Characterization of tau oligomers. Oligomerized tau protein (tau151-391) was prepared by in vitro oligomerization reaction using polyanionic inducer heparin. Progress of tau oligomerization was monitored by thioflavin T fluorescence (A). Neither tau nor heparin alone in the oligomerization reaction produced significant thioflavin T fluorescence over the time course of the experiment. Monomeric tau (25 kDa, apparent molecular weight on SDS-PAGE is 30 kDa) and oligomerized tau (45 to 170 kDA) was analyzed by Western blot and visualized by monoclonal antibody DC25 (epitope 347-354 of human tau isoform Tau40) (B), which revealed multiple SDS-stable oligomeric species. Transmission electron microscopy (C) showed tau oligomers in the form of small round particles and short filaments (arrows). Scale bar represents 200 nm.