Specification of germ cell fate is fundamental in development and heredity. Recent evidence indicates that in mice, specification of primordial germ cells (PGCs), the common source of both oocytes and spermatozoa, occurs through the integration of three key events: repression of the somatic program, reacquisition of potential pluripotency and ensuing genome-wide epigenetic reprogramming. Here we provide genetic evidence that Prdm14, a PR domain-containing transcriptional regulator with exclusive expression in the germ cell lineage and pluripotent cell lines, is critical in two of these events, the reacquisition of potential pluripotency and successful epigenetic reprogramming. In Prdm14 mutants, the failure of these two events manifests even in the presence of Prdm1 (also known as Blimp1), a key transcriptional regulator for PGC specification. Our combined evidence demonstrates that Prdm14 defines a previously unknown genetic pathway, initiating independently from Prdm1, for ensuring the launching of the mammalian germ cell lineage.

We previously reported that primordial germ cells (PGCs) in mice erase genome-wide DNA methylation and histone H3 lysine9 dimethylation (H3K9me2), and instead acquire high levels of tri-methylation of H3K27 (H3K27me3) during their migration, a process that might be crucial for the re-establishment of potential totipotency in the germline. We here explored a cellular dynamics associated with this epigenetic reprogramming. We found that PGCs undergo erasure of H3K9me2 and upregulation of H3K27me3 in a progressive, cell-by-cell manner, presumably depending on their developmental maturation. Before or concomitant with the onset of H3K9 demethylation, PGCs entered the G2 arrest of the cell cycle, which apparently persisted until they acquired high H3K27me3 levels. Interestingly, PGCs exhibited repression of RNA polymerase II-dependent transcription, which began after the onset of H3K9me2 reduction in the G2 phase and tapered off after the acquisition of high-level H3K27me3. The epigenetic reprogramming and transcriptional quiescence were independent from the function of Nanos3. We found that before H3K9 demethylation, PGCs exclusively repress an essential histone methyltransferase, GLP, without specifically upregulating histone demethylases. We suggest the possibility that active repression of an essential enzyme and subsequent unique cellular dynamics ensures successful implementation of genome-wide epigenetic reprogramming in migrating PGCs.

Specification of germ cell fate is fundamental in development. With a highly representative single-cell microarray and rigorous quantitative PCR analysis, we defined the genome-wide transcription dynamics that create primordial germ cells (PGCs) from the epiblast, a process that exclusively segregates them from their somatic neighbors. We also analyzed the effect of the loss of Blimp1, a key transcriptional regulator, on these dynamics. Our analysis revealed that PGC specification involves complex, yet highly ordered regulation of a large number of genes, proceeding under the strong influence of mesoderm induction but specifically avoiding developmental programs such as the epithelial-mesenchymal transition, Hox cluster activation, cell cycle progression, and DNA methyltransferase machinery. Remarkably, Blimp1 is essential for repressing nearly all the genes normally down-regulated in PGCs relative to their somatic neighbors. In contrast, it is dispensable for the activation of approximately half of the genes up-regulated in PGCs, uncovering the Blimp1-independent events for PGC specification. Notably, however, highly PGC-specific genes exhibited distinct correlations to Blimp1 in wild-type embryos, and these correlations faithfully predicted their expression impairments in Blimp1 mutants. Moreover, their expression overlaps within single cells were severely damaged without Blimp1, demonstrating that Blimp1 exerts positive influence on their concerted activation. Thus, Blimp1 is not a single initiator but a dominant coordinator of the transcriptional program for the establishment of the germ cell fate in mice.[Keywords: Primordial germ cells; Blimp1/Prdm1; cell fate specification; single-cell analysis; genome-wide gene expression profiling] Supplemental material is available at http://www.genesdev.org.

Specification of the germ cell lineage is vital to development and heredity. In mice, the germ cell fate is induced in pluripotent epiblast cells by signaling molecules, yet the underlying mechanism remains unknown. Here we demonstrate that germ cell fate in the epiblast is a direct consequence of Bmp4 signaling from the extraembryonic ectoderm (ExE), which is antagonized by the anterior visceral endoderm (AVE). Strikingly, Bmp8b from the ExE restricts AVE development, thereby contributing to Bmp4 signaling. Furthermore, Wnt3 in the epiblast ensures its responsiveness to Bmp4. Serum-free, defined cultures revealed that, in response to Bmp4, competent epiblast cells uniformly expressed key transcriptional regulators Blimp1 and Prdm14 and acquired germ-cell properties, including genome-wide epigenetic reprogramming, in an orderly fashion. Notably, the induced cells contributed to both spermatogenesis and fertility of offspring. By identifying a signaling principle in germ cell specification, our study establishes a robust strategy for reconstituting the mammalian germ cell lineage in vitro.

The ability to monitor the development of a given cell lineage in a non-invasive manner by fluorescent markers both in vivo and in vitro provides a great advantage for the analysis of the lineage of interest. To date, a number of transgenic or knock-in mouse strains, in which developing germ cells are marked with fluorescent reporters, have been generated. We here describe a novel double transgenic reporter mouse strain that expresses membrane-targeted Venus (mVenus), a brighter variant of yellow fluorescent protein (YFP), under the control of Prdm1 (Blimp1) regulatory elements and enhanced cyan fluorescent protein (ECFP) under the control of Dppa3 (Stella/Pgc7). The double transgenic strain unambiguously marked Prdm1 expression in the lineage-restricted precursors of primordial germ cells (PGCs) in the proximal epiblast at embryonic day (E) 6.25 and specifically illuminated Prdm1-and Dppa3-positive migrating PGCs after E8.5. The double transgenic reporter also precisely recapitulated dynamic embryonic expression of Prdm1 outside the germ cell lineage. Moreover, we derived ES cells that bore both transgenes. These cells made a robust contribution both to the germ and somatic cell lineages in chimeras with accurate Prdm1-mVenus and Dppa3-ECFP expression. The transgenic strain and the ES cells will serve as valuable experimental materials not only for analyzing the origin and properties of the germ cell lineage in vivo, but also for establishing a culture system to efficiently induce proper germ cells with temporally coordinated Prdm1 and Dppa3 expression in vitro.