Transforming growth factor beta's 1, 2 and 3 inhibit proliferation of ramified microglia on an astrocyte monolayer

Astrocyte-targeted IL-10 production decreases proliferation and induces a downregulation of activated microglia/macrophages after PPT

When central nervous system (CNS) homeostasis is altered, microglial cells become rapidly activated, proliferate and release a broad range of molecules. Among the plethora of molecules involved in the regulation of microglial activation, cytokines are considered crucial. Although production of interleukin-10 (IL-10) has been demonstrated after different types of CNS injuries and associated with protective functions, the specific role played by IL-10 modulating microglial cells remains unclear. Hence, the objective of this study was to evaluate the effects of transgenic astrocyte IL-10 production on microglial activation associated with axonal anterograde degeneration. To address it, the hippocampal area subjected to perforant pathway transection (PPT) was analyzed by immunohistochemistry (IHC), flow cytometry and protein microarray in transgenic (GFAP-IL10Tg) mice and their corresponding wild types (WT) littermates. Our results demonstrated increased microglial/macrophages density in nonlesioned and PPT-lesioned GFAP-IL10Tg animals when compared with nonlesioned and lesioned WT, respectively. This increase was not due to proliferation, as GFAP-IL10Tg mice showed a reduced proliferation of microglial cells, but was related to an increased population of CD11b+/CD45^high monocyte/macrophages. Despite this higher number, the microglia/macrophage population in transgenic animals displayed a downregulated phenotype characterized by lower MHCII, ICOSL, and CD11c. Moreover, a sustained T-cell infiltration was found in transgenic animals. We strongly suggest these modifications must be associated with indirect effects derived from the influence of IL-10 on astrocytes and/or neurons, which express IL-10R. We finally suggested that TGF-β produced by astrocytes, along with IL-2 and CXCL10 might be crucial molecules mediating the effects of transgenic IL-10.

Comparing Effects of Transforming Growth Factor β1 on Microglia From Rat and Mouse: Transcriptional Profiles and Potassium Channels

The cytokine, transforming growth factor β1 (TGFβ1), is up-regulated after central nervous system (CNS) injuries or diseases involving microglial activation, and it has been proposed as a therapeutic agent for treating neuroinflammation. Microglia can produce and respond to TGFβ1. While rats and mice are commonly used for studying neuroinflammation, very few reports directly compare them. Such studies are important for improving pre-clinical studies and furthering translational progress in developing therapeutic interventions. After intracerebral hemorrhage (ICH) in the rat striatum, the TGFβ1 receptor was highly expressed on microglia/macrophages within the hematoma. We recently found species similarities and differences in response to either a pro-inflammatory (interferon-γ, IFN-γ, +tumor necrosis factor, TNF-α) or anti-inflammatory interleukin-4 (IL-4) stimulus. Here, we assessed whether rat and mouse microglia differ in their responses to TGFβ1. Microglia were isolated from Sprague-Dawley rats and C57BL/6 mice and treated with TGFβ1. We quantified changes in expression of >50 genes, in their morphology, proliferation, apoptosis and in three potassium channels that are considered therapeutic targets. Many inflammatory mediators, immune receptors and modulators showed species similarities, but notable differences included that, for some genes, only one species responded (e.g., Il4r, Il10, Tgfbr2, colony-stimulating factor receptor (Csf1r), Itgam, suppressor of cytokine signaling 1 (Socs1), toll-like receptors 4 (Tlr4), P2rx7, P2ry12), and opposite responses were seen for others (Tgfb1, Myc, Ifngr1). In rat only, TGFβ1 affected microglial morphology and proliferation, but there was no apoptosis in either species. In both species, TGFβ1 dramatically increased Kv1.3 channel expression and current (no effects on Kir2.1). KCa3.1 showed opposite species responses: the current was low in unstimulated rat microglia and greatly increased by TGFβ1 but higher in control mouse cells and decreased by TGFβ1. Finally, we compared TGFβ1 and IL10 (often considered similar anti-inflammatory stimuli) and found many different responses in both species. Overall, the numerous species differences should be considered when characterizing neuroinflammation and microglial activation in vitro and in vivo, and when targeting potassium channels.

Combinatorial identification of DNA methylation patterns over age in the human brain

Background

DNA methylation plays a key role in developmental processes, which is reflected in changing methylation patterns at specific CpG sites over the lifetime of an individual. The underlying mechanisms are complex and possibly affect multiple genes or entire pathways.

Results

We applied a multivariate approach to identify combinations of CpG sites that undergo modifications when transitioning between developmental stages. Monte Carlo feature selection produced a list of ranked and statistically significant CpG sites, while rule-based models allowed for identifying particular methylation changes in these sites.

Our rule-based classifier reports combinations of CpG sites, together with changes in their methylation status in the form of easy-to-read IF-THEN rules, which allows for identification of the genes associated with the underlying sites.

Conclusion

We utilized machine learning and statistical methods to discretize decision class (age) values to get a general pattern of methylation changes over the lifespan. The CpG sites present in the significant rules were annotated to genes involved in brain formation, general development, as well as genes linked to cancer and Alzheimer’s disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-016-1259-3) contains supplementary material, which is available to authorized users.

Changes in Neuronal Excitability by Activated Microglia: Differential Na(+) Current Upregulation in Pyramid-Shaped and Bipolar Neurons by TNF-α and IL-18

Microglia are activated during pathological events in the brain and are capable of releasing various types of inflammatory cytokines. Here, we demonstrate that the addition of 5% microglia activated by 1 μg/ml lipopolysaccharides (LPS) to hippocampal cultures upregulates Na⁺ current densities (I_NavD) of bipolar as well as pyramid-shaped neurons, thereby increasing their excitability. Deactivation of microglia by the addition of 10 ng/ml transforming growth factor-β (TGF-β) decreases I_NavD below control levels suggesting that the residual activated microglial cells influence neuronal excitability in control cultures. Preincubation of hippocampal cultures with 10 ng/ml tumor necrosis factor-α (TNF-α), a major cytokine released by activated microglia, upregulated I_NavD significantly by ~30% in bipolar cells, whereas in pyramid-shaped cells, the upregulation only reached an increase of ~14%. Incubation of the cultures with antibodies against either TNF-receptor 1 or 2 blocked the upregulation of I_NavD in bipolar cells, whereas in pyramid-shaped cells, increases in I_NavD were exclusively blocked by antibodies against TNF-receptor 2, suggesting that both cell types respond differently to TNF-α exposure. Since additional cytokines, such as interleukin-18 (IL-18), are released from activated microglia, we tested potential effects of IL-18 on I_NavD in both cell types. Exposure to 5–10 ng/ml IL-18 for 4 days increased I_NavD in both pyramid-shaped as well as bipolar neurons, albeit the dose–response curves were shifted to lower concentrations in bipolar cells. Our results suggest that by secretion of cytokines, microglial cells upregulate Na⁺ current densities in bipolar and pyramid-shaped neurons to some extent differentially. Depending on the exact cytokine composition and concentration released, this could change the balance between the activity of inhibitory bipolar and excitatory pyramid-shaped cells. Since bipolar cells show a larger upregulation of I_NavD in response to TNF-α as well as respond to smaller concentrations of IL-18, our results offer an explanation for the finding, that in certain conditions of brain inflammations periods of dizziness are followed by epileptic seizures.

Protective Effects of Human iPS-Derived Retinal Pigmented Epithelial Cells in Comparison with Human Mesenchymal Stromal Cells and Human Neural Stem Cells on the Degenerating Retina in rd1 mice

Retinitis pigmentosa (RP) is a group of visual impairments characterized by progressive rod photoreceptor cell loss due to a genetic background. Pigment epithelium-derived factor (PEDF) predominantly secreted by the retinal pigmented epithelium (RPE) has been reported to protect photoreceptors in retinal degeneration models, including rd1. In addition, clinical trials are currently underway outside Japan using human mesenchymal stromal cells and human neural stem cells to protect photoreceptors in RP and dry age-related macular degeneration, respectively. Thus, this study aimed to investigate the rescue effects of induced pluripotent stem (iPS)-RPE cells in comparison with those types of cells used in clinical trials on photoreceptor degeneration in rd1 mice. Cells were injected into the subretinal space of immune-suppressed 2-week-old rd1 mice. The results demonstrated that human iPS-RPE cells significantly attenuated photoreceptor degeneration on postoperative days (PODs) 14 and 21 and survived longer up to at least 12 weeks after operation than the other two types of graft cells with less immune responses and apoptosis. The mean PEDF concentration in the intraocular fluid in RPE-transplanted eyes was more than 1 µg/ml at PODs 14 and 21, and this may have contributed to the protective effect of RPE transplantation. Our findings suggest that iPS-RPE cells serve as a competent source to delay photoreceptor degeneration through stable survival in degenerating ocular environment and by releasing neuroprotective factors such as PEDF.

Increased levels of tumour necrosis factor alpha (TNFα) but not transforming growth factor-beta 1 (TGFβ1) are associated with the severity of congenital hydrocephalus in the hyh mouse

AIMS

Here, we tested the hypothesis that glial responses via the production of cytokines such as transforming growth factor-beta 1 (TGFβ1) and tumour necrosis factor alpha (TNFα), which play important roles in neurodegenerative diseases, are correlated with the severity of congenital hydrocephalus in the hyh mouse model. We also searched for evidence of this association in human cases of primary hydrocephalus.

METHODS

Hyh mice, which exhibit either severe or compensated long-lasting forms of hydrocephalus, were examined and compared with wild-type mice. TGFβ1, TNFα and TNFαR1 mRNA levels were quantified using real-time PCR. TNFα and TNFαR1 were immunolocalized in the brain tissues of hyh mice and four hydrocephalic human foetuses relative to astroglial and microglial reactions.

RESULTS

The TGFβ1 mRNA levels were not significantly different between hyh mice exhibiting severe or compensated hydrocephalus and normal mice. In contrast, severely hydrocephalic mice exhibited four- and two-fold increases in the mean levels of TNFα and TNFαR1, respectively, compared with normal mice. In the hyh mouse, TNFα and TNFαR1 immunoreactivity was preferentially detected in astrocytes that form a particular periventricular reaction characteristic of hydrocephalus. However, these proteins were rarely detected in microglia, which did not appear to be activated. TNFα immunoreactivity was also detected in the glial reaction in the small group of human foetuses exhibiting hydrocephalus that were examined.

CONCLUSIONS

In the hyh mouse model of congenital hydrocephalus, TNFα and TNFαR1 appear to be associated with the severity of the disease, probably mediating the astrocyte reaction, neurodegenerative processes and ischaemia.

The use of the antimicrobial peptide piscidin (PCD)-1 as a novel anti-nociceptive agent

The antimicrobial peptide piscidin (PCD)-1 has been reported to have antibacterial and immunomodulatory functions. Here, we investigated the anti-neuropathic properties of PCD-1, in order to determine its potential as a compound to alleviate pain. Treatment with PCD-1 suppressed the inflammatory proteins COX-2 and iNOS in murine macrophage (RAW264.7) and microglial (BV2) cell lines stimulated by lipopolysaccharide (LPS). For studies of the effect of PCD-1 in vivo, mononeuropathy in rats was induced by chronic constriction injury (CCI), and the resulting anti-nociceptive behaviors were compared between CCI controls and CCI rats given intrathecal injections of PCD-1. Much like gabapentin, PCD-1 exerts anti-nociceptive effects against thermal hyperalgesia, with a median effective dose (ED50) of 9.5 μg in CCI rats. In CCI rats, PCD-1 exerted effects against mechanical and cold allodynia, thermal hyperalgesia, and weight-bearing deficits. Furthermore, CCI-mediated activation of microglia and astrocytes in the dorsal horn of the lumbar spinal cord were decreased by PCD-1. In addition, PCD-1 suppressed up-regulation of interleukin-1β (IL-1β) and phosphorylated mammalian target of rapamycin (phospho-mTOR) in CCI rats. Finally, CCI-induced down-regulation of transforming growth factor-β1 (TGF-β1) in rats was attenuated by injection of PCD-1. Taken together, the present findings demonstrate that the marine antimicrobial peptide PCD-1 has anti-nociceptive effects, and thus may have potential for development as an alternative pain-alleviating agent.

Induction of transforming growth factor beta receptors following focal ischemia in the rat brain

Transforming growth factor-βs (TGF-βs) regulate cellular proliferation, differentiation, and survival. TGF-βs bind to type I (TGF-βRI) and II receptors (TGF-βRII), which are transmembrane kinase receptors, and an accessory type III receptor (TGF-βRIII). TGF-β may utilize another type I receptor, activin-like kinase receptor (Alk1). TGF-β is neuroprotective in the middle cerebral artery occlusion (MCAO) model of stroke. Recently, we reported the expression pattern of TGF-β1-3 after MCAO. To establish how TGF-βs exert their actions following MCAO, the present study describes the induction of TGF-βRI, RII, RIII and Alk1 at 24 h, 72 h and 1 mo after transient 1 h MCAO as well as following 24 h permanent MCAO using in situ hybridization histochemistry. In intact brain, only TGF-βRI had significant expression: neurons in cortical layer IV contained TGF-βRI. At 24 h after the occlusion, no TGF-β receptors showed induction. At 72 h following MCAO, all four types of TGF-β receptors were induced in the infarct area, while TGF-βRI and RII also appeared in the penumbra. Most cells with elevated TGF-βRI mRNA levels were microglia. TGF-βRII co-localized with both microglial and endothelial markers while TGF-βRIII and Alk1 were present predominantly in endothels. All four TGF-β receptors were induced within the lesion 1 mo after the occlusion. In particular, TGF-βRIII was further induced as compared to 72 h after MCAO. At this time point, TGF-βRIII signal was predominantly not associated with blood vessels suggesting its microglial location. These data suggest that TGF-β receptors are induced after MCAO in a timely and spatially regulated fashion. TGF-β receptor expression is preceded by increased TGF-β expression. TGF-βRI and RII are likely to be co-expressed in microglial cells while Alk1, TGF-βRII, and RIII in endothels within the infarct where TGF-β1 may be their ligand. At later time points, TGF-βRIII may also appear in glial cells to potentially affect signal transduction via TGF-βRI and RII.

TGF-β1 attenuates spinal neuroinflammation and the excitatory amino acid system in rats with neuropathic pain

Previous studies have reported that the intrathecal (i.t.) administration of transforming growth factor β1 (TGF-β1) prevents and reverses neuropathic pain. However, only limited information is available regarding the possible role and effects of spinal TGF-β1 in neuropathic pain. We aimed to investigate the antinociceptive effects of exogenous TGF-β1 on chronic constriction injury (CCI)-induced neuropathic pain in rats. We demonstrated that sciatic nerve injury caused a downregulation of endogenous TGF-β1 levels on the ipsilateral side of the lumbar spinal dorsal gray matter, and that the i.t. administration of TGF-β1 (.01-10 ng) significantly attenuated CCI-induced thermal hyperalgesia in neuropathic rats. TGF-β1 significantly inhibited CCI-induced spinal neuroinflammation, microglial and astrocytic activation, and upregulation of tumor necrosis factor-α. Moreover, i.t. TGF-β1 significantly attenuated the CCI-induced downregulation of glutamate transporter 1, the glutamate aspartate transporter, and the excitatory amino acid carrier 1 on the ipsilateral side. Furthermore, i.t. TGF-β1 significantly decreased the concentrations of 2 excitatory amino acids, aspartate and glutamate, in the spinal dialysates in CCI rats. In summary, we conclude that the mechanisms of the antinociceptive effects of i.t. TGF-β1 in neuropathy may include attenuation of spinal neuroinflammation, attenuation, or upregulation of glutamate transporter downregulation, and a decrease of spinal extracellular excitatory amino acids.

PERSPECTIVE

Clinically, medical treatment is usually initiated after the onset of intractable pain. Therefore, in the present study, i.t. TGF-β1 was designed to be administered 2 weeks after the establishment of CCI pain. Compared to the continuous TGF-β1 infusion mode, single-dose administration seems more convenient and practical to use.

Flexibilide obtained from cultured soft coral has anti-neuroinflammatory and analgesic effects through the upregulation of spinal transforming growth factor-β1 in neuropathic rats

Chronic neuroinflammation plays an important role in the development and maintenance of neuropathic pain. The compound flexibilide, which can be obtained from cultured soft coral, possesses anti-inflammatory and analgesic effects in the rat carrageenan peripheral inflammation model. In the present study, we investigated the antinociceptive properties of flexibilide in the rat chronic constriction injury (CCI) model of neuropathic pain. First, we found that a single intrathecal (i.t.) administration of flexibilide significantly attenuated CCI-induced thermal hyperalgesia at 14 days after surgery. Second, i.t. administration of 10-μg flexibilide twice daily was able to prevent the development of thermal hyperalgesia and weight-bearing deficits in CCI rats. Third, i.t. flexibilide significantly inhibited CCI-induced activation of microglia and astrocytes, as well as the upregulated proinflammatory enzyme, inducible nitric oxide synthase, in the ipsilateral spinal dorsal horn. Furthermore, flexibilide attenuated the CCI-induced downregulation of spinal transforming growth factor-β1 (TGF-β1) at 14 days after surgery. Finally, i.t. SB431542, a selective inhibitor of TGF-β type I receptor, blocked the analgesic effects of flexibilide in CCI rats. Our results suggest that flexibilide may serve as a therapeutic agent for neuropathic pain. In addition, spinal TGF-β1 may be involved in the anti-neuroinflammatory and analgesic effects of flexibilide.

Adult human glia, pericytes and meningeal fibroblasts respond similarly to IFNy but not to TGFβ1 or M-CSF

The chemokine Interferon gamma-induced protein 10 (IP-10) and human leukocyte antigen (HLA) are widely used indicators of glial activation and neuroinflammation and are up-regulated in many brain disorders. These inflammatory mediators have been widely studied in rodent models of brain disorders, but less work has been undertaken using human brain cells. In this study we investigate the regulation of HLA and IP-10, as well as other cytokines and chemokines, in microglia, astrocytes, pericytes, and meningeal fibroblasts derived from biopsy and autopsy adult human brain, using immunocytochemistry and a Cytometric Bead Array. Interferonγ (IFNγ) increased microglial HLA expression, but contrary to data in rodents, the anti-inflammatory cytokine transforming growth factor β1 (TGFβ1) did not inhibit this increase in HLA, nor did TGFβ1 affect basal microglial HLA expression or IFNγ-induced astrocytic HLA expression. In contrast, IFNγ-induced and basal microglial HLA expression, but not IFNγ-induced astrocytic HLA expression, were strongly inhibited by macrophage colony stimulating factor (M-CSF). IFNγ also strongly induced HLA expression in pericytes and meningeal fibroblasts, which do not basally express HLA, and this induction was completely blocked by TGFβ1, but not affected by M-CSF. In contrast, TGFβ1 did not block the IFNγ-induced increase in IP-10 in pericytes and meningeal fibroblasts. These results show that IFNγ, TGFβ1 and M-CSF have species- and cell type-specific effects on human brain cells that may have implications for their roles in adult human brain inflammation.

Cudrania cochinchinensis attenuates amyloid β protein-mediated microglial activation and promotes glia-related clearance of amyloid β protein

Background

Microglial inflammation may significantly contribute to the pathology of Alzheimer’s disease. To examine the potential of Cudrania cochinchinensis to ameliorate amyloid β protein (Aβ)-induced microglia activation, BV-2 microglial cell line, and the ramified microglia in the primary glial mixed cultured were employed.

Results

Lipopolysaccharide (LPS), Interferon-γ (IFN-γ), fibrillary Aβ (fAβ), or oligomeric Aβ (oAβ) were used to activate microglia. LPS and IFN-γ, but not Aβs, activated BV-2 cells to produce nitric oxide through an increase in inducible nitric oxide synthase (iNOS) expression without significant effects on cell viability of microglia. fAβ, but not oAβ, enhanced the IFN-γ-stimulated nitric oxide production and iNOS expression.

The ethanol/water extracts of Cudrania cochinchinensis (CC-EW) and the purified isolated components (i.e. CCA to CCF) effectively reduced the nitric oxide production and iNOS expression stimulated by IFN-γ combined with fAβ. On the other hand, oAβ effectively activated the ramified microglia in mixed glial culture by observing the morphological alteration of the microglia from ramified to amoeboid. CC-EW and CCB effectively prohibit the Aβ-mediated morphological change of microglia. Furthermore, CC-EW and CCB effectively decreased Aβ deposition and remained Aβ in the conditioned medium suggesting the effect of CC-EW and CCB on promoting Aβ clearance. Results are expressed as mean ± S.D. and were analyzed by ANOVA with post-hoc multiple comparisons with a Bonferroni test.

Conclusions

The components of Cudrania cochinchinensis including CC-EW and CCB are potential for novel therapeutic intervention for Alzheimer’s disease.

Upregulation of mesencephalic astrocyte-derived neurotrophic factor in glial cells is associated with ischemia-induced glial activation

Background

Mesencephalic astrocyte-derived neurotrophic factor (MANF), a 20 kDa secreted protein, was originally derived from a rat mesencephalic type-1 astrocyte cell line. MANF belongs to a novel evolutionally conserved family of neurotrophic factors along with conserved dopamine neurotrophic factor. In recent years, ever-increasing evidence has shown that both of them play a remarkable protective role against various injuries to neurons in vivo or in vitro. However, the characteristics of MANF expression in the different types of glial cells, especially in astrocytes, remain unclear.

Methods

The model of focal cerebral ischemia was induced by rat middle cerebral artery occlusion. Double-labeled immunofluorescent staining was used to identify the types of neural cells expressing MANF. Primarily cultured glial cells were used to detect the response of glial cells to endoplasmic reticulum stress stimulation. Propidium iodide staining was used to determine dead cells. Reverse transcription PCR and western blotting were used to detect the levels of mRNA and proteins.

Results

We found that MANF was predominantly expressed in neurons in both normal and ischemic cortex. Despite its name, MANF was poorly expressed in glial cells, including astrocytes, in normal brain tissue. However, the expression of MANF was upregulated in the glial cells under focal cerebral ischemia, including the astrocytes. This expression was also induced by several endoplasmic reticulum stress inducers and nutrient deprivation in cultured primary glial cells. The most interesting phenomenon observed in this study was the pattern of MANF expression in the microglia. The expression of MANF was closely associated with the morphology and state of microglia, accompanied by the upregulation of BIP/Grp78.

Conclusions

These results indicate that MANF expression was upregulated in the activated glial cells, which may contribute to the mechanism of ischemia-induced neural injury.

Endogenous transforming growth factor-beta promotes quiescence of primary microglia in vitro

Microglia are the immune cells of the central nervous system (CNS) and play important roles under physiological and pathophysiological conditions. Activation of microglia has been reported for a variety of CNS diseases and is believed to be involved in inflammation-mediated neurodegeneration. Loss of TGFβ1 results in increased microgliosis and neurodegeneration in mice which indicates that TGFβ1 is an important regulator of microglial functions in vivo. Here, we addressed the role of endogenous TGFβ signaling for microglia in vitro. We clearly demonstrate active TGFβ signaling in primary microglia and further introduce Klf10 as a new TGFβ target gene in microglia. Moreover, we provide evidence that microglia express and release TGFβ1 that acts in an autocrine manner to activate microglial TGFβ/Smad signaling in vitro. Using microarrays, we identified TGFβ-regulated genes in microglia that are involved in TGFβ1 processing, its extracellular storage as well as activation of latent TGFβ. Finally, we demonstrate that pharmacological inhibition of microglial TGFβ signaling resulted in upregulation of the proinflammatory markers IL6 and iNOS and downregulation of the alternative activation markers Arg1 and Ym1 in vitro. Together, these data clearly show that endogenous TGFβ1 and autocrine TGFβ signaling is important for microglial quiescence in vitro and further suggest the upregulation of TGFβ1 in neurodegenerative diseases as a mechanism to regulate microglia functions and silence neuroinflammation.

Characteristic response of astrocytes to plasminogen/plasmin to upregulate transforming growth factor beta 3 (TGFbeta3) production/secretion through proteinase-activated receptor-1 (PAR-1) and the downstream phosphatidylinositol 3-kinase (PI3K)-Akt/PKB signaling cascade

The effects of microglia-derived plasminogen (PLGn) on the neurotrophic role of astrocytes were investigated in vitro. The treatment of astrocytes with rat PLGn led to a significant increase in transforming growth factor beta3 (TGFbeta3) in the conditioned medium (CM). This response of astrocytes to PLGn was characteristic and different from that to other stimulators, including lipopolysaccharide, phorbol-12-myristate-13-acetate, interferon-gamma, and ATP. In surveying the signaling molecules that respond to PLGn in astrocytes, we found that Akt/PKB phosphorylation is promoted. The pretreatment of astrocytes with an Akt inhibitor prior to PLGn stimulation resulted in a significant decrease in TGFbeta3 amounts in the CM, suggesting an association of Akt with TGFbeta3 production/secretion. Further survey revealed that phosphatidylinositol 3 kinase (PI3K) is closely associated with TGFbeta3 production/secretion in astrocytes. In fact, PI3K inhibitor clearly depressed the phosphorylation of Akt, indicating that PI3K is localized upstream of Akt. Moreover, the effects of PLGn to increase TGFbeta3 were depressed by pretreatment with a proteinase-activated receptor-1 (PAR-1) inhibitor. Plasmin could mimic the PLGn effects to upregulate TGFbeta3, and the plasmin effects were suppressed by pretreatment with the PAR-1 inhibitor, suggesting the association of PLGn/plasmin effects with PAR-1. In addition, Akt phosphorylation caused by plasmin was inhibited in the presence of PAR-1 inhibitor. We have therefore demonstrated that PLGn/plasmin, probably plasmin, facilitates the production/secretion of TGFbeta3 in astrocytes through both PAR-1 and the subsequent signaling cascade including PI3K and Akt.

Upregulation of p21Cip1 in activated glial cells

The cdk inhibitor p21(Cip1), also named p21(Cip1/Waf1), is intimately involved in coupling growth arrest to cellular differentiation in several cell types. p21(Cip1) is a multifunctional protein that might regulate cell-cycle progression at different levels. In a recent study, we found no differences in the rate of proliferation between glial cells from wild-type and p21(Cip1-/-) mice. In the present study, we examined differences in glial activation between glial cells from wild-type and p21(Cip1-/-) mice, using mixed glial cultures, microglia-enriched cultures, and astrocyte-enriched cultures. We compared the effect of lipopolysaccharide and two forms (oligomeric and fibrillar) of the 1-42 beta-amyloid peptide on glial activation. We observed an attenuation of nuclear translocation of the nuclear factor kappa-B in p21(Cip1-/-) glial cells, when compared with glial cells from wild-type mice. In contrast, tumor necrosis factor-alpha release was enhanced in p21(Cip1-/-)microglial cells. In addition glial activation induced by lipopolysaccharide and the fibrillar form of the 1-42 beta-amyloid peptide upregulated p21(Cip1). Our results support a role for p21(Cip1) in the activation of glial cells, particularly in microglia.

Endogenous transforming growth factor beta 1 suppresses inflammation and promotes survival in adult CNS

Transforming growth factor beta1 (TGFbeta1) is a pleiotropic cytokine with potent neurotrophic and immunosuppressive properties that is upregulated after injury, but also expressed in the normal nervous system. In the current study, we examined the regulation of TGFbeta1 and the effects of TGFbeta1 deletion on cellular response in the uninjured adult brain and in the injured and regenerating facial motor nucleus. To avoid lethal autoimmune inflammation within 3 weeks after birth in TGFbeta1-deficient mice, this study was performed on a T- and B-cell-deficient RAG2-/- background. Compared with wild-type siblings, homozygous deletion of TGFbeta1 resulted in an extensive inflammatory response in otherwise uninjured brain parenchyma. Astrocytes increased in GFAP and CD44 immunoreactivity; microglia showed proliferative activity, expression of phagocytosis-associated markers [alphaXbeta2, B7.2, and MHC1 (major histocompatibility complex type 1)], and reduced branching. Ultrastructural analysis revealed focal blockade of axonal transport, perinodal damming of axonal organelles, focal demyelination, and myelin debris in granule-rich, phagocytic microglia. After facial axotomy, absence of TGFbeta1 led to a fourfold increase in neuronal cell death (52 vs 13%), decreased central axonal sprouting, and significant delay in functional recovery. It also interfered with the microglial response, resulting in a diminished expression of early activation markers [ICAM1 (intercellular adhesion molecule 1), alpha6beta1, and alphaMbeta2] and reduced proliferation. In line with axonal and glial findings in the otherwise uninjured CNS, absence of endogenous TGFbeta1 also caused an approximately 10% reduction in the number of normal motoneurons, pointing to an ongoing and potent trophic role of this anti-inflammatory cytokine in the normal as well as in the injured brain.

Global changes in optic nerve head gene expression after exposure to elevated intraocular pressure in a rat glaucoma model

PURPOSE

In glaucoma, the optic nerve head (ONH) is the likely site of initial injury and elevated intraocular pressure (IOP) is the best-known risk factor. This study determines global gene expression changes in the pressure-injured ONH.

METHODS

Unilateral sustained IOP elevation (glaucoma, n = 46) or optic nerve transection (n = 10) was produced in rats. ONHs were removed, and the retrobulbar optic nerves were graded for degeneration. Gene expression in the glaucomatous ONH with extensive injury was compared with that in the fellow ONH (n = 6/group), by using cDNA microarrays. Data from 12 arrays were normalized, significant differences in gene expression determined, and significantly affected gene classes identified. For the remaining ONH, grouped by experimental condition and degree of injury, quantitative reverse transcriptase-PCR (qPCR) and ANOVA were used to compare selected message levels.

RESULTS

Microarray analysis identified more than 2000 significantly regulated genes. For 225 of these genes, the changes were greater than twofold. The most significantly affected gene classes were cell proliferation, immune response, lysosome, cytoskeleton, extracellular matrix, and ribosome. A 2.7-fold increase in ONH cellularity confirmed glaucoma model cell proliferation. By qPCR, increases in levels of periostin, collagen VI, and transforming growth factor beta1 were linearly correlated to the degree of IOP-induced injury. For cyclinD1, fibulin 2, tenascin C, TIMP1, and aquaporin-4, correlations were significantly nonlinear, displaying maximum change with focal injury.

CONCLUSIONS

In the ONH, pressure-induced injury results in cell proliferation and dramatically altered gene expression. For specific genes, expression levels were most altered by focal injury, suggesting that further array studies may identify initial, and potentially injurious, altered processes.

Emerging roles for TGF-beta1 in nervous system development

Transforming growth factor betas (TGF-betas) are known as multifunctional growth factors, which participate in the regulation of key events of development, disease and tissue repair. In central nervous system (CNS), TGF-beta1 has been widely recognized as an injury-related cytokine, specially associated with astrocyte scar formation in response to brain injury. TGF-betas family is represented by three isoforms: TGF-beta1, -beta 2 and -beta 3, all produced by both glial and neuronal cells. They are involved in essential tissue functions, including cell-cycle control, regulation of early development and differentiation, neuron survival and astrocyte differentiation. TGF-beta signaling is mediated mainly by two serine threonine kinase receptors, TGFRI and TGFRII, which activate Smad 2/3 and Smad 4 transcription factors. Phosphorylation and activation of these proteins is followed by formation of Smad 2/3-4 complex, which translocates to the nucleus regulating transcriptional responses to TGF-beta. Very few data are available concerning the intracellular pathway required for the effect of TGF-beta in brain cells. Recently, emerging data on TGF-beta1 and its signaling molecules have been suggesting that besides its role in brain injury, TGF-beta1 might be a crucial regulator of CNS development. In this review, we will focus on TGF-betas members, specially TGF-beta1, in neuron and astrocyte development. We will discuss some advances concerning the emerging scenario of TGF-beta1 and its signaling pathways as putative modulators of astrocyte biology and their implications as a novel mediator of cellular interactions in the CNS.

Differential effects of interleukin-1beta on neurotoxicity, cytokine induction and glial reaction in specific brain regions

An appropriate inflammatory response is crucial for the maintenance of tissue homeostasis. The inflammatory responses seen in the brain parenchyma differ from those elicited in the periphery, ventricles and meninges. However, although an inflammatory component has been associated with many CNS diseases, the differences among parenchymal inflammatory responses in different brain regions have not yet been fully elucidated. Here, we performed a systematic comparison of the effects of a common pro-inflammatory stimulus, IL-1beta, on the hippocampus, substantia nigra, striatum and cortex. We determined various responses, including cytokine mRNA induction, glial activation, immune cell infiltration and changes in neuronal integrity, in both injected and adjacent regions 1 and 6 days after the injection of IL-1beta. We found that the mRNA for TGF-beta was up-regulated in a region-specific manner after IL-1beta administration. Contrary to its response in the periphery, IL-1alpha showed no detectable induction in the tested parenchymal regions. In addition, cytokine induction was also sometimes observed in regions distant from the site of injection. Interestingly, IL-1beta-mediated neurodegeneration was observed in the dentate gyrus of the hippocampus, but not in the other tested regions. The cellular recruitment mediated by IL-1 beta injection consisted mainly of polymorphonuclear cells (PMN), which correlated with IL-1betamRNA induction even in regions far from the injection site. These results indicate that various parenchymal regions show different inflammatory responses and neurodegeneration in response to IL-1beta. Taken together, our results provide a more precise picture of regional inflammation in the brain and should provide a basis for differential interpretation of results based on regional inflammatory differences.

Microglial cell population dynamics in the injured adult central nervous system

Reactive microgliosis is characteristic of trauma and stroke as well as inflammatory and chronic neurodegenerative disease. A conspicuous feature of the microglial reaction to acute neural injury is a massive expansion of the microglial cell population which peaks a few days following injury. New data based on the use of radiation bone marrow-chimeric mice suggest this expansion also involves recruitment of bone marrow-derived cells, which migrate into the neural parenchyma and differentiate into microglia. Here, we discuss the contribution of bone marrow-derived cells to the injury-induced expansion of the microglial cell population, seen in the dentate gyrus with ongoing anterograde axonal and terminal synaptic degeneration, subsequent to transection of the entorhino-dentate perforant path projection. In this paradigm of minor brain injury, the bone marrow-derived cells are grossly outnumbered by activated resident microglia, which express the stem cell antigen CD34 concurrent to a marked capacity for self-renewal. The observation of a mixed origin of lesion-reactive microglia, consisting of a smaller subpopulation of exogenous bone marrow-derived microglia, and a larger population of activated resident microglia, the majority of which express CD34 and undergo proliferation, suggests that lesion-reactive microglia consist of functionally distinct cell populations. The demonstration of an injury-enhanced recruitment of bone marrow-derived cells into the perforant path-denervated dentate gyrus, raises the possibility of using genetically manipulated cells as vectors for lesion-site-specific gene therapy even in minimally injured areas of the central nervous system.

Activated Microglia Initiate Motor Neuron Injury by a Nitric Oxide and Glutamate-Mediated Mechanism

Recent studies suggest that motor neuron (MN) death may be non-cell autonomous, with cell injury mediated by interactions involving non-neuronal cells, such as microglia and astrocytes. To help define these interactions, we used primary MN cultures to investigate the effects of microglia activated by lipopolysaccharide or IgG immune complexes from patients with amyotrophic lateral sclerosis. Following activation, microglia induced MN injury, which was prevented by a microglial iNOS inhibitor as well as by catalase or glutathione. Glutamate was also required since inhibition of the MN AMPA/kainate receptor by CNQX prevented the toxic effects of activated microglia. Peroxynitrite and glutamate were synergistic in producing MN injury. Their toxic effects were also blocked by CNQX and prevented by calcium removal from the media. The addition of astrocytes to cocultures of MN and activated microglia prevented MN injury by removing glutamate from the media. The protective effects could be reversed by inhibiting astrocytic glutamate transport with dihydrokainic acid or pretreating astrocytes with H2O2. Astrocytic glutamate uptake was also decreased by activated microglia or by added peroxynitrite. These data suggest that free radicals released from activated microglia may initiate MN injury by increasing the susceptibility of the MN AMPA/kainate receptor to the toxic effects of glutamate.

Down-regulation of the expression of rat inhibitor-of-apoptosis protein-1 and -3 during transforming growth factor-beta1-mediated apoptosis in rat brain microglia

Transforming growth factor (TGF)-beta1 is a key regulator of brain response to injury and inflammation. It exerts anti-inflammatory roles by inhibiting microglial proliferation and free radical induction. TGF-beta1 is known to induce apoptotic cell death of microglia in a Bcl-2-independent pathway. The purpose of this study was to examine detailed mechanisms of TGF-beta1-induced microglial apoptosis. Assays for cell viability and DNA fragmentation demonstrated that TGF-beta1 induced apoptotic cell death in primary rat microglial cultures. Reverse transcription (RT)-PCR analysis showed that primary microglial cells expressed mRNAs for rat inhibitor-of-apoptosis protein (RIAP)-1 and RIAP-3 under normal culture conditions and that treatment with TGF-beta1 resulted in a significant reduction in the amounts of RIAP-1 and RIAP-3 mRNAs. Because IAPs are potent suppressor of apoptotic cell death, decrease in IAP expression might provide an important regulatory function in TGF-beta1-mediated microglial death and in attenuation of excessive microglial activation in pathological conditions.

GDNF and TGF-beta1 promote cell survival in serum-free cultures of primary rat microglia

Recent evidence indicates that glial cell line-derived neurotrophic factor (GDNF) may influence microglial survival, proliferation, and activation, but this has not yet been tested on isolated primary microglia. We compared the effects of individual and combined application of 10 ng/ml GDNF and 1 ng/ml transforming growth factor-beta1 (TGF-beta1) on total cell number, 5-bromo-2'-deoxyuridine (BrdU) incorporation, DNA nick-end labelling (TUNEL staining), and nitrite and lactate dehydrogenase (LDH) secretion in serum-free cultures of primary rat microglia. GDNF as well as TGF-beta1 enhanced the total number of lectin-positive cells and decreased the number of TUNEL-positive nuclei, while no effect on proliferation was observed. Both factors suppressed the secretion of nitrite during the first 4 days of culturing, and GDNF but not TGF-beta1 reduced the secretion of LDH in 2-week-old cultures. These findings suggest that GDNF and TGF-beta1 support survival of primary microglia in vitro.

Loss of microglial ramification in microglia-astrocyte cocultures: involvement of adenylate cyclase, calcium, phosphatase, and Gi-protein systems

Reduction in microglial branching is a common feature in brain pathology and culminates in the transformation into small, rounded, microglia-derived phagocytes in the presence of neural debris. The molecular factors responsible for this transformation are unknown. Here we explored the effect of different classes of intra- and extracellular stimuli in vitro on the morphology of ramified microglia cultured on a confluent astrocyte substrate. These studies showed a strong dose-dependent effect for the Ca(2+) ionophore calcimycine/A21837 (50 microM) and for dibutyryl-cAMP (1 mM), with a loss of microglial ramification. Direct activation of the adenylate cyclase with forskolin (0.1 mM) also led to the disappearance of microglial branching. Okadaic acid (70 nM), the inhibitor of protein phosphatases 1 and 2A (PP1/PP2A), and pertussis toxin (12.5 microg/ml), a G(i)-protein inhibitor, also showed similar effects. No effect was observed for dibutyryl-cGMP or for UTP; addition of ATP had a moderate effect, but only at very high, probably nonphysiological concentrations (100 mM). Extracellular matrix components such as keratatan-sulfate, integrin receptor blockers, the disintegrins kistrin, echistatin, and flavoridin, or the serine protease thrombin all had no effect. Addition of prostaglandin D(2) (PGD(2)), a molecule produced by activated microglial cells, had a transforming effect, but at concentrations two orders of magnitude higher than that of established PGD(2) receptors. In summary, addition of agents causing intracellular elevation of Ca(2+) and cAMP or inhibition of G(i)-proteins and phosphatases to ramified microglia cultured on top of confluent astrocytes leads to a rapid loss of microglial branching. Signaling cascades controlled by these molecules may play an important role in the regulation of this common physiological process in the injured brain.

Glial cell line-derived neurotrophic factor enhances survival of GM-CSF dependent rat GMIR1-microglial cells

Microglial activation and proliferation occur in nearly all forms of brain injury. The aim of this study was to investigate the influence of glial cell-line derived neurotrophic factor (GDNF) on proliferation and/or survival in a GMIR1 rat microglial cell line, which proliferates in response to granulocyte-macrophage-colony stimulating factor (GM-CSF). Endogenous GDNF and its receptor, GFRalpha-1, were detected in GMIR1 cells by ELISA and immunohistochemistry/Western blot, respectively. Recombinant GDNF strongly enhanced GMIR1 cell numbers and BrdU-incorporation, an effect inhibited by GDNF blocking antibodies. Inhibition of cAMP/cGMP dependent protein kinase enhanced the GDNF-induced GMIR1 cell number. The results suggest that GDNF has synergistic survival promoting effects on microglia potentially via autocrine mechanisms.

Interleukin-4, interleukin-10, and interleukin-1-receptor antagonist but not transforming growth factor-beta induce ramification and reduce adhesion molecule expression of rat microglial cells

The activity of microglial cells is strictly controlled in order to maintain central nervous system (CNS) immune privilege. We hypothesized that several immunomodulatory factors present in the CNS parenchyma, i.e., the Th2-derived cytokines interleukin (IL)-4 and IL-10, interleukin-1-receptor-antagonist (IL-1-ra), or transforming growth factor (TGF)-beta can modulate microglial morphology and functions. Microglial cells were incubated with IL-4, IL-10, IL-1-ra, TGF-beta, or with astrocyte conditioned media (ACM) and were analyzed for morphological changes, expression of intercellular adhesion molecule (ICAM)-1, and secretion of IL-1beta or tumor necrosis factor (TNF)-alpha. Whereas untreated controls showed an amoeboid morphology both Th2-derived cytokines, IL-1-ra, and ACM induced a morphological transformation to the ramified phenotype. In contrast, TGF-beta-treated microglial cells showed an amoeboid morphology. Even combined with the neutralizing antibodies against IL-4, IL-10, or TGF-beta ACM induced microglial ramification. Furthermore, ACM did not contain relevant amounts of IL-4 and IL-10, as measured by enzyme-linked immunosorbent assay (ELISA). Flow cytometry showed that lipopolysaccharide (LPS)-induced ICAM-1-expression on microglial cells was strongly suppressed by ACM, significantly modulated by IL-4, IL-10, or IL-1-ra, but not influenced by TGF-beta. The LPS-induced secretion of IL-1beta and TNF-alpha was only reduced after application of ACM, whereas IL-4 or IL-10 did not inhibit IL-1beta- or TNF-alpha secretion. TGF-beta enhanced IL-1beta- but not TNF-alpha secretion. In summary, we demonstrate that IL-4, IL-10, and IL-1-ra induce microglial ramification and reduce ICAM-1-expression, whereas the secretion of proinflammatory cytokines is not prevented. TGF-beta has no modulating effects. Importantly, unidentified astrocytic factors that are not identical with IL-4, IL-10, or TGF-beta possess strong immunomodulatory properties.

Expression of phosphatidylserine receptor and down-regulation of pro-inflammatory molecule production by its natural ligand in rat microglial cultures

Exposure of phosphatidylserine (PS), an aminophospholipid normally sequestered in the inner leaflet of plasma membrane, is one of the crucial steps in the recognition and ingestion of apoptotic cells by macrophages. The recognition of PS on apoptotic cells by peripheral macrophages is mediated by a phosphatidylserine-specific receptor (PtdSerR), which has recently been cloned. In spite of the important role of apoptosis in the CNS, the process of apoptotic neuron recognition by microglia is poorly understood. Because recent studies suggest that engagement of PS with a not yet characterized microglial receptor is necessary for apoptotic neuron uptake, we investigated the expression of PtdSer-R and its functional role in neonatal rat brain microglial cultures. Semi-quantitative RT-PCR analysis revealed that PtdSerR mRNA was detectable in unstimulated cultures and enhanced in LPS activated microglia. The presence of PS-liposomes strongly reduced the release of pro-inflammatory molecules such as nitric oxide, interleukin-1beta, and tumor necrosis factor-alpha by LPS-activated microglia. At variance, the immunoregulatory cytokines interleukin-10 and transforming growth factor-beta1 were moderately decreased or unaffected. The activity of PS-liposomes was mimicked by the PS head group phospho-L-serine, but not by phosphatidylcholine-containing liposomes. Our data suggest that, as for peripheral macrophages, PS through its receptor can modulate microglial activation toward an anti-inflammatory phenotype.

Chronic intrathecal infusions after spinal cord injury cause scarring and compression

Intrathecal infusions are used in a number of rodent studies to deliver substances to the injured spinal cord. Whereas this method has been successful in certain paradigms, two potential limitations of this model have not been extensively reported: (1) scar formation at the catheter tip, which can lead to infusion failure, and (2) damage to the spinal cord caused by the catheter itself. Thus, the purpose of the present study was threefold: (1) to determine intrathecal infusion efficiency over 14 days following spinal cord injury; (2) to examine possible secondary damage caused by intrathecal tubing; and (3) to explore whether alternative protocols that avoid such damage are effective. Adult Fischer 344 rats were subjected to spinal cord lesions at T7, followed by placement of an intrathecal catheter attached to an Alzet minipump. Seven or 14 days following injury and catheter placement, tube patency was evaluated by diffusion of Evans Blue dye from the minipump. Results indicate that infusion was efficient 7 days following injury but was markedly reduced after 14 days. Further, histology and immunocytochemistry 14 days after injury demonstrated compression damage to the cord where the tubing rested. Alternative protocols, including intrathecal infusions through metal cannulae, or "drip" infusions directly over the lesion, did not improve delivery. These data suggest that results from rodent studies using infusion from catheters placed adjacent to lesion sites may be attributable to acute or subacute effects of the delivered substance. Future rodent studies using intrathecal infusions should include rigorous evaluation of infusion efficiency and possible secondary tissue damage.

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.

Astrocytic factors protect neuronal integrity and reduce microglial activation in an in vitro model of N-methyl-D-aspartate-induced excitotoxic injury in organotypic hippocampal slice cultures

Acute CNS lesions lead to neuronal injury and a parallel glial activation that is accompanied by the release of neurotoxic substances. The extent of the original neuronal damage can therefore be potentiated in a process called secondary damage. As astrocytes are known to secrete immunomodulatory and neuroprotective substances, we investigated whether astrocytic factors can attenuate the amount of neuronal injury as well as the degree of microglial activation in a model of excitotoxic neurodegeneration. Treatment of organotypic hippocampal slice cultures with N-methyl-D-aspartate (NMDA) resulted in a reproducible loss of viable granule cells, partial destruction of the regular hippocampal cytoarchitecture and a concomitant accumulation of amoeboid microglial cells at sites of neuronal damage. Astrocyte-conditioned media reduced the amount of NMDA-induced neuronal injury by 45.3%, diminished the degree of microglial activation and resulted in an improved preservation of the hippocampal cytoarchitecture. Transforming growth factor (TGF)-beta failed to act as a neuroprotectant and even enhanced the amount of neuronal injury by 52.5%. Direct effects of astrocytic factors on isolated microglial cells consisted of increased microglial ramification and down-regulated expression of intercellular adhesion molecule-1, whereas incubation with TGF-beta had no such effects. In summary, our findings show that hitherto unidentified astrocyte-derived factors that are probably not identical with TGF-beta can substantially enhance neuronal survival, either by eliciting direct neuroprotective effects or by modulating the microglial response to neuronal injury.

In vitro model of microglial deramification: ramified microglia transform into amoeboid phagocytes following addition of brain cell membranes to microglia-astrocyte cocultures

Changes in the morphology of ramified microglia are a common feature in brain pathology and culminate in the appearance of small, rounded, microglia-derived phagocytes in the presence of neural debris. Here, we explored the effect of adding brain cell membranes on the morphology of alphaMbeta2-integrin (CD11b/CD18, CR3) positive microglia cultured on a confluent astrocyte substrate as an in vitro model of deramification. Addition of brain membranes led to a loss of microglial ramification, with full transformation to small, rounded, macrophages at 20-40 microg/ml. Time course studies showed a rapid response, with first effects at 1-3 hours, and full transformation at 24-48 hours. Removal of cell membranes and exchange of the culture medium led to a similarly rapid process of reramification. Comparison of cell membranes from different tissues at 20 microg/ml showed strong transforming effect for the brain, more moderate for kidney and liver, and very weak for spleen and skeletal muscle. Fluorescent labeling of brain membranes revealed uptake by almost all rounded macrophages, by a subpopulation of glial fibrillary acidic protein (GFAP)-positive astrocytes, but not by ramified microglia. Phagocytosis of inert fluorobeads did not lead to a transformation into macrophages but their phagocytosis was inhibited by brain membranes, pointing to a saturable uptake mechanism. In summary, addition of brain cell membranes and their phagocytosis leads to a rapid and reversible loss of ramification. The differences in transforming activity from different tissues and the absence of effect from phagocytosed fluorobeads suggest, however, the need for a second stimulus following the phagocytosis of cell debris.

Effect of lipopolysaccharide on the morphology and integrin immunoreactivity of ramified microglia in the mouse brain and in cell culture

Microglial cells form the first line of defense in brain infection. They are related to monocytes and macrophages and can be readily activated by cell wall components of bacteria such as lipopolysaccharides (LPS). In the present study, we explored the effect of this endotoxin in mouse on the morphology of microglia and their immunoreactivity for the integrin family of cell adhesion molecules in vitro and in vivo. Subcutaneous injection of LPS led to a dose-dependent activation of alpha M beta 2-positive microglia, with a saturating effect at 1 microg LPS in the blood-brain barrier deficient area postrema, at 10 microg in the directly adjacent tissue, and at 100 microg throughout the brainstem and cerebellum. Morphologically, this activation was characterized by the swelling of the microglial cell body, a thickening of the proximal processes, and a reduction in distal ramification. Microglial immunoreactivity for the integrins alpha 4 beta 1, alpha 5 beta 1, alpha 6 beta 1, and alpha M beta 2 was strongly increased. In vitro, ramified microglia were obtained using a coculture on top of a confluent astrocyte monolayer. Two days exposure to LPS resulted in a morphological activation of the cultured cells with an increase of the integrin immunoreactivity for alpha 5 (5.7-fold), alpha 4 (3.1-fold), beta 1 (2.3-fold), and alpha M (1.5-fold), and a decrease in the alpha 6-staining intensity by 39%. Even a sublethal dose of LPS (3 mg in vivo and 500 microg/ml in vitro, respectively) did not induce the phagocyte-associated integrin alpha X beta 2 (CD11c/CD18, p150,95) and did not lead to a morphological transformation of the ramified microglia into phagocytes.

Transforming growth factor-beta1 induces transforming growth factor-beta1 and transforming growth factor-beta receptor messenger RNAs and reduces complement C1qB messenger RNA in rat brain microglia

Transforming growth factor-beta1 is a multifunctional peptide with increased expression during Alzheimer's disease and other neurodegenerative conditions which involve inflammatory mechanisms. We examined the autoregulation of transforming growth factor-beta1 and transforming growth factor-beta receptors and the effects of transforming growth factor-beta1 on complement C1q in brains of adult Fischer 344 male rats and in primary glial cultures. Perforant path transection by entorhinal cortex lesioning was used as a model for the hippocampal deafferentation of Alzheimer's disease. In the hippocampus ipsilateral to the lesion, transforming growth factor-beta1 peptide was increased >100-fold; the messenger RNAs encoding transforming growth factor-beta1, transforming growth factor-beta type I and type II receptors were also increased, but to a smaller degree. In this acute lesion paradigm, microglia are the main cell type containing transforming growth factor-beta1, transforming growth factor-beta type I and II receptor messenger RNAs, shown by immunocytochemistry in combination with in situ hybridization. Autoregulation of the transforming growth factor-beta1 system was examined by intraventricular infusion of transforming growth factor-beta1 peptide, which increased hippocampal transforming growth factor-beta1 messenger RNA levels in a dose-dependent fashion. Similarly, transforming growth factor-beta1 increased levels of transforming growth factor-beta1 messenger RNA and transforming growth factor-beta type II receptor messenger RNA (IC(50), 5pM) and increased release of transforming growth factor-beta1 peptide from primary microglia cultures. Interactions of transforming growth factor-beta1 with complement system gene expression are also indicated, because transforming growth factor-beta1 decreased C1qB messenger RNA in the cortex and hippocampus, after intraventricular infusion, and in cultured glia. These indications of autocrine regulation of transforming growth factor-beta1 in the rodent brain support a major role of microglia in neural activities of transforming growth factor-beta1 and give a new link between transforming growth factor-beta1 and the complement system. The auto-induction of the transforming growth factor-beta1 system has implications for transgenic mice that overexpress transforming growth factor-beta1 in brain cells and for its potential role in amyloidogenesis.

Astrocytic factors down-regulate the expression of major histocompatibility complex-class-II and intercellular adhesion molecule-1 on human monocytes

Several factors contribute to the maintenance of central nervous system immune privilege and astrocytes have been identified as a major source of immunomodulatory cytokines. To investigate whether hematogenous monocytes are immunologically deactivated by astrocyte-derived factors human monocytes were stimulated with lipopolysaccharide or interferon (IFN)-gamma and treated with the supernatant from pure astrocyte cultures, interleukin (IL)-4, IL-10, or with IL-1-receptor antagonist (1L-1-RA). Flow cytometry demonstrated that the supernatant from astrocyte cultures was the most potent agent in reducing the levels of major histocompatibility complex (MHC)-class-II- as well as intercellular adhesion molecule-1-expression, whereas IL-4, IL-10, and IL-1-RA had only marginal effects. The expression of leukocyte function antigen-1 and very late antigen-4 was not modulated by either factor. In conclusion, astrocytes seem to provide soluble factors that have the capacity to deactivate hematogenous monocytes.

Peptides as regulators of the immune system: emphasis on somatostatin

Study of the communication between nervous and immune systems culminated in the understanding that cytokines, formerly considered exclusively as immune system-derived peptides, are endogenous to the brain and display central actions. More recently, immune cells have been recognized as a peripheral source of "brain-specific" peptides with immunomodulatory actions. This article reviews studies concerning reciprocal effects of selected cytokines and neuropeptides in the nervous and immune systems, respectively. The functional equivalence of these two categories of communicators is discussed with reference to the example of the actions of neuropeptide somatostatin in the immune system.

Upregulation of Kv1.3 K(+) channels in microglia deactivated by TGF-beta

Microglial activation is accompanied by changes in K(+) channel expression. Here we demonstrate that a deactivating cytokine changes the electrophysiological properties of microglial cells. Upregulation of delayed rectifier (DR) K(+) channels was observed in microglia after exposure to transforming growth factor-beta (TGF-beta) for 24 h. In contrast, inward rectifier K(+) channel expression was unchanged by TGF-beta. DR current density was more than sixfold larger in TGF-beta-treated microglia than in untreated microglia. DR currents of TGF-beta-treated cells exhibited the following properties: activation at potentials more positive than -40 mV, half-maximal activation at -27 mV, half-maximal inactivation at -38 mV, time dependent and strongly use-dependent inactivation, and a single channel conductance of 13 pS in Ringer solution. DR channels were highly sensitive to charybdotoxin (CTX) and kaliotoxin (KTX), whereas alpha-dendrotoxin had little effect. With RT-PCR, mRNA for Kv1.3 and Kir2.1 was detected in microglia. In accordance with the observed changes in DR current density, the mRNA level for Kv1.3 (assessed by competitive RT-PCR) increased fivefold after treatment of microglia with TGF-beta.

Galectin-3 is upregulated in microglial cells in response to ischemic brain lesions, but not to facial nerve axotomy

We have recently demonstrated that the beta-galactoside-specific lectin galectin-3 is expressed by microglial cells in vitro, but not by normal resting microglia in vivo. In the present study, we have analyzed the expression of galectin-3 by microglia under traumatic conditions in vivo using two experimental rat models which substantially differ in the severity of lesion related to a breakdown of the blood-brain barrier (BBB) and the occurrence of inflammatory processes. These two features are absent after peripheral nerve lesion and present after cerebral ischemia. Here we show that, following facial nerve axotomy under conditions allowing (nerve anastomosis) or not subsequent regeneration (nerve resection), galectin-3 is not expressed by microglia in the corresponding facial nucleus 1-112 days after lesion. Galectin-3 is also absent in microglia at sites of a defective BBB in the normal brain, such as the circumventricular organs. Following experimental ischemia (i.e., permanent occlusion of the middle cerebral artery), in contrast, galectin-3 becomes strongly expressed by activated microglia as early as 48 hours after trauma, as determined by immunohistochemistry and Western blot analysis. Our findings suggest that the expression of galectin-3 by microglia in vivo correlates with the state of microglial activation.

Comparative evaluation of cytokine profiles and reactive gliosis supports a critical role for interleukin-6 in neuron-glia signaling during regeneration

Using reverse transcription polymerase chain reaction (RT-PCR), we have studied the temporal expression of interleukin-1beta (IL-1beta), interleukin-6 (IL-6), transforming growth factor-beta 1 (TGF-beta 1), and tumor necrosis factor-alpha (TNF-alpha) mRNAs in three axotomy paradigms with distinct functional outcomes. Axotomy of adult rat facial motoneurons results in neuronal regeneration, axotomy of neonatal facial motoneurons results in neuronal apoptosis, and axotomy of rubrospinal neurons results in neuronal atrophy. Our RT-PCR findings show that a significant and sustained upregulation of IL-6 mRNA is associated uniquely with the regeneration of adult facial motoneurons. Histochemical studies using IL-6 immunohistochemistry show intense IL-6 immunoreactivity in axotomized adult facial motoneurons. Assessment of reactive glial changes with astroglial and microglial markers reveals that the reactive gliosis following adult facial nerve axotomy is more intense than that observed in either of the other two paradigms. Exposure of cultured microglial cells to IL-6 stimulates microglial proliferation in a dose-dependent manner. Cultured microglia also show expression of IL-6 receptor mRNA, as determined by RT-PCR. Our findings support the idea that reactive gliosis is required for neuron regeneration to occur, and more specifically, they suggest that neuron-derived IL-6 serves as a signalling molecule that induces microglial proliferation during motoneuron regeneration.

Molecular signals for glial activation: pro- and anti-inflammatory cytokines in the injured brain

Injury to the central nervous system leads to cellular changes not only in the affected neurons but also in adjacent glial cells. This neuroglial activation is a consistent feature in almost all forms of brain pathology and appears to reflect an evolutionarily-conserved program which plays an important role for the repair of the injured nervous system. Recent work in mice that are genetically-deficient for different cytokines (M-CSF, IL-6, TNF-alpha, TGF-beta 1) has begun to shed light on the molecular signals that regulate this cellular response. Here, the availability of cytokine-deficient animals with reduced or abolished neuroglial activation provides a direct approach to determine the function of the different components of the cellular response leading to repair and regeneration following neural trauma.

Neuroglial activation repertoire in the injured brain: graded response, molecular mechanisms and cues to physiological function

Damage to the central nervous system (CNS) leads to cellular changes not only in the affected neurons but also in adjacent glial cells and endothelia, and frequently, to a recruitment of cells of the immune system. These cellular changes form a graded response which is a consistent feature in almost all forms of brain pathology. It appears to reflect an evolutionarily conserved program which plays an important role in the protection against infectious pathogens and the repair of the injured nervous system. Moreover, recent work in mice that are genetically deficient for different cytokines (MCSF, IL1, IL6, TNFalpha, TGFbeta1) has begun to shed light on the molecular signals that regulate this cellular response. Here we will review this work and the insights it provides about the biological function of the neuroglial activation in the injured brain.

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.

Glial fibrillary acidic protein (GFAP): modulation by growth factors and its implication in astrocyte differentiation

Intermediate filament (IF) proteins constitute an extremely large multigene family of developmentally and tissue-regulated cytoskeleton proteins abundant in most vertebrate cell types. Astrocyte precursors of the CNS usually express vimentin as the major IF. Astrocyte maturation is followed by a switch between vimentin and glial fibrillary acidic protein (GFAP) expression, with the latter being recognized as an astrocyte maturation marker. Levels of GFAP are regulated under developmental and pathological conditions. Upregulation of GFAP expression is one of the main characteristics of the astrocytic reaction commonly observed after CNS lesion. In this way, studies on GFAP regulation have been shown to be useful to understand not only brain physiology but also neurological disease. Modulators of GFAP expression include several hormones such as thyroid hormone, glucocorticoids and several growth factors such as FGF, CNTF and TGF beta, among others. Studies of the GFAP gene have already identified several putative growth factor binding domains in its promoter region. Data obtained from transgenic and knockout mice have provided new insights into IF protein functions. This review highlights the most recent studies on the regulation of IF function by growth factors and hormones.