Evidence for a mesodermal embryonic regulator of the sea urchin CyIIa gene

An Elk transcription factor is required for Runx-dependent survival signaling in the sea urchin embryo

Elk proteins are Ets family transcription factors that regulate cell proliferation, survival, and differentiation in response to ERK (extracellular-signal regulated kinase)-mediated phosphorylation. Here we report the embryonic expression and function of Sp-Elk, the single Elk gene of the sea urchin Strongylocentrotus purpuratus. Sp-Elk is zygotically expressed throughout the embryo beginning at late cleavage stage, with peak expression occurring at blastula stage. Morpholino antisense-mediated knockdown of Sp-Elk causes blastula-stage developmental arrest and embryo disintegration due to apoptosis, a phenotype that is rescued by wild-type Elk mRNA. Development is also rescued by Elk mRNA encoding a serine to aspartic acid substitution (S402D) that mimics ERK-mediated phosphorylation of a conserved site that enhances DNA binding, but not by Elk mRNA encoding an alanine substitution at the same site (S402A). This demonstrates both that the apoptotic phenotype of the morphants is specifically caused by Elk depletion, and that phosphorylation of serine 402 of Sp-Elk is critical for its anti-apoptotic function. Knockdown of Sp-Elk results in under-expression of several regulatory genes involved in cell fate specification, cell cycle control, and survival signaling, including the transcriptional regulator Sp-Runt-1 and its target Sp-PKC1, both of which were shown previously to be required for cell survival during embryogenesis. Both Sp-Runt-1 and Sp-PKC1 have sequences upstream of their transcription start sites that specifically bind Sp-Elk. These results indicate that Sp-Elk is the signal-dependent activator of a feed-forward gene regulatory circuit, consisting also of Sp-Runt-1 and Sp-PKC1, which actively suppresses apoptosis in the early embryo.

Two ParaHox genes, SpLox and SpCdx, interact to partition the posterior endoderm in the formation of a functional gut

We report the characterization of the ortholog of the Xenopus XlHbox8 ParaHox gene from the sea urchin Strongylocentrotus purpuratus, SpLox. It is expressed during embryogenesis, first appearing at late gastrulation in the posterior-most region of the endodermal tube, becoming progressively restricted to the constriction between the mid- and hindgut. The physiological effects of the absence of the activity of this gene have been analyzed through knockdown experiments using gene-specific morpholino antisense oligonucleotides. We show that blocking the translation of the SpLox mRNA reduces the capacity of the digestive tract to process food, as well as eliminating the morphological constriction normally present between the mid- and hindgut. Genetic interactions of the SpLox gene are revealed by the analysis of the expression of a set of genes involved in endoderm specification. Two such interactions have been analyzed in more detail: one involving the midgut marker gene Endo16, and another involving the other endodermally expressed ParaHox gene, SpCdx. We find that SpLox is able to bind Endo16 cis-regulatory DNA, suggesting direct repression of Endo16 expression in presumptive hindgut territories. More significantly, we provide the first evidence of interaction between ParaHox genes in establishing hindgut identity, and present a model of gene regulation involving a negative-feedback loop.

The surprising complexity of the transcriptional regulation of the spdri gene reveals the existence of new linkages inside sea urchin's PMC and Oral Ectoderm Gene Regulatory Networks

During sea urchin embryogenesis the spdri gene participates in two separate Gene Regulatory Networks (GRNs): the Primary Mesenchyme Cells' (PMCs) and the Oral Ectoderm's one. In both cases, activation of the gene follows initial specification events [Amore, G., Yavrouian, R., Peterson, K., Ransick, A., McClay, D., Davidson, E., 2003. Spdeadringer, a sea urchin embryo gene required separately in skeletogenic and oral ectoderm gene regulatory networks. Dev. Biol. 261, 55-81.]. We identified a portion of genomic DNA ("4.7IL" -3456;+389) which is sufficient to replicate sdpri's expression pattern in experiments of transgenesis, using a GFP reporter. In our experiments, the activation kinetic of 4.7IL-GFP was similar to that of the endogenous gene and the reporter responded to known spdri's transcriptional regulators (Ets1, Alx1, Gsc and Dri). Here we present a dissection of this regulatory region and a description of the modules involved in spdri's transcriptional regulation. Both in the PMCs' and Oral Ectoderm's expression phases, activation of spdri is obtained through the integration of three kinds of inputs: positive and globally distributed ones; negative ones (that prevent ectopic expression); positive and tissue-specific ones. Our results allow to expand the map of the regulatory connections at the spdri node, both in the PMCs and in the Oral Ectoderm Gene Regulatory Networks (GRNs).

Mesenchymal cell fusion in the sea urchin embryo

Mesenchymal cells of the sea urchin embryo provide a valuable experimental model for the analysis of cell-cell fusion in vivo. The unsurpassed optical transparency of the sea urchin embryo facilitates analysis of cell fusion in vivo using fluorescent markers and time-lapse three-dimensional imaging. Two populations of mesodermal cells engage in homotypic cell-cell fusion during gastrulation: primary mesenchyme cells and blastocoelar cells. In this chapter, we describe methods for studying the dynamics of cell fusion in living embryos. These methods have been used to analyze the fusion of primary mesenchyme cells and are also applicable to blastocoelar cell fusion. Although the molecular basis of cell fusion in the sea urchin has not been investigated, tools have recently become available that highlight the potential of this experimental model for integrating dynamic morphogenetic behaviors with underlying molecular mechanisms.

Gene regulatory networks and developmental plasticity in the early sea urchin embryo: alternative deployment of the skeletogenic gene regulatory network

Cell fates in the sea urchin embryo are remarkably labile, despite the fact that maternal polarity and zygotic programs of differential gene expression pattern the embryo from the earliest stages. Recent work has focused on transcriptional gene regulatory networks (GRNs) deployed in specific embryonic territories during early development. The micromere-primary mesenchyme cell(PMC) GRN drives the development of the embryonic skeleton. Although normally deployed only by presumptive PMCs, every lineage of the early embryo has the potential to activate this pathway. Here, we focus on one striking example of regulative activation of the skeletogenic GRN; the transfating of non-skeletogenic mesoderm (NSM) cells to a PMC fate during gastrulation. We show that transfating is accompanied by the de novo expression of terminal,biomineralization-related genes in the PMC GRN, as well as genes encoding two upstream transcription factors, Lvalx1 and Lvtbr. We report that Lvalx1, a key component of the skeletogenic GRN in the PMC lineage, plays an essential role in the regulative pathway both in NSM cells and in animal blastomeres. MAPK signaling is required for the expression of Lvalx1 and downstream skeletogenic genes in NSM cells, mirroring its role in the PMC lineage. We also demonstrate that Lvalx1 regulates the signal from PMCs that normally suppresses NSM transfating. Significantly,misexpression of Lvalx1 in macromeres (the progenitors of NSM cells)is sufficient to activate the skeletogenic GRN. We suggest that NSM cells normally deploy a basal mesodermal pathway and require only an Lvalx1-mediated sub-program to express a PMC fate. Finally, we provide evidence that, in contrast to the normal pathway, activation of the skeletogenic GRN in NSM cells is independent of Lvpmar1. Our studies reveal that, although most features of the micromere-PMC GRN are recapitulated in transfating NSM cells, different inputs activate this GRN during normal and regulative development.

Carbohydrate involvement in cellular interactions in sea urchin gastrulation

The sea urchin embryo is a model for studying cellular interactions that occur in higher organisms because of its availability, transparency, and accessibility to molecular probes. In previous studies, we found that the mannose/glucose-binding lectin Lens culinaris agglutinin entered living sea urchin embryos, bound to specific cell types and caused exogastrulation, when the developing gut (archenteron) falls out of the embryo proper. We have proposed that the lectin bound to sugar-containing ligands, thus preventing attachment of the archenteron to the blastocoel roof, resulting in exogastrulation. Here, we have continued our study of cellular interactions in this model using Lytechinus pictus sea urchin embryos, and have found that inhibitors of glycoprotein/proteoglycan synthesis, tunicamycin and sodium selenate, and the specific glycosidases, beta-amylase, alpha-glucosidase, and alpha-mannosidase, all inhibit archenteron organization, elongation, and attachment to the blastocoel roof in viable swimming embryos. We also show that single cells obtained by disaggregation of 32-h-old sea urchin embryos bind to L. culinaris agglutinin- and concanavalin A-derivatized beads; the binding is blocked by alpha-methyl mannose, but not l-fucose. These cells also bind to beads derivatized with mannan. These results provide evidence for a role of carbohydrate-containing molecules in cellular interactions in sea urchin gastrulation. In a second set of experiments, we found that the supernatant obtained by disaggregation of 24-32-h-old L. pictus embryos in calcium- and magnesium-free sea water contains molecules that cause exogastrulation, archenteron disorganization, inhibition of archenteron elongation and inhibition of archenteron attachment to the blastocoel roof in viable swimming embryos. We propose that the supernatant contains ligands and/or receptors that mediate archenteron development and attachment to the blastocoel roof and are released when embryos are disaggregated into single cells. These studies may lead to a better understanding of the molecular basis of mechanisms that control cellular interactions during development.

Role of the ERK-mediated signaling pathway in mesenchyme formation and differentiation in the sea urchin embryo

Mesoderm and mesodermal structures in the sea urchin embryo are entirely generated by two embryologically distinct populations of mesenchyme cells: the primary (PMC) and the secondary (SMC) mesenchyme cells. We have identified the extracellular signal-regulated kinase (ERK) as a key component of the regulatory machinery that controls the formation of both these cell types. ERK is activated in a spatial-temporal manner, which coincides with the epithelial-mesenchyme transition (EMT) of the prospective PMCs and SMCs. Here, we show that ERK controls EMT of both primary and secondary mesenchyme cells. Loss and gain of function experiments demonstrate that ERK signaling is not required for the early specification of either PMCs or SMCs, but controls the maintenance and/or the enhancement of expression levels of regulatory genes which participate in the process of specification of these cell types. In addition, ERK-mediated signaling is essential for the transcription of terminal differentiation genes encoding proteins that define the final structures generated by PMCs and SMCs. Our findings suggest that ERK has a central pan-mesodermal role in coupling EMT and terminal differentiation of all mesenchymal cell types in the sea urchin embryo.

Using reporter genes to study cis-regulatory elements

This chapter summarizes four powerful assays for analyzing gene expression in cis-regulatory studies. The enzymatic assays (CAT, luciferase, lacZ) are currently limited by their application to embryo homogenates or fixed samples, but offer more robust analysis of gene activity than GFP. Assays based on CAT enzymatic activity or on CAT mRNA detection by WMISH are laborious but are well established for accurately quantifying gene expression and to determine spatial patterns at defined timepoints during development. LacZ assays are the current standard for spatially visualizing gene products in whole-mount fixed embryos. They are very sensitive but they provide limited temporal or quantitative information due to the perdurance of beta-galactosidase and the subtleties of the staining technique. Recently developed luciferase assays promise to be even more sensitive and accurate than the CAT and lacZ assays, and applicable to living cells and embryos. But, they have not yet been well established in invertebrate deuterostome research. GFP allows visualization of gene expression within living embryos. But because this is not an enzymatic assay, sensitivity can be a problem, particularly for weak promoters. Furthermore, imaging live embryos and quantifying gene expression in space and time (due to scattering of light by tissue, the perdurance of GFP, and other experimental details) is currently fraught with challenges. Ongoing improvements in imaging technology and the advent of multiple fluorescent proteins, as well as fluorescent and luminescent assays for vital imaging, will dramatically facilitate studies of gene expression in the coming decade.

Phylogeny and development of marine model species: strongylocentrotid sea urchins

The phylogenetic relationships of ten strongy-locentrotid sea urchin species were determined using mitochondrial DNA sequences. This phylogeny provides a backdrop for the evolutionary history of one of the most studied groups of sea urchins. Our phylogeny indicates that a major revision of this group is in order. All else remaining unchanged, it supports the inclusion of three additional species into the genus Strongylocentrotus (Hemicentrotus pulcherrimus, Allocentrotus fragilis, and Pseudocentrotus depressus). All were once thought to be closely related to this genus, but subsequent revisions separated them into other taxonomic groupings. Most strongylocentrotid species are the result of a recent burst of speciation in the North Pacific that resulted in an ecological diversification. There has been a steady reduction in the complexity of larval skeletons during the expansion of this group. Gamete attributes like egg size, on the other hand, are not correlated with phylogenetic position. In addition, our results indicate that the rate of replacement substitutions is highly variable among phylogenetic lineages. The branches leading to S. purpuratus and S. franciscanus were three to six times longer than those leading to closely related species.

Visualization of molecular and cellular events with green fluorescent proteins in developing embryos: a review

During the past 5 years, green fluorescent protein (GFP) has become one of the most widely used in vivo protein markers for studying a number of different molecular processes during development, such as promoter activation, gene expression, protein trafficking and cell lineage determination. GFP fluorescence allows observation of dynamic developmental processes in real time, in both transiently and stably transformed cells, as well as in live embryos. In this review, we include the most up-to-date use of GFP during embryonic development and point out the unique contribution of GFP visualization, which resulted in novel discoveries.

Functional characterization of Ets-binding sites in the sea urchin embryo: three base pair conversions redirect expression from mesoderm to ectoderm and endoderm

Because of the limited knowledge of target genes for the ets family of transcription factors, it is yet unclear how specificity of biological function among different members is achieved in this class of proteins. In the present study, we compared two Ets-binding sites in two differentially expressed genes of the sea urchin embryo. The first gene examined is the cytoskeletal actin CyIIa, which is transiently expressed in skeletogenic and secondary mesenchyme and in its terminal and permanent phase in the gut. The second one encodes the hatching enzyme gene of Strongylocentrotus purpuratus, and is regulated cell-autonomously and asymmetrically along the maternally determined animal-vegetal axis. The Ets sites within the regulatory regions of these two genes interact and form different binding complexes with proteins present in the nuclei of mesenchyme blastula embryos. We also demonstrated that the DNA binding specificity of the CyIIa Ets-binding site can be converted to the other type of Ets site, as in the hatching enzyme promoter, by changing only three base pairs near the Ets core sequence. Switching of these three base pairs near the central GGA trinucleotide motif characteristic of all Ets-binding targets was also sufficient to redirect expression of a reporter gene construct containing a heterologous basal promoter from mesenchyme to non-mesenchyme cell type in transgenic sea urchin embryos. These observations suggest that binding affinity of ets transcription factors plays an important role in determining cell type-specific gene expression.