Altering cell fates in sea urchin embryos by overexpressing SpOtx, an orthodenticle-related protein

Conditional gene expression in invertebrate animal models

A mechanistic understanding of biology requires appreciating spatiotemporal aspects of gene expression and its functional implications. Conditional expression allows for (ir)reversible switching of genes on or off, with the potential of spatial and/or temporal control. This provides a valuable complement to the more often used constitutive gene (in)activation through mutagenesis, providing tools to answer a wider array of research questions across biological disciplines. Spatial and/or temporal control are granted primarily by (combinations of) specific promoters, temperature regimens, compound addition, or illumination. The use of such genetic tool kits is particularly widespread in invertebrate animal models because they can be applied to study biological processes in short time frames and on large scales, using organisms amenable to easy genetic manipulation. Recent years witnessed an exciting expansion and optimization of such tools, of which we provide a comprehensive overview and discussion regarding their use in invertebrates. The mechanism, applicability, benefits, and drawbacks of each of the systems, as well as further developments to be expected in the foreseeable future, are highlighted.

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

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

Ca²⁺ influx-linked protein kinase C activity regulates the β-catenin localization, micromere induction signalling and the oral-aboral axis formation in early sea urchin embryos

Sea urchin embryos initiate cell specifications at the 16-cell stage by forming the mesomeres, macromeres and micromeres according to the relative position of the cells in the animal-vegetal axis. The most vegetal cells, micromeres, autonomously differentiate into skeletons and induce the neighbouring macromere cells to become mesoendoderm in the β-catenin-dependent Wnt8 signalling pathway. Although the underlying molecular mechanism for this progression is largely unknown, we have previously reported that the initial events might be triggered by the Ca2+ influxes through the egg-originated L-type Ca2+ channels distributed asymmetrically along the animal-vegetal axis and through the stretch-dependent Ca2+channels expressed specifically in the micromere at the 4th cleavage. In this communication, we have examined whether one of the earliest Ca2+ targets, protein kinase C (PKC), plays a role in cell specification upstream of β-catenin. To this end, we surveyed the expression pattern of β-catenin in early embryos in the presence or absence of the specific peptide inhibitor of Hemicentrotus pulcherrimus PKC (HpPKC-I). Unlike previous knowledge, we have found that the initial nuclear entrance of β-catenin does not take place in the micromeres, but in the macromeres at the 16-cell stage. Using the HpPKC-I, we have demonstrated further that PKC not only determines cell-specific nucleation of β-catenin, but also regulates a variety of cell specification events in the early sea urchin embryos by modulating the cell adhesion structures, actin dynamics, intracellular Ca2+ signalling, and the expression of key transcription factors.

Intragenomic evolution of a transcriptional enhancer in the genome of Strongylocentrotus purpuratus

General principles for how genomic regulatory elements evolve to alter patterns of gene expression remain vague. The purpose of this study was to gain insights into the evolution of genomic regulatory elements by investigating the unique features of a transcriptional enhancer that directs Spec2a gene expression in Strongylocentrotus purpuratus. The Spec2a enhancer is embedded in a repetitive sequence family interspersed throughout the genome. We surveyed the genome and identified 274 of these sequences. They displayed a continuum of sequence divergence defining high and low divergence classes. Alignment of 52 most related to the Spec2a sequence revealed a complex pattern of rearrangements, insertions and deletions, and base-pair changes. A distance tree for the 52 sequences was constructed and correlated with enhancer activity. Unexpectedly, we found a wide range of activities. Notably, repetitive sequences lacking essential cis-elements found in the Spec2a enhancer still had strong activity. We identified short, conserved motifs within the repetitive sequences that may represent novel cis-regulatory elements. Many repetitive sequences with enhancer activity were found nearby genes, suggesting that they regulate gene expression. The results show that the repetitive sequences are rapidly evolving in the S. purpuratus genome and may serve as a renewable pool of transcriptional enhancers.

Functional studies of regulatory genes in the sea urchin embryo

Sea urchin embryos are characterized by an extremely simple mode of development, rapid cleavage, high transparency, and well-defined cell lineage. Although they are not suitable for genetic studies, other approaches are successfully used to unravel mechanisms and molecules involved in cell fate specification and morphogenesis. Microinjection is the elective method to study gene function in sea urchin embryos. It is used to deliver precise amounts of DNA, RNA, oligonucleotides, peptides, or antibodies into the eggs or even into blastomeres. Here we describe microinjection as it is currently applied in our laboratory and show how it has been used in gene perturbation analyses and dissection of cis-regulatory DNA elements.

cis-Regulatory sequences driving the expression of the Hbox12 homeobox-containing gene in the presumptive aboral ectoderm territory of the Paracentrotus lividus sea urchin embryo

Embryonic development is coordinated by networks of evolutionary conserved regulatory genes encoding transcription factors and components of cell signalling pathways. In the sea urchin embryo, a number of genes encoding transcription factors display territorial restricted expression. Among these, the zygotic Hbox12 homeobox gene is transiently transcribed in a limited number of cells of the animal-lateral half of the early Paracentrotus lividus embryo, whose descendants will constitute part of the ectoderm territory. To obtain insights on the regulation of Hbox12 expression, we have explored the cis-regulatory apparatus of the gene. In this paper, we show that the intergenic region of the tandem Hbox12 repeats drives GFP expression in the presumptive aboral ectoderm and that a 234 bp fragment, defined aboral ectoderm (AE) module, accounts for the restricted expression of the transgene. Within this module, a consensus sequence for a Sox factor and the binding of the Otx activator are both required for correct Hbox12 gene expression. Spatial restriction to the aboral ectoderm is achieved by a combination of different repressive sequence elements. Negative sequence elements necessary for repression in the endomesoderm map within the most upstream 60 bp region and nearby the Sox binding site. Strikingly, a Myb-like consensus is necessary for repression in the oral ectoderm, while down-regulation at the gastrula stage depends on a GA-rich region. These results suggest a role for Hbox12 in aboral ectoderm specification and represent our first attempt in the identification of the gene regulatory circuits involved in this process.

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).

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.

Strongylocentrotus purpuratus transcription factor GATA-E binds to and represses transcription at an Otx-Goosecoid cis-regulatory element within the aboral ectoderm-specific spec2a enhancer

During Strongylocentrotus purpuratus embryogenesis, aboral ectoderm-specific expression of spec2a relies on an upstream enhancer that confers its spatial specificity largely through repression. The purpose of this study was to determine how spec2a expression is repressed in endoderm and oral ectoderm territories. A 78-base pair DNA sequence within the enhancer contains five tightly spaced cis-regulatory elements including proximal (TAATCT) and distal (TAATCC) elements that bind to both SpOtx, a broadly distributed transcriptional activator, and SpGoosecoid (SpGsc), an oral ectoderm-restricted transcriptional repressor. We show here that these two seemingly redundant Otx/Gsc elements have distinct functions. The proximal element bound to SpGATA-E, an endomesoderm-specific transcription factor. Treatment with SpGATA-E and SpGsc morpholino antisense oligonucleotides (MASOs) resulted in enhanced transcriptional activity from the proximal element, suggesting that both factors functioned as repressors at this site. SpGATA-E MASO-treated embryos failed to express ectoderm markers, indicating a role for SpGATA-E in ectoderm differentiation. The spec2a proximal element was distinct from the corresponding element in the related spec1 enhancer, and swaps between spec1 and spec2a cis-regulatory elements indicated, that for optimal repression, the proximal element had to interact with a nearby CCAAT-binding factor element. Our results show that the recently evolved proximal element contributes to the repression of spec2a in endomesoderm and oral ectoderm territories.

Structure, expression, and transcriptional regulation of the Strongylocentrotus franciscanus spec gene family encoding intracellular calcium-binding proteins

The mechanisms by which gene expression patterns emerge during evolution are poorly understood. The sea urchin spec genes offer a useful means to investigate evolutionary mechanisms. Genes of the spec family from Strongylocentrotus purpuratus and Lytechinus pictus have identical patterns of aboral ectoderm-specific expression but exhibit species-specific differences in copy number, genomic structure, temporal expression, and cis-regulatory architecture. Here, we identify spec genes from a phylogenetic intermediate, Strongylocentrotus franciscanus, to gain insight into the evolution of the spec gene family and its transcriptional regulation. We identified two spec genes in the S. franciscanus genome, sfspec1a and sfspec1b, that were orthologous to spec1 from S. purpuratus. sfspec1b transcripts began to accumulate at the blastula stage and became progressively more abundant; this was reminiscent of spec expression in L. pictus but different from that in S. purpuratus. As expected, sfspec1b expression was restricted to aboral ectoderm cells. The six-exon structure of the sfspec1b genomic locus was identical to that of the S. purpuratus spec genes and was bounded by two repeat-spacer-repeat (RSR) repetitive sequence elements, which are conserved features of S. purpuratus spec genes and function as transcriptional enhancers. The enhancer activity of the sfspec1b RSRs was comparable to that of their S. purpuratus counterparts, although the placement and orientation of crucial cis-regulatory elements within the RSRs differed. We discovered a spec gene in S. franciscanus that was only distantly related to other spec genes but was highly conserved in S. purpuratus. Unexpectedly, this gene was expressed exclusively in endoderm lineages. Our results show that the evolution of spec cis-regulatory elements is highly dynamic and that substantial alterations can occur when maintaining or grossly modifying gene expression patterns.

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.

Creation of cis-regulatory elements during sea urchin evolution by co-option and optimization of a repetitive sequence adjacent to the spec2a gene

The creation, preservation, and degeneration of cis-regulatory elements controlling developmental gene expression are fundamental genome-level evolutionary processes about which little is known. Here, we identify critical differences in cis-regulatory elements controlling the expression of the sea urchin aboral ectoderm-specific spec genes. We found multiple copies of a repetitive sequence element termed RSR in genomes of species within the Strongylocentrotidae family, but RSRs were not detected in genomes of species outside Strongylocentrotidae. spec genes in Strongylocentrotus purpuratus are invariably associated with RSRs, and the spec2a RSR functioned as a transcriptional enhancer and displayed greater activity than did spec1 or spec2c RSRs. Single-base pair differences at two cis-regulatory elements within the spec2a RSR increased the binding affinities of four transcription factors, SpCCAAT-binding factor at one element and SpOtx, SpGoosecoid, and SpGATA-E at another. The cis-regulatory elements to which these four factors bound were recent evolutionary acquisitions that acted to either activate or repress transcription, depending on the cell type. These elements were found in the spec2a RSR ortholog in Strongylocentrotus pallidus but not in RSR orthologs of Strongylocentrotus droebachiensis or Hemicentrotus pulcherrimus. Our results indicated that a dynamic pattern of cis-regulatory element evolution exists for spec genes despite their conserved aboral ectoderm expression.

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.

An ancient role for nuclear beta-catenin in the evolution of axial polarity and germ layer segregation

The human oncogene beta-catenin is a bifunctional protein with critical roles in both cell adhesion and transcriptional regulation in the Wnt pathway. Wnt/beta-catenin signalling has been implicated in developmental processes as diverse as elaboration of embryonic polarity, formation of germ layers, neural patterning, spindle orientation and gap junction communication, but the ancestral function of beta-catenin remains unclear. In many animal embryos, activation of beta-catenin signalling occurs in blastomeres that mark the site of gastrulation and endomesoderm formation, raising the possibility that asymmetric activation of beta-catenin signalling specified embryonic polarity and segregated germ layers in the common ancestor of bilaterally symmetrical animals. To test whether nuclear translocation of beta-catenin is involved in axial identity and/or germ layer formation in 'pre-bilaterians', we examined the in vivo distribution, stability and function of beta-catenin protein in embryos of the sea anemone Nematostella vectensis (Cnidaria, Anthozoa). Here we show that N. vectensis beta-catenin is differentially stabilized along the oral-aboral axis, translocated into nuclei in cells at the site of gastrulation and used to specify entoderm, indicating an evolutionarily ancient role for this protein in early pattern formation.

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

We discuss steps in the specification of major tissue territories of the sea urchin embryo that occur between fertilization and hatching blastula stage and the cellular interactions required to coordinate morphogenetic processes that begin after hatching. We review evidence that has led to new ideas about how this embryo is initially patterned: (1) Specification of most of the tissue territories is not direct, but proceeds gradually by progressive subdivision of broad, maternally specified domains that depend on opposing gradients in the ratios of animalizing transcription factors (ATFs) and vegetalizing (beta-catenin) transcription factors; (2) the range of maternal nuclear beta-catenin extends further than previously proposed, that is, into the animal hemisphere, where it programs many cells to adopt early aboral ectoderm characteristics; (3) cells at the extreme animal pole constitute a unique ectoderm region, lacking nuclear beta-catenin; (4) the pluripotential mesendoderm is created by the combined outputs of ATFs and nuclear beta-catenin, which initially overlap in the macromeres, and by an undefined early micromere signal; (5) later micromere signals, which activate Notch and Wnt pathways, subdivide mesendoderm into secondary mesenchyme and endoderm; and (6) oral ectoderm specification requires reprogramming early aboral ectoderm at about the hatching blastula stage. Morphogenetic processes that follow initial fate specification depend critically on continued interactions among cells in different territories. As illustrations, we discuss the regulation of (1) the ectoderm/endoderm boundary, (2) mesenchyme positioning and skeletal growth, (3) ciliated band formation, and (4) several suppressive interactions operating late in embryogenesis to limit the fates of multipotent cells.

The ascidian tadpole larva: comparative molecular development and genomics

Evolution is of interest not only to developmental biology but also to genetics and genomics. We are witnessing a new era in which evolution, development, genetics and genomics are merging to form a new discipline, a good example of which is the study of the origin and evolution of the chordates. Recent studies on the formation of the notochord and the dorsal neural tube in the increasingly famous Ciona intestinalis tadpole larva, and the availability of its draft genome, show how the combination of comparative molecular development and evolutionary genomics might help us to better understand our chordate ancestor.

Primary mesenchyme cell patterning during the early stages following ingression

Sea urchin primary mesenchyme cells (PMCs) ingress into the blastocoel during an epithelial-to-mesenchymal transition (EMT), migrate along the blastocoelar wall for a period of time, and then settle into a subequatorial ring to form the larval skeleton. Fluorescent-marked blastomeres alone, or in combination with blastomere recombination, were used to track the position of PMCs during the early phases of this movement. Micromeres expressing Golgi-tethered GFP (galtase-GFP) were transplanted onto TRITC-stained hosts (in place of the endogenous micromere) to observe the progeny of a single micromere. Galtase-GFP as a Golgi marker is not transferred between PMCs when the syncytium forms. Thus, the position of cells can be followed relative to beginning position for longer periods than previously reported. The PMC progeny of a single micromere do not disperse upon ingression, but instead remain in a closely associated cluster. Generally, progeny of a single micromere remain in the quadrant of origin. In total, greater than approximately 94% of labeled PMCs remain within the local region of ingression. By contrast, when a transplanted micromere is placed at the vegetal plate after removing all 4 host micromeres, the resultant PMCs ingress and migrate into all 4 quadrants. Similarly, if 1 blastomere is injected at the 2-cell stage, and later the 2 unlabeled micromeres are removed at the 16-cell stage, the remaining PMCs ingress into all 4 quadrants of the vegetal plate. We conclude that the normal restriction of PMCs to a quadrant is due to mechanical constraint from other micromere-PMCs. If a labeled micromere is placed ectopically at the macromere/mesomere boundary, the PMC progeny ingress ectopically and migrate longitudinally along the animal-vegetal axis only. Injection of galtase-GFP into one blastomere at the 4-cell stage shows a 2-step pattern of localization. At late mesenchyme blastula and early gastrula stages, greater than 90% of GFP-expressing PMCs remain in the injected quadrant, while at mid- to late-gastrula stage and beyond, more PMCs are found outside the injected quadrant. The migration that sets up the asymmetry of the larval skeleton first occurs around mid- to late-gastrula stages, when some PMCs from an aboral quadrant migrate to the adjacent oral quadrant. In all, these data combined with previous data suggest that freshly ingressed PMCs migrate along a longitudinal path toward the animal pole and back toward the vegetal pole. Beginning at mid- to late-gastrula stage, PMCs utilize oral-aboral cues from the ectoderm for the first time. At this time, some aboral PMCs migrate into the adjacent oral quadrant to assist in the formation of the ventrolateral cluster.

Cyclin D and cdk4 are required for normal development beyond the blastula stage in sea urchin embryos

cdk4 mRNA and protein are constitutively expressed in sea urchin eggs and throughout embryonic development. In contrast, cyclin D mRNA is barely detectable in eggs and early embryos, when the cell cycles consist of alternating S and M phases. Cyclin D mRNA increases dramatically in embryos at the early blastula stage and remains at a constant level throughout embryogenesis. An increase in cdk4 kinase activity occurs concomitantly with the increase in cyclin D mRNA. Ectopic expression of cyclin D mRNA in eggs arrests development before the 16-cell stage and causes eventual embryonic death, suggesting that activation of cyclin D/cdk4 in cleavage cell cycles is lethal to the embryo. In contrast, blocking cyclin D or cdk4 expression with morpholino antisense oligonucleotides results in normal development of early gastrula-stage embryos but abnormal, asymmetric larvae. These results suggest that in sea urchins, cyclin D and cdk4 are required for normal development and perhaps the patterning of the developing embryo, but may not be directly involved in regulating entry into the cell cycle.

A regulatory gene network that directs micromere specification in the sea urchin embryo

Micromeres and their immediate descendants have three known developmental functions in regularly developing sea urchins: immediately after their initial segregation, they are the source of an unidentified signal to the adjacent veg(2) cells that is required for normal endomesodermal specification; a few cleavages later, they express Delta, a Notch ligand which triggers the conditional specification of the central mesodermal domain of the vegetal plate; and they exclusively give rise to the skeletogenic mesenchyme of the postgastrular embryo. We demonstrate the key components of the zygotic regulatory gene network that accounts for micromere specificity. This network is a subelement of the overall endomesoderm specification network of the Strongylocentrotus purpuratus embryo. A central role is played by a newly discovered gene encoding a paired class homeodomain transcription factor which in micromeres acts as a repressor of a repressor: the gene is named pmar1 (paired-class micromere anti-repressor). pmar1 is expressed only during cleavage and early blastula stages, and exclusively in micromeres. It is initially activated as soon as the micromeres are formed, in response to Otx and beta-Catenin/Tcf inputs. The repressive nature of the interactions mediated by the pmar1 gene product was shown by the identical effect of introducing mRNA encoding the Pmar1 factor, and mRNA encoding an Engrailed-Pmar1 (En-Pmar1) repressor domain fusion. In both cases, the effects are derepression: of the delta gene; and of skeletogenic genes, including several transcription factors normally expressed only in micromere descendants, and also a set of downstream skeletogenic differentiation genes. The spatial phenotype of embryos bearing exogenous mRNA encoding Pmar1 factor or En-Pmar1 is expansion of the domains of expression of the downstream genes over most or all of the embryo. This results in transformation of much of the embryo into skeletogenic mesenchyme cells that express skeletogenic markers. The normal role of pmarl is to prevent, exclusively in the micromeres, the expression of a repressor that is otherwise operative throughout the embryo. This function accounts for the localization of delta transcription in micromeres, and thereby for the conditional specification of the vegetal plate mesoderm. It also explains why skeletogenic differentiation gene batteries normally function only in micromere descendants. More generally, the regulatory network subelement emerging from this work shows how the specificity of micromere function depends on continuing global regulatory interactions, as well as on early localized inputs.

Ectoderm gene activation in sea urchin embryos mediated by the CCAAT-binding factor

Transcriptional enhancers are short stretches of DNA that function to achieve highly specific patterns of gene expression. To identify the mechanisms by which enhancers achieve their specificity, we made use of an enhancer from the aboral ectoderm-specific spec2a gene of the sea urchin Strongylocentrotus purpuratus. The spec2a enhancer contains five cis-regulatory elements within 78 base pairs that interact with five distinct DNA-binding proteins to confer aboral ectoderm expression. Here, we present an analysis of the sea urchin CCAAT binding factor (CBF), which binds to a CCAAT motif within the spec2a enhancer. S. purpuratus CBF and SpOtx, a ubiquitously expressed factor, act together at closely placed cis-regulatory elements to mediate spec2a transcription in the ectoderm. SpCBF was the sole factor that bound to the spec2a CCAAT element, and two of the three subunits that make up the CBF holoprotein were cloned and shown to have high sequence conservation with their vertebrate orthologs. Based on its involvement in the regulation of several other sea urchin genes, SpCBF appears to be a major transcription factor in the sea urchin embryo for positive regulation of ectoderm gene expression. In addition to its role in vertebrate cell growth and proliferation, our results indicate that CBF also functions at the early stages of germ layer formation, namely ectoderm differentiation.

Regulated nuclear trafficking of the homeodomain protein otx1 in cortical neurons

Otx1 is a homeodomain protein required for axon refinement by layer 5 neurons in developing cerebral cortex. Otx1 localizes to the cytoplasm of progenitor cells in the rat ventricular zone, and remains cytoplasmic as neurons migrate and begin to differentiate. Nuclear translocation occurs during the first week of postnatal life, when layer 5 neurons begin pruning their long-distance axonal projections. Deletion analysis reveals that Otx1 is imported actively into cell nuclei, that the N-terminus of Otx1 is necessary for nuclear import, and that a putative nuclear localization sequence within this domain is sufficient to direct nuclear import in a variety of cell lines. In contrast, GFP-Otx1 fusion proteins that contain the N-terminus are retained in the cytoplasm of cortical progenitor cells, mimicking the distribution of Otx1 in vivo. These results suggest that ventricular cells actively sequester Otx1 in the cytoplasm, either by preventing nuclear import or by promoting a balance of export over import signals.

A novel promoter element, photoreceptor conserved element II, directs photoreceptor-specific expression of nocturnin in Xenopus laevis

Nocturnin is a vertebrate circadian clock-regulated gene, and in Xenopus laevis its mRNA is specifically expressed in retinal photoreceptor cells. We have investigated the transcriptional regulatory mechanism that drives this precise spatial expression pattern of the nocturnin gene. A deletion series of the nocturnin 5'-flanking sequence driving the green fluorescence protein (GFP) reporter was used to generate transgenic Xenopus tadpoles. We found that a construct containing 2.6 kilobase pairs of 5'-flanking sequence targeted high level GFP reporter expression specifically to photoreceptor cells, in a pattern identical to endogenous nocturnin. This photoreceptor-specific expression pattern was maintained with several further deletions of 5'-upstream sequence, including a short 59-base pair fragment. Within this region of 59 base pairs, three perfect repeats of a novel protein binding site were identified by electrophoretic mobility shift assay. Competitions using varying oligonucleotide sequences demonstrated that the sequence required for protein binding is CAGACAGGCTATA, designated photoreceptor-conserved element II (PCE II). The protein complex that binds to this element is enriched in retinal extracts, and mutations of PCE II which fail to bind the protein complex also fail to direct GFP reporter expression to photoreceptors. These results indicate that the PCE II in the proximal promoter of the nocturnin gene is sufficient for driving the photoreceptor-specific expression of nocturnin.

Correct Expression of spec2a in the sea urchin embryo requires both Otx and other cis-regulatory elements

Strongylocentrotus purpuratus Otx (SpOtx) is required simultaneously in sea urchin development for the activation of endo16 in the vegetal plate and for the activation of spec2a in the aboral ectoderm. Because Otx binding sites alone do not appear to be responsible for the spatially restricted expression of spec2a, additional DNA elements were sought. We show here that consensus Otx binding sites fused to basal promoters are sufficient to activate CAT reporter gene expression in all cell types, although expression in endomesoderm progenitors is enhanced. On the other hand, three non-Otx elements derived from the spec2a enhancer are needed together with Otx sites for specifically aboral ectoderm expression. A DNA element termed Y/CBF, lying just downstream from an Otx site within the spec2a enhancer, mediates general activation in the ectoderm. A second element lying between the Otx and Y/CBF sites, called OER, functions to prevent expression in the oral ectoderm. A third site, called ENR, overlapping another Otx site, is required to repress endoderm expression. Three distinct DNA binding proteins interact sequence specifically at the Y/CBF, OER, and ENR elements. The spec2a enhancer thus consists of closely linked activator and repressor elements that function collectively to cause expression of the spec2a gene in the aboral ectoderm.

The subcellular localization of Otx2 is cell-type specific and developmentally regulated in the mouse retina

Recent evidence implicates homeodomain-containing proteins in the specification of cell fates in the central nervous system. Here we report that in the embryonic mouse eye Otx2, a paired homeodomain transcription factor, was found in retinal pigment epithelial cells and a restricted subset of retinal neurons, including ganglion cells. In the postnatal and adult eye, however, both the cellular and subcellular distribution of the Otx2 protein were cell type-specific. Otx2 was detected only in the nuclei of retinal pigment epithelial and bipolar cells, but was present in the cytoplasm of rod photoreceptors. Immunohistochemical studies of retinal explants and transfected cell lines both suggested that the retention of Otx2 in the cytoplasm of immature rods is a developmentally regulated process. The differential distribution of Otx2 in the cytoplasm of rods and the nucleus of other cell types, suggests that subcellular localization of this transcription factor may participate cell fate determination during specific phases of retinal development.

T-Brain expression in the apical organ of hemichordate tornaria larvae suggests its evolutionary link to the vertebrate forebrain

T-box genes encode a novel family of sequence-specific activators that appear to play crucial roles in various processes of animal development. Although most of the T-box genes are involved in the mesoderm formation of chordate embryos, mammalian T-Brain is expressed in the developing central nervous system, and defines molecularly distinct domains within the cerebral cortex. Here we report the first invertebrate T-Brain homologue from the hemichordate acorn worm, Ptychodera flava, which we designate Pf-Tbrain. Developmental expression of Pf-Tbrain was examined by whole mount in situ hybridization to various stages of P. flava embryos. A weak, broad in situ hybridization signal of the Pf-Tbrain transcript is first detected during gastrulation in cells around the archenteron, but this signal disappears as gastrulation proceeds. At mid-gastrula an intense signal appears in several apical ectoderm cells of the gastrula. This signal becomes restricted to the apical region, where the eyespots or the light-sensory organ of the tornaria larva form. Expression of Pf-Tbrain in the apical sensory organ of the tornaria and vertebrate T-Brain in the forebrain suggests an evolutionary relationship between the non-chordate deuterostome larval apical sensory organ and the chordate forebrain.

Modularity and dissociation in the evolution of gene expression territories in development

Modularity is a salient feature of development and crucial to its evolution. This paper extends modularity to include the concept of gene expression territory, as established for sea urchin embryos. Territories provide a mechanism for partitioning of the cells of a rapidly developing embryo into functional units of a feeding larva. Territories exhibit the characteristics of modules. The paper asks if the embryo and the nonfeeding larva of the direct-developing sea urchin Heliocidaris erythrogramma are organized into gene expression territories, and if its territories correspond to the canonical territories of the pluteus. An analysis of cell lineage and gene expression data for H. erythrogramma shows that skeletogenic cell, coelomic, and vegetal plate gene expression territories are conserved, although they arise from cell lineages distinct from those of the pluteus, and the overall morphology of the larva differs from that of a pluteus. The ectoderm, as in indirect developers, is divided into territories. However, the oral ectodermal territory characteristic of the pluteus is absent in H. erythrogramma. Oral ectoderm is restored in hybrids of H. erythrogramma eggs fertilized by Heliocidaris tuberculata sperm. This indicates that embryonic modules evolve by changes in expression of dominant regulatory genes within territories and that entire modules can be eliminated in evolution of embryos.

TCF is the nuclear effector of the beta-catenin signal that patterns the sea urchin animal-vegetal axis

The mechanism of animal-vegetal (AV) axis formation in the sea urchin embryo is incompletely understood. Specification of the axis is thought to involve a combination of cell-cell signals and as yet unidentified maternal determinants. In Xenopus the Wnt pathway plays a crucial role in defining the embryonic axes. Recent experiments in sea urchins have shown that at least two components of the Wnt signaling pathway, GSK3beta and beta-catenin, are involved in embryonic AV axis patterning. These results support the notion that the developmental network that regulates axial patterning in deuterostomes is evolutionarily conserved. To further test this hypothesis, we have examined the role of beta-catenin nuclear binding partners, members of the TCF family of transcriptional regulators, in sea urchin AV axis patterning. To test the role of TCFs in mediating beta-catenin signals in sea urchin AV axis development we examined the consequences of microinjecting RNAs encoding altered forms of TCF on sea urchin development. We show that expression of a dominant negative TCF results in a classic "animalized" embryo. In contrast, microinjected RNA encoding an activated TCF produces a highly "vegetalized" embryo. We show that the transactivational activity of endogenous sea urchin TCF is potentiated by LiCl treatment, which vegetalizes embryos by inhibiting GSK3, consistent with an in vivo interaction between endogenous beta-catenin and TCF. We also provide evidence indicating that all of beta-catenin's activity in patterning the sea urchin AV axis is mediated by TCF. Using a glucocorticoid-responsive TCF, we show that TCF transcriptional activity affects specification along the AV axis between fertilization and the 60-cell stage.

Calponin modulates the exclusion of Otx-expressing cells from convergence extension movements

Otx2, a vertebrate homologue of the Drosophila orthodenticle gene, coordinates two processes in early embryonic development. Not only does it specify cell fate in the anterior regions of the embryo, it also prevents the cells that express it from participating in the convergence extension movements that shape the rest of the body axis. Here we show that, in Xenopus, this latter function is mediated by XclpH3, transcription of which is directly stimulated by Xotx2. XclpH3 is a Xenopus homologue of the mammalian calponin gene, the product of which binds both actin and myosin and prevents the generation of contractile force by actin filaments.

A view from the genome: spatial control of transcription in sea urchin development

The process of embryogenesis depends on differential regulation of genes in the spatial components defined by the embryonic cells (blastomeres). Developmental regulation is mediated by complex, hardwired genomic control systems consisting of clusters of multiple target sites at which specific interactions with regionally presented transcription factors occur. In the age of genomics and gene-transfer technology, the sea urchin embryo provides unique opportunities for experimental analysis of these processes. Research on gene regulation in sea urchin embryos in the past year has seen remarkable progress in two large areas: in understanding functional cis-regulatory architecture; and in understanding the mechanism by which the axial coordinates of the egg are transduced into a molecular system for differential gene activation.

Requirement of SpOtx in cell fate decisions in the sea urchin embryo and possible role as a mediator of beta-catenin signaling

We show here that the homeodomain transcription factor SpOtx is required for endoderm and aboral ectoderm formation during sea urchin embryogenesis. SpOtx target genes were repressed by fusing the SpOtx homeodomain to an active repression domain of Drosophila Engrailed. The Engrailed-SpOtx fusion protein reduced the expression of endoderm- and aboral ectoderm-specific genes and inhibited the formation of endoderm and aboral ectoderm cell types. Coexpressing activated beta-catenin with Engrailed-SpOtx did not overcome the inhibition of endoderm and aboral ectoderm formation, suggesting that SpOtx functioned either downstream of or parallel to nuclear beta-catenin. Embryos expressing C-cadherin, which blocks nuclear translocation of beta-catenin, have defects in endoderm and aboral ectoderm formation. Coexpressing SpOtx with C-cadherin restored aboral ectoderm-specific gene expression and aboral ectoderm morphology, but with C-cadherin present, SpOtx was not sufficient for endoderm formation. Our results show that SpOtx plays a key role in the activation of aboral ectoderm- and endoderm-specific gene expression and, in addition, suggest that SpOtx mediates some of beta-catenin's functions in endoderm and aboral ectoderm formation.

Spatially restricted expression of PlOtp, a Paracentrotus lividus orthopedia-related homeobox gene, is correlated with oral ectodermal patterning and skeletal morphogenesis in late-cleavage sea urchin embryos

Several homeobox genes are expressed in the sea urchin embryo but their roles in development have yet to be elucidated. Of particular interest are homologues of homeobox genes that in mouse and Drosophila are involved in patterning the developing central nervous system (CNS). Here, we report the cloning of an orthopedia (Otp)-related gene from Paracentrotus lividus, PlOtp. Otp is a single copy zygotic gene that presents a unique and highly restricted expression pattern. Transcripts were first detected at the mid-gastrula stage in two pairs of oral ectoderm cells located in a ventrolateral position, overlying primary mesenchyme cell (PMC) clusters. Increases in both transcript abundance and the number of Otp-expressing cells were observed at prism and pluteus stages. Otp transcripts are symmetrically distributed in a few ectodermal cells of the oral field. Labelled cells were observed close to sites of active skeletal rod growth (tips of the budding oral and anal arms), and at the juxtaposition of stomodeum and foregut. Chemicals known to perturb PMC patterning along animal-vegetal and oral-aboral axes altered the pattern of Otp expression. Vegetalization by LiCl caused a shift in Otp-expressing cells toward the animal pole, adjacent to shifted PMC aggregates. Nickel treatment induced expression of the Otp gene in an increased number of ectodermal cells, which adopted a radialized pattern. Finally, ectopic expression of Otp mRNA affected patterning along the oral-aboral axis and caused skeletal abnormalities that resembled those exhibited by nickel-treated embryos. From these results, we conclude that the Otp homeodomain gene is involved in short-range cell signalling within the oral ectoderm for patterning the endoskeleton of the larva through epithelial-mesenchymal interactions.

Function and evolution of Otx proteins

Otx proteins comprise an important class of homeodomain-containing transcription factors known for their essential roles in anterior head formation. Here, we briefly review the basic structural features and functional diversity of Otx proteins and describe current views on the evolution of Otx genes in metazoans. A prominent feature of Otx homeodomains is a lysine residue at position 9 of the recognition helix, which confers high-affinity binding to TAATCC/T elements on DNA. Besides their DNA binding properties, surprisingly little is known about how Otx proteins function to activate target genes in selective regions of the embryo. While an essential and ancient role for Otx is to pattern the anterior regions of the head, drawing conclusions about primordial functions is difficult. This is because Otx proteins have been recruited for numerous developmental roles, and derived functions have often evolved to meet the specialized requirements of individual taxonomic groups. In sea urchin embryos, one form of Otx may have been co-opted by the Wnt--catenin signaling pathway. The consequence of such an evolutionary event would be to link a highly conserved signal transduction pathway to a set of novel downstream genes that make use of Otx for their transcription.

LvNotch signaling mediates secondary mesenchyme specification in the sea urchin embryo

Cell-cell interactions are thought to regulate the differential specification of secondary mesenchyme cells (SMCs) and endoderm in the sea urchin embryo. The molecular bases of these interactions, however, are unknown. We have previously shown that the sea urchin homologue of the LIN-12/Notch receptor, LvNotch, displays dynamic patterns of expression within both the presumptive SMCs and endoderm during the blastula stage, the time at which these two cell types are thought to be differentially specified (Sherwood, D. R. and McClay, D. R. (1997) Development 124, 3363–3374). The LIN-12/Notch signaling pathway has been shown to mediate the segregation of numerous cell types in both invertebrate and vertebrate embryos. To directly examine whether LvNotch signaling has a role in the differential specification of SMCs and endoderm, we have overexpressed activated and dominant negative forms of LvNotch during early sea urchin development. We show that activation of LvNotch signaling increases SMC specification, while loss or reduction of LvNotch signaling eliminates or significantly decreases SMC specification. Furthermore, results from a mosaic analysis of LvNotch function as well as endogenous LvNotch expression strongly suggest that LvNotch signaling acts autonomously within the presumptive SMCs to mediate SMC specification. Finally, we demonstrate that the expansion of SMCs seen with activation of LvNotch signaling comes at the expense of presumptive endoderm cells, while loss of SMC specification results in the endoderm expanding into territory where SMCs usually arise. Taken together, these results offer compelling evidence that LvNotch signaling directly specifies the SMC fate, and that this signaling is critical for the differential specification of SMCs and endoderm in the sea urchin embryo.

Nuclear beta-catenin is required to specify vegetal cell fates in the sea urchin embryo

Beta-catenin is thought to mediate cell fate specification events by localizing to the nucleus where it modulates gene expression. To ask whether beta-catenin is involved in cell fate specification during sea urchin embryogenesis, we analyzed the distribution of nuclear beta-catenin in both normal and experimentally manipulated embryos. In unperturbed embryos, beta-catenin accumulates in nuclei that include the precursors of the endoderm and mesoderm, suggesting that it plays a role in vegetal specification. Using pharmacological, embryological and molecular approaches, we determined the function of beta-catenin in vegetal development by examining the relationship between the pattern of nuclear beta-catenin and the formation of endodermal and mesodermal tissues. Treatment of embryos with LiCl, a known vegetalizing agent, caused both an enhancement in the levels of nuclear beta-catenin and an expansion in the pattern of nuclear beta-catenin that coincided with an increase in endoderm and mesoderm. Conversely, overexpression of a sea urchin cadherin blocked the accumulation of nuclear beta-catenin and consequently inhibited the formation of endodermal and mesodermal tissues including micromere-derived skeletogenic mesenchyme. In addition, nuclear beta-catenin-deficient micromeres failed to induce a secondary axis when transplanted to the animal pole of uninjected host embryos, indicating that nuclear beta-catenin also plays a role in the production of micromere-derived signals. To examine further the relationship between nuclear beta-catenin in vegetal nuclei and micromere signaling, we performed both transplantations and deletions of micromeres at the 16-cell stage and demonstrated that the accumulation of beta-catenin in vegetal nuclei does not require micromere-derived cues. Moreover, we demonstrate that cell autonomous signals appear to regulate the pattern of nuclear beta-catenin since dissociated blastomeres possessed nuclear beta-catenin in approximately the same proportion as that seen in intact embryos. Together, these data show that the accumulation of beta-catenin in nuclei of vegetal cells is regulated cell autonomously and that this localization is required for the establishment of all vegetal cell fates and the production of micromere-derived signals.

HpEts, an ets-related transcription factor implicated in primary mesenchyme cell differentiation in the sea urchin embryo

The mechanism of micromere specification is one of the central issues in sea urchin development. In this study we have identified a sea urchin homologue of ets 1 + 2. HpEts, which is maternally expressed ubiquitously during the cleavage stage and which expression becomes restricted to the skeletogenic primary mesenchyme cells (PMC) after the hatching blastula stage. The overexpression of HpEts by mRNA injection into fertilized eggs alters the cell fate of non-PMC to migratory PMC. HpEts induces the expression of a PMC-specific spicule matrix protein, SM50, but suppresses of aboral ectoderm-specific arylsulfatase and endoderm-specific HpEndo16. The overexpression of dominant negative delta HpEts which lacks the N terminal domain, in contrast, specifically represses SM50 expression and development of the spicule. In the upstream region of the SM50 gene there exists an ets binding site that functions as a positive cis-regulatory element. The results suggest that HpEts plays a key role in the differentiation of PMCs in sea urchin embryogenesis.

Specification of cell fate in the sea urchin embryo: summary and some proposed mechanisms

An early set of blastomere specifications occurs during cleavage in the sea urchin embryo, the result of both conditional and autonomous processes, as proposed in the model for this embryo set forth in 1989. Recent experimental results have greatly illuminated the mechanisms of specification in some early embryonic territories, though others remain obscure. We review the progressive process of specification within given lineage elements, and with reference to the early axial organization of the embryo. Evidence for the conditional specification of the veg2 lineage subelement of the endoderm and other potential interblastomere signaling interactions in the cleavage-stage embryo are summarized. Definitive boundaries between mesoderm and endoderm territories of the vegetal plate, and between endoderm and overlying ectoderm, are not established until later in development. These processes have been clarified by numerous observations on spatial expression of various genes, and by elegant lineage labeling studies. The early specification events depend on regional mobilization of maternal regulatory factors resulting at once in the zygotic expression of genes encoding transcription factors, as well as downstream genes encoding proteins characteristic of the cell types that will much later arise from the progeny of the specified blastomeres. This embryo displays a maximal form of indirect development. The gene regulatory network underlying the embryonic development reflects the relative simplicity of the completed larva and of the processes required for its formation. The requirements for postembryonic adult body plan formation in the larval rudiment include engagement of a new level of genetic regulatory apparatus, exemplified by the Hox gene complex.

Crx, a novel Otx-like paired-homeodomain protein, binds to and transactivates photoreceptor cell-specific genes

The otd/Otx gene family encodes paired-like homeodomain proteins that are involved in the regulation of anterior head structure and sensory organ development. Using the yeast one-hybrid screen with a bait containing the Ret 4 site from the bovine rhodopsin promoter, we have cloned a new member of the family, Crx (Cone rod homeobox). Crx encodes a 299 amino acid residue protein with a paired-like homeodomain near its N terminus. In the adult, it is expressed predominantly in photoreceptors and pinealocytes. In the developing mouse retina, it is expressed by embryonic day 12.5 (E12.5). Recombinant Crx binds in vitro not only to the Ret 4 site but also to the Ret 1 and BAT-1 sites. In transient transfection studies, Crx transactivates rhodopsin promoter-reporter constructs. Its activity is synergistic with that of Nrl. Crx also binds to and transactivates the genes for several other photoreceptor cell-specific proteins (interphotoreceptor retinoid-binding protein, beta-phosphodiesterase, and arrestin). Human Crx maps to chromosome 19q13.3, the site of a cone rod dystrophy (CORDII). These studies implicate Crx as a potentially important regulator of photoreceptor cell development and gene expression and also identify it as a candidate gene for CORDII and other retinal diseases.

The SpHE gene is downregulated in sea urchin late blastulae despite persistence of multiple positive factors sufficient to activate its promoter

Previous studies of the regulatory region of the SpHE (hatching enzyme) gene of the sea urchin Strongylocentrotus purpuratus (Wei, Z., Angerer, L.M., Gagnon, M.L. and Angerer, R.C. (1995) Characterization of the SpHE promoter that are spatially regulated along the animal-vegetal axis of the sea urchin embryo. Dev. Biol. 171, 195-211) have shown that approximately 330 bp is necessary and sufficient to promote high level expression in embryos of transgenes that reproduce the spatially asymmetric pattern of endogenous gene activity along the maternally determined animal-vegetal embryonic axis. Furthermore, SpHE regulatory elements appear to be redundant since several different combinations are sufficient to elicit strong promoter activity and many subsets function like the endogenous gene only in non-vegetal cells of the blastula (Wei, Z., Angerer, L.M. and Angerer, R.C. (1997) Multiple positive cis-elements regulate the asymmetric expression of the SpHE gene along the sea urchin embryo animal-vegetal axis. Dev. Biol., 187, 71-88). Here we demonstrate by in vivo footprinting that many cis elements on the endogenous promoter are occupied when the gene is active in early blastulae, but the binding of corresponding trans factors is significantly reduced when the gene becomes inactive in late blastulae. In addition, downregulation of the promoter is accompanied by a transition from a non-nucleosomal to a nucleosome-like chromatin structure. Surprisingly, in vitro DNase I footprints of the 300 bp promoter using nuclear protein extracts from early and late blastulae are not detectably different and neither this sequence, nor a longer one extending to -1255, reproduces the loss of endogenous SpHE transcriptional activity after very early blastula stage. These observations imply that temporal repression of SpHE transcription involves a decrease in accessibility of the promoter to activators that are nevertheless present in nuclei and capable of activating transgene promoters. Temporal, but not spatial, downregulation is therefore likely to be regulated by negative activities functioning outside the -1255 promoter region which may serve as direct repressors or mediate an inactive chromatin structure.

Identification and localization of a sea urchin Notch homologue: insights into vegetal plate regionalization and Notch receptor regulation

The specifications of cell types and germ-layers that arise from the vegetal plate of the sea urchin embryo are thought to be regulated by cell-cell interactions, the molecular basis of which are unknown. The Notch intercellular signaling pathway mediates the specification of numerous cell fates in both invertebrate and vertebrate development. To gain insights into mechanisms underlying the diversification of vegetal plate cell types, we have identified and made antibodies to a sea urchin homolog of Notch (LvNotch). We show that in the early blastula embryo, LvNotch is absent from the vegetal pole and concentrated in basolateral membranes of cells in the animal half of the embryo. However, in the mesenchyme blastula embryo LvNotch shifts strikingly in subcellular localization into a ring of cells which surround the central vegetal plate. This ring of LvNotch delineates a boundary between the presumptive secondary mesoderm and presumptive endoderm, and has an asymmetric bias towards the dorsal side of the vegetal plate. Experimental perturbations and quantitative analysis of LvNotch expression demonstrate that the mesenchyme blastula vegetal plate contains both animal/vegetal and dorsoventral molecular organization even before this territory invaginates to form the archenteron. Furthermore, these experiments suggest roles for the Notch pathway in secondary mesoderm and endoderm lineage segregation, and in the establishment of dorsoventral polarity in the endoderm. Finally, the specific and differential subcellular expression of LvNotch in apical and basolateral membrane domains provides compelling evidence that changes in membrane domain localization of LvNotch are an important aspect of Notch receptor function.

Specification of endoderm in the sea urchin embryo

Expression of the endoderm specific gene Endo16, was used to monitor endoderm specification in developmentally arrested and in dissociated embryos. beta-APN treatment halts gastrulation, however, in two species of sea urchins Endo16 mRNA is still expressed, suggesting that specification of the endodermal lineage has taken place in these developmentally arrested embryos. Endo16 mRNA is not expressed in embryos dissociated at the 4-8-cell stage unless they are reassociated shortly after the 16-cell stage. Interestingly, dissociation after the 16-cell stage also results in a lack of Endo16 expression. Lithium is unable to rescue Endo16 expression in these dissociated embryos. These results indicate that early signaling events mediated by cell-cell contact are required for the initial specification and maintenance of the endoderm in the developing embryo.

Two Otx proteins generated from multiple transcripts of a single gene in Strongylocentrotus purpuratus

Orthodenticle-related (Otx) proteins are a highly conserved class of homeobox-containing transcription factors found in a wide range of organisms. They function in numerous developmental events, most prominently, anterior head patterning in insects and vertebrates. In the sea urchin, Strongylocentrotus purpuratus, an orthodenticle-related protein called SpOtx is believed to direct the activation of the aboral ectoderm-specific Spec2a gene and more generally the differentiation of aboral ectoderm cells. To learn more about the structure, expression, and function of SpOtx and compare its properties with those of orthologs from other species, we isolated cDNA and genomic clones containing SpOtx sequences. Here, we report that SpOtx exists in two forms (alpha and beta) that are generated by alternative RNA splicing from a single SpOtx gene. SpOtx(alpha) and SpOtx(beta) had identical C-termini and homeoboxes but were entirely different in their N-terminal domains. SpOtx(alpha) mRNAs were transcribed from a single start site and accumulated in all cells during cleavage, but were gradually concentrated in oral ectoderm and vegetal plate territories during gastrulation. In contrast, three distinct SpOtx(beta) mRNAs resulted from two separate transcriptional initiation events, and these transcripts began to accumulate at mesenchyme blastula stage primarily in ectoderm and then later were largely restricted to oral ectoderm and vegetal plate territories. DNA-binding activity for SpOtx(beta) appeared later in development than SpOtx(alpha). Overexpression of SpOtx(alpha) and SpOtx(beta) induced in sea urchin embryos by mRNA injection demonstrated that SpOtx(alpha) was able to repress the accumulation of SpOtx(beta) transcripts, whereas SpOtx(beta) had no effect on the accumulation of SpOtx(alpha) transcripts. These results demonstrate that novel forms of Otx are produced in sea urchins by differential promoter utilization and alternative splicing. It may be that similar regulatory mechanisms lead to diverse forms of Otx in vertebrates.

Multiple positive cis elements regulate the asymmetric expression of the SpHE gene along the sea urchin embryo animal-vegetal axis

The mechanism that establishes the maternally determined animal-vegetal axis of sea urchin embryos is unknown. We have analyzed the cis-regulatory elements of the SpHE gene of Strongylocentrotus purpuratus, which is asymmetrically expressed along this axis, in an effort to identify components of maternal positional information. Previously, we defined a regulatory region that is sufficient to provide correct nonvegetal expression of a beta-galactosidase reporter gene (Wei, Z., Angerer, L. M., Gagnon, M. L., and Angerer, R. C., Dev. Biol. 171, 195-211, 1995). We have now analyzed this region intensively in order to determine if the spatial pattern is controlled by nonvegetal-positive activities or by vegetal-negative activities. The regulatory sequences, except the basal promoter, were mutated by either deletion or sequence replacement. None of these mutations resulted in ectopic beta-gal expression in vegetal cells, showing that no single negative cis element is responsible for the lack of vegetal SpHE transcription. Surprisingly, even short segments of the regulatory region containing only several identified cis elements also direct nonvegetal expression. Furthermore, the SpHE basal promoter functions effectively in vegetal cells in combination with cis-acting elements derived from the PMC-specific gene, SM50. We conclude that the spatial pattern of SpHE transcription is achieved by multiple positive activities concentrated in nonvegetal cells. The vegetal expression of SM50 also is regulated only by positive activities (Makabe, K. W., Kirchhamer, C. V., Britten, R. J., and Davidson, E. H., Development 121, 1957-1970, 1995). A chimeric promoter containing both SpHE and SM50 regulatory sequences is active ubiquitously, suggesting that these regulators are not reciprocally repressive. These observations suggest a model in which the SpHE and SM50 genes are activated by separate sets of positive maternal activities concentrated, respectively, in nonvegetal and vegetal domains of the early embryo.

Short-range cell-cell signals control ectodermal patterning in the oral region of the sea urchin embryo

The ectoderm of the sea urchin embryo has been a useful system for understanding how regions of a simple epithelium are specified during early development, as well as how pattern formation leads to the correct localization of mesenchyme cells during morphogenesis. This study examines cell-cell signals that regulate precise patterning of ectoderm within the oral region of embryos of the sea urchin, Lytechinus variegatus. The oral ectoderm contains at least two types of patterned tissues: (1) the ectoderm that forms the stomodeum and (2) ectoderm expressing pattern information required for formation of parallel oral skeletal rods by primary mesenchyme cells (PMCs). Using microsurgical isolations and cell transplantation, we show (1) that cell-cell signaling is capable of producing new oral ectodermal structures until immediately prior to the gastrula stage, (2) that the presumptive oral ectoderm is not committed to produce oral structures until the early gastrula stage, (3) that oral ectodermal patterning cues for PMCs are highly local in character, and (4) that interactions between the tip of the archenteron and the presumptive oral ectoderm are not required for the differentiation of cells within either tissue. These studies suggest that short-range cell-cell signals within the ectoderm are involved in specifying regionalized oral ectodermal tissues immediately prior to gastrulation, and that this patterned ectoderm then influences the localization of skeletogenic mesenchyme cells in the oral region.

Very early and transient vegetal-plate expression of SpKrox1, a Krüppel/Krox gene from Stronglyocentrotus purpuratus

All endodermal and mesenchymal cells of the sea urchin embryo descend from the vegetal plate, a thickened epithelium of approximately 50 cells arising at the early blastula stage. Cell types that derive from the vegetal plate are specified conditionally by inductive interactions with underlying micromeres, but the molecular details of vegetal-plate specification remain unresolved. In a search for regulatory proteins that have roles in vegetal-plate specification, a screen was performed to clone Krüppel/Krox-related genes from a Strongylocentrotus purpuratus embryo cDNA library. One newly identified clone, named SpKrox1, contained four zinc fingers and a leucine zipper domain. SpKrox1 expression was low in unfertilized eggs, increased severalfold to the early blastula stage and decreased between the early gastrula and pluteus stages. SpKrox1 mRNA was first seen in macromeres of 16-cell stage embryos and was restricted to cells of the developing vegetal plate thereafter. Vegetal-plate expression corresponded to a ring of cells around the blastopore and overlapped the expression patterns of other genes with potential roles in vegetal plate-specification. As the vegetal-plate cells invaginated into the blastopore, SpKrox1 expression was lost, suggesting that its role was not in endoderm differentiation per se but rather in the initial establishment of the vegetal plate.

Transient appearance of Strongylocentrotus purpuratus Otx in micromere nuclei: cytoplasmic retention of SpOtx possibly mediated through an alpha-actinin interaction

At the 16-cell stage, the sea urchin embryo is partitioned along the animal-vegetal axis into eight mesomeres, four macromeres, and four micromeres. The micromeres, unlike the other blastomeres, are autonomously specified to produce skeletogenic mesenchymal cells and are also required to induce the vegetal-plate territory. A long-held belief is that micromeres inherit localized maternal determinants that endow them with their cell autonomous behavior and inducing capabilities. Here, we present evidence that an orthodenticle-related protein, SpOtx appears transiently in nuclei of micromeres but not in nuclei of mesomeres and macromeres. At later stages of development, SpOtx was translocated into nuclei of all cells. To address the possibility that SpOtx was retained in the cytoplasm at early developmental stages, we searched for cytoplasmic proteins that interact with SpOtx. A proline-rich region of SpOtx resembling an SH3-binding domain was used to screen an embryo cDNA expression library, and a cDNA clone was isolated and shown to be alpha-actinin. A yeast two-hybrid analysis showed a specific interaction between the proline-rich region of SpOtx and a putative SH3 domain of the sea urchin alpha-actinin. Because micromeres lack an actin-based cytoskeleton, the results suggested that, at the vegetal pole of the 16-cell stage embryo, SpOtx was translocated into micromere nuclei, whereas in other blastomeres SpOtx was actively retained in the cytoplasm by binding to alpha-actinin. The transient appearance of SpOtx in micromere nuclei may be associated with the specification of micromere cell fate.