Specific post-translational modifications of the soluble tau protein distinguish between Alzheimer’s disease, 4R-, and 3R-tauopathies

Abstract The tau protein aggregates in several neurodegenerative disorders, referred to as tauopathies. The type of tau isoforms (4R/3R) observed in brain aggregates is used to classify tauopathies. However, distinguishing tauopathies in cerebrospinal fluid remains challenging. Here, we demonstrated that the difference in tau isoforms between tauopathies is only observed in aggregates, not in soluble brain extracts. We therefore used untargeted mass spectrometry to characterize all post-translational modifications of both the aggregated and the soluble tau protein obtained from human brain tissue of patients with Alzheimer’s disease, cortico-basal degeneration, Pick’s disease, and fronto-temporal lobe degeneration. We found specific soluble signatures predictive of each tauopathy and its peculiar type of aggregated tau isoforms. These findings provide potential targets for developing cerebrospinal fluid assays able to distinguish between tauopathies in vivo. The comparison between soluble and aggregated tau features indicates potential mechanisms of tau aggregation, providing novel therapeutic leads for these diseases.