Oligomers, fact or artefact? SDS-PAGE induces dimerization of [beta]-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 Alzheimerâ[euro](TM)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]^sub 1â[euro]"40^Met^sub 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]^sub 1â[euro]"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.[PUBLICATION ABSTRACT]