Proton conduction through gp91phox

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.

Deficiency of gp91phox inhibits allergic airway inflammation

Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, a multienzyme complex, is the major source for production of reactive oxygen species (ROS). ROS are increased in allergic diseases, such as asthma, but the role of ROS in disease pathogenesis remains uncertain. We hypothesized that mice unable to generate ROS via the NADPH oxidase pathway would have decreased allergic airway inflammation. To test this hypothesis, we studied gp91phox(-/-) mice in a model of allergic airway inflammation after sensitization and challenge with ovalbumin. Serum, bronchoalveolar lavage fluid, and lungs were then examined for evidence of allergic inflammation. We found that mice lacking a functional NADPH oxidase complex had significantly decreased ROS production and allergic airway inflammation, compared with wild-type (WT) control animals. To determine the mechanism by which allergic inflammation was inhibited by gp91phox deficiency, we cultured bone marrow-derived dendritic cells from WT and gp91phox(-/-) mice and activated them with LPS. IL-12 expression was significantly increased in the gp91phox(-/-) bone marrow-derived dendritic cells, suggesting that the cytokine profile produced in the absence of gp91phox enhanced the conditions leading to T helper (Th) type 1 differentiation, while inhibiting Th2 polarization. Splenocytes from sensitized gp91phox(-/-) animals produced significantly less IL-13 in response to ovalbumin challenge in vitro compared with splenocytes from sensitized WT mice, suggesting that NADPH oxidase promotes allergic sensitization. In contrast, inflammatory cytokines produced by T cells cultured from WT and gp91phox(-/-) mice under Th0, Th1, Th2, and Th17 conditions were not significantly different. This study demonstrates the importance of NADPH oxidase activity and ROS production in a murine model of asthma.

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.

A contribution to the history of the proton channel

The low numbers of hydrogen ions in physiological solutions encouraged the assumption that H(+) currents flowing through conductive pathways would be so small as to be unmeasurable even if theoretically possible. Evidence for an H(+)-based action potential in the luminescent dinoflagellate Noctiluca and for an H(+)-conducting channel created by the secretions of the bacterium Bacillus brevis, did little to alter this perception. The clear demonstration of H(+) conduction in molluscan neurons might have provided the breakthrough but the new pathway was without an easily demonstrable function, and escaped general attention. Indeed the extreme measures that must be taken to successfully isolate H(+) currents meant that it was some years before proton channels were identified in mammalian cells. However, with the general availability of patch-clamp techniques and evidence for an important role in mammalian neutrophils, the stage was set for a series of structure/function studies with the potential to make the proton channel the best understood channel of all. In addition, widespread genomic searches have established that proton channels play important roles in processes ranging from fertilization of the human ovum to the progression of breast cancer. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

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.

Neutrophil granules in health and disease

Neutrophil granules store proteins that are critically important for the neutrophil to move from the vascular bed to tissues and to kill microorganisms. This is illustrated in nature when individual proteins are deleted due to inherited mutations of their cognate genes, and such deficiencies result in the conditions leucocyte adhesion deficiency and chronic granulomatous disease. The granules of the neutrophil have traditionally been divided into two or three major types but are instead a continuum where several subtypes can be identified with differences in protein content and propensity for mobilization. This is explained by the 'targeting by timing hypothesis' which states that granules are filled with granule proteins that are synthesized at the time the granule is formed. The heterogeneity of granules arises because the synthesis of granule proteins is individually controlled and major differences exist in the timings of biosynthesis during granulocytopoiesis. This is largely controlled by gene transcription.

Voltage-gated proton channels: what's next?

This review is an attempt to identify and place in context some of the many questions about voltage-gated proton channels that remain unsolved. As the gene was identified only 2 years ago, the situation is very different than in fields where the gene has been known for decades. For the proton channel, most of the obvious and less obvious structure-function questions are still wide open. Remarkably, the proton channel protein strongly resembles the voltage-sensing domain of many voltage-gated ion channels, and thus offers a novel approach to study gating mechanisms. Another surprise is that the proton channel appears to function as a dimer, with two separate conduction pathways. A number of significant biological questions remain in dispute, unanswered, or in some cases, not yet asked. This latter deficit is ascribable to the intrinsic difficulty in evaluating the importance of one component in a complex system, and in addition, to the lack, until recently, of a means of performing an unambiguous lesion experiment, that is, of selectively eliminating the molecule in question. We still lack a potent, selective pharmacological inhibitor, but the identification of the gene has allowed the development of powerful new tools including proton channel antibodies, siRNA and knockout mice.

Voltage-gated proton channels

The history of research on voltage-gated proton channels is recounted, from their proposed existence in dinoflagellates by Hastings in 1972 and their demonstration in snail neurons by Thomas and Meech in 1982 to the discovery in 2006 (after a decade of controversy) of genes that unequivocally code for proton channels. Voltage-gated proton channels are perfectly selective for protons, conduct deuterons half as well, and the conductance is strongly temperature dependent. These properties are consistent with a conduction mechanism involving hydrogen-bonded-chain transfer, in which the selectivity filter is a titratable amino acid residue. Channel opening is regulated stringently by pH such that only outward current is normally activated. Main functions of proton channels include acid extrusion from cells and charge compensation for the electrogenic activity of the phagocyte NADPH oxidase. Genetic approaches hold the promise of rapid progress in the near future.

Phagocytosis and antigen presentation in dendritic cells

Like macrophages and neutrophils, dendritic cells (DCs) are considered professional phagocytes. Even if the three cell types phagocytose parasites, bacteria, cell debris, or even intact cells very efficiently, the functional outcomes of the phagocytic event are quite different. Macrophages and neutrophils scavenge and destroy phagocytosed particles, a critical step in innate immunity. DCs, in contrast, have developed means to 'preserve' useful information from the ingested particles that serve to initiate adaptive immune responses. Thus, both phagosomal degradation and acidification are much lower in DCs than in macrophages or neutrophils. Reduced degradation results in the conservation of antigenic peptides and in their increased presentation on major histocompatibility complex class I and II molecules. In this article, we review the mechanisms that control this delicate equilibrium between phagosomal degradation/cytotoxicity and antigen presentation in the different families of phagocytes.

The function of the NADPH oxidase of phagocytes, and its relationship to other NOXs

The NADPH oxidase of ‘professional’ phagocytic cells transfers electrons across the wall of the phagocytic vacuole, forming superoxide in the lumen. It is generally accepted that this system promotes microbial killing through the generation of reactive oxygen species and through the activity of myeloperoxidase. An alternative scenario exists in which the passage of electrons across the membrane alters the pH and generates a charge that drives ions into, and out of, the vacuole. It is proposed that the primary function of the oxidase is to produce these pH changes and ion fluxes, and the issues surrounding these processes are considered in this review. The neutrophil oxidase is the prototype of a whole family of NOXs (NAPDH oxidases) that exist throughout biology, from plants to humans, which might function, at least in part, in a similar fashion.

The function of the NADPH oxidase of phagocytes and its relationship to other NOXs in plants, invertebrates, and mammals

The NADPH (nicotinamide adenine dinucleotide phosphate) oxidase (NOX) of ‘professional’ phagocytic cells transfers electrons across the wall of the phagocytic vacuole, forming superoxide in the lumen. It is generally accepted that this system promotes microbial killing through the generation of reactive oxygen species and through the activity of myeloperoxidase. An alternative scenario exists in which the passage of electrons across the membrane alters the pH and generates a charge that drives ions into, and out of, the vacuole. It is proposed that the primary function of the oxidase is to produce these pH changes and ion fluxes, and the issues surrounding these processes are considered. The neutrophil oxidase is the prototype of a whole family of NOXs that exist throughout biology, from plants to man, which might function, at least in part, in a similar fashion. Some examples of how these other NOXs might influence ion fluxes are examined.

Evidence for transmembrane proton transfer in a dihaem-containing membrane protein complex

Membrane protein complexes can support both the generation and utilisation of a transmembrane electrochemical proton potential ('proton-motive force'), either by transmembrane electron transfer coupled to protolytic reactions on opposite sides of the membrane or by transmembrane proton transfer. Here we provide the first evidence that both of these mechanisms are combined in the case of a specific respiratory membrane protein complex, the dihaem-containing quinol:fumarate reductase (QFR) of Wolinella succinogenes, so as to facilitate transmembrane electron transfer by transmembrane proton transfer. We also demonstrate the non-functionality of this novel transmembrane proton transfer pathway ('E-pathway') in a variant QFR where a key glutamate residue has been replaced. The 'E-pathway', discussed on the basis of the 1.78-Angstrom-resolution crystal structure of QFR, can be concluded to be essential also for the viability of pathogenic epsilon-proteobacteria such as Helicobacter pylori and is possibly relevant to proton transfer in other dihaem-containing membrane proteins, performing very different physiological functions.

Single-cell optical imaging of the phagocyte NADPH oxidase

The phagocyte NADPH oxidase is a key component of the innate immune response against invading microorganisms, because the generation of superoxide (O(2)(-)) inside the phagocytic vacuole by this enzyme is responsible for microbial killing by mechanisms that are directly or indirectly dependent on reactive oxygen species (ROS) formation. Most of what is known about the membrane-embedded and cytosolic NADPH oxidase subunits and their intricate network of interactions on assembly and activation has been derived from biochemical and biophysical studies involving subcellular fractionation or reconstituted cell-free systems. Such investigations can be complemented by single-cell microscopy on phagocytes, which may reveal spatial and/or temporal details about NADPH oxidase assembly that cannot be obtained from fractionated-cell assays. In recent years, we have investigated the NADPH oxidase in neutrophils using two complementary optical imaging techniques: Raman microscopy, a vibrational spectroscopic technique that does not require protein labeling, and live-cell fluorescence microscopy, which sheds light on the dynamics of NADPH oxidase assembly in individual cells. Here, we briefly introduce these techniques, compare their characteristics, and show their potential for studying NADPH oxidase at the single-cell level. New microscopy data are presented to illustrate the versatility of Raman and fluorescence microscopy on intact neutrophils.

Electron and proton transport by NADPH oxidases

The NADPH oxidase is the main weapon of phagocytic white blood cells that are the first line of defence of our body against invading pathogens, and patients lacking a functional oxidase suffer from severe and recurrent infections. The oxidase is a multisubunit enzyme complex that transports electrons from cytoplasmic NADPH to molecular oxygen in order to generate superoxide free radicals. Electron transport across the plasma membrane is electrogenic and is associated with the flux of protons through voltage-activated proton channels. Both proton and electron currents can be recorded with the patch-clamp technique, but whether the oxidase is a proton channel or a proton channel modulator remains controversial. Recently, we have used the inside-out configuration of the patch-clamp technique to record proton and electron currents in excised patches. This approach allows us to measure the oxidase activity under very controlled conditions, and has provided new information about the enzymatic activity of the oxidase and its coupling to proton channels. In this chapter I will discuss how the unique characteristics of the electron and proton currents associated with the redox activity of the NADPH oxidase have extended our knowledge about the thermodynamics and the physiological regulation of this remarkable enzyme.

Consequences of the electrogenic function of the phagocytic NADPH oxidase

NADPH oxidase of phagocytic cells transfers a single electron from intracellular NADPH to extracellular O2, producing superoxide (O.-2), the precursor to several other reactive oxygen species. The finding that a genetic defect of the enzyme causes chronic granulomatous disease (CGD), characterized by recurrent severe bacterial infections, linked O.-2 generation to destruction of potentially pathogenic micro-organisms. In this review, we focus on the consequences of the electrogenic functioning of NADPH oxidase. We show that enzyme activity depends on the possibilities for compensating charge movements. In resting neutrophils K+ conductance dominates, but upon activation the plasma membrane rapidly depolarizes beyond the opening threshold of voltage-gated H+ channels and H+ efflux becomes the major charge compensating factor. K+ release is likely to contribute to the killing of certain bacteria but complete elimination only occurs if O.-2 production can proceed at full capacity. Finally, the reversed membrane potential of activated neutrophils inhibits Ca2+ entry, thereby preventing overloading the cells with Ca2+. Absence of this limiting mechanism in CGD cells may contribute to the pathogenesis of the disease.

Expression of gp91phox/Nox2 in COS-7 cells: cellular localization of the protein and the detection of outward proton currents

We have reported previously that gp91phox, expressed in CHO (Chinese hamster ovary) cells, functions as a voltage-dependent proton channel. However, others have reported that COS-7 cells expressing gp91phox failed to exhibit outward proton currents, and concluded that gp91phox does not function as a proton channel. To investigate this clear difference in findings, we have examined the expression and cellular localization of the fusion protein EGFP-C-91, in which gp91phox is fused to the C-terminus of enhanced green fluorescent protein. EGFP-C-91 was observed in the plasma membrane and intracellular membranes of 30% of the transfected COS-7 cells. In the remaining COS-7 cells, EGFP-C-91 was detected in the intracellular membranes only. In CHO cells EGFP-C-91 was present in both the plasma membrane and the intracellular membranes of all transfected cells. Under the whole-cell configuration, outward currents were recorded from COS-7 cells expressing gp91phox. These increased in magnitude and lost their 'droop' over time as the pipette solution equilibrated with the cell cytoplasm (50 min). The threshold activation voltage for the currents was shifted by approximately 60 mV for a 1 unit difference in bath pH. Zn2+ inhibited the outward currents observed in COS-7 cells expressing gp91phox. The tail current reversal potential was -64 mV at a pH(o) (external pH) of 8.0, -40 mV at pH(o) 7.4 and -8 mV at pH(o) 7.0, indicating that the current arises from the movement of protons. Outward currents were exhibited by 37.5% of the COS-7 cells expressing gp91phox. Proton currents were recorded following the excision of inside-out patches from cells transfected with gp91phox. The presence of outward proton currents in COS-7 cells expressing gp91phox provides further support for our proposed role for gp91phox as the NADPH oxidase-associated proton channel.

Neutrophils and keratinocytes in innate immunity--cooperative actions to provide antimicrobial defense at the right time and place

The human neutrophil is a professional phagocyte of fundamental importance for defense against microorganisms, as witnessed by the life-threatening infections occurring in patients with neutropenia or with defects that result in decreased microbicidal activity of the neutrophil. Likewise, the skin and mucosal surfaces provide important barriers against infections. Traditionally, these major defense systems, the epithelial cells and the neutrophils, have been viewed as limited in their armory: The epithelial cells provide defense by constituting a physical barrier, and the neutrophils provide instant delivery of preformed antimicrobial substances or on-the-spot assembly of the multicomponent reduced nicotinamide adenine dinucleotide phosphate oxidase from stored components for the generation of reactive oxygen metabolites. Recent research has shown that epithelial cells are highly dynamic and able to generate antimicrobial peptides in response not only to microbial infection itself but more importantly, to the growth factors that are called into play when the physical barrier is broken, and the risk of microbial infection is imminent. Likewise, the neutrophil changes its profile of actively transcribed genes when it diapedeses into wounded skin. This results in generation of signaling molecules, some of which support the growth and antimicrobial potential of keratinocytes and epithelial cells. This paper will highlight some recent advances in this field.

Analysis of mRNA transcripts from the NAD(P)H oxidase 1 (Nox1) gene. Evidence against production of the NADPH oxidase homolog-1 short (NOH-1S) transcript variant

Recent reports indicate that NAD(P)H oxidase 1 (Nox1) mRNA undergoes alternative splicing, producing a short transcript (NOH-1S) encoding a novel H+ channel. Although the H+ transport properties of NOH-1S-transfected cells resemble those of many cells, the production of a NOH-1S protein was never documented. We characterized Nox1 transcripts in colon-derived cells and present evidence that mRNA splicing does not produce NOH-1S; rather, NOH-1S appears to be an artifact of template switching during cDNA synthesis. The NOH-1S transcript was not observed by Northern blotting, despite claims of its abundance based on RNase protection assays. The shortened cDNA was generated by avian myeloblastosis virus reverse transcriptase, but not by thermally stable reverse transcriptase under conditions that produce full-length Nox1. Analysis of shortened cDNAs detected NOH-1S sequence and other variants that differ at the alleged splice junction site. Although no appropriate RNA splicing sites were found within Nox1 to account for NOH-1S formation, we found repetitive sequence elements bordering the deleted region, which could promote intramolecular template switching during cDNA synthesis. Template switching was confirmed in vitro, where the deleted cDNA was generated by avian myeloblastosis virus reverse transcriptase from a synthetic, full-length Nox1 RNA template. A survey of the expressed sequence tags database suggests that similar switching phenomena occur between repetitive elements in other Nox family transcripts, indicating such cloning artifacts are common. In contrast, genuine RNA splicing does account for another Nox1 transcript lacking the entire exon 11, which is abundant in colon cells but encodes a protein incapable of supporting superoxide production.

Interactions between electron and proton currents in excised patches from human eosinophils

The NADPH–oxidase is a plasma membrane enzyme complex that enables phagocytes to generate superoxide in order to kill invading pathogens, a critical step in the host defense against infections. The oxidase transfers electrons from cytosolic NADPH to extracellular oxygen, a process that requires concomitant H⁺ extrusion through depolarization-activated H⁺ channels. Whether H⁺ fluxes are mediated by the oxidase itself is controversial, but there is a general agreement that the oxidase and H⁺ channel are intimately connected. Oxidase activation evokes profound changes in whole-cell H⁺ current (I_H), causing an approximately −40-mV shift in the activation threshold that leads to the appearance of inward I_H. To further explore the relationship between the oxidase and proton channel, we performed voltage-clamp experiments on inside-out patches from both resting and phorbol-12-myristate-13-acetate (PMA)-activated human eosinophils. Proton currents from resting cells displayed slow voltage-dependent activation, long-term stability, and were blocked by micromolar internal [Zn²⁺]. I_H from PMA-treated cells activated faster and at lower voltages, enabling sustained H⁺ influx, but ran down within minutes, regaining the current properties of nonactivated cells. Bath application of NADPH to patches excised from PMA-treated cells evoked electron currents (I_e), which also ran down within minutes and were blocked by diphenylene iodonium (DPI). Run-down of both I_H and I_e was delayed, and sometimes prevented, by cytosolic ATP and GTP-γ-S. A good correlation was observed between the amplitude of I_e and both inward and outward I_H when a stable driving force for e⁻ was imposed. Combined application of NADPH and DPI reduced the inward I_H amplitude, even in the absence of concomitant oxidase activity. The strict correlation between I_e and I_H amplitudes and the sensitivity of I_H to oxidase-specific agents suggest that the proton channel is either part of the oxidase complex or linked by a membrane-limited mediator.

Structure and regulation of the neutrophil respiratory burst oxidase: comparison with nonphagocyte oxidases

Neutrophils play an essential role in the body's innate defense against pathogens and are one of the primary mediators of the inflammatory response. To defend the host, neutrophils use a wide range of microbicidal products, such as oxidants, microbicidal peptides, and lytic enzymes. The generation of microbicidal oxidants by neutrophils results from the activation of a multiprotein enzyme complex known as the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which is responsible for transferring electrons from NADPH to O2, resulting in the formation of superoxide anion. During oxidase activation, cytosolic oxidase proteins translocate to the phagosome or plasma membrane, where they assemble around a central membrane-bound component known as flavocytochrome b. This process is highly regulated, involving phosphorylation, translocation, and multiple conformational changes. Originally, it was thought that the NADPH oxidase was restricted to phagocytes and used solely in host defense. However, recent studies indicate that similar NADPH oxidase systems are present in a wide variety of nonphagocytic cells. Although the nature of these nonphagocyte NADPH oxidases is still being defined, it is clear that they are functionally distinct from the phagocyte oxidases. It should be noted, however, that structural features of many nonphagocyte oxidase proteins do seem to be similar to those of their phagocyte counterparts. In this review, key structural and functional features of the neutrophil NADPH oxidase and its protein components are described, including a consideration of transcriptional and post-translational regulatory features. Furthermore, relevant details about structural and functional features of various nonphagocyte oxidase proteins will be included for comparison.

Oxidative killing of microbes by neutrophils

Neutrophils and other phagocytic leukocytes contain a phagocyte NADPH oxidase enzyme that generates superoxide after cell activation. Reactive oxygen species derived from superoxide, together with proteases liberated from the granules, are used to kill ingested microbes. Dysfunction of the phagocyte NADPH oxidase results in chronic granulomatous disease, with life-threatening infections.

The requirement of cytosolic phospholipase A2 for the PMA activation of proton efflux through the N-terminal 230-amino-acid fragment of gp91phox

The absolute requirement for the 85 kDa cytosolic phospholipase A(2) (cPLA(2)) in the PMA stimulation of proton efflux through the NADPH-oxidase-associated proton channel, has previously been demonstrated using a PLB-985 cell line deficient in cPLA(2) (PLB-D). The flux of protons in Chinese-Hamster ovary (CHO) cells that express the N-terminal 230-amino-acid (NT) fragment of gp91(phox) is activated by arachidonic acid (AA) added externally. To investigate the physiological role of cPLA(2), and the intracellular AA that it releases, in the activation of proton flux through the NT fragment of gp91(phox), this fragment was stably expressed in PLB-985 cells (PLB-985 NT) and in PLB-D cells (PLB-D NT). The expression of the NT fragment of gp91(phox) by itself in PLB-985 did not initiate differentiation and did not alter their ability to undergo differentiation after the addition of DMSO. Addition of PMA induced a proton efflux from undifferentiated PLB-985 NT cells expressing the NT fragment of gp91(phox), which was inhibited by zinc. In contrast, PMA failed to activate proton efflux in undifferentiated PLB-D NT cells, lacking the expression of cPLA(2); however, addition of AA restored the efflux of protons in these cells. These results establish an essential and specific physiological requirement of cPLA(2)-generated AA in the activation of proton flux through the NT fragment of gp91(phox).