The β-amyloid (Aβ) peptide, a major component of senile plaques in Alzheimer's disease brain, has been shown previously to undergo a process of polymerization to produce neurotoxic forms of amyloid. Recent literature has attempted to define precisely the form of Aβ responsible for its neurodegenerative properties. In the present study we describe a novel density-gradient centrifugation method for the isolation and characterization of structurally distinct polymerized forms of Aβ peptide. Fractions containing protofibrils, fibrils, sheet structures and low molecular mass oligomers were prepared. The fractionated forms of Aβ were characterized structurally by transmission electron microscopy. The effects on cell viability of these fractions was determined in the B12 neuronal cell line and hippocampal neurons. Marked effects on cell viability in the cells were found to correspond to the presence of protofibrillar and fibrillar structures, but not to monomeric peptide or sheet-like structures of polymerized Aβ. Biological activity correlated with a positive reaction in an immunoassay that specifically detects protofibrillar and fibrillar Aβ; those fractions that were immunoassay negative had no effect on cell viability. These data suggest that the effect of Aβ on cell viability is not confined to a single conformational form but that both fibrillar and protofibrillar species have the potential to be active in this assay.
Present address: Pfizer Central Research, Ramsgate Road, Sandwich, Kent CT13 9NJ, U.K.
Present address: Cerebrus, Oakdene Court, 613 Reading Road, Winnersh, Wokingham, Surrey RG41 5UA, U.K.