

A Functional Kinase Short Interfering Ribonucleic Acid Screen Using Protease-Activated Receptor 2-Dependent Opening of Transient Receptor Potential Vanilloid-4

Protease-activated receptor 2 (PAR₂) is a proinflammatory G-protein coupled receptor (GPCR) that is activated by inflammatory proteases, and its activation initiates signaling pathways that modulate the nonselective cation channel transient receptor potential vanilloid-4 (TRPV4). PAR₂-dependent opening of TRPV4 has been attributed to kinase activation, but the identity of the responsible enzymes is unknown. Deciphering the signaling pathways involved in the PAR₂-dependent opening of TRPV4 may yield new targets for pain treatment. This study has identified specific kinases that are involved in opening TRPV4, using a selective screen of short interfering ribonucleic acid (siRNA) SMARTpools, which individually targeted all human kinases, in human embryonic kidney 293 (HEK293) cells that stably express inducible TRPV4. This screen is unique because it uses a real-time assay measuring intracellular calcium with Fura-2AM dye. From the primary screen, subsequent confirmation screen, and on-target messenger ribonucleic acid expression analysis, we identified two kinases as crucial to the PAR₂-dependent opening of TRPV4 in HEK293 cells, mitogen-activated protein kinase 13 and with no lysine kinase 4. In conclusion, this study describes a powerful new application of siRNA knockdown to identity signaling molecules that are responsible for the PAR₂-dependent opening of TRPV4, which will help elucidate this signaling process.

Role of TRP Channels in Dinoflagellate Mechanotransduction

Transient receptor potential (TRP) ion channels are common components of mechanosensing pathways, mainly described in mammals and other multicellular organisms. To gain insight into the evolutionary origins of eukaryotic mechanosensory proteins, we investigated the involvement of TRP channels in mechanosensing in a unicellular eukaryotic protist, the dinoflagellate Lingulodinium polyedra. BLASTP analysis of the protein sequences predicted from the L. polyedra transcriptome revealed six sequences with high similarity to human TRPM2, TRPM8, TRPML2, TRPP1, and TRPP2; and characteristic TRP domains were identified in all sequences. In a phylogenetic tree including all mammalian TRP subfamilies and TRP channel sequences from unicellular and multicellular organisms, the L. polyedra sequences grouped with the TRPM, TPPML, and TRPP clades. In pharmacological experiments, we used the intrinsic bioluminescence of L. polyedra as a reporter of mechanoresponsivity. Capsaicin and RN1734, agonists of mammalian TRPV, and arachidonic acid, an agonist of mammalian TRPV, TRPA, TRPM, and Drosophila TRP, all stimulated bioluminescence in L. polyedra. Mechanical stimulation of bioluminescence, but not capsaicin-stimulated bioluminescence, was inhibited by gadolinium (Gd³⁺), a general inhibitor of mechanosensitive ion channels, and the phospholipase C (PLC) inhibitor U73122. These pharmacological results are consistent with the involvement of TRP-like channels in mechanosensing by L. polyedra. The TRP channels do not appear to be mechanoreceptors but rather are components of the mechanotransduction signaling pathway and may be activated via a PLC-dependent mechanism. The presence and function of TRP channels in a dinoflagellate emphasize the evolutionary conservation of both the channel structures and their functions.

Cellular and Molecular Targets of Menthol Actions

Menthol belongs to monoterpene class of a structurally diverse group of phytochemicals found in plant-derived essential oils. Menthol is widely used in pharmaceuticals, confectionary, oral hygiene products, pesticides, cosmetics, and as a flavoring agent. In addition, menthol is known to have antioxidant, anti-inflammatory, and analgesic effects. Recently, there has been renewed awareness in comprehending the biological and pharmacological effects of menthol. TRP channels have been demonstrated to mediate the cooling actions of menthol. There has been new evidence demonstrating that menthol can significantly influence the functional characteristics of a number of different kinds of ligand and voltage-gated ion channels, indicating that at least some of the biological and pharmacological effects of menthol can be mediated by alterations in cellular excitability. In this article, we examine the results of earlier studies on the actions of menthol with voltage and ligand-gated ion channels.

TRPs in Pain Sensation

According to the International Association for the Study of Pain (IASP) pain is characterized as an "unpleasant sensory and emotional experience associated with actual or potential tissue damage". The TRP super-family, compressing up to 28 isoforms in mammals, mediates a myriad of physiological and pathophysiological processes, pain among them. TRP channel might be constituted by similar or different TRP subunits, which will result in the formation of homomeric or heteromeric channels with distinct properties and functions. In this review we will discuss about the function of TRPs in pain, focusing on TRP channles that participate in the transduction of noxious sensation, especially TRPV1 and TRPA1, their expression in nociceptors and their sensitivity to a large number of physical and chemical stimuli.

Primidone inhibits TRPM3 and attenuates thermal nociception in vivo

Supplemental Digital Content is Available in the Text.

The approved antiepileptic drug primidone potently inhibits TRPM3 channels and thereby exerts analgesic properties to chemical pain and thermal hyperalgesia in mice.

The melastatin-related transient receptor potential (TRP) channel TRPM3 is a nonselective cation channel expressed in nociceptive neurons and activated by heat. Because TRPM3-deficient mice show inflammatory thermal hyperalgesia, pharmacological inhibition of TRPM3 may exert antinociceptive properties. Fluorometric Ca²⁺ influx assays and a compound library containing approved or clinically tested drugs were used to identify TRPM3 inhibitors. Biophysical properties of channel inhibition were assessed using electrophysiological methods. The nonsteroidal anti-inflammatory drug diclofenac, the tetracyclic antidepressant maprotiline, and the anticonvulsant primidone were identified as highly efficient TRPM3 blockers with half-maximal inhibition at 0.6 to 6 μM and marked specificity for TRPM3. Most prominently, primidone was biologically active to suppress TRPM3 activation by pregnenolone sulfate (PregS) and heat at concentrations markedly lower than plasma concentrations commonly used in antiepileptic therapy. Primidone blocked PregS-induced Ca²⁺_i influx through TRPM3 by allosteric modulation and reversibly inhibited atypical inwardly rectifying TRPM3 currents induced by coapplication of PregS and clotrimazole. In vivo, analgesic effects of low doses of primidone were demonstrated in mice, applying PregS- and heat-induced pain models, including inflammatory hyperalgesia. Thus, applying the approved drug at concentrations that are lower than those needed to induce anticonvulsive effects offers a shortcut for studying physiological and pathophysiological roles of TRPM3 in vivo.

TRPC Channel Structure and Properties

TRPC channels are the first identified members in the TRP family. They function as either homo- or heterotetramers regulating intracellular Ca²⁺ concentration in response to numerous physiological or pathological stimuli. TRPC channels are nonselective cation channels permeable to Ca²⁺. The properties and the functional domains of TRPC channels have been identified by electrophysiological and biochemical methods. However, due to the large size, instability, and flexibility of their complexes, the structures of the members in TRPC family remain unrevealed. More efforts should be made on structure analysis and generating good tools, including specific antibodies, agonist, and antagonist.

Src-family tyrosine kinases and the Ca2+ signal

In this review, we shall describe the rich crosstalk between non-receptor Src-family kinases (SFKs) and the Ca²⁺ transient generated in activated cells by a variety of extracellular and intracellular stimuli, resulting in diverse signaling events. The exchange of information between SFKs and Ca²⁺ is reciprocal, as it flows in both directions. These kinases are main actors in pathways leading to the generation of the Ca²⁺ signal, and reciprocally, the Ca²⁺ signal modulates SFKs activity and functions. We will cover how SFKs participate in the generation of the cytosolic Ca²⁺ rise upon activation of a series of receptors and the mechanism of clearance of this Ca²⁺ signal. The role of SFKs modulating Ca²⁺-translocating channels participating in these events will be amply discussed. Finally, the role of the Ca²⁺ sensor protein calmodulin on the activity of c-Src, and potentially on other SFKs, will be outlined as well. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.