Identification of BACE1 cleavage sites in human voltage-gated sodium channel beta 2 subunit

Microglia cells are the brain counterpart of macrophages and function as the first defense in the brain. Although they are neuroprotective in the young brain, microglia cells may be primed to react abnormally to stimuli in the aged brain and to become neurotoxic and destructive during neurodegeneration. Aging-induced immune senescence occurs in the brain as age-associated microglia senescence, which renders microglia to function abnormally and may eventually promote neurodegeneration. Microglia senescence is manifested by both morphological changes and alterations in immunophenotypic expression and inflammatory profile. These changes are likely caused by microinvironmental factors, but intrinsic factors cannot yet be completely excluded. Microglia senescence appears to underlie the switching of microglia from neuroprotective in the young brain to neurotoxic in the aged brain. The hypothesis of microglia senescence during aging offers a novel perspective on their roles in aging-related neurodegeneration. In Parkinson's disease and Alzheimer's disease, over-activation of microglia may play an active role in the pathogenesis because microglia senescence primes them to be neurotoxic during the development of the diseases.     

Functions of microglia in the central nervous system--beyond the immune response

Microglia cells are the immune cells of the central nervous system and consequently play important roles in brain infections and inflammation. Recent in vivo imaging studies have revealed that in the resting healthy brain, microglia are highly dynamic, moving constantly to actively survey the brain parenchyma. These active microglia can rapidly respond to pathological insults, becoming activated to induce a range of effects that may contribute to both pathogenesis, or to confer neuronal protection. However, interactions between microglia and neurons are being recognized as important in shaping neural circuit activity under more normal, physiological conditions. During development and neurogenesis, microglia interactions with neurons help to shape the final patterns of neural circuits important for behavior and with implications for diseases. In the mature brain, microglia can respond to changes in sensory activity and can influence neuronal activity acutely and over the long term. Microglia seem to be particularly involved in monitoring the integrity of synaptic function. In this review, we discuss some of these new insights into the involvement of microglia in neural circuits.     

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

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

Role of Microglia in Ataxias

Microglia, the resident macrophages of the central nervous system, critically influence neural function during development and in adulthood. Microglia are also profoundly sensitive to insults to the brain to which they respond with process of activation that includes spectrum of changes in morphology, function, and gene expression. Ataxias are a class of neurodegenerative diseases characterized by motor discoordination and predominant cerebellar involvement. In case of inherited forms of ataxia, mutant proteins are expressed throughout the brain and it is unclear why cerebellum is particularly vulnerable. Recent studies demonstrated that cerebellar microglia have a uniquely hyper-vigilant immune phenotype compared to microglia from other brain regions. These findings may indicate that microglia actively contribute to cerebellar vulnerability in ataxias. Here we review current knowledge about cerebellar microglia, their activation, and their role in the pathogenesis of ataxias. In addition, we briefly review advantages and disadvantages of several experimental approaches available to study microglia.     

Clearance and deposition of extracellular alpha-synuclein aggregates in microglia

Abnormal deposition of α-synuclein in neurons and glia is implicated in many neurological diseases, such as Parkinson’s disease and Dementia with Lewy bodies. Recently, evidence has emerged that this protein and its aggregates are secreted from neuronal cells, and this extracellular protein may contribute to the pathogenic process. Here, we show that all the major brain cell types (neurons, astrocytes, and microglia) are capable of clearing the extracellular α-synuclein aggregates by internalization and degradation. Among these cell types, microglia showed the highest rate of degradation. Upon activation by lipopolysaccharide, the degradation of the internalized α-synuclein aggregates was slowed, causing protein accumulation in the microglial cytoplasm. These results suggest that microglia may be the major scavenger cells for extracellular α-synuclein aggregates in brain parenchyma, and that clearance may be regulated by the activation state of these cells.     

Sex differences in microglial colonization of the developing rat brain

Microglia are the resident immune cells within the brain and their production of immune molecules such as cytokines and chemokines is critical for the processes of normal brain development including neurogenesis, axonal migration, synapse formation, and programmed cell death. Notably, sex differences exist in many of these processes throughout brain development; however, it is unknown whether a sex difference concurrently exists in the colonization, number, or morphology of microglia within the developing brain. We demonstrate for the first time that the number and morphology of microglia throughout development is dependent upon the sex and age of the individual, as well as the brain region of interest. Males have overall more microglia early in postnatal development [postnatal day (P) 4], whereas females have more microglia with an activated/amoeboid morphology later in development, as juveniles and adults (P30-60). Finally, gene expression of a large number of cytokines, chemokines and their receptors shifts dramatically over development, and is highly dependent upon sex. Taken together, these data warrant further research into the role that sex-dependent mechanisms may play in microglial colonization, number, and function, and their potential contribution to neural development, function, or potential dysfunction.     

Human NK cells kill resting but not activated microglia via NKG2D- and NKp46-mediated recognition

Microglia are resident macrophage-like APCs of the CNS. To avoid escalation of inflammatory processes and bystander damage within the CNS, microglia-driven inflammatory responses need to be tightly regulated and both spatially and temporally restricted. Following traumatic, infectious, and autoimmune-mediated brain injury, NK cells have been found in the CNS, but the functional significance of NK cell recruitment and their mechanisms of action during brain inflammation are not well understood. In this study, we investigated whether and by which mechanisms human NK cells might edit resting and activated human microglial cells via killing in vitro. IL-2-activated NK cells efficiently killed both resting allogeneic and autologous microglia in a cell-contact-dependent manner. Activated NK cells rapidly formed synapses with human microglial cells in which perforin had been polarized to the cellular interface. Ab-mediated NKG2D and NKp46 blockade completely prevented the killing of human microglia by activated NK cells. Up-regulation of MHC class I surface expression by TLR4 stimulation protected microglia from NK cell-mediated killing, whereas MHC class I blockade enhanced cytotoxic NK cell activity. These data suggest that brain-infiltrating NK cells might restrict innate and adaptive immune responses within the human CNS via elimination of resting microglia.     

Norepinephrine Modulates the Motility of Resting and Activated Microglia via Different Adrenergic Receptors

Microglia, the resident immune cells of the central nervous system (CNS), monitor the brain for disturbances of tissue homeostasis by constantly moving their fine processes. Microglia respond to tissue damage through activation of ATP/ADP receptors followed by directional process extension to the damaged area. A common feature of several neurodegenerative diseases is the loss of norepinephrine, which might contribute to the associated neuroinflammation. We carried out a high resolution analysis of the effects of norepinephrine (NE) on microglial process dynamics in acute brain slices from mice that exhibit microglia-specific enhanced green fluorescent protein expression. Bath application of NE to the slices resulted in significant process retraction in microglia. Analysis of adrenergic receptor expression with quantitative PCR indicated that resting microglia primarily express β₂ receptors but switch expression to α_2A receptors under proinflammatory conditions modeled by LPS treatment. Despite the differential receptor expression, NE caused process retraction in both resting and LPS-activated microglia cultured in the gelatinous substrate Matrigel in vitro. The use of subtype-selective receptor agonists and antagonists confirmed the involvement of β₂ receptors in mediating microglial process dynamics in resting cells and α_2A receptors in activated cells. Co-application of NE with ATP to resting microglia blocked the ATP-induced process extension and migration in isolated microglia, and β₂ receptor antagonists prolonged ATP effects in brain slice tissues, suggesting the presence of cross-talk between adrenergic and purinergic signaling in microglia. These data show that the neurotransmitter NE can modulate microglial motility, which could affect microglial functions in pathogenic situations of either elevated or reduced NE levels.     

Imaging P2X4 receptor lateral mobility in microglia: regulation by calcium and p38 MAPK

ATP-gated ionotropic P2X4 receptors are up-regulated in activated microglia and are critical for the development of neuropathic pain, a microglia-associated disorder. However, the nature of how plasma membrane P2X4 receptors are regulated in microglia is not fully understood. We used single-molecule imaging to track quantum dot-labeled P2X4 receptors to explore P2X4 receptor mobility in the processes of resting and activated microglia. We find that plasma membrane P2X4 receptor lateral mobility in resting microglial processes is largely random, consisting of mobile and slowly mobile receptors. Moreover, lateral mobility is P2X subunit- and cell-specific, increased in an ATP activation and calcium-dependent manner, and enhanced in activated microglia by the p38 MAPK pathway that selectively regulates slowly mobile receptors. Thus, our data indicate that P2X4 receptors are dynamically regulated mobile ATP sensors, sampling more of the plasma membrane in response to ATP and during the activated state of microglia that is associated with nervous system dysfunction.     

Inflammation alters the expression of DMT1, FPN1 and hepcidin,and it causes iron accumulation in central nervous system cells

Inflammation and iron accumulation are present in a variety of neurodegenerative diseases that include Alzheimer's disease and Parkinson's disease. The study of the putative association between inflammation and iron accumulation in central nervous system cells is relevant to understand the contribution of these processes to the progression of neuronal death. In this study, we analyzed the effects of the inflammatory cytokines tumor necrosis factor alpha (TNF‐α) and interleukin 6 (IL‐6) and of lipopolysaccharide on total cell iron content and on the expression and abundance of the iron transporters divalent metal transporter 1 (DMT1) and Ferroportin 1 (FPN1) in neurons, astrocytes and microglia obtained from rat brain. Considering previous reports indicating that inflammatory stimuli induce the systemic synthesis of the master iron regulator hepcidin, we identified brain cells that produce hepcidin in response to inflammatory stimuli, as well as hepcidin‐target cells. We found that inflammatory stimuli increased the expression of DMT1 in neurons, astrocytes, and microglia. Inflammatory stimuli also induced the expression of hepcidin in astrocytes and microglia, but not in neurons. Incubation with hepcidin decreased the expression of FPN1 in the three cell types. The net result of these changes was increased iron accumulation in neurons and microglia but not in astrocytes. The data presented here establish for the first time a causal association between inflammation and iron accumulation in brain cells, probably promoted by changes in DMT1 and FPN1 expression and mediated in part by hepcidin. This connection may potentially contribute to the progression of neurodegenerative diseases by enhancing iron‐induced oxidative damage.     

The brain's polymath: Emerging roles of microglia throughout brain development

Microglia, the resident brain immune cells, have garnered a reputation as major effectors of circuit wiring due to their ability to prune synapses. Other roles of microglia in regulating neuronal circuit development have so far received comparatively less attention. Here, we review the latest studies that have contributed to our increased understanding of how microglia regulate brain wiring beyond their role in synapse pruning. We summarize recent findings showing that microglia regulate neuronal numbers and influence neuronal connectivity through a bidirectional communication between microglia and neurons, processes regulated by neuronal activity and the remodeling of the extracellular matrix. Finally, we speculate on the potential contribution of microglia to the development of functional networks and propose an integrative view of microglia as active elements of neural circuits.     

Histone acetylation in drug addiction

Regulation of chromatin structure through post-translational modifications of histones (e.g., acetylation) has emerged as an important mechanism to translate a variety of environmental stimuli, including drugs of abuse, into specific changes in gene expression. Since alterations in gene expression are thought to contribute to the development and maintenance of the addicted state, recent efforts are aimed at identifying how drugs of abuse alter chromatin structure and the enzymes which regulate it. This review discusses how drugs of abuse alter histone acetylation in brain reward regions, through which enzymes this occurs, and ultimately what role histone acetylation plays in addiction-related behaviors.     

From synapse to nucleus: calcium-dependent gene transcription in the control of synapse development and function

One of the unique characteristics of higher organisms is their ability to learn and adapt to changes in their environment. This plasticity is largely a result of the brain's ability to convert transient stimuli into long-lasting alterations in neuronal structure and function. This process is complex and involves changes in receptor trafficking, local mRNA translation, protein turnover, and new gene synthesis. Here, we review how neuronal activity triggers calcium-dependent gene expression to regulate synapse development, maturation, and refinement. Interestingly, many components of the activity-dependent gene expression program are mutated in human cognitive disorders, which suggest that this program is essential for proper brain development and function.     

Alterations of CaMKII after hypoxia-ischemia during brain development

Transient brain hypoxia-ischemia (HI) in neonates leads to delayed neuronal death and long-term neurological deficits. However, the underlying mechanisms are incompletely understood. Calcium-calmodulin-dependent protein kinase II (CaMKII) is one of the most abundant protein kinases in neurons and plays crucial roles in synaptic development and plasticity. This study used a neonatal brain HI model to investigate whether and how CaMKII was altered after HI and how the changes were affected by brain development. Expression of CaMKII was markedly up-regulated during brain development. After HI, CaMKII was totally and permanently depleted from the cytosol and concomitantly deposited into a Triton-insoluble fraction in neurons that were undergoing delayed neuronal death. Autophosphorylation of CaMKII-Thr286 transiently increased at 30 min of reperfusion and declined thereafter. All these changes were mild in P7 pups but more dramatic in P26 rats, consistent with the development-dependent CaMKII expression in neurons. The results suggest that long-term CaMKII depletion from the cytosolic fraction and deposition into the Triton-insoluble fraction may disable synaptic development, damage synaptic plasticity, and contribute to delayed neuronal death and long-term synaptic deficits after transient HI.     

Does microglial dysfunction play a role in autism and Rett syndrome?

Autism spectrum disorders (ASDs) including classic autism is a group of complex developmental disabilities with core deficits of impaired social interactions, communication difficulties and repetitive behaviors. Although the neurobiology of ASDs has attracted much attention in the last two decades, the role of microglia has been ignored. Existing data are focused on their recognized role in neuroinflammation, which only covers a small part of the pathological repertoire of microglia. This review highlights recent findings on the broader roles of microglia, including their active surveillance of brain microenvironments and regulation of synaptic connectivity, maturation of brain circuitry and neurogenesis. Emerging evidence suggests that microglia respond to pre- and postnatal environmental stimuli through epigenetic interface to change gene expression, thus acting as effectors of experience-dependent synaptic plasticity. Impairments of these microglial functions could substantially contribute to several major etiological factors of autism, such as environmental toxins and cortical underconnectivity. Our recent study on Rett syndrome, a syndromic autistic disorder, provides an example that intrinsic microglial dysfunction due to genetic and epigenetic aberrations could detrimentally affect the developmental trajectory without evoking neuroinflammation. We propose that ASDs provide excellent opportunities to study the influence of microglia on neurodevelopment, and this knowledge could lead to novel therapies.