Calcium influx recruits an additional class of kinases to hyperphosphorylate tau.

SH-SY-5Y human neuroblastoma cells were treated with combinations of the kinase inhibitors HA-1004, W-7 and H-7 and calcium ionophore A23187. Microdensitometric analyses revealed that, in the absence of ionophore-mediated calcium influx, PHF-1 levels were reduced by approximately half in cultures treated with HA-1004 or W-7, but were not reduced by H-7. By contrast, the doubling in PHF-1 immunoreactivity that resulted following ionophore treatment was prevented by all three inhibitors. These analyses demonstrate the recruitment of an additional kinase or kinases in tau phosphorylation following calcium influx, and underscore the possibility that de novo hyperactivation of calcium-dependent kinases may be involved in the early events that propagate PHF formation.