TRANSCRIPTIONAL REGULATION OF PLURIPOTENCY AND CELL FATE

Abstract

Embryonic stem cells are pluripotent in nature, in that they can self-renew indefinitely, while maintaining the capability to give rise to all adult cell types. This characteristic makes them an attractive avenue for various therapeutic purposes. Therefore, many studies have been devoted to understanding the fundamental nature of these cells and the processes that govern and maintain their pluripotent cell fate. We hypothesized that cell fate is intrinsically regulated by the underlying chromatin and transcriptional machinery of the cells. To test this, we first studied the heterogeneity that exists within pluripotent cell cultures. We showed that pluripotent sub-populations demarcated by expression of REX1, a pluripotency transcription factor, have distinct differentiation propensities. Additionally, we found that chromatin modification via DNA methylation was the underlying cause of this heterogeneity, providing evidence for the major roles that chromatin and transcription play in regulating cell fate. We next studied a fate maintaining mechanism, mitotic bookmarking, as a method of cell fate preservation. During cell division, chromatin structure undergoes significant remodeling, various proteins uncouple from the DNA, and there is a temporary hiatus in transcription. Despite this restructuring the transcriptional memory of the parent cell is faithfully transmitted to daughter cells. We hypothesized that, in ES cells, several chromatin bound factors would be retained on the mitotic chromatin and would act as bookmarks preserving the underlying pluripotent chromatin structure. Using a global unbiased approach, we found that a large number of chromatin regulators are indeed bound to the mitotic chromatin. Additionally, a chromatin accessibility assay revealed that a large number of accessible promoter sites are preserved during mitosis and into G1 of daughter cells. The mitotic chromatin bound factors likely play a role in the maintenance of these protected DNA sites. Our data suggest that, preservation of these sites by various chromatin regulators during mitosis underlies the faithful transmission of cell identity from parent to daughter cells, ergo maintaining the cell’s fate.