Notch signaling modulates the activation of microglial cells

Forced Activation of Notch in Macrophages Represses Tumor Growth by Upregulating miR-125a and Disabling Tumor-Associated Macrophages

Tumor-associated macrophages (TAM) contribute greatly to hallmarks of cancer. Notch blockade was shown to arrest TAM differentiation, but the precise role and underlying mechanisms require elucidation. In this study, we employed a transgenic mouse model in which the Notch1 intracellular domain (NIC) is activated conditionally to define the effects of active Notch1 signaling in macrophages. NIC overexpression had no effect on TAM differentiation, but it abrogated TAM function, leading to repressed growth of transplanted tumors. Macrophage miRNA profiling identified a novel downstream mediator of Notch signaling, miR-125a, which was upregulated through an RBP-J-binding site at the first intronic enhancer of the host gene Spaca6A. miR-125a functioned downstream of Notch signaling to reciprocally influence polarization of M1 and M2 macrophages by regulating factor inhibiting hypoxia inducible factor-1α and IRF4, respectively. Notably, macrophages transfected with miR-125a mimetics increased phagocytic activity and repressed tumor growth by remodeling the immune microenvironment. We also identified a positive feedback loop for miR-125a expression mediated by RYBP and YY1. Taken together, our results showed that Notch signaling not only supported the differentiation of TAM but also antagonized their protumorigenic function through miR-125a. Targeting this miRNA may reprogram macrophages in the tumor microenvironment and restore their antitumor potential.

MicroRNA-34a Negatively Regulates Efferocytosis by Tissue Macrophages in Part via SIRT1

Apoptotic cell (AC) clearance (efferocytosis) is an evolutionarily conserved process essential for immune health, particularly to maintain self-tolerance. Despite identification of many recognition receptors and intracellular signaling components of efferocytosis, its negative regulation remains incompletely understood and has not previously been known to involve microRNAs (miRs). In this article, we show that miR-34a (gene ID 407040), well recognized as a p53-dependent tumor suppressor, mediates coordinated negative regulation of efferocytosis by resident murine and human tissue macrophages (Mø). The miR-34a expression varied greatly between Mø from different tissues, correlating inversely with their capacity for AC uptake. Transient or genetic knockdown of miR-34a increased efferocytosis, whereas miR-34a overexpression decreased efferocytosis, without altering recognition of live, necrotic, or Ig-opsonized cells. The inhibitory effect of miR-34a was mediated both by reduced expression of Axl, a receptor tyrosine kinase known to recognize AC, and of the deacetylase silent information regulator T1, which had not previously been linked to efferocytosis by tissue Mø. Exposure to AC downregulated Mø miR-34a expression, resulting in a positive feedback loop that increased subsequent capacity to engulf AC. These findings demonstrate that miR-34a both specifically regulates and is regulated by efferocytosis. Given the ability of efferocytosis to polarize ingesting Mø uniquely and to reduce their host-defense functions, dynamic negative regulation by miR-34a provides one means of fine-tuning Mø behavior toward AC in specific tissue environments with differing potentials for microbial exposure.

Synergistic Use of Geniposide and Ginsenoside Rg1 Balance Microglial TNF-α and TGF-β1 following Oxygen-Glucose Deprivation In Vitro: A Genome-Wide Survey

Ischemia-activated microglia are like a double-edged sword, characterized by both neurotoxic and neuroprotective effects. The aim of this study was to reveal the synergistic effect of geniposide and ginsenoside Rg1 based on tumor necrosis factor- (TNF-) α and transforming growth factor- (TGF-) β1 balance of microglia. BV2 microglial cells were divided into 5 groups: control, model (oxygen-glucose deprivation (OGD)), geniposide-treated, ginsenoside-Rg1-treated, and combination-treated. A series of assays were used to detect on (i) cell viability; (ii) NO content; (iii) expression (content) of TNF-α and TGF-β1; and (iv) gene expression profiles. The results showed that integrated use of geniposide and ginsenoside Rg1 significantly inhibited NO level and protected cell viability, improved the content and expression of TGF-β1, and reduced the content and expression of TNF-α. Separated use of geniposide or ginsenoside Rg1 showed different effects at different emphases. Next-generation sequencing showed that Fcγ-receptor-mediated phagocytosis pathway played a key regulatory role in the balance of TNF-α and TGF-β1 when cotreated with geniposide and ginsenoside Rg1. These findings suggest that synergistic drug combination of geniposide and ginsenoside Rg1 in the treatment of stroke is a feasible avenue for the application.

Notch signaling activation is critical to the development of neuropathic pain

Background

Nerve injury-induced neuropathic pain is a major health problem worldwide. Notch signaling is a highly conserved pathway in evolution, which has an important role in synaptic plasticity and inflammation in central nervous system. The present study was designed to investigate the potential role of notch signaling in the development of neuropathic pain.

Methods

The neuropathic pain was induced by spared nerve injury (SNI) in rats. The activation of notch signaling in the lumbar spinal dorsal horn was measured. DAPT, an inhibitor of notch signaling, was intrathecally (i.t.) administered before SNI or after appearance of pain sensitivity. Moreover, Jagged-1 (JAG-1) peptide, a ligand of notch signaling, was i.t. administered to normal rats. The mechanical allodynia was assessed by von Frey test.

Results

Here, we found that DAPT administered 0.5 h before SNI operation could significantly prevent the decrease of mechanical paw withdrawal threshold (PWT) for more than 4 weeks (P < 0.05 vs. SNI group). DAPT administered after appearance of pain sensitivity could also significantly reverse the decrease of mechanical PWT in a dose-dependent manner (P < 0.05). In addition, administration of Jagged-1 (JAG-1) peptide significantly decreased the mechanical PWT of normal rats in a dose-dependent manner (P < 0.05).

Conclusions

Therefore, notch signaling activation might contribute to the development of neuropathic pain. This study might provide a new therapeutic target for neuropathic pain.

Scutellarin regulates the Notch pathway and affects the migration and morphological transformation of activated microglia in experimentally induced cerebral ischemia in rats and in activated BV-2 microglia

Background

Activated microglial cells release an excess of inflammatory mediators after an ischemic stroke. We reported previously that scutellarin effectively suppressed the inflammatory response induced by activated microglia in rats subjected to middle cerebral artery occlusion (MCAO); however, the mechanism via which scutellarin exerts its effects on microglial activation has not been explored. This study aimed to elucidate if scutellarin can regulate the Notch pathway that is linked to microglia activation in MCAO rat, and in lipopolysaccharide (LPS)-induced BV-2 microglia. Along with this, we also investigated some characteristic behavioral responses of activated microglia.

Methods

Expression of various members of the Notch pathway, including Notch-1, intracellular Notch receptor domain (NICD), recombining binding protein suppressor of hairless (RBP-JK) and transcription factor hairy and enhancer of split-1 (Hes-1) in activated microglia was assessed by immunofluorescence staining and western blot after experimental MCAO and in vitro LPS activation. The effect of scutellarin on migration of microglia was determined by the transwell chamber assay as well as expression of monocyte chemoattractant protein-1 (MCP-1). The morphological change of microglia induced by scutellarin was detected by F-actin staining and electron microscopy.

Results

Scutellarin markedly attenuated the expression of NF-κB, Notch-1, NICD, RBP-JK and Hes-1 both in vivo and in activated microglia. It decreased the expression of MCP-1 and microglial migration, but increased the ability of microglia adhesion. Scutellarin also altered the phenotype of microglia by causing rearrangement or reorganization of its cytoskeleton.

Conclusions

The results suggest that scutellarin regulates the activation of microglia via the Notch pathway and concurrently induces morphological and functional changes in activated microglia.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-014-0226-z) contains supplementary material, which is available to authorized users.

Myeloid-Specific Blockade of Notch Signaling by RBP-J Knockout Attenuates Spinal Cord Injury Accompanied by Compromised Inflammation Response in Mice

The outcome of spinal cord injury (SCI) is determined by both neural cell-intrinsic survival pathways and tissue microenvironment-derived signals. Macrophages dominating the inflammatory responses in SCI possess both destructive and reparative potentials, according to their activation status. Notch signaling is involved in both cell survival and macrophage-mediated inflammation, but a comprehensive role of Notch signaling in SCI has been elusive. In this study, we compared the effects of general Notch blockade by a pharmaceutical γ-secretase inhibitor (GSI) and myeloid-specific Notch signal disruption by recombination signal binding protein Jκ (RBP-J) knockout on SCI. The administration of Notch signal inhibitor GSI resulted in worsened hind limb locomotion and exacerbated inflammation. However, mice lacking RBP-J, the critical transcription factor mediating signals from all four mammalian Notch receptors, in myeloid lineage displayed promoted functional recovery, attenuated glial scar formation, improved neuronal survival and axon regrowth, and mitigated inflammatory response after SCI. These benefits were accompanied by enhanced AKT activation in the lesion area after SCI. These findings demonstrate that abrogating Notch signal in myeloid cells ameliorates inflammation response post-SCI and promotes functional recovery, but general pharmaceutical Notch interception has opposite effects. Therefore, clinical intervention of Notch signaling in SCI needs to pinpoint myeloid lineage to avoid the counteractive effects of global inhibition.

Abnormal microglial activation in the Cstb(-/-) mouse, a model for progressive myoclonus epilepsy, EPM1

Progressive myoclonus epilepsy of Unverricht-Lundborg type (EPM1) is an autosomal-recessively inherited neurodegenerative disorder characterized by severely incapacitating myoclonus, seizures, and ataxia, and caused by loss-of-function mutations in the cystatin B gene (CSTB). A central neuropathological finding in the Cstb(-/-) mouse, an animal model for EPM1, is early microglial activation, which precedes astroglial activation, neuronal loss, and onset of myoclonus, thus implying a critical role for microglia in EPM1 pathogenesis. Here, we characterized phenotypic and functional properties of microglia from Cstb(-/-) mice utilizing brain tissue, microglia directly isolated from the brain, and primary microglial cultures. Our results show significantly higher Cstb mRNA expression in microglia than in neurons and astrocytes. In Cstb(-/-) mouse brain, expression of the inflammatory marker p-p38 MAPK and the proportion of both pro-inflammatory M1 and anti-inflammatory M2 microglia is higher than in control mice. Moreover, M1/M2 polarization of microglia in presymptomatic Cstb(-/-) mice is, compared to control mice, skewed towards M2 type at postnatal day 14 (P14), but towards M1 type at P30, a time point associated with onset of myoclonus. At this age, the high expression of both pro-inflammatory inducible nitric oxide synthase (iNOS) and anti-inflammatory arginase 1 (ARG1) in Cstb(-/-) mouse cortex is accompanied by the presence of peripheral immune cells. Consistently, activated Cstb(-/-) microglia show elevated chemokine release and chemotaxis. However, their MHCII surface expression is suppressed. Taken together, our results link CSTB deficiency to neuroinflammation with early activation and dysfunction of microglia and will open new avenues for therapeutic interventions for EPM1.

Notch-1 signaling regulates microglia activation via NF-κB pathway after hypoxic exposure in vivo and in vitro

Neuroinflammation mediated by the activated microglia is suggested to play a pivotal role in the pathogenesis of hypoxic brain injury; however, the underlying mechanism of microglia activation remains unclear. Here, we show that the canonical Notch signaling orchestrates microglia activation after hypoxic exposure which is closely associated with multiple pathological situations of the brain. Notch-1 and Delta-1 expression in primary microglia and BV-2 microglial cells was significantly elevated after hypoxia. Hypoxia-induced activation of Notch signaling was further confirmed by the concomitant increase in the expression and translocation of intracellular Notch receptor domain (NICD), together with RBP-Jκ and target gene Hes-1 expression. Chemical inhibition of Notch signaling with N-[N-(3,5-difluorophenacetyl)-1-alany1- S-phenyglycine t-butyl ester (DAPT), a γ-secretase inhibitor, effectively reduced hypoxia-induced upregulated expression of most inflammatory mediators. Notch inhibition also reduced NF-κB/p65 expression and translocation. Remarkably, Notch inhibition suppressed expression of TLR4/MyD88/TRAF6 pathways. In vivo, Notch signaling expression and activation in microglia were observed in the cerebrum of postnatal rats after hypoxic injury. Most interestingly, hypoxia-induced upregulation of NF-κB immunoexpression in microglia was prevented when the rats were given DAPT pretreatment underscoring the interrelationship between Notch signaling and NF-κB pathways. Taken together, we conclude that Notch signaling is involved in regulating microglia activation after hypoxia partly through the cross talk between TLR4/MyD88/TRAF6/NF-κB pathways. Therefore, Notch signaling may serve as a prospective target for inhibition of microglia activation known to be implicated in brain damage in the developing brain.

Tocopherol derivative TFA-12 promotes myelin repair in experimental models of multiple sclerosis

Multiple sclerosis (MS) is an inflammatory disease of the CNS that is associated with demyelination and axonal loss, resulting in severe neurological handicap. Current MS therapies mostly target neuroinflammation but have only a little impact on CNS myelin repair. Progress toward treatments that enhance remyelination would therefore represent major advances in MS treatment. Here, we examined the ability of TFA-12, a new synthetic compound belonging to tocopherol long-chain fatty alcohols, to promote oligodendrocyte regeneration and remyelination in experimental models of MS. We showed that TFA-12 significantly ameliorates neurological deficit and severity of myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis (EAE) in mice. Histological evaluation of mouse EAE spinal cords showed that TFA-12 treatment reduces inflammation, astrogliosis, and myelin loss. Additionally, we demonstrated that TFA-12 accelerates remyelination of focal demyelinated lesions induced by lysolecithin injections. We also found that this compound induces the differentiation of oligodendrocyte precursor cells into mature oligodendrocytes through the inhibition of the Notch/Jagged1 signaling pathway. Altogether, our data provide important proof of principle indicating that TFA-12 could be a potential therapeutic compound for myelin repair in MS.

Notch signaling pathway is activated in motoneurons of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neurodegenerative disease produced by low levels of Survival Motor Neuron (SMN) protein that affects alpha motoneurons in the spinal cord. Notch signaling is a cell-cell communication system well known as a master regulator of neural development, but also with important roles in the adult central nervous system. Aberrant Notch function is associated with several developmental neurological disorders; however, the potential implication of the Notch pathway in SMA pathogenesis has not been studied yet. We report here that SMN deficiency, induced in the astroglioma cell line U87MG after lentiviral transduction with a shSMN construct, was associated with an increase in the expression of the main components of Notch signaling pathway, namely its ligands, Jagged1 and Delta1, the Notch receptor and its active intracellular form (NICD). In the SMNΔ7 mouse model of SMA we also found increased astrocyte processes positive for Jagged1 and Delta1 in intimate contact with lumbar spinal cord motoneurons. In these motoneurons an increased Notch signaling was found, as denoted by increased NICD levels and reduced expression of the proneural gene neurogenin 3, whose transcription is negatively regulated by Notch. Together, these findings may be relevant to understand some pathologic attributes of SMA motoneurons.

Impact of notch signaling on inflammatory responses in cardiovascular disorders

Notch signaling is a major pathway in cell fate decisions. Since the first reports showing the major role of Notch in embryonic development, a considerable and still growing literature further highlights its key contributions in various pathological processes during adult life. In particular, Notch is now considered as a major player in vascular homeostasis through the control of key cellular functions. In parallel, confounding evidence emerged that inflammatory responses regulate Notch signaling in vitro in endothelial cells, smooth muscle cells or vascular infiltrating cells and in vivo in vascular and inflammatory disorders and in cardiovascular diseases. This review presents how inflammation influences Notch in vascular cells and, reciprocally, emphasizes the functional role of Notch on inflammatory processes, notably by regulating key cell functions (differentiation, proliferation, apoptosis/survival, activation). Understanding how the disparity of Notch receptors and ligands impacts on vasculature biology remains critical for the design of relevant and adequate therapeutic strategies targeting Notch in this major pathological context.

Noncanonical Notch signaling modulates cytokine responses of dendritic cells to inflammatory stimuli

Dendritic cell (DC)-derived cytokines play a key role in specifying adaptive immune responses tailored to the type of pathogen encountered and the local tissue environment. However, little is known about how DCs perceive the local environment. We investigated whether endogenous Notch signaling could affect DC responses to pathogenic stimuli. We demonstrate that concurrent Notch and TLR stimulation results in a unique cytokine profile in mouse bone-marrow derived DCs characterized by enhanced IL-10 and IL-2, and reduced IL-12 expression compared with TLR ligation alone. Unexpectedly, modulation of cytokine production occurred through a noncanonical Notch signaling pathway, independent of γ-secretase activity. Modulation required de novo protein synthesis, and PI3K, JNK, and ERK activity were necessary for enhanced IL-2 expression, whereas modulation of IL-10 required only PI3K activity. Further, we show that this γ-secretase-independent Notch pathway can induce PI3K activity. In contrast, expression of the canonical Notch target gene Hes1 was suppressed in DCs stimulated with Notch and TLR ligands simultaneously. Thus, our data suggest that Notch acts as an endogenous signal that modulates cytokine expression of DCs through a noncanonical pathway and therefore has the potential to tailor the subsequent adaptive immune response in a tissue- and/or stage-dependent manner.

Mutant presenilin-1 deregulated peripheral immunity exacerbates Alzheimer-like pathology

Mutations in the presenilin-1 (PS1) gene are independent causes of familial Alzheimer's disease (AD). AD patients have dysregulated immunity, and PS1 mutant mice exhibit abnormal systemic immune responses. To test whether immune function abnormality caused by a mutant human PS1 gene (mhPS1) could modify AD-like pathology, we reconstituted immune systems of AD model mice carrying a mutant human amyloid precursor protein gene (mhAPP; Tg2576 mice) or both mhAPP and mhPS1 genes (PSAPP mice) with allo-geneic bone marrow cells. Here, we report a marked reduction in amyloid-β (Aβ) levels, β-amyloid plaques and brain inflammatory responses in PSAPP mice following strain-matched wild-type PS1 bone marrow reconstitution. These effects occurred with immune switching from pro-inflammatory T helper (Th) 1 to anti-inflammatory Th2 immune responses in the periphery and in the brain, which likely instructed microglia to phagocytose and clear Aβ in an ex vivo assay. Conversely, Tg2576 mice displayed accelerated AD-like pathology when reconstituted with mhPS1 bone marrow. These data show that haematopoietic cells bearing the mhPS1 transgene exacerbate AD-like pathology, suggesting a novel therapeutic strategy for AD based on targeting PS1 in peripheral immune cells.

JNK1 inhibits GluR1 expression and GluR1-mediated calcium influx through phosphorylation and stabilization of Hes-1

The GluR1 subunit of the AMPA receptor plays an important role in excitatory synaptic transmission and synaptic plasticity in the brain, but the regulation mechanism for GluR1 expression is largely unknown. Hairy and enhancer of split 1 (Hes-1) is a mammalian transcription repressor that regulates neuronal differentiation and development, but the role of Hes-1 in differentiated neurons is also less known. Here, we examined the molecular mechanism in regulation of GluR1 expression in rat cultured cortical neurons. We found that Hes-1 suppressed GluR1 promoter activity and decreased GluR1 expression through direct binding to the N-box and through preventing Mash1/E47 from binding to the E-box of GluR1 promoter. We also found that Hes-1 could be regulated by c-Jun N-terminal kinase (JNK1). JNK1 directly phosphorylates Hes-1 at Ser-263. Furthermore, JNK1 phosphorylation of Hes-1 stabilized the Hes-1 protein and enhanced the suppressing effect of Hes-1 on GluR1 expression. These effects were demonstrated both in the soma and at the synapse. Moreover, this JNK1-mediated signaling pathway was found to inhibit AMPA-evoked calcium influx in cortical neurons and this regulation mechanism is Notch independent. Here, we provided the first evidence that Hes-1 plays an important role in synaptic function in differentiated neurons. We also identified a novel JNK1-Hes-1 signaling pathway that regulates GluR1 expression involved in synaptic function in rat cortical neurons.

The γ-secretase blocker DAPT impairs recovery from lipopolysaccharide-induced inflammation in rat brain

γ-Secretase is an important contributing enzyme in Alzheimer's disease and is therefore an important therapeutic target. However, the impact of γ-secretase inhibition is not well studied in acute neuroinflammation induced by systemic infection. In this study the influence of γ-secretase on the expression of some proinflammatory markers was assessed in the acute phase as well as the subsiding phase of neuroinflammation. Cerebral γ-secretase cleavage activity was measured by a fluorometric assay after lipopolysaccharide (LPS) intraperitoneal administration. Time profiles of TNF-α and COX-II expression were then determined to detect the time points relevant to the maximal inflammatory responses and the subsequent recovery phase. γ-Secretase activity coincident with TNF-α protein expression returned to its basal level till 8-12 h after systemic challenge with low dose LPS while COX-II over expression lasted for 48-72 h later. Pharmacological inhibition of γ-secretase with local or systemic administration of DAPT (N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester) was performed to indicate the results on the developmental and sinking phases of inflammatory responses in 6 and 72 h post LPS respectively. Our results demonstrate that both local and systemic modulation of γ-secretase hyper-activity with DAPT increase the duration of TNF-α, COX-II, and NFκB induction. We consistently found mild augmented apoptosis in animals treated with DAPT as determined by measuring cleaved caspase-3 expression and by TUNEL assay 72 h following LPS injection. These results suggest that γ-secretase modulation interferes with certain immune regulatory pathways which may restrict some inflammatory transcription factors such as NFκB.

Immunological characterization and transcription profiling of peripheral blood (PB) monocytes in children with autism spectrum disorders (ASD) and specific polysaccharide antibody deficiency (SPAD): case study

Introduction

There exists a small subset of children with autism spectrum disorders (ASD) characterized by fluctuating behavioral symptoms and cognitive skills following immune insults. Some of these children also exhibit specific polysaccharide antibody deficiency (SPAD), resulting in frequent infection caused by encapsulated organisms, and they often require supplemental intravenous immunoglobulin (IVIG) (ASD/SPAD). This study assessed whether these ASD/SPAD children have distinct immunological findings in comparison with ASD/non-SPAD or non-ASD/SPAD children.

Case description

We describe 8 ASD/SPAD children with worsening behavioral symptoms/cognitive skills that are triggered by immune insults. These ASD/SPAD children exhibited delayed type food allergy (5/8), treatment-resistant seizure disorders (4/8), and chronic gastrointestinal (GI) symptoms (5/8) at high frequencies. Control subjects included ASD children without SPAD (N = 39), normal controls (N = 37), and non-ASD children with SPAD (N = 12).

Discussion and Evaluation

We assessed their innate and adaptive immune responses, by measuring the production of pro-inflammatory and counter-regulatory cytokines by peripheral blood mononuclear cells (PBMCs) in responses to agonists of toll like receptors (TLR), stimuli of innate immunity, and T cell stimulants. Transcription profiling of PB monocytes was also assessed. ASD/SPAD PBMCs produced less proinflammatory cytokines with agonists of TLR7/8 (IL-6, IL-23), TLR2/6 (IL-6), TLR4 (IL-12p40), and without stimuli (IL-1ß, IL-6, and TNF-α) than normal controls. In addition, cytokine production of ASD/SPAD PBMCs in response to T cell mitogens (IFN-γ, IL-17, and IL-12p40) and candida antigen (Ag) (IL-10, IL-12p40) were less than normal controls. ASD/non-SPAD PBMDs revealed similar results as normal controls, while non-ASD/SPAD PBMCs revealed lower production of IL-6, IL-10 and IL-23 with a TLR4 agonist. Only common features observed between ASD/SPAD and non-ASD/SPAD children is lower IL-10 production in the absence of stimuli. Transcription profiling of PB monocytes revealed over a 2-fold up (830 and 1250) and down (653 and 1235) regulation of genes in ASD/SPAD children, as compared to normal (N = 26) and ASD/non-SPAD (N = 29) controls, respectively. Enriched gene expression of TGFBR (p < 0.005), Notch (p < 0.01), and EGFR1 (p < 0.02) pathways was found in the ASD/SPAD monocytes as compared to ASD/non-SPAD controls.

Conclusions

The Immunological findings in the ASD/SPAD children who exhibit fluctuating behavioral symptoms and cognitive skills cannot be solely attributed to SPAD. Instead, these findings may be more specific for ASD/SPAD children with the above-described clinical characteristics, indicating a possible role of these immune abnormalities in their neuropsychiatric symptoms.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Presenilin 2 is the predominant γ-secretase in microglia and modulates cytokine release

Presenilin 1 (PS1) and Presenilin 2 (PS2) are the enzymatic component of the γ-secretase complex that cleaves amyloid precursor protein (APP) to release amyloid beta (Aβ) peptide. PS deficiency in mice results in neuroinflammation and neurodegeneration in the absence of accumulated Aβ. We hypothesize that PS influences neuroinflammation through its γ-secretase action in CNS innate immune cells. We exposed primary murine microglia to a pharmacological γ-secretase inhibitor which resulted in exaggerated release of TNFα and IL-6 in response to lipopolysaccharide. To determine if this response was mediated by PS1, PS2 or both we used shRNA to knockdown each PS in a murine microglia cell line. Knockdown of PS1 did not lead to decreased γ-secretase activity while PS2 knockdown caused markedly decreased γ-secretase activity. Augmented proinflammatory cytokine release was observed after knockdown of PS2 but not PS1. Proinflammatory stimuli increased microglial PS2 gene transcription and protein in vitro. This is the first demonstration that PS2 regulates CNS innate immunity. Taken together, our findings suggest that PS2 is the predominant γ-secretase in microglia and modulates release of proinflammatory cytokines. We propose PS2 may participate in a negative feedback loop regulating inflammatory behavior in microglia.

γ-Secretase component presenilin is important for microglia β-amyloid clearance

OBJECTIVE

The cleavage of amyloid precursor protein by γ-secretase is an important aspect of the pathogenesis of Alzheimer's disease. γ-Secretase also cleaves other membrane proteins (eg, Notch), which control cell development and homeostasis. Presenilin 1 and 2 are considered important determinants of the γ-secretase catalytic site. Our aim was to investigate whether γ-secretase can be important for microglial phagocytosis of Alzheimer's disease β-amyloid.

METHODS

We investigated the role of γ-secretase in microglia activity toward β-amyloid phagocytosis in cell culture using γ-secretase inhibitors and small hairpin RNA and presenilin-deficient mice.

RESULTS

We found that γ-secretase inhibitors impair microglial activity as measured in gene expression, protein levels, and migration ability, which resulted in a reduction of soluble β-amyloid phagocytosis. Moreover, microglia deficient in presenilin 1 and 2 showed impairment in phagocytosis of soluble β-amyloid. Dysfunction in the γ-secretase catalytic site led to an impairment in clearing insoluble β-amyloid from brain sections taken from an Alzheimer's disease mouse model when compared to microglia from wild-type mice.

INTERPRETATION

We suggest for the first time, a dual role for γ-secretase in Alzheimer's disease. One role is the cleavage of the amyloid precursor protein for pathologic β-amyloid production and the other is to regulate microglia activity that is important for clearing neurotoxic β-amyloid deposits. Further studies of γ-secretase-mediated cellular pathways in microglia may provide useful insights into the development of Alzheimer's disease and other neurodegenerative diseases, providing future avenues for therapeutic intervention.

Jagged1 regulates the activation of astrocytes via modulation of NFkappaB and JAK/STAT/SOCS pathways

The Notch pathway is implicated in many aspects of the central nervous system (CNS) development and functions. Recently, we and others identified the Notch pathway to be involved in inflammatory events of the CNS. To understand the implication of this pathway on astrocytes, we have studied the Jagged-Notch-Hes pathway under inflammatory conditions. LPS exposure induced an upregulation of Jagged1 expression on cultured astrocytes. To address the role of Jagged1 in the modulation of inflammation, we used a siRNA mediated silencing of Jagged1 (siRNA J1). Jagged1 inhibition induced important variations on the Notch pathway components like Hes1, Hes5, Notch3, and RBP-Jkappa. siRNA J1 repressed the mRNA expression of genes known as hallmarks of the gliosis like GFAP and endothelin(B) receptor. On activated astrocytes, the inhibition of Jagged1 had antiinflammatory effects and resulted in a decrease of LPS-induced proinflammatory cytokines (IL1beta, IL1alpha, and TNFalpha) as well as the iNOS expression. The inhibition of Jagged1 induced a modulation of the JAK/STAT/SOCS signaling pathway. Most interestingly, the siRNA J1 decreased the LPS-induced translocation of NFkappaB p65 and this could be correlated to the phosphorylation of IkappaBalpha. These results suggest that during inflammatory and gliotic events of the CNS, Jagged1/Notch signaling sustains the inflammation mainly through NFkappaB and in part through JAK/STAT/SOCS signaling pathways.

Notch: a new player in MS mechanisms

Notch is a family of four transmembrane receptors (Notch1-4) that orchestrate differentiation of various cell types, tissues and organs. Recent studies have revealed that Notch, among other processes, regulates immune responses of peripheral T cells, controls oligodendrocyte maturation and myelination of axons and under inflammatory conditions affects activation of macrophages and microglia. Therefore, Notch signaling has been implicated in the differentiation and function of all cell types considered crucial for the development and clinical progression of multiple sclerosis (MS). Importantly, inflammatory/demyelinating lesions in MS and its animal model, autoimmune experimental encephalomyelitis (EAE), abundantly express Notch receptors, their ligands and downstream activation targets. In EAE, in vivo modulation of Notch signaling affects immune responses of myelin-reactive T cells, enhances tissue repair and reduces clinical severity of the disease. In this review, we present recent findings on how Notch signaling may affect function of both immune and glial cells, analyze data implicating the Notch pathway in MS and EAE, and discuss the therapeutic potential of manipulating Notch signaling in MS patients.

The multifaceted profile of activated microglia

Although relatively neglected previously, research efforts in the past decade or so have identified a pivotal role for glial cells in regulating neuronal function. Particular emphasis has been placed on increasing our understanding of the function of microglia because a change from the ramified "resting" state of these cells has been associated with the pathogenesis of several neurodegenerative diseases, notably Alzheimer's disease. However, it is not clear whether activation of microglia and the associated inflammatory changes play a part in triggering disease processes or whether cell activation is a response to the early changes associated with the disease. In either case, the possibility exists that modulation of microglial activation may be beneficial in some circumstances, underlying the need to pursue research in this area. The original morphological categorization of microglia by Del Rio Hortega into ameboid, ramified, and intermediate forms, must now be elaborated to encompass a functional description. The evidence which has been generated recently suggests that microglia are probably never in a "resting" state and that several intermediate transitional states, based on function and morphology, probably exist. A more complete understanding of these states and the triggers which lead to a change from one to another state, and the factors which modulate the molecular switch that determines the persistence of the "activated" state remain to be identified.

Characterization of the microglial phenotype under specific pro-inflammatory and anti-inflammatory conditions: Effects of oligomeric and fibrillar amyloid-beta

M1 and M2 are the extremes of the differentiation spectrum of activated macrophages. Since microglia are members of the same cell lineage, we have characterized their transcription profile and their phagocytic activity under different conditions. LPS or IFN-gamma induce a M1-like phenotype, while IL-10 or IL-4 differentiate microglia towards a M2-deactivated or M2-alternatively-activated phenotype respectively. These differentiation processes also affect the Notch pathway. In order to study the polarization induced by Abeta, microglia was stimulated with different forms of the peptide. The oligomeric Abeta is a stronger M1-inductor than the fibrillar form. Moreover, a cytokine-induced anti-inflammatory environment reduces the microglial reactivity towards oligomeric Abeta.

Increased expression of the gamma-secretase components presenilin-1 and nicastrin in activated astrocytes and microglia following traumatic brain injury

Gamma-secretase is an aspartyl protease composed of four proteins: presenilin (PS), nicastrin (Nct), APH1, and PEN2. These proteins assemble into a membrane complex that cleaves a variety of substrates within the transmembrane domain. The gamma-secretase cleavage products play an important role in various biological processes such as embryonic development and Alzheimer's disease (AD). The major role of gamma-secretase in brain pathology has been linked to AD and to the production of the amyloid beta-peptide. However, little is known about the possible role of gamma-secretase following acute brain insult. Here we examined by immunostaining the expression patterns of two gamma-secretase components, PS1 and Nct, in three paradigms of brain insult in mice: closed head injury, intracerebroventricular injection of LPS, and brain stabbing. Our results show that in naïve and sham-injured brains expression of PS1 and Nct is restricted mainly to neurons. However, following insult, the expression of both proteins is also observed in nonneuronal cells, consisting of activated astrocytes and microglia. Furthermore, the proteins are coexpressed within the same astrocytes and microglia, implying that these cells exhibit an enhanced gamma-secretase activity following brain damage. In view of the important role played by astrocytes and microglia in brain disorders, our findings suggest that gamma-secretase may participate in brain damage and repair processes by regulating astrocyte and microglia activation and/or function.