The nanometer-scale structure of amyloid-Β visualized by atomic force microscopy

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Abstract
Amyloid-Β (AΒ) is the major protein component of neuritic plaques found in Alzheimer's disease. Evidence suggests that the physical aggregation state of AΒ directly influences neurotoxicity and specific cellular biochemical events. Atomic force microscopy (AFM) is used to investigate the three-dimensional structure of aggregated AΒ and characterize aggregate/fibril size, structure, and distribution. Aggregates are characterized by fibril length and packing densities. The packing densities correspond to the differential thickness of fiber aggregates along az axis (fiber height above thex-y imaging surface). Densely packed aggregates (≥100 nm thick) were observed. At the edges of these densely packed regions and in dispersed regions, three types of AΒ fibrils were observed. These were classified by fibril thickness into three size ranges: 2–3 nm thick, 4–6 nm thick, and 8–12 nm thick. Some of the two thicker classes of fibrils exhibited pronounced axial periodicity. Substructural features observed included fibril branching or annealing and a height periodicity which varied with fibril thickness. When identical samples were visualized with AFM and electron microscopy (EM) the thicker fibrils (4–6 nm and 8–12 nm thick) had similar morphology. In comparison, the densely packed regions of ∼≥100 nm thickness observed by AFM were difficult to resolve by EM. The small, 2- to 3-nm-thick, fibrils were not observed by EM even though they were routinely imaged by AFM. These studies demonstrate that AFM imaging of AΒ fibrils can, for the first time, resolve nanometer-scale,z-axis, surface-height (thickness) fibril features. Concurrentx-y surface scans of fibrils reveal the surface submicrometer structure and organization of aggregated AΒ. Thus, when AFM imaging of AΒ is combined with, and correlated to, careful studies of cellular AΒ toxicity it may be possible to relate certain AΒ structural features to cellular neurotoxicity.