Abstract
Alzheimer's disease is characterized by the spontaneous assembly of a cytoskeletal protein into highly insoluble filamentous structures which appear, in conventional transmission electron microscopy (EM), as paired helical filaments (PHFs) or straight filaments (SFs). Recent examination of PHF ultrastructure using EM of vertical PtC replicas of isolated PHF indicate that PHFs more closely resemble twisted ribbons than helically wound pairs of filaments. To re-evaluate the fine structure of PHFs we isolated fractions highly enriched in PHF from Alzheimer diseased brains and characterized the fractions with an atomic force microscope. PHFs adhered to both mica and graphite surfaces without pre-treatment of the surfaces and remained fixed to the substrate even after repeated scanning. Using the AFM, PHFs appeared as twisted ribbons and had dimensions similar to those reported for vertically replicated PHF viewed with EM. Rare SFs were found admixed with typical twisted ribbons. These data indicate that high resolution imaging using both PtC replication and AFM give similar quantitative data with respect to PHF dimensions. We conclude that Alzheimer PHFs should be modelled as a twisted ribbon rather than the structure implied by its name.
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