Amyloid-beta (A beta) is the major protein component of neuritic plaques found in Alzheimer's disease. Evidence suggests that the physical aggregation state of A beta directly influences neurotoxicity and specific cellular biochemical events. Atomic force microscopy (AFM) is used to investigate the three-dimensional structure of aggregated A beta 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 a zeta axis (fiber height above the x-y imaging surface). Densely packed aggregates ( > or = 100 nm thick) were observed. At the edges of these densely packed regions and in dispersed regions, three types of A beta 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 approximately > or = 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 A beta fibrils can, for the first time, resolve nanometer-scale, zeta-axis, surface-height (thickness) fibril features. Concurrent x-y surface scans of fibrils reveal the surface submicrometer structure and organization of aggregated A beta. Thus, when AFM imaging of A beta is combined with, and correlated to, careful studies of cellular A beta toxicity it may be possible to relate certain A beta structural features to cellular neurotoxicity.

The world of nature provides a never-ending set of fascinating problems for the chemist. Many of the most intriguing problems, however, concern compounds available in only truly minute quantities. One solution is to focus on bioassay-guided separations. In so doing one can isolate compounds with novel structures or unsuspected activities from almost any phylum, including tunicates, sponges, insects, or even the much-studied terrestrial plants, as exemplified in several recent studies in our laboratory involving activities ranging from antiviral and antimicrobial activity to cytotoxicity and immunomodulation. Moreover, newer spectroscopic techniques, especially fast atom bombardment mass spectrometry and tandem mass spectrometry, enhance one's ability to study compounds present in minute quantities, including those of importance to the host organism, such as neuropeptides in insects or marine invertebrates.

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