Ingo Roeder

Project Title: Cell fate decisions in mES cells and the role of functional heterogeneity

Mouse embryonic stem (mES) cells are characterized by their potential to self-renew their own population while retaining the capacity to differentiate into a multitude of different cell types (pluripotency). How the balance between self-renewal and differentiation is regulated and how stem cells are directed towards differentiation are central questions in stem cell research. Although several lines of evidence suggest that different types of inter-cellular heterogeneity might play a decisive role, many details about the molecular regulation of pluripotency and spatial organization of mES cells are still unknown.

In our studies we focus on the regulatory mechanisms regarding the prominent transcription factor (TF) Nanog. It has been demonstrated that Nanog expression levels in mES cells are heterogeneous and reversibly changing between low and high states reflecting their propensity for differentiation. To study the effects of such dynamical state transitions on the maintenance and exit of pluripotency we apply a mathematical modelling approach describing the intra-cellular regulation of mES cells. As a first step we establish a network model to identify potential regulatory mechanisms that are in principle able to induce the observed Nanog variations. Subsequently, we refine our network model to describe the dynamical, time-extended exit from pluripotency and the transition into differentiation as the result of a molecular interaction network. In particular we study the role of a differentiation-inducing signal suggesting that differentiation requires a destabilization (transient down-regulation) of the Nanog levels to render mES cells responsive for external signals. Our findings are verified using experimental data on TF expression levels in mES cell cultures. Additionally, we aim to incorporate imaging data to further investigate the spatial arrangement of mES cells.

Concluding from our results we argue that Nanog heterogeneity is a functional element to control the differentiation propensity of mES cell populations supporting the idea of Nanog as a gate-keeper of pluripotency. We furthermore demonstrate the feasibility of our quantitative modelling approach to consistently describe a range of dynamical phenomena associated with maintenance of and exit from mES cell pluripotency.