Monitoring oligomer formation from self-aggregating amylin peptides using ESI-IMS-MS
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Amyloid diseases are a serious cause for concern world-wide. To understand the mechanism of formation of the fibrillar structures associated with such disorders, it is necessary to study the progression from soluble protein or peptide monomer through an array of oligomers to the final, insoluble, fibrils. The protein IAPP is found in vivo in the form of insoluble amyloid deposits in the pancreatic islets of diabetes type II sufferers. Here, we have studied the in vitro self-aggregation of three fibril-forming peptides from the amyloidogenic core of IAPP. Using electrospray ionization—mass spectrometry coupled with ion mobility spectrometry, the mass and cross-sectional area of each oligomer present in the heterogeneous assembly mixtures can be determined individually in a single, rapid experiment over time. For the three peptides studied, oligomers ≤20-mer were characterized. Conversely, no oligomers higher than a dimer were detected for a non-assembling peptide control. The rate in which the cross-sectional area of the oligomers increases with increasing number of peptide sub-units indicates that assembly for the amyloid-forming peptides proceeds in a linear fashion until an oligomer of a certain size is attained. After this, a step increase in cross-sectional area occurs for the next higher-order oligomer. This behaviour can be explained by molecular modelling of singly, doubly, triply and quadruply stacked β-stranded structures. Using one peptide as an example, the cross-sectional areas of the lower order oligomers (dimer to pentamer) were found to be consistent with a single β-sheet model, whereas the higher order oligomers were consistent with double-stranded (hexamer to decamer oligomers), triply-stranded (11-mers to 15-mers) and quadruply-stranded (16-mers to 20-mers) β-sheet models.
KeywordsIon mobility spectrometry-mass spectrometry Islet amyloid polypeptide Amyloid Fibril Molecular modelling
LY is funded by a Biotechnology and Biological Sciences Research Council CASE studentship (Grant Number BB/I015361/1) sponsored by Micromass UK Ltd/Waters Corpn, Manchester, UK; HN and TWK were funded by Engineering and Physical Sciences Research Council White Rose studentships (Grant numbers EP/G500010/1 and EP/E501869/1, respectively). The Synapt HDMS mass spectrometer was purchased with funds from the Biotechnology and Biological Sciences Research Council through its Research Equipment Initiative scheme (BB/E012558/1). We thank all members of the Ashcroft, Harris and Radford groups for helpful discussions.
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