THE NANOMETER-SCALE STRUCTURE OF AMYLOID-BETA VISUALIZED BY ATOMIC-FORCE MICROSCOPY

Amyloid-beta (A beta) is the major protein component of neuritic plaqu es found in Alzheimer's disease. Evidence suggests that the physical a ggregation state of A beta directly influences neurotoxicity and speci fic cellular biochemical events. Atomic force microscopy (AFM) is used to investigate the three-dimensional structure of aggregated A beta a nd characterize aggregate/fibril size, structure, and distribution Agg regates are characterized by fibril length and packing densities. The packing densities correspond to the differential thickness of fiber ag gregates along a z axis (fiber height above the x-y imaging surface) D ensely packed aggregates (greater than or equal to 100 nm thick) were observed. At the edges of these densely packed regions and in disperse d regions, three types of A beta fibrils were observed. These were cla ssified 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 fibril s exhibited pronounced axial periodicity Substructural features observ ed included fibril branching or annealing and a height periodicity whi ch varied with fibril thickness. visualized with AFM and electron micr oscopy (EM) the thicker fibrils (4-6 nm and 8-12 nm thick) had similar morphology In comparison, the densely packed regions of similar to gr eater than or equal to 100 nm thickness observed by AFM were difficult to resolve by EM. The small, 2- to 3-nm-thick, fibrils were not obser ved by EM even though they were routinely imaged by AFM. These studies demonstrate that AFM imaging of A beta fibrils can, for the first tim e, resolve nanometer-scale, z axis, surface height (thickness) fibril features. Concurrent x-y surface scans of fibrils reveal the surface s ubmicrometer structure and organization of aggregated A beta. Thus, wh en AFM imaging of A beta is combined with, and correlated to, careful studies of cellular A beta toxicity it may be possible to relate certa in A beta structural features to cellular neurotoxicity.