Mouse embryonic germ (EG) cell lines: transmission through the germline and differences in the methylation imprint of insulin-like growth factor 2 receptor (Igf2r) gene compared with embryonic stem (ES) cell lines

Recent advances in stem and germ cell research: implications for the derivation of pig pluripotent cells

Pluripotent stem cells have the unique capacity to contribute to all the tissues of an adult animal after transfer into a host embryo. How pluripotency is acquired during early development and how it is maintained in stem cells have attracted the interest of many scientists for over three decades. Much progress in our understanding of how stem cells arise in culture and the signals required for homoeostasis has enabled the derivation of pluripotent cells in multiple species. Here, we discuss recent developments in stem cell biology that will impact the generation of pluripotent cells from different embryonic origins and will contribute to increase our capacity for generating transgenic animals.

Genetics of germ cell development

The germ line represents a continuous cellular link between generations and between species, but the germ cells themselves develop in a specialized, organism-specific context. The model organisms Caenorhabditis elegans, Drosophila melanogaster and the mouse display striking similarities, as well as major differences, in the means by which they control germ cell development. Recent developments in genetic technologies allow a more detailed comparison of the germ cells of these three organisms than has previously been possible, shedding light not only on universal aspects of germline regulation, but also on the control of the pluripotent state in vivo and on the earliest steps of embryogenesis. Here, we highlight themes from the comparison of these three alternative strategies for navigating the fundamental cycle of sexual reproduction.

Pluripotency and its layers of complexity

Pluripotency is depicted by a self-renewing state that can competently differentiate to form the three germ layers. Different stages of early murine development can be captured on a petri dish, delineating a spectrum of pluripotent states, ranging from embryonic stem cells, embryonic germ cells to epiblast stem cells. Anomalous cell populations displaying signs of pluripotency have also been uncovered, from the isolation of embryonic carcinoma cells to the derivation of induced pluripotent stem cells. Gaining insight into the molecular circuitry within these cell types enlightens us about the significance and contribution of each stage, hence deepening our understanding of vertebrate development. In this review, we aim to describe experimental milestones that led to the understanding of embryonic development and the conception of pluripotency. We also discuss attempts at exploring the realm of pluripotency with the identification of pluripotent stem cells within mouse teratocarcinomas and embryos, and the generation of pluripotent cells through nuclear reprogramming. In conclusion, we illustrate pluripotent cells derived from other organisms, including human derivatives, and describe current paradigms in the comprehension of human pluripotency.

DNA methyltransferase 1 (Dnmt1) mutation affects Snrpn imprinting in the mouse male germ line

DNA methylation and DNA methyltransferases are essential for spermatogenesis. Mutations in the DNA methyltransferase Dnmt1 gene exert a paternal effect on epigenetic states and phenotypes of offspring, suggesting that DNMT1 is important for the epigenetic remodeling of the genome that takes place during spermatogenesis. However, the specific role of DNMT1 in spermatogenesis and the establishment of genomic imprints in the male germ line remains elusive. To further characterize the effect of DNMT1 deficiency on the resetting of methylation imprints during spermatogenesis, we analyzed the methylation profiles of imprinted regions in the spermatozoa of mice that were heterozygous for a Dnmt1 loss-of-function mutation. The mutation did not affect the H19 or IG differentially methylated regions (DMRs) that are usually highly methylated but led to a partial hypermethylation of the Snrpn DMR, a region that should normally be unmethylated in mature spermatozoa. This defect does not appear in mouse models with mutations in Dnmt3a and Mthfr genes and, therefore, it is specific for the Dnmt1 gene and is suggestive of a role of DNMT1 in imprint resetting or maintenance in the male germ line.

Reprogramming and the mammalian germline: the Weismann barrier revisited

The germline represents a unique cell type that can transmit genetic material to the next generation. During early embryonic development, somatic cells give rise to a small population of cells known as germ cells, which eventually differentiate into mature gametes. Germ cells undergo a process of removing and resetting relevant epigenetic information, mainly by DNA demethylation. This extensive epigenetic reprogramming leads to the conversion of germ cells into immortal cells that can pass on the genome to the next generation. In the absence of germline-specific reprogramming, germ cells would preserve the old, parental epigenetic memory, which would prevent the transfer of heritable information to the offspring. On the contrary, somatic cells cannot reset epigenetic information by preserving the full methylation pattern on imprinting genes. In this review, we focus on the capacity of germ cells and somatic cells (soma) to transfer genetic information to the next generation, and thus revisit the Weismann theory of heredity.

Cell-intrinsic reprogramming capability: gain or loss of pluripotency in germ cells

In multicellular organisms, germ cells are an extremely specialized cell type with the vital function of transmitting genetic information across generations. In this respect, they are responsible for the perpetuity of species, and are separated from somatic lineages at each generation. Interestingly, in the past two decades research has shown that germ cells have the potential to proceed along two distinct pathways: gametogenesis or pluripotency. Unequivocally, the primary role of germ cells is to produce gametes, the sperm or oocyte, to produce offspring. However, under specific conditions germ cells can become pluripotent, as shown by teratoma formation in vivo or cell culture-induced reprogramming in vitro. This phenomenon seems to be a general propensity of germ cells, irrespective of developmental phase. Recent attempts at cellular reprogramming have resulted in the generation of induced pluripotent stem cells (iPSCs). In iPSCs, the intracellular molecular networks instructing pluripotency have been activated and override the exclusively somatic cell programs that existed. Because the generation of iPSCs is highly artificial and depends on gene transduction, whether the resulting machinery reflects any physiological cell-intrinsic programs is open to question. In contrast, germ cells can spontaneously shift their fate to pluripotency during in-vitro culture. Here, we review the two fates of germ cells, i.e., differentiation and reprogramming. Understanding the molecular mechanisms regulating differentiation versus reprogramming would provide invaluable insight into understanding the mechanisms of cellular reprogramming that generate iPSCs.

Tracing the conversion process from primordial germ cells to pluripotent stem cells in mice

To understand mechanisms underlying acquisition of pluripotency, it is critical to identify cells that can be converted to pluripotent stem cells. For this purpose, we focused on unipotent primordial germ cells (PGCs), which can be reprogrammed into pluripotent embryonic germ (EG) cells under defined conditions. Treatment of PGCs with combinations of signaling inhibitors, including inhibitors of MAP2K (MEK), GSK3B (GSK-3beta), and TGFB (TGFbeta) type 1 receptors, induced cells to enter a pluripotent state at a high frequency (12.1%) by Day 10 of culture. When we employed fluorescence-activated cell sorting to monitor conversion of candidate cells to a pluripotent state, we observed a cell cycle shift to S phase, indicating enrichment of pluripotent cells, during the early phase of EG formation. Transcriptome analysis revealed that PGCs retained expression of some pluripotent stem cell-associated genes, such as Pou5f1 and Sox2, during EG cell formation. On the other hand, PGCs lost their germ lineage characteristics and acquired expression of pluripotent stem cell markers, such as Klf4 and Eras. The overall gene expression profiles revealed by this system provide novel insight into how pluripotency is acquired in germ-committed cells.

Generation of Nkx2.2:lacZ mice using recombination-mediated cassette exchange technology

Nkx2.2 encodes a homeodomain transcription factor required for the correct specification and/or differentiation of cells in the pancreas, intestine, and central nervous system (CNS). To follow the fate of cells deleted for Nkx2.2 within these tissues, we generated Nkx2.2:lacZ knockin mice using a recombination-mediated cassette exchange (RMCE) approach. Expression analysis of lacZ and/or β-galactosidase in Nkx2.2(lacZ/+) heterozygote embryos and adults demonstrates that lacZ faithfully recapitulates endogenous Nkx2.2 expression. Furthermore, the Nkx2.2(lacZ/lacZ) homozygous embryos display phenotypes indistinguishable from the previously characterized Nkx2.2(-/-) strain. LacZ expression analyses in the Nkx2.2(lacZ/lacZ) homozygous embryos indicate that Nkx2.2-expressing progenitor cells within the pancreas are generated in their normal numbers and are not mislocalized within the pancreatic ductal epithelium or developing islets. In the CNS of Nkx2.2(lacZ/lacZ) embryos, LacZ-expressing cells within the ventral P3 progenitor domain display different migration properties depending on the developmental stage and their respective differentiation potential.

C5aR expression in a novel GFP reporter gene knockin mouse: implications for the mechanism of action of C5aR signaling in T cell immunity

C5aR is a G protein-coupled receptor for the anaphylatoxin C5a and mediates many proinflammatory reactions. C5aR signaling also has been shown to regulate T cell immunity, but its sites and mechanism of action in this process remain uncertain. In this study, we created a GFP knockin mouse and used GFP as a surrogate marker to examine C5aR expression. GFP was knocked into the 3'-untranslated region of C5ar1 by gene targeting. We show that GFP is expressed highly on Gr-1(+)CD11b(+) cells in the blood, spleen, and bone marrow and moderately on CD11b(+)F4/80(+) circulating leukocytes and elicited peritoneal macrophages. No GFP is detected on resting or activated T lymphocytes or on splenic myeloid or plasmacytoid dendritic cells. In contrast, 5-25% cultured bone marrow-derived dendritic cells expressed GFP. Interestingly, GFP knockin prevented cell surface but not intracellular C5aR expression. We conclude that C5aR is unlikely to play an intrinsic role on murine T cells and primary dendritic cells. Instead, its effect on T cell immunity in vivo may involve CD11b(+)F4/80(+) or other C5aR-expressing leukocytes. Further, our data reveal a surprising role for the 3'-untranslated region of C5aR mRNA in regulating C5aR protein targeting to the plasma membrane.

LKB1 controls the pluripotent state of human embryonic stem cells

Human embryonic stem cells maintained on human amniotic epithelial cells (hESCs(hAEC)) are better preserved in an undifferentiated state and express pluripotency genes Oct4, Nanog, and Sox2 at higher levels compared with growth on mitotically inactivated mouse embryonic fibroblasts (hESCs(MEF)). Here we report that this correlates with the absence of the tumor suppressor and metabolic balancer gene, LKB1 expression in hESCs(hAEC). RNA interference knockdown of LKB1 in hESCs(MEF) resulted in upregulation of pluripotency marker genes of Oct4 and Nanog, while downregulation of differentiation markers (Runx1, AFP, GATA, Brachyury, Sox17 and Nestin). As in somatic cells, LKB1 controls p21/WAF1 expression by promoter binding in hESCs(MEF). Our results suggested that the absence of LKB1-mediated signaling is an important determinant of feeder cell-mediated support of hESC renewal.

Transcriptional analysis of pluripotency reveals the Hippo pathway as a barrier to reprogramming

Pluripotent stem cells are derived from culture of early embryos or the germline and can be induced by reprogramming of somatic cells. Barriers to reprogramming that stabilize the differentiated state and have tumor suppression functions are expected to exist. However, we have a limited understanding of what such barriers might be. To find novel barriers to reprogramming to pluripotency, we compared the transcriptional profiles of the mouse germline with pluripotent and somatic cells, in vivo and in vitro. There is a remarkable global expression of the transcriptional program for pluripotency in primordial germ cells (PGCs). We identify parallels between PGC reprogramming to pluripotency and human germ cell tumorigenesis, including the loss of LATS2, a tumor suppressor kinase of the Hippo pathway. We show that knockdown of LATS2 increases the efficiency of induction of pluripotency in human cells. LATS2 RNAi, unlike p53 RNAi, specifically enhances the generation of fully reprogrammed iPS cells without accelerating cell proliferation. We further show that LATS2 represses reprogramming in human cells by post-transcriptionally antagonizing TAZ but not YAP, two downstream effectors of the Hippo pathway. These results reveal transcriptional parallels between germ cell transformation and the generation of iPS cells and indicate that the Hippo pathway constitutes a barrier to cellular reprogramming.

A concise review on epigenetic regulation: insight into molecular mechanisms

Epigenetic mechanisms are responsible for the regulation of transcription of imprinted genes and those that induce a totipotent state. Starting just after fertilization, DNA methylation pattern undergoes establishment, reestablishment and maintenance. These modifications are important for normal embryo and placental developments. Throughout life and passing to the next generation, epigenetic events establish, maintain, erase and reestablish. In the context of differentiated cell reprogramming, demethylation and activation of genes whose expressions contribute to the pluripotent state is the crux of the matter. In this review, firstly, regulatory epigenetic mechanisms related to somatic cell nuclear transfer (SCNT) reprogramming are discussed, followed by embryonic development, and placental epigenetic issues.

A predicted hairpin cluster correlates with barriers to PCR, sequencing and possibly BAC recombineering

Formation of higher-order structure of nucleic acids (hairpins or loops, for example) may impact not only gene regulation, but also molecular biology techniques and approaches critical for design and production of vectors needed for genetic engineering approaches. In the course of designing vectors aimed to modify the murine Foxd3 locus through homologous recombination in embryonic stem cells, we discovered a 370 nucleotide segment of DNA resistant to polymerase read-through. In addition to sequencing and PCR disruptions, we were unable to use BAC recombineering strategies to exchange sequences within the Foxd3 locus. This segment corresponds to a putative DNA hairpin region just upstream of the 5' untranslated region of Foxd3. This region is also highly conserved across vertebrate species, suggesting possible functional significance. Our findings provide a cautionary note for researchers experiencing technical challenges with BAC recombineering or other molecular biology methods requiring recombination or polymerase activity.

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.

Quantification of factors influencing fluorescent protein expression using RMCE to generate an allelic series in the ROSA26 locus in mice

Fluorescent proteins (FPs) have great utility in identifying specific cell populations and in studying cellular dynamics in the mouse. To quantify the factors that determine both the expression and relative brightness of FPs in mouse embryonic stem cells (mESCs) and in mice, we generated eight different FP-expressing ROSA26 alleles using recombinase-mediated cassette exchange (RMCE). These alleles enabled us to analyze the effects on FP expression of a translational enhancer and different 3'-intronic and/or polyadenylation sequences, as well as the relative brightness of five different FPs, without the confounding position and copy number effects that are typically associated with randomly inserted transgenes. We found that the expression of a given FP can vary threefold or more depending on the genetic features present in the allele. The optimal FP expression cassette contained both a translational enhancer sequence in the 5'-untranslated region (UTR) and an intron-containing rabbit β-globin sequence within the 3'-UTR. The relative expressed brightness of individual FPs varied up to tenfold. Of the five different monomeric FPs tested, Citrine (YFP) was the brightest, followed by Apple, eGFP, Cerulean (CFP) and Cherry. Generation of a line of Cherry-expressing mice showed that there was a 30-fold variation of Cherry expression among different tissues and that there was a punctate expression pattern within cells of all tissues examined. This study should help investigators make better-informed design choices when expressing FPs in mESCs and mice.

Biallele expression of PEG10 gene in primordial germ cells derived from day 27 porcine fetuses

Primordial germ cells (PGCs) from day 27 porcine fetuses have often been isolated to establish pluripotent embryonic germ (EG) cell lines, but little is known regarding their imprinted gene status. In our study, we attempted to detect the imprinted gene expression of cloned embryos and EG cells derived from individual PGC of day 27 and day 35, using single nucleotide polymorphism (SNP) analysis of the paternally expression gene 10 (PEG10) as a sign of parental-origin-specific expression. The results showed biallelic gene expression of the SNP that occurred in EG cell colonies and almost all of the cloned blastocysts, demonstrating that aberrant imprinted gene expression of PEG10 occurs in the day 27 porcine PGCs, whereas monoallelic expression of the PEG10 gene occurs in all the PGC clones derived from day 35 PGCs. In addition, the same imprinted gene status was observed for blastocysts derived from both male and female PGCs, indicating that the parental genomic imprinting is erased in male and female germlines.

Embryonic Stem Cell Transplantation into Seminiferous Tubules: A Model for the Study of Invasive Germ Cell Tumors of the Testis

Over the last 15 years, cell transplantation into seminiferous tubules has become a valuable tool to study germinal cell biology and related matters. This is particularly so, because the blood–testis permeability barrier establishes a sealed compartment which protect against certain influences such as immunological rejection. In the light of the functional and genetic similarities between carcinoma in situ (CIS) of the testis and embryonic stem (ES) cells, our laboratory has developed a tumor assay to study cancer invasion processes in testicular germ cell tumors (TGCT) based on the transplantation of ES cells into the seminiferous tubules. Here, we describe this new tumor assay and provide additional information regarding the transplantation techniques used and their application for the study of TGCTs. Finally, we discuss the practical implications of our experimental approach and its potential application for the understanding of TGCT invasive processes and the development of new antineoplastic strategies.

Gene from a psoriasis susceptibility locus primes the skin for inflammation

Psoriasis is a common complex genetic disease characterized by hyperplasia and inflammation in the skin; however, the relative contributions of epidermal cells and the immune system to disease pathogenesis remain unclear. Linkage studies have defined a psoriasis susceptibility locus (PSORS4) on 1q21, the epidermal differentiation complex, which includes genes for small S100 calcium-binding proteins. These proteins are involved in extracellular and intracellular signaling during epithelial host defense, linking innate and adaptive immunity. Inflammation-prone psoriatic skin constitutively expresses elevated concentrations of S100A7 (psoriasin) and S100A15 (koebnerisin) in the epidermis. Here, we report that genetically modified mice expressing elevated amounts of doxycycline-regulated mS100a7a15 in skin keratinocytes demonstrated an exaggerated inflammatory response when challenged by exogenous stimuli such as abrasion (Koebner phenomenon). This immune response was characterized by immune cell infiltration and elevated concentrations of T helper 1 (T(H)1) and T(H)17 proinflammatory cytokines, which have been linked to the pathogenesis of psoriasis and were further amplified upon challenge. Both inflammation priming and amplification required mS100a7a15 binding to the receptor of advanced glycation end products (RAGE). mS100a7a15 potentiated inflammation by acting directly as a chemoattractant for leukocytes, further increasing the number of inflammatory cells infiltrating the skin. This study provides a pathogenetic psoriasis model using a psoriasis candidate gene to link the epidermis and innate immune system in inflammation priming, highlighting the S100A7A15-RAGE axis as a potential therapeutic target.

The different shades of mammalian pluripotent stem cells

BACKGROUND

Pluripotent stem cells have been derived from a variety of sources such as from the inner cell mass of preimplantation embryos, from primordial germ cells, from teratocarcinomas and from male germ cells. The recent development of induced pluripotent stem cells demonstrates that somatic cells can be reprogrammed to a pluripotent state in vitro.

METHODS

This review summarizes our current understanding of the origins of mouse and human pluripotent cells. We pay specific attention to transcriptional and epigenetic regulation in pluripotent cells and germ cells. Furthermore, we discuss developmental aspects in the germline that seem to be of importance for the transition of germ cells towards pluripotency. This review is based on literature from the Pubmed database, using Boolean search statements with relevant keywords on the subject.

RESULTS

There are distinct molecular mechanisms involved in the generation and maintenance of the various pluripotent cell types. Furthermore, there are important similarities and differences between the different categories of pluripotent cells in terms of phenotype and epigenetic modifications. Pluripotent cell lines from various origins differ in growth characteristics, developmental potential, transcriptional activity and epigenetic regulation. Upon derivation, pluripotent stem cells generally acquire new properties, but they often also retain a 'footprint' of their tissue of origin.

CONCLUSIONS

In order to further our knowledge of the mechanisms underlying self-renewal and pluripotency, a thorough comparison between different pluripotent stem cell types is required. This will progress the use of stem cells in basic biology, drug discovery and future clinical applications.

The origin and identity of embryonic stem cells

Embryonic stem (ES) cells are used extensively in biomedical research and as a model with which to study early mammalian development, but their exact origin has been subject to much debate. They are routinely derived from pre-implantation embryos, but it has been suggested that the cells that give rise to ES cells might arise from epiblast cells that are already predisposed to a primordial germ cell (PGC) fate, which then progress to ES cell status via the PGC lineage. Based on recent findings, we propose here that ES cells can be derived directly from early epiblast cells and that ES cells might arise via two different routes that are dictated by their culture conditions.

Cell cycle adaptations and maintenance of genomic integrity in embryonic stem cells and induced pluripotent stem cells

Pluripotent stem cells have the capability to undergo unlimited self-renewal and differentiation into all somatic cell types. They have acquired specific adjustments in the cell cycle structure that allow them to rapidly proliferate, including cell cycle independent expression of cell cycle regulators and lax G(1) to S phase transition. However, due to the developmental role of embryonic stem cells (ES) it is essential to maintain genomic integrity and prevent acquisition of mutations that would be transmitted to multiple cell lineages. Several modifications in DNA damage response of ES cells accommodate dynamic cycling and preservation of genetic information. The absence of a G(1)/S cell cycle arrest promotes apoptotic response of damaged cells before DNA changes can be fixed in the form of mutation during the S phase, while G(2)/M cell cycle arrest allows repair of damaged DNA following replication. Furthermore, ES cells express higher level of DNA repair proteins, and exhibit enhanced repair of multiple types of DNA damage. Similarly to ES cells, induced pluripotent stem (iPS) cells are poised to proliferate and exhibit lack of G(1)/S cell cycle arrest, extreme sensitivity to DNA damage, and high level of expression of DNA repair genes. The fundamental mechanisms by which the cell cycle regulates genomic integrity in ES cells and iPS cells are similar, though not identical.

Embryonic germ cells from mice and rats exhibit properties consistent with a generic pluripotent ground state

Mouse and rat embryonic stem cells can be sustained in defined medium by dual inhibition (2i) of the mitogen-activated protein kinase (Erk1/2) cascade and of glycogen synthase kinase 3. The inhibitors suppress differentiation and enable self-renewal of pluripotent cells that are ex vivo counterparts of naïve epiblast cells in the mature blastocyst. Pluripotent stem cell lines can also be derived from unipotent primordial germ cells via a poorly understood process of epigenetic reprogramming. These are termed embryonic germ (EG) cells to denote their distinct origin. Here we investigate whether EG cell self-renewal and derivation are supported by 2i. We report that mouse EG cells can be established with high efficiency using 2i in combination with the cytokine leukaemia inhibitory factor (LIF). Furthermore, addition of fibroblast growth factor or stem cell factor is unnecessary using 2i-LIF. The derived EG cells contribute extensively to healthy chimaeric mice, including to the germline. Using the same conditions, we describe the first derivations of EG cells from the rat. Rat EG cells express a similar marker profile to rat and mouse ES cells. They have a diploid karyotype, can be clonally expanded and genetically manipulated, and are competent for multilineage colonisation of chimaeras. These findings lend support to the postulate of a conserved molecular ground state in pluripotent rodent cells. Future research will determine the extent to which this is maintained in other mammals and whether, in some species, primordial germ cells might be a more tractable source than epiblast for the capture of naïve pluripotent stem cells.

Germ cell specification in mice: signaling, transcription regulation, and epigenetic consequences

The specification of germ cell fate in development initiates mechanisms essential for the perpetuation of genetic information across the generations. Recent studies in mice have shown that germ cell specification requires at least three key molecular/cellular events: repression of the somatic program, re-acquisition of potential pluripotency, and an ensuing genome-wide epigenetic reprogramming. Moreover, a signaling and transcriptional principle governing these processes has been identified, raising the possibility of inducing the germ cell fate precisely from pluripotent stem cells in culture. These advances will in turn serve as a basis to explore the mechanism of germ cell specification in other mammals, including humans. The recapitulation of germ cell development in humans in culture will provide unprecedented opportunities to understand the basis of the propagation of our genome, both under normal and diseased conditions.

Sex determination in mammalian germ cells: extrinsic versus intrinsic factors

Mammalian germ cells do not determine their sexual fate based on their XX or XY chromosomal constitution. Instead, sexual fate is dependent on the gonadal environment in which they develop. In a fetal testis, germ cells commit to the spermatogenic programme of development during fetal life, although they do not enter meiosis until puberty. In a fetal ovary, germ cells commit to oogenesis by entering prophase of meiosis I. Although it was believed previously that germ cells are pre-programmed to enter meiosis unless they are actively prevented from doing so, recent results indicate that meiosis is triggered by a signaling molecule, retinoic acid (RA). Meiosis is avoided in the fetal testis because a male-specifically expressed enzyme actively degrades RA during the critical time period. Additional extrinsic factors are likely to influence sexual fate of the germ cells, and in particular, we postulate that an additional male-specific fate-determining factor or factors is involved. The full complement of intrinsic factors that underlie the competence of gonadal germ cells to respond to RA and other extrinsic factors is yet to be defined.

Stem cells: The therapeutic role in the treatment of diabetes mellitus

The unlimited proliferative ability and plasticity to generate other cell types ensures that stem cells represent a dynamic system apposite for the identification of new molecular targets and the production and development of novel drugs. These cell lines derived from embryos could be used as a model for the study of basic and applied aspects in medical therapeutics, environmental mutagenesis and disease management. As a consequence, these can be tested for safety or to predict or anticipate potential toxicity in humans. Human ES cell lines may, therefore, prove clinically relevant to the development of safer and more effective drugs for patients presenting with diabetes mellitus.

Chicken embryonic stem cells as a non-mammalian embryonic stem cell model

Embryonic stem cells (ESCs) were isolated in the early 1980s from mouse and in the late 1990s from primate and human. These cells present the unique property of self-renewal and the ability to generate differentiated progeny in all embryonic lineages both in vitro and in vivo. The mESCs (mouse embryonic stem cells) can contribute to both somatic and germinal lineages once re-injected into a recipient embryo at the blastocyst stage. In avian species, chicken embryonic stem cells (cESCs) have been isolated from the in vitro culture of early chicken blastodermal cells (cBCs) taken from stage X embryo (EG&K) These cESCs can be maintained under specific culture conditions and have been characterized on the basis of their morphology, biochemical features, in vitro differentiation potentialities and in vivo morphogenetic properties. The relationship between these cESCs and some of the chicken germ cells identified and grown under specific culture conditions are still under debate, in particular with the identification of the Cvh gene as a key factor for germ cell determination. Moreover, by cloning the avian homologue of the Oct4 mammalian gene, we have demonstrated that this gene, as well as the chicken Nanog gene, was involved in the characterization and maintenance of the chicken pluripotency. These first steps toward the understanding of pluripotency control in a non-mammalian species opens the way for the development and characterization of putative new cell types such as chicken EpiSC and raises the question of the existence of reprogramming in avian species. These different points are discussed.

The DM domain protein DMRT1 is a dose-sensitive regulator of fetal germ cell proliferation and pluripotency

Dmrt1 (doublesex and mab-3 related transcription factor 1) is a conserved transcriptional regulator of male differentiation required for testicular development in vertebrates. Here, we show that in mice of the 129Sv strain, loss of Dmrt1 causes a high incidence of teratomas, whereas these tumors do not form in Dmrt1 mutant C57BL/6J mice. Conditional gene targeting indicates that Dmrt1 is required in fetal germ cells but not in Sertoli cells to prevent teratoma formation. Mutant 129Sv germ cells undergo apparently normal differentiation up to embryonic day 13.5 (E13.5), but some cells fail to arrest mitosis and ectopically express pluripotency markers. Expression analysis and chromatin immunoprecipitation identified DMRT1 target genes, whose missexpression may underlie teratoma formation. DMRT1 indirectly activates the GDNF coreceptor Ret, and it directly represses the pluripotency regulator Sox2. Analysis of human germ cell tumors reveals similar gene expression changes correlated to DMRT1 levels. Dmrt1 behaves genetically as a dose-sensitive tumor suppressor gene in 129Sv mice, and natural variation in Dmrt1 activity can confer teratoma susceptibility. This work reveals a genetic link between testicular dysgenesis, pluripotency regulation, and teratoma susceptibility that is highly sensitive to genetic background and to gene dosage.

Generation of primordial germ cells from pluripotent stem cells

Embryonic stem (ES) cells, derived from pre-implantation embryo, embryonic germ (EG) cells, derived from embryonic precursors of gametes, primordial germ cells (PGCs), can differentiate into any cell type in the body. Moreover, ES cells have the capacity to differentiate into PGCs in vitro. In the present study we have shown the differentiation capacity of six EG cell lines to form PGCs in vitro, in comparison to ES cells. Cell lines were differentiated via embryoid body (EB) formation using the co-expression of mouse vasa homolog (Mvh) and Oct-4 to identify newly formed PGCs in vitro. We found an increase of PGC numbers in almost all analysed cell lines in 5-day-old EBs, thus suggesting that EG and ES cells have similar efficiency to generate PGCs. The addition of retinoic acid confirmed that the cultures had attained a PGC-like identity and continued to proliferate. Furthermore we have shown that the expression pattern of Prmt5 and H3K27me3 in newly formed PGCs is similar to that observed in embryonic day E11.5 PGCs in vivo. By co-culturing EBs with Chinese hamster ovary (CHO) cells some of the PGCs entered into meiosis, as judged by Scp3 expression. The derivation of germ cells from pluripotent stem cells in vitro could provide an invaluable model system to study both the genetic and epigenetic programming of germ cell development in vivo.

A self-renewal program controls the expansion of genetically unstable cancer stem cells in pluripotent stem cell-derived tumors

Human germ cell tumors are often metastatic, presumably due to distal site tumor growth by cancer stem cells. To determine whether cancer stem cells can be identified in a transplantation model of testicular germ cell tumor, we transplanted murine embryonic germ cells (EGCs) into the testis of adult severe combined immunodeficient mice. Transplantation resulted in a locally invasive solid tumor, with a cellular component that generated secondary tumors upon serial transplantation. The secondary tumors were invariably metastatic, a feature not observed in the primary tumors derived from EGCs. To characterize the differences between EGCs and the tumor-derived stem cells, we performed karyotype and microarray analysis. Our results show that generation of cancer stem cells is associated with the acquisition of nonclonal genomic rearrangements not found in the originating population. Furthermore, pretreatment of EGCs with a potent inhibitor of self-renewal, retinoic acid, prevented tumor formation and the emergence of these genetically unstable cancer stem cells. Microarray analysis revealed that EGCs and first- and second-generation cancer stem cells were highly similar; however, approximately 1,000 differentially expressed transcripts could be identified corresponding to alterations in oncogenes and genes associated with motility and development. Combined, the data suggest that the activation of oncogenic pathways in a cellular background of genetic instability, coupled with an inherent ability to self-renew, is involved in the acquisition of metastatic behavior in the cancer stem cell population of tumors derived from pluripotent cells.

Cytolytic assessment of hyperacute rejection and production of nuclear transfer embryos using hCD46-transgenic porcine embryonic germ cells

Human complement regulatory protein hCD46 may reduce the hyperacute rejection (HAR) in pig-to-human xenotransplantation. In this study, anhCD46gene was introduced into porcine embryonic germ (EG) cells. Treatment of human serum did not affect the survival of hCD46-transgenic EG cells, whereas the treatment significantly reduced the survival of non-transgenic EG cells (p< 0.01). The transgenic EG cells presumably capable of alleviating HAR were transferred into enucleated oocytes. Among 235 reconstituted oocytes, 35 (14.9%) developed to the blastocyst stage. Analysis of individual embryos indicated that 80.0% (28/35) of embryos contained the transgene hCD46. The result of the present study demonstrates resistance of hCD46-transgenic EG cells against HAR, and the usefulness of the transgenic approach may be predicted by this cytolytic assessment prior to actual production of transgenic pigs. Subsequently performed EG cell nuclear transfer gave rise to hCD46-transgenic embryos. Further study on the transfer of these embryos to recipients may produce hCD46-transgenic pigs.

Instructing an embryonic stem cell-derived oocyte fate: lessons from endogenous oogenesis

Female reproductive potential is limited in the majority of species due to oocyte depletion. Because functional human oocytes are restricted in number and accessibility, a robust system to differentiate oocytes from stem cells would enable a thorough investigation of the genetic, epigenetic, and environmental factors affecting human oocyte development. Also, the differentiation of functional oocytes from stem cells may permit the success of human somatic cell nuclear transfer for reprogramming studies and for the production of patient-specific embryonic stem cells (ESCs). Thus, ESC-derived oocytes could ultimately help to restore fertility in women. Here, we review endogenous and ESC-derived oocyte development, and we discuss the potential and challenges for differentiating functional oocytes from ESCs.

The Ste20 kinases Ste20-related proline-alanine-rich kinase and oxidative-stress response 1 regulate NKCC1 function in sensory neurons

NKCC1 is highly expressed in dorsal root ganglion neurons, where it is involved in gating sensory information. In a recent study, it was shown that peripheral nerve injury results in increased NKCC1 activity, not due to an increase in cotransporter expression, but to increased phosphorylation of the cotransporter (Pieraut, S., Matha, V., Sar, C., Hubert, T., Méchaly, I., Hilaire, C., Mersel, M., Delpire, E., Valmier, J., and Scamps, F. (2007) J. Neurosci. 27, 6751-6759). Our laboratory has also identified two Ste20-like kinases that bind and phosphorylate NKCC1: Ste20-related proline-alanine-rich kinase (SPAK) and oxidative-stress response 1 (OSR1). In this study, we show that both kinases are expressed at similar expression levels in spinal cord and dorsal root ganglion neurons, and that both kinases participate equally in the regulation of NKCC1. Using a novel fluorescence method to assay NKCC1 activity in single cells, we show a 50% reduction in NKCC1 activity in DRG neurons isolated from SPAK knockout mice, indicating that another kinase, e.g. OSR1, is present to phosphorylate and activate the cotransporter. Using a nociceptive dorsal root ganglion sensory neuronal cell line, which expresses the same cation-chloride cotransporters and kinases as native DRG neurons, and gene silencing via short hairpin RNA, we demonstrate a direct relationship between kinase expression and cotransporter activity. We show that inactivation of either kinase significantly affects NKCC1 activity, whereas inactivation of both kinases results in an additive effect. In summary, our study demonstrates redundancy of kinases in the regulation of NKCC1 in dorsal root ganglion neurons.

Abnormal DNA methyltransferase expression in mouse germline stem cells results in spermatogenic defects

Although spermatogonial stem cells (SSCs) are committed to spermatogenesis, they may also convert to an embryonic stem cell-like pluripotent state at a low frequency. Because changes in DNA methylation patterns are associated with this conversion, we examined the effect of manipulating DNA methyltransferase (Dnmt) expression on the fate of cultured SSCs, germline stem (GS) cells. Dnmt1 knockdown induced apoptosis in GS cells, which was attenuated by the loss of Trp53. In contrast, GS cells proliferated normally in vitro after Dnmt3a/Dnmt3b ablation or during Dnmt3l overexpression. However, Dnmt3a/Dnmt3b double-mutant cells showed hypomethylation in the SineB1 repetitive sequence, and Dnmt3l-overexpressing cells showed hypermethylation in major and minor satellite sequences; neither cell type formed teratomas and completed spermatogenesis following transplantation into the seminiferous tubules. Although genetic manipulation did not increase the conversion of GS cells to a pluripotent state, these results underscore the important role of DNMTs in survival and spermatogenic differentiation in SSCs.

Parameters influencing derivation of embryonic stem cells from murine embryos

The derivation of ES cells is poorly understood and varies in efficiency between different strains of mice. We have investigated potential differences between embryos of permissive and recalcitrant strains during diapause and ES cell derivation. We found that in diapause embryos of the recalcitrant C57BL/6 and CBA strains, the epiblast failed to expand during the primary explant phase of ES cell derivation, whereas in the permissive 129 strain, it expanded dramatically. Epiblasts from the recalcitrant strains could be expanded by reducing Erk activation. Isolation of 129 epiblasts facilitated very efficient derivation of ES cell lines in serum- and feeder-free conditions, but reduction of Erk activity was required for derivation of ES cells from isolated C57BL/6 or CBA epiblasts. The results suggest that the discrepancy in ES cell derivation efficiency is not attributable merely to variable prodifferentiative effects of the extra-embryonic lineages but also to an intrinsic variability within the epiblast to maintain pluripotency.

Heritable imprinting defect caused by epigenetic abnormalities in mouse spermatogonial stem cells

Male germ cells undergo dynamic epigenetic reprogramming during fetal development, eventually establishing spermatogonial stem cells (SSCs) that can convert into pluripotent stem cells. However, little is known about the developmental potential of fetal germ cells and how they mature into SSCs. We developed a culture system for fetal germ cells that proliferate for long periods of time. Male germ cells from embryos 12.5-18.5 days postcoitum could expand by glial cell line-derived neurotrophic factor, a self-renewal factor for SSCs. These cells did not form teratomas, but repopulated seminiferous tubules and produced spermatogenesis, exhibiting spermatogonia potential. However, the offspring from cultured cells showed growth abnormalities and were defective in genomic imprinting. The imprinting defect persisted in both the male and female germlines for at least four generations. Moreover, germ cells in the offspring showed abnormal histone modifications and DNA methylation patterns. These results indicate that fetal germ cells have a limited ability to become pluripotent cells and lose the ability to undergo epigenetic reprogramming by in vitro culture.

Reprogramming primordial germ cells (PGC) to embryonic germ (EG) cells

In this unit we describe the derivation of pluripotent embryonic germ (EG) cells from mouse primordial germ cells (PGCs) isolated from both 8.5- and 11.5-days post-coitum (dpc) embryos. Once EG cells are derived we explain how to propagate and characterize the cell lines. We introduce readers to PGCs and explain differences between PGCs and their in vitro derivatives EG cells. Finally, we also compare mouse EG cells with ES cells. This unit will be of great interest to anyone interested in PGCs or studying the behavior of cultured PGCs or the derivation of new EG cell lines.

Pluripotent stem cell lines

The derivation of human embryonic stem cells 10 years ago ignited an explosion of public interest in stem cells, yet this achievement depended on prior decades of research on mouse embryonic carcinoma cells and embryonic stem cells. In turn, the recent derivation of mouse and human induced pluripotent stem cells depended on the prior studies on mouse and human embryonic stem cells. Both human embryonic stem cells and induced pluripotent stem cells can self-renew indefinitely in vitro while maintaining the ability to differentiate into advanced derivatives of all three germ layers, features very useful for understanding the differentiation and function of human tissues, for drug screen and toxicity testing, and for cellular transplantation therapies. Here we review the family of pluripotent cell lines derived from early embryos and from germ cells, and compare them with the more recently described induced pluripotent stem cells.

Global mapping of DNA methylation in mouse promoters reveals epigenetic reprogramming of pluripotency genes

DNA methylation patterns are reprogrammed in primordial germ cells and in preimplantation embryos by demethylation and subsequent de novo methylation. It has been suggested that epigenetic reprogramming may be necessary for the embryonic genome to return to a pluripotent state. We have carried out a genome-wide promoter analysis of DNA methylation in mouse embryonic stem (ES) cells, embryonic germ (EG) cells, sperm, trophoblast stem (TS) cells, and primary embryonic fibroblasts (pMEFs). Global clustering analysis shows that methylation patterns of ES cells, EG cells, and sperm are surprisingly similar, suggesting that while the sperm is a highly specialized cell type, its promoter epigenome is already largely reprogrammed and resembles a pluripotent state. Comparisons between pluripotent tissues and pMEFs reveal that a number of pluripotency related genes, including Nanog, Lefty1 and Tdgf1, as well as the nucleosome remodeller Smarcd1, are hypomethylated in stem cells and hypermethylated in differentiated cells. Differences in promoter methylation are associated with significant differences in transcription levels in more than 60% of genes analysed. Our comparative approach to promoter methylation thus identifies gene candidates for the regulation of pluripotency and epigenetic reprogramming. While the sperm genome is, overall, similarly methylated to that of ES and EG cells, there are some key exceptions, including Nanog and Lefty1, that are highly methylated in sperm. Nanog promoter methylation is erased by active and passive demethylation after fertilisation before expression commences in the morula. In ES cells the normally active Nanog promoter is silenced when targeted by de novo methylation. Our study suggests that reprogramming of promoter methylation is one of the key determinants of the epigenetic regulation of pluripotency genes. Epigenetic reprogramming in the germline prior to fertilisation and the reprogramming of key pluripotency genes in the early embryo is thus crucial for transmission of pluripotency.