Reorganization of enhancer patterns in transition from naïve to primed pluripotency (original) (raw)

Cell Stem Cell. Author manuscript; available in PMC 2015 Jul 6.

Published in final edited form as:

PMCID: PMC4491504

NIHMSID: NIHMS585843

Christa Buecker,1 Rajini Srinivasan,1 Zhixiang Wu,2 Eliezer Calo,1 Dario Acampora,3,4 Tiago Faial,1 Antonio Simeone,3,4 Minjia Tan,2 Tomasz Swigut,1 and Joanna Wysocka1,5

Christa Buecker

1Department of Chemical and Systems Biology, Stanford University School of Medicine, Stanford, California 94305, USA

Rajini Srinivasan

1Department of Chemical and Systems Biology, Stanford University School of Medicine, Stanford, California 94305, USA

Zhixiang Wu

2State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China

Eliezer Calo

1Department of Chemical and Systems Biology, Stanford University School of Medicine, Stanford, California 94305, USA

Dario Acampora

3Institute of Genetics and Biophysics "Adriano Buzzati-Traverso", CNR, Via P. Castellino, 111, 80131 Naples, Italy

4IRCCS Neuromed, 86077 Pozzilli (IS), Italy

Tiago Faial

1Department of Chemical and Systems Biology, Stanford University School of Medicine, Stanford, California 94305, USA

Antonio Simeone

3Institute of Genetics and Biophysics "Adriano Buzzati-Traverso", CNR, Via P. Castellino, 111, 80131 Naples, Italy

4IRCCS Neuromed, 86077 Pozzilli (IS), Italy

Minjia Tan

2State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China

Tomasz Swigut

1Department of Chemical and Systems Biology, Stanford University School of Medicine, Stanford, California 94305, USA

Joanna Wysocka

1Department of Chemical and Systems Biology, Stanford University School of Medicine, Stanford, California 94305, USA

5Department of Developmental Biology, Stanford University School of Medicine, Stanford, California 94305, USA

1Department of Chemical and Systems Biology, Stanford University School of Medicine, Stanford, California 94305, USA

2State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China

3Institute of Genetics and Biophysics "Adriano Buzzati-Traverso", CNR, Via P. Castellino, 111, 80131 Naples, Italy

4IRCCS Neuromed, 86077 Pozzilli (IS), Italy

5Department of Developmental Biology, Stanford University School of Medicine, Stanford, California 94305, USA

Supplementary Materials

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Summary

Naïve and primed pluripotency is characterized by distinct signaling requirements, transcriptomes and developmental properties, but both cellular states share key transcriptional regulators, Oct4, Sox2 and Nanog. Here we demonstrate that transition between these two pluripotent states is associated with widespread Oct4 relocalization, mirrored by global rearrangement of enhancer chromatin landscapes. Our genomic and biochemical analyses identified candidate mediators of primed state-specific Oct4 binding, including Otx2 and Zic2/3. Even when differentiation cues are blocked, premature Otx2 overexpression is sufficient to exit the naïve state, induce transcription of a substantial subset of primed pluripotency-associated genes and redirect Oct4 to previously inaccessible enhancer sites. However, ability of Otx2 to engage new enhancer regions is determined by its levels, cis-encoded properties of the sites and signaling environment. Our results illuminate regulatory mechanisms underlying pluripotency and suggest that capacity of transcription factors such as Otx2 and Oct4 to pioneer new enhancer sites is highly context-dependent.

Keywords: pluripotency, Oct4, enhancers, transcription factors, epigenetic

Introduction

Two different types of pluripotent stem cells have been derived from mouse and, more recently, human that exhibit distinct characteristics including morphology, growth factor dependency and developmental potential (Chan et al., 2013; Gafni et al., 2013; Nichols and Smith, 2009). Mouse embryonic stem cells (ESCs), particularly those grown in the presence of the differentiation stimuli inhibitors, exist in the so-called "naïve" pluripotent state, corresponding to the cells of the inner cell mass (ICM) and early pre-implantation epiblast (Nichols and Smith, 2009). Upon implantation, cells acquire characteristics of "primed" pluripotency and this state can be captured by derivation of the epiblast stem cells (EpiSCs) from the post-implantation epiblast (Nichols and Smith, 2009). Moreover, transition between the two states can be accomplished in vitro, leading to the formation of transient epiblast-like cells (EpiLCs) (Hayashi et al., 2011) or, after prolonged culture adaptation, self-renewing EpiSCs (Guo et al., 2009).

Interestingly, naïve and primed pluripotent cells share multiple key transcriptional players, including Oct4, Sox2 and Nanog (Boiani and Schöler, 2005; Tesar et al., 2007). These transcription factors (TFs) bind predominantly at promoter-distal cis-regulatory regions called enhancers, which play a central role in directing cell type-specific gene expression (Calo and Wysocka, 2013; Gibcus and Dekker, 2013; Spitz and Furlong, 2012). Presence of multiple TFs is typically required for enhancer activation, providing means for integrating different regulatory inputs. As a consequence, key lineage TFs and signaling mediators often co-bind at enhancers, resulting in their well-correlated genomic occupancy profiles in specific cell types (Mullen et al., 2011; Trompouki et al., 2011; Zinzen et al., 2009). This is also true in mouse and human ESCs, where Oct4, Sox2 and Nanog show extensive co-occupancy at regions coinciding with active enhancers (Chen et al., 2008; Kagey et al., 2010; Karwacki-Neisius et al., 2013). However, since mouse and human ESCs are thought to represent distinct pluripotent states (naïve and primed, respectively), it raises an important question as to how distinctive properties and gene expression programs of the two states are mediated by the highly overlapping set of TFs. Furthermore, in the mouse, Oct4 is critical both for the maintenance of naïve pluripotency and for the exit from it (Karwacki-Neisius et al., 2013; Radzisheuskaya et al., 2013), suggesting a context-dependent role of this TF in regulating transcription.

Here we show that the transition between naïve and primed pluripotency is accompanied by global reorganization of Oct4 genomic binding and enhancer chromatin patterns. Oct4 is lost from enhancers associated with key players in naïve pluripotency, and instead it engages new enhancer elements at loci implicated in the development of the post-implantation epiblast. Oct4 relocalization is mediated, at least in part, by cell state-specific cooperation with other TFs, such as Otx2. Although Otx2 is required for transcription of a limited subset of post-implantation epiblast genes, its overexpression in the naïve state is sufficient to recapitulate a large portion of the primed pluripotency gene expression program and to induce widespread Oct4 relocalization to epiblast enhancers.

Results

EpiLCs as a system to model the transition between naïve and primed pluripotency

For differentiation of mouse ESCs to pluripotent post-implantation epiblast like cells, we selected a system reported and validated by the Saitou group (Hayashi et al., 2011; Nakaki et al., 2013), in which ESCs grown under serum-free 2i+LIF conditions were transferred onto fibronectin and stimulated with Fgf2 and Activin A in the presence of 1% KOSR (Figure 1A). Within 48h cells underwent morphological transformation (including flattening, diminished cell-cell interactions and formation of cellular protrusions) and, as determined by RNA-seq analysis, silenced naive pluripotency/ICM genes (e.g. Klf4, Prdm14, Tbx3, Zfp42), induced post-implantation epiblast genes (e.g. Fgf5, Wnt8a, Dnmt3a/b, Oct6), and continued to express key pluripotency genes (e.g. Oct4, Sox2 and Nanog) (Figure 1A and B, Table S1, Hayashi et al., 2011). Longer exposure (72h) did not lead to further induction of post-implantation epiblast-associated transcripts (Figure S1A), but resulted in increased cell death (not shown and Hayashi et al., 2011). Therefore, the 48h time point was selected for further experiments. Single-cell analysis methods, such as flow cytometry against pre-implantation epiblast surface markers (SSEA1 and CD31, Figure S1B) or anti-Oct6 immunofluorescence microscopy (Figure S1C), demonstrated that the majority of cells within the population underwent cell fate change at the 48h time point. We will hereafter refer to this transition as the "ESC to epiblast-like cell (EpiLC) transition". Interestingly, addition of exogenous Activin A was dispensable for the ESC to EpiLC transition, likely due to activity of the TGF-beta pathway in the ESC state, as evidenced by the high expression of Nodal ligands and presence of Smad2 on chromatin in ESCs (Figure S1D). Consistently, RNA-seq analysis of EpiLCs derived in the presence or absence of exogenous Activin A showed nearly identical gene expression patterns (Figure S1E). However, the release from Erk inhibition was absolutely required for the transition to occur (not shown and Kunath et al., 2007).

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Widespread relocalization of Oct4 during ESC to EpiLC transition

(A) ESC to EpiLC transition. Upper panel depicts corresponding cell fate change in vivo, lower panel shows morphology changes during 48h of in vitro differentiation.

(B) Transcriptome changes during ESC to EpiLC transition. Each point represents FPKM values obtained for a given transcript in RNA-seq analysis from ESCs (ordinate) or EpiLC (abscissa). For clarity, only transcripts, which tested positive for differential expression by cuffdiff are plotted (FDR<0.05, exceptions are Oct4 and Sox2). Selected transcripts known to be associated with naive pluripotency are highlighted in blue, those associated with post-implantation epiblast are highlighted in red, and general pluripotency factors are shown in orange.

(C) Expression of pre- or post- primitive streak epiblast markers in EpiLCs and EpiSCs. Stage-specific genes were selected from Kojima et al., 2013. FPKM values from EpiLCs (red) and EpiSCs (green) RNA-seq experiments are shown for each representative gene. For additional genes see Figure S2B.

(D) Oct4 ChIP-seq signal reproducibility and variation in ESCs and EpiLCs. Oct4 ChIP-seq was performed from two biological replicates of ESCs and from EpiLCs derived with and without the addition of exogenous Activin A. A set of 5783 Oct4 sites was selected for further analysis, representing the union of top 3250 ranked sites in each of the four ChIP-seq samples. These sites will hereafter be referred to as "top Oct4 sites". Plotted are peak KDE values for each of the top Oct4 sites (see also Extended Experimental Procedures).

(E) Changes in Oct4 binding are positively correlated with changes in gene expression. Plotted are histograms of fold expression changes for transcripts that tested as significant in the ESC to EpiLC differentiation (cuffdiff, Figure 1B). We used default GREAT association rules to identify genes (usually two) in proximity of Oct4 sites. Oct4 sites defined as unchanged are associated with genes with no net transcriptional difference, while genes associated with EpiLC-specific sites show significant (Wilcoxon U test) net upregulation in EpiLCs. Opposite trend is seen for the ESC-specific Oct4 sites. Three classes of Oct4 sites were defined as shown in Figure 2A.

See also Figure S1 and S2.

We further confirmed that EpiLCs readily form embryoid bodies containing derivatives of all three germ layers (Figure S2A) and are competent to form PGCs, as originally reported (not shown, Hayashi et al., 2011). A recent study characterized transcriptomes of mouse epiblasts at different stages of development and compared them to those of EpiSCs, revealing closest resemblance of EpiSCs to the late, post-streak epiblast (Kojima et al., 2013). We selected a set of transcripts preferentially enriched in the early, pre-streak post-implantation epiblast and in the late, post-streak epiblast and compared their expression in our RNA-seq datasets from EpiLCs and EpiSCs. Consistent with an early post-implantation epiblast identity of EpiLCs and later identity of EpiSCs, pre-streak epiblast-enriched genes were generally expressed at higher levels in EpiLCs than EpiSCs, whereas post-streak epiblast genes were expressed at higher levels in EpiSCs (Figures 1C and S2B). In sum, ESC to EpiLC transition provides a robust and well defined model for studies of gene regulatory changes in naïve and primed pluripotency. This system is characterized by rapid kinetics, minimal cell death and allows for following dynamic changes of chromatin patterns in the differentiating cell population.

Reorganization of Oct4 binding and enhancer chromatin patterns during the ESC to EpiLC transition

We next asked if Oct4 genomic occupancy changes during differentiation. Oct4 ChIP-seq analyses from ESCs (in two biological replicates) and from EpiLC derived at 48h of differentiation in the presence or absence of Activin A, revealed highly reproducible binding between the two ESC replicates, and between EpiLC derived with and without Activin A (Figure 1D). However, comparison between ESC and EpiLC showed substantial reorganization of Oct4 occupancy during differentiation with gain of binding near genes associated with post-implantation epiblast (e.g. Fgf5, Oct6, Wnt8a) and loss near genes associated with naïve pluripotency (e.g. Klf4, Tbx3, Prdm14) (Figure 1D, Figure S2C–D). Based on the relative enrichments between the two cellular states, we defined top Oct4 sites preferentially enriched in ESCs (“ESC-specific”) or EpiLCs (“EpiLC-specific”), and a similar number of sites at which Oct4 occupancy changed least (“shared”, see also Figure 2A and Extended experimental procedures). Next, we associated Oct4-bound regions within each class with nearby genes and investigated changes in expression observed during the ESC to EpiLC transition (Figure 1E). Genes associated with ESC-specific Oct4 binding were commonly downregulated during the transition, whereas genes associated with EpiLC-specific Oct4 binding were commonly upregulated in EpiLCs as compared to ESCs (Figure 1E).

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Changes in enhancer chromatin patterns mirror Oct4 reorganization

(A) Oct4 sites were classified based on change in Oct4 occupancy during differentiation. We estimated FDR based on biological repeats and chosen FDR <0.003 to define ESC-specific sites (blue) and EpiLC-specific sites (red). Least changed sites (orange) were treated as shared.

(B) Changes in p300 occupancy, H3K27ac modification and to lesser extent H3K4me1 modification follow changes in Oct4 occupancy. Plotted are mean peak ChIP-seq KDE values of the respective ChIP-seq signals from ESC (ordinate) or EpiLC (abscissa) at three classes of Oct4 sites defined in (A).

(C) Genome browser representation of QuEST-generated ChIP-seq profiles from ESCs (upper panels) or EpiLCs (lower panels) at a locus representing ESC-specific Oct4 binding (Tbx3, left) or EpiLC-specific Oct4 binding (Oct6, right).

(D) Schematics of the piggyBac transposon-based enhancer-reporter system used to create dual reporter lines: each enhancer was cloned upstream of the minimal TK promoter driving expression of a different fluorescent protein, coupled to a distinct selection cassette.

(E) Changes in reporter activity during differentiation. Representative fluorescent microscopy images of a clonal cell line containing the Oct6 locus enhancer driving GFP expression and the Tbx3 locus enhancer driving RFP expression. ESC to EpiLC differentiation was carried out for 48h.

(F) Quantification of reporter cell lines described in Figure 2E and two additional cell lines by flow cytometry.

See also Figure S3.

More than 97% of the above-defined top Oct4 sites are distal and located more than 1 kb from the nearest annotated transcription start site (TSS), regardless of the class they belong to, suggesting that most of the strong Oct4 binding and its reorganization occurs at enhancers. To examine whether Oct4 relocalization is associated with changes in enhancer chromatin patterns, we performed p300, H3K27ac and H3K4me1 ChIP-seq analyses of ESCs and EpiLCs (with biological replicates, Figure S3A), and analyzed enrichments at three classes of Oct4-bound regions, as defined in Figure 2A (with ESC-specific sites highlighted in blue, EpiLC-specific sites in red, and shared sites in orange). Changes in p300 and H3K27ac levels generally followed those in Oct4 binding: EpiLC-specific Oct4 sites had high levels of p300 and H3K27ac in EpiLCs, but not in ESCs and vice versa, whereas shared Oct4 sites were similarly enriched for these marks in both states (Figure 2B–C, Figure S3C). Additionally, genes associated with the EpiLC-specific gain of H3K27ac were commonly upregulated during the transition, whereas loss of H3K27ac from ESC-specific sites coincided with downregulation of nearby genes (Figure S3B). Cell type-specific dynamics were much less pronounced for H3K4me1 (Figure 2B), which is consistent with previous observations that H3K4me1 both precedes and follows enhancer activation during differentiation (Bonn et al., 2012; Creyghton et al., 2010; Rada-Iglesias et al., 2010). Annotation analysis revealed association of ESC-specific enhancers with genes expressed and implicated in pre-implantation development, whereas EpiLC-specific and shared enhancers associated with genes expressed both during pre- and post-implantation development. Additionally, EpiLC specific enhancers were associated with phenotypes manifesting at early post-implantation stages (Tables S3–5). Taken together, our data demonstrate that transition from naïve to primed pluripotency is associated with global rearrangement of Oct4 occupancy and transformation of the enhancer chromatin landscapes.

Changes in enhancer activity are population-wide

To address whether changes in enhancer utilization patterns occur in the majority of cells within the differentiating population, we isolated dual transgenic enhancer reporter lines, in which ESC-specific or EpiLC-specific enhancers of interest were cloned upstream of a minimal promoter and one of two distinct fluorescent reporters and integrated into the ESC genome using a PiggyBac transposon system (Figure 2D). During 48h of differentiation the majority, if not all, cells gained fluorescence intensity associated with the EpiLC-specific enhancer and lost fluorescence intensity associated with the ESC-specific enhancer (Figure 2E–F). Our observations confirm highly cell state-specific enhancer activity of selected genomic regions and suggest that results obtained in our system with the population-level assays are informative as to the changes in enhancer activity in the majority of individual cells.

Cell state-dependent association of Oct4 with other TFs

What mechanisms can explain the widespread reorganization of Oct4 occupancy? POU family TFs are characterized by plasticity of DNA recognition mediated by distinct arrangements of POU homeodomains (Herr and Cleary, 1995; Pesce and Schöler, 2001), raising the possibility that Oct4 may alter its DNA-binding specificity in response to differentiation-induced cofactors or post-translational modifications. However, the primary sequence motif recovered de novo in our Oct4 ChIP-seq analyses is virtually the same for ESC-specific and EpiLC-specific sites, and corresponds to the Oct4-Sox2 composite recognition site (Figure S4A). Moreover, Oct4 ChIP-exo studies also recovered the same sequence motif in both states, and we generally failed to gain support for the differentiation-induced change in the Oct4 DNA recognition specificity (not shown). Alternatively, combinatorial interplay of multiple TFs at enhancers is emerging as a key mechanism regulating context-specific TF binding (Heinz et al., 2013; Spitz and Furlong, 2012; Yáñez-Cuna et al., 2012; Zinzen et al., 2009). We hypothesized that if such a mechanism is at play, then: (i) due to proximity to Oct4 on chromatin and shared associations with common coactivators, candidate factors could be recovered as (likely sub-stoichiometric) cell state-specific Oct4 interactors, (ii) motifs for such TFs should be differentially enriched within ESC-specific and EpiLC-specific enhancers. To test the former premise, we first showed that Oct4 protein levels are comparable between the two states, with no more than two fold reduction in EpiLCs compared to ESCs (Figure 3A). Next, we immunopurified endogenous Oct4 from nuclear extracts prepared from an equivalent number of ESCs and EpiLCs and identified associated polypeptides by LC-MS/MS mass spectrometry (Figure 3B, Figure S4B). These interactors included 79 Oct4 binders published in previous proteomic studies of Oct4 partners, which were performed in ESCs grown under serum conditions (van den Berg et al., 2010; Ding et al., 2012; Pardo et al., 2010). Not unexpectedly, a large subset of them represented subunits of major coactivator complexes, including the esBAF complex, p400/Tip60 and Mediator/cohesin (Figure 3B, Table S2). Interestingly, some of the chromatin remodeling/modifying proteins were differentially enriched: we recovered PBAF subunit Arid2 and remodeler Chd7 only from EpiLC extracts, and DNA hydroxylase Tet2 only from ESC extracts. With respect to associated TFs, we detected Esrrb, Klf5, and Tcf3 specifically in ESC purifications, Otx2, Zic2, Zfp281, Zscan10 and Oct6 specifically in EpiLC purifications, whereas Sox2 and Sall1 were present in both (Figure 3B). Selected associations were confirmed by IP-Western in an independent experiment (Figure 3C).

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Identification of candidate Oct4 cooperating TFs

(A) Oct4 protein levels in ESCs and EpiLCs. Anti-Oct4 Western blot of a twofold serial dilution of whole cell extract from ESCs (left) or EpiLCs (right). Actin was used as a loading control.

(B) Schematic representation of LC-MS/MS identification of Oct4-associated proteins. Relevant interaction partners are listed. For full results see Table S2.

(C) Validation of Oct4 association with selected partners by IP-Western in an independent experiment.

(D) Oct4 sites that contain nearby consensus motif for Esrrb or Klf4/5 are bound more strongly in ESCs, whereas sites with Otx2 or Zic motif are bound more strongly in EpiLCs. A set of top 5783 Oct4 sites was classified based on the presence of the consensus recognition motifs for candidate TFs. Analysis required that the analyzed TF consensus motif lies within a very short distance (+/− 50bp) from the center of the respective Oct4 ChIP-seq peak. Ratios of EpiLC to ESC Oct4 ChIP enrichments at such sites were calculated and represented as boxplots.

See also Figure S4

Next, we performed de novo sequence motif searches of Oct4-bound regions, which, in addition to the Oct4 and Sox2 motifs, recovered preferential enrichments of the Klf4/5, Esrrb/Rxra and FoxD3 motifs at the ESC-specific sites (Figure S4C), and Otx/Pitx and Zic motifs at the EpiLC-specific sites (Figure S4D). Thus, DNA recognition motifs for many of the biochemically identified cell state-specific Oct4 partners are differentially enriched within ESC- and EpiLC-specific Oct4 sites. In particular, our combined genomic and biochemical analyses converged on Esrrb and Klf4/5, and Otx2 and Zic2/3 as primary candidate mediators of, respectively, ESC-specific and EpiLC-specific Oct4 binding. It has been postulated that cooperativity among TFs can arise through a competition with histones, even in the absence of stereotypic motif arrangement or direct interaction between TFs, but this requires that the recognition motifs for the cooperating factors are located within the length of one (or in some models, one-half) nucleosome (e.g. < 148 bp) (Miller and Widom, 2003; Mirny, 2010; Moyle-Heyrman et al., 2011). We therefore examined if Oct4 sites occurring within close proximity of motifs for candidate TFs show cell state-specific bias in Oct4 binding. To this end, we classified a set of top Oct4 sites based on the presence of the exact consensus recognition motifs for interrogated TFs within +/− 50bp from the center of the respective Oct4 ChIP-seq peak. Subsequently, we examined Oct4 ChIP enrichments at such sites in ESCs and EpiLCs (Figure 3D). Regions with Oct4, Sox2 and Zfp281 consensus motifs were not significantly different in strength of Oct4 binding from the overall population of Oct4 sites (Figure 3D). However, Oct4 sites containing consensus sequences for Esrrb or Klf4/5 were bound significantly stronger in ESCs, whereas Oct4 sites with Otx2 or Zic2/3 motifs were bound significantly stronger in EpiLCs, in agreement with potential cell state-specific cooperative binding.

During the transition to EpiLC, Otx2 and Oct4 co-bind previously inaccessible chromatin sites

Among the identified candidate TFs, Otx2 piqued our interest, as it has been recently implicated in the induction, stability and differentiation of EpiSCs (Acampora et al., 2013). In the mouse embryo, Otx2 is expressed in the ICM, but its expression is strongly upregulated upon implantation (Acampora et al., 2013). This expression pattern is recapitulated in our system: Otx2 is expressed at low levels in ESCs, but its mRNA is upregulated ~ 30 fold in response to Fgf2, resulting in over 10 fold higher protein levels in EpiLCs (Figure 4A). Of note, low average expression of Otx2 in ESCs is not due to cell heterogeneity (Figure S5A).

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Coordinated changes in Otx2 and Oct4 binding during differentiation

(A) Changes in Otx2 levels during the ESC to EpiLC transition. Otx2 mRNA levels were analyzed by RT-qPCR in ESCs and EpiLCs (left panel, signals were normalized to Rpl13a, compare to Fgf5 and Esrrb levels). Error bars indicate SD of three technical qPCR replicates of one representative experiment. Protein levels we analyzed by quantitative Western blotting (right panel) using Licor. An antibody against Tbp was used as a loading control.

(B) Changes in Otx2 occupancy during differentiation. Plotted are peak KDE values of Otx2 ChIP-seq signals from ESCs and EpiLCs. We chose FDR cutoff 0.003 to define 948 ESC specific sites (blue) and 2180 EpiLC specific sites (red). 2000 least changed sites (orange) were treated as shared.

(C) Changes in p300 occupancy, H3K27ac modification and, to a lesser extent, H3K4me1 modification correlate with changes in Otx2 binding. Plotted are peak KDE values of the respective ChIP-seq signals at Otx2 sites as defined and color coded as in (B).

(D) Oct4 and Otx2 ChIP-seq signals in EpiLCs (left panel), as well as fold changes in binding intensity during ESC to EpiLC transition are plotted. R is Spearman correlation coefficient.

(E) EpiLC-specific Otx2 sites are enriched for the Otx2 motif, whereas ESC-specific Otx2 sites are enriched for the CTCF motif. Genomic regions ±1 kb relative to the Otx2 ChIP-seq signal peak position at top 948 Otx2 sites of each class were scanned with FIMO (part of the MEME suite Bailey et al., 2009; Grant et al., 2011) using PWM derived from Jolma et al., 2013. Kernel smoothed aggregate score (−log(p.value)) is plotted.

(F) Aggregate plot of FAIRE-seq signal from ESC (left) or EpiLC (right) over the ESC-specific (blue) or EpiLC-specific (red) Otx2 sites defined as in (B). Plotted are kernel smoothed normalized counts of FAIRE-seq tags relative to Otx2 ChIP peaks in each Otx2 class (reads per kb per 100,000 reads, kernel bandwidth 100bp).

(G) Aggregate plot of DNase-seq signal from ESCs grown under 2i+LIF conditions at Otx2 sites specific for ESC (blue) or EpiLC (red) defined as in D.

See also Figure S5.

Otx2 ChIP-seq analyses from ESCs and EpiLCs revealed substantially more binding events in EpiLCs than in ESCs (Figure 4B), which is consistent with higher Otx2 levels in the latter. Similarly to Oct4, Otx2 binding showed dynamic changes during the transition and coincided with cell type-specific occupancy of p300 and H3K27ac (Figure 4B–C). Sequence analysis revealed that, besides the Otx2 consensus binding motif, Zic and Oct4 motifs were also highly overrepresented at Otx2-bound regions in EpiLCs (Figure S5B). Importantly, not only was occupancy of Oct4 and Otx2 in EpiLCs highly correlated (R=0.78) (Figure 4D, left panel), but changes in binding of the two factors during ESC to EpiLC transition showed even higher concordance (R=0.9), indicating that Otx2 and Oct4 together engage EpiLC enhancer regions (Figure 4D, right panel).

Interestingly, ESC-specific Otx2 sites were not enriched for Otx2 motif and had high enrichment for CTCF motif instead (Figure 4E). This was not due to cross-reactivity of the Otx2 antibody or cross-linking artifacts, as majority of the recovered ChIP-seq peaks were Otx2-dependent (Figure S5C). Further analysis revealed that in ESCs Otx2 indeed bound at a subset of CTCF sites, which were generally Oct4-poor (Figure S5D, green population, Figure S5E), but it was also detected at active ESC enhancers, occupied by Oct4 (Figure S5D and S5F). We therefore hypothesized that in ESCs, Otx2 cannot access its cognate, nucleosome-occupied recognition sites and instead it opportunistically binds at regions nucleosome-depleted by other factors e.g. CTCF or enhancer binding TFs. Once Otx2 levels are induced, it can access target enhancers via sequence-dependent DNA recognition. To explore this possibility, we performed FAIRE-seq from ESCs and EpiLCs and examined average FAIRE signals at ESC- and EpiLC-specific Otx2 regions (Figure 4F). Consistent with our hypothesis, binding of Otx2 in ESCs coincided with high FAIRE signals (blue line), whereas regions bound by Otx2 specifically in EpiLCs showed virtually no FAIRE enrichment in ESCs (red line), but gained FAIRE signal during transition to EpiLCs (Figure 4F). Additionally, we analyzed a publicly available DNase-seq dataset generated from mouse ESCs grown under 2i conditions (ENCODE Project Consortium, 2011). In agreement with our FAIRE results, ESC-specific Otx2 regions showed high DNase hypersensitivity, whereas EpiLC-specific Otx2 regions were generally not hypersensitive in ESCs (Figure 4G). Taken together, our data suggests that during differentiation Otx2, together with Oct4, gains access to previously inaccessible chromatin sites.

Otx2 is required for a limited subset, but sufficient for activation of many EpiLC transcripts

To examine the functional impact of Otx2 on expression of EpiLC-induced transcripts, we used Otx2−/− ESCs, which robustly self-renewed and exhibited no noticeable changes in morphology or expression of select naïve pluripotency genes when grown under 2i+LIF (Figure 5A and Figure S6A). Upon differentiation, Otx2−/− cells underwent morphological changes indistinguishable from those seen in wt EpiLCs, but showed defects in expression of certain epiblast-associated genes, such as Fgf5 and Oct6 (Figure S6A, Figure 5A). RNA-seq analysis of Otx2−/− EpiLCs identified 260 and 141 genes significantly downregulated or upregulated, respectively, as compared to the genetically-matched wt EpiLCs (at FDR < 0.01, Table S6). Most of the downregulated genes were induced during normal ESC to EpiLC transition (Figure 5B), indicating that Otx2 is required for activation of a subset of EpiLC transcripts.

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Effects of loss or gain of Otx2 on EpiLC gene expression program

(A) RT-qPCR analysis of gene expression changes in wt and Otx2−/− ESCs and EpiLCs. Differentiation was carried out for 48h, cDNA levels were normalized against Rpl13a and analyzed transcripts are indicated on top. Error bars indicate SD of three technical replicates from a representative experiment.

(B) Loss of Otx2 affects a subset of genes induced during the ESC to EpiLC transition. RNA-seq transcriptome of Otx2−/− EpiLCs was compared to EpiLCs obtained from a matched wt cell line. Highlighted in red are transcripts that have statistically significant induction of expression during the differentiation from ESC to EpiLC in a wt background (compare to Figure 1B).

(C) Schematic representation of the inducible Otx2 overexpression system.

(D) Morphological changes upon Otx2 overexpression in ESCs are reminiscent of those observed in EpiLCs. Otx2−/− cells reconstituted with inducible Otx2 (Otx2−/−+ tetOn Otx2) and grown under 2i+LIF were treated with 2µg/ml doxycycline for 48h. Morphology was compared to non-induced cells.

(E) Transcripts affected by Otx2 overexpression are also differentially expressed during ESC to EpiLC transition. RNA-seq from tetON Otx2 cells grown in the absence (X axis) or presence for 26h (Y axis) of Dox were compared and expression values (FPKM) of all significantly changed (FDR<0.01, cuffdiff) transcripts were plotted. Those transcripts that were also significantly upregulated or downregulated during the ESC to EpiLC transition are highlighted in red or blue, respectively.

(F) Otx2 overexpression recapitulates a substantial subset of gene expression changes that occur during the ESC to EpiLC transition. Each point represents FPKM values obtained for a given transcript by RNA-seq analysis from ESCs (X axis) or EpiLC (Y axis). For clarity, only transcripts, which tested positive for differential expression by cuffdiff are plotted (FDR<0.01). Those transcripts that were also significantly upregulated or downregulated upon Otx2 overexpression are highlighted in red or blue, respectively.

(G) RNA-seq transcriptome data from ESC vs EpiLC were plotted similar to Figure 1B but marked in red (blue) are transcripts significantly upregulated (downregulated) by overexpression of Otx2.

(H) Quantitative comparisons of expression changes during EpiLC formation and Otx2 overexpression. We identified ~2200 genes that undergo significant expression change during ESC->EpiLC transition and are either upregulated (red) or downregulated (blue). Next we plotted expression change for these genes in RNAseq from independent repeat of differentiation in wild-type (left) and Otx2−/− cells with induced overexpression of Otx2 (right)

See also Figure S6

Next, we asked if premature overexpression of Otx2 is sufficient to induce differentiation and transcription of EpiLC genes, even when the FGF response is blocked. To test this, we reconstituted Otx2−/− cells with a Doxycycline (Dox) inducible Otx2 transgene (Figure 5C), allowing for a tightly controlled overexpression of Otx2 (Figure S6B–C; of note, ectopic Otx2 expression in these cells is up to tenfold higher than natural levels in EpiLCs). Within 24h of addition of Dox, reconstituted cells grown under 2i+LIF started to change morphology; at 48h the interaction between single cells loosened and cells formed visible protrusions similar to those observed in EpiLCs (Figure 5D, compare to Figure 1A). In contrast, cells either not treated with Dox or treated, but not carrying the Otx2 transgene, retained ESC morphology (Figure 5D). Prolonged overexpression of Otx2 in the absence of additional differentiation cues led to cell death, similarly to what has been observed in EpiLCs after 72h of differentiation (Hayashi et al., 2011). Analogous results were obtained when Otx2 was overexpressed in wt ESCs, rather than Otx2−/− cells (not shown). Analysis of molecular markers at 26h after Dox induction showed downregulation of Tbx3 and Esrrb, and upregulation of Fgf5 (Figure S6D), in agreement with the exit from the naïve pluripotency. Under 2i conditions, Mek/Erk signaling is inhibited, but other arms of the FGF response (e.g. Akt pathway) remain active. We therefore additionally employed an Fgf receptor tyrosine kinase inhibitor, which prevents activation of all downstream pathways. Otx2 overexpression in the presence of this inhibitor resulted in morphological changes similar to those observed with the Mek/Erk inhibitor, and was sufficient to downregulate naïve pluripotency genes and induce Fgf5 transcription (Figure S6E–F and not shown).

We next used RNA-seq to examine the global impact of Otx2 overexpression under 2i conditions on gene regulation. We identified genes significantly (FDR<0.01) upregulated and downregulated at 26h after addition of Dox (Figure 5E), and compared them to those which are up-/downregulated during normal ESC to EpiLC transition (highlighted in red and blue, respectively). With some exceptions, genes significantly up- or downregulated by Otx2 overexpression were also respectively up- or downregulated during EpiLC formation (Figure 5E). Importantly, reverse comparison revealed that a large subset of changes in gene expression occuring during EpiLC formation is recapitulated by Otx2 overexpression (Figure 5F). This includes not only induction of EpiLC transcripts, but also repression of ESC transcipts. Quantitative comparisons showed less pronounced changes in transcription upon Otx2 overexpression than during normal differentiation, with a caveat that the measurements were taken at an earlier time point for the former (26h vs 48h) (Figure 5G). In sum, although Otx2 is required for activation of a limited subset of EpiLC transcripts, its overexpression is sufficient to recapitulate a substantial portion of the transcriptional changes that occur during the ESC to EpiLC transition.

Impact of Otx2 loss or gain on the Fgf5 locus enhancers

Our observations are consistent with the idea that partial redundancy between a cohort of EpiLC enhancer-binding TFs exist, but a smaller combination of factors may be sufficient to assemble coactivators and initiate gene expression. Therefore, removal of one factor (e.g. Otx2) may have a relatively minor effect, while supplying high levels of the same factor may suffice for activation, because additional cooperating factors (e.g. Oct4) are already present in ESCs. To explore this, we first focused on an individual locus, encoding Fgf5, an established molecular marker of the post-implantation epiblast. Loss of Otx2 impairs Fgf5 activation, whereas Otx2 overexpression induces Fgf5 transcript in ESCs (Figure 5A and Figure S6D). Interrogation of our ChIP-seq datasets revealed presence of a proximal enhancer (PE) at the Fgf5 locus, which exists in a poised state in ESCs, as well as a cluster of four enhancers (depicted as E1-E4) located within 25–60 kb downstream from the Fgf5 TSS (Figure 6A) that are unmarked in ESCs, but activated de novo during the transition. Moreover, this enhancer cluster bears attributes of a 'super-enhancer', characterized by high levels of TFs, coactivators and H3K27ac (Figure 6A) (Whyte et al., 2013). Analysis of the publicly available DNase hypersensitivity data from ESCs grown in 2i confirmed that all elements of the super-enhancer were largely nuclease-resistant prior to differentiation, in contrast to the poised PE enhancer, which showed hypersensitivity (Figure 6A, bottom track).

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Sensitivity of the Fgf5 locus enhancer cluster to Otx2 perturbations

(A) Enhancer activation at the Fgf5 locus. Genome browser representation of ChIP-seq tracks for Otx2, Oct4, p300, H3K27ac and H3K4me1 in ESCs (upper part) and EpiLC (lower part) at the Fgf5 locus. DNase-seq track from ESCs grown under 2i+LIF (data from ENCODE Project Consortium, 2011) is shown at the bottom. Highlighted are promoter-proximal poised enhancer (PE) and a cluster of four enhancers (E1-E4) activated de novo.

(B) ChIP-qPCR analyses of the Fgf5 locus enhancers from wt or Otx2−/− ESCs and EpiLCs were carried out with indicated antibodies. % input recovery was calculated and normalized to the average of two negative regions. Error bars indicate SD of three technical qPCR replicates from a representative experiment.

(C) Selective activation of the E2 enhancer upon Otx2 overexpression. ChIP-qPCR analyses from Otx2−/− ESCs and Otx2−/−+ tetOn Otx2 cells grown in the absence or presence of Dox, were carried out with indicated antibodies. % input recovery was calculated and normalized to the average of two negative regions. Error bars indicate SD of three technical qPCR replicates from a representative experiment.

We first examined the effect of Otx2 loss on the Fgf5 locus enhancers. ChIP-qPCR analysis from Otx2−/− EpiLCs showed loss of Otx2 binding from all enhancers, and diminished association of Oct4, p300 and H3K27ac with the E1-E4 elements at the de novo cluster, and to a lesser degree with the PE enhancer, which was already pre-bound by Oct4 in ESCs (Figure 6B). Thus, all enhancers at the Fgf5 locus are sensitive to Otx2 loss. Next, we used our Dox-inducible system to test if, upon overexpression, Otx2 can engage Fgf5 locus enhancers. Surprisingly, ChIP-qPCR analysis at 26h after addition of Dox showed that Otx2 was able to selectively access only one element within the cluster, the E2, in addition to the poised PE element (Figure 6C). Otx2 occupancy at the E2 enhancer was associated with the recruitment of Oct4 and p300 and with the activation of Fgf5 expression (Figure 6C and Figure S6D). Even after prolonged exposure to Dox this specificity did not change (Figure S6G). Therefore, although during normal differentiation Otx2 binds and regulates all enhancers at the Fgf5 locus, upon overexpression it selectively engages the E2 element.

Otx2 overexpression results in the global reorganization of Oct4 binding

To further explore the selectivity of Otx2 in target engagement and Oct4 reorganization, we performed Otx2 and Oct4 ChIP-seqs from Otx2 overexpressing ESCs. In agreement with ChIP-qPCR results, we observed binding of Otx2 and Oct4 at E2, but not at other elements of the Fgf5 locus (Figure 7A). Next, we systematically compared Otx2 sites induced upon overexpression to those induced in EpiLCs and defined three classes of sites: (i) induced both by overexpression and during EpiLC formation (yellow), (ii) induced only during EpiLC formation (purple), and (iii) induced only upon Otx2 overexpression (e.g. ectopic sites) (cyan, Figure 7B, S7A). Further analysis of the ectopically-induced sites showed link to developmental genes and phenotypes associated with gastrulation, but no strong enrichment for specific processes orchestrated by Otx2 later in development (e.g. brain patterning or sensory organ formation, not shown). Importantly, nearly half (~46%) of the Otx2-bound, EpiLC-specific enhancers became engaged by Otx2 upon overexpression.

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Otx2 overexpression leads to global reorganization of Oct4 binding pattern

(A) Otx2 and Oct4 binding at the Fgf5 locus upon Otx2 overexpression. Genome browser representation of ChIP-seq tracks for Otx2 and Oct4 in EpiLCs (upper rows), in tetON Otx2 cells that were uninduced (middle rows) or induced (bottom rows) with Dox.

(B) Fold changes in Otx2 ChIP-seq binding signals during overexpression (X axis) and during ESC to EpiLC transition (Y axis) were plotted at each of the top Otx2 sites (defined by absolute signal). Three classes of Otx2 sites were classified as follows: (i) purple- EpiLC only (sites induced in EpiLCs >3.5x and in Dox <0.66x), (ii) yellow -EpiLC and OE (sites induced in both >3.5x and >3x fold, respectively), (iii) cyan –OE only (sites induced in Dox >3x with <2x change in EpiLC).

(C) Oct4 binding follows Otx2. Left panel: Plotted are KDE values of the respective Oct4 ChIP-seq signals from uninduced (X axis) and induced (Y axis) tetON Otx2 cells. Right panel: Plotted are fold changes in Oct4 binding upon Otx2 overexpression (X axis) and during ESC to EpiLC transition (Y axis). Each dot represents Otx2 site defined and color-coded as in (B).

(D) Sites bound by Otx2 upon overexpression are nucleosome-occupied in ESCs. Aggregate plots of ESC FAIRE-seq signals over: the EpiLC only (purple), EpiLC and Otx2 OE (yellow) and Otx2 only (cyan) sites defined as in (B). For a relative comparison, aggregate plot of FAIRE-seq signals over ESC specific Oct4 sites is also shown (dotted grey line). Plotted are kernel smoothed normalized counts of FAIRE tags relative to Otx2 ChIP peaks in each Otx2 class (reads per kb per 100,000 reads, kernel bandwidth 100bp).

(E) Otx2 binding upon overexpression is guided by inherent genetic determinants. Genomic regions ±1 kb relative to the Otx2 ChIP-seq signal peak position at top Otx2 sites of each class were scanned for presence of indicated motifs, as described for Figure 4E.

(F) Model for cooperative enhancer selection during ESC to EpiLC transition. See discussion for further description.

See also Figure S7.

Analysis of Oct4 occupancy in uninduced (Dox-) and induced (Dox+) cells demonstrated global reorganization of Oct4 upon Otx2 overexpression (Figure 7C, left panel). Comparisons with the three classes defined above showed that at Otx2 sites induced by overexpression, Oct4 occupancy was also induced in the presence of Dox, regardless of whether these sites were EpiLC-associated or ectopic (Figure 7C, left panel). We also compared changes in Oct4 occupancy during EpiLC formation and upon Otx2 overexpression (Figure 7C, right panel). Changes in Oct4 binding were concurrent with changes in Otx2 in that: (i) Otx2 sites induced by overexpression and during EpiLC formation gained Oct4 in both conditions, (ii) Otx2 sites induced only in EpiLC gained Oct4 only during differentiation, and (iii) ectopic Otx2 sites gained Oct4 only upon Otx2 overexpression, but not during differentiation (Figure 7C, right panel; Figure S7B). Taken together, our results provide strong evidence that Otx2 is able to globally redirect Oct4 to new genomic targets.

Genetic determinants of Otx2 engagement with new enhancer regions

Although upon overexpression Otx2 can access nearly half of its EpiLC-specific targets and redirect Oct4 to these sites, other EpiLC enhancers normally bound by Otx2 during differentiation remain unengaged, suggesting that genetic or epigenetic determinants of Otx2 binding must exist. We failed to detect epigenetic characteristics that would distinguish the engaged and unengaged EpiLC sites (such as presence or absence of specific histone modifications). Moreover, both classes of elements (and ectopic sites) were inaccessible in ESCs, as measured by FAIRE-seq (Figure 7D). To explore potential genetic determinants, we analyzed Otx2 motif enrichments at the three classes of sites defined in Figure 7B. Sites bound by Otx2 upon overexpression (either EpiLC-associated or ectopic) are characterized by much higher Otx2 motif enrichment than EpiLC sites not engaged upon overexpression (Figure 7E, left panel). On the other hand, the latter, but not the former, show strong enrichment for Oct4 and Sox2 motifs (Figure 7E, middle and right panel). Thus, when overexpressed, Otx2 preferentially engages sites containing strong and/or multiple recognition motifs (such as the E2 enhancer, which has three recognizable Otx2 consensus motifs), but not those with weak affinity for Otx2 (such as the E1, E3 and E4 elements, which have either none or one recognizable consensus motif). However, during differentiation Otx2 also binds these low affinity enhancers, which are instead more strongly enriched in other motifs, including Oct4-Sox2. Our observations suggest that Otx2 can simultaneously act as a 'leader', facilitating engagement of other TFs with the genome, and as a ‘follower’, binding opportunistically at sites engaged by other TFs.

Discussion

The results presented here provide a molecular framework for understanding how Oct4 can regulate both the naïve and primed pluripotent states. We demonstrate that the engagement of Oct4 with the genome is dynamic and modulated by the presence or absence of other cell state-specific TFs, such as Otx2. Indeed, overexpression of Otx2 is sufficient to redirect Oct4 to thousands of new genomic sites, even in the absence of other differentiation cues. Based on these data, we propose a model for de novo activation of enhancers during the ESC to EpiLC transition (Figure 7F). Selection of new enhancer sites requires TFs to overcome the nucleosomal barrier in order to gain access to the underlying DNA. Although in ESCs Oct4, Sox2 (and low levels of Otx2) are already present, they cannot effectively compete with nucleosomes, and thus these sites remain unengaged. However, once additional factors are induced or activated by FGF signaling (Otx2 being one such factor), a cohort of TFs can collaborate to access DNA, leading to enhancer activation (Figure 7F, left panel). Depending on the cis-regulatory architecture of individual elements, some TFs may provide stronger thermodynamic contribution to the competition with nucleosomes than others (e.g. Otx2 on its high affinity sites, left panel). Upon overexpression, Otx2 selectively engages those enhancers that are strongly enriched for its binding motif, likely because only at these sites it can effectively outcompete nucleosomes (Figure 7F, right panel). However, at weak affinity sites, Otx2 binds only during normal ESC to EpiLC transition, presumably as a consequence of other TFs playing a predominant role in facilitating access to DNA (compare left and right panels). Furthermore, selective engagement of the Fgf5 E2 element upon Otx2 overexpression demonstrates that individual elements within a super-enhancer can show different sensitivity to changes in TF levels. Thus, even when super-enhancer elements bind the same TFs (e.g. Oct4 and Otx2), they can distinctly integrate regulatory inputs, depending on the underlying cis-regulatory features. Interestingly, at weak affinity enhancers Otx2 can also productively contribute to transcriptional activation, as evidenced by the dependency of the Fgf5 E1, E3 and E4 enhancers on Otx2 (Figure 6C).

Results presented here are consistent with proposed models of indirect cooperativity among TFs, such as 'collaborative competition' or 'nucleosome-mediated cooperativity', which do not evoke physical association between the TFs or rigid, stereotypic arrangement of their DNA binding motifs (Miller and Widom, 2003; Mirny, 2010; Moyle-Heyrman et al., 2011). Instead, these models postulate that cooperative behavior arises through competition with histones by multiple TFs whose DNA recognition sites are located within the length of DNA wrapped around one nucleosome (Miller and Widom, 2003; Mirny, 2010; Moyle-Heyrman et al., 2011). Indeed, Oct4 binding sites in naïve and primed states coincide with differential enrichment of a multitude of TF binding motifs, which are often found within +/− 50 bp from the Oct4 site, but, with the exception of the Sox2 motif, not arranged in a stereotypic manner. Moreover, Oct4 occupancy at such close proximity sites is stronger in the cell state characterized by higher expression of a given TF. One consequence of indirect cooperativity is that redundancy among TFs in competition with nucleosomes can be pervasive at enhancers, which are densely populated with TF binding motifs, as long as a critical number of TF competing for the same nucleosome is present. Such redundancy during EpiLC formation is suggested by the fact that although Otx2 overexpression is sufficient to engage nearly half of EpiLC enhancers and activate many, if not most, EpiLC genes, the effect of loss of Otx2 EpiLC gene expression is relatively weak.

In addition to selection of new enhancer sites, the transition to primed pluripotency is also associated with decommissioning of naïve pluripotency enhancers. We speculate that this process is, at least in part, a consequence of downregulation of factors such as Esrrb and Klf4, which in turn destabilizes Oct4 binding at these sites. A recent report highlighted one potential mechanism of Esrrb downregulation during the exit from naïve pluripotency via signaling-dependent occlusion of Tfe3 from the nucleus (Betschinger et al., 2013). Interestingly, Otx2 overexpression in the presence of FGF inhibitors leads to transcriptional repression of Esrrb and many other naïve pluripotency transcripts, suggesting a link between ESC enhancer decommissioning and activation of new enhancer sites. Nonetheless, the effect of Otx2 on disengagement of ESC enhancers is likely indirect, because we did not observe Otx2 binding at these sites upon overexpression.

Finally, our results raise the question of whether Otx2 is a pioneer factor (Cirillo et al., 2002). Although we showed that during transition to EpiLC Otx2 can access previously nucleosome-occluded DNA and facilitate binding of Oct4, our data also show that ability of Otx2 to engage a new enhancer site is highly context-specific and dependent on: (i) Otx2 levels, (ii) signaling environment, which likely dictates presence or absence of other cooperating TFs, and (iii) inherent cis-encoded affinity for the enhancer site. Similarly, although Oct4 can act as a pioneer factor during reprogramming, our results clearly demonstrate that its binding can also be directed by other TFs, including Otx2. Perhaps in a cellular context, no one factor acts as a universal pioneer, but the ability of a TF to engage nucleosomal DNA can be defined in the context of a specific TF concentration, enhancer sequence, preexisting chromatin environment and presence of other TFs. These observations may have consequences for understanding cellular reprogramming, because initial Oct4 targeting to genomic sites is likely not only restricted by repressive chromatin domains (Soufi et al., 2012), but also depends on the milieu of other factors present in the somatic donor cell. Even though reprogramming can be deterministic when certain chromatin restrictions are attenuated (Rais et al., 2013), our results predict that the pattern of initial engagement of Oct4 with the genome should be nonetheless donor cell type-dependent.

Experimental Procedures

Cell Culture and Differentiation

All mouse ESC lines were adapted for a minimum of 5 passages to growth in serum free N2B27 based medium supplemented with MEK inhibitor PD0325901 (0.8µM) and GSK3β inhibitor CHIR99021 (3.3µM) in tissue culture (TC) dishes pretreated with 7.5µg/ml polyL-ornithine (Sigma) and 5µg/ml laminine (BD) (Hayashi et al., 2011). To induce EpiLC differentiation, cells were washed with PBS, trypsinized and strained. 200,000 to 300,000 cells per 10cm2 were plated on TC dishes pretreated with 5µg/ml Fibronectin (Millipore) in N2B27 based medium supplemented with 1% KSR (Invitrogen) and 12µg/ml bFGF (Peprotech). Where indicated 20ng/ml Activin A (R&D Scientific) was added. For inhibition of all branches of the FGF signaling pathway, final concentration of 6.3µ_M_ FGF receptor tyrosine kinase inhibitor (EMD) was added to 2i+LIF medium.

RNA-seq

RNAs from two independent biological replicates of ESCs and EpiLCs were extracted with Trizol (Invitrogen), following the manufacturer's recommendations. 10 µg of total RNA were subjected to two rounds purification using Dynaloligo-dT beads (Invitrogen). Purified RNA was fragmented with 10x fragmentation buffer (Ambion) and used for first-strand cDNA synthesis, using random hexamer primers (Invitrogen) and SuperScript II enzyme (Invitrogen). Second strand cDNA was obtained by adding RNaseH (Invitrogen) and DNA Pol I (New England Biolabs). The resulting double-stranded cDNA was used for Illumina library preparation and sequenced with Illumina Genome Analyzer.

ChIP-seq

ChIP assays were performed from approximately 107 ESC or EpiLC per experiment, according to previously described protocol with slight modification (Rada-Iglesias et al. 2010). Briefly, cells were cross-linked with 1% formaldehyde for 10 min at room temperature and the reaction was quenched by glycine at a final concentration of 0.125 M. Chromatin was sonicated to an average size of 0.5-2 kb, using Bioruptor (Diagenode). A total of 5–7.5ug of antibody was added to the sonicated chromatin and incubated overnight at 4°C. Subsequently, 50µl of protein G Dynal magnetic beads were added to the ChIP reactions and incubated for 4–6 hours at 4 °C. Magnetic beads were washed and chromatin eluted, followed by reversal of cross-links and DNA purification. ChIP DNA was dissolved in water. All antibodies used in this study are summarized in the Extended experimental procedures. For FAIRE, sonicated chromatin was prepared as for ChIP and DNA was extracted as previously described (Rada-Iglesias et al. 2009). ChIP-seq, FAIRE-seq and input libraries were prepared according to Illumina protocol and sequenced using Illumina Genome Analyzer.

Quantitative PCR

All primers used in qPCR analysis are listed in the Extended experimental procedures. Primers are named after genes proximal to the investigated enhancers. For each tested genomic element, two sets of primers were used, one set overlapping the peak of maximal p300 enrichment (central primers) and another set overlapping flanking regions with K27ac enrichment (flanking primers). qPCR analysis was performed on Light Cycler 480II (Roche), using technical triplicates and ChIP-qPCR signals were calculated as percentage of input. In a second step the percentage of input for each primer set was then normalized to the average of two negative regions.

For expression analysis, RNA was extracted from Trizol (Invitrogen) following manufacturer’s protocol, DNA was removed using TuboDNase (Ambion) and cDNA was prepared using Superscript Vilo cDNA synthesis kit (Invitrogen).

All genomic dataset are deposited to the GEO repository with the accession number GSE56138.

Other methods and details of data analysis can be found in the Extended experimental procedures.

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Highlights

Supplementary Material

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Acknowledgements

We thank A. Rada-Iglesias for insightful comments on the manuscript, Wysocka lab members for critical reading, Y. Zhang for assistance, Z. Weng, Stanford Sequencing Service Center, and Stanford Functional Genomics Facility for Illumina sequencing. This work was supported by the P01 GM099130 and CIRM RB3-05100 (J.W.), SGTP training grant (5 T32 HG 44–17) (C.B.) and Italian Association for Cancer Research (AIRC) IG2013 N. 14152 and the CNR-MIUR Epigenetics Flagship grants (A.S.).

Footnotes

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References