Project description:We determined genome-wide nucleosome occupancy in mouse embryonic stem cells and their neural progenitor and embryonic fibroblast counterparts to assess features associated with nucleosome positioning during lineage commitment. Cell type and protein specific binding preferences of transcription factors to sites with either low (e.g. Myc, Klf4, Zfx) or high (e.g. Nanog, Oct4 and Sox2) nucleosome occupancy as well as complex patterns for CTCF were identified. Nucleosome depleted regions around transcription start and termination sites were broad and more pronounced for active genes, with distinct patterns for promoters classified according to their CpG-content or histone methylation marks. Throughout the genome nucleosome occupancy was dependent on the presence of certain histone methylation or acetylation modifications. In addition, the average nucleosome-repeat length increased during differentiation by 5-7 base pairs, with local variations for specific genomic regions. Our results reveal regulatory mechanisms of cell differentiation acting through nucleosome repositioning. The Total RNA from ESCs, NPCs and MEFs was extracted by guanidinisothiocyanat/phenol extraction with the Trifast kit (Peqlab). Total RNA preparations were treated with DNase I, phenol/chloroform extracted and precipitated before further processing. RNAs were depleted of 5S, 5.8S, 18S and 28S rRNAs using the Human/Mouse/Rat Ribo-Zero rRNA Removal Kit (Epicentre) according to the manufacturer’s protocol. After rRNA depletion, RNAs were fragmented with a kit from Ambion. Libraries for Solexa sequencing were generated according to the standard Illumina protocol that comprised first strand cDNA synthesis, second strand cDNA synthesis, end repair, addition of a single A base, and adapter ligation. Sequencing was performed on the Illumina GAIIx (replicate 1) and Illumina HiSeq 2000 (replicate 2) platforms at the sequencing core facilities of the BioQuant in Heidelberg, Germany. RNA reads were aligned with TopHat. Further expression analysis was with the Genomatix software suite (Genomatix, Munich, Germany) and the Eldorado gene annotation. For each transcript a normalized expression value was calculated from the read distribution that accounts for the length differences using the program DEseq for the analysis of differential expression.

Project description:We determined genome-wide nucleosome occupancy in mouse embryonic stem cells and their neural progenitor and embryonic fibroblast counterparts to assess features associated with nucleosome positioning during lineage commitment. Cell type and protein specific binding preferences of transcription factors to sites with either low (e.g. Myc, Klf4, Zfx) or high (e.g. Nanog, Oct4 and Sox2) nucleosome occupancy as well as complex patterns for CTCF were identified. Nucleosome depleted regions around transcription start and termination sites were broad and more pronounced for active genes, with distinct patterns for promoters classified according to their CpG-content or histone methylation marks. Throughout the genome nucleosome occupancy was dependent on the presence of certain histone methylation or acetylation modifications. In addition, the average nucleosome-repeat length increased during differentiation by 5-7 base pairs, with local variations for specific genomic regions. Our results reveal regulatory mechanisms of cell differentiation acting through nucleosome repositioning. For chromatin immunoprecipitation, for each sample, 1 x 106 cells were cross-linked with 1% PFA and cell nuclei were prepared using a swelling buffer (25 mM Hepes pH 7.8, 1 mM MgCl2, 10 mM KCl, 0.1% NP-40, 1 mM DTT). Chromatin was sheared to mononucleosomal fragments. After IgG preclearance the sheared chromatin was incubated with 4 µg of either a H3K9ac (Abcam, ab4441), a H3K27ac (Abcam, ab4729), or a H3K9me3 (Abcam ab8898) antibody over night. After washes with sonication (10 mM Tris-HCl, pH 8.0, 200 mM NaCl, 1 mM EDTA, 0.5% N-lauroylsarcosine, 0.1% Na-deoxycholate), high-salt- (50 mM Hepes pH 7.9, 500 mM NaCl, 1mM EDTA, 1% Triton X-100, 0.1% Na-deoxycholate, 0.1% SDS), lithium- (20 mM Tris-HCl pH 8.0, 1mM EDTA, 250 mM LiCl, 0.5% NP-40, 0.5% Na-deoxycholate) and 10 mM Tris-HCl, chromatin was eluted from the protein G magnetic beads and the crosslink was reversed over night. After RNase A and proteinase K digestion, the DNA was purified and subsequently cloned into a multiplexed Illumina library according to standard protocols. Sequenced 50 bp reads were mapped with bowtie and subsequently clustered with MACS 55 implemented in the Genomatix software suite (Genomatix, Munich, Germany) using a p-value of 10-5.

Project description:The fundamental repeating unit of eukaryotic chromatin is the nucleosome. Besides being involved in packaging DNA, nucleosome organization plays an important role in transcriptional regulation and cellular identity. Currently, there is much debate about the major determinants of the nucleosome architecture of a genome and its significance with little being known about its role in stem cells. To address these questions, we performed ultra-deep sequencing of nucleosomal DNA in two human embryonic stem cell lines and integrated our data with numerous epigenomic maps. Our analyses have revealed that the genome is a determinant of nucleosome organization with transcriptionally inactive regions characterized by a "ground state" of nucleosome profiles driven by underlying DNA sequences. DNA sequence preferences are associated with heterogeneous chromatin organization around transcription start sites. Transcription, histone modifications, and DNA methylation alter this "ground state" by having distinct effects on both nucleosome positioning and occupancy. As the transcriptional rate increases, nucleosomes become better positioned. Exons transcribed and included in the final spliced mRNA have distinct nucleosome profiles in comparison to exons not included at exon-exon junctions. Genes marked by the active modification H3K4m3 are characterized by lower nucleosome occupancy before the transcription start site compared to genes marked by the inactive modification H3K27m3, while bivalent domains, genes associated with both marks, lie exactly in the middle. Combinatorial patterns of epigenetic marks (chromatin states) are associated with unique nucleosome profiles. Nucleosome organization varies around transcription factor binding in enhancers versus promoters. DNA methylation is associated with increasing nucleosome occupancy and different types of methylations have distinct location preferences within the nucleosome core particle. Finally, computational analysis of nucleosome organization alone is sufficient to elucidate much of the circuitry of pluripotency. Our results, suggest that nucleosome organization is associated with numerous genomic and epigenomic processes and can be used to elucidate cellular identity.

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Genome-wide nucleosome positioning during embryonic stem cell development

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