Tau filaments from multiple cases of sporadic and inherited Alzheimer's disease adopt a common fold

Abstract

The ordered assembly of tau protein into abnormal filaments is a defining characteristic of Alzheimer's disease (AD) and other neurodegenerative disorders. It is not known if the structures of tau filaments vary within, or between, the brains of individuals with AD. We used a combination of electron cryo-microscopy (cryo-EM) and immuno-gold negative-stain electron microscopy (immuno-EM) to determine the structures of paired helical filaments (PHFs) and straight filaments (SFs) from the frontal cortex of 17 cases of AD (15 sporadic and 2 inherited) and 2 cases of atypical AD (posterior cortical atrophy). The high-resolution structures of PHFs and SFs from the frontal cortex of 3 cases of AD, 2 sporadic and 1 inherited, were determined by cryo-EM. We also used immuno-EM to study the PHFs and SFs from a number of cortical and subcortical brain regions. PHFs outnumbered SFs in all AD cases. By cryo-EM, PHFs and SFs were made of two C-shaped protofilaments with a combined cross-β/β-helix structure, as described previously for one case of AD. The higher resolution structures obtained here showed two additional amino acids at each end of the protofilament. The immuno-EM findings, which indicated the presence of repeats 3 and 4, but not of the N-terminal regions of repeats 1 and 2, of tau in the filament cores of all AD cases, were consistent with the cryo-EM results. These findings show that there is no significant variation in tau filament structures between individuals with AD. This knowledge will be crucial for understanding the mechanisms that underlie tau filament formation and for developing novel diagnostics and therapies.

Keywords: Alzheimer’s disease; Electron cryo-microscopy; Immuno-gold negative-stain electron microscopy; Neurodegenerative diseases; Paired helical filaments; Straight filaments; Tau protein.

Fig. 1

Cryo-EM densities and atomic models of PHFs and SFs from the frontal cortex of AD case 2. PHFs and SFs from case 2 are resolved to 3.2 Å and 3.3 Å, respectively. Sharpened, high-resolution maps are shown in blue (PHF) and green (SF). Unsharpened 4.5 Å low-pass filtered densities are shown in grey. The models comprise G273-E380 of 3R tau and G304-E380 of 4R tau

Fig. 2

Cryo-EM structures of PHFs and SFs from the frontal cortex of AD cases 1, 2, 3, and 16. All structures show identical pairs of C-shaped protofilaments and the same inter-protofilament packing in PHFs and SFs. Cases 1, 2, and 3 had sporadic AD, whereas case 16 had inherited AD (mutation V717F in APP). The filament structures of case 1 are from (14); the structures from cases 2, 3, and 16 are first described here. All cases had a majority of PHFs and a minority of SFs. Yellow arrows indicate the extra densities, which are present in PHFs and SFs from all four cases, bordering the solvent-exposed side chains of R349 and K375, and of H362 and K369. Scale bar, 50 Å

Fig. 3

Immuno-EM of tau filaments from the frontal cortex of sporadic and inherited AD cases. a 441 amino acid isoform of human tau and epitopes of repeat-specific antibodies. The amino-terminal inserts are labelled N1 and N2, with the microtubule-binding repeats being R1–R4. Black lines indicate the epitopes of antibodies BR136 (R1), Anti-4R (R2), BR135 (R3) and TauC4 (R4) that were used for immuno-EM. b, c Representative images from sporadic (b) and inherited (c) cases of AD before (−) and after (+) pronase treatment. Frontal cortex from 15 cases of typical sporadic AD and two cases of inherited AD (mutation V717F in APP) were studied. Tau filaments from all the cases were decorated by BR136 and Anti-4R before pronase treatment, but not by BR135 or TauC4. Pronase treatment abolished labelling by BR136 and Anti-4R. PHFs were in the majority and SFs in the minority. Scale bar, 100 nm

Fig. 4

Immuno-EM of tau filaments from the putamen of AD case 1. Representative images before (−) and after (+) pronase treatment. In addition to frontal cortex and putamen, temporal cortex, parietal cortex, cingulate cortex, thalamus, and substantia innominata from case 1 were also analysed. Tau filaments from all brain regions were decorated by BR136 and Anti-4R before pronase treatment, but not by BR135 or TauC4. Pronase treatment abolished labelling by BR136 and Anti-4R. PHFs were in the majority and SFs in the minority. Scale bar, 100 nm

Fig. 5

Immuno-EM of tau filaments from the occipital cortex of PCA cases. Representative images before (−) and after (+) pronase treatment. Occipital cortex from two cases of PCA, an atypical form of sporadic AD, was studied. Tau filaments from both cases were decorated by BR136 and Anti-4R before pronase treatment, but not by BR135 or TauC4. Pronase treatment abolished labelling by BR136 and Anti-4R. PHFs were in the majority and SFs in the minority. Scale bar, 100 nm