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Activin/Smad2-induced H3K27me3 reduction is crucial to initiate mesendoderm differentiation of ES Cells

ABSTRACT: Mesendoderm (ME) differentiation of human embryonic stem cells (hESCs) is directed by various extrinsic signals together with intrinsic epigenetic modifications. However, the dynamics of epigenetic modifications and their regulation to initiate ME differentiation remain elusive. In this study, we report that H3K27me3 is decreased during ME initiation, which is essential for the subsequent differentiation by collaborative effects of Activin and Wnt signaling. Mechanistically, Activin decreases the H3K27me3 level via disruption of the SUZ12-EZH2 interaction and EZH2 degradation mediated by Smad2. Our data suggest a two-step process of ME initiation: firstly H3K27me3-marked epigenetic priming and secondly transcription activation. Our findings unravel a critical role of H3K27me3 priming and a direct interaction between extrinsic signals and epigenetic modifications during ME initiation.

ORGANISM(S): Homo sapiens

PROVIDER: GSE81617 | GEO | 2016/11/30

SECONDARY ACCESSION(S): PRJNA322087

REPOSITORIES: GEO

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Project description:Cardiovascular diseases are a major cause of life-threatening burden around the world. The heart has a very low regeneration capacity and donor organs for transplantation are scarce. Therefore regeneration of lost myocardium with stem cell-derived cardiomyocytes (CMs) provides an attractive strategy for heart repair. Human pluripotent stem cells (hPSCs) can be efficiently differentiated in vitro into CMs but the molecular mechanisms behind this process are still not fully understood. In particular identification of secreted autocrine and/or paracrine factors that function as important extrinsic signals remained elusive because the mass spectrometry (MS)-based identification of secreted proteins from cell culture supernatants is impeded by high levels of albumin present in common differentiation media. Thus we established an albumin-free cardiomyogenic differentiation medium and performed secretomics at seven different time points during in vitro differentiation. This analysis led to the identification of 4832 proteins with 1802 being significantly altered during differentiation and 431 of these were annotated as secreted according to gene ontology. Bioinformatics revealed enrichment of extrinsic Wnt pathway-related proteins 3 days upon induction of differentiation and of extracellular matrix proteins in the resulting CMs. Numerous extrinsic components of Wnt, Activin A, Nodal, TGFβ, BMP or FGF signaling pathways were quantitatively assessed during differentiation. Notably, the abundance of pathway agonists was generally lower compared to the respective antagonists but their curves of progression over timer were rather similar. We hypothesize that Activin A, Nodal and TGFβ signaling are turned down shortly upon initiation of cardiac differentiation whereas BMP signaling is switched on. Wnt and FGF signaling peaks between d0 and d3 of differentiation and interestingly, Activin A and TGFβ signaling seem to be reactivated at the cardiac progenitor stages and/or in early CMs.

Project description:Epigenetic priming factors establish a permissive epigenetic landscape which is not required until a later developmental or physiological time point, temporally uncoupling the presence of these factors with their phenotypic effects. One classic example of epigenetic priming is in the context of bivalent chromatin, found in pluripotent stem cells and early embryos at key developmental gene promoters marked by both activating-associated H3K4me3 and repressive-associated H3K27me3 histone modifications. It is currently unknown how these bivalent domains are targeted, or precisely how they impact on lineage commitment. Here we show that the small heterodimerising non-enzymatic DNA binding proteins Developmental Pluripotency Associated 2 (Dppa2) and 4 (Dppa4) act as epigenetic priming factors to establish bivalency at a subset of developmental genes. Dppa2/4 localise to the +1 nucleosome position of bivalent genes and while they are not required for pluripotency in embryonic stem cells (ESCs), double knockout cells fail to exit pluripotency and to differentiate efficiently, with delays in upregulating bivalently marked lineage genes. Proteomics reveal that Dppa2/4 interact on chromatin with members of the COMPASS and Polycomb complexes important for H3K4me3 and H3K27me3 deposition, respectively. Epigenetic profiling reveals a striking loss of H3K4me3, H3K27me3, and their associated enzymatic machinery at a significant subset of bivalent promoters in Dppa2/4 mutants, in addition to loss of H2A.Z and chromatin accessibility. In wild-type ESCs, these “Dppa2/4-dependent” bivalent promoters are characterised by low H3K4me3 enrichment and breadth, near-absent expression levels and initiating but not elongating RNA polymerase. Notably, Dppa2/4-dependent promoters are less evolutionarily conserved suggesting that they lack additional safeguard measures to maintain bivalency at these genes in the absence of Dppa2/4. Concomitantly with the loss of bivalency, Dppa2/4-dependent bivalent promoters gain DNA methylation and consequently are no longer able to be effectively activated upon ESC differentiation, leading to defects in cell fate acquisition. Our findings reveal a targeting principle for bivalency to developmental gene promoters poising them for future lineage specific gene activation.

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Activin/Smad2-induced H3K27me3 reduction is crucial to initiate mesendoderm differentiation of ES Cells

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