Never-resting microglia: physiological roles in the healthy brain and pathological implications

MorphoGlia, an interactive method to identify and map microglia morphologies, demonstrates differences in hippocampal subregions of an Alzheimer's disease mouse model

Microglia are dynamic central nervous system cells crucial for maintaining homeostasis and responding to neuroinflammation, as evidenced by their varied morphologies. Existing morphology analysis often fails to detect subtle variations within the full spectrum of microglial morphologies due to their reliance on predefined categories. Here, we present MorphoGlia, an interactive, user-friendly pipeline that objectively characterizes microglial morphologies. MorphoGlia employs a machine learning ensemble to select relevant morphological features of microglia cells, perform dimensionality reduction, cluster these features, and subsequently map the clustered cells back onto the tissue, providing a spatial context for the identified microglial morphologies. We applied this pipeline to compare the responses between saline solution (SS) and scopolamine (SCOP) groups in a SCOP-induced mouse model of Alzheimer's disease, with a specific focus on the hippocampal subregions CA1 and Hilus. Next, we assessed microglial morphologies across four groups: SS-CA1, SCOP-CA1, SS-Hilus, and SCOP-Hilus. The results demonstrated that MorphoGlia effectively differentiated between SS and SCOP-treated groups, identifying distinct clusters of microglial morphologies commonly associated with pro-inflammatory states in the SCOP groups. Additionally, MorphoGlia enabled spatial mapping of these clusters, identifying the most affected hippocampal layers. This study highlights MorphoGlia's capability to provide unbiased analysis and clustering of microglial morphological states, making it a valuable tool for exploring microglial heterogeneity and its implications for central nervous system pathologies.

Potential of Neuroinflammation-Modulating Strategies in Tuberculous Meningitis: Targeting Microglia

Tuberculous meningitis (TBM) is the most severe complication of tuberculosis (TB) and is associated with high rates of disability and mortality. Mycobacterium tuberculosis (M. tb), the infectious agent of TB, disseminates from the respiratory epithelium, breaks through the blood-brain barrier, and establishes a primary infection in the meninges. Microglia are the core of the immune network in the central nervous system (CNS) and interact with glial cells and neurons to fight against harmful pathogens and maintain homeostasis in the brain through pleiotropic functions. However, M. tb directly infects microglia and resides in them as the primary host for bacillus infections. Largely, microglial activation slows disease progression. The non-productive inflammatory response that initiates the secretion of pro-inflammatory cytokines and chemokines may be neurotoxic and aggravate tissue injuries based on damages caused by M. tb. Host-directed therapy (HDT) is an emerging strategy for modulating host immune responses against diverse diseases. Recent studies have shown that HDT can control neuroinflammation in TBM and act as an adjunct therapy to antibiotic treatment. In this review, we discuss the diverse roles of microglia in TBM and potential host-directed TB therapies that target microglia to treat TBM. We also discuss the limitations of applying each HDT and suggest a course of action for the near future.

Interactions between CNS and immune cells in tuberculous meningitis

The central nervous system (CNS) harbors its own special immune system composed of microglia in the parenchyma, CNS-associated macrophages (CAMs), dendritic cells, monocytes, and the barrier systems within the brain. Recently, advances in the immune cells in the CNS provided new insights to understand the development of tuberculous meningitis (TBM), which is the predominant form of Mycobacterium tuberculosis (M.tb) infection in the CNS and accompanied with high mortality and disability. The development of the CNS requires the protection of immune cells, including macrophages and microglia, during embryogenesis to ensure the accurate development of the CNS and immune response following pathogenic invasion. In this review, we summarize the current understanding on the CNS immune cells during the initiation and development of the TBM. We also explore the interactions of immune cells with the CNS in TBM. In the future, the combination of modern techniques should be applied to explore the role of immune cells of CNS in TBM.

Morphology and Fractal-Based Classifications of Neurons and Microglia in Two and Three Dimensions

Microglia and neurons live physically intertwined, intimately related structurally and functionally in a dynamic relationship in which microglia change continuously over a much shorter timescale than do neurons. Although microglia may unwind and depart from the neurons they attend under certain circumstances, in general, together both contribute to the fractal topology of the brain that defines its computational capabilities. Both neuronal and microglial morphologies are well-described using fractal analysis complementary to more traditional measures. For neurons, the fractal dimension has proved valuable for classifying dendritic branching and other neuronal features relevant to pathology and development. For microglia, fractal geometry has substantially contributed to classifying functional categories, where, in general, the more pathological the biological status, the lower the fractal dimension for individual cells, with some exceptions, including hyper-ramification. This chapter provides a review of the intimate relationships between neurons and microglia, by introducing 2D and 3D fractal analysis methodology and its applications in neuron-microglia function in health and disease.

Box-Counting Fractal Analysis: A Primer for the Clinician

This chapter lays out the elementary principles of fractal geometry underpinning much of the rest of this book. It assumes a minimal mathematical background, defines the key principles and terms in context, and outlines the basics of a fractal analysis method known as box counting and how it is used to perform fractal, lacunarity, and multifractal analyses. As a standalone reference, this chapter grounds the reader to be able to understand, evaluate, and apply essential methods to appreciate and heal the exquisitely detailed fractal geometry of the brain.

Microglia and Astrocytes in Amyotrophic Lateral Sclerosis: Disease-Associated States, Pathological Roles, and Therapeutic Potential

Microglial and astrocytic reactivity is a prominent feature of amyotrophic lateral sclerosis (ALS). Microglia and astrocytes have been increasingly appreciated to play pivotal roles in disease pathogenesis. These cells can adopt distinct states characterized by a specific molecular profile or function depending on the different contexts of development, health, aging, and disease. Accumulating evidence from ALS rodent and cell models has demonstrated neuroprotective and neurotoxic functions from microglia and astrocytes. In this review, we focused on the recent advancements of knowledge in microglial and astrocytic states and nomenclature, the landmark discoveries demonstrating a clear contribution of microglia and astrocytes to ALS pathogenesis, and novel therapeutic candidates leveraging these cells that are currently undergoing clinical trials.

Could theropod dinosaurs have evolved to a human level of intelligence?

Noting that some theropod dinosaurs had large brains, large grasping hands, and likely binocular vision, paleontologist Dale Russell suggested that a branch of these dinosaurs might have evolved to a human intelligence level, had dinosaurs not become extinct. I offer reasons why the likely pallial organization in dinosaurs would have made this improbable, based on four assumptions. First, it is assumed that achieving human intelligence requires evolving an equivalent of the about 200 functionally specialized cortical areas characteristic of humans. Second, it is assumed that dinosaurs had an avian nuclear type of pallial organization, in contrast to the mammalian cortical organization. Third, it is assumed that the interactions between the different neuron types making up an information processing unit within pallium are critical to its role in analyzing information. Finally, it is assumed that increasing axonal length between the neuron sets carrying out this operation impairs its efficacy. Based on these assumptions, I present two main reasons why dinosaur pallium might have been unable to add the equivalent of 200 efficiently functioning cortical areas. First, a nuclear pattern of pallial organization would require increasing distances between the neuron groups corresponding to the separate layers of any given mammalian cortical area, as more sets of nuclei equivalent to a cortical area are interposed between the existing sets, increasing axon length and thereby impairing processing efficiency. Second, because of its nuclear organization, dinosaur pallium could not reduce axon length by folding to bring adjacent areas closer together, as occurs in cerebral cortex.

The multifaceted roles of embryonic microglia in the developing brain

Microglia are the resident immune cells of the central nervous system (CNS). Microglia originate from erythromyeloid progenitors in the yolk sac at the early embryonic stage, and these progenitors then colonize the CNS through extensive migration and proliferation during development. Microglia account for 10% of all cells in the adult brain, whereas the proportion of these cells in the embryonic brain is only 0.5-1.0%. Nevertheless, microglia in the developing brain widely move their cell body within the structure by extending filopodia; thus, they can interact with surrounding cells, such as neural lineage cells and vascular-structure-composing cells. This active microglial motility suggests that embryonic microglia play a pivotal role in brain development. Indeed, recent increasing evidence has revealed diverse microglial functions at the embryonic stage. For example, microglia control differentiation of neural stem cells, regulate the population size of neural progenitors and modulate the positioning and function of neurons. Moreover, microglia exert functions not only on neural lineage cells but also on blood vessels, such as supporting vascular formation and integrity. This review summarizes recent advances in the understanding of microglial cellular dynamics and multifaceted functions in the developing brain, with particular focus on the embryonic stage, and discusses the fundamental molecular mechanisms underlying their behavior.

A mathematical model and experimental procedure to analyze the cognitive effects of audio frequency magnetic fields

Audio frequency magnetic fields (20 Hz-20 kHz) are magnetic fields in extremely low frequency-very low frequency (ELF-VLF) bands that are present near audio equipment and acoustic transducers. These devices transform and operate the electrical signal from the recordings or other devices into acoustic and audio signals. The cognitive influence of sound and noise has been widely studied and recognized since the times of ancient Rome; however, the cognitive effects of the magnetic fields of these frequencies have not been studied. Due to the extensive use of audio devices that use this type of transducer near the temporal-parietal area, we believe that it is of interest to study their impact on short-term memory or working memory (WM) and to analyze their potential as they operate as a transcranial magnetic stimulation. In this study, a mathematical model and an experimental tool are introduced to analyze memory performance. The model dissociates the reaction time of a cognitive task. We analyze the model in data from a group of 65 young, healthy subjects. WM is assessed in our experimental setup by means of the Sternberg test (ST), whereby during the ST, one subgroup was exposed to an audio frequency magnetic stimulus, and the other subgroup received a sham stimulus. The magnetic stimulus was ~0.1 μT and was applied to both sides of the head at the frontal cortex near the temporal-parietal area, which is where WM is expected to be located. The ST records reaction times when determining whether an object displayed on the computer screen is one of the objects to be remembered. The results are analyzed within the mathematical model and changes are observed, including the deterioration of WM, which could affect 32% of its operability.

Microglial diversity along the hippocampal longitudinal axis impacts synaptic plasticity in adult male mice under homeostatic conditions

The hippocampus is a plastic brain area that shows functional segregation along its longitudinal axis, reflected by a higher level of long-term potentiation (LTP) in the CA1 region of the dorsal hippocampus (DH) compared to the ventral hippocampus (VH), but the mechanisms underlying this difference remain elusive. Numerous studies have highlighted the importance of microglia-neuronal communication in modulating synaptic transmission and hippocampal plasticity, although its role in physiological contexts is still largely unknown. We characterized in depth the features of microglia in the two hippocampal poles and investigated their contribution to CA1 plasticity under physiological conditions. We unveiled the influence of microglia in differentially modulating the amplitude of LTP in the DH and VH, showing that minocycline or PLX5622 treatment reduced LTP amplitude in the DH, while increasing it in the VH. This was recapitulated in Cx3cr1 knockout mice, indicating that microglia have a key role in setting the conditions for plasticity processes in a region-specific manner, and that the CX3CL1-CX3CR1 pathway is a key element in determining the basal level of CA1 LTP in the two regions. The observed LTP differences at the two poles were associated with transcriptional changes in the expression of genes encoding for Il-1, Tnf-α, Il-6, and Bdnf, essential players of neuronal plasticity. Furthermore, microglia in the CA1 SR region showed an increase in soma and a more extensive arborization, an increased prevalence of immature lysosomes accompanied by an elevation in mRNA expression of phagocytic markers Mertk and Cd68 and a surge in the expression of microglial outward K⁺ currents in the VH compared to DH, suggesting a distinct basal phenotypic state of microglia across the two hippocampal poles. Overall, we characterized the molecular, morphological, ultrastructural, and functional profile of microglia at the two poles, suggesting that modifications in hippocampal subregions related to different microglial statuses can contribute to dissect the phenotypical aspects of many diseases in which microglia are known to be involved.

L-Type Ca2+ Channel Inhibition Rescues the LPS-Induced Neuroinflammatory Response and Impairments in Spatial Memory and Dendritic Spine Formation

Ca2+ signaling is implicated in the transition between microglial surveillance and activation. Several L-type Ca2+ channel blockers (CCBs) have been shown to ameliorate neuroinflammation by modulating microglial activity. In this study, we examined the effects of the L-type CCB felodipine on LPS-mediated proinflammatory responses. We found that felodipine treatment significantly diminished LPS-evoked proinflammatory cytokine levels in BV2 microglial cells in an L-type Ca2+ channel-dependent manner. In addition, felodipine leads to the inhibition of TLR4/AKT/STAT3 signaling in BV2 microglial cells. We further examined the effects of felodipine on LPS-stimulated neuroinflammation in vivo and found that daily administration (3 or 7 days, i.p.) significantly reduced LPS-mediated gliosis and COX-2 and IL-1β levels in C57BL/6 (wild-type) mice. Moreover, felodipine administration significantly reduced chronic neuroinflammation-induced spatial memory impairment, dendritic spine number, and microgliosis in C57BL/6 mice. Taken together, our results suggest that the L-type CCB felodipine could be repurposed for the treatment of neuroinflammation/cognitive function-associated diseases.

Glia from the central and peripheral nervous system are differentially affected by paclitaxel chemotherapy via modulating their neuroinflammatory and neuroregenerative properties

Glia are critical players in defining synaptic contacts and maintaining neuronal homeostasis. Both astrocytes as glia of the central nervous system (CNS), as well as satellite glial cells (SGC) as glia of the peripheral nervous system (PNS), intimately interact with microglia, especially under pathological conditions when glia regulate degenerative as well as regenerative processes. The chemotherapeutic agent paclitaxel evokes peripheral neuropathy and cognitive deficits; however, the mechanisms underlying these diverse clinical side effects are unclear. We aimed to elucidate the direct effects of paclitaxel on the function of astrocytes, microglia, and SGCs, and their glia-glia and neuronal-glia interactions. After intravenous application, paclitaxel was present in the dorsal root ganglia of the PNS and the CNS of rodents. In vitro, SGC enhanced the expression of pro-inflammatory factors and reduced the expression of neurotrophic factor NT-3 upon exposure to paclitaxel, resulting in predominantly neurotoxic effects. Likewise, paclitaxel induced a switch towards a pro-inflammatory phenotype in microglia, exerting neurotoxicity. In contrast, astrocytes expressed neuroprotective markers and increasingly expressed S100A10 after paclitaxel exposure. Astrocytes, and to a lesser extent SGCs, had regulatory effects on microglia independent of paclitaxel exposure. Data suggest that paclitaxel differentially modulates glia cells regarding their (neuro-) inflammatory and (neuro-) regenerative properties and also affects their interaction. By elucidating those processes, our data contribute to the understanding of the mechanistic pathways of paclitaxel-induced side effects in CNS and PNS.

Microglia states and nomenclature: A field at its crossroads

Microglial research has advanced considerably in recent decades yet has been constrained by a rolling series of dichotomies such as "resting versus activated" and "M1 versus M2." This dualistic classification of good or bad microglia is inconsistent with the wide repertoire of microglial states and functions in development, plasticity, aging, and diseases that were elucidated in recent years. New designations continuously arising in an attempt to describe the different microglial states, notably defined using transcriptomics and proteomics, may easily lead to a misleading, although unintentional, coupling of categories and functions. To address these issues, we assembled a group of multidisciplinary experts to discuss our current understanding of microglial states as a dynamic concept and the importance of addressing microglial function. Here, we provide a conceptual framework and recommendations on the use of microglial nomenclature for researchers, reviewers, and editors, which will serve as the foundations for a future white paper.

Voltage-Gated Ion Channels and the Variability in Information Transfer

The prerequisites for neurons to function within a circuit and be able to contain and transfer information efficiently and reliably are that they need to be homeostatically stable and fire within a reasonable range, characteristics that are governed, among others, by voltage-gated ion channels (VGICs). Nonetheless, neurons entail large variability in the expression levels of VGICs and their corresponding intrinsic properties, but the role of this variability in information transfer is not fully known. In this study, we aimed to investigate how this variability of VGICs affects information transfer. For this, we used a previously derived population of neuronal model neurons, each with the variable expression of five types of VGICs, fast Na+, delayed rectifier K+, A-type K+, T-type Ca++, and HCN channels. These analyses showed that the model neurons displayed variability in mutual information transfer, measured as the capability of neurons to successfully encode incoming synaptic information in output firing frequencies. Likewise, variability in the expression of VGICs caused variability in EPSPs and IPSPs amplitudes, reflected in the variability of output firing frequencies. Finally, using the virtual knockout methodology, we show that among the ion channels tested, the A-type K+ channel is the major regulator of information processing and transfer.

Galectin-3 Involvement in Cognitive Processes for New Therapeutic Considerations

Cognitive impairment may be a consequence of the normal aging process, but it may also be the hallmark of various neurodegenerative and psychiatric diseases. Early identification of individuals at particular risk for cognitive decline is critical, as it is imperative to maintain a cognitive reserve in these neuropsychiatric entities. In recent years, galectin-3 (Gal-3), a member of the galectin family, has received considerable attention with respect to aspects of neuroinflammation and neurodegeneration. The mechanisms behind the putative relationship between Gal-3 and cognitive impairment are not yet clear. Intrigued by this versatile molecule and its unique modular architecture, the latest data on this relationship are presented here. This mini-review summarizes recent findings on the mechanisms by which Gal-3 affects cognitive functioning in both animal and human models. Particular emphasis is placed on the role of Gal-3 in modulating the inflammatory response as a fine-tuner of microglia morphology and phenotype. A review of recent literature on the utility of Gal-3 as a biomarker is provided, and approaches to strategically exploit Gal-3 activities with therapeutic intentions in neuropsychiatric diseases are outlined.

Abnormal innate and learned behavior induced by neuron-microglia miscommunication is related to CA3 reconfiguration

Neuron-microglia communication through the Cx3cr1-Cx3cl1 axis is essential for the development and refinement of neural circuits, which determine their function into adulthood. In the present work we set out to extend the behavioral characterization of Cx3cr1^-/- mice evaluating innate behaviors and spatial navigation, both dependent on hippocampal function. Our results show that Cx3cr1-deficient mice, which show some changes in microglial and synaptic terminals morphology and density, exhibit alterations in activities of daily living and in the rapid encoding of novel spatial information that, nonetheless, improves with training. A neural substrate for these cognitive deficiencies was found in the form of synaptic dysfunction in the CA3 region of the hippocampus, with a marked impact on the mossy fiber (MF) pathway. A network analysis of the CA3 microcircuit reveals the effect of these synaptic alterations on the functional connectivity among CA3 neurons with diminished strength and topological reorganization in Cx3cr1-deficient mice. Neonatal population activity of the CA3 region in Cx3cr1-deficient mice shows a marked reorganization around the giant depolarizing potentials, the first form of network-driven activity of the hippocampus, suggesting that alterations found in adult subjects arise early on in postnatal development, a critical period of microglia-dependent neural circuit refinement. Our results show that interruption of the Cx3cr1-Cx3cl1/neuron-microglia axis leads to changes in CA3 configuration that affect innate and learned behaviors.

Microglia and their LAG3 checkpoint underlie the antidepressant and neurogenesis-enhancing effects of electroconvulsive stimulation

Despite evidence implicating microglia in the etiology and pathophysiology of major depression, there is paucity of information regarding the contribution of microglia-dependent molecular pathways to antidepressant procedures. In this study, we investigated the role of microglia in a mouse model of depression (chronic unpredictable stress-CUS) and its reversal by electroconvulsive stimulation (ECS), by examining the effects of microglia depletion with the colony stimulating factor-1 antagonist PLX5622. Microglia depletion did not change basal behavioral measures or the responsiveness to CUS, but it completely abrogated the therapeutic effects of ECS on depressive-like behavior and neurogenesis impairment. Treatment with the microglia inhibitor minocycline concurrently with ECS also diminished the antidepressant and pro-neurogenesis effects of ECS. Hippocampal RNA-Seq analysis revealed that ECS significantly increased the expression of genes related to neurogenesis and dopamine signaling, while reducing the expression of several immune checkpoint genes, particularly lymphocyte-activating gene-3 (Lag3), which was the only microglial transcript significantly altered by ECS. None of these molecular changes occurred in microglia-depleted mice. Immunohistochemical analyses showed that ECS reversed the CUS-induced changes in microglial morphology and elevation in microglial LAG3 receptor expression. Consistently, either acute or chronic systemic administration of a LAG3 monoclonal antibody, which readily penetrated into the brain parenchyma and was found to serve as a direct checkpoint blocker in BV2 microglia cultures, rapidly rescued the CUS-induced microglial alterations, depressive-like symptoms, and neurogenesis impairment. These findings suggest that brain microglial LAG3 represents a promising target for novel antidepressant therapeutics.

The role of the mucin-glycan foraging Ruminococcus gnavus in the communication between the gut and the brain

Ruminococcus gnavus is a prevalent member of the human gut microbiota, which is over-represented in inflammatory bowel disease and neurological disorders. We previously showed that the ability of R. gnavus to forage on mucins is strain-dependent and associated with sialic acid metabolism. Here, we showed that mice monocolonized with R. gnavus ATCC 29149 (Rg-mice) display changes in major sialic acid derivatives in their cecum content, blood, and brain, which is accompanied by a significant decrease in the percentage of sialylated residues in intestinal mucins relative to germ-free (GF) mice. Changes in metabolites associated with brain function such as tryptamine, indolacetate, and trimethylamine N-oxide were also detected in the cecal content of Rg-mice when compared to GF mice. Next, we investigated the effect of R. gnavus monocolonization on hippocampus cell proliferation and behavior. We observed a significant decrease of PSA-NCAM immunoreactive granule cells in the dentate gyrus (DG) of Rg-mice as compared to GF mice and recruitment of phagocytic microglia in the vicinity. Behavioral assessments suggested an improvement of the spatial working memory in Rg-mice but no change in other cognitive functions. These results were also supported by a significant upregulation of genes involved in proliferation and neuroplasticity. Collectively, these data provide first insights into how R. gnavus metabolites may influence brain regulation and function through modulation of granule cell development and synaptic plasticity in the adult hippocampus. This work has implications for further understanding the mechanisms underpinning the role of R. gnavus in neurological disorders.

Complement C3 mediates early hippocampal neurodegeneration and memory impairment in experimental multiple sclerosis

Memory impairment is one of the disabling manifestations of multiple sclerosis (MS) possibly present from the early stages of the disease and for which there is no specific treatment. Hippocampal synaptic dysfunction and dendritic loss, associated with microglial activation, can underlie memory deficits, yet the molecular mechanisms driving such hippocampal neurodegeneration need to be elucidated. In early-stage experimental autoimmune encephalomyelitis (EAE) female mice, we assessed the expression level of molecules involved in microglia-neuron interactions within the dentate gyrus and found overexpression of genes of the complement pathway. Compared to sham immunized mice, the central element of the complement cascade, C3, showed the strongest and 10-fold upregulation, while there was no increase of downstream factors such as the terminal component C5. The combination of in situ hybridization with immunofluorescence showed that C3 transcripts were essentially produced by activated microglia. Pharmacological inhibition of C3 activity, by daily administration of rosmarinic acid, was sufficient to prevent early dendritic loss, microglia-mediated phagocytosis of synapses in the dentate gyrus, and memory impairment in EAE mice, while morphological markers of microglial activation were still observed. In line, when EAE was induced in C3 deficient mice (C3KO), dendrites and spines of the dentate gyrus as well as memory abilities were preserved. Altogether, these data highlight the central role of microglial C3 in early hippocampal neurodegeneration and memory impairment in EAE and, therefore, pave the way toward new neuroprotective strategies in MS to prevent cognitive deficit using complement inhibitors.

Primed for addiction: A critical review of the role of microglia in the neurodevelopmental consequences of adolescent alcohol drinking

Alcohol is one of the most widely used recreational substances worldwide, with drinking frequently initiated during adolescence. The developmental state of the adolescent brain makes it vulnerable to initiating alcohol use, often in high doses, and particularly susceptible to alcohol-induced brain changes. Microglia, the brain parenchymal macrophages, have been implicated in mediating some of these effects, though the role that these cells play in the progression from alcohol drinking to dependence remains unclear. Microglia are uniquely positioned to sense and respond to central nervous system insult, and are now understood to exhibit innate immune memory, or "priming," altering their future functional responses based on prior exposures. In alcohol use disorders (AUDs), the role of microglia is debated. Whereas microglial activation can be pathogenic, contributing to neuroinflammation, tissue damage, and behavioral changes, or protective, it can also engage protective functions, providing support and mediating the resolution of damage. Understanding the role of microglia in adolescent AUDs is complicated by the fact that microglia are thought to be involved in developmental processes such as synaptic refinement and myelination, which underlie the functional maturation of multiple brain systems in adolescence. Thus, the role microglia play in the impact of alcohol use in adolescence is likely multifaceted. Long-term sequelae may be due to a failure to recover from EtOH-induced tissue damage, altered neurodevelopmental trajectories, and/or persistent changes to microglial responsivity and function. Here, we review critically the literature surrounding the effects of alcohol on microglia in models of adolescent alcohol misuse. We attempt to disentangle what is known about microglia from other neuroimmune effectors, to which we apply recent discoveries on the role of microglia in development and plasticity. Considered altogether, these studies challenge assumptions that proinflammatory microglia drive addiction. Alcohol priming microglia and thereby perturbing their homeostatic roles in neurodevelopment, especially during critical periods of plasticity such as adolescence, may have more serious implications for the neuropathogenesis of AUDs in adolescents.

Proneurogenic and neuroprotective effect of a multi strain probiotic mixture in a mouse model of acute inflammation: Involvement of the gut-brain axis

Neuroinflammation can severely affect brain homeostasis and adult hippocampal neurogenesis with detrimental effects on cognitive processes. Brain and gut are intimately connected via the "gut-brain axis", a bidirectional communication system, and the administration of live bacteria (probiotics) has been shown to represent an intriguing approach for the prevention or even the cure of several diseases. In the present study we evaluated the putative neuroprotective effect of 15-days consumption of a multi-strain probiotic formulation based on food-associated strains and human gut bacteria at the dose of 109 CFU/mouse/day in a mouse model of acute inflammation, induced by an intraperitoneal single injection of LPS (0.1 mg/kg) at the end of probiotic administration. The results indicate that the prolonged administration of the multi-strain probiotic formulation not only prevents the LPS-dependent increase of pro-inflammatory cytokines in specific regions of the brain (hippocampus and cortex) and in the gastrointestinal district but also triggers a potent proneurogenic response capable of enhancing hippocampal neurogenesis. This effect is accompanied by a potentiation of intestinal barrier, as documented by the increased epithelial junction expression in the colon. Our hypothesis is that pre-treatment with the multi-strain probiotic formulation helps to create a systemic protection able to counteract or alleviate the effects of LPS-dependent acute pro-inflammatory responses.

Morpho-functional variation and response pattern of microglia through rodent ontogeny showing infant microglia as stable and adaptive than matured

Microglia, myelo-monocytic lineage cells, that enter in the developing brain at early embryonic stages and integrate in CNS, are involved in almost all neuroinflammatory conditions. We studied how microglia change their responses through the development and maturation of brain in normal physiological conditions using an ex situ model to delineate their age-specific morpho-functional responsiveness. Rapidly isolated microglia from different age-matched rats were characterized with Iba1⁺ /CD11b/c⁺ /MHCclassII⁺ , cultured, studied for cell-cycle/proliferative potency, ROS generation and phagocytosis, viability and morphological analysis induced with GMCSF, MCSF, IL-4, IL-6, IL-10, and IFN-γ. The study showed marked differences in cellular properties, stability, and viability of microglia through ontogeny with specific patterns in their studied functions which were coherent with their in situ morpho-functional attributes. Phagocytic behavior showed a notable shift from ROS independence to dependence toward maturation. Perinatal microglia were found persistent in ex situ environment and neonatal microglia qualified as the most potent and versatile responders for morpho-functional variations under cytokine induced conditions. The study identified that microglia from infants were the most stable, adaptive, and better responders, which can perform as an ex situ model system to study microglial biology.

Distinct Features of Brain-Resident Macrophages: Microglia and Non-Parenchymal Brain Macrophages

Tissue-resident macrophages play an important role in maintaining tissue homeostasis and innate immune defense against invading microbial pathogens. Brain-resident macrophages can be classified into microglia in the brain parenchyma and non-parenchymal brain macrophages, also known as central nervous system-associated or border-associated macrophages, in the brain-circulation interface. Microglia and non-parenchymal brain macrophages, including meningeal, perivascular, and choroid plexus macrophages, are mostly produced during embryonic development, and maintained their population by self-renewal. Microglia have gained much attention for their dual roles in the maintenance of brain homeostasis and the induction of neuroinflammation. In particular, diverse phenotypes of microglia have been increasingly identified under pathological conditions. Single-cell phenotypic analysis revealed that microglia are highly heterogenous and plastic, thus it is difficult to define the status of microglia as M1/M2 or resting/activated state due to complex nature of microglia. Meanwhile, physiological function of non-parenchymal brain macrophages remain to be fully demonstrated. In this review, we have summarized the origin and signatures of brain-resident macrophages and discussed the unique features of microglia, particularly, their phenotypic polarization, diversity of subtypes, and inflammasome responses related to neurodegenerative diseases.

Palmitoylethanolamide: A Natural Compound for Health Management

All nations which have undergone a nutrition transition have experienced increased frequency and falling latency of chronic degenerative diseases, which are largely driven by chronic inflammatory stress. Dietary supplementation is a valid strategy to reduce the risk and severity of such disorders. Palmitoylethanolamide (PEA) is an endocannabinoid-like lipid mediator with extensively documented anti-inflammatory, analgesic, antimicrobial, immunomodulatory and neuroprotective effects. It is well tolerated and devoid of side effects in animals and humans. PEA's actions on multiple molecular targets while modulating multiple inflammatory mediators provide therapeutic benefits in many applications, including immunity, brain health, allergy, pain modulation, joint health, sleep and recovery. PEA's poor oral bioavailability, a major obstacle in early research, has been overcome by advanced delivery systems now licensed as food supplements. This review summarizes the functionality of PEA, supporting its use as an important dietary supplement for lifestyle management.

Targeting microglia L-type voltage-dependent calcium channels for the treatment of central nervous system disorders

Calcium (Ca2+ ) is a ubiquitous mediator of a multitude of cellular functions in the central nervous system (CNS). Intracellular Ca2+ is tightly regulated by cells, including entry via plasma membrane Ca2+ permeable channels. Of specific interest for this review are L-type voltage-dependent Ca2+ channels (L-VDCCs), due to their pleiotropic role in several CNS disorders. Currently, there are numerous approved drugs that target L-VDCCs, including dihydropyridines. These drugs are safe and effective for the treatment of humans with cardiovascular disease and may also confer neuroprotection. Here, we review the potential of L-VDCCs as a target for the treatment of CNS disorders with a focus on microglia L-VDCCs. Microglia, the resident immune cells of the brain, have attracted recent attention for their emerging inflammatory role in several CNS diseases. Intracellular Ca2+ regulates microglia transition from a resting quiescent state to an "activated" immune-effector state and is thus a valuable target for manipulation of microglia phenotype. We will review the literature on L-VDCC expression and function in the CNS and on microglia in vitro and in vivo and explore the therapeutic landscape of L-VDCC-targeting agents at present and future challenges in the context of Alzheimer's disease, Parkinson's disease, Huntington's disease, neuropsychiatric diseases, and other CNS disorders.

Microglia Demonstrate Local Mixed Inflammation and a Defined Morphological Shift in an APP/PS1 Mouse Model

Background: Microglia are traditionally described as the immune cells of the brain and have an inflammatory role in Alzheimer’s disease (AD). Microglial morphological and phenotypic shifts in AD have not been fully characterized; however, microglia are often described as either pro- or anti-inflammatory. Objective: To determine microglial if microglial morphology and phenotype changes with disease status. Methods: This study observed morphology through Iba1 immunohistochemistry on tissue sections encompassing the primary motor cortex and somatosensory barrel fields. Immunohistochemistry for pro-inflammatory markers: CD14 and CD40; and anti-inflammatory markers: CD16 and TREM2, was performed at 3, 6, and 12 months of age which correlated with pre-plaque, onset, and significant plaque load in APP/PS1 brains (n = 6) and compared to age-matched littermate controls (n = 6). Results: Microglia demonstrated a defined morphological shift with time. Deramified morphologies increased in the APP/PS1, at both 6 months (p < 0.0001) and 12 months (p < 0.0001). At 12 months, there were significantly lower numbers of ramified microglia (p < 0.001). Results indicated that microglia have a heterogenic marker immunoreactivity as CD16, TREM2, and CD40 were associated with an activated morphology at the same time points. All inflammatory markers were significantly upregulated at 12 months in the APP/PS1 mice (TREM2 (F (2,30)  = 10.75, p = 0.0003), CD40 (F (2,30)  = 15.86, p < 0.0001), CD14 (F (2,30)  = 6.84, p = 0.0036), and CD16 (F (2,30)  = 3.026, p = 0.0635)). Conclusion: Our data indicate that pro- and anti-inflammatory factors of microglia occur in APP/PS1 mice.

A role for the orphan nuclear receptor TLX in the interaction between neural precursor cells and microglia

Microglia are an essential component of the neurogenic niche in the adult hippocampus and are involved in the control of neural precursor cell (NPC) proliferation, differentiation and the survival and integration of newborn neurons in hippocampal circuitry. Microglial and neuronal cross-talk is mediated in part by the chemokine fractalkine/chemokine (C-X3-C motif) ligand 1 (CX3CL1) released from neurons, and its receptor CX3C chemokine receptor 1 (CX3CR1) which is expressed on microglia. A disruption in this pathway has been associated with impaired neurogenesis yet the specific molecular mechanisms by which this interaction occurs remain unclear. The orphan nuclear receptor TLX (Nr2e1; homologue of the Drosophila tailless gene) is a key regulator of hippocampal neurogenesis, and we have shown that in its absence microglia exhibit a pro-inflammatory activation phenotype. However, it is unclear whether a disturbance in CX3CL1/CX3CR1 communication mediates an impairment in TLX-related pathways which may have subsequent effects on neurogenesis. To this end, we assessed miRNA expression of up- and down-stream signalling molecules of TLX in the hippocampus of mice lacking CX3CR1. Our results demonstrate that a lack of CX3CR1 is associated with altered expression of TLX and its downstream targets in the hippocampus without significantly affecting upstream regulators of TLX. Thus, TLX may be a potential participant in neural stem cell (NSC)-microglial cross-talk and may be an important target in understanding inflammatory-associated impairments in neurogenesis.

The impact of hyperpolarization-activated cyclic nucleotide-gated (HCN) and voltage-gated potassium KCNQ/Kv7 channels on primary microglia function

Background

Microglia are essential to maintain cell homeostasis in the healthy brain and are activated after brain injury. Upon activation, microglia polarize towards different phenotypes. The course of microglia activation is complex and depends on signals in the surrounding milieu. Recently, it has been suggested that microglia respond to ion currents, as a way of regulating their activity and function.

Methods and results

Under the hypothesis that HCN and KCNQ/Kv7 channels impact on microglia, we studied primary rat microglia in the presence or absence of specific pharmacological blockade or RNA silencing. Primary microglia expressed the subunits HCN1-4, Kv7.2, Kv7.3, and Kv7.5. The expression of HCN2, as well as Kv7.2 and Kv7.3, varied among different microglia phenotypes. The pharmacological blockade of HCN channels by ZD7288 resulted in cell depolarization with slowly rising intracellular calcium levels, leading to enhanced survival and reduced proliferation rates of resting microglia. Furthermore, ZD7288 treatment, as well as knockdown of HCN2 RNA by small interfering RNA, resulted in an attenuation of later microglia activation—both towards the anti- and pro-inflammatory phenotype. However, HCN channel inhibition enhanced the phagocytic capacity of IL4-stimulated microglia. Blockade of Kv7/KCNQ channel by XE-991 exclusively inhibited the migratory capacity of resting microglia.

Conclusion

These observations suggest that the HCN current contributes to various microglia functions and impacts on the course of microglia activation, while the Kv7/KCNQ channels affect microglia migration. Characterizing the role of HCN channels in microglial functioning may offer new therapeutic approaches for targeted modulation of neuroinflammation as a hallmark of various neurological disorders.

Microglial process convergence on axonal segments in health and disease

Microglia dynamically interact with neurons influencing the development, structure, and function of neuronal networks. Recent studies suggest microglia may also influence neuronal activity by physically interacting with axonal domains responsible for action potential initiation and propagation. However, the nature of these microglial process interactions is not well understood. Microglial-axonal contacts are present early in development and persist through adulthood, implicating microglial interactions in the regulation of axonal integrity in both the developing and mature central nervous system. Moreover, changes in microglial-axonal contact have been described in disease states such as multiple sclerosis (MS) and traumatic brain injury (TBI). Depending on the disease state, there are increased associations with specific axonal segments. In MS, there is enhanced contact with the axon initial segment and node of Ranvier, while, in TBI, microglia alter interactions with axons at the site of injury, as well as at the axon initial segment. In this article, we review the interactions of microglial processes with axonal segments, analyzing their associations with various axonal domains and how these interactions may differ between MS and TBI. Furthermore, we discuss potential functional consequences and molecular mechanisms of these interactions and how these may differ among various types of microglial-axonal interactions.

Neuroinflammation

Neuroinflammation is implicated in contributing to a variety of neurologic and somatic illnesses including Alzheimer's disease (AD), Parkinson's disease (PD), and depression. In this chapter, we focus on the role of neuroinflammation in mediating these three illnesses and portray interactions between the immune response and the central nervous system in the context of sex differences in disease progression. The majority of this chapter is supported by clinical findings; however, we occasionally utilize preclinical models where human studies are currently lacking. We begin by detailing the pathology of neuroinflammation, distinguishing between acute and chronic inflammation, and examining contributions from the innate and adaptive immune systems. Next, we summarize potential mechanisms of immune cell mediators including interleukin-1 beta (IL-1β), tumor necrosis factor α, and IL-6 in AD, PD, and depression development. Given the strong sex bias seen in these illnesses, we additionally examine the role of sex hormones, e.g., estrogen and testosterone in mediating neuroinflammation at the cellular level. Systematically, we detail how sex hormones may contribute to distinct behavioral and clinical symptoms and prognosis between males and females with AD, PD, or depression. Finally, we highlight the possible role of exercise in alleviating neuroinflammation, as well as evidence that antiinflammatory drug therapies improve cognitive symptoms observed in brain-related diseases.

Endogenous neural precursor cells in health and disease

Neurogenesis persists in the adult brain of mammals in the subventricular zone (SVZ) of the lateral ventricles and in the subgranular zone (SGZ) of the dentate gyrus (DG). The complex interactions between intrinsic and extrinsic signals provided by cells in the niche but also from distant sources regulate the fate of neural stem/progenitor cells (NPCs) in these sites. This fine regulation is perturbed in aging and in pathological conditions leading to a different NPC behavior, tailored to the specific physio-pathological features. Indeed, NPCs exert in physiological and pathological conditions important neurogenic and non-neurogenic regulatory functions and participate in maintaining and protecting brain tissue homeostasis. In this review, we discuss intrinsic and extrinsic signals that regulate NPC activation and NPC functional role in various homeostatic and non-homeostatic conditions.

Degeneracy in hippocampal physiology and plasticity

Degeneracy, defined as the ability of structurally disparate elements to perform analogous function, has largely been assessed from the perspective of maintaining robustness of physiology or plasticity. How does the framework of degeneracy assimilate into an encoding system where the ability to change is an essential ingredient for storing new incoming information? Could degeneracy maintain the balance between the apparently contradictory goals of the need to change for encoding and the need to resist change towards maintaining homeostasis? In this review, we explore these fundamental questions with the mammalian hippocampus as an example encoding system. We systematically catalog lines of evidence, spanning multiple scales of analysis that point to the expression of degeneracy in hippocampal physiology and plasticity. We assess the potential of degeneracy as a framework to achieve the conjoint goals of encoding and homeostasis without cross-interferences. We postulate that biological complexity, involving interactions among the numerous parameters spanning different scales of analysis, could establish disparate routes towards accomplishing these conjoint goals. These disparate routes then provide several degrees of freedom to the encoding-homeostasis system in accomplishing its tasks in an input- and state-dependent manner. Finally, the expression of degeneracy spanning multiple scales offers an ideal reconciliation to several outstanding controversies, through the recognition that the seemingly contradictory disparate observations are merely alternate routes that the system might recruit towards accomplishment of its goals.

In vivo imaging reveals transient microglia recruitment and functional recovery of photoreceptor signaling after injury

Microglia, the resident macrophages of the central nervous system, are critical for synaptic pruning and maintenance and for mitigating injury and neurodegeneration. Determining whether microglia–neuron interactions are beneficial in specific instances has been difficult, largely because of the local and transient nature of the interactions. Using simultaneous optical coherence tomography/scanning laser ophthalmoscopy (SLO) and adaptive optics SLO retinal imaging in mice, we show interactions of microglia and photoreceptors over time scales from seconds to months during injury, degeneration, and repair. In vivo optical assessment of photoreceptor signaling in a large neuronal field encompassing the injured area allows us to relate the time course of these microglia movements to that of the tissue remodeling and functional recovery.

Microglia respond to damage and microenvironmental changes within the central nervous system by morphologically transforming and migrating to the lesion, but the real-time behavior of populations of these resident immune cells and the neurons they support have seldom been observed simultaneously. Here, we have used in vivo high-resolution optical coherence tomography (OCT) and scanning laser ophthalmoscopy with and without adaptive optics to quantify the 3D distribution and dynamics of microglia in the living retina before and after local damage to photoreceptors. Following photoreceptor injury, microglia migrated both laterally and vertically through the retina over many hours, forming a tight cluster within the area of visible damage that resolved over 2 wk. In vivo OCT optophysiological assessment revealed that the photoreceptors occupying the damaged region lost all light-driven signaling during the period of microglia recruitment. Remarkably, photoreceptors recovered function to near-baseline levels after the microglia had departed the injury locus. These results demonstrate the spatiotemporal dynamics of microglia engagement and restoration of neuronal function during tissue remodeling and highlight the need for mechanistic studies that consider the temporal and structural dynamics of neuron–microglia interactions in vivo.

Environmental neurotoxicant-induced dopaminergic neurodegeneration: a potential link to impaired neuroinflammatory mechanisms

With the increased incidence of neurodegenerative diseases worldwide, Parkinson's disease (PD) represents the second-most common neurodegenerative disease. PD is a progressive multisystem neurodegenerative disorder characterized by a marked loss of nigrostriatal dopaminergic neurons and the formation of Lewy pathology in diverse brain regions. Although the mechanisms underlying dopaminergic neurodegeneration remain poorly characterized, data from animal models and postmortem studies have revealed that heightened inflammatory responses mediated via microglial and astroglial activation and the resultant release of proinflammatory factors may act as silent drivers of neurodegeneration. In recent years, numerous studies have demonstrated a positive association between the exposure to environmental neurotoxicants and the etiology of PD. Although it is unclear whether neuroinflammation drives pesticide-induced neurodegeneration, emerging evidence suggests that the failure to dampen neuroinflammatory mechanisms may account for the increased vulnerability to pesticide neurotoxicity. Furthermore, recent studies provide additional evidence that shifts the focus from a neuron-centric view to glial-associated neurodegeneration following pesticide exposure. In this review, we propose to summarize briefly the possible factors that regulate neuroinflammatory processes during environmental neurotoxicant exposure with a focus on the potential roles of mitochondria-driven redox mechanisms. In this context, a critical discussion of the data obtained from experimental research and possible epidemiological studies is included. Finally, we hope to provide insights on the pivotal role of exosome-mediated intercellular transmission of aggregated proteins in microglial activation response and the resultant dopaminergic neurodegeneration after exposure to pesticides. Collectively, an improved understanding of glia-mediated neuroinflammatory signaling might provide novel insights into the mechanisms that contribute to neurodegeneration induced by environmental neurotoxicant exposure.

Role of CX3CR1 Signaling on the Maturation of GABAergic Transmission and Neuronal Network Activity in the Neonate Hippocampus

In the developing brain, microglial cells play an important role in shaping neuronal circuits. These immune cells communicate with neurons through fractalkine (CX3CL1), a neuronal cytokine that acts on microglial CX3CR1 receptor. Among various functions, this signaling pathway has been implicated in the postnatal maturation of glutamatergic synapses. Although microglial cells are present in the neonate hippocampus when GABA receptor-mediated synaptic transmission and synchronized oscillatory events take place, it remains unknown whether microglial cells tune the establishment of these activities. Using CX3CR1-deficient mice and electrophysiological means, we investigated in CA3 pyramidal neurons the role of the fractalkine signaling in the maturation of GABA_A receptor-mediated synaptic currents and giant depolarizing potentials (GDPs), a network activity important for shaping synaptic connections. In CX3CR1-deficient mice, GABAergic currents were slightly altered, whereas the developmental changes of these currents were comparable with wild-type animals. Despite these minor changes in GABAergic transmission, the GDP frequency was strikingly reduced in CX3CR1-deficient mice compared to wild-type, with no change in the GDP shape and ending period. Collectively, it emerges that, in the neonate hippocampus, the fractalkine signaling pathway tunes GDP activities and is marginally involved in the maturation of GABAergic synapses, suggesting that microglial cells have distinct impact on maturing GABAergic, glutamatergic, and network functions.

Glial Contribution to Excitatory and Inhibitory Synapse Loss in Neurodegeneration

Synapse loss is an early feature shared by many neurodegenerative diseases, and it represents the major correlate of cognitive impairment. Recent studies reveal that microglia and astrocytes play a major role in synapse elimination, contributing to network dysfunction associated with neurodegeneration. Excitatory and inhibitory activity can be affected by glia-mediated synapse loss, resulting in imbalanced synaptic transmission and subsequent synaptic dysfunction. Here, we review the recent literature on the contribution of glia to excitatory/inhibitory imbalance, in the context of the most common neurodegenerative disorders. A better understanding of the mechanisms underlying pathological synapse loss will be instrumental to design targeted therapeutic interventions, taking in account the emerging roles of microglia and astrocytes in synapse remodeling.

Contribution of Neurons and Glial Cells to Complement-Mediated Synapse Removal during Development, Aging and in Alzheimer's Disease

Synapse loss is an early manifestation of pathology in Alzheimer's disease (AD) and is currently the best correlate to cognitive decline. Microglial cells are involved in synapse pruning during development via the complement pathway. Moreover, recent evidence points towards a key role played by glial cells in synapse loss during AD. However, further contribution of glial cells and the role of neurons to synapse pathology in AD remain not well understood. This review is aimed at comprehensively reporting the source and/or cellular localization in the CNS—in microglia, astrocytes, or neurons—of the triggering components (C1q, C3) of the classical complement pathway involved in synapse pruning in development, adulthood, and AD.

Reduced Microglial Activity and Enhanced Glutamate Transmission in the Basolateral Amygdala in Early CNS Autoimmunity

Emotional dysfunction is common in multiple sclerosis (MS) patients and in mouse models of MS, including experimental autoimmune encephalomyelitis (EAE); however, the etiology of these behaviors is poorly understood. To identify CNS changes associated with these behaviors, we focused on the basolateral amygdala (BLA) because of its central role in the regulation of emotional behavior. Whole-cell recordings were performed in the principal neurons of the BLA in early EAE, before demyelination, T-cell invasion, and motor dysfunction. EAE female mice displayed increased frequency of mEPSCs, with no alteration in amplitude or evoked EPSC paired-pulse ratio compared with controls. We found an increase in the AMPA-NMDA ratio and dendritic spine density, indicating increased numbers of glutamatergic synapses. We saw similar electrophysiological changes in BLA principal neurons after microglia were either inactivated (minocycline) or depleted (Mac1-Saporin) in the BLA. Microglia regulate synapses through pruning, directed by complement protein 3 (C3) expression. C3 was downregulated in the BLA in EAE. Ultrastructural analysis of microglia revealed more complex ramifications and reduced extracellular digestion of cellular elements. We also observed reduced IBA-1 and CD68 staining and lack of proinflammatory cytokine expression in the amygdala. Thus, early EAE is a state of microglial "deactivation" associated with reduced synaptic pruning. This contrasts with the prototypic microglial activation commonly associated with inflammatory CNS disease. Additionally, these data support a role for the acquired immune system to influence both neuronal and microglial function in early CNS autoimmunity.SIGNIFICANCE STATEMENT Microglia help regulate synaptic homeostasis, but there has been little evidence for how this might be important in neuroinflammatory diseases. The data from this study reveal increased synaptic activity and spine density in early stages of experimental autoimmune encephalomyelitis (an animal model of multiple sclerosis) in the basolateral amygdala, a nucleus important in the types of behavioral changes we have previously described. These electrophysiological and morphological effects occurred without significant elevation of local inflammatory cytokines or local demyelination. Unexpectedly, in the context of inflammatory state, we found that microglia were "deactivated." This study provides strong evidence for a link between microglial activity and synaptic function; the conclusions contrast with the generally accepted view that microglia are activated in inflammatory disease.

Dephosphorylated rather than hyperphosphorylated Tau triggers a pro-inflammatory profile in microglia through the p38 MAPK pathway

Tauopathies are a broad set of neurodegenerative dementias characterized by the aggregation of Tau protein. Activated microglia and elevated levels of pro-inflammatory molecules are also pathological hallmarks of tauopathies. In these diseases, intracellular Tau is secreted to the extracellular space, where it interacts with other cells, such as neurons and glia, promoting inflammation. However, the mechanism through which extracellular Tau triggers pro-inflammatory responses in microglia remains unknown. Primary microglia cultures were treated with extracellular Tau in its hyperphosphorylated, dephosphorylated or non-phosphorylated form. Protein cytokine arrays, real-time PCR, inhibition of the p38 MAPK pathway, phosphatase assays, and quantification of proteins through immunoblotting were used to analyze the effect of extracellular Tau on the pro-inflammatory response of microglia. The main finding of this work is that extracellular non-phosphorylated and dephosphorylated forms of Tau, rather than hyperphosphorylated Tau, activate the p38 MAPK pathway in microglia, thus triggering a pro-inflammatory response in these cells.

The plasticity of primary microglia and their multifaceted effects on endogenous neural stem cells in vitro and in vivo

Background

Microglia—the resident immune cells of the brain—are activated after brain lesions, e.g., cerebral ischemia, and polarize towards a classic “M1” pro-inflammatory or an alternative “M2” anti-inflammatory phenotype following characteristic temporo-spatial patterns, contributing either to secondary tissue damage or to regenerative responses. They closely interact with endogenous neural stem cells (NSCs) residing in distinct niches of the adult brain. The current study aimed at elucidating the dynamics of microglia polarization and their differential effects on NSC function.

Results

Primary rat microglia in vitro were polarized towards a M1 phenotype by LPS, or to a M2 phenotype by IL4, while simultaneous exposure to LPS plus IL4 resulted in a hybrid phenotype expressing both M1- and M2-characteristic markers. M2 microglia migrated less but exhibit higher phagocytic activity than M1 microglia. Defined mediators switched microglia from one polarization state to the other, a process more effective when transforming M2 microglia towards M1 than vice versa. Polarized microglia had differential effects on the differentiation potential of NSCs in vitro and in vivo, with M1 microglia promoting astrocytogenesis, while M2 microglia supported neurogenesis. Regardless of their polarization, microglia inhibited NSC proliferation, increased NSC migration, and accelerated NSC differentiation.

Conclusion

Overall, this study shed light on the complex conditions governing microglia polarization and the effects of differentially polarized microglia on critical functions of NSCs in vitro and in vivo. Refining the understanding of microglia activation and their modulatory effects on NSCs is likely to facilitate the development of innovative therapeutic concepts supporting the innate regenerative capacity of the brain.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1261-y) contains supplementary material, which is available to authorized users.

Microglia and the Brain: Complementary Partners in Development and Disease

An explosion of findings driven by powerful new technologies has expanded our understanding of microglia, the resident immune cells of the central nervous system (CNS). This wave of discoveries has fueled a growing interest in the roles that these cells play in the development of the CNS and in the neuropathology of a diverse array of disorders. In this review, we discuss the crucial roles that microglia play in shaping the brain-from their influence on neurons and glia within the developing CNS to their roles in synaptic maturation and brain wiring-as well as some of the obstacles to overcome when assessing their contributions to normal brain development. Furthermore, we examine how normal developmental functions of microglia are perturbed or remerge in neurodevelopmental and neurodegenerative disease.

Microglial TREM2/DAP12 Signaling: A Double-Edged Sword in Neural Diseases

Microglia are activated after neuronal injury and in neurodegenerative diseases, and trigger neuroinflammation in the central nervous system (CNS). Microglia-derived neuroinflammation has both beneficial and detrimental effects on neurons. Because the timing and magnitude of microglial activation is thought to be a critical determinant of neuronal fate, understanding the molecular mechanisms underlying microglial activation is required to enable establishment of microglia-targeted therapies for neural diseases. Plasma membrane receptors play primary roles as activators of microglia and in this review, we focus on a receptor complex involving triggering receptor expressed on myeloid cells 2 (TREM2) and DNAX-activating protein of 12 kDa (DAP12), both of which are causative genes for Nasu-Hakola disease, a dementia with bone cysts. Recent transcriptome approaches demonstrated TREM2/DAP12 signaling as the principal regulator that transforms microglia from a homeostatic to a neural disease-associated state. Furthermore, animal model studies revealed critical roles for TREM2/DAP12 in the regulation of microglial activity, including survival, phagocytosis, and cytokine production, not only in Alzheimer's disease but also in other neural diseases, such as Parkinson's disease, demyelinating disease, ischemia, and peripheral nerve injury. Intriguingly, while TREM2/DAP12-mediated microglial activation is detrimental for some diseases, including peripheral nerve injury, it is beneficial for other diseases. As the role of activated microglia differs among disease models, TREM2/DAP12 signaling may result in different outcomes in different diseases. In this review we discuss recent perspectives on the role of TREM2/DAP12 in microglia and their contribution to neural diseases.

The Role of Microglia in the Spread of Tau: Relevance for Tauopathies

Tauopathies are neurodegenerative diseases which course with the accumulation of Tau, mainly in neurons. In addition, Tau accumulates in a hyperphosphorylated and aggregated form. This protein is released into the extracellular space and spreads following a stereotypical pattern, inducing the development of the disease through connected regions of the brain. Microglia-the macrophages of the brain-are involved in maintaining brain homeostasis. They perform a variety of functions related to the surveillance and clearance of pathological proteins, among other dead cells and debris, from the extracellular space that could compromise brain equilibrium. This review focuses on the role played by microglia in tauopathies, specifically in Alzheimer's disease (AD), and how the uncoupling of activation/phagocytosis functions can have fatal consequences leading to the development of the pathology.

Triggering receptor expressed on myeloid cells 2 (TREM2): a potential therapeutic target for Alzheimer disease?

INTRODUCTION

There are currently no effective therapeutics for Alzheimer disease (AD). Clinical trials targeting amyloid beta thus far have shown very little benefit and only in the earliest stages of disease. These limitations have driven research to identify alternative therapeutic targets, one of the most promising is the triggering receptor expressed on myeloid cells 2 (TREM2). Areas covered: Here, we review the literature to-date and discuss the potentials and pitfalls for targeting TREM2 as a potential therapeutic for AD. We focus on research in animal and cell models for AD and central nervous system injury models which may help in understanding the role of TREM2 in disease. Expert opinion: Studies suggest TREM2 plays a key role in AD pathology; however, results have been conflicting about whether TREM2 is beneficial or harmful. More research is necessary before designing TREM2-targeting therapies. Successful therapeutics will most likely be administered early in disease.

Microglia: Housekeeper of the Central Nervous System

Microglia, of myeloid origin, play fundamental roles in the control of immune responses and the maintenance of central nervous system homeostasis. These cells, just like peripheral macrophages, may be activated into M1 pro-inflammatory or M2 anti-inflammatory phenotypes by appropriate stimuli. Microglia do not respond in isolation, but form part of complex networks of cells influencing each other. This review addresses the complex interaction of microglia with each cell type in the brain: neurons, astrocytes, cerebrovascular endothelial cells, and oligodendrocytes. We also highlight the participation of microglia in the maintenance of homeostasis in the brain, and their roles in the development and progression of age-related neurodegenerative disorders.

Anti-Inflammatory Effects of Omega-3 Fatty Acids in the Brain: Physiological Mechanisms and Relevance to Pharmacology

Classically, polyunsaturated fatty acids (PUFA) were largely thought to be relatively inert structural components of brain, largely important for the formation of cellular membranes. Over the past 10 years, a host of bioactive lipid mediators that are enzymatically derived from arachidonic acid, the main n-6 PUFA, and docosahexaenoic acid, the main n-3 PUFA in the brain, known to regulate peripheral immune function, have been detected in the brain and shown to regulate microglia activation. Recent advances have focused on how PUFA regulate the molecular signaling of microglia, especially in the context of neuroinflammation and behavior. Several active drugs regulate brain lipid signaling and provide proof of concept for targeting the brain. Because brain lipid metabolism relies on a complex integration of diet, peripheral metabolism, including the liver and blood, which supply the brain with PUFAs that can be altered by genetics, sex, and aging, there are many pathways that can be disrupted, leading to altered brain lipid homeostasis. Brain lipid signaling pathways are altered in neurologic disorders and may be viable targets for the development of novel therapeutics. In this study, we discuss in particular how n-3 PUFAs and their metabolites regulate microglia phenotype and function to exert their anti-inflammatory and proresolving activities in the brain.

Dynamic cross-talk between microglia and peripheral monocytes underlies stress-induced neuroinflammation and behavioral consequences

Psychological stress promotes the development and recurrence of anxiety and depressive behavioral symptoms. Basic and clinical research indicates that stress exposure can influence the neurobiology of mental health disorders through dysregulation of neuroimmune systems. Consistent with this idea several studies show that repeated stress exposure causes microglia activation and recruitment of peripheral monocytes to the brain contributing to development of anxiety- and depressive-like behavior. Further studies show that stress-induced re-distribution of peripheral monocytes leads to stress-sensitized neuroimmune responses and recurrent anxiety-like behavior. These stress-associated immune changes are important because brain resident and peripheral immune cells contribute to physiological processes that support neuroplasticity. Thus, perturbations in neuroimmune function can lead to impaired neuronal responses and synaptic plasticity deficits that underlie behavioral symptoms of mental health disorders. In this review we discuss recent advances in neuroimmune regulation of behavior and summarize studies showing that stress-induced microglia activation and monocyte trafficking in the brain contribute to the neurobiology of mental health disorders.

Bioactive lipids as new class of microglial modulators: When nutrition meets neuroimunology

Within the central nervous system the traditional role of microglia has been in brain infection and disease, phagocytosing debris and secreting factors to modify disease progression. More recently, microglia have been found to be important for normal brain development, circuit refinement, and synaptic plasticity in ways that were previously unsuspected. Hence, the brain innate immune system appears to be key in all situations, ranging from physiology to pathology. This unique feature of microglia is established by the wide array of receptors it is equipped with to sense molecular patterns. This includes receptors to most if not all neurotransmitters, neuromodulators and purines. We here review novel, yet extensive literature on a new class of microglia modulators, namely bioactive fatty acids. These lipids are issued from metabolism of nutrients and can cross the blood brain barrier to reach the CNS. They appear to be direct modulators of microglial activity, triggering/inhibiting inflammatory processes or enhancing/inhibiting the ability of these cells to respond to hazardous agents.

Ethanol differentially modulates P2X4 and P2X7 receptor activity and function in BV2 microglial cells

Neuroinflammation is one of the mechanisms leading to neurodegenerative brain damage induced by chronic alcohol (ethanol) exposure. Microglia play a major role in the development of innate immune responses to environmental injuries including ethanol. Adenosine 5″-triphosphate (ATP)-activated purinergic P2X receptor (P2XR) subtypes, P2X4Rs and P2X7Rs, are endogenously expressed in microglia and can modulate their activity. These 2 P2XR subtypes differ pharmacologically and functionally: 1) P2X4Rs are activated at lower (≤0.1 mM) whereas P2X7Rs - at higher (≥1.0 mM) ATP concentrations; 2) P2X4R activation contributes to the release of brain derived neurotrophic factor and its role in tactile allodynia and neuropathic pain is demonstrated; 3) Due to its role in the secretion of pro-inflammatory IL-1β, P2X7Rs have been implicated in the development of neurodegenerative pathologies, pain and morphine tolerance. To date, the roles of individual P2XR subtypes in ethanol effects on microglia and the functional consequences are not completely understood. Based on the existing knowledge on the pharmacological and functional differences between P2X4Rs and P2X7Rs, the present work tested the hypothesis that P2X4Rs and P2X7Rs play differential roles in ethanol action in microglia. Effects of ethanol on P2X4R and P2X7R activity, expression and functional consequences were determined using murine BV2 microglial cells. Ethanol (≥100 mM) inhibited P2X4Rs but was inactive on P2X7 channel activity. Ethanol (25, 100 mM) inhibited P2X4R-mediated microglia migration whereas it potentiated pore formation in P2X7Rs. Furthermore, ethanol (25, 100 mM) potentiated P2X7R-mediated IL-1β secretion from BV2 microglia. Ethanol also induced protein expression for both P2XR subtypes. Overall, the findings identify differential roles for P2X4Rs and P2X7Rs in regards to ethanol effects on microglia which may be linked to different stages of ethanol exposure.

Roles of Microglial Phagocytosis and Inflammatory Mediators in the Pathophysiology of Sleep Disorders

Sleep serves crucial learning and memory functions in both nervous and immune systems. Microglia are brain immune cells that actively maintain health through their crucial physiological roles exerted across the lifespan, including phagocytosis of cellular debris and orchestration of neuroinflammation. The past decade has witnessed an explosive growth of microglial research. Considering the recent developments in the field of microglia and sleep, we examine their possible impact on various pathological conditions associated with a gain, disruption, or loss of sleep in this focused mini-review. While there are extensive studies of microglial implication in a variety of neuropsychiatric and neurodegenerative diseases, less is known regarding their roles in sleep disorders. It is timely to stimulate new research in this emergent and rapidly growing field of investigation.