Differentiation and cell cycle regulation in stem cell have a key function for embryonic development, organ homeostasis and tissue repair. Recent results have shown that these two mechanisms are intrinsically connected. Indeed, cell cycle machinery directly controls maintenance of pluripotency and initiation of differentiation. More precisely, the cell cycle regulator Cyclin D appears to control the transcriptional activity of Activin/Nodal signalling during progression of the cell cycle in human Embryonic Stem Cells (hESCs). As a consequence, hESCs can only differentiate into endoderm in the Early G1 phase when Cyclin Ds are expressed at low levels. These results show the mechanisms by which the cell cycle defines differentiation propensity of stem cells. However, these observations also imply the existence of interplays coordinating extra cellular signalling pathways with the epigenetic state, chromatin structure and transcriptional networks during cell cycle progression and these mechanisms remain to be fully uncovered.
Here, I have utilised the FUCCI reporter system combined with ATAC-Seq to analyse chromatin dynamics during cell cycle progression in hESCs. Furthermore, I performed ChIP-Seq analyses to define the genomic location of transcriptional regulators during cell cycle progression as well as RNA-Seq to confirm variation in gene expression pattern. Integration of these data shows that the chromatin status in hESCs is highly dynamic and the core pluripotency transcription factors and epigenetic modifiers change genomic location during cell cycle progression. I also showed that hESCs in the Late G1 phase accumulate transcripts that are important for differentiation and development; therefore, indicating this phase represents a unique portion of the cell cycle for cell fate decisions.
Taken together, these results uncover that transcriptional networks are unexpectedly dynamic during the progression of cell cycle in stem cells. I hypothesise that these modifications are necessary to prime hESCs for different cell fate choices allowing a diversity of differentiation that is otherwise impossible. Overall these mechanisms underline the need to study transcriptional and epigenetic mechanisms in the dynamic context of the cell cycle and have major implications for adult tissue homeostasis and disease.Wellcome Trust 4-Year (1+3) PhD Programme in Stem Cell Biology & Medicin
