Activin A Balances Sertoli and Germ Cell Proliferation in the Fetal Mouse Testis

Abstract

Activin affects many aspects of cellular development, including those essential for reproductive fitness. This study examined the contribution of activin A to murine fetal testicular development, revealing contrasting outcomes of activin actions on Sertoli cells and gonocytes. Shortly after sex determination, from Embryonic Day 12.5 (E12.5) through to birth (0 dpp), the activin A subunit transcript (Inhba) level rises in testis but not ovary, followed closely by the Inha transcript (encoding the inhibitory inhibin alpha subunit). Activin receptor transcript levels also change, with Acvr1 (encoding ALK2) and Acvr2b (ActRIIB) significantly higher and lower, respectively, at 0 dpp compared with E13.5 and E15.5. Transcripts encoding the signaling mediators Smad1, Smad3, and Smad4 were higher at 0 dpp compared with E13.5 and E15.5, whereas Smad2, Smad5, and Smad7 were lower. Detection of phosphorylated (P-)SMAD2/3 in nearly all testis cell nuclei indicated widespread transforming growth factor beta (TGFB) and/or activin ligand signaling activity. In contrast to wild-type littermates, activin betaA subunit knockout (Inhba(-/-)) mice have significantly smaller testes at birth, attributable to a 50% lower Sertoli cell number and decreased Sertoli cell proliferation from E13.5. Inhba(-/-) testes contained twice the normal gonocyte number at birth, with some appearing to bypass quiescence. Persistence of widespread P-SMAD2/3 in Inhba(-/-) cells indicates other TGFB superfamily ligands are active in fetal testes. Significant differences in Smad and cell cycle regulator transcript levels correlating to Inhba gene dosage correspond to differences in Sertoli and germ cell numbers. In Inhba(-/-) testes, Cdkn1a (encoding p21(cip1)), identified previously in fetal gonocytes, was lower at E13.5, whereas Cdkn1b (encoding p(27kip1) in somatic cells) was lower at birth, and cyclin D2 mRNA and protein were lower at E15.5 and 0 dpp. Thus, activin A dosage contributes to establishing the balance between Sertoli and germ cell number that is ultimately required for adult male fertility.

Mammalian gonocyte and spermatogonia differentiation: recent advances and remaining challenges

“…However, in the same study, RA was reported to slightly increase PND3 gonocyte numbers and have no effect on apoptosis. Moreover, activin A and androgen were reported to be negative regulators of fetal gonocyte proliferation, whereas we did not find any effect of testosterone on PND3 gonocyte proliferation in vitro (Merlet et al 2007, Thuillier et al 2010, Mendis et al 2011.…”

Nodal/Cripto signaling in fetal male germ cell development: implications for testicular germ cell tumors

“…In contrast to these findings however, another study concluded that disruption of Activin/Nodal signaling affected fetal germ cell meiosis (although pluripotency was not investigated) (Souquet et al 2012). In that study, Activin/Nodal inhibitors disrupted Nodal signaling in germ cells but also Activin signaling in somatic cells, so it is possible that the observed meiosis is the result of inadvertent disruption of the gonadal environment: Activin is normally required for the induction of germ cell mitotic arrest and differentiation (Mendis et al 2010). Complete Nodal deletion in germ cells may be required to clarify whether the main role of Nodal signaling is to maintain pluripotency or to prevent meiosis.…”

Prostaglandin D2 acts through the Dp2 receptor to influence male germ cell differentiation in the foetal mouse testis

“…Multiple Tgfβ superfamily members have also been associated with male germ cell differentiation, including Tgfβ2 (Miles et al, 2013;Moreno et al, 2010) and activin β 5 (InhbaMouse Genome Informatics) (Mendis et al, 2011). These factors repress germ cell proliferation, and participate in the entry into quiescence (Moreno et al, 2010) and probably also in its maintenance (Mendis et al, 2011;Moreno et al, 2010).…”