You, Yang

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 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.