Ellison, J. W

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 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.