Structural Studies of Alzheimer's beta-Amyloid Fibrils

National Institute Of Diabetes And Digestive And Kidney Diseases

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Abstract

Progress in FY2021 has been in the following areas: (1) AMYLOID-BETA FIBRIL STRUCTURES FROM CRYO-ELECTRON MICROSCOPY: We have obtained high-quality cryo-EM images of 42-residue amyloid-beta (Ab42) fibrils obtained by seeding with amyloid-enriched extract from human brain tissue. To our knowledge, these are the first cryo-EM images of brain-derived Ab42 fibrils. Preliminary analyses of the images, using improved helical reconstruction software developed in our lab in FY2020, indicates a two-fold-symmetric structure in which each Ab42 molecule adopts a U-shaped conformation, with N-termini exposed on the fibril surface and C-termini buried in the fibril core. This structure closely resembles two-fold structural models for amyloid-beta fibrils grown in vitro that were developed by our lab about 15 years ago from solid state NMR data, but is significantly different from structures of Ab42 fibrils grown in vitro that have been reported by other labs, based on either solid state NMR or cryo-EM measurements. Thus, this is a new fold for Ab42 fibrils, and is especially relevant to Alzheimer's disease (AD) because our fibrils were derived from cortical tissue of AD patients. Additional image analysis and structure refinement are ongoing. We expect to publish these results within the next several months. (2) COMPARISON OF BRAIN-DERIVED AMYLOID-BETA FIBRILS FROM ALZHEIMER'S DISEASE PATIENTS AND FROM NON-DEMENTED ELDERLY INDIVIUALS: Following experimental methods that we have used previously to amplify and isotopically label 40- and 42-residue amyloid-beta (Ab40 and Ab42) fibrils from cortical tissue of AD patients (see Qiang et al., Nature 2017), we have prepared Ab40 and Ab42 fibrils from tissue of non-demented individuals with high amyloid loads, obtained from the Religious Orders Study of the Rush Alzheimer's Disease Center. We have obtained high-quality 2D solid state NMR spectra of all brain-derived fibrils and performed quantitative analyses to compare the 2D spectra of fibrils from non-demented individuals with 2D spectra of fibrils from AD patients. We find that the 2D spectra are similar, but have statistically significant differences on average, especially for Ab42 fibrils. These results indicate that non-demented elderly individuals with high amyloid loads (sometimes called individuals with pathological aging) have the same or similar amyloid-beta fibril polymorphs in their brain tissue, but with subtle differences in the relative populations of certain polymorphs. These results do not provide strong support for the hypothesis that certain individuals develop AD and others remain cognitively normal because the amyloid structures in their brains are qualitatively different. Rather, these results suggest that other factors, such as cognitive reserve, may distinguish those who develop AD from those who do not. These results have been submitted for publication. We expect these results to be published within the next several weeks. (3) MOLECULAR STRUCTURE OF FIBRILS FORMED BY A RACEMIC MIXTURE OF D-AB40 AND L-AB40: In collaboration with Prof. Raskatov of the University of California at Santa Cruz, we have performed solid state NMR measurements on fibrils formed by D,L-Ab40, a 1:1 racemic mixture of D-Ab40 and L-Ab40. Earlier work from the Raskatov lab showed that D,L-Ab40 fibrils form more rapidly and are more stable (i.e., form at lower concentrations) than naturally-occurring L-Ab40. The solid state NMR data support an rippled beta-sheet structure within the D,L-Ab40 fibrils, in which D-Ab40 and L-Ab40 alternate within beta-sheets along the fibril growth direction, with antiparallel alignment of neighboring peptide chains. Such a structure closely resembles that of metastable L-Ab40 protofibrils that our lab had demonstrated previously (Qiang et al., PNAS 2012), but has not been seen previously in thermodynamically stable amyloid-beta fibrils. These results have been published in JACS.