MicroRNA signature in testes-derived male germ-line stem cells

The testis-derived male germ-line stem (GS) cell, the in vitro counterpart of spermatogonial stem cell (SSC), can initiate donor-derived spermatogenesis in recipient testes and therefore, has been viewed as a future therapeutic modality for treatment of male infertility in azoospermic patients and in cancer patients who are expecting chemotherapy. Upon extended in vitro culture, GS cells also generate a second cell type called multipotent adult germ-line stem (maGS) cell which, upon testicular transplantation, produces teratoma instead of initiating spermatogenesis. Here, we show that expressions of both Let-7a and Let-7d were consistently higher while that of miR-294 (embryonic stem cell-cycle-regulating miRNA; ESCC) was lower in GS cells than in maGS cells. Furthermore, among several putative targets of Let-7 identified by in silico bioinformatics, expressions of Igf2 and H19 mRNA targets significantly differed between GS and maGS cells. However, although the CTCF binding factor (a component of DNA methylation machinery at Igf2-H19 cluster) was also a putative target for Let-7, the difference in expressions of Igf2 and H19 between GS and maGS cells was not mediated through a change in DNA methylation. Both GS and maGS cells maintained androgenetic imprinting at the Igf2-H19 imprinting control region and Peg1 differentially methylated region. In conclusion, our study suggests that high Let-7 expression may be a unique property of GS cells and expressions of Let-7 and ESCC miRNAs may serve as miRNA signatures to distinguish them from maGS cells during clinical transplantation, to avoid the likelihood of teratoma formation due to maGS cells generated during extended in vitro culture of GS cells.