The Cell Cycle and Differentiation in Stem Cells

Abstract

The relationship between cellular proliferation and differentiation is a major topic in cell biology. What we know comes from models of somatic cell differentiation, where it is widely viewed that cycling and differentiation are coupled, antagonistic phenomena linked at the G1 phase. The extension of this view to stem cells, however, is unclear. One potential possibility is that stem cells also tightly link their G1 phase with their differentiation, indicating a similarity between the differentiation of stem cells and the differentiation of more mature somatic cells. On the other hand, stem cells may utilize different mechanisms or adaptations that confer on them some aspect of uniqueness or "stemness." In this case, stem cells will not exhibit the same coupling with the cell cycle as in many somatic cell models. In this thesis, we examined mouse embryonic stem cells (mESCs), a stem cell that is pluripotent and rapidly cycling with a highly condensed G1 phase. Direct extension of the somatic view posits that elongation of their G1 phase to somatic lengths by cyclin-dependent kinase (CDK) activity inhibition should induce or increase differentiation of these stem cells. Evidence supporting this claim has been contradictory. We show that elongation of the cell cycle and elongation of G1 to somatic lengths is fully compatible with the pluripotent state of mESCs. Multiple methods that lengthen the cell cycle and that target CDK activity or that trigger putative downstream mechanisms (i.e. Rb and E2F activity) all fail to induce differentiation on their own or even to facilitate differentiation. These results indicates that the model of linkage between the G1 phase and differentiation in mESCs is incorrect and leads us to propose that "stemness" may have a physiological basis in the decoupling of cell cycling and differentiation. In summary, we provide evidence that there is a resistance of mESCs to differentiation induced by lengthening G1 and/or the cell cycle. This could allow for separate control of these events and provide new opportunities for investigation and application.