The HVCN1 voltage-gated proton channel contributes to pH regulation in canine ventricular myocytes

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

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

Transcendent Aspects of Proton Channels

A handful of biological proton-selective ion channels exist. Some open at positive or negative membrane potentials, others open at low or high pH, and some are light activated. This review focuses on common features that result from the unique properties of protons. Proton conduction through water or proteins differs qualitatively from that of all other ions. Extraordinary proton selectivity is needed to ensure that protons permeate and other ions do not. Proton selectivity arises from a proton pathway comprising a hydrogen-bonded chain that typically includes at least one titratable amino acid side chain. The enormously diverse functions of proton channels in disparate regions of the phylogenetic tree can be summarized by considering the chemical and electrical consequences of proton flux across membranes. This review discusses examples of cells in which proton efflux serves to increase pHi, decrease pHo, control the membrane potential, generate action potentials, or compensate transmembrane movement of electrical charge.

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.

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

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

Intrapericardial long non-coding RNA-Tcf21 antisense RNA inducing demethylation administration promotes cardiac repair

AIMS

Epicardium and epicardium-derived cells are critical players in myocardial fibrosis. Mesenchymal stem cell-derived extracellular vesicles (EVs) have been studied for cardiac repair to improve cardiac remodelling, but the actual mechanisms remain elusive. The aim of this study is to investigate the mechanisms of EV therapy for improving cardiac remodelling and develop a promising treatment addressing myocardial fibrosis.

METHODS AND RESULTS

Extracellular vesicles were intrapericardially injected for mice myocardial infarction treatment. RNA-seq, in vitro gain- and loss-of-function experiments, and in vivo studies were performed to identify targets that can be used for myocardial fibrosis treatment. Afterward, a lipid nanoparticle-based long non-coding RNA (lncRNA) therapy was prepared for mouse and porcine models of myocardial infarction treatment. Intrapericardial injection of EVs improved adverse myocardial remodelling in mouse models of myocardial infarction. Mechanistically, Tcf21 was identified as a potential target to improve cardiac remodelling. Loss of Tcf21 function in epicardium-derived cells caused increased myofibroblast differentiation, whereas forced Tcf21 overexpression suppressed transforming growth factor-β signalling and myofibroblast differentiation. LncRNA-Tcf21 antisense RNA inducing demethylation (TARID) that enriched in EVs was identified to up-regulate Tcf21 expression. Formulated lncRNA-TARID-laden lipid nanoparticles up-regulated Tcf21 expression in epicardium-derived cells and improved cardiac function and histology in mouse and porcine models of myocardial infarction.

CONCLUSION

This study identified Tcf21 as a critical target for improving cardiac fibrosis. Up-regulating Tcf21 by using lncRNA-TARID-laden lipid nanoparticles could be a promising way to treat myocardial fibrosis. This study established novel mechanisms underlying EV therapy for improving adverse remodelling and proposed a lncRNA therapy for cardiac fibrosis.

5-Chloro-2-Guanidinobenzimidazole (ClGBI) Is a Non-Selective Inhibitor of the Human HV1 Channel

5-chloro-2-guanidinobenzimidazole (ClGBI), a small-molecule guanidine derivative, is a known effective inhibitor of the voltage-gated proton (H⁺) channel (H_V1, K_d ≈ 26 μM) and is widely used both in ion channel research and functional biological assays. However, a comprehensive study of its ion channel selectivity determined by electrophysiological methods has not been published yet. The lack of selectivity may lead to incorrect conclusions regarding the role of hHv1 in physiological or pathophysiological responses in vitro and in vivo. We have found that ClGBI inhibits the proliferation of lymphocytes, which absolutely requires the functioning of the K_V1.3 channel. We, therefore, tested ClGBI directly on hK_V1.3 using a whole-cell patch clamp and found an inhibitory effect similar in magnitude to that seen on hH_V1 (K_d ≈ 72 μM). We then further investigated ClGBI selectivity on the hK_V1.1, hK_V1.4-IR, hK_V1.5, hK_V10.1, hK_V11.1, hK_Ca3.1, hNa_V1.4, and hNa_V1.5 channels. Our results show that, besides H_V1 and K_V1.3, all other off-target channels were inhibited by ClGBI, with K_d values ranging from 12 to 894 μM. Based on our comprehensive data, ClGBI has to be considered a non-selective hH_V1 inhibitor; thus, experiments aiming at elucidating the significance of these channels in physiological responses have to be carefully evaluated.

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.

The Voltage-Gated Hv1 H+ Channel Is Expressed in Tumor-Infiltrating Myeloid-Derived Suppressor Cells

Myeloid-derived suppressor cells (MDSCs) are key determinants of the immunosuppressive microenvironment in tumors. As ion channels play key roles in the physiology/pathophysiology of immune cells, we aimed at studying the ion channel repertoire in tumor-derived polymorphonuclear (PMN-MDSC) and monocytic (Mo-MDSC) MDSCs. Subcutaneous tumors in mice were induced by the Lewis lung carcinoma cell line (LLC). The presence of PMN-MDSC (CD11b+/Ly6G+) and Mo-MDSCs (CD11b+/Ly6C+) in the tumor tissue was confirmed using immunofluorescence microscopy and cells were identified as CD11b+/Ly6G+ PMN-MDSCs and CD11b+/Ly6C+/F4/80−/MHCII− Mo-MDSCs using flow cytometry and sorting. The majority of the myeloid cells infiltrating the LLC tumors were PMN-MDSC (~60%) as compared to ~10% being Mo-MDSCs. We showed that PMN- and Mo-MDSCs express the Hv1 H+ channel both at the mRNA and at the protein level and that the biophysical and pharmacological properties of the whole-cell currents recapitulate the hallmarks of Hv1 currents: ~40 mV shift in the activation threshold of the current per unit change in the extracellular pH, high H+ selectivity, and sensitivity to the Hv1 inhibitor ClGBI. As MDSCs exert immunosuppression mainly by producing reactive oxygen species which is coupled to Hv1-mediated H+ currents, Hv1 might be an attractive target for inhibition of MDSCs in tumors.

Activation-pathway transitions in human voltage-gated proton channels revealed by a non-canonical fluorescent amino acid

Voltage-dependent gating of the voltage-gated proton channels (H_V1) remains poorly understood, partly because of the difficulty of obtaining direct measurements of voltage sensor movement in the form of gating currents. To circumvent this problem, we have implemented patch-clamp fluorometry in combination with the incorporation of the fluorescent non-canonical amino acid Anap to monitor channel opening and movement of the S4 segment. Simultaneous recording of currents and fluorescence signals allows for direct correlation of these parameters and investigation of their dependence on voltage and the pH gradient (ΔpH). We present data that indicate that Anap incorporated in the S4 helix is quenched by an aromatic residue located in the S2 helix and that motion of the S4 relative to this quencher is responsible for fluorescence increases upon depolarization. The kinetics of the fluorescence signal reveal the existence of a very slow transition in the deactivation pathway, which seems to be singularly regulated by ΔpH. Our experiments also suggest that the voltage sensor can move after channel opening and that the absolute value of the pH can influence the channel opening step. These results shed light on the complexities of voltage-dependent opening of human H_V1 channels.

The role of Phe150 in human voltage-gated proton channel

The voltage-gated proton channel H_v1 is a member of voltage-gated ion channels containing voltage-sensing domains (VSDs). The VSDs are made of four membrane-spanning segments (S1 through S4), and their function is to detect changes in membrane potential in the cells. A highly conserved phenylalanine 150 (F150) is located in the S2 segment of human voltage-gated proton channels. We previously discovered that the F150 is a binding site for the open channel blocker 2GBI. Here, we show that the H_v1 VSD voltage-dependent activation requires a hydrophobic group at position F150. We perform double-mutant cycle analysis to probe interactions between F150 and positively charged arginines in the S4 segment of the channel. Our results indicate that F150 interacts with two arginines (R2 and R3) in the S4 segment and catalyzes the transfer of the S4 arginines in the process of voltage-dependent activation.

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Hydrophobicity of F150 is crucial for human H_v1 channel voltage-dependent activation

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F150 interacts with R2 to stabilize the closed state of the H_v1 channel

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When depolarized, R3 moves upward to interact with F150 stabilizing the open state of H_v1

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F150 is essential for the transfer of the H_v1 arginines in the process of voltage sensing