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First published online 29 May 2007
doi: 10.1242/jcs.004127
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Research Article |
1 Department of Gastroenterological Surgery, Kobe University Graduate School of Medicine, 7-5-1 Kusunoki-cho, Chuo-ku, Kobe 650-0017, Japan
2 21st Century COE Program, Kobe University Graduate School of Medicine, 7-5-1 Kusunoki-cho, Chuo-ku, Kobe 650-0017, Japan
* Author for correspondence (e-mail: horiy{at}med.kobe-u.ac.jp)
Accepted 30 April 2007
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indicated that this protocol for definitive endoderm and mesoderm differentiation is superior to any other available protocol. Our findings also indicated that the Nodal or Nodal-related molecules secreted from Nodal-expressing ES cells could cause genetically unmanipulated ES cells to induce the expression of the Nodal signaling pathway and its downstream targets, which consequently leads to the differentiation of the ES cells into definitive endoderm and mesoderm. Our differentiation system, using tightly regulated Nodal expression, enabled us to investigate the mechanism of ES cell differentiation into definitive endoderm or mesoderm derivatives. Our findings also demonstrate that Nodal-expressing ES cells might be a source of highly active proteins that could be used for developing endoderm or mesoderm tissues in regenerative medicine.
Key words: Nodal, Definitive endoderm, Mesoderm, Embryonic stem (ES) cells, Regenerative medicine, Tetracycline, CXCR4
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; Hori et al., 2002; Lumelsky et al., 2001) and hepatocyte-like cells (Gouon-Evans et al., 2006; Soto-Gutierrez et al., 2006), or mesoderm-derived cells, such as endothelial cells (Yamashita et al., 2000) and cardiomyocytes (Klug et al., 1996), from embryonic stem (ES) cells. These findings support the potential use of definitive endoderm or mesoderm differentiation for developing tissue replacement strategies. However, there are some problems with many of the current protocols, such as the low frequency of differentiated cells and the cellular heterogeneity in the cultures. The most likely reason for these difficulties is that none of the protocols have been able to recreate definitive endoderm or mesoderm differentiation from ES cells.
The definitive endoderm and mesoderm develop in close association with each other in the vertebrate embryo (Wells and Melton, 1999). A fate map of the mouse epiblast was generated using an intracellular tracer, and it shows that most mesendoderm cells originate from the anterior primitive streak (Lawson et al., 1991). Transforming growth factor β (TGFβ) signals are thought to play a crucial role during the earliest stages of this developmental regulation. Nodal, the most notable endogenous molecule in this early period, sets up the embryonic axes, induces mesoderm and endoderm formation, and determines left-right asymmetry in vertebrates (Schier, 2003). Nodal expression is first detected during implantation [embryonic day (E) 4.5 in mouse] in the inner cell mass of the blastocyst (Mesnard et al., 2006) and persists until the extended primitive streak stage (E7.5) (Collignon et al., 1996; Meno et al., 2001). Mouse Nodal mutants do not form the primitive streak (Conlon et al., 1994; Zhou et al., 1993). In Xenopus inhibitors of Nodal signaling such as Cer-S and Lefty also block mesendoderm formation (Agius et al., 2000; Cheng et al., 2000; Tanegashima et al., 2000). However, the misexpression of Nodal in presumptive ectoderm can induce cells to become either mesoderm or endoderm (Schier and Shen, 2000; Whitman, 2001). Activin, another TGFβ family member, also binds the same receptors as Nodal (with the exception of the coreceptor Cripto), triggering similar intracellular events (de Caestecker, 2004). It is used to induce the development of definitive endoderm and mesoderm from ES cells in vitro (D'Amour et al., 2005; Kubo et al., 2004; Tada et al., 2005; Yasunaga et al., 2005). Although activin can induce endoderm and mesoderm formation in other experimental situations, targeting studies in mice suggest that the endogenous factor does not determine this differentiation in the early embryo (Matzuk et al., 1995; Vassalli et al., 1994). However, there is evidence that maternal TGFβ1 can rescue null knockout embryos, so the question remains as to whether or not maternal activin can rescue early activin functions in activin knockout embryos (Letterio et al., 1994). Therefore, it is still unclear whether or not Nodal and/or activin signaling can regulate the differentiation and maturation of definitive endoderm or mesoderm from ES cells as well as the embryo, since a tightly regulated source is still not yet readily available.
To determine whether Nodal signaling could induce ES cells to differentiate into definitive endoderm or mesoderm and consequently establish conditions for the efficient development of specific cell lineages, we generated an in vitro and in vivo ES cell differentiation system with tetracycline-regulated expression of Nodal employing a promoter engineered for expression. We determined that the upregulated Nodal signaling pathway and the downstream transcriptional targets of Nodal induces ES cells to specify the definitive endoderm and mesoderm derivatives, and downregulated Nodal signaling promotes the maturation of definitive endoderm both in vitro and in vivo. Recently, it has been shown that CXCR4 is a surface marker that distinguishes early visceral and definitive endoderm from ES cells (D'Amour et al., 2005; Yasunaga et al., 2005). More recently, CXCR4+ ES cells have been shown to differentiate into either endoderm cells with a hepatocytic phenotype (Gouon-Evans et al., 2006) or pancreatic endocrine cells (D'Amour et al., 2006). In addition, a PDGFR-+ population derived from ES cells has been shown to include paraxial mesoderm cells that can give rise to bone, cartilage and muscle cells (Nakayama et al., 2003; Sakurai et al., 2006). ES cell-derived VEGFR2+ cells can give rise to both endothelial cells and hematopoietic cells that are progenies of the lateral mesoderm (Kabrun et al., 1997; Wang et al., 2004; Yamashita et al., 2000). We examined single cell differentiation and maturation of definitive endoderm and mesoderm from ES cells using CXCR4 for endoderm and VEGFR2 and PDGFR- for mesoderm, using flow cytometry. We observed that, consistent with Nodal signaling expression, CXCR4, VEGFR2 and PDGFR- expression increases with the initial differentiation into definitive endoderm and mesoderm and decreases with their subsequent maturation. We also found that the present protocol for definitive endoderm and mesoderm differentiation is superior to other available protocols such as activin A or recombinant Nodal treatment in both in vitro flow cytometry and in vivo transplant experiments. Assuming that Nodal or Nodal-related molecules secreted from Nodal-expressing ES cells are a source of highly active protein for inducing definitive endoderm and mesoderm, Nodal signaling could serve as the basis for developing replacement tissue from human stem cells. Our findings indicate that it can enable genetically unmanipulated mouse ES cells to promote the expression of Nodal signaling, thus resulting in the differentiation of these cells into definitive endoderm and mesoderm. These results suggest that our ES cell differentiation system, using tightly regulated Nodal signaling, recapitulates definitive endoderm and mesoderm differentiation along developmental cell lineages. Consequently, Nodal-expressing ES cells may be of potential use in regenerative medicine.
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). The expression of exogenous Nodal was suppressed in the presence of Tet and activated when the drug was removed from the medium (Fig. 1A). To simplify the mechanism of differentiation, we transfected the transgene into the feeder-free ES cell line, E14tg2a (Niwa et al., 1998) and obtained six clones. Each clone showed a similar expression pattern of exogenous Nodal-eGfp and similar in vitro differentiation (data not shown).
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Nodal-expressing ES cells differentiate into mesendoderm at the expense of anterior visceral endoderm and neuroectoderm
We initially examined the intracellular Nodal signaling pathway in Nodal-expressing ES cells. Smad2 is an effector of Nodal signaling (Kumar et al., 2001). A high level of Smad2 phosphorylation was detected in the Tet-off/Nodal-on ES cells compared to the wild-type or Tet-on/Nodal-off ES cells. This elevation was blocked by Cripto-neutralizing antibody, thus suggesting that the Nodal signaling pathway is activated in Nodal-expressing ES cells (Fig. 2A). Notably, the administration of recombinant Nodal protein showed a much lower level of Smad2 phosphorylation than that of Nodal-on ES cells (Fig. 2A). We also performed another set of RT-PCR for the TGFβ family to determine whether the enhanced induction of mesoderm and endoderm progenitors reflects direct action of the Nodal ligand. We detected an increased expression of inhibin beta-c (Inhbc), inhibin beta E (Inhbe) and transforming growth factor beta 3 (Tgfb3) but not inhibin beta-A and inhibin beta-B (Inhba and Inhbb, respectively) or Tgfb1 and Tgfb2 in stage 2 Nodal-on ES cells (Fig. S1 in supplementary material). Taken together with a partial inhibition by Cripto antibody in the FACS analysis as described below (Fig. 2I), we were unable to rule out the possibility of the indirect action of Nodal ligand via other TGFβ ligands. Several genetic studies have determined the identity of various downstream transcriptional targets of the Nodal signaling pathway, including Smad2 and Smad3, Foxh1, Foxa2 and Gsc (Gritsman et al., 2000; Hoodless et al., 2001; Toyama et al., 1995). Foxa2 encodes an important transcriptional factor for definitive endoderm development, and it is required for the formation of the foregut and midgut (Ang and Rossant, 1993). Recently, Tada et al. used Gsc as a marker for mesendoderm differentiation from ES cells (Tada et al., 2005). The transcriptional targets of Nodal signaling were also upregulated in Nodal-expressing ES cells (Fig. 2B), and Cripto neutralizing antibody inhibited these effects (data not shown). We also obtained similar results from real-time quantitative RT-PCR. Expression of genes, including Smad2 and Smad3, Foxa2 and Gsc, in Tet-off/Nodal-on ES cells was approximately four to ten times as much as that in Tet-on/Nodal-off cells (Fig. 2C) or wild-type ES cells (data not shown). These results suggest that the Nodal signaling pathway is activated and the downstream targets are upregulated in Nodal-on ES cells at stage 2.
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We also investigated the effect of Nodal expression on mature definitive endoderm and mesoderm. The marker genes of mature definitive endoderm, including the foregut markers thyroid transcription factor 1 (Ttf1) and surfactant-associated protein C (Sftpc), the fore- and mid-gut markers albumin 1 (Alb1) and pancreatic and duodenal homeobox 1 (Pdx1), the hindgut markers intestinal fatty-acid-binding protein (also known as Fabp2 and hereafter referred to as IFABP) and villin 1 (Vil1), and mesoderm markers GATA-binding protein 1 (Gata1) and kinase-insert-domain protein receptor (Kdr, also known as and hereafter referred to as VEGFR2) are obviously upregulated (Fig. 2D). By contrast, the expression of brachyury, an early mesendoderm marker, was downregulated. These results, together with the expression pattern of Gsc, indicate that the Nodal-expressing ES cells at stage 2 include a number of mesendoderm cells, definitive endoderm and mesoderm progenitor cells (Tada et al., 2005). This result is consistent with the FACS results. Recently, it has been shown that CXCR4 is a surface marker that can be used to distinguish early visceral and definitive endoderm in both human and mouse ES cells (D'Amour et al., 2005; Yasunaga et al., 2005). As shown by FACS analysis, we observed an increased level of CXCR4 as well as mesoderm lineage markers such as VEGFR2 (lateral mesoderm progenitor) and PDGFR (paraxial mesoderm progenitor) (Nishikawa et al., 1998) in the Nodal-GFP-expressing ES cells at stage 2 (Fig. 2H). Consistent with the findings of gene expression and the FACS analysis, we also observed a higher expression of Foxa2, CXCR4 and VEGFR2 in Nodal-expressing ES cells at stage 2 by immunohistochemistry (see Fig. S2 in supplementary material). By contrast, the Nodal expression repressed differentiation into anterior visceral endoderm (AVE) and neuroectoderm (Fig. 2E) at the transcription level. It is possible that cerberus (Cer1) and Lefty1 expressed in the AVE could define the extent of the primitive streak in the epiblast by limiting the activity of Nodal during gastrulation. The loss of Lefty function leads to enhanced Nodal signaling during mesendoderm induction (Chen and Schier, 2002; Feldman et al., 2002). It is currently unknown whether AVE forms in response to high or low levels of Nodal signaling (Rodriguez et al., 2005). The failure to detect AVE marker expression could be due to the presence of excessively high Nodal levels. Morphological neuron-like cells with neurite outgrowth were not observed in the stage 2 Nodal-expressing cells. We also examined β-tubulin III (pan-neural marker) expression by immunohistochemistry and detected fewer positive cells in the Nodal-expressing ES cells (Fig. 2F,G), thus suggesting that Nodal signaling represses the neuroectoderm default pathway of ES cells. These results are consistent with the previous report that Nodal inhibits the differentiation of human embryonic stem cells along the neuroectoderm default pathway (Vallier et al., 2004). Moreover, another report, which demonstrated that the absence of Nodal signaling promotes precocious neural differentiation in the mouse embryo, also further supports our results (Camus et al., 2006).
To determine whether the present protocol has any advantages over previous protocols (such as activin A or recombinant Nodal) regarding definitive endoderm and mesoderm differentiation, we examined quantitative single cell analysis by FACS using CXCR4, VEGFR2 and PDGFR-, and compared our results with other available results. ES cells treated with 100 ng/ml activin A differentiate into a larger population of progenitor cells than wild-type ES cells. However, the population of these marker-positive cells in Nodal-on ES cells is significantly higher than that in activin A- or recombinant Nodal-treated ES cells (Fig. 2I). We demonstrated that the present protocol for definitive endoderm and mesoderm differentiation is superior to other available protocols in terms of initial induction of definitive endoderm and mesoderm progenitors from ES cells. Moreover, Cripto neutralizing antibody partially blocked this effect, thus suggesting that the observed effect of Nodal is due to the activation of the Nodal signaling pathway (Fig. 2I).
Previous reports have shown that Nodal ligands can inhibit bone morphogenetic protein (BMP) signaling as well as activating the Smad2 pathway by a Cripto-independent mechanism. This occurs through heterodimerization of Nodal and BMPs at the level of dimeric ligand production (Yeo and Whitman, 2001). Moreover, the formation of endoderm progenitors has been shown to be negatively regulated by BMPs (Poulain et al., 2006). We also attempted to determine whether there were inhibitory effects of BMP signaling in stage 2 Nodal-on ES cells. However, we detected a high level of phosphorylation of Smad1, Smad5 and Smad8 in stage 2 Nodal-on ES cells (Fig. 2A). Surprisingly, BMP ligands including BMP2, BMP 4, BMP 6 and BMP 7 were also upregulated in Nodal-on ES cells (Fig. S1 in supplementary material), thus suggesting that BMP signaling is not suppressed. However, we still do not have sufficient evidence and further molecular studies are thus required to determine whether there are phenotypic consequences of increased BMP signaling.
Mesoderm develops in close association with definitive endoderm both in vivo and in vitro (Kubo et al., 2004; Schuldiner et al., 2000). E-cadherin and Ep-CAM are cell surface proteins that are expressed in definitive endoderm but not in mesoderm. Flow cytometry demonstrated that definitive endoderm was clearly enriched in the E-cadherin-positive population and mesoderm in the E-cadherin-negative population (data not shown).
Maturation of the definitive endoderm by the reduction of Nodal signaling
Nodal-expressing ES cells differentiate into definitive endoderm and mesoderm at stage 2. However, we could not detect any gene products by immunohistochemical studies (data not shown). Nodal expression is first detected during implantation (E4.5) in the inner cell mass of the blastocyst (Mesnard et al., 2006) and thereafter it persists until the extended primitive streak stage (E7.5) (Collignon et al., 1996; Meno et al., 2001), which is the point at which the definitive endoderm has formed. The Nodal expression diminishes during gut tube formation. We therefore hypothesized that it may be necessary for the Nodal expression to be reduced to achieve the maturation of definitive endoderm and mesoderm derivatives. We examined the reduction of Nodal signaling with Tet-on and the formation of a three-dimensional (3D) structure in cell aggregates that enhances cell-cell interactions, which may be important for maturation in tissue culture (stage 3) (Fig. 3A). Notably, in stage 3a (stage 2 Nodal-on-stage 3 Nodal-off) cells, the expression of Titf1 and SftpC (foregut markers), albumin, tyrosine aminotransferase (Tat), Pdx1, glucagon, and amylase (fore and midgut markers) and the gene encoding IFABP and villin (hindgut markers) transcripts were drastically upregulated compared to stage 2 and stage 3b (stage 2 Nodal-on-stage 3 Nodal-on) cells (Fig. 3B). This finding suggested that the stage 3a cells achieved maturation of definitive endoderm. These data are consistent with the expression of gene products, including Foxa2, E-cadherin, pan-cytokeratin, TTF1, pro surfactant protein C (proSftpc), albumin and Pdx1, by immunohistochemistry (Fig. 3C). Mesoderm markers could not be detected in these cell aggregates, indicating that mature mesoderm derivatives might be excluded during the formation of a 3D structure.
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; Robertson et al., 2003). Nodal may provide anti-differentiation signals, due to the fact that it is essential to maintain Oct4 expression during gastrulation (Brennan et al., 2001). To determine how sustained Nodal expression inhibits definitive endoderm maturation, we examined the expression of pluripotent markers [Oct4 and the stage-specific mouse embryonic antigen (SSEA1)] in stage 2 and 3 ES cells. RT-PCR (Fig. 4A) and immunohistochemistry (Fig. 4B) demonstrated that Oct4 and SSEA1 expression in stage 3a cells decreased, in comparison with stage 2 or stage 3b cells. Pluripotent marker-positive cells were located in the inner layer of cell aggregates. This finding is consistent with the data obtained by FACS. The populations of CXCR4+, VEGFR2+ or PDGFR-+ progenitors were significantly reduced in stage 3a cells (Fig. 4C). In the embryo, the early expression of CXCR4 in definitive endoderm development is downregulated during subsequent differentiation (McGrath et al., 1999). As described above, mature mesoderm derivatives might be excluded during the formation of a 3D structure at stage 3a and 3b owing to the lack of adhesion molecules, including E-cadherin and Ep-CAM. We thus assume that this gave rise to the variability across FACS experiments (Fig. 2H, Fig. 4C), and that the relative amounts of midgut and hindgut markers during stage 3b increased in the mixture with pluripotent cells, in comparison to those during stage 2. Our observations showed that the downregulation of Nodal signaling led to the maturation of definitive endoderm, and sustained Nodal expression resulted in the maintenance of the pluripotent cell population.
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Nodal-expressing ES cells were tightly regulated and differentiated into definitive endoderm and mesoderm in vivo
If ES cells are used for regenerative medicine, one possible scenario is the engraftment of differentiated cells derived from ES cells. However, mixing differentiated cells with undifferentiated cells gives rise to teratoma. In addition, it has been shown that ES derivatives such as insulin-producing cells easily become undifferentiated and acquired multipotent undifferentiated potential in vivo (Fujikawa et al., 2005). Another possibility is to coax the differentiation of ES cells with a tightly regulated system in vivo. To examine whether or not our system for the differentiation of definitive endoderm and mesoderm also functions in vivo, we performed a series of grafting experiments and regulated Nodal expression in immunocompromised recipient mice. An analysis of the grafts on kidneys 3 weeks after transplantation clearly revealed that the tumor size of Nodal-expressing ES derivatives (Nodal on/off grafts) was significantly less than that of wild-type ES derivatives or Nodal on/on grafts (2.8±0.5 g versus 4.5±1.5 g or 4.0±1.0 g, respectively; Fig. 5A,B). This finding indicated that in Nodal-expressing ES derivatives, the downregulation of Nodal expression reduced the pluripotency in vivo. This is consistent with the results of gene product assessment in the graft (Fig. 5C). A histological analysis revealed that Nodal-expressing ES derivatives differentiated into mature definitive endoderm in vivo. Compared with wild-type ES transplants or the Nodal on/on group, protein expression of definitive endoderm markers, including FOXA2, proSFTPC, albumin, PDX1, glucagon and IFABP, was enhanced in the Nodal on/off group (Fig. 5E). Notably, FOXA2, PDX1 and IFABP gene products were detected in a tubular structure, that appears to be an early gut tube (Fig. 5E). These results were consistent with the gene expression in the graft tissue (Fig. 5D). The expression of Nodal and its transcriptional targets was significantly diminished by doxycyclin treatment and mature definitive endoderm and mesoderm markers were increased in vivo as well as in vitro. We therefore demonstrated that Nodal signaling was tightly regulated and our protocol resulted in efficient differentiation into definitive endoderm and mesoderm in vivo. As shown by our RT-PCR data, differentiation into neuroectoderm is a default pathway in the wild-type ES cell-derived graft. Moreover, the expression of genes of the Nodal signaling pathway and downstream transcriptional targets in the wild-type ES cell-derived graft is different from that in the Nodal on/off graft. Although we were unable to observe that the course of Nodal expression in the wild-type ES cell-derived graft is the same as that in the actual embryo, we assume that the phenotype of the wild-type ES cell-derived graft is similar to that of Nodal on/on graft rather than Nodal on/off graft in several ways, including teratoma formation and the expression of a pluripotent marker.
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Nodal-expressing ES cells promote differentiation of definitive endoderm and mesoderm from genetically unmanipulated ES cells
In our present study, recombinant Nodal protein at a concentration of 100 ng/ml could induce Nodal signaling activation (Fig. 2A), but not definitive endoderm and mesoderm progenitor cells, whereas Nodal-expressing ES cells could produce both of these results (Fig. 2I).
We also tested whether Nodal and/or Nodal-related molecules from Nodal-expressing ES cells could promote the differentiation of definitive endoderm and mesoderm from genetically unmanipulated ES cells. To simplify the effect of Nodal signaling, we used feeder-free wild-type ES cells in monolayer culture (Fig. 6A). Co-culture with Nodal-expressing ES cells induced a high level of Smad2 phosphorylation and downstream transcriptional targets of Nodal signaling in genetically unmanipulated ES cells. The conditional media (CM) of Nodal-expressing ES cells also induced similar results. Although we were unable to purify Nodal protein or Nodal-related molecules from the conditional media, Cripto neutralizing antibody partially blocked this induction, indicating that the obtained effects are mainly due to the direct effect of Nodal pathway activation (Fig. 6B,C,E). Consequently, the expressions of definitive endoderm markers, including Titf1, albumin, Pdx1 and IFABP (Fig. 6D), and mesoderm markers (data not shown) increased. Furthermore, quantitative single cell analysis by FACS showed that CXCR4 expression was significantly increased in the cells co-cultured with either Nodal-expressing ES cells or CM (Fig. 6E). The population of definitive endoderm progenitor cells increased to a similar extent as that in Nodal-expressing ES cells. This result indicated that our ES cell line is more likely to be a highly active source for the differentiation and maturation of definitive endoderm and mesoderm than recombinant protein. In addition, gene products such as Foxa2 and TTF1 were induced (Fig. 6F).
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We also employed Pdx1-GFP knock-in ES cells and found that there was a higher increase in Pdx1-GFP expression in the cells co-cultured with Nodal-expressing ES cells than in Pdx1-GFP ES cells cultured alone (Fig. 6G). The frequency of GFP+ cells in our culture was 5.4%, which is significantly higher than that of a previous report of a culture treated with retinoic acid (Micallef et al., 2005). Although we could not induce any further maturation of endoderm cells, our ES cell line could potentially be used to enable human ES cells to induce mature definitive endoderm and mesoderm for tissue replacement therapy.
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There is evidence that the high level of Nodal protein, Smad2 phosphorylation, and gene expression of downstream transcriptional targets (including Smad2 and Smad3, Foxh1, Gsc and Foxa2) are tightly controlled in Nodal-expressing ES cells. This regulation allowed us to examine the induction of definitive endoderm and mesoderm and their differentiation and found that Nodal expression induced the expression of more mature definitive endoderm genes, including SftpC, albumin, Pdx1, IFABP, villin and mesoderm genes, including Gata1 and VEGFR2 in stage-2 cells. This finding is surprising because upregulated Nodal signaling alone induced the specification of definitive endoderm and mesoderm from ES cells. Although it may be possible that an alternative signaling pathway including BMP, Wnt and FGF is also upregulated in our cells, we have no definite explanation for this finding. Pedersen et al. reported that the constitutive over-expression of Nodal promotes the differentiation of mouse ES cells into mesoderm and endoderm, but they could not induce the expression of mature definitive endoderm (Pfendler et al., 2005). However, using constitutively Nodal-expressing human ES cell lines, Pedersen et al. also demonstrated that (1) Nodal-expressing embryoid bodies display markers of pluripotency and extraembryonic endoderm, and (2) that they induce anterior visceral endoderm (AVE)-like differentiation and inhibit mesoderm and endoderm differentiation (Vallier et al., 2004). Although the difference of definitive endoderm differentiation between mouse and human ES cells regarding the response to Nodal signaling is still paradoxical, it is clear that Nodal expression represses the default pathway to the neuroectoderm.
Alternative protocols using activin A or recombinant Nodal are reported to induce definitive endoderm and mesoderm differentiation from ES cells (D'Amour et al., 2005; Kubo et al., 2004; Tada et al., 2005; Yasunaga et al., 2005). For quantitative single cell analysis, we next employed flow cytometry to monitor the definitive endoderm and mesoderm progenitor cell proportions. CXCR4, a chemokine receptor, has been recently identified as a cell surface marker to isolate definitive endoderm from ES cell derivatives (D'Amour et al., 2005; Yasunaga et al., 2005). We employed this marker as a definitive endoderm progenitor, VEGFR2 as a lateral mesoderm progenitor, and PDGFR as a paraxial mesoderm progenitor. The proportions of CXCR4+, VEGFR2+ or PDGFR-+ cells are significantly higher in Nodal-on ES cells than in other conditions. Although our system differs significantly from those used in previous studies in that the initial stages of differentiation are carried out in monolayer culture, there is evidence that the present protocol is quantitatively superior to other protocols for the differentiation of definitive endoderm and mesoderm progenitors from ES cells.
We also described that the upregulation and subsequent downregulation of Nodal signaling resulted in the maturation of definitive endoderm and mesoderm both in vitro and in vivo. We could detect expression of definitive endoderm genes but could not detect any gene product during stage 2, suggesting that the gene expression level is still very low. The Nodal expression persists until the extended primitive streak stage (E7.5) (Collignon et al., 1996; Meno et al., 2001), and disappears before the maturation of the gut tube. It must be noted that there are considerable differences in the interpretations of frog and fish experiments regarding whether endoderm formation is promoted by a sustained high level Nodal signaling, or whether Nodal signaling needs to be downregulated for endoderm differentiation. In mouse models, the evidence for concentration-dependent effects is less direct because misexpression experiments have not been performed (Schier, 2003). We expect that the Nodal expression must be reduced in order to achieve the maturation of the definitive endoderm and mesoderm. The downregulation of Nodal signaling by Tet-on significantly promoted the maturation of definitive endoderm as shown by the high expression of fore-, mid- and hindgut genes and gene products. However, mesoderm derivatives were reduced during 3D structural formation. One possible explanation is that some cell surface molecules such as E-cadherin or Ep-CAM that are specific for definitive endoderm may play a pivotal role in forming the 3D structure. CXCR4 expression is frequently associated with less differentiated cell types. In the embryo, the early expression of CXCR4 in lung and thyroid development is downregulated during subsequent differentiation (McGrath et al., 1999). This observation is also consistent with our results obtained by FACS. The limited number of differentiated definitive endoderm cells with upregulated Nodal signaling were CXCR4+ and mature definitive endoderm with downregulated Nodal signaling were CXCR4–. We, therefore, assume that CXCR4 is also an available lineage marker to monitor the differentiation and maturation of definitive endoderm from ES cells. To our knowledge, few differentiation systems have the tightly controlled expression of Nodal/activin signaling and achieve mature definitive endoderm in vitro.
Our differentiation system also functions in vivo. The Nodal on/off graft showed more mature phenotypes and definitive endoderm and mesoderm derivatives compared with the Nodal on/on graft or the wild-type graft, thus indicating that the upregulation and subsequent downregulation of Nodal signaling, is both functional and possible in vivo. Protocols to differentiate mouse or human ES cells into definitive endoderm and mesoderm derivatives in vivo using activin A have been recently described (D'Amour et al., 2005; Kubo et al., 2004). Although they are different from our culture condition, it is possible that definitive endoderm derivatives can be induced by spontaneously diminished activin signaling after engraftment. To clarify this mechanism, an alternative loss-of-function study through the use of RNAi that can be delivered to ES cells with lentiviruses should be conducted. Moreover, the administration of activin A to coax ES cells to differentiate into definitive endoderm in vivo is not realistic. We therefore offer a realistic and efficient protocol to differentiate mouse ES cells into mature definitive endoderm and mesoderm using tightly regulated ES cells, especially in vivo. At present, we cannot induce fully mature and post-mitotic cells such as insulin-producing pancreatic β-cells. We speculate that some other combination of growth factors regulating the anterior-posterior axis or the ventral-dorsal axis is required for further functional maturation. Recently, Kubo et al. demonstrated that definitive endoderm cells could be generated in brachyury-GFP-expressing ES cells in vitro and in vivo (Kubo et al., 2004). Although they could generate hepatocyte-like cells in vitro, this lineage did not persist or expand in the grafts. Moreover, ES derivatives such as insulin-producing cells have been shown to become easily undifferentiated, while also acquiring a multipotent potential in vivo (Fujikawa et al., 2005). Further investigations are needed to enable the ES derivatives of interest to persist and function in vivo.
We questioned how sustained Nodal expression inhibits the maturation of definitive endoderm. Nodal null mutant mice display a very low level of Oct4 expression, a pluripotent marker (Brennan et al., 2001; Robertson et al., 2003). We found that the expression of the pluripotent markers, Oct4 and SSEA1 persists in sustained Nodal-expressing cells or grafts. Nodal/TGFβ ligands promote the maintenance of pluripotency in human ES cells (Amit et al., 2004; Beattie et al., 2005; Vallier et al., 2004). As described above, ES derivatives easily became undifferentiated and acquired pluripotent potential as shown by Oct4 and SSEA1 expression in vivo (Fujikawa et al., 2005). These findings, along with our results concerning teratoma formation, suggest that this is a possible mechanism for the inhibition of definitive endoderm maturation.
Our results also demonstrated that Nodal signaling molecules from Nodal-expressing ES cells allow genetically unmanipulated ES cells to differentiate into mature definitive endoderm and mesoderm, thus indicating that our cell line could be a source of highly active Nodal or Nodal-related protein. Extracellular EGF-CFC proteins, including Cripto and Criptic in mouse and human, chick CFC, frog FRL-1 and zebrafish one-eyed pinhead (Gritsman et al., 1999) are important components of the Nodal signaling pathway. It is possible that they act as coreceptors for Nodal ligands (Shen and Schier, 2000). Because ES cells are known to express Cripto and Criptic (Minchiotti, 2005), we presumed that there was a possibility of inducing definitive endoderm and mesoderm from these cells. Nodal protein has concentration-dependent roles in cell fate in the embryo (Chen and Schier, 2002; Feldman et al., 2002; Meno et al., 1999; Norris et al., 2002; Vincent et al., 2003), and high levels of Nodal-related signaling are required for endoderm or mesoderm development (Schier et al., 1997). Even high doses (100 ng/ml) of exogenous recombinant Nodal protein could not induce any mature definitive endoderm and mesoderm markers in our study. Our observed levels of Smad2 phosphorylation indicated that Nodal or Nodal-related molecules secreted from Nodal-expressing ES cells are more functionally active than recombinant Nodal. It is likely that the concentration, timing and duration of administration of Nodal protein to ES cells might be important factors contributing to differentiation (Gurdon and Bourillot, 2001). We must not forget other reported Nodal-expressing cell lines are also sources of highly active protein (Beck et al., 2002; Yan et al., 2002). Although we agree that human ES cells are dramatically different from mouse ES cells, these Nodal-expressing cell lines could serve as the basis for developing endoderm and mesoderm tissues from human ES cells.
In conclusion, our protocol enables the differentiation and maturation of definitive endoderm and mesoderm derivatives from ES cells and has several advantages over other available protocols. Further research involving ES cell development and the mechanisms governing the differentiation of definitive endoderm and mesoderm cells may provide a basis for producing functional, transplantable tissues.
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). EB5 cells, which were derived from E14tg2a ES cells (a gift from H. Niwa), were maintained on a 0.1% gelatin-coated dish without feeders in GMEM supplemented with 1% FCS, 10% Knockout Serum Replacement (Gibco-BRL), 0.1 mM 2ME, 1 mM sodium pyruvate, 2 mM L-glutamine, 0.1 mM NEAA, 20 µg/ml blasticidin S hydrochloride (Funakoshi), and 1000 IU/ml LIF. Pdx1-GFP knock-in ES cells (a gift from E. Stanley, Monash Immunology and Stem Cell Laboratories, Monash University, Australia) were maintained with mouse embryonic fibroblasts in high glucose DMEM (Gibco-BRL), 0.1 mM 2ME, 2 mM L-glutamine, 0.1 mM NEAA, 1x nucleoside (Chemicon) and 1000 IU/ml LIF (Micallef et al., 2005).
Establishment of ES cell lines with conditional Nodal expression
The tetracycline (Tet) regulatory system was used to induce Nodal expression in ES cell clones as described previously (Era and Witte, 2000). The E14tg2a ES cells were stably transfected with the tetracycline transactivator (tTA). A Tet-regulatable Nodal construct was generated by inserting the mouse Nodal cDNA into the EcoRI site of pUHD 10-3 IRES GFP (a gift from T. Era, Center for Developmental Biology, RIKEN, Kobe, Japan). Tet-inducible Nodal-expressing cell lines were established by stable transfection of pUHD 10-3 Nodal IRES GFP into the parental cells. Regulated Nodal-expressing ES cell clones (Tet-off system) were maintained in the presence of 200 µg/ml G418, 1000 IU/ml LIF and 1 µg/ml tetracycline. In the preliminary studies, we transfected an empty vector to the E14tg2a ES cells. We then treated the cells with tetracycline and demonstrated that the reactions we observed were specific.
In vitro differentiation of definitive endoderm and mesoderm
Before the induction of differentiation, the established ES cells were cultured in the presence or absence of Tet for 3 days as undifferentiated ES cells (stage 1). The cells were dissociated with 0.25% trypsin and 0.04% EDTA in PBS. 1000 cells were plated onto 6-well culture dishes coated by type IV collagen (Biocoat; Becton Dickinson) in -minimum essential medium (-MEM: Gibco-BRL) containing 10% FCS, 0.1 mM 2ME and 200 µg/ml G418 in the presence or absence of Tet, and cultured for 10 days (stage 2; see Fig. 1B). We did not feed the ES cells during stage 2. To determine Nodal pathway activation, we added 10 µM SB431542, the ALK4/5/7 inhibitor (Tocris), or 1.0 µg/ml anti-mouse Cripto antibody (R&D Systems) during stage 2. To achieve more maturation of definitive endoderm derivatives, we examined the reduction of Nodal signaling with Tet-on and the formation of cell aggregates in the new MEM medium containing 10% FCS and 0.1 mM 2ME, and cultured ES cells for an additional 4 days (stage 3; see Fig. 3A). We did not feed the ES cells during stage 3.
In vivo differentiation of definitive endoderm and mesoderm
All animal studies were performed in accordance with Kobe University Animal Care and Use Guidelines. To form the embryoid body, 6x105 Nodal-expressing ES cells were cultured for 4 days in Costar ultra-low attachment dishes (Corning) with LIF and G418 in the presence of Tet. 6x105 E14tg2a cells were used as a control. Under general anesthesia, 5-week-old male nude mice (Crea, Japan) were engrafted with EBs (derived from E14tg2a or Nodal-expressing ES cells) in the left subcapsular renal space. One week after transplantation in the state of Tet-off/Nodal-on, drinking water with 1 mg/ml doxycycline (Sigma) (Nodal on/off group) or without doxycycline (Nodal on/on group) was administered to recipients for an additional 2 weeks. The recipients (n=3) were sacrificed, and the grafts were removed, weighted, fixed in 4% paraformaldehyde, and embedded in OCT compound. They were then sectioned to generate 10-µm-thick tissue sections for immunohistochemistry. Other grafts were homogenized in TRIzol reagent and RT-PCR was performed as described below. All data are given as the mean (from the indicated number of samples) ± standard error of the mean. Two-tailed t-tests were conducted to determine statistical significance.
Differentiation of definitive endoderm and mesoderm from genetically unmanipulated ES cells
Co-culture was performed with a cell culture insert (pore size: 0.4 µm; Falcon) in 12-well plates. 1x104 Nodal-expressing ES cells were plated on an upper insert, and 5x103 genetically unmanipulated ES cells (EB5) were plated on a gelatin-coated lower chamber and cultured for 7 days in -MEM supplemented with 10% FCS, 0.1 mM 2ME, penicillin-streptomycin (see Fig. 6A). We did not feed ES cells for 7 days. We also examined the conditional media (CM) of Nodal-expressing ES cells and replaced half of the medium with CM every other day for 7 days. To determine Nodal pathway activation, we added 10 µM SB431542, the ALK4/5/7 inhibitor, or 1.0 µg/ml anti-mouse Cripto antibody to CM.
Flow cytometry
The cultured cells were harvested with cell dissociation buffer (Gibco-BRL) and analyzed. The dissociated 1x105 cells were incubated with each type of antibody at 4°C for 20 minutes. Nonspecific antibody binding was blocked using purified rat anti-mouse CD16/CD32 monoclonal antibodies (Becton Dickinson) for 15 minutes. The cells were stained with biotin-conjugated anti-CXCR4 monoclonal antibody (mAb), anti-VEGFR2 mAb (Becton Dickinson), or anti-PDGFR mAb (eBioscience, CA). The stained cells were resuspended in Hank's balanced salt solution (Gibco-BRL) containing 1% bovine serum albumin and 1 µg/ml propidium iodide (Sigma) to exclude dead cells. The cells were sorted and analyzed by FACS Aria and Calibur (n=5; Becton Dickinson Immunocytometry Systems).
Western blot analysis
A western blot analysis was performed with mouse monoclonal antibodies. A total of 1x106 cells were resuspended in 100 µl of boiling lysis buffer containing 25 mM Tris (pH 7.5), 100 mM NaCl, 2 mM EDTA, 0.5% Triton X-100, 20% glycerol, 2% SDS, 5% 2ME, 0.1% Bromophenol Blue, and protease inhibitor cocktail (Nacalai, Japan). Phosphatase inhibitor cocktail (Nacalai, Japan) was used for phospho-Smad2 blotting. Each sample (20 µl) was separated on a 12.5% SDS Tris-glycine gel. Proteins were transferred onto a polyvinylidene difluoride membrane and visualized with an enhanced chemiluminescence detection kit (Amersham Pharmacia). The membranes were incubated with Nodal polyclonal antibodies (R&D systems; 1:500), phospho-Smad2 antibodies, or phospho-Smad1/5/8 antibodies (Cell Signaling Technology; 1:200) at 4°C overnight. Either goat anti-mouse IgG or anti-rabbit IgG conjugated to HRP (Bio-Rad) was used as a secondary antibody.
RT-PCR analysis
Total RNA was prepared using the TRIzol reagent (Invitrogen) and RQ1 RNase-free DNase (Promega). For cDNA synthesis, oligo(dT) primers were used to prime the reverse transcription reactions, and the synthesis was carried out using Superscript II (Invitrogen). PCR was performed with Taq polymerase (Promega). The primer sequences are listed in supplementary material Table S1. The primer sequences for the TGFβ superfamily were synthesized as described previously (Oxburgh et al., 2004).
Real-time RT-PCR
mRNA expression was quantified by real-time RT-PCR using a Lightcycler 3 (Roche). One-step RT-PCR reactions were performed in capillary tubes using a Lightcycler RNA amplification kit SYBR Green I. 200 ng total RNA, 0.1 µmol/l of each of the sense and antisense primers, and SYBR Green I mixtures in a total volume of 20 µl were used in each capillary. The RT reaction consisted of 45 cycles of 55°C for 20 minutes, then heating to 95°C for 10 seconds.
Immunohistochemical analysis
Cultured cells were washed three times with PBS, fixed in methanol at –20°C for 10 minutes, and washed three times with PBS. Nonspecific antibody binding was blocked using PBS with 1% Triton X-100 (PBST) and 10% FCS for 30 minutes. The following dilutions of primary antibodies were used: rabbit anti-albumin 1:5000 (Biogenesis, Poole, UK), goat anti-Foxa2 1:100 (Santa Cruz), rabbit anti-IFABP 1:1000 (Green et al., 1992), mouse anti-β-tubulin III 1:1000 (Sigma), rabbit anti-Pdx-1 1:100 (TransGenic, Kobe, Japan), rabbit anti-TTF1 1:1000 (Biopat, Caserta, Italy), rabbit anti-prosurfactant C (proSftpC) 1:1000 (Chemicon), mouse anti-pancytokeratin (panCK) 1:100 (Sigma), mouse anti-E-cadherin 1:100 (Takara, Otsu, Japan), mouse anti-SSEA-1 1:50 (R&D Systems), mouse anti-Oct3 and/or 4 1:50 (Santa Cruz), rabbit anti-glucagon 1:100 (Dako), rat anti-CXCR4 1:100 (BD Bioscience), rat anti-VEGFR2 (Flk1) 1:100 (BD Bioscience), rabbit anti-VE-cadherin 1:100 (Santa Cruz), mouse anti- smooth muscle actin 1:500 (Sigma). The fixed cells were incubated with the primary antibodies at 4°C overnight. The cells were then washed three times with PBST, and incubated with the appropriate secondary antibodies labeled with Alexa Fluor 555- or Alexa Fluor 488-conjugated goat anti-rabbit or anti-mouse IgG or donkey anti-goat IgG 1:200 (Molecular Probes) for 1 hour at room temperature. The cells were washed with PBST and subsequently observed by confocal immunofluorescence microscopy (Zeiss LSM510).
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