Hungry for quick and reliable biomarkers, AD researchers have tried hard to correlate measurements of plasma Aβ species with disease progression. However, studies of Aβ42 have produced conflicting results. Some have suggested that the peptide holds promise as a blood diagnostic marker (see, e.g., Mayeux et al., 2003), while others have not (see, e.g., Blasko et al., 2006). A recent effort to develop a highly sensitive assay for plasma Aβ also came up empty in this regard, failing to show that plasma Aβ levels could predict sporadic AD (Hansson et al., 2008). Yet another study suggested that a drop in plasma Aβ levels could signal impending dementia in familial AD patients (Ringman et al., 2008). “These often inconsistent reports on the association of plasma Aβ levels with AD may reflect the fact that measurements to date only represent the pools of monomeric Aβ and were measured by different Aβ ELISAs (Fullwood et al., 2006),” write Selkoe and colleagues.

Last year, Selkoe’s group showed that oligomeric forms of Aβ, particularly dimers, are in fact more neurotoxic than plaques (see ARF related news story). However, there were no good methods to specifically detect and quantify these soluble rabblerousers—until now. In the new study, lead author Weiming Xia and colleagues devised an ELISA that used a single monoclonal antibody for both capture and detection of Aβ. This ensured that the assay would only recognize Aβ assemblies containing at least two Aβ molecules. The researchers confirmed the ELISA’s specificity for oligomeric Aβ by putting through a synthetic Aβ peptide that could form reversible disulfide-crosslinked dimers, and checking for its monomeric and dimeric species on silver-stained protein gels. The assay “should be useful to detect and quantify soluble, low-n oligomers in a range of samples, including in postmortem human or APP transgenic mouse brains, in cell culture medium, and in human CSF and plasma,” Selkoe wrote in an e-mail to ARF.

Using this new ELISA and existing assays, Selkoe’s team measured oligomeric and several monomeric Aβ species in plasma samples from AD patients and control subjects. They found that oligomeric Aβ levels were closely linked to Aβ42 monomer levels across all subjects, and that levels of both were higher in the AD samples. They also observed that measurements of each species dropped in parallel over time. The findings suggest that “measuring plasma Aβ and oligomeric Aβ over one to two years or more can reveal a significant reduction in plasma Aβ levels, especially Aβ42 levels, and this finding raises the possibility of a direct relationship of plasma Aβ to brain amyloid formation,” the authors write.

Another way to get a handle on how Aβ peptides contribute to AD is to produce big batches of disease-associated forms of the protein for structural analysis. Such efforts require affordable sources of Aβ peptide. In their FEBS Journal paper, first author Walsh and colleagues offer up a promising approach. Using a bacterial expression system, the researchers churned out hefty supplies of very pure Aβ peptide without using specialized equipment. With this procedure, “even a small lab can make its own highly pure Aβ at minimal cost,” Walsh wrote in an e-mail to ARF. He noted that 30 mg of Aβ40 could be produced in less than a week for about $200, while 30 mg of Aβ42 could be made in that timeframe for about $500. “I think it has the potential to make a big difference to many AD investigators studying Aβ,” he wrote.—Esther Landhuis.

References:
Xia W, Yang T, Shankar G, Smith IM, Shen Y, Walsh DM, Selkoe DJ. A specific enzyme-linked immunosorbent assay for measuring beta-amyloid protein oligomers in human plasma and brain tissue of patients with Alzheimer disease. Arch Neurol. 2009 Feb;66(2):190-9. Abstract

Walsh DM, Thulin E, Minogue AM, Gustavsson N, Pang E, Teplow DB, Linse S. A facile method for expression and purification of the Alzheimer's disease-associated amyloid beta-peptide. FEBS J. 2009 Jan 27. Abstract