Zinc inhibition of monomeric and dimeric proton channels suggests cooperative gating

Interior pH-sensing residue of human voltage-gated proton channel Hv1 is histidine 168

The molecular mechanisms governing the human voltage-gated proton channel hHv1 remain elusive. Here, we used membrane-enabled hybrid-solvent continuous constant pH molecular dynamics (CpHMD) simulations with pH replica exchange to further evaluate the structural models of hHv1 in the closed (hyperpolarized) and open (depolarized) states recently obtained with MD simulations and explore potential pH-sensing residues. The CpHMD titration at a set of symmetric pH conditions revealed three residues that can gain or lose protons upon channel depolarization. Among them, residue H168 at the intracellular end of the S3 helix switches from the deprotonated to the protonated state and its protonation is correlated with the increased tilting of the S3 helix during the transition from the closed to the open state. Thus, the simulation data suggest H168 as an interior pH sensor, in support of a recent finding based on electrophysiological experiments of Hv1 mutants. We propose that protonation of H168 acts as a key that unlocks the closed channel configuration by increasing the flexibility of the S2-S3 linker, which increases the tilt angle of S3 and enhances the mobility of the S4 helix, thus promoting channel opening. Our work represents an important step toward deciphering the pH-dependent gating mechanism of hHv1.

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.

The Role of Ion-Transporting Proteins on Crosstalk Between the Skeletal Muscle and Central Nervous Systems Elicited by Physical Exercise

A paradigm shift in the understanding of bidirectional interactions between peripheral and central nervous systems is essential for development of rehabilitation and preventive interventions based on physical exercise. Although a causal relationship has not been completely established, modulation of voltage-dependent ion channels (Ca2+, Cl-, K+, Na+, lactate-, H+) in skeletal and neuronal cells provides opportunities to maintain force production during exercise and reduce the risk of disease. However, there are caveats to consider when interpreting the effects of physical exercise on this bidirectional axis, since exercise protocol details (e.g., duration and intensity) have variable effects on this crosstalk. Therefore, an integrative perspective of the skeletal muscle and brain's communication pathway is discussed, and the role of physical exercise on such communication highway is explained in this review.

Temperature Dependent Activity of the Voltage-Gated Proton Channel

Voltage-gated proton channel (Hv) has activity of proton transport following electrochemical gradient of proton. Hv is expressed in neutrophils and macrophages of which functions are physiologically temperature-sensitive. Hv is also expressed in human sperm cells and regulates their locomotion. H+ transport through Hv is both regulated by membrane potential and pH difference across biological membrane. It is also reported that properties of Hv such as proton conductance and gating are highly temperature-dependent. Hv consists of the N-terminal cytoplasmic domain, the voltage sensor domain (VSD), and the C-terminal coiled-coil domain, and H+ permeates through VSD voltage-dependently. The functional unit of Hv is a dimer via the interaction between C-terminal coiled-coils assembly domain. We have reported that the coiled-coil domain of Hv has the nature of dissociation around our bodily temperature and mutational change of the coiled-coil affected temperature-sensitive gating, especially its temperature threshold. The temperature-sensitive gating is assessed from two separate points: temperature threshold and temperature dependence. In this chapter, I describe physiological roles and molecular structure mechanisms of Hv by mainly focusing on thermosensitive properties.

Voltage-Gated Proton Channels in the Tree of Life

With a single gene encoding H_V1 channel, proton channel diversity is particularly low in mammals compared to other members of the superfamily of voltage-gated ion channels. Nonetheless, mammalian H_V1 channels are expressed in many different tissues and cell types where they exert various functions. In the first part of this review, we regard novel aspects of the functional expression of H_V1 channels in mammals by differentially comparing their involvement in (1) close conjunction with the NADPH oxidase complex responsible for the respiratory burst of phagocytes, and (2) in respiratory burst independent functions such as pH homeostasis or acid extrusion. In the second part, we dissect expression of H_V channels within the eukaryotic tree of life, revealing the immense diversity of the channel in other phylae, such as mollusks or dinoflagellates, where several genes encoding H_V channels can be found within a single species. In the last part, a comprehensive overview of the biophysical properties of a set of twenty different H_V channels characterized electrophysiologically, from Mammalia to unicellular protists, is given.

Microglial reprogramming by Hv1 antagonism protects neurons from inflammatory and glutamate toxicity

Although the precise mechanisms determining the neurotoxic or neuroprotective activation phenotypes in microglia remain poorly characterized, metabolic changes in these cells appear critical for these processes. As cellular metabolism can be tightly regulated by changes in intracellular pH, we tested whether pharmacological targeting of the microglial voltage-gated proton channel 1 (Hv1), an important regulator of intracellular pH, is critical for activated microglial reprogramming. Using a mouse microglial cell line and mouse primary microglia cultures, either alone, or co-cultured with rat cerebrocortical neurons, we characterized in detail the microglial activation profile in the absence and presence of Hv1 inhibition. We observed that activated microglia neurotoxicity was mainly attributable to the release of tumor necrosis factor alpha, reactive oxygen species, and zinc. Strikingly, pharmacological inhibition of Hv1 largely abrogated inflammatory neurotoxicity not only by reducing the production of cytotoxic mediators but also by promoting neurotrophic molecule production and restraining excessive phagocytic activity. Importantly, the Hv1-sensitive change from a pro-inflammatory to a neuroprotective phenotype was associated with metabolic reprogramming, particularly via a boost in NADH availability and a reduction in lactate. Most critically, Hv1 antagonism not only reduced inflammatory neurotoxicity but also promoted microglia-dependent neuroprotection against a separate excitotoxic injury. Our results strongly suggest that Hv1 blockers may provide an important therapeutic tool against a wide range of inflammatory neurodegenerative disorders.

Unexpected expansion of the voltage-gated proton channel family

Voltage-gated ion channels, whose first identified function was to generate action potentials, are divided into subfamilies with numerous members. The family of voltage-gated proton channels (HV ) is tiny. To date, all species found to express HV have exclusively one gene that codes for this unique ion channel. Here we report the discovery and characterization of three proton channel genes in the classical model system of neural plasticity, Aplysia californica. The three channels (AcHV 1, AcHV 2, and AcHV 3) are distributed throughout the whole animal. Patch-clamp analysis confirmed proton selectivity of these channels but they all differed markedly in gating. AcHV 1 gating resembled HV in mammalian cells where it is responsible for proton extrusion and charge compensation. AcHV 2 activates more negatively and conducts extensive inward proton current, properties likely to acidify the cytosol. AcHV 3, which differs from AcHV 1 and AcHV 2 in lacking the first arginine in the S4 helix, exhibits proton selective leak currents and weak voltage dependence. We report the expansion of the proton channel family, demonstrating for the first time the expression of three functionally distinct proton channels in a single species.

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

We designed C6 peptide to address the absence of specific inhibitors of human voltage-gated proton channels (hHv1). Two C6 bind to the two hHv1 voltage sensors at the resting state, inhibiting activation on depolarization. Here, we identify the C6–hHv1 binding interface using tethered-toxin variants and channel mutants, unveil an important role for negatively charged lipids, and present a model of the C6–hHv1 complex. Inspired by nature, we create a peptide with two C6 epitopes (C6₂) that binds to both channel subunits simultaneously, yielding picomolar affinity and significantly improved inhibition at high potentials. C6 and C6₂ are peptides designed to regulate hHv1, a channel involved in innate immune-system inflammatory pathophysiology, sperm capacitation, cancer-cell proliferation, and tissue damage in ischemic stroke.

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.

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

Gating of the voltage-gated proton channel H_V1 is strongly controlled by pH. There is evidence that this involves the sidechains of titratable amino acids that change their protonation state with changes of the pH. Despite experimental investigations to identify the amino acids involved in pH sensing only few progress has been made, including one histidine at the cytoplasmic side of the channel that is involved in sensing cellular pH. We have used constant pH molecular dynamics simulations in symmetrical and asymmetrical pH conditions across the membrane to investigate the pH- and ΔpH-dependent gating of the human H_V1 channel. Therefore, the pK_a of every titratable amino acids has been assessed in single simulations. Our simulations captured initial conformational changes between a deactivated and an activated state of the channel induced solely by changes of the pH. The pH-dependent gating is accompanied by an outward displacement of the three S4 voltage sensing arginines that moves the second arginine past the hydrophobic gasket (HG) which separates the inner and outer pores of the channel. H_V1 activation, when outer pH increases, involves amino acids at the extracellular entrance of the channel that extend the network of interactions from the external solution down to the HG. Whereas, amino acids at the cytoplasmic entrance of the channel are involved in activation, when inner pH decreases, and in a network of interactions that extend from the cytoplasm up to the HG.

Voltage-gated proton channels in polyneopteran insects

Voltage‐gated proton channels (H_V1) are expressed in eukaryotes, including basal hexapods and polyneopteran insects. However, currently, there is little known about H_V1 channels in insects. A characteristic aspartate (Asp) that functions as the proton selectivity filter (SF) and the RxWRxxR voltage‐sensor motif are conserved structural elements in H_V1 channels. By analysing Transcriptome Shotgun Assembly (TSA) databases, we found 33 polyneopteran species meeting these structural requirements. Unexpectedly, an unusual natural variation Asp to glutamate (Glu) at SF was found in Phasmatodea and Mantophasmatodea. Additionally, we analysed the expression and function of H_V1 in the phasmatodean stick insect Extatosoma tiaratum (Et). EtH_V1 is strongly expressed in nervous tissue and shows pronounced inward proton conduction. This is the first study of a natural occurring Glu within the SF of a functional H_V1 and might be instrumental in uncovering the physiological function of H_V1 in insects.

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

Voltage-gated proton channels (H_V 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 H_V 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 H_V 1 N-terminal truncated isoforms expressed in wild-type CHO-K1 and CHO-745 cells. On three different monomeric H_V 1 constructs, no alterations in the biophysical parameters were observed. Moreover, we have shown that H_V 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 H_V 1 channel.

Analysis of an electrostatic mechanism for ΔpH dependent gating of the voltage-gated proton channel, HV1, supports a contribution of protons to gating charge

Voltage-gated proton channels (HV1) resemble the voltage-sensing domain of other voltage-gated ion channels, but differ in containing the conduction pathway. Essential to the functions of HV1 channels in many cells and species is a unique feature called ΔpH dependent gating. The pH on both sides of the membrane strictly regulates the voltage range of channel opening, generally resulting in exclusively outward proton current. Two types of mechanisms could produce ΔpH dependent gating. The "countercharge" mechanism proposes that protons destabilize salt bridges between amino acids in the protein that stabilize specific gating configurations (closed or open). An "electrostatic" mechanism proposes that protons bound to the channel alter the electrical field sensed by the protein. Obligatory proton binding within the membrane electrical field would contribute to measured gating charge. Estimations on the basis of the electrostatic model explain ΔpH dependent gating, but quantitative modeling requires calculations of the electric field inside the protein which, in turn, requires knowledge of its structure. We conclude that both mechanisms operate and contribute to ΔpH dependent gating of HV1.

Engineered high-affinity zinc binding site reveals gating configurations of a human proton channel

The voltage-gated proton channel (H_V1) is a voltage sensor that also conducts protons. The singular ability of protons to penetrate proteins complicates distinguishing closed and open channels. When we replaced valine with histidine at position 116 in the external vestibule of hH_V1, current was potently inhibited by externally applied Zn²⁺ in a construct lacking the two His that bind Zn²⁺ in WT channels. High-affinity binding with profound effects at 10 nM Zn²⁺ at pH_o 7 suggests additional groups contribute. We hypothesized that Asp¹⁸⁵, which faces position 116 in our closed-state model, contributes to Zn²⁺ chelation. Confirming this prediction, V116H/D185N abolished Zn²⁺ binding. Studied in a C-terminal truncated monomeric construct, V116H channels activated rapidly. Anomalously, Zn²⁺ slowed activation, producing a time constant independent of both voltage and Zn²⁺ concentration. We hypothesized that slow turn-on of H⁺ current in the presence of Zn²⁺ reflects the rate of Zn²⁺ unbinding from the channel, analogous to drug-receptor dissociation reactions. This behavior in turn suggests that the affinity for Zn²⁺ is greater in the closed state of hH_V1. Supporting this hypothesis, pulse pairs revealed a rapid component of activation whose amplitude decreased after longer intervals at negative voltages as closed channels bound Zn²⁺. The lower affinity of Zn²⁺ in open channels is consistent with the idea that structural rearrangements within the transmembrane region bring Arg²⁰⁵ near position 116, electrostatically expelling Zn²⁺. This phenomenon provides direct evidence that Asp¹⁸⁵ opposes position 116 in closed channels and that Arg²⁰⁵ moves between them when the channel opens.

A novel Hv1 inhibitor reveals a new mechanism of inhibition of a voltage-sensing domain

Voltage-gated sodium, potassium, and calcium channels consist of four voltage-sensing domains (VSDs) that surround a central pore domain and transition from a down state to an up state in response to membrane depolarization. While many types of drugs bind pore domains, the number of organic molecules known to bind VSDs is limited. The Hv1 voltage-gated proton channel is made of two VSDs and does not contain a pore domain, providing a simplified model for studying how small ligands interact with VSDs. Here, we describe a ligand, named HIF, that interacts with the Hv1 VSD in the up and down states. We find that HIF rapidly inhibits proton conduction in the up state by blocking the open channel, as previously described for 2-guanidinobenzimidazole and its derivatives. HIF, however, interacts with a site slowly accessible in the down state. Functional studies and MD simulations suggest that this interaction traps the compound in a narrow pocket lined with charged residues within the VSD intracellular vestibule, which results in slow recovery from inhibition. Our findings point to a “wrench in gears” mechanism whereby side chains within the binding pocket trap the compound as the teeth of interlocking gears. We propose that the use of screening strategies designed to target binding sites with slow accessibility, similar to the one identified here, could lead to the discovery of new ligands capable of interacting with VSDs of other voltage-gated ion channels in the down state.

Voltage-gated proton channels exist in the plasma membrane of human oocytes

STUDY QUESTION

Do human oocytes express voltage-gated proton channels?

SUMMARY ANSWER

Human oocytes exhibit voltage-gated proton currents.

WHAT IS KNOWN ALREADY

Voltage-gated proton currents have been reported in human sperm, where they contribute to capacitation and motility. No such studies of human oocytes exist.

STUDY DESIGN, SIZE, DURATION

Voltage-clamp studies were undertaken using entire oocytes and vesicles derived from oocytes and in excised patches of membrane from oocytes.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Frozen, thawed human metaphase II oocytes were obtained from material donated to the gamete repository at the Rush Center for Advanced Reproductive Care. Prior to patch clamping, oocytes were warmed and equilibrated. Formation of an electrically tight seal requires exposing bare oolemma. Sections of the zona pellucida (ZP) were removed using a laser, followed by repeated pipetting, to further separate the oocyte from the ZP. Patch-clamp studies were performed using the whole-cell configuration on oocytes or vesicles derived from oocytes, and using inside-out patches of membrane, under conditions optimized to detect voltage-gated proton currents.

MAIN RESULTS AND THE ROLE OF CHANCE

Proton currents are present at significant levels in human oocytes where they exhibit properties similar to those reported in other human cells, as well as those in heterologous expression systems transfected with the HVCN1 gene that codes for the voltage-gated proton channel.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Human oocytes are large cells, which limits our ability to control the intracellular solution. Subtle effects of cryopreservation by vitrification and subsequent warming on properties of HVCN1, the HVCN1 gene product, cannot be ruled out.

WIDER IMPLICATIONS OF THE FINDINGS

Possible functions for voltage-gated proton channels in human oocytes may now be contemplated.

STUDY FUNDING/COMPETING INTEREST(S)

NIH R35GM126902 (TED), Bears Care (DM). No competing interests.

TRIAL REGISTRATION NUMBER

N/A.

The voltage-gated proton channel hHv1 is functionally expressed in human chorion-derived mesenchymal stem cells

The voltage-gated proton channel Hv1 is widely expressed, among others, in immune and cancer cells, it provides an efficient cytosolic H⁺extrusion mechanism and regulates vital functions such as oxidative burst, migration and proliferation. Here we demonstrate the presence of human Hv1 (hHv1) in the placenta/chorion-derived mesenchymal stem cells (cMSCs) using RT-PCR. The voltage- and pH-dependent gating of the current is similar to that of hHv1 expressed in cell lines and that the current is blocked by 5-chloro-2-guanidinobenzimidazole (ClGBI) and activated by arachidonic acid (AA). Inhibition of hHv1 by ClGBI significantly decreases mineral matrix production of cMSCs induced by conditions mimicking physiological or pathological (inorganic phosphate, Pi) induction of osteogenesis. Wound healing assay and single cell motility analysis show that ClGBI significantly inhibits the migration of cMSCs. Thus, seminal functions of cMSCs are modulated by hHv1 which makes this channel as an attractive target for controlling advantages/disadvantages of MSCs therapy.

Role of the time dependence of Boltzmann open probability in voltage-gated proton channels

The modeling and simulation of experimental families of current-time (I-t) curves of dimeric voltage-gated proton channels and of proton-conducting voltage sensing domains (VSDs) with a minimum of free parameters requires the movement of protons to be controlled by the rate of increase of the Boltzmann open probability p over time in passing from the holding to the depolarizing potential. Families of I-t curves of protomers and proton-conducting VSDs can be satisfactorily fitted by the use of a single free parameter expressing the rate constant k_p for the increase of p over time. Families of I-t curves of dimeric H_v1 channels can be fitted by a model that assumes an initial proton current I₁ flowing along the two monomeric units, while they are still operating separately; I₁ is gradually replaced by a slower and more potential-dependent current I₂ flowing when the two monomers start operating jointly under the control of the coiled-coil domain. Here too, p is assumed to increase over time with a rate constant k_p that doubles in passing from I₁ to I₂, with fit requiring three free parameters. Chord conductance yields erroneously high gating charges when fitted by the Boltzmann function, differently from slope conductance.

Zinc modulation of proton currents in a new voltage-gated proton channel suggests a mechanism of inhibition

The H_V1 voltage‐gated proton (H_V1) channel is a key component of the cellular proton extrusion machinery and is pivotal for charge compensation during the respiratory burst of phagocytes. The best‐described physiological inhibitor of H_V1 is Zn²⁺. Externally applied ZnCl₂ drastically reduces proton currents reportedly recorded in Homo sapiens, Rattus norvegicus, Mus musculus, Oryctolagus cuniculus, Rana esculenta, Helix aspersa, Ciona intestinalis, Coccolithus pelagicus, Emiliania huxleyi, Danio rerio, Helisoma trivolvis, and Lingulodinium polyedrum, but with considerable species variability. Here, we report the effects of Zn²⁺ and Cd²⁺ on H_V1 from Nicoletia phytophila, NpH_V1. We introduced mutations at potential Zn²⁺ coordination sites and measured Zn²⁺ inhibition in different extracellular pH, with Zn²⁺ concentrations up to 1000 μm. Zn²⁺ inhibition in NpH_V1 was quantified by the slowing of the activation time constant and a positive shift of the conductance–voltage curve. Replacing aspartate in the S3‐S4 loop with histidine (D145H) enhanced both the slowing of activation kinetics and the shift in the voltage–conductance curve, such that Zn²⁺ inhibition closely resembled that of the human channel. Histidine is much more effective than aspartate in coordinating Zn²⁺ in the S3‐S4 linker. A simple Hodgkin Huxley model of NpH_V1 suggests a decrease in the opening rate if it is inhibited by zinc or cadmium. Limiting slope measurements and high‐resolution clear native gel electrophoresis (hrCNE) confirmed that NpH_V1 functions as a dimer. The data support the hypothesis that zinc is coordinated in between the dimer instead of the monomer. Zinc coordination sites may be potential targets for drug development.

Assessing Structural Determinants of Zn2+ Binding to Human HV1 via Multiple MD Simulations

Voltage-gated proton channels (H_V1) are essential for various physiological tasks but are strongly inhibited by Zn²⁺ cations. Some determinants of Zn²⁺ binding have been elucidated experimentally and in computational studies. However, the results have always been interpreted under the assumption that Zn²⁺ binds to monomeric H_V1 despite evidence that H_V1 expresses as a dimer and that the dimer has a higher affinity for zinc than the monomer and experimental data that suggest coordination in the dimer interface. The results of former studies are also controversial, e.g., supporting either one single or two binding sites. Some structural determinants of the binding are still elusive. We performed a series of molecular dynamics simulations to address different structures of the human proton channel, the monomer and two plausible dimer conformations, to compare their respective potential to interact with and bind Zn²⁺ via the essential histidines. The series consisted of several copies of the system to generate independent trajectories and increase the significance compared to a single simulation. The amount of time simulated totals 29.9 μs for 126 simulations of systems comprising ∼59,000 to ∼187,000 atoms. Our approach confirms the existence of two binding sites in monomeric and dimeric human H_V1. The dimer interface is more efficient for attracting and binding Zn²⁺ via the essential histidines than the monomer or a dimer with the histidines in the periphery. The higher affinity is due to the residues in the dimer interface that create an attractive electrostatic potential funneling the zinc cations toward the binding sites.

Proton channel blockers inhibit Duox activity independent of Hv1 effects

The NADPH oxidase reaction produces protons. In the case of the NADPH oxidase, NOX2, activity depends on secretion of these protons and is inhibited by blockade of the voltage-gated proton channel (Hv1). Duox1 and Duox2 activities similarly produce intracellular protons but synthesize hydrogen peroxide directly instead of superoxide. Hv1 contributes to acid secretion in some epithelia that express Duox. To test the hypothesis that Duox activity is also sensitive to Hv1 channel blockers, Duox was assayed in the presence of either Zn²⁺ or 5-chloro-2-guanidinobenzimidazole (ClGBI). Both compounds inhibited Duox activity in normal human bronchial epithelial cells but with an IC50 over 10-fold higher than that reported for Hv1 (IC50 Zn²⁺ = 0.68 mM; IC50 ClGBI = 0.07–0.14 mM). Homogenized HEK293T cells expressing either Duox1 or Duox2 showed similar IC50 values for ClGBI suggesting these compounds inhibit the enzymes through alternate mechanisms independent of Hv1 proton secretion. Inclusion of superoxide dismutase did not restore Duox hydrogen peroxide synthesis. Addition of nigericin to eliminate any possible transmembrane pH gradients in intracellular membrane-localized Duox did not alter activity in HEK293T homogenates. Extracellular Zn²⁺ blocked intracellular Ca²⁺ increases needed for Duox activity. Together the data suggest that Duox enzyme activities in epithelia are inhibited by compounds that block Hv1 but inhibition occurs through Hv1-independent mechanisms and support the idea that Hv1 is not required for Duox activity.

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Hv1 proton channel inhibitors block Duox in differentiated bronchial epithelial cells.

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Zinc blocks Duox activity concurrently with reduction of calcium transients.

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ClGBI, an inhibitor of Hv1, blocks Duox activity in homogenates of cells lacking Hv1.

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In differentiated bronchial epithelia, Hv1 blockers did not alter intracellular pH.

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H⁺/K⁺ ATPase inhibition acidified cytoplasm but did not block Duox activity.

Hydrophobic gasket mutation produces gating pore currents in closed human voltage-gated proton channels

A large family of membrane proteins, voltage-gated ion channels, regulate a vast array of physiological functions in essentially all life forms. How these molecules sense membrane potential and respond by creating ionic conduction is incompletely understood. The voltage sensors of these channels contain a “hydrophobic gasket,” a ring of hydrophobic amino acids near the center of the membrane, separating internal and external aqueous solutions. Although voltage-gated proton channels, H_V1, resemble voltage-sensing domains of other channels, they differ fundamentally. On depolarization, H_V1 conducts protons, whereas other voltage sensors open a physically distinct pore. We identify Val¹⁰⁹, Phe¹⁵⁰, Val¹⁷⁷, and Val¹⁷⁸ as the hH_V1 hydrophobic gasket. Replacement with less hydrophobic amino acids accelerated channel opening and caused proton-selective leak through closed channels.

The hydrophobic gasket (HG), 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 “gating pore” when the channel opens. S4 movement may occur during gating of the human voltage-gated proton channel, hH_V1, but proton current flows through the same pore in open channels. Here, we replaced putative HG residues with less hydrophobic residues or acidic Asp. Substitution of individuals, pairs, or all 3 HG positions did not impair proton selectivity. Evidently, the HG does not act as a secondary selectivity filter. However, 2 unexpected functions of the HG in H_V1 were discovered. Mutating HG residues independently accelerated channel opening and compromised the closed state. Mutants exhibited open–closed gating, but strikingly, at negative voltages where “normal” gating produces a nonconducting closed state, the channel leaked protons. Closed-channel proton current was smaller than open-channel current and was inhibited by 10 μM Zn²⁺. Extreme hyperpolarization produced a deeper closed state through a weakly voltage-dependent transition. We functionally identify the HG as Val¹⁰⁹, Phe¹⁵⁰, Val¹⁷⁷, and Val¹⁷⁸, which play a critical and exclusive role in preventing H⁺ influx through closed channels. Molecular dynamics simulations revealed enhanced mobility of Arg²⁰⁸ in mutants exhibiting H⁺ leak. Mutation of HG residues produces gating pore currents reminiscent of several channelopathies.

Inhibition of CatSper and Hv1 Channels and NOX5 Enzyme Affect Progesterone-Induced Increase of Intracellular Calcium Concentration and ROS Generation in Human Sperm

BACKGROUND

Normal sperm function depends on appropriate intracellular calcium (Ca_i ²⁺) and reactive oxygen species (ROS) levels. Calcium activates NADPH oxidase-5 (NOX5) that leads to ROS generation. The calcium channel of sperm (CatSper) is activated by progesterone and intracellular alkalization. Herein, the interactive role of CatSper, Hv1 channels, and NOX5 enzyme on Ca_i ²⁺ and ROS generation in human sperm is investigated.

METHODS

The present laboratory in vitro study was carried out in the School of Medicine, Shiraz University of Medical Sciences (Shiraz, Iran) during 2016. Normal semen samples (n=15) were washed and diluted to 20×10⁶ sperm/mL. The diluted samples were divided into 16 groups containing Ham's F-10 (the control group), 2 µM NNC (CatSper inhibitor), 1 mM ZnCl2 (Hv1 inhibitor), 1 µM DPI (NOX5 inhibitor), NNC+Zn, NNC+DPI, and NNC+Zn+DPI. The other 8 groups were the same as the above except that they contained 1 µM progesterone. Cell viability and Ca_i ²⁺ were analyzed by flou-3 AM probe and PI staining, respectively, using flow cytometric method. ROS generation was assessed by chemiluminescence method. Statistical analysis was performed using the one-way ANOVA followed by Tukey's test. P values <0.05 were considered statistically significant.

RESULTS

Progesterone increased Ca_i ²⁺ and ROS generation. The addition of NNC, Zn, or NNC+Zn significantly decreased Ca_i ²⁺ in the control and progesterone containing groups. Progesterone-induced ROS generation was decreased significantly in all groups containing NNC, Zn, or DPI and reached to the control level when DPI was added to NNC or Zn.

CONCLUSION

There is a functional relationship between CatSper and Hv1 channels in increasing Ca_i ²⁺+. The activity of CatSper and Hv1 channels are required for progesterone-induced ROS generation by NOX5 enzyme.

Molecular mechanisms of coupling to voltage sensors in voltage-evoked cellular signals

The voltage sensor domain (VSD) has long been studied as a unique domain intrinsic to voltage-gated ion channels (VGICs). Within VGICs, the VSD is tightly coupled to the pore-gate domain (PGD) in diverse ways suitable for its specific function in each physiological context, including action potential generation, muscle contraction and relaxation, hormone and neurotransmitter secretion, and cardiac pacemaking. However, some VSD-containing proteins lack a PGD. Voltage-sensing phosphatase contains a cytoplasmic phosphoinositide phosphatase with similarity to phosphatase and tensin homolog (PTEN). H_v1, a voltage-gated proton channel, also lacks a PGD. Within H_v1, the VSD operates as a voltage sensor, gate, and pore for both proton sensing and permeation. H_v1 has a C-terminal coiled coil that mediates dimerization for cooperative gating. Recent progress in the structural biology of VGICs and VSD proteins provides insights into the principles of VSD coupling conserved among these proteins as well as the hierarchy of protein organization for voltage-evoked cell signaling.

Optical consequences of a genetically-encoded voltage indicator with a pH sensitive fluorescent protein

Genetically-Encoded Voltage Indicators (GEVIs) are capable of converting changes in membrane potential into an optical signal. Here, we focus on recent insights into the mechanism of ArcLight-type probes and the consequences of utilizing a pH-dependent Fluorescent Protein (FP). A negative charge on the exterior of the β-can of the FP combined with a pH-sensitive FP enables voltage-dependent conformational changes to affect the fluorescence of the probe. This hypothesis implies that interaction/dimerization of the FP creates a microenvironment for the probe that is altered via conformational changes. This mechanism explains why a pH sensitive FP with a negative charge on the outside of the β-can is needed, but also suggests that pH could affect the optical signal as well. To better understand the effects of pH on the voltage-dependent signal of ArcLight, the intracellular pH (pHi) was tested at pH 6.8, 7.2, or 7.8. The resting fluorescence of ArcLight gets brighter as the pHi increases, yet only pH 7.8 significantly affected the ΔF/F. ArcLight could also simultaneously report voltage and pH changes during the acidification of a neuron firing multiple action potentials revealing different buffering capacities of the soma versus the processes of the cell.

Coupling between an electrostatic network and the Zn2+ binding site modulates Hv1 activation

The voltage sensor (VS) domain in Hv1 proton channels mediates a voltage-dependent and H+-selective "aqueous" conductance (GAQ) that is potently modulated by extracellular Zn2+ Although two conserved His residues are required for Zn2+ effects on GAQ gating, the atomic structure of the Zn2+ coordination site and mechanism by which extracellular Zn2+ stabilizes a closed-state conformation remain unknown. Here we use His mutagenesis to identify residues that increase Zn2+ potency and are therefore likely to participate in first solvation shell interactions with Zn2+ Experimental Zn2+-mapping data were then used to constrain the structure of a new resting-state Hv1 model (Hv1 F). Molecular dynamics (MD) simulations show how protein and water atoms directly contribute to octahedral Zn2+ coordination spheres in Zn2+-bound and -unbound Hv1 F models. During MD simulations, we observed correlated movements of Zn2+-interacting side chains and residues in a highly conserved intracellular Coulombic network (ICN) that contains highly conserved Arg "gating charges" in S4 as well as acidic "counter-charges" in S2 and S3 and is known to control VS activation, suggesting that occupancy of the extracellular Zn2+ site is conformationally coupled to reorganization of the ICN. To test this hypothesis, we neutralized an ICN Glu residue (E153) and show that in addition to shifting GAQ activation to more negative voltages, E153A also decreases Zn2+ potency. We speculate that extracellular gating-modifier toxins and other ligands may use a generally similar long-range conformational coupling mechanism to modulate VS activation in related ion channel proteins.

Exotic properties of a voltage-gated proton channel from the snail Helisoma trivolvis

Voltage-gated proton channels, HV1, were first reported in Helix aspersa snail neurons. These H+ channels open very rapidly, two to three orders of magnitude faster than mammalian HV1. Here we identify an HV1 gene in the snail Helisoma trivolvis and verify protein level expression by Western blotting of H. trivolvis brain lysate. Expressed in mammalian cells, HtHV1 currents in most respects resemble those described in other snails, including rapid activation, 476 times faster than hHV1 (human) at pHo 7, between 50 and 90 mV. In contrast to most HV1, activation of HtHV1 is exponential, suggesting first-order kinetics. However, the large gating charge of ∼5.5 e0 suggests that HtHV1 functions as a dimer, evidently with highly cooperative gating. HtHV1 opening is exquisitely sensitive to pHo, whereas closing is nearly independent of pHo Zn2+ and Cd2+ inhibit HtHV1 currents in the micromolar range, slowing activation, shifting the proton conductance-voltage (gH-V) relationship to more positive potentials, and lowering the maximum conductance. This is consistent with HtHV1 possessing three of the four amino acids that coordinate Zn2+ in mammalian HV1. All known HV1 exhibit ΔpH-dependent gating that results in a 40-mV shift of the gH-V relationship for a unit change in either pHo or pHi This property is crucial for all the functions of HV1 in many species and numerous human cells. The HtHV1 channel exhibits normal or supernormal pHo dependence, but weak pHi dependence. Under favorable conditions, this might result in the HtHV1 channel conducting inward currents and perhaps mediating a proton action potential. The anomalous ΔpH-dependent gating of HtHV1 channels suggests a structural basis for this important property, which is further explored in this issue (Cherny et al. 2018. J. Gen. Physiol. https://doi.org/10.1085/jgp.201711968).

Histidine168 is crucial for ΔpH-dependent gating of the human voltage-gated proton channel, hHV1

Voltage-gated proton channels open appropriately in myriad physiological situations because their gating is powerfully modulated by both pH_o and pH_i. Cherny et al. serendipitously identify a histidine at the inner end of the S3 helix that is required for the response to pH_i.

We recently identified a voltage-gated proton channel gene in the snail Helisoma trivolvis, HtH_V1, and determined its electrophysiological properties. Consistent with early studies of proton currents in snail neurons, HtH_V1 opens rapidly, but it unexpectedly exhibits uniquely defective sensitivity to intracellular pH (pH_i). The H⁺ conductance (g_H)-V relationship in the voltage-gated proton channel (H_V1) from other species shifts 40 mV when either pH_i or pH_o (extracellular pH) is changed by 1 unit. This property, called ΔpH-dependent gating, is crucial to the functions of H_V1 in many species and in numerous human tissues. The HtH_V1 channel exhibits normal pH_o dependence but anomalously weak pH_i dependence. In this study, we show that a single point mutation in human hH_V1—changing His¹⁶⁸ to Gln¹⁶⁸, the corresponding residue in HtH_V1—compromises the pH_i dependence of gating in the human channel so that it recapitulates the HtH_V1 response. This location was previously identified as a contributor to the rapid gating kinetics of H_V1 in Strongylocentrotus purpuratus. His¹⁶⁸ mutation in human H_V1 accelerates activation but accounts for only a fraction of the species difference. H168Q, H168S, or H168T mutants exhibit normal pH_o dependence, but changing pH_i shifts the g_H-V relationship on average by <20 mV/unit. Thus, His¹⁶⁸ is critical to pH_i sensing in hH_V1. His¹⁶⁸, located at the inner end of the pore on the S3 transmembrane helix, is the first residue identified in H_V1 that significantly impairs pH sensing when mutated. Because pH_o dependence remains intact, the selective erosion of pH_i dependence supports the idea that there are distinct internal and external pH sensors. Although His¹⁶⁸ may itself be a pH_i sensor, the converse mutation, Q229H, does not normalize the pH_i sensitivity of the HtH_V1 channel. We hypothesize that the imidazole group of His¹⁶⁸ interacts with nearby Phe¹⁶⁵ or other parts of hH_V1 to transduce pH_i into shifts of voltage-dependent gating.

Surface pH changes suggest a role for H+/OH- channels in salinity response of Chara australis

To understand salt stress, the full impact of salinity on plant cell physiology has to be resolved. Electrical measurements suggest that salinity inhibits the proton pump and opens putative H+/OH- channels all over the cell surface of salt sensitive Chara australis (Beilby and Al Khazaaly 2009; Al Khazaaly and Beilby 2012). The channels open transiently at first, causing a characteristic noise in membrane potential difference (PD), and after longer exposure remain open with a typical current-voltage (I/V) profile, both abolished by the addition of 1 mM ZnCl2, the main known blocker of animal H+ channels. The cells were imaged with confocal microscopy, using fluorescein isothiocyanate (FITC) coupled to dextran 70 to illuminate the pH changes outside the cell wall in artificial fresh water (AFW) and in saline medium. In the early saline exposure, we observed alkaline patches (bright fluorescent spots) appearing transiently in random spatial distribution. After longer exposure, some of the spots became fixed in space. Saline also abolished or diminished the pH banding pattern observed in the untreated control cells. ZnCl2 suppressed the alkaline spot formation in saline and the pH banding pattern in AFW. The osmotic component of the saline stress did not produce transient bright spots or affect banding. The displacement of H+ from the cell wall charges, the H+/OH- channel conductance/density, and self-organization are discussed. No homologies to animal H+ channels were found. Salinity activation of the H+/OH- channels might contribute to saline response in roots of land plants and leaves of aquatic angiosperms.

Voltage and pH sensing by the voltage-gated proton channel, HV1

Voltage-gated proton channels are unique ion channels, membrane proteins that allow protons but no other ions to cross cell membranes. They are found in diverse species, from unicellular marine life to humans. In all cells, their function requires that they open and conduct current only under certain conditions, typically when the electrochemical gradient for protons is outwards. Consequently, these proteins behave like rectifiers, conducting protons out of cells. Their activity has electrical consequences and also changes the pH on both sides of the membrane. Here we summarize what is known about the way these proteins sense the membrane potential and the pH inside and outside the cell. Currently, it is hypothesized that membrane potential is sensed by permanently charged arginines (with very high pKa) within the protein, which results in parts of the protein moving to produce a conduction pathway. The mechanism of pH sensing appears to involve titratable side chains of particular amino acids. For this purpose their pKa needs to be within the operational pH range. We propose a 'counter-charge' model for pH sensing in which electrostatic interactions within the protein are selectively disrupted by protonation of internally or externally accessible groups.

Factors and pathways involved in capacitation: how are they regulated?

In mammals, fertilization occurs via a comprehensive progression of events. Freshly ejaculated sperm have yet to acquire progressive motility or fertilization ability. They must first undergo a series of biochemical and physiological changes, collectively known as capacitation. Capacitation is a significant prerequisite to fertilization. During the process of capacitation, changes in membrane properties, intracellular ion concentration and the activities of enzymes, together with other protein modifications, induce multiple signaling events and pathways in defined media in vitro or in the female reproductive tract in vivo. These, in turn, stimulate the acrosome reaction and prepare spermatozoa for penetration of the egg zona pellucida prior to fertilization. In the present review, we conclude all mainstream factors and pathways regulate capacitation and highlight their crosstalk. We also summarize the relationship between capacitation and assisted reproductive technology or human disease. In the end, we sum up the open questions and future avenues in this field.

Insight into the Role of the Hv1 C-Terminal Domain in Dimer Stabilization

The voltage-gated proton-selective channel (Hv1) conducts protons in response to changes in membrane potential. The Hv1 protein forms dimers in the membrane. Crystal structures of Hv1 channels have revealed that the primary contacts between the two monomers are in the C-terminal domain (CTD), which forms a coiled-coil structure. The role of Hv1-CTD in channel assembly and activity is not fully understood. Here, molecular dynamics (MD) simulations of full-length and truncated CTD models of human and mouse Hv1 channels reveal a strong contribution of the CTD to the packing of the transmembrane domains. Simulations of the CTD models highlight four fundamental interactions of the key residues contributing to dimer stability. These include salt bridges, hydrophobic interactions, hydrogen bonds, and a disulfide bond across the dimer interface. At neutral pH, salt-bridge interactions increase dimer stability and the dimer becomes less stable at acidic pH. Hydrophobic core packing of the heptad pattern is important for stability, as shown by favorable nonpolar binding free energies rather than by electrostatic components. Moreover, free-energy calculations indicate that a more uniform hydrophobic core in the coiled-coil structure of the Hv1-NIN, a channel carrying the triple mutation M234N-N235I-V236N, leads to an increase in dimer stability with respect to the wild-type. A Cys disulfide bond has a strong impact on dimer stability by holding the dimer together and facilitating the interactions described above. These results are consistent with dissociative temperatures and energy barriers of dimer dissociation obtained from the temperature-accelerated MD.

Induction of divalent cation permeability by heterologous expression of a voltage sensor domain

The voltage sensor domain (VSD) is a protein domain that confers sensitivity to membrane potential in voltage-gated ion channels as well as the voltage-sensing phosphatase. Although VSDs have long been considered to function as regulatory units acting on adjacent effectors, recent studies have revealed the existence of direct ion permeation paths in some mutated VSDs and in the voltage-gated proton channel. In this study, we show that calcium currents are evoked upon membrane hyperpolarization in cells expressing a VSD derived from an ascidian voltage-gated ion channel superfamily. Unlike the previously reported omega-pore in the Shaker K⁺ channel and rNav1.4, mutations are not required. From electrophysiological experiments in heterologous expression systems, we found that the conductance is directly mediated by the VSD itself and is carried by both monovalent and divalent cations. This is the first report of divalent cation permeation through a VSD-like structure.

Insights into the structure and function of HV1 from a meta-analysis of mutation studies

The voltage-gated proton channel (HV1) is a widely distributed, proton-specific ion channel with unique properties. Since 2006, when genes for HV1 were identified, a vast array of mutations have been generated and characterized. Accessing this potentially useful resource is hindered, however, by the sheer number of mutations and interspecies differences in amino acid numbering. This review organizes all existing information in a logical manner to allow swift identification of studies that have characterized any particular mutation. Although much can be gained from this meta-analysis, important questions about the inner workings of HV1 await future revelation.

The intimate and controversial relationship between voltage-gated proton channels and the phagocyte NADPH oxidase

One of the most fascinating and exciting periods in my scientific career entailed dissecting the symbiotic relationship between two membrane transporters, the Nicotinamide adenine dinucleotide phosphate reduced form (NADPH) oxidase complex and voltage-gated proton channels (HV 1). By the time I entered this field, there had already been substantial progress toward understanding NADPH oxidase, but HV 1 were known only to a tiny handful of cognoscenti around the world. Having identified the first proton currents in mammalian cells in 1991, I needed to find a clear function for these molecules if the work was to become fundable. The then-recent discoveries of Henderson, Chappell, and colleagues in 1987-1988 that led them to hypothesize interactions of both molecules during the respiratory burst of phagocytes provided an excellent opportunity. In a nutshell, both transporters function by moving electrical charge across the membrane: NADPH oxidase moves electrons and HV 1 moves protons. The consequences of electrogenic NADPH oxidase activity on both membrane potential and pH strongly self-limit this enzyme. Fortunately, both consequences specifically activate HV 1, and HV 1 activity counteracts both consequences, a kind of yin-yang relationship. Notwithstanding a decade starting in 1995 when many believed the opposite, these are two separate molecules that function independently despite their being functionally interdependent in phagocytes. The relationship between NADPH oxidase and HV 1 has become a paradigm that somewhat surprisingly has now extended well beyond the phagocyte NADPH oxidase - an industrial strength producer of reactive oxygen species (ROS) - to myriad other cells that produce orders of magnitude less ROS for signaling purposes. These cells with their seven NADPH oxidase (NOX) isoforms provide a vast realm of mechanistic obscurity that will occupy future studies for years to come.

The Hv1 proton channel responds to mechanical stimuli

The voltage-gated proton channel, Hv1, is expressed in tissues throughout the body and plays important roles in pH homeostasis and regulation of NADPH oxidase. Hv1 operates in membrane compartments that experience strong mechanical forces under physiological or pathological conditions. In microglia, for example, Hv1 activity is potentiated by cell swelling and causes an increase in brain damage after stroke. The channel complex consists of two proton-permeable voltage-sensing domains (VSDs) linked by a cytoplasmic coiled-coil domain. Here, we report that these VSDs directly respond to mechanical stimuli. We find that membrane stretch facilitates Hv1 channel opening by increasing the rate of activation and shifting the steady-state activation curve to less depolarized potentials. In the presence of a transmembrane pH gradient, membrane stretch alone opens the channel without the need for strong depolarizations. The effect of membrane stretch persists for several minutes after the mechanical stimulus is turned off, suggesting that the channel switches to a "facilitated" mode in which opening occurs more readily and then slowly reverts to the normal mode observed in the absence of membrane stretch. Conductance simulations with a six-state model recapitulate all the features of the channel's response to mechanical stimulation. Hv1 mechanosensitivity thus provides a mechanistic link between channel activation in microglia and brain damage after stroke.

Zinc as Allosteric Ion Channel Modulator: Ionotropic Receptors as Metalloproteins

Zinc is an essential metal to life. This transition metal is a structural component of many proteins and is actively involved in the catalytic activity of cell enzymes. In either case, these zinc-containing proteins are metalloproteins. However, the amino acid residues that serve as ligands for metal coordination are not necessarily the same in structural proteins compared to enzymes. While crystals of structural proteins that bind zinc reveal a higher preference for cysteine sulfhydryls rather than histidine imidazole rings, catalytic enzymes reveal the opposite, i.e., a greater preference for the histidines over cysteines for catalysis, plus the influence of carboxylic acids. Based on this paradigm, we reviewed the putative ligands of zinc in ionotropic receptors, where zinc has been described as an allosteric modulator of channel receptors. Although these receptors do not strictly qualify as metalloproteins since they do not normally bind zinc in structural domains, they do transitorily bind zinc at allosteric sites, modifying transiently the receptor channel's ion permeability. The present contribution summarizes current information showing that zinc allosteric modulation of receptor channels occurs by the preferential metal coordination to imidazole rings as well as to the sulfhydryl groups of cysteine in addition to the carboxyl group of acid residues, as with enzymes and catalysis. It is remarkable that most channels, either voltage-sensitive or transmitter-gated receptor channels, are susceptible to zinc modulation either as positive or negative regulators.

Direct Interaction between the Voltage Sensors Produces Cooperative Sustained Deactivation in Voltage-gated H+ Channel Dimers

The voltage-gated H(+) channel (Hv) is a voltage sensor domain-like protein consisting of four transmembrane segments (S1-S4). The native Hv structure is a homodimer, with the two channel subunits functioning cooperatively. Here we show that the two voltage sensor S4 helices within the dimer directly cooperate via a π-stacking interaction between Trp residues at the middle of each segment. Scanning mutagenesis showed that Trp situated around the original position provides the slow gating kinetics characteristic of the dimer's cooperativity. Analyses of the Trp mutation on the dimeric and monomeric channel backgrounds and analyses with tandem channel constructs suggested that the two Trp residues within the dimer are functionally coupled during Hv deactivation but are less so during activation. Molecular dynamics simulation also showed direct π-stacking of the two Trp residues. These results provide new insight into the cooperative function of voltage-gated channels, where adjacent voltage sensor helices make direct physical contact and work as a single unit according to the gating process.

Gating mechanisms of voltage-gated proton channels

Hv1 is a voltage-gated proton-selective channel that plays critical parts in host defense, sperm motility, and cancer progression. Hv1 contains a conserved voltage-sensor domain (VSD) that is shared by a large family of voltage-gated ion channels, but it lacks a pore domain. Voltage sensitivity and proton conductivity are conferred by a unitary VSD that consists of four transmembrane helices. The architecture of Hv1 differs from that of cation channels that form a pore in the center among multiple subunits (as in most cation channels) or homologous repeats (as in voltage-gated sodium and calcium channels). Hv1 forms a dimer in which a cytoplasmic coiled coil underpins the two protomers and forms a single, long helix that is contiguous with S4, the transmembrane voltage-sensing segment. The closed-state structure of Hv1 was recently solved using X-ray crystallography. In this article, we discuss the gating mechanism of Hv1 and focus on cooperativity within dimers and their sensitivity to metal ions.

The voltage-gated proton channel Hv1 is expressed in pancreatic islet β-cells and regulates insulin secretion

The voltage-gated proton channel Hv1 is a potent acid extruder that participates in the extrusion of the intracellular acid. Here, we showed for the first time, Hv1 is highly expressed in mouse and human pancreatic islet β-cells, as well as β-cell lines. Imaging studies demonstrated that Hv1 resides in insulin-containing granules in β-cells. Knockdown of Hv1 with RNA interference significantly reduces glucose- and K(+)-induced insulin secretion in isolated islets and INS-1 (832/13) β-cells and has an impairment on glucose- and K(+)-induced intracellular Ca(2+) homeostasis. Our data demonstrated that the expression of Hv1 in pancreatic islet β-cells regulates insulin secretion through regulating Ca(2+) homeostasis.

Interrogation of the intersubunit interface of the open Hv1 proton channel with a probe of allosteric coupling

The Hv1 voltage-gated proton channel is a dimeric complex consisting of two voltage-sensing domains (VSDs), each containing a gated proton permeation pathway. Dimerization is controlled by a cytoplasmic coiled-coil domain. The transitions from the closed to the open state in the two VSDs are known to occur cooperatively; however, the underlying mechanism is poorly understood. Intersubunit interfaces play a critical role in allosteric processes; but, such interfaces have not been determined in the open Hv1 channel. Here we show that 2-guanidinothiazole derivatives block the two Hv1 VSDs in a cooperative way, and use one of the compounds as a probe of allosteric coupling between open subunits. We find that the extracellular ends of the first transmembrane segments of the VSDs form the intersubunit interface that mediates coupling between binding sites, while the coiled-coil domain does not directly participate in the process. We also find strong evidence that the channel’s proton selectivity filter controls blocker binding cooperativity.

Voltage-gated proton (H(v)1) channels, a singular voltage sensing domain

The main role of voltage-gated proton channels (Hv1) is to extrude protons from the intracellular milieu when, mediated by different cellular processes, the H(+) concentration increases. Hv1 are exquisitely selective for protons and their structure is homologous to the voltage sensing domain (VSD) of other voltage-gated ion channels like sodium, potassium, and calcium channels. In clear contrast to the classical voltage-dependent channels, Hv1 lacks a pore domain and thus permeation necessarily occurs through the voltage sensing domain. Hv1 channels are activated by depolarizing voltages, and increases in internal proton concentration. It has been proposed that local conformational changes of the transmembrane segment S4, driven by depolarization, trigger the molecular rearrangements that open Hv1. However, it is still unclear how the electromechanical coupling is achieved between the VSD and the potential pore, allowing the proton flux from the intracellular to the extracellular side. Here we provide a revised view of voltage activation in Hv1 channels, offering a comparative scenario with other voltage sensing channels domains.

Voltage-Gated Proton Channels as Novel Drug Targets: From NADPH Oxidase Regulation to Sperm Biology

SIGNIFICANCE

Voltage-gated proton channels are increasingly implicated in cellular proton homeostasis. Proton currents were originally identified in snail neurons less than 40 years ago, and subsequently shown to play an important auxiliary role in the functioning of reactive oxygen species (ROS)-generating nicotinamide adenine dinucleotide phosphate (NADPH) oxidases. Molecular identification of voltage-gated proton channels was achieved less than 10 years ago. Interestingly, so far, only one gene coding for voltage-gated proton channels has been identified, namely hydrogen voltage-gated channel 1 (HVCN1), which codes for the HV1 proton channel protein. Over the last years, the first picture of putative physiological functions of HV1 has been emerging.

RECENT ADVANCES

The best-studied role remains charge and pH compensation during the respiratory burst of the phagocyte NADPH oxidase (NOX). Strong evidence for a role of HV1 is also emerging in sperm biology, but the relationship with the sperm NOX5 remains unclear. Probably in many instances, HV1 functions independently of NOX: for example in snail neurons, basophils, osteoclasts, and cancer cells.

CRITICAL ISSUES

Generally, ion channels are good drug targets; however, this feature has so far not been exploited for HV1, and hitherto no inhibitors compatible with clinical use exist. However, there are emerging indications for HV1 inhibitors, ranging from diseases with a strong activation of the phagocyte NOX (e.g., stroke) to infertility, osteoporosis, and cancer.

FUTURE DIRECTIONS

Clinically useful HV1-active drugs should be developed and might become interesting drugs of the future.

Hv1 proton channel opening is preceded by a voltage-independent transition

The voltage sensing domain (VSD) of the voltage-gated proton channel Hv1 mediates a H(+)-selective conductance that is coordinately controlled by the membrane potential (V) and the transmembrane pH gradient (ΔpH). Allosteric control of Hv1 channel opening by ΔpH (V-ΔpH coupling) is manifested by a characteristic shift of approximately 40 mV per ΔpH unit in the activation. To further understand the mechanism for V-ΔpH coupling in Hv1, H(+) current kinetics of activation and deactivation in excised membrane patches were analyzed as a function of the membrane potential and the pH in the intracellular side of the membrane (pHI). In this study, it is shown for the first time to our knowledge that the opening of Hv1 is preceded by a voltage-independent transition. A similar process has been proposed to constitute the step involving coupling between the voltage-sensing and pore domains in tetrameric voltage-gated channels. However, for Hv1, the VSD functions as both the voltage sensor and the conduction pathway, suggesting that the voltage independent transition is intrinsic to the voltage-sensing domain. Therefore, this article proposes that the underlying mechanism for the activation of Hv1 involves a process similar to VSD relaxation, a process previously described for voltage-gated channels and voltage-controlled enzymes. Finally, deactivation seemingly occurs as a strictly voltage dependent process, implying that the kinetic event leading to opening of the proton conductance are different than those involved in the closing. Thus, from this work it is proposed that Hv1 activity displays hysteresis.

Proton channel models filling the gap between experimental data and the structural rationale

Voltage-gated proton channels are integral membrane proteins with the capacity to permeate elementary particles in a voltage and pH dependent manner. These proteins have been found in several species and are involved in various physiological processes. Although their primary topology is known, lack of details regarding their structures in the open conformation has limited analyses toward a deeper understanding of the molecular determinants of their function and regulation. Consequently, the function-structure relationships have been inferred based on homology models. In the present work, we review the existing proton channel models, their assumptions, predictions and the experimental facts that support them. Modeling proton channels is not a trivial task due to the lack of a close homolog template. Hence, there are important differences between published models. This work attempts to critically review existing proton channel models toward the aim of contributing to a better understanding of the structural features of these proteins.

The Voltage-Gated Proton Channel: A Riddle, Wrapped in a Mystery, inside an Enigma

The main properties of the voltage-gated proton channel (HV1) are described in this review, along with what is known about how the channel protein structure accomplishes its functions. Just as protons are unique among ions, proton channels are unique among ion channels. Their four transmembrane helices sense voltage and the pH gradient and conduct protons exclusively. Selectivity is achieved by the unique ability of H3O(+) to protonate an Asp-Arg salt bridge. Pathognomonic sensitivity of gating to the pH gradient ensures HV1 channel opening only when acid extrusion will result, which is crucial to most of its biological functions. An exception occurs in dinoflagellates in which influx of H(+) through HV1 triggers the bioluminescent flash. Pharmacological interventions that promise to ameliorate cancer, asthma, brain damage in ischemic stroke, Alzheimer's disease, autoimmune diseases, and numerous other conditions await future progress.