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Biophysical comparison of soluble amyloid-β(1-42) protofibrils, oligomers, and protofilaments.

Some of the pathological hallmarks of the Alzheimer's disease brain are senile plaques composed of insoluble amyloid-β protein (Aβ) fibrils. However, much of the recent emphasis in research has been on soluble Aβ aggregates in response to a growing body of evidence that shows that these species may be more neurotoxic than fibrils. Within this subset of soluble aggregated Aβ are protofibrils and oligomers. Although each species has been widely investigated separately, few studies have directly compared and contrasted their physical properties. In this work, we examined well-recognized preparations of Aβ(1-42) oligomers and protofibrils with multiangle (MALS) and dynamic (DLS) light scattering in line with, or following, size-exclusion chromatography (SEC). Multiple SEC-MALS analyses of protofibrils revealed molecular weight (Mw) gradients ranging from 200 to 2600 kDa. Oligomeric Aβ species are generally considered to be a smaller and more nascent than protofibrils. However, oligomer Mw values ranged from 225 to 3000 kDa, larger than that for protofibrils. Root-mean-square radius (Rg) values correlated with the Mw trends with protofibril Rg values ranging from 16 to 35 nm, while oligomers produced one population at 40-43 nm with a more disperse population from 22 to 39 nm. Hydrodynamic radius (RH) measurements by DLS and thioflavin T fluorescence measurements indicated that protofibrils and oligomers had commonalities, yet electron microscopy revealed morphological differences between the two. SEC-purified Aβ(1-42) monomer at lower concentrations was slower to nucleate but formed protofibrils (1500 kDa) or soluble protofilaments (3000 kDa) depending on the buffer type. The findings from these studies shed new light on the similarities and differences between distinct soluble aggregated Aβ species.