Syk inhibitor reduces oligomeric tau associated with GSK3β inactivation and autophagy activation

<jats:title>Abstract</jats:title><jats:sec><jats:title>Background</jats:title><jats:p>Neurofibrillary tangle (NFT), composed of highly phosphorylated tau is one of the pathological hallmarks of Alzheimer’s disease (AD). Spleen tyrosine kinase (Syk) is non receptor type tyrosine kinase, and has various physiological functions, including histamine release in mast cells, phagocytosis in macrophage, and B cell differentiation. The possibility of Syk inhibitor for clinical application was proposed in allergic rhinitis, rheumatoid arthritis, and immunologic thrombocytopenia. The possible relationship between Syk and AD has been reported recently. We have examined the effects of Syk specific inhibitor BAY61‐3606 on phosphorylation levels of tau protein and oligomeric tau by using cell culture model of tauopathy, M1C cells.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>We utilized human neuroblastoma cell line M1C cells, which expresses wild type tau protein (4R0N) via tetracycline off induction (Tet Off induction). Reduction of phosphorylated tau protein and oligomeric tau protein by Syk inhibitor was examined by using various phospho‐tau antibodies, and tau oligomer specific antibody. Influences of activity of autophagy by Syk inhibitor was also examined by western blot analysis.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>In this cell line, 72kDa of Syk protein was identified by Western blot analysis. Phosphorylated tau detected by various phospho‐tau antibodies, including PHF‐1, CP13, and AT270 was decreased by 1, 10 μM of Syk inhibitor. One of the patholoigical form of tau, caspase cleaved tau detected by TauC3 was also decreased by Syk inhibitor. Oligomeric tau, which detected by tau oligomer specific antibody, TOC1 was also decreased by Syk inhibitor. Syk inhibitor also inactivated glycogen synthase kinase 3β (GSK3β). Syk inhibitor upregulated autophagy detected by increase of LC3‐II, and reduction of P62. In summary, Syk inhibitor decreased phosphorylated tau as well as total tau by upregulating autophagy, and inactivating tau kinases, including GSK3β. Finally, oligomeric tau was reduced by Syk inhibitor.</jats:p></jats:sec><jats:sec><jats:title>Conclusion</jats:title><jats:p>Syk inhibitor may have promising for the treatment of tauopathy, including AD.</jats:p></jats:sec>