Nanos1 functions as a translational repressor in the Xenopus germline

Regulation by the RNA-binding protein Unkempt at its effector interface

How RNA-binding proteins (RBPs) convey regulatory instructions to the core effectors of RNA processing is unclear. Here, we document the existence and functions of a multivalent RBP-effector interface. We show that the effector interface of a conserved RBP with an essential role in metazoan development, Unkempt, is mediated by a novel type of 'dual-purpose' peptide motifs that can contact two different surfaces of interacting proteins. Unexpectedly, we find that the multivalent contacts do not merely serve effector recruitment but are required for the accuracy of RNA recognition by Unkempt. Systems analyses reveal that multivalent RBP-effector contacts can repurpose the principal activity of an effector for a different function, as we demonstrate for the reuse of the central eukaryotic mRNA decay factor CCR4-NOT in translational control. Our study establishes the molecular assembly and functional principles of an RBP-effector interface.

Distribution of XTdrd6/Xtr protein during oogenesis and early development in Xenopus laevis: Zygotic translation begins only in germ cells that have entered the genital ridge

We previously identified Xenopus tudor domain containing 6/Xenopus tudor repeat (Xtdrd6/Xtr), which was exclusively expressed in the germ cells of adult Xenopus laevis. Western blot analysis showed that the XTdrd6/Xtr protein was translated in St. I/II oocytes and persisted as a maternal factor until the tailbud stage. XTdrd6/Xtr has been reported to be essential for the translation of maternal mRNA involved in oocyte meiosis. In the present study, we examined the distribution of the XTdrd6/Xtr protein during oogenesis and early development, to predict the time point of its action during development. First, we showed that XTdrd6/Xtr is localized to germinal granules in the germplasm by electron microscopy. XTdrd6/Xtr was found to be localized to the origin of the germplasm, the mitochondrial cloud of St. I oocytes, during oogenesis. Notably, XTdrd6/Xtr was also found to be localized around the nuclear membrane of St. I oocytes. This suggests that XTdrd6/Xtr may immediately interact with some mRNAs that emerge from the nucleus and translocate to the mitochondrial cloud. XTdrd6/Xtr was also detected in primordial germ cells and germ cells throughout development. Using transgenic Xenopus expressing XTdrd6/Xtr with a C-terminal FLAG tag produced by homology-directed repair, we found that the zygotic translation of the XTdrd6/Xtr protein began at St. 47/48. As germ cells are surrounded by gonadal somatic cells and are considered to enter a new differentiation stage at this phase, the newly synthesized XTdrd6/Xtr protein may regulate the translation of mRNAs involved in the new steps of germ cell differentiation.

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.

A paradigm for regulation at the effector interface with RNA-binding proteins

RNA-binding proteins (RBPs) are key regulators of gene expression, but how RBPs convey regulatory instructions to the core effectors of RNA processing is unclear. Here we document the existence and functions of a multivalent RBP-effector interface. We show that the effector interface of a deeply conserved RBP with an essential role in metazoan development, Unkempt, is mediated by a novel type of 'dual-purpose' peptide motifs that can contact two different surfaces of interacting proteins. Unexpectedly, we find that the multivalent contacts do not merely serve effector recruitment but are required for the accuracy of RNA recognition by the recruiting RBP. Systems analyses reveal that multivalent RBP-effector contacts can repurpose the principal activity of an effector for a different function, as we demonstrate for reuse of the central eukaryotic mRNA decay factor CCR4-NOT in translational control. Our study establishes the molecular assembly and functional principles of an RBP-effector interface, with implications for the evolution and function of RBP-operated regulatory networks.

Molecular Markers of Ovarian Germ Cells of Banana Prawn (Fenneropenaeus merguiensis)

The banana prawn (Fenneropenaeus merguiensis) is a valuable prawn in the worldwide market. However, cultivation of this species is limited owing to the difficulty in culture management and limited knowledge of reproduction. Therefore, we studied the gene expression and molecular mechanisms involved in oogenesis for elucidating ovarian germ cell development in banana prawns. The tissue-specific distribution of certain genes identified from previous transcriptome data showed that FmCyclinB, FmNanos, and nuclear autoantigenic sperm protein (FmNASP) were only expressed in gonads. The in situ hybridization (ISH) of these three genes showed different expression patterns throughout oogenesis. FmCyclinB was highly expressed in pre-vitellogenic oocytes. FmNanos was expressed at almost the same level during oogenesis but showed the most expression in late pre-vitellogenic stages. Based on the highest expression of FmCyclinB and FmNanos in mid pre-vitellogenic and late pre-vitellogenic oocytes, respectively, we suggested that FmNanos may suppress FmCyclinB expression before initiation of vitellogenesis. Meanwhile, FmNASP expression was detected only in pre-vitellogenesis. Moreover, quantitative real-time polymerase chain reaction (qRT-PCR) analysis of FmNASP expression was supported by FmNASP ISH analysis based on high expression of FmNASP in sub-adult ovaries, which contain most of pre-vitellogenic oocytes. In this study, we found three reliable ovarian markers for banana prawns and also found a dynamic change of molecular mechanism during the sub-stage of pre-vitellogenesis. We determined the expression levels of these genes involved in oogenesis. Our findings provide information for further studies on banana prawn reproduction which may assist in their cultivation.

Two RNA binding proteins, ADAD2 and RNF17, interact to form a heterogeneous population of novel meiotic germ cell granules with developmentally dependent organelle association

Mammalian male germ cell differentiation relies on complex RNA biogenesis events, many of which occur in non-membrane bound organelles termed RNA germ cell granules that are rich in RNA binding proteins (RBPs). Though known to be required for male germ cell differentiation, we understand little of the relationships between the numerous granule subtypes. ADAD2, a testis specific RBP, is required for normal male fertility and forms a poorly characterized granule in meiotic germ cells. This work aimed to understand the role of ADAD2 granules in male germ cell differentiation by clearly defining their molecular composition and relationship to other granules. Biochemical analyses identified RNF17, a testis specific RBP that forms meiotic male germ cell granules, as an ADAD2-interacting protein. Phenotypic analysis of Adad2 and Rnf17 mutants identified a rare post-meiotic chromatin defect, suggesting shared biological roles. ADAD2 and RNF17 were found to be dependent on one another for granularization and together form a previously unstudied set of germ cell granules. Based on co-localization studies with well-characterized granule RBPs and organelle-specific markers, a subset of the ADAD2-RNF17 granules are found to be associated with the intermitochondrial cement and piRNA biogenesis. In contrast, a second, morphologically distinct population of ADAD2-RNF17 granules co-localized with the translation regulators NANOS1 and PUM1, along with the molecular chaperone PDI. These large granules form a unique funnel-shaped structure that displays distinct protein subdomains and is tightly associated with the endoplasmic reticulum. Developmental studies suggest the different granule populations represent different phases of a granule maturation process. Lastly, a double Adad2-Rnf17 mutant model suggests the interaction between ADAD2 and RNF17, as opposed to loss of either, is the likely driver of the Adad2 and Rnf17 mutant phenotypes. These findings shed light on the relationship between germ cell granule pools and define new genetic approaches to their study.

Conserved Transcriptome Features Define Prepubertal Primate Spermatogonial Stem Cells as Adark Spermatogonia and Identify Unique Regulators

Antineoplastic treatments for cancer and other non-malignant disorders can result in long-term or permanent male infertility by ablating spermatogonial stem cells (SSCs). SSC transplantation using testicular tissue harvested before a sterilizing treatment is a promising approach for restoring male fertility in these cases, but a lack of exclusive biomarkers to unequivocally identify prepubertal SSCs limits their therapeutic potential. To address this, we performed single-cell RNA-seq on testis cells from immature baboons and macaques and compared these cells with published data from prepubertal human testis cells and functionally-defined mouse SSCs. While we found discrete groups of human spermatogonia, baboon and rhesus spermatogonia appeared less heterogenous. A cross-species analysis revealed cell types analogous to human SSCs in baboon and rhesus germ cells, but a comparison with mouse SSCs revealed significant differences with primate SSCs. Primate-specific SSC genes were enriched for components and regulators of the actin cytoskeleton and participate in cell-adhesion, which may explain why the culture conditions for rodent SSCs are not appropriate for primate SSCs. Furthermore, correlating the molecular definitions of human SSC, progenitor and differentiating spermatogonia with the histological definitions of Adark/Apale spermatogonia indicates that both SSCs and progenitor spermatogonia are Adark, while Apale spermatogonia appear biased towards differentiation. These results resolve the molecular identity of prepubertal human SSCs, define novel pathways that could be leveraged for advancing their selection and propagation in vitro, and confirm that the human SSC pool resides entirely within Adark spermatogonia.

Micromere formation and its evolutionary implications in the sea urchin

The micromeres of the sea urchin embryo are distinct from other blastomeres. After they arise through an asymmetric cell division at the 8- to 16-cell stage, micromeres immediately function as organizers. They also commit themselves to specific cell fates such as larval skeletogenic cells and primordial germ cells, while other blastomeres remain plastic and uncommitted at the 16-cell stage. In the phylum Echinodermata, only the sea urchin (class Echinoidea) embryo forms micromeres that serve as apparent organizers during early embryogenesis. Therefore, it is considered that micromeres are the derived features and that modification(s) of the developmental system allowed evolutionary introduction of this unique cell lineage. In this chapter, we summarize the both historic and recent observations that demonstrate unique properties of micromeres and discuss how this lineage of micromeres may have arisen during echinoderm evolution.

A single cell RNA sequencing resource for early sea urchin development

Identifying cell states during development from their mRNA profiles provides insight into their gene regulatory network. Here, we leverage the sea urchin embryo for its well-established gene regulatory network to interrogate the embryo using single cell RNA sequencing. We tested eight developmental stages in Strongylocentrotus purpuratus, from the eight-cell stage to late in gastrulation. We used these datasets to parse out 22 major cell states of the embryo, focusing on key transition stages for cell type specification of each germ layer. Subclustering of these major embryonic domains revealed over 50 cell states with distinct transcript profiles. Furthermore, we identified the transcript profile of two cell states expressing germ cell factors, one we conclude represents the primordial germ cells and the other state is transiently present during gastrulation. We hypothesize that these cells of the Veg2 tier of the early embryo represent a lineage that converts to the germ line when the primordial germ cells are deleted. This broad resource will hopefully enable the community to identify other cell states and genes of interest to expose the underpinning of developmental mechanisms.

Distinct roles of two eIF4E isoforms in the germline of Caenorhabditis elegans

Germ cells use both positive and negative mRNA translational control to regulate gene expression that drives their differentiation into gametes. mRNA translational control is mediated by RNA-binding proteins, miRNAs and translation initiation factors. We have uncovered the discrete roles of two translation initiation factor eIF4E isoforms (IFE-1, IFE-3) that bind 7-methylguanosine (m7G) mRNA caps during Caenorhabditiselegans germline development. IFE-3 plays important roles in germline sex determination (GSD), where it promotes oocyte cell fate and is dispensable for spermatogenesis. IFE-3 is expressed throughout the germline and localizes to germ granules, but is distinct from IFE-1 and PGL-1, and facilitates oocyte growth and viability. This contrasts with the robust expression in spermatocytes of IFE-1, the isoform that resides within P granules in spermatocytes and oocytes, and promotes late spermatogenesis. Each eIF4E is localized by its cognate eIF4E-binding protein (IFE-1:PGL-1 and IFE-3:IFET-1). IFE-3 and IFET-1 regulate translation of several GSD mRNAs, but not those under control of IFE-1. Distinct mutant phenotypes, in vivo localization and differential mRNA translation suggest independent dormant and active periods for each eIF4E isoform in the germline.

Novel functions of the ubiquitin-independent proteasome system in regulating Xenopus germline development

In most species, early germline development occurs in the absence of transcription with germline determinants subject to complex translational and post-translational regulations. Here, we report for the first time that early germline development is influenced by dynamic regulation of the proteasome system, previously thought to be ubiquitously expressed and to serve 'housekeeping' roles in controlling protein homeostasis. We show that proteasomes are present in a gradient with the highest levels in the animal hemisphere and extending into the vegetal hemisphere of Xenopus oocytes. This distribution changes dramatically during the oocyte-to-embryo transition, with proteasomes becoming enriched in and restricted to the animal hemisphere and therefore separated from vegetally localized germline determinants. We identify Dead-end1 (Dnd1), a master regulator of vertebrate germline development, as a novel substrate of the ubiquitin-independent proteasomes. In the oocyte, ubiquitin-independent proteasomal degradation acts together with translational repression to prevent premature accumulation of Dnd1 protein. In the embryo, artificially increasing ubiquitin-independent proteasomal degradation in the vegetal pole interferes with germline development. Our work thus reveals novel inhibitory functions and spatial regulation of the ubiquitin-independent proteasome during vertebrate germline development.

Cap-Independent mRNA Translation in Germ Cells

Cellular mRNAs in plants and animals have a 5'-cap structure that is accepted as the recognition point to initiate translation by ribosomes. Consequently, it was long assumed that the translation initiation apparatus was built solely for a cap-dependent (CD) mechanism. Exceptions that emerged invoke structural damage (proteolytic cleavage) to eukaryotic initiation factor 4 (eIF4) factors that disable cap recognition. The residual eIF4 complex is thought to be crippled, but capable of cap-independent (CI) translation to recruit viral or death-associated mRNAs begrudgingly when cells are in great distress. However, situations where CI translation coexists with CD translation are now known. In such cases, CI translation is still a minor mechanism in the major background of CD synthesis. In this review, I propose that germ cells do not fit this mold. Using observations from various animal models of oogenesis and spermatogenesis, I suggest that CI translation is a robust partner to CD translation to carry out the translational control that is so prevalent in germ cell development. Evidence suggests that CI translation provides surveillance of germ cell homeostasis, while CD translation governs the regulated protein synthesis that ushers these meiotic cells through the remarkable steps in sperm/oocyte differentiation.

Oocyte maturation and origin of the germline as revealed by the expression of Nanos-like in the Pacific oyster Crassostrea gigas

Nanos gene plays an important role in germline development in animals. However, the molecular mechanisms involved in germline development in Mollusca, the second largest animal phylum, are still poorly understood. Here we identified the Nanos orthologue from the Pacific oyster Crassostrea gigas (Cg-Nanos-like), and investigated the expression patterns of Nanos during gametogenesis and embryogenesis in C. gigas. Tissue expression analysis showed that Cg-Nanos-like was specifically expressed in female gonads. During the reproductive cycle, the expression of Cg-Nanos-like mRNA increased matching the seasonal development of the ovarian tissues in diploids, while the expression levels were significantly lower in the ovaries of sterile triploids compared to diploids. High expression of Cg-Nanos-like transcripts were detected in early embryonic stages, while the expression significantly dropped at gastrulation and was barely detectable in veliger stages. In situ hybridization showed that Cg-Nanos-like was expressed at different stages of developing oocytes, whereas positive signals were detected only in spermatogonia during the spermatogenic cycle. These findings indicated that Cg-Nanos-like was involved in the development of germ cells, and maintenance of oocyte maturation. In early embryogenesis, the transcripts were broadly expressed; following gastrulation, the expression was restricted to two cell clumps, which might be the putative primordial germ cells (PGCs) or their precursors. Based on the results, the formation of the PGCs in C. gigas was consistent with the model of transition from epigenesis to preformation.

Evolutionary conservation and divergence of Vasa, Dazl and Nanos1 during embryogenesis and gametogenesis in dark sleeper (Odontobutis potamophila)

Germline-specific genes, Vasa, Dazl and Nanos1, have highly conserved functions in germline development and fertility across animal phyla. In this study, the full-length sequences of Opvasa, Opdazl and Opnanos1 were cloned and characterized from the dark sleeper (Odontobutis potamophila). Gonad-specific expression patterns of Opvasa and Opdazl were confirmed in adult tissues by quantitative real-time PCR (qRT-PCR). Different from Opvasa and Opdazl, the expression of Opnanos1 was ubiquitously detected in all examined tissues except for the liver and spleen. Time-course dynamic expressions during embryogenesis were assessed, and all three genes (Opvasa, Opdazl and Opnanos1) persisted at a high level until gastrulation. qRT-PCR and Western blotting analyses revealed that all three genes were highly expressed throughout gametogenesis. In testis, the expressions of all three genes at the mRNA and protein levels were down-regulated during spermatogenesis. In ovary, different expression patterns were found, and all three genes had a differential role in translational regulation during oogenesis. The expressions of Opvasa, Opdazl and Opnanos1 at the mRNA but not the protein level were high in stage IV. Different expression patterns were found in premeiotic gonads treated by HPG axis hormones (HCG and LHRH-A). Immunolocalization analysis demonstrated that in testis, Opvasa, Opdazl and Opnanos1 were detected in spermatogonia and spermatocytes but absent in the meiotic products, such as spermatids and spermatozoa. In ovary, Opvasa, Opdazl and Opnanos1 persisted at a high level throughout oogenesis. These findings indicated that Opvasa, Opdazl and Opnanos1 played an important role in mitotic and early meiotic phases of oogenesis and spermatogenesis, and they functioned as maternal factors in early embryogenesis. Their proteins could be used as three new markers for germ cells during gametogenesis in O. potamophila gonad. Our data laid a good foundation for improving the breeding efficiency of O. potamophila.

Nanos genes and their role in development and beyond

The hallmark of Nanos proteins is their typical (CCHC)2 zinc finger motif (zf-nanos). Animals have one to four nanos genes. For example, the fruit fly and demosponge have only one nanos gene, zebrafish and humans have three, and Fugu rubripes has four. Nanos genes are mainly known for their evolutionarily preserved role in germ cell survival and pluripotency. Nanos proteins have been reported to bind the C-terminal RNA-binding domain of Pumilio to form a post-transcriptional repressor complex. Several observations point to a link between the miRNA-mediated repression complex and the Nanos/Pumilio complex. Repression of the E2F3 oncogene product is, indeed, mediated by cooperation between the Nanos/Pumilio complex and miRNAs. Another important interaction partner of Nanos is the CCR4-NOT deadenylase complex. Besides the tissue-specific contribution of Nanos proteins to normal development, their ectopic expression has been observed in several cancer cell lines and various human cancers. An inverse correlation between the expression levels of human Nanos1 and Nanos3 and E-cadherin was observed in several cancer cell lines. Loss of E-cadherin, an important cell-cell adhesion protein, contributes to tumor invasion and metastasis. Overexpression of Nanos3 induces epithelial-mesenchymal transition in lung cancer cell lines partly by repressing E-cadherin. Other than some most interesting data from Nanos knockout mice, little is known about mammalian Nanos proteins, and further research is needed. In this review, we summarize the main roles of Nanos proteins and discuss the emerging concept of Nanos proteins as oncofetal antigens.

Nanos3 of the frog Rana rugosa: Molecular cloning and characterization

Nanos is expressed in the primordial germ cells (PGCs) and also the germ cells of a variety of organisms as diverse as Drosophila, medaka fish, Xenopus and mouse. In Nanos3-deficient mice, PGCs fail to incorporate into the gonad and the size of the testis and ovary is thereby dramatically reduced. To elucidate the role of Nanos in an amphibian species, we cloned Nanos3 cDNA from the testis of the R. rugosa frog. RT-PCR analysis showed strong expression of Nanos3 mRNA in the testis of adult R. rugosa frogs, but expression was not sexually dimorphic during gonadal differentiation. In Nanos3-knockdown tadpoles produced by the CRISPR/Cas9 system, the number of germ cells decreased dramatically in the gonads of both male and female tadpoles before sex determination and thereafter. This was confirmed by three dimensional imaging of wild-type and Nanos3 knockdown gonads using serial sections immunostained for Vasa, a marker specific to germ cells. Taken together, these results suggest that Nanos3 protein function is conserved between R. rugosa and mouse.

Maternal Nanos-Dependent RNA Stabilization in the Primordial Germ Cells of Drosophila Embryos

Nanos (Nos) is an evolutionary conserved protein expressed in the germline of various animal species. In Drosophila, maternal Nos protein is essential for germline development. In the germline progenitors, or the primordial germ cells (PGCs), Nos binds to the 3' UTR of target mRNAs to repress their translation. In contrast to this prevailing role of Nos, here we report that the 3' UTR of CG32425 mRNA mediates Nos-dependent RNA stabilization in PGCs. We found that the level of mRNA expressed from a reporter gene fused to the CG32425 3' UTR was significantly reduced in PGCs lacking maternal Nos (nos PGCs) as compared with normal PGCs. By deleting the CG32425 3' UTR, we identified the region required for mRNA stabilization, which includes Nos-binding sites. In normal embryos, CG32425 mRNA was maternally supplied into PGCs and remained in this cell type during embryogenesis. However, as expected from our reporter assay, the levels of CG32425 mRNA and its protein product expressed in nos PGCs were lower than in normal PGCs. Thus, we propose that Nos protein has dual functions in translational repression and stabilization of specific RNAs to ensure proper germline development.

A quiet space during rush hour: Quiescence in primordial germ cells

Quiescence is a common character in stem cells. Low cellular activity in these cells may function to minimize the potential damaging effects of oxidative stress, reduce the number of cells needed for tissue replenishment, and as a consequence, perhaps occupy unique niches. Quiescent stem cells are found in many adult human tissues, the hematopoietic stem cells are paradigmatic, and more recently it appears that stem cell of the germ line in many animals display quiescence characters. Here we explore the diversity of quiescence phenotypes in primordial germ cells, leveraging the diverse mechanisms of germ cell formation to extract evolutionary significance to common processes.

Maternal Dead-end 1 promotes translation of nanos1 by binding the eIF3 complex

In the developing embryo, primordial germ cells (PGCs) represent the exclusive progenitors of the gametes, and their loss results in adult infertility. During early development, PGCs are exposed to numerous signals that specify somatic cell fates. To prevent somatic differentiation, PGCs must transiently silence their genome, an early developmental process that requires Nanos activity. However, it is unclear how Nanos translation is regulated in developing embryos. We report here that translation of nanos1 after fertilization requires Dead-end 1 (Dnd1), a vertebrate-specific germline RNA-binding protein. We provide evidence that Dnd1 protein, expression of which is low in oocytes, but increases dramatically after fertilization, directly interacts with, and relieves the inhibitory function of eukaryotic initiation factor 3f, a repressive component in the 43S preinitiation complex. This work uncovers a novel translational regulatory mechanism that is fundamentally important for germline development.

Transient translational quiescence in primordial germ cells

Stem cells in animals often exhibit a slow cell cycle and/or low transcriptional activity referred to as quiescence. Here, we report that the translational activity in the primordial germ cells (PGCs) of the sea urchin embryo (Strongylocentrotus purpuratus) is quiescent. We measured new protein synthesis with O-propargyl-puromycin and L-homopropargylglycine Click-iT technologies, and determined that these cells synthesize protein at only 6% the level of their adjacent somatic cells. Knockdown of translation of the RNA-binding protein Nanos2 by morpholino antisense oligonucleotides, or knockout of the Nanos2 gene by CRISPR/Cas9 resulted in a significant, but partial, increase (47%) in general translation specifically in the PGCs. We found that the mRNA of the translation factor eEF1A is excluded from the PGCs in a Nanos2-dependent manner, a consequence of a Nanos/Pumilio response element (PRE) in its 3'UTR. In addition to eEF1A, the cytoplasmic pH of the PGCs appears to repress translation and simply increasing the pH also significantly restores translation selectively in the PGCs. We conclude that the PGCs of this sea urchin institute parallel pathways to quiesce translation thoroughly but transiently.

Divergent Expression Patterns and Function Implications of Four nanos Genes in a Hermaphroditic Fish, Epinephelus coioides

Multiple nanos genes have been characterized in several fishes, but the functional implications of their various expression patterns remain unclear. In this study, we identified and characterized four nanos genes from a hermaphroditic fish orange-spotted grouper, Epinephelus coioides. Ecnanos1a and Ecnanos1b show divergent expression patterns, and the dynamic expression change of Ecnanos1a in pituitaries during sex change is associated with testis differentiation and spermatogenesis. Ecnanos2 and Ecnanos3 might be germline stem cells (GSCs) and primordial germ cells (PGCs)-specific markers, respectively. Significantly, Ecnanos3 3'-untranslated region (UTR) is necessary for PGC specific expression, where a non-canonical "GCACGTTT" sequence is required for miR-430-mediated repression of Ecnanos3 RNA. Furthermore, grouper Dead end (Dnd) can relieve miR-430 repression in PGCs by associating with a 23 bp U-rich region (URR) in Ecnanos3 3'-UTR. The current study revealed the functional association of multiple nanos genes with PGC formation and germ cell development in orange-spotted grouper, and opened up new possibilities for developing biotechnologies through utilizing the associations between Ecnanos3 and PGCs or between Ecnanos2 and GSCs in the hermaphroditic fish.

Mechanisms of Vertebrate Germ Cell Determination

Two unique characteristics of the germ line are the ability to persist from generation to generation and to retain full developmental potential while differentiating into gametes. How the germ line is specified that allows it to retain these characteristics within the context of a developing embryo remains unknown and is one focus of current research. Germ cell specification proceeds through one of two basic mechanisms: cell autonomous or inductive. Here, we discuss how germ plasm driven germ cell specification (cell autonomous) occurs in both zebrafish and the frog Xenopus. We describe the segregation of germ cells during embryonic development of solitary and colonial ascidians to provide an evolutionary context to both mechanisms. We conclude with a discussion of the inductive mechanism as exemplified by both the mouse and axolotl model systems. Regardless of mechanism, several general themes can be recognized including the essential role of repression and posttranscriptional regulation of gene expression.

Differential Nanos 2 protein stability results in selective germ cell accumulation in the sea urchin

Nanos is a translational regulator required for the survival and maintenance of primordial germ cells. In the sea urchin, Strongylocentrotus purpuratus (Sp), Nanos 2 mRNA is broadly transcribed but accumulates specifically in the small micromere (sMic) lineage, in part because of the 3'UTR element GNARLE leads to turnover in somatic cells but retention in the sMics. Here we found that the Nanos 2 protein is also selectively stabilized; it is initially translated throughout the embryo but turned over in the future somatic cells and retained only in the sMics, the future germ line in this animal. This differential stability of Nanos protein is dependent on the open reading frame (ORF), and is independent of the sumoylation and ubiquitylation pathways. Manipulation of the ORF indicates that 68 amino acids in the N terminus of the Nanos protein are essential for its stability in the sMics whereas a 45 amino acid element adjacent to the zinc fingers targets its degradation. Further, this regulation of Nanos protein is cell autonomous, following formation of the germ line. These results are paradigmatic for the unique presence of Nanos in the germ line by a combination of selective RNA retention, distinctive translational control mechanisms (Oulhen et al., 2013), and now also by defined Nanos protein stability.

Distinct modes of recruitment of the CCR4-NOT complex by Drosophila and vertebrate Nanos

Nanos proteins repress the expression of target mRNAs by recruiting effector complexes through non-conserved N-terminal regions. In vertebrates, Nanos proteins interact with the NOT1 subunit of the CCR4-NOT effector complex through a NOT1 interacting motif (NIM), which is absent in Nanos orthologs from several invertebrate species. Therefore, it has remained unclear whether the Nanos repressive mechanism is conserved and whether it also involves direct interactions with the CCR4-NOT deadenylase complex in invertebrates. Here, we identify an effector domain (NED) that is necessary for the Drosophila melanogaster (Dm) Nanos to repress mRNA targets. The NED recruits the CCR4-NOT complex through multiple and redundant binding sites, including a central region that interacts with the NOT module, which comprises the C-terminal domains of NOT1-3. The crystal structure of the NED central region bound to the NOT module reveals an unanticipated bipartite binding interface that contacts NOT1 and NOT3 and is distinct from the NIM of vertebrate Nanos. Thus, despite the absence of sequence conservation, the N-terminal regions of Nanos proteins recruit CCR4-NOT to assemble analogous repressive complexes.

Hermes (Rbpms) is a Critical Component of RNP Complexes that Sequester Germline RNAs during Oogenesis

The germ cell lineage in Xenopus is specified by the inheritance of germ plasm that assembles within the mitochondrial cloud or Balbiani body in stage I oocytes. Specific RNAs, such as nanos1, localize to the germ plasm. nanos1 has the essential germline function of blocking somatic gene expression and thus preventing Primordial Germ Cell (PGC) loss and sterility. Hermes/Rbpms protein and nanos RNA co-localize within germinal granules, diagnostic electron dense particles found within the germ plasm. Previous work indicates that nanos accumulates within the germ plasm through a diffusion/entrapment mechanism. Here we show that Hermes/Rbpms interacts with nanos through sequence specific RNA localization signals found in the nanos-3'UTR. Importantly, Hermes/Rbpms specifically binds nanos, but not Vg1 RNA in the nucleus of stage I oocytes. In vitro binding data show that Hermes/Rbpms requires additional factors that are present in stage I oocytes in order to bind nanos1. One such factor may be hnRNP I, identified in a yeast-2-hybrid screen as directly interacting with Hermes/Rbpms. We suggest that Hermes/Rbpms functions as part of a RNP complex in the nucleus that facilitates selection of germline RNAs for germ plasm localization. We propose that Hermes/Rbpms is required for nanos RNA to form within the germinal granules and in this way, participates in the germline specific translational repression and sequestration of nanos RNA.

The Scd6/Lsm14 protein xRAPB has properties different from RAP55 in selecting mRNA for early translation or intracellular distribution in Xenopus oocytes

Oocytes accumulate mRNAs in the form of maternal ribonucleoprotein (RNP) particles, the protein components of which determine the location and stability of individual mRNAs prior to translation. Scd6/Lsm14 proteins, typified by RAP55, function in a wide range of eukaryotes in repressing translation and relocating mRNPs to processing bodies and stress granules. In Xenopus laevis, the RAP55 orthologue xRAPA fulfils these functions. Here we describe the properties of a variant of xRAPA, xRAPB, which is a member of the Lsm14B group. xRAPB differs from xRAPA in various respects: it is expressed at high concentration earlier in oogenesis; it interacts specifically with the DDX6 helicase Xp54; it is detected in polysomes and stalled translation initiation complexes; its over-expression leads to selective binding to translatable mRNA species without evidence of translation repression or mRNA degradation. Since both Xp54 and xRAPA are repressors of translation, activation appears to be effected through targeting of xRAPB/Xp54.

The Xenopus Maternal-to-Zygotic Transition from the Perspective of the Germline

In Xenopus, the germline is specified by the inheritance of germ-plasm components synthesized at the beginning of oogenesis. Only the cells in the early embryo that receive germ plasm, the primordial germ cells (PGCs), are competent to give rise to the gametes. Thus, germ-plasm components continue the totipotent potential exhibited by the oocyte into the developing embryo at a time when most cells are preprogrammed for somatic differentiation as dictated by localized maternal determinants. When zygotic transcription begins at the mid-blastula transition, the maternally set program for somatic differentiation is realized. At this time, genetic control is ceded to the zygotic genome, and developmental potential gradually becomes more restricted within the primary germ layers. PGCs are a notable exception to this paradigm and remain transcriptionally silent until the late gastrula. How the germ-cell lineage retains full potential while somatic cells become fate restricted is a tale of translational repression, selective degradation of somatic maternal determinants, and delayed activation of zygotic transcription.

Interaction of NANOS2 and NANOS3 with different components of the CNOT complex may contribute to the functional differences in mouse male germ cells

NANOS2 and NANOS3 belong to the Nanos family of proteins that contain a conserved zinc finger domain, which consists of two consecutive CCHC-type zinc finger motifs, and contribute to germ cell development in mice. Previous studies indicate that there are redundant and distinct functions of these two proteins. NANOS2 rescues NANOS3 functions in the maintenance of primordial germ cells, whereas NANOS3 fails to replace NANOS2 functions in the male germ cell pathway. However, the lack of a conditional allele of Nanos3 has hampered delineation of each contribution of NANOS2 and NANOS3 to the male germ cell pathway. In addition, the molecular mechanism underlying the distinct functions of these proteins remains unexplored. Here, we report an unexpected observation of a transgenic mouse line expressing a NANOS2 variant harboring mutations in the zinc finger domain. Transcription of Nanos2 and Nanos3 was strongly compromised in the presence of this transgene, which resulted in the mimicking of the Nanos2/Nanos3 double-null condition in the male gonad. In these transgenic mice, P-bodies involved in RNA metabolism had disappeared and germ cell differentiation was more severely affected than that in Nanos2-null mice, indicating that NANOS3 partially substitutes for NANOS2 functions. In addition, similar to NANOS2, we found that NANOS3 associated with the CCR4-NOT deadenylation complex but via a direct interaction with CNOT8, unlike CNOT1 in the case of NANOS2. This alternate interaction might account for the molecular basis of the functional redundancy and differences in NANOS2 and NANOS3 functions.

Structural basis for the Nanos-mediated recruitment of the CCR4-NOT complex and translational repression

Nanos family RNA-binding proteins play essential roles in metazoan germ cell fate and survival, yet the underlying mechanism has not been elucidated. Bhandari et al. now provide the structural basis for CCR4–NOT complex recruitment by vertebrate Nanos. The authors show that three human Nanos paralogs interact with the CNOT1 C-terminal domain through a short, conserved CNOT1-interacting motif (NIM). This work indicates that NIMs are the major determinants of the translational repression mediated by Nanos and identifies the CCR4–NOT complex as the main effector for Nanos function.

The RNA-binding proteins of the Nanos family play an essential role in germ cell development and survival in a wide range of metazoan species. They function by suppressing the expression of target mRNAs through the recruitment of effector complexes, which include the CCR4–NOT deadenylase complex. Here, we show that the three human Nanos paralogs (Nanos1–3) interact with the CNOT1 C-terminal domain and determine the structural basis for the specific molecular recognition. Nanos1–3 bind CNOT1 through a short CNOT1-interacting motif (NIM) that is conserved in all vertebrates and some invertebrate species. The crystal structure of the human Nanos1 NIM peptide bound to CNOT1 reveals that the peptide opens a conserved hydrophobic pocket on the CNOT1 surface by inserting conserved aromatic residues. The substitutions of these aromatic residues in the Nanos1–3 NIMs abolish binding to CNOT1 and abrogate the ability of the proteins to repress translation. Our findings provide the structural basis for the recruitment of the CCR4–NOT complex by vertebrate Nanos, indicate that the NIMs are the major determinants of the translational repression mediated by Nanos, and identify the CCR4–NOT complex as the main effector complex for Nanos function.

Maternal syntabulin is required for dorsal axis formation and is a germ plasm component in Xenopus

In amphibians and teleosts, early embryonic axial development is driven by maternally deposited mRNAs and proteins, called dorsal determinants, which migrate to the presumptive dorsal side of the embryo in a microtubule-dependent manner after fertilization. Syntabulin is an adapter protein that binds to kinesin KIF5B and to the transmembrane protein Syntaxin1. In zebrafish, a mutation in Syntabulin causes complete embryo ventralization. It is unknown whether Syntabulin plays an analogous role during early development of other species, a question addressed here in Xenopus laevis. in situ hybridization of syntabulin mRNA was carried out at different stages of Xenopus development. In oocytes, syntabulin transcripts were localized to the vegetal cortex of large oocytes and the mitochondrial cloud of very young oocytes. We extended the zebrafish data by finding that during cleavage Xenopus syntabulin mRNA localized to the germ plasm and was later expressed in primordial germ cells (PGCs). This new finding suggested a role for Syntabulin during germ cell differentiation. The functional role of maternal syntabulin mRNA was investigated by knock-down with phosphorothioate DNA antisense oligos followed by oocyte transfer. The results showed that syntabulin mRNA depletion caused the complete loss of dorso-anterior axis formation in frog embryos. Consistent with the ventralized phenotype, syntabulin-depleted embryos displayed severe reduction of dorsal markers and ubiquitous transcription of the ventral marker sizzled. Syntabulin was required for the maternal Wnt/β-Catenin signal, since ventralization could be completely rescued by injection of β-catenin (or syntabulin) mRNA. The data suggest an evolutionarily conserved role for Syntabulin, a protein that bridges microtubule motors and membrane vesicles, during dorso-ventral axis formation in the vertebrates.

Every which way--nanos gene regulation in echinoderms

Nanos is an essential factor of germ line success in all animals tested. This gene encodes a Zn-finger RNA-binding protein that in complex with its partner pumilio binds to and changes the fate of several known transcripts. We summarize here the documented functions of Nanos in several key organisms, and then emphasize echinoderms as a working model for how nanos expression is regulated. Nanos presence outside of the target cells is often detrimental to the animal, and in sea urchins, nanos expression appears to be regulated at every step of transcription, and post-transcriptional activity, making this gene product exciting, every which way.

Restriction of the Xenopus DEADSouth mRNA to the primordial germ cells is ensured by multiple mechanisms

DEADSouth mRNA encoding the RNA helicase DDX25 is a component of the germ plasm in Xenopus laevis. We investigated the mechanisms underlying its specific mRNA expression in primordial germ cells (PGCs). Based on our previous findings of several microRNA miR-427 recognition elements (MREs) in the 3' untranslated region of the mRNA, we first examined whether DEADSouth mRNA was degraded by miR-427 targeting in somatic cells. Injection of antisense miR-427 oligomer and reporter mRNA for mutated MREs revealed that DEADSouth mRNA was potentially degraded in somatic cells via miR-427 targeting, but not in PGCs after the mid-blastula transition (MBT). The expression level of miR-427 was very low in PGCs, which probably resulted in the lack of miR-427-mediated degradation. In addition, the DEADSouth gene was expressed zygotically after MBT. Thus, the predominant expression of DEADSouth mRNA in the PGCs is ensured by multiple mechanisms including zygotic expression and prohibition from miR-427-mediated degradation.

Protein interactions in Xenopus germ plasm RNP particles

Hermes is an RNA-binding protein that we have previously reported to be found in the ribonucleoprotein (RNP) particles of Xenopus germ plasm, where it is associated with various RNAs, including that encoding the germ line determinant Nanos1. To further define the composition of these RNPs, we performed a screen for Hermes-binding partners using the yeast two-hybrid system. We have identified and validated four proteins that interact with Hermes in germ plasm: two isoforms of Xvelo1 (a homologue of zebrafish Bucky ball) and Rbm24b and Rbm42b, both RNA-binding proteins containing the RRM motif. GFP-Xvelo fusion proteins and their endogenous counterparts, identified with antisera, were found to localize with Hermes in the germ plasm particles of large oocytes and eggs. Only the larger Xvelo isoform was naturally found in the Balbiani body of previtellogenic oocytes. Bimolecular fluorescence complementation (BiFC) experiments confirmed that Hermes and the Xvelo variants interact in germ plasm, as do Rbm24b and 42b. Depletion of the shorter Xvelo variant with antisense oligonucleotides caused a decrease in the size of germ plasm aggregates and loosening of associated mitochondria from these structures. This suggests that the short Xvelo variant, or less likely its RNA, has a role in organizing and maintaining the integrity of germ plasm in Xenopus oocytes. While GFP fusion proteins for Rbm24b and 42b did not localize into germ plasm as specifically as Hermes or Xvelo, BiFC analysis indicated that both interact with Hermes in germ plasm RNPs. They are very stable in the face of RNA depletion, but additive effects of combinations of antisense oligos suggest they may have a role in germ plasm structure and may influence the ability of Hermes protein to effectively enter RNP particles.

NANOS2 promotes male germ cell development independent of meiosis suppression

NANOS2 is an RNA-binding protein essential for fetal male germ cell development. While we have shown that the function of NANOS2 is vital for suppressing meiosis in embryonic XY germ cells, it is still unknown whether NANOS2 plays other roles in the sexual differentiation of male germ cells. In this study, we addressed the issue by generating Nanos2/Stra8 double knockout (dKO) mice, whereby meiosis was prohibited in the double-mutant male germ cells. We found that the expression of male-specific genes, which was decreased in the Nanos2 mutant, was hardly recovered in the dKO embryo, suggesting that NANOS2 plays a role in male gene expression other than suppression of meiosis. To investigate the molecular events that may be controlled by NANOS2, we conducted a series of microarray analyses to search putative targets of NANOS2 that fulfilled 2 criteria: (1) increased expression in the Nanos2 mutant and (2) the mRNA associated with NANOS2. Interestingly, the genes predominantly expressed in undifferentiated primordial germ cells (PGCs) were significantly selected, implying the involvement of NANOS2 in the termination of the characteristics of PGCs. Furthermore, we showed that NANOS2 is required for the maintenance of mitotic quiescence, but not for the initiation of the quiescence in fetal male germ cells. These results suggest that NANOS2 is not merely a suppressor of meiosis, but instead plays pivotal roles in the sexual differentiation of male germ cells.

Maternal Dead-End1 is required for vegetal cortical microtubule assembly during Xenopus axis specification

Vertebrate axis specification is an evolutionarily conserved developmental process that relies on asymmetric activation of Wnt signaling and subsequent organizer formation on the future dorsal side of the embryo. Although roles of Wnt signaling during organizer formation have been studied extensively, it is unclear how the Wnt pathway is asymmetrically activated. In Xenopus and zebrafish, the Wnt pathway is triggered by dorsal determinants, which are translocated from the vegetal pole to the future dorsal side of the embryo shortly after fertilization. The transport of dorsal determinants requires a unique microtubule network formed in the vegetal cortex shortly after fertilization. However, molecular mechanisms governing the formation of vegetal cortical microtubule arrays are not fully understood. Here we report that Dead-End 1 (Dnd1), an RNA-binding protein required for primordial germ cell development during later stages of embryogenesis, is essential for Xenopus axis specification. We show that knockdown of maternal Dnd1 specifically interferes with the formation of vegetal cortical microtubules. This, in turn, impairs translocation of dorsal determinants, the initiation of Wnt signaling, organizer formation, and ultimately results in ventralized embryos. Furthermore, we found that Dnd1 binds to a uridine-rich sequence in the 3'-UTR of trim36, a vegetally localized maternal RNA essential for vegetal cortical microtubule assembly. Dnd1 anchors trim36 to the vegetal cortex in the egg, promoting high concentrations of Trim36 protein there. Our work thus demonstrates a novel and surprising function for Dnd1 during early development and provides an important link between Dnd1, mRNA localization, the microtubule cytoskeleton and axis specification.

Localisation of RNAs into the germ plasm of vitellogenic Xenopus oocytes

We have studied the localisation of mRNAs in full-grown Xenopus laevis oocytes by injecting fluorescent RNAs, followed by confocal microscopy of the oocyte cortex. Concentrating on RNA encoding the Xenopus Nanos homologue, nanos1 (formerly Xcat2), we find that it consistently localised into aggregated germ plasm ribonucleoprotein (RNP) particles, independently of cytoskeletal integrity. This implies that a diffusion/entrapment-mediated mechanism is active, as previously reported for previtellogenic oocytes. Sometimes this was accompanied by localisation into scattered particles of the “late”, Vg1/VegT pathway; occasionally only late pathway localisation was seen. The Xpat RNA behaved in an identical fashion and for neither RNA was the localisation changed by any culture conditions tested. The identity of the labelled RNP aggregates as definitive germ plasm was confirmed by their inclusion of abundant mitochondria and co-localisation with the germ plasm protein Hermes. Further, the nanos1/Hermes RNP particles are interspersed with those containing the germ plasm protein Xpat. These aggregates may be followed into the germ plasm of unfertilized eggs, but with a notable reduction in its quantity, both in terms of injected molecules and endogenous structures. Our results conflict with previous reports that there is no RNA localisation in large oocytes, and that during mid-oogenesis even germ plasm RNAs localise exclusively by the late pathway. We find that in mid oogenesis nanos1 RNA also localises to germ plasm but also by the late pathway. Late pathway RNAs, Vg1 and VegT, also may localise into germ plasm. Our results support the view that mechanistically the two modes of localisation are extremely similar, and that in an injection experiment RNAs might utilise either pathway, the distinction in fates being very subtle and subject to variation. We discuss these results in relation to their biological significance and the results of others.

The 3'UTR of nanos2 directs enrichment in the germ cell lineage of the sea urchin

Nanos is a translational regulator required for the survival and maintenance of primordial germ cells during embryogenesis. Three nanos homologs are present in the genome of the sea urchin Strongylocentrotus purpuratus (Sp), and each nanos mRNA accumulates specifically in the small micromere (sMic) lineage. We found that a highly conserved element in the 3' UTR of nanos2 is sufficient for reporter expression selectively in the sMic lineage: microinjection into a Sp fertilized egg of an RNA that contains the GFP open reading frame followed by Sp nanos2 3'UTR leads to selective reporter enrichment in the small micromeres in blastulae. The same result was seen with nanos2 from the sea urchin Hemicentrotus pulcherrimus (Hp). In both species, the 5'UTR alone is not sufficient for the sMic localization but it always increased the sMic reporter enrichment when present with the 3'UTR. We defined an element conserved between Hp and Sp in the nanos2 3'UTR which is necessary and sufficient for protein enrichment in the sMic, and refer to it as GNARLE (Global Nanos Associated RNA Lability Element). We also found that the nanos2 3'UTR is essential for the selective RNA retention in the small micromeres; GNARLE is required but not sufficient for this process. These results show that a combination of selective RNA retention and translational control mechanisms instills nanos accumulation uniquely in the sMic lineage.

Regulation of cell polarity and RNA localization in vertebrate oocytes

It has long been appreciated that the inheritance of maternal cytoplasmic determinants from different regions of the egg can lead to differential specification of blastomeres during cleavage. Localized RNAs are important determinants of cell fate in eggs and embryos but are also recognized as fundamental regulators of cell structure and function. This chapter summarizes recent molecular and genetic experiments regarding: (1) mechanisms that regulate polarity during different stages of vertebrate oogenesis, (2) pathways that localize presumptive protein and RNA determinants within the polarized oocyte and egg, and (3) how these determinants act in the embryo to determine the ultimate cell fates. Emphasis is placed on studies done in Xenopus, where extensive work has been done in these areas, and comparisons are drawn with fish and mammals. The prospects for future work using in vivo genome manipulation and other postgenomic approaches are also discussed.

Principles and roles of mRNA localization in animal development

Intracellular targeting of mRNAs has long been recognized as a means to produce proteins locally, but has only recently emerged as a prevalent mechanism used by a wide variety of polarized cell types. Localization of mRNA molecules within the cytoplasm provides a basis for cell polarization, thus underlying developmental processes such as asymmetric cell division, cell migration, neuronal maturation and embryonic patterning. In this review, we describe and discuss recent advances in our understanding of both the regulation and functions of RNA localization during animal development.

Early gonadogenesis in mammals: significance of long and narrow gonadal structure

In mammalian embryogenesis, the gonadal primordium arises from the thickening of the coelomic epithelium, which results in a pair of extremely long and narrow gonadal structures along the anteroposterior axis. These gonadal structures are conserved in various mammalian species, suggesting a great advantage in properly receiving migrating primordial germ cells (PGCs) that are widely scattered throughout the hindgut tube. Soon after the PGCs settle, the bipotential gonads undergo sex determination into testes or ovaries by the sex-determining gene, Sry, which is expressed in supporting cell precursors in a center-to-pole manner. Such a long, narrow gonadal structure bestows a considerable time lag on Sry expression between the center and pole regions, but testiculogenesis with cord formation and Leydig cell differentiation occurs synchronously throughout the whole organ. This synchronous testiculogenesis could be explained by a positive-feedback mechanism between SOX9 (another SRY-related transcription factor) and FGF9 downstream of Sry. FGF signals are likely secreted from the center region, rapidly diffuse into the poles, and then induce the establishment of SOX9 expression in Sertoli cells in the pole domains. This work focuses on recent knowledge of the molecular and cellular events of PGC migration, gonadogenesis, and testiculogenesis, and their biological significance in mammalian embryogenesis.

Molecular and expression characterization of a nanos1 homologue in Chinese sturgeon, Acipenser sinensis

The nanos gene family was essential for germ line development in diverse organisms. In the present study, the full-length cDNA of a nanos1 homologue in A. sinensis, Asnanos1, was isolated and characterized. The cDNA sequence of Asnanos1 was 1489 base pairs (bp) in length and encoded a peptide of 228 amino acid residues. Multiple sequence alignment showed that the zinc-finger motifs of Nanos1 were highly conserved in vertebrates. By RT-PCR analysis, Asnanos1 mRNAs were ubiquitously detected in all tissues examined except for the fat, including liver, spleen, heart, ovary, kidney, muscle, intestines, pituitary, hypothalamus, telencephalon, midbrain, cerebellum, and medulla oblongata. Moreover, a specific polyclonal antibody was prepared from the in vitro expressed partial AsNanos1 protein. Western blot analysis revealed that the tissue expression pattern of AsNanos1 was not completely coincided with that of its mRNAs, which was not found in fat, muscle and intestines. Additionally, by immunofluoresence localization, it was observed that AsNanos1 protein was in the cytoplasm of primary oocytes and spermatocytes. The presented results indicated that the expression pattern of Asnanos1 was differential conservation and divergence among diverse species.

Ectopic formation of primordial germ cells by transplantation of the germ plasm: direct evidence for germ cell determinant in Xenopus

In many animals, the germ line is specified by a distinct cytoplasmic structure called germ plasm (GP). GP is necessary for primordial germ cell (PGC) formation in anuran amphibians including Xenopus. However, it is unclear whether GP is a direct germ cell determinant in vertebrates. Here we demonstrate that GP acts autonomously for germ cell formation in Xenopus. EGFP-labeled GP from the vegetal pole was transplanted into animal hemisphere of recipient embryos. Cells carrying transplanted GP (T-GP) at the ectopic position showed characteristics similar to the endogenous normal PGCs in subcellular distribution of GP and presence of germ plasm specific molecules. However, T-GP-carrying-cells in the ectopic tissue did not migrate towards the genital ridge. T-GP-carrying cells from gastrula or tailbud embryos were transferred into the endoderm of wild-type hosts. From there, they migrated into the developing gonad. To clarify whether ectopic T-GP-carrying cells can produce functional germ cells, they were identified by changing the recipients, from the wild-type Xenopus to transgenic Xenopus expressing DsRed2. After transferring T-GP carrying cells labeled genetically with DsRed2 into wild-type hosts, we could find chimeric gonads in mature hosts. Furthermore, the spermatozoa and eggs derived from T-GP-carrying cells were fertile. Thus, we have demonstrated that Xenopus germ plasm is sufficient for germ cell determination.

Xenopus Nanos1 is required to prevent endoderm gene expression and apoptosis in primordial germ cells

Nanos is expressed in multipotent cells, stem cells and primordial germ cells (PGCs) of organisms as diverse as jellyfish and humans. It functions together with Pumilio to translationally repress targeted mRNAs. Here we show by loss-of-function experiments that Xenopus Nanos1 is required to preserve PGC fate. Morpholino knockdown of maternal Nanos1 resulted in a striking decrease in PGCs and a loss of germ cells from the gonads. Lineage tracing and TUNEL staining reveal that Nanos1-deficient PGCs fail to migrate out of the endoderm. They appear to undergo apoptosis rather than convert to normal endoderm. Whereas normal PGCs do not become transcriptionally active until neurula, Nanos1-depleted PGCs prematurely exhibit a hyperphosphorylated RNA polymerase II C-terminal domain at the midblastula transition. Furthermore, they inappropriately express somatic genes characteristic of endoderm regulated by maternal VegT, including Xsox17α, Bix4, Mixer, GATA4 and Edd. We further demonstrate that Pumilio specifically binds VegT RNA in vitro and represses, along with Nanos1, VegT translation within PGCs. Repressed VegT RNA in wild-type PGCs is significantly less stable than VegT in Nanos1-depleted PGCs. Our data indicate that maternal VegT RNA is an authentic target of Nanos1/Pumilio translational repression. We propose that Nanos1 functions to translationally repress RNAs that normally specify endoderm and promote apoptosis, thus preserving the germline.

Germ plasm in Eleutherodactylus coqui, a direct developing frog with large eggs

BACKGROUND

RNAs for embryo patterning and for germ cell specification are localized to the vegetal cortex of the oocyte of Xenopus laevis. In oocytes of the direct developing frog Eleutherodactylus coqui, orthologous RNAs for patterning are not localized, raising the question as to whether RNAs and other components of germ plasm are localized in this species.

METHODS

To identify germ plasm, E. coqui embryos were stained with DiOC6(3) or examined by in situ hybridization for dazl and DEADSouth RNAs. The cDNAs for the E. coqui orthologues were cloned by RT-PCR using degenerate primers. To examine activity of the E. coqui orthologues, RNAs, made from constructs of their 3'UTRs with mCherry, were injected into X. laevis embryos.

RESULTS

Both DiOC6(3) and dazl and DEADSouth in situs identified many small islands at the vegetal surface of cleaving E. coqui embryos, indicative of germ plasm. Dazl was also expressed in primordial germ cells in the genital ridge. The 3'UTRs of E. coqui dazl and DEADSouth directed primordial germ cell specific protein synthesis in X. laevis.

CONCLUSIONS

E. coqui utilizes germ plasm with RNAs localized to the vegetal cortex to specify primordial germ cells. The large number of germ plasm islands suggests that an increase in the amount of germ plasm was important in the evolution of the large E. coqui egg.

Xenopus germline nanos1 is translationally repressed by a novel structure-based mechanism

The translational repressor Nanos is expressed in the germline and stem cell populations of jellyfish as well as humans. Surprisingly, we observed that unlike other mRNAs, synthetic nanos1 RNA translates very poorly if at all after injection into Xenopus oocytes. The current model of simple sequestration of nanos1 within germinal granules is insufficient to explain this observation and suggests that a second level of repression must be operating. We find that an RNA secondary structural element immediately downstream of the AUG start site is both necessary and sufficient to prevent ribosome scanning in the absence of a repressor. Accordingly, repression is relieved by small in-frame insertions before this secondary structure, or translational control element (TCE), that provide the 15 nucleotides required for ribosome entry. nanos1 is translated shortly after fertilization, pointing to the existence of a developmentally regulated activator. Oocyte extracts were rendered fully competent for nanos1 translation after the addition of a small amount of embryo extract, confirming the presence of an activator. Misexpression of Nanos1 in oocytes from unlocalized RNA results in abnormal development, highlighting the importance of TCE-mediated translational repression. Although found in prokaryotes, steric hindrance as a mechanism for negatively regulating translation is novel for a eukaryotic RNA. These observations unravel a new mode of nanos1 regulation at the post-transcriptional level that is essential for normal development.