Microtubule associated protein tau is a phosphoprotein which potentially has 80 serine/threonine and 5 tyrosine phosphorylation sites. Normal brain tau contains 2-3 moles of phosphate per mole of the protein. In Alzheimer's disease brain, tau is abnormally hyperphosphorylated to a stoichiometry of at least three-fold greater than normal tau, and in this altered state it is aggregated into paired helical filaments forming neurofibrillary tangles, a histopathological hallmark of the disease. The abnormal hyperphosphorylation of tau is also a hallmark of several other related neurodegenerative disorders, called tauopathies. The density of neurofibrillary tangles in the neocortex correlates with dementia and, hence, is a rational therapeutic target and an area of increasing research interest. Development of rational tau-based therapeutic drugs requires understanding of the role of various phosphorylation sites, protein kinases and phosphatases, and post-translational modifications that regulate the phosphorylation of this protein at various sites, as well as the molecular mechanism by which the abnormally hyperphosphorylated tau leads to neurodegeneration and dementia. In this article we briefly review the progress made in these areas of research.

Microtubule associated protein (MAP) tau is abnormally hyperphosphorylated in Alzheimer's disease (AD) and related tauopathies; in this form it is the major protein subunit of paired helical filaments (PHF)/neurofibrillary tangles. However, the nature of protein kinases and phosphatases and tau sites involved in this lesion has been elusive. We investigated self-assembly and microtubule assembly promoting activities of hyperphosphorylated tau isolated from Alzheimer disease brain cytosol, the AD abnormally hyperphosphorylated tau (AD P-tau) before and after dephosphorylation by phosphoseryl/phosphothreonyl protein phosphatase-2A (PP-2A), and then rephosphorylation by cyclic AMP-dependent protein kinase (PKA), calcium, calmodulin-dependent protein kinase II (CaMKII), glycogen synthase kinase-3beta (GSK-3beta) and cyclin-dependent protein kinase 5 (cdk5) in different kinase combinations. We found that (i) dephosphorylation of AD P-tau by PP-2A inhibits its polymerization into PHF/straight filaments (SF) and restores its binding and ability to promote assembly of tubulin into microtubules; (ii) rephosphorylation of PP-2A-dephosphorylated AD P-tau by sequential phosphorylation by PKA, CaMKII and GSK-3beta or cdk5, and as well as by cdk5 and GSK-3beta, promotes its self-assembly into tangles of PHF similar to those seen in Alzheimer brain, and (iii) phosphorylation of tau sites required for this pathology are Thr231 and Ser262, along with several sites flanking the microtubule binding repeat region. Phosphorylation of recombinant human brain tau(441) yielded similar results as the PP-2A dephosphorylated AD P-tau, except that mostly SF were formed. The conditions for the abnormal hyperphosphorylation of tau that promoted its self-assembly also induced the microtubule assembly inhibitory activity. These findings suggest that activation of PP-2A or inhibition of either both GSK-3beta and cdk5 or one of these two kinases plus PKA or CaMKII might be required to inhibit Alzheimer neurofibrillary degeneration.

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Microtubule-associated protein T is abnormally hyperphosphorylated and aggregated in affected neurons of Alzheimer disease brain . This hyperphosphorylated -r can be dephosphorylated at some of the abnormal phosphorylated sites by purified protein phosphatase-1, 2A, and 213 in vitro . In the present study, we have developed an assay to measure protein phosphatase activity toward -r-1 sites (Ser' 9'/Ser 212) using the hyperphosphorylated T isolated from Alzheimer disease brain as substrate . Using this assay, we have identified that in normal brain, protein phosphatase-2A and 213 and, to a lesser extent, 1 are involved in the dephosphorylation of r. The Km values of dephosphorylation of the hyperphosphorylated T by protein phosphatase-2A and 2B are similar. The T phosphatase activity is decreased by -30% in brain of Alzheimer disease patients compared with those of age-matched controls . These findings suggest that a defect of protein phosphatase could be the cause of the abnormal hyperphosphorylation of r in Alzheimer disease . Key Words : Microtubule-associated protein T-Protein phosphatase-Protein dephosphorylation-Alzheimer disease .

Alzheimer's disease (AD) is characterized by the presence of numerous neurons with neurofibrillary tangles of paired helical filaments (PHFs). The microtubule-associated protein tau in abnormally hyperphosphorylated form is the major protein subunit of the PHF. We now show that PHF tangles isolated from AD brains are glycosylated, whereas no glycan is detected in normal tau. Deglycosylation of PHF tangles by endoglycosidase F/N-glycosidase F converts them into bundles of straight filaments 2.5 +/- 0.5 nm in diameter, similar to those generated by the interaction of normal tau and abnormally hyperphosphorylated tau (AD P-tau). Deglycosylation plus dephosphorylation, but not deglycosylation alone, of AD P-tau and tau from PHF tangles restores their microtubule polymerization activity. Dephosphorylation of deglycosylated PHF tangles results in increased tau release. Thus, although the abnormal phosphorylation might promote aggregation of tau and inhibition of the assembly of microtubules, glycosylation appears to be responsible for the maintenance of the PHF structure.