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Vol. 18, Issue 4, 1348-1358, April 2007
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*Stem Cell and Developmental Biology Group, Gene Regulation Laboratory, and ||Information and Mathematical Sciences Group, Genome Institute of Singapore, Singapore 138672; and Department of Biological Sciences, National University of Singapore, Singapore 117543
Submitted July 24, 2006;
Revised November 9, 2006;
Accepted January 22, 2007
Monitoring Editor: Marianne Bronner-Fraser
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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). In ES cells Oct4, Nanog, and Sox2 co-occupy promoters of hundreds of genes that are both expressed and repressed in the pluripotent state (Boyer et al., 2005; Loh et al., 2006). This suggests complex regulatory circuitry in which Oct4, Nanog, and Sox2 collectively and uniquely regulate downstream genes to control ES cell differentiation. However, it remains unclear what are the downstream effectors of these transcription factors that contribute to maintaining the pluripotent status of ES cells. It also not understood how these "master regulators" of pluripotency are involved in controlling lineage-specific differentiation of ES cells. It is therefore useful to elucidate the transcriptional networks surrounding Oct4, Nanog, and Sox2, where detailed knowledge of these pathways remain key to harnessing the potential to direct differentiation of ES cells into therapeutically useful cell types.
To expand our understanding of the transcriptional networks that control stem cell differentiation, we have looked at transcription factors whose expression is directly regulated by Oct4, Nanog, and Sox2. We have identified Zic3 (Zinc finger protein of the cerebellum 3) as a transcription factor of interest for two main reasons. First, Oct4, Nanog, and Sox2 binding have been mapped to the Zic3 promoter regions in ES cells (Boyer et al., 2005; Loh et al., 2006), implying that these key factors may regulate Zic3 expression. The overlap between mouse and human ES cells further highlights the significance of Zic3 and suggests possible conservation of the gene's pathways between the two species. Second, Zic3 demonstrates differential gene expression between the pluripotent and early differentiation phases, where its expression is higher in the pluripotent state (Brandenberger et al., 2004; Wei et al., 2005). The changes in gene expression between these states suggest a potential role for Zic3 in controlling differentiation of mouse and human ES cells.
Zic3 belongs to the GLI superfamily of transcription factors and is a vertebrate homologue of the Drosophila pair-rule gene odd-paired (opa; Aruga et al., 1996a). The five known mammalian Zic genes (Zic1-5) encode five tandem C2H2 zinc finger domains that are highly conserved across species (Herman and El-Hodiri, 2002; Grinberg and Millen, 2005). Although the expression of Zic3 is restricted to the cerebellum of adult mammals, dynamic patterns of expression have been observed during embryonic development in mouse (Herman and El-Hodiri, 2002), Xenopus (Nakata et al., 1997, 1998), chick (Warner et al., 2003), and zebrafish (Grinblat and Sive, 2001). The expression of Zic3 in the embryonic ectoderm and mesoderm during gastrulation (Kitaguchi et al., 2002; Elms et al., 2004), and throughout the tailbud, retina and limb bud during neurulation and organogenesis (Herman and El-Hodiri, 2002; Orkin, 2005), suggests an important role for this transcription factor in embryonic ectoderm and mesoderm development. This is further supported by molecular pathways in which Zic3 has been implicated. For example the mesoderm-associated gene Brachyury induces Zic3 expression in Xenopus (Kitaguchi et al., 2002), and the embryonic patterning gene Nodal is regulated by Zic3 during gastrulation through interaction with an upstream enhancer region in mouse and Xenopus embryos (Ware et al., 2006a). In ectodermal development, Zic3 is a potent inducer of Xenopus proneural and neural crest genes (Nakata et al., 1997) and is induced directly downstream of transcription factors Pbx1b and Meis1 in the Xenopus ectoderm (Maeda et al., 2002; Kelly et al., 2006).
Zic3 mutations are associated with X-linked heterotaxy, a disorder characterized by disruptions in embryonic laterality and midline developmental field defect (Gebbia et al., 1997). In Zic3 mutant organisms situs ambiguus is frequently observed, encompassing failure in lateralization of internal organs, mirror-image inversions, and left-right isomerism (Aylsworth, 2001). Several mutations have been identified in humans that render the Zic3 protein unstable and absent in cells or incapable of nuclear localization where its transcriptional effect is exerted (Gebbia et al., 1997; Ware et al., 2004).
Consistent with its expression in the involuting mesoderm and presumptive neural plate during gastrulation, Zic3 is involved in regulating left–right asymmetry and neural tube development. Zic3-null mice exhibit a wide spectrum of phenotypes. Fifty percent of null mice succumb to embryonic lethality over different gestational stages, and 30% to perinatal lethality as a result of congenital heart defects, pulmonary isomerism, and defects in the CNS (Purandare et al., 2002). The earliest and most profound Zic3-null defects have been attributed to failure in establishment of the anterior-posterior axis by the anterior visceral endoderm (AVE) before gastrulation (Ware et al., 2006b). In less severely affected embryos, abnormalities are observed at gastrulation in the distribution and accumulation of excess mesoderm tissue. Taken together, the defects in embryonic lethal mice demonstrate a key role for Zic3 in early embryonic patterning that encompasses anterior visceral endoderm formation, initiation of gastrulation, and primitive streak morphogenesis (Ware et al., 2006b).
The varying degrees of severity in failure to complete gastrulation displayed by Zic3 null mice may perhaps be attributed to compensatory mechanisms in developing embryos, as indicated by the distinct and partially overlapping expression patterns exhibited by members of the Zic gene family (Nagai et al., 1997; Elms et al., 2004). It is important to note that Zic3 shares overall 64 and 59% homology with Zic1 and Zic2, respectively, and this homology increases to 91% within the zinc finger domain. Thus members of Zic family are strong candidates for redundancy in molecular signaling owing to the high degree of homology and overlapping expression observed among the members of this family.
Although Zic3 expression has been implicated in embryonic development, still lacking is a detailed understanding of what regulates Zic3 expression and what the downstream effectors of Zic3 are. The Zic3 gene has been identified as a target of Oct4, Nanog, and Sox2 in ES cells (Boyer et al., 2005; Loh et al., 2006), and Zic3 is preferentially expressed in pluripotent state (Brandenberger et al., 2004; Wei et al., 2005). Questions arising from these data are as follows: 1) How do Oct4, Nanog, and Sox2 interact with the Zic3 regulatory region, and what results from this interaction and, 2) what role does Zic3 play in the embryonic stem cell? We have addressed these questions using the loss-of-function approach for Zic3 and the key regulatory genes in ES cells. In this study, we examined the function of Zic3 as a regulatory target of Oct4, Nanog, and Sox2 in ES cells. We report that Zic3 shares significant overlap with the Oct4, Nanog, and Sox2 transcriptional networks and is important in maintaining ES cell pluripotency by preventing differentiation of cells into endodermal lineages. Thus our results extend the current knowledge of the ES cell transcriptional circuitry beyond Oct4, Nanog, and Sox2.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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-mercaptoethanol (Invitrogen), and Chinese hamster ovary-Leukaemia Inhibitory Factor (CHO-LIF) (1000 U/ml). Feeder-free undifferentiated HuES9 human ES cells were maintained on matrigel-coated dishes in conditioned medium containing knockout DMEM/10% serum replacement (Invitrogen), 0.1 mM MEM nonessential amino acids (Invitrogen), 1 mM L-glutamine (Invitrogen), 0.1 mM -mercaptoethanol (Invitrogen), 8% plasmanate (National University Hospital Pharmacy, Singapore), 12 ng/ml LIF, and 10 ng/ml human recombinant basic fibroblast growth factor (bFGF; Invitrogen). Conditioned medium was obtained by culturing mouse embryonic fibroblast (MEF) cells with HuES9 media. The medium was collected at 24 h intervals, filter sterilized, and further supplemented with 8 ng/ml bFGF for HuES9 cell culture.
RNA Interference and Establishment of Clonal Knockdown Lines
Small Interfering RNA (siRNA) Experiments.
RNA interference (RNAi) experiments were performed with Dharmacon siGENOME SMARTpool reagents (Boulder, CO) against human or mouse Zic3. The Dharmacon siCONTROL nontargeting siRNA pool was used as a negative control. Mouse ES cells were transfected according to manufacturer's instructions in 12-well plates at a density of 2 x 105 cells per well. Retransfections were performed on pre-adherent cells at 48-h intervals, and RNA expression analysis was performed on samples from day 5. Human ES cells were transfected in 12-well plates with 2 x 105 cells, in suspension, per well. Subsequent retransfections were performed on adherent cells at 24-h intervals and RNA was harvested for analysis at day 5.
Short Hairpin RNA (shRNA) Experiments.
The Oct4 and Nanog RNAi experiments were previously published (Loh et al., 2006). The Zic3 shRNA construct was designed as described (Chew et al., 2005) with a target sequence of 5'-GAATTCGAAGGCTGTGACA-3'. E14 cells in six-well plates were transfected with 2.0 µg pSUPERpuro vector or Zic3-pSUPER.puro (OligoEngine, Seattle, WA) at a density of 4 x 105 cells per well. Puromycin selection was introduced 1 d after transfection at 1.0 µg/ml and was maintained for 3 d before RNA isolation. ES cells were maintained in proliferative medium at all times.
Clonal Zic3 knockdown lines were established by transfection of shRNA constructs as described above. The Zic3 knockdown and vector control colonies were picked after 7 d of puromycin selection (1.0 µg/ml). Colonies were dissociated into single-cell suspensions by treatment with 0.05% Trypsin (Invitrogen) and plated on puromycin-resistant mitomycin-inactivated DR4 MEFs (ATCC, Manassas, VA). In total, 15 Zic3 clonal knockdown and 7 vector control lines were established and maintained under constant puromycin selection. The lines analyzed in this article were maintained feeder-free in ES cell proliferative media on 0.1% gelatin-coated dishes over a period of eight passages.
Secondary ES Colony-replating Assay
ES cells were transfected with Zic3- or empty pSUPER shRNA constructs and selected 24 h later with puromycin at 1.0 µg/ml over 4 d. At the end of 4 d few cells remained in the untransfected control wells indicating that selection was effective. The surviving cells were trypsinized and resuspended in E14 medium without LIF. Ten thousand or 20,000 cells were plated onto mouse feeder layers in six-well plates for secondary ES cell-colony formation. After 7 d, emerging colonies were stained with the Wright-Giemsa (Sigma, St. Louis, MO) stain. The extent of differentiated colonies was defined as the percentage of unstained colonies out of the total number of colonies in the well.
RNAi Rescue Experiments
The Zic3 open reading frame (ORF; NM_009575
[GenBank]
) was cloned from reverse-transcribed cDNA from mouse embryonic stem cells, using the primers indicated in Supplementary Table 1A. The PCR product was subsequently cloned into a vector driven by the CAG promoter. The RNAi-immune Zic3 ORF R3M (Supplementary Figure 1) was generated from this template using site-specific mutagenesis. To perform the rescue experiments, 4 x 105 mouse ES cells were seeded per well in six-well plates and transfected according to the scheme in Supplementary Table 1B. Hygromycin selection (1.0 µg/ml) was introduced 1 d after transfection.
RNA Extraction, cDNA Synthesis, and Quantitative Real-Time PCR
To minimize genomic DNA contamination, RNA was extracted with TriZol reagent (Invitrogen) and further purified with the RNeasy minikit (Qiagen, Chatsworth, CA). cDNA was synthesized with 1.0 µg total RNA using the High Capacity cDNA Archive kit (Applied Biosystems, Foster City, CA). For each qPCR reaction, cDNA samples diluted 10 times in water were mixed with 5.0 µl TaqMan Universal PCR Master Mix reagent (Applied Biosystems) and 0.5 µl of a single TaqMan probe from the following list: Zic3, Oct4, Nanog, Sox2, or the lineage markers in Table 1(20x TaqMan Gene Expression Assay reagents; Applied Biosystems) with a final volume of 10 µl. Quantitative real-time PCR analysis was conducted in 384-well clear optical reaction plate (Applied Biosystems) on the ABI Prism 7900 machine (Columbia, MD).
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-actin (1:3000; Invitrogen) and goat anti-mouse HRP (1:5000; Santa Cruz Biotechnology). For immunocytochemistry, cells were seeded at a density of 1.0 x 105 cells per well on fibronectin-coated chamber slides, fixed in 4% paraformaldehyde, and permeabilized with 0.3% Triton X-100. Blocking was performed with 5% fetal bovine serum and 1% bovine serum albumin in PBS solution for 30 min. Cells were stained with the following primary antibodies (1:100): goat or mouse anti-Oct4 (Santa Cruz Biotechnology, N-19 and C-10, respectively), rabbit-anti-Nanog (Chemicon, Temecula, CA; AB5731), goat anti-FoxA2 (M-20, Santa Cruz Biotechnology), goat-anti-Gata6 (C-20, Santa Cruz Biotechnology), or mouse anti-CD140a (PDGFRA; eBioscience, San Diego, CA; 16-1401). This was followed by the appropriate secondary antibodies detecting mouse or goat IgG Alexa Fluor 488 (Molecular Probes, Eugene, OR; 1:500) for Oct4 staining, rabbit IgG Alexa Fluor 594 (Molecular Probes; 1:500) for Nanog staining, or Qdot 655 anti-goat or anti-mouse antibodies (Molecular Probes) for FoxA2, Gata6, and PDGFRA staining (1:150) according to the manufacturer's protocol. Images were captured with the Zeiss LSM 5 Duo inverted confocal microscope (Zeiss, Thornwood, NY).
Luciferase Reporter Construct and Assays
The 300-base pair Zic3 enhancer region containing the Nanog-binding site was cloned from mouse genomic DNA. The primers used were as follows: forward, 5' ATATAacgcgtTTAGAGGTCAAACCAT-3' and reverse, 5'-TATATagatctTAGTAGTCAAACTGGATT-3' with restriction sites indicated in lower case letters. The PCR fragment was digested with MluI and BglII and cloned into the pGL3-Basic vector (Promega, Madison, WI) containing a basal promoter comprising the 500-bp region immediately upstream of the mouse Oct4 gene. The following constructs were transfected into cells 2.5 x 104 cells in 96-well plates for the luciferase assay: 100 ng firefly luciferase reporter, 1.0 ng of the Renilla luciferase vector, pRL-SV40 plasmid normalization control, and 250 ng of the respective knock-down construct. Puromycin selection (1.0 µg/ml) was introduced 20 h after transfection and cultured for 2 d. Luciferase activity measured using the Dual Luciferase System (Promega) in a Centro LB960 96-well luminometer (Berthold Technologies, Natick, MA).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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; Wei et al., 2005). Among these genes are transcription factors Oct4, Nanog, and Sox2, which are required to maintain pluripotency of ES cells. Zic3, a zinc-finger transcription factor, was also found to be expressed in undifferentiated ES and suppressed in differentiated cells, and thus, may play a role in regulating ES cell differentiation. We assayed the expression of Zic3 in mouse ES cells induced to differentiate over 6 d by addition of retinoic acid (RA; Figure 1A). Similar to the trends observed for Oct4, Nanog, and Sox2 genes, Zic3 transcript levels decreased between 1.5- and 10-fold for each 2-d interval (D2, D4, and D6), relative to the undifferentiated control. Zic3 RNA levels were also significantly decreased in mouse ES cells differentiated by treatment with HMBA (hexamethylene bisacetamide) or dimethyl sulfoxide, and also by aggregation into embryoid bodies (data not shown). The decrease in Zic3 mRNA correlated with a comparable decrease in protein expression (Figure 1B). Thus, Zic3 gene expression is associated with the mouse ES pluripotent state and its expression decreases as cells differentiate.
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; Loh et al., 2006). The binding of these transcription factors, which are demonstrated regulators of pluripotency, suggests that Zic3 is a direct target for regulation by these TFs and may play a role in regulating ES cell differentiation.
Regulation of Zic3 by Oct4, Sox2, and Nanog
To further validate that Oct4, Sox2, and Nanog regulate Zic3 expression, we performed gene expression knockdown experiments in mouse ES cells using RNA interference. Mouse ES cells were thrice transfected with gene-specific siRNAs against Oct4, Sox2, and Nanog on alternate days to achieve 80–90% reduction in expression of the targeted gene (Figure 2A). Down-regulation of Oct4 and Sox2 reduced the level of endogenous Zic3 to <25%, whereas Nanog RNAi reduced the level of Zic3 to 70% (Figure 2B). These data indicate that Zic3 expression is regulated by Oct4, Sox2, and Nanog.
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). As the endogenous levels of Zic3 decreased in the presence of RA-induced differentiation (Figure 1), we were interested in determining if Nanog overexpression would sustain Zic3 levels under RA treatment. ES cells were stably transfected with a construct that expresses Nanog from a constitutively active promoter. The Nanog-expressing cells and cells transfected with empty vector were treated for 2 d with 0.3 µM RA. Vector-only control cells showed a decrease in Zic3 RNA levels typical of RA-induced differentiation. In contrast, mouse ES cells overexpressing Nanog sustained the level of Zic3 at greater than 80%, relative to the control ES cell line (Figure 2C). Thus, overexpression and knockdown of Nanog in ES cells results in an increase and decrease, respectively, of Zic3, suggesting that Zic3 expression is regulated by Nanog, perhaps directly or indirectly.
Our previous study identified a Nanog binding site in the enhancer region, 16.4 kb upstream of the transcription start site, of the Zic3 gene (Loh et al., 2006). As this DNA region was available for further study in our lab, we sought to determine if Zic3 expression was directly regulated by Nanog. We fused the 292-base pair portion of the Zic3 enhancer that contains the Nanog-binding site upstream of a minimal Pou5f1 promoter driving the firefly luciferase gene (Figure 2D). The minimal promoter was weakly active in ES cells, whereas activity of the Zic3 enhancer region linked to the minimal promoter was ninefold up-regulated as quantified by luciferase (Figure 2E). When the sequences of this putative Nanog binding site were deleted from the Zic3 enhancer the corresponding reporter activity decreased (Figure 2E). We then transfected Nanog RNAi together with the wild-type reporter construct and showed that the activity of the Zic3 enhancer decreased fourfold relative to the controls (Figure 2F). Collectively, our data show that Zic3 expression is directly regulated by Nanog and thus, may be a downstream effector in controlling ES cell differentiation.
Effect of Zic3 Depletion on ES Cell Differentiation
To investigate the role of Zic3 in ES cells, we used RNAi to achieve knockdown of gene expression. Both the siRNA and shRNA methods resulted in a 70% reduction of Zic3 transcript levels relative to the nontargeting controls (Figure 3A). Zic3 protein levels reflect this decrease in gene expression after Zic3 RNAi treatment, whereas protein expression remained high in vector-only–treated cells (Figure 3B).
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It is interesting that targeted repression of Zic3 induced morphological differentiation of ES cells while maintaining the expression of pluripotency marker genes in the transient knockdown experiments. We were interested in assessing the role of Zic3 in the maintenance of pluripotency. To determine the differentiation status of these cells we assayed by Q-RT-PCR for expression of markers that represent lineage-specific ES cell differentiation (Table 1). Zic3 knockdown in mouse and human ES cells resulted in an up-regulation of a panel of endodermal markers: Sox17 (3.5-fold), PDGFRA (3.2- to 5.5-fold in mouse ES cells; 2.7-fold in human ES cells), and Gata6 (2.5- to 3.5-fold; Figure 4). In addition, two more endodermal lineage genes Gata4 and Foxa2 were up-regulated in the E14 RNAi cells (2.5-fold). We also assayed the expression of mesendodermal, mesodermal, ectodermal, trophectodermal and Wnt-pathway markers in Zic3 RNAi cells. These markers remained unchanged relative to the nontargeting control in both mouse and human RNAi experiments (Figure 4). These results indicate that Zic3 could play a specific role in maintaining ES cell pluripotency by suppressing endodermal specification.
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The Zic3 RNAi-immune expression construct was engineered with five silent mutations in protein coding domain sequence (Supplementary Figure 1). As such, this construct (mutZic3) produces functional Zic3 protein, but with the added feature that it is resistant to RNAi targeting and degradation. Using this mutZic3 construct, we determined the specificity of the endodermal lineage specification produced by Zic3 knockdown. First, the expression levels of endodermal markers Foxa2, Gata4, and Sox17 were induced in ES cells cotransfected with empty vector and Zic3-RNAi, compared with cells cotransfeceted with empty vector and GFP-RNAi (6.5-, 10.1-, and 8.7-fold, for Foxa2, Gata4, and Sox17, respectively, Figure 5, A–C). However, ES cells that express the mutZic3 (RNAi immune construct) showed no induction of endodermal markers by Zic3-RNAi. (Figure 5, A–C). These experiments indicate that our RNAi results are not due to off-target effects and further support our conclusions that Zic3 plays a role in maintaining the pluripotency of ES cells.
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; Wei et al., 2005). Zic2 may also be regulated by Oct4, Sox2, and Nanog as binding sites for these TFs have been mapped to the Zic2 gene by chromatin immunoprecipitation (ChIP; Supplementary Figure 2). It was interesting that Zic3 RNAi resulted in a twofold increase in Zic2 (Figure 6A), and this raised the possibility that Zic2 may be compensating for the reduction in Zic3 levels. Knockdown of Zic2 expression by siRNA (75% reduction in RNA levels) did not produce any effect on lineage marker expression (Figure 6B). To determine if Zic2 compensated for the absence of Zic3, we performed a double RNAi experiment with Zic2 and Zic3 in ES cells. The double knockdown prevented Zic2 levels from increasing in a compensatory manner as observed in the Zic3 single knockdown (Figure 6C). Interestingly, endodermal specification was markedly enhanced after the Zic2 and Zic3 double knockdown as demonstrated by increased expression of Sox17 (4.7-fold), PDGFRA (8.7-fold), and Gata4 (3.1-fold), which is more robust than observed for all three markers (Sox17, 3.1-fold; PDGFRA, 3.3-fold; Gata4, 1.5-fold) when Zic3 alone was reduced (Figure 6D). Thus, we demonstrate that in the absence of Zic3, Zic2 is able to compensate at least partially to reduce the extent of endodermal specification in ES cells.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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). The differences we observed in Zic3 expression levels between pluripotent and early differentiation phases imply a potentially significant role for Zic3 in ES cell pluripotency. In addition, ChIP mapping by us and others has revealed Oct4, Nanog, and Sox2 co-occupancy on the Zic3 regulatory region, suggesting that Zic3 may be coordinately regulated by Oct4, Nanog, and Sox2 in mouse and human ES cells (Boyer et al., 2005; Loh et al., 2006; Figure 1C). These observations together led to our hypothesis that Zic3 functions to maintain the pluripotent state of ES cells. Here we characterized the relationship of Zic3 with that of the key stem cell regulatory factors and uncovered a role for Zic3 in the maintenance of ES cell pluripotency.
Our first objective was to assess the nature of interactions between Oct4, Nanog, and Sox2 with the Zic3 regulatory region. In constructing the transcriptional network around the key pluripotency genes, it is important to establish the outcome of transcription factor binding on downstream genes. We addressed this using a combinatorial approach encompassing the results of ChIP mapping and RNAi, demonstrating that ablation of Oct4, Nanog, and Sox2 in mouse ES cells resulted in a significant decrease in Zic3 expression (Figure 3A). Because Zic3 has already been implicated as a target of Oct4, Nanog, and Sox2 in ChIP experiments (Boyer et al., 2005; Loh et al., 2006), the concern of nondirect or secondary effects of RNAi was significantly reduced (Blais and Dynlacht, 2005). We thus concluded that the interaction of Oct4, Nanog, and Sox2 with the regulatory region of the Zic3 gene serves to enhance target gene expression. In other words, the key pluripotency regulators function as transcriptional activators of Zic3 in ES cells (Figure 9). This point is underscored by our results with Nanog overexpression and binding site mutagenesis assays, which demonstrate a positive association between Nanog binding and Zic3 expression. We thus demonstrate positive functional interactions between the key pluripotency regulators and the Zic3 gene regulatory region.
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; Blais and Dynlacht, 2005), we also asked if the inverse relationship was true, that is, whether Zic3 regulates expression of the key regulatory genes. We observed that Oct4 and Sox2 levels remained largely unperturbed by the ablation of Zic3 expression (Figures 3A and 7A). In the absence of clear changes despite a robust Zic3 knockdown, our data places Zic3 downstream of Oct4 and Sox2 in the ES cell transcriptional networks as illustrated in Figure 9. In addition, we found that Nanog expression decreased significantly in the Zic3 clonal knockdown lines (Figures 7A and 8C). It remains to be determined whether Zic3 directly regulates the expression of Nanog in embryonic stem cells.
ES cells are derived from the inner cell mass of the blastocyst and, as such, are able to undergo unlimited self-renewal and differentiation into the three germ layers of the embryo: mesoderm, ectoderm, and endoderm (Evans and Kaufman, 1981; Martin, 1981). In the pluripotent state, ES cells remain undifferentiated and do not express specific lineage markers. We were interested in examining the effect of Zic3 knockdown on the maintenance of ES pluripotency using specific lineage markers as an assessment of differentiation after Zic3 knockdown (Table 1). Here we show that ablation of Zic3 expression in both mouse and human ES cells resulted in a significant increase in markers of endodermal lineage (Figures 4, 7, and 8). These results suggest that Zic3 may have an important role in preventing endodermal specification in ES cells.
Many reports support this observation: First, Zic3 knockdown in ES cells induced expression of Gata4 and Gata6, and forced expression of Gata4 and Gata 6 in ES cells result in differentiation toward extraembryonic endoderm (Fujikura et al., 2002). Further strengthening this association is the fact that all other endodermal markers assayed (PDGFRA, Sox17, and FoxA2) are also expressed in extraembryonic endoderm derivatives (Kunath et al., 2005). Second, Zic3 regulates Nodal expression through direct interaction with its promoter during gastrulation, and it has been shown that Nodal expression is essential in proper specification of the embryonic visceral endoderm (Mesnard et al., 2006). This significance is underscored by studies reporting that the earliest abnormalities observed in Zic3 null mice are defects in proper patterning of the anterior visceral endoderm (Ware et al., 2006b). Finally, Zic3 clonal knockdown lines exhibit a significant decrease in Nanog gene expression (Figures 7A and 8C), and several groups have reported that RNAi-mediated depletion of Nanog expression resulted in an induction of extraembryonic endoderm markers Gata4 and Gata6 (Mitsui et al., 2003; Hyslop et al., 2005; Hough et al., 2006).
Here we have shown that Zic3 functions as a gatekeeper of pluripotency in ES cells by preventing their differentiation into cells that express endodermal markers. Corroborating this, we have found that Nanog expression is significantly reduced in the Zic3 clonal lines. This reduction is noteworthy as Nanog is a key regulator of pluripotency in ES cells (Chambers et al., 2003), and it is well established that disruption of Nanog expression results in development of extraembryonic endoderm character in ES cells (Mitsui et al., 2003; Hyslop et al., 2005; Hough et al., 2006). Thus, we demonstrate here an important role for Zic3 in the maintenance of pluripotency in ES cells through prevention of endodermal lineage specification, and we suggest that its action may in part be mediated through the key pluripotency regulator Nanog (Figure 9).
The role of Zic3 in preventing endodermal specification is further supported by evidence indicating its restricted expression within the mesoderm and ectoderm lineages during gastrulation (Herman and El-Hodiri, 2002). In addition, Zic3 activity has been specifically implicated in the mesodermal and ectodermal molecular pathways in the early developing embryo (Nakata et al., 1997; Kitaguchi et al., 2002; Maeda et al., 2002; Kelly et al., 2006). These data in combination with our results suggest that although Zic3 is instructive for mesodermal and ectodermal specification in embryonic development, it may simultaneously function as a repressor of ectopic endodermal induction in these tissues.
The transcription factor Zic3 shares five highly conserved Zinc finger domains with family members Zic1, Zic2, Zic4, and Zic5 (Aruga et al., 1994, 1996a,b, 2004). Their partially overlapping spatial and temporal patterns of expression during early development suggests potential functional redundancy between the Zic family members (Nagai et al., 1997; Elms et al., 2004). We observed that Zic2 gene levels were up-regulated when Zic3 expression was reduced (Figure 6A). Because Zic2 is also differentially expressed between pluripotent and differentiation states of ES cells (Brandenberger et al., 2004; Wei et al., 2005) and binding of the key pluripotency transcription factor Nanog has been mapped to the Zic2 regulatory region (Supplementary Figure 2), we reasoned that Zic2 may participate in the regulation of ES cell pluripotency along with Zic3. To unveil the possible effects of functional redundancy between Zic2 and Zic3, a double knockdown was performed in mouse ES cells. We report that repression of Zic2 and Zic3 expression significantly enhanced endoderm specification in ES cells (Figure 6C). The evidence that Nanog binds to the Zic2 regulatory region suggests that it may be involved in similar pathways as Zic3 in repressing endoderm expression. Thus, Zic2 and Zic3 may participate in redundant or partially overlapping networks to silence endoderm specifying gene expression and contribute to the maintenance of pluripotency in ES cells.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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ACKNOWLEDGMENTS |
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Footnotes |
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The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org). 
These authors contributed equally to this work.
Address correspondence to: Lawrence W. Stanton (stantonl{at}gis.a-star.edu.sg)
Abbreviations used: ES, embryonic stem; RA, retinoic acid; RNAi, RNA interference; ChIP, chromatin immunoprecipitation; AVE, anterior visceral endoderm.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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