Ikezu, Seiko

Abstract The apolipoprotein E ε4 ( APOE4 ) allele is the strongest genetic risk factor for late-onset Alzheimer’s disease (AD), yet the mechanistic underpinnings by which it alters human microglial function remain poorly understood. Here, we utilize CRISPR/Cas9-engineered isogenic human induced pluripotent stem cell-derived microglia-like cells (iMGs) expressing APOE3 or APOE4 to systematically interrogate genotype-dependent effects on microglial proteomes and their secreted extracellular vesicles (EVs) under basal and fibrillar amyloid-β (fAβ)-challenged conditions. Quantitative proteomics revealed that APOE4 iMGs exhibit downregulated RNA processing, impaired phagocytosis, and exacerbated pro-inflammatory signaling, particularly upon fAβ exposure. Functionally, APOE4 iMGs demonstrated increased NF-κB activation, enhanced neuronal apoptosis and tau pathology in 2D and 3D co-culture systems. Interestingly, APOE4 iMGs released greater amount of EVs enriched in proteasomal components over APOE3 iMG. Integrated proteomic analyses further highlighted APOE4 -specific selective packaging of EV cargo linked to neuron-glia communication, inflammatory signaling, and proteostasis dysregulation, suggesting a potent, non-cell-autonomous mechanism driving neuronal damage. Consistently, these APOE4 iMGEVs were preferentially internalized by neurons, leading to disrupted neuronal proteostasis, impaired neuronal firing, and increased tau phosphorylation, particularly under pathological stress. Notably, pharmacologic inhibition of NF-κB in APOE4 iMGs reduced EV cargo-loading of proteasomal molecules and mitigated iMGEV-driven neuronal dysfunction and tau pathology. These findings establish a mechanistic link between APOE4 , dysregulated microglial homeostasis, and microglial EV-mediated propagation of proteostatic dysfunction and neurodegeneration, advancing our understanding of how APOE4 contributes to AD pathogenesis.

Abstract In Alzheimer’s disease (AD), tau pathology arises in entorhinal cortex layer II (ECII) and advances through defined hippocampal circuits to CA1 and connected neocortical regions, yet the determinants of this hierarchical spread remain unclear. We previously established a circuit-defined propagation model by expressing Cre-inducible human P301L 2N4R tau selectively in Wolframin-1 (Wfs1) + ECII neurons using AAV-FLEX-Tau P301L in Wfs1-Cre mice. Here, to test how amyloid-β (Aβ) and human tau background shape propagation, we generated human MAPT knock-in Wfs1 mice and APP NL-G-F / MAPT double knock-in Wfs1 mice (T-Wfs1 and AT-Wfs1) and induced ECII-restricted Tau P301L expression. Three months after injection, phosphorylated or misfolded tau-positive neurons were enriched in proximal CA1 in Wfs1 and T-Wfs1 mice, resembling primary age-related tauopathy, whereas AT-Wfs1 mice showed preferential accumulation near the CA1/subiculum (Sub) boundary, consistent with an AD-like pattern. In T-Wfs1 and AT-Wfs1 mice, tau spread extended through Sub to neocortical regions, and phosphorylated tau accumulated predominantly in excitatory rather than inhibitory neurons. Electrophysiological analyses revealed increased spontaneous neuronal firing and impaired GABAergic transmission in the CA1/Sub boundary and neocortical areas in T-Wfs1 and AT-Wfs1 mice, indicative of impaired GABAergic input and enhanced neuronal excitability in these regions. Together, these data indicate that human MAPT and Aβ pathology shift the circuit topography of tau propagation and are associated with early network dysfunction, supporting a synergistic interaction that promotes AD-like spread and synaptic imbalance.

Abstract Tau pathology emerges early in Alzheimer’s disease within entorhinal cortex layer II (ECII) and reaches hippocampal CA1, but how this circuit-level spread translates into sex-dependent vulnerability remains unclear. Using a circuit-defined model in which P301L human tau is expressed selectively in Wolframin-1 (Wfs1 + ) ECII neurons and propagates to CA1, we found that the extent and proximal–distal distribution of tau-positive CA1 neurons were comparable in males and females. Despite similar propagation, females exhibited broad hippocampal-dependent cognitive impairment (working memory, object recognition, fear acquisition, trace associative memory, and contextual fear memory), whereas males showed a selective deficit in trace associative memory. Consistent with these behavioral outcomes, CA1 pyramidal neurons in tau-propagated females displayed reduced excitability (slower action potential kinetics, reduced firing during depolarizing steps) and reduced spontaneous excitatory postsynaptic current (EPSC) amplitude, while males showed subtler intrinsic changes with altered EPSC kinetics. Bulk RNA sequencing of entorhinal cortex and CA1 revealed robust immune pathway engagement after tau propagation, with males showing a stronger Th1/Th2 and neuroinflammatory signature and CTLA4-associated signaling changes, whereas females showed prominent complement–phagosome pathway enrichment and a female-specific increase in Clec7a + microglia density in CA1. CD4 + T-cell infiltration into CA1 was detected in both sexes. Together, these results indicate that sex-specific neuroimmune programs, rather than differences in tau propagation load, shape CA1 electrophysiological dysfunction and the breadth of memory impairment following early entorhinal-to-hippocampal tau spread.

Abstract Background Brain cells secrete extracellular vesicles (EVs) containing signaling and pathological proteins related to Alzheimer's disease (AD) and related disorders. Recent studies demonstrate brain‐derived EVs (BDEVs) contain misfolded tau and are transmissible of tau pathology in the brain. However, the underlying mechanism regarding EV‐mediated tau pathology is still largely uncharacterized. Method We conducted the immuno‐affinity purification of tau in human brain‐derived EVs (BDEVs) isolated from age and sex‐matched 14 AD and 14 CTRL cases and performed tandem mass‐tag mass spectrometry to profile unbiased tau interactome in BDEVs. We used Nanotemper Monolith to validate the interaction of EV‐tau interactors with tau, and super‐resolution microscopy (Nanoimager) to detect their colocalization in a single‐EV level. We next designed siRNAs to silence EV‐tau interactors in SH‐SY5Y cells overexpressing human P301L tau (SH‐SY5Y‐P301Ltau) and assessed tau loading to EVs by Flow Nanoanalyzer. These EVs were tested for their uptake and tau seeding by human iPSC‐derived neurons (iNeurons). We finally tested the inhibitors of identified molecules on BDEV‐mediated tau propagation in vivo . Result A total of 764 proteins were identified in BDEV‐associated tau interactome from CTRL and AD patients. Sixty‐five proteins were significantly downregulated in AD BDEVs; whereas 5 proteins were significantly upregulated in AD BDEVs compared to CTRL BDEVs. The most enriched tau‐interacting proteins were significantly positively correlated with Braak stage. We confirmed the direct binding of candidate proteins to tau with micromolar binding affinities by Monolith, and their colocalization with tau in human BDEVs using Nanoimager. Silencing of candidate molecules in human SH‐SY5Y‐P301Ltau cells showed reduced loading of tau in EVs, their reduced uptake by iNeurons, and tau seeding activities. Finally, intracerebroventricular injection of neutralizing antibody against the most promising target results in reduction of tau pathology in PS19 mice expressing P301S human tau mutant. Conclusion Our study identified tau‐interacting molecules highly enriched in AD BDEVs compared to CTRL BDVs. Silencing of candidate molecule reduced tau loading to the EVs and their uptake by iNeurons and neutralizing antibody suppressed tau dissemination, highlighting them as promising therapeutic targets to halt tau pathology in AD and related tauopathy.

Abstract Background Extracellular vesicles (EVs) are key mediators in transferring pathological proteins associated with Alzheimer's disease (AD). The apolipoprotein E (APOE) gene, particularly the ε4 allele, is a major genetic risk factor for late‐onset AD. However, the influence of APOE genotype on the biological characteristics and cargo composition of brain‐derived extracellular vesicles (BDEVs) in AD remains poorly understood. Method In this study, BDEVs were isolated from human AD temporal cortex with APOE3/3 and APOE4/4 genotypes ( N = 20 / group). Nanoflow cytometry and super‐resolution microscopy were used to evaluate the tau load within these BDEVs. EV‐mediated tau transmission was assessed in aged human Tau KI and APP NL‐G‐F : Tau KI mice in vivo . BDEV uptake and tau seeding were evaluated in iPSC‐derived neurons through live imaging. Additionally, shotgun lipidomics and data independent acquisition proteomics were analyzed lipidome and proteome of BDEVs, respectively. Key molecules linked to APOE genotype were identified through integrated weighted gene co‐expression network and trait correlation analysis. Result APOE4/4 BDEVs exhibited a higher proportion of phosphorylated tau (pS396) at single EV levels. Upon injection of 300pg tau of BDEVs into aged Tau KI and APP NL‐G‐F : Tau KI mice, APOE4/4 BDEVs significantly increased AT8 + phosphorylated tau levels, and induced neuroinflammation in vivo . Moreover, APOE4/4 BDEVs demonstrated stronger neuronal uptake and tau transfer compared to APOE3/3 BDEVs, impairing the neuronal activity of iPSC‐derived neurons. Lipidomic revealed that APOE4/4 BDEVs were significantly more enriched in pro‐inflammatory long‐chain polyunsaturated fatty acids (PUFA) compared to APOE3/3 with Braak stage association. Furthermore, gene ontology pathway analysis using differentially expressed proteins in APOE4/4 BDEVs showed increased activity in oxidative phosphorylation and sterol‐binding. Multi‐omics integrated data revealed that upregulation of PUFA was significantly associated with elevated cell adhesion molecules and ATPase transporter activity with neural cell adhesion molecule as one of key molecule. Notably, neutralizing antibody against the molecule effectively reduced APOE4/4 BDEV internalization and mitigated EV‐mediated tau pathology in recipient cells. Conclusion Our comprehensive analysis suggested that AD APOE4/4 BDEV may enhance transmissibility of tau pathology and neuroinflammation via enriched pro‐inflammatory PUFA and cell adhesion molecules. Targeting these pathways presents a promising therapeutic strategy to mitigate disease progression, particularly in APOE4 individuals.

Abstract Background We have recently reported that wolframin‐1‐expressing (Wfs1 + ) pyramidal neurons in the entorhinal cortex layer II (ECII) that projects to the CA1 propagate phosphorylated tau (pTau) via the temporoammonic pathway, mimicking early stages of tau pathology in AD (ECII‐CA1 tau mice). We examined if amyloid pathology and human tau expression with this mouse model may change the character and distribution of tau transfer. Furthermore, using a chemogenetic approach, we aimed to determine the effect of neuronal excitability on tau transfer to the hippocampal regions in mouse brains. Method Cre‐inducible AAV expression P301L tau mutant (AAV‐Flex‐P301Ltau) was injected into the EC II region of APP NL‐G‐F human MAPT double knockin (APP NL‐G‐F : TAUKI) mice crossed with Wfs1‐Cre and Wfs1‐Cre mice at 7 months of age. Tau propagation was evaluated in the hippocampal and cortex regions by immunofluorescence using HT7 (human tau) and AT8 (pS 202 /pT 205 tau) mAbs at one‐ and 3‐months post‐injection. AAV‐Flex‐P301Ltau was co‐injected with Cre‐inducible AAV expressing hM3D(Gq) or hM4D(Gi) DREADD in the ECII in Wfs1‐Cre mice at 4 months of age, followed by subcutaneous infusion of clozapine N ‐oxide or saline via osmotic pumps for 28 days. Immunofluorescence against HT7 mAb was performed at one‐month post‐injection. Result We observed robust HT7 positivity in the CA1 and subiculum in Wfs1‐Cre mice after AAV‐Flex‐P301Ltau injection, although there was little AT8 staining at both one‐ and 3‐month. Interestingly, APP NL‐G‐F :TauKI:Wfs1‐Cre mice showed strong HT7 and AT8 positivity in the subiculum, but not in CA1 at 1‐month. Notably, p ‐tau further advanced into the visual cortex (VC) region at 3‐month in APP NL‐G‐F :TauKI:Wfs1‐Cre mice, while neither HT7 nor AT8 was positive in the VC in Wfs1‐Cre mice. Increased or suppressed neuronal excitability in the medial ECII pyramidal neurons by DREADD approach significantly induced or reduced tau propagation to the CA1 compared to the control group at one‐month post‐injection, respectively. Conclusion Our study demonstrates that APP NL‐G‐F :TAUKI:Wfs1‐Cre mice injected with P301L tau in the ECII recapitulates advanced‐stage AD pathology with amyloid beta navigating tau to the neocortical region and enhancing the maturity of tau pathology. Furthermore, ECII neuronal excitability regulates tau transfer efficiency from the ECII to CA1 region.

Abstract Background We previously identified a novel mechanism describing how pathological tau transfers via extracellular vesicles (EVs) in Alzheimer's disease (AD). Targeting EV secretion to mitigate tau transfer is, therefore, a promising therapeutic approach for AD. P2X purinoreceptor 7 (P2RX7), an ATP‐gated cationic channel predominantly expressed in microglia, regulates the secretion and biogenesis of EVs. Here we investigated the effect of P2rx7 deficiency in PS19 tauopathy mouse model and by proteomic profiling of brain EVs. Method PS19: P2rx7 –/– mice at 9‐10 months of age were tested for fear conditioning and their brain sections were assessed for brain atrophy. Tau pathology was evaluated using ELISA and immunofluorescence against phosphorylated (AT8) and misfolded (Alz50) tau. Brain tissues were further assessed by bulk and single cell RNA‐seq (scRNA‐seq) and brain‐EVs were subjected to proteomic profiling by mass‐spectrometry. P2rx7 –/– mice were intracranially injected with viral vectors expressing P301L tau in neurons and mEmerald‐CD9 in microglia to visualize the secretion of microglial EVs in vivo . Mice conditional knockout (cKO) for P2rx7 in microglia, neurons and astrocytes were injected with a viral vector expressing P301L tau in entorhinal cortex for tau propagation assessment. Result PS19: P2rx7 –/– mice showed significant improvement in contextual and cued memory compared to age‐matched PS19 mice, which was accompanied by preserved cortical and hippocampal volume, and significant reduction of hippocampal synapse loss and tau pathology. By weighted gene co‐expression network analysis (WGCNA) we identified a microglial and EV‐related module strongly correlated with tau pathology which was remarkably downregulated in PS19: P2rx7 –/– mice hippocampus. PS19 mice showed increased secretion of EVs containing tau and mitochondrial proteins in the mouse brain parenchyma, which was significantly downregulated in PS19: P2rx7 –/– mice. scRNA‐seq analysis showed that P2rx7 is required for microglia conversion to inflammatory phenotype with increased expression of EV genes in PS19 mice. P2rx7 deficiency reduced microglial mEmerald‐CD9 + ‐EV secretion in mice expressing P301L tau. Moreover, microglia and neuron‐specific deletion of P2rx7 inhibit tau propagation from the entorhinal cortex to the mouse hippocampus. Conclusion Our study demonstrated a microglial P2RX7‐EV axis with potential implications on neuroinflammation and neurodegeneration associated with tau pathology, further indicating the therapeutic potential of targeting P2rx7 to ameliorate AD progression.

Abstract P2x purinoreceptor 7 (P2RX7), an ATP-gated ion channel, is known to play pivotal roles in the progression of Alzheimer’s disease (AD), although its cell type-specific pathological mechanisms have yet to be elucidated. Here, we show that genetic deletion of P2rx7 mitigates brain atrophy, tau accumulation and cognitive impairment in PS19 tauopathy mice. Specific deletion of P2rx7 in microglia, but not astrocytes, significantly suppresses tau propagation from the entorhinal cortex to CA1 in the hippocampus, an early event in AD pathology. Single-cell (sc)-RNA sequencing of mouse brains revealed specific P2rx7 expression in microglia, inducing inflammatory changes accompanied by elevated extracellular vesicles (EVs) secretion in PS19 mice. Brain-derived EVs (BDEVs) proteome demonstrated that P2RX7 increases EV cargo loading of tau and mitochondrial molecules in BDEVs from PS19 mice, which was further validated by single-molecule super-resolution. Notably, following the injection of BDEVs isolated from PS19 mice with or without P2rx7 deficiency, the microglial transcriptome of recipient mice revealed enriched DNA-sensing and type II interferon signaling in response to BDEVs from PS19 mice, which was diminished in the group injected with P2rx7-deficient BDEVs. Thus, our results indicate that P2RX7 regulates EV-mediated tau and mitochondrial transfer and inflammatory activation in microglia with increased EV secretion, thereby contributing to tauopathy and neurodegeneration, highlighting the therapeutic potential of targeting the P2RX7-EV axis in AD.

Abstract We have previously reported the therapeutic effect of depletion and regeneration of endogenous microglia on autism spectrum disorder-like behaviors in offspring from the dams under maternal immune activation (MIA) by dampening their neuritogenic activation. Here we show a long-lasting pathological effect by MIA, leading to abnormal behaviors in offspring mice in a sex-specific manner at 3 months of age. MIA was induced by injecting Polyinosinic:polycytidylic acid [Poly(I:C)] at 10mg/kg at E9.5 with preselected dams based on the immunoreactivity to low-dose Poly(I:C) injection. MIA offspring show impairments in sociability, repetitive behavior, and spatial working and associative memories in both sexes, whereas only male offspring show social novelty deficit. MIA has no effect on anxiety-like behavior nor sensori-motor or locomotor activity. Administering colony stimulating factor receptor (CSF1R) inhibitor in young adult MIA offspring regenerates microglia and ameliorates sociability deficits in males and spatial working and associative memory impairments in both sexes without effect on social novelty impairments. Transcriptomic analysis of prefrontal cortex and hippocampal tissues reveals MIA and sex-specific pathways and highlights potential key genes involved in the corrective effect of microglial regeneration in MIA offspring. Our results underscore the potential of the sex-specific therapeutic applications of CSF1R inhibitor for MIA-related social and cognitive disorders.

Abstract Brain-derived extracellular vesicles (BDEVs) carry tau filaments and promote tau transmission in Alzheimer’s disease (AD). However, how APOE ε4 allele, a key genetic risk factor for AD, may change BDEV molecular structures thereby facilitate disease progression is poorly understood. Here we report comprehensive analyses of BDEVs isolated from human E3/3 and E4/4 AD brains with a biological multi-omics approach. E4/4 BDEVs significantly enhanced tau propagation in aged human MAPT (Tau) knock-in and APPNL-G-F: Tau double knock-in mouse brains in vivo and increased neuronal uptake and excitability in induced pluripotent stem cell-derived neurons (iNeurons) compared to E3/3 BDEVs in vitro. Notably, correlation analysis of BDEV-lipidome and proteome exhibited synergistic enrichment in unsaturated free fatty acid (FFA)18:2, a precursor of inflammatory w6 FFA, and neural cell adhesion molecule 1 (NCAM1). Treatment of iNeurons with FFA 18:2 induces NCAM1 expression, recruits tau into EVs and enhance their tau seeding activity, which is blocked by NCAM1 antibody in vitro . Finally, intracerebroventricular injection of NCAM1 antibody significantly alleviated pathological tau accumulation and glial inflammation in PS19 tauopathy mouse brains, which had previously been reported to exhibit increased level of 18:2 FFA. This highlights novel pathological mechanism in tau transfer mediated by E4/4 BDEVs and emphasizes strong therapeutic potential of targeting EV molecules in AD progression.