Gdnf-Gfra1 Pathway Is Expressed in a Spermatogenetic-Dependent Manner and Is Regulated by Fsh in a Fish Testis

What makes the spermatogonial stem cells (SSCs) self-renew or differentiate to produce spermatozoa is barely understood, in particular in nonmammalian species. Our research explores possible regulations of the SSC niche in teleost, locally by paracrine factors and peripherally by hormonal regulation. In the present study, we focus on the Gdnf-Gfra1 pathway that plays a major role in the regulation of SSC self-renewal in mammals. We describe a complex evolution of the genes encoding for Gdnf and Gfra1 proteins in trout with the emergence of three gdnf and two gfra1 paralogs. Using quantitative PCR measurements in isolated testicular cell populations, the gdnfb paralog was found expressed in A-spermatogonia and probably in another testicular cell type. In contrast, the transcript of gfra1a, the Gdnf receptor, was preferentially expressed in a population of undifferentiated A-spermatogonia (und A-Spg) separated by centrifugal elutriation. These und A-Spg also demonstrated high stemness potential in transplantation studies and preferentially expressed nanos2, a putative SSC marker in trout (Bellaiche et al., Biol Reprod 2014; 90:79). Flow cytometer experiments demonstrate that only a subfraction of und A-Spg express Gfra1. In trout, spermatogenesis develops along a strict annual cycle, and gdnfb and its receptor were expressed in a spermatogenetic activity-dependent manner. In particular, a dramatic increase of the gdnfb transcript coincided with the progressive cessation of rapid spermatogonial proliferation and of meiosis toward the end of the reproductive cycle. Together these results suggest that, in trout, Gdnfb is involved in the repression of und A-Spg differentiation. Fsh is an endocrine regulator of SSCs self-renewal through the up-regulation of Gdnf in rodents. We demonstrate that in trout, in vitro Fsh treatment stimulated the expression of the gfra1a1 but not of its ligand, gdnfb. Fsh treatment also stimulated the proliferation of und A-Spg cocultured with testicular somatic cells. Based on those results, the Gfra1-positive cells could correspond to the putative SSCs in rainbow trout, and we propose that the balance between SSC self-renewal and differentiation during the trout spermatogenetic cycle is under paracrine regulation by Gdnfb, which represses, and under peripheral regulation by Fsh via the control of gfra1a1 expression.