Species differences in the generation of reactive oxygen species by microglia

A comparative atlas of single-cell chromatin accessibility in the human brain

Recent advances in single-cell transcriptomics have illuminated the diverse neuronal and glial cell types within the human brain. However, the regulatory programs governing cell identity and function remain unclear. Using a single-nucleus assay for transposase-accessible chromatin using sequencing (snATAC-seq), we explored open chromatin landscapes across 1.1 million cells in 42 brain regions from three adults. Integrating this data unveiled 107 distinct cell types and their specific utilization of 544,735 candidate cis-regulatory DNA elements (cCREs) in the human genome. Nearly a third of the cCREs demonstrated conservation and chromatin accessibility in the mouse brain cells. We reveal strong links between specific brain cell types and neuropsychiatric disorders including schizophrenia, bipolar disorder, Alzheimer's disease (AD), and major depression, and have developed deep learning models to predict the regulatory roles of noncoding risk variants in these disorders.

Alzheimer's Disease Protein Relevance Analysis Using Human and Mouse Model Proteomics Data

The principles governing genotype-phenotype relationships are still emerging(1-3), and detailed translational as well as transcriptomic information is required to understand complex phenotypes, such as the pathogenesis of Alzheimer's disease. For this reason, the proteomics of Alzheimer disease (AD) continues to be studied extensively. Although comparisons between data obtained from humans and mouse models have been reported, approaches that specifically address the between-species statistical comparisons are understudied. Our study investigated the performance of two statistical methods for identification of proteins and biological pathways associated with Alzheimer's disease for cross-species comparisons, taking specific data analysis challenges into account, including collinearity, dimensionality reduction and cross-species protein matching. We used a human dataset from a well-characterized cohort followed for over 22 years with proteomic data available. For the mouse model, we generated proteomic data from whole brains of CVN-AD and matching control mouse models. We used these analyses to determine the reliability of a mouse model to forecast significant proteomic-based pathological changes in the brain that may mimic pathology in human Alzheimer's disease. Compared with LASSO regression, partial least squares discriminant analysis provided better statistical performance for the proteomics analysis. The major biological finding of the study was that extracellular matrix proteins and integrin-related pathways were dysregulated in both the human and mouse data. This approach may help inform the development of mouse models that are more relevant to the study of human late-onset Alzheimer's disease.

Neuroinflammation as a Therapeutic Target for Mitigating the Long-Term Consequences of Acute Organophosphate Intoxication

Acute intoxication with organophosphates (OPs) can cause a potentially fatal cholinergic crisis characterized by peripheral parasympathomimetic symptoms and seizures that rapidly progress to status epilepticus (SE). While current therapeutic countermeasures for acute OP intoxication significantly improve the chances of survival when administered promptly, they are insufficient for protecting individuals from chronic neurologic outcomes such as cognitive deficits, affective disorders, and acquired epilepsy. Neuroinflammation is posited to contribute to the pathogenesis of these long-term neurologic sequelae. In this review, we summarize what is currently known regarding the progression of neuroinflammatory responses after acute OP intoxication, drawing parallels to other models of SE. We also discuss studies in which neuroinflammation was targeted following OP-induced SE, and explain possible reasons why such therapeutic interventions have inconsistently and only partially improved long-term outcomes. Finally, we suggest future directions for the development of therapeutic strategies that target neuroinflammation to mitigate the neurologic sequelae of acute OP intoxication.

Glial Cells in Glaucoma: Friends, Foes, and Potential Therapeutic Targets

Glaucoma is the second leading cause of blindness worldwide, affecting ~80 million people by 2020 (1, 2). The condition is characterized by a progressive loss of retinal ganglion cells (RGCs) and their axons accompanied by visual field loss. The underlying pathophysiology of glaucoma remains elusive. Glaucoma is recognized as a multifactorial disease, and lowering intraocular pressure (IOP) is the only treatment that has been shown to slow the progression of the condition. However, a significant number of glaucoma patients continue to go blind despite intraocular pressure-lowering treatment (2). Thus, the need for alternative treatment strategies is indisputable. Accumulating evidence suggests that glial cells play a significant role in supporting RGC function and that glial dysfunction may contribute to optic nerve disease. Here, we review recent advances in understanding the role of glial cells in the pathophysiology of glaucoma. A particular focus is on the dynamic and essential interactions between glial cells and RGCs and potential therapeutic approaches to glaucoma by targeting glial cells.

Velvet Antler Methanol Extracts Ameliorate Parkinson's Disease by Inhibiting Oxidative Stress and Neuroinflammation: From C. elegans to Mice

Velvet antler is the traditional tonic food or medicine used in East Asia for treating aging-related diseases. Herein, we try to dissect the pharmacology of methanol extracts (MEs) of velvet antler on Parkinson's disease (PD). Caenorhabditis elegans studies showed that MEs decreased the aggregation of α-synuclein and protected oxidative stress-induced DAergic neuron degeneration. In vitro cellular data indicated that MEs suppressed the LPS-induced MAPKs and NF-κB activation, therefore inhibiting overproduction of reactive oxygen species, nitric oxide, tumor necrosis factor-α, and interleukin-6; blocking microglia activation; and protecting DAergic neurons from the microglia-mediated neurotoxicity. In vivo MPTP-induced PD mouse investigations found that MEs prevented MPTP-induced neuron loss in the substantia nigra and improved the behavioral rotating rod performance in MPTP-treated mice by increasing the expression level of tyrosine hydroxylase (TH) and downregulating α-synuclein protein expression. In all, these results demonstrate that MEs ameliorate PD by inhibiting oxidative stress and neuroinflammation.

Neuroinflammation Mechanisms and Phytotherapeutic Intervention: A Systematic Review

Neuroinflammation is indicated in the pathogenesis of several acute and chronic neurological disorders. Acute lesions in the brain parenchyma induce intense and highly complex neuroinflammatory reactions with similar mechanisms among various disease prototypes. Microglial cells in the CNS sense tissue damage and initiate inflammatory responses. The cellular and humoral constituents of the neuroinflammatory reaction to brain injury contribute significantly to secondary brain damage and neurodegeneration. Inflammatory cascades such as proinflammatory cytokines from invading leukocytes and direct cell-mediated cytotoxicity between lymphocytes and neurons are known to cause "collateral damage" in models of acute brain injury. In addition to degeneration and neuronal cell loss, there are secondary inflammatory mechanisms that modulate neuronal activity and affect neuroinflammation which can even be detected at the behavioral level. Hence, several of health conditions result from these pathogenetic conditions which are underlined by progressive neuronal function loss due to chronic inflammation and oxidative stress. In the first part of this Review, we discuss critical neuroinflammatory mediators and their pathways in detail. In the second part, we review the phytochemicals which are considered as potential therapeutic molecules for treating neurodegenerative diseases with an inflammatory component.

The Delayed Neuroprotective Effect of Methylene Blue in Experimental Rat Brain Trauma

After traumatic brain injury (TBI), an increase in dysfunction of the limbs contralateral to injury focus was observed. Using different behavioral tests, we found that a single intravenous injection of methylene blue (MB, 1 mg/kg) 30 min after the injury reduced the impairment of the motor functions of the limbs from 7 to 120 days after TBI. Administration of methylene blue 30 min after the injury and then monthly (six injections in total) was the most effective both in terms of preservation of limb function and duration of therapeutic action. This therapeutic effect was clearly manifested from the seventh day and continued until the end of the experiment-by the 180th day after TBI. MB is known to possess antioxidant properties; it has a protective effect against TBI by promoting autophagy and minimizing lesion volume in the first two weeks after TBI. Studies of the brains on the 180th day after TBI demonstrated that the monthly treatment of animals with MB statistically significantly prevented an increase in the density of microglial cells in the ipsilateral hemisphere and a decrease in the thickness of the corpus callosum in the contralateral hemisphere in comparison with untreated animals. However, on the 180th day after TBI, the magnetic resonance imaging scan of the animal brains did not show a significant reduction in the volume of the lesion in MB-treated animals. These findings are important for understanding the development of the long-term effects of TBI and expand the required therapeutic window for targeted neuroprotective interventions.

Evidence for a dynorphin-mediated inner ear immune/inflammatory response and glutamate-induced neural excitotoxicity: an updated analysis

Acoustic overstimulation (AOS) is defined as the stressful overexposure to high-intensity sounds. AOS is a precipitating factor that leads to a glutamate (GLU)-induced Type I auditory neural excitotoxicity and an activation of an immune/inflammatory/oxidative stress response within the inner ear, often resulting in cochlear hearing loss. The dendrites of the Type I auditory neural neurons that innervate the inner hair cells (IHCs), and respond to the IHC release of the excitatory neurotransmitter GLU, are themselves directly innervated by the dynorphin (DYN)-bearing axon terminals of the descending brain stem lateral olivocochlear (LOC) system. DYNs are known to increase GLU availability, potentiate GLU excitotoxicity, and induce superoxide production. DYNs also increase the production of proinflammatory cytokines by modulating immune/inflammatory signal transduction pathways. Evidence is provided supporting the possibility that the GLU-mediated Type I auditory neural dendritic swelling, inflammation, excitotoxicity, and cochlear hearing loss that follow AOS may be part of a brain stem-activated, DYN-mediated cascade of inflammatory events subsequent to a LOC release of DYNs into the cochlea. In support of a DYN-mediated cascade of events are established investigations linking DYNs to the immune/inflammatory/excitotoxic response in other neural systems.

Toxicologic Pathology Analysis for Translational Neuroscience

A half-day American College of Toxicology continuing education course presented key issues often confronted by translational neuroscientists when predicting human risk from animal-derived toxicologic pathology data. Two talks correlated discrete structures with major functions in brains of rodents and nonrodents. The third lecture provided practical advice to obtain highly homologous rodent brain sections for quantitative morphometry in developmental neurotoxicity testing. The last presentation discussed demographic influences (eg, species, strain, sex, age), physiological attributes (eg, body composition, brain vascularity, pharmacokinetic/pharmacodynamic patterns, etc), and husbandry parameters (eg, group housing) recognized to impact the actions of neuroactive chemicals. Speakers described common cases of real-world challenges to animal data interpretation encountered when designing studies or extrapolating biological responses across species. The efficiency of translational neuroscience efforts will likely be enhanced as new methods (eg, high-resolution non-invasive imaging) improve our capability to cross-connect subtle anatomic and/or biochemical lesions with functional changes over time.

Characterization of inflammatory markers and transcriptome profiles of differentially activated embryonic stem cell-derived microglia

Microglia, the immune cells of the CNS, are highly adaptive cells that can acquire different pro- and anti-inflammatory activation states with distinct functions in CNS homeostasis and pathologies. To study microglial function in vitro, primary microglia or immortalized cell lines are commonly used. An alternative to these cells are embryonic stem cell-derived microglia (ESdM). ESdM have previously been shown to be very similar to primary microglia in terms of expression profiles and surface molecules. In this study, ESdM and primary microglia were treated with different inflammatory stimulants to analyze their ability to adopt different activation states. Using quantitative real-time PCR, comparative transcriptomics, ELISA, and flow cytometry, we found that different activation states can be induced in ESdM, which are similar to those found in primary microglia. These states are characterized by specific sets of inflammatory marker molecules and differential transcriptome signatures. Our results show that ESdM are a valuable alternative cell model to study microglial functions and neuroinflammatory mechanisms.

Hydrogen peroxide-induced calcium influx in lung microvascular endothelial cells involves TRPV4

In acute respiratory distress syndrome, both reactive oxygen species (ROS) and increased intracellular calcium ([Ca(2+)]i) are thought to play important roles in promoting endothelial paracellular permeability, but the mechanisms linking ROS and [Ca(2+)]i in microvascular endothelial cells are not known. In this study, we assessed the effect of hydrogen peroxide (H2O2) on [Ca(2+)]i in mouse and human lung microvascular endothelial cells (MLMVEC and HLMVEC, respectively). We found that in both MLMVECs and HLMVECs, exogenously applied H2O2 increased [Ca(2+)]i through Ca(2+) influx and that pharmacologic inhibition of the calcium channel transient receptor potential vanilloid 4 (TRPV4) attenuated the H2O2-induced Ca(2+) influx. Additionally, knockdown of TRPV4 in HLMVEC also attenuated calcium influx following H2O2 challenge. Administration of H2O2 or TRPV4 agonists decreased transmembrane electrical resistance (TER), suggesting increased barrier permeability. To explore the regulatory mechanisms underlying TRPV4 activation by ROS, we examined H2O2-induced Ca(2+) influx in MLMVECs and HLMVECs with either genetic deletion, silencing, or pharmacologic inhibition of Fyn, a Src family kinase. In both MLMVECs derived from mice deficient for Fyn and HLMVECs treated with either siRNA targeted to Fyn or the Src family kinase inhibitor SU-6656 for 24 or 48 h, the H2O2-induced Ca(2+) influx was attenuated. Treatment with SU-6656 decreased the levels of phosphorylated, but not total, TRPV4 protein and had no effect on TRPV4 response to the external agonist, GSK1016790A. In conclusion, our data suggest that application of exogenous H2O2 increases [Ca(2+)]i and decreases TER in microvascular endothelial cells via activation of TRPV4 through a mechanism that requires the Src kinase Fyn.

Arginine deprivation and immune suppression in a mouse model of Alzheimer's disease

The pathogenesis of Alzheimer's disease (AD) is a critical unsolved question; and although recent studies have demonstrated a strong association between altered brain immune responses and disease progression, the mechanistic cause of neuronal dysfunction and death is unknown. We have previously described the unique CVN-AD mouse model of AD, in which immune-mediated nitric oxide is lowered to mimic human levels, resulting in a mouse model that demonstrates the cardinal features of AD, including amyloid deposition, hyperphosphorylated and aggregated tau, behavioral changes, and age-dependent hippocampal neuronal loss. Using this mouse model, we studied longitudinal changes in brain immunity in relation to neuronal loss and, contrary to the predominant view that AD pathology is driven by proinflammatory factors, we find that the pathology in CVN-AD mice is driven by local immune suppression. Areas of hippocampal neuronal death are associated with the presence of immunosuppressive CD11c(+) microglia and extracellular arginase, resulting in arginine catabolism and reduced levels of total brain arginine. Pharmacologic disruption of the arginine utilization pathway by an inhibitor of arginase and ornithine decarboxylase protected the mice from AD-like pathology and significantly decreased CD11c expression. Our findings strongly implicate local immune-mediated amino acid catabolism as a novel and potentially critical mechanism mediating the age-dependent and regional loss of neurons in humans with AD.

Microglia are crucial regulators of neuro-immunity during central nervous system tuberculosis

Mycobacterium tuberculosis (M. tuberculosis) infection of the central nervous system (CNS) is the most devastating manifestation of tuberculosis (TB), with both high mortality and morbidity. Although research has been fueled by the potential therapeutic target microglia offer against neurodegenerative inflammation, their part in TB infection of the CNS has not been fully evaluated nor elucidated. Yet, as both the preferential targets of M. tuberculosis and the immune-effector cells of the CNS, microglia are likely to be key determinants of disease severity and clinical outcomes. Following pathogen recognition, bacilli are internalized and capable of replicating within microglia. Cellular activation ensues, utilizing signaling molecules that may be neurotoxic. Central to initiating, orchestrating and modulating the tuberculous immune response is microglial secretion of cytokines and chemokines. However, the neurological environment is unique in that inflammatory signals, which appear to be damaging in the periphery, could be beneficial by governing neuronal survival, regeneration and differentiation. Furthermore, microglia are important in the recruitment of peripheral immune cells and central to defining the pro-inflammatory milieu of which neurotoxicity may result from many of the participating local or recruited cell types. Microglia are capable of both presenting antigen to infiltrating CD4(+) T-lymphocytes and inducing their differentiation-a possible correlate of protection against M. tuberculosis infection. Clarifying the nature of the immune effector molecules secreted by microglia, and the means by which other CNS-specific cell types govern microglial activation or modulate their responses is critical if improved diagnostic and therapeutic strategies are to be attained. Therefore, this review evaluates the diverse roles microglia play in the neuro-immunity to M. tuberculosis infection of the CNS.

The impact of human and mouse differences in NOS2 gene expression on the brain's redox and immune environment

BACKGROUND

Mouse models are used in the study of human disease. Despite well-known homologies, the difference in immune response between mice and humans impacts the application of data derived from mice to human disease outcomes. Nitric oxide synthase-2 (NOS2) is a key gene that displays species-specific outcomes via altered regulation of the gene promoter and via post-transcriptional mechanisms in humans that are not found in mice. The resulting levels of NO produced by activation of human NOS2 are different from the levels of NO produced by mouse Nos2. Since both tissue redox environment and immune responsiveness are regulated by the level of NO and its interactions, we investigated the significance of mouse and human differences on brain oxidative stress and on immune activation in HuNOS2tg/mNos2-/- mice that express the entire human NOS2 gene and that lack a functional mNos2 compared to wild type (WT) mice that express normal mNos2.

METHODS/RESULTS

Similarly to human, brain tissue from HuNOS2tg/mNos2-/- mice showed the presence of a NOS2 gene 3'UTR binding site. We also identified miRNA-939, the binding partner for this site, in mouse brain lysates and further demonstrated reduced levels of nitric oxide (NO) typical of the human immune response on injection with lipopolysaccharide (LPS). HuNOS2tg/mNos2-/- brain samples were probed for characteristic differences in redox and immune gene profiles compared to WT mice using gene arrays. Selected genes were also compared against mNos2-/- brain lysates. Reconstitution of the human NOS2 gene significantly altered genes that encode multiple anti-oxidant proteins, oxidases, DNA repair, mitochondrial proteins and redox regulated immune proteins. Expression levels of typical pro-inflammatory, anti-inflammatory and chemokine genes were not significantly different with the exception of increased TNFα and Ccr1 mRNA expression in the HuNOS2tg/mNos2-/- mice compared to WT or mNos2-/- mice.

CONCLUSIONS

NO is a principle factor in establishing the tissue redox environment and changes in NO levels impact oxidative stress and immunity, both of which are primary characteristics of neurodegenerative diseases. The HuNOS2tg/mNos2-/- mice provide a potentially useful mechanism to address critical species- specific immune differences that can impact the study of human diseases.

Fundamentals of translational neuroscience in toxicologic pathology: optimizing the value of animal data for human risk assessment

A half-day Society of Toxicologic Pathology continuing education course on "Fundamentals of Translational Neuroscience in Toxicologic Pathology" presented some current major issues faced when extrapolating animal data regarding potential neurological consequences to assess potential human outcomes. Two talks reviewed functional-structural correlates in rodent and nonrodent mammalian brains needed to predict behavioral consequences of morphologic changes in discrete neural cell populations. The third lecture described practical steps for ensuring that specimens from rodent developmental neurotoxicity tests will be processed correctly to produce highly homologous sections. The fourth talk detailed demographic factors (e.g., species, strain, sex, and age); physiological traits (body composition, brain circulation, pharmacokinetic/pharmacodynamic patterns, etc.); and husbandry influences (e.g., group housing) known to alter the effects of neuroactive agents. The last presentation discussed the appearance, unknown functional effects, and potential relevance to humans of polyethylene glycol (PEG)-associated vacuoles within the choroid plexus epithelium of animals. Speakers provided real-world examples of challenges with data extrapolation among species or with study design considerations that may impact the interpretability of results. Translational neuroscience will be bolstered in the future as less invasive and/or more quantitative techniques are devised for linking overt functional deficits to subtle anatomic and chemical lesions.

Hypoxia inducible factor-1α mediates iron uptake which induces inflammatory response in amoeboid microglial cells in developing periventricular white matter through MAP kinase pathway

Iron accumulation occurs in tissues such as periventricular white matter (PWM) in response to hypoxic injuries, and microglial cells sequester excess iron following hypoxic exposure. As hypoxia has a role in altering the expression of proteins involved in iron regulation, this study was aimed at examining the interaction between hypoxia inducible factor (HIF)-1α and proteins involved in iron transport in microglial cells, and evaluating the mechanistic action of deferoxamine and KC7F2 (an inhibitor of HIF-1α) in iron mediated hypoxic injury. Treating the microglial cultures with KC7F2, led to decreased expression of transferrin receptor and divalent metal transporter-1. Administration of deferoxamine or KC7F2 to hypoxic microglial cells enhanced extracellular signal-regulated kinase (ERK) phosphorylation (p-ERK), but decreased the phosphorylation of p38 (p-p38). The increased p-ERK further phosphorylated the cAMP response element-binding protein (p-CREB) which in turn may have resulted in the increased mitogen activated protein kinase (MAPK) phosphatase 1 (MKP1), known to dephosphorylate MAPKs. Consistent with the decrease in p-p38, the production of pro-inflammatory cytokines TNF-α and IL-1β was reduced in hypoxic microglia treated with deferoxamine and SB 202190, an inhibitor for p38. This suggests that the anti-inflammatory effect exhibited by deferoxamine is by inhibition of p-p38 induced inflammation through the pERK-pCREB-MKP1 pathway, whereas that of KC7F2 requires further investigation. The present results suggest that HIF-1α may mediate iron accumulation in hypoxic microglia and KC7F2, similar to deferoxamine, might provide limited protection against iron induced PWMD.

Immune heterogeneity in neuroinflammation: dendritic cells in the brain

Dendritic cells (DC) are critical to an integrated immune response and serve as the key link between the innate and adaptive arms of the immune system. Under steady state conditions, brain DC’s act as sentinels, continually sampling their local environment. They share this function with macrophages derived from the same basic hemopoietic (bone marrow-derived) precursor and with parenchymal microglia that arise from a unique non-hemopoietic origin. While multiple cells may serve as antigen presenting cells (APCs), dendritic cells present both foreign and self-proteins to naïve T cells that, in turn, carry out effector functions that serve to protect or destroy. The resulting activation of the adaptive response is a critical step to resolution of injury or infection and is key to survival. In this review we will explore the critical roles that DCs play in the brain’s response to neuroinflammatory disease with emphasis on how the brain’s microenvironment impacts these actions.

Nitric oxide-mediated regulation of β-amyloid clearance via alterations of MMP-9/TIMP-1

Fibrillar amyloid plaques are largely composed of amyloid-beta (Aβ) peptides that are metabolized into products, including Aβ1-16, by proteases including matrix metalloproteinase 9 (MMP-9). The balance between production and degradation of Aβ proteins is critical to amyloid accumulation and resulting disease. Regulation of MMP-9 and its endogenous inhibitor tissue inhibitor of metalloproteinase (TIMP)-1 by nitric oxide (NO) has been shown. We hypothesize that nitric oxide synthase (NOS2) protects against Alzheimer's disease pathology by increasing amyloid clearance through NO regulation of MMP-9/TIMP-1 balance. We show NO-mediated increased MMP-9/TIMP-1 ratios enhanced the degradation of fibrillar Aβ in vitro, which was abolished when silenced for MMP-9 protein translation. The in vivo relationship between MMP-9, NO and Aβ degradation was examined by comparing an Alzheimer's disease mouse model that expresses NOS2 with a model lacking NOS2. To quantitate MMP-9 mediated changes, we generated an antibody recognizing the Aβ1-16 fragment, and used mass spectrometry multi-reaction monitoring assay for detection of immunoprecipitated Aβ1-16 peptides. Aβ1-16 levels decreased in brain lysates lacking NOS2 when compared with strains that express human amyloid precursor protein on the NOS2 background. TIMP-1 increased in the APPSwDI/NOS2(-/-) mice with decreased MMP activity and increased amyloid burden, thereby supporting roles for NO in the regulation of MMP/TIMP balance and plaque clearance.

Identification of hamster inducible nitric oxide synthase (iNOS) promoter sequences that influence basal and inducible iNOS expression

IFN-γ/LPS-activated hamster (Mesocricetus auratus) macrophages express significantly less iNOS (NOS2) than activated mouse macrophages, which contributes to the hamster's susceptibility to intracellular pathogens. We determined a mechanism responsible for differences in iNOS promoter activity in hamsters and mice. The HtPP (1.2 kb) showed low basal and inducible promoter activity when compared with the mouse, and sequences within a 100-bp region (-233 to -133) of the mouse and hamster promoters influenced this activity. Moreover, within this 100 bp, we identified a smaller region (44 bp) in the mouse promoter, which recovered basal promoter activity when swapped into the hamster promoter. The mouse homolog (100-bp region) contained a cis-element for NF-IL-6 (-153/-142), which was absent in the hamster counterpart. EMSA and supershift assays revealed that the hamster sequence did not support the binding of NF-IL-6. Introduction of a functional NF-IL-6 binding sequence into the hamster promoter or its alteration in the mouse promoter revealed the critical importance of this transcription factor for full iNOS promoter activity. Furthermore, the binding of NF-IL-6 to the iNOS promoter (-153/-142) in vivo was increased in mouse cells but was reduced in hamster cells after IFN-γ/LPS stimulation. Differences in the activity of the iNOS promoters were evident in mouse and hamster cells, so they were not merely a result of species-specific differences in transcription factors. Thus, we have identified unique DNA sequences and a critical transcription factor, NF-IL-6, which contribute to the overall basal and inducible expression of hamster iNOS.

Iron and iron regulatory proteins in amoeboid microglial cells are linked to oligodendrocyte death in hypoxic neonatal rat periventricular white matter through production of proinflammatory cytokines and reactive oxygen/nitrogen species

This study was aimed to examine the role of iron in causing periventricular white matter (PWM) damage following a hypoxic injury in the developing brain. Along with iron, the expression of iron regulatory proteins (IRPs) and transferrin receptor (TfR), which are involved in iron acquisition, was also examined in the PWM by subjecting 1-d-old Wistar rats to hypoxia. Apart from an increase in iron levels in PWM, Perls' iron staining showed an increase of intracellular iron in the preponderant amoeboid microglial cells (AMCs) in the tissue. In response to hypoxia, the protein levels of IRP1, IRP2, and TfR in PWM and AMCs were significantly increased. In primary microglial cultures, administration of iron chelator deferoxamine reduced the generation of iron-induced reactive oxygen and nitrogen species and proinflammatory cytokines such as tumor necrosis factor-α and interleukin-1β. Primary oligodendrocytes treated with conditioned medium from hypoxic microglia exhibited reduced glutathione levels, increased lipid peroxidation, upregulated caspase-3 expression, and reduced proliferation. This was reversed to control levels on treatment with conditioned medium from deferoxamine treated hypoxic microglia; also, there was reduction in apoptosis of oligodendrocytes. The present results suggest that excess iron derived primarily from AMCs might be a mediator of oligodendrocyte cell death in PWM following hypoxia in the neonatal brain.

Paraquat neurotoxicity is mediated by the dopamine transporter and organic cation transporter-3

The herbicide paraquat (PQ) has increasingly been reported in epidemiological studies to enhance the risk of developing Parkinson's disease (PD). Furthermore, case-control studies report that individuals with genetic variants in the dopamine transporter (DAT, SLC6A) have a higher PD risk when exposed to PQ. However, it remains a topic of debate whether PQ can enter dopamine (DA) neurons through DAT. We report here a mechanism by which PQ is transported by DAT: In its native divalent cation state, PQ(2+) is not a substrate for DAT; however, when converted to the monovalent cation PQ(+) by either a reducing agent or NADPH oxidase on microglia, it becomes a substrate for DAT and is accumulated in DA neurons, where it induces oxidative stress and cytotoxicity. Impaired DAT function in cultured cells and mutant mice significantly attenuated neurotoxicity induced by PQ(+). In addition to DAT, PQ(+) is also a substrate for the organic cation transporter 3 (Oct3, Slc22a3), which is abundantly expressed in non-DA cells in the nigrostriatal regions. In mice with Oct3 deficiency, enhanced striatal damage was detected after PQ treatment. This increased sensitivity likely results from reduced buffering capacity by non-DA cells, leading to more PQ(+) being available for uptake by DA neurons. This study provides a mechanism by which DAT and Oct3 modulate nigrostriatal damage induced by PQ(2+)/PQ(+) redox cycling.

Comprehensive analysis of tissue preservation and recording quality from chronic multielectrode implants

Multielectrodes have been used with great success to simultaneously record the activity of neuronal populations in awake, behaving animals. In particular, there is great promise in the use of this technique to allow the control of neuroprosthetic devices by human patients. However, it is crucial to fully characterize the tissue response to the chronic implants in animal models ahead of the initiation of human clinical trials. Here we evaluated the effects of unilateral multielectrode implants on the motor cortex of rats weekly recorded for 1-6 months using several histological methods to assess metabolic markers, inflammatory response, immediate-early gene (IEG) expression, cytoskeletal integrity and apoptotic profiles. We also investigated the correlations between each of these features and firing rates, to estimate the impact of post-implant time on neuronal recordings. Overall, limited neuronal loss and glial activation were observed on the implanted sites. Reactivity to enzymatic metabolic markers and IEG expression were not significantly different between implanted and non-implanted hemispheres. Multielectrode recordings remained viable for up to 6 months after implantation, and firing rates correlated well to the histochemical and immunohistochemical markers. Altogether, our results indicate that chronic tungsten multielectrode implants do not substantially alter the histological and functional integrity of target sites in the cerebral cortex.

Features of microglia and neuroinflammation relevant to environmental exposure and neurotoxicity

Microglia are resident cells of the brain involved in regulatory processes critical for development, maintenance of the neural environment, injury and repair. They belong to the monocytic-macrophage lineage and serve as brain immune cells to orchestrate innate immune responses; however, they are distinct from other tissue macrophages due to their relatively quiescent phenotype and tight regulation by the CNS microenvironment. Microglia actively survey the surrounding parenchyma and respond rapidly to changes such that any disruption to neural architecture or function can contribute to the loss in regulation of the microglia phenotype. In many models of neurodegeneration and neurotoxicity, early events of synaptic degeneration and neuronal loss are accompanied by an inflammatory response including activation of microglia, perivascular monocytes, and recruitment of leukocytes. In culture, microglia have been shown to be capable of releasing several potentially cytotoxic substances, such as reactive oxygen intermediates, nitric oxide, proteases, arachidonic acid derivatives, excitatory amino acids, and cytokines; however, they also produce various neurotrophic factors and quench damage from free radicals and excitotoxins. As the primary source for pro-inflammatory cytokines, microglia are implicated as pivotal mediators of neuroinflammation and can induce or modulate a broad spectrum of cellular responses. Neuroinflammation should be considered as a balanced network of processes whereby subtle modifications can shift the cells toward disparate outcomes. For any evaluation of neuroinflammation and microglial responses, within the framework of neurotoxicity or degeneration, one key question in determining the consequence of neuroinflammation is whether the response is an initiating event or the consequence of tissue damage. As examples of environmental exposure-related neuroinflammation in the literature, we provide an evaluation of data on manganese and diesel exhaust particles.

Characterization of NTera2/D1 cells as a model system for the investigation of cannabinoid function in human neurons and astrocytes

The limited availability and potential to culture primary human brain cells means that there is still a need for cell lines that reliably model human neurons and glial cells. The human-derived NTera2/D1 (NT2) cell line is a promising tool from which both neuronal (NT2N) and astrocytic (NT2A) cells can be derived in vitro. Here we have investigated the potential to use this cell model to investigate the endocannabinoid system in the CNS. Through immunocytochemical characterization with a range of neuronal and glial markers, we found that these cell lines differentiate into cells with immature neuronal and astrocytic phenotypes, respectively. By real-time PCR, immunocytochemistry, and functional inhibition of cAMP accumulation, the cannabinoid 1 receptors were identified only on NT2N cells, consistent with high levels of expression of this receptor in neuronal cells of the CNS. No evidence of cannabinoid 2 receptor expression was found on any of the NT2 cell types. Both the precursors and the differentiated NT2N and NT2A cells demonstrated mRNA expression for the key enzymes involved in endocannabinoid synthesis and degradation. This work establishes a cannabinergic phenotype in NT2N and NT2A cells, providing an alternative human derived renewable cell model for investigation of cannabinoid receptor function and endocannabinoid synthesis and metabolism in the CNS.

Nitric oxide and redox mechanisms in the immune response

The role of redox molecules, such as NO and ROS, as key mediators of immunity has recently garnered renewed interest and appreciation. To regulate immune responses, these species trigger the eradication of pathogens on the one hand and modulate immunosuppression during tissue-restoration and wound-healing processes on the other. In the acidic environment of the phagosome, a variety of RNS and ROS is produced, thereby providing a cauldron of redox chemistry, which is the first line in fighting infection. Interestingly, fluctuations in the levels of these same reactive intermediates orchestrate other phases of the immune response. NO activates specific signal transduction pathways in tumor cells, endothelial cells, and monocytes in a concentration-dependent manner. As ROS can react directly with NO-forming RNS, NO bioavailability and therefore, NO response(s) are changed. The NO/ROS balance is also important during Th1 to Th2 transition. In this review, we discuss the chemistry of NO and ROS in the context of antipathogen activity and immune regulation and also discuss similarities and differences between murine and human production of these intermediates.

Mechanisms of oxidative damage in multiple sclerosis and a cell therapy approach to treatment

Although significant advances have recently been made in the understanding and treatment of multiple sclerosis, reduction of long-term disability remains a key goal. Evidence suggests that inflammation and oxidative stress within the central nervous system are major causes of ongoing tissue damage in the disease. Invading inflammatory cells, as well as resident central nervous system cells, release a number of reactive oxygen and nitrogen species which cause demyelination and axonal destruction, the pathological hallmarks of multiple sclerosis. Reduction in oxidative damage is an important therapeutic strategy to slow or halt disease processes. Many drugs in clinical practice or currently in trial target this mechanism. Cell-based therapies offer an alternative source of antioxidant capability. Classically thought of as being important for myelin or cell replacement in multiple sclerosis, stem cells may, however, have a more important role as providers of supporting factors or direct attenuators of the disease. In this paper we focus on the antioxidant properties of mesenchymal stem cells and discuss their potential importance as a cell-based therapy for multiple sclerosis.

Effects of immunomodulatory substances on phagocytosis of abeta(1-42) by human microglia

Glial activation and increased inflammation characterize neuropathology in Alzheimer's disease (AD). The aim was to develop a model for studying phagocytosis of beta-amyloid (Abeta) peptide by human microglia and to test effects thereupon by immunomodulatory substances. Human CHME3 microglia showed intracellular Abeta(1-42) colocalized with lysosome-associated membrane protein-2, indicating phagocytosis. This was increased by interferon-gamma, and to a lesser degree with Protollin, a proteosome-based adjuvant. Secretion of brain-derived neurotrophic factor (BDNF) was decreased by Abeta(1-42) and by interferon-gamma and interleukin-1beta. These cytokines, but not Abeta(1-42), stimulated interleukin-6 release. Microglia which phagocytosed Abeta(1-42) exhibited a higher degree of expression of interleukin-1 receptor type I and inducible nitric oxide synthase. In conclusion, we show that human microglia are able to phagocytose Abeta(1-42) and that this is associated with expression of inflammatory markers. Abeta(1-42) and interferon-gamma decreased BDNF secretion suggesting a new neuropathological role for Abeta(1-42) and the inflammation accompanying AD.

Effects of chronic low dose rotenone treatment on human microglial cells

Background

Exposure to toxins/chemicals is considered to be a significant risk factor in the pathogenesis of Parkinson's disease (PD); one putative chemical is the naturally occurring herbicide rotenone that is now used widely in establishing PD models. We, and others, have shown that chronic low dose rotenone treatment induces excessive accumulation of Reactive Oxygen Species (ROS), inclusion body formation and apoptosis in dopaminergic neurons of animal and human origin. Some studies have also suggested that microglia enhance the rotenone induced neurotoxicity. While the effects of rotenone on neurons are well established, there is little or no information available on the effect of rotenone on microglial cells, and especially cells of human origin. The aim of the present study was to investigate the effects of chronic low dose rotenone treatment on human microglial CHME-5 cells.

Methods

We have shown previously that rotenone induced inclusion body formation in human dopaminergic SH-SY5Y cells and therefore used these cells as a control for inclusion body formation in this study. SH-SY5Y and CHME-5 cells were treated with 5 nM rotenone for four weeks. At the end of week 4, both cell types were analysed for the presence of inclusion bodies, superoxide dismutases and cell activation (only in CHME-5 cells) using Haematoxylin and Eosin staining, immunocytochemical and western blotting methods. Levels of active caspases and ROS (both extra and intra cellular) were measured using biochemical methods.

Conclusion

The results suggest that chronic low dose rotenone treatment activates human microglia (cell line) in a manner similar to microglia of animal origin as shown by others. However human microglia release excessive amounts of ROS extracellularly, do not show excessive amounts of intracellular ROS and active caspases and most importantly do not show any protein aggregation or inclusion body formation. Human microglia appear to be resistant to rotenone (chronic, low dose) induced damage.

Inflammation in transgenic mouse models of neurodegenerative disorders

Much evidence is available that inflammation contributes to the development of neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease and Huntington's disease. Our review investigates how well current mouse models reflect this aspect of the pathogenesis. Transgenic models of AD have been available for several years and are the most extensively studied. Modulation of cytokine levels, activation of microglia and, to a lesser extent, activation of the complement system have been reported. Mouse models of PD and HD so far show less evidence for the involvement of inflammation. An increasing number of transgenic mouse strains is being created to model human neurodegenerative diseases. A perfect model should reflect all aspects of a disease. It is important to evaluate continuously the models for their match with the human disease and reevaluate them in light of new findings in human patients. Although none of the transgenic mouse models recapitulates all aspects of the human disorder they represent, all models have provided valuable information on basic molecular pathways. In particular, the mouse models of Alzheimer disease have also led to the development of new therapeutic strategies such as vaccination and modulation of microglial activity.

Microglia in infectious diseases of the central nervous system

Microglia are the resident macrophage population in the central nervous system (CNS) parenchyma and, as such, are poised to provide a first line of defense against invading pathogens. Microglia are endowed with a vast repertoire of pattern recognition receptors that include such family members as Toll-like receptors and phagocytic receptors, which collectively function to sense and eliminate microbes invading the CNS parenchyma. In addition, microglial activation elicits a broad range of pro-inflammatory cytokines and chemokines that are involved in the recruitment and subsequent activation of peripheral immune cells infiltrating the infected CNS. Studies from several laboratories have demonstrated the ability of microglia to sense and respond to a wide variety of pathogens capable of colonizing the CNS including bacterial, viral, and fungal species. This review will highlight the role of microglia in microbial recognition and the resultant antipathogen response that ensues in an attempt to clear these infections. Implications as to whether microglial activation is uniformly beneficial to the CNS or in some circumstances may exacerbate pathology will also be discussed.

Heterogeneity of microglial activation in the innate immune response in the brain

The immune response in the brain has been widely investigated and while many studies have focused on the proinflammatory cytotoxic response, the brain’s innate immune system demonstrates significant heterogeneity. Microglia, like other tissue macrophages, participate in repair and resolution processes after infection or injury to restore normal tissue homeostasis. This review examines the mechanisms that lead to reduction of self-toxicity and to repair and restructuring of the damaged extracellular matrix in the brain. Part of the resolution process involves switching macrophage functional activation to include reduction of proinflammatory mediators, increased production and release of anti-inflammatory cytokines, and production of cytoactive factors involved in repair and reconstruction of the damaged brain. Two partially overlapping and complimentary functional macrophage states have been identified and are called alternative activation and acquired deactivation. The immunosuppressive and repair processes of each of these states and how alternative activation and acquired deactivation participate in chronic neuroinflammation in the brain are discussed.

Amyloid reduction by amyloid-beta vaccination also reduces mouse tau pathology and protects from neuron loss in two mouse models of Alzheimer's disease

Shown to lower amyloid deposits and improve cognition in APP transgenic mouse models, immunotherapy appears to be a promising approach for the treatment of Alzheimer's disease (AD). Due to limitations in available animal models, however, it has been unclear whether targeting amyloid is sufficient to reduce the other pathological hallmarks of AD-namely, accumulation of pathological, nonmutated tau and neuronal loss. We have now developed two transgenic mouse models (APPSw/NOS2(-/-) and APPSwDI/NOS2(-/-)) that more closely model AD. These mice show amyloid pathology, hyperphosphorylated and aggregated normal mouse tau, significant neuron loss, and cognitive deficits. A beta(1-42) or KLH vaccinations were started in these animals at 12 months, when disease progression and cognitive decline are well underway, and continued for 4 months. Vaccinated APPSwDI/NOS2(-/-) mice, which have predominantly vascular amyloid pathology, showed a 30% decrease in brain A beta and a 35-45% reduction in hyperphosphorylated tau. Neuron loss and cognitive deficits were partially reduced. In APPSw/NOS2(-/-) vaccinated mice, brain A beta was reduced by 65-85% and hyperphosphorylated tau by 50-60%. Furthermore, neurons were completely protected, and memory deficits were fully reversed. Microhemorrhage was observed in all vaccinated APPSw/NOS2(-/-) mice and remains a significant adverse event associated with immunotherapy. Nevertheless, by providing evidence that reducing amyloid pathology also reduces nonmutant tau pathology and blocks neuron loss, these data support the development of amyloid-lowering therapies for disease-modifying treatment of AD.

Expression of syndecan-2 in the amoeboid microglial cells and its involvement in inflammation in the hypoxic developing brain

The present study examined the expression of heparan sulphate proteoglycan, syndecan-2 (Sdc-2) in the corpus callosum and the amoeboid microglial cells (AMC) in the neonatal rat brain in response to hypoxia. In 1-day old Wistar rats subjected to hypoxia the mRNA and protein expression of Sdc-2 in the corpus callosum, heavily populated by AMC, was increased up to 3 days after the hypoxic exposure. Immunoexpression of Sdc-2 was localized in AMC as confirmed by double labeling using microglial marker. Primary cultures of microglial cells subjected to hypoxia showed a significant increase in Sdc-2 expression. Application of Sdc-2 to microglial cultures under hypoxia increased the release of tumor necrosis factor-alpha, interleukin-1beta, chemokine (C-C motif) ligand 2 (CCL2), and chemokine (C-X-C motif) ligand 12 (CXCL12) by the microglial cells. Additionally, Sdc-2 enhanced the production of reactive oxygen species (ROS) by microglia subjected to hypoxia. Edaravone [3-methyl-1phenyl-2-pyrazolin-5-one], an antioxidant drug, suppressed the hypoxia- and Sdc-2-induced increased production of cytokines, chemokines, and ROS. In the light of these findings, we suggest that Sdc-2 plays an important role in microglial production of inflammatory cytokines, chemokines, and ROS in hypoxic conditions. In this connection, edaravone suppressed the hypoxia- and Sdc-2-induced increased cytokine and ROS production suggesting its therapeutic potential in ameliorating neuroinflammation.

Marked induction of inducible nitric oxide synthase and tumor necrosis factor-alpha in rat CD40+ microglia by comparison to CD40- microglia

There may be two subtypes of microglia (MG) at least in the CNS. We separated the two types from rat mixed glial culture. mRNAs and proteins for inducible nitric oxide synthase (iNOS) and tumor necrosis factor-alpha (TNFalpha) were more induced in CD40(+) MG than CD40(-) MG after LPS stimulation. Although the expression level of LPS receptors showed a little difference between the subtypes, LPS-induced degradation of phosphorylated IkappaBalpha was marked in CD40(+) MG. These results strongly suggest that CD40(+) MG produce larger amount of NO and TNFalpha to exhibit neurotoxic action under certain pathological conditions in brains.

The effects of NOS2 gene deletion on mice expressing mutated human AbetaPP

Nitric oxide synthase 2 (NOS2) and its gene product, inducible NOS (iNOS) play an important role in neuroinflammation by generating nitric oxide (NO), a critical signaling and redox factor in the brain. Although NO is associated with tissue damage, it can also promote cell survival. We hypothesize that during long-term exposure to amyloid-beta (Abeta) in Alzheimer's disease (AD), NO levels fall in the brain to a threshold at which the protective effects of NO cannot be sustained, promoting Abeta mediated damage. Two new mouse models of AD have been developed that utilize this concept of NO's action. These mice express human amyloid-beta protein precursor (AbetaPP) mutations that generate Abeta peptides on a mouse NOS2 knockout background. The APP/NOS2(-/-) bigenic mice progress from Abeta production and amyloid deposition to hyperphosphorylated normal mouse tau at AD-associated epitopes, aggregation and redistribution of tau to somatodendritic regions of neurons and significant neuronal loss including loss of interneurons. This AD-like pathology is accompanied by robust behavioral changes. As APP/NOS2(-/-) bigenic mice more fully model the human AD disease pathology, they may serve as a tool to better understand disease progression in AD and the role of NO in altering chronic neurological disease processes.

Hydroxy-1-aryl-isochromans: protective compounds against lipid peroxidation and cellular nitrosative stress

Hydroxy-1-aryl-isochromans are a novel class of polyphenolic heterocyclic compounds with potent antioxidative potential. Here we investigated the protective effects of hydroxy-1-aryl-isochroman derivatives against lipid peroxidation and nitrosative stress as a function of their degree of hydroxylation. Measurements of thiobarbituric acid reactive substances (TBARS) in rat brain mitochondria, nitric oxide analysis in microglia cultures, and cell viability analysis were performed. We found that hydroxyl-1-aryl-isochroman derivatives with two, three, and four hydroxyl groups (ISO-2, ISO-3, and ISO-4, respectively) display a higher activity against mitochondrial lipid peroxidation than the corresponding derivative without hydroxyl groups (ISO-0) as well as the reference antioxidant Trolox. However, ISO-0 strongly reduced microglial nitric oxide production, whereas ISO-2 and ISO-4 had a less pronounced effect and ISO-3 was even ineffective. ISO-0 and ISO-3 increased the cell viability of activated microglia, whereas a minor effect of ISO-2 and no significant increase after treatment with ISO-4 could be observed. In conclusion, hydroxy-1-aryl-isochromans exhibit cytoprotective properties, besides their action against reactive oxygen/nitrogen species.

Progression of amyloid pathology to Alzheimer's disease pathology in an amyloid precursor protein transgenic mouse model by removal of nitric oxide synthase 2

Alzheimer's disease (AD) is characterized by three primary pathologies in the brain: amyloid plaques, neurofibrillary tangles, and neuron loss. Mouse models have been useful for studying components of AD but are limited in their ability to fully recapitulate all pathologies. We crossed the APPSwDI transgenic mouse, which develops amyloid beta (Abeta)-protein deposits only, with a nitric oxide synthase 2 (NOS2) knock-out mouse, which develops no AD-like pathology. APPSwDI/NOS2(-/-) mice displayed impaired spatial memory compared with the APPSwDI mice, yet they have unaltered levels of Abeta. APPSwDI mice do not show tau pathology, whereas APPSwDI/NOS2(-/-) mice displayed extensive tau pathology associated with regions of dense microvascular amyloid deposition. Also, APPSwDI mice do not have any neuron loss, whereas the APPSwDI/NOS2(-/-) mice have significant neuron loss in the hippocampus and subiculum. Neuropeptide Y neurons have been shown to be particularly vulnerable in AD. These neurons appear to be particularly vulnerable in the APPSwDI/NOS2(-/-) mice as we observe a dramatic reduction in the number of NPY neurons in the hippocampus and subiculum. These data show that removal of NOS2 from an APP transgenic mouse results in development of a much greater spectrum of AD-like pathology and behavioral impairments.

Implications for the kynurenine pathway and quinolinic acid in amyotrophic lateral sclerosis

The kynurenine pathway (KP) is a major route of L-tryptophan catabolism leading to production of several neurobiologically active molecules. Among them is the excitotoxin quinolinic acid (QUIN) that is known to be involved in the pathogenesis of several major inflammatory neurological diseases. In amyotrophic lateral sclerosis (ALS) degeneration of motor neurons is associated with a chronic and local inflammation (presence of activated microglia and astrocytes). There is emerging evidence that the KP is important in ALS. Recently, we demonstrated that QUIN is significantly increased in serum and CSF of ALS patients. Moreover, most of the factors associated with QUIN toxicity are found in ALS, implying that QUIN may play a substantial role in the neuropathogenesis of ALS. This review details the potential role the KP has in ALS and advances a testable hypothetical model.

Up-regulation of PK11195 binding in areas of axonal degeneration coincides with early microglial activation in mouse brain

Increased binding of the peripheral benzodiazepine binding site (PBBS) ligand [(3)H]PK11195 in the central nervous system of patients suffering from acute and chronic neuropathology has been associated with reactive microgliosis. However, it remains uncertain which stages of microglial activation occur in conjunction with the increased [(3)H]PK11195 binding. We used quantitative autoradiography for [(3)H]PK11195 and quantitative polymerase chain reaction for PBBS mRNA and markers of early and late microglial activation to investigate the time-course of cellular responses in the hippocampus of mice with degeneration of the entorhinal-hippocampal perforant path. The axonal lesion evoked an increase in the B(max) for [(3)H]PK11195 in hippocampus which peaked at 2 days post-lesion, remained elevated at day 5 and began to decline at 10 days post-lesion. These changes occurred in the absence of significant changes in affinity in vitro. Quantitative polymerase chain reaction analysis of isolated hippocampi using exon-specific primers indicated the presence of several splice variants of PBBS mRNA, which appeared to be affected differentially by the lesion. The changes in PBBS mRNA and CD11b mRNA levels correlated with the B(max) for [(3)H]PK11195 during 10 days post-lesion, suggesting that microglial activation couples with increases in mRNA levels for these markers. In addition, the onset of changes in PBBS mRNA levels coincided with the significantly elevated tumor necrosis factor mRNA levels present during early microglial activation at 2 days post-lesion. We conclude that up-regulation of [(3)H]PK11195 binding and PBBS mRNA levels coincided with early microglial activation, characterized by concomitantly increased microglial tumor necrosis factor mRNA levels, and persisted throughout the period with reactive microgliosis.

Characterization of NO and cytokine production in immune-activated microglia and peritoneal macrophages derived from a mouse model expressing the human NOS2 gene on a mouse NOS2 knockout background

Significant differences exist in the production and release of nitric oxide (NO) from human macrophages versus macrophages of mouse origin. Human macrophages have been shown to respond poorly to stimuli that provoke strong inflammatory reactions from mouse macrophages. To address the differences in macrophage function in an animal model, a transgenic mouse was created that contained the entire human NOS2 gene, including the human promoter and all of its exons and introns. The huNOS2 transgenic mouse was then mated to mice lacking a functional NOS2 gene (muNOS2(/) or NOS2 knockout mice) to generate a double transgenic mouse (huNOS2(+/0)/muNOS2(/)) that expresses a functional human NOS2 gene in place of the mouse NOS2 gene. These double transgenic mice were found to express only human NOS2 mRNA and human iNOS proteins in response to immune stimulation. The production and release of nitric oxide from isolated macrophages from the doubly transgenic mouse also more closely paralleled human responses rather than mouse. Peritoneal macrophages from double transgenic mice generated nanomolar levels of nitrite in response to inflammatory stimuli, while peritoneal macrophages from wild-type mice generated micromolar levels of nitrite in response to the same inflammatory stimuli. Similarly, microglia from the huNOS2(+/0)/muNOS2(/) mice accumulated nanomolar levels of nitrite following inflammatory stimulation. Reduced nitrite release persisted in spite of normal responsiveness to inflammatory stimulation as measured by tumor necrosis factor alpha and interleukin-6 production and release. These data suggest that the human-specific release of nanomolar levels of nitrite may largely result from differences between the human and mouse NOS2 genes, which may program different degrees of nitric oxide responses to inflammatory signals in humans than in mice.

Demyelination: the role of reactive oxygen and nitrogen species

This review summarises the role that reactive oxygen and nitrogen species play in demyelination, such as that occurring in the inflammatory demyelinating disorders multiple sclerosis and Guillain-Barré syndrome. The concentrations of reactive oxygen and nitrogen species (e.g. superoxide, nitric oxide and peroxynitrite) can increase dramatically under conditions such as inflammation, and this can overwhelm the inherent antioxidant defences within lesions. Such oxidative and/or nitrative stress can damage the lipids, proteins and nucleic acids of cells and mitochondria, potentially causing cell death. Oligodendrocytes are more sensitive to oxidative and nitrative stress in vitro than are astrocytes and microglia, seemingly due to a diminished capacity for antioxidant defence, and the presence of raised risk factors, including a high iron content. Oxidative and nitrative stress might therefore result in vivo in selective oligodendrocyte death, and thereby demyelination. The reactive species may also damage the myelin sheath, promoting its attack by macrophages. Damage can occur directly by lipid peroxidation, and indirectly by the activation of proteases and phospholipase A2. Evidence for the existence of oxidative and nitrative stress within inflammatory demyelinating lesions includes the presence of both lipid and protein peroxides, and nitrotyrosine (a marker for peroxynitrite formation). The neurological deficit resulting from experimental autoimmune demyelinating disease has generally been reduced by trial therapies intended to diminish the concentration of reactive oxygen species. However, therapies aimed at diminishing reactive nitrogen species have had a more variable outcome, sometimes exacerbating disease.

Investigations with cultured human microglia on pathogenic mechanisms of Alzheimer's disease and other neurodegenerative diseases

Inflammation-mediated mechanisms for human neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD) have evolved from being on the fringe of medical hypotheses to mainstream thinking. Pioneering immunopathology studies with human brain tissues identified microglia associated with neuropathologic hallmarks of these diseases. As activated macrophages were known to produce many potential toxic products, this gave rise to the hypothesis that activated microglia (brain resident macrophages) could be contributing to the degeneration of key target neurons in these diseases, as well as potential vascular dysfunction. Studies with microglia derived from different sources, including human brains, have confirmed that activated microglia can mediate neuronal cell death. Based on these theories, a number of human clinical trials with antiinflammatory agents have been carried out on AD patients. Results to date have indicated a lack of effectiveness at slowing disease progression and have begun to cast doubt on the significance of inflammation in AD. It has been shown recently that activating microglia through immunization of amyloid plaque-developing mice with amyloid beta peptide (Abeta) has promise as a therapeutic strategy and despite some setbacks, has potential as a treatment for AD patients. This article will consider experimental data with microglia to determine whether the additional targets need to be investigated. The use of human microglia cultures, in particular those derived from elderly diseased human brains, offers an experimental system that can closely model the cell type activated in human neurodegenerative diseases. Experimental data produced by our laboratory and others is reviewed to determine the contribution of this unique experimental model to understanding disease mechanisms and possibly discovering new therapeutic targets.

White matter injury following prolonged free radical formation in the 0.65 gestation fetal sheep brain

Free radicals seem to be involved in the development of cerebral white matter damage after asphyxia in the premature infant. The immature brain may be at increased risk of free radical mediated injury, as particularly the preterm infant has a relative deficiency in brain antioxidants systems, such as superoxide dismutase and glutathione peroxidase. In vitro studies show that immature oligodendrocytes express an intrinsic vulnerability to reactive oxygen species and free radical scavengers are able to protect immature oligodendrocytes from injury. The aim of this study was to examine the formation of ascorbyl radicals as a marker of oxidative stress in the preterm brain in association with cerebral white matter injury after intrauterine asphyxia. Fetal sheep at 0.65 gestation were chronically instrumented with vascular catheters and an occluder cuff around the umbilical cord. A microdialysis probe was placed in the periventricular white matter. Fetal asphyxia was induced by occlusion of the umbilical cord for 25 min (n = 10). Microdialysis samples were collected for 72 h and analyzed for ascorbyl radicals using electron spin resonance. Five instrumented fetuses served as controls. Three days after the insult, fetal brains were examined for morphologic injury. Umbilical cord occlusion resulted in prolonged and marked increase in ascorbyl radical production in the brain in connection with white matter injury, with activation of microglia cells in periventricular white matter and axonal injury. These data suggest that reperfusion injury following asphyxia in the immature brain is associated with marked free radical production.

The role of oxidative damage in the neuropathology of organic acidurias: insights from animal studies

Organic acidurias represent a group of inherited disorders resulting from deficient activity of specific enzymes of the catabolism of amino acids, carbohydrates or lipids, leading to tissue accumulation of one or more carboxylic (organic) acids. Patients affected by organic acidurias predominantly present neurological symptoms and structural brain abnormalities, of which the aetiopathogenesis is poorly understood. However, in recent years increasing evidence has emerged suggesting that oxidative stress is possibly involved in the pathology of some organic acidurias and other inborn errors of metabolism. This review addresses some of the recent developments obtained mainly from animal studies indicating oxidative damage as an important determinant of the neuropathophysiology of some organic acidurias. Recent data showing that various organic acids are capable of inducing free radical generation and decreasing brain antioxidant defences is presented. The discussion focuses on the relatively low antioxidant defences of the brain and the vulnerability of this tissue to reactive species. This offers new perspectives for potential therapeutic strategies for these disorders, which may include the early use of appropriate antioxidants as a novel adjuvant therapy, besides the usual treatment based on removing toxic compounds and using special diets and pharmacological agents, such as cofactors and L-carnitine.

Beta-amyloid peptide potentiates inflammatory responses induced by lipopolysaccharide, interferon -gamma and 'advanced glycation endproducts' in a murine microglia cell line

beta-Amyloid (Abeta) plaques are characteristic hallmarks of Alzheimer's disease (AD). In AD, it has been suggested that activation of microglial cells might be the link between Abeta deposition and neuronal degeneration. Activated microglia are associated with senile plaques and produce free radicals and inflammatory cytokines. However, it is still not clear whether Abeta needs a prestimulated environment to exert its proinflammatory potential. Advanced glycation endproducts (AGEs), protein-bound oxidation products of sugars, have been shown to accumulate in senile plaques and could induce a silent but chronic inflammation in the AD brain. We tested whether Abeta acts as an amplifier of a submaximal proinflammatory response initiated by exposure to chicken egg albumin-AGE, lipopolysaccharide or interferon-gamma. Synthetic Abeta was used to produce three different samples (Abeta-fibrilar; Abeta-aggregated; Abeta-AGE), which were characterized for beta-sheeted fibrils by the thioflavin-T test and electron microscopy. As markers of microglial activation, nitric oxide, interleukin-6, macrophage-colony stimulation factor and tumour necrosis factor-alpha production was measured. All three Abeta samples alone could not induce a detectable microglial response. The combination of Abeta preparations, however, with the coinducers provoked a strong microglial response, whereby Abeta-AGE and fibrilar Abeta were more potent inflammatory signals than aggregated Abeta. Thus, Abeta in senile plaques can amplify microglial activation by a coexisting submaximal inflammatory stimulus. Hence, anti-inflammatory therapeutics could either target the primary proinflammatory signal (e.g. by limiting AGE-formation by AGE inhibitors or cross-link breakers) or the amplifyer Abeta (e.g. by limiting Abeta production by beta- or gamma-secretase inhibitors).