Scorpion toxin inhibits the voltage-gated proton channel using a Zn2+ -like long-range conformational coupling mechanism

Szabo B, Kothalawala RC, Szekelyhidi V, Nagy P, Varga Z, Panyi G, Zakany F. Inhibition of the HV1 voltage-gated proton channel compromises the viability of human polarized macrophages in a polarization- and ceramide-dependent manner

The human voltage-gated proton channel (HV1) provides an efficient proton extrusion pathway from the cytoplasm contributing to the intracellular pH regulation and the oxidative burst. Although its pharmacological inhibition was previously shown to induce cell death in various cell types, no such effects have been examined in polarized macrophages albeit HV1 was suggested to play important roles in these cells. This study highlights that 5-chloro-2-guanidinobenzimidazole (ClGBI), the most widely applied HV1 inhibitor, reduces the viability of human THP-1-derived polarized macrophages at biologically relevant doses with M1 macrophages being the most, and M2 cells the least sensitive to this compound. ClGBI may exert this effect principally by blocking HV1 since the sensitivity of polarized macrophages correlates well with their HV1 expression levels; inhibitors of other macrophage ion channels that may be susceptible for off-target ClGBI effects cause no viability reductions; and Zn2+, another non-specific HV1 blocker, exerts similar effects. As a potential mechanism behind the ClGBI-induced cell death, we identify a complex pH dysregulation involving acidification of the cytoplasm and alkalinization of the lysosomes, which eventually result in membrane ceramide accumulation. Furthermore, ClGBI effects are alleviated by ARC39, a selective acid sphingomyelinase inhibitor supporting the unequivocal significance of ceramide accumulation in the process. Altogether, our results suggest that HV1 inhibition leads to cellular toxicity in polarized macrophages in a polarization-dependent manner, which occurs due to a pH dysregulation and concomitant ceramide overproduction mainly depending on the activity of acid sphingomyelinase. The reduced macrophage viability and plausible concomitant changes in homeostatic M1-M2 balance could contribute to both the therapeutic and potential side effects of HV1 inhibitors that show great promise in the treatment of neuroinflammation and malignant diseases.

The Emerging Role of Microglial Hv1 as a Target for Immunomodulation in Myelin Repair

In the central nervous system (CNS), the myelin sheath ensures efficient interconnection between neurons and contributes to the regulation of the proper function of neuronal networks. The maintenance of myelin and the well-organized subtle process of myelin plasticity requires cooperation among myelin-forming cells, glial cells, and neural networks. The process of cooperation is fragile, and the balance is highly susceptible to disruption by microenvironment influences. Reactive microglia play a critical and complicated role in the demyelination and remyelination process. Recent studies have shown that the voltage-gated proton channel Hv1 is selectively expressed in microglia in CNS, which regulates intracellular pH and is involved in the production of reactive oxygen species, underlying multifaceted roles in maintaining microglia function. This paper begins by examining the molecular mechanisms of demyelination and emphasizes the crucial role of the microenvironment in demyelination. It focuses specifically on the role of Hv1 in myelin repair and its therapeutic potential in CNS demyelinating diseases.

Molecular action mechanisms of two novel and selective calcium release-activated calcium channel antagonists

The prototypical calcium release-activated calcium (CRAC) channel, composed of STIM1 and Orai1, is a sought-after drug target for treating autoimmune disorders. Herein, we identified two novel and selective CRAC channel inhibitors, the indole-like compound C63368 and pyrazole core-containing compound C79413, potently and reversibly inhibiting the CRAC channel with low micromolar IC50s and sparing various off-target ion channels. These two compounds did not inhibit STIM1 activation or its coupling with Orai1, nor did they affect the channel's calcium-dependent fast inactivation. Instead, they directly acted on the Orai1 protein, with the channel's pore geometry profoundly affecting their potencies. In vitro, C63368 and C79413 effectively inhibited Jurkat cell proliferation and cytokines production in human T lymphocytes. Intragastric administration of C63368 and C79413 to mice yielded great therapeutic benefits in psoriasis and colitis animal models of autoimmune disorders, reducing serum cytokines production and significantly relieving pathological symptoms. It's worth noting, that this study provided the first insight into the characterization and mechanistic investigation of an indole-like CRAC channel antagonist. Altogether, the identification of these two highly selective CRAC channel antagonists, coupled with the elucidation of their action mechanisms, not only provides valuable template molecules but also offers profound insights for drug development targeting the CRAC channel.

A new polymodal gating model of the proton-activated chloride channel

The proton-activated chloride (PAC) channel plays critical roles in ischemic neuron death, but its activation mechanisms remain elusive. Here, we investigated the gating of PAC channels using its novel bifunctional modulator C77304. C77304 acted as a weak activator of the PAC channel, causing moderate activation by acting on its proton gating. However, at higher concentrations, C77304 acted as a weak inhibitor, suppressing channel activity. This dual function was achieved by interacting with 2 modulatory sites of the channel, each with different affinities and dependencies on the channel's state. Moreover, we discovered a protonation-independent voltage activation of the PAC channel that appears to operate through an ion-flux gating mechanism. Through scanning-mutagenesis and molecular dynamics simulation, we confirmed that E181, E257, and E261 in the human PAC channel serve as primary proton sensors, as their alanine mutations eliminated the channel's proton gating while sparing the voltage-dependent gating. This proton-sensing mechanism was conserved among orthologous PAC channels from different species. Collectively, our data unveils the polymodal gating and proton-sensing mechanisms in the PAC channel that may inspire potential drug development.

Discovery and validation of new Hv1 proton channel inhibitors with onco-therapeutic potential

The voltage-gated hydrogen channel Hv1 encoded in humans by the HVCN1 gene is a highly selective proton channel that allows large fluxes of protons across biological membranes. Hv1 form functional dimers of four transmembrane spanning proteins resembling the voltage sensing domain of potassium channels. Each subunit is highly selective for protons and is controlled by changes in the transmembrane voltage and pH gradient. Hv1 is most expressed in phagocytic cells where it sustains NADPH oxidase-dependent bactericidal function and was reported to facilitate antibody production by B cells and to promote the maturation and motility of spermatocytes. Hv1 contributes to neuroinflammation following brain damage and favors cancer progression possibly by extruding protons generated during aerobic glycolysis of cancer cells. Lack of specific Hv1 inhibitors has hampered translation of this knowledge to treat immune, fertility, or malignancy diseases. In this study, we show that the genetic deletion of Hv1 delays tumor development in a mouse model of granulocytic sarcoma and report the discovery and characterization of two novel bioavailable inhibitors of Hv1 channels that we validate by orthogonal assays and electrophysiological recordings.

Role of voltage-gated proton channel (Hv1) in cancer biology

The acid-base characteristics of tumor cells and the other elements that compose the tumor microenvironment have been topics of scientific interest in oncological research. There is much evidence confirming that pH conditions are maintained by changes in the patterns of expression of certain proton transporters. In the past decade, the voltage-gated proton channel (Hv1) has been added to this list and is increasingly being recognized as a target with onco-therapeutic potential. The Hv1 channel is key to proton extrusion for maintaining a balanced cytosolic pH. This protein-channel is expressed in a myriad of tissues and cell lineages whose functions vary from producing bioluminescence in dinoflagellates to alkalizing spermatozoa cytoplasm for reproduction, and regulating the respiratory burst for immune system response. It is no wonder that in acidic environments such as the tumor microenvironment, an exacerbated expression and function of this channel has been reported. Indeed, multiple studies have revealed a strong relationship between pH balance, cancer development, and the overexpression of the Hv1 channel, being proposed as a marker for malignancy in cancer. In this review, we present data that supports the idea that the Hv1 channel plays a significant role in cancer by maintaining pH conditions that favor the development of malignancy features in solid tumor models. With the antecedents presented in this bibliographic report, we want to strengthen the idea that the Hv1 proton channel is an excellent therapeutic strategy to counter the development of solid tumors.

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 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.

Functions and Mechanisms of the Voltage-Gated Proton Channel Hv1 in Brain and Spinal Cord Injury

The voltage-gated proton channel Hv1 is a newly discovered ion channel that is highly conserved among species. It is known that Hv1 is not only expressed in peripheral immune cells but also one of the major ion channels expressed in tissue-resident microglia of the central nervous systems (CNS). One key role for Hv1 is its interaction with NADPH oxidase 2 (NOX2) to regulate reactive oxygen species (ROS) and cytosolic pH. Emerging data suggest that excessive ROS production increases and requires proton currents through Hv1 in the injured CNS, and manipulations that ablate Hv1 expression or induce loss of function may provide neuroprotection in CNS injury models including stroke, traumatic brain injury, and spinal cord injury. Recent data demonstrating microglial Hv1-mediated signaling in the pathophysiology of the CNS injury further supports the idea that Hv1 channel may function as a key mechanism in posttraumatic neuroinflammation and neurodegeneration. In this review, we summarize the main findings of Hv1, including its expression pattern, cellular mechanism, role in aging, and animal models of CNS injury and disease pathology. We also discuss the potential of Hv1 as a therapeutic target for CNS injury.

Voltage-gated proton channels from fungi highlight role of peripheral regions in channel activation

Here, we report the identification and characterization of the first proton channels from fungi. The fungal proteins are related to animal voltage-gated Hv channels and are conserved in both higher and lower fungi. Channels from Basidiomycota and Ascomycota appear to be evolutionally and functionally distinct. Representatives from the two phyla share several features with their animal counterparts, including structural organization and strong proton selectivity, but they differ from each other and from animal Hvs in terms of voltage range of activation, pharmacology, and pH sensitivity. The activation gate of Hv channels is believed to be contained within the transmembrane core of the protein and little is known about contributions of peripheral regions to the activation mechanism. Using a chimeragenesis approach, we find that intra- and extracellular peripheral regions are main determinants of the voltage range of activation in fungal channels, highlighting the role of these overlooked components in channel gating.

Dimer interaction in the Hv1 proton channel

The voltage-gated proton channel Hv1 is a member of the voltage-gated ion channel superfamily, which stands out in design: It is a dimer of two voltage-sensing domains (VSDs), each containing a pore pathway, a voltage sensor (S4), and a gate (S1) and forming its own ion channel. Opening of the two channels in the dimer is cooperative. Part of the cooperativity is due to association between coiled-coil domains that extend intracellularly from the S4s. Interactions between the transmembrane portions of the subunits may also contribute, but the nature of transmembrane packing is unclear. Using functional analysis of a mutagenesis scan, biochemistry, and modeling, we find that the subunits form a dimer interface along the entire length of S1, and also have intersubunit contacts between S1 and S4. These interactions exert a strong effect on gating, in particular on the stability of the open state. Our results suggest that gating in Hv1 is tuned by extensive VSD-VSD interactions between the gates and voltage sensors of the dimeric channel.

Insights on small molecule binding to the Hv1 proton channel from free energy calculations with molecular dynamics simulations

Hv1 is a voltage-gated proton channel whose main function is to facilitate extrusion of protons from the cell. The development of effective channel blockers for Hv1 can lead to new therapeutics for the treatment of maladies related to Hv1 dysfunction. Although the mechanism of proton permeation in Hv1 remains to be elucidated, a series of small molecules have been discovered to inhibit Hv1. Here, we computed relative binding free energies of a prototypical Hv1 blocker on a model of human Hv1 in an open state. We used alchemical free energy perturbation techniques based on atomistic molecular dynamics simulations. The results support our proposed open state model and shed light on the preferred tautomeric state of the channel blocker. This work lays the groundwork for future studies on adapting the blocker molecule for more effective inhibition of Hv1.

Scorpion Venom: Detriments and Benefits

Scorpion venom may cause severe medical complications and untimely death if injected into the human body. Neurotoxins are the main components of scorpion venom that are known to be responsible for the pathological manifestations of envenoming. Besides neurotoxins, a wide range of other bioactive molecules can be found in scorpion venoms. Advances in separation, characterization, and biotechnological approaches have enabled not only the development of more effective treatments against scorpion envenomings, but have also led to the discovery of several scorpion venom peptides with interesting therapeutic properties. Thus, scorpion venom may not only be a medical threat to human health, but could prove to be a valuable source of bioactive molecules that may serve as leads for the development of new therapies against current and emerging diseases. This review presents both the detrimental and beneficial properties of scorpion venom toxins and discusses the newest advances within the development of novel therapies against scorpion envenoming and the therapeutic perspectives for scorpion toxins in drug discovery.

Scorpion toxin inhibits the voltage-gated proton channel using a Zn2+ -like long-range conformational coupling mechanism

BACKGROUND AND PURPOSE

Blocking the voltage-gated proton channel H_V 1 is a promising strategy for the treatment of diseases like ischaemia stroke and cancer. However, few H_V 1 channel antagonists have been reported. Here, we have identified a novel H_V 1 channel antagonist from scorpion venom and have elucidated its action mechanism.

EXPERIMENTAL APPROACH

H_V 1 and NaV channels were heterologously expressed in mammalian cell lines and their currents recorded using whole-cell patch clamp. Site-directed mutagenesis was used to generate mutants. Toxins were recombinantly produced in Escherichia coli. AGAP/W38F-H_V 1 interaction was modelled by molecular dynamics simulations.

KEY RESULTS

The scorpion toxin AGAP (anti-tumour analgesic peptide) potently inhibited H_V 1 currents. One AGAP mutant has reduced Na_V channel activity but intact H_V 1 activity (AGAP/W38F). AGAP/W38F inhibited H_V 1 channel activation by trapping its S4 voltage sensor in a deactivated state and inhibited H_V 1 currents with less pH dependence than Zn²⁺ . Mutation analysis showed that the binding pockets of AGAP/W38F and Zn²⁺ in H_V 1 channel partly overlapped (common sites are His140 and His193). The E153A mutation at the intracellular Coulombic network (ICN) in H_V 1 channel markedly reduced AGAP/W38F inhibition, as observed for Zn²⁺ . Experimental data and MD simulations suggested that AGAP/W38F inhibited H_V 1 channel using a Zn²⁺ -like long-range conformational coupling mechanism.

CONCLUSION AND IMPLICATIONS

Our results suggest that the Zn²⁺ binding pocket in H_V 1 channel might be a hotspot for modulators and valuable for designing H_V 1 channel ligands. Moreover, AGAP/W38F is a useful molecular probe to study H_V 1 channel and a lead compound for drug development.