Primary culture of porcine PGCs requires LIF and porcine membrane-bound stem cell factor

Porcine Primordial Germ Cell-Like Cells Generated from Induced Pluripotent Stem Cells Under Different Culture Conditions

Culture conditions regulate the process of pluripotency acquisition and self-renewal. This study aimed to analyse the influence of the in vitro environment on the induction of porcine induced pluripotent stem cell (piPSCs) differentiation into primordial germ cell-like cells (pPGCLCs). piPSC culture with different supplementation strategies (LIF, bFGF, or LIF plus bFGF) promoted heterogeneous phenotypic profiles. Continuous bFGF supplementation during piPSCs culture was beneficial to support a pluripotent state and the differentiation of piPSCs into pPGCLCs. The pPGCLCs were positive for the gene and protein expression of pluripotent and germinative markers. This study can provide a suitable in vitro model for use in translational studies and to help answer numerous remaining questions about germ cells.

FGF2 Signaling Plays an Important Role in Maintaining Pluripotent State of Pig Embryonic Germ Cells

Germ cells are alternative sources for deriving pluripotent stem cells. Because embryonic germ cells (EGCs) possess physiological and developmental features similar to those of embryonic stem cells, pig EGCs are considered a potential tool for generating transgenic animals for agricultural usage. Therefore, in this study, we attempted to establish and characterize pig EGCs from fetal gonads. EGC lines were derived from the genital ridges of porcine fetuses in media containing leukemia inhibitory factor (LIF), fibroblast growth factor 2 (FGF2), and stem cell factor. After establishment, these cells were cultured and stabilized in LIF- or FGF2-containing media. The cell lines were maintained under both conditions over an extended time period and spontaneously differentiated into the three germ layers in vitro. Interestingly, expression of pluripotency markers showed different patterns between cell lines cultured in LIF or FGF2. SSEA4 was only expressed in FGF2-treated pig EGCs (FGF2-pEGCs), not LIF-treated pig EGCs (LIF-pEGCs). Pluripotency genes were upregulated in FGF2-pEGCs, and germline markers were highly expressed, indicating that FGF2 supplements are more efficient in supporting the pluripotency of pEGCs. In conclusion, we verified that FGF2 signaling plays an important role in reprogramming and maintaining pEGCs from fetal gonads.

Poultry genetic resource conservation using primordial germ cells

The majority of poultry genetic resources are maintained in situ in living populations. However, in situ conservation of poultry genetic resources always carries the risk of loss owing to pathogen outbreaks, genetic problems, breeding cessation, or natural disasters. Cryobanking of germplasm in birds has been limited to the use of semen, preventing conservation of the W chromosome and mitochondrial DNA. A further challenge is posed by the structure of avian eggs, which restricts the cryopreservation of ova and fertilized embryos, a technique widely used for mammalian species. By using a unique biological property and accessibility of avian primordial germ cells (PGCs), precursor cells for gametes, which temporally circulate in the vasculature during early development, an avian PGC transplantation technique has been established. To date, several techniques for PGC manipulation including purification, cryopreservation, depletion, and long-term culture have been developed in chickens. PGC transplantation combined with recent advanced PGC manipulation techniques have enabled ex situ conservation of poultry genetic resources in their complete form. Here, the updated technologies for avian PGC manipulation are introduced, and then the concept of a poultry PGC-bank is proposed by considering the biological properties of avian PGCs.

Derivation of motor neuron-like cells from neonatal mouse testis in a simple culture condition

Embryonic stem cell (ESC) therapy is an exciting way to treat neurodegenerative disease and central nervous system injury. However, many ethical and immunological problems surround the use of embryonic stem cells. Finding an alternative source of stem cells is therefore pertinent. In this study, spermatogonia stem cells (SSCs) were used to generate mature motor neurons. SSCs were extracted from neonatal testes and cultured in DMED/F12 medium for 3 weeks. Characterisation of SSC-derived ESC-like cells was confirmed by RT-qPCR, immunostaining, alkaline phosphatase activity and their ability to form embryoid bodies (EBs). The EBs were induced by retinoic acid and Sonic hedgehog and trypsinised to obtain single induced cells. The single cells were cultured in neural medium for 18 days. Characterisation of neural precursors and motor neuron-like cells was confirmed by RT-qPCR and immunocytochemical analysis at the 7th day (early stage) and 18th day (late stage), respectively, of culturing. The neural precursors were found to be positive for nestin and Sox2, and a small fraction of cells expressed β-tubulin III. Upon further differentiation, multipolar neurons were detected that expressed β-tubulin III and MAP2 markers. Moreover, the expression levels of Olig2 and PAX6 were significantly lower, while HB9, Isl1 and Isl2 expression levels were higher at the late stage when compared to the early stage. These results show that SSCs have the potential to differentiate to motor neuron-like cells and express markers specific for mature motor neurons. However, the functional ability of these cells remains to be evaluated in future studies.

bFGF signaling-mediated reprogramming of porcine primordial germ cells

Primordial germ cells (PGCs) have the ability to be reprogrammed into embryonic germ cells (EGCs) in vitro and are an alternative source of embryonic stem cells. Other than for the mouse, the systematic characterization of mammalian PGCs is still lacking, especially the process by which PGCs convert to pluripotency. This hampers the understanding of germ cell development and the derivation of authenticated EGCs from other species. We observed the morphological development of the genital ridge from Bama miniature pigs and found primary sexual differentiation in the E28 porcine embryo, coinciding with Blimp1 nuclear exclusion in PGCs. To explore molecular events involved in porcine PGC reprogramming, transcriptome data of porcine EGCs and fetal fibroblasts (FFs) were assembled and 1169 differentially expressed genes were used for Gene Ontology analysis. These genes were significantly enriched in cell-surface receptor-linked signal transduction, in agreement with the activation of LIF/Stat3 signaling and FGF signaling during the derivation of porcine EG-like cells. Using a growth-factor-defined culture system, we explored the effects of bFGF on the process and found that bFGF not only functioned at the very beginning of PGC dedifferentiation by impeding Blimp1 nuclear expression via a PI3K/AKT-dependent pathway but also maintained the viability of cultured PGCs thereafter. These results provide further insights into the development of germ cells from livestock and the mechanism of porcine PGC reprogramming.

FGF, Insulin, and SMAD Signaling Cooperate for Avian Primordial Germ Cell Self-Renewal

Precise self-renewal of the germ cell lineage is fundamental to fertility and reproductive success. The early precursors for the germ lineage, primordial germ cells (PGCs), survive and proliferate in several embryonic locations during their migration to the embryonic gonad. By elucidating the active signaling pathways in migratory PGCs in vivo, we were able to create culture conditions that recapitulate this embryonic germ cell environment. In defined medium conditions without feeder cells, the growth factors FGF2, insulin, and Activin A, signaling through their cognate-signaling pathways, were sufficient for self-renewal of germline-competent PGCs. Forced expression of constitutively active MEK1, AKT, and SMAD3 proteins could replace their respective upstream growth factors. Unexpectedly, we found that BMP4 could replace Activin A in non-clonal growth conditions. These defined medium conditions identify the key molecular pathways required for PGC self-renewal and will facilitate efforts in biobanking of chicken genetic resources and genome editing.

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Avian primordial germ cell self-renewal is dependent on FGF2, insulin, and Activin A molecules

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BMP4 can replace Activin A in non-clonal growth conditions

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Defined culture medium conditions will facilitate studies of germ cell self-renewal in other vertebrate species

Pluripotent stem cells and reprogrammed cells in farm animals

Pluripotent cells are unique because of their ability to differentiate into the cell lineages forming the entire organism. True pluripotent stem cells with germ line contribution have been reported for mice and rats. Human pluripotent cells share numerous features of pluripotentiality, but confirmation of their in vivo capacity for germ line contribution is impossible due to ethical and legal restrictions. Progress toward derivation of embryonic stem cells from domestic species has been made, but the derived cells were not able to produce germ line chimeras and thus are termed embryonic stem-like cells. However, domestic animals, in particular the domestic pig (Sus scrofa), are excellent large animals models, in which the clinical potential of stem cell therapies can be studied. Reprogramming technologies for somatic cells, including somatic cell nuclear transfer, cell fusion, in vitro culture in the presence of cell extracts, in vitro conversion of adult unipotent spermatogonial stem cells into germ line derived pluripotent stem cells, and transduction with reprogramming factors have been developed with the goal of obtaining pluripotent, germ line competent stem cells from domestic animals. This review summarizes the present state of the art in the derivation and maintenance of pluripotent stem cells in domestic animals.

Effects of 6-bromoindirubin-3′-oxime on the maintenance of pluripotency of porcine embryonic germ cells in combination with stem cell factor, leukemia inhibitory factor and fibroblast growth factor

6-Bromoindirubin-3′-oxime (BIO), which is one of the glycogen synthase kinase 3 inhibitors and a key regulator of numerous signaling pathways, was reported to be capable of maintaining the pluripotency of human and mouse embryonic stem cells. Presently, it is unknown whether BIO can influence the derivation of porcine embryonic germ (EG) cells. In this study, porcine primordial germ cells (PGCs) were isolated from gonads of 24- and 28-day embryos, and were then treated with BIO either individually or in combination with other cytokines (stem cell factor (SCF), leukemia inhibitory factor (LIF), and fibroblast growth factor (FGF); abbreviated as ‘3F’), and the effects of the treatment on the proliferation ability of porcine PGCs at early stage were examined using 5-bromo-2-deoxyuridine (Brdu) immunostaining assay. After continuous culture, the effects on the efficiency of porcine undifferentiated EG cells in the third passage and differentiated EG cells from embryoid bodies were examined as well. The results obtained through the observation of the Brdu-labeled PGCs indicated that BIO in combination with 3F resulted in a significant increase in the mitosis index, and also indicated that the BIO in combination with 3F had a higher efficiency in promoting the formation of porcine EG colony derived from porcine day 24 PGCs than BIO used either individually or in combination with LIF. In addition, BIO in combination with 3F exhibited the apparent anti-differentiation activity by reversing the differentiated EG cells to the undifferentiated status. Our results demonstrate that BIO in combination with SCF, LIF, and FGF could significantly contribute to the establishment of a porcine EG cell colony and maintain the undifferentiated status.

Effects of growth factors and feeder cells on porcine primordial germ cells in vitro

As embryonic stem (ES) cells are not available in swine, embryonic germ (EG) cells derived from primordial germ cells (PGCs) are an alternate source of pluripotent embryonic cells for genetic modification through homologous recombination. Although morphological and biochemical characteristics are similar between ES and EG cells, culture conditions are quite different. To optimize the culture condition for the establishment of porcine EG cells, porcine PGCs were cultured in vitro with various combinations of growth factors (leukemia inhibitory factor [LIF], stem cell factor [SCF], and basic fibroblast growth factor [bFGF]) and on different kinds of feeder cells (STO, TM(4), Sl/Sl(4) m220, porcine embryonic fibroblasts, and COS-7 cells). Optimal results were obtained when all three growth factors (LIF, SCF, and bFGF) were present in the media. Also, feeder cells expressing membrane-bound SCF are required for survival and establishment of porcine EG cells. Therefore, a combination of growth factors and proper feeder cells are critical for the establishment of undifferentiated porcine EG cells.

Derivation of human embryonic germ cells: an alternative source of pluripotent stem cells

Based on evidence suggesting similarities to human embryonic stem cells, human embryonic germ (hEG) cells have been advocated as an alternative pluripotent stem cell resource but have so far received limited attention. To redress this imbalance, human fetal gonads were collected for the isolation and culture of primordial germ cells at 7-9 weeks postconception. We provide evidence for the derivation, culture, and differentiation of hEG cells in vitro. This evidence includes the expression of markers characteristic of pluripotent cells, the retention of normal XX or XY karyotypes, and the demonstration of pluripotency, as suggested by the expression of markers indicative of differentiation along the three germ lineages (ectoderm, mesoderm, and endoderm) and an associated loss of pluripotent markers. In assessing this differentiation, however, we also demonstrate a hitherto unacknowledged overlap in gene expression profiles between undifferentiated and differentiated cell types, highlighting the difficulty in ascribing cell lineage by gene expression analyses. Furthermore, we draw attention to the problems inherent in the management of these cells in prolonged culture, chiefly the difficulty in preventing spontaneous differentiation, which hinders the isolation of pure, undifferentiated clonal lines. While these data advocate the pursuit of pluripotent hEG cell studies with relevance to early human embryonic development, culture limitations carry implications for their potential applicability to ambitious cell replacement therapies.

Pluripotent stem cells--model of embryonic development, tool for gene targeting, and basis of cell therapy

Embryonic stem (ES) cells are pluripotent cell lines with the capacity of self-renewal and a broad differentiation plasticity. They are derived from pre-implantation embryos and can be propagated as a homogeneous, uncommitted cell population for an almost unlimited period of time without losing their pluripotency and their stable karyotype. Murine ES cells are able to reintegrate fully into embryogenesis when returned into an early embryo, even after extensive genetic manipulation. In the resulting chimeric offspring produced by blastocyst injection or morula aggregation, ES cell descendants are represented among all cell types, including functional gametes. Therefore, mouse ES cells represent an important tool for genetic engineering, in particular via homologous recombination, to introduce gene knock-outs and other precise genomic modifications into the mouse germ line. Because of these properties ES cell technology is of high interest for other model organisms and for livestock species like cattle and pigs. However, in spite of tremendous research activities, no proven ES cells colonizing the germ line have yet been established for vertebrate species other than the mouse (Evans and Kaufman, 1981; Martin, 1981) and chicken (Pain et al., 1996). The in vitro differentiation capacity of ES cells provides unique opportunities for experimental analysis of gene regulation and function during cell commitment and differentiation in early embryogenesis. Recently, pluripotent stem cells were established from human embryos (Thomson et al., 1998) and early fetuses (Shamblott et al., 1998), opening new scenarios both for research in human developmental biology and for medical applications, i.e. cell replacement strategies. At about the same time, research activities focused on characteristics and differentiation potential of somatic stem cells, unravelling an unexpected plasticity of these cell types. Somatic stem cells are found in differentiated tissues and can renew themselves in addition to generating the specialized cell types of the tissue from which they originate. Additional to discoveries of somatic stem cells in tissues that were previously not thought to contain these kinds of cells, they also appear to be capable of developing into cell types of other tissues, but have a reduced differentiation potential as compared to embryo-derived stem cells. Therefore, somatic stem cells are referred to as multipotent rather than pluripotent. This review summarizes characteristics of pluripotent stem cells in the mouse and in selected livestock species, explains their use for genetic engineering and basic research on embryonic development, and evaluates their potential for cell therapy as compared to somatic stem cells.

Differential expression of growth factors in irradiated mouse testes

PURPOSE

By using as an experimental model the male mouse gonad, which contains both radiosensitive (germ) and radioresistant (somatic) cells, we have studied the growth factor (and/or receptor) expression of transforming growth factor-beta receptor (TGFbeta RI), stem cell factor (SCF), c-kit, Fas-L, Fas, tumor necrosis factor receptor (TNF R55), and leukemia inhibiting factor receptor (LIF-R) after local irradiation.

METHODS AND MATERIALS

Adult male mice were locally irradiated on the testes. Induction of apoptosis in the different testicular cell types following X-ray radiation was identified by the TdT-mediated dUTP Nick End Labeling (TUNEL) approach. Growth factor expression was evidenced by semiquantitative RT-PCR and Western blot analyses.

RESULTS

Apoptosis, identified through the TUNEL approach, occurred in radiosensitive testicular (premeotic) germ cells with the following kinetics: the number of apoptotic cells increased after 24 h (p < 0.001) and was maximal 48 h after a 2-Gy ionizing radiation (p < 0.001). Apoptotic cells were no longer observed 72 h after a 2-Gy irradiation. The number of apoptotic cells increased with the dose of irradiation (1-4 Gy). In the seminiferous tubules, the growth factor expression in premeiotic radiosensitive germ cells was modulated by irradiation. Indeed Fas, c-kit, and LIF-R expression, which occurs in (radiosensitive) germ cells, decreased 24 h after a 2-Gy irradiation, and the maximal decrease was observed with a 4-Gy irradiation. The decrease in Stra8 expression occurred earlier, at 4 h after a 2-Gy irradiation. In addition, a significant (p < 0.03) decrease in Stra8 mRNA levels was observed at the lowest dose used (0.5 Gy, 48 h). Moreover, concerning a growth factor receptor, such as TGFbeta RI, which is expressed both in radiosensitive and radioresistant cells, we observed a differential expression depending on the cell radiosensitivity after irradiation. Indeed, TGFbeta RI expression was increased after irradiation in interstitial radioresistant testicular cells in a dose- and time-dependent manner, while it decreased in seminiferous radiosensitive (germ cells) testicular cells. Such a differential expression between radioresistant and radiosensitive cells in TGFbeta RI levels was observed in terms of both mRNA and protein. In contrast, the growth factors specifically expressed in the somatic radioresistant (Sertoli) cells in the seminiferous tubules (SCF, Fas-L, TNF R55) were not affected by ionizing radiation (up to 4 Gy, 72 h).

CONCLUSION

Growth factor expression decreased in the radiosensitive testicular cells after irradiation. Such a decrease occurred before the detection of apoptosis using the TUNEL approach. TGFbeta RI mRNA levels decreased in the radiosensitive cells, whereas it increased in the radioresistant cells, suggesting that TGFbeta RI may represent a biomarker of the intrinsic radiosensitivity of cells.

Establishment of pluripotent cell lines from vertebrate species--present status and future prospects

Pluripotent embryonic stem (ES) cells are undifferentiated cell lines derived from early embryos and are capable of unlimited undifferentiated proliferation in vitro. They retain the ability to differentiate into all cell types including germ cells in chimeric animals in vivo, and can be induced to form derivatives of all three germ layers in vitro. Mouse ES cells represent one of the most important tools in genetic research. Major applications include the targeted mutation of specific genes by homologous recombination and the discovery of new genes by gene trap strategies. These applications would be of high interest for other model organisms and also for livestock species. However, in spite of tremendous research activities, no proven ES cells colonizing the germ line have been established for vertebrate species other than mouse and chicken thus far. This review summarizes the current status of deriving pluripotent embryonic stem cell lines from vertebrates and recent developments in nuclear transfer technology, which may provide an alternative tool for genetic modification of livestock animals.