Immunohistochemical localization of macrophages and microglia in the adult and developing mouse brain

c-Jun and c-Fos regulate the complement factor H promoter in murine astrocytes

The complement system is a critical component of innate immunity that requires regulation to avoid inappropriate activation. This regulation is provided by many proteins, including complement factor H (CFH), a critical regulator of the alternative pathway of complement activation. Given its regulatory function, mutations in CFH have been implicated in diseases such as age-related macular degeneration and membranoproliferative glomerulonephritis, and central nervous system diseases such as Alzheimer's disease, Parkinson's disease, and a demyelinating murine model, experimental autoimmune encephalomyelitis (EAE). There have been few investigations on the transcriptional regulation of CFH in the brain and CNS. Our studies show that CFH mRNA is present in several CNS cell types. The murine CFH (mCFH) promoter was cloned and examined through truncation constructs and we show that specific regions throughout the promoter contain enhancers and repressors that are positively regulated by inflammatory cytokines in astrocytes. Database mining of these regions indicated transcription factor binding sites conserved between different species, which led to the investigation of specific transcription factor binding interactions in a 241 base pair (bp) region at -416 bp to -175 bp that showed the strongest activity. Through supershift analysis, it was determined that c-Jun and c-Fos interact with the CFH promoter in astrocytes in this region. These results suggest a relationship between cell cycle and complement regulation, and how these transcription factors and CFH affect disease will be a valuable area of investigation.

Exploring the neuroimmunopharmacology of opioids: an integrative review of mechanisms of central immune signaling and their implications for opioid analgesia

Vastly stimulated by the discovery of opioid receptors in the early 1970s, preclinical and clinical research was directed at the study of stereoselective neuronal actions of opioids, especially those played in their crucial analgesic role. However, during the past decade, a new appreciation of the non-neuronal actions of opioids has emerged from preclinical research, with specific appreciation for the nonclassic and nonstereoselective sites of action. Opioid activity at Toll-like receptors, newly recognized innate immune pattern recognition receptors, adds substantially to this unfolding story. It is now apparent from molecular and rodent data that these newly identified signaling events significantly modify the pharmacodynamics of opioids by eliciting proinflammatory reactivity from glia, the immunocompetent cells of the central nervous system. These central immune signaling events, including the release of cytokines and chemokines and the associated disruption of glutamate homeostasis, cause elevated neuronal excitability, which subsequently decreases opioid analgesic efficacy and leads to heightened pain states. This review will examine the current preclinical literature of opioid-induced central immune signaling mediated by classic and nonclassic opioid receptors. A unification of the preclinical pharmacology, neuroscience, and immunology of opioids now provides new insights into common mechanisms of chronic pain, naive tolerance, analgesic tolerance, opioid-induced hyperalgesia, and allodynia. Novel pharmacological targets for future drug development are discussed in the hope that disease-modifying chronic pain treatments arising from the appreciation of opioid-induced central immune signaling may become practical.

A transcriptomic atlas of mouse neocortical layers

In the mammalian cortex, neurons and glia form a patterned structure across six layers whose complex cytoarchitectonic arrangement is likely to contribute to cognition. We sequenced transcriptomes from layers 1-6b of different areas (primary and secondary) of the adult (postnatal day 56) mouse somatosensory cortex to understand the transcriptional levels and functional repertoires of coding and noncoding loci for cells constituting these layers. A total of 5,835 protein-coding genes and 66 noncoding RNA loci are differentially expressed ("patterned") across the layers, on the basis of a machine-learning model (naive Bayes) approach. Layers 2-6b are each associated with specific functional and disease annotations that provide insights into their biological roles. This new resource (http://genserv.anat.ox.ac.uk/layers) greatly extends currently available resources, such as the Allen Mouse Brain Atlas and microarray data sets, by providing quantitative expression levels, by being genome-wide, by including novel loci, and by identifying candidate alternatively spliced transcripts that are differentially expressed across layers.

Pleiotrophin over-expression provides trophic support to dopaminergic neurons in parkinsonian rats

Background

Pleiotrophin is known to promote the survival and differentiation of dopaminergic neurons in vitro and is up-regulated in the substantia nigra of Parkinson's disease patients. To establish whether pleiotrophin has a trophic effect on nigrostriatal dopaminergic neurons in vivo, we injected a recombinant adenovirus expressing pleiotrophin in the substantia nigra of 6-hydroxydopamine lesioned rats.

Results

The viral vector induced pleiotrophin over-expression by astrocytes in the substantia nigra pars compacta, without modifying endogenous neuronal expression. The percentage of tyrosine hydroxylase-immunoreactive cells as well as the area of their projections in the lesioned striatum was higher in pleiotrophin-treated animals than in controls.

Conclusions

These results indicate that pleiotrophin over-expression partially rescues tyrosine hydroxylase-immunoreactive cell bodies and terminals of dopaminergic neurons undergoing 6-hydroxydopamine-induced degeneration.

Emerging concepts in myeloid cell biology after spinal cord injury

Traumatic spinal cord injury (SCI) affects the activation, migration, and function of microglia, neutrophils and monocyte/macrophages. Because these myeloid cells can positively and negatively affect survival of neurons and glia, they are among the most commonly studied immune cells. However, the mechanisms that regulate myeloid cell activation and recruitment after SCI have not been adequately defined. In general, the dynamics and composition of myeloid cell recruitment to the injured spinal cord are consistent between mammalian species; only the onset, duration, and magnitude of the response vary. Emerging data, mostly from rat and mouse SCI models, indicate that resident and recruited myeloid cells are derived from multiple sources, including the yolk sac during development and the bone marrow and spleen in adulthood. After SCI, a complex array of chemokines and cytokines regulate myelopoiesis and intraspinal trafficking of myeloid cells. As these cells accumulate in the injured spinal cord, the collective actions of diverse cues in the lesion environment help to create an inflammatory response marked by tremendous phenotypic and functional heterogeneity. Indeed, it is difficult to attribute specific reparative or injurious functions to one or more myeloid cells because of convergence of cell function and difficulties in using specific molecular markers to distinguish between subsets of myeloid cell populations. Here we review each of these concepts and include a discussion of future challenges that will need to be overcome to develop newer and improved immune modulatory therapies for the injured brain or spinal cord.

EAAT expression by macrophages and microglia: still more questions than answers

Glutamate is the main excitatory amino acid, but its presence in the extracellular milieu has deleterious consequences. It may induce excitotoxicity and also compete with cystine for the use of the cystine-glutamate exchanger, blocking glutathione neosynthesis and inducing an oxidative stress-induced cell death. Both mechanisms are critical in the brain where up to 20% of total body oxygen consumption occurs. In normal conditions, the astrocytes ensure that extracellular concentration of glutamate is kept in the micromolar range, thanks to their coexpression of high-affinity glutamate transporters (EAATs) and glutamine synthetase (GS). Their protective function is nevertheless sensitive to situations such as oxidative stress or inflammatory processes. On the other hand, macrophages and microglia do not express EAATs and GS in physiological conditions and are the principal effector cells of brain inflammation. Since the late 1990s, a number of studies have now shown that both microglia and macrophages display inducible EAAT and GS expression, but the precise significance of this still remains poorly understood. Brain macrophages and microglia are sister cells but yet display differences. Both are highly sensitive to their microenvironment and can perform a variety of functions that may oppose each other. However, in the very particular environment of the healthy brain, they are maintained in a repressed state. The aim of this review is to present the current state of knowledge on brain macrophages and microglial cells activation, in order to help clarify their role in the regulation of glutamate under pathological conditions as well as its outcome.

Early microglial colonization of the human forebrain and possible involvement in periventricular white-matter injury of preterm infants

Amoeboid microglial subpopulations visualized by antibodies against ionized calcium-binding adapter molecule 1, CD68, and CD45 enter the forebrain starting at 4.5 postovulatory or gestational weeks (gw). They penetrate the telencephalon and diencephalon via the meninges, choroid plexus, and ventricular zone. Early colonization by amoeboid microglia-macrophages is first restricted to the white matter, where these cells migrate and accumulate in patches at the junctions of white-matter pathways, such as the three junctions that the internal capsule makes with the thalamocortical projection, external capsule and cerebral peduncle, respectively. In the cerebral cortex anlage, migration is mainly radial and tangential towards the immature white matter, subplate layer, and cortical plate, whereas pial cells populate the prospective layer I. A second wave of microglial cells penetrates the brain via the vascular route at about 12-13 gw and remains confined to the white matter. Two main findings deserve emphasis. First, microglia accumulate at 10-12 gw at the cortical plate-subplate junction, where the first synapses are detected. Second, microglia accumulate in restricted laminar bands, most notably around 19-30 gw, at the axonal crossroads in the white matter (semiovale centre) rostrally, extending caudally in the immature white matter to the visual radiations. This accumulation of proliferating microglia is located at the site of white-matter injury in premature neonates. The spatiotemporal organization of microglia in the immature white and grey matter suggests that these cells may play active roles in developmental processes such as axonal guidance, synaptogenesis, and neurodevelopmental apoptosis as well as in injuries to the developing brain, in particular in the periventricular white-matter injury of preterm infants.

A two-way communication between microglial cells and angiogenic sprouts regulates angiogenesis in aortic ring cultures

BACKGROUND

Myeloid cells have been associated with physiological and pathological angiogenesis, but their exact functions in these processes remain poorly defined. Monocyte-derived tissue macrophages of the CNS, or microglial cells, invade the mammalian retina before it becomes vascularized. Recent studies correlate the presence of microglia in the developing CNS with vascular network formation, but it is not clear whether the effect is directly caused by microglia and their contact with the endothelium.

METHODOLOGY/PRINCIPAL FINDINGS

We combined in vivo studies of the developing mouse retina with in vitro studies using the aortic ring model to address the role of microglia in developmental angiogenesis. Our in vivo analyses are consistent with previous findings that microglia are present at sites of endothelial tip-cell anastomosis, and genetic ablation of microglia caused a sparser vascular network associated with reduced number of filopodia-bearing sprouts. Addition of microglia in the aortic ring model was sufficient to stimulate vessel sprouting. The effect was independent of physical contact between microglia and endothelial cells, and could be partly mimicked using microglial cell-conditioned medium. Addition of VEGF-A promoted angiogenic sprouts of different morphology in comparison with the microglial cells, and inhibition of VEGF-A did not affect the microglia-induced angiogenic response, arguing that the proangiogenic factor(s) released by microglia is distinct from VEGF-A. Finally, microglia exhibited oriented migration towards the vessels in the aortic ring cultures.

CONCLUSIONS/SIGNIFICANCE

Microglia stimulate vessel sprouting in the aortic ring cultures via a soluble microglial-derived product(s), rather than direct contact with endothelial cells. The observed migration of microglia towards the growing sprouts suggests that their position near endothelial tip-cells could result from attractive cues secreted by the vessels. Our data reveals a two-way communication between microglia and vessels that depends on soluble factors and should extend the understanding of how microglia promote vascular network formation.

Nitric oxide-mediated tumoricidal activity of murine microglial cells

Experimental metastases in the brain of mice are infiltrated by microglia, and parabiosis experiments of green fluorescent protein (GFP(+)) and GFP(-) mice revealed that these microglia are derived from circulating monocytes (GFP(+), F4/80(+), and CD68(+)). These findings raised the question as to whether microglia (specialized macrophages) possess tumoricidal activity. C8-B4 murine microglia cells were incubated in vitro in medium (control) or in medium containing both lipopolysaccharide and interferon-γ. Control microglia were not tumoricidal against a number of murine and human tumor cells, whereas lipopolysaccharide/interferon-γ-activated microglia lysed murine and human tumor cells by release of nitric oxide. Parallel experiments with murine peritoneal macrophages produced identical results. Neither activated microglia nor activated macrophages lysed nontumorigenic murine or human cells. Collectively, these data demonstrate that brain metastasis-associated microglia are derived from circulating mononuclear cells and exhibit selective and specific tumoricidal activity.

The role of microglia in synaptic stripping and synaptic degeneration: a revised perspective

Chronic neurodegenerative diseases of the CNS (central nervous system) are characterized by the loss of neurons. There is, however, growing evidence to show that an early stage of this process involves degeneration of presynaptic terminals prior to the loss of the cell body. Synaptic plasticity in CNS pathology has been associated with microglia and the phenomenon of synaptic stripping. We review here the evidence for the involvement of microglia in synaptic stripping and synapse degeneration and we conclude that this is a case of guilt by association. In disease models of chronic neurodegeneration, there is no evidence that microglia play an active role in either synaptic stripping or synapse degeneration, but the degeneration of the synapse and the envelopment of a degenerating terminal appears to be a neuron autonomous event. We highlight here some of the gaps in our understanding of synapse degeneration in chronic neurodegenerative disease.

Glial transcripts and immune-challenged glia in the suprachiasmatic nucleus of young and aged mice

Biological rhythms are frequently disturbed with advancing age, and aging-related changes of glia in the hypothalamic suprachiasmatic nucleus (SCN), the master circadian pacemaker, require special attention. In particular, astrocytes contribute to SCN function, and aging is associated with increased inflammatory activity in the brain, in which microglia could be especially implicated. On this basis, we investigated in the SCN of young and old mice glial transcripts and cell features, and the glial cell response to a central inflammatory challenge. Quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR) was used to analyze the expression of mRNAs encoding the astrocytic glial fibrillary acidic protein and the microglial antigen CD11b. Both these transcripts, here investigated in the SCN for the first time, were significantly increased in the old SCN. Glial cell phenotyping with immunohistochemistry revealed hypertrophic and intensely stained astrocytes and microglia in the aged SCN. In both age groups, microglia were scattered throughout the SCN and astrocytes were prominent in the ventral portion, where retinal fibers are densest; in the aged SCN, astrocytes were also numerous in the dorsal portion. After intracerebroventricular injections of a mixture of interferon-gamma and tumor necrosis factor-alpha, or phosphate-buffered saline as control, immunolabeling was evaluated with stereological cell counts and confocal microscopy. Phenotypic features of astrocyte and microglia activation in response to cytokine injections were markedly enhanced in the aged SCN. Subregional variations in glial cell density were also documented in the aged compared to the young SCN. Altogether, the findings show increases in the expression of glial transcripts and hypertrophy of astrocytes and microglia in the aged SCN, as well as age-dependent variation in the responses of immune-challenged SCN glia. The data thus point out an involvement of glia in aging-related changes of the biological clock.

17β-estradiol attenuates injury-induced microglia activation in the oculomotor nucleus

Recent studies provide increasing data indicating the prominent role of estrogens in protecting the nervous system against the noxious consequences of nerve injury. It is also clear that in the process of nerve injury and recovery not only the neurons, but the glial cells are also involved and they are important components of the protective mechanisms. In the present article the effect of 17β-estradiol on injury-induced microglia activation was studied in an animal model. Peripheral axotomy of the oculomotor neurons was achieved by the removal of the right eyeball including the extraocular muscles of ovariectomized adult mice. The time course and the extent of microglia activation was followed by the unbiased morphometric analysis of CD11b immunoreactive structures within the oculomotor nucleus. The first sign of microglia activation appeared after 24 h following injury, the maximal effect was found on the fourth day. In ovariectomized females hormone treatment (daily injection of 17β-estradiol, 5 μg/100 g b.w.) decreased significantly the microglia reaction at postoperative day 4. Our results show that microglia response to nerve injury is affected by estradiol, that is these cells may mediate some of the hormonal effects and may contribute to protective mechanisms resulting in the structural and functional recovery of the nervous system.

Motor neuron-immune interactions: the vicious circle of ALS

Because microglial cells, the resident macrophages of the CNS, react to any lesion of the nervous system, they have for long been regarded as potential players in the pathogenesis of several neurodegenerative disorders including amyotrophic lateral sclerosis, the most common motor neuron disease in the adult. In recent years, this microglial reaction to motor neuron injury, in particular, and the innate immune response, in general, has been implicated in the progression of the disease, in mouse models of ALS. The mechanisms by which microglial cells influence motor neuron death in ALS are still largely unknown. Microglial activation increases over the course of the disease and is associated with an alteration in the production of toxic factors and also neurotrophic factors. Adding to the microglial/macrophage response to motor neuron degeneration, the adaptive immune system can likewise influence the disease process. Exploring these motor neuron-immune interactions could lead to a better understanding in the physiopathology of ALS to find new pathways to slow down motor neuron degeneration.

Microglial response to light-induced photoreceptor degeneration in the mouse retina

The microglial response elicited by degeneration of retinal photoreceptor cells was characterized in BALB/c mice exposed to bright light for 7 hours and then kept in complete darkness for survival times ranging from 0 hours to 10 days. Photodegeneration resulted in extensive cell death in the retina, mainly in the outer nuclear layer (ONL), where the photoreceptor nuclei are located. Specific immunolabeling of microglial cells with anti-CD11b, anti-CD45, anti-F4/80, anti-SRA, and anti-CD68 antibodies revealed that microglial cells were activated in light-exposed retinas. They migrated to the ONL, changed their morphology, becoming rounded cells with short and thick processes, and, finally, showed immunophenotypic changes. Specifically, retinal microglia began to strongly express antigens recognized by anti-CD11b, anti-CD45, and anti-F4/80, coincident with cell degeneration. In contrast, upregulation of the antigen recognized by anti-SRA was not detected by immunocytochemistry until 6 hours after light exposure. Differences were also observed at 10 days after light exposure: CD11b, CD45, and F4/80 continued to be strongly expressed in retinal microglia, whereas the expression of CD68 and SRA had decreased to near-normal values. Therefore, microglia did not return to their original state after photodegeneration and continued to show a degree of activation. The accumulation of activated microglial cells in affected regions simultaneously with photoreceptor degeneration suggests that they play some role in photodegeneration.

Activation and deactivation of periventricular white matter phagocytes during postnatal mouse development

Brain microglia are related to peripheral macrophages but undergo a highly specific process of regional maturation and differentiation inside the brain. Here, we examined this deactivation and morphological differentiation in cerebral cortex and periventricular subcortical white matter, the main "fountain of microglia" site, during postnatal mouse development, 0-28 days after birth (P0-P28). Only macrophages in subcortical white matter but not cortical microglia exhibited strong expression of typical activation markers alpha5, alpha6, alphaM, alphaX, and beta2 integrin subunits and B7.2 at any postnatal time point studied. White matter phagocyte activation was maximal at P0, decreased linearly over P3 and P7 and disappeared at P10. P7 white matter phagocytes also expressed high levels of IGF1 and MCSF, but not TNFalpha mRNA; this expression disappeared at P14. This process of deactivation followed the presence of ingested phagocytic material but correlated only moderately with ramification, and not with the extent of TUNEL+ death in neighboring cells, their ingestion or microglial proliferation. Intravenous fluosphere labeling revealed postnatal recruitment and transformation of circulating leukocytes into meningeal and perivascular macrophages as well as into ramified cortical microglia, but bypassing the white matter areas. In conclusion, this study describes strong and selective activation of postnatally resident phagocytes in the P0-P7 subcortical white matter, roughly equivalent to mid 3rd trimester human fetal development. This presence of highly active and IGF1- and MCSF-expressing phagocytes in the neighborhood of vulnerable white matter could play an important role in the genesis of or protection against axonal damage in the fetus and premature neonate.

Toll-like receptor 2 and facial motoneuron survival after facial nerve axotomy

We have previously demonstrated that CD4(+) Th2 lymphocytes are required to rescue facial motoneuron (FMN) survival after facial nerve axotomy through interaction with peripheral antigen presenting cells, as well as CNS resident microglia. Furthermore, the innate immune molecule, toll-like receptor 2 (TLR2), has been implicated in the development of Th2-type immune responses and can be activated by intracellular components released by dead or dying cells. The role of TLR2 in the FMN response to axotomy was explored in this study, using a model of facial nerve axotomy at the stylomastoid foramen in the mouse, in which blood-brain-barrier (BBB) permeability does not occur. After facial nerve axotomy, TLR2 mRNA was significantly upregulated in the facial motor nucleus and co-immunofluorescence localized TLR2 to CD68(+) microglia, but not GFAP(+) astrocytes. Using TLR2-deficient (TLR2(-/-)) mice, it was determined that TLR2 does not affect FMN survival levels after axotomy. These data contribute to understanding the role of innate immunity after FMN death and may be relevant to motoneuron diseases, such as amyotrophic lateral sclerosis (ALS).

The more you have, the less you get: the functional role of inflammation on neuronal differentiation of endogenous and transplanted neural stem cells in the adult brain

The differentiation of neural stem cells toward a neuronal phenotype is determined by the extracellular and intracellular factors that form the neurogenic niche. In this review, we discuss the available data on the functional role of inflammation and in particular, pro- and anti-inflammatory cytokines, on neuronal differentiation from endogenous and transplanted neural stem/progenitor cells. In addition, we discuss the role of microglial cell activation on these processes and the fact that microglial cell activation is not univocally associated with a pro-inflammatory milieu. We conclude that brain cytokines could be regarded as part of the endogenous neurogenic niche. In addition, we propose that accumulating evidence suggests that pro-inflammatory cytokines have a negative effect on neuronal differentiation, while anti-inflammatory cytokines exert an opposite effect. The clarification of the functional role of cytokines on neuronal differentiation will be relevant not only to better understand adult neurogenesis, but also to envisage complementary treatments to modulate cytokine action that could increase the therapeutic benefit of future progenitor/stem cell-based therapies.

Glial precursors clear sensory neuron corpses during development via Jedi-1, an engulfment receptor

During the development of peripheral ganglia, 50% of the neurons that are generated undergo apoptosis. How the massive numbers of corpses are removed is unknown. We found that satellite glial cell precursors are the primary phagocytic cells for apoptotic corpse removal in developing mouse dorsal root ganglia (DRG). Confocal and electron microscopic analysis revealed that glial precursors, rather than macrophages, were responsible for clearing most of the dead DRG neurons. Moreover, we identified Jedi-1, an engulfment receptor, and MEGF10, a purported engulfment receptor, as homologs of the invertebrate engulfment receptors Draper and CED-1 expressed in the glial precursor cells. Expression of Jedi-1 or MEGF10 in fibroblasts facilitated binding to dead neurons, and knocking down either protein in glial cells or overexpressing truncated forms lacking the intracellular domain inhibited engulfment of apoptotic neurons. Together, these results suggest a cellular and molecular mechanism by which neuronal corpses are culled during DRG development.

Involvement of Fc receptors in disorders of the central nervous system

Immunoglobulins are proteins with a highly variable antigen-binding domain and a constant region (Fc domain) that binds to a cell surface receptor (FcR). Activation of FcRs in immune cells (lymphocytes, macrophages, and mast cells) triggers effector responses including cytokine production, phagocytosis, and degranulation. In addition to their roles in normal responses to infection or tissue injury, and in immune-related diseases, FcRs are increasingly recognized for their involvement in neurological disorders. One or more FcRs are expressed in microglia, astrocytes, oligodendrocytes, and neurons. Aberrant activation of FcRs in such neural cells may contribute to the pathogenesis of major neurodegenerative conditions including Alzheimer's disease, Parkinson's disease, ischemic stroke, and multiple sclerosis. On the other hand, FcRs may play beneficial roles in counteracting pathological processes; for e.g., FcRs may facilitate removal of amyloid peptides from the brain and so protect against Alzheimer's disease. Knowledge of the functions of FcRs in the nervous system in health and disease is leading to novel preventative and therapeutic strategies for stroke, Alzheimer's disease, and other neurological disorders.

Enhanced microglial clearance of myelin debris in T cell-infiltrated central nervous system

Acute multiple sclerosis lesions are characterized by accumulation of T cells and macrophages, destruction of myelin and oligodendrocytes, and axonal damage. There is, however, limited information on neuroimmune interactions distal to sites of axonal damage in the T cell-infiltrated central nervous system. We investigated T-cell infiltration, myelin clearance, microglial activation, and phagocytic activity distal to sites of axonal transection through analysis of the perforant pathway deafferented dentate gyrus in SJL mice that had received T cells specific for myelin basic protein (TMBP) or ovalbumin (TOVA). The axonal lesion of TMBP-recipient mice resulted in lesion-specific recruitment of large numbers of T cells in contrast to very limited T-cell infiltration in TOVA-recipient and -naïve perforant pathway-deafferented mice. By double immunofluorescence and confocal microscopy, infiltration with TMBP but not TOVA enhanced the microglial response to axonal transection and microglial phagocytosis of myelin debris associated with the degenerating axons. Because myelin antigen-specific immune responses may provoke protective immunity, increased phagocytosis of myelin debris might enhance regeneration after a neural antigen-specific T cell-mediated immune response in multiple sclerosis.

Innate immunity in the pathogenesis of polytropic retrovirus infection in the central nervous system

Neuroinflammation, including astrogliosis, microgliosis, and the production of proinflammatory cytokines and chemokines is a common response in the central nervous system (CNS) to virus infection, including retrovirus infection. However, the contribution of this innate immune response in disease pathogenesis remains unresolved. Analysis of the neuroinflammatory response to polytropic retrovirus infection in the mouse has provided insight into the potential contribution of the innate immune response to retrovirus-induced neurologic disease. In this model, retroviral pathogenesis correlates with the induction of neuroinflammatory responses including the activation of astrocytes and microglia, as well as the production of proinflammatory cytokines and chemokines. Studies of the neurovirulent determinants of the polytropic envelope protein as well as studies with knockout mice suggest that retroviral pathogenesis in the brain is multifaceted and that cytokine and chemokine production may be only one mechanism of disease pathogenesis. Analysis of the activation of the innate immune response to retrovirus infection in the CNS indicates that toll-like receptor 7 (TLR7) is a contributing factor to retrovirus-induced neuroinflammation, but that other factors can compensate for the lack of TLR7 in inducing both neuroinflammation and neurologic disease.

Microglial physiology: unique stimuli, specialized responses

Microglia, the macrophages of the central nervous system parenchyma, have in the normal healthy brain a distinct phenotype induced by molecules expressed on or secreted by adjacent neurons and astrocytes, and this phenotype is maintained in part by virtue of the blood-brain barrier's exclusion of serum components. Microglia are continually active, their processes palpating and surveying their local microenvironment. The microglia rapidly change their phenotype in response to any disturbance of nervous system homeostasis and are commonly referred to as activated on the basis of the changes in their morphology or expression of cell surface antigens. A wealth of data now demonstrate that the microglia have very diverse effector functions, in line with macrophage populations in other organs. The term activated microglia needs to be qualified to reflect the distinct and very different states of activation-associated effector functions in different disease states. Manipulating the effector functions of microglia has the potential to modify the outcome of diverse neurological diseases.

Influenza virus- and cytokine-immunoreactive cells in the murine olfactory and central autonomic nervous systems before and after illness onset

Influenza virus invades the olfactory bulb (OB) and enhances cytokine mRNAs therein at the time of illness onset. Here we show that viral antigen immunoreactivity co-localized with glial markers in the OB but could not be detected in other brain areas. Interleukin 1beta- and tumor necrosis factor alpha-immunoreactivity co-localized with neuronal markers in olfactory and central autonomic systems, and the number of cytokine-immunoreactive neurons increased at the time of illness onset [15 h post-inoculation (PI)] but not before (10 h PI). These results suggest that the OB virus influences the brain cytokines and therefore the onset of illness.

Trophic macrophages in development and disease

Specialized phagocytes are found in the most primitive multicellular organisms. Their roles in homeostasis and in distinguishing self from non-self have evolved with the complexity of organisms and their immune systems. Equally important, but often overlooked, are the roles of macrophages in tissue development. As discussed in this Review, these include functions in branching morphogenesis, neuronal patterning, angiogenesis, bone morphogenesis and the generation of adipose tissue. In each case, macrophage depletion impairs the formation of the tissue and compromises its function. I argue that in several diseases, the unrestrained acquisition of these developmental macrophage functions exacerbates pathology. For example, macrophages enhance tumour progression and metastasis by affecting tumour-cell migration and invasion, as well as angiogenesis.

Vitamin E increases S100B-mediated microglial activation in an S100B-overexpressing mouse model of pathological aging

S100B is a calcium-binding protein released by astroglial cells of the brain capable of producing numerous extracellular effects. Although the direct molecular mechanism remains unknown, these effects can be trophic including differentiation, growth, recovery, and survival of neurons when the S100B protein is mainly oxidized and neurotoxic including apoptosis and neuroinflammatory processes marked by microglial activation when in a reduced state. S100B and its receptor RAGE (receptor for advanced glycation end products) have been found to be increased in Alzheimer's disease, Down syndrome, with tissue trauma and ischemia. In the current study, we examined the binding of the S100B receptor (RAGE) on microglial cells and the developmental effects of the antioxidant vitamin E on microglial activation and the upregulation of RAGE in an S100B over-expressing mouse model of pathological aging. We report that RAGE is co-localized on activated microglial cells and vitamin E induced dramatic increases in microglial activation as well as total microglial relative optical density that was accompanied by upregulation of the RAGE receptor, particularly in the CA1 region of the hippocampus. Our findings suggest further investigation into the potential role of vitamin E in reducing the oxidation state of the S100B protein and its influence on neuroinflammatory processes marked by microglial activation in vivo.

NPY and its involvement in axon guidance, neurogenesis, and feeding

OBJECTIVES

The role of neuropeptides in nervous system function is still in many cases undefined. In the present study we examined a possible role of the 36-amino acid neuropeptide Y (NPY) with regard to three functions: axon guidance and attraction/repulsion, adult neurogenesis, and control of food intake.

METHODS

Growth cones from embryonic dorsal root ganglion neurons were studied in culture during asymmetrical gradient application of NPY. Growth cones were monitored over a 60-min period, and final turning angle and growth rate were recorded. In the second part the NPY Y(1) and Y(2) receptors were studied in the subventricular zone, the rostral migratory stream, and the olfactory bulb in normal mice and mice with genetically deleted NPY Y(1) or Y(2) receptors. In the third part an anorectic mouse was analyzed with immunohistochemistry.

RESULTS

1) NPY elicited an attractive turning response and an increase in growth rate, effects exerted via the NPY Y(1) receptor. 2) The NPY Y(1) receptor was expressed in neuroblasts in the anterior rostral migratory stream. Mice deficient in the Y(1) or Y(2) receptor had fewer proliferating precursor cells and neuroblasts in the subventricular zone and rostral migratory stream and fewer neurons in the olfactory bulb expressing calbindin, calretinin or tyrosine hydroxylase. 3) In the anorectic mouse markers for microglia were strongly upregulated in the arcuate nucleus and in projection areas of the NPY/agouti gene-related protein arcuate system.

CONCLUSION

NPY participates in several mechanisms involved in the development of the nervous system and is of importance in the control of food intake.

Flow cytometric characterization of neural precursor cells and their progeny

It is now clear that the adult central nervous system contains a population of neural stem and progenitor cells which act as a reservoir to underpin cell genesis for the lifetime of the animal. Unfortunately, understanding how these cells are activated both under normal conditions and following injury or disease has been a difficult task, owing not only to the rarity of these populations, but also to a paucity of cell type-specific markers. In this chapter, we will discuss in detail the methods involved in generating single cell suspension from the periventricular region of the adult mouse brain appropriate for cell sorting, and how to use negative selection strategies to produce an essentially pure population of neurosphere-forming precursor cells. While these methods have been tailored for the sorting of neural precursor cells, these methods can be easily adapted to sort for any subpopulation of neural cells based on a variety of cell surface antigen expression.

GABAergic activities enhance macrophage inflammatory protein-1alpha release from microglia (brain macrophages) in postnatal mouse brain

Microglial cells (brain macrophages) invade the brain during embryonic and early postnatal development, migrate preferentially along fibre tracts to their final position and transform from an amoeboid to a ramified morphology. Signals by which the invading microglia communicate with other brain cells are largely unknown. Here, we studied amoeboid microglia in postnatal corpus callosum obtained from 6- to 8-day-old mice. These cells accumulated on the surface of acute brain slices. Whole-cell patch-clamp recordings revealed that the specific GABA(A) receptor agonist muscimol triggered a transient increase in conductance typical for inward rectifying potassium channels in microglia. This current increase was not mediated by microglial GABA(A) receptors since microglial cells removed from the slice surface no longer reacted and cultured microglia only responded when a brain slice was placed in their close vicinity. Muscimol triggered a transient increase in extracellular potassium concentration ([K(+)](o)) in brain slices and an experimental elevation of [K(+)](o) mimicked the muscimol response in microglial cells. Moreover, in adult brain slices, muscimol led only to a minute increase in [K(+)](o) and microglial cells failed to respond to muscimol. In turn, an increase in [K(+)](o) stimulated the release of chemokine macrophage inflammatory protein-1alpha (MIP1-alpha) from brain slices and from cultures of microglia but not astrocytes. Our observations indicate that invading microglia in early postnatal development sense GABAergic activities indirectly via sensing changes in [K(+)](o) which results in an increase in MIP1-alpha release.

The complement cascade: Yin-Yang in neuroinflammation--neuro-protection and -degeneration

The complement cascade has long been recognized to play a key role in inflammatory and degenerative diseases. It is a 'double edged' sword as it is necessary to maintain health, yet can have adverse effects when unregulated, often exacerbating disease. The contrasting effects of complement, depending on whether in a setting of health or disease, is the price paid to achieve flexibility in scope and degree of a protective response for the host from infection and injury. Loss or even decreased efficiency of critical regulatory control mechanisms can result in aggravated inflammation and destruction of self-tissue. The role of the complement cascade is poorly understood in the nervous system and neurological disorders. Novel studies have demonstrated that the expression of complement proteins in brain varies in different cell types and the effects of complement activation in various disease settings appear to differ. Understanding the functioning of this cascade is essential, as it has therapeutic implications. In this review, we will attempt to provide insight into how this complex cascade functions and to identify potential strategic targets for therapeutic intervention in chronic diseases as well as acute injury in the CNS.

C1q: the perfect complement for a synaptic feast?

The efficient and selective removal of apoptotic cells is an important feature of tissue development, homeostasis and pathology. In the nervous system, synapses and distal axons are selectively eliminated as part of the remodelling that underpins development and pathology, through a process that has some features in common with apoptotic cell removal. Components of the complement cascade are implicated in the efficient removal of apoptotic cells outside the nervous system, and recent evidence suggests that the complement components C1q and C3 have a role in the selective tagging of supernumerary synapses in the developing visual system and in their efficient removal by as yet unidentified cells.

Role of microglial IKKbeta in kainic acid-induced hippocampal neuronal cell death

Microglial cells are activated during excitotoxin-induced neurodegeneration. However, the in vivo role of microglia activation in neurodegeneration has not yet been fully elucidated. To this end, we used Ikkbeta conditional knockout mice (LysM-Cre/Ikkbeta(F/F)) in which the Ikkbeta gene is specifically deleted in cells of myeloid lineage, including microglia, in the CNS. This deletion reduced IkappaB kinase (IKK) activity in cultured primary microglia by up to 40% compared with wild-type (Ikkbeta(F/F)), and lipopolysaccharide-induced proinflammatory gene expression was also compromised. Kainic acid (KA)-induced hippocampal neuronal cell death was reduced by 30% in LysM-Cre/Ikkbeta(F/F) mice compared with wild-type mice. Reduced neuronal cell death was accompanied by decreased KA-induced glial cell activation and subsequent expression of proinflammatory genes such as tumour necrosis factor (TNF)-alpha and interleukin (IL)-1beta. Similarly, neurons in organotypic hippocampal slice cultures (OHSCs) from LysM-Cre/Ikkbeta(F/F) mouse brain were less susceptible to KA-induced excitotoxicity compared with wild-type OHSCs, due in part to decreased TNF-alpha and IL-1beta expression. Based on these data, we concluded that IKK/nuclear factor-kappaB dependent microglia activation contributes to KA-induced hippocampal neuronal cell death in vivo through induction of inflammatory mediators.

Transient impairment of hippocampus-dependent learning and memory in relatively low-dose of acute radiation syndrome is associated with inhibition of hippocampal neurogenesis

Neurogenesis in the adult hippocampus, which occurs constitutively, is vulnerable to ionizing radiation. In the relatively low-dose exposure of acute radiation syndrome (ARS), the change in the adult hippocampal function is poorly understood. This study analyzed the changes in apoptotic cell death and neurogenesis in the DGs of hippocampi from adult ICR mice with single whole-body gamma-irradiation using the TUNEL method and immunohistochemical markers of neurogenesis, Ki-67 and doublecortin (DCX). In addition, the hippocampus-dependent learning and memory tasks after single whole-body gamma-irradiation were examined in order to evaluate the hippocampus-related behavioral dysfunction in the relatively low-dose exposure of ARS. The number of TUNEL-positive apoptotic nuclei in the dentate gyrus (DG) was increased 6-12 h after acute gamma-irradiation (a single dose of 0.5 to 4 Gy). In contrast, the number of Ki-67- and DCX-positive cells began to decrease significantly 6 h postirradiation, reaching its lowest level 24 h after irradiation. The level of Ki-67 and DCX immunoreactivity decreased in a dose-dependent manner within the range of irradiation applied (0-4 Gy). In passive avoidance and object recognition memory test, the mice trained 1 day after acute irradiation (2 Gy) showed significant memory deficits, compared with the sham controls. In conclusion, the pattern of the hippocampus-dependent memory dysfunction is consistent with the change in neurogenesis after acute irradiation. It is suggested that a relatively low dose of ARS in adult ICR mice is sufficiently detrimental to interrupt the functioning of the hippocampus, including learning and memory, possibly through the inhibition of neurogenesis.

CD11c/EYFP transgene illuminates a discrete network of dendritic cells within the embryonic, neonatal, adult, and injured mouse brain

The CD11c enhanced yellow fluorescent protein (EYFP) transgenic mouse was constructed to identify dendritic cells in the periphery (Lindquist et al. [2004] Nat. Immunol. 5:1243-1250). In this study, we used this mouse to characterize dendritic cells within the CNS. Our anatomic results showed discrete populations of EYFP(+) brain dendritic cells (EYFP(+) bDC) that colocalized with a small fraction of microglia immunoreactive for Mac-1, Iba-1, CD45, and F4/80 but not for NeuN, Dcx, NG2 proteoglycan, or GFAP. EYFP(+) bDC, isolated by fluorescent activated cell sorting (FACS), expressed mRNA for the Itgax (CD11c) gene, whereas FACS anlaysis of EYFP(+) bDC cultures revealed the presence of CD11c protein. Light microscopy studies revealed that EYFP(+) bDC were present in the embryonic CNS when the blood-brain barrier is formed and postnatally when brain cells are amenable to culturing. In adult male mice, EYFP(+) bDC distribution was prominent within regions of the CNS that 1) are subject to structural plasticity and neurogenesis, 2) receive sensory and humoral input from the external environment, and 3) lack a blood-brain barrier. Ultrastructural analysis of EYFP(+) bDC in adult neurogenic niches showed their proximity to developing neurons and a morphology characteristic of immune/microglia cells. Kainic acid-induced seizures revealed that EYFP(+) bDC responded to damage of the hippocampus and displayed morphologies similar to those described for seizure-activated EGFP(+) microglia in the hippocampus of cfms (CSF-1R) EGFP mice. Collectively, these findings suggest a new member of the dendritic cell family residing among the heterogeneous microglia population.

The role of the immune system in clearance of Abeta from the brain

In Alzheimer's disease (AD), there is abnormal accumulation of Abeta and tau proteins in the brain. There is an associated immunological response, but it is still unclear whether this is beneficial or harmful. Inflammation in AD, specifically in the form of microglial activation, has, for many years, been considered to contribute to disease progression. However, two types of evidence suggest that it may be appropriate to revise this view: first, the disappointing results of prospective clinical trials of anti-inflammatory agents and, second, the observation that microglia can clear plaques in AD following Abeta immunization. Although Abeta immunization alters AD pathology, there is limited evidence so far of benefit to cognitive function. Immunization against microorganisms is almost always used as a method of disease prevention rather than to treat a disease process that has already started. In animal models, immunotherapy at an early age can protect against Abeta accumulation and it will be interesting to see if this can usefully be applied to humans to prevent AD.

Localization and function of tissue macrophages

The rat monoclonal antibody F4/80 defines a plasma membrane glycoprotein of about 160 kilodaltons that is expressed by mature mouse macrophages. The antigen has been used to define macrophage distribution within the mouse (normal adult, embryo, infection models) by cytochemistry and quantitative immunochemical analysis. Macrophages migrate into fetal and adult haemopoietic and other tissues in an ordered sequence. The surface properties of 'fixed' macrophages isolated from various organs (bone marrow, liver, spleen) are distinct from those of circulating monocytes or free cells (peritoneal and pleural cavities, alveolar) and may play a role in local adhesion and trophic interactions with other cells.

Aberrant agouti-related protein system in the hypothalamus of the anx/anx mouse is associated with activation of microglia

Agouti-related protein (AgRP) is a key orexigenic neuropeptide expressed in the hypothalamic arcuate nucleus and a marker for neurons conveying hormonal signals of hunger to the brain. Mice homozygous for the anorexia (anx) mutation are characterized by decreased food intake, starvation, and death by 3-5 weeks of age. At this stage immunoreactivity for AgRP is increased in cell bodies but decreased in the nerve terminals. We studied when during early postnatal development the aberrant phenotype of the AgRP system becomes apparent in anx/anx mice and possible underlying mechanisms. AgRP and ionized calcium binding adapter molecule (Iba1), a marker for activated microglia, as well as Toll-like receptor 2 (TLR-2), were studied by immunohistochemistry at postnatal days P1, P5, P10, P12, P15 and P21 in anx/anx and wild-type mice. We found that the AgRP system in the anx/anx mouse develops similarly to the wild type until P12, when AgRP fibers in anx/anx mice cease to increase in density in the main projection areas. At P21, AgRP fiber density in anx/anx mice was significantly reduced vs. P15, in certain regions. At P21, many strongly AgRP-positive cell bodies were observed in the anx/anx arcuate nucleus vs. only few and weakly fluorescent ones in the wild type. The decrease in AgRP fiber density in anx/anx mice overlapped with an increase in Iba1 and TLR-2 immunoreactivities. Thus, the aberrant appearance of the AgRP system in the anx/anx mouse in the early postnatal development could involve a microglia-associated process and the innate immune system.

From bone marrow to microglia: barriers and avenues

Microglia form a unique population of brain-resident macrophages. Although microglia have been involved in multiple disorders of the central nervous system (CNS), the issue of microglial renewal, under normal or pathological conditions, has been controversial. In mice, results from bone marrow chimera studies indicated that microglia are slowly but continuously replenished by bone marrow-derived cells. Moreover, such a microglial turnover was found to be greatly accelerated under multiple neurological conditions. However, recent works questioned the use of irradiation/reconstitution experiments to assess microglial turnover. Based on these different studies, we propose here a re-evaluation of microglia origin(s) in the inflamed CNS. We also discuss the therapeutic perspectives offered by the demonstration of an adult microglial lineage, from bone marrow to brain.

Immunosuppression after traumatic or ischemic CNS damage: it is neuroprotective and illuminates the role of microglial cells

Acute traumatic and ischemic events in the central nervous system (CNS) invariably result in activation of microglial cells as local representatives of the immune system. It is still under debate whether activated microglia promote neuronal survival, or whether they exacerbate the original extent of neuronal damage. Protagonists of the view that microglial cells cause secondary damage have proposed that inhibition of microglial activation by immunosuppression is beneficial after acute CNS damage. It is the aim of this review to analyse the effects of immunosuppressants on isolated microglial cells and neurons, and to scrutinize the effects of immunosuppression in different in vivo models of acute CNS trauma or ischemia. It is found that the immunosuppressants cytosine-arabinoside, different steroids, cyclosporin A, FK506, rapamycin, mycophenolate mofetil, and minocycline all have direct inhibitory effects on microglial cells. These effects are mainly exerted by inhibiting microglial proliferation or microglial secretion of neurotoxic substances such as proinflammatory cytokines and nitric oxide. Furthermore, immunosuppression after acute CNS trauma or ischemia results in improved structure preservation and, mostly, in enhanced function. However, all investigated immunosuppressants also have direct effects on neurons, and some immunosuppressants affect other glial cells such as astrocytes. In summary, it is safe to conclude that immunosuppression after acute CNS trauma or ischemia is neuroprotective. Furthermore, circumferential evidence indicates that microglial activation after traumatic or ischemic CNS damage is not beneficial to adjacent neurons in the immediate aftermath of such acute lesions. Further experiments with more specific agents or genetic approaches that specifically inhibit microglial cells are needed in order to fully answer the question of whether microglial activation is "good or bad".

Spatial arrangement of microglia in the mouse hippocampus: a stereological study in comparison with astrocytes

Microglia are classically considered to be immune cells in the brain, but have now been proven to be involved in neuronal activity as well. Here we stereologically analyzed the spatial arrangement of microglia in the mouse hippocampus. First, we estimated the numerical densities (NDs) of microglia identified by ionized calcium-binding adaptor molecule 1 (Iba1). Despite that microglia appeared to be evenly distributed throughout the hippocampal area, the NDs demonstrated significant dorsoventral, interregional, and interlaminar differences. Briefly, the NDs in the ventral hippocampus were significantly lower in the CA3 region than in the CA1 region and dentate gyrus, although no interregional differences were detectable in the dorsal hippocampus. Both in the CA1 and CA3 regions, the NDs were significantly higher in the stratum lacunosum-moleculare than in the remaining layers. Next, we investigated the spatial patterns of distribution of Iba1-labeled microglia and S100beta-labeled astrocytes. So far as we examined, the somato-somatic contacts were not seen among microglia or among astrocytes, whereas the close apposition between microglia and astrocytes were occasionally detected. The 3D point process analysis showed that the spatial distribution of microglia was significantly repulsive. Because the statistical territory of single microglia was larger than that estimated from process tracing, they are not likely to touch each other with their processes. Astrocytes were distributed slightly repulsively with overlapping areas. The 3D point process analysis also revealed a significant spatial attraction between microglia and astrocytes. The present findings provide a novel anatomical basis for glial research.

Entry and Distribution of Microglial Cells in Human Embryonic and Fetal Cerebral Cortex

Microglial cells penetrate into and scatter throughout the human cortical grey and white matter according to a specific spatiotemporal pattern during the first 2 trimesters of gestation. Routes of entry were quantitatively and qualitatively different from those identified in the diencephalon. Starting at 4.5 gestational weeks, amoeboid microglial cells, characterized by different antibodies as Iba1, CD68, CD45, and MHC-II, entered the cerebral wall from the ventricular lumen and the leptomeninges. Migration was mainly radial and tangential toward the immature white matter, subplate layer, and cortical plate, whereas pial cells populated the prospective layer I. The intraparenchymal vascular route of entry was detectable only from 12 gestational weeks. Interestingly, microglial cells accumulated in restricted laminar bands particularly at 19 to 24 gestational weeks among the corona radiata fibers rostrally, extending caudally in the immature white matter to reach the visual radiations. This accumulation of proliferating MIB1-positive microglia (as shown by MIB1-Iba1 double immunolabeling) was located at the site of white matter injury in premature neonates. The spatiotemporal organization of microglia in the immature white and grey matter suggests that these cells may play active roles in developmental processes and in injury to the developing brain.