Mammalian sex is defined at three developmental stages: First, at fertilization, when either an X-bearing or Y-bearing sperm penetrates the haploid oocyte. XY embryos will develop as males whereas XX embryos will develop as females. Secondly, at E11.5 in the mouse, sex determination occurs with the bipotential gonad committing to either testicular or ovarian cell fates. Lastly, sex differentiation happens at both embryonic stages and puberty, relying on sex hormones secreted from the gonads. Discordance between these three stages can lead to Disorders of Sex Development (DSD) with an overall prevalence of 1 in 4000 newborns. Extensive genetic and molecular studies, in both mouse and human, indicate that the process of sex determination relies on a delicate balance of the expression and activity of several pro-male versus pro-female factors, most of which are transcription factors (TFs) and signalling pathways components. Although we now know many of these players, we still fail to understand the gene regulatory networks operated by these factors, the target genes they activate/repress, the genomic elements they exploit to exert this highly coordinated gene expression regulation, nor do we have an in vitro system to address these questions.
Two of the major drawbacks that prevented our ability to explore this delicate gene regulatory system, and understand cases of DSD patients, are the lack of an in vitro system that can closely model the gonads and the inability to isolate pure population of Sertoli and granulosa cells, the somatic cells of the gonads that determine the sex. In this proposal, we aim to characterize the gene regulatory networks operated by the key factors controlling testis and ovary development. We will identify the target genes activated and repressed by these key factors as well as the genomic elements they use to regulate their target genes. Furthermore, we will develop an in vitro system to model testis development. Altogether, this proposal will provide a systems biology view of the process of sex determination and the regulatory networks governing it. This will allow better diagnosis of DSD patients and modelling of these pathologies in vitro, in a human-related context. It will also provide wide implications for understanding spermatogenesis and potentially offer treatment for infertility. Insights we gain from this complex system can shed light on gene expression regulation and cell fate decisions in other developmental systems.
