Establishment of testis-specific SOX9 activation requires high-glucose metabolism in mouse sex differentiation

Complete male-to-female sex reversal in XY mice lacking the miR-17~92 cluster

Mammalian sex determination is controlled by antagonistic gene cascades operating in embryonic undifferentiated gonads. The expression of the Y-linked gene SRY is sufficient to trigger the testicular pathway, whereas its absence in XX embryos leads to ovarian differentiation. Yet, the potential involvement of non-coding regulation in this process remains unclear. Here we show that the deletion of a single microRNA cluster, miR-17~92, induces complete primary male-to-female sex reversal in XY mice. Sry expression is delayed in XY knockout gonads, which develop as ovaries. Sertoli cell differentiation is reduced, delayed and unable to sustain testicular development. Pre-supporting cells in mutant gonads undergo a transient state of sex ambiguity which is subsequently resolved towards the ovarian fate. The miR-17~92 predicted target genes are upregulated, affecting the fine regulation of gene networks controlling gonad development. Thus, microRNAs emerge as key components for mammalian sex determination, controlling Sry expression timing and Sertoli cell differentiation.

External genitalia phenotypes of a Mab21l1-null mouse model for cerebellar, ocular, craniofacial, and genital (COFG) syndrome

The cerebellar, ocular, craniofacial, and genital (COFG) syndrome is a human genetic disease that is caused by MAB21L1 mutations. A COFG mouse model with Mab21l1-null mutation causes severe microphthalmia and fontanelle dysosteogenesis, similar to the symptoms in human patients. One of the typical symptoms is scrotal agenesis in male infants, while male Mab21l1-null mice show hypoplastic preputial glands, a rodent-specific derivative of the cranial scrotal fold. However, it is still unclear where and how MAB21Ll acts in the external genitalia in both mice and humans. Here we show that, at the neonatal stage, MAB21L1 expression in the external genitalia was restricted to two mesenchymal cell populations-underneath the scrotal and labial skin and around the preputial and clitoral glands (PG/CG). Morphometric analyses of the Mab21l1-/- pups revealed a significant reduction in the external size of the scrotum, vulva, and CG, as well as PG. In the periglandular region around PG and CG, the periglandular mesenchymal cells showed a drastic reduction in both cell density and immunoreactive signals for several extracellular matrix proteins (e.g., collagen I, fibronectin, and proteoglycans), together with their reduced Ki67-positive cell proliferation index. In the Mab21l1-/- PG/CG, together with reduced vascularization, the glandular epithelia displayed atrophy with discontinuous basal lamina along the basal surface and defective glycogen accumulation in their cytoplasm. Under a 5-day organ culture of the isolated PG, the Mab21l1-/- explants showed poor outgrowth and retention of the glandular structure in vitro. However, the addition of exogenous Matrigel could partially rescue such tissue-autonomous phenotypes, showing glandular morphology similar to that of the wild-type explants. These findings suggest that MAB21L1+ mesenchymal cells play a crucial role in providing nutrient ECM support for glandular outgrowth and morphogenesis in the peripheral external genitalia.

Direct diffusion of anti-Müllerian hormone from both the cranial and caudal regions of the testis during early gonadal development in mice

BACKGROUND

The Müllerian duct (MD), the primordium of the female reproductive tract, is also formed in males during the early stage of development, then regresses due to the anti-Müllerian hormone (AMH) secreted from the testes. However, the detailed diffusion pathway of AMH remains unclear. We herein investigated the mechanism by which AMH reaches the middle region of the MD using an organ culture system.

RESULTS

Injection of recombinant human AMH into the testis around the start of MD regression induced diffuse immunoreactivity in the mesonephros near the injection site. When the testis and mesonephros were cultured separately, the diameters of both cranial and middle MDs were significantly increased compared to the control. In the testis-mesonephros complex cultured by inhibiting the diffusion of AMH through the cranial region, the cranial MD diameter was significantly increased compared to the control, and there was no difference in middle MD diameter.

CONCLUSIONS

These results indicate that AMH, which infiltrates from the testis through the cranial region at physiological concentrations, induces regression of the cranial MD at the start of MD regression. They also indicate that AMH infiltrating through the caudal regions induces regression of the middle MD.

Loss of NR5A1 in mouse Sertoli cells after sex determination changes cellular identity and induces cell death by anoikis

To investigate the role of the nuclear receptor NR5A1 in the testis after sex determination, we analyzed mice lacking NR5A1 in Sertoli cells (SCs) from embryonic day (E) 13.5 onwards. Ablation of Nr5a1 impaired the expression of genes characteristic of SC identity (e.g. Sox9 and Amh), caused SC death from E14.5 onwards through a Trp53-independent mechanism related to anoikis, and induced disorganization of the testis cords. Together, these effects caused germ cells to enter meiosis and die. Single-cell RNA-sequencing experiments revealed that NR5A1-deficient SCs changed their molecular identity: some acquired a 'pre-granulosa-like' cell identity, whereas other reverted to a 'supporting progenitor-like' cell identity, most of them being 'intersex' because they expressed both testicular and ovarian genes. Fetal Leydig cells (LCs) did not display significant changes, indicating that SCs are not required beyond E14.5 for their emergence or maintenance. In contrast, adult LCs were absent from postnatal testes. In addition, adult mutant males displayed persistence of Müllerian duct derivatives, decreased anogenital distance and reduced penis length, which could be explained by the loss of AMH and testosterone synthesis due to SC failure.

Intrauterine hyponutrition reduces fetal testosterone production and postnatal sperm count in the mouse

Although intrauterine hyponutrition is regarded as a risk factor for the development of "testicular dysgenesis syndrome" (TDS) in the human, underlying mechanism(s) remain largely unknown. To clarify the underlying mechanism(s), we fed vaginal plug-positive C57BL/6N female mice with regular food ad libitum throughout the pregnant course (control females) (C-females) or with 50% of the mean daily intake of the C-females from 6.5 dpc (calorie-restricted females) (R-females), and compared male reproductive findings between 17.5-dpc-old male mice delivered from C-females (C-fetuses) and those delivered from R-females (R-fetuses) and between 6-week-old male mice born to C-females (C-offspring) and those born to R-females (R-offspring). Compared with the C-fetuses, the R-fetuses had (1) morphologically normal external genitalia with significantly reduced anogenital distance index, (2) normal numbers of testicular component cells, and (3) significantly low intratesticular testosterone, in association with significantly reduced expressions of steroidogenic genes. Furthermore, compared with the C-offspring, the R-offspring had (1) significantly increased TUNEL-positive cells and normal numbers of other testicular component cells, (2) normal intratesticular testosterone, in association with normal expressions of steroidogenic genes, (3) significantly reduced sperm count, and normal testis weight and sperm motility, and (4) significantly altered expressions of oxidation stress-related, apoptosis-related, and spermatogenesis-related genes. The results, together with the previous data including the association between testosterone deprivation and oxidative stress-evoked apoptotic activation, imply that reduced fetal testosterone production is the primary underlying factor for the development of TDS in intrauterine hyponutrition, and that TDS is included in the clinical spectrum of Developmental Origins of Health and Disease.

Assessment of Zearalenone-Induced Cell Survival and of Global Gene Regulation in Mouse TM4 Sertoli Cells

Zearalenone (ZEA) is a non-steroidal xenoestrogen mycotoxin produced by many Fusarium fungal species, which are common contaminants of cereal crops destined for worldwide human and animal consumption. ZEA has been reported in various male reproduction dysfonctions, including decreased fertility potential. In this report, the direct effect of ZEA on the immature Sertoli TM4 cell line was evaluated. The results show that high concentrations of ZEA increase reactive oxygen species via the activation of MAPK signaling. Transcriptome analysis was performed on the TM4 cell line treated with ZEA, and genes involved in sex differentiation (Fgfr2, Igf1, Notch1, Sox9) and extracellular matrix (ECM) formation (Ctgf, Fam20a, Fbn1, Mmp9, Postn, Sparcl1, Spp1) were identified at the center of the functional protein association network, suggesting that ZEA could be detrimental to the early steps of Sertoli cell differentiation.

Transcriptional Regulation of the Y-Linked Mammalian Testis-Determining Gene SRY

Mammalian male sex differentiation is triggered during embryogenesis by the activation of the Y-linked testis-determining gene SRY. Since insufficient or delayed expression of SRY results in XY gonadal sex reversal, accurate regulation of SRY is critical for male development in XY animals. In humans, dysregulation of SRY may cause disorders of sex development. Mouse Sry is the most intensively studied mammalian model of sex determination. Sry expression is controlled in a spatially and temporally stringent manner. Several transcription factors play a key role in sex determination as trans-acting factors for Sry expression. In addition, recent studies have shown that several epigenetic modifications of Sry are involved in sex determination as cis-acting factors for Sry expression. Herein, we review the current understanding of transcription factor- and epigenetic modifier-mediated regulation of SRY/Sry expression.

3D reconstruction and histopathological analyses on murine corporal body

PURPOSE

Erectile dysfunction (ED) is one of the increasing diseases with aging society. The basis of ED derived from local penile abnormality is poorly understood because of the complex three-dimensional (3D) distribution of sinusoids in corpus cavernosum (CC). Understanding the 3D histological structure of penis is thus necessary. Analyses on the status of regulatory signals for such abnormality are also performed.

METHODS

To analyze the 3D structure of sinusoid, 3D reconstruction from serial sections of murine CC were performed. Histological analyses between young (2 months old) and aged (14 months old) CC were performed. As for chondrogenic signaling status of aged CC, SOX9 and RBPJK staining was examined.

RESULTS

Sinusoids prominently developed in the outer regions of CC adjacent to tunica albuginea. Aged CC samples contained ectopic chondrocytes in such regions. Associating with the appearance of chondrocytes, the expression of SOX9, chondrogenic regulator, was upregulated. The expression of RBPJK, one of the Notch signal regulators, was downregulated in the aged CC.

CONCLUSIONS

Prominent sinusoids distribute in the outer region of CC which may possess important roles for erection. A possibility of ectopic chondrogenesis induced by alteration of SOX9/Notch signaling with aging is indicated.

Establishment of an organ culture system to induce Sertoli cell differentiation from undifferentiated mouse gonads

Organ culture systems are useful for elucidating the process of testicular differentiation from mammalian undifferentiated genetically male gonads, as they permit various experiments, including experiments involving the control of gene expression. However, without addition of testicular differentiation-related factors, it is difficult to induce the formation of testis cord from immature gonads by a time point earlier 12 tail somites (ts) that corresponding to 11.0 days post coitum (dpc). In this study, we attempted to establish an organ culture system that induces testis formation from immature gonads (around 8 ts: 10.5 dpc) just before Sry (sex-determining region of the Y chromosome) expression. A paired gonad-mesonephros complex of around 8 ts was placed in the groove of an agarose gel block and put the semi-cylindrical piece of agarose gel to maintain the gonad morphology. The gonads were cultured in the gas phase for 96 hr. As a result, testis cord-like structures appeared in many genetically male gonads. Cells expressing the Sertoli cell markers Sox9 (SRY-box 9) and Amh (anti-Müllerian hormone) were observed, while granulosa cell marker Foxl2 (forkhead box L2) was not detected. In addition, Sox9- and Amh-expressing cells were observed throughout the entire gonad in many individuals. Amh mRNA expression was also upregulated. Surprisingly, formation of a partial testicular structure was observed from more immature gonads (6 ts). These results show that our gonadal organ culture system is useful for elucidating the regulation mechanism of Sry expression in undifferentiated bipotential gonads.

The gene encoding the ketogenic enzyme HMGCS2 displays a unique expression during gonad development in mice

Disorders/differences of sex development (DSD) cause profound psychological and reproductive consequences for the affected individuals, however, most are still unexplained at the molecular level. Here, we present a novel gene, 3-hydroxy-3-methylglutaryl coenzyme A synthase 2 (HMGCS2), encoding a metabolic enzyme in the liver important for energy production from fatty acids, that shows an unusual expression pattern in developing fetal mouse gonads. Shortly after gonadal sex determination it is up-regulated in the developing testes following a very similar spatial and temporal pattern as the male-determining gene Sry in Sertoli cells before switching to ovarian enriched expression. To test if Hmgcs2 is important for gonad development in mammals, we pursued two lines of investigations. Firstly, we generated Hmgcs2-null mice using CRISPR/Cas9 and found that these mice had gonads that developed normally even on a sensitized background. Secondly, we screened 46,XY DSD patients with gonadal dysgenesis and identified two unrelated patients with a deletion and a deleterious missense variant in HMGCS2 respectively. However, both variants were heterozygous, suggesting that HMGCS2 might not be the causative gene. Analysis of a larger number of patients in the future might shed more light into the possible association of HMGCS2 with human gonadal development.

Extracellular enzymatically synthesized glycogen promotes osteogenesis by activating osteoblast differentiation via Akt/GSK-3β signaling pathway

Glycogen is the stored form of glucose and plays a major role in energy metabolism. Recently, it has become clear that enzymatically synthesized glycogen (ESG) has biological functions, such as the macrophage-stimulating activity. This study aimed to evaluate the effect of ESG on osteogenesis. MC3T3-E1 cells were cultured with ESG, and their cell proliferative activity and osteoblast differentiation were measured. An in vivo study was conducted in which ESG pellets with BMP-2 were grafted into mouse calvarial defects and histomorphometrically analyzed for the new bone formation. To confirm the effect of ESG on bone growth in vivo, ESG was orally administered to pregnant mice and the femurs of their pups were examined. We observed that ESG stimulated cell proliferation and enhanced messenger RNA expression of osteocalcin and osteopontin in MC3T3-E1 cells. ESG was taken up by the cells associated with GLUT-1 and activated the Akt/GSK-3β pathway. In vivo, the new bone formation in the calvarial defect was significantly accelerated by ESG and the maternal administration of ESG promoted fetal bone growth. In conclusion, ESG stimulates cell proliferation and differentiation of preosteoblasts via the activation of Akt/GSK-3β signaling and promotes new bone formation in vivo, suggesting that ESG could be a useful stimulant for osteogenesis.

Defects in the first wave of folliculogenesis in mouse XO ovaries

In mouse ovaries, the first wave of folliculogenesis perinatally starts near the medullary region, which directs the initial round of follicular growth soon after birth. At the same time, cortical primordial follicles start forming in the ovarian surface region, and then some are cyclically recruited for the second and subsequent rounds of follicular growth. Recent studies suggest different dynamics between the first and subsequent waves of follicular growth in postnatal ovaries. However, the phenotypic differences between these phases remain unclear. Here, we show direct evidence that XO female mice, a murine model for Turner Syndrome, lack the first wave of folliculogenesis. Our histopathological analyses of XX and XO littermates revealed a lack of anti-Müllerian hormone (AMH)-positive primary follicles in the XO ovaries by 4 days post partum (dpp). This loss of first follicles was also confirmed by histological bioassay for SRY-dependent SOX9 inducibility, a specific marker for the first follicular granulosa cells. In contrast, cortical primordial follicles formed properly in XO ovaries, and some of them formed primary and secondary follicles in the subcortical region by 7 dpp. They rapidly developed into late antral follicles, showing similarities to XX littermate ovaries by 21 dpp. These results suggest distinct X-monosomy effects between the first and subsequent waves of follicular growth, highlighting the high susceptibility to elimination of XO oocytes in the first wave of mammalian folliculogenesis.

Transient Decrease in Circulatory Testosterone and Homocysteine Precedes the Development of Metabolic Syndrome Features in Fructose-Fed Sprague Dawley Rats

Background. Increased fructose consumption is linked to the development of metabolic syndrome (MS). Here we investigated the time course of development of MS features in high-fructose-fed Sprague Dawley rats along with circulatory testosterone and homocysteine levels. Methods. Rats were divided into control and experimental groups and fed with diets containing 54.5% starch and fructose, respectively, for 4, 12, and 24 weeks. Plasma testosterone and homocysteine levels were measured along with insulin, glucose, and lipids. Body composition, insulin resistance, and hepatic lipids were measured. Results. Increase in hepatic triglyceride content was first observed in metabolic disturbance followed by hypertriglyceridemia and systemic insulin resistance in fructose-fed rats. Hepatic lipids were increased in time-dependent manner by fructose-feeding starting from 4 weeks, but circulatory triglyceride levels were increased after 12 weeks. Fasting insulin and Homeostatis Model Assessment of Insulin Resistance (HOMA-IR) were increased after 12 weeks of fructose-feeding. Decreased visceral adiposity, circulatory testosterone, and homocysteine levels were observed after 4 weeks of fructose-feeding, which were normalized at 12 and 24 weeks. Conclusions. We conclude that transient decrease in circulatory testosterone and homocysteine levels and increased hepatic triglyceride content are the earliest metabolic disturbances that preceded hypertriglyceridemia and insulin resistance in fructose-fed SD rats.

The glycogen metabolism via Akt signaling is important for the secretion of enamel matrix in tooth development

Cells alter their energy metabolism depending on the stage of differentiation or various environments. In the ameloblast differentiation of continuous growing mouse incisors, we found temporary glycogen storage in preameloblasts before the start of enamel matrix secretion and investigated the relationship between enamel matrix secretion and glycogen metabolism. Immunohistochemistry showed that in the transitional stage from preameloblasts to secretory ameloblasts, the glycogen synthase changed from the inactive form to the active form, the expression of glycogen phosphorylase increased, and further, the levels of IGF-1, IGF-1 receptor and activated Akt increased. These results suggested that the activation of Akt signaling via IGF is linked to the onset of both glycogen metabolism and enamel matrix deposition. In the experiments using organ culture and ameloblast cell line, the activation of Akt signaling by IGF-1 stimulated glycogen metabolism through the up-regulation of Glut-1,-4 and Gsk-3β and the dephosphorylation of glycogen synthase. Subsequently, they resulted in increased enamel matrix secretion. In contrast, some inhibitors of Akt signals and glycogen synthesis/degradation down-regulated enamel matrix secretion. Taking these findings together, glycogen metabolism via Akt signaling is an essential system for the secretion of enamel matrix in ameloblast differentiation.

Legacy Effect of Foxo1 in Pancreatic Endocrine Progenitors on Adult β-Cell Mass and Function

β-Cell dysfunction in diabetes results from abnormalities of insulin production, secretion, and cell number. These abnormalities may partly arise from altered developmental programming of β-cells. Foxo1 is important to maintain adult β-cells, but little is known about its role in pancreatic progenitor cells as determinants of future β-cell function. We addressed this question by generating an allelic series of somatic Foxo1 knockouts at different stages of pancreatic development in mice. Surprisingly, ablation of Foxo1 in pancreatic progenitors resulted in delayed appearance of Neurogenin3(+) progenitors and their persistence into adulthood as a self-replicating pool, causing a fourfold increase of β-cell mass. Similarly, Foxo1 ablation in endocrine progenitors increased their numbers, extended their survival, and expanded β-cell mass. In contrast, ablation of Foxo1 in terminally differentiated β-cells did not increase β-cell mass nor did it affect Neurogenin3 expression. Despite the increased β-cell mass, islets from mice lacking Foxo1 in pancreatic or endocrine progenitors responded poorly to glucose, resulting in glucose intolerance. We conclude that Foxo1 integrates cues that determine developmental timing, pool size, and functional features of endocrine progenitor cells, resulting in a legacy effect on adult β-cell mass and function. Our results illustrate how developmental programming predisposes to β-cell dysfunction in adults and raise questions on the desirability of increasing β-cell mass for therapeutic purposes in type 2 diabetes.

High-throughput sequencing analyses of XX genital ridges lacking FOXL2 reveal DMRT1 up-regulation before SOX9 expression during the sex-reversal process in goats

FOXL2 loss of function in goats leads to the early transdifferentiation of ovaries into testes, then to the full sex reversal of XX homozygous mutants. By contrast, Foxl2 loss of function in mice induces an arrest of follicle formation after birth, followed by complete female sterility. In order to understand the molecular role of FOXL2 during ovarian differentiation in the goat species, putative FOXL2 target genes were determined at the earliest stage of gonadal sex-specific differentiation by comparing the mRNA profiles of XX gonads expressing the FOXL2 protein or not. Of these 163 deregulated genes, around two-thirds corresponded to testicular genes that were up-regulated when FOXL2 was absent, and only 19 represented female-associated genes, down-regulated in the absence of FOXL2. FOXL2 should therefore be viewed as an antitestis gene rather than as a female-promoting gene. In particular, the key testis-determining gene DMRT1 was found to be up-regulated ahead of SOX9, thus suggesting in goats that SOX9 primary up-regulation may require DMRT1. Overall, our results equated to FOXL2 being an antitestis gene, allowing us to propose an alternative model for the sex-determination process in goats that differs slightly from that demonstrated in mice.

The emerging role of insulin-like growth factors in testis development and function

The insulin-like family of growth factors (IGFs) - composed of insulin, and insulin-like growth factors I (IGF1) and II (IGF2) - provides essential signals for the control of testis development and function. In the testis, IGFs act in an autocrine-paracrine manner but the extent of their actions has been underestimated due to redundancies at both the ligand and receptor levels, and the perinatal lethality of constitutive knockout mice. This review synthesizes the current understanding of how the IGF system regulates biological processes such as primary sex determination, testis development, spermatogenesis and steroidogenesis, and highlights the questions that remain to be explored.

Mitochondria and mammalian reproduction

Mitochondria are cellular organelles with crucial roles in ATP synthesis, metabolic integration, reactive oxygen species (ROS) synthesis and management, the regulation of apoptosis (namely via the intrinsic pathway), among many others. Additionally, mitochondria in different organs or cell types may have distinct properties that can decisively influence functional analysis. In terms of the importance of mitochondria in mammalian reproduction, and although there are species-specific differences, these aspects involve both energetic considerations for gametogenesis and fertilization, control of apoptosis to ensure the proper production of viable gametes, and ROS signaling, as well as other emerging aspects. Crucially, mitochondria are the starting point for steroid hormone biosynthesis, given that the conversion of cholesterol to pregnenolone (a common precursor for all steroid hormones) takes place via the activity of the cytochrome P450 side-chain cleavage enzyme (P450scc) on the inner mitochondrial membrane. Furthermore, mitochondrial activity in reproduction has to be considered in accordance with the very distinct strategies for gamete production in the male and female. These include distinct gonad morpho-physiologies, different types of steroids that are more prevalent (testosterone, estrogens, progesterone), and, importantly, the very particular timings of gametogenesis. While spermatogenesis is complete and continuous since puberty, producing a seemingly inexhaustible pool of gametes in a fixed environment; oogenesis involves the episodic production of very few gametes in an environment that changes cyclically. These aspects have always to be taken into account when considering the roles of any common element in mammalian reproduction.

Insulin and IGF1 receptors are essential for XX and XY gonadal differentiation and adrenal development in mice

Mouse sex determination provides an attractive model to study how regulatory genetic networks and signaling pathways control cell specification and cell fate decisions. This study characterizes in detail the essential role played by the insulin receptor (INSR) and the IGF type I receptor (IGF1R) in adrenogenital development and primary sex determination. Constitutive ablation of insulin/IGF signaling pathway led to reduced proliferation rate of somatic progenitor cells in both XX and XY gonads prior to sex determination together with the downregulation of hundreds of genes associated with the adrenal, testicular, and ovarian genetic programs. These findings indicate that prior to sex determination somatic progenitors in Insr;Igf1r mutant gonads are not lineage primed and thus incapable of upregulating/repressing the male and female genetic programs required for cell fate restriction. In consequence, embryos lacking functional insulin/IGF signaling exhibit (i) complete agenesis of the adrenal cortex, (ii) embryonic XY gonadal sex reversal, with a delay of Sry upregulation and the subsequent failure of the testicular genetic program, and (iii) a delay in ovarian differentiation so that Insr;Igf1r mutant gonads, irrespective of genetic sex, remained in an extended undifferentiated state, before the ovarian differentiation program ultimately is initiated at around E16.5.

Heterogeneity in sexual bipotentiality and plasticity of granulosa cells in developing mouse ovaries

In mammalian sex determination, SRY directly upregulates the expression of SOX9, the master regulatory transcription factor in Sertoli cell differentiation, leading to testis formation. Without SRY action, the bipotential gonadal cells become pre-granulosa cells, which results in ovarian follicle development. When, where and how pre-granulosa cells are determined to differentiate into developing ovaries, however, remains unclear. By monitoring SRY-dependent SOX9-inducibility (SDSI) in a Sry-inducible mouse system, here we show spatiotemporal changes in the sexual bipotentiality/plasticity of ovarian somatic cells throughout a life. The early pre-granulosa cells maintain the SDSI until 11.5 dpc, after which most pre-granulosa cells rapidly lose this ability by 12.0 dpc. Unexpectedly, we found a subpopulation of the pre-granulosa cells near the mesonephric tissue that continuously retains SDSI throughout fetal and early postnatal stages. After birth, these SDSI-positive pre-granulosa cells contribute to the initial round of folliculogenesis by secondary follicle stage. In experimental sex reversal of 13.5-dpc ovaries grafted into adult male nude mice, the differentiated granulosa cells reacquire the SDSI before other signs of masculinization. Our data provide direct evidence of an unexpectedly high sexual heterogeneity of granulosa cells in developing mouse ovaries in a stage- and region-specific manner. Discovery of such sexually bipotential granulosa cells provides a novel entry point to the understanding of masculinization in various cases of XX disorders of sexual development in mammalian ovaries.

Early gonadogenesis in mammals: significance of long and narrow gonadal structure

In mammalian embryogenesis, the gonadal primordium arises from the thickening of the coelomic epithelium, which results in a pair of extremely long and narrow gonadal structures along the anteroposterior axis. These gonadal structures are conserved in various mammalian species, suggesting a great advantage in properly receiving migrating primordial germ cells (PGCs) that are widely scattered throughout the hindgut tube. Soon after the PGCs settle, the bipotential gonads undergo sex determination into testes or ovaries by the sex-determining gene, Sry, which is expressed in supporting cell precursors in a center-to-pole manner. Such a long, narrow gonadal structure bestows a considerable time lag on Sry expression between the center and pole regions, but testiculogenesis with cord formation and Leydig cell differentiation occurs synchronously throughout the whole organ. This synchronous testiculogenesis could be explained by a positive-feedback mechanism between SOX9 (another SRY-related transcription factor) and FGF9 downstream of Sry. FGF signals are likely secreted from the center region, rapidly diffuse into the poles, and then induce the establishment of SOX9 expression in Sertoli cells in the pole domains. This work focuses on recent knowledge of the molecular and cellular events of PGC migration, gonadogenesis, and testiculogenesis, and their biological significance in mammalian embryogenesis.

Analysis of medaka sox9 orthologue reveals a conserved role in germ cell maintenance

The sex determining gene is divergent among different animal species. However, sox9 is up-regulated in the male gonads in a number of species in which it is the essential regulator of testis determination. It is therefore often discussed that the sex determining gene-sox9 axis functions in several vertebrates. In our current study, we show that sox9b in the medaka (Oryzias latipes) is one of the orthologues of mammalian Sox9 at syntenic and expression levels. Medaka sox9b affects the organization of extracellular matrices, which represents a conserved role of sox9, but does not directly regulate testis determination. We made this determination via gene expression and phenotype analyses of medaka with different copy numbers of sox9b. Sox9b is involved in promoting cellular associations and is indispensible for the proper proliferation and survival of germ cells in both female and male medaka gonads. Medaka mutants that lack sox9b function exhibit a seemingly paradoxical phenotype of sex reversal to male. This is explained by a reduction in the germ cell number associated with aberrant extracellular matrices. Together with its identified roles in other vertebrate gonads, a testis-determining role for Sox9 in mammals is likely to have been neofunctionalized and appended to its conserved role in germ cell maintenance.

Sry: the master switch in mammalian sex determination

SRY, the mammalian Y-chromosomal testis-determining gene, induces male sex determination. Recent studies in mice reveal that the major role of SRY is to achieve sufficient expression of the related gene Sox9, in order to induce Sertoli cell differentiation, which in turn drives testis formation. Here, we discuss the cascade of events triggered by SRY and the mechanisms that reinforce the differentiation of the testes in males while actively inhibiting ovarian development.

FGF signaling directs a center-to-pole expansion of tubulogenesis in mouse testis differentiation

In mouse embryogenesis, Sry is transiently activated in a center-to-pole wavelike manner along the anteroposterior (AP) axis of developing XY gonads. However, the mechanism and significance of the center-to-pole expansion of testis initiation pathways downstream of Sry expression remain unclear. Here we demonstrate that FGF9 can act as a diffusible conductor for a poleward expansion of tubulogenic programs at early phases of testis differentiation. In XY genital ridge cultures of anterior, middle and posterior segments at 11.0-11.25 days post-coitum, male-specific activation of Sry and its target gene, Sox9, was still observed in both anterior and posterior pole segments despite their isolation from the central domain. However, high-level Sox9 expression was not maintained, resulting in the failure of testis cord organization in most pole segments. A reconstruction experiment using ROSA:lacZ middle segments showed rescue of the tubulogenic defect in the poles without any appreciable contribution of lacZ-positive gonadal parenchyma cells. A partition culture assay also showed a possible contribution of soluble/diffusible factors secreted from the gonadal center domain to proper tubulogenesis in the poles. Among various signaling factors, Fgf9 expression was significantly lower in both anterior and posterior pole segments than in the central domain. The supportive role of the central domain could be substituted by exogenous FGF9 supply, whereas reduction of Wnt4 activity did not rescue the tubulogenesis defect in the pole segments. These observations imply that center-to-pole FGF9 diffusion directs a poleward expansion of testiculogenic programs along the AP axis of developing XY gonads.

The game plan: cellular and molecular mechanisms of mammalian testis development

In mammals, biological differences between males and females, which influence many aspects of their physical, social, and psychological environments, are solely determined genetically. In the presence of a Y chromosome, the gonadal primordium will differentiate into a testis, whereas in the absence of the Y chromosome an ovary will develop. Testis and ovary subsequently direct the differentiation of all secondary sex characteristics down the male and female pathway, respectively. The male-determining factor on the Y chromosome, SRY, was identified some 20 years ago. Since then, significant progress has been made toward understanding the molecular and cellular pathways that result in the formation of a testis. Here, we review what is known about testis differentiation in mice and humans, with reference to other species where appropriate.

The PGD2 pathway, independently of FGF9, amplifies SOX9 activity in Sertoli cells during male sexual differentiation

Activation by the Y-encoded testis determining factor SRY and maintenance of expression of the Sox9 gene encoding the central transcription factor of Sertoli cell differentiation are key events in the mammalian sexual differentiation program. In the mouse XY gonad, SOX9 upregulates Fgf9, which initiates a Sox9/Fgf9 feedforward loop, and Sox9 expression is stimulated by the prostaglandin D2 (PGD2) producing lipocalin prostaglandin D synthase (L-PGDS, or PTDGS) enzyme, which accelerates commitment to the male pathway. In an attempt to decipher the genetic relationships between Sox9 and the L-Pgds/PGD2 pathway during mouse testicular organogenesis, we found that ablation of Sox9 at the onset or during the time window of expression in embryonic Sertoli cells abolished L-Pgds transcription. By contrast, L-Pgds(-/-) XY embryonic gonads displayed a reduced level of Sox9 transcript and aberrant SOX9 protein subcellular localization. In this study, we demonstrated genetically that the L-Pgds/PGD2 pathway acts as a second amplification loop of Sox9 expression. Moreover, examination of Fgf9(-/-) and L-Pgds(-/-) XY embryonic gonads demonstrated that the two Sox9 gene activity amplifying pathways work independently. These data suggest that, once activated and maintained by SOX9, production of testicular L-PGDS leads to the accumulation of PGD2, which in turn activates Sox9 transcription and nuclear translocation of SOX9. This mechanism participates together with FGF9 as an amplification system of Sox9 gene expression and activity during mammalian testicular organogenesis.