Proton inhibition of unitary currents of vanilloid receptors

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.

Mechanisms of proton inhibition and sensitization of the cation channel TRPV3

TRPV3 is a temperature-sensitive, nonselective cation channel expressed prominently in skin keratinocytes. TRPV3 plays important roles in hair morphogenesis and maintenance of epidermal barrier function. Gain-of-function mutations of TRPV3 have been found in both humans and rodents and are associated with hair loss, pruritus, and dermatitis. Here, we study the mechanisms of acid regulation of TRPV3 by using site-directed mutagenesis, fluorescent intracellular calcium measurement, and whole-cell patch-clamp recording techniques. We show that, whereas extracellular acid inhibits agonist-induced TRPV3 activation through an aspartate residue (D641) in the selectivity filter, intracellular protons sensitize the channel through cytoplasmic C-terminal glutamate and aspartate residues (E682, E689, and D727). Neutralization of the three C-terminal residues presensitizes the channel to agonist stimulation. Molecular dynamic simulations revealed that charge neutralization of the three C-terminal residues stabilized the sensitized channel conformation and enhanced the probability of α-helix formation in the linker between the S6 transmembrane segment and TRP domain. We conclude that acid inhibits TRPV3 function from the extracellular side but facilitates it from the intracellular side. These novel mechanisms of TRPV3 proton sensing can offer new insights into the role of TRPV3 in the regulation of epidermal barrier permeability and skin disorders under conditions of tissue acidosis.

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.

TRPV1 pore turret dictates distinct DkTx and capsaicin gating

Many neurotoxins inflict pain by targeting receptors expressed on nociceptors, such as the polymodal cationic channel TRPV1. The tarantula double-knot toxin (DkTx) is a peptide with an atypical bivalent structure, providing it with the unique capability to lock TRPV1 in its open state and evoke an irreversible channel activation. Here, we describe a distinct gating mechanism of DkTx-evoked TRPV1 activation. Interestingly, DkTx evokes significantly smaller TRPV1 macroscopic currents than capsaicin, with a significantly lower unitary conductance. Accordingly, while capsaicin evokes aversive behaviors in TRPV1-transgenic Caenorhabditis elegans, DkTx fails to evoke such response at physiological concentrations. To determine the structural feature(s) responsible for this phenomenon, we engineered and evaluated a series of mutated toxins and TRPV1 channels. We found that elongating the DkTx linker, which connects its two knots, increases channel conductance compared with currents elicited by the native toxin. Importantly, deletion of the TRPV1 pore turret, a stretch of amino acids protruding out of the channel's outer pore region, is sufficient to produce both full conductance and aversive behaviors in response to DkTx. Interestingly, this deletion decreases the capsaicin-evoked channel activation. Taken together with structure modeling analysis, our results demonstrate that the TRPV1 pore turret restricts DkTx-mediated pore opening, probably through steric hindrance, limiting the current size and mitigating the evoked downstream physiological response. Overall, our findings reveal that DkTx and capsaicin elicit distinct TRPV1 gating mechanisms and subsequent pain responses. Our results also indicate that the TRPV1 pore turret regulates the mechanisms of channel gating and permeation.

Conformational dynamics in TRPV1 channels reported by an encoded coumarin amino acid

TRPV1 channels support the detection of noxious and nociceptive input. Currently available functional and structural data suggest that TRPV1 channels have two gates within their permeation pathway: one formed by a 'bundle-crossing' at the intracellular entrance and a second constriction at the selectivity filter. To describe conformational changes associated with channel gating, the fluorescent non-canonical amino acid coumarin-tyrosine was genetically encoded at Y671, a residue proximal to the selectivity filter. Total internal reflection fluorescence microscopy was performed to image the conformational dynamics of the channels in live cells. Photon counts and optical fluctuations from coumarin encoded within TRPV1 tetramers correlates with channel activation by capsaicin, providing an optical marker of conformational dynamics at the selectivity filter. In agreement with the fluorescence data, molecular dynamics simulations display alternating solvent exposure of Y671 in the closed and open states. Overall, the data point to a dynamic selectivity filter that may serve as a gate for permeation.

Animal Toxins Providing Insights into TRPV1 Activation Mechanism

Beyond providing evolutionary advantages, venoms offer unique research tools, as they were developed to target functionally important proteins and pathways. As a key pain receptor in the nociceptive pathway, transient receptor potential vanilloid 1 (TRPV1) of the TRP superfamily has been shown to be a target for several toxins, as a way of producing pain to deter predators. Importantly, TRPV1 is involved in thermoregulation, inflammation, and acute nociception. As such, toxins provide tools to understand TRPV1 activation and modulation, a critical step in advancing pain research and the development of novel analgesics. Indeed, the phytotoxin capsaicin, which is the spicy chemical in chili peppers, was invaluable in the original cloning and characterization of TRPV1. The unique properties of each subsequently characterized toxin have continued to advance our understanding of functional, structural, and biophysical characteristics of TRPV1. By building on previous reviews, this work aims to provide a comprehensive summary of the advancements made in TRPV1 research in recent years by employing animal toxins, in particular DkTx, RhTx, BmP01, Echis coloratus toxins, APHCs and HCRG21. We examine each toxin’s functional aspects, behavioral effects, and structural features, all of which have contributed to our current knowledge of TRPV1. We additionally discuss the key features of TRPV1’s outer pore domain, which proves to be the target of the currently discussed toxins.

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.

Engineering vanilloid-sensitivity into the rat TRPV2 channel

The TRPV1 channel is a detector of noxious stimuli, including heat, acidosis, vanilloid compounds and lipids. The gating mechanisms of the related TRPV2 channel are poorly understood because selective high affinity ligands are not available, and the threshold for heat activation is extremely high (>50°C). Cryo-EM structures of TRPV1 and TRPV2 reveal that they adopt similar structures, and identify a putative vanilloid binding pocket near the internal side of TRPV1. Here we use biochemical and electrophysiological approaches to investigate the resiniferatoxin(RTx) binding site in TRPV1 and to explore the functional relationships between TRPV1 and TRPV2. Collectively, our results support the interaction of vanilloids with the proposed RTx binding pocket, and demonstrate an allosteric influence of a tarantula toxin on vanilloid binding. Moreover, we show that sensitivity to RTx can be engineered into TRPV2, demonstrating that the gating and permeation properties of this channel are similar to TRPV1.

Selective potentiation of 2-APB-induced activation of TRPV1-3 channels by acid

Temperature-sensitive TRP channels are important for responses to pain and inflammation, to both of which tissue acidosis is a major contributing factor. However, except for TRPV1, acid-sensing by other ThermoTRP channels remains mysterious. We show here that unique among TRPV1–3 channels, TRPV3 is directly activated by protons from cytoplasmic side. This effect is very weak and involves key cytoplasmic residues L508, D512, S518, or A520. However, mutations of these residues did not affect a strong proton induced potentiation of TRPV3 currents elicited by the TRPV1–3 common agonist, 2-aminoethoxydiphenyl borate (2-APB), no matter if the ligand was applied from extracellular or cytoplasmic side. The acid potentiation was common among TRPV1–3 and only seen with 2-APB-related ligands. Using ¹H-nuclear magnetic resonance to examine the solution structures of 2-APB and its analogs, we observed striking structural differences of the boron-containing compounds at neutral/basic as compared to acidic pH, suggesting that a pH-dependent configuration switch of 2-APB-based drugs may underlie their functionality. Supporting this notion, protons also enhanced the inhibitory action of 2-APB on TRPM8. Collectively, our findings reveal novel insights into 2-APB action on TRP channels, which should facilitate the design of new drugs for these channels.

Proton block of proton-activated TRPV1 current

H⁺ not only activates the TRPV1 cation channel but inhibits ion permeation, yielding a prominent OFF current response.

The TRPV1 cation channel is a polymodal nociceptor that is activated by heat and ligands such as capsaicin and is highly sensitive to changes in extracellular pH. In the body core, where temperature is usually stable and capsaicin is normally absent, H⁺ released in response to ischemia, tissue injury, or inflammation is the best-known endogenous TRPV1 agonist, activating the channel to mediate pain and vasodilation. Paradoxically, removal of H⁺ elicits a transient increase in TRPV1 current that is much larger than the initial H⁺-activated current. We found that this prominent OFF response is caused by rapid recovery from H⁺ inhibition of the excitatory current carried by H⁺-activated TRPV1 channels. H⁺ inhibited current by interfering with ion permeation. The degree of inhibition is voltage and permeant ion dependent, and it can be affected but not eliminated by mutations to acidic residues within or near the ion selectivity filter. The opposing H⁺-mediated gating and permeation effects produce complex current responses under different cellular conditions that are expected to greatly affect the response of nociceptive neurons and other TRPV1-expressing cells.

Membrane Protein Properties Revealed through Data-Rich Electrostatics Calculations

The electrostatic properties of membrane proteins often reveal many of their key biophysical characteristics, such as ion channel selectivity and the stability of charged membrane-spanning segments. The Poisson-Boltzmann (PB) equation is the gold standard for calculating protein electrostatics, and the software APBSmem enables the solution of the PB equation in the presence of a membrane. Here, we describe significant advances to APBSmem, including full automation of system setup, per-residue energy decomposition, incorporation of PDB2PQR, calculation of membrane-induced pKa shifts, calculation of non-polar energies, and command-line scripting for large-scale calculations. We highlight these new features with calculations carried out on a number of membrane proteins, including the recently solved structure of the ion channel TRPV1 and a large survey of 1,614 membrane proteins of known structure. This survey provides a comprehensive list of residues with large electrostatic penalties for being embedded in the membrane, potentially revealing interesting functional information.

TRPV1

TRPV1 is a well-characterised channel expressed by a subset of peripheral sensory neurons involved in pain sensation and also at a number of other neuronal and non-neuronal sites in the mammalian body. Functionally, TRPV1 acts as a sensor for noxious heat (greater than ~42 °C). It can also be activated by some endogenous lipid-derived molecules, acidic solutions (pH < 6.5) and some pungent chemicals and food ingredients such as capsaicin, as well as by toxins such as resiniferatoxin and vanillotoxins. Structurally, TRPV1 subunits have six transmembrane (TM) domains with intracellular N- (containing 6 ankyrin-like repeats) and C-termini and a pore region between TM5 and TM6 containing sites that are important for channel activation and ion selectivity. The N- and C- termini have residues and regions that are sites for phosphorylation/dephosphorylation and PI(4,5)P2 binding, which regulate TRPV1 sensitivity and membrane insertion. The channel has several interacting proteins, some of which (e.g. AKAP79/150) are important for TRPV1 phosphorylation. Four TRPV1 subunits form a non-selective, outwardly rectifying ion channel permeable to monovalent and divalent cations with a single-channel conductance of 50-100 pS. TRPV1 channel kinetics reveal multiple open and closed states, and several models for channel activation by voltage, ligand binding and temperature have been proposed. Studies with TRPV1 agonists and antagonists and Trpv1 (-/-) mice have suggested a role for TRPV1 in pain, thermoregulation and osmoregulation, as well as in cough and overactive bladder. TRPV1 antagonists have advanced to clinical trials where findings of drug-induced hyperthermia and loss of heat sensitivity have raised questions about the viability of this therapeutic approach.

Expression-dependent pharmacology of transient receptor potential vanilloid subtype 1 channels in Xenopus laevis oocytes

Transient receptor potential vanilloid subfamily member 1 channels are polymodal sensors of noxious stimuli and integral players in thermosensation, inflammation and pain signaling. It has been shown previously that under prolonged stimulation, these channels show dynamic pore dilation, providing a pathway for large and otherwise relatively impermeant molecules. Further, we have shown recently that these nonselective cation channels, when activated by capsaicin, are potently and reversibly blocked by external application of quaternary ammonium compounds and local anesthetics. Here we describe a novel phenomenon in transient receptor potential channel pharmacology whereby their expression levels in Xenopus laevis oocytes, as assessed by the magnitude of macroscopic currents, are negatively correlated with extracellular blocker affinity: small current densities give rise to nanomolar blockade by quaternary ammoniums and this affinity decreases linearly as current density increases. Possible mechanisms to explain these data are discussed in light of similar observations in other channels and receptors.

Exploring structure-function relationships between TRP and Kv channels

The molecular mechanisms underlying the activation of Transient Receptor Potential (TRP) ion channels are poorly understood when compared to those of the voltage-activated potassium (Kv) channels. The architectural and pharmacological similarities between the members of these two families of channels suggest that their structure-function relationships may have common features. We explored this hypothesis by replacing previously identified domains and critical structural motifs of the membrane-spanning portions of Kv2.1 with corresponding regions of two TRP channels, TRPM8 and TRPV1. Our results show that the S3b-S4 paddle motif of Kv2.1, but not other domains, can be replaced by the analogous regions of both TRP channels without abolishing voltage-activation. In contrast, replacement of portions of TRP channels with those of Kv2.1 consistently yielded non-functional channels. Taken together, these results suggest that most structural elements within TRP channels and Kv channels are not sufficiently related to allow for the creation of hybrid channels.

Crosslinking the ligand-binding domain dimer interface locks kainate receptors out of the main open state

Kainate-selective ionotropic glutamate receptors (iGluRs) fulfil key roles in the CNS, making them the subject of detailed structural and functional analyses. Although they are known to gate a channel pore with high and low ion-permeation rates, it is still not clear how switches between these gating modes are achieved at the structural level. Here, we uncover an unexpected role for the ligand-binding domain (LBD) dimer assembly in this process. Covalent crosslinking of the dimer interface keeps kainate receptors out of the main open state but permits access to lower conductance states suggesting that significant rearrangements of the dimer interface are required for the receptor to achieve full activation. These observations differ from NMDA-selective iGluRs where constraining dimer movement reduces open-channel probability. In contrast, our data show that restricting movement of the dimer interface interferes with conformational changes that underlie both activation and desensitization. Working within the limits of a common architectural design, we propose functionally diverse iGluR families were able to emerge during evolution by re-deploying existing gating structures to fulfil different tasks.

Protons stabilize the closed conformation of gain-of-function mutants of the TRPV1 channel

The vanilloid transient receptor potential channel TRPV1 is a molecular integrator of noxious stimuli, including capsaicin, heat and protons. Despite clear similarities between the overall architecture of TRPV1 and voltage-dependent potassium (Kv) channels, the extent of conservation in the molecular logic for gating is unknown. In Kv channels, a small contact surface between S1 and the pore-helix is required for channel functioning. To explore the function of S1 in TRPV1, we used tryptophan-scanning mutagenesis and characterized the responses to capsaicin and protons. Wild-type-like currents were generated in 9 out of 17 mutants; three mutants (M445W, A452W, R455W) were non-functional. The conservative mutation R455K in the extracellular extent of S1 slowed down capsaicin-induced activation and prevented normal channel closure. This mutant was neither activated nor potentiated by protons, on the contrary, protons promoted a rapid deactivation of its currents. Similar phenotypes were found in two other gain-of-function mutants and also in the pore-helix mutant T633A, known to uncouple proton activation. We propose that the S1 domain contains a functionally important region that may be specifically involved in TRPV1 channel gating, and thus be important for the energetic coupling between S1-S4 sensor activation and gate opening. Analogous to Kv channels, the S1-pore interface might serve to stabilize conformations associated with TRPV1 channel gating.

Calcium-dependent decrease in the single-channel conductance of TRPV1

TRPV1 is a Ca(2+) permeable cation channel gated by multiple stimuli including noxious heat, capsaicin, protons, and extracellular cations. In this paper, we show that Ca(2+) causes a concentration and voltage-dependent decrease in the capsaicin-gated TRPV1 single-channel conductance. This Ca(2+)-dependent effect on conductance was strongest at membrane potentials between -60 and +20 mV, but was diminished at more hyperpolarised potentials. Using simultaneous recordings of membrane current and fura-2 fluorescence to measure the fractional Ca(2+) current of whole-cell currents evoked through wild-type and mutant TRPV1, we investigated a possible link between the mechanisms underlying Ca(2+) permeation and the Ca(2+)-dependent effect on conductance. Surprisingly, we found no evidence of a structural correlation, and observed that the substitution of amino acids known to regulate Ca(2+) permeability had little effect on the ability for Ca(2+) to decrease TRPV1 conductance. However, we did observe that the Ca(2+)-dependent effect on conductance was not diminished by negative hyperpolarisation for a mutant receptor with severely impaired Ca(2+) permeability, TRPV1-D646N/E648Q/E651Q. This would be consistent with the idea that Ca(2+) reduces conductance by interacting with an intra-pore binding site, and that negative hyperpolarization reduces occupancy of this site by speeding the exit of Ca(2+) into the cell. Taken together, our data show that in addition to directly and indirectly regulating channel gating, Ca(2+) also directly reduces the conductance of TRPV1. Surprisingly, the mechanism underlying this Ca(2+)-dependent effect on conductance is largely independent of mechanisms governing Ca(2+) permeability.

Kinetic and energetic analysis of thermally activated TRPV1 channels

Thermal TRP channels are important for thermal sensation and nociception, but their gating mechanisms have remained elusive. With optically generated submillisecond temperature steps from 22°C to >60°C, we have directly measured the activation and deactivation kinetics of TRPV1 channels, and from the measurements we determined the energetics of thermal gating. We show that activation by temperature follows single exponential time courses. It occurs in a few milliseconds and is significantly faster than activation by agonists. The gating has characteristics of a melting process involving large compensatory enthalpy (>100 kcal/mol) and entropy changes with little free energy change. The reaction path is asymmetrical with temperature mainly driving the opening while the closing has nominal but negative temperature dependence (i.e., sensitivity to cold). Both voltage and agonists alter the slope of the temperature-dependent gating curve as well as shifting the midpoint. However, compared to the energetic effect of temperature on gating, the effect of voltage is small. Our data on the interdependence between voltage and direct temperature responses are not fit to a model involving independent stimuli but instead support a temperature-sensing mechanism that is coupled to charge movement or agonist binding.

Investigations of the in vivo requirements of transient receptor potential ion channels using frog and zebrafish model systems

Transient Receptor Potential (TRP) channels are cation channels that serve as cellular sensors on the plasma membrane, and have other less-well defined roles in intracellular compartments. The first TRP channel was identified upon molecular characterization of a fly mutant with abnormal photoreceptor function. More than 20 TRP channels have since been identified in vertebrates and invertebrate model systems, and these are divided into subfamilies based on structural similarities. The biophysical properties of TRP channels have primarily been explored in tissue culture models. The in vivo requirements for TRPs have been studied in invertebrate models like worm and flies, and also in vertebrate models, primarily mice and rats. Frog and zebrafish model systems offer certain experimental advantages relative to mammalian systems, and here a selection of papers which capitalize on these advantages to explore vertebrate TRP channel biology are reviewed. For instance, frog oocytes are useful for biochemistry and for electrophysiology, and these features were exploited in the identifcation TRPC1 as a candidate vertebrate mechanoreceptor. Also, the spinal neurons from frog embryos can be readily grown in culture. This feature was used to establish a role for TRPC1 in axon pathfinding in these neurons, and to explore how TRPC1 activity is regulated in this context. Zebrafish embryos are transparent making them well suited for in vivo imaging studies. This quality was exploited in a study in which the trpc2 gene promoter was used to label and trace the axon pathway of a subset of olfactory sensory neurons. Another experimental advantage of zebrafish is the speed and low cost of manipulating gene expression in embryos. Using these methods, it has been shown that TRPN1 is necessary for mechanosensation in zebrafish hair cells. Frogs and fish genomes have been mined to make inferences regarding evolutionary diversification of the thermosensitive TRP channels. Finally, TRPM7 is required for early morphogenesis in mice but not in fish; the reason for this difference is unclear, but it has caused zebrafish to be favored for exploration of TRPM7's role in later events in embryogenesis. The special experimental attributes of frogs and zebrafish suggest that these animals will continue to play an important role as models in future explorations of TRP channel biology.