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

A rapid and stable spontaneous reprogramming system of Spermatogonial stem cells to Pluripotent State

BACKGROUND

The scarcity of pluripotent stem cells poses a major challenge to the clinical application, given ethical and biosafety considerations. While germline stem cells commit to gamete differentiation throughout life, studies demonstrated the spontaneous acquisition of pluripotency by spermatogonial stem cells (SSCs) from neonatal testes at a low frequency (1 in 1.5 × 107). Notably, this process occurs without exogenous oncogenes or chemical supplementation. However, while knockout of the p53 gene accelerates the transformation of SSCs, it also increases risk and hampers their clinical use.

RESULTS

We report a transformation system that efficiently and stably convert SSCs into pluripotent stem cells around 10 passages with the morphology similar to that of epiblast stem cells, which convert to embryonic stem (ES) cell-like colonies after change with ES medium. Epidermal growth factor (EGF), leukemia inhibitory factor (LIF) and fresh mouse embryonic fibroblast feeder (MEF) are essential for transformation, and addition of 2i (CHIR99021 and PD0325901) further enhanced the pluripotency. Transcriptome analysis revealed that EGF activated the RAS signaling pathway and inhibited p38 to initiate transformation, and synergically cooperated with LIF to promote the transformation.

CONCLUSION

This system established an efficient and safe resource of pluripotent cells from autologous germline, and provide new avenues for regenerative medicine and animal cloning.

The mouse allantois: new insights at the embryonic-extraembryonic interface

During the early development of Placentalia, a distinctive projection emerges at the posterior embryonic-extraembryonic interface of the conceptus; its fingerlike shape presages maturation into the placental umbilical cord, whose major role is to shuttle fetal blood to and from the chorion for exchange with the mother during pregnancy. Until recently, the biology of the cord's vital vascular anlage, called the body stalk/allantois in humans and simply the allantois in rodents, has been largely unknown. Here, new insights into the development of the mouse allantois are featured, from its origin and mechanism of arterial patterning through its union with the chorion. Key to generating the allantois and its critical functions are the primitive streak and visceral endoderm, which together are sufficient to create the entire fetal-placental connection. Their newly discovered roles at the embryonic-extraembryonic interface challenge conventional wisdom, including the physical limits of the primitive streak, its function as sole purveyor of mesoderm in the mouse, potency of visceral endoderm, and the putative role of the allantois in the germ line. With this working model of allantois development, understanding a plethora of hitherto poorly understood orphan diseases in humans is now within reach. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

The foundational framework of tumors: Gametogenesis, p53, and cancer

The completion-of-tumor hypothesis involved in the dynamic interplay between the initiating oncogenic event and progression is essential to better recognize the foundational framework of tumors. Here we review and extend the gametogenesis-related hypothesis of tumors, because high embryonic/germ cell traits are common in tumors. The century-old gametogenesis-related hypothesis of tumors postulated that tumors arise from displaced/activated trophoblasts, displaced (lost) germ cells, and the reprogramming/reactivation of gametogenic program in somatic cells. Early primordial germ cells (PGCs), embryonic stem (ES) cells, embryonic germ cells (EGCs), and pre-implantation embryos at the stage from two-cell stage to blastocysts originating from fertilization or parthenogenesis have the potential to develop teratomas/teratocarcinomas. In addition, the teratomas/teratocarcinomas/germ cells occur in gonads and extra-gonads. Undoubtedly, the findings provide strong support for the hypothesis. However, it was thought that these tumor types were an exception rather than verification. In fact, there are extensive similarities between somatic tumor types and embryonic/germ cell development, such as antigens, migration, invasion, and immune escape. It was documented that embryonic/germ cell genes play crucial roles in tumor behaviors, e.g. tumor initiation and metastasis. Of note, embryonic/germ cell-like tumor cells at different developmental stages including PGC and oocyte to the early embryo-like stage were identified in diverse tumor types by our group. These embryonic/germ cell-like cancer cells resemble the natural embryonic/germ cells in morphology, gene expression, the capability of teratoma formation, and the ability to undergo the process of oocyte maturation and parthenogenesis. These embryonic/germ cell-like cancer cells are derived from somatic cells and contribute to tumor formation, metastasis, and drug resistance, establishing asexual meiotic embryonic life cycle. p53 inhibits the reactivation of embryonic/germ cell state in somatic cells and oocyte-like cell maturation. Based on earlier and our recent studies, we propose a novel model to complete the gametogenesis-related hypothesis of tumors, which can be applied to certain somatic tumors. That is, tumors tend to establish a somatic asexual meiotic embryonic cycle through the activation of somatic female gametogenesis and parthenogenesis in somatic tumor cells during the tumor progression, thus passing on corresponding embryonic/germ cell traits leading to the malignant behaviors and enhancing the cells' independence. This concept may be instrumental to better understand the nature and evolution of tumors. We rationalize that targeting the key events of somatic pregnancy is likely a better therapeutic strategy for cancer treatment than directly targeting cell mitotic proliferation, especially for those tumors with p53 inactivation.

Germ cell determination and the developmental origin of germ cell tumors

In each generation, the germline is tasked with producing somatic lineages that form the body, and segregating a population of cells for gametogenesis. During animal development, when do cells of the germline irreversibly commit to producing gametes? Integrating findings from diverse species, we conclude that the final commitment of the germline to gametogenesis - the process of germ cell determination - occurs after primordial germ cells (PGCs) colonize the gonads. Combining this understanding with medical findings, we present a model whereby germ cell tumors arise from cells that failed to undertake germ cell determination, regardless of their having colonized the gonads. We propose that the diversity of cell types present in these tumors reflects the broad developmental potential of migratory PGCs.

Mouse Primordial Germ Cells: In Vitro Culture and Conversion to Pluripotent Stem Cell Lines

Primordial germ cells (PGCs) are the embryonic precursors of the gametes. Despite decades of research, in vitro culture of PGCs remains a major challenge and has previously relied on undefined components such as serum and feeders. Notably, PGCs cultured for extended periods do not maintain their lineage identity but instead undergo conversion to form pluripotent stem cell lines called embryonic germ (EG) cells in response to LIF/STAT3 signaling. Here we report both established and new methodologies to derive EG cells, in a range of different conditions. We show that basic fibroblast growth factor is not required for EG cell conversion. We detail the steps taken in our laboratory to systematically remove complex components and establish a fully defined protocol that allows efficient conversion of isolated PGCs to pluripotent EG cells. In addition, we demonstrate that PGCs can adhere and proliferate in culture without the support of feeder cells or serum. This may well suggest novel approaches to establishing short-term culture of PGCs in defined conditions.

A most formidable arsenal: genetic technologies for building a better mouse

The mouse is one of the most widely used model organisms for genetic study. The tools available to alter the mouse genome have developed over the preceding decades from forward screens to gene targeting in stem cells to the recent influx of CRISPR approaches. In this review, we first consider the history of mice in genetic study, the development of classic approaches to genome modification, and how such approaches have been used and improved in recent years. We then turn to the recent surge of nuclease-mediated techniques and how they are changing the field of mouse genetics. Finally, we survey common classes of alleles used in mice and discuss how they might be engineered using different methods.

Cardiovascular and body weight regulation changes in transgenic mice overexpressing thyrotropin-releasing hormone (TRH)

Thyrotropin-releasing hormone (TRH) plays several roles as a hormone/neuropeptide. Diencephalic TRH (dTRH) participates in the regulation of blood pressure in diverse animal models, independently of the thyroid status. The present study aimed to evaluate whether chronic overexpression of TRH in mice affects cardiovascular and metabolic variables. We developed a transgenic (TG) mouse model that overexpresses dTrh. Despite having higher food consumption and water intake, TG mice showed significantly lower body weight respect to controls. Also, TG mice presented higher blood pressure, heart rate, and locomotor activity independently of thyroid hormone levels. These results and the higher urine noradrenaline excretion observed in TG mice suggest a higher metabolic rate mediated by sympathetic overflow. Cardiovascular changes were impeded by siRNA inhibition of the diencephalic Trh overexpression. Also, the silencing of dTRH in the TG mice normalized urine noradrenaline excretion, supporting the view that the cardiovascular effects of TRH involve the sympathetic system. Overall, we show that congenital dTrh overexpression leads to an increase in blood pressure accompanied by changes in body weight and food consumption mediated by a higher sympathetic overflow. These results provide new evidence confirming the participation of TRH in cardiovascular and body weight regulation.

Transgenerational inheritance: how impacts to the epigenetic and genetic information of parents affect offspring health

BACKGROUND

A defining feature of sexual reproduction is the transmission of genomic information from both parents to the offspring. There is now compelling evidence that the inheritance of such genetic information is accompanied by additional epigenetic marks, or stable heritable information that is not accounted for by variations in DNA sequence. The reversible nature of epigenetic marks coupled with multiple rounds of epigenetic reprogramming that erase the majority of existing patterns have made the investigation of this phenomenon challenging. However, continual advances in molecular methods are allowing closer examination of the dynamic alterations to histone composition and DNA methylation patterns that accompany development and, in particular, how these modifications can occur in an individual’s germline and be transmitted to the following generation. While the underlying mechanisms that permit this form of transgenerational inheritance remain unclear, it is increasingly apparent that a combination of genetic and epigenetic modifications plays major roles in determining the phenotypes of individuals and their offspring.

OBJECTIVE AND RATIONALE

Information pertaining to transgenerational inheritance was systematically reviewed focusing primarily on mammalian cells to the exclusion of inheritance in plants, due to inherent differences in the means by which information is transmitted between generations. The effects of environmental factors and biological processes on both epigenetic and genetic information were reviewed to determine their contribution to modulating inheritable phenotypes.

SEARCH METHODS

Articles indexed in PubMed were searched using keywords related to transgenerational inheritance, epigenetic modifications, paternal and maternal inheritable traits and environmental and biological factors influencing transgenerational modifications. We sought to clarify the role of epigenetic reprogramming events during the life cycle of mammals and provide a comprehensive review of how the genomic and epigenomic make-up of progenitors may determine the phenotype of its descendants.

OUTCOMES

We found strong evidence supporting the role of DNA methylation patterns, histone modifications and even non-protein-coding RNA in altering the epigenetic composition of individuals and producing stable epigenetic effects that were transmitted from parents to offspring, in both humans and rodent species. Multiple genomic domains and several histone modification sites were found to resist demethylation and endure genome-wide reprogramming events. Epigenetic modifications integrated into the genome of individuals were shown to modulate gene expression and activity at enhancer and promoter domains, while genetic mutations were shown to alter sequence availability for methylation and histone binding. Fundamentally, alterations to the nuclear composition of the germline in response to environmental factors, ageing, diet and toxicant exposure have the potential to become hereditably transmitted.

WIDER IMPLICATIONS

The environment influences the health and well-being of progeny by working through the germline to introduce spontaneous genetic mutations as well as a variety of epigenetic changes, including alterations in DNA methylation status and the post-translational modification of histones. In evolutionary terms, these changes create the phenotypic diversity that fuels the fires of natural selection. However, rather than being adaptive, such variation may also generate a plethora of pathological disease states ranging from dominant genetic disorders to neurological conditions, including spontaneous schizophrenia and autism.

Tissue-Specific Oncogenic Activity of KRASA146T

KRAS is the most frequently mutated oncogene. The incidence of specific KRAS alleles varies between cancers from different sites, but it is unclear whether allelic selection results from biological selection for specific mutant KRAS proteins. We used a cross-disciplinary approach to compare KRASG12D, a common mutant form, and KRASA146T, a mutant that occurs only in selected cancers. Biochemical and structural studies demonstrated that KRASA146T exhibits a marked extension of switch 1 away from the protein body and nucleotide binding site, which activates KRAS by promoting a high rate of intrinsic and guanine nucleotide exchange factor-induced nucleotide exchange. Using mice genetically engineered to express either allele, we found that KRASG12D and KRASA146T exhibit distinct tissue-specific effects on homeostasis that mirror mutational frequencies in human cancers. These tissue-specific phenotypes result from allele-specific signaling properties, demonstrating that context-dependent variations in signaling downstream of different KRAS mutants drive the KRAS mutational pattern seen in cancer. SIGNIFICANCE: Although epidemiologic and clinical studies have suggested allele-specific behaviors for KRAS, experimental evidence for allele-specific biological properties is limited. We combined structural biology, mass spectrometry, and mouse modeling to demonstrate that the selection for specific KRAS mutants in human cancers from different tissues is due to their distinct signaling properties.See related commentary by Hobbs and Der, p. 696.This article is highlighted in the In This Issue feature, p. 681.

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

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

Reprogramming p53-Deficient Germline Stem Cells Into Pluripotent State by Nanog

Cultured mouse spermatogonial stem cells (SSCs), also known as germline stem cells (GSCs), revert back to pluripotent state either spontaneously or upon being modified genetically. However, the reprogramming efficiencies are low, and the underlying mechanism remains poorly understood. In the present study, we conducted transcriptomic analysis and found that many transcription factors and epigenetic modifiers were differentially expressed between GSCs and embryonic stem cells. We failed in reprogramming GSCs to pluripotent state using the Yamanaka 4 Factors, but succeeded when Nanog and Tet1 were included. More importantly, reprogramming was also achieved with Nanog alone in a p53-deficient GSC line with an efficiency of 0.02‰. These GSC-derived-induced pluripotent stem cells possessed in vitro and in vivo differentiation abilities despite the low rate of chimera formation, which might be caused by abnormal methylation in certain paternally imprinted genes. Together, these results show that GSCs can be reprogrammed to pluripotent state via multiple avenues and contribute to our understanding of the mechanisms of GSC reprogramming.

What Can Stem Cell Models Tell Us About Human Germ Cell Biology?

Fusion of sperm and egg generates a totipotent zygote that develops into a whole organism. Accordingly, the "immortal" germline transmits genetic and epigenetic information to subsequent generations with consequences for human health and disease. In mammals, primordial germ cells (PGCs) originate from peri-gastrulation embryos. While early human embryos are inaccessible for research, in vitro model systems using pluripotent stem cells have provided critical insights into human PGC specification, which differs from that in mice. This might stem from significant differences in early embryogenesis at the morphological and molecular levels, including pluripotency networks. Here, we discuss recent advances and experimental systems used to study mammalian germ cell development. We also highlight key aspects of germ cell disorders, as well as mitochondrial and potentially epigenetic inheritance in humans.

EDC IMPACT: Is exposure during pregnancy to acetaminophen/paracetamol disrupting female reproductive development?

Concern has been raised over chemical-induced disruption of ovary development during fetal life resulting in long-lasting consequences only manifesting themselves much later during adulthood. A growing body of evidence suggests that prenatal exposure to the mild analgesic acetaminophen/paracetamol can cause such a scenario. Therefore, in this review, we discuss three recent reports that collectively indicate that prenatal exposure in a period of 13.5 days post coitum in both rats and mouse can result in reduced female reproductive health. The combined data show that the exposure results in the reduction of primordial follicles, irregular menstrual cycle, premature absence of corpus luteum, as well as reduced fertility, resembling premature ovarian insufficiency syndrome in humans that is linked to premature menopause. This could especially affect the Western parts of the world, where the age for childbirth is continuously being increased and acetaminophen is recommended during pregnancy for pain and fever. We therefore highlight an urgent need for more studies to verify these data including both experimental and epidemiological approaches.

Germ cell tumors: Insights from the Drosophila ovary and the mouse testis

Ovarian and testicular germ cell tumors of young adults are thought to arise from defects in germ cell development, but the molecular mechanisms underlying malignant transformation are poorly understood. In this review, we focus on the biology of germ cell tumor formation in the Drosophila ovary and the mouse testis, for which evidence supports common underlying mechanisms, such as blocking initiation into the differentiation pathway, impaired lineage progression, and sexual identity instability. We then discuss how these concepts inform our understanding of the disease in humans. Mol. Reprod. Dev. 84: 200-211, 2017. © 2017 Wiley Periodicals, Inc.

Is there a place for human fetal-derived stem cells for cell replacement therapy in Huntington's disease?

Huntington's disease (HD) is a neurodegenerative disease that offers an excellent paradigm for cell replacement therapy because of the associated relatively focal cell loss in the striatum. The predominant cells lost in this condition are striatal medium spiny neurons (MSNs). Transplantation of developing MSNs taken from the fetal brain has provided proof of concept that donor MSNs can survive, integrate and bring about a degree of functional recovery in both pre-clinical studies and in a limited number of clinical trials. The scarcity of human fetal tissue, and the logistics of coordinating collection and dissection of tissue with neurosurgical procedures makes the use of fetal tissue for this purpose both complex and limiting. Alternative donor cell sources which are expandable in culture prior to transplantation are currently being sought. Two potential donor cell sources which have received most attention recently are embryonic stem (ES) cells and adult induced pluripotent stem (iPS) cells, both of which can be directed to MSN-like fates, although achieving a genuine MSN fate has proven to be difficult. All potential donor sources have challenges in terms of their clinical application for regenerative medicine, and thus it is important to continue exploring a wide variety of expandable cells. In this review we discuss two less well-reported potential donor cell sources; embryonic germ (EG) cells and fetal neural precursors (FNPs), both are which are fetal-derived and have some properties that could make them useful for regenerative medicine applications.

DUSP9 Modulates DNA Hypomethylation in Female Mouse Pluripotent Stem Cells

Blastocyst-derived embryonic stem cells (ESCs) and gonad-derived embryonic germ cells (EGCs) represent two classic types of pluripotent cell lines, yet their molecular equivalence remains incompletely understood. Here, we compare genome-wide methylation patterns between isogenic ESC and EGC lines to define epigenetic similarities and differences. Surprisingly, we find that sex rather than cell type drives methylation patterns in ESCs and EGCs. Cell fusion experiments further reveal that the ratio of X chromosomes to autosomes dictates methylation levels, with female hybrids being hypomethylated and male hybrids being hypermethylated. We show that the X-linked MAPK phosphatase DUSP9 is upregulated in female compared to male ESCs, and its heterozygous loss in female ESCs leads to male-like methylation levels. However, male and female blastocysts are similarly hypomethylated, indicating that sex-specific methylation differences arise in culture. Collectively, our data demonstrate the epigenetic similarity of sex-matched ESCs and EGCs and identify DUSP9 as a regulator of female-specific hypomethylation.

Mesenchymal to Epithelial Transition Mediated by CDH1 Promotes Spontaneous Reprogramming of Male Germline Stem Cells to Pluripotency

Cultured spermatogonial stem cells (GSCs) can spontaneously form pluripotent cells in certain culture conditions. However, GSC reprogramming is a rare event that is largely unexplained. We show GSCs have high expression of mesenchymal to epithelial transition (MET) suppressors resulting in a developmental barrier inhibiting GSC reprogramming. Either increasing OCT4 or repressing transforming growth factor β (TGF-β) signaling promotes GSC reprogramming by upregulating CDH1 and boosting MET. Reducing ZEB1 also enhances GSC reprogramming through its direct effect on CDH1. RNA sequencing shows that rare GSCs, identified as CDH1⁺ after trypsin digestion, are epithelial-like cells. CDH1⁺ GSCs exhibit enhanced reprogramming and become more prevalent during the course of reprogramming. Our results provide a mechanistic explanation for the spontaneous emergence of pluripotent cells from GSC cultures; namely, rare GSCs upregulate CDH1 and initiate MET, processes normally kept in check by ZEB1 and TGF-β signaling, thereby ensuring germ cells are protected from aberrant acquisition of pluripotency.

Mechanisms of Vertebrate Germ Cell Determination

Two unique characteristics of the germ line are the ability to persist from generation to generation and to retain full developmental potential while differentiating into gametes. How the germ line is specified that allows it to retain these characteristics within the context of a developing embryo remains unknown and is one focus of current research. Germ cell specification proceeds through one of two basic mechanisms: cell autonomous or inductive. Here, we discuss how germ plasm driven germ cell specification (cell autonomous) occurs in both zebrafish and the frog Xenopus. We describe the segregation of germ cells during embryonic development of solitary and colonial ascidians to provide an evolutionary context to both mechanisms. We conclude with a discussion of the inductive mechanism as exemplified by both the mouse and axolotl model systems. Regardless of mechanism, several general themes can be recognized including the essential role of repression and posttranscriptional regulation of gene expression.

Basic fibroblast growth factor is critical to reprogramming buffalo (Bubalus bubalis) primordial germ cells into embryonic germ stem cell-like cells

Primordial germ cells (PGCs) are destined to form gametes in vivo, and they can be reprogrammed into pluripotent embryonic germ (EG) cells in vitro. Buffalo PGC have been reported to be reprogrammed into EG-like cells, but the identities of the major signaling pathways and culture media involved in this derivation remain unclear. Here, the effects of basic fibroblast growth factor (bFGF) and downstream signaling pathways on the reprogramming of buffalo PGCs into EG-like cells were investigated. Results showed bFGF to be critical to buffalo PGCs to dedifferentiate into EG-like cells (20 ng/mL is optimal) with many characteristics of pluripotent stem cells, including alkaline phosphatase (AP) activity, expression of pluripotency marker genes such as OCT4, NANOG, SOX2, SSEA-1, CDH1, and TRA-1-81, and the capacity to differentiate into all three embryonic germ layers. After chemically inhibiting pathways or components downstream of bFGF, data showed that inhibition of the PI3K/AKT pathway led to significantly lower EG cell derivation, while inhibition of P53 activity resulted in an efficiency of EG cell derivation comparable to that in the presence of bFGF. These results suggest that the role of bFGF in PGC-derived EG-like cell generation is mainly due to the activation of the PI3K/AKT/P53 pathway, in particular, the inhibition of P53 function.

Critical role of Emx2 in the pluripotency – differentiation transition in male gonocytes via regulation of FGF9/NODAL pathway

Emx2 deletion impairs the growth and maintenance of the genital ridge. However, its role in subsequent germ cell differentiation during embryonic stages is unknown. Using a tamoxifen-inducible Cre-loxP mouse model (Emx2flox/flox, Cre-ERTM, hereafter called as Emx2 knockdown), we showed that germ cell differentiation was impaired in Emx2-knockdown testes. Representative characteristics of male germ cell differentiation, including a reduced ability to form embryonic germ (EG) cell colonies in vitro, down-regulation of pluripotency markers and G1/G0 arrest, did not occur in Emx2-knockdown testes. Furthermore, FGF9 and NODAL signalling occurred at abnormally high levels in Emx2-knockdown testes. Both blocking FGF9 signalling with SU5402 and inhibiting NODAL signalling with SB431542 allowed germ cells from Emx2-knockdown testes to differentiate in vitro. Therefore, EMX2 in somatic cells is required to trigger germ cell differentiation in XY foetuses, posterior to its previously reported role in the growth and maintenance of the genital ridge.

Role of DNA methylation in cardiovascular diseases

A complex interplay between genetic and environmental factors is involved in the pathogenesis of cardiovascular diseases (CVDs). Environmental factors have crucial effects on the epigenetic trait of genes, which refers to a stably heritable phenotype resulting from changes in the chromosomes without alteration of the DNA sequence, but has profound effects on the cellular repertoire. Among the epigenetic patterns, DNA methylation is of great interest. DNA methylation occurs at both global and specific gene promoter levels and relates to atherosclerosis. Aberrant DNA methylation affects the transcription and expression of critical regulatory genes and induces a proatherogenic cellular phenotype, which plays key roles in endothelia cell dysfunction, abnormal vascular smooth muscle cell proliferation, extracellular matrix formation, and inflammation in CVDs. This review focuses on the contribution of DNA methylation in the pathogenesis of CVDs.

Pluripotent stem cells derived from mouse primordial germ cells by small molecule compounds

Primordial germ cells (PGCs) can give rise to pluripotent stem cells known as embryonic germ cells (EGCs) when cultured with basic fibroblast growth factor (bFGF), stem cell factor (SCF), and leukemia inhibitory factor. Somatic cells can give rise to induced pluripotent stem cells (iPSCs) by introduction of the reprogramming transcription factors Oct4, Sox2, and Klf4. The effects of Sox2 and Klf4 on somatic cell reprogramming can be reproduced using the small molecule compounds, transforming growth factor-β receptor (TGFβR) inhibitor and Kempaullone, respectively. Here we examined the effects of TGFβR inhibitor and Kempaullone on EGC derivation from PGCs. Treatment of PGCs with TGFβR inhibitor and/or Kempaullone generated pluripotent stem cells under standard embryonic stem cell (ESC) culture conditions without bFGF and SCF, which we termed induced EGCs (iEGCs). The derivation efficiency of iEGCs was dependent on the differentiation stage and sex. DNA methylation levels of imprinted genes in iEGCs were reduced, with the exception of the H19 gene. The promoters of genes involved in germline development were generally hypomethylated in PGCs, but three germline genes showed comparable DNA methylation levels among iEGs, ESCs, and iPSCs. These results show that PGCs can be reprogrammed into pluripotent state using small molecule compounds, and that DNA methylation of these germline genes is not maintained in iEGCs.

Initial embryology and pluripotent stem cells in the pig--The quest for establishing the pig as a model for cell therapy

The quest for porcine pluripotent stem cells (PSCs) was initiated in the early 90s. Initially, it was the intention to benefit from these cells for production of genetically modified pigs using homologous recombination followed by derivation of chimeric offspring; a technology that has been used to produce genetically modified mice since the mid-80s. However, no convincing reports on the generation of bona fide porcine embryonic stem cells or embryonic germ cells resulted from these activities, and with the advent of somatic cell nuclear transfer during the late 90s, alternative methods for creating genetically modified pigs emerged. Over the past years, renewed interest in porcine PSCs has sparked activities in deriving in particular porcine induced pluripotent stem cells to develop the pig as a faithful model for studying the potentials and risks associated with induced pluripotent stem cell-based human therapy. Here, we review the recent data on establishment of porcine PSCs and the differences in embryonic development between pig and mouse, which may be underlying factors for the continuing challenge to culture and maintain porcine PSCs.

Tet-mediated imprinting erasure in H19 locus following reprogramming of spermatogonial stem cells to induced pluripotent stem cells

Selective methylation of CpG islands at imprinting control regions (ICR) determines the monoparental expression of a subset of genes. Currently, it is unclear whether artificial reprogramming induced by the expression of Yamanaka factors disrupts these marks and whether cell type of origin affects the dynamics of reprogramming. In this study, spermatogonial stem cells (SSC) that harbor paternalized imprinting marks, and fibroblasts were reprogrammed to iPSC (SSCiPSC and fiPSC). The SSCiPSC were able to form teratomas and generated chimeras with a higher skin chimerism than those derived from fiPSC. RNA-seq revealed extensive reprogramming at the transcriptional level with 8124 genes differentially expressed between SSC and SSCiPSC and only 490 between SSCiPSC and fiPSC. Likewise, reprogramming of SSC affected 26 of 41 imprinting gene clusters known in the mouse genome. A closer look at H19 ICR revealed complete erasure in SSCiPSC in contrast to fiPSC. Imprinting erasure in SSCiPSC was maintained even after in vivo differentiation into teratomas. Reprogramming of SSC from Tet1 and Tet2 double knockout mice however lacked demethylation of H19 ICR. These results suggest that imprinting erasure during reprogramming depends on the epigenetic landscape of the precursor cell and is mediated by TETs at the H19 locus.

Primordial germ cells: the first cell lineage or the last cells standing?

Embryos of many animal models express germ line determinants that suppress transcription and mediate early germ line commitment, which occurs before the somatic cell lineages are established. However, not all animals segregate their germ line in this manner. The 'last cell standing' model describes primordial germ cell (PGC) development in axolotls, in which PGCs are maintained by an extracellular signalling niche, and germ line commitment occurs after gastrulation. Here, we propose that this 'stochastic' mode of PGC specification is conserved in vertebrates, including non-rodent mammals. We postulate that early germ line segregation liberates genetic regulatory networks for somatic development to evolve, and that it therefore emerged repeatedly in the animal kingdom in response to natural selection.

PGC Reversion to Pluripotency Involves Erasure of DNA Methylation from Imprinting Control Centers followed by Locus-Specific Re-methylation

Primordial germ cells (PGCs) are fate restricted to differentiate into gametes in vivo. However, when removed from their embryonic niche, PGCs undergo reversion to pluripotent embryonic germ cells (EGCs) in vitro. One of the major differences between EGCs and embryonic stem cells (ESCs) is variable methylation at imprinting control centers (ICCs), a phenomenon that is poorly understood. Here we show that reverting PGCs to EGCs involved stable ICC methylation erasure at Snrpn, Igf2r, and Kcnqot1. In contrast, the H19/Igf2 ICC undergoes erasure followed by de novo re-methylation. PGCs differentiated in vitro from ESCs completed Snrpn ICC erasure. However, the hypomethylated state is highly unstable. We also discovered that when the H19/Igf2 ICC was abnormally hypermethylated in ESCs, this is not erased in PGCs differentiated from ESCs. Therefore, launching PGC differentiation from ESC lines with appropriately methylated ICCs is critical to the generation of germline cells that recapitulate endogenous ICC erasure.

Pluripotent cell derivation from male germline cells by suppression of Dmrt1 and Trp53

Diploid germ cells are thought to have pluripotency potential. We recently described a method to derive pluripotent stem cells (PSCs) from cultured spermatogonial stem cells (SSCs) by depleting Trp53 and Dmrt1, both of which are known suppressors of teratomas. In this study, we used this technique to analyze the effect of this protocol in deriving PSCs from the male germline at different developmental stages. We collected primordial germ cells (PGCs), gonocytes and spermatogonia, and the cells were transduced with lentiviruses expressing short hairpin RNA against Dmrt1 and/or Trp53. We found that PGCs are highly susceptible to reprogramming induction and that only Trp53 depletion was sufficient to induce pluripotency. In contrast, gonocytes and spermatogonia were resistant to reprogramming by double knockdown of Dmrt1 and Trp53. PSCs derived from PGCs contributed to chimeras produced by blastocyst injection, but some of the embryos showed placenta-only phenotypes suggestive of epigenetic abnormalities of PGC-derived PSCs. These results show that PGCs and gonocytes/spermatogonia have distinct reprogramming potential and also suggest that fresh and cultured SSCs do not necessarily have the same properties.

Ex Uno Plures: Molecular Designs for Embryonic Pluripotency

Pluripotent cells in embryos are situated near the apex of the hierarchy of developmental potential. They are capable of generating all cell types of the mammalian body proper. Therefore, they are the exemplar of stem cells. In vivo, pluripotent cells exist transiently and become expended within a few days of their establishment. Yet, when explanted into artificial culture conditions, they can be indefinitely propagated in vitro as pluripotent stem cell lines. A host of transcription factors and regulatory genes are now known to underpin the pluripotent state. Nonetheless, how pluripotent cells are equipped with their vast multilineage differentiation potential remains elusive. Consensus holds that pluripotency transcription factors prevent differentiation by inhibiting the expression of differentiation genes. However, this does not explain the developmental potential of pluripotent cells. We have presented another emergent perspective, namely, that pluripotency factors function as lineage specifiers that enable pluripotent cells to differentiate into specific lineages, therefore endowing pluripotent cells with their multilineage potential. Here we provide a comprehensive overview of the developmental biology, transcription factors, and extrinsic signaling associated with pluripotent cells, and their accompanying subtypes, in vitro heterogeneity and chromatin states. Although much has been learned since the appreciation of mammalian pluripotency in the 1950s and the derivation of embryonic stem cell lines in 1981, we will specifically emphasize what currently remains unclear. However, the view that pluripotency factors capacitate differentiation, recently corroborated by experimental evidence, might perhaps address the long-standing question of how pluripotent cells are endowed with their multilineage differentiation potential.

How are pluripotent cells captured in culture?

In mice, three pluripotent stem cell lines have been established from different stage of developing embryo, which are embryonic stem (ES) cell, post-implantation epiblast stem cell (EpiSC), and embryonic germ (EG) cell. ES cell and EG cell share many common features including factor requirement, colony morphology, and gene expression pattern. On the other hand, EpiSC needs different external signal inputs, exhibits flattened colony morphology, and a different set of gene expression patterns. In addition, the germ line competency of EpiSCs is still unclear. To distinguish the differences between them, they are defined by the words "naïve" and "primed" pluripotent cells, respectively. This article introduces how pluripotent stem cell lines are established in culture, and how much those cells in vitro are similar or relevant to their in vivo origin and the knowledge about transcription factors to support this state.

Pluripotent states of human embryonic stem cells

Since human embryonic stem cells (hESCs) were first isolated and successfully cultured in vitro, the pluripotent potential of hESCs has been underestimated. The pluripotency of mouse embryonic stem cells (mESCs) can be categorized as naïve and primed, depending on their corresponding in vivo developing phases. mESC morphology differs at distinct pluripotent states, which differ in signaling dependence, gene expression, epigenetic features, and developmental potential. hESCs resemble mouse stem cells at primed pluripotency, and consequently are believed to correspond to a later developmental stage in vivo than mESCs. Nevertheless, recent studies indicate that a naïve state of pluripotency may exist in hESCs, and the pluripotency of hESCs also can be enhanced by genetic modification or optimized culture systems. These findings provide novel insight into the properties and differentiation potential of hESCs. Here, we review the recent advances in characterization of ESC states and investigate the mechanisms regulating hESC pluripotency.

Mouse primordial germ cells: a reappraisal

Current dogma is that mouse primordial germ cells (PGCs) segregate within the allantois, or source of the umbilical cord, and translocate to the gonads, differentiating there into sperm and eggs. In light of emerging data on the posterior embryonic-extraembryonic interface, and the poorly studied but vital fetal-umbilical connection, we have reviewed the past century of experiments on mammalian PGCs and their relation to the allantois. We demonstrate that, despite best efforts and valuable data on the pluripotent state, what is and is not a PGC in vivo is obscure. Furthermore, sufficient experimental evidence has yet to be provided either for an extragonadal origin of mammalian PGCs or for their segregation within the posterior region. Rather, most evidence points to an alternative hypothesis that PGCs in the mouse allantois are part of a stem/progenitor cell pool that exhibits all known PGC "markers" and that builds/reinforces the fetal-umbilical interface, common to amniotes. We conclude by suggesting experiments to distinguish the mammalian germ line from the soma.

Nonviral minicircle generation of induced pluripotent stem cells compatible with production of chimeric chickens

Chickens are vitally important in numerous countries as a primary food source and a major component of economic development. Efforts have been made to produce transgenic birds through pluripotent stem cell [primordial germ cells and embryonic stem cells (ESCs)] approaches to create animals with improved traits, such as meat and egg production or even disease resistance. However, these cell types have significant limitations because they are hard to culture long term while maintaining developmental plasticity. Induced pluripotent stem cells (iPSCs) are a novel class of stem cells that have proven to be robust, leading to the successful development of transgenic mice, rats, quail, and pigs and may potentially overcome the limitations of previous pluripotent stem cell systems in chickens. In this study we generated chicken (c) iPSCs from fibroblast cells for the first time using a nonviral minicircle reprogramming approach. ciPSCs demonstrated stem cell morphology and expressed key stem cell markers, including alkaline phosphatase, POU5F1, SOX2, NANOG, and SSEA-1. These cells were capable of rapid growth and expressed high levels of telomerase. Late-passage ciPSCs transplanted into stage X embryos were successfully incorporated into tissues of all three germ layers, and the gonads demonstrated significant cellular plasticity. These cells provide an exciting new tool to create transgenic chickens with broad implications for agricultural and transgenic animal fields at large.

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.

Efficient induction of pluripotency in primordial germ cells by dual inhibition of TGF-β and ERK signaling pathways

Primordial germ cells (PGCs) have the ability to be reprogrammed into a pluripotent state and are defined as embryonic germ cells (EGCs) in vitro. EGC formation is more efficient, has a shorter culture period than somatic cell reprogramming, and does not require exogenous genetic manipulation. Therefore, EGCs are a good model to analyze mechanisms by which committed cells acquire a pluripotent state. In the present study we have attempted to elucidate a more defined and robust protocol that promotes EGC generation through the suppression of transforming growth factor-β (TGF-β) and extracellular signal-regulated kinase (ERK) signaling pathways by SB431542 (SB) and PD0325901 (PD), respectively. Under this condition the efficiency of transformation of PGCs into EGCs was more than the inhibition of glycogen synthase kinase 3 and ERK signaling pathways. Pluripotency of the resultant-derived EGC lines were further confirmed by gene expression, immunofluorescent staining, directed differentiation ability, teratoma formation, and their contribution to chimeric mice and germ-line transmission. These results showed that PGCs from different embryonic stages (E8.5 and E12.5) could be reprogrammed, maintained, and expanded efficiently under feeder- and serum-free chemically defined conditions by dual inhibition of TGF-β and ERK signaling pathways, regardless of the animal's genetic background.

Induction of pluripotent stem cells from primordial germ cells by single reprogramming factors

Germ cells are similar to pluripotent stem cells in terms of gene expression patterns and the capacity to convert to pluripotent stem cells in culture. The factors involved in germ cell development are also able to reprogram somatic cells. This suggests that germ cells are useful tools for investigating the mechanisms responsible for somatic cell reprograming. In this study, the expression of reprograming factors in primordial germ cells (PGCs) was analyzed. PGCs expressed Oct3/4, Sox2, and c-Myc but not Klf4. However, Klf2, Klf5, Essrb, or Essrg, which were expressed in PGCs, could compensate for Klf4 during somatic cell reprograming. Furthermore, PGCs could be converted to a pluripotent state by infection with any of the known reprogramming factors (Oct3/4, Sox2, Klf4, and c-Myc). These cells were designated as multipotent PGCs (mPGCs). Contrary to differences in the origins of somatic cells in somatic cell reprogramming, we hypothesized that the gene expression levels of the reprogramming factors would vary in mPGCs. Candidate genes involved in the regulation of tumorigenicity and/or reprogramming efficiency were identified by comparing the gene expression profiles of mPGCs generated by the exogenous expression of c-Myc or L-Myc.

Spermatogonial stem cell self-renewal and development

Spermatogenesis originates from spermatogonial stem cells (SSCs). Development of the spermatogonial transplantation technique in 1994 provided the first functional assay to characterize SSCs. In 2000, glial cell line-derived neurotrophic factor was identified as a SSC self-renewal factor. This discovery not only provided a clue to understand SSC self-renewing mechanisms but also made it possible to derive germline stem (GS) cell cultures in 2003. In vitro culture of GS cells demonstrated their potential pluripotency and their utility in germline modification. However, in vivo SSC analyses have challenged the traditional concept of SSC self-renewal and have revealed their relationship with the microenvironment. An improved understanding of SSC self-renewal through functional assays promises to uncover fundamental principles of stem cell biology and will enable us to use these cells for applications in animal transgenesis and medicine.

Compensatory proliferation of endogenous chicken primordial germ cells after elimination by busulfan treatment

Introduction

Primordial germ cells (PGCs) are the major population of cells in the developing bilateral embryonic gonads. Little is known about the cellular responses of PGCs after treatment with toxic chemicals such as busulfan during embryo development. In this study, we investigated the elimination, restorative ability, and cell cycle status of endogenous chicken PGCs after busulfan treatment.

Methods

Busulfan was emulsified in sesame oil by a dispersion-emulsifying system and injected into the chick blastoderm (embryonic stage X). Subsequently, we conducted flow cytometry analysis to evaluate changes in the PGC population and cell cycle status, and immunohistochemistry to examine the germ cell proliferation.

Results

Results of flow cytometry and immunohistochemistry analyses after busulfan treatment showed that the proportion of male PGCs at embryonic day 9 and female PGCs at embryonic day 7 were increased by approximately 60% when compared with embryonic day 5.5. This result suggests the existence of a compensatory mechanism in PGCs in response to the cytotoxic effects of busulfan. Results of cell cycling analysis showed that the germ cells in the G₀/G₁ phase were significantly decreased, while S/G₂/M-phase germ cells were significantly increased in the treatment group compared with the untreated control group in both 9-day-old male and female embryos. In addition, in the proliferation analysis with 5-ethynyl-2′-deoxyuridine (EdU) incorporation, we found that the proportion of EdU-positive cells among VASA homolog-positive cells in the 9-day embryonic gonads of the busulfan-treated group was significantly higher than in the control group.

Conclusions

We conclude that PGCs enter a restoration pathway by promoting their cell cycle after experiencing a cytotoxic effect.

A set of genes critical to development is epigenetically poised in mouse germ cells from fetal stages through completion of meiosis

In multicellular organisms, germ cells carry the hereditary material from one generation to the next. Developing germ cells are unipotent gamete precursors, and mature gametes are highly differentiated, specialized cells. However, upon gamete union at fertilization, their genomes drive a totipotent program, giving rise to a complete embryo as well as extraembryonic tissues. The biochemical basis for the ability to transition from differentiated cell to totipotent zygote is unknown. Here we report that a set of developmentally critical genes is maintained in an epigenetically poised (bivalent) state from embryonic stages through the end of meiosis. We performed ChIP-seq and RNA-seq analysis on flow-sorted male and female germ cells during embryogenesis at three time points surrounding sexual differentiation and female meiotic initiation, and then extended our analysis to meiotic and postmeiotic male germ cells. We identified a set of genes that is highly enriched for regulators of differentiation and retains a poised state (high H3K4me3, high H3K27me3, and lack of expression) across sexes and across developmental stages, including in haploid postmeiotic cells. The existence of such a state in embryonic stem cells has been well described. We now demonstrate that a subset of genes is maintained in a poised state in the germ line from the initiation of sexual differentiation during fetal development and into postmeiotic stages. We propose that the epigenetically poised condition of these developmental genes is a fundamental property of the mammalian germ-line nucleus, allowing differentiated gametes to unleash a totipotent program following fertilization.

Regulation of pluripotency in male germline stem cells by Dmrt1

Spermatogonial stem cells (SSCs) have the potential to acquire pluripotency under specific culture conditions. Takashima et al. report that global DNA hypomethylation triggered by Dnmt1 depletion induces pluripotent cell derivation. Dnmt1 depletion down-regulates Dmrt1, a gene involved in sexual differentiation. Dmrt1 depletion up-regulates Sox2, which in turn up-regulates Oct4 and produces pluripotent cells. These results suggest that the Dmrt1–Sox2 axis plays a crucial role in repression of SSC pluripotency.

Spermatogonial stem cells (SSCs) present the potential to acquire pluripotency under specific culture conditions. However, the frequency of pluripotent cell derivation is low, and the mechanism of SSC reprogramming remains unknown. In this study, we report that induction of global DNA hypomethylation in germline stem (GS) cells (cultured SSCs) induces pluripotent cell derivation. When DNA demethylation was triggered by Dnmt1 depletion, GS cells underwent apoptosis. However, GS cells were converted into embryonic stem (ES)-like cells by double knockdown of Dnmt1 and p53. This treatment down-regulated Dmrt1, a gene involved in sexual differentiation, meiosis, and pluripotency. Dmrt1 depletion caused apoptosis of GS cells, but a combination of Dmrt1 and p53 depletion also induced pluripotency. Functional screening of putative Dmrt1 target genes revealed that Dmrt1 depletion up-regulates Sox2. Sox2 transfection up-regulated Oct4 and produced pluripotent cells. This conversion was enhanced by Oct1 depletion, suggesting that the balance of Oct proteins maintains SSC identity. These results suggest that spontaneous SSC reprogramming is caused by unstable DNA methylation and that a Dmrt1–Sox2 cascade is critical for regulating pluripotency in SSCs.

Rebuilding pluripotency from primordial germ cells

Mammalian primordial germ cells (PGCs) are unipotent progenitors of the gametes. Nonetheless, they can give rise directly to pluripotent stem cells in vitro or during teratocarcinogenesis. This conversion is inconsistent, however, and has been difficult to study. Here, we delineate requirements for efficient resetting of pluripotency in culture. We demonstrate that in defined conditions, routinely 20% of PGCs become EG cells. Conversion can occur from the earliest specified PGCs. The entire process can be tracked from single cells. It is driven by leukemia inhibitory factor (LIF) and the downstream transcription factor STAT3. In contrast, LIF signaling is not required during germ cell ontogeny. We surmise that ectopic LIF/STAT3 stimulation reconstructs latent pluripotency and self-renewal. Notably, STAT3 targets are significantly upregulated in germ cell tumors, suggesting that dysregulation of this pathway may underlie teratocarcinogenesis. These findings demonstrate that EG cell formation is a robust experimental system for exploring mechanisms involved in reprogramming and cancer.

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A defined system for generation of pluripotent EG cells at high efficiency

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20% of single primordial germ cells become EG cells

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Stimulation with LIF but not FGF drives conversion to pluripotency

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LIF/STAT3 targets are upregulated in pluripotent germ cell tumors

Minireview: the diverse roles of nuclear receptors in the regulation of embryonic stem cell pluripotency

Extensive research has been devoted to the goal of understanding how a single cell of embryonic origin can give rise to every somatic cell type and the germ cell lineage, a hallmark defined as "pluripotency." The aggregate of this work supports fundamentally important roles for the gene transcription networks inherent to the pluripotent cell. Transcription networks have been identified that are both required for pluripotency, as well as sufficient to reprogram somatic cells to a naive pluripotent state. Several members of the nuclear receptor (NR) superfamily of transcription factors have been identified to play diverse roles in the regulation of pluripotency. The ligand-responsive nature of NRs coupled with the abundance of genetic models available has led to a significant advance in the understanding of NR roles in embryonic stem cell pluripotency. Furthermore, the presence of a ligand-binding domain may lead to development of small molecules for a wide range of therapeutic and research applications, even in cases of NRs that are not known to respond to physiologic ligands. Presented here is an overview of NR regulation of pluripotency with a focus on the transcriptional, proteomic, and epigenetic mechanisms by which they promote or suppress the pluripotent state.

The mammalian germline as a pluripotency cycle

Naive pluripotency refers to the capacity of single cells in regulative embryos to engender all somatic and germline cell types. Only germ cells - conventionally considered to be unipotent - can naturally re-acquire pluripotency, by cycling through fertilisation. Furthermore, primordial germ cells express, and appear to be functionally dependent upon, transcription factors that characterise the pluripotent state. We hypothesise that germ cells require pluripotency factors to control a de-restricted epigenome. Consequently, they harbour latent potential, as manifested in teratocarcinogenesis or direct conversion into pluripotent stem cells in vitro. Thus, we suggest that there exists an unbroken cycle of pluripotency, naive in the early epiblast and latent in the germline, that is sustained by a shared transcription factor network.

Bivalent chromatin marks developmental regulatory genes in the mouse embryonic germline in vivo

Developmental regulatory genes have both activating (H3K4me3) and repressive (H3K27me3) histone modifications in embryonic stem cells (ESCs). This bivalent configuration is thought to maintain lineage commitment programs in a poised state. However, establishing physiological relevance has been complicated by the high number of cells required for chromatin immunoprecipitation (ChIP). We developed a low-cell-number chromatin immunoprecipitation (low-cell ChIP) protocol to investigate the chromatin of mouse primordial germ cells (PGCs). Genome-wide analysis of embryonic day 11.5 (E11.5) PGCs revealed H3K4me3/H3K27me3 bivalent domains highly enriched at developmental regulatory genes in a manner remarkably similar to ESCs. Developmental regulators remain bivalent and transcriptionally silent through the initiation of sexual differentiation at E13.5. We also identified >2,500 "orphan" bivalent domains that are distal to known genes and expressed in a tissue-specific manner but silent in PGCs. Our results demonstrate the existence of bivalent domains in the germline and raise the possibility that the somatic program is continuously maintained as bivalent, potentially imparting transgenerational epigenetic inheritance.

Very small embryonic-like cells: biology and function of these potential endogenous pluripotent stem cells in adult tissues

Very small embryonic-like cells (VSELs), found in murine bone marrow and other adult tissues, are small, non-hematopoietic cells expressing markers of pluripotent embryonic and primordial germ cells. A similar cell type in humans has begun to be characterized, though with a slightly different phenotype and surface markers. Consistent with expression of pluripotency genes, murine VSELs differentiate into cell types from three germ-layer lineages in vitro, though pluripotency has yet to be shown at the single-cell level or in vivo. VSELs appear to be quiescent under steady state conditions, apparently due to partially erased imprinting and overexpression of cell cycle inhibitory genes. In vivo, VSELs can enter the cell cycle under stress conditions, but which factors regulate quiescence versus proliferation and self-renewal versus differentiation are as yet unknown, and in vitro conditions that induce proliferation and self-renewal have yet to be defined. Future experiments are needed to address whether a VSEL niche actively regulates quiescence in vivo or quiescence is cell autonomous under steady state conditions. Insights into these mechanisms may help to address whether or not VSELs could play a role in regenerative medicine in the future.

Different roles for Tet1 and Tet2 proteins in reprogramming-mediated erasure of imprints induced by EGC fusion

Genomic imprinting directs the allele-specific marking and expression of loci according to their parental origin. Differential DNA methylation at imprinted control regions (ICRs) is established in gametes and, although largely preserved through development, can be experimentally reset by fusing somatic cells with embryonic germ cell (EGC) lines. Here, we show that the Ten-Eleven Translocation proteins Tet1 and Tet2 participate in the efficient erasure of imprints in this model system. The fusion of B cells with EGCs initiates pluripotent reprogramming, in which rapid re-expression of Oct4 is accompanied by an accumulation of 5-hydroxymethylcytosine (5hmC) at several ICRs. Tet2 was required for the efficient reprogramming capacity of EGCs, whereas Tet1 was necessary to induce 5-methylcytosine oxidation specifically at ICRs. These data show that the Tet1 and Tet2 proteins have discrete roles in cell-fusion-mediated pluripotent reprogramming and imprint erasure in somatic cells.

Naive pluripotency is associated with global DNA hypomethylation

Naive pluripotent embryonic stem cells (ESCs) and embryonic germ cells (EGCs) are derived from the preimplantation epiblast and primordial germ cells (PGCs), respectively. We investigated whether differences exist between ESCs and EGCs, in view of their distinct developmental origins. PGCs are programmed to undergo global DNA demethylation; however, we find that EGCs and ESCs exhibit equivalent global DNA methylation levels. Inhibition of MEK and Gsk3b by 2i conditions leads to pronounced reduction in DNA methylation in both cell types. This is driven by Prdm14 and is associated with downregulation of Dnmt3a and Dnmt3b. However, genomic imprints are maintained in 2i, and we report derivation of EGCs with intact genomic imprints. Collectively, our findings establish that culture in 2i instills a naive pluripotent state with a distinctive epigenetic configuration that parallels molecular features observed in both the preimplantation epiblast and nascent PGCs.

In vivo epigenomic profiling of germ cells reveals germ cell molecular signatures

The limited number of in vivo germ cells poses an impediment to genome-wide studies. Here, we applied a small-scale chromatin immunoprecipitation sequencing (ChIP-seq) method on purified mouse fetal germ cells to generate genome-wide maps of four histone modifications (H3K4me3, H3K27me3, H3K27ac, and H2BK20ac). Comparison of active chromatin state between somatic, embryonic stem, and germ cells revealed promoters and enhancers needed for stem cell maintenance and germ cell development. We found the nuclear receptor Nr5a2 motif to be enriched at a subset of germ cell cis-regulatory regions, and our results implicate Nr5a2 in germ cell biology. Interestingly, in germ cells, the H3K27me3 histone modification occurs more frequently at regions that are enriched for retrotransposons and MHC genes, indicating that these loci are specifically silenced in germ cells. Together, our study provides genome-wide histone modification maps of in vivo germ cells and reveals the molecular chromatin signatures of germ cells.