Bacterial inclusion bodies of Alzheimer's disease: β-amyloid peptides can be employed to study native-like aggregation intermediate states.

The structures of oligomeric intermediate states in the aggregation process of Alzheimer's disease β-amyloid peptides have been the subject of debate for many years. Bacterial inclusion bodies contain large amounts of small heat shock proteins (sHSPs), which are highly homologous to those found in the plaques of the brains of Alzheimer's disease patients. sHSPs break down amyloid fibril structure in vitro and induce oligomeric assemblies. Prokaryotic protein overexpression thus mimics the conditions encountered in the cell under stress and allows the structures of Aβ aggregation intermediate states to be investigated under native-like conditions, which is not otherwise technically possible. We show that IB40/IB42 fulfil all the requirements to be classified as amyloids: they seed fibril growth, are Congo red positive and show characteristic β-sheet-rich CD spectra. However, IB40 and IB42 are much less stable than fibrils formed in vitro and contain significant amounts of non-β-sheet regions, as seen from FTIR studies. Quantitative analyses of solution-state NMR H/D exchange rates show that the hydrophobic cores involving residues V18-F19-F20 adopt β-sheet conformations, whereas the C termini adopt α-helical coiled-coil structures. In the past, an α-helical intermediate-state structure has been postulated, but could not be verified experimentally. In agreement with the current literature, in which Aβ oligomers are described as the most toxic state of the peptides, we find that IB42 contains SDS-resistant oligomers that are more neurotoxic than Aβ42 fibrils. E. coli inclusion bodies formed by the Alzheimer's disease β-amyloid peptides Aβ40 and Aβ42 thus behave structurally like amyloid aggregation intermediate states and open the possibility of studying amyloids in a native-like, cellular environment