The long-term goal of this project is the understanding of the biology and molecular pathogenesis of Alzheimer's disease (AD), the most common organic dementia seen in old age. AD is characterized by the presence of neurofibrillary tangles which consist primarily of paired helical filaments (PHF). PHF preparations contain aberrant forms of tau named epitopes but differed in reactivity with a monoclonal anti-tau antibody (Tau-1) and antibodies to a region of bovine tau encoded by tau gene exon 2. A pretreatment of PHF-tau with phosphatase improved its binding with these antibodies, suggesting that PHF-tau differed from normal tau in the site/or the extent of phosphorylation. We have shown recently that PHF-tau also differed from normal tau in molecular mass and isoelectric charge. Dephosphorylation had very little effect on the biochemical properties of PHF tau, suggesting that post-translational modifications, besides phosphorylation, and/or alternative gene splicing play a role in the formation of PHF-tau. It remains to be determined if PHF-tau, besides the two regions mentioned above, contain other aberrant regions, and if phosphorylation is the only biochemical modification that distinguishes PHF-tau from normal tau. It is also unknown if PHF-tau contains additional phosphorylation sites and if the aberrant phosphorylation of PHF-tau is catalyzed via a protein kinase (s) in normal brains or only in AD brains. These questions will be answered by comparing PHF-tau, and tau proteins from normal and AD brains for their phosphate content, amino acid composition and sequences, post-translational modifications and by studying the rephosphorylation of dephosphorylated PHF-tau with known kinases and kinases from brain homogenates. A subtoxic level of glutamate or calcium ionophore A23187 was recently shown to alter the immunoreactivity of tau in cultured neurons. It is unclear whether this alteration is similar to that in PHF-tau. Our last specific aim is to study the effect of these treatments on the biochemical and immunocytochemical properties of tau, and on the ultrastructure of the affected neurons.
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