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Determination of the protein interactome of the transcription factor Sox2 in embryonic stem cells engineered for inducible expression of four reprogramming factors

ABSTRACT: Coordinate expression of the somatic cell reprogramming factors Oct4, Sox2, Klf4 and c-Myc within embryonic stem cells preserves the self-renewal of these cells, while allowing for the expression epitope tagged Sox2. Taking advantage of this observation, we engineered embryonic stem cells (i-OSKM-ESC) to inducibly express Oct4, Klf4, c-Myc and an epitope tagged form of Sox2 from a polycistronic element, in the presence of doxycycline. We isolated Sox2 and its associated protein complexes by co-immunoprecipitation. Subsequently, we identified the Sox2-protein interactome in self-renewing embryonic stem cells using an unbiased proteomic screen (Multidimensional Protein Identification Technology [MudPIT]). Affymetrix microarrays were used to characterize the gene expression profile of i-OSKM-ESC in the absence and presence of doxycycline.

ORGANISM(S): Mus musculus

PROVIDER: GSE34801 | GEO | 2012/02/20

SECONDARY ACCESSION(S): PRJNA151129

REPOSITORIES: GEO

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Project description:Derivation and maintenance of pluripotent stem cells (PSCs) including embryonic stem cells(ESCs) and (iPSCs) usually requires optimization of complex culture media, which hinders the generation of PSCs from various species. Expression of Oct4, Sox2, Klf4 and c-Myc (OSKM) can reprogram somatic cells into iPSCs, even for species possessing no optimal culture condition. Here we explored whether expression of OSKM could induce and maintain pluripotency without specific growth factors and signaling inhibitors. Tet-On-OSKM/Oct4-GFP Mouse ESCs (mESCs) were derived from the embryos obtained after mating of Tet-On-OSKM mouse and Oct4-GFP mouse under the 2iL ( N2B27 plus MEK inhibitor PD0325901, GSK3β inhibitor ChIR99021, and leukemia inhibitory factor) condition. Tet-On-OSKM mouse was Gt(ROSA)26Sor tm1(rtTA*M2)Jae Col1a1 tm3(tetO-Pou5f1,-Sox2,-Klf4,-Myc)Jae/J(The Jackson Laboratory,011004), and Oct4-GFP mouse was B6;CBA-Tg(Pou5f1-EGFP)2Mnn/J (The Jackson Laboratory,004654).The culture medium of Tet-On-OSKM/Oct4-GFP mouse ESCs was switched from 2iL to N2B27 plus Doxycycline medium (OSKM medium), under which OSKM trangene was activated. These above cells switched from 2iL to OSKM medium were called OSKM-ESCs. Meanwhile 2iL-ESCs (continuely cultured in 2iL) served as the positive control group. 2iL-ESCs and OSKM-ESCs collected at selective days (d18, 25 and 38) after medium switch were collected for RNA-Seq. Moreover, Tet-On-OSKM mouse embryonic fibroblast (MEF) cells were reprogrammed under OSKM medium (N2B27 plus Doxycycline) and 2iL medium, respectively. The resulting OSKM-iPSCs were reprogrammed and cultured under OSKM medium.The 2iL-iPSCs were reprogrammed under 2iL plus Doxycycline medium and cultured under 2iL medium (without Doxycycline)once clones were picked up.RNA-Seq of these above samples was conducted to analyze gene expression. We found that via continuous expression of OSKM,OSKM-ESCs and OSKM-iPSCs propagated stably, expressed pluripotency marker genes, and formed three germ layers in teratomas. Importantly, OSKM-iPSCs could produce gene-modified animals through germline transmission.Transcriptional landscapes of OSKM-iPSCs resembled 2iL-ESCs, and were more similar to those of ESCs cultured in serum/LIF.

Project description:Cellular reprogramming of somatic cells towards induced Pluripotent Stem Cells (iPSCs) is achieved through the over-expression of the transcription factors Oct4, Sox2, Klf4 and c-Myc (OSKM). Due to the high complexity of the process, the vast number of the involved molecules and its stochastic and inefficient nature, the molecular background of reprogramming is only partially revealed. The aim of this study is the identification of genomic regulatory elements which are induced during cellular reprogramming, the examination of their action and their association with the expression of important genes. First, we constructed a combined ChIP-seq OSKM dataset using experiments on MEFs (Mouse Embryonic Fibroblasts) from multiple published studies, in order to examine the OSKM binding upon the genome, irrespective of minor discrepancies originating from the different reprogramming systems and protocols used. We also performed detailed RNA-seq and ATAC-seq experiments in order to monitor the functional results of the OSKM genomic binding. Initially, we focused on the ESC-related sites (Embryonic Stem Cells, natural equivalent of iPSCs) and found out that almost 1/3 of them are bound by OSKM as early as at day 1, or since MEFs. Our analysis highlighted the significant role of Myc in reprogramming, which is often overlooked. We saw that, along with Klf4, they “mark” positions where Oct4 and/or Sox2 will bind after reprogramming initiation. Furthermore, we discovered that various genes are regulated by multiple genomic elements, where OSKM bind at different timepoints in reprogramming. For example, many genes involved in the MET pathway (Mesenchymal to Epithelial Transition) are located near both ESC-, MEF- and Transient-OSKM sites, resulting in stably high expression levels which temporarily increase during the intermediate timepoints. Finally, we observed that OSKM often bind on the chromatin in a dynamic fashion. This “on-and-off” association of OSKM with the genome is related to the need of chromatin reorganization to achieve pluripotency. It is logical to assume that part of the inherent stochasticity of cellular reprogramming is based on the low percentage of cells that will achieve to reproduce this dynamic OSKM binding to regulate efficiently their transcriptional programs and to finally transform to iPSCs.

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Determination of the protein interactome of the transcription factor Sox2 in embryonic stem cells engineered for inducible expression of four reprogramming factors

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