Germ cells: finding programs of mass repression

Germ plasm dynamics during oogenesis and early embryonic development in Xenopus and zebrafish

Specification of the germline and its segregation from the soma mark one of the most crucial events in the lifetime of an organism. In different organisms, this specification can occur through either inheritance or inductive mechanisms. In species such as Xenopus and zebrafish, the specification of primordial germ cells relies on the inheritance of maternal germline determinants that are synthesized and sequestered in the germ plasm during oogenesis. In this review, we discuss the formation of the germ plasm, how germline determinants are recruited into the germ plasm during oogenesis, and the dynamics of the germ plasm during oogenesis and early embryonic development.

Cellular remodeling and JAK inhibition promote zygotic gene expression in the Ciona germline

Transcription control is a major determinant of cell fate decisions in somatic tissues. By contrast, early germline fate specification in numerous vertebrate and invertebrate species relies extensively on RNA-level regulation, exerted on asymmetrically inherited maternal supplies, with little-to-no zygotic transcription. However delayed, a maternal-to-zygotic transition is nevertheless poised to complete the deployment of pre-gametic programs in the germline. Here, we focus on early germline specification in the tunicate Ciona to study zygotic genome activation. We first demonstrate that a peculiar cellular remodeling event excludes localized postplasmic Pem-1 mRNA, which encodes the general inhibitor of transcription. Subsequently, zygotic transcription begins in Pem-1-negative primordial germ cells (PGCs), as revealed by histochemical detection of elongating RNA Polymerase II, and nascent Mef2 transcripts. In addition, we uncover a provisional antagonism between JAK and MEK/BMPRI/GSK3 signaling, which controls the onset of zygotic gene expression, following cellular remodeling of PGCs. We propose a 2-step model for the onset of zygotic transcription in the Ciona germline and discuss the significance of germ plasm dislocation and remodeling in the context of developmental fate specification.

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.

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.

Global transcriptional repression: An initial and essential step for Plasmodium sexual development

Gametocytes are nonreplicative sexual forms that mediate malaria transmission to a mosquito vector. They are generated from asexual blood-stage parasites that proliferate in the circulation. However, little is known about how this transition is genetically regulated. Here, we report that an Apetala2 (AP2) family transcription factor, AP2-G2, regulates this transition as a transcriptional repressor. Disruption of AP2-G2 in the rodent malaria parasite Plasmodium berghei did not prevent commitment to the sexual stage but did halt development before the appearance of sex-specific morphologies. ChIP-seq analysis revealed that AP2-G2 targeted ∼1,500 genes and recognized a five-base motif in their promoters. Most of these target genes are required for asexual proliferation of the parasites in the blood, suggesting that AP2-G2 blocks the program that precedes asexual replication to promote conversion to the sexual stage. Microarray analysis showed that the identified targets constituted ∼70% of the up-regulated genes in AP2-G2-depleted parasites, suggesting that AP2-G2 actually functions as a repressor in gametocytes. A promoter assay using a centromere plasmid demonstrated that the binding motif functions as a cis-acting negative regulatory element. These results suggest that global transcriptional repression, which occurs during the initial phase of gametocytogenesis, is an essential step in Plasmodium sexual development.

Hematopoietic activity in putative mouse primordial germ cell populations

In the present paper, starting from the observation of heterogeneous expression of the GOF-18ΔPE-GFP Pou5f1 (Oct3/4) transgene in putative mouse PGC populations settled in the aorta-gonad-mesonephros (AGM) region, we identified various OCT3/4 positive populations showing distinct expression of PGC markers (BLIMP-1, AP, TG-1, STELLA) and co-expressing several proteins (CD-34, CD-41, FLK-1) and genes (Brachyury, Hox-B4, Scl/Tal-1 and Gata-2) of hematopoietic precursors. Moreover, we found that Oct3/4-GFP(weak) CD-34(weak/high) cells possess robust hematopoietic colony forming activity (CFU) in vitro. These data indicate that the cell population usually considered PGCs moving toward the gonadal ridges encompasses a subset of cells co-expressing several germ cell and hematopoietic markers and possessing hematopoietic activity. These results are discussed within of the current model of germline segregation.

A non-cell autonomous role of E(z) to prevent germ cells from turning on a somatic cell marker

In many metazoans, germ cells are separated from somatic lineages early in development and maintain their identity throughout life. Here, we show that a Polycomb group (PcG) component, Enhancer of Zeste [E(z)], a histone transferase that generates trimethylation at lysine 27 of histone H3, maintains germline identity in Drosophila adult testes. We find excessive early-stage somatic gonadal cells in E(z) mutant testes, which originate from both overproliferative cyst stem cells and germ cells turning on an early-stage somatic cell marker. Using complementary lineage-tracing experiments in E(z) mutant testes, a portion of excessive early-stage somatic gonadal cells are found to originate from early-stage germ cells, including germline stem cells. Moreover, knocking down E(z) specifically in somatic cells caused this change, which suggests a non-cell autonomous role of E(z) to antagonize somatic identity in germ cells.

Retention of exogenous mRNAs selectively in the germ cells of the sea urchin requires only a 5'-cap and a 3'-UTR

The abundance of an mRNA in a cell depends on its overall rates of synthesis and decay. RNA stability is an important element in the regulation of gene expression, and is achieved by a variety of processes including specific recruitment of nucleases and RNAi-associated mechanisms. These mechanisms are particularly important in stem cells, which, in many cases, have attenuated transcription. Here we report that exogenous mRNA injected into fertilized eggs of the sea urchin is selectively retained in the small micromeres, which contribute to the germ line in this organism, beginning in blastulae, when compared to adjacent somatic cells. We show that modification of this exogenous RNA using cap analogs and poly-adenosine tail deletions do not affect its selective retention in the small micromeres, but removal of the cap or of the 3'-untranslated region eliminates any selective mRNA retention in the presumptive germ line. Our results illuminate a likely ancient mechanism used by stem cells to prolong the lifespan of RNAs-either through RNA protection or by the absence of basic RNA degradation mechanisms, which are employed by most other cells of an organism.

14-3-3ε Is required for germ cell migration in Drosophila

Although 14-3-3 proteins participate in multiple biological processes, isoform-specific specialized functions, as well as functional redundancy are emerging with tissue and developmental stage-specificity. Accordingly, the two 14-3-3ε proteins in Drosophila exhibit functional specificity and redundancy. Homozygotes for loss of function alleles of D14-3-3ε contain significantly fewer germ line cells (pole cells) in their gonads, a phenotype not shared by mutants in the other 14-3-3 gene leo. We show that although D14-3-3ε is enriched within pole cells it is required in mesodermal somatic gonad precursor cells which guide pole cells in their migration through the mesoderm and coalesce with them to form the embryonic gonad. Loss of D14-3-3ε results in defective pole cell migration, reduced pole cell number. We present evidence that D14-3-3ε loss results in reduction or loss of the transcription factor Zfh-1, one of the main regulatory molecules of the pole cell migration, from the somatic gonad precursor cells.

Histone variants in metazoan development

Embryonic development is regulated by both genetic and epigenetic mechanisms, with nearly all DNA-templated processes influenced by chromatin architecture. Sequence variations in histone proteins, core components of chromatin, provide a means to generate diversity in the chromatin structure, resulting in distinct and profound biological outcomes in the developing embryo. Emerging literature suggests that epigenetic contributions from histone variants play key roles in a number of developmental processes such as the initiation and maintenance of pericentric heterochromatin, X-inactivation, and germ cell differentiation. Here, we review the role of histone variants in the embryo with particular emphasis on early mammalian development.

Soma-germline interactions that influence germline proliferation in Caenorhabditis elegans

Caenorhabditis elegans boasts a short lifecycle and high fecundity, two features that make it an attractive and powerful genetic model organism. Several recent studies indicate that germline proliferation, a prerequisite to optimal fecundity, is tightly controlled over the course of development. Cell proliferation control includes regulation of competence to proliferate, a poorly understood aspect of cell fate specification, as well as cell-cycle control. Furthermore, dynamic regulation of cell proliferation occurs in response to multiple external signals. The C. elegans germ line is proving a valuable model for linking genetic, developmental, systemic, and environmental control of cell proliferation. Here, we consider recent studies that contribute to our understanding of germ cell proliferation in C. elegans. We focus primarily on somatic control of germline proliferation, how it differs at different life stages, and how it can be altered in the context of the life cycle and changes in environmental status.

Gene regulation: a tale of germline mRNA tails

Gene regulation often plays by different rules in the germline compared to the soma. In Caenorhabditis elegans, the spatial and temporal expression of germline genes is controlled post-transcriptionally via the 3' UTR rather than transcriptionally via the promoter.

Genetic modification of primordial germ cells by gene trapping, gene targeting, and phiC31 integrase

The genome of germline committed cells is thought to be protected by mechanisms of transcriptional silencing, posing a barrier to transgenesis using cultured germline cells. We found that selection for transgene integration into the primordial germ cell genome required that the transgenes be flanked by the chicken beta-globin insulator. However, integration frequency was low, and sequencing of the insertion sites revealed that the transgenes preferentially inserted into active promoter regions, implying that silencing prohibited recovery of insertions in other regions. Much higher frequencies of integration were achieved when the phiC31 integrase was used to insert transgenes into endogenous pseudo attP sites. Despite the evidence for transcriptional silencing in PGCs, gene targeting of a nonexpressed gene was also achieved. The ability to make genetic modifications in PGCs provides unprecedented opportunities to study the biology of PGCs, as well as produce transgenic chickens for applications in biotechnology and developmental biology.

Ronin is essential for embryogenesis and the pluripotency of mouse embryonic stem cells

Pluripotency is a unique biological state that allows cells to differentiate into any tissue type. Here we describe a candidate pluripotency factor, Ronin, that possesses a THAP domain, which is associated with sequence-specific DNA binding and epigenetic silencing of gene expression. Ronin is expressed primarily during the earliest stages of murine embryonic development, and its deficiency in mice produces periimplantational lethality and defects in the inner cell mass. Conditional knockout of Ronin prevents the growth of ES cells while forced expression of Ronin allows ES cells to proliferate without differentiation under conditions that normally do not promote self-renewal. Ectopic expression also partly compensates for the effects of Oct4 knockdown. We demonstrate that Ronin binds directly to HCF-1, a key transcriptional regulator. Our findings identify Ronin as an essential factor underlying embryogenesis and ES cell pluripotency. Its association with HCF-1 suggests an epigenetic mechanism of gene repression in pluripotent cells.

Development of germ cells in the mouse

In mammals, germ cells are induced from a population of cells at the base of the allantois. This regulative mechanism of germ line induction depends on Bmp signals and a combination of epigenetic changes that silence somatic differentiation genes and activate pluripotency genes. RNA binding proteins are a conserved feature of germ cell development in mammals, and play critical roles in the establishment and maintenance of pluripotency. After their specification, germ cells move through the gut to the gonads under the influence of migratory and attractive cues. In the gonad, germ cells initiate sex-specific differentiation. Germ cells that arrive in the ovary enter meiosis, whereas germ cells that arrive in the testis undergo mitotic arrest. Entry into meiosis is controlled by retinoic acid signals that are blocked in the testis. The signals regulating mitotic arrest in the testis are still not completely understood, but likely involve RNA-binding proteins. Epigenetic reprograming occurs during specification, migratory stages, and sex-specific stages, when maternal and paternal imprints are established. The facility of transitions between germ cells and stem cells suggests a close relationship among their genomic programs.

Early loss of Xist RNA expression and inactive X chromosome associated chromatin modification in developing primordial germ cells

Background

The inactive X chromosome characteristic of female somatic lineages is reactivated during development of the female germ cell lineage. In mouse, analysis of protein products of X-linked genes and/or transgenes located on the X chromosome has indicated that reactivation occurs after primordial germ cells reach the genital ridges.

Principal Findings/Methodology

We present evidence that the epigenetic reprogramming of the inactive X-chromosome is initiated earlier than was previously thought, around the time that primordial germ cells (PGCs) migrate through the hindgut. Specifically, we find that Xist RNA expression, the primary signal for establishment of chromosome silencing, is extinguished in migrating PGCs. This is accompanied by displacement of Polycomb-group repressor proteins Eed and Suz(12), and loss of the inactive X associated histone modification, methylation of histone H3 lysine 27.

Conclusions/Significance

We conclude that X reactivation in primordial germ cells occurs progressively, initiated by extinction of Xist RNA around the time that germ cells migrate through the hindgut to the genital ridges. The events that we observe are reminiscent of X reactivation of the paternal X chromosome in inner cell mass cells of mouse pre-implantation embryos and suggest a unified model in which execution of the pluripotency program represses Xist RNA thereby triggering progressive reversal of epigenetic silencing of the X chromosome.

Inferring developmental constraint and constraint release: Primordial germ cell determination mechanisms as examples

Developmental constraint and its converse constraint release are significant concepts in understanding pattern and process in macroevolution. The purpose of this paper is to propose a two-step method for identifying constraints and constraint release. The first step is a phylogenetic optimization procedure to identify which trait/process is primitive and which is derived. The primitive trait is inferred to be the constraint and the convergently derived trait the release. The second criterion uses sister-clade asymmetry. Clades diagnosed by the constraint will have fewer taxa than clades diagnosed by the release. As an example, we use the process of germ cell specification, in which there are three modes of specification. Our results corroborate previous conclusions that the induced mode is the constraint and the predetermined mode is the release and we speculate on the importance of these two processes in terms of robustness and evolvability.

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.

Pathway to totipotency: lessons from germ cells

Oocytes and sperm are some of the most differentiated cells in our bodies, yet they generate all cell types after fertilization. Accumulating evidence suggests that this extraordinary potential is conferred to germ cells from the time of their formation during embryogenesis. In this Review, we describe common themes emerging from the study of germ cells in vertebrates and invertebrates. Transcriptional repression, chromatin remodeling, and an emphasis on posttranscriptional gene regulation preserve the totipotent genome of germ cells through generations.

Modeling human germ cell development with embryonic stem cells

There has previously been no robust cell-based model for examining the genetic and epigenetic mechanisms of human germ cell formation. Human embryonic stem cells (hESCs) could potentially fill this need, as all cell types analyzed to date (including mature germ cells) can be identified by marker analysis during hESC differentiation. Furthermore, hESCs could also be used to differentiate mature female germ cells (oocytes) in culture as an alternate reprogramming cell for somatic cell nuclear transfer. However, to differentiate and isolate a functional germ cell from hESCs, the mechanisms that regulate germ cell formation need to be understood. The purpose of this review is to summarize the current understanding of the earliest events in human germ cell formation and to describe some of the known genetic pathways that regulate germ cell specification and development in the mouse. Finally, the current literature on the formation of germ cells from ESCs will be described.

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.

Genetic basis of human testicular germ cell cancer: insights from the fruitfly and mouse

The prevalence of tumours of the germ line is increasing in the male population. This complex disease has a complex aetiology. We examine the contribution of genetic mutations to the development of germ line tumours in this review. In particular, we concentrate on fly and mouse experimental systems in order to demonstrate that mutations in some conserved genes cause pathologies typical of certain human germ cell tumours, whereas other mutations elicit phenotypes that are unique to the experimental model. Despite these experimental systems being imperfect, we show that they are useful models of human testicular germ cell tumourigenesis.

Blimp1 is a critical determinant of the germ cell lineage in mice

Germ cell fate in mice is induced in pluripotent epiblast cells in response to signals from extraembryonic tissues. The specification of approximately 40 founder primordial germ cells and their segregation from somatic neighbours are important events in early development. We have proposed that a critical event during this specification includes repression of a somatic programme that is adopted by neighbouring cells. Here we show that Blimp1 (also known as Prdm1), a known transcriptional repressor, has a critical role in the foundation of the mouse germ cell lineage, as its disruption causes a block early in the process of primordial germ cell formation. Blimp1-deficient mutant embryos form a tight cluster of about 20 primordial germ cell-like cells, which fail to show the characteristic migration, proliferation and consistent repression of homeobox genes that normally accompany specification of primordial germ cells. Furthermore, our genetic lineage-tracing experiments indicate that the Blimp1-positive cells originating from the proximal posterior epiblast cells are indeed the lineage-restricted primordial germ cell precursors.

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.

Hold the germ cells, I'm on duty

Germ cell segregation and gamete production are developmental problems that all sexually reproducing species must solve in order to survive. Many people are familiar with the complex social structures of some insect species, where specialised castes of adult insects perform specific tasks, one of which is usually to guard the sexually reproductive queen. The parasitic wasp Copidosoma floridanum adds another level of complexity to the caste system: a fertilised egg produces both sterile, short-lived "soldier" larvae and "reproductive" larvae that complete metamorphosis to produce sexually reproductive adults. How two morphologically and functionally distinct larval castes are produced by genetically identical groups of cells developing under the same environmental conditions is a baffling problem. A recent paper suggests that differential germ cell segregation during embryogenesis may be an event both necessary and sufficient for caste determination.(1)