Activation of microglia in acute hippocampal slices affects activity-dependent long-term potentiation and synaptic tagging and capture in area CA1.

Activity dependent setting of synaptic tags is critical for the establishment and maintenance of long-term plasticity and its associative properties such as synaptic tagging and capture (STC), a widely studied cellular model of associative memory. Although the known mechanisms of STC such as setting of synaptic tags or distribution of plasticity related proteins (PRPs) are the processes mainly happening within the neuronal compartments, the role of non-neuronal components is still elusive. Here, we report that microglia has a specific role in setting the synaptic tags and thus promotes long-term plasticity and STC. Treatment of hippocampal slices with clodronate, a specific inhibitor of microglia, resulted in an activated morphology of microglia but not of the hippocampal pyramidal neurons, oligodendrocytes or astrocytes. Activation of microglia before or 60 min after the induction of long-term plasticity prevented its maintenance and thus the expression of STC. Interestingly, activation of microglia 2 h after the induction of long-term plasticity neither prevented its maintenance nor its associative interaction with activated nearby synaptic populations. Given the half-life of synaptic tags is until about 60-90 min, activation of microglia beyond this time point while the maintenance phase is still unperturbed, suggests a lack of microglial interference in the synthesis or trigger of plasticity related products. Thus, our study provides the first evidence that microglia play a critical role in the setting of synaptic tags during the early phase of activity dependent plasticity.

Recent progress in therapeutic strategies for microglia-mediated neuroinflammation in neuropathologies.

INTRODUCTION: Chronic activation of microglia is the hallmark of numerous neuropathologies such as Alzheimer's disease, Parkinson's disease, and multiple sclerosis. The activated microglia perpetuate inflammation by releasing an array of pro-inflammatory and neurotoxic factors, which eventually exacerbate neurotoxicity and neurodegeneration upon chronic activation of these cells. However, under acute conditions, activated microglia elicit pro-inflammatory as well as anti-inflammatory responses that are associated with neuroprotection. Given the role of microglia in neuroinflammation, recent studies have attempted to unravel the mechanisms that aid to establish microglial cell-based therapy. Areas covered: While total suppression of microglial activation may compromise its beneficial role in tissue repair in the aftermath of an insult, the benefits of modulating microglial activation and promoting microglia polarization to a neuroprotective phenotype have been highlighted recently. Expert opinion: So far, the therapeutic strategy focussed on neutralizing microglia-mediated neuroinflammation using drugs that block the release of pro-inflammatory mediators has limitations, such as unwarranted side effects. Recent advances reveal several alternative molecular targets and potential epi-drugs that are capable of modulating microglial function and promoting neuroprotection. This review discusses the recent progress made in understanding the mechanisms of microglia-mediated neuroinflammation in various neuropathologies, and the emerging anti-inflammatory therapeutic strategies in this field.

Microglial SMAD4 regulated by microRNA-146a promotes migration of microglia which support tumor progression in a glioma environment.

Glioma tumors constitute a significant portion of microglial cells, which are known to support tumor progression. The present study demonstrates that transforming growth factor-ß (TGFß) signaling pathway in microglia in a glioma environment is involved in tumor progression and pathogenesis. It has been shown that the TGFß level is elevated in higher grades of gliomas and its signaling pathway regulates tumor progression through phosphorylation of SMAD2 and SMAD3, which form a complex with SMAD4 to regulate target gene transcription. In an in vitro cell line-based model increased protein levels of pSMAD2/3, total SMAD2/3 and SMAD4 were observed in murine BV2 microglia cultured in glioma conditioned medium (GCM), indicative of the activated TGFß signaling pathway in microglia associated with glioma environment. Immunofluorescence labeling further revealed the expression of SMAD4 in microglial and non-microglial cells of human glioblastomas tissue in vivo. Functional analysis through shRNA-mediated stable knockdown of SMAD4 in microglia revealed the downregulation of the expression of matrix metalloproteinase 9 (MMP9), which has been shown to be involved in tumor progression and cell migration. Further, knockdown of SMAD4 in microglia decreased the migration of microglial cells towards GCM, indicating that SMAD4 promotes microglial migration in glioma environment. In addition, SMAD4 has been shown to be post-transcriptionally regulated by microRNA-146a, which was downregulated in microglia treated with GCM. Overexpression of miR-146a resulted in decreased expression of SMAD4 together with tumor supportive gene MMP9 in microglia, and subsequently suppressed microglial migration towards GCM, possibly through regulation of SMAD4. On the other hand, the cell viability assay revealed decreased viability of glioma cells when they were treated with conditioned medium derived from SMAD4 knockdown microglia or miR-146a overexpressed microglia as compared to glioma cells treated with the medium from control microglial cells. Taken together, the present study suggests that microglial SMAD4 which is epigenetically regulated by miR-146a promotes microglial migration in gliomas and glioma cell viability.

Production of proinflammatory mediators in activated microglia is synergistically regulated by Notch-1, glycogen synthase kinase (GSK-3ß) and NF-κB/p65 signalling.

Microglia activation and associated inflammatory response are involved in the pathogenesis of different neurodegenerative diseases. We have reported that Notch-1 and NF-κB/p65 signalling pathways operate in synergy in regulating the production of proinflammatory mediators in activated microglia. In the latter, there is also evidence by others that glycogen synthase kinase 3ß (GSK-3ß) mediates the release of proinflammatory cytokines but the interrelationships between the three signalling pathways have not been fully clarified. This is an important issue as activated microglia are potential therapeutic target for amelioration of microglia mediated neuroinflammation. Here we show that blocking of Notch-1 with N-[(3,5-Difluorophenyl) acetyl]-L-alanyl-2-phenylglycine-1,1-dimethylethyl ester (DAPT) in LPS activated BV-2 microglia not only suppressed Notch intracellular domain (NICD) and Hes-1 protein expression, but also that of GSK-3ß. Conversely, blocking of the latter with lithium chloride (LiCl) decreased NICD expression in a dose-dependent manner; moreover, Hes-1 immunofluorescence was attenuated. Along with this, the protein expression level of p-GSK-3ß and p-AKT protein expression was significantly increased. Furthermore, DAPT and LiCl decreased production of IL-1ß, TNF-α, IL-6, iNOS, Cox2 and MCP-1; however, IL-10 expression was increased notably in LiCl treated cells. The effects of DAPT and LiCl on changes of the above-mentioned biomarkers were confirmed by immunofluorescence in both BV-2 and primary microglia. Additionally, NF-κB/p65 immunofluorescence was attenuated by DAPT and LiCl; as opposed to this, IκBα protein expression was increased. Taken together, it is suggested that Notch-1, NF-κB/p65 and GSK-3ß operate in synergy to inhibit microglia activation. This may be effected via increased expression of phospho-GSK-3ß (p-GSK-3ß), phospho-protein kinase B (PKB) (p-AKT) and IκBα. It is concluded that the three signalling pathways are functionally interlinked in regulating microglia activation.

Scutellarin Attenuates Microglia-Mediated Neuroinflammation and Promotes Astrogliosis in Cerebral Ischemia - A Therapeutic Consideration.

Neuroinflammation plays an important role in different brain diseases including acute brain injuries such as cerebral ischemic stroke and chronic neurodegenerative diseases e.g. Alzheimer's disease etc. The central player in this is the activated microglia, which produce substantial amounts of proinflammatory mediators that may exacerbate the disease. Associated with microglia activation is astrogliosis characterized by hypertrophic astrocytes with increased expression of proinflammatory cytokines, neurotrophic factors, stem cell, neuronal and proliferation markers, all these are crucial for reconstruction of damaged tissue and ultimate restoration of neurological functions. Here, we review the roles of activated microglia and reactive astrocytes in brain diseases with special reference to cerebral ischemia, and the effects of scutellarin, a Chinese herbal extract on both glial cells. We first reviewed the close spatial relation between activated microglia and reactive astrocytes as it suggests that both glial cells work in concert for tissue reconstruction and repair. Secondly, we have identified scutellarin as a putative therapeutic agent as it has been found to not only suppress microglial activation thus ameliorating neuroinflammation, but also enhance astrocytic reaction. In the latter, scutellarin amplified the astrocytic reaction by upregulating the expression of neurotrophic factors among others thus indicating its neuroprotective role. Remarkably, the effects of scutellarin on reactive astrocytes were mediated by activated microglia supporting a functional "cross-talk" between the two glial types. This review highlights some of our recent findings taking into consideration of others demonstrating the beneficial effects of scutellarin on both glial cell types in cerebral ischemia as manifested by improvement of neurological functions.

MicroRNAs: Key Players in Microglia and Astrocyte Mediated Inflammation in CNS Pathologies.

The significance of microglia and astrocytes in neural development, in maintaining synaptic connections and homeostasis in the healthy brain is well established. Microglia are dynamic immune cells of the brain that elicit an immune response during brain damage and also participate in tissue repair and regeneration, while astrocytes contribute to the local inflammatory response by producing proinflammatory cytokines and resolving neuronal damage through production of anti-inflammatory cytokines and neurotrophic factors. Recent efforts have focused on elucidating the epigenetic mechanisms which regulate glial cell behavior in normal and pathologic states. An important class of epigenetic regulators is microRNAs (miRNAs) which are small non-coding RNA molecules that regulate gene expression posttranscriptionally. Certain dysregulated miRNAs contribute to chronic microglial inflammation in the brain, thereby leading to progression of neurological diseases like Alzheimer's disease, traumatic injury, amyotrophic lateral sclerosis and stroke. Further, several miRNAs are differentially expressed in astrocytes after ischemia and spinal cord injury. Despite knowledge about miRNAs in neuroinflammation, little is known about effective delivery routes and pharmacokinetic data for miRNA based therapeutics. This review summarizes the current research on the role of miRNAs in promoting and inhibiting inflammatory response of microglia and astrocytes in a disease-specific manner. In addition, miRNA delivery as a therapeutic strategy to treat neuroinflammation is discussed.

Role of dietary phenols in mitigating microglia-mediated neuroinflammation.

Chronic neuroinflammation is a pathological feature of a number of central nervous system (CNS) diseases and is mediated by sustained activation of microglial cells, the innate immune cells of the CNS. Studies have mainly focused on identifying the molecular and epigenetic mechanisms of microglial activation. This is crucial in designing therapeutic strategies for neuropathologies in which prolonged microglial activation is known to exacerbate disease condition. In recent years, increasing evidence show that naturally occurring compounds present in regular diet could function as "nutraceuticals," arresting microglial activation, and thus conferring neuroprotection. This review summarizes our understanding of the role of dietary phenolic nutraceuticals in mitigating microglia-mediated neuroinflammation. Studies show that these natural phenols inhibit key signaling pathways in activated microglia such as the NFκB, MAPK and JAK-STAT that trigger microglia-mediated inflammation in various neuropathological conditions such as injury, infection, stroke, autism and neurodegenerative diseases, i.e., Alzheimer's disease and Parkinson's disease. The anti-inflammatory and antioxidant effect exerted by these natural phenols have shown considerable success in improving disease condition in animal models of neuropathologies, and thus seem to be suitable candidates for developing therapeutic strategies.