Jarid2 Methylation via the PRC2 Complex Regulates H3K27me3 Deposition during Cell Differentiation

摘要

class="head no_bottom_margin" id="sec1title">IntroductionAppropriate gene expression patterns in distinct cell lineages need to be set during embryogenesis and perpetuated during the lifespan of an organism. Polycomb group (PcG) proteins are known to take part in the maintenance of gene repression mostly through chromatin regulation (). Polycomb Repressive Complex 2 (PRC2), a key component of the Polycomb machinery, is composed of four core components: the catalytic subunit Ezh1/2, Suz12, Eed, and RbAp46/48. PRC2 is responsible for the di- and tri-methylation of histone H3 at lysine 27 (H3K27me2/3), a histone mark that correlates with silent or poorly transcribed genomic regions (). In addition to the core components of PRC2, several cofactors were shown to interact with this complex and to modulate both its binding to chromatin and its enzymatic activity ().The molecular mechanisms responsible for PRC2 recruitment to chromatin are still unclear. Two models have been proposed (). A first “instructive” model proposes that PRC2 recruitment relies either on transcription factors (TFs) or on long non-coding RNAs (lncRNAs). Several studies support this hypothesis, with the examples of the lncRNAs Xist (), HOTAIR (), or Kcnq1ot1 () and of TFs such as YY1 () or Snail (). However, the nature and the relevance of the interactions between PRC2 and lncRNAs or TFs are not yet clear ().A second “responsive” model relies on the observation that chromatin structure modulates PRC2 recruitment and functions. Several studies have shown that PRC2 activity is regulated by marks already present on chromatin. For example, the H3K4me3 and H3K36me3 marks associated to active transcription are reported to prevent the methylation of H3K27 by PRC2 when present on the same histone tail (). In contrast, PRC2 enzymatic activity is stimulated by H3K27me3 via specific interactions between the methylated lysine 27 and the aromatic cage of Eed () and by H2A ubiquitination (H2AUb) through a less defined molecular mechanism ().Other chromatin features have also been shown to impact PRC2/chromatin interaction such as DNA methylation () and nucleosome density (href="#bib42" rid="bib42 bib50" class=" bibr popnode">Simon and Kingston, 2013; Yuan et al., 2012). In addition, PRC2 cofactors actively contribute to “sensing” chromatin structure. For instance, the PCL proteins were recently reported to recognize H3K36me3 (href="#bib4" rid="bib4 bib6 bib30 bib36" class=" bibr popnode">Brien et al., 2012; Cai et al., 2013; Musselman et al., 2012; Qin et al., 2013) and both the cofactors Aebp2 and Jarid2 have putative DNA binding domains (href="#bib17" rid="bib17 bib18" class=" bibr popnode">Kim et al., 2004, 2009). Of note, transcription can modulate PRC2 function not only through its impact on chromatin but also through PRC2 interaction with nascent RNA transcripts (href="#bib9" rid="bib9 bib14 bib16" class=" bibr popnode">Davidovich et al., 2013; Kaneko et al., 2013; Kanhere et al., 2010).Jarid2, a member of the Jumonji family of proteins (href="#bib20" rid="bib20" class=" bibr popnode">Klose et al., 2006), is a developmental regulator, which is necessary for proper mouse development and embryonic stem cell (ESC) differentiation (href="#bib22" rid="bib22" class=" bibr popnode">Landeira and Fisher, 2011). However, unlike other members of the Jumonji family of proteins, Jarid2 has no histone demethylase activity. Previous studies showed that it interacts with PRC2 complex (href="#bib23" rid="bib23 bib25 bib33 bib34 bib41" class=" bibr popnode">Landeira et al., 2010; Li et al., 2010; Pasini et al., 2010; Peng et al., 2009; Shen et al., 2009). PRC2 and Jarid2 mostly co-localize at chromatin in ESC (href="#bib23" rid="bib23 bib25 bib33 bib34 bib41" class=" bibr popnode">Landeira et al., 2010; Li et al., 2010; Pasini et al., 2010; Peng et al., 2009; Shen et al., 2009) and Jarid2 depletion reduces PRC2 enrichment at chromatin, leading to the hypothesis that Jarid2 may act to recruit PRC2 (href="#bib33" rid="bib33" class=" bibr popnode">Pasini et al., 2010). In support of this, we recently demonstrated that Jarid2 has a nucleosome-binding domain that stabilizes PRC2 binding to chromatin (href="#bib43" rid="bib43" class=" bibr popnode">Son et al., 2013), interaction that could be modulated by lncRNAs (href="#bib15" rid="bib15" class=" bibr popnode">Kaneko et al., 2014). Notably, reduced occupancy of PRC2 at chromatin in the absence of Jarid2 does not translate into substantial decrease of H3K27me3 enrichment, suggesting that Jarid2 could constrain PRC2 enzymatic activity. However, several studies have now shown that Jarid2 positively regulates PRC2 activity (href="#bib25" rid="bib25 bib43 bib51" class=" bibr popnode">Li et al., 2010; Son et al., 2013; Zhang et al., 2011). Overall, although there is a consensus on the importance of Jarid2 as regulator of PRC2, how exactly it modulates PRC2 and H3K27me3 deposition is far from clear.In this study, we investigate the interplay between Jarid2 and PRC2 and explore how this cofactor regulates H3K27me3 deposition and PRC2 function. We show that Jarid2 is a substrate for PRC2 and we characterize Jarid2 methylation in relation to PRC2 activity by a combination of biochemical, genomic, and in vivo approaches. Our study reveals that Jarid2 methylation participates in a unique regulatory mechanism controlling PRC2 catalytic activity and is required for the proper deposition of H3K27me3 during cell differentiation.