The contribution of the cell cycle to reprogramming and the role of Jarid2 in coordinating Nanog expression and Wnt/PCP signalling in mESC differentiation and early development

Abstract

It remains unclear how cellular identities are established during developmental programming and reprogramming and what the contributing factors are. Pluripotent cells were shown to dominantly reprogram somatic nuclei towards pluripotency upon experimental cell fusion, but the role of the cell cycle in determining dominance has not been explored. I compared the reprogramming capacity of two pluripotent cell types, epiblast stem cells (EpiSCs) and embryonic stem cells (ESCs) reported to show different cell cycle structures. I demonstrated that compared to ESCs EpiSCs induce lower levels of pluripotency-associated transcripts in somatic nuclei, but activate the expression of genes associated with early differentiation programs. This reduced pluripotent reprogramming capacity of EpiSCs is not due to differences in the cell cycle as these cell types show similar cell cycle structures. Increasing Nanog expression is not sufficient to enhance the pluripotent reprogramming capacity of EpiSCs. Subsequently, I examined whether fusing S-G/2 somatic cells with G1 mESCs could reverse the dominance and reprogram mESCs towards somatic-like identity. I show that B lymphocytes do not induce reprogramming of ESCs nuclei towards lymphocyte-like state, regardless of cell cycle stages of these cell types. Establishment of new cellular identities can be also examined by ESC differentiation. Jarid2 is a member of Polycomb Repressor Complex 2 (PRC2) maintaining gene repression via chromatin regulation. Jarid2 deficiency does not cause major chromatin alterations, but results in impaired differentiation. In the second part of my thesis, I investigated factors and pathways deregulated in Jarid2-null mESCs. I show that Jarid2-null ESCs express constitutively high levels of Nanog, but reduced β-catenin activity and expression of PCP signalling components. Co-culture of Jarid2-null with wild-type mESCs reinstates normal Nanog expression and β-catenin activity, and partially restores the differentiation capacity of mutant cells. These results reveal a role for Jarid2 in coordinating a core pluripotency and Wnt/PCP signalling network, and facilitating ESC differentiation.