Sox8 and Sox9 act redundantly for ovarian-to-testicular fate reprogramming in the absence of R-spondin1 in mouse sex reversals.

In mammals, testicular differentiation is initiated by transcription factors SRY and SOX9 in XY gonads, and ovarian differentiation involves R-spondin1 (RSPO1) mediated activation of WNT/ß-catenin signaling in XX gonads. Accordingly, the absence of RSPO1/Rspo1 in XX humans and mice leads to testicular differentiation and female-to-male sex reversal in a manner that does not requireSry or Sox9 in mice. Here we show that an alternate testis-differentiating factor exists and that this factor is Sox8. Specifically, genetic ablation of Sox8 and Sox9 prevents ovarian-to-testicular reprogramming observed in XX Rspo1 loss-of-function mice. Consequently, Rspo1 Sox8 Sox9 triple mutant gonads developed as atrophied ovaries. Thus, SOX8 alone can compensate for the loss of SOX9 for Sertoli cell differentiation during female-to-male sex reversal.

In humans, mice and other mammals, genetic sex is determined by the combination of sex chromosomes that each individual inherits. Individuals with two X chromosomes (XX) are said to be chromosomally female, while individuals with one X and one Y chromosome (XY) are chromosomally males. One of the major differences between XX and XY individuals is that they have different types of gonads (the organs that make egg cells or sperm). In mice, for example, before males are born, a gene called Sox9 triggers a cascade of events that result in the gonads developing into testes. In females, on the other hand, another gene called Rspo1 stimulates the gonads to develop into ovaries. Loss of Sox9 in XY embryos, or Rspo1 in XX embryos, leads to mice developing physical characteristics that do not match their genetic sex, a phenomenon known as sex reversal. For example, in XX female mice lacking Rspo1, cells in the gonads reprogram into testis cells known as Sertoli cells just before birth and form male structures known as testis cords. The gonads of female mice missing both Sox9 and Rspo1 (referred to as "double mutants") also develop Sertoli cells and testis cords, suggesting another gene may compensate for the loss of Sox9. Previous studies suggest that a gene known as Sox8, which is closely related to Sox9, may be able to drive sex reversal in female mice. However, it was not clear whether Sox8 is able to stimulate testis to form in female mice in the absence of Sox9. To address this question, Richardson et al. studied mutant female mice lacking Rspo1, Sox8 and Sox9, known as "triple mutants". Just before birth, the gonads in the triple mutant mice showed some characteristics of sex reversal but lacked the Sertoli cells found in the double mutant mice. After the mice were born, the gonads of the triple mutant mice developed as rudimentary ovaries without testis cords, unlike the more testis-like gonads found in the double mutant mice. The findings of Richardson et al. show that Sox8 is able to trigger sex reversal in female mice in the absence of Rspo1 and Sox9. Differences in sexual development in humans affect the appearance of individuals and often cause infertility. Identifying Sox8 and other similar genes in mice may one day help to diagnose people with such conditions and lead to the development of new therapies.

R-spondin1, WNT4, and the CTNNB1 signaling pathway: strict control over ovarian differentiation.

Sex differentiation is a unique developmental process. Starting from a bipotential gonad, it gives rise to the ovary and the testis, two highly specialized organs that differ morphologically and physiologically despite sharing common reproductive and endocrine functions. This highlights the specific plasticity of the gonadal precursors and the existence of complex antagonistic genetic regulation. Mammalian sex determination is controlled by paternal transmission of the Y-linked gene, sex-determining region Y (SRY). Using mouse models, it has been shown that the main role of Sry is to activate the expression of the transcription factor Sox9; either one of these two genes is necessary and sufficient to allow testicular development through Sertoli cell differentiation. Thus, defects in SRY/Sry and/or SOX9/Sox9 expression result in male-to-female sex reversal of XY individuals. Molecular mechanisms governing ovarian differentiation remained unknown for a long time, until the discovery of the roles of R-spondin1 (RSPO1) and WNT4. In XX individuals, activation of the ß-catenin signaling pathway by the secreted proteins RSPO1 and WNT4 is required to allow granulosa cell differentiation and, in turn, ovarian differentiation. Thus, mutations in RSPO1 result in female-to-male sex reversal of XX patients, and mouse models have allowed the identification of genetic cascades activated by RSPO1 and WNT4 to regulate ovarian development. In this review, we will discuss the respective roles of RSPO1, WNT4, and the ß-catenin signaling pathway during ovarian differentiation in mice.

Ret Finger Protein: An E3 Ubiquitin Ligase Juxtaposed to the XY Body in Meiosis.

During prophase I of male meiosis, the sex chromosomes form a compact structure called XY body that associates with the nuclear membrane of pachytene spermatocytes. Ret Finger Protein is a transcriptional repressor, able to interact with both nuclear matrix-associated proteins and double-stranded DNA. We report the precise and unique localization of Ret Finger Protein in pachytene spermatocytes, in which Ret Finger Protein takes place of lamin B1, between the XY body and the inner nuclear membrane. This localization of Ret Finger Protein does not seem to be associated with O-glycosylation or sumoylation. In addition, we demonstrate that Ret Finger Protein contains an E3 ubiquitin ligase activity. These observations lead to an attractive hypothesis in which Ret Finger Protein would be involved in the positioning and the attachment of XY body to the nuclear lamina of pachytene spermatocytes.

RNA-mediated non-mendelian inheritance of an epigenetic change in the mouse.

Paramutation is a heritable epigenetic modification induced in plants by cross-talk between allelic loci. Here we report a similar modification of the mouse Kit gene in the progeny of heterozygotes with the null mutant Kit(tm1Alf) (a lacZ insertion). In spite of a homozygous wild-type genotype, their offspring maintain, to a variable extent, the white spots characteristic of Kit mutant animals. Efficiently inherited from either male or female parents, the modified phenotype results from a decrease in Kit messenger RNA levels with the accumulation of non-polyadenylated RNA molecules of abnormal sizes. Sustained transcriptional activity at the postmeiotic stages--at which time the gene is normally silent--leads to the accumulation of RNA in spermatozoa. Microinjection into fertilized eggs either of total RNA from Kit(tm1Alf/+) heterozygotes or of Kit-specific microRNAs induced a heritable white tail phenotype. Our results identify an unexpected mode of epigenetic inheritance associated with the zygotic transfer of RNA molecules.

Germ cells and fatty acids induce translocation of CD36 scavenger receptor to the plasma membrane of Sertoli cells.

The CD36 scavenger receptor is involved in the uptake and transport of fatty acids, as well as the phagocytosis process in macrophages. We show here that the CD36 protein is expressed by Sertoli cells in the seminiferous epithelium, mainly during the stages where phagocytosis takes place. Using a Sertoli-derived cell line, we show that addition of germ cells and residual bodies triggers a re-localization of CD36 from the cytoplasm to the plasma membrane of the cells, while latex beads do not. Moreover, Sertoli cell phagocytosis of germ cells, but not of latex beads, is reduced by the presence of fatty acids in the culture medium. In the testis, CD36 plays a key role in both phagocytosis and lipid recycling, for constant production of mature spermatozoa.