The Maternal Maverick/GDF15-like TGF-β Ligand Panda Directs Dorsal-Ventral Axis Formation by Restricting Nodal Expression in the Sea Urchin Embryo

Maternal TGF-β ligand Panda breaks the radial symmetry of the sea urchin embryo by antagonizing the Nodal type II receptor ACVRII

In the highly regulative embryo of the sea urchin Paracentrotus lividus, establishment of the dorsal-ventral (D/V) axis critically depends on the zygotic expression of the TGF-β nodal in the ventral ectoderm. nodal expression is first induced ubiquitously in the 32-cell embryo and becomes progressively restricted to the presumptive ventral ectoderm by the early blastula stage. This early spatial restriction of nodal expression is independent of Lefty, and instead relies on the activity of Panda, a maternally expressed TGF-β ligand related to Lefty and Inhibins, which is required maternally for D/V axis specification. However, the mechanism by which Panda restricts the early nodal expression has remained enigmatic and it is not known if Panda works like a BMP ligand by opposing Nodal and antagonizing Smad2/3 signaling, or if it works like Lefty by sequestering an essential component of the Nodal signaling pathway. In this study, we report that Panda functions as an antagonist of the TGF-β type II receptor ACVRII (Activin receptor type II), which is the only type II receptor for Nodal signaling in the sea urchin and is also a type II receptor for BMP ligands. Inhibiting translation of acvrII mRNA disrupted D/V patterning across all 3 germ layers and caused acvrII morphants to develop with a typical Nodal loss-of-function phenotype. In contrast, embryos overexpressing acvrII displayed strong ectopic Smad1/5/8 signaling at blastula stages and developed as dorsalized larvae, a phenotype very similar to that caused by over activation of BMP signaling. Remarkably, embryos co-injected with acvrII mRNA and panda mRNA did not show ectopic Smad1/5/8 signaling and developed with a largely normal dorsal-ventral polarity. Furthermore, using an axis induction assay, we found that Panda blocks the ability of ACVRII to orient the D/V axis when overexpressed locally. Using co-immunoprecipitation, we showed that Panda physically interacts with ACVRII, as well as with the Nodal co-receptor Cripto, and with TBR3 (Betaglycan), which is a non-signaling receptor for Inhibins in mammals. At the molecular level, we have traced back the antagonistic activity of Panda to the presence of a single proline residue, conserved with all the Lefty factors, in the ACVRII binding motif of Panda, instead of a serine as in most of TGF-β ligands. Conversion of this proline to a serine converted Panda from an antagonist that opposed Nodal signaling and promoted dorsalization to an agonist that promoted Nodal signaling and triggered ventralization when overexpressed. Finally, using phylogenomics, we analyzed the emergence of the agonist and antagonist form of Panda in the course of evolution. Our data are consistent with the idea that the presence of a serine at that position, like in most TGF-β, was the ancestral condition and that the initial function of Panda was possibly in promoting and not in antagonizing Nodal signaling. These results highlight the existence of key functional and structural elements conserved between Panda and Lefty, allow to draw an intriguing parallel between sea urchin Panda and mammalian Inhibin α and raise the unexpected possibility that the original function of Panda may have been in activation of the Nodal pathway rather than in its inhibition.

An approach to quantitate maternal transcripts localized in sea urchin egg cortex using RT-qPCR with accurate normalization

The sea urchin egg cortex is a peripheral region of eggs comprising a cell membrane and adjacent cytoplasm, which contains actin and tubulin cytoskeleton, cortical granules and some proteins required for early development. Method for isolation of cortices from sea urchin eggs and early embryos was developed in 1970s. Since then, this method has been reliable tool to study protein localization and cytoskeletal organization in cortex of unfertilized eggs and embryos during first cleavages. This study was aimed to estimate the reliability of RT-qPCR to analyze levels of maternal transcripts that are localized in egg cortex. Firstly, we selected seven potential reference genes, 28S, Cycb, Ebr1, GAPDH, Hmg1, Smtnl1 and Ubb, the transcripts of which are maternally deposited in sea urchin eggs. The candidate reference genes were ranked by five different algorithms (BestKeeper, CV, ΔCt, geNorm and NormFinder) based on calculated level of stability in both eggs as well as isolated cortices. Our results showed that gene ranking differs in total RNA and mRNA samples, though Ubb is most suitable reference gene in both cases. To validate feasibility of comparative analysis of eggs and isolated egg cortices, we selected Daglb-2 as a gene of interest, which transcripts are potentially localized in cortex according to transcriptome analysis, and observed increased level of Daglb-2 in egg cortices by RT-qPCR. This suggests that proposed RNA isolation method with subsequent quantitative RT-qPCR analysis can be used to determine cortical association of transcripts in sea urchin eggs.

Dorsal-ventral axis formation in sea urchin embryos

Most sea urchin species produce planktonic feeding larvae with distinct dorsal-ventral polarity. Such morphological indicators of polarity arise after gastrulation, when several morphogenesis and cell differentiation events occur differentially along the dorsal-ventral axis. For instance, the gut bends toward the ventral side where the mouth will form, skeletogenesis occurs initially near the ventral side with the forming skeleton extending dorsally, and pigment cells differentiate and embed in the dorsal ectoderm. The patterning mechanisms and gene regulatory networks underlying these events have been extensively studied. Two opposing TGF-β signaling pathways, Nodal and BMP, play key roles in all three germ layers to respectively pattern the sea urchin ventral and dorsal sides. In this chapter, I describe our current understanding of sea urchin dorsal-ventral patterning mechanisms. Additionally, differences in the patterning mechanisms observed in lecithotrophic sea urchins (nonfeeding larvae) and in cidaroid sea urchins are also discussed, along with evolutionary insights gained from comparative analyses.

BMP Signaling: Lighting up the Way for Embryonic Dorsoventral Patterning

One of the most significant events during early embryonic development is the establishment of a basic embryonic body plan, which is defined by anteroposterior, dorsoventral (DV), and left-right axes. It is well-known that the morphogen gradient created by BMP signaling activity is crucial for DV axis patterning across a diverse set of vertebrates. The regulation of BMP signaling during DV patterning has been strongly conserved across evolution. This is a remarkable regulatory and evolutionary feat, as the BMP gradient has been maintained despite the tremendous variation in embryonic size and shape across species. Interestingly, the embryonic DV axis exhibits robust stability, even in face of variations in BMP signaling. Multiple lines of genetic, molecular, and embryological evidence have suggested that numerous BMP signaling components and their attendant regulators act in concert to shape the developing DV axis. In this review, we summarize the current knowledge of the function and regulation of BMP signaling in DV patterning. Throughout, we focus specifically on popular model animals, such as Xenopus and zebrafish, highlighting the similarities and differences of the regulatory networks between species. We also review recent advances regarding the molecular nature of DV patterning, including the initiation of the DV axis, the formation of the BMP gradient, and the regulatory molecular mechanisms behind BMP signaling during the establishment of the DV axis. Collectively, this review will help clarify our current understanding of the molecular nature of DV axis formation.

Development of a larval nervous system in the sea urchin

This review reports recent findings on the specification and patterning of neurons that establish the larval nervous system of the sea urchin embryo. Neurons originate in three regions of the embryo. Perturbation analyses enabled construction of gene regulatory networks controlling the several neural cell types. Many of the mechanisms described reflect shared features of all metazoans and others are conserved among deuterostomes. This nervous system with a very small number of neurons supports the feeding and swimming behaviors of the larva until metamorphosis when an adult nervous system replaces that system.

TGFβ family signaling and development

The transforming growth factor β (TGFβ) signaling family is evolutionarily conserved in metazoans. The signal transduction mechanisms of TGFβ family members have been expansively investigated and are well understood. During development and homeostasis, numerous TGFβ family members are expressed in various cell types with temporally changing levels, playing diverse roles in embryonic development, adult tissue homeostasis and human diseases by regulating cell proliferation, differentiation, adhesion, migration and apoptosis. Here, we discuss the molecular mechanisms underlying signal transduction and regulation of the TGFβ subfamily pathways, and then highlight their key functions in mesendoderm induction, dorsoventral patterning and laterality development, as well as in the formation of several representative tissues/organs.

Coup-TF: A maternal factor essential for differentiation along the embryonic axes in the sea urchin Paracentrotus lividus

Coup-TF, a member of the nuclear receptor super-family, is present in the pool of maternal mRNAs and proteins in the sea urchin egg. The presence of this protein seems to be essential for the execution of the early developmental program, leading to all three embryonic layers. Our results demonstrate that Pl-Coup-TF morphants, i.e. Pl-Coup-TF morpholino knockdown embryos, resemble blastulae that lack archenteron at 24 hpf (hours post fertilization), a stage at which normal embryos reach the end of gastrulation in Paracentrotus lividus. At 48 hpf, when normal embryos reach the pluteus larva stage, the morphants are seemingly underdeveloped and lack the characteristic skeletal rods. Nevertheless, the morphant embryos express vegetal endomesodermal marker genes, such as Pl-Blimp1, Pl-Endo16, Pl-Alx1 and Pl-Tbr as judged by in situ hybridization experiments. The anterior neuroectoderm genes, Pl-FoxQ2, Pl-Six3 and Pl-Pax6, are also expressed in the morphant embryos, but Pl-Hbn and Pl-Fez mRNAs, which encode proteins significant for the differentiation of serotonergic neurons, are not detected. Consequently, Pl-Coup-TF morphants at 48 hpf lack serotonergic neurons, whereas normal 48 hpf plutei exhibit the formation of two bilateral pairs of such neurons in the apical organ. Furthermore, genes indicative of the ciliary band formation, Pl-Hnf6, Pl-Dri, Pl-FoxG and Pl-Otx, are not expressed in Pl-Coup-TF morphants, suggesting the disruption of this neurogenic territory as well. In addition, the Pl-SynB gene, a marker of differentiated neurons, is silent leading to the hypothesis that Pl-Coup-TF morphants might lack all types of neurons. On the contrary, the genes expressing signaling molecules, which establish the ventral/dorsal axis, Pl-Nodal and Pl-Lefty show the characteristic ventral lateral expression pattern, Pl-Bmp2/4, which activates the dorsal ectoderm GRN is down-regulated and Pl-Chordin is aberrantly over-expressed in the entire ectoderm. The identity of ectodermal cells in Pl-Coup-TF morphant embryos, was probed for expression of the ventral marker Pl-Gsc which was over-expressed and dorsal markers, Pl-IrxA and Pl-Hox7, which were silent. Therefore, we propose that maternal Pl-Coup-TF is essential for correct dissemination of the early embryonic signaling along both animal/vegetal and ventral/dorsal axes. Limiting Pl-Coup-TF's quantity, results in an embryo without digestive and nervous systems, skeleton and ciliary band that cannot survive past the initial 48 h of development.

Deciphering and modelling the TGF-β signalling interplays specifying the dorsal-ventral axis of the sea urchin embryo

During sea urchin development, secretion of Nodal and BMP2/4 ligands and their antagonists Lefty and Chordin from a ventral organiser region specifies the ventral and dorsal territories. This process relies on a complex interplay between the Nodal and BMP pathways through numerous regulatory circuits. To decipher the interplay between these pathways, we used a combination of treatments with recombinant Nodal and BMP2/4 proteins and a computational modelling approach. We assembled a logical model focusing on cell responses to signalling inputs along the dorsal-ventral axis, which was extended to cover ligand diffusion and enable multicellular simulations. Our model simulations accurately recapitulate gene expression in wild-type embryos, accounting for the specification of ventral ectoderm, ciliary band and dorsal ectoderm. Our model simulations further recapitulate various morphant phenotypes, reveal a dominance of the BMP pathway over the Nodal pathway and stress the crucial impact of the rate of Smad activation in dorsal-ventral patterning. These results emphasise the key role of the mutual antagonism between the Nodal and BMP2/4 pathways in driving early dorsal-ventral patterning of the sea urchin embryo.

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.

Genomic resources for the study of echinoderm development and evolution

Echinoderms are important research models for a wide range of biological questions. In particular, echinoderm embryos are exemplary models for dissecting the molecular and cellular processes that drive development and testing how these processes can be modified through evolution to produce the extensive morphological diversity observed in the phylum. Modern attempts to characterize these processes depend on some level of genomic analysis; from querying annotated gene sets to functional genomics experiments to identify candidate cis-regulatory sequences. Given how essential these data have become, it is important that researchers using available datasets or performing their own genome-scale experiments understand the nature and limitations of echinoderm genomic analyses. In this chapter we highlight the current state of echinoderm genomic data and provide methodological considerations for common approaches, including analysis of transcriptome and functional genomics datasets.

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.

MAPK and GSK3/ß-TRCP-mediated degradation of the maternal Ets domain transcriptional repressor Yan/Tel controls the spatial expression of nodal in the sea urchin embryo

In the sea urchin embryo, specification of the dorsal-ventral axis critically relies on the spatially restricted expression of nodal in the presumptive ventral ectoderm. The ventral restriction of nodal expression requires the activity of the maternal TGF-β ligand Panda but the mechanism by which Panda restricts nodal expression is unknown. Similarly, what initiates expression of nodal in the ectoderm and what are the mechanisms that link patterning along the primary and secondary axes is not well understood. We report that in Paracentrotus lividus, the activity of the maternally expressed ETS-domain transcription factor Yan/Tel is essential for the spatial restriction of nodal. Inhibiting translation of maternal yan/tel mRNA disrupted dorsal-ventral patterning in all germ layers by causing a massive ectopic expression of nodal starting from cleavage stages, mimicking the phenotype caused by inactivation of the maternal Nodal antagonist Panda. We show that like in the fly or in vertebrates, the activity of sea urchin Yan/Tel is regulated by phosphorylation by MAP kinases. However, unlike in the fly or in vertebrates, phosphorylation by GSK3 plays a central role in the regulation Yan/Tel stability in the sea urchin. We show that GSK3 phosphorylates Yan/Tel in vitro at two different sites including a β-TRCP ubiquitin ligase degradation motif and a C-terminal Ser/Thr rich cluster and that phosphorylation of Yan/Tel by GSK3 triggers its degradation by a β-TRCP/proteasome pathway. Finally, we show that, Yan is epistatic to Panda and that the activity of Yan/Tel is required downstream of Panda to restrict nodal expression. Our results identify Yan/Tel as a central regulator of the spatial expression of nodal in Paracentrotus lividus and uncover a key interaction between the gene regulatory networks responsible for patterning the embryo along the dorsal-ventral and animal-vegetal axes.

Specification of the embryonic axes is an essential step during early development of metazoa. In the sea urchin embryo, specification of the dorsal-ventral axis critically relies on the spatial restriction of the expression of the TGF-ß family member Nodal in ventral cells, a process that requires the activity of the maternal determinant Panda. How the spatially restricted expression of nodal is established downstream of Panda is not well understood. We have discovered that, in the Mediterranean sea urchin Paracentrotus lividus, the spatial restriction of nodal on the ventral side of the embryo requires the inhibitory activity of a transcriptional repressor named Yan/Tel. This finding suggests a molecular mechanism for the control of nodal expression by the release of a repression. We found that this release requires the activity of two families of kinases that we identified as the MAP kinases and GSK3, a kinase which, intriguingly, was previously known as a key regulator of patterning along the animal-vegetal axis. We discovered that phosphorylation by MAPK and GSK3 triggers degradation of Yan/Tel by a β-TRCP proteasome pathway. Finally, we find that Yan/Tel likely acts downstream of Panda in the hierarchy of genes required for nodal restriction. Our study therefore identifies Yan/Tel as a new essential regulator of nodal expression downstream of Panda and identifies a novel key interaction between the gene regulatory networks responsible for patterning along the primary and secondary axis of polarity.

Translatome analysis at the egg-to-embryo transition in sea urchin

Early embryogenesis relies on the translational regulation of maternally stored mRNAs. In sea urchin, fertilization triggers a dramatic rise in translation activity, necessary for the onset of cell division. Here, the full spectrum of the mRNAs translated upon fertilization was investigated by polysome profiling and sequencing. The translatome of the early sea urchin embryo gave a complete picture of the polysomal recruitment dynamics following fertilization. Our results indicate that only a subset of maternal mRNAs were selectively recruited onto polysomes, with over-represented functional categories in the translated set. The increase in translation upon fertilization depends on the formation of translation initiation complexes following mTOR pathway activation. Surprisingly, mTOR pathway inhibition differentially affected polysomal recruitment of the newly translated mRNAs, which thus appeared either mTOR-dependent or mTOR-independent. Therefore, our data argue for an alternative to the classical cap-dependent model of translation in early development. The identification of the mRNAs translated following fertilization helped assign translational activation events to specific mRNAs. This translatome is the first step to a comprehensive analysis of the molecular mechanisms governing translation upon fertilization and the translational regulatory networks that control the egg-to-embryo transition as well as the early steps of embryogenesis.

Causes and evolutionary consequences of primordial germ-cell specification mode in metazoans

In animals, primordial germ cells (PGCs) give rise to the germ lines, the cell lineages that produce sperm and eggs. PGCs form in embryogenesis, typically by one of two modes: a likely ancestral mode wherein germ cells are induced during embryogenesis by cell-cell signaling (induction) or a derived mechanism whereby germ cells are specified by using germ plasm-that is, maternally specified germ-line determinants (inheritance). The causes of the shift to germ plasm for PGC specification in some animal clades remain largely unknown, but its repeated convergent evolution raises the question of whether it may result from or confer an innate selective advantage. It has been hypothesized that the acquisition of germ plasm confers enhanced evolvability, resulting from the release of selective constraint on somatic gene networks in embryogenesis, thus leading to acceleration of an organism's protein-sequence evolution, particularly for genes expressed at early developmental stages, and resulting in high speciation rates in germ plasm-containing lineages (denoted herein as the "PGC-specification hypothesis"). Although that hypothesis, if supported, could have major implications for animal evolution, our recent large-scale coding-sequence analyses from vertebrates and invertebrates provided important examples of genera that do not support the hypothesis of liberated constraint under germ plasm. Here, we consider reasons why germ plasm might be neither a direct target of selection nor causally linked to accelerated animal evolution. We explore alternate scenarios that could explain the repeated evolution of germ plasm and propose potential consequences of the inheritance and induction modes to animal evolutionary biology.

Spatial and temporal control of NODAL signaling

Embryonic development is orchestrated by the activity of signal transduction pathways, amongst which are those downstream of the transforming growth factor β (TGF-β) family. Here I focus on signalling by one of these ligands, NODAL, which is essential for early embryonic axis patterning. I review recent advances in our understanding of how NODAL signalling is transduced from the plasma membrane to the nucleus to regulate the transcription of target genes, and how domains of NODAL activity are established and refined during embryonic development. The duration of signalling is emerging as a key determinant of the specificity of downstream responses in terms of cell fate decisions and I will discuss what is currently known about the underlying mechanisms.

Using Morpholinos to Probe Gene Networks in Sea Urchin

The control processes that underlie the progression of development can be summarized in maps of gene regulatory networks (GRNs). A critical step in their assembly is the systematic perturbation of network candidates. In sea urchins the most important method for interfering with expression in a gene-specific way is application of morpholino antisense oligonucleotides (MOs). MOs act by binding to their sequence complement in transcripts resulting in a block in translation or a change in splicing and thus result in a loss of function. Despite the tremendous success of this technology, recent comparisons to mutants generated by genome editing have led to renewed criticism and challenged its reliability. As with all methods based on sequence recognition, MOs are prone to off-target binding that may result in phenotypes that are erroneously ascribed to the loss of the intended target. However, the slow progression of development in sea urchins has enabled extremely detailed studies of gene activity in the embryo. This wealth of knowledge paired with the simplicity of the sea urchin embryo enables careful analysis of MO phenotypes through a variety of methods that do not rely on terminal phenotypes. This article summarizes the use of MOs in probing GRNs and the steps that should be taken to assure their specificity.

Embryonic neurogenesis in echinoderms

The phylogenetic position of echinoderms is well suited to revealing shared features of deuterostomes that distinguish them from other bilaterians. Although echinoderm neurobiology remains understudied, genomic resources, molecular methods, and systems approaches have enabled progress in understanding mechanisms of embryonic neurogenesis. Even though the morphology of echinoderm larvae is diverse, larval nervous systems, which arise during gastrulation, have numerous similarities in their organization. Diverse neural subtypes and specialized sensory neurons have been identified and details of neuroanatomy using neuron-specific labels provide hypotheses for neural function. The early patterning of ectoderm and specification of axes has been well studied in several species and underlying gene regulatory networks have been established. The cells giving rise to central and peripheral neural components have been identified in urchins and sea stars. Neurogenesis includes typical metazoan features of asymmetric division of neural progenitors and in some cases limited proliferation of neural precursors. Delta/Notch signaling has been identified as having critical roles in regulating neural patterning and differentiation. Several transcription factors functioning in pro-neural phases of specification, neural differentiation, and sub-type specification have been identified and structural or functional components of neurons are used as differentiation markers. Several methods for altering expression in embryos have revealed aspects of a regulatory hierarchy of transcription factors in neurogenesis. Interfacing neurogenic gene regulatory networks to the networks regulating ectodermal domains and identifying the spatial and temporal inputs that pattern the larval nervous system is a major challenge that will contribute substantially to our understanding of the evolution of metazoan nervous systems. This article is categorized under: Comparative Development and Evolution > Model Systems Comparative Development and Evolution > Body Plan Evolution Early Embryonic Development > Gastrulation and Neurulation.

The Drosophila Ret gene functions in the stomatogastric nervous system with the Maverick TGFβ ligand and the Gfrl co-receptor

The RET receptor tyrosine kinase is crucial for the development of the enteric nervous system (ENS), acting as a receptor for Glial cell line-derived neurotrophic factor (GDNF) via GFR co-receptors. Drosophila has a well-conserved RET homolog (Ret) that has been proposed to function independently of the Gfr-like co-receptor (Gfrl). We find that Ret is required for development of the stomatogastric (enteric) nervous system in both embryos and larvae, and its loss results in feeding defects. Live imaging analysis suggests that peristaltic waves are initiated but not propagated in mutant midguts. Examination of axons innervating the midgut reveals increased branching but the area covered by the branches is decreased. This phenotype can be rescued by Ret expression. Additionally, Gfrl shares the same ENS and feeding defects, suggesting that Ret and Gfrl might function together via a common ligand. We identified the TGFβ family member Maverick (Mav) as a ligand for Gfrl and a Mav chromosomal deficiency displayed similar embryonic ENS defects. Our results suggest that the Ret and Gfrl families co-evolved before the separation of invertebrate and vertebrate lineages.

Specification of Larval Axes of Partial Embryos in the Temnopleurid Temnopleurus toreumaticus and the Strongylocentroid Hemicentrotus pulcherrimus

Many sea urchins, including the strongylocentroid Hemicentrotus pulcherrimus, produce an amniotic cavity on the left for adult rudiment formation at the late larval stage. In contrast, temnopleurids form a cell mass at the early larval stage instead of an amniotic cavity. Although the mechanisms establishing left-right polarity of the amniotic cavity involve cell-cell interactions and signaling pathways, corresponding pathways for the cell mass are unknown. We analyzed the effects of blastomere isolation on the specification of larval axes in the temnopleurid Temnopleurus toreumaticus and compared them to those in H. pulcherrimus. Blastomere isolation at the two- or four-cell stages in T. toreumaticus disturbed the location of the cell mass and adult rudiment in approximately 10-20% of specimens. In contrast, isolation at the two-cell stage in H. pulcherrimus caused the left-right polarity to become random. When blastomeres isolated at the two-cell stage were cultured as pairs, approximately 20% of pairs had atypical polarity in both species. Following isolation at the four-cell stage, 71.4% of quartets produced larvae with atypical polarity in T. toreumaticus. Thus, cell-cell interaction between two daughter blastomeres after the second cleavage may be involved in the mechanism determining left-right polarity. Dye injection into a blastomere and subsequent observations indicated that the location of the boundary of the first cleavage showed similar patterns in both species. These observations suggest that species-specific mechanisms establish the larval axes and blastomeres at the two- and four-cell stages redistribute their cytoplasm, forming gradients that establish left-right polarity.

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.

p38 MAPK as an essential regulator of dorsal-ventral axis specification and skeletogenesis during sea urchin development: a re-evaluation

Dorsal-ventral axis formation in the sea urchin embryo relies on the asymmetrical expression of the TGFβ Nodal. The p38-MAPK pathway has been proposed to be essential for dorsal-ventral axis formation by acting upstream of nodal expression. Here, we report that, in contrast to previous studies that used pharmacological inhibitors of p38, manipulating the activity of p38 by genetic means has no obvious impact on morphogenesis. Instead, we discovered that p38 inhibitors strongly disrupt specification of all germ layers by blocking signalling from the Nodal receptor and by interfering with the ERK pathway. Strikingly, while expression of a mutant p38 that is resistant to SB203580 did not rescue dorsal-ventral axis formation or skeletogenesis in embryos treated with this inhibitor, expression of mutant Nodal receptors that are resistant to SB203580 fully restored nodal expression in SB203580-treated embryos. Taken together, these results establish that p38 activity is not required for dorsal-ventral axis formation through nodal expression nor for skeletogenesis. Our results prompt a re-evaluation of the conclusions of several recent studies that linked p38 activity to dorsal-ventral axis formation and to patterning of the skeleton.

Diversification of spatiotemporal expression and copy number variation of the echinoid hbox12/pmar1/micro1 multigene family

Changes occurring during evolution in the cis-regulatory landscapes of individual members of multigene families might impart diversification in their spatiotemporal expression and function. The archetypal member of the echinoid hbox12/pmar1/micro1 family is hbox12-a, a homeobox-containing gene expressed exclusively by dorsal blastomeres, where it governs the dorsal/ventral gene regulatory network during embryogenesis of the sea urchin Paracentrotus lividus. Here we describe the inventory of the hbox12/pmar1/micro1 genes in P. lividus, highlighting that gene copy number variation occurs across individual sea urchins of the same species. We show that the various hbox12/pmar1/micro1 genes group into three subfamilies according to their spatiotemporal expression, which ranges from broad transcription throughout development to transient expression in either the animal hemisphere or micromeres of the early embryo. Interestingly, the promoter regions of those genes showing comparable expression patterns are highly similar, while differing from those of the other subfamilies. Strikingly, phylogenetic analysis suggests that the hbox12/pmar1/micro1 genes are species-specific, exhibiting extensive divergence in their noncoding, but not in their coding, sequences across three distinct sea urchin species. In spite of this, two micromere-specific genes of P. lividus possess a TCF/LEF-binding motif in a similar position, and their transcription relies on Wnt/β-catenin signaling, similar to the pmar1 and micro1 genes, which in other sea urchin species are involved in micromere specification. Altogether, our findings suggest that the hbox12/pmar1/micro1 gene family evolved rather rapidly, generating paralogs whose cis-regulatory sequences diverged following multiple rounds of duplication from a common ancestor.

Developmental gene regulatory networks in sea urchins and what we can learn from them

Sea urchin embryos begin zygotic transcription shortly after the egg is fertilized.  Throughout the cleavage stages a series of transcription factors are activated and, along with signaling through a number of pathways, at least 15 different cell types are specified by the beginning of gastrulation.  Experimentally, perturbation of contributing transcription factors, signals and receptors and their molecular consequences enabled the assembly of an extensive gene regulatory network model.  That effort, pioneered and led by Eric Davidson and his laboratory, with many additional insights provided by other laboratories, provided the sea urchin community with a valuable resource.  Here we describe the approaches used to enable the assembly of an advanced gene regulatory network model describing molecular diversification during early development.  We then provide examples to show how a relatively advanced authenticated network can be used as a tool for discovery of how diverse developmental mechanisms are controlled and work.

Quantitative developmental transcriptomes of the Mediterranean sea urchin Paracentrotus lividus

Embryonic development progresses through the timely activation of thousands of differentially activated genes. Quantitative developmental transcriptomes provide the means to relate global patterns of differentially expressed genes to the emerging body plans they generate. The sea urchin is one of the classic model systems for embryogenesis and the models of its developmental gene regulatory networks are of the most comprehensive of their kind. Thus, the sea urchin embryo is an excellent system for studies of its global developmental transcriptional profiles. Here we produced quantitative developmental transcriptomes of the sea urchin Paracentrotus lividus (P. lividus) at seven developmental stages from the fertilized egg to prism stage. We generated de-novo reference transcriptome and identified 29,817 genes that are expressed at this time period. We annotated and quantified gene expression at the different developmental stages and confirmed the reliability of the expression profiles by QPCR measurement of a subset of genes. The progression of embryo development is reflected in the observed global expression patterns and in our principle component analysis. Our study illuminates the rich patterns of gene expression that participate in sea urchin embryogenesis and provide an essential resource for further studies of the dynamic expression of P. lividus genes.

A deuterostome origin of the Spemann organiser suggested by Nodal and ADMPs functions in Echinoderms

During development of chordates, establishment of the body plan relies on the activity of an organizing centre located on the dorsal side of the embryo that patterns the embryo and induces neural tissue. Intriguingly, the evolutionary origin of this crucial signalling centre remains unclear and whether analogous organizers regulate D/V patterning in other deuterostome or protostome phyla is not known. Here we provide evidence that the ventral ectoderm of the sea urchin embryo is a long-range organizing centre that shares several fundamental properties with the Spemann organizer: the ability to induce duplicated embryonic axes when ectopically induced, the ability to induce neural fate in neighbouring tissues and the ability to finely regulate the level of BMP signalling by using an autoregulatory expansion–repression mechanism. These findings suggest that the evolutionary origin of the Spemann organizer is more ancient than previously thought and that it may possibly be traced back to the common ancestor of deuterostomes.

Establishment of the body plan in chordates is determined by an organizing centre located on the dorsal side of the embryo. Here, the authors show that the ventral ectoderm of the sea urchin embryo is an organizing centre that shares several fundamental properties with the amphibian Spemann organizer.