TAU-PHOSPHORYLATION IN HUMAN, PRIMATE, AND RAT-BRAIN - EVIDENCE THAT A POOL OF TAU IS HIGHLY PHOSPHORYLATED IN-VIVO AND IS RAPIDLY DEPHOSPHORYLATED IN-VITRO

The extent of tau phosphorylation is thought to regulate the binding o f tau to microtubules: Highly phosphorylated tau does not bind to tubu les, whereas dephosphorylated tau can bind to microtubules. it is inte resting that the extent of tau phosphorylation in vivo has not been ac curately determined. tau was rapidly isolated from human temporal neoc ortex and hippocampus, rhesus monkey temporal neocortex, and rat tempo ral neocortex and hippocampus under conditions that minimized dephosph orylation. In brain slices, we observed that tau isolated under such c onditions largely existed in several phosphorylated states, including a pool that was highly phosphorylated; this was determined using epito pe-specific monoclonal and polyclonal antibodies. This highly phosphor ylated tau was dephosphorylated during a 120-min time course in vitro, presumably as a result of neuronal phosphatase activity. The slow-mob ility forms of tau were shifted to faster-mobility forms following in vitro incubation with alkaline phosphatase. Laser densitometry was use d to estimate the percent of tau in slow-mobility, highly phosphorylat ed forms. Approximately 25% of immunoreactive tau was present as stow- mobility (66- and 68-kDa) forms of tau. The percentage of immunoreacti ve tau in faster-mobility pools (42-54 kDa) increased in proportion to the decrease in content of 66-68-kDa tau as a function of neuronal ph osphatases or alkaline phosphatase treatment. These data suggest that the turnover of phosphorylated sites on tau is rapid and depends on ne uronal phosphatases. Furthermore, tau is highly phosphorylated in norm al-appearing human, primate, and rodent brain. The presence of a highl y phosphorylated pool of tau in adult brain may modify the present hyp otheses on how paired helical filaments of Alzheimer's disease are for med.