ALZHEIMERS DISEASE-LIKE PHOSPHORYLATION OF THE MICROTUBULE-ASSOCIATEDPROTEIN-TAU BY GLYCOGEN-SYNTHASE KINASE-3 IN TRANSFECTED MAMMALIAN-CELLS

Background: Paired helical filaments (PHFs) are a characteristic patho logical feature of Alzheimer's disease; their principal component is t he microtubule-associated protein tau. The tau in PHFs (PHF-tau) is hy perphosphorylated, but the cellular mechanisms responsible for this hy perphosphorylation have yet to be elucidated. A number of kinases, inc luding mitogen-activated protein (MAP) kinase, glycogen synthase kinas e (GSK)-3 alpha, GSK-3 beta and cyclin-dependent kinase-5, phosphoryla te recombinant tau in vitro so that it resembles PHF-tau as judged by its reactivity with a panel of antibodies capable of discriminating be tween normal tau and PHF-tau, and by a reduced electrophoretic mobilit y that is characteristic of PHF-tau. To determine whether MAP kinase, GSK-3 alpha and GSK-3 beta can also induce Alzheimer's disease-like ph osphorylation of tau in mammalian cells, we studied the phosphorylatio n status of tau in primary neuronal cultures and transfected COS cells following changes in the activities of MAP kinase and GSK-3. Results: Activating MAP kinase in cultures of primary neurons or transfected C OS cells expressing tau isoforms did not increase the level of phospho rylation for any PHF-tau epitope investigated. But elevating GSK-3 act ivity in the COS cells by co-transfection with GSK-3 alpha or GSK-3 be ta decreased the electrophoretic mobility of tau so that it resembled that of PHF-tau, and induced reactivity with eight PHF-tau-selective m onoclonal antibodies. Conclusions: Our data indicate that GSK-3 alpha and/or GSK-3 beta, but not MAP kinase, are good candidates for generat ing PHF-type phosphorylation of tau in Alzheimer's disease. The involv ement of other kinases in the generation of PHFs cannot, however, be e liminated. Our results suggest that aberrant regulation of GSK-3 may b e a pathogenic mechanism in Alzheimer's disease.