A comparison study in the proteomic signatures of multipotent germline stem cells, embryonic stem cells, and germline stem cells

Differential gene expression in mouse spermatogonial stem cells and embryonic stem cells

Mouse spermatogonial stem cells (mSSCs) may be reprogrammed to become pluripotent stem cells under in vitro culture conditions, due to epigenetic modifications, which are closely associated with the expression of transcription factors and epigenetic factors. Thus, this study was conducted to compare the gene expression of transcription factors and epigenetic factors in mSSCs and mouse embryonic stem cells (mESCs). Firstly, the freshly isolated mSSCs [mSSCs (f)] were enriched by magnetic-activated cell sorting with Thy1.2 (CD90.2) microbeads, and the typical morphological characteristics were maintained under in vitro culture conditions for over 5 months to form long-term propagated mSSCs [mSSCs (l)]. These mSSCs (l) expressed pluripotency-associated genes and were induced to differentiate into sperm. Our findings indicated that the mSSCs (l) expressed high levels of the transcription factors, Lin28 and Prmt5, and the epigenetic factors, Tet3, Parp1, Max, Tert and Trf1, in comparison with the mESCs, with the levels of Prmt5, Tet3, Parp1 and Tert significantly higher than those in the mESCs. There was no significant difference in Kdm2b expression between mSSCs (l) and mESCs. Furthermore, the gene expression of N-Myc, Dppa2, Tbx3, Nr5a2, Prmt5, Tet3, Parp1, Max, Tert and Trf1 in the mSSCs (l) was markedly higher in comparison to that in the mSSCs (f). Collectively, our results suggest that the mSSCs and the mESCs displayed differential gene expression profiles, and the mSSCs possessed the potential to acquire pluripotency based on the high expression of transcription factors and epigenetic factors. These data may provide novel insights into the reprogramming mechanism of mSSCs.

miRNA signature in mouse spermatogonial stem cells revealed by high-throughput sequencing

Spermatogonial stem cells (SSCs) play fundamental roles in spermatogenesis. Although a handful of genes have been discovered as key regulators of SSC self-renewal and differentiation, the regulatory network responsible for SSC function remains unclear. In particular, small RNA signatures during mouse spermatogenesis are not yet systematically investigated. Here, using next generation sequencing, we compared small RNA signatures of in vitro expanded SSCs, testis-derived somatic cells (Sertoli cells), developing germ cells, mouse embryonic stem cells (ESCs), and mouse mesenchymal stem cells among mouse embryonic stem cells (ESCs) to address small RNA transition during mouse spermatogenesis. The results manifest that small RNA transition during mouse spermatogenesis displays overall declined expression profiles of miRNAs and endo-siRNAs, in parallel with elevated expression profiles of piRNAs, resulting in the normal biogenesis of sperms. Meanwhile, several novel miRNAs were preferentially expressed in mouse SSCs, and further investigation of their functional annotation will allow insights into the mechanisms involved in the regulation of SSC activities. We also demonstrated the similarity of miRNA signatures between SSCs and ESCs, thereby providing a new clue to understanding the molecular basis underlying the easy conversion of SSCs to ESCs.

Cell-intrinsic reprogramming capability: gain or loss of pluripotency in germ cells

In multicellular organisms, germ cells are an extremely specialized cell type with the vital function of transmitting genetic information across generations. In this respect, they are responsible for the perpetuity of species, and are separated from somatic lineages at each generation. Interestingly, in the past two decades research has shown that germ cells have the potential to proceed along two distinct pathways: gametogenesis or pluripotency. Unequivocally, the primary role of germ cells is to produce gametes, the sperm or oocyte, to produce offspring. However, under specific conditions germ cells can become pluripotent, as shown by teratoma formation in vivo or cell culture-induced reprogramming in vitro. This phenomenon seems to be a general propensity of germ cells, irrespective of developmental phase. Recent attempts at cellular reprogramming have resulted in the generation of induced pluripotent stem cells (iPSCs). In iPSCs, the intracellular molecular networks instructing pluripotency have been activated and override the exclusively somatic cell programs that existed. Because the generation of iPSCs is highly artificial and depends on gene transduction, whether the resulting machinery reflects any physiological cell-intrinsic programs is open to question. In contrast, germ cells can spontaneously shift their fate to pluripotency during in-vitro culture. Here, we review the two fates of germ cells, i.e., differentiation and reprogramming. Understanding the molecular mechanisms regulating differentiation versus reprogramming would provide invaluable insight into understanding the mechanisms of cellular reprogramming that generate iPSCs.

Proteomic analyses reveal common promiscuous patterns of cell surface proteins on human embryonic stem cells and sperms

Background

It has long been proposed that early embryos and reproductive organs exhibit similar gene expression profiles. However, whether this similarity is propagated to the protein level remains largely unknown. We have previously characterised the promiscuous expression pattern of cell surface proteins on mouse embryonic stem (mES) cells. As cell surface proteins also play critical functions in human embryonic stem (hES) cells and germ cells, it is important to reveal whether a promiscuous pattern of cell surface proteins also exists for these cells.

Methods and Principal Findings

Surface proteins of hES cells and human mature sperms (hSperms) were purified by biotin labelling and subjected to proteomic analyses. More than 1000 transmembrane or secreted cell surface proteins were identified on the two cell types, respectively. Proteins from both cell types covered a large variety of functional categories including signal transduction, adhesion and transporting. Moreover, both cell types promiscuously expressed a wide variety of tissue specific surface proteins, and some surface proteins were heterogeneously expressed.

Conclusions/Significance

Our findings indicate that the promiscuous expression of functional and tissue specific cell surface proteins may be a common pattern in embryonic stem cells and germ cells. The conservation of gene expression patterns between early embryonic cells and reproductive cells is propagated to the protein level. These results have deep implications for the cell surface signature characterisation of pluripotent stem cells and germ cells and may lead the way to a new area of study, i.e., the functional significance of promiscuous gene expression in pluripotent and germ cells.

Identification of transcripts commonly expressed in both hematopoietic and germ-line stem cells

Germ-line stem cells (GSCs) constitute a stem cell population with remarkable stability and proliferative potential in vitro and are a useful model for studying the mechanism of self-renewal and "stemness" function of committed tissue stem cells. To identify GSC-specific genes, we performed subtractive hybridization using cDNA from GSCs, testis, and embryonic stem (ES) cells, and successfully identified 11 genes highly expressed in GSCs. Histological analysis confirmed expression of Cry alpha b, Mcpt8, Cxcl5, Fth1, Ctla2 alpha, and Spp1 in undifferentiated spermatogonia on the basement membrane area of the seminiferous epithelium of the testis, where the GSC niche is thought to be located. Among GSC-specific genes, quantitative PCR analysis showed seven genes-Fth1, Cry alpha b, Spp1, Bcap31, Arhgap1, Ctla2 alpha, and Serpina3g-to be common transcripts highly expressed in hematopoietic stem cells (HSCs). Histological analysis confirmed that Ctla2 alpha-, Serpina3g-, and Spp1-expressing cells were observed in the trabecular bone region of the bone marrow, where the HSC niche is located. Furthermore, histological analysis revealed that only Spp1 was expressed in the hair follicle bulge in the area of the hair follicle stem cell niche. Thus, identifying stemness genes by comparative analysis to GSCs is a powerful tool with which to explore the fundamental commonalities of HSCs and other stem cell types.