GM1 Cluster Mediates Formation of Toxic Aβ Fibrils by Providing Hydrophobic Environments

The conversion of soluble, nontoxic amyloid beta-proteins (A beta) to aggregated, toxic forms rich in beta-sheets is considered to be a key step in the development of Alzheimer's disease. Accumulating evidence suggests that lipid-protein interactions play a crucial role in the aggregation of amyloidogenic proteins like A beta. Our group has previously reported that amyloid fibrils of A beta formed on membranes containing clusters of GM1 ganglioside (M-fibrils) exhibit greater cytotoxicity than fibrils formed in aqueous solution (W-fibrils) [Okada et al. (2008) J. Mol. Biol. 382, 1066-1074]. W-fibrils are considered to consist of in-register parallel beta-sheets. However, the precise molecular structure of M-fibrils and force driving the formation of toxic fibrils remain unclear. In this study, we hypothesized that low-polarity environments provided by GM1 clusters drive the formation of toxic fibrils and compared the structure and cytotoxicity of W-fibrils, M-fibrils, and aggregates formed in a low-polarity solution mimicking membrane environments. First, we determined solvent conditions which mimic the polarity of raftlike membranes using A beta-(1-40) labeled with the 7-diethylaminocoumarin-3-carbonyl dye. The polarity of a mixture of 80% 1,4-dioxane and 20% water (v/v) was found to be close to that of raftlike membranes. A beta-(1-40) formed amyloid fibrils within several hours in 80% dioxane (D-fibrils) or in the presence of raftlike membranes, whereas a much longer incubation time was required for fibril formation in a conventional buffer. D-fibrils were morphologically similar to M-fibrils. Fourier-transform infrared spectroscopy suggested that M-fibrils and D-fibrils contained antiparallel beta-sheets. These fibrils had greater surface hydrophobicity and exhibited significant toxicity against human neuroblastoma SH-SYSY cells, whereas W-fibrils with less surface hydrophobicity were not cytotoxic. We concluded that ganglioside clusters mediate the formation of toxic amyloid fibrils of A beta with an antiparallel beta-sheet structure by providing less polar environments.