Immune function of microglia

Cell-cell communication induces random spikes of spontaneous calcium oscillations in multi-BV-2 microglial cells

As the first and main form of active immune defense in the central nervous system, microglial cells usually exhibit complicated intracellular calcium (Ca²⁺) activity that can regulate the downstream components of signaling cascades. In the present work, spontaneous oscillations of the cytosolic calcium concentration ([Ca²⁺]c) in multi-BV-2 microglial cells were observed by video microscopy. These cells exhibited random spikes of Ca²⁺ oscillations. Cross-correlation analysis of the temporal dependence of the oscillations indicated the existence of cell-cell communication mediated by extracellular messengers. Numerical simulations based on a simple mathematical model suggested that these communications could induce random spikes of spontaneous Ca oscillations in the multi-cell system. Short-time imaging analysis of random spikes in different regions of a single cell showed that spontaneous Ca²⁺ oscillations resulted from Ca²⁺ wave generated by other cells as well as from calcium elevation inside the cell. Taken together, our data demonstrate that cell-cell communication existed between the BV-2 microglial cells in vitro and further resulted in the random spikes of spontaneous Ca²⁺ oscillations.

Pesticides, microglial NOX2, and Parkinson's disease

Accumulating evidence indicates that pesticide exposure is associated with an increased risk for developing Parkinson's disease (PD). Several pesticides known to damage dopaminergic (DA) neurons, such as paraquat, rotenone, lindane, and dieldrin also demonstrate the ability to activate microglia, the resident innate immune cell in the brain. While each of these environmental toxicants may impact microglia through unique mechanisms, they all appear to converge on a common final pathway of microglial activation: NADPH oxidase 2 (NOX2) activation. This review will detail the role of microglia in selective DA neurotoxicity, highlight what is currently known about the mechanism of microglial NOX2 activation in these key pesticides, and describe the importance for DA neuron survival and PD etiology.

Interferon γ-dependent migration of microglial cells in the retina after systemic cytomegalovirus infection

Microglial cells are the resident macrophages of the central nervous system and participate in both innate and adaptive immune responses but can also lead to exacerbation of neurodegenerative pathologies after viral infections. Microglia in the outer layers of the retina and the subretinal space are thought to be involved in retinal diseases where low-grade chronic inflammation and oxidative stress play a role. This study investigated the effect of systemic infection with murine cytomegalovirus on the distribution and dynamics of retinal microglia cells. Systemic infection with murine cytomegalovirus elicited a significant increase in the number of microglia in the subretinal space and an accumulation of iris macrophages, along with morphological signs of activation. Interferon γ (IFN-γ)-deficient mice failed to induce changes in microglia distribution. Bone marrow chimera experiments confirmed that microglial cells in the subretinal space were not recruited from the circulating monocyte pool, but rather represented an accumulation of resident microglial cells from within the retina. Our results demonstrate that a systemic viral infection can lead to IFN-γ-mediated accumulation of microglia into the outer retinal layers and offer proof of concept that systemic viral infections alter the ocular microenvironment and therefore, may influence the course of diseases such as macular degeneration, diabetic retinopathy, or autoimmune uveitis, where low-grade inflammation is implicated.

Role of neuroinflammation in morphine tolerance: effect of tumor necrosis factor-α

Opioids have been used as potent analgesics in clinics for decades; however, their long-term administration leads to tolerance. Two possible mechanisms for drug tolerance are postulated as within-system and between-systems adaptation. The within-system tolerance is involved in the signal transduction of opioid receptors, including downregulation of opioid receptors, uncoupling of G-protein from opioid receptors, and β-arrestin recruitment to opioid receptors, which causes receptor desensitization and internalization/endocytosis. The between-systems tolerance comprehends the glutamatergic receptor system and glial activation with the release of proinflammatory cytokines, and thus the analgesic effect of morphine is reduced. Tumor necrosis factor-α (TNF-α) is a vital proinflammatory cytokine and exerts either a neurotoxic or neuroprotective effect on different diseases of the central nervous system. TNF-α has also been demonstrated to correlate with neuronal plasticity via activation of spinal glial cells and enhancement of glutamatergic transmission. Previous studies had revealed an increased expression of TNF-α in morphine tolerance. This review article focuses on the role of TNF-α in neuroinflammation and the glutamatergic receptor system in morphine tolerance. It may provide another adjuvant therapy for morphine tolerance, which extends the effectiveness of opioids in clinical pain management.

Phosphatidylinositol 4-phosphate 5-kinase α facilitates Toll-like receptor 4-mediated microglial inflammation through regulation of the Toll/interleukin-1 receptor domain-containing adaptor protein (TIRAP) location

Phosphatidylinositol (PI) 4,5-bisphosphate (PIP(2)), generated by PI 4-phosphate 5-kinase (PIP5K), regulates many critical cellular events. PIP(2) is also known to mediate plasma membrane localization of the Toll/IL-1 receptor domain-containing adaptor protein (TIRAP), required for the MyD88-dependent Toll-like receptor (TLR) 4 signaling pathway. Microglia are the primary immune competent cells in brain tissue, and TLR4 is important for microglial activation. However, a functional role for PIP5K and PIP(2) in TLR4-dependent microglial activation remains unclear. Here, we knocked down PIP5Kα, a PIP5K isoform, in a BV2 microglial cell line using stable expression of lentiviral shRNA constructs or siRNA transfection. PIP5Kα knockdown significantly suppressed induction of inflammatory mediators, including IL-6, IL-1β, and nitric oxide, by lipopolysaccharide. PIP5Kα knockdown also attenuated signaling events downstream of TLR4 activation, including p38 MAPK and JNK phosphorylation, NF-κB p65 nuclear translocation, and IκB-α degradation. Complementation of the PIP5Kα knockdown cells with wild type but not kinase-dead PIP5Kα effectively restored the LPS-mediated inflammatory response. We found that PIP5Kα and TIRAP colocalized at the cell surface and interacted with each other, whereas kinase-dead PIP5Kα rendered TIRAP soluble. Furthermore, in LPS-stimulated control cells, plasma membrane PIP(2) increased and subsequently declined, and TIRAP underwent bi-directional translocation between the membrane and cytosol, which temporally correlated with the changes in PIP(2). In contrast, PIP5Kα knockdown that reduced PIP(2) levels disrupted TIRAP membrane targeting by LPS. Together, our results suggest that PIP5Kα promotes TLR4-associated microglial inflammation by mediating PIP(2)-dependent recruitment of TIRAP to the plasma membrane.

The role of innate and adaptive immunity in Parkinson's disease

In recent years, inflammation has become implicated as a major pathogenic factor in the onset and progression of Parkinson's disease. Understanding the precise role for inflammation in PD will likely lead to understanding of how sporadic disease arises. In vivo evidence for inflammation in PD includes microglial activation, increased expression of inflammatory genes in the periphery and in the central nervous system (CNS), infiltration of peripheral immune cells into the CNS, and altered composition and phenotype of peripheral immune cells. These findings are recapitulated in various animal models of PD and are reviewed herein. Furthermore, we examine the potential relevance of PD-linked genetic mutations to altered immune function and the extent to which environmental exposures that recapitulate these phenotypes, which may lead to sporadic PD through similar mechanisms. Given the implications of immune system involvement on disease progression, we conclude by reviewing the evidence supporting the potential efficacy of immunomodulatory therapies in PD prevention or treatment. There is a clear need for additional research to clarify the role of immunity and inflammation in this chronic, neurodegenerative disease.

Targeted drug delivery to treat pain and cerebral hypoxia

Limited drug penetration is an obstacle that is often encountered in treatment of central nervous system (CNS) diseases including pain and cerebral hypoxia. Over the past several years, biochemical characteristics of the brain (i.e., tight junction protein complexes at brain barrier sites, expression of influx and efflux transporters) have been shown to be directly involved in determining CNS permeation of therapeutic agents; however, the vast majority of these studies have focused on understanding those mechanisms that prevent drugs from entering the CNS. Recently, this paradigm has shifted toward identifying and characterizing brain targets that facilitate CNS drug delivery. Such targets include the organic anion-transporting polypeptides (OATPs in humans; Oatps in rodents), a family of sodium-independent transporters that are endogenously expressed in the brain and are involved in drug uptake. OATP/Oatp substrates include drugs that are efficacious in treatment of pain and/or cerebral hypoxia (i.e., opioid analgesic peptides, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors). This clearly suggests that OATP/Oatp isoforms are viable transporter targets that can be exploited for optimization of drug delivery to the brain and, therefore, improved treatment of CNS diseases. This review summarizes recent knowledge in this area and emphasizes the potential that therapeutic targeting of OATP/Oatp isoforms may have in facilitating CNS drug delivery and distribution. Additionally, information presented in this review will point to novel strategies that can be used for treatment of pain and cerebral hypoxia.

Establishment and characterization of primary adult microglial culture in mice

Microglial cells account for approximately 12-15 % of the cells in the central nervous system (CNS). Microglial cells are polarized by pathological stimuli such as cytokines, chemokines, and growth factors, and play important roles in the deterioration and repair of the CNS. Here, we established cultures of primary microglial cells isolated from the brains of adult C57/BL6 mice using Percoll density gradients. The cells were cultured and stained with antibodies against CD11b, glial fibrillary acidic protein, myelin basic protein and NeuN to determine microglial, astroglial, oligodendroglial, and neuronal cells respectively. Moreover, the cells were exposed to interferon-γ (IFNγ) plus interleukin-1β (IL-1β) or IL-4 for 24 h to demonstrate the activating phenotypes with inducible nitric oxide synthase (iNOS), Ym1, and Iba-1 immunoblotting. At least 95 % of the cultured cells were CD11b-positive and -negative for astroglial, neuronal, and oligodendrocyte markers. IFNγ plus IL-1β treatment resulted in classical activation, which was represented by an increase in iNOS. The cells also displayed alternative activation, which increased Ym1 when treated with IL-4. The present study indicates that the microglial cells isolated as described here are a useful tool for elucidating adult microglial function.

Blood-brain barrier integrity and glial support: mechanisms that can be targeted for novel therapeutic approaches in stroke

The blood-brain barrier (BBB) is a critical regulator of brain homeostasis. Additionally, the BBB is the most significant obstacle to effective CNS drug delivery. It possesses specific charcteristics (i.e., tight junction protein complexes, influx and efflux transporters) that control permeation of circulating solutes including therapeutic agents. In order to form this "barrier," brain microvascular endothelial cells require support of adjacent astrocytes and microglia. This intricate relationship also occurs between endothelial cells and other cell types and structures of the CNS (i.e., pericytes, neurons, extracellular matrix), which implies existence of a "neurovascular unit." Ischemic stroke can disrupt the neurovascular unit at both the structural and functional level, which leads to an increase in leak across the BBB. Recent studies have identified several pathophysiological mechanisms (i.e., oxidative stress, activation of cytokine-mediated intracellular signaling systems) that mediate changes in the neurovascular unit during ischemic stroke. This review summarizes current knowledge in this area and emphasizes pathways (i.e., oxidative stress, cytokine-mediated intracellular signaling, glial-expressed receptors/targets) that can be manipulated pharmacologically for i) preservation of BBB and glial integrity during ischemic stroke and ii) control of drug permeation and/or transport across the BBB. Targeting these pathways present a novel opportunity for optimization of CNS delivery of therapeutics in the setting of ischemic stroke.

Susceptibility of juvenile and adult blood-brain barrier to endothelin-1: regulation of P-glycoprotein and breast cancer resistance protein expression and transport activity

Background

P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) play a critical role in keeping neurotoxic substances from entering the brain. We and others have previously reported an impact of inflammation on the regulation of adult blood–brain barrier (BBB) efflux transporters. However, studies in children have not been done. From the pediatric clinical perspective, it is important to understand how the central nervous system (CNS) and BBB drug efflux transporters differ in childhood from those of adults under normal and inflammatory conditions. Therefore, we examined and compared the regulation of P-gp and BCRP expression and transport activity in young and adult BBB and investigated the molecular mechanisms underlying inflammatory responses.

Methods

Rats at postnatal day (P) P21 and P84, corresponding to the juvenile and adult stages of human brain maturation, respectively, were treated with endothelin-1 (ET-1) given by the intracerebroventricular (icv) route. Twenty-four hours later, we measured P-gp and BCRP protein expression in isolated brain capillary by immunoblotting as well as by transport activity in vivo by measuring the unbound drug partitioning coefficient of the brain (K_p,uu,brain) of known efflux transporter substrates administered intravenously. Glial activation was measured by immunohistochemistry. The release of cytokines/chemokines (interleukins-1α, 1-β (IL-1β), -6 (IL-6), -10 (IL-10), monocyte chemoattractant protein (MCP-1/CCL2), fractalkine and tissue inhibitor of metalloproteinases-1 (TIMP-1)) were simultaneously measured in brain and serum samples using the Agilent Technology cytokine microarray.

Results

We found that juvenile and adult BBBs exhibited similar P-gp and BCRP transport activities in the normal physiological conditions. However, long-term exposure of the juvenile brain to low-dose of ET-1 did not change BBB P-gp transport activity but tended to decrease BCRP transport activity in the juvenile brain, while a significant increase of the activity of both transporters was evidenced at the BBB in the adult brain. Moreover, juvenile and adult brain showed differences in their expression profiles of cytokines and chemokines mediated by ET-1.

Conclusions

BBB transporter activity during neuroinflammation differs between the juvenile and adult brains. These findings emphasize the importance of considering differential P-gp and BCRP transport regulation mechanisms between adult and juvenile BBB in the context of neuroinflammation.

Aging and the neurovascular unit

With the demonstration that acute recanalization of obstructed symptomatic cerebral arteries during ischemic stroke can result in substantial improvement in clinical outcome, the variability in clinical responses, and in hemorrhagic transformation, requires attention. This short review addresses the effect of aging and amyloid deposition disease on microvessel integrity, interactions within the neurovascular unit, cerebral tissue susceptibility to ischemic injury, and postischemic inflammation, and ultimately on the outcomes and safety of acute recanalization during ischemic stroke. Microvessels and neighboring neurons respond simultaneously to focal ischemia. The cellular components and matrix barriers of the neurovascular unit all respond to ischemia; however, their coordinate interactions are not understood. Furthermore, there is little known about the cell-cell and cell-matrix interactions within the unit, or about the effect of β-amyloid on microvessel responses during ischemia. These considerations indicate the need for a coordinated research effort to understand the origins of the variability in recanalization outcome.

Quantitative proteomic analysis reveals that lipopolysaccharide induces mitogen-activated protein kinase-dependent activation in human microglial cells

Microglial cells act as the first and main form of active immune defense in the central nervous system related to inflammation and neurodegenerative disease. Lipopolysaccharide (LPS) induces many genes encoding inflammatory mediators, including cytokines such as tumor necrosis factor-α, interleukin-1β, (IL-1β), and IL-6, chemokines, and prostaglandins in microglial cells. Quantitative proteomics methods with isobaric chemical labeling using tandem mass tags and 2D-nano LC-ESI-MS/MS were used to systematically analyze proteomic changes in microglia responding to LPS stimulation. As a result, we found that the expression level of 21 proteins in human microglial cells changed after activation. Among those, one of the strong mitogen-activated protein kinase (MAPK) regulator proteins, CMPK1 was highly upregulated after LPS stimulation in human microglial cells. We detected and validated upregulation of MAPK including ERK1/2, p38, and SAPK/JNK by immunohistochemistry and Western blotting. NFκB, strong transcription factor of CMPK1, was translocated to the nucleus from the cytosol by high contents screening after LPS stimulation. Taken together, we conclude that MAPK signaling plays an important role in LPS-induced human microglial activation related to inflammatory response.

Functions of connexins and large pore channels on microglial cells: the gates to environment

Microglial cells are not only sensitive indicators for pathology of the central nervous system (CNS), they are a key factor for neurotoxicity and degeneration in many diseases. Neuronal damage leads to reactive gliosis and to activation of microglia including cytoarchitectonic changes accompanied by alterations in surface receptor and channel expression. In this context, the release of neuroactive soluble factors like pro-inflammatory cytokines can result in increased cellular motility and a higher grade of phagocytotic activity. Ligands including glutamate, tumor necrosis factor alpha (TNF-α), cytokines, superoxide radicals and neurotrophins released by microglia have in turn effects on neuronal function and cell death. The current review focuses on large pore and hemichannel function in microglial cells under different conditions of activation and elucidates the role of these channels in cytokine release, as well as putative targets for clinical intervention in case of inflammatory processes. This article is part of a Special Issue entitled Electrical Synapses.

Building biocompatible hydrogels for tissue engineering of the brain and spinal cord

Tissue engineering strategies employing biomaterials have made great progress in the last few decades. However, the tissues of the brain and spinal cord pose unique challenges due to a separate immune system and their nature as soft tissue. Because of this, neural tissue engineering for the brain and spinal cord may require re-establishing biocompatibility and functionality of biomaterials that have previously been successful for tissue engineering in the body. The goal of this review is to briefly describe the distinctive properties of the central nervous system, specifically the neuroimmune response, and to describe the factors which contribute to building polymer hydrogels compatible with this tissue. These factors include polymer chemistry, polymerization and degradation, and the physical and mechanical properties of the hydrogel. By understanding the necessities in making hydrogels biocompatible with tissue of the brain and spinal cord, tissue engineers can then functionalize these materials for repairing and replacing tissue in the central nervous system.

Recognition of betaine as an inhibitor of lipopolysaccharide-induced nitric oxide production in activated microglial cells

BACKGROUND

Neuroinflammation, as a major outcome of microglia activation, is an important factor for progression of neurodegenerative disorders including Alzheimer's disease and Parkinson's disease. Microglial cells, as the first-line defense in the central nervous system, act as a source of neurotoxic factors such as nitric oxide (NO), a free radical which is involved in neuronal cell death. The aim of this study was to inhibit production of NO in activated microglial cells in order to decrease neurological damages that threat the central nervous system.

METHODS

An in vitro model of a newborn rat brain cell culture was used to examine the effect of betaine on the release of NO induced by lipopolysaccharide (LPS). Briefly, primary microglial cells were stimulated by LPS and after 2 minutes, they were treated by different concentrations of betaine. The production of NO was assessed by the Griess assay while cell viability was determined by the MTT assay.

RESULTS

Our investigations indicated that LPS-induced NO release was attenuated by betaine, suggesting that this compound might inhibit NO release. The effects of betaine on NO production in activated microglial cells after 24 h were "dose-dependent". It means that microglial cells which were treated with higher concentrations of betaine, released lower amounts of NO. Also our observations showed that betaine compound has no toxic effect on microglial cells.

CONCLUSION

Betaine has an inhibitory effect on NO release in activated microglial cells and may be an effective therapeutic component to control neurological disorders.

IL-12p40 deficiency leads to uncontrolled Trypanosoma cruzi dissemination in the spinal cord resulting in neuronal death and motor dysfunction

Chagas’ disease is a protozoosis caused by Trypanosoma cruzi that frequently shows severe chronic clinical complications of the heart or digestive system. Neurological disorders due to T. cruzi infection are also described in children and immunosuppressed hosts. We have previously reported that IL-12p40 knockout (KO) mice infected with the T. cruzi strain Sylvio X10/4 develop spinal cord neurodegenerative disease. Here, we further characterized neuropathology, parasite burden and inflammatory component associated to the fatal neurological disorder occurring in this mouse model. Forelimb paralysis in infected IL-12p40KO mice was associated with 60% (p<0.05) decrease in spinal cord neuronal density, glutamate accumulation (153%, p<0.05) and strong demyelization in lesion areas, mostly in those showing heavy protein nitrosylation, all denoting a neurotoxic degenerative profile. Quantification of T. cruzi 18S rRNA showed that parasite burden was controlled in the spinal cord of WT mice, decreasing from the fifth week after infection, but progressive parasite dissemination was observed in IL-12p40KO cords concurrent with significant accumulation of the astrocytic marker GFAP (317.0%, p<0.01) and 8-fold increase in macrophages/microglia (p<0.01), 36.3% (p<0.01) of which were infected. Similarly, mRNA levels for CD3, TNF-α, IFN-γ, iNOS, IL-10 and arginase I declined in WT spinal cords about the fourth or fifth week after infection, but kept increasing in IL-12p40KO mice. Interestingly, compared to WT tissue, lower mRNA levels for IFN-γ were observed in the IL-12p40KO spinal cords up to the fourth week of infection. Together the data suggest that impairments of parasite clearance mechanisms in IL-12p40KO mice elicit prolonged spinal cord inflammation that in turn leads to irreversible neurodegenerative lesions.

Regulation of oxidative stress in long-lived lipopolysaccharide-activated microglia

1. Previously, we reported that an optimal dose of lipopolysaccharide (LPS) markedly extends the life span of mouse primary-cultured microglia by suppressing apoptotic and autophagic cell death pathways. The aim of the present study was to assess how these cells protect themselves against reactive oxygen species (ROS) generated by LPS treatment. 2. The study was conducted in microglia obtained from murine neonate brain, which are destined to die within a few days under ordinary culture conditions. 3. The generation of ROS was maximal after 15 h LPS treatment (1 ng/mL LPS and 100 ng/mL LPS). The expression of inducible nitric oxide (NO) synthase protein was significantly increased by Day 1 of LPS treatment and was followed by the production of NO. The expression of either Cu/Zn- or Mn-superoxide dismutase protein (SOD) was also increased by 16 h and Day 1 of LPS treatment. LPS did not affect the expression of Cu/Zn- and Mn-SOD proteins, nor did it extend the life span of microglia that had mutated Toll-like receptor (TLR) 4. 4. The findings of the present study suggest that SODs function as a potent barrier to overcome ROS generated in primary-cultured microglia following LPS treatment and that TLR4 may be significantly involved in inducing these proteins. The microglia may be able to protect themselves against oxidative stress, allowing them to live for more than 1 month. Because long-lived microglia may play a critical role in the exacerbation of neurodegeneration, bringing activated microglia back to their resting stage could be a new and promising strategy to inhibit the deterioration underlying neurodegenerative disorders.

Toll-like receptor 2 ligands promote microglial cell death by inducing autophagy

Microglial cells are phagocytes in the central nervous system (CNS) and become activated in pathological conditions, resulting in microgliosis, manifested by increased cell numbers and inflammation in the affected regions. Thus, controlling microgliosis is important to prevent pathological damage to the brain. Here, we evaluated the contribution of Toll-like receptor 2 (TLR2) to microglial survival. We observed that activation of microglial cells with peptidoglycan (PGN) from Staphylococcus aureus and other TLR2 ligands results in cell activation followed by the induction of autophagy and autophagy-dependent cell death. In C57BL/6J mice, intracerebral injection of PGN increased the autophagy of microglial cells and reduced the microglial/macrophage cell number in brain parenchyma. Our results demonstrate a novel role of TLRs in the regulation of microglial cell activation and survival, which are important for the control of microgliosis and associated inflammatory responses in the CNS.

PINK1 stimulates interleukin-1β-mediated inflammatory signaling via the positive regulation of TRAF6 and TAK1

Parkinson's disease (PD) is characterized by a progressive loss of dopaminergic neurons in the substantia nigra. The cause of neuronal death in PD is largely unknown, but several genetic loci, including PTEN-induced putative kinase 1 (PINK1), have been linked to early onset autosomal recessive forms of familial PD. PINK1 encodes a serine/threonine kinase, which phosphorylates several substrates and consequently leads to cell protection against apoptosis induced by various stresses. In addition, research has shown that inflammation largely contributes to the pathogenesis of PD, but the functional link between PINK1 and PD-linked neuroinflammation remains poorly understood. Therefore, in the present study, we investigated the functional role of PINK1 in interleukin (IL)-1β-mediated inflammatory signaling. We show that PINK1 specifically binds to TRAF6 and TAK1, and facilitates the autodimerization and autoubiquitination of TRAF6. PINK1 also enhances the association between TRAF6 and TAK1, phosphorylates TAK1, and stimulates polyubiquitination of TAK1. Furthermore, PINK1 leads to the potentiation of IL-1β-mediated NF-κB activity and cytokine production. These findings suggest that PINK1 positively regulates two key molecules, TRAF6 and TAK1, in the IL-1β-mediated signaling pathway, consequently up-regulating their downstream inflammatory events.

Intravenous multipotent adult progenitor cell therapy after traumatic brain injury: modulation of the resident microglia population

Introduction

We have demonstrated previously that the intravenous delivery of multipotent adult progenitor cells (MAPC) after traumatic brain injury affords neuroprotection via interaction with splenocytes, leading to an increase in systemic anti-inflammatory cytokines. We hypothesize that the observed modulation of the systemic inflammatory milieu is related to T regulatory cells and a subsequent increase in the locoregional neuroprotective M2 macrophage population.

Methods

C57B6 mice were injected with intravenous MAPC 2 and 24 hours after controlled cortical impact injury. Animals were euthanized 24, 48, 72, and 120 hours after injury. In vivo, the proportion of CD4⁺/CD25⁺/FOXP3⁺ T-regulatory cells were measured in the splenocyte population and plasma. In addition, the brain CD86⁺ M1 and CD206⁺ M2 macrophage populations were quantified. A series of in vitro co-cultures were completed to investigate the need for direct MAPC:splenocyte contact as well as the effect of MAPC therapy on M1 and M2 macrophage subtype apoptosis and proliferation.

Results

Significant increases in the splenocyte and plasma T regulatory cell populations were observed with MAPC therapy at 24 and 48 hours, respectively. In addition, MAPC therapy was associated with an increase in the brain M2/M1 macrophage ratio at 24, 48 and 120 hours after cortical injury. In vitro cultures of activated microglia with supernatant derived from MAPC:splenocyte co-cultures also demonstrated an increase in the M2/M1 ratio. The observed changes were secondary to an increase in M1 macrophage apoptosis.

Conclusions

The data show that the intravenous delivery of MAPC after cortical injury results in increases in T regulatory cells in splenocytes and plasma with a concordant increase in the locoregional M2/M1 macrophage ratio. Direct contact between the MAPC and splenocytes is required to modulate activated microglia, adding further evidence to the central role of the spleen in MAPC-mediated neuroprotection.

Effects of the sigma-1 receptor agonist 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)-piperazine dihydro-chloride on inflammation after stroke

Activation of the sigma-1 receptor (Sig-1R) improves functional recovery in models of experimental stroke and is known to modulate microglia function. The present study was conducted to investigate if Sig-1R activation after experimental stroke affects mediators of the inflammatory response in the ischemic hemisphere. Male Wistar rats were subjected to transient occlusion of the middle cerebral artery (MCAO) and injected with the specific Sig-1R agonist 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA4503) or saline for 5 days starting on day 2 after MCAO. Treatment did not affect the increased levels of the pro-inflammatory cytokines interleukin 1 beta (IL-1β), tumor necrosis factor alpha (TNF-α), interferon gamma (IFN-γ), interleukin 4 (IL-4), interleukin 5 (IL-5), and interleukin 13 (IL-13) in the infarct core and peri-infarct area after MCAO. In addition, treatment with SA4503 did not affect elevated levels of nitrite, TNF-α and IL-1β observed in primary cultures of microglia exposed to combined Hypoxia/Aglycemia, while the unspecific sigma receptor ligand 1,3-di-o-tolylguanidine (DTG) significantly decreased the production of nitrite and levels of TNF-α. Analysis of the ischemic hemisphere also revealed increased levels of ionized calcium binding adaptor molecule 1 (Iba1) levels in the infarct core of SA4503 treated animals. However, no difference in Iba1 immunoreactivity was detected in the infarct core. Also, levels of the proliferation marker proliferating cell nuclear antigen (PCNA) and OX-42 were not increased in the infarct core in rats treated with SA4503. Together, our results suggest that sigma-1 receptor activation affects Iba1 expression in microglia/macrophages of the ischemic hemisphere after experimental stroke but does not affect post-stroke inflammatory mediators.

Differential effects of domoic acid and E. coli lipopolysaccharide on tumor necrosis factor-alpha, transforming growth factor-beta1 and matrix metalloproteinase-9 release by rat neonatal microglia: evaluation of the direct activation hypothesis

The excitatory amino acid domoic acid is the causative agent of amnesic shellfish poisoning in humans. The in vitro effects of domoic acid on rat neonatal brain microglia were compared with E. coli lipopolysaccharide (LPS), a known activator of microglia mediator release over a 4 to 24 hour observation period. LPS [3 ng/mL] but not domoic acid [1mM] stimulated a statistically significant increase in TNF-α mRNA and protein generation. Furthermore, both LPS and domoic acid did not significantly affect TGF-β1 gene expression and protein release. Finally, an in vitro exposure of microglia to LPS resulted in statistically significant MMP-9 expression and release, thus extending and confirming our previous observations. However, in contrast, no statistically significant increase in MMP-9 expression and release was observed after domoic acid treatment. Taken together our observations do not support the hypothesis that a short term (4 to 24 hours) in vitro exposure to domoic acid, at a concentration toxic to neuronal cells, activates rat neonatal microglia and the concomitant release of the pro-inflammatory mediators tumor necrosis factor-α (TNF-α) and matrix metalloproteinases-9 (MMP-9), as well as the anti-inflammatory cytokine transforming growth factor β1 (TGF-β1).

The Immunology of Glaucoma

The presence of specific antibodies and T cells that are specific in patients with glaucoma supports the idea that the immune system may play an important role in the initiation and/or sustainment of glaucomatous optic neuropathy, at least in some patients. At present, our understanding regarding immunological mechanisms associated with glaucomatous optic neuropathy is far from satisfactory. In this review, we examined evidence suggesting involvement of autoimmune responses in the pathogenesis of glaucoma. These include detection of autoantibodies and T cells and expression of cytokines and stress proteins in patients with glaucoma. Although immune responses are thought to be detrimental, some responses may exert a protective effect against neurodegenerative damage. Likely, the balance between positive and negative regulators determines the survival or demise of cells. It is vital that research continues to elucidate the roles of the immune system in glaucomatous neurodegeneration and the possibility of alternative modalities of treatment. These studies may also provide valuable molecular biomarkers for the diagnosis and identification of a specific cohort of patients with glaucoma, that is, those with normal-tension glaucoma.

ROCKs as immunomodulators of stroke

INTRODUCTION

Stroke is the third leading cause of death and a major cause of long-term disability in the adult population. Growing evidence suggests that inflammation may play an important role in the evolution of stroke. Because Rho-associated coiled-coil containing kinases (ROCKs) are important mediators of inflammation, they may contribute to stroke and stroke recovery.

AREAS COVERED

The pathophysiological role of ROCKs in mediating inflammation at different phases of stroke, and the therapeutic opportunities for stroke prevention and stroke treatment with ROCK inhibitors will be discussed.

EXPERT OPINION

Inflammation is a double-edged sword during the evolution of stroke. Immunomodulation might provide a novel therapeutic approach for stroke prevention and stroke treatment. ROCK plays an important role in mediating the inflammatory response following vascular injury as well as platelet activation and thrombus formation. ROCK inhibitors have been shown to be beneficial in stroke prevention, acute neuroprotection and chronic stroke recovery by affecting inflammatory-mediated platelet and endothelial function, smooth muscle contraction and neuronal regeneration. Thus, ROCK-mediated inflammation could be a potential therapeutic target for stroke prevention and stroke treatment. However, the mechanism by which ROCKs regulate the inflammatory response is unclear, and the role of the two ROCK isoforms in stroke and stroke recovery remains to be determined.

Vaccination with a non-human random sequence amyloid oligomer mimic results in improved cognitive function and reduced plaque deposition and micro hemorrhage in Tg2576 mice

Background

It is well established that vaccination of humans and transgenic animals against fibrillar Aβ prevents amyloid accumulation in plaques and preserves cognitive function in transgenic mouse models. However, autoimmune side effects have halted the development of vaccines based on full length human Aβ. Further development of an effective vaccine depends on overcoming these side effects while maintaining an effective immune response.

Results

We have previously reported that the immune response to amyloid oligomers is largely directed against generic epitopes that are common to amyloid oligomers of many different proteins and independent of a specific amino acid sequence. Here we have examined whether we can exploit this generic immune response to develop a vaccine that targets amyloid oligomers using a non-human random sequence amyloid oligomer. In order to study the effect of vaccination against generic oligomer epitopes, a random sequence oligomer (3A) was selected as it forms oligomers that react with the oligomer specific A11 antibody. Oligomer mimics from 3A peptide, Aβ, islet amyloid polypeptide (IAPP), and Aβ fibrils were used to vaccinate Tg2576 mice, which develop a progressive accumulation of plaques and cognitive impairment. Vaccination with the 3A random sequence antigen was just as effective as vaccination with the other antigens in improving cognitive function and reducing total plaque load (Aβ burden) in the Tg2576 mouse brains, but was associated with a much lower incidence of micro hemorrhage than Aβ antigens.

Conclusion

These results shows that the amyloid Aβ sequence is not necessary to produce a protective immune response that specifically targets generic amyloid oligomers. Using a non-human, random sequence antigen may facilitate the development of a vaccine that avoids autoimmune side effects.

Proinflammatory cytokine regulation of cyclic AMP-phosphodiesterase 4 signaling in microglia in vitro and following CNS injury

Cyclic AMP suppresses immune cell activation and inflammation. The positive feedback loop of proinflammatory cytokine production and immune activation implies that cytokines may not only be regulated by cyclic AMP but also conversely regulate cyclic AMP. This study examined the effects of tumor necrosis factor (TNF)-α and interleukin (IL)-1β on cyclic AMP-phosphodiesterase (PDE) signaling in microglia in vitro and after spinal cord injury (SCI) or traumatic brain injury (TBI). TNF-α or IL-1β stimulation produced a profound reduction (>90%) of cyclic AMP within EOC2 microglia from 30 min that then recovered after IL-1β but remained suppressed with TNF-α through 24 h. Cyclic AMP was also reduced in TNF-α-stimulated primary microglia, albeit to a lesser extent. Accompanying TNF-α-induced cyclic AMP reductions, but not IL-1β, was increased cyclic AMP-PDE activity. The role of PDE4 activity in cyclic AMP reductions was confirmed by using Rolipram. Examination of pde4 mRNA revealed an immediate, persistent increase in pde4b with TNF-α; IL-1β increased all pde4 mRNAs. Immunoblotting for PDE4 showed that both cytokines increased PDE4A1, but only TNF-α increased PDE4B2. Immunocytochemistry revealed PDE4B nuclear translocation with TNF-α but not IL-1β. Acutely after SCI/TBI, where cyclic AMP levels are reduced, PDE4B was localized to activated OX-42(+) microglia; PDE4B was absent in OX-42(+) cells in uninjured spinal cord/cortex or inactive microglia. Immunoblotting showed PDE4B2 up-regulation from 24 h to 1 wk post-SCI, the peak of microglia activation. These studies show that TNF-α and IL-1β differentially affect cyclic AMP-PDE signaling in microglia. Targeting PDE4B2 may be a putative therapeutic direction for reducing microglia activation in CNS injury and neurodegenerative diseases.

Temporal changes in cell marker expression and cellular infiltration in a controlled cortical impact model in adult male C57BL/6 mice

Background

Traumatic injury to the central nervous system (CNS) triggers a robust inflammatory response that leads to axonal damage and secondary degeneration of spared tissue. In contrast, some immune responses have neuroprotective effects. However, detailed information regarding the dynamics of immune responses after traumatic CNS injury is still unavailable.

Methods

In the present study, changes in the immune cells present in the injured brain, spleen, and cervical lymph nodes (CLNs), which are draining lymphatic organs from the CNS, were analyzed after controlled cortical impact (CCI) by flow cytometry and immunohistochemistry.

Results

The number of neutrophils and macrophages that infiltrated the injured brain immediately increased 1 d post-injury and declined rapidly thereafter. In the injured brain, resident microglia showed a bimodal increase during the first week and in the chronic phase (≥3 weeks) after injury. Increase in the Iba-1⁺ microglia/macrophages was observed around the injured site. Morphologic analysis showed that Iba-1⁺ cells were round at 1 week, whereas those at 3 weeks were more ramified. Furthermore, CD86⁺/CD11b⁺ M1-like microglia increased at 4 weeks after CCI, whereas CD206⁺/CD11b⁺ M2-like microglia increased at 1 week. These results suggest that different subsets of microglia increased in the acute and chronic phases after CCI. Dendritic cells and T cells increased transiently within 1 week in the injured brain. In the CLNs and the spleen, T cells showed dynamic changes after CCI. In particular, the alteration in the number of T cells in the CLNs showed a similar pattern, with a 1-week delay, to that of microglia in the injured brain.

Conclusion

The data from this study provide useful information on the dynamics of immune cells in CNS injuries.

Early steps of microglial activation are directly affected by neuroprotectant FK506 in both in vitro inflammation and in rat model of stroke

Neuroprotective and/or neuroregenerative activity of FK506, its derivatives, and to a lesser extent cyclosporin A (CsA) in animal models of neurodegenerative diseases of different etiology have been reported. Here, we verified a hypothesis that the most likely mechanism of their neuroprotective action is inhibition of the early steps of inflammatory activation of microglia by interference with mitogen-activated protein kinase (MAPK) signaling. The effect of immunosuppressants on lipopolysaccharide (LPS)-induced changes in morphology, proliferation, and motility of rat primary microglial cultures was evaluated. FK506 and CsA directly inhibited LPS-induced microglia activation and inflammatory responses. While both drugs efficiently reduced the expression of iNOS and the release of nitric oxide, only FK506 strongly inhibited the expression of Cox-2 and secretion of the mature form of IL-1β. FK506 strongly reduced LPS-induced activation of MAPK, and its downstream signaling crucial for inflammatory responses. Comparative analysis of global gene expression in rat ischemic brains and in LPS-stimulated microglial cultures revealed many genes and signaling pathways regulated in the same way in both systems. FK506 treatment blocked a majority of genes induced by an ischemic insult in the cortex, in particular inflammatory/innate immunity and apoptosis-related genes. Microglia-mediated inflammation is considered as one of the most important components of brain injury after trauma or stroke; thus, effective and multifaceted blockade of microglial activation by FK506 has clinical relevance and potential therapeutic implications.

Time-dependent effects of hypothermia on microglial activation and migration

Background

Therapeutic hypothermia is one of the neuroprotective strategies that improve neurological outcomes after brain damage in ischemic stroke and traumatic brain injury. Microglial cells become activated following brain injury and play an important role in neuroinflammation and subsequent brain damage. The aim of this study was to determine the time-dependent effects of hypothermia on microglial cell activation and migration, which are accompanied by neuroinflammation.

Methods

Microglial cells in culture were subjected to mild (33 °C) or moderate (29 °C) hypothermic conditions before, during, or after lipopolysaccharide (LPS) or hypoxic stimulation, and the production of nitric oxide (NO), proinflammatory cytokines, reactive oxygen species, and neurotoxicity was evaluated. Effects of hypothermia on microglial migration were also determined in in vitro as well as in vivo settings.

Results

Early-, co-, and delayed-hypothermic treatments inhibited microglial production of inflammatory mediators to varying degrees: early treatment was the most efficient, and delayed treatment showed time-dependent effects. Delayed hypothermia also suppressed the mRNA levels of proinflammatory cytokines and iNOS, and attenuated microglial neurotoxicity in microglia-neuron co-cultures. Furthermore, delayed hypothermia reduced microglial migration in the Boyden chamber assay and wound healing assay. In a stab injury model, delayed local hypothermia reduced migration of microglia toward the injury site in the rat brain.

Conclusion

Taken together, our results indicate that delayed hypothermia is sufficient to attenuate microglial activation and migration, and provide the basis of determining the optimal time window for therapeutic hypothermia. Delayed hypothermia may be neuroprotective by inhibiting microglia-mediated neuroinflammation, indicating the therapeutic potential of post-injury hypothermia for patients with brain damages exhibiting some of the inflammatory components.

Cannabinoid CB(2) receptors modulate ERK-1/2 kinase signalling and NO release in microglial cells stimulated with bacterial lipopolysaccharide

BACKGROUND AND PURPOSE

Cannabinoid (CB) receptor agonists have potential utility as anti-inflammatory drugs in chronic immune inflammatory diseases. In the present study, we characterized the signal transduction pathways affected by CB(2) receptors in quiescent and lipopolysaccharide (LPS)-stimulated murine microglia.

EXPERIMENTAL APPROACH

We examined the effects of the synthetic CB(2) receptor ligand, JWH-015, on phosphorylation of MAPKs and NO production.

KEY RESULTS

Stimulation of CB(2) receptors by JWH-015 activated JNK-1/2 and ERK-1/2 in quiescent murine microglial cells. Furthermore, CB(2) receptor activation increased p-ERK-1/2 at 15 min in LPS-stimulated microglia. Surprisingly, this was reduced after 30 min in the presence of both LPS and JWH-015. The NOS inhibitor L-NAME blocked the ability of JWH-015 to down-regulate the LPS-induced p-ERK increase, indicating that activation of CB(2) receptors reduced effects of LPS on ERK-1/2 phosphorylation through NO. JWH-015 increased LPS-induced NO release at 30 min, while at 4 h CB(2) receptor stimulation had an inhibitory effect. All the effects of JWH-015 were significantly blocked by the CB(2) receptor antagonist AM 630 and, as the inhibition of CB(2) receptor expression by siRNA abolished the effects of JWH-015, were shown to be mediated specifically by activation of CB(2) receptors.

CONCLUSIONS AND IMPLICATIONS

Our results demonstrate that CB(2) receptor stimulation activated the MAPK pathway, but the presence of a second stimulus blocked MAPK signal transduction, inhibiting pro-inflammatory LPS-induced production of NO. Therefore, CB(2) receptor agonists may promote anti-inflammatory therapeutic responses in activated microglia.

Age related changes in microglial phenotype vary between CNS regions: grey versus white matter differences

Subtle regional differences in microglial phenotype exist in the adult mouse brain. We investigated whether these differences were amplified during ageing and following systemic challenge with lipopolysaccharide (LPS). We studied microglial morphology and phenotype in young (4mo) and aged (21mo) C57/BL6 mice using immunohistochemistry and quantified the expression levels of surface molecules on microglia in white and grey matter along the rostral-caudal neuraxis. We detected significant regional, age dependent differences in microglial phenotypes, with the microglia of white matter and caudal areas of the CNS exhibiting greater upregulation of CD11b, CD68, CD11c, F4/80 and FcγRI than grey matter and rostral CNS areas. Upregulation of CD11c with age was restricted to the white matter, as was the appearance of multinucleated giant cells. Systemic LPS caused a subtle upregulation of FcγRI after 24 h, but the other markers examined were not affected. Burrowing behaviour and static rod assays were used to assess hippocampal and cerebellar integrity. Aged mice exhibited exaggerated and prolonged burrowing deficits following systemic LPS injection, while in the absence of an inflammatory challenge aged mice performed significantly worse than young mice in the static rod test. Taken together, these findings show that the effects of age on microglial phenotype and functional integrity vary significantly between CNS compartments, as do, albeit to a lesser extent, the effects of systemic LPS.

Acute neuroimmune modulation attenuates the development of anxiety-like freezing behavior in an animal model of traumatic brain injury

Chronic anxiety is a common and debilitating result of traumatic brain injury (TBI) in humans. While little is known about the neural mechanisms of this disorder, inflammation resulting from activation of the brain's immune response to insult has been implicated in both human post-traumatic anxiety and in recently developed animal models. In this study, we used a lateral fluid percussion injury (LFPI) model of TBI in the rat and examined freezing behavior as a measure of post-traumatic anxiety. We found that LFPI produced anxiety-like freezing behavior accompanied by increased reactive gliosis (reflecting neuroimmune inflammatory responses) in key brain structures associated with anxiety: the amygdala, insula, and hippocampus. Acute peri-injury administration of ibudilast (MN166), a glial cell activation inhibitor, suppressed both reactive gliosis and freezing behavior, and continued neuroprotective effects were apparent several months post-injury. These results support the conclusion that inflammation produced by neuroimmune responses to TBI play a role in post-traumatic anxiety, and that acute suppression of injury-induced glial cell activation may have promise for the prevention of post-traumatic anxiety in humans.

Efficient isolation of live microglia with preserved phenotypes from adult mouse brain

Background

Microglial activation plays a key role in the neuroinflammation associated with virtually all CNS disorders, although their role in normal CNS physiology is becoming increasingly appreciated. Neuroinflammation is often assessed by analyzing pro-inflammatory mediators in CNS tissue homogenates, under the assumption that microglia are the main source of these molecules. However, other cell types in the CNS can also synthesize inflammatory molecules. Hence, to enable direct analysis of microglial activities ex vivo, an efficient, reliable, and reproducible method of microglial isolation is needed.

Methods

After enzymatic digestion of brain tissues and myelin removal, CD11b⁺ cells were isolated using immunomagnetic separation, yielding highly purified microglia without astrocyte or neuronal contamination. We used three methods of myelin removal (30% Percoll, 0.9 mol/l sucrose and anti-myelin magnetic beads), and compared their effects on microglial viability and yield. To determine whether the isolation procedure itself activates microglia, we used flow cytometry to examine microglial properties in brain-tissue homogenates and isolated microglia from control and lipopolysaccharide (LPS) -treated mice.

Results

This method yielded a highly purified CD11b⁺ cell population with properties that reflected their in vivo phenotype. The viability and yield of isolated cells were significantly affected by the myelin removal method. Although the microglial phenotype was comparable in all methods used, the highest viability and number of CD11b⁺ cells was obtained with Percoll. Microglia isolated from LPS-treated mice displayed a pro-inflammatory phenotype as determined by upregulated levels of TNF-α, whereas microglia isolated from control mice did not.

Conclusions

Immunomagnetic separation is an efficient method to isolate microglia from the CNS, and is equally suitable for isolating quiescent and activated microglia. This technique allows evaluation of microglial activities ex vivo, which accurately reflects their activities in vivo. Microglia obtained by this method can be used for multiple downstream applications including qRT-PCR, ELISA, Western blotting, and flow cytometry to analyze microglial activities in any number of CNS pathologies or injuries.

Gemfibrozil, a lipid-lowering drug, upregulates IL-1 receptor antagonist in mouse cortical neurons: implications for neuronal self-defense

Chronic inflammation is becoming a hallmark of several neurodegenerative disorders and accordingly, IL-1β, a proinflammatory cytokine, is implicated in the pathogenesis of neurodegenerative diseases. Although IL-1β binds to its high-affinity receptor, IL-1R, and upregulates proinflammatory signaling pathways, IL-1R antagonist (IL-1Ra) adheres to the same receptor and inhibits proinflammatory cell signaling. Therefore, upregulation of IL-1Ra is considered important in attenuating inflammation. The present study underlines a novel application of gemfibrozil (gem), a Food and Drug Administration-approved lipid-lowering drug, in increasing the expression of IL-1Ra in primary mouse and human neurons. Gem alone induced an early and pronounced increase in the expression of IL-1Ra in primary mouse cortical neurons. Activation of type IA p110α PI3K and Akt by gem and abrogation of gem-induced upregulation of IL-1Ra by inhibitors of PI3K and Akt indicate a role of the PI3K-Akt pathway in the upregulation of IL-1Ra. Gem also induced the activation of CREB via the PI3K-Akt pathway, and small interfering RNA attenuation of CREB abolished the gem-mediated increase in IL-1Ra. Furthermore, gem was able to protect neurons from IL-1β insult. However, small interfering RNA knockdown of neuronal IL-1Ra abrogated the protective effect of gem against IL-1β, suggesting that this drug increases the defense mechanism of cortical neurons via upregulation of IL-1Ra. Taken together, these results highlight the importance of the PI3K-Akt-CREB pathway in mediating gem-induced upregulation of IL-1Ra in neurons and suggest gem as a possible therapeutic treatment for propagating neuronal self-defense in neuroinflammatory and neurodegenerative disorders.

Transcriptome analysis of amoeboid and ramified microglia isolated from the corpus callosum of rat brain

Background

Microglia, the resident immune cells of the central nervous system (CNS), have two distinct phenotypes in the developing brain: amoeboid form, known to be amoeboid microglial cells (AMC) and ramified form, known to be ramified microglial cells (RMC). The AMC are characterized by being proliferative, phagocytic and migratory whereas the RMC are quiescent and exhibit a slow turnover rate. The AMC transform into RMC with advancing age, and this transformation is indicative of the gradual shift in the microglial functions. Both AMC and RMC respond to CNS inflammation, and they become hypertrophic when activated by trauma, infection or neurodegenerative stimuli. The molecular mechanisms and functional significance of morphological transformation of microglia during normal development and in disease conditions is not clear. It is hypothesized that AMC and RMC are functionally regulated by a specific set of genes encoding various signaling molecules and transcription factors.

Results

To address this, we carried out cDNA microarray analysis using lectin-labeled AMC and RMC isolated from frozen tissue sections of the corpus callosum of 5-day and 4-week old rat brain respectively, by laser capture microdissection. The global gene expression profiles of both microglial phenotypes were compared and the differentially expressed genes in AMC and RMC were clustered based on their functional annotations. This genome wide comparative analysis identified genes that are specific to AMC and RMC.

Conclusions

The novel and specific molecules identified from the trancriptome explains the quiescent state functioning of microglia in its two distinct morphological states.

Liposomal clodronate selectively eliminates microglia from primary astrocyte cultures

BACKGROUND

There is increasing interest in astrocyte biology because astrocytes have been demonstrated to play prominent roles in physiological and pathological conditions of the central nervous system, including neuroinflammation. To understand astrocyte biology, primary astrocyte cultures are most commonly used because of the direct accessibility of astrocytes in this system. However, this advantage can be hindered by microglial contamination. Although several authors have warned regarding microglial contamination in this system, complete microglial elimination has never been achieved.

METHODS

The number and proliferative potential of contaminating microglia in primary astrocyte cultures were quantitatively assessed by immunocytologic and flow cytometric analyses. To examine the utility of clodronate for microglial elimination, primary astrocyte cultures or MG-5 cells were exposed to liposomal or free clodronate, and then immunocytologic, flow cytometric, and gene expression analyses were performed. The gene expression profiles of microglia-eliminated and microglia-contaminated cultures were compared after interleukin-6 (IL-6) stimulation.

RESULTS

The percentage of contaminating microglia exceeded 15% and continued to increase because of their high proliferative activity in conventional primary astrocyte cultures. These contaminating microglia were selectively eliminated low concentration of liposomal clodronate. Although primary microglia and MG-5 cells were killed by both liposomal and free clodronate, free clodronate significantly affected the viability of astrocytes. In contrast, liposomal clodronate selectively eliminated microglia without affecting the viability, proliferation or activation of astrocytes. The efficacy of liposomal clodronate was much higher than that of previously reported methods used for decreasing microglial contamination. Furthermore, we observed rapid tumor necrosis factor-α and IL-1b gene induction in conventional primary astrocyte cultures after IL-6 stimulation, which was due to the activation of the Janus kinase/signal transducer and activator of the transcription pathway in contaminating microglia.

CONCLUSIONS

Because contaminating microglia could result in erroneous data regarding the pro-inflammatory properties of astrocytes, astrocyte biology should be studied in the absence of microglial contamination. Our simple method will be widely applicable to experimental studies of astrocyte biology and provide clues for understanding the role of astrocytes in neural development, function and disease.

Microglia/monocytes with NG2 expression have no phagocytic function in the cortex after LPS focal injection into the rat brain

While OX42(+) microglia/macrophages have been considered as a scavenger in the brain, NG2(+) cells are generally considered as oligodendrocyte progenitor cells or function-unknown glial cells. Recent evidence showed that under some pathological conditions, certain cells have become positive for both anti-NG2 and anti-OX42 antibodies. Our results suggested that some OX42(+) microglia or macrophages were induced to express NG2 proteins 3 and 5 days later after focal injection of lipopolysaccharide into the brain cortex of Sprague-Dawley rats. In consideration of the induction of NG2 expression may associate with gaining or losing functions of microglia/macrophages, we further showed that, while OX42(+) or ED1(+) microglia/macrophages presented active phagocytic function, NG2(+) /OX42(+) cells failed to engulf latex beads. The induced expression of NG2 protein may possibly indicate the functional diversity of activated microglia/macrophages in the brain.

A novel in vitro human microglia model: characterization of human monocyte-derived microglia

Microglia are the innate immune cells of the central nervous system. They help maintaining physiological homeostasis and contribute significantly to inflammatory responses in the course of infection, injury and degenerative processes. To date, there is no standardized simple model available to investigate the biology of human microglia. The aim of this study was to establish a new human microglia model. For that purpose, human peripheral blood monocytes were cultured in serum free medium in the presence of M-CSF, GM-CSF, NGF and CCL2 to generate monocyte-derived microglia (M-MG). M-MG were clearly different in morphology, phenotype and function from freshly isolated monocytes, cultured monocytes in the absence of the cytokines and monocyte-derived dendritic cells (M-DC) cultured in the presence of GM-CSF and IL-4. M-MG acquired a ramified morphology with primary and secondary processes. M-MG displayed a comparable phenotype to the human microglia cell line HMC3, expressing very low levels of CD45, CD14 and HLA-DR, CD11b and CD11c; and undetectable levels of CD40, CD80 and CD83, and a distinct pattern of chemokine receptors (positive for CCR1, CCR2, CCR4, CCR5, CXCR1, CXCR3, CX3CR1; negative for CCR6 and CCR7). In comparison with M-DC, M-MG displayed lower T-lymphocyte stimulatory capacity, as well as lower phagocytosis activity. The described protocol for the generation of human monocyte-derived microglia is feasible, well standardized and reliable, as it uses well defined culture medium and recombinant cytokines, but no serum or conditioned medium. This protocol will certainly be very helpful for future studies investigating the biology and pathology of human microglia.

The evolving landscape of neurotoxicity by unconjugated bilirubin: role of glial cells and inflammation

Unconjugated hyperbilirubinemia is a common condition in the first week of postnatal life. Although generally harmless, some neonates may develop very high levels of unconjugated bilirubin (UCB), which may surpass the protective mechanisms of the brain in preventing UCB accumulation. In this case, both short-term and long-term neurodevelopmental disabilities, such as acute and chronic UCB encephalopathy, known as kernicterus, or more subtle alterations defined as bilirubin-induced neurological dysfunction (BIND) may be produced. There is a tremendous variability in babies' vulnerability toward UCB for reasons not yet explained, but preterm birth, sepsis, hypoxia, and hemolytic disease are comprised as risk factors. Therefore, UCB levels and neurological abnormalities are not strictly correlated. Even nowadays, the mechanisms of UCB neurotoxicity are still unclear, as are specific biomarkers, and little is known about lasting sequelae attributable to hyperbilirubinemia. On autopsy, UCB was shown to be within neurons, neuronal processes, and microglia, and to produce loss of neurons, demyelination, and gliosis. In isolated cell cultures, UCB was shown to impair neuronal arborization and to induce the release of pro-inflammatory cytokines from microglia and astrocytes. However, cell dependent sensitivity to UCB toxicity and the role of each nerve cell type remains not fully understood. This review provides a comprehensive insight into cell susceptibilities and molecular targets of UCB in neurons, astrocytes, and oligodendrocytes, and on phenotypic and functional responses of microglia to UCB. Interplay among glia elements and cross-talk with neurons, with a special emphasis in the UCB-induced immunostimulation, and the role of sepsis in BIND pathogenesis are highlighted. New and interesting data on the anti-inflammatory and antioxidant activities of different pharmacological agents are also presented, as novel and promising additional therapeutic approaches to BIND.

The molecular profile of microglia under the influence of glioma

Microglia, which contribute substantially to the tumor mass of glioblastoma, have been shown to play an important role in glioma growth and invasion. While a large number of experimental studies on functional attributes of microglia in glioma provide evidence for their tumor-supporting roles, there also exist hints in support of their anti-tumor properties. Microglial activities during glioma progression seem multifaceted. They have been attributed to the receptors expressed on the microglia surface, to glioma-derived molecules that have an effect on microglia, and to the molecules released by microglia in response to their environment under glioma control, which can have autocrine effects. In this paper, the microglia and glioma literature is reviewed. We provide a synopsis of the molecular profile of microglia under the influence of glioma in order to help establish a rational basis for their potential therapeutic use. The ability of microglia precursors to cross the blood-brain barrier makes them an attractive target for the development of novel cell-based treatments of malignant glioma.

Zaprinast activates MAPKs, NFκB, and Akt and induces the expressions of inflammatory genes in microglia

Previously, the authors reported that zaprinast, an inhibitor of cGMP-selective phosphodiesterases, induced the secretions of TNF-α and IL-1β by microglia and enhanced the induction of iNOS by lipopolysaccharide (LPS). In this study, the signaling mechanism responsible for microglial activation by zaprinast was investigated and the effects of zaprinast and LPS on microglial activation were compared. Zaprinast was found to activate ERK1/2, p38 MAPK, JNK, NFκB, and PI3K/Akt, and subsequently, induce the mRNA expressions of IL-1α, IL-1β, TNF-α, CCL2, CCL4, CXCL1, CXCL2, and CD14. Associations between signaling pathways and gene expressions were examined by treating microglia with signal inhibitors. PDTC inhibited the induction of all the above genes by zaprinast, and SB203580 inhibited all genes except CXCL1. SP600125, PD98059, and LY294002 inhibited the induction of at least CCL2. Microglial activation by zaprinast was then compared with full-blown activation by LPS. The zaprinast-induced phosphorylations of MAPKs and IκB were less prompt than LPS-induced phosphorylations. IκB degradation by LPS was significant at 10min and did not return to normal, whereas zaprinast induced a later, transient degradation. LPS induced the mRNA expressions of IL-1β, TNF-α, IL-6, CCL2, iNOS, and COX-2, and although zaprinast significantly induced the expressions of all except IL-6 and iNOS, these inductions were far less than those induced by LPS. Collectively, zaprinast was found to upregulate microglial activity mainly via NFκB and p38 MAPK signaling and the subsequent expressions of inflammatory genes. Although, zaprinast was found to have obvious effects on microglia, these were weaker than the effects of LPS.

Migration of monocytes after intracerebral injection

Recently, we monitored green fluorescent protein (GFP)-expressing monocytes after injection at the entorhinal cortex lesion (ECL) site in mice. We followed their migration out of the central nervous system (CNS) along olfactory nerve fibers penetrating the lamina cribrosa, within the nasal mucosa, and their subsequent appearance within the deep cervical lymph nodes (CLN), with numbers peaking at day 7. This is the same route activated T cells use for reaching the CLN, as we have shown before. Interestingly, GFP cells injected into the brain and subsequently found in the CLN exhibited ramified morphologies, which are typical of microglia and dendritic cells. To gain more insight into immunity and regeneration within the CNS we want to monitor injected monocytes using magnetic resonance imaging (MRI) after labeling with very small superparamagnetic iron oxide particles (VSOP). Due to their small size, nanoparticles have huge potential for magnetic labeling of different cell populations and their MRI tracking in vivo. So far we have verified that incubation with VSOP particles does not alter their migration pattern after ECL.

Changes of ribosomal protein S3 immunoreactivity and its new expression in microglia in the mice hippocampus after lipopolysaccharide treatment

Lipopolysaccharide (LPS) has been used as a reagent for a model of systemic inflammatory response. Ribosomal protein S3 (rpS3) is a multi-functional protein that is involved in transcription, metastasis, DNA repair, and apoptosis. In the present study, we examined the changes of rpS3 immunoreactivity in the mouse hippocampus after systemic administration of 1 mg/kg of LPS. From 6 h after LPS treatment, rpS3 immunoreactivity was decreased in pyramidale cells of the hippocampus proper (CA1-CA3 regions) and in granule cells of the dentate gyrus. At this point in time, rpS3 immunoreactivity began to increase in non-pyramidal cells and non-granule cells. From 1 day after LPS treatment, rpS3 immunoreactivity in pyramidal and granule cells was hardly detected; however, strong rpS3 immunoreactivity was shown in non-pyramidal and non-granule cells. Based on double immunofluorescence staining for rpS3/ionized calcium-binding adapter 1 (Iba-1, a marker for microglia) and glial fibrillary acidic protein (GFAP, a marker for astrocytes), strong rpS3 immunoreactivity was expressed in Iba-1-immunoreactive microglia, not in GFAP-immunoreactive astrocytes, at 1 and 2 days after LPS treatment. These results indicate that rpS3 immunoreactivity changes only in pyramidal and granule cells, and rpS3 is expressed only in activated microglia after LPS treatment: this may be associated with the neuroinflammatory responses in the brain.

The changing phenotype of microglia from homeostasis to disease

It has been nearly a century since the early description of microglia by Rio-Hortega; since then many more biological and pathological features of microglia have been recognized. Today, microglia are generally considered to be beneficial to homeostasis at the resting state through their abilities to survey the environment and phagocytose debris. However, when activated microglia assume diverse phenotypes ranging from fully inflamed, which involves the release of many pro-inflammatory cytokines, to alternatively activated, releasing anti-inflammatory cytokines or neurotrophins, the consequences to neurons can range from detrimental to supportive. Due to the different experimental sets and conditions, contradictory results have been obtained regarding the controversial question of whether microglia are “good” or “bad.” While it is well understood that the dual roles of activated microglia depend on specific situations, the underlying mechanisms have remained largely unclear, and the interpretation of certain findings related to diverse microglial phenotypes continues to be problematic. In this review we discuss the functions of microglia in neuronal survival and neurogenesis, the crosstalk between microglia and surrounding cells, and the potential factors that could influence the eventual manifestation of microglia.

Neural stem/progenitor cells as a promising candidate for regenerative therapy of the central nervous system

Neural transplantation is a promising therapeutic strategy for neurodegenerative diseases and other disorders of the central nervous system (CNS) such as Parkinson and Huntington diseases, multiple sclerosis or stroke. Although cell replacement therapy already went through clinical trials for some of these diseases using fetal human neuroblasts, several significant limitations led to the search for alternative cell sources that would be more suitable for intracerebral transplantation.Taking into account logistical and ethical issues linked to the use of tissue derived from human fetuses, and the immunologically special status of the CNS allowing the occurrence of deleterious immune reactions, neural stem/progenitor cells (NSPCs) appear to be an interesting cell source candidate. In addition to their ability for replacing cell populations lost during the pathological events, NSPCs also display surprising therapeutic effects of neuroprotection and immunomodulation. A better knowledge of the mechanisms involved in these specific characteristics will hopefully lead in the future to a successful use of NSPCs in regenerative medicine for CNS disorders.

Immune system in the brain: a modulatory role on dendritic spine morphophysiology?

The central nervous system is closely linked to the immune system at several levels. The brain parenchyma is separated from the periphery by the blood brain barrier, which under normal conditions prevents the entry of mediators such as activated leukocytes, antibodies, complement factors, and cytokines. The myeloid cell lineage plays a crucial role in the development of immune responses at the central level, and it comprises two main subtypes: (1) resident microglia, distributed throughout the brain parenchyma; (2) perivascular macrophages located in the brain capillaries of the basal lamina and the choroid plexus. In addition, astrocytes, oligodendrocytes, endothelial cells, and, to a lesser extent, neurons are implicated in the immune response in the central nervous system. By modulating synaptogenesis, microglia are most specifically involved in restoring neuronal connectivity following injury. These cells release immune mediators, such as cytokines, that modulate synaptic transmission and that alter the morphology of dendritic spines during the inflammatory process following injury. Thus, the expression and release of immune mediators in the brain parenchyma are closely linked to plastic morphophysiological changes in neuronal dendritic spines. Based on these observations, it has been proposed that these immune mediators are also implicated in learning and memory processes.

Chemokines and their receptors in intracerebral hemorrhage

Intracerebral hemorrhage (ICH) is a devastating clinical event which results in a high rate of disability and death. At present, no effective treatment is available for ICH. Accumulating evidence suggests that inflammatory responses contribute significantly to the ICH-induced secondary brain outcomes. During ICH, inflammatory cells accumulate at the ICH site attracted by gradients of chemokines. This review summarizes recent progress in ICH studies and the chemoattractants that act during the injury and focuses on and introduces the basic biology of the chemokine monocyte chemoattractant protein-1 (MCP1) and its role in the progression of ICH. Better understanding of MCP1 signaling cascade and the compensation after its inhibition could shed light on the development of effective treatments for ICH.