Dorsal-ventral patterning in Drosophila: DNA binding of snail protein to the single-minded gene

Molecular asymmetry in the cephalochordate embryo revealed by single-blastomere transcriptome profiling

Studies in various animals have shown that asymmetrically localized maternal transcripts play important roles in axial patterning and cell fate specification in early embryos. However, comprehensive analyses of the maternal transcriptomes with spatial information are scarce and limited to a handful of model organisms. In cephalochordates (amphioxus), an early branching chordate group, maternal transcripts of germline determinants form a compact granule that is inherited by a single blastomere during cleavage stages. Further blastomere separation experiments suggest that other transcripts associated with the granule are likely responsible for organizing the posterior structure in amphioxus; however, the identities of these determinants remain unknown. In this study, we used high-throughput RNA sequencing of separated blastomeres to examine asymmetrically localized transcripts in two-cell and eight-cell stage embryos of the amphioxus Branchiostoma floridae. We identified 111 and 391 differentially enriched transcripts at the 2-cell stage and the 8-cell stage, respectively, and used in situ hybridization to validate the spatial distribution patterns for a subset of these transcripts. The identified transcripts could be categorized into two major groups: (1) vegetal tier/germ granule-enriched and (2) animal tier/anterior-enriched transcripts. Using zebrafish as a surrogate model system, we showed that overexpression of one animal tier/anterior-localized amphioxus transcript, zfp665, causes a dorsalization/anteriorization phenotype in zebrafish embryos by downregulating the expression of the ventral gene, eve1, suggesting a potential function of zfp665 in early axial patterning. Our results provide a global transcriptomic blueprint for early-stage amphioxus embryos. This dataset represents a rich platform to guide future characterization of molecular players in early amphioxus development and to elucidate conservation and divergence of developmental programs during chordate evolution.

Specification of the Drosophila CNS midline cell lineage: direct control of single-minded transcription by dorsal/ventral patterning genes

The Drosophila CNS consists of a bilaterally symmetric group of neurons separated by a discrete group of CNS midline cells. The specification of the CNS midline cell lineage requires transcription of the single-minded gene. Genetic evidence suggests that a group of transcription factors, including Dorsal, Snail, Twist, and Daughterless:: Scute, is required for initial single-minded transcription. Comparison of the DNA sequences of the single-minded gene regulatory regions between two Drosophila species reveals conserved sequence elements. Biochemical studies using purified proteins indicate that a number of these conserved sequences represent binding sites for Dorsal, Snail, and Twist. In vitro mutagenesis combined with germline transformation indicates that these binding sites are required in vivo for single-minded mesectodermal transcription. These results show that single-minded transcription and, thus, CNS midline specification is directly controlled by dorsal/ventral patterning transcription factors. They also suggest a model in which multiple transcriptional activators function in a cooperative, concentration-dependent mode in combination with a transcriptional repressor to restrict single-minded transcription to the CNS midline precursor cells.

Uncoupling neurogenic gene networks in the Drosophila embryo

The EGF signaling pathway specifies neuronal identities in the Drosophila embryo by regulating developmental patterning genes such as intermediate neuroblasts defective (ind). EGFR is activated in the ventral midline and neurogenic ectoderm by the Spitz ligand, which is processed by the Rhomboid protease. CRISPR/Cas9 was used to delete defined rhomboid enhancers mediating expression at each site of Spitz processing. Surprisingly, the neurogenic ectoderm, not the ventral midline, was found to be the dominant source of EGF patterning activity. We suggest that Drosophila is undergoing an evolutionary transition in central nervous system (CNS)-organizing activity from the ventral midline to the neurogenic ectoderm.

Redeployment of a conserved gene regulatory network during Aedes aegypti development

Changes in gene regulatory networks (GRNs) underlie the evolution of morphological novelty and developmental system drift. The fruitfly Drosophila melanogaster and the dengue and Zika vector mosquito Aedes aegypti have substantially similar nervous system morphology. Nevertheless, they show significant divergence in a set of genes co-expressed in the midline of the Drosophila central nervous system, including the master regulator single minded and downstream genes including short gastrulation, Star, and NetrinA. In contrast to Drosophila, we find that midline expression of these genes is either absent or severely diminished in A. aegypti. Instead, they are co-expressed in the lateral nervous system. This suggests that in A. aegypti this "midline GRN" has been redeployed to a new location while lost from its previous site of activity. In order to characterize the relevant GRNs, we employed the SCRMshaw method we previously developed to identify transcriptional cis-regulatory modules in both species. Analysis of these regulatory sequences in transgenic Drosophila suggests that the altered gene expression observed in A. aegypti is the result of trans-dependent redeployment of the GRN, potentially stemming from cis-mediated changes in the expression of sim and other as-yet unidentified regulators. Our results illustrate a novel "repeal, replace, and redeploy" mode of evolution in which a conserved GRN acquires a different function at a new site while its original function is co-opted by a different GRN. This represents a striking example of developmental system drift in which the dramatic shift in gene expression does not result in gross morphological changes, but in more subtle differences in development and function of the late embryonic nervous system.

Su(H)-mediated repression positions gene boundaries along the dorsal-ventral axis of Drosophila embryos

In Drosophila embryos, a nuclear gradient of the Dorsal (Dl) transcription factor directs differential gene expression along the dorsoventral (DV) axis, translating it into distinct domains that specify future mesodermal, neural, and ectodermal territories. However, the mechanisms used to differentially position gene expression boundaries along this axis are not fully understood. Here, using a combination of approaches, including mutant phenotype analyses and chromatin immunoprecipitation, we show that the transcription factor Suppressor of Hairless, Su(H), helps define dorsal boundaries for many genes expressed along the DV axis. Synthetic reporter constructs also provide molecular evidence that Su(H) binding sites support repression and act to counterbalance activation through Dl and the ubiquitous activator Zelda. Our study highlights a role for broadly expressed repressors, like Su(H), and organization of transcription factor binding sites within cis-regulatory modules as important elements controlling spatial domains of gene expression to facilitate flexible positioning of boundaries across the entire DV axis.

A conserved role for Snail as a potentiator of active transcription

The transcription factors of the Snail family are key regulators of epithelial-mesenchymal transitions, cell morphogenesis, and tumor metastasis. Since its discovery in Drosophila ∼25 years ago, Snail has been extensively studied for its role as a transcriptional repressor. Here we demonstrate that Drosophila Snail can positively modulate transcriptional activation. By combining information on in vivo occupancy with expression profiling of hand-selected, staged snail mutant embryos, we identified 106 genes that are potentially directly regulated by Snail during mesoderm development. In addition to the expected Snail-repressed genes, almost 50% of Snail targets showed an unanticipated activation. The majority of "Snail-activated" genes have enhancer elements cobound by Twist and are expressed in the mesoderm at the stages of Snail occupancy. Snail can potentiate Twist-mediated enhancer activation in vitro and is essential for enhancer activity in vivo. Using a machine learning approach, we show that differentially enriched motifs are sufficient to predict Snail's regulatory response. In silico mutagenesis revealed a likely causative motif, which we demonstrate is essential for enhancer activation. Taken together, these data indicate that Snail can potentiate enhancer activation by collaborating with different activators, providing a new mechanism by which Snail regulates development.

Epithelial plasticity: a common theme in embryonic and cancer cells

During embryonic development, many cells are born far from their final destination and must travel long distances. To become motile and invasive, embryonic epithelial cells undergo a process of mesenchymal conversion known as epithelial-to-mesenchymal transition (EMT). Likewise, EMT can be seen in cancer cells as they leave the primary tumor and disseminate to other parts of the body to colonize distant organs and form metastases. In addition, through the reverse process (mesenchymal-to-epithelial transition), both normal and carcinoma cells revert to the epithelial phenotype to, respectively, differentiate into organs or form secondary tumors. The parallels in phenotypic plasticity in normal morphogenesis and cancer highlight the importance of studying the embryo to understand tumor progression and to aid in the design of improved therapeutic strategies.

Temporal regulation of single-minded target genes in the ventral midline of the Drosophila central nervous system

Differentiation of a specific organ or tissue requires sequential activation of regulatory genes. However, little is known about how serial gene expression is temporally regulated. Here, we present evidence that differential expression of single-minded (sim) target genes can be attributed, in part, to the number of Sim and Tango (Tgo) heterodimer binding sites within their enhancer regions. The Sim, termed a master regulator, directs ventral midline differentiation of Drosophila central nervous system (CNS). According to data on the onset timing of ventral midline gene expression, sim target genes are classified into at least 2 groups (early and late). The sim and rhomboid (rho) genes are activated during early midline differentiation whereas orthodenticle (otd), CG10249, and slit (sli) genes undergo activation during later stages of midline differentiation. Germline transformation and in situ hybridization with transgenic embryos demonstrate that enhancers activating sim and rho expression contain 4 Sim-Tgo binding sites whereas only 1 Sim-Tgo binding site is found in an enhancer of sli. A mutagenized version of the rho enhancer lacking either 1, 2, or 3 Sim-Tgo binding sites mediated progressively more delayed expression of a lacZ reporter gene in the ventral midline. In contrast, a modified sli enhancer displayed progressively earlier onset of lacZ expression when 1, 2, or 3 more Sim-Tgo binding sites were added. Taken together, these results suggest that the number of Sim-Tgo-binding sites is decisive in determining the timing of gene expression in the developing ventral midline. We also discuss a combinatorial model accounting for the sequential expression of sim target genes.

Mesodermal repression of single-minded in Drosophila embryo is mediated by a cluster of Snail-binding sites proximal to the early promoter

single-minded (sim) is a master regulatory gene that directs differentiation in the central nervous system during Drosophila embryogenesis. Recent identification of the mesectoderm enhancer (MSE) of sim has led to the hypothesis that two Snail (Sna)-binding sites in the MSE may repress sim expression in the presumptive mesoderm. We provide evidence here that three Sna-binding sites proximal to the sim promoter, but not those of the MSE, are responsible for the mesodermal repression of sim in vivo. Using transgenic embryos injected with lacZ transgenes, we showed that sim repression in the mesoderm requires the three promoter-proximal Sna-binding sites. These results suggest that Sna represses the mesectodermal expression of sim by directly repressing the nearby promoter, and not by quenching adjacent transcriptional activators in the MSE. These data also showed how the MSE, lacking the three proximal Sna-binding sites, reproduced the endogenous pattern of sim expression in transgenic embryos.

Analysis of snail genes in the crustacean Parhyale hawaiensis: insight into snail gene family evolution

The transcriptional repressor snail was first discovered in Drosophila melanogaster, where it initially plays a role in gastrulation and mesoderm formation, and later plays a role in neurogenesis. Among arthropods, this role of snail appears to be conserved in the insects Tribolium and Anopheles gambiae, but not in the chelicerates Cupiennius salei and Achaearanea tepidariorum, the myriapod Glomeris marginata, or the Branchiopod crustacean Daphnia magna. These data imply that within arthropoda, snail acquired its role in gastrulation and mesoderm formation in the insect lineage. However, crustaceans are a diverse group with several major taxa, making analysis of more crustaceans necessary to potentially understand the ancestral role of snail in Pancrustacea (crustaceans + insects) and thus in the ancestor of insects as well. To address these questions, we examined the snail family in the Malacostracan crustacean Parhyale hawaiensis. We found three snail homologs, Ph-snail1, Ph-snail2 and Ph-snail3, and one scratch homolog, Ph-scratch. Parhyale snail genes are expressed after gastrulation, during germband formation and elongation. Ph-snail1, Ph-snail2, and Ph-snail3 are expressed in distinct patterns in the neuroectoderm. Ph-snail1 is the only Parhyale snail gene expressed in the mesoderm, where its expression cycles in the mesodermal stem cells, called mesoteloblasts. The mesoteloblasts go through a series of cycles, where each cycle is composed of a migration phase and a division phase. Ph-snail1 is expressed during the migration phase, but not during the division phase. We found that as each mesoteloblast division produces one segment's worth of mesoderm, Ph-snail1 expression is linked to both the cell cycle and the segmental production of mesoderm.

Mastermind mutations generate a unique constellation of midline cells within the Drosophila CNS

Background

The Notch pathway functions repeatedly during the development of the central nervous system in metazoan organisms to control cell fate and regulate cell proliferation and asymmetric cell divisions. Within the Drosophila midline cell lineage, which bisects the two symmetrical halves of the central nervous system, Notch is required for initial cell specification and subsequent differentiation of many midline lineages.

Methodology/Principal Findings

Here, we provide the first description of the role of the Notch co-factor, mastermind, in the central nervous system midline of Drosophila. Overall, zygotic mastermind mutations cause an increase in midline cell number and decrease in midline cell diversity. Compared to mutations in other components of the Notch signaling pathway, such as Notch itself and Delta, zygotic mutations in mastermind cause the production of a unique constellation of midline cell types. The major difference is that midline glia form normally in zygotic mastermind mutants, but not in Notch and Delta mutants. Moreover, during late embryogenesis, extra anterior midline glia survive in zygotic mastermind mutants compared to wild type embryos.

Conclusions/Significance

This is an example of a mutation in a signaling pathway cofactor producing a distinct central nervous system phenotype compared to mutations in major components of the pathway.

The snail repressor inhibits release, not elongation, of paused Pol II in the Drosophila embryo

The development of the precellular Drosophila embryo is characterized by exceptionally rapid transitions in gene activity, with broadly distributed maternal regulatory gradients giving way to precise on/off patterns of gene expression within a one-hour window, between two and three hours after fertilization [1]. Transcriptional repression plays a pivotal role in this process, delineating sharp expression patterns (e.g., pair-rule stripes) within broad domains of gene activation. As many as 20 different sequence-specific repressors have been implicated in this process, yet the mechanisms by which they silence gene expression have remained elusive [2]. Here we report the development of a method for the quantitative visualization of transcriptional repression. We focus on the Snail repressor, which establishes the boundary between the presumptive mesoderm and neurogenic ectoderm [3]. We find that elongating Pol II complexes complete transcription after the onset of Snail repression. As a result, moderately sized genes (e.g., the 22 kb sog locus) are fully silenced only after tens of minutes of repression. We propose that this "repression lag" imposes a severe constraint on the regulatory dynamics of embryonic patterning and further suggest that posttranscriptional regulators, like microRNAs, are required to inhibit unwanted transcripts produced during protracted periods of gene silencing.

Complex interactions between cis-regulatory modules in native conformation are critical for Drosophila snail expression

It has been shown in several organisms that multiple cis-regulatory modules (CRMs) of a gene locus can be active concurrently to support similar spatiotemporal expression. To understand the functional importance of such seemingly redundant CRMs, we examined two CRMs from the Drosophila snail gene locus, which are both active in the ventral region of pre-gastrulation embryos. By performing a deletion series in a ∼25 kb DNA rescue construct using BAC recombineering and site-directed transgenesis, we demonstrate that the two CRMs are not redundant. The distal CRM is absolutely required for viability, whereas the proximal CRM is required only under extreme conditions such as high temperature. Consistent with their distinct requirements, the CRMs support distinct expression patterns: the proximal CRM exhibits an expanded expression domain relative to endogenous snail, whereas the distal CRM exhibits almost complete overlap with snail except at the anterior-most pole. We further show that the distal CRM normally limits the increased expression domain of the proximal CRM and that the proximal CRM serves as a `damper' for the expression levels driven by the distal CRM. Thus, the two CRMs interact in cis in a non-additive fashion and these interactions may be important for fine-tuning the domains and levels of gene expression.

A prominent requirement for single-minded and the ventral midline in patterning the dorsoventral axis of the crustacean Parhyale hawaiensis

In bilaterians, establishing the correct spatial positioning of structures along the dorsoventral (DV) axis is essential for proper embryonic development. Insects such as Drosophila rely on the Dorsal activity gradient and Bone morphogenetic protein (BMP) signaling to establish cell fates along the DV axis, leading to the distinction between tissues such as mesoderm, neurogenic ectoderm and dorsal ectoderm in the developing embryo. Subsequently, the ventral midline plays a more restricted role in DV patterning by establishing differential cell fates in adjacent regions of the neurogenic ectoderm. In this study, we examine the function of the ventral midline and the midline-associated gene single-minded (Ph-sim) in the amphipod crustacean Parhyale hawaiensis. Remarkably, we found that Ph-sim and the ventral midline play a central role in establishing proper fates along the entire DV axis in this animal; laser ablation of midline cells causes a failure to form neurogenic ectoderm and Ph-sim RNAi results in severely dorsalized embryos lacking both neurogenic ectoderm and the appendage-bearing lateral ectoderm. Furthermore, we hypothesize that this role of midline cells was present in the last common ancestor of crustaceans and insects. We predict that the transition to a Dorsal-dependent DV patterning system in the phylogenetically derived insect lineage leading to Drosophila has led to a more restricted role of the ventral midline in patterning the DV axis of these insects.

Shadow enhancers foster robustness of Drosophila gastrulation

Critical developmental control genes sometimes contain "shadow" enhancers that can be located in remote positions, including the introns of neighboring genes [1]. They nonetheless produce patterns of gene expression that are the same as or similar to those produced by more proximal primary enhancers. It was suggested that shadow enhancers help foster robustness in gene expression in response to environmental or genetic perturbations [2, 3]. We critically tested this hypothesis by employing a combination of bacterial artificial chromosome (BAC) recombineering and quantitative confocal imaging methods [2, 4]. Evidence is presented that the snail gene is regulated by a distal shadow enhancer located within a neighboring locus. Removal of the proximal primary enhancer does not significantly perturb snail function, including the repression of neurogenic genes and formation of the ventral furrow during gastrulation at normal temperatures. However, at elevated temperatures, there is sporadic loss of snail expression and coincident disruptions in gastrulation. Similar defects are observed at normal temperatures upon reductions in the levels of Dorsal, a key activator of snail expression (reviewed in [5]). These results suggest that shadow enhancers represent a novel mechanism of canalization whereby complex developmental processes "bring about one definite end-result regardless of minor variations in conditions" [6].

Evolution of insect dorsoventral patterning mechanisms

The dorsoventral (DV) patterning of the early Drosophila embryo depends on Dorsal, a maternal sequence-specific transcription factor related to mammalian NF-kappaB. Dorsal controls DV patterning through the differential regulation of approximately 50 target genes in a concentration-dependent manner. Whole-genome methods, including ChIP-chip and ChIP-seq assays, have identified approximately 100 Dorsal target enhancers, and more than one-third of these have been experimentally confirmed via transgenic embryo assays. Despite differences in DV patterning among divergent insects, a number of the Dorsal target enhancers are located in conserved positions relative to the associated transcription units. Thus, the evolution of novel patterns of gene expression might depend on the modification of old enhancers, rather than the invention of new ones. As many as half of all Dorsal target genes appear to contain "shadow" enhancers: a second enhancer that directs the same or similar expression pattern as the primary enhancer. Preliminary studies suggest that shadow enhancers might help to ensure resilience of gene expression in response to environmental and genetic perturbations. Finally, most Dorsal target genes appear to contain RNA polymerase II (pol II) prior to their activation. Stalled pol II fosters synchronous patterns of gene activation in the early embryo. In contrast, DV patterning genes lacking stalled pol II are initially activated in an erratic or stochastic fashion. It is possible that stalled pol II confers fitness to a population by ensuring coordinate deployment of the gene networks controlling embryogenesis.

Epithelial-mesenchymal transitions: the importance of changing cell state in development and disease

The events that convert adherent epithelial cells into individual migratory cells that can invade the extracellular matrix are known collectively as epithelial-mesenchymal transition (EMT). Throughout evolution, the capacity of cells to switch between these two cellular states has been fundamental in the generation of complex body patterns. Here, we review the EMT events that build the embryo and further discuss two prototypical processes governed by EMT in amniotes: gastrulation and neural crest formation. Cells undergo EMT to migrate and colonize distant territories. Not surprisingly, this is also the mechanism used by cancer cells to disperse throughout the body.

Epithelial-mesenchymal transitions: the importance of changing cell state in development and disease

The events that convert adherent epithelial cells into individual migratory cells that can invade the extracellular matrix are known collectively as epithelial-mesenchymal transition (EMT). Throughout evolution, the capacity of cells to switch between these two cellular states has been fundamental in the generation of complex body patterns. Here, we review the EMT events that build the embryo and further discuss two prototypical processes governed by EMT in amniotes: gastrulation and neural crest formation. Cells undergo EMT to migrate and colonize distant territories. Not surprisingly, this is also the mechanism used by cancer cells to disperse throughout the body.

The C. elegans Snail homolog CES-1 can activate gene expression in vivo and share targets with bHLH transcription factors

Snail-type transcription factors (TFs) are found in numerous metazoan organisms and function in a plethora of cellular and developmental processes including mesoderm and neuronal development, apoptosis and cancer. So far, Snail-type TFs are exclusively known as transcriptional repressors. They repress gene expression by recruiting transcriptional co-repressors and/or by preventing DNA binding of activators from the basic helix-loop-helix (bHLH) family of TFs to CAGGTG E-box sequences. Here we report that the Caenorhabditis elegans Snail-type TF CES-1 can activate transcription in vivo. Moreover, we provide results that suggest that CES-1 can share its binding site with bHLH TFs, in different tissues, rather than only occluding bHLH DNA binding. Together, our data indicate that there are at least two types of CES-1 target genes and, therefore, that the molecular function of Snail-type TFs is more plastic than previously appreciated.

Involvement of co-repressors Groucho and CtBP in the regulation of single-minded in Drosophila

Dorso-ventral patterning results in the establishment of the two germ layers in the Drosophila embryo, mesoderm and mesectoderm, that are separated by a strip of cells giving rise to the mesectoderm and eventually to the ventral midline. The mesectoderm is specified by the expression of single-minded (sim) which is activated through the concerted action of Dorsal and Twist in addition to a Notch signal. In the mesoderm, sim is repressed by Snail together with the co-repressor C-terminal binding protein (CtBP). Here, we address the involvement of the two co-repressors CtBP and Groucho (Gro) in repression of sim in the neuroectoderm. It was shown earlier that sim is restricted in the neuroectoderm with help of Suppressor of Hairless [Su(H)] and Hairless. Using the female sterile technique, we generated germ line clones deficient for Gro, CtBP or Hairless and assayed sim mRNA relative to snail mRNA expression. We show that sim repression requires both co-repressors Gro and CtBP to be fully repressed in the neuroectoderm, suggesting that a repression complex is assembled including Su(H) and Hairless as was shown for other Notch target genes before. Moreover, our work implies that Gro is important for the repression of sim specifically within the mesoderm anlagen, indicating that Snail and CtBP are insufficient to entirely silence sim in this germ layer.

Evolution of the ventral midline in insect embryos

The ventral midline is a source of signals that pattern the nerve cord of insect embryos. In dipterans such as the fruitfly Drosophila melanogaster (D. mel.) and the mosquito Anopheles gambiae (A. gam.), the midline is narrow and spans just 1-2 cells. However, in the honeybee, Apis mellifera (A. mel.), the ventral midline is broad and encompasses 5-6 cells. slit and other midline-patterning genes display a corresponding expansion in expression. Evidence is presented that this difference is due to divergent cis regulation of the single-minded (sim) gene, which encodes a bHLH-PAS transcription factor essential for midline differentiation. sim is regulated by a combination of Notch signaling and a Twist (Twi) activator gradient in D. mel., but it is activated solely by Twi in A. mel. We suggest that the Twi-only mode of regulation--and the broad ventral midline--represents the ancestral form of CNS patterning in Holometabolous insects.

Worniu, a Snail family zinc-finger protein, is required for brain development in Drosophila

The Snail family of zinc-finger transcriptional repressors is essential for morphogenetic cell movements, mesoderm formation, and neurogenesis during embryonic development. These proteins also control cell cycle, cell death, and cancer progression. In Drosophila, three members of this protein family, Snail, Escargot, and Worniu, have essential but redundant functions in asymmetric cell division of neuroblasts. In addition, Snail is critical for early mesoderm formation and Escargot is required for maintaining diploidy in wing imaginal disc cells. In this report, we demonstrate that Worniu plays a role in brain development. We show that alleles of the l(2)35Da complementation group are mutants of worniu. The developing larvae of these mutant alleles fail to shorten their brainstems. The brain phenotype, as well as the lethality, of these mutants can be rescued by worniu transgenes. Moreover, RNAi experiments targeting the worniu transcript show the same nonshortening phenotype in larval brains. worniu is expressed in the neuroblasts of brain hemispheres and ventral ganglions. The results suggest that the loss of Worniu function within the neuroblasts ultimately causes the larval brainstem to fail to go through shortening during development.

Drosophila WntD is a target and an inhibitor of the Dorsal/Twist/Snail network in the gastrulating embryo

The maternal Toll signaling pathway sets up a nuclear gradient of the transcription factor Dorsal in the early Drosophila embryo. Dorsal activates twist and snail, and the Dorsal/Twist/Snail network activates and represses other zygotic genes to form the correct expression patterns along the dorsoventral axis. An essential function of this patterning is to promote ventral cell invagination during mesoderm formation, but how the downstream genes regulate ventral invagination is not known. We show here that wntD is a novel member of the Wnt family. The expression of wntD is activated by Dorsal and Twist, but the expression is much reduced in the ventral cells through repression by Snail. Overexpression of WntD in the early embryo inhibits ventral invagination, suggesting that the de-repressed WntD in snail mutant embryos may contribute to inhibiting ventral invagination. The overexpressed WntD inhibits invagination by antagonizing Dorsal nuclear localization, as well as twist and snail expression. Consistent with the early expression of WntD at the poles in wild-type embryos, loss of WntD leads to posterior expansion of nuclear Dorsal and snail expression, demonstrating that physiological levels of WntD can also attenuate Dorsal nuclear localization. We also show that the de-repressed WntD in snail mutant embryos contributes to the premature loss of snail expression, probably by inhibiting Dorsal. Thus, these results together demonstrate that WntD is regulated by the Dorsal/Twist/Snail network, and is an inhibitor of Dorsal nuclear localization and function.

A systems biology approach to developmental toxicology

Recent advances in developmental biology have yielded detailed models of gene regulatory networks (GRNs) involved in cell specification and other processes in embryonic differentiation. Such networks form the bedrock on which a systems biology approach to developmental toxicology can be built. In this review, an introduction to GRNs in general is followed by a description of specific networks involved in sea urchin and Drosophila development. A hypothesis is presented regarding the role of GRN analysis in the determination of mechanisms of chemical toxicity during embryonic development. Potential for future directions and research approaches in this area is discussed.

Unraveling signalling cascades for the Snail family of transcription factors

During development and carcinogenesis, the gradient of different molecular factors, the availability of corresponding receptors and the interplay between different signalling cascades combine to orchestrate the different stages. A good understanding of both developmental processes and oncogenesis leads to new insights into normal and aberrant regulation, processes that share some mutual key players. In this review, we will focus on the Snail family of transcription factors. These proteins, which share an evolutionarily conserved role in invertebrates and vertebrates, are implicated in several developmental processes, but are involved in carcinogenesis as well. We will highlight the different signalling cascades leading to the expression of Snail and Slug and how these factors are regulated on the transcriptional level. Then we will focus on how these factors execute their functions by repression of the numerous target genes that have been described to date.

Gene expression profiling of the developing Drosophila CNS midline cells

The Drosophila CNS midline cells constitute a specialized set of interneurons, motorneurons, and glia. The utility of the CNS midline cells as a neurogenomic system to study CNS development derives from the ability to easily identify CNS midline-expressed genes. For this study, we used a variety of sources to identify 281 putative midline-expressed genes, including enhancer trap lines, microarray data, published accounts, and the Berkeley Drosophila Genome Project (BDGP) gene expression data. For each gene, we analyzed expression at all stages of embryonic CNS development and categorized expression patterns with regard to specific midline cell types. Of the 281 candidates, we identified 224 midline-expressed genes, which include transcription factors, signaling proteins, and transposable elements. We find that 58 genes are expressed in mesectodermal precursor cells, 138 in midline primordium cells, and 143 in mature midline cells--50 in midline glia, 106 in midline neurons. Additionally, we identified 27 genes expressed in glial and mesodermal cells associated with the midline cells. This work provides the basis for future research that will generate a complete cellular and molecular map of CNS midline development, thus allowing for detailed genetic and molecular studies of neuronal and glial development and function.

A regulatory code for neurogenic gene expression in the Drosophila embryo

Bioinformatics methods have identified enhancers that mediate restricted expression in the Drosophila embryo. However, only a small fraction of the predicted enhancers actually work when tested in vivo. In the present study, co-regulated neurogenic enhancers that are activated by intermediate levels of the Dorsal regulatory gradient are shown to contain several shared sequence motifs. These motifs permitted the identification of new neurogenic enhancers with high precision: five out of seven predicted enhancers direct restricted expression within ventral regions of the neurogenic ectoderm. Mutations in some of the shared motifs disrupt enhancer function, and evidence is presented that the Twist and Su(H) regulatory proteins are essential for the specification of the ventral neurogenic ectoderm prior to gastrulation. The regulatory model of neurogenic gene expression defined in this study permitted the identification of a neurogenic enhancer in the distant Anopheles genome. We discuss the prospects for deciphering regulatory codes that link primary DNA sequence information with predicted patterns of gene expression.

Regulation of tight junctions during the epithelium-mesenchyme transition: direct repression of the gene expression of claudins/occludin by Snail

Snail is a transcription repressor that plays a central role in the epithelium-mesenchyme transition (EMT), by which epithelial cells lose their polarity. Claudins and occludin are integral membrane proteins localized at tight junctions, which are responsible for establishing and maintaining epithelial cell polarity. We examined the relationship between Snail and the promoter activity of claudins and occludin. When Snail was overexpressed in cultured mouse epithelial cells, EMT was induced with concomitant repression of the expression of claudins and occludin not only at the protein but also at the mRNA level. We then isolated the promoters of genes encoding claudins and occludin, in which multiple E-boxes were identified. Transfection experiments with various promoter constructs as well as electrophoretic mobility assays revealed that Snail binds directly to the E-boxes of the promoters of claudin/occludin genes, resulting in complete repression of their promoter activity. Because the gene encoding E-cadherin was also reported to be repressed by Snail, we concluded that EMT was associated with the simultaneous repression of the genes encoding E-cadherin and claudins/occludin (i.e. the expression of adherens and tight junction adhesion molecules, respectively).

Snail is required for Delta endocytosis and Notch-dependent activation of single-minded expression

In the Drosophila embryo, the mesectoderm corresponds to a single row of cells abutting the mesoderm. It is specified by the expression of the single-minded (sim) gene. The information that precisely positions the sim-expressing cells along the dorso-ventral axis is incompletely understood. Previous studies have shown that Dorsal and Twist activate sim expression in a large ventral domain, while two negative regulators, Snail (Sna) and Suppressor of Hairless [Su(H)], repress sim expression in the mesoderm and neuroectoderm, respectively. Repression by Su(H) is relieved in the presumptive mesectoderm by Notch signaling. In this paper, we show that Sna also has a positive regulatory function on sim expression in the presumptive mesectoderm. This positive effect of Sna depends on the Su(H)-binding sites within the sim promoter, suggesting that Sna regulates Notch signaling. In addition, we find that Delta is endocytosed together with the extracellular domain of Notch. The endocytosis of Delta is only seen in the mesoderm and requires the activity of the sna and neuralized (neur) genes. Interestingly, the Neur-mediated endocytosis of Delta has recently been shown to be sufficient for the non-autonomous activation of Notch target genes in wing imaginal discs. We therefore propose a novel model in which Sna positions the mesectoderm via its dual regulatory activity. In this model, Sna cell-autonomously represses sim expression in the mesoderm and relieves Su(H)-dependent repression in a cell non-autonomous fashion by promoting the Neur-dependent endocytosis of Delta in the mesoderm.

Linear signaling in the Toll-Dorsal pathway of Drosophila: activated Pelle kinase specifies all threshold outputs of gene expression while the bHLH protein Twist specifies a subset

Differential activation of the Toll receptor leads to the formation of a broad Dorsal nuclear gradient that specifies at least three patterning thresholds of gene activity along the dorsoventral axis of precellular embryos. We investigate the activities of the Pelle kinase and Twist basic helix-loop-helix (bHLH) transcription factor in transducing Toll signaling. Pelle functions downstream of Toll to release Dorsal from the Cactus inhibitor. Twist is an immediate-early gene that is activated upon entry of Dorsal into nuclei. Transgenes misexpressing Pelle and Twist were introduced into different mutant backgrounds and the patterning activities were visualized using various target genes that respond to different thresholds of Toll-Dorsal signaling. These studies suggest that an anteroposterior gradient of Pelle kinase activity is sufficient to generate all known Toll-Dorsal patterning thresholds and that Twist can function as a gradient morphogen to establish at least two distinct dorsoventral patterning thresholds. We discuss how the Dorsal gradient system can be modified during metazoan evolution and conclude that Dorsal-Twist interactions are distinct from the interplay between Bicoid and Hunchback, which pattern the anteroposterior axis.

Dorsal gradient networks in the Drosophila embryo

Here, we describe one of the major maternal regulatory gradients, Dorsal, and threshold outputs of gene expression that result from the graded distribution of this transcription factor. The analysis of a large number of authentic and synthetic target genes suggests that the Dorsal gradient directly specifies at least four, and possibly as many as seven, different thresholds of gene activity and tissue differentiation. These thresholds initiate the differentiation of the three primary embryonic tissues: the mesoderm, neurogenic ectoderm, and dorsal ectoderm. Moreover, primary readouts of the Dorsal gradient create asymmetries that subdivide each tissue into multiple cell types during gastrulation. Dorsal patterning thresholds represent the culmination of one of the most complete gene regulation network known in development, which begins with the asymmetric positioning of the oocyte nucleus within the egg chamber and leads to the localized activation of the Toll-Dorsal signaling pathway in ventral regions of the early embryo.

The Snail repressor positions Notch signaling in the Drosophila embryo

The maternal Dorsal nuclear gradient initiates the differentiation of the mesoderm, neurogenic ectoderm and dorsal ectoderm in the precellular Drosophila embryo. Each tissue is subsequently subdivided into multiple cell types during gastrulation. We have investigated the formation of the mesectoderm within the ventral-most region of the neurogenic ectoderm. Previous studies suggest that the Dorsal gradient works in concert with Notch signaling to specify the mesectoderm through the activation of the regulatory gene sim within single lines of cells that straddle the presumptive mesoderm. This model was confirmed by misexpressing a constitutively activated form of the Notch receptor, NotchIC, in transgenic embryos using the eve stripe2 enhancer. The NotchIC stripe induces ectopic expression of sim in the neurogenic ectoderm where there are low levels of the Dorsal gradient. sim is not activated in the ventral mesoderm, due to inhibition by the localized zinc-finger Snail repressor, which is selectively expressed in the ventral mesoderm. Additional studies suggest that the Snail repressor can also stimulate Notch signaling. A stripe2-snail transgene appears to induce Notch signaling in ‘naïve’ embryos that contain low uniform levels of Dorsal. We suggest that these dual activities of Snail, repression of Notch target genes and stimulation of Notch signaling, help define precise lines of sim expression within the neurogenic ectoderm.

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 Snail protein family regulates neuroblast expression of inscuteable and string, genes involved in asymmetry and cell division in Drosophila

Delaminated neuroblasts in Drosophila function as stem cells during embryonic central nervous system development. They go through repeated asymmetric divisions to generate multiple ganglion mother cells, which divide only once more to produce postmitotic neurons. Snail, a zinc-finger transcriptional repressor, is a pan-neural protein, based on its extensive expression in neuroblasts. Previous results have demonstrated that Snail and related proteins, Worniu and Escargot, have redundant and essential functions in the nervous system. We show that the Snail family of proteins control central nervous system development by regulating genes involved in asymmetry and cell division of neuroblasts. In mutant embryos that have the three genes deleted, the expression of inscuteable is significantly lowered, while the expression of other genes that participate in asymmetric division, including miranda, staufen and prospero, appears normal. The deletion mutants also have much reduced expression of string, suggesting that a key component that drives neuroblast cell division is abnormal. Consistent with the gene expression defects, the mutant embryos lose the asymmetric localization of prospero RNA in neuroblasts and lose the staining of Prospero protein that is normally present in ganglion mother cells. Simultaneous expression of inscuteable and string in the snail family deletion mutant efficiently restores Prospero expression in ganglion mother cells, demonstrating that the two genes are key targets of Snail in neuroblasts. Mutation of the dCtBP co-repressor interaction motifs in the Snail protein leads to reduction of the Snail function in central nervous system. These results suggest that the Snail family of proteins control both asymmetry and cell division of neuroblasts by activating, probably indirectly, the expression of inscuteable and string.

Transcriptional repression by Suppressor of Hairless involves the binding of a Hairless-dCtBP complex in Drosophila

Notch is the receptor for a conserved signaling pathway that regulates numerous cell fate decisions during development [1]. Signal transduction involves the presenilin-dependent intracellular processing of Notch and the nuclear translocation of the intracellular domain of Notch, NICD [2-6]. NICD associates with Suppressor of Hairless [Su(H)], a DNA binding protein, and Mastermind (Mam), a transcriptional coactivator [7-9]. In the absence of Notch signaling, Su(H) acts as a transcriptional repressor [10, 11]. Repression by Su(H) is relieved by the activation of Notch [12-16]. In the Drosophila embryo, this transcriptional switch from repression to activation is important for patterning the expression of the single-minded (sim) gene along the dorsoventral axis [12]. Here, we investigate the mechanisms by which Su(H) inhibits the expression of Notch target genes in Drosophila. We show that Hairless, an antagonist of Notch signaling [17-19], is required to repress the transcription of the sim gene. Hairless forms a DNA-bound complex with Su(H). Furthermore, it directly binds the Drosophila C-terminal Binding Protein (dCtBP), which acts as a transcriptional corepressor. The dCtBP binding motif of Hairless is essential for the function of Hairless in vivo. We propose that Hairless mediates transcriptional repression by Su(H) via the recruitment of dCtBP.

A family of snail-related zinc finger proteins regulates two distinct and parallel mechanisms that mediate Drosophila neuroblast asymmetric divisions

Three snail family genes snail, escargot and worniu, encode related zinc finger transcription factors that mediate Drosophila central nervous system (CNS) development. We show that simultaneous removal of all three genes causes defective neuroblast asymmetric divisions; inscuteable transcription/translation is delayed/suppressed in the segmented CNS. Further more, defects in localization of cell fate determinants and orientation of the mitotic spindle in dividing neuroblasts are much stronger than those associated with inscuteable loss of function. In inscuteable neuroblasts, cell fate determinants are mislocalized during prophase and metaphase, yet during anaphase and telophase the great majority of mutant neuroblasts localize these determinants as cortical crescents overlying one of the spindle poles. This phenomenon, known as 'telophase rescue', does not occur in the absence of the snail family genes; moreover, in contrast to inscuteable mutants, mitotic spindle orientation is completely randomized. Our data provide further evidence for the existence of two distinct asymmetry-controlling mechanisms in neuroblasts both of which require snail family gene function: an inscuteable-dependent mechanism that functions throughout mitosis and an inscuteable-independent mechanism that acts during anaphase/telophase.

Drosophila single-minded represses gene transcription by activating the expression of repressive factors

The Drosophila single-minded gene controls CNS midline cell development by both activating midline gene expression and repressing lateral CNS gene expression in the midline cells. The mechanism by which Single-minded represses transcription was examined using the ventral nervous system defective gene as a target gene. Transgenic-lacZ analysis of constructs containing fragments of the ventral nervous system defective regulatory region identified sequences required for lateral CNS transcription and midline repression. Elimination of Single-minded:Tango binding sites within the ventral nervous system defective gene did not affect midline repression. Mutants of Single-minded that removed the DNA binding and transcriptional activation regions abolished ventral nervous system defective repression, as well as transcriptional activation of other genes. The replacement of the Single-minded transcriptional activation region with a heterologous VP16 transcriptional activation region restored the ability of Single-minded to both activate and repress transcription. These results indicate that Single-minded indirectly represses transcription by activating the expression of repressive factors. Single-minded provides a model system for how regulatory proteins that act only as transcriptional activators can control lineage-specific transcription in both positive and negative modes.

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.

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.

Repression by suppressor of hairless and activation by Notch are required to define a single row of single-minded expressing cells in the Drosophila embryo

Notch signal transduction appears to involve the ligand-induced intracellular processing of Notch, and the formation of a processed Notch-Suppressor of Hairless complex that binds DNA and activates the transcription of Notch target genes. This suggests that loss of either Notch or Su(H) activities should lead to similar cell fate changes. However, previous data indicate that, in the Drosophila blastoderm embryo, mesectoderm specification requires Notch but not Su(H) activity. The determination of the mesectodermal fate is specified by Single-minded (Sim), a transcription factor expressed in a single row of cells abutting the mesoderm. The molecular mechanisms by which the dorsoventral gradient of nuclear Dorsal establishes the single-cell wide territory of sim expression are not fully understood. We have found that Notch activity is required for sim expression in cellularizing embryos. In contrast, at this stage, Su(H) has a dual function. Su(H) activity was required to up-regulate sim expression in the mesectoderm, and to prevent the ectopic expression of sim dorsally in the neuroectoderm. We have shown that repression of sim transcription by Su(H) is direct and independent of Notch activity. Conversely, activation of sim transcription by Notch requires the Su(H)-binding sites. Thus, Notch signalling appears to relieve the repression exerted by Su(H) and to up-regulate sim transcription in the mesectoderm. We propose a model in which repression by Su(H) and derepression by Notch are essential to allow for the definition of a single row of mesectodermal cells in the blastoderm embryo.

Repression by Suppressor of Hairless and activation by Notch are required to define a single row of single-minded expressing cells in the Drosophila embryo

Notch signal transduction appears to involve the ligand-induced intracellular processing of Notch, and the formation of a processed Notch-Suppressor of Hairless complex that binds DNA and activates the transcription of Notch target genes. This suggests that loss of eitherNotch or Su(H) activities should lead to similar cell fate changes. However, previous data indicate that, in theDrosophila blastoderm embryo, mesectoderm specification requires Notch but not Su(H) activity. The determination of the mesectodermal fate is specified by Single-minded (Sim), a transcription factor expressed in a single row of cells abutting the mesoderm. The molecular mechanisms by which the dorsoventral gradient of nuclear Dorsal establishes the single-cell wide territory of sim expression are not fully understood. We have found that Notch activity is required for simexpression in cellularizing embryos. In contrast, at this stage,Su(H) has a dual function. Su(H) activity was required to up-regulate sim expression in the mesectoderm, and to prevent the ectopic expression of sim dorsally in the neuroectoderm. We have shown that repression of simtranscription by Su(H) is direct and independent of Notchactivity. Conversely, activation of sim transcription by Notch requires the Su(H)-binding sites. Thus, Notch signalling appears to relieve the repression exerted by Su(H) and to up-regulate simtranscription in the mesectoderm. We propose a model in which repression by Su(H) and derepression by Notch are essential to allow for the definition of a single row of mesectodermal cells in the blastoderm embryo.

A binding site for the transcription factor Grainyhead/Nuclear transcription factor-1 contributes to regulation of the Drosophila proliferating cell nuclear antigen gene promoter

The Drosophila proliferating cell nuclear antigen promoter contains multiple transcriptional regulatory elements, including upstream regulatory element (URE), DNA replication-related element, E2F recognition sites, and three common regulatory factor for DNA replication and DNA replication-related element-binding factor genes recognition sites. In nuclear extracts of Drosophila embryos, we detected a protein factor, the URE-binding factor (UREF), that recognizes the nucleotide sequence 5'-AAACCAGTTGGCA located within URE. Analyses in Drosophila Kc cells and transgenic flies revealed that the UREF-binding site plays an important role in promoter activity both in cultured cells and in living flies. A yeast one-hybrid screen using URE as a bait allowed isolation of a cDNA encoding a transcription factor, Grainyhead/nuclear transcription factor-1 (GRH/NTF-1). The nucleotide sequence required for binding to GRH was indistinguishable from that for UREF detected in embryo nuclear extracts. Furthermore, a specific antibody to GRH reacted with UREF in embryo nuclear extracts. From these results we conclude that GRH is identical to UREF. Although GRH has been thought to be involved in regulation of differentiation-related genes, this study demonstrates, for the first time, involvement of a GRH-binding site in regulation of the DNA replication-related proliferating cell nuclear antigen gene.

zfh-1, the Drosophila homologue of ZEB, is a transcriptional repressor that regulates somatic myogenesis

zfh-1 is a member of the zfh family of proteins, which all contain zinc finger and homeodomains. The roles and mechanisms of action of most family members are still unclear. However, we have shown previously that another member of the family, the vertebrate ZEB protein, is a transcriptional repressor that binds E box sequences and inhibits myotube formation in cell culture assays. zfh-1 is downregulated in Drosophila embryos prior to myogenesis. Embryos with zfh-1 loss-of-function mutation show alterations in the number and position of embryonic somatic muscles, suggesting that zfh-1 could have a regulatory role in myogenesis. However, nothing is known about the nature or mechanism of action of zfh-1. Here, we demonstrate that zfh-1 is a transcription factor that binds E box sequences and acts as an active transcriptional repressor. When zfh-1 expression was maintained in the embryo beyond its normal temporal pattern of downregulation, the differentiation of somatic but not visceral muscle was blocked. One potential target of zfh-1 in somatic myogenesis could be the myogenic factor mef2. mef2 is known to be regulated by the transcription factor twist, and we show here that zfh-1 binds to sites in the mef2 upstream regulatory region and inhibits twist transcriptional activation. Even though there is little sequence similarity in the repressor domains of ZEB and zfh-1, we present evidence that zfh-1 is the functional homologue of ZEB and that the role of these proteins in myogenesis is conserved from Drosophila to mammals.

SLUG, a ces-1-related zinc finger transcription factor gene with antiapoptotic activity, is a downstream target of the E2A-HLF oncoprotein

The E2A-HLF fusion gene transforms human pro-B lymphocytes by interfering with an early step in apoptotic signaling. In a search for E2A-HLF-responsive genes, we identified a zinc finger transcription factor, SLUG, whose product belongs to the Snail family of developmental regulatory proteins. Importantly, SLUG bears close homology to the CES-1 protein of C. elegans, which acts downstream of CES-2 in a neuron-specific cell death pathway. Consistent with the postulated role of CES-1 as an antiapoptotic transcription factor, SLUG was nearly as active as Bcl-2 or Bcl-xL in promoting the survival of IL-3-dependent murine pro-B cells deprived of the cytokine. We conclude that SLUG is an evolutionarily conserved transcriptional repressor whose activation by E2A-HLF promotes the aberrant survival and eventual malignant transformation of mammalian pro-B cells otherwise slated for apoptotic death.

dCtBP mediates transcriptional repression by Knirps, Krüppel and Snail in the Drosophila embryo

The pre-cellular Drosophila embryo contains 10 well characterized sequence-specific transcriptional repressors, which represent a broad spectrum of DNA-binding proteins. Previous studies have shown that two of the repressors, Hairy and Dorsal, recruit a common co-repressor protein, Groucho. Here we present evidence that three different repressors, Knirps, Krüppel and Snail, recruit a different co-repressor, dCtBP. Mutant embryos containing diminished levels of maternal dCtBP products exhibit both segmentation and dorsoventral patterning defects, which can be attributed to loss of Krüppel, Knirps and Snail activity. In contrast, the Dorsal and Hairy repressors retain at least some activity in dCtBP mutant embryos. dCtBP interacts with Krüppel, Knirps and Snail through a related sequence motif, PXDLSXK/H. This motif is essential for the repression activity of these proteins in transgenic embryos. We propose that dCtBP represents a major form of transcriptional repression in development, and that the Groucho and dCtBP co-repressors mediate separate pathways of repression.

The knirps and knirps-related genes organize development of the second wing vein in Drosophila

The neighboring homologous knirps (kni) and knirps-related (knrl) genes in Drosophila encode transcription factors in the steroid hormone receptor superfamily. During early embryogenesis, kni functions as a gap gene to control expression of segmentation genes within the abdominal region of the embryo. In this study, we present evidence that kni and knrl link A/P positional information in larval wing imaginal discs to morphogenesis of the second longitudinal wing vein (L2). We show that kni and knrl are expressed in similar narrow stripes corresponding to the position of the L2 primordium. The kni and knrl L2 stripes abut the anterior border of the broad central expression domain of the Dpp target gene spalt major (salm). We provide evidence that radius incompletus (ri), a well-known viable mutant lacking the L2 vein, is a regulatory mutant of the kni/knrl locus. In ri mutant wing discs, kni and knrl fail to be expressed in the L2 primordium. In addition, the positions of molecular breakpoints in the kni/knrl locus indicate that the ri function is provided by cis-acting sequences upstream of the kni transcription unit. Epistasis tests reveal that the kni/knrl locus functions downstream of spalt major (salm) and upstream of genes required to initiate vein-versus-intervein differentiation. Mis-expression experiments suggest that kni and knrl expressing cells inhibit neighboring cells from becoming vein cells. Finally, kni and knrl are likely to refine the L2 position by positively auto-regulating their own expression and by providing negative feedback to repress salm expression. We propose a model in which the combined activities of kni and knrl organize development of the L2 vein in the appropriate position.

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

Previous studies have identified a minimal 434 bp enhancer from the promoter region of the Ciona Brachyury gene (Ci-Bra), which is sufficient to direct a notochord-specific pattern of gene expression. Here we present evidence that a Ciona homolog of snail (Ci-sna) encodes a repressor of the Ci-Bra enhancer in the tail muscles. DNA-binding assays identified four Ci-Sna-binding sites in the Ci-Bra enhancer, and mutations in these sites cause otherwise normal Ci-Bra/lacZ transgenes to be misexpressed in ectopic tissues, particularly the tail muscles. Selective misexpression of Ci-sna using a heterologous promoter results in the repression of Ci-Bra/lacZ transgenes in the notochord. Moreover, the conversion of the Ci-Sna repressor into an activator results in the ectopic induction of Ci-Bra/lacZ transgenes in the muscles, and also causes an intermixing of notochord and muscle cells during tail morphogenesis. These results suggest that Ci-Sna functions as a boundary repressor, which subdivides the mesoderm into separate notochord and tail muscle lineages.

Cis-acting elements conserved between mouse and pufferfish Otx2 genes govern the expression in mesencephalic neural crest cells

Previous studies suggested that the Otx2 gene plays an essential role in the development of cranial skeletons and nerves of mesencephalic neural crest origin. To clarify this role, we have identified the cis-acting elements in mouse and pufferfish Otx2 genes responsible for the expression in the crest cells using a transgenic approach with the lacZ reporter gene. In mouse, 49 bp sequences in the proximal 5′ region upstream were essential and sufficient to direct the transgene expression in the cephalic mesenchyme. In pufferfish, the 1.1 kb distal region, located far downstream (from +14.4 to +15.5 kb), had almost identical activity. Between them, several DNA sequences were conserved, and mutational analyses indicated that motif A was critical for the transgene expression in the premandibular region while motif B was critical in both premandibular and mandibular regions. Motif B, CTAATTA, contains the core motif for binding of homeodomain proteins while motif A, TAAATCTG, does not match any known consensus binding sequences for transcriptional factors. The cephalic mesenchyme that expressed beta-galactosidase under these cis-elements is most likely to correspond to mesencephalic crest cells. Thus the molecular machinery regulating Otx2 expression in these cells appears to be conserved between mouse and fish, implying a crucial role of the Otx2 gene in development of the neural-crest-derived structures of the gnathostome rostral head.

Differential regulation of gastrulation and neuroectodermal gene expression by Snail in the Drosophila embryo

The initiation of mesoderm differentiation in the Drosophila embryo requires the gene products of twist and snail. In either mutant, the ventral cell invagination during gastrulation is blocked and no mesoderm-derived tissue is formed. One of the functions of Snail is to repress neuroectodermal genes and restrict their expressions to the lateral regions. The derepression of the neuroectodermal genes into the ventral region in snail mutant is a possible cause of defects in gastrulation and in mesoderm differentiation. To investigate such possibility, we analysed a series of snail mutant alleles. We found that different neuroectodermal genes respond differently in various snail mutant background. Due to the differential response of target genes, one of the mutant alleles, V2, that has reduced Snail function showed an intermediate phenotype. In V2 embryos, neuroectodermal genes, such as single-minded and rhomboid, are derepressed while ventral invagination proceeds normally. However, the differentiation of these invaginated cells into mesodermal lineage is disrupted. The results suggest that the establishment of mesodermal cell fate requires the proper restriction of neuroectodermal genes, while the ventral cell movement is independent of the expression patterns of these genes. Together with the data showing that the expression of some ventral genes disappear in snail mutants, we propose that Snail may repress or activate another set of target genes that are required specifically for gastrulation.