A transgenic insertion upstream of sox9 is associated with dominant XX sex reversal in the mouse

Deletion of the Parkin coregulated gene causes male sterility in the quaking(viable) mouse mutant

Quaking(viable) (qk(v)) is a recessive neurological mouse mutation with severe dysmyelination of the CNS and spermiogenesis failure. The molecular lesion in the qk(v) mutant is a deletion of approximately 1 Mb on mouse chromosome 17 that alters the expression of the qk gene in oligodendrocytes. Complementation analysis between the qk(v) mutation and qk mutant alleles generated through chemical mutagenesis showed that the male sterility is a distinctive feature of the qk(v) allele. This observation suggested that the sperm differentiation defect in qk(v) is due to the deletion of a gene(s) distinct from qk. Here, we demonstrate that the deletion of Pacrg is the cause of male sterility in the qk(v) mutant. Pacrg is the mouse homologue of the human PARKIN-coregulated gene (PACRG), which encodes for a protein whose biochemical function remains unclear. We show that Pacrg is highly expressed in the testes in both mice and humans. In addition, the expression pattern of Pacrg during spermiogenesis suggests that it plays a role in sperm differentiation. In support of this hypothesis, we show that transgenic expression of Pacrg in testes restores spermiogenesis and fertility in qk(v) males. This finding provides the first in vivo evidence, to our knowledge, for the function of Pacrg in a model organism. Immunolocalization experiments on isolated spermatozoa show that the Pacrg protein is present in mature sperm. Remarkably, the mammalian Pacrg protein shares significant sequence similarities with gene products from flagellated protozoans, suggesting that Pacrg may be necessary for proper flagellar formation in many organisms.

Functional analysis of Sox8 and Sox9 during sex determination in the mouse

Sex determination in mammals directs an initially bipotential gonad to differentiate into either a testis or an ovary. This decision is triggered by the expression of the sex-determining gene Sry, which leads to the activation of male-specific genes including the HMG-box containing gene Sox9. From transgenic studies in mice it is clear that Sox9 is sufficient to induce testis formation. However, there is no direct confirmation for an essential role for Sox9 in testis determination. The studies presented here are the first experimental proof for an essential role for Sox9 in mediating a switch from the ovarian pathway to the testicular pathway. Using conditional gene targeting, we show that homozygous deletion of Sox9 in XY gonads interferes with sex cord development and the activation of the male-specific markers Mis and P450scc, and leads to the expression of the female-specific markers Bmp2 and follistatin. Moreover, using a tissue specific knock-out approach, we show that Sox9 is involved in Sertoli cell differentiation, the activation of Mis and Sox8, and the inactivation of Sry. Finally, double knock-out analyses suggest that Sox8 reinforces Sox9 function in testis differentiation of mice.

Cell aggregation precedes the onset of Sox9-expressing preSertoli cells in the genital ridge of mouse

SOX9 is expressed at the onset of the genital ridge formation in both sexes. It is assumed that SRY, the testis determining gene, turns SOX9 on in male embryos because it is turned off in female embryos. Spatial expression of SRY follows a cranio-caudal pattern. Here, we asked if SOX9 is expressed in the same cell lineage and with a similar pattern as SRY. A correlative study between the structural changes in the genital ridge and the immunocytochemical localization of SOX9-positive cells was undertaken. We used a transgenic strain expressing the green fluorescent protein (GFP) that considerably enhanced the cell context where the first SOX9-positive cells appear. Although SOX9-positive cells are located among loose mesenchymal cells by stages of 8-14 tail somites (ts) in both sexes, they are absent in the thickening coelomic epithelium of females. At 15 ts the first SOX9-positive cells appear within the core of the condensed cells only in male genital ridges. At 17 ts, a gradient of SOX9-positive cells in males is apparent, closely following the cranio-caudal pattern of cell aggregation seen in genital ridges of both sexes. Hence, our results suggest that SOX9 is expressed only in loose mesenchymal cells in both sexes and that expression of SOX9 in males requires the prior aggregation of cells in the genital ridges. The correspondence of SOX9 and SRY pattern of expression supports that both genes are expressed in the preSertoli cell lineage in the core of the genital ridges.

Expression studies of the PIS-regulated genes suggest different mechanisms of sex determination within mammals

In mammals, the Y-located SRY gene is known to induce testis formation from the indifferent gonad. A related gene, SOX9, also plays a critical role in testis differentiation in mammals, in birds and reptiles. It is now assumed that SRY acts upstream of SOX9 in the sex determination cascade, but the regulatory link which should exist between these two genes remains unknown. Studies on XX sex reversal in polled goats (PIS mutation: Polled Intersex Syndrome) have led to the discovery of a female-specific locus crucial for ovarian differentiation. This genomic region is composed of at least two genes, FOXL2 and PISRT1, which share a common transcriptional regulatory region, PIS. In this review, we present the expression pattern of these PIS-regulated genes in mice. The FOXL2 expression profile of mice is similar to that described in goats in accordance with a conserved role of this ovarian differentiating gene in mammals. On the contrary, the PISRT1 expression profile is different between mice and goats, suggesting different mechanisms of the primary switch in the testis determination process within mammals. A model based on two different modes of SOX9 regulation in mice and other mammals is proposed in order to integrate our results into the current scheme of gonad differentiation.

Disturbed Expression of Sox9 in Pre-Sertoli Cells Underlies Sex-Reversal in Mice B6.Ytir1

Sry in some varieties of Mus musculus domesticus fails to form normal testis when introduced into the C57BL/6J (B6) strain. We studied the developmental pattern of pre-Sertoli cells that express Sox9 by immunofluorescence and the profile levels of Sox9 transcripts by semiquantitative reverse transcriptase polymerase chain reaction and in situ hybridization in developing gonads of B6-Ytir mice. Sox9-positive cells (pre-Sertoli cells) appeared in all B6.Ytir genital ridges at 11.5 and 12.5 days postcoitum (dpc). However, at 13.5 dpc, Sox9-positive cells were not detected only in 50% of the B6.Ytir gonads compared with 100% of B6 gonads. Although pre-Sertoli cells formed the seminiferous cords after 14.5 dpc in the medial region of the B6.Ytir gonad, the cranial and caudal regions formed ovarian tissue. Further, B6.Ytir ovaries have lower levels of Sox9 than ovotestes at all fetal stages. These results suggest that although the pre-Sertoli cell lineage forms in B6.Ytir genital ridges, its further differentiation into Sertoli cells is apparently prevented. The cause may be the low levels of Sox9 and down-regulation of its product. Results suggest that inhibitory signals of Sox9 acting along the whole genital ridge or only at its cranial and/or caudal regions underlie formation of B6.Ytir ovaries or ovotestes, respectively. Furthermore, our results suggest that infertility of B6.Ytir females may be due to the abnormal presence of Sox9 transcripts in their ovaries.

The degenerate Y chromosome – can conversion save it?

The human Y chromosome is running out of time. In the last 300 million years, it has lost 1393 of its original 1438 genes, and at this rate it will lose the last 45 in a mere 10 million years. But there has been a proposal that perhaps rescue is at hand in the form of recently discovered gene conversion within palindromes. However, I argue here that although conversion will increase the frequency of variation of the Y (particularly amplification) between Y chromosomes in a population, it will not lead to a drive towards a more functional Y. The forces of evolution have made the Y a genetically isolated, non-recombining entity, vulnerable to genetic drift and selection for favourable new variants sharing the Y with damaging mutations. Perhaps it will even speed up the decline of the Y chromosome and the onset of a new round of sex-chromosome differentiation. The struggle to preserve males may perhaps lead to hominid speciation.

Molecular characterization of three gonad cell lines

To facilitate the study of the regulation and downstream interactions of genes involved in gonad development it is important to have a suitable cell culture model. We therefore aimed to characterize molecularly three different mouse gonad cell lines. TM3 and TM4 cells were originally isolated from prepubertal mouse gonads and were tentatively identified as being of Leydig cell and Sertoli cell origin, respectively, based upon their morphology and hormonal responses. The third line is a conditionally immortalized cell line, derived from 10.5-11.5 days post-coitum (dpc) male gonads of transgenic embryos carrying a temperature-sensitive SV40 large T-antigen. We studied by reverse transcription-polymerase chain reaction (RT-PCR) the expression profiles of a number of genes known to be important for early gonad development. Moreover, we assessed these cell lines for their capacity to induce SOX9 transcription upon expression of SRY, a key molecular event occurring during sex determination. We found that all three cell lines were unable to upregulate SOX9 expression upon transfection of SRY-expression constructs, even though these cells express many of the studied embryonic gonad genes. These observations point to a requirement for SRY cofactors for direct or indirect upregulation of SOX9 expression during testis determination.

Turning on the male – SRY, SOX9 and sex determination in mammals

The decision of the bi-potential gonad to develop into either a testis or ovary is determined by the presence or absence of the Sex-determining Region gene on the Y chromosome (SRY). Since its discovery, almost 13 years ago, the molecular role that SRY plays in initiating the male sexual development cascade has proven difficult to ascertain. While biochemical studies of clinical mutants and mouse genetic models have helped in our understanding of SRY function, no direct downstream targets of SRY have yet been identified. There are, however, a number of other genes of equal importance in determining sexual phenotype, expressed before and after expression of SRY. Of these, one has proven of central importance to mammals and vertebrates, SOX9. This review describes our current knowledge of SRY and SOX9 structure and function in the light of recent key developments.

Origin and possible roles of the SOX8 transcription factor gene during sexual development

SOX8 is a member of the SOX family of developmental transcription factor genes and is closely related to SOX9, a critical gene involved in mammalian sex determination and differentiation. Both genes encode proteins with the ability to bind similar DNA target sequences, and to activate transcription in in vitro assays. Expression studies indicate that the two genes have largely overlapping patterns of activity during mammalian embryonic development. A knockout of SOX8 in mice has no obvious developmental phenotype, suggesting that the two genes are able to act redundantly in a variety of developmental contexts. In particular, both genes are expressed in the developing Sertoli cell lineage of the developing testes in mice, and both proteins are able to activate transcription of the gene encoding anti-Müllerian hormone (AMH), through synergistic action with steroidogenic factor 1 (SF1). We have hypothesized that SOX8 may substitute for SOX9 in species where SOX9 is expressed too late to be involved in sex determination or regulation of AMH expression. However, our studies involving the red-eared slider turtle indicate that SOX8 is expressed at similar levels in males and females throughout the sex-determining period, suggesting that SOX8 is neither a transcriptional regulator for AMH, nor responsible for sex determination or gonad differentiation in that species. Similarly, SOX8 is not expressed in a sexually dimorphic pattern during gonadogenesis in the chicken. Since a functional role(s) for SOX8 is implied by its conservation during evolution, the significance of SOX8 for sexual and other aspects of development will need to be uncovered through more directed lines of experimentation. Copyright 2003 S. Karger AG, Basel

DAX1 and its network partners: exploring complexity in development

DAX1 encoded by NR0B1, when mutated, is responsible for X-linked adrenal hypoplasia congenita (AHC). AHC is due to failure of the adrenal cortex to develop normally and is fatal if untreated. When duplicated, this gene is associated with an XY sex-reversed phenotype. DAX1 expression is present during development of the steroidogenic hypothalamic-pituitary-adrenal-gonadal (HPAG) axis and persists into adult life. Despite recognition of the crucial role for DAX1, its function remains largely undefined. The phenotypes of patients and animal models are complex and not always in agreement. Investigations using cell lines have proved difficult to interpret, possibly reflecting cell line choices and their limited characterization. We will review the efforts of our group and others to identify appropriate cell lines for optimizing ex vivo analysis of NR0B1 function throughout development. We will examine the role of DAX1 and its network partners in development of the hypothalamic-pituitary-adrenal/gonadal axis (HPAG) using a variety of different types of investigations, including those in model organisms. This network analysis will help us to understand normal and abnormal development of the HPAG. In addition, these studies permit identification of candidate genes for human inborn errors of HPAG development.

Sry and Sox9 expression during canine gonadal sex determination assayed by quantitative reverse transcription-polymerase chain reaction

Testis induction is associated with gonadal Sry and Sox9 expression in mammals, and with Sox9 expression in vertebrates where Sry is absent. In mammals, Sry might initiate testis induction by upregulating Sox9 expression; however, direct evidence supporting this hypothesis is lacking. Models of Sry-negative XX sex reversal (XXSR), in which testes develop in the absence of Sry, could provide the link between Sry and Sox9 in testis induction. To define the stages at which testis determination occurs in the canine model, Sry and Sox9 expression were measured in normal urogenital ridges (UGR) and gonads by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Testicular Sry expression rose continuously during canine developmental ages comparable to human carnegie stages (CS) 16-18, with maximal expression at CS 18. Sox9 was expressed in both male and female canine UGR up to CS 17, at which time testis expression became tenfold greater than in the ovary. Although Sox9 was detected by qRT-PCR in ovaries and mesonephroi of both sexes, expression was detected only in canine testes by whole mount in situ hybridization (WMISH). The timing of Sry and Sox9 expression is consistent with a role in testis determination: Sry expression begins at CS 16 in testes, followed by upregulation of Sox9 expression at CS 17. The quantity and temporal and spatial patterns of Sry and Sox9 expression in normal canine gonads are similar to those in humans, sheep, and pigs. These studies should provide the basis for understanding the mechanism of testis induction in the canine model of Sry-negative XXSR.

Candidate testis-determining gene, Maestro (Mro), encodes a novel HEAT repeat protein

Mammalian sex determination depends on the presence or absence of SRY transcripts in the embryonic gonad. Expression of SRY initiates a pathway of gene expression resulting in testis development. Here, we describe a novel gene potentially functioning in this pathway using a cDNA microarray screen for genes exhibiting sexually dimorphic expression during murine gonad development. Maestro (Mro) transcripts are first detected in the developing male gonad before overt testis differentiation. By 12.5 days postcoitus (dpc), Mro transcription is restricted to the developing testis cords and its expression is not germ cell-dependent. No expression is observed in female gonads between 10.5 and 14.5 dpc. Maestro encodes a protein containing HEAT-like repeats that localizes to the nucleolus in cell transfection assays. Maestro maps to a region of mouse chromosome 18 containing a genetic modifier of XX sex reversal. We discuss the possible function of Maestro in light of these data.

The molecular action and regulation of the testis-determining factors, SRY (sex-determining region on the Y chromosome) and SOX9 [SRY-related high-mobility group (HMG) box 9]

Despite 12 yr since the discovery of SRY, little is known at the molecular level about how SRY and the SRY-related protein, SOX9 [SRY-related high-mobility group (HMG) box 9], initiate the program of gene expression required to commit the bipotential embryonic gonad to develop into a testis rather than an ovary. Analysis of SRY and SOX9 clinical mutant proteins and XX mice transgenic for testis-determining genes have provided some insight into their normal functions. SRY and SOX9 contain an HMG domain, a DNA-binding motif. The HMG domain plays a central role, being highly conserved between species and the site of nearly all missense mutations causing XY gonadal dysgenesis. SRY and SOX9 are architectural transcription factors; their HMG domain is capable of directing nuclear import and DNA bending. Whether SRY and SOX9 activate testis-forming genes, repress ovary-forming genes, or both remains speculative until downstream DNA target genes are identified. However, factors that control SRY and SOX9 gene expression have been identified, as have a dozen sex-determining genes, allowing some of the pieces in this molecular genetic puzzle to be connected. Many genes, however, remain unidentified, because in the majority of cases of XY females and in all cases of XX males lacking SRY, the mutated gene is unknown.

Sexy transgenes: the impact of gene transfer and gene inactivation technologies on the understanding of mammalian sex determination

Amongst the various developmental pathways ending in a sound mammal, sex determination presents the peculiarity of a choice between two equally viable options: female or male. Therefore, destroying a 'male-determining gene' or a 'female-determining gene' should generally not be lethal. Genetic sex determination is divided into two consecutive steps: construction of the bipotential gonad, and then sex determination per se. The genes involved in the first step are in fact involved in the development of various body compartments, and their mutation is generally far from innocuous. From transgenic and inactivation studies carried out on the laboratory mouse, a complete picture of the two steps is beginning to emerge, where the gonad itself and the necessary ducts are shown to evolve in a very coordinate way, with well-defined sex-specificities. Compared with testis determination, the ovarian side of the picture is still relatively empty, but this situation can change rapidly as candidate ovarian genes for inactivation studies are beginning to be identified.

Change of the heterogametic sex from male to female in the frog

Two different types of sex chromosomes, XX/XY and ZZ/ZW, exist in the Japanese frog Rana rugosa. They are separated in two local forms that share a common origin in hybridization between the other two forms (West Japan and Kanto) with male heterogametic sex determination and homomorphic sex chromosomes. In this study, to find out how the different types of sex chromosomes differentiated, particularly the evolutionary reason for the heterogametic sex change from male to female, we performed artificial crossings between the West Japan and Kanto forms and mitochondrial 12S rRNA gene sequence analysis. The crossing results showed male bias using mother frogs with West Japan cytoplasm and female bias using those with Kanto cytoplasm. The mitochondrial genes of ZZ/ZW and XX/XY forms, respectively, were similar in sequence to those of the West Japan and Kanto forms. These results suggest that in the primary ZZ/ZW form, the West Japan strain was maternal and thus male bias was caused by the introgression of the Kanto strain while in the primary XX/XY form and vice versa. We therefore hypothesize that sex ratio bias according to the maternal origin of the hybrid population was a trigger for the sex chromosome differentiation and the change of heterogametic sex.

Porcine SRY promoter is a target for steroidogenic factor 1

To study the process of mammalian sex determination and in particular to further understand the mechanisms of transcriptional regulation of the SRY gene, we have isolated a 4.5-kilobase (kb) pig SRY 5' flanking sequence. To facilitate the in vitro analysis of these sequences, we have generated a porcine genital ridge (PGR) cell line (9E11) that expresses SRY as well as SOX9, steroidogenic factor-1 (SF-1), and DAX1. Via primer extension analysis on RNA from this cell line, a transcription start site for porcine SRY was identified at -661 base pairs (bps) 5' from the translation initiation site. Deletion studies of the SRY 5' flanking sequences in PGR 9E11 cells demonstrated that -1.4 kb of 5' flanking sequences retained full transcriptional activity compared with the -4.5 kb fragment, but that transcriptional activity fell when further deletions were made. Sequences downstream of the transcriptional start site are important for promoter activity, because deleting transcribed but not translated sequences eliminated promoter activity. Sequence analysis of the -1.4 kb fragment identified two potential binding sites for SF-1, at -1369 and at -290 from the ATG. To address the role of SF-1 transactivation in SRY promoter activity, mutagenesis studies of the potential SF-1 binding sites were performed and revealed that these sites were indeed important for SRY promoter activity. Cotransfection studies in a heterologous cell system (mouse CV-1 cells) demonstrated that pig SF-1 was able to transactivate the pig SRY promoter. Gel shift assays confirmed that the upstream site was recognized by mouse SF-1 protein. We conclude that two sites for SF-1 transactivation exist within the pig SRY promoter, at -1369 bp and at -290 bp, and that the site at -1369 bp is quantitatively the most important.

FGF signaling antagonizes cytokine-mediated repression of Sox9 in SW1353 chondrosarcoma cells

OBJECTIVE

The Sox9 transcription factor has emerged as an important determinant of chondrocyte differentiation, including the regulation of type II collagen (Col2) and aggrecan gene expression. We sought to identify a human cell line model that conserves the Sox9 regulatory pathways identified in the mouse.

DESIGN

The SW1353 chondrosarcoma cell line was considered to be a candidate for Sox9 studies. The activity of a Sox9 regulated Col2a1 enhancer reporter gene was analyzed in response to treating cells with known regulators of murine Sox9 expression/activity. The effect of treatment on expression of the endogenous Sox9 gene was analyzed by real-time PCR and Western blot.

RESULTS

Col2 enhancer activity was stimulated by fibroblast growth factors (FGF-1 and -2) and repressed by inflammatory cytokines (IL-1beta and TNFalpha) in SW1353 cells. These effects correlated with changes in Sox9 mRNA and protein levels. In addition, FGF-9 was shown to stimulate enhancer activity and Sox9 expression. Cotreatment studies demonstrated that FGFs functionally antagonize the cytokine-mediated repression of Sox9 expression and Col2 enhancer activity.

CONCLUSIONS

SW1353 cells represent a useful human cell model as they conserve many Sox9 signaling pathways previously demonstrated in mouse chondrocytes. We identify FGF-9 as a particularly potent Sox9 agonist. The antagonism between FGFs and cytokines on Sox9 expression and Col2 enhancer activity suggests that Sox9 integrates the opposing activities of FGFs and cytokines. We also find that SW1353 cells respond to very low doses of IL-1 with Col2 enhancer activation, while increasing doses lead to repression.

The sex determination in Ellobius lutescens remains bizarre

Mammalian sex determination and gonad differentiation are the result of a complex interaction of fine-tuned spatial and temporal gene expression with threshold levels of individual genes. The male pathway is initiated by SRY. Some exceptional mammals determine male sex without the SRY gene and even without a Y chromosome. Ellobius lutescens in this report is one example of this "weird" species. We provide key data on the genomic level that there are no coarse differences in the genomes of male and female animals by comparative genomic hybridization. On the gene level we studied the gene Nr5a1 for the orphan nuclear receptor, steroidogenic factor SF-1, a central constituent for gonad differentiation and adrenal gland development. The Ellobius lutescens Nr5a1 gene was mapped to the proximal short arm of chromosome 2 by fluorescence in situ hybridization. In addition, we provide evidence by linkage analysis in two E. lutescens pedigrees that Nr5a1 is not the key male sex-determining gene in Ellobius lutescens.

Expression of Sox9 in granulosa cells lacking the estrogen receptors, ERalpha and ERbeta

Ovaries from adult mice lacking both estrogen receptors ERalpha and ERbeta (ERalphabetaKO mice) contain abnormal cells sharing morphologic features with Sertoli cells, which are located mainly in the interstitial compartment. We show here that these cells express the Sertoli cell markers TIF1beta, TIF2, and Sox9. In ERalphabetaKO ovaries, Sox9 is expressed by granulosa cells before the morphologic appearance of Sertoli cells, but neither by granulosa cell precursors nor by non-Sertolian interstitial cells. These findings suggest that functional Sertoli cells can transdifferentiate from mature granulosa cells devoid of estrogen receptors as a result of Sox9 expression.

The Kruppel-like factor Zf9 and proteins in the Sp1 family regulate the expression of HSP47, a collagen-specific molecular chaperone

In several cells and tissues the synthesis of HSP47, a collagen-specific molecular chaperone in the endoplasmic reticulum, is closely correlated with the synthesis of collagen. We previously reported that the Sp1 binding site at -210 bp in the promoter region and the first and second introns are required for the tissue-specific expression of HSP47 in transgenic mice (Hirata, H., Yamamura, I., Yasuda, K., Kobayashi, A., Tada, N., Suzuki, M., Hirayoshi, K., Hosokawa, N., and Nagata, K. (1999) J. Biol. Chem. 274, 35703-35710). Here, we analyze how these introns influence the transcriptional regulation of the hsp47 gene in BALB/c 3T3 cells, which produce high levels of HSP47. In vitro promoter analysis using a luciferase reporter and gel mobility shift analysis revealed that two cis-acting elements in the first and second introns, BS5-B and EP7-D, respectively, are required for the activation of hsp47 in BALB/c 3T3 cells. Several members of the Kruppel-like factor (KLF) family of proteins were identified as BS5-B-binding proteins by yeast one-hybrid analysis using these elements as baits. One of these proteins, KLF-6/Zf9, binds to the BS5-B element and activates expression of the reporter construct when transfected into cells. Chromatin immunoprecipitation assay analysis revealed that the endogenous KLF-6/Zf9 binds the BS5-B elements that contain the CACCC motif, which is a consensus recognition sequence for other proteins in the KLF family. We also showed that BS5-B and EP7-D are bound by two members of the Sp1 family, Sp2 and Sp3. These results suggest that at least three sequences are required for the constitutive expression of hsp47 in BALB/c 3T3 cells: the -210 bp Sp1 binding site, the BS5-B element in the first intron, and the EP7-D element in the second intron. We suggest that KLF proteins regulate the transcription of hsp47 by binding the BS5-B element in cooperation with Sp2 and/or Sp3.

Biochemical defects in eight SRY missense mutations causing XY gonadal dysgenesis

Sex determining Region of the Y chromosome (SRY) is the Y-borne gene required for male sex determination. Many XY females with complete gonadal dysgenesis carry SRY mutations. We describe here the effects of eight clinically isolated point mutations on the DNA-binding and -bending functions of SRY. We found that the seven mutations in the HMG domain affected the protein's DNA-binding and -bending activities to varying degrees, although all cause complete gonadal dysgenesis. DNA binding was abolished by the R75N and L94P mutations, severely disrupted by the F67V mutation and reduced by the M64R (6-fold), R76P (4-fold), A113T (3-fold), and M78T (1.7-fold) mutations. Of these, variant M64R showed no DNA-bending activity, while M78T caused a mild reduction in DNA bending. The S18N mutation, a familial mutation that lies outside the HMG domain and caused partial gonadal dysgenesis in one patient, had minimal effect on DNA binding and bending. Analysis of the NMR solution structure of the SRY HMG domain bound to DNA suggests that mutations disrupt the protein's conformation (helicity, packing), or interactions at the DNA interface. The degree to which mutations causing complete gonadal dysgenesis affect the DNA-binding activity varies. We propose that there is a threshold level of SRY activity or expression required for testis determination, as we observe that familial mutations have the least effect on SRY activity.

Genetic network identification by high density, multiplexed reversed transcriptional (HD-MRT) analysis in steroidogenic axis model cell lines

Transcriptional network analysis in steroidogenic axis cell lines requires an understanding of cellular network composition and complexity. Previous studies have shown that absence of transcriptional network components in a cell line compromises that cell line's functional capacity for transcriptional regulation. Our goal was to analyze qualitatively steroidogenic axis-derived cell lines' expression of a putative transcriptional network involved in human and mouse development. To pursue this analysis we used Northern blots and a high density-multiplexed reverse transcription-polymerase chain reaction (HD-MRT-PCR) approach. Our results revealed that, while some members of this putative network were universally expressed, only a minority of the non-constitutive targeted transcripts were present in any single line. Based on our data and previously published results for contextual expression of these transcription factors, a model was constructed possessing the topology suggestive of a scale-free network: certain network members were highly connected nodes and would represent critical sites of vulnerability. The importance of these highly connected nodes for network function is supported by the severe phenotypes exhibited by human patients and animal models when these genes are mutated. We conclude that knowledge of network composition in specific cell lines is essential for their use as models to investigate functional interactions within selected subnetworks.

Estrogen regulates development of the somatic cell phenotype in the eutherian ovary

Steroids play a critical role in gonadal differentiation in birds, reptiles, and amphibia whereas gonadal differentiation in mammals is thought to be determined by genetic mechanisms. The gonads of female mice incapable of synthesizing estrogens due to disruption of the aromatase gene (ArKO) provide a unique model to test the role of estrogen in regulating the gonadal phenotype. We have shown that in the absence of estrogen, genetically female mice develop testicular tissue within their ovaries. The ovaries develop cells that possess structural and functional characteristics of testicular interstitial cells and of seminiferous tubule-like structures lined with Sertoli cells. Moreover, the ovaries express mRNA for the testis-specific Sertoli cell transcription factor Sox 9 and espin protein, which is specific for inter-Sertoli cell junctions. The development of the testicular tissue in this model can be reverted/postponed by replacing estrogen. When ArKO female mice were fed a diet containing phytoestrogens, the appearance of Leydig and Sertoli cells was postponed and reduced. Furthermore, administration of estradiol-17beta decreased the number of Sertoli and Leydig cells in the ovaries. These findings constitute definitive evidence that estrogen plays a critical role in maintaining female somatic interstitial and granulosa cells in the eutherian ovary.

A nuclear export signal within the high mobility group domain regulates the nucleocytoplasmic translocation of SOX9 during sexual determination

In mammals, male sex determination starts when the Y chromosome Sry gene is expressed within the undetermined male gonad. One of the earliest effect of Sry expression is to induce up-regulation of Sox9 gene expression in the developing gonad. SOX9, like SRY, contains a high mobility group domain and is sufficient to induce testis differentiation in transgenic XX mice. Before sexual differentiation, SOX9 protein is initially found in the cytoplasm of undifferentiated gonads from both sexes. At the time of testis differentiation and anti-Müllerian hormone expression, it becomes localized to the nuclear compartment in males whereas it is down-regulated in females. In this report, we used NIH 3T3 cells as a model to examine the regulation of SOX9 nucleo-cytoplasmic shuttling. SOX9-transfected cells expressed nuclear and cytoplasmic SOX9 whereas transfected cells treated with the nuclear export inhibitor leptomycin B, displayed an exclusive nuclear localization of SOX9. By using SOX9 deletion constructs in green fluorescent protein fusion proteins, we identified a functional nuclear export signal sequence between amino acids 134 and 147 of SOX9 high mobility group box. More strikingly, we show that inhibiting nuclear export with leptomycin B in mouse XX gonads cultured in vitro induced a sex reversal phenotype characterized by nuclear SOX9 and anti-Müllerian hormone expression. These results indicate that SOX9 nuclear export signal is essential for SOX9 sex-specific subcellular localization and could be part of a regulatory switch repressing (in females) or triggering (in males) male-specific sexual differentiation.

The porcine SRY promoter is transactivated within a male genital ridge environment

In mammals the SRY gene functions as a dominant genetic switch for testis determination (Gubbay et al.: Nature 346:1128-1135, 1990; Koopman et al.: Nature 351:117-121, 1991; Sinclair et al.: Nature 346:240-244, 1990). To study SRY transcriptional regulation within an evolutionary context, we have generated transgenic mice that express green fluorescent protein (GFP) under the control of 4.5 kb of pig SRY 5' flanking sequences (pSRYp-GFP). Autofluorescence was observed in the genital ridges of e11.5 male embryos (18-21 tail somites), and by e12.5 (27 tail somites) autofluorescence was observed within the testes cords. The expression of the transgene did not display the abrupt termination characteristic of endogenous mouse SRY, but rather showed a gradual reduction in expression characteristic of human, pig and sheep SRY. Surprisingly, no autofluorescence was observed in normal XX genital ridges, although more sensitive RT-PCR analysis detected transgene transcription. When the transgene was bred into a constitutively male line of mice (Odsex; Bishop et al.: Nat Genet 26:490-494, 2000), autofluorescence was visible in genital ridges of XX animals, in the genetic absence of Sry protein. Via RT-PCR analysis, purified autofluorescent cells from e12.5 gonadal ridges expressed mouse SRY but not Oct4 transcripts, whereas autofluorescent cells from e14.5 gonadal ridges expressed MIS but not Oct4 transcripts, in each case consistent with a pre-Sertoli cell phenotype. In vitro expression studies performed in CV-1 cells demonstrated that pig SOX9 cDNA transactivated the pig SRY promoter but that pig SRY cDNA did not. When a SOX9 potential binding site identified at -205 of the pig SRY 5' flanking sequences was mutated, the SOX9 transactivation effect was reduced by 70%. This site is conserved in the 5' flanking sequences of bovine and human SRY genes but not in the mouse gene. Gel retardation assays using this binding site showed specific binding to SOX9-enriched nuclear extracts that was competed by excess unlabelled binding site but not by mutated binding site. We suggest that pig SRY gene is responsive to a testicular environment and propose a model of feedback amplification of pig SRY transcription by SOX9.

Depletion of definitive gut endoderm in Sox17-null mutant mice

In the mouse, the definitive endoderm is derived from the epiblast during gastrulation, and, at the early organogenesis stage, forms the primitive gut tube, which gives rise to the digestive tract, liver, pancreas and associated visceral organs. The transcription factors, Sox17 (a Sry-related HMG box factor) and its upstream factors, Mixer (homeobox factor) and Casanova (a novel Sox factor), have been shown to function as endoderm determinants in Xenopus and zebrafish, respectively. However, whether the mammalian orthologues of these genes are also involved with endoderm formation is not known. We show that Sox17–/– mutant embryos are deficient of gut endoderm. The earliest recognisable defect is the reduced occupancy by the definitive endoderm in the posterior and lateral region of the prospective mid- and hindgut of the headfold-stage embryo. The prospective foregut develops properly until the late neural plate stage. Thereafter, elevated levels of apoptosis lead to a reduction in the population of the definitive endoderm in the foregut. In addition, the mid- and hindgut tissues fail to expand. These are accompanied by the replacement of the definitive endoderm in the lateral region of the entire length of the embryonic gut by cells that resemble the visceral endoderm. In the chimeras, although Sox17-null ES cells can contribute unrestrictedly to ectodermal and mesodermal tissues, few of them could colonise the foregut endoderm and they are completely excluded from the mid- and hindgut endoderm. Our findings indicate an important role of Sox17 in endoderm development in the mouse, highlighting the idea that the molecular mechanism for endoderm formation is likely to be conserved among vertebrates.

Sex with two SOX on: SRY and SOX9 in testis development

Although gonads are not required for development or survival, defects in gonadal development undoubtedly have a profound influence on affected individuals. Recent complementary studies in the fields of cytology, biochemistry and molecular genetics have revealed that normal gonad development involves an exquisitely regulated network of gene expression and protein-protein interactions. The initial event of gonadogenesis, in both males and females, involves the formation of a bipotential primordium. A Y chromosome then activates the male-specific pathway. The demonstration that mutations in the SOX proteins, SRY and SOX9, are responsible for disorders associated with male-to-female sex reversal showed dramatically that SRY and SOX9 have an essential role in male sex differentiation. This was emphasized when it was shown that female mice carrying transgenes that encode these proteins developed as males. SRY and SOX9 proteins have been characterized extensively and aspects of their function and regulation are now known.

Regulation of the orphan nuclear receptor steroidogenic factor 1 by Sox proteins

Steroidogenic factor 1 (SF-1) is an essential factor in endocrine proliferation and gene expression. Despite the fact that SF-1 expression is restricted to specialized cells within the endocrine system, the only identified regulatory factors of SF-1 are the ubiquitously expressed E-box proteins (upstream stimulatory factors 1 and 2). Sequence examination of the SF-1 proximal promoter revealed a conserved site of AACAAAG (Sox-BS1), which matches exactly the defined consensus Sox protein binding element. Among the approximately 20 known members of the Sox gene family, we focused on Sox3, Sox8, and Sox9, based on their coexpression with SF-1 in the embryonic testis. Indeed, all three of these Sox proteins were capable of binding the proximal Sox-BS1 within the SF-1 promoter (-110 to -104), albeit with differing affinities. Of the three Sox proteins, Sox9 exhibited high-affinity binding to the Sox-BS1 element and consistently activated SF-1 promoter-reporter constructs. Mutating the Sox-BS1 attenuated SF-1 promoter activity in both embryonic and postnatal Sertoli cells, as well as in the adrenocortical cell line, Y1. Our findings, taken together with the overlapping expression profiles of Sox9 and SF-1, and the similar intersex phenotypes associated with both SOX9 and SF-1 human mutations, suggest that Sox9 up-regulates SF-1 and accounts partially for the sexually dimorphic expression pattern of SF-1 observed during male gonadal differentiation.

Invited review: sex-based differences in gene expression

Certain diseases are more prevalent among women than men. The reasons for this increased prevalence are unknown, but there could be a genetic basis. Increased expression of X-linked genes in females, protective effects of Y-linked genes in males, or sex-limited gene expression that is developmentally or hormonally regulated could all account for these differences. Analysis of individuals with and without genetic sex reversal provides a means for distinguishing between genetic and hormonal causes. This can be complemented by genetic linkage and gene expression profiling to aid in the identification of candidate genes.

46,XX sex reversal

In humans, sexual differentiation is directed by SRY, a master regulatory gene located at the Y chromosome. This gene initiates the male pathway or represses the female pathway by regulating the transcription of downstream genes; however, the precise mechanisms by which SRY acts are largely unknown. Moreover, several genes have recently been implicated in the development of the bipotential gonad even before SRY is expressed. In some individuals, the normal process of sexual differentiation is altered and a sex reversal disorder is observed. These subjects present the chromosomes of one sex but the physical attributes of the other. Over the past years, considerable progress has been achieved in the molecular characterization of these disorders by using a combination of strategies including cell biology, animal models, and by studying patients with these pathologic entities.

Comparative genomics of the SOX9 region in human and Fugu rubripes: conservation of short regulatory sequence elements within large intergenic regions

Campomelic dysplasia (CD), a human skeletal malformation syndrome with XY sex reversal, is caused by heterozygous mutations in and around the gene SOX9. SOX9 has an extended 5' control region, as indicated by CD translocation breakpoints scattered over 1 Mb proximal to SOX9 and by expression data from mice transgenic for human SOX9-spanning yeast artificial chromosomes. To identify long-range regulatory elements within the SOX9 5' control region, we compared approximately 3.7 Mb and 195 kb of sequence around human and Fugu rubripes SOX9, respectively. We identified only seven and five protein-coding genes in the human and F. rubripes sequences, respectively. Four of the F. rubripes genes have been mapped in humans; all reside on chromosome 17 but show extensive intrachromosomal gene shuffling compared with the gene order in F. rubripes. In both species, very large intergenic distances separate SOX9 from its directly flanking genes: 2 Mb and 500 kb on either side of SOX9 in humans, and 68 and 97 kb on either side of SOX9 in F. rubripes. Comparative sequence analysis of the intergenic regions revealed five conserved elements, E1-E5, up to 290 kb 5' to human SOX9 and up to 18 kb 5' to F. rubripes SOX9, and three such elements, E6-E8, 3' to SOX9. Where available, mouse sequences confirm conservation of the elements. From the yeast artificial chromosome transgenic data, elements E3-E5 are candidate enhancers for SOX9 expression in limb and vertebral column, and 8 of 10 CD translocation breakpoints separate these elements from SOX9.

Minireview: sex differentiation

Mammalian sex differentiation is a hormone-dependent process in the male following the determination of a testis from the indifferent gonad through a cascade of genetic events. Female sex differentiation is not dependent on ovarian hormones, yet there is evidence that members of the Wnt family of developmental signaling molecules play a role in Müllerian duct development and in suppressing Leydig cell differentiation in the ovary. The testis induces male sex differentiation (including testis descent) through a time-dependent production of optimal concentrations of anti-Müllerian hormone, insulin-like factor(s) and androgens. Observations in several human syndromes of disordered fetal sex development corroborate findings in murine embryo studies, although there are exceptions in some gene knockout models. The ubiquitously expressed AR interacts in a ligand-dependent manner with coregulators to control the expression of androgen-responsive genes. Preliminary studies suggest the possibility of hormone resistance syndromes associated with coregulator dysfunction. Polymorphic variants in genes controlling androgen synthesis and action may modulate androgenic effects on sex differentiation.

Sox9 induces testis development in XX transgenic mice

Mutations in SOX9 are associated with male-to-female sex reversal in humans. To analyze Sox9 function during sex determination, we ectopically expressed this gene in XX gonads. Here, we show that Sox9 is sufficient to induce testis formation in mice, indicating that it can substitute for the sex-determining gene Sry.

Testis determination in mammals: more questions than answers

In humans, testis development depends on a regulated genetic hierarchy initiated by the Y-linked SRY gene. Failure of testicular determination results in the condition termed 46,XY gonadal dysgenesis (GD). Several components of the testis determining pathway have recently been identified though it has been difficult to articulate a cascade with the known elements of the system. It seems, however, that early gonadal development is the result of a network of interactions instead of the outcome of a linear cascade. Accumulating evidence shows that testis formation in man is sensitive to gene dosage. Haploinsufficiency of SF1, WT1 and SOX9 is responsible for 46,XY gonadal dysgenesis. Besides, data on SRY is consistent with possible dosage anomalies in certain cases of male to female sex reversal. 46,XY GD due to monosomy of distal 9p and 10q might also be associated with an insufficient gene dosage effect. Duplications of the locus DSS can lead to a failure of testicular development and a duplication of the region containing SOX9 has been implicated in XX sex reversal. Transgenic studies in mouse have shown, however, that this mammal is less sensitive to gene dosage than man. Here, we will try to put in place the known pieces of the jigsaw puzzle that is sex determination in mammals, as far as current knowledge obtained from man and animal models allows. We are certain that from this attempt more questions than answers will arise.

Molecular evolution of Sry and Sox gene

The mammalian Sry on the short arm of the Y chromosome encodes a nuclear factor-like protein harboring a DNA-binding domain known as the HMG box. The Sox genes encode similar factor like proteins, but the sequence similarity of the HMG box to that of Sry is variable as being at least 60%. The functional relationship of Sox to Sry genes with special reference to sex determination is unclear except for a few items such as human autosomal Sox9. Thus, it is significant to know more about the evolutionary in addition to the functional relationship between Sry and Sox genes for deepening and broadening our understanding concerning primary sex determination. Therefore, to clarify the ancestry and molecular evolution of the mammalian sex determining gene Sry with its evolutionary relationships to the Sox gene, a molecular phylogenetic tree for the HMG box superfamily was constructed and analyzed, and the following conclusions were reached: (1) The nuclear non histone HMG proteins are supposedly the oldest, appearing at least more than one billion years ago, before the divergence of animals and plants. They diverged into two subgroups: one contains HMG14 and HMG17, and the other one contains HMG1 and HMG2 with various other genes. Subsequent divergences include the nucleolar UBF, nuclear SSRP as well as fungal mating protein Mc, MAT and Ste11. (2) The Sox and Sry genes diverged following the diversification of lymphoid transcription factors TCF and LEF. The Sry gene might have definitely evolved from the Sox gene cluster a few hundred million years ago. Additionally, the marsupial Sry, e.g. from Wallabie's and Dunnart's, is distinguished by being distant from eutherian Sry, but being closely related to the Sox gene cluster. (3) Molecular evolutionary rates estimated in mammalian Sry as the divergent rate per 100 million years are much higher than in Sox genes or other genes from the HMG box superfamily. This rapid evolution of Sry might agree with the fact that the Srys are present not on the pseudoautosomal region but on the distal region with no recombination of the Y chromosomal short arm.

Establishing sexual dimorphism: conservation amidst diversity?

The molecular mechanisms that control sexual dimorphism are very different in distantly related animals. Did sex determination arise several times with different regulatory mechanisms, or is it an ancient process with little surviving evidence of ancestral genes? The recent identification of related sexual regulators in different phyla indicates that some aspects of sexual regulation might be ancient. Studies of sex-determining mechanisms are beginning to reveal how sexual dimorphism arises and evolves.