• 
• 
• 
• 
• 

The Y-linked gene Sry regulates mammalian sex determination in bipotential embryonic gonads. Here we report that the transcription factors Six1 and Six4 are required for male gonadal differentiation. Loss of Six1 and Six4, but neither alone, resulted in a male-to-female sex reversal phenotype in XY-mutant gonads accompanied by a failure in Sry activation. Decreased gonadal precursor cell formation at the onset of Sry expression and a gonadal size reduction in both sexes were also found in mutant embryos. Forced Sry transgene expression in XY-mutant gonads rescued testicular development, but not the initial disruption to precursor growth. Furthermore, we identified two downstream targets of Six1/Six4 in gonadal development, Fog2 (Zfpm2) and Ad4BP ( Nr5a1/Sf1 ). These two distinct Six1/Six4-regulated pathways are crucial for gonadal development. The regulation of Fog2 induces Sry transactivation in male sex determination, and the regulation of Nr5a1 in gonadal precursor formation determines gonadal size. In human patients, deletion at 14q22-23, which includes the genes BMP4, OTX2, RTN1, SIX6, SIX1 and SIX4 , leads to a contiguous gene syndrome comprising anophthalmia, pituitary hypoplasia, ear anomalies and also gonadal underdevelopment. Our findings therefore suggest the possible association of the Six1/Six4-NR5A1 pathway with gonadal developmental defects in human patients.

 

Figure: Six1/Six4 play essential roles in size determination and testis determination of mouse embryonic gonads by regulating two distinct transcriptional networks.

In gonadal sex determination, Six1/Six4 are considered to up-regulate Srythrough Fog2 transactivation. Fog2 interacts with activated Gata4, which might be transiently regulated by its phosphorylation by the Gadd45g/Map3k4 pathway. Fog2/Gata4 then transactivates Sry to induce gonadal differentiation into testis.

In gonadal size determination, Six1/Six4 are considered to control Nr5a1transactivation and the initial growth of Nr5a1- and Gata4-positive precursor cells of gonadal somatic cells before the Sry expression . Nr5a1 is essential for gonadal primordium formation and the dose-dependent expression level of Nr5a1 might be crucial for gonadal precursor growth and size determination.