A Cbx8-Containing Polycomb Complex Facilitates the Transition to Gene Activation during ES Cell Differentiation

Polycomb proteins play an essential role in maintaining the repression of developmental genes in self-renewing embryonic stem cells. The exact mechanism allowing the derepression of polycomb target genes during cell differentiation remains unclear. Our project aimed to identify Cbx8 binding sites in differentiating mouse embryonic stem cells. Therefore, we used a genome-wide chromatin immunoprecipitation of endogenous Cbx8 coupled to direct massive parallel sequencing (ChIP-Seq). Our analysis identified 171 high confidence peaks. By crossing our data with previously published microarray analysis, we show that several differentiation genes transiently recruit Cbx8 during their early activation. Depletion of Cbx8 partially impairs the transcriptional activation of these genes. Both interaction analysis, as well as chromatin immunoprecipitation experiments support the idea that activating Cbx8 acts in the context of an intact PRC1 complex. Prolonged gene activation results in eviction of PRC1 despite persisting H3K27me3 and H2A ubiquitination. The composition of PRC1 is highly modular and changes when embryonic stem cells commit to differentiation. We further demonstrate that the exchange of Cbx7 for Cbx8 is required for the effective activation of differentiation genes. Taken together, our results establish a function for a Cbx8-containing complex in facilitating the transition from a Polycomb-repressed chromatin state to an active state. As this affects several key regulatory differentiation genes this mechanism is likely to contribute to the robust execution of differentiation programs.
Author Summary Cell fate transitions have long been known to be accompanied by alterations in chromatin structure. But only during the last few years has it become clear that chromatin modifications form the molecular basis of an epigenetic memory that defines cell identity. The Polycomb Group Proteins (PcGs) form two major protein complexes known as polycomb repressive complexes 1 and 2 (PRC1 and PRC2). Their function is essential for the maintenance of transcriptional repression during embryogenesis through the methylation of the lysine 27 on histone H3 and the subsequent ubiquitination of histone H2A. The chromobox homolog 8, Cbx8, which is part of the PRC1 complex, is therefore generally defined as a repressor of gene transcription. The genome wide profiling of Cbx8 during the early steps of mouse embryonic stem (mES) cells differentiation provided us with surprising results involving Cbx8 in gene activation. Our results point out that Cbx8 is part of a PRC1 complex involved in the transition from a Polycomb repressed state to an active state.