Conserved and non-conserved characteristics of porcine glial cell line-derived neurotrophic factor expressed in the testis.

Glial cell line-derived neurotrophic factor (GDNF) is essential for the self-renewal and proliferation of spermatogonial stem cells (SSCs) in mice, rats, and rabbits. Although the key extrinsic factors essential for spermatogonial proliferation in other mammals have not been determined, GDNF is one of the potential candidates. In this study, we isolated porcine GDNF (pGDNF) cDNAs from neonatal testis and generated recombinant pGDNF to investigate its biological activity on gonocytes/undifferentiated spermatogonia, including SSCs. In porcine testis, long and short forms of GDNF transcripts, the counterparts of pre-(α)pro and pre-(β)pro GDNF identified in humans and rodents, were expressed. The two transcripts encode identical mature proteins. Recombinant pGDNF supported proliferation of murine SSCs in culture, and their stem cell activity was confirmed by a transplantation assay. Subsequently, porcine gonocytes/undifferentiated spermatogonia were cultured with pGDNF; however, pGDNF did not affect their proliferation. Furthermore, GDNF expression was localised to the vascular smooth muscle cells, and its cognate receptor GFRA1 expression was negligible during spermatogonial proliferation in the testes. These results indicate that although pGDNF retains structural similarity with those of other mammals and conserves the biological activity on the self-renewal of murine SSCs, porcine SSCs likely require extrinsic factors other than GDNF for their proliferation.