Stem Cell Rev and Rep DOI 10.1007/s12015-014-9509-0

The Generation of Definitive Endoderm from Human Embryonic Stem Cells is Initially Independent from Activin A but Requires Canonical Wnt-Signaling Ortwin Naujok & Ulf Diekmann & Sigurd Lenzen

# Springer Science+Business Media New York 2014

Abstract The activation of the TGF-beta pathway by activin A directs ES cells into the definitive endoderm germ layer. However, there is evidence that activin A/TGF-beta is not solely responsible for differentiation into definitive endoderm. GSK3beta inhibition has recently been shown to generate definitive endoderm-like cells from human ES cells via activation of the canonical Wnt-pathway. The GSK3beta inhibitor CHIR-99021 has been reported to generate mesoderm from human iPS cells. Thus, the specific role of the GSK3beta inhibitor CHIR-99021 was analyzed during the differentiation of human ES cells and compared against a classic endoderm differentiation protocol. At high concentrations of CHIR99021, the cells were directed towards mesodermal cell fates, while low concentrations permitted mesodermal and endodermal differentiation. Finally, the analyses revealed that GSK3beta inhibition rapidly directed human ES cells into a primitive streak-like cell type independently from the TGFbeta pathway with mesoderm and endoderm differentiation potential. Addition of low activin A concentrations effectively differentiated these primitive streak-like cells into definitive endoderm. Thus, the in vitro differentiation of human ES cells into definitive endoderm is initially independent from the activin A/TGF-beta pathway but requires high canonical Wnt-signaling activity.

This work has been supported by the German Research Foundation within the framework of the Cluster of Excellence REBIRTH. Electronic supplementary material The online version of this article (doi:10.1007/s12015-014-9509-0) contains supplementary material, which is available to authorized users. O. Naujok (*) : U. Diekmann : S. Lenzen Institute of Clinical Biochemistry, Hannover Medical School, Carl-Neuberg-Str. 1, 30625 Hannover, Lower Saxony, Germany e-mail: [email protected]

Keywords Human ESCs . Definitive endoderm . GSK3beta inhibition . Wnt/ß-catenin pathway . Primitive streak

Introduction The generation of surrogate cells for treatment of liver, lung and pancreas diseases such as diabetes mellitus is of great interest [1]. These organs are derived from the endoderm germ layer. Thus, the definitive endoderm (DE) represents an important intermediate stage during in vitro differentiation towards the aforementioned tissues [2]. It is generally accepted that treatment of human and mouse embryonic stem (ES) cells with high concentrations of activin A, by activation of the TGF-beta pathway, triggers the DE differentiation [3, 4]. However, activin A/TGF-beta might not be solely responsible for the lineage specification of ES cells into the DE. In vivo the DE formation requires at least four distinct events. The nidation of the blastula in the endometrium causes a change from the embryoblast to the epiblast stage. Further migration of epiblast cells towards the embryonic disk’s median line results in the formation of the primitive streak (PS) and ventral migration of primitive streak cells causes the formation of the short-lived mesendoderm predecessor [5]. Finally, the separation of the mesendoderm into the mesoderm and definitive endoderm germ layer takes place [6]. Human ES cells are considered to be a derivative of the epiblast stage [7]; consequently the directed differentiation into epiblast cells is not necessarily required. It is unlikely that the well concerted events directing ES cells into the DE are controlled by one pathway alone so that many research groups combined additional growth factors or small molecules with activin A, e.g. wnt3a [8–10], PI3K-inhibitors [11], HDAC inhibitors [12] or staurosporine family members [13]. The canonical Wnt-pathway controls biological processes during embryogenesis and tumor formation [14]. During

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embryonic development, high Wnt-activity can be detected in the PS of mouse embryos [15] and embryos with a homozygous knockout of the signaling factor wnt3 failed to induce PS [16]. However, its role during in vitro culture of mouse ES cells is not well understood because high Wnt-activity may enhance pluripotency [17], generate primitive endoderm-like cells [18] or induce cells of the anterior and posterior primitive streak [19]. In human ES cells the canonical Wnt-pathway is thought to promote differentiation [20] and specifies synergistically with nodal/TGF beta-signaling the anterior PS and subsequently the DE [21]. This dissimilarity could be a result of the naïve and primed state of mouse and human ES cells [22]. Recently, a new approach has been published in which DE was generated from human ES cells by chemical inhibition of the glycogen synthase kinase 3 beta (GSK3beta) [23]. This kinase plays an inhibitory role in the canonical Wnt-pathway by targeting beta-catenin via phosphorylation to trigger the proteasomal degradation [24]. Thus, effective inhibition of GSK3beta leads to an activation of this pathway by accumulation of free beta-catenin, which can translocate to the nucleus and induce Wnt-target genes [25, 26]. Conversely, a recent publication reported robust differentiation of human iPS cells into the mesoderm [26]. Then the cells could be further directed into cardiac progenitors after initial treatment with the GSK3beta inhibitor CHIR-99021 followed by a betacatenin siRNA knockdown [26]. This suggests a dual function of the Wnt/beta-catenin pathway during early cell lineage specification. We analyzed the effect of different CHIR-99021 concentrations on the cell lineage commitment of human ES cells using the Hues8 cell line, which has been reported to by suited for endoderm and pancreatic differentiation [27]. We tested the ability of CHIR-99021 to induce the DE formation alone, sequentially or in combination with activin A. In this study we show that CHIR-99021 directed human ES cells within 24 h into PS-like population and addition of low concentrations of activin A could effectively differentiate them into DE-cells, whereas prolonged treatment yielded mesodermal cell fates. Thus, in contrast to the current concept of the decisive roles of the BMP/TGF-beta pathway, our results strongly suggest a dominant role for the Wnt/beta-catenin pathway on the early cell lineage specification.

Materials and Methods Materials RPMI1640 advanced, RPMI1640, B27 and glutamax were obtained from Life Technologies (Darmstadt, Germany). Wnt3a and KGF was from Reliatech (Wolfenbüttel, Germany) and CHIR-99021 from Biozol (Eching, Germany). SB-

431542 and dorsomorphin was obtained from Tocris Bioscience (Bristol, United Kingdom). Activin A and FGF-10 was from Peprotech (Hamburg, Germany) and BMP4 from Life Technologies. All primers were synthesized by Life Technologies. The RevertAid™ H-Minus M-MuLV reverse transcriptase was purchased from Thermo Fisher Scientific (Braunschweig, Germany). The GoTaq® Taq polymerase was from Promega (Mannheim, Germany) and dNTPs were purchased from Genecraft (Münster, Germany). TaqMan assays, TaqMan qPCR-reagents and TaqMan® array cards were from Life Technologies. If not mentioned otherwise, chemicals were obtained from Sigma-Aldrich (Taufkirchen, Germany) or Merck (Darmstadt, Germany). ES Cell Culture Routine feeder-free culture of the ES cell lines Hues8 and Hues4 was performed with minor modifications as described earlier [12]. Briefly, ES cells were cultured on matrigel-coated 6-well plates (BD Biosciences, Heidelberg, Germany) in mTeSR™1 medium (Stemcell Technologies, Köln, Germany). The cells were grown in colonies and passaged once a week. For passaging, the cells were enzymatically treated with 1 mg/ml collagenase (Stemcell Technologies), colonies were detached by scrapping, and re-seeded in a ratio of 1:4 on freshly matrigel-coated 6-well plates. Differentiation Experiments Differentiation was started with ES cell colonies approximately 1–2 days prior to passaging. All differentiation protocols were performed with the Hues8 cell line [27] unless otherwise stated in the legend to the figure. Different protocols were used to generate cells of the definitive endoderm germ layer and the primitive gut tube: classical induction of DE and primitive gut tube was conducted as described earlier [9] using a sequential approach with 25 ng/ml wnt3a and 100 ng/ml activin A for one day, followed by a 2 day treatment with 100 ng/ml activin A, and finally 3 day treatment with 50 ng/ml KGF in RPMI advanced. The basal medium for other differentiation experiments was RPMI advanced supplemented with 0.2 % FCS, penicillin/streptomycin and 1-fold glutamax, whereas randomized differentiation was done with basal medium without growth factors and/or small molecules. Differentiation by GSK3beta inhibition was performed with 1–10 μM CHIR-99021 and differentiation via activin Asignaling was conducted with 100 ng/ml activin A. Combinations of CHIR-99021 and 100 ng/ml activin A and dilution experiments of CHIR-99021 and activin A were carried out in basal medium with concentrations ranging from 0 to 10 μM for CHIR-99021 and 0 to 100 ng/ml for activin A. Pancreatic differentiation was carried out in DMEM plus 1-fold glutamax, 1x B27, 50 ng/ml FGF-10, 2 μM retinoic acid,

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1 μM dorsomorphin and 10 μM SB-431542 after endoderm induction with Chir+ActA [11, 28]. Gene Expression Analyses Total RNA was isolated from the cells using the RNeasy Kit (Qiagen, Hilden, Germany). The cells were lysed in Qiazol (Qiagen), the hydrophilic phase was loaded onto RNA spin columns, and RNA was then prepared as instructed. The cDNA synthesis was performed with random hexamers to prime the reaction of the Reverse Transcriptase. Array cards were loaded in each lane with 500 ng cDNA and the TaqMan® gene expression master mix according to the manufacturer’s instructions. Reactions were performed on a ViiA7 real-time PCR system (Life Technologies) with standard TaqMan PCR conditions. Each PCR was performed in triplicate using specific primers (Supplementary information, Table S1). Data normalization was performed with qBasePlus (Biogazelle, Zwijnaarde, Belgium) against the geometric mean of the four housekeeping genes G6PD, TBP, TUBA1A, and GAPDH. Standard qRT-PCRs were performed with cDNA prepared as described above. Here, 10–20 ng of cDNA was loaded in each well of a 384-well plate and specific primers (Supplementary information, Table S2) were mixed with the GoTaq® PCR master mix according to the manufacturer’s instructions. Each amplification was performed in triplicate and the gathered data were normalized with qBasePlus against the housekeeping genes G6PD, TBP, TUBA1A. Flow Cytometry Analysis and Cell Sorting For flow cytometry in vitro differentiated cells were washed with PBS, dissociated using trypsin/EDTA, and resuspended in PBS plus 2 % FCS. Surface antigen staining was performed according to standard procedures for 30–45 min on ice in the dark. The following conjugated antibodies were used: antihuman CD49e-FITC (Biolegend, London, UK) and antihuman anti-human CXCR4-PE (Neuromics, Minneapolis, USA). The cells were then washed twice and measured in a flow cytometer (CyFlow ML, Partec, Münster, Germany). For cell sorting 2–5×106 cells were stained with CD49e-FITC/ CXCR4-PE in PBS supplemented with 20 % FCS and 1 mM EDTA. Cell sorting was conducted at the central cell sorting facility of Hannover Medical School. Immunocytochemistry Immunocytochemistry was performed according to standard procedures. Human ES cells were stained on matrigel-coated glass slides (Zellkontakt, Nörten-Hardenberg, Germany). The cells were fixed in 4 % (w/v) paraformaldehyde for 1 h at 4 °C. Subsequently, the cells were blocked for 20 min in PBS plus 0.2 % Triton X-100, 6 % BSA, and 1 mg/ml NaBH4. Primary

and secondary antibodies were diluted in PBS with 0.1 % Triton X-100 and 0.1 % BSA. Primary antibodies were incubated on the slides for 1–3 h at room temperature (RT) or overnight at 4 °C. Secondary antibodies were incubated on the slides for 1 h at RT. The following primary antibodies were used: anti-SOX17 (AF1924, R&D Systems, Minneapolis, USA), anti-FOXA2 (07–633, MerckMillipore, Schwalbach, Germany), anti-OCT3/4 (sc-5,279, Santa Cruz Biotechnology, Heidelberg, Germany), anti-BRACHYURY (sc-20,109, Santa Cruz Biotechnology and AF2085 from R&D systems), Anti active beta-catenin (05–665, MerckMillipore, Billerica, MA, USA) and antiGOOSECOID (AF4086, R&D Systems). Secondary antibodies were obtained from Dianova (Hamburg, Germany) and conjugated with DyLight 488, Cy3 or DyLight 649. Finally, the slides were mounted with immunoselect antifading mounting medium containing DAPI (Dianova). The stained cells were examined with Xcellence RT on an Olympus IX81 inverted microscope (Olympus, Hamburg, Germany). Western Blot Analyses Cellular proteins were extracted using RIPA buffer (SigmaAldrich) and the protein concentration was determined by a BCA assay. 20 μg of total protein was separated on a 12.5 % SDS-PAGE followed by electroblotting onto a PVDF membrane. Then, the membrane was blocked with 5 % non-fat dry milk or 2 % ECL select specific dry milk for 60 min at RT. Blots were incubated with primary antibodies for 1–3 h at RT or overnight at 4 °C. The following antibodies were used: phospho-β-CATENIN Ser33/37/Thr41 (9,561, Cell Signaling Technology, Frankfurt am Main, Germany), β-CATENIN (sc7,199, Santa Cruz Biotechnology), phospho-SMAD2 ser465/ 467 (3,108, Cell Signaling Technology), SMAD2/3 (sc-6,032, Santa Cruz Biotechnology), GSK3beta (sc-9,166, Santa Cruz Biotechnology), phospho-GSK3beta tyr216 (sc-135,653, Santa Cruz Biotechnology), phospho-GSK3 beta ser9 (9,322, Cell Signaling Technology), and GAPDH (sc137,179, Santa Cruz Biotechnology). Western blot membranes were subsequently incubated with peroxidaseconjugated secondary antibodies (Dianova). Protein bands were visualized by chemiluminescence using ECL Plus (Pierce, Bonn, Germany) or the ECL select detection system (GE Healthcare Lifesciences, Freiburg, Germany). Protein loading was quantified by densitometry and normalized against GAPDH using the Gel-Pro Analyzer 6.0 (Media Cybernetics, Silver Spring, MD, USA). Statistics Unless stated otherwise the data values were expressed as mean ± SEM. Statistical analyses were performed using the GraphPad Prism software (Graphpad, San Diego, CA, USA)

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applying Student’s t-test or ANOVA followed by Bonferroni’s or Dunnett’s post hoc test for multiple comparisons.

Results Gene Expression Profile of Wnt3a/Activin A or CHIR-99021 Induced ES Cells To survey the potential of Chir to direct human ES cells into DE and potentially primitive gut tube derivatives via canonical Wnt-signaling, the cells were cultured for 6 days according to a previously published protocol [10], with 10 μM Chir or without any directive cues (random). The control protocol comprised an initial treatment with 25 ng/ml wnt3a plus 100 ng/ml activin A for one day and a further 2 day treatment with 100 ng/ml activin A alone (abbreviated as Wnt3a/ActA). Subsequently the cells were cultured for 3 days with 50 ng/ml KGF (primitive gut tube stage). Samples were taken for each treatment initially (d0), after 3 (DE stage), and 6 days (primitive gut tube stage) of differentiation. qRT-PCR analysis with TaqMan® array cards for pluripotency markers showed a significant decrease of POU5F1 (OCT3/4), NANOG, SOX2 in Chir-treated samples (Fig. 1a). These genes were highly expressed during pluripotency and reduced >60-fold at day 3 and >400-fold at day 6 compared to cells before differentiation. Reduction in random samples and Wnt3a/ActA samples were less pronounced and less significant. Analysis of the PS markers T and MIXL1 showed a significant increase in gene expression in Chir-induced samples, whereas T and MIXL1 were not significantly increased in Wnt3a/ActA-treated samples compared to the randomized differentiation (Fig. 1b). Please note: gene expression at this time point has most likely already passed the expression peak in Wnt3a/ActA-treated samples. In contrast, GSC (goosecoid) expression was prominently expressed in Wnt3a/ActA-treated samples, especially at day 3 from where the expression declined again. GSC expression in Chir-induced samples was elevated, albeit not significantly (Fig. 1b). Genes expressed in DE cells, namely SOX17, FOXA2, and GATA3/4/5, were exclusively and significantly increased in the control protocol using Wnt3a/ActA, whereas Chir-induced samples failed to express relevant levels of DE-markers above the undifferentiated sample (d0) or the randomly differentiated samples. SOX7 expression was 2.6-fold increased in Wnt3a/ActA-treated cells, most likely as a result of minor differentiation towards parietal/visceral endoderm (Fig. 1c). Not surprisingly Chir-treated samples did not express any markers of the primitive gut tube or foregut after further differentiation for 3 days (Supplementary information, Fig. S1a and b). In contrast, gene expression of HNF1B, HNF4A, SHH, FOXA1, HHEX, and HNF6 were prominently expressed in the control protocol after 6 days, with HNF1b and HNF4A being induced >1.000-fold

compared to undifferentiated cells (Supplementary information, Fig. S1a and b) indicating further differentiation into cells of the foregut. CDX family members 1, 2, and 4 were significantly increased in Chir-treated cells with little induction in randomly differentiated and Wnt3a/ActA-treated cells (Supplementary information, Fig. S1c). The ability of CHIR99021 to induce canonical Wnt-signaling was assessed by staining wnt3a and CHIR-99021-treated cells with an antibody directed against the active form of beta-catenin (Supplementary information, Fig. S2a and b). Nuclear translocation of beta-catenin upon wnt3a treatment was 1.3-fold increased and in case of CHIR-99021 1.9-fold increased compared to undifferentiated cells (Supplementary information, Fig. S2c). Concentration-Dependent Effects of CHIR-99021 on Human ES Cells To assess whether the lineage specification of ES cells might be attributed to the stringency of canonical Wnt-signaling, the effect of different Chir concentrations on the gene expression of endodermal and mesodermal genes was measured (Fig. 2). Human ES cells were cultured for three days with 1 to 7.5 μM Chir. Real-time RT-PCR results showed that T and MIXL1 expression positively correlated with the Chir concentration (Fig. 2a). Both genes already peaked at 5 μM Chir. Noteworthy, 5 and 7.5 μM caused an expression peak already after day one, whereas lower concentrations delayed the expression peak to day two. SOX17 transcripts were reliably detected at low Chir concentrations from 1 to 3 μM and were increased at day two or three of the differentiation compared to undifferentiated cells (Fig. 2b). At higher Chir concentrations, SOX17 was missing or as low as in undifferentiated ES cells. GSC and FOXA2 expression showed a comparable pattern and especially GSC was readily detected at all concentrations and at all analyzed time points (Fig. 2b). Low Chir concentrations at 1 or 2 μM caused the expression peak to appear after 2 days, whereas from 3 to 7.5 μM FOXA2 and GSC expression already peaked after day one. Then, both genes were slightly (in case of GSC) or strongly (in case of FOXA2) down regulated. PDGFRA and FLK1 are described as target genes of MIXL1 and were analyzed as representatives of the mesodermal, hematopoietic lineage [29]. FLK1 was low to absent at all analyzed time points but PDGFRA, expressed in lateral plate mesoderm, displayed an expression pattern dependent on the Chir concentration (Fig. 2c). In order to elucidate whether the prominent gene expression of SOX17 and FOXA2 at low Chir concentrations could translate into SOX17/ FOXA2 double-positive DE-cells, immunofluorescence staining for both proteins was performed (Fig. 2d). SOX17/ FOXA2 double-positive cells were readily detected after a three day treatment with 1 μM Chir alone. Higher concentrations reduced the number of SOX17/FOXA2 double-positive cells. At 3 μM Chir SOX17/FOXA2 double-positive cells

Stem Cell Rev and Rep Fig. 1 Gene expression profile of pluripotency, primitive streak, and endoderm markers in wnt3a/ activin A and CHIR-99021 induced human ES cells. ES cells were differentiated either randomly, with a combination of wnt3a and activin A (Wnt3a/ ActA) for 3 days and followed by a 3 day treatment with KGF or with 10 μM CHIR-99021 (Chir) for 6 days. As a comparison, the relative gene expression of undifferentiated human ES cells is depicted (d0). Gene expression of marker genes was measured with TaqMan® array cards and calibrated normalized relative quantitates (CNRQ) were calculated after normalization to four stably expressed housekeeping genes. a Relative gene expression of the pluripotency markers POU5F1, NANOG, and SOX2, b of marker genes of the primitive streak and mesendoderm, and c of endodermal genes with SOX17, FOXA2 and GATA-family members representing the definitive endoderm and SOX7 representing extra-embryonic endoderm. For a better comparison T, MIXL1, GSC, and SOX17 are presented with a twosegmented y-axis. Values are means ± SEM, n=3-4. ANOVA plus Bonferroni’s post-hoc test. ** p≤0.01, * p≤0.05 compared to the undifferentiated sample. ## p≤0.01, # p≤0.05 compared to d6 Wnt3a/ActA

were scarcely detectable (Fig. 2d). Thus, high concentrations of CHIR-99021 yielded in mesodermal cell types when treated longer than one day, whereas low concentrations permitted expression of endodermal and mesodermal genes. Notably, T and MIXL1 always preceded the expression of these germ layer markers. Comparative Flow Cytometry and qRT-PCR-Analysis of DE Development After Treatment With Wnt3a, Activin A, and CHIR-99021 In order to quantify DE-cells, the surface markers CD49e [30] and CXCR4 [12, 31–35] were tested in a control experiment. Human ES cells were differentiated with wnt3a and activin A ES cells and stained for CXCR4 (PE) and CD49e (FITC)

(Supplementary information, Fig. S3a). Two distinct populations were observed and the cells in the denoted regions were sorted (Supplementary information, Fig. S3a). CD49elow/ CXCR4neg cells exhibited a low gene expression of GSC, FOXA2 and SOX17 compared to undifferentiated ES cells and unsorted cells (Supplementary information, Fig. S3b). In contrast, CD49ehigh/CXCR4high cells showed an increased gene expression of endodermal marker genes, which were robustly induced 328-fold for FOXA2, 250-fold for GSC, and 280-fold for SOX17 compared to CD49elow/CXCR4neg cells, respectively (Supplementary information, Fig. S3b). Thus, the CD49ehigh/CXCR4high can be regarded as endoderm committed cells. Next we refined our experimental strategy and analyzed surface markers and the gene expression profile during the

Stem Cell Rev and Rep Fig. 2 Concentration-dependent effect of CHIR-99021 on the protein and gene expression of human ES cells. ES cells were differentiated for 3 days with 1– 7.5 μM CHIR-99021. Gene expression analysis of a T and MIXL1, b SOX17, FOXA2, and GSC and c the MIXL1-dependent mesoderm marker genes FLK1 and PDGFRA as measured by qRT-PCR. Calibrated normalized relative quantitates (CNRQ) were calculated after normalization to three stably expressed housekeeping genes and scaling to undifferentiated cells. Values are means ± SEM, n=4–5. d Fluorescence images showing the protein expression of SOX17/ FOXA2 after a three day differentiation with 1, 2, and 3 μM CHIR-99021. Nuclei were counterstained with DAPI. Scale bar=100 μm

first days of differentiation. We compared the effect of randomly differentiated cells, activin A (ActA) or Chir-treated cells (Chir) and cells treated with activin A and wnt3a (Wnt3a/ ActA) [10]. We hypothesized that the activation of the Wnt/ beta-catenin signaling in human ES cells caused a rapid generation of a primitive streak-like stage, as shown by the high gene expression of T and MIXL1. Thus, we used 5 μM Chir either sequentially or in combination with 100 ng/ml activin A

(Chir/ActA + ActA and Chir + ActA, respectively) to simulate conditions of combined Wnt- and activin A signaling or sequential activation of these pathways (Fig. 3a). Fig. 3b depicts representative flow cytometry dot plots of the CXCR4/CD49e staining (Fig. 3b). In Fig. 3c a statistical analysis is presented comparing all culture conditions. Analysis of CXCR4 and CD49e expression after 3 days revealed low effects in randomly differentiated cells with the mean of

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only 2.4±0.5 % double-positive cells (Fig. 3c). ActA treatment alone was equally effective with 2.2±0.5 % positive cells. The treatment with Wnt3a/ActA in the control protocol yielded 22±3.5 % positive cells and was therewith clearly inferior to cells in which Chir was added for one day either alone (57.0±7.9 %) or in combination with activin A (59.5± 7.2 %) followed by a two day treatment with activin A (Fig. 3c). Chir alone without any activation of the TGF-beta pathway resulted in >85 % double-positive cells with an almost homogeneous staining for CXCR4 and CD49e (Fig. 3c), however, without a prominent expression of DEmarker genes indicating that both genes might be regulated by Wnt/beta-catenin. Thus, we performed a second control experiment by treating ES cells initially with 5 μM CHIR-99021 for one day followed by a two day treatment with either 100 ng/ml activin A or 25 ng/ml BMP4 (Supplementary information, Fig. S4). ES cells treated with BMP4 did not acquire doublepositivity for CXCR4 and CD49e but were only singlepositive for CD49e (Supplementary information, Fig. S4a) and showed upregulation of the mesodermal marker genes PDGFRA, FLK1 (lateral plate mesoderm), and MEOX1 (paraxial mesoderm) (Supplementary information, Fig. S4b). Only upon treatment with activin A double positivity for CXCR4/CD49e was detectable and the endoderm marker genes SOX17, FOXA2, and GSC were increased (Supplementary information, Fig. S4a and b). Gene expression analysis of the aforementioned protocols showed that ActA treated cells did not express significant amounts of T, MIXL1, GSC, FOXA2, and SOX17 (Fig. 3d). The treatment with Wnt3a/ActA revealed in tendency an expression pattern similar to developmental kinetics with T and MIXL1 expressed after induction for one day from where the expression of both genes decreased in the next two days (Fig. 3d). In opposite, GSC, FOXA2 and SOX17 gene expression continually increased and peaked after 3 days of treatment with Wnt3a/ActA. When human ES cells were exposed to Chir/ActA + ActA or Chir + ActA the expression pattern was remarkably similar without any significant difference (Fig. 3d). In detail, T expression after one day was significantly higher when compared to the control protocol by 20.2-fold and 16.6-fold for Chir/ActA + ActA and Chir + ActA, respectively. After changing to activin A supplemented medium, T expression diminished under both conditions. A similar expression pattern was detected for MIXL1, which was increased 9.8-fold and 7.0-fold for Chir/ActA + ActA and Chir + ActA, respectively, and subsequently decreased in expression strength parallel to T. GSC, FOXA2, and SOX17 expression gradually increased with the highest expression after 3 days of differentiation. In summary the expression was 3.9-fold (GSC), 2.9-fold (FOXA2), and 7.1-fold (SOX17) higher at day three compared to Wnt3a/ActA-treated cells for Chir/ActA + ActA and 4.0-fold (GSC), 3.0-fold (FOXA2),

and 8.4-fold (SOX17) higher for Chir + ActA, respectively (Fig. 3d). The same gene and protein expression kinetics were detected for the second human ES cell line, Hues4, used in this study (Supplementary information, Fig. S5a and b). The changes in gene expression after a three day treatment with Chir alone is presented for comparison and confirmation of the array card data from Fig. 1. Briefly, treatment with Chir alone induced a strong T and MIXl1 gene expression, but low GSC and FOXA2 gene expression compared to Chir/ActA + ActA and Chir + ActA treated cells. SOX17 was detectable only after day one but not significantly higher than in undifferentiated ES cells (Fig. 3d). Both combinations of Chir and activin A resulted in no obvious difference. Thus, we concluded that activin A/TGF-beta signaling is initially not required and further experimentation was carried out with 5 μM Chir for one day followed by an activin A treatment.

Transition From Pluripotency to Endodermal Progeny Via A PS-Like State Fluorescence images of protein expression of PS and DE marker genes during differentiation showed in the control protocol (Wnt3a/ActA) after one day predominantly clustered single-positive cells for FOXA2. The first SOX17/FOXA2 double-positive cells were detected after 2 days and increased thereafter until day 3 (Fig. 3e). Nonetheless large areas still remained negative for SOX17/FOXA2. BRACHYURY was detected in scattered single cells after day one from where it significantly decreased and remained undetectable at day three (Fig. 3e). The protein expression of the pluripotency marker OCT3/4 was present from day 1 to 3. The cells were nearly homogenously positive for OCT3/4 at the start of the differentiation with some single cells in addition double-positive for BRACHYURY. OCT3/4 protein then declined; nonetheless many areas of the cell culture slide remained positive for this marker even after 3 days of differentiation (Fig. 3e). Protein expression kinetics of Chir + ActA treated cells displayed a very similar pattern but with different quantities. SOX17/ FOXA2 double-positive cells were not detected before day two from where they increased until near homogeneity at day three (Fig. 3f). BRACHYURY expression was initiated at the rim of a stem cell colony co-localizing with OCT3/4. Later, BRACHYURY was scarcely detected in the outgrowth of differentiated cell colonies (Fig. 3f). A regular phenomenon after 3 days was a structure comprising a core region with BRACHYURY single-positive cells, encircled by FOXA2/ BRACHYURY double-positive cells well defined off a zone from which homogenous outgrowth of SOX17/FOXA2 double-positive cells occurred (Supplementary information, Fig. S6a and b). The transition zone between the core region and the endodermal outgrowth was defined by a fraction of GSC-positive cells (an anterior primitive streak marker).

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GSC-positive cells were scarcely detected inside the core region (Supplementary information, Fig. S6c). In contrast, treatment with Chir at a 5 μM concentration failed to differentiate the cells into SOX17/FOXA2 doublepositive cells in a quantitative manner. Double-positive cells were scarcely detected, but the majority of the cells were positive for FOXA2 after day one. Thereafter FOXA2positivity significantly declined (Fig. 3g). OCT3/4 protein expression was readily detected after day one and the ES colonies displayed a distinct rim of BRACHYURY/OCT3/4-

positive cells. At later time points OCT3/4 protein expression was hardly detectable (Fig. 3g). Activation of Signaling Pathways Analysis of protein expression by Western blot showed the effect of the different protocols on key downstream signaling molecules of the canonical Wnt pathway, activin A/TGF-beta pathway, and GSK3beta (Supplementary information, Fig. S7a). The protein amounts of GSK3beta did not change

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ƒF i g .

3 C o m p a r a t i v e f l o w c y t o m e t r y, q RT- P C R , a n d immunofluorescence analysis of definitive endoderm development after treatment of human embryonic stem cells with wnt3a, activin A or CHIR99021. a Scheme of the various differentiation protocols used in a comparative analysis of endoderm development after induction with activin A (ActA) or CHIR-99021 (Chir) alone, a combined treatment with wnt3a and activin A (Wnt3a/ActA), and two combinations of CHIR99021 with activin A (Chir/ActA + ActA and Chir + ActA). b Representative dot plots of human ES cells 3 days after the specified treatment. Depicted is the flow cytometric staining of the markers CXCR4 and CD49e for six different treatments. The upper right quadrant denotes the number of CXCR4/CD49e double-positive cells (in %) in this particular experiment. c Summary of the absolute cell number of CXCR4/CD49e double-positive cells after 3 days of differentiation with the specified treatment. Values are means ± SEM, n=4–7. ANOVA plus Bonferroni’s post-hoc test. ** p≤0.01 compared to Wnt3a/ActA-treated cells. d Relative gene expression of the primitive streak marker genes T and MIXL1 and of the definitive endoderm related markers SOX17, FOXA2, and GSC. Values are calibrated normalized relative quantities (CNRQ) after normalization to three stably expressed housekeeping genes and scaling to undifferentiated ES cells. Values are means ± SEM, n=4–8. ANOVA plus Bonferroni’s post-hoc test. ** p≤ 0.01, * p≤0.05 compared to the Wnt3a/ActA differentiated sample, ## p ≤ 0.01, # p ≤ 0.05 compared to d1 in each treatment group. e–g Fluorescence micrographs showing the protein expression of SOX17/ FOXA2 and BRACHYURY/OCT4 after a 3 day differentiation with e Wnt3a/ActA, f Chir + ActA, and g Chir-only. Nuclei were counterstained with DAPI. SOX17/FOXA2 double-positive cells were scarcely detected at day one after the treatment with Wnt3a/ActA or Chir + ActA. SOX17/ FOXA2 double-positive cells were abundant in Chir + ActA treated cells after 3 days but virtually undetectable in Chir-treated cells. BRACHY URY-positive cells transiently appeared after day one and almost disappeared after 3 days of differentiation. In Chir only treated cells, loss of BRACHYURY protein expression was low and OCT3/4 protein was hardly detectable after day one. Original magnification 200x. Scale bars=100 μm. BRA = BRACHYURY

under any of the treatment conditions. In contrast, the activating tyr216 phosphorylation of GSK3beta was nearly absent in Chir-treated cells, whereas in Chir + ActA cells the phosphorylation site was absent for only 1 day. The inactivating ser9 phosphorylation was present without any changes in Chirtreated cells, whereas in the Chir + ActA treatment the phosphorylation site was present for two days, though the cells had been treated for only 1 day. No changes were observed for both phosphorylation sites after treatment with Wnt3a/ActA. Of note is the presence of a ser9 and tyr216 phosphorylation in undifferentiated human ES cells (d0), indicating a tight regulation of GSK3beta activity. SMAD2/3 and phosphorylated SMAD2/3 (p-Smad2/3) revealed a low protein expression in undifferentiated human ES cells, which gradually increased upon treatment with Chir + ActA. Protein bands upon Wnt3a/ ActA were generally lower in intensity and showed only a slight increase (Supplementary information, Fig. S7a). In Chir-treated samples SMAD2/3 was present, whereas pSMAD2/3 was near the detection limit of the Western blot. Quantification of the protein bands verified the stronger phosphorylation SMAD2/3 of Chir + ActA treated samples compared to the control protocol (Supplementary information,

Fig. S7b). Changes in the protein expression of beta-catenin and phosphorylated beta-catenin were notable one day after treatment with Chir and to a lesser extent also with Wnt3a. Upon longer exposure in Chir-treated samples, both betacatenin and phosphorylated beta-catenin levels remained high. When Chir or Wnt3a was removed from the differentiation medium after day one, beta-catenin and phosphorylated betacatenin decreased again (Supplementary information, Fig. S7b). Concentration-Dependent Effect of Activin A After Initial Induction With CHIR-99021 Next we analyzed whether the efficient induction into a PSlike cell type would reduce the requirement for activin A during differentiation. Human ES cells were differentiated in total for three days with 5 μM Chir for one day followed by a two day treatment with 0 to 100 ng/ml activin A. The control group was initially treated with 25 ng/ml wnt3a plus 0 to 100 ng/ml activin A for one day followed by activin A at the same concentration. Representative dot plots for the CXCR4/ CD49e staining for 0, 25 and 100 ng/ml activin A are presented in Fig. 4a. The control protocol resulted in a continuous increase of CXCR4/CD49e double-positive cells parallel to the activin A concentration (Fig. 4b). When primed with 5 μM Chir, already low concentrations of activin A were required to induce a significant increase of CXCR4/CD49e doublepositive cells. This effect became statistically significant with 10 ng/ml activin A in the medium and the curve showed saturation of this effect from ≥25 ng/ml activin A onwards (Fig. 4b). This was confirmed by qRT-PCR analyses. We found an elevated SOX17 and FOXA2 gene expression with increasing activin A concentrations in the medium after initial use of Wnt3a (Fig. 4c). Priming with Chir yielded an increased expression similar to the flow cytometry curve; the CNRQ values showed saturation from ≥25 ng/ml activin A onwards (Fig. 4c). Gene expression of these markers was significantly increased between 3.7–6.1-fold for FoxA2 and 3.2–7.8-fold for SOX17, when compared to the control protocol. Remarkably, PDGRFa gene expression was significantly elevated in cells only primed with Chir, whilst SOX17 and FOXA2 gene expression was low (Fig. 4c). Thus, GSK3beta inhibition alone, at these tested concentration, favors mesoderm development, but Chir-primed cells showed a higher sensitivity to the inductive signal of activin A/TGF-beta to differentiate into the DE-stage. Pancreatic Differentiation Potential Finally we assessed whether cells committed into the definitive endoderm linage are capable of further differentiation into endodermal somatic cells (Fig. 4c). Upon further differentiation in presence of directive cues known to promote pancreas

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Fig. 4 Concentration-dependent effect of activin A after initial induction with CHIR-99021. Human ES cells were differentiated with 5 μM CHIR99021 (Chir) for one day and subsequently treated with 0–100 ng/ml activin A for additional 2 days. As a comparison the control protocol with wnt3a was used with identical activin A concentrations. a Representative flow cytometry dot plots of human ES cells double-stained for CD49e (FITC) and CXCR4 (PE) after a 3 day treatment with Chir + ActA and Wnt3a/ActA using 0, 25 and 100 ng/ml activin A. The number in the upper right quadrant denotes the cell percentage of this particular experiment. b Summary of the flow cytometry analysis of CXCR4-positive cells after 3 days of differentiation. Values are means ± SEM, n=5–7. ANOVA plus Bonferroni’s post-hoc test. ** p≤0.01, * p≤0.05 compared to d0 undifferentiated samples. Students’s t-test. ## p≤0.01 Chir + ActA

treated cells compared to the corresponding cells of the control protocol. c qRT-PCR analysis of SOX17, FOXA2, T, PDGFRA after treatment with different activin A concentrations. Calibrated normalized relative quantitates (CNRQ) were calculated after normalization to three stably expressed housekeeping genes. Values are means ± SEM, n=3–6. ANOVA plus Dunnett’s post-hoc test. ** p≤0.01, * p≤0.05 compared to d0 undifferentiated samples. Students’s t-test. ## p≤0.01, # p≤0.05 Chir + ActA treated cells compared to the corresponding cells of the control protocol. NS = no significant difference to the control protocol. d qRT-PCR analysis of PDX1, HLXB9, NKX2.2, and NGN3 after a 14 day differentiation with Chir + ActA (4 days) plus 10 days of pancreatic induction with FGF-10, retinoic acid, dorsomorphin, and SB-431542. Values are means ± SEM, n=3

specification, the cells expressed the pancreatic markers PDX1 and HLXB9 succeeded by expression of the endocrine markers NKX2.2 and NGN3 in a timed fashion reminiscent of pancreatic development (Fig. 4c).

formation by GSK3beta inhibition using 1m [23]. Another group recently published a study in which the GSK3beta inhibitor CHIR-99021 was used to generate the mesoderm germ layer to robustly produce cardiomyocytes from human pluripotent cells [26]. This comprised the differentiation into a BRACHYURY-positive primitive streak-like (PS-like) cell type. In light of these contrasting results it is obvious that GSK3beta inhibition may act in multiple ways. We observed that differentiation of human ES cells, cultivated under feeder-free conditions, with high CHIR-99021 concentrations yielded in cells with an overlapping high

Discussion We studied whether lineage selection towards the definitive endoderm (DE) could be chemically induced via GSK3beta inhibition. Bone and co-workers described a novel way of DE

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expression of T, MIXL1 followed by PDGFRA. T and MIXL1 are both considered to be expressed in the PS and later in the mesoderm [36], whereas PDGFRA marks the lateral plate mesoderm. Hence differentiated human ES cells did not acquire a typical endoderm expression pattern under this condition, providing strong evidence that sustained and strong GSK3beta inhibition is not suited to generate significant amounts of DE-cells. This is additionally approved by the strong expression of CDX-family members, which are reported to be controlled by T in early mesodermal cells [37]. Incubation of human ES cells with different concentrations of CHIR-99021, simulating conditions of strong and weak Wnt/beta-catenin-signaling, revealed quick cell fate decisions. Strong and prolonged GSK3beta inhibition favors mesoderm differentiation [14, 21], whereas weak GSK3beta inhibition permitted expression of endoderm and mesoderm related genes. Thus, the SOX17/FOXA2 double-positive cells detected after treatment with low Chir concentrations might represent the same cell population reported by Bone and coworkers using the inhibitor 1m [23]. This concentrationdependent effect is most likely caused by human ES cells primed into a PS-like cell type that randomly progresses into both germ layers under less stringent Wnt-signaling conditions or no inductive signals. Thus, these findings confirm the work by Tan and co-workers [38] and reveal that a pretreatment with CHIR-99021 enables human ES cells to progress into the endoderm and mesoderm germ layer. Importantly, this effect is highly dependent on the concentration of the GSK3beta inhibitor and its incubation time. Thus, in contrast to the current concept TGF-beta signaling by activin A or nodal is obviously not solely responsible for efficient endoderm differentiation. In 2004 Kubo and co-workers reported that mouse ES cells can be forced into DE by treatment with activin A via TGF beta signaling [3]. The transient expression of T upon activin A treatment was accounted to the active nodal/TGF beta signaling pathway [3, 4]. However, in our study the changes in gene expression of T and MIXL1 in activin A treated cells were surprisingly low and not significantly higher when additionally combined with wnt3a. Since T is transactivated via TCF/LEF binding sites downstream of Wnt/beta-catenin signaling [39], we consider GSK3beta inhibition as a stronger indirect chemical activator of the canonical Wnt-pathway than wnt3a circumventing the use of recombinant proteins with variable activities. Thus, the endoderm differentiation protocol was split into two distinct parts; first forcing ES cells into a PS-like cell type independently of the activin A/TGF beta signaling pathway but by activation of the canonical Wnt-pathway and second treating PS-like cells with activin A. This new protocol (Chir + ActA) was in addition compared to the study by Kunisada and coworkers, who used CHIR-99021 and activin A in combination [28] but did not provide a reasonable explanation for this

treatment. The combined protocol with Chir + ActA proved to be superior in many aspects. In quantitative terms Chir + ActA almost tripled the number of DE-cells compared to the control protocol by D’Amour and co-workers [10]. The expression of marker genes was significantly increased and closely resembled a transient PS-like expression pattern followed by strong induction of DE-related genes. Immunofluorescence revealed tissue structures that appeared as gastrulation embryos with defined regions of anterior and posterior primitive streak and a distinct definitive endoderm outgrowth from the anterior primitive streak region marked by high GSC-positivity. Importantly, this was achieved without inhibition of the PI3-kinase activity, which is commonly used to disrupt ES cell self-renewal [11, 40]. The combination of CHIR-99021 plus activin A on the first day of differentiation [28] showed no significant difference in gene and protein expression to our protocol in which CHIR-99021 was applied initially alone. These data confirm the important role of the canonical Wntsignaling pathway [21, 28, 41] with the crucial difference that extrinsic activin A/TGF-beta signaling is expandable for the formation of the PS during the in vitro differentiation of human ES cells. Further pancreatic differentiation revealed that DE generated by this new protocol is physiologically relevant. Thus, the synergistic activity of Wnt/beta-catenin signaling and activin A/TGF-beta signaling reported by other groups [21] is not required under our differentiation conditions. Remarkably, prolonged GSK3beta inhibition caused a rapid and homogenous expression of CD49e and CXCR4 on cells that have not yet been committed to the endoderm. For CXCR4 it is known that its promoter region contains TCF/ LEF binding sites along with SMAD2/3 and SOX17 binding sites [42]. Sumi and co-workers already demonstrated that CXCR4 protein expression can be induced by activation of the canonical Wnt-pathway alone [21]. Sequence analysis of the 1 kb DNA region upstream of the transcription start site of the human ITGA5 gene (CD49e) revealed at least three putative TCF/LEF binding sites (Supplementary information, Fig. S8). Thus, it is likely that a strong and prolonged canonical Wnt-pathway activity is sufficient to induce CXCR4 and ITGA5 gene expression. However, a short exposure to CHIR99021 of only 24 h is not sufficient to induce CXCR4/CD49e double-positivity in BMP4 treated cells; once activin A is supplemented to the medium the cells acquire both markers (Supplementary information, Fig. S4). We also found a tight regulation of the phosphorylation status of GSK3beta via tyrosine 216 and serine 9 phosphorylation sites in the presence of Chir. Additionally, a subtle increase in the amount of unphosphorylated beta-catenin was detected. Moreover, Chir primed cells showed after supplementation with activin A an overall better sensitivity to activin A/TGF-beta signaling as seen by the increased

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phospho-SMAD2/3 protein levels. Finally this study reveals that activin A concentrations in the media can be reduced up to 75 % after priming of human ES cells with CHIR-99021. This is an important finding since the expenses for this growth factor represent a high cost pressure for ES cell research laboratories worldwide. Mechanistically the overall better efficiency of endoderm differentiation by initial GSK3beta inhibition may be explained by the fact that T gene expression mainly depends on the canonical Wnt-pathway [39]. T is an important transactivator of Wnt3a in the mouse primitive streak, thereby amplifying the Wnt/beta-catenin pathway [43]. Furthermore, T and MIXL1 are co-expressed in differentiating mouse ES cells and may likely control similar processes during endoderm and mesoderm specification [44]. The control of MIXL1 gene expression has been primarily attributed to the BMP- and activin A-signaling pathway [36]. However, our results strongly suggest a dominant role for Wnt/beta-catenin induction since T and MIXL1 gene expression showed an overlapping and strong expression upon CHIR-99021 treatment, which significantly decreased once the differentiation protocol changed from GSK3beta inhibition to activin A/TGF-beta signaling. MIXL1 is a strong inducer of GSC [45]. Interestingly, the anterior primitive streak marker GSC represses T and MIXl1 in mouse ES cells [46, 47], thus acting as a key regulator to facilitate endoderm development in the anterior primitive streak via suppression of T and MIXL1. In case of a strong and prolonged Wnt/beta-catenin signaling and absent activin A/TGF-beta signaling, the lower expression of GSC would permit mesoderm formation. In contrast, activin A/TGF-beta signaling induced a strong GSC gene expression after initial Wnt/beta-catenin-signaling. Whether GSC directly controls SOX17 and FOXA2 gene expression and is itself activated by SMAD2/3 has to be defined in the future. Thus, T, MIXL1, and GSC may likely form a regulatory network controlling early mesoderm or endoderm cell fate decisions during the PS-stage of differentiated human ES cells. In this study the Hues8 cell line has been used primarily and in addition certain control experiments were performed also with a second line, Hues4, for verification (Supplementary information, Fig. S5). Although both lines have been reported to be suited for endoderm differentiation [27], the results presented in this study, those from a previous report [12], and the results from the scientists, who established these human ES cell lines [27, 48] showed that Hues8 cells are superior in terms of gene and protein expression after endoderm induction. In competition with activin A based differentiation media, alternatives have recently been published to commit ES cells in the definitive endoderm [49, 50]. Zhu and co-workers [13] differentiated human ES cells into endoderm by initial stauprimide treatment followed by low activin A

concentrations. Many staurosporine family members are well-known GSK3 inhibitors [51], suggesting that the effects reported by Zhu and co-workers were a likely result of GSK3beta inhibition [13].

Summary In conclusion, the differentiation of human ES cells into cells of the definitive endoderm requires in the first instance a transient and efficient differentiation into a PS-like stage using either Wnt ligands or by GSK3beta inhibition. This does, in contrast to the current concept, not necessarily require synergistic activin A/TGF-beta signaling. These PS-like cells can be considered as a bipotential cell population and are responsive to BMP4, resulting in mesodermal cell derivatives, or activin A resulting in DE-cells. Thus, this study shows that the differentiation into the DE is initially independent from the activin A/TGF-beta pathway but requires short and transiently high canonical Wnt-pathway activity. Acknowledgments This work has been supported by the Deutsche Forschungsgemeinschaft (German Research Foundation) within the framework of the Cluster of Excellence REBIRTH. The skillful technical assistance of J. Kresse and R. Strauss is gratefully acknowledged. We would like to acknowledge the assistance of the Cell Sorting Facility of Hannover Medical School supported by the Braukmann-WittenbergHerz-Stiftung and the Deutsche Forschungsgemeinschaft. Conflict of Interest interest.

The authors declare no potential conflicts of

References 1. Naujok, O., & Lenzen, S. (2012). Pluripotente Stammzellen zur Zellersatztherapie des Diabetes mellitus. Deutsche Medizinische Wochenschrift, 137, 1062–1066. 2. Naujok, O., Burns, C., Jones, P. M., & Lenzen, S. (2011). Insulinproducing surrogate beta-cells from embryonic stem cells: are we there yet? Molecular Therapy, 19, 1759–1768. 3. Kubo, A., Shinozaki, K., Shannon, J. M., Kouskoff, V., Kennedy, M., Woo, S., et al. (2004). Development of definitive endoderm from embryonic stem cells in culture. Development, 131, 1651–1662. 4. D’Amour, K. A., Agulnick, A. D., Eliazer, S., Kelly, O. G., Kroon, E., & Baetge, E. E. (2005). Efficient differentiation of human embryonic stem cells to definitive endoderm. Nature Biotechnology, 23, 1534–1541. 5. Rodaway, A., & Patient, R. (2001). Mesendoderm. an ancient germ layer? Cell, 105, 169–172. 6. Technau, U., & Scholz, C. B. (2003). Origin and evolution of endoderm and mesoderm. International Journal of Developmental Biology, 47, 531–539. 7. Hanna, J. H., Saha, K., & Jaenisch, R. (2010). Pluripotency and cellular reprogramming: facts, hypotheses, unresolved issues. Cell, 143, 508–525. 8. Mfopou, J. K., Chen, B., Mateizel, I., Sermon, K., & Bouwens, L. (2010). Noggin, retinoids, and fibroblast growth factor regulate

Stem Cell Rev and Rep

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24. 25.

hepatic or pancreatic fate of human embryonic stem cells. Gastroenterology, 138, 2233–2245. Kroon, E., Martinson, L. A., Kadoya, K., Bang, A. G., Kelly, O. G., Eliazer, S., et al. (2008). Pancreatic endoderm derived from human embryonic stem cells generates glucose-responsive insulin-secreting cells in vivo. Nature Biotechnology, 26, 443–452. D’Amour, K. A., Bang, A. G., Eliazer, S., Kelly, O. G., Agulnick, A. D., Smart, N. G., et al. (2006). Production of pancreatic hormoneexpressing endocrine cells from human embryonic stem cells. Nature Biotechnology, 24, 1392–1401. Cho, C. H., Hannan, N. R., Docherty, F. M., Docherty, H. M., Joao Lima, M., Trotter, M. W., et al. (2012). Inhibition of activin/nodal signalling is necessary for pancreatic differentiation of human pluripotent stem cells. Diabetologia, 55, 3284– 3295. Naujok, O., & Lenzen, S. (2012). A critical re-evaluation of CD24positivity of human embryonic stem cells differentiated into pancreatic progenitors. Stem Cell Reviews and Reports, 8, 779–791. Zhu, S., Wurdak, H., Wang, J., Lyssiotis, C. A., Peters, E. C., Cho, C. Y., et al. (2009). A small molecule primes embryonic stem cells for differentiation. Cell Stem Cell, 4, 416–426. Holland, J. D., Klaus, A., Garratt, A. N., & Birchmeier, W. (2013). Wnt signaling in stem and cancer stem cells. Current Opinion in Cell Biology, 25, 254–264. ten Berge, D., Koole, W., Fuerer, C., Fish, M., Eroglu, E., & Nusse, R. (2008). Wnt signaling mediates self-organization and axis formation in embryoid bodies. Cell Stem Cell, 3, 508–518. Barrow, J. R., Howell, W. D., Rule, M., Hayashi, S., Thomas, K. R., Capecchi, M. R., et al. (2007). Wnt3 signaling in the epiblast is required for proper orientation of the anteroposterior axis. Developmental Biology, 312, 312–320. Kirby, L. A., Schott, J. T., Noble, B. L., Mendez, D. C., Caseley, P. S., Peterson, S. C., et al. (2012). Glycogen synthase kinase 3 (GSK3) inhibitor, SB-216763, promotes pluripotency in mouse embryonic stem cells. PLoS ONE, 7, e39329. Price, F. D., Yin, H., Jones, A., van Ijcken, W., Grosveld, F., & Rudnicki, M. A. (2012). Canonical Wnt signaling induces a primitive endoderm metastable state in mouse embryonic stem cells. Stem Cells, 31, 752–764. Nakanishi, M., Kurisaki, A., Hayashi, Y., Warashina, M., Ishiura, S., Kusuda-Furue, M., et al. (2009). Directed induction of anterior and posterior primitive streak by Wnt from embryonic stem cells cultured in a chemically defined serum-free medium. FASEB Journal, 23, 114–122. Davidson, K. C., Adams, A. M., Goodson, J. M., McDonald, C. E., Potter, J. C., Berndt, J. D., et al. (2012). Wnt/beta-catenin signaling promotes differentiation, not self-renewal, of human embryonic stem cells and is repressed by Oct4. Proceedings of the National Academy of Sciences of the United States of America, 109, 4485–4490. Sumi, T., Tsuneyoshi, N., Nakatsuji, N., & Suemori, H. (2008). Defining early lineage specification of human embryonic stem cells by the orchestrated balance of canonical Wnt/ beta-catenin, Activin/Nodal and BMP signaling. Development, 135, 2969–2979. Brons, I. G., Smithers, L. E., Trotter, M. W., Rugg-Gunn, P., Sun, B., de Sousa, C., Lopes, S. M., et al. (2007). Derivation of pluripotent epiblast stem cells from mammalian embryos. Nature, 448, 191–195. Bone, H. K., Nelson, A. S., Goldring, C. E., Tosh, D., & Welham, M. J. (2011). A novel chemically directed route for the generation of definitive endoderm from human embryonic stem cells based on inhibition of GSK-3. Journal of Cell Science, 124, 1992–2000. Doble, B. W., & Woodgett, J. R. (2003). GSK-3: tricks of the trade for a multi-tasking kinase. Journal of Cell Science, 116, 1175–1186. Ying, Q. L., Wray, J., Nichols, J., Batlle-Morera, L., Doble, B., Woodgett, J., et al. (2008). The ground state of embryonic stem cell self-renewal. Nature, 453, 519–523.

26. Lian, X., Hsiao, C., Wilson, G., Zhu, K., Hazeltine, L. B., Azarin, S. M., et al. (2012). Robust cardiomyocyte differentiation from human pluripotent stem cells via temporal modulation of canonical Wnt signaling. Proceedings of the National Academy of Sciences of the United States of America, 109, 1848–1857. 27. Osafune, K., Caron, L., Borowiak, M., Martinez, R. J., Fitz-Gerald, C. S., Sato, Y., et al. (2008). Marked differences in differentiation propensity among human embryonic stem cell lines. Nature Biotechnology, 26, 313–315. 28. Kunisada, Y., Tsubooka-Yamazoe, N., Shoji, M., & Hosoya, M. (2012). Small molecules induce efficient differentiation into insulinproducing cells from human induced pluripotent stem cells. Stem Cell Research, 8, 274–284. 29. Pereira, L. A., Wong, M. S., Mossman, A. K., Sourris, K., Janes, M. E., Knezevic, K., et al. (2012). Pdgfralpha and Flk1 are direct target genes of Mixl1 in differentiating embryonic stem cells. Stem Cell Research, 8, 165–179. 30. Wang, P., Rodriguez, R. T., Wang, J., Ghodasara, A., & Kim, S. K. (2011). Targeting SOX17 in human embryonic stem cells creates unique strategies for isolating and analyzing developing endoderm. Cell Stem Cell, 8, 335–346. 31. Jiang, W., Wang, J., & Zhang, Y. (2013). Histone H3K27me3 demethylases KDM6A and KDM6B modulate definitive endoderm differentiation from human ESCs by regulating WNT signaling pathway. Cell Research, 23, 122–130. 32. Nostro, M. C., Sarangi, F., Ogawa, S., Holtzinger, A., Corneo, B., Li, X., et al. (2011). Stage-specific signaling through TGFbeta family members and WNT regulates patterning and pancreatic specification of human pluripotent stem cells. Development, 138, 861–871. 33. Jiang, W., Zhang, D., Bursac, N., & Zhang, Y. (2013). WNT3 Is a Biomarker Capable of Predicting the Definitive Endoderm Differentiation Potential of hESCs. Stem Cell Reports, 1, 46–52. 34. Rezania, A., Bruin, J. E., Riedel, M. J., Mojibian, M., Asadi, A., Xu, J., et al. (2012). Maturation of human embryonic stem cell-derived pancreatic progenitors into functional islets capable of treating pre-existing diabetes in mice. Diabetes, 61, 2016–2029. 35. Bruin, J. E., Rezania, A., Xu, J., Narayan, K., Fox, J. K., O’Neil, J. J., et al. (2013). Maturation and function of human embryonic stem cellderived pancreatic progenitors in macroencapsulation devices following transplant into mice. Diabetologia, 56, 1987–1998. 36. Pereira, L. A., Wong, M. S., Mei Lim, S., Stanley, E. G., & Elefanty, A. G. (2012). The Mix family of homeobox genes-key regulators of mesendoderm formation during vertebrate development. Developmental Biology, 367, 163–177. 37. Bernardo, A. S., Faial, T., Gardner, L., Niakan, K. K., Ortmann, D., Senner, C. E., et al. (2011). BRACHYURY and CDX2 mediate BMP-induced differentiation of human and mouse pluripotent stem cells into embryonic and extraembryonic lineages. Cell Stem Cell, 9, 144–155. 38. Tan, J. Y., Sriram, G., Rufaihah, A. J., Neoh, K. G., & Cao, T. (2013). Efficient derivation of lateral plate and paraxial mesoderm subtypes from human embryonic stem cells through GSKi-mediated differentiation. Stem Cells and Development, 22, 1893–1906. 39. Arnold, S. J., Stappert, J., Bauer, A., Kispert, A., Herrmann, B. G., & Kemler, R. (2000). Brachyury is a target gene of the Wnt/beta-catenin signaling pathway. Mechanisms of Development, 91, 249–258. 40. McLean, A. B., D’Amour, K. A., Jones, K. L., Krishnamoorthy, M., Kulik, M. J., Reynolds, D. M., et al. (2007). Activin a efficiently specifies definitive endoderm from human embryonic stem cells only when phosphatidylinositol 3-kinase signaling is suppressed. Stem Cells, 25, 29–38. 41. Gadue, P., Huber, T. L., Paddison, P. J., & Keller, G. M. (2006). Wnt and TGF-beta signaling are required for the induction of an in vitro model of primitive streak formation using embryonic stem cells.

Stem Cell Rev and Rep

42.

43.

44.

45.

46.

Proceedings of the National Academy of Sciences of the United States of America, 103, 16806–16811. Katoh, M., & Katoh, M. (2010). Integrative genomic analyses of CXCR4: transcriptional regulation of CXCR4 based on TGFbeta, Nodal, Activin signaling and POU5F1, FOXA2, FOXC2, FOXH1, SOX17, and GFI1 transcription factors. International Journal of Oncology, 36, 415–420. Evans, A. L., Faial, T., Gilchrist, M. J., Down, T., Vallier, L., Pedersen, R. A., et al. (2012). Genomic targets of Brachyury (T) in differentiating mouse embryonic stem cells. PLoS ONE, 7, e33346. Pereira, L. A., Wong, M. S., Lim, S. M., Sides, A., Stanley, E. G., & Elefanty, A. G. (2011). Brachyury and related Tbx proteins interact with the Mixl1 homeodomain protein and negatively regulate Mixl1 transcriptional activity. PLoS ONE, 6, e28394. Zhang, H., Fraser, S. T., Papazoglu, C., Hoatlin, M. E., & Baron, M. H. (2009). Transcriptional activation by the Mixl1 homeodomain protein in differentiating mouse embryonic stem cells. Stem Cells, 27, 2884–2895. Boucher, D. M., Schaffer, M., Deissler, K., Moore, C. A., Gold, J. D., & Burdsal, C. A. (2000). goosecoid expression represses Brachyury in embryonic stem cells and affects craniofacial development in

47.

48.

49.

50.

51.

chimeric mice. International Journal of Developmental Biology, 44, 279–288. Izzi, L., Silvestri, C., von Both, I., Labbe, E., Zakin, L., Wrana, J. L., et al. (2007). Foxh1 recruits Gsc to negatively regulate Mixl1 expression during early mouse development. The EMBO Journal, 26, 3132–3143. Cowan, C. A., Klimanskaya, I., McMahon, J., Atienza, J., Witmyer, J., Zucker, J. P., et al. (2004). Derivation of Embryonic Stem-Cell Lines from Human Blastocysts. New England Journal of Medicine, 350, 1353–1356. Borowiak, M., Maehr, R., Chen, S., Chen, A. E., Tang, W., Fox, J. L., et al. (2009). Small molecules efficiently direct endodermal differentiation of mouse and human embryonic stem cells. Cell Stem Cell, 4, 348–358. Diekmann, U., Naujok, O., Blasczyk, R., & Müller, T. (2013). Embryonic stem cells of the non-human primate Callithrix jacchus can be differentiated into definitive endoderm by Activin-A but not IDE-1/2. Journal of Tissue Engineering and Regenerative Medicine. Chichester, West Sussex, UK: John Wiley & Sons. Kramer, T., Schmidt, B., & Lo Monte, F. (2012). Small-Molecule Inhibitors of GSK-3: Structural Insights and Their Application to Alzheimer’s Disease Models. International Journal of Alzheimer's Disease, 2012, 381029.