Self-Replication of Transthyretin Amyloid Aggregates from Native Tetramers in vitro

Abstract

The conversion of proteins from their native conformations into amyloid aggregates with their beta -sheet enriched structures is associated with an increasing number of pathological conditions including neurodegenerative disease and systemic amyloidosis. Transthyretin (TTR) is a soluble human plasma protein that is linked to both senile systemic amyloidosis (SSA) and familial amyloid polyneuropathy (FAP) in humans. In vitro experiments have indicated that TTR amyloid aggregation requires the dissociation of the native TTR tetramer into a non-native monomer state. However, typical in-vitro experiments require the use of non-physiological, acidic solution conditions. This raises the question whether the mechanisms observed for TTR amyloid formation under harsh in-vitro conditions match those active in vivo.We have previously reported that lysozyme oligomers and curvilinear fibrils were capable of prion-like self-replication from native monomers near physiological solution conditions. We therefore investigated whether TTR amyloid oligomers and their curvilinear fibrils were capable of similar self-replication from their native tetrameric counterparts in vitro. We induced the formation of TTR oligomers and curvilinear fibrils during transient thermal denaturation in physiological saline. The morphologies and tinctorial characteristics of the resulting aggregates match those for TTR oligomers and curvilinear fibrils obtained by in vitro growth from acidic solutions. After mechanical separation of these aggregates, they were seeded into solutions of native TTR tetramers at physiological conditions (pH, temperature, ionic strength). Light scattering and thioflavin T responses indicated that TTR oligomers and their curvilinear fibrils, similar to our prior observations with lysozyme, induced conversion of the native TTR tetramers into amyloids. Our observations suggest that amyloid oligomers of different proteins have the capability for autocatalytic self-replication from their native counterparts, thereby resembling the behavior of prion proteins.