Functional 3'-UTR Variants Identify Regulatory Mechanisms Impacting Alcohol Use Disorder and Related Traits.

Although genome-wide association studies (GWAS) have identified loci associated with alcohol consumption and alcohol use disorder (AUD), they do not identify which variants are functional. To approach this, we evaluated the impact of variants in 3' untranslated regions (3'-UTRs) of genes in loci associated with substance use and neurological disorders using a massively parallel reporter assay (MPRA) in neuroblastoma and microglia cells. Functionally impactful variants explained a higher proportion of heritability of alcohol traits than non-functional variants. We identified genes whose 3'UTR activities are associated with AUD and alcohol consumption by combining variant effects from MPRA with GWAS results. We examined their effects by evaluating gene expression after CRISPR inhibition of neuronal cells and stratifying brain tissue samples by MPRA-derived 3'-UTR activity. A pathway analysis of differentially expressed genes identified inflammation response pathways. These analyses suggest that variation in response to inflammation contributes to the propensity to increase alcohol consumption.

Therapeutic targeting of immunometabolism in Alzheimer's disease reveals a critical reliance on Hexokinase 2 dosage on microglial activation and disease progression.

Microgliosis and neuroinflammation are prominent features of Alzheimer's disease (AD). Disease-responsive microglia meet their increased energy demand by reprogramming metabolism, specifically, switching to favor glycolysis over oxidative phosphorylation. Thus, targeting of microglial immunometabolism might be of therapeutic benefit for treating AD, providing novel and often well understood immune pathways and their newly recognized actions in AD. We report that in the brains of 5xFAD mice and postmortem brains of AD patients, we found a significant increase in the levels of Hexokinase 2 (HK2), an enzyme that supports inflammatory responses by rapidly increasing glycolysis. Moreover, binding of HK2 to mitochondria has been reported to regulate inflammation by preventing mitochondrial dysfunction and NLRP3 inflammasome activation, suggesting that its inflammatory role extends beyond its glycolytic activity. Here we report, that HK2 antagonism selectively affects microglial phenotypes and disease progression in a gene-dose dependent manner. Paradoxically, complete loss of HK2 fails to improve AD progression by exacerbating inflammasome activity while its haploinsufficiency results in reduced pathology and improved cognition in the 5XFAD mice. We propose that the partial antagonism of HK2, is effective in slowed disease progression and inflammation through a non-metabolic mechanism associated with the modulation of NFKß signaling, through its cytosolic target IKBα. The complete loss of HK2 affects additional inflammatory mechanisms associated to mitochondrial dysfunction.

Deep learning-driven adaptive optics for single-molecule localization microscopy.

The inhomogeneous refractive indices of biological tissues blur and distort single-molecule emission patterns generating image artifacts and decreasing the achievable resolution of single-molecule localization microscopy (SMLM). Conventional sensorless adaptive optics methods rely on iterative mirror changes and image-quality metrics. However, these metrics result in inconsistent metric responses and thus fundamentally limit their efficacy for aberration correction in tissues. To bypass iterative trial-then-evaluate processes, we developed deep learning-driven adaptive optics for SMLM to allow direct inference of wavefront distortion and near real-time compensation. Our trained deep neural network monitors the individual emission patterns from single-molecule experiments, infers their shared wavefront distortion, feeds the estimates through a dynamic filter and drives a deformable mirror to compensate sample-induced aberrations. We demonstrated that our method simultaneously estimates and compensates 28 wavefront deformation shapes and improves the resolution and fidelity of three-dimensional SMLM through >130-µm-thick brain tissue specimens.

Genetic variants of phospholipase C-γ2 alter the phenotype and function of microglia and confer differential risk for Alzheimer's disease.

Genetic association studies have demonstrated the critical involvement of the microglial immune response in Alzheimer's disease (AD) pathogenesis. Phospholipase C-gamma-2 (PLCG2) is selectively expressed by microglia and functions in many immune receptor signaling pathways. In AD, PLCG2 is induced uniquely in plaque-associated microglia. A genetic variant of PLCG2, PLCG2P522R, is a mild hypermorph that attenuates AD risk. Here, we identified a loss-of-function PLCG2 variant, PLCG2M28L, that confers an increased AD risk. PLCG2P522R attenuated disease in an amyloidogenic murine AD model, whereas PLCG2M28L exacerbated the plaque burden associated with altered phagocytosis and Aß clearance. The variants bidirectionally modulated disease pathology by inducing distinct transcriptional programs that identified microglial subpopulations associated with protective or detrimental phenotypes. These findings identify PLCG2M28L as a potential AD risk variant and demonstrate that PLCG2 variants can differentially orchestrate microglial responses in AD pathogenesis that can be therapeutically targeted.

Control of the temporal development of Alzheimer's disease pathology by the MR1/MAIT cell axis.

BACKGROUND: Neuroinflammation is an important feature of Alzheimer's disease (AD). Understanding which aspects of the immune system are important in AD may lead to new therapeutic approaches. We study the major histocompatibility complex class I-related immune molecule, MR1, which is recognized by an innate-like T cell population called mucosal-associated invariant T (MAIT) cells. METHODS: Having found that MR1 gene expression is elevated in the brain tissue of AD patients by mining the Agora database, we sought to examine the role of the MR1/MAIT cell axis in AD pathology. Brain tissue from AD patients and the 5XFAD mouse model of AD were used to analyze MR1 expression through qPCR, immunofluorescence, and flow cytometry. Furthermore, mice deficient in MR1 and MAIT cells were crossed with the 5XFAD mice to produce a model to study how the loss of this innate immune axis alters AD progression. Moreover, 5XFAD mice were also used to study brain-resident MAIT cells over time. RESULTS: In tissue samples from AD patients and 5XFAD mice, MR1 expression was substantially elevated in the microglia surrounding plaques vs. those that are further away (human AD: P < 0.05; 5XFAD: P < 0.001). In 5XFAD mice lacking the MR1/MAIT cell axis, the development of amyloid-beta plaque pathology occurred at a significantly slower rate than in those mice with MR1 and MAIT cells. Furthermore, in brain tissue from 5XFAD mice, there was a temporal increase in MAIT cell numbers (P < 0.01) and their activation state, the latter determined by detecting an upregulation of both CD69 (P < 0.05) and the interleukin-2 receptor alpha chain (P < 0.05) via flow cytometry. CONCLUSIONS: Together, these data reveal a previously unknown role for the MR1/MAIT cell innate immune axis in AD pathology and its potential utility as a novel therapeutic target.

TREM2 splice isoforms generate soluble TREM2 species that disrupt long-term potentiation.

BACKGROUND: TREM2 is a transmembrane receptor expressed by myeloid cells and acts to regulate their immune response. TREM2 governs the response of microglia to amyloid and tau pathologies in the Alzheimer's disease (AD) brain. TREM2 is also present in a soluble form (sTREM2), and its CSF levels fluctuate as a function of AD progression. Analysis of stroke and AD mouse models revealed that sTREM2 proteins bind to neurons, which suggests sTREM2 may act in a non-cell autonomous manner to influence neuronal function. sTREM2 arises from the proteolytic cleavage of the membrane-associated receptor. However, alternatively spliced TREM2 species lacking a transmembrane domain have been postulated to contribute to the pool of sTREM2. Thus, both the source of sTREM2 species and its actions in the brain remain unclear. METHODS: The expression of TREM2 isoforms in the AD brain was assessed through the analysis of the Accelerating Medicines Partnership for Alzheimer's Disease Consortium transcriptomics data, as well as qPCR analysis using post-mortem samples of AD patients and of the AD mouse model 5xFAD. TREM2 cleavage and secretion were studied in vitro using HEK-293T and HMC3 cell lines. Synaptic plasticity, as evaluated by induction of LTP in hippocampal brain slices, was employed as a measure of sTREM2 actions. RESULTS: Three distinct TREM2 transcripts, namely ENST00000373113 (TREM2230), which encodes the full-length transmembrane receptor, and the alternatively spliced isoforms ENST00000373122 (TREM2222) and ENST00000338469 (TREM2219), are moderately increased in specific brain regions of patients with AD. We provide experimental evidence that TREM2 alternatively spliced isoforms are translated and secreted as sTREM2. Furthermore, our functional analysis reveals that all sTREM2 species inhibit LTP induction, and this effect is abolished by the GABAA receptor antagonist picrotoxin. CONCLUSIONS: TREM2 transcripts can give rise to a heterogeneous pool of sTREM2 which acts to inhibit LTP. These results provide novel insight into the generation, regulation, and function of sTREM2 which fits into the complex biology of TREM2 and its role in human health and disease. Given that sTREM2 levels are linked to AD pathogenesis and progression, our finding that sTREM2 species interfere with LTP furthers our understanding about the role of TREM2 in AD.

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

INTRODUCTION: 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. METHODS: We used the 5xFAD mouse model of amyloidosis to assess how Inpp5d haplodeficiency regulates amyloid pathogenesis. RESULTS: Inpp5d haplodeficiency perturbs the microglial intracellular signaling pathways regulating the immune response, including phagocytosis and clearing of amyloid beta (Aß). 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. CONCLUSION: 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.

Integrating multimodality magnetic resonance imaging to the Allen Mouse Brain Common Coordinate Framework.

High-resolution magnetic resonance imaging (MRI) affords unique image contrasts to nondestructively probe the tissue microstructure; validation of MRI findings with conventional histology is essential to better understand the MRI contrasts. However, the dramatic difference in the spatial resolution and image contrast of these two techniques impedes accurate comparison between MRI metrics and traditional histology. To better validate various MRI metrics, we acquired whole mouse brain multigradient recalled-echo and multishell diffusion MRI datasets at 25-µm isotropic resolution. The recently developed Allen Mouse Brain Common Coordinate Framework (CCFv3) provides opportunities to integrate multimodal and multiscale datasets of the whole mouse brain in a common three-dimensional (3D) space. The T2*, quantitative susceptibility mapping, diffusion tensor imaging, and neurite orientation dispersion and density imaging parameters were compared with both serial two-photon tomography images and 3D Nissl staining images in the CCFv3 at the same spatial resolution. The correlation between MRI and Nissl staining strongly depends on different metrics and different regions of the brain. Integrating different imaging modalities to the same space may substantially improve our understanding of the complexity of the brain at different scales.

Bassoon contributes to tau-seed propagation and neurotoxicity.

Tau aggregation is a defining histopathological feature of Alzheimer's disease and other tauopathies. However, the cellular mechanisms involved in tau propagation remain unclear. Here, we performed an unbiased quantitative proteomic study to identify proteins that specifically interact with this tau seed. We identified Bassoon (BSN), a presynaptic scaffolding protein, as an interactor of the tau seed isolated from a mouse model of tauopathy, and from Alzheimer's disease and progressive supranuclear palsy postmortem samples. We show that BSN exacerbates tau seeding and toxicity in both mouse and Drosophila models for tauopathy, and that BSN downregulation decreases tau spreading and overall disease pathology, rescuing synaptic and behavioral impairments and reducing brain atrophy. Our findings improve the understanding of how tau seeds can be stabilized by interactors such as BSN. Inhibiting tau-seed interactions is a potential new therapeutic approach for neurodegenerative tauopathies.

The intersection of metabolism and inflammation is governed by the intracellular topology of hexokinases and the metabolic fate of glucose.

Hexokinases (HKs) catalyze the first and irreversible step of glucose metabolism. Its product, glucose-6-phosphate (G-6P) serves as a precursor for catabolic processes like glycolysis for adenosine 5'-triphosphate (ATP) production and anabolic pathways including the pentose phosphate pathway (PPP) for the generation of intermediaries like nicotinamide adenine dinucleotide phosphate (NADPH) and ribulose-5-P. Thus, the cellular fate of glucose is important not only for growth and maintenance, but also to determine different cellular activities. Studies in immune cells have demonstrated an intimate linkage between metabolic pathways and inflammation, however the precise molecular mechanisms that determine the cellular fate of glucose during inflammation or aging are not completely understood. Here we discuss a study by De Jesus et al that describes the role of HK1 cytosolic localization as a critical regulator of glucose flux by shunting glucose into the PPP at the expense of glycolysis, exacerbating the inflammatory response of macrophages. The authors convincingly demonstrate a novel mechanism that is independent of its mitochondrial functions, but involve the association to a protein complex that inhibits glycolysis at the level of glyceraldehyde 3-phosphate dehydrogenase. We expand the discussion by comparing previous studies related to the HK2 isoform and how cells have evolved to regulate the mitochondrial association of these two isoforms by non-redundant mechanism.

Corrigendum: Age-dependent microstructure alterations in 5xFAD mice by high-resolution diffusion tensor imaging.

[This corrects the article DOI: 10.3389/fnins.2022.964654.].

Age-dependent microstructure alterations in 5xFAD mice by high-resolution diffusion tensor imaging.

Purpose: To evaluate the age-dependent microstructure changes in 5xFAD mice using high-resolution diffusion tensor imaging (DTI). Methods: The 5xFAD mice at 4, 7.5, and 12 months and the wild-type controls at 4 months were scanned at 9.4T using a 3D echo-planar imaging (EPI) pulse sequence with the isotropic spatial resolution of 100 µm. The b-value was 3000 s/mm2 for all the diffusion MRI scans. The samples were also acquired with a gradient echo pulse sequence at 50 µm isotropic resolution. The microstructure changes were quantified with DTI metrics, including fractional anisotropy (FA) and mean diffusivity (MD). The conventional histology was performed to validate with MRI findings. Results: The FA values (p = 0.028) showed significant differences in the cortex between wild-type (WT) and 5xFAD mice at 4 months, while hippocampus, anterior commissure, corpus callosum, and fornix showed no significant differences for either FA and MD. FA values of 5xFAD mice gradually decreased in cortex (0.140 ± 0.007 at 4 months, 0.132 ± 0.008 at 7.5 months, 0.126 ± 0.013 at 12 months) and fornix (0.140 ± 0.007 at 4 months, 0.132 ± 0.008 at 7.5 months, 0.126 ± 0.013 at 12 months) with aging. Both FA (p = 0.029) and MD (p = 0.037) demonstrated significant differences in corpus callosum between 4 and 12 months age old. FA and MD were not significantly different in the hippocampus or anterior commissure. The age-dependent microstructure alterations were better captured by FA when compared to MD. Conclusion: FA showed higher sensitivity to monitor amyloid deposition in 5xFAD mice. DTI may be utilized as a sensitive biomarker to monitor beta-amyloid progression for preclinical studies.

CX3CR1 deficiency aggravates amyloid driven neuronal pathology and cognitive decline in Alzheimer's disease.

BACKGROUND: Despite its identification as a key checkpoint regulator of microglial activation in Alzheimer's disease, the overarching role of CX3CR1 signaling in modulating mechanisms of Aß driven neurodegeneration, including accumulation of hyperphosphorylated tau is not well understood. METHODOLOGY: Accumulation of soluble and insoluble Aß species, microglial activation, synaptic dysregulation, and neurodegeneration is investigated in 4- and 6-month old 5xFAD;Cx3cr1+/+ and 5xFAD;Cx3cr1-/- mice using immunohistochemistry, western blotting, transcriptomic and quantitative real time PCR analyses of purified microglia. Flow cytometry based, in-vivo Aß uptake assays are used for characterization of the effects of CX3CR1-signaling on microglial phagocytosis and lysosomal acidification as indicators of clearance of methoxy-X-04+ fibrillar Aß. Lastly, we use Y-maze testing to analyze the effects of Cx3cr1 deficiency on working memory. RESULTS: Disease progression in 5xFAD;Cx3cr1-/- mice is characterized by increased deposition of filamentous plaques that display defective microglial plaque engagement. Microglial Aß phagocytosis and lysosomal acidification in 5xFAD;Cx3cr1-/- mice is impaired in-vivo. Interestingly, Cx3cr1 deficiency results in heighted accumulation of neurotoxic, oligomeric Aß, along with severe neuritic dystrophy, preferential loss of post-synaptic densities, exacerbated tau pathology, neuronal loss and cognitive impairment. Transcriptomic analyses using cortical RNA, coupled with qRT-PCR using purified microglia from 6 month-old mice indicate dysregulated TGFß-signaling and heightened ROS metabolism in 5xFAD;Cx3cr1-/- mice. Lastly, microglia in 6 month-old 5xFAD;Cx3cr1-/- mice express a 'degenerative' phenotype characterized by increased levels of Ccl2, Ccl5, Il-1ß, Pten and Cybb along with reduced Tnf, Il-6 and Tgfß1 mRNA. CONCLUSIONS: Cx3cr1 deficiency impairs microglial uptake and degradation of fibrillar Aß, thereby triggering increased accumulation of neurotoxic Aß species. Furthermore, loss of Cx3cr1 results in microglial dysfunction typified by dampened TGFß-signaling, increased oxidative stress responses and dysregulated pro-inflammatory activation. Our results indicate that Aß-driven microglial dysfunction in Cx3cr1-/- mice aggravates tau hyperphosphorylation, neurodegeneration, synaptic dysregulation and impairs working memory.

The niacin receptor HCAR2 modulates microglial response and limits disease progression in a mouse model of Alzheimer's disease.

Increased dietary intake of niacin has been correlated with reduced risk of Alzheimer's disease (AD). Niacin serves as a high-affinity ligand for the receptor HCAR2 (GPR109A). In the brain, HCAR2 is expressed selectively by microglia and is robustly induced by amyloid pathology in AD. The genetic inactivation of Hcar2 in 5xFAD mice, a model of AD, results in impairment of the microglial response to amyloid deposition, including deficits in gene expression, proliferation, envelopment of amyloid plaques, and uptake of amyloid-ß (Aß), ultimately leading to exacerbation of amyloid burden, neuronal loss, and cognitive deficits. In contrast, activation of HCAR2 with an FDA-approved formulation of niacin (Niaspan) in 5xFAD mice leads to reduced plaque burden and neuronal dystrophy, attenuation of neuronal loss, and rescue of working memory deficits. These data provide direct evidence that HCAR2 is required for an efficient and neuroprotective response of microglia to amyloid pathology. Administration of Niaspan potentiates the HCAR2-mediated microglial protective response and consequently attenuates amyloid-induced pathology, suggesting that its use may be a promising therapeutic approach to AD that specifically targets the neuroimmune response.

PLCG2 is associated with the inflammatory response and is induced by amyloid plaques in Alzheimer's disease.

BACKGROUND: Alzheimer's disease (AD) is characterized by robust microgliosis and phenotypic changes that accompany disease pathogenesis. Accumulating evidence from genetic studies suggests the importance of phospholipase C γ 2 (PLCG2) in late-onset AD (LOAD) pathophysiology. However, the role of PLCG2 in AD is still poorly understood. METHODS: Using bulk RNA-Seq (N=1249) data from the Accelerating Medicines Partnership-Alzheimer's Disease Consortium (AMP-AD), we investigated whether PLCG2 expression increased in the brains of LOAD patients. We also evaluated the relationship between PLCG2 expression levels, amyloid plaque density, and expression levels of microglia specific markers (AIF1 and TMEM119). Finally, we investigated the longitudinal changes of PLCG2 expression in the 5xFAD mouse model of AD. To further understand the role of PLCG2 in different signaling pathways, differential gene expression and co-expression network analyses were performed using bulk RNA-Seq and microglial single-cell RNA-Seq data. To substantiate the human analyses, we performed differential gene expression analysis on wild-type (WT) and inactivated Plcg2 mice and used immunostaining to determine if the differentially expressed genes/pathways were altered by microglial cell coverage or morphology. RESULTS: We observed significant upregulation of PLCG2 expression in three brain regions of LOAD patients and significant positive correlation of PLCG2 expression with amyloid plaque density. These findings in the human brain were validated in the 5xFAD amyloid mouse model, which showed disease progression-dependent increases in Plcg2 expression associated with amyloid pathology. Of note, increased Plcg2 expression levels in 5xFAD mice were abolished by reducing microglia. Furthermore, using bulk RNA-Seq data, we performed differential expression analysis by comparing cognitively normal older adults (CN) with 75th percentile (high) and 25th percentile (low) PLCG2 gene expression levels to identify pathways related to inflammation and the inflammatory response. The findings in the human brain were validated by differential expression analyses between WT and plcg2 inactivated mice. PLCG2 co-expression network analysis of microglial single-cell RNA-Seq data identified pathways related to the inflammatory response including regulation of I-kappaB/NF-kappa B signaling and response to lipopolysaccharide. CONCLUSIONS: Our results provide further evidence that PLCG2 plays an important role in AD pathophysiology and may be a potential target for microglia-targeted AD therapies.

The P522R protective variant of PLCG2 promotes the expression of antigen presentation genes by human microglia in an Alzheimer's disease mouse model.

The P522R variant of PLCG2, expressed by microglia, is associated with reduced risk of Alzheimer's disease (AD). Yet, the impact of this protective mutation on microglial responses to AD pathology remains unknown. Chimeric AD and wild-type mice were generated by transplanting PLCG2-P522R or isogenic wild-type human induced pluripotent stem cell microglia. At 7 months of age, single-cell and bulk RNA sequencing, and histological analyses were performed. The PLCG2-P522R variant induced a significant increase in microglial human leukocyte antigen (HLA) expression and the induction of antigen presentation, chemokine signaling, and T cell proliferation pathways. Examination of immune-intact AD mice further demonstrated that the PLCG2-P522R variant promotes the recruitment of CD8+ T cells to the brain. These data provide the first evidence that the PLCG2-P522R variant increases the capacity of microglia to recruit T cells and present antigens, promoting a microglial transcriptional state that has recently been shown to be reduced in AD patient brains.

TREM2 splicing emerges as crucial aspect to understand TREM2 biology.

Discussion on TREM2 splicing and its different transcripts with regard to the physiological function of TREM2 and its role in disease.

INPP5D expression is associated with risk for Alzheimer's disease and induced by plaque-associated microglia.

Alzheimer'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), also known as SHIP1, is selectively expressed in brain microglia and has been reported to be associated with LOAD. Although INPP5D is likely a crucial player in AD pathophysiology, its role in disease onset and progression remains unclear. We performed differential gene expression analysis to investigate INPP5D expression in AD and its association with plaque density and microglial markers using transcriptomic (RNA-Seq) 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 in the 5xFAD amyloid mouse model. Differential gene expression analysis found that INPP5D expression was upregulated in LOAD and positively correlated with amyloid plaque density. In addition, in 5xFAD mice, Inpp5d expression increased as the disease progressed, and selectively in plaque-associated microglia. Increased Inpp5d expression levels in 5xFAD mice were abolished entirely by depleting microglia with the colony-stimulating factor receptor-1 antagonist PLX5622. Our findings show that INPP5D expression increases as AD progresses, predominantly in plaque-associated microglia. Importantly, we provide the first evidence that increased INPP5D expression might be a risk factor in AD, highlighting INPP5D as a potential therapeutic target. Moreover, we have shown that the 5xFAD mouse model is appropriate for studying INPP5D in AD.

CNS-Native Myeloid Cells Drive Immune Suppression in the Brain Metastatic Niche through Cxcl10.

Brain metastasis (br-met) develops in an immunologically unique br-met niche. Central nervous system-native myeloid cells (CNS-myeloids) and bone-marrow-derived myeloid cells (BMDMs) cooperatively regulate brain immunity. The phenotypic heterogeneity and specific roles of these myeloid subsets in shaping the br-met niche to regulate br-met outgrowth have not been fully revealed. Applying multimodal single-cell analyses, we elucidated a heterogeneous but spatially defined CNS-myeloid response during br-met outgrowth. We found Ccr2+ BMDMs minimally influenced br-met while CNS-myeloid promoted br-met outgrowth. Additionally, br-met-associated CNS-myeloid exhibited downregulation of Cx3cr1. Cx3cr1 knockout in CNS-myeloid increased br-met incidence, leading to an enriched interferon response signature and Cxcl10 upregulation. Significantly, neutralization of Cxcl10 reduced br-met, while rCxcl10 increased br-met and recruited VISTAHi PD-L1+ CNS-myeloid to br-met lesions. Inhibiting VISTA- and PD-L1-signaling relieved immune suppression and reduced br-met burden. Our results demonstrate that loss of Cx3cr1 in CNS-myeloid triggers a Cxcl10-mediated vicious cycle, cultivating a br-met-promoting, immune-suppressive niche.

Trem2 Y38C mutation and loss of Trem2 impairs neuronal synapses in adult mice.

BACKGROUND: Triggering receptor expressed on myeloid cells 2 (TREM2) is expressed in the brain exclusively on microglia and genetic variants are linked to neurodegenerative diseases including Alzheimer's disease (AD), frontotemporal dementia (FTD) and Nasu Hakola Disease (NHD). The Trem2 variant R47H, confers substantially elevated risk of developing late onset Alzheimer's disease, while NHD-linked Trem2 variants like Y38C, are associated with development of early onset dementia with white matter pathology. However, it is not known how these Trem2 species, predisposes individuals to presenile dementia. METHODS: To investigate if Trem2 Y38C or loss of Trem2 alters neuronal function we generated a novel mouse model to introduce the NHD Trem2 Y38C variant in murine Trem2 using CRISPR/Cas9 technology. Trem2Y38C/Y38C and Trem2-/- mice were assessed for Trem2 expression, differentially expressed genes, synaptic protein levels and synaptic plasticity using biochemical, electrophysiological and transcriptomic approaches. RESULTS: While mice harboring the Trem2 Y38C exhibited normal expression levels of TREM2, the pathological outcomes phenocopied Trem2-/- mice at 6 months. Transcriptomic analysis revealed altered expression of neuronal and oligodendrocytes/myelin genes. We observed regional decreases in synaptic protein levels, with the most affected synapses in the hippocampus. These alterations were associated with reduced synaptic plasticity. CONCLUSION: Our findings provide in vivo evidence that Trem2 Y38C disrupts normal TREM2 functions. Trem2Y38C/Y38C and Trem2-/- mice demonstrated altered gene expression, changes in microglia morphology, loss of synaptic proteins and reduced hippocampal synaptic plasticity at 6 months in absence of any pathological triggers like amyloid. This suggests TREM2 impacts neuronal functions providing molecular insights on the predisposition of individuals with TREM2 variants resulting in presenile dementia.