Ziomik bitcoin

Marker and imprinting profiles

The majority of cells in the mGC colonies expressed POU5F1, NANOG, SSEA-1, and VASA (Fig. 4a–d). They also expressed pluripotent genes Sox2, Dppa5, Rex1, eRas, and Crypto along with germ line-specific markers, including Stella, Dazl, Vasa, and cRet (Fig. 4q). In addition, the expression of POU5F1, NANOG, and SOX2 was confirmed by Western blot analysis (Fig. 4p). Among the three mGSC lines tested in this study, only one line showed Nanog expression and this is the line that has been used for Western blot analysis. The mouse cell line at passage 20 showed high telomerase activity (similar to ES cells) and normal karyotype (40, XY) (Fig. 5).

We also analyzed the global gene expression and imprinting patterns of the mGCs before and after culture and compared that with ES cells. Interestingly, our culture condition did not change the imprinting pattern of the mGCs in all the DMR sites tested. In contrast to the mouse ES cells that showed only a partial androgenetic imprinting, the mGCs clearly exhibited a 100% androgenetic imprinting pattern (Fig. 6). Somewhat surprisingly, our microarray analysis showed that the global gene expression pattern of the mGCs had 87% similarity before and after culture.

Differentiation potential

When mGCs were aggregated to form embryoid bodies (EBs), gastrulation was observed within 9–15 days (Fig. 7a). Cells in the EBs expressed early developmental markers including E-cadherin and laminin-1 (markers of polarized epithelium, Fig. 7b and c); ZIC1, PAX6, and SOX1 (early ectoderm markers, Fig. 7d and f); GATA4 and FOXA2 (early endoderm markers, Fig. 7e and f); and BRACHURY, BMP4, and COL2A1 (early mesoderm markers, Fig. 7f). In culture, mGC colonies spontaneously differentiated into phenotypes expressing markers of cardiomyocytes (Fig. 7g–j), adipocytes (Fig. 7k), and neural cells (Fig. 7l and m). Some of the cells that spontaneously differentiated to cardiomyocytes exhibited rhythmic contractions for up to 3 days (Fig. 7i, the beating sequence of representative cells are shown in the Supplementary Video, which can be viewed online at www.reproduction-online.org/supplemental). Using directed differentiation protocols, mGC lines could be induced to differentiate into neural cells representing neural progenitors (nestin, NEUROD1), neurons (MAP2, NF-68, GAD67), and glial cells (GFAP, MBP, A2B5, O4, NG2), as shown in Fig. 8a–g and j. They could also be induced to form cardiomyocytes (troponin-1, cardiac myosin, desmin, NKX2.5, GATA4) or chondrocytes (collagen Xa1 and staining by alcian blue) (Fig. 8h–I and k–l).

In a separate differentiation study with mGCs, we counted the number of cells (nuclei) with and without staining of neural markers in seven colonies within a culture, and the average percentage was estimated as 17.6% for GFAP⁺ cells, 2.5% for Tuj-1⁺ cells, and 2.3% for MAP2⁺ cells. In general, the efficiency of induced differentiation by these protocols was much higher in ES cells compared with the mGCs.

Testes regeneration

Four weeks after transplantation, testes of the control animals as well as those that received POU5F1-positive c-Kit-positive cells showed no spermatogenesis in the majority of the seminiferous tubules. On the contrary, 80% of those that received freshly isolated POU5F1-positive c-Kit-negative cells showed some degrees of spermatogenesis throughout the testes, indicating the presence of functional SSCs in the cell suspension. Similarly, our short-term transplantation experiment using the cell trace marker carboxyfluorescein diacetate succinimidyl ester (CSFE) showed that only the c-Kit-negative subpopulation of germ line stem cells colonized the recipient testes (Fig. 9). No spermatogenesis was found in the majority of seminiferous tubules of the recipient mice testes transplanted with the mGCs, indicating that these cells do not have SSC properties (Table 1).

Teratomas and chimera formation

For teratoma formation, we injected equal numbers of mouse ES cells (as positive control) or POU5F1-GFP/LacZ mGS cells into the skin, muscle, and testis of different groups of nude mice (one million cells/site). All recipient mice (6/6) receiving ES cells developed teratomas in all three tissue types. By contrast, none of the mice (0/20) receiving mGSCs gave rise to teratomas (Fig. 9a–f; Supplementary Fig. 2, which can be viewed online at www.reproduction-online.org/supplemental). Six weeks after transplantation, POU5F1-GFP/LacZ cells were found in skin, muscle, and testicular tissues (Fig. 9g–l). These data show that mGCs, unlike ES cells, are non-tumorigenic. We tested chimera formation by injecting cultured POU5F1-GFP/LacZ cells into eight-cell embryos and blastocysts of CD-1 mice, an approach considered by many to be the highest standard for proving the pluripotency of cells. As shown in Fig. 10a–d, POU5F1-GFP/LacZ cells were incorporated into the inner cell mass of the mouse blastocysts. The embryos were transferred to the uterus of pseudopregnant mice (a total of 45 fetuses from 119 transferred embryos). At 12.5 dpc, staining of whole embryos for LacZ (β-galactosidase activity) showed distinctive patterns in the eye, brain, and limbs (Fig. 10e). The intensity of LacZ staining was much higher in chimeric embryos receiving mouse ES cells than those injected with mGC lines. The distribution of chimeric cells is also demonstrated in histological sections of the brain, heart, gonadal ridge, and liver (Fig. 10l–o). The intensity and the number of LacZ-positive cells were much higher in chimeric embryos injected with LacZ-ES cells than those injected with LacZ-GS cells (data not shown). Confirmation of POU5F1-GFP/LacZ chimeric tissues was supported by the presence of GFP DNA sequence in the ectodermal (brain), mesodermal (heart), endodermal (liver), and testis of the chimeric pups (Fig. 10p), as well as the presence of LacZ DNA (Fig. 10q) in all four tissues. These combined results clearly demonstrate that cultured mGCs are non-teratogenic stem cells with some pluripotent characteristics.

Cell isolation and culture

Mouse testicular stem cells were isolated from neonatal OG2 or OG2-LacZ males (0–3 days old, ∼30 pups/trial) or adult OG2 mice (2–5 months old, 1 male/trial). After mincing, the testes were digested in DPBS (Dulbecco's 10 mM phosphate-buffered (pH 7.2), 0.14 M saline) containing collagenase (1 mg/ml), DNase-1 (1 μg/ml), and EDTA (5 mM). Testicular cell culture was performed according to the previously published protocols (Izadyar et al. 2003, Shinohara et al. 2003, Kanatsu-Shinohara et al. 2004). In brief, germ cell enrichment was accomplished by differential adhesion. The cells were dispensed into gelatin-coated (0.1%) culture dishes. The following day the floating cells were collected and passed to a secondary culture plate (1×10⁵ cells per 1.2 cm²) in a culture medium used by Kanatsu-Shinohara et al. (2004) with modifications (Supplementary Table, which can be viewed online at www.reproduction-online.org/supplemental). After 2–4 weeks in culture, GFP-positive colonies were mechanically transferred to culture dishes containing mitomycin C-treated murine embryonic fibroblast (MEF) feeder layers (see below). After passage for three to four times, via mechanical transfer, to new MEF cultures, the colonies were established and could be removed from the culture plate enzymatically (collagenase 1 mg/ml, 5–10 min) for further expansion and/or storage in liquid nitrogen. To enhance germ line stem cell derivation, spermatogenesis was arrested in adult OG2 mice (4–6 weeks old, n=4), i.e., testes were surgically secured to the abdominal wall to become cryptorchid as described previously (de Rooij et al. 1999). mGC lines were also generated from neonatal and non-cryptorchid adult OG2 mice by sorting the GFP-positive cells. To further study which subpopulation of germ line stem cells generate these cell lines, the GFP-positive cells were sorted based on their c-Kit expression and were cultured on MEF feeders as described above. Finally, the effect of the growth factor removal on self-renewal or differentiation of mGC lines was investigated.

Preparation of MEF feeders

MEF feeders were made by the standard procedures using 12.5 dpc CD-1 mouse embryos. The embryos were eviscerated before trypsinization and the dissociated cells plated onto 150 mm plates at a plating density of ∼1.5 embryos per plate. After initial plating, MEFs were split 1:5 and then frozen/thawed (passage 1). Thawed MEFs (P1) were passed only once for expansion purposes prior to mitomycin C treatment. MEF feeders were plated at a density of 50×10³ to 60×10³ per cm². New MEF feeders were used for pluripotent germ cell culture every 7–10 days. All animal experiments were conducted in accordance with the National Research Council's Guidelines for the Care and Use of Laboratory Animals.

Evaluation of telomerase activity and karyotyping

For the determination of telomerase activity, cell extracts were isolated from germ cell lines (passage 10 and higher), freshly isolated POU5F1+/c-Kit+ sorted, and POU5F1+/c-Kit− sorted cells using CHAPS lysis buffer containing 150 U/ml RNase. Cell lysates were centrifuged for 20 min at 12 000 g, 4 °C, and the supernatants were stored at −80 °C. Protein concentration was measured by Bradford assay using BSA as the standard. Telomerase activity was measured by PCR-based assay using TRAPEZE detection kit (Chemicon, Temecula, CA, USA). Two microliters of the cell extract at 750 μg/μl were added to a total volume of 50 μl PCR mix containing the TRAP reaction buffer, dNTPs, substrate oligonucleotide, telomerase primer, internal standard primer, and Taq polymerase. Two microliters of mESC cell extract were added to the reaction mix as positive control, and CHAPS lysis buffer and heat-inactivated telomerase were used as negative control for each experimental sample. Each sample was incubated at 30 °C for 30 min for telomerase extension, followed by PCR amplification. Karyotyping was performed at Coriell Cell Repositories, Cytogenetics Laboratory. For karyotyping, proliferating cells were incubated in culture with 0.1 μg/ml KaryoMAX Colcemid (Invitrogen) for 3–4 h before they were resuspended in hypotonic solution (0.075 M KCl) and incubated at room temperature for 10 min. The cells were then resuspended in a cold fixative (3:1 methanol:acetic acid) and stored at 4 °C for at least 30 min. After washing with the fixative, the cells were applied to clean glass slides and air dried. Metaphase chromosomes were prepared and karyotypes created using an Applied Spectral Imaging Band View digital imaging system.

In vitro differentiation

For generating EBs, mGSC colonies were dissociated with collagenase and plated at a concentration of 1×10⁶/well to 2×10⁶/well on 6-well non-adhesive culture plates in the complete medium containing 15% FBS (Hyclone, Logan ,UT, USA). In some experiments, EBs were formed in hanging drops by aggregating 50×10³ to 100×10³ cells in 50 μl medium. For differentiation into cells representing the three germ layers, the EBs were cultured for 15 days, and every 3 days 20–30 EBs were collected for RT-PCR analysis and 20–30 EBs were used for histological examinations. For induced differentiation, the EBs were cultured in the complete medium for 4 days before they were cultured in the serum-free N1 medium for lineage selection, i.e., DMEM/F12 (Invitrogen) supplemented with ITS (insulin, 10 mg/l; transferrin, 5.5 mg/l; selenium, 0.67 mg/l) and fibronectin (50 μg/ml). After 5–7 days, N1-treated cell aggregates were transferred to gelatin-coated culture plates (Ying et al. 2003) in the N2 medium for expansion of neural progenitor cells, i.e., N1 medium with ITS, without fibronectin, and supplemented with bFGF (10 ng/ml). For differentiation into cardiomyocytes, the EBs were cultured for 2 weeks in the presence of different cardiogenic compounds including DMSO (0.06 M), 5′-aza-2′-deoxycytidine AZA (5 mM), and Cardiogenol C (25–50 μM; Calbiochem, San Diego, CA, USA) (Paquin et al. 2002, Choi et al. 2004). During the differentiation process, the morphology of cells was analyzed and the samples were taken both for gene expression analysis by RT-PCR and immunohistochemical (IHC) staining (see below). Chondrocyte differentiation (Lee et al. 2004) of mGSCs was induced by adding a chondrogenic induction medium (Chondrogenic SingleQuots, Cambrex, Walkersville, MD, USA) supplemented with transforming growth factor-3β (10 ng/ml) and 20% FBS.

Immunocytochemical and IHC staining

Cultured cells were fixed in 4% paraformaldehyde for 10–30 min at room temperature and stored in PBS at 4 °C. For fluorescent immunocytochemistry, the cells were permeabilized with 1×Cytoperm (BD Biosciences, San Jose, CA, USA) or 0.2% Triton X-100 (Sigma) for 15 min and subsequently incubated in 2% (w/v) BSA (Sigma) and 2% (v/v) normal goat serum (GS)/1× Cytoperm–PBS for 30–60 min both at room temperature. Primary antibody was either diluted at the optimal concentration in 2% BSA and 2% GS/1× Cytoperm–PBS and incubated for 3 h at 4 °C or diluted in blocking buffer overnight at 4 °C. After two washes, fluorescent secondary antibody was diluted accordingly (Supplementary Fig. 2) in 2% BSA and 2% goat serum/1× Cytoperm–PBS and incubated for 1 h at 4 °C in the dark. The cells were washed twice with PBS, wrapped in foil, and stored at 4 °C until microscopic analysis. Images were recorded using an Olympus IX71 microscope or Zeiss LSM510 confocal microscope equipped with digital image hardware and software.

For bright-field immunocytochemistry, the cells were washed once with 1× PBS. Endogenous peroxidase activity was blocked with 3% (v/v) H₂O₂ for 15 min followed by permeabilization – blocking with 2% BSA and 2% GS/1× Cytoperm–PBS for 30 min. The primary antibody was diluted accordingly (Supplementary Fig. 2) in 2% BSA and 2% GS/1× Cytoperm–PBS and incubated for 3 h at 4 °C. The remainder of the staining was accomplished using ABC staining kits (Vector Labs, Burlingame, CA, USA), according to the manufacturer's instructions. Visualization was done with enhanced diaminobenzidine substrate (Sigma) tablet dissolved in purified water and incubated for 5–10 min. For negative controls, the primary antibody was omitted. The primary antibodies used in this study were obtained from various companies. Each antibody was validated and the concentrations optimized in our laboratory. The source and working dilutions of these antibodies are presented in Table 2.

Flow cytometry

Specific antibodies, including SSEA-1 and c-Kit, were optimized in our laboratory for FACS analysis. Cells were sorted on the inFlux Cell Sorter (Cytopeia, Seattle, WA, USA). For c-Kit sorting, freshly isolated testicular cells containing the POU5F1-GFP construct were stained with CD117 APC (BD Pharmingen, San Diego, CA, USA). GFP excitation was attained with 488 nm solid-state Coherent laser (Coherent, Santa Clara, CA, USA) and emission light collected through a 530-40 dichroic mirror. APC excitation was attained with 638 nm solid-state Coherent laser (Coherent) and emission light collected through a 710-40 dichroic mirror. The cells were stained at a final dilution of 1:200 c-Kit APC in the complete culture medium for 45 min on ice in the dark. After washing twice with the fresh cold complete medium, the cells were concentrated at 10×10⁶ cells/ml and kept on ice for sorting. To sort for GFP/c-Kit+/− cells, first we gated the whole GFP population with the non-stained cells to establish a baseline for APC channel. A region was placed based on the baseline to discriminate between negative and positive c-Kit-stained cells. Any cells/events falling into this region was considered to be positively stained for c-Kit and was sorted from the negative c-Kit cells. For some experiments, fresh germ cell colonies were dissociated and the cells were stained with anti-SSEA-1 antibody followed by goat-anti mouse IGM conjugated with PE-Cy7 (BD Biosciences).

Gene expression, imprinting analysis, and GFP amplification

Total RNA was isolated using RNeasy mini kit (Qiagen) and the RNA was used for RT-PCR, quantitative PCR, or microarray analysis. For RT-PCR, cDNA was synthesized with the Sensiscript RT kit (Qiagen), and the PCR was performed with HotStarTaq DNA Polymerase (Qiagen). All PCRs began with an initial incubation at 95 °C for 15 min to activate the enzyme. This was followed by 35 cycles of 95 °C for 15 s, the appropriate annealing temperature for 1 min, and 72 °C for 1 min, which was then followed by 1 cycle of 72 °C for 10 min for final extension. The reactions were carried out using an iCycler Thermal Cycler (Bio-Rad). RT-PCR was carried out using specific primers including Pou5f1, Nanog, Rex1, Dppa5, Dazl, β-actin (Actb), Nkx2.5, nestin, Mab2, and Gfap (primer sequences are presented in Table 3). For internal controls, Gapdh was used as a housekeeping gene for cellular samples and β-actin or interleukin-2 (IL2) was used in mouse embryos.

Imprinting patterns in mGSCs and mESCs were determined by a PCR-based analysis. PCR amplification of each dimethylated region (DMR) from bisulfite-treated DNAs was carried out by specific primers as described (Kanatsu-Shinohara et al. 2004). For analysis of the imprinted genes, we used the UVP image software to quantify the band intensity. For GFP and LacZ amplification, individual tissues from chimeric embryos were carefully collected by dissection, minced into small pieces, and placed in DNA extraction buffer (DNeasy kit, Qiagen) for DNA isolation and purification, according to the manufacturer's protocol. PCR (1 μl DNA template containing 100 ng DNA) was carried out as described previously (Izadyar et al. 2000).

Microarray analysis

Total cellular RNA was isolated using RNeasy mini kit (Qiagen Inc.), according to the manufacturer's recommendations. To eliminate DNA contamination, the samples were treated with 2.0 U DNase I (Amplification grade, Invitrogen) at 37 °C for 15 min, and the enzyme is inactivated by the addition of EDTA (2 mM final) at 65 °C for 10 min. The samples were concentrated by ethanol precipitation and resuspended in RNase-free water. Two micrograms of total RNA of each sample were sent to the UCI DNA Microarray Facility, where the samples were prepared and hybridized to Affymetrix Mouse Genome 430 2.0 GeneChips (Affymetrix Inc., Santa Clara, CA, USA) and scanned, according to the manufacturer's protocols.

SSC transplantation

To test the functionality of the subpopulations of germ line stem cells and our mGCs for regeneration of spermatogenesis, we used the SSC transplantation technique. Sixteen 6–8 weeks immune-deficient nude male mice (Harlan, Indianapolis, IN, USA) have been treated with busulfan (40 mg/kg) and used as recipients. One month after busulfan treatment, 2×10⁵ cells were transplanted into the seminiferous tubules via rete testis injection as described previously (Ogawa et al. 2000). Four mice were transplanted with freshly isolated POU5F1-positive c-Kit-positive sorted cells, four mice injected with freshly isolated POU5F1-positive c-Kit-negative sorted cells, and four mice transplanted with mGCs. As the mGCs are cultured on MEFs, they were sorted for GFP to avoid MEF contamination during transplantation. The remaining four mice served as sham control and not injected. One month after transplantation, the animals were killed and the testes harvested and used for histological evaluations. To evaluate the efficiency of transplantation, 100–150 tubule cross sections of each animal was examined and the number of tubules with different stages of spermatogenesis was counted. Statistical analysis was carried out using ANOVA and P<0.05 was considered significant. To better identify the transplanted cells in the recipient testes, a fluorescent cell trace marker, CSFE, (Invitrogen) was used. CSFE is colorless and non-fluorescent until the acetate groups are cleaved by intracellular esterases to yield highly fluorescent product. This fluorescent product is well retained and can be fixed with an aldehyde fixative; however, it diminishes following multiple cell divisions and can be used successfully only in short-term studies. Two mice were transplanted with POU5F1-positive c-Kit-negative cells and two other mice received POU5F1-positive c-Kit-positive cells. Ten days after transplantation, the mice were killed and the number of CSFE-positive colonies was determined. Considering 72 h for each cell doubling of germ line stem cells (Fig. 2), at this time after transplantation, SSCs could have gone through two to three cell doublings. Therefore, colonies of four to eight cells could have been formed.

Tests for teratoma and chimera formation

To test the ability of the mGCs to form teratomas or chimeras, OG2 mice (Jackson Laboratories, Bar Harbor, ME, USA) were bred with Rosa26 mice (Jackson Laboratories) and a new strain (OG2-R26) was generated. These mice have both GFP and LacZ constructs in their germ cells. Culture was performed as described and the new POU5F1-GFP/LacZ germ cell lines were produced for testing teratoma and chimera formation. Mouse POU5F1-GFP/LacZ mGSCs were examined for their ability to form teratomas in vivo by s.c., i.m., or injection into the seminiferous tubules of nude mice (Harlan). As positive controls for teratoma formation, the ES cells were injected into some mice. For s.c., i.m., or testicular injections, ∼1×10⁶ cells were injected. Mice were killed 6 weeks later and the tissues harvested for morphological and histological analysis.

The ability of mouse POU5F1-GFP/LacZ GSCs to form chimeric cell populations was determined after injection into host blastocysts, or by their aggregation with morula stage embryos or eight-cell stage embryos (Bradley 1987). Blastocyst injections of 15–20 cells were administered using 3.5-day blastocysts collected from CD-1 mice following the procedure as described (Chatot et al. 1990). After injection, the blastocysts were transferred (7–8 blastocysts in each horn of the uterus) to 2.5-day pseudopregnant CD-1 females, previously mated with vasectomized males. Incorporation of LacZ cells was examined in different areas of the chimeric 12.5 dpc embryos by the β-galactosidase staining kit (Sigma). In addition, LacZ and GFP PCRs were performed in DNAs isolated from the brain, heart, liver, and gonadal ridges of the chimeric embryos formed from POU5F1-GFP/LacZ cells.