Spatial and temporal expression analysis of BMP signal modifiers, Smoc1 and Smoc2, from postnatal to adult developmental stages in the mouse testis

Smoc1 and Smoc2, members of the SPARC family of genes, encode signaling molecules downstream of growth factors such as the TGF-β, FGF, and PDGF families. Smoc1 has been implicated in playing a crucial role in microphthalmia with limb anomalies in humans and mice, while Smoc2 deficiency causes dental developmental defects. Although developmental cytokines/growth factors including TGF-β superfamily have been shown to play critical roles in postnatal spermatogenesis, there are no reports analyzing the spatial and temporal expression of Smoc1 and Smoc2 in the postnatal testis. In this study, we investigated the mRNA and protein expression of Smoc1 and Smoc2 in neonatal, juvenile, and adult mouse testes by RNA in situ hybridization, immunofluorescence, and single-cell RNA-seq analysis. We show that Smoc1 and Smoc2 have distinct expression patterns in male germ cells: Smoc1 is more highly expressed than Smoc2 in the germline. In contrast, Smoc2 is highly expressed in testicular somatic cells from neonatal to juvenile stages. The Smoc2-expressing cells then switch from somatic cells to germ cells in adults. Thus, although SMOC1 and SMOC2 proteins are structurally very similar, their spatial and temporal expression patterns in the postnatal testis differ significantly, suggesting their distinct roles in reproduction.

Comprehensive profiling of migratory primordial germ cells reveals niche-specific differences in non-canonical Wnt and Nodal-Lefty signaling in anterior vs posterior migrants

Mammalian primordial germ cells (PGCs) migrate asynchronously through the embryonic hindgut and dorsal mesentery to reach the gonads. We previously found that interaction with different somatic niches regulates PGC proliferation along the migration route. To characterize transcriptional heterogeneity of migrating PGCs and their niches, we performed single-cell RNA sequencing of 13,262 mouse PGCs and 7,868 surrounding somatic cells during migration (E9.5, E10.5, E11.5) and in anterior versus posterior locations to enrich for leading and lagging migrants. Analysis of PGCs by position revealed dynamic gene expression changes between faster or earlier migrants in the anterior and slower or later migrants in the posterior at E9.5; these differences include migration-associated actin polymerization machinery and epigenetic reprogramming-associated genes. We furthermore identified changes in signaling with various somatic niches, notably strengthened interactions with hindgut epithelium via non-canonical WNT (ncWNT) in posterior PGCs compared to anterior. Reanalysis of a previously published dataset suggests that ncWNT signaling from the hindgut epithelium to early migratory PGCs is conserved in humans. Trajectory inference methods identified putative differentiation trajectories linking cell states across timepoints and from posterior to anterior in our mouse dataset. At E9.5, we mainly observed differences in cell adhesion and actin cytoskeletal dynamics between E9.5 posterior and anterior migrants. At E10.5, we observed divergent gene expression patterns between putative differentiation trajectories from posterior to anterior including Nodal signaling response genes Lefty1, Lefty2, and Pycr2 and reprogramming factors Dnmt1, Prc1, and Tet1. At E10.5, we experimentally validated anterior migrant-specific Lefty1/2 upregulation via whole-mount immunofluorescence staining for LEFTY1/2 proteins, suggesting that elevated autocrine Nodal signaling accompanies the late stages of PGC migration. Together, this positional and temporal atlas of mouse PGCs supports the idea that niche interactions along the migratory route elicit changes in proliferation, actin dynamics, pluripotency, and epigenetic reprogramming.

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.

The developmental dynamics of the human male germline

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

The RNA-binding protein Adad1 is necessary for germ cell maintenance and meiosis in zebrafish

The double stranded RNA binding protein Adad1 (adenosine deaminase domain containing 1) is a member of the adenosine deaminase acting on RNAs (Adar) protein family with germ cell-specific expression. In mice, Adad1 is necessary for sperm differentiation, however its function outside of mammals has not been investigated. Here, through an N-ethyl-N-nitrosourea (ENU) based forward genetic screen, we identified an adad1 mutant zebrafish line that develops as sterile males. Further histological examination revealed complete lack of germ cells in adult mutant fish, however germ cells populated the gonad, proliferated, and entered meiosis in larval and juvenile fish. Although meiosis was initiated in adad1 mutant testes, the spermatocytes failed to progress beyond the zygotene stage. Thus, Adad1 is essential for meiosis and germline maintenance in zebrafish. We tested if spermatogonial stem cells were affected using nanos2 RNA FISH and a label retaining cell (LRC) assay, and found that the mutant testes had fewer LRCs and nanos2-expressing cells compared to wild-type siblings, suggesting that failure to maintain the spermatogonial stem cells resulted in germ cell loss by adulthood. To identify potential molecular processes regulated by Adad1, we sequenced bulk mRNA from mutants and wild-type testes and found mis-regulation of genes involved in RNA stability and modification, pointing to a potential broader role in post-transcriptional regulation. Our findings suggest that the RNA regulatory protein Adad1 is required for fertility through regulation of spermatogonial stem cell maintenance in zebrafish.

Testicular Germ Cell Tumor Tissue Biomarker Analysis: A Comparison of Human Protein Atlas and Individual Testicular Germ Cell Tumor Component Immunohistochemistry

The accurate management of testicular germ cell tumors (TGCTs) depends on identifying the individual histological tumor components. Currently available data on protein expression in TGCTs are limited. The human protein atlas (HPA) is a comprehensive resource presenting the expression and localization of proteins across tissue types and diseases. In this study, we have compared the data from the HPA with our in-house immunohistochemistry on core TGCT diagnostic genes to test reliability and potential biomarker genes. We have compared the protein expression of 15 genes in TGCT patients and non-neoplastic testicles with the data from the HPA. Protein expression was converted into diagnostic positivity. Our study discovered discrepancies in three of the six core TGCT diagnostic genes, POU5F1, KIT and SOX17 in HPA. DPPA3, CALCA and TDGF1 were presented as potential novel TGCT biomarkers. MGMT was confirmed while RASSF1 and PRSS21 were identified as biomarkers of healthy testicular tissue. Finally, SALL4, SOX17, RASSF1 and PRSS21 dysregulation in the surrounding testicular tissue with complete preserved spermatogenesis of TGCT patients was detected, a potential early sign of neoplastic transformation. We highlight the importance of a multidisciplinary collaborative approach to fully understand the protein landscape of human testis and its pathologies.

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.

Generation and characterization of a Ddx4-iCre transgenic line for deletion in the germline beginning at genital ridge colonization

Germline-specific Cre lines are useful for analyses of primordial germ cell, spermatogonial and oogonial development, but also for whole-body deletions when transmitted through subsequent generations. Several germ cell specific Cre mouse strains exist, with various degrees of specificity, efficiency, and temporal activation. Here, we describe the CRISPR/Cas9 targeted insertion of an improved Cre (iCre) sequence in-frame at the 3' end of the Ddx4 locus to generate the Ddx4-P2A-iCre allele. Our functional assessment of this new allele, designated Ddx4^iCreJoBo , reveals that Cre activity begins in PGCs from at least E10.5, and that it achieves higher efficiency for early gonadal (E10.5-12.5) germline deletion when compared to the inducible Oct4^CreERT2 line. We found the Ddx4^iCreJoBo allele to be hypomorphic for Ddx4 expression and homozygous males, but not females, were infertile. Using two reporter lines (R26R^LacZ and R26R^tdTomato ) and a floxed gene of interest (Cripto^flox ) we found ectopic activity in multiple organs; global recombination (a common feature of germline Cre alleles) varies from 10 to 100%, depending on the particular floxed allele. There is a strong maternal effect, and therefore it is preferable for Ddx4^iCreJoBo to be inherited from the male parent if ubiquitous deletion is not desired. With these limitations considered, we describe the Ddx4^iCreJoBo line as useful for germline studies in which early gonadal deletion is required.

Germ Cell Maintenance and Sustained Testosterone and Precursor Hormone Production in Human Prepubertal Testis Organ Culture with Tissues from Boys 7 Years+ under Conditions from Adult Testicular Tissue

Human prepubertal testicular tissues are rare, but organ culture conditions to develop a system for human in vitro-spermatogenesis are an essential option for fertility preservation in prepubertal boys subjected to gonadotoxic therapy. To avoid animal testing in line with the 3Rs principle, organ culture conditions initially tested on human adult testis tissue were applied to prepubertal samples (n = 3; patient ages 7, 9, and 12 years). Tissues were investigated by immunostaining and transmission electron microscopy (TEM), and the collected culture medium was profiled for steroid hormones by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Culture conditions proved suitable for prepubertal organ culture since SSCs and germ cell proliferation could be maintained until the end of the 3-week-culture. Leydig cells (LCs) were shown to be competent for steroid hormone production. Three additional testis tissues from boys of the same age were examined for the number of germ cells and undifferentiated spermatogonia (SPG). Using TEM micrographs, eight tissues from patients aged 1.5 to 13 years were examined, with respect to the sizes of mitochondria (MT) in undifferentiated SPG and compared with those from two adult testicular tissues. Mitochondrial sizes were shown to be comparable between adults and prepubertal boys from approximately 7 years of age, which suggests the transition of SSCs from normoxic to hypoxic metabolism at about or before this time period.

Deciphering Sex-Specific Differentiation of Human Fetal Gonads: Insight From Experimental Models

Sex-specific gonadal differentiation is initiated by the expression of SRY in male foetuses. This promotes a signalling pathway directing testicular development, while in female foetuses the absence of SRY and expression of pro-ovarian factors promote ovarian development. Importantly, in addition to the initiation of a sex-specific signalling cascade the opposite pathway is simultaneously inhibited. The somatic cell populations within the gonads dictates this differentiation as well as the development of secondary sex characteristics via secretion of endocrine factors and steroid hormones. Opposing pathways SOX9/FGF9 (testis) and WNT4/RSPO1 (ovary) controls the development and differentiation of the bipotential mouse gonad and even though sex-specific gonadal differentiation is largely considered to be conserved between mice and humans, recent studies have identified several differences. Hence, the signalling pathways promoting early mouse gonad differentiation cannot be directly transferred to human development thus highlighting the importance of also examining this signalling in human fetal gonads. This review focus on the current understanding of regulatory mechanisms governing human gonadal sex differentiation by combining knowledge of these processes from studies in mice, information from patients with differences of sex development and insight from manipulation of selected signalling pathways in ex vivo culture models of human fetal gonads.

Identification and functional analysis of foxl2 and nodal in sea cucumber, Apostichopus japonicus

Sea cucumber (Apostichopus japonicus) is an important mariculture species in China. To date, the mechanisms of sex determination and differentiation in sea cucumber remain unclear. Identifying sex-specific molecular markers is an effective method for revealing the genetic basis of sex determination and sex differentiation. In this study, foxl2 and nodal homologous genes were identified in A. japonicus. Foxl2 exhibited dynamic and sexually dimorphic expression patterns in the gonads, with prominent expression in the ovaries and minimal expression in the testis according to real-time quantitative PCR (RT-qPCR) study. As nodal was specifically expressed in the ovary, it could serve as an ovary-specific marker in sea cucumber. Additionally, knockdown of foxl2 or nodal using RNA interference (RNAi) led to the down-regulation of piwi, germ cell-less, and dmrt1, suggesting that foxl2 and nodal may play important roles in gonad maintenance of sea cucumber. Overall, this study adds to our understanding of the roles of foxl2 and nodal in the gonadal development of A. japonicus, which provides further insight into the mechanisms of sea cucumber sex determination and differentiation.

The Impact of Activin A on Fetal Gonocytes: Chronic Versus Acute Exposure Outcomes

Activin A, a TGFβ superfamily member, is important for normal testis development through its actions on Sertoli cell development. Our analyses of altered activin A mouse models indicated gonocyte abnormalities, implicating activin A as a key determinant of early germline formation. Whether it acts directly or indirectly on germ cells is not understood. In humans, the fetal testis may be exposed to abnormally elevated activin A levels during preeclampsia, maternal infections, or following ingestion of certain medications. We hypothesized that this may impact fetal testis development and ultimately affect adult fertility. Germ cells from two mouse models of altered activin bioactivity were analysed. RNA-Seq of gonocytes purified from E13.5 and E15.5 Inhba KO mice (activin A subunit knockout) identified 46 and 44 differentially expressed genes (DEGs) respectively, and 45 in the E13.5 Inha KO (inhibin alpha subunit knockout; increased activin A) gonocytes. To discern direct effects of altered activin bioactivity on germline transcripts, isolated E13.5 gonocytes were cultured for 24h with activin A or with the activin/Nodal/TGFβ inhibitor, SB431542. Gonocytes responded directly to altered signalling, with activin A promoting a more differentiated transcript profile (increased differentiation markers Dnmt3l, Nanos2 and Piwil4; decreased early germ cell markers Kit and Tdgf1), while SB431542 had a reciprocal effect (decreased Nanos2 and Piwil4; increased Kit). To delineate direct and indirect effects of activin A exposure on gonocytes, whole testes were cultured 48h with activin A or SB431542 and collected for histological and transcript analyses, or EdU added at the end of culture to measure germ and Sertoli cell proliferation using flow cytometry. Activin increased, and SB431542 decreased, Sertoli cell proliferation. SB431542-exposure resulted in germ cells escaping mitotic arrest. Analysis of FACS-isolated gonocytes following whole testis culture showed SB431542 increased the early germ cell marker Kit, however there was a general reduction in the impact of altered activin A bioavailability in the normal somatic cell environment. This multifaceted approach identifies a capacity for activin A to directly influence fetal germ cell development, highlighting the potential for altered activin A levels in utero to increase the risk of testicular pathologies that arise from impaired germline maturation.

Single-cell transcriptomics reveal temporal dynamics of critical regulators of germ cell fate during mouse sex determination

Despite the importance of germ cell (GC) differentiation for sexual reproduction, the gene networks underlying their fate remain unclear. Here, we comprehensively characterize the gene expression dynamics during sex determination based on single-cell RNA sequencing of 14 914 XX and XY mouse GCs between embryonic days (E) 9.0 and 16.5. We found that XX and XY GCs diverge transcriptionally as early as E11.5 with upregulation of genes downstream of the bone morphogenic protein (BMP) and nodal/Activin pathways in XY and XX GCs, respectively. We also identified a sex-specific upregulation of genes associated with negative regulation of mRNA processing and an increase in intron retention consistent with a reduction in mRNA splicing in XY testicular GCs by E13.5. Using computational gene regulation network inference analysis, we identified sex-specific, sequential waves of putative key regulator genes during GC differentiation and revealed that the meiotic genes are regulated by positive and negative master modules acting in an antagonistic fashion. Finally, we found that rare adrenal GCs enter meiosis similarly to ovarian GCs but display altered expression of master genes controlling the female and male genetic programs, indicating that the somatic environment is important for GC function. Our data are available on a web platform and provide a molecular roadmap of GC sex determination at single-cell resolution, which will serve as a valuable resource for future studies of gonad development, function, and disease.

Germ cell determination and the developmental origin of germ cell tumors

In each generation, the germline is tasked with producing somatic lineages that form the body, and segregating a population of cells for gametogenesis. During animal development, when do cells of the germline irreversibly commit to producing gametes? Integrating findings from diverse species, we conclude that the final commitment of the germline to gametogenesis - the process of germ cell determination - occurs after primordial germ cells (PGCs) colonize the gonads. Combining this understanding with medical findings, we present a model whereby germ cell tumors arise from cells that failed to undertake germ cell determination, regardless of their having colonized the gonads. We propose that the diversity of cell types present in these tumors reflects the broad developmental potential of migratory PGCs.

Potential biomarkers for testicular germ cell tumour: Risk assessment, diagnostic, prognostic and monitoring of recurrence

Testicular germ cell tumour (TGCT) is considered a relatively rare malignancy usually occurring in young men between 15 and 35 years of age, and both genetic and environmental factors contribute to its development. The majority of patients are diagnosed in an early-stage of TGCTs with an elevated 5-year survival rate after therapy. However, approximately 25% of patients show an incomplete response to chemotherapy or tumours relapse. The current therapies are accompanied by several adverse effects, including infertility. Aside from classical serum biomarker, many studies reported novel biomarkers for TGCTs, but without proper validation. Cancer cells share many similarities with embryonic stem cells (ESCs), and since ESC genes are not transcribed in most adult tissues, they could be considered ideal candidate targets for cancer-specific diagnosis and treatment. Added to this, several microRNAs (miRNA) including miRNA-371-3p can be further investigated as a molecular biomarker for diagnosis and monitoring of TGCTs. In this review, we will illustrate the findings of recent investigations in novel TGCTs biomarkers applicable for risk assessment, screening, diagnosis, prognosis, prediction and monitoring of the relapse.

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.

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.

Molecular and epigenetic pathogenesis of germ cell tumors

The development of germ cell tumors (GCTs) is a unique pathogenesis occurring at an early developmental stage during specification, migration or colonization of primordial germ cells (PGCs) in the genital ridge. Since driver mutations could not be identified so far, the involvement of the epigenetic machinery during the pathogenesis seems to play a crucial role. Currently, it is investigated whether epigenetic modifications occurring between the omnipotent two-cell stage and the pluripotent implanting PGCs might result in disturbances eventually leading to GCTs. Although progress in understanding epigenetic mechanisms during PGC development is ongoing, little is known about the complete picture of its involvement during GCT development and eventual classification into clinical subtypes. This review will shed light into the current knowledge of the complex epigenetic and molecular contribution during pathogenesis of GCTs by emphasizing on early developmental stages until arrival of late PGCs in the gonads. We questioned how misguided migrating and/or colonizing PGCs develop to either type I or type II GCTs. Additionally, we asked how pluripotency can be regulated during PGC development and which epigenetic changes contribute to GCT pathogenesis. We propose that SOX2 and SOX17 determine either embryonic stem cell-like (embryonal carcinoma) or PGC-like cell fate (seminoma). Finally, we suggest that factors secreted by the microenvironment, i.e. BMPs and BMP inhibiting molecules, dictate the fate decision of germ cell neoplasia in situ (into seminoma and embryonal carcinoma) and seminomas (into embryonal carcinoma or extraembryonic lineage), indicating an important role of the microenvironment on GCT plasticity.

Influence of Nodal signalling on pluripotency factor expression, tumour cell proliferation and cisplatin-sensitivity in testicular germ cell tumours

BACKGROUND

Testicular germ cell tumours (TGCTs) are characterised by an overall high cisplatin-sensitivity which has been linked to their continued expression of pluripotency factors. Recently, the Nodal signalling pathway has been implicated in the regulation of pluripotency factor expression in fetal germ cells, and the pathway could therefore also be involved in regulating expression of pluripotency factors in malignant germ cells, and hence cisplatin-sensitivity in TGCTs.

METHODS

We used in vitro culture of the TGCT-derived cell line NTera2, ex vivo tissue culture of primary TGCT specimens and xenografting of NTera2 cells into nude mice in order to investigate the consequences of manipulating Nodal and Activin signalling on pluripotency factor expression, apoptosis, proliferation and cisplatin-sensitivity.

RESULTS

The Nodal signalling factors were markedly expressed concomitantly with the pluripotency factor OCT4 in GCNIS cells, seminomas and embryonal carcinomas. Despite this, inhibition of Nodal and Activin signalling either alone or simultaneously did not affect proliferation or apoptosis in malignant germ cells in vitro or ex vivo. Interestingly, inhibition of Nodal signalling in vitro reduced the expression of pluripotency factors and Nodal pathway genes, while stimulation of the pathway increased their expression. However, cisplatin-sensitivity was not affected following pharmacological inhibition of Nodal/Activin signalling or siRNA-mediated knockdown of the obligate co-receptor CRIPTO in NTera2 cells in vitro or in a xenograft model.

CONCLUSION

Our findings suggest that the Nodal signalling pathway may be involved in regulating pluripotency factor expression in malignant germ cells, but manipulation of the pathway does not appear to affect cisplatin-sensitivity or tumour cell proliferation.

CRIPTO and miR-371a-3p Are Serum Biomarkers of Testicular Germ Cell Tumors and Are Detected in Seminal Plasma from Azoospermic Males

miR-371a-3p is currently the most informative reported biomarker for germ cell tumors (GCTs). Another developmental-related biomarker, CRIPTO, is involved in the regulation of pluripotency and germ cell fate commitment. We aimed to assess the value of CRIPTO as a diagnostic and prognostic biomarker of testicular GCTs (TGCTs) and also to assess its presence in seminal plasma samples, compared with miR-371a-3p. In total, 217 and 94 serum/seminal plasma samples were analyzed. CRIPTO was quantified using ELISA and miR-371a-3p using bead-based isolation followed by RT-qPCR. Methylation profiling (EPIC array) for the CRIPTO promoter region was undertaken in 35 TGCT tissues plus four (T)GCT cell lines. Significantly higher CRIPTO concentration was found in sera of non-seminomas compared to controls (p = 0.0297), and in stage II/III disease compared to stage I (p = 0.0052, p = 0.0097). CRIPTO concentration was significantly positively correlated with miR-371a-3p levels in serum (r = 0.16) and seminal plasma (r = 0.40). CRIPTO/miR-371a-3p levels were significantly higher in seminal plasma controls when compared to serum controls (p = 0.0001, p < 0.0001). CRIPTO/miR-371a-3p were detected both in normospermic and azoospermic males, and levels were higher in TGCTs compared to GCNIS-only. We have provided the largest dataset of evaluation of CRIPTO in serum and seminal plasma of GCTs, showing its potential value as a biomarker of the disease.

Dysregulation of lncRNA and circRNA Expression in Mouse Testes after Exposure to Triptolide

BACKGROUND

Triptolide has been shown to exert various pharmacological effects on systemic autoimmune diseases and cancers. However, its severe toxicity, especially reproductive toxicity, prevents its widespread clinical use for people with fertility needs. Noncoding RNAs including lncRNAs and circRNAs are novel regulatory molecules that mediate a wide variety of physiological activities; they are crucial for spermatogenesis and their dysregulation might cause male infertility. However, whether they are involved in triptolide-induced reproductive toxicity is completely unknown.

METHODS

After exposure of mice to triptolide, the total RNAs were used to investigate lncRNA/circRNA/mRNA expression profiles by strand-specific RNA sequencing at the transcriptome level to help uncover RNA-related mechanisms in triptolide-induced toxicity.

RESULTS

Triptolide significantly decreased testicular weight, damaged testis and sperm morphology, and reduced sperm motility and density. Remarkable deformities in sperm head and tail were also found in triptolide-exposed mice. At the transcriptome level, the triptolide-treated mice exhibited aberrant expression profiles of lncRNAs/circRNAs/mRNAs. Gene Ontology and pathway analyses revealed that the functions of the differentially expressed lncRNA targets, circRNA cognate genes, and mRNAs were closely linked to many processes involved in spermatogenesis. In addition, some lncRNAs/circRNAs were greatly upregulated or inducibly expressed, implying their potential value as candidate markers for triptolide-induced male reproductive toxicity.

CONCLUSION

This study provides a preliminary database of triptolide-induced transcriptome, promotes understanding of the reproductive toxicity of triptolide, and highlights the need for research on increasing the medical efficacy of triptolide and decreasing its toxicity.

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.

Targeting Nodal and Cripto-1: Perspectives Inside Dual Potential Theranostic Cancer Biomarkers

BACKGROUND

Elucidating the mechanisms of recurrence of embryonic signaling pathways in tumorigenesis has led to the discovery of onco-fetal players which have physiological roles during normal development but result aberrantly re-activated in tumors. In this context, Nodal and Cripto-1 are recognized as onco-developmental factors, which are absent in normal tissues but are overexpressed in several solid tumors where they can serve as theranostic agents.

OBJECTIVE

To collect, review and discuss the most relevant papers related to the involvement of Nodal and Cripto-1 in the development, progression, recurrence and metastasis of several tumors where they are over-expressed, with a particular attention to their occurrence on the surface of the corresponding sub-populations of cancer stem cells (CSC).

RESULTS

We have gathered, rationalized and discussed the most interesting findings extracted from some 370 papers related to the involvement of Cripto-1 and Nodal in all tumor types where they have been detected. Data demonstrate the clear connection between Nodal and Cripto-1 presence and their multiple oncogenic activities across different tumors. We have also reviewed and highlighted the potential of targeting Nodal, Cripto-1 and the complexes that they form on the surface of tumor cells, especially of CSC, as an innovative approach to detect and suppress tumors with molecules that block one or more mechanisms that they regulate.

CONCLUSION

Overall, Nodal and Cripto-1 represent two innovative and effective biomarkers for developing potential theranostic anti-tumor agents that target normal as well as CSC subpopulations and overcome both pharmacological resistance and tumor relapse.

An evaluation of different Cripto-1 antibodies and their variable results

Cripto-1 is a protein expressed during embryonal development and has been linked to several malignant processes in cancer. Since the discovery of cripto-1 in the late 1980s, it has become a subject of biomarker investigation in several types of cancer which in many cases relies on immunolocalization of cripto-1 using antibodies. Investigating cripto-1 expression and localization in primary glioblastoma cells, we discovered nonspecific binding of cripto-1 antibody to the extracellular matrix Geltrex. A panel of four cripto-1 antibodies was investigated with respect to their binding to the Geltrex matrix and to the cripto-1 positive control cells NTERA2. The cripto-1 expression was varied for the different antibodies with respect to cellular localization and fixation methods. To further elaborate on these findings, we present a systematic review of cripto-1 antibodies found in the literature and highlight some possible cross reactants with data on sequence alignments and structural comparison of EGF domains.

Testicular germ cell tumor: a comprehensive review

Testicular tumors are the most common tumors in adolescent and young men and germ cell tumors (TGCTs) account for most of all testicular cancers. Increasing incidence of TGCTs among males provides strong motivation to understand its biological and genetic basis. Gains of chromosome arm 12p and aneuploidy are nearly universal in TGCTs, but TGCTs have low point mutation rate. It is thought that TGCTs develop from premalignant intratubular germ cell neoplasia that is believed to arise from the failure of normal maturation of gonocytes during fetal or postnatal development. Progression toward invasive TGCTs (seminoma and nonseminoma) then occurs after puberty. Both inherited genetic factors and environmental risk factors emerge as important contributors to TGCT susceptibility. Genome-wide association studies have so far identified more than 30 risk loci for TGCTs, suggesting that a polygenic model fits better with the genetic landscape of the disease. Despite high cure rates because of its particular sensitivity to platinum-based chemotherapy, exploration of mechanisms underlying the occurrence, progression, metastasis, recurrence, chemotherapeutic resistance, early diagnosis and optional clinical therapeutics without long-term side effects are urgently needed to reduce the cancer burden in this underserved age group. Herein, we present an up-to-date review on clinical challenges, origin and progression, risk factors, TGCT mouse models, serum diagnostic markers, resistance mechanisms, miRNA regulation, and database resources of TGCTs. We appeal that more attention should be paid to the basic research and clinical diagnosis and treatment of TGCTs.

Perspectives of small molecule inhibitors of activin receptor‑like kinase in anti‑tumor treatment and stem cell differentiation (Review)

Activin receptor‑like kinases (ALKs), members of the type I activin receptor family, belong to the serine/threonine kinase receptors of the transforming growth factor‑β (TGF‑β) superfamily. ALKs mediate the roles of activin/TGF‑β in a wide variety of physiological and pathological processes, ranging from cell differentiation and proliferation to apoptosis. For example, the activities of ALKs are associated with an advanced tumor stage in prostate cancer and the chondrogenic differentiation of mesenchymal stem cells. Therefore, potent and selective small molecule inhibitors of ALKs would not only aid in investigating the function of activin/TGF‑β, but also in developing treatments for these diseases via the disruption of activin/TGF‑β. In recent studies, several ALK inhibitors, including LY‑2157299, SB‑431542 and A‑83‑01, have been identified and have been confirmed to affect stem cell differentiation and tumor progression in animal models. This review discusses the therapeutic perspective of small molecule inhibitors of ALKs as drug targets in tumor and stem cells.

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.

Transcriptomic analysis of mRNA expression and alternative splicing during mouse sex determination

Mammalian sex determination hinges on sexually dimorphic transcriptional programs in developing fetal gonads. A comprehensive view of these programs is crucial for understanding the normal development of fetal testes and ovaries and the etiology of human disorders of sex development (DSDs), many of which remain unexplained. Using strand-specific RNA-sequencing, we characterized the mouse fetal gonadal transcriptome from 10.5 to 13.5 days post coitum, a key time window in sex determination and gonad development. Our dataset benefits from a greater sensitivity, accuracy and dynamic range compared to microarray studies, allows global dynamics and sex-specificity of gene expression to be assessed, and provides a window to non-transcriptional events such as alternative splicing. Spliceomic analysis uncovered female-specific regulation of Lef1 splicing, which may contribute to the enhanced WNT signaling activity in XX gonads. We provide a user-friendly visualization tool for the complete transcriptomic and spliceomic dataset as a resource for the field.

Nodal Signaling Regulates Germ Cell Development and Establishment of Seminiferous Cords in the Human Fetal Testis

Disruption of human fetal testis development is widely accepted to underlie testicular germ cell cancer (TGCC) origin and additional disorders within testicular dysgenesis syndrome (TDS). However, the mechanisms for the development of testicular dysgenesis in humans are unclear. We used ex vivo culture and xenograft approaches to investigate the importance of Nodal and Activin signaling in human fetal testis development. Inhibition of Nodal, and to some extent Activin, signaling disrupted seminiferous cord formation, abolished AMH expression, reduced androgen secretion, and decreased gonocyte numbers. Subsequent xenografting of testicular tissue rescued the disruptive effects on seminiferous cords and somatic cells but not germ cell effects. Stimulation of Nodal signaling increased the number of germ cells expressing pluripotency factors, and these persisted after xenografting. Our findings suggest a key role for Nodal signaling in the regulation of gonocyte differentiation and early human testis development with implications for the understanding of TGCC and TDS origin.

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.

Delayed male germ cell sex-specification permits transition into embryonal carcinoma cells with features of primed pluripotency

Testicular teratomas result from anomalies in embryonic germ cell development. In 129 inbred mice, teratoma initiation coincides with germ cell sex-specific differentiation and the mitotic-meiotic switch: XX and XY germ cells repress pluripotency, XX germ cells initiate meiosis, and XY germ cells activate male-specific differentiation and mitotic arrest. Here, we report that expression of Nanos2, a gene that is crucial to male sex specification, is delayed in teratoma-susceptible germ cells. Decreased expression of Nanos2 was found to be due, in part, to the Nanos2 allele present in 129 mice. In teratoma-susceptible germ cells, diminished expression of genes downstream of Nanos2 disrupted processes that were crucial to male germ cell differentiation. Deficiency for Nanos2 increased teratoma incidence in 129 mice and induced developmental abnormalities associated with tumor initiation in teratoma-resistant germ cells. Finally, in the absence of commitment to the male germ cell fate, we discovered that a subpopulation of teratoma-susceptible germ cells transition into embryonal carcinoma (EC) cells with primed pluripotent features. We conclude that delayed male germ cell sex-specification facilitates the transformation of germ cells with naïve pluripotent features into primed pluripotent EC cells.

Analysis of Endocannabinoid System in Rat Testis During the First Spermatogenetic Wave

Endocannabinoids are lipid mediators, enzymatically synthesized and hydrolyzed, that bind cannabinoid receptors. Together with their receptors and metabolic enzymes, they form the "endocannabinoid system" (ECS). Anandamide (AEA) and 2-arachidonoylglycerol (2-AG) are the main endocannabinoids studied in testis. In this study, using the first wave of spermatogenesis as an in vivo model to verify the progressive appearance of germ cells in seminiferous tubules [i.e., spermatogonia, spermatocytes, and spermatids], we analyzed the expression of the main enzymes and receptors of ECS in rat testis. In particular, the expression profile of the main enzymes metabolizing AEA and 2-AG as well as the expression of cannabinoid receptors, such as CB1 and CB2, and specific markers of mitotic, meiotic, and post-meiotic germ cell appearance or activities have been analyzed by RT-PCR and appropriately correlated. Our aim was to envisage a relationship between expression of ECS components and temporal profile of germ cell appearance or activity as well as among ECS components. Results show that expression of ECS components is related to germ cell progression. In particular, CB2 and 2-AG appear to be related to mitotic/meiotic stages, while CB1 and AEA appear to be related to spermatogonia stem cells activity and spermatids appearance, respectively. Our data also suggest that a functional interaction among ECS components occurs in the testis. Indeed, in vitro-incubated testis show that AEA-CB2 activity affects negatively monoacylglycerol-lipase levels via upregulation of CB1 suggesting a CB1/CB2-mediated relationship between AEA and 2-AG. Finally, we provide the first evidence that CB1 is present in fetal gonocytes, during mitotic arrest.

SOX30 is required for male fertility in mice

Male infertility is a major and growing problem and, in most cases, the specific root cause is unknown. Here we show that the transcription factor SOX30 plays a critical role in mouse spermatogenesis. Sox30-null mice are healthy and females are fertile, but males are sterile. In the absence of Sox30 meiosis initiates normally in both sexes but, in males, germ cell development arrests during the post-meiotic round spermatid period. In the mutant testis, acrosome and axoneme development are aberrant, multinucleated germ cells (symplasts) form and round spermatids unable to process beyond step 3 of spermiogenesis. No elongated spermatids nor spermatozoa are produced. Thus, Sox30 represents a rare example of a gene for which loss of function results in a complete arrest of spermatogenesis at the onset of spermiogenesis. Our results suggest that SOX30 mutations may underlie some instances of unexplained non-obstructive azoospermia in humans.

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.

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.

DMRT proteins and coordination of mammalian spermatogenesis

DMRT genes encode a deeply conserved family of transcription factors that share a unique DNA binding motif, the DM domain. DMRTs regulate development in a broad variety of metazoans and they appear to have controlled sexual differentiation for hundreds of millions of years. In mice, starting during embryonic development, three Dmrt genes act sequentially to help establish and maintain spermatogenesis. Dmrt1 has notably diverse functions that include repressing pluripotency genes and promoting mitotic arrest in embryonic germ cells, reactivating prospermatogonia perinatally, establishing and maintaining spermatogonial stem cells (SSCs), promoting spermatogonial differentiation, and controlling the mitosis/meiosis switch. Dmrt6 acts in differentiating spermatogonia to coordinate an orderly exit from the mitotic/spermatogonial program and allow proper timing of entry to the meiotic/spermatocyte program. Finally, Dmrt7 takes over during the first meiotic prophase to help choreograph a transition in histone modifications that maintains transcriptional silencing of the sex chromosomes. The combined action of these three Dmrt genes helps ensure robust and sustainable spermatogenesis.

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.

Involvement of the DNA mismatch repair system in cisplatin sensitivity of testicular germ cell tumours

BACKGROUND

Testicular germ cell tumours (TGCT) are highly sensitive to cisplatin-based chemotherapy, but patients with tumours containing differentiated teratoma components are less responsive to this treatment. The cisplatin sensitivity in TGCT has previously been linked to the embryonic phenotype in the majority of tumours, although the underlying mechanism largely remains to be elucidated. The aim of this study was to investigate the role of the DNA mismatch repair (MMR) system in the cisplatin sensitivity of TGCT.

METHODS

The expression pattern of key MMR proteins, including MSH2, MSH6, MLH1 and PMS2, were investigated during testis development and in the pathogenesis of TGCT, including germ cell neoplasia in situ (GCNIS). The TGCT-derived cell line NTera2 was differentiated using retinoic acid (10 μM, 6 days) after which MMR protein expression and activity, as well as cisplatin sensitivity, were investigated in both undifferentiated and differentiated cells. Finally, the expression of MSH2 was knocked down by siRNA in NTera2 cells after which the effect on cisplatin sensitivity was examined.

RESULTS

MMR proteins were expressed in proliferating cells in the testes, while in malignant germ cells MMR protein expression was found to coincide with the expression of the pluripotency factor OCT4, with no or low expression in the more differentiated yolk sac tumours, choriocarcinomas and teratomas. In differentiated NTera2 cells we found a significantly (p < 0.05) lower expression of the MMR and pluripotency factors, as well as a reduced MMR activity and cisplatin sensitivity, compared to undifferentiated NTera2 cells. Also, we found that partial knockdown of MSH2 expression in undifferentiated NTera2 cells resulted in a significantly (p < 0.001) reduced cisplatin sensitivity.

CONCLUSION

This study reports, for the first time, expression of the MMR system in fetal gonocytes, from which GCNIS cells are derived. Our findings in primary TGCT specimens and TGCT-derived cells suggest that a reduced sensitivity to cisplatin in differentiated TGCT components could result from a reduced expression of MMR proteins, in particular MSH2 and MLH1, which are involved in the recognition of cisplatin adducts and in activation of the DNA damage response pathway to initiate apoptosis.

DND1 maintains germline stem cells via recruitment of the CCR4-NOT complex to target mRNAs

The vertebrate-conserved RNA-binding protein DND1 is required for the survival of primordial germ cells (PGCs), as well as the suppression of germ cell tumours in mice. Here we show that in mice DND1 binds a UU(A/U) trinucleotide motif predominantly in the 3' untranslated regions of mRNA, and destabilizes target mRNAs through direct recruitment of the CCR4-NOT deadenylase complex. Transcriptomic analysis reveals that the extent of suppression is dependent on the number of DND1-binding sites. This DND1-dependent mRNA destabilization is required for the survival of mouse PGCs and spermatogonial stem cells by suppressing apoptosis. The spectrum of target RNAs includes positive regulators of apoptosis and inflammation, and modulators of signalling pathways that regulate stem-cell pluripotency, including the TGFβ superfamily, all of which are aberrantly elevated in DND1-deficient PGCs. We propose that the induction of the post-transcriptional suppressor DND1 synergizes with concurrent transcriptional changes to ensure precise developmental transitions during cellular differentiation and maintenance of the germ line.

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.

Plasticity underlies tumor progression: role of Nodal signaling

The transforming growth factor beta (TGFβ) superfamily member Nodal is an established regulator of early embryonic development, with primary roles in endoderm induction, left-right asymmetry, and primitive streak formation. Nodal signals through TGFβ family receptors at the plasma membrane and induces signaling cascades leading to diverse transcriptional regulation. While conceptually simple, the regulation of Nodal and its molecular effects are profoundly complex and context dependent. Pioneering work by developmental biologists has characterized the signaling pathways, regulatory components, and provided detailed insight into the mechanisms by which Nodal mediates changes at the cellular and organismal levels. Nodal is also an important factor in maintaining pluripotency of embryonic stem cells through regulation of core transcriptional programs. Collectively, this work has led to an appreciation for Nodal as a powerful morphogen capable of orchestrating multiple cellular phenotypes. Although Nodal is not active in most adult tissues, its reexpression and signaling have been linked to multiple types of human cancer, and Nodal has emerged as a driver of tumor growth and cellular plasticity. In vitro and in vivo experimental evidence has demonstrated that inhibition of Nodal signaling reduces cancer cell aggressive characteristics, while clinical data have established associations with Nodal expression and patient outcomes. As a result, there is great interest in the potential targeting of Nodal activity in a therapeutic setting for cancer patients that may provide new avenues for suppressing tumor growth and metastasis. In this review, we evaluate our current understanding of the complexities of Nodal function in cancer and highlight recent experimental evidence that sheds light on the therapeutic potential of its inhibition.

Long non-coding RNA GAS5 controls human embryonic stem cell self-renewal by maintaining NODAL signalling

Long non-coding RNAs (lncRNAs) are known players in the regulatory circuitry of the self-renewal in human embryonic stem cells (hESCs). However, most hESC-specific lncRNAs remain uncharacterized. Here we demonstrate that growth-arrest-specific transcript 5 (GAS5), a known tumour suppressor and growth arrest-related lncRNA, is highly expressed and directly regulated by pluripotency factors OCT4 and SOX2 in hESCs. Phenotypic analysis shows that GAS5 knockdown significantly impairs hESC self-renewal, but its overexpression significantly promotes hESC self-renewal. Using RNA sequencing and functional analysis, we demonstrate that GAS5 maintains NODAL signalling by protecting NODAL expression from miRNA-mediated degradation. Therefore, we propose that the above pluripotency factors, GAS5 and NODAL form a feed-forward signalling loop that maintains hESC self-renewal. As this regulatory function of GAS5 is stem cell specific, our findings also indicate that the functions of lncRNAs may vary in different cell types due to competing endogenous mechanisms.

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.

Activin pathway enhances colorectal cancer stem cell self-renew and tumor progression

Activin belongs to transforming growth factor (TGF)-β super family of growth and differentiation factors and activin pathway participated in broad range of cell process. Studies elaborated activin pathway maintain pluripotency in human stem cells and suggest that the function of activin/nodal signaling in self-renew would be conserved across embryonic and adult stem cells. In this study, we tried to determine the effect of activin signaling pathway in regulation of cancer stem cells as a potential target for cancer therapy in clinical trials. A population of colorectal cancer cells was selected by the treatment of activin A. This population of cell possessed stem cell character with sphere formation ability. We demonstrated activin pathway enhanced the colorectal cancer stem cells self-renew and contribute to colorectal cancer progression in vivo. Targeting activin pathway potentially provide effective strategy for colorectal cancer therapy.

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.