Activation and assembly of the NADPH oxidase: a structural perspective

Reactive oxygen species in phagocytic leukocytes

Phagocytic leukocytes consume oxygen and generate reactive oxygen species in response to appropriate stimuli. The phagocyte NADPH oxidase, a multiprotein complex, existing in the dissociated state in resting cells becomes assembled into the functional oxidase complex upon stimulation and then generates superoxide anions. Biochemical aspects of the NADPH oxidase are briefly discussed in this review; however, the major focus relates to the contributions of various modes of microscopy to our understanding of the NADPH oxidase and the cell biology of phagocytic leukocytes.

Regulation of superoxide production in neutrophils: role of calcium influx

Upon stimulation, activation of NADPH oxidase complexes in neutrophils produces a burst of superoxide anions contributing to oxidative stress and the development of inflammatory process. Store-operated calcium entry (SOCE), whereby the depletion of intracellular stores induces extracellular calcium influx, is known to be a crucial element of NADPH oxidase regulation. However, the mechanistic basis mediating SOCE is still only partially understood, as is the signal-coupling pathway leading to modulation of store-operated channels. This review emphasizes the role of calcium influx in the control of the NADPH oxidase and summarizes the current knowledge of pathways mediating this extracellular calcium entry in neutrophils. Such investigations into the cross-talk between NADPH oxidase and calcium might allow the identification of novel pharmacological targets with clinical use, particularly in inflammatory diseases.

Priming of the neutrophil NADPH oxidase activation: role of p47phox phosphorylation and NOX2 mobilization to the plasma membrane

Neutrophils play an essential role in host defense against microbial pathogens and in the inflammatory reaction. Upon activation, neutrophils produce superoxide anion (O*2), which generates other reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), hydroxyl radical (OH*) and hypochlorous acid (HOCl), together with microbicidal peptides and proteases. The enzyme responsible for O2* production is called the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase or respiratory burst oxidase. This multicomponent enzyme system is composed of two trans-membrane proteins (p22phox and gp91phox/NOX2, which form the cytochrome b558), three cytosolic proteins (p47phox, p67phox, p40phox) and a GTPase (Rac1 or Rac2), which assemble at membrane sites upon cell activation. NADPH oxidase activation in phagocytes can be induced by a large number of soluble and particulate factors. Three major events accompany NAPDH oxidase activation: (1) protein phosphorylation, (2) GTPase activation, and (3) translocation of cytosolic components to the plasma membrane to form the active enzyme. Actually, the neutrophil NADPH oxidase exists in different states: resting, primed, activated, or inactivated. The resting state is found in circulating blood neutrophils. The primed state can be induced by neutrophil adhesion, pro-inflammatory cytokines, lipopolysaccharide, and other agents and has been characterized as a "ready to go" state, which results in a faster and higher response upon exposure to a second stimulus. The active state is found at the inflammatory or infection site. Activation is induced by the pathogen itself or by pathogen-derived formylated peptides and other agents. Finally, inactivation of NADPH oxidase is induced by anti-inflammatory agents to limit inflammation. Priming is a "double-edged sword" process as it contributes to a rapid and efficient elimination of the pathogens but can also induce the generation of large quantities of toxic ROS by hyperactivation of the NADPH oxidase, which can damage surrounding tissues and participate to inflammation. In order to avoid extensive damage to host tissues, NADPH oxidase priming and activation must be tightly regulated. In this review, we will discuss some of the mechanisms of NADPH oxidase priming in neutrophils and the relevance of this process to physiology and pathology.

Structure, regulation and evolution of Nox-family NADPH oxidases that produce reactive oxygen species

NADPH oxidases of the Nox family exist in various supergroups of eukaryotes but not in prokaryotes, and play crucial roles in a variety of biological processes, such as host defense, signal transduction, and hormone synthesis. In conjunction with NADPH oxidation, Nox enzymes reduce molecular oxygen to superoxide as a primary product, and this is further converted to various reactive oxygen species. The electron-transferring system in Nox is composed of the C-terminal cytoplasmic region homologous to the prokaryotic (and organelle) enzyme ferredoxin reductase and the N-terminal six transmembrane segments containing two hemes, a structure similar to that of cytochrome b of the mitochondrial bc(1) complex. During the course of eukaryote evolution, Nox enzymes have developed regulatory mechanisms, depending on their functions, by inserting a regulatory domain (or motif) into their own sequences or by obtaining a tightly associated protein as a regulatory subunit. For example, one to four Ca(2+)-binding EF-hand motifs are present at the N-termini in several subfamilies, such as the respiratory burst oxidase homolog (Rboh) subfamily in land plants (the supergroup Plantae), the NoxC subfamily in social amoebae (the Amoebozoa), and the Nox5 and dual oxidase (Duox) subfamilies in animals (the Opisthokonta), whereas an SH3 domain is inserted into the ferredoxin-NADP(+) reductase region of two Nox enzymes in Naegleria gruberi, a unicellular organism that belongs to the supergroup Excavata. Members of the Nox1-4 subfamily in animals form a stable heterodimer with the membrane protein p22(phox), which functions as a docking site for the SH3 domain-containing regulatory proteins p47(phox), p67(phox), and p40(phox); the small GTPase Rac binds to p67(phox) (or its homologous protein), which serves as a switch for Nox activation. Similarly, Rac activates the fungal NoxA via binding to the p67(phox)-like protein Nox regulator (NoxR). In plants, on the other hand, this GTPase directly interacts with the N-terminus of Rboh, leading to superoxide production. Here I describe the regulation of Nox-family oxidases on the basis of three-dimensional structures and evolutionary conservation.

Dissociation of Rac1(GDP).RhoGDI complexes by the cooperative action of anionic liposomes containing phosphatidylinositol 3,4,5-trisphosphate, Rac guanine nucleotide exchange factor, and GTP

Rac plays a pivotal role in the assembly of the superoxide-generating NADPH oxidase of phagocytes. In resting cells, Rac is found in the cytosol in complex with Rho GDP dissociation inhibitor (RhoGDI). NADPH oxidase assembly involves dissociation of the Rac.RhoGDI complex and translocation of Rac to the membrane. We reported that liposomes containing high concentrations of monovalent anionic phospholipids cause Rac.RhoGDI complex dissociation ( Ugolev, Y., Molshanski-Mor, S., Weinbaum, C., and Pick, E. (2006) J. Biol. Chem. 281, 19204-19219 ). We now designed an in vitro model mimicking membrane phospholipid remodeling during phagocyte stimulation in vivo. We showed that liposomes of "resting cell membrane" composition (less than 20 mol % monovalent anionic phospholipids), supplemented with 1 mol % of polyvalent anionic phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) in conjunction with constitutively active forms of the guanine nucleotide exchange factors (GEFs) for Rac, Trio, or Tiam1 and a non-hydrolyzable GTP analogue, cause dissociation of Rac1(GDP).RhoGDI complexes, GDP to GTP exchange on Rac1, and binding of Rac1(GTP) to the liposomes. Complexes were not dissociated in the absence of GEF and GTP, and optimal dissociation required the presence of PtdIns(3,4,5)P(3) in the liposomes. Dissociation of Rac1(GDP).RhoGDI complexes was correlated with the affinity of particular GEF constructs, via the N-terminal pleckstrin homology domain, for PtdIns(3,4,5)P(3) and involved GEF-mediated GDP to GTP exchange on Rac1. Phagocyte membranes enriched in PtdIns(3,4,5)P(3) responded by NADPH oxidase activation upon exposure in vitro to Rac1(GDP).RhoGDI complexes, p67(phox), GTP, and Rac GEF constructs with affinity for PtdIns(3,4,5)P(3) at a level superior to that of native membranes.

Reactive oxygen species and glucocorticoid-induced hypertension

1. There is increasing evidence for a role of oxidative stress and nitric oxide deficiency in experimental glucocorticoid-induced hypertension, as evidenced by increased biomarkers of oxidative stress; the effectiveness of antioxidants or reduced NADPH oxidase antagonists in lowering blood pressure; and secondary upregulation of endogenous antioxidant enzymes in response to oxidative stress. 2. In the vasculature, the main sources of superoxide are NADPH oxidase, xanthine oxidase, uncoupled endothelial nitric oxide synthase (eNOS) and mitochondria. 3. NADPH oxidase plays a significant role in the pathogenesis of glucocorticoid-induced hypertension in the rats, but xanthine oxidase and uncoupled eNOS pathways are not important sources of reactive oxygen species in these models. The role of mitochondrial reactive oxygen species in glucocorticoid-induced hypertension remains to be clarified.

Protein tyrosine phosphorylation and protein tyrosine nitration in redox signaling

Reversible phosphorylation of protein tyrosine residues by polypeptide growth factor-receptor protein tyrosine kinases is implicated in the control of fundamental cellular processes including the cell cycle, cell adhesion, and cell survival, as well as cell proliferation and differentiation. During the last decade, it has become apparent that receptor protein tyrosine kinases and the signaling pathways they activate belong to a large signaling network. Such a network can be regulated by various extracellular cues, which include cell adhesion, agonists of G protein-coupled receptors, and oxidants. It is well documented that signaling initiated by receptor protein tyrosine kinases is directly dependent on the intracellular production of oxidants, including reactive oxygen and nitrogen species. Accumulated evidence indicates that the intracellular redox environment plays a major role in the mechanisms underlying the actions of growth factors. Oxidation of cysteine thiols and nitration of tyrosine residues on signaling proteins are described as posttranslational modifications that regulate, positively or negatively, protein tyrosine phosphorylation (PTP). Early observations described the inhibition of PTP activities by oxidants, resulting in increased levels of proteins phosphorylated on tyrosine. Therefore, a redox circuitry involving the increasing production of intracellular oxidants associated with growth-factor stimulation/cell adhesion, oxidative reversible inhibition of protein tyrosine phosphatases, and the activation of protein tyrosine kinases can be delineated.

Apocynin improves outcome in experimental stroke with a narrow dose range

Inflammation following ischemic stroke is known to contribute to injury. NADPH oxidase (NOX) is a major enzyme system originally studied in immune cells that leads to superoxide (O.*) generation. Apocynin is a NOX inhibitor that has been studied as a potential treatment in experimental stroke. Here we explored the effect of different doses of apocynin in a mouse model of 2 h transient middle cerebral artery occlusion (tMCAO) followed by 22 h reperfusion. Apocynin, given i.v. at a dose of 2.5 mg/kg 30 min before reperfusion, improved neurological function (P<0.01), reduced infarct volume (P<0.05), and reduced the incidence of cerebral hemorrhage (P<0.05), but not at higher doses of 3.75 and 5 mg/kg, where it actually increased brain hemorrhage. Apocynin also tended to reduce mortality at the lower dose, but not at higher doses. Using hydroethine fluorescence to delineate O.* in the brain, neurons and some microglia/macrophages, but not vascular endothelial cells were found to contain O.*. Apocynin at protective doses markedly prevented ischemia-induced increases in O.*. Our data suggested that apocynin can protect against experimental stroke, but with a narrow therapeutic window.

Volume-sensitive NADPH oxidase activity and taurine efflux in NIH3T3 mouse fibroblasts

Reactive oxygen species (ROS) are produced in NIH3T3 fibroblasts during hypotonic stress, and H(2)O(2) potentiates the concomitant release of the organic osmolyte taurine (Lambert IH. J Membr Biol 192: 19-32, 2003). The increase in ROS production [5-(and-6)-carboxy-2', 7'-dichlorodihydrofluorescein diacetate fluorescence] is detectable after a reduction in the extracellular osmolarity from 335 mosM (isotonic) to 300 mosM and reaches a maximal value after a reduction to 260 mosM. The swelling-induced ROS production is reduced by the flavoprotein inhibitor diphenylene iodonium chloride (25 microM) but is unaffected by the nitric oxide synthase inhibitor N omega-nitro-l-arginine methyl ester, indicating that the volume-sensitive ROS production is NADPH oxidase dependent. NIH3T3 cells express the NADPH oxidase components: p22 phox, a NOX4 isotype; p47 phox; and p67 phox (real-time PCR). Exposure to the Ca2+-mobilizing agonist ATP (10 microM) potentiates the release of taurine but has no effect on ROS production under hypotonic conditions. On the other hand, addition of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA, 100 nM) or the lipid messenger lysophosphatidic acid (LPA, 10 nM) potentiates the swelling-induced taurine release as well as the ROS production. Overexpression of Rac1 or p47 phox or p47 phox knockdown [small interfering (si)RNA] had no effect on the swelling-induced ROS production or taurine release. NOX4 knockdown (siRNA) impairs the increase in the ROS production and the concomitant taurine release following osmotic exposure. It is suggested that a NOX4 isotype plus p22 phox account for the swelling-induced increase in the ROS production in NIH3T3 cells and that the oxidase activity is potentiated by PKC and LPA but not by Ca2+.

A case-control study of rheumatoid arthritis identifies an associated single nucleotide polymorphism in the NCF4 gene, supporting a role for the NADPH-oxidase complex in autoimmunity

Rheumatoid arthritis (RA) is a chronic inflammatory disease with a heritability of 60%. Genetic contributions to RA are made by multiple genes, but only a few gene associations have yet been confirmed. By studying animal models, reduced capacity of the NADPH-oxidase (NOX) complex, caused by a single nucleotide polymorphism (SNP) in one of its components (the NCF1 gene), has been found to increase severity of arthritis. To our knowledge, however, no studies investigating the potential role played by reduced reactive oxygen species production in human RA have yet been reported. In order to examine the role played by the NOX complex in RA, we investigated the association of 51 SNPs in five genes of the NOX complex (CYBB, CYBA, NCF4, NCF2, and RAC2) in a Swedish case-control cohort consisting of 1,842 RA cases and 1,038 control individuals. Several SNPs were found to be mildly associated in men in NCF4 (rs729749, P = 0.001), NCF2 (rs789181, P = 0.02) and RAC2 (rs1476002, P = 0.05). No associations were detected in CYBA or CYBB. By stratifying for autoantibody status, we identified a strong association for rs729749 (in NCF4) in autoantibody negative disease, with the strongest association detected in rheumatoid factor negative men (CT genotype versus CC genotype: odds ratio 0.34, 95% confidence interval 0.2 to 0.6; P = 0.0001). To our knowledge, this is the first genetic association identified between RA and the NOX complex, and it supports previous findings from animal models of the importance of reactive oxygen species production capacity to the development of arthritis.

Priming of neutrophil oxidative burst in diabetes requires preassembly of the NADPH oxidase

Hyperglycemia associated with diabetes mellitus results in the priming of neutrophils leading to oxidative stress that is, in part, responsible for diabetic complications. p47phox, a NADPH oxidase cytosolic subunit, is a key protein in the assembly of the NADPH oxidase leading to superoxide generation. Little is known about the priming mechanism of oxidative pathways in neutrophils of people with diabetes. In this study, the kinetics of p47phox activation was investigated by comparing neutrophils from diabetic and healthy subjects, and the mechanism of hyperglycemia-induced changes was studied by using neutrophil-like HL-60 cells as a model. In resting neutrophils from diabetic subjects, p47phox prematurely translocates to the cell membrane and preassembles with p22phox, a NADPH oxidase membrane subunit. This premature p47phox translocation and preassembly with p22phox were also observed in HL-60 cells cultured with high glucose (HG; 25 mM) and with the specific ligand for the receptor for advanced glycation end products (RAGE), S100B. Phosphorylation of ERK1/2, but not p38 MAPK, was the primary signaling pathway, as evidenced by PD98059 suppressing the translocation of p47phox in HL-60 cells incubated with HG and S100B. HL-60 cells cultured in HG and S100B exhibited a 1.8-fold increase in fMLP-induced superoxide generation compared with those cultured in normal glucose (5.5 mM). These data suggest that HG and increased AGE prime neutrophils and increase oxidative stress inducing the translocation of p47phox to the cell membrane and preassembly with p22phox by stimulating a RAGE-ERK1/2 pathway.

Identification of a conserved Rac-binding site on NADPH oxidases supports a direct GTPase regulatory mechanism

The NADPH oxidases (Noxs) are a family of superoxide-generating enzymes implicated in a variety of biological processes. Full activity of Nox1, -2, and -3 requires the action of a Rac GTPase. A direct regulatory interaction of Rac with Nox2 has been proposed as part of a two-step mechanism for regulating electron transfer during superoxide formation. Using truncation analysis of Rac binding to the cytoplasmic tail of Nox2, along with peptides derived from this region in cell-free assays, we identify a Rac interaction site within amino acids 419-430 of Nox2. This region is required for binding Rac2 but not p47(phox) or p67(phox) cytosolic regulatory factors. A cell-permeant version of the peptide encompassing amino acids 419-430 specifically inhibits NADPH oxidase activation in intact human neutrophils. Mutational analysis of the putative Rac-binding site revealed specific residues, particularly Lys-421, Tyr-425, and Lys-426, individually required for Rac-dependent NADPH oxidase activity that are conserved in the Rac-regulated Nox1, Nox2, and Nox3 enzymes but not in Nox4 or Nox5. Mutation of the conserved residues in the Rac-binding site of Nox1 also result in the loss of Rac-dependent activity. Our data identify a functional Rac interaction site conserved in Rac-dependent Noxs and support a direct regulatory interaction of Rac GTPases to promote activation of these NADPH oxidases.

Stimulus-dependent impairment of the neutrophil oxidative burst response in lactoferrin-deficient mice

Lactoferrin (LF) is an iron-binding protein found in milk, mucosal secretions, and the secondary granules of neutrophils in which it is considered to be an important factor in the innate immune response against microbial infections. Moreover, LF deficiency in the secondary granules of neutrophils has long been speculated to contribute directly to the hypersusceptibility of specific granule deficiency (SGD) patients to severe, life-threatening bacterial infections. However, the exact physiological significance of LF in neutrophil-mediated host defense mechanisms remains controversial and has not yet been clearly established in vivo using relevant animal models. In this study, we used lactoferrin knockout (LFKO) mice to directly address the selective role of LF in the host defense response of neutrophils and to determine its contribution, if any, to the phenotype of SGD. Neutrophil maturation, migration, phagocytosis, granule release, and antimicrobial response to bacterial challenge were unaffected in LFKO mice. Interestingly, a stimulus-dependent defect in the oxidative burst response of LFKO neutrophils was observed in that normal activation was seen in response to opsonized bacteria whereas an impaired response was evident after phorbol myristate-13-acetate stimulation. Taken together, these results indicate that although LF deficiency alone is not a primary cause of the defects associated with SGD, this protein does play an immunomodulatory role in the oxidative burst response of neutrophils.

Superoxide and derived reactive oxygen species in the regulation of hypoxia-inducible factors

Superoxide and its derived reactive oxygen species (ROS) have been considered for a long time to be generated as toxic byproducts of metabolic events. More recently, it has been acknowledged that ROS generated in low amounts are also able to act as signaling molecules in a variety of responses. One of the major pathways regulated by the ambient concentration of oxygen relies on the activity of hypoxia-inducible transcription factors (HIF). Originally described to be only induced and activated under hypoxia, accumulating evidence suggests that HIFs play a more general role in the response to a variety of cellular activators and stressors, many of which use ROS as signal transducers. Indeed, ROS have been found to modulate the levels of HIF not only under hypoxia, but also in response to many factors and under different stress conditions. However, the underlying regulatory mechanisms by which superoxide and derived ROS control HIF are only slowly beginning to be elucidated. We summarize here current knowledge about the mechanisms by which ROS can regulate HIF and give additional information about useful methods to determine ROS under various conditions.

How human neutrophils kill and degrade microbes: an integrated view

Neutrophils constitute the dominant cell in the circulation that mediates the earliest innate immune human responses to infection. The morbidity and mortality from infection rise dramatically in patients with quantitative or qualitative neutrophil defects, providing clinical confirmation of the important role of normal neutrophils for human health. Neutrophil-dependent anti-microbial activity against ingested microbes represents the collaboration of multiple agents, including those prefabricated during granulocyte development in the bone marrow and those generated de novo following neutrophil activation. Furthermore, neutrophils cooperate with extracellular agents as well as other immune cells to optimally kill and degrade invading microbes. This brief review focuses attention on two examples of the integrated nature of neutrophil-mediated anti-microbial action within the phagosome. The importance and complexity of myeloperoxidase-mediated events illustrate a collaboration of anti-microbial responses that are endogenous to the neutrophil, whereas the synergy between the phagocyte NADPH (nicotinamide adenine dinucleotide phosphate) oxidase and plasma-derived group IIA phospholipase A(2) exemplifies the collective effects of the neutrophil with an exogenous factor to achieve degradation of ingested staphylococci.

Reactive oxygen species and p47phox activation are essential for the Mycobacterium tuberculosis-induced pro-inflammatory response in murine microglia

Background

Activated microglia elicits a robust amount of pro-inflammatory cytokines, which are implicated in the pathogenesis of tuberculosis in the central nervous system (CNS). However, little is known about the intracellular signaling mechanisms governing these inflammatory responses in microglia in response to Mycobacterium tuberculosis (Mtb).

Methods

Murine microglial BV-2 cells and primary mixed glial cells were stimulated with sonicated Mtb (s-Mtb). Intracellular ROS levels were measured by staining with oxidative fluorescent dyes [2',7'-Dichlorodihydrofluorescein diacetate (H₂DCFDA) and dihydroethidium (DHE)]. NADPH oxidase activities were measured by lucigenin chemiluminescence assay. S-Mtb-induced MAPK activation and pro-inflammatory cytokine release in microglial cells were measured using by Western blot analysis and enzyme-linked immunosorbent assay, respectively.

Results

We demonstrate that s-Mtb promotes the up-regulation of reactive oxygen species (ROS) and the rapid activation of mitogen-activated protein kinases (MAPKs), including p38 and extracellular signal-regulated kinase (ERK) 1/2, as well as the secretion of tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-12p40 in murine microglial BV-2 cells and primary mixed glial cells. Both NADPH oxidase and mitochondrial electron transfer chain subunit I play an indispensable role in s-Mtb-induced MAPK activation and pro-inflammatory cytokine production in BV-2 cells and mixed glial cells. Furthermore, the activation of cytosolic NADPH oxidase p47phox and MAPKs (p38 and ERK1/2) is mutually dependent on s-Mtb-induced inflammatory signaling in murine microglia. Neither TLR2 nor dectin-1 was involved in s-Mtb-induced inflammatory responses in murine microglia.

Conclusion

These data collectively demonstrate that s-Mtb actively induces the pro-inflammatory response in microglia through NADPH oxidase-dependent ROS generation, although the specific pattern-recognition receptors involved in these responses remain to be identified.

ASK1-p38 MAPK-p47phox activation is essential for inflammatory responses during tuberculosis via TLR2-ROS signalling

The roles of intracellular reactive oxygen species (ROS) and related signalling pathways in mycobacterial infection are largely unknown. Here we show that tuberculin purified protein derivative (PPD)/Toll-like receptor (TLR) 2/ROS signalling through activation of apoptosis-regulating signal kinase (ASK) 1 and p47phox pathways is responsible for the induction of proinflammatory responses during tuberculosis (TB) infection. Tuberculin PPD stimulation resulted in rapid activation of mitogen-activated protein kinases (MAPKs) and an early burst of ROS in monocytes/macrophages in a TLR2-dependent manner. PPD-induced ROS production led to robust activation of ASK1 upstream of p38 MAPK, via TLR2. Interestingly, phosphorylation of the cytosolic NADPH oxidase subunit p47phox and ASK1 activation are mutually dependent on PPD/TLR2-mediated signalling. Furthermore, active pulmonary TB patients showed upregulated ROS generation, as well as enhanced activation of ASK1/p38/p47phox pathways in their primary monocytes compared with healthy controls, which suggests a systemic primed status during TB. Taken together, these results indicate that activation of the ASK1/p38 MAPK/p47phox cascade plays a central role in PPD/TLR2-induced ROS generation and suggests the existence of a 'ROS/ASK1' inflammatory amplification feedback loop in monocytes/macrophages. The altered regulation of this axis with an increasing free-radical burden may contribute to the immunopathogenesis of human TB.

Single-step immunoaffinity purification and functional reconstitution of human phagocyte flavocytochrome b

Human neutrophil flavocytochrome b (Cyt b) is a heterodimeric, integral membrane protein that generates high levels of superoxide in the multisubunit NADPH oxidase complex. Since Cyt b is currently isolated in limited quantities, improved methods for purification from low levels of starting membranes (from both neutrophils and other expressing cell types) are important for the analysis of structure and catalytic mechanism. In the present study, the epitope-mapped monoclonal antibody CS9 was coupled to Sepharose beads and used as an affinity matrix for single-step immunoaffinity purification of Cyt b. Following solubilization of both human neutrophil and PLB-985 membrane fractions in the nonionic detergent octylglucoside, Cyt b was absorbed on the CS9-Sepharose affinity matrix and purified protein was eluted under non-denaturing conditions with an epitope-mimicking peptide. The high efficiency of this isolation procedure allowed Cyt b to be reproducibly purified from readily obtainable levels of starting membrane fractions (9x10(8) cell equivalents of neutrophil membranes and 2x10(9) cell equivalents of PLB-985 membranes). Since Cyt b could be affinity-purified in the detergent octylglucoside, high-level functional reconstitution was carried out directly on elution fractions by simple addition of solubilized phospholipid and subsequent dialysis for detergent removal. To our knowledge, this study describes the most efficient method for generating purified, functionally-reconstituted Cyt b and should facilitate analyses that require a highly-defined NADPH oxidase system.

Regulation of Nox1 activity via protein kinase A-mediated phosphorylation of NoxA1 and 14-3-3 binding

Nox activator 1 (NoxA1) is a homologue of p67(phox) that acts in conjunction with Nox organizer 1 (NoxO1) to regulate reactive oxygen species (ROS) production by the NADPH oxidase Nox1. The phosphorylation of cytosolic regulatory components by multiple kinases plays important roles in assembly and activity of the phagocyte NADPH oxidase (Nox2) system, but little is known about regulation by phosphorylation in the Nox1 system. Here we identify Ser(172) and Ser(461) of NoxA1 as phosphorylation sites for protein kinase A (PKA). A consequence of this phosphorylation was the enhancement of NoxA1 complex formation with 14-3-3 proteins. Using both a transfected human embryonic kidney 293 cell Nox1 model system and endogenous Nox1 in colon cell lines, we showed that the elevation of cAMP inhibits, whereas the inhibition of PKA enhances, Nox1-dependent ROS production through effects on NoxA1. Inhibition of Nox1 activity was intensified by the availability of 14-3-3zeta protein, and this regulatory interaction was dependent on PKA-phosphorylatable sites at Ser(172) and Ser(461) in NoxA1. We showed that phosphorylation and 14-3-3 binding induce the dissociation of NoxA1 from the Nox1 complex at the plasma membrane, suggesting a mechanism for the inhibitory effect on Nox1 activity. Our data establish that PKA-phosphorylated NoxA1 is a new binding partner of 14-3-3 protein(s) and that this forms the basis of a novel mechanism regulating the formation of ROS by Nox1 and, potentially, other NoxA1-regulated Nox family members.

Rheumatoid arthritis: the role of reactive oxygen species in disease development and therapeutic strategies

Autoimmune diseases such as rheumatoid arthritis (RA) are chronic diseases that cannot be prevented or cured If the pathologic basis of such disease would be known, it might be easier to develop new drugs interfering with critical pathway. Genetic analysis of animal models for autoimmune diseases can result in discovery of proteins and pathways that play key function in pathogenesis, which may provide rationales for new therapeutic strategies. Currently, only the MHC class II is clearly associated with human RA and animal models for RA. However, recent data from rats and mice with a polymorphism in Ncf1, a member of the NADPH oxidase complex, indicate a role for oxidative burst in protection from arthritis. Oxidative burst-activating substances can treat and prevent arthritis in rats, as efficiently as clinically applied drugs, suggesting a novel pathway to a therapeutic target in human RA. Here, the authors discuss the role of oxygen radicals in regulating the immune system and autoimmune disease. It is proposed that reactive oxygen species set the threshold for T cell activation and thereby regulate chronic autoimmune inflammatory diseases like RA. In the light of this new hypothesis, new possibilities for preventive and therapeutic treatment of chronic inflammatory diseases are discussed.

Sinomenine, a natural dextrorotatory morphinan analog, is anti-inflammatory and neuroprotective through inhibition of microglial NADPH oxidase

Background

The mechanisms involved in the induction and regulation of inflammation resulting in dopaminergic (DA) neurotoxicity in Parkinson's disease (PD) are complex and incompletely understood. Microglia-mediated inflammation has recently been implicated as a critical mechanism responsible for progressive neurodegeneration.

Methods

Mesencephalic neuron-glia cultures and reconstituted cultures were used to investigate the molecular mechanisms of sinomenine (SN)-mediated anti-inflammatory and neuroprotective effects in both the lipopolysaccharide (LPS)- and the 1-methyl-4-phenylpyridinium (MPP⁺)-mediated models of PD.

Results

SN showed equivalent efficacy in protecting against DA neuron death in rat midbrain neuron-glial cultures at both micro- and sub-picomolar concentrations, but no protection was seen at nanomolar concentrations. The neuroprotective effect of SN was attributed to inhibition of microglial activation, since SN significantly decreased tumor necrosis factor-α (TNF-α, prostaglandin E₂ (PGE₂) and reactive oxygen species (ROS) production by microglia. In addition, from the therapeutic point of view, we focused on sub-picomolar concentration of SN for further mechanistic studies. We found that 10^-14 M of SN failed to protect DA neurons against MPP⁺-induced toxicity in the absence of microglia. More importantly, SN failed to show a protective effect in neuron-glia cultures from mice lacking functional NADPH oxidase (PHOX), a key enzyme for extracellular superoxide production in immune cells. Furthermore, we demonstrated that SN reduced LPS-induced extracellular ROS production through the inhibition of the PHOX cytosolic subunit p47^phoxtranslocation to the cell membrane.

Conclusion

Our findings strongly suggest that the protective effects of SN are most likely mediated through the inhibition of microglial PHOX activity. These findings suggest a novel therapy to treat inflammation-mediated neurodegenerative diseases.

NEUTROPHIL NAD(P)H OXIDASE IS REQUIRED FOR HEMORRHAGIC SHOCK-ENHANCED TLR2 UP-REGULATION IN ALVEOLAR MACROPHAGES IN RESPONSE TO LPS

Alveolar macrophages (AMs) play an important role in the development of posttrauma lung inflammation through initiating polymorphonuclear neutrophil (PMN) migration by direct interactions with PMN, which is in turn mediated by the expression of chemokines and cytokines. We have recently reported that hemorrhagic shock-activated PMN sensitize AM to bacteria LPS for the up-regulation of Toll-like receptor (TLR)2; in turn, this TLR2 up-regulation results in the amplification of expression of cytokines and chemokines in the AM in response to the bacterial products LPS and peptidoglycan, associated with enhanced PMN sequestration in the lung. We sought to address the mechanism underlying the augmentation of TLR2 in AM by shock-activated PMN. We found that hemorrhagic shock/resuscitation (shock) followed by a low dose of i.t. LPS markedly increased TLR2 mRNA expression in AM in wild-type (WT) mice. In contrast, in mice lacking the gp91 subunit of nicotinamide adenine dinucleotide phosphate (reduced form) oxidase (gp91) or in neutropenic WT mice, the increase in TLR2 mRNA was attenuated. Coculture of AM with PMN derived from WT-shocked mice caused a significantly higher level of TLR2 expression in the AM in response to LPS. However, this increase in TLR2 expression was less evident when the AMs were cocultured with PMN derived from gp91 mice subjected to shock. The antioxidant polyethylene glycol catalase markedly decreased MyD88-dependent activation of IL-1 receptor associated kinase 4 and TLR2 expression in the AM in response to LPS. Thus, PMN nicotinamide adenine dinucleotide phosphate (reduced form) oxidase sensitizes hemorrhagic shock-primed AM to LPS, at least in part via enhancing IL-1 receptor associated kinase 4 activity.

Redox-active cysteines of a membrane electron transporter DsbD show dual compartment accessibility

The membrane-embedded domain of the unusual electron transporter DsbD (DsbDbeta) uses two redox-active cysteines to catalyze electron transfer between thioredoxin-fold polypeptides on opposite sides of the bacterial cytoplasmic membrane. How the electrons are transferred across the membrane is unknown. Here, we show that DsbDbeta displays an inherent functional and structural symmetry: first, the two cysteines of DsbDbeta can be alkylated from both the cytoplasm and the periplasm. Second, when the two cysteines are disulfide-bonded, cysteine scanning shows that the C-terminal halves of the cysteine-containing transmembrane segments 1 and 4 are exposed to the aqueous environment while the N-terminal halves are not. Third, proline residues located pseudo-symmetrically around the two cysteines are required for redox activity and accessibility of the cysteines. Fourth, mixed disulfide complexes, apparent intermediates in the electron transfer process, are detected between DsbDbeta and thioredoxin molecules on each side of the membrane. We propose a model where the two redox-active cysteines are located at the center of the membrane, accessible on both sides of the membrane to the thioredoxin proteins.

Activation of NADPH oxidase by transforming growth factor-beta in hepatocytes mediates up-regulation of epidermal growth factor receptor ligands through a nuclear factor-kappaB-dependent mechanism

The TGF-beta (transforming growth factor-beta) induces survival signals in foetal rat hepatocytes through transactivation of EGFR (epidermal growth factor receptor). The molecular mechanism is not completely understood, but both activation of the TACE (tumour necrosis factor alpha-converting enzyme)/ADAM17 (a disintegrin and metalloproteinase 17; one of the metalloproteases involved in shedding of the EGFR ligands) and up-regulation of TGF-alpha and HB-EGF (heparin-binding epidermal growth factor-like growth factor) appear to be involved. In the present study, we have analysed the molecular mechanisms that mediate up-regulation of the EGFR ligands by TGF-beta in foetal rat hepatocytes. The potential involvement of ROS (reactive oxygen species), an early signal induced by TGF-beta, and the existence of an amplification loop triggered by initial activation of the EGFR, have been studied. Results indicate that DPI (diphenyleneiodonium) and apocynin, two NOX (NADPH oxidase) inhibitors, and SB431542, an inhibitor of the TbetaR-I (TGF-beta receptor I), block up-regulation of EGFR ligands and Akt activation. Different members of the NOX family of genes are expressed in hepatocytes, included nox1, nox2 and nox4. TGF-beta up-regulates nox4 and increases the levels of Rac1 protein, a known regulator of both Nox1 and Nox2, in a TbetaR-I-dependent manner. TGF-beta mediates activation of the nuclear factor-kappaB pathway, which is inhibited by DPI and is required for up-regulation of TGF-alpha and HB-EGF. In contrast, EGFR activation is not required for TGF-beta-induced up-regulation of those ligands. Considering previous work that has established the role of ROS in apoptosis induced by TGF-beta in hepatocytes, the results of the present study indicate that ROS might mediate both pro- and anti-apoptotic signals in TGF-beta-treated cells.

Hemorrhagic shock induces NAD(P)H oxidase activation in neutrophils: role of HMGB1-TLR4 signaling

Hemorrhagic shock/resuscitation (HS/R)-induced generation of reactive oxygen species (ROS) plays an important role in posthemorrhage inflammation and tissue injury. We have recently reported that HS/R-activated neutrophils (PMN), through release of ROS, serve an important signaling function in mediating alveolar macrophage priming and lung inflammation. PMN NAD(P)H oxidase has been thought to be an important source of ROS following HS/R. TLR4 sits at the interface of microbial and sterile inflammation by mediating responses to both bacterial endotoxin and multiple endogenous ligands, including high-mobility group box 1 (HMGB1). Recent studies have implicated HMGB1 as an early mediator of inflammation after HS/R and organ ischemia/reperfusion. In the present study, we tested the hypothesis that HS/R activates NAD(P)H oxidase in PMN through HMGB1/TLR4 signaling. We demonstrated that HS/R induced PMN NAD(P)H oxidase activation, in the form of phosphorylation of p47phox subunit of NAD(P)H oxidase, in wild-type mice; this induction was significantly diminished in TLR4-mutant C3H/HeJ mice. HMGB1 levels in lungs, liver, and serum were increased as early as 2 h after HS/R. Neutralizing Ab to HMGB1 prevented HS/R-induced phosphorylation of p47phox in PMN. In addition, in vitro stimulation of PMN with recombinant HMGB1 caused TLR4-dependent activation of NAD(P)H oxidase as well as increased ROS production through both MyD88-IRAK4-p38 MAPK and MyD88-IRAK4-Akt signaling pathways. Thus, PMN NAD(P)H oxidase activation, induced by HS/R and as mediated by HMGB1/TLR4 signaling, is an important mechanism responsible for PMN-mediated inflammation and organ injury after hemorrhage.

Tripartite chimeras comprising functional domains derived from the cytosolic NADPH oxidase components p47phox, p67phox, and Rac1 elicit activator-independent superoxide production by phagocyte membranes: an essential role for anionic membrane phospholipids

The superoxide-generating NADPH oxidase is converted to an active state by the assembly of a membrane-localized cytochrome b(559) with three cytosolic components: p47(phox), p67(phox), and GTPase Rac1 or Rac2. Assembly involves two sets of protein-protein interactions: among cytosolic components and among cytosolic components and cytochrome b(559) within its lipid habitat. We circumvented the need for interactions among cytosolic components by constructing a recombinant tripartite chimera (trimera) consisting of the Phox homology (PX) and Src homology 3 (SH3) domains of p47(phox), the tetratricopeptide repeat and activation domains of p67(phox), and full-length Rac1. Upon addition to phagocyte membrane, the trimera was capable of oxidase activation in vitro in the presence of an anionic amphiphile. The trimera had a higher affinity (lower EC(50)) for and formed a more stable complex (longer half-life) with cytochrome b(559) compared with the combined individual components, full-length or truncated. Supplementation of membrane with anionic but not neutral phospholipids made activation by the trimera amphiphile-independent. Mutagenesis, truncations, and domain replacements revealed that oxidase activation by the trimera was dependent on the following interactions: 1) interaction with anionic membrane phospholipids via the poly-basic stretch at the C terminus of the Rac1 segment; 2) interaction with p22(phox) via Trp(193) in the N-terminal SH3 domain of the p47(phox) segment, supplementing the electrostatic attraction; and 3) an intrachimeric bond among the p67(phox) and Rac1 segments complementary to their physical fusion. The PX domain of the p47(phox) segment and the insert domain of the Rac1 segment made only minor contributions to oxidase assembly.

Hypoglycemic neuronal death is triggered by glucose reperfusion and activation of neuronal NADPH oxidase

Hypoglycemic coma and brain injury are potential complications of insulin therapy. Certain neurons in the hippocampus and cerebral cortex are uniquely vulnerable to hypoglycemic cell death, and oxidative stress is a key event in this cell death process. Here we show that hypoglycemia-induced oxidative stress and neuronal death are attributable primarily to the activation of neuronal NADPH oxidase during glucose reperfusion. Superoxide production and neuronal death were blocked by the NADPH oxidase inhibitor apocynin in both cell culture and in vivo models of insulin-induced hypoglycemia. Superoxide production and neuronal death were also blocked in studies using mice or cultured neurons deficient in the p47(phox) subunit of NADPH oxidase. Chelation of zinc with calcium disodium EDTA blocked both the assembly of the neuronal NADPH oxidase complex and superoxide production. Inhibition of the hexose monophosphate shunt, which utilizes glucose to regenerate NADPH, also prevented superoxide formation and neuronal death, suggesting a mechanism linking glucose reperfusion to superoxide formation. Moreover, the degree of superoxide production and neuronal death increased with increasing glucose concentrations during the reperfusion period. These results suggest that high blood glucose concentrations following hypoglycemic coma can initiate neuronal death by a mechanism involving extracellular zinc release and activation of neuronal NADPH oxidase.

NADPH oxidase- and mitochondrion-derived superoxide at rostral ventrolateral medulla in endotoxin-induced cardiovascular depression

We evaluated the contribution of superoxide anion (O2*-) generated by NADPH oxidase or mitochondria in the rostral ventrolateral medulla (RVLM), where sympathetic premotor neurons for arterial pressure maintenance are located, on cardiovascular depression induced by inducible nitric oxide synthase-derived NO after Escherichia coli lipopolysaccharide (LPS) treatment. In Sprague-Dawley rats maintained under propofol anesthesia, microinjection of LPS bilaterally into the RVLM induced progressive hypotension, bradycardia, and reduction in sympathetic vasomotor outflow over our 240-min observation period. This was accompanied by an increase in O2*- production (60-240 min) in the RVLM, alongside phosphorylation of p47(phox) or p67(phox), upregulation of gp91(phox) or p47(phox) protein, and increase in Rac-1 or NADPH oxidase activity (60-120 min), and a depression of mitochondrial respiratory enzyme activity (120-240 min). Whereas inhibition of NADPH oxidase or knockdown of the gp91(phox) or p47(phox) gene blunted the early phase (60-150 min), coenzyme Q10 or mitochondrial K(ATP) channel inhibitor antagonized the delayed phase (120-240 min) of LPS-induced increase in O2*- production in RVLM and cardiovascular depression. We conclude that, whereas NADPH oxidase-derived O2*- in RVLM participates predominantly in the early phase, O2*- generated by depression in mitochondrial respiratory enzyme activity or opening of mitoK(ATP) channels mediates the delayed phase of LPS-induced cardiovascular depression.

The reactivity of ortho-methoxy-substituted catechol radicals with sulfhydryl groups: contribution for the comprehension of the mechanism of inhibition of NADPH oxidase by apocynin

Redox processes are involved in the mechanism of action of NADPH oxidase inhibitors such as diphenyleneiodonium and apocynin. Here, we studied the structure-activity relationship for apocynin and analogous ortho-methoxy-substituted catechols as inhibitors of the NADPH oxidase in neutrophils and their reactivity with peroxidase. Aiming to alter the reduction potential, the ortho-methoxy-catechol moiety was kept constant and the substituents at para position related to the hydroxyl group were varied. Two series of compounds were employed: methoxy-catechols bearing electron-withdrawing groups (MC-W) such as apocynin, vanillin, 4-nitroguaiacol, 4-cyanoguaiacol, and methoxy-catechol bearing electron-donating groups (MC-D) such as 4-methylguaiacol and 4-ethylguaiacol. We found that MC-D were weaker inhibitors compared to MD-W. Furthermore, the radicals generated by oxidation of MC-W via MPO/H(2)O(2), but not for MC-D, were able to oxidize glutathione (GSH) as verified by the formation of thiyl radicals, depletion of GSH, and recycling of the ortho-methoxy-catechols during their oxidations. The capacity of oxidizing sulfhydryl (SH) groups was also verified when ovalbumin was incubated with MC-W, but not for MC-D. Since the effect of apocynin has been correlated with inactivation of the cytosolic fractions of the NADPH oxidase complex and its oxidation during the inhibitory process develops a special role in this process, we suggest that the close relationship between the reactivity of the radicals of MC-W compounds with thiol groups and their efficacy as NADPH oxidase inhibitor could be the chemical pathway behind the mechanism of action of apocynin and should be taken into account in the design of new and specific NADPH oxidase inhibitors.

NADPH oxidase 5 (NOX5) interacts with and is regulated by calmodulin

Superoxide generation by NADPH oxidase 5 (NOX5) is regulated by Ca(2+) through intramolecular activation of the C-terminal catalytic domain by the EF-hand-containing N-terminal regulatory domain. The C terminus contains a consensus calmodulin-binding domain (CaMBD), which, however, is not the binding site of the N-terminal regulatory domain. Here we show by pull down, cross-linking, fluorimetry and by enzymatic assays, that calmodulin binds to this CaMBD in a Ca(2+)-dependent manner, changes its conformation and increases the Ca(2+) sensitivity of the N terminus-regulated enzymatic activity. This mechanism represents an additional sophistication in the regulation of superoxide production by NOX5.

The inflammatory response in stroke

Recent works in the area of stroke and brain ischemia has demonstrated the significance of the inflammatory response accompanying necrotic brain injury. Acutely, this response appears to contribute to ischemic pathology, and anti-inflammatory strategies have become popular. This chapter will discuss the current knowledge of the contribution of systemic and local inflammation in experimental stroke. It will review the role of specific cell types including leukocytes, endothelium, glia, microglia, the extracellular matrix and neurons. Intracellular inflammatory signaling pathways such as nuclear factor kappa beta and mitogen-activated protein kinases, and mediators produced by inflammatory cells such as cytokines, chemokines, reactive oxygen species and arachidonic acid metabolites will be reviewed as well as the potential for therapy in stroke and hypoxic-ischemic injury.

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.

Nucleotide receptor signalling and the generation of reactive oxygen species

Elevated levels of extracellular nucleotides are present at sites of inflammation, platelet degranulation and cellular damage or lysis. These extracellular nucleotides can lead to the activation of purinergic (nucleotide) receptors on various leukocytes, including monocytes, macrophages, eosinophils, and neutrophils. In turn, nucleotide receptor activation has been linked to increased cellular production and release of multiple inflammatory mediators, including superoxide anion, nitric oxide and other reactive oxygen species (ROS). In the present review, we will summarize the evidence that extracellular nucleotides can facilitate the generation of multiple ROS by leukocytes. In addition, we will discuss several potential mechanisms by which nucleotide-enhanced ROS production may occur. Delineation of these mechanisms is important for understanding the processes associated with nucleotide-induced antimicrobial activities, cell signalling, apoptosis, and pathology.

The oxidation of apocynin catalyzed by myeloperoxidase: Proposal for NADPH oxidase inhibition

Apocynin has been used as an efficient inhibitor of the NADPH oxidase complex and its mechanism of inhibition is linked to prior activation through the action of peroxidases. Here we studied the oxidation of apocynin catalyzed by myeloperoxidase (MPO) and activated neutrophils. We found that apocynin is easily oxidized by MPO/H2O2 or activated neutrophils and has as products dimer and trimer derivatives. Since apocynin impedes the migration of the cytosolic component p47phox to the membrane and this effect could be related to its conjugation with essential thiol groups, we studied the reactivity of apocynin and its MPO-catalyzed oxidation products with glutathione (GSH). We found that apocynin and its oxidation products do not react with GSH. However, this thiol compound was efficiently oxidized by the apocynin radical during the MPO-catalyzed oxidation. We suggest that the reactivity of apocynin radical with thiol compounds could be involved in the inhibitory effect of this methoxy-catechol on NADPH oxidase complex.

Bone-marrow derived hematopoietic stem/progenitor cells express multiple isoforms of NADPH oxidase and produce constitutively reactive oxygen species

Consolidated evidence highlights the importance of redox signalling in poising the balance between self-renewal and differentiation in adult stem cells. The present study shows that human hematopoietic stem/progenitor cells (HSCs) constitutively generate low levels of hydrogen peroxide whose production is inhibited by DPI, apocynin, catalase, and LY294002 and scarcely stimulated by PMA. Moreover, it is shown that HSCs express at the mRNA and protein levels the catalytic subunits of NOX1, NOX2, and NOX4 isoforms of the NADPH oxidase family along with the complete battery of the regulatory subunits p22, p40, p47, p67, rac1, rac2, NOXO1, and NOXA1 as well as the splicing variant NOX2s and that the three NOX isoforms are largely co-expressed in the same HSC. These findings are interpreted in terms of a positive feed-back mechanism of NOXs activation enabling a fine tuning of the ROS level to be possibly used in redox-mediated signalling for growth and differentiation of HSCs.

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.

Inflammation in adult and neonatal stroke

This chapter will discuss the current knowledge of the contribution of systemic and local inflammation in acute and sub-chronic stages of experimental stroke in both the adult and neonate. It will review the role of specific cell types and interactions among blood cells, endothelium, glia, microglia, the extracellular matrix and neurons - cumulatively called "neurovascular unit" - in stroke induction and evolution. Intracellular inflammatory signaling pathways such as nuclear factor kappa beta and mitogen-activated protein kinases, and mediators produced by inflammatory cells such as cytokines, chemokines, reactive oxygen species and arachidonic acid metabolites, as well as the modifying role of age on these mechanisms, will be reviewed as well as the potential for therapy in stroke and hypoxic-ischemic injury.

Leu505 of Nox2 is crucial for optimal p67phox-dependent activation of the flavocytochrome b558 during phagocytic NADPH oxidase assembly

The role of Leu505 of Nox2 on the NADPH oxidase activation process was investigated. An X-CGD PLB-985 cell line expressing the Leu505Arg Nox2 mutant was obtained, exactly mimicking the phenotype of a previously published X91+-CGD case. In a reconstituted cell-free system (CFS), NADPH oxidase and iodonitrotetrazolium (INT) reductase activities were partially maintained concomitantly with a partial cytosolic factors translocation to the plasma membrane. This suggests that assembly and electron transfer from NADPH occurred partially in the Leu505Arg Nox2 mutant. Moreover, in a simplified CFS using purified mutant cytochrome b558 and recombinant p67phox, p47phox, and Rac1proteins, we found that the Km for NADPH and for NADH was about three times higher than those of purified WT cytochrome b558, indicating that the Leu505Arg mutation induces a slight decrease of the affinity for NADPH and NADH. In addition, oxidase activity can be extended by increasing the amount of p67phox in the simplified CFS assay. However, the maximal reconstituted oxidase activity using WT purified cytochrome b558 could not be reached using mutant cytochrome b558. In a three-dimensional model of the C-terminal tail of Nox2, Leu505 appears to have a strategic position just at the entry of the NADPH binding site and at the end of the alpha-helical loop (residues 484-504), a potential cytosolic factor binding region. The Leu505Arg mutation seems to affect the oxidase complex activation process through alteration of cytosolic factors binding and more particularly the p67phox interaction with cytochrome b558, thus affecting NADPH access to its binding site.

The Rac effector p67phox regulates phagocyte NADPH oxidase by stimulating Vav1 guanine nucleotide exchange activity

The phagocyte NADPH oxidase catalyzes the reduction of molecular oxygen to superoxide and is essential for microbial defense. Electron transport through the oxidase flavocytochrome is activated by the Rac effector p67(phox). Previous studies suggest that Vav1 regulates NADPH oxidase activity elicited by the chemoattractant formyl-Met-Leu-Phe (fMLP). We show that Vav1 associates with p67(phox) and Rac2, but not Rac1, in fMLP-stimulated human neutrophils, correlating with superoxide production. The interaction of p67(phox) with Vav1 is direct and activates nucleotide exchange on Rac, which enhances the interaction between p67(phox) and Vav1. This provides new molecular insights into regulation of the neutrophil NADPH oxidase, suggesting that chemoattractant-stimulated superoxide production can be amplified by a positive feedback loop in which p67(phox) targets Vav1-mediated Rac activation.

Tubulin ligands suggest a microtubule–NADPH oxidase relationship in postischemic cardiomyocytes

Alterations of the microtubule network, which is involved in many vital processes, occur in several pathological conditions, such as cardiac ischemia. However, the connection between the microtubule assembly state and the factors affecting myocardial reperfusion injury, especially oxidative stress, is unknown. We aimed thus to study the effects of different tubulin ligands on the changes in the microtubule network and in several markers of cell injury and oxidative activity in cardiac muscle cells submitted to a reversible substrate-free, hypoxia-reoxygenation model of ischemia-reperfusion. The microtubule network was visualized by immunocytochemistry. Cell injury was evaluated via lactate dehydrogenase release and the mitochondrial function by the MTT test. Superoxide production was detected using dihydroethidium. The activity of NADPH oxidase and mRNA subunit expression were investigated. The microtubule disassembly induced by simulated ischemia was reversed by placing cardiomyocytes under normoxic conditions. This post-"ischemic" restoration of microtubule assembly was modulated by microtubule stabilizers (taxol: paclitaxel) and by microtubule disrupting drugs (nocodazole, colchicine). In addition, nocodazole decreased superoxide anion production as well as NADPH oxidase activity and mRNA expression of the NADPH oxidase subunit p22phox. These results demonstrated that the "ischemia"-induced microtubule network alteration is reversible and suggest a possible relationship between "reperfusion"-induced reassembly of microtubules and free radical generation in post-"ischemic" cardiomyocytes.

PtdIns3P binding to the PX domain of p40phox is a physiological signal in NADPH oxidase activation

The production of reactive oxygen species by the NADPH oxidase complex of phagocytes plays a critical role in our defence against bacterial and fungal infections. The PX domains of two oxidase components, p47(phox) and p40(phox), are known to bind phosphoinositide products of PI3Ks but the physiological roles of these interactions are unclear. We have created mice which carry an R58A mutation in the PX domain of their p40(phox) gene, which selectively prevents binding to PtdIns3P. p40(phoxR58A/R58A) embryos do not develop normally but p40(phoxR58A/-) mice are viable and neutrophils from these animals exhibit significantly reduced oxidase responses compared to those from their p40(phox+/-) siblings (e.g. 60% reduced in response to phagocytosis of Staphylococcus aureus). Wortmannin inhibition of the S. aureus oxidase response correlates with inhibition of phagosomal PtdIns3P accumulation and overlaps with the reduction in this response caused by the R58A mutation, suggesting PI3K regulation of this response is substantially dependent on PtdIns3P-binding to p40(phox). p40(phoxR58A/-) mice are significantly compromised in their ability to kill S. aureus in vivo, defining the physiological importance of this interaction.

Synaptic plasticity deficits and mild memory impairments in mouse models of chronic granulomatous disease

Reactive oxygen species (ROS) are required in a number of critical cellular signaling events, including those underlying hippocampal synaptic plasticity and hippocampus-dependent memory; however, the source of ROS is unknown. We previously have shown that NADPH oxidase is required for N-methyl-D-aspartate (NMDA) receptor-dependent signal transduction in the hippocampus, suggesting that NADPH oxidase may be required for NMDA receptor-dependent long-term potentiation (LTP) and hippocampus-dependent memory. Herein we present the first evidence that NADPH oxidase is involved in hippocampal synaptic plasticity and memory. We have found that pharmacological inhibitors of NADPH oxidase block LTP. Moreover, mice that lack the NADPH oxidase proteins gp91(phox) and p47(phox), both of which are mouse models of human chronic granulomatous disease (CGD), also lack LTP. We also found that the gp91(phox) and p47(phox) mutant mice have mild impairments in hippocampus-dependent memory. The gp91(phox) mutant mice exhibited a spatial memory deficit in the Morris water maze, and the p47(phox) mutant mice exhibited impaired context-dependent fear memory. Taken together, our results are consistent with NADPH oxidase being required for hippocampal synaptic plasticity and memory and are consistent with reports of cognitive dysfunction in patients with CGD.

Intracellular redox regulation by the family of small GTPases

There is growing experimental evidence that the production of intracellular reactive oxygen species (ROS) represents a tightly regulated process. In particular, numerous observations have suggested a role for the Ras superfamily of small GTPases in redox regulation. This article reviews the evidence that ROS can serve as important downstream effectors for both Ras and Rac proteins. Given the prominent role these proteins play in regulating growth, senescence, and transformation, understanding the role of the small GTPase family in redox regulation may significantly alter our current concepts as to how free radicals contribute to diverse processes from aging to cancer.

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.

Regulation of the phagocyte NADPH oxidase by Rac GTPase

Phagocytic leukocytes generate reactive oxygen species important for the killing of invading microorganisms. The source of these oxidants is the NADPH oxidase, a tightly controlled multicomponent enzyme made up of a membrane-associated catalytic moiety and cytosolic regulatory components that must assemble to form the active oxidase. The phagocyte NADPH oxidase was the first mammalian system shown to be directly regulated by a Rac GTPase. We review here our understanding of NADPH oxidase regulation by Rac, as well as the regulation of Rac itself, in phagocytic leukocytes. Rather than viewing Rac as a "cog" in the NADPH oxidase machinery, we argue for a view of Rac GTPases as critical "molecular switches" regulating the formation of ROS by phagocytic leukocytes under physiologic and pathologic conditions.

Regulation of novel superoxide-producing NAD(P)H oxidases

Deliberate production of reactive oxygen species (ROS) are catalyzed by enzymes that belong to the NAD(P)H oxidase (Nox) family. The human genome contains seven members of the Nox family: the superoxide-producing enzymes Nox1 through Nox5 and the dual oxidases Duox1 and Duox2 that release hydrogen peroxide but not superoxide. Among them, the classical member gp91( phox )/Nox2 functions as the phagocyte NADPH oxidase, playing a crucial role in host defense. Although Nox2, heterodimerized with its membrane-spanning partner p22( phox ), is inactive in resting cells, during phagocytosis it forms an active complex with soluble regulatory proteins such as the organizer p47( phox ), the activator p67( phox ), and the small GTPase Rac. Here the authors describe how the novel superoxide-producing Nox oxidases (Nox1, 3, 4, and 5) with different functions are regulated by p22( phox ), the Nox organizers, the Nox activators, and Rac, and how their expression is controlled at the transcriptional level.

Taurine chloramine inhibits PMA-stimulated superoxide production in human neutrophils perhaps by inhibiting phosphorylation and translocation of p47phox

Neutrophils produce microbicidal oxidants to destroy the invading pathogens using nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, a membrane-associated enzyme complex that generates superoxide anion (O(2)(-)). Upon stimulation, the cytosolic components of NADPH oxidase, p47(phox) and p67(phox) and the small GTPase Rac move to phagosomal and plasma membranes where they become associated with the membrane components of NADPH oxidase, gp91(phox) and p22(phox) and express enzyme activity. We previously showed that taurine chloramine (Tau-Cl) inhibits O(2)(-) production in mouse peritoneal neutrophils (Kim, 1996). In the present study, we investigated the mechanisms underlying Tau-Cl-derived inhibition on O(2)(-) production using a human myeloid leukemia cell line, PLB-985 cell, which has been differentiated into neutrophil-like cell. Tau-Cl inhibited the phorbol myristate acetate (PMA)-elicited O(2)(-) production as previously observed in murine peritoneal neutrophils. Translocation of p47(phox), p67(phox) and Rac was increased in response to PMA, and Tau-Cl inhibited the PMA-stimulated translocation of p47(phox) and p67(phox) to plasma membrane without affecting the translocation of Rac. In addition, Tau-Cl inhibited the PMA-derived phosphorylation of p47(phox), a requirement for the translocation of cytosolic NADPH oxidase component to the plasma membrane. These results suggest that Tau-Cl inhibits PMA-elicited O(2)(-) production in PLB-985 granulocytes by inhibiting phosphorylation of p47(phox) and translocation of p47(phox) and p67(phox), eventually blocking the assembly of NADPH oxidase complex.