Triggering Receptor Expressed On Myeloid Cells 2 Research Articles

1,25-Dihydroxy Vitamin D3 Facilitates the M2 Polarization and β-Amyloid Uptake by Human Microglia in a TREM2-Dependent Manner.

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by dementia as the primary clinical symptom. The production and accumulation of aggregated β-amyloid (Aβ) in patient brain tissues is one of the hallmarks of AD pathogenesis. Microglia, brain-resident macrophages, produce inflammatory cytokines in response to Aβ oligomers or fibrils exacerbating Aβ pathology in AD. HMO6 cells were treated with Aβ42 in the presence or absence of 1,25-dihydroxy vitamin D3 (1,25(OH)2D3) to determine its potential immunomodulatory effects, and the expression of pro-/anti-inflammatory cytokines, M1/M2-associated markers, Toll-like receptors (TLRs), and triggering receptor expressed on myeloid cells 2 (TREM2) was examined. 1,25(OH)2D3 was found to suppress Aβ-induced expression of proinflammatory cytokines (TNF-α, IL-1β, and IL-6), M1 markers (CD86 and iNOS), and TLR2/4, whilst increasing the expression of anti-inflammatory cytokines (IL-4, IL-10, and CCL17) and M2 markers (CD206 and Arg-1). Furthermore, 1,25(OH)2D3 promoted TREM2 expression and Aβ uptake by HMO6 cells, and the enhancement of Aβ uptake and M2 polarization was revealed to be TREM2-dependent. The findings of this study suggest that 1,25(OH)2D3 facilitates M2 polarization and Aβ uptake in a TREM2-dependent manner.

The TgF344-AD rat: behavioral and proteomic changes associated with aging and protein expression in a transgenic rat model of Alzheimer's disease

Animal models of Alzheimer's Disease (AD) are attractive tools for preclinical, prodromal drug testing. The TgF344-AD (Tg) rat exhibits cognitive deficits and 5 major hallmarks of AD. Here we show that spatial water maze (WMZ) memory deficits and proteomic differences in dorsal CA1 were present in young Tg rats. Aged learning-unimpaired (AU) and aged learning-impaired (AI) proteome associated changes were identified and differed by sex. Levels of phosphorylated tau, reactive astrocytes and microglia were significantly increased in aged Tg rats and correlated with the WMZ learning index (LI); in contrast, no significant correlation was present between amyloid plaques or insoluble Aβ levels and LI. Neuroinflammatory markers were also significantly correlated with LI and increased in female Tg rats. The anti-inflammatory marker, triggering receptor expressed on myeloid cells-2 (TREM2), was significantly reduced in aged impaired Tg rats and correlated with LI. Identifying and understanding mechanisms that allow for healthy aging by overcoming genetic drivers for AD, and/or promoting drivers for successful aging, are important for developing successful therapeutics against AD.

INPP5D deficiency attenuates amyloid pathology in a mouse model of Alzheimer's disease.

Inositol polyphosphate-5-phosphatase (INPP5D) is a microglia-enriched lipid phosphatase in the central nervous system. A non-coding variant (rs35349669) in INPP5D increases the risk for Alzheimer's disease (AD), and elevated INPP5D expression is associated with increased plaque deposition. INPP5D negatively regulates signaling via several microglial cell surface receptors, including triggering receptor expressed on myeloid cells 2 (TREM2); however, the impact of INPP5D inhibition on AD pathology remains unclear. We used the 5xFAD mouse model of amyloidosis to assess how Inpp5d haplodeficiency regulates amyloid pathogenesis. Inpp5d haplodeficiency perturbs the microglial intracellular signaling pathways regulating the immune response, including phagocytosis and clearing of amyloid beta (Aβ). It is important to note that Inpp5d haploinsufficiency leads to the preservation of cognitive function. Spatial transcriptomic analysis revealed that pathways altered by Inpp5d haploinsufficiency are related to synaptic regulation and immune cell activation. These data demonstrate that Inpp5d haplodeficiency enhances microglial functions by increasing plaque clearance and preserves cognitive abilities in 5xFAD mice. Inhibition of INPP5D is a potential therapeutic strategy for AD.

Platinum-Based TREM2 Inhibitor Suppresses Tumors by Remodeling the Immunosuppressive Microenvironment.

Triggering receptor expressed on myeloid cells-2 (TREM2) is a key pro-tumorigenic marker of tumor-infiltrating macrophages, showing potent immunosuppressive activity in tumor microenvironment. A platinum(IV) complex OPA derived from oxaliplatin (OP) and artesunate (ART) exhibited direct cytotoxicity against human colon cancer cells and immunomodulatory activity to inhibit TREM2 on macrophages in vitro and vivo. Furthermore, OPA deterred the tumor growth in mouse models bearing MC38 colorectal tumor by reducing the number of CD206+ and CX3 CR1+ immunosuppressive macrophages; it also promoted the expansion and infiltration of immunostimulatory dendritic, cytotoxic T, and natural killer cells. OPA is the first small-molecular TREM2 inhibitor capable of relieving immunosuppressive tumor microenvironment and enhancing chemical anticancer efficiency of a platinum drug, thus showing typical characteristics of a chemoimmunotherapeutic agent.

Inflammatory role of microglia in brain injury caused by subarachnoid hemorrhage.

Early brain injury is a series of pathophysiological changes and direct damage of brain tissue within 72 h after subarachnoid hemorrhage before cerebral vasospasm occurs. Early brain injury is a key factor affecting the prognosis of subarachnoid hemorrhage, and its possible pathological mechanisms include oxidative stress, cell apoptosis, autophagy, and immune inflammation. Microglia are important immune cells of the central nervous system. Microglia play a dual role in protection and injury. Microglia are involved in the occurrence of brain edema, the processes of neuronal apoptosis, and the blood-brain barrier disruption after subarachnoid hemorrhage (SAH) through the signaling pathways mediated by receptors such as Toll-like receptor 4 (TLR4), calcium-sensing receptor (CaSR), and triggering receptor expressed on myeloid cells-1 (TREM-1), which secrete pro-inflammatory cytokines such as interleukins and tumor necrosis factor α. Conversely, they exert their anti-inflammatory and protective effects by expressing substances such as neuroglobin and heme oxygenase-1. This article reviews the latest developments in single-cell transcriptomics for microglia in early brain injury after subarachnoid hemorrhage and its inflammatory role.

Electroacupuncture pretreatment induces ischemic tolerance by neuronal TREM2-mediated enhancement of autophagic flux

The mechanism of electroacupuncture (EA) pretreatment-induced neuroprotection remains unclear. In this study, we found that neuronal Triggering receptor expressed on myeloid cells 2 (TREM2) expression was increased and peaked at 48 h and 72 h after ischemia/reperfusion. After specific knockdown of TREM2 in excitatory neurons, neurological function was damaged, and the infarct volume was enlarged. Furthermore, the expression of LC3II/LC3I and Beclin1 was decreased, while the expression of p62 was increased. EA pretreatment enhanced TREM2, LC3II/LC3I and Beclin1 expression while reducing p62 in the ischemic penumbra area. The EA-induced neuroprotective effects and improvements in autophagic flux were abolished by specific knockdown of TREM2 in excitatory neurons. Taken together, our findings provide novel mechanistic insight into EA-induced ischemic tolerance and suggest a promising therapeutic strategy of targeting neuronal TREM2 to treat brain ischemia.

TREM2 activation alleviates neural damage via Akt/CREB/BDNF signalling after traumatic brain injury in mice

BackgroundNeuroinflammation is one of the most important processes in secondary injury after traumatic brain injury (TBI). Triggering receptor expressed on myeloid cells 2 (TREM2) has been proven to exert neuroprotective effects in neurodegenerative diseases and stroke by modulating neuroinflammation, and promoting phagocytosis and cell survival. However, the role of TREM2 in TBI has not yet been elucidated. In this study, we are the first to use COG1410, an agonist of TREM2, to assess the effects of TREM2 activation in a murine TBI model.MethodsAdult male wild-type (WT) C57BL/6 mice and adult male TREM2 KO mice were subjected to different treatments. TBI was established by the controlled cortical impact (CCI) method. COG1410 was delivered 1 h after CCI via tail vein injection. Western blot analysis, immunofluorescence, laser speckle contrast imaging (LSCI), neurological behaviour tests, brain electrophysiological monitoring, Evans blue assays, magnetic resonance imaging (MRI), and brain water content measurement were performed in this study.ResultsThe expression of endogenous TREM2 peaked at 3 d after CCI, and it was mainly expressed on microglia and neurons. We found that COG1410 improved neurological functions within 3 d, as well as neurological functions and brain electrophysiological activity at 2 weeks after CCI. COG1410 exerted neuroprotective effects by inhibiting neutrophil infiltration and microglial activation, and suppressing neuroinflammation after CCI. In addition, COG1410 treatment alleviated blood brain barrier (BBB) disruption and brain oedema; furthermore, COG1410 promoted cerebral blood flow (CBF) recovery at traumatic injury sites after CCI. In addition, COG1410 suppressed neural apoptosis at 3 d after CCI. TREM2 activation upregulated p-Akt, p-CREB, BDNF, and Bcl-2 and suppressed TNF-α, IL-1β, Bax, and cleaved caspase-3 at 3 d after CCI. Moreover, TREM2 knockout abolished the effects of COG1410 on vascular phenotypes and microglial states. Finally, the neuroprotective effects of COG1410 were suppressed by TREM2 depletion.ConclusionsAltogether, we are the first to demonstrate that TREM2 activation by COG1410 alleviated neural damage through activation of Akt/CREB/BDNF signalling axis in microglia after CCI. Finally, COG1410 treatment improved neurological behaviour and brain electrophysiological activity after CCI.Graphical

Biophysical analysis of TREM2 interactions reveals a shared binding site for Alzheimer’s disease ligands apoE and oligomeric amyloid beta

AbstractBackgroundThe development of new innovative treatments to prevent and ameliorate AD requires knowledge of molecular mechanisms that are critical to neuronal health. The triggering receptor expressed on myeloid cells 2 (TREM2) receptor is part of a signaling complex that modulates inflammatory responses, phagocytosis and cell survival in microglia, resident immune cells in the brain that play a critical role in clearing misfolded aggregates. Examples include neurotoxic aggregates consisting largely of amyloid beta (Ab) and apoliproteinE (apoE). Both molecules have emerged as important signaling ligands for TREM2. Although TREM2 signaling in microglia is generally associated with beneficial outcomes, intense or prolonged signaling may produce overactivated microglia leading to neuronal damage. Such events may also contribute to other diseases such as Parkinson’s or cancers. Furthermore, rare TREM2 variants, most notably R47H and R62H, have been identified that are associated with a significantly increased risk of developing AD. Given these significant roles, TREM2 has emerged as an important yet challenging therapeutic target for AD. Although a number of ligands for TREM2 with relevance to AD have been identified, little is known regarding the molecular details of how TREM2 engages them. Detailed knowledge of the molecular mechanisms underlying TREM2 signaling in microglia is urgently needed to facilitate the development of specific, potent, safe and efficacious therapies for AD that target the TREM2 signaling pathway. The objective of this study was to identify the critical surfaces on TREM2 that mediate interactions with both apolipoproteinE (apoE) and oligomeric amyloid b1‐42 (oAb42) using rigorous structural and biophysical methods.MethodWe used structure‐based mutations and comprehensive biolayer interferometry (BLI) analysis to investigate TREM2 interactions with apoE and oAb42.ResultWe found that TREM2 utilizes a distal hydrophobic surface, consisting of the CDR1, CDR2, and CDR3 loops, to engage apoE. Surprisingly, we found that this same surface is utilized to engage oAb42.ConclusionOur results indicate that the hydrophobic site on TREM2 (consisting of the CDR1, CDR2, and CDR3 loops) is the critical ligand‐engaging surface on TREM2. They also indicate that therapeutics that either directly bind or allosterically alter the hydrophobic site on TREM2 should modulate TREM2 signaling as potential AD treatments.

A clinically relevant mouse model of the Trem2*R47H Alzheimer’s disease risk variant

AbstractBackgroundThe TREM2 (triggering receptor expressed on myeloid cells 2) locus harbors numerous relatively rare variants associated with Alzheimer’s disease risk. The TREM2*R47H allele is one of the strongest genetic risk factors for developing late‐onset Alzheimer’s disease; patients exhibit both earlier onset of symptoms and rapid cognitive decline. TREM2 is expressed by microglia in the brain and plays a key role in modulating microglial activity. TREM2 expression is increased in AD and is associated with recruitment of microglia to amyloid plaques. An experimental model is therefore critical to study mechanisms of late‐onset AD, and the MODEL‐AD set out to create a clinically relevant mouse model.MethodWe used CRISPR to engineer the R47H variant into the mouse Trem2 locus; this model is available as JAX #27918. This model, as well as two other CRISPR‐generated Trem2*R47H models, display a splicing event seen in mice, but not humans, that leads to a novel splice form with a deletion of 119bp at the 5’ end of exon 2 (Xiang et al, Molecular Neurodegeneration, 2018). This leads to a decrease in ∼50% of both transcript and protein levels in brain homogenate in the mouse, indicating that the model expresses mutant Trem2 protein, but is also a hypomorph. To generate a more clinically relevant model, we have modified the existing Trem2*R47H mouse allele by using CRISPR to humanize the exon 2 splice site. Expression in these models was compared to normal Trem2 levels using both RNA‐seq and Western blot.ResultThe new model, which we are calling Trem2*R47HHSS for humanized splice site (JAX #33781), showed normal levels of expression. Relative to B6J controls, this new model expresses normal levels of each of the four transcripts. No truncated transcript was detected. Protein levels were restored to wild‐type levels in the model with the humanized splice site.ConclusionWe expect that this model Trem2*R47H model will facilitate research into the mechanisms and pathways related to TREM2 genetic risk in late‐onset AD. For more information see www.model-ad.org.

TREM2 differentially modulates parenchymal and vascular pathology in a mouse model of cerebral amyloid angiopathy

AbstractBackgroundCerebral amyloid angiopathy (CAA) features cerebral vascular deposition of pathological amyloid‐beta protein (Aβ) and plays an important role in the multimorbidity of aging brains including Alzheimer’s disease (AD), but it is less understood compared to parenchymal Aβ plaques. Triggering receptor expressed on myeloid cells 2 (TREM2), a key player in innate immune response, is exclusively expressed in microglia in the brain and mediates Aβ clearance and subsequently alleviates neuroinflammation. Loss‐of‐function mutations in TREM2 increase the risk of AD. Recent studies on TREM2 function using TREM2 knockout (KO) in transgenic (Tg) AD mice have shown that TREM2 modulates parenchymal Aβ deposition in the brain, although the direction of the effect depends on the specific Tg background and the stage of the disease development. However, the impact of TREM2 on CAA had not been investigated.MethodsTREM2 KO mice were crossed with SwDI mice (C57BL/6‐Tg(Thy1‐APPSwDutIowa)BWevn/Mmjax), a model of CAA and AD, to generate SwDI/TREM2WT, SwDI/TREM2Het, and SwDI/TREM2KO mice. At the age of 16 months, the mice were euthanized for tissue collection. ELISA was utilized for the assessment of Aβ40 and Aβ42 levels in brain tissue homogenates, and histochemical and immunohistochemical or immunofluorescent assays were used to evaluate amyloid deposition, astrogliosis, and microgliosis. All results were analyzed using GraphPad Prism 9.ResultsTREM2 deficiency markedly increased overall Aβ load in SwDI/TREM2KO mice compared to SwDI/TREM2WT or SwDI/TREM2Het in the cortex, hippocampus, and thalamus. Intriguingly, TREM2 deletion led to a dramatic decrease in CAA in vasculature‐enriched regions, such as thalamus, along with reduced microglial association with CAA. Consistent with previous reports, Aβ plaque‐associated microglia were significantly reduced in SwDI/TREM2KO mice. Additional experiments are underway to explore the molecular mechanisms underlying the distinct impact of TREM2 deficiency on Aβ plaques and CAA.ConclusionDeletion of TREM2 increases the overall brain Aβ load whereas decreases CAA, suggesting the differential role of TREM2 in the regulation of parenchymal versus vascular deposition of Aβ in the brain.

VGL101 Rescues CSF1R Dysfunction in Human Microglia And Macrophages: Evaluation of In Vitro Trem2 Agonism in Models Of A CSF1R‐Dependent Leukodystrophy

AbstractBackgroundAdult‐onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) is an autosomal dominant primary microgliopathy caused by kinase loss‐of‐function mutations in the CSF1 receptor (CSF1R). The cardinal features of ALSP include demyelination, axonopathy, and symptoms ranging from frontal temporal dementia‐like behavior, movement disorders, epilepsy, and psychiatric manifestations. Many of these hallmarks further resemble Nasu‐Hakola disease, another rare leukoencephalopathy associated with homozygous mutations in triggering receptor expressed on myeloid cells 2 (TREM2) or its tyrosine kinase binding adaptor (TYROBP) which result in complete loss‐of‐function of TREM2 or its signaling partner DAP12. Following binding to its cognate ligands, IL‐34 or CSF‐1, CSF1R dimerization initiates a signaling cascade both through its kinase domain and through DAP12, which together activate SYK to promote myeloid cell survival, proliferation, and morphological changes. While analogous to CSF1R in cellular expression and interaction with DAP12, TREM2 lacks intrinsic signaling capacity and therefore requires DAP12 to translate extracellular damage‐associated cues and orchestrate a constellation of intracellular and functional responses which phenocopy CSF1R activation. Accordingly, we hypothesize TREM2 activation may rescue CSF1R loss‐of‐function and ameliorate ALSP‐associated pathology.MethodsTo test whether TREM2 gain‐of‐function could compensate for CSF1R loss‐of‐function, we developed multiple approaches to model ALSP in human myeloid cells by CSF‐1/IL‐34 depletion or CSF1R kinase inhibition using PLX‐5622 and explored functional intervention via a human TREM2 agonistic antibody, VGL101, currently in Phase‐1 clinical development. Rescue for CSF1R was quantified using a combination of live‐cell morphology captured via automated longitudinal microscopy, viability monitored by ATP Cell‐Titer Glo, and apoptosis quantified by Caspase‐3/7 fluorogenic‐imaging.ResultsIn vitro phospho‐SYK profiling of VGL101 in iPSC‐derived microglia is consistent with its agonist profile (EC50: 0.77 ± 0.08 nM), while the absence of pSYK induction in TREM2‐knockout microglia demonstrates exquisite on‐target selectively. In both CSF‐1/IL‐34 depletion and CSF1R kinase inhibition paradigms of ALSP, VGL101 demonstrated full rescue of microglial and monocyte‐derived macrophage viability, inhibited apoptosis, and promoted morphological changes resembling intact CSF1R activation.ConclusionCollectively, our studies support the hypothesis that TREM2 agonism can compensate for CSF1R dysfunction and highlight the therapeutic potential for treating ALSP with TREM2 agonism via VGL101 passive immunotherapy.

Trem2R47H NSS; 5xFAD mice display age/disease progression‐dependent changes in plaques and plaque‐associated microglia, and increased plasma neurofilament light chain

AbstractBackgroundGenome‐Wide Association Studies revealed Triggering receptor expressed on myeloid cells 2 (TREM2) R47H mutation as one of the strongest genetic risk factors for late‐onset Alzheimer’s Disease (AD). In the brain, TREM2 is a transmembrane receptor expressed exclusively by myeloid cells. Many current TREM2*R47H mouse models have observed evidence of cryptic splicing products of the mutant allele, resulting in artefactually reduced protein product. Model Organism Development & Evaluation for Late‐Onset Alzheimer’s Disease (MODEL‐AD) consortium at University of California ‐ Irvine has developed the Trem2 R47H NSS (Normal Splice Site) mouse model where the Trem2 allele is expressed at a level similar to the wild‐type Trem2 allele, with no evidence of cryptic splicing products from the mutant allele.MethodWe generated four groups – WT (C57BL6/J), Trem2 R47H homozygous (HO), 5xFAD, and 5xFAD/ Trem2 R47H HO. Coronal brain slices of 4 and 12‐month‐old mice (n=5/sex/genotype/age) were immunolabeled with Thioflavin‐S for dense‐core plaques, ionized calcium binding adaptor molecule 1 (IBA1) for microglia and lysosome‐associated membrane glycoprotein 1 (LAMP1) for neuritic dystrophy. We also measured plasma neurofilament light chain (NfL) as a surrogate measure of brain damage via Meso Scale Discovery (MSD) technology.ResultSex‐related differences in dense‐core Aß plaque burden and size are found in 4‐month 5xFAD/ Trem2 R47H, where female 5xFAD/ Trem2 R47H mice exhibit increased plaque load compared to 5xFAD mice. In males, stark reductions in plaque load are observed. This sex‐difference is not observed at 12‐month. Examination of IBA1+ microglia at 4‐month reveals significant reductions in microglia volume in both Trem2 R47H and 5xFAD/ Trem2 R47H mice compared to their controls. Notably, there is a significant impairment of plaque‐microglia interaction in 5xFAD/ Trem2 R47H at 4‐month that is rescued at 12‐month. Moreover, we find a significant increase in plaque‐associated neuritic damage in 5xFAD/ Trem2 R47H when immunolabeled with LAMP1 at 4‐month that was also absent at 12‐month. Similarly, plasma NfL levels revealed significant increases in brain damage of 5xFAD/ Trem2 R47H compared to 5xFAD at 4‐month and a trending increase at 12‐month.ConclusionCollectively, these results show the effects of the Trem2 R47H variant on plaque development and downstream pathology, highlighting sex differences as well as age/ disease progression‐dependent changes.

INPP5D inhibition attenuates amyloid pathology through the regulation of microglial functions.

AbstractBackgroundAlzheimer’s disease (AD) is a progressive neurodegenerative disorder characterized by cognitive decline, robust microgliosis, neuroinflammation, and neuronal loss. Genome‐wide association studies recently highlighted a prominent role for microglia in late‐onset AD (LOAD). Specifically, inositol polyphosphate‐5‐phosphatase (INPP5D) is selectively expressed in brain microglia and one of its common intronic variants (rs35349669; OR=1.08, 95%CI=1.06‐1.11) has been reported to be associated with increased risk of LOAD. INPP5D is linked to the triggering receptor expressed on myeloid cells 2 (TREM2) signaling, but little is known about the function of INPP5D in microglia and how INPP5D regulates TREM2‐related AD pathogenesis. Therefore, we aim to understand the role of INPP5D in microglia and AD pathology.MethodWe performed differential gene expression analysis to investigate INPP5D expression in LOAD and its association with plaque density using transcriptomic data from the Accelerating Medicines Partnership for Alzheimer’s Disease (AMP‐AD) cohort. We also performed quantitative real‐time PCR, immunoblotting, and immunofluorescence assays to assess INPP5D expression and microglial markers in the 5xFAD amyloid mouse model with INPP5D deficiency. Behavioral deficits were assessed by the Y‐maze assay in all animal cohorts at 4‐ and 6‐month of age. The primary microglial culture was used the evaluate the amyloid uptake ability of microglia.Result INPP5D gene expression was upregulated in LOAD and positively correlated with amyloid plaque density. Inpp5d expression increased as the disease progressed in the 5xFAD mice, and selectively in plaque‐associated microglia. Inpp5d deficiency mitigated the plaque burdens and neuropathology in the 5xFAD mice and further protected against behavioral deficits induced by amyloid pathology. Inpp5d deficiency modulates the microglial signaling, including Trem2 expression and Syk phosphorylation, and further upregulates the expression of the post‐synaptic marker. In the primary microglial culture, Inpp5d inhibition increased the uptake of fibril amyloid beta‐peptide.ConclusionOur findings show that INPP5D expression increases throughout the AD progression and is predominantly in plaque‐associated microglia. Importantly, INPP5D deficiency reduces Abeta pathology through the regulation of microglial functions, highlighting INPP5D as a potential therapeutic target.

TMIC-29. CHARACTERIZING THE ROLE OF TRIGGERING RECEPTOR EXPRESSED ON MYELOID CELLS 2 (TREM2)-EXPRESSING MICROGLIA/MACROPHAGES IN GLIOBLASTOMA

Abstract The tumor microenvironment in glioblastoma (GBM) contains numerous cell types, notably a rich immune compartment dominated by myeloid cells, such as brain-resident microglia and infiltrating macrophages. However, current immunotherapies target lymphoid components like T cells, which are sparse in the brain and the tumors originating there. Therefore, immune checkpoint drugs designed for systemic cancers have not yielded significant benefits for patients with glioblastoma, and the efficacy of immunotherapy for central nervous system malignancies lags behind treatments for other solid tumors. Further understanding of the myeloid compartment is necessary for developing glioblastoma immunotherapies. Our recent single cell RNA-seq (scRNA-seq) analyses have identified microglial clusters that are enriched for antigen presentation and phagocytic gene expression modules. These clusters were restricted to microglia and macrophages seen only in glioma patients, but not in myeloid cells derived from non-glioma brain controls. Triggering receptor expressed on myeloid cells 2 (TREM2), a receptor abundant on microglia and vital for the detection and phagocytosis of pathogens, was highly expressed in these clusters. Because microglia expressing TREM2 play a protective role in neurodegenerative diseases like Alzheimer's disease, we examined the function of TREM2+ myeloid cells in GBM using mice lacking TREM2. Implantation of GL261 cells in TREM2-/- mice showed significantly reduced survival relative to wild-type mice, suggesting a protective role of TREM2+ myeloid cells in glioma. Data on immunophenotyping of glioma xenografts from WT and TREM2-/- mice brains using state of the art technologies (e.g. Cytek) will be presented. We conclude that TREM2 is a phagocytic immunoregulator in glioblastoma, manifesting anti-glioma properties to TREM2+ myeloid cells.

Triggering Receptor Expressed on Myeloid Cell-2 Protects PC12 Cells Injury by Inhibiting BV2 Microglial Activation.

Microglia play a crucial role in the activation of immune defense mechanism as the resident macrophages in the central nervous system (CNS). Microglia can eliminate damaged neurons, plaques, and other infectious agents. Triggering receptor expressed on myeloid cell-2 (TREM-2) speculates to be beneficial in preventing inflammation-induced bystander damage of neurons. However, the precise molecular mechanisms underlying the regulation of TREM-2 on neurons are not clarified. We cultured PC12 cells with conditioned medium which was the supernatant of LPS-treated BV2 cells and six groups of PC12 cells (control group, LPS group, TREM-2 WT + LPS group, TREM-2 over-expression + LPS group, siRNA control + LPS group, and siRNA TREM-2 + LPS group) were investigated. The mRNA levels of inflammatory mediators: Nitric oxide synthase (iNOS) and Arginase-1(Arg-1) were quantified by using RT-PCR. Assessment of apoptosis in PC12 cells mediated by BV2 microglia was analyzed using TUNEL assays. The result showed that LPS stimulation significantly enhanced inducible iNOS (M1) production in BV2 cells (P < 0.01), and increased PC12 cells apoptosis (P < 0.01), while reduced the production of Arg-1 (M2) in BV2 cells (P < 0.01). These effects were attenuated by TREM-2 over-expression, but enhanced by TREM-2 silencing. It indicated that TREM-2 inhibited LPS-mediated neuronal apoptosis by down-regulating iNOS and up-regulating the expression of Arg-1 in BV2 microglia. Therefore, our findings may provide new insights in the regulation of TREM-2 on neuronal apoptosis via BV2 microglial M1/M2 modulation.

TREM2 and Microglia Contribute to the Synaptic Plasticity: from Physiology to Pathology.

Synapses are bridges for information transmission in the central nervous system (CNS), and synaptic plasticity is fundamental for the normal function of synapses, contributing substantially to learning and memory. Numerous studies have proven that microglia can participate in the occurrence and progression of neurodegenerative diseases (NDDs), such as Alzheimer's disease (AD), by regulating synaptic plasticity. In this review, we summarize the main characteristics of synapses and synaptic plasticity under physiological and pathological conditions. We elaborate the origin and development of microglia and the two well-known microglial signaling pathways that regulate synaptic plasticity. We also highlight the unique role of triggering receptor expressed on myeloid cells 2 (TREM2) in microglia-mediated regulation of synaptic plasticity and its relationship with AD. Finally, we propose four possible ways in which TREM2 is involved in regulating synaptic plasticity. This review will help researchers understand how NDDs develop from the perspective of synaptic plasticity.

Triggering receptor expressed on myeloid cells 2 deficiency exacerbates injury-induced inflammation in a mouse model of tauopathy.

Traumatic brain injury (TBI) promotes several Alzheimer's disease-like pathological features, including microtubule-associated protein tau (MAPT) accumulation within neurons. Macrophage activation in the injured hTau mouse model of tauopathy raises the question whether there is a relationship between MAPT pathology and alterations in macrophage activation following TBI. Triggering receptor expressed on myeloid cells 2 (TREM2) is a critical regulator of microglia and macrophage phenotype, but its mechanisms on TBI remain unclear. To address the association with TREM2 in TBI and MAPT pathology, we studied TREM2 deficiency in hTau mice (hTau;Trem2-/- ) 3 (acute phase) and 120 (chronic phase) days after experimental TBI. At three days following injury, hTau;Trem2-/- mice exhibited reduced macrophage activation both in the cortex and hippocampus. However, to our surprise, hTau;Trem2-/- mice exposed to TBI augments macrophage accumulation in the corpus callosum and white matter near the site of tissue damage in a chronic phase, which results in exacerbated axonal injury, tau aggregation, and impaired neurogenesis. We further demonstrate that TREM2 deficiency in hTau injured mice promotes neuronal dystrophy in the white matter due to impaired phagocytosis of apoptotic cells. Remarkably, hTau;Trem2-/- exposed to TBI failed to restore blood-brain barrier integrity. These findings imply that TREM2 deficiency accelerates inflammation and neurodegeneration, accompanied by attenuated microglial phagocytosis and continuous blood-brain barrier (BBB) leakage, thus exacerbating tauopathy in hTau TBI mice.

TREM2 knockdown improves the therapeutic effect of PD-1 blockade in hepatocellular carcinoma

Triggering receptor expressed on myeloid cells 2 (TREM2), which is typically expressed in microglia/macrophage, is a crucial receptor modulating the activation of immune cells. Recent single-cell RNA sequencing studies of the tumor microenvironment (TME) have found that TREM2 is highly expressed in several subgroups of tumor-associated macrophages (TAMs) with immunosuppressive activity. TREM2 knockout mice are more resistant to tumor growth than wild-type mice in multiple mice tumor models. But the function of TREM2 expression in TAMs of hepatocellular carcinoma (HCC) is still unclear. Here we used a self-complementary adeno-associated virus (scAAV) with macrophage tropism to efficiently knockdown TREM2 in TAMs in vivo and found that the growth of hepatocellular carcinoma was suppressed by TREM2 knockdown. Moreover, we found that TREM2 knockdown remodeled TAMs to an immune-stimulating status and enhanced the therapeutic effect of PD-1 immune-checkpoint blockade in HCC.

TREM2 improves neurological dysfunction and attenuates neuroinflammation, TLR signaling and neuronal apoptosis in the acute phase of intracerebral hemorrhage.

Neuroinflammation contributes to secondary brain injury following intracerebral hemorrhage (ICH). Triggering receptor expressed on myeloid cells 2 (TREM2) confers strong neuroprotective effect by suppressing neuroinflammatory response in experimental ischemic stroke. This study aimed to clarify the neuroprotective role of TREM2 and potential underlying mechanism in a mouse model of ICH and in vitro. Adeno-associated virus (AAV) and green fluorescent protein-lentivirus (GFP-LV) strategies were employed to enhance TREM2 expression in the C57/BL6 mice and BV2 cells, respectively. The adult male C57/BL6 mice were subjected to ICH by administration of collagenase-IV in 1 month after the AAV particles injection. An in vitro ICH model was performed with oxygen hemoglobin in BV2 cells. Toll-like receptor 4 (TLR4) antagonist TAK242 was applied at 6 h following ICH. Neurological function, TREM2, pro-inflammatory cytokines, brain water content and Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining were evaluated at 24 h following ICH. TLR4, NF-κB and mitogen-activated protein kinases (MAPK) signaling pathways were also determined by Western blot analysis at the same time point. The levels of TREM2 were increased at 12 h, peaked at 24 h and recovered on 7d following ICH. TREM2 overexpression ameliorated ICH induced neurological dysfunction, inhibited neuroinflammation, and attenuated apoptosis and brain edema. Further mechanistic study revealed that TREM2 overexpression inhibited TLR4 activation and NF-κB and MAPK signaling pathways. ICH increased the percentage of TUNEL-positive cells, which was markedly decreased by TREM2 overexpression. A similar improvement was also observed by the administration of TAK242 following ICH. TREM2 improves neurological dysfunction and attenuates neuroinflammation and neuronal apoptosis in the acute phase of ICH, which is, at least in part, mediated by negatively regulating TLR4 signaling pathway. These findings highlight TREM2 as a potential target for early brain injury following ICH.

Role of TREM2 in the Development of Neurodegenerative Diseases After Traumatic Brain Injury.

Traumatic brain injury (TBI) has been found as the primary cause of morbidity and disability worldwide, which has posed a significant social and economic burden. The first stage of TBI produces brain edema, axonal damage, and hypoxia, thus having an effect on the blood-brain barrier function, promoting inflammatory responses, and increasing oxidative stress. Patients with TBI are more likely to develop post-traumatic epilepsy, behavioral issues, as well as mental illnesses. The long-term effects arising from TBI have aroused rising attention over the past few years. Microglia in the brain can express the triggering receptor expressed on myeloid cells 2 (TREM2), which is a single transmembrane receptor pertaining to the immunoglobulin superfamily. The receptor has been correlated with a number of neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, and other relevant diseases. In this review, it is demonstrated that TREM2 is promising to serve as a neuroprotective factor for neurodegenerative disorders following TBI by modulating the function of microglial cells. Accordingly, it has potential avenues for TREM2-related therapies to improve long-term recovery after TBI.