Francisco Barrionuevo

Homozygous Inactivation of Sox9 Causes Complete XY Sex Reversal in Mice

Abstract In the presence of the Y-chromosomal gene Sry, the bipotential mouse gonads develop as testes rather than as ovaries. The autosomal gene Sox9, a likely and possibly direct Sry target, can induce testis development in the absence of Sry. Sox9 is thus sufficient but not necessarily essential for testis induction. Mutational inactivation of one allele of SOX9/Sox9 causes sex reversal in humans but not in mice. Because Sox9–/– embryos die around Embryonic Day 11.5 (E11.5) at the onset of testicular morphogenesis, differentiation of the mutant XY gonad can be analyzed only ex vivo in organ culture. We have therefore conditionally inactivated both Sox9 alleles in the gonadal anlagen using the CRE/loxP recombination system, whereby CRE recombinase is under control of the cytokeratin 19 promoter. Analysis of resulting Sox9–/– XY gonads up to E15.5 reveals immediate, complete sex reversal, as shown by expression of the early ovary-specific markers Wnt4 and Foxl2 and by lack of testis cord and Leydig cell formation. Sry expression in mutant XY gonads indicates that downregulation of Wnt4 and Foxl2 is dependent on Sox9 rather than on Sry. Our results provide in vivo proof that, in contrast to the situation in humans, complete XY sex reversal in mice requires inactivation of both Sox9 alleles and that Sox9 is essential for testogenesis in mice.

Testis cord differentiation after the sex determination stage is independent of Sox9 but fails in the combined absence of Sox9 and Sox8

Sox9 and Sox8 are transcription factors expressed in embryonic and postnatal Sertoli cells of the mouse testis. Sox9 inactivation prior to the sex determination stage leads to complete XY sex reversal. In contrast, there is normal embryonic testis development in Sox8 mutants which are initially fertile, but later develop progressive seminiferous tubule failure and infertility. To determine whether Sox9 is required for testis development after the initial steps of sex determination, we crossed Sox9(flox) mice with an AMH-Cre transgenic line thereby completely deleting Sox9 in Sertoli cells by E14.0. Conditional Sox9 null mutants show normal embryonic testis development and are initially fertile, but, like Sox8(-/-) mutants, become sterile from dysfunctional spermatogenesis at about 5 months. To see whether Sox8 may compensate for the absence of Sox9 during embryonic testis differentiation, we generated a Sox9 conditional knockout on a Sox8 mutant background. In the double mutants, differentiation of testis cords into seminiferous testis tubules ceases after P6 in the absence of one Sox8 allele, and after P0 in the absence of both Sox8 alleles, leading to complete primary infertility. Sox9,Sox8 double nullizygous testes show upregulation of early ovary-specific markers and downregulation of Sertoli intercellular junctions at E15.5. Their very low Amh levels still cause complete regression of the Müllerian duct but with reduced penetrance. This study shows that testis cord differentiation is independent of Sox9, and that concerted Sox9 and Sox8 function in post E14.0 Sertoli cells is essential for the maintenance of testicular function.

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.

Dicer is required for Sertoli cell function and survival.

Dicer is a key enzyme that processes microRNA precursors into their mature form, enabling them to regulate gene expression. Dicer null mutants die before gastrulation. To study Dicer function in testis development, we crossed mice carrying a conditional Dicer allele with an AMH-Cre transgenic line, thereby inactivating Dicer in Sertoli cells around embryonic day 14.0 (E14.0). Dicer null Sertoli cells show normal embryonic development, and at postnatal day 0 (P0), testis tubules are normal in number and histologically undistinguishable from controls. Subsequently, Dicer-mutant testes show a progressively aberrant development, so that at P6, they contain a reduced number of disorganized testis tubules leading to primary sterility. Apoptosis and prophase I assays reveal a massive wave of apoptosis starting at P3, causing progressive loss of Sertoli cells, but also of germ cells, resulting in drastically reduced testis size. Expression of genes that play crucial roles in testis development, structural integrity and spermatogenesis is downregulated at P0, before morphological changes become apparent, indicating that Dicer-mutant testes are already transcriptionally compromised at this stage. Taken together, the results of this study show that Dicer is required for Sertoli cell function and survival and for spermatogenesis in mice.

L-Sox5 and Sox6 proteins enhance chondrogenic miR-140 microRNA expression by strengthening dimeric Sox9 activity.

Background: miR-140 is a critical regulator of cartilage development and homeostasis. Results: The proximal upstream region of miR-140 has in vivo chondrogenic promoter activity and an L-Sox5/Sox6/Sox9 (Sox trio) response element. Conclusion: We reveal that L-Sox5 and Sox6 control miR-140 expression together with Sox9. Significance: Uncovering molecular mechanisms of chondrogenesis has implications for cartilage repair and restoration of tissue function. Sox9 plays a critical role in early chondrocyte initiation and promotion as well as repression of later maturation. Fellow Sox family members L-Sox5 and Sox6 also function as regulators of cartilage development by boosting Sox9 activation of chondrocyte-specific genes such as Col2a1 and Agc1; however, the regulatory mechanism and other target genes are largely unknown. MicroRNAs are a class of short, non-coding RNAs that act as negative regulators of gene expression by promoting target mRNA degradation and/or repressing translation. Analysis of genetically modified mice identified miR-140 as a cartilage-specific microRNA that could be a critical regulator of cartilage development and homeostasis. Recent findings suggest Sox9 promotes miR-140 expression, although the detailed mechanisms are not fully understood. In this study we demonstrate that the proximal upstream region of pri-miR-140 has chondrogenic promoter activity in vivo. We found an L-Sox5/Sox6/Sox9 (Sox trio) response element and detailed binding site in the promoter region. Furthermore, detailed analysis suggests the DNA binding and/or transactivation ability of Sox9 as a homodimer is boosted by L-Sox5 and Sox6. These findings provide new insight into cartilage-specific gene regulation by the Sox trio.

Sox9 is required for invagination of the otic placode in mice.

The HMG-domain-containing transcription factor Sox9 is an important regulator of chondrogenesis, testis formation and development of several other organs. Sox9 is expressed in the otic placodes, the primordia of the inner ear, and studies in Xenopus have provided evidence that Sox9 is required for otic specification. Here we report novel and different functions of Sox9 during mouse inner ear development. We show that in mice with a Foxg1(Cre)-mediated conditional inactivation of Sox9 in the otic ectoderm, otic placodes form and express markers of otic specification. However, mutant placodes do not attach to the neural tube, fail to invaginate, and subsequently degenerate by apoptosis, resulting in a complete loss of otic structures. Transmission-electron microscopic analysis suggests that cell-cell contacts in the Sox9 mutant placodes are abnormal, although E-cadherin, N-cadherin, and beta-catenin protein expression are unchanged. In contrast, expression of Epha4 was downregulated in mutant placodes. In embryos with a Keratin-19(Cre)-mediated mosaic inactivation of Sox9, Sox9-negative and Sox9-positive cells in the otic ectoderm sort out from one another. In these embryos only Sox9-positive cells invaginate and form one or several micro-vesicles, whereas Sox9-negative cells stay behind and die. Our findings demonstrate that, in contrast to Xenopus, Sox9 is not required for the initial specification of the otic placode in the mouse, but instead controls adhesive properties and invagination of placodal cells in a cell-autonomous manner.

Sall1, Sall2, and Sall4 Are Required for Neural Tube Closure in Mice

Four homologs to the Drosophila homeotic gene spalt (sal) exist in both humans and mice (SALL1 to SALL4/Sall1 to Sall4, respectively). Mutations in both SALL1 and SALL4 result in the autosomal-dominant developmental disorders Townes-Brocks and Okihiro syndrome, respectively. In contrast, no human diseases have been associated with SALL2 to date, and Sall2-deficient mice have shown no apparent abnormal phenotype. We generated mice deficient in Sall2 and, contrary to previous reports, 11% of our Sall2-deficient mice showed background-specific neural tube defects, suggesting that Sall2 has a role in neurogenesis. To investigate whether Sall4 may compensate for the absence of Sall2, we generated compound Sall2 knockout/Sall4 genetrap mutant mice. In these mutants, the incidence of neural tube defects was significantly increased. Furthermore, we found a similar phenotype in compound Sall1/4 mutant mice, and in vitro studies showed that SALL1, SALL2, and SALL4 all co-localized in the nucleus. We therefore suggest a fundamental and redundant function of the Sall proteins in murine neurulation, with the heterozygous loss of a particular SALL protein also possibly compensated in humans during development.

Sox9 and Sox8 Are Required for Basal Lamina Integrity of Testis Cords and for Suppression of FOXL2 During Embryonic Testis Development in Mice

ABSTRACT The sex-determining gene Sry and its target gene Sox9 initiate the early steps of testis development in mammals. Of the related Sox genes Sox8, Sox9, and Sox10, all expressed during Sertoli cell differentiation, only inactivation of Sox9 before the sex determination stage at Embryonic Day 11.5 (E11.5) causes XY sex reversal, while Sox9 inactivation after this stage has no effect on testis cord differentiation. We have previously shown that both Sox9 and Sox8 are essential for maintaining testicular function in post-E14.0 Sertoli cells. To gain insight into the molecular and cellular processes underlying the abnormal development of Sox9 and Sox8 mutant testes, we performed a detailed developmental study of embryonic and neonatal stages. We observe a progressive disruption of the basal lamina surrounding the testis cords that starts at E17.5 and already at E15.5 reduced expression levels of collagen IV, collagen IXa3 and testatin, structural components of the basal lamina, and the extracellular matrix transcriptional regulator Scleraxis. Lineage tracing reveals that mutant Sertoli cells delaminate from testis cords and are present as isolated cells between remaining cords. Also, Sox10 expression is strongly reduced in the absence of Sox9 and/or Sox8. Finally, we document increasing expression of the ovarian marker FOXL2 in mutant cords starting at E15.5, indicating progressive transdifferentiation of mutant Sertoli cells. This study shows that Sox9 and Sox8 maintain integrity of the basal lamina to prevent testis cord disintegration and that both factors actively suppress the ovarian program during early testis development.

Role of Apoptosis and Cell Proliferation in the Testicular Dynamics of Seasonal Breeding Mammals: A Study in the Iberian Mole, Talpa occidentalis

Apoptosis and cell proliferation are two important cellular processes known to be involved in the normal functioning of the testis in nonseasonally breeding mammals, but there is some controversy concerning their roles in the gonads of males from seasonally breeding species. We have studied the processes of apoptosis and cell proliferation in the testes of males of the Iberian mole (Talpa occidentalis), a species showing a strict seasonal reproduction pattern. Both males and females are sexually active during the winter and completely inactive in the summer, with two transitional periods, in the autumn and the spring. Adult males from these four reproductive stages were captured, and their testes were immunohistochemically studied for the presence of apoptotic and proliferation molecular markers as well for other testicular and meiotic cell-specific markers. We found that apoptosis varies in a season-dependent manner in the testes of male moles, affecting mainly late zygotene and pachytene cells during the period of sexual inactivity, but it does not differentially affect the number of Sertoli cells. More interestingly, apoptosis is not responsible for the massive germ-cell depletion occurring during mole testis regression. In addition, a wave of spermatogonial cell proliferation appears to restore the number of spermatogonia lost during the period of testis inactivity. According to current knowledge, data from moles indicate that mammals do not form a homogeneous group regarding the mechanisms by which the cell-content dynamics are regulated in the testes of males from seasonally breeding species.

Developmental Stages and Growth Rate of the Mole Talpa occidentalis (Insectivora, Mammalia)

Abstract Moles are the only fertile true hermaphrodites described up to now among mammals. This paper investigates the development and growth of the mole Talpa occidentalis. Fifteen developmental stages (8 prenatal and 7 postnatal) were established and 12 of them, ranging from early gestation to weaning, are described in detail. The growth of moles was shown to fit triphasic curves for both males and females. The coefficient of reproductive effort of females, defined as mass of the litter at weaning relative to maternal body mass, is inversely proportional to mass of the female, according to an exponential function. Compared with other insectivores with lower body mass (shrews), this coefficient in female moles (2.69) is significantly higher than expected (1.91), which implies that reproductive effort of moles is 36% greater. This study provides the chronological criteria (based on body mass, crown–rump length, and major external morphological features) needed for age determination of individual developing moles and thus represents a useful tool for further studies in these mammals.