Vasa protein expression is restricted to the small micromeres of the sea urchin, but is inducible in other lineages early in development

mRNA splicing variants of the transcription factor Blimp1 differentially regulate germline genes in echinoderms

Germ cell specification is an essential step in sexually reproducing animals. Echinoderms possess diverse representatives of the main mechanisms that result in this cell fate determination. Sea urchins use an inherited mechanism, whereas sea stars rely on the ancestral, induced mechanism. Blimp1 (B lymphocyte-induced maturation protein-1) is a transcriptional regulator reported in mice to function in the induction of germline cells. Here, we identify the dynamic function of Blimp1 during development in a comparative approach using the purple sea urchin, Strongylocentrotus purpuratus (inherited germline) and the batstar, Patiria miniata (induced germline). We found that Blimp1 is important for germ cell specification in both species and that multiple Blimp1 isoforms result from differential mRNA splicing in each animal. Each isoform of Blimp1 functions in distinct expression of germline determinants, including Vasa and Nanos. These results show that Blimp1 is a conserved and key regulator for germ cell specification, but divergent in function as a result of post-transcriptional modification. Overall, we conclude that Blimp1 is an intersectional node in diverse germline specification strategies and supports the concept that differential mRNA splicing is an essential mechanism in germ cell formation.

Visualizing metabolic regulation using metabolic biosensors during sea urchin embryogenesis

Growing evidence suggests that metabolic regulation directly influences cellular function and development and thus may be more dynamic than previously expected. In vivo and in real-time analysis of metabolite activities during development is crucial to test this idea directly. In this study, we employ two metabolic biosensors to track the dynamics of pyruvate and oxidative phosphorylation (Oxphos) during the early embryogenesis of the sea urchin. A pyruvate sensor, PyronicSF, shows the signal enrichment on the mitotic apparatus, which is consistent with the localization patterns of the corresponding enzyme, pyruvate kinase (PKM). The addition of pyruvate increases the PyronicSF signal, while PKM knockdown decreases its signal, responding to the pyruvate level in the cell. Similarly, a ratio-metric sensor, Grx-roGFP, that reads the redox potential of the cell responds to DTT and H2O2, the known reducer and inducer of Oxphos. These observations suggest that these metabolic biosensors faithfully reflect the metabolic status in the cell during embryogenesis. The time-lapse imaging of these biosensors suggests that pyruvate and Oxphos levels change both spatially and temporarily during embryonic development. Pyruvate level is increased first in micromeres compared to other blastomeres at the 16-cell stage and remains high in ectoderm while decreasing in endomesoderm during gastrulation. In contrast, the Oxphos signal first decreases in micromeres at the 16-cell stage, while it increases in the endomesoderm during gastrulation, showing the opposite trend of the pyruvate signal. These results suggest that metabolic regulation is indeed both temporally and spatially dynamic during embryogenesis, and these biosensors are a valuable tool to monitor metabolic activities in real-time in developing embryos.

The evolutionary modifications of a GoLoco motif in the AGS protein facilitate micromere formation in the sea urchin embryo

The evolutionary introduction of asymmetric cell division (ACD) into the developmental program facilitates the formation of a new cell type, contributing to developmental diversity and, eventually, to species diversification. The micromere of the sea urchin embryo may serve as one of those examples: An ACD at the 16-cell stage forms micromeres unique to echinoids among echinoderms. We previously reported that a polarity factor, Activator of G-protein Signaling (AGS), plays a crucial role in micromere formation. However, AGS and its associated ACD factors are present in all echinoderms and across most metazoans. This raises the question of what evolutionary modifications of AGS protein or its surrounding molecular environment contributed to the evolutionary acquisition of micromeres only in echinoids. In this study, we learned that the GoLoco motifs at the AGS C-terminus play critical roles in regulating micromere formation in sea urchin embryos. Further, other echinoderms' AGS or chimeric AGS that contain the C-terminus of AGS orthologs from various organisms showed varied localization and function in micromere formation. In contrast, the sea star or the pencil urchin orthologs of other ACD factors were consistently localized at the vegetal cortex in the sea urchin embryo, suggesting that AGS may be a unique variable factor that facilitates ACD diversity among echinoderms. Consistently, sea urchin AGS appears to facilitate micromere-like cell formation and accelerate the enrichment timing of the germline factor Vasa during early embryogenesis of the pencil urchin, an ancestral type of sea urchin. Based on these observations, we propose that the molecular evolution of a single polarity factor facilitates ACD diversity while preserving the core ACD machinery among echinoderms and beyond during evolution.

Vasa nucleates asymmetric translation along the mitotic spindle during unequal cell divisions

mRNA translation on the spindle is hypothesized to be an essential strategy for the localized production of cell regulators. This mechanism may be important particularly in early embryonic cells, which have a large diffusion volume and that undergo rapid cell divisions. Evidence to test such a hypothesis has been, however, limited. Here, we use an embryo with both symmetric and asymmetric cell divisions and manipulate Vasa protein, an RNA-helicase, on the spindle in live sea urchin embryos. We learned that the spindle serves as a major site of translation and that protein synthesis within a single spindle can be unequal and help drive asymmetric cell divisions during embryogenesis. Recruiting Vasa to the ectopic sub-cellular region induced a new site of translation, disturbed asymmetric translation on the spindle, and changed the cell fate. Based on these observations, we conclude that Vasa functions in localized translation, which provides a spatiotemporal control in protein synthesis and is essential for rapidly developing embryonic cells.

Association of mRNA translation with the mitotic spindle is thought to be involved in localized production of cell fate determinants. Here, the authors show Vasa facilitates asymmetric translation, which contributes to differential regulation during sea urchin embryogenesis.

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.

A conserved node in the regulation of Vasa between an induced and an inherited program of primordial germ cell specification

Primordial germ cells (PGCs) are specified by diverse mechanisms in early development. In some animals, PGCs are specified via inheritance of maternal determinants, while in others, in a process thought to represent the ancestral mode, PGC fate is induced by cell interactions. Although the terminal factors expressed in specified germ cells are widely conserved, the mechanisms by which these factors are regulated can be widely diverse. Here we show that a post-translational mechanism of germ cell specification is conserved between two echinoderm species thought to employ divergent germ line segregation strategies. Sea urchins segregate their germ line early by an inherited mechanism. The DEAD-box RNA - helicase Vasa, a conserved germline factor, becomes enriched in the PGCs by degradation in future somatic cells by the E3-ubiquitin-ligase Gustavus (Gustafson et al., 2011). This post-translational activity occurs early in development, substantially prior to gastrulation. Here we test this process in germ cell specification of sea star embryos, which use inductive signaling mechanisms after gastrulation for PGC fate determination. We find that Vasa-GFP protein becomes restricted to the PGCs in the sea star even though the injected mRNA is present throughout the embryo. Gustavus depletion, however, results in uniform accumulation of the protein. These data demonstrate that Gustavus-mediated Vasa turnover in somatic cells is conserved between species with otherwise divergent PGC specification mechanisms. Since Gustavus was originally identified in Drosophila melanogaster to have similar functions in Vasa regulation (Kugler et al., 2010), we conclude that this node of Vasa regulation in PGC formation is ancestral and evolutionarily transposable from the ancestral, induced PGC specification program to an inherited PGC specification mechanism.

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.

Somatic cell conversion to a germ cell lineage: A violation or a revelation?

The germline is unique and immortal (or at least its genome is). It is able to perform unique jobs (meiosis) and is selected for genetic changes. Part of being this special also means that entry into the germline club is restricted and cells of the soma are always left out. However, the recent evidence from multiple animals now suggests that somatic cells may join the club and become germline cells in an animal when the original germline is removed. This "violation" may have garnered acceptance by the observation that iPScells, originating experimentally from somatic cells of an adult, can form reproductively successful eggs and sperm, all in vitro. Each of the genes and their functions used to induce pluripotentiality are found normally in the cell and the in vitro conditions to direct germline commitment replicate conditions in vivo. Here, we discuss evidence from three different animals: an ascidian, a segmented worm, and a sea urchin; and that the cells of a somatic cell lineage can convert into the germline in vivo. We discuss the consequences of such transitions and provide thoughts as how this process may have equal precision to the original germline formation of an embryo.

Developmental single-cell transcriptomics in the Lytechinus variegatus sea urchin embryo

Using scRNA-seq coupled with computational approaches, we studied transcriptional changes in cell states of sea urchin embryos during development to the larval stage. Eighteen closely spaced time points were taken during the first 24 h of development of Lytechinus variegatus (Lv). Developmental trajectories were constructed using Waddington-OT, a computational approach to 'stitch' together developmental time points. Skeletogenic and primordial germ cell trajectories diverged early in cleavage. Ectodermal progenitors were distinct from other lineages by the 6th cleavage, although a small percentage of ectoderm cells briefly co-expressed endoderm markers that indicated an early ecto-endoderm cell state, likely in cells originating from the equatorial region of the egg. Endomesoderm cells also originated at the 6th cleavage and this state persisted for more than two cleavages, then diverged into distinct endoderm and mesoderm fates asynchronously, with some cells retaining an intermediate specification status until gastrulation. Seventy-nine out of 80 genes (99%) examined, and included in published developmental gene regulatory networks (dGRNs), are present in the Lv-scRNA-seq dataset and are expressed in the correct lineages in which the dGRN circuits operate.

Light-induced, spatiotemporal control of protein in the developing embryo of the sea urchin

Differential protein regulation is a critical biological process that regulates cellular activity and controls cell fate determination. It is especially important during early embryogenesis when post-transcriptional events predominate differential fate specification in many organisms. Light-induced approaches have been a powerful technology to interrogate protein functions with temporal and spatial precision, even at subcellular levels within a cell by controlling laser irradiation on the confocal microscope. However, application and efficacy of these tools need to be tested for each model system or for the cell type of interest because of the complex nature of each system. Here, we introduce two types of light-induced approaches to track and control proteins at a subcellular level in the developing embryo of the sea urchin. We found that the photoconvertible fluorescent protein Kaede is highly efficient to distinguish pre-existing and newly synthesized proteins with no apparent phototoxicity, even when interrogating proteins associated with the mitotic spindle. Further, chromophore-assisted light inactivation (CALI) using miniSOG successfully inactivated target proteins of interest in the vegetal cortex and selectively delayed or inhibited asymmetric cell division. Overall, these light-induced manipulations serve as important molecular tools to identify protein function for for subcellular interrogations in developing embryos.

CRISPR-Cas9 editing of non-coding genomic loci as a means of controlling gene expression in the sea urchin

We seek to manipulate gene function here through CRISPR-Cas9 editing of cis-regulatory sequences, rather than the more typical mutation of coding regions. This approach would minimize secondary effects of cellular responses to nonsense mediated decay pathways or to mutant protein products by premature stops. This strategy also allows for reducing gene activity in cases where a complete gene knockout would result in lethality, and it can be applied to the rapid identification of key regulatory sites essential for gene expression. We tested this strategy here with genes of known function as a proof of concept, and then applied it to examine the upstream genomic region of the germline gene Nanos2 in the sea urchin, Strongylocentrotus purpuratus. We first used CRISPR-Cas9 to target established genomic cis-regulatory regions of the skeletogenic cell transcription factor, Alx1, and the TGF-β signaling ligand, Nodal, which produce obvious developmental defects when altered in sea urchin embryos. Importantly, mutation of cis-activator sites (Alx1) and cis-repressor sites (Nodal) result in the predicted decreased and increased transcriptional output, respectively. Upon identification of efficient gRNAs by genomic mutations, we then used the same validated gRNAs to target a deadCas9-VP64 transcriptional activator to increase Nodal transcription directly. Finally, we paired these new methodologies with a more traditional, GFP reporter construct approach to further our understanding of the transcriptional regulation of Nanos2, a key gene required for germ cell identity in S. purpuratus. With a series of reporter assays, upstream Cas9-promoter targeted mutagenesis, coupled with qPCR and in situ RNA hybridization, we concluded that the promoter of Nanos2 drives strong mRNA expression in the sea urchin embryo, indicating that its primordial germ cell (PGC)-specific restriction may rely instead on post-transcriptional regulation. Overall, we present a proof-of-principle tool-kit of Cas9-mediated manipulations of promoter regions that should be applicable in most cells and embryos for which CRISPR-Cas9 is employed.

Evidence for BMP-mediated specification of primordial germ cells in an indirect-developing hemichordate

Primordial germ cells (PGCs) are specified during development by either one of two major mechanisms, the preformation mode or the inductive mode. Because the inductive mode is widely employed by many bilaterians and early branching metazoan lineages, it has been postulated as an ancestral mechanism. However, among the deuterostome species that have been studied, invertebrate chordates use the preformation mode, while many vertebrate and echinoderm species are known to utilize an inductive mechanism, thus leaving the evolutionary history of PGC specification in the deuterostome lineage unclear. Hemichordates are the sister phylum of echinoderms, and together they form a clade called Ambulacraria that represents the closest group to the chordates. Thus, research in hemichordates is highly informative for resolving this issue. In this study, we investigate the developmental process of PGCs in an indirect-developing hemichordate, Ptychodera flava. We show that maternal transcripts of the conserved germline markers vasa, nanos, and piwi1 are ubiquitously distributed in early P. flava embryos, and these genes are coexpressed specifically in the dorsal hindgut starting from the gastrula stage. Immunostaining revealed that Vasa protein is concentrated toward the vegetal pole in early P. flava embryos, and it is restricted to cells in the dorsal hindgut of gastrulae and newly hatched larvae. The Vasa-positive cells later contribute to the developing trunk coeloms of the larvae and eventually reside in the adult gonads. We further show that bone morphogenetic protein (BMP) signaling is required to activate expression of the germline determinants in the gastrula hindgut, suggesting that PGC specification is induced by BMP signaling in P. flava. Our data support the hypothesis that the inductive mode is a conserved mechanism in Ambulacraria, which might even trace back to the common ancestor of Deuterostomes.

Hedgehog signaling is required for endomesodermal patterning and germ cell development in the sea anemone Nematostella vectensis

Two distinct mechanisms for primordial germ cell (PGC) specification are observed within Bilatera: early determination by maternal factors or late induction by zygotic cues. Here we investigate the molecular basis for PGC specification in Nematostella, a representative pre-bilaterian animal where PGCs arise as paired endomesodermal cell clusters during early development. We first present evidence that the putative PGCs delaminate from the endomesoderm upon feeding, migrate into the gonad primordia, and mature into germ cells. We then show that the PGC clusters arise at the interface between hedgehog1 and patched domains in the developing mesenteries and use gene knockdown, knockout and inhibitor experiments to demonstrate that Hh signaling is required for both PGC specification and general endomesodermal patterning. These results provide evidence that the Nematostella germline is specified by inductive signals rather than maternal factors, and support the existence of zygotically-induced PGCs in the eumetazoan common ancestor.

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.

Maternal control of early patterning in sea urchin embryos

Sea urchin development has been studied extensively for more than a century and considered regulative since the first experimental evidence. Further investigations have repeatedly supported this standpoint by revealing the presence of inductive mechanisms that alter cell fate decisions at early cleavage stages and flexibility of development in response to environmental conditions. Some features indicate that sea urchin development is not completely regulative, but actually includes determinative events. In 16-cell embryos, mesomeres and macromeres represent multipotency, while the cell fate of most vegetal micromeres is restricted. It is known that the mature sea urchin eggs are polarized by the asymmetrical distribution of some maternal mRNAs and proteins. Spatially-distributed maternal factors are necessary for the orientation of the primary animal-vegetal axis, which is established by both maternal and zygotic mechanisms later in development. The secondary dorsal-ventral axis is conditionally specified later in development. Dorsal-ventral polarity is very liable during the early cleavages, though more recent data argue that its direction may be oriented by maternal asymmetry. In this review, we focus on the role of maternal factors in initial embryonic patterning during the first cleavages of sea urchin embryos before activation of the embryonic genome.

The gene regulatory control of sea urchin gastrulation

The cell behaviors associated with gastrulation in sea urchins have been well described. More recently, considerable progress has been made in elucidating gene regulatory networks (GRNs) that underlie the specification of early embryonic territories in this experimental model. This review integrates information from these two avenues of work. I discuss the principal cell movements that take place during sea urchin gastrulation, with an emphasis on molecular effectors of the movements, and summarize our current understanding of the gene regulatory circuitry upstream of those effectors. A case is made that GRN biology can provide a causal explanation of gastrulation, although additional analysis is needed at several levels of biological organization in order to provide a deeper understanding of this complex morphogenetic process.

A tumor suppressor Retinoblastoma1 is essential for embryonic development in the sea urchin

BACKGROUND

Embryonic cells and cancer cells share various cellular characteristics important for their functions. It has been thus proposed that similar mechanisms of regulation may be present in these otherwise disparate cell types.

RESULTS

To explore how regulative embryonic cells are fundamentally different from cancerous cells, we report here that a fine balance of a tumor suppressor protein Retinoblastoma1 (Rb1) and a germline factor Vasa are important for proper cell proliferation and differentiation of the somatic cells during embryogenesis of the sea urchin. Rb1 knockdown blocked embryonic development and induced Vasa accumulation in the entire embryo, while its overexpression resulted in a smaller-sized embryo with differentiated body structures. These results suggest that a titrated level of Rb1 protein may be essential for a proper balance of cell proliferation and differentiation during development. Vasa knockdown or overexpression, on the other hand, reduced or increased Rb1 protein expression, respectively.

CONCLUSIONS

Taken together, it appears that Vasa protein positively regulates Rb1 protein while Rb1 protein negatively regulates Vasa protein, balancing the act of these two antagonistic molecules in somatic cells. This mechanism may provide a fine control of cell proliferation and differentiation, which is essential for regulative embryonic development.

Evolutionary modification of AGS protein contributes to formation of micromeres in sea urchins

Evolution is proposed to result, in part, from acquisition of new developmental programs. One such example is the appearance of the micromeres in a sea urchin that form by an asymmetric cell division at the 4th embryonic cleavage and function as a major signaling center in the embryo. Micromeres are not present in other echinoderms and thus are  considered as a derived feature, yet its acquisition mechanism is unknown. Here, we report that the polarity factor AGS and its associated proteins are responsible for micromere formation. Evolutionary modifications of AGS protein seem to have provided the cortical recruitment and binding of AGS to the vegetal cortex, contributing to formation of micromeres in the sea urchins. Indeed, introduction of sea urchin AGS into the sea star embryo induces asymmetric cell divisions, suggesting that the molecular evolution of AGS protein is key in the transition of echinoderms to micromere formation and the current developmental style of sea urchins not seen in other echinoderms.

Distinct transcriptional regulation of Nanos2 in the germ line and soma by the Wnt and delta/notch pathways

Specification of the primordial germ cells (PGCs) is essential for sexually reproducing animals. Although the mechanisms of PGC specification are diverse between organisms, the RNA binding protein Nanos is consistently required in the germ line in all species tested. How Nanos is selectively expressed in the germ line, however, remains largely elusive. We report that in sea urchin embryos, the early expression of Nanos2 in the PGCs requires the maternal Wnt pathway. During gastrulation, however, Nanos2 expression expands into adjacent somatic mesodermal cells and this secondary Nanos expression instead requires Delta/Notch signaling through the forkhead family member FoxY. Each of these transcriptional regulators were tested by chromatin immunoprecipitation analysis and found to directly interact with a DNA locus upstream of Nanos2. Given the conserved importance of Nanos in germ line specification, and the derived character of the micromeres and small micromeres in the sea urchin, we propose that the ancestral mechanism of Nanos2 expression in echinoderms was by induction in mesodermal cells during gastrulation.

Methods to label, isolate, and image sea urchin small micromeres, the primordial germ cells (PGCs)

Small micromeres of the sea urchin are believed to be primordial germ cells (PGCs), fated to give rise to sperm or eggs in the adult. Sea urchin PGCs are formed at the fifth cleavage, undergo one additional division during blastulation, and migrate to the coelomic pouches of the pluteus larva. The goal of this chapter is to detail classical and modern techniques used to analyze primordial germ cell specification, gene expression programs, and cell behaviors in fixed and live embryos. The transparency of the sea urchin embryo enables both live imaging techniques and in situ RNA hybridization and immunolabeling for a detailed molecular characterization of these cells. Four approaches are presented to highlight small micromeres with fluorescent molecules for analysis by live and fixed cell microscopy: (1) small molecule dye accumulation during cleavage and blastula stages, (2) primordial germ cell targeted RNA expression using the Nanos untranslated regions, (3) fusing genes of interest with a Nanos2 targeting peptide, and (4) EdU and BrdU labeling. Applications of the live labeling techniques are discussed, including sorting by fluorescence-activated cell sorting for transcriptomic analysis, and, methods to image small micromere behavior in whole and dissociated embryos by live confocal microscopy. Finally, summary table of antibody and RNA probes as well as small molecule dyes to label small micromeres at a variety of developmental stages is provided.

Trapping, tagging and tracking: Tools for the study of proteins during early development of the sea urchin

The exquisite synchronicity of sea urchin development provides a reliable model for studying maternal proteins in the haploid egg as well as those involved in egg activation, fertilization and early development. Sea urchin eggs are released by the millions, enabling the quantitative evaluation of maternally stored and newly synthesized proteins over a range of time (seconds to hours post fertilization). During this window of development exist many hallmark and unique biochemical interactions that can be investigated for the purpose of characterizing profiles of kinases and other signaling proteins, manipulated using pharmacology to test sufficiency and necessity, for identification of post translational modifications, and for capturing protein-protein interactions. Coupled with the fact that sea urchin eggs and embryos are transparent, this synchronicity also results in large populations of cells that can be evaluated for newly synthesized protein localization and identification through use of the Click-iT technology. We provide basic protocols for these approaches and direct readers to the appropriate literature for variations and examples.

Expression of exogenous mRNAs to study gene function in echinoderm embryos

With the completion of the genome sequencing projects, a new challenge for developmental biologists is to assign a function to the thousands of genes identified. Expression of exogenous mRNAs is a powerful, versatile and rapid technique that can be used to study gene function during development of the sea urchin. This chapter describes how this technique can be used to analyze gene function in echinoderm embryos, how it can be combined with cell transplantation to perform mosaic analysis and how it can be applied to identify downstream targets genes of transcription factors and signaling pathways. We describe specific examples of the use of overexpression of mRNA to analyze gene function, mention the benefits and current limitations of the technique and emphasize the importance of using different controls to assess the specificity of the effects observed. Finally, this chapter details the different steps, vectors and protocols for in vitro production of mRNA and phenotypic analysis.

Early development of the feeding larva of the sea urchin Heliocidaris tuberculata: role of the small micromeres

The two modes of development in sea urchins are direct development, in which the adult develops directly from the gastrula to the adult and does not feed, and indirect development, in which the adult develops indirectly through a feeding larva. In this account of the indirect, feeding larva of Heliocidaris tuberculata, the question raised is whether an evolutionary difference of unequal cell divisions contributes to the development of feeding structures in the indirect larva. In indirect development, the cell divisions at the fourth and fifth cell cycles of the zygote are unequal, with four small micromeres formed at the vegetal pole at the fifth cell division. In direct development, these cell divisions are not unequal. From their position at the head of the archenteron, the small micromeres are strategically located to contribute to the feeding tissues of the larva and the adult of H. tuberculata.

An optogenetic approach to control protein localization during embryogenesis of the sea urchin

Light inducible protein-protein interactions have been used to manipulate protein localization and function in the cell with utmost spatial and temporal precision. In this technical report, we use a recently developed optogenetic approach to manipulate protein localization in the developing sea urchin embryo. A photosensitive LOV domain from Avena sativa phototropin1 cages a small peptide that binds the engineered PDZ domain (ePDZ) upon blue light irradiation. Using this system, mCherry tagged proteins fused with the LOV domain were recruited to ectopic sub-cellular regions such as the membrane, microtubules, or actin by GFP tagged proteins fused with the ePDZ domain upon blue light irradiation within 1-3 min in the sea urchin embryo. The efficiency and speed of recruitment of each protein to its respective subcellular region appeared to be dependent on the power and duration of laser irradiation, as well as the respective level of affinity to the tagged location. Controlled laser irradiation allowed partial recruitment of the spindle to the membrane, and resulted in cell blebbing. Vasa, a cell cycle and germline factor that localizes on the spindle and enriches in the micromeres at 8-16 cell stage was recruited to ectopic sites, preventing normal enrichment. Continuous blue light activation with a regular blue aquarium light over two days of culture successfully induced LOV-ePDZ binding in the developing embryos, resulting in continued ectopic recruitment of Vasa and failure in gastrulation at Day 2. Although some cytotoxicity was observed with prolonged blue light irradiation, this optogenetic system provides a promising approach to test the sub-cellular activities of developmental factors, as well as to alter protein localization and development during embryogenesis.

Xenopus Vasa Homolog XVLG1 is Essential for Migration and Survival of Primordial Germ Cells

Xenopus vasa-like gene 1 (XVLG1), a DEAD-Box Helicase 4 (DDX4) gene identified as a vertebrate vasa homologue, is required for the formation of primordial germ cells (PGCs). However, it remains to be clarified when and how XVLG1 functions in the formation of the germ cells. To gain a better understanding of the molecular mechanisms underlying XVLG1 during PGC development, we injected XVLG1 morpholino oligos into germ-plasm containing blastomeres of 32-cell stage of Xenopus embryos, and traced cell fates of the injected blastomere-derived PGCs. As a result of this procedure, migration of the PGCs was impaired and the number of PGCs derived from the blastomeres was significantly decreased. In addition, TUNEL staining in combination with in situ hybridization revealed that the loss of PGCs peaked at stage 27 was caused by apoptosis. This data strongly suggests an essential role for XVLG1 in migration and survival of the germ cells.

Asymmetric distribution of hypoxia-inducible factor α regulates dorsoventral axis establishment in the early sea urchin embryo

Hypoxia signaling is an ancient pathway by which animals can respond to low oxygen. Malfunction of this pathway disturbs hypoxic acclimation and can result in various diseases, including cancers. The role of hypoxia signaling in early embryogenesis remains unclear. Here, we show that in the blastula of the sea urchin Strongylocentrotus purpuratus, hypoxia-inducible factor α (HIFα), the downstream transcription factor of the hypoxia pathway, is localized and transcriptionally active on the future dorsal side. This asymmetric distribution is attributable to its oxygen-sensing ability. Manipulations of the HIFα level entrained the dorsoventral axis, as the side with the higher level of HIFα tends to develop into the dorsal side. Gene expression analyses revealed that HIFα restricts the expression of nodal to the ventral side and activates several genes encoding transcription factors on the dorsal side. We also observed that intrinsic hypoxic signals in the early embryos formed a gradient, which was disrupted under hypoxic conditions. Our results reveal an unprecedented role of the hypoxia pathway in animal development.

Germline factor DDX4 functions in blood-derived cancer cell phenotypes

DDX4 (the human ortholog of Drosophila Vasa) is an RNA helicase and is present in the germ lines of all metazoans tested. It was historically thought to be expressed specifically in germline, but with additional organisms studied, it is now clear that in some animals DDX4/Vasa functions outside of the germline, in a variety of somatic cells in the embryo and in the adult. In this report, we document that DDX4 is widely expressed in soma-derived cancer cell lines, including myeloma (IM-9) and leukemia (THP-1) cells. In these cells, the DDX4 protein localized to the mitotic spindle, consistent with findings in other somatic cell functions, and its knockout in IM-9 cells compromised cell proliferation and migration activities, and downregulated several cell cycle/oncogene factors such as CyclinB and the transcription factor E2F1. These results suggest that DDX4 positively regulates cell cycle progression of diverse somatic-derived blood cancer cells, implying its broad contributions to the cancer cell phenotype and serves as a potential new target for chemotherapy.

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.

C-terminal residues specific to Vasa among DEAD-box helicases are required for its functions in piRNA biogenesis and embryonic patterning

The DEAD-box RNA helicase Vasa (Vas, also known as DDX4) is required for germ cell development. In Drosophila, analysis of hypomorphic mutations has implicated maternally expressed Vas in germ cell formation and posterior embryonic patterning. vas-null females, which rarely complete oogenesis, exhibit defects in mitotic progression of germline stem cells, Piwi-interacting RNA (piRNA)-mediated transposon silencing, and translation of Gurken (Grk), an EGFR ligand. The carboxy-terminal region of Vas orthologs throughout the animal kingdom consists of several acidic residues as well as an invariant tryptophan in the penultimate or ultimate position (Trp660 in Drosophila melanogaster). Using CRISPR/Cas9 gene editing, we made a substitution mutant in this residue. Replacing Trp660 by Glu (W660E) abolishes the ability of Vas to support germ cell formation and embryonic patterning and greatly reduces Vas activity in piRNA biogenesis, as measured by transposon silencing, and in activating Grk translation. A conservative substitution (W660F) has much milder phenotypic consequences. In addition, females expressing only a form of Vas in which the seven C-terminal amino acids were replaced with the corresponding residues from Belle (Bel, also known as DDX3) show defects in perinuclear nuage assembly and transposon silencing. Oogenesis in females expressing only the chimeric Vas arrests early; however, in a vas 1 background, in which early expression of endogenous Vas supports oogenesis, the chimeric protein supports posterior patterning and germ cell specification. These results indicate that the unique C-terminus of Vas is essential for its function in piRNA biogenesis and that the conserved Trp660 residue has an important functional role.

Maintenance of somatic tissue regeneration with age in short- and long-lived species of sea urchins

Aging in many animals is characterized by a failure to maintain tissue homeostasis and the loss of regenerative capacity. In this study, the ability to maintain tissue homeostasis and regenerative potential was investigated in sea urchins, a novel model to study longevity and negligible senescence. Sea urchins grow indeterminately, regenerate damaged appendages and reproduce throughout their lifespan and yet different species are reported to have very different life expectancies (ranging from 4 to more than 100 years). Quantitative analyses of cell proliferation and apoptosis indicated a low level of cell turnover in tissues of young and old sea urchins of species with different lifespans (Lytechinus variegatus, Strongylocentrotus purpuratus and Mesocentrotus franciscanus). The ability to regenerate damaged tissue was maintained with age as assessed by the regrowth of amputated spines and tube feet (motor and sensory appendages). Expression of genes involved in cell proliferation (pcna), telomere maintenance (tert) and multipotency (seawi and vasa) was maintained with age in somatic tissues. Immunolocalization of the Vasa protein to areas of the tube feet, spines, radial nerve, esophagus and a sub-population of circulating coelomocytes suggests the presence of multipotent cells that may play a role in normal tissue homeostasis and the regenerative potential of external appendages. The results indicate that regenerative potential was maintained with age regardless of lifespan, contrary to the expectation that shorter lived species would invest less in maintenance and repair.

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.

The diversity of nanos expression in echinoderm embryos supports different mechanisms in germ cell specification

Specification of the germ cell lineage is required for sexual reproduction in all animals. However, the timing and mechanisms of germ cell specification is remarkably diverse in animal development. Echinoderms, such as sea urchins and sea stars, are excellent model systems to study the molecular and cellular mechanisms that contribute to germ cell specification. In several echinoderm embryos tested, the germ cell factor Vasa accumulates broadly during early development and is restricted after gastrulation to cells that contribute to the germ cell lineage. In the sea urchin, however, the germ cell factor Vasa is restricted to a specific lineage by the 32-cell stage. We therefore hypothesized that the germ cell specification program in the sea urchin/Euechinoid lineage has evolved to an earlier developmental time point. To test this hypothesis we determined the expression pattern of a second germ cell factor, Nanos, in four out of five extant echinoderm clades. Here we find that Nanos mRNA does not accumulate until the blastula stage or later during the development of all other echinoderm embryos except those that belong to the Echinoid lineage. Instead, Nanos is expressed in a restricted domain at the 32-128 cell stage in Echinoid embryos. Our results support the model that the germ cell specification program underwent a heterochronic shift in the Echinoid lineage. A comparison of Echinoid and non-Echinoid germ cell specification mechanisms will contribute to our understanding of how these mechanisms have changed during animal evolution.

An unregulated regulator: Vasa expression in the development of somatic cells and in tumorigenesis

Growing evidence in diverse organisms shows that genes originally thought to function uniquely in the germ line may also function in somatic cells, and in some cases even contribute to tumorigenesis. Here we review the somatic functions of Vasa, one of the most conserved "germ line" factors among metazoans. Vasa expression in somatic cells is tightly regulated and often transient during normal development, and appears to play essential roles in regulation of embryonic cells and regenerative tissues. Its dysregulation, however, is believed to be an important element of tumorigenic cell regulation. In this perspectives paper, we propose how some conserved functions of Vasa may be selected for somatic cell regulation, including its potential impact on efficient and localized translational activities and in some cases on cellular malfunctioning and tumorigenesis.

Cyclin B Translation Depends on mTOR Activity after Fertilization in Sea Urchin Embryos

The cyclin B/CDK1 complex is a key regulator of mitotic entry. Using PP242, a specific ATP-competitive inhibitor of mTOR kinase, we provide evidence that the mTOR signalling pathway controls cyclin B mRNA translation following fertilization in Sphaerechinus granularis and Paracentrotus lividus. We show that PP242 inhibits the degradation of the cap-dependent translation repressor 4E-BP (eukaryotic initiation factor 4E-Binding Protein). PP242 inhibits global protein synthesis, delays cyclin B accumulation, cyclin B/CDK1 complex activation and consequently entry into the mitotic phase of the cell cycle triggered by fertilization. PP242 inhibits cyclin B mRNA recruitment into active polysomes triggered by fertilization. An amount of cyclin B mRNA present in active polysomes appears to be insensitive to PP242 treatment. Taken together, our results suggest that, following sea urchin egg fertilization, cyclin B mRNA translation is controlled by two independent mechanisms: a PP242-sensitive and an additional PP242-insentitive mechanism.

Germ Line Mechanics--And Unfinished Business

Primordial germ cells are usually made early in the development of an organism. These are the mother of all stem cells that are necessary for propagation of the species, yet use highly diverse mechanisms between organisms. How they are specified, and when and where they form, are central to developmental biology. Using diverse organisms to study this development is illuminating for understanding the mechanics these cells use in this essential function and for identifying the breadth of evolutionary changes that have occurred between species. This essay emphasizes how echinoderms may contribute to the patchwork quilt of our understanding of germ line formation during embryogenesis.

Primordial Germ Cell Specification and Migration

Primordial germ cells are the progenitor cells that give rise to the gametes. In some animals, the germline is induced by zygotic transcription factors, whereas in others, primordial germ cell specification occurs via inheritance of maternally provided gene products known as germ plasm. Once specified, the primordial germ cells of some animals must acquire motility and migrate to the gonad in order to survive. In all animals examined, perinuclear structures called germ granules form within germ cells. This review focuses on some of the recent studies, conducted by several groups using diverse systems, from invertebrates to vertebrates, which have provided mechanistic insight into the molecular regulation of germ cell specification and migration.

Localization of germ plasm-related structures during sea urchin oogenesis

BACKGROUND

Animal germ cells have specific organelles that are similar to ribonucleoprotein complex, called germ plasm, which is accumulated in eggs. Germ plasm is essential for inherited mechanism of germ line segregation in early embryogenesis. Sea urchins have early germ line segregation in early embryogenesis. Nevertheless, organization of germ plasm-related organelles and their molecular composition are still unclear. Another issue is whether maternally accumulated germ plasm exists in the sea urchin eggs.

RESULTS

I analyzed intracellular localization of germ plasm during oogenesis in sea urchin Strongylocentrotus intermedius by using morphological approach and immunocytochemical detection of Vasa, a germ plasm marker. All ovarian germ cells have germ plasm-related organelles in the form of germ granules, Balbiani bodies, and perinuclear nuage found previously in germ cells in other animals. Maternal germ plasm is accumulated in late oogenesis at the cell periphery. Cytoskeletal drug treatment showed an association of Vasa-positive granules with actin filaments in the egg cortex.

CONCLUSIONS

All female germ cells of sea urchins have germ plasm-related organelles. Eggs have a maternally accumulated germ plasm associated with cortical cytoskeleton. These findings correlate with early segregation of germ line in sea urchins.

Deployment of a retinal determination gene network drives directed cell migration in the sea urchin embryo

Gene regulatory networks (GRNs) provide a systems-level orchestration of an organism's genome encoded anatomy. As biological networks are revealed, they continue to answer many questions including knowledge of how GRNs control morphogenetic movements and how GRNs evolve. The migration of the small micromeres to the coelomic pouches in the sea urchin embryo provides an exceptional model for understanding the genomic regulatory control of morphogenesis. An assay using the robust homing potential of these cells reveals a ‘coherent feed-forward’ transcriptional subcircuit composed of Pax6, Six3, Six1/2, Eya, and Dach1 that is responsible for the directed homing mechanism of these multipotent progenitors. The linkages of that circuit are strikingly similar to a circuit involved in retinal specification in Drosophila suggesting that systems-level tasks can be highly conserved even though the tasks drive unrelated processes in different animals.

DOI:http://dx.doi.org/10.7554/eLife.08827.001

Within an animal embryo, groups of cells tend to move, or migrate, between different areas before they form into tissues and organs. These cell migrations are regulated by hundreds of genes, which must be expressed at the right time and in the right place. Cells use proteins called transcription factors to regulate the expression of genes. These proteins work together in circuit board-like networks called gene regulatory networks in order to drive different aspects of development, including cell migration.

The sea urchin is a useful model organism to study how animals develop. This is because these marine animals express many of the same genes as humans, but they can be easily manipulated and studied in the laboratory. In a developing sea urchin embryo, cells called the small micromeres move towards one end of animal and get incorporated into a pocket-like structure known as the coelomic pouch. From this pouch, these cells mature and eventually contribute to the adult germ cells (the precursors to the sperm and eggs).

Martik and McClay have now analyzed how small micromeres make their way to their final location in the coelomic pouch. Micromeres were labeled with a dye that fluoresces green so that they could be tracked under a microscope. This revealed that, like other moving cells, micromeres actively change their shape as they migrate. Furthermore, when micromeres were experimentally moved to abnormal locations in the sea urchin embryo, they were still able to actively home in on the coelomic pouch no matter their starting location.

Martik and McClay then identified five transcription factors expressed in the coelomic pouch in the sea urchin that are involved in this homing activity. Reducing the expression of any of these transcription factors was enough to hinder the ability of the micromeres to find their way to the coelomic pouch. Further experiments and analysis then revealed that these five transcription factors work together in a sub-circuit, which is in turn embedded in a larger gene regulatory network.

This sub-circuit that drives cell migration is unexpectedly similar to another circuit in the fruit fly Drosophila. Intriguingly, the sub-circuit in the fly controls eye development, which is unrelated to cell homing and migration. These observations raise the possibility that this circuit has been conserved as a unit over millions of years of evolution and redeployed in new networks under completely different circumstances. The data also suggest the possibility that additional conserved sub-circuits will be identified as more systems are analyzed in detail.

DOI:http://dx.doi.org/10.7554/eLife.08827.002

The Expression of a Novel Mitochondrially-Encoded Gene in Gonadic Precursors May Drive Paternal Inheritance of Mitochondria

Mitochondria have an active role in germ line development, and their inheritance dynamics are relevant to this process. Recently, a novel protein (RPHM21) was shown to be encoded in sperm by the male-transmitted mtDNA of Ruditapes philippinarum, a species with Doubly Uniparental Inheritance (DUI) of mitochondria. In silico analyses suggested a viral origin of RPHM21, and we hypothesized that the endogenization of a viral element provided sperm mitochondria of R. philippinarum with the ability to invade male germ line, thus being transmitted to the progeny. In this work we investigated the dynamics of germ line development in relation to mitochondrial transcription and expression patterns using qPCR and specific antibodies targeting the germ line marker VASPH (R. philippinarum VASA homolog), and RPHM21. Based on the experimental results we conclude that both targets are localized in the primordial germ cells (PGCs) of males, but while VASPH is detected in all PGCs, RPHM21 appears to be expressed only in a subpopulation of them. Since it has been predicted that RPHM21 might have a role in cell proliferation and migration, we here suggest that PGCs expressing it might gain advantage over others and undertake spermatogenesis, accounting for RPHM21 presence in all spermatozoa. Understanding how foreign sequence endogenization and co-option can modify the biology of an organism is of particular importance to assess the impact of such events on evolution.

Germ Line Versus Soma in the Transition from Egg to Embryo

With few exceptions, all animals acquire the ability to produce eggs or sperm at some point in their life cycle. Despite this near-universal requirement for sexual reproduction, there exists an incredible diversity in germ line development. For example, animals exhibit a vast range of differences in the timing at which the germ line, which retains reproductive potential, separates from the soma, or terminally differentiated, nonreproductive cells. This separation may occur during embryonic development, after gastrulation, or even in adults, depending on the organism. The molecular mechanisms of germ line segregation are also highly diverse, and intimately intertwined with the overall transition from a fertilized egg to an embryo. The earliest embryonic stages of many species are largely controlled by maternally supplied factors. Later in development, patterning control shifts to the embryonic genome and, concomitantly with this transition, the maternally supplied factors are broadly degraded. This chapter attempts to integrate these processes--germ line segregation, and how the divergence of germ line and soma may utilize the egg to embryo transitions differently. In some embryos, this difference is subtle or maybe lacking altogether, whereas in other embryos, this difference in utilization may be a key step in the divergence of the two lineages. Here, we will focus our discussion on the echinoderms, and in particular the sea urchins, in which recent studies have provided mechanistic understanding in germ line determination. We propose that the germ line in sea urchins requires an acquisition of maternal factors from the egg and, when compared to other members of the taxon, this appears to be a derived mechanism. The acquisition is early--at the 32-cell stage--and involves active protection of maternal mRNAs, which are instead degraded in somatic cells with the maternal-to-embryonic transition. We collectively refer to this model as the Time Capsule method for germ line determination.

Tissue regeneration and biomineralization in sea urchins: role of Notch signaling and presence of stem cell markers

Echinoderms represent a phylum with exceptional regenerative capabilities that can reconstruct both external appendages and internal organs. Mechanistic understanding of the cellular pathways involved in regeneration in these animals has been hampered by the limited genomic tools and limited ability to manipulate regenerative processes. We present a functional assay to investigate mechanisms of tissue regeneration and biomineralization by measuring the regrowth of amputated tube feet (sensory and motor appendages) and spines in the sea urchin, Lytechinus variegatus. The ability to manipulate regeneration was demonstrated by concentration-dependent inhibition of regrowth of spines and tube feet by treatment with the mitotic inhibitor, vincristine. Treatment with the gamma-secretase inhibitor DAPT resulted in a concentration-dependent inhibition of regrowth, indicating that both tube feet and spine regeneration require functional Notch signaling. Stem cell markers (Piwi and Vasa) were expressed in tube feet and spine tissue, and Vasa-positive cells were localized throughout the epidermis of tube feet by immunohistochemistry, suggesting the existence of multipotent progenitor cells in these highly regenerative appendages. The presence of Vasa protein in other somatic tissues (e.g. esophagus, radial nerve, and a sub-population of coelomocytes) suggests that multipotent cells are present throughout adult sea urchins and may contribute to normal homeostasis in addition to regeneration. Mechanistic insight into the cellular pathways governing the tremendous regenerative capacity of echinoderms may reveal processes that can be modulated for regenerative therapies, shed light on the evolution of regeneration, and enable the ability to predict how these processes will respond to changing environmental conditions.

Essential elements for translation: the germline factor Vasa functions broadly in somatic cells

Vasa is a conserved RNA-helicase found in the germ lines of all metazoans tested. Whereas Vasa presence is often indicated as a metric for germline determination in animals, it is also expressed in stem cells of diverse origin. Recent research suggests, however, that Vasa has a much broader function, including a significant role in cell cycle regulation. Results herein indicate that Vasa is utilized widely, and often induced transiently, during development in diverse somatic cells and adult precursor tissues. We identified that Vasa in the sea urchin is essential for: (1) general mRNA translation during embryogenesis, (2) developmental re-programming upon manipulations to the embryo and (3) larval wound healing. We also learned that Vasa interacted with mRNAs in the perinuclear area and at the spindle in an Importin-dependent manner during cell cycle progression. These results suggest that, when present, Vasa functions are essential to contributing to developmental regulation.

vasa and piwi are required for mitotic integrity in early embryogenesis in the spider Parasteatoda tepidariorum

Studies in vertebrate and invertebrate model organisms on the molecular basis of primordial germ cell (PGC) specification have revealed that metazoans can specify their germ line either early in development by maternally transmitted cytoplasmic factors (inheritance), or later in development by signaling factors from neighboring tissues (induction). Regardless of the mode of PGC specification, once animal germ cells are specified, they invariably express a number of highly conserved genes. These include vasa and piwi, which can play essential roles in any or all of PGC specification, development, or gametogenesis. Although the arthropods are the most speciose animal phylum, to date there have been no functional studies of conserved germ line genes in species of the most basally branching arthropod clade, the chelicerates (which includes spiders, scorpions, and horseshoe crabs). Here we present the first such study by using molecular and functional tools to examine germ line development and the roles of vasa and piwi orthologues in the common house spider Parasteatoda (formerly Achaearanea) tepidariorum. We use transcript and protein expression patterns of Pt-vasa and Pt-piwi to show that primordial germ cells (PGCs) in the spider arise during late embryogenesis. Neither Pt-vasa nor Pt-piwi gene products are localized asymmetrically to any embryonic region before PGCs emerge as paired segmental clusters in opisthosomal segments 2-6 at late germ band stages. RNA interference studies reveal that both genes are required maternally for egg laying, mitotic progression in early embryos, and embryonic survival. Our results add to the growing body of evidence that vasa and piwi can play important roles in somatic development, and provide evidence for a previously hypothesized conserved role for vasa in cell cycle progression.

The biology of the germ line in echinoderms

The formation of the germ line in an embryo marks a fresh round of reproductive potential. The developmental stage and location within the embryo where the primordial germ cells (PGCs) form, however, differs markedly among species. In many animals, the germ line is formed by an inherited mechanism, in which molecules made and selectively partitioned within the oocyte drive the early development of cells that acquire this material to a germ-line fate. In contrast, the germ line of other animals is fated by an inductive mechanism that involves signaling between cells that directs this specialized fate. In this review, we explore the mechanisms of germ-line determination in echinoderms, an early-branching sister group to the chordates. One member of the phylum, sea urchins, appears to use an inherited mechanism of germ-line formation, whereas their relatives, the sea stars, appear to use an inductive mechanism. We first integrate the experimental results currently available for germ-line determination in the sea urchin, for which considerable new information is available, and then broaden the investigation to the lesser-known mechanisms in sea stars and other echinoderms. Even with this limited insight, it appears that sea stars, and perhaps the majority of the echinoderm taxon, rely on inductive mechanisms for germ-line fate determination. This enables a strongly contrasted picture for germ-line determination in this phylum, but one for which transitions between different modes of germ-line determination might now be experimentally addressed.

Deadenylase depletion protects inherited mRNAs in primordial germ cells

A crucial event in animal development is the specification of primordial germ cells (PGCs), which become the stem cells that create sperm and eggs. How PGCs are created provides a valuable paradigm for understanding stem cells in general. We find that the PGCs of the sea urchin Strongylocentrotus purpuratus exhibit broad transcriptional repression, yet enrichment for a set of inherited mRNAs. Enrichment of several germline determinants in the PGCs requires the RNA-binding protein Nanos to target the transcript that encodes CNOT6, a deadenylase, for degradation in the PGCs, thereby creating a stable environment for RNA. Misexpression of CNOT6 in the PGCs results in their failure to retain Seawi transcripts and Vasa protein. Conversely, broad knockdown of CNOT6 expands the domain of Seawi RNA as well as exogenous reporters. Thus, Nanos-dependent spatially restricted CNOT6 differential expression is used to selectively localize germline RNAs to the PGCs. Our findings support a 'time capsule' model of germline determination, whereby the PGCs are insulated from differentiation by retaining the molecular characteristics of the totipotent egg and early embryo.

Protein degradation machinery is present broadly during early development in the sea urchin

Ubiquitin-dependent proteosome-mediated proteolysis is an important pathway of degradation that controls the timed destruction of cellular proteins in all tissues. All intracellular proteins and many extracellular proteins are continually being hydrolyzed to their constituent amino acids as a result of their recognition by E3 ligases for specific targeting of ubiquitination. Gustavus is a member of an ECS-type E3 ligase which interacts with Vasa, a DEAD-box RNA helicase, to regulate its localization during sea urchin embryonic development, and Gustavus mRNA accumulation is highly localized and dynamic during development. We tested if the core complex for Gustavus function was present in the embryo and if other SOCS box proteins also had restricted expression profiles that would inform future research. Expression patterns of the key members of the proteasomal function, such as the E3 core complex which interacts with Gustavus, and other E3-SOCS box proteins, are widely spread and dynamic in early development of the embryo suggesting broad core complex availability in the proteasome degradation pathway and temporal/spatial enrichments of various E3 ligase dependent targeting mechanisms.

The DEAD Box RNA helicase VBH-1 is a new player in the stress response in C. elegans

For several years, DEAD box RNA helicase Vasa (DDX4) has been used as a bona fide germline marker in different organisms. C. elegans VBH-1 is a close homolog of the Vasa protein, which plays an important role in gametogenesis, germ cell survival and embryonic development. Here, we show that VBH-1 protects nematodes from heat shock and oxidative stress. Using the germline-defective mutant glp-4(bn2) we found that a potential somatic expression of vbh-1 might be important for stress survival. We also show that the VBH-1 paralog LAF-1 is important for stress survival, although this protein is not redundant with its counterpart. Furthermore, we observed that the mRNAs of the heat shock proteins hsp-1 and sip-1 are downregulated when vbh-1 or laf-1 are silenced. Previously, we reported that in C. elegans, VBH-1 was primarily expressed in P granules of germ cells and in the cytoplasm of all blastomeres. Here we show that during stress, VBH-1 co-localizes with CGH-1 in large aggregates in the gonad core and oocytes; however, VBH-1 aggregates do not overlap with CGH-1 foci in early embryos under the same conditions. These data demonstrate that, in addition to the previously described role for this protein in the germline, VBH-1 plays an important role during the stress response in C. elegans through the potential direct or indirect regulation of stress response mRNAs.