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

: 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, 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. Amyloid (cid:1) -protein (A (cid:1) ) is linked to neuronal injury and death in Alzheimer’s disease (AD). Of particular relevance for elucidating the role of A (cid:1) in AD is new evidence that oligomeric forms of A (cid:1) are potent neurotoxins that play a major role in neurodegeneration and the strong association of the 42-residue form of A (cid:1) , A (cid:1) 42, with the disease. Detailed knowledge of the structure and assembly dynamics of A (cid:1) thus is important for the development of properly targeted AD therapeutics. Recently, we have shown that A (cid:1) 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 A (cid:1) 40andA (cid:1) 42.Carefullypreparedaggregate-freeA (cid:1) 40existedas monomers, dimers, trimers, and tetramers, in rapid equilibrium. In contrast, A (cid:1) 42 preferentially formed pentamer (cid:1) hexamer units (paranuclei) that assembled further to form beaded superstructures similar to early protoﬁbrils. Addition of Ile-41 to A (cid:1) 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 (cid:1) 42 assembly involves formation of several distinct transient structures that gradually rearrange into protoﬁ-brils. The strong etiologic association of A (cid:1) 42 with AD may thus be a result of assemblies formed at the earliest stages of peptide oligomerization. provides an the comprise multiples octadecamer) of dominant hexamers