Thrombin induces in vivo degeneration of nigral dopaminergic neurones along with the activation of microglia

Blood-brain barrier alterations and their impact on Parkinson's disease pathogenesis and therapy

There is increasing evidence for blood-brain barrier (BBB) alterations in Parkinson's disease (PD), the second most common neurodegenerative disorder with rapidly rising prevalence. Altered tight junction and transporter protein levels, accumulation of α-synuclein and increase in inflammatory processes lead to extravasation of blood molecules and vessel degeneration. This could result in a self-perpetuating pathophysiology of inflammation and BBB alteration, which contribute to neurodegeneration. Toxin exposure or α-synuclein over-expression in animal models has been shown to initiate similar pathologies, providing a platform to study underlying mechanisms and therapeutic interventions. Here we provide a comprehensive review of the current knowledge on BBB alterations in PD patients and how rodent models that replicate some of these changes can be used to study disease mechanisms. Specific challenges in assessing the BBB in patients and in healthy controls are discussed. Finally, a potential role of BBB alterations in disease pathogenesis and possible implications for therapy are explored. The interference of BBB alterations with current and novel therapeutic strategies requires more attention. Brain region-specific BBB alterations could also open up novel opportunities to target specifically vulnerable neuronal subpopulations.

Thrombin Priming Promotes the Neuroprotective Effects of Human Wharton's Jelly-Derived Mesenchymal Stem Cells Via the HGF/AKT/STAT3 Signaling Pathway

Mesenchymal stem cells (MSCs) directly differentiate into neurons and endothelial cells after transplantation, and their secretome has considerable potential for treating brain injuries. Previous studies have suggested that the effects of MSCs priming with exposure to hypoxia, cytokines, growth factors, or chemical agents could optimize the paracrine potency and therapeutic potential of MSCs. Studies have suggested that thrombin-primed Wharton's Jelly-derived mesenchymal stem cells (Th.WJ-MSCs) significantly enhance the neuroprotective beneficial effects of naive MSCs in brain injury such as hypoxic-ischemic brain injury (HIE) and intraventricular hemorrhage (IVH). This study aimed to characterize WJ-MSCs in terms of stem cell markers, differentiation, cell proliferation, and paracrine factors by comparing naive and Th.WJ-MSCs. We demonstrated that compared with naive MSCs, Th.MSCs significantly enhanced the neuroprotective effects in vitro. Moreover, we identified differentially expressed proteins in the conditioned media of naive and Th.WJ-MSCs by liquid chromatography-tandem mass spectrometry analysis. Secretome analysis of the conditioned medium of WJ-MSCs revealed that such neuroprotective effects were mediated by paracrine effects with secretomes of Th.WJ-MSCs, and hepatocyte growth factor was identified as a key paracrine mediator. These results can be applied further in the preclinical and clinical development of effective and safe cell therapeutics for brain injuries such as HIE and IVH.

Thrombin-Induced Microglia Activation Modulated through Aryl Hydrocarbon Receptors

Thrombin is a multifunctional serine protein which is closely related to neurodegenerative disorders. The Aryl hydrocarbon receptor (AhR) is well expressed in microglia cells involving inflammatory disorders of the brain. However, it remains unclear as to how modulation of AhR expression by thrombin is related to the development of neurodegeneration disorders. In this study, we investigated the role of AhR in the development of thrombin-induced neurodegenerative processes, especially those concerning microglia. The primary culture of either wild type or AhR deleted microglia, as well as BV-2 cell lines, was used for an in vitro study. Hippocampal slice culture and animals with either wild type or with AhR deleted were used for the ex vivo and in vivo studies. Simulations of ligand protein docking showed a strong integration between the thrombin and AhR. In thrombin-triggered microglia cells, deleting AhR escalated both the NO release and iNOS expression. Such effects were abolished by the administration of the AhR agonist. In thrombin-activated microglia cells, downregulating AhR increased the following: vascular permeability, pro-inflammatory genetic expression, MMP-9 activity, and the ratio of M1/M2 phenotype. In the in vivo study, thrombin induced the activation of microglia and their volume, thereby contributing to the deterioration of neurobehavior. Deleting AhR furthermore aggravated the response in terms of impaired neurobehavior, increasing brain edema, aggregating microglia, and increasing neuronal death. In conclusion, thrombin caused the activation of microglia through increased vessel permeability, expression of inflammatory response, and phenotype of M1 microglia, as well the MMP activity. Deleting AhR augmented the above detrimental effects. These findings indicate that the modulation of AhR is essential for the regulation of thrombin-induced brain damages and that the AhR agonist may harbor the potentially therapeutic effect in thrombin-induced neurodegenerative disorder.

Inflammatory Animal Models of Parkinson’s Disease

Accumulating evidence suggests that microglia and peripheral immune cells may play determinant roles in the pathogenesis of Parkinson’s disease (PD). Consequently, there is a need to take advantage of immune-related models of PD to study the potential contribution of microglia and peripheral immune cells to the degeneration of the nigrostriatal system and help develop potential therapies for PD. In this review, we have summarised the main PD immune models. From a historical perspective, we highlight first the main features of intranigral injections of different pro-inflammogens, including lipopolysaccharide (LPS), thrombin, neuromelanin, etc. The use of adenoviral vectors to promote microglia-specific overexpression of different molecules in the ventral mesencephalon, including α-synuclein, IL-1β, and TNF, are also presented and briefly discussed. Finally, we summarise different models associated with peripheral inflammation whose contribution to the pathogenesis of neurodegenerative diseases is now an outstanding question. Illustrative examples included systemic LPS administration and dextran sulfate sodium-induced colitis in rodents.

Microglia Phenotypes in Aging and Neurodegenerative Diseases

Neuroinflammation is a hallmark of many neurodegenerative diseases (NDs) and plays a fundamental role in mediating the onset and progression of disease. Microglia, which function as first-line immune guardians of the central nervous system (CNS), are the central drivers of neuroinflammation. Numerous human postmortem studies and in vivo imaging analyses have shown chronically activated microglia in patients with various acute and chronic neuropathological diseases. While microglial activation is a common feature of many NDs, the exact role of microglia in various pathological states is complex and often contradictory. However, there is a consensus that microglia play a biphasic role in pathological conditions, with detrimental and protective phenotypes, and the overall response of microglia and the activation of different phenotypes depends on the nature and duration of the inflammatory insult, as well as the stage of disease development. This review provides a comprehensive overview of current research on the various microglia phenotypes and inflammatory responses in health, aging, and NDs, with a special emphasis on the heterogeneous phenotypic response of microglia in acute and chronic diseases such as hemorrhagic stroke (HS), Alzheimer's disease (AD), and Parkinson's disease (PD). The primary focus is translational research in preclinical animal models and bulk/single-cell transcriptome studies in human postmortem samples. Additionally, this review covers key microglial receptors and signaling pathways that are potential therapeutic targets to regulate microglial inflammatory responses during aging and in NDs. Additionally, age-, sex-, and species-specific microglial differences will be briefly reviewed.

The pleiotropic effects of antithrombotic drugs in the metabolic-cardiovascular-neurodegenerative disease continuum: impact beyond reduced clotting

Antithrombotic drugs are widely used for primary and secondary prevention, as well as treatment of many cardiovascular disorders. Over the past few decades, major advances in the pharmacology of these agents have been made with the introduction of new drug classes as novel therapeutic options. Accumulating evidence indicates that the beneficial outcomes of some of these antithrombotic agents are not solely related to their ability to reduce thrombosis. Here, we review the evidence supporting established and potential pleiotropic effects of four novel classes of antithrombotic drugs, adenosine diphosphate (ADP) P2Y12-receptor antagonists, Glycoprotein IIb/IIIa receptor Inhibitors, and Direct Oral Anticoagulants (DOACs), which include Direct Factor Xa (FXa) and Direct Thrombin Inhibitors. Specifically, we discuss the molecular evidence supporting such pleiotropic effects in the context of cardiovascular disease (CVD) including endothelial dysfunction (ED), atherosclerosis, cardiac injury, stroke, and arrhythmia. Importantly, we highlight the role of DOACs in mitigating metabolic dysfunction-associated cardiovascular derangements. We also postulate that DOACs modulate perivascular adipose tissue inflammation and thus, may reverse cardiovascular dysfunction early in the course of the metabolic syndrome. In this regard, we argue that some antithrombotic agents can reverse the neurovascular damage in Alzheimer's and Parkinson's brain and following traumatic brain injury (TBI). Overall, we attempt to provide an up-to-date comprehensive review of the less-recognized, beneficial molecular aspects of antithrombotic therapy beyond reduced thrombus formation. We also make a solid argument for the need of further mechanistic analysis of the pleiotropic effects of antithrombotic drugs in the future.

Role of Thrombin in Central Nervous System Injury and Disease

Thrombin is a Na+-activated allosteric serine protease of the chymotrypsin family involved in coagulation, inflammation, cell protection, and apoptosis. Increasingly, the role of thrombin in the brain has been explored. Low concentrations of thrombin are neuroprotective, while high concentrations exert pathological effects. However, greater attention regarding the involvement of thrombin in normal and pathological processes in the central nervous system is warranted. In this review, we explore the mechanisms of thrombin action, localization, and functions in the central nervous system and describe the involvement of thrombin in stroke and intracerebral hemorrhage, neurodegenerative diseases, epilepsy, traumatic brain injury, and primary central nervous system tumors. We aim to comprehensively characterize the role of thrombin in neurological disease and injury.

Cerebral venous congestion promotes blood-brain barrier disruption and neuroinflammation, impairing cognitive function in mice

Cognitive impairment is one of the most common co-occurring chronic conditions among elderly heart failure patients (incidence: up to ~ 80%); however, the underlying mechanisms are not completely understood. It is hypothesized that in addition to decreased cardiac output, increases in central-and consequentially, cerebral-venous pressure (backward failure) also contribute significantly to the genesis of cognitive impairment. To test this hypothesis and elucidate the specific pathogenic role of venous congestion in the brain, we have established a novel model of increased cerebral venous pressure: mice with jugular vein ligation (JVL). To test the hypothesis that increased venous pressure in the brain contributes to the development of cognitive deficits by causing blood-brain barrier disruption, dysregulation of blood flow, and/or promoting neuroinflammation, in C57BL/6 mice, the internal and external jugular veins were ligated. Cognitive function (radial arm water maze), gait function (CatWalk), and motor coordination (rotarod) were tested post-JVL. Neurovascular coupling responses were assessed by measuring changes in cerebral blood flow in the whisker barrel cortex in response to contralateral whisker stimulation by laser speckle contrast imaging through a closed cranial window. Blood-brain barrier integrity (IgG extravasation) and microglia activation (Iba1 staining) were assessed in brain slices by immunohistochemistry. Neuroinflammation-related gene expression profile was assessed by a targeted qPCR array. After jugular vein ligation, mice exhibited impaired spatial learning and memory, altered motor coordination, and impaired gait function, mimicking important aspects of altered brain function observed in human heart failure patients. JVL did not alter neurovascular coupling responses. In the brains of mice with JVL, significant extravasation of IgG was detected, indicating blood-brain barrier disruption, which was associated with histological markers of neuroinflammation (increased presence of activated microglia) and a pro-inflammatory shift in gene expression profile. Thus, cerebral venous congestion per se can cause blood-brain barrier disruption and neuroinflammation, which likely contribute to the genesis of cognitive impairment. These findings have relevance to the pathogenesis of cognitive decline associated with heart failure as well as increased cerebal venous pressure due to increased jugular venous reflux in elderly human patients.

Neuroinflammation in hypertension: the renin-angiotensin system versus pro-resolution pathways

Overstimulation of the pro-inflammatory pathways within brain areas responsible for sympathetic outflow is well evidenced as a primary contributing factor to the establishment and maintenance of neurogenic hypertension. However, the precise mechanisms and stimuli responsible for promoting a pro-inflammatory state are not fully elucidated. Recent work has unveiled novel compounds derived from omega-3 polyunsaturated fatty acids (ω-3 PUFAs), termed specialized pro-resolving mediators (SPMs), which actively regulate the resolution of inflammation. Failure or dysregulation of the resolution process has been linked to a variety of chronic inflammatory and neurodegenerative diseases. Given the pathologic role of neuroinflammation in the hypertensive state, SPMs and their associated pathways may provide a link between hypertension and the long-standing association of dietary ω-3 PUFAs with cardioprotection. Herein, we review recent progress in understanding the RAS-driven pathophysiology of neurogenic hypertension, particularly in regards to the chronic low-grade neuroinflammatory response. In addition, we examine the potential for an impaired resolution of inflammation process in the context of hypertension.

Divergent Effects of Metformin on an Inflammatory Model of Parkinson's Disease

The oral antidiabetic drug metformin is known to exhibit anti-inflammatory properties through activation of AMP kinase, thus protecting various brain tissues as cortical neurons, for example. However, the effect of metformin on the substantia nigra (SN), the main structure affected in Parkinson’s disease (PD), has not yet been studied in depth. Inflammation is a key feature of PD and it may play a central role in the neurodegeneration that takes place in this disorder. The aim of this work was to determine the effect of metformin on the microglial activation of the SN of rats using the animal model of PD based on the injection of the pro-inflammogen lipopolysaccharide (LPS). In vivo and in vitro experiments were conducted to study the activation of microglia at both the cellular and molecular levels. Our results indicate that metformin overall inhibits microglia activation measured by OX-6 (MHCII marker), IKKβ (pro-inflammatory marker) and arginase (anti-inflammatory marker) immunoreactivity. In addition, qPCR experiments reveal that metformin treatment minimizes the expression levels of several pro- and anti-inflammatory cytokines. Mechanistically, the drug decreases the phosphorylated forms of mitogen-activated protein kinases (MAPKs) as well as ROS generation through the inhibition of the NADPH oxidase enzyme. However, metformin treatment fails to protect the dopaminergic neurons of SN in response to intranigral LPS. These findings suggest that metformin could have both beneficial and harmful pharmacological effects and raise the question about the potential use of metformin for the prevention and treatment of PD.

Hypertension impairs neurovascular coupling and promotes microvascular injury: role in exacerbation of Alzheimer's disease

Hypertension in the elderly substantially increases both the risk of vascular cognitive impairment (VCI) and Alzheimer's disease (AD); however, the underlying mechanisms are not completely understood. This review discusses the effects of hypertension on structural and functional integrity of cerebral microcirculation, including hypertension-induced alterations in neurovascular coupling responses, cellular and molecular mechanisms involved in microvascular damage (capillary rarefaction, blood-brain barrier disruption), and the genesis of cerebral microhemorrhages and their potential role in exacerbation of cognitive decline associated with AD. Understanding and targeting the hypertension-induced cerebromicrovascular alterations that are involved in the onset and progression of AD and contribute to cognitive impairment are expected to have a major role in preserving brain health in high-risk older individuals.

Role of thrombin in the pathogenesis of central nervous system inflammatory diseases

Thrombin initiates proinflammatory signaling responses through activation of protease-activated receptors (PARs) in in vitro and in vivo systems. Proinflammatory signaling function of thrombin increases secretion of proinflammatory cytokines and chemokines, triggers vascular permeability, promotes leukocyte migration, and induces adhesion molecule expression. Thrombin as a potent signaling molecule is strongly implicated in a number of proinflammatory disorders including severe sepsis, cancer, cardiovascular disease, and of special interest in this review neurodegenerative disorders. This review summarizes the role of thrombin in the pathogenesis of central nervous system (CNS) inflammatory diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD), promoting greater understanding and clinical management of these diseases. J. Cell. Physiol. 232: 482-485, 2017. © 2016 Wiley Periodicals, Inc.

Protease-Activated Receptors in Neuronal Development, Neurodegeneration, and Neuroprotection: Thrombin as Signaling Molecule in the Brain

Protease-activated receptors (PARs) belong to the superfamily of seven transmembrane domain G protein-coupled receptors. Four PAR subtypes are known, PAR-1 to -4. PARs are highly homologous between the species and are expressed in a wide variety of tissues and cell types. Of particular interest is the role which these receptors play in the brain, with regard to neuroprotection or degeneration under pathological conditions. The main agonist of PARs is thrombin, a multifunctional serine protease, known to be present not only in blood plasma but also in the brain. PARs possess an irreversible activation mechanism. Binding of agonist and subsequent cleavage of the extracellular N-terminus of the receptor results in exposure of a so-called tethered ligand domain, which then binds to extracellular loop 2 of the receptor leading to receptor activation. PARs exhibit an extensive expression pattern in both the central and the peripheral nervous system. PARs participate in several mechanisms important for normal cellular functioning and during critical situations involving cellular survival and death. In the last few years, research on Alzheimer’s disease and stroke has linked PARs to the pathophysiology of these neurodegenerative disorders. Actions of thrombin are concentration-dependent, and therefore, depending on cellular function and environment, serve as a double-edged sword. Thrombin can be neuroprotective during stress conditions, whereas under normal conditions high concentrations of thrombin are toxic to cells.

The Importance of Thrombin in Cerebral Injury and Disease

There is increasing evidence that prothrombin and its active derivative thrombin are expressed locally in the central nervous system. So far, little is known about the physiological and pathophysiological functions exerted by thrombin in the human brain. Extra-hepatic prothrombin expression has been identified in neuronal cells and astrocytes via mRNA measurement. The actual amount of brain derived prothrombin is expected to be 1% or less compared to that in the liver. The role in brain injury depends upon its concentration, as higher amounts cause neuroinflammation and apoptosis, while lower concentrations might even be cytoprotective. Its involvement in numerous diseases like Alzheimer’s, multiple sclerosis, cerebral ischemia and haemorrhage is becoming increasingly clear. This review focuses on elucidation of the cerebral thrombin expression, local generation and its role in injury and disease of the central nervous system.

Relevance of chronic stress and the two faces of microglia in Parkinson's disease

This review is aimed to highlight the importance of stress and glucocorticoids (GCs) in modulating the inflammatory response of brain microglia and hence its potential involvement in Parkinson’s disease (PD). The role of inflammation in PD has been reviewed extensively in the literature and it is supposed to play a key role in the course of the disease. Historically, GCs have been strongly associated as anti-inflammatory hormones. However, accumulating evidence from the peripheral and central nervous system have clearly revealed that, under specific conditions, GCs may promote brain inflammation including pro-inflammatory activation of microglia. We have summarized relevant data linking PD, neuroinflamamation and chronic stress. The timing and duration of stress response may be critical for delineating an immune response in the brain thus probably explain the dual role of GCs and/or chronic stress in different animal models of PD.

Three-Dimensional Electron Microscopy Reconstruction of Degenerative Dopaminergic Neurons Surrounded by Activated Microglia in Substantia Nigra

There is an urgent need to investigate the reason for the pathogenic mechanism of intractable central neurological diseases such as Parkinson's disease. It has been reported that the activation of microglial cells is involved in the pathology of these diseases. However, due to technical difficulties, the relationship between degenerative neurons and activated microglial cells remains unclear. Therefore, we tried the improved analysis technique to clarify the spatial relationship between these cell types. We were able to establish an analysis technique that consists of a three-dimensional reconstruction method using serial immunoelectron micrographs after having identified both degenerative neurons and activated microglial cells under optical microscope. Using this technique, we have relatively easily been able to clarify the spatial relationship between degenerative neurons and activated microglial cells. Furthermore, using this technique it is possible to determine the neuronal degeneration process in detail, because it is able to identify structures implicated in degeneration, such as accumulation of lipofuscin in degenerated neuronal somata and phagocytotic structures of microglial cells. In future, this technical approach may be applied to elucidate the relationship between degenerative neurons and activated glial cells in human diseases.

Chronic stress as a risk factor for Alzheimer's disease

This review aims to point out that chronic stress is able to accelerate the appearance of Alzheimer's disease (AD), proposing the former as a risk factor for the latter. Firstly, in the introduction we describe some human epidemiological studies pointing out the possibility that chronic stress could increase the incidence, or the rate of appearance of AD. Afterwards, we try to justify these epidemiological results with some experimental data. We have reviewed the experiments studying the effect of various stressors on different features in AD animal models. Moreover, we also point out the data obtained on the effect of chronic stress on some processes that are known to be involved in AD, such as inflammation and glucose metabolism. Later, we relate some of the processes known to be involved in aging and AD, such as accumulation of β-amyloid, TAU hyperphosphorylation, oxidative stress and impairement of mitochondrial function, emphasizing how they are affected by chronic stress/glucocorticoids and comparing with the description made for these processes in AD. All these data support the idea that chronic stress could be considered a risk factor for AD.

Preconditioning provides neuroprotection in models of CNS disease: paradigms and clinical significance

Preconditioning is a phenomenon in which brief episodes of a sublethal insult induce robust protection against subsequent lethal injuries. Preconditioning has been observed in multiple organisms and can occur in the brain as well as other tissues. Extensive animal studies suggest that the brain can be preconditioned to resist acute injuries, such as ischemic stroke, neonatal hypoxia/ischemia, surgical brain injury, trauma, and agents that are used in models of neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease. Effective preconditioning stimuli are numerous and diverse, ranging from transient ischemia, hypoxia, hyperbaric oxygen, hypothermia and hyperthermia, to exposure to neurotoxins and pharmacological agents. The phenomenon of "cross-tolerance," in which a sublethal stress protects against a different type of injury, suggests that different preconditioning stimuli may confer protection against a wide range of injuries. Research conducted over the past few decades indicates that brain preconditioning is complex, involving multiple effectors such as metabolic inhibition, activation of extra- and intracellular defense mechanisms, a shift in the neuronal excitatory/inhibitory balance, and reduction in inflammatory sequelae. An improved understanding of brain preconditioning should help us identify innovative therapeutic strategies that prevent or at least reduce neuronal damage in susceptible patients. In this review, we focus on the experimental evidence of preconditioning in the brain and systematically survey the models used to develop paradigms for neuroprotection, and then discuss the clinical potential of brain preconditioning.

Obesity in aging exacerbates blood-brain barrier disruption, neuroinflammation, and oxidative stress in the mouse hippocampus: effects on expression of genes involved in beta-amyloid generation and Alzheimer's disease

There is growing evidence that obesity has deleterious effects on the brain and cognitive function in the elderly population. However, the specific mechanisms through which aging and obesity interact to promote cognitive decline remain unclear. To test the hypothesis that aging exacerbates obesity-induced cerebromicrovascular damage and neuroinflammation, we compared young (7 months) and aged (24 months) high fat diet-fed obese C57BL/6 mice. Aging exacerbated obesity-induced systemic inflammation and blood-brain barrier disruption, as indicated by the increased circulating levels of proinflammatory cytokines and increased presence of extravasated immunoglobulin G in the hippocampus, respectively. Obesity-induced blood-brain barrier damage was associated with microglia activation, upregulation of activating Fc-gamma receptors and proinflammatory cytokines, and increased oxidative stress. Treatment of cultured primary microglia with sera derived from aged obese mice resulted in significantly more pronounced microglia activation and oxidative stress, as compared with treatment with young sera. Serum-induced activation and oxidative stress were also exacerbated in primary microglia derived from aged animals. Hippocampal expression of genes involved in regulation of the cellular amyloid precursor protein-dependent signaling pathways, beta-amyloid generation, and the pathogenesis of tauopathy were largely unaffected by obesity in aged mice. Collectively, obesity in aging is associated with a heightened state of systemic inflammation, which exacerbates blood-brain barrier disruption. The resulting neuroinflammation and oxidative stress in the mouse hippocampus likely contribute to the significant cognitive decline observed in aged obese animals.

Immunology primer for neurosurgeons and neurologists part 2: Innate brain immunity

Over the past several decades we have learned a great deal about microglia and innate brain immunity. While microglia are the principle innate immune cells, other cell types also play a role, including invading macrophages, astrocytes, neurons, and endothelial cells. The fastest reacting cell is the microglia and despite its name, resting microglia (also called ramified microglia) are in fact quite active. Motion photomicrographs demonstrate a constant movement of ramified microglial foot processes, which appear to be testing the microenvironment for dangerous alteration in extracellular fluid content. These foot processes, in particular, interact with synapses and play a role in synaptic function. In event of excitatory overactivity, these foot processes can strip selected synapses, thus reducing activation states as a neuroprotective mechanism. They can also clear extracellular glutamate so as to reduce the risk of excitotoxicity. Microglia also appear to have a number of activation phenotypes, such as: (1) phagocytic, (2) neuroprotective and growth promoting, or (3) primarily neurodestructive. These innate immune cells can migrate a great distance under pathological conditions and appear to have anatomic specificity, meaning they can accumulate in specifically selected areas of the brain. There is some evidence that there are several types of microglia. Macrophage infiltration into the embryonic brain is the source of resident microglia and in adulthood macrophages can infiltrate the brain and are for the most part pathologically indistinguishable from resident microglia, but may react differently. Activation itself does not imply a destructive phenotype and can be mostly neuroprotective via phagocytosis of debris, neuron parts and dying cells and by the release of neurotrophins such as nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF). Evidence is accumulating that microglia undergo dynamic fluctuations in phenotype as the neuropathology evolves. For example, in the early stages of neurotrauma and stroke, microglia play a mostly neuroprotective role and only later switch to a neurodestructive mode. A great number of biological systems alter microglia function, including neurohormones, cannabinoids, other neurotransmitters, adenosine triphosphate (ATP), adenosine, and corticosteroids. One can appreciate that with aging many of these systems are altered by the aging process itself or by disease thus changing the sensitivity of the innate immune system.

Microglia and neurodegeneration: the role of systemic inflammation

It is well accepted that CNS inflammation has a role in the progression of chronic neurodegenerative disease, although the mechanisms through which this occurs are still unclear. The inflammatory response during most chronic neurodegenerative disease is dominated by the microglia and mechanisms by which these cells contribute to neuronal damage and degeneration are the subject of intense study. More recently it has emerged that systemic inflammation has a significant role to play in the progression of these diseases. Well-described adaptive pathways exist to transduce systemic inflammatory signals to the brain, but activation of these pathways appears to be deleterious to the brain if the acute insult is sufficiently robust, as in severe sepsis, or sufficiently prolonged, as in repeated stimulation with robust doses of inflammogens such as lipopolysaccharide (LPS). Significantly, moderate doses of inflammogens produce new pathology in the brain and exacerbate or accelerate features of disease when superimposed upon existing pathology or in the context of genetic predisposition. It is now apparent in multiple chronic disease states, and in ageing, that microglia are primed by prior pathology, or by genetic predisposition, to respond more vigorously to subsequent inflammatory stimulation, thus transforming an adaptive CNS inflammatory response to systemic inflammation, into one that has deleterious consequences for the individual. In this review, the preclinical and clinical evidence supporting a significant role for systemic inflammation in chronic neurodegenerative diseases will be discussed. Mechanisms by which microglia might effect neuronal damage and dysfunction, as a consequence of systemic stimulation, will be highlighted.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Manganese-induced NF-kappaB activation and nitrosative stress is decreased by estrogen in juvenile mice

Manganese toxicity can cause a neurodegenerative disorder affecting cortical and basal ganglia structures with a neurological presentation resembling features of Parkinson's disease. Children are more sensitive to Mn-induced neurological dysfunction than adults, and recent studies from our laboratory revealed a marked sensitivity of male juvenile mice to neuroinflammatory injury from Mn, relative to females. To determine the role of estrogen (E2) in mediating sex-dependent vulnerability to Mn-induced neurotoxicity, we exposed transgenic mice expressing an NF-κB-driven enhanced green fluorescent protein (EGFP) reporter construct (NF-κB-EGFP mice) to Mn, postulating that supplementing male mice with E2 during juvenile development would attenuate neuroinflammatory changes associated with glial activation, including expression of inducible nitric oxide synthase (NOS2) and neuronal protein nitration. Juvenile NF-κB-EGFP mice were separated in groups composed of females, males, and males surgically implanted with Silastic capsules containing 25 μg of 17-β-estradiol (E2) or vehicle control. Mice were then treated with 0 or 100 mg/Kg MnCl(2) by intragastric gavage from postnatal days 21-34. Manganese treatment caused alterations in levels of striatal dopamine, as well as increases in NF-κB reporter activity and NOS2 expression in both microglia and astrocytes that were prevented by supplementation with E2. E2 also decreased neuronal protein nitration in Mn-treated mice and inhibited apoptosis in striatal neurons cocultured with Mn-treated astrocytes in vitro. These data indicate that E2 protects against Mn-induced neuroinflammation in developing mice and that NF-κB is an important regulator of neuroinflammatory gene expression in glia associated with nitrosative stress in the basal ganglia during Mn exposure.

Inflammatory Animal Model for Parkinson's Disease: The Intranigral Injection of LPS Induced the Inflammatory Process along with the Selective Degeneration of Nigrostriatal Dopaminergic Neurons

We have developed an animal model of degeneration of the nigrostriatal dopaminergic neurons, the neuronal system involved in Parkinson's disease (PD). The implication of neuroinflammation on this disease was originally established in 1988, when the presence of activated microglia in the substantia nigra (SN) of parkinsonians was reported by McGeer et al. Neuroinflammation could be involved in the progression of the disease or even has more direct implications. We injected 2 μg of the potent proinflammatory compound lipopolysaccharide (LPS) in different areas of the CNS, finding that SN displayed the highest inflammatory response and that dopaminergic (body) neurons showed a special and specific sensitivity to this process with the induction of selective dopaminergic degeneration. Neurodegeneration is induced by inflammation since it is prevented by anti-inflammatory compounds. The special sensitivity of dopaminergic neurons seems to be related to the endogenous dopaminergic content, since it is overcome by dopamine depletion. Compounds that activate microglia or induce inflammation have similar effects to LPS. This model suggest that inflammation is an important component of the degeneration of the nigrostriatal dopaminergic system, probably also in PD. Anti-inflammatory treatments could be useful to prevent or slow down the rate of dopaminergic degeneration in this disease.

Peripheral inflammation increases the damage in animal models of nigrostriatal dopaminergic neurodegeneration: possible implication in Parkinson's disease incidence

Inflammatory processes described in Parkinson's disease (PD) and its animal models appear to be important in the progression of the pathogenesis, or even a triggering factor. Here we review that peripheral inflammation enhances the degeneration of the nigrostriatal dopaminergic system induced by different insults; different peripheral inflammations have been used, such as IL-1β and the ulcerative colitis model, as well as insults to the dopaminergic system such as 6-hydroxydopamine or lipopolysaccharide. In all cases, an increased loss of dopaminergic neurons was described; inflammation in the substantia nigra increased, displaying a great activation of microglia along with an increase in the production of cytokines such as IL-1β and TNF-α. Increased permeability or disruption of the BBB, with overexpression of the ICAM-1 adhesion molecule and infiltration of circulating monocytes into the substantia nigra, is also involved, since the depletion of circulating monocytes prevents the effects of peripheral inflammation. Data are reviewed in relation to epidemiological studies of PD.

Developmental exposure to manganese increases adult susceptibility to inflammatory activation of glia and neuronal protein nitration

Chronic exposure to manganese (Mn) produces a neurodegenerative disorder affecting the basal ganglia characterized by reactive gliosis and expression of neuroinflammatory genes including inducible nitric oxide synthase (NOS2). Induction of NOS2 in glial cells causes overproduction of nitric oxide (NO) and injury to neurons that is associated with parkinsonian-like motor deficits. Inflammatory activation of glia is believed to be an early event in Mn neurotoxicity, but specific responses of microglia and astrocytes to Mn during development remain poorly understood. In this study, we investigated the effect of juvenile exposure to Mn on the activation of glia and production of NO in C57Bl/6J mice, postulating that developmental Mn exposure would lead to heightened sensitivity to gliosis and increased expression of NOS2 in adult mice exposed again later in life. Immunohistochemical analysis indicated that Mn exposure caused increased activation of both microglia and astrocytes in the striatum (St), globus pallidus (Gp), and substantia nigra pars reticulata (SNpr) of treated mice compared with controls. More robust activation of microglia was observed in juveniles, whereas astrogliosis was more prominent in adult mice preexposed during development. Co-immunofluorescence studies demonstrated increased expression of NOS2 in glia located in the Gp and SNpr. Additionally, greater increases in the level of 3-nitrotyrosine protein adducts were detected in dopamine- and cAMP-regulated phosphoprotein-32-positive neurons of the St of Mn-treated adult mice preexposed as juveniles. These data indicate that subchronic exposure to Mn during development leads to temporally distinct patterns of glial activation that result in elevated nitrosative stress in distinct populations of basal ganglia neurons.

Zocor Forte (simvastatin) has a neuroprotective effect against LPS striatal dopaminergic terminals injury, whereas against MPP+ does not

Due to their potential role in preventing further deterioration of Parkinson's disease, anti-inflammatory strategies have attracted great interest. In this context, some studies point out the possible protective effect of anti-inflammatory compounds against the in vivo degeneration of dopaminergic neurons produced by lipopolysaccharide (LPS)-induced inflammatory processes and others. We have investigated the effect of the treatment of Zocor Forte (simvastatin) in LPS and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurodegenerative models to identify neuroprotective drugs for Parkinson's disease. We have perfused different concentrations of LPS or 1 mM 1-methyl-4-phenylpyridinium ion (MPP+) in the rat's striatum, 24 h after implanting a brain microdialysis probe, both with and without Zocor Forte (simvastatin) treatment. Results show that LPS perfusion produced a decrease in the basal release of dopamine. Forty-eight hours after implanting the probe, we have perfused 1 mM MPP+ to check the integrity of the dopaminergic terminals present around the cannula. Our model to study toxicity in the striatal dopaminergic terminals suggests that Zocor Forte (simvastatin) could prevent the neurotoxic damage produced by LPS, but not that produced by MPP+.

The intranigral injection of tissue plasminogen activator induced blood-brain barrier disruption, inflammatory process and degeneration of the dopaminergic system of the rat

Tissue-type plasminogen activator (tPA) is the only drug approved for the treatment of thromboembolic stroke, but it might lead to some neurotoxic side effects. tPA is a highly specific serine proteinase, one of the two principal plasminogen activators and one of the three trypsin-like serine proteinases of the tissue kallikrein family. We have observed that tPA injection in the SN leads to the degeneration of the dopaminergic neurons in a dose-dependent manner, without affecting the GABAergic neurons. We also found that tPA injected in the substantia nigra of rats produced the disruption of the blood-brain barrier (BBB) integrity, the induction of microglial activation, the loss of astroglia and the expression of aquaporin 4 (AQP4), as well as an increase in the expression of NMDA receptors and the brain derived neurothrophic factor (BDNF). All these effects, along with the changes produced in the phosphorylated forms of several MAP kinases and the transcription factor CREB, and the increase in the expression of nNOS and iNOS observed under our experimental conditions, could be involved in the loss of dopaminergic neurons.

Detection, purification and identification of an endogenous inhibitor of L-Dopa decarboxylase activity from human placenta

An endogenous inhibitor of L-Dopa decarboxylase activity was identified and purified from human placenta. The endogenous inhibitor of L-Dopa decarboxylase (Ddc) was localized in the membrane fraction of placental tissue. Treatment of membranes with phosphatidylinositol-specific phospholipase C or proteinase K did not affect membrane-associated Ddc inhibitory activity, suggesting that a population of the inhibitor is embedded within membranes. Purification was achieved by extraction from a nondenaturing polyacrylamide gel. The purification scheme resulted in the isolation of a single 35 kDa band, bearing L-Dopa decarboxylase inhibitory activity. The purified inhibitor was identified as Annexin V. The elucidation of the biological importance of the presence of an L-Dopa decarboxylase activity inhibitor in normal human tissues could provide us with new information leading to the better understanding of the biological pathways that Ddc is involved in.

Thrombin-induced microglial activation contributes to the degeneration of nigral dopaminergic neurons in vivo

OBJECTIVE

To evaluate the role of thrombin-activated microglia in the neurodegeneration of nigral dopaminergic neurons in the rat substantia nigra (SN) in vivo.

METHODS

After stereotaxic thrombin injection into unilateral SN of rats, immunostaining, reverse transcription polymerase chain reaction (RT-PCR) and biochemical methods were used to observe tyrosine hydroxylase (TH) immunoreactive positive cells, microglia activation, nitric oxide (NO) amount and inducible nitric-oxide synthase (iNOS) expression.

RESULTS

(1) Selective damage to dopaminergic neurons was produced after thrombin injection, which was evidenced by loss of TH immunostaining in time-dependent manner; (2) Strong microglial activation was observed in the SN; (3) RT-PCR demonstrated the early and transient expression of neurotoxic factors iNOS mRNA in the SN. Immunofluorescence results found that thrombin induced expression of iNOS in microglia. The NO production in the thrombin-injected rats was significantly higher than that of controls (P < 0.05).

CONCLUSION

Thrombin intranigral injection can injure the dopaminergic neurons in the SN. Thrombin-induced microglia activation precedes dopaminergic neuron degeneration, which suggest that activation of microglia and release of NO may play important roles in dopaminergic neuronal death in the SN.

Simvastatin prevents the inflammatory process and the dopaminergic degeneration induced by the intranigral injection of lipopolysaccharide

Anti-inflammatory strategies have attracted much interest for their potential to prevent further deterioration of Parkinson's disease. Recent experimental and clinical evidence indicate that statins - extensively used in medical practice as effective lipid-lowering agents - have also anti-inflammatory effects. In this study, we investigated the influence of simvastatin on the degenerative process of the dopaminergic neurons of the rat following intranigral injection of lipopolysaccharide (LPS), a potent inductor of inflammation that we have previously used as an animal model of Parkinson's disease. We evaluated TH positive neurons, astroglial, and microglial populations and found that simvastatin prevented the inflammatory processes, as the induction of interleukin-1beta, tumor necrosis factor-alpha, and iNOS and the consequent dopaminergic degeneration induced by LPS. Moreover, simvastatin produced the activation of the neurotrophic factor BDNF, along with the prevention of the oxidative damage to proteins. Moreover, it also prevents the main changes produced by LPS on different mitogen-activated protein kinases, featured as increases of P-c-Jun N-terminal protein kinase, P-extracellular signal-regulated kinase, p-38, and P-glycogen synthase kinase and the decrease of the promotion of cell survival signals such as cAMP response element-binding protein and Akt. Our results suggest that statins could delay the progression of dopaminergic degeneration in disorders involving inflammatory processes.

Protease-activated receptors in the brain: receptor expression, activation, and functions in neurodegeneration and neuroprotection

Protease-activated receptors (PARs) are G protein-coupled receptors that regulate the cellular response to extracellular serine proteases, like thrombin, trypsin, and tryptase. The PAR family consists of four members: PAR-1, -3, and -4 as thrombin receptors and PAR-2 as the trypsin/tryptase receptor, which are abundantly expressed in the brain throughout development. Recent evidence has supported the direct involvement of PARs in brain development and function. The expression of PARs in the brain is differentially upregulated or downregulated under pathological conditions in neurodegenerative disorders, like Parkinson's disease, Alzheimer's disease, multiple sclerosis, stroke, and human immunodeficiency virus-associated dementia. Activation of PARs mediates cell death or cell survival in the brain, depending on the amplitude and the duration of agonist stimulation. Interference or potentiation of PAR activation is beneficial in animal models of neurodegenerative diseases. Therefore, PARs mediate either neurodegeneration or neuroprotection in neurodegenerative diseases and represent attractive therapeutic targets for treatment of brain injuries. Here, we review the abnormal expression of PARs in the brain under pathological conditions, the functions of PARs in neurodegenerative disorders, and the molecular mechanisms involved.

Exacerbation of Dopaminergic Terminal Damage in a Mouse Model of Parkinson's Disease by the G-Protein-Coupled Receptor Protease-Activated Receptor 1

Protease-activated receptor 1 (PAR1) is a G-protein-coupled receptor activated by serine proteases and expressed in astrocytes, microglia, and specific neuronal populations. We examined the effects of genetic deletion and pharmacologic blockade of PAR1 in the mouse 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease, a neurodegenerative disease characterized by nigrostriatal dopamine damage and gliosis. After MPTP injection, PAR1-/- mice showed significantly higher residual levels of dopamine, dopamine transporter, and tyrosine hydroxylase and diminished microgliosis compared with wild-type mice. Comparable levels of dopaminergic neuroprotection from MPTP-induced toxicity were obtained by infusion of the PAR1 antagonist, BMS-200261 into the right lateral cerebral ventricle. MPTP administration caused changes in the brain protease system, including increased levels of mRNA for two PAR1 activators, matrix metalloprotease-1 and Factor Xa, suggesting a mechanism by which MPTP administration could lead to overactivation of PAR1. We also report that PAR1 is expressed in human substantia nigra pars compacta glia as well as tyrosine hydroxylase-positive neurons. Together, these data suggest that PAR1 might be a target for therapeutic intervention in Parkinson's disease.

The effect of thrombin on a 6-hydroxydopamine model of Parkinson's disease depends on timing

Recent results in animal models suggest that thrombin may modulate brain injury in Parkinson's disease (PD). High doses of thrombin ( approximately 20U) can damage dopaminergic neurons, while we have found that low dose thrombin (1U), given several days before a brain insult (thrombin preconditioning), is protective in models of PD and stroke. However, the effects of such low levels of thrombin at the time of, or after, exposure to the dopamine neurotoxin 6-hydroxydopamine (6-OHDA) have not been examined and are the focus of this study. In the first set of experiments, rats received co-administration of thrombin (1U) or saline and 6-OHDA (5microg) into the medial forebrain bundle. 6-OHDA+thrombin resulted in striking increases in behavioral deficits, compared to 6-OHDA+saline. Similarly, co-administration of an agonist to protease-activated receptor (PAR)-1, a thrombin receptor, also resulted in significantly greater behavioral deficits. In a second set of experiments, thrombin (1U) or saline was administered 1 or 7 days after 6-OHDA to determine the effects of thrombin after 6-OHDA. Surprisingly, the rats that received saline had strikingly increased behavioral and neurochemical deficits resulting from the 6-OHDA lesion, while delayed thrombin administration prevented this effect. The results indicate that thrombin has differential effects in the 6-OHDA model, dependent on the time of administration. The ability of a second cannula insertion with saline infusion to increase dramatically deficits raises questions as to what role physical injury to already susceptible cells might play in the pathogenesis of some cases of PD.

Blood-brain barrier disruption induces in vivo degeneration of nigral dopaminergic neurons

We have evaluated the possibility that changes in the vascular system may constitute a contributing factor for the death of nigral dopaminergic neurons in Parkinson's disease. Thus, we have employed intranigral injections of vascular endothelial growth factor (VEGF), the most potent inducer of blood-brain barrier (BBB) permeability. A single dose of 1 mug of VEGF, chosen from a dose-response study, highly disrupted the BBB in the ventral mesencephalon in a time-dependent manner. A strong regional correlation between BBB disruption and loss of tyrosine hydroxylase-positive neurons was evident. Moreover, Fluoro-Jade B labelling showed the presence of dying neurons in the substantia nigra in response to VEGF injection. High number of TUNEL-positive nuclei was observed in this area along with activation of caspase 3 within nigral dopaminergic neurons. Analysis of the glial population demonstrated a strong inflammatory response and activation of astroglia in response to BBB disruption. We conclude that disruption of the BBB may be a causative factor for degeneration of nigral dopaminergic neurons.

Interferon-γ induced nitric oxide mediates in vitro neuronal damage by Trypanosoma cruzi-infected macrophages

Neuronal lesions and peripheral denervation in Chagas' disease are related to local inflammation; however, the pathogenic mechanisms of neuronal lesions in the heart and megavisceras are still unclear. We investigated the involvement of nitric oxide (NO) on neuronal lesion in co-cultures of neurons and macrophages. Trypanosoma cruzi-infected and interferon-gamma (IFN-gamma)-activated co-cultures of neurons and wild-type (WT) macrophages showed significant reduction of both neuronal survival and neurite density. These findings correlated with the levels of NO and the expression of inducible nitric oxide synthase (iNOS). Accordingly, neuronal survival rate in the co-cultures was recovered to control levels by treatment of the cultures with the iNOS inhibitor, aminoguanidine. Moreover, neither neuronal survival nor the neurite density was affected in the co-cultures when the macrophages were harvested from iNOS-deficient mice. These results demonstrate that iNOS-derived NO is the major molecule involved in neuronal damage mechanism in our in vitro model of Chagas' disease neuropathology.

Resveratrol protects dopaminergic neurons in midbrain slice culture from multiple insults

Increasing lines of evidence show that resveratrol, a polyphenol compound contained in several dietary products, exhibits cytoprotective actions. Notably, resveratrol activates sirtuin family of NAD-dependent histone deacetylases implicated in regulation of various cellular processes including gene transcription, DNA repair and apoptosis. Here we examined neuroprotective effect of resveratrol on dopaminergic neurons in organotypic midbrain slice culture. Resveratrol and quercetin, another sirtuin-activating polyphenol, prevented the decrease of dopaminergic neurons and the increase of propidium iodide uptake into slices induced by a dopaminergic neurotoxin 1-methyl-4-phenyl pyridinium (MPP(+)). Resveratrol also provided concentration-dependent neuroprotective effects against sodium azide, a mitochondrial complex IV inhibitor, and thrombin (EC number 3.4.21.5), a microglia-activating agent. Sirtuin inhibitors such as nicotinamide and sirtinol did not attenuate the protective effect of resveratrol against MPP(+) cytotoxicity. Instead, we found that resveratrol prevented accumulation of reactive oxygen species, depletion of cellular glutathione, and cellular oxidative damage induced by MPP(+), suggesting involvement of antioxidative properties in the neuroprotective action of resveratrol. On the other hand, resveratrol as well as a sirtuin activator NAD inhibited dopaminergic neurotoxicity of a DNA alkylating agent, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Moreover, MNNG-induced increase in acetylation of p53, a representative target of sirtuin deacetylase activity, was suppressed by resveratrol. These results indicate that resveratrol can exert neuroprotective actions in dopaminergic neurons. Either antioxidative activity or sirtuin-activating potential may play an important role in the neuroprotectice actions of resveratrol against different kinds of insults.

Protease-activated receptor-1 mediates protection elicited by thrombin preconditioning in a rat 6-hydroxydopamine model of Parkinson's disease

The etiology of Parkinson's disease remains poorly understood, and current treatment options do not slow disease progression. Recently, chemical (thrombin) preconditioning (TPC) was found to be protective in a 6-hydroxydopamine (6-OHDA) model of the disease. It is important to understand the mechanisms behind these thrombin-induced protective effects. The current study was conducted in the rat to determine whether the protective effects of TPC are mediated via activation of protease-activated receptors (PARs). Preconditioning with specific local infusion of agonist peptides for PAR-1 and PAR-4 3 days before unilateral 6-OHDA administration (10 microg into the medial forebrain bundle) was tested. In addition, co-administration of a PAR-1 antagonist with TPC was examined. In a neurobehavioral assessment battery, PAR-1 agonist preconditioning provided protection in a vibrissae-elicited forelimb placing test, a forelimb-use asymmetry test, and a corner turn test. In addition, inclusion of a PAR-1 antagonist prevented the protective effects elicited by TPC. In contrast to the effects of the PAR-1 agonist, PAR-4 agonist preconditioning afforded no such protection. Indeed, in a lower-dose model of 6-OHDA (5 microg), PAR-4 preconditioning significantly increased behavioral deficits. These results indicate that the protective effects of TPC in this model are mediated through PAR-1 activation. Neither the effects of PAR-1 nor TPC on later 6-OHDA-induced behavioral deficits appeared to be mediated through (DA) content sparing. Further mechanistic studies on the actions of PAR-1 and PAR-4 as detrimental in experimental models of Parkinson's disease are warranted.

Modulation of thrombin-induced neuroinflammation in BV-2 microglia by carbon monoxide-releasing molecule 3

Carbon monoxide-releasing molecules are emerging as a new class of pharmacological agents that regulate important cellular function by liberating CO in biological systems. Here, we examined the role of carbon monoxide-releasing molecule 3 (CORM-3) in modulating neuroinflammatory responses in BV-2 microglial cells, considering its practical application as a novel therapeutic alternative in the treatment of stroke. BV-2 microglia cells were incubated for 24 h in normoxic conditions with thrombin alone or in combination with interferon-gamma to simulate the inflammatory response. Cells were also subjected to 12 h of hypoxia and reoxygenated for 24 h in the presence of thrombin and interferon-gamma. In both set of experiments, the anti-inflammatory action of CORM-3 was evaluated by assessing its effect on nitric oxide production (nitrite levels) and tumor necrosis factor (TNF)-alpha release. CORM-3 (75 microM) did not show any cytotoxicity and markedly attenuated the inflammatory response to thrombin and interferon-gamma in normoxia and to a lesser extent in hypoxia as evidenced by a reduction in nitrite levels and TNF-alpha production. Inactive CORM-3, which does not liberate CO and is used as a negative control, failed to prevent the increase in inflammatory mediators. Blockade of endogenous CO production by tin protoporphyrin-IX did not change the anti-inflammatory activity of CORM-3, suggesting that CO liberated from the compound is responsible for the observed effects. In addition, inhibition of the mitogen-activated protein kinases phosphatidyl inositol 3 kinase and extracellular signal-regulated kinase amplified the anti-inflammatory effect of CORM-3. These results suggest that the anti-inflammatory activity of CORM-3 could be exploited to mitigate microglia activity in stroke and other neuroinflammatory diseases.

Thrombin induces neurodegeneration and microglial activation in the cortex in vivo and in vitro: proteolytic and non-proteolytic actions

The present study evaluated the role of thrombin and its receptors in neurodegeneration and microglial activation. Immunocytochemical evidence indicated that intracortical injection of thrombin resulted in a significant loss of neurons and the activation of microglia in the rat cortex in vivo. Reverse transcription PCR and double-label immunocytochemistry further demonstrated the early and transient expression of pro-inflammatory cytokines and neurotoxic factors as well as their colocalization within activated microglia. The thrombin-induced loss of cortical neurons was partially blocked by N(G)-nitro-L-arginine methyl ester hydrochloride, a nitric oxide synthase inhibitor, and by NS-398, a cyclooxygenase-2 inhibitor, indicating that the activation of microglia is involved in the neurotoxicity of thrombin in the cortex in vivo. In addition, thrombin activated cortical microglia in culture, as indicated by the expression of several pro-inflammatory cytokines and produced cell death in microglia-free, neuron-enriched cortical cultures. However, agonist peptides for thrombin receptors, including protease-activated receptor-1 (SFLLRN), -3 (TFRGAP), and -4 (GYPGKF), failed to activate microglia and were not neurotoxic in culture. Intriguingly, morphological and biochemical evidence indicated that thrombin-induced neurotoxicity but not microglial activation was prevented by hirudin, a specific inhibitor of thrombin. Collectively, the present data suggest that a non-proteolytic activity of thrombin activates microglia and that the proteolytic activity mediates its neurotoxicity.

Nitric oxide-producing microglia mediate thrombin-induced degeneration of dopaminergic neurons in rat midbrain slice culture

Activated microglia are considered to play important roles in degenerative processes of midbrain dopaminergic neurons. Here we examined mechanisms of neurotoxicity of thrombin, a protease known to trigger microglial activation, in organotypic midbrain slice cultures. Thrombin induced a progressive decline in the number of dopaminergic neurons, an increase in nitric oxide (NO) production, and whole tissue injury indicated by lactate dehydrogenase release and propidium iodide uptake. Microglia expressed inducible NO synthase (iNOS) in response to thrombin, and inhibition of iNOS rescued dopaminergic neurons without affecting whole tissue injury. Inhibitors of mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulated kinase (ERK), p38 MAPK and c-Jun N-terminal kinase (JNK) attenuated thrombin-induced iNOS induction and dopaminergic cell death. Whole tissue injury was also attenuated by inhibition of ERK and p38 MAPK. Moreover, depletion of resident microglia from midbrain slices abrogated thrombin-induced NO production and dopaminergic cell death, but did not inhibit tissue injury. Finally, antioxidative drugs prevented thrombin-induced dopaminergic cell death without affecting whole tissue injury. Hence, NO production resulting from MAPK-dependent microglial iNOS induction is a crucial event in thrombin-induced dopaminergic neurodegeneration, whereas damage of other midbrain cells is MAPK-dependent but is NO-independent.

Upregulation of protease-activated receptor-1 in astrocytes in Parkinson disease: astrocyte-mediated neuroprotection through increased levels of glutathione peroxidase

In the present study, we investigated the expression of protease-activated receptors (PARs), receptors for thrombin, in substantia nigra pars compacta (SNpc) of Parkinson disease (PD) brains and cultures of human neurons, astrocytes, oligodendrocytes, and microglia as determined by immunocytochemistry and reverse transcriptase-polymerase chain reaction (RT-PCR). Expression of PAR-1 was demonstrated only in glial fibrillary acidic protein-positive astrocytes in SNpc, and the number of astrocytes expressing PAR-1 increased in SNpc of PD as compared with nonneurologic control brain. Immunoreactivity for thrombin and prothrombin was stronger in astrocytes and the vessel walls in SNpc of PD brains. PAR-1 was expressed in human astrocytes and neurons, but not in oligodendrocytes or microglia as determined by RT-PCR. We investigated thrombin-mediated activation of human astrocytes. Thrombin treatment activates human astrocytes and induces morphologic change and a marked increase in proliferation of astrocytes. Increased expression of glial cell line-derived growth factor and glutathione peroxidase (GPx) but no change in the expression of nerve growth factor and inflammatory cytokines/chemokine (IL-1beta, IL-6, IL-8, MCP-1) was found in thrombin/PAR-activated astrocytes. Next, we studied the neuroprotective effect exerted by thrombin-activated astrocytes in human cerebral neuron x human neuroblastoma hybrid neurons. Although thrombin showed neurotoxicity against human hybrid neurons in a dose-dependent manner, the conditioned media derived from thrombin-pretreated astrocyte cultures promoted the survival of human hybrid neurons. The protective effect was completely inhibited with a GPx inhibitor, mercaptosuccinic acid, indicating that GPx released from thrombin/PAR-activated astrocytes is responsible for neuroprotection of hybrid neurons against thrombin cytotoxicity. The present study suggests that the increased expression of PAR-1 in astrocytes in SNpc of PD brain is the restorative move taken by the brain to provide neuroprotection against neuronal degeneration and cell death of dopaminergic neurons caused by noxious insults during the progression of PD pathology.

Thrombin-induced oxidative stress contributes to the death of hippocampal neurons in vivo: role of microglial NADPH oxidase

The present study investigated whether thrombin, a potent microglial activator, can induce reactive oxygen species (ROS) generation through activation of microglial NADPH oxidase and if this may contribute to oxidative damage and consequent neurodegeneration. Seven days after intrahippocampal injection of thrombin, Nissl staining and immunohistochemistry using the neuronal-specific nuclear protein NeuN revealed a significant loss in hippocampal CA1 neurons. In parallel, thrombin-activated microglia, assessed by OX-42 and OX-6 immunohistochemistry, and ROS production, assessed by hydroethidine histochemistry, were observed in the hippocampal CA1 area in which degeneration of hippocampal neurons occurred. Reverse transcription-PCR at various time points after thrombin administration demonstrated an early and transient expression of inducible nitric oxide synthase (iNOS) and several proinflammatory cytokines. Western blot analysis and double-label immunohistochemistry showed an increase in the expression of and the localization of iNOS within microglia. Additional studies demonstrated that thrombin induced the upregulation of membrane (gp91(phox)) and cytosolic (p47(phox) and p67(phox)) components, translocation of cytosolic proteins (p47(phox), p67(phox), and Rac1) to the membrane, and p67(phox) expression of the NADPH oxidase in microglia in the hippocampus in vivo, indicating the activation of NADPH oxidase. The thrombin-induced oxidation of proteins and loss of hippocampal CA1 neurons were partially inhibited by an NADPH oxidase inhibitor and by an antioxidant. To our knowledge, the present study is the first to demonstrate that thrombin-induced neurotoxicity in the hippocampus in vivo is caused by microglial NADPH oxidase-mediated oxidative stress. This suggests that thrombin inhibition or enhancing antioxidants may be beneficial for the treatment of neurodegenerative diseases, such as Alzheimer's disease, that are associated with microglial-derived oxidative damage.

Dopamine changes in a rat model of intracerebral hemorrhage

Recent case reports suggest that dopamine (DA) replacement may reduce behavioral deficits resulting from hemorrhages along the nigrostriatal tract. In the rat model of intracerebral hemorrhage (ICH), behavioral deficits are first evident on day 1, with return to near control levels by day 28. The current study was conducted to determine if striatal dopamine alterations are correlated with behavioral deficits. Gamma-aminobutyric acid (GABA) levels were measured to determine selectivity. Striatal DA, DA metabolites, and GABA were determined at days 1, 3, 7, and 28 after ICH by high-pressure liquid chromatography with electrochemical detection. ICH resulted in significant increases above control in DA contralateral to the lesion (177 to 361% above control, days 1 to 28). There were also significant, but much less marked changes in GABA. In the ipsilateral striatum, significant DA increases also occurred (approximately 200%, at day 3 and approximately 275% day 28), while GABA alterations were not significant. These results indicate that the striatal DA system is selectively altered after ICH. Further studies will be needed to determine if regional dopamine alterations occur relative to the location of the hematoma.

Thrombin-activated microglia contribute to death of dopaminergic neurons in rat mesencephalic cultures: dual roles of mitogen-activated protein kinase signaling pathways

This study evaluated the role of thrombin-activated microglia in the neurodegeneration of mesencephalic cultures. Immunocytochemical and biochemical evidence indicated that in co-cultures consisting of rat cortical microglia and mesencephalic neurons, thrombin led to nonselective loss of mesencephalic neurons. Accompanying neurodegeneration, microglial activation was obvious, evidenced by expression of tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, IL-1beta, inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2) and by increasing production of TNF-alpha and nitric oxide (NO). In mesencephalic neurons treated with conditioned media (CM) taken from thrombin-activated microglia, the number of dopaminergic neurons was significantly attenuated. The neurotoxicity of the CM was diminished when it was derived from microglia co-treated with thrombin and either an extracellular signal-regulated kinase 1/2 (ERK1/2) pathway inhibitor (PD98059) or a p38-mitogen-activated protein kinase (p38-MAPK) inhibitor (SB203580). Moreover, jun N-terminal kinase (JNK) and p38-MAPK were activated in mesencephalic neurons treated with CM of thrombin-activated microglia. Inhibition of JNK and p38-MAPK rescued the dopaminergic neurons. Collectively, these results indicate that thrombin-activated microglia induce neurodegeneration in cultured mesencephalic neurons and that the MAPKs actively participate in both microglial activation and neurodegeneration. The present data carefully suggest that microglial activation triggered by thrombin may be involved in the neuropathological processes of dopaminergic neuronal cell death that occur in Parkinson's disease.