Gradual inhibition of inducible nitric oxide synthase but not of interleukin-1 beta production in rat microglial cells of endotoxin-treated mixed glial cell cultures

Amplification and propagation of interleukin-1β signaling by murine brain endothelial and glial cells

BACKGROUND

During acute infections and chronic illnesses, the pro-inflammatory cytokine interleukin-1β (IL-1β) acts within the brain to elicit metabolic derangements and sickness behaviors. It is unknown which cells in the brain are the proximal targets for IL-1β with respect to the generation of these illness responses. We performed a series of in vitro experiments to (1) investigate which brain cell populations exhibit inflammatory responses to IL-1β and (2) examine the interactions between different IL-1β-responsive cell types in various co-culture combinations.

METHODS

We treated primary cultures of murine brain microvessel endothelial cells (BMEC), astrocytes, and microglia with PBS or IL-1β, and then performed qPCR to measure inflammatory gene expression or immunocytochemistry to evaluate nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation. To evaluate whether astrocytes and/or BMEC propagate inflammatory signals to microglia, we exposed microglia to astrocyte-conditioned media and co-cultured endothelial cells and glia in transwells. Treatment groups were compared by Student's t tests or by ANOVA followed by Bonferroni-corrected t tests.

RESULTS

IL-1β increased inflammatory gene expression and NF-κB activation in primary murine-mixed glia, enriched astrocyte, and BMEC cultures. Although IL-1β elicited minimal changes in inflammatory gene expression and did not induce the nuclear translocation of NF-κB in isolated microglia, these cells were more robustly activated by IL-1β when co-cultured with astrocytes and/or BMEC. We observed a polarized endothelial response to IL-1β, because the application of IL-1β to the abluminal endothelial surface produced a more complex microglial inflammatory response than that which occurred following luminal IL-1β exposure.

CONCLUSIONS

Inflammatory signals are detected, amplified, and propagated through the CNS via a sequential and reverberating signaling cascade involving communication between brain endothelial cells and glia. We propose that the brain's innate immune response differs depending upon which side of the blood-brain barrier the inflammatory stimulus arises, thus allowing the brain to respond differently to central vs. peripheral inflammatory insults.

Modulating the delicate glial-neuronal interactions in neuropathic pain: promises and potential caveats

During neuropathic pain, glial cells (mainly astrocytes and microglia) become activated and initiate a series of signaling cascades that modulate pain processing at both spinal and supraspinal levels. It has been generally accepted that glial cell activation contributes to neuropathic pain because glia release proinflammatory cytokines, chemokines, and factors such as calcitonin gene-related peptide, substance P, and glutamate, which are known to facilitate pain signaling. However, recent research has shown that activation of glia also leads to some beneficial outcomes. Glia release anti-inflammatory factors that protect against neurotoxicity and restore normal pain. Accordingly, use of glial inhibitors might compromise the protective functions of glia in addition to suppressing their detrimental effects. With a better understanding of how different conditions affect glial cell activation, we may be able to promote the protective function of glia and pave the way for future development of novel, safe, and effective treatments of neuropathic pain.

Astrocytes inhibit nitric oxide-dependent Ca(2+) dynamics in activated microglia: involvement of ATP released via pannexin 1 channels

Under inflammatory conditions, microglia exhibit increased levels of free intracellular Ca(2+) and produce high amounts of nitric oxide (NO). However, whether NO, Ca(2+) dynamics, and gliotransmitter release are reciprocally modulated is not fully understood. More importantly, the effect of astrocytes in the potentiation or suppression of such signaling is unknown. Our aim was to address if astrocytes could regulate NO-dependent Ca(2+) dynamics and ATP release in LPS-stimulated microglia. Griess assays and Fura-2AM time-lapse fluorescence images of microglia revealed that LPS produced an increased basal [Ca(2+) ]i that depended on the sequential activation of iNOS, COXs, and EP1 receptor. TGFβ1 released by astrocytes inhibited the abovementioned responses and also abolished LPS-induced ATP release by microglia. Luciferin/luciferase assays and dye uptake experiments showed that release of ATP from LPS-stimulated microglia occurred via pannexin 1 (Panx1) channels, but not connexin 43 hemichannels. Moreover, in LPS-stimulated microglia, exogenous ATP triggered activation of purinergic P2Y1 receptors resulting in Ca(2+) release from intracellular stores. Interestingly, TGFβ1 released by astrocytes inhibited ATP-induced Ca(2+) response in LPS-stimulated microglia to that observed in control microglia. Finally, COX/EP1 receptor signaling and activation of P2 receptors via ATP released through Panx1 channels were critical for the increased NO production in LPS-stimulated microglia. Thus, Ca(2+) dynamics depended on the inflammatory profile of microglia and could be modulated by astrocytes. The understanding of mechanisms underlying glial cell regulatory crosstalk could contribute to the development of new treatments to reduce inflammatory cytotoxicity in several brain pathologies.

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.

Long term potentiation is impaired in membrane glycoprotein CD200-deficient mice: a role for Toll-like receptor activation

The membrane glycoprotein CD200 is expressed on several cell types, including neurons, whereas expression of its receptor, CD200R, is restricted principally to cells of the myeloid lineage, including microglia. The interaction between CD200 and CD200R maintains microglia and macrophages in a quiescent state; therefore, CD200-deficient mice express an inflammatory phenotype exhibiting increased macrophage or microglial activation in models of arthritis, encephalitis, and uveoretinitis. Here, we report that lipopolysaccharide (LPS) and Pam(3)CysSerLys(4) exerted more profound effects on release of the proinflammatory cytokines, interleukin (IL)-1β, IL-6, and tumor necrosis factor-α (TNFα), in glia prepared from CD200(-/-) mice compared with wild type mice. This effect is explained by the loss of CD200 on astrocytes, which modulates microglial activation. Expression of Toll-like receptors 4 and 2 (TLR4 and -2) was increased in glia prepared from CD200(-/-) mice, and the evidence indicates that microglial activation, assessed by the increased numbers of CD11b(+) cells that stained positively for both MHCII and CD40, was enhanced in CD200(-/-) mice compared with wild type mice. These neuroinflammatory changes were associated with impaired long term potentiation (LTP) in CA1 of hippocampal slices prepared from CD200(-/-) mice. One possible explanation for this is the increase in TNFα in hippocampal tissue prepared from CD200(-/-) mice because TNFα application inhibited LTP in CA1. Significantly, LPS and Pam(3)CysSerLys(4), at concentrations that did not affect LTP in wild type mice, inhibited LTP in slices prepared from CD200(-/-) mice, probably due to the accompanying increase in TLR2 and TLR4. Thus, the neuroinflammatory changes that result from CD200 deficiency have a negative impact on synaptic plasticity.

The multifaceted profile of activated microglia

Although relatively neglected previously, research efforts in the past decade or so have identified a pivotal role for glial cells in regulating neuronal function. Particular emphasis has been placed on increasing our understanding of the function of microglia because a change from the ramified "resting" state of these cells has been associated with the pathogenesis of several neurodegenerative diseases, notably Alzheimer's disease. However, it is not clear whether activation of microglia and the associated inflammatory changes play a part in triggering disease processes or whether cell activation is a response to the early changes associated with the disease. In either case, the possibility exists that modulation of microglial activation may be beneficial in some circumstances, underlying the need to pursue research in this area. The original morphological categorization of microglia by Del Rio Hortega into ameboid, ramified, and intermediate forms, must now be elaborated to encompass a functional description. The evidence which has been generated recently suggests that microglia are probably never in a "resting" state and that several intermediate transitional states, based on function and morphology, probably exist. A more complete understanding of these states and the triggers which lead to a change from one to another state, and the factors which modulate the molecular switch that determines the persistence of the "activated" state remain to be identified.

Differential neutrophil infiltration contributes to regional differences in brain inflammation in the substantia nigra pars compacta and cortex

Brain inflammation is a suggested risk factor for neurodegenerative disease. Interestingly, severe inflammation in the substantia nigra pars compacta (SNpc) accelerates the onset and progression of Parkinson's disease. In this study, we examined the underlying mechanisms of severe inflammation in the SNpc by comparing the inflammatory process with that in the cortex. In intact brain, the densities of CD11b(+) microglia were similar in the SNpc and cortex. However, lipopolysaccharide injection enhanced the CD11b(+) cell number in the SNpc, but not in the cortex. Previously, we reported that CD11b and myeloperoxidase (MPO) double-positive neutrophils infiltrate the SNpc following LPS injection (GLIA 55:1577-88). Notably, the MPO(+) neutrophil number increased dramatically in the SNpc, but only slightly in the cortex. The extent of neutrophil infiltration appeared to correlate with neuronal damage. We confirmed that loss of neurons in the SNpc was significantly reduced in neutropenic rats versus normal rats following LPS injection. In addition, the densities of astrocytes were much lower in the intact SNpc, compared with the cortex. Furthermore, after LPS injection, damage of endothelial cells and astrocytes, and blood-brain barrier (BBB) permeability was more pronounced in the SNpc. These results collectively suggest that excessive neutrophil infiltration and environmental factors, such as lower astrocyte density and higher BBB permeability, contribute to severe inflammation and neuronal death in the SNpc.

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.

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.

Characterization of the temporo-spatial effects of chronic bilateral intrahippocampal cannulae on interleukin-1beta

The implantation of a foreign object in the brain produces an acute neuroinflammatory state in which glia (astrocytes and microglia) may remain chronically activated in response to the inert foreign object. Activated glia can exhibit a sensitized pro-inflammatory response to immunogenic stimuli. This may be relevant to intracranial cannula implantation, which is commonly used to administer substances directly into the brain. If intracranial cannulation activates glia, a subsequent neuroinflammatory stimulus might induce a potentiated pro-inflammatory response, thereby introducing a potential experimental confound. We tested the temporal and spatial responses of interleukin-1beta (IL-1beta) to an acute immune challenge produced by lipopolysaccharide (LPS) in animals with chronic bilateral intrahippocampal cannulae implants (stainless steel). Cannulation increased the gene expression of the microglia activation antigens MHC II and CD11b, but not the astrocyte antigen GFAP. Moreover, this activation was temporally and spatially dependent. In addition, IL-1beta mRNA, but not IL-1beta protein, was significantly elevated in cannulated animals. Administration of LPS, however, significantly potentiated the brain IL-1beta response in cannulated animals, but not in stab wounded or naïve animals. This IL-1beta response was also temporo-spatially dependent. Thus, the pro-inflammatory sequelae of intracranial cannulation should be considered when designing studies of neuroinflammatory processes.

IL-1beta, an immediate early protein secreted by activated microglia, induces iNOS/NO in C6 astrocytoma cells through p38 MAPK and NF-kappaB pathways

In the present study we sought to examine cell-cell interactions by investigating the effects of factors released by stimulated microglia on inducible nitric oxide (NO) synthase (iNOS) induction in astrocytoma cells. After examining the temporal profiles of proinflammatory molecules induced by lipopolysaccharide (LPS) stimulation in BV2 microglial cells, iNOS and IL-1beta were observed to be the first immediate-response molecules. Removal of LPS after 3 hr stimulation abrogated NO release, whereas a full induction of IL-1beta was retained in BV2 cells. We observed consistently that conditioned medium (CM) from activated microglia resulted in the induction of iNOS in C6 cells, and IL-1beta was shown to be a key regulator of iNOS induction. An IL-1beta-neutralizing antibody diminished NO induction. Incubation with recombinant IL-1beta stimulated NO release to a lesser extent compared to microglial CM; co-treatment of LPS and IL-1beta had a potent, synergistic effect on NO release from C6 cells. Transient transfection with MEK kinase 1 (MEKK1) or nuclear factor-kappa B (NF-kappaB) expression plasmids induced iNOS, and IL-1beta further enhanced the MEKK1 response. Furthermore, IL-1beta-mediated NO release from C6 cells was significantly suppressed by inhibition of p38 mitogen activated protein kinase (MAPK) or NF-kappaB by specific chemical inhibitors. Both IL-1beta and MEKK1 stimulated p38 and JNK MAPKs, as well as the NF-kappaB pathway, to induce iNOS in C6 cells. Microglia may represent an anti-tumor response in the central nervous system, which is potentiated by the local secretion of immunomodulatory factors that in turn affects astrocytoma (glioma) cells. A better understanding of microglia-glioma or microglia-astrocyte interactions will help in the design of novel immune-based therapies for brain tumors or neuronal diseases.

Astrocytes induce hemeoxygenase-1 expression in microglia: a feasible mechanism for preventing excessive brain inflammation

Microglia are the major inflammatory cells in the brain, in which microglial inflammatory responses are modulated by interactions with other brain cells. Here, we show that astrocytes, the most abundant cells in the brain, can secrete one or more factors capable of modulating microglial activation by regulating the microglial levels of reactive oxygen species (ROS). Treatment of microglia with astrocyte culture-conditioned media (ACM) increased the expression level and activity of hemeoxygenase-1 (HO-1). ACM also induced nuclear translocation of the nuclear factor E2-related factor 2 transcription factor, increased the binding activity of the antioxidant response element (ARE), and enhanced HO-1 promoter activity in an ARE-dependent manner. Furthermore, treatment with ACM suppressed interferon-gamma (IFN-gamma)-induced ROS production, leading to reduced inducible nitric oxide synthase (iNOS) expression and nitric oxide (NO) release. In agreement with these results, mimickers of HO-1 products, such as bilirubin, ferrous iron, and a carbon monoxide-releasing molecule, reduced IFN-gamma-induced iNOS expression and/or NO release. Finally, we found that the active component(s) in ACM was heat labile and smaller than 3 kDa. Together, these results suggest that astrocytes could cooperate with microglia to prevent excessive inflammatory responses in the brain by regulating microglial expression of HO-1 and production of ROS.

Detoxication enzyme inducers modify cytokine production in rat mixed glial cells

Pro-inflammatory cytokines, e.g. interleukin-1beta (IL-1beta), interleukin-6 (IL-6), tumor necrosis factor-alpha (TNFalpha) as well as neurotoxic molecules such as nitric oxide (NO), that are produced and released by activated glial cells, play an important role in inflammation and oxidative stress occurring during Multiple Sclerosis (MS). Reduction of these processes could therefore be of therapeutic interest. Dimethylfumarate (DMF) and sulforaphane (SP) are well known for their detoxicating properties. Furthermore, they have anti-inflammatory effects as shown clinically by the treatment of inflammatory skin diseases. However, their detoxication and anti-inflammatory action on brain-derived cells is unknown. In the present study we have studied, within the same concentration range, the anti-inflammatory and detoxicating effects of DMF and SP on the production and release of mediators of inflammation and detoxication from lipopolysaccharide (LPS) activated primary co-cultures of rat microglial and astroglial cells. DMF and SP attenuated the LPS-induced production and release of TNFalpha, IL-1beta, IL-6 and NO. In addition, DMF and SP increase both mRNA level and activity of NAD(P)H:quinone reductase (NQO-1), a detoxication enzyme, as well as the cellular glutathione content. We conclude that DMF or SP simultaneously can (1) reduce mediators of inflammation and (2) enhance detoxication enzymes in LPS stimulated co-cultures of astroglial and microglial cells. This double-sided effect could potentially be of therapeutic interest.

Inhibition of microglial inflammation by the MLK inhibitor CEP-1347

CEP-1347 is a potent inhibitor of the mixed lineage kinases (MLKs), a distinct family of mitogen-activated protein kinase kinase kinases (MAPKKK). It blocks the activation of the c-Jun/JNK apoptotic pathway in neurons exposed to various stressors and attenuates neurodegeneration in animal models of Parkinson's disease (PD). Microglial activation may involve kinase pathways controlled by MLKs and might contribute to the pathology of neurodegenerative diseases. Therefore, the possibility that CEP-1347 modulates the microglial inflammatory response [tumour necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), and monocyte chemotactic protein-1 (MCP-1)] was explored. Indeed, the MLK inhibitor CEP-1347 reduced cytokine production in primary cultures of human and murine microglia, and in monocyte/macrophage-derived cell lines, stimulated with various endotoxins or the plaque forming peptide Abeta1-40. Moreover, CEP-1347 inhibited brain TNF production induced by intracerebroventricular injection of lipopolysaccharide in mice. As expected from a MLK inhibitor, CEP-1347 acted upstream of p38 and c-Jun activation in microglia by dampening the activity of both pathways. These data imply MLKs as important, yet unrecognized, modulators of microglial inflammation, and demonstrate a novel anti-inflammatory potential of CEP-1347.

Modulation of cerebral vascular tone by activated glia: involvement of nitric oxide

The ability of activated glia to affect cerebral vascular tone has been evaluated using an in vitro experimental system in which basilar arteries were incubated with glial cultures activated by treatment with lipopolysaccharide (LPS). Vascular tone was measured with an isometric myograph. Contraction in response to high KCl and serotonin was reduced in arteries co-incubated for 24 h with LPS-activated glia, whereas the response to acetylcholine was not modified. The reduced contraction was prevented when the nitric oxide synthase (NOS) inhibitor L-N-nitro-arginine (L-NNA) was added throughout the whole incubation time (activation of glial cells with LPS + co-incubation of glial cells with cerebral arteries). Under these conditions, nitrite levels were drastically reduced. A reduced contraction to KCl was also observed after treatment of the cerebral vessel with sodium nitroprusside. In contrast, L-NNA added to the vessel did not modify the response to contracting stimuli and the expression of endothelial NOS was not modified in cerebral arteries pre-incubated with activated glia. These results suggest that activated glia, which finds an in vivo correlate in several neuropathological conditions, can contribute to changes of vascular tone by modifying the levels of nitric oxide (NO) to which the vessel is exposed.

Pathological consequences of inducible nitric oxide synthase expression in hippocampal slice cultures

The generation of toxic concentrations of nitric oxide by the inducible nitric oxide synthase expressed in microglia and other brain cell types is frequently invoked as a causative factor in neurodegeneration. Experiments were carried out on slice cultures of rat hippocampus to test this hypothesis. Exposure of the slices to bacterial lipopolysaccharide plus interferon-gamma led to a time-dependent expression of functional inducible nitric oxide synthase that was found only in microglia. Microglial activation by other means, such as physical damage, was not associated with inducible nitric oxide synthase expression. Damage and cell death in slices expressing inducible nitric oxide synthase was evaluated over a period of 6 days, but none was found. Consistent with this result, cGMP measurements indicated that the average local nitric oxide concentration remained in the low nanomolar range. When the microglial population was expanded to a density three-fold above normal by applying granulocyte-macrophage colony stimulating factor, however, lipopolysaccharide plus interferon-gamma provoked neurodegeneration that could be blocked by an inducible nitric oxide synthase inhibitor. The associated nitric oxide concentration in the slices was saturating for guanylyl cyclase-coupled nitric oxide receptors, signifying at least 10 nM. It is concluded that inducible nitric oxide synthase is expressed in microglia only in response to specific stimuli involving the innate immune system, and that the resulting level of nitric oxide in intact brain tissue is normally too low to inflict damage directly. Quantities of nitric oxide sufficient to contribute directly or indirectly to pathology could be produced should the density of microglia become high enough, although caution must be exercised in extrapolating this finding to the human brain in vivo.

Inhibition of microglial inflammatory responses by norepinephrine: effects on nitric oxide and interleukin-1beta production

BACKGROUND: Under pathological conditions, microglia produce proinflammatory mediators which contribute to neurologic damage, and whose levels can be modulated by endogenous factors including neurotransmitters such as norepinephrine (NE). We investigated the ability of NE to suppress microglial activation, in particular its effects on induction and activity of the inducible form of nitric oxide synthase (NOS2) and the possible role that IL-1beta plays in that response. METHODS: Rat cortical microglia were stimulated with bacterial lipopolysaccharide (LPS) to induce NOS2 expression (assessed by nitrite and nitrate accumulation, NO production, and NOS2 mRNA levels) and IL-1beta release (assessed by ELISA). Effects of NE were examined by co-incubating cells with different concentrations of NE, adrenergic receptor agonists and antagonists, cAMP analogs, and protein kinase (PK) A and adenylate cyclase (AC) inhibitors. Effects on the NFkappaB:IkappaB pathway were examined by using selective a NFkappaB inhibitor and measuring IkappaBalpha protein levels by western blots. A role for IL-1beta in NOS2 induction was tested by examining effects of caspase-1 inhibitors and using caspase-1 deficient cells. RESULTS: LPS caused a time-dependent increase in NOS2 mRNA levels and NO production; which was blocked by a selective NFkappaB inhibitor. NE dose-dependently reduced NOS2 expression and NO generation, via activation of beta2-adrenergic receptors (beta2-ARs), and reduced loss of inhibitory IkBalpha protein. NE effects were replicated by dibutyryl-cyclic AMP. However, co-incubation with either PKA or AC inhibitors did not reverse suppressive effects of NE, but instead reduced nitrite production. A role for IL-1beta was suggested since NE potently blocked microglial IL-1beta production. However, incubation with a caspase-1 inhibitor, which reduced IL-1beta levels, had no effect on NO production; incubation with IL-receptor antagonist had biphasic effects on nitrite production; and NE inhibited nitrite production in caspase-1 deficient microglia. CONCLUSIONS: NE reduces microglial NOS2 expression and IL-1beta production, however IL-1beta does not play a critical role in NOS2 induction nor in mediating NE suppressive effects. Changes in magnitude or kinetics of cAMP may modulate NOS2 induction as well as suppression by NE. These results suggest that dysregulation of the central cathecolaminergic system may contribute to detrimental inflammatory responses and brain damage in neurological disease or trauma.

Effects of β-AP peptides on activation of the transcription factor NF-κB and in cell proliferation in glial cell cultures

The effect of two beta amyloid peptides (Abeta 25/35 and Abeta 1/42) on the activation of the transcription factor kappaB (NF-kappaB) in pure astroglial, pure microglial and mixed glial cell cultures was compared by means of single or double immunofluorescence and Western blot techniques. We also studied the effect of both peptides in cell proliferation in mixed glial cultures and pure astrocytes. The Abeta 1/42 peptide induced the activation of NF-kappaB in all studied cell cultures and its effect was potentiated by interferon-gamma (IFN-gamma). Abeta 25/35 alone did not induce NF-kappaB activation, but Abeta 25/35 plus IFN-gamma induced the activation of the transcription factor in the mixed and pure microglial cultures, although not in pure astroglia. The Abeta 1/42 peptide, but not Abeta 25/35, induced proliferation in pure astroglial and mixed glial cell cultures. The results suggest that the state of peptide aggregation is related to their ability to activate glial cells.

Glial activation modulates glutamate neurotoxicity in cerebellar granule cell cultures

We studied the influence of glial cells on the neuronal response to glutamate toxicity in cerebellar granule cell cultures. We compared the effect of glutamate on neuronal viability in neuronal vs. neuronal-glial cultures and determined this effect after pretreating the cultures with the lipopolysaccharide (LPS) of Escherichia coli, agent widely used to induce glial activation. Morphological changes in glial cells and nitric oxide (NO) production were evaluated as indicators of glial activation. We observed that glutamate neurotoxicity in neuronal-glial cultures was attenuated in a certain range of glutamate concentration when compared to neuronal cultures, but it was enhanced at higher glutamate concentrations. This enhanced neurotoxicity was associated with morphological changes in astrocytes and microglial cells in the absence of NO production. LPS treatment induced morphological changes in glial cells in neuronal-glial cultures as well as NO production. These effects occurred in the absence of significant neuronal death. However, when LPS-pretreated cultures were treated with glutamate, the sensitivity of neuronal-glial cultures to glutamate neurotoxicity was increased. This was accompanied by additional morphological changes in glial cells in the absence of a further increase in NO production. These results suggest that quiescent glial cells protect neuronal cells from glutamate neurotoxicity, but reactive glial cells increase glutamate neurotoxicity. Therefore, glial cells play a key role in the neuronal response to a negative stimulus, suggesting that this response can be modified through an action on glial cells.

Wortmannin enhances lipopolysaccharide-induced inducible nitric oxide synthase expression in microglia in the presence of astrocytes in rats

Microglia, the primary inflammatory cells in the brain, are activated upon brain injury. Activated microglia produce nitric oxide (NO), a major toxin to neuronal cells. It has been reported that astrocytes inhibit microglial activation. In this study, we found that wortmannin, a natural inhibitor of phosphatidylinositol 3-kinase, significantly increased lipopolysaccharide (LPS)-induced NO release and inducible nitric oxide synthase (iNOS) expression in microglia in the presence but not in the absence of astrocytes. In response to LPS even in the presence of wortmannin, iNOS immunoreactivity was detected in microglia but not in astrocytes. These results suggest that astrocytes could regulate microglia-mediated brain inflammation by inhibiting microglial NO release/iNOS expression via a wortmannin-sensitive mechanism.

Expression and regulation of interleukin-10 and interleukin-10 receptor in rat astroglial and microglial cells

Activated glial cells crucially contribute to brain inflammatory responses. Interleukin-10 (IL-10) is an important modulator of glial cell responses in the brain. In the present study we describe the expression of IL-10 and the IL-10 receptor (IL-10R1) in primary cocultures of rat microglial and astroglial cells. Using quantitative RT-PCR and ELISA, we show that IL-10 mRNA expression and subsequent IL-10 secretion is time-dependently induced by lipopolysaccharide (LPS). IL-10R1, however, is constitutively expressed in glial cell cocultures, as shown by RT-PCR and immunocytochemistry. Radioligand binding studies using 125I-IL-10 reveal that rat glial cells express a single binding site with an apparent affinity of approximately 600 pm for human IL-10. Observations in enriched cultures of either microglial or astroglial cells indicate that both cell types express IL-10 mRNA and are capable of secreting IL-10. Both cell types also express IL-10R1 mRNA and protein. However, in glial cell cocultures immunoreactive IL-10R1 protein is predominantly observed in astrocytes, suggesting that microglial expression of IL-10R1 in cocultures is suppressed by astrocytes. In addition, exogenous IL-10 is highly potent in down-regulating LPS-induced IL-1beta and IL-10 mRNA, and, at a higher dose, IL-10R1 mRNA in untreated and LPS-treated cultures, suggesting that IL-10 autoregulates its expression and inhibits that of IL-1beta at the transcriptional level. Together the findings support the concept that IL-10, produced by activated microglial and astroglial cells, modulates glia-mediated inflammatory responses through high-affinity IL-10 receptors via paracrine and autocrine interactions.

Astrocytes enhance lipopolysaccharide-induced nitric oxide production by microglial cells

Several stimuli result in glial activation and induce nitric oxide (NO) production in microglial and astroglial cells. The bacterial endotoxin lipopolysaccharide (LPS) has been widely used to achieve glial activation in vitro, and several studies show that both microglial and, to a lesser extent, astroglial cell cultures produce NO after LPS treatment. However, NO production in endotoxin-treated astrocyte cultures is controversial. We characterized NO production in microglial, astroglial and mixed glial cell cultures treated with lipopolysaccharide, measured as nitrite accumulation in the culture media. We also identified the NO-producing cells by immunocytochemistry, using specific markers for the inducible NO synthase (iNOS) isoform, microglial and astroglial cells. Only microglial cells showed iNOS immunoreactivity. Thus, contaminating microglial cells were responsible for NO production in the secondary astrocyte cultures. We then analysed the effect of astrocytes on NO production by microglial cells using microglial-astroglial cocultures, and we observed that this production was clearly enhanced in the presence of astroglial cells. Soluble factors released by astrocytes did not appear to be directly responsible for such an effect, whereas nonsoluble factors present in the cell membrane of LPS-treated astrocytes could account, at least in part, for this enhancement.

Relationship between beta-AP peptide aggregation and microglial activation

We compared the relationship between the state of aggregation of two peptides (beta-AP 25-35 and beta-AP 1-42) and microglial activation. After 7 days at 37 degrees C beta-AP 25-35 was in an amorphous state and did not activate microglial cells. In the same conditions, aggregated beta-AP 1-42 activated these cells and caused changes in microglial ramification, increasing the proliferation index and inducing tumor necrosis factor alpha (TNF alpha) release. Neither peptide induced a release of nitric oxide (NO). As the toxicity of beta-AP peptides in cell culture is associated with the formation of amyloid fibrils, we also examined the toxicity of both peptides in microglial cell cultures and in PC 12 cell cultures. The results suggest that the two beta-AP fragments studied have similar neurotoxic effects but different pro-inflammatory activities.

Microglia in neurodegeneration: molecular aspects

Inflammatory events in the CNS are associated with injuries as well as with well-known chronic degenerative diseases, such as Multiple Sclerosis, Parkinson's, or Alzheimer's disease. Compared to inflammation in peripheral tissues, inflammation in brain appears to follow distinct pathways and time-courses, which likely has to do with a relatively strong immunosuppression in that organ. For this reason, it is of great importance to get insights into the molecular mechanism governing immune reactions in brain tissue. This task is hard to achieve in vivo, but can be approached by studying the major cell type responsible for brain inflammation, the microglia, in culture. Since these cells are the only professional antigen-presenting cells resident in brain parenchyma, molecular mechanisms of antigen presentation are being discussed first. After covering the expression and regulation of anti- and proinflammatory cytokines, induction and regulation of two key enzymes and their products-COX-2 and iNOS-are summarized. Possibly, pivotal molecular targets for drug therapies of brain disorders will be discovered in intracellular signaling pathways leading to activation of transcription factors. Finally, the impact of growth factors, of neurotrophins in particular, is highlighted. It is concluded that the presently available data on the molecular level is far from being statisfying, but that only from better insights into molecular events will we obtain the information required for more specific therapies.

Role of calmodulin in the differentiation/activation of microglial cells

In the present work the role of calmodulin (CaM) in regulating lipopolysaccharide (LPS)-induced microglial activation and in the spontaneous microglial differentiation has been investigated. We used pure rat microglial cell cultures to examine the effects of W13, a specific inhibitor of CaM, on microglial activation produced by LPS and the effect of CaM inhibition on microglial proliferation induced by the macrophage colony-stimulating factor (M-CSF). Microglial morphological transformation, inducible nitric oxide synthase (iNOS) activity and proliferating cell nuclear antigen (PCNA) immunostaining were determinate. Results show that CaM does not participate in the microglial increase of iNOS produced by LPS. In contrast, it is involved in spontaneous microglial ramification and in the activation of proliferation from quiescence. Multiple second-messenger pathways are involved in the transduction of signals initiated by LPS. The study of these mechanisms may allow us to extend our knowledge of the signals controlling the expression of these mediators.

Involvement of interleukin-1 in glial responses to lipopolysaccharide: endogenous versus exogenous interleukin-1 actions

Interleukin-1beta (IL-1beta) participates in neuroinflammation and neurodegeneration. Its mechanisms of action are not fully understood, but appear to involve complex interactions between neurons and glia. The objective of this study was to determine the involvement of endogenous IL-1beta in inflammatory responses to LPS in cultured mouse glial cells, and compare this to the effects of exogenous IL-1beta. Activation of primary mixed glial cultures by incubation with LPS (1 microgram/ml, 24 h), caused marked (approximately ten-fold) increases in release of NO, twenty-fold increases in PGE(2) and ninety-fold increases of IL-6 release. Incubation with human recombinant IL-1beta (100 ng/ml) also stimulated NO and IL-6 release to a similar extent to LPS, but IL-1beta (1 or 100 ng/ml) caused only modest increases (approximately seven-fold) in PGE(2) release. Co-incubation with IL-1ra inhibited the effects of LPS on NO release (-65%) and IL-6 production (-30%), but failed to reduce PGE(2) release. These results indicate that exogenous IL-1beta induces release of NO, PGE(2) and IL-6 in mixed glial cultures, and that endogenous IL-1beta mediates inflammatory actions of LPS on NO and to a lesser extent IL-6, but not on PGE(2) release in mixed glial cultures. Indeed endogenous IL-1beta appears to inhibit LPS-induced PGE(2) release.

Nitric oxide as a mediator of nucleus pulposus-induced effects on spinal nerve roots

Nerve root dysfunction and sciatic pain in disc herniation are considered to be caused by mechanical compression and related to the presence of nucleus pulposus in the epidural space. Autologous nucleus pulposus has been shown to induce endoneural edema and to decrease nerve-conduction velocity in spinal nerve roots in experimental disc herniation models, and inflammatory mediators have been suggested to be involved in these mechanisms. Nitric oxide, a potent inflammatory mediator, is implicated in vasoregulation, neurotransmission, and neuropathic pain. Nitric oxide synthesis can be induced by different cytokines, e.g., tumor necrosis factor-alpha, which recently was shown to be of pathophysiological importance in experimental disc herniation. The enzyme nitric oxide synthase mediates the production of nitric oxide. Three series of experiments were performed in rat and pig disc herniation models to (a) investigate nitric oxide synthase activity in spinal nerve roots after exposure to autologous nucleus pulposus and (b) evaluate the effects of systemic treatment with aminoguanidine, a nitric oxide synthase inhibitor, on vascular permeability and nerve-conduction velocity. In a disc herniation model in the rat, calcium-independent nitric oxide synthase activity was measured in nerve roots exposed to nucleus pulposus; however, no nitric oxide synthase activity was detected in nerve roots from animals that underwent a sham operation, reflecting increased inducible nitric oxide synthase activity. In nucleus pulposus-exposed spinal nerve roots in the pig, the edema was less severe after systemic aminoguanidine administration than without aminoguanidine treatment. Aminoguanidine treatment also significantly reduced the negative effect of nucleus pulposus on nerve-conduction velocity in spinal nerve roots in the pig. These results demonstrate that nucleus pulposus increases inducible nitric oxide synthase activity in spinal nerve roots and that nitric oxide synthase inhibition reduces nucleus pulposus-induced edema and prevents reduction of nerve-conduction velocity. Furthermore, the results suggest that nitric oxide is involved in the pathophysiological effects of nucleus pulposus in disc herniation.

Interleukin-10, interleukin-4, and transforming growth factor-beta differentially regulate lipopolysaccharide-induced production of pro-inflammatory cytokines and nitric oxide in co-cultures of rat astroglial and microglial cells

The pro-inflammatory cytokines interleukin-1beta (IL-1beta), IL-6, tumor necrosis factor-alpha (TNF-alpha), and nitric oxide (NO) can be produced by activated glial cells and play a critical role in various neurological diseases. Using primary co-cultures of rat microglial and astroglial cells, we investigated the effects of the anti-inflammatory cytokines transforming growth factor-beta1 (TGF-beta1)/beta2, IL-4, and IL-10 on the production of (pro-) inflammatory mediators after stimulation of the cells with lipopolysaccharide (LPS; 0.1 micrograms/ml, 24 h). IL-10 (10 and 100 ng/ml) and IL-4 (5 and 50 U/ml) suppressed the LPS-induced production of NO, IL-6, and TNF-alpha in a dose-dependent manner, whereas TGF-beta1/beta2 (2 and 20 ng/ml) only suppressed NO production. LPS-induced levels of IL-1beta were suppressed by IL-10, but not by IL-4 and TGF-beta1/beta2. Conversely, co-incubation of the glial cells with LPS and antibodies to TGF-beta1/beta2 selectively enhanced LPS-induced NO production, whereas co-incubation with antibody to IL-10 enhanced LPS-induced production of all pro-inflammatory cytokines and NO. This finding strongly suggests that effective concentrations of TGF-beta1/beta2 and IL-10 are produced by LPS-stimulated glial cell co-cultures. Production of IL-10 in these co-cultures was confirmed by measurement of rat IL-10 by radioimmunoassay. We conclude that anti-inflammatory cytokines affect the production of inflammatory mediators in LPS-activated co-cultures of microglial and astroglial cells differentially.

Expression of nitric oxide synthase-2 (NOS-2) in reactive astrocytes of the human glaucomatous optic nerve head

Inducible nitric oxide synthase (NOS-2) is abundantly present in the optic nerve heads of glaucoma patients. To determine the regulation of NOS-2 expression in the glaucomatous optic nerve head, the specific cells that express NOS-2 in the optic nerve heads of patients with primary open-angle glaucoma were studied by immunohistochemical double-labeling of NOS-2 and one of the characteristic cell markers for different cell types. Most of the labeling for NOS-2 was identified in reactive astrocytes that were clustered in the areas of nerve damage in the prelaminar and lamina cribrosa regions of the glaucomatous optic nerve heads. In vitro, the expression of GFAP and NOS-2 by reactive astrocytes of human optic nerve heads was demonstrated by immunocytochemistry and Western blot. In primary cultures of human lamina cribrosa astrocytes, stimulation by interferon-gamma and interleukin-1beta upregulated GFAP and induced expression of NOS-2 protein. At 24, 48 and 72 h of stimulation, NOS-2 appeared first in the Golgi body and then was sent out into the cytoplasm in granules. These results demonstrated that the astrocytes of human optic nerve head are capable of inducing the expression of NOS-2. Reactive astrocytes in the glaucomatous optic nerve heads apparently play an important role in local neurotoxicity to the axons of the retinal ganglion cells by producing excessive nitric oxide in glaucomatous optic neuropathy.

The effect of microglia on embryonic dopaminergic neuronal survival in vitro: diffusible signals from neurons and glia change microglia from neurotoxic to neuroprotective

When embryonic dopaminergic neurons are transplanted into the adult brain, approximately 95% die within a few days. To assess whether microglia activated during transplantation might be responsible for this rapid death, we examined the effect of microglia on rat embryonic dopaminergic neurons in vitro. Conditioned medium from 7-day-old microglia was found to decrease the number of dopamine neurons surviving in primary culture, but activation of the microglia with N-formyl-methionyl-leucyl-phenylalanine (FMLP) or Zymosan A did not increase the toxicity of the conditioned medium. We next tested the effect of coculturing microglia and dopaminergic neurons by placing microglia in semipermeable well inserts over the neuronal cultures. The presence of microglia now increased dopaminergic neuronal survival, microglial activation again having no effect. To increase yet further the possible interactions between microglia and neurons, the mesencephalic cells and microglia were mixed together and placed as a tissue in three-dimensional culture, and here again the presence of microglia increased dopaminergic neuronal survival with no effect of activation. Contact of microglia with the mesencephalic cells therefore converted them from being toxic to dopaminergic neurons to promoting their survival. The change in microglial effect from toxic to protective was caused by soluble molecules secreted by cells in the neuronal cultures, as conditioned medium derived from microglia-neuronal cocultures also had a dopaminergic neuron survival effect, indicating that microglia in cocultures behave differently from microglia removed from neuronal and glial influence. Microglia cocultured with either neurons or astrocytes downregulated inducible nitric oxide synthase (iNOS), indicating a decrease in the production of nitric oxide and possibly other toxic molecules. These findings indicate that in their natural environment, microglia are likely to be beneficial for the survival of embryonic dopaminergic grafts.

N-Acetyl-cysteine inhibition of encephalomyelitis Theiler's virus-induced nitric oxide and tumour necrosis factor-alpha production by murine astrocyte cultures

The pathological mechanisms that cause central nervous system (CNS) dysfunction in most neurological diseases are not well established. Theiler's murine encephalomyelitis virus (TMEV) is known to interact with cells of the CNS and its intracerebral inoculation to susceptible mice strains causes neurological disorders resembling multiple sclerosis (MS). In this study, we reported that primary astrocyte cultures from SJL/J susceptible mice when infected with TMEV released important amounts of nitrites (NO2-) to the culture medium, as measured in the supernatants 24 hours after infection. In addition, we observed an increment in the production of tumour necrosis factor alpha (TNF-alpha) by susceptible SJL/J strain derived astrocytes infected with TMEV. The treatment with the thiolic antioxidant N-acetyl-cysteine partially suppressed the virus-stimulated production of nitric oxide and TNF-alpha, in a dose response fashion. These results indicate that during viral infection astrocytes are an important cellular source of nitric oxide and TNF-alpha, substances which play important roles during CNS inflammatory events. The effects of the antioxidant N-acetyl-cysteine, modulating the production of the above compounds by TMEV-infected astrocytes may be a significant factor in preventing CNS demyelination.

Inhibition of endotoxin-induced nitric oxide synthase production in microglial cells by the presence of astroglial cells: a role for transforming growth factor beta

In mixed glial cell cultures from cerebral cortices of newborn rats, endotoxin induces inducible nitric oxide (iNOS), nitric oxide (NO), and interleukin-1 beta (IL-1 beta) production in microglial cells. Earlier we demonstrated that endotoxin induced iNOS but not IL-1 beta expression in microglial cells is inhibited by the presence of astroglial cells. In the present paper we describe studies on the mechanism by which astroglial cells exert selective suppressive action on iNOS expression by microglial cells. Expression of iNOS and IL-1 beta was studied by single or double label immunocytochemical techniques and cell identification was performed with GSA-I-B4-isolectin and an antibody against GFAP. Production of IL-1 beta and NO was determined by measurement of IL-1 beta and nitrite concentrations in cell lysates and the culture medium, respectively. TGF beta, a cytokine known to inhibit NO production by endotoxin challenged macrophages, was measured in culture medium of mixed glial cell cultures using a bioassay. Microglial, astroglial, and mixed glial cell cultures produced similar concentrations of TGF beta. The potential effect of TGF beta was studied by using immunoneutralizing antibodies against TGF beta 1 and TGF beta 2 on the induction of iNOS in microglial cells in the presence of astroglial cells. Incubation of the mixed glial cell culture with these TGF beta antibodies (3 micrograms/ml) markedly increased endotoxin-induced NO production and iNOS expression in microglial cells, whereas the production of IL-1 beta was not affected. The antibodies against TGF beta 1 and TGF beta 2 marginally increased NO production in pure microglial cell cultures, nonetheless in cultures of purified microglial cells recombinant TGF beta 1 and TGF beta 2 together with endotoxin inhibited NO production. We conclude that the presence of astroglial cells is essential for the inhibitory effect of TGF beta on NO production by microglial cells (possibly) by activation of TGF beta or by increasing the sensitivity of microglial cells for TGF beta.