One of the possible mechanisms of amyloid fibrils formation based on the sizes of primary and secondary folding nuclei of Aß40 and Aß42.

In the presented paper, theoretical as well as electron microscopy and X-ray diffraction experimental approaches were employed for studding the process of Aß amyloid formation. Using quantitative estimates of a number of monomers which form the nuclei of amyloid fibrils the sizes of folding nuclei of amyloid fibrils for Aß40 and 42 have been determined for the first time. We have shown that the size of the primary nucleus of Aß42 peptide fibrils corresponds to 3 monomers, the size of the secondary nucleus for this peptide is 2 monomers. Applying the same analysis to Aß40 we conclude that the size of the primary nucleus is 2 monomers, and the size of the secondary nucleus is one monomer. Summation of our theoretical and experimental results has allowed us to propose a new model of the structural organization of amyloid fibrils. Our model suggests that the generation of fibrils takes place along the following simplified pathway: a monomer→a ring oligomer→a mature fibril consisting of ring oligomers. These data shed more light upon our understanding of what sizes of the oligomers could represent main targets for future therapies (tetramers for Aß42 and trimers for Aß40), and aid in the development of inhibitors of Aß40 and 42 oligomer formation.