Localization of NADPH oxidase subunits in neonatal sympathetic neurons

A Role for Second Messengers in Axodendritic Neuronal Polarity

Neuronal polarization is a complex molecular process regulated by intrinsic and extrinsic mechanisms. Nerve cells integrate multiple extracellular cues to generate intracellular messengers that ultimately control cell morphology, metabolism, and gene expression. Therefore, second messengers' local concentration and temporal regulation are crucial elements for acquiring a polarized morphology in neurons. This review article summarizes the main findings and current understanding of how Ca2+, IP3, cAMP, cGMP, and hydrogen peroxide control different aspects of neuronal polarization, and highlights questions that still need to be resolved to fully understand the fascinating cellular processes involved in axodendritic polarization.

Neuronal NADPH oxidase 2 regulates growth cone guidance downstream of slit2/robo2

NADPH oxidases (Nox) are membrane-bound multi-subunit protein complexes producing reactive oxygen species (ROS) that regulate many cellular processes. Emerging evidence suggests that Nox-derived ROS also control neuronal development and axonal outgrowth. However, whether Nox act downstream of receptors for axonal growth and guidance cues is presently unknown. To answer this question, we cultured retinal ganglion cells (RGCs) derived from zebrafish embryos and exposed these neurons to netrin-1, slit2, and brain-derived neurotrophic factor (BDNF). To test the role of Nox in cue-mediated growth and guidance, we either pharmacologically inhibited Nox or investigated neurons from mutant fish that are deficient in Nox2. We found that slit2-mediated growth cone collapse, and axonal retraction were eliminated by Nox inhibition. Though we did not see an effect of either BDNF or netrin-1 on growth rates, growth in the presence of netrin-1 was reduced by Nox inhibition. Furthermore, attractive and repulsive growth cone turning in response to gradients of BDNF, netrin-1, and slit2, respectively, were eliminated when Nox was inhibited in vitro. ROS biosensor imaging showed that slit2 treatment increased growth cone hydrogen peroxide levels via mechanisms involving Nox2 activation. We also investigated the possible relationship between Nox2 and slit2/Robo2 signaling in vivo. astray/nox2 double heterozygote larvae exhibited decreased area of tectal innervation as compared to individual heterozygotes, suggesting both Nox2 and Robo2 are required for establishment of retinotectal connections. Our results provide evidence that Nox2 acts downstream of slit2/Robo2 by mediating growth and guidance of developing zebrafish RGC neurons.

Developmental Axon Degeneration Requires TRPV1-Dependent Ca2+ Influx

Development of the nervous system relies on a balance between axon and dendrite growth and subsequent pruning and degeneration. The developmental degeneration of dorsal root ganglion (DRG) sensory axons has been well studied in part because it can be readily modeled by removing the trophic support by nerve growth factor (NGF) in vitro. We have recently reported that axonal fragmentation induced by NGF withdrawal is dependent on Ca²⁺, and here, we address the mechanism of Ca²⁺ entry required for developmental axon degeneration of mouse embryonic DRG neurons. Our results show that the transient receptor potential vanilloid family member 1 (TRPV1) cation channel plays a critical role mediating Ca²⁺ influx in DRG axons withdrawn from NGF. We further demonstrate that TRPV1 activation is dependent on reactive oxygen species (ROS) generation that is driven through protein kinase C (PKC) and NADPH oxidase (NOX)-dependent pathways that become active upon NGF withdrawal. These findings demonstrate novel mechanistic links between NGF deprivation, PKC activation, ROS generation, and TRPV1-dependent Ca²⁺ influx in sensory axon degeneration.

Reactive oxygen species are involved in BMP-induced dendritic growth in cultured rat sympathetic neurons

Previous studies have shown that bone morphogenetic proteins (BMPs) promote dendritic growth in sympathetic neurons; however, the downstream signaling molecules that mediate the dendrite promoting activity of BMPs are not well characterized. Here we test the hypothesis that reactive oxygen species (ROS)-mediated signaling links BMP receptor activation to dendritic growth. In cultured rat sympathetic neurons, exposure to any of the three mechanistically distinct antioxidants, diphenylene iodinium (DPI), nordihydroguaiaretic acid (NGA) or desferroxamine (DFO), blocked de novo BMP-induced dendritic growth. Addition of DPI to cultures previously induced with BMP to extend dendrites caused dendritic retraction while DFO and NGA prevented further growth of dendrites. The inhibition of the dendrite promoting activity of BMPs by antioxidants was concentration-dependent and occurred without altering axonal growth or neuronal cell survival. Antioxidant treatment did not block BMP activation of SMAD 1,5 as determined by nuclear localization of these SMADs. While BMP treatment did not cause a detectable increase in intracellular ROS in cultured sympathetic neurons as assessed using fluorescent indicator dyes, BMP treatment increased the oxygen consumption rate in cultured sympathetic neurons as determined using the Seahorse XF24 Analyzer, suggesting increased mitochondrial activity. In addition, BMPs upregulated expression of NADPH oxidase 2 (NOX2) and either pharmacological inhibition or siRNA knockdown of NOX2 significantly decreased BMP-7 induced dendritic growth. Collectively, these data support the hypothesis that ROS are involved in the downstream signaling events that mediate BMP7-induced dendritic growth in sympathetic neurons, and suggest that ROS-mediated signaling positively modulates dendritic complexity in peripheral neurons.

Intestinal NADPH oxidase 2 activity increases in a neonatal rat model of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a complication of prematurity. The etiology is unknown, but is related to enteral feeding, ischemia, infection, and inflammation. Reactive oxygen species production, most notably superoxide, increases in NEC. NADPH oxidase (NOX) generates superoxide, but its activity in NEC remains unknown. We hypothesize that NOX-derived superoxide production increases in NEC. Newborn Sprague-Dawley rats were divided into control, formula-fed, formula/LPS, formula/hypoxia, and NEC (formula, hypoxia, and LPS). Intestinal homogenates were analyzed for NADPH-dependent superoxide production. Changes in superoxide levels on days 0-4 were measured. Inhibitors for nitric oxide synthase (L-NAME) and NOX2 (GP91-ds-tat) were utilized. RT-PCR for eNOS, NOX1, GP91^phox expression was performed. Immunofluorescence studies estimated the co-localization of p47^phox and GP91^phox in control and NEC animals on D1, D2, and D4. NEC pups generated more superoxide than controls on D4, while all other groups were unchanged. NADPH-dependent superoxide production was greater in NEC on days 0, 3, and 4. GP91-ds-tat decreased superoxide production in both groups, with greater inhibition in NEC. L-NAME did not alter superoxide production. Temporally, superoxide production varied minimally in controls. In NEC, superoxide generation was decreased on day 1, but increased on days 3-4. GP91^phox expression was higher in NEC on days 2 and 4. NOX1 and eNOS expression were unchanged from controls. GP91^phox and p47^phox had minimal co-localization in all control samples and NEC samples on D1 and D2, but had increased co-localization on D4. In conclusion, this study proves that experimentally-induced NEC increases small intestinal NOX activity. All components of NEC model are necessary for increased NOX activity. NOX2 is the major source, especially as the disease progresses.

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.

Generation of reactive oxygen species in 1-methyl-4-phenylpyridinium (MPP+) treated dopaminergic neurons occurs as an NADPH oxidase-dependent two-wave cascade

BACKGROUND

Reactive oxygen species (ROS), superoxide and hydrogen peroxide (H2O2), are necessary for appropriate responses to immune challenges. In the brain, excess superoxide production predicts neuronal cell loss, suggesting that Parkinson's disease (PD) with its wholesale death of dopaminergic neurons in substantia nigra pars compacta (nigra) may be a case in point. Although microglial NADPH oxidase-produced superoxide contributes to dopaminergic neuron death in an MPTP mouse model of PD, this is secondary to an initial die off of such neurons, suggesting that the initial MPTP-induced death of neurons may be via activation of NADPH oxidase in neurons themselves, thus providing an early therapeutic target.

METHODS

NADPH oxidase subunits were visualized in adult mouse nigra neurons and in N27 rat dopaminergic cells by immunofluorescence. NADPH oxidase subunits in N27 cell cultures were detected by immunoblots and RT-PCR. Superoxide was measured by flow cytometric detection of H2O2-induced carboxy-H2-DCFDA fluorescence. Cells were treated with MPP+ (MPTP metabolite) following siRNA silencing of the Nox2-stabilizing subunit p22phox, or simultaneously with NADPH oxidase pharmacological inhibitors or with losartan to antagonize angiotensin II type 1 receptor-induced NADPH oxidase activation.

RESULTS

Nigral dopaminergic neurons in situ expressed three subunits necessary for NADPH oxidase activation, and these as well as several other NADPH oxidase subunits and their encoding mRNAs were detected in unstimulated N27 cells. Overnight MPP+ treatment of N27 cells induced Nox2 protein and superoxide generation, which was counteracted by NADPH oxidase inhibitors, by siRNA silencing of p22phox, or losartan. A two-wave ROS cascade was identified: 1) as a first wave, mitochondrial H2O2 production was first noted at three hours of MPP+ treatment; and 2) as a second wave, H2O2 levels were further increased by 24 hours. This second wave was eliminated by pharmacological inhibitors and a blocker of protein synthesis.

CONCLUSIONS

A two-wave cascade of ROS production is active in nigral dopaminergic neurons in response to neurotoxicity-induced superoxide. Our findings allow us to conclude that superoxide generated by NADPH oxidase present in nigral neurons contributes to the loss of such neurons in PD. Losartan suppression of nigral-cell superoxide production suggests that angiotensin receptor blockers have potential as PD preventatives.

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.

Reactive oxygen species regulate F-actin dynamics in neuronal growth cones and neurite outgrowth

Reactive oxygen species are well known for their damaging effects due to oxidation of lipids, proteins and DNA that ultimately result in cell death. Accumulating evidence indicates that reactive oxygen species also have important signaling functions in cell proliferation, differentiation, cell motility and apoptosis. Here, we tested the hypothesis whether reactive oxygen species play a physiological role in regulating F-actin structure and dynamics in neuronal growth cones. Lowering cytoplasmic levels of reactive oxygen species with a free radical scavenger, N-tert-butyl-alpha-phenylnitrone, or by inhibiting specific sources of reactive oxygen species, such as NADPH oxidases or lipoxygenases, reduced the F-actin content in the peripheral domain of growth cones. Fluorescent speckle microscopy revealed that these treatments caused actin assembly inhibition, reduced retrograde actin flow and increased contractility of actin structures in the transition zone referred to as arcs, possibly by activating the Rho pathway. Reduced levels of reactive oxygen species ultimately resulted in disassembly of the actin cytoskeleton. When neurons were cultured overnight in conditions of reduced free radicals, growth cone formation and neurite outgrowth were severely impaired. Therefore, we conclude that physiological levels of reactive oxygen species are critical for maintaining a dynamic F-actin cytoskeleton and controlling neurite outgrowth.

Aligned electrospun nanofibers specify the direction of dorsal root ganglia neurite growth

Nerve injury, a significant cause of disability, may be treated more effectively using nerve guidance channels containing longitudinally aligned fibers. Aligned, electrospun nanofibers direct the neurite growth of immortalized neural stem cells, demonstrating potential for directing regenerating neurites. However, no study of neurite guidance on these fibers has yet been performed with primary neurons. Here, we examined neurites from dorsal root ganglia explants on electrospun poly-L-lactate nanofibers of high, intermediate, and random alignment. On aligned fibers, neurites grew radially outward from the ganglia and turned to follow the fibers upon contact. Neurite guidance was robust, with neurites never leaving the fibers to grow on the surrounding cover slip. To compare the alignment of neurites to that of the nanofiber substrates, Fourier methods were used to quantify the alignment. Neurite alignment, however striking, was inferior to fiber alignment on all but the randomly aligned fibers. Neurites on highly aligned substrates were 20 and 16% longer than neurites on random and intermediate fibers, respectively. Schwann cells on fibers assumed a very narrow morphology compared to those on the surrounding coverslip. The robust neurite guidance demonstrated here is a significant step toward the use of aligned, electrospun nanofibers for nerve regeneration. (c) 2007 Wiley Periodicals, Inc. J Biomed Mater Res, 2007.

The roles of NADPH oxidase and phospholipases A2 in oxidative and inflammatory responses in neurodegenerative diseases

Reactive oxygen species (ROS) are produced in mammalian cells through enzymic and non-enzymic mechanisms. Although some ROS production pathways are needed for specific physiological functions, excessive production is detrimental and is regarded as the basis of numerous neurodegenerative diseases. Among enzymes producing superoxide anions, NADPH oxidase is widespread in mammalian cells and is an important source of ROS in mediating physiological and pathological processes in the cardiovascular and the CNS. ROS production is linked to the alteration of intracellular calcium homeostasis, activation of Ca(2+)-dependent enzymes, alteration of cytoskeletal proteins, and degradation of membrane glycerophospholipids. There is evolving evidence that ROS produced by NADPH oxidase regulate neuronal functions and degrade membrane phospholipids through activation of phospholipases A(2) (PLA(2)). This review is intended to cover recent studies describing ROS generation from NADPH oxidase in the CNS and its downstream activation of PLA(2), namely, the group IV cytosolic cPLA(2) and the group II secretory sPLA(2). A major focus is to elaborate the dual role of NADPH oxidase and PLA(2) in mediating the oxidative and inflammatory responses in neurodegenerative diseases, including cerebral ischemia and Alzheimer's disease. Elucidation of the signaling pathways linking NADPH oxidase with the multiple forms of PLA(2) will be important in understanding the oxidative and degradative mechanisms that underline neuronal damage and glial activation and will facilitate development of therapeutic intervention for prevention and treatment of these and other neurodegenerative diseases.

Antioxidant and antiplatelet effects of rosuvastatin in a hamster model of prediabetes

The objectives of this study were to determine the relationships among Type II diabetes (T2DM)-dependent elevations in platelet-derived reactive oxygen species (ROS), platelet-surface protein disulfide isomerase (psPDI) NO-releasing activity, and platelet aggregation and to evaluate the efficacy of rosuvastatin in normalizing these parameters in primary cells derived from a hamster model of prediabetic insulin resistance induced by fructose feeding. Platelets from rosuvastatin-treated non-fructose-fed (NFF) and fructose-fed (FF) hamsters were analyzed for aggregability and psPDI-denitrosation activity. Platelets from NFF animals treated with xanthine/xanthine oxidase (X/XO) were assessed for the same parameters and primary aortic endothelial cells (AEC) cultivated with a range of [rosuvastatin] +/- mevalonate were analyzed for ROS production. Platelets from FF hamsters displayed statistically significant enhanced ROS production, diminished psPDI-mediated NO-releasing activity, and hyperaggregability. Suggestively, platelets from NFF animals treated with X/XO displayed characteristics similar to platelets from FF animals. Rosuvastatin elicited a normalizing effect on all parameters measured in platelets from FF animals. Further, ROS production in primary AEC from FF animals could be blunted to that of NFF animals by concentrations of rosuvastatin in the range of those achieved in the bloodstream. Diminished psPDI-dependent NO-releasing activity and increased initial aggregation rates of FF platelets may result from elevated vascular ROS production under conditions of insulin resistance. Normalization of ROS production and platelet aggregation by rosuvastatin indicates its potential use as a vasculoprotective agent.

NAD(P)H oxidase-induced oxidative stress in sympathetic ganglia of apolipoprotein E deficient mice

Superoxide anion (O2*-) is increased throughout the arterial wall in atherosclerosis. The oxidative stress contributes to lesion formation and vascular dysfunction. In the present study, we tested the hypothesis that NAD(P)H oxidase-derived O2*- is increased in nodose sensory ganglia and sympathetic ganglia of apolipoprotein E deficient (apoE-/-) mice, an established animal model of atherosclerosis. O2*- measured ex vivo by L-012-enhanced chemiluminescence was increased by 79+/-17% in whole sympathetic ganglia from apoE-/- mice (n=5) compared with sympathetic ganglia from control mice (n=5) (P<0.05). In contrast, O2*- was not elevated in nodose ganglia from apoE-/- mice. Dihydroethidium staining confirmed the selective increase in O2*- in sympathetic ganglia of apoE-/- mice, and revealed the contribution of both neurons and non-neuronal cells to the O2*- generation. We investigated the enzymatic source of increased O2*- in sympathetic ganglia of apoE-/- mice. The mRNA expression of gp91phox, p22phox, p67phox, and p47phox subunits of NAD(P)H oxidase measured by real time RT-PCR was increased approximately 3-4 fold in sympathetic ganglia of apoE-/- mice (n=5) compared with control ganglia (n=5). NADPH oxidase activity measured by lucigenin chemiluminescence was increased by 68+/-12% in homogenates of sympathetic ganglia from apoE-/- mice (n=7) compared with control ganglia (n=7) (P<0.05). The results identify sympathetic ganglia as a novel site of oxidative stress in atherosclerosis, and suggest that upregulation of NAD(P)H oxidase is the source of increased O2*- generation. We speculate that oxidative stress in sympathetic ganglia may contribute to impaired baroreflex control of sympathetic nerve activity.