Astrocytes enhance lipopolysaccharide-induced nitric oxide production by microglial cells

Human Microglia-Like Cells Differentiated from Monocytes with GM-CSF and IL-34 Show Phagocytosis of α-Synuclein Aggregates and C/EBPβ-Dependent Proinflammatory Activation

Microglia, the main resident immune cells in the central nervous system, are implicated in the pathogenesis of various neurological disorders. Much of our knowledge on microglial biology was obtained using rodent microglial cultures. To understand the role of microglia in human disease, reliable in vitro models of human microglia are necessary. Monocyte-derived microglia-like cells (MDMi) are a promising approach. This study aimed to characterize MDMi cells generated from adult human monocytes using granulocyte-macrophage colony-stimulating factor and interleukin-34. To this end, 49 independent cultures of MDMI were prepared, and various methodological and functional studies were performed. We show that with this protocol, adult human monocytes develop into microglia-like cells, a coating is unnecessary, and high cell density seeding is preferable. When compared to monocytes, MDMi upregulate the expression of many, but not all, microglial markers, indicating that, although these cells display a microglia-like phenotype, they cannot be considered bona fide human microglia. At the functional level, MDMi phagocytose α-synuclein aggregates and responds to lipopolysaccharide (LPS) by nuclear translocation of the transcription factor nuclear factor-kappaB (NFkappaB) and the upregulation of proinflammatory genes. Finally, a long-lasting silencing of the transcription factor CCAAT/enhancer protein β (C/EBPβ) was achieved by small interfering RNA, resulting in the subsequent downregulation of proinflammatory genes. This supports the hypothesis that C/EBPβ plays a key role in proinflammatory gene program activation in human microglia. Altogether, this study sheds new light on the properties of MDMi cells and supports these cells as a promising in vitro model for studying adult human microglia-like cells.

Lack of Direct Effects of Neurotrophic Factors in an In Vitro Model of Neuroinflammation

Neuroinflammation is associated with several neurological disorders including temporal lobe epilepsy. Seizures themselves can induce neuroinflammation. In an in vivo model of epilepsy, the supplementation of brain-derived neurotropic factor (BDNF) and fibroblast growth factor-2 (FGF-2) using a Herpes-based vector reduced epileptogenesis-associated neuroinflammation. The aim of this study was to test whether the attenuation of the neuroinflammation obtained in vivo with BDNF and FGF-2 was direct or secondary to other effects, for example, the reduction in the severity and frequency of spontaneous recurrent seizures. An in vitro model of neuroinflammation induced by lipopolysaccharide (LPS, 100 ng/mL) in a mouse primary mixed glial culture was used. The releases of cytokines and NO were analyzed via ELISA and Griess assay, respectively. The effects of LPS and neurotrophic factors on cell viability were determined by performing an MTT assay. BDNF and FGF-2 were tested alone and co-administered. LPS induced a significant increase in pro-inflammatory cytokines (IL-1β, IL-6, and TNF-α) and NO. BDNF, FGF-2, and their co-administration did not counteract these LPS effects. Our study suggests that the anti-inflammatory effect of BDNF and FGF-2 in vivo in the epilepsy model was indirect and likely due to a reduction in seizure frequency and severity.

Sex and APOE Genotype Alter the Basal and Induced Inflammatory States of Primary Astrocytes from Humanized Targeted Replacement Mice

Apolipoprotein E4 (APOE4) genotype and sex are significant risk factors for Alzheimer's disease (AD), with females demonstrating increased risk modulated by APOE genotype. APOE is predominantly expressed in astrocytes, however, there is a lack of comprehensive assessments of sex differences in astrocytes stratified by APOE genotype. Here, we examined the response of mixed-sex and sex-specific neonatal APOE3 and APOE4 primary mouse astrocytes (PMA) to a cytokine mix of IL1b, TNFa, and IFNg. Pro-inflammatory and anti-inflammatory cytokine profiles were assessed by qRT-PCR and Meso Scale Discovery multiplex assay. Mixed-sex APOE4 PMA were found to have higher basal messenger RNA expression of several pro-inflammatory cytokines including Il6, Tnfa, Il1b, Mcp1, Mip1a, and Nos2 compared to APOE3 PMA, which was accompanied by increased levels of these secreted cytokines. In sex-specific cultures, basal expression of Il1b, Il6, and Nos2 was 1.5 to 2.5 fold higher in APOE4 female PMA compared to APOE4 males, with both being higher than APOE3 PMA. Similar results were found for secreted levels of these cytokines. Together, these findings indicate that APOE4 genotype and female sex, contribute to a greater inflammatory response in primary astrocytes and these data may provide a framework for investigating the mechanisms contributing to genotype and sex differences in AD-related neuroinflammation.

Distinct and Dynamic Transcriptome Adaptations of iPSC-Generated Astrocytes after Cytokine Stimulation

Astrocytes (ACs) do not only play a role in normal neurogenesis and brain homeostasis, but also in inflammatory and neurodevelopmental disorders. We studied here the different patterns of inflammatory activation triggered by cytokines in human induced pluripotent stem cell (iPSC)-derived ACs. An optimized differentiation protocol provided non-inflamed ACs. These cells reacted to TNFα with a rapid translocation of NFκB, while AC precursors showed little response. Transcriptome changes were quantified at seven time points (2–72 h) after stimulation with TNFα, IFNγ or TNFα plus IFNγ. TNFα triggered a strong response within 2 h. It peaked from 12–24 h and reverted towards the ground state after 72 h. Activation by IFNγ was also rapid, but the response pattern differed from that of TNFα. For instance, several chemokines up-regulated by TNFα were not affected by IFNγ. Instead, MHC-II-related antigen presentation was drastically enhanced. The combination of the two cytokines led to a stronger and more persistent response. For instance, TRIB3 up-regulation by the combination of TNFα plus IFNγ may have slowed NFκB inactivation. Additionally, highly synergistic regulation was observed for inflammation modifiers, such as CASP4, and for STAT1-controlled genes. The combination of the cytokines also increased oxidative stress markers (e.g., CHAC1), led to phenotypic changes in ACs and triggered markers related to cell death. In summary, these data demonstrate that there is a large bandwidth of pro-inflammatory AC states, and that single markers are not suitable to describe AC activation or their modulation in disease, development and therapy.

Mesoporous silica particles are phagocytosed by microglia and induce a mild inflammatory response in vitro

Aim: Mesoporous silica particles (MSPs) are broadly used drug delivery carriers. In this study, the authors analyzed the responses to MSPs of astrocytes and microglia, the two main cellular players in neuroinflammation. Materials & methods: Primary murine cortical mixed glial cultures were treated with rhodamine B-labeled MSPs. Results: MSPs are avidly internalized by microglial cells and remain inside the cells for at least 14 days. Despite this, MSPs do not affect glial cell viability or morphology, basal metabolic activity or oxidative stress. MSPs also do not affect mRNA levels of key proinflammatory genes; however, in combination with lipopolysaccharide, they significantly increase extracellular IL-1β levels. Conclusion: These results suggest that MSPs could be novel tools for specific drug delivery to microglial cells.

Pharmaceutical Potential of Casein-Derived Tripeptide Met-Lys-Pro: Improvement in Cognitive Impairments and Suppression of Inflammation in APP/PS1 Mice

Background:

Tripeptide Met-Lys-Pro (MKP), a component of casein hydrolysates, has effective angiotensin-converting enzyme (ACE) inhibitory activity. Brain angiotensin II enzyme activates the NADPH oxidase complex via angiotensin II receptor type 1 (AT1) and enhances oxidative stress injury. ACE inhibitors improved cognitive function in Alzheimer’s disease (AD) mouse models and previous clinical trials. Thus, although undetermined, MKP may be effective against pathological amyloid-β (Aβ) accumulation-induced cognitive impairment.

Objective:

The current study aimed to investigate the potential of MKP as a pharmaceutical against AD by examining MKP’s effect on cognitive function and molecular changes in the brain using double transgenic (APP/PS1) mice.

Methods:

Experimental procedures were conducted in APP/PS1 mice (n = 38) with a C57BL/6 background. A novel object recognition test was used to evaluate recognition memory. ELISA was used to measure insoluble Aβ₄₀, Aβ₄₂, and TNF-α levels in brain tissue. Immunohistochemical analysis allowed the assessment of glial cell activation in MKP-treated APP/PS1 mice.

Results:

The novel object recognition test revealed that MKP-treated APP/PS1 mice showed significant improvement in recognition memory. ELISA of brain tissue showed that MKP significantly reduced insoluble Aβ₄₀, Aβ₄₂, and TNF-α levels. Immunohistochemical analysis indicated the suppression of the marker for microglia and reactive astrocytes in MKP-treated APP/PS1 mice.

Conclusion:

Based on these results, we consider that MKP could ameliorate pathological Aβ accumulation-induced cognitive impairment in APP/PS1 mice. Furthermore, our findings suggest that MKP potentially contributes to preventing cognitive decline in AD.

Novel protocol for the isolation of highly purified neonatal murine microglia and astrocytes

BACKGROUND

The crosstalk and reactivity of the cell type glia, especially microglia and astrocytes, have progressively gathered research attention in understanding proper brain function regulated by the innate immune response. Therefore, methods to isolate highly viable and pure glia for the analysis on a cell-specific level are indispensable.

NEW METHOD

We modified previously established techniques: Animal numbers were reduced by multiple microglial harvests from the same mixed glial culture, thereby maximizing microglial yields following the principles of the 3Rs (replacement, reduction, and refinement). We optimized Magnetic-activated cell sorting (MACS®) of microglia and astrocytes by applying cultivated primary glial cell suspensions instead of directly sorting dissociated single cell suspension.

RESULTS

We generated highly viable and pure microglia and astrocytes derived from a single mixed culture with a purity of ~99%, as confirmed by FACS analysis. Field emission scanning electron microscopy (FESEM) demonstrated integrity of the MACS-purified glial cells. Tumor necrosis factor (TNF) and Interleukin-10 (IL-10) ELISA confirmed pro- and anti-inflammatory responses to be functional in purified glia, but significantly weakened compared to non-purified cells, further highlighting the importance of cellular crosstalk for proper immune activation.

COMPARISON WITH EXISTING METHOD(S)

Unlike previous studies that either isolated a single type of glia or displayed a substantial proportion of contamination with other cell types, we achieved isolation of both microglia and astrocytes at an increased purity (99-100%).

CONCLUSIONS

We have created an optimized protocol for the efficient purification of both primary microglia and astrocytes. Our results clearly demonstrate the importance of purity in glial cell cultivation in order to examine immune responses, which particularly holds true for astrocytes. We propose the novel protocol as a tool to investigate the cell type-specific crosstalk between microglia and astrocytes in the frame of CNS diseases.

Chemogenetic stimulation of the Gi pathway in astrocytes suppresses neuroinflammation

Engineered G protein-coupled receptors (GPCRs) are commonly used in chemogenetics as designer receptors exclusively activated by designer drugs (DREADDs). Although several GPCRs have been studied in astrocytes using a chemogenetic approach, the functional role of the astrocytic Gi pathway is not clear, as the literature is conflicting depending on the brain regions or behaviors investigated. In this study, we evaluated the role of the astrocytic Gi pathway in neuroinflammation using a Gi -coupled DREADD (hM4Di). Gi -DREADD was expressed in hippocampal astrocytes of a lipopolysaccharide (LPS)-induced neuroinflammation mouse model using adeno-associated viruses. We found that astrocyte Gi -DREADD stimulation using clozapine N-oxide (CNO) inhibits neuroinflammation, as characterized by decreased levels of proinflammatory cytokines, glial activation, and cognitive impairment in mice. Subsequent experiments using primary astrocyte cultures revealed that Gi -DREADD stimulation significantly downregulated LPS-induced expression of Nos2 mRNA and nitric oxide production. Similarly, in vitro calcium imaging showed that activation of the astrocytic Gi pathway attenuated intracellular calcium transients triggered by LPS treatment, suggesting a positive correlation between enhanced calcium transients and the inflammatory phenotype of astrocytes observed in the inflamed brain. Taken together, our results indicate that the astrocytic Gi pathway plays an inhibitory role in neuroinflammation, providing an opportunity to identify potential cellular and molecular targets to control neuroinflammation.

lncRNA MEG3 aggravated neuropathic pain and astrocyte overaction through mediating miR-130a-5p/CXCL12/CXCR4 axis

OBJECTIVE

Long non-coding RNAs (lncRNAs) exert a critical function in mediating neuropathic pain (NP). MEG3, a novel lncRNA, contributes to astrocyte activation and inflammation. However, its role in NP remains unclear.

METHODS

The chronic constriction injury (CCI) method was employed to construct an NP rat model. Astrocyte activation was induced by lipopolysaccharide (LPS). The profiles of MEG3, microRNA (miR)-130a-5p, CXC motif chemokine receptor 12 (CXCL12)/CXC motif chemokine receptor 4 (CXCR4), and the Rac1/NF-κB pathway in CCI rats' spinal cord tissues and astrocytes were monitored by reverse transcription-quantitative PCR (RT-qPCR) and western blot (WB). Pain scores of CCI rats were assessed. Enzyme-linked immunosorbent assay (ELISA) was adopted to monitor neuroinflammation alteration. The glial fibrillary acidic protein (GFAP)-labeled astrocytes were tested by immunohistochemistry (IHC). Bioinformatics, dual-luciferase reporter assay and RNA immunoprecipitation (RIP) were utilized to verify the molecular mechanism between MEG3 and miR-130a-3p.

RESULTS

MEG3, CXCL12 and CXCR4 were overexpressed and miR-130a-5p was knocked down in CCI rats and LPS-induced astrocytes. Up-regulating MEG3 aggravated NP, enhanced inflammatory cytokines interleukin-1β (IL-1β), tumor necrosis factor (TNF)-α, and interleukin-6 (IL-6) expression and release in CCI rats and LPS-induced astrocytes. Up-regulating miR-130-5p repressed LPS-induced inflammation in astrocytes. AS verified by the dual-luciferase reporter assay and RIP assay, MEG3 sponged miR-130a-5p as a competitive endogenous RNA (ceRNA). What's more, miR-130a-5p up-regulation weakened the MEG3-induced proinflammatory effects on LPS-induced astrocytes.

CONCLUSIONS

MEG3 aggravates NP and astrocyte activation via the miR-130a-5p/CXCL12/CXCR4 axis, which is a potential therapeutic target for NP.

Osteopontin regulates proliferation, migration, and survival of astrocytes depending on their activation phenotype

The glycoprotein osteopontin is highly upregulated in central nervous system (CNS) disorders such as ischemic stroke. Osteopontin regulates cell growth, cell adhesion, homeostasis, migration, and survival of various cell types. Accordingly, osteopontin is considered an essential regulator of regeneration and repair in the ischemic milieu. Astrocytes are the most abundant cells in the CNS and play significant roles in health and disease. Astrocytes are involved in homeostasis, promote neuroprotection, and regulate synaptic plasticity. Upon activation, astrocytes may adopt different phenotypes, termed A1 and A2. The direct effects of osteopontin on astrocytes, especially in distinct activation states, are yet unknown. The current study aimed to elucidate the impact of osteopontin on resting and active astrocytes. We established an inflammatory in vitro model of activated (A1) primary astrocytes derived from neonatal wistar rats by exposure to a distinct combination of proinflammatory cytokines. To model ischemic stroke in vitro, astrocytes were subjected to oxygen and glucose deprivation (OGD) in the presence or absence of osteopontin. Osteopontin modulated the activation phenotype by attenuating A1- and restoring A2-marker expression without compromising the active astrocytes' immunocompetence. Osteopontin promoted the proliferation of active and the migration of resting astrocytes. Following transient OGD, osteopontin mitigated the delayed ongoing death of primary astrocytes, promoting their survival. Data suggest that osteopontin differentially regulates essential functions of resting and active astrocytes and confirm a significant regulatory role of osteopontin in an in vitro ischemia model. Furthermore, the data suggest that osteopontin constitutes a promising target for experimental therapies modulating neuroregeneration and repair.

Role of astroglial toll-like receptors (TLRs) in central nervous system infections, injury and neurodegenerative diseases

Central nervous system (CNS) innate immunity plays essential roles in infections, neurodegenerative diseases, and brain or spinal cord injuries. Astrocytes and microglia are the principal cells that mediate innate immunity in the CNS. Pattern recognition receptors (PRRs), expressed by astrocytes and microglia, sense pathogen-derived or endogenous ligands released by damaged cells and initiate the innate immune response. Toll-like receptors (TLRs) are a well-characterized family of PRRs. The contribution of microglial TLR signaling to CNS pathology has been extensively investigated. Even though astrocytes assume a wide variety of key functions, information about the role of astroglial TLRs in CNS disease and injuries is limited. Because astrocytes display heterogeneity and exhibit phenotypic plasticity depending on the effectors present in the local milieu, they can exert both detrimental and beneficial effects. TLRs are modulators of these paradoxical astroglial properties. The goal of the current review is to highlight the essential roles played by astroglial TLRs in CNS infections, injuries and diseases. We discuss the contribution of astroglial TLRs to host defense as well as the dissemination of viral and bacterial infections in the CNS. We examine the link between astroglial TLRs and the pathogenesis of neurodegenerative diseases and present evidence showing the pivotal influence of astroglial TLR signaling on sterile inflammation in CNS injury. Finally, we define the research questions and areas that warrant further investigations in the context of astrocytes, TLRs, and CNS dysfunction.

Dynamic indentation reveals differential viscoelastic properties of white matter versus gray matter-derived astrocytes upon treatment with lipopolysaccharide

Astrocytes in white matter (WM) and gray matter (GM) brain regions have been reported to have different morphology and function. Previous single cell biomechanical studies have not differentiated between WM- and GM-derived samples. In this study, we explored the local viscoelastic properties of isolated astrocytes and show that astrocytes from rat brain WM-enriched areas are ~1.8 times softer than astrocytes from GM-enriched areas. Upon treatment with pro-inflammatory lipopolysaccharide, GM-derived astrocytes become significantly softer in the nuclear and the cytoplasmic regions, where the F-actin network appears rearranged, whereas WM-derived astrocytes preserve their initial mechanical features and show no alteration in the F-actin cytoskeletal network. We hypothesize that the flexibility in biomechanical properties of GM-derived astrocytes may contribute to promote regeneration of the brain under neuroinflammatory conditions.

Pharmacological approaches to mitigate neuroinflammation in Alzheimer's disease

Alzheimer's disease (AD) is one of the most prevalent neurodegenerative diseases characterized by the formation of extracellular amyloid beta (Aβ) plaques and intracellular neurofibrillary tangles (NFTs). Growing evidence suggested that there is an association between neuronal dysfunction and neuroinflammation (NI) in AD, coordinated by the chronic activation of astrocytes and microglial cells along with the subsequent excessive generation of the proinflammatory molecule. Therefore, a better understanding of the relationship between the nervous and immune systems is important in order to delay or avert the neurodegenerative events of AD. The inflammatory/immune pathways and the mechanisms to control these pathways may provide a novel arena to develop new drugs in order to target NI in AD. In this review, we represent the influence of cellular mediators which are involved in the NI process, with regards to the progression of AD. We also discuss the processes and the current status of multiple anti-inflammatory agents which are used in AD and have gone through or going through clinical trials. Moreover, new prospects for targeting NI in the development of AD drugs have also been highlighted.

Early and late neuropathological features of meningoencephalitis associated with Maraba virus infection

Maraba virus is a member of the genus Vesiculovirus of the Rhabdoviridae family that was isolated in 1983 from sandflies captured in the municipality of Maraba, state of Pará, Amazônia, Brazil. Despite 30 years having passed since its isolation, little is known about the neuropathology induced by the Maraba virus. Accordingly, in this study the histopathological features, inflammatory glial changes, cytokine concentrations, and nitric oxide activity in the encephalon of adult mice subjected to Maraba virus nostril infection were evaluated. The results showed that 6 days after intranasal inoculation, severe neuropathological-associated disease signs appeared, including edema, necrosis and pyknosis of neurons, generalized congestion of encephalic vessels, and intra- and perivascular meningeal lymphocytic infiltrates in several brain regions. Immunolabeling of viral antigens was observed in almost all central nervous system (CNS) areas and this was associated with intense microglial activation and astrogliosis. Compared to control animals, infected mice showed significant increases in interleukin (IL)-6, tumor necrosis factor (TNF)-α, interferon (INF)-γ, MCP-1, nitric oxide, and encephalic cytokine levels. We suggest that an exacerbated inflammatory response in several regions of the CNS of adult BALB/c mice might be responsible for their deaths.

Microglial responses after phagocytosis: Escherichia coli bioparticles, but not cell debris or amyloid beta, induce matrix metalloproteinase-9 secretion in cultured rat primary microglial cells

Upon infection or brain damage, microglia are activated to play roles in immune responses, including phagocytosis and soluble factor release. However, little is known whether the event of phagocytosis could be a trigger for releasing soluble factors from microglia. In this study, we tested if microglia secrete a neurovascular mediator matrix metalloproteinase-9 (MMP-9) after phagocytosis in vitro. Primary microglial cultures were prepared from neonatal rat brains. Cultured microglia phagocytosed Escherichia coli bioparticles within 2 hr after incubation and started to secrete MMP-9 at around 12 hr after the phagocytosis. A TLR4 inhibitor TAK242 suppressed the E. coli-bioparticle-induced MMP-9 secretion. However, TAK242 did not change the engulfment of E. coli bioparticles in microglial cultures. Because lipopolysaccharides (LPS), the major component of the outer membrane of E. coli, also induced MMP-9 secretion in a dose-response manner and because the response was inhibited by TAK242 treatment, we assumed that the LPS-TLR4 pathway, which was activated by adhering to the substance, but not through the engulfing process of phagocytosis, would play a role in releasing MMP-9 from microglia after E. coli bioparticle treatment. To support the finding that the engulfing step would not be a critical trigger for MMP-9 secretion after the event of phagocytosis in microglia, we confirmed that cell debris and amyloid beta were both captured into microglia via phagocytosis, but neither of them induced MMP-9 secretion from microglia. Taken together, these data demonstrate that microglial response in MMP-9 secretion after phagocytosis differs depending on the types of particles/substances that microglia encountered.

Abnormal Cannabidiol Affects Production of Pro-Inflammatory Mediators and Astrocyte Wound Closure in Primary Astrocytic-Microglial Cocultures

Abnormal cannabidiol (abn-CBD) exerts neuroprotective effects in vivo and in vitro. In the present study, we investigated the impact of abn-CBD on the glial production of proinflammatory mediators and scar formation within in vitro models. Primary astrocytic-microglial cocultures and astrocytic cultures from neonatal C57BL/6 mice and CB2 receptor knockout mice were stimulated with lipopolysaccharide (LPS), and the concentrations of tumor necrosis factor α (TNFα), interleukin-6 (IL-6) and nitrite were determined. Furthermore, we performed a live cell microscopy-based scratch-wound assay. After LPS stimulation, TNFα, IL-6 and nitrite production was more strongly increased in cocultures than in isolated astrocytes. Abn-CBD treatment attenuated the LPS-induced production of TNFα and nitrite in cocultures, while IL-6 production remained unaltered. In isolated astrocytes, only LPS-induced TNFα production was reduced by abn-CBD. Similar effects were observed after abn-CBD application in cocultures of CB2 knockout mice. Interestingly, LPS-induced TNFα and nitrite levels were far lower in CB2 knockout cultures compared to wildtypes, while IL-6 levels did not differ. In the scratch-wound assay, treatment with abn-CBD decelerated wound closure when microglial cells were present. Our data shows a differential role of abn-CBD for modulation of glial inflammation and astrocytic scar formation. These findings provide new explanations for mechanisms behind the neuroprotective potential of abn-CBD.

Serum amyloid A primes microglia for ATP-dependent interleukin-1β release

BACKGROUND

Acute-phase response is a systemic reaction to environmental/inflammatory insults and involves production of acute-phase proteins, including serum amyloid A (SAA). Interleukin-1β (IL-1β), a master regulator of neuroinflammation produced by activated inflammatory cells of the myeloid lineage, in particular microglia, plays a key role in the pathogenesis of acute and chronic diseases of the peripheral nervous system and CNS. IL-1β release is promoted by ATP acting at the purinergic P2X₇ receptor (P2X₇R) in cells primed with toll-like receptor (TLR) ligands.

METHODS

Purified (> 99%) microglia cultured from neonatal rat cortex and cerebellum were first primed with the putative TLR4/TLR2 agonist SAA (recombinant human Apo-SAA) or the established TLR4 agonist lipopolysaccharide (LPS) followed by addition of ATP. Expression of genes for the NLRP3 inflammasome, IL-1β, tumor necrosis factor-α (TNF-α), and SAA1 was measured by quantitative real-time polymerase chain reaction (q-PCR). Intracellular and extracellular amounts of IL-1β were determined by ELISA.

RESULTS

Apo-SAA stimulated, in a time-dependent manner, the expression of NLRP3, IL-1β, and TNF-α in cortical microglia, and produced a concentration-dependent increase in the intracellular content of IL-1β in these cells. A 2-h 'priming' of the microglia with Apo-SAA followed by addition of ATP for 1 h, resulting in a robust release of IL-1β into the culture medium, with a concomitant reduction in its intracellular content. The selective P2X₇R antagonist A740003 blocked ATP-dependent release of IL-1β. Microglia prepared from rat cerebellum displayed similar behaviors. As with LPS, Apo-SAA upregulated SAA1 and TLR2 mRNA, and downregulated that of TLR4. LPS was less efficacious than Apo-SAA, perhaps reflecting an action of the latter at TLR4 and TLR2. The TLR4 antagonist CLI-095 fully blocked the action of LPS, but only partially that of Apo-SAA. Although the TLR2 antagonist CU-CPT22 was inactive against Apo-SAA, it also failed to block the TLR2 agonist Pam₃CSK₄.

CONCLUSIONS

Microglia are central to the inflammatory process and a major source of IL-1β when activated. P2X₇R-triggered IL-1β maturation and export is thus likely to represent an important contributor to this cytokine pool. Given that SAA is detected in Alzheimer disease and multiple sclerosis brain, together with IL-1β-immunopositive microglia, these findings propose a link between P2X₇R, SAA, and IL-1β in CNS pathophysiology.

Sphingosine 1-phosphate receptors regulate TLR4-induced CXCL5 release from astrocytes and microglia

Sphingosine 1-phosphate receptors (S1PR) are G protein-coupled and compose a family with five subtypes, S1P1R-S1P5R. The drug Gilenya^® (Novartis, Basel, Switzerland) (Fingolimod; FTY720) targets S1PRs and was the first oral therapy for patients with relapsing-remitting multiple sclerosis (MS). The phosphorylated form of FTY720 (pFTY720) binds S1PRs causing initial agonism, then subsequent receptor internalization and functional antagonism. Internalization of S1P1R attenuates sphingosine 1-phosphate (S1P)-mediated egress of lymphocytes from lymph nodes, limiting aberrant immune function in MS. pFTY720 also exerts direct actions on neurons and glial cells which express S1PRs. In this study, we investigated the regulation of pro-inflammatory chemokine release by S1PRs in enriched astrocytes and microglial cultures. Astrocytes and microglia were stimulated with lipopolysaccharide (LPS) and increases in C-X-C motif chemokine 5 (CXCL5), also known as LIX (lipopolysaccharide-induced CXC chemokine) expression were quantified. Results showed that pFTY720 attenuated LPS-induced CXCL5 (LIX) protein release from astrocytes, as did the S1P1R selective agonist, SEW2871. In addition, pFTY720 blocked messenger ribonucleic acid (mRNA) transcription of the chemokines, (i) CXCL5/LIX, (ii) C-X-C motif chemokine 10 (CXCL10) also known as interferon gamma-induced protein 10 (IP10) and (iii) chemokine (C-C motif) ligand 2 (CCL2) also known as monocyte chemoattractant protein 1 (MCP1). Interestingly, inhibition of sphingosine kinase attenuated LPS-induced increases in mRNA levels of all three chemokines, suggesting that LPS-TLR4 (Toll-like receptor 4) signalling may enhance chemokine expression via S1P-S1PR transactivation. Lastly, these observations were not limited to astrocytes since we also found that pFTY720 attenuated LPS-induced release of CXCL5 from microglia. These data highlight a role for S1PR signalling in regulating the levels of chemokines in glial cells and support the notion that pFTY720 efficacy in multiple sclerosis may involve the direct modulation of astrocytes and microglia.

Green tea extract attenuates LPS-induced retinal inflammation in rats

Inflammation is in a wide spectrum of retinal diseases, causing irreversible blindness and visual impairment. We have previously demonstrated that Green Tea Extract (GTE) is a potent anti-inflammatory agent for anterior uveitis. Here we investigated the anti-inflammatory effect of GTE on lipopolysaccharides (LPS)-induced retinal inflammation in rats and explored the underlying mechanism. Adult rats were injected with LPS and GTE was administered intra-gastrically at 2, 8, 26 and 32 hours post-injection. Staining of whole-mount retina showed that the number of activated microglia cells was significantly increased at 48 hours post-injection, which was suppressed after GTE treatment in a dose-dependent manner. Activation of astrocytes and Müller glia in the retina was also suppressed after GTE treatment. Meanwhile, GTE reduced the expression of pro-inflammatory cytokines including IL-1β, TNF-α and IL-6 in retina and vitreous humor. These anti-inflammatory effects were associated with a reduced phosphorylation of STAT3 and NF-κB in the retina. Furthermore, the surface receptor of EGCG, 67LR, was localized on the neurons and glia in the retina. These findings demonstrate that GTE is an effective agent in suppressing LPS-induced retinal inflammation, probably through its potent anti-oxidative property and a receptor-mediated action on transcription factors that regulate production of pro-inflammatory cytokines.

Astrocyte/Microglia Cocultures as a Model to Study Neuroinflammation

Glial cell activation plays an important role in the pathogenesis of various neurodegenerative disorders as well as in chronic and neuropathic pain. This chapter describes a model which allows one to assess the individual and combined contributions of astrocytes and microglia in response to a pro-inflammatory stimulus, with emphasis on ionotropic purinergic receptors.

Microglia and Astrocytes in Alzheimer's Disease: Implications for Therapy

BACKGROUND

Alzheimer's disease (AD) is a complex neurodegenerative disorder characterized by the progressive loss of neurons, which typically leads to severe impairments in cognitive functions including memory and learning. Key pathological features of this disease include the deposition of highly insoluble amyloid β peptides and the formation of neurofibrillary tangles (NFTs) in the brain. Mounting evidence also implicates sustained glial-mediated inflammation as a major contributor of the neurodegenerative processes and cognitive deficits observed in AD.

METHODS

This paper provides an overview of findings from both human and animal studies investigating the role of microglia and astrocytes in AD, and discusses potential avenues for therapeutic intervention.

RESULTS

Glial-mediated inflammation is a 'double-edged sword', performing both detrimental and beneficial functions in AD. Despite tremendous effort in elucidating the molecular and cellular mechanisms underlying AD pathology, to date, there is no treatment that could prevent or cure this disease. Current treatments are only useful in slowing down the progression of AD and helping patients manage some of their behavioral and cognitive symptoms.

CONCLUSION

A better understanding of the role of microglia and astrocytes in the regulation of AD pathology is needed as this could pave the way for new therapeutic strategies.

AM404, paracetamol metabolite, prevents prostaglandin synthesis in activated microglia by inhibiting COX activity

Background

N-arachidonoylphenolamine (AM404), a paracetamol metabolite, is a potent agonist of the transient receptor potential vanilloid type 1 (TRPV1) and low-affinity ligand of the cannabinoid receptor type 1 (CB1). There is evidence that AM404 exerts its pharmacological effects in immune cells. However, the effect of AM404 on the production of inflammatory mediators of the arachidonic acid pathway in activated microglia is still not fully elucidated.

Method

In the present study, we investigated the effects of AM404 on the eicosanoid production induced by lipopolysaccharide (LPS) in organotypic hippocampal slices culture (OHSC) and primary microglia cultures using Western blot, immunohistochemistry, and ELISA.

Results

Our results show that AM404 inhibited LPS-mediated prostaglandin E₂ (PGE₂) production in OHSC, and LPS-stimulated PGE₂ release was totally abolished in OHSC if microglial cells were removed. In primary microglia cultures, AM404 led to a significant dose-dependent decrease in the release of PGE₂, independent of TRPV1 or CB1 receptors. Moreover, AM404 also inhibited the production of PGD₂ and the formation of reactive oxygen species (8-iso-PGF₂ alpha) with a reversible reduction of COX-1- and COX-2 activity. Also, it slightly decreased the levels of LPS-induced COX-2 protein, although no effect was observed on LPS-induced mPGES-1 protein synthesis.

Conclusions

This study provides new significant insights about the potential anti-inflammatory role of AM404 and new mechanisms of action of paracetamol on the modulation of prostaglandin production by activated microglia.

Electronic supplementary material

The online version of this article (10.1186/s12974-017-1014-3) contains supplementary material, which is available to authorized users.

(E)-2-Methoxy-4-(3-(4-methoxyphenyl) prop-1-en-1-yl) Phenol Ameliorates LPS-Mediated Memory Impairment by Inhibition of STAT3 Pathway

Alzheimer’s disease (AD) is pathologically characterized by an excessive accumulation of amyloid-beta (Aβ) fibrils within the brain. We tested the anti-inflammatory and anti-amyloidogenic effects of (E)-2-methoxy-4-(3-(4-methoxyphenyl) prop-1-en-1-yl) phenol (MMPP), a selective signal transducer and activator of transcription 3 (STAT3) inhibitor. We examined whether MMPP (5 mg/kg in drinking water for 1 month) prevents amyloidogenesis and cognitive impairment on AD model mice induced by intraperitoneal LPS (250 μg/kg daily 7 times) injections. Additionally, we investigated the anti-neuroinflammatory and anti-amyloidogenic effect of MMPP (1, 5, and 10 μg/mL) in LPS (1 μg/mL)-treated cultured astrocytes and microglial BV-2 cells. MMPP treatment reduced LPS-induced memory loss. This memory recovery effect was associated with the reduction of LPS-induced inflammatory proteins; cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) as well as activation of microglial cells and astrocytes in the brain. Furthermore, MMPP reduced LPS-induced β-secretase and Aβ generation. In in vitro study, LPS-induced expression of inflammatory proteins and amyloidogenic proteins was decreased in microglial BV-2 cells and cultured astrocytes by MMPP treatment. Moreover, MMPP treatment suppressed DNA binding activities of the activation of STAT3 in in vivo and in vitro. These results indicated that MMPP inhibits LPS-induced amyloidogenesis and neuroinflammation via inhibition of STAT3.

Expression and Differential Responsiveness of Central Nervous System Glial Cell Populations to the Acute Phase Protein Serum Amyloid A

Acute-phase response is a systemic reaction to environmental/inflammatory insults and involves hepatic production of acute-phase proteins, including serum amyloid A (SAA). Extrahepatically, SAA immunoreactivity is found in axonal myelin sheaths of cortex in Alzheimer's disease and multiple sclerosis (MS), although its cellular origin is unclear. We examined the responses of cultured rat cortical astrocytes, microglia and oligodendrocyte precursor cells (OPCs) to master pro-inflammatory cytokine tumour necrosis factor (TNF)-α and lipopolysaccaride (LPS). TNF-α time-dependently increased Saa1 (but not Saa3) mRNA expression in purified microglia, enriched astrocytes, and OPCs (as did LPS for microglia and astrocytes). Astrocytes depleted of microglia were markedly less responsive to TNF-α and LPS, even after re-addition of microglia. Microglia and enriched astrocytes showed complementary Saa1 expression profiles following TNF-α or LPS challenge, being higher in microglia with TNF-α and higher in astrocytes with LPS. Recombinant human apo-SAA stimulated production of both inflammatory mediators and its own mRNA in microglia and enriched, but not microglia-depleted astrocytes. Co-ultramicronized palmitoylethanolamide/luteolin, an established anti-inflammatory/ neuroprotective agent, reduced Saa1 expression in OPCs subjected to TNF-α treatment. These last data, together with past findings suggest that co-ultramicronized palmitoylethanolamide/luteolin may be a novel approach in the treatment of inflammatory demyelinating disorders like MS.

Astrocytes Enhance Streptococcus suis-Glial Cell Interaction in Primary Astrocyte-Microglial Cell Co-Cultures

Streptococcus (S.) suis infections are the most common cause of meningitis in pigs. Moreover, S. suis is a zoonotic pathogen, which can lead to meningitis in humans, mainly in adults. We assume that glial cells may play a crucial role in host-pathogen interactions during S. suis infection of the central nervous system. Glial cells are considered to possess important functions during inflammation and injury of the brain in bacterial meningitis. In the present study, we established primary astrocyte-microglial cell co-cultures to investigate interactions of S. suis with glial cells. For this purpose, microglial cells and astrocytes were isolated from new-born mouse brains and characterized by flow cytometry, followed by the establishment of astrocyte and microglial cell mono-cultures as well as astrocyte-microglial cell co-cultures. In addition, we prepared microglial cell mono-cultures co-incubated with uninfected astrocyte mono-culture supernatants and astrocyte mono-cultures co-incubated with uninfected microglial cell mono-culture supernatants. After infection of the different cell cultures with S. suis, bacteria-cell association was mainly observed with microglial cells and most prominently with a non-encapsulated mutant of S. suis. A time-dependent induction of NO release was found only in the co-cultures and after co-incubation of microglial cells with uninfected supernatants of astrocyte mono-cultures mainly after infection with the capsular mutant. Only moderate cytotoxic effects were found in co-cultured glial cells after infection with S. suis. Taken together, astrocytes and astrocyte supernatants increased interaction of microglial cells with S. suis. Astrocyte-microglial cell co-cultures are suitable to study S. suis infections and bacteria-cell association as well as NO release by microglial cells was enhanced in the presence of astrocytes.

Ligand engagement of Toll-like receptors regulates their expression in cortical microglia and astrocytes

Background

Toll-like receptor (TLR) activation on microglia and astrocytes are key elements in neuroinflammation which accompanies a number of neurological disorders. While TLR activation on glia is well-established to up-regulate pro-inflammatory mediator expression, much less is known about how ligand engagement of one TLR may affect expression of other TLRs on microglia and astrocytes.

Methods

In the present study, we evaluated the effects of agonists for TLR2 (zymosan), TLR3 (polyinosinic-polycytidylic acid (poly(I:C)), a synthetic analogue of double-stranded RNA) and TLR4 (lipopolysaccaride (LPS)) in influencing expression of their cognate receptor as well as that of the other TLRs in cultures of rat cortical purified microglia (>99.5 %) and nominally microglia-free astrocytes. Elimination of residual microglia (a common contaminant of astrocyte cultures) was achieved by incubation with the lysosomotropic agent l-leucyl-l-leucine methyl ester (L-LME).

Results

Flow cytometric analysis confirmed the purity (essentially 100 %) of the obtained microglia, and up to 5 % microglia contamination of astrocytes. L-LME treatment effectively removed microglia from the latter (real-time polymerase chain reaction). The three TLR ligands robustly up-regulated gene expression for pro-inflammatory markers (interleukin-1 and interleukin-6, tumor necrosis factor) in microglia and enriched, but not purified, astrocytes, confirming cellular functionality. LPS, zymosan and poly(I:C) all down-regulated TLR4 messenger RNA (mRNA) and up-regulated TLR2 mRNA at 6 and 24 h. In spite of their inability to elaborate pro-inflammatory mediator output, the nominally microglia-free astrocytes (>99 % purity) also showed similar behaviours to those of microglia, as well as changes in TLR3 gene expression. LPS interaction with TLR4 activates downstream mitogen-activated protein kinase and nuclear factor-κB signalling pathways and subsequently causes inflammatory mediator production. The effects of LPS on TLR2 mRNA in both cell populations were antagonized by a nuclear factor-κB inhibitor.

Conclusions

TLR2 and TLR4 activation in particular, in concert with microglia and astrocytes, comprise key elements in the initiation and maintenance of neuropathic pain. The finding that both homologous (zymosan) and heterologous (LPS, poly(I:C)) TLR ligands are capable of regulating TLR2 gene expression, in particular, may have important implications in understanding the relative contributions of different TLRs in neurological disorders associated with neuroinflammation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0458-6) contains supplementary material, which is available to authorized users.

APOE-modulated Aβ-induced neuroinflammation in Alzheimer's disease: current landscape, novel data, and future perspective

Chronic glial activation and neuroinflammation induced by the amyloid-β peptide (Aβ) contribute to Alzheimer's disease (AD) pathology. APOE4 is the greatest AD-genetic risk factor; increasing risk up to 12-fold compared to APOE3, with APOE4-specific neuroinflammation an important component of this risk. This editorial review discusses the role of APOE in inflammation and AD, via a literature review, presentation of novel data on Aβ-induced neuroinflammation, and discussion of future research directions. The complexity of chronic neuroinflammation, including multiple detrimental and beneficial effects occurring in a temporal and cell-specific manner, has resulted in conflicting functional data for virtually every inflammatory mediator. Defining a neuroinflammatory phenotype (NIP) is one way to address this issue, focusing on profiling the changes in inflammatory mediator expression during disease progression. Although many studies have shown that APOE4 induces a detrimental NIP in peripheral inflammation and Aβ-independent neuroinflammation, data for APOE-modulated Aβ-induced neuroinflammation are surprisingly limited. We present data supporting the hypothesis that impaired apoE4 function modulates Aβ-induced effects on inflammatory receptor signaling, including amplification of detrimental (toll-like receptor 4-p38α) and suppression of beneficial (IL-4R-nuclear receptor) pathways. To ultimately develop APOE genotype-specific therapeutics, it is critical that future studies define the dynamic NIP profile and pathways that underlie APOE-modulated chronic neuroinflammation. In this editorial review, we present data supporting the hypothesis that impaired apoE4 function modulates Aβ-induced effects on inflammatory receptor signaling, including amplification of detrimental (TLR4-p38α) and suppression of beneficial (IL-4R-nuclear receptor) pathways, resulting in an adverse NIP that causes neuronal dysfunction. NIP, Neuroinflammatory phenotype; P.I., pro-inflammatory; A.I., anti-inflammatory.

Toll-like receptors 2, -3 and -4 prime microglia but not astrocytes across central nervous system regions for ATP-dependent interleukin-1β release

Interleukin-1β (IL-1β) is a crucial mediator in the pathogenesis of inflammatory diseases at the periphery and in the central nervous system (CNS). Produced as an unprocessed and inactive pro-form which accumulates intracellularly, release of the processed cytokine is strongly promoted by ATP acting at the purinergic P2X₇ receptor (P2X₇R) in cells primed with lipopolysaccharide (LPS), a Toll-like receptor (TLR) 4 ligand. Microglia are central to the inflammatory process and a major source of IL-1β when activated. Here we show that purified (>99%) microglia cultured from rat cortex, spinal cord and cerebellum respond robustly to ATP-dependent IL-1β release, upon priming with a number of TLR isoform ligands (zymosan and Pam3CSK4 for TLR2, poly(I:C) for TLR3). Cytokine release was prevented by a P2X₇R antagonist and inhibitors of stress-activated protein kinases. Enriched astrocytes (≤5% microglia) from these CNS regions displayed responses qualitatively similar to microglia but became unresponsive upon eradication of residual microglia with the lysosomotropic agent Leu-Leu-OMe. Activation of multiple TLR isoforms in nervous system pathology, coupled with elevated extracellular ATP levels and subsequent P2X₇R activation may represent an important route for microglia-derived IL-1β. This phenomenon may have important consequences for neuroinflammation and its position to the common pathology of CNS diseases.

Microglial regulation of immunological and neuroprotective functions of astroglia

Microglia and astroglia play critical roles in the development, function, and survival of neurons in the CNS. However, under inflammatory conditions the role of astrogliosis in the inflammatory process and its effects on neurons remains unclear. Here, we used several types of cell cultures treated with the bacterial inflammogen LPS to address these questions. We found that the presence of astroglia reduced inflammation-driven neurotoxicity, suggesting that astrogliosis is principally neuroprotective. Neutralization of supernatant glial cell line-derived neurotrophic factor (GDNF) released from astroglia significantly reduced this neuroprotective effect during inflammation. To determine the immunological role of astroglia, we optimized a highly-enriched astroglial culture protocol and demonstrated that LPS failed to induce the synthesis and release of TNF-α and iNOS/NO. Instead we found significant enhancement of TNF-α and iNOS expression in highly-enriched astroglial cultures required the presence of 0.5-1% microglia, respectively. Thus suggesting that microglial-astroglial interactions are required for LPS to induce the expression of pro-inflammatory factors and GDNF from astroglia. Specifically, we found that microglia-derived TNF-α plays a pivotal role as a paracrine signal to regulate the neuroprotective functions of astrogliosis. Taken together, these findings suggest that astroglia may not possess the ability to directly recognize the innate immune stimuli LPS, but rather depend on crosstalk with microglia to elicit release of neurotrophic factors as a counterbalance to support neuronal survival from the collateral damage generated by activated microglia during neuroinflammation.

Biological role of Toll-like receptor-4 in the brain

The Toll-like receptors (TLRs) are a family of microbe-sensing receptors that play a central role in the regulation of the host immune system. TLR4 has been described in the brain and seems to regulate some physiological processes, such as neurogenesis. TLR4 has also been reported to play a role during neurodegenerative disorders, including Alzheimer's disease, amyotrophic lateral sclerosis, multiple sclerosis and Parkinson's disease. This review is focused on reports concerning recent insights into the role and activation mechanisms of TLR4 in the brain, in pathological and physiological conditions, as well as the therapeutic benefit that could derive from TLR4 modulation.

Comparison of primary and secondary rat astrocyte cultures regarding glucose and glutathione metabolism and the accumulation of iron oxide nanoparticles

Astrocyte-rich primary cultures (APCs) are frequently used as a model system for the investigation of properties of brain astrocytes. However, as APCs contain a substantial number of microglial and oligodendroglial cells, biochemical parameters determined for such cultures may at least in part reflect also the presence of the contaminating cell types. To lower the potential contributions of microglial and oligodendroglial cells on properties of the astrocytes in APCs we prepared rat astrocyte-rich secondary cultures (ASCs) by subculturing of APCs and compared these ASCs with APCs regarding basal metabolic parameters, specific enzyme activities and the accumulation of iron oxide nanoparticles. Immunocytochemical characterization revealed that ASCs contained only minute amounts of microglial and oligodendroglial cells. ASCs and APCs did not significantly differ in their specific glucose consumption and lactate production rates, in their specific iron and glutathione contents, in their specific activities of various enzymes involved in glucose and glutathione metabolism nor in their accumulation of iron oxide nanoparticles. Thus, the absence or presence of some contaminating microglial and oligodendroglial cells appears not to substantially modulate the investigated metabolic parameters of astrocyte cultures.

Microglial disruption in young mice with early chronic lead exposure

The mechanisms by which early chronic lead (Pb) exposure alter brain development have not been identified. We examined neuroimmune system effects in C57BL/6J mice with Pb exposure, including levels that may be common among children in lower socioeconomic income environments. Pups were exposed via dams' drinking water from birth to post-natal day 28 to low, high or no Pb conditions. We compared gene expression of neuroinflammatory markers (study 1); and microglial mean cell body volume and mean cell body number in dentate gyrus, and dentate gyrus volume (study 2). Blood Pb levels in exposed animals at sacrifice (post-natal day 28) ranged from 2.66 to 20.31μg/dL. Only interleukin-6 (IL6) differed between groups and reductions were dose-dependent. Microglia cell body number also differed between groups and reductions were dose-dependent. As compared with controls, microglia cell body volume was greater but highly variable in only low-dose animals; dentate gyri volumes in low- and high-dose animals were reduced. The results did not support a model of increased neuroinflammation. Instead, early chronic exposure to Pb disrupted microglia via damage to, loss of, or lack of proliferation of microglia in the developing brains of Pb-exposed animals.

Depletion of microglia from primary cellular cultures

Primary cultures are an important in vitro tool to study cellular processes and interactions. These cultures are complex systems, composed of many cell types, including neurons, astrocytes, oligodendrocytes, microglia, NG2 cells, and endothelial cells. For some studies it is necessary to be able to study a pure culture of one cell type, or eliminate a particular cell type, to better understand its function. There exist cell culture protocols for making pure astrocyte or microglia cultures. Here we present two protocols to produce cultures depleted for microglia: in the first case, from a mixed astrocyte-microglia culture and, in the second, for eliminating microglia from neuronal cultures.

Astrocyte-derived thrombospondins mediate the development of hippocampal presynaptic plasticity in vitro

Astrocytes contribute to many neuronal functions, including synaptogenesis, but their role in the development of synaptic plasticity remains unclear. Presynaptic muting of hippocampal glutamatergic terminals defends against excitotoxicity. Here we studied the role of astrocytes in the development of presynaptic muting at glutamatergic synapses in rat hippocampal neurons. We found that astrocytes were critical for the development of depolarization-dependent and G(i/o)-dependent presynaptic muting. The ability of cAMP analogues to modulate presynaptic function was also impaired by astrocyte deficiency. Although astrocyte deprivation resulted in postsynaptic glutamate receptor deficits, this effect appeared independent of astrocytes' role in presynaptic muting. Muting was restored with chronic, but not acute, treatment with astrocyte-conditioned medium, indicating that a soluble factor is permissive for muting. Astrocyte-derived thrombospondins (TSPs) are likely responsible because TSP1 mimicked the effect of conditioned medium, and gabapentin, a high-affinity antagonist of TSP binding to the α2δ-1 calcium channel subunit, mimicked astrocyte deprivation. We found evidence that protein kinase A activity is abnormal in astrocyte-deprived neurons but restored by TSP1, so protein kinase A dysfunction may provide a mechanism by which muting is disrupted during astrocyte deficiency. In summary our results suggest an important role for astrocyte-derived TSPs, acting through α2δ-1, in maturation of a potentially important form of presynaptic plasticity.

Microglia are required for astroglial Toll-like receptor 4 response and for optimal TLR2 and TLR3 response

Within the central nervous system, astrocytes and microglia are the primary responders to endogenous ligands released upon injury and stress, as well as to infectious pathogens. Toll-like receptors (TLRs) are implicated in recognition of both types of stimulus. Whether astrocytes respond as strongly as microglia to TLR agonists remains contentious. In this study, we have rigorously purified astrocytes to determine their capacity for autonomous TLR response, in absence of microglia. We used flow cytometry and differential adhesion as well as a myeloid lineage-specific suicide gene to purify astrocytes from mixed glial cultures and measured their response to TLR agonists. Our results show that the response of astrocytes to TLR2 and TLR3 agonists is greatly enhanced by, and response to TLR4 agonists is completely dependent on, the presence of functional microglia. In the case of the TLR4 response to lipopolysaccharide, microglia exert their effect on astrocytes at least partially through release of soluble mediators that directly activate or facilitate astrocyte responses. Our findings underline the contribution of glial crosstalk in CNS responses to injury or inflammation.

Pro-inflammatory gene expression and neurotoxic effects of activated microglia are attenuated by absence of CCAAT/enhancer binding protein β

Background

Microglia and astrocytes respond to homeostatic disturbances with profound changes of gene expression. This response, known as glial activation or neuroinflammation, can be detrimental to the surrounding tissue. The transcription factor CCAAT/enhancer binding protein β (C/EBPβ) is an important regulator of gene expression in inflammation but little is known about its involvement in glial activation. To explore the functional role of C/EBPβ in glial activation we have analyzed pro-inflammatory gene expression and neurotoxicity in murine wild type and C/EBPβ-null glial cultures.

Methods

Due to fertility and mortality problems associated with the C/EBPβ-null genotype we developed a protocol to prepare mixed glial cultures from cerebral cortex of a single mouse embryo with high yield. Wild-type and C/EBPβ-null glial cultures were compared in terms of total cell density by Hoechst-33258 staining; microglial content by CD11b immunocytochemistry; astroglial content by GFAP western blot; gene expression by quantitative real-time PCR, western blot, immunocytochemistry and Griess reaction; and microglial neurotoxicity by estimating MAP2 content in neuronal/microglial cocultures. C/EBPβ DNA binding activity was evaluated by electrophoretic mobility shift assay and quantitative chromatin immunoprecipitation.

Results

C/EBPβ mRNA and protein levels, as well as DNA binding, were increased in glial cultures by treatment with lipopolysaccharide (LPS) or LPS + interferon γ (IFNγ). Quantitative chromatin immunoprecipitation showed binding of C/EBPβ to pro-inflammatory gene promoters in glial activation in a stimulus- and gene-dependent manner. In agreement with these results, LPS and LPS+IFNγ induced different transcriptional patterns between pro-inflammatory cytokines and NO synthase-2 genes. Furthermore, the expressions of IL-1β and NO synthase-2, and consequent NO production, were reduced in the absence of C/EBPβ. In addition, neurotoxicity elicited by LPS+IFNγ-treated microglia co-cultured with neurons was completely abolished by the absence of C/EBPβ in microglia.

Conclusions

These findings show involvement of C/EBPβ in the regulation of pro-inflammatory gene expression in glial activation, and demonstrate for the first time a key role for C/EBPβ in the induction of neurotoxic effects by activated microglia.

c-Jun is essential for the induction of Il-1β gene expression in in vitro activated Bergmann glial cells

In the central nervous system (CNS), the c-Jun transcription factor has been mainly studied in neuronal cells and coupled to apoptotic and regenerative pathways following brain injury. Besides, several studies have shown a transcriptional role of c-Jun in activated cortical and spinal astrocytes. In contrast, little is known about c-Jun expression and transactivation in Bergmann glial (BG) cells, the radial cerebellar astrocytes playing crucial roles in cerebellar development and physiology. Here, we used neuronal/glial cerebellar cultures from neonatal mice to assess putative functions of c-Jun in BG cells. By performing double immunocytochemical staining of c-Jun and two BG specific markers, S100 and glutamate aspartate transporter (GLAST), we show that c-Jun was highly expressed in radial glial cells derived from Bergmann glia. Bergmann glia-derived cells expressed toll-like receptor 4 and treatment with bacterial lipopolysaccharide (LPS)-induced c-Jun phosphorylation at serine 63, a hallmark of c-Jun transactivation, exclusively in BG cells. Moreover, LPS-induced IL-1β expression and inhibition of c-Jun N-terminal kinase (JNK) activity abolished both c-Jun phosphorylation and the increase of IL-1β mRNA. Notably, LPS failed to induce IL-1β mRNA in neuronal/glial cerebellar cultures generated from conditional knockout mice lacking c-Jun expression in the CNS, indicating the essential role of c-Jun in astroglial-specific induction of IL-1β. Immunohistochemical analyses of c-Jun-expressing cells in the early postnatal cerebellum confirmed in vivo the expression of c-Jun in BG cells and uncovered a dynamic expression of c-Jun during the formation of the BG monolayer. Altogether, our finding underlines a putative role of c-Jun in astroglia-mediated neuroinflammatory dysfunctions of the cerebellum.

The ribotoxin deoxynivalenol affects the viability and functions of glial cells

Glial cells are responsible for maintaining brain homeostasis. Modification of the viability and functions of glial cells, including astrocytes and microglia, are associated with neuronal death and neurological diseases. Many toxins (heavy metals, pesticides, bacterial or viral toxins) are known to impact on brain cell viability and functions. Although recent publications suggest a potential link between environmental exposure of humans to mycotoxins and neurological diseases, data regarding the effects of fungal toxins on brain cells are scarce. In the present study, we looked at the impact of deoxynivalenol (DON), a fungal ribotoxin, on glial cells from animal and human origin. We found that DON decreased the viability of glial cells with a higher toxicity against microglial cells compared with astrocytes. In addition to cellular toxicity, DON affected key functions of glial cells. Thus, DON caused a biphasic effect on the neuroinflammatory response of microglia to lipopolysaccharide (LPS), while sublethal doses of DON increased the LPS-induced secretion of TNF-α and nitric oxide, toxic doses inhibited it. In addition to affecting microglial functions, sublethal doses of DON also suppressed the uptake of L-glutamate by astrocytes. This inhibition was associated with a modification of the expression of the glutamate transporters at the plasma membrane. Our results suggest that environmental ribotoxins such as DON could, at low doses, cause modifications of brain homeostasis and possibly participate in the etiology of neurological diseases in which alterations of the glia are involved.

CCAAT/enhancer binding protein delta in microglial activation

The transcription factor CCAAT/enhancer binding protein delta (C/EBP delta) regulates transcription of genes that play important roles in glial activation. Previous studies have shown the astroglial expression of C/EBP delta but the microglial expression of C/EBP delta remains virtually unexplored, with the exception of two microarray studies. In this report, using murine primary cultures and BV2 cells we clearly demonstrate that C/EBP delta is expressed by microglia and it is upregulated in microglial activation. Lipopolysaccharide upregulates C/EBP delta both in microglia and in astrocytes. This effect is time-dependent, with a maximum effect at 3 hr at mRNA level and at 4-8 hr at protein level, and concentration-dependent, with a maximum effect at 100 ng/mL. The lipopolysaccharide-induced C/EBP delta upregulation in BV2 microglia is mimicked by agonists of the toll-like receptors 2, 3 and 9 and can be prevented by an inhibitor of extracellular signal-regulated kinase activation. C/EBP delta from activated BV2 microglia binds to the cyclooxygenase-2 promoter and forms complexes with C/EBP beta isoforms. These results point to C/EBP delta as a putative key regulator of proinflammatory gene expression in microglial activation.

Astrocytes play a key role in activation of microglia by persistent Borna disease virus infection

Neonatal Borna disease virus (BDV) infection of the rat brain is associated with microglial activation and damage to certain neuronal populations. Since persistent BDV infection of neurons is nonlytic in vitro, activated microglia have been suggested to be responsible for neuronal cell death in vivo. However, the mechanisms of activation of microglia in neonatally BDV-infected rat brains remain unclear. Our previous studies have shown that activation of microglia by BDV in culture requires the presence of astrocytes as neither the virus nor BDV-infected neurons alone activate microglia. Here, we evaluated the mechanisms whereby astrocytes can contribute to activation of microglia in neuron-glia-microglia mixed cultures. We found that persistent infection of neuronal cells leads to activation of uninfected astrocytes as measured by elevated expression of RANTES. Activation of astrocytes then produces activation of microglia as evidenced by increased formation of round-shaped, MHCI-, MHCII- and IL-6-positive microglia cells. Our analysis of possible molecular mechanisms of activation of astrocytes and/or microglia in culture indicates that the mediators of activation may be soluble heat-resistant, low molecular weight factors. The findings indicate that astrocytes may mediate activation of microglia by BDV-infected neurons. The data are consistent with the hypothesis that microglia activation in the absence of neuronal damage may represent initial steps in the gradual neurodegeneration observed in brains of neonatally BDV-infected rats.

Activated microglia modulate astroglial enzymes involved in oxidative and inflammatory stress and increase the resistance of astrocytes to oxidative stress in vitro

Neuropathological processes in the central nervous system are commonly accompanied by an activation of microglia and astrocytes. The involvement of both cell populations in the onset and progress of neurological disorders has been widely documented, implicating both beneficial and detrimental influences on the neural tissue. Nevertheless, little is known about the interplay of these glial cell populations, especially under diseased conditions. To examine the effects of activated microglia on astrocytes purified rat astroglial cell cultures were treated with medium conditioned by purified quiescent (MCM[-]) or lipopolysaccharide (LPS)-activated rat microglia (MCM[+]) and subjected to a comparative proteome analysis based on two-dimensional gel electrophoresis. No significant down regulation of proteins was observed. The majority of the 19 proteins identified by means of nano HPLC/ESI-MS/MS in the 12 most prominent protein spots significantly overexpressed (> or =2-fold) in MCM[+] treated astrocytes are involved in inflammatory processes and oxidative stress response: superoxide dismutases (Sod), peroxiredoxins, glutathione S-transferases (Gst), nucleoside diphosphate kinase B, argininosuccinate synthase (Ass), and cellular retinol-binding protein I (Rbp1). Sod2, Rbp1, Gstp1, and Ass were also significantly increased on the mRNA level determined by quantitative RT-PCR. The upregulation of antioxidative enzymes in astrocytes was accompanied by a higher resistance to oxidative stress induced by H2O2. These results show that activated microglia change the expression of antioxidative proteins in astrocytes and protect them against oxidative stress, which might be an effective way to increase the neuroprotective potential of astrocytes under pathological conditions associated with oxidative stress and inflammation.

The ANP-cGMP-protein kinase G pathway induces a phagocytic phenotype but decreases inflammatory gene expression in microglial cells

Reactive gliosis is a prominent feature of CNS injury that involves dramatic changes in glial cell morphology together with increased motility, phagocytic activity, and release of inflammatory mediators. We have recently demonstrated that stimulation of the cGMP-protein kinase G (PKG) pathway by NO or atrial natriuretic peptide (ANP) regulates cytoskeleton dynamics and motility in rat astrocytes in culture. In this work, we show that the cGMP-PKG pathway stimulated by ANP, but not by NO, regulates microglial cell morphology by inducing a dramatic reorganization in the actin cytoskeleton. Both ANP (0.01-1.0 microM) and the permeable cGMP analog, dibutyryl-cGMP (1-100 microM), promote a rapid (maximal at 30 min) and concentration-dependent increase in size, rounding, and lamellipodia and filopodia formation in rat brain cultured microglia. These morphological changes involve an augment and redistribution of F-actin and result in increased phagocytic activity. ANP-induced rearrangements in actin cytoskeleton and inert particle phagocytosis are prevented by the PKG inhibitor, Rp-8-Br-PET-cGMPS (0.5 microM), and involve inhibition of RhoA GTPase and activation of Rac1 and Cdc42. However, ANP does not induce NO synthase Type 2 (NOS-2) or tumor necrosis factor-alpha expression and is able to decrease lipopolysaccharide (LPS)-elicited induction of these inflammatory genes. The morphological changes and the decrease of LPS-induced NOS-2 expression produced by ANP in cultured microglia are also observed by immunostaining in organotypic cultures from rat hippocampus. These results suggest that stimulation of the ANP-cGMP-PKG pathway in microglia could play a beneficial role in the resolution of neuroinflammation by removing dead cells and decreasing levels of proinflammatory mediators.

Angiotensin type 1 receptor antagonist inhibits lipopolysaccharide-induced stimulation of rat microglial cells by suppressing nuclear factor kappaB and activator protein-1 activation

We investigated whether angiotensin (ANG) II and its receptors contribute to lipopolysaccharide (LPS)-induced microglial activation through activation of the proinflammatory transcription factors nuclear factor kappaB (NF-kappaB) and activator protein-1 (AP-1). Using primary microglial cell cultures, we examined whether losartan [ANG type 1 receptor (AT(1)) antagonist] alters the effects of LPS on: the production of interleukin-1 (IL-1) and nitric oxide, cell morphology, and NF-kappaB and AP-1 activities. Reverse transcription-polymerase chain reaction revealed that LPS-stimulated microglial cells exhibited marked mRNA expression for AT(1), ANG type 2 receptor (AT(2)) and the ANG II precursor angiotensinogen, whereas non-stimulated microglial cells expressed only those for AT(2) and angiotensinogen. We further demonstrated marked peptide/protein expression for AT(1) and ANG II in LPS-activated microglial cells. LPS (100 ng/mL)-stimulated microglial cells showed increased concentrations of IL-1 and nitrite (a relatively stable metabolite of nitric oxide), and increased expression of IL-1 mRNA as well as a morphological change from an amoeboid shape to a multipolar (mostly bipolar but sometimes tripolar) rod shape. These effects were all significantly inhibited by losartan treatment (10(-5) M or less). NF-kappaB and AP-1 activities were enhanced in LPS-stimulated microglial cells, effects that were significantly suppressed by losartan (10(-5) M). ANG II application enhanced the LPS-induced increases in IL-1 and nitrite concentrations, as well as the LPS-induced morphological changes and AP-1 activation, and these enhancements were inhibited by losartan (10(-5) M). These results suggest that endogenous ANG II enhances LPS-induced microglial activities through stimulation of the microglial AT(1), which itself evokes activation of the transcription factors NF-kappaB and AP-1.

Translocator protein 18 kDa is involved in the regulation of reactive gliosis

Translocator protein (18 kDa) (TSPO), previously known as peripheral-type benzodiazepine receptor, is a critical component of the mitochondrial permeability transition pore. Brain inflammation results in the induction of the expression of TSPO in glial cells and some TSPO ligands decrease reactive gliosis after brain injury. However, since some TSPO ligands are neuroprotective, their effects on reactive gliosis may be the consequence of a reduced neurodegeneration. To assess whether TSPO ligands can modulate reactive gliosis in absence of neuronal death, we have tested their effects on the inflammatory response induced in the hippocampus of male rats by the intracerebroventricular infusion of lipopolysaccharide (LPS). LPS treatment did not induce neuronal death, assessed by Fluoro jade-B staining, but increased the number of cells immunoreactive for vimentin and MHC-II, used as markers of reactive astrocytes and reactive microglia, respectively. Furthermore, LPS produced an increase in the number of proliferating microglia. The TSPO ligand PK11195 reduced the number of MHC-II immunoreactive cells and the proliferation of microglia in LPS treated rats. In contrast, another TSPO ligand, Ro5-4864, did not significantly affect the response of microglia to LPS. Neither PK11195 nor Ro5-4864 affected the LPS-mediated increase in the number of vimentin-immunoreactive astrocytes at the time point studied, although PK11195 reduced vimentin immunoreactivity. These findings identify TSPO as a potential target for controlling neural inflammation, showing that the TSPO ligand PK11195 may reduce microglia activation by a mechanism that is independent of the regulation of neuronal survival.

Spatial arrangement of microglia in the mouse hippocampus: a stereological study in comparison with astrocytes

Microglia are classically considered to be immune cells in the brain, but have now been proven to be involved in neuronal activity as well. Here we stereologically analyzed the spatial arrangement of microglia in the mouse hippocampus. First, we estimated the numerical densities (NDs) of microglia identified by ionized calcium-binding adaptor molecule 1 (Iba1). Despite that microglia appeared to be evenly distributed throughout the hippocampal area, the NDs demonstrated significant dorsoventral, interregional, and interlaminar differences. Briefly, the NDs in the ventral hippocampus were significantly lower in the CA3 region than in the CA1 region and dentate gyrus, although no interregional differences were detectable in the dorsal hippocampus. Both in the CA1 and CA3 regions, the NDs were significantly higher in the stratum lacunosum-moleculare than in the remaining layers. Next, we investigated the spatial patterns of distribution of Iba1-labeled microglia and S100beta-labeled astrocytes. So far as we examined, the somato-somatic contacts were not seen among microglia or among astrocytes, whereas the close apposition between microglia and astrocytes were occasionally detected. The 3D point process analysis showed that the spatial distribution of microglia was significantly repulsive. Because the statistical territory of single microglia was larger than that estimated from process tracing, they are not likely to touch each other with their processes. Astrocytes were distributed slightly repulsively with overlapping areas. The 3D point process analysis also revealed a significant spatial attraction between microglia and astrocytes. The present findings provide a novel anatomical basis for glial research.

C6 glioma cell insoluble matrix components enhance interferon-gamma-stimulated inducible nitric-oxide synthase/nitric oxide production in BV2 microglial cells

Microglia are the primary central nervous system immune effector cells. Microglial activation is linked to interactions with extracellular cytokines and the extracellular matrix (ECM). Astrocytomas are characterized by their diffuse nature, which is regulated by insoluble ECM components produced by the tumor cells that are largely absent from normal central nervous system tissue. The present study examined the influence of astrocytoma (C6 rat glioma) insoluble matrix components on interferon-gamma (IFN-gamma)-mediated inducible nitric-oxide synthase (iNOS) induction in microglial cells. We found that IFN-gamma-stimulated iNOS induction and nitric oxide release was greater in microglia cultured on C6 glioma cell-derived matrices compared with microglia cultured on primary rat astrocyte-derived matrices. Culture of microglia on C6 glioma cell-derived matrices also led to activation of STAT1, augmentation of IFN-gamma-induced STAT-3 activation, and an increase in IFN-gamma-activated site (GAS)-luciferase reporter activity. In addition, culture of microglia on C6 glioma cell-derived matrices activated NF-kappaB DNA binding activity and transcriptional activity. The results suggest that insoluble matrix components derived from malignant glioma cells can regulate microglia activation. These factors may include ECM components, such as fibronectin, collagen, laminin, vitronectin, and other nondiffusible compounds, and laminin seems to a critical regulator of this process. Microglia activation and subsequent brain inflammation may influence tumor growth, treatment, and metastasis. Better understanding of the regulation of microglial activation by astrocytoma-derived insoluble matrix components may be important in the development of immune-based treatment strategies against malignant brain tumors.

Microglial cells in astroglial cultures: a cautionary note

Primary rodent astroglial-enriched cultures are the most popular model to study astroglial biology in vitro. From the original methods described in the 1970's a great number of minor modifications have been incorporated into these protocols by different laboratories. These protocols result in cultures in which the astrocyte is the predominant cell type, but astrocytes are never 100% of cells in these preparations. The aim of this review is to bring attention to the presence of microglia in astroglial cultures because, in my opinion, the proportion of and the role that microglial cells play in astroglial cultures are often underestimated. The main problem with ignoring microglia in these cultures is that relatively minor amounts of microglia can be responsible for effects observed on cultures in which the astrocyte is the most abundant cell type. If the relative contributions of astrocytes and microglia are not properly assessed an observed effect can be erroneously attributed to the astrocytes. In order to illustrate this point the case of NO production in activated astroglial-enriched cultures is examined. Lipopolysaccharide (LPS) induces nitric oxide (NO) production in astroglial-enriched cultures and this effect is very often attributed to astrocytes. However, a careful review of the published data suggests that LPS-induced NO production in rodent astroglial-enriched cultures is likely to be mainly microglial in origin. This review considers cell culture protocol factors that can affect the proportion of microglial cells in astroglial cultures, strategies to minimize the proportion of microglia in these cultures, and specific markers that allow the determination of such microglial proportions.

The ERK/MAPK pathway, as a target for the treatment of neuropathic pain

Peripheral nerve injury produces neuropathic pain as well as phosphorylation of mitogen activated protein kinase (MAPK) family in dorsal root ganglia (DRG) and dorsal horn. Following nerve injury, phosphorylation of extracellular signal-regulated protein kinase (ERK), an important member of this family, is sequentially increased in neurons, microglia and astrocytes of the dorsal horn and gracile nucleus, and in injured large DRG neurons. Nerve injury-induced phosphorylation of ERK occurs early and is long-lasting. In several animal models of neuropathic pain, MEK inhibitors, known to suppress the synthesis of ERK, have proven effective to alleviate pain at various time points. Thus, the regulation of ERK/MAPK can be considered as a promising therapeutic target for the treatment of neuropathic pain.