Heme histidine ligands within gp91(phox) modulate proton conduction by the phagocyte NADPH oxidase

Mechanistic Insights on Heme-to-Heme Transmembrane Electron Transfer Within NADPH Oxydases From Atomistic Simulations

NOX5 is a member of the NADPH oxidase family which is dedicated to the production of reactive oxygen species. The molecular mechanisms governing transmembrane electron transfer (ET) that permits to shuttle electrons over the biological membrane have remained elusive for a long time. Using computer simulations, we report conformational dynamics of NOX5 embedded within a realistic membrane environment. We assess the stability of the protein within the membrane and monitor the existence of cavities that could accommodate dioxygen molecules. We investigate the heme-to-heme electron transfer. We find a reaction free energy of a few tenths of eV (ca. −0.3 eV) and a reorganization free energy of around 1.1 eV (0.8 eV after including electrostatic induction corrections). The former indicates thermodynamically favorable ET, while the latter falls in the expected values for transmembrane inter-heme ET. We estimate the electronic coupling to fall in the range of the μeV. We identify electron tunneling pathways showing that not only the W378 residue is playing a central role, but also F348. Finally, we reveal the existence of two connected O₂₋binding pockets near the outer heme with fast exchange between the two sites on the nanosecond timescale. We show that when the terminal heme is reduced, O₂ binds closer to it, affording a more efficient tunneling pathway than when the terminal heme is oxidized, thereby providing an efficient mechanism to catalyze superoxide production in the final step. Overall, our study reveals some key molecular mechanisms permitting reactive oxygen species production by NOX5 and paves the road for further investigation of ET processes in the wide family of NADPH oxidases by computer simulations.

Monoclonal antibody 7D5 recognizes the R147 epitope on the gp91phox , phagocyte flavocytochrome b558 large subunit

Human phagocyte flavocytochrome b₅₅₈ (Cyt b), the catalytic center of nicotinamide adenine dinucleotide phosphate oxidase, consists of a heavily glycosylated large subunit (gp91^phox ; Nox2) and a small subunit (p22^phox ). Cyt b is a membrane-spanning complex enzyme. Chronic granulomatous disease (CGD) is predominantly caused by a mutation in the CYBB gene encoding gp91^phox on the X-chromosome. Because the phagocytes of patients with CGD are not able to generate the superoxide anion, these patients are susceptible to severe infections that can be fatal. It has been suggested that the extracellular region of gp91^phox is necessary for and critical to forming the epitope of mAb 7D5 and that 7D5 provides a useful tool for rapid screening of X-linked CGD by FACS. To further elucidate the mAb 7D5 epitope on human gp91^phox , chimeric DNA expressed human and mouse gp91^phox recombinant protein were constructed. The fusion proteins were immunostained for mAb 7D5 and analyzed by FACS and western blot analysis. The ¹⁴³ ELGDRQNES¹⁵¹ region was found to reside at the extracellular surface on human gp91^phox and to be an important epitope for the interaction with mAb 7D5, as analyzed by FACS analysis. In particular, amino acid R147 is a unique epitope on the membrane-associated Cyt b for mAb 7D5. In conclusion, it is proposed that FACS analysis using mAb 7D5 is a valuable tool for early diagnosis of CGD.

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.

Redox signaling in cardiovascular health and disease

Spatiotemporal regulation of the activity of a vast array of intracellular proteins and signaling pathways by reactive oxygen species (ROS) governs normal cardiovascular function. However, data from experimental and animal studies strongly support that dysregulated redox signaling, resulting from hyperactivation of various cellular oxidases or mitochondrial dysfunction, is integral to the pathogenesis and progression of cardiovascular disease (CVD). In this review, we address how redox signaling modulates the protein function, the various sources of increased oxidative stress in CVD, and the labyrinth of redox-sensitive molecular mechanisms involved in the development of atherosclerosis, hypertension, cardiac hypertrophy and heart failure, and ischemia-reperfusion injury. Advances in redox biology and pharmacology for inhibiting ROS production in specific cell types and subcellular organelles combined with the development of nanotechnology-based new in vivo imaging systems and targeted drug delivery mechanisms may enable fine-tuning of redox signaling for the treatment and prevention of CVD.

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.

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.

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.

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.

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: 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.

Detailed comparison of expressed and native voltage-gated proton channel currents

Two years ago, genes coding for voltage-gated proton channels in humans, mice and Ciona intestinalis were discovered. Transfection of cDNA encoding the human HVCN1 (H(V)1) or mouse (mVSOP) ortholog of HVCN1 into mammalian cells results in currents that are extremely similar to native proton currents, with a subtle, but functionally important, difference. Expressed proton channels exhibit high H(+) selectivity, voltage-dependent gating, strong temperature sensitivity, inhibition by Zn(2+), and gating kinetics similar to native proton currents. Like native channels, expressed proton channels are regulated by pH, with the proton conductance-voltage (g(H)-V) relationship shifting toward more negative voltages when pH(o) is increased or pH(i) is decreased. However, in every (unstimulated) cell studied to date, endogenous proton channels open only positive to the Nernst potential for protons, E(H). Consequently, only outward H(+) currents exist in the steady state. In contrast, when the human or mouse proton channel genes are expressed in HEK-293 or COS-7 cells, sustained inward H(+) currents can be elicited, especially with an inward proton gradient (pH(o) < pH(i)). Inward current is the result of a negative shift in the absolute voltage dependence of gating. The voltage dependence at any given pH(o) and pH(i) is shifted by about -30 mV compared with native H(+) channels. Expressed H(V)1 voltage dependence was insensitive to interventions that promote phosphorylation or dephosphorylation of native phagocyte proton channels, suggesting distinct regulation of expressed channels. Finally, we present additional evidence that speaks against a number of possible mechanisms for the anomalous voltage dependence of expressed H(+) channels.

Biodiversity of voltage sensor domain proteins

The six-transmembrane type voltage-gated ion channels play an essential role in neuronal excitability, muscle contraction, and secretion. The voltage sensor domain (VSD) is the key element of voltage-gated ion channels for sensing transmembrane potential, and has been studied at the levels of both biophysics and protein structure. Two recently identified proteins containing VSD without a pore domain showed unexpected biological roles: regulation of phosphatase activity and proton permeation. These proteins not only provide novel platforms to understand mechanisms of voltage sensing and ion permeation but also highlight previously unappreciated roles of membrane potential in non-neuronal cells.

The NOX family of ROS-generating NADPH oxidases: physiology and pathophysiology

For a long time, superoxide generation by an NADPH oxidase was considered as an oddity only found in professional phagocytes. Over the last years, six homologs of the cytochrome subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1, and DUOX2. Together with the phagocyte NADPH oxidase itself (NOX2/gp91(phox)), the homologs are now referred to as the NOX family of NADPH oxidases. These enzymes share the capacity to transport electrons across the plasma membrane and to generate superoxide and other downstream reactive oxygen species (ROS). Activation mechanisms and tissue distribution of the different members of the family are markedly different. The physiological functions of NOX family enzymes include host defense, posttranlational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. NOX enzymes also contribute to a wide range of pathological processes. NOX deficiency may lead to immunosuppresion, lack of otoconogenesis, or hypothyroidism. Increased NOX activity also contributes to a large number or pathologies, in particular cardiovascular diseases and neurodegeneration. This review summarizes the current state of knowledge of the functions of NOX enzymes in physiology and pathology.

Role of nucleotides and phosphoinositides in the stability of electron and proton currents associated with the phagocytic NADPH oxidase

The phagocytic NADPH oxidase (phox) moves electrons across cell membranes to kill microbes. The activity of this lethal enzyme is tightly regulated, but the mechanisms that control phox inactivation are poorly understood for lack of appropriate assays. The phox generates measurable electron currents, I(e), that are associated with inward proton currents, I(H). To study the inactivation of the phox and of its associated proton channel, we determined which soluble factors can stabilize I(e) (induced by the addition of NADPH) and I(H) (initiated by small depolarizing voltage steps) in inside-out patches from PMA-activated human eosinophils. I(e) decayed rapidly in the absence of nucleotides (tau approximately 6 min) and was maximally stabilized by the combined addition of 5 mM ATP and 50 microM of the non-hydrolysable GTP analogue GTP[S] (guanosine 5'-[gamma-thio]triphosphate) (tau approximately 57 min), but not by either ATP or GTP[S] alone. I(H) also decayed rapidly and was stabilized by the ATP/GTP[S] mixture, but maximal stabilization of I(H) required further addition of 25 muM PI(3,4)P2 (phosphoinositide 3,4-bisphosphate) to the cytosolic side of the patch. PI(3,4)P2 had no effect on I(e) and its stabilizing effect on I(H) could not be mimicked by other phosphoinositides. Reducing the ATP concentration below millimolar levels decreased I(H) stability, an effect that was not prevented by phosphatase inhibitors but by the non-hydrolysable ATP analogue ATP[S] (adenosine 5'-[gamma-thio]triphosphate). Our data indicate that the assembled phox complex is very stable in eosinophil membranes if both ATP and GTP[S] are present, but inactivates within minutes if one of the nucleotides is removed. Stabilization of the phox-associated proton channel in a highly voltage-sensitive conformation does not appear to involve phosphorylation but ATP binding, and requires not only ATP and GTP[S] but also PI(3,4)P2, a protein known to anchor the cytosolic phox subunit p47(phox) to the plasma membrane.

Expression of Nox1 in 3T3 cells increases cellular acid production but not proton conductance

The role of the NADPH oxidase homolog 1 (Nox1) in plasma membrane H(+) conductance and cellular H(+) production was investigated in 3T3 cells stably expressing Nox1 (Nox1 3T3) compared to vector-expressing control cells (mock 3T3). In whole cell patch clamp experiments both Nox1 and mock 3T3 expressed a similar H(+) conductance (Nox1 3T3, 13.2+/-8.6 pS/pF; mock 3T3, 16.6+/-13.4 pS/pF) with a number of similar characteristics (e.g., current-voltage relations, current activation kinetics, Zn(2+)-sensitivity). When the intracellular pH of cells was alkalinized with NH(4)Cl, rates of intracellular acidification were significantly higher in Nox1 3T3 compared to mock 3T3. Nox1 3T3 showed a time course of acidification that followed a double-exponential function with a fast and a slow component of, on average, tau=165 s and 1780 s, whereas mock 3T3 showed only a single slow tau of 1560 s. Expression of Nox1 also caused cells to acidify the extracellular medium at higher rates than control cells; Nox1 3T3 released 96+/-19 fmol h(-1)cell(-1) of acid equivalents compared to 19+/-12 fmol h(-1)cell(-1) in mock 3T3. These data show that expression of Nox1 results in a mechanism that has the capacity to rapidly acidify the cytosol and generate significant amounts of acid. No significant effect of Nox1 expression on the plasma membrane H(+) conductance was found.

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.

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.

How neutrophils kill microbes

Neutrophils provide the first line of defense of the innate immune system by phagocytosing, killing, and digesting bacteria and fungi. Killing was previously believed to be accomplished by oxygen free radicals and other reactive oxygen species generated by the NADPH oxidase, and by oxidized halides produced by myeloperoxidase. We now know this is incorrect. The oxidase pumps electrons into the phagocytic vacuole, thereby inducing a charge across the membrane that must be compensated. The movement of compensating ions produces conditions in the vacuole conducive to microbial killing and digestion by enzymes released into the vacuole from the cytoplasmic granules.

Reactive oxygen species as mediators of angiogenesis signaling: role of NAD(P)H oxidase

Angiogenesis, a process of new blood vessel growth, contributes to various pathophysiologies such as cancer, diabetic retinopathy and atherosclerosis. Accumulating evidence suggests that cardiovascular diseases are associated with increased oxidative stress in blood vessels. Reactive oxygen species (ROS) such as superoxide and H2O2 cause blood vessels to thicken, produce inflammation in the vessel wall, and thus are regarded as "risk factors" for vascular disease, whereas ROS also act as signaling molecules in many aspects of growth factor-mediated physiological responses. Recent reports suggest that ROS play an important role in angiogenesis; however, its underlying molecular mechanisms remain unknown. Vascular endothelial growth factor (VEGF) induces angiogenesis by stimulating endothelial cell (EC) proliferation and migration primarily through the receptor tyrosine kinase VEGF receptor2 (Flk1/KDR). VEGF binding initiates tyrosine phosphorylation of KDR, which results in activation of downstream signaling enzymes including ERK1/2, Akt and eNOS, which contribute to angiogenic-related responses in EC. Importantly, the major source of ROS in EC is a NAD(P)H oxidase and EC express all the components of phagocytic NAD(P)H oxidase including gp91phox, p22phox, p47phox, p67phox and the small G protein Rac1. We have recently demonstrated that ROS derived from NAD(P)H oxidase are critically important for VEGF signaling in vitro and angiogenesis in vivo. Furthermore, a peptide hormone, angiotensin II, a major stimulus for vascular NAD(P)H oxidase, also plays an important role in angiogenesis. Because EC migration and proliferation are primary features of the process of myocardial angiogenesis, we would like to focus on the recent progress that has been made in the emerging area of NAD(P)H oxidase-derived ROS-dependent signaling in ECs, and discuss the possible roles in angiogenesis. Understanding these mechanisms may provide insight into the components of NAD(P)H oxidase as potential therapeutic targets for treatment of angiogenesis-dependent diseases such as cancer and atherosclerosis and for promoting myocardial angiogenesis in ischemic heart diseases.

Aberrant [correction of Abberant] cytosolic calcium ion mobilization in chronic granulomatous disease neutrophils

Beside the inability to produce superoxide ions, neutrophils (PMN) from chronic granulomatous disease (CGD) patients show other functional defects, e.g. abnormal membrane potential reactions. We observed that PMN from a female CGD carrier, with a discrete mutation in one allele of the pg91(phox) gene and exhibiting extreme lyonization, showed a consistently and remarkably delayed PMN cytosolic calcium response to the tripeptide fMLP or leukotriene B4 (LTB4). In keeping with results from other CGD patients, membrane potential changes were abnormal, whereas chemotaxis and adherence was normal. Since phospholipase D-generated metabolites are important for calcium transients we examined the generation of phosphatidic acid, but found that to be normal. A male CGD patient with pg91(phox) deficiency exhibited a trend toward prolongation of this calcium response, whereas two other CGD patients (one with p47 and one with 67(phox) deficiencies) had normal calcium transients. Thus, our finding points to a defect of the stimulus response coupling for fMLP and LTB4, which is supposed to be characteristic for this patient or a subset of CGD patients.

Overexpression of human heme oxygenase-1 attenuates endothelial cell sloughing in experimental diabetes

Heme oxygenase (HO)-1 represents a key defense mechanism against oxidative injury. Hyperglycemia produces oxidative stress and various perturbations of cell physiology. The effect of streptozotocin (STZ)-induced diabetes on aortic HO activity, heme content, the number of circulating endothelial cells, and urinary 8-epi-isoprostane PGF2alpha (8-Epi) levels in control rats and rats overexpressing or underexpressing HO-1 was measured. HO activity was decreased in hyperglycemic rats. Hyperglycemia increased urinary 8-Epi, and this increase was augmented in rats underexpressing HO-1 and diminished in rats overexpressing HO-1. The number of detached endothelial cells and O2- formation increased in diabetic rats and in hyperglycemic animals underexpressing HO-1 and decreased in diabetic animals overexpressing HO-1 compared with controls. These data demonstrate that HO-1 gene transfer in hyperglycemic rats brings about a reduction in O2- production and a decrease in endothelial cell sloughing. Upregulation of HO-1 decreases oxidant production and endothelial cell damage and shedding and may attenuate vascular complications in diabetes.

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.

The NADPH oxidase of professional phagocytes--prototype of the NOX electron transport chain systems

The NADPH oxidase is an electron transport chain in "professional" phagocytic cells that transfers electrons from NADPH in the cytoplasm, across the wall of the phagocytic vacuole, to form superoxide. The electron transporting flavocytochrome b is activated by the integrated function of four cytoplasmic proteins. The antimicrobial function of this system involves pumping K+ into the vacuole through BKCa channels, the effect of which is to elevate the vacuolar pH and activate neutral proteases. A number of homologous systems have been discovered in plants and lower animals as well as in man. Their function remains to be established.

NADPH oxidase-dependent acid production in airway epithelial cells

The purpose of this study was to determine the role of NADPH oxidase in H(+) secretion by airway epithelia. In whole cell patch clamp recordings primary human tracheal epithelial cells (hTE) and the human serous gland cell line Calu-3 expressed a functionally similar zinc-blockable plasma membrane H(+) conductance. However, the rate of H(+) secretion of confluent epithelial monolayers measured in Ussing chambers was 9-fold larger in hTE compared with Calu-3. In hTE H(+) secretion was blocked by mucosal ZnCl(2) and the NADPH oxidase blockers acetovanillone and 4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF), whereas these same blockers had no effect in Calu-3. We determined levels of transcripts for the NADPH oxidase transmembrane isoforms (Nox1 through -5, Duox1 and -2, and p22(phox)) and found Duox1, -2, and p22(phox) to be highly expressed in hTE, as well as the intracellular subunits p40(phox), p47(phox), and p67(phox). In contrast, Calu-3 lacked transcripts for Duox1, p40(phox), and p47(phox). Anti-Duox antibody staining resulted in prominent apical staining in hTE but no significant staining in Calu-3. When treated with amiloride to block the Na(+)/H(+) exchanger, intracellular pH in hTE acidified at significantly higher rates than in Calu-3, and treatment with AEBSF blocked acidification. These data suggest a role for an apically located Duox-based NADPH oxidase during intracellular H(+) production and H(+) secretion, but not in H(+) conduction.

Reactive oxygen species generated by microbial NADPH oxidase NoxA regulate sexual development in Aspergillus nidulans

NADPH oxidases (Nox) have been characterized as higher eukaryotic enzymes used deliberately to produce reactive oxygen species (ROS). The recent discovery of new functional members of the Nox family in plants and animals has led to the recognition of the increasing importance of ROS as signals involved in regulation of diverse cellular processes such as defence, growth and signalling. Here, we address the role of NADPH oxidase-generated ROS in the biology of the filamentous fungus Aspergillus nidulans. We characterize the noxA gene and show that it encodes a member of a novel NADPH oxidase subfamily ubiquitous in lower eukaryotes. Deletion of noxA specifically blocks differentiation of sexual fruit bodies (cleistothecia), without affecting hyphal growth or asexual development. Accordingly, the noxA gene is induced during sexual development, peaking at the time of cleistothecia differentiation and in parallel with the hülle cell-associated catalase peroxidase gene cpeA. This expression pattern is not dependent on transcription factors SteA and StuA, which are essential for cleistothecia formation. In contrast, noxA-dependent premature sexual development correlates with noxA derepression in DeltasakA null mutants, connecting stress MAPK signalling to the regulated production of ROS. Using a nitroblue tetrazolium (NBT) assay to detect superoxide, we found that hülle cells and cleistothecia initials produce superoxide in a process inhibited by NADPH oxidase inhibitor DPI and markedly reduced in DeltanoxA mutants. Furthermore, using H2DCFDA, we detected that H2O2 and possibly other ROS are generated in a NoxA-dependent fashion, mainly in the external walls from cleistothecia initials. The essential role of NoxA-generated ROS in A. nidulans sexual differentiation and the presence of one or two noxA homologues in all analysed filamentous fungi suggest that NADPH oxidase-generated ROS play important roles in fungal physiology and differentiation.

Heme oxygenase-1 prevents superoxide anion-associated endothelial cell sloughing in diabetic rats

Heme oxygenase-1 (HO-1) represents a key defense mechanism against oxidative injury. Hyperglycemia has been linked to increased oxidative stress, leading to endothelial dysfunction, delayed cell replication, and enhanced apoptosis. The effect of streptozotocin (STZ)-induced diabetes on HO activity, HO-1 promoter activity, superoxide anion (O*-2, and the number of circulating endothelial cells was measured. The expression of HO-1/HO-2 protein was unchanged, but HO activity was decreased in aortas of diabetic rats compared with control (p < 0.05). High glucose decreased HO-1 promoter activity (p < 0.05). Hyperglycemia increased O*-2 and this increase was augmented with HO-1 inhibition and diminished with HO-1 upregulation (p < 0.05). Circulating endothelial cells were significantly higher in diabetic rats and were decreased or increased with administration of the HO-1 inducer (CoPP) or inhibitor (SnMP), respectively (p<0.05). In conclusion, HO-1 upregulation in diabetic rats brings about an increase in serum bilirubin, a reduction in O*-2 production, and a decrease in endothelial cell sloughing.

Interactions between NADPH oxidase and voltage-gated proton channels: why electron transport depends on proton transport

Leukocytes kill microbes by producing reactive oxygen species, using a multi-component enzyme complex, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Electrons pass from intracellular NADPH through a redox chain within the enzyme, to reduce extracellular O2 to O2-. Electron flux is electrogenic, and rapidly depolarizes the membrane potential. Excessive depolarization can turn off electron transport by self-inhibition, but this is prevented by proton flux that balances the electron flux. Although the membrane potential depolarizes by approximately 100 mV during the respiratory burst (NADPH oxidase activity), NADPH oxidase activity is independent of voltage in this range, which permits optimal function and prevents self-inhibition.

Transplasma membrane electron transport: enzymes involved and biological function

The notion of transmembrane electron transport is usually associated with mitochondria and chloroplasts. However, since the early 1970s, it has been known that this phenomenon also occurs at the level of the plasma membrane. Ever since, evidence has accumulated for the existence of a plethora of transplasma membrane electron transport enzymes. In this review, we discuss the various enzymes known, their molecular characteristics and their biological functions.

Vascular NAD(P)H oxidases: specific features, expression, and regulation

The importance of reactive oxygen species (ROS) in vascular physiology and pathology is becoming increasingly evident. All cell types in the vascular wall produce ROS derived from superoxide-generating protein complexes similar to the leukocyte NADPH oxidase. Specific features of the vascular enzymes include constitutive and inducible activities, substrate specificity, and intracellular superoxide production. Most phagocyte enzyme subunits are found in vascular cells, including the catalytic gp91phox (aka, nox2), which was the earliest member of the newly discovered nox family. However, smooth muscle frequently expresses nox1 rather than gp91phox, and nox4 is additionally present in all cell types. In cell culture, agonists increase ROS production by activating multiple signals, including protein kinase C and Rac, and by upregulating oxidase subunits. The oxidases are also upregulated in vascular disease and are involved in the development of atherosclerosis and a significant part of angiotensin II-induced hypertension, possibly via nox1 and nox4. Likewise, enhanced vascular oxidase activity is associated with diabetes. Therefore, members of this enzyme family appear to be important in vascular biology and disease and constitute promising targets for future therapeutic interventions.

Voltage-gated proton channels and other proton transfer pathways

Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H(+) channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H(+) channels are expressed in many cells. During the respiratory burst in phagocytes, H(+) current compensates for electron extrusion by NADPH oxidase. Most evidence indicates that the H(+) channel is not part of the NADPH oxidase complex, but rather is a distinct and as yet unidentified molecule.

Two novel proteins activate superoxide generation by the NADPH oxidase NOX1

NOX1, an NADPH oxidase expressed predominantly in colon epithelium, shows a high degree of similarity to the phagocyte NADPH oxidase. However, superoxide generation by NOX1 has been difficult to demonstrate. Here we show that NOX1 generates superoxide when co-expressed with the p47(phox) and p67(phox) subunits of the phagocyte NADPH oxidase but not when expressed by itself. Since p47(phox) and p67(phox) are restricted mainly to myeloid cells, we searched for their homologues and identified two novel cDNAs. The mRNAs of both homologues were found predominantly in colon epithelium. Differences between the homologues and the phagocyte NADPH oxidase subunits included the lack of the autoinhibitory domain and the protein kinase C phosphorylation sites in the p47(phox) homologue as well as the absence of the first Src homology 3 domain and the presence of a hydrophobic stretch in the p67(phox) homologue. Co-expression of NOX1 with the two novel proteins led to stimulus-independent high level superoxide generation. Stimulus dependence of NOX1 was restored when p47(phox) was used to replace its homologue. In conclusion, NOX1 is a superoxide-generating enzyme that is activated by two novel proteins, which we propose to name NOXO1 (NOX organizer 1) and NOXA1 (NOX activator 1).

Structural changes are induced in human neutrophil cytochrome b by NADPH oxidase activators, LDS, SDS, and arachidonate: intermolecular resonance energy transfer between trisulfopyrenyl-wheat germ agglutinin and cytochrome b(558)

Anionic amphiphiles such as sodium- and lithium dodecyl sulfate (SDS, LDS), or arachidonate (AA) initiate NADPH oxidase and proton channel activation in cell-free systems and intact neutrophils. To investigate whether these amphiphiles exert allosteric effects on cytochrome b, trisulfopyrenyl-labeled wheat germ agglutinin (Cascade Blue-wheat germ agglutinin, CCB-WGA) was used as an extrinsic fluorescence donor for resonance energy transfer (RET) to the intrinsic heme acceptors of detergent-solubilized cytochrome b. In solution, cytochrome b complexed with the CCB-WGA causing a rapid, saturable, carbohydrate-dependent quenching of up to approximately 55% of the steady-state fluorescence. Subsequent additions of SDS, LDS, or AA to typical cell-free oxidase assay concentrations completely relaxed the fluorescence quenching. The relaxation effects were specific, and not caused by dissociation of the CCB-WGA-cytochrome b complex or alterations in the spectral properties of the chromophores. In contrast, addition of the oxidase antagonist, arachidonate methyl ester, caused an opposite effect and was able to partially reverse the activator-induced relaxation. We conclude that the activators induce a cytochrome b conformation wherein the proximity or orientation between the hemes and the extrinsic CCB fluorescence donors has undergone a significant change. These events may be linked to NADPH oxidase assembly and activation or proton channel induction.

Absence of proton channels in COS-7 cells expressing functional NADPH oxidase components

Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is an enzyme of phagocytes that produces bactericidal superoxide anion (O(2)(-)) via an electrogenic process. Proton efflux compensates for the charge movement across the cell membrane. The proton channel responsible for the H(+) efflux was thought to be contained within the gp91(phox) subunit of NADPH oxidase, but recent data do not support this idea (DeCoursey, T.E., V.V. Cherny, D. Morgan, B.Z. Katz, and M.C. Dinauer. 2001. J. Biol. Chem. 276:36063-36066). In this study, we investigated electrophysiological properties and superoxide production of COS-7 cells transfected with all NADPH oxidase components required for enzyme function (COS(phox)). The 7D5 antibody, which detects an extracellular epitope of the gp91(phox) protein, labeled 96-98% of COS(phox) cells. NADPH oxidase was functional because COS(phox) (but not COS(WT)) cells stimulated by phorbol myristate acetate (PMA) or arachidonic acid (AA) produced superoxide anion. No proton currents were detected in either wild-type COS-7 cells (COS(WT)) or COS(phox) cells studied at pH(o) 7.0 and pH(i) 5.5 or 7.0. Anion currents that decayed at voltages positive to 40 mV were the only currents observed. PMA or AA did not elicit detectable H(+) current in COS(WT) or COS(phox) cells. Therefore, gp91(phox) does not function as a proton channel in unstimulated cells or in activated cells with a demonstrably functional oxidase.

Molecular and functional characterization of a new X-linked chronic granulomatous disease variant (X91+) case with a double missense mutation in the cytosolic gp91phox C-terminal tail

We report here two atypical cases of X-linked CGD patients (first cousins) in which cytochrome b(558) is present at a normal level but is not functional (X91+). The mutations were localized by single-strand conformational polymorphism of reverse transcriptase-polymerase chain reaction amplified fragments and then identified by sequence analysis. They consisted in two base substitutions (C919 to A and C923 to G), changing His303 to Asn and Pro304 to Arg in the cytosolic gp91phox C-terminal tail. Mismatched polymerase chain reaction and genomic DNA sequencing showed that mothers had both wild-type and mutated alleles, confirming that this case was transmitted in an X-linked fashion. A normal amount of FAD was found in neutrophil membranes, both in the X91+ patients and their parents. Epstein-Barr virus-transformed B lymphocytes from the X91+ patients acidified normally upon stimulation with arachidonic acid, indicating that the mutated gp91phox still functioned as a proton channel. A cell-free translocation assay demonstrated that the association of the cytosolic factors p47phox and p67phox with the membrane fraction was strongly disrupted. We concluded that residues 303 and 304 are crucial for the stable assembly of the NADPH oxidase complex and for electron transfer, but not for its proton channel activity.

Killing activity of neutrophils is mediated through activation of proteases by K+ flux

According to the hitherto accepted view, neutrophils kill ingested microorganisms by subjecting them to high concentrations of highly toxic reactive oxygen species (ROS) and bringing about myeloperoxidase-catalysed halogenation. We show here that this simple scheme, which for many years has served as a satisfactory working hypothesis, is inadequate. We find that mice made deficient in neutrophil-granule proteases but normal in respect of superoxide production and iodinating capacity, are unable to resist staphylococcal and candidal infections. We also show that activation provokes the influx of an enormous concentration of ROS into the endocytic vacuole. The resulting accumulation of anionic charge is compensated for by a surge of K+ ions that cross the membrane in a pH-dependent manner. The consequent rise in ionic strength engenders the release of cationic granule proteins, including elastase and cathepsin G, from the anionic sulphated proteoglycan matrix. We show that it is the proteases, thus activated, that are primarily responsible for the destruction of the bacteria.

Modulation of the cytosolic and phagosomal pH by the NADPH oxidase

Proton (equivalents) are primary participants in the control and potency of the NADPH oxidase. Both the cytosolic and intraphagosomal pH are influenced during oxidase activation, and maintenance of the optimal environment requires the coordinated action of a series of sophisticated, highly regulated H(+) transporters, including the Na(+)/H(+) exchange, vacuolar H(+)-ATPase, and H(+)-conductive pathway(s). In addition, protons that are produced during some of the NADPH oxidase reactions then are substrates for the dismutation of superoxide, which precedes production of additional bactericidal agents. In this review, pH homeostasis is shown in conjunction with the NADPH oxidase to present an integrated picture of leukocyte physiology during the phagocytic response.

The neutrophil NADPH oxidase

The NADPH oxidase of phagocytes catalyzes the conversion of oxygen to O2(-). This multicomponent enzyme complex contains five essential protein components, two in the membrane and three in the cytosol. Unassembled and inactive in resting phagocytes, the oxidase becomes active after translocation of cytosolic components to the membrane to assemble a functional oxidase. Multiple factors regulate its assembly and activity, thus serving to maintain this highly reactive system under spatial and temporal control until recruited for antimicrobial or proinflammatory events. The recent identification of homologs of one of the membrane components in nonphagocytic cells will expand understanding of the biological contexts in which this system may function.

A Ca(2+)-activated NADPH oxidase in testis, spleen, and lymph nodes

Superoxide and its derivatives are increasingly implicated in the regulation of physiological functions from oxygen sensing and blood pressure regulation to lymphocyte activation and sperm-oocyte fusion. Here we describe a novel superoxide-generating NADPH oxidase referred to as NADPH oxidase 5 (NOX5). NOX5 is distantly related to the gp91(phox) subunit of the phagocyte NADPH oxidase with conserved regions crucial for the electron transport (NADPH, FAD and heme binding sites). However, NOX5 has a unique N-terminal extension that contains three EF hand motifs. The mRNA of NOX5 is expressed in pachytene spermatocytes of testis and in B- and T-lymphocyte-rich areas of spleen and lymph nodes. When heterologously expressed, NOX5 was quiescent in unstimulated cells. However, in response to elevations of the cytosolic Ca(2+) concentration it generated large amounts of superoxide. Upon Ca(2+) activation, NOX5 also displayed a second function: it became a proton channel, presumably to compensate charge and pH alterations due to electron export. In summary, we have identified a novel NADPH oxidase that generates superoxide and functions as a H(+) channel in a Ca(2+)-dependent manner. NOX5 is likely to be involved in Ca(2+)-activated, redox-dependent processes of spermatozoa and lymphocytes such as sperm-oocyte fusion, cell proliferation, and cytokine secretion.

The gp91phox component of NADPH oxidase is not the voltage-gated proton channel in phagocytes, but it helps

During the "respiratory burst," the NADPH oxidase complex of phagocytes produces reactive oxygen species that kill bacteria and other invaders (Babior, B. M. (1999) Blood 93, 1464-1476). Electron efflux through NADPH oxidase is electrogenic (Henderson, L. M., Chappell, J. B., and Jones, O. T. G. (1987) Biochem. J. 246, 325-329) and is compensated by H(+) efflux through proton channels that reportedly are contained within the gp91(phox) subunit of NADPH oxidase. To test whether gp91(phox) functions as a proton channel, we studied H(+) currents in granulocytes from X-linked chronic granulomatous disease patients lacking gp91(phox) (X-CGD), the human myelocytic PLB-985 cell line, PLB-985 cells in which gp91(phox) was knocked out by gene targeting (PLB(KO)), and PLB-985 knockout cells re-transfected with gp91(phox) (PLB(91)). H(+) currents in unstimulated PLB(KO) cells had amplitude and gating kinetics similar to PLB(91) cells. Furthermore, stimulation with the phorbol ester phorbol 12-myristate 13-acetate increased H(+) currents to a similar extent in X-CGD, PLB(KO), and PLB(91) cells. Thus, gp91(phox) is not the proton channel in unstimulated phagocytes and does not directly mediate the increase of proton conductance during the respiratory burst. Changes in H(+) channel gating kinetics during NADPH oxidase activity are likely crucial to the activation of H(+) flux during the respiratory burst.