TREM2 regulates microglial cell activation in response to demyelination in vivo

Microglia depletion exacerbates demyelination and impairs remyelination in a neurotropic coronavirus infection

Microglia are considered both pathogenic and protective during recovery from demyelination, but their precise role remains ill defined. Here, using an inhibitor of colony stimulating factor 1 receptor (CSF1R), PLX5622, and mice infected with a neurotropic coronavirus (mouse hepatitis virus [MHV], strain JHMV), we show that depletion of microglia during the time of JHMV clearance resulted in impaired myelin repair and prolonged clinical disease without affecting the kinetics of virus clearance. Microglia were required only during the early stages of remyelination. Notably, large deposits of extracellular vesiculated myelin and cellular debris were detected in the spinal cords of PLX5622-treated and not control mice, which correlated with decreased numbers of oligodendrocytes in demyelinating lesions in drug-treated mice. Furthermore, gene expression analyses demonstrated differential expression of genes involved in myelin debris clearance, lipid and cholesterol recycling, and promotion of oligodendrocyte function. The results also demonstrate that microglial functions affected by depletion could not be compensated by infiltrating macrophages. Together, these results demonstrate that microglia play key roles in debris clearance and in the initiation of remyelination following infection with a neurotropic coronavirus but are not necessary during later stages of remyelination.

Dynamic Responses of Microglia in Animal Models of Multiple Sclerosis

Microglia play an essential role in maintaining central nervous system (CNS) homeostasis, as well as responding to injury and disease. Most neurological disorders feature microglial activation, a process whereby microglia undergo profound morphological and transcriptional changes aimed at containing CNS damage and promoting repair, but often resulting in overt inflammation that sustains and propagates the neurodegenerative process. This is especially evident in multiple sclerosis (MS), were microglial activation and microglia-driven neuroinflammation are considered key events in the onset, progression, and resolution of the disease. Our understanding of microglial functions in MS has widened exponentially in the last decade by way of new tools and markers to discriminate microglia from other myeloid populations. Consequently, the complex functional and phenotypical diversity of microglia can now be appreciated. This, in combination with a variety of animal models that mimic specific features and processes of MS, has contributed to filling the gap of knowledge in the cascade of events underlying MS pathophysiology. The purpose of this review is to present the most up to date knowledge of the dynamic responses of microglia in the commonly used animal models of MS, specifically the immune-mediated experimental autoimmune encephalomyelitis (EAE) model, and the chemically-induced cuprizone and lysolecithin models. Elucidating the spectrum of microglial functions in these models, from detrimental to protective, is essential to identify emerging targets for therapy and guide drug discovery efforts.

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.

Connecting Immune Cell Infiltration to the Multitasking Microglia Response and TNF Receptor 2 Induction in the Multiple Sclerosis Brain

Signaling from central nervous system (CNS)-infiltrating lymphocytes and macrophages is critical to activate microglia and cause tissue damage in multiple sclerosis (MS). We combined laser microdissection with high-throughput real time RT-PCR to investigate separately the CNS exogenous and endogenous inflammatory components in postmortem brain tissue of progressive MS cases. A previous analysis of immune infiltrates isolated from the white matter (WM) and the meninges revealed predominant expression of genes involved in antiviral and cytotoxic immunity, including IFNγ and TNF. Here, we assessed the expression of 71 genes linked to IFN and TNF signaling and microglia/macrophage activation in the parenchyma surrounding perivascular cuffs at different stages of WM lesion evolution and in gray matter (GM) lesions underlying meningeal infiltrates. WM and GM from non-neurological subjects were used as controls. Transcriptional changes in the WM indicate activation of a classical IFNγ-induced macrophage defense response already in the normal-appearing WM, amplification of detrimental (proinflammatory/pro-oxidant) and protective (anti-inflammatory/anti-oxidant) responses in actively demyelinating WM lesions and persistence of these dual features at the border of chronic active WM lesions. Transcriptional changes in chronic subpial GM lesions indicate skewing toward a proinflammatory microglia phenotype. TNF receptor 2 (TNFR2) mediating TNF neuroprotective functions was one of the genes upregulated in the MS WM. Using immunohistochemistry we show that TNFR2 is highly expressed in activated microglia in the normal-appearing WM, at the border of chronic active WM lesions, and in foamy macrophages in actively demyelinating WM and GM lesions. In lysolecithin-treated mouse cerebellar slices, a model of demyelination and remyelination, TNFR2 RNA and soluble protein increased immediately after toxin-induced demyelination along with transcripts for microglia/macrophage-derived pro- and anti-inflammatory cytokines. TNFR2 and IL10 RNA and soluble TNFR2 protein remained elevated during remyelination. Furthermore, myelin basic protein expression was increased after selective activation of TNFR2 with an agonistic antibody. This study highlights the key role of cytotoxic adaptive immunity in driving detrimental microglia activation and the concomitant healing response. It also shows that TNFR2 is an early marker of microglia activation and promotes myelin synthesis, suggesting that microglial TNFR2 activation can be exploited therapeutically to stimulate CNS repair.

Echinacoside promotes the proliferation of human renal tubular epithelial cells by blocking the HBX/TREM2‑mediated NF‑κB signalling pathway

Hepatitis B virus X (HBX) protein is required for the replication of HBV and plays a role in the progression of hepatitis in humans. However, the underlying function of HBX during HBV‑induced chronic glomerulonephritis (HBV‑GN) is unknown. Echinacoside (ECH) is a phenylethanoid glycoside from the Cistanche genus, which possesses strong antiapoptosis and neuroprotective activities. In the present study, the function of HBX and the relationship between HBX and ECH in human renal tubular epithelial cells (RTECs; HK‑2 cell line) were explored. Reverse transcription‑quantitative PCR and western blot analyses were used to quantify the mRNA and protein expression levels of HBX in HK‑2 cells, respectively. The Cell Counting Kit‑8 assay was performed to analyse cell proliferation. Flow cytometry analysis was used to determine the rate of apoptosis. HBX showed antiproliferative and proapoptotic effects in HK‑2 cells and was positively associated with triggering receptor expressed on myeloid cells 2 (TREM2) expression. Furthermore, ECH disrupted the function of HBX in HK‑2 cells, functioning as an HBX suppressor. Moreover, a specific NF‑κB inhibitor, PDTC, was used to further examine the relationship between HBX and NF‑κB. The results suggested that NF‑κB was involved in the HBX/TREM2 signaling pathway and negatively regulated TREM2 expression in RTECs. The present study provided novel insights into the function of HBX, and also indicated the potential value of ECH as a therapeutic agent for HBV‑GN.

Differential accumulation of storage bodies with aging defines discrete subsets of microglia in the healthy brain

To date, microglia subsets in the healthy CNS have not been identified. Utilizing autofluorescence (AF) as a discriminating parameter, we identified two novel microglia subsets in both mice and non-human primates, termed autofluorescence-positive (AF+) and negative (AF-). While their proportion remained constant throughout most adult life, the AF signal linearly and specifically increased in AF+ microglia with age and correlated with a commensurate increase in size and complexity of lysosomal storage bodies, as detected by transmission electron microscopy and LAMP1 levels. Post-depletion repopulation kinetics revealed AF- cells as likely precursors of AF+ microglia. At the molecular level, the proteome of AF+ microglia showed overrepresentation of endolysosomal, autophagic, catabolic, and mTOR-related proteins. Mimicking the effect of advanced aging, genetic disruption of lysosomal function accelerated the accumulation of storage bodies in AF+ cells and led to impaired microglia physiology and cell death, suggestive of a mechanistic convergence between aging and lysosomal storage disorders.

Shifting paradigms: The central role of microglia in Alzheimer's disease

Recent human genetic studies have challenged long standing hypotheses about the chain of events in Alzheimer's disease (AD), as the identification of genetic risk factors in microglial genes supports a causative role for microglia in the disease. Parallel transcriptome and histology studies at the single-cell level revealed a rich palette of microglial states affected by disease status and genetic risk factors. Taken together, those findings support microglia dysfunction as a central mechanism in AD etiology and thus the therapeutic potential of modulating microglial activity for AD treatment. Here we review how human genetic studies discovered microglial AD risk genes, such as TREM2, CD33, MS4A and APOE, and how experimental studies are beginning to decipher the cellular functions of some of these genes. Our review also focuses on recent transcriptomic studies of human microglia from postmortem tissue to critically assess areas of similarity and dissimilarity between human and mouse models currently in use in order to better understand the biology of innate immunity in AD.

Alzheimer's-associated PLCγ2 is a signaling node required for both TREM2 function and the inflammatory response in human microglia

Human genetic data indicate that microglial dysfunction contributes to the pathology of Alzheimer's disease (AD), exemplified by the identification of coding variants in triggering receptor expressed on myeloid cells 2 (TREM2) and, more recently, in PLCG2, a phospholipase-encoding gene expressed in microglia. Although studies in mouse models have implicated specific Trem2-dependent microglial functions in AD, the underlying molecular mechanisms and translatability to human disease remain poorly defined. In this study, we used genetically engineered human induced pluripotent stem cell-derived microglia-like cells to show that TREM2 signals through PLCγ2 to mediate cell survival, phagocytosis, processing of neuronal debris, and lipid metabolism. Loss of TREM2 or PLCγ2 signaling leads to a shared signature of transcriptional dysregulation that underlies these phenotypes. Independent of TREM2, PLCγ2 also signals downstream of Toll-like receptors to mediate inflammatory responses. Therefore, PLCγ2 activity regulates divergent microglial functions via distinct TREM2-dependent and -independent signaling and might be involved in the transition to a microglial state associated with neurodegenerative disease.

Triggering receptor expressed on myeloid cells 2 activation downregulates toll-like receptor 4 expression and ameliorates cognitive impairment in the Aβ1-42 -induced Alzheimer's disease mouse model

Increasing evidence suggests that changes in the triggering receptor expressed on myeloid cells 2 (TREM2) is closely correlated with the pathological development of Alzheimer's disease (AD). However, the biological function and related role of this change remain poorly understood. Higher TREM2 expression has been reported in the brain of AD patients than in normal controls. Here, levels of TREM2 gene and protein levels were observed to be higher in both cortex and hippocampus of the Aβ_1-42 -induced AD mice than in those of the wild type mice. Together with in vitro experimental data, we found that the anti-inflammatory role of TREM2 was, to some extent, limited and potentially counteracted by the hyperactive toll-like receptor 4 (TLR4) in the AD mice. In this context, Interleukin 4 (IL-4), as an agonist of TREM2, was administered to the AD mice to persistently activate TREM2. Interestingly, TREM2 activation in IL-4-treated AD mice led to an elevation in lysosomes and microtubule-associated protein 1 light chain 3 (LC3) II/I expression, demonstrating that the level of microglia autophagy was increased. Increased autophagy significantly downregulated the expression levels of caspase recruitment domain-containing protein 9 (CARD9) and TLR4, potentially weakening the CARD9-TLR4 pathway and suppressing the TLR4-mediated pro-inflammatory effect in IL-4-treated AD mice. Furthermore, data acquired from Morris water maze testing indicated that IL-4 administration could ameliorate cognitive impairment in the AD mice. In conclusion, the findings from in vitro and in vivo experiments suggest that TREM2 might represent a potential drug target to treat neuroinflammation in AD.

Enhancing protective microglial activities with a dual function TREM2 antibody to the stalk region

Triggering receptor expressed on myeloid cells 2 (TREM2) is essential for the transition of homeostatic microglia to a disease-associated microglial state. To enhance TREM2 activity, we sought to selectively increase the full-length protein on the cell surface via reducing its proteolytic shedding by A Disintegrin And Metalloproteinase (i.e., α-secretase) 10/17. We screened a panel of monoclonal antibodies against TREM2, with the aim to selectively compete for α-secretase-mediated shedding. Monoclonal antibody 4D9, which has a stalk region epitope close to the cleavage site, demonstrated dual mechanisms of action by stabilizing TREM2 on the cell surface and reducing its shedding, and concomitantly activating phospho-SYK signaling. 4D9 stimulated survival of macrophages and increased microglial uptake of myelin debris and amyloid β-peptide in vitro. In vivo target engagement was demonstrated in cerebrospinal fluid, where nearly all soluble TREM2 was 4D9-bound. Moreover, in a mouse model for Alzheimer's disease-related pathology, 4D9 reduced amyloidogenesis, enhanced microglial TREM2 expression, and reduced a homeostatic marker, suggesting a protective function by driving microglia toward a disease-associated state.

Protective Microglial Subset in Development, Aging, and Disease: Lessons From Transcriptomic Studies

Microglial heterogeneity has been the topic of much discussion in the scientific community. Elucidation of their plasticity and adaptability to disease states triggered early efforts to characterize microglial subsets. Over time, their phenotypes, and later on their homeostatic signature, were revealed, through the use of increasingly advanced transcriptomic techniques. Recently, an increasing number of these "microglial signatures" have been reported in various homeostatic and disease contexts. Remarkably, many of these states show similar overlapping microglial gene expression patterns, both in homeostasis and in disease or injury. In this review, we integrate information from these studies, and we propose a unique subset, for which we introduce a core signature, based on our own research and reports from the literature. We describe that this subset is found in development and in normal aging as well as in diverse diseases. We discuss the functions of this subset as well as how it is induced.

The pro-remyelination properties of microglia in the central nervous system

Microglia are resident macrophages of the CNS that are involved in its development, homeostasis and response to infection and damage. Microglial activation is a common feature of neurological disorders, and although in some instances this activation can be damaging, protective and regenerative functions of microglia have been revealed. The most prominent example of the regenerative functions is a role for microglia in supporting regeneration of myelin after injury, a process that is critical for axonal health and relevant to numerous disorders in which loss of myelin integrity is a prevalent feature, such as multiple sclerosis, Alzheimer disease and motor neuron disease. Although drugs that are intended to promote remyelination are entering clinical trials, the mechanisms by which remyelination is controlled and how microglia are involved are not completely understood. In this Review, we discuss work that has identified novel regulators of microglial activation - including molecular drivers, population heterogeneity and turnover - that might influence their pro-remyelination capacity. We also discuss therapeutic targeting of microglia as a potential approach to promoting remyelination.

Brain Parenchymal and Extraparenchymal Macrophages in Development, Homeostasis, and Disease

Microglia are parenchymal macrophages of the CNS; as professional phagocytes they are important for maintenance of the brain's physiology. These cells are generated through primitive hematopoiesis in the yolk sac and migrate into the brain rudiment after establishment of embryonic circulation. Thereafter, microglia develop in a stepwise fashion, reaching complete maturity after birth. In the CNS, microglia self-renew without input from blood monocytes. Recent RNA-sequencing studies have defined a molecular signature for microglia under homeostasis. However, during disease, microglia undergo remarkable phenotypic changes, which reflect the acquisition of specialized functions tailored to the pathological context. In addition to microglia, the brain-border regions host populations of extraparenchymal macrophages with disparate origins and phenotypes that have recently been delineated. In this review we outline recent findings that provide a deeper understanding of both parenchymal microglia and extraparenchymal brain macrophages in homeostasis and during disease.

Longitudinal transcriptomic analysis of altered pathways in a CHMP2Bintron5-based model of ALS-FTD

Amyotrophic lateral sclerosis and frontotemporal dementia are two neurodegenerative diseases with currently no cure. These two diseases share a clinical continuum with overlapping genetic causes. Mutations in the CHMP2B gene are found in patients with ALS, FTD and ALS-FTD. To highlight deregulated mechanisms occurring in ALS-FTD linked to the CHMP2B gene, we performed a whole transcriptomic study on lumbar spinal cord from CHMP2B^intron5 mice, a model that develops progressive motor alterations associated with dementia symptoms reminiscent of both ALS and FTD. To gain insight into the transcriptomic changes taking place during disease progression this study was performed at three stages: asymptomatic, symptomatic and end stage. We showed that before appearance of motor symptoms, the major disrupted mechanisms were linked with the immune system/inflammatory response and lipid metabolism. These processes were progressively replaced by alterations of neuronal electric activity as motor symptoms appeared, alterations that could lead to motor neuron dysfunction. To investigate overlapping alterations in gene expression between two ALS-causing genes, we then compared the transcriptome of symptomatic CHMP2B^intron5 mice with the one of symptomatic SOD1^G86R mice and found the same families deregulated providing further insights into common underlying dysfunction of biological pathways, disrupted or disturbed in ALS. Altogether, this study provides a database to explore potential new candidate genes involved in the CHMP2B^intron5-based pathogenesis of ALS, and provides molecular clues to further understand the functional consequences that diseased neurons expressing CHMP2B mutant may have on their neighbor cells.

The Emerging Roles and Therapeutic Potential of Soluble TREM2 in Alzheimer's Disease

Alzheimer’s disease (AD) is the most common form of dementia characterized by the deposition of extracellular amyloid-β (Aβ)-containing plaques, the formation of intraneuronal neurofibrillary tangles as well as neuroinflammatory changes. As the key player in the brain innate immune system, microglia has now taken a center stage in AD research. A large number of AD risk loci identified by genome-wide association studies are located in or near the genes highly expressed in microglia. Among them, the triggering receptor expressed on myeloid cells 2 (TREM2) has drawn much attention. A rare variant in TREM2 increases AD risk with an odds ratio comparable to the strongest genetic risk factor apolipoprotein ε4 allele. In the past 6 years, extensive studies have dissected the mechanisms by which TREM2 and its variants modulate microglial functions impacting amyloid and tau pathologies in both animal models and human studies. In addition to the full-length TREM2, research on the soluble form of TREM2 (sTREM2) has facilitated the translation of preclinical findings on TREM2. In this review, we summarize our current understanding of the biology and pathobiology of sTREM2 including its origin, its emergence as a disease biomarker, and its potential neuroprotective functions. These aspects are important for understanding the involvement of sTREM2 in AD pathogenesis and may provide novel insights into applying sTREM2 for AD diagnosis and therapy.

EphA4 Negatively Regulates Myelination by Inhibiting Schwann Cell Differentiation in the Peripheral Nervous System

Myelin plays a crucial role in axon function recovery following nerve damage, and the interaction between Schwann cells (SCs) and regenerating axons profoundly affects myelin formation. Eph receptor A4 (EphA4), a member of the Eph tyrosine kinase receptor family, regulates cell-cell interactions via its ligand ephrins. However, our current knowledge on how EphA4 regulates the formation of myelin sheaths remains limited. In order to explore the roles of EphA4 in myelination in the peripheral nervous system, we used a combination of (1) a co-culture model of dorsal root ganglion (DRG) explants and SCs, (2) a SC differentiation model induced by db-cAMP, and (3) a regeneration model of crushed sciatic nerves in rats. Our results demonstrated that EphA4 inhibited myelination by inhibiting SC differentiation and facilitating SC proliferation in vitro. The in vivo experiments revealed that EphA4 expression in SCs is upregulated following nerve crush injury and then downregulated during remyelination. Moreover, silencing of EphA4 by siRNA or overexpression of EphA4 by genetic manipulation can accelerate or slow down nerve remyelination in crushed sciatic nerves. Taken together, our results suggest that EphA4 may negatively regulate myelination by abrogating SC differentiation.

Distinct Signaling Pathways Regulate TREM2 Phagocytic and NFκB Antagonistic Activities

Several genetic variants of the Triggering Receptor Expressed on Myeloid Cells-2 (TREM2) have been shown to increase the risk of developing Alzheimer’s disease (AD) supporting a role of microglia and immune cells in the pathobiology of AD. We have employed an ectopic model of TREM2 and DAP12 expression in HEK293 cells to study selectively TREM2 dependent signaling and phagocytic functions and evaluated the effects of some of the TREM2 mutations associated with AD. We show that shedding of the TREM2 N-terminal domain does not affect the inhibition of NFκB activation induced by TREM2 while it completely blocks phagocytosis suggesting that TREM2 anti-inflammatory properties can be mediated by the TREM2 C-terminal fragment while the phagocytic activity requires the full-length receptor. In addition, we confirm in that model that apolipoprotein E (APOE) is a ligand for TREM2 and triggers TREM2 signaling. In particular, we show that APOE4 stimulates spleen tyrosine kinase (SYK) activation more potently than APOE2 in a TREM2 dependent manner. Interestingly, TREM2 appears to antagonize NFκB activation induced by phorbol ester but is unable to prevent TNFα induction of NFκB activation suggesting that TREM2 antagonizes inflammatory events triggered downstream of PKC. TREM2 mutations drastically impact TREM2 phagocytosis as well as its ability to antagonize NFκB activation and notably prevent the activation of the PI3K/AKT pathway observed with wild-type TREM2. Overall our data suggest that TREM2 dependent phagocytosis requires an activation of the SYK/PI3K/AKT/PLCγ pathways while the suppression of NFκB activation by TREM2 is independent of SYK, PI3K, and PLCγ activities. This model of ectopic TREM2-DAP12 co-expression appears suitable to study TREM2 signaling as several biological functions of TREM2 and TREM2 mutations that have been previously described in myeloid and microglial cells were also replicated in this model.

DAMP-sensing receptors in sterile inflammation and inflammatory diseases

The innate immune system has the capacity to detect 'non-self' molecules derived from pathogens, known as pathogen-associated molecular patterns, via pattern recognition receptors. In addition, an increasing number of endogenous host-derived molecules, termed damage-associated molecular patterns (DAMPs), have been found to be sensed by various innate immune receptors. The recognition of DAMPs, which are produced or released by damaged and dying cells, promotes sterile inflammation, which is important for tissue repair and regeneration, but can also lead to the development of numerous inflammatory diseases, such as metabolic disorders, neurodegenerative diseases, autoimmune diseases and cancer. Here we examine recent discoveries concerning the roles of DAMP-sensing receptors in sterile inflammation and in diseases resulting from dysregulated sterile inflammation, and then discuss insights into the cross-regulation of these receptors and their ligands.

Myelin in the Central Nervous System: Structure, Function, and Pathology

Oligodendrocytes generate multiple layers of myelin membrane around axons of the central nervous system to enable fast and efficient nerve conduction. Until recently, saltatory nerve conduction was considered the only purpose of myelin, but it is now clear that myelin has more functions. In fact, myelinating oligodendrocytes are embedded in a vast network of interconnected glial and neuronal cells, and increasing evidence supports an active role of oligodendrocytes within this assembly, for example, by providing metabolic support to neurons, by regulating ion and water homeostasis, and by adapting to activity-dependent neuronal signals. The molecular complexity governing these interactions requires an in-depth molecular understanding of how oligodendrocytes and axons interact and how they generate, maintain, and remodel their myelin sheaths. This review deals with the biology of myelin, the expanded relationship of myelin with its underlying axons and the neighboring cells, and its disturbances in various diseases such as multiple sclerosis, acute disseminated encephalomyelitis, and neuromyelitis optica spectrum disorders. Furthermore, we will highlight how specific interactions between astrocytes, oligodendrocytes, and microglia contribute to demyelination in hereditary white matter pathologies.

Microglia in Brain Development, Homeostasis, and Neurodegeneration

Advances in human genetics have implicated a growing number of genes in neurodegenerative diseases, providing insight into pathological processes. For Alzheimer disease in particular, genome-wide association studies and gene expression studies have emphasized the pathogenic contributions from microglial cells and motivated studies of microglial function/dysfunction. Here, we summarize recent genetic evidence for microglial involvement in neurodegenerative disease with a focus on Alzheimer disease, for which the evidence is most compelling. To provide context for these genetic discoveries, we discuss how microglia influence brain development and homeostasis, how microglial characteristics change in disease, and which microglial activities likely influence the course of neurodegeneration. In all, we aim to synthesize varied aspects of microglial biology and highlight microglia as possible targets for therapeutic interventions in neurodegenerative disease.

The MS4A gene cluster is a key modulator of soluble TREM2 and Alzheimer's disease risk

Soluble triggering receptor expressed on myeloid cells 2 (sTREM2) in cerebrospinal fluid (CSF) has been associated with Alzheimer's disease (AD). TREM2 plays a critical role in microglial activation, survival, and phagocytosis; however, the pathophysiological role of sTREM2 in AD is not well understood. Understanding the role of sTREM2 in AD may reveal new pathological mechanisms and lead to the identification of therapeutic targets. We performed a genome-wide association study (GWAS) to identify genetic modifiers of CSF sTREM2 obtained from the Alzheimer's Disease Neuroimaging Initiative. Common variants in the membrane-spanning 4-domains subfamily A (MS4A) gene region were associated with CSF sTREM2 concentrations (rs1582763; P = 1.15 × 10-15); this was replicated in independent datasets. The variants associated with increased CSF sTREM2 concentrations were associated with reduced AD risk and delayed age at onset of disease. The single-nucleotide polymorphism rs1582763 modified expression of the MS4A4A and MS4A6A genes in multiple tissues, suggesting that one or both of these genes are important for modulating sTREM2 production. Using human macrophages as a proxy for microglia, we found that MS4A4A and TREM2 colocalized on lipid rafts at the plasma membrane, that sTREM2 increased with MS4A4A overexpression, and that silencing of MS4A4A reduced sTREM2 production. These genetic, molecular, and cellular findings suggest that MS4A4A modulates sTREM2. These findings also provide a mechanistic explanation for the original GWAS signal in the MS4A locus for AD risk and indicate that TREM2 may be involved in AD pathogenesis not only in TREM2 risk-variant carriers but also in those with sporadic disease.

Peripheral TREM1 responses to brain and intestinal immunogens amplify stroke severity

Stroke is a multiphasic process in which initial cerebral ischemia is followed by secondary injury from immune responses to ischemic brain components. Here we demonstrate that peripheral CD11b+CD45+ myeloid cells magnify stroke injury via activation of triggering receptor expressed on myeloid cells 1 (TREM1), an amplifier of proinflammatory innate immune responses. TREM1 was induced within hours after stroke peripherally in CD11b+CD45+ cells trafficking to ischemic brain. TREM1 inhibition genetically or pharmacologically improved outcome via protective antioxidant and anti-inflammatory mechanisms. Positron electron tomography imaging using radiolabeled antibody recognizing TREM1 revealed elevated TREM1 expression in spleen and, unexpectedly, in intestine. In the lamina propria, noradrenergic-dependent increases in gut permeability induced TREM1 on inflammatory Ly6C+MHCII+ macrophages, further increasing epithelial permeability and facilitating bacterial translocation across the gut barrier. Thus, following stroke, peripheral TREM1 induction amplifies proinflammatory responses to both brain-derived and intestinal-derived immunogenic components. Critically, targeting this specific innate immune pathway reduces cerebral injury.

TREM2 deficiency aggravates α-synuclein-induced neurodegeneration and neuroinflammation in Parkinson's disease models

Variants in the gene encoding the triggering receptor expressed on myeloid cells 2 (TREM2) are known to increase the risk of developing Alzheimer disease and Parkinson's disease (PD). However, the potential role of TREM2 effect on synucleinopathy has not been characterized. In this study, we investigated whether loss of TREM2 function affects α-synucleinopathy both in vitro and in vivo. In vitro, BV2 microglial cells were exposed to α-synuclein (α-syn) in the presence or absence of TREM2 small interference RNA. For in vivo studies, wild-type controls and TREM2 gene knockout mice were intracranially injected in the substantia nigra with adeno-associated viral vectors expressing human α-syn (AAV-SYN) to induce PD. Our results revealed that knockdown of TREM2 aggravated α-syn-induced inflammatory responses in BV2 cells and caused greater apoptosis in SH-SY5Y cells treated with BV2-conditioned medium. In mice, TREM2 knockout exacerbated dopaminergic neuron loss in response to AAV-SYN. Moreover, both in vitro and in vivo TREM2 deficiency induced a shift from an anti-inflammatory toward a proinflammatory activation status of microglia. These data suggest that impairing microglial TREM2 signaling aggravates proinflammatory responses to α-syn and exacerbates α-syn-induced neurodegeneration by modulating microglial activation state.-Guo, Y., Wei, X., Yan, H., Qin, Y., Yan, S., Liu, J., Zhao, Y., Jiang, F., Lou, H. TREM2 deficiency aggravates α-synuclein-induced neurodegeneration and neuroinflammation in Parkinson's disease models.

The Emerging Role of Triggering Receptor Expressed on Myeloid Cells 2 as a Target for Immunomodulation in Ischemic Stroke

Stroke is the second most common cause of death and permanent disability. It is characterized by loss of neural tissue in which inflammation plays a crucial role in both the acute contribution to ischemic damage as in the late-stage impact on post-ischemic tissue regeneration. Microglia play a key role in the inflammatory stroke microenvironment as they can adapt a disease-promoting pro-inflammatory- or pro-regenerative phenotype thereby contributing to the exacerbation or alleviation of ischemic damage, respectively. Triggering receptor expressed on myeloid cells 2 (TREM2) is a cell surface receptor which in the central nervous system is mainly expressed on microglia. This receptor has been shown to play an essential role in microglial phagocytosis and function but its contribution in stroke pathobiology remains unclear. TREM2 was shown to be activated by nucleotides and lipid mediators, key factors that are secreted in the extracellular stroke environment by apoptotic neurons and cell/myelin debris. These factors in turn stimulate TREM2 signaling which mediates microglial migration toward- and phagocytosis of myelin debris and apoptotic cells. Moreover, microglial TREM2 appears to counteract the toll-like receptor response, thereby decreasing the production of pro-inflammatory cytokines. Finally, TREM2 is involved in microglial migration, survival, and is suggested to directly stimulate pro-regenerative phenotype shift. Therefore, this receptor is an attractive target for microglial modulation in the treatment of ischemic stroke and it provides additional information on microglial effector functions. This short review aims to elaborate on these TREM2-mediated microglial functions in the pathobiology and resolving of ischemic stroke.

Direct and indirect effects of lipids on microglia function

Microglia are key players in brain function by maintaining brain homeostasis across lifetime. They participate to brain development and maturation through their ability to release neurotrophic factors, to remove immature synapses or unnecessary neural progenitors. They modulate neuronal activity in healthy adult brains and they also orchestrate the neuroinflammatory response in various pathophysiological contexts such as aging and neurodegenerative diseases. One of the main features of microglia is their high sensitivity to environmental factors, partly via the expression of a wide range of receptors. Recent data pinpoint that dietary fatty acids modulate microglia function. Both the quantity and the type of fatty acid are potent modulators of microglia physiology. The present review aims at dissecting the current knowledge on the direct and indirect mechanisms (focus on gut microbiota and hormones) through which fatty acids influence microglial physiology. We summarize main discoveries from in vitro and in vivo models on fatty acid-mediated microglial modulation. All these studies represent a promising field of research that could promote using nutrition as a novel therapeutic or preventive tool in diseases involving microglia dysfunctions.

Phagocytosis in the Brain: Homeostasis and Disease

Microglia are resident macrophages of the central nervous system and significantly contribute to overall brain function by participating in phagocytosis during development, homeostasis, and diseased states. Phagocytosis is a highly complex process that is specialized for the uptake and removal of opsonized and non-opsonized targets, such as pathogens, apoptotic cells, and cellular debris. While the role of phagocytosis in mediating classical innate and adaptive immune responses has been known for decades, it is now appreciated that phagocytosis is also critical throughout early neural development, homeostasis, and initiating repair mechanisms. As such, modulating phagocytic processes has provided unexplored avenues with the intent of developing novel therapeutics that promote repair and regeneration in the CNS. Here, we review the functional consequences that phagocytosis plays in both the healthy and diseased CNS, and summarize how phagocytosis contributes to overall pathophysiological mechanisms involved in brain injury and repair.

Exercise as a potential modulator of inflammation in patients with Alzheimer's disease measured in cerebrospinal fluid and plasma

BACKGROUND

Neuroinflammation is recognized as part of the pathological progression of Alzheimer's disease (AD), but the molecular mechanisms are still not entirely clear. Systemically, physical exercise has shown to have a positive modulating effect on markers of inflammation. It is not known if this general effect also takes place in the central nervous system in AD. The aim of this study was to investigate the effect of 16 weeks of moderate to high-intensity physical exercise on selected biomarkers of inflammation both systemically and in the CNS, in patients with AD.

METHODS

Plasma and cerebrospinal fluid (CSF) from 198 patients with Alzheimer's disease participating in the Preserving Cognition, Quality of Life, Physical Health and Functional Ability in Alzheimer's Disease: The Effect of Physical Exercise (ADEX) study were analyzed for concentrations of 8‑isoprostane, soluble trigger receptor expressed on myeloid cells 2 (sTREM2), and the MSD v-plex proinflammation panel 1 human containing interferon gamma (IFNγ), Interleukin-10 (IL10), IL12p70, IL13, IL1β, IL2, IL4, IL6, IL8, and tumor necrosis factor alpha (TNFα), before and after a 16-week intervention with physical exercise, and we studied whether changes were modulated by the patients' APOE genotype.

RESULTS

Most inflammatory markers remained unchanged after exercise. We found an increasing effect of 16 weeks of physical exercise on sTREM2 measured in CSF. Further, IL6 in plasma increased in the exercise group after physical exercise (mean relative change 41.03, SD 76.7), compared to controls (-0.97, SD 49.4). In a sub-analysis according to APOE genotype, we found that in ε4 carriers, exercise had a stabilizing effect on IFNγ concentration with a mean relative change of 7.84 (SD 42.6), as compared to controls (114.7 (SD 188.3), p = 0.038.

CONCLUSION

Our findings indicate an effect of physical exercise on markers of neuroinflammation in CSF measured by an increase in sTREM2 in patients with AD. Further, there may be a small inflammatory systemic effect related to physical exercise in patients with AD.

Microglial SIRPα regulates the emergence of CD11c+ microglia and demyelination damage in white matter

A characteristic subset of microglia expressing CD11c appears in response to brain damage. However, the functional role of CD11c⁺ microglia, as well as the mechanism of its induction, are poorly understood. Here we report that the genetic ablation of signal regulatory protein α (SIRPα), a membrane protein, induced the emergence of CD11c⁺ microglia in the brain white matter. Mice lacking CD47, a physiological ligand of SIRPα, and microglia-specific SIRPα-knockout mice exhibited the same phenotype, suggesting that an interaction between microglial SIRPα and CD47 on neighbouring cells suppressed the emergence of CD11c⁺ microglia. A lack of SIRPα did not cause detectable damage to the white matter, but resulted in the increased expression of genes whose expression is characteristic of the repair phase after demyelination. In addition, cuprizone-induced demyelination was alleviated by the microglia-specific ablation of SIRPα. Thus, microglial SIRPα suppresses the induction of CD11c⁺ microglia that have the potential to accelerate the repair of damaged white matter.

Quantification of white matter cellularity and damage in preclinical and early symptomatic Alzheimer's disease

Interest in understanding the roles of white matter (WM) inflammation and damage in the pathophysiology of Alzheimer disease (AD) has been growing significantly in recent years. However, in vivo magnetic resonance imaging (MRI) techniques for imaging inflammation are still lacking. An advanced diffusion-based MRI method, neuro-inflammation imaging (NII), has been developed to clinically image and quantify WM inflammation and damage in AD. Here, we employed NII measures in conjunction with cerebrospinal fluid (CSF) biomarker classification (for β-amyloid (Aβ) and neurodegeneration) to evaluate 200 participants in an ongoing study of memory and aging. Elevated NII-derived cellular diffusivity was observed in both preclinical and early symptomatic phases of AD, while disruption of WM integrity, as detected by decreased fractional anisotropy (FA) and increased radial diffusivity (RD), was only observed in the symptomatic phase of AD. This may suggest that WM inflammation occurs earlier than WM damage following abnormal Aβ accumulation in AD. The negative correlation between NII-derived cellular diffusivity and CSF Aβ42 level (a marker of amyloidosis) may indicate that WM inflammation is associated with increasing Aβ burden. NII-derived FA also negatively correlated with CSF t-tau level (a marker of neurodegeneration), suggesting that disruption of WM integrity is associated with increasing neurodegeneration. Our findings demonstrated the capability of NII to simultaneously image and quantify WM cellularity changes and damage in preclinical and early symptomatic AD. NII may serve as a clinically feasible imaging tool to study the individual and composite roles of WM inflammation and damage in AD.

Non-parenchymal TREM-2 protects the liver from immune-mediated hepatocellular damage

OBJECTIVE

Liver injury impacts hepatic inflammation in part via Toll-like receptor (TLR) signalling. Triggering receptor expressed on myeloid cells 2 (TREM-2) modulates TLR4-mediated inflammation in bone marrow (BM)-derived macrophages but its function in liver injury is unknown. Here we hypothesised that the anti-inflammatory effects of TREM-2 on TLR signalling may limit hepatic injury.

DESIGN

TREM-2 expression was analysed in livers of humans with various forms of liver injury compared with control individuals. Acute and chronic liver injury models were performed in wild type and Trem-2-/- mice. Primary liver cells from both genotypes of mice were isolated for in vitro experiments.

RESULTS

TREM-2 was expressed on non-parenchymal hepatic cells and induced during liver injury in mice and man. Mice lacking TREM-2 exhibited heightened liver damage and inflammation during acute and repetitive carbon tetrachloride and acetaminophen (APAP) intoxication, the latter of which TREM-2 deficiency was remarkably associated with worsened survival. Liver damage in Trem-2-/- mice following chronic injury and APAP challenge was associated with elevated hepatic lipid peroxidation and macrophage content. BM transplantation experiments and cellular reactive oxygen species assays revealed effects of TREM-2 in the context of chronic injury depended on both immune and resident TREM-2 expression. Consistent with effects of TREM-2 on inflammation-associated injury, primary hepatic macrophages and hepatic stellate cells lacking TREM-2 exhibited augmented TLR4-driven proinflammatory responses.

CONCLUSION

Our data indicate that by acting as a natural brake on inflammation during hepatocellular injury, TREM-2 is a critical regulator of diverse types of hepatotoxic injury.

Up-regulation of TREM2 accelerates the reduction of amyloid deposits and promotes neuronal regeneration in the hippocampus of amyloid beta1-42 injected mice

Alzheimer's disease (AD) is characterized by a robust inflammatory response elicited by the accumulation and subsequently deposition of amyloid beta (Aβ) within the brain. The immune cells of brain migrate to and invest their processes within Aβ plaques and clear plaques from the brain. Previous studies have shown that treatment of myeloid cell with nuclear factor inhibitor increases expression of phagocytesis-related genes, such as triggering receptor expressed on myeloid cells 2 (TREM2). In myeloid cells, TREM2 has been involved in the regulation of phagocytosis, cell proliferation as well as inflammatory response in vitro. The purpose of this study was to further investigate microglial proliferation, phagocytosis and the expression of brain derived neurotrophic factor (BDNF) induced by up-regulation of TREM2 in Aβ1-42 injected mice. We first singly injected Aβ1-42 into the hippocampus of mice to build the model of AD-like symptoms. Subsequently, ammonium pyrrolidinedithiocarbamate (PDTC) was injected into the lateral ventricle of mice. Various immunohistochemical techniques and Western blot analyses were applied to examine expressions of TREM2, microglia, Aβ, Neuronal migration protein doublecortin (DCX) and BDNF in the hippocampus of mice. In the present study, we found the plaques-associated microglia lowly expressed TREM2 and BDNF in Aβ1-42 intra-hippocampal injected mice. Treatment of the models with a nuclear factor inhibitor, PDTC, further induced the expression of TREM2 and enhanced microglial phagocytosis, coincident with the rapid reduction in plaque burden. The expression of BDNF was up-regulated and the expression of DCX was partly restored. This means that up-regulation of TREM2 might induce the microglia to express the BDNF. These findings further indicate that the level of TREM2 may affect the microglia response to pathological process induced by Aβ.

An in vitro model for studying CNS white matter: functional properties and experimental approaches

The normal development and maintenance of CNS white matter, and its responses to disease and injury, are defined by synergies between axons, oligodendrocytes, astrocytes and microglia, and further influenced by peripheral components such as the gut microbiome and the endocrine and immune systems. Consequently, mechanistic insights, therapeutic approaches and safety tests rely ultimately on in vivo models and clinical trials. However, in vitro models that replicate the cellular complexity of the CNS can inform these approaches, reducing costs and minimising the use of human material or experimental animals; in line with the principles of the 3Rs. Using electrophysiology, pharmacology, time-lapse imaging, and immunological assays, we demonstrate that murine spinal cord-derived myelinating cell cultures recapitulate spinal-like electrical activity and innate CNS immune functions, including responses to disease-relevant myelin debris and pathogen associated molecular patterns (PAMPs).  Further, we show they are (i) amenable to siRNA making them suitable for testing gene-silencing strategies; (ii) can be established on microelectrode arrays (MEAs) for electrophysiological studies; and (iii) are compatible with multi-well microplate formats for semi-high throughput screens, maximising information output whilst further reducing animal use. We provide protocols for each of these. Together, these advances increase the utility of this in vitro tool for studying normal and pathological development and function of white matter, and for screening therapeutic molecules or gene targets for diseases such as multiple sclerosis, motor neuron disease or spinal cord injury, whilst avoiding in vivo approaches on experimental animals.

Inflammatory expression profile in peripheral blood mononuclear cells from patients with Nasu-Hakola Disease

Homozygous mutations in Triggering Receptor Expressed on Myeloid cells 2 gene (TREM2) are one of the major causes of Nasu Hakola Disease (NHD). We analysed Peripheral Blood Mononuclear Cells (PBMC) profile of 164 inflammatory factors in patients with NHD carrying the TREM2 Q33X mutation as compared with heterozygous and wild type individuals. Several molecules related to bone formation and angiogenesis were altered in NHD compared to non-carriers: Bone Morphogenetic Protein (BMP)-1 mRNA levels were significantly increased in PBMC (2.32 fold-increase; P = 0.01), as were Transforming Growth Factor Beta (TGFB)3 levels (1.51 fold-increase; P = 0.02). Conversely, CXCL5 and Pro Platelet Basic Protein (PPBP) were strongly downregulated (-28.26, -9.85 fold-decrease over non-carriers, respectively, P = 0.01), as well as Platelet Factor 4 Variant 1 (PF4V1; -41.44, P = 0.03). Among other inflammatory factors evaluated, Interleukin (IL)-15 and Tumor Necrosis Factor Superfamily Member (TNFSF)4 mRNA levels were decreased in NHD as compared with non-carriers (-2.25 and -3.87 fold-decrease, P = 0.01 and 0.001, respectively). In heterozygous individuals, no significant differences were observed, apart from IL-15 mRNA levels, that were decreased at the same extent as NHD (-2.05 fold-decrease over non-carriers, P = 0.002). We identified a signature in PBMC from patients with NHD consisting of strongly decreased mRNA levels of CXCL5, PPBP, PF4V1, mildly decreased IL-15 and TNFSF4 and mildly increased BMP-1 and TGFB3.

Erythromyeloid-Derived TREM2: A Major Determinant of Alzheimer's Disease Pathology in Down Syndrome

Background:

Down syndrome (DS; trisomy 21) individuals have a spectrum of hematopoietic and neuronal dysfunctions and by the time they reach the age of 40 years, almost all develop Alzheimer’s disease (AD) neuropathology which includes senile plaques and neurofibrillary tangles. Inflammation and innate immunity are key players in AD and DS. Triggering receptor expressed in myeloid cells-2 (TREM2) variants have been identified as risk factors for AD and other neurodegenerative diseases.

Objective:

To investigate the effects of TREM2 and the AD-associated R47H mutation on brain pathology and hematopoietic state in AD and DS.

Methods:

We analyzed peripheral blood, bone marrow, and brain tissue from DS, AD, and age-matched control subjects by immunohistochemistry and western blotting. TREM2-related phagocytosis was investigated using a human myeloid cell line.

Results:

TREM2 protein levels in brain and sera declined with age and disease progression in DS. We observed soluble TREM2 in brain parenchyma that may be carried by a subset of microglia, macrophages, or exosomes. Two DS cases had the AD-associated TREM2-R47H mutation, which manifested a morphologically extreme phenotype of megakaryocytes and erythrocytes in addition to impaired trafficking of TREM2 to the erythroid membrane. TREM2 was shown to be involved in phagocytosis of red blood cells. TREM2 was seen in early and late endosomes. Silencing TREM2 using siRNA in THP1 cells resulted in significant cell death.

Conclusion:

We provide evidence that peripheral TREM2 originating from erythromyeloid cells significantly determines AD neuropathology in DS subjects. Understanding the molecular signaling pathways mediated by TREM2 may reveal novel therapeutic targets.

Steered migration and changed morphology of human astrocytes by an applied electric field

Direct current electric field (DC EF) plays a role in influencing the biological behaviors and functions of cells. We hypothesize that human astrocytes (HAs) could also be influenced in EF. Astrocytes, an important type of nerve cells with a high proportion quantitatively, are generally activated and largely decide the brain repair results after brain injury. So far, no electrotaxis study on HAs has been performed. We here obtained HAs derived from brain trauma patients. After purification and identification, HAs were seeded in the EF chamber and recorded in a time-lapse image system. LY294002 and U0126 were then used to probe the role of PI3K or ERK signaling pathway on cellular behaviors. The results showed that HAs could be guided to migrate to the anode in DC EFs, in a voltage-dependent manner. The HAs displayed elongated cell bodies and reoriented perpendicularly to the EF in morphology. When treated with LY294002 or U0126, alternation of parameters such as cellular verticality, track speed, displacement speed, long axis, vertical length and circularity were inhibited partly as expected, while the EF-induced directedness was not terminated even at a high drug dosage which was not consistent with previous electrotaxis studies. In conclusion, applied EFs steered the patient-derived HAs directional migration and changed morphology, in which PI3K and ERK pathways at least partially participate. The characteristics of HAs to EF stimulation may be involved in wound healing and neural regeneration, which could be utilized as a novel treatment strategy in brain injury.

Isolation and Culture of Microglia

Microglia represent 5-10% of cells in the central nervous system and contribute to the development, homeostasis, injury, and repair of neural tissues. As the tissue-resident macrophages of the central nervous system, microglia execute core innate immune functions such as detection of pathogens/damage, cytokine secretion, and phagocytosis. However, additional properties that are specific to microglia and their neural environment are beginning to be appreciated. This article describes approaches for purification of microglia by fluorescence-activated cell sorting using microglia-specific surface markers and for enrichment of microglia by magnetic sorting and immunopanning. Detailed information about culturing primary microglia at various developmental stages is also provided. Throughout, we focus on special considerations for handling microglia and compare the relative strengths or disadvantages of different protocols. © 2018 by John Wiley & Sons, Inc.

Chronic stress as a risk factor for Alzheimer's disease: Roles of microglia-mediated synaptic remodeling, inflammation, and oxidative stress

Microglia are the predominant immune cells of the central nervous system (CNS) that exert key physiological roles required for maintaining CNS homeostasis, notably in response to chronic stress, as well as mediating synaptic plasticity, learning and memory. The repeated exposure to stress confers a higher risk of developing neurodegenerative diseases including sporadic Alzheimer's disease (AD). While microglia have been causally linked to amyloid beta (Aβ) accumulation, tau pathology, neurodegeneration, and synaptic loss in AD, they were also attributed beneficial roles, notably in the phagocytic elimination of Aβ. In this review, we discuss the interactions between chronic stress and AD pathology, overview the roles played by microglia in AD, especially focusing on chronic stress as an environmental risk factor modulating their function, and present recently-described microglial phenotypes associated with neuroprotection in AD. These microglial phenotypes observed under both chronic stress and AD pathology may provide novel opportunities for the development of better-targeted therapeutic interventions.

Inflammation as a central mechanism in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is characterized by cognitive decline and the presence of two core pathologies, amyloid β plaques and neurofibrillary tangles. Over the last decade, the presence of a sustained immune response in the brain has emerged as a third core pathology in AD. The sustained activation of the brain's resident macrophages (microglia) and other immune cells has been demonstrated to exacerbate both amyloid and tau pathology and may serve as a link in the pathogenesis of the disorder. In the following review, we provide an overview of inflammation in AD and a detailed coverage of a number of microglia-related signaling mechanisms that have been implicated in AD. Additional information on microglia signaling and a number of cytokines in AD are also reviewed. We also review the potential connection of risk factors for AD and how they may be related to inflammatory mechanisms.

The Trem2 R47H Alzheimer's risk variant impairs splicing and reduces Trem2 mRNA and protein in mice but not in humans

BACKGROUND

The R47H variant of the Triggering Receptor Expressed on Myeloid cells 2 (TREM2) significantly increases the risk for late onset Alzheimer's disease. Mouse models accurately reproducing phenotypes observed in Alzheimer' disease patients carrying the R47H coding variant are required to understand the TREM2 related dysfunctions responsible for the enhanced risk for late onset Alzheimer's disease.

METHODS

A CRISPR/Cas9-assisted gene targeting strategy was used to generate Trem2 R47H knock-in mice. Trem2 mRNA and protein levels as well as Trem2 splicing patterns were assessed in these mice, in iPSC-derived human microglia-like cells, and in human brains from Alzheimer's patients carrying the TREM2 R47H risk factor.

RESULTS

Two independent Trem2 R47H knock-in mouse models show reduced Trem2 mRNA and protein production. In both mouse models Trem2 haploinsufficiency was due to atypical splicing of mouse Trem2 R47H, which introduced a premature stop codon. Cellular splicing assays using minigene constructs demonstrate that the R47H variant induced abnormal splicing only occurs in mice but not in humans. TREM2 mRNA levels and splicing patterns were both normal in iPSC-derived human microglia-like cells and patient brains with the TREM2 R47H variant.

CONCLUSIONS

The Trem2 R47H variant activates a cryptic splice site that generates miss-spliced transcripts leading to Trem2 haploinsufficiency only in mice but not in humans. Since Trem2 R47H related phenotypes are mouse specific and do not occur in humans, humanized TREM2 R47H knock-in mice should be generated to study the cellular consequences caused by the human TREM2 R47H coding variant. Currently described phenotypes of Trem2 R47H knock-in mice can therefore not be translated to humans.

Loss of Trem2 in microglia leads to widespread disruption of cell coexpression networks in mouse brain

Rare heterozygous coding variants in the triggering receptor expressed in myeloid cells 2 (TREM2) gene, conferring increased risk of developing late-onset Alzheimer's disease, have been identified. We examined the transcriptional consequences of the loss of Trem2 in mouse brain to better understand its role in disease using differential expression and coexpression network analysis of Trem2 knockout and wild-type mice. We generated RNA-Seq data from cortex and hippocampus sampled at 4 and 8 months. Using brain cell-type markers and ontology enrichment, we found subnetworks with cell type and/or functional identity. We primarily discovered changes in an endothelial gene-enriched subnetwork at 4 months, including a shift toward a more central role for the amyloid precursor protein gene, coupled with widespread disruption of other cell-type subnetworks, including a subnetwork with neuronal identity. We reveal an unexpected potential role of Trem2 in the homeostasis of endothelial cells that goes beyond its known functions as a microglial receptor and signaling hub, suggesting an underlying link between immune response and vascular disease in dementia.

Orthologous proteins of experimental de- and remyelination are differentially regulated in the CSF proteome of multiple sclerosis subtypes

OBJECTIVE

Here, we applied a multi-omics approach (i) to examine molecular pathways related to de- and remyelination in multiple sclerosis (MS) lesions; and (ii) to translate these findings to the CSF proteome in order to identify molecules that are differentially expressed among MS subtypes.

METHODS

To relate differentially expressed genes in MS lesions to de- and remyelination, we compared transcriptome of MS lesions to transcriptome of cuprizone (CPZ)-induced de- and remyelination. Protein products of the overlapping orthologous genes were measured within the CSF by quantitative proteomics, parallel reaction monitoring (PRM). Differentially regulated proteins were correlated with molecular markers of inflammation by using MesoScale multiplex immunoassay. Expression kinetics of differentially regulated orthologous genes and proteins were examined in the CPZ model.

RESULTS

In the demyelinated and remyelinated corpus callosum, we detected 1239 differentially expressed genes; 91 orthologues were also differentially expressed in MS lesions. Pathway analysis of these orthologues suggested that the TYROBP (DAP12)-TREM2 pathway, TNF-receptor 1, CYBA and the proteasome subunit PSMB9 were related to de- and remyelination. We designed 129 peptides representing 51 orthologous proteins, measured them by PRM in 97 individual CSF, and compared their levels between relapsing (n = 40) and progressive MS (n = 57). Four proteins were differentially regulated among relapsing and progressive MS: tyrosine protein kinase receptor UFO (UFO), TIMP-1, apolipoprotein C-II (APOC2), and beta-2-microglobulin (B2M). The orthologous genes/proteins in the mouse brain peaked during acute remyelination. UFO, TIMP-1 and B2M levels correlated inversely with inflammation in the CSF (IL-6, MCP-1/CCL2, TARC/CCL17). APOC2 showed positive correlation with IL-2, IL-16 and eotaxin-3/CCL26.

CONCLUSIONS

Pathology-based multi-omics identified four CSF markers that were differentially expressed in MS subtypes. Upregulated TIMP-1, UFO and B2M orthologues in relapsing MS were associated with reduced inflammation and reflected reparatory processes, in contrast to the upregulated orthologue APOC2 in progressive MS that reflected changes in lipid metabolism associated with increased inflammation.

Microglial TREM2/DAP12 Signaling: A Double-Edged Sword in Neural Diseases

Microglia are activated after neuronal injury and in neurodegenerative diseases, and trigger neuroinflammation in the central nervous system (CNS). Microglia-derived neuroinflammation has both beneficial and detrimental effects on neurons. Because the timing and magnitude of microglial activation is thought to be a critical determinant of neuronal fate, understanding the molecular mechanisms underlying microglial activation is required to enable establishment of microglia-targeted therapies for neural diseases. Plasma membrane receptors play primary roles as activators of microglia and in this review, we focus on a receptor complex involving triggering receptor expressed on myeloid cells 2 (TREM2) and DNAX-activating protein of 12 kDa (DAP12), both of which are causative genes for Nasu-Hakola disease, a dementia with bone cysts. Recent transcriptome approaches demonstrated TREM2/DAP12 signaling as the principal regulator that transforms microglia from a homeostatic to a neural disease-associated state. Furthermore, animal model studies revealed critical roles for TREM2/DAP12 in the regulation of microglial activity, including survival, phagocytosis, and cytokine production, not only in Alzheimer's disease but also in other neural diseases, such as Parkinson's disease, demyelinating disease, ischemia, and peripheral nerve injury. Intriguingly, while TREM2/DAP12-mediated microglial activation is detrimental for some diseases, including peripheral nerve injury, it is beneficial for other diseases. As the role of activated microglia differs among disease models, TREM2/DAP12 signaling may result in different outcomes in different diseases. In this review we discuss recent perspectives on the role of TREM2/DAP12 in microglia and their contribution to neural diseases.

Role of metabolic programming in the modulation of microglia phagocytosis by lipids

Microglia phagocytosis is an essential process to maintain lifelong brain homeostasis and clear potential toxic factors from the neuropil. Microglia can engulf cells or part of cells through the expression of specific receptors at their surface and activation of downstream signaling pathways to engulf material. Microglia phagocytosis is finely regulated and is under the dependence of many factors, including environmental cues such as dietary lipids. Yet, the molecular mechanisms implicated are still largely unknown. The present publication is a 'hypothesis review', assessing the possibility that lipid-mediated modulation of phagocytosis occurs by affecting bioenergetic pathways within microglia. I assess our present knowledge and the elements that allow drawing such hypothesis. I also list some of the important gaps in the literature that need to be filled in. I also consider opportunities for future therapeutic target including nutritional interventions.

Triggering receptor expressed on myeloid cells 2 (TREM2): a potential therapeutic target for Alzheimer disease?

INTRODUCTION

There are currently no effective therapeutics for Alzheimer disease (AD). Clinical trials targeting amyloid beta thus far have shown very little benefit and only in the earliest stages of disease. These limitations have driven research to identify alternative therapeutic targets, one of the most promising is the triggering receptor expressed on myeloid cells 2 (TREM2). Areas covered: Here, we review the literature to-date and discuss the potentials and pitfalls for targeting TREM2 as a potential therapeutic for AD. We focus on research in animal and cell models for AD and central nervous system injury models which may help in understanding the role of TREM2 in disease. Expert opinion: Studies suggest TREM2 plays a key role in AD pathology; however, results have been conflicting about whether TREM2 is beneficial or harmful. More research is necessary before designing TREM2-targeting therapies. Successful therapeutics will most likely be administered early in disease.

Distinct Gene Profiles of Bone Marrow-Derived Macrophages and Microglia During Neurotropic Coronavirus-Induced Demyelination

Multiple Sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) characterized by demyelination and axonal loss. Demyelinating lesions are associated with infiltrating T lymphocytes, bone marrow-derived macrophages (BMDM), and activated resident microglia. Tissue damage is thought to be mediated by T cell produced cytokines and chemokines, which activate microglia and/or BMDM to both strip myelin and produce toxic factors, ultimately damaging axons and promoting disability. However, the relative contributions of BMDM and microglia to demyelinating pathology are unclear, as their identification in MS tissue is difficult due to similar morphology and indistinguishable surface markers when activated. The CD4 T cell-induced autoimmune murine model of MS, experimental autoimmune encephalitis (EAE), in which BMDM are essential for demyelination, has revealed pathogenic and repair-promoting phenotypes associated with BMDM and microglia, respectively. Using a murine model of demyelination induced by a gliatropic coronavirus, in which BMDM are redundant for demyelination, we herein characterize gene expression profiles of BMDM versus microglia associated with demyelination. While gene expression in CNS infiltrating BMDM was upregulated early following infection and subsequently sustained, microglia expressed a more dynamic gene profile with extensive mRNA upregulation coinciding with peak demyelination after viral control. This delayed microglia response comprised a highly pro-inflammatory and phagocytic profile. Furthermore, while BMDM exhibited a mixed phenotype of M1 and M2 markers, microglia repressed the vast majority of M2-markers. Overall, these data support a pro-inflammatory and pathogenic role of microglia temporally remote from viral control, whereas BMDM retained their gene expression profile independent of the changing environment. As demyelination is caused by multifactorial insults, our results highlight the plasticity of microglia in responding to distinct inflammatory settings, which may be relevant for MS pathogenesis.

The Microglial Response to Neurodegenerative Disease

Microglia are a subset of tissue macrophages that constitute the major immune cell type of the central nervous system. These cells have long been known to change their morphology and functions in response to various neurological insults. Recently, a plethora of unbiased transcriptomics studies have revealed that across a broad spectrum of neurodegeneration-like disease models, microglia adopt a similar activation signature and perform similar functions. Despite these commonalities in response, the role of microglia has been described as both positive and negative in different murine disease models. In humans, genetic association studies have revealed strong connections between microglia genes and various neurodegenerative diseases, and mechanistic investigations of these mutations have added another layer of complexity. Here, we provide an overview of studies that have built a case for a common microglial response to neurodegeneration and discuss pathways that may be important to initiate and sustain this response; delineate the multifaceted functions of activated microglia spanning different diseases; and discuss insights from studying genes associated with disease in humans. We argue that strong evidence causally links activated microglia function to neurodegeneration and discuss what seems to be a conflict between mouse models and human genetics.

The Microglial Innate Immune Receptor TREM2 Is Required for Synapse Elimination and Normal Brain Connectivity

The triggering receptor expressed on myeloid cells 2 (TREM2) is a microglial innate immune receptor associated with a lethal form of early, progressive dementia, Nasu-Hakola disease, and with an increased risk of Alzheimer's disease. Microglial defects in phagocytosis of toxic aggregates or apoptotic membranes were proposed to be at the origin of the pathological processes in the presence of Trem2 inactivating mutations. Here, we show that TREM2 is essential for microglia-mediated synaptic refinement during the early stages of brain development. The absence of Trem2 resulted in impaired synapse elimination, accompanied by enhanced excitatory neurotransmission and reduced long-range functional connectivity. Trem2^-/- mice displayed repetitive behavior and altered sociability. TREM2 protein levels were also negatively correlated with the severity of symptoms in humans affected by autism. These data unveil the role of TREM2 in neuronal circuit sculpting and provide the evidence for the receptor's involvement in neurodevelopmental diseases.