The activity of protein phosphatase (PP)-2A, which regulates tau 
phosphorylation, is compromised in Alzheimer disease brain. Here we show that 
the transient transfection of PC12 cells with inhibitor-2 (I2PP2A) of PP2A 
causes abnormal hyperphosphorylation of tau at Ser396/Ser404 and Ser262/Ser356. 
This hyperphosphorylation of tau is observed only when a sub-cellular shift of 
I2PP2A takes place from the nucleus to the cytoplasm and is accompanied by 
cleavage of I2PP2A into a 20 kDa fragment. Memantine, an un-competitive 
inhibitor of N-methyl-D-aspartate receptors, inhibits this abnormal 
phosphorylation of tau and cell death and prevents the I2PP2A-induced inhibition 
of PP2A activity in vitro. These findings demonstrate novel mechanisms by which 
I2PP2A regulates the intracellular activity of PP2A and phosphorylation of tau, 
and by which Memantine modulates PP2A signaling and inhibits neurofibrillary 
degeneration.