Paired helical filaments in corticobasal degeneration: the fine fibrillary structure with NanoVan

Paired helical filaments (PHF) composed of hyperphosphorylated tau proteins are characteristic findings in neurodegenerative disorders, including Alzheimer's disease (AD) and corticobasal degeneration (CBD). The filaments in CBD differ from those in AD by a reduced number of tau isoforms and less stable ultrastructure. To further compare the ultrastructure of both filaments, we employed a novel staining reagent, NanoVan, as well as aurothioglucose and uranyl acetate. With commonly used uranyl acetate, both kinds of filaments appeared as twisted ribbons 15–20-nm and 21–23-nm wide, respectively, without significant internal substructure. With application of aurothioglucose, only few structural details were apparent. With NanoVan, AD filaments showed similar structure to that with uranyl acetate but CBD filaments displayed a highly heterogeneous appearance consistent with the dissociation of the 20–25-nm-wide filaments along two longitudinal axes. This was evident by the presence of thinner, 12–13-nm-wide filaments and filaments that splayed into two 20–25-nm-wide components at one or both ends. Moreover, detection of a prominent, 7–8-nm-wide axial region distinguished up to four protofilaments per one filament. Each protofilament appeared to contain two 3–5-nm-wide fibrils separated by an approximately 1-nm-wide axial region. The results suggest that 3–5-nm fibrils are the smallest structural subunits of filaments in CBD and that NanoVan may be an unique reagent in detecting eight-fibril organization in these less stable filaments.