back to Fifth International Conference

• Enter the seminar

We have purified two enzymes which can phosphorylate tau protein and form PHF epitope. The Nomenclature Committee of the International Union of Biochemistry and Molecular Biology has named these enzymes tau protein kinase (TPK). Of the two TPKs, TPKI and TPKII, TPKI was found to be identical with glycogen synthase kinase 3ß (GSK3ß). TPKII consists of two subunits, of which the larger was identical with cdk5, while but the smaller was a new protein. Each of the TPKs phosphorylated 4 Ser or Thr residues in tau protein. Moreover, prior phosphorylation of tau by TPKII facilitated the phosphorylation by TPKI. These 8 phosphorylation sites of the two TPKs appear to correspond to PHF sites, supporting the idea that TPKs are involved in the formation of PHF epitope. To confirm this, we raised an antibody against TPKI and found that it strongly stained the hippocampal region of AD brain, but scarcely stained the same region of normal brain. Next, we wanted to see whether this TPKI oper-ates pathologically in AD brain to genrate PHF epitope. Among many phos-phorylation sites of PHF, Ser-413 is characteristic for TPKI. We raised an anti-body highly specific to this phosphorylated site, and foud that it strongly stained NFT, as expected.

The next question was how TPKI is induced specifically in AD brain. We assumed that ß-amyloid (Aß ) might be the inducer, since deposition of Ab precedes appearance of PHF in AD brain. In hippocampal culture the addition of Ab induced TPKI and formed PHF epitope, detected with the respective antibod-ies. Moreover, hippocampal cells were killed by Aß, but this could be prevented by anti-sense oligonucleotide of TPKI gene. We tried to clarify the mechanism by which TPKI kills cells. Using a yeast two-hybrid system, we identified pyruvate dehydrogenase (PDH) as a substrate of TPKI, besides tau. When PDH was treated with TPKI in the in vitro system, its activity decreased in paral-lel with its phosphorylation. In the in vivo system, when neuronal cells were treated with Aß, PDH was inactivated in parallel with the activation of TPKI. This inactivation of PDH can account for the accumulation of pyruvate and lactate, and the decrease in glucose metabolism and ATP production, which characterize AD brain. All of these results suggest that TPKI plays a crucial role in the pathogenesis of AD.