Marie-Christine Chaboissier

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.

R-spondin1 is essential in sex determination, skin differentiation and malignancy

R-spondins are a recently characterized small family of growth factors. Here we show that human R-spondin1 (RSPO1) is the gene disrupted in a recessive syndrome characterized by XX sex reversal, palmoplantar hyperkeratosis and predisposition to squamous cell carcinoma of the skin. Our data show, for the first time, that disruption of a single gene can lead to complete female-to-male sex reversal in the absence of the testis-determining gene, SRY.

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.

Fgf9 and Wnt4 Act as Antagonistic Signals to Regulate Mammalian Sex Determination

The genes encoding members of the wingless-related MMTV integration site (WNT) and fibroblast growth factor (FGF) families coordinate growth, morphogenesis, and differentiation in many fields of cells during development. In the mouse, Fgf9 and Wnt4 are expressed in gonads of both sexes prior to sex determination. Loss of Fgf9 leads to XY sex reversal, whereas loss of Wnt4 results in partial testis development in XX gonads. However, the relationship between these signals and the male sex-determining gene, Sry, was unknown. We show through gain- and loss-of-function experiments that fibroblast growth factor 9 (FGF9) and WNT4 act as opposing signals to regulate sex determination. In the mouse XY gonad, Sry normally initiates a feed-forward loop between Sox9 and Fgf9, which up-regulates Fgf9 and represses Wnt4 to establish the testis pathway. Surprisingly, loss of Wnt4 in XX gonads is sufficient to up-regulate Fgf9 and Sox9 in the absence of Sry. These data suggest that the fate of the gonad is controlled by antagonism between Fgf9 and Wnt4. The role of the male sex-determining switch— Sry in the case of mammals—is to tip the balance between these underlying patterning signals. In principle, sex determination in other vertebrates may operate through any switch that introduces an imbalance between these two signaling pathways.

Activation of β-catenin signaling by Rspo1 controls differentiation of the mammalian ovary

The sex of an individual is determined by the fate of the gonad. While the expression of Sry and Sox9 is sufficient to induce male development, we here show that female differentiation requires activation of the canonical beta-catenin signaling pathway. beta-catenin activation is controlled by Rspo1 in XX gonads and Rspo1 knockout mice show masculinized gonads. Molecular analyses demonstrate an absence of female-specific activation of Wnt4 and as a consequence XY-like vascularization and steroidogenesis. Moreover, germ cells of XX knockout embryos show changes in cellular adhesions and a failure to enter XX specific meiosis. Sex cords develop around birth, when Sox9 becomes strongly activated. Thus, a balance between Sox9 and beta-catenin activation determines the fate of the gonad, with Rspo1 acting as a crucial regulator of canonical beta-catenin signaling required for female development.

Sox9 is essential for outer root sheath differentiation and the formation of the hair stem cell compartment

BACKGROUND The mammalian hair represents an unparalleled model system to understand both developmental processes and stem cell biology. The hair follicle consists of several concentric epithelial sheaths with the outer root sheath (ORS) forming the outermost layer. Functionally, the ORS has been implicated in the migration of hair stem cells from the stem cell niche toward the hair bulb. However, factors required for the differentiation of this critical cell lineage remain to be identified. Here, we describe an unexpected role of the HMG-box-containing gene Sox9 in hair development. RESULTS Sox9 expression can be first detected in the epithelial component of the hair placode but then becomes restricted to the outer root sheath (ORS) and the hair stem cell compartment (bulge). Using tissue-specific inactivation of Sox9, we demonstrate that this gene serves a crucial role in hair differentiation and that skin deleted for Sox9 lacks external hair. Strikingly, the ORS acquires epidermal characteristics with ectopic expression of GATA3. Moreover, Sox9 knock hair show severe proliferative defects and the stem cell niche never forms. Finally, we show that Sox9 expression depends on sonic hedgehog (Shh) signaling and demonstrate overexpression in skin tumors in mouse and man. CONCLUSIONS We conclude that although Sox9 is dispensable for hair induction, it directs differentiation of the ORS and is required for the formation of the hair stem cell compartment. Our genetic analysis places Sox9 in a molecular cascade downstream of sonic hedgehog and suggests that this gene is involved in basal cell carcinoma.

Sox9 is expressed in mouse multipotent retinal progenitor cells and functions in Müller glial cell development

It is widely accepted that the process of retinal cell fate determination is under tight transcriptional control mediated by a combinatorial code of transcription factors. However, the exact repertoire of factors necessary for the genesis of each retinal cell type remains to be fully defined. Here we show that the HMG‐box transcription factor, Sox9, is expressed in multipotent mouse retinal progenitor cells throughout retinogenesis. We also find that Sox9 is downregulated in differentiating neuronal populations, yet expression in Müller glial cells persists into adulthood. Furthermore, by employing a conditional knockout approach, we show that Sox9 is essential for the differentiation and/or survival of postnatal Müller glial cells. J. Comp. Neurol. 510:237–250, 2008.

RSPO1/β-Catenin signaling pathway regulates oogonia differentiation and entry into meiosis in the mouse fetal ovary

Differentiation of germ cells into male gonocytes or female oocytes is a central event in sexual reproduction. Proliferation and differentiation of fetal germ cells depend on the sex of the embryo. In male mouse embryos, germ cell proliferation is regulated by the RNA helicase Mouse Vasa homolog gene and factors synthesized by the somatic Sertoli cells promote gonocyte differentiation. In the female, ovarian differentiation requires activation of the WNT/β-catenin signaling pathway in the somatic cells by the secreted protein RSPO1. Using mouse models, we now show that Rspo1 also activates the WNT/β-catenin signaling pathway in germ cells. In XX Rspo1−/− gonads, germ cell proliferation, expression of the early meiotic marker Stra8, and entry into meiosis are all impaired. In these gonads, impaired entry into meiosis and germ cell sex reversal occur prior to detectable Sertoli cell differentiation, suggesting that β-catenin signaling acts within the germ cells to promote oogonial differentiation and entry into meiosis. Our results demonstrate that RSPO1/β-catenin signaling is involved in meiosis in fetal germ cells and contributes to the cellular decision of germ cells to differentiate into oocyte or sperm.

SOX9 Controls Epithelial Branching by Activating RET Effector Genes during Kidney Development

Congenital abnormalities of the kidney and urinary tract are some of the most common defects detected in the unborn child. Kidney growth is controlled by the GDNF/RET signalling pathway, but the molecular events required for the activation of RET downstream targets are still poorly understood. Here we show that SOX9, a gene involved in campomelic dysplasia (CD) in humans, together with its close homologue SOX8, plays an essential role in RET signalling. Expression of SOX9 can be found from the earliest stages of renal development within the ureteric tip, the ureter mesenchyme and in a segment-specific manner during nephrogenesis. Using a tissue-specific knockout approach, we show that, in the ureteric tip, SOX8 and SOX9 are required for ureter branching, and double-knockout mutants exhibit severe kidney defects ranging from hypoplastic kidneys to renal agenesis. Further genetic analysis shows that SOX8/9 are required downstream of GDNF signalling for the activation of RET effector genes such as Sprouty1 and Etv5. At later stages of development, SOX9 is required to maintain ureteric tip identity and SOX9 ablation induces ectopic nephron formation. Taken together, our study shows that SOX9 acts at multiple steps during kidney organogenesis and identifies SOX8 and SOX9 as key factors within the RET signalling pathway. Our results also explain the aetiology of kidney hypoplasia found in a proportion of CD patients.

Genetics of Ovarian Differentiation: Rspo1 , a Major Player

In mammals, the sex of the embryo is determined during development by its commitment either to the male or female genetic program regulating testicular or ovarian organogenesis. Major steps towards unraveling sex determination in mammals are achieved by the identification of key genes involved in human pathologies and the application of mouse genetics to analyze their function. While the expression of Sry and Sox9 is sufficient to induce the male devel- opmental program, the molecular pathways that specify ovarian differentiation were unclear before the recent demonstration that mutations in the RSPO1 gene induce female-to-male sex reversal in XX patients. By generating the corresponding mouse model, we have shown that Rspo1 is so far the earliest known gene controlling the female genetic developmental program. Rspo1 activates the canonical β-catenin signaling pathway required for female somatic cell differentiation and germ cell commitment into meiosis. The aim of this review is to describe the roles of R-spondins (Rspo)in developmental processes and disorders and the current knowledge obtained from murine models. A particular focus will be on Rspo1 and its crucial function in sex determination.