PROJECT SUMMARY Alzheimer's disease (AD) is defined by the appearance of two defining pathologies, namely Aβ-amyloid plaques and neurofibrillary tangles enriched with hyperphosphorylated Tau. Accumulation of Aβ precedes the appearance of pathological Tau, and although correlative evidence indicates that Aβ proteotoxicity and Tau pathology, molecular mechanisms defining how Aβ can directly drive Tau pathogenesis are yet elusive. Recent correlative evidence indicates a role for dysfunction of the microglial immune receptor, Trem2 in enhancing Tau pathogenesis in regions enriched with Aβ plaques in AD mouse models. Interestingly, our previous results indicate that TREM2 is a potential Aβ receptor that directly binds and transduces proteotoxic Aβ signals to drive microglial activation. Given that Trem2 (and the R47H TREM2 variant in humans) is a potent risk factor for AD onset, it seems likely that TREM2 can be a potential link between Aβ and Tau pathology, and potentially modulates Tau pathogenesis with Aβ exposure. Here, we present preliminary results suggesting that Trem2 deletion (KO) in microglia can enhance Tau dispersion from the medial entorhinal cortex (MEC) to the hippocampus, which manifests in behavioral memory impairment and synaptic dysfunction. Transcriptomic analysis of Trem2 KO microglia indicates differential expression of exosomal components, and upregulation of machinery such as Atg12 which drive endosome trafficking and exosomal biogenesis. Our preliminary results in vitro also indicate that microglial Trem2 deletion can enhance transneuronal Tau transduction, which implicates a model where Trem2 deletion may enhance intraneuronal Tau dispersion during AD onset. Our previous results indicate that Aβ oligomers can induce Syk activation, whereas prolonged Aβ exposure can result in progressive renormalization of Syk activity, suggesting that chronic Aβ exposure can “desensitize” microglial TREM2 signaling. Given that the TREM2 R47H likely confers loss-of-function, we will determine whether enhancement of exosome pathways, namely upregulation of Atg12, and/or suppression of the mTOR pathway mediate enhanced Tau pathogenesis with Trem2 deletion, or TREM2 R47H knock-in (KI) in microglia. We will also establish whether long-term Aβ treatment can affect Tau uptake, enhance sorting into exosomes, and extrusion in microglia, and compare differences in exosomal Tau trafficking in WT, Trem2 KO and R47H KI microglia. Using a Tau FRET biosensor cell line system (Tau RD), we will also assay potency of extruded exosomal Tau with long-term Aβ treatment in WT, Trem2 KO and R47H KI backgrounds, and determine whether alterations in Atg12 or mTOR pathways can affect Tau seeding potency. Together, completion of these Aims will provide insight into TREM2 as an intermediary Aβ sensor which initially suppresses Tau dispersion with acute Aβ exposure. Chronic Aβ exposure, however, desensitizes the TREM2 signaling pathway, thereby potentially aggravating Ta...