Oligomers, fact or artefact? SDS-PAGE induces dimerization of β-amyloid in human brain samples

The formation of low-order oligomers of beta-amyloid (A beta) within the brain is widely believed to be a central component of Alzheimer's disease (AD) pathogenesis. However, despite advances in high-throughput and high-resolution techniques such as xMAP and mass spectrometry (MS), investigations into these oligomeric species have remained reliant on low-resolution Western blots and enzyme-linked immunosorbent assays. The current investigation compared A beta profiles within human cortical tissue using sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis (PAGE), xMAP and surface enhanced laser desorption/ionization time-of-flight MS and found that whilst there was significant correlation across the techniques regarding levels of monomeric A beta, only SDS-PAGE was capable of detecting dimeric isoforms of A beta. The addition of synthetic di-tyrosine cross-linked A beta(1-40)Met(35)(O) to the AD tissue demonstrated that the MS methodology was capable of observing dimeric A beta at femto-molar concentrations, with no noticeable effect on monomeric A beta levels. Focus turned to the association between SDS-PAGE and levels of observable dimeric A beta within the AD brain tissue. These investigations revealed that increased levels of dimeric A beta were observed with increasing concentrations of SDS in the sample buffer. This finding was subsequently confirmed using synthetic A beta(1-42) and suggests that SDS was inducing the formation of dimeric A beta. The findings that SDS promotes A beta dimerization have significant implications for the putative role of low-order oligomers in AD pathogenesis and draw into question the utility of oligomeric A beta as a therapeutic target.