Abstract
Neurofibrillary tangles, one of the pathological hallmarks of Alzheimer's disease, consist of highly phosphorylated tau proteins. Tau protein binds to microtubules and is best known for its role in regulating microtubule dynamics. However, if tau protein is phosphorylated by activated major tau kinases, including glycogen synthase kinase 3β or cyclin-dependent kinase 5, or inactivated tau phosphatase, including protein phosphatase 2A, its affinity for microtubules is reduced, and the free tau is believed to aggregate, thereby forming neurofibrillary tangles. We previously reported that pitavastatin decreases the total and phosphorylated tau protein using a cellular model of tauopathy. The reduction of tau was considered to be due to Rho-associated coiled-coil protein kinase (ROCK) inhibition by pitavastatin. ROCK plays important roles to organize the actin cytoskeleton, an expected therapeutic target of human disorders. Several ROCK inhibitors are clinically applied to prevent vasospasm postsubarachnoid hemorrhage (fasudil) and for the treatment of glaucoma (ripasudil). We have examined the effects of ROCK inhibitors (H1152, Y-27632, and fasudil [HA-1077]) on tau protein phosphorylation in detail. A human neuroblastoma cell line (M1C cells) that expresses wild-type tau protein (4R0N) by tetracycline-off (TetOff) induction, primary cultured mouse neurons, and a mouse model of tauopathy (rTG4510 line) were used. The levels of phosphorylated tau and caspase-cleaved tau were reduced by the ROCK inhibitors. Oligomeric tau levels were also reduced by ROCK inhibitors. After ROCK inhibitor treatment, glycogen synthase kinase 3β, cyclin-dependent kinase 5, and caspase were inactivated, protein phosphatase 2A was activated, and the levels of IFN-γ were reduced. ROCK inhibitors activated autophagy and proteasome pathways, which are considered important for the degradation of tau protein. Collectively, these results suggest that ROCK inhibitors represent a viable therapeutic route to reduce the pathogenic forms of tau protein in tauopathies, including Alzheimer's disease.