Junjun Ding

NANOG-dependent function of TET1 and TET2 in establishment of pluripotency.

Molecular control of the pluripotent state is thought to reside in a core circuitry of master transcription factors including the homeodomain-containing protein NANOG, which has an essential role in establishing ground state pluripotency during somatic cell reprogramming. Whereas the genomic occupancy of NANOG has been extensively investigated, comparatively little is known about NANOG-associated proteins and their contribution to the NANOG-mediated reprogramming process. Using enhanced purification techniques and a stringent computational algorithm, we identify 27 high-confidence protein interaction partners of NANOG in mouse embryonic stem cells. These consist of 19 previously unknown partners of NANOG that have not been reported before, including the ten-eleven translocation (TET) family methylcytosine hydroxylase TET1. We confirm physical association of NANOG with TET1, and demonstrate that TET1, in synergy with NANOG, enhances the efficiency of reprogramming. We also find physical association and reprogramming synergy of TET2 with NANOG, and demonstrate that knockdown of TET2 abolishes the reprogramming synergy of NANOG with a catalytically deficient mutant of TET1. These results indicate that the physical interaction between NANOG and TET1/TET2 proteins facilitates reprogramming in a manner that is dependent on the catalytic activity of TET1/TET2. TET1 and NANOG co-occupy genomic loci of genes associated with both maintenance of pluripotency and lineage commitment in embryonic stem cells, and TET1 binding is reduced upon NANOG depletion. Co-expression of NANOG and TET1 increases 5-hydroxymethylcytosine levels at the top-ranked common target loci Esrrb and Oct4 (also called Pou5f1), resulting in priming of their expression before reprogramming to naive pluripotency. We propose that TET1 is recruited by NANOG to enhance the expression of a subset of key reprogramming target genes. These results provide an insight into the reprogramming mechanism of NANOG and uncover a new role for 5-methylcytosine hydroxylases in the establishment of naive pluripotency.

Oct4 Links Multiple Epigenetic Pathways to the Pluripotency Network

Oct4 is a well-known transcription factor that plays fundamental roles in stem cell self-renewal, pluripotency, and somatic cell reprogramming. However, limited information is available on Oct4-associated protein complexes and their intrinsic protein-protein interactions that dictate Oct4s critical regulatory activities. Here we employed an improved affinity purification approach combined with mass spectrometry to purify Oct4 protein complexes in mouse embryonic stem cells (mESCs), and discovered many novel Oct4 partners important for self-renewal and pluripotency of mESCs. Notably, we found that Oct4 is associated with multiple chromatin-modifying complexes with documented as well as newly proved functional significance in stem cell maintenance and somatic cell reprogramming. Our study establishes a solid biochemical basis for genetic and epigenetic regulation of stem cell pluripotency and provides a framework for exploring alternative factor-based reprogramming strategies.

Zfp281 mediates Nanog autorepression through recruitment of the NuRD complex and inhibits somatic cell reprogramming

The homeodomain transcription factor Nanog plays an important role in embryonic stem cell (ESC) self-renewal and is essential for acquiring ground-state pluripotency during reprogramming. Understanding how Nanog is transcriptionally regulated is important for further dissecting mechanisms of ESC pluripotency and somatic cell reprogramming. Here, we report that Nanog is subjected to a negative autoregulatory mechanism, i.e., autorepression, in ESCs, and that such autorepression requires the coordinated action of the Nanog partner and transcriptional repressor Zfp281. Mechanistically, Zfp281 recruits the NuRD repressor complex onto the Nanog locus and maintains its integrity to mediate Nanog autorepression and, functionally, Zfp281-mediated Nanog autorepression presents a roadblock to efficient somatic cell reprogramming. Our results identify a unique transcriptional regulatory mode of Nanog gene expression and shed light into the mechanistic understanding of Nanog function in pluripotency and reprogramming.

Tex10 Coordinates Epigenetic Control of Super-Enhancer Activity in Pluripotency and Reprogramming.

Super-enhancers (SEs) are large clusters of transcriptional enhancers that are co-occupied by multiple lineage-specific transcription factors driving expression of genes that define cell identity. In embryonic stem cells (ESCs), SEs are highly enriched for the core pluripotency factors Oct4, Sox2, and Nanog. In this study, we sought to dissect the molecular control mechanism of SE activity in pluripotency and reprogramming. Starting from a protein interaction network surrounding Sox2, we identified Tex10 as a key pluripotency factor that plays a functionally significant role in ESC self-renewal, early embryo development, and reprogramming. Tex10 is enriched at SEs in a Sox2-dependent manner and coordinates histone acetylation and DNA demethylation at SEs. Tex10 activity is also important for pluripotency and reprogramming in human cells. Our study therefore highlights Tex10 as a core component of the pluripotency network and sheds light on its role in epigenetic control of SE activity for cell fate determination.

RNA-dependent chromatin targeting of TET2 for endogenous retrovirus control in pluripotent stem cells

Ten-eleven translocation (TET) proteins play key roles in the regulation of DNA-methylation status by oxidizing 5-methylcytosine (5mC) to generate 5-hydroxymethylcytosine (5hmC), which can both serve as a stable epigenetic mark and participate in active demethylation. Unlike the other members of the TET family, TET2 does not contain a DNA-binding domain, and it remains unclear how it is recruited to chromatin. Here we show that TET2 is recruited by the RNA-binding protein Paraspeckle component 1 (PSPC1) through transcriptionally active loci, including endogenous retroviruses (ERVs) whose long terminal repeats (LTRs) have been co-opted by mammalian genomes as stage- and tissue-specific transcriptional regulatory modules. We found that PSPC1 and TET2 contribute to ERVL and ERVL-associated gene regulation by both transcriptional repression via histone deacetylases and post-transcriptional destabilization of RNAs through 5hmC modification. Our findings provide evidence for a functional role of transcriptionally active ERVs as specific docking sites for RNA epigenetic modulation and gene regulation.The authors show that TET2 is recruited to chromatin by the RNA-binding protein PSPC1. PSPC1 and TET2 contribute to ERV and ERV-associated gene regulation by both transcriptional repression via histone deacetylases and post-transcriptional destabilization of ERV RNAs through 5hmC modification.

YY1 Positively Regulates Transcription by Targeting Promoters and Super-Enhancers through the BAF Complex in Embryonic Stem Cells

Summary Yin Yang 1 (YY1) regulates early embryogenesis and adult tissue formation. However, the role of YY1 in stem cell regulation remains unclear. YY1 has a Polycomb group (PcG) protein-dependent role in mammalian cells. The PcG-independent functions of YY1 are also reported, although their underlying mechanism is still undefined. This paper reports the role of YY1 and BAF complex in the OCT4-mediated pluripotency network in mouse embryonic stem cells (mESCs). The interaction between YY1 and BAF complex promotes mESC proliferation and pluripotency. Knockdown of Yy1 or Smarca4, the core component of the BAF complex, downregulates pluripotency markers and upregulates several differentiation markers. Moreover, YY1 enriches at both promoter and super-enhancer regions to stimulate transcription. Thus, this study elucidates the role of YY1 in regulating pluripotency through its interaction with OCT4 and the BAF complex and the role of BAF complex in integrating YY1 into the core pluripotency network.

NAC1 Regulates Somatic Cell Reprogramming by Controlling Zeb1 and E-cadherin Expression

Summary Reprogramming somatic cells to induced pluripotent stem cells (iPSCs) is a long and inefficient process. A thorough understanding of the molecular mechanisms underlying reprogramming is paramount for efficient generation and safe application of iPSCs in medicine. While intensive efforts have been devoted to identifying reprogramming facilitators and barriers, a full repertoire of such factors, as well as their mechanistic actions, is poorly defined. Here, we report that NAC1, a pluripotency-associated factor and NANOG partner, is required for establishment of pluripotency during reprogramming. Mechanistically, NAC1 is essential for proper expression of E-cadherin by a dual regulatory mechanism: it facilitates NANOG binding to the E-cadherin promoter and fine-tunes its expression; most importantly, it downregulates the E-cadherin repressor ZEB1 directly via transcriptional repression and indirectly via post-transcriptional activation of the miR-200 miRNAs. Our study thus uncovers a previously unappreciated role for the pluripotency regulator NAC1 in promoting efficient somatic cell reprogramming.

Endothelial cells instruct liver specification of embryonic stem cell-derived endoderm through endothelial VEGFR2 signaling and endoderm epigenetic modifications

Liver organogenesis requires complex cell-cell interactions between hepatic endoderm cells and adjacent cell niches. Endothelial cells are key players for endoderm hepatic fate decision. We previously demonstrated that the endothelial cell niche promotes hepatic specification of mouse embryonic stem cell(ESC)-derived endoderm through dual repression of Wnt and Notch pathways in endoderm cells. In the present study, we dissected further the mechanisms by which endothelial cells trigger endoderm hepatic specification. Using our previously established in vitro mouse ESC system mimicking the early hepatic specification process, endoderm cells were purified and co-cultured with endothelial cells to induce hepatic specification. The comparison of transcriptome profiles between hepatic endoderm cells isolated from co-cultures and endoderm cells cultured alone revealed that VEGF signaling instructs hepatic specification of endoderm cells through endothelial VEGFR2 activation. Additionally, epigenetic mark inhibition assays upon co-cultures uncovered that histone acetylation and DNA methylation promote hepatic specification while histone methylation inhibits it. This study provides an efficient 2D platform modelling the endothelial cell niche crosstalk with endoderm, and reveals mechanisms by which endothelial cells promote hepatic specification of mouse ESC-derived endoderm cells through endothelial VEGFR2 activation and endoderm epigenetic modifications.

Context-Dependent Functions of NANOG Phosphorylation in Pluripotency and Reprogramming

Summary The core pluripotency transcription factor NANOG is critical for embryonic stem cell (ESC) self-renewal and somatic cell reprogramming. Although NANOG is phosphorylated at multiple residues, the role of NANOG phosphorylation in ESC self-renewal is incompletely understood, and no information exists regarding its functions during reprogramming. Here we report our findings that NANOG phosphorylation is beneficial, although nonessential, for ESC self-renewal, and that loss of phosphorylation enhances NANOG activity in reprogramming. Mutation of serine 65 in NANOG to alanine (S65A) alone has the most significant impact on increasing NANOG reprogramming capacity. Mechanistically, we find that pluripotency regulators (ESRRB, OCT4, SALL4, DAX1, and TET1) are transcriptionally primed and preferentially associated with NANOG S65A at the protein level due to presumed structural alterations in the N-terminal domain of NANOG. These results demonstrate that a single phosphorylation site serves as a critical interface for controlling context-dependent NANOG functions in pluripotency and reprogramming.