Molecular mechanisms of mouse embryonic stem cell differentiation

Mouse embryonic stem (ES) cells are pluripotent cells, meaning that they can give rise to all tissues in the body. This has catalyzed research in both early embryogenesis as a model system for mammalian development as well as regenerative medicine as a renewable source of unspecialized cells which can be converted into nearly any cell type required by a patient. ES cells have been an invaluable resource for advancing fundamental understanding of global transcriptional and epigenetic regulations, signaling pathways, and noncoding RNA in mammalian systems. However, the molecular mechanisms of how ES cells are differentiated remain much less understood. Differentiation is a complex process involving actions of ES cell core factors, lineage specific regulators, epigenetic modifications, and chromatin remodelers. Thus, a single reporter-based screen would have been inappropriate to identify novel regulators of ES cell differentiation. To overcome the problems, we have developed a unique signature-based screen. This screen is capable of analyzing the expression of 48 genes simultaneously across dozens of different samples, and our gene list covers all three germ layers that arise during normal embryonic development, the trophectoderm, and epigenetic regulators of chromatin status. Our signature-based screen established several categories of genes based on their comparative functions during the differentiation of ES cells. This will be a valuable information for other researchers interested in ES cell differentiation from various perspectives. We have identified two novel regulators of ES cell differentiation – Yap1 and Rbpj. Yap1 is a transcriptional co-activator of Hippo signaling pathway. We disproved past misconceptions in the field about the role of Yap1 concerning its function in ES cell self-renewal, showing that like the inner cell mass, Yap1 is dispensable for long-term maintenance in culture. Conversely, we found that Yap1 is essential for proper ES cell differentiation. Rbpj is a transcriptional regulator of Notch signaling pathway. Consistent with previous observations of repressive role of Rbpj, Rbpj serves as a repressor of ES cell core factors in the absence of Notch signaling pathway. Repressive role of Rbpj is also required for proper differentiation of ES cells by silencing core factors.