Molecular competition in G1 controls when cells simultaneously commit to terminally differentiate and exit the cell-cycle

Abstract

Terminal differentiation is essential for the development and maintenance of tissues in all multi-cellular organisms and is associated with a permanent exit from the cell cycle. Failure to permanently exit the cell cycle can result in cancer and disease. However, the molecular mechanisms and timing that coordinates differentiation commitment and cell cycle exit are not yet understood. Here using adipogenesis as a model system to track differentiation commitment in live cells, we show that a rapid switch mechanism engages exclusively in G1 to trigger a simultaneous commitment to differentiate and permanently exit from the cell cycle. We identify a signal integration mechanism whereby the strengths of both mitogen and differentiation stimuli control a molecular competition between cyclin D1 and PPARG-induced expression of the CDK inhibitor p21 which in turn regulates if and when the differentiation switch is triggered and when the proliferative window closes. In this way, the differentiation control system is able to couple mitogen and differentiation stimuli to sustain a long-term balance between terminally differentiating cells and maintaining the progenitor cell pool, a parameter of critical importance for enabling proper development of tissue domains and organs. HIGHLIGHTSO_LIProgenitor cells both commit to terminally differentiate and permanently exit the cell cycle at a precise time in G1 as a result of a competition process that can last over multiple cell cycles. C_LIO_LIPositive-feedback driven expression of PPARG and the parallel induction of p21 triggers a rapid commitment to terminally differentiate and then maintains a postmitotic adipocyte state. C_LIO_LIOpposing mitogen and adipogenic signals are funneled into a molecular competition in G1 phase that controls if and when cells commit to differentiate, which in turn regulates the number of differentiated cells produced while allowing for the maintenance of sufficient progenitor cells. C_LI