Effects of cell cycle inhibitors on tau phosphorylation in N2aTau3R cells

Neurofibrillary tangles are one of the pathologic hallmarks of Alzheimer's disease (AD). They are composed of paired helical filaments (PHF) containing hyperphosphorylated forms of tau. Hyperphosphorylation of certain tau sites favors its dissociation from the microtubules (MT), interfering with axonal transport and compromising the function and viability of neurons. Reappearance of cell cycle proteins have been reported in neurons exhibiting tau aggregation, suggesting that an aberrant cell cycle occurs before neurons die. Cell cycle suppression in neurons is crucial to survival, thus prevention of progression through the cell cycle may offer a therapeutic approach. Using a neuroblastoma cell line overexpressing 3-repeat (3R) tau, we investigated the effects of cell cycle inhibitors on tau phosphorylation. G2/M phase inhibitors did not alter phosphorylation of tau at Ser-202 and Ser-396/404 at the lower doses, but did at higher doses. Ser-202 and Ser-396/404 are phosphorylation sites of early and late neurofibrillary tangles, respectively, in AD. Cisplatin, a G1 phase inhibitor, did not phosphorylate tau. Cyclophosphamide and phosphoramide mustard, DNA cross-linking agents, decreased tau phosphorylation at Ser-396/404 site, but increased phosphorylation at Ser-202. These studies demonstrate that the G2/M blockers have a dose-dependent effect on tau phosphorylation. This seems to be a consequence of both the disruption of MT-organization and MT-dynamics when doses are higher, but only a disruption of MT-dynamics with lower doses. These results are also in agreement with the lack of phosphorylation seen for cisplatin, another inhibitor that produces disruption of the MT-dynamics.