The genetic network that underlies the first steps of sex determination and gonad differentiation has been elucitated over the 15 past years. During the last few years, we have contributed to major advances in our understanding of genes driving testicular differentiation in mammals and largely contributed to demonstrate, by biochemical and genetic approaches, that Sox9 is the master gene controlling Sertoli cell fate and testis differentiation, although detailed cellular and molecular processes downstream of Sox9 are still largely unelucidated. - Our previous experiments demonstrated the crucial function of Sox9 in the sex determination pathway, and suggested that Amh/Mis and Sox8 represent direct targets of Sox9. However, since neither Sox8 nor Amh/Mis knockout mice show XY sex reversal, it is clear that important additional targets of Sox9 must exist. Furthermore, we demonstrated that nuclear import of Sox9 was sufficient to induce testis differentiation. Besides the Sox9/Fgf9 regulatory loop, we characterized the functional relationships between Sox9 and Pgds by analysing Sox9 and Pgds knockout mice and identified the Pgds gene as a new gene of the sex determination cascade. - - In this project, we will decipher in more detail the molecular and cellular mechanisms leading to the formation of a functional testis. Sox9, the effector of the testicular differentiation, must activate a complex program of gene transcription resulting in expression of secreted signalling molecules, receptors, hormones, matrix molecules and enzymes that should drive testis differentiation. - Using our established cellular and animal models and the complementarities of both partners, we will thus identify and analyse new genes involved in early sex determination by the available ChIP on chip technique, with a particular focus on Sox9 target genes and we will define new signalling pathways activated during the sex differentiation period. - We will thus directly link Sox9 to key transcription factors and signalling molecules that could serve as amplifiers of differentiation signals and effectors of specific Sertoli functions. Signalling is crucial for key cellular and developmental decisions. A number of signalling pathways involved in the regulation of Sox9 activity have been described. By identifying new genes, we will clarify the relationships and the hierarchy of these molecular pathways; this will also bring new insights since these genes could potentially represent excellent candidates to explain the genetic causes of many human sex disorders that are still unclear. - The aims of this project are thus : to find the direct target genes for Sox9 by the high-thoughput Chip on chip technique, to validate this approach by using transcriptomic approach of established mouse models, to study the functional analysis of the Sox9 target genes in genetically modified mice and by biochemical approaches, to elucidate the functional interplay of these new genes with known pathways: Fgf9, Pgds, SOX9 and Wnt4 and finally to screen for mutation(s) in panels of human sex reversal patients. - ...