Rare bioactive tau oligomers from Alzheimer brain support both templated misfolding and fibril formation.

The accumulation of hyperphosphorylated tau aggregates is a hallmark of Alzheimer's disease (AD). In addition to long-recognized tau deposits in neurofibrillary tangles, recent studies suggest that diffusible, aqueous soluble (High Molecular Weight, or HMW) species are also bioactive, i.e., able to seed templated misfolding. The characteristics of the diffusible misfolded proteins are largely unknown, and their relationship to classical fibrillar structures is unclear. Using sequential size exclusion and anion exchange chromatography, we fractionated the HMW tau population and identified multiple subspecies varying in retention properties. The subspecies that elute early from the size column, and are retained on the anion exchange column are seed competent, whereas the other soluble fractions are not. Biophysical analyses using super resolution, atomic force, and immunogold electron microscopy confirmed that the size and conformation of both bioactive and non-bioactive tau oligomers are similar, with dimers, trimers, and tetramers predominating. The presence of surface phosphorylations, as detected by recently developed single molecule array (SIMOA) analyses, correlates with seeding capacity. Single bioactive tau oligomers at fMol concentrations can induce seeding and templated misfolding in a reporter cell. The bioactive species can alternatively support aggregation of a truncated repeat domain tau construct into thioflavin T positive fibrils in a real-time quacking-induced conversion (RT-QuIC) assay, whereas full length recombinant tau yields oligomers. These findings provide structural insights into bioactive oligomeric tau species, emphasize the small concentrations necessary for bioactivity, and highlight the possibility that, under different conditions, they can seed either oligomeric or fibrillar structures.