GDNF family receptor alpha 1 is a reliable marker of undifferentiated germ cells in bulls

Undifferentiated germ cells, including spermatogonial stem cells (SSCs), make up only a very small proportion of germ cells within the testis; for example, 0.03% of germ cells in the mouse testis are SSCs. In this study, we investigated the characteristics of bovine undifferentiated germ cells and developed an enrichment procedure for these cells on the basis of fluorescence-activated cell sorting (FACS), using the specific cell surface marker glial cell line-derived neurotrophic factor family receptor alpha 1 (GFRα1). FACS analysis showed that only 0.6% of the total testicular cells were GFRα1-positive. These GFRα1-positive cells had a significantly higher expression of UCHL1, ZBTB16, and DDX4 (all markers of undifferentiated spermatogonial and germ cells) than that of fresh testicular cells. Quantitative reverse-transcription PCR analyses also indicated that the gene expression of BCL6B and NANOS2 was significantly higher in GFRα1-positive cells. Furthermore, xenogeneic transplantation of bovine testicular cells into immunodeficient mice resulted in 4.4-fold more colonies of GFRα1-positive cells than those of fresh testicular cells, indicating that FACS with antibodies to GFRα1 had efficiently enriched putative SSCs from total testicular cells. Collectively, these results demonstrate that GFRα1 could be used as a marker of bovine undifferentiated germ cells, including putative SSCs, and that its expression on SSCs has important implications for the further development of techniques for enriching stem cells from other species.