Ubiquitin E3 Ligase Ring1b/Rnf2 of Polycomb Repressive Complex 1 Contributes to Stable Maintenance of Mouse Embryonic Stem Cells

Abstract

BackgroundPolycomb repressive complex 1 (PRC1) core member Ring1b/Rnf2, with ubiquitin E3 ligase activity towards histone H2A at lysine 119, is essential for early embryogenesis. To obtain more insight into the role of Ring1b in early development, we studied its function in mouse embryonic stem (ES) cells.Methodology/Principal FindingsWe investigated the effects of Ring1b ablation on transcriptional regulation using Ring1b conditional knockout ES cells and large-scale gene expression analysis. The absence of Ring1b results in aberrant expression of key developmental genes and deregulation of specific differentiation-related pathways, including TGFbeta signaling, cell cycle regulation and cellular communication. Moreover, ES cell markers, including Zfp42/Rex-1 and Sox2, are downregulated. Importantly, retained expression of ES cell regulators Oct4, Nanog and alkaline phosphatase indicates that Ring1b-deficient ES cells retain important ES cell specific characteristics. Comparative analysis of our expression profiling data with previously published global binding studies shows that the genes that are bound by Ring1b in ES cells have bivalent histone marks, i.e. both active H3K4me3 and repressive H3K27me3, or the active H3K4me3 histone mark alone and are associated with CpG-‘rich’ promoters. However, deletion of Ring1b results in deregulation, mainly derepression, of only a subset of these genes, suggesting that additional silencing mechanisms are involved in repression of the other Ring1b bound genes in ES cells.ConclusionsRing1b is essential to stably maintain an undifferentiated state of mouse ES cells by repressing genes with important roles during differentiation and development. These genes are characterized by high CpG content promoters and bivalent histone marks or the active H3K4me3 histone mark alone.

Dual role of Med12 in PRC1-dependent gene repression and ncRNA-mediated transcriptional activation

“…In Ring1b depleted mESCs only Sox2 levels were compromised while the expression of the other ES regulators remained unchanged in line with earlier reports. 49,50 Depletion of Med12 resulted in reduced levels of Oct4, Nanog and Sox2 in accordance with previous findings. 44 In addition we observed reduced levels of Klf4 in Med12 knockdown mESCs.…”

Distinct histone modifications in stem cell lines and tissue lineages from the early mouse embryo

“…The H3K27me3 mark is deposited by PRC2 and functions at bivalent domains in ES cells by recruiting PRC1, which maintains the underlying promoter in a poised transcriptional state (34)(35)(36). Using ChIP-qPCR, we detected binding of PRC2 components Ezh2 and Eed above background at H3K27me3-marked genes in TS and XEN cells, suggesting that H3K27me3 is actively maintained in extraembryonic stem cells (Fig.…”

SetDB1 contributes to repression of genes encoding developmental regulators and maintenance of ES cell state

“…They also occupy genes encoding a large set of developmental regulators that are silent in ES cells, but whose expression is associated with lineage commitment and cellular differentiation. Polycomb-repressive complexes (PRCs) cooccupy the genes encoding these developmental regulators to help maintain a silent transcriptional state in ES cells (Azuara et al 2006;Boyer et al 2006; Lee et al 2006;Stock et al 2007; van der Stoop et al 2008).…”

“…The PcG protein complex PRC2 catalyzes H3K27me3, which contributes to repression of genes encoding key developmental regulators. Disruption of PcG function in ES cells leads to derepression of these developmental genes, alteration of the ES cell transcriptional program, and a loss of the ES cell state Lee et al 2006;Pasini et al 2007;Stock et al 2007;Jaenisch and Young 2008;van der Stoop et al 2008). In ES cells, genes encoding lineage-specific developmental regulators contain nucleosomes with both H3K4me3 and H3K27me3 modifications, and have thus been called ''bivalent'' (Bernstein et al 2006).…”