Abstract
The activin/inhibin system regulates follicle-stimulating hormone (FSH) synthesis and release by pituitary gonadotrope cells in mammals. In vitro cell line data suggest that activins stimulate FSH β-subunit (Fshb) transcription via complexes containing the receptor-regulated SMAD proteins SMAD2 and SMAD3. Here, we used a Cre-loxP approach to determine the necessity for SMAD2 and/or SMAD3 in FSH synthesis in vivo. Surprisingly, mice with conditional mutations in both Smad2 and Smad3 specifically in gonadotrope cells are fertile and produce FSH at quantitatively normal levels. Notably, however, we discovered that the recombined Smad3 allele produces a transcript that encodes the entirety of the SMAD3 C-terminal Mad homology 2 (MH2) domain. This protein behaves similarly to full-length SMAD3 in Fshb transcriptional assays. As the truncated protein lacks the N-terminal Mad homology 1 (MH1) domain, these results show that SMAD3 DNA-binding activity as well as SMAD2 are dispensable for normal FSH synthesis in vivo. Furthermore, the observation that deletion of proximal exons does not remove all SMAD3 function may facilitate interpretation of divergent phenotypes previously described in different Smad3 knockout mouse lines.