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

DMRT1 regulates human germline commitment

Germline commitment following primordial germ cell (PGC) specification during early human development establishes an epigenetic programme and competence for gametogenesis. Here we follow the progression of nascent PGC-like cells derived from human embryonic stem cells in vitro. We show that switching from BMP signalling for PGC specification to Activin A and retinoic acid resulted in DMRT1 and CDH5 expression, the indicators of migratory PGCs in vivo. Moreover, the induction of DMRT1 and SOX17 in PGC-like cells promoted epigenetic resetting with striking global enrichment of 5-hydroxymethylcytosine and locus-specific loss of 5-methylcytosine at DMRT1 binding sites and the expression of DAZL representing DNA methylation-sensitive genes, a hallmark of the germline commitment programme. We provide insight into the unique role of DMRT1 in germline development for advances in human germ cell biology and in vitro gametogenesis.

Origin and segregation of the human germline

Human germline-soma segregation occurs during weeks 2-3 in gastrulating embryos. Although direct studies are hindered, here, we investigate the dynamics of human primordial germ cell (PGCs) specification using in vitro models with temporally resolved single-cell transcriptomics and in-depth characterisation using in vivo datasets from human and nonhuman primates, including a 3D marmoset reference atlas. We elucidate the molecular signature for the transient gain of competence for germ cell fate during peri-implantation epiblast development. Furthermore, we show that both the PGCs and amnion arise from transcriptionally similar TFAP2A-positive progenitors at the posterior end of the embryo. Notably, genetic loss of function experiments shows that TFAP2A is crucial for initiating the PGC fate without detectably affecting the amnion and is subsequently replaced by TFAP2C as an essential component of the genetic network for PGC fate. Accordingly, amniotic cells continue to emerge from the progenitors in the posterior epiblast, but importantly, this is also a source of nascent PGCs.

Prospects for fertility preservation: the ovarian organ function reconstruction techniques for oogenesis, growth and maturation in vitro

Today, fertility preservation is receiving more attention than ever. Cryopreservation, which preserves ovarian tissue to preserve fertility in young women and reduce the risk of infertility, is currently the most widely practiced. Transplantation, however, is less feasible for women with blood-borne leukemia or cancers with a high risk of ovarian metastasis because of the risk of cancer recurrence. In addition to cryopreservation and re-implantation of embryos, in vitro ovarian organ reconstruction techniques have been considered as an alternative strategy for fertility preservation. In vitro culture of oocytes in vitro Culture, female germ cells induction from pluripotent stem cells (PSC) in vitro, artificial ovary construction, and ovaria-related organoids construction have provided new solutions for fertility preservation, which will therefore maximize the potential for all patients undergoing fertility preservation. In this review, we discussed and thought about the latest ovarian organ function reconstruction techniques in vitro to provide new ideas for future ovarian disease research and fertility preservation of patients with cancer and premature ovarian failure.

Systems-Level Mapping of Cancer Testis Antigen 1b/a to Sarcoma Pathways Identifies Activated Ran Binding-2 E3 SUMO-Protein Ligase and Transducin-Like Enhancer Protein 1

Here we describe the identification of genes and their encoded proteins that are expressed in advanced grade tumors by reconstruction of a sarcoma cancer testis gene 1b/a (catg1b/a) network. CTAG1B/A is an ortholog of the yeast/Drosophila transcription factor Pcc1p, and a member of the KEOPS transcription complex. It has been implicated in telomere maintenance and transcriptional regulation through association with chromatin remodeling factors and is only expressed during adult testis germ cell differentiation. Ctag1b/a is re-activated in synovial sarcomas and myxoid liposarcomas but not in differentiated liposarcomas. We mapped CTAG1B/A protein to sarcoma transcription pathways with gene set expression analysis (GSEA) and using independent samples, we immunohistochemically identified expression of at least two network neighbors, RANBP2, and TLE1, thus validating our approach. This work demonstrates that mapping unknown genes to functional pathways by network re-construction is a powerful tool that can be used to identify candidate oncoproteins.

Selective elimination of pluripotent stem cells by PIKfyve specific inhibitors

Inhibition of PIKfyve phosphoinositide kinase selectively kills autophagy-dependent cancer cells by disrupting lysosome homeostasis. Here, we show that PIKfyve inhibitors can also selectively eliminate pluripotent embryonal carcinoma cells (ECCs), embryonic stem cells, and induced pluripotent stem cells under conditions where differentiated cells remain viable. PIKfyve inhibitors prevented lysosome fission, induced autophagosome accumulation, and reduced cell proliferation in both pluripotent and differentiated cells, but they induced death only in pluripotent cells. The ability of PIKfyve inhibitors to distinguish between pluripotent and differentiated cells was confirmed with xenografts derived from ECCs. Pretreatment of ECCs with the PIKfyve specific inhibitor WX8 suppressed their ability to form teratocarcinomas in mice, and intraperitoneal injections of WX8 into mice harboring teratocarcinoma xenografts selectively eliminated pluripotent cells. Differentiated cells continued to proliferate, but at a reduced rate. These results provide a proof of principle that PIKfyve specific inhibitors can selectively eliminate pluripotent stem cells in vivo as well as in vitro.

Long Noncoding RNA Mediated Regulation in Human Embryogenesis, Pluripotency, and Reproduction

Long noncoding RNAs (lncRNAs), a class of noncoding RNAs with more than 200 bp in length, are produced by pervasive transcription in mammalian genomes and regulate gene expression through various action mechanisms. Accumulating data indicate that lncRNAs mediate essential biological functions in human development, including early embryogenesis, induction of pluripotency, and germ cell development. Comprehensive analysis of sequencing data highlights that lncRNAs are expressed in a stage-specific and human/primate-specific pattern during early human development. They contribute to cell fate determination through interacting with almost all classes of cellular biomolecules, including proteins, DNA, mRNAs, and microRNAs. Furthermore, the expression of a few of lncRNAs is highly associated with the pathogenesis and progression of many reproductive diseases, suggesting that they could serve as candidate biomarkers for diagnosis or novel targets for treatment. Here, we review research on lncRNAs and their roles in embryogenesis, pluripotency, and reproduction. We aim to identify the underlying molecular mechanisms essential for human development and provide novel insight into the causes and treatments of human reproductive diseases.

Heading towards a dead end: The role of DND1 in germ line differentiation of human iPSCs

The DND microRNA-mediated repression inhibitor 1 (DND1) is a conserved RNA binding protein (RBP) that plays important roles in survival and fate maintenance of primordial germ cells (PGCs) and in the development of the male germline in zebrafish and mice. Dead end was shown to be expressed in human pluripotent stem cells (PSCs), PGCs and spermatogonia, but little is known about its specific role concerning pluripotency and human germline development. Here we use CRISPR/Cas mediated knockout and PGC-like cell (PGCLC) differentiation in human iPSCs to determine if DND1 (1) plays a role in maintaining pluripotency and (2) in specification of PGCLCs. We generated several clonal lines carrying biallelic loss of function mutations and analysed their differentiation potential towards PGCLCs and their gene expression on RNA and protein levels via RNA sequencing and mass spectrometry. The generated knockout iPSCs showed no differences in pluripotency gene expression, proliferation, or trilineage differentiation potential, but yielded reduced numbers of PGCLCs as compared with their parental iPSCs. RNAseq analysis of mutated PGCLCs revealed that the overall gene expression remains like non-mutated PGCLCs. However, reduced expression of genes associated with PGC differentiation and maintenance (e.g., NANOS3, PRDM1) was observed. Together, we show that DND1 iPSCs maintain their pluripotency but exhibit a reduced differentiation to PGCLCs. This versatile model will allow further analysis of the specific mechanisms by which DND1 influences PGC differentiation and maintenance.

Insights from the Applications of Single-Cell Transcriptomic Analysis in Germ Cell Development and Reproductive Medicine

Mechanistic understanding of germ cell formation at a genome-scale level can aid in developing novel therapeutic strategies for infertility. Germ cell formation is a complex process that is regulated by various mechanisms, including epigenetic regulation, germ cell-specific gene transcription, and meiosis. Gonads contain a limited number of germ cells at various stages of differentiation. Hence, genome-scale analysis of germ cells at the single-cell level is challenging. Conventional genome-scale approaches cannot delineate the landscape of genomic, transcriptomic, and epigenomic diversity or heterogeneity in the differentiating germ cells of gonads. Recent advances in single-cell genomic techniques along with single-cell isolation methods, such as microfluidics and fluorescence-activated cell sorting, have helped elucidate the mechanisms underlying germ cell development and reproductive disorders in humans. In this review, the history of single-cell transcriptomic analysis and their technical advantages over the conventional methods have been discussed. Additionally, recent applications of single-cell transcriptomic analysis for analyzing germ cells have been summarized.

Advances in Female Germ Cell Induction from Pluripotent Stem Cells

Germ cells are capable of maintaining species continuity through passing genetic and epigenetic information across generations. Female germ cells mainly develop during the embryonic stage and pass through subsequent developmental stages including primordial germ cells, oogonia, and oocyte. However, due to the limitation of using early human embryos as in vivo research model, in vitro research models are needed to reveal the early developmental process and related mechanisms of female germ cells. After birth, the number of follicles gradually decreases with age. Various conditions which damage ovarian functions would cause premature ovarian failure. Alternative treatments to solve these problems need to be investigated. Germ cell differentiation from pluripotent stem cells in vitro can simulate early embryonic development of female germ cells and clarify unresolved issues during the development process. In addition, pluripotent stem cells could potentially provide promising applications for female fertility preservation after proper in vitro differentiation. Mouse female germ cells have been successfully reconstructed in vitro and delivered to live offspring. However, the derivation of functional human female germ cells has not been fully achieved due to technical limitations and ethical issues. To provide an updated and comprehensive information, this review centers on the major studies on the differentiation of mouse and human female germ cells from pluripotent stem cells and provides references to further studies of developmental mechanisms and potential therapeutic applications of female germ cells.

Environmentally Induced Epigenetic Transgenerational Inheritance and the Weismann Barrier: The Dawn of Neo-Lamarckian Theory

For the past 120 years, the Weismann barrier and associated germ plasm theory of heredity have been a doctrine that has impacted evolutionary biology and our concepts of inheritance through the germline. Although August Weismann in his 1872 book was correct that the sperm and egg were the only cells to transmit molecular information to the subsequent generation, the concept that somatic cells do not impact the germline (i.e., the Weismann barrier) is incorrect. However, the doctrine or dogma of the Weismann barrier still influences many scientific fields and topics. The discovery of epigenetics, and more recently environmentally induced epigenetic transgenerational inheritance of phenotypic variation and pathology, have had significant impacts on evolution theory and medicine today. Environmental epigenetics and the concept of epigenetic transgenerational inheritance refute aspects of the Weismann barrier and require a re-evaluation of both inheritance theory and evolution theory.

A critical role of PRDM14 in human primordial germ cell fate revealed by inducible degrons

PRDM14 is a crucial regulator of mouse primordial germ cells (mPGCs), epigenetic reprogramming and pluripotency, but its role in the evolutionarily divergent regulatory network of human PGCs (hPGCs) remains unclear. Besides, a previous knockdown study indicated that PRDM14 might be dispensable for human germ cell fate. Here, we decided to use inducible degrons for a more rapid and comprehensive PRDM14 depletion. We show that PRDM14 loss results in significantly reduced specification efficiency and an aberrant transcriptome of hPGC-like cells (hPGCLCs) obtained in vitro from human embryonic stem cells (hESCs). Chromatin immunoprecipitation and transcriptomic analyses suggest that PRDM14 cooperates with TFAP2C and BLIMP1 to upregulate germ cell and pluripotency genes, while repressing WNT signalling and somatic markers. Notably, PRDM14 targets are not conserved between mouse and human, emphasising the divergent molecular mechanisms of PGC specification. The effectiveness of degrons for acute protein depletion is widely applicable in various developmental contexts.

Genetic basis for primordial germ cells specification in mouse and human: Conserved and divergent roles of PRDM and SOX transcription factors

Primordial germ cells (PGCs) are embryonic precursors of sperm and egg that pass on genetic and epigenetic information from one generation to the next. In mammals, they are induced from a subset of cells in peri-implantation epiblast by BMP signaling from the surrounding tissues. PGCs then initiate a unique developmental program that involves comprehensive epigenetic resetting and repression of somatic genes. This is orchestrated by a set of signaling molecules and transcription factors that promote germ cell identity. Here we review significant findings on mammalian PGC biology, in particular, the genetic basis for PGC specification in mice and human, which has revealed an evolutionary divergence between the two species. We discuss the importance and potential basis for these differences and focus on several examples to illustrate the conserved and divergent roles of critical transcription factors in mouse and human germline.