Supplementary Material for: Direct Evidence for Self-Propagation of Different Amyloid-β Fibril Conformations

<b><i>Background:</i></b> Amyloid fibrils formed by amyloid-β (Aβ) peptides are associated with Alzheimer's disease and can occur in a range of distinct morphologies that are not uniquely determined by the Aβ sequence. Whether distinct conformations of Aβ fibrils can be stably propagated over multiple cycles of seeding and fibril growth has not been established experimentally. <b><i>Objective:</i></b> The ability of the 40-residue peptide Aβ<sub>1-40</sub> to assemble into fibrils with the conformation of the mutant Aβ<sub>1-40</sub> peptide containing the ‘Osaka' mutation E22Δ was investigated. <b><i>Methods:</i></b> Fibril formation of highly pure, recombinant Aβ<sub>1-40</sub> in the presence of distinct, preformed seeds in vitro was recorded with thioflavin T fluorescence, and distinct fibrillar structures were identified and distinguished by fluorescence spectroscopy and electron microscopy. <b><i>Results:</i></b> We propagated the specific quaternary structure of Aβ<sub>1-40</sub> E22Δ fibrils with wild-type Aβ<sub>1-40</sub> over up to seven cycles of seeding and fibril elongation. As a result of a 10<sup>7</sup>-fold dilution of the initially present Aβ<sub>1-40</sub> E22Δ seeds, the vast majority of fibrils formed after the seventh propagation cycle with Aβ<sub>1-40</sub> did not contain a single molecule of Aβ<sub>1-40</sub> E22Δ, but still retained the conformation of the initial Aβ<sub>1-40</sub> E22Δ seeds. Increased critical concentrations of Aβ<sub>1-40</sub> fibrils formed in the presence of Aβ<sub>1-40</sub> E22Δ nuclei suggest that these fibrils are less stable than homologously seeded Aβ<sub>1-40</sub> fibrils, consistent with a kinetically controlled mechanism of fibril formation. <b><i>Conclusion:</i></b> The propagation of a distinct Aβ fibril conformation over multiple cycles of seeded fibril growth demonstrates the basic ability of the Aβ peptide to form amyloid strains that in turn may cause phenotypes in Alzheimer's disease.