A germ cell origin of embryonic stem cells?

Developmental expression of the pluripotency factor sal-like protein 4 in the monkey, human and mouse testis: restriction to premeiotic germ cells

SALL4 (sal-like protein 4) is a pluripotency transcription factor, which is highly expressed in embryonic stem (ES) cells and which is essential for mouse preimplantation development. In adult mouse organs, Sall4 mRNA is highly expressed in the testis and ovary, while there is only little or no expression in other organs. There is also a high expression of SALL4 in human testicular germ cell tumors. However, there is as yet no detailed analysis of SALL4 expression during mammalian testicular development. We analyzed SALL4 expression in ES cells, preimplantation embryos, and the developing and adult testis of a nonhuman primate (NHP) species, the common marmoset monkey (Callithrix jacchus). Immunofluorescence revealed SALL4 in the nuclei of marmoset ES cells and preimplantation embryos. Marmoset SALL4 isoform analysis in ES cells and newborn and adult testis by RT- PCR and Western blotting showed two different isoforms, SALL4-A and SALL4-B. Immunohistochemistry localized this transcription factor to the nuclei of primordial germ cells and most gonocytes in the prenatal and early postnatal marmoset testis. In the pubertal and adult testis SALL4 was present in undifferentiated spermatogonia. In the developing and adult human and mouse testis SALL4 expression mimicked the pattern in the marmoset. Adult testes from additional NHP species, the treeshrew, the cat and the dog also exhibited SALL4 in undifferentiated spermatogonia, indicating a conserved expression in the mammalian testis. Taking into account the importance of SALL4 for mouse development, we conclude that SALL4 may play an important role during mammalian germ cell development and is involved in the regulation of spermatogonial proliferation in the adult testis.

Germ cell pluripotency, premature differentiation and susceptibility to testicular teratomas in mice

Testicular teratomas result from anomalies in germ cell development during embryogenesis. In the 129 family of inbred strains of mice, teratomas initiate around embryonic day (E) 13.5 during the same developmental period in which female germ cells initiate meiosis and male germ cells enter mitotic arrest. Here, we report that three germ cell developmental abnormalities, namely continued proliferation, retention of pluripotency, and premature induction of differentiation, associate with teratoma susceptibility. Using mouse strains with low versus high teratoma incidence (129 versus 129-Chr19(MOLF/Ei)), and resistant to teratoma formation (FVB), we found that germ cell proliferation and expression of the pluripotency factor Nanog at a specific time point, E15.5, were directly related with increased tumor risk. Additionally, we discovered that genes expressed in pre-meiotic embryonic female and adult male germ cells, including cyclin D1 (Ccnd1) and stimulated by retinoic acid 8 (Stra8), were prematurely expressed in teratoma-susceptible germ cells and, in rare instances, induced entry into meiosis. As with Nanog, expression of differentiation-associated factors at a specific time point, E15.5, increased with tumor risk. Furthermore, Nanog and Ccnd1, genes with known roles in testicular cancer risk and tumorigenesis, respectively, were co-expressed in teratoma-susceptible germ cells and tumor stem cells, suggesting that retention of pluripotency and premature germ cell differentiation both contribute to tumorigenesis. Importantly, Stra8-deficient mice had an 88% decrease in teratoma incidence, providing direct evidence that premature initiation of the meiotic program contributes to tumorigenesis. These results show that deregulation of the mitotic-meiotic switch in XY germ cells contributes to teratoma initiation.

Pluripotency and nuclear reprogramming

Pluripotency is a "blank" cellular state characteristic of specific cells within the early embryo (e.g., epiblast cells) and of certain cells propagated in vitro (e.g., embryonic stem cells, ESCs). The terms pluripotent cell and stem cell are often used interchangeably to describe cells capable of differentiating into multiple cell types. In this review, we discuss the prevailing molecular and functional definitions of pluripotency and the working parameters employed to describe this state, both in the context of cells residing within the early embryo and cells propagated in vitro.

Technological progress in generation of induced pluripotent stem cells for clinical applications

Reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) is achieved by viral-mediated transduction of defined transcription factors. Generation of iPSCs is of great medical interest as they have the potential to be a source of patient-specific cells. For the eventual goal of clinical application, it is necessary to overcome the limitations of low reprogramming efficiency and chromosomal abnormalities due to viral DNA integration. In this paper, we summarize the current state of reprogramming technology for generation of iPSCs and also discuss potential approaches to the development of safe iPSCs for personalized cell-based replacement therapy.

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

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

Single cell analysis facilitates staging of Blimp1-dependent primordial germ cells derived from mouse embryonic stem cells

The cell intrinsic programming that regulates mammalian primordial germ cell (PGC) development in the pre-gonadal stage is challenging to investigate. To overcome this we created a transgene-free method for generating PGCs in vitro (iPGCs) from mouse embryonic stem cells (ESCs). Using labeling for SSEA1 and cKit, two cell surface molecules used previously to isolate presumptive iPGCs, we show that not all SSEA1+/cKit+ double positive cells exhibit a PGC identity. Instead, we determined that selecting for cKit^bright cells within the SSEA1+ fraction significantly enriches for the putative iPGC population. Single cell analysis comparing SSEA1+/cKit^bright iPGCs to ESCs and embryonic PGCs demonstrates that 97% of single iPGCs co-express PGC signature genes Blimp1, Stella, Dnd1, Prdm14 and Dazl at similar levels to e9.5–10.5 PGCs, whereas 90% of single mouse ESC do not co-express PGC signature genes. For the 10% of ESCs that co-express PGC signature genes, the levels are significantly lower than iPGCs. Microarray analysis shows that iPGCs are transcriptionally distinct from ESCs and repress gene ontology groups associated with mesoderm and heart development. At the level of chromatin, iPGCs contain 5-methyl cytosine bases in their DNA at imprinted and non-imprinted loci, and are enriched in histone H3 lysine 27 trimethylation, yet do not have detectable levels of Mvh protein, consistent with a Blimp1-positive pre-gonadal PGC identity. In order to determine whether iPGC formation is dependent upon Blimp1, we generated Blimp1 null ESCs and found that loss of Blimp1 significantly depletes SSEA1/cKit^bright iPGCs. Taken together, the generation of Blimp1-positive iPGCs from ESCs constitutes a robust model for examining cell-intrinsic regulation of PGCs during the Blimp1-positive stage of development.

Blimp1 expression predicts embryonic stem cell development in vitro

Despite recent critical insights into the pluripotent state of embryonic stem cells (ESCs), there is little agreement over the inaugural and subsequent steps leading to its generation [1-4]. Here we show that inner cell mass (ICM)-generated cells expressing Blimp1, a key transcriptional repressor of the somatic program during germ cell specification [5, 6], emerge on day 2 of blastocyst culture. Single-cell gene expression profiling indicated that many of these Blimp1-positive cells coexpress other genes typically associated with early germ cell specification. When genetically traced in vitro, these cells acquired properties normally associated with primordial germ cells. Importantly, fate-mapping experiments revealed that ESCs commonly arise from Blimp1-positive precursors; indeed, prospective sorting of such cells from ICM outgrowths increased the rate of ESC derivation more than 9-fold. Finally, using genetic ablation or distinct small molecules [7, 8], we show that epiblast cells can become ESCs without first acquiring Blimp1 positivity. Our findings suggest that the germ cell-like state is facultative for the stabilization of pluripotency in vitro. Thus, the association of Blimp1 expression with ESC development furthers understanding of how the pluripotent state of these cells is established in vitro and suggests a means to enhance the generation of new stem cell lines from blastocysts.

Isolation of epiblast stem cells from preimplantation mouse embryos

Pluripotent stem cells provide a platform to interrogate control elements that function to generate all cell types of the body. Despite their utility for modeling development and disease, the relationship of mouse and human pluripotent stem cell states to one another remains largely undefined. We have shown that mouse embryonic stem (ES) cells and epiblast stem cells (EpiSCs) are distinct, pluripotent states isolated from pre- and post-implantation embryos respectively. Human ES cells are different than mouse ES cells and share defining features with EpiSCs, yet are derived from pre-implantation human embryos. Here we show that EpiSCs can be routinely derived from pre-implantation mouse embryos. The preimplantation-derived EpiSCs exhibit molecular features and functional properties consistent with bona fide EpiSCs. These results provide a simple method for isolating EpiSCs and offer direct insight into the intrinsic and extrinsic mechanisms that regulate the acquisition of distinct pluripotent states.

Transcripts that associate with the RNA binding protein, DEAD-END (DND1), in embryonic stem (ES) cells

Background

The RNA binding protein, DEAD END (DND1), is essential for maintaining viable germ cells in vertebrates. It is also a testicular germ cell tumor susceptibility factor in mice. DND1 has been shown to interact with the 3'-untranslated region (3'-UTR) of mRNAs such as P27 and LATS2. Binding of DND1 to the 3'-UTRs of these transcripts blocks the inhibitory function of microRNAs (miRNA) from these transcripts and in this way DND1 helps maintain P27 and LATS2 protein expression. We found that DND1 is also expressed in embryonic stem (ES) cells. Because ES cells share similar gene expression patterns as germ cells, we utilized ES cells to identify additional candidate mRNAs that associate with DND1.

Results

ES cells are readily amenable to genetic modification and easier to culture in vitro compared to germ cells. Therefore, for the purpose of our study, we made a genetically modified, stable, human embryonic stem (hES) cell line that expresses hemagluttinin (HA)-tagged DND1 in a doxycycline (dox) regulatable manner. This line expresses modest levels of HA-DND1 and serves as a good system to study DND1 function in vitro. We used this stable cell line to identify the transcripts that physically interact with DND1. By performing ribonucleoprotein immunoprecipitation (RIP) followed by RT-PCR, we identified that transcripts encoding pluripotency factors (OCT4, SOX2, NANOG, LIN28), cell cycle regulators (TP53, LATS2) and apoptotic factors (BCLX, BAX) are specifically associated with the HA-DND1 ribonucleoprotein complex. Surprisingly, in many cases, bioinformatics analysis of the pulled-down transcripts did not reveal the presence of known DND1 interacting motifs.

Conclusions

Our results indicate that the inducible ES cell line system serves as a suitable in vitro system to identify the mRNA targets of DND1. The RIP-RT results hint at the broad spectrum of mRNA targets that interact with DND1 in ES cells. Based on what is known about DND1 function, our results suggest that DND1 may impose another level of translational regulation to modulate expression of critical factors in ES cells.

Stage-specific germ-cell marker genes are expressed in all mouse pluripotent cell types and emerge early during induced pluripotency

Embryonic stem cells (ESCs) generated from the in-vitro culture of blastocyst stage embryos are known as equivalent to blastocyst inner cell mass (ICM) in-vivo. Though several reports have shown the expression of germ cell/pre-meiotic (GC/PrM) markers in ESCs, their functional relevance for the pluripotency and germ line commitment are largely unknown. In the present study, we used mouse as a model system and systematically analyzed the RNA and protein expression of GC/PrM markers in ESCs and found them to be comparable to the expression of cultured pluripotent cells originated from the germ line. Further, siRNA knockdown experiments have demonstrated the parallel maintenance and independence of pluripotent and GC/PrM networks in ESCs. Through chromatin immunoprecipitation experiments, we observed that pluripotent cells exhibit active chromatin states at GC marker genes and a bivalent chromatin structure at PrM marker genes. Moreover, gene expression analysis during the time course of iPS cells generation revealed that the expression of GC markers precedes pluripotency markers. Collectively, through our observations we hypothesize that the chromatin state and the expression of GC/PrM markers might indicate molecular parallels between in-vivo germ cell specification and pluripotent stem cell generation.

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.

In vitro and in vivo germ line potential of stem cells derived from newborn mouse skin

We previously reported that fetal porcine skin-derived stem cells were capable of differentiation into oocyte-like cells (OLCs). Here we report that newborn mice skin-derived stem cells are also capable of differentiating into early OLCs. Using stem cells from mice that are transgenic for Oct4 germline distal enhancer-GFP, germ cells resulting from their differentiation are expected to be GFP(+). After differentiation, some GFP(+) OLCs reached 40-45 µM and expressed oocyte markers. Flow cytometric analysis revealed that ∼ 0.3% of the freshly isolated skin cells were GFP(+). The GFP-positive cells increased to ∼ 7% after differentiation, suggesting that the GFP(+) cells could be of in vivo origin, but are more likely induced upon being cultured in vitro. To study the in vivo germ cell potential of skin-derived cells, they were aggregated with newborn ovarian cells, and transplanted under the kidney capsule of ovariectomized mice. GFP(+) oocytes were identified within a subpopulation of follicles in the resulting growth. Our finding that early oocytes can be differentiated from mice skin-derived cells in defined medium may offer a new in vitro model to study germ cell formation and oogenesis.

Sertoli cell-conditioned medium induces germ cell differentiation in human embryonic stem cells

PURPOSE

To investigate the spontaneous germ cell differentiation capacity of VUB hESC lines, develop a protocol for the induction of germ cell differentiation using conditioned medium from Sertoli cells (SCCM) and compare it to existing protocols.

METHODS

hESC were allowed to differentiate spontaneously or after the addition of bone morphogenetic proteins (BMPs) and/or SCCM. VASA transcripts were measured by relative quantification real-time RT-PCR to determine the efficiency of germ cell differentiation.

RESULTS

VUB hESC lines can differentiate spontaneously towards the germ cell lineage, however, more consistently in an embryoid body approach than in monolayer cultures. BMPs and SCCM significantly improve VASA expression, but do not have a synergistic effect. Direct contact of differentiating hESC with Sertoli cells does not improve VASA expression.

CONCLUSIONS

SCCM contains inductive factors for germ cell differentiation and could represent an element for in-vitro differentiation to germ cells.

The origin and identity of embryonic stem cells

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

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.

Developmental plasticity of stem cells and diseases

Stem cells, which reside in most tissues with high plasticity of self-renewal and differentiation, play a critical role in the development and maintenance processes of many tissues. It is reasonable to suspect that many undefined chronic diseases are caused by the malfunction of stem cells. In this hypothesis, we try to explain the origin of undefined chronic diseases by the developmental plasticity of stem cells. Chronic diseases are phenotypes of stem cells' malfunction. Mutations of multipotent stem cells at the prenatal stage can be used to explain phenomena of early development of diseases. In addition, stem cells accumulate mutations from fetus stage to aging period and usually develop chronic diseases in late life periods.

Survival and death of epiblast cells during embryonic stem cell derivation revealed by long-term live-cell imaging with an Oct4 reporter system

Despite the broad literature on embryonic stem cells (ESCs), their derivation process remains enigmatic. This may be because of the lack of experimental systems that can monitor this prolonged cellular process. Here we applied a live-cell imaging technique to monitor the process of ESC derivation over 10 days from morula to outgrowth phase using an Oct4/eGFP reporter system. Our imaging reflects the 'natural' state of ESC derivation, as the ESCs established after the imaging were both competent in chimeric mice formation and germ-line transmission. Using this technique, ESC derivation in conventional conditions was imaged. After the blastocoel was formed, the intensity of Oct4 signals attenuated in the trophoblast cells but was maintained in the inner cell mass (ICM). Thereafter, the Oct4-positive cells scattered and their number decreased along with apoptosis of the other Oct4-nagative cells likely corresponds to trophoblast and hypoblast cells, and then only the surviving Oct4-positive cells proliferated and formed the colony. All embryos without exception passed through this cell death phase. Importantly, the addition of caspase inhibitor Z-VAD-FMK to the medium dramatically suppressed the loss of Oct4-positive cells and also other embryo-derived cells, suggesting that the cell deaths was induced by a caspase-dependent apoptotic pathway. Next we imaged the ESC derivation in 3i medium, which consists of chemical compounds that can suppress differentiation. The most significant difference between the conventional and 3i methods was that there was no obvious cell death in 3i, so that the colony formation was rapid and all of the Oct4-positive cells contributed to the formation of the outgrown colony. These data indicate that the prevention of cell death in epiblast cells is one of the important events for the successful establishment of ESCs. Thus, our imaging technique can advance the understanding of the time-dependent cellular changes during ESC derivation.

Pluripotent embryonic stem cells and multipotent adult germline stem cells reveal similar transcriptomes including pluripotency-related genes

DNA microarray analysis was performed with mouse multipotent adult germline stem cells (maGSCs) and embryonic stem cells (ESCs) from different genetic backgrounds cultured under standard ESC-culture conditions and under differentiation-promoting conditions by the withdrawal of the leukemia inhibitory factor (LIF) and treatment with retinoic acid (RA). The analyzed undifferentiated cell lines are very similar based on their global gene expression pattern and show 97-99% identity dependent on the analyzed background. Only 621 genes are differentially expressed in cells derived from mouse 129SV-background and 72 genes show differences in expression in cells generated from transgenic Stra8-EGFP/Rosa26-LacZ-background. Both maGSCs and ESCs express the same genes involved in the regulation of pluripotency and even show no differences in the expression level of these genes. When comparing maGSCs with previously published signature genes of other pluripotent cell lines, we found that maGSCs shared a very similar gene expression pattern with embryonic germ cells (EGCs). Also after differentiation of maGSCs and ESCs the transcriptomes of the cell lines are nearly identical which suggests that both cell types differentiate spontaneously in a very similar way. This is the first study, at transcriptome level, to compare ESCs and a pluripotent cell line derived from an adult organism (maGSCs).

Epiblast/germ line hypothesis of cancer development revisited: lesson from the presence of Oct-4+ cells in adult tissues

The morphology of several tumors mimics developmentally early tissues; tumors often express early developmental markers characteristic for the germ line lineage. Recently, our group identified a population of very small stem cells (SCs) in murine bone marrow (BM) and other adult organs that express several markers characteristic for epiblast/germ line-derived SCs. We named these rare cells "Very Small Embryonic/Epiblast-like Stem Cells (VSELs)." We hypothesized that these cells that express both epiblast and germ line markers are deposited during early gastrulation in developing tissues and organs and play an important role in the turnover of tissue-committed (TC) SCs. To support this, we envision that the germ line is not only the origin of SCs, but also remains as a scaffold or back-up for the SC compartment in adult life. Furthermore, we noticed that VSELs are protected from uncontrolled proliferation and teratoma formation by a unique DNA methylation pattern in some developmentally crucial imprinted genes, which show hypomethylation or erasure of imprints in paternally methylated genes and hypermethylation of imprints in the maternally methylated. In pathological situations, however, we hypothesize that VSELs could be involved in the development of several malignancies. Therefore, potential involvement of VSELs in cancerogenesis could support century-old concepts of embryonic rest- or germ line-origin hypotheses of cancer development. However, we are aware that this working hypothesis requires further direct experimental confirmation.

Tracing the derivation of embryonic stem cells from the inner cell mass by single-cell RNA-Seq analysis

During the transition from the inner cell mass (ICM) cells of blastocysts to pluripotent embryonic stem cells (ESCs) in vitro, a normal developmental program is replaced in cells that acquire a capacity for infinite self-renewal and pluripotency. We explored the underlying mechanism of this switch by using RNA-Seq transcriptome analysis at the resolution of single cells. We detected significant molecular transitions and major changes in transcript variants, which include genes for general metabolism. Furthermore, the expression of repressive epigenetic regulators increased with a concomitant decrease in gene activators that might be necessary to sustain the inherent plasticity of ESCs. Furthermore, we detected changes in microRNAs (miRNAs), with one set that targets early differentiation genes while another set targets pluripotency genes to maintain the unique ESC epigenotype. Such genetic and epigenetic events may contribute to a switch from a normal developmental program in adult cells during the formation of diseased tissues, including cancers.

Caenorhabditis elegans as a model for stem cell biology

We review the application of Caenorhabditis elegans as a model system to understand key aspects of stem cell biology. The only bona fide stem cells in C. elegans are those of the germline, which serves as a valuable paradigm for understanding how stem-cell niches influence maintenance and differentiation of stem cells and how somatic differentiation is repressed during germline development. Somatic cells that share stem cell-like characteristics also provide insights into principles in stem-cell biology. The epidermal seam cell lineages lend clues to conserved mechanisms of self-renewal and expansion divisions. Principles of developmental plasticity and reprogramming relevant to stem-cell biology arise from studies of natural transdifferentiation and from analysis of early embryonic progenitors, which undergo a dramatic transition from a pluripotent, reprogrammable condition to a state of committed differentiation. The relevance of these developmental processes to our understanding of stem-cell biology in other organisms is discussed.

Spermatogonial stem cells, in vivo transdifferentiation and human regenerative medicine

IMPORTANCE OF THE FIELD

Embryonic stem (ES) cells have potential for use in regenerative medicine, but use of these cells is hindered by moral, legal and ethical issues. Induced pluripotent cells have promise in regenerative medicine. However, since generation of these cells involves genetic manipulation, it also faces significant hurdles before clinical use. This review discusses spermatogonial stem cells (SSCs) as a potential alternative source of pluripotent cells for use in human regenerative medicine.

AREAS COVERED IN THE REVIEW

The potential of SSCs to give rise to a wide range of other cell types either directly, when recombined with instructive inducers, or indirectly, after being converted to ES-like cells. Current understanding of the differentiation potential of murine SSCs and recent progress in isolating and culturing human SSCs and demonstrating their properties is also discussed.

WHAT THE READER WILL GAIN

Insight into the plasticity of SSCs and the unique properties of these cells for regenerative applications, the limitations of SSCs for stem-cell-based therapy and the potential alternatives available.

TAKE HOME MESSAGE

If methodologies for isolation and conversion of adult human SSCs directly into other cell types can be effectively developed, SSCs could represent an important alternate source of pluripotent cells that can be used in human tissue repair and/or regeneration.

Differential expression and regulation by activin of the neurotrophins BDNF and NT4 during human and mouse ovarian development

The tropomyosin-related kinase (Trk) B neurotrophin receptor is essential for ovarian germ cell survival and primordial follicle formation, but the contributions of its ligands, brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4), are unknown. We have investigated their expression and regulation in developing human and mouse ovaries. BDNF expression increased with increasing gestation, expression of human NTF4 and of both Ntf5 and Bdnf in the mouse was unchanged. Bdnf expression was dramatically lower than Ntf5 in the mouse, but levels were comparable in the human. Human fetal ovarian somatic cells expressed BDNF. Activin A selectively regulated BDNF and Ntf5 expression in human and mouse, respectively, identifying an oocyte/somatic signaling pathway which might mediate the pro-survival effects of activin. These data reveal that expression and regulation of the TrkB ligands are differentially controlled in the developing ovaries of humans and mice, and identify BDNF as a potential regulator of germ cell fate in the human fetal ovary.

Cyclin D2 and the CDK substrate p220(NPAT) are required for self-renewal of human embryonic stem cells

Self-renewal of pluripotent human embryonic stem (hES) cells utilizes an abbreviated cell cycle that bypasses E2F/pRB-dependent growth control. We investigated whether self-renewal is alternatively regulated by cyclin/CDK phosphorylation of the p220(NPAT)/HiNF-P complex to activate histone gene expression at the G1/S phase transition. We show that cyclin D2 is prominently expressed in pluripotent hES cells, but cyclin D1 eclipses cyclin D2 during differentiation. Depletion of cyclin D2 or p220(NPAT) causes a cell cycle defect in G1 reflected by diminished phosphorylation of p220(NPAT), decreased cell cycle dependent histone H4 expression and reduced S phase progression. Thus, cyclin D2 and p220(NPAT) are principal cell cycle regulators that determine competency for self-renewal in pluripotent hES cells. While pRB/E2F checkpoint control is relinquished in human ES cells, fidelity of physiological regulation is secured by cyclin D2 dependent activation of the p220(NPAT)/HiNF-P mechanism that may explain perpetual proliferation of hES cells without transformation or tumorigenesis.

Suppression of Erk signalling promotes ground state pluripotency in the mouse embryo

Embryonic stem (ES) cells can be derived and propagated from multiple strains of mouse and rat through application of small-molecule inhibitors of the fibroblast growth factor (FGF)/Erk pathway and of glycogen synthase kinase 3. These conditions shield pluripotent cells from differentiation-inducing stimuli. We investigate the effect of these inhibitors on the development of pluripotent epiblast in intact pre-implantation embryos. We find that blockade of Erk signalling from the 8-cell stage does not impede blastocyst formation but suppresses development of the hypoblast. The size of the inner cell mass (ICM) compartment is not reduced, however. Throughout the ICM, the epiblast-specific marker Nanog is expressed, and in XX embryos epigenetic silencing of the paternal X chromosome is erased. Epiblast identity and pluripotency were confirmed by contribution to chimaeras with germline transmission. These observations indicate that segregation of hypoblast from the bipotent ICM is dependent on FGF/Erk signalling and that in the absence of this signal, the entire ICM can acquire pluripotency. Furthermore, the epiblast does not require paracrine support from the hypoblast. Thus, naïve epiblast and ES cells are in a similar ground state, with an autonomous capacity for survival and replication, and high vulnerability to Erk signalling. We probed directly the relationship between naïve epiblast and ES cells. Dissociated ICM cells from freshly harvested late blastocysts gave rise to up to 12 ES cell clones per embryo when plated in the presence of inhibitors. We propose that ES cells are not a tissue culture creation, but are essentially identical to pre-implantation epiblast cells.

Transplantation directs oocyte maturation from embryonic stem cells and provides a therapeutic strategy for female infertility

Ten to 15% of couples are infertile, with the most common causes being linked to the production of few or no oocytes or sperm. Yet, our understanding of human germ cell development is poor, at least in part due to the inaccessibility of early stages to genetic and developmental studies. Embryonic stem cells (ESCs) provide an in vitro system to study oocyte development and potentially treat female infertility. However, most studies of ESC differentiation to oocytes have not documented fundamental properties of endogenous development, making it difficult to determine the physiologic relevance of differentiated germ cells. Here, we sought to establish fundamental parameters of oocyte development during ESC differentiation to explore suitability for basic developmental genetic applications using the mouse as a model prior to translating to the human system. We demonstrate a timeline of definitive germ cell differentiation from ESCs in vitro that initially parallels endogenous oocyte development in vivo by single-cell expression profiling and analysis of functional milestones including responsiveness to defined maturation media, shared genetic requirement of Dazl, and entry into meiosis. However, ESC-derived oocyte maturation ultimately fails in vitro. To overcome this obstacle, we transplant ESC-derived oocytes into an ovarian niche to direct their functional maturation and, thereby, present rigorous evidence of oocyte physiologic relevance and a potential therapeutic strategy for infertility.

Development and spindle formation in rat somatic cell nuclear transfer (SCNT) embryos in vitro using porcine recipient oocytes

Cloning that uses somatic cell nuclear transfer (SCNT) technology with gene targeting could be a potential alternative approach to obtain valuable rat models. In the present study, we determined the developmental competence of rat SCNT embryos constructed using murine and porcine oocytes at metaphase II (MII). Further, we assessed the effects of certain factors, such as: (i) the donor cell type (fetal fibroblasts or cumulus cells); and (ii) premature chromosome condensation (PCC) with normal spindle formation, on the developmental competence of rat interspecies SCNT (iSCNT) embryos. iSCNT embryos that had been constructed using porcine oocytes developed to the blastocyst stage, while those embryos made using murine MII oocytes did not. Rat iSCNT embryos constructed with green fluorescent protein (GFP)-expressing fetal fibroblasts injected into porcine oocytes showed considerable PCC with a normal bipolar spindle formation. The total cell number of iSCNT blastocyst derived from GFP-expressing fetal fibroblasts was higher than the number derived from cumulus cells. In addition, these embryos expressed GFP at the blastocyst stage. This paper is the first report to show that rat SCNT embryos constructed using porcine MII oocytes have the potential to develop to the blastocyst stage in vitro. Thus the iSCNT technique, when performed using porcine MII oocytes, could provide a new bioassay system for the evaluatation of the developmental competence of rat somatic cells.

Direct transdifferentiation of stem/progenitor spermatogonia into reproductive and nonreproductive tissues of all germ layers

Pluripotent stem cells have great clinical potential for tissue regeneration/repair in humans. The use of embryonic stem (ES) cells is ethically controversial, leading to searches for other sources of pluripotent stem cells. Testicular spermatogonial stem cells (SSCs) produce the spermatogenic lineage. Under in vitro conditions, SSCs have the ability to give rise to pluripotent ES-like cells. We hypothesized that stem/progenitor spermatogonia could directly transdifferentiate into different tissue types if they were recombined with inductive mesenchymes from fetal/neonatal organs using a tissue separation/recombination methodology and grown in vivo. Green fluorescent protein transgenic mice were used to track cell lineages. Our results indicate that stem/progenitor spermatogonia recombined with the appropriate mesenchyme can directly transdifferentiate in vivo into tissues of all germ layers, including prostatic, uterine, and skin epithelium. In addition, transdifferentiated tissue expressed molecular, histological, and functional markers of the appropriate epithelium. The ability of stem/progenitor spermatogonia to directly generate various epithelia emphasizes their clinical potential, and if adult human SSCs have similar properties, this may have applications in human regenerative medicine.

Stem cells, pluripotency and nuclear reprogramming

Reprogramming of somatic cells to a pluripotent embryonic stem cell-like state has been achieved by nuclear transplantation of a somatic nucleus into an enucleated egg and most recently by introducing defined transcription factors into somatic cells. Nuclear reprogramming is of great medical interest as it has the potential to generate a source of patient-specific cells. This short review summarizes strategies to reprogram somatic cells to a pluripotent embryonic state and discuss the implications of this technology for transplantation medicine.

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

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

Very small embryonic-like stem cells in adult tissues-potential implications for aging

Recently our group identified in murine bone marrow (BM) and human cord blood (CB), a rare population of very small embryonic-like (VSEL) stem cells. We hypothesize that these cells are deposited during embryonic development in BM as a mobile pool of circulating pluripotent stem cells (PSC) that play a pivotal role in postnatal tissue turnover both of non-hematopoietic and hematopoietic tissues. During in vitro co-cultures with murine myoblastic C2C12 cells, VSELs form spheres that contain primitive stem cells. Cells isolated from these spheres may give rise to cells from all three germ layers when plated in tissue specific media. The number of murine VSELs and their ability to form spheres decreases with the age and is reduced in short-living murine strains. Thus, developmental deposition of VSELs in adult tissues may potentially play an underappreciated role in regulating the rejuvenation of senescent organs. We envision that the regenerative potential of these cells could be harnessed to decelerate aging processes.

Dynamic equilibrium and heterogeneity of mouse pluripotent stem cells with distinct functional and epigenetic states

Embryonic stem cells (ESCs) are apparently homogeneous self-renewing cells, but we observed heterogeneous expression of Stella in ESCs, which is a marker of pluripotency and germ cells. Here we show that, whereas Stella-positive ESCs were like the inner cell mass (ICM), Stella-negative cells were like the epiblast cells. These states were interchangeable, which reflects the metastability and plasticity of ESCs. The established equilibrium was skewed reversibly in the absence of signals from feeder cells, which caused a marked shift toward an epiblast-like state, while trichostatin A, an inhibitor of histone deactelylase, restored Stella-positive population. The two populations also showed different histone modifications and striking functional differences, as judged by their potential for differentiation. The Stella-negative ESCs were more like the postimplantation epiblast-derived stem cells (EpiSCs), albeit the stella locus was repressed by DNA methylation in the latter, which signifies a robust epigenetic boundary between ESCs and EpiSCs.

On the origin of the term "stem cell"

Stem cells have fascinated both biologists and clinicians for over a century. Here, we discuss the origin of the term "stem cell," which can be traced back to the late 19th century. The term stem cell originated in the context of two major embryological questions of that time: the continuity of the germ-plasm and the origin of the hematopoietic system. Theodor Boveri and Valentin Häcker used the term stem cell to describe cells committed to give rise to the germline. In parallel, Artur Pappenheim, Alexander Maximow, Ernst Neumann, and others used it to describe a proposed progenitor of the blood system. The original meanings of the term stem cell, rather than being historical relics, continue to capture important aspects of the biology of stem cells as we see them today.

Markers that define stemness in ESC are unable to identify the totipotent cells in human preimplantation embryos

BACKGROUND

During human preimplantation development, early blastomeres are believed to be totipotent. It is likely, however, that blastomeres are allocated to a specific lineage prior to any morphological differentiation. NANOG, SOX2 and SALL4 are transcription factors that play a key role in controlling stemness in embryonic stem cells (ESC) and are therefore candidate markers for developmental triggers in early embryos. KRT18, a trophoblast-determining gene, may mark early differentiation. Examining the expression pattern of these genes may inform us about when and in which cells totipotency is lost during early human development.

METHODS

Thirtheen oocytes, 124 preimplantation embryos and 7 human embryonic stem cell (hESC) lines were examined for the presence of NANOG, SOX2, SALL4 or KRT18 proteins using immunostaining and confocal microscopy.

RESULTS

All stemness markers were expressed in the hESC, but none of them was specific for totipotent cells during human preimplantation development, and none of them seemed to mark cells allocated to the inner cell mass (ICM) or trophectoderm. After lineage specification, only the nuclear expression of NANOG and SOX2 became restricted to the ICM, at least to some cells because only a subpopulation expressed NANOG. KRT18 expression was seen for the first time during compaction in some outer cells. KRT18 was not expressed in hESC.

CONCLUSION

We conclude that the protein expression patterns of markers that define stemness in ESC do not identify the totipotent cells in human preimplantation embryos. Assessing the presence of KRT18 proteins implied that the outer cells of compacting embryos have probably lost their totipotent competence prior to any visible differentiation.

Modelling germ cell development in vitro

Germ cells have a critical role in mediating the generation of genetic diversity and transmitting this information across generations. Furthermore, gametogenesis is unique as a developmental process in that it generates highly-specialized haploid gametes from diploid precursor stem cells through meiosis. Despite the importance of this process, progress in elucidating the molecular mechanisms underpinning mammalian germ cell development has been retarded by the lack of an efficient and reproducible system of in vitro culture for the expansion and trans-meiotic differentiation of germline cells. The dearth of such a culture system has rendered the study of germ cell biology refractory to the application of new high-throughput technologies such as RNA interference, leaving in vivo gene-targeting approaches as the only option to determine the function of genes believed to be involved in gametogenesis. Recent reports detailing the derivation of gametes in vitro from stem cells may provide the first steps in developing new tools to solve this problem. This review considers the developments made in modelling germ cell development using stem cells, and some of the challenges that need to be overcome to make this a useful tool for studying gametogenesis and to realize any future clinical application.

Recent advances in the derivation of germ cells from the embryonic stem cells

In recent years, considerable progress has been made in the establishment and differentiation of human embryonic stem (ES) cell lines. The primordial germ cells (PGCs) and embryonic germ (EG) cells derived from them share many of their properties with ES cells. ES cell lines have now been derived from different stages of germ cell development and they have differentiated into gametes and shown embryonic development in mice, including the production of live pups. Conversely, germ cells can also be derived from ES cells. It has been demonstrated that murine (m) ES cells can differentiate into PGCs and subsequently into early gametes (oocytes and sperms) and blastocysts. Recently, immature sperm cells derived from mES cells in culture have produced live offspring. Preliminary research has indicated that human (h) ES cells probably have the potential to differentiate into germ cells. Adult stem cells have been reported to differentiate into mature germ cells in vitro. Therefore, stem cells may offer a valuable in vitro model for the investigation of germ cell development and the early stages of human gametogenesis, including epigenetic modifications of the germ line. This review discusses recent developments in the derivation and specification of mammalian germ cells from ES cells and describes some of the mechanisms of germ cell development.

The tumorigenicity of human embryonic stem cells

Human embryonic stem cells (HESCs) are the in vitro descendants of the pluripotent inner cell mass (ICM) of human blastocyst stage embryos. HESCs can be kept undifferentiated in culture or be differentiated to tissues representing all three germ layers, both in vivo and in vitro. These properties make HESC-based therapy remarkably appealing for the treatment of various disorders. Upon transplantation in vivo, undifferentiated HESCs rapidly generate the formation of large tumors called teratomas. These are benign masses of haphazardly differentiated tissues. Teratomas also appear spontaneously in humans and in mice. When they also encompass a core of malignant undifferentiated cells, these tumors are defined as teratocarcinomas. These malignant undifferentiated cells are termed embryonic carcinoma (EC), and are the malignant counterparts of embryonic stem cells. Here we review the history of experimental teratomas and teratocarcinomas, from spontaneous teratocarcinomas in mice to induced teratomas by HESC transplantation. We then discuss cellular and molecular aspects of the tumorigenicity of HESCs. We also describe the utilization of HESC-induced teratomas for the modeling of early human embryogenesis and for modeling developmental diseases. The problem of HESC-induced teratomas may also impede or prevent future HESC-based therapies. We thus conclude with a survey of approaches to evade HESC-induced tumor formation.