Inhibition of PHF-like tau hyperphosphorylation in SH-SY5Y cells and rat brain slices by K252a.

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.

Microtubule-associated protein tau as a therapeutic target in neurodegenerative disease.

Interest in the biology of the microtubule-associated protein tau, not only as a pathologic marker, but as a therapeutic target has surged considerably over the last few years. This is due, in part, to the discovery of mutations in tau causing a group of aggressively degenerative neurologic disorders characterized by abnormalities of tau very similar to what is seen in Alzheimer's disease where mutations in tau are absent. As these same mutations also precipitate authentic forms of neurofibrillary degeneration in tau transgenic mice, the gateways to testing therapeutic ideas preclinically have opened. Other Alzheimer's disease animal models have been notoriously bare of this feature, limiting their predictive power for clinical success. In this review, the authors discuss some of the main therapeutic ideas presently advanced in the field and their molecular rationales.

An inhibitor of tau hyperphosphorylation prevents severe motor impairments in tau transgenic mice.

An orally bioavailable and blood-brain barrier penetrating analog of the kinase inhibitor K252a was able to prevent the typical motor deficits in the tau (P301L) transgenic mouse model (JNPL3) and markedly reduce soluble aggregated hyperphosphorylated tau. However, neurofibrillary tangle counts were not reduced in the successfully treated cohort, suggesting that the main cytotoxic effects of tau are not exerted by neurofibrillary tangles but by lower molecular mass aggregates of tau. Our findings strongly suggest that abnormal tau hyperphosphorylation plays a critical role in the development of tauopathy and suggest a previously undescribed treatment strategy for neurodegenerative diseases involving tau pathology.

Prospect of therapeutic approaches to tauopathies.

Alois Alzheimer described the concurrence of two conspicuous proteinacious aggregates in 1906. Today it is clear that the two types of protein aggregates are fundamentally different. One consists of a short fragment (Abeta peptide) of a membrane protein APP and is found mainly outside of cells and the other is formed from a biochemically modified cytoskeleton-associated protein known as tau. The latter is found exclusively inside cells. Aggregated tau in all tauopathies including AD is abnormally modified by the excess incorporation of chemical groups called phosphates (hyperphosphorylation), and the appearance of this biochemical change coincides with the onset all tauopathies, suggesting that it is both a necessary and sufficient cause for such diseases. Data indicate that the basis for tau hyperphosphorylation is the dysregulation of key enzymes known as kinases. These enzymes are therapeutic targets for AD and other neurodegenerative diseases, which feature pathological tau structures in brain