Selective upregulation of scavenger receptors in and around demyelinating areas in multiple sclerosis

The OLR1/NF-κB feedback loop exacerbates HIV-1 Tat-induced microglial inflammatory response and neuronal apoptosis

Oxidized low density lipoprotein receptor 1 (OLR1), a type II integral membrane glycoprotein, is involved in multiple neurological diseases. However, the roles and mechanisms of OLR1 in HIV-associated neurocognitive disorder (HAND) remain unclear. In the central nervous system, Transactivator of transcription (Tat) induces inflammatory response in microglia, thereby leading to neuronal apoptosis. In the present study, we demonstrated that OLR1 expression was upregulated during ectopic expression of Tat or soluble Tat stimulus in BV-2 microglial cells. Moreover, OLR1 signaling was proved to facilitate Tat-triggered inflammatory response and alleviated the microglia-derived conditioned media-mediated HT-22 neural cells apoptosis in a NF-κB-dependent manner. Conversely, Tat augmented OLR1 expression via NF-κB signaling pathway. Finally, in mouse models, we determined that silencing of OLR1 significantly ameliorated Tat‑induced neuroinflammation and hippocampal neuronal death. Taken together, our study clarifies the potential role of the OLR1/NF-κB feedback loop in Tat-induced microglial inflammatory response and neuronal apoptosis, which could be a novel therapeutic target for relief of HAND.

Role of innate immune cells in multiple sclerosis

Multiple sclerosis (MS) is a chronic autoimmune, inflammatory and neurodegenerative disease affecting the central nervous system (CNS). MS is associated with a complex interplay between neurodegenerative and inflammatory processes, mostly attributed to pathogenic T and B cells. However, a growing body of preclinical and clinical evidence indicates that innate immunity plays a crucial role in MS promotion and progression. Accordingly, preclinical and clinical studies targeting different innate immune cells to control MS are currently under study, highlighting the importance of innate immunity in this pathology. Here, we reviewed recent findings regarding the role played by innate immune cells in the pathogenesis of MS. Additionally, we discuss potential new treatments for MS based on targets against innate immune components.

miR-147-3p in pathogenic CD4 T cells controls chemokine receptor expression for the development of experimental autoimmune diseases

Incomplete Freund's adjuvant (IFA) has long been used to trigger autoimmune diseases in animal models, such as experimental autoimmune encephalitis and collagen-induced arthritis. However, the molecular mechanisms that control CD4 T cell effector functions and lead to the development of autoimmune diseases are not well understood. A self-antigen and heat-killed Mycobacterium tuberculosis emulsified in IFA augmented the activation of CD4 T cells, leading to the differentiation of pathogenic CD4 T cells in the draining lymph nodes. In contrast, IFA emulsification did not elicit Foxp3+ regulatory T cell expansion. We found that pathogenic Th1 cells expressed miR-147-3p, which targets multiple genes to affect T cell function. Finally, miR-147-3p expressed in CXCR6+SLAMF6- Th1 cells was required for the onset of neurological symptoms through the control of CXCR3 expression. Our findings demonstrate that miR-147-3p expressed in pathogenic CD4 T cells regulates the migratory potential in peripheral tissues and impacts the development of autoimmune diseases.

Myelin debris phagocytosis in demyelinating disease

Demyelinating diseases are often caused by a variety of triggers, including immune responses, viral infections, malnutrition, hypoxia, or genetic factors, all of which result in the loss of myelin in the nervous system. The accumulation of myelin debris at the lesion site leads to neuroinflammation and inhibits remyelination; therefore, it is crucial to promptly remove the myelin debris. Initially, Fc and complement receptors on cellular surfaces were the primary clearance receptors responsible for removing myelin debris. However, subsequent studies have unveiled the involvement of additional receptors, including Mac-2, TAM receptors, and the low-density lipoprotein receptor-related protein 1, in facilitating the removal process. In addition to microglia and macrophages, which serve as the primary effector cells in the disease phase, a variety of other cell types such as astrocytes, Schwann cells, and vascular endothelial cells have been demonstrated to engage in the phagocytosis of myelin debris. Furthermore, we have concluded that oligodendrocyte precursor cells, as myelination precursor cells, also exhibit this phagocytic capability. Moreover, our research group has innovatively identified the low-density lipoprotein receptor as a potential phagocytic receptor for myelin debris. In this article, we discuss the functional processes of various phagocytes in demyelinating diseases. We also highlight the alterations in signaling pathways triggered by phagocytosis, and provide a comprehensive overview of the various phagocytic receptors involved. Such insights are invaluable for pinpointing potential therapeutic strategies for the treatment of demyelinating diseases by targeting phagocytosis.

Profiling of microglia nodules in multiple sclerosis reveals propensity for lesion formation

Microglia nodules (HLA-DR+ cell clusters) are associated with brain pathology. In this post-mortem study, we investigated whether they represent the first stage of multiple sclerosis (MS) lesion formation. We show that microglia nodules are associated with more severe MS pathology. Compared to microglia nodules in stroke, those in MS show enhanced expression of genes previously found upregulated in MS lesions. Furthermore, genes associated with lipid metabolism, presence of T and B cells, production of immunoglobulins and cytokines, activation of the complement cascade, and metabolic stress are upregulated in microglia nodules in MS. Compared to stroke, they more frequently phagocytose oxidized phospholipids and possess a more tubular mitochondrial network. Strikingly, in MS, some microglia nodules encapsulate partially demyelinated axons. Taken together, we propose that activation of microglia nodules in MS by cytokines and immunoglobulins, together with phagocytosis of oxidized phospholipids, may lead to a microglia phenotype prone to MS lesion formation.

The Heterogeneous Multiple Sclerosis Lesion: How Can We Assess and Modify a Degenerating Lesion?

Multiple sclerosis (MS) is a heterogeneous disease of the central nervous system that is governed by neural tissue loss and dystrophy during its progressive phase, with complex reactive pathological cellular changes. The immune-mediated mechanisms that promulgate the demyelinating lesions during relapses of acute episodes are not characteristic of chronic lesions during progressive MS. This has limited our capacity to target the disease effectively as it evolves within the central nervous system white and gray matter, thereby leaving neurologists without effective options to manage individuals as they transition to a secondary progressive phase. The current review highlights the molecular and cellular sequelae that have been identified as cooperating with and/or contributing to neurodegeneration that characterizes individuals with progressive forms of MS. We emphasize the need for appropriate monitoring via known and novel molecular and imaging biomarkers that can accurately detect and predict progression for the purposes of newly designed clinical trials that can demonstrate the efficacy of neuroprotection and potentially neurorepair. To achieve neurorepair, we focus on the modifications required in the reactive cellular and extracellular milieu in order to enable endogenous cell growth as well as transplanted cells that can integrate and/or renew the degenerative MS plaque.

Maternal immune activation alters fetal and neonatal microglia phenotype and disrupts neurogenesis in mice

BACKGROUND

Activation of microglia, increase in cortical neuron density, and reduction in GABAergic interneurons are some of the key findings in postmortem autism spectrum disorders (ASD) subjects. The aim of this study was to investigate how maternal immune activation (MIA) programs microglial phenotypes and abnormal neurogenesis in offspring mice.

METHODS

MIA was induced by injection of lipopolysaccharide (LPS, i.p.) to pregnant mice at embryonic (E) day 12.5. Microglial phenotypes and neurogenesis were investigated between E15.5 to postnatal (P) day 21 by immunohistochemistry, flow cytometry, and cytokine array.

RESULTS

MIA led to a robust increase in fetal and neonatal microglia in neurogenic regions. Homeostatic E15.5 and P4 microglia are heterogeneous, consisting of M1 (CD86+/CD206-) and mixed M1/M2 (CD86+/CD206+)-like subpopulations. MIA significantly reduced M1 but increased mixed M1/M2 microglia, which was associated with upregulation of numerous cytokines with pleotropic property. MIA resulted in a robust increase in Ki67+/Nestin+ and Tbr2+ neural progenitor cells in the subventricular zone (SVZ) of newborn mice. At juvenile stage, a male-specific reduction of Parvalbumin+ but increase in Reelin+ interneurons in the medial prefrontal cortex was found in MIA offspring mice.

CONCLUSIONS

MIA programs microglia towards a pleotropic phenotype that may drive excessive neurogenesis in ASD patients.

IMPACT

Maternal immune activation (MIA) alters microglial phenotypes in the brain of fetal and neonatal mouse offspring. MIA leads to excessive proliferation and overproduction of neural progenitors in the subventricular zone (SVZ). MIA reduces parvalbumin+ while increases Reelin+ interneurons in the prefrontal cortex. Our study sheds light on neurobiological mechanisms of abnormal neurogenesis in certain neurodevelopmental disorders, such as autism spectrum disorder (ASD).

Immune Regulatory Functions of Macrophages and Microglia in Central Nervous System Diseases

Macrophages can be characterized as a very multifunctional cell type with a spectrum of phenotypes and functions being observed spatially and temporally in various disease states. Ample studies have now demonstrated a possible causal link between macrophage activation and the development of autoimmune disorders. How these cells may be contributing to the adaptive immune response and potentially perpetuating the progression of neurodegenerative diseases and neural injuries is not fully understood. Within this review, we hope to illustrate the role that macrophages and microglia play as initiators of adaptive immune response in various CNS diseases by offering evidence of: (1) the types of immune responses and the processes of antigen presentation in each disease, (2) receptors involved in macrophage/microglial phagocytosis of disease-related cell debris or molecules, and, finally, (3) the implications of macrophages/microglia on the pathogenesis of the diseases.

IL-17A expression by both T cells and non-T cells contribute to HSV-IL-2-induced CNS demyelination

Previously we reported that a recombinant HSV-1 expressing murine IL-2 (HSV-IL-2) causes CNS demyelination in different strains of mice and in a T cell-dependent manner. Since TH17 cells have been implicated in CNS pathology, in the present study, we looked into the effects of IL-17A-/- and three of its receptors on HSV-IL-2-induced CNS demyelination. IL-17A-/- mice did not develop CNS demyelination, while IL-17RA-/-, IL-17RC-/-, IL-17RD-/- and IL-17RA-/-RC-/- mice developed CNS demyelination. Adoptive transfer of T cells from wild-type (WT) mice to IL-17A-/- mice or T cells from IL-17A-/- mice to Rag-/- mice induced CNS demyelination in infected mice. Adoptive T cell experiments suggest that both T cells and non-T cells expressing IL-17A contribute to HSV-IL-2-induced CNS demyelination with no difference in the severity of demyelination between the two groups of IL-17A producing cells. IL-6, IL-10, or TGFβ did not contribute to CNS demyelination in infected mice. Transcriptome analysis between IL-17A-/- brain and spinal cord of infected mice with and without T cell transfer from WT mice revealed that "neuron projection extension involved in neuron projection guidance" and "ensheathment of neurons" pathways were associated with CNS demyelination. Collectively, the results indicate the importance of IL-17A in CNS demyelination and the possible involvement of more than three of IL-17 receptors in CNS demyelination.

Guiding Epilepsy Surgery with an LRP1-Targeted SPECT/SERRS Dual-Mode Imaging Probe

Accurate identification of the resectable epileptic lesion is a precondition of operative intervention to drug-resistant epilepsy (DRE) patients. However, even when multiple diagnostic modalities are combined, epileptic foci cannot be accurately identified in ∼30% of DRE patients. Inflammation-associated low-density lipoprotein receptor-related protein-1 (LRP1) has been validated to be a surrogate target for imaging epileptic foci. Here, we reported an LRP1-targeted dual-mode probe that is capable of providing comprehensive epilepsy information preoperatively with SPECT imaging while intraoperatively delineating epileptic margins in a sensitive high-contrast manner with surface-enhanced resonance Raman scattering (SERRS) imaging. Notably, a novel and universal strategy for constructing self-assembled monolayer (SAM)-based Raman reporters was proposed for boosting the sensitivity, stability, reproducibility, and quantifiability of the SERRS signal. The probe showed high efficacy to penetrate the blood-brain barrier. SPECT imaging showed the probe could delineate the epileptic foci clearly with a high target-to-background ratio (4.11 ± 0.71, 2 h). Further, with the assistance of the probe, attenuated seizure frequency in the epileptic mouse models was achieved by using SPECT together with Raman images before and during operation, respectively. Overall, this work highlights a new strategy to develop a SPECT/SERRS dual-mode probe for comprehensive epilepsy surgery that can overcome the brain shift by the co-registration of preoperative SPECT and SERRS intraoperative images.

Drug-induced microglial phagocytosis in multiple sclerosis and experimental autoimmune encephalomyelitis and the underlying mechanisms

Microglia are resident macrophages of the central nervous system (CNS). It plays a significant role in immune surveillance under physiological conditions. On stimulation by pathogens, microglia change their phenotypes, phagocytize toxic molecules, secrete pro-inflammatory/anti-inflammatory factors, promotes tissue repair, and maintain the homeostasis in CNS. Accumulation of myelin debris in multiple sclerosis (MS)/experimental autoimmune encephalomyelitis (EAE) inhibits remyelination by decreasing the phagocytosis by microglia and prevent the recovery of MS/EAE. Drug induced microglia phagocytosis could be a novel therapeutic intervention for the treatment of MS/EAE. But the abnormal phagocytosis of neurons and synapses by activated microglia will lead to neuronal damage and degeneration. It indicates that the phagocytosis of microglia has many beneficial and harmful effects in central neurodegenerative diseases. Therefore, simply promoting or inhibiting the phagocytic activity of microglia may not achieve ideal therapeutic results. However, limited reports are available to elucidate the microglia mediated phagocytosis and its underlying molecular mechanisms. On this basis, the present review describes microglia-mediated phagocytosis, drug-induced microglia phagocytosis, molecular mechanism, and novel approach for MS/EAE treatment.

Brain proteome-wide association study linking-genes in multiple sclerosis pathogenesis

OBJECTIVES

To identify genes that confer MS risk via the alteration of cis-regulated protein abundance and verify their aberrant expression in human brain.

METHODS

Utilizing a two-stage proteome-wide association study (PWAS) design, MS GWAS data (N = 41,505) was respectively integrated with two distinct human brain proteomes from the dorsolateral prefrontal cortex, including ROSMAP (N = 376) in the discovery stage and Banner (N = 152) in the confirmation stage. In the following, Bayesian colocalization analysis was conducted for GWAS and protein quantitative trait loci signals to prioritize candidate genes. Differential expression analysis was then used to verify the dysregulation of risk genes in white matter and gray matter for evidence at the transcription level.

RESULTS

A total of 51 genes whose protein abundance had association with the MS risk were identified, of which 18 genes overlapped in the discovery and confirmation PWAS. Bayesian colocalization indicated six causal genes with genetic risk variants for the MS risk. The differential expression analysis of SHMT1 (P_FDR  = 4.82 × 10^-2 ), FAM120B (P_FDR  = 8.13 × 10^-4 ) in white matter and ICA1L (P_FDR  = 3.44 × 10^-2 ) in gray matter confirmed the dysregulation at the transcription level. Further investigation of expression found SHMT1 significantly up-regulated in white matter lesion, and FAM120B up-regulated in both white matter lesion and normal appearing white matter. ICA1L was down-regulated in both gray matter lesion and normal appearing gray matter.

INTERPRETATION

Dysregulation of SHMT1, FAM120B and ICA1L may confer MS risk. Our findings shed new light on the pathogenesis of MS and prioritized promising targets for future therapy research.

Trem2 deficiency impairs recovery and phagocytosis and dysregulates myeloid gene expression during virus-induced demyelination

Background

Triggering receptor expressed on myeloid cells 2 (Trem2) plays a protective role in neurodegenerative diseases. By contrast, Trem2 functions can exacerbate tissue damage during respiratory viral or liver infections. We, therefore, investigated the role of Trem2 in a viral encephalomyelitis model associated with prominent Th1 mediated antiviral immunity leading to demyelination.

Methods

Wild-type (WT) and Trem2 deficient (Trem2^−/−) mice were infected with a sublethal glia tropic murine coronavirus (MHV–JHM) intracranially. Disease progression and survival were monitored daily. Leukocyte accumulation and pathological features including demyelination and axonal damage in spinal cords (SC) were determined by flow cytometry and tissue section immunofluorescence analysis. Expression of select inflammatory cytokines and chemokines was measured by RT-PCR and global myeloid cell gene expression in SC-derived microglia and infiltrated bone-marrow-derived macrophages (BMDM) were determined using the Nanostring nCounter platform.

Results

BMDM recruited to SCs in response to infection highly upregulated Trem2 mRNA compared to microglia coincident with viral control. Trem2 deficiency did not alter disease onset or severity, but impaired clinical recovery after onset of demyelination. Disease progression in Trem2^−/− mice could not be attributed to altered virus control or an elevated proinflammatory response. A prominent difference was increased degenerated myelin not associated with the myeloid cell markers IBA1 and/or CD68. Gene expression profiles of SC-derived microglia and BMDM further revealed that Trem2 deficiency resulted in impaired upregulation of phagocytosis associated genes Lpl and Cd36 in microglia, but a more complex pattern in BMDM.

Conclusions

Trem2 deficiency during viral-induced demyelination dysregulates expression of other select genes regulating phagocytic pathways and lipid metabolism, with distinct effects on microglia and BMDM. The ultimate failure to remove damaged myelin is reminiscent of toxin or autoimmune cell-induced demyelination models and supports that Trem2 function is regulated by sensing tissue damage including a dysregulated lipid environment in very distinct inflammatory environments.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02629-1.

NLRP3-Mediated Piezo1 Upregulation in ACC Inhibitory Parvalbumin-Expressing Interneurons Is Involved in Pain Processing after Peripheral Nerve Injury

The anterior cingulate cortex (ACC) is particularly critical for pain information processing. Peripheral nerve injury triggers neuronal hyper-excitability in the ACC and mediates descending facilitation to the spinal dorsal horn. The mechanically gated ion channel Piezo1 is involved in the transmission of pain information in the peripheral nervous system. However, the pain-processing role of Piezo1 in the brain is unknown. In this work, we found that spared (sciatic) nerve injury (SNI) increased Piezo1 protein levels in inhibitory parvalbumin (PV)-expressing interneurons (PV-INs) but not in glutaminergic CaMKⅡ+ neurons, in the bilateral ACC. A reduction in the number of PV-INs but not in the number of CaMKⅡ+ neurons and a significant reduction in inhibitory synaptic terminals was observed in the SNI chronic pain model. Further, observation of morphological changes in the microglia in the ACC showed their activated amoeba-like transformation, with a reduction in process length and an increase in cell body area. Combined with the encapsulation of Piezo1-positive neurons by Iba1+ microglia, the loss of PV-INs after SNI might result from phagocytosis by the microglia. In cellular experiments, administration of recombinant rat TNF-α (rrTNF) to the BV2 cell culture or ACC neuron primary culture elevated the protein levels of Piezo1 and NOD-like receptor (NLR) family pyrin domain containing 3 (NLRP3). The administration of the NLRP3 inhibitor MCC950 in these cells blocked the rrTNF-induced expression of caspase-1 and interleukin-1β (key downstream factors of the activated NLRP3 inflammasome) in vitro and reversed the SNI-induced Piezo1 overexpression in the ACC and alleviated SNI-induced allodynia in vivo. These results suggest that NLRP3 may be the key factor in causing Piezo1 upregulation in SNI, promoting an imbalance between ACC excitation and inhibition by inducing the microglial phagocytosis of PV-INs and, thereby, facilitating spinal pain transmission.

The validity of animal models to explore the pathogenic role of the complement system in multiple sclerosis: A review

Animal models of multiple sclerosis (MS) have been extensively used to characterize the disease mechanisms in MS, as well as to identify potential pharmacologic targets for this condition. In recent years, the immune complement system has gained increased attention as an important effector in the pathogenesis of MS. Evidence from histological, serum, and CSF studies of patients supports an involvement of complement in both relapsing-remitting and progressive MS. In this review, we discuss the history and advances made on the use of MS animal models to profile the effects of the complement system in this condition. The first studies that explored the complement system in the context of MS used cobra venom factor (CVF) as a complement depleting agent in experimental autoimmune encephalomyelitis (EAE) Lewis rats. Since then, multiple mice and rat models of MS have revealed a role of C3 and the alternative complement cascade in the opsonization and phagocytosis of myelin by microglia and myeloid cells. Studies using viral vectors, genetic knockouts and pharmacologic complement inhibitors have also shown an effect of complement in synaptic loss. Antibody-mediated EAE models have revealed an involvement of the C1 complex and the classical complement as an effector of the humoral response in this disease. C1q itself may also be involved in modulating microglia activation and oligodendrocyte differentiation in these animals. In addition, animal and in vitro models have revealed that multiple complement factors may act as modulators of both the innate and adaptive immune responses. Finally, evidence gathered from mice models suggests that the membrane attack complex (MAC) may even exert protective roles in the chronic stages of EAE. Overall, this review summarizes the importance of MS animal models to better characterize the role of the complement system and guide future therapeutic approaches in this condition.

Regulation of activated microglia and macrophages by systemically administered DNA/RNA heteroduplex oligonucleotides

Microglial activation followed by recruitment of blood-borne macrophages into the central nervous system (CNS) aggravates neuroinflammation. Specifically, in multiple sclerosis (MS) as well as in experimental autoimmune encephalomyelitis (EAE), a rodent model of MS, activated microglia and macrophages (Mg/Mφ) promote proinflammatory responses and expand demyelination in the CNS. However, a potent therapeutic approach through the systemic route for regulating their functions has not yet been developed. Here, we demonstrate that a systemically injected DNA/RNA heteroduplex oligonucleotide (HDO), composed of an antisense oligonucleotide (ASO) and its complementary RNA, conjugated to cholesterol (Chol-HDO) distributed more efficiently to demyelinating lesions of the spinal cord in EAE mice with significant gene silencing than the parent ASO. Importantly, systemic administration of Cd40-targeting Chol-HDO improved clinical signs of EAE with significant downregulation of Cd40 in Mg/Mφ. Furthermore, we successfully identify that macrophage scavenger receptor 1 (MSR1) is responsible for the uptake of Chol-HDO by Mg/Mφ of EAE mice. Overall, our findings demonstrate the therapeutic potency of systemically administered Chol-HDO to regulate activated Mg/Mφ in neuroinflammation.

Exploring the Pro-Phagocytic and Anti-Inflammatory Functions of PACAP and VIP in Microglia: Implications for Multiple Sclerosis

Multiple sclerosis (MS) is a chronic neuroinflammatory and demyelinating disease of the central nervous system (CNS), characterised by the infiltration of peripheral immune cells, multifocal white-matter lesions, and neurodegeneration. In recent years, microglia have emerged as key contributors to MS pathology, acting as scavengers of toxic myelin/cell debris and modulating the inflammatory microenvironment to promote myelin repair. In this review, we explore the role of two neuropeptides, pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP), as important regulators of microglial functioning during demyelination, myelin phagocytosis, and remyelination, emphasising the potential of these neuropeptides as therapeutic targets for the treatment of MS.

Carbohydrate and lipid metabolism in multiple sclerosis: Clinical implications for etiology, pathogenesis, diagnosis, prognosis, and therapy

Multiple sclerosis (MS) is a complicated autoimmune disease characterized by inflammatory and demyelinating events in the central nervous system. The exact etiology and pathogenesis of MS have not been elucidated. However, a set of metabolic changes and their effects on immune cells and neural functions have been explained. This review highlights the contribution of carbohydrates and lipids metabolism to the etiology and pathogenesis of MS. Then, we have proposed a hypothetical relationship between such metabolic changes and the immune system in patients with MS. Finally, the potential clinical implications of these metabolic changes in diagnosis, prognosis, and discovering therapeutic targets have been discussed. It is concluded that research on the pathophysiological alterations of carbohydrate and lipid metabolism may be a potential strategy for paving the way toward MS treatment.

The Unique Phenotype of Lipid-Laden Macrophages

Macrophages are key multi-talented cells of the innate immune system and are equipped with receptors involved in damage and pathogen recognition with connected immune response guiding signaling systems. In addition, macrophages have various systems that are involved in the uptake of extracellular and intracellular cargo. The lysosomes in macrophages play a central role in the digestion of all sorts of macromolecules and the entry of nutrients to the cytosol, and, thus, the regulation of endocytic processes and autophagy. Simplistically viewed, two macrophage phenotype extremes exist. On one end of the spectrum, the classically activated pro-inflammatory M1 cells are present, and, on the other end, alternatively activated anti-inflammatory M2 cells. A unique macrophage population arises when lipid accumulation occurs, either caused by flaws in the catabolic machinery, which is observed in lysosomal storage disorders, or as a result of an acquired condition, which is found in multiple sclerosis, obesity, and cardiovascular disease. The accompanying overload causes a unique metabolic activation phenotype, which is discussed here, and, consequently, a unifying phenotype is proposed.

Tissue-resident memory T cells invade the brain parenchyma in multiple sclerosis white matter lesions

Multiple sclerosis is a chronic inflammatory, demyelinating disease, although it has been suggested that in the progressive late phase, inflammatory lesion activity declines. We recently showed in the Netherlands Brain Bank multiple sclerosis-autopsy cohort considerable ongoing inflammatory lesion activity also at the end stage of the disease, based on microglia/macrophage activity. We have now studied the role of T cells in this ongoing inflammatory lesion activity in chronic multiple sclerosis autopsy cases. We quantified T cells and perivascular T-cell cuffing at a standardized location in the medulla oblongata in 146 multiple sclerosis, 20 neurodegenerative control and 20 non-neurological control brain donors. In addition, we quantified CD3+, CD4+, and CD8+ T cells in 140 subcortical white matter lesions. The location of CD8+ T cells in either the perivascular space or the brain parenchyma was determined using CD8/laminin staining and confocal imaging. Finally, we analysed CD8+ T cells, isolated from fresh autopsy tissues from subcortical multiple sclerosis white matter lesions (n = 8), multiple sclerosis normal-appearing white matter (n = 7), and control white matter (n = 10), by flow cytometry. In normal-appearing white matter, the number of T cells was increased compared to control white matter. In active and mixed active/inactive lesions, the number of T cells was further augmented compared to normal-appearing white matter. Active and mixed active/inactive lesions were enriched for both CD4+ and CD8+ T cells, the latter being more abundant in all lesion types. Perivascular clustering of T cells in the medulla oblongata was only found in cases with a progressive disease course and correlated with a higher percentage of mixed active/inactive lesions and a higher lesion load compared to cases without perivascular clusters in the medulla oblongata. In all white matter samples, CD8+ T cells were located mostly in the perivascular space, whereas in mixed active/inactive lesions, 16.3% of the CD8+ T cells were encountered in the brain parenchyma. CD8+ T cells from mixed active/inactive lesions showed a tissue-resident memory phenotype with expression of CD69, CD103, CD44, CD49a, and PD-1 and absence of S1P1. They upregulated markers for homing (CXCR6), reactivation (Ki-67), and cytotoxicity (GPR56), yet lacked the cytolytic enzyme granzyme B. These data show that in chronic progressive multiple sclerosis cases, inflammatory lesion activity and demyelinated lesion load is associated with an increased number of T cells clustering in the perivascular space. Inflammatory active multiple sclerosis lesions are populated by CD8+ tissue-resident memory T cells, which show signs of reactivation and infiltration of the brain parenchyma.

Imaging epileptic foci in mouse models via a low-density lipoprotein receptor-related protein-1 targeting strategy

Background

In the setting of drug-resistant epilepsy (DRE), the success of surgery depends on the ability to accurately locate the epileptic foci to be resected or disconnected. However, the epileptic foci in a considerable percentage of the DRE patients cannot be adequately localised. This warrants the need for a reliable imaging strategy to identify the “concealed” epileptic regions.

Methods

Brain specimens from DRE patients and kainate-induced epileptic mouse models were immuno-stained to evaluate the integrity of the blood-brain barrier (BBB). The expression of low-density lipoprotein receptor-related protein-1 (LRP1) in the epileptic region of DRE patients and kainate models was studied by immunofluorescence. A micellar-based LRP1-targeted paramagnetic probe (Gd³⁺-LP) was developed and its ability to define the epileptic foci was investigated by magnetic resonance imaging (MRI).

Findings

The integrity of the BBB in the epileptic region of DRE patients and kainate mouse models were demonstrated. LRP1 expression levels in the epileptic foci of DRE patients and kainate models were 1.70–2.38 and 2.32–3.97 folds higher than in the control brain tissues, respectively. In vivo MRI demonstrated that Gd³⁺-LP offered 1.68 times higher (P < 0.05) T1-weighted intensity enhancement in the ipsilateral hippocampus of chronic kainite models than the control probe without LRP1 specificity.

Interpretation

The expression of LRP1 is up-regulated in vascular endothelium, activated glia in both DRE patients and kainate models. LRP1-targeted imaging strategy may provide an alternative strategy to define the “concealed” epileptic foci by overcoming the intact BBB.

Funding

This work was supported by the National Natural Science Foundation, Shanghai Science and Technology Committee, Shanghai Municipal Science and Technology, Shanghai Municipal Health and Family Planning Commission and the National Postdoctoral Program for Innovative Talents.

Retinoic acid increases phagocytosis of myelin by macrophages

Traumatic injuries of the central nervous system (CNS) are followed by the accumulation of cellular debris including proteins and lipids from myelinated fiber tracts. Insufficient phagocytic clearance of myelin debris influences the pathological process because it induces inflammation and blocks axonal regeneration. We investigated whether ligands of nuclear receptor families retinoic acid receptors (RARs), retinoid X receptors, peroxisome proliferator-activated receptors, lipid X receptors, and farnesoid X receptors increase myelin phagocytosis by murine bone marrow-derived macrophages and Raw264.7 cells. Using in vitro assays with 3,3'-dioctadecyloxacarbocyanine perchlorate- and pHrodo-labeled myelin we found that the transcriptional activator all-trans retinoic acid (RA)enhanced endocytosis of myelin involving the induction of tissue transglutaminase-2. The RAR-dependent increase of phagocytosis was not associated with changes in gene expression of receptors FcγR1, FcγR2b, FcγR3, TREM2, DAP12, CR3, or MerTK. The combination of RA and myelin exposure significantly reduced the expression of M1 marker genes inducible nitric oxide synthase and interleukin-1β and increased expression of transmembrane proteins CD36 and ABC-A1, which are involved in lipid transport and metabolism. The present results suggest an additional mechanism for therapeutic applications of RA after CNS trauma. It remains to be studied whether endogenous RA-signaling regulates phagocytosis in vivo.

Salvianolate reduces neuronal apoptosis by suppressing OGD-induced microglial activation

AIMS

The aim of this study was to investigate the mechanism of pro-inflammatory phenotype transformation of microglia induced by oxygen-glucose deprivation (OGD), and how salvianolate regulates the polarization of microglia to exert neuroprotective effects.

MAIN METHODS

The immunofluorescence and western blot experiments were used to verify the injury effect on neuronal cells after inflammatory polarization of microglia. Secondly, immunofluorescence staining and western blot were analyzed inflammatory phenotype of microglia and TLR4 signaling pathway after salvianolate treatment. RT-qPCR and ELISA assays were showed the levels of RNA and proteins of inflammatory factors in microglia. Finally, flow cytometry and western blot assay proved that salvianolate had a certain protective effect on neuronal injury after inhibiting the phenotype of microglia.

KEY FINDINGS

The OGD condition could promote inflammation and activate of TLR4 signal pathway in microglia, and the polarization of microglia triggered caspase-3 signal pathway of neuronal cell. The optimal concentrations of salvianolate were incubated with microglia under OGD condition, which could reduce the reactive oxygen species (ROS) expression (P = 0.002) and also regulate the activity of SOD, CAT and GSH-px enzymes (P < 0.05). Moreover, salvianolate treatment could inhibit TLR4 signal pathway (P = 0.012), suppress the pro-inflammatory phenotype of microglia in OGD condition (P = 0.018), and reduce the expression of IL-6 and TNF-α (P < 0.05). Finally, neuronal damage induced by microglia under OGD condition was reversed after administration of the microglia supernatant after salvianolate treatment.

SIGNIFICANCE

Salvianolate, as an antioxidant, plays a neuroprotective role by inhibiting the pro-inflammatory phenotype and decreasing the expression of ROS in microglia.

Microglial Phagocytosis-Rational but Challenging Therapeutic Target in Multiple Sclerosis

Multiple sclerosis (MS) is the most common autoimmune and demyelinating disease of the central nervous system (CNS), characterized, in the majority of cases, by initial relapses that later evolve into progressive neurodegeneration, severely impacting patients’ motor and cognitive functions. Despite the availability of immunomodulatory therapies effective to reduce relapse rate and slow disease progression, they all failed to restore CNS myelin that is necessary for MS full recovery. Microglia are the primary inflammatory cells present in MS lesions, therefore strongly contributing to demyelination and lesion extension. Thus, many microglial-based therapeutic strategies have been focused on the suppression of microglial pro-inflammatory phenotype and neurodegenerative state to reduce disease severity. On the other hand, the contribution of myelin phagocytosis advocating the neuroprotective role of microglia in MS has been less explored. Indeed, despite the presence of functional oligodendrocyte precursor cells (OPCs), within lesioned areas, MS plaques fail to remyelinate as a result of the over-accumulation of myelin-toxic debris that must be cleared away by microglia. Dysregulation of this process has been associated with the impaired neuronal recovery and deficient remyelination. In line with this, here we provide a comprehensive review of microglial myelin phagocytosis and its involvement in MS development and repair. Alongside, we discuss the potential of phagocytic-mediated therapeutic approaches and encourage their modulation as a novel and rational approach to ameliorate MS-associated pathology.

Single-cell mass cytometry reveals complex myeloid cell composition in active lesions of progressive multiple sclerosis

Myeloid cells contribute to inflammation and demyelination in the early stages of multiple sclerosis (MS), but it is still unclear to what extent these cells are involved in active lesion formation in progressive MS (PMS). Here, we have harnessed the power of single-cell mass cytometry (CyTOF) to compare myeloid cell phenotypes in active lesions of PMS donors with those in normal-appearing white matter from the same donors and control white matter from non-MS donors. CyTOF measurements of a total of 74 targeted proteins revealed a decreased abundance of homeostatic and TNFhi microglia, and an increase in highly phagocytic and activated microglia states in active lesions of PMS donors. Interestingly, in contrast to results obtained from studies of the inflammatory early disease stages of MS, infiltrating monocyte-derived macrophages were scarce in active lesions of PMS, suggesting fundamental differences of myeloid cell composition in advanced stages of PMS.

Time-dependent cytokine and chemokine changes in mouse cerebral cortex following a mild traumatic brain injury

Traumatic brain injury (TBI) is a serious global health problem, many individuals live with TBI-related neurological dysfunction. A lack of biomarkers of TBI has impeded medication development. To identify new potential biomarkers, we time-dependently evaluated mouse brain tissue and neuronally derived plasma extracellular vesicle proteins in a mild model of TBI with parallels to concussive head injury. Mice (CD-1, 30–40 g) received a sham procedure or 30 g weight-drop and were euthanized 8, 24, 48, 72, 96 hr, 7, 14 and 30 days later. We quantified ipsilateral cortical proteins, many of which differed from sham by 8 hours post-mTBI, particularly GAS-1 and VEGF-B were increased while CXCL16 reduced, 23 proteins changed in 4 or more of the time points. Gene ontology pathways mapped from altered proteins over time related to pathological and physiological processes. Validation of proteins identified in this study may provide utility as treatment response biomarkers.

Perivascular tissue resident memory T cells as therapeutic target in multiple sclerosis

INTRODUCTION

Multiple sclerosis (MS) is characterized by inflammatory attacks of infiltrating leukocytes at onset but evolves into a smoldering, progressive disease within the central nervous system at its later stages. The authors discuss the contribution of white matter lesions to the pathology of advanced MS, thereby paying particular attention to the role of T cells.

AREAS COVERED

Diagnostic biopsy and autopsy studies of white matter lesions in early MS show different pathological patterns of demyelination and leukocyte infiltration. Brain autopsies from advanced MS display substantial inflammation without distinct patterns and suggest a role for perivascular CD8⁺ tissue-resident memory T (T_RM) cells in active and mixed active/inactive MS white matter lesions. When compared to control and normal-appearing white matter, these lesions are enriched for parenchymal CD8⁺ T cells. In the perivascular space, cuffs containing CD8⁺ T_RM cells are observed also in progressive MS, and could be sites of local reactivation.

EXPERT OPINION

Recent findings point toward the perivascular space as an immunological hotspot, which could be targeted in order to suppress a contribution of T_RM cells to ongoing white matter lesion activity in advanced progressive MS. The authors discuss approaches, which may be explored to suppress T_RM-cell reactivation in the perivascular space.

Neutrophils: Underestimated Players in the Pathogenesis of Multiple Sclerosis (MS)

Neutrophils are the most abundant circulating and first-responding innate myeloid cells and have so far been underestimated in the context of multiple sclerosis (MS). MS is the most frequent, immune-mediated, inflammatory disease of the central nervous system. MS is treatable but not curable and its cause(s) and pathogenesis remain elusive. The involvement of neutrophils in MS pathogenesis has been suggested by the use of preclinical animal disease models, as well as on the basis of patient sample analysis. In this review, we provide an overview of the possible mechanisms and functions by which neutrophils may contribute to the development and pathology of MS. Neutrophils display a broad variety of effector functions enabling disease pathogenesis, including (1) the release of inflammatory mediators and enzymes, such as interleukin-1β, myeloperoxidase and various proteinases, (2) destruction and phagocytosis of myelin (as debris), (3) release of neutrophil extracellular traps, (4) production of reactive oxygen species, (5) breakdown of the blood-brain barrier and (6) generation and presentation of autoantigens. An important question relates to the issue of whether neutrophils exhibit a predominantly proinflammatory function or are also implicated in the resolution of chronic inflammatory responses in MS.

The role of CXC chemokine ligand 16 in physiological and pathological pregnancies

The survival and development of a semi-allogeneic fetus during pregnancy require the involvement of a series of cytokines and immune cells. Chemokines are a type of special cytokine those were originally described as having a role in leukocyte trafficking. CXC chemokine ligand (CXCL) 16 is a member of the chemokine family, and CXC chemokine receptor (CXCR) 6 is its sole receptor. Emerging evidence has shown that CXCL16/CXCR6 is expressed at the maternal-fetal interface, by cell types that include trophoblast cells, decidual stroma cells, and decidual immune cells (eg, monocytes, γδT cells, and natural killer T (NKT) cells). The regulation of expression of CXCL16 is quite complex, and this process involves a multitude of factors. CXCL16 exerts a critical role in the establishment of a successful pregnancy through a series of molecular interactions at the maternal-fetal interface. However, an abnormal expression of CXCL16 is associated with certain pathological states associated with pregnancy, including recurrent miscarriage, pre-eclampsia, and gestational diabetes mellitus (GDM). In the present review, the expression and pleiotropic roles of CXCL16 under conditions of physiological and pathological pregnancy are systematically discussed.

Cranial irradiation mediated spine loss is sex-specific and complement receptor-3 dependent in male mice

Cranial irradiation is the main therapeutic treatment for primary and metastatic malignancies in the brain. However, cranial radiation therapy produces long-term impairment in memory, information processing, and attention that contribute to a decline in quality of life. The hippocampal neural network is fundamental for proper storage and retrieval of episodic and spatial memories, suggesting that hippocampal signaling dysfunction could be responsible for the progressive memory deficits observed following irradiation. Previous rodent studies demonstrated that irradiation induces significant loss in dendritic spine number, alters spine morphology, and is associated with behavioral task deficits. Additionally, the literature suggests a common mechanism in which synaptic elimination via microglial-mediated phagocytosis is complement dependent and associated with cognitive impairment in aging as well as disease. We demonstrate sexual dimorphisms in irradiation-mediated alterations of microglia activation markers and dendritic spine density. Further, we find that the significant dendritic spine loss observed in male mice following irradiation is microglia complement receptor 3 (CR3)-dependent. By identifying sex-dependent cellular and molecular factors underlying irradiation-mediated spine loss, therapies can be developed to counteract irradiation-induced cognitive decline and improve patient quality of life.

Interferon-β corrects massive gene dysregulation in multiple sclerosis: Short-term and long-term effects on immune regulation and neuroprotection

Background

In multiple sclerosis (MS), immune up-regulation is coupled to subnormal immune response to interferon-β (IFN-β) and low serum IFN-β levels. The relationship between the defect in IFN signalling and acute and long-term effects of IFN-β on gene expression in MS is inadequately understood.

Methods

We profiled IFN-β-induced transcriptome shifts, using high-resolution microarrays on 227 mononuclear cell samples from IFN-β-treated MS Complete Responders (CR) stable for five years, and stable and active Partial Responders (PR), stable and active untreated MS, and healthy controls.

Findings

IFN-β injection induced short-term changes in 1,200 genes compared to baseline expression after 4-day IFN washout. Pre-injection after washout, and in response to IFN-β injections, PR more frequently had abnormal gene expression than CR. Surprisingly, short-term IFN-β induced little shift in Th1/Th17/Th2 gene expression, but up-regulated immune-inhibitory genes (ILT, IDO1, PD-L1). Expression of 8,800 genes was dysregulated in therapy-naïve compared to IFN-β-treated patients. These long-term changes in protein-coding and long non-coding RNAs affect immunity, synaptic transmission, and CNS cell survival, and correct the disordered therapy-naïve transcriptome to near-normal. In keeping with its impact on clinical course and brain repair in MS, long-term IFN-β treatment reversed the overexpression of proinflammatory and MMP genes, while enhancing genes involved in the oligodendroglia-protective integrated stress response, neuroprotection, and immunoregulation. In the rectified long-term signature, 277 transcripts differed between stable PR and CR patients.

Interpretation

IFN-β had minimal short-term effects on Th1 and Th2 pathways, but long-term it corrected gene dysregulation and induced immunoregulatory and neuroprotective genes. These data offer new biomarkers for IFN-β responsiveness.

Funding

Unrestricted grants from the US National MS Society, NMSS RG#4509A, and Bayer Pharmaceuticals

STAT1 drives M1 microglia activation and neuroinflammation under hypoxia

Microglia are resident immune cells that act as the first active defence in the central nervous system. These cells constantly monitor the tissue microenvironment and rapidly react in response to hypoxia, infection and injuries. Hypoxia in the brain has been detected in several neurodegenerative disorders such as Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease and Huntington's disease. Hypoxic conditions activate microglia cells towards M1 phenotype resulting in oxidative stress and the release of pro-inflammatory cytokines. Recently, we have demonstrated that oxidative stress induces S-glutathionylation of the STAT1 and hyper-activates its signaling in microglia BV2 cells pointing out the importance of this transcription factor in neuroinflammation. In this paper we analyse the cellular mechanisms that drive M1 microglia activation in BV2 cells in response to hypoxia correlating it to STAT1 activation. The analysis of the molecular mechanism of STAT1 signaling reveals that hypoxia generates oxidative stress and induces both phosphorylation and S-glutathionylation of STAT1 that are responsible of its aberrant activation. The silencing of STAT1 protein expression counteracts hypoxia-M1 microglia phenotype suggesting the strong link between hypoxia-STAT1 and STAT1-microglia activation.

Reduced Angiopoietin-Like 4 Expression in Multiple Sclerosis Lesions Facilitates Lipid Uptake by Phagocytes via Modulation of Lipoprotein-Lipase Activity

Multiple sclerosis (MS) is a chronic inflammatory disorder of the central nervous system (CNS) characterized by the presence of focal demyelinated plaques. Sufficient clearance of myelin and cellular debris is one of the requirements for proper tissue repair and remyelination. The mechanisms underlying the clearance of such debris by phagocytes are not fully understood, but recent findings suggest a prominent role for lipoprotein-lipase (LPL) in this process. Here, we demonstrate that angiopoietin-like 4 (ANGPTL4), a potent inhibitor of LPL, is abundantly expressed in astrocytes in control white matter tissue and its expression is markedly reduced in active MS lesions. We provide evidence that ANGPTL4 inhibits the uptake of myelin-derived lipids by LPL-immunoreactive phagocytes. Taken together, our data suggest that the strong reduction in astrocytic ANGPTL4 expression in active demyelinating MS lesions enables phagocytes to adequately clear myelin debris, setting the stage for remyelination.

Transcriptome analysis of normal-appearing white matter reveals cortisol- and disease-associated gene expression profiles in multiple sclerosis

Inter-individual differences in cortisol production by the hypothalamus–pituitary–adrenal (HPA) axis are thought to contribute to clinical and pathological heterogeneity of multiple sclerosis (MS). At the same time, accumulating evidence indicates that MS pathogenesis may originate in the normal-appearing white matter (NAWM). Therefore, we performed a genome-wide transcriptional analysis, by Agilent microarray, of post-mortem NAWM of 9 control subjects and 18 MS patients to investigate to what extent gene expression reflects disease heterogeneity and HPA-axis activity. Activity of the HPA axis was determined by cortisol levels in cerebrospinal fluid and by numbers of corticotropin-releasing neurons in the hypothalamus, while duration of MS and time to EDSS6 served as indicator of disease severity. Applying weighted gene co-expression network analysis led to the identification of a range of gene modules with highly similar co-expression patterns that strongly correlated with various indicators of HPA-axis activity and/or severity of MS. Interestingly, molecular profiles associated with relatively mild MS and high HPA-axis activity were characterized by increased expression of genes that actively regulate inflammation and by molecules involved in myelination, anti-oxidative mechanism, and neuroprotection. Additionally, group-wise comparisons of gene expression in white matter from control subjects and NAWM from (subpopulations of) MS patients uncovered disease-associated gene expression as well as strongly up- or downregulated genes in patients with relatively benign MS and/or high HPA-axis activity, with many differentially expressed genes being previously undescribed in the context of MS. Overall, the data suggest that HPA-axis activity strongly impacts on molecular mechanisms in NAWM of MS patients, but partly also independently of disease severity.

Transcriptional profiling of human microglia reveals grey-white matter heterogeneity and multiple sclerosis-associated changes

Here we report the transcriptional profile of human microglia, isolated from normal-appearing grey matter (GM) and white matter (WM) of multiple sclerosis (MS) and non-neurological control donors, to find possible early changes related to MS pathology. Microglia show a clear region-specific profile, indicated by higher expression of type-I interferon genes in GM and higher expression of NF-κB pathway genes in WM. Transcriptional changes in MS microglia also differ between GM and WM. MS WM microglia show increased lipid metabolism gene expression, which relates to MS pathology since active MS lesion-derived microglial nuclei show similar altered gene expression. Microglia from MS GM show increased expression of genes associated with glycolysis and iron homeostasis, possibly reflecting microglia reacting to iron depositions. Except for ADGRG1/GPR56, expression of homeostatic genes, such as P2RY12 and TMEM119, is unaltered in normal-appearing MS tissue, demonstrating overall preservation of microglia homeostatic functions in the initiation phase of MS.

LRP1 Controls TNF Release via the TIMP-3/ADAM17 Axis in Endotoxin-Activated Macrophages

The metalloproteinase ADAM17 plays a pivotal role in initiating inflammation by releasing TNF from its precursor. Prolonged TNF release causes many chronic inflammatory diseases, indicating that tight regulation of ADAM17 activity is essential for resolution of inflammation. In this study, we report that the endogenous ADAM17 inhibitor TIMP-3 inhibits ADAM17 activity only when it is bound to the cell surface and that cell surface levels of TIMP-3 in endotoxin-activated human macrophages are dynamically controlled by the endocytic receptor LRP1. Pharmacological blockade of LRP1 inhibited endocytic clearance of TIMP-3, leading to an increase in cell surface levels of the inhibitor that blocked TNF release. Following LPS stimulation, TIMP-3 levels on the surface of macrophages increased 4-fold within 4 h and continued to accumulate at 6 h, before a return to baseline levels at 8 h. This dynamic regulation of cell surface TIMP-3 levels was independent of changes in TIMP-3 mRNA levels, but correlated with shedding of LRP1. These results shed light on the basic mechanisms that maintain a regulated inflammatory response and ensure its timely resolution.

The physiology of foamy phagocytes in multiple sclerosis

Multiple sclerosis (MS) is a chronic disease of the central nervous system characterized by massive infiltration of immune cells, demyelination, and axonal loss. Active MS lesions mainly consist of macrophages and microglia containing abundant intracellular myelin remnants. Initial studies showed that these foamy phagocytes primarily promote MS disease progression by internalizing myelin debris, presenting brain-derived autoantigens, and adopting an inflammatory phenotype. However, more recent studies indicate that phagocytes can also adopt a beneficial phenotype upon myelin internalization. In this review, we summarize and discuss the current knowledge on the spatiotemporal physiology of foamy phagocytes in MS lesions, and elaborate on extrinsic and intrinsic factors regulating their behavior. In addition, we discuss and link the physiology of myelin-containing phagocytes to that of foamy macrophages in other disorders such atherosclerosis.

P2X7 as a scavenger receptor for innate phagocytosis in the brain

The P2X7 receptor has been widely studied for its ATP-induced pro-inflammatory effect, but in the absence of a ligand, P2X7 has a second function as a scavenger receptor, which is active in the development of the human brain. The scavenger activity of P2X7 is only evident in the absence of serum but is fully active in cerebrospinal fluid. P2X7 on the cell surface is present as a membrane complex, and an attachment to non-muscle myosin of the cytoskeleton is required for particle engulfment. Selective antagonists of P2X7 pro-inflammatory function have little effect on phagocytosis, but inheritance of a variant haplotype spanning the P2RX7 and P2RX4 genes has been associated with loss of P2X7-mediated phagocytosis. Recent studies in mice suggest that the innate phagocytosis mediated by P2X7 receptors declines with ageing. Thus, defective P2X7-mediated phagocytosis may contribute to age-related neuro-degenerative diseases including Alzheimer's disease, age-related macular degeneration and primary progressive multiple sclerosis.

Progressive multiple sclerosis patients show substantial lesion activity that correlates with clinical disease severity and sex: a retrospective autopsy cohort analysis

Multiple sclerosis (MS) is a highly heterogeneous disease with large inter-individual differences in disease course. MS lesion pathology shows considerable heterogeneity in localization, cellular content and degree of demyelination between patients. In this study, we investigated pathological correlates of disease course in MS using the autopsy cohort of the Netherlands Brain Bank (NBB), containing 182 MS brain donors. Using a standardized autopsy procedure including systematic dissection from standard locations, 3188 tissue blocks containing 7562 MS lesions were dissected. Unbiased measurements of lesion load were made using the tissue from standard locations. Lesion demyelinating and innate inflammatory activity were visualized by immunohistochemistry for proteolipid protein and human leukocyte antigen. Lesions were classified into active, mixed active/inactive (also known as chronic active), inactive or remyelinated, while microglia/macrophage morphology was classified as ramified, amoeboid or foamy. The severity score was calculated from the time from first symptoms to EDSS-6. Lesion type prevalence and microglia/macrophage morphology were analyzed in relation to clinical course, disease severity, lesion load and sex, and in relation to each other. This analysis shows for the first time that (1) in progressive MS, with a mean disease duration of 28.6 ± 13.3 years (mean ± SD), there is substantial inflammatory lesion activity at time to death. 57% of all lesions were either active or mixed active/inactive and 78% of all patients had a mixed active/inactive lesion present; (2) patients that had a more severe disease course show a higher proportion of mixed active/inactive lesions (p = 6e-06) and a higher lesion load (p = 2e-04) at the time of death, (3) patients with a progressive disease course show a higher lesion load (p = 0.001), and a lower proportion of remyelinated lesions (p = 0.03) compared to patients with a relapsing disease course, (4) males have a higher incidence of cortical grey matter lesions (p = 0.027) and a higher proportion of mixed active/inactive lesions compared to females across the whole cohort (p = 0.007). We confirm that there is a higher proportion of mixed active/inactive lesions (p = 0.006) in progressive MS compared to relapsing disease. Identification of mixed active/inactive lesions on MRI is necessary to determine whether they can be used as a prognostic tool in living MS patients.

Gene Expression Profiling of Multiple Sclerosis Pathology Identifies Early Patterns of Demyelination Surrounding Chronic Active Lesions

In multiple sclerosis (MS), activated microglia and infiltrating macrophages phagocytose myelin focally in (chronic) active lesions. These demyelinating sites expand in time, but at some point turn inactive into a sclerotic scar. To identify molecular mechanisms underlying lesion activity and halt, we analyzed genome-wide gene expression in rim and peri-lesional regions of chronic active and inactive MS lesions, as well as in control tissue. Gene clustering revealed patterns of gene expression specifically associated with MS and with the presumed, subsequent stages of lesion development. Next to genes involved in immune functions, we found regulation of novel genes in and around the rim of chronic active lesions, such as NPY, KANK4, NCAN, TKTL1, and ANO4. Of note, the presence of many foamy macrophages in active rims was accompanied by a congruent upregulation of genes related to lipid binding, such as MSR1, CD68, CXCL16, and OLR1, and lipid uptake, such as CHIT1, GPNMB, and CCL18. Except CCL18, these genes were already upregulated in regions around active MS lesions, showing that such lesions are indeed expanding. In vitro downregulation of the scavenger receptors MSR1 and CXCL16 reduced myelin uptake. In conclusion, this study provides the gene expression profile of different aspects of MS pathology and indicates that early demyelination, mediated by scavenger receptors, is already present in regions around active MS lesions. Genes involved in early demyelination events in regions surrounding chronic active MS lesions might be promising therapeutic targets to stop lesion expansion.

Shedding of membrane-associated LDL receptor-related protein-1 from microglia amplifies and sustains neuroinflammation

In the CNS, microglia are activated in response to injury or infection and in neurodegenerative diseases. The endocytic and cell signaling receptor, LDL receptor-related protein-1 (LRP1), is reported to suppress innate immunity in macrophages and oppose microglial activation. The goal of this study was to identify novel mechanisms by which LRP1 may regulate microglial activation. Using primary cultures of microglia isolated from mouse brains, we demonstrated that LRP1 gene silencing increases expression of proinflammatory mediators; however, the observed response was modest. By contrast, the LRP1 ligand, receptor-associated protein (RAP), robustly activated microglia, and its activity was attenuated in LRP1-deficient cells. An important element of the mechanism by which RAP activated microglia was its ability to cause LRP1 shedding from the plasma membrane. This process eliminated cellular LRP1, which is anti-inflammatory, and generated a soluble product, shed LRP1 (sLRP1), which is potently proinflammatory. Purified sLRP1 induced expression of multiple proinflammatory cytokines and the mRNA encoding inducible nitric-oxide synthase in both LRP1-expressing and -deficient microglia. LPS also stimulated LRP1 shedding, as did the heat-shock protein and LRP1 ligand, calreticulin. Other LRP1 ligands, including α₂-macroglobulin and tissue-type plasminogen activator, failed to cause LRP1 shedding. Treatment of microglia with a metalloproteinase inhibitor inhibited LRP1 shedding and significantly attenuated RAP-induced cytokine expression. RAP and sLRP1 both caused neuroinflammation in vivo when administered by stereotaxic injection into mouse spinal cords. Collectively, these results suggest that LRP1 shedding from microglia may amplify and sustain neuroinflammation in response to proinflammatory stimuli.

Microglia Polarization with M1/M2 Phenotype Changes in rd1 Mouse Model of Retinal Degeneration

Microglia activation is recognized as the hallmark of neuroinflammation. However, the activation profile and phenotype changes of microglia during the process of retinal degeneration are poorly understood. This study aimed to elucidate the time-spatial pattern of microglia distribution and characterize the polarized phenotype of activated microglia during retinal neuroinflammation and degeneration in rd1 (Pde6β^rd1/rd1) mice, the classic model of inherited retinal degeneration. Retinae of rd1 mice at different postnatal days (P7, P14, P21, P28, P56, and P180) were prepared for further analysis. We found most CD11b⁺ or IBA1⁺ microglia expressed Ki-67 and CD68 in rd1 mice and these cells migrated toward the layer of degenerative photoreceptors at the rapid rods degeneration phase from P14 to P28. These microglia exhibited typical ameboid activated shape with round bodies and scarce dendrites, while at late phase at P180, they displayed resting ramified morphology with elongated dendrites. Flow cytometry revealed that the percentage of CD86⁺CD206^- M1 microglia increased markedly in rd1 retinae, however, no significant change was observed in CD206⁺CD86^- M2 microglia. Interestingly, CD86⁺CD206⁺ microglia, an intermediate state between the two extremes of M1 and M2, increased markedly at the rapid rods degeneration phase. The immunofluorescence images revealed that microglia in rd1 mice highly expressed M1 markers including CD16/32, CD86, and CD40. In addition, increased expression of pro-inflammatory cytokines (TNF-α, IL-6, and CCL2) was observed in rd1 mice. Our findings unfolded a panorama for the first time that microglia conducted distinctive behaviors with the progression of retinal degeneration in rd1 mice. Microglia is activated and particularly polarized to a pro-inflammatory M1 phenotype at the rapid rods degenerative phase, suggesting that the involvement of M1 microglia in the retinal neuroinflammation and degeneration. Most microglia adopted an intermediate polarization “M1½” state in rd1, revealing that microglia orchestrated a complicated continuous spectrum in degenerative retina.

Staining of HLA-DR, Iba1 and CD68 in human microglia reveals partially overlapping expression depending on cellular morphology and pathology

HLA-DR, Iba1 and CD68 are widely used microglia markers in human tissue. However, due to differences in gene regulation, they may identify different activation stages of microglia. Here, we directly compared the expression of HLA-DR, Iba1 and CD68 in microglia with different phenotypes, ranging from ramified to amoeboid, to foamy phagocytizing macrophages, in adjacent sections immunocytochemically double stained for two of the markers. Material was used from patients diagnosed with multiple sclerosis (MS) and Alzheimer's disease (AD) patients and control subjects because together they contain all the microglia activation stages in an acute and a chronic inflammatory setting. We found a similar, yet not identical, overall expression pattern. All three markers were expressed by ramified/amoeboid microglia around chronic active MS lesions, but overlap between HLA-DR and Iba1 was limited. Foamy macrophages in the demyelinating rims of active MS lesions of MS expressed more HLA-DR and CD68 than Iba1. All markers were expressed by small microglia accumulations (nodules) in MS NAWM. Dense core AD plaques in the hippocampus were mostly associated with microglia expressing HLA-DR. Diffuse AD plaques were not specifically associated with microglia at all. These results indicate that microglia markers have different potential for neuropathological analysis, with HLA-DR and CD68 reflecting immune activation and response to tissue damage, and Iba1 providing a marker more suited for structural studies in the absence of pathology.

Pharmacological Tools to Activate Microglia and their Possible use to Study Neural Network Patho-physiology

BACKGROUND

Microglia are the resident immunocompetent cells of the CNS and also constitute a unique cell type that contributes to neural network homeostasis and function. Understanding microglia cell-signaling not only will reveal their diverse functions but also will help to identify pharmacological and non-pharmacological tools to modulate the activity of these cells.

METHODS

We undertook a search of bibliographic databases for peer-reviewed research literature to identify microglial activators and their cell-specificity. We also looked for their effects on neural network function and dysfunction.

RESULTS

We identified several pharmacological targets to modulate microglial function, which are more or less specific (with the proper control experiments). We also identified pharmacological targets that would require the development of new potent and specific modulators. We identified a wealth of evidence about the participation of microglia in neural network function and their alterations in pathological conditions.

CONCLUSION

The identification of specific microglia-activating signals provides experimental tools to modulate the activity of this heterogeneous cell type in order to evaluate its impact on other components of the nervous system, and it also helps to identify therapeutic approaches to ease some pathological conditions related to microglial dysfunction.

Myelin phagocytosis by astrocytes after myelin damage promotes lesion pathology

Astrocytes are key players in the pathology of multiple sclerosis and can assume beneficial and detrimental roles during lesion development. The triggers and timing of the different astroglial responses in acute lesions remain unclear. Astrocytes in acute multiple sclerosis lesions have been shown previously to contain myelin debris, although its significance has not been examined. We hypothesized that myelin phagocytosis by astrocytes is an early event during lesion formation and leads to astroglial immune responses. We examined multiple sclerosis lesions and other central nervous system pathologies with prominent myelin injury, namely, progressive multifocal leukoencephalopathy, metachromatic leukodystrophy and subacute infarct. In all conditions, we found that myelin debris was present in most astrocytes at sites of acute myelin breakdown, indicating that astroglial myelin phagocytosis is an early and prominent feature. Functionally, myelin debris was taken up by astrocytes through receptor-mediated endocytosis and resulted in astroglial NF-κB activation and secretion of chemokines. These in vitro results in rats were validated in human disease where myelin-positive hypertrophic astrocytes showed increased nuclear localization of NF-κB and elevated chemokine expression compared to myelin-negative, reactive astrocytes. Thus, our data suggest that myelin uptake is an early response of astrocytes in diseases with prominent myelin injury that results in recruitment of immune cells. This first line response of astrocytes to myelin injury may exert beneficial or detrimental effects on the lesion pathology, depending on the inflammatory context. Modulating this response might be of therapeutic relevance in multiple sclerosis and other demyelinating conditions.

LRP1 expression in microglia is protective during CNS autoimmunity

Multiple sclerosis is a devastating neurological disorder characterized by the autoimmune destruction of the central nervous system myelin. While T cells are known orchestrators of the immune response leading to MS pathology, the precise contribution of CNS resident and peripheral infiltrating myeloid cells is less well described. Here, we explore the myeloid cell function of Low-density lipoprotein receptor-related protein-1 (LRP1), a scavenger receptor involved in myelin clearance and the inflammatory response, in the context of Multiple sclerosis. Supporting its central role in Multiple sclerosis pathology, we find that LRP1 expression is increased in Multiple sclerosis lesions in comparison to the surrounding healthy tissue. Using two genetic mouse models, we show that deletion of LRP1 in microglia, but not in peripheral macrophages, negatively impacts the progression of experimental autoimmune encephalomyelitis, an animal model of Multiple sclerosis. We further show that the increased disease severity in experimental autoimmune encephalomyelitis is not due to haplodeficiency of the Cx3cr1 locus. At the cellular level, microglia lacking LRP1 adopt a pro-inflammatory phenotype characterized by amoeboid morphology and increased production of the inflammatory mediator TNF-α. We also show that LRP1 functions as a robust inhibitor of NF-kB activation in myeloid cells via a MyD88 dependent pathway, potentially explaining the increase in disease severity observed in mice lacking LRP1 expression in microglia. Taken together, our data suggest that the function of LRP1 in microglia is to keep these cells in an anti-inflammatory and neuroprotective status during inflammatory insult, including experimental autoimmune encephalomyelitis and potentially in Multiple sclerosis.