| Summary: | During normal testis development, SRY and SF1 up-regulate Sox9 expression via its testis-specific enhancer element (TESCO), which is crucial for the establishment of the male supporting cell lineage, the Sertoli cells. In contrast, during normal ovarian development, Sox9 expression needs to be down-regulated in the female supporting cells and failure results in XX female-to-male sex reversal. It was not known whether the repression of Sox9 expression is mediated via the TESCO regulatory element in XX gonads. FOXL2 is a forkhead transcription factor, expressed in the supporting cells of the ovary, but not testis, from 12.5~dpc onwards. Homozygous mutation of Foxl2 results in the up-regulation of Sox9 and other testis-specific genes in postnatal gonads, suggesting that FOXL2 plays an important role in proper ovarian development by maintaining the repression of Sox9. In this thesis, in vitro analyses demonstrate that FOXL2 can repress TESCO activity (as can other candidate ovarian determining genes, such as Dax1 and Sox4). The repressing effect of FOXL2 is more severe in the presence of ER. Several forkhead factor binding sites and EREs are present in the TESCO sequence, however mutation analyses showed that the repression effect is not mediated only through these. Moreover, FOXl2 can interact with SF1 and interfere with its ability to activate TESCO in vitro. In vivo studies showed that homozygous loss of Foxl2 results in the de-repression of TESCO activity in the remaining granulosa-like cells in the XX gonads, which correlates with the expression of endogenous Sox9. However, this de-repression occurs only after birth, getting more severe with age, suggesting that Foxl2 is the critical factor to repress TESCO activity in the adult ovary. Moreover, evidence is provided that the de-repression of Sox9 is independent from oocyte-depletion and solely due to the loss of Foxl2 as targeted deletion of oocytes did not result in an up-regulation of Sox9 expression. Further in vivo analyses indicate the involvement of Wnt signalling in the repression of TESCO activity during embryonic development, as TESCO becomes partially de-repressed in Wnt4 homozygous mutant mice before birth.
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