Induction of gp91-phox, a component of the phagocyte NADPH oxidase, in microglial cells during central nervous system inflammation

Calorie restriction and stroke

Stroke, a major cause of disability and mortality in the elderly, occurs when a cerebral blood vessel is occluded or ruptured, resulting in ischemic damage and death of brain cells. The injury mechanism involves metabolic and oxidative stress, excitotoxicity, apoptosis and inflammatory processes, including activation of glial cells and infiltration of leukocytes. In animal models, dietary energy restriction, by daily calorie reduction (CR) or intermittent fasting (IF), extends lifespan and decreases the development of age-related diseases. Dietary energy restriction may also benefit neurons, as suggested by experimental evidence showing that CR and IF protect neurons against degeneration in animal models. Recent findings by our group and others suggest the possibility that dietary energy restriction may protect against stroke induced brain injury, in part by inducing the expression of neurotrophic factors, such as brain-derived neurotrophic factor (BDNF) and basic fibroblast growth factor (bFGF); protein chaperones, including heat shock protein 70 (Hsp70) and glucose regulated protein 78 (GRP78); antioxidant enzymes, such as superoxide dismutases (SOD) and heme oxygenase-1 (HO-1), silent information regulator T1 (SIRT1), uncoupling proteins and anti-inflammatory cytokines. This article discusses the protective mechanisms activated by dietary energy restriction in ischemic stroke.

Roles of reactive oxygen and nitrogen species in pain

Peroxynitrite (PN; ONOO⁻) and its reactive oxygen precursor superoxide (SO; O₂•⁻) are critically important in the development of pain of several etiologies including pain associated with chronic use of opiates such as morphine (also known as opiate-induced hyperalgesia and antinociceptive tolerance). This is now an emerging field in which considerable progress has been made in terms of understanding the relative contributions of SO, PN, and nitroxidative stress in pain signaling at the molecular and biochemical levels. Aggressive research in this area is poised to provide the pharmacological basis for development of novel nonnarcotic analgesics that are based upon the unique ability to selectively eliminate SO and/or PN. As we have a better understanding of the roles of SO and PN in pathophysiological settings, targeting PN may be a better therapeutic strategy than targeting SO. This is because, unlike PN, which has no currently known beneficial role, SO may play a significant role in learning and memory. Thus, the best approach may be to spare SO while directly targeting its downstream product, PN. Over the past 15 years, our team has spearheaded research concerning the roles of SO and PN in pain and these results are currently leading to the development of solid therapeutic strategies in this important area.

NADPH oxidase mediates striatal neuronal injury after transient global cerebral ischemia

Medium spiny neurons (MSNs) constitute most of the striatal neurons and are known to be vulnerable to ischemia; however, the mechanisms of the vulnerability remain unclear. Activated forms of nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase (NOX), which require interaction between cytosolic and membrane-bound subunits, are among the major sources of superoxide in the central nervous system. Although increasing evidence suggests that NOX has important roles in neurodegenerative diseases, its roles in MSN injury after transient global cerebral ischemia (tGCI) have not been elucidated. To clarify this issue, C57BL/6 mice were subjected to tGCI by bilateral common carotid artery occlusion for 22 minutes. Western blot analysis revealed upregulation of NOX subunits and recruitment of cytosolic subunits to the cell membrane at early (3 to 6 hours) and late (72 hours) phases after tGCI. Taken together with immunofluorescent studies, this activation arose in MSNs and endothelial cells at the early phase, and in reactive microglia at the late phase. Pharmacological and genetic inhibition of NOX attenuated oxidative injury, microglial activation, and MSN death after tGCI. These findings suggest that NOX has pivotal roles in MSN injury after tGCI and could be a therapeutic target for brain ischemia.

Microglial activation and chronic neurodegeneration

Microglia, the resident innate immune cells in the brain, have long been implicated in the pathology of neurodegenerative diseases. Accumulating evidence points to activated microglia as a chronic source of multiple neurotoxic factors, including tumor necrosis factor-α, nitric oxide, interleukin-1β, and reactive oxygen species (ROS), driving progressive neuron damage. Microglia can become chronically activated by either a single stimulus (e.g., lipopolysaccharide or neuron damage) or multiple stimuli exposures to result in cumulative neuronal loss with time. Although the mechanisms driving these phenomena are just beginning to be understood, reactive microgliosis (the microglial response to neuron damage) and ROS have been implicated as key mechanisms of chronic and neurotoxic microglial activation, particularly in the case of Parkinson's disease. We review the mechanisms of neurotoxicity associated with chronic microglial activation and discuss the role of neuronal death and microglial ROS driving the chronic and toxic microglial phenotype.

Gp91phox (NOX2) in classically activated microglia exacerbates traumatic brain injury

Background

We hypothesized that gp91^phox (NOX2), a subunit of NADPH oxidase, generates superoxide anion (O₂^-) and has a major causative role in traumatic brain injury (TBI). To evaluate the functional role of gp91^phox and reactive oxygen species (ROS) on TBI, we carried out controlled cortical impact in gp91^phox knockout mice (gp91^phox-/-). We also used a microglial cell line to determine the activated cell phenotype that contributes to gp91^phox generation.

Methods

Unilateral TBI was induced in gp91^phox-/- and wild-type (Wt) mice (C57/B6J) (25-30 g). The expression and roles of gp91^phox after TBI were investigated using immunoblotting and staining techniques. Levels of O₂^- and peroxynitrite were determined in situ in the mouse brain. The activated phenotype in microglia that expressed gp91^phox was determined in a microglial cell line, BV-2, in the presence of IFNγ or IL-4.

Results

Gp91^phox expression increased mainly in amoeboid-shaped microglial cells of the ipsilateral hemisphere of Wt mice after TBI. The contusion area, number of TUNEL-positive cells, and amount of O₂^- and peroxynitrite metabolites produced were less in gp91^phox-/- mice than in Wt. In the presence of IFNγ, BV-2 cells had increased inducible nitric oxide synthase and nitric oxide levels, consistent with a classical activated phenotype, and drastically increased expression of gp91^phox.

Conclusions

Classical activated microglia promote ROS formation through gp91^phox and have an important role in brain damage following TBI. Modulating gp91^phox and gp91^phox -derived ROS may provide a new therapeutic strategy in combating post-traumatic brain injury.

Spinal NADPH oxidase is a source of superoxide in the development of morphine-induced hyperalgesia and antinociceptive tolerance

The role of superoxide and its active byproduct peroxynitrite as mediators of nociceptive signaling is emerging. We have recently reported that nitration and inactivation of spinal mitochondrial superoxide dismutase (MnSOD) provides a critical source of these reactive oxygen and nitrogen species during central sensitization associated with the development of morphine-induced hyperalgesia and antinociceptive tolerance. In this study, we demonstrate that activation of spinal NADPH oxidase is another critical source for superoxide generation. Indeed, the development of morphine-induced hyperalgesia and antinociceptive tolerance was associated with increased activation of NADPH oxidase and superoxide release. Co-administration of morphine with systemic delivery of two structurally unrelated NADPH oxidase inhibitors namely apocynin or diphenyleneiodonium (DPI), blocked NADPH oxidase activation and the development of hyperalgesia and antinociceptive tolerance at doses devoid of behavioral side effects. These results suggest that activation of spinal NADPH oxidase contributes to the development of morphine-induced hyperalgesia and antinociceptive tolerance. The role of spinal NADPH oxidase was confirmed by showing that intrathecal delivery of apocynin blocked these events. Our results are the first to implicate the contribution of NADPH oxidase as an enzymatic source of superoxide and thus peroxynitrite in the development of central sensitization associated with morphine-induced hyperalgesia and antinociceptive tolerance. These results continue to support the critical role of these reactive oxygen and nitrogen species in pain while advancing our knowledge of their biomolecular sources.

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.

NADPH oxidase plays a central role in cone cell death in retinitis pigmentosa

Retinitis pigmentosa (RP) is a collection of diseases in which rod photoreceptors die from a variety of mutations. After rods die, the level of tissue oxygen in the outer retina becomes elevated and there is progressive oxidative damage to cones that ultimately triggers apoptosis. In this study, we investigated the hypothesis that NADPH oxidase (Nox) and/or xanthine oxidase serve as critical intermediaries between increased tissue oxygen and the generation of excessive reactive oxygen species that cause oxidative damage to cones. Apocynin, a blocker of Nox, but not allopurinol, a blocker of xanthine oxidase, markedly reduced the superoxide radicals visualized by hydroethidine in the outer retina in the retinal degeneration-1 (rd1(+/+)) model of RP. Compared to rd1(+/+) mice treated with vehicle, those treated with apocynin, but not those treated with allopurinol, had significantly less oxidative damage in the retina measured by ELISA for carbonyl adducts. Apocynin-treated, but not allopurinol-treated, rd1(+/+) mice had preservation of cone cell density, increased mRNA levels for m- and s-cone opsin, and increased mean photopic b-wave amplitude. In Q344ter mice, a model of dominant RP in which mutant rhodopsin is expressed, apocynin treatment preserved photopic electroretinogram b-wave amplitude compared to vehicle-treated controls. These data indicate that Nox, but not xanthine oxidase, plays a critical role in generation of the oxidative stress that leads to cone cell death in RP and inhibition of Nox provides a new treatment strategy.

NADPH oxidase activity is higher in cerebral versus systemic arteries of four animal species: role of Nox2

We previously reported that NADPH oxidase activity is greater in intracranial cerebral versus systemic arteries of the rat. Here, we first tested whether NADPH oxidase activity is also greater in intracranial cerebral than systemic arteries of three other animal species, i.e., mouse, rabbit, and pig. Second, using Nox2-deficient mice, we evaluated the involvement of Nox2-containing NADPH oxidases in any such regional differences. NADPH-stimulated superoxide (O2−) production by basilar, middle cerebral arteries (MCA), and common carotid arteries (CA) and thoracic aorta (AO) from rat, mouse, rabbit, and pig was measured using lucigenin-enhanced chemiluminescence. Basal production of O2− and hydrogen peroxide (H2O2) by cerebral arteries, AO, and CA from wild-type (WT) and Nox2−/− mice was measured using L-012-enhanced chemiluminescence and Amplex Red fluorescence, respectively. Western blotting was used to measure Nox2 and SOD1–3 protein expression, and immunofluorescence was used to localize Nox2, in mouse arteries. In rats, WT mice, rabbits, and pigs, NADPH-stimulated O2− production by cerebral arteries was up to 40-fold greater than that in AO and CA. In WT mice, basal O2− and H2O2 production by cerebral arteries was ninefold and ∼2.5-fold higher, respectively, than that in AO and CA and was associated with ∼40% greater expression of Nox2 protein. Nox2 immunofluorescence was localized to the endothelium, and to a lesser extent the adventitia, in all mouse arteries and appeared to be more intense in endothelium of MCA than AO or CA. In Nox2−/− mice, NADPH-stimulated O2− production by cerebral arteries was ∼35% lower than that in WT mice, whereas Nox2 deletion had no significant effect on O2− production by AO or CA. Thus NADPH oxidase activity is greater in intracranial cerebral versus systemic arteries of several animal species and is associated with higher cerebrovascular expression and activity of Nox2.

Early increase of Nox4 NADPH oxidase and superoxide generation following endothelin-1-induced stroke in conscious rats

Oxidative stress contributes to the progression of brain injury following ischemic stroke and reperfusion. NADPH oxidase is a well-established source of superoxide in vascular disease, but its contribution to tissue injury following ischemic stroke has yet to be fully elucidated. Here we show the spatiotemporal profile of NADPH oxidase subunits Nox2 and Nox4 and concurrent superoxide generation following stroke induced by middle cerebral artery constriction in conscious rats. Nox2 mRNA was progressively up-regulated in both the ipsilateral cortex and the striatum from 6 hr to 7 days poststroke and reperfusion. Nox4 mRNA was also up-regulated transiently in the cortex at 6 hr poststroke but returned to control levels after this time. In situ detection of superoxide generation with dihydroethidium fluorescence revealed an increase in superoxide within the ischemic core at 6 hr poststroke that was mostly colocalized with the neuronal marker NeuN. By 24 hr, this increase in superoxide production had spread to the boundary zone of the infarct, whereas it disappeared in the ischemic core as neuronal numbers declined. Subsequently, superoxide within the ischemic core again increased at 7 days and was mostly colocalized with the activated microglia/macrophage marker OX-42. Immunoreactivity to Nox2 followed the same spatiotemporal pattern as that of OX-42 immunostaining poststroke. Clearly, NADPH oxidase is an important mediator of oxidative stress and contributes to the progression of brain damage beyond the infarct core, via the activation of two catalytic subunits, Nox2 and Nox4. Selectively blocking these subunits might be useful for intervening in the progression of stroke brain injury.

Oxidative stress through activation of NAD(P)H oxidase in hypertensive mice with spontaneous intracranial hemorrhage

We have developed an experimental model of spontaneous intracranial hemorrhage (ICH) in transgenic mice expressing human renin and human angiotensinogen (R+/A+) treated with high-salt diet and N(omega)-nitro-L-arginine methyl ester (L-NAME). We investigated whether oxidative stress is associated with spontaneous ICH in R+/A+ mice. R+/A+ mice on high-salt diet and L-NAME presented neurologic signs 57+/-13 (mean+/-s.e.m.) days after the start of treatment. Intracranial hemorrhage was shown with histologic examination. Levels of superoxide in brain homogenate were significantly increased in R+/A+ mice with ICH (118+/-10 RLU per sec per mg; RLU, relative light unit) compared with age-matched control mice (19+/-1) and R+/A+ mice without ICH (53+/-3). NAD(P)H oxidase activity was significantly higher in R+/A+ mice with ICH (34,933+/-2,420 RLU per sec per mg) than in control mice (4,984+/-248) and R+/A+ mice without ICH (15,069+/-917). These results suggest that increased levels of superoxide are due, at least in part, to increased NAD(P)H oxidase activity. Increased NAD(P)H oxidase activity preceded signs of ICH, and increased further when R+/A+ mice developed ICH. These findings suggest that oxidative stress may contribute to spontaneous ICH in chronic hypertension.

Nox enzymes, ROS, and chronic disease: an example of antagonistic pleiotropy

Reactive oxygen species (ROS) are considered to be chemically reactive with and damaging to biomolecules including DNA, protein, and lipid, and excessive exposure to ROS induces oxidative stress and causes genetic mutations. However, the recently described family of Nox and Duox enzymes generates ROS in a variety of tissues as part of normal physiological functions, which include innate immunity, signal transduction, and biochemical reactions, e.g., to produce thyroid hormone. Nature's "choice" of ROS to carry out these biological functions seems odd indeed, given its predisposition to cause molecular damage. This review describes normal biological roles of Nox enzymes as well as pathological conditions that are associated with ROS production by Nox enzymes. By far the most common conditions associated with Nox-derived ROS are chronic diseases that tend to appear late in life, including atherosclerosis, hypertension, diabetic nephropathy, lung fibrosis, cancer, Alzheimer's disease, and others. In almost all cases, with the exception of a few rare inherited conditions (e.g., related to innate immunity, gravity perception, and hypothyroidism), diseases are associated with overproduction of ROS by Nox enzymes; this results in oxidative stress that damages tissues over time. I propose that these pathological roles of Nox enzymes can be understood in terms of antagonistic pleiotropy: genes that confer a reproductive advantage early in life can have harmful effects late in life. Such genes are retained during evolution despite their harmful effects, because the force of natural selection declines with advanced age. This review discusses some of the proposed physiologic roles of Nox enzymes, and emphasizes the role of Nox enzymes in disease and the likely beneficial effects of drugs that target Nox enzymes, particularly in chronic diseases associated with an aging population.

Ion channel blockade attenuates aggregated alpha synuclein induction of microglial reactive oxygen species: relevance for the pathogenesis of Parkinson's disease

Brain mononuclear phagocyte (perivascular macrophage and microglia, MG) inflammatory neurotoxins play a principal role in the pathogenesis of Parkinson's disease; chief among these are reactive oxygen species (ROS). We posit that aggregated, misfolded and oxidized alpha-synuclein (a major constituent of Lewy bodies), released or secreted from dying dopaminergic neurons, induces microglial ROS production that is regulated by ion channels and as such affects disease progression. To address this hypothesis, we performed patch clamp recordings of outward ionic currents in murine microglia and characterized their links to ROS production during alpha-synuclein stimulation. Aggregated nitrated alpha-synuclein induced ROS production in a dose-dependent manner that was inhibited by voltage-gated potassium current blockade, and to a more limited degree, by chloride current blockade. Interestingly, ROS produced in MG primed with tumor necrosis factor alpha and activated with phorbol myristate acetate was attenuated by voltage-gated potassium current blockade and more completely by chloride current blockade. In contrast, amyloid beta or cell membrane extract failed to induce microglial ROS production. Similar results were obtained using bone marrow-derived macrophages. The association of ROS production with specific plasma membrane ion currents provides a link between regulation of microglial ion transport and oxygen free radical production. Understanding these linkages may lead to novel therapeutics for Parkinson's disease where modulation of redox-related stress may slow disease progression.

Oxidative stress and the pathogenesis of neurodegenerative disorders

Microglia-derived inflammatory neurotoxins play a principal role in the pathogenesis of neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and HIV-associated dementia; chief among these is reactive oxygen species. The detrimental effects of oxidative stress in the brain and nervous system are primarily a result of the diminished capacity of the central nervous system to prevent ongoing oxidative damage. A spectrum of environmental cues, mitochondrial dysfunction, accumulation of aberrant misfolded proteins, inflammation, and defects in protein clearance are known to evolve and form as a result of disease progression. These factors likely affect glial function serving to accelerate the tempo of disease. Understanding the relationships between disease progression, free radical formation, neuroinflammation, and neurotoxicity is critical to elucidating disease mechanisms and the development of therapeutic modalities to combat disease processes. In an era where populations continue to age, the prevalence and incidence of age-related neurodegenerative diseases are on the rise; therefore, the need for novel therapeutic strategies that attenuate neuroinflammation and protect neurons against oxidative stress is ever more immediate.

Cytokines, neurotrophins, and oxidative stress in brain disease from mucopolysaccharidosis IIIB

Mucopolysaccharidosis IIIB (MPS IIIB; Sanfilippo syndrome type B) is characterized by profound neurological deterioration. Because a murine model of MPS IIIB disease is available, we focused on analysis of gene expression in the brain and cerebellum of 7-month-old MPS IIIB mice by pathway-specific filter microarrays designed to probe apoptotic-related, neurotrophic signalling molecules and inflammatory cytokines and receptors. Moreover, we extended the analysis with real-time PCR performed at 1, 3, 7 months after birth. Bdnf was down-regulated in the brain but up-regulated in the cerebellum at 7 months of age, both at RNA and at protein levels. Cbln1 presented a threefold increase in the oldest brains while remaining unaltered in the cerebellum. Ccl3, Casp11, gp91(phox), p67(phox), and p47(phox) showed an increased expression in both brain and cerebellum at each examined time point. Ccl3, in particular, exhibited in both organs and at all times tested approximately a tenfold increase in its expression. Insofar as p47(phox), p67(phox), and gp91(phox) are all components of the phagocyte NADPH oxidase, our results suggest the possible involvement of the reactive oxygen species in the genesis of neurodegeneration in MPS IIIB disease.

NADPH-oxidase activity is elevated in penumbral and non-ischemic cerebral arteries following stroke

Reactive oxygen species play a role in neuronal damage following cerebral ischemia-reperfusion. We tested whether activity of the superoxide-generating enzyme, NADPH-oxidase, is enhanced in cerebral arteries within, adjacent and distant from the ischemic core. The right middle cerebral artery (MCA) of conscious rats was temporarily occluded by perivascular injection of endothelin-1 to induce stroke (ET-1; n=19). Control rats were injected with saline (n=9). At 24 h or 72 h post-administration of ET-1, the MCA and its branches within the ipsilateral penumbra and infarcted core, corresponding arteries in the contralateral hemisphere, and basilar artery were excised. Anatomically similar arteries were excised from saline-injected rats. At 24 h after stroke, NADPH-stimulated superoxide production by arteries from the infarcted core did not differ from levels generated by arteries from control rats, whereas levels were significantly lower 72 h after stroke. However, at both time points after stroke, superoxide production by arteries from the ischemic penumbra was 8-fold greater than levels generated by arteries from control rats. Surprisingly, even in the non-ischemic arteries from the contralateral hemisphere and in the basilar artery, superoxide production was increased approximately 4- to 6-fold at 24 h, but had returned to normal 72 h after stroke. The NADPH-oxidase inhibitor, diphenyleneiodonium, virtually abolished superoxide production by all arteries. Thus, the activity of NADPH-oxidase is enhanced in cerebral arteries from the ischemic penumbra at 24 h and 72 h following cerebral ischemia. Additionally, NADPH-oxidase activity is temporarily enhanced after cerebral ischemia within arteries from non-ischemic parts of the brain.

Novel isoforms of NADPH-oxidase in cerebral vascular control

Reactive oxygen species (ROS) are thought to play an important role in the initiation and progression of a variety of vascular diseases. Furthermore, accumulating evidence indicates that ROS may also serve as important cell signalling molecules for the regulation of normal vascular function. Recently, a novel family of proteins (Nox1, 2 and 4) that act as the catalytic subunit of the superoxide (O2-) producing enzyme NADPH-oxidase has been discovered in vascular cells. There is now preliminary evidence suggesting that NADPH-oxidase-derived ROS may serve as a physiological vasodilator mechanism in the cerebral circulation. Moreover, the activity of NADPH-oxidase is profoundly greater in cerebral versus systemic arteries. Studies have shown that Nox1, Nox2 (also known as gp91phox) and Nox4 are all expressed in cerebral arteries, suggesting that multiple isoforms of NADPH-oxidase may be important for ROS production by cerebral arteries. Enhanced NADPH-oxidase activity is associated with several vascular-related diseases, including hypertension, stroke, subarachnoid haemorrhage and Alzheimer's dementia; however, the consequences of this for cerebral vascular function are controversial. For example, there is some evidence suggesting that NADPH-oxidase-derived O2- may play a role in endothelial dysfunction of cerebral arteries and a subsequent rise in cerebral vascular tone, associated with hypertension. However, activation of NADPH-oxidase elicits cerebral vasodilatation in vivo, and this mechanism is enhanced in chronic hypertension. While further supportive evidence is needed, it is an intriguing possibility that NADPH-oxidase-derived ROS may play a protective role in regulating cerebral vascular tone during disease.

NADPH oxidases of the brain: distribution, regulation, and function

The NADPH oxidase is a multi-subunit enzyme that catalyzes the reduction of molecular oxygen to form superoxide (O(2)(-)). While classically linked to the respiratory burst in neutrophils, recent evidence now shows that O(2)(-) (and associated reactive oxygen species, ROS) generated by NADPH oxidase in nonphagocytic cells serves myriad functions in health and disease. An entire new family of NADPH Oxidase (Nox) homologues has emerged, which vary widely in cell and tissue distribution, as well as in function and regulation. A major concept in redox signaling is that while NADPH oxidase-derived ROS are necessary for normal cellular function, excessive oxidative stress can contribute to pathological disease. This certainly is true in the central nervous system (CNS), where normal NADPH oxidase function appears to be required for processes such as neuronal signaling, memory, and central cardiovascular homeostasis, but overproduction of ROS contributes to neurotoxicity, neurodegeneration, and cardiovascular diseases. Despite implications of NADPH oxidase in normal and pathological CNS processes, still relatively little is known about the mechanisms involved. This paper summarizes the evidence for NADPH oxidase distribution, regulation, and function in the CNS, emphasizing the diversity of Nox isoforms and their new and emerging role in neuro-cardiovascular function. In addition, perspectives for future research and novel therapeutic targets are offered.

Apocynin protects against global cerebral ischemia-reperfusion-induced oxidative stress and injury in the gerbil hippocampus

Increased production of reactive oxygen species (ROS) following cerebral ischemia-reperfusion (I/R) is an important underlying cause for neuronal injury leading to delayed neuronal death (DND). In this study, apocynin, a specific inhibitor for NADPH oxidase, was used to test whether suppression of ROS by the NADPH oxidase inhibitor can protect against ischemia-induced ROS generation and decrease DND. Global cerebral ischemia was induced in gerbils by a 5-min occlusion of bilateral common carotid arteries (CCA). Using measurement of 4-hydroxy-2-nonenal (HNE) as a marker for lipid peroxidation, apocynin (5 mg/kg body weight) injected i.p. 30 min prior to ischemia significantly attenuated the early increase in HNE in hippocampus measured at 3 h after I/R. Apocynin also protected against I/R-induced neuronal degeneration and DND, oxidative DNA damage, and glial cell activation. Taken together, the neuroprotective effects of apocynin against ROS production during early phase of I/R and subsequent I/R-induced neuronal damage provide strong evidence that inhibition of NADPH oxidase could be a promising therapeutic mechanism to protect against stroke damage in the brain.

Microglia potentiate damage to blood-brain barrier constituents: improvement by minocycline in vivo and in vitro

BACKGROUND

Blood-brain barrier (BBB) disruption after stroke can worsen ischemic injury by increasing edema and causing hemorrhage. We determined the effect of microglia on the BBB and its primary constituents, endothelial cells (ECs) and astrocytes, after ischemia using in vivo and in vitro models.

METHODS AND RESULTS

Primary astrocytes, ECs, or cocultures were prepared with or without added microglia. Primary ECs were more resistant to oxygen-glucose deprivation/reperfusion than astrocytes. ECs plus astrocytes showed intermediate vulnerability. Microglia added to cocultures nearly doubled cell death. This increase was prevented by minocycline and apocynin. In vivo, minocycline reduced infarct volume and neurological deficits and markedly reduced BBB disruption and hemorrhage in mice after experimental stroke.

CONCLUSIONS

Inhibition of microglial activation may protect the brain after ischemic stroke by improving BBB viability and integrity. Microglial inhibitors may prove to be an important treatment adjunct to fibrinolysis.

Microglia and inflammation-mediated neurodegeneration: multiple triggers with a common mechanism

Inflammation, a common denominator among the diverse list of neurodegenerative diseases, has recently been implicated as a critical mechanism responsible for the progressive nature of neurodegeneration. Microglia are the resident innate immune cells in the central nervous system and produce a barrage of factors (IL-1, TNFalpha, NO, PGE2, superoxide) that are toxic to neurons. Evidence supports that the unregulated activation of microglia in response to environmental toxins, endogenous proteins, and neuronal death results in the production of toxic factors that propagate neuronal injury. In the following review, we discuss the common thread of microglial activation across numerous neurodegenerative diseases, define current perceptions of how microglia are damaging neurons, and explain how the microglial response to neuronal damage results in a self-propelling cycle of neuron death.

Activation of microglial NADPH oxidase is synergistic with glial iNOS expression in inducing neuronal death: a dual-key mechanism of inflammatory neurodegeneration

Background

Inflammation-activated glia are seen in many CNS pathologies and may kill neurons through the release of cytotoxic mediators, such as nitric oxide from inducible NO synthase (iNOS), and possibly superoxide from NADPH oxidase (NOX). We set out to determine the relative role of these species in inducing neuronal death, and to test the dual-key hypothesis that the production of both species simultaneously is required for significant neuronal death.

Methods

Primary co-cultures of cerebellar granule neurons and glia from rats were used to investigate the effect of NO (from iNOS, following lipopolysaccharide (LPS) and/or cytokine addition) or superoxide/hydrogen peroxide (from NOX, following phorbol 12-myristate 13-acetate (PMA), ATP analogue (BzATP), interleukin-1β (IL-1β) or arachidonic acid (AA) addition) on neuronal survival.

Results

Induction of glial iNOS caused little neuronal death. Similarly, activation of NOX alone resulted in little or no neuronal death. However, if NOX was activated (by PMA or BzATP) in the presence of iNOS (induced by LPS and interferon-γ) then substantial delayed neuronal death occurred over 48 hours, which was prevented by inhibitors of iNOS (1400W), NOX (apocynin) or a peroxynitrite decomposer (FeTPPS). Neurons and glia were also found to stain positive for nitrotyrosine (a putative marker of peroxynitrite) only when both iNOS and NOX were simultaneously active. If NOX was activated by weak stimulators (IL-1β, AA or the fibrillogenic prion peptide PrP106-126) in the presence of iNOS, it caused microglial proliferation and delayed neurodegeneration over 6 days, which was prevented by iNOS or NOX inhibitors, a peroxynitrite decomposer or a NMDA-receptor antagonist (MK-801).

Conclusion

These results suggest a dual-key mechanism, whereby glial iNOS or microglial NOX activation alone is relatively benign, but if activated simultaneously are synergistic in killing neurons, through generating peroxynitrite. This mechanism may mediate inflammatory neurodegeneration in response to cytokines, bacteria, ATP, arachidonate and pathological prions, in which case neurons may be protected by iNOS or NOX inhibitors, or scavengers of NO, superoxide or peroxynitrite.

Oxidative and antioxidative potential of brain microglial cells

Microglial cells are the resident immune cells of the central nervous system. These cells defend the central nervous system against invading microorganisms and clear the debris from damaged cells. Upon activation, microglial cells produce a large number of neuroactive substances that include cytokines, proteases, and prostanoids. In addition, activated microglial cells release radicals, such as superoxide and nitric oxide, that are products of the enzymes NADPH oxidase and inducible nitric oxide synthase, respectively. Microglia-derived radicals, as well as their reactive reaction products hydrogen peroxide and peroxynitrite, have the potential to harm cells and have been implicated in contributing to oxidative damage and neuronal cell death in neurological diseases. For self-protection against oxidative damage, microglial cells are equipped with efficient antioxidative defense mechanisms. These cells contain glutathione in high concentrations, substantial activities of the antioxidative enzymes superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase, as well as NADPH-regenerating enzymes. Their good antioxidative potential protects microglial cells against oxidative damage that could impair important functions of these cells in defense and repair of the brain.

Peroxynitrite generated by inducible nitric oxide synthase and NADPH oxidase mediates microglial toxicity to oligodendrocytes

Reactive microglia in the CNS have been implicated in the pathogenesis of white matter disorders, such as periventricular leukomalacia and multiple sclerosis. However, the mechanism by which activated microglia kill oligodendrocytes (OLs) remains elusive. Here we show that lipopolysaccharide (LPS)-induced death of developing OLs is caused by microglia-derived peroxynitrite, the reaction product of nitric oxide (NO) and superoxide anion. Blocking peroxynitrite formation with nitric oxide synthase inhibitors, superoxide dismutase mimics, or a decomposition catalyst abrogated the cytotoxicity. Only microglia, but not OLs, expressed inducible NO synthase (iNOS) after LPS challenge; microglia from iNOS knockout mice were not cytotoxic upon activation. The molecular source for superoxide was identified as the superoxide-generating enzyme NADPH oxidase. The oxidase was activated upon LPS exposure, and its inhibition prevented microglial toxicity toward OLs. Furthermore, microglia isolated from mice deficient in the catalytic component of the oxidase, gp91(phox), failed to induce cell death. Our results reveal a role for NADPH oxidase in LPS-induced OL death and suggest that peroxynitrite produced by iNOS and NADPH oxidase in activated microglia may play an important role in the pathogenesis of white matter disorders.

Role of NADPH oxidase in the brain injury of intracerebral hemorrhage

The major risk factors for intracerebral hemorrhage (ICH) are hypertension and aging. A fundamental mechanism for hypertension- and aging-induced vascular injury is oxidative stress. We hypothesize that oxidative stress has a crucial role in ICH. To test our hypothesis, we used bacterial collagenase to produce ICH in wild-type C57BL/6 and gp91phox knockout (gp91phox KO) mice (deficient in gp91phox subunit of the superoxide-producing enzyme NADPH oxidase). All animals were studied at 20-35 weeks of age, resembling an older patient population. We found that collagenase produced less bleeding in gp91phox KO mice than wild-type mice. Total oxidative product was lower in gp91phox KO mice than in wild-type mice, both under basal conditions and after ICH. Consistent with the ICH volume, brain edema formation, neurological deficit and a high mortality rate was noted in wild-type but not in gp91phox KO mice. This ICH-induced brain injury in wild-type mice is associated with enhanced expression of the gp91phox subunit of NADPH oxidase. In conclusion, the oxidative stress resulting from activation of NADPH oxidase contributes to ICH induced by collagenase and promotes brain injury.

Tuftsin fragment 1-3 is beneficial when delivered after the induction of intracerebral hemorrhage

BACKGROUND AND PURPOSE

Microglial activation may contribute to the pathogenesis of the brain injury in intracerebral hemorrhage (ICH). We have reported that the tripeptide macrophage/microglial inhibitory factor (MIF), Thr-Lys-Pro, inhibits microglial activation and results in functional improvement when given before the onset of hemorrhage. In this study, we investigate the protection and efficacy of treatment when MIF is administered 2 hours after collagenase injection.

METHODS

ICH was induced by injecting bacterial collagenase into the caudate nucleus; 100 microL MIF (500 micromol/L) was delivered via a micro-osmotic pump. Infusion of MIF or saline (control) was initiated 2 hours after collagenase injection and continued for 24 or 72 hours. Microglial activation and macrophage infiltration were assessed by 5-d-4 and F4/80 immunofluorescence, respectively. Production of reactive oxygen species was visualized by in situ detection of ethidium. Degenerating neurons were assessed by Fluoro-Jade B staining. Neurological deficits, brain injury volumes, and brain edema were assessed at 24 and 72 hours after MIF/saline treatment.

RESULTS

MIF can inhibit microglial activation and macrophage infiltration, attenuate the numbers of ethidium-positive cells compared with the saline-treated control mice, reduce the injury volume, edema, and degenerating neurons, and improve the neurological functional outcome.

CONCLUSIONS

Activated microglia/macrophages are important contributors to brain injury after ICH. MIF could be a valuable neuroprotective agent for the treatment of ICH, if treatment is initiated soon after the onset of hemorrhage.

Redox regulation of glial inflammatory response to lipopolysaccharide and interferongamma

Astrocytes and microglia, the two immune-regulatory cells of the central nervous system (CNS), are activated by a variety of pathogens and cytokines to elicit rapid transcriptional responses. This program of activation is initiated by a set of intracellular signaling cascades that includes mitogen-activated protein kinase (MAPK), nuclear factor (NF) kappaB, and Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathways. This study defines the critical role that NADPH oxidase(Phox)-derived reactive oxygen species (ROS) play in lipopolysaccharide (LPS)- and interferon (IFN)gamma-induced signaling cascades leading to gene expression in glial cells. Treatment of rat microglia and astrocytes with LPS and IFNgamma resulted in a rapid activation of Phox and the release of ROS followed by an induction of inducible nitric oxide synthase (iNOS) expression. iNOS induction was blocked by inhibitors of Phox, i.e., diphenylene iodonium chloride (DPI) and 4-(2-aminoethyl) benzenesulfonylfluoride (AEBSF), suggesting an involvement of ROS signaling in iNOS gene expression. Exogenous catalase but not superoxide dismutase suppressed the basal activity and completely blocked induced levels of NO/iNOS, suggesting that hydrogen peroxide is the ROS involved. Phox inhibitors and catalase also suppressed LPS/IFNgamma-induced expression of cytokines, i.e., interleukin (IL)-1, IL-6, and tumor necrosis factor (TNF)alpha and blocked LPS activation of MAP kinases (i.e., p38 MAPK, c-Jun N-terminal kinase and extracellular signal-regulated kinase), NFkappaB, and IFNgamma-induced STAT1 phosphorylation. A microglial cell line stably transfected with a mutant form of Phox subunit, i.e., p47(phox) W(193)R, and primary astrocytes derived from Phox-deficient mice showed attenuated ROS production and induction of iNOS in response to LPS/IFNgamma, further strengthening the notion that Phox-derived ROS are crucial for proinflammatory gene expression in glial cells.

Exogenous NADPH increases cerebral blood flow through NADPH oxidase-dependent and -independent mechanisms

OBJECTIVE

NADPH, a substrate for the superoxide-producing enzyme NADPH oxidase, produces vasodilation in the cerebral circulation. However, the mechanisms of the effect have not been fully elucidated. We used a peptide inhibitor of NADPH oxidase (gp91ds-tat) and null mice lacking the gp91phox subunit of NADPH oxidase to examine the mechanisms of the cerebrovascular effects of exogenous NADPH.

METHODS AND RESULTS

Cerebral blood flow (CBF) was assessed by laser-Doppler flowmetry in anesthetized mice equipped with a cranial window. Superfusion with NADPH increased CBF (27% at 100 micromol/L) without affecting the EEG. The CBF increase was attenuated by the free-radical scavenger MnTBAP (-54%, P<0.05) but not by the H2O2 scavenger catalase. The response was also attenuated by gp91ds-tat (-64%, P<0.05) and by the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (-44%, P<0.05). The increase in CBF produced by NADPH was attenuated in gp91-null mice (-41%, P<0.05). NADPH increased production of reactive oxygen species, assessed by hydroethidine microfluorography, an effect blocked by MnTBAP or gp91ds-tat and not observed in gp91-null mice.

CONCLUSIONS

These data suggest that the mechanisms of the CBF increases produced by exogenous NADPH are multifactorial and include NADPH oxidase-dependent and -independent factors.

Neuroprotective effect of dextromethorphan in the MPTP Parkinson's disease model: role of NADPH oxidase

Parkinson's disease (PD) is a neurodegenerative movement disorder characterized by a progressive loss of dopaminergic neurons in the substantia nigra and depletion of the neurotransmitter dopamine in the striatum. Progress in the search for effective therapeutic strategies that can halt this degenerative process remains limited. Mechanistic studies using animal systems such as the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) rodent PD model have revealed the involvement of the brain's immune cells and free radical-generating processes. We recently reported that dextromethorphan (DM), a widely used anti-tussive agent, attenuated endotoxin-induced dopaminergic neurodegeneration in vitro. In the current study, we investigated the potential neuroprotective effect of DM and the underlying mechanism of action in the MPTP rodent PD model. Mice (C57BL/6J) that received daily MPTP injections (15 mg free base/kg body weight, s.c.) for 6 consecutive days exhibited significant degeneration of the nigrostriatal dopaminergic pathway. However, the MPTP-induced loss of nigral dopaminergic neurons was significantly attenuated in those mice receiving DM (10 mg/kg body weight, s.c.). In mesencephalic neuron-glia cultures, DM significantly reduced the MPTP-induced production of both extracellular superoxide free radicals and intracellular reactive oxygen species (ROS). Because NADPH oxidase is the primary source of extracellular superoxide and intracellular ROS, we investigated the involvement of NADPH oxidase in the neuroprotective effect of DM. Indeed, the neuroprotective effect of DM was only observed in the wild-type but not in the NADPH oxidase-deficient mice, indicating that NADPH oxidase is a critical mediator of the neuroprotective activity of DM. More importantly, due to its proven safety record of long-term clinical use in humans, DM may be a promising agent for the treatment of degenerative neurological disorders such as PD.

NADPH oxidase mediates lipopolysaccharide-induced neurotoxicity and proinflammatory gene expression in activated microglia

Parkinson's disease is characterized by the progressive degeneration of dopaminergic neurons in the substantia nigra. We have previously reported that lipopolysaccharide (LPS)-induced degeneration of dopaminergic neurons is mediated by the release of proinflammatory factors from activated microglia. Here, we report the pivotal role of NADPH oxidase in inflammation-mediated neurotoxicity, where the LPS-induced loss of nigral dopaminergic neurons in vivo was significantly less pronounced in NADPH oxidase-deficient (PHOX-/-) mice when compared with control (PHOX+/+) mice. Dopaminergic neurons in primary mensencephalic neuron-glia cultures from PHOX+/+ mice were significantly more sensitive to LPS-induced neurotoxicity in vitro when compared with PHOX-/- mice. Further, PHOX+/+ neuron-glia cultures chemically depleted of microglia failed to show dopaminergic neurotoxicity with the addition of LPS. Neuron-enriched cultures from both PHOX+/+ mice and PHOX-/- mice also failed to show any direct LPS-induced dopaminergic neurotoxicity. However, the addition of PHOX+/+ microglia to neuron-enriched cultures from either strain resulted in reinstatement of LPS-induced dopaminergic neurotoxicity, supporting the role of microglia as the primary source of NADPH oxidase-generated insult and neurotoxicity. Immunostaining for F4/80 in mensencephalic neuron-glia cultures revealed that PHOX-/- microglia failed to show activated morphology at 10 h, suggesting an important role of reactive oxygen species (ROS) generated from NADPH oxidase in the early activation of microglia. LPS also failed to elicit extracellular superoxide and produced low levels of intracellular ROS in microglia-enriched cultures from PHOX-/- mice. Gene expression and release of tumor necrosis factor alpha was much lower in PHOX-/- mice than in control PHOX+/+ mice. Together, these results demonstrate the dual neurotoxic functions of microglial NADPH oxidase: 1) the production of extracellular ROS that is toxic to dopamine neurons and 2) the amplification of proinflammatory gene expression and associated neurotoxicity.

Migration of enhanced green fluorescent protein expressing bone marrow-derived microglia/macrophage into the mouse brain following permanent focal ischemia

Brain ischemia induces a marked response of resident microglia and hematopoietic cells including monocytes/macrophages. The present study was designed to assess the distribution of microglia/macrophages in cerebral ischemia using bone marrow chimera mice known to express enhanced green fluorescent protein (EGFP). At 24 h after middle cerebral artery occlusion (MCAO), many round-shaped EGFP-positive cells migrated to the ischemic core and peri-infarct area. At 48-72 h after MCAO, irregular round- or oval-shaped EGFP/ionized calcium-binding adapter molecule 1 (Iba 1)-positive cells increased in the transition zone, while many amoeboid-shaped or large-cell-body EGFP/Iba 1-positive cells were increased in number in the innermost area of ischemia. At 7 days after MCAO, many process-bearing ramified shaped EGFP/Iba 1-positive cells were detected in the transition to the peri-infarct area, while phagocytic cells were distributed in the transition to the core area of the infarction. The distribution of these morphologically variable EGFP/Iba 1-positive cells was similar up to 14 days from MCAO. The present study directly showed the migration and distribution of bone marrow-derived monocytes/macrophages and the relationship between resident microglia and infiltrated hematogenous element in ischemic mouse brain. It is important to study the distribution of intrinsic and extrinsic microglia/macrophage in ischemic brain, since such findings may allow the design of appropriate gene-delivery system using exogenous microglia/macrophages to the ischemic brain area.

Regulating factors for microglial activation

Microglia, residential macrophages in the central nervous system, can release a variety of factors including cytokines, chemokines, etc. to regulate the communication among neuronal and other types of glial cells. Microglia play immunological roles in mechanisms underlying the phagocytosis of invading microorganisms and removal of dead or damaged cells. When microglia are hyperactivated due to a certain pathological imbalance, they may cause neuronal degeneration. Pathological activation of microglia has been reported in a wide range of conditions such as cerebral ischemia, Alzheimer's disease, prion diseases, multiple sclerosis, AIDS dementia, and others. Nearly 5000 papers on microglia can be retrieved on the Web site PubMed at present (November 2001) and half of them were published within the past 5 years. Although it is not possible to read each paper in detail, as many factors as possible affecting microglial functions in in vitro culture systems are presented in this review. The factors are separated into "activators" and "inhibitors," although it is difficult to classify many of them. An overview on these factors may help in the development of a new strategy for the treatment of various neurodegenerative diseases.