High-molecular weight Aβ oligomers and protofibrils are the predominant Aβ species in the native soluble protein fraction of the AD brain

Alzheimers disease (AD) is characterized by the aggregation and deposition of amyloid beta protein (A beta) in the brain. Soluble A beta oligomers are thought to be toxic. To investigate the predominant species of A beta protein that may play a role in AD pathogenesis, we performed biochemical analysis of AD and control brains. Sucrose buffer-soluble brain lysates were characterized in native form using blue native (BN)-PAGE and also in denatured form using SDS-PAGE followed by Western blot analysis. BN-PAGE analysis revealed a high-molecular weight smear (>1000 kD) of A beta 42-positive material in the AD brain, whereas low-molecular weight and monomeric A beta species were not detected. SDS-PAGE analysis, on the other hand, allowed the detection of prominent A beta monomer and dimer bands in AD cases but not in controls. Immunoelectron microscopy of immunoprecipitated oligomers and protofibrils/fibrils showed spherical and protofibrillar A beta-positive material, thereby confirming the presence of high-molecular weight A beta (hiMWA beta) aggregates in the AD brain. In vitro analysis of synthetic A beta 40- and A beta 42 preparations revealed A gamma fibrils, protofibrils, and hiMWA beta oligomers that were detectable at the electron microscopic level and after BN-PAGE. Further, BN-PAGE analysis exhibited a monomer band and less prominent low-molecular weight A beta (loMWA beta) oligomers. In contrast, SDS-PAGE showed large amounts of loMWA beta but no hiMWA beta 40 and strikingly reduced levels of hiMWA beta 42. These results indicate that hiMWA beta aggregates, particularly A beta 42 species, are most prevalent in the soluble fraction of the AD brain. Thus, soluble hiMWA beta aggregates may play an important role in the pathogenesis of AD either independently or as a reservoir for release of loMWA beta oligomers.