Dynamics and epigenetic control of PcG proteins during skeletal muscle cell differentiation
Prezioso, Carolina (2010) Dynamics and epigenetic control of PcG proteins during skeletal muscle cell differentiation. [Tesi di dottorato] (Inedito)
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During skeletal muscle differentiation, signal dependent switches in cell differentiation programs require global rearrangements in repression and activation of lineage specific genes, hence the importance of unravelling epigenetic mechanisms that control these dynamics and their integration with signaling pathways. Polycomb Group (PcG) proteins are transcriptional repressors that modify chromatin through epigenetic modifications that prevent changes in cell identity by maintaining transcription patterns, throughout development and in adulthood. PcG proteins form two major multiprotein complexes, Polycomb repressive complex 1 and 2 (PRC1 and PRC2, respectively), the latter containing the catalytic subunit, EZH2 that modifies histone H3 by trimethylation of lysine 27 (H3K27me3). While EZH2 promotes transcriptional repression of muscle specific genes, transcriptional activation that accompanies skeletal muscle differentiation is characterized by loss of EZH2 and recruitment of transcriptional activators, such as MyoD and SRF, at muscle regulatory regions(Caretti et al. 2004). Although much is known about the processes regulated by PcG proteins, little is known about signaling dependent pathway mechanisms that regulate PcG dynamics onto the chromatin. We used C2C12 mouse cell line as a skeletal muscle differentiation model to gain insight into the role of different PRC2 components during this process and into the the signaling pathways that regulates PRC2 dynamics at muscle target loci. We report that two different PRC2 complexes are present during skeletal muscle differentiation: PRC2-EZH2, that is predominant in proliferating myoblasts and PRC2-EZH1 that is specific for postmitotic myotubes. We show that these two complexes are differentially involved in the regulation of skeletal muscle differentiation. We demonstrate that the opposite dynamics of PRC2-EZH2 and PRC2-EZH1 at muscle regulatory regions is differentially regulated at the chromatin level by MAPK- (MSK1) dependent phospho/methyl switch mechanism involving phosphorylation of the lysine 28 of the histone H3 (H3Ser28ph), enabling to counteract the function of the PRC2 complex docking site H3K27me3. We report that, while MSK1/H3Ser28ph is crucial for the displacement of the PRC2-EZH2 from muscle regulatory regions,allowing gene activation and muscle differentiation, this pathway does not influence PRC2-EZH1 binding to chromatin, anticipating a novel PcG function in post-mitotic cells. Taken together, our results suggest that the phosphorylation of H3Ser28 by MSK1 plays a key role in epigenetic gene regulation and this modification is necessary for skeletal muscle differentiation.
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