The role of brain protein phosphatases 1 and 2A in the regulation of microtubule assembly

The regulation and function of protein phosphatases in the brain

Data emerging from a number of different systems indicate that protein phosphatases are highly regulated and potentially responsive to changes in the levels of intracellular second messengers produced by extracellular stimulation. They may therefore be involved in the regulation of many cell functions. The protein phosphatases in the nervous system have not been well studied. However, a number of neuronal-specific regulators (such as DARPP-32 and G-substrate) exist, and brain protein phosphatases appear to have particularly low specific activity, suggesting that neuronal protein phosphatases possess considerable and unique potential for regulation. Several early events following depolarization or receptor activation appear to involve specific dephosphorylations, indicating that regulation of protein phosphatase activity is important for the control of many neuronal functions. This article reviews the current literature concerning the identification, regulation, and function of serine/threonine protein phosphatases in the brain, with particular emphasis on the regulation of the major protein phosphatases, PP1 and PP2A, and their potential roles in modulating neurotransmitter release and postsynaptic responses.

Dephosphorylation of tau factor by protein phosphatase 2A in synaptosomal cytosol fractions, and inhibition by aluminum

When the synaptosomal cytosol fraction from rat brain was chromatographed on a DEAE-cellulose column and assayed for protein phosphatases for tau factor and histone H1, two peaks of activities, termed peak 1 (major) and peak 2 (minor), were separated. Each peak was in a single form 2 (minor), were separated. Each peak was in a single form on Sephacryl S-300 column chromatography. Both peaks 1 and 2 dephosphorylated tau factor phosphorylated by Ca2+/calmodulin-dependent protein kinase II and the catalytic subunit of cyclic AMP-dependent protein kinase. The Km values were in the range of 0.42-0.84 microM for tau factor. There were no differences in kinetic properties of dephosphorylation between the substrates phosphorylated by the two kinases. The phosphatase activities did not depend on Ca2+, Mn2+ and Mg2+. Immunoprecipitation and immunoblotting analysis using polyclonal antibodies to the catalytic subunit of brain protein phosphatase 2A revealed that both protein phosphatases are the holoenzymic forms of protein phosphatase 2A. Aluminum chloride inhibited the activities of both peaks 1 and 2 with IC50 values of 40-60 microM. These results suggest that dephosphorylation of tau factor in presynaptic nerve terminals is controlled mainly by protein phosphatase 2A and that the neurotoxic effect of aluminum seems to be related mostly to inhibition of dephosphorylation of tau factor.