A novel H(+) conductance in eosinophils: unique characteristics and absence in chronic granulomatous disease

The formation and function of the neutrophil phagosome

Neutrophils are the most abundant circulating leukocyte and are crucial to the initial innate immune response to infection. One of their key pathogen-eliminating mechanisms is phagocytosis, the process of particle engulfment into a vacuole-like structure called the phagosome. The antimicrobial activity of the phagocytic process results from a collaboration of multiple systems and mechanisms within this organelle, where a complex interplay of ion fluxes, pH, reactive oxygen species, and antimicrobial proteins creates a dynamic antimicrobial environment. This complexity, combined with the difficulties of studying neutrophils ex vivo, has led to gaps in our knowledge of how the neutrophil phagosome optimizes pathogen killing. In particular, controversy has arisen regarding the relative contribution and integration of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-derived antimicrobial agents and granule-delivered antimicrobial proteins. Clinical syndromes arising from dysfunction in these systems in humans allow useful insight into these mechanisms, but their redundancy and synergy add to the complexity. In this article, we review the current knowledge regarding the formation and function of the neutrophil phagosome, examine new insights into the phagosomal environment that have been permitted by technological advances in recent years, and discuss aspects of the phagocytic process that are still under debate.

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.

Intracellular pH regulation: characterization and functional investigation of H+ transporters in Stylophora pistillata

BACKGROUND

Reef-building corals regularly experience changes in intra- and extracellular H⁺ concentrations ([H⁺]) due to physiological and environmental processes. Stringent control of [H⁺] is required to maintain the homeostatic acid-base balance in coral cells and is achieved through the regulation of intracellular pH (pH_i). This task is especially challenging for reef-building corals that share an endosymbiotic relationship with photosynthetic dinoflagellates (family Symbiodinaceae), which significantly affect the pH_i of coral cells. Despite their importance, the pH regulatory proteins involved in the homeostatic acid-base balance have been scarcely investigated in corals. Here, we report in the coral Stylophora pistillata a full characterization of the genomic structure, domain topology and phylogeny of three major H⁺ transporter families that are known to play a role in the intracellular pH regulation of animal cells; we investigated their tissue-specific expression patterns and assessed the effect of seawater acidification on their expression levels.

RESULTS

We identified members of the Na⁺/H⁺ exchanger (SLC9), vacuolar-type electrogenic H⁺-ATP hydrolase (V-ATPase) and voltage-gated proton channel (H_vCN) families in the genome and transcriptome of S. pistillata. In addition, we identified a novel member of the H_vCN gene family in the cnidarian subclass Hexacorallia that has not been previously described in any species. We also identified key residues that contribute to H⁺ transporter substrate specificity, protein function and regulation. Last, we demonstrated that some of these proteins have different tissue expression patterns, and most are unaffected by exposure to seawater acidification.

CONCLUSIONS

In this study, we provide the first characterization of H⁺ transporters that might contribute to the homeostatic acid-base balance in coral cells. This work will enrich the knowledge of the basic aspects of coral biology and has important implications for our understanding of how corals regulate their intracellular environment.

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.

Voltage-Sensing Phosphatases: Biophysics, Physiology, and Molecular Engineering

Voltage-sensing phosphatase (VSP) contains a voltage sensor domain (VSD) similar to that in voltage-gated ion channels, and a phosphoinositide phosphatase region similar to phosphatase and tensin homolog deleted on chromosome 10 (PTEN). The VSP gene is conserved from unicellular organisms to higher vertebrates. Membrane depolarization induces electrical driven conformational rearrangement in the VSD, which is translated into catalytic enzyme activity. Biophysical and structural characterization has revealed details of the mechanisms underlying the molecular functions of VSP. Coupling between the VSD and the enzyme is tight, such that enzyme activity is tuned in a graded fashion to the membrane voltage. Upon VSP activation, multiple species of phosphoinositides are simultaneously altered, and the profile of enzyme activity depends on the history of the membrane potential. VSPs have been the obvious candidate link between membrane potential and phosphoinositide regulation. However, patterns of voltage change regulating VSP in native cells remain largely unknown. This review addresses the current understanding of the biophysical biochemical properties of VSP and provides new insight into the proposed functions of VSP.

Complement C5a Alters the Membrane Potential of Neutrophils during Hemorrhagic Shock

Background

Polymorphonuclear granulocytes (PMN) play a crucial role in host defense. Physiologically, exposure of PMN to the complement activation product C5a results in a protective response against pathogens, whereas in the case of systemic inflammation, excessive C5a substantially impairs neutrophil functions. To further elucidate the inability of PMN to properly respond to C5a, this study investigates the role of the cellular membrane potential of PMN in response to C5a.

Methods

Electrophysiological changes in cellular and mitochondrial membrane potential and intracellular pH of PMN from human healthy volunteers were determined by flow cytometry after exposure to C5a. Furthermore, PMN from male Bretoncelles-Meishan-Willebrand cross-bred pigs before and three hours after severe hemorrhagic shock were analyzed for their electrophysiological response.

Results

PMN showed a significant dose- and time-dependent depolarization in response to C5a with a strong response after one minute. The chemotactic peptide fMLP also evoked a significant shift in the membrane potential of PMN. Acidification of the cellular microenvironment significantly enhanced depolarization of PMN. In a clinically relevant model of porcine hemorrhagic shock, the C5a-induced changes in membrane potential of PMN were markedly diminished compared to healthy littermates. Overall, these membrane potential changes may contribute to PMN dysfunction in an inflammatory environment.

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.

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.

Extracellular phosphates enhance activities of voltage-gated proton channels and production of reactive oxygen species in murine osteoclast-like cells

Osteoclasts are highly differentiated bone-resorbing cells and play a significant role in bone remodelling. In the resorption pit, inorganic phosphate (Pi) concentrations increase because of degradation of hydroxyapatite. We studied effects of extracellular Pi on voltage-gated H⁺ channels in osteoclast-like cells derived from a macrophage cell line (RAW264). Extracellular Pi (1.25-20 mM) increased the H⁺ channel currents dose dependently and reversibly. The Pi-induced increases were attenuated by removal of extracellular Na⁺ and by phosphonoformic acid, a blocker of Na⁺-dependent Pi transporters. Pi increased the maximal conductance, decreased activation time constant, increased deactivation time constant, and shifted the conductance-voltage relationship to more negative voltages. The most marked change was enhanced gating which was mainly caused by elevation of intracellular Pi levels. The Pi-induced enhanced gating was partially inhibited by protein kinase C (PKC) inhibitors, GF109203X and staurosporine, indicating that PKC-mediated phosphorylation was involved in part. The increase in the maximal conductance was mainly due to accompanying decrease in intracellular pH. These effects of Pi were not affected by intracellular Mg²⁺, bafilomycin A₁ (V-ATPase inhibitor) and removal of intracellular ATP. Extracellular Pi also upregulated reactive oxygen species (ROS). Diphenyleneiodonium chloride, an inhibitor of NADPH oxidases, decreased ROS production and partially attenuated the enhanced gating. In the cells during later passages where osteoclastogenesis declined, H⁺ channel activities and ROS production were both modest. These results suggest that, in osteoclasts, ambient Pi is a common enhancer for H⁺ channels and ROS production and that potentiation of H⁺ channels may help ROS production.

β-eudesmol, a sesquiterpene from Teucrium ramosissimum, inhibits superoxide production, proliferation, adhesion and migration of human tumor cell

Reactive oxygen species are well-known mediators of various biological responses. Recently, new homologues of the catalytic subunit of NADPH oxidase have been discovered in non phagocytic cells. These new homologues (Nox1-Nox5) produce low levels of superoxides compared to the phagocytic homologue Nox2/gp91phox. In this study we examined the effect of β-eudesmol, a sesquiterpenoid alcohol isolated from Teucrium ramosissimum leaves, on proliferation, superoxide anion production, adhesion and migration of human lung (A549) and colon (HT29 and Caco-2) cancer cell lines. Proliferation of tumor cells was inhibited by β-eudesmol. It also significantly inhibited superoxide production in A549 cells. Furthermore, β-eudesmol inhibited adhesion and migration of A549 and HT29 cell. These results demonstrate that β-eudesmol may be a novel anticancer agent for the treatment of lung and colon cancer by different ways: by inhibition of superoxide production or by blocking proliferation, adhesion and migration.

Proton channels and renal hypertensive injury: a key piece of the Dahl salt-sensitive rat puzzle?

Hv1 is a voltage-gated proton channel highly expressed in phagocytic cells, where it participates in the NADPH oxidase-dependent respiratory burst. We have recently identified Hv1 as a novel renal channel, expressed in the renal medullary thick ascending limb that appears to importantly contribute to the pathogenesis of renal hypertensive injury in the Dahl salt-sensitive rat model. The purpose of this review is to describe the experimental approaches that we have undertaken to identify the source of excess reactive oxygen species production in the renal outer medulla of Dahl salt-sensitive rats and the resulting evidence that the voltage-gated proton channel Hv1 mediates augmented superoxide production and contributes to renal medullary oxidative stress and renal injury. In addition, we will attempt to point out areas of current controversy, as well as propose areas in which further experimental studies are likely to move the field forward. The content of the following review was presented as part of the Water and Electrolyte Homeostasis Section New Investigator Award talk at Experimental Biology 2014.

Effects of unsaturated fatty acids on the kinetics of voltage-gated proton channels heterologously expressed in cultured cells

KEY POINTS

Arachidonic acid (AA) greatly enhances the activity of the voltage-gated proton (Hv) channel, although its mechanism of action and physiological function remain unclear. In the present study, we analysed the effects of AA on proton currents through Hv channels heterologously expressed in HEK293T cells. The dramatic increase in proton current amplitude elicited by AA was accompanied by accelerated activation kinetics and a leftward shift in the voltage-dependence of activation. Mutagenesis studies suggest the two aforementioned effects of AA reflect two distinct structural mechanisms. Application of phospholipase A2 , which liberates AA from phospholipids in the membrane, also enhances Hv channel activity, supporting the idea that AA modulates Hv channel activity within physiological contexts. Unsaturated fatty acids are key components of the biological membranes of all cells, and precursors of mediators for cell signalling. Arachidonic acid (AA) is an unsaturated fatty acid known to modulate the activities of various ion channels, including the voltage-gated proton (Hv) channel, which supports the rapid production of reactive oxygen species (ROS) in phagocytes through regulation of pH and membrane potential. However, the molecular mechanisms and physiological functions of the effects of AA on Hv channels remain unclear. In the present study, we report an electrophysiological analysis of the effects of AA on the mouse Hv channel (mHv1) heterologously expressed in HEK293T cells. Application of AA to excised inside-out patch membranes rapidly induced a robust increase in the amplitude of the proton current through mHv1. The current increase was accompanied by accelerated activation kinetics and a small leftward shift of the current-voltage relationship. In monomeric channels lacking the coiled-coil region of the channel protein, the shift in the current-voltage relationship was diminished but activation and deactivation remained accelerated. Studies with several AA derivatives showed that double bonds and hydrophilic head groups are essential for the effect of AA, although charge was not important. The application of phospholipase A2 (PLA2), which generates AA from cell membrane phospholipids, stimulated mHv1 activity to a similar extent as direct application of ∼ 20 μM AA, suggesting that endogenous AA may regulate Hv channel activity.

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.

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.

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.

Enhanced activation of an amino-terminally truncated isoform of the voltage-gated proton channel HVCN1 enriched in malignant B cells

HVCN1 (Hydrogen voltage-gated channel 1) is the only mammalian voltage-gated proton channel. In human B lymphocytes, HVCN1 associates with the B-cell receptor (BCR) and is required for optimal BCR signaling and redox control. HVCN1 is expressed in malignant B cells that rely on BCR signaling, such as chronic lymphocytic leukemia (CLL) cells. However, little is known about its regulation in these cells. We found that HVCN1 was expressed in B cells as two protein isoforms. The shorter isoform (HVCN1S) was enriched in B cells from a cohort of 76 CLL patients. When overexpressed in a B-cell lymphoma line, HVCN1S responded more profoundly to protein kinase C-dependent phosphorylation. This more potent enhanced gating response was mediated by increased phosphorylation of the same residue responsible for enhanced gating in HVCN1L, Thr(29). Furthermore, the association of HVCN1S with the BCR was weaker, which resulted in its diminished internalization upon BCR stimulation. Finally, HVCN1S conferred a proliferative and migratory advantage as well as enhanced BCR-dependent signaling. Overall, our data show for the first time, to our knowledge, the existence of a shorter isoform of HVCN1 with enhanced gating that is specifically enriched in malignant B cells. The properties of HVCN1S suggest that it may contribute to the pathogenesis of BCR-dependent B-cell malignancies.

Expression of the voltage-sensing phosphatase gene in the chick embryonic tissues and in the adult cerebellum

Voltage-sensing phosphatase (VSP) consists of a transmembrane voltage sensor domain (VSD) and the cytoplasmic domain with phosphoinositide-phosphatase activities. It operates as the voltage sensor and directly translates membrane potential into phosphoinositide turnover by coupling VSD to the cytoplasmic domain. VSPs are evolutionarily conserved from marine invertebrate up to humans. Recently, we demonstrated that ectopic expression of the chick ortholog of VSP, Gg-VSP, in a fibroblast cell line caused characteristic cell process outgrowths. Co-expression of chick PTEN suppressed such morphological change, suggesting that VSP regulates cell shape by increasing PI(3,4)P₂. However, the in vivo function of Gg-VSP remains unclear. Here, we showed that in chick embryos Gg-VSP is expressed in the stomach, mesonephros, pharyngeal arch, limb bud, somites, floor plate of neural tube, and notochord. In addition, both Gg-VSP transcripts and the protein were found in the cerebellar Purkinje neurons. These findings provide an insight into the physiological functions of VSP.

Regulation of immune responses by proton channels

The identification of the HVCN1 gene, encoding the only mammalian voltage-gated proton channel, prompted a number of studies on how proton channels affect cellular functions. As their expression is mainly restricted to immune cells, it is not surprising that proton channels regulate different aspects of immune responses. In this review, I will examine the current knowledge of voltage-gated proton channels in both innate and adaptive responses and assess the remaining outstanding questions.

Philosophy of voltage-gated proton channels

In this review, voltage-gated proton channels are considered from a mainly teleological perspective. Why do proton channels exist? What good are they? Why did they go to such lengths to develop several unique hallmark properties such as extreme selectivity and ΔpH-dependent gating? Why is their current so minuscule? How do they manage to be so selective? What is the basis for our belief that they conduct H(+) and not OH(-)? Why do they exist in many species as dimers when the monomeric form seems to work quite well? It is hoped that pondering these questions will provide an introduction to these channels and a way to logically organize their peculiar properties as well as to understand how they are able to carry out some of their better-established biological functions.

Analysis of electrophysiological properties and responses of neutrophils

The past decade has seen increasing use of the patch-clamp technique on neutrophils and eosinophils. The main goal of these electrophysiological studies has been to elucidate the mechanisms underlying the phagocyte respiratory burst. NADPH oxidase activity, which defines the respiratory burst in granulocytes, is electrogenic because electrons from NADPH are transported across the cell membrane, where they reduce oxygen to form superoxide anion (O2 (-)). This passage of electrons comprises an electrical current that would rapidly depolarize the membrane if the charge movement were not balanced by proton efflux. The patch-clamp technique enables simultaneous recording of NADPH oxidase-generated electron current and H(+) flux through the closely related H(+) channel. Increasing evidence suggests that other ion channels may play crucial roles in degranulation, phagocytosis, and chemotaxis, highlighting the importance of electrophysiological studies to advance knowledge of granulocyte function. Several configurations of the patch-clamp technique exist. Each has advantages and limitations that are discussed here. Meaningful measurements of ion channels cannot be achieved without an understanding of their fundamental properties. We describe the types of measurements that are necessary to characterize a particular ion channel.

Temperature-sensitive gating of voltage-gated proton channels

The voltage-gated proton channel (Hv) mediates robust proton transport down the proton electrochemical gradient. Hv is mainly expressed in immune cells, including neutrophils and macrophages, the physiological functions of which are temperature sensitive. In those cells, Hv plays key roles in the regulation of reactive oxygen species production and pH homeostasis. Proton transport through Hv is regulated by both the membrane potential and the pH difference across the cell membrane. Earlier studies showed that the properties of Hv, including proton conductance and gating, are highly temperature dependent. Hv consists of a voltage sensor domain involved in both voltage sensing and proton permeation and a C-terminal coiled coil region. Although the channel's activities are innate to the protomers, normally two protomers assemble as a dimer via interaction between C-terminal coiled coils. We recently discovered that the coiled-coil region of Hv dissociates at around room temperature, and that subtle changes in the coiled-coil region affect temperature-sensitive gating. In this chapter, we describe the physiological functions and molecular mechanisms of Hv, focusing mainly on the structure and thermosensitive properties of Hv.

Voltage-gated proton channels: molecular biology, physiology, and pathophysiology of the H(V) family

Voltage-gated proton channels (H(V)) are unique, in part because the ion they conduct is unique. H(V) channels are perfectly selective for protons and have a very small unitary conductance, both arguably manifestations of the extremely low H(+) concentration in physiological solutions. They open with membrane depolarization, but their voltage dependence is strongly regulated by the pH gradient across the membrane (ΔpH), with the result that in most species they normally conduct only outward current. The H(V) channel protein is strikingly similar to the voltage-sensing domain (VSD, the first four membrane-spanning segments) of voltage-gated K(+) and Na(+) channels. In higher species, H(V) channels exist as dimers in which each protomer has its own conduction pathway, yet gating is cooperative. H(V) channels are phylogenetically diverse, distributed from humans to unicellular marine life, and perhaps even plants. Correspondingly, H(V) functions vary widely as well, from promoting calcification in coccolithophores and triggering bioluminescent flashes in dinoflagellates to facilitating killing bacteria, airway pH regulation, basophil histamine release, sperm maturation, and B lymphocyte responses in humans. Recent evidence that hH(V)1 may exacerbate breast cancer metastasis and cerebral damage from ischemic stroke highlights the rapidly expanding recognition of the clinical importance of hH(V)1.

Consequences of dimerization of the voltage-gated proton channel

The human voltage-gated proton channel, hHV1, appears to exist mainly as a dimer. Teleologically, this is puzzling because each protomer retains the main properties that characterize this protein: proton conduction that is regulated by conformational (channel opening and closing) changes that occur in response to both voltage and pH. The HV1 dimer is mainly linked by C-terminal coiled-coil interactions. Several types of mutations produce monomeric constructs that open approximately five times faster than the wild-type dimeric channel but with weaker voltage dependence. Intriguingly, the quintessential function of the HV1 dimer, opening to allow H(+) conduction, occurs cooperatively. Both protomers undergo a conformational change, but both must undergo this transition before either can conduct. The teleological purpose of dimerization may be to steepen the voltage dependence of channel opening, at least in phagocytes. In other cells, the purpose is not understood. Finally, several single-celled species have HV that are likely monomeric.

Biophysical properties of the voltage gated proton channel H(V)1

The biophysical properties of the voltage gated proton channel (H(V)1) are the key elements of its physiological function. The voltage gated proton channel is a unique molecule that in contrast to all other ion channels is exclusively selective for protons. Alone among proton channels, it has voltage and time dependent gating like other "classical" ion channels. H(V)1 is furthermore a sensor for the pH in the cell and the surrounding media. Its voltage dependence is strictly coupled to the pH gradient across the membrane. This regulation restricts opening of the channel to specific voltages at any given pH gradient, therefore allowing H(V)1 to perform its physiological task in the tissue it is expressed in. For H(V)1 there is no known blocker. The most potent channel inhibitor is zinc (Zn(2+)) which prevents channel opening. An additional characteristic of H(V)1 is its strong temperature dependence of both gating and conductance. In contrast to single-file water filled pores like the gramicidin channel, H(V)1 exhibits pronounced deuterium effects and temperature effects on conduction, consistent with a different conduction mechanism than other ion channels. These properties may be explained by the recent identification of an aspartate in the pore of H(V)1 that is essential to its proton selectivity.

Do Hv1 proton channels regulate the ionic and redox homeostasis of phagosomes?

Recent work on animal models has revealed the important role played by the voltage-gated proton channel Hv1 during bacterial killing by innate immune cells. Studies from mice lacking Hv1 channels showed that Hv1 proton channels are required for high-level production of reactive oxygen species (ROS) by the NADPH oxidase of phagocytes (NOX2) in two ways. First, Hv1 channels maintain a physiological membrane potential during the respiratory burst of neutrophils by providing a compensating charge for the electrons transferred by NOX2 from NADPH to superoxide. Second, Hv1 channels maintain a physiological cytosolic pH by extruding the acid generated by the NOX2-dependent consumption of NADPH. The two mechanisms directly sustain the activity of the NOX2 enzyme and indirectly sustain other neutrophil functions by enhancing the driving force for the entry of calcium into cells, thereby boosting cellular calcium signals. The increased depolarization of Hv1-deficient neutrophils aborted calcium responses to chemoattractants and revealed adhesion and migration defects that were associated with an impaired depolymerization of the cortical actin cytoskeleton. Current research aims to transpose these findings to phagosomes, the phagocytic vacuoles where bacterial killing takes place. However, the mechanisms that control the phagosomal pH appear to vary greatly between phagocytes: phagosomes rapidly acidify in macrophages but remain neutral for several minutes in neutrophils following ingestion of solid particles, whereas in dendritic cells phagosomes alkalinize, a mechanism thought to promote antigen cross-presentation. In this review, we discuss how the knowledge gained on the role of Hv1 channels at the plasma membrane of neutrophils can be used to study the regulation of the phagosomal pH, ROS, membrane potential, and calcium fluxes in different phagocytic cells.

NADPH oxidase 1 overexpression enhances invasion via matrix metalloproteinase-2 and epithelial-mesenchymal transition in melanoma cells

NADPH oxidase 1 (Nox1) is a member of the NADPH oxidase family that has not been well characterized in the melanocytic cell lineage. Here we demonstrated that Nox1 and Nox4 were detected in melanocytic lineage, with only Nox1 detected in normal human melanocytes and Nox4 in a subset of metastatic melanoma cell lines. The protein level and enzymatic activity of Nox1 was elevated in all melanoma cells as compared with normal melanocytes. Overexpression of GFP-Nox1 protein in Wm3211 primary melanoma cells increased invasion rate by 4- to 6-fold as measured by Matrigel invasion assay, whereas knocking down or inhibiting Nox1 decreased invasion by approximately 40-60% in Wm3211 and SK-Mel-28 cells. Matrix metalloproteinase-2 (MMP-2) was increased by Nox1 overexpression at the mRNA, protein, and activity levels, and decreased by Nox1 knockdown. MMP-2 promoter activity was also regulated by Nox1 knockdown. In addition, stable clones overexpressing Nox1 exhibited an epithelial-mesenchymal transition (EMT) as examined by cell morphology and EMT markers; knockdown or inhibiting Nox1 led to a reversal of EMT. Supplementing MMP-2 to culture media did not induce EMT, suggesting that EMT induction by Nox1 was not through MMP-2 upregulation. In summary, Nox1 was overexpressed in all melanoma cell lines examined, and enhanced cell invasion by MMP-2 upregulation and EMT induction.

Voltage-gated proton channels

Voltage-gated proton channels, HV1, have vaulted from the realm of the esoteric into the forefront of a central question facing ion channel biophysicists, namely, the mechanism by which voltage-dependent gating occurs. This transformation is the result of several factors. Identification of the gene in 2006 revealed that proton channels are homologues of the voltage-sensing domain of most other voltage-gated ion channels. Unique, or at least eccentric, properties of proton channels include dimeric architecture with dual conduction pathways, perfect proton selectivity, a single-channel conductance approximately 10(3) times smaller than most ion channels, voltage-dependent gating that is strongly modulated by the pH gradient, ΔpH, and potent inhibition by Zn(2+) (in many species) but an absence of other potent inhibitors. The recent identification of HV1 in three unicellular marine plankton species has dramatically expanded the phylogenetic family tree. Interest in proton channels in their own right has increased as important physiological roles have been identified in many cells. Proton channels trigger the bioluminescent flash of dinoflagellates, facilitate calcification by coccolithophores, regulate pH-dependent processes in eggs and sperm during fertilization, secrete acid to control the pH of airway fluids, facilitate histamine secretion by basophils, and play a signaling role in facilitating B-cell receptor mediated responses in B-lymphocytes. The most elaborate and best-established functions occur in phagocytes, where proton channels optimize the activity of NADPH oxidase, an important producer of reactive oxygen species. Proton efflux mediated by HV1 balances the charge translocated across the membrane by electrons through NADPH oxidase, minimizes changes in cytoplasmic and phagosomal pH, limits osmotic swelling of the phagosome, and provides substrate H(+) for the production of H2O2 and HOCl, reactive oxygen species crucial to killing pathogens.

NOX5 in human spermatozoa: expression, function, and regulation

Physiological and pathological processes in spermatozoa involve the production of reactive oxygen species (ROS), but the identity of the ROS-producing enzyme system(s) remains a matter of speculation. We provide the first evidence that NOX5 NADPH oxidase is expressed and functions in human spermatozoa. Immunofluorescence microscopy detected NOX5 protein in both the flagella/neck region and the acrosome. Functionally, spermatozoa exposed to calcium ionophore, phorbol ester, or H(2)O(2) exhibited superoxide anion production, which was blocked by addition of superoxide dismutase, a Ca(2+) chelator, or inhibitors of either flavoprotein oxidases (diphenylene iododonium) or NOX enzymes (GKT136901). Consistent with our previous overexpression studies, we found that H(2)O(2)-induced superoxide production by primary sperm cells was mediated by the non-receptor tyrosine kinase c-Abl. Moreover, the H(V)1 proton channel, which was recently implicated in spermatozoa motility, was required for optimal superoxide production by spermatozoa. Immunoprecipitation experiments suggested an interaction among NOX5, c-Abl, and H(V)1. H(2)O(2) treatment increased the proportion of motile sperm in a NOX5-dependent manner. Statistical analyses showed a pH-dependent correlation between superoxide production and enhanced sperm motility. Collectively, our findings show that NOX5 is a major source of ROS in human spermatozoa and indicate a role for NOX5-dependent ROS generation in human spermatozoa motility.

Strong glucose dependence of electron current in human monocytes

Reactive oxygen species (ROS) production by human monocytes differs profoundly from that by neutrophils and eosinophils in its dependence on external media glucose. Activated granulocytes produce vast amounts of ROS, even in the absence of glucose. Human peripheral blood monocytes (PBM), in contrast, are suspected not to be able to produce any ROS if glucose is absent from the media. Here we compare ROS production by monocytes and neutrophils, measured electrophysiologically on a single-cell level. Perforated-patch-clamp measurements revealed that electron current appeared after stimulation of PBM with phorbol myristate acetate. Electron current reflects the translocation of electrons through the NADPH oxidase, the main source of ROS production. The electron current was nearly abolished by omitting glucose from the media. Furthermore, in preactivated glucose-deprived cells, electron current appeared immediately with the addition of glucose to the bath. To characterize glucose dependence of PBM further, NADPH oxidase activity was assessed as hydrogen peroxide (H(2)O(2)) production and was recorded fluorometrically. H(2)O(2) production exhibited similar glucose dependence as did electron current. We show fundamental differences in the glucose dependence of ROS in human monocytes compared with human neutrophils.

ALS genetic modifiers that increase survival of SOD1 mice and are suitable for therapeutic development

Amyotrophic lateral sclerosis (ALS) is a frequently fatal motor neuron disease without any cure. To find molecular therapeutic targets, several studies crossed transgenic ALS murine models with animals transgenic for some ALS target genes. We aimed to revise the new discoveries and new works in this field. We selected the 10 most promising genes, according to their capability when down-regulated or up-regulated in ALS animal models, for increasing life span and mitigating disease progression: XBP-1, NogoA and NogoB, dynein, heavy and medium neurofilament, NOX1 and NOX2, MLC-mIGF-1, NSE-VEGF, and MMP-9. Interestingly, some crucial modifier genes have been described as being involved in common pathways, the most significant of which are inflammation and cytoskeletal activities. The endoplasmic reticulum also seems to play an important role in ALS pathogenesis, as it is involved in different selected gene pathways. In addition, these genes have evident links to each other, introducing the hypothesis of a single unknown, common pathway involving all of these identified genes and others to be discovered.

Coupling of the phosphatase activity of Ci-VSP to its voltage sensor activity over the entire range of voltage sensitivity

The voltage sensing phosphatase Ci-VSP is composed of a voltage sensor domain (VSD) and a cytoplasmic phosphatase domain. Upon membrane depolarization, movement of the VSD triggers the enzyme's phosphatase activity. To gain further insight into its operating mechanism, we studied the PI(4,5)P2 phosphatase activity of Ci-VSP expressed in Xenopus oocytes over the entire range of VSD motion by assessing the activity of coexpressed Kir2.1 channels or the fluorescence signal from a pleckstrin homology domain fused with green fluorescent protein (GFP) (PHPLC-GFP). Both assays showed greater phosphatase activity at 125 mV than at 75 mV, which corresponds to 'sensing' charges that were 90% and 75% of maximum, respectively. On the other hand, the activity at 160 mV (corresponding to 98% of the maximum 'sensing' charge) was indistinguishable from that at 125 mV. Modelling the kinetics of the PHPLC-GFP fluorescence revealed that its time course was dependent on both the level of Ci-VSP expression and the diffusion of PHPLC-GFP beneath the plasma membrane. Enzyme activity was calculated by fitting the time course of PHPLC-GFP fluorescence into the model. The voltage dependence of the enzyme activity was superimposable on the Q-V curve, which is consistent with the idea that the enzyme activity is tightly coupled to VSD movement over the entire range of membrane potentials that elicit VSD movement.

Gene expression profile of Ci-VSP in juveniles and adult blood cells of ascidian

VSP is a transmembrane protein whose cytoplasmic region shows significant similarity to phosphatase and tensin homolog deleted on chromosome 10 (PTEN). Notably, VSP exhibits a unique ability to transduce electrical signals into phosphoinositide turnover by coupling a transmembrane voltage sensor domain to the PTEN-like phosphoinositide phosphatase domain. Moreover, VSP gene is known to be widely conserved among deuterostome genomes, though the function of VSP in vivo remains largely unknown. In the present study, the expression pattern of ascidian VSP(Ci-VSP) was examined in embryos and juveniles of a marine invertebrate chordate, Ciona intestinalis. RT-PCR showed that Ci-VSP is expressed at the larval stage and that expression persists in juveniles. Whole mount in situ hybridization showed that Ci-VSP is expressed in cells of the stomach, intestine and blood cells of 2- to 3-week-old juveniles. Moreover, double staining blood cells from 2-month-old adults with Ci-VSP and Ci-PTEN probes showed that Ci-VSP-positive cells are a distinct population, separate from cells expressing Ci-PTEN. These findings suggest that in addition to its previously suggested roles in testis or sperm, Ci-VSP plays a key role in voltage-induced signal transduction in cells of the digestive system and blood.

Molecular and functional characterization of Hv1 proton channel in human granulocytes

Voltage-gated proton current (I_Hv) has been characterized in several cell types, but the majority of the data was collected in phagocytes, especially in human granulocytes. The prevailing view about the role of I_Hv in phagocytes is that it is an essential supporter of the intense and sustained activity of Nox2 (the core enzyme of the phagocyte NADPH oxidase complex) during respiratory burst. Recently H_v1, a voltage-gated proton channel, was cloned, and leukocytes from H_v1 knockout mice display impaired respiratory burst. On the other hand, hardly anything is known about H_v1 in human granulocytes. Using qPCR and a self made antibody, we detected a significant amount of H_v1 in human eosinophil and neutrophil granulocytes and in PLB-985 leukemia cells. Using different crosslinking agents and detergents in reducing and non-reducing PAGE, significant expression of H_v1 homodimers, but not that of higher-order multimers, could be detected in granulocytes. Results of subcellular fractionation and confocal imaging indicate that H_v1 is resident in both plasmalemmal and granular membrane compartments of resting neutrophils. Furthermore, it is also demonstrated that H_v1 accumulates in phagosome wall during zymosan engulfment together with, but independently of Nox2. During granulocytic differentiation early and parallel upregulation of H_v1 and Nox2 expression was observed in PLB-985 cells. The upregulation of H_v1 or Nox2 expression did not require the normal expression of the other molecule. Using RNA interference, we obtained strong correlation between H_v1 expression and I_Hv density in PLB-985 cells. It is also demonstrated that a massive reduction in H_v1 expression can limit the Nox2 mediated superoxide production of PLB-985 granulocytes. In summary, beside monomers native H_v1 forms stable proton channel dimer in resting and activated human granulocytes. The expression pattern of H_v1 in granulocytes is optimized to support intense NADPH oxidase activity.

pH regulation and beyond: unanticipated functions for the voltage-gated proton channel, HVCN1

Electrophysiological studies have implicated voltage-gated proton channels in several specific cellular contexts. In neutrophils, they mediate charge compensation that is associated with the oxidative burst of phagocytosis. Molecular characterization of the hydrogen voltage-gated channel 1 (HVCN1) has enabled identification of unanticipated and diverse functions: HVCN1 not only modulates signaling from the B-cell receptor following B-cell activation and histamine release from basophils, but also mediates pH-dependent activation of spermatozoa, as well as acid secretion by tracheal epithelium. The importance of HVCN1 in pH regulation during phagocytosis was established by surprising evidence that indicated its first-responder role. In this review, we discuss recent findings from a functional perspective, and the potential of HVCN1 as a therapeutic target for autoimmune and other diseases.

Physiological roles of voltage-gated proton channels in leukocytes

Voltage-gated proton channels are designed to extrude large quantities of cytosolic acid in response to depolarising voltages. The discovery of the Hvcn1 gene and the generation of mice lacking the channel molecule have confirmed several postulated functions of proton channels in leukocytes. In neutrophils and macrophages, proton channels are required for high-level production of superoxide anions by the phagocytic NADPH oxidase, a bactericidal enzyme essential for host defence against infections. In B lymphocytes, proton channels are required for low-level production of superoxide that boosts the production of antibodies. Proton channels sustain the activity of immune cells in several ways. By extruding excess cytosolic acid, proton channels prevent deleterious acidification of the cytosol and at the same time deliver protons required for chemical conversion of the superoxide secreted by membrane oxidases. By moving positive charges across membranes, proton channels limit the depolarisation of the plasma membrane, promoting the electrogenic activity of NADPH oxidases and the entry of calcium ions into cells. Acid extrusion by proton channels is not restricted to leukocytes but also mediates the intracellular alkalinisation required for the activation of spermatozoids. Proton channels are therefore multitalented channels that control male fertility as well as our innate and adaptive immunity.

Oligomerization of the voltage-gated proton channel

The voltage-gated proton channel exists as a dimer, although each protomer has a separate conduction pathway, and when forced to exist as a monomer, most major functions are retained. However, the proton channel protomers appear to interact during gating. Proton channel dimerization is thought to result mainly from coiled-coil interaction of the intracellular C-termini. Several types of evidence are discussed that suggest that the dimer conformation may not be static, but is dynamic and can sample different orientations. Zn(2+) appears to link the protomers in an orientation from which the channel(s) cannot open. A tandem WT-WT dimer exhibits signs of cooperative gating, indicating that despite the abnormal linkage, the correct orientation for opening can occur. We propose that C-terminal interaction functions mainly to tether the protomers together. Comparison of the properties of monomeric and dimeric proton channels speaks against the hypothesis that enhanced gating reflects monomer-dimer interconversion.

The role of calcium signaling in phagocytosis

Immune cells kill microbes by engulfing them in a membrane-enclosed compartment, the phagosome. Phagocytosis is initiated when foreign particles bind to receptors on the membrane of phagocytes. The best-studied phagocytic receptors, those for Igs (FcgammaR) and for complement proteins (CR), activate PLC and PLD, resulting in the intracellular production of the Ca(2+)-mobilizing second messengers InsP3 and S1P, respectively. The ensuing release of Ca(2+) from the ER activates SOCE channels in the plasma and/or phagosomal membrane, leading to sustained or oscillatory elevations in cytosolic Ca(2+) concentration. Cytosolic Ca(2+) elevations are required for efficient ingestion of foreign particles by some, but not all, phagocytic receptors and stringently control the subsequent steps involved in the maturation of phagosomes. Ca(2+) is required for the solubilization of the actin meshwork that surrounds nascent phagosomes, for the fusion of phagosomes with granules containing lytic enzymes, and for the assembly and activation of the superoxide-generating NADPH oxidase complex. Furthermore, Ca(2+) entry only occurs at physiological voltages and therefore, requires the activity of proton channels that counteract the depolarizing action of the phagocytic oxidase. The molecules that mediate Ca(2+) ion flux across the phagosomal membrane are still unknown but likely include the ubiquitous SOCE channels and possibly other types of Ca(2+) channels such as LGCC and VGCC. Understanding the molecular basis of the Ca(2+) signals that control phagocytosis might provide new, therapeutic tools against pathogens that subvert phagocytic killing.

Zinc inhibition of monomeric and dimeric proton channels suggests cooperative gating

Voltage-gated proton channels are strongly inhibited by Zn(2+), which binds to His residues. However, in a molecular model, the two externally accessible His are too far apart to coordinate Zn(2+). We hypothesize that high-affinity Zn(2+) binding occurs at the dimer interface between pairs of His residues from both monomers. Consistent with this idea, Zn(2+) effects were weaker in monomeric channels. Mutation of His(193) and His(140) in various combinations and in tandem dimers revealed that channel opening was slowed by Zn(2+) only when at least one His was present in each monomer, suggesting that in wild-type (WT) H(V)1, Zn(2+) binding between His of both monomers inhibits channel opening. In addition, monomeric channels opened exponentially, and dimeric channels opened sigmoidally. Monomeric channel gating had weaker temperature dependence than dimeric channels. Finally, monomeric channels opened 6.6 times faster than dimeric channels. Together, these observations suggest that in the proton channel dimer, the two monomers are closely apposed and interact during a cooperative gating process. Zn(2+) appears to slow opening by preventing movement of the monomers relative to each other that is prerequisite to opening. These data also suggest that the association of the monomers is tenuous and allows substantial freedom of movement. The data support the idea that native proton channels are dimeric. Finally, the idea that monomer-dimer interconversion occurs during activation of phagocytes appears to be ruled out.

Voltage-gated proton channels find their dream job managing the respiratory burst in phagocytes

The voltage-gated proton channel bears surprising resemblance to the voltage-sensing domain (S1-S4) of other voltage-gated ion channels but is a dimer with two conduction pathways. The proton channel seems designed for efficient proton extrusion from cells. In phagocytes, it facilitates the production of reactive oxygen species by NADPH oxidase.

Modulation of cellular redox state underlies antagonism between oxaliplatin and cetuximab in human colorectal cancer cell lines

BACKGROUND AND PURPOSE

Oxaliplatin is the first platinum-based compound effective in the treatment of colorectal cancer. Oxaliplatin combined with cetuximab for metastatic colorectal cancer is under evaluation. The preliminary results seem controversial, particularly for the use of cetuximab in K-Ras mutated patients. K-Ras mutation is known to affect redox homeostasis. Here we evaluated how the efficacy of oxaliplatin alone or combined with cetuximab varied according to the Ras mutation and redox status in a panel of colorectal tumour cell lines.

EXPERIMENTAL APPROACH

Viability was evaluated by methylthiazoletetrazolium assay, reactive oxygen species production by DCFDA and lucigenin on HT29-D4, Caco-2, SW480 and SW620 cell lines.

KEY RESULTS

Combination of oxaliplatin and cetuximab was less cytotoxic than oxaliplatin alone in colorectal cells harbouring wild-type Ras and membrane expression of receptors for epidermal growth factor receptor (EGFR), such as HT29-D4 and Caco-2 cells. In contrast, cetuximab did not affect oxaliplatin efficiency in cells harbouring K-Ras(V12) mutation, irrespective of membrane EGFR expression (SW620 and SW480 cells). Transfection of HT29-D4 with K-Ras(V12) decreased oxaliplatin IC(50) and impaired cetuximab sensitivity, without affecting expression of membrane EGFR compared with HT29-D4 control. Oxaliplatin efficacy relies on endogenous production of H(2)O(2). Cetuximab inhibits H(2)O(2) production inhibiting the EGFR/Nox1 NADPH oxidase pathway. Oxaliplatin efficacy was impaired by short hairpin RNA for Nox1 and by catalase (H(2)O(2) scavenger).

CONCLUSIONS AND IMPLICATIONS

Cetuximab limited oxaliplatin efficiency by affecting the redox status of cancer cells through Nox1. Such combined therapy might be improved by controlling H(2)O(2) elimination.

Identification of Thr29 as a critical phosphorylation site that activates the human proton channel Hvcn1 in leukocytes

Voltage-gated proton channels and NADPH oxidase function cooperatively in phagocytes during the respiratory burst, when reactive oxygen species are produced to kill microbial invaders. Agents that activate NADPH oxidase also enhance proton channel gating profoundly, facilitating its roles in charge compensation and pH(i) regulation. The "enhanced gating mode" appears to reflect protein kinase C (PKC) phosphorylation. Here we examine two candidates for PKC-delta phosphorylation sites in the human voltage-gated proton channel, H(V)1 (Hvcn1), Thr(29) and Ser(97), both in the intracellular N terminus. Channel phosphorylation was reduced in single mutants S97A or T29A, and further in the double mutant T29A/S97A, by an in vitro kinase assay with PKC-delta. Enhanced gating was evaluated by expressing wild-type (WT) or mutant H(V)1 channels in LK35.2 cells, a B cell hybridoma. Stimulation by phorbol myristate acetate enhanced WT channel gating, and this effect was reversed by treatment with the PKC inhibitor GF109203X. The single mutant T29A or double mutant T29A/S97A failed to respond to phorbol myristate acetate or GF109203X. In contrast, the S97A mutant responded like cells transfected with WT H(V)1. We conclude that under these conditions, direct phosphorylation of the proton channel molecule at Thr(29) is primarily responsible for the enhancement of proton channel gating. This phosphorylation is crucial to activation of the proton conductance during the respiratory burst in phagocytes.

VSOP/Hv1 proton channels sustain calcium entry, neutrophil migration, and superoxide production by limiting cell depolarization and acidification

Neutrophils kill microbes with reactive oxygen species generated by the NADPH oxidase, an enzyme which moves electrons across membranes. Voltage-gated proton channels (voltage-sensing domain only protein [VSOP]/Hv1) are required for high-level superoxide production by phagocytes, but the mechanism of this effect is not established. We show that neutrophils from VSOP/Hv1^−/− mice lack proton currents but have normal electron currents, indicating that these cells have a fully functional oxidase that cannot conduct protons. VSOP/Hv1^−/− neutrophils had a more acidic cytosol, were more depolarized, and produced less superoxide and hydrogen peroxide than neutrophils from wild-type mice. Hydrogen peroxide production was rescued by providing an artificial conductance with gramicidin. Loss of VSOP/Hv1 also aborted calcium responses to chemoattractants, increased neutrophil spreading, and decreased neutrophil migration. The migration defect was restored by the addition of a calcium ionophore. Our findings indicate that proton channels extrude the acid and compensate the charge generated by the oxidase, thereby sustaining calcium entry signals that control the adhesion and motility of neutrophils. Loss of proton channels thus aborts superoxide production and causes a severe signaling defect in neutrophils.

Voltage-gated proton channels maintain pH in human neutrophils during phagocytosis

Phagocytosis of microbial invaders represents a fundamental defense mechanism of the innate immune system. The subsequent killing of microbes is initiated by the respiratory burst, in which nicotinamide adenine dinucleotide phosphate (NADPH) oxidase generates vast amounts of superoxide anion, precursor to bactericidal reactive oxygen species. Cytoplasmic pH regulation is crucial because NADPH oxidase functions optimally at neutral pH, yet produces enormous quantities of protons. We monitored pH(i) in individual human neutrophils during phagocytosis of opsonized zymosan, using confocal imaging of the pH sensing dye SNARF-1, enhanced by shifted excitation and emission ratioing, or SEER. Despite long-standing dogma that Na(+)/H(+) antiport regulates pH during the phagocyte respiratory burst, we show here that voltage-gated proton channels are the first transporter to respond. During the initial phagocytotic event, pH(i) decreased sharply, and recovery required both Na(+)/H(+) antiport and proton current. Inhibiting myeloperoxidase attenuated the acidification, suggesting that diffusion of HOCl into the cytosol comprises a substantial acid load. Inhibiting proton channels with Zn(2+) resulted in profound acidification to levels that inhibit NADPH oxidase. The pH changes accompanying phagocytosis in bone marrow phagocytes from HVCN1-deficient mice mirrored those in control mouse cells treated with Zn(2+). Both the rate and extent of acidification in HVCN1-deficient cells were twice larger than in control cells. In summary, acid extrusion by proton channels is essential to the production of reactive oxygen species during phagocytosis.

NOX enzymes in the central nervous system: from signaling to disease

Oxidative stress has been implicated in the pathogenesis of neurologic and psychiatric diseases. The brain is particularly vulnerable to oxidative damage due to high oxygen consumption, low antioxidant defense, and an abundance of oxidation-sensitive lipids. Production of reactive oxygen species (ROS) by mitochondria is generally thought to be the main cause of oxidative stress. However, a role for ROS-generating NADPH oxidase NOX enzymes has recently emerged. Activation of the phagocyte NADPH oxidase NOX2 has been studied mainly in microglia, where it plays a role in inflammation, but may also contribute to neuronal death in pathologic conditions. However, NOX-dependent ROS production can be due to the expression of other NOX isoforms, which are detected not only in microglia, but also in astrocytes and neurons. The physiologic and pathophysiologic roles of such NOX enzymes are only partially understood. In this review, we summarize the present knowledge about NOX enzymes in the central nervous system and their involvement in neurologic and psychiatric diseases.

The intimate and mysterious relationship between proton channels and NADPH oxidase

Voltage gated proton channels and NADPH oxidase function cooperatively in phagocytes during the respiratory burst, when reactive oxygen species are produced to kill microbial invaders. Although these molecules are distinct entities, with no proven physical interaction, their presence and activity in many cells appears to be coordinated. We describe these interactions and discuss several types of mechanisms that might explain them.