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Specificity, Propagation, and Memory of Pericentric Heterochromatin

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Specificity, Propagation, and Memory of Pericentric Heterochromatin Article Specificity, propagation, and memory of pericentric heterochromatin Katharina Müller-Ott1, Fabian Erdel1, Anna Matveeva2, Jan-Philipp Mallm1, Anne Rademacher1, Matthias Hahn3, Caroline Bauer1, Qin Zhang2, Sabine Kaltofen1,†, Gunnar Schotta3, Thomas Höfer2 & Karsten Rippe1,* Abstract (Berger et al, 2009). These chromatin signals in turn recruit archi- tectural chromatin components or chromatin remodeling factors in The cell establishes heritable patterns of active and silenced chro- a highly dynamic manner and regulate genome access (McBryant matin via interacting factors that set, remove, and read epigenetic et al, 2006; Taverna et al, 2007; Campos & Reinberg, 2009; Clapier marks. To understand how the underlying networks operate, we & Cairns, 2009; Erdel et al, 2011a). On a global scale, the concerted have dissected transcriptional silencing in pericentric heterochro- and targeted activity of these networks results in the formation of matin (PCH) of mouse fibroblasts. We assembled a quantitative the denser, transcriptionally repressed heterochromatin state and map for the abundance and interactions of 16 factors related to the more open and biologically active euchromatin, which can be PCH in living cells and found that stably bound complexes of the distinguished at the resolution of the light microscope (Grewal & histone methyltransferase SUV39H1/2 demarcate the PCH state. Jia, 2007; Eissenberg & Reuter, 2009). A prototypic example for a From the experimental data, we developed a predictive mathe- constitutive heterochromatin domain is pericentric heterochromatin matical model that explains how chromatin-bound SUV39H1/2 (PCH) in mouse cells (Probst & Almouzni, 2008). It is characterized complexes act as nucleation sites and propagate a spatially by its high content of repetitive major satellite repeats and repres- confined PCH domain with elevated histone H3 lysine 9 trimethyla- sive epigenetic marks such as 5-methylcytosine (5meC) the binding tion levels via chromatin dynamics. This “nucleation and looping” of proteins with a methyl-CpG-binding domain that recognize this mechanism is particularly robust toward transient perturbations modification, trimethylation of histone H3 lysine residue 9 and stably maintains the PCH state. These features make it an (H3K9me3), and histone H4 lysine residue 20 (H4K20me3), as well attractive model for establishing functional epigenetic domains as hypoacetylation of histones (Probst & Almouzni, 2008). The throughout the genome based on the localized immobilization of H3K9me3 modification is set by the histone methylases SUV39H1 chromatin-modifying enzymes. and SUV39H2 (in the following “SUV39H” refers to both isoforms), while SUV4-20H1 and SUV4-20H2 set H4K20me2 and promote Keywords FRAP/FCS; heterochromatin protein 1; histone methylation; H4K20me3 (“SUV4-20H” for both isoforms) (Kwon & Workman, pericentric heterochromatin; protein network 2008; Schotta et al, 2008; Eissenberg & Reuter, 2009; Byrum et al, Subject Categories Quantitative Biology & Dynamical Systems; Chromatin, 2013). Epigenetics, Genomics & Functional Genomics A central protein component of PCH is heterochromatin protein 1 DOI 10.15252/msb.20145377 | Received 19 April 2014 | Revised 10 July 2014 | (HP1) that is present in three very similar isoforms HP1a, HP1b, Accepted 15 July 2014 and HP1c in mice and humans (Maison & Almouzni, 2004; Mol Syst Biol. (2014) 10: 746 Hiragami & Festenstein, 2005; Kwon & Workman, 2008). HP1 contains an N-terminal chromodomain (CD) and a C-terminal chro- moshadow-domain (CSD) connected by a flexible linker region. The Introduction CD interacts specifically with H3 histone tails that carry the K9me3 modification (Jacobs & Khorasanizadeh, 2002; Fischle et al, 2003). Epigenetic networks control the accessibility of DNA for transcrip- HP1 is able to form homo- and heterodimers (Nielsen et al, 2001; tion, DNA repair, and replication machineries. They establish and Yamamoto & Sonoda, 2003; Rosnoblet et al, 2011), interacts with maintain different functional chromatin states through cell division SUV39H1 (Aagaard et al, 1999; Yamamoto & Sonoda, 2003), SUV4- via protein factors that set or remove specific modifications of 20H2 (Schotta et al, 2004; Souza et al, 2009), the DNA methylase histones and DNA in the absence of alterations of the DNA sequence DNMT1 (Fuks et al, 2003; Lehnertz et al, 2003; Smallwood et al, 1 Deutsches Krebsforschungszentrum (DKFZ) and BioQuant, Research Group Genome Organization & Function, Heidelberg, Germany 2 Deutsches Krebsforschungszentrum (DKFZ) and BioQuant, Division Theoretical Systems Biology, Heidelberg, Germany 3 Munich Center for Integrated Protein Science and Adolf Butenandt Institute, Ludwig Maximilians University, Munich, Germany *Corresponding author. Tel: +49 6221 5451376; Fax: +49 6221 5451487; E-mail: [email protected] †Present address: Biochemistry & Structural Biology, Lund University, Lund, Sweden ª 2014 The Authors. Published under the terms of the CC BY 4.0 license Molecular Systems Biology 10: 746 | 2014 1 Molecular Systems Biology Dissecting the pericentric heterochromatin network Katharina Müller-Ott et al 2007) as well as the methyl-CpG-binding proteins MBD1 and MECP2 as the component that defines the PCH state. We further demon- (Fujita et al, 2003; Agarwal et al, 2007). SUV39H1 interacts with the strate that these SUV39H complexes represent “nucleation sites” DNA methylation-associated proteins DNMT1, MBD1, and MECP2 that are sufficient to provide specificity, confined propagation of the (Lunyak et al, 2002; Fujita et al, 2003; Fuks et al, 2003; Esteve et al, H3K9me3 mark as well as cellular memory to transmit the PCH state 2006). Thus, a complex protein–protein interaction network exists through the cell cycle. in PCH. The interactions constituting this network in mammalian cells have been studied mostly in vitro or via immunoprecipitation experiments and have not been probed comprehensively in living Results cells. Since HP1 interacts with SUV39H via its CSD, a feedback loop of The repressive PCH state is defined by enrichment of 5meC, HP1 binding-mediated H3K9 methylation has been proposed as a MECP2, MBD1, and SUV39H mechanism for propagating the H3K9me3 mark to adjacent nucleo- somes (Schotta et al, 2002; Grewal & Jia, 2007; Eissenberg & Reuter, We quantitated the enrichment of the PCH-associated histone modi- 2009). Theoretical models based on a combination of such feedback fications H3K9me3, H4K20me3, the DNA methylation 5meC and the loops have suggested the existence of two discrete chromatin states known proteins that set, remove, or recognize these modifications that can stably co-exist (“bistability”) for a certain range of condi- in the context of PCH in mouse NIH-3T3 fibroblasts: the histone tions (Schreiber & Bernstein, 2002; Dodd et al, 2007; Angel et al, H3K9 and histone H4K20 specific methylases SUV39H1, SUV39H2, 2011). Hathaway et al have proposed an alternative, “monostable” SUV4-20H1, SUV4-20H2, all three isoforms of the H3K9me3-reader model of heterochromatin propagation through interactions between HP1 (HP1a, HP1b, and HP1c), the histone demethylases JMJD2B neighboring nucleosomes (Hathaway et al, 2012). However, direct and JMJD2C that remove H3K9me3, the DNA methylase DNMT1 as evidence on how such epigenetic networks might operate in living well as the methyl-CpG binding domain (MBD)-containing proteins cells is lacking. In particular, three crucial questions remained MECP2, MBD1, MBD2, and MBD3 [see (Fodor et al, 2010) for a unanswered: (i) How is the separation of the genome in active and review of previously identified mammalian PCH components]. silenced chromatin states established and maintained and what are Furthermore, we included the transcription factors PAX3, PAX5, the factors that provide specificity for distinct euchromatic and PAX7, and PAX9 in our analysis since a role for PAX3 in PCH heterochromatic states? (ii) How is the confinement of a given chro- assembly has been reported (Bulut-Karslioglu et al, 2012). Although matin state to a certain genomic locus achieved? For the case of a the colocalization of these factors and histone marks with PCH was feedback loop between SUV39H, HP1, and H3K9me3 in PCH, it is studied previously, a comprehensive quantitative analysis of their elusive why the H3K9me3 does not spread throughout the whole PCH-specificity and abundance has been lacking. Thus, we fluores- genome. (iii) How is a given chromatin state like that of PCH trans- cently labeled the factors of interest (Supplementary Fig S1) and mitted through the cell cycle? determined the enrichment of GFP-tagged proteins in PCH using the Here, we have set out to address these issues by dissecting the workflow shown in Fig 1A: For each factor, the fluorescence inten- mouse pericentric heterochromatin network centered around the sity was measured in PCH as defined by foci with intense DAPI H3K9 and H4K20 methylation. This model system has the advantage (40,6-diamidino-2-phenylindole) staining and in the surrounding that the corresponding heterochromatin domains can be readily euchromatin of G1 phase cells. The enrichment of these factors in identified on fluorescence microscopy images as chromatin-dense PCH was calculated, followed by normalization to the enrichment of spots, the chromocenters. Accordingly, we were able to distinguish core histone H2A and DAPI in PCH. Chromatin binding states were the features of PCH from the surrounding bona fide euchromatin. identified by fluorescence recovery after photobleaching
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