Human Primordial Germ Cell-like Cells Research Articles

Generation of mouse and rat xenogeneic ovaries in vitro for production of mouse oocyte

ABSTRACT The system forming ovarian follicles is developed to investigate in vitro folliculogenesis in a confined environment to obtain functional oocytes. Several studies have reported the successful generation of fully functional oocytes using mouse-induced pluripotent stem cells (iPSCs) and mouse female germline stem cells (fGSCs) as sources of stem cells for in vitro gametogenesis models. In addition, human oogonia have been generated through heterologous co-culture of differentiated human primordial germ cell-like cells (hPGCLCs) with mouse germline somatic cells, although oocyte formation remains challenging. Thus, studies on in vitro ovarian formation in other species are utilized as an introductory approach for in vitro mammalian gametogenesis by understanding the differences in culture systems between species and underlying mechanisms. In this study, we optimized the method of the entire oogenesis process from rat embryonic gonads. We identified well-maturated MII oocytes from rat gonads using our constructed method. Moreover, we generated the first successful in vitro reconstitution of xenogeneic follicles from mouse primordial germ cells (PGCs) and rat somatic cells. We also established an appropriate culture medium and incubation period for xenogeneic follicles. This method will be helpful in studies of xenogeneic follicular development and oocyte generation.

Derivation of human primordial germ cell-like cells in an embryonic-like culture

Primordial germ cells (PGCs) are the embryonic precursors of sperm and eggs. They transmit genetic and epigenetic information across generations. Given the prominent role of germline defects in diseases such as infertility, detailed understanding of human PGC (hPGC) development has important implications in reproductive medicine and studying human evolution. Yet, hPGC specification remains an elusive process. Here, we report the induction of hPGC-like cells (hPGCLCs) in a bioengineered human pluripotent stem cell (hPSC) culture that mimics peri-implantation human development. In this culture, amniotic ectoderm-like cells (AMLCs), derived from hPSCs, induce hPGCLC specification from hPSCs through paracrine signaling downstream of ISL1. Our data further show functional roles of NODAL, WNT, and BMP signaling in hPGCLC induction. hPGCLCs are successfully derived from eight non-obstructive azoospermia (NOA) participant-derived hPSC lines using this biomimetic platform, demonstrating its promise for screening applications.

TET1 facilitates specification of early human lineages including germ cells

Ten Eleven Translocation 1 (TET1) is a regulator of localized DNA demethylation through the conversion of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC). To examine DNA demethylation in human primordial germ cell-like cells (hPGCLCs) induced from human embryonic stem cells (hESCs), we performed bisulfite-assisted APOBEC coupled epigenetic sequencing (bACEseq) followed by integrated genomics analysis. Our data indicates that 5hmC enriches at hPGCLC-specific NANOG, SOX17 or TFAP2C binding sites on hPGCLC induction, and this is accompanied by localized DNA demethylation. Using CRISPR-Cas9, we show that deleting the catalytic domain of TET1 reduces hPGCLC competency when starting with hESC cultured on mouse embryonic fibroblasts, and this phenotype can be rescued after transitioning hESCs to defined media and a recombinant substrate. Taken together, our study demonstrates the importance of 5hmC in facilitating hPGCLC competency, and the role of hESC culture conditions in modulating this effect.

Efficient and scalable generation of primordial germ cells in 2D culture using basement membrane extract overlay

Current methods to generate human primordial germ cell-like cells (hPGCLCs) from human pluripotent stem cells (hPSCs) can be inefficient, and it is challenging to generate sufficient hPGCLCs to optimize invitro gametogenesis. We present a differentiation method that uses diluted basement membrane extract (BMEx) and low BMP4 concentration to efficiently induce hPGCLC differentiation in scalable 2D cell culture. We show that BMEx overlay potentiated BMP/SMAD signaling, induced lumenogenesis, and increased expression of key hPGCLC-progenitor markers such as TFAP2A and EOMES. hPGCLCs that were generated using the BMEx overlay method were able to upregulate more mature germ cell markers, such as DAZL and DDX4, in human fetal ovary reconstitution culture. These findings highlight the importance of BMEx during hPGCLC differentiation and demonstrate the potential of the BMEx overlay method to interrogate the formation of PGCs and amnion in humans, as well as to investigate the next steps to achieve invitro gametogenesis.

Directed differentiation of human iPSCs to functional ovarian granulosa-like cells via transcription factor overexpression.

An in vitro model of human ovarian follicles would greatly benefit the study of female reproduction. Ovarian development requires the combination of germ cells and several types of somatic cells. Among these, granulosa cells play a key role in follicle formation and support for oogenesis. Whereas efficient protocols exist for generating human primordial germ cell-like cells (hPGCLCs) from human induced pluripotent stem cells (hiPSCs), a method of generating granulosa cells has been elusive. Here, we report that simultaneous overexpression of two transcription factors (TFs) can direct the differentiation of hiPSCs to granulosa-like cells. We elucidate the regulatory effects of several granulosa-related TFs and establish that overexpression of NR5A1 and either RUNX1 or RUNX2 is sufficient to generate granulosa-like cells. Our granulosa-like cells have transcriptomes similar to human fetal ovarian cells and recapitulate key ovarian phenotypes including follicle formation and steroidogenesis. When aggregated with hPGCLCs, our cells form ovary-like organoids (ovaroids) and support hPGCLC development from the premigratory to the gonadal stage as measured by induction of DAZL expression. This model system will provide unique opportunities for studying human ovarian biology and may enable the development of therapies for female reproductive health.

Generation of NANOS3-mCherry reporter human embryonic stem cell line SYSUe-009-A using CRISPR/Cas9

NANOS3 is a zinc-finger containing RNA-binding protein that has been demonstrated to be highly expressed in human primordial germ cell(hPGC), thus NANOS3 can serve as a marker for hPGC development. Due to the ethical and technical restrictions, it is difficult to get primary human germline cells. Human primordial germ cell-like cells (hPGCLCs) generated from pluripotent stem cells is an excellent alternatives in human germ cell-related studies. This hESC line with an mCherry knock-in at the site before NANOS3′s stop codon serves as a useful tool to learn human PGC specification.

Maintenance of Human Primordial Germ Cell-Like Cells in a Long-Term Culture System.

Primordial germ cells (PGCs) are the earliest form of mammalian germ lineage. In humans, PGCs are present during a very early and limited window in development, limiting the ability to study fundamental developmental steps in human reproductive biology. However, recent advancements in generating in-vitro models of gametogenesis have allowed the field to generate human primordial germ cell-like cells (hPGCLCs). In this chapter, we will review the generation of hPGCLCs using the incipient mesoderm-like cell (iMeLC) protocol and the subsequent expansion of hPGCLCs in a long-term culture system.

Sorting and Manipulation of Human PGC-LC Using PDPN and Hanging Drop Cultures.

The generation of oocytes from induced pluripotent stem cells (iPSCs) was proven efficient with mouse cells. However, no human iPSCs have yet been reported to generate cells able to complete oogenesis. Additionally, efficient sorting of human Primordial Germ Cell-like Cells (hPGC-LCs) without genomic integration of fluorescent reporter for their downstream manipulation is still lacking. Here, we aimed to develop a model that allows human germ cell differentiation in vitro in order to study the developing human germline. The hPGC-LCs specified from two iPS cell lines were sorted and manipulated using the PDPN surface marker without genetic modification. hPGC-LCs obtained remain arrested at early stages of maturation and no further differentiation nor meiotic onset occurred when these were cultured with human or mouse fetal ovarian somatic cells. However, when cultured independently of somatic ovarian cells, using BMP4 and the hanging drop-transferred EBs system, early hPGC-LCs further differentiate efficiently and express late PGC (DDX4) and meiotic gene markers, although no SYCP3 protein was detected. Altogether, we characterized a tool to sort hPGC-LCs and an efficient in vitro differentiation system to obtain pre-meiotic germ cell-like cells without using a gonadal niche.

The President's Hat.

<h3>ABSTRACT</h3> The hominoid-specific endogenous retrovirus LTR5_Hs is transcriptionally activated in human primordial germ cell-like cells (hPGCLCs), a pluripotent stem cell-derived cell culture model of PGCs. Here, taking the unique advantage of our novel cell culture method to obtain large amounts of pure hPGCLCs, we performed proteomics profiling of hPGCLCs and detected various viral proteins produced from the LTR5_Hs RNA <i>via</i> ribosomal frameshifting. We also present transmission electron microscopy images of 100-nm diameter virus-like particles (VLPs) assembled at the surface of hPGCLCs. Compared to hPGCLCs, expression of LTR5_Hs RNA is far weaker in human seminomas, the germ cell tumors resembling PGCs. Re-analysis of published single cell RNA-seq data of human embryos revealed strong activation of LTR5_Hs in migrating PGCs but suppressed in PGCs upon they reach the gonadal anlagen. In the microfluidics-supported polarized embryoids mimicking peri-implantation stages of human embryos, LTR5_Hs RNA was detected by RNA in situ hybridization in NANOG⁺/TFAP2C⁺/SOX17⁺ cells resembling freshly emerged PGCs. These results support that human germ cells produce LTR5_Hs proteins and VLPs during their earliest stages of normal development until their settlement in the gonadal anlagen. <h3>SUMMARY STATEMENT</h3> The hominoid-specific endogenous retrovirus LTR5_Hs is activated in a cell culture model resembling early-stage human primordial germ cells, producing not only viral RNA but also retrovirus proteins and virus-like particles.

Expanding homogeneous culture of human primordial germ cell-like cells maintaining germline features without serum or feeder layers.

SummaryIn vitro expansion of human primordial germ cell-like cells (hPGCLCs), a pluripotent stem cell-derived PGC model, has proved challenging due to rapid loss of primordial germ cell (PGC)-like identity and limited cell survival/proliferation. Here, we describe long-term culture hPGCLCs (LTC-hPGCLCs), which actively proliferate in a serum-free, feeder-free condition without apparent limit as highly homogeneous diploid cell populations maintaining transcriptomic and epigenomic characteristics of hPGCLCs. Histone proteomics confirmed reduced H3K9me2 and increased H3K27me3 marks in LTC-hPGCLCs compared with induced pluripotent stem cells (iPSCs). LTC-hPGCLCs established from multiple human iPSC clones of both sexes were telomerase positive, senescence-free cells readily passaged with minimal cell death or deviation from the PGC-like identity. LTC-hPGCLCs are capable of differentiating to DAZL-positive M-spermatogonia-like cells in the xenogeneic reconstituted testis (xrTestis) organ culture milieu as well as efficiently producing fully pluripotent embryonic germ cell-like cells in the presence of stem cell factor and fibroblast growth factor 2. Thus, LTC-hPGCLCs provide convenient access to unlimited amounts of high-quality and homogeneous hPGCLCs.

Differentiation of Human-Induced Pluripotent Stem Cells (hiPSCs) into Human Primordial Germ Cell-like Cells (hPGCLCs) In Vitro.

In humans, germ cells are specified in the extraembryonic yolk sac, at proximity of allantois, around the second week of gestation. Derivation of human germ cell-like cells (hPGCLCs) from human pluripotent cells in vitro is of a great importance for research purposes, such as disease modeling, or studying the early human germ cell development and the effect of environmental factors on this development. As it is not possible to access human embryos at early developmental stages, a two-step protocol has been proposed by Sasaki and colleagues to differentiate hPGCLCs in vitro from human pluripotent stem cells. Here, we report a detailed protocol for in vitro hPGCLCs differentiation from induced pluripotent stem cells (iPSCs).

NANOS3 downregulation in Down syndrome hiPSCs during primordial germ cell-like cell differentiation.

Human infertility is a complex disorder at the genetic, molecular, cellular, organ, and hormonal levels. New developing technology based on the generation of human primordial germ cell-like cells (hPGCLCs) from induced pluripotent stem cells (hiPSCs) might improve understanding of early germ cell development (specification, migration, gametogenesis, and epigenetic reconstitutions), as well as offering a solution for infertility and hereditary disorders. In this study, we differentiated hiPSCs with trisomy 21 into hPGCLCs. In vitro-derived germ cells from hiPSCs with Down syndrome (DS) express hPGCLC core circuitry, EOMES, SOX17, and PRDM14 at relatively low levels. TFAP2C and PRDM1 were expressed and remained elevated, whereas NANOS3 and NANOG were downregulated in BMP4-induced hiPSCs with DS. The low level of NANOG and NANOS3 expression might negatively influence hPGCLC generation in DS hiPSCs. We suggest that DS hPGCLCs could be a suitable model for studying human early germ cell development, the epigenetic and molecular mechanisms of PGC specification and formation, as well as related infertility disorders, such as azoospermia and teratozoospermia.

Tissue of Origin, but Not XCI State, Influences Germ Cell Differentiation from Human Pluripotent Stem Cells.

Human pluripotent stem cells (hPSCs) are not only a promising tool to investigate differentiation to many cell types, including the germline, but are also a potential source of cells to use for regenerative medicine purposes in the future. However, current in vitro models to generate human primordial germ cell-like cells (hPGCLCs) have revealed high variability regarding differentiation efficiency depending on the hPSC lines used. Here, we investigated whether differences in X chromosome inactivation (XCI) in female hPSCs could contribute to the variability of hPGCLC differentiation efficiency during embryoid body (EB) formation. For this, we first characterized the XCI state in different hPSC lines by investigating the expression of XIST and H3K27me3, followed by differentiation and quantification of hPGCLCs. We observed that the XCI state did not influence the efficiency to differentiate to hPGCLCs; rather, hPSCs derived from cells isolated from urine showed an increased trend towards hPGCLCs differentiation compared to skin-derived hPSCs. In addition, we also characterized the XCI state in the generated hPGCLCs. Interestingly, we observed that independent of the XCI state of the hPSCs used, both hPGCLCs and soma cells in the EBs acquired XIST expression, indicative of an inactive X chromosome. In fact, culture conditions for EB formation seemed to promote XIST expression. Together, our results contribute to understanding how epigenetic properties of hPSCs influence differentiation and to optimize differentiation methods to obtain higher numbers of hPGCLCs, the first step to achieve human in vitro gametogenesis.

Sex-Specific Isolation and Propagation of Human Premeiotic Fetal Germ Cells and Germ Cell-Like Cells.

The second trimester of human development is marked by asynchronous gonadal development hampering the isolation of homogenous populations of early and late fetal germ cells (FGCs). We evaluated the feasibility of using surface markers TNAP, PDPN, EPCAM and ITGA6 to isolate FGCs as well as human primordial germ cell-like cells (hPGCLCs) derived from embryonic stem cells (hESCs) from both sexes by fluorescence-activated cell sorting (FACS). Our results suggest that a combination of TNAP and PDPN was sufficient to separate populations of premeiotic FGCs and hPGCLCs in both sexes. This combination of antibodies also proved efficient in separating ‘mitotic’ from ‘retinoic-acid responsive’ female FGCs. Furthermore, we report that the differentiation efficiency of TNAP+PDPN+ hPGCLCs from hESCs was sex-independent, but the ability to propagate differed considerably between the sexes. In contrast to male, female hPGCLCs retained their characteristics and exhibited robust colony-forming ability when cultured for five days in medium containing LIF, forskolin and FGF2. We conclude that marked sex differences exist in the isolation and propagation of human FGCs and hPGCLCs. Our study provides novel insights relevant for the optimization of in vitro gametogenesis in humans.

The chromatin accessibility landscape reveals distinct transcriptional regulation in the induction of human primordial germ cell-like cells from pluripotent stem cells.

SummaryIn vitro induction of human primordial germ cell-like cells (hPGCLCs) provides an ideal platform to recapitulate hPGC development. However, the detailed molecular mechanisms regulating the induction of hPGCLCs remain largely uncharacterized. Here, we profiled the chromatin accessibility and transcriptome dynamics throughout the process of hPGCLC induction. Genetic ablation of SOX15 indicated the crucial roles of SOX15 in the maintenance of hPGCLCs. Mechanistically, SOX15 exerted its roles via suppressing somatic gene expression and sustaining latent pluripotency. Notably, ETV5, a downstream regulator of SOX15, was also uncovered to be essential for hPGCLC maintenance. Finally, a stepwise switch of OCT4/SOX2, OCT4/SOX17, and OCT4/SOX15 binding motifs were found to be enriched in closed-to-open regions of human embryonic stem cells, and early- and late-stage hPGCLCs, respectively. Collectively, our data characterized the chromatin accessibility and transcriptome landscapes throughout hPGCLC induction and defined the SOX15-mediated regulatory networks underlying this process.

The histone demethylase KDM2B regulates human primordial germ cell-like cells specification

Germline specification is a fundamental step for human reproduction and this biological phenomenon possesses technical challenges to study in vivo as it occurs immediately after blastocyst implantation. The establishment of in vitro human primordial germ cell-like cells (hPGCLCs) induction system allows sophisticated characterization of human primordial germ cells (hPGCs) development. However, the underlying molecular mechanisms of hPGCLC specification are not fully elucidated. Here, we observed particularly high expression of the histone demethylase KDM2B in male fetal germ cells (FGCs) but not in male somatic cells. Besides, KDM2B shared similar expression pattern with hPGC marker genes in hPGCLCs, suggesting an important role of KDM2B in germ cell development. Although deletion of KDM2B had no significant effects on human embryonic stem cell (hESC)'s pluripotency, loss of KDM2B dramatically impaired hPGCLCs differentiation whereas ectopically expressed KDM2B could efficiently rescue such defect, indicating this defect was due to KDM2B's loss in hPGCLC specification. Mechanistically, as revealed by the transcriptional profiling, KDM2B suppressed the expression of somatic genes thus inhibited somatic differentiation during hPGCLC specification. These data collectively indicate that KDM2B is an indispensable epigenetic regulator for hPGCLC specification, shedding lights on how epigenetic regulations orchestrate transcriptional events in hPGC development for future investigation.

In vitro derived female hPGCLCs are unable to complete meiosis in embryoid bodies

The fundamental question about the functionality of in vitro derived human primordial germ cell-like cells remains unanswered, despite ongoing research in this area. Attempts have been made to imitate the differentiation of human primordial germ cells (hPGCs) and meiocytes in vitro from human pluripotent stem cells (hPSCs). A defined system for developing human haploid cells in vitro is the challenge that scientists face to advance the knowledge of human germ cell development. To develop human primordial germ cell-like cells (hPGCLCs) from human pluripotent stem cells (hPSCs) that are capable of giving rise to haploid cells, we applied a sequential induction protocol via the early mesodermal push of female human embryonic and induced pluripotent stem cells. BMP4-induced early mesoderm-like cells showed significant alterations in their expression profiles toward early (PRDM1 and NANOS3) and late (VASA and DAZL) germ cell markers. Furthermore, using retinoic acid (RA), we induced hPGCLCs in embryoid bodies and identified positive staining for the meiotic initiation marker STRA8. Efforts to find the cells exhibiting progression to meiosis were unsuccessful. The validation by the expression of SCP3 did not correspond to the natural pattern. Regarding the 20-day meiotic induction, the derived hPGCLCs containing two X-chromosomes were unable to complete the meiotic division. We observed the expression of the oocyte marker PIWIL1 and PIWIL4. RNAseq analysis and cluster dendrogram showed a similar clustering of hPGCLC groups and meiotic like cell groups as compared to previously published data. This reproducible in vitro model for deriving hPGCLCs provides opportunities for studying the molecular mechanisms involved in the specification of hPGCs. Moreover, our results will support a further elucidation of gametogenesis and meiosis of female hPGCs.

Long-term expansion with germline potential of human primordial germ cell-like cells invitro.

Human germ cells perpetuate human genetic and epigenetic information. However, the underlying mechanism remains elusive, due to a lack of appropriate experimental systems. Here, we show that human primordial germ cell-like cells (hPGCLCs) derived from human-induced pluripotent stem cells (hiPSCs) can be propagated to at least ~106 -fold over a period of 4months under a defined condition invitro. During expansion, hPGCLCs maintain an early hPGC-like transcriptome and preserve their genome-wide DNA methylation profiles, most likely due to retention of maintenance DNA methyltransferase activity. These characteristics contrast starkly with those of mouse PGCLCs, which, under an analogous condition, show a limited propagation (up to ~50-fold) and persist only around 1week, yet undergo cell-autonomous genome-wide DNA demethylation. Importantly, upon aggregation culture with mouse embryonic ovarian somatic cells in xenogeneic-reconstituted ovaries, expanded hPGCLCs initiate genome-wide DNA demethylation and differentiate into oogonia/gonocyte-like cells, demonstrating their germline potential. By creating a paradigm for hPGCLC expansion, our study uncovers critical divergences in expansion potential and the mechanism for epigenetic reprogramming between the human and mouse germ cell lineage.

Reproduction reimagined

Reproduction reimagined

An Extended Culture System that Supports Human Primordial Germ Cell-like Cell Survival and Initiation of DNA Methylation Erasure.

SummaryThe development of an in vitro system in which human primordial germ cell-like cells (hPGCLCs) are generated from human pluripotent stem cells (hPSCs) has been invaluable to further our understanding of human primordial germ cell (hPGC) specification. However, the means to evaluate the next fundamental steps in germ cell development have not been well established. In this study we describe a two dimensional extended culture system that promotes proliferation of specified hPGCLCs, without reversion to a pluripotent state. We demonstrate that hPGCLCs in extended culture undergo partial epigenetic reprogramming, mirroring events described in hPGCs in vivo, including a genome-wide reduction in DNA methylation and maintenance of depleted H3K9me2. This extended culture system provides a new approach for expanding the number of hPGCLCs for downstream technologies, including transplantation, molecular screening, or possibly the differentiation of hPGCLCs into gametes by in vitro gametogenesis.