Neurofilament Monoclonal Antibodies RT97 and 8D8 Recognize Different Modified Epitopes in Paired Helical Filament-τ in Alzheimer's Disease

Abstract: Neurofibrillary tangles in Alzheimer's disease have been previously found to be labeled by some neurofilament antibodies that also recognize τ proteins. We have studied the reactivity of two such monoclonal antibodies, RT97 and 8D8, and of an anti-ubiquitin serum with the abnormal paired helical filaments (PHF)-τ (A68) polypeptides known to be the main component of the PHFs constituting the neurofibrillary tangles. 8D8 recognized the three major PHF-τ polypeptides, but RT97 reacted only with the two larger PHF-τ species. PHF-τ polypeptides were labeled by 8D8 and RT97 much more strongly than normal human τ and this labeling was decreased after alkaline phosphatase treatment. Anti-ubiquitin and anti-phosphotyrosine antibodies did not label PHF-τ polypeptides. The immunoreactivity of proteolytic fragments of PHF-τ polypeptides was studied with RT97, 8D8, and a panel of τ antibodies. The epitope for 8D8 on PHF-τ was localized between amino acids 222 and 427 in the carboxyl half of τ. The RT97 epitope on PHF-τ was localized in the amino domain of τ, probably in the 29-amino-acid insertion (insert 1) found towards the amino terminus of some τ isoforms. These results show that the basis for the labeling of neurofibrillary tangles by antibodies 8D8 and RT97 to neurofilament is their ability to react with PHF-τ polypeptides by recognizing sites specifically modified on PHF-τ, including a site specific to some τ isoforms.