Forschungsgruppenleiter, Systemische Zellbiologie
Dynamik der Zelldifferenzierung
The emergence of cells with discrete fates from undifferentiated precursor cells is a central theme in development and tissue homeostasis. What are the mechanisms that guide the underlying cell fate decisions and lead to differentiation?
We study the role of cell signaling in this process, using mouse embryonic stem cells (mESCs) as a model system. These cells can be maintained in culture in an undifferentiated state for unlimited periods of time, while retaining the ability to differentiate into all embryonic cell types in vitro and in vivo (Fig. 2). This makes them an ideal system to follow the molecular dynamics that underlie cell fate decisions of mammalian development in individual live cells. Furthermore, ESC assemblies have the remarkable property to self-organize into embryo-like structures, containing defined sets of differentiated cell types in reproducible spatial arrangements. In our work, we seek to address the operational properties of signaling networks in cellular differentiation. Genetic and biochemical approaches over the last decades have revealed both the parts-list of signaling components required for specific fate decisions and the regulatory links between them. Now is the time to study the functional properties of these networks – how do they process and encode information, and how does this encoding control fate decisions? Addressing this knowledge gap requires quantitative, dynamic measurements of the molecular processes underlying cell fate decisions in single cells. To achieve this, we apply a range of live reporter systems combined with long-term time-lapse imaging and tracking of mESCs (Movie 1). We use our quantitative, time-resolved datasets to inform and constrain mathematical models of the molecular circuits that control cell fate decisions. Applying this quantitative approach to stem cell biology, we expect to identify general principles of developmental cell fate decisions. Our results may also inform new approaches for the targeted differentiation of stem cells in the field of regenerative medicine.
The goals of our research are to understand
- How quantitative aspects of growth factor signaling are encoded in the dynamic activation of signaling networks within single cells
- How cell signaling coordinates fate decisions in cell populations
- How cell signaling leads to the self-organizing properties of ESC assemblies
Our current work is mainly focused on understanding the functions of fibroblast growth factor (FGF) signaling in these contexts. We are also extending these investigations to other signaling systems with functions in maintaining pluripotency and inducing differentiation, such as TGF- and BMP signals
1. Quantitative encoding and decoding of extracellular signals
Understanding how cell communication controls fates requires knowing how cells quantitatively process information contained in the identity and concentration of extracellular ligands. Although the components and molecular architecture of the signaling systems downstream of specific ligands are well established, we still know surprisingly little about the functional properties of these networks - do they simply detect the presence or absence of ligands, or do they read their concentration, their dynamic changes, or a combination of the two? And how are these quantitative aspects of ligand exposure encoded in signal transduction activity? We explore these questions in the context of signal transduction and transcriptional response downstream of fibroblast growth factor (FGF) ligands, because of their importance for many cell fate decisions of early development. Our initial results indicate complex dynamics of ERK activation and transcriptional response following FGF exposure in single cells (Fig. 1), raising the attractive hypothesis that ESCs store information about extracellular ligand concentrations in the dynamic activation pattern of signaling pathways. In collaboration with the group of Luis Morelli at the IBioBA (Buenos Aires, Argentina), we develop time series analysis approaches to determine dynamic signatures and characteristic timescales of these events, and to test how they relate to ligand concentration or time-course of exposure. By extending these investigations to studying signaling dynamics downstream of other ligands with functions in maintaining pluripotency and inducing differentiation, we hope to identify generic principles for information encoding by intracellular signaling pathways in cell fate decisions.
2. Coupling cell fate decisions with FGF/ERK signaling
In this project we ask how cell communication mediated by FGF coordinates fate decisions in a cell population. We build on previous work where we developed an mESC model for the fate decision between the epiblast (Epi) and the primitive endoderm (PrE) fate that occurs in the preimplantation blastocyst (Schröter et al., 2015; Fig. 1A). We have shown that the transcriptional networks underlying the two fates form a mutual repression circuit that functions as a bistable switch, and demonstrated that FGF signaling sets the switching threshold of this circuit in individual cells. Recently, we have found that expression of the Fgf4 gene is under the control of the genes that control the Epi versus PrE fate decision, thereby coupling the transcription-factor based bistable switches in individual cells at the population level. Our experimental and theoretical analysis (together with the group of A Koseska) of this “network of networks” suggest that it has the potential to generate stereotyped proportions of Epi- and PrE-like cells. To further explore this idea, we are developing tools to visualize and measure FGF signaling events in this system. We expect that this work will describe novel roles for FGF signaling that may be relevant beyond cell fate decisions of preimplantation development.
3. Cell communication and the self-organization of ESC assemblies
A remarkable property of ESCs that has emerged over the last few years is their ability to self-organize into embryo-like structures with a reproducible arrangement of several more differentiated cell types. To understand how cell-cell communication controls this process, we are planning to extend our investigations from two-dimensional culture conditions to three-dimensional embryonic organoid systems. Our goal here is to visualize and measure critical parameters of cell communication in three-dimensional cell aggregates, such as ligand production and spatial range of signals. This knowledge can inform spatial models of cell-fate decision-making in cell aggregates. To start this line of investigation we have, together our collaborator S. Fischer in the group of Ernst Stelzer in Frankfurt, developed an embryonic organoid system to model the fate decision between the Epi and the PrE fate in an ICM-like three-dimensional geometry. In the future, we will also be using organoid systems to study cell fate decisions of post-implantation development in three-dimensional cell aggregates. Our work in this area shall contribute to a mechanistic understanding of self-organization in ESC assemblies.