While the core subunits of Polycomb group (PcG) complexes are well characterized, little is known about the dynamics of these protein complexes during cellular differentiation. We used quantitative interaction proteomics and genome-wide profiling to study PcG proteins in mouse embryonic stem cells (ESCs) and neural progenitor cells (NPCs). We found the stoichiometry and genome-wide binding of PRC1 and PRC2 to be highly dynamic during neural differentiation. Intriguingly, we observed a downregulation and loss of PRC2 from H3K27me3-marked chromatin during differentiation, whereas PRC1 was retained at these sites. Additionally, we found PRC1 at enhancer and promoter regions independent of PRC2 binding and H3K27me3. Finally, overexpression of NPC-specific PRC1 interactors in ESCs led to increased Ring1b binding to and decreased expression of NPC-enriched Ring1b target genes. In summary, our integrative analyses have uncovered dynamic PcG subcomplexes and widespread co-localization with active chromatin marks during differentiation.

Polycomb complexes (PRC1 and PRC2) are essential regulators of epigenetic gene silencing in embryonic and adult stem cells. Emerging evidence suggests that the core subunit composition regulates distinct biological processes, yet little is known about the mechanistic underpinnings of how differently composed Polycomb complexes instruct and maintain cell fate. Here we find that Mel18, also known as Pcgf2 and one of six Pcgf paralogs, uniquely regulates PRC1 to specify mesoderm cell fate in embryonic stem cells. Mechanistically, Mel18 functions as a classical Polycomb protein during early cardiac mesoderm differentiation by repressing pluripotency, lineage specification, late cardiac differentiation, and negative regulators of the BMP pathway. However, Mel18 also positively regulates expression of key mesoderm transcription factors, revealing an unexpected function of Mel18 in gene activation during cardiac differentiation. Taken together, our findings reveal that Mel18 is required to specify PRC1 function in both a context- and stage-specific manner.

c Embryonic stem cells (ESCs) are characterized by their ability to self-renew and to differentiate into all cell types of a given organism. Understanding the molecular mechanisms that govern the ESC state is of great interest not only for basic research-for instance, ESCs represent a perfect system to study cellular differentiation in vitro-but also for their potential implications in human health, as these mechanisms are likewise involved in cancer progression and could be exploited in regenerative medicine. In this minireview, we focus on the latest insights into the molecular mechanisms mediated by the pluripotency factors as well as their roles during differentiation. We also discuss recent advances in understanding the function of the epigenetic regulators, Polycomb and MLL complexes, in ESC biology. Within 2 days after fertilization, a mouse oocyte has undergone a series of cellular divisions and has developed into the morula embryo. The totipotent cells within the morula then divide and further specialize to form the hollow blastocyst sphere. The outer layer of the blastocyst contains the trophectoderm cells, while the inner cell mass (ICM) contains the pluripotent embryonic stem cells (ESCs) that will give rise in the developing embryo to all cell types of the three germ layers-ectoderm, mesoderm, and endoderm. Mouse ESCs were first isolated by Evans and Kaufman in 1981 (1) and have since been extensively studied. Under proper cell culture conditions, ESCs can divide and self-renew indefinitely, yet under differentiation stimuli, ESCs can also differentiate into virtually all cell types of the organism.Which molecular mechanisms control the decision of ESCs to self-renew or to differentiate? During the last decades, several transcription factors have been identified to be essential for ESC pluripotency. These transcription factors regulate pluripotency by a so-called "pluripotency network" that regulates their own expression and coregulates the expression of other key transcription factors through multiple mechanisms. Interestingly, pluripotency is controlled at the transcriptional levels of genes through specific signaling pathways and epigenetic factors.Epigenetics is the study of heritable changes in gene expression that are not caused by changes at the DNA sequence level. The Polycomb and MLL (myeloid-lineage leukemia) complexes are two of the best-characterized epigenetic machineries implicated in ESC pluripotency and differentiation. Although pluripotency factors do not physically interact with Polycomb and MLL complexes, they coregulate lineage-specific genes important for ESC differentiation.In this review, we discuss the most recent advances in understanding mouse ESC pluripotency and differentiation, paying particular attention to Oct4, Nanog, and Sox2, as well as to factors involved in the exit from pluripotency. Finally, we discuss the function and the molecular mechanisms of the Polycomb and MLL complexes in mouse ESC pluripotency. MASTER REGULATORS OF ESC IDENTITY: THE Oct4, Sox2, AND Nano...