Characterization of tau fibrillization in vitro

Abstract

Background The assembly of tau proteins into paired helical filaments, the building blocks of neurofibrillary tangles, is linked to neurodegeneration in Alzheimer's disease and related tauopathies. A greater understanding of this assembly process could identify targets for the discovery of drugs to treat Alzheimer's disease and related disorders. By using recombinant human tau, we have delineated events leading to the conversion of normal soluble tau into tau fibrils. Methods Atomic force microscopy and transmission electron microscopy methodologies were used to determine the structure of tau assemblies that formed when soluble tau was incubated with heparin for increasing lengths of time. Results Tau initially oligomerizes into spherical nucleation units of 18- to 21-nm diameter that appear to assemble linearly into nascent fibrils. Among the earliest tau fibrils are species that resemble a string of beads formed by linearly aligned spheres that with time seem to coalesce to form straight and twisted ribbon-like filaments, as well as paired helical filaments similar to those found in human tauopathies. An analysis of fibril cross sections at later incubation times revealed three fundamental axial structural features. Conclusions By monitoring tau fibrillization, we showed that different tau filament morphologies coexist. Temporal changes in the predominant tau structural species suggest that tau fibrillization involves the generation of structural intermediates, resulting in the formation of tau fibrils with verisimilitude to their authentic human counterparts.