Capturing transient peptide assemblies associated with Alzheimer's disease : Native mass spectrometry studies of amyloid-β oligomerization

Abstract: Correct folding of proteins is essential for maintaining a functional living cell. Misfolding and aggregation of proteins, where non-native intermolecular interactions form large and highly ordered amyloid aggregates with low free energy, is hence associated with multiple diseases. One example is Alzheimer’s disease (AD) where the Amyloid-β (Aβ) peptide aggregates into amyloid fibrils, which deposit as neuritic plaques in the brains of AD patients. Nucleation of amyloid fibrils takes place via formation of smaller pre-nucleation clusters, so-called oligomers, which are considered to be especially toxic and are therefore potentially important in AD pathology. Detailed mechanistic molecular knowledge of Aβ aggregation is important for design of AD treatments that target these processes. The oligomeric species are however challenging to study experimentally due to their low abundance and high polydispersity.  Aβ oligomers are in this thesis studied under controlled in vitro conditions using bottom-up biophysics. Highly pure recombinant Aβ peptides are studied, primarily using native ion-mobility mass spectrometry, to monitor the spontaneous formation of oligomers in aqueous solution. Mass spectrometry is capable of resolving individual oligomeric states, while ion mobility provides low-resolution structure information. This is complemented with other biophysical techniques, as well as theoretical modeling. The oligomers are also studied upon modulating intrinsic factors, such as peptide length and sequence, or extrinsic factors, such as the chemical environment. Interactions with two important biological interaction partners are studied: chaperone proteins and cell membranes.  We show how Aβ oligomers assemble, and form extended structures which may be linked to continued growth into amyloid fibrils. We also show how different amyloid chaperone proteins interact with growing aggregates, which modifies and delays the aggregation process. These interactions are shown to depend on specific sequence-motifs in the chaperones and client peptides. Membrane-mimicking micelles are on the other hand able to stabilize globular compact forms of the Aβ oligomers and to inhibit the formation of extended structures which nucleate into amyloid fibrils. This may contribute to enrichment of toxic species in vivo. Interactions with membrane-mimicking systems are shown to be highly dependent on both the Aβ peptide isoform and the properties of the membrane environment, such as headgroup charges. It is also demonstrated how addition of a designed small peptide construct can inhibit formation of Aβ oligomers in the membrane environment.