Structural Studies of Alzheimer's beta-Amyloid Fibrils

National Institute Of Diabetes And Digestive And Kidney Diseases

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Abstract

Progress in FY2025 has been in the following area: (1) NEW MOLECULAR STRUCTURES OF BRAIN-SEEDED AMYLOID-BETA FIBRILS: In previous years, we developed methods for generating 42-residue and 40-residue amyloid-beta fibrils (Abeta42 and Abeta40) by seeded growth, using amyloid fibrils from autopsied brain tissue from Alzheimer's disease (AD) patients as the seeds. We had also shown that molecular structural differences between amyloid fibril polymorphs propagate themselves in seeded fibril growth. Therefore, we believe that brain-seeded fibrils, grown from fibrils in human brain tissue, allow us to probe amyloid-beta structures that develop in human AD tissue. In earlier work, we used cryogenic electron microscopy (cryo-EM) to solve the structures of two prevalent Abeta42 fibril polymorphs from AD tissue. These two polymorph structures were markedly different from Abeta42 polymorph structures reported by researchers at the MRC laboratory in Cambridge, UK, based on cryo-EM studies of fibrils that were directly extracted from brain tissue, rather than seeded with fibrils from brain tissue. (Although direct extraction could in principle be the preferred approach, in practice direct extraction requires that most of the amyloid fibril material in brain tissue be discarded or remain unanalyzed, raising questions about selectivity.) In FY2025, we have determined two additional polymorph structures in our brain-seeded Abeta42 fibril samples. The two new structures resemble those reported by the MRC scientists, but are still different in a number of ways. A manuscript describing these results will be submitted for publication early in FY2026. This work contributes to our understanding of the range of possible Abeta42 polymorph structures and our understanding of the relationship between brain-seeded and brain-extracted fibrils. (2) NEW MOLECULAR STRUCTURES OF AMYLOID-BETA FIBRILS GROWN IN VITRO: In FY2025, we have also performed new cryo-EM measurements on Abeta40 fibrils that were grown entirely in vitro, without involvement of brain material. The focus of this work is on rapidly-twisting Abeta40 fibril polymorphs, which had not been characterized structurally in any previous work. For this work, we developed in vitro fibril growth conditions that produce a high percentage of rapidly-twisting fibrils, which had been a minor species in earlier studies. The cryo-EM images, surprisingly, showed that there are several distinct varieties of rapidly-twisting fibrils and that the Abeta40 monomer conformations within all of these polymorphs are similar. In one rapidly-twisting polymorph, the two cross-beta subunits of the fibril structure are inequivalent, rather than being related by two-fold rotational or screw symmetry. This inequivalence of subunits had not been seen in any previous Abeta40 or Abeta42 structure. A manuscript describing these results will be submitted for publication early in FY2026.