Activin Bioactivity Affects Germ Cell Differentiation in the Postnatal Mouse Testis In Vivo1

Abstract

The transforming growth factor beta superfamily ligand activin A controls juvenile testis growth by stimulating Sertoli cell proliferation. Testicular levels are highest in the first postnatal week, when Sertoli cells are proliferating and spermatogonial stem cells first form. Levels decrease sharply as Sertoli cell proliferation ceases and spermatogenic differentiation begins. We hypothesized that changing activin levels also affect germ cell maturation. We detected an acute and developmentally regulated impact of activin on Kit mRNA in cocultures of Sertoli cells and germ cells from Day 8, but not Day 4, mice. Both stereological and flow cytometry analyses identified an elevated spermatogonium:Sertoli cell ratio in Day 7 testes from Inhba(BK/BK) mice, which have decreased bioactive activin, and the germ cell markers Sycp3, Dazl, and Ccnd3 were significantly elevated in Inhba(BK/BK) mice. The flow cytometry measurements demonstrated that surface KIT protein is significantly higher in Day 7 Inhba(BK/BK) germ cells than in wild-type littermates. By Day 14, the germ cell:Sertoli cell ratio did not differ between genotypes, but the transition of type A spermatogonia into spermatocytes was altered in Inhba(BK/BK) testes. We conclude that regulated activin signaling not only controls Sertoli cell proliferation, as previously described, but also influences the in vivo progression of germ cell maturation in the juvenile testis at the onset of spermatogenesis.

Endogenous Nodal signaling regulates germ cell potency during mammalian testis development

“…1F). Given the restricted expression of Cripto, the low level of P-Smad2 detected in somatic cells is likely to reflect non-Nodal TGF signaling in these cells (Mendis et al, 2010). Together, these results indicate that activin/Nodal signaling is active in fetal XY germ cells, but not XX germ cells or gonadal somatic cells, in the crucial 12.5-14.5 dpc period when their proliferation is arrested and commitment to spermatogenic differentiation is stimulated.…”

Fsh and Lh have common and distinct effects on gene expression in rainbow trout testis

“…In rodents, numerous in vivo and in vitro studies have demonstrated the importance of activin A and its regulators (inhibin and follistatin) in the control of spermatogenesis (de Kretser et al 2004, Barakat et al 2008. In fetal and postnatal testis, activin A promotes both Sertoli cell and gonocyte proliferation but hampers the differentiation of gonocytes in spermatogonia (Mithraprabhu et al 2010, Fan et al 2012. Meehan et al (2000) hypothesized that a reduction of activin A bioactivity is required for the onset of spermatogenesis in the early postnatal testis, to drive the differentiation of spermatogonial germ cells.…”

Mammalian gonocyte and spermatogonia differentiation: recent advances and remaining challenges

“…As activin A has been previously shown to induce the proliferation of spermatogonia in vitro (Mather et al 1990), the authors proposed that activin A may decrease the SSC pool by increasing their recruitment to the subset of cells committed to differentiation and induced to mitosis by activin A (Nagano et al 2003). However, another group using PND0-PND7 Inhba knockin mice (Inhba BK/BK ) in which the mature coding subunit sequence of Inhba had been replaced by the lower affinity Inhbb domain sequence proposed that a reduction of bioactive activin A levels led to the premature increase in markers of differentiated germ cells such as KIT, and suggested that activin A is critical for the proper coordination of germ cell maturation, as well as for Sertoli cell proliferation (Mithraprabhu et al 2010). Interestingly, differentiating neonatal gonocytes were shown to be negative for activin A protein expression, with Inhba mRNA presenting a sharp decrease between PND3 and PND6 in mice, and instead to express the activin inhibitor follistatin (Meehan et al 2000, Mithraprabhu et al 2010.…”

“…However, another group using PND0-PND7 Inhba knockin mice (Inhba BK/BK ) in which the mature coding subunit sequence of Inhba had been replaced by the lower affinity Inhbb domain sequence proposed that a reduction of bioactive activin A levels led to the premature increase in markers of differentiated germ cells such as KIT, and suggested that activin A is critical for the proper coordination of germ cell maturation, as well as for Sertoli cell proliferation (Mithraprabhu et al 2010). Interestingly, differentiating neonatal gonocytes were shown to be negative for activin A protein expression, with Inhba mRNA presenting a sharp decrease between PND3 and PND6 in mice, and instead to express the activin inhibitor follistatin (Meehan et al 2000, Mithraprabhu et al 2010. Simultaneous to the decrease in activin A during the first postnatal week, there was a gradual increase in its inhibitor follistatin, followed during the second postnatal week by an increase in inhibins (Barakat et al 2008).…”