Triggering Receptor Expressed On Myeloid Cells 2 Function Research Articles

Multimerization of TREM2 is impaired by Alzheimer's disease-associated variants.

The immune receptor triggering receptor expressed on myeloid cells 2 (TREM2) is among the strongest genetic risk factors for Alzheimer's disease (AD) and is a therapeutic target. TREM2 multimers have been identified in crystallography and implicated in the efficacy of antibody therapeutics; however, the molecular basis for TREM2 multimerization remains poorly understood. We used molecular dynamics simulations and binding energy analysis to determine the effects of AD-associated variants on TREM2 multimerization and validated with experimental results. TREM2 trimers remained stably bound, driven primarily by salt bridge between residues D87 and R76 at the interface of TREM2 units. This salt bridge was disrupted by the AD-associated variants R47H and R98W and nearly ablated by the D87N variant. This decreased binding among TREM2 multimers was validated with co-immunoprecipitation assays. This study uncovers a molecular basis for TREM2 forming stable trimers and unveils a novel mechanism by which TREM2 variants may increase AD risk by disrupting TREM2 oligomerization to impair TREM2 normal function. Triggering receptor expressed on myeloid cells 2 (TREM2) multimerization could regulate TREM2 activation and function. D87-R76 salt bridges at the interface of TREM2 units drive the formation of stable TREM2 dimers and trimers. Alzheimer's disease (AD)-associated R47H and R98W variants disrupt the D87-R76 salt bridge. The AD-associated D87N variant leads to complete loss of the D87-R76 salt bridge.

Intermittent hypoxia training enhances Aβ endocytosis by plaque associated microglia via VPS35-dependent TREM2 recycling in murine Alzheimer’s disease

BackgroundBeta-amyloid (Aβ) deposition in the brain parenchyma is a crucial initiating step in the amyloid cascade hypothesis of Alzheimer’s disease (AD) pathology. Furthermore, dysfunction of plaque-associated microglia, also known as disease-associated microglia (DAM) has been reported to accelerate Aβ deposition and cognitive impairment. Our previous research demonstrated that intermittent hypoxia training (IHT) improved AD pathology by upregulating autophagy in DAM, thereby enhancing oligomeric Aβ (oAβ) clearance. Considering that oAβ internalization is the initial stage of oAβ clearance, this study focused on the IHT mechanism involved in upregulating Aβ uptake by DAM.MethodsIHT was administered to 8-month-old APP/PS1 mice or 6-month-old microglial vacuolar protein sorting 35 (VPS35) knockout mice in APP/PS1 background (MG VPS35 KO: APP/PS1) for 28 days. After the IHT, the spatial learning-memory capacity of the mice was assessed. Additionally, AD pathology was determined by estimating the nerve fiber and synapse density, Aβ plaque deposition, and Aβ load in the brain. A model of Aβ-exposed microglia was constructed and treated with IHT to explore the related mechanism. Finally, triggering receptor expressed on myeloid cells 2 (TREM2) intracellular recycling and Aβ internalization were measured using a fluorescence tracing technique.ResultsOur results showed that IHT ameliorated cognitive function and Aβ pathology. In particular, IHT enhanced Aβ endocytosis by augmenting the intracellular transport function of microglial TREM2, thereby contributing to Aβ clearance. Furthermore, IHT specifically upregulated VPS35 in DAM, the primary cause for the enhanced intracellular recycling of TREM2. IHT lost ameliorative effect on Aβ pathology in MG VPS35 KO: APP/PS1 mice brain. Lastly, the IHT mechanism of VPS35 upregulation in DAM was mediated by the transcriptional regulation of VPS35 by transcription factor EB (TFEB).ConclusionIHT enhances Aβ endocytosis in DAM by upregulating VPS35-dependent TREM2 recycling, thereby facilitating oAβ clearance and mitigation of Aβ pathology. Moreover, the transcriptional regulation of VPS35 by TFEB demonstrates a close link between endocytosis and autophagy in microglia. Our study further elucidates the IHT mechanism in improving AD pathology and provides evidence supporting the potential application of IHT as a complementary therapy for AD.Graphical abstract

Distinct roles of TREM2 in central nervous system cancers and peripheral cancers

Glioblastomas (GBM) are incurable central nervous system (CNS) cancers characterized by substantial myeloid cell infiltration. Whether myeloid cell-directed therapeutic targets identified in peripheral non-CNS cancers are applicable to GBM requires further study. Here, we identify that the critical immunosuppressive target in peripheral cancers, triggering receptor expressed on myeloid cells-2 (TREM2), is immunoprotective in GBM. Genetic or pharmacological TREM2 deficiency promotes GBM progression invivo. Single-cell and spatial sequencing reveals downregulated TREM2 in GBM-infiltrated myeloid cells. TREM2 negatively correlates with immunosuppressive myeloid and Tcell exhaustion signatures in GBM. We further demonstrate that during GBM progression, CNS-enriched sphingolipids bind TREM2 on myeloid cells and elicit antitumor responses. Clinically, high TREM2 expression in myeloid cells correlates with better survival in GBM. Adeno-associated virus-mediated TREM2 overexpression impedes GBM progression and synergizes with anti-PD-1 therapy. Our results reveal distinct functions of TREM2 in CNS cancers and support organ-specific myeloid cell remodeling in cancer immunotherapy.

Dihydroquercetin alleviates dopamine neuron loss via regulating TREM2 activation

BackgroundParkinson's disease (PD) is a prevalent neurodegenerative disorder, marked by the degeneration of dopamine (DA) neurons in the substantia nigra (SN). Current evidence strongly suggests that neuroinflammation, primarily mediated by microglia, contributes to PD pathogenesis. Triggering receptor expressed on myeloid cells 2 (TREM2) might serve as a promising therapeutic target for PD due to its ability to suppress neuroinflammation. Dihydroquercetin (DHQ) is an important natural dihydroflavone and confers apparent anti-inflammatory, antioxidant and anti-fibrotic effects. Recently, DHQ-mediated neuroprotection was exhibited. However, the specific mechanisms of its neuroprotective effects remain incompletely elucidated. MethodsIn this study, rat models were utilized to induce damage to DA neurons using lipopolysaccharide (LPS) and 6-hydroxydopamine (6-OHDA) to assess the impacts of DHQ on the loss of DA neurons. Furthermore, DA neuronal MN9D cells and microglial BV2 cells were employed to investigate the function of TREM2 in DHQ-mediated DA neuroprotection. Finally, TREM2 knockout mice were used to investigate whether the neuroprotective effects mediated by DHQ through a mechanism dependent on TREM2. ResultsThe main findings demonstrated that DHQ effectively protected DA neurons against neurotoxicity induced by LPS and 6-OHDA and inhibited microglia-elicited neuroinflammation. Meanwhile, DHQ promoted microglial TREM2 signaling activation. Notably, DHQ failed to reduce inflammatory cytokines release and further present neuroprotection from DA neurotoxicity upon TREM2 silencing. Similarly, DHQ didn't exert DA neuroprotection in TREM2 knockout mice. ConclusionsThese findings suggest that DHQ exerted DA neuroprotection by regulating microglia TREM2 activation.

TREM2 deficiency impairs the energy metabolism of Schwann cells and exacerbates peripheral neurological deficits

Triggering receptor expressed on myeloid cells-2 (TREM2) has been implicated in susceptibility to neurodegenerative disease. Schwann cells (SCs), the predominant glial cell type in the peripheral nervous system (PNS), play a crucial role in myelination, providing trophic support for neurons and nerve regeneration. However, the function of TREM2 in SCs has not been fully elucidated. Here, we found that TREM2 is expressed in SCs but not in neurons in the PNS. TREM2 deficiency leads to disruption of glycolytic flux and oxidative metabolism in SCs, impairing cell proliferation. The energy crisis caused by TREM2 deficiency triggers mitochondrial damage and autophagy by activating AMPK and impairing PI3K-AKT-mTOR signaling. Combined metabolomic analysis demonstrated that energic substrates and energy metabolic pathways were significantly impaired in TREM2-deficient SCs. Moreover, TREM2 deficiency impairs energy metabolism and axonal growth in sciatic nerve, accompanied by exacerbation of neurological deficits and suppression of nerve regeneration in a mouse model of acute motor axonal neuropathy. These results indicate that TREM2 is a critical regulator of energy metabolism in SCs and exerts neuroprotective effects on peripheral neuropathy.TREM2 deficiency impairs glycolysis and oxidative metabolism in Schwann cells, resulting in compromised cell proliferation. The energy crisis caused by TREM2 deficiency induces mitochondrial damage and autophagy by activating AMPK and impairing PI3K-AKT-mTOR signaling. Moreover, TREM2 deficiency disrupts the energy metabolism of the sciatic nerve and impairs support for axonal regeneration, accompanied by exacerbation of neurological deficits and suppression of nerve regeneration in a mouse model of acute motor axonal neuropathy (by FigDraw).

Function and mechanism of TREM2 in bacterial infection.

Triggering receptor expressed on myeloid cells 2 (TREM2), which is a lipid sensing and phagocytosis receptor, plays a key role in immunity and inflammation in response to pathogens. Here, we review the function and signaling of TREM2 in microbial binding, engulfment and removal, and describe TREM2-mediated inhibition of inflammation by negatively regulating the Toll-like receptor (TLR) response. We further illustrate the role of TREM2 in restoring organ homeostasis in sepsis and soluble TREM2 (sTREM2) as a diagnostic marker for sepsis-associated encephalopathy (SAE). Finally, we discuss the prospect of TREM2 as an interesting therapeutic target for sepsis.

TREM2 mediates MHCII-associated CD4+ T-cell response against gliomas.

Myeloid cells comprise up to 50% of the total tumor mass in glioblastoma (GBM) and have been implicated in promoting tumor progression and immunosuppression. Modulating the response of myeloid cells to the tumor has emerged as a promising new approach for cancer treatment. In this regard, we focus on the Triggering Receptor Expressed on Myeloid Cells 2 (TREM2), which has recently emerged as a novel immune modulator in peripheral tumors. We studied the TREM2 expression profile in various patient tumor samples and conducted single-cell transcriptomic analysis in both GBM patients and the GL261 mouse glioma model. We utilized multiple mouse glioma models and employed state-of-the-art techniques such as invivo 2-photon imaging, spectrum flow cytometry, and in vitro co-culture assays to study TREM2 function in myeloid cell-mediated phagocytosis of tumor cells, antigen presentation, and response of CD4+ T cells within the tumor hemispheres. Our research revealed significantly elevated levels of TREM2 expression in brain tumors compared to other types of tumors in patients. TREM2 was predominantly localized in tumor-associated myeloid cells and was highly expressed in nearly all microglia, as well as various subtypes of macrophages. Surprisingly, in preclinical glioma models, TREM2 deficiency did not confer a beneficial effect; instead, it accelerated glioma progression. Through detailed investigations, we determined that TREM2 deficiency impaired the ability of tumor-myeloid cells to phagocytose tumor cells and led to reduced expression of MHCII. This deficiency further significantly decreased the presence of CD4+ T cells within the tumor hemispheres. Our study unveiled a previously unrecognized protective role of tumor-myeloid TREM2. Specifically, we found that TREM2 enhances the phagocytosis of tumor cells and promotes an immune response by facilitating MHCII-associated CD4+ T-cell responses against gliomas.

The Pathogenesis in Alzheimer's Disease: TREM2 as a Potential Target.

Alzheimer's disease (AD) is ranked as the third-most expensive illness and sixth leading cause of mortality. It is associated with the deposition of extracellular amyloid-β (Aβ) in neural plaques (NPs), as well as intracellular hyperphosphorylated tau proteins that form neurofibrillary tangles (NFTs). As a new target in regulating neuroinflammation in AD, triggering receptor expressed on myeloid cells 2 (TREM2) is highly and exclusively expressed on the microglial surface. TREM2 interacts with adaptor protein DAP12 to initiate signal pathways that mainly dominant microglia phenotype and phagocytosis mobility. Furthermore, TREM2 gene mutations confer increased AD risk, and TREM2 deficiency exhibits more dendritic spine loss around neural plaques. Mechanisms for regulating TREM2 to alleviate AD has evolved as an area of AD research in recent years. Current medications targeting Aβ or tau proteins are unable to reverse AD progression. Emerging evidence implicating neuroinflammation may provide novel insights, as early microglia-related inflammation can be induced decades prior to the commencement of AD-related cognitive damage. Physical exercise can exert a neuroprotective effect over the course of AD progression. This review aims to (1) summarize the pathogenesis of AD and recent updates in the field, (2) assess the concept that AD cognitive impairment is closely correlated with microglia-related inflammation, and (3) review TREM2 functions and its role between exercise and AD, which is likely to be an ideal candidate target.

The dynamic functional role of triggering receptor expressed on myeloid cells 2 (TREM2) assessed with cerebrospinal fluid and positron emission tomography measures

AbstractBackgroundTriggering receptor expressed on myeloid cells 2 (TREM2) regulates several microglial activities and is essential for microglia to switch to an activated state. The role of TREM2 in the pathogenesis of Alzheimer’s disease (AD) is unclear, however. Following its role in microglial activity, the ectodomain of TREM2 is cleaved into the soluble proteolytic end product of TREM2 (sTREM2), which is released from the transmembrane domain into the cerebrospinal fluid (CSF) and, therefore, CSF sTREM2 is considered a biomarker of TREM2 activity.MethodTo examine the role of TREM2, we explored the relationships between CSF sTREM2, amyloid‐beta (Aβ42), phosphorylated tau‐181 (p‐tau‐181), and total‐tau (t‐tau) in a cohort of 495 participants (100 patients with AD, 257 patients with MCI, and 138 controls) whose data were obtained from the Alzheimer’s Disease Neuroimaging Initiative (ADNI) database. Associations between CSF sTREM2 and 18F‐fluorodeoxyglucose (18F‐FDG) positron emission tomography (PET), 18F‐florbetapir (18F‐AV45) PET, and grey matter volume (GMV) were also investigated.ResultCSF sTREM2 levels were stable across AD, MCI and cognitively normal subgroups. In the whole cohort, CSF sTREM2 positively correlated with age (r = 0.214,p<0.001), CSF Aβ42 (r = 0.118,p = 0.009), CSF t‐tau (r = 0.423,p<0.001), and CSF p‐tau‐181 (r = 0.396,p<0.001), and negatively correlated with MMSE score (r = ‐0.105,p = 0.019). CSF sTREM2, t‐tau and p‐tau‐181 could significantly predict CSF Aβ42 levels, while CSF sTREM2 and Aβ42 could significantly predict CSF p‐tau‐181 and t‐tau levels. Higher CSF sTREM2 was associated with greater amyloid load, decreased glucose metabolism, and reduced GMV in MCI, but not in AD, in certain brain regions.ConclusionCSF sTREM2 levels were comparable across clinical groups, and with CSF sTREM2 correlating with markers of AD and neuronal damage, we predict that changes in neuropathological processes occurring in relation to changes in CSF sTREM2 levels are a result of alterations in TREM2 function, rather than quantity. Taken together with the relationships between CSF sTREM2 and imaging findings, these results suggest that sTREM2 appears early on in the AD trajectory, probably in response to early microglial activation in MCI in response to evolving pathological processes.

TREM2 expression on the microglia resolved lead exposure-induced neuroinflammation by promoting anti-inflammatory activities

Neurotoxicity caused by environmental lead (Pb) pollution is a worldwide public health concern, and developing a therapeutic strategy against Pb-induced neurotoxicity is an important area in the current research. Our prior research has demonstrated the significant involvement of microglia-mediated inflammatory responses in the manifestation of Pb-induced neurotoxicity. Additionally, the suppression of proinflammatory mediator activity significantly mitigated the toxic effects associated with Pb exposure. Recent studies have highlighted the critical role of the triggering receptor expressed on myeloid cells 2 (TREM2) in the pathogenesis of neurodegenerative disorders. TREM2 exerted protective effects on inflammation, but whether TREM2 is involved in Pb-induced neuroinflammation is poorly understood. In the present study, cell culture experiments and animal models were designed to investigate the role of TREM2 in Pb’s neuroinflammation. We examined the impact of pro- and anti-inflammatory cytokines involved in Pb-induced neuroinflammation. Flow cytometry and microscopy techniques were applied to detect microglia phagocytosis and migration ability. Our results showed that Pb treatment significantly downregulated TREM2 expression and altered the localization of TREM2 expression in microglia. The protein expression of TREM2 was restored, and the inflammatory responses provoked by Pb exposure were ameliorated upon the overexpression of TREM2. Furthermore, the phagocytosis and migratory capabilities of microglia, which were impaired due to Pb exposure, were alleviated by TREM2 overexpression. Our in vitro findings were corroborated in vivo, demonstrating that TREM2 regulates the anti-inflammatory functions of microglia, thereby mitigating Pb-induced neuroinflammation. Our results provide insights into the detailed mechanism by which TREM2 alleviates Pb-induced neuroinflammation and suggest that activating the anti-inflammatory functions of TREM2 may represent a potential therapeutic strategy against environmental Pb-induced neurotoxicity.

TREM2 is associated with tumor immunity and implies poor prognosis in glioma.

Triggering receptor expressed on myeloid cells 2 (TREM2) is expressed in myeloid cells of the central nervous system (CNS), which mediate the immunological response in a variety of diseases. Uncertain is the function of TREM2 in glioma and tumor immune responses. In this research, the expression patterns of TREM2 in glioma were analyzed, along with its prognostic value and functional roles. TREM2 expression is increased in glioblastomas, gliomas with a mesenchymal subtype, gliomas with wild-type isocitrate dehydrogenase, and gliomas without 1p/19q deletion, all of which suggest the aggressiveness and poor prognosis of gliomas. Gene ontology, KEGG, and Gene set variation analyses indicated that TREM2 may serve as an immune response mediator. However, the function of T cells against tumor cells was negatively correlated with TREM2, suggesting that TREM2 may suppress tumor immunity. Further investigation demonstrated a correlation between TREM2 expression and immune checkpoint expression. CIBERSORT research revealed a link between a higher TREM2 expression level and the enrichment of tumor-associated macrophages, especially M2 subtype. Single-cell analysis and multiple immunohistochemical staining results showed that microglia and macrophage cells expressed TREM2. Immunofluorescent staining indicated that knocking down the expression of TREM2 would result in a decrease in M2 polarization. TREM2 was discovered to be an independent prognostic factor in glioma. In conclusion, our findings revealed that TREM2 was significantly expressed in microglia and macrophage cells and was intimately associated with the tumor immune microenvironment. Thus, it is expected that small-molecule medications targeting TREM2 or monoclonal antibodies would enhance the efficacy of glioma immunotherapy.

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.

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.

Chronic TREM2 activation exacerbates Aβ-associated tau seeding and spreading.

Variants in the triggering receptor expressed on myeloid cells 2 (TREM2) gene are associated with increased risk for late-onset AD. Genetic loss of or decreased TREM2 function impairs the microglial response to amyloid-β (Aβ) plaques, resulting in more diffuse Aβ plaques and increased peri-plaque neuritic dystrophy and AD-tau seeding. Thus, microglia and TREM2 are at a critical intersection of Aβ and tau pathologies in AD. Since genetically decreasing TREM2 function increases Aβ-induced tau seeding, we hypothesized that chronically increasing TREM2 signaling would decrease amyloid-induced tau-seeding and spreading. Using a mouse model of amyloidosis in which AD-tau is injected into the brain to induce Aβ-dependent tau seeding/spreading, we found that chronic administration of an activating TREM2 antibody increases peri-plaque microglial activation but surprisingly increases peri-plaque NP-tau pathology and neuritic dystrophy, without altering Aβ plaque burden. Our data suggest that sustained microglial activation through TREM2 that does not result in strong amyloid removal may exacerbate Aβ-induced tau pathology, which may have important clinical implications.

Microglial response in triggering receptor expressed on myeloid cells 2 (TREM2) knock-out mice after systemic stimulation with Escherichia coli

BackgroundSystemic infection is an important risk factor for delirium, associated with neurodegeneration and subsequent cognitive impairment in older people. Microglial cell response is a known key player in this process and we hypothesize that the triggering receptor expressed on myeloid cells 2 (TREM2) plays an important role in the regulation of this response. Methods8- to 10-week old male wild-type (WT) and TREM2 knock-out (Trem2-/-) mice were intraperitoneally inoculated with live Escherichia coli (E. coli) or saline. After inoculation, all mice were treated with ceftriaxone (an antimicrobial drug) at 12 and 24 h and were sacrificed after 2 and 3 days. Microglial response was determined by immunohistochemical staining with an ionized calcium-binding adaptor molecule 1 (Iba-1) antibody and flow cytometry. mRNA expression of pro- and anti-inflammatory mediators was measured to quantify the inflammatory response. ResultsWe observed increased Iba-1 positive cells number in thalamus of Trem2-/- mice at 3d after inoculation compared to WT mice (mean 120 cell/mm2 [SD 8] vs 105 cell/mm2 [SD 11]; p = 0.03). Flow cytometry showed no differences in forward scatter or expression of CD11b, CD45 and CD14 between WT and Trem2-/- mice. The brain mRNA expression levels of tumor necrosis factor alpha (TNF-α) of Trem2-/- mice at 2d were higher compared to WT mice (p = 0.003). Higher mRNA expression of interleukin 1 beta (IL-1β), Iba-1, CD11b and mitogen-activated protein kinase 1 (MAPK-1) was found in brain of WT mice at 2d compared to Trem2-/- mice (respectively p = 0.02; p = 0.001; p = 0.03 and p = 0.02). In spleen there were no differences in inflammatory mediators, between WT and Trem2-/- mice. InterpretationAlthough the loss of function of TREM2 during systemic infection led to an increased number of activated microglia in the thalamus, we did not observe a consistent increase in expression of inflammatory genes in the brain. The role of TREM2 in the neuro-inflammatory response following systemic infection therefore appears to be limited.

TREM2 Deficiency Disrupts Network Oscillations Leading to Epileptic Activity and Aggravates Amyloid-β-Related Hippocampal Pathophysiology in Mice.

Genetic mutations in triggering receptor expressed on myeloid cells-2 (TREM2) have been strongly associated with increased risk of developing Alzheimer's disease (AD) and other progressive dementias. In the brain, TREM2 protein is specifically expressed on microglia suggesting their active involvement in driving disease pathology. Using various transgenic AD models to interfere with microglial function through TREM2, several recent studies provided important data indicating a causal link between TREM2 and underlying amyloid-β (Aβ) and tau pathology. However, mechanisms by which TREM2 contributes to increased predisposition to clinical AD and influences its progression still remain largely unknown. Our aim was to elucidate the potential contribution of TREM2 on specific oscillatory dynamic changes associated with AD pathophysiology. Spontaneous and brainstem nucleus pontis oralis stimulation-induced hippocampal oscillation paradigm was used to investigate the impact of TREM2 haploinsufficiency TREM2(Het) or total deficiency TREM2(Hom) on hippocampal network function in wild-type and Aβ overproducing Tg2576 mice under urethane anesthesia. Partial (TREM2(Het)) or total (TREM2(Hom)) deletion of TREM2 led to increased incidence of spontaneous epileptiform seizures in both wild-type and Tg2576 mice. Importantly, deficiency of TREM2 in Tg2576 mice significantly diminished power of theta oscillation in the hippocampus elicited by brainstem-stimulation compared to wild-type mice. However, it did not affect hippocampal theta-phase gamma-amplitude coupling significantly, since over a 60%reduction was found in coupling in Tg2576 mice regardless of TREM2 function. Our findings indicate a role for TREM2-dependent microglial function in the hippocampal neuronal excitability in both wild type and Aβ overproducing mice, whereas deficiency in TREM2 function exacerbates disruptive effects of Aβ on hippocampal network oscillations.

LILRB2-mediated TREM2 signaling inhibition suppresses microglia functions

BackgroundMicroglia plays crucial roles in Alzheimer’s disease (AD) development. Triggering receptor expressed on myeloid cells 2 (TREM2) in association with DAP12 mediates signaling affecting microglia function. Here we study the negative regulation of TREM2 functions by leukocyte immunoglobulin-like receptor subfamily B member 2 (LILRB2), an inhibitory receptor bearing ITIM motifs.MethodsTo specifically interrogate LILRB2-ligand (oAβ and PS) interactions and microglia functions, we generated potent antagonistic LILRB2 antibodies with sub-nanomolar level activities. The biological effects of LILRB2 antagonist antibody (Ab29) were studied in human induced pluripotent stem cell (iPSC)–derived microglia (hMGLs) for migration, oAβ phagocytosis, and upregulation of inflammatory cytokines. Effects of the LILRB2 antagonist antibody on microglial responses to amyloid plaques were further studied in vivo using stereotaxic grafted microglia in 5XFAD mice.ResultsWe confirmed the expression of both LILRB2 and TREM2 in human brain microglia using immunofluorescence. Upon co-ligation of the LILRB2 and TREM2 by shared ligands oAβ or PS, TREM2 signaling was significantly inhibited. We identified a monoclonal antibody (Ab29) that blocks LILRB2/ligand interactions and prevents TREM2 signaling inhibition mediated by LILRB2. Further, Ab29 enhanced microglia phagocytosis, TREM2 signaling, migration, and cytokine responses to the oAβ-lipoprotein complex in hMGL and microglia cell line HMC3. In vivo studies showed significantly enhanced clustering of microglia around plaques with a prominent increase in microglial amyloid plaque phagocytosis when 5XFAD mice were treated with Ab29.ConclusionsThis study revealed for the first time the molecular mechanisms of LILRB2-mediated inhibition of TREM2 signaling in microglia and demonstrated a novel approach of enhancing TREM2-mediated microglia functions by blocking LILRB2-ligand interactions. Translationally, a LILRB2 antagonist antibody completely rescued the inhibition of TREM2 signaling by LILRB2, suggesting a novel therapeutic strategy for improving microglial functions.

Dynamic insights into the effects of nonsynonymous polymorphisms (nsSNPs) on loss of TREM2 function

Single nucleotide variations in Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) are associated with many neurodegenerative diseases, including Nasu-Hakola disease (NHD), frontotemporal dementia (FTD), and late-onset Alzheimer's disease because they disrupt ligand binding to the extracellular domain of TREM2. However, the effects of nonsynonymous single nucleotide polymorphisms (nsSNPs) in TREM2 on disease progression remain unknown. In this study, we identified several high-risk nsSNPs in the TREM2 gene using various deleterious SNP predicting algorithms and analyzed their destabilizing effects on the ligand recognizing region of the TREM2 immunoglobulin (Ig) domain by molecular dynamics (MD) simulation. Cumulative prediction by all tools employed suggested the three most deleterious nsSNPs involved in loss of TREM2 function are rs549402254 (W50S), rs749358844 (R52C), and rs1409131974 (D104G). MD simulation showed that these three variants cause substantial structural alterations and conformational remodeling of the apical loops of the TREM2 Ig domain, which is responsible for ligand recognition. Detailed analysis revealed that these variants substantially increased distances between apical loops and induced conformation remodeling by changing inter-loop nonbonded contacts. Moreover, all nsSNPs changed the electrostatic potentials near the putative ligand-interacting region (PLIR), which suggested they might reduce specificity or loss of binding affinity for TREM2 ligands. Overall, this study identifies three potential high-risk nsSNPs in the TREM2 gene. We propose further studies on the molecular mechanisms responsible for loss of TREM2 function and the associations between TREM2 nsSNPs and neurodegenerative diseases.

Developing a functionally valid model of the TREM2-APOE complex to better understand its role in Alzheimer's disease.

Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) and Apolipoprotein E (ApoE) are the most potent genetic risk factors for late-onset Alzheimer's disease (AD). ApoE can act as a ligand of TREM2, activating an immune response around AD-associated pathology in the brain; however, without a robust model of the binding between the two proteins, it is difficult to study the mechanism of this interaction or to target it for drug development. To define the structural mechanism by which AD-associated variants impaired TREM2's function as an innate immune receptor, we used a combination of sequence-based binding site prediction, in silico molecular modelling, and biophysical validation with biolayer interferometry (BLI) to develop an initial all-atom model for the TREM2-ApoE complex. Sequence-based binding site prediction and BLI both identify the apical hydrophobic site of TREM2 and hinge region of ApoE as the likely binding sites for the complex. Early models from in silico docking and molecular simulations at these sites suggest formation of a stable complex model and reveal TREM2 variant-dependent changes in the conformation and dynamics of ApoE. Extended simulations using site-specific mutations of TREM2 in models with restraints optimized based on BLI results are also in progress, which will be further validated by comparing predicted binding affinities from the simulated complex to experimental affinities measured with BLI. Our work linking AD-associated variants to instability in TREM2's hydrophobic site, where we find ApoE to bind, provides further evidence that the loss-of-function caused by these variants is likely driven by impaired binding of ligands like ApoE. This work also provides further justification for targeting the TREM2-ApoE interaction as a potential therapeutic for AD, while simultaneously providing a model that may be useful for developing and refining such therapeutics.

Differential interaction with TREM2 modulates microglial uptake of modified Aβ species.

Rare coding variants of the microglial triggering receptor expressed on myeloid cells 2 (TREM2) confer an increased risk for Alzheimer's disease (AD) characterized by the progressive accumulation of aggregated forms of amyloid β peptides (Aβ). Aβ peptides are generated by proteolytic processing of the amyloid precursor protein (APP). Heterogeneity in proteolytic cleavages and additional post-translational modifications result in the production of several distinct Aβ variants that could differ in their aggregation behavior and toxic properties. Here, we sought to assess whether post-translational modifications of Aβ affect the interaction with TREM2. Biophysical and biochemical methods revealed that TREM2 preferentially interacts with oligomeric Aβ, and that phosphorylation of Aβ increases this interaction. Phosphorylation of Aβ also affected the TREM2 dependent interaction and phagocytosis by primary microglia and in APP transgenic mouse models. Thus, TREM2 function is important for sensing phosphorylated Aβ variants in distinct aggregation states and reduces the accumulation and deposition of these toxic Aβ species in preclinical models of Alzheimer's disease.