Lipopolysaccharide-induced expression of IP-10 mRNA in rat brain and in cultured rat astrocytes and microglia

Time and age dependent regulation of neuroinflammation in a rat model of mesial temporal lobe epilepsy: Correlation with human data

Acute brain insults trigger diverse cellular and signaling responses and often precipitate epilepsy. The cellular, molecular and signaling events relevant to the emergence of the epileptic brain, however, remain poorly understood. These multiplex structural and functional alterations tend also to be opposing - some homeostatic and reparative while others disruptive; some associated with growth and proliferation while others, with cell death. To differentiate pathological from protective consequences, we compared seizure-induced changes in gene expression hours and days following kainic acid (KA)-induced status epilepticus (SE) in postnatal day (P) 30 and P15 rats by capitalizing on age-dependent differential physiologic responses to KA-SE; only mature rats, not immature rats, have been shown to develop spontaneous recurrent seizures after KA-SE. To correlate gene expression profiles in epileptic rats with epilepsy patients and demonstrate the clinical relevance of our findings, we performed gene analysis on four patient samples obtained from temporal lobectomy and compared to four control brains from NICHD Brain Bank. Pro-inflammatory gene expressions were at higher magnitudes and more sustained in P30. The inflammatory response was driven by the cytokines IL-1β, IL-6, and IL-18 in the acute period up to 72 h and by IL-18 in the subacute period through the 10-day time point. In addition, a panoply of other immune system genes was upregulated, including chemokines, glia markers and adhesion molecules. Genes associated with the mitogen activated protein kinase (MAPK) pathways comprised the largest functional group identified. Through the integration of multiple ontological databases, we analyzed genes belonging to 13 separate pathways linked to Classical MAPK ERK, as well as stress activated protein kinases (SAPKs) p38 and JNK. Interestingly, genes belonging to the Classical MAPK pathways were mostly transiently activated within the first 24 h, while genes in the SAPK pathways had divergent time courses of expression, showing sustained activation only in P30. Genes in P30 also had different regulatory functions than in P15: P30 animals showed marked increases in positive regulators of transcription, of signaling pathways as well as of MAPKKK cascades. Many of the same inflammation-related genes as in epileptic rats were significantly upregulated in human hippocampus, higher than in lateral temporal neocortex. They included glia-associated genes, cytokines, chemokines and adhesion molecules and MAPK pathway genes. Uniquely expressed in human hippocampus were adaptive immune system genes including immune receptors CDs and MHC II HLAs. In the brain, many immune molecules have additional roles in synaptic plasticity and the promotion of neurite outgrowth. We propose that persistent changes in inflammatory gene expression after SE leads not only to structural damage but also to aberrant synaptogenesis that may lead to epileptogenesis. Furthermore, the sustained pattern of inflammatory genes upregulated in the epileptic mature brain was distinct from that of the immature brain that show transient changes and are resistant to cell death and neuropathologic changes. Our data suggest that the epileptogenic process may be a result of failed cellular signaling mechanisms, where insults overwhelm the system beyond a homeostatic threshold.

Human Primary Astrocytes Differently Respond to Pro- and Anti-Inflammatory Stimuli

For a long time, astrocytes were considered a passive brain cell population. However, recently, many studies have shown that their role in the central nervous system (CNS) is more active. Previously, it was stated that there are two main functional phenotypes of astrocytes. However, nowadays, it is clear that there is rather a broad spectrum of these phenotypes. The major goal of this study was to evaluate the production of some inflammatory chemokines and neurotrophic factors by primary human astrocytes after pro- or anti-inflammatory stimulation. We observed that only astrocytes induced by inflammatory mediators TNFα/IL-1a/C1q produced CXCL10, CCL1, and CXCL13 chemokines. Unstimulated astrocytes and those cultured with anti-inflammatory cytokines (IL-4, IL-10, or TGF-β1) did not produce these chemokines. Interestingly, astrocytes cultured in proinflammatory conditions significantly decreased the release of neurotrophic factor PDGF-A, as compared to unstimulated astrocytes. However, in response to anti-inflammatory cytokine TGF-β1, astrocytes significantly increased PDGF-A production compared to the medium alone. The production of another studied neurotrophic factor BDNF was not influenced by pro- or anti-inflammatory stimulation. The secretory response was accompanied by changes in HLA-DR, CD83, and GFAP expression. Our study confirms that astrocytes differentially respond to pro- and anti-inflammatory stimuli, especially to inflammatory cytokines TNF-α, IL-1a, and C1q, suggesting their role in leukocyte recruitment.

Potential neurotoxic activity of diverse molecules released by microglia

Microglia are the professional immune cells of the brain, which support numerous physiological processes. One of the defensive functions provided by microglia involves secretion of cytotoxins aimed at destroying invading pathogens. It is also recognized that the adverse activation of microglia in diseased brains may lead to secretion of cytotoxic molecules, which could be damaging to the surrounding cells, including neurons. Several of these toxins, such as reactive oxygen and nitrogen species, L-glutamate, and quinolinic acid, are widely recognized and well-studied. This review is focused on a structurally diverse group of less-established microglia neurotoxins, which were selected by applying the two criteria that these molecules 1) can be released by microglia, and 2) have the potential to be directly harmful to neurons. The following 11 molecules are discussed in detail: amyloid beta peptides (Aβ); cathepsin (Cat)B and CatD; C-X-C motif chemokine ligand (CXCL)10 and CXCL12 (5-67); high mobility group box (HMGB)1; lymphotoxin (LT)-α; matrix metalloproteinase (MMP)-2 and MMP-9; platelet-activating factor (PAF); and prolyl endopeptidase (PEP). Molecular mechanisms of their release by microglia and neurotoxicity, as well as available evidence implicating their involvement in human neuropathologies are summarized. Further studies on several of the above molecules are warranted to confirm either their microglial origin in the brain or direct neurotoxic effects. In addition, investigations into the differential secretion patterns of neurotoxins by microglia in response to diverse stimuli are required. This research could identify novel therapeutic targets for neurological disorders involving adverse microglial activation.

Roles of Microglial and Monocyte Chemokines and Their Receptors in Regulating Alzheimer's Disease-Associated Amyloid-β and Tau Pathologies

Chemokines and their receptors have been shown to affect amyloid-β (Aβ) and tau pathologies in mouse models of Alzheimer's disease (AD) by regulating microglia and monocyte-associated neuroinflammation, microglial movement and monocyte recruitment into the brain. These cells in turn can promote and mediate Aβ phagocytosis and degradation and tau phosphorylation. In this review we discuss published work in this field in mouse models of AD and review what is known about the contributions of microglial and monocyte chemokines and their receptors to amyloid and tau pathologies. We focus on the roles of the chemokine/chemokine receptor pairs CCL2/CCR2, CX3CL1/CX3CR1, CCL5/CCR5, CXCL10/CXCR3 and CXCL1/CXCR2, highlighting important knowledge gaps in this field. A full understanding of the functions of chemokines and their receptors in AD may guide the development of novel immunotherapies for this devastating disease.

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

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

Selective estrogen receptor modulators decrease the production of interleukin-6 and interferon-gamma-inducible protein-10 by astrocytes exposed to inflammatory challenge in vitro

Expression of proinflammatory molecules by glial cells is involved in the pathophysiological changes associated with chronic neurological diseases. Under pathological conditions, astrocytes release a number of proinflammatory molecules, such as interleukin-6 (IL-6) and interferon-gamma-inducible protein-10 (IP-10). The ovarian hormone estradiol exerts protective effects in the central nervous system that, at least in part, may be mediated by a reduction of local inflammation. This study was designed to assess whether estradiol affects the production of IL-6 and IP-10 by primary cultures of newborn mice astrocytes exposed to lipopolysaccharide (LPS), a bacterial endotoxin known to cause neuroinflammation. In addition, the possible anti-inflammatory effect of several selective estrogen receptor modulators (SERMs) was also assessed. LPS induced an increase in the expression of IL-6 and IP-10 mRNA levels in astrocytes and an increase in IL-6 and IP-10 protein levels in the culture medium. These effects of LPS were impaired by estradiol and by the four SERMs tested in our study: tamoxifen, raloxifene, ospemifene, and bazedoxifene. All SERMs tested showed a similar effect on IL-6 and IP-10 mRNA levels, but raloxifene and ospemifene were more effective than tamoxifen and bazedoxifene in reducing protein levels in LPS-treated cultures. Finally, we report that news SERMs, ospemifene and bazedoxifene, exert anti-inflammatory actions by a mechanism involving classical estrogen receptors and by the inhibition of LPS-induced NFkappaB p65 transactivation. The results suggest that estrogenic compounds may be candidates to counteract brain inflammation under neurodegenerative conditions by targeting the production and release of proinflammatory molecules by astrocytes.

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.

MAP kinase regulation of IP10/CXCL10 chemokine gene expression in microglial cells

Interferongamma inducible protein-10 (IP10 or CXCL10), a Th-1 affiliated chemokine, is expressed by activated glial cells and may contribute to the trafficking of immune cells in the inflamed central nervous system. This study examines the regulation of the expression of this chemokine in cultured microglial cells focusing on the roles of mitogen-activated protein (MAP) kinase cascades. Exposure of a mouse microglial cell line, BV-2, to lipopolysaccharide (LPS) and IFNgamma led to an induction of IP10 mRNA and protein as determined by RT-PCR and ELISA, respectively. This induction was suppressed by pharmacological inhibitors of p38 MAPK (i.e., SB203580) and c-Jun N-terminal kinase (JNK, SP600125), suggesting the involvement of the two kinases in IP10 expression. LPS also induced the activity of an IP10 promoter reporter (luciferase) construct transfected into BV-2 cells in a MAP kinase- and NFkappaB-dependent manner. The use of deletion constructs revealed that the kinase-targeted sequences were within the region between -533 bp and -332 bp upstream of the transcriptional start site. Co-transfection of IP10 luciferase with the active forms of the upstream kinases in the MAP kinase cascades, i.e., MAPK kinase-3 (MKK3), MKK6 (the immediately upstream activators of p38 kinase) and a MAP3K, i.e., TGFbeta-activated kinase-1 (TAK1), produced a marked stimulation of the promoter activity. The results of this study indicate that the MAP kinase cascades prominently regulate IP10 gene expression in microglial cells.

Changes in hippocampal IL-15, related cytokines, and neurogenesis in IL-2 deficient mice

Previous studies have demonstrated that interleukin-2 knockout (KO) mice exhibit alterations in hippocampal cytoarchitecture. Several lines of evidence suggest that these variations may result from immune dysregulation and/or autoimmunity. Thus, this study sought to compare adult IL-2 KO mice and wild-type littermates (8-12 weeks of age), the age where differences in hippocampal cytoarchitecture have previously been observed, for differences in measures of neuroimmunological status in the hippocampus. Furthermore, because IL-15 shares the same receptor subunits for signal transduction as IL-2 (IL-2/15Rbeta and gammac) that are enriched in the hippocampus and may induce inflammatory processes in IL-2 KO mice, we sought to test the hypothesis that IL-15 is elevated in the hippocampus of IL-2 KO mice. Compared to wild-type mice, IL-2 KO mice exhibited increased hippocampal protein concentrations of IL-15 as well as IL-12, IP-10, and MCP-1. These cytokine changes, however, did not correlate with levels in the peripheral circulation, and there were no T cells or an increase in MHCII-positive microglia in the hippocampus of IL-2 KO mice. Since elevated levels of certain inflammatory cytokines may impair hippocampal neurogenesis, we also tested the hypothesis that changes in neuroimmunological status would be associated with reductions in neurogenesis of neurons in the dentate gyrus of IL-2 KO mice. Contrary to this hypothesis, compared to wild-type mice, male IL-2 KO mice exhibited increased neurogenesis in both the infrapyramidal and suprapyramidal limbs of the granule cell layer of the dentate gyrus, differences that were not observed between females. These findings indicate that IL-2 gene deletion alters the neuroimmunological status of the mouse hippocampus through a dysregulation of cytokines produced by CNS cells, and in males, these changes are associated with increased hippocampal neurogenesis.

Expression profiling methods used in drug abuse research

A variety of analytical methodologies to investigate gene expression patterns in cells or tissues have been developed. For screening purposes, a large number of target mRNAs have to be interrogated simultaneously. These requirements have been met more or less comprehensively by Differential Display (DD) RT-PCR, Suppression Subtractive Hybridization (SSH), Serial Analysis of Gene Expression (SAGE), and DNA chips. The ultimate goal to cover any gene transcript potentially expressed by a given cell is on the way to be achieved by microbead arrays and by Affymetrix gene chips. Once targets of interest are identified, techniques employing low degrees of multiplexing, such as RNAse protection assays or some bead-based techniques (Luminex) eventually provide extremely fast results on the diagnostic level. With the aid of powerful computer programs, expression profiling technologies have opened intriguing new insights into the complex world of gene regulation. These new techniques have also been applied in drug abuse research recently and some examples of such approaches are described.

Expression of rat I-TAC/CXCL11/SCYA11 during central nervous system inflammation: comparison with other CXCR3 ligands

The chemokines are a large gene superfamily with critical roles in development and immunity. The chemokine receptor CXCR3 appears to play a major role in the trafficking of activated Th1 lymphocytes. There are at least three major ligands for CXCR3: mig/CXCL9, IP-10/CXCL10 and I-TAC/CXCL11, and of these three ligands, CXCL11 is the least well-characterized. In this study, we have cloned a rat ortholog of CXCL11, evaluated its function, and examined its expression in the Th-1-mediated disease, experimental autoimmune encephalomyelitis (EAE) in the rat. Based on its predicted primary amino-acid sequence, rat I-TAC/CXCL11 was synthesized and shown to induce chemotaxis of activated rat T lymphocytes in vitro and the in vivo migration of T lymphocytes when injected into the skin. I-TAC/CXCL11 expression, as determined by RT-PCR, increased in lymph node and spinal cord tissue collected from rats in which EAE had been actively induced, and in spinal cord tissue from rats in which EAE had been passively induced. The kinetics of expression were similar to that of CXCR3 and IP-10/CXCL10, although expression of both CXCR3 and IP-10/CXCL10 was more intense than that of I-TAC/CXCL11 and increased more rapidly in both lymph nodes and the spinal cord. Only minor levels of expression of the related chemokine mig/CXCL9 were observed. Immunohistochemistry revealed that the major cellular source of I-TAC/CXCL11 in the central nervous system (CNS) during EAE is likely to be the astrocyte. Together, these data indicate that I-TAC/CXCL11 is expressed in the CNS during the clinical phase of EAE. However, the observation that I-TAC/CXCL11 is expressed after receptor expression is detected suggests that it is not essential for the initial migration of CXCR3-bearing cells into the CNS.

Role of microglia in central nervous system infections

The nature of microglia fascinated many prominent researchers in the 19th and early 20th centuries, and in a classic treatise in 1932, Pio del Rio-Hortega formulated a number of concepts regarding the function of these resident macrophages of the brain parenchyma that remain relevant to this day. However, a renaissance of interest in microglia occurred toward the end of the 20th century, fueled by the recognition of their role in neuropathogenesis of infectious agents, such as human immunodeficiency virus type 1, and by what appears to be their participation in other neurodegenerative and neuroinflammatory disorders. During the same period, insights into the physiological and pathological properties of microglia were gained from in vivo and in vitro studies of neurotropic viruses, bacteria, fungi, parasites, and prions, which are reviewed in this article. New concepts that have emerged from these studies include the importance of cytokines and chemokines produced by activated microglia in neurodegenerative and neuroprotective processes and the elegant but astonishingly complex interactions between microglia, astrocytes, lymphocytes, and neurons that underlie these processes. It is proposed that an enhanced understanding of microglia will yield improved therapies of central nervous system infections, since such therapies are, by and large, sorely needed.

Effect of a short-term in vitro exposure to the marine toxin domoic acid on viability, tumor necrosis factor-alpha, matrix metalloproteinase-9 and superoxide anion release by rat neonatal microglia

BACKGROUND

The excitatory amino acid domoic acid, a glutamate and kainic acid analog, is the causative agent of amnesic shellfish poisoning in humans. No studies to our knowledge have investigated the potential contribution to short-term neurotoxicity of the brain microglia, a cell type that constitutes circa 10% of the total glial population in the brain. We tested the hypothesis that a short-term in vitro exposure to domoic acid, might lead to the activation of rat neonatal microglia and the concomitant release of the putative neurotoxic mediators tumor necrosis factor-alpha (TNF-alpha), matrix metalloproteinases-2 and-9 (MMP-2 and -9) and superoxide anion (O2-).

RESULTS

In vitro, domoic acid [10 microM-1 mM] was significantly neurotoxic to primary cerebellar granule neurons. Although neonatal rat microglia expressed ionotropic glutamate GluR4 receptors, exposure during 6 hours to domoic acid [10 microM-1 mM] had no significant effect on viability. By four hours, LPS (10 ng/mL) stimulated an increase in TNF-alpha mRNA and a 2,233 % increase in TNF-alpha protein In contrast, domoic acid (1 mM) induced a slight rise in TNF-alpha expression and a 53 % increase (p < 0.01) of immunoreactive TNF-alpha protein. Furthermore, though less potent than LPS, a 4-hour treatment with domoic acid (1 mM) yielded a 757% (p < 0.01) increase in MMP-9 release, but had no effect on MMP-2. Finally, while PMA (phorbol 12-myristate 13-acetate) stimulated O2- generation was elevated in 6 hour LPS-primed microglia, a similar pretreatment with domoic acid (1 mM) did not prime O2- release.

CONCLUSIONS

To our knowledge this is the first experimental evidence that domoic acid, at in vitro concentrations that are toxic to neuronal cells, can trigger a release of statistically significant amounts of TNF-alpha and MMP-9 by brain microglia. These observations are of considerable pathophysiological significance because domoic acid activates rat microglia several days after in vivo administration.

CX3CL1 and CX3CR1 expression in human brain tissue: noninflammatory control versus multiple sclerosis

An important role for CX3CL1 in neuroinflammation and neurodegeneration has been suggested in recent publications. In this study, we compared the expression of CX3CL1 and its receptor CX3CR1 in human brain tissue derived from control patients without neurological complications and in multiple sclerosis (MS) patients. Results from this study demonstrate that CX3CL1 is constitutively expressed in human central nervous system (CNS) astrocytes in vivo and under basal conditions in human adult astrocyte cultures. CX3CR1 is expressed on astrocytes and microglial cells both in vivo and in vitro. Chemotaxis assay shows a functional response upon CX3CR1 signaling in microglial cells. Although CX3CL1 expression is upregulated in cultured astrocytes in response to proinflammatory cytokines, no evidence for expression differences of CX3CL1 between control patients and MS patients was found. Our data suggest that CX3CL1 has more general physiological functions, which occur also in the absence of proinflammatory conditions.

Inhibition of interferon (IFN) gamma-induced Jak-STAT1 activation in microglia by vasoactive intestinal peptide: inhibitory effect on CD40, IFN-induced protein-10, and inducible nitric-oxide synthase expression

Interferon (IFN)-gamma is one of the most important microglia stimulators in vivo participating in inflammation and Th1 activation/differentiation. IFN-gamma-mediated signaling involves the activation of the Jak/STAT1 pathway. The neuropeptides vasoactive intestinal peptide (VIP) and the pituitary adenylate cyclase activating polypeptide (PACAP) are two potent microglia-deactivating factors that inhibit the production of proinflammatory mediators in vitro and in vivo. The present study investigated the molecular mechanisms involved in the VIP/PACAP regulation of several IFN-gamma-induced microglia-derived factors, including IFN-gamma-inducible protein-10 (IP-10), inducible nitric-oxide synthase (iNOS), and CD40. The results indicate that VIP/PACAP inhibit Jak1-2 and STAT1 phosphorylation, and the binding of activated STAT1 to the IFN-gamma activated site motif in the IFN regulatory factor-1 and CD40 promoter and to the IFN-stimulated response element motif of the IP-10 promoter. Through its effect in the IFN-gamma-induced Jak/STAT1 pathway, VIP and PACAP are able to control the gene expression of IP-10, CD40, and iNOS, three microglia-derived mediators that play an essential role in several pathologies, i.e. inflammation and autoimmune disorders. The effects of VIP/PACAP are mediated through the specific receptor VPAC1 and the cAMP/protein kinase A transduction pathway. Because IFN-gamma is a major stimulator of innate and adaptive immune responses in vivo, the down-regulation of IFN-gamma-induced gene expression by VIP and PACAP could represent a significant element in the regulation of the inflammatory response in the central nervous system by endogenous neuropeptides.

Induced neuronal expression of class I major histocompatibility complex mRNA in acute and chronic inflammation models

Studies have demonstrated neuronal expression of class I major histocompatibility complex (MHC) mRNA and protein in normal and developing brain and in response to injury or viral infection. We report neuronal expression of class I MHC mRNA in hypothalamic paraventricular nucleus (PVN) neurons in rats following systemic infection with Trypanosoma brucei brucei parasites (chronic) and in response to intravenous 1 mg/kg lipopolysaccharide administration (acute peripheral) and in striatal neurons following intrastriatal 5 microg lipopolysaccharide injection (acute central). These results demonstrate that neurons can be a source of immune signaling molecules and establish class I MHC as part of the neuronal component of immune responses.

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.

Immunoregulation of microglial functional properties

Microglia are the resident tissue macrophages of the central nervous system (CNS) parenchyma and are key players in the initiation of an inflammatory response. Microglia rapidly transform from a resting to an activated morphology in response to a variety of disease states. However, they can also be the target of infections, as in the case of HIV. Many of the effector properties of microglia can be attributed to the array of substances they secrete in response to stimuli such as bacterial lipopolysaccharide, cytokines, and chemokines. The products of activated microglia include: cytokines (pro- and anti-inflammatory), chemokines, nitric oxide, superoxide radicals, and proteases. Furthermore, microglia have the ability to present antigen to T cells, migrate in response to chemotactic stimuli, and phagocytose cell debris. This report focuses on the immunomodulatory functions of microglia, with particular attention to chemokines, and highlights their pivotal role in the CNS.

Chemokines and viral diseases of the central nervous system

This chapter discusses chemokines and their receptors in the evolution of viral infectious diseases of the central nervous system (CNS). Infection of the human CNS with many different viruses or infection of the rodent CNS induces vigorous host-inflammatory responses with recruitment of large numbers of leukocytes, particularly T lymphocytes and macrophages. Chemokines coordinate trafficking of peripheral blood leukocytes by stimulating their chemotaxis, adhesion, extravasation, and other effector functions. In view of these properties, research efforts have turned increasingly to the possible involvement of chemokines in regulating both peripheral tissue and CNS leukocyte migration during viral infection. The biological effects of chemokines are mediated via their interaction with receptors belonging to the family of seven transmembrane (7TM)-spanning, G-protein coupled receptors (GPCRs). In the normal mammalian CNS, the number of leukocytes present in the brain is scant. However, these cells are attracted to, and accumulate in, a variety of pathologic states, many involving viral infection. Although leukocyte migration into local tissue compartments, such as the CNS, is a multifactorial process, it has become clear that chemokines are pivotal components of this process, providing a necessary chemotactic signal for leukocyte recruitment.

Differential fibronectin expression in activated C6 glial cells treated with ethanol

The central nervous system is particularly susceptible to alcohol effects and toxicity. Glial cells constitute the most common cell type in the brain and play critical roles in normal brain function and during infection and injury. Astrocytes in particular seem to be important targets for alcohol neurotoxicity during both development and in adulthood. To gain more insight into alcohol-mediated effects on astrocytes at the molecular level, gene expression in rat C6 glial cells was studied in the presence or absence of ethanol. The differential display of mRNA technique was used to screen the expressed genes in ethanol-treated rat C6 cells before and after treatment with lipopolysaccharide (LPS) combined with phorbol-12-myristate-13-acetate (PMA), conditions that mimic an infectious inflammatory state and cause immunologic activation. The present data show that fibronectin appeared as a major gene whose expression is increased in C6 cells by LPS plus PMA stimulation and decreased by chronic ethanol exposure, both in mRNA and protein levels. Fibronectin is a dimeric glycoprotein found in the extracellular matrix of most tissues, in the blood, and on cell surfaces and is involved in many cellular processes. These results show that chronic exposure to ethanol is associated with changes in astrocyte properties during immunologic activation that reduce fibronectin expression. The discovery of astrocyte fibronectin expression as a potential regulated target for chronic alcohol abuse may be useful in understanding, preventing, and treating some brain disorders associated with alcohol abuse and alcoholism.

Interleukin-4-inhibited mRNA expression in mixed rat glial and in isolated microglial cultures

Interleukin-4 (IL-4) likely is one of the key players in the concert of immunosuppressive factors in brain. Therefore, influences of the cytokine on mRNA expression of endogenous mediators of inflammation, such as cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), tumor necrosis factor-alpha (TNF-alpha), interferon-inducible protein 10 (IP-10), interleukin-3 receptor-beta (IL-3R-beta), and of another antiinflammatory cytokine, IL-10, have been evaluated in the present study by semi-quantitative RT-PCR. Primary rat mixed glial cultures and isolated microglial cells, the resident immunocytes of the brain, have been used as rich sources of these mRNAs in response to the bacterial cell wall component lipopolysaccharide (LPS). Time-course studies showed peak levels of LPS-increased mRNAs at approximately 4 h. Interestingly, IL-10 mRNA was elevated also upon the LPS-stimulus. IL-4, given 30 min before LPS, inhibited increases of all mRNAs significantly, including IL-10 mRNA. IL-4, however, induced peroxisome proliferator-activated receptor (PPAR)-gamma in cultured microglia. This induction was completely inhibited by simultaneous administration of LPS. The data confirms IL-4 as an important antiinflammatory cytokine and gives some idea of cross-talk between intracellular signaling evoked by pro- and antiinflammatory substances.

Distinct patterns of stimulus-inducible chemokine mRNA accumulation in human fetal astrocytes and microglia

Interferon-gamma-inducible 10 kd protein (IP-10) is an ELR (Glu-Leu-Arg)(-) alpha chemokine with known chemotactic effects on T cells and monocytes, as well as anti-viral, anti-angiogenic, and anti-tumor effects. Previous studies have demonstrated that in cultured rat astrocytes and microglia, stimulation with LPS or virus can induce the expression of IP-10. In this study, we determined the pattern of IP-10 gene induction in primary human microglia and astrocytes by cytokines and LPS using ribonuclease protection assay. The expression of IP-10 mRNA was compared with that of other alpha (IL-8) and beta chemokines. The results showed that in human microglia, IP-10 expression was induced equally potently by LPS, IFNbeta or IFNgamma. "Proinflammatory" cytokines IL-1beta or TNFalpha also induced small amounts of IP-10 mRNA. "Anti-inflammatory" cytokines IL-4, IL-10 and TGFbeta were ineffective in inducing IP-10 in microglia. In human astrocytes, induction of IP-10 mRNA by cytokines was similar to that in microglia. LPS, however, was ineffective in inducing IP-10 in human astrocytes. The monocyte chemoattractant beta-chemokine I-309 mRNA was induced in human astrocytes and microglia by IFNbeta or IFNgamma, or by LPS in microglia, showing a tight co-regulation with IP-10 mRNA expression. In contrast to the potent induction of IP-10 and I-309 by IFNs in human glia, the ELR(+) alpha chemokine IL-8 mRNA was induced by IL-1beta and TNFalpha, and to a lesser extent by IFNbeta in microglia. IFNbeta but not IFNgamma was effective in inducing the expression of beta chemokines MIP-1alpha and MIP-1beta in human microglia, with the levels of mRNA similar to those induced by IL-1beta or TNFalpha. Neither MIP-1alpha nor MIP-1beta mRNAs were induced by any stimulation in human astrocytes. The induction of RANTES mRNA in microglia by IFNbeta, IL-1beta or TNFalpha was variable, showing no to low level expression depending on the case, whereas LPS provided a consistent inducing signal. In astrocytes, only cytokine combinations (IFN + IL-1beta) effectively induced the RANTES mRNA. These results demonstrate that distinct sets of chemokine genes are induced in human glial cells by cytokines and interferons. These results may have wide implications for inflammatory, vascular and neoplastic diseases of the CNS.