The RNA-binding protein DND1 acts sequentially as a negative regulator of pluripotency and a positive regulator of epigenetic modifiers required for germ cell reprogramming

Transcription and chromatin regulation by TAF4b during cellular quiescence of developing prospermatogonia

Prospermatogonia (ProSpg) link the embryonic development of male primordial germ cells to the healthy establishment of postnatal spermatogonia and spermatogonial stem cells. While these spermatogenic precursor cells undergo the characteristic transitions of cycling and quiescence, the transcriptional events underlying these developmental hallmarks remain unknown. Here, we investigated the expression and function of TBP-associated factor 4b (Taf4b) in the timely development of quiescent mouse ProSpg using an integration of gene expression profiling and chromatin mapping. We find that Taf4b mRNA expression is elevated during the transition of mitotic-to-quiescent ProSpg and Taf4b-deficient ProSpg are delayed in their entry into quiescence. Gene ontology, protein network analysis, and chromatin mapping demonstrate that TAF4b is a direct and indirect regulator of chromatin and cell cycle-related gene expression programs during ProSpg quiescence. Further validation of these cell cycle mRNA changes due to the loss of TAF4b was accomplished via immunostaining for proliferating cell nuclear antigen (PCNA). Together, these data indicate that TAF4b is a key transcriptional regulator of the chromatin and quiescent state of the developing mammalian spermatogenic precursor lineage.

Oxygen availability influences the incidence of testicular teratoma in Dnd1Ter/+ mice

Testicular teratomas and teratocarcinomas are the most common testicular germ cell tumors in early childhood and young men, and they are frequently found unilaterally in the left testis. In 129/SvJ mice carrying a heterozygous copy of the potent modifier of tumor incidence Ter, a point mutation in the dead-end homolog one gene (Dnd1 Ter/+), ∼70% of the unilateral teratomas arise in the left testis. We previously showed that in mice, left/right differences in vascular architecture are associated with reduced hemoglobin saturation and increased levels of the hypoxia inducible factor-1 alpha (HIF-1α) in the left compared to the right testis. To test the hypothesis that systemic reduction of oxygen availability in Dnd1 Ter/+ mice would lead to an increased incidence of bilateral tumors, we placed pregnant females from 129/SvJ Dnd1 Ter/+ intercross matings in a hypobaric chamber for 12-h intervals. Our results show that in 129/SvJ Dnd1 Ter/+ male gonads, the incidence of bilateral teratoma increased from 3.3% to 64% when fetuses were exposed to acute low oxygen conditions for 12-h between E13.8 and E14.3. The increase in tumor incidence correlated with the maintenance of high expression of pluripotency genes Oct4, Sox2 and Nanog, elevated activity of the Nodal signaling pathway, and suppression of germ cell mitotic arrest. We propose that the combination of heterozygosity for the Ter mutation and hypoxia causes a delay in male germ cell differentiation that promotes teratoma initiation.

Loss of Dead end1 induces testicular teratomas from primordial germ cells that failed to undergo sexual differentiation in embryonic testes

Spontaneous testicular teratomas (STTs) are tumours comprising a diverse array of cell and tissue types, which are derived from pluripotent stem-like cells called embryonal carcinoma cells (ECCs). Although mouse ECCs originate from primordial germ cells (PGCs) in embryonic testes, the molecular basis underlying ECC development remains unclear. This study shows that the conditional deletion of mouse Dead end1 (Dnd1) from migrating PGCs leads to STT development. In Dnd1-conditional knockout (Dnd1-cKO) embryos, PGCs colonise the embryonic testes but fail to undergo sexual differentiation; subsequently, ECCs develop from a portion of the PGCs. Transcriptomic analyses reveal that PGCs not only fail to undergo sexual differentiation but are also prone to transformation into ECCs by upregulating the expression of marker genes for primed pluripotency in the testes of Dnd1-cKO embryos. Thus, our results clarify the role of Dnd1 in developing STTs and developmental process of ECC from PGC, providing novel insights into pathogenic mechanisms of STTs.

Prenatal and postnatal exposure to polystyrene microplastics induces testis developmental disorder and affects male fertility in mice

Polystyrene microplastics (PS-MPs) can threaten human health, especially male fertility. However, most existing studies have focused on the adulthood stage of male reproduction toxicity caused by relatively short-term PS-MP exposure. This study aimed to investigate the toxic effect of PS-MPs on testicular development and reproductive function upon prenatal and postnatal exposure. Pregnant mice and their offspring were exposed to 0, 0.5 mg/L, 5 mg/L, and 50 mg/L PS-MPs through their daily drinking water from gestational day 1 to postnatal day (PND) 35 or PND70. We found that PS-MP exposure induced testis development disorder by PND35 and spermatogenesis dysfunction by PND70. By combining RNA sequencing results and bioinformatics analysis, the hormone-mediated signaling pathway, G1/S transition of the mitotic cell cycle, coregulation of androgen receptor activity, and Hippo signaling pathway were shown to be involved in testis development on PND35. The meiotic cell cycle, regulation of the immune effector process, neutrophil degranulation, and inflammation mediated by chemokine and cytokine signaling pathways were associated with disturbed spermatogenesis on PND70. These findings show that prenatal and postnatal exposure to PS-MPs resulted in testis development disorder and male subfertility, which may be regulated by the Hippo signaling pathway and involve an immune reaction.

The solution structure of Dead End bound to AU-rich RNA reveals an unusual mode of tandem RRM-RNA recognition required for mRNA regulation

Dead End (DND1) is an RNA-binding protein essential for germline development through its role in post-transcriptional gene regulation. The molecular mechanisms behind selection and regulation of its targets are unknown. Here, we present the solution structure of DND1's tandem RNA Recognition Motifs (RRMs) bound to AU-rich RNA. The structure reveals how an NYAYUNN element is specifically recognized, reconciling seemingly contradictory sequence motifs discovered in recent genome-wide studies. RRM1 acts as a main binding platform, including atypical extensions to the canonical RRM fold. RRM2 acts cooperatively with RRM1, capping the RNA using an unusual binding pocket, leading to an unusual mode of tandem RRM-RNA recognition. We show that the consensus motif is sufficient to mediate upregulation of a reporter gene in human cells and that this process depends not only on RNA binding by the RRMs, but also on DND1's double-stranded RNA binding domain (dsRBD), which is dispensable for binding of a subset of targets in cellulo. Our results point to a model where DND1 target selection is mediated by a non-canonical mode of AU-rich RNA recognition by the tandem RRMs and a role for the dsRBD in the recruitment of effector complexes responsible for target regulation.

A homozygous missense variant in DND1 causes non-obstructive azoospermia in humans

Non-obstructive azoospermia (NOA) is a severe factor of male infertility; it affects approximately 1% of the global male population and accounts for 40% of male infertility cases. However, the majority of NOA cases remain idiopathic. This is the first study using whole-exome sequencing (WES) to identify a novel missense mutation in the DND1 gene (c.212A>C, p. E71A) from a Pakistani family, that includes three males with NOA. This mutation is predicted to cause DND1 protein misfolding and weaken the DND1 interaction with NANOS2, a significant regulator in primordial germ cell development. Our study identified a DND1 pathogenic mutation in NOA patients and highlighted its critical role in male fertility in humans.

Early Gonadal Development and Sex Determination in Mammal

Sex determination is crucial for the transmission of genetic information through generations. In mammal, this process is primarily regulated by an antagonistic network of sex-related genes beginning in embryonic development and continuing throughout life. Nonetheless, abnormal expression of these sex-related genes will lead to reproductive organ and germline abnormalities, resulting in disorders of sex development (DSD) and infertility. On the other hand, it is possible to predetermine the sex of animal offspring by artificially regulating sex-related gene expression, a recent research hotspot. In this paper, we reviewed recent research that has improved our understanding of the mechanisms underlying the development of the gonad and primordial germ cells (PGCs), progenitors of the germline, to provide new directions for the treatment of DSD and infertility, both of which involve manipulating the sex ratio of livestock offspring.

A cooperative mechanism of target RNA selection via germ-cell-specific RNA-binding proteins NANOS2 and DND1

The germ-cell-specific RNA-binding protein (RBP) NANOS2 plays a pivotal role in male gonocyte differentiation and spermatogonial stem cell maintenance. Although NANOS2 interacts with the CNOT deadenylation complex and Dead end 1 (DND1) to repress target RNAs, the molecular mechanisms underlying target mRNA selection remain unclear because of the limited cell resource in vivo. Here, we demonstrate that exogenous NANOS2-DND1 suppresses target mRNAs in somatic cells. Using this somatic cell system, we find that NANOS2 interacts with RNA-bound DND1 and recruits the CNOT complex to the mRNAs. However, a fusion construct composed of the CNOT1-binding site of NANOS2 (NIM) and DND1 fails to repress the target gene expression. Therefore, NANOS2 is required not only for recruitment of the CNOT complex but also for selecting the target mRNA with DND1. This study reveals that NANOS2 functions as a second-layer RBP for the target recognition and functional adaptation of DND1.

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.

Instructing Mouse Germ Cells to Adopt a Female Fate

BACKGROUND

Germ cells are critical for the survival of our species. They are the only cells that undergo meiosis - the reductive form of cell division that is necessary for genetic reassortment of chromosomes and production of the haploid gametes, the sperm and eggs. Remarkably, the initial female/male fate decision in fetal germ cells does not depend on whether they are chromosomally XX or XY; rather, initial sexual fate is imposed by influences from the surrounding tissue. In mammals, the female germline is particularly precious: despite recent suggestions that germline stem cells exist in the ovary, it is still generally accepted that the ovarian reserve is finite, and its size is dependant on germ cells of the fetal ovary initiating meiosis in a timely manner.

SUMMARY

Prior to 2006, evidence suggested that gonadal germ cells initiate meiotic prophase I by default, but more recent data support a key role for the signalling molecule retinoic acid (RA) in instructing female germ cell fate. Newer findings also support a key meiosis-inducing role for another signalling molecule, bone morphogenic protein (BMP). Nonetheless, many questions remain.

KEY MESSAGES

Here, we review knowledge thus far regarding extrinsic and intrinsic determinants of a female germ cell fate, focusing on the mouse model.

Post-transcriptional regulation in spermatogenesis: all RNA pathways lead to healthy sperm

Multiple RNA pathways are required to produce functional sperm. Here, we review RNA post-transcriptional regulation during spermatogenesis with particular emphasis on the role of 3' end modifications. From early studies in the 1970s, it became clear that spermiogenesis transcripts could be stored for days only to be translated at advanced stages of spermatid differentiation. The transition between the translationally repressed and active states was observed to correlate with the shortening of the transcripts' poly(A) tail, establishing a link between RNA 3' end metabolism and male germ cell differentiation. Since then, numerous RNA metabolic pathways have been implicated not only in the progression through spermatogenesis, but also in the maintenance of genomic integrity. Recent studies have characterized the elusive 3' biogenesis of Piwi-interacting RNAs (piRNAs), identified a critical role for messenger RNA (mRNA) 3' uridylation in meiotic progression, established the mechanisms that destabilize transcripts with long 3' untranslated regions (3'UTRs) in post-mitotic cells, and defined the physiological relevance of RNA exonucleases and deadenylases in male germ cells. In this review, we discuss RNA processing in the male germline in the light of the most recent findings. A brief recollection of different RNA-processing events will aid future studies exploring post-transcriptional regulation in spermatogenesis.

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.

RNA-binding protein ELAVL2 plays post-transcriptional roles in the regulation of spermatogonia proliferation and apoptosis

Objectives

RNA‐binding proteins (RBPs) play essential post‐transcriptional roles in regulating spermatogonial stem cells (SSCs) maintenance and differentiation. We identified a conserved and SSCs‐enriched RBP ELAVL2 from our single‐cell sequencing data, but its function and mechanism in SSCs were unclear.

Materials and methods

Expressions of ELAVL2 during human and mouse testis development were validated. Stable C18‐4 and TCam‐2 cell lines with overexpression and knockdown of ELAVL2 were established, which were applied to proliferation and apoptosis analysis. RNA immunoprecipitation and sequencing were used to identify ELAVL2 targets, and regulatory functions of ELAVL2 on target mRNAs were studied. Proteins interacting with ELAVL2 in human and mouse testes were identified using immunoprecipitation and mass spectrometric, which were validated by in vivo and in vitro experiments.

Results

ELAVL2 was testis‐enriched and preferentially expressed in human and mouse SSCs. ELAVL2 was down‐regulated in NOA patients. ELAVL2 promoted proliferation and inhibited apoptosis of C18‐4 and TCam‐2 cell lines via activating ERK and AKT pathways. ELAVL2 associated with mRNAs encoding essential regulators of SSCs proliferation and survival, and promoted their protein expression at post‐transcriptional level. ELAVL2 interacted with DAZL in vivo and in vitro in both human and mouse testes.

Conclusions

Taken together, these results indicate that ELAVL2 is a conserved SSCs‐enriched RBP that down‐regulated in NOA, which regulates spermatogonia proliferation and apoptosis by promoting protein expression of targets.

The Role of DND1 in Cancers

The Ter mutation in Dead-End 1 (Dnd1), Dnd1^Ter, which leads to a premature stop codon, has been determined to be the cause for primordial germ cell deficiency, accompanied with a high incidence of congenital testicular germ cell tumors (TGCTs) or teratomas in the 129/Sv-Ter mice. As an RNA-binding protein, DND1 can bind the 3'-untranslated region (3'-UTR) of mRNAs and function in translational regulation. DND1 can block microRNA (miRNA) access to the 3'-UTR of target mRNAs, thus inhibiting miRNA-mediated mRNA degradation and up-regulating translation or can also function to degrade or repress mRNAs. Other mechanisms of DND1 activity include promoting translation initiation and modifying target protein activity. Although Dnd1^Ter mutation causes spontaneous TGCT only in male 129 mice, it can also cause ovarian teratomas in mice when combined with other genetic defects or cause germ cell teratomas in both genders in the WKY/Ztm rat strain. Furthermore, studies on human cell lines, patient cancer tissues, and the use of human cancer genome analysis indicate that DND1 may possess either tumor-suppressive or -promoting functions in a variety of somatic cancers. Here we review the involvement of DND1 in cancers, including what appears to be its emerging role in somatic cancers.

Establishing and maintaining fertility: the importance of cell cycle arrest

In this review, Frost et al. summarize the current knowledge on the Cip/Kip family of cyclin-dependent kinase inhibitors in mouse gonad development and highlight new roles for cell cycle inhibitors in controlling and maintaining female fertility.

Development of the ovary or testis is required to establish reproductive competence. Gonad development relies on key cell fate decisions that occur early in embryonic development and are actively maintained. During gonad development, both germ cells and somatic cells proliferate extensively, a process facilitated by cell cycle regulation. This review focuses on the Cip/Kip family of cyclin-dependent kinase inhibitors (CKIs) in mouse gonad development. We particularly highlight recent single-cell RNA sequencing studies that show the heterogeneity of cyclin-dependent kinase inhibitors. This diversity highlights new roles for cell cycle inhibitors in controlling and maintaining female fertility.

A transgenic DND1GFP fusion allele reports in vivo expression and RNA-binding targets in undifferentiated mouse germ cells†

In vertebrates, the RNA-binding protein (RBP) dead end 1 (DND1) is essential for primordial germ cell (PGC) survival and maintenance of cell identity. In multiple species, Dnd1 loss or mutation leads to severe PGC loss soon after specification or, in some species, germ cell transformation to somatic lineages. Our investigations into the role of DND1 in PGC specification and differentiation have been limited by the absence of an available antibody. To address this problem, we used CRISPR/Cas9 gene editing to establish a transgenic mouse line carrying a DND1GFP fusion allele. We present imaging analysis of DND1GFP expression showing that DND1GFP expression is heterogeneous among male germ cells (MGCs) and female germ cells (FGCs). DND1GFP was detected in MGCs throughout fetal life but lost from FGCs at meiotic entry. In postnatal and adult testes, DND1GFP expression correlated with classic markers for the premeiotic spermatogonial population. Utilizing the GFP tag for RNA immunoprecipitation (RIP) analysis in MGCs validated this transgenic as a tool for identifying in vivo transcript targets of DND1. The DND1GFP mouse line is a novel tool for isolation and analysis of embryonic and fetal germ cells, and the spermatogonial population of the postnatal and adult testis.

Mouse dead end1 acts with Nanos2 and Nanos3 to regulate testicular teratoma incidence

Spontaneous testicular teratomas (STTs) derived from primordial germ cells (PGCs) in the mouse embryonic testes predominantly develop in the 129 family inbred strain. Ter (spontaneous mutation) is a single nucleotide polymorphism that generates a premature stop codon of Dead end1 (Dnd1) and increases the incidence of STTs in the 129 genetic background. We previously found that DND1 interacts with NANOS2 or NANOS3 and that these complexes play a vital role in male embryonic germ cells and adult spermatogonia. However, the following are unclear: (a) whether DND1 works with NANOS2 or NANOS3 to regulate teratoma incidence, and (b) whether Ter simply causes Dnd1 loss or produces a short mutant DND1 protein. In the current study, we newly established a conventional Dnd1-knockout mouse line and found that these mice showed phenotypes similar to those of Ter mutant mice in spermatogenesis, oogenesis, and teratoma incidence, with a slight difference in spermiogenesis. In addition, we found that the amount of DND1 in Dnd1+/Ter embryos decreased to half of that in wild-type embryos, while the expression of the short mutant DND1 was not detected. We also found that double mutants for Dnd1 and Nanos2 or Nanos3 showed synergistic increase in the incidence of STTs. These data support the idea that Ter causes Dnd1 loss, leading to an increase in STT incidence, and that DND1 acts with NANOS2 and NANOS3 to regulate the development of teratoma from PGCs in the 129 genetic background. Thus, our results clarify the role of Dnd1 in the development of STTs and provide a novel insight into its pathogenic mechanism.

WOMEN IN REPRODUCTIVE SCIENCE: To be or not to be a testis

Work that established the testis as the driver of male development, and the Y chromosome as the bearer of the male-determining gene, established a working model, and set the stage for the molecular age of mammalian sex determination. The discovery and characterization of Sry/SRY at the top of the hierarchy in mammals launched the field in two major directions. The first was to identify the downstream transcription factors and other molecular players that drive the bifurcation of Sertoli and granulosa cell differentiation. The second major direction was to understand organogenesis of the early bipotential gonad, and how divergence of its two distinct morphogenetic pathways (testis and ovary) is regulated at the cellular level. This review will summarize the early discoveries soon after Sry was identified and focus on my study of the gonad as a model of organogenesis.

Generation of a novel mouse strain with conditional, cell-type specific, expression of DND1

Dead-End 1 (DND1) encodes an RNA binding protein critical for viable primordial germ cells in vertebrates. When introduced into cancer cell lines, DND1 suppresses cell proliferation and enhances apoptosis. However, the molecular function of mammalian wild-type DND1 has mostly been studied in cell lines and not verified in the organism. To facilitate study of wild-type DND1 function in mammalian systems, we generated a novel transgenic mouse line, LSL-FM-DND1 ^flox/+ , which conditionally expresses genetically engineered, FLAG-tagged and myc-tagged DND1 in a cell type-specific manner. We report that FLAG-myc-DND1 is indeed expressed in specific tissues of the mouse when LSL-FM-DND1 ^flox/+ is combined with mouse strains expressing Cre-recombinase. LSL-FM-DND1 ^flox/+ mice are fertile with no overt health effects. We expressed FLAG-myc-DND1 in the pancreas and found that chronic, ectopic expression of FLAG-myc-DND1 led to increase in fasting glucose levels in older mice. Thus, this novel LSL-FM-DND1 ^flox/+ mouse strain will facilitate studies on the biological and molecular function of wild-type DND1.