In situ atomic force microscopy study of Alzheimer's β-amyloid peptide on different substrates:: New insights into mechanism of β-sheet formation

We have applied irt situ atomic force microscopy to directly observe the aggregation of Alzheimer's beta-amyloid peptide (A beta) in contact with two model solid surfaces: hydrophilic mica and hydrophobic graphite, The time course of aggregation was followed by continuous imaging of surfaces remaining in contact with 10-500 mu M solutions of A beta in PBS (pH 7.4). Visualization of fragile nanoscale aggregates of A beta was made possible by the application of a tapping mode of imaging, which minimizes the lateral forces between the probe tip and the sample, The size and the shape of A beta aggregates, as well as the kinetics of their formation, exhibited pronounced dependence on the physicochemical nature of the surface. On hydrophilic mica, A beta formed particulate, pseudo-micellar aggregates, which at higher A beta concentration had the tendency to form linear assemblies, reminiscent of protofibrillar species described recently in the literature. In contrast, on hydrophobic graphite A beta formed uniform, elongated sheets. The dimensions of those sheets were consistent with the dimensions of beta-sheets with extended peptide chains perpendicular to the long axis of the aggregate. The sheets of A beta were oriented along three directions at 120 degrees to each other, resembling the crystallographic symmetry of a graphite surface, Such substrate-templated self-assembly may be the distinguishing feature of beta-sheets in comparison with alpha-helices. These studies show that in situ atomic force microscopy enables direct assessment of amyloid aggregation in physiological fluids and suggest that A beta fibril formation may be driven by interactions at the interface of aqueous solutions and hydrophobic substrates, as occurs in membranes and lipoprotein particles in vivo.