Human Voltage-gated Proton Channel Research Articles

Interaction with stomatin directs human proton channels into cholesterol-dependent membrane domains

Many membrane proteins are modulated by cholesterol. Here we report profound effects of cholesterol depletion and restoration on the human voltage-gated proton channel, hHV1, in excised patches but negligible effects in the whole-cell configuration. Despite the presence of a putative cholesterol-binding site, a CARC motif in hHV1, mutation of this motif did not affect cholesterol effects. The murine HV1 lacks a CARC sequence but displays similar cholesterol effects. These results argue against a direct effect of cholesterol on the HV1 protein. However, the data are fully explainable if HV1 preferentially associates with cholesterol-dependent lipid domains, or “rafts.” The rafts would be expected to concentrate in the membrane/glass interface and to be depleted from the electrically accessible patch membrane. This idea is supported by evidence that HV1 channels can diffuse between seal and patch membranes when suction is applied. Simultaneous truncation of the large intracellular N and C termini of hHV1 greatly attenuated the cholesterol effect, but C truncation alone did not; this suggests that the N terminus is the region of attachment to lipid domains. Searching for abundant raft-associated proteins led to stomatin. Co-immunoprecipitation experiment results were consistent with hHV1 binding to stomatin. The stomatin-mediated association of HV1 with cholesterol-dependent lipid domains provides a mechanism for cells to direct HV1 to subcellular locations where it is needed, such as the phagosome in leukocytes.

Introducing NZ-58, a potent inhibitor of the human voltage-gated proton channel

Introducing NZ-58, a potent inhibitor of the human voltage-gated proton channel

Trp207 regulation of voltage-dependent activation of human Hv1 proton channel

In voltage-gated Na+ and K+ channels, the hydrophobicity of non-charged residues in the S4 helix has been shown to regulate the S4 movement underlying the process of voltage-sensing domain (VSD) activation. In voltage-gated proton channel Hv1, there is a bulky non-charged tryptophan residue located at the S4 transmembrane segment. This tryptophan remains entirely conserved across all Hv1 members but is not seen in other voltage-gated ion channels, indicating that the tryptophan contributes different roles in VSD activation. The conserved tryptophan of human voltage-gated proton channel Hv1 is Trp207 (W207). Here we showed that W207 modifies human Hv1 voltage-dependent activation, and small residues replacement at position 207 strongly perturbs Hv1 channel opening and closing, the size of the side chain instead of the hydrophobic group of W207 regulates the transition between closed and open states of the channel. We conclude that the large side chain of tryptophan controls the energy barrier during the Hv1 voltage-sensing domain transition.

Structural and functional characterisation of Tst2, a novel TRPV1 inhibitory peptide from the Australian sea anemone Telmatactis stephensoni

Sea anemone venoms are complex mixtures of biologically active compounds, including disulfide-rich peptides, some of which have found applications as research tools, and others as therapeutic leads. Our recent transcriptomic and proteomic studies of the Australian sea anemone Telmatactis stephensoni identified a transcript for a peptide designated Tst2. Tst2 is a 38-residue peptide showing sequence similarity to peptide toxins known to interact with a range of ion channels (NaV, TRPV1, KV and CaV). Recombinant Tst2 (rTst2, which contains an additional Gly at the N-terminus) was produced by periplasmic expression in Escherichia coli, enabling the production of both unlabelled and uniformly 13C,15N–labelled peptide for functional assays and structural studies. The LC-MS profile of the recombinant Tst2 showed a pure peak with molecular mass 6 Da less than that of the reduced form of the peptide, indicating the successful formation of three disulfide bonds from its six cysteine residues. The solution structure of rTst2 was determined using multidimensional NMR spectroscopy and revealed that rTst2 adopts an inhibitor cystine knot (ICK) structure. rTst2 was screened using various functional assays, including patch–clamp electrophysiological and cytotoxicity assays. rTst2 was inactive against voltage-gated sodium channels (NaV) and the human voltage-gated proton (hHv1) channel. rTst2 also did not possess cytotoxic activity when assessed against Drosophila melanogaster flies. However, the recombinant peptide at 100 nM showed >50% inhibition of the transient receptor potential subfamily V member 1 (TRPV1) and slight (∼10%) inhibition of transient receptor potential subfamily A member 1 (TRPA1). Tst2 is thus a novel ICK inhibitor of the TRPV1 channel.

Arachidonic acid reverses cholesterol and zinc inhibition of human voltage-gated proton channels

Unlike other members of the voltage-gated ion channel superfamily, voltage-gated proton (Hv) channels are solely composed of voltage sensor domains without separate ion-conducting pores. Due to their unique dependence on both voltage and transmembrane pH gradients, Hv channels normally open to mediate proton efflux. Multiple cellular ligands were also found to regulate the function of Hv channels, including Zn2+, cholesterol, polyunsaturated arachidonic acid, and albumin. Our previous work showed that Zn2+ and cholesterol inhibit the human voltage-gated proton channel hHv1 by stabilizing its S4 segment at resting state conformations. Released from phospholipids by phospholipase A2 in cells upon infection or injury, arachidonic acid regulates the function of many ion channels, including hHv1. In the present work, we examined the effects of arachidonic acid on purified hHv1 channels using liposome flux assays and revealed underlying structural mechanisms using single-molecule Fluorescence Resonance Energy Transfer (smFRET). Our data indicated that arachidonic acid strongly activates hHv1 channels by promoting transitions of the S4 segment towards opening or 'pre-opening' conformations. Moreover, we found that arachidonic acid even activates hHv1 channels inhibited by Zn2+ and cholesterol, providing a biophysical mechanism to activate hHv1 channels in non-excitable cells upon infection or injury.

Identification of a new family of inhibitors of the human Hv1 proton channel.

The human voltage-gated proton channel (hHv1) plays an important role in immune and cancer cells being involved in functions such as proliferation, migration, and oxidative burst. The structure of this protein differs from that of other voltage-gated ion channels in that it has no conventional ion-conducting pore, conduction occurs through the voltage-sensing domain itself. This dissemblance may be the reason for the lack of selective hHv1 inhibitors. Our aim was to find potent and selective inhibitors for hHv1, which could be useful research tools and serve as lead molecules for the development of drug molecules targeting Hv1 for therapeutic goals. We used manual patch-clamp to test the Hv1-affinity and selectivity of potential inhibitors on CHO (Chinese hamster ovary) and HEK (human embryonic kidney) cells expressing hHv1 and other channels. Proliferation assays were performed to test possible off-target effects of the molecules. Over 30 small molecules were tested, of which seven turned out to be effective on hHv1 (i.e. blocked more than 50% of the current in 50 µM). Six of them had the same scaffold in their structure. The scaffold itself, named NZ-13, was also one of the hit molecules, and had an IC50 value of 26 µM for hHv1. Most of the hit molecules turned out to have low selectivity: they had comparable or even higher affinity for the other tested Kv and NaV ion channels than for Hv1. However, NZ-13 had lower affinity for the other channels than for Hv1 and also proved to have the smallest effect on T-cell proliferation. Based on our results, we have successfully identified a scaffold, which can be a starting point for creating new, potent and selective hHv1 inhibitors.

Mechanisms of ligand regulation in human voltage-gated proton channel.

Voltage-gated proton (Hv) channels are standalone voltage sensors without separate pore-forming domains in canonical voltage-gated ion channels. They normally function as acid extruders for their unique voltage- and pH-dependent gating. Aside from voltage and pH, Hv channels are also gated by multiple cellular ligands, including Zn2+, polyunsaturated arachidonic acid, albumin, and membrane cholesterol, identified in our recent work. Our previous results have established the S4 segment as the central gating machinery in hHv1 channels by showing that voltage activation enriched its opening conformations while inhibitory ligands, including proton, Zn2+ and cholesterol, stabilized it at resting state conformations. How is the function of hHv1 channels regulated in cells? Our functional characterizations on purified hHv1 channels and those expressed in cells indicated that arachidonic acid has very strong activating effects on hHv1 channels, even being able to reverse both Zn2+ and cholesterol inhibition at physiological levels. Examining the structural effects of arachidonic acid on hHv1 channels using single-molecule FRET further revealed that it activates hHv1 channels by promoting ‘pre-opening’ rather than the opening conformations. In many cells, hHv1 channels are closely associated with NADPH oxidases to compensate for charge/pH imbalances, and both were found in cholesterol-rich lipid rafts. Our results propose that cholesterol perhaps determines the basal activities of the hHv1 channel in cells, while releases of arachidonic acid following irritation or injury may serve as a major mechanism to activate hHv1 channels to promote respiration burst mediating by NADPH oxidases.

Molecular origin of the proton selectivity in the human voltage-gated proton channel.

The human voltage-gated proton channel hHv1 is a ubiquitous membrane protein involved in a large range of cellular processes including proton extrusion, pH regulation, reactive oxygen species production and cancer cell proliferation. The proton channel, once activated, conducts a drastic shift in one of the four transmembrane helices aligned with the direction of the membrane potential. Mutagenesis studies have suggested that the proton selectivity of this channel is directly related to a key residue, Asp112, but the mechanism by which Asp112 controls proton selectivity remains elusive. The goal of this study is to understand the proton permeation mechanism and the associated proton selectivity of hHv1 via quantum-mechanics-based simulations of explicit proton migration over the whole channel. We computed a quantitative free energy profile characterizing the proton transport process, which, when paired with Markov state kinetics modeling, clarifies the critical involvement of Asp112 in proton selectivity.

Voltage sensor dynamics of a bacterial voltage-gated sodium channel NavAb reveal three conformational states

High-resolution structures of voltage-gated sodium channels (Nav) were first obtained from a prokaryotic ortholog NavAb, which provided important mechanistic insights into Na+ selectivity and voltage gating. Unlike eukaryotic Navs, the NavAb channel is formed by four identical subunits, but its ion selectivity and pharmacological profiles are very similar to eukaryotic Navs. Recently, the structures of the NavAb voltage sensor at resting and activated states were obtained by cryo-EM, but its intermediate states and transition dynamics remain unclear. In the present work, we used liposome flux assays to show that purified NavAb proteins were functional to conduct both H+ and Na+ and were blocked by the local anesthetic lidocaine. Additionally, we examined the real-time conformational dynamics of the NavAb voltage sensor using single-molecule FRET. Our single-molecule FRET measurements on the tandem NavAb channel labeled with Cy3/5 FRET fluorophore pair revealed spontaneous transitions of the NavAb S4 segment among three conformational states, which fitted well with the kinetic model developed for the S4 segment of the human voltage-gated proton channel hHv1. Interestingly, even under strong activating voltage, the NavAb S4 segment seems to adopt a conformational distribution similar to that of the hHv1 S4 segment at a deep resting state. The conformational behaviors of the NavAb voltage sensor under different voltages need to be further examined to understand the mechanisms of voltage sensing and gating in the canonical voltage-gated ion channel superfamily.

The role of Phe150 in human voltage-gated proton channel

The voltage-gated proton channel Hv1 is a member of voltage-gated ion channels containing voltage-sensing domains (VSDs). The VSDs are made of four membrane-spanning segments (S1 through S4), and their function is to detect changes in membrane potential in the cells. A highly conserved phenylalanine 150 (F150) is located in the S2 segment of human voltage-gated proton channels. We previously discovered that the F150 is a binding site for the open channel blocker 2GBI. Here, we show that the Hv1 VSD voltage-dependent activation requires a hydrophobic group at position F150. We perform double-mutant cycle analysis to probe interactions between F150 and positively charged arginines in the S4 segment of the channel. Our results indicate that F150 interacts with two arginines (R2 and R3) in the S4 segment and catalyzes the transfer of the S4 arginines in the process of voltage-dependent activation.

Cholesterol inhibits human voltage-gated proton channel hHv1

Although human sperm is morphologically mature in the epididymis, it cannot fertilize eggs before capacitation. Cholesterol efflux from the sperm plasma membrane is a key molecular event essential for cytoplasmic alkalinization and hyperactivation, but the underlying mechanism remains unclear. The human voltage-gated proton (hHv1) channel functions as an acid extruder to regulate intracellular pHs of many cell types, including sperm. Aside from voltage and pH, Hv channels are also regulated by distinct ligands, such as Zn2+ and albumin. In the present work, we identified cholesterol as an inhibitory ligand of the hHv1 channel and further investigated the underlying mechanism using the single-molecule fluorescence resonance energy transfer (smFRET) approach. Our results indicated that cholesterol inhibits the hHv1 channel by stabilizing the voltage-sensing S4 segment at resting conformations, a similar mechanism also utilized by Zn2+. Our results suggested that the S4 segment is the central gating machinery in the hHv1 channel, on which voltage and distinct ligands are converged to regulate channel function. Identification of membrane cholesterol as an inhibitory ligand provides a mechanism by which the hHv1 channel regulates fertilization by linking the cholesterol efflux with cytoplasmic alkalinization, a change that triggers calcium influx through the CatSper channel. These events finally lead to hyperactivation, a remarkable change in the mobility pattern indicating fertilization competence of human sperm.

Novel Dimeric hHv1 Model and Structural Bioinformatic Analysis Reveal an ATP-Binding Site Resulting in a Channel Activating Effect.

The human voltage-gated proton channel (hHv1) is a highly selective ion channel codified by the HVCN1 gene. It plays a fundamental role in several physiological processes such as innate and adaptive immunity, insulin secretion, and sperm capacitation. Moreover, in humans, a higher hHv1 expression/function has been reported in several types of cancer cells. Here we report a multitemplate homology model of the hHv1 channel, built and refined as a dimer in Rosetta. The model was then subjected to extensive Gaussian accelerated molecular dynamics (GaMD) for enhanced conformational sampling, and representative snapshots were extracted by clustering analysis. Combining different structure- and sequence-based methodologies, we predicted a putative ATP-binding site located on the intracellular portion of the channel. Furthermore, GaMD simulations of the ATP-bound dimeric hHv1 model showed that ATP interacts with a cluster of positively charged residues from the cytoplasmic N and C terminal segments. According to the in silico predictions, we found that 3 mM intracellular ATP significantly increases the H+ current mediated by the hHv1 channel expressed in HEK293 cells and measured by the patch-clamp technique in an inside-out configuration (2.86 ± 0.63 fold over control at +40 mV). When ATP was added on the extracellular side, it was not able to activate the channel supporting the idea that the ATP-binding site resides in the intracellular face of the hHV1 channel. In a physiological and pathophysiological context, this ATP-mediated modulation could integrate the cell metabolic state with the H+ efflux, especially in cells where hHv1 channels are relevant for pH regulation, such as pancreatic β-cells, immune cells, and cancer cells.

Molecular determinants of inhibition of the human proton channel hHv1 by the designer peptide C6 and a bivalent derivative

The human voltage-gated proton channel (hHv1) is important for control of intracellular pH. We designed C6, a specific peptide inhibitor of hHv1, to evaluate the roles of the channel in sperm capacitation and in the inflammatory immune response of neutrophils [R. Zhao et al., Proc. Natl. Acad. Sci. U.S.A. 115, E11847–E11856 (2018)]. One C6 binds with nanomolar affinity to each of the two S3–S4 voltage-sensor loops in hHv1 in cooperative fashion so that C6-bound channels require greater depolarization to open and do so more slowly. As depolarization drives hHv1 sensors outwardly, C6 affinity decreases, and inhibition is partial. Here, we identified residues essential to C6–hHv1 binding by scanning mutagenesis, five in the hHv1 S3–S4 loops and seven on C6. A structural model of the C6–hHv1 complex was then generated by molecular dynamics simulations and validated by mutant-cycle analysis. Guided by this model, we created a bivalent C6 peptide (C62) that binds simultaneously to both hHv1 subunits and fully inhibits current with picomolar affinity. The results help delineate the structural basis for C6 state-dependent inhibition, support an anionic lipid-mediated binding mechanism, and offer molecular insight into the effectiveness of engineered C6 as a therapeutic agent or lead.

Structural dynamics determine voltage and pH gating in human voltage-gated proton channel.

Voltage-gated proton (Hv) channels are standalone voltage sensors without separate ion conductive pores. They are gated by both voltage and transmembrane proton gradient (i.e., ∆pH), serving as acid extruders in most cells. Like the canonical voltage sensors, Hv channels are a bundle of four helices (named S1 -S4), with the S4 segment carrying three positively charged Arg residues. Extensive structural and electrophysiological studies on voltage-gated ion channels, in general, agree on an outwards movement of the S4 segment upon activating voltage, but the real-time conformational transitions are still unattainable. With purified human voltage-gated proton (hHv1) channels reconstituted in liposomes, we have examined its conformational dynamics, including the S4 segment at different voltage and pHs using single-molecule fluorescence resonance energy transfer (smFRET). Here, we provide the first glimpse of real-time conformational trajectories of the hHv1 voltage sensor and show that both voltage and pH gradient shift the conformational dynamics of the S4 segment to control channel gating. Our results indicate that the S4 segment transits among three major conformational states and only the transitions between the inward and outward conformations are highly dependent on voltage and pH. Altogether, we propose a kinetic model that explains the mechanisms underlying voltage and pH gating in Hv channels, which may also serve as a general framework for understanding the voltage sensing and gating in other voltage-gated ion channels.

The role of Phe150 in human voltage-gated proton channel

The voltage-gated proton channel is a member of the voltage-gated ion channels containing voltage-sensing domains (VSDs). The VSDs are made of four membrane spanning segments (S1 through S4), and their function is to detect changes in membrane potential in the cells. A highly conserved phenylalanine (Phe) is located in the S2 segment of the voltage-gated ion channel and acts as a “gating charge transfer center”. In the voltage-gated shaker K+ channel, the large side chain of the conserved Phe was crucial to stabilize the depolarized VSD conformation.

The pH-dependent gating of the human voltage-gated proton channel from computational simulations

(Δ)pH induces conformational change from a deactivated to an activated state of the hHv1 voltage-gated proton channel.

Cell glycosaminoglycans content modulates human voltage-gated proton channel (HV 1) gating.

Voltage-gated proton channels (HV 1) have been found in many mammalian cells and play a crucial role in the immune system, male fertility, and cancer progression. Glycosaminoglycans play a significant role in various aspects of cell physiology, including the modulation of membrane receptors and ion channel function. We present here evidence that mechanosensitivity of the dimeric HV 1 channel transduce changes on cell membrane fluidity related to the defective biosynthesis of chondroitin sulfate and heparan sulfate in Chinese Hamster Ovary (CHO-745) cells into a leftward shift in the activation voltage dependence. This effect was accompanied by an increase in the H+ current, and an acceleration of the activation kinetics, under symmetrical or asymmetrical pH gradient (ΔpH) conditions. Similar gating alterations were evoked by two naturally occurring HV 1 N-terminal truncated isoforms expressed in wild-type CHO-K1 and CHO-745 cells. On three different monomeric HV 1 constructs, no alterations in the biophysical parameters were observed. Moreover, we have shown that HV 1 gating can be modulated by manipulating CHO-K1 cell membrane fluidity. Our results suggest that the defective biosynthesis of chondroitin sulfate and heparan sulfate on CHO-745 cell increases membrane fluidity and allosterically modulates the coupling between voltage- and ΔpH-sensing through the dimeric HV 1 channel.

HIFs: New arginine mimic inhibitors of the Hv1 channel with improved VSD-ligand interactions.

The human voltage-gated proton channel Hv1 is a drug target for cancer, ischemic stroke, and neuroinflammation. It resides on the plasma membrane and endocytic compartments of a variety of cell types, where it mediates outward proton movement and regulates the activity of NOX enzymes. Its voltage-sensing domain (VSD) contains a gated and proton-selective conduction pathway, which can be blocked by aromatic guanidine derivatives such as 2-guanidinobenzimidazole (2GBI). Mutation of Hv1 residue F150 to alanine (F150A) was previously found to increase 2GBI apparent binding affinity more than two orders of magnitude. Here, we explore the contribution of aromatic interactions between the inhibitor and the channel in the presence and absence of the F150A mutation, using a combination of electrophysiological recordings, classic mutagenesis, and site-specific incorporation of fluorinated phenylalanines via nonsense suppression methodology. Our data suggest that the increase in apparent binding affinity is due to a rearrangement of the binding site allowed by the smaller residue at position 150. We used this information to design new arginine mimics with improved affinity for the nonrearranged binding site of the wild-type channel. The new compounds, named "Hv1 Inhibitor Flexibles" (HIFs), consist of two "prongs," an aminoimidazole ring, and an aromatic group connected by extended flexible linkers. Some HIF compounds display inhibitory properties that are superior to those of 2GBI, thus providing a promising scaffold for further development of high-affinity Hv1 inhibitors.

A Novel Model of the Dimeric Human Hv1 Channel Reveals a Putative ATP Binding Site

A multi-template model of the human voltage-gated proton channel (hHv1) was developed by combining the recently solved NMR structural ensemble of the hHv1 (PDBID: 5OQK) with the coiled-coil C-terminal domain (PDBID: 3A2A), bridging them through the chimeric mHv1cc structure (PDBID: 3WKV). The model was built and refined as a dimer within Rosetta's Membrane framework, using the symmetry extracted from the coiled-coil and including experimentally derived constraints during the relaxation. The dimeric model was then subjected to extensive Gaussian Accelerated Molecular Dynamics simulations in a POPC bilayer, and representative snapshots were extracted from the trajectory by clustering analysis. Blind docking simulations of an ATP molecule on this structural ensemble revealed a putative ATP binding site located on the intracellular portion of the permeation pathway, below the hydrophobic gasket. The same region was identified by two sequence-based ATP binding site predictors: ATPSite and ATPint; and a third meta-predictor which combines sequence and structure-based features: ATPBind. According to the in-silico predictions, we found that whole-cell hHv1 current measured in presence of intracellular 3 mM ATP (patch-clamp, Jurkat T cells, n=7) showed a higher amplitude (148.3±27.0 vs 35.6±4.6 pA at 20 mV p<0.05) and a hyperpolarized Vthreshold (−25.7±2.0 vs −12.0±3.7 mV, p<0.05), compared to control cells (without ATP, n=5). The combination of several in-silico methodologies allowed us to develop a novel model of dimeric hHv1 channel, and predict its direct modulation by ATP. Moreover, experimental results confirmed that ATP is able to activate the channel. In a physiological context, this ATP-mediated modulation could integrate the cell metabolic state with the H+ efflux, especially in cells where hHv1 channels are relevant for pH regulation such as pancreatic β-cells, cancer cells or immune cells.

Hydrophobic Gasket Mutation Produces Proton Selective Gating Pore Currents in Closed Human Voltage-Gated Proton Channels

A ring of hydrophobic amino acids in the voltage-sensing domain of most voltage-gated ion channels forms a constriction between internal and external aqueous vestibules. Cationic Arg or Lys side chains lining the S4 helix move through this apparent “hydrophobic gasket” (HG) when the channel opens. In the human voltage-gated proton channel, hHV1, not only does S4 movement occur during gating, but proton current also flows through this “gating pore” in open channels. To probe the role of the HG in the function of hHV1, we replaced putative HG residues with less hydrophobic residues or acidic Asp.