SMAD3 Regulates Follicle-Stimulating Hormone Synthesis by Pituitary Gonadotrope Cells In Vivo

Abstract Pituitary follicle-stimulating hormone (FSH) is an essential regulator of fertility in females and of quantitatively normal spermatogenesis in males. Pituitary-derived activins are thought to act as major stimulators of FSH synthesis by gonadotrope cells. In vitro, activins signal via SMAD3, SMAD4, and forkhead box L2 (FOXL2) to regulate transcription of the FSHβ subunit gene (Fshb). Consistent with this model, gonadotrope-specific Smad4 or Foxl2 knockout mice have greatly reduced FSH and are subfertile. SMAD3's role in vivo is unresolved; however, residual FSH production in Smad4 conditional knockout mice may derive from partial compensation by SMAD3 and its ability to bind DNA in the absence of SMAD4. To test this hypothesis and determine SMAD3's role in FSH biosynthesis, we generated mice lacking both the SMAD3 DNA binding domain and SMAD4 specifically in gonadotropes. Conditional knockout females were hypogonadal, acyclic, sterile, and had thread-like uteri; their ovaries lacked antral follicles and corpora lutea. Knockout males were fertile but had reduced testis weights and epididymal sperm counts. These phenotypes were consistent with those of Fshb knockout mice. Indeed, pituitary Fshb mRNA levels were nearly undetectable in both male and female knockouts. In contrast, gonadotropin-releasing hormone receptor mRNA levels were significantly elevated in knockouts in both sexes. Interestingly, luteinizing hormone production was altered in a sex-specific fashion. Overall, our analyses demonstrate that SMAD3 is required for FSH synthesis in vivo.