Nonoverlapping Functions of the Polycomb Group Cbx Family of Proteins in Embryonic Stem Cells

Abstract

Polycomb group proteins are essential regulators of cell fate decisions during embryogenesis. In mammals, at least five different Cbx proteins (Cbx2, Cbx4, Cbx6, Cbx7, and Cbx8) are known to associate with the core Polycomb repressive complex 1 (PRC1). Here we show that pluripotency and differentiation of mouse embryonic stem cells (ESCs) is regulated by different Cbx-associated PRC1 complexes with unique functions. Maintenance of pluripotency primarily depends on Cbx7, while lineage commitment is orchestrated by Cbx2 and Cbx4. At the molecular level, we have uncovered a Polycomb autoregulatory loop in which Cbx7 represses the expression of prodifferentiation Cbx proteins, thereby maintaining the pluripotent state. We additionally show that the occupancy of Cbx7 on promoters is completely dependent on PRC2 activity but only partially dependent on a functional PRC1 complex. Thus, Cbx proteins confer distinct target selectivity to the PRC1 complex, achieving a balance between the self-renewal and the differentiation of ESCs.

CBX7 and miR‐9 are part of an autoregulatory loop controlling p16INK4a

“…The regulation of CBX7 is particularly interesting because CBX7 is implicated in senescence (Gil et al ., 2004) and cancer (Bernard et al ., 2005; Scott et al ., 2007; Pallante et al ., 2008; Karamitopoulou et al ., 2010). In addition, CBX7 levels and consequently the composition of PRC1 complexes change dramatically during ES cell differentiation (Morey et al ., 2012; O'Loghlen et al ., 2012). In a screen for transcription factors that regulate CBX7 (O'Loghlen et al ., 2015), we identified E2F family members and the nuclear receptor NR2E1 as positive regulators of CBX7 expression (O'Loghlen et al ., 2015).…”

“…Indeed, CBX7 was first identified in a screen for bypass of replicative senescence (Gil et al ., 2004). More recently, CBX7 was recognized as the main orthologue of Drosophila Polycomb implicated in maintaining the self‐renewal of embryonic stem (ES) cells (Morey et al ., 2012; O'Loghlen et al ., 2012). In ES cells, the miR‐125 and miR‐181 families regulate CBX7 expression levels and impact on the balance between self‐renewal and differentiation (O'Loghlen et al ., 2012).…”

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

“…We observed that knockdown of Ring1b caused misregulation, either up-or down-regulation, of selected marker genes during EB differentiation in agreement with earlier reports. 10,51 In comparison, depletion of Cdk8 led only to mild alterations of the expression levels (Afp, Brachyury, Mixl1, Nestin) or caused increased expression (Gata6, VE-Cadherin, Claudin6, Gsc, Hand1, Otx2, Bmp4) of the marker genes when compared to control cells. Importantly, knockdown of Med12 caused a sustained repression of most of the developmental marker genes tested suggesting that Med12 plays a decisive role in the activation of these genes during differentiation.…”

“…4B and S5B) as previously reported. 10 Likewise, EBs generated from shMed12 cells exhibit smaller diameters as compared to the control EBs. In contrast, the diameters of Cdk8 knockdown and control EBs were heterogeneous in size.…”

“…In the course of differentiation Cbx7 is replaced by other proteins of the Cbx family (Cbx2 and Cbx4), which play a role in later stages of development. 10,11 Another type of PRC1 in mESCs, which is recruited to chromatin independent of the H3K27me3 mark, comprises RYBP instead of Cbx7. 12 PRC1 is thought to repress transcription via direct interaction with the general transcriptional machinery and it has been proposed that Mediator subunits might facilitate this repressive function.…”