Human Tau Isoforms Assemble into Ribbon-like Fibrils That Display Polymorphic Structure and Stability

Abstract

Fibrous aggregates of Tau protein are characteristic features of Alzheimer disease. We applied high resolution atomic force and EM microscopy to study fibrils assembled from different human Tau isoforms and domains. All fibrils reveal structural polymorphism; the "thin twisted" and "thin smooth" fibrils resemble flat ribbons (cross-section ϳ10 ؋ 15 nm) with diverse twist periodicities. "Thick fibrils" show periodicities of ϳ65-70 nm and thicknesses of ϳ9 -18 nm such as routinely reported for "paired helical filaments" but structurally resemble heavily twisted ribbons. Therefore, thin and thick fibrils assembled from different human Tau isoforms challenge current structural models of paired helical filaments. Furthermore, all Tau fibrils reveal axial subperiodicities of ϳ17-19 nm and, upon exposure to mechanical stress or hydrophobic surfaces, disassemble into uniform fragments that remain connected by thin thread-like structures (ϳ2 nm). This hydrophobically induced disassembly is inhibited at enhanced electrolyte concentrations, indicating that the fragments resemble structural building blocks and the fibril integrity depends largely on hydrophobic and electrostatic interactions. Because full-length Tau and repeat domain constructs assemble into fibrils of similar thickness, the "fuzzy coat" of Tau protein termini surrounding the fibril axis is nearly invisible for atomic force microscopy and EM, presumably because of its high flexibility.

The fuzzy coat of pathological human Tau fibrils is a two-layered polyelectrolyte brush

“…FV-AFM topographs (Fig. 2C) recorded at imaging forces of 300 pN at pH 7.4 in presence of 50 mM KCl showed the typical appearance of TauRD fibrils and hTau40 fibrils (16). Cross-sections taken perpendicular to the fibril center (Fig.…”

“…In this case, all N-terminal ends protrude to the same side in straight fibrils and form a twist-dependent brush surrounding twisted fibrils. However, the assembly of Tau molecules may vary among Tau fibrils, thereby inducing a high degree of structural polymorphism (16).…”

“…We previously used high-resolution atomic force microscopy (AFM) to observe the structural polymorphism of Tau fibrils in physiological solution (16). Fibrils of recombinant full-length hTau40 showed a thickness (i.e., height measured by AFM) of ≈18.5 nm, whereas the thickness of fibril cores assembled from recombinant TauRDs that lack most of the terminal Tau domains (Fig.…”

Biochemistry and Cell Biology of Tau Protein in Neurofibrillary Degeneration

“…An alternative description is that of twisted ribbons, 22 nm wide and 10 nm thick (Moreno-Herrero et al 2004;Pollanen et al 1997;Wegmann et al 2010). There is little evidence for the existence of physically separate subfibers; on the contrary, AFM imaging and force application suggest that subunits are stacked axially, not side by side (Wegmann et al 2010). Thus, the term "Tau fibers" is more appropriate than "PHFs," although the latter is probably too established to fade away.…”

“…This coat is still highly mobile even in the aggregated state (Sillen et al 2005), similar to Tau itself and the projection domain of MT-bound Tau. It can be viewed as a "soft polymer brush" extending from the PHF core, and able to enter multiple interactions with other cellular components (Wegmann et al 2010). …”