Formation, release, and internalization of stable tau oligomers in cells.

Tau is a neuronal microtubule-binding protein that, in Alzheimer's disease and other neurodegenerative diseases, can form oligomeric and large fibrillar aggregates, which deposit in neurofibrillary tangles. Tau's physiological state of multimerization appears to vary across conditions, and a stable dimeric form of soluble tau has been suggested from experiments using recombinant tau in vitro. We tested if tau dimerization or oligomerization, also occurs in cells, and if soluble tau oligomers are relevant for the release and internalization of tau. We developed a sensitive tau split-luciferase assay to show the rapid intracellular formation of stable tau dimers that are released and taken up by cells. Our data further suggest that tau dimerization can be accelerated slightly by aggregation catalysts. We conclude that tau oligomers are a stable physiological form of tau, and that tau oligomerization does not necessarily lead to tau aggregation.