Amyloid -protein (A ) assembly: A 40 and A 42 oligomerize through distinct pathways

Amyloid -protein (A ) is linked to neuronal injury and death in Alzheimer’s disease (AD). Of particular relevance for elucidating the role of A in AD is new evidence that oligomeric forms of A are potent neurotoxins that play a major role in neurodegeneration and the strong association of the 42-residue form of A , A 42, with the disease. Detailed knowledge of the structure and assembly dynamics of A thus is important for the development of properly targeted AD therapeutics. Recently, we have shown that A 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, sizeexclusion chromatography, dynamic light scattering, circular dichroism spectroscopy, and electron microscopy have been combined to elucidate fundamental features of the early assembly of A 40 and A 42. Carefully prepared aggregate-free A 40 existed as monomers, dimers, trimers, and tetramers, in rapid equilibrium. In contrast, A 42 preferentially formed pentamer hexamer units (paranuclei) that assembled further to form beaded superstructures similar to early protofibrils. Addition of Ile-41 to A 40 was sufficient to induce formation of paranuclei, but the presence of Ala-42 was required for their further association. These data demonstrate that A 42 assembly involves formation of several distinct transient structures that gradually rearrange into protofibrils. The strong etiologic association of A 42 with AD may thus be a result of assemblies formed at the earliest stages of peptide oligomerization.