Turning germ cells into stem cells

SDF-1/CXCR4 signal is involved in the induction of Primordial Germ Cell migration in a model marine fish, Japanese anchovy (Engraulis japonicus)

Primordial germ cells (PGCs) are pivotal for gonadal development and reproductive success. Though artificial induction of sterility by targeting PGCs are gaining popularity due to its advantages in fish surrogacy and biodiversity management, it is often skill and time intensive. In this study, we have focused on understanding the role of PGCs and the chemotactic SDF-1/CXCR4 signaling on gonad development of Japanese anchovy (JA, Engraulis japonicus), an upcoming marine model organism with eco-commercial values, with an aim to develop a novel, easy, and versatile gonad sterilization method. Our data showed that PGC migration related genes, i.e., sdf-1a, sdf-1b, cxcr4a, cxcr4b and vasa, are phylogenetically closer relatives of respective herring (Clupea harengus) and zebrafish (Danio rerio) homolog. Subsequently, PGC marking and live tracing experiments confirmed that PGC migration in JA initiates from 16 hours post fertilization (hpf) followed by PGC settlement in the gonadal ridge at 44 hpf. We found that overexpression of zebrafish sdf-1a mRNA in the germ cell suppresses cxcr4a and increases cxcr4b transcription at 8 hpf, dose dependently disrupts PGC migration at 24-48 hpf, induces PGC death and upregulates sdf-1b at 5 days post hatching. 48 h of immersion treatment with CXCR4 antagonist (AMD3100, Abcam) also accelerated PGC mismigration and pushed the PGC away from gonadal ridge in a dose responsive manner, and further when grown to adulthood caused germ cell less gonad formation in some individuals. Cumulatively, our data, for the first time, suggests that JA PGC migration is largely regulated by SDF1/CXCR4 signaling, and modulation of this signaling has strong potential for sterile, germ cell less gonad preparation at a mass scale. However, further in-depth analysis is pertinent to apply this methodology in marine fish species to successfully catapult Japanese anchovy into a true marine fish model.

The Beginning of Meiosis in Mammalian Female Germ Cells: A Never-Ending Story of Intrinsic and Extrinsic Factors

Meiosis is the unique division of germ cells resulting in the recombination of the maternal and paternal genomes and the production of haploid gametes. In mammals, it begins during the fetal life in females and during puberty in males. In both cases, entering meiosis requires a timely switch from the mitotic to the meiotic cell cycle and the transition from a potential pluripotent status to meiotic differentiation. Revealing the molecular mechanisms underlying these interrelated processes represents the essence in understanding the beginning of meiosis. Meiosis facilitates diversity across individuals and acts as a fundamental driver of evolution. Major differences between sexes and among species complicate the understanding of how meiosis begins. Basic meiotic research is further hindered by a current lack of meiotic cell lines. This has been recently partly overcome with the use of primordial-germ-cell-like cells (PGCLCs) generated from pluripotent stem cells. Much of what we know about this process depends on data from model organisms, namely, the mouse; in mice, the process, however, appears to differ in many aspects from that in humans. Identifying the mechanisms and molecules controlling germ cells to enter meiosis has represented and still represents a major challenge for reproductive medicine. In fact, the proper execution of meiosis is essential for fertility, for maintaining the integrity of the genome, and for ensuring the normal development of the offspring. The main clinical consequences of meiotic defects are infertility and, probably, increased susceptibility to some types of germ-cell tumors. In the present work, we report and discuss data mainly concerning the beginning of meiosis in mammalian female germ cells, referring to such process in males only when pertinent. After a brief account of this process in mice and humans and an historical chronicle of the major hypotheses and progress in this topic, the most recent results are reviewed and discussed.

Transcriptome and methylome analysis of CNS germ cell tumor finds its cell-of-origin in embryogenesis and reveals shared similarities with testicular counterparts

BACKGROUND

CNS germ cell tumors (GCTs) predominantly develop in pediatric and young adult patients with variable responses to surgery, radiation, and chemotherapy. This study aimed to examine the complex and largely unknown pathogenesis of CNS GCTs.

METHODS

We used a combined transcriptomic and methylomic approach in 84 cases and conducted an integrative analysis of the normal cells undergoing embryogenesis and testicular GCTs.

RESULTS

Genome-wide transcriptome analysis in CNS GCTs indicated that germinoma had a transcriptomic profile representative of primitive cells during early embryogenesis with high meiosis/mitosis potentials, while nongerminomatous GCTs (NGGCTs) had differentiated phenotypes oriented toward tissue formation and organogenesis. Co-analysis with the transcriptome of human embryonic cells revealed that germinomas had expression profiles similar to those of primordial germ cells, while the expression profiles of NGGCTs were similar to those of embryonic stem cells. Some germinoma cases were characterized by extensive immune-cell infiltration and high expression of cancer-testis antigens. NGGCTs had significantly higher immune-cell infiltration, characterized by immune-suppression phenotype. CNS and testicular GCTs (TGCTs) had similar mutational profiles; TGCTs showed enhanced copy number alterations. Methylation analysis clustered germinoma/seminoma and nongerminoma/nonseminoma separately. Germinoma and seminoma were co-categorized based on the degree of the tumor microenvironment balance.

CONCLUSIONS

These results suggested that the pathophysiology of GCTs was less dependent on their site of origin and more dependent on the state of differentiation as well as on the tumor microenvironment balance. This study revealed distinct biological properties of GCTs, which will hopefully lead to future treatment development.

Histopathology and prognosis of germ cell tumors metastatic to brain: cohort study

INTRODUCTION

Germ cell tumors (GCTs) are uncommon neoplasms predominantly arising in midline tissues. The prognostic significance of histopathology in predicting metastatic GCT behavior is poorly understood.

METHODS

Multicenter international cohort study including 29 patients with GCTs metastatic to brain were retrospectively investigated (18 patients from Mayo Clinic and 11 patients from the intracranial germ cell tumor genome analysis consortium in Japan). Clinical characteristics were analyzed using the Chi-square test (two-tailed) for categorical variables and using the log-rank test for survival data.

RESULTS

Median age at treatment was 31 years (range 14-58). Primary disease sites were testis (71%), mediastinum (18%), and female reproductive organs (11%). Median metastatic interval was 223 days (range, 6-6124). Median follow-up was 346 days (range, 1-5356), with 16 deaths (57%) occurring after the median overall survival of 455 days. Actuarial one-year survival was 51%; 12-of-16 deaths (75%) were attributed to intracranial disease. Appearance of the same GCT subtype at the metastatic site as the primary was high for non-seminomatous GCT (NSGCT, 64-100%), but low for seminoma/dysgerminoma and mature teratoma (MT, 14, 17%, respectively). Gain of a new component was seen in 4 (20%)-3 of which included embryonal carcinoma (EC) at the primary site (75%). Incidence of cases without seminoma/dysgerminoma increased significantly after metastasis (p = 0.02). Metastatic interval was shorter in cases with histological change (199 vs 454 days, p = 0.009). Overall survival was associated with MT primary histopathology (p = 0.02).

CONCLUSION

Histological differentiation at the primary GCT site influences metastatic prognosis. Aggressive behavior is associated with NSGCT, while EC frequently demonstrates multi-directional histological differentiation after brain metastasis, and such histological dynamism is associated with shorter metastatic interval. Most metastases occurred within one year of diagnosis, emphasizing the need for close surveillance in newly diagnosed extra-cranial GCT.

Induction of mesenchymal stem cell‑like transformation in rat primary glial cells using hypoxia, mild hypothermia and growth factors

The transformation of rat primary glial cells into mesenchymal stem cells (MSCs) is intriguing as more seed cells can be harvested. The present study aimed to evaluate the effects of growth factors, hypoxia and mild hypothermia on the transformation of primary glial cells into MSCs. Rat primary glial cells were induced to differentiate by treatment with hypoxia, mild hypothermia and basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF). Immunohistochemistry and western blotting were then used to determine the expression levels of glial fibrillary acidic protein (GFAP), nestin, musashi-1, neuron specific enolase (NSE) and neuronal nuclei (NeuN), in each treatment group. bFGF and EGF increased the proportion of CD44⁺ and CD105⁺ cells, while anaerobic mild hypothermia increased the proportion of CD90⁺ cells. The combination of bFGF and EGF, and anaerobic mild hypothermia increased the proportion of CD29⁺ cells and significantly decreased the proportions of GFAP⁺ cells and NSE⁺ cells. Treatment of primary glial cells with bFGF and EGF increased the expression levels of nestin, Musashi-1, NSE and NeuN. Anaerobic mild hypothermia increased the expression levels of Musashi-1 and decreased the expression levels of NSE and NeuN in glial cells. The results of the present study demonstrated that bFGF, EGF and anaerobic mild hypothermia treatments may promote the transformation of glial cells into MSC-like cells, and that the combination of these two treatments may have the optimal effect.

Some of the Factors Involved in Male Infertility: A Prospective Review

Infertility is defined as the inability of couples to have a baby after one year of regular unprotected intercourse, affecting 10 to 15% of couples. According to the latest WHO statistics, approximately 50-80 million people worldwide sufer from infertility, and male factors are responsible for approximately 20-30% of all infertility cases. The diagnosis of infertility in men is mainly based on semen analysis. The main parameters of semen include: concentration, appearance and motility of sperm. Causes of infertility in men include a variety of things including hormonal disorders, physical problems, lifestyle problems, psychological issues, sex problems, chromosomal abnormalities and single-gene defects. Despite numerous efforts by researchers to identify the underlying causes of male infertility, about 70% of cases remain unknown. These statistics show a lack of understanding of the mechanisms involved in male infertility. This article focuses on the histology of testicular tissue samples, the male reproductive structure, factors affecting male infertility, strategies available to find genes involved in infertility, existing therapeutic methods for male infertility, and sperm recovery in infertile men.

Evolving Role of RING1 and YY1 Binding Protein in the Regulation of Germ-Cell-Specific Transcription

Separation of germline cells from somatic lineages is one of the earliest decisions of embryogenesis. Genes expressed in germline cells include apoptotic and meiotic factors, which are not transcribed in the soma normally, but a number of testis-specific genes are active in numerous cancer types. During germ cell development, germ-cell-specific genes can be regulated by specific transcription factors, retinoic acid signaling and multimeric protein complexes. Non-canonical polycomb repressive complexes, like ncPRC1.6, play a critical role in the regulation of the activity of germ-cell-specific genes. RING1 and YY1 binding protein (RYBP) is one of the core members of the ncPRC1.6. Surprisingly, the role of Rybp in germ cell differentiation has not been defined yet. This review is focusing on the possible role of Rybp in this process. By analyzing whole-genome transcriptome alterations of the Rybp^-/- embryonic stem (ES) cells and correlating this data with experimentally identified binding sites of ncPRC1.6 subunits and retinoic acid receptors in ES cells, we propose a model how germ-cell-specific transcription can be governed by an RYBP centered regulatory network, underlining the possible role of RYBP in germ cell differentiation and tumorigenesis.

Seminoma component of mixed testicular germ cell tumor shows a higher incidence of loss of heterozygosity than pure-type seminoma

Using analysis of allelic loss (loss of heterozygosity [LOH]), we previously reported a putative progression pathway from germ cell neoplasia in situ (GCNIS) to seminoma and then to embryonal carcinoma in mixed-type testicular germ cell tumors. To identify the genetic backgrounds related to the progression of nonseminomatous germ cell tumor, patterns of LOH were studied in seminoma components in mixed tumors (18 cases), pure seminomas (20 cases), and coexisting GCNIS lesions. Each tumor was assessed for LOH at 22 polymorphic loci located on 12 chromosomal arms: 3q, 5q, 6p, 9p, 10q, 11p, 12p, 12q, 13q, 17p, 17q, and 18q. For all informative loci, the frequency of LOH in seminoma components in mixed tumors was significantly higher than that in pure seminomas (32% [96/302 loci] versus 19% [60/323 loci], P < .0001). The frequency of LOH in GCNIS lesions was not significantly different between the 2 tumor groups. The frequencies of LOH at chromosomes 6p and 10q were significantly higher in seminoma components in mixed tumors than in pure seminomas (P = .020 and P = .0041, respectively). Immunohistochemical analysis demonstrated a close association between the allelic status of the 10q23 locus and levels of phosphatase and tensin homolog deleted from chromosome 10 protein expression in seminoma (P = .00051). These data indicate that the seminoma, which has a potential to progress to nonseminomatous germ cell tumor, already exhibits several genetic changes including allelic losses of 6p and 10q, unlike pure seminoma.

Pluripotency and differentiation of cells from human testicular sperm extraction: An investigation of cell stemness

Human male germ-line stem cells (hmGSCs) and human testis-derived embryonic stem cell-like (htESC-like) cells are claimed to be in vitro pluripotent counterparts of spermatogonial stem cells (SSCs), but the origin and pluripotency of human testis-derived cell cultures are still under debate. The aim of this study was to generate putative pluripotent stem cells in vitro from human testicular sperm-extracted (TESE) samples of infertile men, and to assess their pluripotency and capacity to differentiate. TESE samples were minced, enzymatically disaggregated and dispersed into single-cell or cluster suspensions, and then cultured. Initially, cell clusters resembled those described for hmGSCs and htESC-like cells, and were positive for markers such as OCT4/POU5F1, NANOG, and TRA-2-54. Prolonged propagation of cell clusters expressing pluripotency markers did not thrive; instead, the cells that emerged possessed characteristics of mesenchymal stromal cells (MSCs) such as STRO-1, CD105/EGLN1, CD13/ANPEP, SOX9, vimentin, and fibronectin. KIT, SOX2, and CD44 were not expressed by these MSCs. The multipotential differentiation capacity of these cells was confirmed using Oil Red-O and Alizarin Red staining after induction with specific culture conditions. It is therefore concluded that pluripotent stem cells could not be derived using the conditions previously reported to be successful for TESE samples.

Direct Reprogramming of Human Primordial Germ Cells into Induced Pluripotent Stem Cells: Efficient Generation of Genetically Engineered Germ Cells

Primordial germ cells (PGCs) share many properties with embryonic stem cells (ESCs) and innately express several key pluripotency-controlling factors, including OCT4, NANOG, and LIN28. Therefore, PGCs may provide a simple and efficient model for studying somatic cell reprogramming to induced pluripotent stem cells (iPSCs), especially in determining the regulatory mechanisms that fundamentally define pluripotency. Here, we report a novel model of PGC reprogramming to generate iPSCs via transfection with SOX2 and OCT4 using integrative lentiviral. We also show the feasibility of using nonintegrative approaches for generating iPSC from PGCs using only these two factors. We show that human PGCs express endogenous levels of KLF4 and C-MYC protein at levels similar to embryonic germ cells (EGCs) but lower levels of SOX2 and OCT4. Transfection with both SOX2 and OCT4 together was required to induce PGCs to a pluripotent state at an efficiency of 1.71%, and the further addition of C-MYC increased the efficiency to 2.33%. Immunohistochemical analyses of the SO-derived PGC-iPSCs revealed that these cells were more similar to ESCs than EGCs regarding both colony morphology and molecular characterization. Although leukemia inhibitory factor (LIF) was not required for the generation of PGC-iPSCs like EGCs, the presence of LIF combined with ectopic exposure to C-MYC yielded higher efficiencies. Additionally, the SO-derived PGC-iPSCs exhibited differentiation into representative cell types from all three germ layers in vitro and successfully formed teratomas in vivo. Several lines were generated that were karyotypically stable for up to 24 subcultures. Their derivation efficiency and survival in culture significantly supersedes that of EGCs, demonstrating their utility as a powerful model for studying factors regulating pluripotency in future studies.

Cancer stem cells and therapeutic targets: an emerging field for cancer treatment

Recent paradigm in the field of cancer defines its origin from a small population of fast growing cells known as cancer stem cells (CSCs), and they are mainly responsible for disease aggressiveness, drug resistance and tumor relapse. The existence of CSCs has been proven in different types of cancer and possesses characteristic expression of a wide array of cell surface markers specific to the type of cancer. CSCs have been isolated and enriched using several surface markers in different cancer types. Self-renewal, drug resistance and the ability to transition from epithelial to mesenchymal phenotype are the major features attributed to this fraction of mutated stem cells. The CSC hypothesis proposes that these CSCs mimic stem cells by sharing similar pathways, such as Wnt, SHH, Notch and others. Further, the niche, which in this case is the tumor microenvironment, plays a very important role in the maintenance of CSCs. Altogether, this emerging field of research on CSCs is expected to unveil answers to the most difficult issues of one of the most dreadful diseases called cancer.

Hypoxia induces pluripotency in primordial germ cells by HIF1α stabilization and Oct4 deregulation

AIMS

To study the mechanisms of pluripotency induction, we compared gene expression in pluripotent embryonic germ cells (EGCs) and unipotent primordial germ cells (PGCs).

RESULTS

We found 11 genes ≥1.5-fold overexpressed in EGCs. None of the genes identified was the Yamanaka genes but instead related to glycolytic metabolism. The prospect of pluripotency induction by cell metabolism manipulation was investigated by hypoxic culturing. Hypoxia induced a glycolytic program in PGCs in detriment of mitochondrial oxidative phosphorylation. We demonstrate that hypoxia alone induces reprogramming in PGCs, giving rise to hypoxia-induced EGC-like cells (hiEGLs), which differentiate into cells of the three germ layers in vitro and contribute to the internal cell mass of the blastocyst in vivo, demonstrating pluripotency. The mechanism of hypoxia induction involves HIF1α stabilization and Oct4 deregulation. However, hiEGL cannot be passaged long term. Self-renewal capacity is not achieved by hypoxia likely due to the lack of upregulation of c-Myc and Klf4. Gene expression analysis of hypoxia signaling suggests that hiEGLs have not reached the stabilization phase of cell reprogramming.

INNOVATION AND CONCLUSION

Our data suggest that the two main properties of stemness, pluripotency and self-renewal, are differentially regulated in PGC reprogramming induced by hypoxia.

Germ-cell culture conditions facilitate the production of mouse embryonic stem cells

The derivation of embryonic stem-cell (ESC) lines from blastocysts is a very inefficient process. Murine ESCs are thought to arise from epiblast cells that are already predisposed to a primordial-germ-cell fate. During the process of ESC derivation from B6D2 F1 hybrid mice, if we first culture the embryo from the two-cell stage in medium supplemented with LIF, we improve the quality of the blastocyst. When the blastocyst is then cultured in a germ-line stem-cell culture medium (GSCm), we are able to more efficiently (28.3%) obtain quality ESC lines that have a normal karyotype, proper degree of chimerism, and exhibit germ-line transmission when microinjected into blastocysts. Although germ-cell-specific genes were expressed in all culture medium conditions, GSCm did not shift the transcriptome towards germ-cell specification. A correlation was further observed between ESC derivation efficiency and the expression of some imprinted genes and retrotransposable elements. In conclusion, the combination of LIF supplementation followed by culture in GSCm establishes a higher efficiency method for ESC derivation.

Primordial germ-cell development and epigenetic reprogramming in mammals

Primordial germ cells (PGCs) are the embryonic precursors of the gametes and represent the founder cells of the germline. Specification of PGCs is a critical divergent point during embryogenesis. Whereas the somatic lineages will ultimately perish, cells of the germline have the potential to form a new individual and hence progress to the next generation. It is therefore critical that the genome emerges intact and carrying the appropriate epigenetic information during its passage through the germline. To ensure this fidelity of transmission, PGC development encompasses extensive epigenetic reprogramming. The low cell numbers and relative inaccessibility of PGCs present a challenge to those seeking mechanistic understanding of the crucial developmental and epigenetic processes in this most fascinating of lineages. Here, we present an overview of PGC development in the mouse and compare this with the limited information available for other mammalian species. We believe that a comparative approach will be increasingly important to uncover the extent to which mechanisms are conserved and reveal the critical steps during PGC development in humans.

Seeking the origin of female germline stem cells in the mammalian ovary

The function of female germline stem cells (FGSCs, also called oogonial stem cells) in the adult mammalian ovary is currently debated in the scientific community. As the evidence to support or discard the possible crucial role of this new class of germ cells in mammals has been extensively discussed, in this review, we wonder which could be their origin. We will assume that FGSCs are present in the post-natal ovaries and speculate as to what origin and characteristics such cells could have. We believe that the definition of these features might shed light on future experimental approaches that could clarify the ongoing debate.

Reprogramming and transdifferentiation shift the landscape of regenerative medicine

Regenerative medicine is a new interdisciplinary field in biomedical science, which aims at the repair or replacement of the defective tissue or organ by congenital defects, age, injury, or disease. Various cell-related techniques such as stem cell-based biotherapy are a hot topic in the current press, and stem cell research can help us to expand our understanding of development as well as the pathogenesis of disease. In addition, new technology such as reprogramming or dedifferentiation and transdifferentiation open a new area for regenerative medicine. Here we review new approaches of these technologies used for cell-based therapy and discuss future directions and challenges in the field of regeneration.

Small RNAs in germline development

One of the most important and evolutionarily conserved strategies to control gene expression in higher metazoa is posttranscriptional regulation via small regulatory RNAs such as microRNAs (miRNAs), endogenous small interfering RNAs (endo-siRNAs), and piwi-interacting RNAs (piRNAs). Primordial germ cells, which are defined by their totipotent potential and noted for their dependence on posttranscriptional regulation by RNA-binding proteins, rely on these small regulatory RNAs for virtually every aspect of their development, including specification, migration, and differentiation into competent gametes. Here, we review current knowledge of the roles miRNAs, endo-siRNAs, and piRNAs play at all stages of germline development in various organisms, focusing on studies in the mouse.

Induced pluripotent stem cells in reproductive medicine

Despite recent advances in reproductive medicine, there are still no effective treatments for severe infertility caused by congenital absence of germ cells or gonadotoxic treatments during prepubertal childhood. However, the development of technologies for germ cell formation from stem cells in vitro, induction of pluripotency from somatic cells, and production of patient-specific pluripotent stem cells may provide new solutions for treating these severe fertility problems. It may be possible to produce germ cells in vitro from our own somatic cells that can be used to restore fertility. In addition, these technologies may also bring about novel therapies by helping to elucidate the mechanisms of human germ cell development. In this review, we describe the current approaches for obtaining germ cells from pluripotent stem cells, and provide basic information about induction of pluripotency and germ cell development.

Spermatogonial stem cells (SSCs) in buffalo (Bubalus bubalis) testis

BACKGROUND

Water buffalo is an economically important livestock species and about half of its total world population exists in India. Development of stem cell technology in buffalo can find application in targeted genetic modification of this species. Testis has emerged as a source of pluripotent stem cells in mice and human; however, not much information is available in buffalo.

OBJECTIVES AND METHODS

Pou5f1 (Oct 3/4) is a transcription factor expressed by pluripotent stem cells. Therefore, in the present study, expression of POU5F1 transcript and protein was examined in testes of both young and adult buffaloes by semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and immunohistochemical analysis. Further, using the testis transplantation assay, a functional assay for spermatogonial stem cells (SSCs), stem cell potential of gonocytes/spermatogonia isolated from prepubertal buffalo testis was also determined.

RESULTS

Expression of POU5F1 transcript and protein was detected in prepubertal and adult buffalo testes. Western blot analysis revealed that the POU5F1 protein in the buffalo testis exists in two isoforms; large (∼47 kDa) and small (∼21 kDa). Immunohistochemical analysis revealed that POU5F1 expression in prepubertal buffalo testis was present in gonocytes/spermatogonia and absent from somatic cells. In the adult testis, POU5F1 expression was present primarily in post-meiotic germ cells such as round spermatids, weakly in spermatogonia and spermatocytes, and absent from elongated spermatids. POU5F1 protein expression was seen both in cytoplasm and nuclei of the stained germ cells. Stem cell potential of prepubertal buffalo gonocytes/spermatogonia was confirmed by the presence of colonized DBA-stained cells in the basal membrane of seminiferous tubules of xenotransplanted mice testis.

CONCLUSION/SIGNIFICANCE

These findings strongly indicate that gonocytes/spermatogonia, isolated for prepubertal buffalo testis can be a potential target for establishing a germ stem cell line that would enable genetic modification of buffaloes.

Nuclear reprogramming in mouse primordial germ cells: epigenetic contribution

The unique capability of germ cells to give rise to a new organism, allowing the transmission of primary genetic information from generation to generation, depends on their epigenetic reprogramming ability and underlying genomic totipotency. Recent studies have shown that genome-wide epigenetic modifications, referred to as “epigenetic reprogramming”, occur during the development of the gamete precursors termed primordial germ cells (PGCs) in the embryo. This reprogramming is likely to be critical for the germ line development itself and necessary to erase the parental imprinting and setting the base for totipotency intrinsic to this cell lineage. The status of genome acquired during reprogramming and the associated expression of key pluripotency genes render PGCs susceptible to transform into pluripotent stem cells. This may occur in vivo under still undefined condition, and it is likely at the origin of the formation of germ cell tumors. The phenomenon appears to be reproduced under partly defined in vitro culture conditions, when PGCs are transformed into embryonic germ (EG) cells. In the present paper, I will try to summarize the contribution that epigenetic modifications give to nuclear reprogramming in mouse PGCs.

Ovarian stem cell niche and follicular renewal in mammals

Stem cell niche consists of perivascular compartment, which connects the stem cells to the immune and vascular systems. During embryonic period, extragonadal primordial germ cells colonize coelomic epithelium of developing gonads. Subsequently, ovarian stem cells (OSC) produce secondary germ cells under the influence of OSC niche, including immune system-related cells and hormonal signaling. The OSC in fetal and adult human ovaries serve as a source of germ and granulosa cells. Lack of either granulosa or germ cell niche will result in premature ovarian failure in spite of the presence of OSC. During perinatal period, the OSC transdifferentiate into fibroblast-like cells forming the ovarian tunica albuginea resistant to environmental threats. They represent mesenchymal precursors of epithelial OSC during adulthood. The follicular renewal during the prime reproductive period (PRP) ensures that there are fresh eggs available for a healthy progeny. End of PRP is followed by exponentially growing fetal genetic abnormalities. The OSC are present in adult, aging, and postmenopausal ovaries, and differentiate in vitro into new oocytes. During in vitro development of large isolated oocytes reaching 200 μm in diameter, an ancestral mechanism of premeiotic nurse cells, which operates during oogenesis in developing ovaries from invertebrates to mammalian species, is utilized. In vitro developed eggs could be used for autologous IVF treatment of premature ovarian failure. Such eggs are also capable to produce parthenogenetic embryos like some cultured follicular oocytes. The parthenotes produce embryonic stem cells derived from inner cell mass, and these cells can serve as autologous pluripotent stem cells.

Mammalian artificial chromosomes and clinical applications for genetic modification of stem cells: an overview

Modifying multipotent, self-renewing human stem cells with mammalian artificial chromosomes (MACs), present a promising clinical strategy for numerous diseases, especially ex vivo cell therapies that can benefit from constitutive or overexpression of therapeutic gene(s). MACs are nonintegrating, autonomously replicating, with the capacity to carry large cDNA or genomic sequences, which in turn enable potentially prolonged, safe, and regulated therapeutic transgene expression, and render MACs as attractive genetic vectors for "gene replacement" or for controlling differentiation pathways in progenitor cells. The status quo is that the most versatile target cell would be one that was pluripotent and self-renewing to address multiple disease target cell types, thus making multilineage stem cells, such as adult derived early progenitor cells and embryonic stem cells, as attractive universal host cells. We will describe the progress of MAC technologies, the subsequent modifications of stem cells, and discuss the establishment of MAC platform stem cell lines to facilitate proof-of-principle studies and preclinical development.

Bone morphogenetic protein 4 mediates human embryonic germ cell derivation

Human primordial germ cells (PGCs) have proven to be a source of pluripotent stem cells called embryonic germ cells (EGCs). Unlike embryonic stem cells, virtually little is known regarding the factors that regulate EGC survival and maintenance. In mice, the growth factor bone morphogenetic protein 4 (BMP4) has been shown to be required for maintaining mouse embryonic stem cells, and disruptions in this gene lead to defects in mouse PGC specification. Here, we sought to determine whether recombinant human BMP4 could influence EGC derivation and/or human PGC survival. We found that the addition of recombinant BMP4 increased the number of human PGCs after 1 week of culture in a dose-responsive manner. The efficiency of EGC derivation and maintenance in culture was also enhanced by the presence of recombinant BMP4 based on alkaline phosphatase and OCT4 staining. In addition, an antagonist of the BMP4 pathway, Noggin, decreased PGC proliferation and led to an increase in cystic embryoid body formation. Quantitative real-time (qRT)-polymerase chain reaction analyses and immunostaining confirmed that the constituents of the BMP4 pathway were upregulated in EGCs versus PGCs. Downstream activators of the BMP4 pathway such as ID1 and phosphorylated SMADs 1 and 5 were also expressed, suggesting a role of this growth factor in EGC pluripotency.

BMP signaling controls formation of a primordial germ cell niche within the early genital ridges

Stem cells are necessary to maintain tissue homeostasis and the microenvironment (a.k.a. the niche) surrounding these cells controls their ability to self-renew or differentiate. For many stem cell populations it remains unclear precisely what cells and signals comprise a niche. Here we identify a possible PGC niche in the mouse genital ridges. Conditional ablation of Bmpr1a was used to demonstrate that BMP signaling is required for PGC survival and migration as these cells colonize the genital ridges. Reduced BMP signaling within the genital ridges led to increased somatic cell death within the mesonephric mesenchyme. Loss of these supporting cells correlated with decreased levels of the mesonephric marker, Pax2, as well as a reduction in genes expressed in the coelomic epithelium including the putative PGC chemo-attractants Kitl and Sdf1a. We propose that BMP signaling promotes mesonephric cell survival within the genital ridges and that these cells support correct development of the coelomic epithelium, the target of PGC migration. Loss of BMP signaling leads to the loss of the PGC target resulting in reduced PGC numbers and disrupted PGC migration.

Pluripotent stem cells derived from adult human testes

Recent reports have demonstrated that adult tissue cells can be induced to pluripotency, the iPS cells, mostly with the addition of genes delivered using viruses. Also, several publications both in mouse and in human have demonstrated that spermatogonial stem cells (SSCs) from testes can convert back to embryonic stem (ES)-like cells without the addition of genes. Furthermore, these pluripotent ES-like cells can differentiate into all three germ layers and organ lineages. Thus, SSCs have great potential for cell-based, autologous organ regeneration therapy for various diseases. We obtained testes from organ donors and using 1 g pieces of tissue (biopsy size) we demonstrate that testis germ cells (putative SSCs and/or their progenitors) reprogram to pluripotency when removed from their stem cell niche and when appropriate growth factors and reagents in embryonic stem cell medium are added. In addition, our method of obtaining pluripotent ES-like cells from germ cells is simpler than the described methods and may be more suitable if this procedure is developed for the clinic to obtain pluripotent cells to cure disease.

Isolation and derivation of mouse embryonic germinal cells

The ability of embryonic germinal cells (EG) to differentiate into primordial germinal cells (PGCs) and later into gametes during early developmental stages is a perfect model to address our hypothesis about cancer and infertility. This protocol shows how to isolate primordial germinal cells from developing gonads in 10.5-11.5 days post coitum (dpc) mouse embryos. Developing gonadal ridges from mouse embryos (C57BL6J) were dissociated by mechanical disruption with collagenase, then plated in a mouse embryo fibroblast feeder layer (MEF-CF1) that was previously mitotically inactivated with mitomycin C in the presence of knockout media and supplemented with Leukemia Inhibitor Factor (LIF), basic Fibroblast Growth Factor (bFGF), and Stem Cell Factor (SCF). Using these optimized methods for PCG identification, isolation, and establishment of culture conditions permits long term cultures of EG cells for more than 40 days. The embryonic germinal cell lines showed embryonic phenotype and expression of common used markers of the pluripotent state. Isolation and derivation of germinal cells in culture provide a tool to understand their development in vitro and offer the opportunity to monitor cumulative damage at genetic and epigenetic levels after exposure to oxidative stress.

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 role for Lin28 in primordial germ-cell development and germ-cell malignancy

The rarity and inaccessibility of the earliest primordial germ cells (PGCs) in the mouse embryo thwart efforts to investigate molecular mechanisms of germ-cell specification. stella (also called Dppa3) marks the rare founder population of the germ lineage. Here we differentiate mouse embryonic stem cells carrying a stella transgenic reporter into putative PGCs in vitro. The Stella(+) cells possess a transcriptional profile similar to embryo-derived PGCs, and like their counterparts in vivo, lose imprints in a time-dependent manner. Using inhibitory RNAs to screen candidate genes for effects on the development of Stella(+) cells in vitro, we discovered that Lin28, a negative regulator of let-7 microRNA processing, is essential for proper PGC development. Furthermore, we show that Blimp1 (also called Prdm1), a let-7 target and a master regulator of PGC specification, can rescue the effect of Lin28 deficiency during PGC development, thereby establishing a mechanism of action for Lin28 during PGC specification. Overexpression of Lin28 promotes formation of Stella(+) cells in vitro and PGCs in chimaeric embryos, and is associated with human germ-cell tumours. The differentiation of putative PGCs from embryonic stem cells in vitro recapitulates the early stages of gamete development in vivo, and provides an accessible system for discovering novel genes involved in germ-cell development and malignancy.

Isolation and characterization of pluripotent human spermatogonial stem cell-derived cells

Several reports have documented the derivation of pluripotent cells (multipotent germline stem cells) from spermatogonial stem cells obtained from the adult mouse testis. These spermatogonia-derived stem cells express embryonic stem cell markers and differentiate to the three primary germ layers, as well as the germline. Data indicate that derivation may involve reprogramming of endogenous spermatogonia in culture. Here, we report the derivation of human multipotent germline stem cells (hMGSCs) from a testis biopsy. The cells express distinct markers of pluripotency, form embryoid bodies that contain derivatives of all three germ layers, maintain a normal XY karyotype, are hypomethylated at the H19 locus, and express high levels of telomerase. Teratoma assays indicate the presence of human cells 8 weeks post-transplantation but limited teratoma formation. Thus, these data suggest the potential to derive pluripotent cells from human testis biopsies but indicate a need for novel strategies to optimize hMGSC culture conditions and reprogramming.

A mini-review of familial ovarian germ cell tumors: an additional manifestation of the familial testicular germ cell tumor syndrome

INTRODUCTION

While testicular germ cell tumors (TGCTs) are the most common malignancy in young men, germ cell tumors in women are uncommon. Familial clustering, epidemiologic evidence of increased risk with family or personal history of TGCT, and associations with genitourinary tract anomalies suggest an underlying genetic predisposition to TGCT, but traditional linkage studies have yet to identify a highly penetrant TGCT cancer susceptibility gene. In this paper, we investigate the familial occurrence of testicular and ovarian germ cell tumors.

METHODS

We report a family in which a TGCT and an ovarian germ cell tumor (OGCT) occurred in two siblings, summarize the existing literature on familial occurrences of OGCT, either alone or in combination with extragonadal or TGCTs, and compare the incidence of familial and sporadic testicular and ovarian GCTs. Sporadic GCT data were obtained from the US Surveillance Epidemiology and End Results (SEER) registry.

RESULTS

We identified 16 reports of OGCT occurring in conjunction with either ovarian, testicular or extragonadal GCT. In these familial cases, the mean age at onset of female dysgerminoma was younger than that noted in the general population (age 17 vs. age 24, p=0.01). In SEER, the incidence of TGCT was 15 times higher than that of OGCT. Histologic distributions in males and females showed distinctly different patterns.

DISCUSSION

Although the incidence of OGCTs in the general population is quite low, its occurrence in multiple members of the same family and in families with TGCT suggests that a gene conferring susceptibility to GCTs may exist in some families.

Patient-specific pluripotent stem cells: promises and challenges

Tissue transplantation is a well-established tool for the treatment of degenerative and malignant disorders, yet its use in clinical practice is hampered by the need for human-leukocyte-antigen-compatible donors and a shortage of suitable graft tissue. The discovery of human embryonic stem cells a decade ago raised hopes that a universal resource for the cell-based treatment of various conditions would soon become available. Embryonic stem cells derived by somatic-cell nuclear transfer or parthenogenesis can provide human-leukocyte-antigen-matched cells, which may be transplanted without the need for immunosuppressive treatment. However, technical hurdles and ethical concerns about use of oocytes and involvement of embryos have limited the clinical use of these cells. An alternative approach involves adult somatic cells being reprogrammed to enter a pluripotent state. Such manipulation of these readily available cells has enabled derivation of patient-specific, pluripotent stem-cell lines, without progression through the blastocyst stage. This Review critically analyzes the currently available methods for the generation of pluripotent stem cells, and discusses prospects for their clinical use.

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.

Potency of germ cells and its relevance for regenerative medicine

Germline stem cells, which can self-renew and generate gametes, are unique stem cells in that they are solely dedicated to transmit genetic information from generation to generation. The germ cells have a special place in the life cycle because they must be able to retain the ability to recreate the organism, a property known as developmental totipotency. Several lines of evidence have suggested the extensive proliferation activity and pluripotency of prenatal, neonatal and adult germline stem cells. We showed that adult male germline stem cells, spermatogonial stem cells, can be converted into embryonic stem cell-like cells, which can differentiate into the somatic stem cells of three germ layers. Different cell types such as vascular, heart, liver, pancreatic and blood cells could also be obtained from these stem cells. Understanding how spermatogonial stem cells can give rise to pluripotent stem cells and how somatic stem cells differentiate into germ cells could give significant insight into the regulation of developmental totipotency as well as having important implications for male fertility and regenerative medicine.

Stem cells for reproductive medicine

The generation of various pluripotent stem cell lines provides a new route to investigate developmental process of germ cell and embryo development, which until now was difficult to access in the human. In the future these cells may be used for new therapies in reproductive medicine. This brief review outlines the development of germ cells and their pluripotent capabilities, how embryonic and germline stem cells can mimic developmental processes in vitro and generate gamete and trophoblast phenotypes for research and potential treatments.

Zebrafish primordial germ cell cultures derived from vasa::RFP transgenic embryos

Although embryonic germ (EG) cell-mediated gene transfer has been successful in the mouse for more than a decade, this approach is limited in other species due to the difficulty of isolating the small numbers of progenitors of germ cell lineage (PGCs) from early-stage embryos and the lack of information on the in vitro culture requirements of the cells. In this study, methods were established for the culture of PGCs obtained from zebrafish embryos. Transgenic embryos that express the red fluorescent protein (RFP) under the control of the PGC-specific vasa promoter were used, making it possible to isolate pure populations of PGCs by fluorescence-activated cell sorting (FACS) and to optimize the culture conditions by counting the number of fluorescent PGC colonies produced in different media. Cultures initiated from 26-somite-stage embryos contained the highest percentage of PGCs that proliferated in vitro to generate colonies. The effect of growth factors, including Kit ligand a and b (Kitlga and Kitlgb) and stromal cell-derived factor 1a and 1b (Sdf-1a and Sdf-1b), on PGC proliferation was studied. Optimal in vitro growth and survival of the zebrafish PGCs was achieved when recombinant Kitlga and Sdf-1b were added to the culture medium through transfected feeder cells, resulting in a doubling of the number of PGC colonies. Results from RT-PCR and in situ hybridization analysis demonstrated that PGCs maintained in culture expressed the kita receptor, even though receptor expression was not detected in PGCs isolated by FACS directly from dissociated embryos. In optimal growth conditions, the PGCs continued to proliferate for at least 4 months in culture. The capacity to establish long-term PGC cultures from zebrafish will make it possible to conduct in vitro studies of germ cell differentiation and EG cell pluripotency in this model species and may be valuable for the development of a cell-mediated gene transfer approach.

Expression of NANOG, but not POU5F1, points to the stem cell potential of primitive germ cells in neonatal pig testis

Gonocytes are primitive germ cells that are present in the neonatal testis and are committed to male germline development. Gonocytes differentiate to spermatogonia, which establish and maintain spermatogenesis in the postnatal testis. However, it is unknown whether large animal species have pluripotency-specific proteins in the testis. Nanog and Pou5f1 (Oct3/4) have been identified as transcription factors essential for maintaining pluripotency of embryonic stem cells in mice. Here, we show that NANOG protein was expressed in the germ cells of neonatal pig testes, but was progressively lost with age. NANOG was expressed in most of the lectin Dolichos biflorus agglutinin- and ZBTB16-positive gonocytes, which are known gonocyte-specific markers in pigs. NANOG was also expressed in Sertoli and interstitial cells of neonatal testes. Interestingly, POU5F1 expression was not detected at either the transcript or the protein level in neonatal pig testis. In the prepubertal testis, NANOG and POU5F1 proteins were primarily detected in differentiated germ cells, such as spermatocytes and spermatids, and rarely in undifferentiated spermatogonia. By using a testis transplantation assay, we found that germ cells from 2- to 4-day-old pigs could colonize and proliferate in the testes of the recipient mice, suggesting that primitive germ cells from neonatal pig testes have stem cell potential.

Systematic identification of cis-regulatory sequences active in mouse and human embryonic stem cells

Understanding the transcriptional regulation of pluripotent cells is of fundamental interest and will greatly inform efforts aimed at directing differentiation of embryonic stem (ES) cells or reprogramming somatic cells. We first analyzed the transcriptional profiles of mouse ES cells and primordial germ cells and identified genes upregulated in pluripotent cells both in vitro and in vivo. These genes are enriched for roles in transcription, chromatin remodeling, cell cycle, and DNA repair. We developed a novel computational algorithm, CompMoby, which combines analyses of sequences both aligned and non-aligned between different genomes with a probabilistic segmentation model to systematically predict short DNA motifs that regulate gene expression. CompMoby was used to identify conserved overrepresented motifs in genes upregulated in pluripotent cells. We show that the motifs are preferentially active in undifferentiated mouse ES and embryonic germ cells in a sequence-specific manner, and that they can act as enhancers in the context of an endogenous promoter. Importantly, the activity of the motifs is conserved in human ES cells. We further show that the transcription factor NF-Y specifically binds to one of the motifs, is differentially expressed during ES cell differentiation, and is required for ES cell proliferation. This study provides novel insights into the transcriptional regulatory networks of pluripotent cells. Our results suggest that this systematic approach can be broadly applied to understanding transcriptional networks in mammalian species.

Embryonic stem cells have two remarkable properties: they can proliferate very rapidly, and they can give rise to all of the body's cell types. Understanding how gene activity is regulated in embryonic stem cells will be an important step towards therapeutic applications. The activity of genes is regulated by proteins called transcription factors, which bind to stretches of DNA sequences that act as on or off switches. We identified genes that are active in mouse embryonic stem cells but not in differentiated cells. We reasoned that if these genes have similar patterns of activity, they may be regulated by the same transcription factors. We therefore developed a computational approach that takes information on gene activity and predicts DNA sequences that may act as switches. Using this approach, we discovered new DNA switches that regulate gene activity in mouse and human embryonic stem cells. Furthermore, we identified a transcription factor that binds to one of these DNA switches and is important for the rapid proliferation of embryonic stem cells. Our approach sheds light on the genetic regulation of embryonic stem cells and will be broadly applicable to questions of how gene activity is regulated in other cell types of interest.