Opposing effects of retinoic acid and FGF9 on Nanos2 expression and meiotic entry of mouse germ cells

Meiosis and retinoic acid in the mouse fetal gonads: An unforeseen twist

In mammals, differentiation of germ cells is crucial for sexual reproduction, involving complex signaling pathways and environmental cues defined by the somatic cells of the gonads. This review examines the long-standing model positing that all-trans retinoic acid (ATRA) acts as a meiosis-inducing substance (MIS) in the fetal ovary by inducing expression of STRA8 in female germ cells, while CYP26B1 serves as a meiosis-preventing substance (MPS) in the fetal testis by degrading ATRA and preventing STRA8 expression in the male germ cells until postnatal development. Recent genetic studies in the mouse challenge this paradigm, revealing that meiosis initiation in female germ cells can occur independently of ATRA signaling, with key roles played by other intrinsic factors like DAZL and DMRT1, and extrinsic signals such as BMPs and vitamin C. Thus, ATRA can no longer be considered as 'the' long-searched MIS. Furthermore, evidence indicates that CYP26B1 does not prevent meiosis by degrading ATRA in the fetal testis, but acts by degrading an unidentified MIS or synthesizing an equally unknown MPS. By emphasizing the necessity of genetic loss-of-function approaches to accurately delineate the roles of signaling molecules such ATRA in vivo, this chapter calls for a reevaluation of the mechanisms instructing and preventing meiosis initiation in the fetal ovary and testis, respectively. It highlights the need for further research into the molecular identities of the signals involved in these processes.

A comprehensive review on p38MAPK signaling as a potent radioprotector in testis

BACKGROUND

Previous studies have shown that the activation of p38MAPK signaling plays a crucial role in regulating gonadal cell fate decisions in both mouse and human. Excessive activation of p38MAPK by radiation significantly causes testicular damage and negatively affects the male reproductive function. Therefore, fine-tuned regulation of p38MAPK signaling is critical in both physiological and pathological conditions.

RESULT

This review summarizes the impact of p38MAPK signaling on testicular germ cells and microenvironment under normal condition. The relationship between radiation, reactive oxygen species (ROS), and p38MAPK is summarized. In conclusion, radiation exposure triggers the overactivation of p38MAPK, which is regulated by ROS, resulting in testicular damage. Various p38MAPK-targeting agents are discussed, providing guidance for developing new strategies.

The proliferation and differentiation of spermatogonial stem cells in the frist wave of spermatogenesis in rats with Trib3 gene knockout

This study explores the effects of Trib3 gene knockout on adult male rat spermatogenesis. Using CRISPR/Cas9, we knocked out the Trib3 gene in Wistar rats. Results indicate altered expression of PLZF, ID4, and c-KIT in knockout rats, suggesting impaired spermatogonial stem cell proliferation and differentiation. Histological analysis reveals reduced seminiferous tubule area and decreased spermatocyte numbers. Mating experiments demonstrate reduced offspring rates after the second self-mating in knockout rats. SYCP3, a meiosis marker, shows elevated expression in knockout rat testes at 14 days postpartum, suggesting an impact on reproductive processes. ELISA results indicate decreased testosterone, FSH, and FGF9 levels in knockout rat testicular tissues. In conclusion, Trib3 gene deletion may impede spermatogonial self-renewal and promote differentiation through the FSH-FGF9- c-KIT interaction and p38MAPK pathway, affecting reproductive capacity. These findings contribute to understanding the molecular mechanisms regulating spermatogenesis.

The fundamentals of fibroblast growth factor 9

Fibroblast growth factor 9 (FGF9) was first identified during a screen for factors acting on cells of the central nervous system (CNS). Research over the subsequent two decades has revealed this protein to be a critically important and elegantly regulated growth factor. A hallmark control feature is reciprocal compartmentalization, particularly during development, with epithelium as a dominant source and mesenchyme a prime target. This mesenchyme selectivity is accomplished by the high affinity of FGF9 to the IIIc isoforms of FGFR1, 2, and 3. FGF9 is expressed widely in the embryo, including the developing heart and lungs, and more selectively in the adult, including the CNS and kidneys. Global Fgf9-null mice die shortly after birth due to respiratory failure from hypoplastic lungs. As well, their hearts are dilated and poorly vascularized, the skeleton is small, the intestine is shortened, and male-to-female sex reversal can be found. Conditional Fgf9-null mice have revealed CNS phenotypes, including ataxia and epilepsy. In humans, FGF9 variants have been found to underlie multiple synostoses syndrome 3, a syndrome characterized by multiple joint fusions. Aberrant FGF9 signaling has also been implicated in differences of sex development and cancer, whereas vascular stabilizing effects of FGF9 could benefit chronic diseases. This primer reviews the attributes of this vital growth factor.

FGF-independent MEK1/2 signalling in the developing foetal testis is essential for male germline differentiation in mice

BACKGROUND

Disrupted germline differentiation or compromised testis development can lead to subfertility or infertility and are strongly associated with testis cancer in humans. In mice, SRY and SOX9 induce expression of Fgf9, which promotes Sertoli cell differentiation and testis development. FGF9 is also thought to promote male germline differentiation but the mechanism is unknown. FGFs typically signal through mitogen-activated protein kinases (MAPKs) to phosphorylate ERK1/2 (pERK1/2). We explored whether FGF9 regulates male germline development through MAPK by inhibiting either FGF or MEK1/2 signalling in the foetal testis immediately after gonadal sex determination and testis cord formation, but prior to male germline commitment.

RESULTS

pERK1/2 was detected in Sertoli cells and inhibition of MEK1/2 reduced Sertoli cell proliferation and organisation and resulted in some germ cells localised outside of the testis cords. While pERK1/2 was not detected in germ cells, inhibition of MEK1/2 after somatic sex determination profoundly disrupted germ cell mitotic arrest, dysregulated a broad range of male germline development genes and prevented the upregulation of key male germline markers, DPPA4 and DNMT3L. In contrast, while FGF inhibition reduced Sertoli cell proliferation, expression of male germline markers was unaffected and germ cells entered mitotic arrest normally. While male germline differentiation was not disrupted by FGF inhibition, a range of stem cell and cancer-associated genes were commonly altered after 24 h of FGF or MEK1/2 inhibition, including genes involved in the maintenance of germline stem cells, Nodal signalling, proliferation, and germline cancer.

CONCLUSIONS

Together, these data demonstrate a novel role for MEK1/2 signalling during testis development that is essential for male germline differentiation, but indicate a more limited role for FGF signalling. Our data indicate that additional ligands are likely to act through MEK1/2 to promote male germline differentiation and highlight a need for further mechanistic understanding of male germline development.

Mammal Reproductive Homeobox (Rhox) Genes: An Update of Their Involvement in Reproduction and Development

The reproductive homeobox on the X chromosome (RHOX) genes were first identified in the mouse during the 1990s and have a crucial role in reproduction. In various transcription factors with a key regulatory role, the homeobox sequence encodes a "homeodomain" DNA-binding motif. In the mouse, there are three clusters of Rhox genes (α, β, and γ) on the X chromosome. Each cluster shows temporal and/or quantitative collinearity, which regulates the progression of the embryonic development process. Although the RHOX family is conserved in mammals, the interspecies differences in the number of RHOX genes and pseudogenes testifies to a rich evolutionary history with several relatively recent events. In the mouse, Rhox genes are mainly expressed in reproductive tissues, and several have a role in the differentiation of primordial germ cells (Rhox1, Rhox6, and Rhox10) and in spermatogenesis (Rhox1, Rhox8, and Rhox13). Despite the lack of detailed data on human RHOX, these genes appear to be involved in the formation of germ cells because they are predominantly expressed during the early (RHOXF1) and late (RHOXF2/F2B) stages of germ cell development. Furthermore, the few variants identified to date are thought to induce or predispose to impaired spermatogenesis and severe oligozoospermia or azoospermia. In the future, research on the pathophysiology of the human RHOX genes is likely to confirm the essential role of this family in the reproductive process and might help us to better understand the various causes of infertility and characterize the associated human phenotypes.

The transcription factors Creb1 and Cebpb regulate Sox9 promoter activity in TM4 Sertoli cells

In Sertoli cells, the Sox9 gene is essential for testicular development and normal spermatogenesis. SOX9 is critical for postnatal Sertoli cells differentiation and proliferation in the testis. However, the molecular mechanisms that specifically regulate its expression are not entirely understood. Sox9 expression is regulated by CREB1 and CEBPB in other biological contexts such as during chondrogenesis and in rat thyroid follicular cells. We hypothesized that Sox9 promoter activity is regulated by CREB1 and CEBPB in Sertoli cells. Our results show that Sox9 expression is dependent on the activation of these transcription factors by the cAMP/PKA signaling pathway in TM4 Sertoli cells. Chromatin immunoprecipitation and promoter/reporter luciferase assays with 5' promoter deletions and site-directed mutagenesis demonstrated that CREB1 is being recruited to a DNA regulatory element at -141 bp of the Sox9 promoter region. Such regulation is dependent on the cAMP/PKA signaling pathway, resulting in phosphorylation of CREB1. Activation of Sox9 expression by CEBPB may involve its recruitment to the proximal promoter region by protein-protein interaction with CREB1. Thus, we have shown that the Sox9 promoter is being regulated by the transcription factors CREB1 and CEBPB in TM4 Sertoli cells and involve their recruitment to the proximal promoter region.

Endocannabinoid system and epigenetics in spermatogenesis and testicular cancer

In mammals, male germ cell development starts during fetal life and is carried out in postnatal life with the formation of sperms. Spermatogenesis is the complex and highly orderly process during which a group of germ stem cells is set at birth, starts to differentiate at puberty. It proceeds through several stages: proliferation, differentiation, and morphogenesis and it is strictly regulated by a complex network of hormonal, autocrine and paracrine factors and it is associated with a unique epigenetic program. Altered epigenetic mechanisms or inability to respond to these factors can impair the correct process of germ development leading to reproductive disorders and/or testicular germ cell cancer. Among factors regulating spermatogenesis an emerging role is played by the endocannabinoid system (ECS). ECS is a complex system comprising endogenous cannabinoids (eCBs), their synthetic and degrading enzymes, and cannabinoid receptors. Mammalian male germ cells have a complete and active ECS which is modulated during spermatogenesis and that crucially regulates processes such as germ cell differentiation and sperm functions. Recently, cannabinoid receptor signaling has been reported to induce epigenetic modifications such as DNA methylation, histone modifications and miRNA expression. Epigenetic modifications may also affect the expression and function of ECS elements, highlighting the establishment of a complex mutual interaction. Here, we describe the developmental origin and differentiation of male germ cells and testicular germ cell tumors (TGCTs) focusing on the interplay between ECS and epigenetic mechanisms involved in these processes.

Sertoli cells-derived exosomal miR-30a-5p regulates ubiquitin E3 ligase Zeb2 to affect the spermatogonial stem cells proliferation and differentiation

The role of spermatogonial stem cells (SSCs) is crucial in spermatogenesis, and extracellular vesicles (EVs) have been the focus of research as an important intercellular communication mechanism. Various endogenous regulatory factors secreted by Sertoli cells (SCs) can affect the self-maintenance and regeneration of SSCs, but little is known about the roles of SCs-derived exosomal microRNAs (miRNAs) on SSCs. In this study, we aimed to explore the regulation of the SCs-derived exosomal miR-30a-5p on SSCs proliferation and differentiation. EVs from the SCs were detected by electron microscopy and nanoparticle tracking analysis (NTA). Subsequently, the SSCs were treated with the SCs-derived extracellular vesicles (SCs-EVs). CCK-8 assay and EdU staining was applied to detect the cell proliferation, and the results indicated that SCs-EVs promoted the SSCs proliferation. Western blot detection of the SSCs markers (Gfrα1, Plzf, Stra8, and C-kit) indicated that SCs-EVs promoted the SSCs differentiation. Additionally, we found that SCs-EVs secreted miR-30a-5p to show the promoting effects. Besides, we discovered that miR-30a-5p targeted zinc finger E-box binding homeobox 2 (Zeb2) to regulate the ubiquitination of fibroblast growth factor 9 (Fgf9) in SSCs. miR-30a-3p/Zeb2/Fgf9 promoted the SSCs proliferation and differentiation by activating the mitogen‑activated protein kinase (MAPK) signaling pathway. Taken together, our study showed that SCs-EVs can transport miR-30a-5p to SSCs and affect SSCs proliferation and differentiation by regulating the MAPK signaling pathway via Zeb2/Fgf9. This paper disclosed a novel molecular mechanism that regulates SSCs proliferation and differentiation, which could be valuable for the treatment of male infertility.

The Beginning of Meiosis in Mammalian Female Germ Cells: A Never-Ending Story of Intrinsic and Extrinsic Factors

Meiosis is the unique division of germ cells resulting in the recombination of the maternal and paternal genomes and the production of haploid gametes. In mammals, it begins during the fetal life in females and during puberty in males. In both cases, entering meiosis requires a timely switch from the mitotic to the meiotic cell cycle and the transition from a potential pluripotent status to meiotic differentiation. Revealing the molecular mechanisms underlying these interrelated processes represents the essence in understanding the beginning of meiosis. Meiosis facilitates diversity across individuals and acts as a fundamental driver of evolution. Major differences between sexes and among species complicate the understanding of how meiosis begins. Basic meiotic research is further hindered by a current lack of meiotic cell lines. This has been recently partly overcome with the use of primordial-germ-cell-like cells (PGCLCs) generated from pluripotent stem cells. Much of what we know about this process depends on data from model organisms, namely, the mouse; in mice, the process, however, appears to differ in many aspects from that in humans. Identifying the mechanisms and molecules controlling germ cells to enter meiosis has represented and still represents a major challenge for reproductive medicine. In fact, the proper execution of meiosis is essential for fertility, for maintaining the integrity of the genome, and for ensuring the normal development of the offspring. The main clinical consequences of meiotic defects are infertility and, probably, increased susceptibility to some types of germ-cell tumors. In the present work, we report and discuss data mainly concerning the beginning of meiosis in mammalian female germ cells, referring to such process in males only when pertinent. After a brief account of this process in mice and humans and an historical chronicle of the major hypotheses and progress in this topic, the most recent results are reviewed and discussed.

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.

LncRNA HOTAIR promotes proliferation and suppresses apoptosis of mouse spermatogonium GC-1 cells by sponging miR-761 to modulate NANOS2 expression

LncRNA HOX antisense intergenic RNA (HOTAIR) can regulate cancer-related gene expression and promote stem cell and tumor cell proliferation via mechanisms including the competing endogenous RNA (ceRNA) mechanism. HOTAIR is abundantly expressed in the genital tubercle of E11.5, E12.5, and E13.5 embryos, whereas it became barely detectable at E13.5 and expressed again in adult mouse testis. However, the underlying function and mechanism of HOTAIR in spermatogenesis have not been elucidated. Interestingly, other researchers reported that the function of gene Nanos C2HC-Type Zinc Finger 2 (nanos2) includes the maintenance of both the primordial germ cells (PGCs) and germline stem cells, and Nanos2 protein and transcripts (NANOS2) were detected only in PGCs from day E11.5 and undifferentiated spermatogonia in spermatogenesis. We therefore investigated the relationship between HOTAIR and NANOS2 in maintaining spermatogonial stem cell population. We found that, compared to the adult mouse, the expression levels of HOTAIR and NANOS2 in embryo mouse were significantly higher and miR-761expression level was lower. In mouse GC-1 spermatogonia cells, overexpression of miRNA-761 significantly inhibited the expression of NANOS2 and HOTAIR, suppressed the proliferation, and promotes apoptosis of cells. Knock down and overexpression of HOTAIR indicated that HOTAIR expression was positively correlated with NANOS2 expression; overexpressed HOTAIR could promote proliferation and suppresses apoptosis of GC-1 cells. By a rescue experiment and dual luciferase reporter assay, miR-761 was identified as a direct target of HOTAIR, and NANOS2 was identified as the direct target of miR-761. The above results indicate that HOTAIR promotes proliferation and suppresses apoptosis of mouse spermatogonium GC-1 cells by sponging miR-761 to modulate NANOS2 expression. Our findings elucidate one of possible mechanisms and importance of HOTAIR in maintaining spermatogonial stem cell population, and provide new candidate genes and possible pathogenesis for male infertility.

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.

Sexual determination in zebrafish

Zebrafish have emerged as a major model organism to study vertebrate reproduction due to their high fecundity and external development of eggs and embryos. The mechanisms through which zebrafish determine their sex have come under extensive investigation, as they lack a definite sex-determining chromosome and appear to have a highly complex method of sex determination. Single-gene mutagenesis has been employed to isolate the function of genes that determine zebrafish sex and regulate sex-specific differentiation, and to explore the interactions of genes that promote female or male sexual fate. In this review, we focus on recent advances in understanding of the mechanisms, including genetic and environmental factors, governing zebrafish sex development with comparisons to gene functions in other species to highlight conserved and potentially species-specific mechanisms for specifying and maintaining sexual fate.

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.

The homeobox gene TGIF1 is required for chicken ovarian cortical development and generation of the juxtacortical medulla

During early embryogenesis in amniotic vertebrates, the gonads differentiate into either ovaries or testes. The first cell lineage to differentiate gives rise to the supporting cells: Sertoli cells in males and pre-granulosa cells in females. These key cell types direct the differentiation of the other cell types in the gonad, including steroidogenic cells. The gonadal surface epithelium and the interstitial cell populations are less well studied, and little is known about their sexual differentiation programs. Here, we show the requirement of the homeobox transcription factor gene TGIF1 for ovarian development in the chicken embryo. TGIF1 is expressed in the two principal ovarian somatic cell populations: the cortex and the pre-granulosa cells of the medulla. TGIF1 expression is associated with an ovarian phenotype in estrogen-mediated sex reversal experiments. Targeted misexpression and gene knockdown indicate that TGIF1 is required, but not sufficient, for proper ovarian cortex formation. In addition, TGIF1 is identified as the first known regulator of juxtacortical medulla development. These findings provide new insights into chicken ovarian differentiation and development, specifically cortical and juxtacortical medulla formation.

Zebrafish dazl regulates cystogenesis and germline stem cell specification during the primordial germ cell to germline stem cell transition

Fertility and gamete reserves are maintained by asymmetric divisions of the germline stem cells to produce new stem cells or daughters that differentiate as gametes. Before entering meiosis, differentiating germ cells (GCs) of sexual animals typically undergo cystogenesis. This evolutionarily conserved process involves synchronous and incomplete mitotic divisions of a GC daughter (cystoblast) to generate sister cells connected by intercellular bridges that facilitate the exchange of materials to support rapid expansion of the gamete progenitor population. Here, we investigated cystogenesis in zebrafish and found that early GCs are connected by ring canals, and show that Deleted in azoospermia-like (Dazl), a conserved vertebrate RNA-binding protein (Rbp), is a regulator of this process. Analysis of dazl mutants revealed the essential role of Dazl in regulating incomplete cytokinesis, germline cyst formation and germline stem cell specification before the meiotic transition. Accordingly, dazl mutant GCs form defective ring canals, and ultimately remain as individual cells that fail to differentiate as meiocytes. In addition to promoting cystoblast divisions and meiotic entry, dazl is required for germline stem cell establishment and fertility.

Low retinoic acid levels mediate regionalization of the Sertoli valve in the terminal segment of mouse seminiferous tubules

In mammalian testes, undifferentiated spermatogonia (A_undiff) undergo differentiation in response to retinoic acid (RA), while their progenitor states are partially maintained by fibroblast growth factors (FGFs). Sertoli valve (SV) is a region located at the terminal end of seminiferous tubule (ST) adjacent to the rete testis (RT), where the high density of A_undiff is constitutively maintained with the absence of active spermatogenesis. However, the molecular and cellular characteristics of SV epithelia still remain unclear. In this study, we first identified the region-specific AKT phosphorylation in the SV Sertoli cells and demonstrated non-cell autonomous specialization of Sertoli cells in the SV region by performing a Sertoli cell ablation/replacement experiment. The expression of Fgf9 was detected in the RT epithelia, while the exogenous administration of FGF9 caused ectopic AKT phosphorylation in the Sertoli cells of convoluted ST. Furthermore, we revealed the SV region-specific expression of Cyp26a1, which encodes an RA-degrading enzyme, and demonstrated that the increased RA levels in the SV region disrupt its pool of A_undiff by inducing their differentiation. Taken together, RT-derived FGFs and low levels of RA signaling contribute to the non-cell-autonomous regionalization of the SV epithelia and its local maintenance of A_undiff in the SV region.

Sempervirine inhibits RNA polymerase I transcription independently from p53 in tumor cells

In the search of small molecules that can target MDM2/p53 pathway in testicular germ cell tumors (TGCTs), we identified sempervirine (2,3,4,13-tetrahydro-1H-benz[g]indolo[2,3-a]quinolizin-6-ium), an alkaloid of Gelsemium sempervirens, that has been previously proposed as an inhibitor of MDM2 that targets p53-wildtype (wt) tumor cells. We found that sempervirine not only affects cell growth of p53-wt cancer cells, but it is also active in p53-mutated and p53-null cells by triggering p53-dependent and independent pathways without affecting non-transformed cells. To understand which mechanism/s could be activated both in p53-wt and -null cells, we found that sempervirine induced nucleolar remodeling and nucleolar stress by reducing protein stability of RPA194, the catalytic subunit of RNA polymerase I, that led to rRNA synthesis inhibition and to MDM2 block. As shown for other cancer cell models, MDM2 inhibition by nucleolar stress downregulated E2F1 protein levels both in p53-wt and p53-null TGCT cells with the concomitant upregulation of unphosphorylated pRb. Finally, we show that sempervirine is able to enter the nucleus and accumulates within the nucleolus where it binds rRNA without causing DNA damage. Our results identify semperivirine as a novel rRNA synthesis inhibitor and indicate this drug as a non-genotoxic anticancer small molecule.

FGF9 promotes mouse spermatogonial stem cell proliferation mediated by p38 MAPK signalling

Objectives

Fibroblast growth factor 9 (FGF9) is expressed by somatic cells in the seminiferous tubules, yet little information exists about its role in regulating spermatogonial stem cells (SSCs).

Materials and Methods

Fgf9 overexpression lentivirus was injected into mouse testes, and PLZF immunostaining was performed to investigate the effect of FGF9 on spermatogonia in vivo. Effect of FGF9 on SSCs was detected by transplanting cultured germ cells into tubules of testes. RNA‐seq of bulk RNA and single cell was performed to explore FGF9 working mechanisms. SB203580 was used to disrupt p38 MAPK pathway. p38 MAPK protein expression was detected by Western blot and qPCR was performed to determine different gene expression. Small interfering RNA (siRNA) was used to knock down Etv5 gene expression in germ cells.

Results

Overexpression of Fgf9 in vivo resulted in arrested spermatogenesis and accumulation of undifferentiated spermatogonia. Exposure of germ cell cultures to FGF9 resulted in larger numbers of SSCs over time. Inhibition of p38 MAPK phosphorylation negated the SSC growth advantage provided by FGF9. Etv5 and Bcl6b gene expressions were enhanced by FGF9 treatment. Gene knockdown of Etv5 disrupted the growth effect of FGF9 in cultured SSCs along with downstream expression of Bcl6b.

Conclusions

Taken together, these data indicate that FGF9 is an important regulator of SSC proliferation, operating through p38 MAPK phosphorylation and upregulating Etv5 and Bcl6b in turn.

Expression and possible roles of extracellular signal-related kinases 1-2 (ERK1-2) in mouse primordial germ cell development

In the present work, we described the expression and activity of extracellular signal-related kinases 1-2 (ERK1-2) in mouse primordial germ cells (PGCs) from 8.5–14.5 days post coitum (dpc) and investigated whether these kinases play a role in regulating the various processes of PGC development. Using immunofluorescence and immunoblotting to detect the active phosphorylated form of ERK1-2 (p-ERK1-2), we found that the kinases were present in most proliferating 8.5–10.5 dpc PGCs, low in 11.5 dpc PGCs, and progressively increasing between 12.5–14.5 dpc both in female and male PGCs. In vitro culture experiments showed that inhibiting activation of ERK1-2 with the MEK-specific inhibitor U0126 significantly reduced the growth of 8.5 dpc PGCs in culture but had little effect on 11.5–12.5 dpc PGCs. Moreover, we found that the inhibitor did not affect the adhesion of 11.5 dpc PGCs, but it significantly reduced their motility features onto a cell monolayer. Further, while the ability of female PGCs to begin meiosis was not significantly affected by U0126, their progression through meiotic prophase I was slowed down. Notably, the activity of ERK1-2 was necessary for maintaining the correct expression of oocyte-specific genes crucial for germ cells survival and the formation of primordial follicles.

Dysregulation of FGFR signalling by a selective inhibitor reduces germ cell survival in human fetal gonads of both sexes and alters the somatic niche in fetal testes

STUDY QUESTION

Does experimental manipulation of fibroblast growth factor 9 (FGF9)-signalling in human fetal gonads alter sex-specific gonadal differentiation?

SUMMARY ANSWER

Inhibition of FGFR signalling following SU5402 treatment impaired germ cell survival in both sexes and severely altered the developing somatic niche in testes, while stimulation of FGF9 signalling promoted Sertoli cell proliferation in testes and inhibited meiotic entry of germ cells in ovaries.

WHAT IS KNOWN ALREADY

Sex-specific differentiation of bipotential gonads involves a complex signalling cascade that includes a combination of factors promoting either testicular or ovarian differentiation and inhibition of the opposing pathway. In mice, FGF9/FGFR2 signalling has been shown to promote testicular differentiation and antagonize the female developmental pathway through inhibition of WNT4.

STUDY DESIGN, SIZE, DURATION

FGF signalling was manipulated in human fetal gonads in an established ex vivo culture model by treatments with recombinant FGF9 (25 ng/ml) and the tyrosine kinase inhibitor SU5402 (10 μM) that was used to inhibit FGFR signalling. Human fetal testis and ovary tissues were cultured for 14 days and effects on gonadal development and expression of cell lineage markers were determined.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Gonadal tissues from 44 male and 33 female embryos/fetuses from first trimester were used for ex vivo culture experiments. Tissues were analyzed by evaluation of histology and immunohistochemical analysis of markers for germ cells, somatic cells, proliferation and apoptosis. Culture media were collected throughout the experimental period and production of steroid hormone metabolites was analyzed in media from fetal testis cultures by liquid chromatography-tandem mass spectrometry (LC-MS/MS).

MAIN RESULTS AND THE ROLE OF CHANCE

Treatment with SU5402 resulted in near complete loss of gonocytes (224 vs. 14 OCT4+ cells per mm2, P < 0.05) and oogonia (1456 vs. 28 OCT4+ cells per mm2, P < 0.001) in human fetal testes and ovaries, respectively. This was a result of both increased apoptosis and reduced proliferation in the germ cells. Addition of exogenous FGF9 to the culture media resulted in a reduced number of germ cells entering meiosis in fetal ovaries (102 vs. 60 γH2AX+ germ cells per mm2, P < 0.05), while in fetal testes FGF9 stimulation resulted in an increased number of Sertoli cells (2503 vs. 3872 SOX9+ cells per mm2, P < 0.05). In fetal testes, inhibition of FGFR signalling by SU5402 treatment altered seminiferous cord morphology and reduced the AMH expression as well as the number of SOX9-positive Sertoli cells (2503 vs. 1561 SOX9+ cells per mm2, P < 0.05). In interstitial cells, reduced expression of COUP-TFII and increased expression of CYP11A1 and CYP17A1 in fetal Leydig cells was observed, although there were no subsequent changes in steroidogenesis.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Ex vivo culture may not replicate all aspects of fetal gonadal development and function in vivo. Although the effects of FGF9 were studied in ex vivo culture experiments, there is no direct evidence that FGF9 acts in vivo during human fetal gonadogenesis. The FGFR inhibitor (SU5402) used in this study is not specific to FGFR2 but inhibits all FGF receptors and off-target effects on unrelated tyrosine kinases should be considered.

WIDER IMPLICATIONS OF THE FINDINGS

The findings of this study suggest that dysregulation of FGFR-mediated signalling may affect both testicular and ovarian development, in particular impacting the fetal germ cell populations in both sexes.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported in part by an ESPE Research Fellowship, sponsored by Novo Nordisk A/S to A.JØ. Additional funding was obtained from the Erichsen Family Fund (A.JØ.), the Aase and Ejnar Danielsens Fund (A.JØ.), the Danish Government's support for the EDMaRC programme (A.JU.) and a Wellcome Trust Intermediate Clinical Fellowship (R.T.M., Grant no. 098522). The Medical Research Council (MRC) Centre for Reproductive Health (R.T.M.) is supported by an MRC Centre Grant (MR/N022556/1). The authors have no conflict of interest to disclose.

Conversion from spermatogonia to spermatocytes: Extracellular cues and downstream transcription network

Spermatocyte (spc) formation from spermatogonia (spg) differentiation is the first step of spermatogenesis which produces prodigious spermatozoa for a lifetime. After decades of studies, several factors involved in the functioning of a mouse were discovered both inside and outside spg. Considering the peculiar expression and working pattern of each factor, this review divides the whole conversion of spg to spc into four consecutive development processes with a focus on extracellular cues and downstream transcription network in each one. Potential coordination among Dmrt1, Sohlh1/2 and BMP families mediates Ngn3 upregulation, which marks progenitor spg, with other changes. After that, retinoic acid (RA), as a master regulator, promotes A1 spg formation with its helpers and Sall4. A1-to-B spg transition is under the control of Kitl and impulsive RA signaling together with early and late transcription factors Stra8 and Dmrt6. Finally, RA and its responsive effectors conduct the entry into meiosis. The systematic transcription network from outside to inside still needs research to supplement or settle the controversials in each process. As a step further ahead, this review provides possible drug targets for infertility therapy by cross-linking humans and mouse model.

In-vitro spermatogenesis through testis modelling: Toward the generation of testicular organoids

Background

The testicular organoid concept has recently been introduced in tissue engineering to refer to testicular cell organizations modeling testicular architecture and function. The testicular organoid approach gives control over which and how cells reaggregate, which is not possible in organotypic cultures, thereby extending the applicability of in‐vitro spermatogenesis (IVS) systems. However, it remains unclear which culture method and medium allow reassociation of testicular cells into a functional testicular surrogate in‐vitro.

Objective

The aim of this paper is to review the different strategies that have been used in an attempt to create testicular organoids and generate spermatozoa. We want to provide an up‐to‐date list on culture methodologies and media compositions that have been used and determine their role in regulating tubulogenesis and differentiation of testicular cells.

Search method

A literature search was conducted in PubMed, Web of Science, and Scopus to select studies reporting the reorganization of testicular cell suspensions in‐vitro, using the keywords: three‐dimensional culture, in‐vitro spermatogenesis, testicular organoid, testicular scaffold, and tubulogenesis. Papers published before the August 1, 2019, were selected.

Outcome

Only a limited number of studies have concentrated on recreating the testicular architecture in‐vitro. While some advances have been made in the testicular organoid research in terms of cellular reorganization, none of the described culture systems is adequate for the reproduction of both the testicular architecture and IVS.

Conclusion

Further improvements in culture methodology and medium composition have to be made before being able to provide both testicular tubulogenesis and spermatogenesis in‐vitro.

Testis Development

Production of sperm and androgens is the main function of the testis. This depends on normal development of both testicular somatic cells and germ cells. A genetic program initiated from the Y chromosome gene sex-determining region Y (SRY) directs somatic cell specification to Sertoli cells that orchestrate further development. They first guide fetal germ cell differentiation toward spermatogenic destiny and then take care of the full service to spermatogenic cells during spermatogenesis. The number of Sertoli cells sets the limits of sperm production. Leydig cells secrete androgens that determine masculine development. Testis development does not depend on germ cells; that is, testicular somatic cells also develop in the absence of germ cells, and the testis can produce testosterone normally to induce full masculinization in these men. In contrast, spermatogenic cell development is totally dependent on somatic cells. We herein review germ cell differentiation from primordial germ cells to spermatogonia and development of the supporting somatic cells. Testicular descent to scrota is necessary for normal spermatogenesis, and cryptorchidism is the most common male birth defect. This is a mild form of a disorder of sex differentiation. Multiple genetic reasons for more severe forms of disorders of sex differentiation have been revealed during the last decades, and these are described along with the description of molecular regulation of testis development.

Postnatal testis development in the collared peccary (Tayassu tajacu), with emphasis on spermatogonial stem cells markers and niche

Collared peccaries (Tayassu tajacu) present a unique testis cytoarchitecture, where Leydig cells (LC) are mainly located in cords around the seminiferous tubules (ST) lobes. This peculiar arrangement is very useful to better investigate and understand the role of LC in spermatogonial stem cells (SSCs) biology and niche. Recent studies from our laboratory using adult peccaries have shown that the undifferentiated type A spermatogonia (A_und or SSCs) are preferentially located in ST regions adjacent to the intertubular compartment without LC. Following these studies, our aims were to investigate the collared peccary postnatal testis development, from birth to adulthood, with emphasis on the establishment of LC cytoarchitecture and the SSCs niche. Our findings demonstrated that the unique LC cytoarchitecture is already present in the neonate peccary's testis, indicating that this arrangement is established during fetal development. Based on the most advanced germ cell type present at each time period evaluated, puberty (the first sperm release in the ST lumen) in this species was reached at around one year of age, being preceded by high levels of estradiol and testosterone and the end of Sertoli cell proliferation. Almost all gonocytes and SSCs expressed Nanos1, Nanos2 and GFRA1. The analysis of SSCs preferential location indicated that the establishment of SSCs niche is coincident with the occurrence of puberty. Taken together, our findings reinforced and extended the importance of the collared peccary as an animal model to investigate testis function in mammals, particularly the aspects related to testis organogenesis and the SSCs biology and niche.

Sexually dimorphic germ cell identity in mammals

Germ cells are the stem cells of the species. Thus, it is critical that we have a good understanding of how they are specified, how the somatic cells instruct and support them, how they commit to one or other sex, and how they ultimately develop into functional gametes. Here, we focus on specifics of how sexual fate is determined during fetal life. Because the majority of relevant experimental work has been done using the mouse model, we focus on that species. We review evidence regarding the identity of instructive signals from the somatic cells, and the molecular responses that occur in germ cells in response to those extrinsic signals. In this way we aim to clarify progress to date regarding the mechanisms underlying the mitotic to meiosis switch in germ cells of the fetal ovary, and those involved in adopting and securing male fate in germ cells of the fetal testis.

Post-transcriptional Regulatory Functions of Mammalian Pumilio Proteins

Mammalian Pumilio proteins, PUM1 and PUM2, are members of the PUF family of sequence-specific RNA-binding proteins. In this review, we explore their mechanisms, regulatory networks, biological functions, and relevance to diseases. Pumilio proteins bind an extensive network of mRNAs and repress protein expression by inhibiting translation and promoting mRNA decay. Opposingly, in certain contexts, they can activate protein expression. Pumilio proteins also regulate noncoding (nc)RNAs. The ncRNA, ncRNA activated by DNA damage (NORAD), can in turn modulate Pumilio activity. Genetic analysis provides new insights into Pumilio protein function. They are essential for growth and development. They control diverse processes, including stem cell fate, and neurological functions, such as behavior and memory formation. Novel findings show that their dysfunction contributes to neurodegeneration, epilepsy, movement disorders, intellectual disability, infertility, and cancer.

Royal jelly protects male mice against nicotine-induced reproductive failure

This study evaluated the possible protective effect of royal jelly (RJ) on sperm parameters and sperm malondialdehyde (MDA) concentration and in vitro fertilizing potential in nicotine (NIC) exposed male mice. Thrtiy-six male BALB/c mice were randomly divided into six groups (n = 6). Group 1 received normal saline, group 2 received 100 mg kg-1 per day RJ, groups 3 and 4 received NIC at doses of 0.50 and 1.00 mg kg-1 per day, respectively and groups 5 and 6 received NIC at doses of 0.50 and 1.00 mg kg-1 per day, respectively plus RJ. Caudal epididymal sperm characteristics, lipid peroxidation and in vitro fertilizing capacity and embryo development were evaluated after 35 days. The NIC treatment caused a significant decrease in sperm motility and viability and fertilization rate along with poor blastocyst formation and increased sperm DNA damage and MDA levels. Moreover, the incidences of chromatin abnormality in spermatozoa were significantly higher in NIC-exposed mice than those of control. Nevertheless, RJ treatment improved sperm parameters and in vitro fertilization outcome as well as sperm lipid peroxidation level. Data from the current study suggest that RJ has a potential repro-protective action against NIC-induced sperm abnormalities and embryotoxicity in mice.

Intergenerational effects on mouse sperm quality after in utero exposure to acetaminophen and ibuprofen

Nonsteroidal antiinflammatory drugs and analgesic drugs, such as N-acetyl- p-aminophenol (APAP; acetaminophen, paracetamol), are widely used by pregnant women. Accumulating evidence has indicated that these molecules can favor genital malformations in newborn boys and reproductive disorders in adults. However, the consequences on postnatal testis development and adult reproductive health after exposure during early embryogenesis are still unknown. Using the mouse model, we show that in utero exposure to therapeutic doses of the widely used APAP-ibuprofen combination during the sex determination period leads to early differentiation and decreased proliferation of male embryonic germ cells, and early 5-methylcytosine and extracellular matrix protein deposition in 13.5 d postcoitum exposed testes. Consequently, in postnatal testes, Sertoli-cell maturation is delayed, the Leydig-cell compartment is hyperplasic, and the spermatogonia A pool is decreased. This results in a reduced production of testosterone and in epididymal sperm parameter defects. We observed a reduced sperm count (19%) in utero-exposed (F0) adult males and also a reduced sperm motility (40%) in their offspring (F1) when both parents were exposed, which leads to subfertility among the 6 mo old F1 animals. Our study suggests that the use of these drugs during the critical period of sex determination affects the germ-line development and leads to adverse effects that could be passed to the offspring.-Rossitto, M., Marchive, C., Pruvost, A., Sellem, E., Ghettas, A., Badiou, S., Sutra, T., Poulat, F., Philibert, P., Boizet-Bonhoure, B. Intergenerational effects on mouse sperm quality after in utero exposure to acetaminophen and ibuprofen.

Human foetal ovary shares meiotic preventing factors with the developing testis

STUDY QUESTION

How can pre-meiotic germ cells persist in the human foetal ovary?

SUMMARY ANSWER

Numerous oogonia escaping meiotic entry were retrieved throughout human ovarian development simultaneously with the expression of signalling pathways preventing meiosis, typically described in the rodent embryonic testis.

WHAT IS KNOWN ALREADY

The transition from mitosis to meiosis is a key event in female germ cells that remains poorly documented in research on the human ovary. Previous reports described a strikingly asynchronous differentiation in the human female germ line during development, with the persistence of oogonia among oocytes and follicles during the second and third trimesters. The possible mechanisms allowing some cells to escape meiosis remain elusive.

STUDY DESIGN SIZE, DURATION

In order to document the extent of this phenomenon, we detailed the expression profile of germ cell differentiation markers using 73 ovaries ranging from 6.4 to 35 weeks post-fertilization.

PARTICIPANTS/MATERIALS SETTING, METHODS

Pre-meiotic markers were detected by immunohistochemistry or qRT-PCR. The expression of the main meiosis-preventing factors identified in mice was analysed, and their functionality assessed using organ cultures.

MAIN RESULTS AND THE ROLE OF CHANCE

Oogonia stained for AP2γ could be traced from the first trimester until the end of the third trimester. Female germ cell differentiation is organized both in time and space in a centripetal manner in the foetal human ovary. Unexpectedly, some features usually ascribed to rodent pre-spermatogonia could be observed in human foetal ovaries, such as NANOS2 expression and quiescence in some germ cells. The two main somatic signals known to inhibit meiosis in the mouse embryonic testis, CYP26B1 and FGF9, were detected in the human ovary and act simultaneously to repress STRA8 and meiosis in human foetal female germ cells.

LARGE SCALE DATA

N/A.

LIMITATIONS REASON FOR CAUTION

Our conclusions relied partly on in vitro experiments. Germ cells were not systematically identified with immunostaining and some may have thus escaped analysis.

WIDER IMPLICATIONS OF THE FINDINGS

We found evidence that a robust repression of meiotic entry is taking place in the human foetal ovary, possibly explaining the exceptional long-lasting presence of pre-meiotic germ cells until late gestational age. This result calls for a redefinition of the markers known as classical male markers, which may in fact characterize mammalian developing gonads irrespectively of their sex.

STUDY FUNDING/COMPETING INTEREST(S)

This research was supported by the Université Paris Diderot-Paris 7 and Université Paris-Sud, CEA, INSERM, and Agence de la Biomédecine. The authors declare no conflict of interest.

Expression profiles of Sox transcription factors within the postnatal rodent testes

SRY-related box (Sox) transcription factors are conserved among vertebrate species. These proteins regulate multiple processes including sex determination and testis differentiation of the male embryo. Members of the Sox family have been identified in pre- and postnatal testis and are known to play an important role in sex determination (Sry, Sox9), male gonadal development, and fertility (Sox4, Sox8, Sox30). However, their expression profiles per cell types remain elusive. The objectives of this research were to characterize the expression profiles of Sox family members within adult testes using publically available datasets and to determine whether these findings are consistent with literature as well as immunofluorescence and in situ hybridization results. We have found that Sox4, Sox8, Sox9, and Sox12 are highly expressed in Sertoli cells, whereas Sox5, Sox6, and Sox30 were typically expressed in spermatocytes and spermatids. Spermatogonia were characterized by the expressions of Sox3, Sox4, Sox12, Sox13, and Sox18. Hence, these results suggest that Sox transcription factors may play different roles according to cell types of the adult mammalian testis.

Reconstitution of Female Germ Cell Fate Determination and Meiotic Initiation in Mammals

Meiosis is a fundamental process that underpins sexual reproduction. In mammals, the execution of meiosis is tightly integrated within the complex processes of oogenesis and spermatogenesis, and elucidation of the molecular mechanisms regulating meiotic initiation remains challenging. We have recently developed in vitro culture strategies to induce mouse pluripotent stem cells into germ cells, which successfully contribute to both oogenesis and spermatogenesis and to fertile offspring. The culture strategies faithfully recapitulate transcriptional and epigenetic dynamics as well as signaling principles for germ cell specification, proliferation, and female sex determination/meiotic induction, providing a valuable platform for studies to illuminate the molecular mechanisms underlying such critical processes. Here, we review mammalian gametogenesis with a focus on the implementation of meiosis and, based on our recent studies, discuss new insights into the mechanisms for meiotic initiation and germ cell sex determination in mice.

Dose-dependent functions of fibroblast growth factor 9 regulate the fate of murine XY primordial germ cells

Male differentiation of primordial germ cells (PGCs) is initiated by the inhibition of entry into meiosis and exposure to male-inducing factor(s), which are regulated by somatic elements of the developing gonad. Fibroblast growth factor 9 (FGF9) produced by pre-Sertoli cells is essential for male gonadal differentiation and also contributes to survival and male differentiation of XY PGCs. However, it is not clear how FGF9 regulates PGC fate. Using a PGC culture system, we identified dose-dependent, fate-determining functions of FGF9 in XY PGCs. Treatment with low levels of FGF9 (0.2 ng/ml) increased expression of male-specific Dnmt3L and Nanos2 in XY PGCs. Conversely, treatment with high levels of FGF9 (25 ng/ml) suppressed male-specific gene expression and stimulated proliferation of XY PGCs. Western blotting showed that low FGF9 treatment enhanced p38 MAPK (mitogen-activated protein kinase) phosphorylation in the same cells. In contrast, high FGF9 treatment significantly stimulated the ERK (extracellular signal-regulated kinase)1/2 signaling pathway in XY PGCs. We investigated the relationship between the ERK1/2 signaling pathway stimulated by high FGF9 and regulation of PGC proliferation. An ERK1/2 inhibitor (U0126) suppressed the PGC proliferation that would otherwise be stimulated by high FGF9 treatment, and increased Nanos2 expression in XY PGCs. Conversely, a p38 MAPK inhibitor (SB202190) significantly suppressed Nanos2 expression that would otherwise be stimulated by low FGF9 in XY PGCs. Taken together, our results suggest that stage-specific expression of FGF9 in XY gonads regulates the balance between proliferation and differentiation of XY PGCs in a dose-dependent manner.

Expression and cellular localization of double sex and mab-3 related transcription factor 1 in testes of postnatal Small-Tail Han sheep at different developmental stages

Double sex and mab-3 related transcription factor 1 (Dmrt1), an evolutionarily conserved gene, is a sex-related gene expressed in male gonads, that is involved in the regulation of sex differentiation, testicular development and reproductive function maintenance. Until now, functional studies on the Dmrt1 gene in sheep (Ovis aries) have been lacking. In this study, testis, heart, liver, spleen, lung, kidney and longissimus dorsi muscle tissues were collected from Small-Tail Han sheep at 0, 2, 5, 12 and 24months after birth (mab). Dmrt1 expression and cellular localization were detected in various testicular tissues by quantitative real time PCR (qRT-PCR), western blot and immunohistochemistry methods. The morphological structures of testicular tissues at different developmental stages were observed by hematoxylin & eosin (HE) staining. The Dmrt1 mRNA expression levels in 12 and 24 mab sheep were significantly higher than those in 0 and 2 mab sheep (P<0.05), and Dmrt1 protein expression showed a similar trend. The qRT-PCR results in various tissues at 12 mab showed that Dmrt1 mRNA was predominantly expressed in testes. Immunohistochemical staining in testes at different developmental stages showed that Dmrt1 protein immunoreactive responses were mainly localized in Sertoli cells and gonocytes at 0, 2 and 5 mab, while they were localized in spermatocytes, sperm cells and some spermatogonia and Sertoli cells at 12 and 24 mab. We speculate that the Dmrt1 gene plays a vital role in postnatal sheep spermatogenesis, perhaps by regulating the maturation and functional maintenance of Sertoli cells, the proliferation and differentiation of gonocytes in prepubertal sheep testes, and the mitosis and meiosis of germ cells in adult sheep, but the specific mechanisms underlying these phenomena must be further studied and verified.

ABBREVIATIONS

DMRTC2, PAX7, BRACHYURY/T and TERT Are Implicated in Male Germ Cell Development Following Curative Hormone Treatment for Cryptorchidism-Induced Infertility

Defective mini-puberty results in insufficient testosterone secretion that impairs the differentiation of gonocytes into dark-type (Ad) spermatogonia. The differentiation of gonocytes into Ad spermatogonia can be induced by administration of the gonadotropin-releasing hormone agonist, GnRHa (Buserelin, INN)). Nothing is known about the mechanism that underlies successful GnRHa treatment in the germ cells. Using RNA-sequencing of testicular biopsies, we recently examined RNA profiles of testes with and without GnRHa treatment. Here, we focused on the expression patterns of known gene markers for gonocytes and spermatogonia, and found that DMRTC2, PAX7, BRACHYURY/T, and TERT were associated with defective mini-puberty and were responsive to GnRHa. These results indicate novel testosterone-dependent genes and provide valuable insight into the transcriptional response to both defective mini-puberty and curative GnRHa treatment, which prevents infertility in man with one or both undescended (cryptorchid) testes.

Overactive type 2 cannabinoid receptor induces meiosis in fetal gonads and impairs ovarian reserve

Type 2 cannabinoid receptor (CB₂R) has been proposed to promote in vitro meiotic entry of postnatal male germ cells and to maintain the temporal progression of spermatogenesis in vivo. However, no information is presently available on the role played by CB₂R in male and female fetal gonads. Here we show that in vitro pharmacological stimulation with JWH133, a CB₂R agonist, induced activation of the meiotic program in both male and female fetal gonads. Upon stimulation, gonocytes initiated the meiotic program but became arrested at early stages of prophase I, while oocytes showed an increased rate of meiotic entry and progression toward more advanced stage of meiosis. Acceleration of meiosis in oocytes was accompanied by a strong increase in the percentage of γ-H2AX-positive pachytene and diplotene cells, paralleled by an increase of TUNEL-positive cells, suggesting that DNA double-strand breaks were not correctly repaired during meiosis, leading to oocyte apoptosis. Interestingly, in vivo pharmacological stimulation of CB₂R in fetal germ cells through JWH133 administration to pregnant females caused a significant reduction of primordial and primary follicles in the ovaries of newborns with a consequent depletion of ovarian reserve and reduced fertility in adult life, while no alterations of spermatogenesis in the testis of the offspring were detected. Altogether our findings highlight a pro-meiotic role of CB₂R in male and female germ cells and suggest that the use of cannabis in pregnant female might represent a risk for fertility and reproductive lifespan in female offspring.

Unraveling transcriptome dynamics in human spermatogenesis

Spermatogenesis is a dynamic developmental process that includes stem cell proliferation and differentiation, meiotic cell divisions and extreme chromatin condensation. Although studied in mice, the molecular control of human spermatogenesis is largely unknown. Here, we developed a protocol that enables next-generation sequencing of RNA obtained from pools of 500 individually laser-capture microdissected cells of specific germ cell subtypes from fixed human testis samples. Transcriptomic analyses of these successive germ cell subtypes reveals dynamic transcription of over 4000 genes during human spermatogenesis. At the same time, many of the genes encoding for well-established meiotic and post-meiotic proteins are already present in the pre-meiotic phase. Furthermore, we found significant cell type-specific expression of post-transcriptional regulators, including expression of 110 RNA-binding proteins and 137 long non-coding RNAs, most of them previously not linked to spermatogenesis. Together, these data suggest that the transcriptome of precursor cells already contains the genes necessary for cellular differentiation and that timely translation controlled by post-transcriptional regulators is crucial for normal development. These established transcriptomes provide a reference catalog for further detailed studies on human spermatogenesis and spermatogenic failure.

Highlighted Article: Using laser capture microscopy, a comprehensive transcriptomic dataset of well-defined and distinct germ cell subtypes based on morphology and localization in the human testis is generated.

Sex Determination in the Mammalian Germline

Sexual reproduction crucially depends on the production of sperm in males and oocytes in females. Both types of gamete arise from the same precursor, the germ cells. We review the events that characterize the development of germ cells during fetal life as they commit to, and prepare for, oogenesis or spermatogenesis. In females, fetal germ cells enter meiosis, whereas in males they delay meiosis and instead lose pluripotency, activate an irreversible program of prospermatogonial differentiation, and temporarily cease dividing. Both pathways involve sex-specific molecular signals from the somatic cells of the developing gonads and a suite of intrinsic receptors, signal transducers, transcription factors, RNA stability factors, and epigenetic modulators that act in complex, interconnected positive and negative regulatory networks. Understanding these networks is important in the contexts of the etiology, diagnosis, and treatment of infertility and gonadal cancers, and in efforts to augment human and animal fertility using stem cell approaches.

Regulation of fetal male germ cell development by members of the TGFβ superfamily

There is now substantial evidence that members of the transforming growth factor-β (TGFβ family) regulate germ cell development in the mouse fetal testis. Correct development of germ cells during fetal life is critical for establishment of effective spermatogenesis and for avoiding the formation of testicular germ cell cancer in later life. Here we consider the evidence for involvement of various TGFβ family members, attempt to reconcile discrepancies and clarify what we believe to be the likely in vivo roles of these factors.

SOX4 regulates gonad morphogenesis and promotes male germ cell differentiation in mice

The group C SOX transcription factors SOX4, -11 and -12 play important and mutually overlapping roles in development of a number of organs. Here, we examined the role of SoxC genes during gonadal development in mice. All three genes were expressed in developing gonads of both sexes, predominantly in somatic cells, with Sox4 being most strongly expressed. Sox4 deficiency resulted in elongation of both ovaries and testes, and an increased number of testis cords. While female germ cells entered meiosis normally, male germ cells showed reduced levels of differentiation markers Nanos2 and Dnmt3l and increased levels of pluripotency genes Cripto and Nanog, suggesting that SOX4 may normally act to restrict the pluripotency period of male germ cells and ensure their proper differentiation. Finally, our data reveal that SOX4 (and, to a lesser extent, SOX11 and -12) repressed transcription of the sex-determining gene Sox9 via an upstream testis-specific enhancer core (TESCO) element in fetal gonads, raising the possibility that SOXC proteins may function as transcriptional repressors in a context-dependent manner.

Translational Control of Germ Cell Decisions

Germline poses unique challenges to gene expression control at the transcriptional level. While the embryonic germline maintains a global hold on new mRNA transcription, the female adult germline produces transcripts that are not translated into proteins until embryogenesis of subsequent generation. As a consequence, translational control plays a central role in governing various germ cell decisions including the formation of primordial germ cells, self-renewal/differentiation decisions in the adult germline, onset of gametogenesis and oocyte maturation. Mechanistically, several common themes such as asymmetric localization of mRNAs, conserved RNA-binding proteins that control translation by 3' UTR binding, translational activation by the cytoplasmic elongation of the polyA tail and the assembly of mRNA-protein complexes called mRNPs have emerged from the studies on Caenorhabditis elegans, Xenopus and Drosophila. How mRNPs assemble, what influences their dynamics, and how a particular 3' UTR-binding protein turns on the translation of certain mRNAs while turning off other mRNAs at the same time and space are key challenges for future work.

Molecular Signatures of Primary Human Spermatogonial Progenitors and Its Neighboring Peritubular Stromal Compartment

In-depth understanding of human spermatogenesis requires studying specific molecular signatures and interactions of spermatogonia with other testicular cell populations, for which isolation of pure populations of different cell types is crucial. Here, we describe a technique to simultaneously enrich pure, multiple testicular cell populations, including spermatogonia, endothelial (TECs), and perivascular mesenchymal stem/stromal cells (TMSCs), from testicular tissue by flow cytometry using a combination of defined markers. Immunohistochemical studies, multicolor staining, and cell sorting followed by multiplex quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that spermatogonia were highly enriched in the CD49f⁺CD49a^-HLA-ABC^-SSEA-4⁺ fraction of primary testicular cells. In contrast to spermatogonia, TMSCs and TECs were highly enriched in the CD49f⁺CD49a⁺HLA-ABC⁺CD144^- and CD49f⁺CD49a⁺HLA-ABC⁺CD144⁺subsets, respectively. The delineation was confirmed by the expression of specific stromal and endothelial key markers as well as by the differentiation and angiogenic capacity of the sorted populations. In this article, for the first time, we performed transcriptome profiling of highly enriched, freshly isolated human spermatogonia and compared their expression profile with that of TMSCs. Our RNA sequencing data favor the hypothesis that TMSCs are candidate niche components for spermatogonia. The composite genotype and phenotype of defined testicular cell populations combined with a robust isolation procedure from small biopsies contributes to a better understanding of cellular interactions and for the establishment of efficient culture techniques to maintain spermatogonial progenitors.

FGF9, activin and TGFβ promote testicular characteristics in an XX gonad organ culture model

Testis development is dependent on the key sex-determining factors SRY and SOX9, which activate the essential ligand FGF9. Although FGF9 plays a central role in testis development, it is unable to induce testis formation on its own. However, other growth factors, including activins and TGFβs, also present testis during testis formation. In this study, we investigated the potential of FGF9 combined with activin and TGFβ to induce testis development in cultured XX gonads. Our data demonstrated differing individual and combined abilities of FGF9, activin and TGFβ to promote supporting cell proliferation, Sertoli cell development and male germ line differentiation in cultured XX gonads. FGF9 promoted proliferation of supporting cells in XX foetal gonads at rates similar to those observed in vivo during testis cord formation in XY gonads but was insufficient to initiate testis development. However, when FGF9, activin and TGFβ were combined, aspects of testicular development were induced, including the expression of Sox9, morphological reorganisation of the gonad and deposition of laminin around germ cells. Enhancing β-catenin activity diminished the testis-promoting activities of the combined growth factors. The male promoting activity of FGF9 and the combined growth factors directly or indirectly extended to the germ line, in which a mixed phenotype was observed. FGF9 and the combined growth factors promoted male germ line development, including mitotic arrest, but expression of pluripotency genes was maintained, rather than being repressed. Together, our data provide evidence that combined signalling by FGF9, activin and TGFβ can induce testicular characteristics in XX gonads.

Germline stem cells are critical for sexual fate decision of germ cells

Egg or sperm? The mechanism of sexual fate decision in germ cells has been a long‐standing issue in biology. A recent analysis identified foxl3 as a gene that determines the sexual fate decision of germ cells in the teleost fish, medaka. foxl3/Foxl3 acts in female germline stem cells to repress commitment into male fate (spermatogenesis), indicating that the presence of mitotic germ cells in the female is critical for continuous sexual fate decision of germ cells in medaka gonads. Interestingly, foxl3 is found in most vertebrate genomes except for mammals. This provides the interesting possibility that the sexual fate of germ cells in mammals is determined in a different way compared to foxl3‐possessing vertebrates. Considering the fact that germline stem cells are the cells where foxl3 begins to express and sexual fate decision initiates and mammalian ovary does not have typical germline stem cells, the mechanism in mammals may have been co‐evolved with germline stem cell loss in mammalian ovary.

Sexual Fate Change of XX Germ Cells Caused by the Deletion of SMAD4 and STRA8 Independent of Somatic Sex Reprogramming

The differential programming of sperm and eggs in gonads is a fundamental topic in reproductive biology. Although the sexual fate of germ cells is believed to be determined by signaling factors from sexually differentiated somatic cells in fetal gonads, the molecular mechanism that determines germ cell fate is poorly understood. Herein, we show that mothers against decapentaplegic homolog 4 (SMAD4) in germ cells is required for female-type differentiation. Germ cells in Smad4-deficient ovaries respond to retinoic acid signaling but fail to undergo meiotic prophase I, which coincides with the weaker expression of genes required for follicular formation, indicating that SMAD4 signaling is essential for oocyte differentiation and meiotic progression. Intriguingly, germline-specific deletion of Smad4 in Stra8-null female germ cells resulted in the up-regulation of genes required for male gonocyte differentiation, including Nanos2 and PLZF, suggesting the initiation of male-type differentiation in ovaries. Moreover, our transcriptome analyses of mutant ovaries revealed that the sex change phenotype is achieved without global gene expression changes in somatic cells. Our results demonstrate that SMAD4 and STRA8 are essential factors that regulate the female fate of germ cells.

Double ablation of SMAD4 and STRA8 causes female-to-male switching of XX germ cells without affecting somatic cell fate. This suggests that SMAD4 and STRA8 are essential intrinsic factors that determine the female fate of germ cells, collaborating to suppress expression of male genes.

Mammalian sex depends on a male-specific gene, sex-determining region Y (SRY), which is located on the Y chromosome. Individuals lacking this gene will develop as female. Accordingly, germ cell fate also changes from male to female in the absence of SRY. Therefore, it is thought that somatic cells regulate germ cells to become sperm or oocytes. However, it is largely unknown what factor is responsible for sexual fate determination in germ cells. In fetal ovaries, retinoic acid (RA) initiates STRA8 expression in germ cells and induces meiosis. Female germ cells without STRA8 fail to enter meiosis but still progress to oogenesis and form oocyte-like cells, indicating that RA is not the regulator of oogenesis. Here, we found that female germ cells lacking both SMAD4 and STRA8 (but not a single knockout) develop as male gonocyte-like cells in ovaries, indicating that these two factors work as female germ cell determinants. To our surprise, the sexual fate switch observed in the double knockout ovary is not accompanied by gene expression changes in somatic cells, revealing the unexpected finding that somatic factors controlled by SRY are dispensable for the upregulation of male-specific genes in germ cells.

Regulation of Translocator Protein 18 kDa (TSPO) Expression in Rat and Human Male Germ Cells

Translocator protein 18 kDa (TSPO) is a high affinity cholesterol- and drug-binding protein highly expressed in steroidogenic cells, such as Leydig cells, where it plays a role in cholesterol mitochondrial transport. We have previously shown that TSPO is expressed in postnatal day 3 rat gonocytes, precursors of spermatogonial stem cells. Gonocytes undergo regulated phases of proliferation and migration, followed by retinoic acid (RA)-induced differentiation. Understanding these processes is important since their disruption may lead to the formation of carcinoma in situ, a precursor of testicular germ cell tumors (TGCTs). Previously, we showed that TSPO ligands do not regulate gonocyte proliferation. In the present study, we found that TSPO expression is downregulated in differentiating gonocytes. Similarly, in F9 embryonal carcinoma cells, a mouse TGCT cell line with embryonic stem cell properties, there is a significant decrease in TSPO expression during RA-induced differentiation. Silencing TSPO expression in gonocytes increased the stimulatory effect of RA on the expression of the differentiation marker Stra8, suggesting that TSPO exerts a repressive role on differentiation. Furthermore, in normal human testes, TSPO was located not only in Leydig cells, but also in discrete spermatogenic phases such as the forming acrosome of round spermatids. By contrast, seminomas, the most common type of TGCT, presented high levels of TSPO mRNA. TSPO protein was expressed in the cytoplasmic compartment of seminoma cells, identified by their nuclear expression of the transcription factors OCT4 and AP2G. Thus, TSPO appears to be tightly regulated during germ cell differentiation, and to be deregulated in seminomas, suggesting a role in germ cell development and pathology.