Resting microglia directly monitor the functional state of synapses in vivo and determine the fate of ischemic terminals. J Neurosci. 2009;29:3974-3980. [PMID: 19339593 DOI: 10.1523/jneurosci.4363-08.2009]

Microglia: an active player in the regulation of synaptic activity

Synaptic plasticity is critical for elaboration and adaptation in the developing and developed brain. It is well established that astrocytes play an important role in the maintenance of what has been dubbed “the tripartite synapse”. Increasing evidence shows that a fourth cell type, microglia, is critical to this maintenance as well. Microglia are the resident macrophages of the central nervous system (CNS). Because of their well-characterized inflammatory functions, research has primarily focused on their innate immune properties. The role of microglia in the maintenance of synapses in development and in homeostasis is not as well defined. A number of significant findings have shed light on the critical role of microglia at the synapse. It is becoming increasingly clear that microglia play a seminal role in proper synaptic development and elimination.

Retinal connectomics: towards complete, accurate networks

Connectomics is a strategy for mapping complex neural networks based on high-speed automated electron optical imaging, computational assembly of neural data volumes, web-based navigational tools to explore 10(12)-10(15) byte (terabyte to petabyte) image volumes, and annotation and markup tools to convert images into rich networks with cellular metadata. These collections of network data and associated metadata, analyzed using tools from graph theory and classification theory, can be merged with classical systems theory, giving a more completely parameterized view of how biologic information processing systems are implemented in retina and brain. Networks have two separable features: topology and connection attributes. The first findings from connectomics strongly validate the idea that the topologies of complete retinal networks are far more complex than the simple schematics that emerged from classical anatomy. In particular, connectomics has permitted an aggressive refactoring of the retinal inner plexiform layer, demonstrating that network function cannot be simply inferred from stratification; exposing the complex geometric rules for inserting different cells into a shared network; revealing unexpected bidirectional signaling pathways between mammalian rod and cone systems; documenting selective feedforward systems, novel candidate signaling architectures, new coupling motifs, and the highly complex architecture of the mammalian AII amacrine cell. This is but the beginning, as the underlying principles of connectomics are readily transferrable to non-neural cell complexes and provide new contexts for assessing intercellular communication.

Contributions of microglia to structural synaptic plasticity

Synaptic plasticity critically depends on reciprocal interactions between neurons and glia. Among glial cells, microglia represent approximately 10% of the total brain cell population serve as the brain’s resident macrophage, and help to modulate neural activity. Because of their special role in the brain’s immune response, microglia are involved in the pathological progression of neurodegenerative disorders such as Alzheimer’s disease (AD). However, microglia also are surveyors of the brain’s health and continuously contact dendritic spines to regulate structural synaptic changes. This review summarizes our current understanding of neuronal-microglial signals that affect neural function at the synapse. Here, we examine the role of microglia in neuronal synapses in pathological brains and specifically focus on in vivo studies using 2-photon microscopy. Furthermore, because the role of microglia in AD progression is controversial, we outline the interaction between neurons and microglia in pathological conditions such as AD.

Proteome of brain glia: the molecular basis of diverse glial phenotypes

Several different types of nonneuronal glial cells with diverse phenotypes are present in the CNS, and all have distinct indispensible functions. Although glial cells primarily provide neurons with metabolic and structural support in the healthy brain, they may switch phenotype from a "resting" to a "reactive" state in response to pathological insults. Furthermore, this reactive gliosis is an invariant feature of the pathogeneses of CNS maladies. The glial proteome serves as a signature of glial phenotype, and not only executes physiological functions, but also acts as a molecular mediator of the reactive glial phenotype. The glial proteome is also involved in intra- and intercellular communications as exemplified by glia-glia and neuron-glia interactions. The utilization of authoritative proteomic tools and the bioinformatic analyses have helped to profile the brain glial proteome and explore the molecular mechanisms of diverse glial phenotypes. Furthermore, technologic innovations have equipped the field of "glioproteomics" with refined tools for studies of the expression, interaction, and function of glial proteins in the healthy and in the diseased CNS. Glioproteomics is expected to contribute to the elucidation of the molecular mechanisms of CNS pathophysiology and to the discovery of biomarkers and theragnostic targets in CNS disorders.

Amyloid β peptide-mediated neurotoxicity is attenuated by the proliferating microglia more potently than by the quiescent phenotype

Background

The specific role of microglia on Aβ-mediated neurotoxicity is difficult to assign in vivo due to their complicated environment in the brain. Therefore, most of the current microglia-related studies employed the isolated microglia. However, the previous in vitro studies have suggested either beneficial or destructive function in microglia. Therefore, to investigate the phenotypes of the isolated microglia which exert activity of neuroprotective or destructive is required.

Results

The present study investigates the phenotypes of isolated microglia on protecting neuron against Aβ-mediated neurotoxicity. Primary microglia were isolated from the mixed glia culture, and were further cultured to distinct phenotypes, designated as proliferating amoeboid microglia (PAM) and differentiated process-bearing microglia (DPM). Their inflammatory phenotypes, response to amyloid β (Aβ), and the beneficial or destructive effects on neurons were investigated. DPM may induce both direct neurotoxicity without exogenous stimulation and indirect neurotoxicity after Aβ activation. On the other hand, PAM attenuates Aβ-mediated neurotoxicity through Aβ phagocytosis and/or Aβ degradation.

Conclusions

Our results suggest that the proliferating microglia, but not the differentiated microglia, protect neurons against Aβ-mediated neurotoxicity. This discovery may be helpful on the therapeutic investigation of Alzheimer’s disease.

Phagocytic glial cells: sculpting synaptic circuits in the developing nervous system

In the developing nervous system, synaptic connections are formed in excess and must remodel to achieve the precise synaptic connectivity characteristic of the mature organism. Synaptic pruning is a developmental process in which subsets of synapses are eliminated while the remaining synapses are preserved and strengthened. Recent findings have demonstrated unexpected roles for glial cells in this developmental process. These data demonstrate that phagocytic glia engulf synaptic and/or axonal elements in the developing nervous system and disruptions in this process result in sustained deficits in synaptic connectivity. These new findings highlight the importance of glia for nervous system development and function and may shed new light on mechanisms underlying nervous system disease.

Neuron-glia interaction as a possible glue to translate the mind-brain gap: a novel multi-dimensional approach toward psychology and psychiatry

Neurons and synapses have long been the dominant focus of neuroscience, thus the pathophysiology of psychiatric disorders has come to be understood within the neuronal doctrine. However, the majority of cells in the brain are not neurons but glial cells including astrocytes, oligodendrocytes, and microglia. Traditionally, neuroscientists regarded glial functions as simply providing physical support and maintenance for neurons. Thus, in this limited role glia had been long ignored. Recently, glial functions have been gradually investigated, and increasing evidence has suggested that glial cells perform important roles in various brain functions. Digging up the glial functions and further understanding of these crucial cells, and the interaction between neurons and glia may shed new light on clarifying many unknown aspects including the mind-brain gap, and conscious-unconscious relationships. We briefly review the current situation of glial research in the field, and propose a novel translational research with a multi-dimensional model, combining various experimental approaches such as animal studies, in vitro & in vivo neuron-glia studies, a variety of human brain imaging investigations, and psychometric assessments.

Minocycline, a microglial inhibitor, reduces 'honey trap' risk in human economic exchange

Recently, minocycline, a tetracycline antibiotic, has been reported to improve symptoms of psychiatric disorders and to facilitate sober decision-making in healthy human subjects. Here we show that minocycline also reduces the risk of the ‘honey trap’ during an economic exchange. Males tend to cooperate with physically attractive females without careful evaluation of their trustworthiness, resulting in betrayal by the female. In this experiment, healthy male participants made risky choices (whether or not to trust female partners, identified only by photograph, who had decided in advance to exploit the male participants). The results show that trusting behaviour in male participants significantly increased in relation to the perceived attractiveness of the female partner, but that attractiveness did not impact trusting behaviour in the minocycline group. Animal studies have shown that minocycline inhibits microglial activities. Therefore, this minocycline effect may shed new light on the unknown roles microglia play in human mental activities.

The intrinsic microglial molecular clock controls synaptic strength via the circadian expression of cathepsin S

Microglia are thought to play important roles in the maintenance of neuronal circuitry and the regulation of behavior. We found that the cortical microglia contain an intrinsic molecular clock and exhibit a circadian expression of cathepsin S (CatS), a microglia-specific lysosomal cysteine protease in the brain. The genetic deletion of CatS causes mice to exhibit hyperlocomotor activity and removes diurnal variations in the synaptic activity and spine density of the cortical neurons, which are significantly higher during the dark (waking) phase than the light (sleeping) phase. Furthermore, incubation with recombinant CatS significantly reduced the synaptic activity of the cortical neurons. These results suggest that CatS secreted by microglia during the dark-phase decreases the spine density of the cortical neurons by modifying the perisynaptic environment, leading to downscaling of the synaptic strength during the subsequent light-phase. Disruption of CatS therefore induces hyperlocomotor activity due to failure to downscale the synaptic strength.

Learning and memory ... and the immune system

The nervous system and the immune system are two main regulators of homeostasis in the body. Communication between them ensures normal functioning of the organism. Immune cells and molecules are required for sculpting the circuitry and determining the activity of the nervous system. Within the parenchyma of the central nervous system (CNS), microglia constantly monitor synapses and participate in their pruning during development and possibly also throughout life. Classical inflammatory cytokines, such as interleukin (IL)-1β and tumor necrosis factor (TNF), are released during neuronal activity and play a crucial role in regulating the strength of synaptic transmission. Systemically, proper functioning of the immune system is critical for maintaining normal nervous system function. Disruption of the immune system functioning leads to impairments in cognition and in neurogenesis. In this review we provide examples of the communication between the nervous and the immune systems in the interest of normal CNS development and function.

Modulation of adult-born neurons in the inflamed hippocampus

Throughout life new neurons are continuously added to the hippocampal circuitry involved with spatial learning and memory. These new cells originate from neural precursors in the subgranular zone of the dentate gyrus, migrate into the granule cell layer, and integrate into neural networks encoding spatial and contextual information. This process can be influenced by several environmental and endogenous factors and is modified in different animal models of neurological disorders. Neuroinflammation, as defined by the presence of activated microglia, is a common key factor to the progression of neurological disorders. Analysis of the literature shows that microglial activation impacts not only the production, but also the migration and the recruitment of new neurons. The impact of microglia on adult-born neurons appears much more multifaceted than ever envisioned before, combining both supportive and detrimental effects that are dependent upon the activation phenotype and the factors being released. The development of strategies aimed to change microglia toward states that promote functional neurogenesis could therefore offer novel therapeutic opportunities against neurological disorders associated with cognitive deficits and neuroinflammation. The present review summarizes the current knowledge on how production, distribution, and recruitment of new neurons into behaviorally relevant neural networks are modified in the inflamed hippocampus.

P2Y4 receptor-mediated pinocytosis contributes to amyloid beta-induced self-uptake by microglia

Brain disturbances, like injuries or aberrant protein deposits, evoke nucleotide release or leakage from cells, leading to microglial chemotaxis and ingestion. Recent studies have identified P2Y12 purinergic receptors as triggers for microglial chemotaxis and P2Y6 receptors as mediators for phagocytosis. However, pinocytosis, known as the internalization of fluid-phase materials, has received much less attention. We found that ATP efficiently triggered pinocytosis in microglia. Pharmacological analysis and knockdown experiments demonstrated the involvement of P2Y4 receptors and the phosphatidylinositol 3-kinase/Akt cascade in the nucleotide-induced pinocytosis. Further evidence indicated that soluble amyloid beta peptide 1-42 induced self-uptake in microglia through pinocytosis, a process involving activation of P2Y4 receptors by autocrine ATP signaling. Our results demonstrate a previously unknown function of ATP as a "drink me" signal for microglia and P2Y4 receptors as a potential therapeutic target for the treatment of Alzheimer's disease.

Bidirectional microglia-neuron communication in the healthy brain

Unlike other resident neural cells that are of neuroectodermal origin, microglia are resident neural cells of mesodermal origin. Traditionally recognized for their immune functions during disease, new roles are being attributed to these cells in the development and maintenance of the central nervous system (CNS) including specific communication with neurons. In this review, we highlight some of the recent findings on the bidirectional interaction between neurons and microglia. We discuss these interactions along two lines. First, we review data that suggest that microglial activity is modulated by neuronal signals, focusing on evidence that (i) neurons are capable of regulating microglial activation state and influence basal microglial activities; (ii) classic neurotransmitters affect microglial behavior; (iii) chemotactic signals attract microglia during acute neuronal injury. Next, we discuss some of the recent data on how microglia signal to neurons. Signaling mechanisms include (i) direct physical contact of microglial processes with neuronal elements; (ii) microglial regulation of neuronal synapse and circuit by fractalkine, complement, and DAP12 signaling. In addition, we discuss the use of microglial depletion strategies in studying the role of microglia in neuronal development and synaptic physiology. Deciphering the mechanisms of bidirectional microglial-neuronal communication provides novel insights in understanding microglial function in both the healthy and diseased brain.

Microglia during development and aging

Microglia are critical nervous system-specific cells influencing brain development, maintenance of the neural environment, response to injury, and repair. They contribute to neuronal proliferation and differentiation, pruning of dying neurons, synaptic remodeling and clearance of debris and aberrant proteins. Colonization of the brain occurs during gestation with an expansion following birth with localization stimulated by programmed neuronal death, synaptic pruning, and axonal degeneration. Changes in microglia phenotype relate to cellular processes including specific neurotransmitter, pattern recognition, or immune-related receptor activation. Upon activation, microglia cells have the capacity to release a number of substances, e.g., cytokines, chemokines, nitric oxide, and reactive oxygen species, which could be detrimental or beneficial to the surrounding cells. With aging, microglia shift their morphology and may display diminished capacity for normal functions related to migration, clearance, and the ability to shift from a pro-inflammatory to an anti-inflammatory state to regulate injury and repair. This shift in microglia potentially contributes to increased susceptibility and neurodegeneration as a function of age. In the current review, information is provided on the colonization of the brain by microglia, the expression of various pattern recognition receptors to regulate migration and phagocytosis, and the shift in related functions that occur in normal aging.

Novel objective classification of reactive microglia following hypoglossal axotomy using hierarchical cluster analysis

A total of 136 microglia were intracellularly labeled and their morphological features were evaluated by 3D morphometric measurement. According to hierarchical cluster analysis, microglia were objectively categorized into four groups termed types I-IV. The validity of this classification was confirmed by principal component analysis and linear discriminant analysis. Type I microglia were found in sham-operated mice and in mice sacrificed 28 days (D28) after axotomy. The appearance of type I cells was similar to so-called ramified microglia in a resting state. Type II microglia were mainly seen in D14 mice, which exhibited small cell bodies with thin and short processes. Interestingly, none of the already-known morphological types of microglia seemed to be comparable to type II cells. We thus named type II microglia "small ramified" cells. Types III and IV microglia were mainly seen in D3 and D7 mice and their appearances were similar to hypertrophied and bushy cells, respectively. Proliferating cell nuclear antigen (PCNA), a mitosis marker, was almost exclusively expressed in D3 mice. On the other hand, voltage-dependent potassium channels (Kv1.3/1.5), neurotoxicity-related molecules, were most highly expressed in D14 mice. Increased expression of Kv1.3/1.5 in D14 mice was suppressed by minocycline treatment. These findings indicate that type II and III microglia may be involved in neurotoxicity and mitosis, respectively. Type IV microglial cells are assumed to be in the process of losing mitotic activity and gaining neurotoxicity. Our data also suggest that type II microglia can be a potential therapeutic target against neurodegenerative diseases.

Microglia and spinal cord synaptic plasticity in persistent pain

Microglia are regarded as macrophages in the central nervous system (CNS) and play an important role in neuroinflammation in the CNS. Microglial activation has been strongly implicated in neurodegeneration in the brain. Increasing evidence also suggests an important role of spinal cord microglia in the genesis of persistent pain, by releasing the proinflammatory cytokines tumor necrosis factor-alpha (TNFα), Interleukine-1beta (IL-1β), and brain derived neurotrophic factor (BDNF). In this review, we discuss the recent findings illustrating the importance of microglial mediators in regulating synaptic plasticity of the excitatory and inhibitory pain circuits in the spinal cord, leading to enhanced pain states. Insights into microglial-neuronal interactions in the spinal cord dorsal horn will not only further our understanding of neural plasticity but may also lead to novel therapeutics for chronic pain management.

New players in the neurovascular unit: insights from experimental and clinical epilepsy

The conventional notion that neurons are exclusively responsible for brain signaling is increasingly challenged by the idea that brain function in fact depends on a complex interplay between neurons, glial cells, vascular endothelium, and immune-related blood cells. Recent data demonstrates that neuronal activity is profoundly affected by an entire cellular and extracellular 'orchestra', the so-called neurovascular unit (NVU). Among the 'musical instruments' of this orchestra, there may be molecules long-known in biomedicine as important mediators of inflammatory and immune responses in the organism, as well as non-neuronal cells, e.g., leukocytes. We here review recent evidence on the structure and function of the NVU, both in the healthy brain and in pathological conditions, such as the abnormal NVU activation observed in epilepsy. We will argue that a better understanding of NVU function will require the addition of new players to the 'orchestra'.

Acute isolation and transcriptome characterization of cortical astrocytes and microglia from young and aged mice

Astrocytes and microglia become reactive in many neurological disorders resulting in phenotypic and functional alterations. Both cell types might also display functional changes during normal aging. To identify gene signatures and changes in basal cellular functions of astrocytes and microglia in relation to aging, we isolated viable astrocytes and microglia from young adult and aged mouse cortices and determined their gene expression profile. Aged astrocytes, compared with young astrocytes, showed an increased inflammatory phenotype and increased 'zinc ion binding.' Young astrocytes showed higher expression of genes involved in 'neuronal differentiation' and hemoglobin synthesis. Astrocyte expression of genes involved in neuronal signaling remains high throughout age. Aged microglia had higher expression of genes involved in 'vesicle release,' 'zinc ion binding,' and genes within the tumor necrosis factor-ligand family and young microglia had increased transcript levels of C-C motif chemokines. These data provide a transcriptome database of cell-type enriched genes of astrocytes and microglia from adult mice and give insight into the differential gene signature of astrocytes and microglia in relation to normal aging.

Hypoxia Attenuates Purinergic P2X Receptor-Induced Inflammatory Gene Expression in Brainstem Microglia

Hypoxia and increased extracellular nucleotides are frequently coincident in the brainstem. Extracellular nucleotides are potent modulators of microglial inflammatory gene expression via P2X purinergic receptor activation. Although hypoxia is also known to modulate inflammatory gene expression, little is known about how hypoxia or P2X receptor activation alone affects inflammatory molecule production in brainstem microglia, nor how hypoxia and P2X receptor signaling interact when they occur together. In the study reported here, we investigated the ability of a brief episode of hypoxia (2 hours) in the presence and absence of the nonselective P2X receptor agonist 2′(3′)-O-(4-benzoylbenzoyl)adenosine-5′-triphosphate (BzATP) to promote inflammatory gene expression in brainstem microglia in adult rats. We evaluated inducible nitric oxide synthase (iNOS), tumor necrosis factor alpha (TNFα), and interleukin (IL)-6 messenger RNA levels in immunomagnetically isolated brainstem microglia. While iNOS and IL-6 gene expression increased with hypoxia and BzATP alone, TNFα expression was unaffected. Surprisingly, BzATP-induced inflammatory effects were lost after hypoxia, suggesting that hypoxia impairs proinflammatory P2X-receptor signaling. We also evaluated the expression of key P2X receptors activated by BzATP, namely P2X1, P2X4, and P2X7. While hypoxia did not alter their expression, BzATP upregulated P2X4 and P2X7 mRNAs; these effects were ablated in hypoxia. Although both P2X4 and P2X7 receptor expression correlated with increased microglial iNOS and IL-6 levels in microglia from normoxic rats, in hypoxia, P2X7 only correlated with IL-6, and P2X4 correlated only with iNOS. In addition, correlations between P2X7 and P2X4 were lost following hypoxia, suggesting that P2X4 and P2X7 receptor signaling differs in normoxia and hypoxia. Together, these data suggest that hypoxia suppresses P2X receptor-induced inflammatory gene expression, indicating a potentially immunosuppressive role of extracellular nucleotides in brainstem microglia following exposure to hypoxia.

Resting microglia respond to and regulate neuronal activity in vivo

Microglia are the primary immune cells in the brain. Under pathological conditions, they become activated and participate in scavenging, inflammation and tissue repair in response to brain injury. While the function and underlying mechanism of activated microglia have been intensively studied in the past decades, physiological functions of resting microglia remain largely underestimated. In our recent work, by simultaneously monitoring both the motility of resting microglial processes and the activity of surrounding neurons in intact zebrafish optic tectum, we examined the interaction between resting microglia and neurons. Local increase in neuronal activity attracts resting microglial processes and drives them to contact neurons with high levels of activity. This process is mediated by neuronal release of "find-me" signals such as ATP via pannexin-1 hemichannels and requires small Rho GTPase Rac in microglia. Reciprocally, the microglia-neuron contact reduces both the spontaneous and visually evoked activities of contacted neurons. We here summarize and explain the key results in the context of our previous work.

Contribution of neuronal and glial circuit in intact hemisphere for functional remodeling after focal ischemia

The number of people who suffer from disabilities such as aphasia and/or paralysis after a focal brain stroke has not markedly decreased even in countries with established medical care systems. Functions such as speech can be lost following a stroke; however, such functions can sometimes be recovered. In this review, we focus on functional compensation that was achieved by the intact region contralateral to the stroke region. Using a mice stroke model, we used in vivo imaging techniques in combination with conventional electrophysiology and behavior tests, which showed that functional recovery was achieved through the specific synaptic (neuronal circuit) remodeling at the region contralateral to the focal stroke region 1 week after the stroke. During this period, astrocytes play a critical role in reducing the accumulation of synaptically released glutamate in the extracellular space, which would otherwise cause excitotoxicity. These findings indicate that the hemisphere that was intact after a stroke can potentially achieve bilateral functions even in adults when proper remodeling of neuronal circuits occurs. Activating the intact hemisphere may become a new therapeutic strategy for stroke patients.

Review: microglia of the aged brain: primed to be activated and resistant to regulation

Innate immunity within the central nervous system (CNS) is primarily provided by resident microglia. Microglia are pivotal in immune surveillance and also facilitate the co-ordinated responses between the immune system and the brain. For example, microglia interpret and propagate inflammatory signals that are initiated in the periphery. This transient microglial activation helps mount the appropriate physiological and behavioural response following peripheral infection. With normal ageing, however, microglia develop a more inflammatory phenotype. For instance, in several models of ageing there are increased pro-inflammatory cytokines in the brain and increased expression of inflammatory receptors on microglia. This increased inflammatory status of microglia with ageing is referred to as primed, reactive or sensitized. A modest increase in the inflammatory profile of the CNS and altered microglial function in ageing has behavioural and cognitive consequences. Nonetheless, there are major differences in microglial biology between young and old age when the immune system is challenged and microglia are activated. In this context, microglial activation is amplified and prolonged in the aged brain compared with adults. The cause of this amplified microglial activation may be related to impairments in several key regulatory systems with age that make it more difficult to resolve microglial activation. The consequences of impaired regulation and microglial hyper-activation following immune challenge are exaggerated neuroinflammation, sickness behaviour, depressive-like behaviour and cognitive deficits. Therefore the purpose of this review is to discuss the current understanding of age-associated microglial priming, consequences of priming and reactivity, and the impairments in regulatory systems that may underlie these age-related deficits.

Missing and Possible Link between Neuroendocrine Factors, Neuropsychiatric Disorders, and Microglia

Endocrine systems have long been suggested to be one of the important factors in neuropsychiatric disorders, while the underlying mechanisms have not been well understood. Traditionally, neuropsychiatric disorders have been mainly considered the consequence of abnormal conditions in neural circuitry. Beyond the neuronal doctrine, microglia, one of the glial cells with inflammatory/immunological functions in the central nervous system (CNS), have recently been suggested to play important roles in neuropsychiatric disorders. However, the crosstalk between neuroendocrine factors, neuropsychiatric disorders, and microglia has been unsolved. Therefore, we herein introduce and discuss a missing and possible link between these three factors; especially highlighting the following hormones; (1) Hypothalamic-Pituitary-Adrenal (HPA) axis-related hormones such as corticotropin-releasing hormone (CRH) and glucocorticoids, (2) sex-related hormones such as estrogen and progesterone, and (3) oxytocin. A growing body of evidence has suggested that these hormones have a direct effect on microglia. We hypothesize that hormone-induced microglial activation and the following microglia-derived mediators may lead to maladaptive neuronal networks including synaptic dysfunctions, causing neuropsychiatric disorders. Future investigations to clarify the correlation between neuroendocrine factors and microglia may contribute to a novel understanding of the pathophysiology of neuropsychiatric disorders.

Microglia: a new frontier for synaptic plasticity, learning and memory, and neurodegenerative disease research

We focus on emerging roles for microglia in synaptic plasticity, cognition and disease. We outline evidence that ramified microglia, traditionally thought to be functionally "resting" (i.e. quiescent) in the normal brain, in fact are highly dynamic and plastic. Ramified microglia continually and rapidly extend processes, contact synapses in an activity and experience dependent manner, and play a functionally dynamic role in synaptic plasticity, possibly through release of cytokines and growth factors. Ramified microglial also contribute to structural plasticity through the elimination of synapses via phagocytic mechanisms, which is necessary for normal cognition. Microglia have numerous mechanisms to monitor neuronal activity and numerous mechanisms also exist to prevent them transitioning to an activated state, which involves retraction of their surveying processes. Based on the evidence, we suggest that maintaining the ramified state of microglia is essential for normal synaptic and structural plasticity that supports cognition. Further, we propose that change of their ramified morphology and function, as occurs in inflammation associated with numerous neurological disorders such as Alzheimer's and Parkinson's disease, disrupts their intricate and essential synaptic functions. In turn altered microglia function could cause synaptic dysfunction and excess synapse loss early in disease, initiating a range of pathologies that follow. We conclude that the future of learning and memory research depends on an understanding of the role of non-neuronal cells and that this should include using sophisticated molecular, cellular, physiological and behavioural approaches combined with imaging to causally link the role of microglia to brain function and disease including Alzheimer's and Parkinson's disease and other neuropsychiatric disorders.

Brain inflammation and microglia: facts and misconceptions

THE INFLAMMATION THAT ACCOMPANIES ACUTE INJURY HAS DUAL FUNCTIONS: bactericidal action and repair. Bactericidal functions protect damaged tissue from infection, and repair functions are initiated to aid in the recovery of damaged tissue. Brain injury is somewhat different from injuries in other tissues in two respects. First, many cases of brain injury are not accompanied by infection: there is no chance of pathogens to enter in ischemia or even in traumatic injury if the skull is intact. Second, neurons are rarely regenerated once damaged. This raises the question of whether bactericidal inflammation really occurs in the injured brain; if so, how is this type of inflammation controlled? Many brain inflammation studies have been conducted using cultured microglia (brain macrophages). Even where animal models have been used, the behavior of microglia and neurons has typically been analyzed at or after the time of neuronal death, a time window that excludes the inflammatory response, which begins immediately after the injury. Therefore, to understand the patterns and roles of brain inflammation in the injured brain, it is necessary to analyze the behavior of all cell types in the injured brain immediately after the onset of injury. Based on our experience with both in vitro and in vivo experimental models of brain inflammation, we concluded that not only microglia, but also astrocytes, blood inflammatory cells, and even neurons participate and/or regulate brain inflammation in the injured brain. Furthermore, brain inflammation played by these cells protects neurons and repairs damaged microenvironment but not induces neuronal damage.

Inflammation in Alzheimer disease-a brief review of the basic science and clinical literature

Biochemical and neuropathological studies of brains from individuals with Alzheimer disease (AD) provide clear evidence for an activation of inflammatory pathways, and long-term use of anti-inflammatory drugs is linked with reduced risk to develop the disease. As cause and effect relationships between inflammation and AD are being worked out, there is a realization that some components of this complex molecular and cellular machinery are most likely promoting pathological processes leading to AD, whereas other components serve to do the opposite. The challenge will be to find ways of fine tuning inflammation to delay, prevent, or treat AD.

Microglial activation underlies cerebellar deficits produced by repeated cannabis exposure

Chronic cannabis exposure can lead to cerebellar dysfunction in humans, but the neurobiological mechanisms involved remain incompletely understood. Here, we found that in mice, subchronic administration of the psychoactive component of cannabis, delta9-tetrahydrocannabinol (THC), activated cerebellar microglia and increased the expression of neuroinflammatory markers, including IL-1β. This neuroinflammatory phenotype correlated with deficits in cerebellar conditioned learning and fine motor coordination. The neuroinflammatory phenotype was readily detectable in the cerebellum of mice with global loss of the CB1 cannabinoid receptor (CB1R, Cb1(-/-) mice) and in mice lacking CB1R in the cerebellar parallel fibers, suggesting that CB1R downregulation in the cerebellar molecular layer plays a key role in THC-induced cerebellar deficits. Expression of CB2 cannabinoid receptor (CB2R) and Il1b mRNA was increased under neuroinflammatory conditions in activated CD11b-positive microglial cells. Furthermore, administration of the immunosuppressant minocycline or an inhibitor of IL-1β receptor signaling prevented the deficits in cerebellar function in Cb1(-/-) and THC-withdrawn mice. Our results suggest that cerebellar microglial activation plays a crucial role in the cerebellar deficits induced by repeated cannabis exposure.

From development to dysfunction: microglia and the complement cascade in CNS homeostasis

Of the many mysteries that surround the brain, few surpass the awe-inspiring complexity of its development. The intricate wiring of the brain at both the system and molecular level is both spatially and temporally regulated in perfect synchrony. How such a delicate, yet elegant, system arises from an embryo's most basic cells remains at the forefront of neuroscientific research. At the cellular level, the competitive dance between synapses struggling to gain dominance seems to be refereed by both neurons themselves and microglia, the innate immune cells of the nervous system. Additionally, the unexpected complement cascade, a major effecter arm of the innate immune system, is almost certainly involved in synaptic remodeling by tagging destined neurons and synapses for destruction. As suddenly as they appear, the mechanisms of neurogenesis recede entering into adulthood. However, with age and insult, these mechanisms boisterously return, resulting in neurodegeneration. This review describes some of the mechanisms involved in synaptogenesis and wiring of the brain from the point of view of the innate immune system and then covers how similar molecular processes return with age and disease, specifically in the context of Alzheimer's disease.

Stimulus-evoked calcium transients in somatosensory cortex are temporarily inhibited by a nearby microhemorrhage

Although microhemorrhages are common in the brain of the elderly, the direct impact of these lesions on neural function remains unclear. In this work, we used femtosecond laser irradiation to rupture the wall of single arterioles in the brain of anesthetized rodents, producing a hematoma of ∼100-µm diameter. Our objective was to study the impact of these microhemorrhages on cortical activity using cell-resolved two-photon imaging of bulk-loaded calcium-sensitive dye. We monitored peripheral sensory stimulus-induced calcium transients from individual neuronal cell bodies, regions of neuropil, and astrocytes at different distances from the microhemorrhage before and 0.5, 2, and 4 hours after the creation of the lesion. We found that immediately after the hemorrhage the average amplitude of the stimulus-induced calcium response was reduced to about half within 150 µm from the hematoma. Beyond 300 µm, there was little effect on cell response, with a smooth increase in response amplitude from 150 µm to 300 µm from the lesion. Cortical function gradually improved with time and by four hours after the lesion the response from neurons and astrocytes had recovered to baseline everywhere but within 150 µm from the hematoma. To assess whether the cells closest to the microhemorrhage recovered over a longer timeframe, we developed a re-openable chronic cranial window preparation that allowed reinjection of calcium-sensitive fluorescent dye. We found that the response largely recovered by one day after the microhemorrhage even within 150 µm from the hematoma. This work suggests that neuronal and astrocyte function is transiently lost near a microhemorrhage, but recovers within one day after the lesion.

The p38α MAPK regulates microglial responsiveness to diffuse traumatic brain injury

Neuropathology after traumatic brain injury (TBI) is the result of both the immediate impact injury and secondary injury mechanisms. Unresolved post-traumatic glial activation is a secondary injury mechanism that contributes to a chronic state of neuroinflammation in both animal models of TBI and human head injury patients. We recently demonstrated, using in vitro models, that p38α MAPK signaling in microglia is a key event in promoting cytokine production in response to diverse disease-relevant stressors and subsequent inflammatory neuronal dysfunction. From these findings, we hypothesized that the p38α signaling pathway in microglia could be contributing to the secondary neuropathologic sequelae after a diffuse TBI. Mice where microglia were p38α-deficient (p38α KO) were protected against TBI-induced motor deficits and synaptic protein loss. In wild-type (WT) mice, diffuse TBI produced microglia morphological activation that lasted for at least 7 d; however, p38α KO mice failed to activate this response. Unexpectedly, we found that the peak of the early, acute phase cytokine and chemokine levels was increased in injured p38α KO mice compared with injured WT mice. The increased cytokine levels in the p38α KO mice could not be accounted for by more infiltration of macrophages or neutrophils, or increased astrogliosis. By 7 d after injury, the cytokine and chemokine levels remained elevated in injured WT mice but not in p38α KO mice. Together, these data suggest that p38α balances the inflammatory response by acutely attenuating the early proinflammatory cytokine surge while perpetuating the chronic microglia activation after TBI.

Microglia express distinct M1 and M2 phenotypic markers in the postnatal and adult central nervous system in male and female mice

Although microglial activation is associated with all CNS disorders, many of which are sexually dimorphic or age-dependent, little is known about whether microglial basal gene expression is altered with age in the healthy CNS or whether it is sex dependent. Analysis of microglia from the brains of 3-day (P3)- to 12-month-old male and female C57Bl/6 mice revealed distinct gene expression profiles during postnatal development that differ significantly from those in adulthood. Microglia at P3 are characterized by relatively high iNOS, TNFα and arginase-I mRNA levels, whereas P21 microglia have increased expression of CD11b, TLR4, and FcRγI. Adult microglia (2-4 months) are characterized by low proinflammatory cytokine expression, which increases by 12 months of age. Age-dependent differences in gene expression suggest that microglia likely undergo phenotypic changes during ontogenesis, although in the healthy brain they did not express exclusively either M1 or M2 phenotypic markers at any time. Interestingly, microglia were sexually dimorphic only at P3, when females had higher expression of inflammatory cytokines than males, although there were no sex differences in estrogen receptor expression at this or any other time evaluated here. Compared with microglia in vivo, primary microglia prepared from P3 mice had considerably altered gene expression, with higher levels of TNFα, CD11b, arginase-I, and VEGF, suggesting that culturing may significantly alter microglial properties. In conclusion, age- and sex-specific variances in basal gene expression may allow differential microglial responses to the same stimulus at different ages, perhaps contributing to altered CNS vulnerabilities and/or disease courses.

The brain's best friend: microglial neurotoxicity revisited

One long standing aspect of microglia biology was never questioned; their involvement in brain disease. Based on morphological changes (retracted processes and amoeboid shape) that inevitably occur in these cells in case of damage in the central nervous system, microglia in the diseased brain were called “activated.” Because “activated” microglia were always found in direct neighborhood to dead or dying neuron, and since it is known now for more than 20 years that cultured microglia release numerous factors that are able to kill neurons, microglia “activation” was often seen as a neurotoxic process. From an evolutionary point of view, however, it is difficult to understand why an important, mostly post-mitotic and highly vulnerable organ like the brain would host numerous potential killers. This review is aimed to critically reconsider the term microglia neurotoxicity and to discuss experimental problems around microglia biology, that often have led to the conclusion that microglia are neurotoxic cells.

Microglia and synapse interactions: fine tuning neural circuits and candidate molecules

Brain function depends critically on the interactions among the underlying components that comprise neural circuits. This includes coordinated activity in pre-synaptic and postsynaptic neuronal elements, but also in the non-neuronal elements such as glial cells. Microglia are glial cells in the central nervous system (CNS) that have well-known roles in neuronal immune function, responding to infections and brain injury and influencing the progress of neurodegenerative disorders. However, microglia are also surveyors of the healthy brain, continuously extending and retracting their processes and making contacts with pre- and postsynaptic elements of neural circuits, a process that clearly consumes considerable energy. Pruning of synapses during development and in response to injury has also been documented, and we propose that this extensive surveillance of the brain parenchyma in adult healthy brain results in similar “fine-tuning” of neural circuits. A reasonable extension is that a dysfunction of such a homeostatic role of microglia could be a primary cause of neuronal disease. Indeed, neuronal functions including cognition, personality, and information processing are affected by immune status. In this review we focus on the interactions between microglia and synapses, the possible cellular and molecular mechanisms that mediate such contacts, and the possible implications these interactions may have in the fine tuning of neural circuits that is so important for physiological brain function.

Functional diversity of microglia - how heterogeneous are they to begin with?

Microglia serve in the surveillance and maintenance, protection and restoration of the central nervous system (CNS) homeostasis. By their parenchymal location they differ from other CNS-associated myeloid cells, and by origin as well as functional characteristics they are also–at least in part–distinct from extraneural tissue macrophages. Nevertheless, microglia themselves may not comprise a uniform cell type. CNS regions vary by cellular and chemical composition, including white matter (myelin) content, blood–brain barrier properties or prevailing neurotransmitters. Such a micromilieu could instruct as well as require local adaptions of microglial features. Yet even cells within circumscribed populations may reveal some specialization by subtypes, regarding house-keeping duties and functional capacities upon challenges. While diversity of reactive phenotypes has been established still little is known as to whether all activated cells would respond with the same program of induced genes and functions or whether responder subsets have individual contributions. Preferential synthesis of a key cytokine could asign a master control to certain cells among a pool of activated microglia. Critical functions could be sequestered to discrete microglial subtypes in order to avoid interference, such as clearance of endogenous material and presentation of antigens. Indeed, several and especially a number of recent studies provide evidence for the constitutive and reactive heterogeneity of microglia by and within CNS regions. While such a principle of “division of labor” would influence the basic notion of “the” microglia, it could come with the practival value of addressing separate microglia types in experimental and therapeutic manipulations.

GPR18 in microglia: implications for the CNS and endocannabinoid system signalling

A review of what is presently known about the G protein coupled receptor GPR18 in terms of its expression and distribution, pharmacology and potential implications for central nervous system and endocannabinoid system signalling. LINKED ARTICLES This article is part of a themed section on Cannabinoids. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.167.issue-8.

Classical and unconventional pathways of vesicular release in microglia

Emerging evidence indicates that activation of microglia, the immune cells of the brain, is strictly associated to both secretion of soluble molecules and release of extracellular membrane vesicles (EMVs) into the pericellular space. Through these processes, microglia heavily influence brain cell functions, either propagating inflammation and causing damage to neurons or playing a supportive, neuroprotective role. In this review, we highlight the emerging concepts related to vesicular mechanisms of secretion operating in microglial cells, with the aim of dissecting how microglia communicate with other cell types within the brain microenvironment in health and disease.

Microglia in the cerebral cortex in autism

We immunocytochemically identified microglia in fronto-insular (FI) and visual cortex (VC) in autopsy brains of well-phenotyped subjects with autism and matched controls, and stereologically quantified the microglial densities. Densities were determined blind to phenotype using an optical fractionator probe. In FI, individuals with autism had significantly more microglia compared to controls (p = 0.02). One such subject had a microglial density in FI within the control range and was also an outlier behaviorally with respect to other subjects with autism. In VC, microglial densities were also significantly greater in individuals with autism versus controls (p = 0.0002). Since we observed increased densities of microglia in two functionally and anatomically disparate cortical areas, we suggest that these immune cells are probably denser throughout cerebral cortex in brains of people with autism.

Neurotransmitter signaling in the pathophysiology of microglia

Microglial cells are the resident immune cells of the central nervous system. In the resting state, microglia are highly dynamic and control the environment by rapidly extending and retracting motile processes. Microglia are closely associated with astrocytes and neurons, particularly at the synapses, and more recent data indicate that neurotransmission plays a role in regulating the morphology and function of surveying/resting microglia, as they are endowed with receptors for most known neurotransmitters. In particular, microglia express receptors for ATP and glutamate, which regulate microglial motility. After local damage, the release of ATP induces microgliosis and activated microglial cells migrate to the site of injury, proliferate, and phagocytose cells, and cellular compartments. However, excessive activation of microglia could contribute to the progression of chronic neurodegenerative diseases, though the underlying mechanisms are still unclear. Microglia have the capacity to release a large number of substances that can be detrimental to the surrounding neurons, including glutamate, ATP, and reactive oxygen species. However, how altered neurotransmission following acute insults or chronic neurodegenerative conditions modulates microglial functions is still poorly understood. This review summarizes the relevant data regarding the role of neurotransmitter receptors in microglial physiology and pathology.

Gene expression changes in aging retinal microglia: relationship to microglial support functions and regulation of activation

Microglia, the resident immune cells of the central nervous system (CNS), are thought to contribute to the pathogenesis of age-related neurodegenerative disorders. It has been hypothesized that microglia undergo age-related changes in gene expression patterns that give rise to pathogenic phenotypes. We compared the gene expression profiles in microglia isolated ex vivo from the retinas of mice ranging from early adulthood to late senescence. We discovered that microglial gene expression demonstrated progressive change with increasing age, and involved genes that regulate microglial supportive functions and immune activation. Molecular pathways involving immune function and regulation, angiogenesis, and neurotrophin signaling demonstrated age-related change. In particular, expression levels of complement genes, C3 and CFB, previously associated with age-related macular degeneration (AMD), increased with aging, suggesting that senescent microglia may contribute to complement dysregulation during disease pathogenesis. Taken together, senescent microglia demonstrate age-related gene expression changes capable of altering their constitutive support functions and regulation of their activation status in ways relating to neuroinflammation and neurodegeneration in the CNS.

Rare contacts between synapses and microglial processes containing high levels of Iba1 and actin--a postembedding immunogold study in the healthy rat brain

Although microglia is recognised as the cell-mediating innate immunity in the brain, emerging evidence suggests a role of microglia in synaptic communication and modulation. The ability of microglia to move in the neuropil and contact synapses is crucial for such a function. However, the frequency of microglial contact with synapses is not known. Microglia motility is regulated by actin polymerisation and its interaction with ionising calcium-binding adaptor protein 1 (Iba1). In order to move and make contact with synapses, delicate microglial processes should contain high levels of actin and Iba1. To study this we refined an electron microscopic postembedding immunogold method enabling us to identify and quantitatively study different microglial constituents in intact brain tissue. We show that Iba1 and actin were colocalised at high densities in delicate processes in the rat frontal cortex, and that these delicate processes of microglia contact synaptic elements. About 3.5% of the synapses received direct contact from microglia. There was a marked inverse correlation between the densities of Iba1/actin gold particles and the area of the microglial processes, suggesting that the most delicate processes possess the machinery to provide movement in the neuropil. The low frequency of microglia interaction with synaptic elements suggests that microglia have a limited role in overall regulation of synaptic activity.

Efficiently stimulated adult microglia cross-prime naive CD8+ T cells injected in the brain

Microglia are the major myeloid-immune cells of the brain parenchyma. In a steady state, microglia monitor their environment for pathogens or damaged cells. In response to neural injury or inflammation, microglia become competent APCs able to prime CD4(+) and CD8(+) T lymphocytes. We previously demonstrated that neonatal and adult microglia cross-present exogenous soluble Ags in vitro. However, whether microglia are able to cross-present Ag to naive CD8(+) T cells in vivo, within the brain microenvironment, remains undetermined. Here, we have designed an original protocol in order to exclude the involvement in cross-presentation activity of peripheral migrating APCs and of CNS-associated APCs. In C57Bl/6 mice, in which the body but not the head has been properly irradiated, we analyzed the ability of resident microglia to stimulate intracerebrally injected CD8(+) T cells in vivo. This study demonstrates for the first time that adult microglia cross-present Ag to naive CD8(+) T cells in vivo and that full microglia activation is required to overcome the inhibitory constrains of the brain and to render microglia able to cross-prime naive CD8(+) T cells injected in the brain. These observations offer new insights in brain-tumor immunotherapy based on the induction of cytotoxic antitumoral T cells.

Functional impairment of microglia coincides with Beta-amyloid deposition in mice with Alzheimer-like pathology

Microglial cells closely interact with senile plaques in Alzheimer’s disease and acquire the morphological appearance of an activated phenotype. The significance of this microglial phenotype and the impact of microglia for disease progression have remained controversial. To uncover and characterize putative changes in the functionality of microglia during Alzheimer’s disease, we directly assessed microglial behavior in two mouse models of Alzheimer’s disease. Using in vivo two-photon microscopy and acute brain slice preparations, we found that important microglial functions - directed process motility and phagocytic activity - were strongly impaired in mice with Alzheimer’s disease-like pathology compared to age-matched non-transgenic animals. Notably, impairment of microglial function temporally and spatially correlated with Aβ plaque deposition, and phagocytic capacity of microglia could be restored by interventionally decreasing amyloid burden by Aβ vaccination. These data suggest that major microglial functions progressively decline in Alzheimer’s disease with the appearance of Aβ plaques, and that this functional impairment is reversible by lowering Aβ burden, e.g. by means of Aβ vaccination.

Microglia and monocyte-derived macrophages: functionally distinct populations that act in concert in CNS plasticity and repair

Functional macrophage heterogeneity is recognized outside the central nervous system (CNS), where alternatively activated macrophages can perform immune-resolving functions. Such functional heterogeneity was largely ignored in the CNS, with respect to the resident microglia and the myeloid-derived cells recruited from the blood following injury or disease, previously defined as blood-derived microglia; both were indistinguishably perceived detrimental. Our studies have led us to view the myeloid-derived infiltrating cells as functionally distinct from the resident microglia, and accordingly, to name them monocyte-derived macrophages (mo-MΦ). Although microglia perform various maintenance and protective roles, under certain conditions when they can no longer provide protection, mo-MΦ are recruited to the damaged CNS; there, they act not as microglial replacements but rather assistant cells, providing activities that cannot be timely performed by the resident cells. Here, we focus on the functional heterogeneity of microglia/mo-MΦ, emphasizing that, as opposed to the mo-MΦ, microglia often fail to timely acquire the phenotype essential for CNS repair.

Glial responses after chorda tympani nerve injury

The chorda tympani (CT) nerve innervates lingual taste buds and is susceptible to damage during dental and inner ear procedures. Interruption of the CT results in a disappearance of taste buds, which can be accompanied by taste disturbances. Because the CT usually regenerates to reinnervate taste buds successfully within a few weeks, a persistence of taste disturbances may indicate alterations in central nervous function. Peripheral injury to other sensory nerves leads to glial responses at central terminals, which actively contribute to abnormal sensations arising from nerve damage. Therefore, the current study examined microglial and astrocytic responses in the first central gustatory relay, the nucleus of the solitary tract (nTS), after transection of the CT. Damage to the CT resulted in significant microglial responses in terms of morphological reactivity and an increased density of microglial cells from 2 to 20 days after injury. This increased microglial population resulted primarily from microglial proliferation from 1.5 to 3 days, which was supplemented by microglial migration within subdivisions of the nTS between days 2 and 3. Unlike other nerve injuries, CT injury did not result in recruitment of bone marrow-derived precursors. Astrocytes also reacted in the nTS with increased levels of glial fibrillary acidic protein (GFAP) by 3 days, although none showed evidence of cell division. GFAP levels remained increased at 30 days, by which time microglial responses had resolved. These results show that nerve damage to the CT results in central glial responses, which may participate in long-lasting taste alterations following CT lesion.

Optogenetic manipulation of neural and non-neural functions

Optogenetic manipulation of the neuronal activity enables one to analyze the neuronal network both in vivo and in vitro with precise spatio-temporal resolution. Channelrhodopsins (ChRs) are light-sensitive cation channels that depolarize the cell membrane, whereas halorhodopsins and archaerhodopsins are light-sensitive Cl(-) and H(+) transporters, respectively, that hyperpolarize it when exogenously expressed. The cause-effect relationship between a neuron and its function in the brain is thus bi-directionally investigated with evidence of necessity and sufficiency. In this review we discuss the potential of optogenetics with a focus on three major requirements for its application: (i) selection of the light-sensitive proteins optimal for optogenetic investigation, (ii) targeted expression of these selected proteins in a specific group of neurons, and (iii) targeted irradiation with high spatiotemporal resolution. We also discuss recent progress in the application of optogenetics to studies of non-neural cells such as glial cells, cardiac and skeletal myocytes. In combination with stem cell technology, optogenetics may be key to successful research using embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) derived from human patients through optical regulation of differentiation-maturation, through optical manipulation of tissue transplants and, furthermore, through facilitating survival and integration of transplants.