APOE and microglia-mediated progression in tau pathology in AD

We have previously reported that microglia and extracellular vesicles (EVs) play pivotal roles in tau propagation. Our findings have been reproduced in numerous studies showing involvement of microglia and EVs in the development of tau pathology in animal models and human biospecimens. During the current funded period (1RF1 AG054199), we identified: 1) Enrichment of glial and disease-associated proteins in Alzheimer’s disease brain-derived EVs (AD EVs) compared to control EVs, 2) Highly transmissible nature of AD EVs demonstrated by increased neuronal uptake and tau aggregation potency, 3) Robust tau propagation throughout the entire hippocampus by inoculating AD EVs containing just 300 pg of tau in aged C57BL/6 mouse brains. We have yet to elucidate the underlying mechanism of AD EVs gaining pathogenic functions. We hypothesize that Ab-induced inflammatory conditions in AD leads to microglial activation, which alters molecular compositions of microglial EVs, leading to their increased transmission to neurons. APOE4 enhances this process by augmented Ab deposition and MGnD induction. The hypothesis has been developed based on the previous studies and our preliminary data showing that (1) Clec7a+ neurodegenerative microglia (MGnD) surrounding Ab plaques play a key role in accelerating tau propagation in APPNL-G-F/NL-G-F knock-in (APP KI) mice; (2) MGnD upregulates EV markers; (3) AD EVs and APP mouse brain-derived EVs show elevated MGnD and cell adhesion molecules compared to controls; (4) AD EVs show increased tau seeding activity and neuronal uptake compared to controls, which is exacerbated in APOE4 over non-APOE4 AD cohorts; and (5) APOE4 knock-in APP/PS1 mice accelerates Ab deposition, microglial activation, and expression of MGnD markers compared to APOE3 knock-in APP/PS1 mice. The rationale of the proposed research is that identification of EV protein and lipid composition uniquely expressed in AD and APOE4 background will serve as the basis of molecular understanding of EV-mediated disease progression. Our study will fill the missing link between MGnD induction and tau propagation by microglial tau-seeding EV production. In Aim 1, we will delineate the chemical and molecular structure of aggregation-prone tau in AD EVs and tau- interacting molecules. We will also characterize the protein and lipid compositions of EVs from APOE4/4 and APOE3/3 EVs to characterize the difference in cellular origin, involvement of inflammatory and toxic molecules, and tau seeding activity in vitro and in vivo. In Aim 2, we will knock down Apoe, involved in microglial activation and Smpd3, involved in EV synthesis, to determine the role of MGnD induction and glial EV production on tau propagation. In Aim 3, we will differentiate isogenic human iPSCs in APOE3/3 and APOE4/4 background to microglia-like cells (iMGL), and characterize pathogenicity of iMGL EVs under MGnD condition. We will perform proteomic and lipidomic profiling of iMGL EVs to validate our findings from human AD EVs in Aim 1. The proposed work is expected to obtain the fundamental molecular bases for EV biology in tauopathy.