Mixtures of Wild-type and a Pathogenic (E22G) Form of Aβ40 in Vitro Accumulate Protofibrils, Including Amyloid Pores

Although APP mutations associated with inherited forms of Alzheimerʹs disease (AD) are relatively rare, detailed studies of these mutations may prove critical for gaining important insights into the mechanism(s) and etiology of AD. Here, we present a detailed biophysical characterization of the structural properties of protofibrils formed by the Arctic variant (E22G) of amyloid-β protein (Aβ40ARC) as well as the effect of Aβ40WT on the distribution of the protofibrillar species formed by Aβ40ARC by characterizing biologically relevant mixtures of both proteins that may mimic the situation in the heterozygous patients. These studies revealed that the Arctic mutation accelerates both Aβ oligomerization and fibrillogenesis in vitro. In addition, Aβ40ARC was observed to affect both the morphology and the size distribution of Aβ protofibrils. Electron microscopy examination of the protofibrils formed by Aβ40ARC revealed several morphologies, including: (1) relatively compact spherical particles roughly 4–5 nm in diameter; (2) annular pore-like protofibrils; (3) large spherical particles 18–25 nm in diameter; and (4) short filaments with chain-like morphology. Conversion of Aβ40ARC protofibrils to fibrils occurred more rapidly than protofibrils formed in mixed solutions of Aβ40WT/Aβ40ARC, suggesting that co-incubation of Aβ40ARC with Aβ40WT leads to kinetic stabilization of Aβ40ARC protofibrils. An increase in the ratio of AβWT/AβMUT(Arctic), therefore, may result in the accumulation of potential neurotoxic protofibrils and acceleration of disease progression in familial Alzheimerʹs disease mutation carriers.