Bone marrow-derived mesenchymal stem cells (BMMSCs) are multipotent stem cells capable of differentiation into a variety of cell types, proliferation, and production of clinically useful secretory factors. These advantages make BMMSCs highly useful for cell transplantation therapy. However, the molecular network underlying BMMSC proliferation remains poorly understood. Here, we showed that TGFβ-activated kinase 1 (Tak1) is a critical molecule that regulates the activation of cell cycling and that Tak1 inhibition leads to quiescence in BMMSCs both in vivo and in vitro. Mechanistically, Tak1 was phosphorylated by growth factor stimulations, allowing it to bind and stabilize Yap1/Taz, which could then be localized to the nucleus. We also demonstrated that the quiescence induction by inhibiting Tak1 increased oxidized stress tolerance and improved BMMSC engraftment in intramuscular and intrabone marrow cell transplantation models. This study reveals a novel pathway controlling BMMSC proliferation and suggests a useful method to improve the therapeutic effect of BMMSC transplantation. Stem Cells  2019;37:1595–1605

This study demonstrates that TGFβ-activated kinase 1 (Tak1) is a critical regulator of bone marrow-derived mesenchymal stem cell (BMMSC) proliferation. Tak1 was activated by various mitogenic cytokines and could then induce stabilization and nuclear localization of Yap1/Taz by interacting with these proteins. Tak1 inhibition brought about the quiescent state in BMMSCs both in vivo and in vitro. Reversible quiescence induced by Tak1 inhibition provided stress tolerance and improved engraftment for BMMSCs in cell transplantation models.