Tau profiling of brain extracellular vesicles reveals PHF6 peptide as core for pathological tau seeding in Alzheimer's disease

BACKGROUND: Tauopathies are neurodegenerative diseases all characterized by tau lesions in the brain. Nevertheless, a clinical and pathophysiological heterogeneity is present among them. This includes the dominant tau isoform found within aggregates (3R and/or 4R tau) along with different brain regions being affected. For some tauopathies, especially in Alzheimer's disease, a specific spatio-temporal staging of tau lesions is present. This staging has been the basis for the prion-like propagation hypothesis, which describes a cell-to-cell transfer of pathological tau species resulting in new aggregates formation in recipient neurons. Human extracellular vesicles isolated from the brain-derived fluid (BD-EVs) of Alzheimer's disease patients contain seeds that contribute to this tau pathology spreading. However, the nature of these tau species responsible for this nucleation activity remains unknown. Additionally, heterogeneity in seeding activity of BD-EVs of Alzheimer's disease, progressive supranuclear palsy and Pick's disease patients is known. METHODS: Here, EVs were isolated from human frozen tissue (Alzheimer's disease, Progressive Supranuclear Palsy, Pick disease and non-demented controls). We used a tau immunoprecipitation followed by high-resolution mass spectrometry to define their proteomic profile and test their seeding capacity in vitro. RESULTS: We show that the tau profile present within BD-EVs is different among tauopathies. Interestingly, multiple tau peptides located in the microtubule binding region were specifically enriched in Alzheimer's disease extracellular vesicles. Of these, mainly the PHF6 (VQIVYK) containing proteins mediate tau seeding activity. CONCLUSIONS: PHF6 is a driver for the higher EVs-mediated tau propagation in AD patients, revealing an interesting therapeutic target to prevent tau pathology spreading.