Female fertility and activin-stimulated Fshb expression do not require SMAD2/3 in murine gonadotropes
J. Fortin & D. Bernard
In mammals, follicle-stimulating hormone (FSH), produced by pituitary gonadotropes, is required for proper gonadal function and female fertility. The TGFβ superfamily members, activins, are critical regulators of FSH synthesis. Activins signal through heteromeric type I/type II receptor complexes and the effector proteins SMADs 2 and 3 to regulate gene transcription. Numerous studies in gonadotrope-like cell lines suggest that activin-stimulated FSH β subunit (Fshb) transcription is mediated by SMAD2/3. To test the hypothesis that SMAD2/3 are required in gonadotropes for proper FSH synthesis and reproductive axis activity in vivo, we generated mice with gonadotrope-specific deletion of Smad2/3 (hereafter Smad2/3KO) by crossing Gnrhr-IRES-Cre (GRIC) mice with those carrying conditional (floxed) Smad2 and Smad3 alleles. Female Smad2/3KO mice had diminished estrous cycle frequency and spent more time in estrus than control females as assessed by daily inspection of vaginal cytology. However, in metestrus/diestrus, Smad2/3KO ovarian and uterine weights were comparable to those of control mice. Importantly, the frequency and size of litters produced over a six month period by Smad2/3KO females paired with wild-type males were indistinguishable from controls. Smad2/3KO males had slightly reduced testes weights but were fertile. Surprisingly, pituitary Fshb expression was normal in both male and female Smad2/3KO mice. In light of these results, we examined whether SMAD2/3 are required for activin A-regulated Fshb expression in gonadotropes ex vivo. Primary pituitary cells from mice carrying floxed Smad2/3 alleles were infected with control or Cre-expressing adenovirus. Despite >95% depletion of Smad2/3 mRNA in Cre infected cultures, inhibitory effects on basalFshb and activin A-induced Fshb expression were modest. Together, our data challenge the current model that FSH synthesis depends on SMAD2/3-mediated signaling.
The authors thank Drs Ulrich Boehm, Jonathan Graff, Martin Matzuk and Michael Weinstein for their generous gift of the mouse lines used in this study.
Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.
Funding: This work was supported, however funding details are unavailable.