Nr5a1 suppression during the fetal period optimizes ovarian development by fine-tuning of Notch signaling

The nuclear receptor NR5A1 (also known as Ad4BP, or SF1) is essential for the initial steps of mammalian gonadal development. The Nr5a1 gene is equally expressed in XX and XY gonadal primordia, but after sex determination, is up-regulated in XY and down-regulated in XX gonads. We recently reported a case of 46, XX disorder of sex development (DSD) in which ectopically expressed NR5A1 in XX gonads led to an ovo-testicular phenotype, suggesting that excess NR5A1 can direct the development of immature XX gonads towards testicular formation. However, a direct causal relationship has not been demonstrated in an animal model. Here, using a Wt1-BAC (bacterial artificial chromosome) transgene system, we generated two lines of mice overexpressing Nr5a1 in the fetal gonads at different levels. One of these lines (Tg-S), highly expressing Nr5a1, revealed that enforced Nr5a1 expression alone is insufficient to switch the fate of the 46,XX gonads toward testicular formation in mice. In the other line (Tg-A) expressing Nr5a1 at lower level, ovarian development was compromised, with multi-oocyte follicles, reduced number of matured follicles, and impaired expression of Wnt4, resulting in late onset infertility at 20 weeks after birth. The phenotype was similar to that of genetically modified mice with impaired Notch signaling. Indeed, the expression level of Notch2 and 3 was significantly reduced in Tg-A mice, and the ovarian phenotype in Tg-A mice was almost completely rescued by in utero treatment with a Notch2 agonist HMN2-29. We conclude that suppression of Nr5a1 during the fetal period optimizes ovarian development by fine tuning of Notch signaling levels.