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Microglia: Phagocytosing to Clear, Sculpt, and Eliminate

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Abstract

Microglia are the primary phagocytes of the central nervous system. They eliminate excess functional connections between neurons to sculpt neuronal circuits during development and throughout adulthood. Understanding how microglia recognize and prune synapses during development is providing insight into synapse loss and dysfunction in disease.

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... Microglia, the resident macrophages of the central nervous system (CNS), are essential participants in the development of the brain and its neuronal circuits. During development, microglia both remove and promote the formation of dendritic spines, synapses, and live and dead cells to establish connections between neurons [1][2][3][4][5][6][7][8]. One mechanism by which microglia efficiently mediate these diverse effects is phagocytosis, a process in which cell particles or entire cell nuclei are engulfed and ingested [9]. ...
... Both ramified and non-ramified microglia are capable of phagocytosis [2,5,[13][14][15][16]. Local environmental signals, associated with chemotaxis, cell death, injury, inflammation, as well as synaptic connectivity, can all regulate microglial phagocytosis [1,2,4,17,18]. Moreover, the phagocytic profile of microglia varies according to developmental stage, brain region, and sex [5,16,19]. ...
... Treatment with PGE 2 had no effect on the number of phagocytic cups [ F [1,20] = 0.091, p = 0.766] (Fig. 6b) or the density of microglia with thick (Fig. 6c) or thin processes (Fig. 6d). ...
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Microglia are essential to sculpting the developing brain, and they achieve this in part through the process of phagocytosis which is regulated by microenvironmental signals associated with cell death and synaptic connectivity. In the rat cerebellum, microglial phagocytosis reaches its highest activity during the third postnatal week of development but the factors regulating this activity are unknown. A signaling pathway, involving prostaglandin E2 (PGE2) stimulation of the estrogen synthetic enzyme aromatase, peaks during the 2nd postnatal week and is a critical regulator of Purkinje cell maturation. We explored the relationship between the PGE2-estradiol pathway and microglia in the maturing cerebellum. Toward that end, we treated developing rat pups with pharmacological inhibitors of estradiol and PGE2 synthesis and then stained microglia with the universal marker Iba1 and quantified microglia engaged in phagocytosis as well as phagocytic cups in the vermis and cerebellar hemispheres. Inhibition of aromatase reduced the number of phagocytic cups in the vermis, but not in the cerebellar hemisphere at postnatal day 17. Similar results were found after treatment with nimesulide and indomethacin, inhibitors of the PGE2-producing enzymes cyclooxygenase 1 and 2. In contrast, treatment with estradiol or PGE2 had little effect on microglial phagocytosis in the developing cerebellum. Thus, endogenous estrogens and prostaglandins upregulate the phagocytic activity of microglia during a select window of postnatal cerebellar development, but exogenous treatment with these same signaling molecules does not further increase the already high levels of phagocytosis. This may be due to an upper threshold or evidence of resistance to exogenous perturbation.
... Median [68] High IL-4 [49,52]; Low P2ry12 [58] Low P2ry12 Low Sirpa [136] Others (NF-κB, CD11b, MHCII, Tim3, etc.) Higher in human WM [66,75,76] Median [59,62,63,136] High [49] Median [59,62,63,136] Median [59,62,63,136] High [59,62,63,136] Cellular functions: Proliferation/replenish after ablation Both fast [42] Both fast [46,48] Both fast [44,45,104] Replenish fast [42] Fast [48] /replenish slower [42] Replenish fast [42] Protrusion toward ATP/ Phagocytosis/pruning Fast protrusion [52]/Low lysosome content in NAc [65] High surveillance [74,82] Slow protrusion [52] High lysosome content in SNr [65]/High pruning in thalamus [117] Less surveillance [34]/High clearance [77] High pruning [139] Ontogenesis Hoxb8 ± [101]; Sensitive to IL-34 [111,112] but not CSF1 [113] Sensitive to IL-34 [111,112]; ...
... Encouragingly, evidences from multiple studies jointly indicate that for instance microglia in the cortical GM and WM are phenotypically and functionally different. As microglia are the primary phagocytes of the CNS, their actions of tissue debris clearance and pruning of neuronal circuits during development and throughout adulthood provide a key mechanism for CNS structural and functional plasticity, and have been a major focus in studies of various brain diseases [74]. In this regard, microglia were previously found to express high levels of MHCII [75] and Tim-3 (a cell surface protein that regulates macrophage activation and promotes immunological tolerance) [76] in the corpus callosum compared with the cerebral cortex in humans. ...
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Microglia have been recently shown to manifest a very interesting phenotypical heterogeneity across different regions in the mammalian central nervous system (CNS). However, the underlying mechanism and functional meaning of this phenomenon are currently unclear. Baseline diversities of adult microglia in their cell number, cellular and subcellular structures, molecular signature as well as relevant functions have been discovered. But recent transcriptomic studies using bulk RNAseq and single-cell RNAseq have produced conflicting results on region-specific signatures of microglia. It is highly speculative whether such spatial heterogeneity contributes to varying sensitivities of individual microglia to the same physiological and pathological signals in different CNS regions, and hence underlie their functional relevance for CNS disease development. This review aims to thoroughly summarize up-to-date knowledge on this specific topic and provide some insights on the potential underlying mechanisms, starting from microgliogenesis. Understanding regional heterogeneity of microglia in the context of their diverse neighboring neurons and other glia may provide an important clue for future development of innovative therapies for neuropsychiatric disorders.
... For instance, in a model of viral encephalitis, virally infected neurons express chemokines, such as CCL2, as 'attack-me signals' , to attract the phagocytes that strip the neuron of its synapses 42 . Moreover, in development and neurodegeneration, dysfunctional synapses are tagged by components of the complement system, which phagocytes recognize as 'eat-me signals' to initiate engulfment 44 . To these executive mechanisms of phagocyte-mediated synapse stripping, we now add localized calcium dyshomeostasis as an early predictor of which synapses are fated for removal. ...
... The following aspects could contribute to such a collaborative interaction: the expression of eat-me signals in neurons could be directly regulated by calcium signals, as has been shown, for example, for the membrane exposure of phosphatidylserine 47 . Indeed, many aspects of developmental synapse pruning are activity, and hence probably calcium, dependent, including complement-mediated synapse removal 44 . Furthermore, accumulation of calcium could lead to a local destabilization of the cytoskeleton, for example, by the activation of calcium-dependent proteases 48 or phosphatases 12 , which could facilitate subsequent spine removal-especially, if such cytoskeletal remodeling became more widespread. ...
Article
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Cortical pathology contributes to chronic cognitive impairment of patients suffering from the neuroinflammatory disease multiple sclerosis (MS). How such gray matter inflammation affects neuronal structure and function is not well understood. In the present study, we use functional and structural in vivo imaging in a mouse model of cortical MS to demonstrate that bouts of cortical inflammation disrupt cortical circuit activity coincident with a widespread, but transient, loss of dendritic spines. Spines destined for removal show local calcium accumulations and are subsequently removed by invading macrophages or activated microglia. Targeting phagocyte activation with a new antagonist of the colony-stimulating factor 1 receptor prevents cortical synapse loss. Overall, our study identifies synapse loss as a key pathological feature of inflammatory gray matter lesions that is amenable to immunomodulatory therapy.
... Turning from observational comparison to the broader AD literature, we find validation of our complement cascade and TYROBP inferences. The complement cascade has been reported to mediate microglial pruning of neuronal synapses in development and behaves aberrantly in neurodegenerative disease (Schafer et al., 2012;Hong and Stevens, 2016;. Specific to genes enriched in our analysis, elevation of C1q has been reported to precede plaque deposition in AD mouse models. ...
... Complement cascade and TREM2/TYROBP inferences from our study were validated in integrative computational AD literature and in literature oriented toward LOAD (Cribbs et al., 2012;Schafer et al., 2012;Zhang et al., 2013;Hong and Stevens, 2016;Keren-Shaul et al., 2017;Gratuze et al., 2018;Bartels et al., 2020). The ability of TransComp-R to identify well-known hub genes, such as TREM2 and TYROBP, identified through other computational methods provided a positive confirmation of our modeling methodology, especially given analysis using a traditional amyloid mouse model dataset. ...
Article
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Mouse models are vital for preclinical research on Alzheimer’s disease (AD) pathobiology. Many traditional models are driven by autosomal dominant mutations identified from early onset AD genetics whereas late onset and sporadic forms of the disease are predominant among human patients. Alongside ongoing experimental efforts to improve fidelity of mouse model representation of late onset AD, a computational framework termed Translatable Components Regression (TransComp-R) offers a complementary approach to leverage human and mouse datasets concurrently to enhance translation capabilities. We employ TransComp-R to integratively analyze transcriptomic data from human postmortem and traditional amyloid mouse model hippocampi to identify pathway-level signatures present in human patient samples yet predictive of mouse model disease status. This method allows concomitant evaluation of datasets across different species beyond observational seeking of direct commonalities between the species. Additional linear modeling focuses on decoupling disease signatures from effects of aging. Our results elucidated mouse-to-human translatable signatures associated with disease: excitatory synapses, inflammatory cytokine signaling, and complement cascade- and TYROBP-based innate immune activity; these signatures all find validation in previous literature. Additionally, we identified agonists of the Tyro3 / Axl / MerTK (TAM) receptor family as significant contributors to the cross-species innate immune signature; the mechanistic roles of the TAM receptor family in AD merit further dedicated study. We have demonstrated that TransComp-R can enhance translational understanding of relationships between AD mouse model data and human data, thus aiding generation of biological hypotheses concerning AD progression and holding promise for improved preclinical evaluation of therapies.
... Furthermore, 97 genetic association studies have demonstrated a link between Alzheimer's disease and 98 polymorphisms or mutations in genes linked to immune responses ( Villegas-Llerena et al., 99 2016). Although the mechanisms and mediators of inflammatory risk in Alzheimer's disease 100 are not fully understood, synaptic and neuronal injury may arise from the release of 101 cytokines and pro-inflammatory molecules such as interleukin-1ß and TGF-ß ( Fernandez- 102 Botran et al., 2011), or direct microglial injury to synapses (Hong and Stevens, 2016;Hong et 103 al., 2016). These, in turn, impair synaptic function, network communication, and may 104 accelerate neurodegeneration and synaptic loss (Heppner et al., 2015;Hoeijmakers et al., 105 2016;Villegas-Llerena et al., 2016;Li et al., 2018;Wang et al., 2018). ...
... We argue that the cognitive deficits in Alzheimer's disease can be directly related to 409 changes in functional connectivity which in turn are mediated by microglia activation, 410 although we acknowledge that there are several mechanisms by which neuroinflammation 411 can alter brain functional connectivity and vice versa (i.e., the ways in which synaptic firing 412 can influence microglia). Microglia are important contributors in the process of synaptic 413 pruning and regulation of synaptic function (Hong et al., 2016). The microglia's highly mobile 414 and ramified branches can reach and surround synaptic terminals to promote phagocytosis 415 and synaptic demise (Hong et al., 2016). ...
Article
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Neuroinflammation is a key part of the etio-pathogenesis of Alzheimer's disease. We test the relationship between neuroinflammation and the disruption of functional connectivity in large-scale networks, and their joint influence on cognitive impairment.We combined [11C]PK11195 positron emission tomography (PET) and resting-state functional magnetic resonance imaging (rs-fMRI) in 28 humans (12 females/16 males) with clinical diagnosis of probable Alzheimer's disease or mild cognitive impairment with positive PET biomarker for amyloid, and 14 age-, sex-, and education-matched healthy humans (8 females/6 males). Source-based 'inflammetry' was used to extract principal components of [11C]PK11195 PET signal variance across all participants. rs-fMRI data were pre-processed via independent component analyses to classify neuronal and non-neuronal signals. Multiple linear regression models identified sources of signal co-variance between neuroinflammation and brain connectivity profiles, in relation to group and cognitive status.Patients showed significantly higher [11C]PK11195 binding relative to controls, in a distributed spatial pattern including the hippocampus, medial, and inferior temporal cortex. Patients with enhanced loading on this [11C]PK11195 binding distribution displayed diffuse abnormal functional connectivity. The expression of a stronger association between such abnormal connectivity and higher levels of neuroinflammation correlated with worse cognitive deficits.Our study suggests that neuroinflammation relates to the pathophysiological changes in network function that underlie cognitive deficits in Alzheimer's disease. Neuroinflammation, and its association with functionally-relevant reorganisation of brain networks, is proposed as a target for emerging immuno-therapeutic strategies aimed at preventing or slowing the emergence of dementia.SIGNIFICANCE STATEMENTNeuroinflammation is an important aspect of Alzheimer's disease (AD), but it was not known whether the influence of neuroinflammation on brain network function in humans was important for cognitive deficit.Our study provides clear evidence that in vivo neuroinflammation in AD impairs large-scale network connectivity; and that the link between inflammation and functional network connectivity is relevant to cognitive impairment.We suggest that future studies should address how neuroinflammation relates to network function as AD progresses; and whether the neuroinflammation in AD is reversible, as the basis of immunotherapeutic strategies to slow the progression of AD.
... Microglial cells are the professional phagocytes of the brain and as such, they are capable of eliminating entire cells or cellular substructures [84]. But if one had to pinpoint the origin of the renewed interest in microglia, it would be for their involvement in synaptic pruning. ...
Article
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Microglia are the only immune cell population present in the brain parenchyma. Their vantage position in the central nervous system (CNS) enables these myeloid cells to perform the most disparate of tasks: from the classical immune functions of fighting infections and surveilling the extracellular space for pathogens and damage, to sculpting the neuronal circuitry by pruning unnecessary synapses and assisting neurons in spine formation, aiding in the maintenance of brain homeostasis. The neurotrophin field has always been dominated by the neurocentric view that the primary target of these molecules must be neurons: this holds true even for the Nerve Growth Factor (NGF), which owes its popularity in the neuroscience community to its trophic and tropic activity towards sensory and sympathetic neurons in the peripheral nervous system, and cholinergic neurons in the CNS. The increasing evidence that microglia are an integral part of neuronal computation calls for a closer look as to whether these glial cells are capable of responding directly to NGF. In this review, we will first outline evidence in support of a role for NGF as a molecule mediating neuroimmune communication. Then, we will illustrate some of those non-immune features that have made microglial cells one of the hottest topics of this last decade. In conclusion, we will discuss evidence in support of a microglial function for NGF.
... T he interplay among different cells in a tissue is crucial for maintaining homeostasis. Although many diseases have been traditionally perceived as the result of the malfunction of a particular cell or cell type, mounting evidence [1][2][3][4][5] and new therapeutic strategies 3,6,7 have demonstrated the pivotal role of multicellular action in health and diseases, opening new opportunities for intervention, diagnosis, disease monitoring and prevention. In parallel, advances in single-cell RNA sequencing (scRNA-seq) 8,9 and spatial transcriptomics [10][11][12] now allow the systematic exploration of molecular profiles at single-cell resolution across cell types 13 , tissues 14,15 and disease states [16][17][18] , in both isolated cells and intact tissues [10][11][12] . ...
Article
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Deciphering the functional interactions of cells in tissues remains a major challenge. Here we describe DIALOGUE, a method to systematically uncover multicellular programs (MCPs)—combinations of coordinated cellular programs in different cell types that form higher-order functional units at the tissue level—from either spatial data or single-cell data obtained without spatial information. Tested on spatial datasets from the mouse hypothalamus, cerebellum, visual cortex and neocortex, DIALOGUE identified MCPs associated with animal behavior and recovered spatial properties when tested on unseen data while outperforming other methods and metrics. In spatial data from human lung cancer, DIALOGUE identified MCPs marking immune activation and tissue remodeling. Applied to single-cell RNA sequencing data across individuals or regions, DIALOGUE uncovered MCPs marking Alzheimer’s disease, ulcerative colitis and resistance to cancer immunotherapy. These programs were predictive of disease outcome and predisposition in independent cohorts and included risk genes from genome-wide association studies. DIALOGUE enables the analysis of multicellular regulation in health and disease. Coordinated gene programs spanning multiple different cell types are identified in healthy and diseased tissues.
... Our microglia analysis focused on the PVN, which is part of the hypothalamic-pituitary-adrenal (HPA) axis and is considered the major neuroendocrine system that regulates the body's response to stress [80]. Since microglia are crucial in early life for synaptic pruning, maternal antibiotic use could alter the rates of synaptogenesis during this critical window of development, which then persists long after the offspring have been weaned [81,82]. Here, we found that maternal antibiotic use leads to altered neuroimmune responses in their offspring at 10 weeks of age, as seen by longer processes and greater dendrite complexity in response to LPS. ...
Article
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Microbial colonization of the gut early in life is crucial for the development of the immune and nervous systems, as well as influencing metabolism and weight gain. While early life exposure to antibiotics can cause microbial dysbiosis, prebiotics are non-digestible substrates that selectively promote the growth of beneficial gut microbiota. Our objective was to examine the effects of dietary prebiotic administration on the consequences of maternal antibiotic intake on offspring body weight, behavior, and neuroimmune responses later in life. Sprague-Dawley rat dams were given low-dose penicillin (LDP), prebiotic fiber (10% oligofructose), or both, during the third week of pregnancy and throughout lactation. Anxiety-like behavior, weight gain, body composition, cecal microbiota composition, and microglial responses to lipopolysaccharide (LPS) were assessed in offspring. Male and female prebiotic offspring had lower body weight compared to antibiotic offspring. Maternal antibiotic exposure resulted in lasting effects on select offspring microbiota including a lower relative abundance of Streptococcus, Lactococcus, and Eubacterium at 10 weeks of age. Maternal antibiotic use impaired microglial response to LPS in the hypothalamus compared to control, and this phenotype was reversed with prebiotic. Prebiotic fiber warrants further investigation as an adjunct to antibiotic use during pregnancy.
... Excess exposure to proinflammatory cytokines in the developing brain induces improper activation of microglia, resulting in improper support and regulation of neuronal activity, impeding neuron differentiation and survival [113,115,116]. Rodent studies showed that dysregulation of microglia is associated with autism and schizophrenia [117], and microglia play an active part in obesity-related cognitive decline by phagocytosis of synapses [118]. ...
Article
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Obesity and diabetes is a worldwide public health problem among women of reproductive age. This narrative review highlights recent epidemiological studies regarding associations of maternal obesity and diabetes with neurodevelopmental and psychiatric disorders in offspring, and provides an overview of plausible underlying mechanisms and challenges for future human studies. A comprehensive search strategy selected terms that corresponded to the domains of interest (maternal obesity, different types of diabetes, offspring cognitive functions and neuropsychiatric disorders). The databases searched for articles published between January 2010 and April 2019 were PubMed, Web of Science and CINAHL. Evidence from epidemiological studies strongly suggests that maternal pre-pregnancy obesity is associated with increased risks for autism spectrum disorder, attention-deficit hyperactivity disorder and cognitive dysfunction with modest effect sizes, and that maternal diabetes is associated with the risk of the former two disorders. The influence of maternal obesity on other psychiatric disorders is less well studied, but there are reports of associations with increased risks for offspring depression, anxiety, schizophrenia and eating disorders, at modest effect sizes. It remains unclear whether these associations are due to intrauterine mechanisms or explained by confounding family-based sociodemographic, lifestyle and genetic factors. The plausible underlying mechanisms have been explored primarily in animal models, and are yet to be further investigated in human studies.
... Male MG 4E microglia with altered phagocytosis and motility. Microglia are the primary phagocytes in the brain that engulf invading pathogens and cellular debris 56 . Upregulation of antimicrobial pathways including the phagosome pathway (Fig. 7c) prompted us to hypothesize that the phagocytic capacity of microglia is enhanced in 2-week-old male MG 4E mice. ...
Article
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Mutations that inactivate negative translation regulators cause autism spectrum disorders (ASD), which predominantly affect males and exhibit social interaction and communication deficits and repetitive behaviors. However, the cells that cause ASD through elevated protein synthesis resulting from these mutations remain unknown. Here we employ conditional overexpression of translation initiation factor eIF4E to increase protein synthesis in specific brain cells. We show that exaggerated translation in microglia, but not neurons or astrocytes, leads to autism-like behaviors in male mice. Although microglial eIF4E overexpression elevates translation in both sexes, it only increases microglial density and size in males, accompanied by microglial shift from homeostatic to a functional state with enhanced phagocytic capacity but reduced motility and synapse engulfment. Consequently, cortical neurons in the mice have higher synapse density, neuroligins, and excitation-to-inhibition ratio compared to control mice. We propose that functional perturbation of male microglia is an important cause for sex-biased ASD. The main cell types involved in autism spectrum disorders through elevated protein synthesis are not well identified. Here, the authors show that overexpression of translation initiation factor eIF4E in microglia results in autism-like behaviour in male, but not female, mice.
... During phagocytosis, proteins on the microglial cell surface, such as the Toll-Like Receptors (TLRs), Fc receptors, and scavenger receptors including CD36 and the receptor for advanced glycation end products (RAGE) among others, recognize the "eat-me" signals and engulf the target substrates into intracellular compartments called phagosomes [7][8][9][10] . The phagosomes mature by fusing with lysosomes to form highly acidic phagolysosomes and mobilize the phagocytosed material for enzymatic degradation. ...
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Phagocytosis by glial cells is essential to regulate brain function during development and disease. Given recent interest in using amyloid β (Aβ)-targeted antibodies as a therapy for patients with Alzheimer's disease, removal of Aβ by phagocytosis is likely protective early in Alzheimer's disease, but remains poorly understood. Impaired phagocytic function of glial cells surrounding Aβ plaques during later stages in Alzheimer's disease likely contributes to worsened disease outcomes, but the underlying mechanisms of how this occurs remain unknown. We have developed a human Aβ 1-42 analogue (Aβ pH ) that exhibits green fluorescence upon internalization into the acidic phagosomes of cells but is non-fluorescent at physiological pH. This allowed us to image, for the first time, glial uptake of Aβ pH in real time in live animals. Microglia phagocytose more Aβ pH than astrocytes in culture, in brain slices and in vivo . Aβ pH can be used to investigate the phagocytic mechanisms removing Aβ from the extracellular space, and thus could become a useful tool to study Aβ clearance at different stages of Alzheimer's disease.
... anti-inflammatory cytokines depending on the different phenotypes. 6,7 On one hand, activated microglia can secrete a series of proinflammatory cytokines to aggravate the destruction of the blood-brain barrier and tissue damage. 3,8 On the other hand, they promote tissue repair by phagocytosis or secreting neurotrophic factors. ...
Article
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Introduction: Clearance of damaged cells and debris is beneficial for the functional recovery after ischemic brain injury. However, the specific phagocytic receptor that mediates microglial phagocytosis after ischemic stroke is unknown. Aim: To investigate whether P2Y6 receptor-mediated microglial phagocytosis is beneficial for the debris clearance and functional recovery after ischemic stroke. Results: The expression of the P2Y6 receptor in microglia increased within 3 days after transient middle cerebral artery occlusion. Inhibition of microglial phagocytosis by the selective inhibitor MRS2578 enlarged the brain atrophy and edema volume after ischemic stroke, subsequently aggravated neurological function as measured by modified neurological severity scores and Grid walking test. MRS2578 treatment had no effect on the expression of IL-1α, IL-1β, IL-6, IL-10, TNF-α, TGF-β, and MPO after ischemic stroke. Finally, we found that the expression of myosin light chain kinase decreased after microglial phagocytosis inhibition in the ischemic mouse brain, which suggested that myosin light chain kinase was involved in P2Y6 receptor-mediated phagocytosis. Conclusion: Our results indicate that P2Y6 receptor-mediated microglial phagocytosis plays a beneficial role during the acute stage of ischemic stroke, which can be a therapeutic target for ischemic stroke.
... Other genes, such Ador2a, Car3, Chat, Drd2, Egr2, Hif3a, Notch, Penk, Tac1 and Ttr, which were also differentially expressed after Prl treatment, are involved in maintaining microglial functions. Interestingly, these genes have been reported to be altered in neuroinflammation, neurodegenerative diseases, and psychiatric disorders [31][32][33][34][35][36][37][38][39] . It is possible that genes related to microglial functions induced by Prl might play roles in neuro-immunomodulation in the hippocampus or in preventing neuronal cell damage. ...
Article
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Prolactin (Prl) is a pleiotropic hormone with multiple functions in several tissues and organs, including the brain. In the hippocampus, Prl has been implicated in several functions, including neuroprotection against excitotoxicity in lactating rats and in Prl-treated ovariectomized animals. However, the molecular mechanisms involved in Prl actions in the hippocampus have not been completely elucidated. The aim of this study was to analyse the hippocampal transcriptome of female Prl-treated ovariectomized rats. Transcriptomic analysis by RNASeq revealed 162 differentially expressed genes throughout 24 h of Prl treatment. Gene Ontology analysis of those genes showed that 37.65% were involved in brain processes that are regulated by the hippocampus, such as learning, memory and behaviour, as well as new processes that we did not foresee, such as glial differentiation, axogenesis, synaptic transmission, postsynaptic potential, and neuronal and glial migration. Immunodetection analysis demonstrated that Prl significantly modified microglial morphology, reduced the expression of Cd11b/c protein, and altered the content and location of the neuronal proteins Tau, Map2 and Syp, which are involved in axogenic and synaptic functions. This novel delineation of Prl activity in the hippocampus highlights its importance as a neuroactive hormone, opens a new avenue for understanding its actions and supports its participation in neuronal plasticity of this brain area.
... Initially, recruitment of microglia is beneficial to the inflammatory response of a dying cell. Prolongation of the response is likely to be detrimental to neighboring neurons and synapses (Hong and Stevens, 2016). ITI-214 inhibits ADPdependent migration of BV2 cells without affecting migration or underlying cAMP dependent signaling in the absence of the ADP trigger, and thus may serve to slow the unchecked immune response. ...
Article
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A novel, potent, and highly specific inhibitor of calcium-calmodulin-dependent phosphodiesterases (PDE) of the PDE1 family, ITI-214, was used to investigate the role of PDE1 in inflammatory responses. ITI-214 dose-dependently suppressed lipopolysaccharide (LPS)-induced gene expression of pro-inflammatory cytokines in an immortalized murine microglial cell line, BV2 cells. RNA profiling (RNA-Seq) was used to analyze the impact of ITI-214 on the BV2 cell transcriptome in the absence and the presence of LPS. ITI-214 was found to regulate classes of genes that are involved in inflammation and cell migration responses to LPS exposure. The gene expression changes seen with ITI-214 treatment were distinct from those elicited by inhibitors of other PDEs with anti-inflammatory activity (e.g., a PDE4 inhibitor), indicating a distinct mechanism of action for PDE1. Functionally, ITI-214 inhibited ADP-induced migration of BV2 cells through a P2Y12-receptor-dependent pathway, possibly due to increases in the extent of cAMP and VASP phosphorylation downstream of receptor activation. Importantly, this effect was recapitulated in P2 rat microglial cells in vitro, indicating that these pathways are active in native microglial cells. These studies are the first to demonstrate that inhibition of PDE1 exerts anti-inflammatory effects through effects on microglia signaling pathways. The ability of PDE1 inhibitors to prevent or dampen excessive inflammatory responses of BV2 cells and microglia provides a basis for exploring their therapeutic utility in the treatment of neurodegenerative diseases associated with increased inflammation and microglia proliferation such as Parkinson's disease and Alzheimer's disease.
... Reactive microglia display dual roles in pathological nervous tissues, buy first being neuroprotective and later develop into a phagocytic and neurotoxic phenotype (Chen and Trapp, 2016). Different damages to nervous tissues induce reactive gliosis, which also include aggregation of reactive microglia to synaptic areas (Bisht et al., 2016b;Hong and Stevens, 2016;Lafrenaye et al., 2015). In recent studies by Bisht and colleagues (Bisht et al., 2016a,b), a novel microglia phenotype referred to as dark microglia was described at the ultrastructural level. ...
Article
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In the adult retina, ramifying microglia interact with the outer plexiform layer (OPL) monitoring the synaptic integrity between photoreceptors and post-synaptic target cells. Microglia are reactive during photoreceptor diseases, but their disease-related function(s) are not fully understood. Retinal explant cultures are model systems used to study degenerative events including photoreceptor degeneration and gliosis. Our culture paradigm, with adult porcine retinas subjected to coculture with human A-retinal pigment epithelia-19 (ARPE) cells, is an experimental approach resulting in improved photoreceptor survival and reduced gliosis. Under the in vitro pathological conditions with photoreceptor degeneration, reactive Iba1-and CD11b-immunoreactive microglia and their processes positioned in proximity with the OPL and among photoreceptor outer segments. Coculture for 3 days with ARPE-cells resulted in a significantly increased density of microglia at the OPL. After 5 days of culture, the density of microglia at the OPL was similar between coculture and control specimens. Electron microscopy revealed the presence of two subtypes of microglia: one exhibiting a dark nucleus and cytosol with dilated endoplasmic reticulum, vacuoles, endosomes and mitochondrial variations. This subtype localized close to synaptic structures in the OPL. The other subtype appeared as pale phagocytic microglia localized among degenerating outer segments. The Iba1-and CD11b-immunoreactive microglia in degenerating retina may be of two separate subtypes, which differ in localization, subcellular morphology and perhaps function.
... Differentiated oligodendrocytes insulate neuronal axons with a myelin sheath to guarantee proper conductance of neuronal signals [12]. A growing emphasis is now placed on the role of astrocytes and microglia in facilitating synaptic pruning during early life through adolescence, allowing later in life the fine tuning of complex circuits [13]. Plasticity is a key feature of the standard neurodevelopmental trajectory and modulates the dynamics of synaptic connections and neural circuitry formation. ...
Article
Pregnancy and early life are characterized by marked changes in body microbial composition. Intriguingly, these changes take place simultaneously with neurodevelopmental plasticity, suggesting a complex dialogue between the microbes that inhabit the gastrointestinal tract and the brain. The purpose of this chapter is to describe the natural trajectory of microbiota during pregnancy and early life, as well as review the literature available on its interaction with neurodevelopment. Several lines of evidence show that the gut microbiota interacts with diet, drugs and stress both prenatally and postnatally. Clinical and preclinical studies are illuminating how these disruptions result in different developmental outcomes. Understanding the role of the microbiota in neurodevelopment may lead to novel approaches to the study of the pathophysiology and treatment of neuropsychiatric disorders.
... Findings of this study are inline with a second recent report, from different investigators, which showed that TREM2 intracellular signaling functions to maintain the metabolic fitness and phagocytic responses of microglia operating to defend the brain in AD (91,92). Since microglia are active participants in the formation, remodeling, and elimination of synapses, this research may also shed light on the mechanisms which underlie synapse loss in AD (93,94). This research also raises the issue of whether NF-κB-dependent regulation of microglia TREM2 expression might also play an as-yet unexplored role in the synaptic plasticity associated with learning and memory. ...
Article
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Nuclear Factor Kappa B (NF-κB) is a ubiquitously expressed transcription factor with key functions in a wide array of biological systems. While the role of NF-κB in processes, such as host immunity and oncogenesis has been more clearly defined, an understanding of the basic functions of NF-κB in the nervous system has lagged behind. The vast cell-type heterogeneity within the central nervous system (CNS) and the interplay between cell-type specific roles of NF-κB contributes to the complexity of understanding NF-κB functions in the brain. In this review, we will focus on the emerging understanding of cell-autonomous regulation of NF-κB signaling as well as the non-cell-autonomous functional impacts of NF-κB activation in the mammalian nervous system. We will focus on recent work which is unlocking the pleiotropic roles of NF-κB in neurons and glial cells (including astrocytes and microglia). Normal physiology as well as disorders of the CNS in which NF-κB signaling has been implicated will be discussed with reference to the lens of cell-type specific responses.
... Microglia serve many functions in the healthy CNS and are considered the first line of defense responding to infection and damaged tissue [9][10][11][12][13]. These glial cells constantly survey their microenvironment, using a cadre of receptors on their surface recognizing pathogen- ...
Article
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Neuroinflammation and neurodegeneration are common during prion infection, but the mechanisms that underlie these pathological features are not well understood. Several components of innate immunity, such as Toll-like receptor (TLR) 4 and Complement C1q, have been shown to influence prion disease. To identify additional components of innate immunity that might impact prion disease within the central nervous system (CNS), we screened RNA from brains of pre-clinical and clinical 22L-infected mice for alterations in genes associated with innate immunity. Transcription of several genes encoding damage-associated molecular pattern (DAMP) proteins and receptors were increased in the brains of prion-infected mice. To investigate the role of some of these proteins in prion disease of the CNS, we infected mice deficient in DAMP receptor genes Tlr2, C3ar1, and C5ar1 with 22L scrapie. Elimination of TLR2 accelerated disease by a median of 10 days, while lack of C3aR1 or C5aR1 had no effect on disease tempo. Histopathologically, all knockout mouse strains tested were similar to infected control mice in gliosis, vacuolation, and PrPSc deposition. Analysis of proinflammatory markers in the brains of infected knockout mice indicated only a few alterations in gene expression suggesting that C5aR1 and TLR2 signaling did not act synergistically in the brains of prion-infected mice. These results indicate that signaling through TLR2 confers partial neuroprotection during prion infection.
... In addition, our demonstration of PS exposure occurring specifically in axonal sub-domains is a strong indication that there is a tight spatial regulation of PS exposure in the cell membrane, allowing it to serve as a selective "eat me" signal targeting axons or parts of axons for removal without cell death. This suggests that PS exposure could serve in other spatially-regulated processes executed by microglia and astrocytes, such as synapse elimination [53][54][55][56] . ...
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Apoptotic cells expose Phosphatidylserine (PS), that serves as an “eat me” signal for engulfing cells. Previous studies have shown that PS also marks degenerating axonsduring developmental pruning or in response to insults (Wallerian degeneration), but the pathways that control PS exposure on degenerating axons are largely unknown. Here, we used a series of in vitro assays to systematically explore the regulation of PS exposure during axonal degeneration. Our results show that PS exposure is regulated by the upstream activators of axonal pruning and Wallerian degeneration. However, our investigation of signaling further downstream revealed divergence between axon degeneration and PS exposure. Importantly, elevation of the axonal energetic status hindered PS exposure, while inhibition of mitochondrial activity caused PS exposure, without degeneration. Overall, our results suggest that the levels of PS on the outer axonal membrane can be dissociated from the degeneration process and that the axonal energetic status plays a key role in the regulation of PS exposure.
... But overproduction of cytokines by microglia is also harmful. And excessive engulfment of synapses by microglia might contribute to cognitive impairment in AD. 20,47,48 Synapse loss is in fact a hallmark of AD and many other neurodegenerative diseases, and can occur years before clinical symptoms-and fewer synapses in the AD's brain correlate with cognitive decline. 49,50 The mechanisms underlying synapse loss and dysfunction are poorly understood, although there are clues. ...
Article
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New knowledge about microglia is so fresh that it's not even in the textbooks yet. Microglia are cells that help guide brain development and serve as its immune system helpers by gobbling up diseased or damaged cells and discarding cellular debris. Our authors believe that microglia might hold the key to understanding not just normal brain development, but also what causes Alzheimer's disease, Huntington's disease, autism, schizophrenia, and other intractable brain disorders.
... But overproduction of cytokines by microglia is also harmful. And excessive engulfment of synapses by microglia might contribute to cognitive impairment in AD. 20,47,48 Synapse loss is in fact a hallmark of AD and many other neurodegenerative diseases, and can occur years before clinical symptoms-and fewer synapses in the AD's brain correlate with cognitive decline. 49,50 The mechanisms underlying synapse loss and dysfunction are poorly understood, although there are clues. ...
Article
Editor’s Note New knowledge about microglia is so fresh that it’s not even in the textbooks yet. Microglia are cells that help guide brain development and serve as its immune system helpers by gobbling up diseased or damaged cells and discarding cellular debris. Our authors believe that microglia might hold the key to understanding not just normal brain development, but also what causes Alzheimer’s disease, Huntington’s disease, autism, schizophrenia, and other intractable brain disorders.
... The interplay between different cells in a tissue ecosystem is crucial for maintaining tissue homeostasis. While many diseases have been traditionally perceived as the result of the malfunction of a particular cell or cell type, mounting evidence (1)(2)(3)(4)(5) and new therapeutic strategies (3,6,7) have demonstrated the pivotal role of multicellular action in maintaining homeostasis and its dysregulation in a wide-range of diseases, thus opening new opportunities for intervention, diagnosis, disease monitoring and prevention. In parallel, advances in single cell RNA-seq (scRNA-seq) (8,9) and spatial transcriptomics (10)(11)(12) now allow us to systematically explore molecular profiles at single cell resolution across cell types (13), tissues (14,15), and disease states (16)(17)(18), in both isolated cells and intact tissues (10)(11)(12). ...
Preprint
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Tissue homeostasis relies on orchestrated multicellular circuits, where interactions between different cell types dynamically balance tissue function. While single-cell genomics identifies tissues' cellular components, deciphering their coordinated action remains a major challenge. Here, we tackle this problem through a new framework of multicellular programs: combinations of distinct cellular programs in different cell types that are coordinated together in the tissue, thus forming a higher order functional unit at the tissue, rather than only cell, level. We develop the open-access DIALOGUE algorithm to systematically uncover such multi-cellular programs not only from spatial data, but even from tissue dissociated and profiled as single cells, e.g., by single-cell RNA-Seq. Tested on spatial transcriptomes from the mouse hypothalamus, DIALOGUE recovered spatial information, predicted the properties of a cell's environment only based on its transcriptome, and identified multicellular programs that mark animal behavior. Applied to brain samples and colon biopsies profiled by scRNA-Seq, DIALOGUE identified multicellular configurations that mark Alzheimer's disease and ulcerative colitis (UC), including a program spanning five cell types that is predictive of response to anti-TNF therapy in UC patients and enriched for UC risk genes from GWAS, each acting in different cell types, but all cells acting in concert. Taken together, our study provides a novel conceptual and methodological framework to unravel multicellular regulation in health and disease.
... Our microglia analysis focused on the PVN, which is part of the hypothalamic-pituitary-adrenal (HPA) axis and is considered the major neuroendocrine system that regulates the body's response to stress [80]. Since microglia are crucial in early life for synaptic pruning, maternal antibiotic use could alter the rates of synaptogenesis during this critical window of development, which then persists long after the offspring have been weaned [81,82]. Here, we found that maternal antibiotic use leads to altered neuroimmune responses in their offspring at 10 weeks of age, as seen by longer processes and greater dendrite complexity in response to LPS. ...
Article
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Microbial colonization of the gut early in life is crucial for the development of the immune and nervous systems, as well as influencing metabolism and weight gain. While early life exposure to antibiotics can cause microbial dysbiosis, prebiotics are non-digestible substrates that selectively promote the growth of beneficial gut microbiota. Our objective was to examine the effects of dietary prebiotic administration on the consequences of maternal antibiotic intake on offspring body weight, behavior, and neuroimmune responses later in life. Sprague-Dawley rat dams were given low-dose penicillin (LDP), prebiotic fiber (10% oligofructose), or both, during the third week of pregnancy and throughout lactation. Anxiety-like behavior, weight gain, body composition, cecal microbiota composition, and microglial responses to lipopolysaccharide (LPS) were assessed in offspring. Male and female prebiotic offspring had lower body weight compared to antibiotic offspring. Maternal antibiotic exposure resulted in lasting effects on select offspring microbiota including a lower relative abundance of Streptococcus, Lactococcus, and Eubacterium at 10 weeks of age. Maternal antibiotic use impaired microglial response to LPS in the hypothalamus compared to control, and this phenotype was reversed with prebiotic. Prebiotic fiber warrants further investigation as an adjunct to antibiotic use during pregnancy.
... The latter are further divided into microglia, astrocyte, and oligodendrocyte. Microglia are intrinsic macrophages of the CNS, which have the function of clearing damaged and apoptotic neurons and pruning synapses [4]. Studies have demonstrated that microglia cleared red blood cells after ICH [5]. ...
Article
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Intracerebral hemorrhage (ICH) is a cerebrovascular disease with high mortality and morbidity for which effective treatments are currently lacking. Wogonin is a major flavonoid compound isolated from Scutellaria radix. Accumulating evidence suggests that wogonin plays a crucial role in anti-inflammatory and anti-oxidative stress. Treatment of microglia with nuclear receptor agonists augments the expression of phagocytosis-related genes. However, the neuroprotective effects of wogonin in ICH remain obscure. In this study, we elucidated an innovative mechanism by which wogonin acts to enhance phagocytosis in a murine model of ICH. Wogonin promoted hematoma clearance and improved neurological recovery after ICH by upregulating the expression of Axl, MerTK, CD36, and LAMP2 in perihematomal microglia and BV2 cells. Treatment of a murine model of ICH with wogonin stimulated microglial phagocytosis in vitro. Further, we demonstrated that wogonin dramatically attenuated inflammatory and oxidative stress responses in a murine model of ICH by reducing the expression of pro-inflammatory cytokines and pro-oxidant enzymes such as TNF-α, IL-1β, and inducible nitric oxide synthase (iNOS) after ICH. The effects of wogonin were abolished by administration of the PPAR-γ inhibitor GW9662. In conclusion, our data suggest that wogonin facilitates hematoma clearance and neurobehavioral recovery by targeting PPAR-γ.
... A study in the hippocampus of adult virgin female Wistar rat showed that several genes (Adora2a, Car3, Chat, Drd2, Egr2, Hif3a, Notch, Penk, Tac1, Ttr, Sema3A, and Penk) induced by PRL are related to the regulation of brain functions and processes associated with axons, synaptic transmission, and microglial regulation [104], and the expression of these genes had been previously shown to be altered in neuroinflammatory diseases and NDDs [35,[105][106][107][108][109][110][111][112]. Of note, Adora2a is related to synaptic im-pairment in neurons, and high Adora2a expression is related to neurogenesis and hippocampal volume in AD [113,114]. ...
Article
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Background: Prolactin (PRL) is one of the most diverse pituitary hormones and is known to modulate normal neuronal function and neurodegenerative conditions. Many studies have described the influence PRL has on the central nervous system (CNS) and addressed its contribution to neurodegeneration, but little is known of the mechanisms responsible for the effects of PRL on neurodegenerative disorders, especially on Alzheimer’s disease (AD) and Parkinson’s disease (PD). Summary: we review and summarize existing literature and current understanding of the roles of PRL on various PRL aspects of AD and PD. Key Messages: In general, PRL is viewed as a promising molecule for the treatment of AD and PD. Modulation of PRL functions and targeting of immune mechanisms are needed to devise preventive or therapeutic strategies.
... These glial cells have immunocompetent potential and are known as phagocytic cells of the CNS (Prinz et al., 2019). Microglia originate from yolk sac progenitors and contribute to synaptic pruning during development and synaptic modulation (Wu et al., 2015;Hong and Stevens, 2016). NG2 glial cells, also known as oligodendrocytes progenitor cells (OPCs) (Ffrench-Constant et al., 1986;Dimou and Götz, 2014), first appear in the early phases of development and are present in the adult CNS (Kirdajova and Anderova, 2020). ...
Article
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Cross talk between glia and neurons is crucial for a variety of biological functions, ranging from nervous system development, axonal conduction, synaptic transmission, neural circuit maturation, to homeostasis maintenance. Extracellular vesicles (EVs), which were initially described as cellular debris and were devoid of biological function, are now recognized as key components in cell-cell communication and play a critical role in glia-neuron communication. EVs transport the proteins, lipids, and nucleic acid cargo in intercellular communication, which alters target cells structurally and functionally. A better understanding of the roles of EVs in glia-neuron communication, both in physiological and pathological conditions, can aid in the discovery of novel therapeutic targets and the development of new biomarkers. This review aims to demonstrate that different types of glia and neuronal cells secrete various types of EVs, resulting in specific functions in intercellular communications.
... This aligns with human molecular and anatomical neuropathology studies of brains from individuals with ASD, identifying reactive microglial accumulation and an increase in related gene expression (Gupta et al. 2014;Morgan et al. 2012;Pinto et al. 2014;Vargas et al. 2005;Voineagu et al. 2011). This convergence on microglia and developmental neuroinflammation as risks for ASD reflects an important role for this immunocompetent cell in typical brain development, with disruption altering connectivity and functions that are related to characteristics of ASD (Hong and Stevens 2016;Prinz and Priller 2014;Zhan et al. 2014). PM 2:5 exposure also may increase ASD risk by its impact on placental health, which is essential for proper fetal brain development (Zeltser and Leibel 2011). ...
Article
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Background: Studies have shown that air pollution exposures during pregnancy are associated with an increased risk of autism spectrum disorder (ASD) in children, and the risk appears to be greater for boys. However, studies assessing gestational windows of susceptibility have been mostly limited by trimesters. Objective: We identified sensitive windows of exposure to regional air pollution and risk of ASD and examined sex differences in a large birth cohort. Methods: This population-based retrospective cohort study included 294,937 mother-child pairs with singleton deliveries in Kaiser Permanente Southern California (KPSC) hospitals from 2001 to 2014. Children were followed using electronic medical records until clinical ASD diagnosis, non-KPSC membership, death, or 31 December 2019, whichever came first. Weekly mean fine particulate matter [PM with an aerodynamic diameter of ≤2.5μm (PM2.5)], nitrogen dioxide (NO2), and ozone (O3) pregnancy exposures were estimated using spatiotemporal prediction models. Cox proportional hazard models with distributed lags were used to estimate weekly pollutant exposure associations with ASD risk for the entire cohort, and separately for boys and for girls. Models were adjusted for child sex (for full cohort), maternal race/ethnicity, maternal age at delivery, parity, maternal education, maternal comorbidities, medical center, census tract median household income, birth year, and season. Results: There were 5,694 ASD diagnoses (4,636 boys, 1,058 girls). Sensitive PM2.5 exposure windows associated with ASD were found early in pregnancy, statistically significant throughout the first two trimesters [1-27 wk of gestation, cumulative hazard ratio (HR)=1.14 [95% confidence interval (CI): 1.06, 1.23] per interquartile range (IQR) (7.4-μg/m3) increase]. O3 exposure during 34-37 wk of gestation was associated with increased risk [HR=1.06 (95% CI: 1.01, 1.11) per IQR (17.4 ppb) increase] but with reduced risk during 20-28 wk of gestation [HR=0.93 (95% CI: 0.89, 0.98)]. No associations were observed with NO2. Sex-stratified early gestational PM2.5 associations were stronger among boys [boys HR=1.16 (95% CI: 1.08, 1.26); girls HR=1.06 (95% CI: 0.89, 1.26)]. O3 associations in later gestation were observed only in boys [boys HR=1.10 (95% CI: 1.04, 1.16); girls HR=0.94 (95% CI: 0.84, 1.05)]. Conclusions: Exposures to PM2.5 in the first two gestational trimesters were associated with increased ASD risk in children, with stronger associations observed for boys. The role of O3 exposure on ASD risk merits further investigation. https://doi.org/10.1289/EHP9509.
... When microglia cells are settled in the embryonic brain, they constitute a dynamic and highly activated network that shape and remodel the CNS (Hong and Stevens, 2016). While the brain develops, microglia cells also undergo gene transcriptional changes that give them distinct functions throughout life. ...
Conference Paper
Alzheimer´s disease (AD) is the most common type of dementia representing an estimated 60-80% of all cases, and no cure or successful therapy has been found. Two main hallmarks have been identified in AD histopathology: senile plaques, composed of amyloid-β protein (Aβ), and neurofibrillary tangles, composed of phosphorylated TAU protein. Additionally, genetic studies have shown that immune processes play important roles in AD. Microglial gene expression and function are closely correlated to amyloid pathology and are therefore potential targets for altering the progression of AD. Recently an Amyloid Precursor Protein knock-in line was generated, which, in contrast to transgenic AD mice shows an Aβ pathology without overexpression. This project aims to analyse if microglial cells are active modulators of Aβ plaques and synaptic changes in APP knock-in mice at the early stages of the pathology. Initial characterisation of electrophysiological phenotypes for APPNL-G-F and APPNL-F were studied. Also, dose- and time-dependent effects of the drug PLX5622, which has been shown to specifically deplete microglia, were analysed. APPNL-G-F mice exhibited unaltered synaptic transmission at 3.5 months of age regardless a clear accumulation of hippocampal Aβ plaques. APPNL-F mice showed increased glutamate release probability, unchanged spontaneous excitatory activity and little accumulation of Aβ plaques at 10 months of age. After PLX5622 treatments, surviving microglia tended to be CD68+ in both APP knock-in models. Partial microglia ablation led to aged but not young wild type animals mimicking the increased glutamate release probability and exacerbated the APP knock-in phenotype. Complete ablation was less effective in altering synaptic function, while neither treatment altered plaque load. It is suggested that alteration of surviving microglia towards a phagocytic phenotype, rather than microglial loss, drives age-dependent effects on glutamate release that become exacerbated in AD.
... clearance of cellular debris, but if not controlled this process can result in aberrant synaptic engulfment [4][5][6][7]. Temporary depletion of microglia can be achieved by using pharmacologic inhibitors of the colony-stimulating factor 1 receptor (CSF-1R) [8]. In the normal brain, treatment with CSF-1R inhibitors (CSF-1Ri) can deplete up to 99% of microglia without causing detectable changes to cognitive functions [8,9]. ...
Article
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Background Brain-resident microglia have a distinct origin compared to macrophages in other organs. Under physiological conditions, microglia are maintained by self-renewal from the local pool, independent of hematopoietic progenitors. Pharmacological depletion of microglia during whole-brain radiotherapy prevents synaptic loss and long-term recognition memory deficits. However, the origin or repopulated cells and the mechanisms behind these protective effects are unknown. Methods CD45 low/int /CD11b ⁺ cells from naïve brains, irradiated brains, PLX5622-treated brains and PLX5622 + whole-brain radiotherapy-treated brains were FACS sorted and sequenced for transcriptomic comparisons. Bone marrow chimeras were used to trace the origin and long-term morphology of repopulated cells after PLX5622 and whole-brain radiotherapy. FACS analyses of intrinsic and exotic synaptic compartments were used to measure phagocytic activities of microglia and repopulated cells. In addition, concussive brain injuries were given to PLX5622 and brain-irradiated mice to study the potential protective functions of repopulated cells after PLX5622 + whole-brain radiotherapy. Results After a combination of whole-brain radiotherapy and microglia depletion, repopulated cells are brain-engrafted macrophages that originate from circulating monocytes. Comparisons of transcriptomes reveal that brain-engrafted macrophages have an intermediate phenotype that resembles both monocytes and embryonic microglia. In addition, brain-engrafted macrophages display reduced phagocytic activity for synaptic compartments compared to microglia from normal brains in response to a secondary concussive brain injury. Importantly, replacement of microglia by brain-engrafted macrophages spare mice from whole-brain radiotherapy-induced long-term cognitive deficits, and prevent concussive injury-induced memory loss. Conclusions Brain-engrafted macrophages prevent radiation- and concussion-induced brain injuries and cognitive deficits.
... However, another study revealed elevated serum PRL levels in patients with AD, but significantly higher levels only in patients with severe dementia, indicating dementia is associated with dysfunction of the tuberoinfundibular pathway that regulates PRL secretion [32]. Several genes induced by PRL were identified to be altered in neuroinflammation and NDs [33][34][35][36][37][38][39][40][41]. Of note, Adora2a in neurons is connected to synaptic impairment and increased Adora2a gene expression related to neurogenesis and hippocampal volume in patients with AD [42,43]. ...
Article
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Introduction: Prolactin (PRL) exerts inflammatory and anti-inflammatory properties and is also thought to play an important role in the pathogenesis of neurodegenerative diseases (NDs). However, serum PRL levels in patients with NDs were inconsistent in the research literature. Objective: We aimed to assess the serum PRL levels in patients with NDs. Methods: Electronic databases, including MEDLINE, Embase, Cochrane Library database, clinicaltrials.gov, Web of Science, and Google Scholar, and reference lists of articles were searched up to December 31, 2020. Pooled standard mean difference (SMD) with 95% confidence interval (CI) was calculated by fixed-effect or random-effect model analysis. Results: A total of 36 comparisons out of 29 studies (3 RCTs and 26 case controls) focusing on NDs (including Parkinson's disease, Alzheimer's disease, Huntington's disease [HD], multiple sclerosis [MS], and epilepsy) were reported. The meta-analysis showed that there was no statistically significant difference in serum PRL levels between patients with NDs and healthy controls (SMD = 0.40, 95% CI: -0.16 to 0.96, p = 0.16). Subgroup analysis showed that serum PRL levels in patients with HD and MS were higher than those of healthy controls. Furthermore, patients with NDs aged <45 years had higher serum PRL levels (SMD = 0.97, 95% CI: 0.16-1.78, p = 0.018) than healthy controls. High serum PRL levels were found in subgroups such as the microenzymatic method, Asia, and the Americas. Conclusions: Our meta-analysis showed serum PRL levels in patients with HD and MS were significantly higher than those in healthy controls. Serum PRL levels were associated with age, region, and detection method. Other larger sample studies using more uniform detection methods are necessary to confirm our results.
... These cells are important in host defense against infection and in responding to cellular damage in the CNS. Furthermore, they play diverse roles in the CNS that include neurodevelopment, phagocytosing dead or dying cells, and maintenance of homeostasis in the brain [3][4][5]. Microglia also influence astrocytes in the CNS in response to lipopolysaccharide (LPS) by secretion of immune effectors (TNFα, IL-1α, and C1q) that induce astrocytes to assume a proinflammatory phenotype, termed A1-astrocytes [6,7]. Thus, microglia can directly and indirectly influence neuroinflammation and neurodegeneration. ...
Article
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Background Past experiments studying innate immunity in the central nervous system (CNS) utilized microglia obtained from neonatal mouse brain, which differ developmentally from adult microglia. These differences might impact our current understanding of the role of microglia in CNS development, function, and disease. Methods Cytokine protein secretion was compared in ex vivo P3 and adult microglial cultures after exposure to agonists for three different toll-like receptors (TLR4, lipopolysaccharide [LPS]; TLR7, imiquimod [IMQ]; and TLR9, CpG Oligodeoxynucleotide [CpG-ODN] 1585). In addition, changes in inflammatory gene expression in ex vivo adult microglia in response to the TLR agonists was assessed. Furthermore, in vivo experiments evaluated changes in gene expression associated with inflammation and TLR signaling in brains of mice with or without treatment with PLX5622 to reduce microglia. Results Ex vivo adult and P3 microglia increased cytokine secretion when exposed to TLR4 agonist LPS and to TLR7 agonist IMQ. However, adult microglia decreased expression of numerous genes after exposure to TLR 9 agonist CpG-ODN 1585. In contrast, in vivo studies indicated a core group of inflammatory and TLR signaling genes increased when each of the TLR agonists was introduced into the CNS. Reducing microglia in the brain led to decreased expression of various inflammatory and TLR signaling genes. Mice with reduced microglia showed extreme impairment in upregulation of genes after exposure to TLR7 agonist IMQ. Conclusions Cultured adult microglia were more reactive than P3 microglia to LPS or IMQ exposure. In vivo results indicated microglial influences on neuroinflammation were agonist specific, with responses to TLR7 agonist IMQ more dysregulated in mice with reduced microglia. Thus, TLR7-mediated innate immune responses in the CNS appeared more dependent on the presence of microglia. Furthermore, partial responses to TLR4 and TLR9 agonists in mice with reduced microglia suggested other cell types in the CNS can compensate for their absence.
... High concentrations of this pro-inflammatory cytokine during pregnancy were associated with differences in newborn functional brain networks and social, emotional, and cognitive development, and it has been reported that changes in its levels can lead to cognitive and behavioral deficits in offspring by altering synapse formation (Rudolph et al., 2018). Also, excessive exposure to pro-inflammatory cytokines causes inadequate activation of the microglia, altering neuronal activity, which prevents the differentiation and survival of neurons (Cope et al., 2018;Hong and Stevens, 2016). ...
Article
The early stage of development is a vulnerable period for progeny neurodevelopment, altering cytogenetic and correct cerebral functionality. The exposure High-Fat Diet (HFD) is a factor that impacts the future mental health of individuals. This review analyzes possible mechanisms involved in the development of mood disorders in adulthood because of maternal HFD intake during gestation and lactation, considering previously reported findings in the last five years, both in humans and animal models. Maternal HFD could induce alterations in mood regulation, reported as increased stress response, anxiety-like behavior, and depressive-like behavior. These changes were mostly related to HPA axis dysregulations and neuroinflammatory responses. In conclusion, there could be a relationship between HFD consumption during the early stages of life and the development of psychopathologies during adulthood. These findings provide guidelines for the understanding of possible mechanisms involved in mood disorders, however, there is still a need for more human clinical studies that provide evidence to improve the understanding of maternal nutrition and future mental health outcomes in the offspring.
... Redundant cells and synapses are removed by microglia in a process called synaptic pruning. 31,32 The final stage in fetal brain development is the formation of myelin sheaths, which creates an insulating layer that permits fast signal transduction. Even in adult life, stem cells in the central nervous system remain active, 33 differentiating into neural or glial cells that migrate into the cerebral white or grey matter. ...
Article
The performance of the human brain is based on an interplay between the inherited genotype and external environmental factors, including diet. Food and nutrition, essential in maintenance of brain performance, also aid in prevention and treatment of mental disorders. Both the overall composition of the human diet and specific dietary components have been shown to have an impact on brain function in various experimental models and epidemiological studies. This narrative review provides an overview of the role of diet in 5 key areas of brain function related to mental health and performance, including: (1) brain development, (2) signaling networks and neurotransmitters in the brain, (3) cognition and memory, (4) the balance between protein formation and degradation, and (5) deteriorative effects due to chronic inflammatory processes. Finally, the role of diet in epigenetic regulation of brain physiology is discussed.
... 5,6 During phagocytosis, proteins on the microglial cell surface, such as the Toll-Like Receptors (TLRs), Fc receptors, and scavenger receptors including CD36 and the receptor for advanced glycation end products (RAGE) among others, recognize the "eat-me" signals and engulf the target substrates into intracellular compartments called phagosomes. [7][8][9][10] The phagosomes mature by fusing with lysosomes to form highly acidic phagolysosomes and mobilize the phagocytosed material for enzymatic degradation. The pH of phagosomal organelles during this maturation process is progressively reduced 11 from 6.0 to around 5.0-4.5. ...
Article
Full-text available
Phagocytosis by glial cells is essential to regulate brain function during health and disease. Therapies for Alzheimer's disease (AD) have primarily focused on targeting antibodies to amyloid β (Aβ) or inhibitng enzymes that make it, and while removal of Aβ by phagocytosis is protective early in AD it remains poorly understood. Impaired phagocytic function of glial cells during later stages of AD likely contributes to worsened disease outcome, but the underlying mechanisms of how this occurs remain unknown. We have developed a human Aβ1-42 analogue (AβpH) that exhibits green fluorescence upon internalization into the acidic organelles of cells but is non-fluorescent at physiological pH. This allowed us to image, for the first time, glial uptake of AβpH in real time in live animals. We find that microglia phagocytose more AβpH than astrocytes in culture, in brain slices and in vivo. AβpH can be used to investigate the phagocytic mechanisms responsible for removing Aβ from the extracellular space, and thus could become a useful tool to study Aβ clearance at different stages of AD.
... Infancy is a period of heightened neuroplasticity as it is characterized by rapid brain growth (Knickmeyer et al., 2008;Tau & Peterson, 2010), massive outgrowth of dendrites and axons, and synaptogenesis alongside synaptic pruning (Huttenlocher & Dabholkar, 1997;Petanjek, Judas, Kostović, & Uylings, 2008). Glial cells proliferate in the subventricular zone of the forebrain, migrate across brain regions, and differentiate into oligodendrocytes and astrocytes (Menn et al., 2006;Sharon, Sampson, Geschwind, & Mazmanian, 2016), which facilitates synaptic pruning by complement activation and phagocytosis (Hong & Stevens, 2016). ...
Article
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Perturbations to the gut microbiome are implicated in altered neurodevelopmental trajectories that may shape life span risk for emotion dysregulation and affective disorders. However, the sensitive periods during which the microbiome may influence neurodevelopment remain understudied. We investigated relationships between gut microbiome composition across infancy and temperament at 12 months of age. In 67 infants, we examined if gut microbiome composition assessed at 1–3 weeks, 2, 6, and 12 months of age was associated with temperament at age 12 months. Stool samples were sequenced using the 16S Illumina MiSeq platform. Temperament was assessed using the Infant Behavior Questionnaire-Revised (IBQ-R). Beta diversity at age 1–3 weeks was associated with surgency/extraversion at age 12 months. Bifidobacterium and Lachnospiraceae abundance at 1–3 weeks of age was positively associated with surgency/extraversion at age 12 months. Klebsiella abundance at 1–3 weeks was negatively associated with surgency/extraversion at 12 months. Concurrent composition was associated with negative affectivity at 12 months, including a positive association with Ruminococcus-1 and a negative association with Lactobacillus . Our findings support a relationship between gut microbiome composition and infant temperament. While exploratory due to the small sample size, these results point to early and late infancy as sensitive periods during which the gut microbiome may exert effects on neurodevelopment.
... In addition, they can alter their gene expression patterns and release functional molecules such as pro-and anti-inflammatory cytokines, reactive oxygen species, and nitric oxide 6,7 . To exert their protective roles, microglia migrate towards the injury site directed by chemotactic signals, as well as phagocytose pathogens, microbes, toxic molecules, and cell debris 8,9 . With all these functions, microglia become the first line of defense in protecting central nervous system against harmful insults by modulating local immune responses. ...
Article
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Microglia, CNS resident innate immune cells, respond strongly to activation of TLR3 and TLR4, which recognize viral dsRNA poly(I:C) and bacterial endotoxin LPS, respectively. However, few studies have thoroughly and parallelly compared functional phenotypes and downstream mechanisms between LPS- and poly(I:C)-exposed primary microglia. Here, we investigated the responses of mouse primary microglia upon LPS and poly(I:C) stimulation by detecting various phenotypes ranging from morphology, proliferation, secretion, chemotaxis, to phagocytosis. Furthermore, we explored their sequential gene expression and the downstream signal cascades. Interestingly, we found that the microglial activation pattern induced by LPS was distinguished from that induced by poly(I:C). Regarding microglial morphology, LPS caused an ameboid-like shape while poly(I:C) induced a bushy shape. Microglial proliferation was also facilitated by LPS but not by poly(I:C). In addition, LPS and poly(I:C) modulated microglial chemotaxis and phagocytosis differently. Furthermore, genome-wide analysis provided gene-level support to these functional differences, which may be associated with NF-κb and type I interferon pathways. Last, LPS- and poly(I:C)-activated microglia mediated neurotoxicity in a co-culture system. This study extends our understanding of TLR roles in microglia and provides insights into selecting proper inflammatory microglial models, which may facilitate identification of new targets for therapeutic application.
... 81,85e87 From a genetic and proteomic perspective, genes and proteins that were dysregulated in the aging brain were found to be connected to immune pathways. Highly enriched in microglia were particularly elements of the complement pathway 88 which label cells for microglia-mediated elimination, 89,90 and of the microglial sensome involved in the sensing of endogenous ligands and pathogens. 91,92 In addition, two subpopulations of reactive microglia in the aging brain have been further characterized by a specific genetic signature, namely the chemokine ligand 4 (CCL4) gene, as well as the upregulation of inflammatory signals (IL-1b, CCL3), interferon-response genes (Ifitm3, Rtp4) and transcription factors (Id2, ATF3). ...
Article
Neurodegenerative diseases and their associated cognitive decline are known to be more prevalent during aging. Recent evidence has uncovered the role of microglia, the immunocompetent cells of the brain, in dysfunctions linked to neurodegenerative diseases such as Alzheimer’s disease (AD). Similar to other pathologies, AD is shown to be sex-biased, with females being more at risk compared to males. While the mechanisms driving this prevalence are still unclear, emerging data suggest the sex differences present in microglia throughout life might lead to different responses of these cells in both health and disease. Furthermore, microglial cells have recently been recognized as a deeply heterogeneous population, with multiple subsets and/or phenotypes stemming from diverse parameters such as age, sex or state of health. Therefore, this review discusses microglial heterogeneity during aging in both basal conditions and AD with a focus on existing sex differences in this process.
... Microglia are the resident immune cells of the central nervous system. In contrast to the other phagocytotic cells in mammals, microglia display strong interactions with neurons, astrocytes, and oligodendrocytes leading to a prominent role of microglia in neuronal development and plasticity [84,85]. As initially documented in mammals, the most striking characteristic of microglia is their high degree of plasticity, which enables them to switch from a resting state (quiescent) to a phagocytotic state (ameboid) in response to injury [86][87][88], a phenomenon also observed in zebrafish after telencephalic injury (Figure 4). ...
Article
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Adult neurogenesis is an evolutionary conserved process occurring in all vertebrates. However, striking differences are observed between the taxa, considering the number of neurogenic niches, the neural stem cell (NSC) identity, and brain plasticity under constitutive and injury-induced conditions. Zebrafish has become a popular model for the investigation of the molecular and cellular mechanisms involved in adult neurogenesis. Compared to mammals, the adult zebrafish displays a high number of neurogenic niches distributed throughout the brain. Furthermore, it exhibits a strong regenerative capacity without scar formation or any obvious disabilities. In this review, we will first discuss the similarities and differences regarding (i) the distribution of neurogenic niches in the brain of adult zebrafish and mammals (mainly mouse) and (ii) the nature of the neural stem cells within the main telencephalic niches. In the second part, we will describe the cascade of cellular events occurring after telencephalic injury in zebrafish and mouse. Our study clearly shows that most early events happening right after the brain injury are shared between zebrafish and mouse including cell death, microglia, and oligodendrocyte recruitment, as well as injury-induced neurogenesis. In mammals, one of the consequences following an injury is the formation of a glial scar that is persistent. This is not the case in zebrafish, which may be one of the main reasons that zebrafish display a higher regenerative capacity.
... are the resident immune cells of the central nervous system. In contrast to other phagocytotic cells in mammals, microglia display strong interactions with neurons, astrocytes and oligodendrocytes leading to a prominent role of microglia in neuronal development and plasticity (Frost and Schafer, 2016;Hong and Stevens, 2016). As shown in mammals, their most striking characteristic is their high degree of plasticity which enables them to switch from a resting state (quiescent) to a phagocytotic state (ameboid) in response to injury (Davalos et al., 2005;Nimmerjahn et al., 2005;Morrison and Filosa, 2013), a phenomenon also observed in zebrafish after telencephalic injury ( Figure 4). ...
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Adult neurogenesis is an evolutionary conserved process occurring in all vertebrates. However, striking differences are observed between the taxa, considering the number of neurogenic niches, the neural stem cell (NSC) identity and brain plasticity under constitutive and injury-induced conditions. Zebrafish has become a popular model for the investigation of the molecular and cellular mechanisms involved in adult neurogenesis. Compared to mammals, the adult zebrafish displays a high number of neurogenic niches distributed throughout the brain. Furthermore, it exhibits a strong regenerative capacity without scar formation or any obvious disabilities. In this review, we will first discuss the similarities and differences regarding (i) the distribution of neurogenic niches in the brain of adult zebrafish and mammals (mainly mouse) and (ii) the nature of the neural stem cells within the main telencephalic niches. In the second part, we will describe the cascade of cellular events occurring after telencephalic injury in zebrafish and mouse. Our study clearly shows that most early events happening right after the brain injury are shared between zebrafish and mouse including cell death, microglia and oligodendrocyte recruitment, as well as injury-induced neurogenesis. In mammals one of the consequences following an injury is the formation of a glial scar that is persistent. This is not the case in zebrafish, which may be one of the main reasons that zebrafish display a higher regenerative capacity.
... They have a crucial role in the correct neurodevelopment of the child: they modulate astrocytic differentiation from neuronal precursor cells and, later, contribute to synaptic pruning and clearance of apoptotic neuronal precursors. [38][39][40] Thus, a cyclic activation of microglia is positively required for a typical neurodevelopment to occur; on the contrary, persistent microglial activation causes brain cell death and reduced or abnormal interneuronal connectivity. 41,42 The 2005 paper by Vargas and coll. 1 might represent the foundational moment in the theorisation of the inflammatory causes of ASD. ...
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Autism spectrum disorder (ASD) is a neurodevelopmental disorder defined by deficits in social communication and stereotypical behaviours. ASD's aetiology remains mostly unclear, because of a complex interaction between genetic and environmental factors. Recently, a strong consensus has developed around ASD's immune-mediated pathophysiology, which is the subject of this review. For many years, neuroimmunological studies tried to understand ASD as a prototypical antibody-or cell-mediated disease. Other findings indicated the importance of autoimmune mechanisms such as familial and individual autoimmunity, adaptive immune abnormalities and the influence of infections during gestation. However, recent studies have challenged the idea that autism may be a classical autoimmune disease. Modern neurodevelopmental immunology shows the double-edged nature of many immune effectors, which can be either beneficial or detrimental depending on tissue homeostasis, stressors, neurodevelopmental stage, inherited and de novo gene mutations and other variables. Nowadays, mother-child interactions in the prenatal environment appear to be crucial for the occurrence of ASD. Studies of animal maternal-foetal immune interaction are being fruitfully carried out using different combinations of type and timing of infection, of maternal immune response and foetal vulnerability and of resilience factors to hostile events. The derailed neuroimmune crosstalk through the placenta initiates and maintains a chronic foetal neuroglial activation, eventually causing the alteration of neurogenesis, migration, synapse formation and pruning. The importance of pregnancy can also allow early immune interventions, which can significantly reduce the increasing risk of ASD and its heavy social burden.
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The decrease of microglia in the hippocampus is a novel mechanism for depression onset. Reversal of this decrease can ameliorate stress-induced depression-like behaviors in rodents. However, the property of this therapeutic strategy remains unclear. We addressed this issue by designing a series of behavioral experiments. Results showed that a single lipopolysaccharide (LPS) injection at the dose of 75 and 100 μg/kg, but not at 30 or 50 μg/kg, produced obvious antidepressant effects in chronic unpredictable stress (CUS) mice at 5 h after the drug administration. In the time-dependent experiment, a single LPS injection (100 μg/kg) ameliorated the CUS-induced depression-like behaviors in mice at 5 and 8 h, but not at 3 h, after the drug administration. The antidepressant effect of a single LPS injection persisted at least 10 days and disappeared at 14 days after the drug administration. 14 days after the first injection, a second LPS injection (100 μg/kg) still produced antidepressant effects in chronically-stressed mice who re-displayed depression-like behaviors at 5 h after the drug administration. The antidepressant effect of LPS appears to be dependent on microglia, as at 5 h after LPS administration (100 μg/kg), the CUS-induced decrease in microglial numbers and Iba-1 mRNA levels in the hippocampus was reversed markedly, and inhibition of microglia by minocycline (40 mg/kg) or PLX33297 (290 mg/kg) prevented the antidepressant effect of LPS in CUS mice. These results indicate that a single LPS injection displays rapid and sustained antidepressant effects in chronically stressed mice likely through stimulating hippocampal microglia.
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The complement system, an essential tightly regulated innate immune system, is a key regulator of normal CNS development and function. However, aberrant complement component expression and activation in the brain may culminate into marked neuroinflammatory response, neurodegenerative processes, and cognitive impairment. Over the years, complement‐mediated neuroinflammatory responses and complement‐driven neurodegeneration have been increasingly implicated in the pathogenesis of a wide spectrum of CNS disorders. This review describes how complement system contributes to normal brain development and function. We also discuss how pathologic insults such as misfolded proteins, lipid droplet/lipid droplet‐associated protein or glycosaminoglycan accumulation, could trigger complement‐mediated neuroinflammatory responses and neurodegenerative process in neurodegenerative proteinopathies, age‐related macular degeneration, and neurodegenerative lysosomal storage disorders.
Thesis
Growing experimental evidences highlight microglial alterations in psychiatric diseases, including depression and stress-related disorders. The central goal of my thesis was to investigate microglia in a mouse model of chronic-stress consisting of 5 days of Repeated Forced Swim Stress (RFSS). Mice exposed to RFSS the paradigm exhibit behavioral changes resembling depressive-like behavior. I first analyzed microglia by confocal imaging in the hippocampus. My analysis showed that the number of microglial cells was unaltered in mice undergoing the RFSS paradigm. However, microglia exhibited an increased ramification of the processes (hyper-ramified morphology) in the hippocampal regions DG, CA1 and CA3. By contrast, I found no change in microglial morphology or cell number in brain regions that are presumably not affected by chronic-stress (somatosensory cortex, caudate-putamen striatal nucleus, cerebellum). Since the phenotypic and functional properties of hyper-ramified microglia are unknown, I analyzed the expression of cytokines and microglial activation markers. By qPCR, I showed that the expression of the classical pro-inflammatory cytokines CCL2, IL6 and IL1β, and of the macrophage-activation marker CD11b was increased in hippocampal microglia upon the RFSS paradigm. By confocal imaging, I showed increased CD11b immunoreactivity and phagosomes volume in hippocampal microglia from RFSS mice. These data suggested that RFSS promotes a pro-inflammatory phenotype in hippocampal microglia. Next, I assessed the motility of microglia in CA1 by in vivo two-photon microscopy. Time-lapse imaging in Cx3cr1GFP/+ mice (with GFP-positive microglia) before and immediately after the RFSS paradigm showed that RFSS markedly reduced the motility of microglial processes. Together, these data showed that RFSS affects the homeostatic functions of microglia in the hippocampus, however, it was not yet clear whether microglia are important for the behavioral phenotype of the RFSS model. To answer this question, I investigated the IL34 knock-out mouse line, exhibiting a 50% reduction in the number of microglial cells. The cytokine IL34 is a ligand for CSF1-receptor, expressed in microglia and required for their survival. My data showed that IL34 knock-out mice did not develop RFSS-induced behavioral changes, whereas learning and memory skills at the Morris-Water Maze were unaffected. These data suggest that hyper-ramified microglia play a crucial role for the behavioral phenotype of the RFSS model. In line with this assumption, I could show that LPS injection in RFSS mice induced a rapid de-ramification of microglial processes, along with a partial “rescue” of the RFSS-induced behavioral changes. Further studies are needed to understand the biological underpinnings of this mechanism. Because TNFα was found to be increased in depressed patients, and microglia are the main producer of TNFα in the bran, I hypothesized that microglia may elicit RFSS-induced behavioral changes in a TNFα-dependent manner. To test this, I analyzed mice harboring a conditional genetic deletion of the TNFα locus in microglia. My results showed that mice with TNFα-deficient microglia exhibit RFSS-induced behavioral changes indistinguishable from wild-type. These data suggested that microglia-derived TNFα does not play an important role at the RFSS paradigm. I then hypothesized that hyper-ramified microglia in the hippocampus may affect synaptic function during the RFSS paradigm. Hence, I analyzed the number of glutamatergic synapses in CA1 and somatosensory cortex in RFSS and control mice by confocal imaging. In CA1 of RFSS mice the number of glutamatergic synapses was significantly reduced whereas the somatosensory cortex was unaffected. Given that hyper-ramified microglia were found in the CA1, but not in somatosensory cortex, it is possible that microglia played a role in the synaptic loss. It was shown that microglia can sense the activity of glutamatergic synapses via the purinergic receptors. Moreover, growing evidences suggest that the brain’s purinergic signaling is involved in chronic-stress. I then set out to investigate the role of P2Y12R (a microglial purinergic receptor) in the RFSS model. To do so, I tested both P2Y12R knock-out and wild-type mice to the RFSS paradigm. Interestingly, P2Y12R knock-out mice did not develop RFSS-induced behavioral changes. Moreover, RFSS-induced microglia hyper-ramification and synaptic loss in CA1 were partially reduced in P2Y12R-deficient mice. With a following experiment, I showed that wild-type mice treated with a P2Y12R-inhibitor did not exhibit RFSS-induced behavioral changes, indicating that pharmacological blockage of the P2Y12R-signalling in wild-types recapitulates the RFSS-resilient phenotype of the P2Y12R knock-out mice. Together, these data emphasize the importance of microglia in this model of RFSS and reveal a previously unappreciated role for the P2Y12R signaling during chronic-stress.
Article
Sensitive periods are times of development during which the effects of experience are unusually strong and long lasting. The peripubertal period has emerged as one such sensitive period, and a single administration of lipopolysaccharide (LPS) during this time reduces hormone-induced sexual behavior in adult female mice. During periods of high synaptic turnover, maturation, and elimination, as occurs during this sensitive period, microglia are particularly active. Estradiol also regulates microglial numbers, morphology, and activation. In addition, a good deal of evidence suggests that estradiol may confer this vulnerability to the effects of a stressor during the peripubertal period. Therefore, we investigated the effects of estradiol on microglial morphology, cytokine levels, and the sickness response to LPS. Estradiol levels were manipulated by implanting an estradiol-filled SILASTIC capsule (or oil-filled control) in ovariectomized mice or by administering the aromatase inhibitor, formestane (or oil control), to ovary-intact mice. We found that (1) estradiol elevates basal microglial Iba1 immunoreactivity in the ventromedial nucleus of the hypothalamus (VMH), (2) LPS induces higher levels of proinflammatory cytokines in the presence of estradiol, and (3) LPS causes hypothermia in the presence of estradiol. Taken together, these data suggest that estradiol enhances the effect of LPS during the pubertal sensitive period.
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Bruton’s tyrosine kinase (BTK), a critical component of B cell receptor signaling, has recently been implicated in regulation of the peripheral innate immune response. However, the role of BTK in microglia, the resident innate immune cells of the central nervous system, and its involvement in the pathobiology of neurodegenerative disease has not been explored. Here we found that BTK is a key regulator of microglial phagocytosis. Using potent BTK inhibitors and small interfering RNA (siRNA) against BTK, we observed that blockade of BTK activity decreased activation of phospholipase gamma 2, a recently identified genetic risk factor in Alzheimer’s disease (AD), and reduced phagocytosis in rodent microglia and human monocyte-derived macrophages. Inhibition of BTK signaling also decreased microglial uptake of synaptosomes but did not have major impacts on other key microglial functions such as migration and cytokine release. Similarly, blocking BTK function ex vivo in acute brain slices reduced microglial phagocytosis and maintained numbers of resting microglia. In brain tissues from the 5xFAD mouse model of AD, levels of microglial BTK were elevated while in two gene expression datasets of post-mortem AD patient brain tissues, upregulation of BTK transcript was observed. Our study provides novel insights into the role of BTK in regulating microglial phagocytosis and uptake of synaptic structures and suggests that inhibiting microglial BTK may improve cognition in AD by preventing microglial activation and synaptic loss. Graphical AbstractMicroglial-mediated synapse loss has been implicated in AD pathogenesis. Inhibition of BTK decreases activation of PLCγ2, a genetic risk factor in AD, and reduces microglial phagocytosis and uptake of synaptic structures. As such BTK inhibition may represent a therapeutic route to prevent microglial activation and synapse loss in AD
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Neuroinflammation is a key part of the etio-pathogenesis of Alzheimer’s disease. We test the relationship between neuroinflammation and the disruption of functional connectivity in large-scale networks, and their joint influence on cognitive impairment. We combined [ ¹¹ C]PK11195 positron emission tomography (PET) and resting-state functional magnetic resonance imaging (rs-fMRI) in 28 humans (13 females/15 males) with clinical diagnosis of probable Alzheimer’s disease or mild cognitive impairment with positive PET biomarker for amyloid, and 14 age-, sex-, and education-matched healthy humans (8 females/6 males). Source-based ‘inflammetry’ was used to extract principal components of [ ¹¹ C]PK11195 PET signal variance across all participants. rs-fMRI data were pre-processed via independent component analyses to classify neuronal and non-neuronal signals. Multiple linear regression models identified sources of signal co-variance between neuroinflammation and brain connectivity profiles, in relation to group and cognitive status. Patients showed significantly higher [ ¹¹ C]PK11195 binding relative to controls, in a distributed spatial pattern including the hippocampus, medial, and inferior temporal cortex. Patients with enhanced loading on this [ ¹¹ C]PK11195 binding distribution displayed diffuse abnormal functional connectivity. The expression of a stronger association between such abnormal connectivity and higher levels of neuroinflammation correlated with worse cognitive deficits. Our study suggests that neuroinflammation relates to the pathophysiological changes in network function that underlie cognitive deficits in Alzheimer’s disease. Neuroinflammation, and its association with functionally-relevant reorganisation of brain networks, is proposed as a target for emerging immuno-therapeutic strategies aimed at preventing or slowing the emergence of dementia. Significance Statement Neuroinflammation is an important aspect of Alzheimer’s disease (AD), but it was not known whether the influence of neuroinflammation on brain network function in humans was important for cognitive deficit. Our study provides clear evidence that in vivo neuroinflammation in AD impairs large-scale network connectivity; and that the link between inflammation and functional network connectivity is relevant to cognitive impairment. We suggest that future studies should address how neuroinflammation relates to network function as AD progresses; and whether the neuroinflammation in AD is reversible, as the basis of immunotherapeutic strategies to slow the progression of AD.
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Tau pathology and neuroinflammation are key etio-pathogenetic mediators of Alzheimer’s disease (AD). Network dysfunction has also been reported in AD and linked to cognitive impairment. However, it is unclear how tau pathology and neuroinflammation contribute to network dysfunction and cognitive deficit in AD. I study these issues by combining: 1) positron emission tomography imaging using the [18F-AV]-1451 tracer (measuring in vivo tau pathology) or [11C]PK11195 ligand (indexing in vivo neuroinflammation), with 2) connectivity measures of resting-state functional magnetic resonance imaging. I found increased [18F-AV]-1451 binding (reflecting tau pathology) in AD patients, relative to controls, in the medial/lateral temporal and parietal cortices. In terms of functional connectivity, more strongly connected brain regions accrued more tau pathology. Increasing tau burden was also linked to progressive weakening of the connectivity across the same regions. I also found increased [11C]PK11195 binding (reflecting neuroinflammation) in the medial/lateral temporal and parietal cortices in AD patients, relative to controls. [11C]PK11195 binding in the cuneus/precuneus correlated with episodic memory deficits in AD patients. This pattern of neuroinflammation was linked to large-scale network’ dysfunction and cognitive deficit. AD patients with enhanced neuroinflammation showed more abnormal connectivity across the whole-brain. The expression of a stronger association between altered functional connectivity and high levels of neuroinflammation related to cognitive deficit in AD. My studies have wide-ranging implications that include: 1) the validation of animal models of tau propagation in living patients with AD; 2) improvements in our understanding of the relationship between in vivo tau pathology and brain functioning; 3) evidence for a primary role of neuroinflammation in mediating network dysfunction in AD; 4) support to the notion that immune-therapeutic strategies targeting tau pathology and neuroinflammation may be useful in AD.
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Morphological and molecular characteristics determine the function of biological tissues. Attempts to combine immunofluorescence and electron microscopy invariably compromise the quality of the ultrastructure of tissue sections. We developed NATIVE, a correlated light and electron microscopy approach that preserves ultrastructure while showing the locations of multiple molecular moieties, even deep within tissues. This technique allowed the large-scale 3D reconstruction of a volume of mouse hippocampal CA3 tissue at nanometer resolution. © 2018, The Author(s), under exclusive licence to Springer Nature America, Inc.
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The overarching objective is to review how early exposure to adversity interacts with inflammation to alter brain maturation. Both adversity and inflammation are significant risk factors for psychopathology. Literature relevant to the effects of adversity in children and adolescents on brain development is reviewed. These studies are supported by research in animals exposed to species-relevant stressors during development. While it is known that exposure to adversity at any age increases inflammation, the effects of inflammation are exacerbated at developmental stages when the immature brain is uniquely sensitive to experiences. Microglia play a vital role in this process, as they scavenge cellular debris and prune synapses to optimize performance. In essence, microglia modify the synapse to match environmental demands, which is necessary for someone with a history of adversity. Overall, by piecing together clinical and preclinical research areas, what emerges is a picture of how adversity uniquely sculpts the brain. Microglia interactions with the inhibitory neurotransmitter GABA (specifically, the subtype expressing parvalbumin) are discussed within contexts of development and adversity. A review of inflammation markers in individuals with a history of abuse is combined with preclinical studies to describe their effects on maturation. Inconsistencies within the literature are discussed, with a call for standardizing methodologies relating to the age of assessing adversity effects, measures to quantify stress and inflammation, and more brain-based measures of biochemistry. Preclinical studies pave the way for interventions using anti-inflammation-based agents (COX-2 inhibitors, CB2 agonists, meditation/yoga) by identifying where, when, and how the developmental trajectory goes awry.
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Alzheimer's disease (AD), an age-related neurodegenerative condition, is the most common cause of dementia among the elder people, but currently there is no treatment. A number of putative pathogenic events, particularly amyloid β peptide (Aβ) accumulation, are believed to be early triggers that initiate AD. However, thus far targeting Aβ generation/aggregation as the mainstay strategy of drug development has not led to effective AD-modifying therapeutics. Oxidative damage is a conspicuous feature of AD, but this remains poorly defined phenomenon and mechanistically ill understood. The TRPM2 channel has emerged as a potentially ubiquitous molecular mechanism mediating oxidative damage and thus plays a vital role in the pathogenesis and progression of diverse neurodegenerative diseases. This article will review the emerging evidence from recent studies and propose a novel 'hypothesis' that multiple TRPM2-mediated cellular and molecular mechanisms cascade Aβ and/or oxidative damage to AD pathologies. The 'hypothesis' based on these new findings discusses the prospect of considering the TRPM2 channel as a novel therapeutic target for intervening AD and age-related dementia.
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Microglia are phagocytic cells that infiltrate the brain during development and have a role in the elimination of synapses during brain maturation. Changes in microglial morphology and gene expression have been associated with neurodevelopmental disorders. However, it remains unknown whether these changes are a primary cause or a secondary consequence of neuronal deficits. Here we tested whether a primary deficit in microglia was sufficient to induce some autism-related behavioral and functional connectivity deficits. Mice lacking the chemokine receptor Cx3cr1 exhibit a transient reduction of microglia during the early postnatal period and a consequent deficit in synaptic pruning. We show that deficient synaptic pruning is associated with weak synaptic transmission, decreased functional brain connectivity, deficits in social interaction and increased repetitive-behavior phenotypes that have been previously associated with autism and other neurodevelopmental and neuropsychiatric disorders. These findings open the possibility that disruptions in microglia-mediated synaptic pruning could contribute to neurodevelopmental and neuropsychiatric disorders.
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Microglia are highly motile phagocytic cells that infiltrate and take up residence in the developing brain, where they are thought to provide a surveillance and scavenging function. However, although microglia have been shown to engulf and clear damaged cellular debris after brain insult, it remains less clear what role microglia play in the uninjured brain. Here, we show that microglia actively engulf synaptic material and play a major role in synaptic pruning during postnatal development in mice. These findings link microglia surveillance to synaptic maturation and suggest that deficits in microglia function may contribute to synaptic abnormalities seen in some neurodevelopmental disorders.
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Microglia are the immune cells of the brain. In the absence of pathological insult, their highly motile processes continually survey the brain parenchyma and transiently contact synaptic elements. Aside from monitoring, their physiological roles at synapses are not known. To gain insight into possible roles of microglia in the modification of synaptic structures, we used immunocytochemical electron microscopy, serial section electron microscopy with three-dimensional reconstructions, and two-photon in vivo imaging to characterize microglial interactions with synapses during normal and altered sensory experience, in the visual cortex of juvenile mice. During normal visual experience, most microglial processes displayed direct apposition with multiple synapse-associated elements, including synaptic clefts. Microglial processes were also distinctively surrounded by pockets of extracellular space. In terms of dynamics, microglial processes localized to the vicinity of small and transiently growing dendritic spines, which were typically lost over 2 d. When experience was manipulated through light deprivation and reexposure, microglial processes changed their morphology, showed altered distributions of extracellular space, displayed phagocytic structures, apposed synaptic clefts more frequently, and enveloped synapse-associated elements more extensively. While light deprivation induced microglia to become less motile and changed their preference of localization to the vicinity of a subset of larger dendritic spines that persistently shrank, light reexposure reversed these behaviors. Taken together, these findings reveal different modalities of microglial interactions with synapses that are subtly altered by sensory experience. These findings suggest that microglia may actively contribute to the experience-dependent modification or elimination of a specific subset of synapses in the healthy brain.
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Microglia, the resident myeloid cells of the central nervous system, play important roles in life-long brain maintenance and in pathology. Despite their crucial role, their regulatory dynamics during brain development have not been fully elucidated. Genome-wide chromatin and expression profiling coupled with single-cell transcriptomic analysis throughout development reveal that microglia undergo three temporal developmental stages in synchrony with the brain: early, pre-, and adult microglia, which are under distinct regulatory circuits. Knockout of the adult microglia transcription factor MafB and environmental perturbations, such as those affecting the microbiome or prenatal immune activation, led to disruption of developmental genes and immune response pathways. Together, our work identifies a stepwise developmental program of microglia integrating immune response pathways that may be associated with several neurodevelopmental disorders.
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Over 50% of patients who survive neuroinvasive infection with West Nile virus (WNV) exhibit chronic cognitive sequelae. Although thousands of cases of WNV-mediated memory dysfunction accrue annually, the mechanisms responsible for these impairments are unknown. The classical complement cascade, a key component of innate immune pathogen defence, mediates synaptic pruning by microglia during early postnatal development. Here we show that viral infection of adult hippocampal neurons induces complement-mediated elimination of presynaptic terminals in a murine WNV neuroinvasive disease model. Inoculation of WNV-NS5-E218A, a WNV with a mutant NS5(E218A) protein leads to survival rates and cognitive dysfunction that mirror human WNV neuroinvasive disease. WNV-NS5-E218A-recovered mice (recovery defined as survival after acute infection) display impaired spatial learning and persistence of phagocytic microglia without loss of hippocampal neurons or volume. Hippocampi from WNV-NS5-E218A-recovered mice with poor spatial learning show increased expression of genes that drive synaptic remodelling by microglia via complement. C1QA was upregulated and localized to microglia, infected neurons and presynaptic terminals during WNV neuroinvasive disease. Murine and human WNV neuroinvasive disease post-mortem samples exhibit loss of hippocampal CA3 presynaptic terminals, and murine studies revealed microglial engulfment of presynaptic terminals during acute infection and after recovery. Mice with fewer microglia (Il34(-/-) mice with a deficiency in IL-34 production) or deficiency in complement C3 or C3a receptor were protected from WNV-induced synaptic terminal loss. Our study provides a new murine model of WNV-induced spatial memory impairment, and identifies a potential mechanism underlying neurocognitive impairment in patients recovering from WNV neuroinvasive disease.
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Microglia maintain homeostasis in the brain, but whether aberrant microglial activation can cause neurodegeneration remains controversial. Here, we use transcriptome profiling to demonstrate that deficiency in frontotemporal dementia (FTD) gene progranulin (Grn) leads to an age-dependent, progressive upregulation of lysosomal and innate immunity genes, increased complement production, and enhanced synaptic pruning in microglia. During aging, Grn(-/-) mice show profound microglia infiltration and preferential elimination of inhibitory synapses in the ventral thalamus, which lead to hyperexcitability in the thalamocortical circuits and obsessive-compulsive disorder (OCD)-like grooming behaviors. Remarkably, deleting C1qa gene significantly reduces synaptic pruning by Grn(-/-) microglia and mitigates neurodegeneration, behavioral phenotypes, and premature mortality in Grn(-/-) mice. Together, our results uncover a previously unrecognized role of progranulin in suppressing aberrant microglia activation during aging. These results represent an important conceptual advance that complement activation and microglia-mediated synaptic pruning are major drivers, rather than consequences, of neurodegeneration caused by progranulin deficiency.
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Synapse loss in Alzheimer's disease (AD) correlates with cognitive decline. Involvement of microglia and complement in AD has been attributed to neuroinflammation, prominent late in disease. Here we show in mouse models that complement and microglia mediate synaptic loss early in AD. C1q, the initiating protein of the classical complement cascade, is increased and associated with synapses before overt plaque deposition. Inhibition of C1q, C3 or the microglial complement receptor CR3, reduces the number of phagocytic microglia as well as the extent of early synapse loss. C1q is necessary for the toxic effects of soluble β-amyloid (Aβ) oligomers on synapses and hippocampal long-term potentiation (LTP). Finally, microglia in adult brains engulf synaptic material in a CR3-dependent process when exposed to soluble Aβ oligomers. Together, these findings suggest that the complement-dependent pathway and microglia that prune excess synapses in development are inappropriately activated and mediate synapse loss in AD.
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Microglia are resident macrophages of the central nervous system (CNS), representing 5-10% of total CNS cells. Recent findings reveal that microglia enter the embryonic brain, take up residence before the differentiation of other CNS cell types, and become critical regulators of CNS development. Here, we discuss exciting new work implicating microglia in a range of developmental processes, including regulation of cell number and spatial patterning of CNS cells, myelination, and formation and refinement of neural circuits. Furthermore, we review studies suggesting that these cellular functions result in the modulation of behavior, which has important implications for a variety of neurological disorders.
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Schizophrenia is a heritable brain illness with unknown pathogenic mechanisms. Schizophrenia's strongest genetic association at a population level involves variation in the major histocompatibility complex (MHC) locus, but the genes and molecular mechanisms accounting for this have been challenging to identify. Here we show that this association arises in part from many structurally diverse alleles of the complement component 4 (C4) genes. We found that these alleles generated widely varying levels of C4A and C4B expression in the brain, with each common C4 allele associating with schizophrenia in proportion to its tendency to generate greater expression of C4A. Human C4 protein localized to neuronal synapses, dendrites, axons, and cell bodies. In mice, C4 mediated synapse elimination during postnatal development. These results implicate excessive complement activity in the development of schizophrenia and may help explain the reduced numbers of synapses in the brains of individuals with schizophrenia.
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Microglia have critical roles in neural development, homeostasis and neuroinflammation and are increasingly implicated in age-related neurological dysfunction. Neurodegeneration often occurs in disease-specific, spatially restricted patterns, the origins of which are unknown. We performed to our knowledge the first genome-wide analysis of microglia from discrete brain regions across the adult lifespan of the mouse, and found that microglia have distinct region-dependent transcriptional identities and age in a regionally variable manner. In the young adult brain, differences in bioenergetic and immunoregulatory pathways were the major sources of heterogeneity and suggested that cerebellar and hippocampal microglia exist in a more immune-vigilant state. Immune function correlated with regional transcriptional patterns. Augmentation of the distinct cerebellar immunophenotype and a contrasting loss in distinction of the hippocampal phenotype among forebrain regions were key features during aging. Microglial diversity may enable regionally localized homeostatic functions but could also underlie region-specific sensitivities to microglial dysregulation and involvement in age-related neurodegeneration.
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Microglia, the major myeloid cells of the central nervous system (CNS) are implicated in physiologic processes and in the pathogenesis of several CNS disorders. Since their initial description early in the 20th century, our ability to identify and isolate microglia has significantly improved and new research is providing insight into the functions of these cells in sickness and in health. Here, we review recent advances in our understanding of the role of microglia in physiological and pathological processes of the CNS with a focus on multiple sclerosis and Alzheimer's disease. Because of the prominent roles CX3CR1 and its ligand fractalkine played in bringing about these advances, we discuss the physiological and pathological roles of microglia as viewed from the CX3CR1-fractalkine perspective, providing a unique viewpoint. Based on the most recent studies of molecular profiling of microglia, we also propose a molecular and functional definition of microglia that incorporates the properties attributed to these cells in recent years.
Article
The human body contains several hundred cell types, all of which share the same genome. In metazoans, much of the regulatory code that drives cell type-specific gene expression is located in distal elements called enhancers. Although mammalian genomes contain millions of potential enhancers, only a small subset of them is active in a given cell type. Cell type-specific enhancer selection involves the binding of lineage-determining transcription factors that prime enhancers. Signal-dependent transcription factors bind to primed enhancers, which enables these broadly expressed factors to regulate gene expression in a cell type-specific manner. The expression of genes that specify cell type identity and function is associated with densely spaced clusters of active enhancers known as super-enhancers. The functions of enhancers and super-enhancers are influenced by, and affect, higher-order genomic organization.
Article
Microglia are the resident CNS immune cells and active surveyors of the extracellular environment. While past work has focused on the role of these cells during disease, recent imaging studies reveal dynamic interactions between microglia and synaptic elements in the healthy brain. Despite these intriguing observations, the precise function of microglia at remodeling synapses and the mechanisms that underlie microglia-synapse interactions remain elusive. In the current study, we demonstrate a role for microglia in activity-dependent synaptic pruning in the postnatal retinogeniculate system. We show that microglia engulf presynaptic inputs during peak retinogeniculate pruning and that engulfment is dependent upon neural activity and the microglia-specific phagocytic signaling pathway, complement receptor 3(CR3)/C3. Furthermore, disrupting microglia-specific CR3/C3 signaling resulted in sustained deficits in synaptic connectivity. These results define a role for microglia during postnatal development and identify underlying mechanisms by which microglia engulf and remodel developing synapses.
Article
During development, the formation of mature neural circuits requires the selective elimination of inappropriate synaptic connections. Here we show that C1q, the initiating protein in the classical complement cascade, is expressed by postnatal neurons in response to immature astrocytes and is localized to synapses throughout the postnatal CNS and retina. Mice deficient in complement protein C1q or the downstream complement protein C3 exhibit large sustained defects in CNS synapse elimination, as shown by the failure of anatomical refinement of retinogeniculate connections and the retention of excess retinal innervation by lateral geniculate neurons. Neuronal C1q is normally downregulated in the adult CNS; however, in a mouse model of glaucoma, C1q becomes upregulated and synaptically relocalized in the adult retina early in the disease. These findings support a model in which unwanted synapses are tagged by complement for elimination and suggest that complement-mediated synapse elimination may become aberrantly reactivated in neurodegenerative disease.
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