Protein aggregation is problematic in the manufacturing of therapeutic proteins and in human disease states. The aggregation and deposition of amyloid-β protein (Aβ) into fibrillar deposits is widely accepted as the primary event driving the pathogenesis of Alzheimer's disease. However, the mechanism of Aβ fibril formation in vivo is still unclear. In vitro studies have shown that Aβ fibril formation is a nucleation-dependent polymerization process characterized by an initial lag phase, during which fibril nuclei emerge from Aβ micelles at peptide concentrations above 50μM. However, Aβ concentration in vivo is orders of magnitude lower, approximately 10nM. Clearly, Aβ micelle induced fibrillogenesis cannot be the dominant pathway of Aβ aggregation in vivo. We show here that the cell membrane may exert a potent templating effect on Aβ fibrillogenesis through specific lipid-Aβ interactions.

Model membrane systems – Langmuir lipid monolayers and vesicles – were used to probe Aβ-lipid interactions and their effects on Aβ fibril formation. Aβ-lipid interactions were measured as the extent of Aβ insertion into lipid monolayers and the effect of lipid membrane on Aβ fibril formation was monitored by incubating Aβ with lipid vesicles. The effect of lipid head group charge and the glycolipid ganglioside G_M1 on Aβ interaction with lipid monolayer and fibril formation were investigated. Electrostatic interactions between Aβ and the phospholipid head groups were found to modulate Aβ insertion into lipid monolayers. Specifically, Aβ exhibited higher insertion into anionic lipids, which normally only resides in the inner leaflet of the cell membrane. Furthermore, from X-ray scattering experiments, the anionic lipid was found to induce molecular ordering of Aβ at the membrane surface that closely mimic β-sheet ordering of Aβ in amyloid fibrils, revealing an intriguing templating effect of the anionic lipid on Aβ. Aβ also showed direct and favorable interaction with ganglioside G_M1 and a similar templating effects was also measured. Incubating Aβ with lipid vesicles containing either negatively charged lipids or G_M1 exhibited greatly enhanced formation of Aβ fibrils. Our results implicate that adsorption of Aβ to negatively charged lipids, which could be exposed to the outer membrane leaflet by cell injury, or physiological levels of G_M1 on neuronal cell surface may serve as an in vivo mechanism of templated aggregation and drive the pathogenesis of Alzheimer's disease.