Forskolin Induces Hyperphosphorylation of Tau Accompanied by Cell Cycle Reactivation in Primary Hippocampal Neurons.

Abnormally hyperphosphorylated tau is the major protein constituent of neurofibrillary tangles (NFTs) in the brain of Alzheimer disease (AD) patients. Cell cycle reactivation is considered an important neuropathological feature of AD, and re-expression and activation of cell cycle regulators are known to occur in neurons containing NFTs. The aim of the present study was to investigate cell cycle reactivation during tau hyperphosphorylation in primary hippocampal neurons. We used forskolin, a specific activator of PKA, to induce tau hyperphosphorylation in cultured primary hippocampal neurons, and then measured levels of cyclin D1 and cyclin B1. We found that forskolin induced hyperphosphorylation of tau at Ser214, Ser396, and Ser202/Thr205 sites, attaining peak levels at 6, 12, and 12 h, respectively, while returning to normal levels at 24 h. Forskolin also induced a sustained cAMP elevation and PKA activation, which peaked at 6 h, in association with activation and overexpression of protein phosphatase-2A (PP-2A) at 24 h. The tau hyperphosphorylation was accompanied by increases in cyclin D1 and cyclin B1 levels; immunostaining showed overlapping distribution of hyperphosphorylated tau and cyclin D1 and cyclin B1 in primary hippocampal neurons. Forskolin induced hyperphosphorylation of tau and increased cyclin D1 and cyclin B1 protein levels in HEK293/tau441 cells, but not in the HEK293/vector cells, whereas the PKA inhibitor H89 inhibited the effects of forskolin on tau hyperphosphorylation and cyclin D1 and cyclin B1 protein levels. These findings suggest that forskolin induces tau hyperphosphorylation, which is itself necessary for the subsequent increases of cyclin D1 and cyclin B1 levels.