Isolation of fetal gonads from embryos of timed-pregnant mice for morphological and molecular studies

The single-cell chromatin landscape in gonadal cell lineage specification

Gonad development includes sex determination and divergent maturation of the testes and ovaries. Recent advances in measuring gene expression in single cells are providing new insights into this complex process. However, the underlying epigenetic regulatory mechanisms remain unclear. Here, we profiled chromatin accessibility in mouse gonadal cells of both sexes from embryonic day 11.5 to 14.5 using single-cell assay for transposase accessible chromatin by sequencing (scATAC-seq). Our results showed that individual cell types can be inferred by the chromatin landscape, and that cells can be temporally ordered along developmental trajectories. Integrative analysis of transcriptomic and chromatin-accessibility maps identified multiple putative regulatory elements proximal to key gonadal genes Nr5a1, Sox9 and Wt1. We also uncover cell type-specific regulatory factors underlying cell type specification. Overall, our results provide a better understanding of the epigenetic landscape associated with the progressive restriction of cell fates in the gonad.

Single-cell transcriptomic profiling to evaluate the effects of Di(2-ethylhexyl)phthalate exposure on early meiosis of female mouse germ cells

Di(2-ethylhexyl)phthalate (DEHP) has proven characteristics of an endocrine-disrupting compound (EDC), which can threaten the reproductive health of humans and other animals. In mammals, a series of chromosomal events occur during the meiotic stage of oocytes. External toxins may enter the body and cause infertility and other related diseases. Therefore, it is crucial to explore the influence of DEHP exposure on the molecular mechanism of germ cell meiosis. We used single-cell RNA sequencing (scRNA-seq) to analyse the ovaries of foetal mice at embryonic day 12.5 (E12.5) and E14.5 after maternal DEHP exposure. DEHP exposure further activated the pathways related to DNA repair in germ cells, increased the expression of genes related to DNA damage and changed the developmental trajectory of germ cells. DEHP exposure may affect the proliferation of pregranulosa (PG) cells. Moreover, DEHP exposure altered the signal transduction between PG cells and germ cells. We showed that DEHP affects meiosis by causing DNA damage in oocytes and disrupting the signal transduction between PG cells and germ cells. These results provide a strong theoretical basis for the prevention and treatment of DEHP-mediated female reproductive health problems.

MAP3K4 kinase activity dependent control of mouse gonadal sex determination†

Sex determination requires the commitment of bipotential gonads to either a testis or an ovarian fate. Gene deletion of the kinase Map3k4 results in gonadal sex reversal in XY mice, and transgenic re-expression of Map3k4 rescues the sex reversal phenotype. Map3k4 encodes a large, multi-functional protein possessing a kinase domain and several, additional protein-protein interaction domains. Although MAP3K4 plays a critical role in male gonadal sex determination, it is unknown if the kinase activity of MAP3K4 is required. Here, we use mice expressing full-length, kinase-inactive MAP3K4 from the endogenous Map3k4 locus to examine the requirement of MAP3K4 kinase activity in sex determination. Although homozygous kinase-inactivation of MAP3K4 (Map3k4KI/KI) is lethal, a small fraction survive to adulthood. We show Map3k4KI/KI adults exhibit a 4:1 female-biased sex ratio. Many adult Map3k4KI/KI phenotypic females have a Y chromosome. XY Map3k4KI/KI adults with sex reversal display female mating behavior, but do not give rise to offspring. Reproductive organs are overtly female, but there is a broad spectrum of ovarian phenotypes, including ovarian absence, primitive ovaries, reduced ovarian size, and ovaries having follicles in all stages of development. Further, XY Map3k4KI/KI adults are smaller than either male or female Map3k4WT/WT mice. Examination of the critical stage of gonadal sex determination at E11.5 shows that loss of MAP3K4 kinase activity results in the loss of Sry expression in XY Map3k4KI/KI embryos, indicating embryonic male gonadal sex reversal. Together, these findings demonstrate the essential role for kinase activity of MAP3K4 in male gonadal sex determination.

MicroRNAs: potential targets and agents of endocrine disruption in female reproduction

MicroRNAs are short non-coding RNAs that have been widely recognized as key mediators in the epigenetic control of gene expression and which are present in virtually all cells and tissues studied. These regulatory molecules are generated in multiple steps in a process called microRNA biogenesis. Distinct microRNA expression patterns during the different stages of oocyte and embryo development suggest important regulatory roles for these small RNAs. Moreover, studies antagonizing specific microRNAs and enzymes in microRNA biogenesis pathways have demonstrated that interference with normal miRNA function leads to infertility and is associated with some reproductive abnormalities. Endocrine disrupting chemicals such as Bisphenol A (BPA) are synthetic hormone mimics that have been found to negatively impact reproductive health. In addition to their direct effects on gene expression, these chemicals are widely implicated in the disruption of epigenetic pathways, including the expression and activity of miRNAs, thereby altering gene expression. In this review, the roles of microRNAs during mammalian oocyte and embryo development are outlined and the different mechanisms by which endocrine disruptors such as BPA interfere with these epigenetic regulators to cause reproductive problems is explored.

Comparative analysis demonstrates cell type-specific conservation of SOX9 targets between mouse and chicken

SRY (sex-determining region Y)-box 9 (SOX9) is a transcription factor regulating both chondrogenesis and sex determination. Among vertebrates, SOX9's functions in chondrogenesis are well conserved, while they vary in sex determination. To investigate the conservation of SOX9's regulatory functions in chondrogenesis and gonad development among species, we performed chromatin immunoprecipitation sequencing (ChIP-seq) using developing limb buds and male gonads from embryos of two vertebrates, mouse and chicken. In both mouse and chicken, SOX9 bound to intronic and distal regions of genes more frequently in limb buds than in male gonads, while SOX9 bound to the proximal upstream regions of genes more frequently in male gonads than in limb buds. In both species, SOX palindromic repeats were identified more frequently in SOX9 binding regions in limb bud genes compared with those in male gonad genes. The conservation of SOX9 binding regions was significantly higher in limb bud genes. In addition, we combined RNA expression analysis (RNA sequencing) with the ChIP-seq results at the same stage in developing chondrocytes and Sertoli cells and determined SOX9 target genes in these cells of the two species and disclosed that SOX9 targets showed high similarity of targets in chondrocytes, but not in Sertoli cells.

Prenatal stress affects placental cytokines and neurotrophins, commensal microbes, and anxiety-like behavior in adult female offspring

Recent studies demonstrate that exposure to stress changes the composition of the intestinal microbiota, which is associated with development of stress-induced changes to social behavior, anxiety, and depression. Stress during pregnancy has also been related to the emergence of these disorders; whether commensal microbes are part of a maternal intrauterine environment during prenatal stress is not known. Here, we demonstrate that microbiome changes are manifested in the mother, and also found in female offspring in adulthood, with a correlation between stressed mothers and female offspring. Alterations in the microbiome have been shown to alter immune responses, thus we examined cytokines in utero. IL-1β was increased in placenta and fetal brain from offspring exposed to the prenatal stressor. Because IL-1β has been shown to prevent induction of brain derived neurotrophic factor (BDNF), we examined BDNF and found a reduction in female placenta and adult amygdala, suggesting in utero impact on neurodevelopment extending into adulthood. Furthermore, gastrointestinal microbial communities were different in adult females born from stressed vs. non-stressed pregnancies. Adult female offspring also demonstrated increased anxiety-like behavior and alterations in cognition, suggesting a critical window where stress is able to influence the microbiome and the intrauterine environment in a deleterious manner with lasting behavioral consequences. The microbiome may be a key link between the intrauterine environment and adult behavioral changes.

Endocrine Regulation in the Ovary by MicroRNA during the Estrous Cycle

Hormonal control of the estrous cycle that occurs in therian mammals is essential for the production of a functional egg. Supporting somatic cell types found within the ovary, such as granulosa and theca cells, respond to endocrine signals to support oocyte maturation and ovulation. Following the release of the egg, now available for fertilization, coordinated hormonal signaling between the mother and putative embryo are required for the establishment of pregnancy. If no conception occurs, both the ovary and uterus are "reset" in preparation for another cycle. The complex molecular changes that occur within cells in response to hormone signaling include a network of non-coding microRNAs (miRNAs) that posttranscriptionally regulate gene expression. They are thus able to fine-tune cellular responses to hormones and confer robustness in gene regulation. In this review, we outline the important roles established for miRNAs in regulating female reproductive hormone signaling during estrus, with a particular focus on signaling pathways in the ovary. Understanding this miRNA network can provide important insights to improving assisted reproductive technologies and may be useful in the diagnosis of female reproductive disorders.

Methods for the Study of Gonadal Development

Current knowledge on gonadal development and sex determination is the product of many decades of research involving a variety of scientific methods from different biological disciplines such as histology, genetics, biochemistry, and molecular biology. The earliest embryological investigations, followed by the invention of microscopy and staining methods, were based on histological examinations. The most robust development of histological staining techniques occurred in the second half of the nineteenth century and resulted in structural descriptions of gonadogenesis. These first studies on gonadal development were conducted on domesticated animals; however, currently the mouse is the most extensively studied species. The next key point in the study of gonadogenesis was the advancement of methods allowing for the in vitro culture of fetal gonads. For instance, this led to the description of the origin of cell lines forming the gonads. Protein detection using antibodies and immunolabeling methods and the use of reporter genes were also invaluable for developmental studies, enabling the visualization of the formation of gonadal structure. Recently, genetic and molecular biology techniques, especially gene expression analysis, have revolutionized studies on gonadogenesis and have provided insight into the molecular mechanisms that govern this process. The successive invention of new methods is reflected in the progress of research on gonadal development.

The sex-determining factors SRY and SOX9 regulate similar target genes and promote testis cord formation during testicular differentiation

Male sex determination is mediated sequentially by sex-determining region Y (SRY) and related SRY-box 9 (SOX9) transcription factors. To understand the gene regulatory hierarchy for SRY and SOX9, a series of chromatin immunoprecipitation and whole-genome promoter tiling microarray (ChIP-Chip) experiments were conducted with mouse gonadal cells at the time of sex determination. SRY and SOX9 bind to the promoters of many common targets involved in testis differentiation and regulate their expression in Sertoli cells. SRY binds to various ovarian differentiation genes and represses their activation through WNT/β-catenin signaling. Sertoli cell-Sertoli cell junction signaling, important for testis cord formation, is the top canonical pathway among the SRY and SOX9 targets. Hence, SRY determines Sertoli cell fate by repressing ovarian and activating testicular differentiation genes, promotes early Sertoli cells to form testis cord, and then passes on its functions to SOX9, which regulates common targets and activates its own gene regulatory program, beyond SRY actions, in sex determination.

Gadd45g is essential for primary sex determination, male fertility and testis development

In humans and most mammals, differentiation of the embryonic gonad into ovaries or testes is controlled by the Y-linked gene SRY. Here we show a role for the Gadd45g protein in this primary sex differentiation. We characterized mice deficient in Gadd45a, Gadd45b and Gadd45g, as well as double-knockout mice for Gadd45ab, Gadd45ag and Gadd45bg, and found a specific role for Gadd45g in male fertility and testis development. Gadd45g-deficient XY mice on a mixed 129/C57BL/6 background showed varying degrees of disorders of sexual development (DSD), ranging from male infertility to an intersex phenotype or complete gonadal dysgenesis (CGD). On a pure C57BL/6 (B6) background, all Gadd45g(-/-) XY mice were born as completely sex-reversed XY-females, whereas lack of Gadd45a and/or Gadd45b did not affect primary sex determination or testis development. Gadd45g expression was similar in female and male embryonic gonads, and peaked around the time of sex differentiation at 11.5 days post-coitum (dpc). The molecular cause of the sex reversal was the failure of Gadd45g(-/-) XY gonads to achieve the SRY expression threshold necessary for testes differentiation, resulting in ovary and Müllerian duct development. These results identify Gadd45g as a candidate gene for male infertility and 46,XY sex reversal in humans.