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

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.

Identification and Characterization of C-Mos in Pearl Mussel Hyriopsis cumingii and Its Role in Gonadal Development

C-Mos, a proto-oncogene, regulates oocyte maturation by activating the classical MAPK pathway in cells. To examine the function of C-Mos in Hyriopsis cumingii, C-Mos was identified in this study. The full-length cDNA of C-Mos was 2213 bp, including 144 bp in the 5' UTR, 923 bp in 3' the UTR, and 1146 bp in the open reading frame (ORF) region. During early gonad development, the expression of C-Mos from 4 to 6 months of age in H. cumingii was significantly higher than that in other months, with the highest expression in 6-month-old H. cumingii, suggesting that C-Mos may be involved in early gonadal development in H. cumingii. Clear hybridization signals were found by in situ hybridization in the oocytes, oocyte nucleus and oogonium, and a small number of hybridization signals were found in the follicular wall of the male gonads. In addition, the C-Mos RNA interference (RNAi) assay results showed that the knockdown of C-Mos caused a down-regulation of ERK and P90rsk. In summary, these results indicate that C-Mos has a crucial part to play in gonadal development in H. cumingii.

Fetal germ cell development in humans, a link with infertility

Gametes are cells that have the unique ability to give rise to new individuals as well as transmit (epi)genetic information across generations. Generation of functionally competent gametes, oocytes and sperm cells, depends to some extent on several fundamental processes that occur during fetal development. Direct studies on human fetal germ cells remain hindered by ethical considerations and inaccessibility to human fetal material. Therefore, the majority of our current knowledge of germ cell development still comes from an invaluable body of research performed using different mammalian species. During the last decade, our understanding of human fetal germ cells has increased due to the successful use of human pluripotent stem cells to model aspects of human early gametogenesis and advancements on single-cell omics. Together, this has contributed to determine the cell types and associated molecular signatures in the developing human gonads. In this review, we will put in perspective the knowledge obtained from several mammalian models (mouse, monkey, pig). Moreover, we will discuss the main events during human fetal (female) early gametogenesis and how the dysregulation of this highly complex and lengthy process can link to infertility later in life.

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.

Mapk activation drives male and female mouse teratocarcinomas from late PGCs

Germ cell tumors (GCTs) are rare tumors that can develop in both sexes, peaking in adolescents. To understand the mechanisms that underlie germ cell transformation, we established a GCT mouse model carrying germ cell-specific BRafV600E mutation with or without heterozygous Pten deletion. Both male and female mice developed monolateral teratocarcinomas containing embryonal carcinoma (EC) cells that showed an aggressive phenotype and metastatic ability. Germ cell transformation started in fetal gonads and progressed after birth leading to gonadal invasion. Early postnatal testes showed foci of tumor transformation, while ovaries showed increased number of follicles, multi-ovular follicles (MOFs) and scattered metaphase I oocytes containing follicles. Our results indicate that Mapk over-activation in fetal germ cells of both sexes can expand their proliferative window leading to neoplastic transformation and metastatic behavior.

Regulation of mouse primordial follicle formation by signaling through the PI3K pathway

Cell signaling mediated by the KIT receptor is critical for many aspects of oogenesis including the proliferation and migration of primordial germ cells, as well as the survival, growth, and maturation of ovarian follicles. We previously showed that KIT regulates cyst breakdown and primordial follicle formation, and in this study, have investigated the mechanisms downstream of the receptor by modulating the activity of two downstream signaling cascades: the phosphoinositide 3-kinase (PI3K) and the mitogen-activated protein kinase (MAPK) pathways. E17.5 ovaries were cultured for five days with a daily dose of media supplemented with either the PI3K inhibitor LY294002, the MEK inhibitor U0126, or a DMSO vehicle control. Our histological observations aligned with the established role of PI3K in oocyte growth and primordial follicle activation but also revealed that LY294002 treatment delayed the processes of cyst breakdown and primordial follicle formation. U0126 treatment also led to a reduction in oocyte growth and follicle development but did not appear to affect cyst breakdown. The delay in cyst breakdown was mitigated when ovaries were dually dosed with LY294002 and KITL, suggesting that while KIT may signal through PI3K to promote cyst breakdown, other signaling networks downstream of the receptor could compensate. These observations unearth a role for PI3K signaling in the establishment of the ovarian reserve and suggest that PI3K might be the primary mediator of KIT-induced cyst breakdown and primordial follicle formation in the mouse ovary.