Microglial Response Research Articles

CircTrim37 Ameliorates Intracerebral Hemorrhage Outcomes by Modulating Microglial Polarization via the miR-30c-5p/SOCS3 Axis.

Circular RNAs (circRNAs) have been progressively recognized as critical regulators in the pathology and pathophysiology of central nervous system disease. However, the potential role of circRNAs in intracerebral hemorrhage (ICH) is still largely unclear. Here, we demonstrate that circTrim37 expression was significantly upregulated at 3days after ICH by circular RNA microarray and qPCR assays. Overexpression of circTrim37 could significantly ameliorate brain injury volume, brain edema, neurologic deficits, and inflammation in vivo after ICH. CircTrim37 promotes M2 polarization while restrains M1 polarization in vitro. Furthermore, circTrim37 acts as an endogenous sponge for miR-30c-5p, thereby inhibiting miR-30c-5p activity, leading to the upregulation of SOCS3 and making the balance of microglial response towards an M2 phenotype. Taken together, our results indicate the participation of circTrim37 and its coupling mechanism in ICH and provide a novel therapeutic target for ICH.

Maternal suboptimal selenium intake and low-level lead exposure affect offspring’s microglial immune profile and its reactivity to a subsequent inflammatory hit

Micronutrients such as selenium (Se) are essentials since prenatal life to support brain and cognitive development. Se deficiency, which affects up to 1 billion people worldwide, can interact with common adverse environmental challenges including (Pb), exacerbating their toxic effects. Exploiting our recently validated rat model of maternal Se restriction and developmental low Pb exposure, our aims were to investigate: (i) the early consequences of suboptimal Se intake and low-Pb exposure on neuroinflammation in neonates’ whole brains; (ii) the potential priming effect of suboptimal Se and low-Pb exposure on offspring’s glial reactivity to a further inflammatory hit. To these aims female rats were fed with suboptimal (0.04 mg/kg; Subopt) and optimal (0.15 mg/kg; Opt) Se dietary levels throughout pregnancy and lactation and exposed or not to environmentally relevant Pb dose in drinking water (12.5 µg/mL) since 4 weeks pre-mating. We found an overall higher basal expression of inflammatory markers in neonatal brains, as well as in purified microglia and organotypic hippocampal slice cultures, from the Subopt Se offspring. Subopt/Pb cultures were highly activated than Subopt cultures and showed a higher susceptibility to the inflammatory challenge lipopolysaccharide than cultures from the Opt groups. We demonstrate that even a mild Se deficiency and low-Pb exposure during brain development can influence the neuroinflammatory tone of microglia, exacerbate the toxic effects of Pb and prime microglial reactivity to subsequent inflammatory stimuli. These neuroinflammatory changes may be responsible, at least in part, for adverse neurodevelopmental outcomes.

Microglia Polarization and Antiglioma Effects Fostered by Dual Cell Membrane-Coated Doxorubicin-Loaded Hexagonal Boron Nitride Nanoflakes.

Microglial cells play a critical role in glioblastoma multiforme (GBM) progression, which is considered a highly malignant brain cancer. The activation of microglia can either promote or inhibit GBM growth depending on the stage of the tumor development and on the microenvironment conditions. The current treatments for GBM have limited efficacy; therefore, there is an urgent need to develop novel and efficient strategies for drug delivery and targeting: in this context, a promising strategy consists of using nanoplatforms. This study investigates the microglial response and the therapeutic efficacy of dual-cell membrane-coated and doxorubicin-loaded hexagonal boron nitride nanoflakes tested on human microglia and GBM cells. Obtained results show promising therapeutic effects on glioma cells and an M2 microglia polarization, which refers to a specific phenotype or activation state that is associated with anti-inflammatory and tissue repair functions, highlighted through proteomic analysis.

Cx3cr1 deficiency interferes with learning- and direct current stimulation-mediated neuroplasticity of the motor cortex.

Microglia are essential contributors to synaptic transmission and stability and communicate with neurons via the fractalkine pathway. Transcranial direct current stimulation [(t)DCS], a form of non-invasive electrical brain stimulation, modulates cortical excitability and promotes neuroplasticity, which has been extensively demonstrated in the motor cortex and for motor learning. The role of microglia and their fractalkine receptor CX3CR1 in motor cortical neuroplasticity mediated by DCS or motor learning requires further elucidation. We demonstrate the effects of pharmacological microglial depletion and genetic Cx3cr1 deficiency on the induction of DCS-induced long-term potentiation (DCS-LTP) ex vivo. The relevance of microglia-neuron communication for DCS response and structural neuroplasticity underlying motor learning are assessed via 2-photon in vivo imaging. The behavioural consequences of impaired CX3CR1 signalling are investigated for both gross and fine motor learning. We show that DCS-mediated neuroplasticity in the motor cortex depends on the presence of microglia and is driven in part by CX3CR1 signalling ex vivo and provide the first evidence of microglia interacting with neurons during DCS in vivo. Furthermore, CX3CR1 signalling is required for motor learning and underlying structural neuroplasticity in concert with microglia interaction. Although we have recently demonstrated the microglial response to DCS in vivo, we now provide a link between microglial integrity and neuronal activity for the expression of DCS-dependent neuroplasticity. In addition, we extend the knowledge on the relevance of CX3CR1 signalling for motor learning and structural neuroplasticity. The underlying molecular mechanisms and the potential impact of DCS in rescuing CX3CR1 deficits remain to be addressed in the future.

Aging drives chronic exacerbated interactions of disease associated microglia and CD8+ T cells following traumatic brain injury

AbstractBackgroundAged individuals are highly susceptible to adverse outcomes following traumatic brain injury (TBI). Despite worsened correlates related to functional outcomes following injury, as well as increased likelihood to acquire a TBI, our understanding of the cellular mechanisms governing the poorer outcomes associated with aged individuals post‐TBI are not well‐characterized.MethodIn the current study, we examined microglial heterogeneity following TBI, in our mouse model of focal TBI across both acute (3d) and chronic (28d) intervals in both young (4m) and aged (18m) male C57B6J mice. At the appropriate interval, brains were collected and processed for single cell RNAseq (scRNAseq), spatial transcriptomics, and histology. A follow up study examining the interaction of TBI and aging upon T cell receptor (TCR) sequences was utilized to examine clonal expansion and lymphocyte heterogeneity in young and aged mice chronically after TBI.ResultOur findings demonstrate that TBI drives diverse transcriptional heterogeneity of microglia in both the young and aged condition. Acutely at 3 days post injury an inflammatory response is seen both in the young and aged mice. However, chronically at 28 days post injury, the microglia from young animals more closely resemble their pre‐injury (i.e. Sham) phenotypes, in comparison to the aged brain where there is a persistence of Apoe‐linked ‘DAM‐like’ microglial response. Inferential analyses point toward these aging‐related chronic populations of microglia as having distinct transcription factor utilization and metabolic pathway enrichment, including Hif1α upregulation and interferon gamma (INFγ) signaling. Furthermore, linkage with chronically accumulated CD8+ T cells and interferon signaling may underlie these phenotypes. Using single cell TCR sequencing, we demonstrate stark contrasts in the responses due to aging and TBI in the clonal expansion of infiltrated CD8+ T cells chronically following TBI.ConclusionOur studies demonstrate that aging is a centrally associated with the persistence of chronically reactive microglia in the brain and that these responses are linked, in part, with the protracted accumulation of CD8+ T cells following TBI.

Omics analysis of Alzheimer’s disease stratified by the microglial polygenic effect

AbstractBackgroundRecent single‐cell omics analyses have revealed that microglia change into reactive microglia when Aβ accumulates in the brain and exhibit Aβ phagocytosis. However, reactive microglia are less likely to be induced in TREM2 mutation carriers. This microglia‐centred pathological mechanism may be considered one of the pathologies of Alzheimer’s disease (AD). However, a single gene mutation, such as a TREM2 mutation, can rarely explain the pathology of AD. Therefore, polygenic effects involving multiple gene mutations have been proposed. In this study, we propose a polygenic risk score (PRS) to quantify the microglial polygenic effect. We stratified patient groups by this PRS and examined differences in pathogenesis based on omics analysis using postmortem brains.MethodWhole‐genome sequencing and bulk brain tissue RNA‐seq of the frontal cortex were performed using postmortem brain samples from 100 subjects with Braak senile plaque (SP) and neurofibrillary tangle (NFT) staging information. In addition, single‐nuclei RNA‐seq (snRNA‐seq) was performed on 15 of these subjects (healthy subjects: 8 subjects, AD patients: 7 subjects).ResultWe divided AD patients into low and high groups by microglial PRS and performed bulk brain tissue RNA‐seq analysis. We identified 112 differentially expressed genes and found that those genes were related to autophagy and inflammation. To further validate these findings at the single‐cell level, we identified cell clusters containing microglia based on snRNA‐seq. Gene expression analysis of microglia in 3 AD patients with low PRS and 4 AD patients with high PRS suggested altered transcriptional profiles of immune cells and phagocytosis. The expression of APOE and TYROBP, signature genes of reactive microglia, was significantly upregulated in the AD patients with low PRS compared to the AD patients with high PRS and healthy subjects. In contrast, CX3CR1 and P2RY12, signature genes of homeostatic microglia, were downregulated.ConclusionThe stratification analysis of AD by PRS suggests that microglial reactivity differs among AD patient groups. Future detailed microglial observations in postmortem brains from AD patients with low PRS and those with high PRS will further elaborate our results.

A new strategy for Alzheimer’s disease: low‐dose combination of radiotherapy and pharmacological epigenetic perturbation

AbstractBackgroundMost Alzheimer’s disease (AD) drug discovery studies focus exclusively on one specific target. This approach has to date shown limited success in AD, mainly because this complex disease is multifactorial and involves numerous pathways. In light of the large number of disappointing clinical trials, the National Institute on Aging (NIA) and the Alzheimer’s Association (AA) published, in 2018, new guidelines recommending focusing on biomarkers of the AD continuum. These guidelines advise targeting Amyloid‐β (A), neurofibrillary tangles (T) and neurodegeneration markers (N), designated “ATN”, to advance AD research. Using 2 different approaches, we have been able to both reverse and prevent ATN burden in mouse models of AD. Indeed, our group has previously demonstrated that histone deacetylase inhibition (HDACi) and cranial radiotherapy (RT) independently improve (AD)‐like pathogenesis in cell and animal models. In the present study, we investigate the synergistic potential of using both RT and HDACi as a low‐dose combination therapy (LDCT) to further improve molecular and behavioral AD‐like phenotypes in AD models.MethodLDCT consisted of daily administration of the HDAC3 inhibitor RGFP‐966 and/or bi‐weekly cranial x‐irradiation. Behavioral analyses, real‐time qPCR, NanoString technology, ELISAs and Western blots were used to assess the effects of RT, RGFP‐966 and LDCT on AD‐like pathogenesis in AD models. Additionally, innate immune response to LDCT was also assessed in vitro and in vivo using human and murine cell models and 3xTg‐AD mice.ResultLDCT induced a tolerant, anti‐inflammatory innate immune response in microglia and increased non‐amyloidogenic APP processing. Both RT and LDCT improved spatial learning and memory in the Barnes maze test. No sensorimotor deficits were observed. LDCT induced a unique anti‐AD gene expression profile in the brain, that included upregulation of neurotrophic genes and downregulation of inflammation‐related genes. Brain region specific observations included lowered Aβ42/40 ratio, decreased Bace1, and reduced p‐tau181 levels.ConclusionThis work shows that it is possible to attenuate AD‐related pathologies through the synergy of low doses of RT and HDACi treatment. This project supports a paradigm shift to a multipronged approach to AD therapeutics.

Single‐nuclei RNA sequencing provides evidence for glial senescence in Alzheimer’s disease

AbstractBackgroundAgeing is the greatest risk factor for Alzheimer’s disease (AD). While accumulation of senescent cells is one of the major hallmarks of ageing, the effect of the senescence burden is yet to be explored in AD. The aims of this study were to i) quantify senescent microglia, oligodendrocyte and astrocyte in human brain in AD, ii) determine the driver of cell senescence in AD, and iii) AD pathology associated senescence.MethodWe multiplexed Image Mass Cytometry (IMC) to co‐localise cell type markers (IBA1, OLIG2, GFAP), senescence markers (GLB1 and p16) and β‐amyloid (4G8) to detect senescence in tissues from Middle Temporal Gyrus (MTG) of 10 AD and 10 non‐disease control (NDC). We also generated snRNA‐seq from three different brain regions of the same subject namely Entorhinal (EC), MTG (Mid‐temporal Cortex) and Somatosensory Cortex (SSC) of 9 AD and 9 NDC. We performed geneset enrichment analysis using a set of curated senescence associated genesets in snRNA‐seq using AUcell and dream (variancePartition). We further performed trajectory analysis using Monocle3 to characterise β‐amyloid associated senescence signatures in the snRNAseq data.ResultMore than 25% microglia, oligodendrocyte and astrocyte were positive for GLB1 in AD brains while only around 5% glia showed GLB1 expression in NDC. Around 25% peri‐plaque microglia within 10 µm around β‐amyloid plaques in AD showed expression of GLB1 while only 3% of microglia >10 µm had associated senescence markers. Senescence genes were significantly upregulated in AD compared to NDC and were positively correlated with increase amyloid densities in all three glial cells (not in neurons). Trajectories of the glial nuclei described increased expression of pathways related to replicative senescence, cellular activation and stress response in microglia and oxidative stress in oligodendroglia, and astrocyte. Senescence gene signatures were positively associated with both β‐amyloid and pTau.ConclusionOur results highlight the high burden of senescent glia and provide evidence for distinct mechanisms of senescence in microglia (both replicative and stress‐induced) relative to oligodendroglia and astrocyte (stress‐induced) in AD. Both β‐amyloid and pTau associated increase in senescence gene signatures suggest that senescence is an important feature in the late stage of AD.

Roles of HDAC6 in the pathogenesis of Alzheimer’s disease

AbstractBackgroundIt has recently been demonstrated that HDAC6 depletion ameliorates the impairment of memory function and mitochondrial axonal transport induced by the accumulation of Aβ. However, very little is known about molecular mechanisms through which HDAC6 regulates APP processing. Also, it remains controversial whether HDAC6 induces inflammation or reduces it.MethodWe analyzed the expression of HDAC6 in AD patients and 5xFAD mice. We confirmed that HDAC6 regulates BACE1 in primary neurons and NLRP3 inflammasome in primary microglia. And we crossed HDAC6 knock‐out mice with 5xFAD mice and got 5xFAD/HDAC6 KO. We compared 5xFAD (n = 11) and 5xFAD/HDAC6 KO (n = 13). We did a nest building test, open field test, Morris water maze, Y‐maze, passive avoidance, and elevated plus maze. After finishing all these behaviour tests, we sacrificed the mouse and checked protein and mRNA. We did Immunohistochemistry using Aβ plaque, NLRP3 inflammasome and synapse‐related markers. We also did RNA‐seq analysis in these mice’s brains.ResultWe confirmed that the expression of HDAC6 was increased in the AD cortex compared to neuropathologically normal brains. The stability of BACE1 is regulated by its C‐terminal lysine, and HDAC6 deacetylates the C‐terminal lysine of BACE1 by direct binding. Also, HDAC6 deficiency reduced NLRP3 inflammasome activation‐, and subsequent inflammatory responses in microglia. By crossing HDAC6 knock‐out mice with 5xFAD mice, the role of HDAC6 was confirmed in an animal model of AD. HDAC6 deletion improved cognitive function and reduced the protein expression of BACE1 and inflammatory factors in 5xFAD mice. Also, HDAC6‐deficient 5xFAD mice developed fewer dense‐core plaques, IL‐1β, and ASC specks than 5xFAD mice. Furthermore, RNA‐sequencing was analyzed from the hippocampus of 5xFAD and 5xFAD/HDAC6 KO mice. Synapse and neuron development‐related genes were significantly up‐regulated, and type I interferon, immune response, and prion disease‐related genes were down‐regulated in HDAC6‐depleted mice. Finally, we confirmed that the ratio of PSD95 colocalized with C1q was significantly decreased in 5xFAD/HDAC6 KO compared to 5xFAD mice, which suggested that synapse loss was inhibited in AD mice.ConclusionThis study reveals distinct functions of HDAC6 and suggests HDAC6 as a significant regulator in the pathogenesis of AD that could be exploited therapeutically.

Xenografted human microglia in response to Alzheimer’s disease Aβ pathology

AbstractBackgroundNeuroinflammation is a pathological hallmark of Alzheimer’s disease (AD). Murine studies have revealed that mouse microglia can adopt several cell states in response to disease insults. However, single‐nuclei RNA sequencing studies have found divergent results when recalling those cell states in human post‐mortem tissue.MethodTo overcome this limitation, we have sequenced 127,000 single cell human microglia isolated from an AD xenotransplantation model.ResultIn response to plaques, human microglia can adopt several cell states including a disease‐associated microglia (DAM) and an antigen‐presenting response (HLA). When reacting to Aβ soluble aggregates, human microglia produces a pro‐inflammatory response cytokine response (CRM). TREM2 and APOE polymorphisms affect the development of the DAM‐HLA response, suggesting an important role of this response in AD.ConclusionOverall, this multi‐faceted response to amyloid pathologies highlights a complex scenario where these distinct phenotypes might influence the outcome on the disease.

Sulfanegen, a 3‐mercaptopyruvate sulfurtransferase substrate, as a therapeutic option for Alzheimer’s disease

AbstractBackgroundThe oxidative stress evident in the Alzheimer’s disease (AD) brain leads to compromised 3‐mercaptopyruvate sulfurtransferase (3MST) function, one of the endogenous hydrogen sulfide (H2S) producing enzymes.1,2 Impaired 3MST function is expected to contribute toward potentiation of oxidative stress by depleting cellular glutathione (GSH) stores, and exacerbation of AD pathology by enhancing Aβ aggregation resulting in neurodegeneration. Preclinical studies with H2S donors such as NaSH have been promising,3,4 however further clinical translation efforts are thwarted due to their poor chemical stability, bioavailability, and dose‐limiting toxicities. We have developed a stable, bioavailable prodrug of 3MST substrate 3‐mercaptopyruvate, named sulfanegen, which can supplement the enzymatic reaction releasing H2S.2 Thus provision of sulfanegen could prove beneficial in mitigating AD‐related neurodegenerative consequences.MethodAged, symptomatic cohorts APP/PS1 mice (7 and 12 month old) were treated with saline (N = 8) or sulfanegen (75 mg/kg, i.p., N = 8) for 12 weeks. Cognitive behavioral assessments (T‐maze) were conducted 10‐11 weeks after treatment initiation. Equal numbers of both sexes of mice were used as much as possible. The harvested brain tissues were analyzed biochemically for cortical/hippocampal Aβ1‐42 burden, oxidative stress and inflammatory markers (protein carbonyls, lipid peroxidation, glutathione assay, TNF‐α and IL6. Quantitative stereological analysis of Aβ pathology, neuroinflammation and progressive neurodegeneration were also conducted.ResultTreatment of symptomatic Alzheimer’s mice with sulfanegen prevented cognitive impairment resulting from AD pathology (Figure 1).2 This was corroborated by reduced brain amyloid load as well as the reversal of oxidative stress and neuroinflammation. Neuropathological analysis also displayed modest, but significant effect of sulfanegen on Aβ levels, reactive astrogliosis and microglial reactivity. More importantly, sulfanegen caused attenuation of progressive degeneration of noradrenergic system in the LC. Additionally, administration of sulfanegen supplemented brain 3MP levels, resulting in restoration of brain 3MST function.ConclusionCollectively, our results show that the provision of a 3MST substrate, sulfanegen, exhibits neuroprotective effects, along with restoration of cognitive function in an AD mouse model. This study suggests an important role of 3MST in AD pathophysiology and potential therapeutic utility of sulfanegen as a disease‐modifying treatment.

Distinct immune signatures associated with sex and genotype in the 12m novel hAPP hAPOE mouse risk model of Alzheimer’s disease

AbstractBackgroundUnderstanding the biology of model systems is critically important in the identification of biomarkers to delay or prevent Alzheimer’s disease. Previous findings in our group demonstrate sex differences in immune transcripts in the AD brain and in the humanized apolipoprotein E (APOE) mouse risk model. With this study, we extend these discoveries to further investigate the effect of APOE genotype using a novel risk mouse model with humanized amyloid precursor protein (APP) and humanized‐APOE transgenes (Jackson Laboratory). We test the hypothesis that during midlife aging, hAPP hAPOE ε4/4 genotype, and female sex will show increased risk profiles for Aβ 42:40 ratio and immunophenotypes associated with Alzheimer’s disease.MethodTo identify midlife (12 months; human correlate 44 years) immunophenotypes that may contribute to AD 3 different risk genotypes were studied: hAPP (6 male /6 female), hAPP hAPOE ε3/3 (16 male/16 female), and hAPP hAPOE ε4/4 (16 male /16 female). The left hemisphere of the brain was collected and prepared for multi‐color flow cytometry. Microglial reactivity (MHCII, CD68), phagocytosis (pHrodo), oxidative stress (CellRox), and the presence of lymphocytes (CD3, CD4, CD8, CD19) were determined. Peripheral blood was collected and assessed for monocytes and lymphocytes while plasma was isolated and ELISA was used to detect the presence of Aβ 40‐42, and levels of pro‐inflammatory cytokines. Female animals were monitored daily by lavage to determine midlife endocrine status.ResulthAPP hAPOE ε4/4 animals demonstrated accelerated endocrine aging compared to hAPP hAPOE ε3/3 animals. Plasma beta‐amyloid 42:40 ratio, and levels of IL‐1B, and IL‐6 showed a significant interaction between sex and hAPP hAPOE genotype (p<0.05, p<0.05, p<0.05). Plasma levels of IL‐5 and CXCL1 trended toward genotype significance with p = 0.0544 and p = 0.0556, respectively. Percentage of MHCII+ activated microglia was significantly impacted by sex (p = 0.0005) and by genotype (p<0.0001).ConclusionThe interaction between hAPP and hAPOE genotype, in coordination with biological sex significantly impacts immunophenotypes of the CNS and periphery during midlife aging. Precision interventions targeting sex and genotype‐dependent prodromal immune biomarkers may serve as an opportunity to support homeostatic immune milieu or to modulate harmful inflammatory signaling and reduce risk of AD or delay AD onset.

Association of reactive astrogliosis and microglial activation with tau pathology in Alzheimer’s disease

AbstractBackgroundGlial contributions to Alzheimer’s disease (AD) etiology and progression have been increasingly recognized. Experimental and clinical evidence suggests that both microglial activation and astrocyte reactivity have pivotal roles in the progression of tau pathology. However, it remains to be elucidated how microglia and astrocytes complement each other in AD pathogenesis. Here, we tested the associations between reactive astrogliosis and microglial activation with tau phosphorylation and aggregation in individuals across the aging and AD spectrum.MethodWe studied 95 participants (14 cognitively unimpaired [CU] young, 46 CU elderly, and 35 cognitively impaired [CI]) from the Translational Biomarkers in Aging and Dementia (TRIAD) cohort. Individuals had available positron emission tomography (PET) for Aβ ([18F]AZD4694), tau tangles ([18F]MK6240), and microglial activation ([11C]PBR28). We further assessed reactive astrogliosis (plasma glial fibrillary acidic protein [GFAP]) and pathological tau phosphorylation (plasma phosphorylated tau [p‐tau] at threonine 231, 181, and 217).ResultDemographic characteristics of the study population are reported in Table 1. Regression analyses revealed that reactive astrogliosis and microglial activation were synergistically associated with higher plasma phosphorylated tau levels independently of Aβ pathology in CU but not CI individuals (Figure 1A). On the other hand, no interactive effects of reactive astrogliosis and microglial activation on tau‐PET burden were observed in either CU or CI individuals (Figure 1B). Similar findings were observed in sensitivity analyses excluding CU young participants.ConclusionOur results suggest that reactive astrocytes and activated microglia have Aβ‐independent synergistic effects on the early progression of AD pathophysiology by contributing to tau phosphorylation but not tau aggregation. These findings can help to better understand the complementary roles of glial cells in neurodegenerative diseases, as well as provide insights for the development novel therapeutic strategies for AD targeting the interplay between astrocyte and microglial reactivity.

Association of reactive astrogliosis and microglial activation with tau pathology in Alzheimer’s disease

AbstractBackgroundGlial contributions to Alzheimer’s disease (AD) etiology and progression have been increasingly recognized. Experimental and clinical evidence suggests that both microglial activation and astrocyte reactivity have pivotal roles in the progression of tau pathology. However, it remains to be elucidated how microglia and astrocytes complement each other in AD pathogenesis. Here, we tested the associations between reactive astrogliosis and microglial activation with tau phosphorylation and aggregation in individuals across the aging and AD spectrum.MethodWe studied 95 participants (14 cognitively unimpaired [CU] young, 46 CU elderly, and 35 cognitively impaired [CI]) from the Translational Biomarkers in Aging and Dementia (TRIAD) cohort. Individuals had available positron emission tomography (PET) for Aß ([18F]AZD4694), tau tangles ([18F]MK6240), and microglial activation ([11C]PBR28). We further assessed reactive astrogliosis (plasma glial fibrillary acidic protein [GFAP]) and pathological tau phosphorylation (plasma phosphorylated tau [p‐tau] at threonine 231, 181, and 217).ResultDemographic characteristics of the study population are reported in Table 1. Regression analyses revealed that reactive astrogliosis and microglial activation were synergistically associated with higher plasma phosphorylated tau levels independently of Aß pathology in CU but not CI individuals (Figure 1A). On the other hand, no interactive effects of reactive astrogliosis and microglial activation on tau‐PET burden were observed in either CU or CI individuals (Figure 1B). Similar findings were observed in sensitivity analyses excluding CU young participants.ConclusionOur results suggest that reactive astrocytes and activated microglia have Aß‐independent synergistic effects on the early progression of AD pathophysiology by contributing to tau phosphorylation but not tau aggregation. These findings can help to better understand the complementary roles of glial cells in neurodegenerative diseases, as well as provide insights for the development novel therapeutic strategies for AD targeting the interplay between astrocyte and microglial reactivity.

Astrocyte reactivity is associated with synaptic dysfunction across the aging and Alzheimer’s disease spectrum, whereas microglial reactivity is specifically associated with synaptic dysfunction related to cognitive impairment

AbstractBackgroundHealthy synapses are the key to proper brain function, ensuring communication between neurons. Recent studies have associated synaptic dysfunction with Alzheimer’s disease (AD) proteins, however, little is known about the role of glial reactivity, another pathology closely linked to AD, in brain synaptic dysfunction (Figure 1).MethodWe evaluated 123 individuals (67 cognitively unimpaired (CU) and 56 cognitively impaired (CI)) who had available Aβ‐ and Tau‐PET as well as cerebrospinal fluid measures of glial fibrillary acidic protein (GFAP), chitinase‐3‐like protein 1 (YKL‐40), soluble triggering receptor expressed on myeloid cells 2 (sTREM2), synaptic markers (growth‐associated protein 43 (GAP‐43), neurogranin (Ng), synaptotagmin 1 (SYT1), and presynaptic protein synaptosomal‐associated protein 25 (SNAP‐25)). ANCOVA adjusted for clinical diagnosis, age, and sex was used to compare levels of CSF biomarkers; whereas linear regressions adjusted for age, sex, clinical diagnosis, and Aβ/tau‐PET were used to test the associations between glial reactivity and synaptic markers.ResultDemographic information is shown in Table 1. Increased levels of GAP‐43, SNAP‐25, and Ng were observed in CI compared to CU individuals. CSF GFAP was highly associated with both presynaptic and postsynaptic biomarkers in CU and CI groups. CSF YKL‐40 was associated only with presynaptic biomarkers in both clinical groups. On the other hand, CSF sTREM2 showed an association with all synaptic markers but only in the CI group (Figure 2).ConclusionWe found a heterogeneous association between synaptic markers and glial activation. The presence of GFAP+ astrocytes was associated with dysfunction of presynaptic/postsynaptic markers, whereas YKL‐40+ astrocytes specifically reflected presynaptic dysfunction across aging and AD spectrums. On the other hand, microglial activation reflected synaptic dysfunction associated with dementia symptoms. Our results support recent experimental observations suggesting that clarifying the heterogeneity of different glial cell phenotypes is crucial to advancing our understanding of the role of immune cells in cognitive decline.

Astrocyte reactivity is associated with synaptic dysfunction across the aging and Alzheimer’s disease spectrum, whereas microglial reactivity is specifically associated with synaptic dysfunction related to cognitive impairment

AbstractBackgroundHealthy synapses are the key to proper brain function, ensuring communication between neurons. Recent studies have associated synaptic dysfunction with Alzheimer’s disease (AD) proteins, however, little is known about the role of glial reactivity, another pathology closely linked to AD, in brain synaptic dysfunction (Figure 1).MethodWe evaluated 123 individuals (67 cognitively unimpaired (CU) and 56 cognitively impaired (CI)) who had available Aß‐ and Tau‐PET as well as cerebrospinal fluid measures of glial fibrillary acidic protein (GFAP), chitinase‐3‐like protein 1 (YKL‐40), soluble triggering receptor expressed on myeloid cells 2 (sTREM2), synaptic markers (growth‐associated protein 43 (GAP‐43), neurogranin (Ng), synaptotagmin 1 (SYT1), and presynaptic protein synaptosomal‐associated protein 25 (SNAP‐25)). ANCOVA adjusted for clinical diagnosis, age, and sex was used to compare levels of CSF biomarkers; whereas linear regressions adjusted for age, sex, clinical diagnosis, and Aß/tau‐PET were used to test the associations between glial reactivity and synaptic markers.ResultDemographic information is shown in Table 1. Increased levels of GAP‐43, SNAP‐25, and Ng were observed in CI compared to CU individuals. CSF GFAP was highly associated with both presynaptic and postsynaptic biomarkers in CU and CI groups. CSF YKL‐40 was associated only with presynaptic biomarkers in both clinical groups. On the other hand, CSF sTREM2 showed an association with all synaptic markers but only in the CI group (Figure 2).ConclusionWe found a heterogeneous association between synaptic markers and glial activation. The presence of GFAP+ astrocytes was associated with dysfunction of presynaptic/postsynaptic markers, whereas YKL‐40+ astrocytes specifically reflected presynaptic dysfunction across aging and AD spectrums. On the other hand, microglial activation reflected synaptic dysfunction associated with dementia symptoms. Our results support recent experimental observations suggesting that clarifying the heterogeneity of different glial cell phenotypes is crucial to advancing our understanding of the role of immune cells in cognitive decline.

CSF sTREM2 and YKL‐40 modify the balance between soluble and fibrillary β‐amyloid in non‐demented individuals

AbstractBackgroundCerebrospinal fluid (CSF) Aβ concentrations and Positron Emission Tomography (PET) imaging provide information of two different Aβ pools. Identifying the factors affecting the balance between soluble and deposited Aβ may inform on the mechanism of cerebral Aβ aggregation. Microglia and astroglia respond to Aβ pathology, yet their role in promoting the aggregation of Aβ plaques is unclear. We addressed whether CSF sTREM2 and YKL‐40 modulate the association between soluble and deposited Aβ in a cohort of middle‐aged cognitively unimpaired (CU) individuals.MethodThe first consecutive 320 CU individuals of the ALFA+ study with a [18F]flutemetamol (FTM) PET scan and CSF samples were included. CSF Aβ40, Aβ42, sTREM2, and YKL‐40 were measured with the Roche NeuroToolKit robust prototype immunoassays (Roche Diagnostics International Ltd). Standardized uptake value ratio images were calculated and a voxel‐wise regression was performed to predict FTM uptake. The interaction terms between CSF Aβ42/40 and either CSF sTREM2 or YKL‐40 served as predictors, while adjusting for age, sex, years of education, and APOE‐ε4. Statistical threshold was set to p<0.001 with a cluster extent correction of 100 voxels.ResultTable 1 describes the sample’s characteristics. As expected, CSF Aβ42/40 concentrations were negatively associated with Aβ PET uptake in widespread areas of the brain, but this association was modified by CSF sTREM2 and YKL‐40. Higher levels of CSF sTREM2 were associated with a reduced Aβ PET uptake for any level of CSF Aβ42/40 in the hippocampus bilaterally. Conversely, higher CSF YKL‐40 related to a higher Aβ PET uptake as a function of CSF Aβ42/40 in the precuneus, prefrontal cortex, and inferior parietal areas.ConclusionIn CU individuals, microglia and astroglia reactivity modify the association between soluble (CSF) and deposited (amyloid PET) Aβ. While TREM2‐mediated microglial response (as measured by CSF sTREM2) may prevent Aβ deposition in individuals with decreased soluble Aβ, astroglial reactivity (as measured by CSF YKL‐40) may have the opposite effect.

Human interlaminar astrocytes with different APOE variants differentially contribute to Alzheimer’s disease pathology

AbstractBackgroundIncreasing evidence for a direct contribution of astrocytes as well as different APOE variants to Alzheimer’s disease (AD) main pathological hallmarks comes from cellular and molecular studies in rodent models. Yet these models cannot fully mimic how human astrocytes affect AD pathology as human and rodent astrocytes differ considerably in morphology, functionality, and gene expression.MethodTo address these challenges, we established an approach to study human astrocytes carrying APOE3 and APOE4 isoforms within the AD brain environment by transplanting human induced pluripotent stem cell (hiPSC)‐derived astrocyte progenitors into neonatal brains of suitable AD model mice.ResultAPOE3 and APOE4 transplanted cells similarly differentiate into astrocytes within the mouse host brain and very often integrate in upper layers of one cortical hemisphere, where they acquire morphological phenotypes resembling human interlaminar astrocytes (h‐ILAs). Interestingly, in AD model mice APOE3 and APOE4 h‐ILAs differentially express and secrete the apoE protein (Figure 1), and differentially affect amyloid‐beta load (Figure 2), microglial responses (Figure 3) and neuritic dystrophy.ConclusionWe describe here a promising model that allows for the first time studying h‐ILAs in the AD brain environment. Importantly, our model reveals a differential contribution of h‐ILAs with different APOE variants to main pathological hallmarks of AD, which can potentially have important implications when developing novel astrocyte‐specific targeting therapeutics.

Neuroinflammation and cognitive decline: a TSPO‐PET imaging study.

AbstractBackgroundNeuroinflammation is a key driving factor of the pathophysiology of neurodegenerative diseases, including Alzheimer’s disease (AD). Microglia, a major immune cell of the central nervous system, may exert protective effects in the early stages of AD, while may damage the brain milieu in later stages of the disease continuum. The microglial response is mirrored by an increase in the detection of translocator protein (TSPO), a mitochondrial membrane protein that can be targeted with positron emission tomography (PET) imaging. Here, we aim to assess the impact of neuroinflammation on cognition across the AD continuum.Method48 participants (23 patients with AD, 14 patients with mild cognitive impairment (MCI), and 11 cognitively normal controls (CN)) underwent TSPO‐PET (flutriciclamide, [18F]GE180) imaging, magnetic resonance imaging (MRI), and neuropsychological testing. All participants were either high‐ or mixed‐affinity binders for TSPO‐PET tracers, as revealed by TSPO genotyping. The spectral analysis generated a flutriciclamide impulse response function at 90 minutes (IRF90) image, which reflects its volume of distribution. Region of interest and voxel‐wise analyses were conducted to explore the relationship between IRF90 and neuropsychological assessment performance on a group‐level.ResultTSPO‐PET binding was significantly higher in patients with AD compared to CNs across the whole brain, while patients with MCI demonstrated greater TSPO‐PET binding in the hippocampus and medial temporal lobe, compared to CNs. Significant negative associations were observed between neuropsychological assessment scores and IRF90, particularly in the temporal lobe (table 1). At a voxel level, negative correlations were seen between mini‐mental state examination score and IRF90 in temporoparietal regions (figure 1).ConclusionMicroglial response, a marker of neuroinflammation, is elevated in patients with AD and MCI, as measured using TSPO‐PET (flutriciclamide, [18F]GE180) imaging. Heightened neuroinflammation, measured by TSPO‐PET, may exert damaging effects on the brain, as highlighted here by the negative association between TSPO‐PET binding and neuropsychometric assessment performance. This study emphasises the requirement to improve our understanding of the role of neuroinflammation in AD in order to identify pharmacological targets in the future.

CSF sTREM2 and YKL‐40 modify the balance between soluble and fibrillary β‐amyloid in non‐demented individuals

AbstractBackgroundCerebrospinal fluid (CSF) Aβ concentrations and Positron Emission Tomography (PET) imaging provide information of two different Aβ pools. Identifying the factors affecting the balance between soluble and deposited Aβ may inform on the mechanism of cerebral Aβ aggregation. Microglia and astroglia respond to Aβ pathology, yet their role in promoting the aggregation of Aβ plaques is unclear. We addressed whether CSF sTREM2 and YKL‐40 modulate the association between soluble and deposited Aβ in a cohort of middle‐aged cognitively unimpaired (CU) individuals.MethodThe first consecutive 320 CU individuals of the ALFA+ study with a [18F]flutemetamol (FTM) PET scan and CSF samples were included. CSF Aβ40, Aβ42, sTREM2, and YKL‐40 were measured with the Roche NeuroToolKit robust prototype immunoassays (Roche Diagnostics International Ltd). Standardized uptake value ratio images were calculated and a voxel‐wise regression was performed to predict FTM uptake. The interaction terms between CSF Aβ42/40 and either CSF sTREM2 or YKL‐40 served as predictors, while adjusting for age, sex, years of education, and APOE‐ε4. Statistical threshold was set to p<0.001 with a cluster extent correction of 100 voxels.ResultTable 1 describes the sample’s characteristics. As expected, CSF Aβ42/40 concentrations were negatively associated with Aβ PET uptake in widespread areas of the brain, but this association was modified by CSF sTREM2 and YKL‐40. Higher levels of CSF sTREM2 were associated with a reduced Aβ PET uptake for any level of CSF Aβ42/40 in the hippocampus bilaterally. Conversely, higher CSF YKL‐40 related to a higher Aβ PET uptake as a function of CSF Aβ42/40 in the precuneus, prefrontal cortex, and inferior parietal areas.ConclusionIn CU individuals, microglia and astroglia reactivity modify the association between soluble (CSF) and deposited (amyloid PET) Aβ. While TREM2‐mediated microglial response (as measured by CSF sTREM2) may prevent Aβ deposition in individuals with decreased soluble Aβ, astroglial reactivity (as measured by CSF YKL‐40) may have the opposite effect.