ALZHEIMER PAIRED HELICAL FILAMENTS - A COMPARISON WITH THE TWISTED RIBBON MODEL

To investigate if Alzheimer paired helical filaments (PHF) closely res emble twisted ribbons, as indicated by recent high-resolution ultrastr uctural studies, we compared physical models of twisted ribbons with e lectron microscopic images of PHF Uranyl-acetate-stained, isolated PHF with one or two helical turns were compared with scale models of twis ted ribbons with one and two helical turns rotated at different angles . The various rotations of the twisted ribbon model corresponded well with the different orientations of randomly dispersed PHF. The electro n-dense regions of individual PHF turns previously thought to represen t a cross-over site of paired filaments corresponded to the edge of th e twisted ribbon when the ribbon was oriented perpendicular to the fil ament axis. These data indicate that the overall configuration of PHF is a twisted ribbon but does not exclude possible configuration restri ctions due to an ordered arrangement of sub-units.