Patterning the sea urchin embryo: gene regulatory networks, signaling pathways, and cellular interactions

The active evolutionary lives of echinoderm larvae

Echinoderms represent a researchable subset of a dynamic larval evolutionary cosmos. Evolution of echinoderm larvae has taken place over widely varying time scales from the origins of larvae of living classes in the early Palaeozoic, approximately 500 million years ago, to recent, rapid and large-scale changes that have occurred within living genera within a span of less than a million years to a few million years. It is these recent evolutionary events that offer a window into processes of larval evolution operating at a micro-evolutionary level of evolution of discrete developmental mechanisms. We review the evolution of the diverse larval forms of living echinoderms to outline the origins of echinoderm larval forms, their diversity among living echinoderms, molecular clocks and rates of larval evolution, and finally current studies on the roles of developmental regulatory mechanisms in the rapid and radical evolutionary changes observed between closely related congeneric species.

Expression and function of blimp1/krox, an alternatively transcribed regulatory gene of the sea urchin endomesoderm network

The blimp1/krox gene of Strongylocentrotus purpuratus, formerly krox1, encodes zinc finger transcription factors which play a central role in both early and late endomesoderm specification. Here we show that there are two alternative splice forms transcribed under the control of different regulatory regions. The blimp/krox1b form was previously unknown, and is the form expressed during cleavage, beginning 6-9 h postfertilization. This form is required for the early events of endomesoderm specification. A different splice variant, blimp1/krox1a, is expressed only from gastrula stage onward. During cleavage stages the blimp1/krox gene is expressed in the large micromeres and veg2 descendents. Soon after, it is expressed in the ring of specified mesoderm cells at the vegetal pole of the blastula. Its expression is later restricted to the blastopore region and the posterior of the invaginating archenteron, and finally to the midgut and hindgut of the pluteus larva. The expression of blimp1/krox is dynamic, and involves several distinct spatial territories. A GFP recombinant BAC was created by substituting the GFP coding sequence for that of the second exon (1b), in order to distinguish the expression pattern of the early form from that of the late form. This construct closely mimics blimp1/krox1b expression during early stages of sea urchin development. To expand our knowledge of the downstream linkages of this gene, additional experiments were carried out using antisense morpholino oligos (MASO). We confirmed previously published data that blimp1/krox autoregulates its own expression, but discovered, surprisingly, that this gene represses rather than activates itself. This negative autoregulation is restricted to the mesodermal and probably skeletogenic territories during the blastula stage, as shown by in situ hybridization analysis of MASO injected embryos. The MASO perturbation analysis also revealed blimp1/krox inputs into other genes of the endomesoderm regulatory network.

Sea urchin embryo as a model organism for the rapid functional screening of tubulin modulators

Identification of antimitotic molecules that affect tubulin dynamics is a multistep procedure. It includes in vitro tubulin polymerization assay, studies of a cell cycle effect, and general cytotoxicity assessment. To simplify this lengthy screening protocol, we have introduced and validated an assay system based on the sea urchin embryos. The proposed two-step procedure involves the fertilized egg test for mitotic arrest and the behavioral assessment of a free-swimming blastula. In order to validate the assay, we have analyzed the effect of a panel of known antiproliferative agents on the sea urchin embryo. For all tubulin destabilizing drugs, we observed rapid spinning and lack of forward movement of an embryo. Both effects are likely to result from the in vivo microtubule disassembly caused by test molecules. Notably, the described assay yields rapid information on antiproliferative, antimitotic, cytotoxic, and tubulin destabilizing activities of the molecules along with their solubility and permeability potential. Moreover, measured potencies of the test articles correlated well with the reported values in both in vitro and cell based assays.

Specification of ectoderm restricts the size of the animal plate and patterns neurogenesis in sea urchin embryos

The animal plate of the sea urchin embryo becomes the apical organ, a sensory structure of the larva. In the absence of vegetal signaling, an expanded and unpatterned apical organ forms. To investigate the signaling that restricts the size of the animal plate and patterns neurogenesis, we have expressed molecules that regulate specification of ectoderm in embryos and chimeras. Enhancing oral ectoderm suppresses serotonergic neuron differentiation, whereas enhancing aboral or ciliary band ectoderm increases differentiation of serotonergic neurons. In embryos in which vegetal signaling is blocked, Nodal expression does not reduce the size of the thickened animal plate; however, almost no neurons form. Expression of BMP in the absence of vegetal signaling also does not restrict the size of the animal plate, but abundant serotonergic neurons form. In chimeras in which vegetal signaling is blocked in the entire embryo, and one half of the embryo expresses Nodal, serotonergic neuron formation is suppressed in both halves. In similar chimeras in which vegetal signaling is blocked and one half of the embryo expresses Goosecoid (Gsc), serotonergic neurons form only in the half of the embryo not expressing Gsc. We propose that neurogenesis is specified by a maternal program that is restricted to the animal pole by signaling that is dependent on nuclearization of beta-catenin and specifies ciliary band ectoderm. Subsequently, neurogenesis in the animal plate is patterned by suppression of serotonergic neuron formation by Nodal. Like other metazoans, echinoderms appear to have a phase of neural development during which the specification of ectoderm restricts and patterns neurogenesis.

Developmental expression of HpNanos, the Hemicentrotus pulcherrimus homologue of nanos

The Hemicentrotus pulcherrimus homologue of nanos (HpNanos), that encodes a protein containing two CCHC zinc finger motifs, was isolated from a gastrula cDNA library. The accumulation of HpNanos mRNA during embryonic development and the spatial expression pattern are reported. Developmental northern blot analysis revealed that HpNanos mRNA markedly accumulated during the blastula stages, and then decreased in abundance at the mesenchyme blastula stage. The second phase of HpNanos mRNA expression occurred during gastrulation, after which the expression returned to a low level. Whole-mount in situ hybridization showed that the HpNanos was exclusively expressed in four to six small micromere-descendant cells at the blastula stage. The expression of HpNanos was restricted to the coelomic pouch, which gives rise to the mesoderm of the ventral surface of the adult rudiment, at the prism stage. These results suggest that HpNanos expression will be instrumental for future analyses of the function of small micromere-descendant cells and of the origin of germ cells during sea urchin development.

Defining the origins and evolution of the chemokine/chemokine receptor system

The chemokine system has a critical role in mammalian immunity, but the evolutionary history of chemokines and chemokine receptors are ill-defined. We used comparative whole genome analysis of fruit fly, sea urchin, sea squirt, pufferfish, zebrafish, frog, and chicken to identify chemokines and chemokine receptors in each species. We report 127 chemokine and 70 chemokine receptor genes in the 7 species, with zebrafish having the most chemokines, 63, and chemokine receptors, 24. Fruit fly, sea urchin, and sea squirt have no identifiable chemokines or chemokine receptors. This study represents the most comprehensive analysis of the chemokine system to date and the only complete characterization of chemokine systems outside of mouse and human. We establish a clear evolutionary model of the chemokine system and trace the origin of the chemokine system to approximately 650 million years ago, identifying critical steps in their evolution and demonstrating a more extensive chemokine system in fish than previously thought.

Subequatorial cytoplasm plays an important role in ectoderm patterning in the sea urchin embryo

To gain information on the process of ectoderm patterning, the animal halves of sea urchin embryos were isolated at various stages, and their morphology was examined when control embryos developed into pluteus larvae. The animal halves separated at the 8-cell stage developed into 'dauerblastula', without showing any conspicuous ectoderm differentiation. In contrast, some of the animal halves isolated at the 60-cell stage (after the sixth cleavage) formed a ciliated band and oral opening, suggesting that some patterning signal was transmitted from the vegetal to animal hemisphere during early cleavage. Further patterning of the animal hemisphere did not seem to occur until hatching, since both the animal halves isolated at the 60-cell stage and hatching stage showed the same degree of ectoderm patterning. After hatching, the later animal halves were isolated, the more patterned ectoderm they formed. The animal halves isolated just prior to gastrulation differentiated well-patterned ectoderm. It is of note, however, that the level of separation was a more crucial factor than the timing of separation; even the animal fragments of newly hatched embryos differentiated well-patterned ectoderm if they had been separated at a subequatorial level. This suggests that the signal for ectoderm patterning is transmitted over the equator after hatching, and once the cells in the supra-equatorial region receive the signal, they, in turn, can transmit the signal upwardly. Interestingly, if the third cleavage plane was shifted toward the vegetal pole, the isolated animal pole-side fragments developed into 'embryoids' with fully patterned ectoderm. These results indicate that not the micromere descendants but the subequatorial cytoplasm plays an important role in ectoderm patterning.

Larval ectoderm, organizational homology, and the origins of evolutionary novelty

Comprehending the origin of marine invertebrate larvae remains a key domain of research for evolutionary biologists, including the repeated origin of direct developmental modes in echinoids. In order to address the latter question, we surveyed existing evidence on relationships of homology between the ectoderm territories of two closely related sea urchin species in the genus Heliocidaris that differ in their developmental mode. Additionally, we explored a recently articulated idea about homology called 'organizational homology' (Müller 2003. In: Müller GB, Newman SA, editors. Origination of organismal form: beyond the gene in developmental and evolutionary biology. Cambridge, MA: A Bradford Book, The MIT Press. p 51-69. ) in the context of this specific empirical case study. Applying the perspective of organizational homology to our experimental system of congeneric echinoids has led us to a new hypothesis concerning the ectoderm evolution in these species. The extravestibular ectoderm of the direct developer Heliocidaris erythrogramma is a novel developmental territory that arose as a fusion of the oral and aboral ectoderm territories found in indirect developing echinoids such as Heliocidaris tuberculata. This hypothesis instantiates a theoretical principle concerning the origin of developmental modules, 'integration', which has been neglected because the opposite theoretical principle, 'parcellation', is more readily observable in events such as gene duplication and divergence (Wagner 1996. Am Zool 36:36-43).

Frizzled5/8 is required in secondary mesenchyme cells to initiate archenteron invagination during sea urchin development

Wnt signaling pathways play key roles in numerous developmental processes both in vertebrates and invertebrates. Their signals are transduced by Frizzled proteins, the cognate receptors of the Wnt ligands. This study focuses on the role of a member of the Frizzled family, Fz5/8, during sea urchin embryogenesis. During development, Fz5/8 displays restricted expression, beginning at the 60-cell stage in the animal domain and then from mesenchyme blastula stage, in both the animal domain and a subset of secondary mesenchyme cells (SMCs). Loss-of-function analyses in whole embryos and chimeras reveal that Fz5/8 is not involved in the specification of the main embryonic territories. Rather, it appears to be required in SMCs for primary invagination of the archenteron, maintenance of endodermal marker expression and apical localization of Notch receptors in endodermal cells. Furthermore, among the three known Wnt pathways, Fz5/8 appears to signal via the planar cell polarity pathway. Taken together, the results suggest that Fz5/8 plays a crucial role specifically in SMCs to control primary invagination during sea urchin gastrulation.

Canonical Notch signaling is dispensable for early cell fate specifications in mammals

The canonical Notch signaling pathway mediated by Delta- and Jagged-like Notch ligands determines a variety of cell fates in metazoa. In Caenorhabditis elegans and sea urchins, canonical Notch signaling is essential for different cell fate specifications during early embryogenesis or the formation of endoderm, mesoderm, or ectoderm germ layers. Transcripts of Notch signaling pathway genes are present during mouse blastogenesis, suggesting that the canonical Notch signaling pathway may also function in early mammalian development. To test this directly, we used conditional deletion in oocytes carrying a ZP3Cre recombinase transgene to generate mouse embryos lacking both maternal and zygotic protein O-fucosyltransferase 1, a cell-autonomous and essential component of canonical Notch receptor signaling. Homozygous mutant embryos derived from eggs lacking Pofut1 gene transcripts developed indistinguishably from the wild type until approximately embryonic day 8.0, a postgastrulation stage after the formation of the three germ layers. Thus, in contrast to the case with C. elegans and sea urchins, canonical Notch signaling is not required in mammals for earliest cell fate specifications or for formation of the three germ layers. The use of canonical Notch signaling for early cell fate specifications by lower organisms may represent co-option of a regulatory pathway originally used later in development by all metazoa.

Regional specification in the early embryo of the brittle star Ophiopholis aculeata

Early embryogenesis has been examined experimentally in several echinoderm and hemichordate classes. Although these studies suggest that the mechanisms which underlie regional specification have been highly conserved within the echinoderm + hemichordate clade, nothing is known about these mechanisms in several other echinoderm classes, including the Ophiuroidea. In this study, early embryogenesis was examined in a very little studied animal, the ophiuroid Ophiopholis aculeata. In O. aculeata, the first two cleavage planes do not coincide with the animal-vegetal axis but rather form approximately 45 degrees off this axis. A fate map of the early embryo was constructed using microinjected lineage tracers. Most significantly, this fate map indicates that there is a major segregation of ectodermal from endomesodermal fates at first cleavage. The distribution of developmental potential in the early embryo was also examined by isolating different regions of the early embryo and following these isolates though larval development. These analyses indicate that endomesodermal developmental potential segregates unequally at first, second, and third cleavage in O. aculeata. These results provide insight into the mechanisms of regional specification in O. aculeata and yield new material for the study of the evolution of echinoderm development.

P16 is an essential regulator of skeletogenesis in the sea urchin embryo

The primary mesenchyme cells (PMCs) of the sea urchin embryo undergo a dramatic sequence of morphogenetic behaviors that culminates in the formation of the larval endoskeleton. Recent studies have identified components of a gene regulatory network that underlies PMC specification and differentiation. In previous work, we identified novel gene products expressed specifically by PMCs (Illies, M.R., Peeler, M.T., Dechtiaruk, A.M., Ettensohn, C.A., 2002. Identification and developmental expression of new biomineralization proteins in the sea urchin, Strongylocentrotus purpuratus. Dev. Genes Evol. 212, 419-431). Here, we show that one of these gene products, P16, plays an essential role in skeletogenesis. P16 is not required for PMC specification, ingression, migration, or fusion, but is essential for skeletal rod elongation. We have compared the predicted sequences of P16 from two species and show that this small, acidic protein is highly conserved in both structure and function. The predicted amino acid sequence of P16 and the subcellular localization of a GFP-tagged form of the protein suggest that P16 is enriched in the plasma membrane. It may function to receive signals required for skeletogenesis or may play a more direct role in the deposition of biomineral. Finally, we place P16 downstream of Alx1 in the PMC gene network, thereby linking the network to a specific "effector" protein involved in biomineralization.

A novel approach to study adhesion mechanisms by isolation of the interacting system

For decades most investigations into mechanisms of adhesive interactions have examined whole organisms or single cells. Results using whole organisms are often unclear because it may not be known if a probe used in an experiment is directly affecting the cellular interaction under study or if it is an indirect effect resulting from action on some other structure or pathway. Here we develop a novel approach to isolate the structural components of a cellular interaction by dissecting them out of the organism to study them in a pristine environment away from all confounding factors. We used the adhesion between the archenteron and blastocoel roof of the sea urchin gastrula stage embryo as a model that can be replicated in many other developmental and pathological systems. The isolated components of the cellular interaction and those in the whole organism possessed identical cell surface receptors and adhesive affinities.

The micro1 gene is necessary and sufficient for micromere differentiation and mid/hindgut-inducing activity in the sea urchin embryo

In the sea urchin embryo, micromeres have two distinct functions: they differentiate cell autonomously into the skeletogenic mesenchyme cells and act as an organizing center that induces endomesoderm formation. We demonstrated that micro1 controls micromere specification as a transcriptional repressor. Because micro1 is a multicopy gene with at least six polymorphic loci, it has been difficult to consistently block micro1 function by morpholino-mediated knockdown. Here, to block micro1 function, we used an active activator of micro1 consisting of a fusion protein of the VP16 activation domain and the micro1 homeodomain. Embryos injected with mRNA encoding the fusion protein exhibited a phenotype similar to that of micromere-less embryos. To evaluate micro1 function in the micromere, we constructed chimeric embryos composed of animal cap mesomeres and a micromere quartet from embryos injected with the fusion protein mRNA. The chimeras developed into dauerblastulae with no vegetal structures, in which the micromere progeny constituted the blastula wall. We also analyzed the phenotype of chimeras composed of an animal cap and a mesomere expressing micro1. These chimeras developed into pluteus larvae, in which the mesomere descendants ingressed as primary mesenchyme cells and formed a complete set of skeletal rods. The hindgut and a part of the midgut were also generated from host mesomeres. However, the foregut and nonskeletogenic mesoderm were not formed in the larvae. From these observations, we conclude that micro1 is necessary and sufficient for both micromere differentiation and mid/hindgut-inducing activity, and we also suggest that micro1 may not fulfill all micromere functions.

Major regulatory factors in the evolution of development: the roles of goosecoid and Msx in the evolution of the direct-developing sea urchin Heliocidaris erythrogramma

The transcription factors Gsc and Msx are expressed in the oral ectoderm of the indirect-developing sea urchin Heliocidaris tuberculata. Their patterns of expression are highly modified in the direct developer Heliocidaris erythrogramma, which lacks an oral ectoderm. We here test the hypothesis that they are large effect genes responsible for the loss of the oral ectoderm module in the direct-developing larva of H. erythrogramma as well as for the restoration of an overt oral ectoderm in H.e. xH.t. hybrids. We undertook misexpression/overexpression and knockdown assays in the two species and in hybrids by mRNA injection. The results indicate that dramatic changes of function of these transcription factors has occurred. One of these genes, Gsc, has the ability when misexpressed to partially restore oral ectoderm in H. erythrogramma. On the other hand, Msx has lost any oral function and instead has a role in mesoderm proliferation and patterning. In addition, we found that the H. tuberculataGsc is up regulated in H.e. xH.t. hybrids, showing a preferential use of the indirect developing parental gene in the development of the hybrid. We suggest that Gsc qualifies as a gene of large evolutionary effect and is partially responsible for the evolution of direct development of H. erythrogramma. We discuss these results in light of modularity and genetic networks in development, as well as in their implications for the rapid evolution of large morphological changes in development.

A Fringe-modified Notch signal affects specification of mesoderm and endoderm in the sea urchin embryo

Fringe proteins are O-fucose-specific beta-1,3 N-acetylglucosaminyltransferases that glycosylate the extracellular EGF repeats of Notch and enable Notch to be activated by the ligand Delta. In the sea urchin, signaling between Delta and Notch is known to be necessary for specification of secondary mesenchyme cells (SMCs). The Lytechinus variegatus Fringe homologue is expressed in both the signaling and receiving cells during this first Delta-Notch signal. Perturbation of Fringe expression through morpholino antisense oligonucleotide (MO) injection results in fewer SMCs but also causes decreased and delayed archenteron invagination. Partial endoderm specification occurs but expression of some endoderm genes is compromised. The data are consistent with a Fringe-requiring Notch signal as one upstream component of archenteron morphogenesis. Finally, Fringe perturbations result in more severe phenotypes than those previously reported for Notch dominant-negative (LvN(neg)) injections or reported here for Notch MO (NMO) injections. Injecting a combination of LvN(neg) and NMO results in a more severe phenotype than either treatment alone, and this combination phenocopies the fringe MO embryos. Taken together, the results show that Fringe is necessary both for maternal and zygotic Notch signals, and these Notch signals affect specification of mesoderm and endoderm.

From larval bodies to adult body plans: patterning the development of the presumptive adult ectoderm in the sea urchin larva

Echinoderms are unique among bilaterians for their derived, nonbilateral adult body plan. Their radial symmetry emerges from the bilateral larval body plan by the establishment of a new axis, the adult oral-aboral axis, involving local mesoderm-ectoderm interactions. We examine the mechanisms underlying this transition in the direct-developing sea urchin Heliocidaris erythrogramma. Adult ectoderm arises from vestibular ectoderm in the left vegetal quadrant. Inductive signals from the left coelom are required for adult ectodermal development but not for initial vestibule formation. We surgically removed gastrula archenteron, making whole-ectoderm explants, left-, right-, and animal-half ectoderm explants, and recombinants of these explants with left coelom. Vestibule formation was analyzed morphologically and with radioactive in situ hybridization with HeET-1, an ectodermal marker. Whole ectodermal explants in the absence of coelom developed vestibules on the left side or ventrally but not on the right side, indicating that left-right polarity is ectoderm autonomous by the gastrula stage. However, right-half ectodermal explants robustly formed vestibules that went on to form adult structures when recombined with the left coelom, indicating that the right side retains vestibule-forming potential that is normally suppressed by signals from the left-side ectoderm. Animal-half explants formed vestibules only about half the time, demonstrating that animal-vegetal axis determination occurs earlier. However, when combined with the left coelom, animal-half ectoderm always formed a vestibule, indicating that the left coelom can induce vestibule formation. This suggests that although coelomic signals are not required for vestibule formation, they may play a role in coordinating the coelom-vestibule interaction that establishes the adult oral-aboral axis.

Co-option of an oral-aboral patterning mechanism to control left-right differentiation: the direct-developing sea urchin Heliocidaris erythrogramma is sinistralized, not ventralized, by NiCl2

Larval dorsoventral (DV) and left-right (LR) axial patterning unfold progressively in sea urchin development, leading to commitment of the major embryonic regions by the gastrula stage. The direct-developing sea urchin Heliocidaris erythrogramma has lost oral-aboral differentiation along the DV axis but has accelerated vestibular ectoderm development on the left side. NiCl(2) radializes indirect-developing sea urchins by shifting cells toward a ventral fate (oral ectoderm). We treated embryos of H. erythrogramma and the indirect-developing H. tuberculata with NiCl(2). H. tuberculata was ventralized exactly like other indirect developers, establishing that basic patterning mechanisms are conserved in this genus. H. erythrogramma was also radialized; timing, dosage response, and some morphological features were similar to those in other sea urchins. Ectodermal explant and recombination experiments demonstrate that the effect of nickel is autonomous to the ectoderm, another feature in common with indirect developers. However, H. erythrogramma is distinctly sinistralized rather than ventralized, its cells shifting toward a left-side fate (vestibular ectoderm). This geometric contrast in the midst of pervasive functional similarity suggests that nickel-sensitive processes in H. erythrogramma axial patterning, homologous to those in indirect developers, have been redeployed, and hence co-opted, from their ancestral role in DV axis determination to a new role in LR axis determination. We discuss DV and LR axial patterning and their evolutionary transformation.

Induction and the Turing-Child field in development

The central problem in biological development is the understanding of epigenesis. The dominant theory of development in the last 80 years that also purports to explain epigenesis is induction theory. It suggests that development is driven by sequential inductions where each "induction" (in one sense of the word induction) is effected by the action of an inducing part of the embryo on a responding part of the embryo. The theory stems from Spemann and Mangold (W.Roux' Arch.f.Entw.d.Organis.u.mikrosk.Anat.100 (1924) 599) who transplanted a tissue from the dorsal blastopore lip of Triturus into the ventral ectoderm of another gastrula and thus initiated and "induced" (in another sense of the word induction) gastrulation and embryogenesis in the ventral side of the host that became a double embryo (siamese twins). We explain this induction, i.e. the formation of the double embryo, according to the Child theory and the Turing-Gierer-Meinhardt theory when it is also assumed that cAMP and ATP are the Turing activator and inhibitor, respectively. Spemann and Mangold (W.Roux' Arch.f.Entw.d.Organis.u.mikrosk.Anat.100 (1924) 599) also suggested that the ingressing mesoderm induces the overlying ectoderm to form the neural plate and neural tube. This 'neural induction', the 'primary embryonic induction', became the cornerstone of induction theory, i.e. of the sequential induction concept referred to above. But we argue that the metabolic gradients that precede and accompany neurulation, as obtained by Child, also for Triturus, arise through a Turing self-organization if it is assumed that cAMP and ATP are the Turing morphogens, and these gradients are the cause and primary event of neurulation. Thus there is no need to invoke the 'neural induction'. It is argued that fundamental events such as gastrulation and also organ formation are caused by the Turing-Child field and not by sequential induction. Similar principles, such as bud formation caused by a radial metabolic pattern that transforms to a longitudinal pattern, govern the formation, for example, of the mouth and the gut. The formation and localization of bottle cells is explained according to the Child-Turing field and modern biochemistry. The chemical metabolic pre-pattern precedes, and causes, morphogenesis and differentiation as envisaged by Turing. The Spemann and Mangold (W.Roux' Arch.f.Entw.d.Organis.u.mikrosk.Anat.100 (1924) 599) transplantation experiment when performed on a sea urchin duplicates not only the phenotype but also the metabolic (reduction) pattern. These experimental results, by Horstadius, predicted by Child, follow from the Turing-Gierer-Meinhardt theory if it is assumed that cAMP and ATP are the Turing morphogens. If the transplantation is performed not onto the whole sea urchin but onto only a part of it, that manifests only a part of the metabolic pattern, then from the part a phenotypic whole underlain by a normal and a whole metabolic pattern can be rescued. These experimental results of Horstadius follow from Turing theory if cAMP and ATP are the Turing morphogens. Understanding how to transform a part into a whole can be valuable in regenerative medicine. Unspecific induction of a secondary amphibian embryo is similar to the induction of posterior structures at the anterior pole of an insect, and the "double abdomen" (and Kalthoff's experimental results) of the midge Smittia resulting from UV irradiation of the anterior pole, can be explained by Meinhardt theory of unspecific induction if ATP is the Turing morphogen. When not working on regeneration, Child investigated intact living organisms and his observation method was not disruptive to normal development, whereas workers in induction theory work with pieces and in general disrupt normal development. We conclude that the Turing-Child field causes all development and explains epigenesis. Sequential induction does not explain epigenesis and does not exist in normal development. But induction in the sense of a transplantation leading to double embryo or rescuing a whole phenotype from a part is of interest.

Developmental biology meets materials science: Morphogenesis of biomineralized structures

Biomineralization is the process by which metazoa form hard minerals for support, defense, and feeding. The minerals so formed, e.g., teeth, bones, shells, carapaces, and spicules, are of considerable interest to chemists and materials scientists. The cell biology underlying biomineralization is not well understood. The study of the formation of mineralized structures in developing organisms offers opportunities for understanding some intriguing aspects of cell and developmental biology. Five examples of biomineralization are presented: (1) the formation of siliceous spicules and frustules in sponges and diatoms, respectively; (2) the structure of skeletal spicules composed of amorphous calcium carbonate in some tunicates; (3) the secretion of the prism and nacre of some molluscan shells; (4) the development of skeletal spicules of sea urchin embryos; and (5) the formation of enamel of vertebrate teeth. Some speculations on the cellular and molecular mechanisms that support biomineralization, and their evolutionary origins, are discussed.

Embryonic-stage-dependent changes in the level of eIF4E-binding proteins during early development of sea urchin embryos

The eukaryotic initiation factor 4E (eIF4E)-binding proteins (4E-BPs) inhibit translation initiation by binding eIF4E and preventing recruitment of the translation machinery to mRNA. We have previously shown that fertilization of sea urchin eggs triggers eIF4E-4E-BP complex dissociation and 4E-BP degradation. Here, we show that microinjection of eIF4E-binding motif peptide into unfertilized eggs delays the onset of the first mitosis triggered by fertilization, demonstrating that dissociation of the eIF4E-4E-BP complex is functionally important for the first mitotic division in sea urchin embryos. We also show by gel filtration analyses that eIF4E is present in unfertilized eggs as an 80 kDa molecular mass complex containing 4E-BP and a new 4E-BP of 40 kDa. Fertilization triggers the dissociation of eIF4E from these two 4E-BPs and triggers the rapid recruitment of eIF4E into a high-molecular-mass complex. Release of eIF4E from the two 4E-BPs is correlated with a decrease in the total level of both 4E-BPs following fertilization. Abundance of the two 4E-BPs has been monitored during embryonic development. The level of the two proteins remains very low during the rapid cleavage stage of early development and increases 8 hours after fertilization. These results demonstrate that these two 4E-BPs are down- and upregulated during the embryonic development of sea urchins. Consequently, these data suggest that eIF4E availability to other partners represents an important determinant of the early development of sea urchin embryos.

Gastrulation Movements: the Logic and the Nuts and Bolts

Gastrulation, the period during the early development of animals when major cell and tissue movements remodel an initially unstructured group of cells, requires coordinated control of different types of cellular activities in different cell populations. A hierarchy of genetic control mechanisms, involving cell signaling and transcriptional regulation, sets up the embryonic axes and specify the territories of the future germ layers. Cells in these territories modulate their cytoskeleton and their adhesive behavior, resulting in shape changes and movement. Similarities among different species in patterning and cell biological mechanisms are beginning to allow us to recognize general, conserved principles and speculate on possible ancestral mechanisms of gastrulation.

Nuclear beta-catenin-dependent Wnt8 signaling in vegetal cells of the early sea urchin embryo regulates gastrulation and differentiation of endoderm and mesodermal cell lineages

The entry of beta-catenin into vegetal cell nuclei beginning at the 16-cell stage is one of the earliest known molecular asymmetries seen along the animal-vegetal axis in the sea urchin embryo. Nuclear beta-catenin activates a vegetal signaling cascade that mediates micromere specification and specification of the endomesoderm in the remaining cells of the vegetal half of the embryo. Only a few potential target genes of nuclear beta-catenin have been functionally analyzed in the sea urchin embryo. Here, we show that SpWnt8, a Wnt8 homolog from Strongylocentrotus purpuratus, is zygotically activated specifically in 16-cell-stage micromeres in a nuclear beta-catenin-dependent manner, and its expression remains restricted to the micromeres until the 60-cell stage. At the late 60-cell stage nuclear beta-catenin-dependent SpWnt8 expression expands to the veg2 cell tier. SpWnt8 is the only signaling molecule thus far identified with expression localized to the 16-60-cell stage micromeres and the veg2 tier. Overexpression of SpWnt8 by mRNA microinjection produced embryos with multiple invagination sites and showed that, consistent with its localization, SpWnt8 is a strong inducer of endoderm. Blocking SpWnt8 function using SpWnt8 morpholino antisense oligonucleotides produced embryos that formed micromeres that could transmit the early endomesoderm-inducing signal, but these cells failed to differentiate as primary mesenchyme cells. SpWnt8-morpholino embryos also did not form endoderm, or secondary mesenchyme-derived pigment and muscle cells, indicating a role for SpWnt8 in gastrulation and in the differentiation of endomesodermal lineages. These results establish SpWnt8 as a critical component of the endomesoderm regulatory network in the sea urchin embryo.

Sea urchin vault structure, composition, and differential localization during development

Background

Vaults are intriguing ribonucleoprotein assemblies with an unknown function that are conserved among higher eukaryotes. The Pacific coast sea urchin, Strongylocentrotus purpuratus, is an invertebrate model organism that is evolutionarily closer to humans than Drosophila and C. elegans, neither of which possesses vaults. Here we compare the structures of sea urchin and mammalian vaults and analyze the subcellular distribution of vaults during sea urchin embryogenesis.

Results

The sequence of the sea urchin major vault protein (MVP) was assembled from expressed sequence tags and genome traces, and the predicted protein was found to have 64% identity and 81% similarity to rat MVP. Sea urchin MVP includes seven ~50 residue repeats in the N-terminal half of the protein and a predicted coiled coil domain in the C-terminus, as does rat MVP. A cryoelectron microscopy (cryoEM) reconstruction of isolated sea urchin vaults reveals the assembly to have a barrel-shaped external structure that is nearly identical to the rat vault structure. Analysis of the molecular composition of the sea urchin vault indicates that it contains components that may be homologs of the mammalian vault RNA component (vRNA) and protein components (VPARP and TEP1). The sea urchin vault appears to have additional protein components in the molecular weight range of 14–55 kDa that might correspond to molecular contents. Confocal experiments indicate a dramatic relocalization of MVP from the cytoplasm to the nucleus during sea urchin embryogenesis.

Conclusions

These results are suggestive of a role for the vault in delivering macromolecules to the nucleus during development.

Unexpected complexity of the Wnt gene family in a sea anemone

The Wnt gene family encodes secreted signalling molecules that control cell fate in animal development and human diseases. Despite its significance, the evolution of this metazoan-specific protein family is unclear. In vertebrates, twelve Wnt subfamilies were defined, of which only six have counterparts in Ecdysozoa (for example, Drosophila and Caenorhabditis). Here, we report the isolation of twelve Wnt genes from the sea anemone Nematostella vectensis, a species representing the basal group within cnidarians. Cnidarians are diploblastic animals and the sister-group to bilaterian metazoans. Phylogenetic analyses of N. vectensis Wnt genes reveal a thus far unpredicted ancestral diversity within the Wnt family. Cnidarians and bilaterians have at least eleven of the twelve known Wnt gene subfamilies in common; five subfamilies appear to be lost in the protostome lineage. Expression patterns of Wnt genes during N. vectensis embryogenesis indicate distinct roles of Wnts in gastrulation, resulting in serial overlapping expression domains along the primary axis of the planula larva. This unexpectedly complex inventory of Wnt family signalling factors evolved in early multi-cellular animals about 650 million years (Myr) ago, predating the Cambrian explosion by at least 100 Myr (refs 5, 8). It emphasizes the crucial function of Wnt genes in the diversification of eumetazoan body plans.

Oral-aboral axis specification in the sea urchin embryo II. Mitochondrial distribution and redox state contribute to establishing polarity in Strongylocentrotus purpuratus

The initial asymmetry that specifies the oral-aboral (OA) axis of the sea urchin embryo has long been a mystery. It was shown previously that OA polarity can be entrained in embryos by imposing a respiratory asymmetry, with the most oxidizing side of the embryo tending to develop as the oral pole. This suggests that one of the earliest observable asymmetries along the incipient OA axis, a redox gradient established by a higher density and/or activity of mitochondria on the prospective oral side of the embryo, might play a causal role in establishing the axis. Here, we examine the origin and functional significance of this early redox gradient. Using MitoTracker Green, we show that mitochondria are asymmetrically distributed in the unfertilized egg of Strongylocentrotus purpuratus, and that the polarity of the maternal asymmetry is maintained in the zygote. Vital staining indicates that the side of the embryo that inherits the highest density of mitochondria tends to develop into the oral pole. This correlation holds when mitochondria are redistributed by centrifugation of eggs or by transfer of purified mitochondria into zygotes, indicating that an asymmetric mitochondrial distribution can entrain OA polarity, possibly through effects on intracellular redox state. In support of this possibility, we find that specification of oral ectoderm is suppressed when embryos are cultured under hypoxic conditions that enforce a relatively reducing redox state. This effect is reversed by overexpression of nodal, an early zygotic marker of oral specification whose localized expression suffices to organize the entire OA axis, indicating that redox state is upstream of nodal expression. We therefore propose that a threshold level of intracellular oxidation is required to effectively activate nodal, and that precocious attainment of this threshold within the blastomeres containing the highest density of mitochondria results in asymmetric nodal activity and consequent specification of the OA axis.

Expression of an NK2 homeodomain gene in the apical ectoderm defines a new territory in the early sea urchin embryo

We have identified an NK2 family homeodomain transcription factor, SpNK2.1, in the sea urchin Strongylocentrotus purpuratus whose transcripts are initially detected within the apical plate ectoderm of the hatching blastula and are confined to the apical organ at least through 2 weeks of development. Protein localization studies demonstrate that SpNK2.1 is restricted to the apical plate epithelium, but is excluded from the nucleus of serotonergic neurons. The expression profile of SpNK2.1 is dictated via two separate regulatory systems. Initially, SpNK2.1 is restricted to the apical pole domain by beta-catenin-dependent processes operating along the animal-vegetal axis, as evidenced by an expansion of SpNK2.1 expression upon cadherin overexpression. Starting at gastrulation, expression in the apical plate is maintained by SpDri, the sea urchin orthologue of dead ringer. Abrogation of SpDri results in the downregulation of SpNK2.1 after gastrulation, but SpDri is not necessary for the initial activation of SpNK2.1. Loss of function experiments using SpNK2.1-specific morpholino antisense oligonucleotides and SpNK2.1 overexpression experiments do not disrupt embryonic development and have no effect upon the development of neuronal components of the apical organ. Nonetheless, SpNK2.1 defines a new early territory of the sea urchin embryo.

Nodal and BMP2/4 signaling organizes the oral-aboral axis of the sea urchin embryo

In the sea urchin embryo, the oral-aboral axis is specified after fertilization by mechanisms that are largely unknown. We report that early sea urchin embryos express Nodal and Antivin in the presumptive oral ectoderm and demonstrate that these genes control formation of the oral-aboral axis. Overexpression of nodal converted the whole ectoderm into oral ectoderm and induced ectopic expression of the orally expressed genes goosecoid, brachyury, BMP2/4, and antivin. Conversely, when the function of Nodal was blocked, by injection of an antisense Morpholino oligonucleotide or by injection of antivin mRNA, neither the oral nor the aboral ectoderm were specified. Injection of nodal mRNA into Nodal-deficient embryos induced an oral-aboral axis in a largely non-cell-autonomous manner. These observations suggest that the mechanisms responsible for patterning the oral-aboral axis of the sea urchin embryo may share similarities with mechanisms that pattern the dorsoventral axis of other deuterostomes.

Paracentrotus lividus eggs contain different RNAs at the animal and vegetal poles

Paracentrotus lividus eggs were divided by centrifugation into nucleated and anucleated halves. Fertilization and development of the two halves permitted us to establish that nucleated and anucleated fragments correspond, respectively, to the animal and vegetal parts. RNA was extracted from both egg halves and submitted to differential display. Northern blot analysis confirmed their maternal origin and showed that each transcript has a different expression pattern during development. By Northern blot and in situ hybridization experiments we ascertained that Bep2 and PlAn1 are localized in the animal part, whereas 16S rRNA, Plveg1, and L27 in the vegetal part, and that Plun1 is uniformly distributed. Moreover, by treating P. lividus eggs with detergent, in presence or not of drugs such as colchicine and cytochalasin B, we demonstrated the involvement of the cytoskeleton only in localization of Bep2, PlAn1, and Plun1, suggesting that different mechanisms are utilized for animal and vegetal distribution.

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.

A Raf/MEK/ERK signaling pathway is required for development of the sea urchin embryo micromere lineage through phosphorylation of the transcription factor Ets

In the sea urchin embryo, the skeleton of the larva is built from a population of mesenchymal cells known as the primary mesenchyme cells (PMCs). These derive from the large micromeres that originate from the vegetal pole at fourth cleavage. At the blastula stage, the 32 cells of this lineage detach from the epithelium and ingress into the blastocoel by a process of epithelial-mesenchymal transition. We report that shortly before ingression,there is a transient and highly localized activation of the MAP-kinase ERK in the micromere lineage. We show that ingression of the PMCs requires the activity of ERK, MEK and Raf, and depends on the maternal Wnt/β-catenin pathway. Dissociation experiments and injection of mRNA encoding a dominant-negative form of Ras indicated that this activation is probably cell autonomous. We identified the transcription factors Ets1 and Alx1 as putative targets of the phosphorylation by ERK. Both proteins contain a single consensus site for phosphorylation by the MAP kinase ERK. In addition, the Ets1 protein sequence contains a putative ERK docking site. Overexpression of ets1 by injection of synthetic mRNA in the egg caused a dramatic increase in the number of cells becoming mesenchymal at the blastula stage. This effect could be largely inhibited by treating embryos with the MEK inhibitor U0126. Moreover, mutations in the consensus phosphorylation motif substituting threonine 107 by an aspartic or an alanine residue resulted respectively in a constitutively active form of Ets1 that could not be inhibited by U0126 or in an inactive form of Ets1. These results show that the MAP kinase pathway, working through phosphorylation of Ets1, is required for full specification of the PMCs and their subsequent transition from epithelial to mesenchymal state.

Generation, annotation, evolutionary analysis, and database integration of 20,000 unique sea urchin EST clusters

Together with the hemichordates, sea urchins represent basal groups of nonchordate invertebrate deuterostomes that occupy a key position in bilaterian evolution. Because sea urchin embryos are also amenable to functional studies, the sea urchin system has emerged as one of the leading models for the analysis of the function of genomic regulatory networks that control development. We have analyzed a total of 107,283 cDNA clones of libraries that span the development of the sea urchin Strongylocentrotus purpuratus. Normalization by oligonucleotide fingerprinting, EST sequencing and sequence clustering resulted in an EST catalog comprised of 20,000 unique genes or gene fragments. Around 7000 of the unique EST consensus sequences were associated with molecular and developmental functions. Phylogenetic comparison of the identified genes to the genome of the urochordate Ciona intestinalis indicate that at least one quarter of the genes thought to be chordate specific were already present at the base of deuterostome evolution. Comparison of the number of gene copies in sea urchins to those in chordates and vertebrates indicates that the sea urchin genome has not undergone extensive gene or complete genome duplications. The established unique gene set represents an essential tool for the annotation and assembly of the forthcoming sea urchin genome sequence. All cDNA clones and filters of all analyzed libraries are available from the resource center of the German genome project at http://www.rzpd.de.

Echinoderm eggs and embryos: procurement and culture

The protocols outlined here hopefully will provide researchers with healthy, beautiful echinoderm oocytes, eggs, and embryos for experimental use. The large size of echinoderm oocytes and eggs, the ease with which they can be manipulated, and (in many species) their optical clarity, make them an ideal model system for studying not only the events specific to oocyte maturation and fertilization, but also for investigating more general questions regarding cell cycle regulation in an in vivo system. The quick rate at which development proceeds after fertilization to produce transparent embryos and larva makes the echinoderm an advantageous organism for studying deuterostome embryogenesis. Continued use of the echinoderms as model systems will undoubtedly uncover exciting answers to questions regarding fertilization, cell cycle regulation, morphogenesis, and how developmental events are controlled.

Patterning mechanisms in the evolution of derived developmental life histories: the role of Wnt signaling in axis formation of the direct-developing sea urchin Heliocidaris erythrogramma

A number of echinoderm species have replaced indirect development with highly modified direct-developmental modes, and provide models for the study of the evolution of early embryonic development. These divergent early ontogenies may differ significantly in life history, oogenesis, cleavage pattern, cell lineage, and timing of cell fate specification compared with those of indirect-developing species. No direct-developing echinoderm species has been studied at the level of molecular specification of embryonic axes. Here we report the first functional analysis of Wnt pathway components in Heliocidaris erythrogramma, a direct-developing sea urchin. We show by misexpression and dominant negative knockout construct expression that Wnt8 and TCF are functionally conserved in the generation of the primary (animal/vegetal) axis in two independently evolved direct-developing sea urchins. Thus, Wnt pathway signaling is an overall deeply conserved mechanism for axis formation that transcends radical changes to early developmental ontogenies. However, the timing of expression and linkages between Wnt8, TCF, and components of the PMC-specification pathway have changed. These changes correlate with the transition from an indirect- to a direct-developing larval life history.

Impairing Otp homeodomain function in oral ectoderm cells affects skeletogenesis in sea urchin embryos

In the sea urchin embryo skeletogenesis is the result of a complex series of molecular and cellular events that coordinate the morphogenetic process. Past and recent evidence strongly indicate that skeletal initiation and growth are strictly dependent on signals emanating from the oral ectodermal wall. As previously suggested, Orthopedia (Otp), a homeodomain-containing transcription factor specifically expressed in a small subset of oral ectoderm cells, might be implicated in this signalling pathway. In this study, we utilize three different strategies to address the issue of whether Otp is an upstream regulator of sketelogenesis. We describe the effects of microinjection of Otp morpholino-substituted antisense oligonucleotides and dominant-negative Otp-engrailed mRNA in Paracentrotus lividus embryos. We demonstrate that inhibition of Otp expression completely abolishes skeletal synthesis. By contrast, coinjection of Otp mRNA and the morpholino antisense oligonucleotide specifically rescues the skeletogenic program. In addition, localized ectodermal expression of the Otp-GFP fusion gene construct driven by the hatching enzyme promoter, induces ectopic and abnormal spiculogenesis. We further show that an indirect target of this homeoprotein is the skeletogenic specific gene SM30, whose expression is known to be under the strict control of the oral ectoderm territory. Based on these results, we conclude that Otp triggers the ectoderm-specific signal that promotes skeletogenesis.

Tight regulation of SpSoxB factors is required for patterning and morphogenesis in sea urchin embryos

Previous studies in sea urchin embryos have demonstrated that nuclearization of beta-catenin is essential for initial steps in the specification of endoderm and mesenchyme, which are derived from vegetal blastomeres. This process begins at the 4th and extends through the 9th cleavage stage, an interval in which the SpSoxB1 transcription regulator is downregulated by beta-catenin-dependent gene products that include the transcription repressor SpKrl. These observations raise the possibility that SpSoxB1 removal is required to allow vegetal development to proceed. Here we show that elevated and ectopic expression of this factor suppresses differentiation of all vegetal cell types, a phenotype that is very similar to that caused by the suppression of beta-catenin nuclear function by cadherin overexpression. Suppression of vegetal fates involves interference at the protein-protein level because a mutation of SpSoxB1 that prevents its binding to DNA does not significantly reduce this activity. Reduction in SpSoxB1 level results in elevated TCF/Lef-beta-catenin-dependent expression of a luciferase reporter gene in vivo, indicating that in the normal embryo this protein suppresses the primary vegetal signaling mechanism that is required for specification of mesenchyme and endoderm. Surprisingly, normal expression of SpSoxB1 is required for gastrulation and endoderm differentiation, as shown by both morpholino-mediated translational interference and expression of a dominant negative protein. Similar gain-of-function and loss-of-function assays of a closely related factor, SpSoxB2, demonstrate that it, too, is required for gastrulation and that its overexpression can suppress vegetal development. However, significant phenotypic differences are apparent in the two perturbations, indicating that SpSoxB1 and SpSoxB2 have at least some distinct developmental functions. The results of all these studies support a model in which the concentration of SpSoxB factors must be tightly regulated along the animal-vegetal axis of the early sea urchin embryo to allow beta-catenin-dependent specification of endoderm and mesenchyme cell fates as well as to activate target genes required for gastrulation.

Alx1, a member of the Cart1/Alx3/Alx4 subfamily of Paired-class homeodomain proteins, is an essential component of the gene network controlling skeletogenic fate specification in the sea urchin embryo

In the sea urchin embryo, the large micromeres and their progeny function as a critical signaling center and execute a complex morphogenetic program. We have identified a new and essential component of the gene network that controls large micromere specification, the homeodomain protein Alx1. Alx1 is expressed exclusively by cells of the large micromere lineage beginning in the first interphase after the large micromeres are born. Morpholino studies demonstrate that Alx1 is essential at an early stage of specification and controls downstream genes required for epithelial-mesenchymal transition and biomineralization. Expression of Alx1 is cell autonomous and regulated maternally through beta-catenin and its downstream effector, Pmar1. Alx1 expression can be activated in other cell lineages at much later stages of development, however, through a regulative pathway of skeletogenesis that is responsive to cell signaling. The Alx1 protein is highly conserved among euechinoid sea urchins and is closely related to the Cart1/Alx3/Alx4 family of vertebrate homeodomain proteins. In vertebrates, these proteins regulate the formation of skeletal elements of the limbs, face and neck. Our findings suggest that the ancestral deuterostome had a population of biomineral-forming mesenchyme cells that expressed an Alx1-like protein.

Development of calcareous skeletal elements in invertebrates

Most metazoans require skeletal support systems. While the formation of bones and teeth in vertebrates has been well studied, endo- and exoskeleton development of non-vertebrates, especially calcification during terminal differentiation, has been neglected. Biomineralization of skeletons in invertebrates presents interesting research opportunities. We undertake here to survey some of the better understood examples of skeletal development in selected invertebrates. The differentiation of the skeletal spicules of euechinoid larvae and other non-vertebrate deuterostomes, the shells of molluscs, and the calcification of crustacean carapaces are surveyed. The diversity of these different kinds of animals and our present limited understanding make it difficult to identify unifying themes, but there certainly are unifying questions: How is the mineral precursor secreted? What is the nature of the interaction of mineral with the matrix proteins of the skeleton? Is there any conservation of protein domains in matrix proteins found in skeletal elements from different phyla? Are there common strategies in the development of organs that form mineralized structures?