EMBRYONIC STEM CELLS/INDUCED PLURIPOTENT STEM CELLS
1
Graduate School of Frontier Biosci‐ 2 ences, Department of Pathology, 5 Department of General Thoracic Sur‐ 6 gery, Medical School, Research Insti‐ tute for Microbial Diseases, Osaka University, 2‐2 Yamada‐oka, Suita, Osaka 565‐0871, Japan; 3Laboratory of 4 Molecular Embryology, Laboratory of Stem Cell Biology, Kitasato University School of Science, 1‐15‐1, Kitasato, Minami‐ku, Sagamihara, Kanagawa 252‐0373, Japan; 7Technology and Development Team for Mammalian Genome Dynamics, RIKEN BioResource Center, 3‐1‐1 Koyadai, Tsukuba City, 8 Ibaraki 305‐0074, Japan; Core Re‐ search for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), 7 Gobancho, Chiyoda‐ku, Tokyo 102‐0075, Japan. Correspondence: Tohru KIMURA, PhD, Laboratory of Molecular Embryology, Laboratory of Stem Cell Biology, De‐ partment of Biosciences, Kitasato Uni‐ versity School of Science, 1‐15‐1, Kitasato, Minami‐ku, Sagamihara, Kanagawa 252‐0373, Japan, Tel: +81‐ 042‐778‐8864; Fax: +81‐042‐778‐8864, E‐mail: tkimura@ kitasato‐u.ac.jp; Grant support: Supported in part by the Ministry of Education, Science, Sports and Culture, and by CREST of JST; The English in this document has been checked by at least two profes‐ sional editors, both native speakers of English Received August 30, 2013; accepted for publication August 18, 2014 ©AlphaMed Press 1066‐5099/2014/$30.00/0 This article has been accepted for pub‐ lication and undergone full peer review but has not been through the copyedit‐ ing, typesetting, pagination and proof‐ reading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/stem.1838
Pluripotent Stem Cells Derived From Mouse Primordial Germ Cells By Small Molecule Com‐ pounds TOHRU KIMURA1,2,3,4, YOSHIAKI KAGA1, YOICHI SEKITA2, KEITA FUJIKAWA3,4, TSUNETOSHI NAKATANI2, MIKA ODAMOTO3,4, SOICHIRO FUNAKI2,5, MASAHITO IKAWA6, KUNIYA ABE7, AND TORU NAKANO1,2,8 Key words. Primordial Germ Cells • Embryonic Germ Cells • Small Molecule Compund • induced Pluripotent Stem Cells • DNA Methylation • Ep‐ igenetic Memory ABSTRACT Primordial germ cells (PGCs) can give rise to pluripotent stem cells known as embryonic germ cells (EGCs) when cultured with basic fibroblast growth factor (bFGF), stem cell factor (SCF), and leukemia inhibitory factor (LIF). Somatic cells can give rise to induced pluripotent stem cells (iPSCs) by in‐ troduction of the reprogramming transcription factors Oct4, Sox2, and Klf4. The effects of Sox2 and Klf4 on somatic cell reprogramming can be repro‐ duced using the small molecule compounds, transforming growth factor‐β receptor (TGFβR) inhibitor and Kempaullone, respectively. Here we exam‐ ined the effects of TGFβR inhibitor and Kempaullone on EGC derivation from PGCs. Treatment of PGCs with TGFβR inhibitor and/or Kempaullone generated pluripotent stem cells under standard embryonic stem cell (ESC) culture conditions without bFGF and SCF, which we termed induced EGCs (iEGCs). The derivation efficiency of iEGCs was dependent on the differen‐ tiation stage and sex. DNA methylation levels of imprinted genes in iEGCs were reduced, with the exception of the H19 gene. The promoters of genes involved in germline development were generally hypomethylated in PGCs, but three germline genes showed comparable DNA methylation levels among iEGs, ESCs, and iPSCs. These results show that PGCs can be repro‐ grammed into pluripotent state using small molecule compounds, and that DNA methylation of these germline genes is not maintained in iEGCs. STEM CELLS 2014; 00:000–000
INTRODUCTION Primordial germ cells (PGCs) are embryonic germ cell precursors of all gametes [1, 2]. PGCs are regarded as STEM CELLS 2014;00:00‐00 www.StemCells.com
unipotent because they eventually differentiate into only sperm or oocytes, depending on the sex, in vivo. However, when cultured in the presence of basic fibro‐ blast growth factor (bFGF), stem cell factor (SCF), and ©AlphaMed Press 2014
The EG cells induced by small molecule compounds
2 leukemia inhibitory factor (LIF), PGCs can give rise to pluripotent stem cells known as embryonic germ cells (EGCs) [3, 4]. EGCs have developmental potency equiva‐ lent to embryonic stem cells (ESCs) derived from epiblast cells of blastocysts. EGCs form three germ layer tissues upon graft into nude mice, as well as contribute to the cells of the germ layers and germ cells upon transfer into blastocysts. In contrast to EGCs, PGCs do not exhibit such broad differentiation potency before culture [5, 6]. In addition, PGCs are originators of testic‐ ular teratomas [7]. These facts demonstrate that PGCs can de‐differentiate or can be reprogrammed into pluripotency under appropriate conditions. Somatic cells can be reprogrammed into pluripotent stem cells known as induced pluripotent stem cells (iPSCs) when the reprogramming transcription factors Oct4, Sox2, and Klf4 are introduced into cells [8]. Small molecule compounds, including histone deacetylase inhibitors, can augment the efficiency of iPSC produc‐ tion by these transcription factors [9]. In addition, it has been demonstrated that the effects of Sox2 and Klf4 can be reproduced by the small molecule compounds SB431542 [transforming growth factor‐β receptor (TGFβR) inhibitor] and Kempaullone [inhibitor of glyco‐ gen synthase kinase‐3 (GSK3) and cyclin‐dependent kinases (CDKs)] [10‐12], respectively. Furthermore, a recent report showed that iPSCs could be derived from mouse somatic cells by treatment with these small mol‐ ecule compounds alone, without introduction of Oct4, Sox2, or Klf4 [13]. Comprehensive genetic and epigenetic analyses of iPSCs revealed some characteristic features of iPSCs. One of the epigenetic features common to iPSCs is “epi‐ genetic memory”, which is characterized by mainte‐ nance of residual DNA methylation patterns of the orig‐ inal cells during early passages of iPSC clones [14‐16]. Therefore, there is a tendency for iPSCs to differentiate preferentially into lineage‐related original cell types, but not into other lineages, under differentiation condi‐ tions. Global DNA demethylation occurs during PGC differ‐ entiation [17‐22]. CpG methylation is decreased in short interspersed nuclear elements (SINEs) in migrating PGCs around embryonic day 8.5 (E8.5). In contrast, demethylation of germline genes, imprinted genes, and long interspersed nuclear elements (LINEs) commences in gonadal PGCs after E10.5‐E11.5. The differentially methylated regions (DMRs) of imprinted genes as well as the promoters of germline genes are completely demethylated until E13.5. Accumulating evidence has shown that the DNA methylation status of imprinted genes is to some extent maintained in EGC lines. For example, EGCs derived from E8.5 PGCs showed relatively higher, but heteroge‐ neous, methylation levels of the imprinted genes, whereas EGCs derived from gonadal PGCs showed low‐ er methylation levels [23‐26]. It has also been reported that repetitive elements, such as intracisternal A parti‐ cle (IAP) and minor satellite repeats, are www.StemCells.com
hypomethylated in EGCs [25]. Taken together, these data suggest that EGCs may be a good source for in vitro generation of germ cells, based on their “epigenet‐ ic memory”. However, it remains unknown whether the methylation status of germline genes is maintained dur‐ ing EGC derivation from PGCs. In this study, we first showed that PGCs could give rise to pluripotent stem cells, which we designated in‐ duced EGCs (iEGCs), using the Sox2 replacement SB431542 and the Klf4 replacement Kempaullone. iEGCs could be derived under standard ESC culture con‐ ditions using mouse embryonic fibroblasts (MEFs) and LIF, but not bFGF and SCF, which are essential growth factors for derivation of standard EGCs. In addition, we showed that the DNA methylation levels of some im‐ printed and germline genes in PGCs were not main‐ tained in cultured iEGCs.
MATERIALS AND METHODS
Animals Oct4‐EGFP transgenic mice were maintained in a mixed background of the DBA2 and C57BL/6 strains [27]. Oct4‐ EGFP transgenic mice were crossed to C57/BL6 mice (SLC, Shizuoka, Japan) to isolate two‐cell embryos, germ cells, and mouse embryonic fibroblasts (MEFs). Animal care was in accordance with the guidelines of Osaka University.
Derivation of induced embryonic germ cells (iEGCs) Germ cells from Oct4‐EGFP transgenic mice were seed‐ ed onto mitomycin C‐treated MEFs and cultured with Dulbecco’s Modified Eagle’s Medium (DMEM; Gibco, Gaithersburg, MD, USA) supplemented with 15% knockout serum replacement (Gibco), 1% non‐essential amino acids solution (NEAA) (Gibco), 1 mM sodium py‐ ruvate (Gibco), and 1,000 U/ml LIF. To induce iEGC lines, the media was supplemented with 25 μM of the Sox2 replacement SB431542 (Wako Pure Chemical, Osaka, Japan) and/or 5 μM of the Klf4 replacement Kempaullone (Wako Pure Chemical). Unlike standard EGC‐derivation culture, bFGF and SCF were not supple‐ mented in the iEGC‐derivation media. Oct4‐EGFP– positive iEGC colonies were picked 5–7 days after seed‐ ing and passaged into secondary culture. After second‐ ary cultures, the iEGCs were maintained on MEFs in ESC‐maintenance media without SB431542 and Kempaullone. ESC‐maintenance media was Glasgow Minimum Essential Medium (Sigma Aldrich, St. Louis, MO) supplemented with 10% fetal calf serum (FCS) (JRH, Lenexa, KS), 1% NEAA, 1 mM sodium pyruvate, 1 μM 2‐mercaptoethanol, and 1,000 U/ml LIF.
Derivation of embryonic stem cells (ESCs) ESC lines were established as described previously [28]. Briefly, Oct4‐EGFP transgenic two‐cell embryos were cultured to the blastocyst stage. Blastocysts were trans‐ ©AlphaMed Press 2014
The EG cells induced by small molecule compounds
3 ferred onto mitomycin C‐treated MEFs and cultured with ESC‐derivation media. ESC‐derivation media was DMEM supplemented with 10% knockout serum re‐ placement, 0.3% FCS, 1% NEAA, 1 mM sodium pyruvate, 1 μM 2‐mercaptoethanol, 1,000 U/ml LIF, and two in‐ hibitors (2i; 1 μM MEK inhibitor PD0325901 and 3 μM GSK3 inhibitor CHIR99021) (Stemgent, Cambridge, MA). Primary ESC colonies were picked and cultured on MEFs in the ESC‐maintenance media described above.
Derivation of induced pluripotent stem cells (iPSCs) iPSC lines were established from MEFs prepared from the E13.5 Oct4‐EGFP transgenic embryos as described previously [29]. After retroviral transduction of Oct4, Sox2, Klf4, and c‐Myc, iPSCs were derived using the ESC‐ maintenance media. iPSC colonies were picked 2–3 weeks after retrovirus transduction and were cultured on MEFs in the ESC‐maintenance media.
qRT‐PCR Total RNA was isolated using the RNeasy Mini Kit (Qiagen, Valencia, CA). Reverse transcription (RT) was performed using the Thermo Script RT‐PCR system (Invi‐ trogen, Carlsbad, CA). Quantitative polymerase chain reaction (qPCR) was performed with an ABI 7900HT‐3 (SDS 2.3) (Applied Biosystems, Foster City, CA). Expres‐ sion levels of each gene were normalized to expression of the housekeeping gene Rplp0 (Arbp: ribosomal pro‐ tein large P0). Primer sequences are listed in Supple‐ mentary Table S1.
Microarray Global gene expression profiles of the cell samples were analyzed by using Agilent SurePrint G3 8x60K microar‐ ray (Agilent Technologies). The microarray experiments were performed using biologically duplicated samples. Qualities of total RNA isolated by RNAeasy kit (Qiagen) were checked by Agilent BioAnalyzer, and the RNAs were labelled with Cy3‐CTP using a Low Input Quick Amp Labeling Kit, One Color (Agilent Technologies). Microarray hybridization was performed according to the protocol suggested by the supplier. After washing, the hybridized slides were scanned using an Agilent microarray scanner (Agilent Technologies), and numeric data of hybridization signals were obtained with the Feature Extraction software ver. 10.5.1.1 (Agilent Tech‐ nologies). The processed hybridization signal data were normalized and analysed by the Gene Spring GX11.5 software (Agilent Technologies).
Teratoma formation iEGCs, ESCs, and iPSCs (100,000 cells) were injected subcutaneously into nude mice (SLC). After 3–4 weeks, teratomas were fixed in 4% paraformaldehyde (PFA) in phosphate buffered saline (PBS), embedded in OCT compound, frozen, and sectioned at 7 µm using a CM3050 cryostat (Leica Microsystems, Wetzlar, Germa‐ www.StemCells.com
ny). Sections were subjected to histologic staining with hematoxylin and eosin, and observed under an AX80 microscope (Olympus, Tokyo, Japan).
Production of chimera mice The iEGCs were injected into the blastocysts isolated from ICR mice. The embryos were transferred into uter‐ us of the foster mothers. The chimeric embryos were analyzed at E13.5. The chimeras were also delivered by a Caesarean section.
Combined bisulfite restriction analysis (COBRA) Genomic DNAs were bisulfite‐treated using the EpiTecht Bisulfite Kit (Qiagen). Fully or semi‐nested PCR was per‐ formed to amplify the differentially methylated regions (DMRs) of imprinted genes. The first and second PCR rounds were performed using Advantage Taq (Clontech, Palo Alto, CA). PCR conditions were: a first round of 1 min at 94°C followed by 30 cycles of 30 s at 94°C, 30 s at 60°C, and 30 s at 68°C, and the second round of 2 min at 95°C followed by 15 cycles of 30 s at 95°C, 30 s at 50°C, and 30 s at 68°C. PCR amplification of germline gene promoters was conducted using Advantage Taq under the following conditions: 1 min at 94°C followed by 35 cycles of 30 sec at 94°C, 30 sec at 60°C, and 30 sec at 68°C. The second round of PCR was performed for the PGC samples as follows: 2 min at 95°C followed by 15 cycles of 30 sec at 95°C, 30 sec at 50°C, and 30 sec at 68°C. PCR products were purified using the QIAquick Gel Extraction Kit (Qiagen), digested with the methyla‐ tion‐sensitive restriction enzymes BstUI or Taq I (New England BioLabs, Ipswich, MA), and applied to 2% agarose gels. Band intensity of digested (methylated) and undigested (unmethylated) DNA fragments was quantified using the ImageJ software (NIH, Bethesda, MD). Sequences of the PCR primers and restriction en‐ zymes used are listed in Table S2.
RESULTS
Derivation of iEGCs from PGCs SB431542 and Kempaullone can reproduce the effects of Sox2 and Klf4, respectively, in terms of inducing iPSCs from somatic cells [10‐12]. To examine the effects of the small molecule compounds on iEGC derivation, gonadal PGCs from E11.5 Oct4‐GEFP embryos were seeded onto mitomycin C‐treated MEFs and cultured using standard ESC culture media supplemented with knockout serum replacement and LIF. Because bFGF and SCF are required for EGC derivation, no Oct4‐GEFP‐ positive colonies emerged under these culture condi‐ tions (Table 1). When the media were supplemented with Kempaullone, no Oct4‐GEFP‐positive colonies emerged. However, when the PGCs were cultured with SB431542, we detected a small number of Oct4‐GEFP‐ positive colonies 5–7 days after seeding. Furthermore, the number of Oct4‐GEFP‐positive colonies increased ©AlphaMed Press 2014
4 upon the simultaneous addition of SB431542 and Kempaullone (Table 1, Fig. 1A). The iEGC colonies gen‐ erated by SB431542 alone and the combination of SB431542 and Kempaullone could be passaged to sec‐ ondary cultures and maintained in ESC‐maintenance media (without compounds) after secondary culture. In contrast to E11.5 PGCs, male PGCs from E13.5 embryonic testes gave rise to iEGC lines in response to Kempaullone alone (Table 1, Fig. 1B). In addition, the combination of SB431542 and Kempaullone decreased the efficiency of iEGC derivation from E13.5 male PGCs (p