A Mass Spectrometric Approach for Characterization of Amyloid-β Aggregates and Identification of Their Post-Translational Modifications

Endogenous amyloid-beta (A beta) oligomeric aggregates have been proposed as toxic agents in Alzheimer's disease (AD). Knowledge of their structures not only may provide insight into the basis of their neurotoxicities but also may reveal new targets for therapeutic drugs and diagnostic tools. However, the low levels of these A beta oligomers have impeded structural characterization. Evidence suggests that the endogenous oligomers are covalently modified in vivo. In this report, we demonstrate an established mass spectrometry (MS) methodology called precursor ion mapping (PIM) that potentially may be applied to endogenous oligomer characterization. First, we illustrate the use of this PIM technique with a synthetic A beta(1-40) monomer sample that had been cross-linked with transglutaminase (TGase) and digested with pepsin. From PIM analysis of an A beta(4-13) MS/MS fragment, precursor ions were identified that corresponded to peptic fragments of three TGase cross-linked species: A beta(4-19)-(4-19), A beta(4-19)-(20-34), and A beta(1-19)-(20-34). Next, we demonstrate the applicability of the PIM technique to an endogenous A beta sample that had been purified and concentrated by immunoaffinity chromatography. Without pepsin digestion, we successfully identified the full length and C-terminally truncated monomeric A beta species 1-35 to 1-42, along with select methionine-oxidized counterparts. Because PIM focuses only on a subpopulation of ions, namely the related precursor ions, the resulting spectra are of increased specificity and sensitivity. Therefore, this methodology shows great promise for structural analysis and identification of post-translational modification(s) in endogenous A beta oligomers.