Cryo-EM studies of amyloid-β fibrils from human and murine brains carrying the Uppsala APP mutation (Δ690-695)

Abstract

Today, 13 intra-amyloid-β (Aβ) amyloid precursor protein (APP) gene mutations are known to cause familial Alzheimer's disease (AD). Most of them are point mutations causing an increased production or a change in the conformation of Aβ. The Uppsala APP mutation (Δ690-695 in APP, Δ19-24 in Aβ) is the first known multi-codon deletion causing autosomal dominant AD. Here, we applied cryo-electron microscopy (cryo-EM) to investigate the structure of Aβ fibrils with the Uppsala APP mutation from tg-UppSwe mouse brain tissue. Murine AβUpp(1-42)_Δ19-24 are made of two identical S-shaped protofilaments with an ordered fibril core of S8-A42. The murine Aβ fold is almost identical to previously described human type II filaments, although the amino acid sequences differ considerably. In addition, we report the cryo-EM structure of Aβ fibrils from the temporal cortex of a patient with the Uppsala APP mutation. The observed structure of the human Aβ fold closely resembles previously described type I fibrils. Structural modeling suggests that these fibrils are composed of wild-type Aβ, which implies that AβUpp may be less soluble and thus not readily accessible for cryo-EM image processing and structure determination. Additionally, from the human sample we determined the structures of tau paired helical filaments and tau straight filaments, which are identical to those found in sporadic AD cases. Finally, we present the 3D cryo-EM structures of four dominant AβUpp(1-42)_Δ19-24 fibril polymorphs, formed in vitro. All four polymorphs differ from the observed folds of Uppsala Aβ in murine and human brain tissue, respectively.

Fig. 1

The 3D structure of AβUpp(1–42)_Δ19–24 purified from tg-UppSwe mouse brain tissue. a The cryo-EM density map (in transparent gray) with the atomic model (green). The Uppsala APP deletion (Δ19–24) is marked in red within the Aβ42 sequence. b Cross-section through the reconstructed fibril density. c A schematic of the fold, produced with atom2svg.py [22] (red: acidic; blue: basic; green: hydrophilic; white: hydrophobic; pink: glycine; yellow: sulfur containing). d Overlay of the cryo-EM structures of murine Aβ(1–42)_Δ19–24 fibrils (green) with the cryo-EM structure of human brain-derived type II Aβ42 filaments (gray, PDB 7Q4M). e Overlay of the cryo-EM structures of murine Aβ(1–42)_Δ19–24 protofilaments with the cryo-EM structure of human brain-derived type I Aβ42 filaments (gray, PDB 7Q4B)

Fig. 2

Tau fibrils purified from human brain tissue of an individual with the Uppsala APP mutation. a Cross-section through the reconstructed fibril density of PHFs. b Reconstructed cryo-EM density map of PHFs (gray) and the corresponding atomic model (blue). c Comparison of PHFs purified from human Uppsala APP mutation brain tissue (blue) and from sAD brain tissue (gray, PDB 5O3L). d Cross-section through the reconstructed fibril density of SFs. e Overlay of the reconstructed cryo-EM density map of SFs (gray) and the atomic model of SFs from sAD brain tissue (dark blue, PDB 6HRF). f Comparison of SFs purified from human Uppsala APP mutation brain tissue (blue) and from sAD brain tissue (gray, PDB 6HRF)

Fig. 3

Aβ fibrils extracted from human brain tissue of an individual with the Uppsala APP mutation. a Cross-section through the reconstructed fibril density. b Reconstructed cryo-EM density map (gray) and the atomic model of type I wild-type Aβ42 filaments (orange, PDB 7Q4B). c Overlay of the atomic model of type I filaments (orange) and a homology model of AβUpp(1–42)_Δ19–24 using the atomic model of type I filaments as a template (burgundy). d Reconstructed cryo-EM density map (gray) and the fitted homology model of AβUpp(1–42)_Δ19–24 (burgundy)

Fig. 4

The 3D structure of in vitro AβUpp(1–42)_Δ19–24. a–d Cross-sections through the reconstructed fibril densities of a PM1, b PM2, c PM3, and d PM4. e–f The cryo-EM density maps (in transparent gray) of e PM1, f PM2, g PM3, and h PM4 with the corresponding atomic models in purple, rose, melon, and lavender, respectively