What controls cell fate?

Both intracellular transcriptional factors and extracellular signalling pathways contribute to cell differentiation. During early mammalian embryogenesis, the cells of the blastocyst's inner cell mass (ICM) differentiate either into epiblast or to the primitive endoderm (PrE). Initially, individual ICM cells co-express the transcription factors affecting this differentiation. Later, the expression patterns become mutually exclusive, responding to the extracellular signalling.

The EU-funded project CELLSTATETRANSITIONS (Capturing transition states associated with lineage decisions in the early mouse embryo) took a closer look at these events. The project used mouse ESCs as a tissue culture model. The researchers engineered cell lines carrying doxycycline-inducible genes, encoding fluorescently tagged GATA transcription factors. These DNA-binding proteins control differentiation processes in the cell by activating or repressing transcription. The project studied the interplay between the transcriptional regulation and a signalling pathway, involving fibroblast growth factors (FGFs) and mitogen-activated protein kinases (MAPKs).

CELLSTATETRANSITIONS showed how the levels of GATA factor in individual cells influenced the cell fate to undergo PrE-like differentiation. Scientists used time-lapse imaging of the fluorescently tagged GATA factors in individual cells, followed by immunostaining for fate markers. They found that PrE-like differentiation required a threshold level of GATA factor expression in individual cells. Differentiation experiments at different signalling levels revealed that FGF/MAPK signalling determined the proportion of differentiating cells by setting the threshold of GATA factors.

The project demonstrated that both transcription factor expression levels and signalling control the proportion of cells differentiating along the PrE lineage. The team suggested a simple mathematical model to describe the events underlying fate choice and validated this model by comparing simulated expression dynamics with experimentally measured processes. The resulting three scientific publications uncover a new principle for signalling in cell fate decisions, controlling the number of cells in a given lineage. In addition to new scientific understanding, the project pioneered multi-colour live cell imaging elucidating the structure of the gene regulatory networks.