Amyloid β-protein (Aβ) assembly: Aβ40 and Aβ42 oligomerize through distinct pathways

TLDR

Amyloid β‑protein (Aβ) is implicated in neuronal injury and death in Alzheimer’s disease, with oligomeric forms—especially the 42‑residue Aβ42—acting as potent neurotoxins and strongly associated with the disease. The study aims to elucidate the structure and assembly dynamics of Aβ to inform the design of targeted Alzheimer’s therapeutics. Using photo‑induced cross‑linking of unmodified proteins (PICUP) alongside size‑exclusion chromatography, dynamic light scattering, circular dichroism, and electron microscopy, the authors investigated the early assembly of Aβ40 and Aβ42. Aβ40 exists as monomers, dimers, trimers, and tetramers in rapid equilibrium, whereas Aβ42 preferentially forms pentamer/hexamer paranuclei that assemble into beaded protofibril‑like superstructures, with Ile‑41 inducing paranuclei in Aβ40 and Ala‑42 required for further association, indicating that distinct transient structures in Aβ42 assembly may underlie its stronger pathogenicity.

Abstract

Amyloid β-protein (Aβ) is linked to neuronal injury and death in Alzheimer's disease (AD). Of particular relevance for elucidating the role of Aβ in AD is new evidence that oligomeric forms of Aβ are potent neurotoxins that play a major role in neurodegeneration and the strong association of the 42-residue form of Aβ, Aβ42, with the disease. Detailed knowledge of the structure and assembly dynamics of Aβ thus is important for the development of properly targeted AD therapeutics. Recently, we have shown that Aβ oligomers can be cross-linked efficiently, and their relative abundances quantified, by using the technique of photo-induced cross-linking of unmodified proteins (PICUP). Here, PICUP, size-exclusion chromatography, dynamic light scattering, circular dichroism spectroscopy, and electron microscopy have been combined to elucidate fundamental features of the early assembly of Aβ40 and Aβ42. Carefully prepared aggregate-free Aβ40 existed as monomers, dimers, trimers, and tetramers, in rapid equilibrium. In contrast, Aβ42 preferentially formed pentamer/hexamer units (paranuclei) that assembled further to form beaded superstructures similar to early protofibrils. Addition of Ile-41 to Aβ40 was sufficient to induce formation of paranuclei, but the presence of Ala-42 was required for their further association. These data demonstrate that Aβ42 assembly involves formation of several distinct transient structures that gradually rearrange into protofibrils. The strong etiologic association of Aβ42 with AD may thus be a result of assemblies formed at the earliest stages of peptide oligomerization.