Microglial dynamics and role in the healthy and diseased brain: a paradigm of functional plasticity

Nanoparticle-microglial interaction in the ischemic brain is modulated by injury duration and treatment

Cerebral ischemia is a major cause of death in both neonates and adults, and currently has no cure. Nanotechnology represents one promising area of therapeutic development for cerebral ischemia due to the ability of nanoparticles to overcome biological barriers in the brain. ex vivo injury models have emerged as a high-throughput alternative that can recapitulate disease processes and enable nanoscale probing of the brain microenvironment. In this study, we used oxygen-glucose deprivation (OGD) to model ischemic injury and studied nanoparticle interaction with microglia, resident immune cells in the brain that are of increasing interest for therapeutic delivery. By measuring cell death and glutathione production, we evaluated the effect of OGD exposure time and treatment with azithromycin (AZ) on slice health. We found a robust injury response with 0.5 hr of OGD exposure and effective treatment after immediate application of AZ. We observed an OGD-induced shift in microglial morphology toward increased heterogeneity and circularity, and a decrease in microglial number, which was reversed after treatment. OGD enhanced diffusion of polystyrene-poly(ethylene glycol) (PS-PEG) nanoparticles, improving transport and ability to reach target cells. While microglial uptake of dendrimers or quantum dots (QDs) was not enhanced after injury, internalization of PS-PEG was significantly increased. For PS-PEG, AZ treatment restored microglial uptake to normal control levels. Our results suggest that different nanoparticle platforms should be carefully screened before application and upon doing so; disease-mediated changes in the brain microenvironment can be leveraged by nanoscale drug delivery devices for enhanced cell interaction.

Long-term environmental enrichment reduces microglia morphological diversity of the molecular layer of dentate gyrus

We investigated long-term environmental influences on morphology of microglia from the outer and middle thirds of molecular layer of the dentate gyrus (MolDG), and on microglia from dorsal and ventral dentate gyrus molecular layer. We also estimated the total number of MolDG microglia using stereology. For this purpose, microglia of the molecular layer of the dentate gyrus of 20-month-old female Swiss albino mice, housed from 21st postnatal day onwards, in the impoverished environment of the standard laboratory cages (SEA), or in a cage with an enriched environment (EEA), were reconstructed microscopically in three dimensions and compared with each other and with microglia of 6-month-old female Swiss albino mice, also housed from weaning onwards in an enriched cage (EEY). All mice had their brains sectioned and processed for immunolabeling for IBA-1, a selective microglia marker. Random and systematic microglia samples were reconstructed in three dimensions and classified morphologically using hierarchical cluster analysis, followed by discriminant function analysis. SEA and EEY showed two morphological phenotypes of microglia in both the outer and middle thirds of MolDG. EEA mice showed such a reduction in the morphological diversity of microglia that essentially a single morphotype was found. EEA mouse microglia showed an intermediate morphological complexity between types I and II SE microglia. We suggest that type I and type II microglia in SE mice may have different physiological roles and that long-term EE may be associated with adaptive responses of microglial phenotypes to somatomotor and cognitive stimuli.

Ketamine/xylazine and barbiturates modulate microglial morphology and motility differently in a mouse model

Microglia, the resident immune cells of the brain, are highly ramified and motile and their morphology is strongly linked to their function. Microglia constantly monitor the brain parenchyma and are crucial for maintaining brain homeostasis and fine-tuning neuronal networks. Besides affecting neurons, anesthetics may have wide-ranging effects mediated by non-neuronal cells and in particular microglia. We thus examined the effect of two commonly used anesthetic agents, ketamine/xylazine and barbiturates, on microglial motility and morphology. A combination of two-photon in vivo imaging and electroencephalography (EEG) recordings in unanesthetized and anesthetized mice as well as automated analysis of ex vivo sections were used to assess morphology and dynamics of microglia. We found that administration of ketamine/xylazine and pentobarbital anesthesia resulted in quite distinct EEG profiles. Both anesthetics reduced microglial motility, but only ketamine/xylazine administration led to reduction of microglial complexity in vivo. The change of cellular dynamics in vivo was associated with a region-dependent reduction of several features of microglial cells ex vivo, such as the complexity index and the ramification length, whereas thiopental altered the size of the cytoplasm. Our results show that anesthetics have considerable effects on neuronal activity and microglial morphodynamics and that barbiturates may be a preferred anesthetic agent for the study of microglial morphology. These findings will undoubtedly raise compelling questions about the functional relevance of anesthetics on microglial cells in neuronal physiology and anesthesia-induced neurotoxicity.

Neuroinflammation and protein aggregation co-localize across the frontotemporal dementia spectrum

The clinical syndromes of frontotemporal dementia are clinically and neuropathologically heterogeneous, but processes such as neuroinflammation may be common across the disease spectrum. We investigated how neuroinflammation relates to the localization of tau and TDP-43 pathology, and to the heterogeneity of clinical disease. We used PET in vivo with (i) 11C-PK-11195, a marker of activated microglia and a proxy index of neuroinflammation; and (ii) 18F-AV-1451, a radioligand with increased binding to pathologically affected regions in tauopathies and TDP-43-related disease, and which is used as a surrogate marker of non-amyloid-β protein aggregation. We assessed 31 patients with frontotemporal dementia (10 with behavioural variant, 11 with the semantic variant and 10 with the non-fluent variant), 28 of whom underwent both 18F-AV-1451 and 11C-PK-11195 PET, and matched control subjects (14 for 18F-AV-1451 and 15 for 11C-PK-11195). We used a univariate region of interest analysis, a paired correlation analysis of the regional relationship between binding distributions of the two ligands, a principal component analysis of the spatial distributions of binding, and a multivariate analysis of the distribution of binding that explicitly controls for individual differences in ligand affinity for TDP-43 and different tau isoforms. We found significant group-wise differences in 11C-PK-11195 binding between each patient group and controls in frontotemporal regions, in both a regions-of-interest analysis and in the comparison of principal spatial components of binding. 18F-AV-1451 binding was increased in semantic variant primary progressive aphasia compared to controls in the temporal regions, and both semantic variant primary progressive aphasia and behavioural variant frontotemporal dementia differed from controls in the expression of principal spatial components of binding, across temporal and frontotemporal cortex, respectively. There was a strong positive correlation between 11C-PK-11195 and 18F-AV-1451 uptake in all disease groups, across widespread cortical regions. We confirmed this association with post-mortem quantification in 12 brains, demonstrating strong associations between the regional densities of microglia and neuropathology in FTLD-TDP (A), FTLD-TDP (C), and FTLD-Pick's. This was driven by amoeboid (activated) microglia, with no change in the density of ramified (sessile) microglia. The multivariate distribution of 11C-PK-11195 binding related better to clinical heterogeneity than did 18F-AV-1451: distinct spatial modes of neuroinflammation were associated with different frontotemporal dementia syndromes and supported accurate classification of participants. These in vivo findings indicate a close association between neuroinflammation and protein aggregation in frontotemporal dementia. The inflammatory component may be important in shaping the clinical and neuropathological patterns of the diverse clinical syndromes of frontotemporal dementia.

Tripartite-motif family protein 35-28 regulates microglia development by preventing necrotic death of microglial precursors in zebrafish

Microglia are tissue-resident macrophages in the central nervous system (CNS) that play essential roles in the regulation of CNS development and homeostasis. Yet, the genetic networks governing microglia development remain incompletely defined. Here, we report the identification and characterization of a microglia-defective zebrafish mutant wulong^hkz12 (wul^hkz12 ) isolated from an ethylnitrosourea (ENU)-based genetic screen. We show that wul^hkz12 mutants harbors a missense point mutation in the gene region encoding the PRY/SPRY domain of the tripartite-motif family protein 35-28 (trim35-28) gene. Time-lapse imaging revealed that the loss of Trim35-28 function causes lytic necrosis of microglial precursors/peripheral macrophages, as indicated by cytoplasmic swelling and membrane rupture of these precursors and accompanied by neutrophil infiltration and systemic inflammation. Intriguingly, the lytic necrosis of microglial precursors in trim35-28-deficient mutants appeared to depend neither on the canonical pyroptotic nor necroptotic pathways, as inhibition of the key component in each pathway could not rescue the microglia phenotype in trim35-28-deficient mutants. Finally, results from tissue-specific rescue experiments suggested that Trim35-28 acts cell-autonomously in the survival of microglial precursors. Taken together, the findings of our study reveal Trim35-28 as a regulatory protein essential for microglia development.

Apamin Suppresses LPS-Induced Neuroinflammatory Responses by Regulating SK Channels and TLR4-Mediated Signaling Pathways

Neuroinflammation plays a vital role in neurodegenerative conditions. Microglia are a key component of the neuroinflammatory response. There is a growing interest in developing drugs to target microglia and thereby control neuroinflammatory processes. Apamin (APM) is a specifically selective antagonist of small conductance calcium-activated potassium (SK) channels. However, its effect on neuroinflammation is largely unknown. We examine the effects of APM on lipopolysaccharide (LPS)-stimulated BV2 and rat primary microglial cells. Regarding the molecular mechanism by which APM significantly inhibits proinflammatory cytokine production and microglial cell activation, we found that APM does so by reducing the expression of phosphorylated CaMKII and toll-like receptor (TLR4). In particular, APM potently suppressed the translocation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB)/signal transducer and activator of transcription (STAT)3 and phosphorylated mitogen-activated protein kinases (MAPK)-extracellular signal-regulated kinase (ERK). In addition, the correlation of NF-κB/STAT3 and MAPK-ERK in the neuroinflammatory response was verified through inhibitors. The literature and our findings suggest that APM is a promising candidate for an anti-neuroinflammatory agent and can potentially be used for the prevention and treatment of various neurological disorders.

Targeting Microglial Population Dynamics in Alzheimer's Disease: Are We Ready for a Potential Impact on Immune Function?

Alzheimer’s disease (AD) is the most common form of dementia, affecting two-thirds of people with dementia in the world. To date, no disease-modifying treatments are available to stop or delay the progression of AD. This chronic neurodegenerative disease is dominated by a strong innate immune response, whereby microglia plays a central role as the main resident macrophage of the brain. Recent genome-wide association studies (GWASs) have identified single-nucleotide polymorphisms (SNPs) located in microglial genes and associated with a delayed onset of AD, highlighting the important role of these cells on the onset and/or progression of the disease. These findings have increased the interest in targeting microglia-associated neuroinflammation as a potentially disease-modifying therapeutic approach for AD. In this review we provide an overview on the contribution of microglia to the pathophysiology of AD, focusing on the main regulatory pathways controlling microglial population dynamics during the neuroinflammatory response, such as the colony-stimulating factor 1 receptor (CSF1R), its ligands (the colony-stimulating factor 1 and interleukin 34) and the transcription factor PU.1. We also discuss the current therapeutic strategies targeting proliferation to modulate microglia-associated neuroinflammation and their potential impact on peripheral immune cell populations in the short and long-term. Understanding the effects of immunomodulatory approaches on microglia and other immune cell types might be critical for developing specific, effective, and safe therapies for neurodegenerative diseases.

Inhibition of the Dead Box RNA Helicase 3 Prevents HIV-1 Tat and Cocaine-Induced Neurotoxicity by Targeting Microglia Activation

HIV-1 Associated Neurocognitive Disorder (HAND) is a common and clinically detrimental complication of HIV infection. Viral proteins, including Tat, released from infected cells, cause neuronal toxicity. Substance abuse in HIV-infected patients greatly influences the severity of neuronal damage. To repurpose small molecule inhibitors for anti-HAND therapy, we employed MOLIERE, an AI-based literature mining system that we developed. All human genes were analyzed and prioritized by MOLIERE to find previously unknown targets connected to HAND. From the identified high priority genes, we narrowed the list to those with known small molecule ligands developed for other applications and lacking systemic toxicity in animal models. To validate the AI-based process, the selective small molecule inhibitor of DDX3 helicase activity, RK-33, was chosen and tested for neuroprotective activity. The compound, previously developed for cancer treatment, was tested for the prevention of combined neurotoxicity of HIV Tat and cocaine. Rodent cortical cultures were treated with 6 or 60 ng/ml of HIV Tat and 10 or 25 μM of cocaine, which caused substantial toxicity. RK-33 at doses as low as 1 μM greatly reduced the neurotoxicity of Tat and cocaine. Transcriptome analysis showed that most Tat-activated transcripts are microglia-specific genes and that RK-33 blocks their activation. Treatment with RK-33 inhibits the Tat and cocaine-dependent increase in the number and size of microglia and the proinflammatory cytokines IL-6, TNF-α, MCP-1/CCL2, MIP-2, IL-1α and IL-1β. These findings reveal that inhibition of DDX3 may have the potential to treat not only HAND but other neurodegenerative diseases. Graphical Abstract RK-33, selective inhibitor of Dead Box RNA helicase 3 (DDX3) protects neurons from combined Tat and cocaine neurotoxicity by inhibition of microglia activation and production of proinflammatory cytokines.

Mitochondrial Dynamics and Microglia as New Targets in Metabolism Regulation

Energy homeostasis regulation is essential for the maintenance of life. Neuronal hypothalamic populations are involved in the regulation of energy balance. In order play this role, they require energy: mitochondria, indeed, have a key role in ensuring a constant energy supply to neurons. Mitochondria are cellular organelles that are involved in dynamic processes; their dysfunction has been associated with many diseases, such as obesity and type 2 diabetes, indicating their importance in cellular metabolism and bioenergetics. Food intake excess can induce mitochondrial dysfunction with consequent production of reactive oxygen species (ROS) and oxidative stress. Several studies have shown the involvement of mitochondrial dynamics in the modulation of releasing agouti-related protein (AgRP) and proopiomelanocortin (POMC) neuronal activity, although the mechanisms are still unclear. However, recent studies have shown that changes in mitochondrial metabolism, such as in inflammation, can contribute also to the activation of the microglial system in several diseases, especially degenerative diseases. This review is aimed to summarize the link between mitochondrial dynamics and hypothalamic neurons in the regulation of glucose and energy homeostasis. Furthermore, we focus on the importance of microglia activation in the pathogenesis of many diseases, such as obesity, and on the relationship with mitochondrial dynamics, although this process is still largely unknown.

Ten Unsolved Questions About Neuroinflammation in Parkinson's Disease

Parkinson's disease is a progressive and debilitating disorder that has so far eluded attempts to develop disease-modifying treatment. Both epidemiological and genetic studies support a role of neuroinflammation in the pathophysiology of Parkinson's disease. Postmortem studies and experimental analyses suggest the involvement of both innate and adaptive immunity in the degenerative process. There is also some circumstantial evidence for effects of immune therapies on the disease. In the present article, we review 10 unanswered questions related to neuroinflammatory processes in Parkinson's disease with the goal of stimulating research in the field and accelerating the clinical development of neuroprotective therapies based on anti-inflammatory strategies. © 2020 International Parkinson and Movement Disorder Society.

Regulation of microglia by glutamate and its signal pathway in neurodegenerative diseases

Microglia are an essential component of the central nervous system (CNS) and are involved in the primary response to microorganisms, neuroinflammation, homeostasis, and tissue regeneration, as well as contributing to the pathogenesis of neurodegenerative diseases. Research has shown that microglial diversity, multifunctionality, and their relationship with glutamate are crucial to determining their roles in these diseases. In this review, we focus on recent progress in determining microglial characteristics and the role of glutamate and its receptors in microglia regulation, which could be a novel therapeutic strategy for neurodegenerative diseases.

Similar Microglial Cell Densities across Brain Structures and Mammalian Species: Implications for Brain Tissue Function

Microglial cells play essential volume-related actions in the brain that contribute to the maturation and plasticity of neural circuits that ultimately shape behavior. Microglia can thus be expected to have similar cell sizes and even distribution both across brain structures and across species with different brain sizes. To test this hypothesis, we determined microglial cell densities (the inverse of cell size) using immunocytochemistry to Iba1 in samples of free cell nuclei prepared with the isotropic fractionator from brain structures of 33 mammalian species belonging to males and females of five different clades. We found that microglial cells constitute ∼7% of non-neuronal cells in different brain structures as well as in the whole brain of all mammalian species examined. Further, they vary little in cell density compared with neuronal cell densities within the cerebral cortex, across brain structures, across species within the same clade, and across mammalian clades. As a consequence, we find that one microglial cell services as few as one and as many as 100 neurons in different brain regions and species, depending on the local neuronal density. We thus conclude that the addition of microglial cells to mammalian brains is governed by mechanisms that constrain the size of these cells and have remained conserved over 200 million years of mammalian evolution. We discuss the probable consequences of such constrained size for brain function in health and disease.SIGNIFICANCE STATEMENT Microglial cells are resident macrophages of the CNS, with key functions in recycling synapses and maintaining the local environment in health and disease. We find that microglial cells occur in similar densities in the brains of different species and in the different structures of each individual brain, which indicates that these cells maintain a similar average size in mammalian evolution, suggesting in turn that the volume monitored by each microglial cell remains constant across mammals. Because the density of neurons is highly variable across the same brain structures and species, our finding implies that microglia-dependent functional recovery may be particularly difficult in those brain structures and species with high neuronal densities and therefore fewer microglial cells per neuron.

Plasma membrane receptors of tissue macrophages: functions and role in pathology

The cells of the mononuclear phagocyte system (MPS) constitute a dispersed organ, which is distributed throughout the body. Macrophages in different tissues display distinctive mosaic phenotypes as resident and recruited cells of embryonic and bone marrow origin, respectively. They help to maintain homeostasis during development and throughout adult life, yet contribute to the pathogenesis of many disease processes, including inflammation, innate and adaptive immunity, metabolic disorders, and cancer. Heterogeneous tissue macrophage populations display a wide variety of surface molecules to recognise and respond to host, microbial, and exogenous ligands in their environment; their receptors mediate the uptake and destruction of effete and dying host cells and pathogens, as well as contribute trophic and secretory functions within every organ in the body. Apart from local cellular interactions, macrophage surface molecules and products serve to mobilise and coordinate systemic humoral and cellular responses. Their use as antigen markers in pathogenesis and as potential drug targets has lagged in clinical pathology and human immunotherapy. In this review, we summarise the properties of selected surface molecules expressed on macrophages in different tissues and disease processes, to provide a functional basis for diagnosis, further research, and treatment. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Microglia control vascular architecture via a TGFβ1 dependent paracrine mechanism linked to tissue mechanics

Tissue microarchitecture and mechanics are important in development and pathologies of the Central Nervous System (CNS); however, their coordinating mechanisms are unclear. Here, we report that during colonization of the retina, microglia contacts the deep layer of high stiffness, which coincides with microglial bipolarization, reduction in TGFβ1 signaling and termination of vascular growth. Likewise, stiff substrates induce microglial bipolarization and diminish TGFβ1 expression in hydrogels. Both microglial bipolarization in vivo and the responses to stiff substrates in vitro require intracellular adaptor Kindlin3 but not microglial integrins. Lack of Kindlin3 causes high microglial contractility, dysregulation of ERK signaling, excessive TGFβ1 expression and abnormally-patterned vasculature with severe malformations in the area of photoreceptors. Both excessive TGFβ1 signaling and vascular defects caused by Kindlin3-deficient microglia are rescued by either microglial depletion or microglial knockout of TGFβ1 in vivo. This mechanism underlies an interplay between microglia, vascular patterning and tissue mechanics within the CNS.

Neuroinflammation after Intracerebral Hemorrhage and Potential Therapeutic Targets

Spontaneous intracerebral hemorrhage (ICH) is a catastrophic illness causing significant morbidity and mortality. Despite advances in surgical technique addressing primary brain injury caused by ICH, little progress has been made treating the subsequent inflammatory cascade. Pre-clinical studies have made advancements identifying components of neuroinflammation, including microglia, astrocytes, and T lymphocytes. After cerebral insult, inflammation is initially driven by the M1 microglia, secreting cytokines (e.g., interleukin-1β [IL-1β] and tumor necrosis factor-α) that are involved in the breakdown of the extracellular matrix, cellular integrity, and the blood brain barrier. Additionally, inflammatory factors recruit and induce differentiation of A1 reactive astrocytes and T helper 1 (Th1) cells, which contribute to the secretion of inflammatory cytokines, augmenting M1 polarization and potentiating inflammation. Within 7 days of ICH ictus, the M1 phenotype coverts to a M2 phenotype, key for hematoma removal, tissue healing, and overall resolution of inflammation. The secretion of anti-inflammatory cytokines (e.g., IL-4, IL-10) can drive Th2 cell differentiation. M2 polarization is maintained by the secretion of additional anti-inflammatory cytokines by the Th2 cells, suppressing M1 and Th1 phenotypes. Elucidating the timing and trigger of the anti-inflammatory phenotype may be integral in improving clinical outcomes. A challenge in current translational research is the absence of an equivalent disease animal model mirroring the patient population and comorbid pathophysiologic state. We review existing data and describe potential therapeutic targets around which we are creating a bench to bedside translational research model that better reflects the pathophysiology of ICH patients.

Region-specific glial homeostatic signature in prion diseases is replaced by a uniform neuroinflammation signature, common for brain regions and prion strains with different cell tropism

Chronic neuroinflammation is recognized as a major neuropathological hallmark in a broad spectrum of neurodegenerative diseases including Alzheimer's, Parkinson's, Frontal Temporal Dementia, Amyotrophic Lateral Sclerosis, and prion diseases. Both microglia and astrocytes exhibit region-specific homeostatic transcriptional identities, which under chronic neurodegeneration, transform into reactive phenotypes in a region- and disease-specific manner. Little is known about region-specific identity of glia in prion diseases. The current study was designed to determine whether the region-specific homeostatic signature of glia changes with the progression of prion diseases, and whether these changes occur in a region-dependent or universal manner. Also of interest was whether different prion strains give rise to different reactive phenotypes. To answer these questions, we analyzed gene expression in the thalamus, cortex, hypothalamus and hippocampus of mice infected with 22L and ME7 prion strains using a Nanostring Neuroinflammation panel at the subclinical, early clinical and advanced stages of the disease. We found that at the preclinical stage of the disease, the region-specific homeostatic identities were preserved. However, with the appearance of clinical signs, the region-specific signatures were partially lost and replaced with a neuroinflammation signature. While the same sets of genes were activated by both prion strains, the timing of neuroinflammation and the degree of activation in different brain regions was strain-specific. Changes in astrocyte function scored at the top of the activated pathways. Moreover, clustering analysis suggested that the astrocyte function pathway responded to prion infection prior to the Activated Microglia or Neuron and Neurotransmission pathways. The current work established neuroinflammation gene expression signature associated with prion diseases. Our results illustrate that with the disease progression, the region-specific homeostatic transcriptome signatures are replaced by the region-independent neuroinflammation signature, which is common for prion strains with different cell tropism. The prion-associated neuroinflammation signature identified in the current study overlapped only partially with the microglia degenerative phenotype and the disease-associated microglia phenotype reported for animal models of other neurodegenerative diseases.

Activation of PPARA-mediated autophagy reduces Alzheimer disease-like pathology and cognitive decline in a murine model

Alzheimer disease (AD) is the most common neurodegenerative disease. An imbalance between the production and clearance of Aβ (amyloid beta) is considered to be actively involved in AD pathogenesis. Macroautophagy/autophagy is a major cellular pathway leading to the removal of aggregated proteins, and upregulation of autophagy represents a plausible therapeutic strategy to combat overproduction of neurotoxic Aβ. PPARA/PPARα (peroxisome proliferator activated receptor alpha) is a transcription factor that regulates genes involved in fatty acid metabolism and activates hepatic autophagy. We hypothesized that PPARA regulates autophagy in the nervous system and PPARA-mediated autophagy affects AD. We found that pharmacological activation of PPARA by the PPARA agonists gemfibrozil and Wy14643 induces autophagy in human microglia (HM) cells and U251 human glioma cells stably expressing the human APP (amyloid beta precursor protein) mutant (APP-p.M671L) and this effect is PPARA-dependent. Administration of PPARA agonists decreases amyloid pathology and reverses memory deficits and anxiety symptoms in APP-PSEN1ΔE9 mice. There is a reduced level of soluble Aβ and insoluble Aβ in hippocampus and cortex tissues from APP-PSEN1ΔE9 mice after treatment with either gemfibrozil or Wy14643, which promoted the recruitment of microglia and astrocytes to the vicinity of Aβ plaques and enhanced autophagosome biogenesis. These results indicated that PPARA is an important factor regulating autophagy in the clearance of Aβ and suggested gemfibrozil be assessed as a possible treatment for AD.Abbreviation: Aβ: amyloid beta; ACTB: actin beta; ADAM10: ADAM metallopeptidase domain 10; AD: Alzheimer disease; AIF1/IBA1: allograft inflammatory factor 1; ANOVA: analysis of variance; APOE: apolipoprotein E; APP: amyloid beta precursor protein; APP-PSEN1ΔE9: APPswe/PSEN1dE9; BAFA1: bafilomycin A1; BDNF: brain derived neurotrophic factor; BECN1: beclin 1; CD68: CD68 molecule; CREB1: cAMP responsive element binding protein 1; DAPI: 4',6-diamidino-2-phenylindole; DLG4/PSD-95: discs large MAGUK scaffold protein 4; DMSO: dimethyl sulfoxide; ELISA: enzyme linked immunosorbent assay; FDA: U.S. Food and Drug Administration; FKBP5: FK506 binding protein 5; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; gemfibrozil: 5-(2,5-dimethylphenoxy)-2,2-dimethylpentanoic acid; GFAP: glial fibrillary acidic protein; GLI2/THP1: GLI family zinc finger 2; HM: human microglia; IL6: interleukin 6; LAMP1: lysosomal associated membrane protein 1; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MTOR: mechanistic target of rapamycin kinase; NC: negative control; OQ: opposite quadrant; PPARA/PPARα, peroxisome proliferator activated receptor alpha; PSEN1/PS1: presenilin 1; SEM: standard error of the mean; SQSTM1: sequestosome 1; SYP: synaptophysin; TFEB: transcription factor EB; TNF/TNF-α: tumor necrosis factor; TQ: target quadrant; WT: wild type; Wy14643: 2-[4-chloro-6-(2,3-dimethylanilino)pyrimidin-2-yl]sulfanylacetic acid.

7-methoxyflavanone alleviates neuroinflammation in lipopolysaccharide-stimulated microglial cells by inhibiting TLR4/MyD88/MAPK signalling and activating the Nrf2/NQO-1 pathway

OBJECTIVES

Neuroprotective potential of 7-methoxyflavanone (7MF) and its underlying mechanism was investigated.

METHODS

Inhibitory effects of 7MF on microglial activation and neuroinflammation were evaluated by employment of lipopolysaccharide (LPS)-induced BV2 microglial cells. Changes in expression of genes and proteins of interest were investigated by RT-qPCR analysis and Western blot analysis. Inhibitory effects of 7MF on microglial overactivation were verified in LPS-treated C57BL/6J mice using ionized calcium-binding adaptor molecule-1 (Iba1) in the brain and interleukin-6 (IL-6) in serum as indicators.

KEY FINDINGS

In BV2 cells, pretreatment with 7MF antagonized LPS-induced production of inflammatory factors IL-6, tumour necrosis factor-α (TNF-α), cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), intercellular adhesion molecule-1 (ICAM-1) and monocyte chemoattractant protein-1 (MCP-1). Mechanistic studies revealed reduced expression of Toll-like receptor 4 (TLR4), myeloid differentiation factor-88 (MyD88), phosphorylated forms of c-Jun N-terminal kinase (p-JNK) and extracellular signal-regulated kinases 1/2 (p-ERK) but increased nuclear accumulation of nuclear factor erythroid 2-related factor 2 (Nrf2) and cellular expression of NAD(P)H quinone dehydrogenase-1 (NQO-1) by 7MF. In LPS-treated mice, pretreatment with 7MF reduced the brain level of Iba1 and serum level of IL-6.

CONCLUSIONS

7-methoxyflavanone inhibited LPS-stimulated TLR4/MyD88/MAPK signalling and activated Nrf2-mediated transcription of antioxidant protein NQO-1, showing antineuroinflammatory effect, so it is a potential neuroprotective agent.

Spatiotemporal analysis of impaired microglia process movement at sites of secondary neurodegeneration post-stroke

It has recently been identified that after motor cortex stroke, the ability of microglia processes to respond to local damage cues is lost from the thalamus, a major site of secondary neurodegeneration (SND). In this study, we combine a photothrombotic stroke model in mice, acute slice and fluorescent imaging to analyse the loss of microglia process responsiveness. The peri-infarct territories and thalamic areas of SND were investigated at time-points 3, 7, 14, 28 and 56 days after stroke. We confirmed the highly specific nature of non-responsive microglia processes to sites of SND. Non-responsiveness was at no time observed at the peri-infarct but started in the thalamus seven days post-stroke and persisted for 56 days. Loss of directed process extension is not a reflection of general functional paralysis as phagocytic function continued to increase over time. Additionally, we identified that somal P₂Y₁₂ was present on non-responsive microglia in the first two weeks after stroke but not at later time points. Finally, both classical microglia activation and loss of process extension are highly correlated with neuronal damage. Our findings highlight the importance of microglia, specifically microglia dynamic functions, to the progression of SND post-stroke, and their potential relevance as modulators or therapeutic targets during stroke recovery.

Glial neuroimmune signaling in opioid reward

The opioid epidemic is a growing public concern affecting millions of people worldwide. Opioid-induced reward is the initial and key process leading to opioid abuse and addiction. Therefore, a better understanding of opioid reward may be helpful in developing a treatment for opioid addiction. Emerging evidence suggests that glial cells, particularly microglia and astrocytes, play an essential role in modulating opioid reward. Indeed, glial cells and their associated immune signaling actively regulate neural activity and plasticity, and directly modulate opioid-induced rewarding behaviors. In this review, we describe the neuroimmune mechanisms of how glial cells affect synaptic transmission and plasticity as well as how opioids can activate glial cells affecting the glial-neuronal interaction. Last, we summarize current attempts of applying glial modulators in treating opioid reward.

Microglial Morphometric Parameters Correlate With the Expression Level of IL-1β, and Allow Identifying Different Activated Morphotypes

Microglia are the resident macrophages in the brain. Traditionally, two forms of microglia have been described: one considered as a resting/surveillant state in which cells have a highly branched morphology, and another considered as an activated state in which they acquire a de-ramified or amoeboid form. However, many studies describe intermediate microglial morphologies which emerge during pathological processes. Since microglial form and function are closely related, it is of interest to correlate microglial morphology with the extent of its activation. To address this issue, we used a rat model of neuroinflammation consisting in a single injection of the enzyme neuraminidase (NA) within the lateral ventricle. Sections from NA-injected animals were co-immunolabeled with the microglial marker IBA1 and the cytokine IL-1β, which highlight features of the cell’s shape and inflammatory activation, respectively. Activated (IL-1β positive) microglial cells were sampled from the dorsal hypothalamus nearby the third ventricle. Images of single microglial cells were processed in two different ways to obtain (1) an accurate measure of the level of expression of IL-1β (indicating the degree of activation), and (2) a set of 15 morphological parameters to quantitatively and objectively describe the cell’s shape. A simple regression analysis revealed a dependence of most of the morphometric parameters on IL-1β expression, demonstrating that the morphology of microglial cells changes progressively with the degree of activation. Moreover, a hierarchical cluster analysis pointed out four different morphotypes of activated microglia, which are characterized not only by morphological parameters values, but also by specific IL-1β expression levels. Thus, these results demonstrate in an objective manner that the activation of microglial cells is a gradual process, and correlates with their morphological change. Even so, it is still possible to categorize activated cells according to their morphometric parameters, each category presenting a different activation degree. The physiological relevance of those activated morphotypes is an issue worth to be assessed in the future.

Region-Specific Response of Astrocytes to Prion Infection

Chronic neuroinflammation involves reactive microgliosis and astrogliosis, and is regarded as a common pathological hallmark of neurodegenerative diseases including Alzheimer’s, Parkinson’s, ALS and prion diseases. Reactive astrogliosis, routinely observed immunohistochemically as an increase in glial fibrillary acidic protein (GFAP) signal, is a well-documented feature of chronic neuroinflammation associated with neurodegenerative diseases. Recent studies on single-cell transcriptional profiling of a mouse brain revealed that, under normal conditions, several distinct subtypes of astrocytes with regionally specialized distribution exist. However, it remains unclear whether astrocytic response to pro-inflammatory pathological conditions is uniform across whole brain or is region-specific. The current study compares the response of microglia and astrocytes to prions in mice infected with 22L mouse-adapted prion strain. While the intensity of reactive microgliosis correlated well with the extent of PrP^Sc deposition, reactive astrogliosis displayed a different, region-specific pattern. In particular, the thalamus and stratum oriens of hippocampus, which are both affected by 22L prions, displayed strikingly different response of astrocytes to PrP^Sc. Astrocytes in stratum oriens of hippocampus responded to accumulation of PrP^Sc with visible hypertrophy and increased GFAP, while in the thalamus, despite stronger PrP^Sc signal, the increase of GFAP was milder than in hippocampus, and the change in astrocyte morphology was less pronounced. The current study suggests that astrocyte response to prion infection is heterogeneous and, in part, defined by brain region. Moreover, the current work emphasizes the needs for elucidating region-specific changes in functional states of astrocytes and exploring the impact of these changes to chronic neurodegeneration.

Microglial response patterns following damage to the zebrafish olfactory bulb

The inherent plasticity of the zebrafish olfactory system serves as a useful model for examining immune cell responses after injury. Microglia are the resident immune cells of the CNS that respond to damage by migrating to the site of injury and phagocytizing neuronal debris. While the olfactory system is renowned for its ability to recover from damage, the specific mechanisms of microglial involvement in olfactory system plasticity are unknown. To approach the potentially time-dependent effects of microglial activation after injury, we performed a time course analysis of microglial response profiles and patterns following different forms of damage: deafferentation by cautery ablation of the olfactory organ, deafferentation by chemical ablation of the olfactory epithelium, and direct lesioning of the olfactory bulb. Our aim was to demonstrate that immunocytochemistry and microscopy methods in zebrafish can be used to determine the timing of distinct microglial response patterns following various forms of injury. We found that permanent and temporary forms of damage to the olfactory bulb resulted in different microglial response profiles from 1 to 72 h after injury, suggesting that there may be critical timepoints in which microglia are activated that contribute to tissue and neuronal repair with a regenerative outcome versus a degenerative outcome. These distinctions between the different forms of damage suggest temporal changes relative to the potential for regeneration, since cautery deafferentation is permanent and unrecoverable while chemical ablation deafferentation and direct lesioning is reversible and can be used to observe the microglial relationship in neural regeneration and functional recovery in future studies.

Rodent models for Alzheimer disease

Animal models are indispensable tools for Alzheimer disease (AD) research. Over the course of more than two decades, an increasing number of complementary rodent models has been generated. These models have facilitated testing hypotheses about the aetiology and progression of AD, dissecting the associated pathomechanisms and validating therapeutic interventions, thereby providing guidance for the design of human clinical trials. However, the lack of success in translating rodent data into therapeutic outcomes may challenge the validity of the current models. This Review critically evaluates the genetic and non-genetic strategies used in AD modelling, discussing their strengths and limitations, as well as new opportunities for the development of better models for the disease.

Microglial signatures and their role in health and disease

Microglia are the primary innate immune cells in the CNS. In the healthy brain, they exhibit a unique molecular homeostatic 'signature', consisting of a specific transcriptional profile and surface protein expression pattern, which differs from that of tissue macrophages. In recent years, there have been a number of important advances in our understanding of the molecular signatures of homeostatic microglia and disease-associated microglia that have provided insight into how these cells are regulated in health and disease and how they contribute to the maintenance of the neural environment.

Neuroimmune responses in the developing brain following traumatic brain injury

Traumatic brain injury (TBI) is one of the leading causes of both acute and long-term morbidity in the pediatric population, leading to a substantial, long-term socioeconomic burden. Despite the increase in the amount of pre-clinical and clinical research, treatment options for TBI rely heavily on supportive care with very limited targeted interventions that improve the acute and chronic sequelae of TBI. Other than injury prevention, not much can be done to limit the primary injury, which consists of tissue damage and cellular destruction. Secondary injury is the result of the ongoing complex inflammatory pathways that further exacerbate tissue damage, resulting in the devastating chronic outcomes of TBI. On the other hand, some level of inflammation is essential for neuronal regeneration and tissue repair. In this review article we discuss the various stages of the neuroimmune response in the immature, pediatric brain in the context of normal maturation and development of the immune system. The developing brain has unique features that distinguish it from the adult brain, and the immune system plays an integral role in CNS development. Those features could potentially make the developing brain more susceptible to worse outcomes, both acutely and in the long-term. The neuroinflammatory reaction which is triggered by TBI can be described as a highly intricate interaction between the cells of the innate and the adaptive immune systems. The innate immune system is triggered by non-specific danger signals that are released from damaged cells and tissues, which in turn leads to neutrophil infiltration, activation of microglia and astrocytes, complement release, as well as histamine release by mast cells. The adaptive immune response is subsequently activated leading to the more chronic effects of neuroinflammation. We will also discuss current attempts at modulating the TBI-induced neuroinflammatory response. A better understanding of the role of the immune system in normal brain development and how immune function changes with age is crucial for designing therapies to appropriately target the immune responses following TBI in order to enhance repair and plasticity.

Microglial activation after ischaemic stroke

Ischaemic stroke can induce rapid activation of microglia. As the resident immune cells of the central nervous system, microglial activation is believed to play a central role in neuroinflammation and pathological progression of ischaemic tissue. The activation of microglia after ischaemia involves several stereotypical events including morphological transformation, proliferation and polarisation. Studies using confocal or two-photon imaging techniques have revealed that the degree of microglial activation is correlated with the degree of ischaemia. Activated microglia display diverse polarisation phenotypes. It remains largely unclear regarding whether activated microglia are beneficial or detrimental to poststroke recovery. This mini-review focuses on the morphological and functional aspects of microglial activation, with particular attention to progress in two-photon imaging studies.

MicroRNA‑195 triggers neuroinflammation in Parkinson's disease in a Rho‑associated kinase 1‑dependent manner

Parkinson's disease (PD) is a common progressive neurodegenerative disorder occurring in older individuals. Mechanistically, neuroinflammation is a central pathological change in the progression of PD. Activation of microglia is widely considered to be a major trigger for neuroinflammation. Certain microRNAs (miRs) are key factors in inhibiting or stimulating inflammation during the occurrence and development of PD, among which miR‑195 may be a potential crucial biomarker. However, the underlying pathological mechanisms remain unclear. To investigate the pathogenesis of PD, lipopolysaccharide (LPS) was used to establish an in vitro model of microglia activation in the present study. It was revealed that miR‑195 expression was decreased in LPS‑stimulated BV2 cells, suggesting a potential mechanism of action of miR‑195 on microglia activation. Furthermore, gain‑ and loss‑of‑function experiments were performed by successful transfection of microglia with miR‑195 mimics or inhibitors. The results demonstrated that miR‑195 overexpression inhibited the release of pro‑inflammatory cytokines, including inducible nitric oxide synthase, interleukin‑6 (IL‑6) and tumor necrosis factor‑α, but induced the release of anti‑inflammatory cytokines in LPS‑treated BV2 cells, including IL‑4 and IL‑10. In addition, Rho‑associated kinase 1 (ROCK1), which is negatively regulated by miR‑195, was increased in LPS‑stimulated BV2 cells. ROCK1 knockdown with small interfering RNA exhibited the same effect as miR‑195 overexpression on regulating microglia status, suggesting that the miR‑195/ROCK1 interaction serves a central role in inducing microglia activation. Furthermore, inhibition of ROCK1 impaired cell viability and proliferation but induced cell apoptosis in LPS‑treated miR‑195‑deficient BV2 cells. The present results suggest that miR‑195 is a potential therapeutic target for PD.

Differential Microglial Morphological Response, TNFα, and Viral Load in Sedentary-like and Active Murine Models After Systemic Non-neurotropic Dengue Virus Infection

Peripheral inflammatory stimuli increase proinflammatory cytokines in the bloodstream and central nervous system and activate microglial cells. Here we tested the hypothesis that contrasting environments mimicking sedentary and active lives would be associated with differential microglial morphological responses, inflammatory cytokines concentration, and virus load in the peripheral blood. For this, mice were maintained either in standard (standard environment) or enriched cages (enriched environment) and then subjected to a single (DENV1) serotype infection. Blood samples from infected animals showed higher viral loads and higher tumor necrosis factor-α (TNFα) mRNA concentrations than control subjects. Using an unbiased stereological sampling approach, we selected 544 microglia from lateral septum for microscopic 3D reconstruction. Morphological complexity contributed most to cluster formation. Infected groups exhibited significant increase in the microglia morphological complexity and number, despite the absence of dengue virus antigens in the brain. Two microglial phenotypes (type I with lower and type II with higher morphological complexity) were found in both infected and control groups. However, microglia from infected mice maintained in enriched environment showed only one morphological phenotype. Two-way ANOVA revealed that environmental changes and infection influenced type-I and II microglial morphologies and number. Environmental enrichment and infection interactions may contribute to microglial morphological change to a point that type-I and II morphological phenotypes could no longer be distinguished in infected mice from enriched environment. Significant linear correlation was found between morphological complexity and TNFα peripheral blood. Our findings demonstrated that sedentary-like and active murine models exhibited differential microglial responses and peripheral inflammation to systemic non-neurotropic infections with DENV1 virus.

Activation of microglia and astrocytes: a roadway to neuroinflammation and Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease that is of high importance to the neuroscience world, yet the complex pathogenicity is not fully understood. Inflammation is usually observed in AD and could implicate both beneficial or detrimental effects depending on the severity of the disease. During initial AD pathology, microglia and astrocyte activation is beneficial since they are involved in amyloid-beta clearance. However, with the progression of the disease, activated microglia elicit detrimental effects by the overexpression of pro-inflammatory cytokines such as interleukin (IL)-1β, IL-6, and tumor necrosis factor-α (TNF-α) bringing forth neurodegeneration in the surrounding brain regions. This results in decline in Aβ clearance by microglia; Aβ accumulation thus increases in the brain resulting in neuroinflammation. Thus, Aβ accumulation is the effect of increased release of pro-inflammatory molecules. Reactive astrocytes acquire gain of toxic function and exhibits neurotoxic effects with loss of neurotrophic functions. Astrocyte dysfunctioning results in increased release of cytokines and inflammatory mediators, neurodegeneration, decreased glutamate uptake, loss of neuronal synapses, and ultimately cognitive deficits in AD. We discuss the role of intracellular signaling pathways in the inflammatory responses produced by astrocytes and microglial activation, including the glycogen synthase kinase-3β, nuclear factor kappa B cascade, mitogen-activated protein kinase pathways and c-Jun N-terminal kinase. In this review, we describe the role of neuroinflammation in the chronicity of AD pathogenesis and an overview of the recent research towards the development of new therapies to treat this disorder.

Dietary Iron Deficiency Impaired Peripheral Immunity but Did Not Alter Brain Microglia in PRRSV-Infected Neonatal Piglets

During the postnatal period the developing brain is vulnerable to insults including nutrient insufficiency and infection that may lead to disrupted development and cognitive dysfunction. Since iron deficiency (ID) often presents with immunodeficiency, the objective of this study was to investigate peripheral viremia and inflammation as well as brain microglial phenotype and function when ID and respiratory infection occur simultaneously in a neonatal piglet model. On postnatal day 2 (PD 2) male and female piglets were assigned to one of four treatments and fed either control or ID milk replacer. On PD 8 half the pigs on each diet were inoculated with either vehicle or porcine reproductive and respiratory syndrome virus (PRRSV; P-129). Blood samples were collected prior to inoculation (PD 7) and repeated once weekly. Rectal temperature, feeding score, and sickness behavior were measured daily until PD 28. Hematocrit, hemoglobin, and serum iron were reduced by ID but not PRRSV infection. PRRSV-infected piglets displayed viremia by PD 14; however, those fed control diet had lower viral titer on PD 28, while circulating virus remained elevated in those fed an ID diet, suggesting that ID either impaired immune function necessary for viral clearance or increased viral replication. ID piglets infected with PRRSV displayed reduced sickness behavior compared to those fed control diet on PD 13-15 and 18-20. While ID piglet sickness behavior progressively worsened, piglets fed control diet displayed improved sickness score after PD 21. Microglia isolated from PRRSV piglets had increased MHCII expression and phagocytic activity ex vivo compared to uninfected piglets. ID did not alter microglial activation or phagocytic activity. Similarly, microglial cytokine expression was increased by PRRSV but unaffected by ID, in stark contrast to peripheral blood mononuclear cell (PBMC) cytokine expression, which was increased by infection and generally decreased by ID. Taken together, these data suggest that ID decreases peripheral immune function leading to increased viremia, but immune activity in the brain is protected from acute ID.

Lipoxin A4 Regulates Lipopolysaccharide-Induced BV2 Microglial Activation and Differentiation via the Notch Signaling Pathway

Inflammatory responses contribute to the pathogenesis of various neurological diseases, and microglia plays an important role in the process. Activated microglia can differentiate into the pro-inflammatory, tissue-damaging M1 phenotype or the anti-inflammatory, tissue-repairing M2 phenotype. Regulating microglia differentiation, hence limiting a harmful response, might help improve the prognosis of inflammation-related nervous system diseases. The present study aimed 1. to observe the anti-inflammatory effect of lipoxin A4 (LXA4) on the inflammatory response associated to lipopolysaccharide (LPS)-induced microglia activation, 2. to clarify that LXA4 modulates the activation and differentiation of microglia induced by LPS stimulation, 3. to determine whether LXA4 regulates the activation and differentiation of microglia through the Notch signaling pathway, 4. to provide a foundation for the use of LXA4 for the treatment of inflammatory related neurological diseases. To construct a model of cellular inflammation, immortalized murine BV2 microglia cells were provided 200 ng/ml LPS. To measure the mRNA and protein levels of inflammatory factors (interleukin [IL]-1β, IL-10, and tumor necrosis factor [TNF]-α) and M1 and M2 microglia markers (inducible nitric oxide synthase [iNOS], cluster of differentiation [CD]32, arginase [Arg]1, and CD206), we performed quantitative reverse transcription polymerase chain reaction (qRT-PCR) and enzyme-linked immunosorbent assay (ELISA), immunofluorescence, or flow cytometry. To determine the mRNA and protein levels of Notch signaling components (Notch1, Hes1, and Hes5), we performed qRT-PCR and western blot. LXA4 inhibits the expression of Notch1 and Hes1 associated with M1 type microglial differentiation and decreases the M1 type microglia marker iNOS and related inflammatory factors IL-1β and TNF-α. Moreover, LXA4 upregulates the expression of the M2-associated Hes5, as well as the expression of the M2 microglia marker Arg1 and the associated inflammatory factor IL-10. These effects are blocked by the administration of the γ-secretase inhibitor DAPT, a specific blocker of the Notch signaling pathway. LXA4 inhibits the microglia activation induced by LPS and the differentiation into M1 type with pro-inflammatory effect, while promoting the differentiation to M2 type with anti-inflammatory effect. LXA4 downregulates the inflammatory mediators IL-1β, TNF-α, and iNOS, while upregulating the anti-inflammatory mediator IL-10, which acts through the Notch signaling pathway.

Microglial Phagocytosis of Neurons: Diminishing Neuronal Loss in Traumatic, Infectious, Inflammatory, and Autoimmune CNS Disorders

Errors in neuron-microglial interaction are known to lead to microglial phagocytosis of live neurons and excessive neuronal loss, potentially yielding poorer clinical outcomes. Factors that affect neuron-microglial interaction have the potential to influence the error rate. Clinical comorbidities that unfavorably impact neuron-microglial interaction may promote a higher rate of neuronal loss, to the detriment of patient outcome. This paper proposes that many common, clinically modifiable comorbidities have a common thread, in that they all influence neuron-microglial interactions. Comorbidities like traumatic brain injury, infection, stress, neuroinflammation, loss of neuronal metabolic integrity, poor growth factor status, and other factors, all have the potential to alter communication between neurons and microglia. When this occurs, microglial phagocytosis of live neurons can increase. In addition, microglia can shift into a morphological form in which they express major histocompatibility complex II (MHC-II), allowing them to function as antigen presenting cells that present neuronal debris as antigen to invading T cells. This can increase risk for the development of CNS autoimmunity, or can exacerbate existing CNS autoimmunity. The detrimental influence of these comorbidities has the potential to contribute to the mosaic of factors that determine patient outcome in some CNS pathologies that have neuropsychiatric involvement, including TBI and CNS disorders with autoimmune components, where excessive neuronal loss can yield poorer clinical outcomes. Recognition of the impact of these comorbidities may contribute to an understanding of the common clinical observation that many seemingly disparate factors contribute to the overall picture of case management and clinical outcome in these complex disorders. In a clinical setting, knowing how these comorbidities can influence neuron-microglial interaction can help focus surveillance and care on a broader group of potential therapeutic targets. Accordingly, an interest in the mechanisms underlying the influence of these factors on neuron-microglial interactions is appropriate. Neuron-microglial interaction is reviewed, and the various mechanisms by which these potential comorbidities influence neuro-microglial interaction are described.

Basic Concept of Microglia Biology and Neuroinflammation in Relation to Psychiatry

The hypothesis that the neuroimmune system plays a role in the pathogenesis of different psychiatric disorders, including schizophrenia, depression, and bipolar disease, has attained increasing interest over the past years. Previously thought to have the sole purpose of protecting the central nervous system (CNS) from harmful stimuli, it is now known that the central immune system is critically involved in regulating physiological processes including neurodevelopment, synaptic plasticity, and circuit maintenance. Hence, alterations in microglia - the main immune cell of the CNS - and/or inflammatory factors do not unequivocally connote ongoing neuroinflammation or neuroinflammatory processes per se but rather might signify changes in brain homoeostasis. Despite this, psychiatric research tends to equate functional changes in microglia or alterations in other immune mediators with neuroinflammation. It is the main impetus of this chapter to overcome some of the current misconceptions and possible oversimplifications with respect to neuroinflammation and microglia activity in psychiatry. In order to do so, we will first provide an overview of the basic concepts of neuroinflammation and neuroinflammatory processes. We will then focus on microglia with respect to their ontogeny and immunological and non-immunological functions presenting novel insights on how microglia communicate with other cell types of the central nervous system to ensure proper brain functioning. And lastly, we will delineate the non-immunological functions of inflammatory cytokines in order to address the possible misconception of equating alterations in central cytokine levels with ongoing central inflammation. We hereby hope to help unravel the functional relevance of neuroimmune dysfunctions in psychiatric illnesses and provide future research directions in the field of psychoneuroimmunology.

Microglia in Neurological Diseases: A Road Map to Brain-Disease Dependent-Inflammatory Response

Microglia represent a specialized population of macrophages-like cells in the central nervous system (CNS) considered immune sentinels that are capable of orchestrating a potent inflammatory response. Microglia are also involved in synaptic organization, trophic neuronal support during development, phagocytosis of apoptotic cells in the developing brain, myelin turnover, control of neuronal excitability, phagocytic debris removal as well as brain protection and repair. Microglial response is pathology dependent and affects to immune, metabolic. In this review, we will shed light on microglial activation depending on the disease context and the influence of factors such as aging, environment or cell-to-cell interaction.

Kinase-Based Taming of Brain Microglia Toward Disease-Modifying Therapy

Microglia are the primary immune cells residing in the central nervous system (CNS), where they play essential roles in the health and disease. Depending on the CNS inflammatory milieu, they exist in either resting or activated states. Chronic neuroinflammation mediated by activated microglia is now considered to be a common characteristic shared by many neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis, which currently pose a significant socioeconomic burden to the global healthcare system. Accumulating evidence has indicated protein kinases (PKs) as important drug targets for therapeutic interventions of these detrimental diseases. Here, we review recent findings suggesting that selected PKs potentially participate in microglia-mediated neuroinflammation. Taming microglial phenotypes by modulating the activity of these PKs holds great promise for the development of disease-modifying therapies for many neurodegenerative diseases.

In vivo molecular imaging of neuroinflammation in Alzheimer's disease

It has become increasingly evident that neuroinflammation plays a critical role in the pathophysiology of Alzheimer's disease (AD) and other neurodegenerative disorders. Increased glial cell activation is consistently reported in both rodent models of AD and in AD patients. Moreover, recent genome wide association studies have revealed multiple genes associated with inflammation and immunity are significantly associated with an increased risk of AD development (e.g. TREM2). Non‐invasive in vivo detection and tracking of neuroinflammation is necessary to enhance our understanding of the contribution of neuroinflammation to the initiation and progression of AD. Importantly, accurate methods of quantifying neuroinflammation may aid early diagnosis and serve as an output for therapeutic monitoring and disease management. This review details current in vivo imaging biomarkers of neuroinflammation being explored and summarizes both pre‐clinical and clinical results from molecular imaging studies investigating the role of neuroinflammation in AD, with a focus on positron emission tomography and magnetic resonance spectroscopy (MRS).

Regulation of microglial process elongation, a featured characteristic of microglial plasticity

Microglia, a type of glia within the brain characterized by a ramified morphology, are essential for removing neuronal debris and restricting the expansion of a lesion site. Upon moderate activation, they undergo a transformation in morphology inducing beneficial responses. However, upon strong stimulation, they mediate neuronal damage via production of pro-inflammatory cytokines. The inhibition of this cascade is considered an effective strategy for neuroinflammation-associated disorder therapy. During this pathological activation microglia also undergo a shortening of process length which contributes to the pathogenesis of such disorders. Thus, microglial plasticity should be considered to have two components: one is the production of inflammatory mediators, and the other is the dynamic changes in their processes. The former role has been well-documented in previous studies, while the latter one remains largely unknown. Recently, we and others have reported that the elongation of microglial process is associated with the transformation of microglia from a pro-inflammatory to an anti-inflammatory state, suggesting that the shortening of process length would make the microglia lose their ability to restrict pathological injury, while the elongation of microglial process would help attenuate neuroinflammation. Compared with the traditional anti-neuroinflammatory strategy, stimulating elongation of microglial process not only reduces the production of pro-inflammatory cytokines, but restores the ability of microglia to scan their surrounding environments, thus rendering their homeostasis regulation more effective. In this review, we provide a discussion of the factors that regulate microglial process elongation in vitro and in vivo, aiming to further drive the understanding of microglial process plasticity.

Isorhamnetin alleviates lipopolysaccharide-induced inflammatory responses in BV2 microglia by inactivating NF-κB, blocking the TLR4 pathway and reducing ROS generation

Isorhamnetin, which is a flavonoid predominantly found in fruits and leaves of various plants, including Hippophae rhamnoides L. and Oenanthe javanica (Blume) DC, is known to possess various pharmacological effects. However, the anti‑inflammatory potential of isorhamnetin remains poorly studied. Therefore, the present study aimed to investigate the inhibitory potential of isorhamnetin against inflammatory responses in lipopolysaccharide (LPS)‑stimulated BV2 microglia. To measure the effects of isorhamnetin on inflammatory mediators and cytokines, and reactive oxygen species (ROS) generation, the following methods were used: cell viability assay, griess assay, ELISA, reverse transcriptase‑polymerase chain reaction, flow cytometry, western blotting and immunofluorescence staining. The results revealed that isorhamnetin significantly suppressed LPS‑induced secretion of pro‑inflammatory mediators, including nitric oxide (NO) and prostaglandin E2, without exhibiting significant cytotoxicity. Consistent with these results, isorhamnetin inhibited LPS‑stimulated expression of regulatory enzymes, including inducible NO synthase and cyclooxygenase‑2 in BV2 cells. Isorhamnetin also downregulated LPS‑induced production and expression of pro‑inflammatory cytokines, such as tumor necrosis factor‑α and interleukin‑1β. The mechanism underlying the anti‑inflammatory effects of isorhamnetin was subsequently evaluated; this flavonoid inhibited the nuclear factor (NF)‑κB signaling pathway by disrupting degradation and phosphorylation of inhibitor κB‑α in the cytoplasm and blocking translocation of NF‑κB p65 into the nucleus. In addition, isorhamnetin effectively suppressed LPS‑induced expression of Toll‑like receptor 4 (TLR4) and myeloid differentiation factor 88. It also suppressed the binding of LPS with TLR4 in BV2 cells. Furthermore, isorhamnetin markedly reduced LPS‑induced generation of ROS in BV2 cells, thus indicating a strong antioxidative effect. Collectively, these results suggested that isorhamnetin may suppress LPS‑mediated inflammatory action in BV2 microglia through inactivating the NF‑κB signaling pathway, antagonizing TLR4 and eliminating ROS accumulation. Further studies are required to fully understand the anti‑inflammatory effects associated with the antioxidant capacity of isorhamnetin; however, the findings of the present study suggested that isorhamnetin may have potential benefits in inhibiting the onset and treatment of neuroinflammatory diseases.

Metabotropic Glutamate Receptors in Glial Cells: A New Potential Target for Neuroprotection?

Neurodegenerative disorders are characterized by excitotoxicity and neuroinflammation that finally lead to slow neuronal degeneration and death. Although neurons are the principal target, glial cells are important players as they contribute by either exacerbating or dampening the events that lead to neuroinflammation and neuronal damage. A dysfunction of the glutamatergic system is a common event in the pathophysiology of these diseases. Metabotropic glutamate (mGlu) receptors belong to a large family of G protein-coupled receptors largely expressed in neurons as well as in glial cells. They often appear overexpressed in areas involved in neurodegeneration, where they can modulate glutamatergic transmission. Of note, mGlu receptor upregulation may involve microglia or, even more frequently, astrocytes, where their activation causes release of factors potentially able to influence neuronal death. The expression of mGlu receptors has been also reported on oligodendrocytes, a glial cell type specifically involved in the development of multiple sclerosis. Here we will provide a general overview on the possible involvement of mGlu receptors expressed on glial cells in the pathogenesis of different neurodegenerative disorders and the potential use of subtype-selective mGlu receptor ligands as candidate drugs for the treatment of neurodegenerative disorders. Negative allosteric modulators (NAM) of mGlu5 receptors might represent a relevant pharmacological tool to develop new neuroprotective strategies in these diseases. Recent evidence suggests that targeting astrocytes and microglia with positive allosteric modulators (PAM) of mGlu3 receptor or oligodendrocytes with mGlu4 PAMS might represent novel pharmacological approaches for the treatment of neurodegenerative disorders.

RNA-Seq transcriptomic profiling of primary murine microglia treated with LPS or LPS + IFNγ

Microglia, the main resident immune cells in the CNS, are thought to participate in the pathogenesis of various neurological disorders. LPS and LPS + IFNγ are stimuli that are widely used to activate microglia. However, the transcriptomic profiles of microglia treated with LPS and LPS + IFNγ have not been properly compared. Here, we treated murine primary microglial cultures with LPS or LPS + IFNγ for 6 hours and then performed RNA-Sequencing. Gene expression patterns induced by the treatments were obtained by WGCNA and 11 different expression profiles were found, showing differential responses to LPS and LPS + IFNγ in many genes. Interestingly, a subset of genes involved in Parkinson’s, Alzheimer’s and Huntington’s disease were downregulated by both treatments. By DESeq analysis we found differentially upregulated and downregulated genes that confirmed LPS and LPS + IFNγ as inducers of microglial pro-inflammatory responses, but also highlighted their involvement in specific cell functions. In response to LPS, microglia tended to be more proliferative, pro-inflammatory and phagocytic; whereas LPS + IFNγ inhibited genes were involved in pain, cell division and, unexpectedly, production of some inflammatory mediators. In summary, this study provides a detailed description of the transcriptome of LPS- and LPS + IFNγ treated primary microglial cultures. It may be useful to determine whether these in vitro phenotypes resemble microglia in in vivo pathological conditions.

Distinct microglia profile in Creutzfeldt-Jakob disease and Alzheimer's disease is independent of disease kinetics

Activated microglia represent a common pathological feature of neurodegenerative diseases. Sporadic Creutzfeldt-Jakob disease (sCJD) patients show more pronounced microglial activation than Alzheimer's disease (AD) patients. Whether these differences are due to differences in disease kinetics or represent disease-specific changes is unknown. We investigated microglial phenotypes in brains of rapidly progressive AD (rpAD) and sCJD patients matched for clinical presentation, including disease duration. We immunostained the frontal cortex, basal ganglia and cerebellum in 16 patients with rpAD and sCJD using antibodies against markers of microglia and recruited monocytes (ionized calcium-binding adaptor molecule 1, human leukocyte antigen DPQR, Cluster of Differentiation 68), an antibody unique to brain-resident microglia (transmembrane protein 119 (TMEM119)), in addition to antibodies against a marker of astrocytes (glial fibrillary acidic protein), amyloid-β (Aβ) and pathological prion protein. rpAD patients showed a distinct microglial phenotype with a high abundance of TMEM119-positive microglia in all investigated regions. Presence of Aβ deposits seen in a sCJD patient with concomitant deposition of Aβ led to increase of TMEM119-positive microglia. Our data suggest that in rpAD, activation of brain-resident microglia significantly contributes to microgliosis, whereas in sCJD the TMEM119 signature of resident microglial cells is barely detectable. This is irrespective of disease duration and may indicate disease-specific microglial reaction.

Diallyl Trisulfide Suppresses the Production of Lipopolysaccharide-induced Inflammatory Mediators in BV2 Microglia by Decreasing the NF-κB Pathway Activity Associated With Toll-like Receptor 4 and CXCL12/CXCR4 Pathway Blockade

Background

Diallyl trisulfide (DATS), a garlic-derived organosulfuric compound, has been documented for potential anti-inflammatory effects. However, the mechanism in microglia remains unknown. In this study, we investigated the anti-inflammatory effects of DATS in lipopolysaccharide (LPS)-stimulated BV2 microglial cells.

Methods

The effects of DATS on LPS-induced pro-inflammatory mediators such as nitric oxide (NO) and prostaglandin E₂ (PGE₂) were assessed under conditions not in the cytotoxicity of DATS. The protein expression of inflammation regulatory genes was measured by Western blot analysis.

Results

DATS significantly inhibited the LPS-induced secretion of NO and PGE₂, which was associated with the suppression of their regulatory genes, inducible NO synthase and COX-2. DATS had been shown to inhibit nuclear translocation of NF-κB by destroying the degradation and phosphorylation of IκB-α inhibitors in the cytoplasm. In addition, DATS effectively inhibited the expression of LPS-induced toll-like receptor 4 (TLR4) and myeloid differentiation factor 88. Furthermore, DATS markedly reduced the LPS-induced expression of chemokine (CXC motif) ligand (CXCL) 12 and CXC receptor (CXCR) 4, demonstrating its capacity to block chemo-attractive activity.

Conclusions

These results indicate that DATS inhibits the activation of the CXCL12/CXCR4 axis associated with antagonizing effect on TLR4 and blocks NF-κB signaling, thus demonstrating anti-inflammatory effects against LPS stimulation.

Environmentally enriched pigs have transcriptional profiles consistent with neuroprotective effects and reduced microglial activity

Environmental enrichment (EE) is widely used to study the effects of external factors on brain development, function and health in rodent models, but very little is known of the effects of EE on the brain in a large animal model such as the pig. Twenty-four young pigs (aged 5 weeks at start of study, 1:1 male: female ratio) were housed in environmentally enriched (EE) pens and provided with additional enrichment stimulation (a bag filled with straw) once daily. Litter, weight and sex matched controls n= (24) were housed in barren (B) conditions. Behaviour was recorded on alternate days from study day 10. After 21 days, RNA-sequencing of the frontal cortex of male piglets culled one hour after the enrichment stimulation, but not those at 4 h after stimulation, showed upregulation of genes involved in neuronal activity and synaptic plasticity in the EE compared to the B condition. This result is mirrored in the behavioural response to the stimulation which showed a peak in activity around the 1 h time-point. By contrast, EE piglets displayed a signature consistent with a relative decrease in microglial activity compared to those in the B condition. These results confirm those from rodents, suggesting that EE may also confer neuronal health benefits in large mammal models, through a potential relative reduction in neuroinflammatory process and increase in neuroprotection driven by an enrichment-induced increase in behavioural activity.