Generation of genetically modified rats from embryonic stem cells

Communicated by Takashi Sugimura, National Cancer Center, Tokyo, Japan, June 30, 2010 (received for review April 22, 2010)
July 26, 2010
107 (32) 14223-14228

Abstract

At present, genetically modified rats have not been generated from ES cells because stable ES cells and a suitable injection method are not available. To monitor the pluripotency of rat ES cells, we generated Oct4-Venus transgenic (Tg) rats via a conventional method, in which Venus is expressed by the Oct4 promoter/enhancer. This monitoring system enabled us to define a significant condition of culture to establish authentic rat ES cells based on a combination of 20% FBS and cell signaling inhibitors for Rho-associated kinase, mitogen-activated protein kinase, TGF-β, and glycogen synthase kinase-3. The rat ES cells expressed ES cell markers such as Oct4, Nanog, Sox2, and Rex1 and retained a normal karyotype. Embryoid bodies and teratomas were also produced from the rat ES cells. All six ES cell lines derived from three different rat strains successfully achieved germline transmission, which strongly depended on the presence of the inhibitors during the injection process. Most importantly, high-quality Tg rats possessing a correct transgene expression pattern were successfully generated via the selection of gene-manipulated ES cell clones through germline transmission. Our rat ES cells should be sufficiently able to receive gene targeting as well as Tg manipulation, thus providing valuable animal models for the study of human diseases.
The laboratory rat was the earliest mammalian species domesticated for scientific research and has been used as an animal model in physiology, toxicology, nutrition, behavior, immunology, and neoplasia for over 150 y (1). Despite this history, rats lag far behind mice in functional genetic studies and the generation of knockout animal models reflecting human diseases because of the absence of germline-competent rat ES cells, which are vital in a reverse genetics approach (2, 3). Recently, gene-targeting rats were created by the zinc finger nuclease strategy (4). However, the system is not available for most researchers because a special technique is required to make algorithm-based sequence-specific DNA nucleases. Thus, establishment of rat ES cells has been desired to produce gene-targeting rats, such as mutant mice, routinely.
Although we established rat ES cell lines with chimeric contribution, none could complete germline transmission (5). Soon after our report, other groups succeeded in establishing rat ES cells with germline transmission by using 2i, mitogen-activated protein kinase (MEK) inhibitor PD0325901, and glycogen synthase kinase-3 (GSK3) inhibitor CHIR99021 (6, 7). The 2i is widely used in the establishment of ES cells or induced pluripotent stem (iPS) cells in mice (8, 9), rats (6, 7, 10), and humans (10). Thus, the inhibition of MEK and GSK3 has been thought to maintain a ground state of pluripotency in various species. Rat iPS cells with chimeric contribution were established by using an inhibitor of type 1 TGF-β receptor Alk5 (A-83-01) with the 2i, although germline transmission was not accomplished (10). Furthermore, a combination of MEK and ALK5 inhibitors dramatically improved the efficiency of iPS cell generation from human fibroblasts (11). These reports indicate that the inhibition of TGF-β signaling also plays a key role in pluripotency.
It is known that rat ES cells present critical problems in that undifferentiated cells cannot proliferate from single cells after enzymatic dissociation (5) and that chromosomal instability is caused by long-term culture, resulting in the failure of germline transmission (57). Recently, Watanabe et al. found that a Rho-associated kinase inhibitor Y-27632 (12) blocks apoptosis and enhances the proliferation of human ES cells after their dissociation into single cells by enzymatic treatment (13). The propagated ES cells cultured by Y-27632 were positive for alkaline phosphatase (ALP) and marker genes such as E-cadherin, Oct4, and SSEA4, and the number of chromosomes was normally maintained during long-term culture (13). These recent reports indicate the suitability of cell signaling inhibitors in the establishment of rat ES cells.
To generate genetically modified rats, highly potent ES cells that can stably contribute to germline chimeras have to be established. As a first step, we generated Oct4-Venus transgenic (Tg) rats, in which Venus [YFP mutant (14)] is expressed by the Oct4 promoter/enhancer. This Tg line enables us to monitor the pluripotency of rat ES cells during the process of establishment. We addressed suitable combinations of the signaling inhibitors based on a culture medium that included 20% FBS. As a result, the use of a combination of four inhibitors, Y-27632, PD0325901, A-83-01, and CHIR99021 (termed YPAC), allowed the establishment of authentic rat ES cells and appeared necessary in the blastocyst injection process for the generation of germline chimeras. Finally, we report that high-quality Tg rats retaining reproductive ability can be generated from rat ES cells.

Results

YPAC Maintains Pluripotency in the Outgrowths of Oct4-Venus Tg Blastocysts.

We first generated a Tg rat carrying an Oct4-Venus fluorescence reporter to monitor pluripotency during establishment of rat ES cells and to investigate development of the ES cells into germ cells in fetal gonads of chimeras. The 3.9-kb Oct4 (also known as Pou5f1) promoter includes both the proximal enhancer and distal enhancer, which gives Oct4 expression in morula, inner cell mass (ICM), epiblast, primordial germ cells (PGCs), and ES cells (15). In the Tg embryo, Venus was detected specifically in PGCs in the gonad (Fig. S1). This result corresponds to previous reports regarding Oct4-reporter Tg mice (16).
Outgrowths were examined from the Tg blastocysts in a basic medium containing 20% FBS, which is generally used for mouse ES cell culture, with or without YPAC. In its absence, Venus fluorescence was decreased at day 3 after plating and disappeared at day 7 despite the fact that ES-like cells propagated and formed a domed structure similar to the mouse ES cell colony (Fig. 1A). In the presence of YPAC, ICM cells rapidly propagated while maintaining Venus fluorescence even at day 7. The fluorescence was not observed in differentiated cells (Fig. 1B). The expression levels of ES cell marker genes Oct4, Nanog, Sox2, and Rex1 in ICM cells with YPAC were higher than those without YPAC (Fig. 1C). In its absence, the decrease of Oct4 mRNA was parallel to that of Venus mRNA and fluorescence. In the YPAC condition, blastocyst outgrowth was observed in 51 samples for all the tested embryos regardless of the strains (Table 1). The blastocyst strains were derived from a hybrid of Tg Wistar and wild-type Wistar (TgWW, albino), wild-type Wistar (WW, albino), Long–Evans agouti [LEA (LL, agouti)], or a hybrid of Tg Wistar and LEA (TgWL, agouti).
Fig. 1.
Outgrowth of ICM cells in YPAC medium. Outgrowth of blastocysts in −YPAC (A) or +YPAC (B) medium. E4.5 blastocysts were plated onto mitotically inactivated MEFs. (C) qPCR analysis of Venus, Oct4, Nanog, Sox2, and Rex1 in ICM cells. Seven days after plating, RNAs were extracted from domed segments of ICM cells derived from seven or four blastocysts in −YPAC or +YPAC medium, respectively. Transcript levels were normalized to Gapdh levels. Data are the mean ± SD of one biological sample assayed in three independent experiments and represent relative expression levels of indicated genes in REFs, ICM (−YPAC), and ICM (+YPAC). (Scale bars: 100 μm.)
Table 1.
Establishment of rat ES cells from blastocysts in YPAC medium
StrainNo. ICMsOutgrowthContinueCell line
TgWL2222
TgWW151511
WW9911
LL191922
TgWW*3322
WW*3300
Total5151 (100%)88 (100%)
*Specific serum was used (FBS for MEF culture; EQUITECH-BIO, Inc.).
Outgrowth refers to the expansion of the ICM.
Cell line refers to continuous culture of at least seven passages. Single-cell passage was begun at passages 1–3. Domed colonies with undifferentiated cells are continuously formed from single cells.

Small Molecules Enable Efficient Derivation and Maintenance of Rat ES Cells.

The outgrowths were dissociated into small pieces and replated in the same mouse embryonic fibroblasts (MEFs)/YPAC condition. After undifferentiated colonies appeared, they were split into single cells by Accutase (Innovative Cell Technologies, Inc.). These cells attached on the MEFs and formed domed colonies, which can be passaged continuously (Fig. 2A Upper Left). Although most of the ES cells showed ALP activity (Fig. 2B Left) and Oct4 protein expression (Fig. 2D Left) even after long passages, Venus fluorescence became weak or negative (Fig. 2A Lower Left). The expression pattern of Venus mRNA was not parallel to that of Oct4 between TgWL1 and TgWW1 cell lines (Fig. 2C). These results suggest that the function of the Oct4-Venus transgene is unavailable in rat ES cell lines. The long-passaged rat ES cells might receive epigenetic silencing effects.
Fig. 2.
Characterization of rat ES cells. Effect of Y-27632 (A) and ALP (B) staining. Dissociated single cells (1 × 105 TgWW1, passage 6) were plated into a well of six-well plates under the condition of MEFs with YPAC (Left), PCA (Center), or Y (Right). (B) ALP staining in these cells. (C) qPCR analysis of Venus, Oct4, Nanog, Sox2, and Rex1 in rat ES cell lines. Transcript levels were normalized to Gapdh levels. Data are the mean ± SD of one biological sample assayed in three independent experiments and represent the relative expression levels of indicated genes in REF, TgWL1, TgWW1, and LL1. (D) Immunofluorescence staining for Oct4, Nanog, and Sox2 in rat ES cells. (E) Cytogenetic analysis in rat ES cells by G-band staining. Representative data of TgWL2 at passage 7 indicate a chromosomal number of 42, including an XX gender chromosome. (F) Histological sections of a teratoma derived from a TgWW1 ES cell line showing three germ layers. Embryoid bodies (TgWL1) were produced in a basic ES cell medium with (H) or without (G) three PAC inhibitors, excluding Y-27632. A time-course experiment was performed, and the EBs were observed at days 3 and 7. (I) qPCR analysis of Venus, Oct4, Nanog, Sox2, and Rex1 in EBs. Transcript levels were normalized to Gapdh levels. Data are the mean ± SD of one biological sample assayed in three independent experiments and represent the relative expression levels of indicated genes in EBs produced without inhibitors (○) or with PAC (▪) at days 0, 3, and 7. (Scale bars: 100 μm.)
The ES cell lines maintained higher mRNA levels of ES cell marker genes Oct4, Sox2, Nanog, and Rex1 compared with rat embryonic fibroblasts (REFs) (Fig. 2C). Microarray analyses also indicated that global gene expression was remarkably different between ES cells and REFs but similar between the three ES cell lines TgWL1, TgWW1, and LL1 (Fig. S2). Nanog and Sox2 proteins were also detected in ES cells (Fig. 2D). The karyotypes of 50 cells were analyzed by G-band staining. Most of the cells exhibited a normal chromosomal number of 42 in TgWL1 (70%, XX, P14), TgWL2 (84%, XX, P7; Fig. 2E), TgWW1 (92%, XX, P5), and LL1 (84%, XX, P6). TgWW1 cells (2.6 × 106) could form a teratoma 34 d after transplantation under the skin of an immunodeficient SCID mouse. A histological examination showed that the tumor contained all three germ layers, including the intestinal epithelium (endoderm), cartilage (mesoderm), and neuronal rosette (ectoderm) (Fig. 2F).
To confirm the effect of Y, the rat ES cells were cultured in a PAC medium. Under this condition, sparse colonies appeared because of a failure in the adherence process of single cells on MEFs, although the colonies kept undifferentiated morphology and ALP activity (Fig. 2 A Middle and B Middle). Although only Y enabled most of the single cells to adhere on MEFs and to proliferate, they were differentiated and did not show ALP activity (Fig. 2 A Right and B Right).
The classic method to induce ES cell differentiation is to allow the cells to grow in suspension and to form 3D aggregates known as embryoid bodies (EBs) (17). Dissociated ES cells were plated into low-cell-binding dishes in the basal medium. EBs could be formed from the ES cells at a much lower efficiency compared with their formation from mouse ES cells (Fig. 2E). The expression of marker genes was decreased during the process of EB differentiation (Fig. 2I). In the presence of PAC, cells aggregated with high efficiency and formed a clear 3D structure (Fig. 2F). The EBs with PAC at day 7 still sustained high expression levels of the marker genes (Fig. 2I). These results suggest that PAC enables ES cells to maintain pluripotency, whereas for rat ES cells to adhere on MEFs, Y is necessary.

YPAC Injection Engenders Germline Chimeras.

First, we produced stable transfectant ES cells expressing cyan fluorescence from a CAG-AmCyan1 transgene to monitor cell fate in the blastocyst after injection or chimerism in fetuses (Fig. 3A). Before generation of the chimera, the potential of YPAC was investigated during the injection and blastocyst incubation processes because the rat ES cells tended to differentiate easily in the absence of inhibitors (Fig. 2 A, B, and E). There was no difference between normal and YPAC injection 5 h after incubation; in both cases, several cyan-positive cells adhered on the ICM and trophectoderm. However, 30 h after incubation, few cyan-positive cells existed in the blastocysts in the absence of YPAC, whereas in its presence, several cells remained on the ICM surface. Furthermore, blastocyst shape was maintained by the addition of YPAC even after incubation for 30 h (Fig. 3B). This result suggests that administration of YPAC during the injection process causes both ES cells and recipient blastocysts to block differentiation or apoptosis. This YPAC injection method enabled generation of chimeric embryos showing positivity for cyan but negativity for Venus in the surface of skin and kidney. Venus-positive cells were detected specifically in the gonads, showing the successful development of the ES cells into germ cells (Fig. 3C). We also succeeded in generating germline chimeras using all other cell lines by detecting Venus fluorescence in the fetal gonad (Table 2). The germline chimeras were detected in 2 of 12 fetuses by using long-cultured TgWL2 cells at passage 22 (Table 2 and Fig. S3).
Fig. 3.
Generation of germline chimeras by YPAC injection method. (A) Expression of AmCyan1 in stable transfectant clones (TgWW1 + C) generated by nucleofection with CAG-AmCyan1 plasmids. (B) Effect of YPAC during injection process. The basic ES cell medium with (Right) or without (Left) YPAC was used during the processes: injection of TgWW1 + C cells into blastocysts and incubation of the blastocysts for 30 h. (C) Generation of germline chimeras in embryos. TgWW1 + C cells were injected into Wistar blastocysts. Venus or AmCyan1 fluorescence was observed in E18.0 whole embryo, kidney, and testis. (D and E) Generation of germline chimeras by single-cell injection. (D) A single cell (TgWW1) was injected into a blastocyst. The image shows the injected blastocyst 3 h after incubation. The arrowhead indicates the injected single cell. (E) Venus-positive germ cells were detected in gonad at E16.0 (arrow). (F) Generation of coat-color chimeras by the YPAC injection. (G) Germline transmission in adult chimeras. The chimera (TgWL1) was mated with a Wistar male rat. Germline pups (4 of 16) were confirmed by an agouti coat color. (H) Genotyping analysis of F1 offspring of female chimera (TgWL2). The Venus region was amplified by PCR from genomic DNA of tail. M, 100-bp DNA marker; 1, 2, and 3, germline offspring with an agouti coat color; 4, 5, and 6, coat color-negative (albino) offspring. (Scale bars, A, B, and D: 100 μm; C and E: 300 μm.)
Table 2.
Summary of germline chimeras: Germ cell development in fetal gonad judged by Venus fluorescence
Cell line (gender)Passage no.Host blastocystInjected embryosFetal no.Germline chimera
TgWL2 (XX)6LEA4391M1F
22Wistar1331F
 Wistar/LEA2391F
TgWW1 (XX)6, 7Wistar5392M7F
TgWW1 + C (XX)8Wistar4691M1M or F
TgWW1s (XX)9Wistar3581F
TgWW2* (XX)8Wistar2811M
F, female; M, male.
*ES cell line established by using specific serum. TgWW1 + C refers to a stable transfectant possessing CAG-AmCyan1 transgene.
To investigate the pluripotent ability of ES cells, we carried out a single-cell injection into a blastocyst. After injection of the TgWW1 cell at passage 9, the single cell attached to the internal surface of the blastocyst (Fig. 3D). In an embryo day (E) 16.0 fetus gonad, germ cell differentiation was confirmed by the detection of Venus fluorescence (Fig. 3E).
To generate coat-color chimeras, TgWL1 cells were injected into Wistar blastocysts using the YPAC injection method. Eight of 23 coat-color chimeras were obtained from the TgWL1 cell line at passage 11 or 12 (Fig. 3F Right and Table 3). Without the YPAC injection method, a coat-color chimera was hardly generated despite the fact that the same cell line, TgWL1, was used at earlier passages 6–8 (Fig. 3F Left and Table 3). Only 1 male chimera of 44 pups was obtained, but the chimerism was very sparse (Fig. S4). The generation of coat-color chimeras was successful in all six cell lines (Table 3). Those from cell line TgWW1 or LL1 are shown in Fig. S5. After mating with male rats, germline transmission was accomplished in adult female chimeras derived from all six cell lines independent of rat strains (Fig. 3G and Table 3). Genotyping analysis indicated that the Oct4-Venus transgene of the ES cells (TgWL2) was transmitted to filial (F)1 germline offspring with an agouti coat color (Fig. 3H).
Table 3.
Summary of germline chimeras: Chimeras and germline transmission judged by coat color of F1 pups
Cell line (gender)Passage no.Host blastocystInjected embryosPup no.Chimera no.Mating no.Germline chimera
−YPAC injection
 TgWL1 (XX)6–8Wistar226441M*0
+YPAC injection
 TgWL1 (XX)11, 12Wistar123233M5F1M3F1F
 TgWL2 (XX)4, 6Wistar70102M3F1M3F2F
 TgWW1 (XX)9Wistar/LEA79195M3F3F1F
 WW1 (XX)10LEA2772M1F1F1F
 LL1 (XX)4, 6Wistar107133F2F1F
 LL2 (XX)9Wistar5263F3F2F
F, female; M, male.
*Coat-color contribution is sparse (Fig. S5).

Generation of ES Cell-Derived Tg Rats.

We proposed to generate ES cell-derived transgenic (esTg) rats harboring the Oct4-Venus transgene, which shows a correct Venus expression pattern similar to Oct4 protein (Fig. 2D). After introduction of the Oct4-Venus transgene containing the same Oct4 promoter/enhancer region as used in the generation of the conventional transgenic (cvTg) rats, 15 Venus-positive colonies (LL2 line) were picked up. After two passages, silencing of Venus gene expression occurred in 13 of 15 clones, resulting in an apparent heterogeneity in the fluorescence of Venus-positive clones (Fig. 4A, arrowheads), whereas only 2 clones kept homogenous Venus fluorescence (Fig. 4B). Chimeric rats were produced via injection of the stable clone into Wistar blastocysts. Furthermore, germline transmission with the Oct4-Venus transgene was accomplished in the female chimeras (Fig. 4C and Table 3). The esTg embryos at 16.0 days postcoitum (dpc) exhibited Venus fluorescence in the gonads (Fig. 4D). The esTg rats were able to mature to adults without apparent abnormalities and had normal reproductive ability. We established ES cell lines from esTg blastocysts to confirm an expression pattern of Venus fluorescence. During outgrowth, their Venus expression pattern (Fig. 4E) was similar to that of cvTg blastocysts (Fig. 1B). However, long-passaged esTg cell lines (n = 3) maintained stable Venus expression in the ES cells (Fig. 4F). This result indicates our success in generating high-quality esTg rats possessing a correct expression pattern of Venus under the Oct4 promoter/enhancer.
Fig. 4.
Generation of Tg rats from ES cells. (A and B) Cloning and expansion of Oct4-Venus transfectants. An Oct4-Venus transgene was introduced into ES cells (LL2) at passage 5. Venus-positive clones were passaged without drug selection. (A) Arrowheads indicate ES cells with Venus expression silenced. (B) ES cells with homogeneous expression of Oct4-Venus. (C) Generation of Tg rats from ES cell clone displaying homogeneous expression of Oct4-Venus. An arrow indicates esTg rats through germline transmission from the chimeric rat. (D) Venus fluorescence in gonads of an esTg embryo at 16.0 days postcoitum (dpc). (E) Outgrowth of esTg blastocyst in YPAC medium. (F) Rat ES cell line derived from an esTg blastocyst. The expression of Oct4-Venus did not receive a silencing effect even after 10 passages. (Scale bars, A, B, and D: 300 μm; E and F: 100 μm.)

Discussion

Our results demonstrated that the use of a combination of serum and cell signaling inhibitors during outgrowth, cell culture, and blastocyst injection leads to the generation of germline chimeras with extremely high efficiency. Furthermore, we generated genetically modified rats from ES cells, termed esTg rats, growing up healthily and retaining reproductive ability. The advantage of this technology of using rat ES cells is that we can select Tg ES cell clones that possess a correct gene expression pattern of the transgene. Our Oct4-Venus esTg rats will be useful for the generation of iPS cells as a pluripotency monitoring system with respect to previous work in mice (9, 18, 19). For further study, addressing the mechanism of the silencing effect on the transgenes should be crucial.
The complete generation of esTg rats might be based on the use of a culture medium containing 20% serum and YPAC, which might provide strong protectection from cell damage during gene introduction with electrical stimuli and maintain pluripotency with a stable karyotype during the cloning and expansion process. To support viability, serum was temporally used in a previous report when rat ES cells were electroporated and cultured overnight in a 2i medium (6). Such efforts are not necessary with our rat ES cells, which are tolerant to the damage induced by gene introduction because of the presence of 20% serum in the YPAC medium. Furthermore, we have confirmed that drug selection through the use of G418 is efficient in rat ES cells for generating genetically modified rats.
Previous work has suggested that the failure in the establishment of authentic rat ES cells over the past 2 decades was attributable to the presence of serum (6, 7). Indeed, serum may contain various kinds of nutrient factors as well as differentiation factors for rat ES cells (20). The reason why we succeeded in the establishment of such significant pluripotent cell lines might be attributable not only to the signaling inhibitors shielding ES cells from differentiation but to the use of the nutrients in the serum. Monitoring serum quality for better ES cell culturing is extremely important. Nevertheless, our combination of YPAC and different serum, which is used for culturing MEFs, allowed stable establishment of rat ES cell lines. Our success is thus partly attributable to the strong potential of YPAC in the maintenance of ES cells regardless of differentiation factors under different culture conditions.
It is noteworthy that leukemia inhibitory factor (LIF) was not necessary in our culture medium, although recent reports have suggested that its addition improved rat ES or rat iPS cell ability to suppress differentiation (6, 7, 10). It has been shown that LIF is the key cytokine secreted by feeders in supporting mouse ES cell self-renewal (21, 22) and that LIF was able to replace the requirement for feeders in propagation (23, 24). Ying et al. (8) demonstrated that a combination of PD0325901 and CHIR99021 enabled mouse ES cells to maintain pluripotency by substituting LIF, feeders, and serum. Considering these reports, the addition of LIF in our culture condition might be dispensable because of the inclusion of serum, MEFs, and the two inhibitors. Actually, the expression level of Tbx3, which is involved in mediating LIF signaling (25), was up-regulated in the rat ES cells. Moreover, we found that the expression of a suppressor of cytokine signaling 3 (SOCS3), which is one of the STAT3’s direct target genes (26), was up-regulated after stimulation with rat LIF (27) even in the YPAC medium. Thus, it seems possible to improve the culture condition further by the administration of rat LIF.
The six established ES cell lines in this work were all female. This result does not correspond to mice, because most of the mouse ES cell lines are male. In our present study, we continued to culture rat ES cell lines exhibiting rapid cell proliferation, resulting in the establishment of six female lines. Thus, we speculate that female blastocysts are suitable for the establishment of rat ES cells or that the addition of MEK and GSK3 inhibitors to the culture medium facilitates female-specific rapid cell growth in rat ES cells. A previous study using MEK and GSK3 inhibitors also reported that six of seven rat ES cell lines were female (6).
Although two groups reported the establishment of authentic rat ES cells, only one of several cell lines accomplished germline transmission in each group (6, 7). So far, there is no report of successful generation of knockout/knockin rats from ES cells. Thus, trials to produce more potent cell lines and to find the optimal combination of rat strains for donor ES cells, host blastocysts, and recipient foster female animals remain to be addressed (6, 7). In this study, our YPAC culture and injection method overcame the difficulty of completing germline transmission in all six ES cell lines independent of rat strains. The YPAC condition will enable the selection of preferable rat strains for the generation of genetically modified rats from ES cells, bringing great advantages to research for strain-specific disease models. We believe that the availability of our rat ES cells and the YPAC injection technique will also open up a valuable platform for routinely generating knockout/knockin rats, holding out the promise for generation of previously undescribed disease models.

Materials and Methods

Media, Feeder, Animals, and Primers.

YPAC medium was prepared by the addition of the four respective inhibitors [10 μM Y-27632 (WAKO), 1 μM PD0325901 (Axon Medchem), 0.5 μM A-83-01 (TOCRIS), and 3 μM CHIR99021 (Axon Medchem)] to basic medium. The basic medium is composed of DMEM [including 110 mg/L sodium pyruvate and 200 mM GlutaMAX (GIBCO)], 20% (vol/vol) FBS (ES Cell Qualified FBS, Lot No. 1204059; GIBCO), 0.1 mM 2-mercaptoethanol (SIGMA), 1% nonessential amino acid stock (GIBCO), and 1× antibiotic antimycotic (GIBCO). Mitomycin C-treated MEFs resistant to neomycin (Millipore) were used as feeders and maintained in DMEM/10% (vol/vol) FBS (Lot No. SFB30-1502; EQUITECH-BIO, Inc.) medium with 1× antibiotic antimycotic. Animal experiments were performed in compliance with the guidelines of the Institute for Laboratory Animal Research, National Cancer Center Research Institute. The Wistar strain, LEA strain, or a hybrid of the Wistar and LEA strain was used in this work. All the primer sequences are listed in Table S1.

Generation of Oct4-Venus Tg Rats via a Conventional Method.

The DNA fragment of the Oct4 promoter region (3.9 kb) was obtained by PCR using KOD Version 2 DNA polymerase (Toyobo) from Wistar rat genomic DNA and was inserted into a pCS2-Venus plasmid (14). The Oct4 promoter-Venus (Oct4-Venus) DNA fragment was injected into pronuclei of fertilized eggs in a Wistar rat strain (Oriental Yeast Co., Ltd.). Six Tg-positive founders were obtained from 222 injected fertilized eggs.

Establishment of Rat ES Cells from Blastocysts.

Rat blastocysts were gently flushed out from the uteri of E4.5- or E5.0-timed pregnant rats with basic ES medium. After removal of the zona with acid Tyrode's solution (Ark Resource Co., Ltd.), whole blastocysts were plated onto six-well plates and cultured on MEFs in the basic ES medium with or without YPAC. After around 7 d, the blastocyst outgrowths were cut into pieces and replated under the same YPAC conditions. Emerging ES cell colonies were then dissociated using Accutase and were expanded. Established ES cell lines were routinely maintained under MEF-YPAC conditions and passaged every 3–4 d. Floated colonies were also passaged. Cells were cryopreserved and recovered by conventional procedures using YPAC medium and DMSO as a cryoprotectant. In the cell line of TgWL1 or TgWL2, 1,000 U/mL rat LIF (25) was added to the YPAC medium until passage 4 or 3, respectively.

Quantitative PCR Analysis.

Total RNA was isolated using ISOGEN (Nippongene). cDNA was synthesized with 2 μg of the total RNA using Super Script III RT (Invitrogen) and oligo-dT primer (Invitrogen). cDNAs were used for PCR utilizing Platinum SYBR Green qPCR SuperMix UDG (Invitrogen). Optimization of the quantitative (q)RT-PCR was performed according to the manufacturer's instructions (PE Applied Biosystems). All quantitations were normalized to an endogenous control GAPDH.

ALP and Immunofluorescent Staining.

Cells were fixed in 4% (wt/vol) paraformaldehyde. ALP staining was performed with Vector Blue substrate (Vector Labs) according to the manufacturer's instructions. Primary antibodies used include the following: Oct4 (C-10, 1:20; Santa Cruz), Nanog (1:20; ReproCell), and Sox2 (1:20; BioLegend). Alexa Fluor fluorescent secondary antibodies (Invitrogen) were used at a 1:500 dilution. Nuclei were visualized with DAPI staining.

Teratoma Formation.

The 2.6 × 106 TgWW1 cells (passage 5) were injected under the skin of immunodeficient mice. Teratomas were obtained 34 d after the injection. They were embedded in paraffin wax and stained with H&E.

EB Formation.

After ES cells were split into single cells using Accutase, they were cultured in a basal ES medium with or without three-inhibitor PAC, excluding Y-27632, on a low-cell-binding dish (NUNC). RNAs were extracted from the EBs at day 3 or 7, followed by qPCR examination.

Blastocyst Injection.

The blastocysts from E4.5-timed pregnant rats were placed into 500 μL of injection medium composed of YPAC (or PAC) and basal ES cell medium without antibiotic antimycotic, and they were then incubated for 2–3 h. The well-expanded blastocysts were used for microinjection. For ES cell preparation, 10–20 domed or floated colonies were picked up by hand-made capillary and treated with an Accutase droplet for 5 min, followed by being split into single cells in a droplet of injection medium. The cells were transferred in 500 μL of the injection medium and incubated for 30–60 min at room temperature. After centrifugation, ES cells were transferred into a droplet of the injection medium under mineral oil (SIGMA). Ten to 15 ES cells were injected into each blastocyst and incubated at 37 °C for 3–5 h in the injection medium to allow the recovery of embryos. Ten to 20 embryos were then transferred into the uterine horn of each E3.5-timed pseudopregnant female rat. Chimeric rats were identified by coat color. Germline transmission was confirmed by the F1 rat coat color resulting from mating of chimera or Oct4-Venus fluorescence in germ cells in the fetal gonad. Genotyping of animals was carried out by PCR on tail DNA.

Gene Transfection of Rat ES Cells.

For nucleofection, 5 μg of CAG-AmCyan1 or 10 μg of Oct4-Venus plasmid linearized by SalI was transfected into 3.2 × 106 TgWW1 or 3 × 106 LL2 rat ES cells, respectively, using a Mouse ES Cell Nucleofector Kit (Amaxa, Inc.). The cells were plated on MEFs in the YPAC medium with 2% (vol/vol) matrigel (BD Biosciences). Three combined colonies of CAG-AmCyan1 or a single colony of Oct4-Venus transfectant, positive for cyan or green fluorescence, respectively, was picked up by hand-made capillary and expanded without drug selection.

Acknowledgments

We thank Shinobu Ueda, Takumi Teratani, Yoshitaka Tamai, and Taku Shimizu for technical advice; Luc Gailhouste for comments on the manuscript; Atsushi Miyawaki (RIKEN) for pCSII-Venus plasmid; Katsuyuki Hayashi and DNA Chip Research, Inc., for microarray analysis; and Setsuo Hirohashi and Masaaki Terada for great support of our project. This work was supported by a Grant-in-Aid for the Third-Term Comprehensive 10-Year Strategy for Cancer Control.

Supporting Information

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Information & Authors

Information

Published in

Go to Proceedings of the National Academy of Sciences
Proceedings of the National Academy of Sciences
Vol. 107 | No. 32
August 10, 2010
PubMed: 20660726

Classifications

Submission history

Published online: July 26, 2010
Published in issue: August 10, 2010

Keywords

  1. genetic engineering
  2. rat
  3. embryonic stem cells

Acknowledgments

We thank Shinobu Ueda, Takumi Teratani, Yoshitaka Tamai, and Taku Shimizu for technical advice; Luc Gailhouste for comments on the manuscript; Atsushi Miyawaki (RIKEN) for pCSII-Venus plasmid; Katsuyuki Hayashi and DNA Chip Research, Inc., for microarray analysis; and Setsuo Hirohashi and Masaaki Terada for great support of our project. This work was supported by a Grant-in-Aid for the Third-Term Comprehensive 10-Year Strategy for Cancer Control.

Authors

Affiliations

Masaki Kawamata
Section for Studies on Metastasis, National Cancer Center Research Institute, 1-1 Tsukiji, 5-chome, Chuo-ku, Tokyo 104-0045, Japan
Takahiro Ochiya1 [email protected]
Section for Studies on Metastasis, National Cancer Center Research Institute, 1-1 Tsukiji, 5-chome, Chuo-ku, Tokyo 104-0045, Japan

Notes

1
To whom correspondence should be addressed. E-mail: [email protected].
Author contributions: M.K. and T.O. designed research; M.K. performed research; M.K. analyzed data; and M.K. and T.O. wrote the paper.

Competing Interests

The authors declare no conflict of interest.

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    Generation of genetically modified rats from embryonic stem cells
    Proceedings of the National Academy of Sciences
    • Vol. 107
    • No. 32
    • pp. 13973-14514

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