The snail repressor establishes a muscle/notochord boundary in the Ciona embryo

Patterning the marginal zone of early ascidian embryos: localized maternal mRNA and inductive interactions

Early animal embryos are patterned by localized egg cytoplasmic factors and cell interactions. In invertebrate chordate ascidians, larval tail muscle originates from the posterior marginal zone of the early embryo. It has recently been demonstrated that maternal macho-1 mRNA encoding transcription factor acts as a localized muscle determinant. Other mesodermal tissues such as notochord and mesenchyme are also derived from the vegetal marginal zone. In contrast, formation of these tissues requires induction from endoderm precursors at the 32-cell stage. FGF-Ras-MAPK signaling is involved in the induction of both tissues. The responsiveness for induction to notochord or mesenchyme depends on the inheritance of localized egg cytoplasmic factors. Previous studies also point to critical roles of directed signaling in polarization of induced cells and in subsequent asymmetric divisions resulting in the formation of two daughter cells with distinct fates. One cell adopts an induced fate, while the other assumes a default fate. A simple model of mesoderm patterning in ascidian embryos is proposed in comparison with that of vertebrates.

The snail superfamily of zinc-finger transcription factors

The Snail superfamily of zinc-finger transcription factors is involved in processes that imply pronounced cell movements, both during embryonic development and in the acquisition of invasive and migratory properties during tumour progression. Different family members have also been implicated in the signalling cascade that confers left right identity, as well as in the formation of appendages, neural differentiation, cell division and cell survival.

The mouse snail gene encodes a key regulator of the epithelial-mesenchymal transition

Snail family genes encode DNA binding zinc finger proteins that act as transcriptional repressors. Mouse embryos deficient for the Snail (Sna) gene exhibit defects in the formation of the mesoderm germ layer. In Sna(-/-) mutant embryos, a mesoderm layer forms and mesodermal marker genes are induced but the mutant mesoderm is morphologically abnormal. Lacunae form within the mesoderm layer of the mutant embryos, and cells lining these lacunae retain epithelial characteristics. These cells resemble a columnar epithelium and have apical-basal polarity, with microvilli along the apical surface and intercellular electron-dense adhesive junctions that resemble adherens junctions. E-cadherin expression is retained in the mesoderm of the Sna(-/-) embryos. These defects are strikingly similar to the gastrulation defects observed in snail-deficient Drosophila embryos, suggesting that the mechanism of repression of E-cadherin transcription by Snail family proteins may have been present in the metazoan ancestor of the arthropod and mammalian lineages.

Ascidian embryos as a model system to analyze expression and function of developmental genes

Ascidians have served as an appropriate experimental system in developmental biology for more than a century. The fertilized egg develops quickly into a tadpole larva, which consists of a small number of organs including epidermis, central nervous system with two sensory organs, endoderm and mesenchyme in the trunk, and notochord and muscle in the tail. This configuration of the ascidian tadpole is thought to represent the most simplified and primitive chordate body plan. Their embryogenesis is simple, and lineage of embryonic cells is well documented. The ascidian genome contains a basic set of genes with less redundancy compared to the vertebrate genome. Cloning and characterization of developmental genes indicate that each gene is expressed under discrete spatio-temporal pattern within their lineage. In addition, the use of various molecular techniques in the ascidian embryo system highlights its advantages as a future experimental system to explore the molecular mechanisms underlying the expression and function of developmental genes as well as genetic circuitry responsible for the establishment of the basic chordate body plan. This review is aimed to highlight the recent advances in ascidian embryology.

Involvement of Rel/NF-kappaB in regulation of ascidian notochord formation

The Rel/NF-kappaB family is known to be involved in a wide variety of biological processes, including morphogenesis. In the present study, two protochordate cDNA clones encoding Rel/NF-kappaB proteins, named As-rel1 and As-rel2, were isolated from a fertilized egg cDNA library of the ascidian Halocynthia roretzi. The As-rel1 protein is a typical Rel/NF-kappaB family member, containing a Rel homology domain, a nuclear localization sequence and a C-terminal putative transcription activation domain, while the As-rel2 protein is a novel Rel/NF-kappaB family member that lacks a nuclear localization sequence and the C-terminal domain. Northern blot analyses showed that both transcripts were maternally expressed and that their expression changed during development of H. roretzi embryos. Although injection of the As-rel2 mRNA into H. roretzi fertilized eggs had little effect on embryonic development, injection of the As-rel1 mRNA interfered greatly with notochord formation, resulting in a shortened tail with a reduced number of notochord cells. In contrast, embryos co-injected with As-rel1 and As-rel2 mRNA developed normally, indicating that the As-rel2 protein rescued the defect in notochord formation induced by the injection of As-rel1 mRNA alone. These results strongly suggest that the As-rel1 protein functions as a suppressor in ascidian notochord formation, while the As-rel2 protein has an antagonistic effect on the action of the As-rel1 protein.

Identification of essential sequence motifs in the node/notochord enhancer of Foxa2 (Hnf3beta) gene that are conserved across vertebrate species

The expression of a winged-helix transcription factor, Foxa2/HNF3beta, is essential for development of the node and the notochord. We examined the node/notochord enhancer of mouse Foxa2 for sequence motifs conserved across vertebrate species. We cloned Foxa2 genes from chicken and fish, and identified the respective node/notochord enhancers that were active in transgenic mouse embryos. Comparison of the sequences of the enhancers revealed three evolutionally conserved sequence motifs, CS1, CS2 and CS3. Mutational analysis of the mouse enhancer indicated that CS3 is indispensable for gene expression in the node and the notochord, while CS1 and CS2 are required to augment enhancer activity. These motifs do not correspond to the consensus binding sequences of transcription factors known to be involved in node/notochord development.

The regulation of forkhead/HNF-3beta expression in the Ciona embryo

The Ciona forkhead/HNF-3beta gene (Ci-fkh) is expressed in the primary axial tissues of the developing tadpole, including the notochord, endoderm, and rudimentary floor plate of the CNS. In an effort to determine the basis for this complex pattern of expression we have conducted a detailed analysis of the Ci-fkh 5'-regulatory region. Different 5' sequences were attached to a lacZ reporter gene and analyzed in electroporated Ciona embryos. A short regulatory sequence (AS) located approximately 1.7 kb upstream of the transcribed region is shown to be essential for expression in all three axial tissues. The proximal 20 bp of the AS contains overlapping Snail repressor elements and a T-box motif. Deleting these sequences causes the loss of reporter gene expression in the endoderm, as well as expanded expression in the neural tube. These results suggest that a T-box gene such as Ci-VegTR activates Ci-fkh expression in the endoderm, while the Ci-Sna repressor excludes expression from the lateral ependymal cells and restricts the Ci-fkh pattern to the rudimentary floor plate in ventral regions of the neural tube. We also present evidence for Ci-fkh positive autofeedback, whereby the Ci-Fkh protein binds to critical activator sites within the Ci-fkh 5'-regulatory region and helps maintain high levels of expression. We discuss these results with respect to forkhead/HNF-3beta regulation in vertebrates.

Snail/slug family of repressors: slowly going into the fast lane of development and cancer

The existence of homologous genes in diverse species is intriguing. A detailed comparison of the structure and function of gene families may provide important insights into gene regulation and evolution. An unproven assumption is that homologous genes have a common ancestor. During evolution, the original function of the ancestral gene might be retained in the different species which evolved along separate courses. In addition, new functions could have developed as the sequence began to diverge. This may also explain partly the presence of multipurpose genes, which have multiple functions at different stages of development and in different tissues. The Drosophila gene snail is a multipurpose gene; it has been demonstrated that snail is critical for mesoderm formation, for CNS development, and for wing cell fate determination. The related vertebrate Snail and Slug genes have also been proposed to participate in mesoderm formation, neural crest cell migration, carcinogenesis, and apoptosis. In this review, we will discuss the Snail/Slug family of regulators in species ranging from insect to human. We will present the protein structures, expression patterns, and functions based on molecular genetic analyses. We will also include the studies that helped to elucidate the molecular mechanisms of repression and the relationship between the conserved and divergent functions of these genes. Moreover, the studies may enable us to trace the evolution of this gene family.

Characterization of a maternal T-Box gene in Ciona intestinalis

A new T-box gene, CiVegTR, was isolated in the ascidian Ciona intestinalis. CiVegTR maternal RNAs become localized to the vegetal cytoplasm of fertilized eggs and are incorporated into muscle lineages derived from the B4.1 blastomere. The CiVegTR protein binds to specific sequences within a minimal, 262-bp enhancer that mediates Ci-snail expression in the tail muscles. Mutations in these binding sites abolish expression from an otherwise normal lacZ reporter gene in electroporated embryos. In addition to the previously identified AC-core E-box sequences, T-box recognition sequences are conserved in the promoter regions of many genes expressed in B4.1 lineages in both Ciona and the distantly related ascidian Halocynthia. These results suggest that CiVegTR encodes a component of the classical muscle determinant that was first identified in ascidians nearly 100 years ago.

An FGF signal from endoderm and localized factors in the posterior-vegetal egg cytoplasm pattern the mesodermal tissues in the ascidian embryo

The major mesodermal tissues of ascidian larvae are muscle, notochord and mesenchyme. They are derived from the marginal zone surrounding the endoderm area in the vegetal hemisphere. Muscle fate is specified by localized ooplasmic determinants, whereas specification of notochord and mesenchyme requires inducing signals from endoderm at the 32-cell stage. In the present study, we demonstrated that all endoderm precursors were able to induce formation of notochord and mesenchyme cells in presumptive notochord and mesenchyme blastomeres, respectively, indicating that the type of tissue induced depends on differences in the responsiveness of the signal-receiving blastomeres. Basic fibroblast growth factor (bFGF), but not activin A, induced formation of mesenchyme cells as well as notochord cells. Treatment of mesenchyme-muscle precursors isolated from early 32-cell embryos with bFGF promoted mesenchyme fate and suppressed muscle fate, which is a default fate assigned by the posterior-vegetal cytoplasm (PVC) of the eggs. The sensitivity of the mesenchyme precursors to bFGF reached a maximum at the 32-cell stage, and the time required for effective induction of mesenchyme cells was only 10 minutes, features similar to those of notochord induction. These results support the idea that the distinct tissue types, notochord and mesenchyme, are induced by the same signaling molecule originating from endoderm precursors. We also demonstrated that the PVC causes the difference in the responsiveness of notochord and mesenchyme precursor blastomeres. Removal of the PVC resulted in loss of mesenchyme and in ectopic notochord formation. In contrast, transplantation of the PVC led to ectopic formation of mesenchyme cells and loss of notochord. Thus, in normal development, notochord is induced by an FGF-like signal in the anterior margin of the vegetal hemisphere, where PVC is absent, and mesenchyme is induced by an FGF-like signal in the posterior margin, where PVC is present. The whole picture of mesodermal patterning in ascidian embryos is now known. We also discuss the importance of FGF induced asymmetric divisions, of notochord and mesenchyme precursor blastomeres at the 64-cell stage.

Human Slug is a repressor that localizes to sites of active transcription

Snail/Slug family proteins have been identified in diverse species of both vertebrates and invertebrates. The proteins contain four to six zinc fingers and function as DNA-binding transcriptional regulators. Various members of the family have been demonstrated to regulate cell movement, neural cell fate, left-right asymmetry, cell cycle, and apoptosis. However, the molecular mechanisms of how these regulators function and the target genes involved are largely unknown. In this report, we demonstrate that human Slug (hSlug) is a repressor and modulates both activator-dependent and basal transcription. The repression depends on the C-terminal DNA-binding zinc fingers and on a separable repression domain located in the N terminus. This domain may recruit histone deacetylases to modify the chromatin and effect repression. Protein localization study demonstrates that hSlug is present in discrete foci in the nucleus. This subnuclear pattern does not colocalize with the PML foci or the coiled bodies. Instead, the hSlug foci overlap extensively with areas of the SC-35 staining, some of which have been suggested to be sites of active splicing or transcription. These results lead us to postulate that hSlug localizes to target promoters, where activation occurs, to repress basal and activator-mediated transcription.

FGF and genes encoding transcription factors in early limb specification

SnR, twist and Fgf10 are expressed in presumptive limb territories of early chick embryos. When FGF-2/FGF-8 beads are implanted in chick flank, an ectopic limb develops and SnR is irreversibly activated as early as 1 h. Ectopic Fgf10 and twist expression are activated much later at 17 and 20 h, respectively. FGF-10 can also induce SnR, but much later, and in this case activation occurs simultaneously with that of twist and Fgf10 via the Fgf8- expressing ridge. Tbx-4 and Tbx-5 are expressed in leg and wing forming regions, respectively, in a similar pattern to SnR and twist. FGF-2 leads to ectopic expression of Tbx-4 and Tbx-5 as rapidly as ectopic expression of SnR, but the patterns of ectopic transcripts suggest that induction of SnR and Tbx gene expression occur via different pathways.

Temporal expression patterns of 39 Brachyury-downstream genes associated with notochord formation in the Ciona intestinalis embryo

Expression of the Brachyury (Ci-Bra) gene of the ascidian Ciona intestinalis is initiated at the 64-cell stage. Gene expression is restricted to notochord precursor cells, and Ci-Bra plays a key role in notochord differentiation. In a previous study, nearly 50 cDNA clones for potential Ci-Bra-downstream genes that are expressed in notochord cells were isolated. The present determination, by whole-mount in situ hybridization, of the temporal expression patterns of 19 notochord-specific and 20 notochord-predominant genes demonstrated that the timings of initiation of the expression of various genes was not identical. The expression of several genes was initiated as early as the gastrula stage. However, the expression of most of the notochord-specific genes commenced at the neural plate stage. Partial nucleotide sequence data of these clones suggest that genes expressed earlier encode potential transcriptional factors and/or nuclear proteins, while those expressed later encode proteins implicated in cell adhesion, signal transduction, regulation of the cytoskeleton, and components of the extracellular matrix. These gene activities may be associated with changes in cell shape and adhesion during the intercalation and extension of the notochord cells.

Developmental gene activities in ascidian embryos

The fertilized egg of ascidians develops quickly into a tadpole-type larva consisting of several distinct types of tissues including epidermis, central nervous system, endoderm, mesenchyme, notochord, and muscle. This architecture of the ascidian larva represents the most simplified chordate body plan. Taking advantage of simple, well-defined cell lineages, the expression of developmental genes is analyzed at single-cell level. Advances in the methodology promote the ascidian embryo as a useful system for studying transcriptional control involved in the specification of embryonic cells and pattern formation of the embryo.

Suppression of muscle fate by cellular interaction is required for mesenchyme formation during ascidian embryogenesis

The tadpole larva of the ascidian Halocynthia roretzi has several hundred mesenchyme cells in its trunk. Mesenchyme cells are exclusively derived from the B8.5 and B7.7 blastomere pairs of the 110-cell embryo. It has been believed that specification of mesenchyme cells is an autonomous process. In the present study, we have demonstrated that presumptive-mesenchyme blastomeres isolated from early 32-cell embryos did not express mesenchyme-specific features, whereas those isolated after the late 64-cell stage developed mesenchyme markers autonomously. Results of experiments involving coisolation and recombination of blastomeres showed that cellular interaction with adjacent presumptive-endoderm blastomeres during the late 32- and early 64-cell stages is required for mesenchyme formation. When such interaction was absent, the presumptive-mesenchyme blastomeres developed into muscle cells. Therefore, a signal from endoderm precursor blastomeres promotes mesenchyme fate, suppressing the muscle fate that is specified by ooplasmic muscle determinants. In Halocynthia, the muscle actin gene was precociously activated in mesenchyme-muscle precursor blastomeres at the 32-cell stage, and the mesenchyme and muscle fates were separated into two daughter blastomeres at the next cleavage. In presumptive-mesenchyme blastomeres at the 64-cell stage, expression of the muscle actin gene was immediately down-regulated by the signal from the neighboring endoderm precursor blastomeres. Thus, mesenchyme formation involves a novel mechanism of fate specification in ascidians, where formation of mesenchyme cells requires cellular interaction that suppresses muscle fate in the mesenchyme precursor blastomeres.

Evolutionary alterations of the minimal promoter for notochord-specific Brachyury expression in ascidian embryos

The Brachyury genes of two divergent ascidians, As-T of Halocynthia roretzi and Ci-Bra of Ciona intestinalis, are expressed exclusively in notochord precursor cells. A previous study showed that the notochord-specific expression of Ci-Bra is controlled by a minimal promoter that is composed of three distinct regions: a region responsible for repression of expression in non-notochord mesoderm cells, a region for activation of expression in notochord cells, and a region for activation of expression in non-notochord mesoderm cells, distal to proximal to the transcription initiation site, respectively. We examined various deletion constructs of the As-T/lacZ fusion gene and demonstrate that a module between −289 and −250 bp of the 5′-flanking region is responsible for notochord-specific expression of the reporter gene. Gel-shift assays suggested the binding of nuclear protein(s) to this module. The 5′-flanking region of As-T contains a potential T-binding motif (-ACCTAGGT-) around −160 bp. Deletion of this motif from the p(−289)As-T/lacZ diminished the reporter gene expression. In addition, coinjection of p(−289)As-T/lacZ and synthetic As-T mRNA resulted in ectopic expression of lacZ in non-notochord cells, suggesting that the T-binding motif is responsible for autoactivation of the gene. These findings revealed striking differences between the minimal promoters of As-T and Ci-Bra so far revealed, with respect to their notochord-specific expression. Furthermore, reciprocal injections of reporter gene constructs, namely As-T/lacZ into Ciona eggs and Ci-Bra/lacZ into Halocynthia eggs, suggest alterations in the cis-regulatory elements and trans-activation factors that have occurred during evolution of the two ascidian species.

Vegetal cell fate specification and anterior neuroectoderm formation by Hroth, the ascidian homologue of orthodenticle/otx

To obtain insights into the mechanisms of gastrulation and neural tube formation, we studied the function and regulation of expression of Hroth, the ascidian homologue of orthodenticle/otx, during embryogenesis. Microinjection of synthetic Hroth mRNA into fertilized eggs led to embryos with an expanded trunk and a reduced tail. In these embryos, development of notochord and muscle was effected. Also, Hroth overexpression caused ectopic formation of anterior neuroectoderm, along with suppression of epidermis development, even in the absence of cell-cell interaction. Furthermore, we demonstrated that ectodermal expression of Hroth requires an inductive influence from the vegetal hemisphere cells. These data suggest roles of Hroth in both specification of mesoendodermal cells and anterior neuroectoderm formation.

Brachyury regulatory region active in embryonal carcinoma P19 cells

Brachyury (T) is involved in mesoderm induction during early mouse development. We analyzed the region regulating expression of T in embryonal carcinoma P19 cells, which differentiate into mesoderm derivatives in vitro. Transfection of plasmids encoding reporter genes under the control of the 5'-flanking region showed positive regulatory elements between -298 and -129 bp are responsible for driving T expression in mesodermal cells.

A PEA3 site flanked by SP1, SP4, and GATA sites positively regulates the differentiation-dependent expression of Brachyury in embryonal carcinoma P19 cells

The promoter sequence of Brachyury was analyzed in mouse embryonal carcinoma P19 cells. The sequence up to -267 bp relative to the transcription start site was sufficient to enhance reporter gene expression depending on the mesodermal differentiation of P19 cells. Footprint analysis by nuclear extract showed binding of a GATA protein and SP4 and mutation of their sites reduced reporter gene expression. Gel-retardation assay in the presence of a series of double-stranded DNA fragments as the competitors showed SP1 and Est sites additionally. Deletion of either sites reduced the reporter gene expression, showing that they are cooperative. Depletion of PEA3 (a transcription factor of the Est family) with a specific antibody diminished the retarded bands only for the nuclear extract from differentiated P19 cells. Thus, the PEA3 site supported by SP1, SP4, and GATA sites positively regulates the differentiation-dependent expression of Brachyury in P19 cells.

Suppressor of hairless activates brachyury expression in the Ciona embryo

The Ciona Brachyury gene (Ci-Bra) is regulated, in part, by a 434-bp enhancer that mediates restricted expression in the notochord. Here we present evidence that a Ciona Suppressor of Hairless ¿Ci-Su(H)¿ protein functions as an activator of this enhancer. Point mutations that reduce the binding of a GST/Ci-Su(H) fusion protein in vitro diminish the expression of mutagenized Ci-Bra/lacZ transgenes in electroporated embryos. Overexpression of a Ci-Su(H) fusion protein containing the Drosophila Hairy repression domain interferes with notochord differentiation, producing mutant tadpoles with shortened tails. Expression of a constitutively activated Xotch receptor in the notochord, endoderm, and CNS also alters tail morphogenesis. These results suggest that a Notch-Su(H) pathway might participate in notochord differentiation in Ciona.

Ascidian embryogenesis and the origins of the chordate body plan

For more than a century, ascidians have been a widely used system for classic embryological studies. Ascidians possess simple, well-defined cell-lineages, compact genomes, rapid development and world-wide distribution. Transgenic DNA can be introduced into developing embryos using simple electroporation methods. The ascidian larva represents the most simplified chordate body plan and provides a useful model for studying the molecular pathways underlying the morphogenesis and differentiation of the notochord and neural tube.