Antiparallel Triple-strand Architecture for Prefibrillar Aβ42 Oligomers

Abstract

Background: Soluble A␤42 oligomers, rather than insoluble amyloid fibrils, are toxic species in Alzheimer's disease. Results: We obtained structural restraints at all 42 residue positions in A␤42 oligomers and performed structural modeling. Conclusion: In oligomers, each A␤42 protein forms a single ␤-sheet with three antiparallel ␤-strands. Significance: Our novel structural model provides new structural framework for understanding oligomer-fibril interconversion and designing oligomer-targeted therapeutics.

Protein structural and surface water rearrangement constitute major events in the earliest aggregation stages of tau

“…This approach allows us to differentiate between the spin-exchanged and simply immobile spectral features, thereby confirming the origin of the characteristic spectral component as caused by β-sheet packing (36)(37)(38)(39)(40)(41). Single-line, spin-exchanged, ESR spectra of protein fibrils have been previously assigned to parallel β-sheet structures (29,(36)(37)(38)(39)(40)(41), including the mature fibrils of full-length tau proteins that were found to stack in parallel and in-register particularly around the PHF6 region (29). By combining ODNP and cw ESR measurements performed in situ and in solution together with conventional techniques of turbidimetry, Thioflavin T (ThT) staining fluorescence, transmission electron microscopy (TEM), and chemical cross-linking to probe aggregation of Δtau187, we aim to answer the following key questions: Results and Discussions Verifying Fibril Formation of Δtau187 by Global Measurement.…”

“…We can verify the assignment of this spectral signature to parallel β-sheet stacking by systematic "spin dilution" to yield a mixture where only 20% of the tau strands are labeled with paramagnetic R1 labels and 80% are labeled with the diamagnetic analog of R1 labels, designated here as R1', at a given site of Δtau187. This approach allows us to differentiate between the spin-exchanged and simply immobile spectral features, thereby confirming the origin of the characteristic spectral component as caused by β-sheet packing (36)(37)(38)(39)(40)(41). Single-line, spin-exchanged, ESR spectra of protein fibrils have been previously assigned to parallel β-sheet structures (29,(36)(37)(38)(39)(40)(41), including the mature fibrils of full-length tau proteins that were found to stack in parallel and in-register particularly around the PHF6 region (29).…”

“…Specifically, we determined the β-sheet content from the population of the single-line, spin-exchanged component as derived from the three-component ESR analysis of 100% R1-labeled tau (see SI ESR Simulation). The assignment of spin-exchanged spectral features to parallel β-sheet arrangement, in which the R1-labeled sites of adjacent protein strands approach each other within 5-8 Å interspin label distance, has been established in the literature (29,(36)(37)(38)(39)(40)(41), and, specifically, has been demonstrated on mature fibrils of full length tau (29). However, this method has not been used to track tau aggregation intermediates in situ and in solution as performed in this study.…”

“…If R1 of Δtau187 at a specific position gets embedded in β-sheet structures, the evolving content of the β-sheets in which R1 labels of adjacent protein strands stack in parallel can be estimated by ESR line shape analysis. This is because, when packing in parallel β-sheet structures, the same R1-labeled sites from different protein strands come into intimate contact within 5-8 Å, yielding a characteristic single-line ESR spectral feature due to spin exchange between overlapping electron spin orbitals of the R1 labels (29,(36)(37)(38)(39)(40)(41). We can verify the assignment of this spectral signature to parallel β-sheet stacking by systematic "spin dilution" to yield a mixture where only 20% of the tau strands are labeled with paramagnetic R1 labels and 80% are labeled with the diamagnetic analog of R1 labels, designated here as R1', at a given site of Δtau187.…”

“…Therefore, the C terminus serves as an important control in this study. The spin-labeled Δtau187 samples were prepared either as undiluted samples, i.e., 100% R1-labeled, or as spin-diluted samples, i.e., by mixing 20% R1-labeled and 80% R1'-labeled Δtau187, all labeled at the same site (36)(37)(38)(39)(40)(41).…”