Phagocytosis of microglia in the central nervous system diseases

Optogenetic Glia Manipulation: Possibilities and Future Prospects

Our brains are composed of two distinct cell types: neurons and glia. Emerging data from recent investigations show that glial cells, especially astrocytes and microglia, are able to regulate synaptic transmission and thus brain information processing. This suggests that, not only neuronal activity, but communication between neurons and glia also plays a key role in brain function. Thus, it is currently well known that the physiology and pathophysiology of brain function can only be completely understood by considering the interplay between neurons and glia. However, it has not yet been possible to dissect glial cell type-specific roles in higher brain functions in vivo. Meanwhile, the recent development of optogenetics techniques has allowed investigators to manipulate neural activity with unprecedented temporal and spatial precision. Recently, a series of studies suggested the possibility of applying this cutting-edge technique to manipulate glial cell activity. This review briefly discusses the feasibility of optogenetic glia manipulation, which may provide a technical innovation in elucidating the in vivo role of glial cells in complex higher brain functions.

Uptake of parasite-derived vesicles by astrocytes and microglial phagocytosis of infected erythrocytes may drive neuroinflammation in cerebral malaria

Astrocytes and microglia are activated during cerebral malaria (CM) and contribute to the production and release of several mediators during neuroinflammatory processes. Whether these changes are the consequence of a direct crosstalk between glial cells and the malarial parasite and how these cells participate in the pathogenesis of CM is not yet clear. We therefore examined the interaction of astrocytes and microglia with Plasmodium berghei ANKA-infected red blood cells using primary cell cultures derived from newborn C57BL/6 mice. We observed a dynamic transfer of vesicles from the parasite to astrocytes within minutes of contact, and the phagocytosis of infected red blood cells by microglia. Differential gene expression studies using the Affymetrix GeneChip^® microarray, and quantitative PCR analyses showed the increase in expression of the set of genes belonging to the immune response network in parasite activated astrocytes and microglia. Interestingly, expression of these genes was also significantly upregulated in brains of mice dying from CM compared with uninfected mice or infected mice that did not develop the neuropathology. Accumulation of parasite-derived vesicles within astrocytes, and the phagocytosis of infected red blood cells by microglia induced a subsequent increase in interferon gamma inducible protein 10 (IP10) in both the brain and plasma of infected mice at the onset of CM, confirming a role for this molecule in CM pathogenesis. Altogether, these observations point to a possible role for glial cells in the neuropathological processes leading to CM. GLIA 2016 GLIA 2017;65:75-92.

Interactions between inflammation, sex steroids, and Alzheimer's disease risk factors

Alzheimer's disease (AD) is an age-related neurodegenerative disorder for which there are no effective strategies to prevent or slow its progression. Because AD is multifactorial, recent research has focused on understanding interactions among the numerous risk factors and mechanisms underlying the disease. One mechanism through which several risk factors may be acting is inflammation. AD is characterized by chronic inflammation that is observed before clinical onset of dementia. Several genetic and environmental risk factors for AD increase inflammation, including apolipoprotein E4, obesity, and air pollution. Additionally, sex steroid hormones appear to contribute to AD risk, with age-related losses of estrogens in women and androgens in men associated with increased risk. Importantly, sex steroid hormones have anti-inflammatory actions and can interact with several other AD risk factors. This review examines the individual and interactive roles of inflammation and sex steroid hormones in AD, as well as their relationships with the AD risk factors apolipoprotein E4, obesity, and air pollution.

Anti CD163+, Iba1+, and CD68+ Cells in the Adult Human Inner Ear: Normal Distribution of an Unappreciated Class of Macrophages/Microglia and Implications for Inflammatory Otopathology in Humans

HYPOTHESIS

Identification, characterization, and location of cells involved in the innate immune defense system of the human inner ear may lead to a better understanding of many otologic diseases and new treatments for hearing and balance-related disorders.

BACKGROUND

Many otologic disorders are thought to have, as part of their disease process, an immune component. Although resident macrophages are known to exist in the mouse inner ear, the innate immune cells in the human inner ear are, to date, unknown.

METHODS

Primary antibodies against CD163, Iba1, and CD68 (markers known to be specific for macrophages/microglia) were used to immunohistochemically stain celloidin embedded archival temporal bone tissue of normal individuals with no known otologic disorders other than changes associated with age.

RESULTS

Cells were positively stained throughout the temporal bone within the connective tissue and supporting cells with all three markers. They were often associated with neurons and on occasion entered the sensory cell areas of the auditory and vestibular epithelium.

CONCLUSIONS

We have immunohistochemically identified an unappreciated class of cells in the normal adult inner ear consistent in staining characteristics and morphology with macrophages/microglia. As in other organ systems, it is likely these cells play an essential role in organ homeostasis that has not yet been elucidated within the ear.

Defective Phagocytic Corpse Processing Results in Neurodegeneration and Can Be Rescued by TORC1 Activation

The removal of apoptotic cell corpses is important for maintaining homeostasis. Previously, defects in apoptotic cell clearance have been linked to neurodegeneration. However, the mechanisms underlying this are still poorly understood. In this study, we report that the absence of the phagocytic receptor Draper in glia leads to a pronounced accumulation of apoptotic neurons in the brain of Drosophila melanogaster. These dead cells persist in the brain throughout the lifespan of the organism and are associated with age-dependent neurodegeneration. Our data indicate that corpses persist because of defective phagosome maturation, rather than recognition defects. TORC1 activation, or inhibition of Atg1, in glia is sufficient to rescue corpse accumulation as well as neurodegeneration. These results suggest that phagocytosis of apoptotic neurons by glia during development is essential for brain homeostasis in adult flies. Furthermore, it suggests that TORC1 regulates Draper-mediated phagosome maturation.

SIGNIFICANCE STATEMENT

Previously, defects in dead cell clearance were linked to neurodegeneration, but the exact mechanisms are not well understood. In this study, we report that the absence of an engulfment receptor leads to a pronounced accumulation of dead neurons in the brain of the fruit fly Drosophila melanogaster. These dead cells persist in the brain throughout the lifespan of the organism and are associated with age-dependent neurodegeneration. Our data indicate that corpses persist because of defective degradation of cells rather than recognition of dead cells.

Priming the inflammatory pump of the CNS after traumatic brain injury

Traumatic brain injury (TBI) can lead to secondary neuropsychiatric problems that develop and persist years after injury. Mounting evidence indicates that neuroinflammatory processes progress after the initial head injury and worsen with time. Microglia contribute to this inflammation by maintaining a primed profile long after the acute effects of the injury have dissipated. This may set the stage for glial dysfunction and hyperactivity to challenges including subsequent head injury, stress, or induction of a peripheral immune response. This review discusses the evidence that microglia become primed following TBI and how this corresponds with vulnerability to a 'second hit' and subsequent neuropsychiatric and neurodegenerative complications.

Microglia Ontology and Signaling

Microglia constitute the powerhouse of the innate immune system in the brain. It is now widely accepted that they are monocytic-derived cells that infiltrate the developing brain at the early embryonic stages, and acquire a resting phenotype characterized by the presence of dense branching processes, called ramifications. Microglia use these dynamic ramifications as sentinels to sense and detect any occurring alteration in brain homeostasis. Once a danger signal is detected, such as molecular factors associated to brain damage or infection, they get activated by acquiring a less ramified phenotype, and mount adequate responses that range from phagocyting cell debris to secreting inflammatory and trophic factors. Here, we review the origin of microglia and we summarize the main molecular signals involved in controlling their function under physiological conditions. In addition, their implication in the pathogenesis of multiple sclerosis and stress is discussed.

Glucocorticoid-Potentiated Spinal Microglia Activation Contributes to Preoperative Anxiety-Induced Postoperative Hyperalgesia

Clinically, preoperative anxiety adversely affected postoperative hyperalgesia. As stress-induced glucocorticoids (GCs) were reported to sensitize the activation of microglia, the present study investigated whether and how GCs and microglia played in the process of preoperative anxiety-induced postoperative hyperalgesia. The study used an animal model that exposed rats to single prolonged stress (SPS) procedure to induce preoperative anxiety-like behaviors 24 h before the plantar incisional surgery. Behavioral testing revealed that preoperative SPS enhanced the mechanical allodynia induced by plantar incision. SPS was also found to induce elevated circulating corticosterone levels, potentiate the activation of spinal microglia, and increase the expression of spinal proinflammatory cytokines. Inhibition of microglia by pretreatment with minocycline attenuated the SPS-enhanced mechanical allodynia, and this was accompanied by decreased activation of spinal microglia and expression of proinflammatory cytokines. Another experiment was conducted by administering RU486, the GC receptor (GR) antagonist, to rats. The results showed that RU486 suppressed SPS-induced and SPS-potentiated proinflammatory activation of spinal microglia and revealed analgesic effects. Together, these data indicated that inhibition of stress-induced GR activation attenuated the preoperative anxiety-induced exacerbation of postoperative pain, and the suppression of spinal microglia activation may underlie this anti-hyperalgesia effect. Pending further studies, these findings suggested that GR and spinal microglia may play important roles in the development of preoperative anxiety-induced postoperative hyperalgesia and may serve as novel targets to prevent this phenomenon.

Friend or Foe? Resident Microglia vs Bone Marrow-Derived Microglia and Their Roles in the Retinal Degeneration

Microglia are immune cells in the central nervous system (CNS) that originate from the yolk sac in an embryo. The renewal of the microglia pool in the adult eye consists of two components. In addition to the self-proliferation of resident cells, microglia in the CNS also derive from the bone marrow (BM). BM-derived cells pass through the blood-brain barrier (BBB) or blood-retina barrier (BRB) and differentiate into microglia under specific conditions which involves a complex mechanism. Recent studies have widely investigated the role of resident microglia and BM-derived microglia in the retinal degenerative disease. Both two cell types play dual roles and share many similar functions. However, resident microglia tend to polarize to the M1 phenotype which is pro-inflammatory and neurotoxic, whereas BM-derived microglia mainly polarize to the neuroprotective M2 phenotype in retinal degeneration. The molecular mechanism that underlines the invasion of peripheral cells has led to extensive discussions. In addition to the BBB and BRB disruption, many signaling pathways are involved in this process. Based on these studies, we discuss the roles of these two types of microglia in retinal degeneration disease and the potential clinical application of BM-derived microglia, which may benefit future therapies.

Indirect effects of TiO2 nanoparticle on neuron-glial cell interactions

Although, titanium dioxide nanoparticles (TiO2NPs) are nanomaterials commonly used in consumer products, little is known about their hazardous effects, especially on central nervous systems. To examine this issue, ALT astrocyte-like, BV-2 microglia and differentiated N2a neuroblastoma cells were exposed to 6 nm of 100% anatase TiO2NPs. A lipopolysaccharide (LPS) was pre-treated to activate glial cells before NP treatment for mimicking NP exposure under brain injury. We found that ALT and BV-2 cells took up more NPs than N2a cells and caused lower cell viability. TiO2NPs induced IL-1β in the three cell lines and IL-6 in N2a. LPS-activated BV-2 took up more TiO2NPs than normal BV-2 and released more intra/extracellular reactive oxygen species (ROS), IL-1β, IL-6 and MCP-1 than did activated BV-2. Involvement of clathrin- and caveolae-dependent endocytosis in ALT and clathrin-dependent endocytosis and phagocytosis in BV-2 both had a slow NP translocation rate to lysosome, which may cause slow ROS production (after 24 h). Although TiO2NPs did not directly cause N2a viability loss, by indirect NP exposure to the bottom chamber of LPS-activated BV-2 in the Transwell system, they caused late apoptosis and loss of cell viability in the upper N2a chamber due to H2O2 and/or TNF-α release from BV-2. However, none of the adverse effects in N2a or BV-2 cells was observed when TiO2NPs were exposed to ALT-N2a or ALT-BV-2 co-culture. These results demonstrate that neuron damage can result from TiO2NP-mediated ROS and/or cytokines release from microglia, but not from astrocytes.

Astrocyte-Microglia Cross Talk through Complement Activation Modulates Amyloid Pathology in Mouse Models of Alzheimer's Disease

Increasing evidence supports a role of neuroinflammation in the pathogenesis of Alzheimer's disease (AD). Previously, we identified a neuron-glia signaling pathway whereby Aβ acts as an upstream activator of astroglial nuclear factor kappa B (NF-κB), leading to the release of complement C3, which acts on the neuronal C3a receptor (C3aR) to influence dendritic morphology and cognitive function. Here we report that astrocytic complement activation also regulates Aβ dynamics in vitro and amyloid pathology in AD mouse models through microglial C3aR. We show that in primary microglial cultures, acute C3 or C3a activation promotes, whereas chronic C3/C3a treatment attenuates, microglial phagocytosis and that the effect of chronic C3 exposure can be blocked by cotreatment with a C3aR antagonist and by genetic deletion of C3aR. We further demonstrate that Aβ pathology and neuroinflammation in amyloid precursor protein (APP) transgenic mice are worsened by astroglial NF-κB hyperactivation and resulting C3 elevation, whereas treatment with the C3aR antagonist (C3aRA) ameliorates plaque load and microgliosis. Our studies define a complement-dependent intercellular cross talk in which neuronal overproduction of Aβ activates astroglial NF-κB to elicit extracellular release of C3. This promotes a pathogenic cycle by which C3 in turn interacts with neuronal and microglial C3aR to alter cognitive function and impair Aβ phagocytosis. This feedforward loop can be effectively blocked by C3aR inhibition, supporting the therapeutic potential of C3aR antagonists under chronic neuroinflammation conditions.

SIGNIFICANCE STATEMENT

The complement pathway is activated in Alzheimer's disease. Here we show that the central complement factor C3 secreted from astrocytes interacts with microglial C3a receptor (C3aR) to mediate β-amyloid pathology and neuroinflammation in AD mouse models. Our study provides support for targeting C3aR as a potential therapy for Alzheimer's disease.

Complex molecular and functional outcomes of single versus sequential cytokine stimulation of rat microglia

BACKGROUND

Microglia are the "professional" phagocytes of the CNS. Phagocytosis is crucial for normal CNS development and maintenance, but it can be either beneficial or detrimental after injury or disease. For instance, white matter damage releases myelin debris that must be cleared by microglia in order for re-myelination to occur. However, phagocytosis can also produce damaging reactive oxygen species (ROS). Furthermore, microglia can acquire pro-inflammatory (M1) or anti-inflammatory (M2) activation states that affect cell functions. Although microglia are exposed to a changing cytokine environment after injury or disease, little is known about the molecular and functional consequences. Therefore, we applied several microglial activation paradigms, with or without myelin debris. We assessed (i) gene expression changes reflecting microglial activation and inflammatory states, and receptors and enzymes related to phagocytosis and ROS production, (ii) myelin phagocytosis and production of ROS, and (iii) expression and contributions of several ion channels that are considered potential targets for regulating microglial behavior.

METHODS

Primary rat microglia were exposed to cytokines, individually or sequentially. First, responses to individual M1 or M2 stimuli were compared: IFN-γ plus TNF-α ("I + T"; M1 activation), interleukin-4 (M2a/alternative activation), and interleukin-10 (M2c/acquired deactivation). Second, sequential cytokine addition was used to assess microglia repolarization and cell functions. The paradigms were M2a→M1, M2c→M1, M1→M2a, and M1→M2c.

RESULTS

M1 stimulation increased pro-inflammatory genes, phagocytosis, and ROS, as well as expression of Kv1.3, KCa3.1, and Kir2.1 channels. M2a stimulation increased anti-inflammatory genes, ROS production, and Kv1.3 and KCa3.1 expression. Myelin phagocytosis enhanced the M1 profile and dampened the M2a profile, and both phagocytosis and ROS production were dependent on NOX enzymes and Kir2.1 and CRAC channels. Importantly, microglia showed some capacity for re-polarization between M1 and M2a states, based on gene expression changes, myelin phagocytosis, and ROS production.

CONCLUSIONS

In response to polarizing and re-polarizing cytokine treatments, microglia display complex changes in gene transcription profiles, phagocytic capacity, NOX-mediated ROS production, and in ion channels involved in microglial activation. Because these changes might affect microglia-mediated CNS inflammation, they should be considered in future experimental, pre-clinical studies.

Neonatal binge alcohol exposure increases microglial activation in the developing rat hippocampus

Aberrant activation of the developing immune system can have long-term negative consequences on cognition and behavior. Teratogens, such as alcohol, activate microglia, the brain's resident immune cells, which could contribute to the lifelong deficits in learning and memory observed in humans with Fetal Alcohol Spectrum Disorders (FASD) and in rodent models of FASD. The current study investigates the microglial response of the brain 24 h following neonatal alcohol exposure (postnatal days (PDs) 4-9, 5.25 g/kg/day). On PD10, microglial cell counts and area of cell territory were assessed using unbiased stereology in the hippocampal subfields CA1, CA3 and dentate gyrus (DG), and hippocampal expression of pro- and anti-inflammatory genes was analyzed. A significant decrease in microglial cell counts in CA1 and DG was found in alcohol-exposed and sham-intubated (SI) animals compared to undisturbed suckle controls (SCs), suggesting overlapping effects of alcohol exposure and intubation alone on the neuroimmune response. Cell territory was decreased in alcohol-exposed animals in CA1, CA3, and DG compared to controls, suggesting the microglia have shifted to a more activated state following alcohol treatment. Furthermore, both alcohol-exposed and SI animals had increased levels of pro-inflammatory cytokines IL-1β, TNF-α, CD11b, and CCL4; in addition, CCL4 was significantly increased in alcohol-exposed animals compared to SI as well. Alcohol-exposed animals also showed increased levels of anti-inflammatory cytokine TGF-β compared to both SI and SCs. In summary, the number and activation of microglia in the neonatal hippocampus are both affected in a rat model of FASD, along with increased gene expression of pro- and anti-inflammatory cytokines. This study shows that alcohol exposure during development induces a neuroimmune response, potentially contributing to long-term alcohol-related changes to cognition, behavior and immune function.

Graphene Functionalized Scaffolds Reduce the Inflammatory Response and Supports Endogenous Neuroblast Migration when Implanted in the Adult Brain

Electroactive materials have been investigated as next-generation neuronal tissue engineering scaffolds to enhance neuronal regeneration and functional recovery after brain injury. Graphene, an emerging neuronal scaffold material with charge transfer properties, has shown promising results for neuronal cell survival and differentiation in vitro. In this in vivo work, electrospun microfiber scaffolds coated with self-assembled colloidal graphene, were implanted into the striatum or into the subventricular zone of adult rats. Microglia and astrocyte activation levels were suppressed with graphene functionalization. In addition, self-assembled graphene implants prevented glial scarring in the brain 7 weeks following implantation. Astrocyte guidance within the scaffold and redirection of neuroblasts from the subventricular zone along the implants was also demonstrated. These findings provide new functional evidence for the potential use of graphene scaffolds as a therapeutic platform to support central nervous system regeneration.

Microglia-blood vessel interactions: a double-edged sword in brain pathologies

Microglia are long-living resident immune cells of the brain, which secure a stable chemical and physical microenvironment necessary for the proper functioning of the central nervous system (CNS). These highly dynamic cells continuously scan their environment for pathogens and possess the ability to react to damage-induced signals in order to protect the brain. Microglia, together with endothelial cells (ECs), pericytes and astrocytes, form the functional blood-brain barrier (BBB), a specialized endothelial structure that selectively separates the sensitive brain parenchyma from blood circulation. Microglia are in bidirectional and permanent communication with ECs and their perivascular localization enables them to survey the influx of blood-borne components into the CNS. Furthermore, they may stimulate the opening of the BBB, extravasation of leukocytes and angiogenesis. However, microglia functioning requires tight control as their dysregulation is implicated in the initiation and progression of numerous neurological diseases. Disruption of the BBB, changes in blood flow, introduction of pathogens in the sensitive CNS niche, insufficient nutrient supply, and abnormal secretion of cytokines or expression of endothelial receptors are reported to prime and attract microglia. Such reactive microglia have been reported to even escalate the damage of the brain parenchyma as is the case in ischemic injuries, brain tumors, multiple sclerosis, Alzheimer's and Parkinson's disease. In this review, we present the current state of the art of the causes and mechanisms of pathological interactions between microglia and blood vessels and explore the possibilities of targeting those dysfunctional interactions for the development of future therapeutics.

The role of autophagy in modulation of neuroinflammation in microglia

Microglia have multiple functions in regulating homeostasis in the central nervous system (CNS), and microglial inflammation is thought to play a role in the etiology of the neurodegenerative diseases. When endogenous or exogenous stimuli trigger disorders in microenvironmental homeostasis in CNS, microglia critically determine the fate of other neural cells. Recently, it was reported that autophagy might influence inflammation and activation of microglia. Though the interaction between autophagy and macrophages has been reported and reviewed in length, the role of autophagy in microglia has yet to be reviewed. Herein, we will highlight recent advances on the emerging role of autophagy in microglia, focusing on the regulation of autophagy during microglial inflammation, and the possible mechanism involved.

Cofilin as a Promising Therapeutic Target for Ischemic and Hemorrhagic Stroke

Neurovascular unit (NVU) is considered as a conceptual framework for investigating the mechanisms as well as developing therapeutic targets for ischemic and hemorrhagic stroke. From a molecular perspective, oxidative stress, excitotoxicity, inflammation, and disruption of the blood brain barrier are broad pathophysiological frameworks on the basis on which potential therapeutic candidates for ischemic and hemorrhagic stroke could be discussed. Cofilin is a potent actin-binding protein that severs and depolymerizes actin filaments in order to generate the dynamics of the actin cytoskeleton. Although studies of the molecular mechanisms of cofilin-induced reorganization of the actin cytoskeleton have been ongoing for decades, the multicellular functions of cofilin and its regulation in different molecular pathways are expanding beyond its primary role in actin cytoskeleton. This review focuses on the role of cofilin in oxidative stress, excitotoxicity, inflammation, and disruption of the blood brain barrier in the context of NVU as well as how and why cofilin could be studied further as a potential target for ischemic and hemorrhagic stroke.

USPIO-Enhanced MRI Neuroimaging: A Review

MRI is a powerful tool for the diagnosis and management for a variety of central nervous system (CNS) diseases. Ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles are a novel category of MRI contrast agents that seem to play a crucial role in the imaging of CNS. Due to their physical properties, USPIOs act as blood pool agents. USPIOs improve visualization of tumor vasculature and relative cerebral blood volume measurements, tumor-associated inflammation, inflammatory-immune mediated disorders, stroke and vascular malformations. Ferumoxytol, a new type of USPIO agent, appears to have ideal characteristics for the imaging of CNS. The last few years, ferumoxytol has been successfully used to image CNS neoplasms, CNS inflammations and cerebral malformations offering useful information on cellular and molecular level. In addition, ferumoxytol studies focused on the pathophysiology of other CNS disorders like multiple sclerosis and epilepsy are already in progress. Aim of this review article is to provide the potential role of USPIO-enhanced MRI and the latest clinical applications of ferumoxytol agent in CNS imaging.

Extracellular Membrane Vesicles as Vehicles for Brain Cell-to-Cell Interactions in Physiological as well as Pathological Conditions

Extracellular vesicles are involved in a great variety of physiological events occurring in the nervous system, such as cross talk among neurons and glial cells in synapse development and function, integrated neuronal plasticity, neuronal-glial metabolic exchanges, and synthesis and dynamic renewal of myelin. Many of these EV-mediated processes depend on the exchange of proteins, mRNAs, and noncoding RNAs, including miRNAs, which occurs among glial and neuronal cells. In addition, production and exchange of EVs can be modified under pathological conditions, such as brain cancer and neurodegeneration. Like other cancer cells, brain tumours can use EVs to secrete factors, which allow escaping from immune surveillance, and to transfer molecules into the surrounding cells, thus transforming their phenotype. Moreover, EVs can function as a way to discard material dangerous to cancer cells, such as differentiation-inducing proteins, and even drugs. Intriguingly, EVs seem to be also involved in spreading through the brain of aggregated proteins, such as prions and aggregated tau protein. Finally, EVs can carry useful biomarkers for the early diagnosis of diseases. Herein we summarize possible roles of EVs in brain physiological functions and discuss their involvement in the horizontal spreading, from cell to cell, of both cancer and neurodegenerative pathologies.

The Potential Roles of Aquaporin 4 in Alzheimer's Disease

Aquaporin 4 (AQP4) is the major water channel expressed in the central nervous system (CNS), and it is primarily expressed in astrocytes. It has been studied in various brain pathological conditions. However, the potential for AQP4 to influence Alzheimer's disease (AD) is still unclear. Research regarding AQP4 functions related to AD can be traced back several years and has gradually progressed toward a better understanding of the potential mechanisms. Currently, it has been suggested that AQP4 influences synaptic plasticity, and AQP4 deficiency may impair learning and memory, in part, through glutamate transporter-1 (GLT-1). AQP4 may mediate the clearance of amyloid beta peptides (Aβ). In addition, AQP4 may influence potassium (K(+)) and calcium (Ca(2+)) ion transport, which could play decisive roles in the pathogenesis of AD. Furthermore, AQP4 knockout is involved in neuroinflammation and interferes with AD. To date, no specific therapeutic agents have been developed to inhibit or enhance AQP4. However, experimental results strongly emphasize the importance of this topic for future investigations.

Modulation of BV-2 microglia functions by novel quercetin pivaloyl ester

Chronic inflammation in brain plays a critical role in major neurodegenerative diseases such as Alzheimer's, Parkinson's disease, stroke or multiple sclerosis. Microglia, resident macrophages and intristinc components of CNS, appear to be main effectors in this pathological process. Quercetin, a naturally occurring flavonoid, was proven to downregulate inflammatory genes in microglia. Synthetically modified quercetin, 3'-O-(3-chloropivaloyl) quercetin (CPQ), is assumed to possess better biological availability and enhanced antioxidant properties. In the present study, antineuroinflammatory capability of the novel compound CPQ was assessed in BV-2 microglial cells. Our data show that treatment with CPQ attenuated the production of the inflammatory mediators, nitric oxide (NO) and tumour necrosis factor-α (TNF-α), in LPS-stimulated microglia somewhat more efficiently than did quercetin (p > 0.05 for CPQ vs. quercetin-treated group). Also, protein level of inducible NO synthase (iNOS) in LPS-activated BV-2 microglia was to some extent more effectively supressed by CPQ than by unmodified flavonoid. In consistence with the extent of their effects on pro-inflammatory markers, CPQ and quercetin showed down-regulation of NFκB activation. This quercetin analogue caused also a decline in BV-2 microglia proliferation with interfering with cell cycle progression (p < 0.001 for CPQ vs. quercetin-treated group). However, CPQ did not remarkably affect cell viability. In addition, CPQ showed a minor better suppression of PMA-induced generation of superoxide than did quercetin. Neither CPQ nor quercetin influenced phagocytosis of BV-2 cells. These results point to the therapeutic potential of 3'-O-(3-chloropivaloyl)quercetin (CPQ) as a novel antiinflammatory drug in neurodegenerative diseases, mediating favourable modulation of pro-inflammatory functions of microglia.

FcγRIIB mediates the inhibitory effect of aggregated α-synuclein on microglial phagocytosis

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease. Although the etiology of PD has not yet been fully understood, accumulating evidence indicates that neuroinflammation plays a critical role in the progression of PD. α-Synuclein (α-Syn) has been considered to be a key player of the pathogenesis of PD, and recent reports that prion-like propagation of misfolded α-syn released from neurons may play an important role in the progression of PD have led to increased attention to the studies elucidating the roles of extracellular α-syn in the CNS. Extracellular α-syn has also been reported to regulate microglial inflammatory response. In this study, we demonstrated that aggregated α-syn inhibited microglial phagocytosis by activating SHP-1. SHP-1 activation was also observed in A53T α-syn transgenic mice. In addition, aggregated α-syn bound to FcγRIIB on microglia, inducing SHP-1 activation, further inhibiting microglial phagocytosis. Aggregated α-syn upregulated FcγRIIB expression in microglia and upregulated FcγRIIB was also observed in A53T α-syn transgenic mice. These data suggest that aggregated α-syn released from neurons dysregulates microglial immune response through inhibiting microglial phagocytosis, further causing neurodegeneration observed in PD. The interaction of aggregated α-syn and FcγRIIB and further SHP-1 activation can be a new therapeutic target against PD.

Validation of Flow Cytometry and Magnetic Bead-Based Methods to Enrich CNS Single Cell Suspensions for Quiescent Microglia

Microglia are resident mononuclear phagocytes within the CNS parenchyma that intimately interact with neurons and astrocytes to remodel synapses and extracellular matrix. We briefly review studies elucidating the molecular pathways that underlie microglial surveillance, activation, chemotaxis, and phagocytosis; we additionally place these studies in a clinical context. We describe and validate an inexpensive and simple approach to obtain enriched single cell suspensions of quiescent parenchymal and perivascular microglia from the mouse cerebellum and hypothalamus. Following preparation of regional CNS single cell suspensions, we remove myelin debris, and then perform two serial enrichment steps for cells expressing surface CD11b. Myelin depletion and CD11b enrichment are both accomplished using antigen-specific magnetic beads in an automated cell separation system. Flow cytometry of the resultant suspensions shows a significant enrichment for CD11b(+)/CD45(+) cells (perivascular microglia) and CD11b(+)/CD45(-) cells (parenchymal microglia) compared to starting suspensions. Of note, cells from these enriched suspensions minimally express Aif1 (aka Iba1), suggesting that the enrichment process does not evoke significant microglial activation. However, these cells readily respond to a functional challenge (LPS) with significant changes in the expression of molecules specifically associated with microglia. We conclude that methods employing a combination of magnetic-bead based sorting and flow cytometry produce suspensions highly enriched for microglia that are appropriate for a variety of molecular and cellular assays.

Triggering receptor expressed on myeloid cells 2 (TREM2) deficiency attenuates phagocytic activities of microglia and exacerbates ischemic damage in experimental stroke

Clearing cellular debris after brain injury represents an important mechanism in regaining tissue homeostasis and promoting functional recovery. Triggering receptor expressed on myeloid cells-2 (TREM2) is a newly identified receptor expressed on microglia and is thought to phagocytose damaged brain cells. The precise role of TREM2 during ischemic stroke has not been fully understood. We explore TREM2 in both in vitro and in vivo stroke models and identify a potential endogenous TREM2 ligand. TREM2 knockdown in microglia reduced microglial activation to an amoeboid phenotype and decreased the phagocytosis of injured neurons. Phagocytosis and infarcted brain tissue resorption was reduced in TREM2 knock-out (KO) mice compared with wild-type (WT) mice. TREM2 KO mice also had worsened neurological recovery and decreased viable brain tissue in the ipsilateral hemisphere. The numbers of activated microglia and phagocytes in TREM2 KO mice were decreased compared with WT mice, and foamy macrophages were nearly absent in the TREM2 KO mice. Postischemia, TREM2 was highly expressed on microglia and TREM2-Fc fusion protein (used as a probe to identify potential TREM2 binding partners) bound to an unknown TREM2 ligand that colocalized to neurons. Oxygen glucose deprivation-exposed neuronal media, or cellular fractions containing nuclei or purified DNA, but not cytosolic fractions, stimulated signaling through TREM2. TREM2-Fc fusion protein pulled down nucleic acids from ischemic brain lysate. These findings establish the relevance of TREM2 in the phagocytosis of the infarcted brain and emphasize its role in influencing neurological outcomes following stroke. Further, nucleic acids may be one potential ligand of TREM2 in brain ischemia.

P2Y6 Receptor-Mediated Microglial Phagocytosis in Radiation-Induced Brain Injury

Microglia are the resident immune cells and the professional phagocytic cells of the CNS, showing a multitude of cellular responses after activation. However, how microglial phagocytosis changes and whether it is involved in radiation-induced brain injury remain unknown. In the current study, we found that microglia were activated and microglial phagocytosis was increased by radiation exposure both in cultured microglia in vitro and in mice in vivo. Radiation increased the protein expression of the purinergic receptor P2Y6 receptor (P2Y6R) located on microglia. The selective P2Y6 receptor antagonist MRS2578 suppressed microglial phagocytosis after radiation exposure. Inhibition of microglial phagocytosis increased inhibitory factor Nogo-A and exacerbated radiation-induced neuronal apoptosis and demyelination. We also found that the levels of protein expression for phosphorylated Ras-related C3 botulinum toxin substrate 1 (Rac1) and myosin light chain kinase (MLCK) were elevated, indicating that radiation exposure activated Rac1 and MLCK. The Rac1 inhibitor NSC23766 suppressed expression of MLCK, indicating that the Rac1-MLCK pathway was involved in microglial phagocytosis. Taken together, these findings suggest that the P2Y6 receptor plays a critical role in mediating microglial phagocytosis in radiation-induced brain injury, which might be a potential strategy for therapeutic intervention to alleviate radiation-induced brain injury.

Microglia in action: how aging and injury can change the brain's guardians

Neuroinflammation, the inflammatory response in the central nervous system (CNS), is a major determinant of neuronal function and survival during aging and disease progression. Microglia, as the resident tissue-macrophages of the brain, provide constant support to surrounding neurons in healthy brain. Upon any stress signal (such as trauma, ischemia, inflammation) they are one of the first cells to react. Local and/or peripheral signals determine microglia stress response, which can vary within a continuum of states from beneficial to detrimental for neuronal survival, and can be shaped by aging and previous insults. In this review, we discuss the roles of microglia upon an ischemic or traumatic injury, and give our perspective how aging may contribute to microglia behavior in the injured brain. We speculate that a deeper understanding of specific microglia identities will pave the way to develop more potent therapeutics to treat the diseases of aging brain.

Inhibition of the activation and recruitment of microglia-like cells protects against neomycin-induced ototoxicity

One of the most unfortunate side effects of aminoglycoside (AG) antibiotics such as neomycin is that they target sensory hair cells (HCs) and can cause permanent hearing impairment. We have observed HC loss and microglia-like cell (MLC) activation in the inner ear (cochlea) following neomycin administration. We focused on CX3CL1, a membrane-bound glycoprotein expressed on neurons and endothelial cells, as a way to understand how the MLCs are activated and the role these cells play in HC loss. CX3CL1 is the exclusive ligand for CX3CR1, which is a chemokine receptor expressed on the surface of macrophages and MLCs. In vitro experiments showed that the expression levels of CX3CL1 and CX3CR1 increased in the cochlea upon neomycin treatment, and CX3CL1 was expressed on HCs, while CX3CR1 was expressed on MLCs. When cultured with 1 μg/mL exogenous CX3CL1, MLCs were activated by CX3CL1, and the cytokine level was increased in the cochleae leading to apoptosis in the HCs. In CX3CR1 knockout mice, a significantly greater number of cochlear HCs survived than in wild-type mice when the cochlear explants were cultured with neomycin in vitro. Furthermore, inhibiting the activation of MLCs with minocycline reduced the neomycin-induced HC loss and improved the hearing function in neomycin-treated mice in vivo. Our results demonstrate that CX3CL1-induced MLC activation plays an important role in the induction of HC death and provide evidence for CX3CL1 and CX3CR1 as promising new therapeutic targets for the prevention of hearing loss.

Mechanisms and potential therapeutic applications of microglial activation after brain injury

As the resident immune cells of the central nervous system, microglia rapidly respond to brain insults, including stroke and traumatic brain injury. Microglial activation plays a major role in neuronal cell damage and death by releasing a variety of inflammatory and neurotoxic mediators. Their activation is an early response that may exacerbate brain injury and many other stressors, especially in the acute stages, but are also essential to brain recovery and repair. The full range of microglial activities is still not completely understood, but there is accumulating knowledge about their role following brain injury. We review recent progress related to the deleterious and beneficial effects of microglia in the setting of acute neurological insults and the current literature surrounding pharmacological interventions for intervention.

Interferon regulatory factor 7 participates in the M1-like microglial polarization switch

Microglia are generally considered the immune cells of the central nervous system. Recent studies have demonstrated that under specific polarization conditions, microglia develop into two different phenotypes, termed M1-like and M2-like microglia. However, the phenotypic characteristics of M1-like- and M2-like-polarized microglia and the mechanisms that regulate polarization are largely unknown. In this study, we characterized lipopolysaccharide-treated M1-like and IL-4-treated M2-like microglia and investigated the mechanisms that regulate phenotypic switching. The addition of M2-like microglial conditioned medium (CM) to primary neurons resulted in an increase in neurite length when compared with neurons treated with M1-like microglial CM, possibly because of the enhanced secretion of neurotrophic factors by M2-like microglia. M1-like microglia were morphologically characterized by larger soma, whereas M2-like microglia were characterized by long processes. M2-like microglia exhibited greater phagocytic capacity than M1-like microglia. These features switched in response to polarization cues. We found that expression of interferon regulatory factor 7 (IRF7) increased during the M2-like to M1-like switch in microglia in vitro and in vivo. Knockdown of IRF7 using siRNA suppressed the expression of M1 marker mRNA and reduced phosphorylation of STAT1. Our findings suggest that IRF7 signaling may play an important role in microglial polarization switching.

Mechanisms of intimate and long-distance cross-talk between glioma and myeloid cells: how to break a vicious cycle

Glioma-associated microglia and macrophages (GAMs) and myeloid-derived suppressor cells (MDSCs) condition the glioma microenvironment to generate an immunosuppressed niche for tumour expansion. This immunosuppressive microenvironment is considered to be shaped through a complex multi-step interactive process between glioma cells, GAMs and MDSCs. Glioma cells recruit GAMs and MDSCs to the tumour site and block their maturation. Glioma cell-derived factors subsequently skew these cells towards an immunosuppressive, tumour-promoting phenotype. Finally, GAMs and MDSCs enhance immune suppression in the glioma microenvironment and promote glioma growth, invasiveness, and neovascularization. The local and distant cross-talk between glioma cells and GAMs and MDSCs is regulated by a plethora of soluble proteins and cell surface-bound factors, and possibly via extracellular vesicles and platelets. Importantly, GAMs and MDSCs have been reported to impair the efficacy of glioma therapy, in particular immunotherapeutic approaches. Therefore, advancing our understanding of the function of GAMs and MDSCs in brain tumours and targeted intervention of their immunosuppressive function may benefit the treatment of glioma.

The complex relationships between microglia, alpha-synuclein, and LRRK2 in Parkinson's disease

The proteins alpha-synuclein (αSyn) and leucine rich repeat kinase 2 (LRRK2) are both key players in the pathogenesis of the neurodegenerative disorder Parkinson's disease (PD), but establishing a functional link between the two proteins has proven elusive. Research studies for these two proteins have traditionally and justifiably focused in neuronal cells, but recent studies indicate that each protein could play a greater pathological role elsewhere. αSyn is expressed at high levels within neurons, but they also secrete the protein into the extracellular milieu, where it can have broad ranging effects in the nervous system and relevance to disease etiology. Similarly, low neuronal LRRK2 expression and activity suggests that LRRK2-related functions could be more relevant in cells with higher expression, such as brain-resident microglia. Microglia are monocytic immune cells that protect neurons from noxious stimuli, including pathological αSyn species, and microglial activation is believed to contribute to neuroinflammation and neuronal death in PD. Interestingly, both αSyn and LRRK2 can be linked to microglial function. Secreted αSyn can directly activate microglia, and can be taken up by microglia for clearance, while LRRK2 has been implicated in the intrinsic regulation of microglial activation and of lysosomal degradation processes. Based on these observations, the present review will focus on how PD-associated mutations in LRRK2 could potentially alter microglial biology with respect to neuronally secreted αSyn, resulting in cell dysfunction and neurodegeneration.

The interaction between microglia and neural stem/precursor cells

In the adult mammalian brain, neurogenesis from neural stem/precursor cell occurs within two regions, the subgranular zone (SGZ) in the dentate gyrus (DG) and the subventricular zone (SVZ) lining the lateral ventricles. The function of neural stem cell is enhanced by external stimuli, such as injury and inflammation. Microglia, as the main immune modulating cells, play important roles in the central nervous system (CNS). Recently, select discoveries reported that microglia might influence the proliferation, differentiation and survival of neural precursor cells (NPCs). Other studies revealed that NPCs might reversibly regulate the function of microglia. Accordingly, in this review we focus on the interaction between microglia and NPCs.

Activation of intrinsic immune responses and microglial phagocytosis in an ex vivo spinal cord slice culture model of West Nile virus infection

West Nile virus (WNV) is a neurotropic flavivirus that causes significant neuroinvasive disease involving the brain and/or spinal cord. Experimental mouse models of WNV infection have established the importance of innate and adaptive immune responses in controlling the extent and severity of central nervous system (CNS) disease. However, differentiating between immune responses that are intrinsic to the CNS and those that are dependent on infiltrating inflammatory cells has proven difficult. We used a murine ex vivo spinal cord slice culture (SCSC) model to determine the innate immune processes specific to the CNS during WNV infections. By 7 days after ex vivo infection of SCSCs, the majority of neurons and a substantial percentage of astrocytes were infected with WNV, resulting in apoptotic cell death and astrogliosis. Microglia, the resident immune cells of the CNS, were activated by WNV infection, as exemplified by their amoeboid morphology, the development of filopodia and lamellipodia, and phagocytosis of WNV-infected cells and debris. Microglial cell activation was concomitant with increased expression of proinflammatory cytokines and chemokines, including CXCL10, CXCL1, CCL5, CCL3, CCL2, tumor necrosis factor alpha (TNF-α), TNF-related apoptosis-inducing ligand (TRAIL), and interleukin-6 (IL-6). The application of minocycline, an inhibitor of neuroinflammation, altered the WNV-induced proinflammatory cytokine/chemokine expression profile, with inhibited production of CCL5, CCL2, and IL-6. Our findings establish that CNS-resident cells have the capacity to initiate a robust innate immune response against WNV infection in the absence of infiltrating inflammatory cells and systemic immune responses.

IMPORTANCE

There are no specific treatments of proven efficacy available for WNV neuroinvasive disease. A better understanding of the pathogenesis of WNV CNS infection is crucial for the rational development of novel therapies. Development of a spinal cord slice culture (SCSC) model facilitates the study of WNV pathogenesis and allows investigation of the intrinsic immune responses of the CNS. Our studies demonstrate that robust CNS innate immune responses, including microglial activation and proinflammatory cytokine/chemokine production, develop independently of contributions from the peripheral immune system and CNS-infiltrating inflammatory cells.

Do glial cells play an anti-oxidative role in Huntington's disease?

Oxidative stress is a condition of imbalance between reactive oxygen species (ROS) formation and antioxidant capacity as a result of dysfunction of the antioxidant system. ROS can be served as a second messenger at low or moderate concentration, while excessive amount of ROS under oxidative stress condition would destroy macromolecules like proteins, DNA, and lipids, finally leading to cell apoptosis or necrosis. Changes in these macromolecules are involved in various pathological changes and progression of diseases, especially neurodegenerative diseases. Neurodegenerative diseases are morphologically featured by progressive neuronal cell loss, accompanied with inclusions formed by protein aggregates in neurons or glial cells. Neurons have always received much more attention than glial cells in neurodegenerative diseases. Actually, glial cells might play a key role in the functioning of neurons and cellular survival through an antioxidant way. Additionally, neurons can modulate the activities of glia either. Herein, the main purposes of this review are to mention the connection between Huntington's disease (HD) and oxidative stress, to summarize the characteristics and functions of glial cells in HD, to state the cross talk between neurons and glial cells, and to emphasize the conclusive role of activation of Keap1-Nrf2-ARE pathway in glial cells against oxidative stress in HD.

Ventricular shunt infections: immunopathogenesis and clinical management

Ventricular shunts are the most common neurosurgical procedure performed in the United States. This hydrocephalus treatment is often complicated by infection of the device with biofilm-forming bacteria. In this review, we discuss the pathogenesis of shunt infection, as well as the implications of the biofilm formation on treatment and prevention of these infections. Many questions remain, including the contribution of glia and the impact of inflammation on developmental outcomes following infection. Immune responses within the CNS must be carefully regulated to contain infection while minimizing bystander damage; further study is needed to design optimal treatment strategies for these patients.

Differences in the phagocytic response of microglia and peripheral macrophages after spinal cord injury and its effects on cell death

Macrophages in the injured spinal cord arise from resident microglia and infiltrating, peripherally derived monocytes. It is still not clear if macrophages derived from these two populations differ in their roles after CNS injury. The aims of this study are to investigate the phagocytic response and clearance of damaged axons and tissue debris by these distinct subsets of macrophages and assess their viability after spinal cord injury (SCI). The lysozyme M EGFP-knockin mouse tags hematogenous macrophages, but not microglia. Using a combination of immunofluorescence, flow cytometry, and neuronal tracing techniques, we show that microglia contact damaged axons early (24 h) after SCI and are the main type of macrophage to contain phagocytic material at 3 d. Thereafter, infiltrating macrophages become the predominant cell in contact with degenerating axons and contain more phagocytic material, which in contrast to microglia, persists for up to 42 d. Furthermore, after phagocytosis of myelin in vitro, bone marrow-derived macrophages are much more susceptible to apoptotic and necrotic cell death than CNS microglia, which is mirrored in vivo with apoptotic TUNEL-positive cells of infiltrating macrophage origin. This work suggests that microglia play a major role in the early response to SCI, by phagocytosing damaged and degenerating tissue, processing phagocytic material efficiently, and remaining viable. Later, macrophages of peripheral origin contribute predominantly to phagocytosis but are less efficient at processing CNS debris, and their death, in situ, may contribute to the secondary damage after CNS injury.

Microglial phagocytosis of live neurons

Microglia, the brain's professional phagocytes, can remove dead and dying neurons as well as synapses and the processes of live neurons. However, we and others have recently shown that microglia can also execute neuronal death by phagocytosing stressed-but-viable neurons - a process that we have termed phagoptosis. In this Progress article, we discuss evidence suggesting that phagoptosis may contribute to neuronal loss during brain development, inflammation, ischaemia and neurodegeneration.