Gdnf Acts as a Germ Cell-Derived Growth Factor and Regulates the Zebrafish Germ Stem Cell Niche in Autocrine- and Paracrine-Dependent Manners

Glial cell line-derived neurotrophic factor (GDNF) and its receptor (GDNF Family Receptor α1-GFRα1) are well known to mediate spermatogonial stem cell (SSC) proliferation and survival in mammalian testes. In nonmammalian species, Gdnf and Gfrα1 orthologs have been found but their functions remain poorly investigated in the testes. Considering this background, this study aimed to understand the roles of the Gdnf-Gfrα1 signaling pathway in zebrafish testes by combining in vivo, in silico and ex vivo approaches. Our analysis showed that zebrafish exhibit two paralogs for Gndf (gdnfa and gdnfb) and its receptor, Gfrα1 (gfrα1a and gfrα1b), in accordance with a teleost-specific third round of whole genome duplication. Expression analysis further revealed that both ligands and receptors were expressed in zebrafish adult testes. Subsequently, we demonstrated that gdnfa is expressed in the germ cells, while Gfrα1a/Gfrα1b was detected in early spermatogonia (mainly in types A_und and A_diff) and Sertoli cells. Functional ex vivo analysis showed that Gdnf promoted the creation of new available niches by stimulating the proliferation of both type A_und spermatogonia and their surrounding Sertoli cells but without changing pou5f3 mRNA levels. Strikingly, Gdnf also inhibited late spermatogonial differentiation, as shown by the decrease in type B spermatogonia and down-regulation of dazl in a co-treatment with Fsh. Altogether, our data revealed that a germ cell-derived factor is involved in maintaining germ cell stemness through the creation of new available niches, supporting the development of spermatogonial cysts and inhibiting late spermatogonial differentiation in autocrine- and paracrine-dependent manners.

Boosting effect of testosterone on GDNF expression in Sertoli cell line (TM4); comparison between TM3 cells-produced and exogenous testosterone

The Glial cell-derived neurotrophic factor (Gdnf) and testosterone induce the spermatogonial stem cells (SSCs) self-renewal and spermatogenesis, respectively. In present study the stimulating role of testosterone on Sertoli cells to produce Gdnf, and the possible effect of Gdnf on Gfrα1 and c-RET expressions were investigated. The TM4 cells (line Sertoli cells) were co-cultured with [0.1, 0.2 and 0.4 (ng/ml)] of exogenous and TM3 (line Leydig cells)-produced testosterones, and consequently the TM4-produced Gdnf concentration was evaluated. Next, the SSCs were co-cultured with the TM-4 derived media (endogenous Gdnf) and exogenous Gdnf [0.1, 0.2, and 0.4 ng/ml)]. The 0.1 and 0.2 ng/ml endogenous and 3 concentrations of exogenous testosterone up-regulated the Gdnf expression versus non-treated Sertoli cells. The TM4-produced and exogenous Gdnfs, in all concentrations, up-regulated the receptors expression. In conclusion, the testosterone, solely, stimulates the Gdnf synthesis and the Gdnf, individually, amplifies its receptor's expression at mRNA and protein levels.

Both Gfrα1a and Gfrα1b Are Involved in the Self-renewal and Maintenance of Spermatogonial Stem Cells in Medaka

Glial cell-derived neurotrophic factor family receptor alpha-1 (GFRα1) plays a crucial role in the self-renewal and maintenance of spermatogonial stem cells (SSCs) from mammals. However, to date, our knowledge about its role in fish SSCs is limited. In the present study, the medaka (Oryzias latipes) gfrα1 duplicate genes, Olgfrα1a and Olgfrα1b, were cloned and characterized. Furthermore, their expression profile and biological activity were investigated. OlGfrα1a and OlGfrα1b predict 524 and 466 amino acid residues, respectively. Both are orthologous to mammalian Gfrα1 by sequence analyses and appear high in spermatogonia by in situ hybridization assay. The knockdown of OlGfrα1a and/or OlGfrα1b via Vivo-Morpholino oligos significantly inhibited the self-renewal and maintenance of SSCs, as evidenced by the decreased proliferation activity of SG3 cells (a spermatogonial stem cell line derived from adult medaka testis) as well as spermatogonia in the testicular organ culture and by the decreased survival rate and expression levels of pluripotency-related genes (klf4, lin28b, bcl6b, and etv5) in SG3 cells. Additionally, our study indicates that OlGfrα1a might function by binding either Gdnfa or Gdnfb (the two medaka Gdnf homologs), whereas OlGfrα1b function by binding Gdnfa not Gdnfb. Taken together, our study indicates that both OlGfrα1a and OlGfrα1b are involved in the self-renewal and maintenance of SSCs by binding Gdnfa and/or Gdnfb, respectively. These findings suggest that the GDNF/GFRα1 signaling pathway might be conserved from mammals to fish species.

GDNF fails to inhibit LPS-mediated activation of mouse microglia

GDNF is a potent neuroprotective factor for midbrain dopaminergic (mDA) neurons. In LPS-mediated models for mDA degeneration GDNF increases neuron survival and further reduces microglia activation. To elucidate the effects of GDNF on LPS-induced activation, primary microglia from C57BL/6 and NMRI mice have been analysed. In this study we demonstrate that GDNF is not able to inhibit LPS-mediated upregulation and release of the proinflammatory factors IL6 and TNFα. Moreover, we provide evidence that mouse microglia, in contrast to rat microglia, lack expression of the GDNF signalling receptor c-Ret resulting in abrogated activation of downstream signalling kinases Akt and Erk1/2.

Stage specific requirement of Gfrα1 in the ureteric epithelium during kidney development

Glial cell line-derived neurotrophic factor (GDNF) binds a coreceptor GDNF family receptor α1 (GFRα1) and forms a signaling complex with the receptor tyrosine kinase RET. GDNF-GFRα1-RET signaling activates cellular pathways that are required for normal induction of the ureteric bud (UB) from the Wolffian duct (WD). Failure of UB formation results in bilateral renal agenesis and perinatal lethality. Gfrα1 is expressed in both the epithelial and mesenchymal compartments of the developing kidney while Ret expression is specific to the epithelium. The biological importance of Gfrα1's wider tissue expression and its role in later kidney development are unclear. We discovered that conditional loss of Gfrα1 in the WD epithelium prior to UB branching is sufficient to cause renal agenesis. This finding indicates that Gfrα1 expressed in the nonepithelial structures cannot compensate for this loss. To determine Gfrα1's role in branching morphogenesis after UB induction we used an inducible Gfrα1-specific Cre-deletor strain and deleted Gfrα1 from the majority of UB tip cells post UB induction in vivo and in explant kidney cultures. We report that Gfrα1 excision from the epithelia compartment after UB induction caused a modest reduction in branching morphogenesis. The loss of Gfrα1 from UB-tip cells resulted in reduced cell proliferation and decreased activated ERK (pERK). Further, cells without Gfrα1 expression are able to populate the branching UB tips. These findings delineate previously unclear biological roles of Gfrα1 in the urinary tract and demonstrate its cell-type and stage-specific requirements in kidney development.