Bulk and single-cell transcriptome datasets of the mouse fetal and adult rete ovarii and surrounding tissues

The rete ovarii (RO) is an epithelial structure that arises during development in close proximity to the ovary and persists throughout adulthood. However, the functional significance of the RO remains elusive, and it is absent from recent discussions of female reproductive anatomy. The RO comprises three regions: the intraovarian rete within the ovary, the extraovarian rete in the periovarian tissue, and the connecting rete linking the two. We hypothesize that the RO plays a pivotal role in ovarian homeostasis and responses to physiological changes. To begin to uncover the nature and function of RO cells, we conducted transcriptomic profiling of the RO. This study presents three datasets, and reports our analysis and quality control approaches for bulk, single-cell, and nucleus-level transcriptomics of the fetal and adult RO tissues using the Pax8-rtTA; Tre-H2B-GFP mouse line, where all RO regions express nuclear GFP. The integration and rigorous validation of these datasets will advance our understanding of the RO's roles in ovarian development, female maturation, and adult female fertility.

Rediscovering the Rete Ovarii: a secreting auxiliary structure to the ovary

The rete ovarii (RO) is an appendage of the ovary that has been given little attention. Although the RO appears in drawings of the ovary in early versions of Gray's Anatomy, it disappeared from recent textbooks, and is often dismissed as a functionless vestige in the adult ovary. Using PAX8 immunostaining and confocal microscopy, we characterized the fetal development of the RO in the context of the ovary. The RO consists of three distinct regions that persist in adult life, the intraovarian rete (IOR), the extraovarian rete (EOR), and the connecting rete (CR). While the cells of the IOR appear to form solid cords within the ovary, the EOR rapidly develops into a convoluted tubular epithelium ending in a distal dilated tip. Cells of the EOR are ciliated and exhibit cellular trafficking capabilities. The CR, connecting the EOR to the IOR, gradually acquires tubular epithelial characteristics by birth. Using microinjections into the distal dilated tip of the EOR, we found that luminal contents flow towards the ovary. Mass spectrometry revealed that the EOR lumen contains secreted proteins potentially important for ovarian function. We show that the cells of the EOR are closely associated with vasculature and macrophages, and are contacted by neuronal projections, consistent with a role as a sensory appendage of the ovary. The direct proximity of the RO to the ovary and its integration with the extraovarian landscape suggest that it plays an important role in ovary development and homeostasis.

Transcriptome analysis of the mouse fetal and adult rete ovarii and surrounding tissues

The rete ovarii (RO) is an epithelial structure that arises during fetal development in close proximity to the ovary and persists throughout adulthood in mice. However, the functional significance of the RO remains elusive, and it has been absent from recent discussions of female reproductive anatomy. The RO comprises three distinct regions: the intraovarian rete (IOR) within the ovary, the extraovarian rete (EOR) in the periovarian tissue, and the connecting rete (CR) linking the EOR and IOR. We hypothesize that the RO plays a pivotal role in maintaining ovarian homeostasis and responding to physiological changes. To uncover the nature and function of RO cells, we conducted transcriptome analysis, encompassing bulk, single-cell, and nucleus-level sequencing of both fetal and adult RO tissues using the Pax8-rtTA; Tre-H2B-GFP mouse line, where all RO regions express nuclear GFP. This study presents three datasets, which highlight RO-specific gene expression signatures and reveal differences in gene expression across the three RO regions during development and in adulthood. The integration and rigorous validation of these datasets will advance our understanding of the RO's roles in ovarian development, female maturation, and adult female fertility.

Higher temperatures directly increase germ cell number, promoting feminization of red-eared slider turtles

In many reptile species, gonadal sex is affected by environmental temperature during a critical period of embryonic development-a process known as temperature-dependent sex determination (TSD).¹ The oviparous red-eared slider turtle, Trachemys scripta, has a warm-female/cool-male TSD system and is among the best-studied members of this group.² When incubated at low temperatures, the somatic cells of the bipotential gonad differentiate into Sertoli cells, the support cells of the testis, whereas at high temperatures, they differentiate into granulosa cells, the support cells of the ovary.³ Here, we report the unexpected finding that temperature independently affects the number of primordial germ cells (GCs) in the embryonic gonad at a time before somatic cell differentiation has initiated. Specifically, embryos incubated at higher, female-inducing temperatures have more GCs than those incubated at the male-inducing temperature. Furthermore, elimination of GCs in embryos incubating at intermediate temperatures results in a strong shift toward male-biased sex ratios. This is the first evidence that temperature affects GC number and the first evidence that GC number influences sex determination in amniotes. This observation has two important implications. First, it supports a new model in which temperature can impact sex determination in incremental ways through multiple cell types. Second, the findings have important implications for a major unresolved question in the fields of ecology and evolutionary biology-the adaptive significance of TSD. We suggest that linking high GC number with female development improves female reproductive potential and provides an adaptive advantage for TSD.

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.

Biased precursor ingression underlies the center-to-pole pattern of male sex determination in mouse

During mammalian development, gonadal sex determination results from the commitment of bipotential supporting cells to Sertoli or granulosa cell fates. Typically, this decision is coordinated across the gonad to ensure commitment to a single organ fate. When unified commitment fails in an XY mouse, an ovotestis forms in which supporting cells in the center of the gonad typically develop as Sertoli cells, while supporting cells in the poles develop as granulosa cells. This central bias for Sertoli cell fate was thought to result from the initial expression of the drivers of Sertoli cell fate, SRY and/or SOX9, in the central domain, followed by paracrine expansion to the poles. However, we show here that the earliest cells expressing SRY and SOX9 are widely distributed across the gonad. In addition, Sertoli cell fate does not spread among supporting cells through paracrine relay. Instead, we uncover a center-biased pattern of supporting cell precursor ingression that occurs in both sexes and results in increased supporting cell density in the central domain. Our findings prompt a new model of gonad patterning in which a density-dependent organizing principle dominates Sertoli cell fate stabilization.

The RNA binding protein DND1 is elevated in a subpopulation of pro-spermatogonia and targets chromatin modifiers and translational machinery during late gestation

DND1 is essential to maintain germ cell identity. Loss of Dnd1 function results in germ cell differentiation to teratomas in some inbred strains of mice or to somatic fates in zebrafish. Using our knock-in mouse line in which a functional fusion protein between DND1 and GFP is expressed from the endogenous locus (Dnd1GFP), we distinguished two male germ cell (MGC) populations during late gestation cell cycle arrest (G0), consistent with recent reports of heterogeneity among MGCs. Most MGCs express lower levels of DND1-GFP (DND1-GFP-lo), but some MGCs express elevated levels of DND1-GFP (DND1-GFP-hi). A RNA-seq time course confirmed high Dnd1 transcript levels in DND1-GFP-hi cells along with 5-10-fold higher levels for multiple epigenetic regulators. Using antibodies against DND1-GFP for RNA immunoprecipitation (RIP)-sequencing, we identified multiple epigenetic and translational regulators that are binding targets of DND1 during G0 including DNA methyltransferases (Dnmts), histone deacetylases (Hdacs), Tudor domain proteins (Tdrds), actin dependent regulators (Smarcs), and a group of ribosomal and Golgi proteins. These data suggest that in DND1-GFP-hi cells, DND1 hosts coordinating mRNA regulons that consist of functionally related and localized groups of epigenetic enzymes and translational components.

Integration of mouse ovary morphogenesis with developmental dynamics of the oviduct, ovarian ligaments, and rete ovarii

Morphogenetic events during the development of the fetal ovary are crucial to the establishment of female fertility. However, the effects of structural rearrangements of the ovary and surrounding reproductive tissues on ovary morphogenesis remain largely uncharacterized. Using tissue clearing and lightsheet microscopy, we found that ovary folding correlated with regionalization into cortex and medulla. Relocation of the oviduct to the ventral aspect of the ovary led to ovary encapsulation, and mutual attachment of the ovary and oviduct to the cranial suspensory ligament likely triggered ovary folding. During this process, the rete ovarii (RO) elaborated into a convoluted tubular structure extending from the ovary into the ovarian capsule. Using genetic mouse models in which the oviduct and RO are perturbed, we found the oviduct is required for ovary encapsulation. This study reveals novel relationships among the ovary and surrounding tissues and paves the way for functional investigation of the relationship between architecture and differentiation of the mammalian ovary.

Origin, specification and differentiation of a rare supporting-like lineage in the developing mouse gonad

Gonadal sex determination represents a unique model for studying cell fate decisions. However, a complete understanding of the different cell lineages forming the developing testis and ovary remains elusive. Here, we investigated the origin, specification, and subsequent sex-specific differentiation of a previously uncharacterized population of supporting-like cells (SLCs) in the developing mouse gonads. The SLC lineage is closely related to the coelomic epithelium and specified as early as E10.5, making it the first somatic lineage to be specified in the bipotential gonad. SLC progenitors are localized within the genital ridge at the interface with the mesonephros and initially coexpress Wnt4 and Sox9. SLCs become sexually dimorphic around E12.5, progressively acquire a more Sertoli- or pregranulosa-like identity and contribute to the formation of the rete testis and rete ovarii. Last, we found that WNT4 is a crucial regulator of the SLC lineage and is required for normal development of the rete testis.

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.

Concerted morphogenesis of genital ridges and nephric ducts in the mouse captured through whole-embryo imaging

During development of the mouse urogenital complex, the gonads undergo changes in three-dimensional structure, body position and spatial relationship with the mesonephric ducts, kidneys and adrenals. The complexity of genital ridge development obscures potential connections between morphogenesis and gonadal sex determination. To characterize the morphogenic processes implicated in regulating gonad shape and fate, we used whole-embryo tissue clearing and light sheet microscopy to assemble a time course of gonad development in native form and context. Analysis revealed that gonad morphology is determined through anterior-to-posterior patterns as well as increased rates of growth, rotation and separation in the central domain that may contribute to regionalization of the gonad. We report a close alignment of gonad and mesonephric duct movements as well as delayed duct development in a gonad dysgenesis mutant, which together support a mechanical dependency linking gonad and mesonephric duct morphogenesis.

A brief review of vertebrate sex evolution with a pledge for integrative research: towards 'sexomics'

Triggers and biological processes controlling male or female gonadal differentiation vary in vertebrates, with sex determination (SD) governed by environmental factors or simple to complex genetic mechanisms that evolved repeatedly and independently in various groups. Here, we review sex evolution across major clades of vertebrates with information on SD, sexual development and reproductive modes. We offer an up-to-date review of divergence times, species diversity, genomic resources, genome size, occurrence and nature of polyploids, SD systems, sex chromosomes, SD genes, dosage compensation and sex-biased gene expression. Advances in sequencing technologies now enable us to study the evolution of SD at broader evolutionary scales, and we now hope to pursue a sexomics integrative research initiative across vertebrates. The vertebrate sexome comprises interdisciplinary and integrated information on sexual differentiation, development and reproduction at all biological levels, from genomes, transcriptomes and proteomes, to the organs involved in sexual and sex-specific processes, including gonads, secondary sex organs and those with transcriptional sex-bias. The sexome also includes ontogenetic and behavioural aspects of sexual differentiation, including malfunction and impairment of SD, sexual differentiation and fertility. Starting from data generated by high-throughput approaches, we encourage others to contribute expertise to building understanding of the sexomes of many key vertebrate species. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

Sex determination without sex chromosomes

With or without sex chromosomes, sex determination is a synthesis of many molecular events that drives a community of cells towards a coordinated tissue fate. In this review, we will consider how a sex determination pathway can be engaged and stabilized without an inherited genetic determinant. In many reptilian species, no sex chromosomes have been identified, yet a conserved network of gene expression is initiated. Recent studies propose that epigenetic regulation mediates the effects of temperature on these genes through dynamic post-transcriptional, post-translational and metabolic pathways. It is likely that there is no singular regulator of sex determination, but rather an accumulation of molecular events that shift the scales towards one fate over another until a threshold is reached sufficient to maintain and stabilize one pathway and repress the alternative pathway. Investigations into the mechanism underlying sex determination without sex chromosomes should focus on cellular processes that are frequently activated by multiple stimuli or can synthesize multiple inputs and drive a coordinated response. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

Differentiation of fetal sertoli cells in the adult testis

Sertoli cells proliferate and construct seminiferous tubules during fetal life, then undergo differentiation and maturation in the prepubertal testes. In the adult testes, mature Sertoli cells maintain spermatogonia and support spermatogenesis during the entire lifetime. Although Sertoli-like cells have been derived from iPS cells, they tend to remain immature. To investigate whether Sertoli cells can spontaneously acquire the ability to support spermatogenesis when transferred into the adult testis, we transplanted mouse fetal testicular cells into a Sertoli-depleted adult testis. We found that donor E12.5, E14.5 and E16.5 Sertoli cells colonized adult seminiferous tubules and supported host spermatogenesis 2 months after transplantation, demonstrating that immature fetal Sertoli cells can undergo sufficient maturation in the adult testis to become functional. This technique will be useful to analyze the developmental process of Sertoli cell maturation and to investigate the potential of iPS-derived Sertoli cells to colonize, undergo maturation, and support spermatogenesis within the testis environment.

Mapping the peripheral nervous system in the whole mouse via compressed sensing tractography

Objective.The peripheral nervous system (PNS) connects the central nervous system with the rest of the body to regulate many physiological functions and is therapeutically targeted to treat diseases such as epilepsy, depression, intestinal dysmotility, chronic pain, and more. However, we still lack understanding of PNS innervation in most organs because the large span, diffuse nature, and small terminal nerve bundle fibers have precluded whole-organism, high resolution mapping of the PNS. We sought to produce a comprehensive peripheral nerve atlas for use in future interrogation of neural circuitry and selection of targets for neuromodulation.Approach.We used diffusion tensor magnetic resonance imaging (DT-MRI) with high-speed compressed sensing to generate a tractogram of the whole mouse PNS. The tractography generated from the DT-MRI data is validated using lightsheet microscopy on optically cleared, antibody stained tissue.Main results.Herein we demonstrate the first comprehensive PNS tractography in a whole mouse. Using this technique, we scanned the whole mouse in 28 h and mapped PNS innervation and fiber network in multiple organs including heart, lung, liver, kidneys, stomach, intestines, and bladder at 70µm resolution. This whole-body PNS tractography map has provided unparalleled information; for example, it delineates the innervation along the gastrointestinal tract by multiple sacral levels and by the vagal nerves. The map enabled a quantitative tractogram that revealed relative innervation of the major organs by each vertebral foramen as well as the vagus nerve.Significance.This novel high-resolution nerve atlas provides a potential roadmap for future neuromodulation therapies and other investigations into the neural circuits which drive homeostasis and disease throughout the body.

Loss of Mafb and Maf distorts myeloid cell ratios and disrupts fetal mouse testis vascularization and organogenesis†

Testis differentiation is initiated when Sry in pre-Sertoli cells directs the gonad toward a male-specific fate. Sertoli cells are essential for testis development, but cell types within the interstitial compartment, such as immune and endothelial cells, are also critical for organ formation. Our previous work implicated macrophages in fetal testis morphogenesis, but little is known about genes underlying immune cell development during organogenesis. Here we examine the role of the immune-associated genes Mafb and Maf in mouse fetal gonad development, and we demonstrate that deletion of these genes leads to aberrant hematopoiesis manifested by supernumerary gonadal monocytes. Mafb; Maf double knockout embryos underwent initial gonadal sex determination normally, but exhibited testicular hypervascularization, testis cord formation defects, Leydig cell deficit, and a reduced number of germ cells. In general, Mafb and Maf alone were dispensable for gonad development; however, when both genes were deleted, we observed significant defects in testicular morphogenesis, indicating that Mafb and Maf work redundantly during testis differentiation. These results demonstrate previously unappreciated roles for Mafb and Maf in immune and vascular development and highlight the importance of interstitial cells in gonadal differentiation.

A transgenic DND1GFP fusion allele reports in vivo expression and RNA-binding targets in undifferentiated mouse germ cells†

In vertebrates, the RNA-binding protein (RBP) dead end 1 (DND1) is essential for primordial germ cell (PGC) survival and maintenance of cell identity. In multiple species, Dnd1 loss or mutation leads to severe PGC loss soon after specification or, in some species, germ cell transformation to somatic lineages. Our investigations into the role of DND1 in PGC specification and differentiation have been limited by the absence of an available antibody. To address this problem, we used CRISPR/Cas9 gene editing to establish a transgenic mouse line carrying a DND1GFP fusion allele. We present imaging analysis of DND1GFP expression showing that DND1GFP expression is heterogeneous among male germ cells (MGCs) and female germ cells (FGCs). DND1GFP was detected in MGCs throughout fetal life but lost from FGCs at meiotic entry. In postnatal and adult testes, DND1GFP expression correlated with classic markers for the premeiotic spermatogonial population. Utilizing the GFP tag for RNA immunoprecipitation (RIP) analysis in MGCs validated this transgenic as a tool for identifying in vivo transcript targets of DND1. The DND1GFP mouse line is a novel tool for isolation and analysis of embryonic and fetal germ cells, and the spermatogonial population of the postnatal and adult testis.

Temperature-dependent sex determination is mediated by pSTAT3 repression of Kdm6b

In many reptiles, including the red-eared slider turtle Trachemys scripta elegans (T. scripta), sex is determined by ambient temperature during embryogenesis. We previously showed that the epigenetic regulator Kdm6b is elevated at the male-producing temperature and essential to activate the male pathway. In this work, we established a causal link between temperature and transcriptional regulation of Kdm6b. We show that signal transducer and activator of transcription 3 (STAT3) is phosphorylated at the warmer, female-producing temperature, binds the Kdm6b locus, and represses Kdm6b transcription, blocking the male pathway. Influx of Ca2+, a mediator of STAT3 phosphorylation, is elevated at the female temperature and acts as a temperature-sensitive regulator of STAT3 activation.

Intravital imaging of mouse embryos

Embryonic development is a complex process that is unamenable to direct observation. In this study, we implanted a window to the mouse uterus to visualize the developing embryo from embryonic day 9.5 to birth. This removable intravital window allowed manipulation and high-resolution imaging. In live mouse embryos, we observed transient neurotransmission and early vascularization of neural crest cell (NCC)-derived perivascular cells in the brain, autophagy in the retina, viral gene delivery, and chemical diffusion through the placenta. We combined the imaging window with in utero electroporation to label and track cell division and movement within embryos and observed that clusters of mouse NCC-derived cells expanded in interspecies chimeras, whereas adjacent human donor NCC-derived cells shrank. This technique can be combined with various tissue manipulation and microscopy methods to study the processes of development at unprecedented spatiotemporal resolution.

Combined iDISCO and CUBIC tissue clearing and lightsheet microscopy for in toto analysis of the adult mouse ovary†

At any given time, the ovary contains a number of follicles in distinct growth stages, each with a set of identifying characteristics. Although follicle counting and staging using histological stains on paraffin-embedded ovary sections has been the gold standard in assessing ovarian health in fertility studies, the final counts rely on extrapolation factors that diverge greatly among studies. These methods also limit our ability to investigate spatial aspects of ovary organization. Recent advances in optical tissue clearing and lightsheet microscopy have permitted comprehensive analysis of intact tissues. In this study, we set out to determine the best clearing and imaging methods to generate 3D images of the complete adult mouse ovary that could be used for accurate assessments of ovarian follicles. We found that a combination of iDISCO and CUBIC was the best method to clear the immunostained ovary. Using lightsheet microscopy, we generated 3D images of the intact ovary and performed qualitative assessments of follicles at all stages of development. This study is an important step toward developing quantitative computational models that allow rapid and accurate assessments of growing and quiescent primordial follicles, and to investigate the integrity of extrinsic ovarian components including vascular and neuronal networks.

Sertoli cell ablation and replacement of the spermatogonial niche in mouse

Spermatogonia, which produce sperm throughout the male lifetime, are regulated inside a niche composed of Sertoli cells, and other testis cell types. Defects in Sertoli cells often lead to infertility, but replacement of defective cells has been limited by the inability to deplete the existing population. Here, we use an FDA-approved non-toxic drug, benzalkonium chloride (BC), to deplete testis cell types in vivo. Four days after BC administration, Sertoli cells are preferentially depleted, and can be replaced to promote spermatogenesis from surviving (host) spermatogonia. Seven days after BC treatment, multiple cell types can be engrafted from fresh or cryopreserved testicular cells, leading to complete spermatogenesis from donor cells. These methods will be valuable for investigation of niche-supporting cell interactions, have the potential to lead to a therapy for idiopathic male infertility in the clinic, and could open the door to production of sperm from other species in the mouse.

Sertoli cells and other somatic cells of the testis comprise the germ cell niche and are critical to regulate spermatogenesis. Here the authors present a method in which Sertoli cells are selectively targeted for ablation by the compound benzalkonium chloride (BC) in mice, and the spermatogenic niche is subsequently repopulated in regions that have been affected by BC treatment.

WOMEN IN REPRODUCTIVE SCIENCE: To be or not to be a testis

Work that established the testis as the driver of male development, and the Y chromosome as the bearer of the male-determining gene, established a working model, and set the stage for the molecular age of mammalian sex determination. The discovery and characterization of Sry/SRY at the top of the hierarchy in mammals launched the field in two major directions. The first was to identify the downstream transcription factors and other molecular players that drive the bifurcation of Sertoli and granulosa cell differentiation. The second major direction was to understand organogenesis of the early bipotential gonad, and how divergence of its two distinct morphogenetic pathways (testis and ovary) is regulated at the cellular level. This review will summarize the early discoveries soon after Sry was identified and focus on my study of the gonad as a model of organogenesis.

The RNA-binding protein DND1 acts sequentially as a negative regulator of pluripotency and a positive regulator of epigenetic modifiers required for germ cell reprogramming

The adult spermatogonial stem cell population arises from pluripotent primordial germ cells (PGCs) that enter the fetal testis around embryonic day (E)10.5. PGCs undergo rapid mitotic proliferation, then enter prolonged cell cycle arrest (G1/G0), during which they transition to pro-spermatogonia. In mice homozygous for the Ter mutation in the RNA-binding protein Dnd1 (Dnd1Ter/Ter ), many male germ cells (MGCs) fail to enter G1/G0 and instead form teratomas: tumors containing many embryonic cell types. To investigate the origin of these tumors, we sequenced the MGC transcriptome in Dnd1Ter/Ter mutants at E12.5, E13.5 and E14.5, immediately prior to teratoma formation, and correlated this information with DO-RIP-Seq-identified DND1 direct targets. Consistent with previous results, we found DND1 controls downregulation of many genes associated with pluripotency and active cell cycle, including mTor, Hippo and Bmp/Nodal signaling pathway elements. However, DND1 targets also include genes associated with male differentiation, including a large group of chromatin regulators activated in wild-type but not mutant MGCs during the E13.5 and E14.5 transition. Results suggest multiple DND1 functions and link DND1 to initiation of epigenetic modifications in MGCs.

CBX2 is required to stabilize the testis pathway by repressing Wnt signaling

XX and XY fetal gonads are initially bipotential, poised between the ovary and testis fate. Multiple lines of evidence suggest that commitment to testis fate requires the repression of genes associated with ovary fate. It was previously shown that loss of CBX2, the subunit of the Polycomb Repressive Complex 1 (PRC1) that binds H3K27me3 and mediates silencing, leads to ovary development in XY mice and humans. While it had been proposed that CBX2 is an activator of the testis-determining gene Sry, we investigated the alternative possibility that CBX2 has a direct role as a repressor of the antagonistic ovary-promoting pathway. To investigate this possibility, we developed a quantitative genome-wide profile of the repressive histone mark H3K27me3 and its active counterpart H3K4me3 in isolated XY and XX gonadal supporting cells before and after sex determination. We show that testis and ovary sex-determining (SD) genes are bivalent before sex determination, providing insight into how the bipotential state of the gonad is established at the epigenetic level. After sex determination, many SD genes of the alternate pathway remain bivalent, possibly contributing to the ability of these cells to transdifferentiate even in adults. The finding that many genes in the Wnt signaling pathway were targeted for H3K27me3-mediated repression in Sertoli cells led us to test whether deletion of Wnt4 could rescue testis development in Cbx2 mutants. We show that Sry expression and testis development were rescued in XY Cbx2^-/-;Wnt4^-/- mice. Furthermore, we show that CBX2 directly binds the downstream Wnt signaler Lef1, an ovary-promoting gene that remains bivalent in Sertoli cells. Our results suggest that stabilization of the testis fate requires CBX2-mediated repression of bivalent ovary-determining genes, which would otherwise block testis development.

During development, the bipotential fetal gonad can commit to the testis fate or to the ovary fate. Mutation of the epigenetic regulator CBX2 leads to ovary development in XY embryos, suggesting a critical role for chromatin remodeling during sex determination. However, the epigenetic mechanisms that regulate the testis vs. ovary cell-fate decision in the mammalian bipotential gonad are poorly understood. In this study, we developed a genome-wide profile of two histone modifications that play critical roles during development: H3K27me3 (repressive) and H3K4me3 (active). We find that sex-determining genes that are initially co-expressed in XX and XY bipotential gonads are bivalent (marked by both H3K4me3 and H3K27me3) prior to sex determination, poised to engage either the testis or ovary fate. Remarkably, after sex determination, repressed genes that promote the alternate fate remain bivalent. We show that stabilization of the testis fate requires CBX2-mediated repression of bivalent ovary-determining genes, which would otherwise block testis development. Our study provides insight into how the bipotential state of the gonad is established at the epigenetic level, and how the testis fate is stabilized by repression of the ovary fate during sex determination.

Epigenetic regulation of male fate commitment from an initially bipotential system

A fundamental goal in biology is to understand how distinct cell types containing the same genetic information arise from a single stem cell throughout development. Sex determination is a key developmental process that requires a unidirectional commitment of an initially bipotential gonad towards either the male or female fate. This makes sex determination a unique model to study cell fate commitment and differentiation in vivo. We have focused this review on the accumulating evidence that epigenetic mechanisms contribute to the bipotential state of the fetal gonad and to the regulation of chromatin accessibility during and immediately downstream of the primary sex-determining switch that establishes the male fate.

Dmrt1 induces the male pathway in a turtle species with temperature-dependent sex determination

The molecular mechanism underlying temperature-dependent sex determination (TSD) has been a long-standing mystery; in particular, the thermosensitive genetic triggers for gonadal sex differentiation are largely unknown. Here, we have characterized a conserved DM domain gene, Dmrt1, in the red-eared slider turtle Trachemys scripta (T. scripta), which exhibits TSD. We found that Dmrt1 has a temperature-dependent, sexually dimorphic expression pattern, preceding gonadal sex differentiation, and is capable of responding rapidly to temperature shifts and aromatase inhibitor treatment. Most importantly, loss- and gain-of-function analyses provide solid evidence that Dmrt1 is both necessary and sufficient to initiate male development in T. scripta Furthermore, the DNA methylation dynamics of the Dmrt1 promoter are tightly correlated with temperature and could mediate the impact of temperature on sex determination. Collectively, our findings demonstrate that Dmrt1 is a candidate master male sex-determining gene in this TSD species, consistent with the idea that DM domain genes are conserved during the evolution of sex determination mechanisms.

Numb regulates somatic cell lineage commitment during early gonadogenesis in mice

During early gonadogenesis, proliferating cells in the coelomic epithelium (CE) give rise to most of the somatic cells in both XX and XY gonads. Previous dye-labeling experiments showed that a single CE cell could give rise to additional CE cells and to both supporting and interstitial cell lineages, implying that cells in the CE domain are multipotent progenitors, and suggesting that an asymmetric division is involved in the acquisition of gonadal cell fates. We found that NUMB is asymmetrically localized in CE cells, suggesting that it might be involved. To test this hypothesis, we conditionally deleted Numb on a Numbl mutant background just prior to gonadogenesis. Mutant gonads showed a loss of cell polarity in the surface epithelial layers, large interior cell patches expressing the undifferentiated cell marker LHX9, and a loss of differentiated cells in somatic cell lineages. These results indicate that NUMB is necessary for establishing polarity in CE cells, and that asymmetric divisions resulting from CE polarity are required for commitment to differentiated somatic cell fates. Surprisingly, germ cells, which do not arise from the CE, were also affected in mutants, which may be a direct or indirect effect of loss of Numb.

Genome-wide identification of regulatory elements in Sertoli cells

A current goal of molecular biology is to identify transcriptional networks that regulate cell differentiation. However, identifying functional gene regulatory elements has been challenging in the context of developing tissues where material is limited and cell types are mixed. To identify regulatory sites during sex determination, we subjected Sertoli cells from mouse fetal testes to DNaseI-seq and ChIP-seq for H3K27ac. DNaseI-seq identified putative regulatory sites around genes enriched in Sertoli and pregranulosa cells; however, active enhancers marked by H3K27ac were enriched proximal to only Sertoli-enriched genes. Sequence analysis identified putative binding sites of known and novel transcription factors likely controlling Sertoli cell differentiation. As a validation of this approach, we identified a novel Sertoli cell enhancer upstream of Wt1, and used it to drive expression of a transgenic reporter in Sertoli cells. This work furthers our understanding of the complex genetic network that underlies sex determination and identifies regions that potentially harbor non-coding mutations underlying disorders of sexual development.

A grafted ovarian fragment rescues host fertility after chemotherapy

STUDY QUESTION

Can host fertility be rescued by grafting of a fragment of a healthy ovary soon after chemotherapy?

SUMMARY ANSWER

We found that grafting a green fluorescent protein (GFP)-positive fragment from a healthy isogenic ovary to the left ovary of a chemo-treated host rescued function and fertility of the grafted host ovary, and resulted in the production of host-derived offspring as late as the sixth litter after chemotherapy (CTx) treatment, whereas none of the ungrafted controls produced a second litter.

WHAT IS KNOWN ALREADY

In women and girls undergoing chemotherapy, infertility and premature ovarian failure are frequent outcomes. There are accumulating reports of improved endocrine function after autotransplantation of an ovarian fragment, raising the possibility that the transplant is beneficial to the endogenous ovary.

STUDY DESIGN, SIZE, DURATION

We first established a CTx treatment regimen that resulted in the permanent loss of fertility in 100% of female mice of the FVB inbred strain. We grafted an isogenic ovary fragment from a healthy female homozygous for a GFP transgene to the left ovary of 100 CTx-treated hosts, and compared fertility to 39 ungrafted controls in 6 months of continuous matings, using GFP to distinguish offspring derived from the graft, and those derived from the host.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Immunofluoresece and western blot analysis of 39 treated ovaries during and 15 days after CTx treatment revealed elevated apoptosis, rapid loss of granulosa cells and an increased recruitment of growing follicles. Using immunofluorescence and confocal imaging, we tracked the outcome of the grafted tissue over 4 months and its effect on the adjacent and contralateral ovary of the host.

MAIN RESULTS AND THE ROLE OF CHANCE

Fifty-three percent of grafted females produced a second litter whereas none of the ungrafted females produced a second litter. The likelihood that this could occur by chance is very low (P < 0.0001).

LIMITATIONS, REASONS FOR CAUTION

These results are shown only in mice, and whether or how they might apply to chemotherapy patients subjected to different CTx regimens is not yet clear.

WIDER IMPLICATIONS OF THE FINDINGS

Our experiments prove that rescue of a chemo-treated ovary is possible, and establish a system to investigate the mechanism of rescue and to identify the factors responsible with the long-term goal of developing therapies for preservation of ovarian endocrine function and fertility in women undergoing chemotherapy.

LARGE SCALE DATA

No large datasets were produced.

STUDY FUNDING/COMPETING INTERESTS

Duke University Medical Center Chancellor's Discovery Grant to BC; ESJ was supported by an NRSA 5F31CA165545; SK was supported by NIH RO1 GM08033; RWT was supported by the Duke University School of Medicine Ovarian Cancer Research Fellowship; XBM was supported by CONICYT. The authors have no conflicts of interest to declare.

A timecourse analysis of systemic and gonadal effects of temperature on sexual development of the red-eared slider turtle Trachemys scripta elegans

Temperature dependent sex determination (TSD) is the process by which the environmental temperature experienced during embryogenesis influences the sex of an organism, as in the red-eared slider turtle Trachemys scripta elegans. In accord with current paradigms of vertebrate sex determination, temperature is believed to exert its effects on sexual development in T. scripta entirely within the middle third of development, when the gonad is forming. However, whether temperature regulates the transcriptome in T. scripta early embryos in a manner that could influence secondary sex characteristics or establish a pro-male or pro-female environment has not been investigated. In addition, apart from a handful of candidate genes, very little is known about potential similarities between the expression cascade during TSD and the genetic cascade that drives mammalian sex determination. Here, we conducted an unbiased transcriptome-wide analysis of the effects of male- and female-promoting temperatures on the turtle embryo prior to gonad formation, and on the gonad during the temperature sensitive period. We found sexually dimorphic expression reflecting differences in steroidogenic enzymes and brain development prior to gonad formation. Within the gonad, we mapped a cascade of differential expression similar to the genetic cascade established in mammals. Using a Hidden Markov Model based clustering approach, we identified groups of genes that show heterochronic shifts between M. musculus and T. scripta. We propose a model in which multiple factors influenced by temperature accumulate during early gonadogenesis, and converge on the antagonistic regulation of aromatase to canalize sex determination near the end of the temperature sensitive window of development.

Cycling in the Cell Fate Landscape

The causal relationship between the cell cycle and cell fate has recently been reconsidered in several developmental systems, now including sex determination. Direct links between cell cycle machinery and developmental factors demonstrate how modulating the cell cycle can have a profound influence on cellular decisions, through phenomena largely divided into phase-specific interactions seen as "priming" and "competence," or global changes in transcription or histone modifications during transitions. This integration challenges the common view of cell fate landscapes.

Left-Biased Spermatogenic Failure in 129/SvJ Dnd1Ter/+ Mice Correlates with Differences in Vascular Architecture, Oxygen Availability, and Metabolites

Homozygosity for the Ter mutation in the RNA-binding protein Dead end 1 (Dnd1(Ter/Ter)) sensitizes germ cells to degeneration in all mouse strains. In 129/SvJ mice, approximately 10% of Dnd1(Ter/+) heterozygotes develop spermatogenic failure, and 95% of unilateral cases occur in the left testis. The first differences between right and left testes were detected at Postnatal Day 15 when many more spermatogonial stem cells (SSCs) were undergoing apoptosis in the left testis compared to the right. As we detected no significant left/right differences in the molecular pathway associated with body axis asymmetry or in the expression of signals known to promote proliferation, differentiation, and survival of germ cells, we investigated whether physiological differences might account for asymmetry of the degeneration phenotype. We show that left/right differences in vascular architecture are associated with a decrease in hemoglobin saturation and increased levels of HIF-1alpha in the left testis compared to the right. In Dnd1 heterozygotes, lower oxygen availability was associated with metabolic differences, including lower levels of ATP and NADH in the left testis. These experiments suggest a dependence on oxygen availability and metabolic substrates for SSC survival and suggest that Dnd1(Ter/+) SSCs may act as efficient sensors to detect subtle environmental changes that alter SSC fate.

Cell fate commitment during mammalian sex determination

The gonads form bilaterally as bipotential organs that can develop as testes or ovaries. All secondary sex characteristics that we associate with 'maleness' or 'femaleness' depend on whether testes or ovaries form. The fate of the gonads depends on a cell fate decision that occurs in a somatic cell referred to as the 'supporting cell lineage'. Once supporting cell progenitors commit to Sertoli (male) or granulosa (female) fate, they propagate this decision to the other cells within the organ. In this review, we will describe what is known about the bipotential state of somatic and germ cell lineages in the gonad and the transcriptional and antagonistic signaling networks that lead to commitment, propagation, and maintenance of testis or ovary fate.

Conserved action of β-catenin during female fate determination in the red-eared slider turtle

In reptiles such as the red-eared slider turtle Trachemys scripta, development of an ovary from the bipotential gonad requires a coordinated expansion of the cortical domain and regression of the medulla. While estrogen, which is necessary and sufficient for ovarian development in non-mammalian vertebrates, is thought to feminize both compartments, it remains unclear whether there is a signaling relationship between the two domains that coordinates their fates. We show that aromatase, the estrogen-synthesizing enzyme, is localized to the medulla of the differentiating turtle ovary and that differentiation of the medulla precedes and is independent of cortical expansion. Coordinated feminization of the two domains may therefore rely on an estrogenic signal from the differentiating medulla. In eutherian mammals, where estrogen is dispensable for early ovary development, the canonical Wnt signaling pathway is critical for female fate determination. Whether this function is conserved among vertebrates and how it is potentially integrated with estrogen signaling are uncertain. Using a novel in vitro turtle gonad culture system, we demonstrate that ectopic activation of the canonical Wnt signaling pathway in presumptive male gonads induced a partial sex-reversal of the medulla, but inhibition of the pathway was not sufficient to sex-reverse differentiating ovaries. These patterns are similar to those previously observed in mice. Wnt signaling appears to function downstream of estrogen, as ectopic activation of the pathway rescued female development when estrogen synthesis was inhibited. Our findings therefore suggest that the ovary-promoting effects of the Wnt signaling pathway may be functionally conserved between mammals and reptiles.

Fine time course expression analysis identifies cascades of activation and repression and maps a putative regulator of mammalian sex determination

In vertebrates, primary sex determination refers to the decision within a bipotential organ precursor to differentiate as a testis or ovary. Bifurcation of organ fate begins between embryonic day (E) 11.0-E12.0 in mice and likely involves a dynamic transcription network that is poorly understood. To elucidate the first steps of sexual fate specification, we profiled the XX and XY gonad transcriptomes at fine granularity during this period and resolved cascades of gene activation and repression. C57BL/6J (B6) XY gonads showed a consistent ~5-hour delay in the activation of most male pathway genes and repression of female pathway genes relative to 129S1/SvImJ, which likely explains the sensitivity of the B6 strain to male-to-female sex reversal. Using this fine time course data, we predicted novel regulatory genes underlying expression QTLs (eQTLs) mapped in a previous study. To test predictions, we developed an in vitro gonad primary cell assay and optimized a lentivirus-based shRNA delivery method to silence candidate genes and quantify effects on putative targets. We provide strong evidence that Lmo4 (Lim-domain only 4) is a novel regulator of sex determination upstream of SF1 (Nr5a1), Sox9, Fgf9, and Col9a3. This approach can be readily applied to identify regulatory interactions in other systems.

Testicular teratomas: an intersection of pluripotency, differentiation and cancer biology

Teratomas represent a critical interface between stem cells, differentiation and tumorigenesis. These tumors are composed of cell types representing all three germ layers reflecting the pluripotent nature of their cell of origin. The study of these curious tumors became possible when Leroy Stevens identified the 129 mouse strain as a model of spontaneous testicular teratoma and later isolated a substrain carrying the Ter mutation, a potent modifier of tumor incidence. Early studies with 129 mice lead to the discovery of embryonal carcinoma (EC) cells which played a foundational role in the embryonic stem (ES) cell field and the study of pluripotency. The cells of origin of testicular teratomas are germ cells. During early development, primordial germ cells diverge from somatic differentiation and establish their pluripotent nature, maintaining or re-expressing core pluripotency genes; Oct4, Sox2 and Nanog. It is believed that a misregulation of male germ cell pluripotency plays a critical role in teratoma development. Several mouse models of teratoma development have now been identified, including a chromosome substitution strain, 129-Chr19(MOLF), conditional Dmrt1 and Pten alleles and the Ter mutation in the Dnd1 gene. However, it is still unknown what role somatic cells and/or physiology play in the sensitivity to teratoma development. These unusual tumors may hold the key to understanding how pluripotency is regulated in vivo.

Testosterone levels influence mouse fetal Leydig cell progenitors through notch signaling

Leydig cells are the steroidogenic lineage of the mammalian testis that produces testosterone, a key hormone required throughout male fetal and adult life for virilization and spermatogenesis. Both fetal and adult Leydig cells arise from a progenitor population in the testis interstitium but are thought to be lineage-independent of one another. Genetic evidence indicates that Notch signaling is required during fetal life to maintain a balance between differentiated Leydig cells and their progenitors, but the elusive progenitor cell type and ligands involved have not been identified. In this study, we show that the Notch pathway signals through the ligand JAG1 in perivascular interstitial cells during fetal life. In the early postnatal testis, we show that circulating levels of testosterone directly affect Notch signaling, implicating a feedback role for systemic circulating factors in the regulation of progenitor cells. Between Postnatal Days 3 and 21, as fetal Leydig cells disappear from the testis and are replaced by adult Leydig cells, the perivascular population of interstitial cells active for Notch signaling declines, consistent with distinct regulation of adult Leydig progenitors.

The western painted turtle genome, a model for the evolution of extreme physiological adaptations in a slowly evolving lineage

BACKGROUND

We describe the genome of the western painted turtle, Chrysemys picta bellii, one of the most widespread, abundant, and well-studied turtles. We place the genome into a comparative evolutionary context, and focus on genomic features associated with tooth loss, immune function, longevity, sex differentiation and determination, and the species' physiological capacities to withstand extreme anoxia and tissue freezing.

RESULTS

Our phylogenetic analyses confirm that turtles are the sister group to living archosaurs, and demonstrate an extraordinarily slow rate of sequence evolution in the painted turtle. The ability of the painted turtle to withstand complete anoxia and partial freezing appears to be associated with common vertebrate gene networks, and we identify candidate genes for future functional analyses. Tooth loss shares a common pattern of pseudogenization and degradation of tooth-specific genes with birds, although the rate of accumulation of mutations is much slower in the painted turtle. Genes associated with sex differentiation generally reflect phylogeny rather than convergence in sex determination functionality. Among gene families that demonstrate exceptional expansions or show signatures of strong natural selection, immune function and musculoskeletal patterning genes are consistently over-represented.

CONCLUSIONS

Our comparative genomic analyses indicate that common vertebrate regulatory networks, some of which have analogs in human diseases, are often involved in the western painted turtle's extraordinary physiological capacities. As these regulatory pathways are analyzed at the functional level, the painted turtle may offer important insights into the management of a number of human health disorders.

Constitutive activation of NOTCH1 signaling in Sertoli cells causes gonocyte exit from quiescence

Notch signaling components have long been detected in Sertoli and germ cells in the developing and mature testis. However, the role of this pathway in testis development and spermatogenesis remains unknown. Using reporter mice expressing green fluorescent protein following Notch receptor activation, we found that Notch signaling was active in Sertoli cells at various fetal, neonatal, and adult stages. Since Notch signaling specifies stem cell fate in many developing and mature organ systems, we hypothesized that maintenance and differentiation of gonocytes and/or spermatogonial stem cells would be modulated through this pathway in Sertoli cells. To this end, we generated mutant mice constitutively expressing the active, intracellular domain of NOTCH1 (NICD1) in Sertoli cells. We found that mutant Sertoli cells were morphologically normal before and after birth, but presented a number of functional changes that drastically affected gonocyte numbers and physiology. We observed aberrant exit of gonocytes from mitotic arrest, migration toward cord periphery, and premature differentiation before birth. These events, presumably unsupported by the cellular microenvironment, were followed by gonocyte apoptosis and near complete disappearance of the gonocytes by day 2 after birth. Molecular analysis demonstrated that these effects are correlated with a dysregulation of Sertoli-expressed genes that are required for germ cell maintenance, such as Cyp26b1 and Gdnf. Taken together, our results demonstrate that Notch signaling is active in Sertoli cells throughout development and that proper regulation of Notch signaling in Sertoli cells is required for the maintenance of gonocytes in an undifferentiated state during fetal development.

Abstract 228: Transcriptional Networks Involved in Differentiation of Renal Progenitor Cells to the Juxtaglomerular Cell Fate

Background: The molecular processes regulating the development of renin producing juxtaglomerular (JG) cells are not fully understood. We have previously shown that LXRα and cAMP treatment can induce mesenchymal stromal cells (MSCs) from different sources to differentiate into renin expressing JG-like cells. Here, we investigated the LXRα/cAMP induced transcriptional networks involved in the commitment of progenitor cells to the JG lineage.

METHODS: Adult renal CD44+ MSCs were isolated from C57BL/6 Ren1c YFP mice. MSCs were differentiated into YFP+ renin expressing cells by treatment with LXRα ligand (T0901317) or cAMP. The Affymetrix Mouse 430 2.0 array was used for profiling. Data analysis was focused on transcriptional and epigenetic regulators and performed by using Dchip, David or Toppgene.

Results: Differentiated YFP+ cells from either cAMP or LXRα treatment expressed high levels of renin and renin regulators such as Hoxd8 and Cebp. Untreated MSCs showed expression of metanephric markers such as Foxd1, Sox11 and Eya. Treatment with cAMP resulted in differential expression of 108 transcripts in YFP+ cells as compared to control MSC. The majority of these genes (82) were down-regulated and involved in chromatin acetylation (23 out of 82, P<2E-5) or embryonic and hematopoietic development (36 out of 82 respectively, P<2E-5). Mesenchymal markers (i.e. Sox11, FC= -3) and repressors of differentiation (i.e. Id2, FC= -2) were also decreased significantly. Similarly, MSC activation by LXRα resulted in differential expression of 98 transcripts, 67 of which were decreased compared to baseline. As with cAMP, the majority of genes were involved in differentiation and chromatin remodeling (i.e Tbx3, Klfs and Mll); still, the overlap of genes between the two treatments was minimal (24 genes) suggesting that cAMP and LXRα might act complementary to each other.

Conclusion: Our data indicate that cAMP and LXRα, rather than directly define JG cell fate, modulate the epigenetic status of MSCs and prevent differentiation to other lineages. Systems biology approaches are under way to evaluate the importance of the transcriptional networks identified. We anticipate that these results will provide novel insights into the mechanisms regulating JG cell fate.