SITE-SPECIFIC REGULATION OF ALZHEIMER-LIKE TAU-PHOSPHORYLATION IN LIVING NEURONS

The microtubule-associated protein tau is more highly phosphorylated a t certain residues in developing brain and in Alzheimer's disease pair ed helical filaments than in adult brain. We examined the regulation o f tau phosphorylation at some of these sites in rat brain using the ph osphorylation state-dependent anti-tau antibodies AT8, Taul, and PHF1. The AT8 and PHF1 antibodies bind to phosphorylated tau, while Taul bi nds to unphosphorylated tau. Levels of tau reactive for AT8 were high only during the first postnatal week, with levels in adult declining t o approximately 5% of the levels in neonates. In neonatal forebrain sl ices, tau became rapidly dephosphorylated at the AT8 and Taul sites du ring incubation at 34 degrees C, but was incompletely dephosphorylated at the PHF1 site. Dephosphorylation at AT8 sites, but not at Taul or PHF1 sites, was completely inhibited by 1 mu M okadaic acid. Hence the regulation of tau phosphorylation by okadaic acid-sensitive phosphata se(s) was site-specific. Addition of 1 mu M okadaic acid after dephosp horylation at the AT8 locus yielded a partial recovery of AT8 immunore activity, and incubation with basic fibroblast growth factor increased phosphorylation at the AT8 site in a dose-dependent manner. These res ults indicate that endogenously active and basic fibroblast growth fac tor stimulated tau kinases directed toward an Alzheimer's disease-rela ted site were present in the dices. In adult brain slices, the small p ool of AT8-reactive tau was remarkably insensitive to dephosphorylatio n during incubation, and okadaic acid treatment induced only small inc reases in AT8 immunoreactivity. These results suggest that tau phospho rylation in adult brain is less dynamic than in neonatal brain. These findings indicate that neonatal tau is not only phosphorylated more hi ghly than adult tau, but also more dynamically regulated by protein ph osphatases and protein kinases than adult tau. The inability of okadai c acid to induce large increases in tau phosphorylation in adult rat b rain slices suggests that a loss of protein phosphatase activity alone would not be sufficient to produce the hyperphosphorylation observed in Alzheimer's disease paired helical filaments. Stimulation of kinase activity by basic fibroblast growth factor is likely to modulate tau function during development, and may contribute to the genesis of hype rphosphorylated tau in Alzheimer's disease.