Abnormal hyperphosphorylation of tau is believed to constitute a critical biochemical event in the process of neurofibrillary degeneration of Alzheimer's disease. We have developed a cellular model where apparently authentic PHF-like tau hyperphosphorylation is induced by okadaic acid. To gain deeper insight into the complex mechanisms of this pathological process we tested a variety of kinase inhibitors in this model. We found that K252a is differentiated from staurosporine by its inhibition of ERK2: both compounds are structurally related microbial metabolites generally believed to have only moderate kinase selectivity. However, since ERK2 inhibitors are exceedingly rare, we used this differential inhibitory property of K252a to demonstrate the involvement of ERK2 in PHF-type tau hyperphosphorylation. K252a was uniquely able to completely suppress the okadaic acid-induced tau hyperphosphorylation in SH-SY5Y cells and rat brain slices by way of including ERK2 in its inhibitory spectrum, and to conserve the normal binding of tau to tubulin. GSK3 inhibitors partially affected the normal state of tau phosphorylation in SH-SY5Y cells, but had no impact on okadaic acid-induced tau hyperhosphorylation. As K252a is the first molecule identified capable of preventing the spectrum of PHF-like tau hyperphosphorylation markers, it may represent a conceptual starting point for therapeutic development of suitable spectrum kinase inhibitors.

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