New ELISAs with high specificity for soluble oligomers of amyloid β-protein detect natural Aβ oligomers in human brain but not CSF

Background: Soluble oligomers of amyloid beta-protein (AB) have been increasingly linked to synaptic dysfunction, tau alteration, and neuritic dystrophy in Alzheimer's disease (AD) and mouse models. There is a great need for assays that quantify A beta oligomers with high specificity and sensitivity. Methods: We designed and validated two oligomer-specific (o-) enzyme-linked immunoassays (ELISAs) using either an A beta aggregate-selective monoclonal for capture and a monoclonal to the free N-terminus for detection, or the latter antibody for both capture and detection. Results: The o-ELISAs specifically quantified pure oligomers of synthetic A beta with sizes from dimers up to much larger assemblies and over a wide dynamic range of concentrations, whereas A beta monomers were undetectable. Natural A beta oligomers of similarly wide size and concentration ranges were measured in extracts of AD and control brains, revealing >1000-fold higher concentrations of A beta oligomers than monomers in the soluble fraction of AD cortex. The assays quantified the age-related rise in oligomers in hAPP transgenic mice. Unexpectedly, none of 90 human cerebrospinal fluid (CSF) samples gave a specific signal in either o-ELISA. Conclusions: These new o-ELISAs with rigorously confirmed specificity can quantify oligomer burden in human and mouse brains for diagnostic and mechanistic studies and for AD biomarker development. However, our data raise the likelihood that the hydrophobicity of A beta oligomers makes them very low in number or absent in aqueous CSF. (c) 2013 The Alzheimer's Association. All rights reserved.