RESUMO
Morphology of aggregation intermediates, polymorphism of amyloid fibrils and aggregation kinetics of the "Arctic" mutant of the Alzheimer's amyloid ß-peptide, Aß((1-40))(E22G), in a physiologically relevant Tris buffer (pH 7.4) were thoroughly explored in comparison with the human wild type Alzheimer's amyloid peptide, wt-Aß((1-40)), using both in situ atomic force and electron microscopy, circular dichroism and thioflavin T fluorescence assays. For arc-Aß((1-40)) at the end of the 'lag'-period of fibrillization an abrupt appearance of ≈ 3 nm size 'spherical aggregates' with a homogeneous morphology, was identified. Then, the aggregation proceeds with a rapid growth of amyloid fibrils with a variety of morphologies, while the spherical aggregates eventually disappeared during in situ measurements. Arc-Aß((1-40)) was also shown to form fibrils at much lower concentrations than wt-Aß((1-40)): ≤ 2.5 µM and 12.5 µM, respectively. Moreover, at the same concentration, 50 µM, the aggregation process proceeds more rapidly for arc-Aß((1-40)): the first amyloid fibrils were observed after c.a. 72 h from the onset of incubation as compared to approximately 7 days for wt-Aß((1-40)). Amyloid fibrils of arc-Aß((1-40)) exhibit a large variety of polymorphs, at least five, both coiled and non-coiled distinct fibril structures were recognized by AFM, while at least four types of arc-Aß((1-40)) fibrils were identified by TEM and STEM and their mass-per-length statistics were collected suggesting supramolecular structures with two, four and six ß-sheet laminae. Our results suggest a pathway of fibrillogenesis for full-length Alzheimer's peptides with small and structurally ordered transient spherical aggregates as on-pathway immediate precursors of amyloid fibrils.