Cell cycle gene-specific control of transcription has a critical role in proliferation of primordial germ cells.

Transcription elongation is stimulated by positive transcription elongation factor b (P-TEFb), for which activity is repressed in the 7SK small nuclear ribonucleoprotein (7SK snRNP) complex. We show here a critical role of 7SK snRNP in growth control of primordial germ cells (PGCs). The expression of p15(INK4b), a cyclin-dependent kinase inhibitor (CDKI) gene, in PGCs is selectively activated by P-TEFb and its recruiting molecule, Brd4, when the amount of active P-TEFb is increased due to reduction of the 7SK snRNP, and PGCs consequently undergo growth arrest. These results indicate that CDKI gene-specific control of transcription by 7SK snRNP plays a pivotal role in the maintenance of PGC proliferation.

The majority of early primordial germ cells acquire pluripotency by AKT activation.

Primordial germ cells (PGCs) are undifferentiated germ cells in embryos, the fate of which is to become gametes; however, mouse PGCs can easily be reprogrammed into pluripotent embryonic germ cells (EGCs) in culture in the presence of particular extracellular factors, such as combinations of Steel factor (KITL), LIF and bFGF (FGF2). Early PGCs form EGCs more readily than do later PGCs, and PGCs lose the ability to form EGCs by embryonic day (E) 15.5. Here, we examined the effects of activation of the serine/threonine kinase AKT in PGCs during EGC formation; notably, AKT activation, in combination with LIF and bFGF, enhanced EGC formation and caused ∼60% of E10.5 PGCs to become EGCs. The results indicate that the majority of PGCs at E10.5 could acquire pluripotency with an activated AKT signaling pathway. Importantly, AKT activation did not fully substitute for bFGF and LIF, and AKT activation without both LIF and bFGF did not result in EGC formation. These findings indicate that AKT signal enhances and/or collaborates with signaling pathways of bFGF and of LIF in PGCs for the acquisition of pluripotency.

Transgenic expression of Telomerase reverse transcriptase (Tert) improves cell proliferation of primary cells and enhances reprogramming efficiency into the induced pluripotent stem cell.

The enzymatic activity of telomerase is important for the extension of the telomere repeat sequence and overcoming cellular senescence. We generated a conditional transgenic mouse line, carrying the telomerase reverse transcriptase (Tert) expression cassette, controlled by the Cre-loxP-mediated recombination. In our study, Cre recombinase expression efficiently activated Tert expression, resulting in its increased enzymatic activity, which extended the period of cellular proliferation until the keratinocytes entered senescence. This suggests that transgenic Tert expression is effective in enhancing primary cell proliferation. Notably, Tert expression increased colony formation of induced pluripotent stem (iPS) cells after the introduction of four reprogramming factors, Oct-4, klf4, SOX-2, and c-Myc into the transgenic fibroblasts. To the best of our knowledge, this is the first study to show that the transgenic Tert expression enhances reprogramming efficiency of iPS cells, which indicates a critical role for Tert in the reprogramming process.

REST and its downstream molecule Mek5 regulate survival of primordial germ cells.

In mouse embryos, some primordial germ cells (PGCs) are eliminated by apoptosis, but the molecular pathways that lead to PGC survival versus apoptosis have not been fully characterized. Here, we found that REST (repressor element 1-silencing transcription factor), a transcription factor that binds a conserved regulatory element, NRSE/RE1, played a role in PGC survival. REST expression was higher in PGCs than in surrounding somatic cells. Moreover, in mouse embryos with a PGC-specific conditional REST mutation, the PGC population experienced more apoptosis and was significantly smaller than that in control embryos; these findings indicated that REST functioned in a cell-autonomous fashion that was critical for PGC survival. Several anti-apoptotic genes were among the previously identified REST-target gene candidates; moreover, some of these genes were downregulated in the REST-deficient PGCs. Mek5, which encodes a component in the a MAP kinase cascade, was one of these downregulated REST-target gene candidates, and a Mek5 mutation, like the REST mutation, caused an increase in PGC apoptosis; these finding suggested that REST promoted PGC survival via regulation of the Mek5 expression. Importantly, there were a normal number of PGCs in the REST mutants at birth, and both the male and female REST-mutant adults were fertile; these final observations revealed that the PGC population was very robust and could recover from a genetically induced reduction in cell number.

Requirement of Oct3/4 function for germ cell specification.

In mammalian embryos, PGCs (primordial germ cells) are specified from a pluripotent epiblast cell population after implantation. In this study, we demonstrated an essential role for the germline-specific transcription factor Oct3/4 in PGC specification. We generated chimeric embryos with ZHBTc4 ES cells lacking both alleles of the Oct3/4 gene (pou5f1). Pluripotency was maintained by an Oct3/4 transgene, and its expression was suppressed by doxycycline (Dox). Transcription of the Oct3/4 transgene in the ES-derived cells unexpectedly suffered constitutive suppression in chimeric embryos without Dox, and the ES-derived cells contributed to PGC precursor-like cells, but failed to form PGCs. We then attempted to rescue Oct3/4 expression in the ES-derived cells in the chimeric embryos by introducing an additional Oct3/4 transgene. The ES cell-derived cells indeed recovered Oct3/4 transcription in these chimeric embryos, and were successfully specified to PGCs. We further confirmed the requirement of Oct3/4 by using another derivative of ZHBTc4 ES cells in which a Dex (dexamethasone)-dependent Oct3/4 transgene was introduced. In the presence of Dox, Oct3/4 protein was absent in the nuclei of the ES-derived cells, which failed to form PGCs. In contrast, the ES-derived cells could be specified to PGCs after activation of Oct3/4 function in the presence of Dex.

Primordial germ cells contain subpopulations that have greater ability to develop into pluripotential stem cells.

Primordial germ cells (PGCs) are undifferentiated germ cells in embryos. We previously found that some mouse PGCs develop into pluripotential cells (EG cells) when cultured on a feeder layer expressing the membrane bound form of Steel factor with culture medium containing leukemia inhibitory factor and basic fibroblast growth factor. To understand the mechanisms of the conversion of PGCs into EG cells, we attempted to identify PGC subpopulations that have the ability to develop into EG cells. Using flow cytometry, we fractionated PGCs by the expression of the cell surface antigen integrin alpha6, as well as by the detection of side-population (SP) cells in which stem cells are enriched in various tissues. PGCs with negative or low integrin alpha6 expression and with SP cell phenotype showed higher potential to convert to EG cells. Negative or low integrin alpha6 expression in PGCs was also correlated with lower expression of Ddx4, which is specifically expressed in PGCs after embryonic day 10.5. The results indicate that the primitive PGC population showing the SP cell phenotype among undifferentiated PGCs has a higher ability of being converted into EG cells. Thus, conversion of PGCs into pluripotential stem cells may be regulated by being influenced by the natural status of individual PGCs as well as the reprogramming process after starting culture.

Heterogeneity of mouse primordial germ cells reflecting the distinct status of their differentiation, proliferation and apoptosis can be classified by the expression of cell surface proteins integrin α6 and c-Kit.

Primordial germ cells (PGCs) in mouse embryos likely include heterogeneous cells having distinct cellular properties. In the present study, we found that heterogeneity of PGCs can be defined by the expression of integrin α6 and c-Kit. The changes in integrin α6 and c-Kit expression in PGCs were obvious as embryonic development progressed, and the PGCs became a mixture of populations consisting of cells with distinct levels of cell surface protein expression. The changes and heterogeneity of cell surface protein expression mainly reflected asynchronous differentiation of PGCs. Apoptosis of PGCs was biased in populations of c-Kit or integrin α6 negative PGCs at particular developmental stages, suggesting possible linkage between PGC apoptosis and the levels of expression of these cell surface proteins. Histochemical analysis confirmed the heterogeneous expression of c-Kit and integrin α6 in PGCs in embryonic gonads, and revealed that PGCs showing different levels of integrin α6 or c-Kit expression and the apoptotic PGCs were scattered and did not show specific localization within gonads. The present study enables us to analyze and isolate populations of living PGCs showing a distinct status of differentiation, or different properties of proliferation or of cell death in individual embryos, and provides a new strategy to examine the mechanisms of PGC development.

Max is a repressor of germ cell-related gene expression in mouse embryonic stem cells.

Embryonic stem cells and primordial germ cells (PGCs) express many pluripotency-associated genes, but embryonic stem cells do not normally undergo conversion into primordial germ cells. Thus, we predicted that there is a mechanism that represses primordial germ cell-related gene expression in embryonic stem cells. Here we identify genes involved in this putative mechanism, by using an embryonic stem cell line with a Vasa reporter in an RNA interference screen of transcription factor genes expressed in embryonic stem cells. We identify five genes that result in the expression of Vasa when silenced. Of these, Max is the most striking. Transcriptome analysis reveals that Max knockdown in embryonic stem cells results in selective, global derepression of germ cell-specific genes. Max interacts with histone H3K9 methyltransferases and associates with the germ cell-specific genes in embryonic stem cells. In addition, Max knockdown results in a decrease in histone H3K9 dimethylation at their promoter regions. We propose that Max is part of protein complex that acts as a repressor of germ cell-related genes in embryonic stem cells.

Implication of DNA demethylation and bivalent histone modification for selective gene regulation in mouse primordial germ cells.

Primordial germ cells (PGCs) sequentially induce specific genes required for their development. We focused on epigenetic changes that regulate PGC-specific gene expression. mil-1, Blimp1, and Stella are preferentially expressed in PGCs, and their expression is upregulated during PGC differentiation. Here, we first determined DNA methylation status of mil-1, Blimp1, and Stella regulatory regions in epiblast and in PGCs, and found that they were hypomethylated in differentiating PGCs after E9.0, in which those genes were highly expressed. We used siRNA to inhibit a maintenance DNA methyltransferase, Dnmt1, in embryonic stem (ES) cells and found that the flanking regions of all three genes became hypomethylated and that expression of each gene increased 1.5- to 3-fold. In addition, we also found 1.5- to 5-fold increase of the PGC genes in the PGCLCs (PGC-like cells) induced form ES cells by knockdown of Dnmt1. We also obtained evidence showing that methylation of the regulatory region of mil-1 resulted in 2.5-fold decrease in expression in a reporter assay. Together, these results suggested that DNA demethylation does not play a major role on initial activation of the PGC genes in the nascent PGCs but contributed to enhancement of their expression in PGCs after E9.0. However, we also found that repression of representative somatic genes, Hoxa1 and Hoxb1, and a tissue-specific gene, Gfap, in PGCs was not dependent on DNA methylation; their flanking regions were hypomethylated, but their expression was not observed in PGCs at E13.5. Their promoter regions showed the bivalent histone modification in PGCs, that may be involved in repression of their expression. Our results indicated that epigenetic status of PGC genes and of somatic genes in PGCs were distinct, and suggested contribution of epigenetic mechanisms in regulation of the expression of a specific gene set in PGCs.

Regulation of expression of mouse interferon-induced transmembrane protein like gene-3, Ifitm3 (mil-1, fragilis), in germ cells.

Mouse interferon-induced transmembrane protein (IFITM) gene, Ifitm3 (previously known as mil-1 and fragilis), is expressed in primordial germ cells (PGCs), in their precursors, and in germ cells of the fetal gonads (Saitou et al. [2002] Nature 418:293-300; Tanaka and Matsui [2002] Mech Dev 119S:S261-S267). By examining the expression of green fluorescent protein transgene under the control of DNA sequences flanking exon 1, we have identified domains that direct Ifitm3 transcription in PGCs and their precursors in gastrula stage and 13.5 days post coitum embryos. Germ cell-specific expression is achieved by the activity of a consensus element unique to the Ifitm genes, which may act to suppress Ifitm3 expression in somatic tissues. The lack of any influence of the interferon-stimulable response elements on transgene expression in the germ-line suggests that interferon-mediated response is not critical for activating Ifitm3.