Role of NADPH oxidase in the apoptotic death of cultured cerebellar granule neurons

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.

Role of Oxygen and Its Radicals in Peripheral Nerve Regeneration: From Hypoxia to Physoxia to Hyperoxia

Oxygen is compulsory for mitochondrial function and energy supply, but it has numerous more nuanced roles. The different roles of oxygen in peripheral nerve regeneration range from energy supply, inflammation, phagocytosis, and oxidative cell destruction in the context of reperfusion injury to crucial redox signaling cascades that are necessary for effective axonal outgrowth. A fine balance between reactive oxygen species production and antioxidant activity draws the line between physiological and pathological nerve regeneration. There is compelling evidence that redox signaling mediated by the Nox family of nicotinamide adenine dinucleotide phosphate (NADPH) oxidases plays an important role in peripheral nerve regeneration. Further research is needed to better characterize the role of Nox in physiological and pathological circumstances, but the available data suggest that the modulation of Nox activity fosters great therapeutic potential. One of the promising approaches to enhance nerve regeneration by modulating the redox environment is hyperbaric oxygen therapy. In this review, we highlight the influence of various oxygenation states, i.e., hypoxia, physoxia, and hyperoxia, on peripheral nerve repair and regeneration. We summarize the currently available data and knowledge on the effectiveness of using hyperbaric oxygen therapy to treat nerve injuries and discuss future directions.

Txnip expression promotes JNK-mediated neuronal death in response to reactive oxygen species

TXNIP is a protein sensitive to oxidant conditions whose expression is related to the progression of death in cancer, diabetes, ischemia, and neurodegenerative diseases, among others. Because of this, many studies propose TXNIP as a therapeutic target in several diseases. Exposure of cerebellar granule neurons to staurosporine or low potassium leads to apoptotic death. Both conditions generate an early production of reactive oxygen species (ROS) that induces the activation of the ASK1 pathway and the apoptotic machinery. In these models, it has been shown an increase in TXNIP protein mediated by ROS. Here, we evaluated the molecular mechanisms involved in the regulation of the Txnip expression during neuronal death, as well as the role of the protein in the progression of cell death induced by these two apoptotic conditions. In cultured cerebellar granule neurons, we observed that low potassium and staurosporine induced an early increase in ROS that correlated with an increase in Txnip mRNA. When we evaluated the promoter of the gene, we found that the JASPAR-reported FOXO1/3 transcription factor motifs are close to the transcription start site (TSS). We then verified through the Chromatin immunoprecipitation technique (ChIP) that FOXO3 interacts with the Txnip promoter after 1 h of low potassium treatment. We also detected FOXO3 nuclear translocation by low potassium and staurosporine treatments. Finally, by using shRNA in the neuroblastoma MSN cell line, we found that Txnip downregulation decreased neuronal death induced by staurosporine stimulus. Together, these results suggest that ROS promotes the expression of Txnip through the activation of the FOXO3 transcription factor mediated by Akt inhibition. We also demonstrated that TXNIP is necessary for neuronal death progression.

Creatine in the fetal brain: A regional investigation of acute global hypoxia and creatine supplementation in a translational fetal sheep model

Background

Creatine supplementation during pregnancy is a promising prophylactic treatment for perinatal hypoxic brain injury. Previously, in near-term sheep we have shown that fetal creatine supplementation reduces cerebral metabolic and oxidative stress induced by acute global hypoxia. This study investigated the effects of acute hypoxia with or without fetal creatine supplementation on neuropathology in multiple brain regions.

Methods

Near-term fetal sheep were administered continuous intravenous infusion of either creatine (6 mg kg^-1 h^-1) or isovolumetric saline from 122 to 134 days gestational age (dGA; term is approx. 145 dGA). At 131 dGA, global hypoxia was induced by a 10 min umbilical cord occlusion (UCO). Fetuses were then recovered for 72 h at which time (134 dGA) cerebral tissue was collected for either RT-qPCR or immunohistochemistry analyses.

Results

UCO resulted in mild injury to the cortical gray matter, thalamus and hippocampus, with increased cell death and astrogliosis and downregulation of genes involved in regulating injury responses, vasculature development and mitochondrial integrity. Creatine supplementation reduced astrogliosis within the corpus callosum but did not ameliorate any other gene expression or histopathological changes induced by hypoxia. Of importance, effects of creatine supplementation on gene expression irrespective of hypoxia, including increased expression of anti-apoptotic (BCL-2) and pro-inflammatory (e.g., MPO, TNFa, IL-6, IL-1β) genes, particularly in the gray matter, hippocampus, and striatum were identified. Creatine treatment also effected oligodendrocyte maturation and myelination in white matter regions.

Conclusion

While supplementation did not rescue mild neuropathology caused by UCO, creatine did result in gene expression changes that may influence in utero cerebral development.

Unravelling the toxic effects mediated by the neurodegenerative disease-associated S375G mutation of TDP-43 and its S375E phosphomimetic variant

TAR DNA-binding protein 43 (TDP-43) is a nucleic acid–binding protein found in the nucleus that accumulates in the cytoplasm under pathological conditions, leading to proteinopathies, such as frontotemporal dementia and ALS. An emerging area of TDP-43 research is represented by the study of its post-translational modifications, the way they are connected to disease-associated mutations, and what this means for pathological processes. Recently, we described a novel mutation in TDP-43 in an early onset ALS case that was affecting a potential phosphorylation site in position 375 (S375G). A preliminary characterization showed that both the S375G mutation and its phosphomimetic variant, S375E, displayed altered nuclear–cytoplasmic distribution and cellular toxicity. To better investigate these effects, here we established cell lines expressing inducible WT, S375G, and S375E TDP-43 variants. Interestingly, we found that these mutants do not seem to affect well-studied aspects of TDP-43, such as RNA splicing or autoregulation, or protein conformation, dynamics, or aggregation, although they do display dysmorphic nuclear shape and cell cycle alterations. In addition, RNA-Seq analysis of these cell lines showed that although the disease-associated S375G mutation and its phosphomimetic S375E variant regulate distinct sets of genes, they have a common target in mitochondrial apoptotic genes. Taken together, our data strongly support the growing evidence that alterations in TDP-43 post-translational modifications can play a potentially important role in disease pathogenesis and provide a further link between TDP-43 pathology and mitochondrial health.

NADPH Oxidases in the Central Nervous System: Regional and Cellular Localization and the Possible Link to Brain Diseases

Significance: The significant role of reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox) in signal transduction is mediated by the production of reactive oxygen species (ROS), especially in the central nervous system (CNS). The pathogenesis of some neurologic and psychiatric diseases is regulated by ROS, acting as a second messenger or pathogen. Recent Advances: In the CNS, the involvement of Nox-derived ROS has been implicated in the regulation of multiple signals, including cell survival/apoptosis, neuroinflammation, migration, differentiation, proliferation, and synaptic plasticity, as well as the integrity of the blood/brain barrier. In these processes, the intracellular signals mediated by the members of the Nox family vary among different tissues. The present review illuminates the regions and cellular, subcellular localization of Nox isoforms in the brain, the signal transduction, and the role of NOX enzymes in pathophysiology, respectively. Critical Issues: Different signal transduction cascades are coupled to ROS derived from various Nox homologues with varying degrees. Therefore, a critical issue worth noting is the varied role of the homologues of NOX enzymes in different signaling pathways and also they mediate different phenotypes in the diverse pathophysiological condition. This substantiates the effectiveness of selective Nox inhibitors in the CNS. Future Directions: Further investigation to elucidate the role of various homologues of NOX enzymes in acute and chronic brain diseases and signaling mechanisms, and the development of more specific NOX inhibitors for the treatment of CNS disease are urgently needed. Antioxid. Redox Signal. 35, 951-973.

Role of Viral Protein U (Vpu) in HIV-1 Infection and Pathogenesis

Human immunodeficiency virus (HIV)-1 and HIV-2 originated from cross-species transmission of simian immunodeficiency viruses (SIVs). Most of these transfers resulted in limited spread of these viruses to humans. However, one transmission event involving SIVcpz from chimpanzees gave rise to group M HIV-1, with M being the principal strain of HIV-1 responsible for the AIDS pandemic. Vpu is an HIV-1 accessory protein generated from Env/Vpu encoded bicistronic mRNA and localized in cytosolic and membrane regions of cells capable of being infected by HIV-1 and that regulate HIV-1 infection and transmission by downregulating BST-2, CD4 proteins levels, and immune evasion. This review will focus of critical aspects of Vpu including its zoonosis, the adaptive hurdles to cross-species transmission, and future perspectives and broad implications of Vpu in HIV-1 infection and dissemination.

Differential ROS-Mediated Phosphorylation of Drp1 in Mitochondrial Fragmentation Induced by Distinct Cell Death Conditions in Cerebellar Granule Neurons

Reactive oxygen species (ROS) production has been associated with neuronal death. ROS are also involved in mitochondrial fission, which is mediated by Dynamin-related protein 1 (Drp1). The regulation of mitochondrial fragmentation mediated by Drp1 and its relationship to mitochondrial ROS (mtROS) in neuronal death have not been completely clarified. The aim of this study is to evaluate the role of mtROS in cell death and their involvement in the activation of Drp1 and mitochondrial fission in a model of cell death of cultured cerebellar granule neurons (CGN). Neuronal death of CGN induced by potassium deprivation (K5) and staurosporine (ST) triggers mitochondrial ROS production and mitochondrial fragmentation. K5 condition evoked an increase of Drp1 phosphorylation at Ser616, but ST treatment led to a decrease of Drp1 phosphorylation. Moreover, the death of CGN induced by both K5 and ST was markedly reduced in the presence of MitoTEMPO; however, mitochondrial morphology was not recovered. Here, we show that the mitochondria are the initial source of ROS involved in the neuronal death of CGN and that mitochondrial fragmentation is a common event in cell death; however, this process is not mediated by Drp1 phosphorylation at Ser616.

Mechanisms of Oxidative Stress and Therapeutic Targets following Intracerebral Hemorrhage

Oxidative stress (OS) is induced by the accumulation of reactive oxygen species (ROS) following intracerebral hemorrhage (ICH) and plays an important role in secondary brain injury caused by the inflammatory response, apoptosis, autophagy, and blood-brain barrier (BBB) disruption. This review summarizes the current state of knowledge regarding the pathogenic mechanisms of brain injury after ICH, markers for detecting OS, and therapeutic strategies that target OS to mitigate brain injury.

Nicotinamide Adenine Dinucleotide Phosphate Oxidase and Neurodegenerative Diseases: Mechanisms and Therapy

Significance: The growing incidence of neurodegenerative diseases significantly impacts the individuals who suffer from these disorders and is a major health concern globally. Although the specific mechanisms of neurodegenerative diseases are still far from being acknowledged, it is becoming clear that oxidative stress and neuroinflammation are critical contributing factors to the progression of neurodegeneration. Thus, it is conceivable that the inhibition of oxidative stress and neuroinflammation may represent promising therapeutic targets for the treatment of neurodegenerative diseases. Recent Advances: Recently, the strategy for neurodegenerative disease therapy has shifted from the use of antioxidants and conventional anti-inflammatory targets to upstream mediators due to the failure of most antioxidants and nonsteroidal anti-inflammatory drugs in clinical trials. Nicotinamide adenine dinucleotide phosphate oxidases (NOXs), a family of superoxide-producing enzyme complexes, have been identified as an upstream factor that controls both oxidative stress and neuroinflammation. Genetic inactivation or pharmacological inhibition of NOX enzymes displays potent neuroprotective effects in a broad spectrum of neurodegenerative disease models. Critical Issues: The detailed mechanisms of how NOX enzymes regulate oxidative stress and neuroinflammation still remain unclear. Moreover, the currently available inhibitors of NOX enzymes exhibit nonspecificity, off-target effects, unsuitable pharmacokinetic properties, and even high toxicity, markedly limiting their potential clinical applications. Future Directions: This review provides novel insights into the roles of NOXs in neurodegenerative pharmacology, and indicates the types of NOX enzyme inhibitors that should be identified and developed as candidates for future applications, which might reveal novel neurodegenerative disease therapies based on NOXs.

The role of NADPH oxidases in neuronal development

Reactive oxygen species (ROS) are critical for maintaining cellular homeostasis and function when produced in physiological ranges. Important sources of cellular ROS include NADPH oxidases (Nox), which are evolutionary conserved multi-subunit transmembrane proteins. Nox-mediated ROS regulate variety of biological processes including hormone synthesis, calcium signaling, cell migration, and immunity. ROS participate in intracellular signaling by introducing post-translational modifications to proteins and thereby altering their functions. The central nervous system (CNS) expresses different Nox isoforms during both development and adulthood. Here, we review the role of Nox-mediated ROS during CNS development. Specifically, we focus on how individual Nox isoforms contribute to signaling in neural stem cell maintenance and neuronal differentiation, as well as neurite outgrowth and guidance. We also discuss how ROS regulates the organization and dynamics of the actin cytoskeleton in the neuronal growth cone. Finally, we review recent evidence that Nox-derived ROS modulate axonal regeneration upon nervous system injury.

Inhibition of NADPH oxidase alleviates germ cell apoptosis and ER stress during testicular ischemia reperfusion injury

Testicular torsion and detorsion (TTD) is a serious urological condition affecting young males that is underlined by an ischemia reperfusion injury (tIRI) to the testis as the pathophysiological mechanism. During tIRI, uncontrolled production of oxygen reactive species (ROS) causes DNA damage leading to germ cell apoptosis (GCA). The aim of the study is to explore whether inhibition of NADPH oxidase (NOX), a major source of intracellular ROS, will prevent tIRI-induced GCA and its association with endoplasmic reticulum (ER) stress. Sprague-Dawley rats (n = 36) were divided into three groups: sham, tIRI only and tIRI treated with apocynin (a NOX inhibitor). Rats undergoing tIRI endured an ischemic injury for 1 h followed by 4 h of reperfusion. Spermatogenic damage was evaluated histologically, while cellular damages were assessed using real time PCR, immunofluorescence staining, Western blot and biochemical assays. Disrupted spermatogenesis was associated with increased lipid and protein peroxidation and decreased antioxidant activity of the enzyme superoxide dismutase (SOD) as a result of tIRI. In addition, increased DNA double strand breaks and formation of 8-OHdG adducts associated with increased phosphorylation of the DNA damage response (DDR) protein H2AX. The ASK1/JNK apoptosis signaling pathway was also activated in response to tIRI. Finally, increased immuno-expression of the unfolded protein response (UPR) downstream targets: GRP78, eIF2-α1, CHOP and caspase 12 supported the presence of ER stress. Inhibition of NOX by apocynin protected against tIRI-induced GCA and ER stress. In conclusion, NOX inhibition minimized tIRI-induced intracellular oxidative damages leading to GCA and ER stress.

Early Celastrol Administration Prevents Ketamine-Induced Psychotic-Like Behavioral Dysfunctions, Oxidative Stress and IL-10 Reduction in The Cerebellum of Adult Mice

Administration of subanesthetic doses of ketamine during brain maturation represents a tool to mimic an early insult to the central nervous system (CNS). The cerebellum is a key player in psychosis pathogenesis, to which oxidative stress also contributes. Here, we investigated the impact of early celastrol administration on behavioral dysfunctions in adult mice that had received ketamine (30 mg/kg i.p.) at postnatal days (PNDs) 7, 9, and 11. Cerebellar levels of 8-hydroxydeoxyguanosine (8-OHdG), NADPH oxidase (NOX) 1 and NOX2, as well as of the calcium-binding protein parvalbumin (PV), were also assessed. Furthermore, celastrol effects on ketamine-induced alterations of proinflammatory (TNF-α, IL-6 and IL-1β) and anti-inflammatory (IL-10) cytokines in this brain region were evaluated. Early celastrol administration prevented ketamine-induced discrimination index decrease at adulthood. The same was found for locomotor activity elevations and increased close following and allogrooming, whereas no beneficial effects on sniffing impairment were detected. Ketamine increased 8-OHdG in the cerebellum of adult mice, which was also prevented by early celastrol injection. Cerebellar NOX1 levels were enhanced at adulthood following postnatal ketamine exposure. Celastrol per se induced NOX1 decrease in the cerebellum. This effect was more significant in animals that were early administered with ketamine. NOX2 levels did not change. Ketamine administration did not affect PV amount in the cerebellum. TNF-α levels were enhanced in ketamine-treated animals; however, this was not prevented by early celastrol administration. While no changes were observed for IL-6 and IL-1β levels, ketamine determined a reduction of cerebellar IL-10 expression, which was prevented by early celastrol treatment. Our results suggest that NOX inhibition during brain maturation prevents the development of psychotic-like behavioral dysfunctions, as well as the increased cerebellar oxidative stress and the reduction of IL-10 in the same brain region following ketamine exposure in postnatal life. This opens novel neuroprotective opportunities against early detrimental insults occurring during brain development.

Role of NADPH oxidase-2 in the progression of the inflammatory response secondary to striatum excitotoxic damage

Background

During excitotoxic damage, neuronal death results from the increase in intracellular calcium, the induction of oxidative stress, and a subsequent inflammatory response. NADPH oxidases (NOX) are relevant sources of reactive oxygen species (ROS) during excitotoxic damage. NADPH oxidase-2 (NOX-2) has been particularly related to neuronal damage and death, as well as to the resolution of the subsequent inflammatory response. As ROS are crucial components of the regulation of inflammatory response, in this work, we evaluated the role of NOX-2 in the progression of inflammation resulting from glutamate-induced excitotoxic damage of the striatum in an in vivo model.

Methods

The striata of wild-type C57BL/6 J and NOX-2 KO mice (gp91^Cybbtm1Din/J) were stereotactically injected with monosodium glutamate either alone or in combination with IL-4 or IL-10. The damage was evaluated in histological sections stained with cresyl violet and Fluoro-Jade B. The enzymatic activity of caspase-3 and NOX were also measured. Additionally, the cytokine profile was identified by ELISA and motor activity was verified by the tests of the cylinder, the adhesive tape removal, and the inverted grid.

Results

Our results show a neuroprotective effect in mice with a genetic inhibition of NOX-2, which is partially due to a differential response to excitotoxic damage, characterized by the production of anti-inflammatory cytokines. In NOX-2 KO animals, the excitotoxic condition increased the production of interleukin-4, which could contribute to the production of interleukin-10 that decreased neuronal apoptotic death and the magnitude of striatal injury. Treatment with interleukin-4 and interleukin-10 protected from excitotoxic damage in wild-type animals.

Conclusions

The release of proinflammatory cytokines during the excitotoxic event promotes an additional apoptotic death of neurons that survived the initial damage. During the subsequent inflammatory response to excitotoxic damage, ROS generated by NOX-2 play a decisive role in the extension of the lesion and consequently in the severity of the functional compromise, probably by regulating the anti-inflammatory cytokines production.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1478-4) contains supplementary material, which is available to authorized users.

A combination antioxidant therapy to inhibit NOX2 and activate Nrf2 decreases secondary brain damage and improves functional recovery after traumatic brain injury

Uncontrolled oxidative stress contributes to the secondary neuronal death that promotes long-term neurological dysfunction following traumatic brain injury (TBI). Surprisingly, both NADPH oxidase 2 (NOX2) that increases and transcription factor Nrf2 that decreases reactive oxygen species (ROS) are induced after TBI. As the post-injury functional outcome depends on the balance of these opposing molecular pathways, we evaluated the effect of TBI on the motor and cognitive deficits and cortical contusion volume in NOX2 and Nrf2 knockout mice. Genetic deletion of NOX2 improved, while Nrf2 worsened the post-TBI motor function recovery and lesion volume indicating that decreasing ROS levels might be beneficial after TBI. Treatment with either apocynin (NOX2 inhibitor) or TBHQ (Nrf2 activator) alone significantly improved the motor function after TBI, but had no effect on the lesion volume, compared to vehicle control. Whereas, the combo therapy (apocynin + TBHQ) given at either 5 min/24 h or 2 h/24 h improved motor and cognitive function and decreased cortical contusion volume compared to vehicle group. Thus, both the generation and disposal of ROS are important modulators of oxidative stress, and a combo therapy that prevents ROS formation and potentiates ROS disposal concurrently is efficacious after TBI.

The fungal neurotoxin penitrem A induces the production of reactive oxygen species in human neutrophils at submicromolar concentrations

Penitrem A is a fungal neurotoxin that recurrently causes intoxication in animals, and occasionally also in humans. We have previously reported that penitrem A induced the production of reactive oxygen species (ROS) in rat cerebellar granule cells, opening for a new mechanism of action for the neurotoxin. The aim of this study was to examine the potential of penitrem A to induce ROS production in isolated human neutrophil granulocytes, and to study possible mechanisms involved. Penitrem A significantly increased the production of ROS in human neutrophils at concentrations as low as 0.25μM (40% increase over basal levels), as measured with the DCF fluorescence assay. The EC₅₀ determined for the production of ROS by penitrem A was 3.8μM. The maximal increase in ROS production was approximately 330% over basal levels at a concentration of 12.5μM. ROS formation was significantly inhibited by the antioxidant vitamin E (50μM), the intracellular Ca⁺² chelator BAPTA-AM (5μM), the mitogen activated protein kinase kinase (MEK) 1/2 and 5 inhibitor U0126 (1 and 10μM), the p38 mitogen activated protein kinase (MAPK) inhibitor SB203580 (1μM), the c-Jun amino-terminal kinase (JNK) inhibitor SP600125 (10μM), and the calcineurin inhibitors FK-506 and cyclosporine A (1.5 and 0.5μM, respectively). These finding suggest that penitrem A is able to induce an increase in ROS production in neutrophils via the activation of several MAPK-signalling pathways. We suggest that this increase may partly explain the pathophysiology generated by penitrem A neuromycotoxicosis in both humans and animals.

NADPH oxidase in brain injury and neurodegenerative disorders

Oxidative stress is a common denominator in the pathology of neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and multiple sclerosis, as well as in ischemic and traumatic brain injury. The brain is highly vulnerable to oxidative damage due to its high metabolic demand. However, therapies attempting to scavenge free radicals have shown little success. By shifting the focus to inhibit the generation of damaging free radicals, recent studies have identified NADPH oxidase as a major contributor to disease pathology. NADPH oxidase has the primary function to generate free radicals. In particular, there is growing evidence that the isoforms NOX1, NOX2, and NOX4 can be upregulated by a variety of neurodegenerative factors. The majority of recent studies have shown that genetic and pharmacological inhibition of NADPH oxidase enzymes are neuroprotective and able to reduce detrimental aspects of pathology following ischemic and traumatic brain injury, as well as in chronic neurodegenerative disorders. This review aims to summarize evidence supporting the role of NADPH oxidase in the pathology of these neurological disorders, explores pharmacological strategies of targeting this major oxidative stress pathway, and outlines obstacles that need to be overcome for successful translation of these therapies to the clinic.

NADPH Oxidase: A Potential Target for Treatment of Stroke

Stroke is the third leading cause of death in industrialized nations. Oxidative stress is involved in the pathogenesis of stroke, and excessive generation of reactive oxygen species (ROS) by mitochondria is thought to be the main cause of oxidative stress. NADPH oxidase (NOX) enzymes have recently been identified and studied as important producers of ROS in brain tissues after stroke. Several reports have shown that knockout or deletion of NOX exerts a neuroprotective effect in three major experimental stroke models. Recent studies also confirmed that NOX inhibitors ameliorate brain injury and improve neurological outcome after stroke. However, the physiological and pathophysiological roles of NOX enzymes in the central nervous system (CNS) are not known well. In this review, we provide a comprehensive summary of our current understanding about expression and physiological function of NOX enzymes in the CNS and its pathophysiological roles in the three major types of stroke: ischemic stroke, intracerebral hemorrhage, and subarachnoid hemorrhage.

Effect of staurosporine in the morphology and viability of cerebellar astrocytes: role of reactive oxygen species and NADPH oxidase

Cell death implies morphological changes that may contribute to the progression of this process. In astrocytes, the mechanisms involving the cytoskeletal changes during cell death are not well explored. Although NADPH oxidase (NOX) has been described as being a critical factor in the production of ROS, not much information is available about the participation of NOX-derived ROS in the cell death of astrocytes and their role in the alterations of the cytoskeleton during the death of astrocytes. In this study, we have evaluated the participation of ROS in the death of cultured cerebellar astrocytes using staurosporine (St) as death inductor. We found that astrocytes express NOX1, NOX2, and NOX4. Also, St induced an early ROS production and NOX activation that participate in the death of astrocytes. These findings suggest that ROS produced by St is generated through NOX1 and NOX4. Finally, we showed that the reorganization of tubulin and actin induced by St is ROS independent and that St did not change the level of expression of these cytoskeletal proteins. We conclude that ROS produced by a NOX is required for cell death in astrocytes, but not for the morphological alterations induced by St.

NADPH oxidase 1, a novel molecular source of ROS in hippocampal neuronal death in vascular dementia

AIMS

Chronic cerebral hypoperfusion (CCH) is a common pathological factor that contributes to neurodegenerative diseases such as vascular dementia (VaD). Although oxidative stress has been strongly implicated in the pathogenesis of VaD, the molecular mechanism underlying the selective vulnerability of hippocampal neurons to oxidative damage remains unknown. We assessed whether the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox) complex, a specialized superoxide generation system, plays a role in VaD by permanent ligation of bilateral common carotid arteries in rats.

RESULTS

Male Wistar rats (10 weeks of age) were subjected to bilateral occlusion of the common carotid arteries (two-vessel occlusion [2VO]). Nox1 expression gradually increased in hippocampal neurons, starting at 1 week after 2VO and for approximately 15 weeks after 2VO. The levels of superoxide, DNA oxidation, and neuronal death in the CA1 subfield of the hippocampus, as well as consequential cognitive impairment, were increased in 2VO rats. Both inhibition of Nox by apocynin, a putative Nox inhibitor, and adeno-associated virus-mediated Nox1 knockdown significantly reduced 2VO-induced reactive oxygen species generation, oxidative DNA damage, hippocampal neuronal degeneration, and cognitive impairment.

INNOVATION AND CONCLUSION

We provided evidence that neuronal Nox1 is activated in the hippocampus under CCH, causing oxidative stress and consequential hippocampal neuronal death and cognitive impairment. This evidence implies that Nox1-mediated oxidative stress plays an important role in neuronal cell death and cognitive dysfunction in VaD. Nox1 may serve as a potential therapeutic target for VaD.

P21-activated kinase in inflammatory and cardiovascular disease

P-21 activated kinases, or PAKs, are serine-threonine kinases that serve a role in diverse biological functions and organ system diseases. Although PAK signaling has been the focus of many investigations, still our understanding of the role of PAK in inflammation is incomplete. This review consolidates what is known about PAK1 across several cell types, highlighting the role of PAK1 and PAK2 in inflammation in relation to NADPH oxidase activation. This review explores the physiological functions of PAK during inflammation, the role of PAK in several organ diseases with an emphasis on cardiovascular disease, and the PAK signaling pathway, including activators and targets of PAK. Also, we discuss PAK1 as a pharmacological anti-inflammatory target, explore the potentials and the limitations of the current pharmacological tools to regulate PAK1 activity during inflammation, and provide indications for future research. We conclude that a vast amount of evidence supports the idea that PAK is a central molecule in inflammatory signaling, thus making PAK1 itself a promising prospective pharmacological target.

Diazoxide pretreatment prevents Aβ1-42 induced oxidative stress in cholinergic neurons via alleviating NOX2 expression

The aggregation and accumulation of amyloid-β (Aβ) plays a significant role in the pathogenesis of Alzheimer's disease. Aβ is known to increase free radical production in neuronal cells, leading to oxidative stress and cell death. Diazoxide (DZ), a highly selective drug capable of opening mitochondrial ATP-sensitive potassium channels, has neuroprotective effects against neuronal cell death. However, the mechanism through which DZ protects cholinergic neurons against Aβ-induced oxidative injury is still unclear. The present study was designed to investigate the effects of DZ pretreatment against Aβ1-42 induced oxidative damage and cytotoxicity. Through measures of DZ effects on Aβ1-42 induced cellular damage, reactive oxygen species (ROS) and MDA generation and expressions of gp91phox and p47phox in cholinergic neurons, new insights into the neuroprotective mechanisms can be derived. Aβ1-42 significantly decreased 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide levels and increased ROS and MDA production; all effects were attenuated by pretreatment with DZ or diphenyleneiodonium chloride (a NOX2 inhibitor). Pretreatment with DZ also attenuated the upregulation of NOX2 subunits (gp91phox and p47phox) induced by Aβ1-42. Since NOX2 is one of the main sources of free radicals, these results suggest that DZ can counteract Aβ1-42 induced oxidative stress and associated cell death by reducing the level of ROS and MDA, in part, by alleviating NOX2 expression.

Bidirectional interactions between NOX2-type NADPH oxidase and the F-actin cytoskeleton in neuronal growth cones

NADPH oxidases are important for neuronal function but detailed subcellular localization studies have not been performed. Here, we provide the first evidence for the presence of functional NADPH oxidase 2 (NOX2)-type complex in neuronal growth cones and its bidirectional relationship with the actin cytoskeleton. NADPH oxidase inhibition resulted in reduced F-actin content, retrograde F-actin flow, and neurite outgrowth. Stimulation of NADPH oxidase via protein kinase C activation increased levels of hydrogen peroxide in the growth cone periphery. The main enzymatic NADPH oxidase subunit NOX2/gp91(phox) localized to the growth cone plasma membrane and showed little overlap with the regulatory subunit p40(phox) . p40(phox) itself exhibited colocalization with filopodial actin bundles. Differential subcellular fractionation revealed preferential association of NOX2/gp91(phox) and p40(phox) with the membrane and the cytoskeletal fraction, respectively. When neurite growth was evoked with beads coated with the cell adhesion molecule apCAM, we observed a significant increase in colocalization of p40(phox) with NOX2/gp91(phox) at apCAM adhesion sites. Together, these findings suggest a bidirectional functional relationship between NADPH oxidase activity and the actin cytoskeleton in neuronal growth cones, which contributes to the control of neurite outgrowth. We have previously shown that reactive oxygen species (ROS) are critical for actin organization and dynamics in neuronal growth cones as well as neurite outgrowth. Here, we report that the cytosolic subunit p40(phox) of the NOX2-type NADPH oxidase complex is partially associated with F-actin in neuronal growth cones, while ROS produced by this complex regulates F-actin dynamics and neurite growth. These findings provide evidence for a bidirectional relationship between NADPH oxidase activity and the actin cytoskeleton in neuronal growth cones.

Which NADPH oxidase isoform is relevant for ischemic stroke? The case for nox 2

Significance and Recent Advances: Ischemic stroke is the leading cause of disability and third in mortality in industrialized nations. Immediate restoration of cerebral blood flow is crucial to salvage brain tissue, but only few patients are eligible for recanalization therapy. Thus, the need for alternative neuroprotective strategies is huge, and antioxidant interventions have long been studied in this context. Reactive oxygen species (ROS) physiologically serve as signaling molecules, but excessive amounts of ROS, as generated during ischemia/reperfusion (I/R), contribute to tissue injury.

CRITICAL ISSUES

Nevertheless and despite a strong rational of ROS being a pharmacological target, all antioxidant interventions failed to improve functional outcome in human clinical trials. Antioxidants may interfere with physiological functions of ROS or do not reach the crucial target structures of ROS-induced injury effectively.

FUTURE DIRECTIONS

Thus, a potentially more promising approach is the inhibition of the source of disease-promoting ROS. Within recent years, NADPH oxidases (Nox) of the Nox family have been identified as mediators of neuronal pathology. As, however, several Nox homologs are expressed in neuronal tissue, and as many of the pharmacological inhibitors employed are rather unspecific, the concept of Nox as mediators of brain damage is far from being settled. In this review, we will discuss the contribution of Nox homologs to I/R injury at large as well as to neuronal damage in particular. We will illustrate that the current data provide evidence for Nox2 as the most important NADPH oxidase mediating cerebral injury.

Role of reactive oxygen species and NADPH-oxidase in the development of rat cerebellum

Experimental evidence suggests that reactive oxygen species (ROS) could participate in the regulation of some physiological conditions. In the nervous system, ROS have been suggested to act as signaling molecules involved in several developmental processes including cell differentiation, proliferation and programmed of cell death. Although ROS can be generated by several sources, it has been suggested that NADPH oxidase (NOX) could be critical in the production of ROS acting as a signal in some of these events. It has been reported that ROS production by NOX enzymes participate in neuronal maturation and differentiation during brain development. In the present study, we found that during rat cerebellar development there was a differential ROS generation at different ages and areas of the cerebellum. We also found a differential expression of NOX homologues during rat cerebellar development. When we treated developing rats with an antioxidant or with apocynin, an inhibitor of NOX, we found a marked decrease of the ROS levels in all the cerebellar layers at all the ages tested. Both treatments also induced a significant change in the cerebellar foliation as well as an alteration in motor behavior. These results suggest that both ROS and NOX have a critical role during cerebellar development.

Entamoeba histolytica induces cell death of HT29 colonic epithelial cells via NOX1-derived ROS

Entamoeba histolytica, which causes amoebic colitis and occasionally liver abscess in humans, is able to induce host cell death. However, signaling mechanisms of colon cell death induced by E. histolytica are not fully elucidated. In this study, we investigated the signaling role of NOX in cell death of HT29 colonic epithelial cells induced by E. histolytica. Incubation of HT29 cells with amoebic trophozoites resulted in DNA fragmentation that is a hallmark of apoptotic cell death. In addition, E. histolytica generate intracellular reactive oxygen species (ROS) in a contact-dependent manner. Inhibition of intracellular ROS level with treatment with DPI, an inhibitor of NADPH oxidases (NOXs), decreased Entamoeba-induced ROS generation and cell death in HT29 cells. However, pan-caspase inhibitor did not affect E. histolytica-induced HT29 cell death. In HT29 cells, catalytic subunit NOX1 and regulatory subunit Rac1 for NOX1 activation were highly expressed. We next investigated whether NADPH oxidase 1 (NOX1)-derived ROS is closely associated with HT29 cell death induced by E. histolytica. Suppression of Rac1 by siRNA significantly inhibited Entamoeba-induced cell death. Moreover, knockdown of NOX1 by siRNA, effectively inhibited E. histolytica-triggered DNA fragmentation in HT29 cells. These results suggest that NOX1-derived ROS is required for apoptotic cell death in HT29 colon epithelial cells induced by E. histolytica.

Non-antioxidant properties of α-tocopherol reduce the anticancer activity of several protein kinase inhibitors in vitro

The antioxidant properties of α-tocopherol have been proposed to play a beneficial chemopreventive role against cancer. However, emerging data also indicate that it may exert contrasting effects on the efficacy of chemotherapeutic treatments when given as dietary supplement, being in that case harmful for patients. This dual role of α-tocopherol and, in particular, its effects on the efficacy of anticancer drugs remains poorly documented. For this purpose, we studied here, using high throughput flow cytometry, the direct impact of α-tocopherol on apoptosis and cell cycle arrest induced by different cytotoxic agents on various models of cancer cell lines in vitro. Our results indicate that physiologically relevant concentrations of α-tocopherol strongly compromise the cytotoxic and cytostatic action of various protein kinase inhibitors (KI), while other classes of chemotherapeutic agents or apoptosis inducers are unaffected by this vitamin. Interestingly, these anti-chemotherapeutic effects of α-tocopherol appear to be unrelated to its antioxidant properties since a variety of other antioxidants were completely neutral toward KI-induced cell cycle arrest and cell death. In conclusion, our data suggest that dietary α-tocopherol could limit KI effects on tumour cells, and, by extent, that this could result in a reduction of the clinical efficacy of anti-cancer treatments based on KI molecules.

NADPH oxidases: novel therapeutic targets for neurodegenerative diseases

Oxidative stress is a key pathologic factor in neurodegenerative diseases such as Alzheimer and Parkinson diseases (AD, PD). The failure of free-radical-scavenging antioxidants in clinical trials pinpoints an urgent need to identify and to block major sources of oxidative stress in neurodegenerative diseases. As a major superoxide-producing enzyme complex in activated phagocytes, phagocyte NADPH oxidase (PHOX) is essential for host defense. However, recent preclinical evidence has underscored a pivotal role of overactivated PHOX in chronic neuroinflammation and progressive neurodegeneration. Deficiency in PHOX subunits mitigates neuronal damage induced by diverse insults/stresses relevant to neurodegenerative diseases. More importantly, suppression of PHOX activity correlates with reduced neuronal impairment in models of neurodegenerative diseases. The discovery of PHOX and non-phagocyte NADPH oxidases in astroglia and neurons further reinforces the crucial role of NADPH oxidases in oxidative stress-mediated chronic neurodegeneration. Thus, proper modulation of NADPH oxidase activity might hold therapeutic potential for currently incurable neurodegenerative diseases.

Cellular viability effects of fatty acid amide hydrolase inhibition on cerebellar neurons

The endocannabinoid anandamide (ANA) participates in the control of cell death inducing the formation of apoptotic bodies and DNA fragmentation. The aim of this study was to evaluate whether the ANA degrading enzyme, the fatty acid amide hydrolase (FAAH), would induce cellular death. Experiments were performed in cerebellar granule neurons cultured with the FAAH inhibitor, URB597 (25, 50 or 100 nM) as well as endogenous lipids such as oleoylethanolamide (OEA) or palmitoylethanolamide (PEA) and cellular viability was determined by MTT test. Neurons cultured with URB597 (25, 50 or 100 nM) displayed a decrease in cellular viability. In addition, if cultured with OEA (25 nM) or PEA (100 nM), cellular death was found. These results further suggest that URB597, OEA or PEA promote cellular death.

NOX1 participates in ROS-dependent cell death of colon epithelial Caco2 cells induced by Entamoeba histolytica

Entamoeba histolytica, which causes amebic colitis and occasional liver abscesses in humans, can induce host cell death through apoptosis and necrosis. Recently, we have demonstrated that E. histolytica can induce cell death in neutrophils via diphenyleneiodonium-sensitive NADPH oxidase (NOX)-derived reactive oxygen species (ROS). Although there are enzyme systems similar to the phagocyte NADPH oxidase system in many non-phagocytic cell types, the signaling role of NOX-derived ROS in cell death of human colon epithelial cells induced by E. histolytica remains obscure. Incubation of colon epithelial Caco2 tumor cell lines with amebic trophozoites resulted in intracellular ROS generation and cell death in a caspase-independent manner. Pretreatment with DPI, an inhibitor of NOX, strongly decreased E. histolytica-induced cell death in Caco2 cells. As identified by RT-PCR, NOX1 transcripts were highly expressed in Caco2 cells. siRNA-mediated suppression of NOX1 protein significantly inhibited E. histolytica-induced cell death and ROS response in Caco2 cells. These results suggest that NOX1 participates in the ROS-dependent cell death of colon epithelial cells induced by amebic adhesion during the early phase of intestinal amebiasis.

Anti-inflammatory effects of apocynin, an inhibitor of NADPH oxidase, in airway inflammation

Oxidative stress is implicated in the pathogenesis of allergic asthma and remains an attractive target for the prevention of the disease. Herein, we investigated the anti-inflammatory effects of apocynin, a NADPH oxidase (NOX) inhibitor, in both in vitro and in vivo allergen-induced experimental asthma mediated by Th2 hyperresponsiveness. Apocynin showed potential antioxidant activities and inhibitory effects on the activation of redox-sensitive transcription factors, such as NF-κB and AP-1, induced by pro-inflammatory stimuli, such as TNF-α, lipopolysaccharide and Poly I:C, and that inhibited the production of pro-inflammatory cytokines, such as TNF-α, IL-1β and IL-6. In in vivo experimental asthma model, moreover, apocynin significantly attenuated ovalbumin-induced airway hyperresponsiveness and inflammation, as shown by the attenuation of total inflammatory cell and soluble product influx into bronchoalveolar lavage fluid, such as macrophages, eosinophils, IL-4, IL-5, IL-12, IL-13 and TNF-α. Apocynin also significantly reduced lung inflammation in the tissues. Altogether, these results suggest that apocynin may be useful in the treatment of inflammatory diseases induced by oxidative stress through NOX activity.

Reactive oxygen species are related to ionic fluxes and volume decrease in apoptotic cerebellar granule neurons: role of NOX enzymes

Reactive oxygen species (ROS) are produced early during apoptosis of cerebellar granule neurons induced by low potassium (K5) and staurosporine (Sts). In addition, K5 and Sts activate NADPH oxidases (NOX). Recently, we described that K5 and Sts induce apoptotic volume decrease (AVD) at a time when ROS generation and NOX activity occur. In the present study, we evaluated the relationship between ROS generation and ionic fluxes during AVD. Here, we showed that K5- and Sts-induced AVD was inhibited by antioxidants and that direct ROS production induced AVD. Moreover, NOX inhibitors eliminated AVD induced by both K5 and Sts. Sts, but not K5, failed to induce AVD in cerebellar granule neurons from NOX2 knockout mice. These findings suggest that K5- and Sts-induced AVD is largely mediated by ROS produced by NOX. On the other hand, we also found that the blockage of ionic fluxes involved in AVD inhibited both ROS generation and NOX activity. These findings suggest that ROS generation and NOX activity are involved in ionic fluxes activation, which in turn could maintain ROS generation by activating NOX, leading to a self-amplifying cycle.

NAD(P)H oxidase contributes to neurotoxicity in an excitotoxic/prooxidant model of Huntington's disease in rats: protective role of apocynin

Intrastriatal injection of quinolinic acid (QUIN) to rodents reproduces some biochemical, morphological, and behavioral characteristics of Huntington's disease. NAD(P)H oxidase is an enzymatic complex that catalyzes superoxide anion (O(2).(-)) production from O(2) and NADPH. The present study evaluated the role of NAD(P)H oxidase in the striatal damage induced by QUIN (240 nmol/microl) in adult male Wistar rats by means of apocynin (APO; 5 mg/kg i.p.), a specific NAD(P)H oxidase inhibitor. Rats were given APO 30 min before and 1 hr after QUIN injection or only 30 min after QUIN injection. NAD(P)H oxidase activity was measured in striatal homogenates by O2(*)(-) production. QUIN infusion to rats significantly increased striatal NAD(P)H oxidase activity (2 hr postlesion), whereas APO treatments decreased the QUIN-induced enzyme activity (2 hr postlesion), lipid peroxidation (3 hr postlesion), circling behavior (6 days postlesion), and histological damage (7 days postlesion). The addition of NADH to striatal homogenates increased NAD(P)H oxidase activity in striata from QUIN-treated animals but not from sham rats. Interestingly, O2(*)(-) production in QUIN-lesioned striata was unaffected by the addition of substrates for intramitochondrial O2(*)(-) production, xanthine oxidase and nitric oxide synthase, suggesting that NAD(P)H oxidase may be the main source of O2(*)(-) in QUIN-treated rats. Moreover, the administration of MK-801 to rats as a pretreatment resulted in a complete prevention of the QUIN-induced NAD(P)H activation, suggesting that this toxic event is completely dependent on N-methyl-D-aspartate receptor overactivation. Our results also suggest that NAD(P)H oxidase is involved in the pathogenic events linked to excitotoxic/prooxidant conditions.

Apoptosis and autophagy in rat cerebellar granule neuron death: Role of reactive oxygen species

Programmed cell death (PCD) has been defined as an active, controlled process in which cells participate in their own demise. Apoptosis, or type I PCD, has been widely characterized, both morphologically and biochemically. More recently, autophagy, the self-digesting mechanism involved in the removal of cytoplasmic long-lived proteins, has been involved in cell death, and type II PCD is defined as cell death occurring with autophagic features. Neurons can undergo more than one type of PCD as a backup mechanism when the traditional death pathway is inhibited or in response to a particular death-inducing stimulus. Reactive oxygen species (ROS) have been shown to be important signaling molecules in the execution of apoptosis and, more recently, in the autophagic pathway. In this work, we characterize apoptotic and autophagic cell death in rat cerebellar granule neuron (CGN) culture, a widespread model for the study of neuronal death. Potassium deprivation (K5) and staurosporine (STS) were used for death induction. We found apoptotic and autophagic features under both conditions. Caspase inhibition as well as autophagy inhibition by 3-methyl adenine decreased cell death. Moreover, CGN can undergo the alternative type of cell death when the other one is inhibited. An antioxidant or NADPH oxidase inhibitors delayed apoptosis and had no effect in autophagic features. Thus, we found that autophagy plays a role in cell death of CGN and that, when cells are treated with K5 or STS, both autophagy and ROS seem to promote apoptosis by independent mechanisms.

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

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

NOX2 mediates apoptotic death induced by staurosporine but not by potassium deprivation in cerebellar granule neurons

Neuronal apoptotic death involves the participation of reactive oxygen species (ROS), but their sources have not been completely elucidated. Previous studies have demonstrated that the ROS-producing enzyme NADPH oxidase is present in neuronal cells and that this enzyme could participate in the apoptotic neuronal death. Cerebellar granule neurons (CGN) undergo apoptosis when cells are transferred from a medium with 25 mM KCl (K25) to a 5 mM KCl (K5) medium or when they are treated with staurosporine (ST). Under these conditions, apoptotic death of CGN is dependent on ROS production. In this study, we evaluated the role of NOX2, an NADPH oxidase homolog, in the apoptotic death of CGN induced by two different conditions. In CGN from NOX2-deficient (ko) mice, a significantly lower rate of apoptotic death occurs compared with wild-type (wt) CGN. Also, caspase-3 activation, NADPH oxidase activity, and superoxide anion production induced by ST were markedly lower in ko neurons than in wt CGN. In contrast to the case with ST, when CGN were treated with K5, no differences were observed between ko and wt cells in any of the parameters measured. However, all NADPH oxidase inhibitors tested noticeably reduced cell death and apoptotic parameters induced by K5 in both wt and ko CGN. These results suggest that NOX2 could be necessary for apoptotic death induced by ST, but not by K5, which could require other member of the NOX family in the apoptotic process.

The insert region of the Rac GTPases is dispensable for activation of superoxide-producing NADPH oxidases

Rac1 and Rac2, which belong to the Rho subfamily of Ras-related GTPases, play an essential role in activation of gp91phox/Nox2 (cytochrome b-245, beta polypeptide; also known as Cybb), the catalytic core of the superoxide-producing NADPH oxidase in phagocytes. Rac1 also contributes to activation of the non-phagocytic oxidases Nox1 (NADPH oxidase 1) and Nox3 (NADPH oxidase 3), each related closely to gp91phox/Nox2. It has remained controversial whether the insert region of Rac (amino acids 123-135), unique to the Rho subfamily proteins, is involved in gp91phox/Nox2 activation. In the present study we show that removal of the insert region from Rac1 neither affects activation of gp91phox/Nox2, which is reconstituted under cell-free and whole-cell conditions, nor blocks its localization to phagosomes during ingestion of IgG-coated beads by macrophage-like RAW264.7 cells. The insert region of Rac2 is also dispensable for gp91phox/Nox2 activation at the cellular level. Although Rac2, as well as Rac1, is capable of enhancing superoxide production by Nox1 and Nox3, the enhancements by the two GTPases are both independent of the insert region. We also demonstrate that Rac3, a third member of the Rac family in mammals, has an ability to activate the three oxidases and that the activation does not require the insert region. Thus the insert region of the Rac GTPases does not participate in regulation of the Nox family NADPH oxidases.