Overlapping mechanisms function to establish transcriptional quiescence in the embryonic Drosophila germline

Chromatin and gene expression changes during female Drosophila germline stem cell development illuminate the biology of highly potent stem cells

Highly potent animal stem cells either self renew or launch complex differentiation programs, using mechanisms that are only partly understood. Drosophila female germline stem cells (GSCs) perpetuate without change over evolutionary time and generate cystoblast daughters that develop into nurse cells and oocytes. Cystoblasts initiate differentiation by generating a transient syncytial state, the germline cyst, and by increasing pericentromeric H3K9me3 modification, actions likely to suppress transposable element activity. Relatively open GSC chromatin is further restricted by Polycomb repression of testis or somatic cell-expressed genes briefly active in early female germ cells. Subsequently, Neijre/CBP and Myc help upregulate growth and reprogram GSC metabolism by altering mitochondrial transmembrane transport, gluconeogenesis, and other processes. In all these respects GSC differentiation resembles development of the totipotent zygote. We propose that the totipotent stem cell state was shaped by the need to resist transposon activity over evolutionary timescales.

Germ Granule Evolution Provides Mechanistic Insight into Drosophila Germline Development

The copackaging of mRNAs into biomolecular condensates called germ granules is a conserved strategy to posttranscriptionally regulate germline mRNAs. In Drosophila melanogaster, mRNAs accumulate in germ granules by forming homotypic clusters, aggregates containing multiple transcripts from the same gene. Nucleated by Oskar (Osk), homotypic clusters are generated through a stochastic seeding and self-recruitment process that requires the 3' untranslated region (UTR) of germ granule mRNAs. Interestingly, the 3' UTR belonging to germ granule mRNAs, such as nanos (nos), have considerable sequence variations among Drosophila species and we hypothesized that this diversity influences homotypic clustering. To test our hypothesis, we investigated the homotypic clustering of nos and polar granule component (pgc) in four Drosophila species and concluded that clustering is a conserved process used to enrich germ granule mRNAs. However, we discovered germ granule phenotypes that included significant changes in the abundance of transcripts present in species' homotypic clusters, which also reflected diversity in the number of coalesced primordial germ cells within their embryonic gonads. By integrating biological data with computational modeling, we found that multiple mechanisms underlie naturally occurring germ granule diversity, including changes in nos, pgc, osk levels and/or homotypic clustering efficacy. Furthermore, we demonstrated how the nos 3' UTR from different species influences nos clustering, causing granules to have ∼70% less nos and increasing the presence of defective primordial germ cells. Our results highlight the impact that evolution has on germ granules, which should provide broader insight into processes that modify compositions and activities of other classes of biomolecular condensate.

Evolutionary changes in germ granule mRNA content are driven by multiple mechanisms in Drosophila

The co-packaging of mRNAs into biomolecular condensates called germ granules is a conserved strategy to post-transcriptionally regulate mRNAs that function in germline development and maintenance. In D. melanogaster, mRNAs accumulate in germ granules by forming homotypic clusters, aggregates that contain multiple transcripts from a specific gene. Nucleated by Oskar (Osk), homotypic clusters in D. melanogaster are generated through a stochastic seeding and self-recruitment process that requires the 3' UTR of germ granule mRNAs. Interestingly, the 3' UTR belonging to germ granule mRNAs, such as nanos (nos), have considerable sequence variations among Drosophila species. Thus, we hypothesized that evolutionary changes in the 3' UTR influences germ granule development. To test our hypothesis, we investigated the homotypic clustering of nos and polar granule component (pgc) in four Drosophila species and concluded that homotypic clustering is a conserved developmental process used to enrich germ granule mRNAs. Additionally, we discovered that the number of transcripts found in nos and/or pgc clusters could vary significantly among species. By integrating biological data with computational modeling, we determined that multiple mechanisms underlie naturally occurring germ granule diversity, including changes in nos, pgc, osk levels, and/or homotypic clustering efficacy. Finally, we found that the nos 3' UTR from different species can alter the efficacy of nos homotypic clustering, resulting in germ granules with reduced nos accumulation. Our findings highlight the impact that evolution has on the development of germ granules and may provide insight into processes that modify the content of other classes of biomolecular condensates.

Germline/soma distinction in Drosophila embryos requires regulators of zygotic genome activation

In Drosophila melanogaster embryos, somatic versus germline identity is the first cell fate decision. Zygotic genome activation (ZGA) orchestrates regionalized gene expression, imparting specific identity on somatic cells. ZGA begins with a minor wave that commences at nuclear cycle (NC)8 under the guidance of chromatin accessibility factors (Zelda, CLAMP, GAF), followed by the major wave during NC14. By contrast, primordial germ cell (PGC) specification requires maternally deposited and posteriorly anchored germline determinants. This is accomplished by a centrosome coordinated release and sequestration of germ plasm during the precocious cellularization of PGCs in NC10. Here, we report a novel requirement for Zelda and CLAMP during the establishment of the germline/soma distinction. When their activity is compromised, PGC determinants are not properly sequestered, and specification is disrupted. Conversely, the spreading of PGC determinants from the posterior pole adversely influences transcription in the neighboring somatic nuclei. These reciprocal aberrations can be correlated with defects in centrosome duplication/separation that are known to induce inappropriate transmission of the germ plasm. Interestingly, consistent with the ability of bone morphogenetic protein (BMP) signaling to influence specification of embryonic PGCs, reduction in the transcript levels of a BMP family ligand, decapentaplegic (dpp), is exacerbated at the posterior pole.

Preformation and epigenesis converge to specify primordial germ cell fate in the early Drosophila embryo

A critical step in animal development is the specification of primordial germ cells (PGCs), the precursors of the germline. Two seemingly mutually exclusive mechanisms are implemented across the animal kingdom: epigenesis and preformation. In epigenesis, PGC specification is non-autonomous and depends on extrinsic signaling pathways. The BMP pathway provides the key PGC specification signals in mammals. Preformation is autonomous and mediated by determinants localized within PGCs. In Drosophila, a classic example of preformation, constituents of the germ plasm localized at the embryonic posterior are thought to be both necessary and sufficient for proper determination of PGCs. Contrary to this longstanding model, here we show that these localized determinants are insufficient by themselves to direct PGC specification in blastoderm stage embryos. Instead, we find that the BMP signaling pathway is required at multiple steps during the specification process and functions in conjunction with components of the germ plasm to orchestrate PGC fate.

Proper specification of primordial germ cells (PGCs) is crucial as PGCs serve as the precursors of germline stem cells. To specify PGC fate, invertebrates rely upon cell autonomous preformation involving maternally deposited germ plasm. In Drosophila melanogaster, to insulate newly formed PGCs from the adverse effects of the cell-cell signaling pathways, germ plasm determinants silence transcription and attenuate the cell cycle. However, our data on the BMP signaling pathway challenge this long-held view of PGC specification and suggest that appropriate specification of embryonic PGCs is sensitive to the BMP ligand, decapentaplegic (dpp), and its cognate receptor, thickveins. We find that PGCs are not only capable of responding to BMP signals from the soma, but also that these signals impact the proper determination of the germ cells. Based on these unanticipated similarities between mammals and flies, we propose a model integrating contribution of both the cell-autonomous (preformation) and non-autonomous (epigenesis) pathways during PGC determination. Consistent with the model, we have observed dominant genetic interactions between, oskar, the maternal determinant of PGC fate, and the BMP pathway ligand dpp.

Antagonism between germ cell-less and Torso receptor regulates transcriptional quiescence underlying germline/soma distinction

Transcriptional quiescence, an evolutionarily conserved trait, distinguishes the embryonic primordial germ cells (PGCs) from their somatic neighbors. In Drosophila melanogaster, PGCs from embryos maternally compromised for germ cell-less (gcl) misexpress somatic genes, possibly resulting in PGC loss. Recent studies documented a requirement for Gcl during proteolytic degradation of the terminal patterning determinant, Torso receptor. Here we demonstrate that the somatic determinant of female fate, Sex-lethal (Sxl), is a biologically relevant transcriptional target of Gcl. Underscoring the significance of transcriptional silencing mediated by Gcl, ectopic expression of a degradation-resistant form of Torso (torso^Deg) can activate Sxl transcription in PGCs, whereas simultaneous loss of torso-like (tsl) reinstates the quiescent status of gcl PGCs. Intriguingly, like gcl mutants, embryos derived from mothers expressing torso^Deg in the germline display aberrant spreading of pole plasm RNAs, suggesting that mutual antagonism between Gcl and Torso ensures the controlled release of germ-plasm underlying the germline/soma distinction.

Reactive oxygen species signaling in primordial germ cell development in Drosophila embryos

REDOX mechanisms that induce biosynthesis of the reactive oxygen species (ROS) have attracted considerable attention due to both the deleterious and beneficial responses elicited by the reactive radical. In several organisms including Drosophila melanogaster, modulation of ROS activity is thought to be crucial for the maintenance of cell fates in developmental contexts. Interestingly, REDOX mechanisms have been shown to be involved in maintaining progenitor fate of stem cells as well as their proliferation and differentiation. Here, we have explored the possible functions of ROS during proper specification and developmental progression of embryonic primordial germ cells (PGCs). Indicating its potential involvement in these processes, ROS can be detected in the embryonic PGCs and the surrounding somatic cells from very early stages of embryogenesis. Using both "loss" and "gain" of function mutations in two different components of the REDOX pathway, we show that ROS levels are likely to be critical in maintaining germ cell behavior, including their directed migration. Altering the activity of a putative regulator of ROS also adversely influences the ability of PGCs to adhere to one another in cellular blastoderm embryos, suggesting potential involvement of this pathway in orchestrating different phases of germ cell migration.

Transcriptional quiescence in primordial germ cells

In most animal species, newly formed primordial germ cells (PGCs) acquire the special characteristics that distinguish them from the surrounding somatic cells. Proper fate specification of the PGCs is coupled with transcriptional quiescence, whether they are segregated by determinative or inductive mechanisms. Inappropriate differentiation of PGCs into somatic cells is thought to be prevented due to repression of RNA polymerase (Pol) II-dependent transcription. In the case of a determinative mode of PGC formation (Drosophila, Caenorhabditis elegans, etc.), there is a broad downregulation of Pol II activity. By contrast, PGCs display only gene-specific repression in organisms that rely on inductive signaling-based mechanism (e.g., mice). In addition to the global block of Pol II activity in PGCs, gene expression can be suppressed in other ways, such as chromatin remodeling and Piwi-mediated RNAi. Here, we discuss the mechanisms responsible for the transcriptionally silent state of PGCs in common experimental animals, such as Drosophila, C. elegans, Danio rerio, Xenopus, and mouse. While a PGC-specific downregulation of transcription is a common feature among these organisms, the diverse nature of underlying mechanisms suggests that this functional trait likely evolved independently on several instances. We discuss the possible biological relevance of these silencing mechanisms vis-a-vis fate determination of PGCs.

A Transcriptional Lineage of the Early C. elegans Embryo

During embryonic development, cells must establish fates, morphologies, and behaviors in coordination with one another to form a functional body. A prevalent hypothesis for how this coordination is achieved is that each cell's fate and behavior is determined by a defined mixture of RNAs. Only recently has it become possible to measure the full suite of transcripts in a single cell. Here we quantify genome-wide mRNA abundance in each cell of the Caenorhabditis elegans embryo up to the 16-cell stage. We describe spatially dynamic expression, quantify cell-specific differential activation of the zygotic genome, and identify genes that were previously unappreciated as being critical for development. We present an interactive data visualization tool that allows broad access to our dataset. This genome-wide single-cell map of mRNA abundance, alongside the well-studied life history and fate of each cell, describes at a cellular resolution the mRNA landscape that guides development.

Germline and Pluripotent Stem Cells

Epigenetic mechanisms play an essential role in the germline and imprinting cycle. Germ cells show extensive epigenetic programming in preparation for the generation of the totipotent state, which in turn leads to the establishment of pluripotent cells in blastocysts. The latter are the cells from which pluripotent embryonic stem cells are derived and maintained in culture. Following blastocyst implantation, postimplantation epiblast cells develop, which give rise to all somatic cells as well as primordial germ cells, the precursors of sperm and eggs. Pluripotent stem cells in culture can be induced to undergo differentiation into somatic cells and germ cells in culture. Understanding the natural cycles of epigenetic reprogramming that occur in the germline will allow the generation of better and more versatile stem cells for both therapeutic and research purposes.

Repression of somatic cell fate in the germline

Germ cells must transmit genetic information across generations, and produce gametes while also maintaining the potential to form all cell types after fertilization. Preventing the activation of somatic programs is, therefore, crucial to the maintenance of germ cell identity. Studies in Caenorhabditis elegans, Drosophila melanogaster, and mouse have revealed both similarities and differences in how somatic gene expression is repressed in germ cells, thereby preventing their conversion into somatic tissues. This review will focus on recent developments in our understanding of how global or gene-specific transcriptional repression, chromatin regulation, and translational repression operate in the germline to maintain germ cell identity and repress somatic differentiation programs.

Maintaining sufficient nanos is a critical function for polar granule component in the specification of primordial germ cells

Primordial germ cells (PGC) are the precursors of germline stem cells. In Drosophila, PGC specification is thought to require transcriptional quiescence and three genes, polar granule component (pgc), nanos (nos), and germ cell less (gcl) function to downregulate Pol II transcription. While it is not understood how nos or gcl represses transcription, pgc does so by inhibiting the transcription elongation factor b (P-TEFb), which is responsible for phosphorylating Ser2 residues in the heptad repeat of the C-terminal domain (CTD) of the largest Pol II subunit. In the studies reported here, we demonstrate that nos are a critical regulatory target of pgc. We show that a substantial fraction of the PGCs in pgc embryos have greatly reduced levels of Nos protein and exhibit phenotypes characteristic of nos PGCs. Lastly, restoring germ cell–specific expression of Nos is sufficient to ameliorate the pgc phenotype.

Genetics of germ cell development

The germ line represents a continuous cellular link between generations and between species, but the germ cells themselves develop in a specialized, organism-specific context. The model organisms Caenorhabditis elegans, Drosophila melanogaster and the mouse display striking similarities, as well as major differences, in the means by which they control germ cell development. Recent developments in genetic technologies allow a more detailed comparison of the germ cells of these three organisms than has previously been possible, shedding light not only on universal aspects of germline regulation, but also on the control of the pluripotent state in vivo and on the earliest steps of embryogenesis. Here, we highlight themes from the comparison of these three alternative strategies for navigating the fundamental cycle of sexual reproduction.

Repression of early zygotic transcription in the germline

Germ cells, the progenitors of gametes, are often specified and segregated from somatic lineages early in embryogenesis. As germ cells are essential to create the next generation in sexually reproducing organisms, they must be prevented from differentiating inappropriately into somatic cells. In Drosophila and Caenorhabditis elegans embryos, this is governed by the transient and global repression of mRNA transcription. Furthermore, the inhibition of somatic transcriptional programs is also crucial for germ cell specification in the mouse. Therefore, the active repression of somatic transcriptional programs appears to be a common mechanism for launching the germline. In this review, we will discuss the mechanisms of transcriptional repression during germ cell specification and their interspecies similarities and differences.

Trans-generational epigenetic regulation of C. elegans primordial germ cells

Background

The processes through which the germline maintains its continuity across generations has long been the focus of biological research. Recent studies have suggested that germline continuity can involve epigenetic regulation, including regulation of histone modifications. However, it is not clear how histone modifications generated in one generation can influence the transcription program and development of germ cells of the next.

Results

We show that the histone H3K36 methyltransferase maternal effect sterile (MES)-4 is an epigenetic modifier that prevents aberrant transcription activity in Caenorhabditis elegans primordial germ cells (PGCs). In mes-4 mutant PGCs, RNA Pol II activation is abnormally regulated and the PGCs degenerate. Genetic and genomewide analyses of MES-4-mediated H3K36 methylation suggest that MES-4 activity can operate independently of ongoing transcription, and may be predominantly responsible for maintenance methylation of H3K36 in germline-expressed loci.

Conclusions

Our data suggest a model in which MES-4 helps to maintain an 'epigenetic memory' of transcription that occurred in germ cells of previous generations, and that MES-4 and its epigenetic product are essential for normal germ cell development.

Mechanisms guiding primordial germ cell migration: strategies from different organisms

The regulated migration of cells is essential for development and tissue homeostasis, and aberrant cell migration can lead to an impaired immune response and the progression of cancer. Primordial germ cells (PGCs), precursors to sperm and eggs, have to migrate across the embryo to reach somatic gonadal precursors, where they carry out their function. Studies of model organisms have revealed that, despite important differences, several features of PGC migration are conserved. PGCs require an intrinsic motility programme and external guidance cues to survive and successfully migrate. Proper guidance involves both attractive and repulsive cues and is mediated by protein and lipid signalling.

A functional antagonism between the pgc germline repressor and torso in the development of somatic cells

Segregation of the germline is a fundamental event during early development. In Drosophila, germ cells are specified at the posterior pole of the embryo by the germplasm. As zygotic expression is activated, germ cells remain transcriptionally silent owing to the polar granule component (Pgc), a small peptide present in germ cells. Somatic cells at both the embryonic ends are specified by the torso (Tor) receptor tyrosine kinase, and in tor mutants the somatic cells closer to the germ cells fail to cellularize correctly. Here, we show that extra wild-type gene copies of pgc cause a similar cellularization phenotype, and that both excessive pgc and a lack of tor are associated with an impairment of transcription in somatic cells. Moreover, a lack of pgc partly ameliorates the cellularization defect of tor mutants, thus revealing a functional antagonism between pgc and tor in the specification of germline and somatic properties. As transcriptional quiescence is a general feature of germ cells, similar mechanisms might operate in many organisms to 'protect' somatic cells that adjoin germ cells from inappropriately succumbing to such quiescence.

Less is more: specification of the germline by transcriptional repression

In animals, the germline is the only lineage that transmits genetic information to the next generation. Although the founder cells of this lineage are specified differently in invertebrates and vertebrates, recent studies have shown that germline specification in C. elegans, Drosophila and mouse depends on the global inhibition of mRNA transcription. Different strategies are used in each organism, but remarkably most target the same two processes: transcriptional elongation and chromatin remodeling. This convergence suggests that a repressed genome is essential to preserve the unique developmental potential of the germline.

A highly conserved cis-regulatory motif directs differential gonadal synexpression of Dmrt1 transcripts during gonad development

Differential gene expression largely accounts for the coordinated manifestation of the genetic programme underlying embryonic development and cell differentiation. The 3′ untranslated region (3′-UTR) of eukaryotic genes can contain motifs involved in regulation of gene expression at the post-transcriptional level. In the 3′-UTR of dmrt1, a key gene that functions in gonad development and differentiation, an 11-bp protein-binding motif was identified that mediates gonad-specific mRNA localization during embryonic and larval development of fish. Mutations that disrupt the 11-bp motif leading to in vitro protein-binding loss and selective transcript stabilization failure indicate a role for this motif in RNA stabilization through protein binding. The sequence motif was found to be conserved in most of the dmrt1 homologous genes from flies to humans suggesting a widespread conservation of this specific mechanism.

Global transcriptional repression in C. elegans germline precursors by regulated sequestration of TAF-4

In C. elegans, four asymmetric divisions, beginning with the zygote (P0), generate transcriptionally repressed germline blastomeres (P1-P4) and somatic sisters that become transcriptionally active. The protein PIE-1 represses transcription in the later germline blastomeres but not in the earlier germline blastomeres P0 and P1. We show here that OMA-1 and OMA-2, previously shown to regulate oocyte maturation, repress transcription in P0 and P1 by binding to and sequestering in the cytoplasm TAF-4, a component critical for assembly of TFIID and the pol II preinitiation complex. OMA-1/2 binding to TAF-4 is developmentally regulated, requiring phosphorylation by the DYRK kinase MBK-2, which is activated at meiosis II after fertilization. OMA-1/2 are normally degraded after the first mitosis, but ectopic expression of wild-type OMA-1 is sufficient to repress transcription in both somatic and later germline blastomeres. We propose that phosphorylation by MBK-2 serves as a developmental switch, converting OMA-1/2 from oocyte to embryo regulators.

Repression of RNA polymerase II transcription by a Drosophila oligopeptide

Background

Germline progenitors resist signals that promote differentiation into somatic cells. This occurs through the transient repression in primordial germ cells of RNA polymerase II, specifically by disrupting Ser2 phosphorylation on its C-terminal domain.

Methodology/Principal Findings

Here we show that contrary to expectation the Drosophila polar granule component (pgc) gene functions as a protein rather than a non-coding RNA. Surprisingly, pgc encodes a 71-residue, dimeric, alpha-helical oligopeptide repressor. In vivo data show that Pgc ablates Ser2 phosphorylation of the RNA polymerase II C-terminal domain and completely suppresses early zygotic transcription in the soma.

Conclusions/Significance

We thus identify pgc as a novel oligopeptide that readily inhibits gene expression. Germ cell repression of transcription in Drosophila is thus catalyzed by a small inhibitor protein.

Inhibition of transcription by the Caenorhabditis elegans germline protein PIE-1: genetic evidence for distinct mechanisms targeting initiation and elongation

In Caenorhabditis elegans embryos, specification of the germ lineage depends on PIE-1, a maternal protein that blocks mRNA transcription in germline blastomeres. Studies in mammalian cell culture have suggested that PIE-1 inhibits P-TEFb, a kinase that phosphorylates serine 2 in the carboxyl-terminal domain (CTD) repeats of RNA polymerase II during transcriptional elongation. We have tested this hypothesis using an in vivo complementation assay for PIE-1 function. Our results support the view that PIE-1 inhibits P-TEFb using the CTD-like motif YAPMAPT. This activity is required to block serine 2 phosphorylation in germline blastomeres, but unexpectedly is not essential for transcriptional repression or specification of the germline. We find that sequences outside of the YAPMAPT are required to inhibit serine 5 phosphorylation, and that this second inhibitory mechanism is essential for transcriptional repression and specification of the germ lineage. Our results suggest that PIE-1 uses partially redundant mechanisms to block transcription by targeting both the initiation and elongation phases of the transcription cycle.

Drosophila Pgc protein inhibits P-TEFb recruitment to chromatin in primordial germ cells

Germ cells are the only cells that transmit genetic information to the next generation, and they therefore must be prevented from differentiating inappropriately into somatic cells. A common mechanism by which germline progenitors are protected from differentiation-inducing signals is a transient and global repression of RNA polymerase II (RNAPII)-dependent transcription. In both Drosophila and Caenorhabditis elegans embryos, the repression of messenger RNA transcription during germ cell specification correlates with an absence of phosphorylation of Ser 2 residues in the carboxy-terminal domain of RNAPII (hereafter called CTD), a critical modification for transcriptional elongation. Here we show that, in Drosophila embryos, a small protein encoded by polar granule component (pgc) is essential for repressing CTD Ser 2 phosphorylation in newly formed pole cells, the germline progenitors. Ectopic Pgc expression in somatic cells is sufficient to repress CTD Ser 2 phosphorylation. Furthermore, Pgc interacts, physically and genetically, with positive transcription elongation factor b (P-TEFb), the CTD Ser 2 kinase complex, and prevents its recruitment to transcription sites. These results indicate that Pgc is a cell-type-specific P-TEFb inhibitor that has a fundamental role in Drosophila germ cell specification. In C. elegans embryos, PIE-1 protein segregates to germline blastomeres, and is thought to repress mRNA transcription through interaction with P-TEFb. Thus, inhibition of P-TEFb is probably a common mechanism during germ cell specification in the disparate organisms C. elegans and Drosophila.

The development of germline stem cells in Drosophila

Germline stem cells (GSCs) in Drosophila are a valuable model to explore of how adult stem cells are regulated in vivo. Genetic dissection of this system has shown that stem cell fate is determined and maintained by the stem cell's somatic microenvironment or niche. In Drosophila gonads, the stem cell niche -- the cap cell cluster in females and the hub in males -- acts as a signaling center to recruit GSCs from among a small population of undifferentiated primordial germ cells (PGCs). Short-range signals from the niche specify and regulate stem cell fate by maintaining the undifferentiated state of the PGCs next to the niche. Germline cells that do not receive the niche signals because of their location assume the default fate and differentiate. Once GSCs are specified, adherens junctions maintain close association between the stem cells and their niche and help to orient stem cell division so that one daughter is displaced from the niche and differentiates. In females, stem cell fate depends on bone morphogenetic protein (BMP) signals from the cap cells; in males, hub cells express the cytokine-like ligand Unpaired, which activates the Janus kinase-signal transducers and activators of transcription (Jak-Stat) pathway in stem cells. Although the signaling pathways operating between the niche and stem cells are different, there are common general features in both males and females, including the arrangement of cell types, many of the genes used, and the logic of the system that maintains stem cell fate.

Cellular dynamics associated with the genome-wide epigenetic reprogramming in migrating primordial germ cells in mice

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.

Genetic and epigenetic regulators of pluripotency

Genetic and epigenetic mechanisms regulate the transition from the totipotent zygote to pluripotent primitive ectoderm cells in the inner cell mass of mouse blastocysts. These pluripotent cells can be propagated indefinitely in vitro, underpinned by a unique epigenetic state. Following implantation of the blastocyst, diverse epigenetic modifiers control differentiation of pluripotent epiblast cells into somatic cells, while specification of germ cells requires repression of the somatic program. Regenerating totipotency during development of germ cells entails re-expression of pluripotency-specific genes and extensive erasure of epigenetic modifications. Increasing knowledge of key underlying mechanisms heightens prospects for creating pluripotent cells directly from adult somatic cells.

Eukaryotic regulatory RNAs: an answer to the 'genome complexity' conundrum

A large portion of the eukaryotic genome is transcribed as noncoding RNAs (ncRNAs). While once thought of primarily as "junk," recent studies indicate that a large number of these RNAs play central roles in regulating gene expression at multiple levels. The increasing diversity of ncRNAs identified in the eukaryotic genome suggests a critical nexus between the regulatory potential of ncRNAs and the complexity of genome organization. We provide an overview of recent advances in the identification and function of eukaryotic ncRNAs and the roles played by these RNAs in chromatin organization, gene expression, and disease etiology.

The drosophila fragile X protein dFMR1 is required during early embryogenesis for pole cell formation and rapid nuclear division cycles

The FMR family of KH domain RNA-binding proteins is conserved from invertebrates to humans. In humans, inactivation of the X-linked FMR gene fragile X is the most common cause of mental retardation and leads to defects in neuronal architecture. While there are three FMR family members in humans, there is only a single gene, dfmr1, in flies. As in humans, inactivation of dfmr1 causes defects in neuronal architecture and in behavior. dfmr1 has other functions in the fly in addition to neurogenesis. Here we have analyzed its role during early embryonic development. We found that dfmr1 embryos display defects in the rapid nuclear division cycles that precede gastrulation in nuclear migration and in pole cell formation. While the aberrations in nuclear division are correlated with a defect in the assembly of centromeric/centric heterochromatin, the defects in pole cell formation are associated with alterations in the actin-myosin cytoskeleton.

The Sm proteins regulate germ cell specification during early C. elegans embryogenesis

Sm and Sm-like proteins are core components of the splicesome but have other functions distinct from pre-mRNA processing. Here, we show that Sm proteins also regulate germ cell specification during early C. elegans embryogenesis. SmE and SmG were required to maintain transcriptional quiescence in embryonic germ cell precursors. In addition, depletion of SmE inhibited expression of the germ lineage-specific proteins PIE-1, GLD-1, and NOS-2, but did not affect maintenance of several maternal mRNAs. PIE-1 had previously been shown to activate transcriptional silencing and NOS-2 expression. We found that PIE-1 also promotes GLD-1 expression by a process that is independent of transcriptional silencing. Thus, Sm proteins could control transcriptional silencing and maternal protein expression by regulating PIE-1. However, loss of SmE function also caused defects in P granule localization and premature division in early germline blastomeres, processes that are independent of PIE-1 function. Therefore, the Sm proteins control multiple aspects of germ cell precursor development. Because depletion of several other core splicing factors did not affect these events, these Sm functions are likely distinct from pre-mRNA splicing. Sm family proteins assemble into ribonucleoprotein complexes (RNPs) that control RNA activities. We suggest that novel Sm RNPs directly or indirectly influence posttranscriptional control of maternal mRNAs to promote germ cell specification in the early C. elegans embryo.

Germline recruitment in mice: a genetic program for epigenetic reprogramming

Germ cells provide an enduring link between generations and therefore must possess the fundamental ability of reprogramming their genome to generate a totipotent state. We wish to understand the molecular basis of the unique properties of the mammalian germ line. Recently we identified Blimp1, a potent transcriptional repressor of a histone methyltransferase subfamily, as a critical determinant of the germ cell lineage in mice. Surprisingly, Blimp1 expression marks the origin of the germ line in proximal epiblast cells in pregastrulation embryos, substantially earlier than previously thought. Furthermore, we showed that established primordial germ cells undergo extensive erasure of genome-wide histone H3 lysine 9 dimethylation (H3K9me2) and DNA methylation, two major repressive epigenetic modifications, and instead acquire high levels of H3-K27 trimethylation (H3K27me3) in their migration period. We suggest that germline specification is a genetic system for the orderly reprogramming of the cells' epigenome toward a totipotent state, with reacquisition of totipotency-associated transcription factors and continued Blimp1 expression preventing their reversion to an explicit pluripotent state or somatic differentiation.

Tracking nucleolar dynamics with GFP-Nopp140 during Drosophila oogenesis and embryogenesis

We expressed two green fluorescent protein (GFP)-tagged Nopp140 isoforms in transgenic Drosophila melanogaster to study nucleolar dynamics during oogenesis and early embryogenesis. Specifically, we wanted to test whether the quiescent oocyte nucleus stored maternal Nopp140 and then to determine precisely when nucleoli formed during embryogenesis. During oogenesis nurse cell nucleoli accumulated GFP-Nopp140 gradually such that posterior nurse cell nucleoli in egg chambers at stage 10 were usually brighter than the more anterior nurse cell nucleoli. Nucleoli within apoptotic nurse cells disassembled in stages 12 and 13, but not all GFP-Nopp140 entered the oocyte through inter-connecting cytoplasmic bridges. Oocytes, on the other hand, lost their nucleoli by stage 3, but GFP-Nopp140 gradually accumulated in oocyte nuclei during stages 8-13. Most oocyte nuclei at stage 10 stored GFP-Nopp140 uniformly, but many stage 10 oocytes accumulated GFP-Nopp140 in presumed endobodies or in multiple smaller spheres. All oocyte nuclei at stages 11-12 were uniformly labeled, and GFP-Nopp140 diffused to the cytoplasm upon nuclear disassembly in stage 13. GFP-Nopp140 reappeared during embryogenesis; initial nucleologenesis occurred in peripheral somatic nuclei during embryonic stage 13, one stage earlier than reported previously. These GFP-Nopp140-containing foci disassembled at the 13th syncytial mitosis, and a second nucleologenesis occurred in early stage 14. The resulting nucleoli occupied nuclear regions closest to the periphery of the embryos. Pole cells contained GFP-Nopp140 during the syncytial embryonic stages, but their nucleologenesis started at gastrulation.

The Role of Mitochondrial rRNAs and Nanos Protein in Germline Formation in Drosophila Embryos

Germ cells, represented by male sperm and female eggs, are specialized cells that transmit genetic material from one generation to the next during sexual reproduction. The mechanism by which multicellular organisms achieve the proper separation of germ cells and somatic cells is one of the longest standing issues in developmental biology. In many animal groups, a specialized portion of the egg cytoplasm, or germ plasm, is inherited by the cell lineage that gives rise to the germ cells (germline). Germ plasm contains maternal factors that are sufficient for germline formation. In the fruit fly, Drosophila, germ plasm is referred to as polar plasm and is distinguished histologically by the presence of polar granules, which act as a repository for the maternal factors required for germline formation. Molecular screens have so far identified several of these factors that are enriched in the polar plasm. This article focuses on the molecular functions of two such factors in Drosophila, mitochondrial ribosomal RNAs and Nanos protein, which are required for the formation and differentiation of the germline progenitors, respectively.

Functions and mechanisms of receptor tyrosine kinase Torso signaling: lessons from Drosophila embryonic terminal development

The Torso receptor tyrosine kinase (RTK) is required for cell fate specification in the terminal regions (head and tail) of the early Drosophila embryo. Torso contains a split tyrosine kinase domain and belongs to the type III subgroup of the RTK superfamily that also includes the platelet-derived growth factor receptors, stem cell or steel factor receptor c-Kit proto-oncoprotein, colony-stimulating factor-1 receptor, and vascular endothelial growth factor receptor. The Torso pathway has been a model system for studying RTK signal transduction. Genetic and biochemical studies of Torso signaling have provided valuable insights into the biological functions and mechanisms of RTK signaling during early Drosophila embryogenesis.

Germ cell-autonomous Wunen2 is required for germline development in Drosophila embryos

In many animals, primordial germ cells (PGCs) migrate through the embryo towards the future gonad, a process guided by attractive and repulsive cues provided from surrounding somatic cells. In Drosophila, the two related lipid phosphate phosphatases (LPPs), Wunen (Wun) and Wun2, are thought to degrade extracellular substrates and to act redundantly in somatic cells to provide a repulsive environment to steer the migration of PGCs, or pole cells. Wun and Wun2 also affect the viability of pole cells, because overexpression of either one in somatic cells causes pole cell death. However, the means by which they regulate pole cell migration and survival remains elusive. We report that Wun2 has a maternal function required for the survival of pole cells during their migration to the gonad. Maternal wun2 RNA was found to be concentrated in pole cells and pole cell-specific expression of wun2 rescued the pole cell death phenotype of the maternal wun2 mutant, suggesting that wun2 activity in pole cells is required for their survival. Furthermore, we obtained genetic evidence that pole cell survival requires a proper balance of LPP activity in pole cells and somatic cells. We propose that Wun2 in pole cells competes with somatic Wun and Wun2 for a common lipid phosphate substrate, which is required by pole cells to produce their survival signal. In somatic cells, Wun and Wun2 may provide a repulsive environment for pole cell migration by depleting this extracellular substrate.

Germ Cell Specification and Migration in Drosophila and beyond

The passage of an individual's genome to future generations is essential for the maintenance of species and is mediated by highly specialized cells, the germ cells. Genetic studies in a number of model organisms have provided insight into the molecular mechanisms that control specification, migration and survival of early germ cells. Focusing on Drosophila, we will discuss the mechanisms by which germ cells initially form and remain transcriptionally silent while somatic cells are transcriptionally active. We will further discuss three separate attractive and repellent guidance pathways, mediated by a G-protein coupled receptor, two lipid phosphate phosphohydrolases, and isoprenylation. We will compare and contrast these findings with those obtained in other organisms, in particular zebrafish and mice. While aspects of germ cell specification are strikingly different between these species, germ cell specific gene functions have been conserved. In particular, mechanisms that sense directional cues during germ cell migration seem to be shared between invertebrates and vertebrates.