Role of the outer pore domain in transient receptor potential vanilloid 1 dynamic permeability to large cations

Tubulin interaction at tubulin-binding sequence 1 (TBS1) is required for proper surface expression and TRPV1 channel activity

TRPV1, a polymodal and nonselective cation channel has unique gating mechanisms which is regulated by supramolecular complexes at the plasma membrane formed with membrane proteins, lipids and kinase pathways. Crosstalk between microtubule cytoskeleton with TRPV1 at various level has been established. Previously we demonstrated that the positively-charged residues present at specific tubulin-binding stretch sequences (i.e. TBS1 and TBS2, AA 710-730 and 770-797 respectively) located at the C-terminus of TRPV1 are crucial for tubulin interaction and such sequences have evolutionary origin. The nature of TRPV1-tubulin complex and its functional importance remain poorly understood. Here, we made several mutations in the TBS1 and TBS2 regions and characterized such mutants. Though these mutations reduce tubulin interaction drastically, a low and basal-level of tubulin interaction remains with these mutants. Substitution of positively-charged residues (Lys and Arg) to Ala in the TBS1, but not in TBS2 region results in reduced ligand-sensitivity. Such ligand-sensitivity is altered in response to Taxol or Nocodazole. We suggest that tubulin interaction at the TBS1 region favours channel opening while interaction in TBS2 favours channel closure. We demonstrate for the first time the functional significance of TRPV1-tubulin complex and endorse microtubule dynamics as a parameter that can alter TRPV1 channel functions. These findings can be relevant for several physiological functions and also in the context of chemotherapy-induced neuropathic pain caused by various microtubule stabilizing chemotherapeutic drugs. Thus, this characterization may indicate TRPV1 as a potential therapeutic target relevant for chemotherapeutic drug-induced peripheral neuropathies, neurodegeneration and other neurological disorders.

Capsaicin-insensitivity of TRPV1-R575D mutant located at the lipid-water-interface region can be rescued by either extracellular Ca2+-chelation or cholesterol reduction

TRPV1 acts as a unique polymodal ion channel having distinct structure and gating properties. In this context, TRPV1-R575D represents a special mutant located at the inner lipid-water-interface (LWI) region that has less possibility of interaction with membrane cholesterol. In control conditions, this lab-generated mutant of TRPV1 shows no "ligand-sensitivity", reduced surface expression, reduced localization in the lipid rafts, yet induces high cellular lethality. Notably, the cellular lethality induced by TRPV1-R575D expression can be rescued by adding 5'I-RTX (a specific inhibitor of TRPV1) or by introducing another mutation in the next position, i.e. in TRPV1-R575D/D576R. In this work we characterized TRPV1-R575D and TRPV1-R575D/D576R mutants in different cellular conditions and compared with the TRPV1-WT. We report that the "ligand-insensitivity" of TRPV1-R575D can be rescued in certain conditions, such as by chelation of extracellular Ca2+, or by reduction of the membrane cholesterol. Here we show that Ca2+ plays an important role in the channel gating of TRPV1-WT as well as LWI mutants (TRPV1-R575D, TRPV1-R575D/D576R). However, chelation of intracellular Ca2+ or depletion of ER Ca2+ did not have a significant effect on the TRPV1-R575D. Certain properties related to channel gating of mutant TRPV1-R575D/D576R can be rescued partially or fully in a context -dependent manner. Cholesterol depletion also alters these properties. Our data suggests that lower intracellular basal Ca2+ acts as a pre-requisite for further opening of TRPV1-R575D. These findings enable better understanding of the structure-function relationship of TRPV1 and may be critical in comprehending the channelopathies induced by other homologous thermosensitive TRPVs.

Dilation of ion selectivity filters in cation channels

Ion channels establish the voltage gradient across cellular membranes by providing aqueous pathways for ions to selectively diffuse down their concentration gradients. The selectivity of any given channel for its favored ions has conventionally been viewed as a stable property, and in many cation channels, it is determined by an ion-selectivity filter within the external end of the ion-permeation pathway. In several instances, including voltage-activated K+ (Kv) channels, ATP-activated P2X receptor channels, and transient receptor potential (TRP) channels, the ion-permeation pathways have been proposed to dilate in response to persistent activation, dynamically altering ion permeation. Here, we discuss evidence for dynamic ion selectivity, examples where ion selectivity filters exhibit structural plasticity, and opportunities to fill gaps in our current understanding.

Insights from molecular dynamics simulations of TRPV1 channel modulators in pain

TRPV1 is a nonselective cation channel vital for detecting noxious stimuli (heat, acid, capsaicin). Its role in pain makes it a potential drug target for chronic pain management, migraines, and related disorders. This review updates molecular dynamics (MD) simulation studies on the TRPV1 channel, focusing on its gating mechanism, ligand-binding sites, and implications for drug design. The article also explores challenges in developing modulators, SAR optimization, and clinical trial studies. Efforts have been undertaken to concisely present MD simulation findings, with a focus on their relevance to drug discovery.

Blockade of TRPV channels by intracellular spermine

The Vanilloid thermoTRP (TRPV1-4) subfamily of TRP channels are involved in thermoregulation, osmoregulation, itch and pain perception, (neuro)inflammation and immune response, and tight control of channel activity is required for perception of noxious stimuli and pain. Here we report voltage-dependent modulation of each of human TRPV1, 3, and 4 by the endogenous intracellular polyamine spermine. As in inward rectifier K channels, currents are blocked in a strongly voltage-dependent manner, but, as in cyclic nucleotide-gated channels, the blockade is substantially reduced at more positive voltages, with maximal blockade in the vicinity of zero voltage. A kinetic model of inhibition suggests two independent spermine binding sites with different affinities as well as different degrees of polyamine permeability in TRPV1, 3, and 4. Given that block and relief occur over the physiological voltage range of action potentials, voltage-dependent polyamine block may be a potent modulator of TRPV-dependent excitability in multiple cell types.

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.

Ratio of Hydrophobic-Hydrophilic and Positive-Negative Residues at Lipid-Water-Interface Influences Surface Expression and Channel Gating of TRPV1

During evolution, TRPV1 has lost, retained or selected certain residues at Lipid-Water-Interface (LWI) and formed specific patterns there. The ratio of "hydrophobic-hydrophilic" and "positive-negative-charged" residues at the inner LWI remains conserved throughout vertebrate evolution and plays important role in regulating TRPV1 trafficking and localization. Arg575 is an important residue as Arg575Asp mutant has reduced surface expression, co-localization with lipid raft markers, cell area and increased cell lethality. This lethality is most likely due to the disruption of the ratio between positive-negative charges caused by the mutation. Such lethality can be rescued by either using TRPV1-specfic inhibitor 5'-IRTX or by restoring the positive-negative charge ratio at that position, i.e. by introducing Asp576Arg mutation in Arg575Asp backbone. We propose that Arg575Asp mutation confers TRPV1 in a "constitutive-open-like" condition. These findings have broader implication in understanding the molecular evolution of thermo-sensitive ion channels and the micro-environments involved in processes that goes erratic in different diseases. The segment of TRPV1 that is present at the inner lipid-water-interface (LWI) has a specific pattern of amino acid combinations. The overall ratio of +ve charge /-ve charge and the ratio of hydrophobicity/hydrophilicity remain constant throughout the vertebrate evolution (ca 450 million years). This specific pattern is not observed in the outer LWI region of TRPV1.

Nonselective TRPC channel inhibition and suppression of aminoglycoside-induced premature termination codon readthrough by the small molecule AC1903

Nonsense mutations, which occur in ∼11% of patients with genetic disorders, introduce premature termination codons (PTCs) that lead to truncated proteins and promote nonsense-mediated mRNA decay. Aminoglycosides such as G418 permit PTC readthrough and so may be used to address this problem. However, their effects are variable between patients, making clinical use of aminoglycosides challenging. In this study, we tested whether TRPC nonselective cation channels contribute to the variable PTC readthrough effect of aminoglycosides by controlling their cellular uptake. Indeed, a recently reported selective TRPC5 inhibitor, AC1903, consistently suppressed G418 uptake and G418-induced PTC readthrough in the DMS-114 cancer cell line and junctional epidermolysis bullosa (JEB) patient-derived keratinocytes. Interestingly, the effect of AC1903 in DMS-114 cells was mimicked by nonselective TRPC inhibitors, but not by well-characterized inhibitors of TRPC1/4/5 (Pico145, GFB-8438) or TRPC3/6/7 (SAR7334), suggesting that AC1903 may work through additional or undefined targets. Indeed, in our experiments, AC1903 inhibited multiple TRPC channels including TRPC3, TRPC4, TRPC5, TRPC6, TRPC4-C1, and TRPC5-C1, as well as endogenous TRPC1:C4 channels in A498 renal cancer cells, all with low micromolar IC50 values (1.8-18 μM). We also show that AC1903 inhibited TRPV4 channels, but had weak or no effects on TRPV1 and no effect on the nonselective cation channel PIEZO1. Our study reveals that AC1903 has previously unrecognized targets, which need to be considered when interpreting results from experiments with this compound. In addition, our data strengthen the hypothesis that nonselective calcium channels are involved in aminoglycoside uptake.

Involvement of P2X7 receptors in chronic pain disorders

Chronic pain is caused by cellular damage with an obligatory inflammatory component. In response to noxious stimuli, high levels of ATP leave according to their concentration gradient, the intracellular space through discontinuities generated in the plasma membrane or diffusion through pannexin-1 hemichannels, and activate P2X7Rs localized at peripheral and central immune cells. Because of the involvement of P2X7Rs in immune functions and especially the initiation of macrophage/microglial and astrocytic secretion of cytokines, chemokines, prostaglandins, proteases, reactive oxygen, and nitrogen species as well as the excitotoxic glutamate/ATP, this receptor type has a key role in chronic pain processes. Microglia are equipped with a battery of pattern recognition receptors that detect pathogen-associated molecular patterns (PAMPs) such as lipopolysaccharide (LPS) from bacterial infections or danger associated molecular patterns (DAMPs) such as ATP. The co-stimulation of these receptors leads to the activation of the NLRP3 inflammasome and interleukin-1β (IL-1β) release. In the present review, we invite you to a journey through inflammatory and neuropathic pain, primary headache, and regulation of morphine analgesic tolerance, in the pathophysiology of which P2X7Rs are centrally involved. P2X7R bearing microglia and astrocyte-like cells playing eminent roles in chronic pain will be also discussed.

Structural snapshots of TRPV1 reveal mechanism of polymodal functionality

Many transient receptor potential (TRP) channels respond to diverse stimuli and conditionally conduct small and large cations. Such functional plasticity is presumably enabled by a uniquely dynamic ion selectivity filter that is regulated by physiological agents. What is currently missing is a "photo series" of intermediate structural states that directly address this hypothesis and reveal specific mechanisms behind such dynamic channel regulation. Here, we exploit cryoelectron microscopy (cryo-EM) to visualize conformational transitions of the capsaicin receptor, TRPV1, as a model to understand how dynamic transitions of the selectivity filter in response to algogenic agents, including protons, vanilloid agonists, and peptide toxins, permit permeation by small and large organic cations. These structures also reveal mechanisms governing ligand binding substates, as well as allosteric coupling between key sites that are proximal to the selectivity filter and cytoplasmic gate. These insights suggest a general framework for understanding how TRP channels function as polymodal signal integrators.

Differential modulation of thermal preference after sensitization by optogenetic or pharmacological activation of heat-sensitive nociceptors

Common approaches to studying mechanisms of chronic pain and sensory changes in pre-clinical animal models involve measurement of acute, reflexive withdrawal responses evoked by noxious stimuli. These methods typically do not capture more subtle changes in sensory processing nor report on the consequent behavioral changes. In addition, data collection and analysis protocols are often labour-intensive and require direct investigator interactions, potentially introducing bias. In this study, we develop and characterize a low-cost, easily assembled behavioral assay that yields self-reported temperature preference from mice that is responsive to peripheral sensitization. This system uses a partially automated and freely available analysis pipeline to streamline the data collection process and enable objective analysis. We found that after intraplantar administration of the TrpV1 agonist, capsaicin, mice preferred to stay in cooler temperatures than saline injected mice. We further observed that gabapentin, a non-opioid analgesic commonly prescribed to treat chronic pain, reversed this aversion to higher temperatures. In contrast, optogenetic activation of the central terminals of TrpV1+ primary afferents via in vivo spinal light delivery did not induce a similar change in thermal preference, indicating a possible role for peripheral nociceptor activity in the modulation of temperature preference. We conclude that this easily produced and robust sensory assay provides an alternative approach to investigate the contribution of central and peripheral mechanisms of sensory processing that does not rely on reflexive responses evoked by noxious stimuli.

In vitro exposure to Hymenoptera venom and constituents activates discrete ionotropic pathways in mast cells

Calcium entry is central to the functional processes in mast cells and basophils that contribute to the induction and maintenance of inflammatory responses. Mast cells and basophils express an array of calcium channels, which mediate responses to diverse stimuli triggered by small bioactive molecules, physicochemical stimuli and immunological inputs including antigens and direct immune cell interactions. These cells are also highly responsive to certain venoms (such as Hymenoptera envenomations), which cause histamine secretion, cytokine release and an array of pro-inflammatory functional responses. There are gaps in our understanding of the coupling of venom exposure to specific signaling pathways such as activation of calcium channels. In the present study, we performed a current survey of a model mast cell line selected for its pleiotropic responsiveness to multiple pro-inflammatory inputs. As a heterogenous stimulus, Hymenoptera venom activates multiple classes of conductance at the population level but tend to lead to the measurement of only one type of conductance per cell, despite the cell co-expressing multiple channel types. The data show that I_CRAC, I_ARC, and TRPV-like currents are present in the model mast cell populations and respond to venom exposure. We further assessed individual venom components, specifically secretagogues and arachidonic acid, and identified the conductances associated with these stimuli in mast cells. Single-cell calcium assays and immunofluorescence analysis show that there is heterogeneity of channel expression across the cell population, but this heterogeneity does not explain the apparent selectivity for specific channels in response to exposure to venom as a composite stimulus.

Endothelial TRPV1 as an Emerging Molecular Target to Promote Therapeutic Angiogenesis

Therapeutic angiogenesis represents an emerging strategy to treat ischemic diseases by stimulating blood vessel growth to rescue local blood perfusion. Therefore, injured microvasculature may be repaired by stimulating resident endothelial cells or circulating endothelial colony forming cells (ECFCs) or by autologous cell-based therapy. Endothelial Ca²⁺ signals represent a crucial player in angiogenesis and vasculogenesis; indeed, several angiogenic stimuli induce neovessel formation through an increase in intracellular Ca²⁺ concentration. Several members of the Transient Receptor Potential (TRP) channel superfamily are expressed and mediate Ca²⁺-dependent functions in vascular endothelial cells and in ECFCs, the only known truly endothelial precursor. TRP Vanilloid 1 (TRPV1), a polymodal cation channel, is emerging as an important player in endothelial cell migration, proliferation, and tubulogenesis, through the integration of several chemical stimuli. Herein, we first summarize TRPV1 structure and gating mechanisms. Next, we illustrate the physiological roles of TRPV1 in vascular endothelium, focusing our attention on how endothelial TRPV1 promotes angiogenesis. In particular, we describe a recent strategy to stimulate TRPV1-mediated pro-angiogenic activity in ECFCs, in the presence of a photosensitive conjugated polymer. Taken together, these observations suggest that TRPV1 represents a useful target in the treatment of ischemic diseases.

Probing temperature and capsaicin-induced activation of TRPV1 channel via computationally guided point mutations in its pore and TRP domains

In a recent computational study, we revealed some mechanistic aspects of TRPV1 (transient receptor potential channel 1) thermal activation and gating and proposed a set of probable functionally important residues - "hot spots" that have not been characterized experimentally yet. In this work, we analyzed TRPV1 point mutants G643A, I679A + A680G, and K688G/P combining molecular modeling, biochemistry, and electrophysiology. The substitution G643A reduced maximal conductivity that resulted in a normal response to moderate stimuli, but a relatively weak response to more intensive activation. I679A + A680G channel was severely toxic for oocytes most probably due to abnormally increased basal activity of the channel ("always open" gates). The replacement K688G presumably facilitated movements of TRP domain and disturbed its coupling to the pore, thus leading to spontaneous activation and enhanced desensitization of the channel. Finally, mutation K688P was suggested to impair TRP domain directed movement, and the mutated channel showed ~100-fold less sensitivity to the capsaicin, enhanced desensitization and weaker activation by the heat. Our results provide a better understanding of TRPV1 thermal and capsaicin-induced activation and gating. These observations provide a structural basis for understanding some aspects of TRPV1 channel functioning and depict potentially pathogenic mutations.

TRP ion channels: Proteins with conformational flexibility

Ion channels display conformational changes in response to binding of their agonists and antagonists. The study of the relationships between the structure and the function of these proteins has witnessed considerable advances in the last two decades using a combination of techniques, which include electrophysiology, optical approaches (i.e. patch clamp fluorometry, incorporation of non-canonic amino acids, etc.), molecular biology (mutations in different regions of ion channels to determine their role in function) and those that have permitted the resolution of their structures in detail (X-ray crystallography and cryo-electron microscopy). The possibility of making correlations among structural components and functional traits in ion channels has allowed for more refined conclusions on how these proteins work at the molecular level. With the cloning and description of the family of Transient Receptor Potential (TRP) channels, our understanding of several sensory-related processes has also greatly moved forward. The response of these proteins to several agonists, their regulation by signaling pathways as well as by protein-protein and lipid-protein interactions and, in some cases, their biophysical characteristics have been studied thoroughly and, recently, with the resolution of their structures, the field has experienced a new boom. This review article focuses on the conformational changes in the pores, concentrating on some members of the TRP family of ion channels (TRPV and TRPA subfamilies) that result in changes in their single-channel conductances, a phenomenon that may lead to fine-tuning the electrical response to a given agonist in a cell.

Structural insights into the gating mechanisms of TRPV channels

Transient Receptor Potential channels from the vanilloid subfamily (TRPV) are a group of cation channels modulated by a variety of endogenous stimuli as well as a range of natural and synthetic compounds. Their roles in human health make them of keen interest, particularly from a pharmacological perspective. However, despite this interest, the complexity of these channels has made it difficult to obtain high resolution structures until recently. With the cryo-EM resolution revolution, TRPV channel structural biology has blossomed to produce dozens of structures, covering every TRPV family member and a variety of approaches to examining channel modulation. Here, we review all currently available TRPV structures and the mechanistic insights into gating that they reveal.

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.

Glia Accumulate Evidence that Actions Are Futile and Suppress Unsuccessful Behavior

When a behavior repeatedly fails to achieve its goal, animals often give up and become passive, which can be strategic for preserving energy or regrouping between attempts. It is unknown how the brain identifies behavioral failures and mediates this behavioral-state switch. In larval zebrafish swimming in virtual reality, visual feedback can be withheld so that swim attempts fail to trigger expected visual flow. After tens of seconds of such motor futility, animals became passive for similar durations. Whole-brain calcium imaging revealed noradrenergic neurons that responded specifically to failed swim attempts and radial astrocytes whose calcium levels accumulated with increasing numbers of failed attempts. Using cell ablation and optogenetic or chemogenetic activation, we found that noradrenergic neurons progressively activated brainstem radial astrocytes, which then suppressed swimming. Thus, radial astrocytes perform a computation critical for behavior: they accumulate evidence that current actions are ineffective and consequently drive changes in behavioral states. VIDEO ABSTRACT.

TRPC-mediated Ca2+ signaling and control of cellular functions

Canonical members of the TRP superfamily of ion channels have long been recognized as key elements of Ca²⁺ handling in a plethora of cell types. The emerging role of TRPC channels in human physiopathology has generated considerable interest in their pharmacological targeting, which requires detailed understanding of their molecular function. Although consent has been reached that receptor-phospholipase C (PLC) pathways and generation of lipid mediators constitute the prominent upstream signaling process that governs channel activity, multimodal sensing features of TRPC complexes have been demonstrated repeatedly. Downstream signaling by TRPC channels is similarly complex and involves the generation of local and global cellular Ca²⁺ rises, which are well-defined in space and time to govern specific cellular functions. These TRPC-mediated Ca²⁺ signals rely in part on Ca²⁺ permeation through the channels, but are essentially complemented by secondary mechanisms such as Ca²⁺ mobilization from storage sites and Na⁺/Ca²⁺ exchange, which involve coordinated interaction with signaling partners. Consequently, the control of cell functions by TRPC molecules is critically determined by dynamic assembly and subcellular targeting of the TRPC complexes. The very recent availability of high-resolution structure information on TRPC channel complexes has paved the way towards a comprehensive understanding of signal transduction by TRPC channels. Here, we summarize current concepts of cation permeation in TRPC complexes, TRPC-mediated shaping of cellular Ca²⁺ signals and the associated control of specific cell functions.

Structures of TRPV2 in distinct conformations provide insight into role of the pore turret

Cation channels of the TRP family serve important physiological roles by opening in response to diverse intra-and extra-cellular stimuli which regulate their lower or upper gates. Despite extensive studies, the mechanism coupling these gates has remained obscure. Previous structures have failed to resolve extracellular loops, known in the TRPV subfamily as “pore turrets,” which are proximal to the upper gates. We establish the importance of the pore turret through activity assays and by solving structures of rat TRPV2 both with and without an intact turret at resolutions of 4.0 Å and 3.6 Å respectively. These structures resolve the full-length pore turret and reveal fully open and partially open states of TRPV2, both with unoccupied vanilloid pockets. Our results suggest a mechanism by which physiological signals, such as lipid binding, can regulate the lower gate and couple to the upper gate through a pore turret-facilitated mechanism.

Novobiocin, a Newly Found TRPV1 Inhibitor, Attenuates the Expression of TRPV1 in Rat Intestine and Intestinal Epithelial Cell Line IEC-6

Background and Purpose: Novobiocin (NOVO), an ABC transporter inhibitor, decreases intestinal wall permeability of capsaicin (CAP), an ABC transporter substrate. However, the mechanism of this effect is not consistent with the action of NOVO as an ABC transporter inhibitor. We previously found that CAP can also be transported via TRPV1, which was site-specific in the permeability of CAP across the intestine. We explored the regulation by NOVO of TRPV1 in the present study. Methods: Rats and transfected IEC-6 cells were used as the models to assess intestinal permeability and expression of TRPV1. Ussing chamber and intracellular accumulation were used to evaluate the influence of NOVO on the transport of CAP in vitro. The expression of TRPV1 was detected after administration of NOVO by qRT-PCR, western blot and immunofluorescent imaging. In addition, MTT and lactate dehydrogenase (LDH) were used to evaluate the cytotoxicity of NOVO in both rat and cell models. Finally, the effect of NOVO on the absorption of CAP in vivo was studied by LC-MS/MS. Results: In vitro data showed that there existed a dose-dependent relationship in the range of concentration between 5 and 50 μM, and even 5 μM NOVO could decrease intestinal permeability of CAP across the intestine. Meanwhile, cytosolic accumulation of CAP decreased when NOVO was used simultaneously or 24 h in advance. NOVO exhibited an inhibition level similar to that of ruthenium red (RR) or SB-705498, a TRPV1-specific inhibitor. NOVO down-regulated TRPV1 expression in the intestine and in transfected cells in a concentration-dependent fashion, hinting that its inhibition of the permeability of CAP is due to its inhibition of TRPV1 expression. Immunofluorescent imaging data showed that the fluorescence intensity of TRPV1 was reduced after pre-treatment with NOVO and SB-705498. In vivo data further demonstrated that oral co-administration of NOVO decreased C_max and AUC of CAP in dosage-dependent ways, consistent with its role as a TRPV1 inhibitor. Conclusion: NOVO could be a potential TRPV1 inhibitor by attenuating the expression of TRPV1 and may be used to attenuate permeability of TRPV1 substrates.

Conformational plasticity in the selectivity filter of the TRPV2 ion channel

Transient receptor potential vanilloid (TRPV) channels are activated by ligands and heat and are involved in various physiological processes. In contrast to the architecturally related voltage-gated cation channels, TRPV1 and TRPV2 subtypes possess another activation gate at the selectivity filter that can open widely enough to permeate large organic cations. Despite recent structural advances, the mechanism of selectivity filter gating and permeation for both metal ions and large molecules by TRPV1 or TRPV2 is not well known. Here, we determined two crystal structures of rabbit TRPV2 in its Ca²⁺-bound and resiniferatoxin (RTx)- and Ca²⁺-bound forms, to 3.9 Å and 3.1 Å, respectively. Notably, our structures show that RTx binding leads to two-fold symmetric opening of the selectivity filter of TRPV2 that is wide enough for large organic cation permeation. Combined with functional characterizations, our studies reveal a structural basis for permeation of Ca²⁺ and large organic cations in TRPV2.

RNA expression profiling in sulfamethoxazole-treated patients with a range of in vitro lymphocyte cytotoxicity phenotypes

The lymphocyte toxicity assay (LTA) is a proposed surrogate marker of sulfonamide antibiotic hypersensitivity. In the LTA, peripheral blood mononuclear cells (PBMCs) undergo apoptosis more readily in hypersensitive versus tolerant patients when exposed to drug-hydroxylamine metabolites in vitro. The purpose of this study was to identify key gene transcripts associated with increased cytotoxicity from sulfamethoxazole-hydroxylamine in human PBMCs in the LTA. The LTA was performed on PBMCs of 10 patients hypersensitive to trimethoprim-sulfamethoxazole (HS) and 10 drug-tolerant controls (TOL), using two cytotoxicity assays: YO-PRO (n = 20) and MTT (n = 12). mRNA expression profiles of PBMCs, enriched for CD8+ T cells, were compared between HS and TOL patients. Transcript expression was interrogated for correlation with % cytotoxicity from YO-PRO and MTT assays. Correlated transcripts of interest were validated by qPCR. LTA results were not significantly different between HS and TOL patients, and no transcripts were found to be differentially expressed between the two groups. 96 transcripts were correlated with cytotoxicity by YO-PRO (r = ±.63-.75, FDR 0.188). Transcripts were selected for validation based on mechanistic plausibility and three were significantly over-expressed by qPCR in high cytotoxicity patients: multi-specific organic anion transporter C (ABCC5), mitoferrin-1 (SLC25A37), and Porimin (TMEM123). These data identify novel transcripts that could contribute to sulfonamide-hydroxylamine induced cytotoxicity. These include SLC25A37, encoding a mitochondrial iron transporter, ABCC5, encoding an arylamine drug transporter, and TMEM123, encoding a transmembrane protein that mediates cell death.

Direct Anandamide Activation of TRPV1 Produces Divergent Calcium and Current Responses

In the brainstem nucleus of the solitary tract (NTS), primary vagal afferent neurons express the transient receptor potential vanilloid subfamily member 1 (TRPV1) at their central terminals where it contributes to quantal forms of glutamate release. The endogenous membrane lipid anandamide (AEA) is a putative TRPV1 agonist in the brain, yet the extent to which AEA activation of TRPV1 has a neurophysiological consequence is not well established. We investigated the ability of AEA to activate TRPV1 in vagal afferent neurons in comparison to capsaicin (CAP). Using ratiometric calcium imaging and whole-cell patch clamp recordings we confirmed that AEA excitatory activity requires TRPV1, binds competitively at the CAP binding site, and has low relative affinity. While AEA-induced increases in peak cytosolic calcium were similar to CAP, AEA-induced membrane currents were significantly smaller. Removal of bath calcium increased the AEA current with no change in peak CAP currents revealing a calcium sensitive difference in specific ligand activation of TRPV1. Both CAP- and AEA-activated TRPV1 currents maintained identical reversal potentials, arguing against a major difference in ion selectivity to resolve the AEA differences in signaling. In contrast with CAP, AEA did not alter spontaneous glutamate release at NTS synapses. We conclude: (1) AEA activation of TRPV1 is markedly different from CAP and produces different magnitudes of calcium influx from whole-cell current; and (2) exogenous AEA does not alter spontaneous glutamate release onto NTS neurons. As such, AEA may convey modulatory changes to calcium-dependent processes, but does not directly facilitate glutamate release.

Transient receptor potential vanilloid 4 (TRPV4) channel as a target of crotamiton and its bimodal effects

The sensation of itching can be defined as "an unpleasant cutaneous sensation that provokes a desire to scratch." The perception of itching is not critical for the maintenance of life, but persistent itching can be extremely irritating and decreases the quality of life. Crotamiton (N-ethyl-o-crotonotoluidide) has been used as an anti-itch agent for humans for around 70 years. In spite of the long use of crotamiton, its mechanism of action remains unknown. We hypothesized that crotamiton might have effects on transient receptor potential (TRP) channels expressed in the peripheral nervous system and the skin. We first examined the effects of crotamiton on TRP channels by whole-cell patch-clamp recordings. We found that crotamiton strongly inhibited TRPV (vanilloid) 4 channels followed by large currents after crotamiton washout. In mice, crotamiton inhibited itch-related behaviors induced by a TRPV4-selective agonist (GSK1016790A). We biophysically investigated the large TRPV4 currents after crotamiton washout. Comparing single-channel open probabilities and current amplitudes of TRPV4, increases in both parameters were found to contribute to the large washout currents of TRPV4. Because the change in current amplitudes suggested pore dilation of TRPV4, we examined this possibility with cation replacement experiments and by measuring changes in reversal potentials. Greater cation influxes and changes in reversal potentials upon crotamiton washout were observed, suggesting that the TRPV4 pore dilated in its uninhibited state. From these results, we identified the molecular target of crotamiton as TRPV4 and demonstrated pore dilation of TRPV4 upon crotamiton washout.

Cellular permeation of large molecules mediated by TRPM8 channels

While most membrane channels are only capable of passing small ions, certain non-selective cation channels have been recently shown to have the capacity to permeate large cations. The mechanisms underlying large molecule permeation are unclear, but this property has been exploited pharmacologically to target molecules, such as nerve conduction blockers, to specific subsets of pain-sensing neurons (nociceptors) expressing the heat-gated transient receptor potential (TRP) channel TRPV1. However, it is not clear if the principal mediator of cold stimuli TRPM8 is capable of mediating the permeation large molecules across cell membranes, suggesting that TRPM8-positive nerves cannot be similarly targeted. Here we show that both heterologous cells and native sensory neurons expressing TRPM8 channels allow the permeation of the large fluorescent cation Po-Pro3. Po-Pro3 influx is blocked by TRPM8-specific antagonism and when channel activity is desensitized. The effects of the potent agonist WS-12 are TRPM8-specific and dye uptake mediated by TRPM8 channels is similar to that observed with TRPV1. Lastly, we find that as with TRPV1, activation of TRPM8 channels can be used as a means to target intracellular uptake of cell-impermeable sodium channel blockers. In a neuronal cell line expressing TRPM8 channels, voltage-gated sodium currents are blocked in the presence of the cell-impermeable, charged lidocaine derivative QX-314 and WS-12. These results show that the ability of somatosensory TRP channels to promote the permeation of large cations also includes TRPM8, thereby suggesting that novel approaches to alter cold pain can also be employed via conduction block in TRPM8-positive sensory neurons.

Medicinal Chemistry, Pharmacology, and Clinical Implications of TRPV1 Receptor Antagonists

Transient receptor potential vanilloid 1 (TRPV1) is an ion channel expressed on sensory neurons triggering an influx of cations. TRPV1 receptors function as homotetramers responsive to heat, proinflammatory substances, lipoxygenase products, resiniferatoxin, endocannabinoids, protons, and peptide toxins. Its phosphorylation increases sensitivity to both chemical and thermal stimuli, while desensitization involves a calcium-dependent mechanism resulting in receptor dephosphorylation. TRPV1 functions as a sensor of noxious stimuli and may represent a target to avoid pain and injury. TRPV1 activation has been associated to chronic inflammatory pain and peripheral neuropathy. Its expression is also detected in nonneuronal areas such as bladder, lungs, and cochlea where TRPV1 activation is responsible for pathology development of cystitis, asthma, and hearing loss. This review offers a comprehensive overview about TRPV1 receptor in the pathophysiology of chronic pain, epilepsy, cough, bladder disorders, diabetes, obesity, and hearing loss, highlighting how drug development targeting this channel could have a clinical therapeutic potential. Furthermore, it summarizes the advances of medicinal chemistry research leading to the identification of highly selective TRPV1 antagonists and their analysis of structure-activity relationships (SARs) focusing on new strategies to target this channel.

Kunitz-Type Peptide HCRG21 from the Sea Anemone Heteractis crispa Is a Full Antagonist of the TRPV1 Receptor

Sea anemone venoms comprise multifarious peptides modulating biological targets such as ion channels or receptors. The sequence of a new Kunitz-type peptide, HCRG21, belonging to the Heteractis crispa RG (HCRG) peptide subfamily was deduced on the basis of the gene sequence obtained from the Heteractis crispa cDNA. HCRG21 shares high structural homology with Kunitz-type peptides APHC1-APHC3 from H. crispa, and clusters with the peptides from so named "analgesic cluster" of the HCGS peptide subfamily but forms a separate branch on the NJ-phylogenetic tree. Three unique point substitutions at the N-terminus of the molecule, Arg1, Gly2, and Ser5, distinguish HCRG21 from other peptides of this cluster. The trypsin inhibitory activity of recombinant HCRG21 (rHCRG21) was comparable with the activity of peptides from the same cluster. Inhibition constants for trypsin and α-chymotrypsin were 1.0 × 10-7 and 7.0 × 10-7 M, respectively. Electrophysiological experiments revealed that rHCRG21 inhibits 95% of the capsaicin-induced current through transient receptor potential family member vanilloid 1 (TRPV1) and has a half-maximal inhibitory concentration of 6.9 ± 0.4 μM. Moreover, rHCRG21 is the first full peptide TRPV1 inhibitor, although displaying lower affinity for its receptor in comparison with other known ligands. Macromolecular docking and full atom Molecular Dynamics (MD) simulations of the rHCRG21-TRPV1 complex allow hypothesizing the existence of two feasible, intra- and extracellular, molecular mechanisms of blocking. These data provide valuable insights in the structural and functional relationships and pharmacological potential of bifunctional Kunitz-type peptides.

TRP channels in calcium homeostasis: from hormonal control to structure-function relationship of TRPV5 and TRPV6

Maintaining plasma calcium levels within a narrow range is of vital importance for many physiological functions. Therefore, calcium transport processes in the intestine, bone and kidney are tightly regulated to fine-tune the rate of absorption, storage and excretion. The TRPV5 and TRPV6 calcium channels are viewed as the gatekeepers of epithelial calcium transport. Several calciotropic hormones control the channels at the level of transcription, membrane expression, and function. Recent technological advances have provided the first near-atomic resolution structural models of several TRPV channels, allowing insight into their architecture. While this field is still in its infancy, it has increased our understanding of molecular channel regulation and holds great promise for future structure-function studies of these ion channels. This review will summarize the mechanisms that control the systemic calcium balance, as well as extrapolate structural views to the molecular functioning of TRPV5/6 channels in epithelial calcium transport.

Lactate is a potent inhibitor of the capsaicin receptor TRPV1

Tissue ischemia results in an accumulation of lactate and local or systemic lactic acidosis. In nociceptive sensory neurons, lactate was reported to sensitize or activate the transient receptor potential ion channel TRPA1 and acid-sensing ion channels (ASICs). However, it is unclear how lactate modulates the TRPV1 regarded as the main sensor for acidosis in sensory neurons. In this study we investigated the effects of lactate (LA) on recombinant and native TRPV1 channels and on TRPV1-mediated release of neuropeptides from mouse nerves. TRPV1-mediated membrane currents evoked by protons, capsaicin or heat are inhibited by LA at concentrations ranging from 3 μM to 100 mM. LA inhibits TRPV1-mediated proton-induced Ca²⁺-influx in dorsal root ganglion neurons as well as proton-evoked neuropeptide release from mouse nerves. Inhibition of TRPV1 by LA is significantly stronger on inward currents as compared to outward currents since LA affects channel gating, shifting the activation curve towards more positive potentials. The mutation I680A in the pore lower gate displays no LA inhibition. Cell-attached as well as excised inside- and outside-out patches suggest an interaction through an extracellular binding site. In conclusion, our data demonstrate that lactate at physiologically relevant concentrations is a potent endogenous inhibitor of TRPV1.

Temperature-sensitive gating of TRPV1 channel as probed by atomistic simulations of its trans- and juxtamembrane domains

Heat-activated transient receptor potential channel TRPV1 is one of the most studied eukaryotic proteins involved in temperature sensation. Upon heating, it exhibits rapid reversible pore gating, which depolarizes neurons and generates action potentials. Underlying molecular details of such effects in the pore region of TRPV1 is of a crucial importance to control temperature responses of the organism. Despite the spatial structure of the channel in both open (O) and closed (C) states is known, microscopic nature of channel gating and mechanism of thermal sensitivity are still poorly understood. In this work, we used unrestrained atomistic molecular dynamics simulations of TRPV1 (without N- and C-terminal cytoplasmic domains) embedded into explicit lipid bilayer in its O- and C-states. We found that the pore domain with its neighboring loops undergoes large temperature-dependent conformational transitions in an asymmetric way, when fragments of only one monomer move with large amplitude, freeing the pore upon heating. Such an asymmetrical gating looks rather biologically relevant because it is faster and more reliable than traditionally proposed “iris-like” symmetric scheme of channel opening. Analysis of structural, dynamic, and hydrophobic organization of the pore domain revealed entropy growth upon TRPV1 gating, which is in line with current concepts of thermal sensitivity.

Structure of the full-length TRPV2 channel by cryo-EM

Transient receptor potential (TRP) proteins form a superfamily Ca²⁺-permeable cation channels regulated by a range of chemical and physical stimuli. Structural analysis of a ‘minimal' TRP vanilloid subtype 1 (TRPV1) elucidated a mechanism of channel activation by agonists through changes in its outer pore region. Though homologous to TRPV1, other TRPV channels (TRPV2–6) are insensitive to TRPV1 activators including heat and vanilloids. To further understand the structural basis of TRPV channel function, we determined the structure of full-length TRPV2 at ∼5 Å resolution by cryo-electron microscopy. Like TRPV1, TRPV2 contains two constrictions, one each in the pore-forming upper and lower gates. The agonist-free full-length TRPV2 has wider upper and lower gates compared with closed and agonist-activated TRPV1. We propose these newly revealed TRPV2 structural features contribute to diversity of TRPV channels.

Transient receptor potential (TRP) proteins are Ca²⁺-permeable cation channels activated by a range of chemical and physical stimuli. Here the authors describe a cryo-EM structure of the full-length TRPV2 channel that provides insight into the regulation of the TRPV subfamily of channels.

TRPing on the pore phenomenon: what do we know about transient receptor potential ion channel-related pore dilation up to now?

Ion channels allow for rapid ion diffusion through the plasma membrane. In some conditions, ion channels induce changes in the critical plasma membrane permeability that permit 900-Da solutes to enter cells. This process is known as the pore phenomenon. Some transient receptor potential (TRP) channel subtypes have been highlighted such as the P2X7 receptor, plasma membrane VDAC-1 channel, and pannexin hemichannels. The TRP ion channels are considered multimodal transducers that respond to several kinds of stimuli. In addition, many TRP channel subtypes are involved in physiological and pathophysiological processes such as inflammation, pain, and cancer. The TRPA1, TRPM8, and TRPV1-4 subtypes have been shown to promote large-molecular-weight solute uptake, including impermeable fluorescent dyes, QX-314 hydrophilic lidocaine derivative, gabapentin, and antineoplastic drugs. This review discusses the current knowledge of TRP-associated pores and encourages scientists to study their features and explore them as novel therapeutic tools.

Measurement of relative Ca²⁺ permeability during sustained activation of TRPV1 receptors

Some cation permeable ligand-gated ion channels, including the capsaicin-sensitive TRPV1, have been reported to exhibit a time-dependent increase in permeability to large inorganic cations during sustained activation, a phenomenon termed "pore dilation." TRPV1 conducts substantial Ca(2+) entry, and it has been suggested that this channel undergoes a time-dependent change in Ca(2+) permeability relative to Na(+) (P Ca/P Na) that parallels pore dilation. However, our experiments employing whole cell patch clamp photometry and single channel recordings to directly measure relative Ca(2+) current in TRPV1 expressing HEK293 cells show that relative Ca(2+) influx remains constant for the duration of capsaicin-evoked channel activation. Further, we present evidence from patch clamp photometry experiments suggesting that sustained activation of Ca(2+) permeable ion channels in the voltage-clamp configuration leads to rapid saturation of the pipette Ca(2+) chelator, and that subsequent observed shifts in the current reversal potentials in the presence of extracellular Ca(2+) are likely due to intracellular accumulation of this ion and a movement of the Ca(2+) equilibrium potential (E Ca) towards zero. Finally, using an adapted reversal potential-based protocol in which cells are only exposed to Ca(2+) after sustained capsaicin exposure in the absence of added extracellular Ca(2+), we demonstrate that the calculated P Ca/P Na is unaffected by duration of TRPV1 activation. In conclusion, we find no evidence in support of a time-dependent change in P Ca/P Na for TRPV1. Our data further urges caution in estimating relative Ca(2+) permeability using reversal potentials, as there is a limited time window in which the cytosolic Ca(2+) chelator included in the patch pipette can prevent localised elevations in cytosolic free Ca(2+) and thus allow for an accurate estimate of this important channel permeability parameter.

Physical basis of apparent pore dilation of ATP-activated P2X receptor channels

The selectivity of ion channels is fundamental for their roles in electrical and chemical signaling and in ion homeostasis. Although most ion channels exhibit stable ion selectivity, the prevailing view of purinergic P2X receptor channels, transient receptor potential V1 (TRPV1) channels and acid-sensing ion channels (ASICs) is that their ion conduction pores dilate upon prolonged activation. We investigated this mechanism in P2X receptors and found that the hallmark shift in equilibrium potential observed with prolonged channel activation does not result from pore dilation, but from time-dependent alterations in the concentration of intracellular ions. We derived a physical model to calculate ion concentration changes during patch-clamp recordings, which validated our experimental findings and provides a quantitative guideline for effectively controlling ion concentration. Our results have fundamental implications for understanding ion permeation and gating in P2X receptor channels, as well as more broadly for using patch-clamp techniques to study ion channels and neuronal excitability.