Structure of Tau filaments in Prion protein amyloidoses

Abstract

In human neurodegenerative diseases associated with the intracellular aggregation of Tau protein, the ordered cores of Tau filaments adopt distinct folds. Here, we analyze Tau filaments isolated from the brain of individuals affected by Prion-Protein cerebral amyloid angiopathy (PrP-CAA) with a nonsense mutation in the PRNP gene that leads to early termination of translation of PrP (Q160Ter or Q160X), and Gerstmann-Sträussler-Scheinker (GSS) disease, with a missense mutation in the PRNP gene that leads to an amino acid substitution at residue 198 (F198S) of PrP. The clinical and neuropathologic phenotypes associated with these two mutations in PRNP are different; however, the neuropathologic analyses of these two genetic variants have consistently shown the presence of numerous neurofibrillary tangles (NFTs) made of filamentous Tau aggregates in neurons. We report that Tau filaments in PrP-CAA (Q160X) and GSS (F198S) are composed of 3-repeat and 4-repeat Tau isoforms, having a striking similarity to NFTs in Alzheimer disease (AD). In PrP-CAA (Q160X), Tau filaments are made of both paired helical filaments (PHFs) and straight filaments (SFs), while in GSS (F198S), only PHFs were found. Mass spectrometry analyses of Tau filaments extracted from PrP-CAA (Q160X) and GSS (F198S) brains show the presence of post-translational modifications that are comparable to those seen in Tau aggregates from AD. Cryo-EM analysis reveals that the atomic models of the Tau filaments obtained from PrP-CAA (Q160X) and GSS (F198S) are identical to those of the Tau filaments from AD, and are therefore distinct from those of Pick disease, chronic traumatic encephalopathy, and corticobasal degeneration. Our data support the hypothesis that in the presence of extracellular amyloid deposits and regardless of the primary amino acid sequence of the amyloid protein, similar molecular mechanisms are at play in the formation of identical Tau filaments.

Keywords: APrP; Cryo-EM; GSS; Neurodegeneration; PrP-CAA; Tau.

Fig. 1

Immunohistochemistry in PrP-CAA (Q160X) and GSS (F198S) compared to AD. Hemispheric coronal sections show the distribution of Tau pathology at the level of the cerebral cortex, caudate nucleus and putamen in a case of AD (Case 1, a), cerebral cortex, amygdala, and caudate nucleus in PrP-CAA (Q160X) b, and cerebral cortex, caudate nucleus, putamen, and claustrum in GSS (F198S) (Case 1, c). Double immunohistochemistry of Tau and Aβ in AD (d), Tau and PrP in PrP-CAA (Q160X) (e), and Tau and PrP in GSS (F198S) (f). Nerve cell bodies and NTs are reactive for Tau (red) and are seen in the vicinity of parenchymal Aβ plaques (brown) in AD (d), blood vessels with APrP angiopathy (black) in PrP-CAA (Q160X) (e), or parenchymal APrP plaques (black) in GSS (F198S) (f). a–f, Anti-Tau antibody AT8; d Anti-Aβ antibody NAB 228; e, f Anti-PrP antibody 95–108. Scale bar, 50 µm

Fig. 2

PrP-CAA (Q160X) and GSS (F198S) Tau are indistinguishable from AD Tau by Western blot or seeding assay. Western blots of sarkosyl-insoluble Tau fractions from AD (Case 2), PrP-CAA (Q160X), and GSS (F198S) (Case 2) show that Tau aggregates consist of full length, hyperphosphorylated Tau with an identical electrophoretic pattern, consisting of major bands of 60, 64, and 68 kDa. No PrP immunoreactivity is seen in the sarkosyl-insoluble Tau fractions of PrP-CAA (Q160X) and GSS (F198S). Total brain homogenate from PrP-CAA (Q160X*) was used as positive control for PrP (a). Tau biosensor cells incubated with the sarkosyl-insoluble fraction obtained from frontal cortex (FC) of AD (Case 2), PrP-CAA (Q160X) and GSS (F198S) (Case 2) show Tau seeding activity, whereas the insoluble fraction of FC from control brain, and cerebellum (CB) from PrP-CAA (Q160X) and GSS do not seed Tau aggregation in vitro (b). Scale bar: 20 µm

Fig. 3

Cryo-EM reconstructions of PHF and SF from PrP-CAA (Q160X) and PHF from GSS (F198S). The structures show identical pairs of C-shaped protofilaments and the same inter-protofilament packing between PHFs (a, b) but different packing for SFs (c). The rectangular boxes in the vertical, surface view of the helical reconstructions of PHFs (left, green), and SFs (right, blue) show the location of the cross-sectional densities (a–c). 2D class averages of PHF (d) and SF (e) of PrP-CAA (Q160X). Fourier Shell Correlation (FSC) curves for independently refined half-maps of PrP-CAA (Q160X) PHFs and SFs, and GSS (F198S) PHFs (f)

Fig. 4

Cryo-EM densities and atomic models of PHFs and SFs. Sharpened, high-resolution maps are shown in green (PHF, PrP-CAA (Q160X)), red (PHF, GSS (F198S)), and blue (SF, PrP-CAA (Q160X)). The blurred map regions represent extra densities low-pass-filtered to 5 Å (a). PrP-CAA (Q160X) PHF extra density (b) and GSS (F198S) extra density (c) around the core have different locations and orientations to that of the extra density of AD PHF (d). Schematic view of the PrP-CAA (Q160X) (e) and GSS (F198S) (f) PHF protofilament cores compared to AD (g) PHF (PDB: 5o3l) showing the similarities in the folds. The colors represent the polarity of the amino acids (red: negatively charged, blue: positively charged, white: non-polar)

Fig. 5

Different folds of Tau that have been identified to-date. Two different Tau folds are associated with 3R and 4R-Tau and make the Tau aggregates in AD, PrP-CAA (Q160X), GSS (F198S), PART, and CTE. A Tau fold associated with 3R-Tau makes the inclusions of Pick disease and a Tau fold associated with 4R-Tau makes the aggregates in CBD. Among the four folds, one known as the Alzheimer Tau fold can occur in the presence or in the absence of an extracellular amyloid deposition. Analysis of additional diseases characterized by 3R-, 4R-, and 3R and 4R-Tau will determine whether this is the complete Tau fold landscape or if additional folds may be found associated with different diseases