Ubiquitin immunoreactivity of paired helical filaments differs in Alzheimer's disease and corticobasal degeneration.

To establish whether there is a relationship between ubiquitination and ultrastructural appearance of filaments, we compared the ubiquitin immunoreactivity of paired helical filaments (PHFs) in Alzheimer's disease (AD) and corticobasal degeneration (CBD). PHFs in these disorders share a limited similarity since filaments in CBD are wider and twisted at longer intervals than those in AD, and also display less ultrastructural stability. Preparations enriched in SDS-soluble filaments were isolated from AD and CBD brains and subjected to tau and ubiquitin immunogold labeling. Both preparations contained mostly dispersed individual PHFs, which labeled for the amino and carboxyl termini of tau. Immunolabeling of ubiquitin was variable, however, being more intense in AD than CBD samples. SDS-insoluble filaments were prepared from PHFs by boiling in the presence of SDS and 2-mercaptoethanol and collected by sedimentation. In both disorders, the pellets contained highly aggregated and bundled filaments, which were devoid of the amino but not the carboxyl terminal region of tau. Again, ubiquitin labeling was more intense in AD than CBD filaments. The present results suggest that ubiquitination has limited influence on SDS solubility, aggregation and bundling of PHFs; however, it may be one of the factors responsible for the ultrastructural variability and/or stability of filaments.