Elucidating Molecular Mass and Shape of a Neurotoxic Aβ Oligomer

Alzheimer's disease (AD), the most prevalent type of dementia, has been associated with the accumulation of amyloid beta oligomers (A beta Os) in the central nervous system. A beta Os vary widely in size, ranging from dimers to larger than 100 kDa. Evidence indicates that not all oligomers are toxic, and there is yet no consensus on the size of the actual toxic oligomer. Here we used NU4, a conformation-dependent anti-A beta O monoclonal antibody, to investigate size and shape of a toxic A beta O assembly. By using size-exclusion chromatography and immuno-based detection, we isolated an A beta O-NU4 complex amenable for biochemical and morphological studies. The apparent molecular mass of the NU4-targeted oligomer was 80 kDa. Atomic force microscopy imaging of the A beta O-NU4 complex showed a size distribution centered at 5.37 rim, an increment of 1.5 nm compared to the size of A beta Os (3.85 urn). This increment was compatible with the size of NU4 (1.3 nm), suggesting a 1:1 oligomer to NU4 ratio. NU4-reactive oligomers extracted from AD human brain concentrated in a molecular mass range similar to that found for in vitro prepared oligomers, supporting the relevance of the species herein studied. These results represent an important step toward understanding the connection between A beta O size and toxicity.