Gametogenesis from Pluripotent Stem Cells

MCM8 interacts with DDX5 to promote R-loop resolution

MCM8 has emerged as a core gene in reproductive aging and is crucial for meiotic homologous recombination repair. It also safeguards genome stability by coordinating the replication stress response during mitosis, but its function in mitotic germ cells remains elusive. Here we found that disabling MCM8 in mice resulted in proliferation defects of primordial germ cells (PGCs) and ultimately impaired fertility. We further demonstrated that MCM8 interacted with two known helicases DDX5 and DHX9, and loss of MCM8 led to R-loop accumulation by reducing the retention of these helicases at R-loops, thus inducing genome instability. Cells expressing premature ovarian insufficiency-causative mutants of MCM8 with decreased interaction with DDX5 displayed increased R-loop levels. These results show MCM8 interacts with R-loop-resolving factors to prevent R-loop-induced DNA damage, which may contribute to the maintenance of genome integrity of PGCs and reproductive reserve establishment. Our findings thus reveal an essential role for MCM8 in PGC development and improve our understanding of reproductive aging caused by genome instability in mitotic germ cells.

HELQ deficiency impairs the induction of primordial germ cell-like cells

Helicase POLQ-like (HELQ) is a DNA helicase essential for the maintenance of genome stability. A recent study identified two HELQ missense mutations in some cases of infertile men. However, the functions of HELQ in the process of germline specification are not well known and whether its function is conserved between mouse and human remains unclear. Here, we revealed that Helq knockout (Helq-/-) could significantly reduce the efficiency of mouse primordial germ cell-like cell (PGCLC) induction. In addition, Helq-/- embryonic bodies exhibited a severe apoptotic phenotype on day 6 of mouse PGCLC induction. p53 inhibitor treatment could partially rescue the generation of mouse PGCLCs from Helq mutant mouse embryonic stem cells. Finally, the genetic ablation of HELQ could also significantly impede the induction of human PGCLCs. Collectively, our study sheds light on the involvement of HELQ in the induction of both mouse and human PGCLCs, providing new insights into the mechanisms underlying germline differentiation and the genetic studies of human fertility.

The gonadal niche safeguards human fetal germline cell development following maternal SARS-CoV-2 infection

During pregnancy, germline development is vital for maintaining the continuation of species. Recent studies have shown increased pregnancy risks in COVID-19 patients at the perinatal stage. However, the potential consequence of infection for reproductive quality in developing fetuses remains unclear. Here, we analyze the transcriptome and DNA methylome of the fetal germline following maternal severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. We find that infection at early gestational age, a critical period of human primordial germ cell specification and epigenetic reprogramming, trivially affects fetal germ cell (FGC) development. Additionally, FGC-niche communications are not compromised by maternal infection. Strikingly, both general and SARS-CoV-2-specific immune pathways are greatly activated in gonadal niche cells to protect FGCs from maternal infection. Notably, there occurs an "in advance" development tendency in FGCs after maternal infection. Our study provides insights into the impacts of maternal SARS-CoV-2 infection on fetal germline development and serves as potential clinical guidance for future pandemics.

Emerging Contributions of Pluripotent Stem Cells to Reproductive Technologies in Veterinary Medicine

The generation of mature gametes and competent embryos in vitro from pluripotent stem cells has been successfully achieved in a few species, mainly in mice, with recent advances in humans and scarce preliminary reports in other domestic species. These biotechnologies are very attractive as they facilitate the understanding of developmental mechanisms and stages that are generally inaccessible during early embryogenesis, thus enabling advanced reproductive technologies and contributing to the generation of animals of high genetic merit in a short period. Studies on the production of in vitro embryos in pigs and cattle are currently used as study models for humans since they present more similar characteristics when compared to rodents in both the initial embryo development and adult life. This review discusses the most relevant biotechnologies used in veterinary medicine, focusing on the generation of germ-cell-like cells in vitro through the acquisition of totipotent status and the production of embryos in vitro from pluripotent stem cells, thus highlighting the main uses of pluripotent stem cells in livestock species and reproductive medicine.

Determining the potency of primordial germ cells by injection into early mouse embryos

Primordial germ cells (PGCs) are the earliest precursors of the gametes. During normal development, PGCs only give rise to oocytes or spermatozoa. However, PGCs can acquire pluripotency in vitro by forming embryonic germ (EG) cells and in vivo during teratocarcinogenesis. Classic embryological experiments directly assessed the potency of PGCs by injection into the pre-implantation embryo. As no contribution to embryos or adult mice was observed, PGCs have been described as unipotent. Here, we demonstrate that PGCs injected into 8-cell embryos can initially survive, divide, and contribute to the developing inner cell mass. Apoptosis-deficient PGCs exhibit improved survival in isolated epiblasts and can form naive pluripotent embryonic stem cell lines. However, contribution to the post-implantation embryo is limited, with no functional incorporation observed. In contrast, PGC-like cells show an extensive contribution to mid-gestation chimeras. We thus propose that PGC formation in vivo establishes a latent form of pluripotency that restricts chimera contribution.

GWAS reveals determinants of mobilization rate and dynamics of an active endogenous retrovirus of cattle

Five to ten percent of mammalian genomes is occupied by multiple clades of endogenous retroviruses (ERVs), that may count thousands of members. New ERV clades arise by retroviral infection of the germline followed by expansion by reinfection and/or retrotransposition. ERV mobilization is a source of deleterious variation, driving the emergence of ERV silencing mechanisms, leaving "DNA fossils". Here we show that the ERVK[2-1-LTR] clade is still active in the bovine and a source of disease-causing alleles. We develop a method to measure the rate of ERVK[2-1-LTR] mobilization, finding an average of 1 per ~150 sperm cells, with >10-fold difference between animals. We perform a genome-wide association study and identify eight loci affecting ERVK[2-1-LTR] mobilization. We provide evidence that polymorphic ERVK[2-1-LTR] elements in four of these loci cause the association. We generate a catalogue of full length ERVK[2-1-LTR] elements, and show that it comprises 15% of C-type autonomous elements, and 85% of D-type non-autonomous elements lacking functional genes. We show that >25% of the variance of mobilization rate is determined by the number of C-type elements, yet that de novo insertions are dominated by D-type elements. We propose that D-type elements act as parasite-of-parasite gene drives that may contribute to the observed demise of ERV elements.

Source cell-type epigenetic memory persists in induced pluripotent cells but is lost in subsequently derived germline cells

Introduction: Retention of source cell-type epigenetic memory may mitigate the potential for induced pluripotent stem cells (iPSCs) to fully achieve transitions in cell fate in vitro. While this may not preclude the use of iPSC-derived somatic cell types for therapeutic applications, it becomes a major concern impacting the potential use of iPSC-derived germline cell types for reproductive applications. The transition from a source somatic cell type to iPSCs and then on to germ-cell like cells (GCLCs) recapitulates two major epigenetic reprogramming events that normally occur during development in vivo-embryonic reprogramming in the epiblast and germline reprogramming in primordial germ cells (PGCs). We examined the extent of epigenetic and transcriptomic memory persisting first during the transition from differentiated source cell types to iPSCs, and then during the transition from iPSCs to PGC-like cells (PGCLCs). Methods: We derived iPSCs from four differentiated mouse cell types including two somatic and two germ cell types and tested the extent to which each resulting iPSC line resembled a) a validated ES cell reference line, and b) their respective source cell types, on the basis of genome-wide gene expression and DNA methylation patterns. We then induced each iPSC line to form PGCLCs, and assessed epigenomic and transcriptomic memory in each compared to endogenous PGCs/M-prospermatogonia. Results: In each iPSC line, we found residual gene expression and epigenetic programming patterns characteristic of the corresponding source differentiated cell type from which each was derived. However, upon deriving PGCLCs, we found very little evidence of lingering epigenetic or transcriptomic memory of the original source cell type. Discussion: This result indicates that derivation of iPSCs and then GCLCs from differentiated source cell types in vitro recapitulates the two-phase epigenetic reprogramming that normally occurs in vivo, and that, to a significant extent, germline cell types derived in vitro from pluripotent cells accurately recapitulate epigenetic programming and gene expression patterns corresponding to equivalent endogenous germ cell types, suggesting that they have the potential to form the basis of in vitro gametogenesis as a useful therapeutic strategy for treatment of infertility.

Single-cell multi-omics analysis of human testicular germ cell tumor reveals its molecular features and microenvironment

Seminoma is the most common malignant solid tumor in 14 to 44 year-old men. However, its molecular features and tumor microenvironment (TME) is largely unexplored. Here, we perform a series of studies via genomics profiling (single cell multi-omics and spatial transcriptomics) and functional examination using seminoma samples and a seminoma cell line. We identify key gene expression programs share between seminoma and primordial germ cells, and further characterize the functions of TFAP2C in promoting tumor invasion and migration. We also identify 15 immune cell subtypes in TME, and find that subtypes with exhaustion features were located closer to the tumor region through combined spatial transcriptome analysis. Furthermore, we identify key pathways and genes that may facilitate seminoma disseminating beyond the seminiferous tubules. These findings advance our knowledge of seminoma tumorigenesis and produce a multi-omics atlas of in situ human seminoma microenvironment, which could help discover potential therapy targets for seminoma.

Genetic clues to reprogramming power and formation of mouse oocyte

Oocyte features the unique capacity to reprogram not only sperm but also somatic nuclei to totipotency, yet the scarcity of oocytes has hindered the exploration and application of their reprogramming ability. In the meanwhile, the formation of oocytes, which involves extensive intracellular alterations and interactions, has also attracted tremendous interest. This review discusses developmental principles and regulatory mechanisms associated with ooplasm reprogramming and oocyte formation from a genetic perspective, with knowledge derived from mouse models. We also discuss future directions, especially to address the lack of insight into the regulatory networks that shape the identity of female germ cells or drive transitions in their developmental programs.

The developmental dynamics of the human male germline

Male germ cells undergo a complex sequence of developmental events throughout fetal and postnatal life that culminate in the formation of haploid gametes: the spermatozoa. Errors in these processes result in infertility and congenital abnormalities in offspring. Male germ cell development starts when pluripotent cells undergo specification to sexually uncommitted primordial germ cells, which act as precursors of both oocytes and spermatozoa. Male-specific development subsequently occurs in the fetal testes, resulting in the formation of spermatogonial stem cells: the foundational stem cells responsible for lifelong generation of spermatozoa. Although deciphering such developmental processes is challenging in humans, recent studies using various models and single-cell sequencing approaches have shed new insight into human male germ cell development. Here, we provide an overview of cellular, signaling and epigenetic cascades of events accompanying male gametogenesis, highlighting conserved features and the differences between humans and other model organisms.

FAAP100 is required for the resolution of transcription-replication conflicts in primordial germ cells

BACKGROUND

The maintenance of genome stability in primordial germ cells (PGCs) is crucial for the faithful transmission of genetic information and the establishment of reproductive reserve. Numerous studies in recent decades have linked the Fanconi anemia (FA) pathway with fertility, particularly PGC development. However, the role of FAAP100, an essential component of the FA core complex, in germ cell development is unexplored.

RESULTS

We find that FAAP100 plays an essential role in R-loop resolution and replication fork protection to counteract transcription-replication conflicts (TRCs) during mouse PGC proliferation. FAAP100 deletion leads to FA pathway inactivation, increases TRCs as well as cotranscriptional R-loops, and contributes to the collapse of replication forks and the generation of DNA damage. Then, the activated p53 signaling pathway triggers PGC proliferation defects, ultimately resulting in insufficient establishment of reproductive reserve in both sexes of mice.

CONCLUSIONS

Our findings suggest that FAAP100 is required for the resolution of TRCs in PGCs to safeguard their genome stability.

Porcine Germ Cells Phenotype during Embryonic and Adult Development

Primordial germ cells (PGCs) are the precursors of gametes. Due to their importance for the formation and reproduction of an organism, understanding the mechanisms and pathways of PGCs and the differences between males and females is essential. However, there is little research in domestic animals, e.g., swine, regarding the epigenetic and pluripotency profiles of PGCs during development. This study analyzed the expression of epigenetic and various pluripotent and germline markers associated with the development and differentiation of PGCs in porcine (pPGCs), aiming to understand the different gene expression profiles between the genders. The analysis of gonads at different gestational periods (from 24 to 35 days post fertilization (dpf) and in adults) was evaluated by immunofluorescence and RT-qPCR and showed phenotypic differences between the gonads of male and female embryos. In addition, the pPGCs were positive for OCT4 and VASA; some cells were H3k27me3 positive in male embryos and adult testes. In adults, the cells of the testes were positive for germline markers, as confirmed by gene expression analysis. The results may contribute to understanding the pPGC pathways during reproductive development, while also contributing to the knowledge needed to generate mature gametes in vitro.

Developmental origins of mammalian spermatogonial stem cells: New perspectives on epigenetic regulation and sex chromosome function

Male and female germ cells undergo genome-wide reprogramming during their development, and execute sex-specific programs to complete meiosis and successfully generate healthy gametes. While sexually dimorphic germ cell development is fundamental, similarities and differences exist in the basic processes governing normal gametogenesis. At the simplest level, male gamete generation in mammals is centred on the activity of spermatogonial stem cells (SSCs), and an equivalent cell state is not present in females. Maintaining this unique SSC epigenetic state, while keeping to germ cell-intrinsic developmental programs, poses challenges for the correct completion of spermatogenesis. In this review, we highlight the origins of spermatogonia, comparing and contrasting them with female germline development to emphasize specific developmental processes that are required for their function as germline stem cells. We identify gaps in our current knowledge about human SSCs and further discuss the impact of the unique regulation of the sex chromosomes during spermatogenesis, and the roles of X-linked genes in SSCs.

Review: A barnyard in the lab: prospect of generating animal germ cells for breeding and conservation

In vitro gametogenesis (IVG) offers broad opportunities for gaining detailed new mechanistic knowledge of germ cell biology that will enable progress in the understanding of human infertility, as well as for applications in the conservation of endangered species and for accelerating genetic selection of livestock. The realisation of this potential depends on overcoming key technical challenges and of gaining more detailed knowledge of the ontogeny and developmental programme in different species. Important differences in the molecular mechanisms of germ cell determination and epigenetic reprogramming between mice and other animals have been elucidated in recent years. These must be carefully considered when developing IVG protocols, as cellular kinetics in mice may not accurately reflect mechanisms in other mammals. Similarly, diverse stem cell models with potential for germ cell differentiation may reflect alternative routes to successful IVG. In conclusion, the fidelity of the developmental programme recapitulated during IVG must be assessed against reference information from each species to ensure the production of healthy animals using these methods, as well as for developing genuine models of gametogenesis.

UBE2T resolves transcription-replication conflicts and protects common fragile sites in primordial germ cells

The proper development of primordial germ cells (PGCs) is an essential prerequisite for gametogenesis and mammalian fertility. The Fanconi anemia (FA) pathway functions in maintaining the development of PGCs. FANCT/UBE2T serves as an E2 ubiquitin-conjugating enzyme that ubiquitylates the FANCD2-FANCI complex to activate the FA pathway, but its role in the development of PGCs is not clear. In this study, we found that Ube2t knockout mice showed defects in PGC proliferation, leading to severe loss of germ cells after birth. Deletion of UBE2T exacerbated DNA damage and triggered the activation of the p53 pathway. We further demonstrated that UBE2T counteracted transcription-replication conflicts by resolving R-loops and stabilizing replication forks, and also protected common fragile sites by resolving R-loops in large genes and promoting mitotic DNA synthesis to maintain the genome stability of PGCs. Overall, these results provide new insights into the function and regulatory mechanisms of the FA pathway ensuring normal development of PGCs.

Short telomeres impede germ cell specification by upregulating MAPK and TGFβ signaling

Functional telomeres protect chromosome ends and play important roles in stem cell maintenance and differentiation. Short telomeres negatively impact germ cell development and can contribute to age-associated infertility. Moreover, telomere syndrome resulting from mutations of telomerase or telomere-associated genes exhibits short telomeres and reduced fertility. It remains elusive whether and how telomere lengths affect germ cell specification. We report that functional telomere is required for the coordinated germ cell and somatic cell fate decisions. Using telomerase gene Terc deficient mice as a model, we show that short telomeres restrain germ cell specification of epiblast cells but promote differentiation towards somatic lineage. Short telomeres increase chromatin accessibility to elevate TGFβ and MAPK/ERK signaling for somatic cell differentiation. Notably, elevated Fst expression in TGFβ pathway represses the BMP4-pSmad signaling pathway, thus reducing germ cell formation. Re-elongation of telomeres by targeted knock-in of Terc restores normal chromatin accessibility to suppress TGFβ and MAPK signaling, thereby facilitating germ cell formation. Taken together, our data reveal that functional telomeres are required for germ cell specification by repressing TGFβ and MAPK signaling.

Efficient generation of marmoset primordial germ cell-like cells using induced pluripotent stem cells

Reconstitution of germ cell fate from pluripotent stem cells provides an opportunity to understand the molecular underpinnings of germ cell development. Here, we established robust methods for induced pluripotent stem cell (iPSC) culture in the common marmoset (Callithrix jacchus [cj]), allowing stable propagation in an undifferentiated state. Notably, iPSCs cultured on a feeder layer in the presence of a WNT signaling inhibitor upregulated genes related to ubiquitin-dependent protein catabolic processes and enter a permissive state that enables differentiation into primordial germ cell-like cells (PGCLCs) bearing immunophenotypic and transcriptomic similarities to pre-migratory cjPGCs in vivo. Induction of cjPGCLCs is accompanied by transient upregulation of mesodermal genes, culminating in the establishment of a primate-specific germline transcriptional network. Moreover, cjPGCLCs can be expanded in monolayer while retaining the germline state. Upon co-culture with mouse testicular somatic cells, these cells acquire an early prospermatogonia-like phenotype. Our findings provide a framework for understanding and reconstituting marmoset germ cell development in vitro, thus providing a comparative tool and foundation for a preclinical modeling of human in vitro gametogenesis.

Ovarian aging: mechanisms and intervention strategies

Ovarian reserve is essential for fertility and influences healthy aging in women. Advanced maternal age correlates with the progressive loss of both the quantity and quality of oocytes. The molecular mechanisms and various contributing factors underlying ovarian aging have been uncovered. In this review, we highlight some of critical factors that impact oocyte quantity and quality during aging. Germ cell and follicle reserve at birth determines reproductive lifespan and timing the menopause in female mammals. Accelerated diminishing ovarian reserve leads to premature ovarian aging or insufficiency. Poor oocyte quality with increasing age could result from chromosomal cohesion deterioration and misaligned chromosomes, telomere shortening, DNA damage and associated genetic mutations, oxidative stress, mitochondrial dysfunction and epigenetic alteration. We also discuss the intervention strategies to delay ovarian aging. Both the efficacy of senotherapies by antioxidants against reproductive aging and mitochondrial therapy are discussed. Functional oocytes and ovarioids could be rejuvenated from pluripotent stem cells or somatic cells. We propose directions for future interventions. As couples increasingly begin delaying parenthood in life worldwide, understanding the molecular mechanisms during female reproductive aging and potential intervention strategies could benefit women in making earlier choices about their reproductive health.

Decoding the Spermatogenesis Program: New Insights from Transcriptomic Analyses

Spermatogenesis is a complex differentiation process coordinated spatiotemporally across and along seminiferous tubules. Cellular heterogeneity has made it challenging to obtain stage-specific molecular profiles of germ and somatic cells using bulk transcriptomic analyses. This has limited our ability to understand regulation of spermatogenesis and to integrate knowledge from model organisms to humans. The recent advancement of single-cell RNA-sequencing (scRNA-seq) technologies provides insights into the cell type diversity and molecular signatures in the testis. Fine-grained cell atlases of the testis contain both known and novel cell types and define the functional states along the germ cell developmental trajectory in many species. These atlases provide a reference system for integrated interspecies comparisons to discover mechanistic parallels and to enable future studies. Despite recent advances, we currently lack high-resolution data to probe germ cell-somatic cell interactions in the tissue environment, but the use of highly multiplexed spatial analysis technologies has begun to resolve this problem. Taken together, recent single-cell studies provide an improvedunderstanding of gametogenesis to examine underlying causes of infertility and enable the development of new therapeutic interventions.

Stem Cell-Based Therapeutic Strategies for Premature Ovarian Insufficiency and Infertility: A Focus on Aging

Reproductive aging is on the rise globally and inseparable from the entire aging process. An extreme form of reproductive aging is premature ovarian insufficiency (POI), which to date has mostly been of idiopathic etiology, thus hampering further clinical applications and associated with enormous socioeconomic and personal costs. In the field of reproduction, the important functional role of inflammation-induced ovarian deterioration and therapeutic strategies to prevent ovarian aging and increase its function are current research hotspots. This review discusses the general pathophysiology and relative causes of POI and comprehensively describes the association between the aging features of POI and infertility. Next, various preclinical studies of stem cell therapies with potential for POI treatment and their molecular mechanisms are described, with particular emphasis on the use of human induced pluripotent stem cell (hiPSC) technology in the current scenario. Finally, the progress made in the development of hiPSC technology as a POI research tool for engineering more mature and functional organoids suitable as an alternative therapy to restore infertility provides new insights into therapeutic vulnerability, and perspectives on this exciting research on stem cells and the derived exosomes towards more effective POI diagnosis and treatment are also discussed.

Tissue hydraulics in reproduction

The development of functional eggs and sperm are critical processes in mammalian development as they ensure successful reproduction and species propagation. While past studies have identified important genes that regulate these processes, the roles of luminal flow and fluid stress in reproductive biology remain less well understood. Here, we discuss recent evidence that support the diverse functions of luminal fluid in oogenesis, spermatogenesis and embryogenesis. We also review emerging techniques that allow for precise quantification and perturbation of tissue hydraulics in female and male reproductive systems, and propose new questions and approaches in this field. We hope this review will provide a useful resource to inspire future research in tissue hydraulics in reproductive biology and diseases.

Female Germ Cell Development in Chickens and Humans: The Chicken Oocyte Enriched Genes Convergent and Divergent with the Human Oocyte

The development of germ cells and other physiological events in the differentiated ovary of humans are highly conserved with several mammalian species, except for the differences in timing. However, comparative knowledge on this topic is very scarce with respect to humans and lower vertebrates, such as chickens. In chickens, female germ cells enter into meiosis around embryonic day (E) 15.5 and are arrested in meiotic prophase I as primary oocytes. The oocytes arrested in meiosis I are accumulated in germ-cell cysts; shortly after hatching, they are enclosed by flattened granulosa cells in order to form primordial follicles. In humans, the process of meiotic recombination in female germ cells begins in the 10–11th week of gestation, and primordial follicles are formed at around week 20. In this review, we comprehensively elucidate both the conservation and the species-specific differences between chickens and humans with respect to germ cell, oocyte, and follicle development. Importantly, we provide functional insights into a set of chicken oocyte enriched genes (from E16 to 1 week post-hatch) that show convergent and divergent expression patterns with respect to the human oocyte (from week 11 to 26).

Stem Cells and Organs-on-chips: New Promising Technologies for Human Infertility Treatment

Having biological children remains an unattainable dream for most couples with reproductive failure or gonadal dysgenesis. The combination of stem cells with gene editing technology and organ-on-a-chip models provides a unique opportunity for infertile patients with impaired gametogenesis caused by congenital disorders in sex development or cancer survivors. But how will these technologies overcome human infertility? This review discusses the regenerative mechanisms, applications, and advantages of different types of stem cells for restoring gametogenesis in infertile patients, as well as major challenges that must be overcome before clinical application. The importance and limitations of in vitro generation of gametes from patient-specific human-induced pluripotent stem cells (hiPSCs) will be discussed in the context of human reproduction. The potential role of organ-on-a-chip models that can direct differentiation of hiPSC-derived primordial germ cell-like cells to gametes and other reproductive organoids is also explored. These rapidly evolving technologies provide prospects for improving fertility to individuals and couples who experience reproductive failure.

Oocyte Casein kinase 1α deletion causes defects in primordial follicle formation and oocyte loss by impairing oocyte meiosis and enhancing autophagy in developing mouse ovary

Casein kinase 1α is a member of CK1 family, which is ubiquitously expressed and plays multiple functions, including its potential roles in regulating cell division. But the functions of CK1α in mammalian oogenesis and folliculogenesis remain elusive. In this study, we assayed the cell type of CK1α expression in the developing mouse ovary and confirmed that CK1α was highly expressed in ovaries after birth. The oocyte-specific CK1α knockout (cKO) mouse model was then established by crossing Ddx4-Cre mice with Csnk1a1-floxp mice, and the effects of CK1α deletion on oogenesis and folliculogenesis were identified. The results showed that oocyte CK1α deletion impaired the progression of oocyte meiosis and primordial follicle formation during meiotic prophase I, which subsequently caused oocyte loss and mouse infertility. Further, the in vivo CK1α deletion and in vitro inhibition of CK1 activity resulted in the defects of DNA double-strand break (DSB) repair, whereas apoptosis and autophagy were enhanced in the developing ovary. These may contribute to oocyte loss and infertility in cKO mice. It is thus concluded that CK1α is essential for mouse oogenesis and folliculogenesis by involving in regulating the processes of oocyte meiosis and DNA DSB repair during meiotic prophase I of mouse oocytes. However, the related signaling pathway and molecular mechanisms need to be elucidated further.

Exclusive generation of rat spermatozoa in sterile mice utilizing blastocyst complementation with pluripotent stem cells

Blastocyst complementation denotes a technique that aims to generate organs, tissues, or cell types in animal chimeras via injection of pluripotent stem cells (PSCs) into genetically compromised blastocyst-stage embryos. Here, we report on successful complementation of the male germline in adult chimeras following injection of mouse or rat PSCs into mouse blastocysts carrying a mutation in Tsc22d3, an essential gene for spermatozoa production. Injection of mouse PSCs into Tsc22d3-Knockout (KO) blastocysts gave rise to intraspecies chimeras exclusively embodying PSC-derived functional spermatozoa. In addition, injection of rat embryonic stem cells (rESCs) into Tsc22d3-KO embryos produced interspecies mouse-rat chimeras solely harboring rat spermatids and spermatozoa capable of fertilizing oocytes. Furthermore, using single-cell RNA sequencing, we deconstructed rat spermatogenesis occurring in a mouse-rat chimera testis. Collectively, this study details a method for exclusive xenogeneic germ cell production in vivo, with implications that may extend to rat transgenesis, or endangered animal species conservation efforts.

Single-cell RNA sequencing of mitotic-arrested prospermatogonia with DAZL::GFP chickens and revealing unique epigenetic reprogramming of chickens

Background

Germ cell mitotic arrest is conserved in many vertebrates, including birds, although the time of entry or exit into quiescence phase differs. Mitotic arrest is essential for the normal differentiation of male germ cells into spermatogonia and accompanies epigenetic reprogramming and meiosis inhibition from embryonic development to post-hatch. However, mitotic arrest was not well studied in chickens because of the difficulty in obtaining pure germ cells from relevant developmental stage.

Results

We performed single-cell RNA sequencing to investigate transcriptional dynamics of male germ cells during mitotic arrest in DAZL::GFP chickens. Using differentially expressed gene analysis and K-means clustering to analyze cells at different developmental stages (E12, E16, and hatch), we found that metabolic and signaling pathways were regulated, and that the epigenome was reprogrammed during mitotic arrest. In particular, we found that histone H3K9 and H3K14 acetylation (by HDAC2) and DNA demethylation (by DNMT3B and HELLS) led to a transcriptionally permissive chromatin state. Furthermore, we found that global DNA demethylation occurred gradually after the onset of mitotic arrest, indicating that the epigenetic-reprogramming schedule of the chicken genome differs from that of the mammalian genome. DNA hypomethylation persisted after hatching, and methylation was slowly re-established 3 weeks later.

Conclusions

We found a unique epigenetic-reprogramming schedule of mitotic-arrested chicken prospermatogonia and prolonged hypomethylation after hatching. This will provide a foundation for understanding the process of germ-cell epigenetic regulation in several species for which this process is not clearly described. Our findings on the biological processes related to sex-specific differentiation of prospermatogonia could help studying germline development in vitro more elaborately.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40104-022-00712-4.

De novo programming: establishment of epigenome in mammalian oocytes

Innovations in ultrasensitive and single-cell measurements enable us to study layers of genome regulation in the view of cellular and regulatory heterogeneity. Genome-scale mapping allows to evaluate epigenetic features and dynamics in different genomic contexts, including genebodies, CGIs, ICRs, promoters, PMDs, and repetitive elements. The epigenome of early embryos, fetal germ cells, and sperm has been extensively studied for the past decade, while oocytes remain less clear. Emerging evidence now supports the notion that transcription and chromatin accessibility precede de novo DNA methylation in both human and mouse oocytes. Recent studies also start to chart correlations among different histone modifications and DNA methylation. We discussed the potential mechanistic hierarchy by which shapes oocyte DNA methylome, also provided insights into the convergent and divergent features between human and mice.

Mysteries and unsolved problems of mammalian fertilization and related topics

Mammalian fertilization is a fascinating process that leads to the formation of a new individual. Eggs and sperm are complex cells that must meet at the appropriate time and position within the female reproductive tract for successful fertilization. I have been studying various aspects of mammalian fertilization over 60 years. In this review, I discuss many different aspects of mammalian fertilization, some of my laboratory's contribution to the field, and discuss enigmas and mysteries that remain to be solved.

Proliferation of human spermatogonial stem cells on optimized PCL/Gelatin nanofibrous scaffolds

Improvement of culture system and increasing the proliferation of spermatogonia stem cells under in vitro condition are the essential treatment options for infertility before autologous transplantation. Therefore, the present study aimed to evaluate the proliferation of human spermatogonia stem cells on the electrospun polycaprolactone/gelatin nanocomposite. Therefore, for this purpose, nanofiber porous scaffolds were prepared using the electrospinning method and their structures were then confirmed by SEM. After performing swelling, biodegradability and cell adhesion tests, human spermatogonia stem cells were cultured on scaffolds. In addition, both cell viability and proliferation were assessed using immunocytochemistry, flow cytometry and real-time PCR techniques in culturing during a 3-week period. SEM images indicated the presence of fibres with suitable diameters and arrangement as well as a sufficient porosity in nanocomposite scaffolds, showing good biocompatibility and biodegradability. The results show a significant increase in the number of spermatogonia stem cells in the cultured group on scaffold compared with the control group (p ≤ 0.05). As well, the results show that the expressions of integrin ɑ6 and β1 and Plzf genes estimated using real-time PCR in nanofiber scaffolds were significantly higher than those of the control group (p ≤ 0.05). However, the expression of c-Kit gene in the 3D group showed a significant decrease compared with the 2D group. Flow cytometry analysis also showed that the number of Plzf-positive cells was significantly higher in nanofiber porous scaffolds compared with the control group (p ≤ 0.05). Additionally, immunocytochemistry findings confirmed the presence of human spermatogonia stem cell colonies. In general, it seems that the designed nanocomposite scaffold could provide a suitable capacity for self-renewal of human spermatogonia stem cells, which can have a good application potential in research and reconstructive medicine related to the field of male infertility.

A Single-Cell Omics Network Model of Cell Crosstalk during the Formation of Primordial Follicles

The fate of fetal germ cells (FGCs) in primordial follicles is largely determined by how they interact with the surrounding granulosa cells. However, the molecular mechanisms underlying this interactive process remain poorly understood. Here, we develop a computational model to characterize how individual genes program and rewire cellular crosstalk across FGCs and somas, how gene regulatory networks mediate signaling pathways that functionally link these two cell types, and how different FGCs diversify and evolve through cooperation and competition during embryo development. We analyze single-cell RNA-seq data of human female embryos using the new model, identifying previously uncharacterized mechanisms behind follicle development. The majority of genes (70%) promote FGC–soma synergism, only with a small portion (4%) that incur antagonism; hub genes function reciprocally between the FGC network and soma network; and germ cells tend to cooperate between different stages of development but compete in the same stage within a developmental embryo. Our network model could serve as a powerful tool to unravel the genomic signatures that mediate folliculogenesis from single-cell omics data.

Fish Pluripotent Stem-Like Cell Line Induced by Small-Molecule Compounds From Caudal Fin and its Developmental Potentiality

The technique of induced pluripotent stem cells has significant application value in breeding and preserving the genetic integrity of fish species. However, it is still unclear whether the chemically induced pluripotent stem cells can be induced from non-mammalian cells or not. In this article, we first verify that fibroblasts of fish can be chemically reprogrammed into pluripotent stem cells. These induced pluripotent stem-like cells possess features of colony morphology, expression of pluripotent marker genes, formation of embryoid bodies, teratoma formation, and the potential to differentiate into germ cell-like cells in vitro. Our findings will offer a new way to generate induced pluripotent stem cells in teleost fish and a unique opportunity to breed commercial fish and even save endangered fish species.

Generation of Miniaturized Ovaries by In Vitro Culture from Mouse Gonads

The incidence of infertility among individuals of reproductive age has been growing due to genetic and environmental factors, and considerable research efforts are focused on solving this issue. Ovarian development is an overly complex process in the body, involving the interaction between primordial germ cells and gonad somatic cells. However, follicles located in the center of the in vitro ovary are poorly formed owing to ovarian complexity, nutrient deficiency, and signaling deficiency. In the present study, we optimized methods for dissociating gonads and culture conditions for the in vitro generation of miniaturized ovaries. The gonads from embryos were dissociated into cell masses and cultured on a Transwell-COL membrane for 3–5 weeks. Approximately 12 follicles were present per in vitro ovary. We observed that miniaturized ovaries successfully matured to MII oocytes in vitro from 150 to 100 µm gonad masses. This method will be useful for investigating follicle development and oocyte production.

Induction of meiosis by embryonic gonadal somatic cells differentiated from pluripotent stem cells

Background

Depletion of oocytes leads to ovarian aging-associated infertility, endocrine disruption and related diseases. Excitingly, unlimited oocytes can be generated by differentiation of primordial germ cell like cells (PGCLCs) from pluripotent stem cells. Nevertheless, development of oocytes and follicles from PGCLCs relies on developmentally matched gonadal somatic cells, only available from E12.5 embryos in mice. It is therefore imperative to achieve an in vitro source of E12.5 gonadal somatic cells.

Methods

We explored to identify small molecules, which can induce female embryonic stem cells (ESCs) into gonadal somatic cell like cells.

Results

Using RNA-sequencing, we identified signaling pathways highly upregulated in E12.5_gonadal somatic cells (E12.5_GSCs). Through searching for the activators of these pathways, we identified small-molecule compounds Vitamin C (Vc) and AM580 in combination (V580) for inducing differentiation of female embryonic stem cells (ESCs) into E12.5_GSC-like cells (E12.5_GSCLCs). After V580 treatment for 6 days and sorted by a surface marker CD63, the cell population yielded a transcriptome profile similar to that of E12.5_GSCs, which promoted meiosis progression and folliculogenesis of primordial germ cells. This approach will contribute to the study of germ cell and follicle development and oocyte production and have implications in potentially treating female infertility.

Conclusion

ESCs can be induced into embryonic gonadal somatic cell like cells by small molecules.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02672-4.

The Dynamic Regulation of mRNA Translation and Ribosome Biogenesis During Germ Cell Development and Reproductive Aging

The regulation of mRNA translation, both globally and at the level of individual transcripts, plays a central role in the development and function of germ cells across species. Genetic studies using flies, worms, zebrafish and mice have highlighted the importance of specific RNA binding proteins in driving various aspects of germ cell formation and function. Many of these mRNA binding proteins, including Pumilio, Nanos, Vasa and Dazl have been conserved through evolution, specifically mark germ cells, and carry out similar functions across species. These proteins typically influence mRNA translation by binding to specific elements within the 3′ untranslated region (UTR) of target messages. Emerging evidence indicates that the global regulation of mRNA translation also plays an important role in germ cell development. For example, ribosome biogenesis is often regulated in a stage specific manner during gametogenesis. Moreover, oocytes need to produce and store a sufficient number of ribosomes to support the development of the early embryo until the initiation of zygotic transcription. Accumulating evidence indicates that disruption of mRNA translation regulatory mechanisms likely contributes to infertility and reproductive aging in humans. These findings highlight the importance of gaining further insights into the mechanisms that control mRNA translation within germ cells. Future work in this area will likely have important impacts beyond germ cell biology.

The Role of BMP Signaling in Female Reproductive System Development and Function

Bone morphogenetic proteins (BMPs) are a group of multifunctional growth factors that belong to the transforming growth factor-β (TGF-β) superfamily of proteins. Originally identified by their ability to induce bone formation, they are now known as essential signaling molecules that regulate the development and function of the female reproductive system (FRS). Several BMPs play key roles in aspects of reproductive system development. BMPs have also been described to be involved in the differentiation of human pluripotent stem cells (hPSCs) into reproductive system tissues or organoids. The role of BMPs in the reproductive system is still poorly understood and the use of FRS tissue or organoids generated from hPSCs would provide a powerful tool for the study of FRS development and the generation of new therapeutic perspectives for the treatment of FRS diseases. Therefore, the aim of this review is to summarize the current knowledge about BMP signaling in FRS development and function.

Mammalian in vitro gametogenesis

[Figure: see text].

In vitro reconstitution of the whole male germ-cell development from mouse pluripotent stem cells

Mammalian male germ-cell development consists of three distinct phases: primordial germ cell (PGC) development, male germ-cell specification for spermatogonium development, and ensuing spermatogenesis. Here, we show an in vitro reconstitution of whole male germ-cell development by pluripotent stem cells (PSCs). Mouse embryonic stem cells (mESCs) are induced into PGC-like cells (mPGCLCs), which are expanded for epigenetic reprogramming. In reconstituted testes under an optimized condition, such mPGCLCs differentiate into spermatogonium-like cells with proper developmental transitions, gene expression, and cell-cycle dynamics and are expanded robustly as germline stem cell-like cells (GSCLCs) with an appropriate androgenetic epigenome. Importantly, GSCLCs show vigorous spermatogenesis, not only upon transplantation into testes in vivo but also under an in vitro culture of testis transplants, and the resultant spermatids contribute to fertile offspring. By uniting faithful recapitulations of the three phases of male germ-cell development, our study creates a paradigm for the in vitro male gametogenesis by PSCs.

Improving In Vitro Culture of Human Male Fetal Germ Cells

Male human fetal germ cells (hFGCs) give rise to spermatogonial stem cells (SSCs), which are the adult precursors of the male gametes. Human SSCs are a promising (autologous) source of cells for male fertility preservation; however, in contrast to mouse SSCs, we are still unable to culture them in the long term. Here, we investigated the effect of two different culture media and four substrates (laminin, gelatin, vitronectin and matrigel) in the culture of dissociated second trimester testes, enriched for hFGCs. After 6 days in culture, we quantified the presence of POU5F1 and DDX4 expressing hFGCs. We observed a pronounced difference in hFGC number in different substrates. The combination of gelatin-coated substrate and medium containing GDNF, LIF, FGF2 and EGF resulted in the highest percentage of hFGCs (10% of the total gonadal cells) after 6 days of culture. However, the vitronectin-coated substrate resulted in a comparable percentage of hFGCs regardless of the media used (3.3% of total cells in Zhou-medium and 4.8% of total cells in Shinohara-medium). We provide evidence that not only the choices of culture medium but also choices of the adequate substrate are crucial for optimizing culture protocols for male hFGCs. Optimizing culture conditions in order to improve the expansion of hFGCs will benefit the development of gametogenesis assays in vitro.

DNA Methylation Dynamics in the Female Germline and Maternal-Effect Mutations That Disrupt Genomic Imprinting

Genomic imprinting is an epigenetic marking process that results in the monoallelic expression of a subset of genes. Many of these ‘imprinted’ genes in mice and humans are involved in embryonic and extraembryonic growth and development, and some have life-long impacts on metabolism. During mammalian development, the genome undergoes waves of (re)programming of DNA methylation and other epigenetic marks. Disturbances in these events can cause imprinting disorders and compromise development. Multi-locus imprinting disturbance (MLID) is a condition by which imprinting defects touch more than one locus. Although most cases with MLID present with clinical features characteristic of one imprinting disorder. Imprinting defects also occur in ‘molar’ pregnancies-which are characterized by highly compromised embryonic development-and in other forms of reproductive compromise presenting clinically as infertility or early pregnancy loss. Pathogenic variants in some of the genes encoding proteins of the subcortical maternal complex (SCMC), a multi-protein complex in the mammalian oocyte, are responsible for a rare subgroup of moles, biparental complete hydatidiform mole (BiCHM), and other adverse reproductive outcomes which have been associated with altered imprinting status of the oocyte, embryo and/or placenta. The finding that defects in a cytoplasmic protein complex could have severe impacts on genomic methylation at critical times in gamete or early embryo development has wider implications beyond these relatively rare disorders. It signifies a potential for adverse maternal physiology, nutrition, or assisted reproduction to cause epigenetic defects at imprinted or other genes. Here, we review key milestones in DNA methylation patterning in the female germline and the embryo focusing on humans. We provide an overview of recent findings regarding DNA methylation deficits causing BiCHM, MLID, and early embryonic arrest. We also summarize identified SCMC mutations with regard to early embryonic arrest, BiCHM, and MLID.

Mir-106b Cluster Regulates Primordial Germ Cells Differentiation from Human Mesenchymal Stem Cells

Objective

Numerous evidence indicates that microRNAs (miRNAs) are critical regulators in the spermatogenesis process. The aim of this study was to investigate Mir-106b cluster regulates primordial germ cells (PGCs) differentiation from human mesenchymal stem cells (MSCs).

Materials and Methods

In this experimental study, samples containing male adipose (n: 9 samples- age: 25-40 years) were obtained from cosmetic surgeries performed for the liposuction in Imam Khomeini Hospital. The differentiation of MSCs into PGCs was accomplished by transfection of a lentivector expressing miR-106b. The transfection of miR- 106b was also confirmed by the detection of a clear green fluorescent protein (GFP) signal in MSCs. MSCs were treated with bone morphogenic factor 4 (BMP4) protein, as a putative inducer of PGCs differentiation, to induce the differentiation of MSCs into PGCs (positive control). After 4 days of transfection, the expression of miR-106b, STELLA, and FRAGILIS genes was evaluated by real-time polymerase chain reaction (PCR). Also, the levels of thymocyte differentiation antigen 1 (Thy1) protein was assessed by the western blot analysis. The cell surface expression of CD90 was also determined by immunocytochemistry method. The cytotoxicity of miR-106b was examined in MSCs after 24, 48, and 72 hours using the MTT assay.

Results

MSCs treated with BMP4 or transfected by miR-106b were successfully differentiated into PGCs. The results of this study also showed that the expression of miR-106b was significantly increased after 48 hours from transfection. Also, we showed STELLA, FARGILIS, as well as the protein expression of Thy1, was significantly higher in MSCs transfected by lentivector expressing miR-106b in comparison with MSCs treated with BMP4 (P≤0.05). MTT assay showed miR-106b was no toxic during 72 hours in 1 µg/ml dose, that this amount could elevated germ cells marker significantly higher than other experimental groups (P≤0.05).

Conclusion

According to this findings, it appears that miR-106b plays an essential role in the differentiation of MSCs into PGCs.

Speculative feminism and the shifting frontiers of bioscience: envisioning reproductive futures with synthetic gametes through the ethnographic method

Scientists are developing a technique called in vitro gametogenesis or IVG to generate synthetic gametes for research and, potentially, for treating infertility. What would it mean for feminist concerns over the future of reproductive practice and biotechnological development if egg and sperm cells could be produced in laboratory conditions? In this article, I take on the question by discussing the emerging technique of IVG through the speculative feminist analysis of ambiguous reproductive futures. Feminist cultural and science studies scholars have explored the transformative effects of biomedicine on reproduction through science fiction novels and other cultural products. I theorise the speculative and visionary in biomedicine in the context of ethnographic methodology by drawing on ‘thought experiments’ conducted with stem cell scientists as shared acts of future-oriented contemplation. I develop the figure of SF proposed by Donna Haraway to investigate how science facts and speculative fabulation together shape futurities of reproduction. I propose including shifting frontiers in feminist thinking of the SFs in bioscience. Biomedical research aims to shift the borders between what is known and not known in reproductive biology, subsequently raising new technical, ethical and political issues in terms of stratified reproduction. The article shows that synthetic gametes are anticipated to intensify selective procreation. Simultaneously, IVG is seen to forge new biogenetic relationships and possibilities for non-normative reproduction and kin-making. Following Haraway, I argue that by ‘staying with the trouble’ of these biotechnological visions, feminist speculative analytics on technoscience offers a valuable tool to envision more hopeful and equal futures together with scientists.

Overexpression of DAZL, STRA8, and BOULE Genes and Treatment With BMP4 or Retinoic Acid Modulate the Expression of MSC Overexpressing Germ Cell Genes

In vitro gamete derivation from stem cells has potential applications in animal reproduction as an alternative method for the dissemination of elite animal genetics, production of transgenic animals, and conservation of endangered species. Mesenchymal stem cells (MSCs) may be suitable candidates for in vitro gamete derivation considering their differentiative capacity and their potential for cell therapy. Due to its relevance in gametogenesis, it has been reported that retinoic acid (RA) and bone morphogenetic protein (BMP) 4 are able to upregulate the expression of specific markers associated to the early stages of germ cell (GCs) differentiation in bovine fetal MSCs (bfMSCs). In the present study, we used polycistronic vectors containing combinations of GC genes DAZL, STRA8, and BOULE followed by exposure to BMP4 or RA to induce GC differentiation of bovine fetal adipose tissue-derived MSC (AT-MSCs). Cells samples at Day 14 were analyzed according to the expression of pluripotent genes NANOG and OCT4 and GC genes DAZL, STRA8, BOULE, PIWI, c-KIT, and FRAGILIS using Q-PCR. Fetal and adult testis and AT-MSCs samples were also analyzed for the expression of DAZL, STRA8, and NANOG using immunofluorescence. Increased gene expression levels in the adult testis and cell-specific distribution of DAZL, STRA8, and NANOG in the fetal testis suggest that these markers are important components of the regulatory network that control the in vivo differentiation of bovine GCs. Overexpression of DAZL and STRA8 in bi-cistronic and DAZL, STRA8, and BOULE in tri-cistronic vectors resulted in the upregulation of OCT4, NANOG, and PIWIL2 in bovine fetal AT-MSCs. While BMP4 repressed NANOG expression, this treatment increased DAZL and c-KIT and activated FRAGILIS expression in bovine fetal AT-MSCs. Treatment with RA for 14 days increased the expression of DAZL and FRAGILIS and maintained the mRNA levels of STRA8 in bovine fetal AT-MSCs transfected with bi-cistronic and tri-cistronic vectors. Moreover, RA treatment repressed the expression of OCT4 and NANOG in these cells. Thus, overexpression of DAZL, STRA8, and BOULE induced the upregulation of the pluripotent markers and PIWIL2 in transfected bovine fetal AT-MSCs. The partial activation of GC gene expression by BMP4 and RA suggests that both factors possess common targets but induce different gene expression effects during GC differentiation in overexpressing bovine fetal AT-MSCs.

Non-human primates as a model for human development

Human development has been studied for over a century, but the molecular mechanisms underlying human embryogenesis remain largely unknown due to technical difficulties and ethical issues. Accordingly, mice have been used as a model for mammalian development and studied extensively to infer human biology based on the conservation of fundamental processes between the two species. As research has progressed, however, species-specific differences in characteristics between rodents and primates have become apparent. Non-human primates (NHPs) have also been used for biomedical research, and are now attracting attention as a model for human development. Here, we summarize primate species from the evolutionary and genomic points of view. Then we review the current issues and progress in gene modification technology for NHPs. Finally, we discuss recent studies on the early embryogenesis of primates and future perspectives.

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

In 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.

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Chromatin accessibility landscape is revealed throughout hPGCLC induction

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SOX15 is involved in hPGCLC maintenance via dual effects

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ETV5, a downstream regulator of SOX15, is essential for hPGCLC maintenance

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A stepwise OCT4:SOX motifs switch is uncovered throughout hPGCLC induction

Testicular germ cell tumors arise in the absence of sex-specific differentiation

In response to signals from the embryonic testis, the germ cell intrinsic factor NANOS2 coordinates a transcriptional program necessary for the differentiation of pluripotent-like primordial germ cells toward a unipotent spermatogonial stem cell fate. Emerging evidence indicates that genetic risk factors contribute to testicular germ cell tumor initiation by disrupting sex-specific differentiation. Here, using the 129.MOLF-Chr19 mouse model of testicular teratomas and a NANOS2 reporter allele, we report that the developmental phenotypes required for tumorigenesis, including failure to enter mitotic arrest, retention of pluripotency and delayed sex-specific differentiation, were exclusive to a subpopulation of germ cells failing to express NANOS2. Single-cell RNA sequencing revealed that embryonic day 15.5 NANOS2-deficient germ cells and embryonal carcinoma cells developed a transcriptional profile enriched for MYC signaling, NODAL signaling and primed pluripotency. Moreover, lineage-tracing experiments demonstrated that embryonal carcinoma cells arose exclusively from germ cells failing to express NANOS2. Our results indicate that NANOS2 is the nexus through which several genetic risk factors influence tumor susceptibility. We propose that, in the absence of sex specification, signals native to the developing testis drive germ cell transformation.

TOR targets an RNA processing network to regulate facultative heterochromatin, developmental gene expression and cell proliferation

Cell proliferation and differentiation require signalling pathways that enforce appropriate and timely gene expression. We find that Tor2, the catalytic subunit of the TORC1 complex in fission yeast, targets a conserved nuclear RNA elimination network, particularly the serine and proline-rich protein Pir1, to control gene expression through RNA decay and facultative heterochromatin assembly. Phosphorylation by Tor2 protects Pir1 from degradation by the ubiquitin-proteasome system involving the polyubiquitin Ubi4 stress-response protein and the Cul4-Ddb1 E3 ligase. This pathway suppresses widespread and untimely gene expression and is critical for sustaining cell proliferation. Moreover, we find that the dynamic nature of Tor2-mediated control of RNA elimination machinery defines gene expression patterns that coordinate fundamental chromosomal events during gametogenesis, such as meiotic double-strand-break formation and chromosome segregation. These findings have important implications for understanding how the TOR signalling pathway reprogrammes gene expression patterns and contributes to diseases such as cancer.

GATA transcription factors, SOX17 and TFAP2C, drive the human germ-cell specification program

This work shows that GATA transcription factors transduce the BMP signaling and, with SOX17 and TFAP2C, induce the human germ-cell fate, delineating the mechanism for human germ-cell specification.

The in vitro reconstitution of human germ-cell development provides a robust framework for clarifying key underlying mechanisms. Here, we explored transcription factors (TFs) that engender the germ-cell fate in their pluripotent precursors. Unexpectedly, SOX17, TFAP2C, and BLIMP1, which act under the BMP signaling and are indispensable for human primordial germ-cell-like cell (hPGCLC) specification, failed to induce hPGCLCs. In contrast, GATA3 or GATA2, immediate BMP effectors, combined with SOX17 and TFAP2C, generated hPGCLCs. GATA3/GATA2 knockouts dose-dependently impaired BMP-induced hPGCLC specification, whereas GATA3/GATA2 expression remained unaffected in SOX17, TFAP2C, or BLIMP1 knockouts. In cynomolgus monkeys, a key model for human development, GATA3, SOX17, and TFAP2C were co-expressed exclusively in early PGCs. Crucially, the TF-induced hPGCLCs acquired a hallmark of bona fide hPGCs to undergo epigenetic reprogramming and mature into oogonia/gonocytes in xenogeneic reconstituted ovaries. By uncovering a TF circuitry driving the germ line program, our study provides a paradigm for TF-based human gametogenesis.

The role of m6A modification in the biological functions and diseases

N⁶-methyladenosine (m6A) is the most prevalent, abundant and conserved internal cotranscriptional modification in eukaryotic RNAs, especially within higher eukaryotic cells. m6A modification is modified by the m6A methyltransferases, or writers, such as METTL3/14/16, RBM15/15B, ZC3H3, VIRMA, CBLL1, WTAP, and KIAA1429, and, removed by the demethylases, or erasers, including FTO and ALKBH5. It is recognized by m6A-binding proteins YTHDF1/2/3, YTHDC1/2 IGF2BP1/2/3 and HNRNPA2B1, also known as "readers". Recent studies have shown that m6A RNA modification plays essential role in both physiological and pathological conditions, especially in the initiation and progression of different types of human cancers. In this review, we discuss how m6A RNA methylation influences both the physiological and pathological progressions of hematopoietic, central nervous and reproductive systems. We will mainly focus on recent progress in identifying the biological functions and the underlying molecular mechanisms of m6A RNA methylation, its regulators and downstream target genes, during cancer progression in above systems. We propose that m6A RNA methylation process offer potential targets for cancer therapy in the future.