More than a colour; how pigment influences colourblind microbes

Many animals have pigments when they themselves cannot see colour. Perhaps those pigments enable the animal to avoid predators, or to attract mates. Maybe even those pigmented surfaces are hosts for microbes, even when the microbes do not see colour. Do some pigments then serve as a chemical signal for a good or bad microbial substrate? Maybe pigments attract or repel various microbe types? Echinoderms serve as an important model to test the mechanisms of pigment-based microbial interactions. Echinoderms are marine benthic organisms, ranging from intertidal habitats to depths of thousands of metres and are exposed to large varieties of microbes. They are also highly pigmented, with a diverse variety of colours between and even within species. Here we focus on one type of pigment (naphthoquinones) made by polyketide synthase, modified by flavin-dependent monoxygenases, and on one type of function, microbial interaction. Recent successes in targeted gene inactivation by CRISPR/Cas9 in sea urchins supports the contention that colour is more than it seems. Here we dissect the players, and their interactions to better understand how such host factors influence a microbial colonization. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

The Essence of Quiescence

Historically hematopoietic stem cells are believed to be predominantly dormant but could be induced into active cell cycle under specific conditions. This review, coupled with years of research from our laboratory, challenges this belief by demonstrating a significant portion of hematopoietic stem cells are actively cycling rather than quiescent. This addresses a major heuristic error in the understanding of hematopoietic stem cells that has shaped this field for decades. By evaluating the cycle status of engraftable hematopoietic stem cells in whole unseparated bone marrow, we demonstrated that a significant portion of these cells are actively cycling, and further confirmed by tritiated thymidine suicide and bromodeoxyuridine labeling assays. Moreover, by analyzing both whole unseparated bone marrow and purified lineage-negative hematopoietic stem cells in murine models, our findings indicate that lineage-positive cells, usually discarded during purification, actually contain actively cycling stem cells. Taken together, our findings highlight that hematopoietic stem cells are characterized as actively cycling and expressing differentiation epitopes. This corrects a basic mistake in stem cell biology. Furthermore, these findings provide valuable insights for a better understanding of the actively cycling hematopoietic stem cells in the field of stem cell biology.

Conservation and contrast in cell states of echinoderm ovaries

Echinoderms produce functional gametes throughout their lifespan, in some cases exceeding 200 years. The histology and ultrastructure of echinoderm ovaries has been described but how these ovaries function and maintain the production of high-quality gametes remains a mystery. Here, we present the first single cell RNA sequencing data sets of mature ovaries from two sea urchin species (Strongylocentrotus purpuratus [Sp] and Lytechinus variegatus [Lv]), and one sea star species (Patiria miniata [Pm]). We find 14 cell states in the Sp ovary, 16 cell states in the Lv ovary and 13 cell states in the ovary of the sea star. This resource is essential to understand the structure and functional biology of the ovary in echinoderms, and better informs decisions in the utilization of in situ RNA hybridization probes selective for various cell types. We link key genes with cell clusters in validation of this approach. This resource also aids in the identification of the stem cells for prolonged and continuous gamete production, is a foundation for testing changes in the annual reproductive cycle, and is essential for understanding the evolution of reproduction of this important phylum.

Identification of the genes encoding candidate septate junction components expressed during early development of the sea urchin, Strongylocentrotus purpuratus, and evidence of a role for Mesh in the formation of the gut barrier

Septate junctions (SJs) evolved as cell-cell junctions that regulate the paracellular barrier and integrity of epithelia in invertebrates. Multiple morphological variants of SJs exist specific to different epithelia and/or phyla but the biological significance of varied SJ morphology is unclear because the knowledge of the SJ associated proteins and their functions in non-insect invertebrates remains largely unknown. Here we report cell-specific expression of nine candidate SJ genes in the early life stages of the sea urchin Strongylocentrotus purpuratus. By use of in situ RNA hybridization and single cell RNA-seq we found that the expression of selected genes encoding putatively SJ associated transmembrane and cytoplasmic scaffold molecules was dynamically regulated during epithelial development in the embryos and larvae with different epithelia expressing different cohorts of SJ genes. We focused a functional analysis on SpMesh, a homolog of the Drosophila smooth SJ component Mesh, which was highly enriched in the endodermal epithelium of the mid- and hindgut. Functional perturbation of SpMesh by both CRISPR/Cas9 mutagenesis and vivo morpholino-mediated knockdown shows that loss of SpMesh does not disrupt the formation of the gut epithelium during gastrulation. However, loss of SpMesh resulted in a severely reduced gut-paracellular barrier as quantitated by increased permeability to 3-5 ​kDa FITC-dextran. Together, these studies provide a first look at the molecular SJ physiology during the development of a marine organism and suggest a shared role for Mesh-homologous proteins in forming an intestinal barrier in invertebrates. Results have implications for consideration of the traits underlying species-specific sensitivity of marine larvae to climate driven ocean change.

Gene regulatory divergence amongst echinoderms underlies appearance of pigment cells in sea urchin development

Larvae of the sea urchin, Strongylocentrotus purpuratus, have pigmented migratory cells implicated in immune defense and gut patterning. The transcription factor SpGcm activates the expression of many pigment cell-specific genes, including those involved in pigment biosynthesis (SpPks1 and SpFmo3) and immune related genes (e.g. SpMif5). Despite the importance of this cell type in sea urchins, pigmented cells are absent in larvae of the sea star, Patiria miniata. In this study, we tested the premises that sea stars lack genes to synthesize echinochrome pigment, that the genes are present but are not expressed in the larvae, or rather that the homologous gene expression does not contribute to echinochrome synthesis. Our results show that orthologs of sea urchin pigment cell-specific genes (PmPks1, PmFmo3-1 and PmMifL1-2) are present in the sea star genome and expressed in the larvae. Although no cell lineage homologous to migratory sea urchin pigment cells is present, dynamic gene activation accomplishes a similar spatial and temporal expression profile. The mechanisms regulating the expression of these genes, though, is highly divergent. In sea stars, PmGcm lacks the central role in pigment gene expression since it is not expressed in PmPks1 and PmFmo3-1-positive cells, and knockdown of Gcm does not abrogate pigment gene expression. Pigment genes are instead expressed in the coelomic mesoderm early in development before later being expressed in the ectoderm. These findings were supported by in situ RNA hybridization and comparative scRNA-seq analyses. We conclude that simply the coexpression of Pks1 and Fmo3 orthologs in cells of the sea star is not sufficient to underlie the emergence of the larval pigment cell in the sea urchin.

Timing and cell specificity of senescence drives postnatal lung development and injury

Senescence causes age-related diseases and stress-related injury. Paradoxically, it is also essential for organismal development. Whether senescence contributes to lung development or injury in early life remains unclear. Here, we show that lung senescence occurred at birth and decreased throughout the saccular stage in mice. Reducing senescent cells at this stage disrupted lung development. In mice (<12 h old) exposed to hyperoxia during the saccular stage followed by air recovery until adulthood, lung senescence increased particularly in type II cells and secondary crest myofibroblasts. This peaked during the alveolar stage and was mediated by the p53/p21 pathway. Decreasing senescent cells during the alveolar stage attenuated hyperoxia-induced alveolar and vascular simplification. Conclusively, early programmed senescence orchestrates postnatal lung development whereas later hyperoxia-induced senescence causes lung injury through different mechanisms. This defines the ontogeny of lung senescence and provides an optimal therapeutic window for mitigating neonatal hyperoxic lung injury by inhibiting senescence.

Single-cell RNA-sequencing analysis of early sea star development

Echinoderms represent a broad phylum with many tractable features to test evolutionary changes and constraints. Here, we present a single-cell RNA-sequencing analysis of early development in the sea star Patiria miniata, to complement the recent analysis of two sea urchin species. We identified 20 cell states across six developmental stages from 8 hpf to mid-gastrula stage, using the analysis of 25,703 cells. The clusters were assigned cell states based on known marker gene expression and by in situ RNA hybridization. We found that early (morula, 8-14 hpf) and late (blastula-to-mid-gastrula) cell states are transcriptionally distinct. Cells surrounding the blastopore undergo rapid cell state changes that include endomesoderm diversification. Of particular import to understanding germ cell specification is that we never see Nodal pathway members within Nanos/Vasa-positive cells in the region known to give rise to the primordial germ cells (PGCs). The results from this work contrast the results of PGC specification in the sea urchin, and the dataset presented here enables deeper comparative studies in tractable developmental models for testing a variety of developmental mechanisms.

Identification of Heme Oxygenase-1 as a Putative DNA-Binding Protein

Heme oxygenase-1 (HO-1) is a rate-limiting enzyme in degrading heme into biliverdin and iron. HO-1 can also enter the nucleus and regulate gene transcription independent of its enzymatic activity. Whether HO-1 can alter gene expression through direct binding to target DNA remains unclear. Here, we performed HO-1 CHIP-seq and then employed 3D structural modeling to reveal putative HO-1 DNA binding domains. We identified three probable DNA binding domains on HO-1. Using the Proteinarium, we identified several genes as the most highly connected nodes in the interactome among the HO-1 gene binding targets. We further demonstrated that HO-1 modulates the expression of these key genes using Hmox1 deficient cells. Finally, mutation of four conserved amino acids (E215, I211, E201, and Q27) within HO-1 DNA binding domain 1 significantly increased expression of Gtpbp3 and Eif1 genes that were identified within the top 10 binding hits normalized by gene length predicted to bind this domain. Based on these data, we conclude that HO-1 protein is a putative DNA binding protein, and regulates targeted gene expression. This provides the foundation for developing specific inhibitors or activators targeting HO-1 DNA binding domains to modulate targeted gene expression and corresponding cellular function.

Optimizing CRISPR/Cas9-based gene manipulation in echinoderms

The impact of new technology can be appreciated by how broadly it is used. Investigators that previously relied only on pharmacological approaches or the use of morpholino antisense oligonucleotide (MASO) technologies are now able to apply CRISPR-Cas9 to study biological problems in their model organism of choice much more effectively. The transitions to new CRISPR-based approaches could be enhanced, first, by standardized protocols and education in their applications. Here we summarize our results for optimizing the CRISPR-Cas9 technology in a sea urchin and a sea star, and provide advice on how to set up CRISPR-Cas9 experiments and interpret the results in echinoderms. Our goal through these protocols and sharing examples of success by other labs is to lower the activation barrier so that more laboratories can apply CRISPR-Cas9 technologies in these important animals.

A Review of Asteroid Biology in the Context of Sea Star Wasting: Possible Causes and Consequences

AbstractSea star wasting-marked in a variety of sea star species as varying degrees of skin lesions followed by disintegration-recently caused one of the largest marine die-offs ever recorded on the west coast of North America, killing billions of sea stars. Despite the important ramifications this mortality had for coastal benthic ecosystems, such as increased abundance of prey, little is known about the causes of the disease or the mechanisms of its progression. Although there have been studies indicating a range of causal mechanisms, including viruses and environmental effects, the broad spatial and depth range of affected populations leaves many questions remaining about either infectious or non-infectious mechanisms. Wasting appears to start with degradation of mutable connective tissue in the body wall, leading to disintegration of the epidermis. Here, we briefly review basic sea star biology in the context of sea star wasting and present our current knowledge and hypotheses related to the symptoms, the microbiome, the viruses, and the associated environmental stressors. We also highlight throughout the article knowledge gaps and the data needed to better understand sea star wasting mechanistically, its causes, and potential management.

A single-cell RNA-seq analysis of Brachyury-expressing cell clusters suggests a morphogenesis-associated signal center of oral ectoderm in sea urchin embryos

Brachyury is a T-box family transcription factor and plays pivotal roles in morphogenesis. In sea urchin embryos, Brachyury, is expressed in the invaginating endoderm, and in the oral ectoderm of the invaginating mouth opening. The oral ectoderm is hypothesized to serve as a signaling center for oral (ventral)-aboral (dorsal) axis formation and to function as a ventral organizer. Our previous results of a single-cell RNA-seq (scRNA-seq) atlas of early Strongylocentrotus purpuratus embryos categorized the constituent cells into 22 clusters, in which the endoderm consists of three clusters and the oral ectoderm four clusters (Foster et al., 2020). Here we examined which clusters of cells expressed Brachyury in relation to the morphogenesis and the identity of the ventral organizer. Our results showed that cells of all three endoderm clusters expressed Brachyury in blastulae. Based on expression profiles of genes involved in the gene regulatory networks (GRNs) of sea urchin embryos, the three clusters are distinguishable, two likely derived from the Veg2 tier and one from the Veg1 tier. On the other hand, of the four oral-ectoderm clusters, cells of two clusters expressed Brachyury at the gastrula stage and genes that are responsible for the ventral organizer at the late blastula stage, but the other two clusters did not. At a single-cell level, most cells of the two oral-ectoderm clusters expressed organizer-related genes, nearly a half of which coincidently expressed Brachyury. This suggests that the ventral organizer contains Brachyury-positive cells which invaginate to form the stomodeum. This scRNA-seq study therefore highlights significant roles of Brachyury-expressing cells in body-plan formation of early sea urchin embryos, though cellular and molecular mechanisms for how Brachyury functions in these processes remain to be elucidated in future studies.

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.

Single-cell transcriptomics reveals lasting changes in the lung cellular landscape into adulthood after neonatal hyperoxic exposure

Ventilatory support, such as supplemental oxygen, used to save premature infants impairs the growth of the pulmonary microvasculature and distal alveoli, leading to bronchopulmonary dysplasia (BPD). Although lung cellular composition changes with exposure to hyperoxia in neonatal mice, most human BPD survivors are weaned off oxygen within the first weeks to months of life, yet they may have persistent lung injury and pulmonary dysfunction as adults. We hypothesized that early-life hyperoxia alters the cellular landscape in later life and predicts long-term lung injury. Using single-cell RNA sequencing, we mapped lung cell subpopulations at postnatal day (pnd)7 and pnd60 in mice exposed to hyperoxia (95% O2) for 3 days as neonates. We interrogated over 10,000 cells and identified a total of 45 clusters within 32 cell states. Neonatal hyperoxia caused persistent compositional changes in later life (pnd60) in all five type II cell states with unique signatures and function. Premature infants requiring mechanical ventilation with different durations also showed similar alterations in these unique signatures of type II cell states. Pathologically, neonatal hyperoxic exposure caused alveolar simplification in adult mice. We conclude that neonatal hyperoxia alters the lung cellular landscape in later life, uncovering neonatal programing of adult lung dysfunction.

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.

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.

Regulation of dynamic pigment cell states at single-cell resolution

Cells bearing pigment have diverse roles and are often under strict evolutionary selection. Here, we explore the regulation of pigmented cells in the purple sea urchin Strongylocentrotus purpuratus, an emerging model for diverse pigment function. We took advantage of single cell RNA-seq (scRNAseq) technology and discovered that pigment cells in the embryo segregated into two distinct populations, a mitotic cluster and a post-mitotic cluster. Gcm is essential for expression of several genes important for pigment function, but is only transiently expressed in these cells. We discovered unique genes expressed by pigment cells and test their expression with double fluorescence in situ hybridization. These genes include new members of the fmo family that are expressed selectively in pigment cells of the embryonic and in the coelomic cells of the adult - both cell-types having immune functions. Overall, this study identifies nodes of molecular intersection ripe for change by selective evolutionary pressures.

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.

Dysfunctional MDR-1 disrupts mitochondrial homeostasis in the oocyte and ovary

Multidrug resistance transporters (MDRs) are best known for their pathological role in neoplastic evasion of chemotherapeutics and antibiotics. Here we show that MDR-1 is present in the oocyte mitochondrial membrane, and it protects the female gamete from oxidative stress. Female mdr1a mutant mice have no significant difference in ovarian follicular counts and stages, nor in reproductively functioning hormone levels, yet these mice are significantly more vulnerable to gonadotoxic chemotherapy, have chronically elevated reactive oxygen species in immature germinal vesicle oocytes, exhibit a significant over-accumulation of metabolites involved in the tricarboxylic acid cycle (TCA), and have abnormal mitochondrial membrane potential. The mdr1a mutant ovaries have a dramatically different transcriptomic profile with upregulation of genes involved in metabolism. Our findings indicate that functionality of MDR-1 reveals a critical intersection of metabolite regulation, oxidative stress, and mitochondrial dysfunction that has direct implications for human infertility, premature reproductive aging due to oxidative stress, and gonadoprotection.

Single cell RNA-seq in the sea urchin embryo show marked cell-type specificity in the Delta/Notch pathway

Sea urchin embryos are excellent for in vivo functional studies because of their transparency and tractability in manipulation. They are also favorites for pharmacological approaches since they develop in an aquatic environment and addition of test substances is straightforward. A concern in many pharmacological tests though is the potential for pleiotropic effects that confound the conclusions drawn from the results. Precise cellular interpretations are often not feasible because the impact of the perturbant is not known. Here we use single-cell mRNA (messenger RNA) sequencing as a metric of cell types in the embryo and to determine the selectivity of two commonly used inhibitors, one each for the Wnt and the Delta-Notch pathways, on these nascent cell types. We identified 11 distinct cell types based on mRNA profiling, and that the cell lineages affected by Wnt and Delta/Notch inhibition were distinct from each other. These data support specificity and distinct effects of these signaling pathways in the embryo and illuminate how these conserved pathways selectively regulate cell lineages at a single cell level. Overall, we conclude that single cell RNA-seq analysis in this embryo is revealing of the cell types present during development, of the changes in the gene regulatory network resulting from inhibition of various signaling pathways, and of the selectivity of these pathways in influencing developmental trajectories.

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.

Identifying gene expression from single cells to single genes

Gene regulatory networks reveal how transcription factors contribute to a dynamic cascade of cellular information processing. Recent advances in technologies have enhanced the toolkit for testing GRN mechanisms and connections. Here we emphasize three approaches that we have found important for interrogating transcriptional mechanisms in echinoderms: single cell mRNA sequencing (drop-seq), nascent RNA detection and identification, and chromatin immunoprecipitation (ChIP). We present these applications in order since it is a logical experimental protocol. With preliminary information from bulk mRNA transcriptome analysis and differential gene expression studies (DE-seq), one may need to test in what specific cells important genes may be expressed and to use single cell sequencing to define such links. Nascent RNA analysis with the Click-iT chemistry allows the investigator to deduce when the RNA was transcribed, not just identify its presence, and ChIP allows the investigator to study direct interactions of putative transcriptional regulators with the gene promoter of interest. This flow of thinking, and the technologies to support it, is presented here for echinoderms. While many of the procedures are general and applicable to many organisms and cell types, we emphasize unique aspects of the protocols for consideration in using echinoderm embryos, larvae, and adult tissues.

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.

CRISPR/Cas9-mediated genome editing in sea urchins

The CRISPR (clustered regularly interspaced short palindromic repeat)/Cas9 (CRISPR-associated nuclease 9) technology enables rapid, targeted, and efficient changes in the genomes of various model organisms. The short guide RNAs (gRNAs) of the CRISPR/Cas9 system can be designed to recognize target DNA within coding regions for functional gene knockouts. Several studies have demonstrated that the CRISPR/Cas9 system efficiently and specifically targets sea urchin genes and results in expected mutant phenotypes. In addition to disrupting gene functions, modifications and additions to the Cas9 protein enable alternative activities targeted to specific sites within the genome. This includes a fusion of cytidine deaminase to Cas9 (Cas9-DA) for single nucleotide conversion in targeted sites. In this chapter, we describe detailed methods for the CRISPR/Cas9 application in sea urchin embryos, including gRNA design, in vitro synthesis of single guide RNA (sgRNA), and the usages of the CRISPR/Cas9 technology for gene knockout and single nucleotide editing. Methods for genotyping the resultant embryos are also provided for assessing efficiencies of gene editing.

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

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

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.

Regeneration in bipinnaria larvae of the bat star Patiria miniata induces rapid and broad new gene expression

BACKGROUND

Some metazoa have the capacity to regenerate lost body parts. This phenomenon in adults has been classically described in echinoderms, especially in sea stars (Asteroidea). Sea star bipinnaria larvae can also rapidly and effectively regenerate a complete larva after surgical bisection. Understanding the capacity to reverse cell fates in the larva is important from both a developmental and biomedical perspective; yet, the mechanisms underlying regeneration in echinoderms are poorly understood.

RESULTS

Here, we describe the process of bipinnaria regeneration after bisection in the bat star Patiria miniata. We tested transcriptional, translational, and cell proliferation activity after bisection in anterior and posterior bipinnaria halves as well as expression of SRAP, reported as a sea star regeneration associated protease (Vickery et al., 2001b). Moreover, we found several genes whose transcripts increased in abundance following bisection, including: Vasa, dysferlin, vitellogenin 1 and vitellogenin 2.

CONCLUSION

These results show a transformation following bisection, especially in the anterior halves, of cell fate reassignment in all three germ layers, with clear and predictable changes. These results define molecular events that accompany the cell fate changes coincident to the regenerative response in echinoderm larvae.

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.

Complexity of Yolk Proteins and Their Dynamics in the Sea Star Patiria miniata

Oviparous animals store yolk proteins within the developing oocyte. These proteins are used in gametogenesis and as a nutritional source for embryogenesis. Vitellogenin and the major yolk protein are two of the most important yolk proteins among diverse species of invertebrates and vertebrates. Among the echinoderms, members of the subphyla Echinozoa (sea urchins and sea cucumbers) express the major yolk protein (MYP) but not vitellogenin (Vtg), while an initial report has documented that two Asterozoa (sea stars) express a vitellogenin. Our results show that sea stars contain two vitellogenins, Vtg 1 and Vtg 2, and MYP. In Patiria miniata, these genes are differentially expressed in the somatic and germ cells of the ovary: Vtg 1 is enriched in the somatic cells of the ovary but not in the oocytes, and Vtg 2 accumulates in both oocytes and somatic cells; MYP is not robustly present in either. Remarkably, Vtg 2 and MYP mRNA reappear in larvae; Vtg 2 is detected within cells of the ectoderm, and MYP accumulates in the coelomic pouches, the intestine, and the posterior enterocoel (PE), the site of germ line formation in this animal. Additionally, the Vtg 2 protein is present in oocytes, follicle cells, and developing embryos, but becomes undetectable following gastrulation. These results help elucidate the mechanisms involved in yolk dynamics, and provide molecular information that allows for greater understanding of the evolution of these important gene products.

Simple perfusion apparatus for manipulation, tracking, and study of oocytes and embryos

OBJECTIVE

To develop and implement a device and protocol for oocyte analysis at a single cell level. The device must be capable of high resolution imaging, temperature control, perfusion of media, drugs, sperm, and immunolabeling reagents all at defined flow rates. Each oocyte and resultant embryo must remain spatially separated and defined.

DESIGN

Experimental laboratory study.

SETTING

University and academic center for reproductive medicine.

PATIENT(S)/ANIMAL(S)

Women with eggs retrieved for intracytoplasmic sperm injection (ICSI) cycles, adult female FVBN and B6C3F1 mouse strains, sea stars.

INTERVENTION(S)

Real-time, longitudinal imaging of oocytes after fluorescent labeling, insemination, and viability tests.

MAIN OUTCOME MEASURE(S)

Cell and embryo viability, immunolabeling efficiency, live cell endocytosis quantification, precise metrics of fertilization, and embryonic development.

RESULT(S)

Single oocytes were longitudinally imaged after significant changes in media, markers, endocytosis quantification, and development, all with supreme control by microfluidics. Cells remained viable, enclosed, and separate for precision measurements, repeatability, and imaging.

CONCLUSION(S)

We engineered a simple device to load, visualize, experiment, and effectively record individual oocytes and embryos without loss of cells. Prolonged incubation capabilities provide longitudinal studies without need for transfer and potential loss of cells. This simple perfusion apparatus provides for careful, precise, and flexible handling of precious samples facilitating clinical IVF approaches.

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.

Conservation of sequence and function in fertilization of the cortical granule serine protease in echinoderms

Conservation of the cortical granule serine protease during fertilization in echinoderms was tested both functionally in sea stars, and computationally throughout the echinoderm phylum. We find that the inhibitor of serine protease (soybean trypsin inhibitor) effectively blocks proper transition of the sea star fertilization envelope into a protective sperm repellent, whereas inhibitors of the other main types of proteases had no effect. Scanning the transcriptomes of 15 different echinoderm ovaries revealed sequences of high conservation to the originally identified sea urchin cortical serine protease, CGSP1. These conserved sequences contained the catalytic triad necessary for enzymatic activity, and the tandemly repeated LDLr-like repeats. We conclude that the protease involved in the slow block to polyspermy is an essential and conserved element of fertilization in echinoderms, and may provide an important reagent for identification and testing of the cell surface proteins in eggs necessary for sperm binding.

Migration of sea urchin primordial germ cells

BACKGROUND

Small micromeres are produced at the fifth cleavage of sea urchin development. They express markers of primordial germ cells (PGCs), and are required for the production of gametes. In most animals, PGCs migrate from sites of formation to the somatic gonad. Here, we investigated whether they also exhibit similar migratory behaviors using live-cell imaging of small micromere plasma membranes.

RESULTS

Early in gastrulation, small micromeres transition from non-motile epithelial cells, to motile quasi-mesenchymal cells. Late in gastrulation, at 43 hr post fertilization (HPF), they are embedded in the tip of the archenteron, but remain motile. From 43-49 HPF, they project numerous cortical blebs into the blastocoel, and filopodia that contact ectoderm. By 54 HPF, they begin moving in the plane of the blastoderm, often in a directed fashion, towards the coelomic pouches. Isolated small micromeres also produced blebs and filopodia.

CONCLUSIONS

Previous work suggested that passive translocation governs some of the movement of small micromeres during gastrulation. Here we show that small micromeres are motile cells that can traverse the archenteron, change position along the left-right axis, and migrate to coelomic pouches. These motility mechanisms are likely to play an important role in their left-right segregation.

Every which way--nanos gene regulation in echinoderms

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

Dysferlin is essential for endocytosis in the sea star oocyte

Dysferlin is a calcium-binding transmembrane protein involved in membrane fusion and membrane repair. In humans, mutations in the dysferlin gene are associated with muscular dystrophy. In this study, we isolated plasma membrane-enriched fractions from full-grown immature oocytes of the sea star, and identified dysferlin by mass spectrometry analysis. The full-length dysferlin sequence is highly conserved between human and the sea star. We learned that in the sea star Patiria miniata, dysferlin RNA and protein are expressed from oogenesis to gastrulation. Interestingly, the protein is highly enriched in the plasma membrane of oocytes. Injection of a morpholino against dysferlin leads to a decrease of endocytosis in oocytes, and to a developmental arrest during gastrulation. These results suggest that dysferlin is critical for normal endocytosis during oogenesis and for embryogenesis in the sea star and that this animal may be a useful model for studying the relationship of dysferlin structure as it relates to its function.

Meeting report – Oocyte maturation and fertilization: lessons from canonical and emerging models

The EMBO workshop ‘Oocyte maturation and fertilization: lessons from canonical and emerging models’ was held at the Oceanologic Observatory of Banyuls in France in June 2013 and was organized by Anne-Marie Geneviere, Olivier Haccard, Peter Lenart and Alex McDougall. A total of 78 participants shared their research on germline formation, oocyte development, sperm, fertilization and early development. Here, we report the highlights of this meeting.

Multidrug-resistant transport activity protects oocytes from chemotherapeutic agents and changes during oocyte maturation

OBJECTIVE

To determine the multidrug-resistant transporter (MDR) activity in oocytes and their potential role in oocyte susceptibility to chemotherapy.

DESIGN

Experimental laboratory study.

SETTING

University and academic center for reproductive medicine.

SUBJECT(S)

Women with eggs retrieved for intracytoplasmic sperm injection cycles and adult female FVBN and B6C3F1 mouse strains.

INTERVENTION(S)

Inhibition of MDR activity in oocytes.

MAIN OUTCOME MEASURE(S)

Efflux activity of MDRs with the use of quantitative fluorescent dye efflux, and oocyte cell death when exposed to chemotherapy.

RESULT(S)

Oocytes effluxed fluorescent reporters, and this activity was significantly reduced in the presence of the MDR inhibitor PSC 833. Geminal vesicle oocytes were more efficient at efflux than metaphase 2 oocytes. Human oocytes exposed to cyclophosphamide and PSC 833 showed cell death with the use of two different viability assays compared with control samples and those exposed to cyclophosphamide alone. Immunoblots detected MDR-1 in all oocytes, with the greatest accumulation in the geminal vesicle stage.

CONCLUSION(S)

Oocytes have a vast repertoire of active MDRs. The implications of this study are that these protective mechanisms are important during oogenesis and that these activities change with maturation, increasing susceptibility to toxicants. Future directions may exploit the up-regulation of these transporters during gonadotoxic therapy.

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

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

Diversity in the fertilization envelopes of echinoderms

Cell surface changes in an egg at fertilization are essential to begin development and for protecting the zygote. Most fertilized eggs construct a barrier around themselves by modifying their original extracellular matrix. This construction usually results from calcium-induced exocytosis of cortical granules, the contents of which in sea urchins function to form the fertilization envelope (FE), an extracellular matrix of cortical granule contents built upon a vitelline layer scaffold. Here, we examined the molecular mechanism of this process in sea stars, a close relative of the sea urchins, and analyze the evolutionary changes that likely occurred in the functionality of this structure between these two organisms. We find that the FE of sea stars is more permeable than in sea urchins, allowing diffusion of molecules in excess of 2 megadaltons. Through a proteomic and transcriptomic approach, we find that most, but not all, of the proteins present in the sea urchin envelope are present in sea stars, including SFE9, proteoliaisin, and rendezvin. The mRNAs encoding these FE proteins accumulated most densely in early oocytes, and then beginning with vitellogenesis, these mRNAs decreased in abundance to levels nearly undetectable in eggs. Antibodies to the SFE9 protein of sea stars showed that the cortical granules in sea star also accumulated most significantly in early oocytes, but different from sea urchins, they translocated to the cortex of the oocytes well before meiotic initiation. These results suggest that the preparation for cell surface changes in sea urchins has been shifted to later in oogenesis, and perhaps reflects the meiotic differences among the species-sea star oocytes are stored in prophase of meiosis and fertilized during the meiotic divisions, as in most animals, whereas sea urchins are one of the few taxons in which eggs have completed meiosis prior to fertilization.

mRNA-selective translation induced by FSH in primary Sertoli cells

FSH is a key hormonal regulator of Sertoli cell secretory activity, required to optimize sperm production. To fulfil its biological function, FSH binds a G protein-coupled receptor, the FSH-R. The FSH-R-transduced signaling network ultimately leads to the transcription or down-regulation of numerous genes. In addition, recent evidence has suggested that FSH might also regulate protein translation. However, this point has never been demonstrated conclusively yet. Here we have addressed this issue in primary rat Sertoli cells endogenously expressing physiological levels of FSH-R. We observed that, within 90 min of stimulation, FSH not only enhanced overall protein synthesis in a mammalian target of rapamycin-dependent manner but also increased the recruitment of mRNA to polysomes. m(7)GTP pull-down experiments revealed the functional recruitment of mammalian target of rapamycin and p70 S6 kinase to the 5'cap, further supported by the enhanced phosphorylation of one of p70 S6 kinase targets, the eukaryotic initiation factor 4B. Importantly, the scaffolding eukaryotic initiation factor 4G was also recruited, whereas eukaryotic initiation factor 4E-binding protein, the eukaryotic initiation factor 4E generic inhibitor, appeared to play a minor role in translational regulations induced by FSH, in contrast to what is generally observed in response to anabolic factors. This particular regulation of the translational machinery by FSH stimulation might support mRNA-selective translation, as shown here by quantitative RT-PCR amplification of the c-fos and vascular endothelial growth factor mRNA but not of all FSH target mRNA, in polysomal fractions. These findings add a new level of complexity to FSH biological roles in its natural target cells, which has been underappreciated so far.

The translational repressor 4E-BP called to order by eIF4E: new structural insights by SAXS

eIF4E binding protein (4E-BP) inhibits translation of capped mRNA by binding to the initiation factor eIF4E and is known to be mostly or completely unstructured in both free and bound states. Using small angle X-ray scattering (SAXS), we report here the analysis of 4E-BP structure in solution, which reveals that while 4E-BP is intrinsically disordered in the free state, it undergoes a dramatic compaction in the bound state. Our results demonstrate that 4E-BP and eIF4E form a ‘fuzzy complex’, challenging current visions of eIF4E/4E-BP complex regulation.

A variant mimicking hyperphosphorylated 4E-BP inhibits protein synthesis in a sea urchin cell-free, cap-dependent translation system

Background

4E-BP is a translational inhibitor that binds to eIF4E to repress cap-dependent translation initiation. This critical protein:protein interaction is regulated by the phosphorylation of 4E-BP. Hypophosphorylated 4E-BP binds to eIF4E and inhibits cap-dependent translation, whereas hyperphosphorylated forms do not. While three 4E-BP proteins exist in mammals, only one gene encoding for 4E-BP is present in the sea urchin genome. The protein product has a highly conserved core domain containing the eIF4E-binding domain motif (YxxxxLΦ) and four of the regulatory phosphorylation sites.

Methodology/Principal Findings

Using a sea urchin cell-free cap-dependent translation system prepared from fertilized eggs, we provide the first direct evidence that the sea urchin 4E-BP inhibits cap-dependent translation. We show here that a sea urchin 4E-BP variant, mimicking phosphorylation on four core residues required to abrogate binding to eIF4E, surprisingly maintains physical association to eIF4E and inhibits protein synthesis.

Conclusions/Significance

Here, we examine the involvement of the evolutionarily conserved core domain and phosphorylation sites of sea urchin 4E-BP in the regulation of eIF4E-binding. These studies primarily demonstrate the conserved activity of the 4E-BP translational repressor and the importance of the eIF4E-binding domain in sea urchin. We also show that a variant mimicking hyperphosphorylation of the four regulatory phosphorylation sites common to sea urchin and human 4E-BP is not sufficient for release from eIF4E and translation promotion. Therefore, our results suggest that there are additional mechanisms to that of phosphorylation at the four critical sites of 4E-BP that are required to disrupt binding to eIF4E.

[Gene expression regulation at the translational level: contribution of marine organisms]

Gene expression regulation is crucial for organism survival. Each step has to be regulated, from the gene to the protein. mRNA can be stored in the cell without any direct translation. This process is used by the cell to control protein synthesis rapidly at the right place, at the right time. Protein synthesis costs a lot of energy for the cell, so that a precise control of this process is required. Translation initiation represents an important step to regulate gene expression. Many factors that can bind mRNA and recruit different partners are involved in the inhibition or stimulation of protein synthesis. Oceans contain an important diversity of organisms that are used as important models to analyse gene expression at the translational level. These are useful to study translational control in different physiological processes for instance cell cycle (meiosis during meiotic maturation of starfish oocytes, mitosis following fertilization of sea urchin eggs) or to understand nervous system mechanisms (aplysia). All these studies will help finding novel actors involved in translational control and will thus be useful to discover new targets for therapeutic treatments against human diseases.

After fertilization of sea urchin eggs, eIF4G is post-translationally modified and associated with the cap-binding protein eIF4E

Release of eukaryotic initiation factor 4E (eIF4E) from its translational repressor eIF4E-binding protein (4E-BP) is a crucial event for the first mitotic division following fertilization of sea urchin eggs. Finding partners of eIF4E following fertilization is crucial to understand how eIF4E functions during this physiological process. The isolation and characterization of cDNA encoding Sphaerechinus granularis eIF4G (SgIF4G) are reported. mRNA of SgIF4G is present as a single 8.5-kb transcript in unfertilized eggs, suggesting that only one ortholog exists in echinoderms. The longest open reading frame predicts a sequence of 5235 nucleotides encoding a deduced polypeptide of 1745 amino acids with a predicted molecular mass of 192 kDa. Among highly conserved domains, SgIF4G protein possesses motifs that correspond to the poly(A) binding protein and eIF4E protein-binding sites. A specific polyclonal antibody was produced and used to characterize the SgIF4G protein in unfertilized and fertilized eggs by SDS-PAGE and western blotting. Multiple differentially migrating bands representing isoforms of sea urchin eIF4G are present in unfertilized eggs. Fertilization triggers modifications of the SgIF4G isoforms and rapid formation of the SgIF4G-eIF4E complex. Whereas rapamycin inhibits the formation of the SgIF4G-eIF4E complex, modification of these SgIF4G isoforms occurs independently from the rapamycin-sensitive pathway. Microinjection of a peptide corresponding to the eIF4E-binding site derived from the sequence of SgIF4G into unfertilized eggs affects the first mitotic division of sea urchin embryos. Association of SgIF4G with eIF4E is a crucial event for the onset of the first mitotic division following fertilization, suggesting that cap-dependent translation is highly regulated during this process. This hypothesis is strengthened by the evidence that microinjection of the cap analog m(7)GDP into unfertilized eggs inhibits the first mitotic division.