Alzheimer's β-peptide oligomer formation at physiologic concentrations

When diluted from dimethyl sulfoxide or 1,1,1,3,3,3-hexafluoro-2-propanol, synthetic human Aβ(1-42) readily forms oligomeric structures at near physiologic concentrations (1-20 nM). Oligomers ≥40 kDa are detected in a sandwich enzyme-linked immunosorbant assay where the capture and detection antibodies recognize the same primary sequence epitope. Monomeric peptide with a single epitope does not react in this format. Aβ(1-40) peptide does not oligomerize readily under these conditions. The rate of oligomer formation has a steep linear temperature dependence but is weakly affected by ionic strength up to 0.5 M NaCl or KCl. Oligomer formation is inhibited by concentrations of Tween 20 and several other detergents well below their critical micelle concentrations. Once formed, high-molecular-weight oligomers are stabilized by Tween 20. Gel permeation chromatography of an oligomer preparation formed at nanomolar concentrations indicates that the majority of the Aβ(1-42) peptide chromatographs as monomers/dimers of apparent mw ∼10 kDa. The most abundant oligomers have apparent mobilities corresponding to 220 kDa (48-mer) and higher multiples of this without detectable concentrations of intermediate low-molecular-weight species. Very little immunoreactive peptide appears in the void volume (>1.5 MDa) of a Superose 12 column. The oligomers are stable, rechromatographing at their original position. Aβ(1-42) oligomer formation at physiologic concentrations is a reproducible process that is amenable to kinetic analysis and inhibition.