Regionally distinct potencies of mouse XY genital ridge to initiate testis differentiation dependent on anteroposterior axis

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.

FGF9 is a downstream target of SRY and sufficient to determine male sex fate in ex vivo XX gonad culture

Fibroblast growth factor 9 (FGF9) is an autocrine/paracrine growth factor that plays critical roles in embryonic and organ developments and is involved in diverse physiological events. Loss of function of FGF9 exhibits male-to-female sex reversal in the transgenic mouse model and gain of FGF9 copy number was found in human 46, XX sex reversal patient with disorders of sex development. These results suggested that FGF9 plays a vital role in male sex development. Nevertheless, how FGF9/Fgf9 expression is regulated during testis determination remains unclear. In this study, we demonstrated that human and mouse SRY bind to -833 to -821 of human FGF9 and -1010 to -998 of mouse Fgf9, respectively, and control FGF9/Fgf9 mRNA expression. Interestingly, we showed that mouse SRY cooperates with SF1 to regulate Fgf9 expression, whereas human SRY-mediated FGF9 expression is SF1 independent. Furthermore, using an ex vivo gonadal culture system, we showed that FGF9 expression is sufficient to switch cell fate from female to male sex development in 12-16 tail somite XX mouse gonads. Taken together, our findings provide evidence to support the SRY-dependent, fate-determining role of FGF9 in male sex development.

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.

Sox17-mediated maintenance of fetal intra-aortic hematopoietic cell clusters

During mouse development, definitive hematopoiesis is first detected around embryonic day 10.5 (E10.5) in the aorta-gonad-mesonephros (AGM) region, which exhibits intra-aortic cell clusters. These clusters are known to contain hematopoietic stem cells (HSCs). On the other hand, it is not clear how the cells in such clusters maintain their HSC phenotype and how they are triggered to differentiate. Here we found that an endodermal transcription factor marker, Sox17, and other F-group (SoxF) proteins, Sox7 and Sox18, were expressed in E10.5 intra-aortic cell clusters. Forced expression of any of these SoxF proteins, particularly Sox17, in E10.5 AGM CD45(low) c-Kit(high) cells, which are the major component of intra-aortic clusters, led to consistent formation of cell clusters in vitro during several passages of cocultures with stromal cells. Cluster-forming cells with constitutive Sox17 expression retained long-term bone marrow reconstitution activity in vivo. Notably, shutdown of exogenously introduced Sox17 gene expression resulted in immediate hematopoietic differentiation. These results indicate that SoxF proteins, especially Sox17, contribute to the maintenance of cell clusters containing HSCs in the midgestation AGM region. Furthermore, SoxF proteins play a pivotal role in controlling the HSC fate decision between indefinite self-renewal and differentiation during fetal hematopoiesis.

CYP26B1 promotes male germ cell differentiation by suppressing STRA8-dependent meiotic and STRA8-independent mitotic pathways

Germ cell sex is defined by factors derived from somatic cells. CYP26B1 is known to be a male sex-promoting factor that inactivates retinoic acid (RA) in somatic cells. In CYP26B1-null XY gonads, germ cells are exposed to a higher level of RA than in normal XY gonads and this activates Stra8 to induce meiosis while male-specific gene expression is suppressed. However, it is unknown whether meiotic entry by an elevated level of RA is responsible for the suppression of male-type gene expression. To address this question, we have generated Cyp26b1/Stra8 double knockout (dKO) embryos. We successfully suppressed the induction of meiosis in CYP26B1-null XY germ cells by removing the Stra8 gene. Concomitantly, we found that the male genetic program represented by the expression of NANOS2 and DNMT3L was totally rescued in about half of dKO germ cells, indicating that meiotic entry causes the suppression of male differentiation. However, half of the germ cells still failed to enter the appropriate male pathway in the dKO condition. Using microarray analyses together with immunohistochemistry, we found that KIT expression was accompanied by mitotic activation, but was canceled by inhibition of the RA signaling pathway. Taken together, we conclude that inhibition of RA is one of the essential factors to promote male germ cell differentiation, and that CYP26B1 suppresses two distinct genetic programs induced by RA: a Stra8-dependent meiotic pathway, and a Stra8-independent mitotic pathway.

Analyzing gene function in whole mouse embryo and fetal organ in vitro

A well-established experimental paradigm to analyze gene function in development is to elucidate the impact of gain and loss of gene activity on cell differentiation, tissue modelling, organogenesis, and morphogenesis. This chapter describes the experimental protocols to study gene function by means of electroporation and lipofection to manipulate genetic activity in whole embryos and fetal organs in vitro. These techniques allow for more precise control of the timing, with reference to developmental age or stage, and the cell/tissue-specificity of the changes in gene activity. They provide an alternative strategy that can expedite the analysis of gene function before resorting to the conventional means of transgenesis and gene targeting in the whole organism.

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.

Cyclical and patch-like GDNF distribution along the basal surface of Sertoli cells in mouse and hamster testes

Background and Aims

In mammalian spermatogenesis, glial cell line-derived neurotrophic factor (GDNF) is one of the major Sertoli cell-derived factors which regulates the maintenance of undifferentiated spermatogonia including spermatogonial stem cells (SSCs) through GDNF family receptor α1 (GFRα1). It remains unclear as to when, where and how GDNF molecules are produced and exposed to the GFRα1-positive spermatogonia in vivo.

Methodology and Principal Findings

Here we show the cyclical and patch-like distribution of immunoreactive GDNF-positive signals and their close co-localization with a subpopulation of GFRα1-positive spermatogonia along the basal surface of Sertoli cells in mice and hamsters. Anti-GDNF section immunostaining revealed that GDNF-positive signals are mainly cytoplasmic and observed specifically in the Sertoli cells in a species-specific as well as a seminiferous cycle- and spermatogenic activity-dependent manner. In contrast to the ubiquitous GDNF signals in mouse testes, high levels of its signals were cyclically observed in hamster testes prior to spermiation. Whole-mount anti-GDNF staining of the seminiferous tubules successfully visualized the cyclical and patch-like extracellular distribution of GDNF-positive granular deposits along the basal surface of Sertoli cells in both species. Double-staining of GDNF and GFRα1 demonstrated the close co-localization of GDNF deposits and a subpopulation of GFRα1-positive spermatogonia. In both species, GFRα1-positive cells showed a slender bipolar shape as well as a tendency for increased cell numbers in the GDNF-enriched area, as compared with those in the GDNF-low/negative area of the seminiferous tubules.

Conclusion/Significance

Our data provide direct evidence of regionally defined patch-like GDNF-positive signal site in which GFRα1-positive spermatogonia possibly interact with GDNF in the basal compartment of the seminiferous tubules.

SRY upregulation of SOX9 is inefficient and delayed, allowing ovarian differentiation, in the B6.Y(TIR) gonad

SRY on the Y-chromosome acts as a transcription factor to initiate testicular differentiation in mammals. Sox9 is a SRY target gene, upregulated immediately after Sry expression, and plays a key role in testicular differentiation. In the present study, we examined the expression of SRY and SOX9 proteins in the B6.Y(TIR) gonad, which undergoes partial or complete sex reversal. The results show that the ontogeny of SRY expression in the B6.Y(TIR) gonad was comparable with that in the B6.XY gonad. On the other hand, while SOX9 expression immediately followed SRY expression in the B6.XY gonad, it was considerably delayed compared to SRY expression in the B6.Y(TIR) gonad or SOX9 expression in the B6.XY gonad. Although SOX9 expression reached the entire gonad at a time point, it was downregulated and became restricted to the central area in which testis cords were organized. MIS, a marker of Sertoli cells, appeared only in well-organized testis cords. We speculate that the SRY protein from the Y(TIR)-chromosome is inefficient in upregulating the Sox9 gene on the B6 background, allowing the initiation of ovarian differentiation.

Identification of SOX3 as an XX male sex reversal gene in mice and humans

Sex in mammals is genetically determined and is defined at the cellular level by sex chromosome complement (XY males and XX females). The Y chromosome-linked gene sex-determining region Y (SRY) is believed to be the master initiator of male sex determination in almost all eutherian and metatherian mammals, functioning to upregulate expression of its direct target gene Sry-related HMG box-containing gene 9 (SOX9). Data suggest that SRY evolved from SOX3, although there is no direct functional evidence to support this hypothesis. Indeed, loss-of-function mutations in SOX3 do not affect sex determination in mice or humans. To further investigate Sox3 function in vivo, we generated transgenic mice overexpressing Sox3. Here, we report that in one of these transgenic lines, Sox3 was ectopically expressed in the bipotential gonad and that this led to frequent complete XX male sex reversal. Further analysis indicated that Sox3 induced testis differentiation in this particular line of mice by upregulating expression of Sox9 via a similar mechanism to Sry. Importantly, we also identified genomic rearrangements within the SOX3 regulatory region in three patients with XX male sex reversal. Together, these data suggest that SOX3 and SRY are functionally interchangeable in sex determination and support the notion that SRY evolved from SOX3 via a regulatory mutation that led to its de novo expression in the early gonad.

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.

Expression and function of mouse Sox17 gene in the specification of gallbladder/bile-duct progenitors during early foregut morphogenesis

In early-organogenesis-stage mouse embryos, the posteroventral foregut endoderm adjacent to the heart tube gives rise to liver, ventral pancreas and gallbladder. Hepatic and pancreatic primordia become specified in the posterior segment of the ventral foregut endoderm at early somite stages. The mechanisms for demarcating gallbladder and bile duct primordium, however, are poorly understood. Here, we demonstrate that the gallbladder and bile duct progenitors are specified in the paired lateral endoderm domains outside the heart field at almost the same timing as hepatic and pancreatic induction. In the anterior definitive endoderm, Sox17 reactivation occurs in a certain population within the most lateral domains posterolateral to the anterior intestinal portal (AIP) lip on both the left and right sides. During foregut formation, the paired Sox17-positive domains expand ventromedially to merge in the midline of the AIP lip and become localized between the liver and pancreatic primordia. In Sox17-null embryos, these lateral domains are missing, resulting in a complete loss of the gallbladder/bile-duct structure. Chimera analyses revealed that Sox17-null endoderm cells in the posteroventral foregut do not display any gallbladder/bile-duct molecular characters. Our findings show that Sox17 functions cell-autonomously to specify gallbladder/bile-duct in the mouse embryo.

A critical time window of Sry action in gonadal sex determination in mice

In mammals, the Y-linked sex-determining gene Srycell-autonomously promotes Sertoli cell differentiation from bipotential supporting cell precursors through SRY-box containing gene 9 (Sox9),leading to testis formation. Without Sry action, the supporting cells differentiate into granulosa cells, resulting in ovarian development. However,how Sry acts spatiotemporally to switch supporting cells from the female to the male pathway is poorly understood. We created a novel transgenic mouse line bearing an inducible Sry transgene under the control of the Hsp70.3 promoter. Analysis of these mice demonstrated that the ability of Sry to induce testis development is limited to approximately 11.0-11.25 dpc, corresponding to a time window of only 6 hours after the normal onset of Sry expression in XY gonads. If Sry was activated after 11.3 dpc, Sox9 activation was not maintained, resulting in ovarian development. This time window is delimited by the ability to engage the high-FGF9/low-WNT4 signaling states required for Sertoli cell establishment and cord organization. Our results indicate the overarching importance of Sry action in the initial 6-hour phase for the female-to-male switching of FGF9/WNT4 signaling patterns.

SOX9 Regulates Prostaglandin D Synthase Gene Transcription in Vivo to Ensure Testis Development

In mammals, male sex is determined by the Y-chromosomal gene Sry (sex-determining region of Y chromosome). The expression of Sry and subsequently Sox9 (SRY box containing gene 9) in precursors of the supporting cell lineage results in the differentiation of these cells into Sertoli cells. Sertoli cells in turn orchestrate the development of all other male-specific cell types. To ensure that Sertoli cells differentiate in sufficient numbers to induce normal testis development, the early testis produces prostaglandin D(2) (PGD(2)), which recruits cells of the supporting cell lineage to a Sertoli cell fate. Here we show that the gene encoding prostaglandin D synthase (Pgds), the enzyme that produces PGD(2), is expressed in Sertoli cells immediately after the onset of Sox9 expression. Promoter analysis in silico and in vitro identified a paired SOX/SRY binding site. Interestingly, only SOX9, and not SRY, was able to bind as a dimer to this site and transactivate the Pgds promoter. In line with this, a transgenic mouse model showed that Pgds expression is not affected by ectopic Sry expression. Finally, chromatin immunoprecipitation proved that SOX9 but not SRY binds to the Pgds promoter in vivo.

From SRY to SOX9: Mammalian Testis Differentiation

Sry (sex-determining region on the Y chromosome) is a master gene that initiates testis differentiation of the bipotential indifferent gonad in mammals. In mice, Sry expression is transiently activated in a center-to-pole wave along the anteroposterior (AP) axis of developing XY gonads. Shortly after the onset of Sry activation, Sox9 (Sry-related HMG box-9), a fundamental testis-differentiation gene common to all vertebrates, is also activated in a center-to-pole pattern similar to the initial Sry expression profile. Several male-specific cellular events, such as glycogenesis, coelomic epithelium proliferation, mesonephric migration and vasculogenesis, are induced in XY gonads following the onset of Sry and Sox9 expression. This paper mainly focuses on recent advances in elucidating the regulatory mechanisms of Sry and Sox9 expression and male-specific cellular events immediately downstream of SRY action during the initial phases of testis differentiation.

Influence on spatiotemporal patterns of a male-specific Sox9 activation by ectopic Sry expression during early phases of testis differentiation in mice

Testis induction is associated with gonadal Sry and Sox9 expression in mammals. This study investigated whether Sry expression directly induces male-specific Sox9 activation during early phases of testis differentiation. We have established an XX sex-reversal mouse line carrying the Sry transgene driven by a weak basal promoter of the Hsp70.3 gene (Hsp-Sry), whereby the transgene was activated in the gonads along the entire anteroposterior axis from earlier stages. The effects of misexpression and overexpression of Sry on the spatiotemporal pattern of Sox9 expression were examined using both XX and XY gonads of Hsp-Sry transgenic embryos. It was shown that ectopic expression of Sry transcripts in the entire gonadal area from earlier stages promotes neither any advance in the timing nor any appreciable ectopic activation of endogenous Sox9 expression. Immediately after the onset of Sox9 activation, however, both the level of Sox9 expression and the number of SOX9-positive cells were significantly enhanced in Hsp-Sry/XY gonads, as compared with those in wild-type/XY and Hsp-Sry/XX gonads. These findings suggest that, although Sry is capable of up-regulating Sox9 expression dose-dependently, Sry mRNA expression alone is not likely to provide positional or timing information needed for male-specific Sox9 activation in developing XY gonads.

A novel Sry-downstream cellular event which preserves the readily available energy source of glycogen in mouse sex differentiation

Sry is transiently activated in pre-Sertoli cells of the gonadal ridge to initiate testis differentiation in mice. In pre-Sertoli cells, however, the cellular events induced immediately after the onset of Sry expression remain largely unknown. Here we show that testis-specific glycogen accumulation in pre-Sertoli cells is one of the earliest cellular events downstream of Sry action. In developing XY gonads, glycogen accumulation starts to occur in pre-Sertoli cells from around 11.15 dpc (tail somite 14 stage) in a center-to-pole pattern similar to the initial Sry expression profile. Glycogen accumulation was also found in XX male gonads of Sry-transgenic embryos, but not in XX female gonads of wildtype embryos at any developmental stage. In vitro analyses using various culture conditions suggest that testis-specific glycogen deposition is a tissue-autonomous event that can be induced even in serum-free conditions and in a culture of gonadal explants without adjacent mesonephros. Moreover, glycogen accumulation in pre-Sertoli cells was significantly inhibited in vitro by the PI3K inhibitor LY294002, but not by the MEK inhibitor PD98059. Active phospho-AKT (PI3K effector) showed a high degree of accumulation in gonadal somatic cells of genital ridges in a testis-specific manner, both in vitro and in vivo. Therefore, these findings suggest that immediately after the onset of Sry expression, activation of the PI3K-AKT pathway promotes testis-specific glycogen storage in pre-Sertoli cells. To the best of our knowledge, this is a novel Sry-downstream cellular event which preserves this readily available energy source in Sertoli cells for testis-specific morphogenesis and hormone production.