Elevated p16Ink4a Expression Enhances Tau Phosphorylation in Neurons Differentiated From Human-Induced Pluripotent Stem Cells.

Increased expression of the cyclin-dependent kinase inhibitor p16Ink4a (p16) is detected in neurons of human Alzheimer's disease (AD) brains and during normal aging. Importantly, selective eliminating p16-expressing cells in AD mouse models attenuates tau pathologies and improves cognition. But whether and how p16 contributes to AD pathogenesis remains unclear. To address this question, we tested whether induction of p16 expression in neurons exacerbates AD pathologies. We created a doxycycline-inducible system to trigger p16 up-regulation in human-induced pluripotent stem cells (iPSCs) and neurons differentiated from iPSCs. We demonstrated that up-regulated p16 expression in iPSCs reduces cell proliferation, down-regulates cell cycle genes, and up-regulates genes involved in focal adhesion, interferon α response and PI3K-Akt signaling. Our approach enables temporal control of p16 induction upon differentiation from iPSCs to neurons. In differentiated cortical neurons, we found that up-regulation of p16 increases tau phosphorylation at Ser202/Thr205 and Thr231 in a cell-autonomous manner, while amyloid beta secretion is not affected. These data suggest a critical role of p16 in regulating tau phosphorylation in neurons, and thereby contributing to pathological progression of AD. As pathological tau tangles have been shown to induce p16 expression, our studies suggest a positive feedback loop between p16 and tau to exacerbate tau pathologies.