ABSTRACT
Brain-derived amyloid-ß (Aß) dimers are associated with Alzheimer's disease (AD). However, their covalent nature remains controversial. This feature is relevant, as a covalent cross-link has been proposed to make brain-derived dimers (brain dimers) more synaptotoxic than Aß monomers and would also make them suitable candidates for biomarker development. To resolve this controversy, we here present a three-step approach. First, we validated a type of synthetic cross-linked Aß (CL Aß) dimers, obtained by means of the photoinduced cross-linking of unmodified proteins (PICUP) reaction, as well-defined mimics of putative brain CL Aß dimers. Second, we used these PICUP CL Aß dimers as standards to improve the isolation of brain Aß dimers and to develop state-of-the-art mass spectrometry (MS) strategies to allow their characterization. Third, we applied these MS methods to the analysis of brain Aß dimer samples allowing the detection of the CL [Aß(6-16)]2 peptide comprising a dityrosine cross-link. This result demonstrates the presence of CL Aß dimers in the brains of patients with AD and opens up avenues for establishing new therapeutic targets and developing novel biomarkers for this disease.
Subject(s)
Alzheimer Disease/metabolism , Amyloid beta-Peptides/chemistry , Brain Chemistry , Brain/metabolism , Brain/pathology , Protein Multimerization , Alzheimer Disease/pathology , Amyloid beta-Peptides/metabolism , Humans , Mass Spectrometry , Tyrosine/analogs & derivatives , Tyrosine/chemistryABSTRACT
The characterization of amyloid-beta peptide (Aß) oligomer forms and structures is crucial to the advancement in the field of Alzheimer´s disease (AD). Here we report a critical evaluation of two methods used for this purpose, namely sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), extensively used in the field, and ion mobility coupled to electrospray ionization mass spectrometry (ESI-IM-MS), an emerging technique with great potential for oligomer characterization. To evaluate their performance, we first obtained pure cross-linked Aß40 and Aß42 oligomers of well-defined order. Analysis of these samples by SDS-PAGE revealed that SDS affects the oligomerization state of Aß42 oligomers, thus providing flawed information on their order and distribution. In contrast, ESI-IM-MS provided accurate information, while also reported on the chemical nature and on the structure of the oligomers. Our findings have important implications as they challenge scientific paradigms in the AD field built upon SDS-PAGE characterization of Aß oligomer samples.
