Development of microglia in the chick embryo spinal cord: Implications in the regulation of motoneuronal survival and death

LPS-stimulated microglial cells promote ganglion cell death in organotypic cultures of quail embryo retina

During development microglia colonize the central nervous system (CNS) and play an important role in programmed cell death, not only because of their ability to remove dead cells by phagocytosis, but also because they can promote the death of neuronal and glial cells. To study this process, we used as experimental systems the developing in situ quail embryo retina and organotypic cultures of quail embryo retina explants (QEREs). In both systems, immature microglia show an upregulation of certain inflammatory markers, e.g., inducible NO synthase (iNOS), and nitric oxide (NO) under basal conditions, which can be further enhanced with LPS-treatment. Hence, we investigated in the present study the role of microglia in promoting ganglion cell death during retinal development in QEREs. Results showed that LPS-stimulation of microglia in QEREs increases (i) the percentage of retinal cells with externalized phosphatidylserine, (ii) the frequency of phagocytic contacts between microglial and caspase-3-positive ganglion cells, (iii) cell death in the ganglion cell layer, and (iv) microglial production of reactive oxygen/nitrogen species, such as NO. Furthermore, iNOS inhibition by L-NMMA decreases cell death of ganglion cells and increases the number of ganglion cells in LPS-treated QEREs. These data demonstrate that LPS-stimulated microglia induce ganglion cell death in cultured QEREs by a NO-dependent mechanism. The fact that phagocytic contacts between microglial and caspase-3-positive ganglion cells increase suggests that this cell death might be mediated by microglial engulfment, although a phagocytosis-independent mechanism cannot be excluded.

Brain effects of gestating germ-free persist in mouse neonates despite acquisition of a microbiota at birth

At birth, mammals experience a massive colonization by microorganisms. We previously reported that newborn mice gestated and born germ-free (GF) have increased microglial labeling and alterations in developmental neuronal cell death in the hippocampus and hypothalamus, as well as greater forebrain volume and body weight when compared to conventionally colonized (CC) mice. To test whether these effects are solely due to differences in postnatal microbial exposure, or instead may be programmed in utero, we cross-fostered GF newborns immediately after birth to CC dams (GF→CC) and compared them to offspring fostered within the same microbiota status (CC→CC, GF→GF). Because key developmental events (including microglial colonization and neuronal cell death) shape the brain during the first postnatal week, we collected brains on postnatal day (P) 7. To track gut bacterial colonization, colonic content was also collected and subjected to 16S rRNA qPCR and Illumina sequencing. In the brains of GF→GF mice, we replicated most of the effects seen previously in GF mice. Interestingly, the GF brain phenotype persisted in GF→CC offspring for almost all measures. In contrast, total bacterial load did not differ between the CC→CC and GF→CC groups on P7, and bacterial community composition was also very similar, with a few exceptions. Thus, GF→CC offspring had altered brain development during at least the first 7 days after birth despite a largely normal microbiota. This suggests that prenatal influences of gestating in an altered microbial environment programs neonatal brain development.

Mechanisms underlying microglial colonization of developing neural retina in zebrafish

Microglia are brain-resident macrophages that function as the first line of defense in brain. Embryonic microglial precursors originate in peripheral mesoderm and migrate into the brain during development. However, the mechanism by which they colonize the brain is incompletely understood. The retina is one of the first brain regions to accommodate microglia. In zebrafish, embryonic microglial precursors use intraocular hyaloid blood vessels as a pathway to migrate into the optic cup via the choroid fissure. Once retinal progenitor cells exit the cell cycle, microglial precursors associated with hyaloid blood vessels start to infiltrate the retina preferentially through neurogenic regions, suggesting that colonization of retinal tissue depends upon the neurogenic state. Along with blood vessels and retinal neurogenesis, IL34 also participates in microglial precursor colonization of the retina. Altogether, CSF receptor signaling, blood vessels, and neuronal differentiation function as cues to create an essential path for microglial migration into developing retina.

The immune system is comprised of many different cells which protect our bodies from infection and other illnesses. The brain contains its own population of immune cells called microglia. Unlike neurons, these cells form outside the brain during development. They then travel to the brain and colonize specific regions like the retina, the light-sensing part of the eye in vertebrates.

It is poorly understood how newly formed microglia migrate to the retina and whether their entry depends on the developmental state of nerve cells (also known as neurons) in this region. To help answer these questions, Ranawat and Masai attached fluorescent labels that can be seen under a microscope to microglia in the embryos of zebrafish. Developing zebrafish are transparent, making it easy to trace the fluorescent microglia as they travel to the retina and insert themselves among its neurons.

Ranawat and Masai found that blood vessels around the retina act as a pathway that microglia move along. Once they reach the retina, the microglia remain attached and only enter the retina at sites where brain cells are starting to mature in to adult neurons. Further experiments showed that microglia fail to infiltrate and colonize the retina when blood vessels are damaged or neuron maturation is blocked.

These findings reveal some of the key elements that guide microglia to the retina during development. However, further work is needed to establish the molecular and biochemical processes that allow microglia to attach to blood vessels and detect when cells in the retina are starting to mature.

More Than Mortar: Glia as Architects of Nervous System Development and Disease

Glial cells are an essential component of the nervous system of vertebrates and invertebrates. In the human brain, glia are as numerous as neurons, yet the importance of glia to nearly every aspect of nervous system development has only been expounded over the last several decades. Glia are now known to regulate neural specification, synaptogenesis, synapse function, and even broad circuit function. Given their ubiquity, it is not surprising that the contribution of glia to neuronal disease pathogenesis is a growing area of research. In this review, we will summarize the accumulated evidence of glial participation in several distinct phases of nervous system development and organization-neural specification, circuit wiring, and circuit function. Finally, we will highlight how these early developmental roles of glia contribute to nervous system dysfunction in neurodevelopmental and neurodegenerative disorders.

Spinal Cord Microglia in Health and Disease

The review summarizes data of recent experimental studies on spinal microglia, the least explored cells of the spinal cord. It focuses on the origin and function of microglia in mammalian spinal cord embryogenesis. The main approaches to the classification of microgliocytes based on their structure, function, and immunophenotypic characteristics are analyzed. We discuss the results of studies conducted on experimental models of spinal cord diseases such as multiple sclerosis, amyotrophic lateral sclerosis, systemic inflammation, and some others, with special emphasis on the key role of microglia in the pathogenesis of these diseases. The review highlights the need to detect the new microglia-specific marker proteins expressed at all stages of ontogeny. New sensitive and selective microglial markers are necessary in order to improve identification of spinal cord microgliocytes in normal and pathological conditions. Possible morphometric methods to assess the functional activity of microglial cells are presented.

Microglia along sex lines: From brain colonization, maturation and function, to implication in neurodevelopmental disorders

In addition to their traditional role as immune sentinels, recent discoveries over the last decade have shown that microglial functions now include regulation of neuronal/glial cell migration, differentiation and maturation, as well as neuronal network formation. It was thus proposed that disruption of these microglial roles, during critical periods of brain development, could lead to the pathological onset of several neurodevelopmental disorders, including autism spectrum disorder, attention deficit hyperactivity disorder, epilepsy, schizophrenia, and major depressive disorder. The prevalence of these disorders exhibits a clear distinction along sex lines with very little known about the mechanisms underlying this difference. One of the fundamental discoveries that arose from recent research into the physiological roles of microglia in neurodevelopment is their sexual dimorphism, raising the intriguing possibility that sex differences in microglial colonization, maturation and/or function in the developing brain could underlie the emergence of various neurodevelopmental disorders. This review discusses the physiological roles of microglia across neurodevelopment, these roles in the two sexes, and the recent evidence that microglial sexually dimorphic nature may contribute, at least partially, to neurodevelopmental disorders.

Transferrin Enhances Microglial Phagocytic Capacity

Transferrin (Tf) is a glycoprotein playing a critical role in iron homeostasis and transport and distribution throughout the body and within tissues and cells. This molecule has been shown to accelerate the process of myelination and remyelination in the central nervous system (CNS) in vivo and induce oligodendroglial cell maturation in vitro. While the mechanisms involved in oligodendroglial precursor cell (OPC) differentiation have not been fully elucidated yet, our group has previously described the first molecular events taking place in OPC in response to extracellular Tf. Here, we show the effect of Tf on the different glial cell populations. We demonstrate that, after a CNS demyelinating injury, Tf can be incorporated by all glial cells-i.e., microglia, astrocytes, and OPC-and that, acting on microglial cells in vitro, Tf increases microglial proliferation rates and phagocytic capacity. It may be then speculated that the in vivo correlation of this process could generate a favorable microenvironment for OPC maturation and remyelination.

Microglia in the Retina: Roles in Development, Maturity, and Disease

Microglia, the primary resident immune cell type, constitute a key population of glia in the retina. Recent evidence indicates that microglia play significant functional roles in the retina at different life stages. During development, retinal microglia regulate neuronal survival by exerting trophic influences and influencing programmed cell death. During adulthood, ramified microglia in the plexiform layers interact closely with synapses to maintain synaptic structure and function that underlie the retina's electrophysiological response to light. Under pathological conditions, retinal microglia participate in potentiating neurodegeneration in diseases such as glaucoma, retinitis pigmentosa, and age-related neurodegeneration by producing proinflammatory neurotoxic cytokines and removing living neurons via phagocytosis. Modulation of pathogenic microglial activation states and effector mechanisms has been linked to neuroprotection in animal models of retinal diseases. These findings have led to the design of early proof-of-concept clinical trials with microglial modulation as a therapeutic strategy.

Emerging Developments in Microbiome and Microglia Research: Implications for Neurodevelopmental Disorders

From immunology to neuroscience, interactions between the microbiome and host are increasingly appreciated as potent drivers of health and disease. Epidemiological studies previously identified compelling correlations between perinatal microbiome insults and neurobehavioral outcomes, the mechanistic details of which are just beginning to take shape thanks to germ-free and antibiotics-based animal models. This review summarizes parallel developments from clinical and preclinical research that suggest neuroactive roles for gut bacteria and their metabolites. We also examine the nascent field of microbiome-microglia crosstalk research, which includes pharmacological and genetic strategies to inform functional capabilities of microglia in response to microbial programming. Finally, we address an emerging hypothesis behind neurodevelopmental disorders, which implicates microbiome dysbiosis in the atypical programming of neuroimmune cells, namely microglia.

CD200-CD200R imbalance correlates with microglia and pro-inflammatory activation in rat spinal cords exposed to amniotic fluid in retinoic acid-induced spina bifida

Spina bifida aperta is a congenital malformation characterized by the failure of neural tube closure resulting in an unprotected fetal spinal cord. The spinal cord then undergoes progressive damage, likely due to chemical and mechanical factors related to exposure to the intrauterine environment. Astrogliosis in exposed spinal cords has been described in animal models of spina bifida during embryonic life but its relationship with neuroinflammatory processes are completely unknown. Using a retinoic acid-induced rat model of spina bifida we demonstrated that, when exposed to amniotic fluid, fetal spinal cords showed progressive astrogliosis with neuronal loss at mid-gestation (E15) compared to unexposed spinal cords. The number of microglial cells with a reactive phenotype and activation marker expression increased during gestation and exhibited progressive disruption in the inhibitory immune ligand-receptor system. Specifically we demonstrate down-regulation of CD200 expression and up-regulation of CD200R. Exposed spinal cords demonstrated neuroinflammation with increased tissue water content and cytokine production by the end of gestation (E20), which correlated with active Caspase3 expression in the exposed layers. Our findings provide new evidence that microglia activation, including the disruption of the endogenous inhibitory system (CD200-CD200R), may participate in the pathogenesis of spina bifida through late gestation.

Onset of microglial entry into developing quail retina coincides with increased expression of active caspase-3 and is mediated by extracellular ATP and UDP

Microglial cell precursors located in the area of the base of the pecten and the optic nerve head (BP/ONH) start to enter the retina of quail embryos at the 7^th day of incubation (E7), subsequently colonizing the entire retina by central-to-peripheral tangential migration, as previously shown by our group. The present study demonstrates a precise chronological coincidence of the onset of microglial cell entry into the retina with a striking increase in death of retinal cells, as revealed by their active caspase-3 expression and TUNEL staining, in regions dorsal to the BP/ONH area, suggesting that dying retinal cells would contribute to the microglial cell inflow into the retina. However, the molecular mechanisms involved in this inflow are currently unclear. Extracellular nucleotides, such as ATP and UDP, have previously been shown to favor migration of microglia towards brain injuries because they are released by apoptotic cells and stimulate both chemotaxis and chemokinesis in microglial cells via signaling through purinergic receptors. Hence, we tested here the hypothesis that ATP and UDP play a role in the entry and migration of microglial precursors into the developing retina. For this purpose, we used an experimental model system based on organotypic cultures of E6.5 quail embryo retina explants, which mimics the entry and migration of microglial precursors in the in situ developing retina. Inhibition of purinergic signaling by treating retina explants with either apyrase, a nucleotide-hydrolyzing enzyme, or suramin, a broad spectrum antagonist of purinergic receptors, significantly prevents the entry of microglial cells into the retina. In addition, treatment of retina explants with either exogenous ATP or UDP results in significantly increased numbers of microglial cells entering the retina. In light of these findings, we conclude that purinergic signaling by extracellular ATP and UDP is necessary for the entry and migration of microglial cells into the embryonic retina by inducing chemokinesis in these cells.

Characteristics of primary rat microglia isolated from mixed cultures using two different methods

Background

Microglial cultures comprise a critically important model system for investigating inflammatory mechanisms in almost all CNS disorders. Mild trypsinization and shaking are the two most commonly used methods to isolate primary microglia from mixed glial cultures. In this study, we characterized and compared microglia obtained using these two methods.

Methods

Primary rat microglia cultures were prepared from cerebral cortices of 1–2-day-old neonatal Sprague-Dawley rats. After achieving confluency at about 14 days in vitro, microglia were isolated from mixed glial cultures via either mild trypsinization or shaking. The purity of microglia was estimated by flow cytometry. Quantitative real-time PCR was used to measure mRNA expression. TNFα, IL-1β, IL-10, and IGF-1 in cell culture supernatant were measured using ELISA kits. Phagocytic function was assessed using fluorescein-labeled Escherichia coli K-12 BioParticles.

Results

Mild trypsinization generated a higher yield and purity than shaking. Microglia isolated by mild trypsinization appeared to be in a quiescent state with ramified morphology. Microglia isolated by shaking showed a more heterogenous morphology, including cells with rounded shapes suggestive of activation. Compared with shaking, microglia isolated by trypsinization also had lower baseline phenotype markers (iNOS, CD86, CD206, and arginase 1) and lower levels of cytokines (TNFα, IL-1β, IL-10, and IGF-1) as well as reduced phagocytic capability. Both methods yielded microglia that were responsive to various stimuli such as IL-4, lipopolysaccharide (LPS), or interferon-γ (IFNγ). Although stimulated patterns of gene expression and cytokine release were generally similar, there were also significant differences in terms of absolute response. LPS treatment induced significantly higher levels of TNFα and IL-10 in microglia isolated by mild trypsinization versus shaking. IFNγ induced a lower response in TNFα in microglia obtained by mild trypsinization versus shaking.

Conclusions

Our results suggest that isolating microglia with the shaking method may induce slight activation even at baseline, and this may affect stimulus responses in subsequent experiments. Caution and attention should be warranted when choosing isolation protocols for primary microglia cultures.

The Indispensable Roles of Microglia and Astrocytes during Brain Development

Glia are essential for brain functioning during development and in the adult brain. Here, we discuss the various roles of both microglia and astrocytes, and their interactions during brain development. Although both cells are fundamentally different in origin and function, they often affect the same developmental processes such as neuro-/gliogenesis, angiogenesis, axonal outgrowth, synaptogenesis and synaptic pruning. Due to their important instructive roles in these processes, dysfunction of microglia or astrocytes during brain development could contribute to neurodevelopmental disorders and potentially even late-onset neuropathology. A better understanding of the origin, differentiation process and developmental functions of microglia and astrocytes will help to fully appreciate their role both in the developing as well as in the adult brain, in health and disease.

Hippocampal adult neurogenesis: Does the immune system matter?

Adult hippocampal neurogenesis involves proliferation, survival, differentiation and integration of newborn neurons into pre-existing neuronal networks. Although its functional significance in the central nervous system (CNS) has not comprehensively elucidated, adult neurogenesis has been attributed a role in cognition, learning and memory. There is a growing body of evidence that CNS resident as well as peripheral immune cells participate in regulating hippocampal adult neurogenesis. Microglial cells are closely associated with neural stem/progenitor cell (NSPC) in the neurogenic niche engaged in a bidirectional communication with neurons, which may be important for adult neurogenesis. Microglial and neuronal crosstalk is mediated in part by CX3CL1/CX3CR1 signaling and a disruption in this pathway has been associated with impaired neurogenesis. It has been also reported that microglial neuroprotective or neurotoxic effects in adult neurogenesis occur in a context-dependent manner. Apart from microglia other brain resident and peripheral immune cells including pericytes, perivascular macrophages, mast cells and T-cells also modulate this phenomenon. It is worth mentioning that under some physiological circumstances such as normal aging there is a significant decrease in hippocampal neurogenesis. A role for innate and adaptive immune system in adult neurogenesis has been also reported during aging. Here, we review the current evidence regarding neuro-immune interactions in the regulation of neurogenesis under distinct conditions, including aging.

Linking Activation of Microglia and Peripheral Monocytic Cells to the Pathophysiology of Psychiatric Disorders

A wide variety of studies have identified microglial activation in psychiatric disorders, such as schizophrenia, bipolar disorder, and major depressive disorder. Relatively fewer, but robust, studies have detected activation of peripheral monocytic cells in psychiatric disorders. Considering the origin of microglia, as well as neuropsychoimmune interactions in the context of the pathophysiology of psychiatric disorders, it is reasonable to speculate that microglia interact with peripheral monocytic cells in relevance with the pathogenesis of psychiatric disorders; however, these interactions have drawn little attention. In this review, we summarize findings relevant to activation of microglia and monocytic cells in psychiatric disorders, discuss the potential association between these cell types and disease pathogenesis, and propose perspectives for future research on these processes.

The autistic brain in the context of normal neurodevelopment

The etiology of autism spectrum disorders (ASDs) is complex and largely unclear. Among various lines of inquiry, many have suggested convergence onto disruptions in both neural circuitry and immune regulation/glial cell function pathways. However, the interpretation of the relationship between these two putative mechanisms has largely focused on the role of exogenous factors and insults, such as maternal infection, in activating immune pathways that in turn result in neural network abnormalities. Yet, given recent insights into our understanding of human neurodevelopment, and in particular the critical role of glia and the immune system in normal brain development, it is important to consider these putative pathological processes in their appropriate normal neurodevelopmental context. In this review, we explore the hypothesis that the autistic brain cellular phenotype likely represents intrinsic abnormalities of glial/immune processes constitutively operant in normal brain development that result in the observed neural network dysfunction. We review recent studies demonstrating the intercalated role of neural circuit development, the immune system, and glial cells in the normal developing brain, and integrate them with studies demonstrating pathological alterations in these processes in autism. By discussing known abnormalities in the autistic brain in the context of normal brain development, we explore the hypothesis that the glial/immune component of ASD may instead be related to intrinsic exaggerated/abnormal constitutive neurodevelopmental processes such as network pruning. Moreover, this hypothesis may be relevant to other neurodevelopmental disorders that share genetic, pathologic, and clinical features with autism.

Microglia: multitasking specialists of the brain

Microglia are macrophages that colonize the brain during development to establish a resident population of professional phagocytes that protect against invading pathogens and contribute to brain development and homeostasis. As such, these cells sit at the interface between immunology and neurobiology. In addition to their key roles in brain physiology, microglia offer a great opportunity to address central questions in biology relating to how migrating cells find their positions in the embryo, adopt a behavior that is appropriate for that position, and interact with their local environment. We aim, in this review, to survey key recent advances in microglial research.

Potential therapeutic strategies for Alzheimer's disease targeting or beyond β-amyloid: insights from clinical trials

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with two hallmarks: β-amyloid plagues and neurofibrillary tangles. It is one of the most alarming illnesses to elderly people. No effective drugs and therapies have been developed, while mechanism-based explorations of therapeutic approaches have been intensively investigated. Outcomes of clinical trials suggested several pitfalls in the choice of biomarkers, development of drug candidates, and interaction of drug-targeted molecules; however, they also aroused concerns on the potential deficiency in our understanding of pathogenesis of AD, and ultimately stimulated the advent of novel drug targets tests. The anticipated increase of AD patients in next few decades makes development of better therapy an urgent issue. Here we attempt to summarize and compare putative therapeutic strategies that have completed clinical trials or are currently being tested from various perspectives to provide insights for treatments of Alzheimer's disease.

Mechanisms involved in spinal cord central synapse loss in a mouse model of spinal muscular atrophy

Motoneuron (MN) cell death is the histopathologic hallmark of spinal muscular atrophy (SMA), although MN loss seems to be a late event. Conversely, disruption of afferent synapses on MNs has been shown to occur early in SMA. Using a mouse model of severe SMA (SMNΔ7), we examined the mechanisms involved in impairment of central synapses. We found that MNs underwent progressive degeneration in the course of SMA, with MN loss still occurring at late stages. Loss of afferent inputs to SMA MNs was detected at embryonic stages, long before MN death. Reactive microgliosis and astrogliosis were present in the spinal cord of diseased animals after the onset of MN loss. Ultrastructural observations indicate that dendrites and microglia phagocytose adjacent degenerating presynaptic terminals. Neuronal nitric oxide synthase was upregulated in SMNΔ7 MNs, and there was an increase in phosphorylated myosin light chain expression in synaptic afferents on MNs; these observations implicate nitric oxide in MN deafferentation and suggest that the RhoA/ROCK pathway is activated. Together, our observations suggest that the earliest change occurring in SMNΔ7 mice is the loss of excitatory glutamatergic synaptic inputs to MNs; reduced excitability may enhance their vulnerability to degeneration and death.

Neuregulin-1 is concentrated in the postsynaptic subsurface cistern of C-bouton inputs to α-motoneurons and altered during motoneuron diseases

C boutons are large, cholinergic, synaptic terminals that arise from local interneurons and specifically contact spinal α-motoneurons (MNs). C boutons characteristically display a postsynaptic specialization consisting of an endoplasmic reticulum-related subsurface cistern (SSC) of unknown function. In the present work, by using confocal microscopy and ultrastructural immunolabeling, we demonstrate that neuregulin-1 (NRG1) accumulates in the SSC of mouse spinal MNs. We also show that the NRG1 receptors erbB2 and erbB4 are presynaptically localized within C boutons, suggesting that NRG1-based retrograde signaling may occur in this type of synapse. In most of the cranial nuclei, MNs display the same pattern of NRG1 distribution as that observed in spinal cord MNs. Conversely, MNs in oculomotor nuclei, which are spared in amyotrophic lateral sclerosis (ALS), lack both C boutons and SSC-associated NRG1. NRG1 in spinal MNs is developmentally regulated and depends on the maintenance of nerve-muscle interactions, as we show after nerve transection experiments. Changes in NRG1 in C boutons were also investigated in mouse models of MN diseases: i.e., spinal muscular atrophy (SMNΔ7) and ALS (SOD1(G93A)). In both models, a transient increase in NRG1 in C boutons occurs during disease progression. These data increase our understanding of the role of C boutons in MN physiology and pathology.

Activity blockade and GABAA receptor blockade produce synaptic scaling through chloride accumulation in embryonic spinal motoneurons and interneurons

Synaptic scaling represents a process whereby the distribution of a cell's synaptic strengths are altered by a multiplicative scaling factor. Scaling is thought to be a compensatory response that homeostatically controls spiking activity levels in the cell or network. Previously, we observed GABAergic synaptic scaling in embryonic spinal motoneurons following in vivo blockade of either spiking activity or GABA_A receptors (GABA_ARs). We had determined that activity blockade triggered upward GABAergic scaling through chloride accumulation, thus increasing the driving force for these currents. To determine whether chloride accumulation also underlies GABAergic scaling following GABA_AR blockade we have developed a new technique. We expressed a genetically encoded chloride-indicator, Clomeleon, in the embryonic chick spinal cord, which provides a non-invasive fast measure of intracellular chloride. Using this technique we now show that chloride accumulation underlies GABAergic scaling following blockade of either spiking activity or the GABA_AR. The finding that GABA_AR blockade and activity blockade trigger scaling via a common mechanism supports our hypothesis that activity blockade reduces GABA_AR activation, which triggers synaptic scaling. In addition, Clomeleon imaging demonstrated the time course and widespread nature of GABAergic scaling through chloride accumulation, as it was also observed in spinal interneurons. This suggests that homeostatic scaling via chloride accumulation is a common feature in many neuronal classes within the embryonic spinal cord and opens the possibility that this process may occur throughout the nervous system at early stages of development.

[Microglial cells and development of the embryonic central nervous system]

Microglia cells are the macrophages of the central nervous system with a crucial function in the homeostasis of the adult brain. However, recent studies showed that microglial cells may also have important functions during early embryonic central nervous system development. In this review we summarize recent works on the extra embryonic origin of microglia, their progenitor niche, the pattern of their invasion of the embryonic central nervous system and on interactions between embryonic microglia and their local environment during invasion. We describe microglial functions during development of embryonic neuronal networks, including their roles in neurogenesis, in angiogenesis and developmental cell death. These recent discoveries open a new field of research on the functions of neural-microglial interactions during the development of the embryonic central nervous system.

Microglia development and function

Proper development and function of the mammalian central nervous system (CNS) depend critically on the activity of parenchymal sentinels referred to as microglia. Although microglia were first described as ramified brain-resident phagocytes, research conducted over the past century has expanded considerably upon this narrow view and ascribed many functions to these dynamic CNS inhabitants. Microglia are now considered among the most versatile cells in the body, possessing the capacity to morphologically and functionally adapt to their ever-changing surroundings. Even in a resting state, the processes of microglia are highly dynamic and perpetually scan the CNS. Microglia are in fact vital participants in CNS homeostasis, and dysregulation of these sentinels can give rise to neurological disease. In this review, we discuss the exciting developments in our understanding of microglial biology, from their developmental origin to their participation in CNS homeostasis and pathophysiological states such as neuropsychiatric disorders, neurodegeneration, sterile injury responses, and infectious diseases. We also delve into the world of microglial dynamics recently uncovered using real-time imaging techniques.

Adaptive phenotype of microglial cells during the normal postnatal development of the somatosensory "Barrel" cortex

Accumulative evidence indicates that microglial cells influence the normal development of central nervous system (CNS) synapses. Yet, the functional properties of microglia in relation with synapse development remain unclear. We recently showed that in layer 4 of the whisker-related barrel field of the mouse somatosensory cortex, microglial cells are recruited only after postnatal day (P)5 in the center of the barrels where thalamo-cortical synapses are concentrated and begin their maturation. In the present study, we analyzed the phenotype of microglia during this developmental process. We show that between P5 and P7 microglial cells acquire a more ramified morphology with a smaller soma, they express classical markers of microglia (Iba1, CD11b, and CD68) but never markers of activation (Mac-2 and MHCII) and rarely the proliferation marker Ki67. Electrophysiological recordings in acute cortical slices showed that at P5 a proportion of layer 4 microglia transiently express voltage-dependant potassium currents of the delayed rectifier family, mostly mediated by Kv1.3 subunits, which are usually expressed by activated microglia under pathological conditions. This proportion of cells with rectifying properties doubles between P5 and P6, in concomitance with the beginning of microglia invasion of the barrel centers. Finally, analysis of the responses mediated by purinergic receptors indicated that a higher percentage of rectifying microglia expressed functional P2Y6 and P2Y12 receptors, as compared with nonrectifying cells, whereas all cells expressed functional P2X7 receptors. Our results indicate that during normal cortical development distinct microglia properties mature differentially, some of them being exquisitely influenced by the local environment of the maturating neuronal network.

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.

Microglia: key elements in neural development, plasticity, and pathology

A century after Cajal identified a "third element" of the nervous system, many issues have been clarified about the identity and function of one of its major components, the microglia. Here, we review recent findings by microgliologists, highlighting results from imaging studies that are helping provide new views of microglial behavior and function. In vivo imaging in the intact adult rodent CNS has revolutionized our understanding of microglial behaviors in situ and has raised speculation about their function in the uninjured adult brain. Imaging studies in ex vivo mammalian tissue preparations and in intact model organisms including zebrafish are providing insights into microglial behaviors during brain development. These data suggest that microglia play important developmental roles in synapse remodeling, developmental apoptosis, phagocytic clearance, and angiogenesis. Because microglia also contribute to pathology, including neurodevelopmental and neurobehavioral disorders, ischemic injury, and neuropathic pain, promising new results raise the possibility of leveraging microglia for therapeutic roles. Finally, exciting recent work is addressing unanswered questions regarding the nature of microglial-neuronal communication. While it is now apparent that microglia play diverse roles in neural development, behavior, and pathology, future research using neuroimaging techniques will be essential to more fully exploit these intriguing cellular targets for effective therapeutic intervention applied to a variety of conditions.

Pathophysiologic cascades in ischemic stroke

Many advances have been achieved in terms of understanding the molecular and cellular mechanisms of ischemic stroke. But thus far, clinically effective neuroprotectants remain elusive. In this minireview, we summarize the basics of ischemic cascades after stroke, covering neuronal death mechanisms, white matter pathophysiology, and inflammation with an emphasis on microglia. Translating promising mechanistic knowledge into clinically meaningful stroke drugs is very challenging. An integrative approach that encompasses the multimodal and multicell signaling phenomenon of stroke will be required to move forward.

The immune theory of psychiatric diseases: a key role for activated microglia and circulating monocytes

This review describes a key role for mononuclear phagocytes in the pathogenesis of major psychiatric disorders. There is accumulating evidence for activation of microglia (histopathology and PET scans) and circulating monocytes (enhanced gene expression of immune genes, an overproduction of monocyte/macrophage-related cytokines) in patients with bipolar disorder, major depressive disorder, and schizophrenia. These data are strengthened by observations in animal models, such as the MIA models, the chronic stress models, and the NOD mouse model. In these animal models of depressive-, anxiety-, and schizophrenia-like behavior, similar activations of microglia and circulating monocytes can be found. These animal models also make in-depth pathogenic studies possible and show that microglia activation impacts neuronal development and function in brain areas congruent with the altered depressive and schizophrenia-like behaviors.

Microglia and neuronal cell death

Microglia, the brain's innate immune cell type, are cells of mesodermal origin that populate the central nervous system (CNS) during development. Undifferentiated microglia, also called ameboid microglia, have the ability to proliferate, phagocytose apoptotic cells and migrate long distances toward their final destinations throughout all CNS regions, where they acquire a mature ramified morphological phenotype. Recent studies indicate that ameboid microglial cells not only have a scavenger role during development but can also promote the death of some neuronal populations. In the mature CNS, adult microglia have highly motile processes to scan their territorial domains, and they display a panoply of effects on neurons that range from sustaining their survival and differentiation contributing to their elimination. Hence, the fine tuning of these effects results in protection of the nervous tissue, whereas perturbations in the microglial response, such as the exacerbation of microglial activation or lack of microglial response, generate adverse situations for the organization and function of the CNS. This review discusses some aspects of the relationship between microglial cells and neuronal death/survival both during normal development and during the response to injury in adulthood.

Physiological roles of microglia during development

In all the species examined thus far, the behavior of microglia during development appears to be highly stereotyped. This reproducibility supports the notion that these cells have a physiological role in development. Microglia are macrophages that migrate from the yolk sac and colonize the central nervous system early during development. The first invading yolk-sac macrophages are highly proliferative and their role has not yet been addressed. At later developmental stages, microglia can be found throughout the brain and tend to preferentially reside at specific locations that are often associated with known developmental processes. Thus, it appears that microglia concentrate in areas of cell death, in proximity of developing blood vessels, in the marginal layer, which contains developing axon fascicles, and in close association with radial glial cells. This review describes the main features of brain colonization by microglia and discusses the possible physiological roles of these cells during development.

Pattern of invasion of the embryonic mouse spinal cord by microglial cells at the time of the onset of functional neuronal networks

Microglial cells invade the central nervous system during embryonic development, but their developmental functional roles in vivo remain largely unknown. Accordingly, their invasion pattern during early embryonic development is still poorly understood. To address this issue, we analyzed the initial developmental pattern of microglial cell invasion in the spinal cord of CX3CR1-eGFP mouse embryos using immunohistochemistry. Microglial cells began to invade the mouse embryonic spinal cord at a developmental period corresponding to the onset of spontaneous electrical activity and of synaptogenesis. Microglial cells reached the spinal cord through the peripheral vasculature and began to invade the parenchyma at 11.5 days of embryonic age (E11.5). Remarkably, at E12.5, activated microglial cells aggregated in the dorsolateral region close to terminals of dying dorsal root ganglia neurons. At E13.5, microglial cells in the ventral marginal zone interacted with radial glial cells, whereas ramified microglial cells within the parenchyma interacted with growing capillaries. At this age, activated microglial cells (Mac-2 staining) also accumulated within the lateral motor columns at the onset of the developmental cell death of motoneurons. This cell aggregation was still observed at E14.5, but microglial cells no longer expressed Mac-2. At E15.5, microglial cells were randomly distributed within the parenchyma. Our results provide the essential basis for further studies on the role of microglial cells in the early development of spinal cord neuronal networks in vivo.

Microglia: biology and pathology

The past 20 years have seen a gain in knowledge on microglia biology and microglia functions in disease that exceeds the expectations formulated when the microglia "immune network" was introduced. More than 10,000 articles have been published during this time. Important new research avenues of clinical importance have opened up such as the role of microglia in pain and in brain tumors. New controversies have also emerged such as the question of whether microglia are active or reactive players in neurodegenerative disease conditions, or whether they may be victims themselves. Premature commercial interests may be responsible for some of the confusion that currently surrounds microglia in both the Alzheimer and Parkinson's disease research fields. A critical review of the literature shows that the concept of "(micro)glial inflammation" is still open to interpretation, despite a prevailing slant towards a negative meaning. Perhaps the most exciting foreseeable development concerns research on the role of microglia in synaptic plasticity, which is expected to yield an answer to the question whether microglia are the brain's electricians. This review provides an analysis of the latest developments in the microglia field.