Origin of microglia

Monocytes release cystatin F dimer to associate with Aβ and aggravate amyloid pathology and cognitive deficits in Alzheimer's disease

BACKGROUND

Understanding the molecular mechanisms of Alzheimer's disease (AD) has important clinical implications for guiding therapy. Impaired amyloid beta (Aβ) clearance is critical in the pathogenesis of sporadic AD, and blood monocytes play an important role in Aβ clearance in the periphery. However, the mechanism underlying the defective phagocytosis of Aβ by monocytes in AD remains unclear.

METHODS

Initially, we collected whole blood samples from sporadic AD patients and isolated the monocytes for RNA sequencing analysis. By establishing APP/PS1 transgenic model mice with monocyte-specific cystatin F overexpression, we assessed the influence of monocyte-derived cystatin F on AD development. We further used a nondenaturing gel to identify the structure of the secreted cystatin F in plasma. Flow cytometry, enzyme-linked immunosorbent assays and laser scanning confocal microscopy were used to analyse the internalization of Aβ by monocytes. Pull down assays, bimolecular fluorescence complementation assays and total internal reflection fluorescence microscopy were used to determine the interactions and potential interactional amino acids between the cystatin F protein and Aβ. Finally, the cystatin F protein was purified and injected via the tail vein into 5XFAD mice to assess AD pathology.

RESULTS

Our results demonstrated that the expression of the cystatin F protein was specifically increased in the monocytes of AD patients. Monocyte-derived cystatin F increased Aβ deposition and exacerbated cognitive deficits in APP/PS1 mice. Furthermore, secreted cystatin F in the plasma of AD patients has a dimeric structure that is closely related to clinical signs of AD. Moreover, we noted that the cystatin F dimer blocks the phagocytosis of Aβ by monocytes. Mechanistically, the cystatin F dimer physically interacts with Aβ to inhibit its recognition and internalization by monocytes through certain amino acid interactions between the cystatin F dimer and Aβ. We found that high levels of the cystatin F dimer protein in blood contributed to amyloid pathology and cognitive deficits as a risk factor in 5XFAD mice.

CONCLUSIONS

Our findings highlight that the cystatin F dimer plays a crucial role in regulating Aβ metabolism via its peripheral clearance pathway, providing us with a potential biomarker for diagnosis and potential target for therapeutic intervention.

Interrogations of single-cell RNA splicing landscapes with SCASL define new cell identities with physiological relevance

RNA splicing shapes the gene regulatory programs that underlie various physiological and disease processes. Here, we present the SCASL (single-cell clustering based on alternative splicing landscapes) method for interrogating the heterogeneity of RNA splicing with single-cell RNA-seq data. SCASL resolves the issue of biased and sparse data coverage on single-cell RNA splicing and provides a new scheme for classifications of cell identities. With previously published datasets as examples, SCASL identifies new cell clusters indicating potentially precancerous and early-tumor stages in triple-negative breast cancer, illustrates cell lineages of embryonic liver development, and provides fine clusters of highly heterogeneous tumor-associated CD4 and CD8 T cells with functional and physiological relevance. Most of these findings are not readily available via conventional cell clustering based on single-cell gene expression data. Our study shows the potential of SCASL in revealing the intrinsic RNA splicing heterogeneity and generating biological insights into the dynamic and functional cell landscapes in complex tissues.

Role of the NF-kB/parkin/vegfr-1 pathway associated with hypoxic-ischemic insult in germinal matrix samples of newborn infants

OBJECTIVE

Given the high proliferative activity of germinal matrix and its direct correlation with hypoxemia, it is necessary to investigate the possible molecular regulation pathways, to understand the existing clinical relationship between the hypoxic-ischemic insult and the biomarkers NF-kB, AKT-3, Parkin, TRK-C and VEGFR-1.

METHODS

A hundred and eighteen germinal matrix samples of the central nervous system of patients who died in the first 28 days of life were submitted to histological and immunohistochemistry analysis to identify the tissue immunoexpression of those biomarkers related to asphyxia, prematurity, and death events within 24h.

RESULTS

A significantly increased tissue immunoexpression of NF-kB, AKT-3 and Parkin was observed in the germinal matrix of preterm infants. In addition, significantly decreased tissue immunoexpression of VEGFR-1 and NF-kB was observed in patients who experienced asphyxia followed by death within 24 hours.

CONCLUSIONS

The results suggest a direct involvement between the hypoxic-ischemic insult and NF-kB and VEGFR-1 markers since a decreased immunoexpression of these biomarkers was observed in asphyxiated patients. Furthermore, it is suggested that there was not enough time for VEGFR-1 to be transcribed, translated and expressed on the surface of the plasma membrane. This temporality can be observed in the relationship between NF-kB expression and the survival time of individuals who died within 24 hours, suggesting that this factor is essential for the production of VEGFR-1 and, therefore, to carry out the necessary remodeling effect to neovascularize the affected region.

Contextual fear conditioning regulates synapse-related gene transcription in mouse microglia

Microglia have been suggested to be involved in the underlying mechanism of conditional fear memory formation by regulating inflammatory cytokines. However, the mechanism linking microglia and neuronal activity related to fear conditioning remains unclear. This study characterized the transcription profile of microglia in a fear memory conditional mouse model. Compared with those in control mice microglia, the most significantly induced genes were synapse-related, whereas immune-related genes were reduced due to fear memory consolidation. Whilst the increased expression of synapse-related genes was reversed after fear memory extinction, that of immunological genes was not, strongly suggesting a connection between microglia, neurons, and a dysregulated immune response following contextual fear conditioning. Furthermore, in the hippocampal microglia, we found that the expression of neurotransmitter release regulators, γ-aminobutyric acid (GABA) receptor GABRB3 and synapsin 1/2, increased under fear memory consolidation and restored (decreased) after extinction. In addition, compared with the transcription profile in peripheral monocytes, few overlapping genes were not enriched in biological processes. Taken together, the identified conditional fear stress-induced changes in mouse microglial transcription profiles suggest that microglia-neuron communication mediates contextual fear conditioning.

Microglia and Microglia-Like Cells: Similar but Different

Microglia are the tissue-resident macrophages of the central nervous parenchyma. In mammals, microglia are thought to originate from yolk sac precursors and posteriorly maintained through the entire life of the organism. However, the contribution of microglial cells from other sources should also be considered. In addition to “true” or “bona-fide” microglia, which are of embryonic origin, the so-called “microglia-like cells” are hematopoietic cells of bone marrow origin that can engraft the mature brain mainly under pathological conditions. These cells implement great parts of the microglial immune phenotype, but they do not completely adopt the “true microglia” features. Because of their pronounced similarity, true microglia and microglia-like cells are usually considered together as one population. In this review, we discuss the origin and development of these two distinct cell types and their differences. We will also review the factors determining the appearance and presence of microglia-like cells, which can vary among species. This knowledge might contribute to the development of therapeutic strategies aiming at microglial cells for the treatment of diseases in which they are involved, for example neurodegenerative disorders like Alzheimer’s and Parkinson’s diseases.

Modulation of Glial Function in Health, Aging, and Neurodegenerative Disease

In the central nervous system (CNS), glial cells, such as microglia and astrocytes, are normally associated with support roles including contributions to energy metabolism, synaptic plasticity, and ion homeostasis. In addition to providing support for neurons, microglia and astrocytes function as the resident immune cells in the brain. The glial function is impacted by multiple aspects including aging and local CNS changes caused by neurodegeneration. During aging, microglia and astrocytes display alterations in their homeostatic functions. For example, aged microglia and astrocytes exhibit impairments in the lysosome and mitochondrial function as well as in their regulation of synaptic plasticity. Recent evidence suggests that glia can also alter the pathology associated with many neurodegenerative disorders including Alzheimer's disease (AD) and Parkinson's disease (PD). Shifts in the microbiome can impact glial function as well. Disruptions in the microbiome can lead to aberrant microglial and astrocytic reactivity, which can contribute to an exacerbation of disease and neuronal dysfunction. In this review, we will discuss the normal physiological functions of microglia and astrocytes, summarize novel findings highlighting the role of glia in aging and neurodegenerative diseases, and examine the contribution of microglia and astrocytes to disease progression.

[Central Nervous System Developmental Regulation of Microglia via Cytokines and Chemokines]

Microglia are immune cells resident in the central nervous system (CNS). It has been gradually clarified that microglia play various roles at the developmental stage of the CNS. From embryonic to early postnatal age, microglia remove apoptotic cells by phagocytosis and refine the neural circuits by synaptic pruning. In addition, microglia promote the proliferation and differentiation of neural stem cells by releasing physiologically active substances. Our group has focused on the physiological actions of microglia via cytokines and chemokines at the early postnatal developmental stage. We found that a large number of activated microglia accumulate in the early postnatal subventricular zone (SVZ). We demonstrated that the these SVZ microglia facilitate neurogenesis and oligodendrogenesis via inflammatory cytokines including IL-1β, TNFα, IL-6, IFNγ. We have also found that microglia regulate the functional maturation of the blood brain barrier (BBB) and identified the cytokines and chemokines involved in the effects of microglia. These findings indicate that microglia are physiologically more important than ever thought to reveal robust brain functions. Furthermore, the new mode of microglial action may lead to the discovery of drug targets of the incurable CNS diseases.

Microglial signatures and their role in health and disease

Microglia are the primary innate immune cells in the CNS. In the healthy brain, they exhibit a unique molecular homeostatic 'signature', consisting of a specific transcriptional profile and surface protein expression pattern, which differs from that of tissue macrophages. In recent years, there have been a number of important advances in our understanding of the molecular signatures of homeostatic microglia and disease-associated microglia that have provided insight into how these cells are regulated in health and disease and how they contribute to the maintenance of the neural environment.

Retinal microglia - A key player in healthy and diseased retina

Microglia, the resident immune cells of the brain and retina, are constantly engaged in the surveillance of their surrounding neural tissue. During embryonic development they infiltrate the retinal tissues and participate in the phagocytosis of redundant neurons. The contribution of microglia in maintaining the purposeful and functional histo-architecture of the adult retina is indispensable. Within the retinal microenvironment, robust microglial activation is elicited by subtle changes caused by extrinsic and intrinsic factors. When there is a disturbance in the cell-cell communication between microglia and other retinal cells, for example in retinal injury, the activated microglia can manifest actions that can be detrimental. This is evidenced by activated microglia secreting inflammatory mediators that can further aggravate the retinal injury. Microglial activation as a harbinger of a variety of retinal diseases is well documented by many studies. In addition, a change in the microglial phenotype which may be associated with aging, may predispose the retina to age-related diseases. In light of the above, the focus of this review is to highlight the role played by microglia in the healthy and diseased retina, based on findings of our own work and from that of others.

Biology of Microglia in the Developing Brain

Microglia exist in different morphological forms in the developing brain. They show a small cell body with scanty cytoplasm with many branching processes in the grey matter of the developing brain. However, in the white matter such as the corpus callosum where the unmyelinated axons are loosely organized, they appear in an amoeboid form having a round cell body endowed with copious cytoplasm rich in organelles. The amoeboid cells eventually transform into ramified microglia in the second postnatal week when the tissue becomes more compact with the onset of myelination. Microglia serve as immunocompetent macrophages that act as neuropathology sensors to detect and respond swiftly to subtle changes in the brain tissues in pathological conditions. Microglial functions are broadly considered as protective in the normal brain development as they phagocytose dead cells and sculpt neuronal connections by pruning excess axons and synapses. They also secrete a number of trophic factors such as insulin-like growth factor-1 and transforming growth factor-β among many others that are involved in neuronal and oligodendrocyte survival. On the other hand, microglial cells when activated produce a plethora of molecules such as proinflammatory cytokines, chemokines, reactive oxygen species, and nitric oxide that are implicated in the pathogenesis of many pathological conditions such as epilepsy, cerebral palsy, autism, and perinatal hypoxic-ischemic brain injury. Although many studies have investigated the origin and functions of the microglia in the developing brain, in-depth in vivo studies along with analysis of their transcriptome and epigenetic changes need to be undertaken to elucidate their full potential be it protective or neurotoxic. This would lead to a better understanding of their roles in the healthy and diseased developing brain and advancement of therapeutic strategies to target microglia-mediated neurotoxicity.

Inhibiting purinergic P2X7 receptors with the antagonist brilliant blue G is neuroprotective in an intranigral lipopolysaccharide animal model of Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder, which is characterized by the selective and progressive death of dopaminergic (DA) neurons in the substantia nigra. Increasing evidence suggests that inflammation is important in the degeneration of DA neurons. The purinergic receptor subtype P2X7 receptor (P2X7R) is key in the activation and proliferation of microglia. The present study aimed to examine whether inhibiting purinergic P2X7 receptors is neuroprotective in a rat model of PD, specifically via inhibiting p38 mitogen-activated protein kinase (MAPK). In an intranigral lipopolysaccharide (LPS) rat model of PD, immunohistochemical analysis revealed enhanced expression of P2X7R was observed in microglia. The administration of the P2X7R antagonist, brilliant blue G (BBG), reduced activation of the microglia and the loss of nigral DA neurons. In addition, immunohistochemistry and western blot analysis revealed the phosphorylation level of p38 MAPK increased in the microglia of the LPS-injected rats, which was inhibited by BBG treatment. The p38 MAPK inhibitor, SB203580, reduced microglial activation and the loss of DA neurons. Thus, these findings suggested that inhibition of P2X7R by BBG attenuated microglial activation and the loss of substantia nigra DA neurons via p38 MAPK in the rat LPS model of PD.

Unraveling the Molecular Details Involved in the Intimate Link between the Immune and Neuroendocrine Systems

During systemic infections, the immune system can signal the brain and act on different neuronal circuits via soluble molecules, such as proinflammatory cytokines, that act on the cells forming the blood-brain barrier and the circumventricular organs. These activated cells release prostaglandin of the E2 type (PGE2), which is the endogenous ligand that triggers the pathways involved in the control of autonomic functions necessary to restore homeostasis and provide inhibitory feedback to innate immunity. Among these neurophysiological functions, activation of the circuits that control the plasma release of glucocorticoids is probably the most critical to the survival of the host in the presence of pathogens. This review revisits this issue and describes in depth the molecular details (including the emerging role of Toll-like receptors during inflammation) underlying the influence of circulating inflammatory molecules on the cerebral tissue, focusing on their contribution in the synthesis and action PGE2 in the brain. We also provide an innovative view supporting the concept of “fast and delayed response” involving the same ligands but different groups of cells, signal transduction pathways, and target genes.

The Indispensable Roles of Microglia and Astrocytes during Brain Development

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

Regulation of brain microglia by female gonadal steroids

Microglial cells are the primary mediators of the CNS immune defense system and crucial for shaping inflammatory responses. They represent a highly dynamic cell population which is constantly moving and surveying their environment. Acute brain damage causes a local attraction and activation of this immune cell type which involves neuron-to-glia and glia-to-glia interactions. The prevailing view attributes microglia a "negative" role such as defense and debris elimination. More topical studies also suggest a protective and "positive" regulatory function. Estrogens and progestins exert anti-inflammatory and neuroprotective effects in the CNS in acute and chronic brain diseases. Recent work revealed that microglial cells express subsets of classical and non-classical estrogen and progesterone receptors in a highly dynamic way. In this review article, we would like to stress the importance of microglia for the spreading of neural damage during hypoxia, their susceptibility to functional modulation by sex steroids, the potency of sex hormones to switch microglia from a pro-inflammatory M1 to neuroprotective M2 phenotype, and the regulation of pro- and anti-inflammatory properties including the inflammasome. We will further discuss the possibility that the neuroprotective action of sex steroids in the brain involves an early and direct modulation of local microglia cell function. This article is part of a Special Issue entitled 'Sex steroids and brain disorders'.

Aggravation of Alzheimer's disease due to the COX-2-mediated reciprocal regulation of IL-1β and Aβ between glial and neuron cells

Alzheimer's disease (AD) is the most common form of dementia and displays the characteristics of chronic neurodegenerative disorders; amyloid plaques (AP) that contain amyloid β-protein (Aβ) accumulate in AD, which is also characterized by tau phosphorylation. Epidemiological evidence has demonstrated that long-term treatment with nonsteroidal anti-inflammatory drugs (NSAIDs) markedly reduces the risk of AD by inhibiting the expression of cyclooxygenase 2 (COX-2). Although the levels of COX-2 and its metabolic product prostaglandin (PG)E2 are elevated in the brain of AD patients, the mechanisms for the development of AD remain unknown. Using human- or mouse-derived glioblastoma and neuroblastoma cell lines as model systems, we delineated the signaling pathways by which COX-2 mediates the reciprocal regulation of interleukin-1β (IL-1β) and Aβ between glial and neuron cells. In glioblastoma cells, COX-2 regulates the synthesis of IL-1β in a PGE2 -dependent manner. Moreover, COX-2-derived PGE2 signals the activation of the PI3-K/AKT and PKA/CREB pathways via cyclic AMP; these pathways transactivate the NF-κB p65 subunit via phosphorylation at Ser 536 and Ser 276, leading to IL-1β synthesis. The secretion of IL-1β from glioblastoma cells in turn stimulates the expression of COX-2 in human or mouse neuroblastoma cells. Similar regulatory mechanisms were found for the COX-2 regulation of BACE-1 expression in neuroblastoma cells. More importantly, Aβ deposition mediated the inflammatory response of glial cells via inducing the expression of COX-2 in glioblastoma cells. These findings not only provide new insights into the mechanisms of COX-2-induced AD but also initially define the therapeutic targets of AD.

Bidirectional tuning of microglia in the developing brain: from neurogenesis to neural circuit formation

The developing brain employs multi-step processes to construct neural circuitry. Recent studies have highlighted that microglia, traditionally known to be the resident immune cells in the brain, have essential roles in these processes, which range from neurogenesis to establishing synaptic connections. Microglia play bidirectional roles for maintaining proper circuitry: eliminating unnecessary cells, axons, and synapses, while supporting the neighboring ones. Although these processes are performed in different parts of the neuron, similar molecular mechanisms are possibly involved. This paper reviews recent progress on the knowledge of the roles of microglia in brain development, and further discusses the application of this knowledge in therapies for brain disorders and injuries.

Dissociation of innate immune responses in microglia infected with Listeria monocytogenes

Microglia, the innate immune cells of the brain, plays a central role in cerebral listeriosis. Here, we present evidence that microglia control Listeria infection differently than macrophages. Infection of primary microglial cultures and murine cell lines with Listeria resulted in a dual function of the two gene expression programmes involved in early and late immune responses in macrophages. Whereas the bacterial gene hly seems responsible for both transcriptional programmes in macrophages, Listeria induces in microglia only the tumor necrosis factor (TNF)-regulated transcriptional programme. Listeria also represses in microglia the late immune response gathered in two clusters, microbial degradation, and interferon (IFN)-inducible genes. The bacterial gene actA was required in microglia to induce TNF-regulated responses and to repress the late response. Isolation of microglial phagosomes revealed a phagosomal environment unable to destroy Listeria. Microglial phagosomes were also defective in several signaling and trafficking components reported as relevant for Listeria innate immune responses. This transcriptional strategy in microglia induced high levels of TNF-α and monocyte chemotactic protein-1 and low production of other neurotoxic compounds such as nitric oxide, hydrogen peroxide, and Type I IFNs. These cytokines and toxic microglial products are also released by primary microglia, and this cytokine and chemokine cocktail display a low potential to trigger neuronal apoptosis. This overall bacterial strategy strongly suggests that microglia limit Listeria inflammation pattern exclusively through TNF-mediated responses to preserve brain integrity.

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

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

Microglia-mediated neuroinflammation is an amplifier of virus-induced neuropathology

Microglia, the major resident immune cells in the central nervous system (CNS) are considered as the key cellular mediators of neuroinflammatory processes. In the past few years, microglial research has become a main focus in cellular neuroimmunology and neuroinflammation. Chronic/remitting neurological disease such as multiple sclerosis (MS) has long been considered an inflammatory autoimmune disease with the infiltration of peripheral myelin-specific T cells into the CNS. With the rapid advancement in the field of microglia and astrocytic neurobiology, the term neuroinflammation progressively started to denote chronic CNS cell-specific inflammation in MS. The direct glial responses in MS are different from conventional peripheral immune responses. This review attempts to summarize current findings of neuroinflammatory responses within the CNS by direct infection of neural cells by mouse hepatitis virus (MHV) and the mechanisms by which glial cell responses ultimately contribute to the neuropathology on demyelination. Microglia can be persistently infected by MHV. Microglial activation and phagocytosis are recognized to be critically important in the pathogenesis of demyelination. Emerging evidence for the pathogenic role of microglia and the activation of inflammatory pathways in these cells in MHV infection supports the concept that microglia induced neuroinflammation is an amplifier of virus-induced neuropathology.

Factors regulating microglia activation

Microglia are resident macrophages of the central nervous system (CNS) that display high functional similarities to other tissue macrophages. However, it is especially important to create and maintain an intact tissue homeostasis to support the neuronal cells, which are very sensitive even to minor changes in their environment. The transition from the “resting” but surveying microglial phenotype to an activated stage is tightly regulated by several intrinsic (e.g., Runx-1, Irf8, and Pu.1) and extrinsic factors (e.g., CD200, CX₃CR1, and TREM2). Under physiological conditions, minor changes of those factors are sufficient to cause fatal dysregulation of microglial cell homeostasis and result in severe CNS pathologies. In this review, we discuss recent achievements that gave new insights into mechanisms that ensure microglia quiescence.

Microglia activation in the extremely preterm human brain

BACKGROUND

The periventricular white matter (PVWM) of the immature preterm brain is selectively vulnerable to a spectrum of injury. Although essential for normal brain development, the presence of resident microglia may exacerbate PVWM injury.

METHODS

We used immunohistochemistry to investigate microglia profile in human preterm noninjured control brains and in brains with evidence of germinal matrix hemorrhage/intraventricular hemorrhage (GMH/IVH), with median gestational age (GA) of 24.1 and 25.4 wk, respectively.

RESULTS

The number of microglia in the PVWM was higher than the other brain regions in both the control and GMH/IVH groups. Microglial density increased further in the PVWM of GMH/IVH brains, regardless of hemorrhage severity and despite normal macroscopic and imaging appearances to the PVWM. This was due to an increase in activated Iba1/CD68- and not Iba/CD45-immunopositive microglia. However, there were very few CD68/Ki67 colocalized cells, suggesting that the source of this increase may be due to a quick transformation of CD45-immunopositive hematopoietic microglia into CD68-immunopositive microglia. There was also increased apoptosis in the PVWM of all cases of GMH/IVH, with axonal injury and increased tumor necrosis factor-α (TNF-α) expression evident in the most severe cases.

CONCLUSION

Isolated GMH/IVH may influence ongoing brain development, with a significant role played by microglial activation.

Increased expression of CD200 on circulating CD11b+ monocytes in patients with neovascular age-related macular degeneration

OBJECTIVE

Dysregulation of retinal microglial activity has been implicated in the pathogenesis of neovascular age-related macular degeneration. Microglia activity can be regulated through the membrane protein CD200 and its corresponding receptor, the CD200 receptor (CD200R). Because both the ligand and the receptor are expressed on a broad spectrum of cell types, we set out to study the expression of CD200 and CD200R on CD11b+ monocytes, granulocytes, and subsets of T lymphocytes.

DESIGN

Prospective, case-control study.

PARTICIPANTS

The study population consisted of 62 patients with neovascular age-related macular degeneration (AMD) and 44 age-matched controls without AMD.

METHODS

The participants were aged 60 years or older, had no history of immune dysfunction or cancer, and were not receiving immune-modulating therapy. All participants were subjected to a structured interview, and detailed retinal imaging was performed: fundus autofluorescence imaging, digital color fundoscopy, and spectral-domain optical coherence tomography. Fluorescein and indocyanine green angiography were performed in patients with suspected neovascular AMD. Visual acuity was measured in both eyes. Fresh venous blood was obtained and stained with monoclonal antibodies and analyzed using flow cytometry within 6 hours of phlebotomy.

MAIN OUTCOME MEASURES

The percentage of CD11b+ monocytes, granulocytes, and CD4+/CD8+ T lymphocytes positive for CD200 or CD200R in patients and controls, respectively.

RESULTS

Patients with neovascular AMD had a higher percentage of CD11b+CD200+ monocytes and CD200+ monocytes compared with controls. Multiple regression analysis revealed that the intergroup differences observed were independent of age. Moreover, an age-related increment in CD200 expression on monocytes was observed in controls with healthy eyes, but not in patients with neovascular AMD. We did not find any differences in CD200 and CD200R expression between patients with subretinal fibrosis and patients without subretinal fibrosis.

CONCLUSIONS

The surface expression of CD200 on circulating CD11b+ monocytes was found to be increased in patients with neovascular AMD compared with controls with healthy eyes. This novel finding supports the notion that altered regulation of the inflammatory response plays an integral role in the pathogenesis of AMD.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Roles of activated microglia in hypoxia induced neuroinflammation in the developing brain and the retina

Amoeboid microglial cells (AMCs) in the developing brain display surface receptors and antigens shared by the monocyte-derived tissue macrophages. Activation of AMCs in the perinatal brain has been associated with periventricular white matter damage in hypoxic-ischemic conditions. The periventricular white matter, where the AMCs preponderate, is selectively vulnerable to hypoxia as manifested by death of premyelinating oligodendrocytes and degeneration of axons leading to neonatal mortality and long-term neurodevelopmental deficits. AMCs respond vigorously to hypoxia by producing excess amounts of inflammatory cytokines e.g. the tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) along with glutamate, nitric oxide (NO) and reactive oxygen species which collectively cause oligodendrocyte death, axonal degeneration as well as disruption of the immature blood brain barrier. A similar phenomenon is observed in the hypoxic developing cerebellum in which activated AMCs induced Purkinje neuronal death through production of TNF-α and IL-1β via their respective receptors. Hypoxia is also implicated in retinopathy of prematurity in which activation of AMCs has been shown to cause retinal ganglion cell death through production of TNF-α and IL-1β and NO. Because AMCs play a pivotal role in hypoxic injuries in the developing brain affecting both neurons and oligodendrocytes, a fuller understanding of the underlying molecular mechanisms of microglial activation under such conditions would be desirable for designing of a novel therapeutic strategy for management of hypoxic damage.

Slitrks as emerging candidate genes involved in neuropsychiatric disorders

Slitrks are a family of structurally related transmembrane proteins belonging to the leucine-rich repeat (LRR) superfamily. Six family members exist (Slitrk1-6) and all are highly expressed in the central nervous system (CNS). Slitrks have been implicated in mediating basic neuronal processes, ranging from neurite outgrowth and dendritic elaboration to neuronal survival. Recent studies in humans and genetic mouse models have led to the identification of Slitrks as candidate genes that might be involved in the development of neuropsychiatric conditions, such as obsessive compulsive spectrum disorders and schizophrenia. Although these system-level approaches have suggested that Slitrks play prominent roles in CNS development, key questions remain regarding the molecular mechanisms through which they mediate neuronal signaling and connectivity.

Interactions between prostaglandins, leukotrienes and HIV-1: possible implications for the central nervous system

In HIV-1-infected individuals, there is often discordance between viremia in peripheral blood and viral load found in the central nervous system (CNS). Although the viral burden is often lower in the CNS compartment than in the plasma, neuroinflammation is present in most infected individuals, albeit attenuated by the current combined antiretroviral therapy. The HIV-1-associated neurological complications are thought to result not only from direct viral replication, but also from the subsequent neuroinflammatory processes. The eicosanoids - prostanoids and leukotrienes - are known as potent inflammatory lipid mediators. They are often present in neuroinflammatory diseases, notably HIV-1 infection. Their exact modulatory role in HIV-1 infection is, however, still poorly understood, especially in the CNS compartment. Nonetheless, a handful of studies have provided evidence as to how these lipid mediators can modulate HIV-1 infection. This review summarizes findings indicating how eicosanoids may influence the progression of neuroAIDS.

Analysis of behavior and trafficking of dendritic cells within the brain during toxoplasmic encephalitis

Under normal conditions the immune system has limited access to the brain; however, during toxoplasmic encephalitis (TE), large numbers of T cells and APCs accumulate within this site. A combination of real time imaging, transgenic reporter mice, and recombinant parasites allowed a comprehensive analysis of CD11c⁺ cells during TE. These studies reveal that the CNS CD11c⁺ cells consist of a mixture of microglia and dendritic cells (DCs) with distinct behavior associated with their ability to interact with parasites or effector T cells. The CNS DCs upregulated several chemokine receptors during TE, but none of these individual receptors tested was required for migration of DCs into the brain. However, this process was pertussis toxin sensitive and dependent on the integrin LFA-1, suggesting that the synergistic effect of signaling through multiple chemokine receptors, possibly leading to changes in the affinity of LFA-1, is involved in the recruitment/retention of DCs to the CNS and thus provides new insights into how the immune system accesses this unique site.

Toxoplasmic encephalitis (TE), caused by the protozoan parasite Toxoplasma gondii, can be potentially life threatening especially in immuno-compromised individuals. Immune cells including dendritic cells have been shown to accumulate in the brain during chronic toxoplasmosis; however, little is known about their function, their behavior in vivo, and the mechanisms by which they migrate into the brain. In the present studies, we utilize a combination of real time imaging, transgenic reporter mice, and recombinant parasites to reveal the distinct behavior and morphologies of dendritic cells within the brain and their ability to interact with parasites and effector T cells during TE. The CNS DCs were also found to exhibit a unique chemokine receptor expression pattern during infection, and the migration of DCs into the brain was mediated through a pertussis toxin (which blocks signaling downstream of several chemokine receptors) sensitive process and dependent on the integrin LFA-1. There is currently a poor understanding of the events that lead to DC recruitment to the CNS during inflammation in general, and our studies provide new insights into the mechanisms by which antigen-presenting cells gain access to the brain during infection.

Interleukin-10 overexpression does not synergize with the neuroprotective action of RGD-containing vectors after postnatal brain excitotoxicity but modulates the main inflammatory cell responses

Antiinflammatory cytokines such as interleukin-10 (IL-10) have been used to modulate and terminate inflammation and provide neuroprotection. Recently, we reported that the modular recombinant transfection vector NLSCt is an efficient tool for transgene overexpression in vivo, which induces neuroprotection as a result of its RGD-mediated integrin-interacting capacity. We here sought to evaluate the putative synergic neuroprotective action exerted by IL-10 overexpression using NLSCt as a transfection vector after an excitotoxic injury to the postnatal rat brain. For this purpose, lesion volume, neurodegeneration, astroglial and microglial responses, neutrophil infiltration, and proinflammatory cytokine production were analyzed at several survival times after intracortical NMDA injection in postnatal day 9 rats, followed by injection of NLSCt combined with the IL-10 gene, a control transgene, or saline vehicle solution. Our results show no combined neuroprotective effect between RGD-interacting vectors and IL-10 gene therapy; instead, IL-10 overexpression using NLSCt as transfection vector increased lesion volume and neuronal degeneration at 12 hr and 3 days postlesion. In parallel, NLSCt/IL-10 treated animals displayed increased density of neutrophils and microglia/macrophages, and a reduced astroglial content of GFAP and vimentin. Moreover, NLSCt/IL-10 treated animals did not show any variation in interleukin-1β or tumor necrosis factor-α expression but a slight increase in interleukin-6 content at 7 days postlesion. In conclusion, overexpression of IL-10 by using NLSCt transfection vector did not synergistically neuroprotect the excitotoxically damaged postnatal rat brain but induced changes in the astroglial and microglial and inflammatory cell response.

The yin and yang of microglia

Microglia, the resident immune cells of the mammalian central nervous system (CNS), play a pivotal role in both physiological and pathological conditions such as the restoration of CNS integrity and the progression of neurodegenerative disorders. Extensive data have been published that describe neuroinflammation by microglial activation to have detrimental consequences on the developing and mature brain. On the other hand, a properly directed and limited inflammatory response is known to be a natural healing process after an insult in several other tissues. Thus, it is not surprising that research results illustrating benefits of neuroinflammation have been emerging over the past decade. Inflammation-mediated benefits for CNS outcomes include mechanisms such as neuroprotection, mobilization of neural precursors for repair, remyelination and axonal regeneration. Here, we review data that highlight the dual aspects of microglia with a focus on the developing brain, i.e. as aggressors potentiating damage and as helpers in the recovery process following CNS damage.

Therapeutic targets for neuroprotection and/or enhancement of functional recovery following traumatic brain injury

Traumatic brain injury (TBI) is a significant public health concern. The number of injuries that occur each year, the cost of care, and the disabilities that can lower the victim's quality of life are all driving factors for the development of therapy. However, in spite of a wealth of promising preclinical results, clinicians are still lacking a therapy. The use of preclinical models of the primary mechanical trauma have greatly advanced our knowledge of the complex biochemical sequela that follow. This cascade of molecular, cellular, and systemwide changes involves plasticity in many different neurochemical systems, which represent putative targets for remediation or attenuation of neuronal injury. The purpose of this chapter is to highlight some of the promising molecular and cellular targets that have been identified and to provide an up-to-date summary of the development of therapeutic compounds for those targets.

Hematopoietic origin of pathological grooming in Hoxb8 mutant mice

Mouse Hoxb8 mutants show unexpected behavior manifested by compulsive grooming and hair removal, similar to behavior in humans with the obsessive-compulsive disorder spectrum disorder trichotillomania. As Hox gene disruption often has pleiotropic effects, the root cause of this behavioral deficit was unclear. Here we report that, in the brain, Hoxb8 cell lineage exclusively labels bone marrow-derived microglia. Furthermore, transplantation of wild-type bone marrow into Hoxb8 mutant mice rescues their pathological phenotype. It has been suggested that the grooming dysfunction results from a nociceptive defect, also exhibited by Hoxb8 mutant mice. However, bone marrow transplant experiments and cell type-specific disruption of Hoxb8 reveal that these two phenotypes are separable, with the grooming phenotype derived from the hematopoietic lineage and the sensory defect derived from the spinal cord cells. Immunological dysfunctions have been associated with neuropsychiatric disorders, but the causative relationships are unclear. In this mouse, a distinct compulsive behavioral disorder is associated with mutant microglia.

Intravitreous delivery of the corticosteroid fluocinolone acetonide attenuates retinal degeneration in S334ter-4 rats

PURPOSE

To study the neuroprotective properties of low-dose, sustained-release intravitreous fluocinolone acetonide (FA) in transgenic S334ter-4 rats.

METHODS

S334ter-4 rats aged 4 weeks were divided into four groups: 0.5 microg/d FA-loaded intravitreous drug delivery implant (IDDI); 0.2 microg/d FA-loaded IDDI; inactive IDDI; and unoperated controls. Electroretinography (ERG) was performed before surgery and every 2 weeks after surgery for 8 weeks. When the rats were 12 weeks of age, outer nuclear layer (ONL) and inner nuclear layer (INL) thicknesses were measured. Microglial cell counts were obtained from retinal wholemounts labeled for Iba-1.

RESULTS

At the end of the study, unoperated and inactive IDDI-implanted rats demonstrated 50% to 60% reductions in ERG amplitudes compared with those recorded at 4 weeks (P < 0.001 for both groups). FA 0.2-microg/d animals demonstrated 15% amplitude attenuation, while FA 0.5-microg/d animals showed 30% reduction. ONL thickness in FA 0.2-microg/d-treated eyes was 25.8% +/- 2.3% higher than in control group eyes (P < 0.001) and 30.0% +/- 2.1% higher than in inactive IDDI-implanted eyes (P < 0.001). In FA 0.5-microg/d-treated eyes, ONL thickness was 22.4% +/- 2.8% higher than in control group eyes (P < 0.001) and 22.3% +/- 3.7% higher than in inactive IDDI-implanted eyes (P < 0.01). No statistically significant difference was observed between the two control groups. No statistically significant difference between the two FA-treated groups was found. FA-treated groups demonstrated significantly fewer activated microglial cells than control groups.

CONCLUSIONS

Chronic intravitreous infusion of FA preserves ONL cell morphology and ERG a- and b-wave amplitudes and reduces retinal neuroinflammation in S334ter rats. Based on these findings, the synthetic corticosteroid FA may promise a therapeutic role in patients with retinal degeneration.

Sphingosine kinase 1 regulates the expression of proinflammatory cytokines and nitric oxide in activated microglia

Microglial activation has been implicated as one of the causative factors for neuroinflammation in various neurodegenerative diseases. The sphingolipid metabolic pathway plays an important role in inflammation, cell proliferation, survival, chemotaxis, and immunity in peripheral macrophages. In this study, we demonstrate that sphingosine kinase1 (SphK1), a key enzyme of the sphingolipid metabolic pathway, and its receptors are expressed in the mouse BV2 microglial cells and SphK1 alters the expression and production of proinflammatory cytokines and nitric oxide in microglia treated with lipopolysaccharide (LPS). LPS treatment increased the SphK1 mRNA and protein expression in microglia as revealed by the RT-PCR, Western blot and immunofluorescence. Suppression of SphK1 by its inhibitor, N, N Dimethylsphingosine (DMS), or siRNA resulted in decreased mRNA expression of TNF-alpha, IL-1beta, and iNOS and release of TNF-alpha and nitric oxide (NO) in LPS-activated microglia. Moreover, addition of sphingosine 1 phosphate (S1P), a breakdown product of sphingolipid metabolism, increased the expression levels of TNF-alpha, IL-1beta and iNOS and production of TNF-alpha and NO in activated microglia. Hence to summarize, suppression of SphK1 in activated microglia inhibits the production of proinflammatory cytokines and NO and the addition of exogenous S1P to activated microglia enhances their inflammatory responses. Since the chronic proinflammatory cytokine production by microglia has been implicated in neuroinflammation, modulation of SphK1 and S1P in microglia could be looked upon as a future potential therapeutic method in the control of neuroinflammation in neurodegenerative diseases.

Inhibition of inducible NO synthase, cyclooxygenase-2 and interleukin-1beta by torilin is mediated by mitogen-activated protein kinases in microglial BV2 cells

BACKGROUND AND PURPOSE

Traditionally, the stem and root bark of Ulmus davidiana var. japonica (Ulmaceae) have been known to be anti-inflammatory in Korea. Anti-inflammatory effects of torilin, isolated from this plant and the underlying mechanisms were examined by using lipopolysaccharide (LPS)-stimulated microglial BV2 cells.

EXPERIMENTAL APPROACH

The cells were treated with torilin prior to LPS exposure and the effects on pro-inflammatory enzymes, inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2), and a pro-inflammatory cytokine, interleukin-1beta (IL-1beta) were analysed by RT-PCR, Western blot or elisa. To reveal the mechanism of action of torilin we investigated the involvement of mitogen-activated protein kinase (MAPK) cascades and their downstream transcription factors, nuclear factor-kappaB (NF-kappaB) and cyclic AMP-responsive element (CRE)-binding protein (CREB).

KEY RESULTS

Torilin significantly reduced the LPS-induced expression of iNOS, COX-2 and IL-1beta, and the subsequent release of NO, prostaglandin E(2) and IL-1beta into culture medium. LPS stimulation of extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 MAPK was inhibited by torilin. In addition, the inhibitory effect of torilin on NF-kappaB and CREB was shown by torilin-mediated recovery of LPS-induced degradation of inhibitor kappaB-alpha and suppression of LPS-induced phosphorylation of CREB respectively.

CONCLUSION AND IMPLICATIONS

This study indicates that torilin inhibited LPS-induced iNOS, COX-2 and IL-1beta via down-regulation of ERK1/2, p38 MAPK, NF-kappaB and CREB and suggests that torilin has a potential as an anti-inflammatory drug candidate.

Oligonucleotide microarray analysis of apoptosis induced by 15-methoxypinusolidic acid in microglial BV2 cells

BACKGROUND AND PURPOSE

We conducted a genome wide gene expression analysis to explore the biological aspects of 15-methoxypinusolidic acid (15-MPA) isolated from Biota orientalis and tried to confirm the suitability of 15-MPA as a therapeutic candidate for CNS injuries focusing on microglia.

EXPERIMENTAL APPROACH

Murine microglial BV2 cells were treated with 15-MPA, and their transcriptome was analysed by using oligonucleotide microarrays. Genes differentially expressed upon 15-MPA treatment were selected for RT-PCR (reverse transcription-polymerase chain reaction) analysis to confirm the gene expression. Inhibition of cell proliferation and induction of apoptosis by 15-MPA were examined by bromodeoxyuridine assay, Western blot analysis of poly-ADP-ribose polymerase and flow cytometry.

KEY RESULTS

A total of 514 genes were differentially expressed by 15-MPA treatment. Biological pathway analysis revealed that 15-MPA induced significant changes in expression of genes in the cell cycle pathway. Genes involved in growth arrest and DNA damage [gadd45alpha, gadd45gamma and ddit3 (DNA damage-inducible transcript 3)] and cyclin-dependent kinase inhibitor (cdkn2b) were up-regulated, whereas genes involved in cell cycle progression (ccnd1, ccnd3 and ccne1), DNA replication (mcm4, orc1l and cdc6) and cell proliferation (fos and jun) were down-regulated. RT-PCR analysis for representative genes confirmed the expression levels. 15-MPA significantly reduced bromodeoxyuridine incorporation, increased poly-ADP-ribose polymerase cleavage and the number of apoptotic cells, indicating that 15-MPA induces apoptosis in BV2 cells.

CONCLUSION AND IMPLICATIONS

15-MPA induced apoptosis in murine microglial cells, presumably via inhibition of the cell cycle progression. As microglial activation is detrimental in CNS injuries, these data suggest a strong therapeutic potential of 15-MPA.

The activation of P2X7 receptor impairs lysosomal functions and stimulates the release of autophagolysosomes in microglial cells

Recently, autophagy has been associated with the TLR signaling pathway to eliminate intracellular pathogens in the innate immune system. However, it is unknown if other pathways regulate autophagy during the immunologic response. Given the critical role of the purinergic P2X7 receptor (P2X7R) pathway during various immunologic functions (i.e., caspase activation and IL-1beta secretion), the principal objective here was to determine whether the P2X7R pathway may regulate autophagy in immune cells. We observed in both MG6 mouse microglial cells and primary microglia that activation of P2X7R by ATP increases the expression of microtubule-associated protein 1 light chain 3 (LC3)-II, the autophagosomal membrane-associated form of LC3, in an extracellular Ca(2+)-dependent manner. Consistent with this, immunohistochemistry showed extensive formation of LC3-immunopositive dots, and electron microscopy demonstrated accumulation of autophagosomes and autophagolysosomes in ATP-treated cells. Importantly, the up-regulation of LC3-II by P2X7R activation was not affected by autophagy inhibitors, such as 3-methyladenine and PI3K inhibitors. Furthermore, while lysosomal functions were impaired by ATP treatment, autophagolysosomal components were released into the extracellular space. Similarly, a phagocytosis assay using Escherichia coli BioParticles showed that phagosome maturation was impaired in ATP-treated cells and a robust release of LC3-immunopositive phagolysosomes was induced along with a radial extension of microtubule bundles. Taken together, the data suggest a novel mechanism whereby the P2X7R signaling pathway may negatively regulate autophagic flux through the impairment of lysosomal functions, leading to stimulation of a release of autophagolysosomes/phagolysosomes into the extracellular space.