Figure 3. Released tau-L1/L2 is mostly soluble and oligomeric.

A) Western Blot for human tau in cells and medium from HEK tau-L1/L2 (40 hours expression). Human tau in the culture medium was further fractionated into soluble (Sol) and extracellular vesicle bound (Vesicles) tau by ultracentrifugation.

B) gLuc assay of cells and different medium fractions shown in (A) reveal that ~90% of tau-L1/L2 in the medium is soluble rather than vesicle/membrane-associated. Mean±SEM, n=2 experiments, 6 repeats.

C) Size exclusion chromatography (SEC) was performed on HEK tau-L1/L2 medium (40 hours) and the collected fractions (0.5 ml) were analyzed by gLuc assay to detect dimeric and oligomeric tau-L1/L2 but not monomeric tau. Most tau-L1/L2 appeared at a MW of truncated oligomeric forms of tau (elution vol. 14–16 ml), and a small amount existed as higher molecular weight oligomeric tau-L1/L2 (elution vol. 7–9 ml). Mean±SEM, n=5 experiments.

D) AFM topographies of human tau isolated from HEK tau-L1/L2 medium (40 h) by immuno-precipitation reveal particles sizes (= AFM heights) of mostly H₁=2.5±0.3 nm and some of H₂=5.2±2.6 nm. Mean±SD, n=269 particles.