Chapter 5 Sexual Development of the Soma in the Mouse

Two redundant transcription factor binding sites in a single enhancer are essential for mammalian sex determination

Male development in mammals depends on the activity of the two SOX gene: Sry and Sox9, in the embryonic testis. As deletion of Enhancer 13 (Enh13) of the Sox9 gene results in XY male-to-female sex reversal, we explored the critical elements necessary for its function and hence, for testis and male development. Here, we demonstrate that while microdeletions of individual transcription factor binding sites (TFBS) in Enh13 lead to normal testicular development, combined microdeletions of just two SRY/SOX binding motifs can alone fully abolish Enh13 activity leading to XY male-to-female sex reversal. This suggests that for proper male development to occur, these few nucleotides of non-coding DNA must be intact. Interestingly, we show that depending on the nature of these TFBS mutations, dramatically different phenotypic outcomes can occur, providing a molecular explanation for the distinct clinical outcomes observed in patients harboring different variants in the same enhancer.

In vitro cellular reprogramming to model gonad development and its disorders

During embryonic development, mutually antagonistic signaling cascades determine gonadal fate toward a testicular or ovarian identity. Errors in this process result in disorders of sex development (DSDs), characterized by discordance between chromosomal, gonadal, and anatomical sex. The absence of an appropriate, accessible in vitro system is a major obstacle in understanding mechanisms of sex-determination/DSDs. Here, we describe protocols for differentiation of mouse and human pluripotent cells toward gonadal progenitors. Transcriptomic analysis reveals that the in vitro-derived murine gonadal cells are equivalent to embryonic day 11.5 in vivo progenitors. Using similar conditions, Sertoli-like cells derived from 46,XY human induced pluripotent stem cells (hiPSCs) exhibit sustained expression of testis-specific genes, secrete anti-Müllerian hormone, migrate, and form tubular structures. Cells derived from 46,XY DSD female hiPSCs, carrying an NR5A1 variant, show aberrant gene expression and absence of tubule formation. CRISPR-Cas9-mediated variant correction rescued the phenotype. This is a robust tool to understand mechanisms of sex determination and model DSDs.

The Chromatin State during Gonadal Sex Determination

At embryonic day (E) 10.5, prior to gonadal sex determination, XX and XY gonads are bipotential and able to differentiate into either a testis or an ovary. At this point, they are transcriptionally and morphologically indistinguishable. Sex determination begins around E11.5 in the mouse when the supporting cell lineage commits to either Sertoli or granulosa cell fate. Testis-specific factors such as SRY and SOX9 drive differentiation of bipotential-supporting cells into the Sertoli cell pathway, whereas ovary-specific factors like WNT4 and FOXL2 guide differentiation into granulosa cells. It is known that these 2 pathways are mutually antagonistic, and repression of the alternative fate is critical for maintenance of the testis or ovary programs. While we understand much about the transcription factor networks guiding the process of sex determination, it is only more recently that we have begun to understand how this process is epigenetically controlled. Studies in the past decade have demonstrated the importance of the chromatin state for gene expression and cell fate commitment, with histone modifications and DNA accessibility having a direct role in gene regulation. It is now clear that the chromatin state during sex determination is dynamic and likely critical for the establishment and/or maintenance of the transcriptional programs. Prior to sex determination, supporting cells have similar chromatin structure and histone modification profiles, reflecting the bipotential nature of these cells. After differentiation to Sertoli or granulosa cells, the chromatin state acquires sex-specific profiles. The proteins that regulate the deposition of histone modifications or the opening of compact chromatin likely play an important role in Sertoli and granulosa cell fate commitment and gonad development. Here, we describe studies profiling the chromatin state during gonadal sex determination and one example in which depletion of Cbx2, a member of the Polycomb Repressive Complex 1 (PRC1), causes male-to-female sex reversal due to a failure to repress the ovarian pathway.

Cis-Regulatory Control of Mammalian Sex Determination

Sex determination is the process by which an initial bipotential gonad adopts either a testicular or ovarian cell fate. The inability to properly complete this process leads to a group of developmental disorders classified as disorders of sex development (DSD). To date, dozens of genes were shown to play roles in mammalian sex determination, and mutations in these genes can cause DSD in humans or gonadal sex reversal/dysfunction in mice. However, exome sequencing currently provides genetic diagnosis for only less than half of DSD patients. This points towards a major role for the non-coding genome during sex determination. In this review, we highlight recent advances in our understanding of non-coding, cis-acting gene regulatory elements and discuss how they may control transcriptional programmes that underpin sex determination in the context of the 3-dimensional folding of chromatin. As a paradigm, we focus on the Sox9 gene, a prominent pro-male factor and one of the most extensively studied genes in gonadal cell fate determination.

The regulation of Sox9 expression in the gonad

The bipotential nature of cell types in the early developing gonad and the process of sex determination leading to either testis or ovary differentiation makes this an interesting system in which to study transcriptional regulation of gene expression and cell fate decisions. SOX9 is a transcription factor with multiple roles during development, including being a key player in mediating testis differentiation and therefore subsequent male development. Loss of Sox9 expression in both humans and mice results in XY female development, whereas its inappropriate activation in XX embryonic gonads can give male development. Multiple cases of Disorders of Sex Development in human patients or sex reversal in mice and other vertebrates can be explained by mutations affecting upstream regulators of Sox9 expression, such as the product of the Y chromosome gene Sry that triggers testis differentiation. Other cases are due to mutations in the Sox9 gene itself, including its own regulatory region. Indeed, rearrangements in and around the Sox9 genomic locus indicate the presence of multiple critical enhancers and the complex nature of its regulation. Here we summarize what is known about the role of Sox9 and its regulation during gonad development, including recently discovered critical enhancers. We also discuss higher order chromatin organization and how this might be involved. We end with some interesting future directions that have the potential to further enrich our understanding on the complex, multi-layered regulation controlling Sox9 expression in the gonads.

Identification of a Novel Zebrafish Mutant Line that Develops Testicular Germ Cell Tumors

Testicular tumors are the most common solid malignant tumors in men 20-35 years of age. Although most of testicular tumors are curable, current treatments still fail in 15%-20% of patients. However, insufficient understanding of the molecular basis and lack of animal models limit development of more effective treatments. This study reports the identification of a novel zebrafish mutant line, ns1402, which develops testicular germ cell tumors (TGCTs). While both male and female ns1402 mutants were fertile at young age, male ns1402 mutants became infertile as early as 9 months of age. This infertility was associated with progressive loss of mature sperm. Failure of spermatogenesis was, at least in part, explained by progressive loss of mature Leydig cells, a source of testosterone that is essential for spermatogenesis. Interestingly, TGCTs in ns1402 mutants contained a large number of Sertoli cells and gene expression profiles of Sertoli cells were altered before loss of mature Leydig cells. This suggests that changes in Sertoli cell properties happened first, followed by loss of mature Leydig cells and failure of spermatogenesis. Taken together, this study emphasizes the importance of cell-cell interactions and cell signaling in the testis for spermatogenesis and tissue homeostasis.

Cep55 overexpression causes male-specific sterility in mice by suppressing Foxo1 nuclear retention through sustained activation of PI3K/Akt signaling

Spermatogenesis is a dynamic process involving self-renewal and differentiation of spermatogonial stem cells, meiosis, and ultimately, the differentiation of haploid spermatids into sperm. Centrosomal protein 55 kDa (CEP55) is necessary for somatic cell abscission during cytokinesis. It facilitates equal segregation of cytoplasmic contents between daughter cells by recruiting endosomal sorting complex required for transport machinery (ESCRT) at the midbody. In germ cells, CEP55, in partnership with testes expressed-14 (TEX14) protein, has also been shown to be an integral component of intercellular bridge before meiosis. Various in vitro studies have demonstrated a role for CEP55 in multiple cancers and other diseases. However, its oncogenic potential in vivo remains elusive. To investigate, we generated ubiquitously overexpressing Cep55 transgenic ( Cep55^Tg/Tg) mice aiming to characterize its oncogenic role in cancer. Unexpectedly, we found that Cep55^Tg/Tg male mice were sterile and had severe and progressive defects in spermatogenesis related to spermatogenic arrest and lack of spermatids in the testes. In this study, we characterized this male-specific phenotype and showed that excessively high levels of Cep55 results in hyperactivation of PI3K/protein kinase B (Akt) signaling in testis. In line with this finding, we observed increased phosphorylation of forkhead box protein O1 (FoxO1), and suppression of its nuclear retention, along with the relative enrichment of promyelocytic leukemia zinc finger (PLZF) -positive cells. Independently, we observed that Cep55 amplification favored upregulation of ret ( Ret) proto-oncogene and glial-derived neurotrophic factor family receptor α-1 ( Gfra1). Consistent with these data, we observed selective down-regulation of genes associated with germ cell differentiation in Cep55-overexpressing testes at postnatal day 10, including early growth response-4 ( Egr4) and spermatogenesis and oogenesis specific basic helix-loop-helix-1 ( Sohlh1). Thus, Cep55 amplification leads to a shift toward the initial maintenance of undifferentiated spermatogonia and ultimately results in progressive germ cell loss. Collectively, our findings demonstrate that Cep55 overexpression causes change in germ cell proportions and manifests as a Sertoli cell only tubule phenotype, similar to that seen in many azoospermic men.-Sinha, D., Kalimutho, M., Bowles, J., Chan, A.-L., Merriner, D. J., Bain, A. L., Simmons, J. L., Freire, R., Lopez, J. A., Hobbs, R. M., O'Bryan, M. K., Khanna, K. K. Cep55 overexpression causes male-specific sterility in mice by suppressing Foxo1 nuclear retention through sustained activation of PI3K/Akt signaling.

Sexual Differentiation and Sex Differences in Neural Development

Sex determination occurs at the moment of conception, as a result of XX or XY chromosome pairing. From that point, the body undergoes the process of sexual differentiation, inducing the development of physical characteristics that are easily distinguishable between the sexes and are often reflected in one's physical appearance and gender identity. Although less apparent, the brain also undergoes sexual differentiation. Sex differences in the brain are organized during a critical period of neural development and have an instrumental role in determining the physiology and behavior of an individual throughout the lifespan. Understanding the extent of sex differences in neurodevelopment also influences our understanding of the potential risk for a number of neurodevelopmental, neurological, and mental health disorders that exhibit strong sex biases. Advances made in our understanding of sexually dimorphic brain nuclei, sex differences in neural cell communication, and sex differences in the communication between the brain and peripheral organs are all research fields that have provided valuable information related to the physiological and behavioral outcomes of sex differences in brain development. More recently, investigations into the impact of epigenetic mechanisms on sexual differentiation of the brain have indicated that changes in gene expression, via epigenetic modifications, also contribute to sexual differentiation of the developing brain. Still, there are a number of important questions and ideas that have arisen from our current understanding of sex differences in neurodevelopmental processes that necessitate more time and attention in this field.

Geography of follicle formation in the embryonic mouse ovary impacts activation pattern during the first wave of folliculogenesis

During embryonic development, mouse female germ cells enter meiosis in an anterior-to-posterior wave believed to be driven by retinoic acid. It has been proposed that ovarian follicle formation and activation follow the same general wave of meiotic progression; however, the precise anatomic specification of these processes has not been delineated. Here, we created a mouse line using Mvh, Gdf9, and Zp3 promoters to drive distinct temporal expression of three fluorescent proteins in the oocytes and to identify where the first follicle cohort develops. The fluorescent profile revealed that the first growing follicles consistently appeared in a specific region of the ovary, the anterior-dorsal region, which led us to analyze if meiotic onset occurred earlier in the dorsal ovarian region. Surprisingly, in addition to the anterior-to-posterior wave, we observed an early meiotic entry in the ventral region of the ovary. This additional anatomic stratification of meiosis contrasts with the localization of the initial follicle formation and activation in the dorsal region of the ovary. Therefore, our study suggests that the specification of cortical and medullar areas in the ventral and dorsal regions on the ovary, rather than the onset of meiosis, impacts where the first follicle activation event occurs.

Gonad differentiation in the rabbit: evidence of species-specific features

The rabbit is an attractive species for the study of gonad differentiation because of its 31-day long gestation, the timing of female meiosis around birth and the 15-day delay between gonadal switch and the onset of meiosis in the female. The expression of a series of genes was thus determined by qPCR during foetal life until adulthood, completed by a histological analysis and whenever possible by an immunohistological one. Interesting gene expression profiles were recorded. Firstly, the peak of SRY gene expression that is observed in early differentiated XY gonads in numerous mammals was also seen in the rabbit, but this expression was maintained at a high level until the end of puberty. Secondly, a peak of aromatase gene expression was observed at two-thirds of the gestation in XX gonads as in many other species except in the mouse. Thirdly, the expression of STRA8 and DMC1 genes (which are known to be specifically expressed in germ cells during meiosis) was enhanced in XX gonads around birth but also slightly and significantly in XY gonads at the same time, even though no meiosis occurs in XY gonad at this stage. This was probably a consequence of the synchronous strong NANOS2 gene expression in XY gonad. In conclusion, our data highlighted some rabbit-specific findings with respect to the gonad differentiation process.

A comparative, developmental, and clinical perspective of neurobehavioral sexual dimorphisms

Women and men differ in a wide variety of behavioral traits and in their vulnerability to developing certain mental disorders. This review endeavors to explore how recent preclinical and clinical research findings have enhanced our understanding of the factors that underlie these disparities. We start with a brief overview of some of the important genetic, molecular, and hormonal determinants that contribute to the process of sexual differentiation. We then discuss the importance of animal models in studying the mechanisms responsible for sex differences in neuropsychiatric disorders (e.g., drug dependence) - with a special emphasis on experimental models based on the neurodevelopmental and "three hits" hypotheses. Next, we describe the most common brain phenotypes observed in vivo with magnetic resonance imaging. We discuss the challenges in interpreting these phenotypes vis-à-vis the underlying neurobiology and revisit the known sex differences in brain structure from birth, through adolescence, and into adulthood. This is followed by a presentation of pertinent clinical and epidemiological data that point to important sex differences in the prevalence, course, and expression of psychopathologies such as schizophrenia, and mood disorders including major depression and posttraumatic stress disorder. Recent evidence implies that mood disorders and psychosis share some common genetic predispositions and neurobiological bases. Therefore, modern research is emphasizing dimensional representation of mental disorders and conceptualization of schizophrenia and major depression as a continuum of cognitive deficits and neurobiological abnormalities. Herein, we examine available evidence on cerebral sexual dimorphism to verify if sex differences vary quantitatively and/or qualitatively along the psychoses-depression continuum. Finally, sex differences in the prevalence of posttraumatic disorder and drug abuse have been described, and we consider the genomic and molecular data supporting these differences.

Molecular control of rodent spermatogenesis

Spermatogenesis is a complex developmental process that ultimately generates mature spermatozoa. This process involves a phase of proliferative expansion, meiosis, and cytodifferentiation. Mouse models have been widely used to study spermatogenesis and have revealed many genes and molecular mechanisms that are crucial in this process. Although meiosis is generally considered as the most crucial phase of spermatogenesis, mouse models have shown that pre-meiotic and post-meiotic phases are equally important. Using knowledge generated from mouse models and in vitro studies, the current review provides an overview of the molecular control of rodent spermatogenesis. Finally, we briefly relate this knowledge to fertility problems in humans and discuss implications for future research. This article is part of a Special Issue entitled: Molecular Genetics of Human Reproductive Failure.

Sex determination and the control of Sox9 expression in mammals

In mouse sex determination, the presence or absence of Sertoli cells in the developing gonad is essential for the decision to form either a testis or an ovary. The transcription factor SOX9 has emerged as the master regulator of Sertoli cell differentiation during testis development and thus the crucial gene to determine sex. It is the target of two sets of regulatory controls, one positive and one negative, where one set tries to gain dominance over the other in the early gonad and then to establish and maintain the activity or silence of Sox9 throughout life. The data reveal the importance of the positive regulatory loops to reinforce initial decisions, whereas the maintenance of the gonadal phenotype appears to rely on the active repression of the opposite pathway.

Expression of miRNAs in ovine fetal gonads: potential role in gonadal differentiation

BACKGROUND

Gonadal differentiation in the mammalian fetus involves a complex dose-dependent genetic network. Initiation and progression of fetal ovarian and testicular pathways are accompanied by dynamic expression patterns of thousands of genes. We postulate these expression patterns are regulated by small non-coding RNAs called microRNAs (miRNAs). The aim of this study was to identify the expression of miRNAs in mammalian fetal gonads using sheep as a model.

METHODS

We determined the expression of 128 miRNAs by real time PCR in early-gestational (gestational day (GD) 42) and mid-gestational (GD75) sheep ovaries and testes. Expression data were further examined and validated by bioinformatic analysis.

RESULTS

Expression analysis revealed significant differences between ovaries and testes among 24 miRNAs at GD42, and 43 miRNAs at GD75. Bioinformatic analysis revealed that a number of differentially expressed miRNAs are predicted to target genes known to be important in mammalian gonadal development, including ESR1, CYP19A1, and SOX9. In situ hybridization revealed miR-22 localization within fetal testicular cords. As estrogen signaling is important in human and sheep ovarian development, these data indicate that miR-22 is involved in repressing estrogen signaling within fetal testes.

CONCLUSIONS

Based on our results we postulate that gene expression networks underlying fetal gonadal development are regulated by miRNAs.

GATA transcription factors in the developing reproductive system

Previous work has firmly established the role for both GATA4 and FOG2 in the initial global commitment to sexual fate, but their (joint or individual) function in subsequent steps remained unknown. Hence, gonad-specific deletions of these genes in mice were required to reveal their roles in sexual development and gene regulation. The development of tissue-specific Cre lines allowed for substantial advances in the understanding of the function of GATA proteins in sex determination, gonadal differentiation and reproductive development in mice. Here we summarize the recent work that examined the requirement of GATA4 and FOG2 proteins at several critical stages in testis and ovarian differentiation. We also discuss the molecular mechanisms involved in this regulation through the control of Dmrt1 gene expression in the testis and the canonical Wnt/ß-catenin pathway in the ovary.

Germ cell sex determination: a collaboration between soma and germline

Sex determination is regulated very differently in the soma vs. the germline, yet both processes are critical for the creation of the male and female gametes. In general, the soma plays an essential role in regulating sexual identity of the germline. However, in some species, such as Drosophila and mouse, the sex chromosome constitution of the germ cells makes an autonomous contribution to germline sexual development. Here we review how the soma and germline cooperate to determine germline sexual identity for some important model systems, the fly, the worm and the mouse, and discuss some of the implications of 'dual control' (soma plus germline) as compared to species where germline sex is dictated only by the surrounding soma.