Structural Changes of Amyloid β-Protein Fibrils on Neuronal Cells

The abnormal aggregation of amyloid β-protein (Aβ) and resultant neuronal damage are central to the pathogenesis of Alzheimer's disease. Accumulating evidence suggests that neuronal cell membranes play a pivotal role in Aβ self-aggregation. We have shown that fibrils formed by Aβ-(1-40) and Aβ-(1-42) on GM1-containing model membranes (M-fibrils) as well as on neuronal cell membranes (C-fibrils) are more toxic compared with fibrils formed in water (W-fibrils), and that Aβ-(1-40) M-fibrils contain both in-register parallel and 2-residue-shifted antiparallel β-sheets. However, structural information on C-fibrils is lacking. In this study, structural changes of Aβ fibrils on living neuronal cells were detected by Fourier-transform infrared attenuated total reflection spectroscopy. Early Aβ-(1-40) C-fibrils contained antiparallel β-sheet structures, which changed to parallel β-sheet structures similar to W-fibrils as the fibril deposition proceeded. Pulse-chase experiments using two fluorescent-labeled Aβs suggested that these structural transitions occurred continuously from the existing fibrils. Considering the fact that Aβ fibrils start to form on the cell surface and extend into the aqueous phase, such environmental changes around fibrils may be coupled with structural alterations. In contrast, Aβ-(1-42) C-fibrils retained their antiparallel β-sheet structures. For both Aβ-(1-40) and Aβ-(1-42), cell viability continued to decrease even during the structural alterations of Aβ-(1-40), indicating that the antiparallel β-sheet-containing fibrils attaching to the cell surface were responsible for the lasting cytotoxicity. Thus, the antiparallel β-sheet-containing early fibrils are a promising target for anti-Aβ therapy.