Proliferation maintains the undifferentiated status of stem cells: the role of the planarian cell cycle regulator Cdh1

The coincidence of cell cycle arrest and differentiation has been described in a wide variety of stem cells and organisms for decades, but the causal relationship is still unclear due to the complicated regulation of the cell cycle. Here, we used the planarian Dugesia japonica since it possesses a quite simple cell cycle regulation in which cdh1 is the only factor that arrests the cell cycle. When cdh1 was functionally inhibited, the planarians could not maintain their tissue homeostasis and could not regenerate their missing body parts. While the ablation of cdh1 caused pronounced propagation of the stem cells, the progenitor and differentiated cells were decreased. Further analysis indicated that the stem cells without cdh1 expression did not undergo differentiation even though they received ERK signaling activation as an induction signal. These results suggested that stem cells could not acquire differentiation competence without cell cycle arrest. Thus, we propose that cell cycle regulation determines the differentiation competence and that cell cycle exit to G0 enables stem cells to undergo differentiation. Summary statement By using planarians, which have quite simple cell cycle regulation, we revealed that stem cells with cell cycle progression could not undergo differentiation even though an induction signal was activated.