Activation of the superoxide-producing phagocyte NADPH oxidase requires co-operation between the tandem SH3 domains of p47phox in recognition of a polyproline type II helix and an adjacent alpha-helix of p22phox

NADPH Oxidase 3: Beyond the Inner Ear

Reactive oxygen species (ROS) were formerly known as mere byproducts of metabolism with damaging effects on cellular structures. The discovery and description of NADPH oxidases (Nox) as a whole enzyme family that only produce this harmful group of molecules was surprising. After intensive research, seven Nox isoforms were discovered, described and extensively studied. Among them, the NADPH oxidase 3 is the perhaps most underrated Nox isoform, since it was firstly discovered in the inner ear. This stigma of Nox3 as "being only expressed in the inner ear" was also used by me several times. Therefore, the question arose whether this sentence is still valid or even usable. To this end, this review solely focuses on Nox3 and summarizes its discovery, the structural components, the activating and regulating factors, the expression in cells, tissues and organs, as well as the beneficial and detrimental effects of Nox3-mediated ROS production on body functions. Furthermore, the involvement of Nox3-derived ROS in diseases progression and, accordingly, as a potential target for disease treatment, will be discussed.

NADPH Oxidases (NOX): An Overview from Discovery, Molecular Mechanisms to Physiology and Pathology

The reactive oxygen species (ROS)-producing enzyme NADPH oxidase (NOX) was first identified in the membrane of phagocytic cells. For many years, its only known role was in immune defense, where its ROS production leads to the destruction of pathogens by the immune cells. NOX from phagocytes catalyzes, via one-electron trans-membrane transfer to molecular oxygen, the production of the superoxide anion. Over the years, six human homologs of the catalytic subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1, and DUOX2. Together with the NOX2/gp91phox component present in the phagocyte NADPH oxidase assembly itself, the homologs are now referred to as the NOX family of NADPH oxidases. NOX are complex multidomain proteins with varying requirements for assembly with combinations of other proteins for activity. The recent structural insights acquired on both prokaryotic and eukaryotic NOX open new perspectives for the understanding of the molecular mechanisms inherent to NOX regulation and ROS production (superoxide or hydrogen peroxide). This new structural information will certainly inform new investigations of human disease. As specialized ROS producers, NOX enzymes participate in numerous crucial physiological processes, including host defense, the post-translational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. These diversities of physiological context will be discussed in this review. We also discuss NOX misregulation, which can contribute to a wide range of severe pathologies, such as atherosclerosis, hypertension, diabetic nephropathy, lung fibrosis, cancer, or neurodegenerative diseases, giving this family of membrane proteins a strong therapeutic interest.

Class I phosphoinositide 3-kinases control sustained NADPH oxidase activation in adherent neutrophils

Phagocytes, especially neutrophils, can produce reactive oxygen species (ROS), through the activation of the NADPH oxidase (NOX2). Although this enzyme is crucial for host-pathogen defense, ROS production by neutrophils can be harmful in several pathologies such as cardiovascular diseases or chronic pulmonary diseases. The ROS production by NOX2 involves the assembly of the cytosolic subunits (p67^phox, p47^phox, and p40^phox) and Rac with the membrane subunits (gp91^phox and p22^phox). Many studies are devoted to the activation of NOX2. However, the mechanisms that cause NADPH oxidase deactivation and thus terminate ROS production are not well known. Here we investigated the ability of class I phosphoinositide 3-kinases (PI3Ks) to sustain NADPH oxidase activation. The NADPH oxidase activation was triggered by seeding neutrophil-like PLB-985 cells, or human neutrophils on immobilized fibrinogen. Adhesion of the neutrophils, mediated by β2 integrins, induced activation of the NADPH oxidase and translocation of the cytosolic subunits at the plasma membrane. Inhibition of class I PI3Ks, and especially PI3Kβ, terminated ROS production. This deactivation of NOX2 is due to the release of the cytosolic subunits, p67^phox and p47^phox from the plasma membrane. Overexpression of an active form of Rac 1 did not prevent the drop of ROS production upon inhibition of class I PI3Ks. Moreover, the phosphorylation of p47^phox at S328, a potential target of kinases activated by the PI3K pathway, was unchanged. Our results indicate that the experimental downregulation of class I PI3K products triggers the plasma membrane NADPH oxidase deactivation. Release of p47^phox from the plasma membrane may involve its PX domains that bind PI3K products.

The Emerging Roles of Nicotinamide Adenine Dinucleotide Phosphate Oxidase 2 in Skeletal Muscle Redox Signaling and Metabolism

Significance: Skeletal muscle is a crucial tissue to whole-body locomotion and metabolic health. Reactive oxygen species (ROS) have emerged as intracellular messengers participating in both physiological and pathological adaptations in skeletal muscle. A complex interplay between ROS-producing enzymes and antioxidant networks exists in different subcellular compartments of mature skeletal muscle. Recent evidence suggests that nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) are a major source of contraction- and insulin-stimulated oxidants production, but they may paradoxically also contribute to muscle insulin resistance and atrophy. Recent Advances: Pharmacological and molecular biological tools, including redox-sensitive probes and transgenic mouse models, have generated novel insights into compartmentalized redox signaling and suggested that NOX2 contributes to redox control of skeletal muscle metabolism. Critical Issues: Major outstanding questions in skeletal muscle include where NOX2 activation occurs under different conditions in health and disease, how NOX2 activation is regulated, how superoxide/hydrogen peroxide generated by NOX2 reaches the cytosol, what the signaling mediators are downstream of NOX2, and the role of NOX2 for different physiological and pathophysiological processes. Future Directions: Future research should utilize and expand the current redox-signaling toolbox to clarify the NOX2-dependent mechanisms in skeletal muscle and determine whether the proposed functions of NOX2 in cells and animal models are conserved into humans.

Recombinant form of mammalian gp91phox is active in the absence of p22phox

In phagocytes, gp91phox is the key membrane component of the NADPH oxidase complex. In contrast with what was known from studies in mammalian phagocytes, in Pichia pastoris we succeeded in producing an active catalytic subunit gp91phox in absence of its membrane partner.

Effects of angiotensin II on the cerebral circulation: role of oxidative stress

Oxidative stress has emerged as a key component of many diseases that affect the vasculature. Oxidative stress is characterized as a cellular environment where the generation of oxidant molecules overwhelms endogenous anti-oxidant defense mechanisms. NADPH oxidases are a family of enzymes whose primary purpose is generation of reactive oxygen species (oxidant molecules) and therefore are likely to be key contributors to oxidative stress. Hypertension is associated with oxidative stress in the vasculature and is a major risk factor for stroke and cognitive abnormalities. Angiotensin II (Ang II) is the main effector peptide of the renin-angiotensin system (RAS) and plays a critical role in promoting oxidative stress in the vasculature. In the cerebral circulation, Ang II has been implicated in reactive oxygen species generation, alterations to vasomotor function, impaired neurovascular coupling, inflammation, and vascular remodeling. Furthermore, studies in humans have shown that cerebral blood flow is altered during hypertension and therapeutically targeting the RAS improves cerebral blood flow. Importantly, many of the aforementioned effects have been shown to be dependent on NADPH oxidases. Thus, Ang II, NADPH oxidases and oxidative stress are likely to play key roles in the pathogenesis of hypertension and associated cerebrovascular disease. This review will focus on our current understanding of the contribution of Ang II and NADPH oxidases to oxidative stress in the cerebral circulation.

Identification of a region in p47phox/NCF1 crucial for phagocytic NADPH oxidase (NOX2) activation

A point mutation in the mouse Ncf1(m1J) gene decreases production of ROS by the phagocytic NOX2 complex. Three mRNA splice variants are expressed, but only one is expressed as a protein, although at lower levels than the WT NCF1 (also known as p47phox). Our aim was to investigate whether the mutant p47phox, lacking 8 aa, is active, but as a result of its low expression, ROS production is decreased in Ncf1(m1J) mice, or whether the mutant p47phox completely lacks the capability to activate the NOX2 complex. The p47phox mutant (Δ228-235), which was equal to the protein in Ncf1(m1J) mice, failed to activate the NOX2 complex. When the deleted region was narrowed down to 2 aa, the p47phox protein remained inactive and failed to translocate to the membrane upon activation. Single amino acid substitutions revealed Thr233 to be vital for ROS production. Residues Tyr231 and Val232 also seemed to be important for p47phox function, as p47phox_Y231G and p47phox_V232G resulted in a >50% decrease in ROS production by the NOX2 complex. In addition, we identified the epitope of the D-10 anti-p47phox mAb. In conclusion, the p47phox protein variant expressed in Ncf1(m1J) mice is completely defective in activating the NOX2 complex to produce ROS, and the effect is dependent on SH3 region amino acids at positions 231-233, which are vital for the proper assembly of the NOX2 complex.

Transient structure and SH3 interaction sites in an intrinsically disordered fragment of the hepatitis C virus protein NS5A

Understanding the molecular mechanisms involved in virus replication and particle assembly is of primary fundamental and biomedical importance. Intrinsic conformational disorder plays a prominent role in viral proteins and their interaction with other viral and host cell proteins via transiently populated structural elements. Here, we report on the results of an investigation of an intrinsically disordered 188-residue fragment of the hepatitis C virus non-structural protein 5A (NS5A), which contains a classical poly-proline Src homology 3 (SH3) binding motif, using sensitivity- and resolution-optimized multidimensional NMR methods, complemented by small-angle X-ray scattering data. Our study provides detailed atomic-resolution information on transient local and long-range structure, as well as fast time scale dynamics in this NS5A fragment. In addition, we could characterize two distinct interaction modes with the SH3 domain of Bin1 (bridging integrator protein 1), a pro-apoptotic tumor suppressor. Despite being largely disordered, the protein contains three regions that transiently adopt α-helical structures, partly stabilized by long-range tertiary interactions. Two of these transient α-helices form a noncanonical SH3-binding motif, which allows low-affinity SH3 binding. Our results contribute to a better understanding of the role of the NS5A protein during hepatitis C virus infection. The present work also highlights the power of NMR spectroscopy to characterize multiple binding events including short-lived transient interactions between globular and highly disordered proteins.

NADPH oxidase (NOX) isoforms are inhibited by celastrol with a dual mode of action

BACKGROUND

Celastrol is one of several bioactive compounds extracted from the medicinal plant Tripterygium wilfordii. Celastrol is used to treat inflammatory conditions, and shows benefits in models of neurodegenerative disease, cancer and arthritis, although its mechanism of action is incompletely understood.

EXPERIMENTAL APPROACH

Celastrol was tested on human NADPH oxidases (NOXs) using a panel of experiments: production of reactive oxygen species and oxygen consumption by NOX enzymes, xanthine oxidase activity, cell toxicity, phagocyte oxidase subunit translocation, and binding to cytosolic subunits of NOX enzymes. The effect of celastrol was compared with diphenyleneiodonium, an established inhibitor of flavoproteins.

KEY RESULTS

Low concentrations of celastrol completely inhibited NOX1, NOX2, NOX4 and NOX5 within minutes with concentration-response curves exhibiting higher Hill coefficients and lower IC₅₀ values for NOX1 and NOX2 compared with NOX4 and NOX5, suggesting differences in their mode of action. In a cell-free system, celastrol had an IC₅₀ of 1.24 and 8.4 µM for NOX2 and NOX5, respectively. Cytotoxicity, oxidant scavenging, and inhibition of p47(phox) translocation could not account for NOX inhibition. Celastrol bound to a recombinant p47(phox) and disrupted the binding of the proline rich region of p22(phox) to the tandem SH3 domain of p47(phox) and NOXO1, the cytosolic subunits of NOX2 and NOX1, respectively.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that celastrol is a potent inhibitor of NOX enzymes in general with increased potency against NOX1 and NOX2. Furthermore, inhibition of NOX1 and NOX2 was mediated via a novel mode of action, namely inhibition of a functional association between cytosolic subunits and the membrane flavocytochrome.

Positioning of a polymorphic quantitative trait nucleotide in the Ncf1 gene controlling oxidative burst response and arthritis severity in rats

The Ncf1 gene, encoding the P47(PHOX) protein that regulates production of reactive oxygen species (ROS) by the phagocyte NADPH oxidase (NOX2) complex, is associated with autoimmunity and arthritis severity in rats. We have now identified that the single-nucleotide polymorphism (SNP) resulting in an M153T amino acid substitution mediates arthritis resistance and thus explains the molecular polymorphism underlying the earlier identified Ncf1 gene effect. We identified the SNP in position 153 to regulate ROS production using COS(PHOX) cells transfected with mutated Ncf1. To determine the role of this SNP for control of arthritis, we used the Wistar strain, identified to carry only the postulated arthritis resistant SNP in position 153. When this Ncf1 allele was backcrossed to the arthritis susceptible DA strain, both granulocyte ROS production and arthritis resistance were restored. Position 153 is located in the hinge region between the PX and SH3 domains of P47(PHOX). Mutational analysis of this position revealed a need for an -OH group in the side chain but we found no evidence for phosphorylation. The polymorphism did not affect assembly of the P47(PHOX)/P67(PHOX) complex in the cytosol or membrane localization, but is likely to operate downstream of assembly, affecting activity of the membrane NOX2 complex.

Combating oxidative stress in vascular disease: NADPH oxidases as therapeutic targets

NADPH oxidases are a family of enzymes that generate reactive oxygen species (ROS). The NOX1 (NADPH oxidase 1) and NOX2 oxidases are the major sources of ROS in the artery wall in conditions such as hypertension, hypercholesterolaemia, diabetes and ageing, and so they are important contributors to the oxidative stress, endothelial dysfunction and vascular inflammation that underlies arterial remodelling and atherogenesis. In this Review, we advance the concept that compared to the use of conventional antioxidants, inhibiting NOX1 and NOX2 oxidases is a superior approach for combating oxidative stress. We briefly describe some common and emerging putative NADPH oxidase inhibitors. In addition, we highlight the crucial role of the NADPH oxidase regulatory subunit, p47phox, in the activity of vascular NOX1 and NOX2 oxidases, and suggest how a better understanding of its specific molecular interactions may enable the development of novel isoform-selective drugs to prevent or treat cardiovascular diseases.

Characterization of superoxide overproduction by the D-Loop(Nox4)-Nox2 cytochrome b(558) in phagocytes-Differential sensitivity to calcium and phosphorylation events

NADPH oxidase is a crucial element of phagocytes involved in microbicidal mechanisms. It becomes active when membrane-bound cytochrome b(558), the redox core, is assembled with cytosolic p47(phox), p67(phox), p40(phox), and rac proteins to produce superoxide, the precursor for generation of toxic reactive oxygen species. In a previous study, we demonstrated that the potential second intracellular loop of Nox2 was essential to maintaining NADPH oxidase activity by controlling electron transfer from FAD to O(2). Moreover, replacement of this loop by the Nox4-D-loop (D-loop(Nox4)-Nox2) in PLB-985 cells induced superoxide overproduction. In the present investigation, we demonstrated that both soluble and particulate stimuli were able to induce this superoxide overproduction. Superoxide overproduction was also observed after phosphatidic acid activation in a purified cell-free-system assay. The highest oxidase activity was obtained after ionomycin and fMLF stimulation. In addition, enhanced sensitivity to Ca(2+) influx was shown by thapsigargin, EDTA, or BTP2 treatment before fMLF activation. Mutated cytochrome b(558) was less dependent on phosphorylation triggered by ERK1/2 during fMLF or PMA stimulation and by PI3K during OpZ stimulation. The superoxide overproduction of the D-loop(Nox4)-Nox2 mutant may come from a change of responsiveness to intracellular Ca(2+) level and to phosphorylation events during oxidase activation. Finally the D-loop(Nox4)-Nox2-PLB-985 cells were more effective against an attenuated strain of Pseudomonas aeruginosa compared to WT-Nox2 cells. The killing mechanism was biphasic, an early step of ROS production that was directly bactericidal, and a second oxidase-independent step related to the amount of ROS produced in the first step.

p47phox molecular activation for assembly of the neutrophil NADPH oxidase complex

The p47(phox) cytosolic factor from neutrophilic NADPH oxidase has always been resistant to crystallogenesis trials due to its modular organization leading to relative flexibility. Hydrogen/deuterium exchange coupled to mass spectrometry was used to obtain structural information on the conformational mechanism that underlies p47(phox) activation. We confirmed a relative opening of the protein with exposure of the SH3 Src loops that are known to bind p22(phox) upon activation. A new surface was shown to be unmasked after activation, representing a potential autoinhibitory surface that may block the interaction of the PX domain with the membrane in the resting state. Within this surface, we identified 2 residues involved in the interaction with the PX domain. The double mutant R162A/D166A showed a higher affinity for specific phospholipids but none for the C-terminal part of p22(phox), reflecting an intermediate conformation between the autoinhibited and activated forms.

Phosphorylation of p22phox on threonine 147 enhances NADPH oxidase activity by promoting p47phox binding

NADPH oxidase comprises both cytosolic and membrane-bound subunits, which, when assembled and activated, initiate the transfer of electrons from NADPH to molecular oxygen to form superoxide. This activity, known as the respiratory burst, is extremely important in the innate immune response as indicated by the disorder chronic granulomatous disease. The regulation of this enzyme complex involves protein-protein and protein-lipid interactions as well as phosphorylation events. Previously, our laboratory demonstrated that the small membrane subunit of the oxidase complex, p22(phox), is phosphorylated in neutrophils and that its phosphorylation correlates with NADPH oxidase activity. In this study, we utilized site-directed mutagenesis in a Chinese hamster ovarian cell system to determine the phosphorylation sites within p22(phox). We also explored the mechanism by which p22(phox) phosphorylation affects NADPH oxidase activity. We found that mutation of threonine 147 to alanine inhibited superoxide production in vivo by more than 70%. This mutation also blocked phosphorylation of p22(phox) in vitro by both protein kinase C-alpha and -delta. Moreover, this mutation blocked the p22(phox)-p47(phox) interaction in intact cells. When phosphorylation was mimicked in vivo through mutation of Thr-147 to an aspartyl residue, NADPH oxidase activity was recovered, and the p22(phox)-p47(phox) interaction in the membrane was restored. Maturation of gp91(phox) was not affected by the alanine mutation, and phosphorylation of the cytosolic component p47(phox) still occurred. This study directly implicates threonine 147 of p22(phox) as a critical residue for efficient NADPH oxidase complex formation and resultant enzyme activity.

Detection of non-DeltaGT NCF-1 mutations in chronic granulomatous disease

AIMS

Chronic granulomatous disease (CGD) is a rare inherited disorder caused by mutations in the subunits of the NADPH oxidase complex, leaving phagocytes unable to produce superoxide and thereby unable to kill invading microorganisms. A subgroup of CGD patients (approximately 20%) is reported to have mutations in NCF-1 encoding p47-phox, which is part of the cytosolic component of NADPH oxidase. More than 94% of these patients share the same mutation, a 2 bp GT deletion in the GTGT dinucleotide repeat in the start of exon 2. The presence of two pseudogenes more than 98% homologous to the functional NCF-1 has complicated the identification of other mutations in the gene. The aim of this study was to find a general technique for detection of non-GT deletion mutations in the coding region of NCF-1.

RESULTS

A technique involving GeneScan analysis followed by amplification of cDNA with intact dinucleotide repeat was set up and used to identify a novel mutation in exon 7 of NCF-1 in a patient with autosomal recessive CGD, explaining the disease by changing a UGG codon to a premature UGA STOP codon.

CONCLUSION

The method is generally applicable for the detection of NCF-1 mutations in patients with suspected CGD.

A region N-terminal to the tandem SH3 domain of p47phox plays a crucial role in the activation of the phagocyte NADPH oxidase

The superoxide-producing NADPH oxidase in phagocytes is crucial for host defence; its catalytic core is the membrane-integrated protein gp91phox [also known as Nox2 (NADPH oxidase 2)], which forms a stable heterodimer with p22phox. Activation of the oxidase requires membrane translocation of the three cytosolic proteins p47phox, p67phox and the small GTPase Rac. At the membrane, these proteins assemble with the gp91phox–p22phox heterodimer and induce a conformational change of gp91phox, leading to superoxide production. p47phox translocates to membranes using its two tandemly arranged SH3 domains, which directly interact with p22phox, whereas p67phox is recruited in a p47phox-dependent manner. In the present study, we show that a short region N-terminal to the bis-SH3 domain is required for activation of the phagocyte NADPH oxidase. Alanine substitution for Ile152 in this region, a residue that is completely conserved during evolution, results in a loss of the ability to activate the oxidase; and the replacement of Thr153 also prevents oxidase activation, but to a lesser extent. In addition, the corresponding isoleucine residue (Ile155) of the p47phox homologue Noxo1 (Nox organizer 1) participates in the activation of non-phagocytic oxidases, such as Nox1 and Nox3. The I152A substitution in p47phox, however, does not affect its interaction with p22phox or with p67phox. Consistent with this, a mutant p47phox (I152A), as well as the wild-type protein, is targeted upon cell stimulation to membranes, and membrane recruitment of p67phox and Rac normally occurs in p47phox (I152A)-expressing cells. Thus the Ile152-containing region of p47phox plays a crucial role in oxidase activation, probably by functioning at a process after oxidase assembly.

Conformational changes in p47(phox) upon activation highlighted by mass spectrometry coupled to hydrogen/deuterium exchange and limited proteolysis

The neutrophil NADPH oxidase is an enzymatic complex involved in innate immunity. Phosphorylation of p47(phox) promotes its translocation with p67(phox) and p40(phox), followed by membrane interaction and assembly with flavocytochrome b(558) into a functional complex. To characterise p47(phox) conformational changes during activation, we used wild-type and the S303/304/328E triple mutant mimicking the phosphorylated state. Hydrogen/deuterium exchange and limited proteolysis coupled to mass spectrometry were used to discriminate between the various structural models. An increase in deuteration confirmed that p47(phox) adopts an open and more flexible conformation after activation. Limited proteolysis correlated this change with increased auto-inhibitory region (AIR) accessibility. These results establish a structural link between the AIR release and the exposure of the Phox homology (PX) domain.

Fc gamma R-stimulated activation of the NADPH oxidase: phosphoinositide-binding protein p40phox regulates NADPH oxidase activity after enzyme assembly on the phagosome

The phagocyte NADPH oxidase generates superoxide for microbial killing, and includes a membrane-bound flavocytochrome b(558) and cytosolic p67(phox), p47(phox), and p40(phox) subunits that undergo membrane translocation upon cellular activation. The function of p40(phox), which binds p67(phox) in resting cells, is incompletely understood. Recent studies showed that phagocytosis-induced superoxide production is stimulated by p40(phox) and its binding to phosphatidylinositol-3-phosphate (PI3P), a phosphoinositide enriched in membranes of internalized phagosomes. To better define the role of p40(phox) in FcgammaR-induced oxidase activation, we used immunofluorescence and real-time imaging of FcgammaR-induced phagocytosis. YFP-tagged p67(phox) and p40(phox) translocated to granulocyte phagosomes before phagosome internalization and accumulation of a probe for PI3P. p67(phox) and p47(phox) accumulation on nascent and internalized phagosomes did not require p40(phox) or PI3 kinase activity, although superoxide production before and after phagosome sealing was decreased by mutation of the p40(phox) PI3P-binding domain or wortmannin. Translocation of p40(phox) to nascent phagosomes required binding to p67(phox) but not PI3P, although the loss of PI3P binding reduced p40(phox) retention after phagosome internalization. We conclude that p40(phox) functions primarily to regulate FcgammaR-induced NADPH oxidase activity rather than assembly, and stimulates superoxide production via a PI3P signal that increases after phagosome internalization.

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.

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.

Molecular evolution of Phox-related regulatory subunits for NADPH oxidase enzymes

Background

The reactive oxygen-generating NADPH oxidases (Noxes) function in a variety of biological roles, and can be broadly classified into those that are regulated by subunit interactions and those that are regulated by calcium. The prototypical subunit-regulated Nox, Nox2, is the membrane-associated catalytic subunit of the phagocyte NADPH-oxidase. Nox2 forms a heterodimer with the integral membrane protein, p22phox, and this heterodimer binds to the regulatory subunits p47phox, p67phox, p40phox and the small GTPase Rac, triggering superoxide generation. Nox-organizer protein 1 (NOXO1) and Nox-activator 1 (NOXA1), respective homologs of p47phox and p67phox, together with p22phox and Rac, activate Nox1, a non-phagocytic homolog of Nox2. NOXO1 and p22phox also regulate Nox3, whereas Nox4 requires only p22phox. In this study, we have assembled and analyzed amino acid sequences of Nox regulatory subunit orthologs from vertebrates, a urochordate, an echinoderm, a mollusc, a cnidarian, a choanoflagellate, fungi and a slime mold amoeba to investigate the evolutionary history of these subunits.

Results

Ancestral p47phox, p67phox, and p22phox genes are broadly seen in the metazoa, except for the ecdysozoans. The choanoflagellate Monosiga brevicollis, the unicellular organism that is the closest relatives of multicellular animals, encodes early prototypes of p22phox, p47phox as well as the earliest known Nox2-like ancestor of the Nox1-3 subfamily. p67phox- and p47phox-like genes are seen in the sea urchin Strongylocentrotus purpuratus and the limpet Lottia gigantea that also possess Nox2-like co-orthologs of vertebrate Nox1-3. Duplication of primordial p47phox and p67phox genes occurred in vertebrates, with the duplicated branches evolving into NOXO1 and NOXA1. Analysis of characteristic domains of regulatory subunits suggests a novel view of the evolution of Nox: in fish, p40phox participated in regulating both Nox1 and Nox2, but after the appearance of mammals, Nox1 (but not Nox2) became independent of p40phox. In the fish Oryzias latipes, a NOXO1 ortholog retains an autoinhibitory region that is characteristic of mammalian p47phox, and this was subsequently lost from NOXO1 in later vertebrates. Detailed amino acid sequence comparisons identified both putative key residues conserved in characteristic domains and previously unidentified conserved regions. Also, candidate organizer/activator proteins in fungi and amoeba are identified and hypothetical activation models are suggested.

Conclusion

This is the first report to provide the comprehensive view of the molecular evolution of regulatory subunits for Nox enzymes. This approach provides clues for understanding the evolution of biochemical and physiological functions for regulatory-subunit-dependent Nox enzymes.

A predicted N-terminal helical domain of a Group 1 LEA protein is required for protection of enzyme activity from drying

Late embryogenesis abundant (LEA) proteins have been repeatedly implicated in the acquisition of desiccation tolerance in angiosperm seed embryos. However, the mechanism(s) by which protection occurs is not well understood. While the Group 1 LEA proteins are predicted to be largely unordered in solution, there is strong evidence that upon drying these proteins undergo a structural transition that leads to an increase in alpha-helical content. Several studies also suggest there is a direct interaction between Group 1 LEA proteins and other molecules in the cytoplasm that may be critical for the establishment of desiccation tolerance during embryo maturation. We have produced a recombinant Group 1 LEA protein and show that it is capable of protecting the enzyme lactate dehydrogenase from the deleterious effects of drying. We have also evaluated the ability of various altered recombinant Group 1 LEA proteins to protect in the same assay. Our results suggest that the highly conserved 20 amino acid Group 1 LEA signature motif is not required for protection in our in vitro assay. However, introduction of two juxtaposed proline residues into an N-terminal helical domain predicted to exist in the hydrated structure significantly compromises the ability of the recombinant protein to provide protection from drying. These results suggest that the N-terminal domain of Group 1 LEA proteins may be important for proper folding during dehydration.

Role of the small GTPase Rac in p22phox-dependent NADPH oxidases

The superoxide-producing phagocyte NADPH oxidase gp91(phox)/Nox2 and the non-phagocytic oxidases Nox1 and Nox3 each form a complex in the membrane with p22(phox), which provides both stabilization and a docking site for organizer proteins. The p22(phox)-complexed Nox2 and Nox1 are dormant on their own, and their activation requires soluble supportive proteins such as a Nox organizer (p47(phox) or Noxo1) and a Nox activator (p67(phox) or Noxa1). The small GTPase Rac directly binds to the activators, and thus plays an essential role in the Nox2-based oxidase containing p47(phox) and p67(phox) or a positive role in Nox1 activity supported by Noxo1 and Noxa1. Although Nox3 complexed with p22(phox) constitutively produce superoxide, the production can be enhanced by supportive proteins. Here we compare the roles of Rac in these p22(phox)-dependent oxidases using the organizer and activator in different combinations. Expression of constitutively active Rac1(Q61L) is essential for activation of the Nox2- or Nox1-based oxidase containing the organizer p47(phox) and either p67(phox) or Noxa1. When these oxidases use Noxo1 as an organizer instead of p47(phox), they produce a small but significant amount of superoxide without expression of Rac1(Q61L), although the production is enhanced by Rac1(Q61L). Thus p47(phox) is likely related to strict dependence on Rac. The Nox3-based oxidase has a similar tendency in the change of the dependence: Rac plays a positive role in Nox3 activation in the presence of p47(phox) and either p67(phox) or Noxa1, whereas Rac fails to upregulate Nox3 activity when p47(phox) is replaced with Noxo1. We also demonstrate that, in the Nox3-based oxidase containing solely p67(phox) as supportive protein, expression of Rac1(Q61L) enhances not only superoxide production but also membrane translocation of p67(phox). Since the enhancements are not observed with a mutant p67(phox) defective in binding to Rac, this GTPase appear to directly recruit p67(phox) to the membrane.

Phagocytosis-coupled activation of the superoxide-producing phagocyte oxidase, a member of the NADPH oxidase (nox) family

The phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase plays a crucial role in host defense by neutrophils and macrophages. When cells ingest invading microbes, this enzyme becomes activated to reduce molecular oxygen to superoxide, a precursor of microbicidal oxidants, in the phagosome. The catalytic core of the oxidase is membrane-bound cytochrome b558, which comprises gp91phox and p22phox. gp91phox belongs to the NADPH oxidase (Nox) family, which contains the entire electron-transporting apparatus from NADPH to molecular oxygen. In resting neutrophils, cytochrome b558 is mainly present in the membrane of the specific granule, an intracellular component, and is targeted to the phagosomal membrane during phagocytosis. Activation of gp91phox involves the integrated function of cytoplasmic proteins such as p47phox, p67phox, p40phox, and the small guanosine triphosphatase Rac; these proteins translocate to the phagosomal membrane to interact with cytochrome b558, leading to superoxide production. Here we describe a current molecular model for phagocytosis-coupled activation of the NADPH oxidase.

Interaction between the SH3 domains and C-terminal proline-rich region in NADPH oxidase organizer 1 (Noxo1)

NADPH oxidase organizer 1 (Noxo1), harboring a PX domain, two SH3 domains, and a proline-rich region (PRR), participates in activation of superoxide-producing Nox-family NADPH oxidases. Here, we show that Noxo1 supports superoxide production in a cell-free system for gp91(phox)/Nox2 activation by arachidonic acid. This lipid enhances an SH3-mediated binding of Noxo1 to p22(phox), a protein complexed with Nox oxidases; the binding is known to be required for Nox activation. We also demonstrate that the bis-SH3 domain directly interacts with the Noxo1 PRR. The interaction appears to prevent the bis-SH3 domain and PRR from binding to their target proteins; disruption of the intramolecular interaction facilitates Noxo1 binding to p22(phox) and also allows the PRR to associate with the Nox activator Noxa1, which association is crucial for Nox activation as well. These findings suggest that Nox activation involves a conformational change leading to disruption of the bis-SH3-PRR interaction in Noxo1.

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.

Deletion mutagenesis of p22phox subunit of flavocytochrome b558: identification of regions critical for gp91phox maturation and NADPH oxidase activity

The heterodimeric flavocytochrome b558, comprised of the two integral membrane proteins p22phox and gp91phox, mediates the transfer of electrons from NADPH to molecular oxygen in the phagocyte NADPH oxidase to generate the superoxide precursor of microbicidal oxidants. This study uses deletion mutagenesis to identify regions of p22phox required for maturation of gp91phox and for NADPH oxidase activity. N-terminal, C-terminal, or internal deletions of human p22phox were generated and expressed in Chinese hamster ovary cells with transgenes for gp91phox and two other NADPH oxidase subunits, p47phox, and p67phox. The results demonstrate that p22phox-dependent maturation of gp91phox carbohydrate, cell surface expression of gp91phox, and the enzymatic function of flavocytochrome b558 are closely correlated. Whereas the 5 N-terminal and 25 C-terminal amino acids are dispensable for these functions, the N-terminal 11 amino acids of p22phox are required, as is a hydrophilic region between amino acids 65 and 90. Upon deletion of 54 residues at the C terminus of p22phox (amino acids 142-195), maturation and cell surface expression of gp91phox was still preserved, although NADPH oxidase activity was absent, as expected, due to removal of a proline-rich domain between amino acids 151-160 that is required for recruitment of p47phox. Antibody binding studies indicate that the extreme N terminus of p22phox is inaccessible in the absence of cell permeabilization, supporting a model in which both the N- and C-terminal domains of p22phox extend into the cytoplasm, anchored by two membrane-embedded regions.

Direct involvement of the small GTPase Rac in activation of the superoxide-producing NADPH oxidase Nox1

Activation of the non-phagocytic superoxide-producing NADPH oxidase Nox1, complexed with p22(phox) at the membrane, requires its regulatory soluble proteins Noxo1 and Noxa1. However, the role of the small GTPase Rac remained to be clarified. Here we show that Rac directly participates in Nox1 activation via interacting with Noxa1. Electropermeabilized HeLa cells, ectopically expressing Nox1, Noxo1, and Noxa1, produce superoxide in a GTP-dependent manner, which is abrogated by expression of a mutant Noxa1(R103E), defective in Rac binding. Superoxide production in Nox1-expressing HeLa and Caco-2 cells is decreased by depletion or sequestration of Rac; on the other hand, it is enhanced by expression of the constitutively active Rac1(Q61L), but not by that of a mutant Rac1 with the A27K substitution, deficient in binding to Noxa1. We also demonstrate that Nox1 activation requires membrane recruitment of Noxa1, which is normally mediated via Noxa1 binding to Noxo1, a protein tethered to the Nox1 partner p22(phox): the Noxa1-Noxo1 and Noxo1-p22(phox) interactions are both essential for Nox1 activity. Rac likely facilitates the membrane localization of Noxa1: although Noxa1(W436R), defective in Noxo1 binding, neither associates with the membrane nor activates Nox1, the effects of the W436R substitution are restored by expression of Rac1(Q61L). The Rac-Noxa1 interaction also serves at a step different from the Noxa1 localization, because the binding-defective Noxa1(R103E), albeit targeted to the membrane, does not support superoxide production by Nox1. Furthermore, a mutant Noxa1 carrying the substitution of Ala for Val-205 in the activation domain, which is expected to undergo a conformational change upon Rac binding, fully localizes to the membrane but fails to activate Nox1.