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Cross-seeding Controls Aβ Fibril Populations and Resulting Function

Amyloid peptides nucleate from monomers to aggregate into fibrils through primary nucleation; pre-existing fibrils can then act as seeds for additional monomers to fibrillize through secondary nucleation. Both nucleation processes can occur simultaneously, yielding a distribution of fibril polymorphs that can generate a spectrum of neurodegenerative effects. Understanding the mechanisms driving polymorph structural distribution during both nucleation processes is important for uncovering fibril structure-function relationships, as well creating polymorph distributions in vitro that better match distributions found in vivo. Here, we explore how cross-seeding WT Aβ1-40 with Aβ1-40 mutants E22G (Arctic) and E22Δ (Osaka), as well as with WT Aβ1-42 affects the distribution of fibril structural polymorphs, and how changes in structural distribution impact toxicity. Transmission electron microscopy analysis reveals that fibril seeds derived from mutants of Aβ1-40 impart their structure to WT Aβ1-40 monomer during secondary nucleation, but WT Aβ1-40 fibril seeds do not affect the structure of fibrils assembled from mutant Aβ1-40 monomers, despite kinetics data indicating accelerated aggregation when cross-seeding of any combination of mutants. Additionally, WT Aβ1-40 fibrils seeded with mutant fibrils to produce similar structural distributions to the mutant seeds also produced similar cytotoxicity on neuroblastoma cell lines. This indicates that mutant fibril seeds not only impart their structure to growing WT Aβ1-40 aggregates, but they also impart cytotoxic properties. Our findings provide clear evidence that there is a relationship between fibril structure and phenotype on a polymorph population level, and that these properties can be passed on through secondary nucleation of succeeding generations of fibrils.