Characterization of Bovine <i>NANOG</i>5’-flanking Region during  Differentiation of Mouse Embryonic Stem Cells

Hye-Jeong Jang; Hwan Hee Park; Tran Thi Thuy Linh; Hak-Kyo Lee; Ki-Duk Song; Woon Kyu Lee

doi:10.5713/ajas.15.0497

Anim Biosci > Volume 28(12); 2015 > Article

Jang, Park, Linh, Lee, Song, and Lee: Characterization of Bovine NANOG5′-flanking Region during Differentiation of Mouse Embryonic Stem Cells

Article

Asian-Australasian Journal of Animal Sciences (AJAS) 2015; 28(12): 1721-1728.

https://doi.org/10.5713/ajas.15.0497

Characterization of Bovine NANOG5′-flanking Region during Differentiation of Mouse Embryonic Stem Cells

Hye-Jeong Jang, Hwan Hee Park, Tran Thi Thuy Linh, Hak-Kyo Lee¹, Ki-Duk Song^1,^*, Woon Kyu Lee^*

¹Department of Animal Biotechnology, Chonbuk National University, Jeonju 561-756, Korea.

^*Corresponding Authors: Ki-Duk Song.
Tel: +82-63-270-4748, Fax: +82-63-270-2614, E-mail: kiduk.song@gmail.com / Woon Kyu Lee.
Tel: +82-32-890-0946, Fax: +82-32-890-1199, E-mail: wklee@inha.ac.kr

Laboratory of Developmental Genetics, Department of Biomedical Sciences, Inha University School of Medicine, Incheon 400-712, Korea

Received June 08, 2015 Revised July 25, 2015 Accepted August 12, 2015

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License http://creativecommons.org/licenses/by-nc/3.0/ which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Embryonic stem cells (ESCs) have been used as a powerful tool for research including gene manipulated animal models and the study of developmental gene regulation. Among the critical regulatory factors that maintain the pluripotency and self-renewal of undifferentiated ESCs, NANOG plays a very important role. Nevertheless, because pluripotency maintaining factors and specific markers for livestock ESCs have not yet been probed, few studies of the NANOG gene from domestic animals including bovine have been reported. Therefore, we chose mouse ESCs in order to understand and compare NANOG expression between bovine, human, and mouse during ESCs differentiation. We cloned a 600 bp (−420/+181) bovine NANOG 5′-flanking region, and tagged it with humanized recombinant green fluorescent protein (hrGFP) as a tracing reporter. Very high GFP expression for bovine NANOG promoter was observed in the mouse ESC line. GFP expression was monitored upon ESC differentiation and was gradually reduced along with differentiation toward neurons and adipocyte cells. Activity of bovine NANOG (−420/+181) promoter was compared with already known mouse and human NANOG promoters in mouse ESC and they were likely to show a similar pattern of regulation. In conclusion, bovine NANOG 5-flanking region functions in mouse ES cells and has characteristics similar to those of mouse and human. These results suggest that bovine gene function studied in mouse ES cells should be evaluated and extrapolated for application to characterization of bovine ES cells.

Keywords: Bovine; Embryonic Stem Cell; NANOG; Promoter; Neural Differentiation; Adipocyte Differentiation

INTRODUCTION

Zygote and morula stage embryos possess a totipotency for generation of a whole organism. Only cells of the inner cell mass (ICM) from blastocyst have the pluripotent capacity for formation of all three primary germ layers, endoderm, mesoderm, and ectoderm. In adult tissues, multipotent stem and progenitor cells exist in tissues and organs for replacement of lost or injured cells (Wobus and Boheler, 2005). Embryonic stem cells (ESCs) are derived from the ICM in the blastocyst stage embryo and can rise to all cells of the embryo (Suda et al., 1987). Once established in vitro, ESCs can be cultured indefinitely without losing their pluripotency, and have the capacity to develop into any cell type in the body, and self-renewal (Okita and Yamanaka, 2006). In addition, development of undifferentiated ESCs occurs in vitro via three-dimensional aggregates (embryoid body [EB]) into differentiated cell types of all three primary germ layers (Kim et al., 2009).

The capacity for self-renewal and the pluripotency of ESCs is known to be controlled by the transcription factors, Nanog homeobox (NANOG), octamer binding transcription factor-4 (OCT4) and sex determining region-Y box-2 (SOX2), and signaling pathways like leukemia inhibitory factor (LIF)-signal transducer and activator of transcription 3 (STAT3) and bone morphogenic protein (BMP)-Mothers against decapentaplegic homolog (SMAD) 1/4/5/8 (Rodda et al., 2005). Function and capability of these transcriptional genes and signaling pathways depend on the stage of development of the self-renewal and pluripotent cells, indicating that these factors function in combination with other processes (Chickarmane et al., 2006). These transcription factors activate repressed pattern of gene expression like GATA binding transcription factor 4/6 (GATA4/6) and caudal type homeobox 2 (CDX), which mediate phenotypic changes toward endoderm during stem cell differentiation, and repress activated pattern of gene expression for differentiation to mesoderm and ectoderm like T-box transcription factor 3 (TBX3) and estrogen-related receptor beta (ESRRB) (Boyer et al., 2005).

Among ESCs transcription factors, NANOG is a homeobox-containing a transcription factor with an essential role in maintaining the pluripotent cells of the ICM and ESCs (Liu et al., 2007). It is expressed in pluripotent cells and is absent from differentiated cells. Both NANOG and OCT4 promoter-driven enhanced green fluorescent protein (EGFP) have been monitored in undifferentiated state ESCs, but not detected following differentiation (Gerrard et al., 2005). Disruption of NANOG in ESCs results in differentiation to endoderm lineages (Hamazaki et al., 2004), while over-expression of NANOG in mouse ESCs renders LIF dependent ESCs self-renewal, although the self-renewal capacity of the cells is reduced, suggesting that NANOG is a major regulator of the pluripotent state. The NANOG over-expression effect is neither dependent on STAT3 activation nor requires presence of BMP4 (Liu et al., 2007). However, little is known about regulation of NANOG gene expression with various transcriptional regulators.

So far, validated ESC lines that can contribute to the germline in chimeras have only been established in mice, rats, and chickens (Pain et al., 1996; Buehr et al., 2008; Li et al., 2008). While ESC lines have also been established in other species, they have not been fully validated due to either ethical reasons in the case of human (Thomson et al., 1998) or technical reasons in the case of mink (Sukoyan et al., 1993), hamster (Doetschman et al., 1988), rhesus monkey (Thompson et al., 1995), canine (Hatoya et al., 2006; Hayes et al., 2008), and various large domestic species. Bovine induced pluripotent stem cells (iPSCs) and ESC-like cells have been established, but not maintained long-term (Keefer et al., 2001; Huang et al., 2011; Sumer et al., 2011). For these reasons, the molecular characteristics of NANOG from domestic animals including bovine have not yet been fully determined. The bovine NANOG promoter sequences (−420/+181) were isolated and their promoter activity was examined in mouse ESCs as a heterologous system. In transfection study, the bovine NANOG promoter exhibited strong activity in mouse ESCs as a heterologous system and its promoter activity was down-regulated during ESCs differentiation, suggesting that NANOG promoter activity is conserved across the mammalian species and bovine NANOG promoter will be useful in development of bovine ESCs.

MATERIALS AND METHODS

Isolation of bovine NANOG 5′-flanking region and construction of bovine NANOG promoter-green fluorescent protein reporter recombinant plasmid

Bovine genomic DNA was isolated from ear cells of Korean beef cattle using the Wizard Genomic DNA purification kit (Promega, Madison, WI, USA). The bovine NANOG promoter region was amplified from the +181 bp region (anti-sense primer: 5′-CTGCTTGCTCACCAT GTTGCTGAGTTGAAGGAG-3′) to the −420 bp region (sense primer: 5′-CCGGAAGCTTAAAGAGGAAAAT GGAGTTAG-3′). Polymerase chain reaction (PCR) amplification was performed using 10 ng of genomic DNA template in final volumes of 50μL using AccuPower Pfu PCR PreMix (Bioneer, Daejeon, Korea). The PCR conditions were as follows: 94°C for 5 min, followed by 35 cycles of denaturation at 94°C for 30 s, annealing at 64°C for 45 s, extension at 72°C for 1 min, and the final extension step was prolonged to 5 min. The PCR product was digested with HindIII (TAKARA, Shiga, Japan) and sticky/blunt ligated into the EcoRV and HindIII site of phrGFP II-C (Agilent Technologies Genomics, Santa Clara, CA, USA) using a ligase kit (ELPIS, Daejeon, Korea).

Cell culture and transgenic cell line

Mouse ESC lines (R1) were cultured on inactivated mouse primary embryonic fibroblast feeder cells with mitomycin C (Sigma, St. Louis, MO, USA) and grown in Dulbecco’s Modified Eagle’s Medium (DMEM) (Gibco, USA), 15% fetal calf serum (HyClone, Logan, UT, USA), 0.1 mM 2-mercaptoethanol (Gibco, USA), 1% non-essential amino acids (Gibco, USA), 2 mM L-glutamine (Gibco, USA), 1 mM MEM sodium pyruvate solution (Gibco, USA), 1% penicillin/streptomycin (Gibco, USA), and 1,000 U/mL LIF (ES-GRO, Chemicon, Billerica, MA, USA). Cells were maintained at 37°C with 5% CO₂. Recombinant plasmids of mouse NANOG (−332/+50) and human NANOG (−380/+24) tagged with pEGFP-N1 as an expression reporter were generously donated by Dr. Takashi Tada, Laboratory of Stem Cell Engineering at Kyoto University (Kyoto, Japan). Mouse, human, and bovine NANOG promoter-GFP reporter vectors were linearized, and each 4.7 kb, 4.5 kb, and 4.6 kb transgene was electroporated into 1×10⁷ R1 cells at 1,400 V, 10 ms, and pulse number 3 using a Neon Transfection System (Life Technologies, Grand Island, NY, USA). After positive selection with G418 (Sigma, USA) for 1 week, integration of the transgenes in the genome of ESC was assessed by fluorescence with an FITC filter.

Neural differentiation

For differentiation, mouse embryonic fibroblasts (MEF) from ESC culture were removed using 0.1% gelatin coated 10 cm petri dishes. MEF free ESCs were cultured in aggregated EBs. Mouse ESCs were cultured in EB formation medium in the absence of LIF in a non-adhesive 10 cm petri dish for 4 days by suspension of 2~3×10⁶ MEF free ESCs. More directed differentiation of ESCs to the neuronal lineage occurred upon addition of 0.5 μM all-trans-retinoic acid (RA, Millipore, Billerica, MA, USA) to the culture for an additional 4 days. Neural induced EBs were subsequently transferred to poly-L-ornithine (10 mg/mL, Millipore, USA) and laminin (1 mg/mL, Millipore, USA) coated slides and cultured in EB formation medium for an additional 8 days. Medium was changed every two days.

Adipocyte differentiation

MEF free mouse ESCs were cultured in EB formation medium in a non-adhesive 10 cm petri dish for 2 days in the absence of LIF to help in formation of EBs. More directed differentiation of ESCs to the adipocyte lineage occurred upon the addition of 0.1 μM RA to the culture for an additional 3 days. Induced EBs were subsequently transferred to 0.1% gelatin coated wells and cultured in freshly prepared adipocyte differentiation medium including 20 nM 3, 3′, 5-triiodo-L-thyronine (T3) solution and 850 nM insulin solution (Millipore, USA) in the EB formation medium for an additional 21 days. Medium was changed every two days.

Immunofluorescence staining

Monolayer cultures of differentiated cells were rinsed with phosphate-buffered saline (PBS) and incubated for 10 min at room temperature in PBS (pH7.4) containing 4% paraformaldehyde. After each of three rinses with PBS and 5% normal donkey serum (Abcam, Cambridge, MA, USA) in PBS, the cells were blocked for 2 h at room temperature with PBS containing 5% donkey serum and 0.3% Triton X-100. The cells were then incubated with mouse NESTIN (1:200), mouse C/EBPβ (1:100) (Abcam, USA) primary antibodies in blocking solution at 4°C overnight. After each of three rinses with PBS and blocking solution, the cells were incubated in blocking solution for 30 min at room temperature. After incubation of the cells in blocking solution with donkey anti-mouse IgG Cy3 conjugated secondary antibody (1:500) (Millipore, USA) for 2 h at room temperature in the dark, the cells were washed with PBS. Finally, the cells were counterstained with 4′,6-diamidino-2-phenylindole (DAPI)(1 μg/mL) (Invitrogen, Waltham, MA, USA), and images were acquired using a LSM 510 META Confocal Laser Scanning Microscope.

Western blot analysis

The cells were harvested and lysed at 4°C in radioimmunoprecipitation assay buffer and 100× protease inhibitor cocktail (Thermo Scientific, Waltham, MA, USA) for 1h. The lysate was centrifuged at 14,000 rpm, 30 min at 4°C. The total protein concentration was quantified using a Pierce BCA protein assay kit (Thermo Scientific, USA). Cell extracts (30 μg/lane) were separated by 12% polyacrylamide by SDS-polyacrylamide gel electrophoresis and transferred to polyvinylidene difluoride (PVDF) membranes using an iBlot transfer device (Invitrogen, USA). After blocking with 5% skimmed milk in Tris-buffered saline containing 0.1% Tween 20 (TBST) for 1.5 h, the membranes were incubated with anti-hrGFP (1:1,000 dilution, Stratagene), anti-EGFP (1:1,000, Clontech), and anti-β actin (1:1,000, Cell Signaling) primary antibodies in TBST overnight at 4°C. After washing with TBST, the membranes were incubated with rabbit horseradish peroxidase-conjugated secondary antibody (1:1,000) and mouse horseradish peroxidase-conjugated secondary antibody (1:1,000) (cell signaling) for 90 min. Hybridization signals were detected using a Super Signal West Pico Chemiluminescent Substrate kit (Thermo Scientific, USA). Density of each protein was determined using the BIO-PROFIL Bio-1D ver.99.04 program. GFP expression was normalized by β-actin at each time point to monitor the NANOG promoters activity during differentiation into either adipocyte or neuronal lineages.

RESULTS

Establishment of green fluorescent protein positive transgenic cell line

To establish the transgenic cell lines of the GFP reporter system tracing expressions of bovine NANOG promoter (−420/+181), mouse NANOG promoter (−332/+50) and human NANOG promoter (−380/+24), linear forms of cytomegalovirus (CMV) promoterless recombinant plasmids were integrated into genomic DNA of mouse R1 ESCs by electroporation. After neomycin positive selection, each positive transgenic ESC line was verified for GFP expression by confocal fluorescence microscopy. Bovine NANOG-GFP transgenic mouse ESC line expressed a strong level of GFP. Similarly, strong levels of GFP expression were observed in mouse and human NANOG transgenic cell lines (Figure 1).

Down-regulation of bovine NANOG gene during neural differentiation for 16 days

To determine whether NANOG-GFP transgene expression was down-regulated in mouse ESC differentiation, we examined NANOG-GFP transgene expression during differentiation toward neurons for 16 days without MEF and LIF. Down-regulation of bovine NANOG was detected by both fluorescence microscopy (Figure 2A) and western blot analysis (Figure 2D). Neuron precursor marker NESTIN was detected from differentiated adhesive cells, but GFP expression was not detected on day 16. On day 8 after RA induction for neural differentiation, the level of GFP expression was decreased by approximately 66% compared with that of day 0. Significantly down-regulated expression of mouse and human NANOG as reflected by GFP expression was also observed on day 8 of neuron differentiation by fluorescence microscopy (Figures 2B and 2C) and Western blot (Figures 2E and 2F). Mouse and human NANOG-GFP expression was decreased by approximately 71% and 66%, respectively, on day 8 of neuron differentiation when compared with GFP expression on day 0. The expression patterns of the bovine NANOG during neural differentiation were similar to those of mouse and human NANOG (Table 1).

Down-regulation of bovine NANOG during adipocyte differentiation for 26 days

To determine whether NANOG-GFP transgene expression was down- regulated during differentiation of another cell type, GFP expression was monitored during the differentiation of adipocytes for 26 days without MEF and LIF. Down-regulation of bovine NANOG was detected by both fluorescence microscopy (Figure 3A) and Western blot analysis (Figure 3D). Adipocyte marker C/EBPβ was detected from differentiated adhesive cells, but GFP expression was undetectable on day 26. As expected, bovine NANOG expression was gradually reduced during adipogenesis. In particular, 53% decrease was observed on day 5 by induction of RA for 3 days on 2-day-old EB when compared with GFP expression on day 0. In addition, the expression of GFP driven by mouse and human NANOG promoter was decreased by approximately 53% and 58%, respectively, on day 5, compared with GFP expression on day 0, as observed by both fluorescence microscopy (Figures 3B and 3C) and western blot (Figures 3E and 3F). The expression levels of GFP driven by the bovine NANOG promoter were comparable with those of GFP expression by both human and murine NANOG promoters (Table 2).

DISCUSSION

The critical features of ESCs provide an invaluable tool for studies related to developmental regulation, functional of genomics, and generation of gene manipulated animals (Hatano et al., 2005). NANOG is a crucial transcriptional factor for maintenance of pluripotency in mammalian embryos and ESCs (Vallier et al., 2009). NANOG expression is restricted to pluripotent cells and is down-regulated upon differentiation (Mitsui et al., 2003). Therefore, understanding the action of NANOG expression in maintaining pluripotency and undifferentiated state of ESCs is important. Unfortunately, efforts made to establish validated bovine ESC lines have been fruitless (Chambers et al., 2003). Therefore, further improvements may be needed for studies on bovine development. In this study, we examined the function of bovine NANOG promoter using mouse ESCs. To determine whether the bovine NANOG gene promoter would have transcriptional activity in mouse ESCs, the construct of the 5′-flanking region of the bovine NANOG promoter (−420/+181) was transfected with hrGFP into mouse ESCs and neomycin resistant clones were selected and used for further study. Expression of bovine NANOG gene reflected by GFP expression in mouse ESCs was comparable with mouse and human NANOG gene promoter driven EGFP. This result demonstrates that the mechanism underlying transcriptional regulation might be conserved across the mammalian species. A previous study reported that the mouse and human NANOG promoter regions were capable of expressing GFP reporter construct in both mouse and human ESCs, and differentiation by chemical inducers such as retinoic acid decreased transcriptional activity of NANOG promoters. This phenomenon was also observed in another study, where treatment with 0.1 μM RA for 3 days resulted in drastically reduced EGFP tagged mouse NANOG protein expression (Rodda et al., 2005). In the same study, FACS analysis indicated that mouse NANOG expression was reduced by 50% at 4 days of EB formation, and treatment with 0.1 μM RA for 4 days resulted in 88% reduction of NANOG expression as well (Rodda et al., 2005).

As in previous studies (Kuroda et al., 2005; Rodda et al., 2005), in our study, GFP expression driven by mouse, human, and bovine NANOG promoters was down-regulated by 26%, 22%, and 19%, respectively, at 4 days of neural differentiation, and by 20%, 4%, and 10%, respectively, at 2 days of adipocyte differentiation. In addition, mouse, human, and bovine NANOG gene expression showed more rapid down-regulation at 4 days of 0.5 μM RA induced neural differentiation (46%, 45%, and 47%, respectively) and at 2 days of 0.1 μM RA induced adipocyte differentiation (34%, 54%, and 46%, respectively). Exposure of growing EBs to high-RA is known to markedly increase the rate of neural differentiation, whereas exposure to low-RA induces more mesodermal cells like adipocytes (Rohweddel et al., 1999). Higher concentrations of RA promote more rapid differentiation of ESCs, as indicated by the pattern of OCT4 expression, which was down-regulated more rapidly in EBs exposed to high-RA (Okada et al., 2004). In addition, OCT4 and NANOG expressions were directly repressed by GCNF during ESCs differentiation when induced by RA (Gu et al., 2005). GCNF, a transcriptional repressor, down-regulates target gene expression. Therefore, these results indicate that the mechanism for repression of NANOG expression in bovine NANOG during RA-induced differentiation may also be mediated by GCNF, however further study is warranted for investigation of molecular mechanisms for the conservation of transcriptional regulation of NANOG transcription during differentiation.

Conclusion

In this study, we report that the bovine NANOG 5′-flanking region from −420 to +181 has an essential role in NANOG transcriptional activity, which is important for maintenance of ESCs pluripotency. Therefore, this system has a potential impact on trace NANOG expression during bovine embryonic development and will contribute to establishment of bovine ESC lines in in vitro cultures using this construct as a potential marker for pluripotency.

Notes

CONFLICT OF INTEREST

We certify that there is no conflict of interest with any financial organization regarding the material discussed in the manuscript.

ACKNOWLEDGMENTS

This work was supported by a grant from the Next-Generation BioGreen 21 Program (Project No. PJ01109301), Rural Development Administration, Republic of Korea and Inha University Research Grant (INHA-50486).

Figure 1

GFP positive transgenic cell lines. Fixed cells were detected under FITC and DAPI filters using a confocal microscope. (A) Bovine NANOG promoter-GFP transgenic mouse ESCs. (B) Mouse NANOG promoter-GFP transgenic mouse ESCs. (C) Human NANOG promoter-GFP transgenic mouse ESCs. Original magnification ×200. (a) Bright field, (b) FITC, (c) DAPI, (d) merge FITC and DAPI.

Figure 2

Down-regulation of GFP expression from GFP positive transgenic cell lines during neuron differentiation. (A–C) Morphological change and GFP expression of differentiated EB cells from bovine, mouse, and human NANOG transgenic ESC lines for 16 days of neural differentiation (original magnification ×100), and immunofluorescence staining of differentiated adhesive cells on day 16 with NESTIN antibody (original magnification ×200). (a,c,e) Bright field, (b,d,f) DAPI, g: merge Rodamine and DAPI. (D–F) Each GFP expression was detected by western blot on day 0, 4, 8, and 16.

Figure 3

Down-regulation of GFP from GFP positive transgenic cell lines during adipocyte differentiation. (A–C) Morphological change and GFP expression of differentiated EB cells from bovine, mouse, and human NANOG transgenic ESC lines for 26 days of adipocyte differentiation (original magnification ×100), and immunofluorescence staining of differentiated adhesive cells on day 26 with C/EBPβ antibody (original magnification ×200). (D–F) Each GFP expression was detected by western blot on day 0, 2, 5, and 26.

Table 1

Densities of proteins during neuron differentiation

Neuron differentiation	Bovine NANOG/β-Actin*	Mouse NANOG/β-Actin*	Human NANOG/β-Actin*
Day 0	66.00 (100%)	56.86 (100%)	67.45 (100%)
Day 4	53.63 (81.26%)	42.21 (74.24%)	53.24 (78.93%)
Day 8	22.68 (34.37%)	16.25 (28.58%)	22.65 (33.58%)
Day 16	0 (0%)	0 (0%)	0.00 (0%)

^* Expression of GFPs were normalized β-actin level. Protein bands were quantified as described in Methods and Materials. After quantification, the values of GFP bands were divided by those of β-actin.

Table 2

Densities of proteins during adipocyte differentiation

Adipocyte differentiation	Bovine NANOG/β-Actin*	Mouse NANOG/β-Actin*	Human NANOG/β-Actin*
Day 0	59.53 (100%)	55.97 (100%)	67.79 (100%)
Day 4	53.69 (90.19%)	44.89 (80.20%)	65.14 (96.10%)
Day 8	27.80 (46.70%)	26.18 (46.77%)	28.57 (42.15%)
Day 16	0 (0%)	0 (0%)	0 (0%)

REFERENCES

Boyer LA, Lee TI, Cole MF, Johnstone SE, Levine SS, Zucker JP, Guenther MG, Kumar RM, Murray HL, Jenner RG, Gifford DK, Melton DA, Jaenisch R, Young RA. 2005. Core transcriptional regulatory circuitry in human embryonic stem. Cell 122:947–956.

Buehr M, Meek S, Blair K, Yang J, Ure J, Silva J, McLay R, Hall J, Ying QL, Smith A. 2008. Capture of authentic embryonic stem cells from rat blastocysts. Cell 135:1287–1298.

Chambers I, Colby D, Robertson M, Nichols J, Lee S, Tweedie S, Smith A. 2003. Functional expression cloning of Nanog, a pluripotency sustaining factor in embryonic stem cells. Cell 113:643–655.

Chickarmane V, Troein C, Nuber UA, Sauro HM, Peterson C. 2006. Transcriptional dynamics of the embryonic stem cell switch. PLoS Comput Biol 2:e123.

Doetschman T, Williams P, Maeda N. 1988. Establishment of hamster blastocyst-derived embryonic stem (ES) cells. Dev Biol 127:224–227.

Gerrard L, Zhao D, Clark AJ, Cui W. 2005. Stably transfected human embryonic stem cell clones express OCT4-specific green fluorescent protein and maintain self-renewal and pluripotency. Stem Cells 23:124–133.

Gu P, LeMenuet D, Chung ACK, Mancini M, Wheeler DA, Cooney AJ. 2005. Orphan nuclear receptor GCNF is required for the repression of pluripotency genes during retinoic acid-induced embryonic stem cell differentiation. Mol Cell Biol 25:8507–8019.

Hatano SY, Tada M, Kimura H, Yamaguchi S, Kono T, Nakano T, Suemori H, Nakatsuji N, Tada T. 2005. Pluripotential competence of cells associated with Nanog activity. Mech Dev 122:67–79.

Hatoya S, Torii R, Kondo Y, Okuno T, Kobayashi K, Wijewardana V, Kawate N, Tamada H, Sawada T, Kumagai D, Sugiura K, Inaba T. 2006. Isolation and characterization of embryonic stemlike cells from canine blastocysts. Mol Reprod Dev 73:298–305.

Hayes B, Fagerlie SR, Ramakrishnan A, Baran S, Harkey M, Graf L, Bar M, Bendoraite A, Tewari M, Torok-Storb B. 2008. Derivation, characterization, and in vitro differentiation of canine embryonic stem cells. Stem Cells 26:465–473.

Huang B, Li T, Alonso-Gonzalez L, Gorre R, Keatley S, Green A, Tumer P, Kallingappa PK, Verma V, Oback B. 2011. A virus-free poly-promoter vector induces pluripotency in quiescent bovine cells under chemically defined conditions of dual kinase inhibition. PLoS ONE 6:e24501.

Hamazaki T, Oka M, Yamanaka S, Terada N. 2004. Aggregation of embryonic stem cells induces Nanog repression and primitive endoderm differentiation. J Cell Sci 117:5681–5686.

Kim MJ, Habiba A, Doherty JM, Mills JC, Mercer RW, Huettner JE. 2009. Regulation of mouse embryonic stem cell neural differentiation by retinoic acid. Dev Biol 328:456–471.

Keefer CL, Baldassarre H, Keyston R, Wang B, Bhatia B, Bilodeau AS, Zhou JF, Leduc M, Downey BR, Lazaris A, Karatzas CN. 2001. Generation of dwarf goat (Capra hircus) clones following nuclear transfer with transfected and nontransfected fetal fibroblasts and in vitro-matured oocytes. Biol Reprod 64:849–856.

Kuroda T, Tada M, Kubota H, Kimura H, Hantano SY, Suemori H, Nakatsuji N, Tada T. 2005. Octamer and sox elements are required for transcriptional cis regulation of Nanog gene expression. Mol Cell Biol 25:2475–2485.

Li P, Tong C, Mehrian-Shai R, Jia L, Wu N, Yan Y, Maxson RE, Schulze EN, Song H, Hsieh CL, Pera MF, Ying QL. 2008. Germline competent embryonic stem cells derived from rat blastocysts. Cell 135:1299–1310.

Liu N, Lu M, Tian X, Han Z. 2007. Molecular mechanisms involved in self-renewal and pluripotency of embryonic stem cells. J Cell Physiol 211:279–286.

Mitsui K, Tokuzawa Y, Itoh H, Segawa K, Murakami M, Takahashi K, Maruyama M, Maeda M, Yamanaka S. 2003. The homeoprotein Nanog is required for maintenance of pluripotency in mouse epiblast and ES cells. Cell 113:631–642.

Pain B, Clark ME, Shen M, Nakazawa H, Sakurai M, Samarut J, Etches RJ. 1996. Long-term in vitro culture and characterization of avian embryonic stem cells with multiple morphogenetic potentialities. Development 122:2339–2348.

Okada Y, Shimazaki T, Sobue G, Okano H. 2004. Retinoic-acid-concentration-dependent acquisition of neural cell identity during in vitro differentiation of mouse embryonic stem cells. Dev Biol 275:124–142.

Okita K, Yamanaka S. 2006. Intracellular signaling pathways regulating pluripotency of embryonic stem cells. Curr Stem Cell Res Ther 1:103–111.

Rohweddel J, Guan K, Wobus AM. 1999. Induction of cellular differentiation by retinoic acid in vitro. Cells Tissues Organs 165:190–202.

Rodda DJ, Chew JL, Lim LH, Loh YH, Wang B, Ng HH, Robson P. 2005. Transcriptional Regulation of Nanog by OCT₄ and SOX₂. J Biol Chem 280:24731–24737.

Suda Y, Suzuki M, Ikawa Y, Aizawa S. 1987. Mouse embryonic stem cells exhibit indefinite proliferative potential. J Cell Physiol 133:197–201.

Sukoyan MA, Vatolin SY, Golubitsa AN, Zhelezova AI, Semenova LA, Serov OL. 1993. Embryonic stem cells derived from morulae, inner cell mass, and blastocysts of mink: Comparisons of their pluripotencies. Mol Reprod Dev 36:148–158.

Sumer H, Liu J, Malaver-Ortega LF, Lim ML, Khodadadi K, Verma PJ. 2011. Nanog is a key factor for induction of pluripotency in bovine adult fibroblasts. J Anim Sci 89:2708–2716.

Thomson JA, Kalishman J, Golos TG, Durning M, Harris CP, Becker RA, Heam JP. 1995. Isolation of a primate embryonic stem cell line. Proc Natl Acad Sci USA 92:7844–7848.

Thomson JA, Itskovitz-Eldor J, Shapiro SS, Waknitz MA, Swiergiel JJ, Marshall VS, Jones JM. 1998. Embryonic stem cell lines derived from human blastocysts. Science 282:1145–1147.

Vallier L, Mendjan S, Brown S, Chng Z, Teo A, Smithers LE, Trotter MW, Cho CH, Martinez A, Rugg-Gunn P, Brons G, Pedersen RA. 2009. Activin/Nodal signaling maintains pluripotency by controlling Nanog expression. Development 136:1339–1349.

Wobus AM, Boheler KR. 2005. Embryonic stem cells: Prospects for developmental biology and cell therapy. Physiol Rev 85:635–678.