TGFβ signalling plays an important role in IL4-induced alternative activation of microglia

The impact of microglial activation on blood-brain barrier in brain diseases

The blood-brain barrier (BBB), constituted by an extensive network of endothelial cells (ECs) together with neurons and glial cells, including microglia, forms the neurovascular unit (NVU). The crosstalk between these cells guarantees a proper environment for brain function. In this context, changes in the endothelium-microglia interactions are associated with a variety of inflammation-related diseases in brain, where BBB permeability is compromised. Increasing evidences indicate that activated microglia modulate expression of tight junctions, which are essential for BBB integrity and function. On the other hand, the endothelium can regulate the state of microglial activation. Here, we review recent advances that provide insights into interactions between the microglia and the vascular system in brain diseases such as infectious/inflammatory diseases, epilepsy, ischemic stroke and neurodegenerative disorders.

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.

TGFβ1 increases microglia-mediated engulfment of apoptotic cells via upregulation of the milk fat globule-EGF factor 8

Milk fat globule-epidermal growth factor-factor 8 (Mfge8) has been described as an essential molecule during microglia-mediated clearance of apoptotic cells via binding to phosphatidylserine residues and subsequent phagocytosis. Impaired uptake of apoptotic cells by microglia results in prolonged inflammatory responses and damage of healthy cells. Although the mechanisms of Mfge8-mediated engulfment of apoptotic cells are well understood, endogenous or exogenous factors that regulate Mfge8 expression remain elusive. Here, we describe that TGFβ1 increases the expression of Mfge8 and enhances the engulfment of apoptotic cells by primary mouse microglia in a Mfge8-dependent manner. Further, apoptotic cells are capable of increasing microglial TGFβ expression and release and shift the microglia phenotype toward alternative activation. Moreover, we provide evidence that Mfge8 expression is differentially regulated in microglia after classical and alternative activation and that Mfge8 is not able to exert direct antiinflammatory effects on LPS-treated primary microglia. Together, these results underline the importance of TGFβ1 as a regulatory factor for microglia and suggest that increased TGFβ1 expression in models of neurodegeneration might be involved in clearance of apoptotic cells via regulation of Mfge8 expression.

IL-4 type 1 receptor signaling up-regulates KCNN4 expression, and increases the KCa3.1 current and its contribution to migration of alternative-activated microglia

The Ca²⁺-activated K⁺ channel, KCa3.1 (KCNN4/IK1/SK4), contributes to “classical,” pro-inflammatory activation of microglia, and KCa3.1 blockers have improved the outcome in several rodent models of CNS damage. For instance, blocking KCa3.1 with TRAM-34 rescued retinal ganglion neurons after optic nerve damage in vivo and, reduced p38 MAP kinase activation, production of reactive oxygen and nitrogen species, and neurotoxicity by microglia in vitro. In pursuing the therapeutic potential of KCa3.1 blockers, it is crucial to assess KCa3.1 contributions to other microglial functions and activation states, especially the IL-4-induced “alternative” activation state that can counteract pro-inflammatory states. We recently found that IL-4 increases microglia migration – a crucial function in the healthy and damaged CNS – and that KCa3.1 contributes to P₂Y₂ receptor-stimulated migration. Here, we discovered that KCa3.1 is greatly increased in alternative-activated rat microglia and then contributes to an enhanced migratory capacity. IL-4 up-regulated KCNN4 mRNA (by 6 h) and greatly increased the KCa3.1 current by 1 day, and this required de novo protein synthesis. The increase in current was sustained for at least 6 days. IL-4 increased microglial migration and this was reversed by blocking KCa3.1 with TRAM-34. A panel of inhibitors of signal-transduction mediators was used to analyze contributions of IL-4-related signaling pathways. Induction of KCNN4 mRNA and KCa3.1 current was mediated specifically through IL-4 binding to the type I receptor and, surprisingly, it required JAK3, Ras/MEK/ERK signaling and the transcription factor, activator protein-1, rather than JAK2, STAT6, or phosphatidylinositol 3-kinase.The same receptor subtype and pathway were required for the enhanced KCa3.1-dependent migration. In providing the first direct signaling link between an IL-4 receptor, expression and roles of an ion channel, this study also highlights the potential importance of KCa3.1 in alternative-activated microglia.

A novel cell line from spontaneously immortalized murine microglia

BACKGROUND

Purified microglia cultures are useful tools to study microglial behavior in vitro. Microglial cell lines serve as an attractive alternative to primary microglia culture, circumventing the costly and lengthy preparation of the latter. However, immortalization by genetic or pharmacologic manipulations may show altered physiology from primary microglia.

NEW METHOD

A novel microglial cell line was isolated from a primary glial culture of postnatal murine cerebral cortices. The culture contained a population of spontaneously transformed microglia that continued to divide without genetic or pharmacological manipulations. After several clones were isolated, one particular clone, SIM-A9, was analyzed for its microglial characteristics.

RESULTS

SIM-A9 cells expressed macrophage/microglia-specific proteins, CD68 and Iba1. SIM-A9 cells were responsive to exogenous inflammatory stimulation with lipopolysaccharide and β-amyloid, triggering tyrosine kinase-based and NFκB signaling cascades as well as TNFα secretion. SIM-A9 cells also exhibited phagocytic uptake of fluorescent labeled β-amyloid and bacterial bioparticles. Furthermore, lipopolysaccharide increased the levels of inducible nitric oxide synthase and cyclooxygenase-2, whereas IL-4 stimulation increased arginase-1 levels demonstrating that SIM-A9 cells are capable of switching their profiles to pro- or anti-inflammatory phenotypes, respectively.

COMPARISON WITH EXISTING METHODS

The use of SIM-A9 cells avoids expensive and lengthy procedures required for the preparation of primary microglia. Spontaneously immortalized SIM-A9 cells are expected to behave more comparably to primary microglia than virally transformed or pharmacologically induced microglial cell lines.

CONCLUSIONS

SIM-A9 cells exhibit key characteristics of cultured primary microglia and may serve as a valuable model system for the investigation of microglial behavior in vitro.

Therapeutically targeting neuroinflammation and microglia after acute ischemic stroke

Inflammation has a pivotal role in the pathogenesis of ischemic stroke, and recent studies posit that inflammation acts as a double-edged sword, not only detrimentally augmenting secondary injury, but also potentially promoting recovery. An initial event of inflammation in ischemic stroke is the activation of microglia, leading to production of both pro- and anti-inflammatory mediators acting through multiple receptor signaling pathways. In this review, we discuss the role of microglial mediators in acute ischemic stroke and elaborate on preclinical and clinical studies focused on microglia in stroke models. Understanding how microglia can lead to both pro- and anti-inflammatory responses may be essential to implement therapeutic strategies using immunomodulatory interventions in ischemic stroke.

Resolvin D1 promotes the interleukin-4-induced alternative activation in BV-2 microglial cells

Background

Microglia play key roles in innate immunity, homeostasis, and neurotropic support in the central nervous system. Similar to macrophages, microglia adopt two different activation phenotypes, the classical and alternative activation. Resolvin D1 (RvD1) is considered to display potent anti-inflammatory and pro-resolving actions in inflammatory models. In this present study, we investigate the effect of RvD1 on IL-4-induced alternative activation in murine BV-2 microglial cells.

Methods

BV-2 cells were incubated with RvD1 alone, IL-4 alone, or the combination of RvD1 and IL-4. Western blot and immunofluorescence were performed to detect protein levels of alternative activation markers arginase 1 (Arg1), chitinase 3-like 3 (Ym1). Moreover, we investigated the effects of RvD1 on IL-4-induced activation of signal transducer and activators of transcription 6 (STAT6) and peroxisome proliferator-activated receptor gamma (PPARγ).

Results

RvD1 promoted IL-4-induced microglia alternative activation by increasing the expression of Arg1 and Ym1. RvD1 also enhanced phosphorylation of STAT6, nuclear translocation of PPARγ and the DNA binding activity of STAT6 and PPARγ. These effects were reversed by butyloxycarbonyl-Phe-Leu-Phe-Leu-Phe (a formyl peptide receptor 2 antagonist). Further, the effects of RvD1 and IL-4 on Arg1 and Ym1 were blocked by the application of leflunomide (a STAT6 inhibitor) or GW9662 (a PPARγ antagonist).

Conclusions

Our studies demonstrate that RvD1 promotes IL-4-induced alternative activation via STAT6 and PPARγ signaling pathways in microglia. These findings suggest that RvD1 may have therapeutic potential for neuroinflammatory diseases.

Tuftsin signals through its receptor neuropilin-1 via the transforming growth factor beta pathway

Tuftsin (Thr-Lys-Pro-Arg) is a natural immunomodulating peptide found to stimulate phagocytosis in macrophages/microglia. Tuftsin binds to the receptor neuropilin-1 (Nrp1) on the surface of cells. Nrp1 is a single-pass transmembrane protein, but its intracellular C-terminal domain is too small to signal independently. Instead, it associates with a variety of coreceptors. Despite its long history, the pathway through which tuftsin signals has not been described. To investigate this question, we employed various inhibitors to Nrp1's coreceptors to determine which route is responsible for tuftsin signaling. We use the inhibitor EG00229, which prevents tuftsin binding to Nrp1 on the surface of microglia and reverses the anti-inflammatory M2 shift induced by tuftsin. Furthermore, we demonstrate that blockade of transforming growth factor beta (TGFβ) signaling via TβR1 disrupts the M2 shift similar to EG00229. We report that tuftsin promotes Smad3 phosphorylation and reduces Akt phosphorylation. Taken together, our data show that tuftsin signals through Nrp1 and the canonical TGFβ signaling pathway. Despite the 40-year history of the tetrapeptide tuftsin (TKPR), a macrophage and microglial activator, its mechanism of action has not been defined. Here, we report that the tuftsin-mediated anti-inflammatory M2 shift in microglia is caused specifically by tuftsin binding to the receptor neuropilin-1 (Nrp1) and signaling through TGFβ receptor-1, a coreceptor of Nrp1. We further show that tuftsin signals via the canonical TGFβ pathway and promotes TGFβ release from target cells.

Microglial phenotype and adaptation

Microglia are the prime innate immune cells of the central nervous system. They can transit from a (so-called) resting state under homeostatic conditions towards a pro-inflammatory activation state upon homeostatic disturbances. Under neurodegenerative conditions, microglia have been largely perceived as neurotoxic cells. It is now becoming clear that resting microglia are not inactive but that they serve house-keeping functions. Moreover, microglia activity is not limited to proinflammatory responses, but covers a spectrum of reactive profiles. Depending on the actual situation, activated microglia display specific effector functions supporting inflammation, tissue remodeling, synaptic plasticity and neurogenesis. Many of these functions not only relate to the current state of the local neural environment but also depend on previous experience. In this review, we address microglia functions with respect to determining factors, phenotypic presentations, adaptation to environmental signals and aging. Finally, we point out primary mechanisms of microglia activation, which may comprise therapeutic targets to control neuro-inflammatory and neurodegenerative activity.

The microglial activation state regulates migration and roles of matrix-dissolving enzymes for invasion

Background

Microglial cells are highly mobile under many circumstances and, after central nervous system (CNS) damage, they must contend with the dense extracellular matrix (ECM) in order to reach their target sites. In response to damage or disease, microglia undergo complex activation processes that can be modulated by environmental cues and culminate in either detrimental or beneficial outcomes. Thus, there is considerable interest in comparing their pro-inflammatory (‘classical’ activation) and resolving ‘alternative’ activation states. Almost nothing is known about how these activation states affect the ability of microglia to migrate and degrade ECM, or the enzymes used for substrate degradation. This is the subject of the present study.

Methods

Primary cultured rat microglial cells were exposed to lipopolysaccharide (LPS) to evoke classical activation or IL4 to evoke alternative activation. High-resolution microscopy was used to monitor changes in cell morphology and aspects of the cytoskeleton. We quantified migration in a scratch-wound assay and through open filter holes, and invasion through Matrigel™. A panel of inhibitors was used to analyze contributions of different matrix-degrading enzymes to migration and invasion, and quantitative real-time reverse transcriptase PCR (qRT-PCR) was used to assess changes in their expression.

Results

Vinculin- and F-actin-rich lamellae were prominent in untreated and IL4-treated microglia (but not after LPS). IL4 increased the migratory capacity of microglia but eliminated the preferential anterior nuclear-centrosomal axis polarity and location of the microtubule organizing center (MTOC). Microglia degraded fibronectin, regardless of treatment, but LPS-treated cells were relatively immobile and IL4-treated cells invaded much more effectively through Matrigel™. For invasion, untreated microglia primarily used cysteine proteases, but IL4-treated cells used a wider range of enzymes (cysteine proteases, cathepsin S and K, heparanase, and matrix metalloproteases). Untreated microglia expressed MMP2, MMP12, heparanase, and four cathepsins (B, K, L1, and S). Each activation stimulus upregulated a different subset of enzymes. IL4 increased MMP2 and cathepsins S and K; whereas LPS increased MMP9, MMP12, MMP14 (MT1-MMP), heparanase, and cathepsin L1.

Conclusions

Microglial cells migrate during CNS development and after CNS damage or disease. Thus, there are broad implications of the finding that classically and alternatively activated microglia differ in morphology, cytoskeleton, migratory and invasive capacity, and in the usage of ECM-degrading enzymes.

Microglia and ischemic stroke: a double-edged sword

Inflammatory processes have a fundamental role in the pathophysiology of stroke. A key initial event is the rapid activation of resident immune cells, primarily microglia. This cell population is an important target for new therapeutic approaches to limit stroke damage. Activation of microglia is normally held in check by strictly controlled mechanisms involving neuronal-glial communication. Ischemic stroke is a powerful stimulus that disables the endogenous inhibitory signaling and triggers microglial activation. Once activated, microglia exhibit a spectrum of phenotypes, release both pro- and anti-inflammatory mediators, and function to either exacerbate ischemic injury or help repair depending on different molecular signals the microglial receptors receive. Various ligands and receptors have been identified for microglial activation. Experimental tools to detect these inflammatory signals are being increasingly developed in an effort to define the functional roles of microglia. Fine-tuning immunomodulatory interventions based on the heterogeneous profiles of microglia are urgently needed for ischemic stroke.

Involvement of microRNA in microglia-mediated immune response

MicroRNAs (miRNAs) are an abundant class of small noncoding RNA molecules that play an important role in the regulation of gene expression at the posttranscriptional level. Due to their ability to simultaneously modulate the fate of different genes, these molecules are particularly well suited to act as key regulators during immune cell differentiation and activation, and their dysfunction can contribute to pathological conditions associated with neuroinflammation. Recent studies have addressed the role of miRNAs in the differentiation of progenitor cells into microglia and in the activation process, aiming at clarifying the origin of adult microglia cells and the contribution of the central nervous system (CNS) environment to microglia phenotype, in health and disease. Altered expression of several miRNAs has been associated with Alzheimer's disease, multiple sclerosis, and ischemic injury, hence strongly advocating the use of these small molecules as disease markers and new therapeutic targets. This review summarizes the recent advances in the field of miRNA-mediated regulation of microglia development and activation. We discuss the role of specific miRNAs in the maintenance and switching of microglia activation states and illustrate the potential of this class of nucleic acids both as biomarkers of inflammation and new therapeutic tools for the modulation of microglia behavior in the CNS.

Functional diversity of microglia - how heterogeneous are they to begin with?

Microglia serve in the surveillance and maintenance, protection and restoration of the central nervous system (CNS) homeostasis. By their parenchymal location they differ from other CNS-associated myeloid cells, and by origin as well as functional characteristics they are also–at least in part–distinct from extraneural tissue macrophages. Nevertheless, microglia themselves may not comprise a uniform cell type. CNS regions vary by cellular and chemical composition, including white matter (myelin) content, blood–brain barrier properties or prevailing neurotransmitters. Such a micromilieu could instruct as well as require local adaptions of microglial features. Yet even cells within circumscribed populations may reveal some specialization by subtypes, regarding house-keeping duties and functional capacities upon challenges. While diversity of reactive phenotypes has been established still little is known as to whether all activated cells would respond with the same program of induced genes and functions or whether responder subsets have individual contributions. Preferential synthesis of a key cytokine could asign a master control to certain cells among a pool of activated microglia. Critical functions could be sequestered to discrete microglial subtypes in order to avoid interference, such as clearance of endogenous material and presentation of antigens. Indeed, several and especially a number of recent studies provide evidence for the constitutive and reactive heterogeneity of microglia by and within CNS regions. While such a principle of “division of labor” would influence the basic notion of “the” microglia, it could come with the practival value of addressing separate microglia types in experimental and therapeutic manipulations.

Microglial dysregulation in psychiatric disease

Microglia, the brain's resident immune cells, are phagocytes of the macrophage lineage that have a key role in responding to inflammation and immune challenge in the brain. More recently, they have been shown to have a number of important roles beyond immune surveillance and response, including synaptic pruning during development and the support of adult neurogenesis. Microglial abnormalities have been found in several neuropsychiatric conditions, though in most cases it remains unclear whether these are causative or are a reaction to some other underlying pathophysiology. Here we summarize postmortem, animal, neuroimaging, and other evidence for microglial pathology in major depression, schizophrenia, autism, obsessive-compulsive disorder, and Tourette syndrome. We identify gaps in the existing literature and important areas for future research. If microglial pathology proves to be an important causative factor in these or other neuropsychiatric diseases, modulators of microglial function may represent a novel therapeutic strategy.