nos-1 and nos-2, two genes related to Drosophila nanos, regulate primordial germ cell development and survival in Caenorhabditis elegans

From embryos to condensates: A developmental biologist's journey

I am the Huntington Sheldon Professor of Medical Discovery in the Department of Molecular Biology and Genetics in the School of Medicine at the Johns Hopkins University, where I have been running a lab for 30 years. Our research focusses on the molecular control of embryonic polarity and germline development, with an emphasis on asymmetric cell division and biomolecular condensates. We have uncovered mechanisms that localize proteins and RNAs in the cytoplasm by controlling protein diffusion and RNA condensation. My lab has also characterized a repressive program that launches the germline by inhibiting somatic gene expression in germline progenitors.

PRDM14 is essential for vertebrate gastrulation and safeguards avian germ cell identity

The zinc finger transcription factor PRDM14, part of the PR domain containing protein family, is critical for mammalian primordial germ cell (PGC) specification, epigenetic reprogramming and maintaining naïve pluripotency in stem cells. However, PRDM14's role in other species is not well understood. In chicken, PRDM14 is broadly expressed in the early embryo, before becoming restricted to the forming neural plate, migratory PGCs, and later, in the adult testes. To investigate the role of PRDM14 we generated two independent targeted chicken lines and bred homozygous knockout embryos. Strikingly, we found that gastrulation was disrupted in PRDM14-/- embryos, which lacked a definitive primitive streak. Transcriptomic and in situ hybridisation analyses revealed a broad loss of anterior primitive streak marker genes, coupled with downregulation of the multifunctional antagonists CHRD and CER1, and expansion of the NODAL expression domain. Further analysis of PRDM14-/- embryos revealed PGCs were still specified but significantly reduced in number, and PRDM14-/- PGCs could not be propagated in vitro. Knockdown studies in vitro confirmed that PRDM14 is essential for PGC survival and antagonises FGF-induced somatic differentiation, similar to PRDM14's role in mammalian stem cells. Taken together, our results show that in chicken, PRDM14 plays a multifunctional and essential role during embryonic development.

How germ granules promote germ cell fate

Germ cells are the only cells in the body capable of giving rise to a new organism, and this totipotency hinges on their ability to assemble membraneless germ granules. These specialized RNA and protein complexes are hallmarks of germ cells throughout their life cycle: as embryonic germ granules in late oocytes and zygotes, Balbiani bodies in immature oocytes, and nuage in maturing gametes. Decades of developmental, genetic and biochemical studies have identified protein and RNA constituents unique to germ granules and have implicated these in germ cell identity, genome integrity and gamete differentiation. Now, emerging research is defining germ granules as biomolecular condensates that achieve high molecular concentrations by phase separation, and it is assigning distinct roles to germ granules during different stages of germline development. This organization of the germ cell cytoplasm into cellular subcompartments seems to be critical not only for the flawless continuity through the germline life cycle within the developing organism but also for the success of the next generation.

Widespread regulation of the maternal transcriptome by Nanos in Drosophila

The translational repressor Nanos (Nos) regulates a single target, maternal hunchback (hb) mRNA, to govern abdominal segmentation in the early Drosophila embryo. Nos is recruited to sites in the 3' UTR of hb mRNA in collaboration with the sequence-specific RNA-binding protein Pumilio (Pum); on its own, Nos has no binding specificity. Nos is expressed at other stages of development, but very few mRNA targets that might mediate its action at these stages have been described. Nor has it been clear whether Nos is targeted to other mRNAs in concert with Pum or via other mechanisms. In this report, we identify mRNAs targeted by Nos via 2 approaches. First, we identify mRNAs depleted upon expression of a chimera bearing Nos fused to the nonsense mediated decay (NMD) factor Upf1. We find that, in addition to hb, Upf1-Nos depletes approximately 2,600 mRNAs from the maternal transcriptome in early embryos. Virtually all of these appear to be targeted in a canonical, hb-like manner in concert with Pum. In a second, more conventional approach, we identify mRNAs that are stabilized during the maternal zygotic transition (MZT) in embryos from nos- females. Most (86%) of the 1,185 mRNAs regulated by Nos are also targeted by Upf1-Nos, validating use of the chimera. Previous work has shown that 60% of the maternal transcriptome is degraded in early embryos. We find that maternal mRNAs targeted by Upf1-Nos are hypoadenylated and inefficiently translated at the ovary-embryo transition; they are subsequently degraded in the early embryo, accounting for 59% of all destabilized maternal mRNAs. We suggest that the late ovarian burst of Nos represses a large fraction of the maternal transcriptome, priming it for later degradation by other factors in the embryo.

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.

Caenorhabditis elegans germ granules accumulate hundreds of low translation mRNAs with no systematic preference for germ cell fate regulators

In animals with germ plasm, embryonic germline precursors inherit germ granules, condensates proposed to regulate mRNAs coding for germ cell fate determinants. In Caenorhabditis elegans, mRNAs are recruited to germ granules by MEG-3, a sequence non-specific RNA-binding protein that forms stabilizing interfacial clusters on germ granules. Using fluorescence in situ hybridization, we confirmed that 441 MEG-3-bound transcripts are distributed in a pattern consistent with enrichment in germ granules. Thirteen are related to transcripts reported in germ granules in Drosophila or Nasonia. The majority, however, are low-translation maternal transcripts required for embryogenesis that are not maintained preferentially in the nascent germline. Granule enrichment raises the concentration of certain transcripts in germ plasm but is not essential to regulate mRNA translation or stability. Our findings suggest that only a minority of germ granule-associated transcripts contribute to germ cell fate in C. elegans and that the vast majority function as non-specific scaffolds for MEG-3.

P-body-like condensates in the germline

P-bodies are cytoplasmic condensates that accumulate low-translation mRNAs for temporary storage before translation or degradation. P-bodies have been best characterized in yeast and mammalian tissue culture cells. We describe here related condensates in the germline of animal models. Germline P-bodies have been reported at all stages of germline development from primordial germ cells to gametes. The activity of the universal germ cell fate regulator, Nanos, is linked to the mRNA decay function of P-bodies, and spatially-regulated condensation of P-body like condensates in embryos is required to localize mRNA regulators to primordial germ cells. In most cases, however, it is not known whether P-bodies represent functional compartments or non-functional condensation by-products that arise when ribonucleoprotein complexes saturate the cytoplasm. We speculate that the ubiquity of P-body-like condensates in germ cells reflects the strong reliance of the germline on cytoplasmic, rather than nuclear, mechanisms of gene regulation.

Germ Line/Multipotency Genes Show Differential Expression during Embryonic Development of the Annelid Enchytraeus coronatus

Germ line development and the origin of the primordial germ cells (PGCs) are very variable and may occur across a range of developmental stages and in several developmental contexts. In establishing and maintaining germ line, a conserved set of genes is involved. On the other hand, these genes are expressed in multipotent/pluripotent cells that may give rise to both somatic and germline cells. To begin elucidating mechanisms by which the germ line is specified in Enchytraeus coronatus embryos, we identified twenty germline/multipotency genes, homologs of Vasa, PL10, Piwi, Nanos, Myc, Pumilio, Tudor, Boule, and Bruno, using transcriptome analysis and gene cloning, and characterized their expression by whole-mount in situ hybridization. To answer the question of the possible origin of PGCs in this annelid, we carried out an additional description of the early embryogenesis. Our results suggest that PGCs derive from small cells originating at the first two divisions of the mesoteloblasts. PGCs form two cell clusters, undergo limited proliferation, and migrate to the developing gonadal segments. In embryos and juvenile E. coronatus, homologs of the germline/multipotency genes are differentially expressed in both germline and somatic tissue including the presumptive germ cell precursors, posterior growth zone, developing foregut, and nervous system.

ZIF-1-mediated degradation of zinc finger proteins in the Caenorhabditis elegans germ line

Rapid and conditional protein depletion is the gold standard genetic tool for deciphering the molecular basis of developmental processes. Previously, we showed that by conditionally expressing the E3 ligase substrate adaptor ZIF-1 in Caenorhabditis elegans somatic cells, proteins tagged with the first CCCH Zn finger 1 (ZF1) domain from the germline regulator PIE-1 degrade rapidly, resulting in loss-of-function phenotypes. The described role of ZIF-1 is to clear PIE-1 and several other CCCH Zn finger proteins from early somatic cells, helping to enrich them in germline precursor cells. Here, we show that proteins tagged with the PIE-1 ZF1 domain are subsequently cleared from primordial germ cells (PGCs) in embryos and from undifferentiated germ cells in larvae and adults by ZIF-1. We harness germline ZIF-1 activity to degrade a ZF1-tagged fusion protein from PGCs and show that its depletion produces phenotypes equivalent to those of a null mutation. Our findings reveal that ZIF-1 transitions from degrading CCCH Zn finger proteins in somatic cells to clearing them from undifferentiated germ cells, and that ZIF-1 activity can be harnessed as a new genetic tool to study the early germline.

Widespread regulation of the maternal transcriptome by Nanos in Drosophila

The translational repressor Nanos (Nos) regulates a single target, maternal hunchback (hb) mRNA, to govern abdominal segmentation in the early Drosophila embryo. Nos is recruited specifically to sites in the 3'-UTR of hb mRNA in collaboration with the sequence-specific RNA-binding protein Pumilio (Pum); on its own, Nos has no binding specificity. Nos is expressed at other stages of development, but very few mRNA targets that might mediate its action at these stages have been described. Nor has it been clear whether Nos is targeted to other mRNAs in concert with Pum or via other mechanisms. In this report, we identify mRNAs targeted by Nos via two approaches. In the first method, we identify mRNAs depleted upon expression of a chimera bearing Nos fused to the nonsense mediated decay (NMD) factor Upf1. We find that, in addition to hb, Upf1-Nos depletes ~2600 mRNAs from the maternal transcriptome in early embryos. Virtually all of these appear to be targeted in a canonical, hb-like manner in concert with Pum. In a second, more conventional approach, we identify mRNAs that are stabilized during the maternal zygotic transition (MZT) in embryos from nos- females. Most (86%) of the 1185 mRNAs regulated by Nos are also targeted by Upf1-Nos, validating use of the chimera. Approximately 60% of mRNAs targeted by Upf1-Nos are not stabilized in the absence of Nos. However, Upf1-Nos mRNA targets are hypo-adenylated and inefficiently translated at the ovary-embryo transition, whether or not they suffer Nos-dependent degradation in the embryo. We suggest that the late ovarian burst of Nos represses a large fraction of the maternal transcriptome, priming it for later degradation by other factors during the MZT in the embryo.

Spatial organization and function of RNA molecules within phase-separated condensates in zebrafish are controlled by Dnd1

Germ granules, condensates of phase-separated RNA and protein, are organelles that are essential for germline development in different organisms. The patterning of the granules and their relevance for germ cell fate are not fully understood. Combining three-dimensional in vivo structural and functional analyses, we study the dynamic spatial organization of molecules within zebrafish germ granules. We find that the localization of RNA molecules to the periphery of the granules, where ribosomes are localized, depends on translational activity at this location. In addition, we find that the vertebrate-specific Dead end (Dnd1) protein is essential for nanos3 RNA localization at the condensates' periphery. Accordingly, in the absence of Dnd1, or when translation is inhibited, nanos3 RNA translocates into the granule interior, away from the ribosomes, a process that is correlated with the loss of germ cell fate. These findings highlight the relevance of sub-granule compartmentalization for post-transcriptional control and its importance for preserving germ cell totipotency.

ZIF-1-mediated degradation of endogenous and heterologous zinc finger proteins in the C. elegans germ line

Rapid and conditional protein depletion is the gold standard genetic tool for deciphering the molecular basis of developmental processes. Previously, we showed that by conditionally expressing the E3 ligase substrate adaptor ZIF-1 in Caenorhabditis elegans somatic cells, proteins tagged with the first CCCH Zn finger (ZF1) domain from the germline regulator PIE-1 degrade rapidly, resulting in loss-of-function phenotypes. The described role of ZIF-1 is to clear PIE-1 and several other CCCH Zn finger proteins from early somatic cells, helping to enrich them in germline precursor cells. Here, we show that proteins tagged with the PIE-1 ZF1 domain are subsequently cleared from primordial germ cells in embryos and from undifferentiated germ cells in larvae and adults by ZIF-1. We harness germline ZIF-1 activity to degrade a ZF1-tagged heterologous protein from PGCs and show that its depletion produces phenotypes equivalent to those of a null mutation. Our findings reveal that ZIF-1 switches roles from degrading CCCH Zn finger proteins in somatic cells to clearing them from undifferentiated germ cells, and that ZIF-1 activity can be harnessed as a new genetic tool to study the early germ line.

Spatial organization and function of RNA molecules within phase-separated condensates are controlled by Dnd1

Germ granules, condensates of phase-separated RNA and protein, are organelles essential for germline development in different organisms The patterning of the granules and its relevance for germ cell fate are not fully understood. Combining three-dimensional in vivo structural and functional analyses, we study the dynamic spatial organization of molecules within zebrafish germ granules. We find that localization of RNA molecules to the periphery of the granules, where ribosomes are localized depends on translational activity at this location. In addition, we find that the vertebrate-specific Dead end (Dnd1) protein is essential for nanos3 RNA localization at the condensates' periphery. Accordingly, in the absence of Dnd1, or when translation is inhibited, nanos3 RNA translocates into the granule interior, away from the ribosomes, a process that is correlated with loss of germ cell fate. These findings highlight the relevance of sub-granule compartmentalization for posttranscriptional control, and its importance for preserving germ cell totipotency.

Transcriptomic analysis of Mythimna separata ovaries and identification of genes involved in reproduction

The migratory insect Mythimna separata is a major pest of grain crops in Asia. Unfortunately, the molecular mechanisms that control and regulate reproduction in this species remain unclear. In this study, transcriptome sequencing was utilized to identify genes associated with ovary development and oogenesis. Clean sequences totaling 117.71 Gb were assembled into 178,534 unigenes with a mean length of 647.37 bp and N50 length of 837 bp. Transcriptome analysis showed that 7921 unigenes were significantly expressed in ovaries with 4403 and 3518 unigenes up- and down-regulated, respectively. Enrichment analysis with the Kyoto Encyclopedia of Genes and Genomes database suggested that 729 differentially expressed genes were significantly enriched in the top 20 pathways (q-values <0.05). Twenty genes were associated with ovary development and oogenesis and included lipases, Nanos, small heat shock proteins (sHsps) and histones; these were further verified by qRT-PCR and may play essential roles in M. separata reproduction. Collectively, our findings reveal underlying mechanisms of M.separata reproduction and may lead to RNAi-based management strategies targeting reproductive physiology.

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.

Analysis of the C. elegans Germline Stem Cell Pool

The Caenorhabditis elegans germline is an excellent model for studying the genetic and molecular regulation of stem cell self-renewal and progression of cells from a stem cell state to a differentiated state. The germline tissue is organized in an assembly line with the germline stem cell (GSC) pool at one end and differentiated gametes at the other. A simple mesenchymal niche caps the GSC pool and maintains GSCs in an undifferentiated state by signaling through the conserved Notch pathway. Notch signaling activates transcription of the key GSC regulators lst-1 and sygl-1 proteins in a gradient through the GSC pool. LST-1 and SYGL-1 proteins work with PUF RNA regulators in a self-renewal hub to maintain the GSC pool. In this chapter, we present methods for characterizing the C. elegans GSC pool and early stages of germ cell differentiation. The methods include examination of germlines in living and fixed worms, cell cycle analysis, and analysis of markers. We also discuss assays to separate mutant phenotypes that affect the stem cell vs. differentiation decision from those that affect germ cell processes more generally.

Specialized germline P-bodies are required to specify germ cell fate in Caenorhabditis elegans embryos

In animals with germ plasm, specification of the germline involves 'germ granules', cytoplasmic condensates that enrich maternal transcripts in the germline founder cells. In Caenorhabditis elegans embryos, P granules enrich maternal transcripts, but surprisingly P granules are not essential for germ cell fate specification. Here, we describe a second condensate in the C. elegans germ plasm. Like canonical P-bodies found in somatic cells, 'germline P-bodies' contain regulators of mRNA decapping and deadenylation and, in addition, the intrinsically-disordered proteins MEG-1 and MEG-2 and the TIS11-family RNA-binding protein POS-1. Embryos lacking meg-1 and meg-2 do not stabilize P-body components, misregulate POS-1 targets, mis-specify the germline founder cell and do not develop a germline. Our findings suggest that specification of the germ line involves at least two distinct condensates that independently enrich and regulate maternal mRNAs in the germline founder cells. This article has an associated 'The people behind the papers' interview.

Germline immortality relies on TRIM32-mediated turnover of a maternal mRNA activator in C. elegans

How the germ line achieves a clean transition from maternal to zygotic gene expression control is a fundamental problem in sexually reproducing organisms. Whereas several mechanisms terminate the maternal program in the soma, this combined molecular reset and handover are poorly understood for primordial germ cells (PGCs). Here, we show that GRIF-1, a TRIM32-related and presumed E3 ubiquitin ligase in Caenorhabditis elegans, eliminates the maternal cytoplasmic poly(A) polymerase (cytoPAP) complex by targeting the germline-specific intrinsically disordered region of its enzymatic subunit, GLD-2, for proteasome-mediated degradation. Interference with cytoPAP turnover in PGCs causes frequent transgenerational sterility and, eventually, germline mortality. Hence, positively acting maternal RNA regulators are cleared via the proteasome system to avoid likely interference between maternal and zygotic gene expression programs to maintain transgenerational fertility and acquire germline immortality. This strategy is likely used in all animals that preform their immortal germ line via maternally inherited germplasm determinants.

Germline stem cells in human

The germline cells are essential for the propagation of human beings, thus essential for the survival of mankind. The germline stem cells, as a unique cell type, generate various states of germ stem cells and then differentiate into specialized cells, spermatozoa and ova, for producing offspring, while self-renew to generate more stem cells. Abnormal development of germline stem cells often causes severe diseases in humans, including infertility and cancer. Primordial germ cells (PGCs) first emerge during early embryonic development, migrate into the gentile ridge, and then join in the formation of gonads. In males, they differentiate into spermatogonial stem cells, which give rise to spermatozoa via meiosis from the onset of puberty, while in females, the female germline stem cells (FGSCs) retain stemness in the ovary and initiate meiosis to generate oocytes. Primordial germ cell-like cells (PGCLCs) can be induced in vitro from embryonic stem cells or induced pluripotent stem cells. In this review, we focus on current advances in these embryonic and adult germline stem cells, and the induced PGCLCs in humans, provide an overview of molecular mechanisms underlying the development and differentiation of the germline stem cells and outline their physiological functions, pathological implications, and clinical applications.

A Comparative Cross-Platform Analysis to Identify Potential Biomarker Genes for Evaluation of Teratozoospermia and Azoospermia

Male infertility is a global public health concern. Teratozoospermia is a qualitative anomaly of spermatozoa morphology, contributing significantly to male infertility, whereas azoospermia is the complete absence of spermatozoa in the ejaculate. Thus, there is a serious need for unveiling the common origin and/or connection between both of these diseases, if any. This study aims to identify common potential biomarker genes of these two diseases via an in silico approach using a meta-analysis of microarray data. In this study, a differential expression analysis of genes was performed on four publicly available RNA microarray datasets, two each from teratozoospermia (GSE6872 and GSE6967) and azoospermia (GSE145467 and GSE25518). From the analysis, 118 DEGs were found to be common to teratozoospermia and azoospermia, and, interestingly, sperm autoantigenic protein 17 (SPA17) was found to possess the highest fold change value among all the DEGs (9.471), while coiled-coil domain-containing 90B (CCDC90B) and coiled-coil domain-containing 91 (CCDC91) genes were found to be common among three of analyses, i.e., Network Analyst, ExAtlas, and GEO2R. This observation indicates that SPA17, CCDC90B, and CCDC91 genes might have significant roles to play as potential biomarkers for teratozoospermia and azoospermia. Thus, our study opens a new window of research in this area and can provide an important theoretical basis for the diagnosis and treatment of both these diseases.

Single-cell transcriptomics identifies conserved regulators of neuroglandular lineages

Communication in bilaterian nervous systems is mediated by electrical and secreted signals; however, the evolutionary origin and relation of neurons to other secretory cell types has not been elucidated. Here, we use developmental single-cell RNA sequencing in the cnidarian Nematostella vectensis, representing an early evolutionary lineage with a simple nervous system. Validated by transgenics, we demonstrate that neurons, stinging cells, and gland cells arise from a common multipotent progenitor population. We identify the conserved transcription factor gene SoxC as a key upstream regulator of all neuroglandular lineages and demonstrate that SoxC knockdown eliminates both neuronal and secretory cell types. While in vertebrates and many other bilaterians neurogenesis is largely restricted to early developmental stages, we show that in the sea anemone, differentiation of neuroglandular cells is maintained throughout all life stages, and follows the same molecular trajectories from embryo to adulthood, ensuring lifelong homeostasis of neuroglandular cell lineages.

Maternal H3K36 and H3K27 HMTs protect germline development via regulation of the transcription factor LIN-15B

Maternally synthesized products play critical roles in the development of offspring. A premier example is the Caenorhabditis elegans H3K36 methyltransferase MES-4, which is essential for germline survival and development in offspring. How maternal MES-4 protects the germline is not well understood, but its role in H3K36 methylation hinted that it may regulate gene expression in primordial germ cells (PGCs). We tested this hypothesis by profiling transcripts from nascent germlines (PGCs and their descendants) dissected from wild-type and mes-4 mutant (lacking maternal and zygotic MES-4) larvae. mes-4 nascent germlines displayed downregulation of some germline genes, upregulation of some somatic genes, and dramatic upregulation of hundreds of genes on the X chromosome. We demonstrated that upregulation of one or more genes on the X is the cause of germline death by generating and analyzing mes-4 mutants that inherited different endowments of X chromosome(s). Intriguingly, removal of the THAP transcription factor LIN-15B from mes-4 mutants reduced X misexpression and prevented germline death. lin-15B is X-linked and misexpressed in mes-4 PGCs, identifying it as a critical target for MES-4 repression. The above findings extend to the H3K27 methyltransferase MES-2/3/6, the C. elegans version of polycomb repressive complex 2. We propose that maternal MES-4 and PRC2 cooperate to protect germline survival by preventing synthesis of germline-toxic products encoded by genes on the X chromosome, including the key transcription factor LIN-15B.

It’s Just a Phase: Exploring the Relationship Between mRNA, Biomolecular Condensates, and Translational Control

Cells spatially organize their molecular components to carry out fundamental biological processes and guide proper development. The spatial organization of RNA within the cell can both promote and result from gene expression regulatory control. Recent studies have demonstrated diverse associations between RNA spatial patterning and translation regulatory control. One form of patterning, compartmentalization in biomolecular condensates, has been of particular interest. Generally, transcripts associated with cytoplasmic biomolecular condensates—such as germ granules, stress granules, and P-bodies—are linked with low translational status. However, recent studies have identified new biomolecular condensates with diverse roles associated with active translation. This review outlines RNA compartmentalization in various condensates that occur in association with repressed or active translational states, highlights recent findings in well-studied condensates, and explores novel condensate behaviors.

NANOS3 suppresses premature spermatogonial differentiation to expand progenitors and fine-tunes spermatogenesis in mice

In the mouse testis, sperm originate from spermatogonial stem cells (SSCs). SSCs give rise to spermatogonial progenitors, which expand their population until entering the differentiation process that is precisely regulated by a fixed time-scaled program called the seminiferous cycle. Although this expansion process of progenitors is highly important, its regulatory mechanisms remain unclear. NANOS3 is an RNA-binding protein expressed in the progenitor population. We demonstrated that the conditional deletion of Nanos3 at a later embryonic stage results in the reduction of spermatogonial progenitors in the postnatal testis. This reduction was associated with the premature differentiation of progenitors. Furthermore, this premature differentiation caused seminiferous stage disagreement between adjacent spermatogenic cells, which influenced spermatogenic epithelial cycles, leading to disruption of the later differentiation pathway. Our study suggests that NANOS3 plays an important role in timing progenitor expansion to adjust to the proper differentiation timing by blocking the retinoic acid (RA) signaling pathway.

Germ granules and gene regulation in the Caenorhabditis elegans germline

The transparency of Caenorhabditis elegans provides a unique window to observe and study the function of germ granules. Germ granules are specialized ribonucleoprotein (RNP) assemblies specific to the germline cytoplasm, and they are largely conserved across Metazoa. Within the germline cytoplasm, they are positioned to regulate mRNA abundance, translation, small RNA production, and cytoplasmic inheritance to help specify and maintain germline identity across generations. Here we provide an overview of germ granules and focus on the significance of more recent observations that describe how they further demix into sub-granules, each with unique compositions and functions.

Post-transcriptional regulation of factors important for the germ line

Echinoderms are a major model system for many general aspects of biology, including mechanisms of gene regulation. Analysis of transcriptional regulation (Gene regulatory networks, direct DNA-binding of proteins to specific cis-elements, and transgenesis) has contributed to our understanding of how an embryo works. This chapter looks at post-transcriptional gene regulation in the context of how the primordial germ cells are formed, and how the factors essential for this process are regulated. Important in echinoderms, as in many embryos, is that key steps of fate determination are made post-transcriptionally. This chapter highlights these steps uncovered in sea urchins and sea stars, and links them to a general theme of how the germ line may regulate its fate differently than many of the embryo's somatic cell lineages.

Nanos Is Expressed in Somatic and Germline Tissue during Larval and Post-Larval Development of the Annelid Alitta virens

Nanos is a translational regulator that is involved in germline development in a number of diverse animals and is also involved in somatic patterning in several model organisms, including insects. Neither germline development nor somatic stem cell lines/undifferentiated multipotent cells have been characterized in the development of the annelid Alitta virens, nor is the mechanism of germ/stem-line specification generally well-understood in annelids. Here, I have cloned an Avi-nanos ortholog from A. virens and determined the spatial and temporal expression of Nanos. The results revealed that transcripts of nanos are expressed during differentiation of multiple tissues, including those that are derived from the 2d and 4d cells. In late embryonic stages and during larval development, these transcripts are expressed in the presumptive brain, ventral nerve cord, mesodermal bands, putative primordial germ cells (PGCs), and developing foregut and hindgut. During metamorphosis of the nectochaete larva into a juvenile worm, a posterior growth zone consisting of nanos-positive cells is established, and the PGCs begin to migrate. Later, the PGCs stop migrating and form a cluster of four nanos-expressing cells located immediately behind the jaws (segments 4–5). During posterior regeneration following caudal amputation, a robust Avi-nanos expression appears de novo at the site of injury and further accompanies all steps of regeneration. The obtained data suggest that blastemal cells are mostly derived from cells of the segment adjacent to the amputation site; this is consistent with the idea that the cluster of PGCs do not participate in regeneration.

The Dynamic Regulation of mRNA Translation and Ribosome Biogenesis During Germ Cell Development and Reproductive Aging

The regulation of mRNA translation, both globally and at the level of individual transcripts, plays a central role in the development and function of germ cells across species. Genetic studies using flies, worms, zebrafish and mice have highlighted the importance of specific RNA binding proteins in driving various aspects of germ cell formation and function. Many of these mRNA binding proteins, including Pumilio, Nanos, Vasa and Dazl have been conserved through evolution, specifically mark germ cells, and carry out similar functions across species. These proteins typically influence mRNA translation by binding to specific elements within the 3′ untranslated region (UTR) of target messages. Emerging evidence indicates that the global regulation of mRNA translation also plays an important role in germ cell development. For example, ribosome biogenesis is often regulated in a stage specific manner during gametogenesis. Moreover, oocytes need to produce and store a sufficient number of ribosomes to support the development of the early embryo until the initiation of zygotic transcription. Accumulating evidence indicates that disruption of mRNA translation regulatory mechanisms likely contributes to infertility and reproductive aging in humans. These findings highlight the importance of gaining further insights into the mechanisms that control mRNA translation within germ cells. Future work in this area will likely have important impacts beyond germ cell biology.

Conserved germ plasm characteristics across the Danio and Devario lineages

In many animal species, germ cell specification requires the inheritance of germ plasm, a biomolecular condensate containing maternally derived RNAs and proteins. Most studies of germ plasm composition and function have been performed in widely evolutionarily divergent model organisms, such as Caenorhabditis elegans, Drosophila, Xenopus laevis, and Danio rerio (zebrafish). In zebrafish, 12 RNAs localize to germ plasm at the furrows of the early embryo. Here, we tested for the presence of these RNAs in three additional species within the Danionin clade: Danio kyathit, Danio albolineatus, and Devario aequipinnatus. By visualizing nanos RNA, we find that germ plasm segregation patterns during early embryogenesis are conserved across these species. Ten additional germ plasm RNAs exhibit localization at the furrows of early embryos in all three non-zebrafish Danionin species, consistent with germ plasm localization. One component of zebrafish germ plasm, ca15b, lacked specific localization in embryos of the more distantly related D. aequipinnatus. Our findings show that within a subset of closely related Danionin species, the vast majority of germ plasm RNA components are conserved. At the same time, the lack of ca15b localization in D. aequipinnatus germ plasm highlights the potential for the divergence of germ plasm composition across a restricted phylogenetic space.

Pharyngeal endoderm expression of nanos1 is dispensable for craniofacial development

Nanos proteins are essential for developing primordial germ cells (PGCs) in both invertebrates and vertebrates. In invertebrates, also contribute to the patterning of the anterior-posterior axis of the embryo and the neural development. In vertebrates, however, besides the role of Nanos proteins in PGC development, the biological functions of the proteins in normal development have not yet been identified. Here, we analyzed the expression and function of nanos1 during craniofacial development in zebrafish. nanos1 was expressed in the pharyngeal endoderm and endodermal pouches essential for the development of facial skeletons and endocrine glands in the vertebrate head. However, no craniofacial defects, such as abnormal pouches, hypoplasia of the thymus, malformed facial skeletons, have been found in nanos1 knockout animals. The normal craniofacial development of nanos1 knockout animals is unlikely a consequence of the genetic redundancy of Nanos1 with Nanos2 or Nanos3 or a result of the genetic compensation for the loss of Nanos1 by Nanos2 or Nanos3 because the expression of nanos2 and nanos3 was rarely seen in the pharyngeal endoderm and endodermal pouches in wild-type and nanos1 mutant animals during craniofacial development. Our findings suggest that nanos1 expression in the pharyngeal endoderm might be dispensable for craniofacial development in zebrafish.

DAF-18/PTEN inhibits germline zygotic gene activation during primordial germ cell quiescence

Quiescence, an actively-maintained reversible state of cell cycle arrest, is not well understood. PTEN is one of the most frequently lost tumor suppressors in human cancers and regulates quiescence of stem cells and cancer cells. The sole PTEN ortholog in Caenorhabditis elegans is daf-18. In a C. elegans loss-of-function mutant for daf-18, primordial germ cells (PGCs) divide inappropriately in L1 larvae hatched into starvation conditions, in a TOR-dependent manner. Here, we further investigated the role of daf-18 in maintaining PGC quiescence in L1 starvation. We found that maternal or zygotic daf-18 is sufficient to maintain cell cycle quiescence, that daf-18 acts in the germ line and soma, and that daf-18 affects timing of PGC divisions in fed animals. Importantly, our results also implicate daf-18 in repression of germline zygotic gene activation, though not in germline fate specification. However, TOR is less important to germline zygotic gene expression, suggesting that in the absence of food, daf-18/PTEN prevents inappropriate germline zygotic gene activation and cell division by distinct mechanisms.

An improved in vivo tethering assay with single molecule FISH reveals that a nematode Nanos enhances reporter expression and mRNA stability

Robust methods are critical for testing the in vivo regulatory mechanism of RNA binding proteins. Here we report improvement of a protein-mRNA tethering assay to probe the function of an RNA binding protein in its natural context within the C. elegans adult germline. The assay relies on a dual reporter expressing two mRNAs from a single promoter and resolved by trans-splicing. The gfp reporter 3'UTR harbors functional binding elements for λN22 peptide, while the mCherry reporter 3'UTR carries mutated nonfunctional elements. This strategy enables internally controlled quantitation of reporter protein by immunofluorescence and mRNA by smFISH. To test the new system, we analyzed a C. elegans Nanos protein, NOS-3, which serves as a post-transcriptional regulator of germ cell fate. Unexpectedly, tethered NOS-3 enhanced reporter expression. We confirmed this enhancement activity with a second reporter engineered at an endogenous germline gene. NOS-3 enhancement of reporter expression was associated with its amino-terminal intrinsically disordered region, not its carboxy-terminal zinc fingers. RNA quantitation revealed that tethered NOS-3 enhances stability of the reporter mRNA. We suggest that this direct NOS-3 enhancement activity may explain a paradox: Classically Nanos proteins are expected to repress RNA, but nos-3 had been found to promote gld-1 expression, an effect that could be direct. Regardless, the new dual reporter dramatically improves in situ quantitation of reporter expression after RNA binding protein tethering to determine its molecular mechanism in a multicellular tissue.

Germ cell ribonucleoprotein granules in different clades of life: From insects to mammals

Ribonucleoprotein (RNP) granules are no newcomers in biology. Found in all life forms, ranging across taxa, these membrane-less "organelles" have been classified into different categories based on their composition, structure, behavior, function, and localization. Broadly, they can be listed as stress granules (SGs), processing bodies (PBs), neuronal granules (NGs), and germ cell granules (GCGs). Keeping in line with the topic of this review, RNP granules present in the germ cells have been implicated in a wide range of cellular functions including cellular specification, differentiation, proliferation, and so forth. The mechanisms used by them can be diverse and many of them remain partly obscure and active areas of research. GCGs can be of different types in different organisms and at different stages of development, with multiple types coexisting in the same cell. In this review, the different known subcategories of GCGs have been studied with respect to five distinct model organisms, namely, Drosophila, Caenorhabditis elegans, Xenopus, Zebrafish, and mammals. Of them, the cytoplasmic polar granules in Drosophila, P granules in C. elegans, balbiani body in Xenopus and Zebrafish, and chromatoid bodies in mammals have been specifically emphasized upon. A descriptive account of the same has been provided along with insights into our current understanding of their functional significance with respect to cellular events relating to different developmental and reproductive processes. This article is categorized under: RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Export and Localization > RNA Localization RNA in Disease and Development > RNA in Disease.

Nutritional control of postembryonic development progression and arrest in Caenorhabditis elegans

Developmental programs are under strict genetic control that favors robustness of the process. In order to guarantee the same outcome in different environmental situations, development is modulated by input pathways, which inform about external conditions. In the nematode Caenorhabditis elegans, the process of postembryonic development involves a series of stereotypic cell divisions, the progression of which is controlled by the nutritional status of the animal. C. elegans can arrest development at different larval stages, leading to cell arrest of the relevant divisions of the stage. This means that studying the nutritional control of development in C. elegans we can learn about the mechanisms controlling cell division in an in vivo model. In this work, we reviewed the current knowledge about the nutrient sensing pathways that control the progression or arrest of development in response to nutrient availability, with a special focus on the arrest at the L1 stage.

Phase Separation in Germ Cells and Development

The animal germline is an immortal cell lineage that gives rise to eggs and/or sperm each generation. Fusion of an egg and sperm, or fertilization, sets off a cascade of developmental events capable of producing an array of different cell types and body plans. How germ cells develop, function, and eventually give rise to entirely new organisms is an important question in biology. A growing body of evidence suggests that phase separation events likely play a significant and multifaceted role in germ cells and development. Here, we discuss the organization, dynamics, and potential functions of phase-separated compartments in germ cells and examine the various ways in which phase separation might contribute to the development of multicellular organisms.

An Interaction Network of RNA-Binding Proteins Involved in Drosophila Oogenesis

During Drosophila oogenesis, the localization and translational regulation of maternal transcripts relies on RNA-binding proteins (RBPs). Many of these RBPs localize several mRNAs and may have additional direct interaction partners to regulate their functions. Using immunoprecipitation from whole Drosophila ovaries coupled to mass spectrometry, we examined protein-protein associations of 6 GFP-tagged RBPs expressed at physiological levels. Analysis of the interaction network and further validation in human cells allowed us to identify 26 previously unknown associations, besides recovering several well characterized interactions. We identified interactions between RBPs and several splicing factors, providing links between nuclear and cytoplasmic events of mRNA regulation. Additionally, components of the translational and RNA decay machineries were selectively co-purified with some baits, suggesting a mechanism for how RBPs may regulate maternal transcripts. Given the evolutionary conservation of the studied RBPs, the interaction network presented here provides the foundation for future functional and structural studies of mRNA localization across metazoans.

mRNA localization is linked to translation regulation in the Caenorhabditis elegans germ lineage

Caenorhabditis elegans early embryos generate cell-specific transcriptomes despite lacking active transcription, thereby presenting an opportunity to study mechanisms of post-transcriptional regulatory control. We observed that some cell-specific mRNAs accumulate non-homogenously within cells, localizing to membranes, P granules (associated with progenitor germ cells in the P lineage) and P-bodies (associated with RNA processing). The subcellular distribution of transcripts differed in their dependence on 3'UTRs and RNA binding proteins, suggesting diverse regulatory mechanisms. Notably, we found strong but imperfect correlations between low translational status and P granule localization within the progenitor germ lineage. By uncoupling translation from mRNA localization, we untangled a long-standing question: Are mRNAs directed to P granules to be translationally repressed, or do they accumulate there as a consequence of this repression? We found that translational repression preceded P granule localization and could occur independently of it. Further, disruption of translation was sufficient to send homogenously distributed mRNAs to P granules. These results implicate transcriptional repression as a means to deliver essential maternal transcripts to the progenitor germ lineage for later translation.

Biology of the Caenorhabditis elegans Germline Stem Cell System

Stem cell systems regulate tissue development and maintenance. The germline stem cell system is essential for animal reproduction, controlling both the timing and number of progeny through its influence on gamete production. In this review, we first draw general comparisons to stem cell systems in other organisms, and then present our current understanding of the germline stem cell system in Caenorhabditis elegans In contrast to stereotypic somatic development and cell number stasis of adult somatic cells in C. elegans, the germline stem cell system has a variable division pattern, and the system differs between larval development, early adult peak reproduction and age-related decline. We discuss the cell and developmental biology of the stem cell system and the Notch regulated genetic network that controls the key decision between the stem cell fate and meiotic development, as it occurs under optimal laboratory conditions in adult and larval stages. We then discuss alterations of the stem cell system in response to environmental perturbations and aging. A recurring distinction is between processes that control stem cell fate and those that control cell cycle regulation. C. elegans is a powerful model for understanding germline stem cells and stem cell biology.

Identification of a germline-expression promoter for genome editing in Bombyx mori

Identification of stage- and tissue-specific cis-regulatory elements will enable more precise genomic editing. In previous studies of the silkworm Bombyx mori, we identified and characterized several tissue- and sex-specific cis-regulatory elements using transgenic technology, including a female- and fat body-specific promoter, vitellogenin, testis-specific promoters, Radial spoke head 1 (BmR1) and beta-tubulin 4 (Bmβ4). Here we report a cis-regulatory element specific for a somatic and germ cell-expressed promoter, nanos (Bmnos). We investigated activities of three truncated promoter sequences upstream of the transcriptional initiation site sequences of Bmnos in vitro (nos-0.6kb, nos-1kb and nos-2kb) and in vivo (nos-2kb). In BmN cultured cells, all three lengths drove expression of the gene encoding enhanced green fluorescence protein (EGFP), although nos-2kb had the highest fluorescence activity. In transgenic silkworms, nos-2kb drove EGFP expression at the early embryonic stage, and fluorescence was concentrated in the gonads at later embryonic stages. In addition, this cis-regulatory element was not sex differentiated. The fluorescence intensity gradually weakened following the larval developmental stage in the gonads and were broadly expressed in the whole body. The nos-2kb promoter drove the Cas9 system with efficiency comparable to that of the broad-spectrum strong IE1 promoter. These results indicate that Bmnos is an effective endogenous cis-regulatory element in the early embryo and in the gonad that can be used in applications involving the clustered, regularly interspaced, short palindromic repeats (CRISPR)/Cas9 system.

Centrosome Aurora A regulates RhoGEF ECT-2 localisation and ensures a single PAR-2 polarity axis in C. elegans embryos

Proper establishment of cell polarity is essential for development. In the one-cell C. elegans embryo, a centrosome-localised signal provides spatial information for polarity establishment. It is hypothesised that this signal causes local inhibition of the cortical actomyosin network, and breaks symmetry to direct partitioning of the PAR proteins. However, the molecular nature of the centrosomal signal that triggers cortical anisotropy in the actomyosin network to promote polarity establishment remains elusive. Here, we discover that depletion of Aurora A kinase (AIR-1 in C. elegans) causes pronounced cortical contractions on the embryo surface, and this creates more than one PAR-2 polarity axis. This function of AIR-1 appears to be independent of its role in microtubule nucleation. Importantly, upon AIR-1 depletion, centrosome positioning becomes dispensable in dictating the PAR-2 axis. Moreover, we uncovered that a Rho GEF, ECT-2, acts downstream of AIR-1 in regulating contractility and PAR-2 localisation, and, notably, AIR-1 depletion influences ECT-2 cortical localisation. Overall, this study provides a novel insight into how an evolutionarily conserved centrosome Aurora A kinase inhibits promiscuous PAR-2 domain formation to ensure singularity in the polarity establishment axis.

P Granules Protect RNA Interference Genes from Silencing by piRNAs

P granules are perinuclear condensates in C. elegans germ cells proposed to serve as hubs for self/non-self RNA discrimination by Argonautes. We report that a mutant (meg-3 meg-4) that does not assemble P granules in primordial germ cells loses competence for RNA-interference over several generations and accumulates silencing small RNAs against hundreds of endogenous genes, including the RNA-interference genes rde-11 and sid-1. In wild type, rde-11 and sid-1 transcripts are heavily targeted by piRNAs and accumulate in P granules but maintain expression. In the primordial germ cells of meg-3 meg-4 mutants, rde-11 and sid-1 transcripts disperse in the cytoplasm with the small RNA biogenesis machinery, become hyper-targeted by secondary sRNAs, and are eventually silenced. Silencing requires the PIWI-class Argonaute PRG-1 and the nuclear Argonaute HRDE-1 that maintains trans-generational silencing of piRNA targets. These observations support a "safe harbor" model for P granules in protecting germline transcripts from piRNA-initiated silencing.

Molecular Cloning and Sexually Dimorphic Expression Analysis of nanos2 in the Sea Urchin, Mesocentrotus nudus

Sea urchin (Mesocentrotus nudus) is an economically important mariculture species in China and the gonads are the solely edible parts to human. The molecular mechanisms of gonad development have attracted increasing attention in recent years. Although the nanos2 gene has been identified as a germ cell marker in several invertebrates, little is known about nanos2 in adult sea urchins. Hereinto, we report the characterization of Mnnano2, an M. nudus nanos2 homology gene. Mnnanos2 is a maternal factor and can be detected continuously during embryogenesis and early ontogeny. Real-time quantitative PCR (RT-qPCR) and section in situ hybridization (ISH) analysis revealed a dynamic and sexually dimorphic expression pattern of Mnnano2 in the gonads. Its expression reached the maximal level at Stage 2 along with the gonad development in both ovary and testis. In the ovary, Mnnanos2 is specifically expressed in germ cells. In contrast, Mnnanos2 is expressed in both nutritive phagocytes (NP) cells and male germ cells in testis. Moreover, knocking down of Mnnanos2 by means of RNA interference (RNAi) reduced nanos2 and boule expression but conversely increased the expression of foxl2. Therefore, our data suggest that Mnnanos2 may serve as a female germ cell marker during gametogenesis and provide chances to uncover its function in adult sea urchin.

MRG-1 is required for both chromatin-based transcriptional silencing and genomic integrity of primordial germ cells in Caenorhabditis elegans

In Caenorhabditis elegans, germline cells remain transcriptionally silenced during embryogenesis. The transcriptional silencing is achieved by two different mechanisms: One is the inhibition of RNA polymerase II in P2-P4 cells at the establishment stage, and another is chromatin-based silencing in two primordial germ cells (PGCs) at the maintenance stage; however, the molecular mechanism underlying chromatin-based silencing is less understood. We investigated the role of the chromodomain protein MRG-1, which is an essential maternal factor for germline development, in transcriptional silencing in PGCs. PGCs lacking maternal MRG-1 showed increased levels of two histone modifications (H3K4me2 and H4K16ac), which are epigenetic markers for active transcription, and precocious activation of germline promoters. Loss of MES-4, a H3K36 methyltransferase, also caused similar derepression of the germline genes in PGCs, suggesting that both MRG-1 and MES-4 function in chromatin-based silencing in PGCs. In addition, the mrg-1 null mutant showed abnormal chromosome structures and a decrease in homologous recombinase RAD-51 foci in PGCs, but the mes-4 null mutant did not show such phenotypes. Taken together, we propose that MRG-1 has two distinct functions: chromatin-based transcriptional silencing and preserving genomic integrity at the maintenance stage of PGCs.

Developmental Control of the Cell Cycle: Insights from Caenorhabditis elegans

During animal development, a single fertilized egg forms a complete organism with tens to trillions of cells that encompass a large variety of cell types. Cell cycle regulation is therefore at the center of development and needs to be carried out in close coordination with cell differentiation, migration, and death, as well as tissue formation, morphogenesis, and homeostasis. The timing and frequency of cell divisions are controlled by complex combinations of external and cell-intrinsic signals that vary throughout development. Insight into how such controls determine in vivo cell division patterns has come from studies in various genetic model systems. The nematode Caenorhabditis elegans has only about 1000 somatic cells and approximately twice as many germ cells in the adult hermaphrodite. Despite the relatively small number of cells, C. elegans has diverse tissues, including intestine, nerves, striated and smooth muscle, and skin. C. elegans is unique as a model organism for studies of the cell cycle because the somatic cell lineage is invariant. Somatic cells divide at set times during development to produce daughter cells that adopt reproducible developmental fates. Studies in C. elegans have allowed the identification of conserved cell cycle regulators and provided insights into how cell cycle regulation varies between tissues. In this review, we focus on the regulation of the cell cycle in the context of C. elegans development, with reference to other systems, with the goal of better understanding how cell cycle regulation is linked to animal development in general.

Expression and intracellular localization of Nanos2-homologue protein in primordial germ cells and spermatogonial stem cells

SummaryThe decision by germ cells to differentiate and undergo either oogenesis or spermatogenesis takes place during embryonic development and Nanos plays an important role in this process. The present study was designed to investigate the expression patterns in rat of Nanos2-homologue protein in primordial germ cells (PGCs) over different embryonic developmental days as well as in spermatogonial stem cells (SSCs). Embryos from three different embryonic days (E8.5, E10.5, E11.5) and SSCs were isolated and used to detect Nanos2-homologue protein using immunocytochemistry, western blotting, reverse transcription polymerase chain reaction (RT-PCR) and flow cytometry. Interestingly, Nanos2 expression was detected in PGCs at day E11.5 onwards and up to colonization of PGCs in the genital ridge of fetal gonads. No Nanos2 expression was found in PGCs during early embryonic days (E8.5 and 10.5). Furthermore, immunohistochemical and immunofluorescence data revealed that Nanos2 expression was restricted within a subpopulation of undifferentiated spermatogonia (As, single type A SSCs and Apr, paired type A SSCs). The same results were confirmed by our western blot and RT-PCR data, as Nanos2 protein and transcripts were detected only in PGCs from day E11.5 and in undifferentiated spermatogonia (As and Apr). Furthermore, Nanos2-positive cells were also immunodetected and sorted using flow cytometry from the THY1-positive SSCs population, and this strengthened the idea that these cells are stem cells. Our findings suggested that stage-specific expression of Nanos2 occurred on different embryonic developmental days, while during the postnatal period Nanos2 expression is restricted to As and Apr SSCs.

Drosophila mRNA Localization During Later Development: Past, Present, and Future

Multiple mechanisms tightly regulate mRNAs during their transcription, translation, and degradation. Of these, the physical localization of mRNAs to specific cytoplasmic regions is relatively easy to detect; however, linking localization to functional regulatory roles has been more difficult to establish. Historically, Drosophila melanogaster is a highly effective model to identify localized mRNAs and has helped identify roles for this process by regulating various cell activities. The majority of the well-characterized functional roles for localizing mRNAs to sub-regions of the cytoplasm have come from the Drosophila oocyte and early syncytial embryo. At present, relatively few functional roles have been established for mRNA localization within the relatively smaller, differentiated somatic cell lineages characteristic of later development, beginning with the cellular blastoderm, and the multiple cell lineages that make up the gastrulating embryo, larva, and adult. This review is divided into three parts—the first outlines past evidence for cytoplasmic mRNA localization affecting aspects of cellular activity post-blastoderm development in Drosophila. The majority of these known examples come from highly polarized cell lineages such as differentiating neurons. The second part considers the present state of affairs where we now know that many, if not most mRNAs are localized to discrete cytoplasmic regions in one or more somatic cell lineages of cellularized embryos, larvae or adults. Assuming that the phenomenon of cytoplasmic mRNA localization represents an underlying functional activity, and correlation with the encoded proteins suggests that mRNA localization is involved in far more than neuronal differentiation. Thus, it seems highly likely that past-identified examples represent only a small fraction of localization-based mRNA regulation in somatic cells. The last part highlights recent technological advances that now provide an opportunity for probing the role of mRNA localization in Drosophila, moving beyond cataloging the diversity of localized mRNAs to a similar understanding of how localization affects mRNA activity.

ES-mediated chimera analysis revealed requirement of DDX6 for NANOS2 localization and function in mouse germ cells

In embryonic male germ cells, the RNA-binding protein NANOS2 recruits its target RNAs to processing bodies (P-bodies), where they are repressed. This process is necessary to promote male-type germ cell differentiation. However, it remains unclear whether all NANOS2 functions depend on P-bodies. To address this question, we established ES cell lines containing a germ cell-specific inducible Cre and reporter together with the floxed Ddx6 allele. We deleted the Ddx6 gene by administering tamoxifen to chimeric embryos containing germ cells derived from recombinant ES cells. DDX6-null germ cells exhibited both similar and distinct defects from those observed in NANOS2-null germ cells. These results demonstrate that NANOS2 function is carried out via both P-body-dependent and -independent mechanisms. RNA-seq analyses further supported the phenotypic differences between DDX6-null and NANOS2-null germ cells, and indicated distinct molecular cascades involved in NANOS2-mediated gene regulation.

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.