Endocrine Abstracts

The decision to be male or female takes place in the developing gonad. Evidence suggests that a number of testis and ovary promoting genes, such as Sox9, Fgf9 and Wnt4, are active at a low level in the indifferent gonad. In XX development Wnt4 expression is maintained and Sox9 and Fgf9 are extinguished. In an XY embryo, Sry is expressed at a critical time to boost Sox9 expression above a critical threshold level, such that it can act on its downstream targets. This may include Sox9 itself, but also Fgf9, which serves both to indirectly maintain high levels of Sox9 and to repress Wnt4. Sox9 is also known to activate other genes critical for Sertoli cell differentiation and function, such as Amh. Finally, other factors, such as prostaglandin D2, also step in to reinforce the initial decision and to ensure testis morphogenesis occurs throughout the gonad. The importance of Sox9 is evident from loss of function mutations in the gene, in both humans and mice, which lead to XY female sex reversal, while constitutive expression or gain of function mutations can lead to XX male development. Moreover, as the genetic pathways involved in sex determination, acting to initiate or reinforce the male pathway or to counteract it in female development, all appear to converge on Sox9, it is critical to understand how Sox9 expression is regulated in the gonad. We have defined an enhancer element upstream of Sox9 that appears sufficient to drive expression in transgenic mice in a manner that mimics that of the endogenous gene. A core region of this element shows high conservation between the mouse, rat and human genes, and includes consensus binding sites for SRY/SOX proteins, as well as for SF1, a factor required for the early expression of several genes in the early gonad including Sox9. Data will be presented to show that SRY and SF1 are bound to the core regulatory sequences in vivo. Moreover, co-transfection data reveal a synergistic effect of SRY and SF1 on activating expression of reporter genes linked to the enhancer. This data, plus additional evidence that will be presented, suggests that this enhancer is important for integrating most, if not all, the regulatory input required to establish or repress Sox9 expression in the gonad.