wingless acts through the shaggy/zeste-white 3 kinase to direct dorsal-ventral axis formation in the Drosophila leg

The pleiotropic functions of Pri smORF peptides synchronize leg development regulators

The last decade witnesses the emergence of the abundant family of smORF peptides, encoded by small ORF (<100 codons), whose biological functions remain largely unexplored. Bioinformatic analyses here identify hundreds of putative smORF peptides expressed in Drosophila imaginal leg discs. Thanks to a functional screen in leg, we found smORF peptides involved in morphogenesis, including the pioneer smORF peptides Pri. Since we identified its target Ubr3 in the epidermis and pri was known to control leg development through poorly understood mechanisms, we investigated the role of Ubr3 in mediating pri function in leg. We found that pri plays several roles during leg development both in patterning and in cell survival. During larval stage, pri activates independently of Ubr3 tarsal transcriptional programs and Notch and EGFR signaling pathways, whereas at larval pupal transition, Pri peptides cooperate with Ubr3 to insure cell survival and leg morphogenesis. Our results highlight Ubr3 dependent and independent functions of Pri peptides and their pleiotropy. Moreover, we reveal that the smORF peptide family is a reservoir of overlooked developmental regulators, displaying distinct molecular functions and orchestrating leg development.

Essential long-range action of Wingless/Wnt in adult intestinal compartmentalization

Signal transduction activated by Wingless/Wnt ligands directs cell proliferation and fate specification in metazoans, and its overactivation underlies the development of the vast majority of colorectal cancers. In the conventional model, the secretion and movement of Wingless to cells distant from its source of synthesis are essential for long-range signaling in tissue patterning. However, this model was upended recently by an unanticipated finding: replacement of wild-type Drosophila Wingless with a membrane-tethered form produced viable adults with largely normal external morphology, which suggested that Wingless secretion and movement are dispensable for tissue patterning. Herein, we tested this foundational principle in the adult intestine, where Wingless signaling gradients coincide with all major boundaries between compartments. We find that the critical roles of Wingless during adult intestinal development, which include regulation of target gene activation, boundary formation, stem cell proliferation, epithelial cell fate specification, muscle differentiation, gut folding, and signaling crosstalk with the Decapentaplegic pathway, are all disrupted by Wingless tethering. These findings provide new evidence that supports the requirement for the direct, long-range action of Wingless in tissue patterning, with relevance for animal development, tissue homeostasis and Wnt-driven disease.

Glycogen Synthase Kinase 3: A Kinase for All Pathways?

Glycogen synthase kinase-3 (GSK-3) is an unusual protein-serine kinase in that it is primarily regulated by inhibition and lies downstream of multiple cell signaling pathways. This raises a variety of questions in terms of its physiological role(s), how signaling specificity is maintained and why so many eggs have been placed into one basket. There are actually two baskets, as there are two isoforms, GSK-3α and β, that are highly related and largely redundant. Their many substrates range from regulators of cellular metabolism to molecules that control growth and differentiation. In this chapter, we review the characteristics of GSK-3, update progress in understanding the kinase, and try to answer some of the questions raised by its unusual properties. Indeed, the kinase may trigger transformation in our thinking of how cellular signals are organized and controlled.

The expression of the T-box selector gene midline in the leg imaginal disc is controlled by both transcriptional regulation and cell lineage

The Drosophila Tbx20 homologs midline and H15 act as selector genes for ventral fate in Drosophila legs. midline and H15 expression defines the ventral domain of the leg and the two genes are necessary and sufficient for the development of ventral fate. Ventral-specific expression of midline and H15 is activated by Wingless (Wg) and repressed by Decapentaplegic (Dpp). Here we identify VLE, a 5 kb enhancer that drives ventral specific expression in the leg disc that is very similar to midline expression. Subdivision of VLE identifies two regions that mediate both activation and repression and third region that only mediates repression. Loss- and gain-of-function genetic mosaic analysis shows that the activating and repressing regions respond to Wg and Dpp signaling respectively. All three repression regions depend on the activity of Mothers-against-decapentaplegic, a Drosophila r-Smad that mediates Dpp signaling, and respond to ectopic expression of the Dpp target genes optomoter-blind and Dorsocross 3. However, only one repression region is responsive to loss of schnurri, a co-repressor required for direct repression by Dpp-signaling. Thus, Dpp signaling restricts midline expression through both direct repression and through the activation of downstream repressors. We also find that midline and H15 expression are both subject to cross-repression and feedback inhibition. Finally, a lineage analysis indicates that ventral midline-expressing cells and dorsal omb-expressing cells do not mix during development. Together this data indicates that the ventral-specific expression of midline results from both transcriptional regulation and from a lack of cell-mixing between dorsal and ventral cells.

Wnt affects symmetry and morphogenesis during post-embryonic development in colonial chordates

Background

Wnt signaling is one of the earliest and most highly conserved regulatory pathways for the establishment of the body axes during regeneration and early development. In regeneration, body axes determination occurs independently of tissue rearrangement and early developmental cues. Modulation of the Wnt signaling in either process has shown to result in unusual body axis phenotypes. Botryllus schlosseri is a colonial ascidian that can regenerate its entire body through asexual budding. This processes leads to an adult body via a stereotypical developmental pathway (called blastogenesis), without proceeding through any embryonic developmental stages.

Results

In this study, we describe the role of the canonical Wnt pathway during the early stages of asexual development. We characterized expression of three Wnt ligands (Wnt2B, Wnt5A, and Wnt9A) by in situ hybridization and qRT-PCR. Chemical manipulation of the pathway resulted in atypical budding due to the duplication of the A/P axes, supernumerary budding, and loss of the overall cell apical-basal polarity.

Conclusions

Our results suggest that Wnt signaling is used for equivalent developmental processes both during embryogenesis and asexual development in an adult organism, suggesting that patterning mechanisms driving morphogenesis are conserved, independent of embryonic, or regenerative development.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-015-0009-3) contains supplementary material, which is available to authorized users.

A context-dependent combination of Wnt receptors controls axis elongation and leg development in a short germ insect

Short germ embryos elongate their primary body axis by consecutively adding segments from a posteriorly located growth zone. Wnt signalling is required for axis elongation in short germ arthropods, including Tribolium castaneum, but the precise functions of the different Wnt receptors involved in this process are unclear. We analysed the individual and combinatorial functions of the three Wnt receptors, Frizzled-1 (Tc-Fz1), Frizzled-2 (Tc-Fz2) and Frizzled-4 (Tc-Fz4), and their co-receptor Arrow (Tc-Arr) in the beetle Tribolium. Knockdown of gene function and expression analyses revealed that Frizzled-dependent Wnt signalling occurs anteriorly in the growth zone in the presegmental region (PSR). We show that simultaneous functional knockdown of the Wnt receptors Tc-fz1 and Tc-fz2 via RNAi resulted in collapse of the growth zone and impairment of embryonic axis elongation. Although posterior cells of the growth zone were not completely abolished, Wnt signalling within the PSR controls axial elongation at the level of pair-rule patterning, Wnt5 signalling and FGF signalling. These results identify the PSR in Tribolium as an integral tissue required for the axial elongation process, reminiscent of the presomitic mesoderm in vertebrates. Knockdown of Tc-fz1 alone interfered with the formation of the proximo-distal and the dorso-ventral axes during leg development, whereas no effect was observed with single Tc-fz2 or Tc-fz4 RNAi knockdowns. We identify Tc-Arr as an obligatory Wnt co-receptor for axis elongation, leg distalisation and segmentation. We discuss how Wnt signalling is regulated at the receptor and co-receptor levels in a dose-dependent fashion.

Separable functions of wingless in distal and ventral patterning of the Tribolium leg

The gene wingless (wg) in Drosophila is an important factor in leg development. During embryonic development wg is involved in the allocation of the limb primordia. During imaginal disk development wg is involved in distal development and it has a separate role in ventral development. The expression pattern of wg is highly conserved in all arthropods (comprising data from insects, myriapods, crustaceans, and chelicerates), suggesting that its function in leg development is also conserved. However, recent work in other insects (e.g. the milkweed bug Oncopeltus fasciatus) argued against a role of wg in leg development. We have studied the role of wg in leg development of the flour beetle Tribolium castaneum. Using stage-specific staggered embryonic RNAi in wild-type and transgenic EGFP expressing enhancer trap lines we are able to demonstrate separable functions of Tribolium wg in distal and in ventral leg development. The distal role affects all podomeres distal to the coxa, whereas the ventral role is restricted to cells along the ventral midline of the legs. In addition, severe leg defects after injection into early embryonic stages are evidence that wg is also involved in proximal development and limb allocation in Tribolium. Our data suggest that the roles of wg in leg development are highly conserved in the holometabolous insects. Further studies will reveal the degree of conservation in other arthropod groups.

A Wnt survival guide: from flies to human disease

It has been two decades since investigators discovered the link between the Drosophila wingless (Wg) gene and the vertebrate oncogene int-1, thus establishing the family of signaling proteins known as Wnts. Since the inception of the Wnt signaling field, there have been 19 Wnt isoforms identified in humans. These secreted glycoproteins can activate at least two distinct signaling pathways in vertebrate cells, leading to cellular changes that regulate a vast array of biological processes, including embryonic development, cell fate, cell proliferation, cell migration, stem cell maintenance, tumor suppression, and oncogenesis. In certain contexts, one subset of Wnt isoforms activates the canonical Wnt/beta-catenin pathway that is characterized by the activation of certain beta-catenin-responsive target genes in response to the binding of Wnt ligand to its cognate receptors. Similarly, a second subset of Wnt isoforms activates beta-catenin-independent pathways, including the Wnt/calcium (Wnt/Ca) pathway and the Wnt/planar cell polarity (Wnt/PCP) pathway, in certain cellular contexts. In addition, research has identified several secreted proteins known to regulate Wnt signaling, including the Dickkopf (DKK) family, secreted Frizzled-related proteins (sFRPs), and Wnt inhibitory factor-1 (WIF-1). The advent of technologies that can provide genome-wide expression data continues to implicate Wnts and proteins that regulate Wnt signaling pathways in a growing number of disease processes. The aim of this review is to provide a context on the Wnt field that will facilitate the interpretation and study of Wnt signaling in the context of human disease.

The Drosophila gene zfh2 is required to establish proximal-distal domains in the wing disc

Three main events characterize the development of the proximal-distal axis of the Drosophila wing disc: first, generation of nested circular domains defined by different combinations of gene expression; second, activation of wingless (wg) gene expression in a ring of cells; and third, an increase of cell number in each domain in response to Wg. The mechanisms by which these domains of gene expression are established and maintained are unknown. We have analyzed the role of the gene zinc finger homeodomain 2 (zfh2). We report that in discs lacking zfh2 the limits of the expression domains of the genes tsh, nub, rn, dve and nab coincide, and expression of wg in the wing hinge, is lost. We show that zfh2 expression is delimited distally by Vg, Nub and Dpp signalling, and proximally by Tsh and Dpp. Distal repression of zfh2 permits activation of nab in the wing blade and wg in the wing hinge. We suggest that the proximal-most wing fate, the hinge, is specified first and that later repression of zfh2 permits specification of the distal-most fate, the wing blade. We propose that proximal-distal axis development is achieved by a combination of two strategies: on one hand a process involving proximal to distal specification, with the wing hinge specified first followed later by the distal wing blade; on the other hand, early specification of the proximal-distal domains by different combinations of gene expression. The results we present here indicate that Zfh2 plays a critical role in both processes.

Secretion of Wnt Ligands Requires Evi, a Conserved Transmembrane Protein

Wnt signaling pathways are important for multiple biological processes during development and disease. Wnt proteins are secreted factors that activate target-gene expression in both a short- and long-range manner. Currently, little is known about how Wnts are released from cells and which factors facilitate their secretion. Here, we identify a conserved multipass transmembrane protein, Evenness interrupted (Evi/Wls), through an RNAi survey for transmembrane proteins involved in Drosophila Wingless (Wg) signaling. During development, evi mutants have patterning defects that phenocopy wg loss-of-function alleles and fail to express Wg target genes. evi's function is evolutionarily conserved as depletion of its human homolog disrupts Wnt signaling in human cells. Epistasis experiments and clonal analysis place evi in the Wg-producing cell. Our results show that Wg is retained by evi mutant cells and suggest that evi is the founding member of a gene family specifically required for Wg/Wnt secretion.

Skeletal muscle hypertrophy and atrophy signaling pathways

Skeletal muscle hypertrophy is defined as an increase in muscle mass, which in the adult animal comes as a result of an increase in the size, as opposed to the number, of pre-existing skeletal muscle fibers. The protein growth factor insulin-like growth factor 1 (IGF-1) has been demonstrated to be sufficient to induce skeletal muscle hypertrophy. Over the past few years, signaling pathways which are activated by IGF-1, and which are responsible for regulating protein synthesis pathways, have been defined. More recently, it has been show that IGF-1 can also block the transcriptional upregulation of key mediators of skeletal muscle atrophy, the ubiquitin-ligases MuRF1 and MAFbx (also called Atrogin-1). Further, it has been demonstrated recently that activation of the NF-kappaB transcription pathway, activated by cachectic factors such as TNFalpha, is sufficient to induce skeletal muscle atrophy, and this atrophy occurs in part via NF-kappaB-mediated upregulation of MuRF1. Further work has demonstrated a trigger for MAFbx expression upon treatment with TNFalpha--the p38 MAPK pathway. This review will focus on the recent progress in the understanding of molecular signalling, which governs skeletal muscle atrophy and hypertrophy, and the known instances of cross-regulation between the two systems.

Developmental genetics and arthropod evolution: part 1, on legs

Developmental genetic information as it relates to the ontogeny of limbs can help evaluate various scenarios of arthropod evolution proposed in the past, as well as help frame other alternatives. First, the cascade of genetic expressions, which controls the development of the arthropod limb, suggests that a postulated evolution of the crustacean coxa from a proximal endite, a structure seen on certain Cambrian crustaceomorphs, might not be correct. Alternative hypotheses could explain the fossil anatomy, and the genetic patterns of expression demand that we at least be cautious in interpreting the Orsten material. Second, recognition of three distinct models of limb formation in arthropods would appear to preclude Rehbachiella, from the Cambrian Orsten, and Lepidocaris, from the Devonian Rhynie Chert, as members of the crown-group Branchiopoda. The recognition of a distinct Artemia Model of limb induction within living anostracans, notostracans, cladocerans, and conchostracans requires that such a model be part of the ground pattern of the Branchiopoda, a pattern that does not appear to have been possible in the fossil species. Finally, the suggestion that a large number of leg segments must be a plesiomorphic condition in arthropods should be considered cautiously. A sequential occurrence of mutations including, for example, a recessive loss-of-function mutant of a Hox-gene like Antennapedia could have resulted in the apomorphic evolution of long, multisegmented limbs within different groups of arthropods. The need for more comprehensive phylogenetic studies using as many taxa and characters possible is obvious both for the generation of scenarios of evolution, as well as in testing multiple alternative hypotheses of relationships.

The hiiragi gene encodes a poly(A) polymerase, which controls the formation of the wing margin in Drosophila melanogaster

The hiiragi (hrg) gene plays a key role in the development of the wing margin in Drosophila melanogaster. A mutation in the hrg gene resulted in a decrease in the level of the hrg transcript and was associated with a notched wing phenotype. We report here that the hrg gene encodes a poly(A) polymerase (PAP). The bovine cDNA for PAP type II reversed the phenotype due to mutation of the hrg gene, suggesting that hrg might encode a functional homolog of PAP. A mutation that reduced the enzymatic activity of Hrg failed to reverse the phenotype of hrg mutants, suggesting that the enzymatic activity of Hrg was required to rescue the wing phenotype. The levels of expression of wingless and cut at the presumptive wing margins were reduced in the late third-instar larvae of hrg mutants. These results suggest that the product of hrg is required for the normal expression of a series of genes in this region. Our results provide the first evidence that a PAP in Drosophila plays a key role in the early development of the wing margin, acting to regulate the specific expression of a series of genes via, perhaps, control of the processing of the 3' ends of transcripts.

Regulatory mutations of the Drosophila Sox gene Dichaete reveal new functions in embryonic brain and hindgut development

Sox domain proteins encompass a conserved family of transcriptional regulators that are implicated in a variety of developmental processes in eukaryotes from worm to man. The Dichaete gene of Drosophila encodes a group B Sox protein related to mammalian Sox1, -2, and -3 and, like these proteins, it is widely and dynamically expressed throughout embryogenesis. In order to unravel new Dichaete functions, we characterized the organization of the Dichaete gene using a combination of regulatory mutant alleles and reporter gene constructs. Dichaete expression is tightly controlled during embryonic development by a complex of regulatory elements distributed over 25 kb downstream and 3 kb upstream of the transcription unit. A series of regulatory alleles which affect tissue-specific domains of Dichaete were used to demonstrate that Dichaete has functions in addition to those during segmentation and midline development previously described. First, Dichaete has functions in the developing brain. A specific group of neural cells in the tritocerebrum fails to develop correctly in the absence of Dichaete, as revealed by reduced expression of labial, zfh-2, wingless, and engrailed. Second, Dichaete is required for the correct differentiation of the hindgut. The Dichaete requirement in hindgut morphogenesis is, in part, via regulation of dpp, since ectopically supplied dpp can rescue Dichaete phenotypes in the hindgut. Taken together, there are now four distinct in vivo functions described for Dichaete that can be used as models for context-dependent comparative studies of Sox function.

Integration of signaling networks that regulate Dictyostelium differentiation

In Dictyostelium amoebae, cell-type differentiation, spatial patterning, and morphogenesis are controlled by a combination of cell-autonomous mechanisms and intercellular signaling. A chemotactic aggregation of approximately 10(5) cells leads to the formation of a multicellular organism. Cell-type differentiation and cell sorting result in a small number of defined cell types organized along an anteroposterior axis. Finally, a mature fruiting body is created by the terminal differentiation of stalk and spore cells. Analysis of the regulatory program demonstrates a role for several molecules, including GSK-3, signal transducers and activators of transcription (STAT) factors, and cAMP-dependent protein kinase (PKA), that control spatial patterning in metazoans. Unexpectedly, two component systems containing histidine kinases and response regulators also play essential roles in controlling Dictyostelium development. This review focuses on the role of cAMP, which functions intracellularly to mediate the activity of PKA, an essential component in aggregation, cell-type specification, and terminal differentiation. Cytoplasmic cAMP levels are controlled through both the regulated activation of adenylyl cyclases and the degradation by a phosphodiesterase containing a two-component system response regulator. Extracellular cAMP regulates G-protein-dependent and -independent pathways to control aggregation as well as the activity of GSK-3 and the transcription factors GBF and STATa during multicellular development. The integration of these pathways with others regulated by the morphogen DIF-1 to control cell fate decisions are discussed.

Regeneration in insects

@9cIntroduction@21T issues exhibit an impressive ability to respond to a myriad of insults by repairing and regenerating complex structures. The elegant and orderly process of regeneration provides clues to the mechanisms of pattern formation but also offers the hope that the process might one day be manipulated to replace damaged body parts. To manipulate the process, it will be necessary to understand the genetic basis of the process. In the case of the insect leg, we are coming close to such a level of understanding and many of the lessons learned are relevant to vertebrate systems. A dynamic web of gene regulatory networks appears to create a robust self-organizing system that is at once extremely intricate but also perhaps simple in its reliance on a few key signaling pathways and a few simple processes, e.g. autoactivation and lateral inhibition. Here we will summarize what has been learned about the networks of gene regulation present in the Drosophila leg discs and then we will explore how the regenerative responses to different insults can be understood as predictable responses to these networks. Each of the regulatory networks could themselves serve as the subject of a detailed review and that is beyond the scope of this discussion. Here we will focus on the interplay between the regulatory networks in patterning the tissue.

Regulation of the protein kinase activity of Shaggy(Zeste-white3) by components of the wingless pathway in Drosophila cells and embryos

The protein-serine kinase Shaggy(Zeste-white3) (Sgg(Zw3)) is the Drosophila homolog of mammalian glycogen synthase kinase-3 and has been genetically implicated in signal transduction pathways necessary for the establishment of patterning. Sgg(Zw3) is a putative component of the Wingless (Wg) pathway, and epistasis analyses suggest that Sgg(Zw3) function is repressed by Wg signaling. Here, we have investigated the biochemical consequences of Wg signaling with respect to the Sgg(Zw3) protein kinase in two types of Drosophila cell lines and in embryos. Our results demonstrate that Sgg(Zw3) activity is inhibited following exposure of cells to Wg protein and by expression of downstream components of Wg signaling, Drosophila frizzled 2 and dishevelled. Wg-dependent inactivation of Sgg(Zw3) is accompanied by serine phosphorylation. We also show that the level of Sgg(Zw3) activity regulates the stability of Armadillo protein and modulates the level of phosphorylation of D-Axin and Armadillo. Together, these results provide direct biochemical evidence in support of the genetic model of Wg signaling and provide a model for dissecting the molecular interactions between the signaling proteins.

The armadillo family of structural proteins

The armadillo gene is a segment polarity gene of Drosophila involved in signal transduction through wingless. Since the mid-1980s, a growing number of related proteins have been identified based on sequence homologies. These proteins share a central domain that is composed of a series of imperfect 45 amino acid repeats. Armadillo family members reveal diverse cellular locations reflecting their diverse functions. A single protein exerts several functions through interactions of its armadillo repeat domain with diverse binding partners. The proteins combine structural roles as cell-contact and cytoskeleton-associated proteins and signaling functions by generating and transducing signals affecting gene expression. The study of armadillo family members has made it increasingly clear that a distinction between structural proteins on the one hand and signaling molecules on the other is rather artificial. Instead armadillo family members exert both functions by interacting with a number of distinct cellular-binding partners.

WNT-1 and HGF regulate GSK3 beta activity and beta-catenin signaling in mammary epithelial cells

Wnt-1, a secreted glycoprotein, participates in development of the nervous system and contributes to mammary oncogenesis when overexpressed. We show that GSK3 activity is decreased in mouse mammary cells transformed by Wnt-1. These cells also exhibit a substantial Wnt-1 dependent increase in the uncomplexed population of beta-catenin. Wnt-1 signaling does not change the steady state level of either GSK3 alpha or GSK3 beta but instead leads to an increased association between GSK3 beta and beta-catenin. HGF/SF treatment of mouse mammary cells also leads to a transient decrease in GSK3 activity and a parallel, selective increase in the uncomplexed pool of beta-catenin. Both Wnt-1 and HGF/SF lead to nuclear accumulation of beta-catenin and activation of a LEF/Tcf responsive reporter gene. This study defines a pivotal signal transduction pathway, activated by both Wnt-1 and HGF/SF, leading to decreased GSK3 beta activity and consequently an increase in the free pool and nuclear accumulation of beta-catenin and changes in gene expression.

Spitz and Wingless, emanating from distinct borders, cooperate to establish cell fate across the Engrailed domain in the Drosophila epidermis

A key step in development is the establishment of cell type diversity across a cellular field. Segmental patterning within the Drosophila embryonic epidermis is one paradigm for this process. At each parasegment boundary, cells expressing the Wnt family member Wingless confront cells expressing the homeoprotein Engrailed. The Engrailed-expressing cells normally differentiate as one of two alternative cell types. In investigating the generation of this cell type diversity among the 2-cell-wide Engrailed stripe, we previously showed that Wingless, expressed just anterior to the Engrailed cells, is essential for the specification of anterior Engrailed cell fate. In a screen for additional mutations affecting Engrailed cell fate, we identified anterior open/yan, a gene encoding an inhibitory ETS-domain transcription factor that is negatively regulated by the Rasl-MAP kinase signaling cascade. We find that Anterior Open must be inactivated for posterior Engrailed cells to adopt their correct fate. This is achieved by the EGF receptor (DER), which is required autonomously in the Engrailed cells to trigger the Ras1-MAP kinase pathway. Localized activation of DER is accomplished by restricted processing of the activating ligand, Spitz. Processing is confined to the cell row posterior to the Engrailed domain by the restricted expression of Rhomboid. These cells also express the inhibitory ligand Argos, which attenuates the activation of DER in cell rows more distant from the ligand source. Thus, distinct signals flank each border of the Engrailed domain, as Wingless is produced anteriorly and Spitz posteriorly. Since we also show that En cells have the capacity to respond to either Wingless or Spitz, these cells must choose their fate depending on the relative level of activation of the two pathways.

Patterning the Xenopus blastula

This review starts from the classical standpoint that there are at least two separable processes acting with respect to axis formation and tissue specification in the early Xenopus embryo: a UV-insensitive event establishing a postgastrula embryo consisting of three concentric germ layers, ectoderm, mesoderm and endoderm, all of a ventral character; and a UV-sensitive event producing tissue of a dorsal type, including somites, notochord and neural tissue, and concomitantly establishing the dorsoventral and anteroposterior axes. The experimental evidence suggesting the molecular basis of the dorsal and ventral pathways is reviewed.

The homeobox gene Distal-less induces ventral appendage development in Drosophila

This study investigates the role of the homeobox gene Distal-less (Dll) in the development of the legs, antennae, and wings of Drosophila. Lack of Dll function causes a change in the identity of ventral appendage cells (legs and antennae) that often results in the loss of the appendage. Ectopic Dll expression in the proximal region of ventral appendages induces nonautonomous duplication of legs and antennae by the activation of wingless and decapentaplegic. Ectopic Dll expression in dorsal appendages produces transformation into corresponding ventral appendages; wings and halteres develop ectopic legs and the head-eye region develops ectopic antennae. In the wing, the exogenous Dll product induces this transformation by activating the endogenous Dll gene and repressing the wing determinant gene vestigial. It is proposed that Dll induces the development of ventral appendages and also participates in a genetic address that specifies the identity of ventral appendages and discriminates the dorsal versus the ventral appendages in the adult. However, unlike other homeotic genes, Dll expression and function is not defined by a cell lineage border. Dll also performs a secondary and late function required for the normal patterning of the wing.

Xwnt-8 and lithium can act upon either dorsal mesodermal or neurectodermal cells to cause a loss of forebrain in Xenopus embryos

When Xenopus gastrulae are made to misexpress Xwnt-8, or are exposed to lithium ions, they develop with a loss of anterior structures. In the current study, we have characterized the neural defects produced by either Xwnt-8 or lithium and have examined potential cellular mechanisms underlying this anterior truncation. We find that the primary defect in embryos exposed to lithium at successively earlier stages during gastrulation is a progressive rostral to caudal deletion of the forebrain, while hindbrain and spinal regions of the CNS remain intact. Misexpression of Xwnt-8 during gastrulation produces an identical loss of forebrain. Our results demonstrate that lithium and Wnts can act upon either prospective neural ectodermal cells, or upon dorsal mesodermal cells, to cause a loss of anterior pattern. Specifically, ectodermal cells isolated from lithium- or Wnt-exposed embryos are unable to form anterior neural tissue in response to inductive signals from normal dorsal mesoderm. In addition, although dorsal mesodermal cells from lithium- or Wnt-exposed embryos are specified properly, and produce normal levels of the anterior neural inducing molecules noggin and chordin, they show a greatly reduced capacity to induce anterior neural tissue in conjugated ectoderm. Taken together, our results are consistent with a model in which Wnt- or lithium-mediated signals can induce either mesodermal or ectodermal cells to produce a dominant posteriorizing morphogen which respecifies anterior neural tissue as posterior.

Shaggy and dishevelled exert opposite effects on Wingless and Decapentaplegic expression and on positional identity in imaginal discs

The finding that Wingless (WG) and Decapentaplegic (DPP) suppress each others transcription provides a mechanism for creating developmental territories in fields of cells. Here, we address the mechanism of that antagonism. The dishevelled (dsh) and shaggy (sgg) genes encode intracellular proteins generally thought of as downstream of WG signaling. We have investigated the effects of changing either DSH or SGG activity on both cell fate and wg and dpp expression. At the level of cell fate in discs, DSH antagonizes SGG activity. At the level of gene expression, SGG positively regulates dpp expression and negatively regulates wg expression while DSH activity suppresses dpp expression and promotes wg expression. Sharp borders of gene expression correlating precisely with clone boundaries suggest that the effects of DSH and SGG on transcription of wg and dpp are not mediated by secreted factors but rather act through intracellular effectors. The interactions described here suggest a model for the antagonism between WG and DPP that is mediated via SGG. The model incorporates autoactivation and lateral inhibition, which are properties required for the production of stable patterns. The regulatory interactions described exhibit extensive ability to organize new pattern in response to manipulation or injury.

Insect—crustacean relationships: insights from comparative developmental and molecular studies

The phylogenetic relationships between the major arthropod groups are still far from being resolved. Phylogenetic analyses have usually relied on detailed morphological comparisons which are confounded by the extensive occurrence of convergence. We examine the available morphological evidence in the light of recent comparative developmental and molecular studies and suggest ways in which genetic-developmental information could help assess homology and overcome the problem of convergence. On the basis of such considerations we support the common origin of crustaceans and insects from a crustaceanlike mandibulate ancestor. Focusing on the specific relationships between crustaceans, myriapods and insects, we suggest that insects could emerge from this crustacean-like ancestor independently from myriapods, and after the major crustacean radiations.

Compartments, wingless and engrailed: patterning the ventral epidermis of Drosophila embryos

Recent experiments on the wing disc of Drosophila have shown that cells at the interface between the anterior and posterior compartments drive pattern formation by becoming the source of a morphogen. Here we ask whether this model applies to the ventral embryonic epidermis. First, we show that interfaces between posterior (engrailed ON) and anterior (engrailed OFF) cells are required for pattern formation. Second, we provide evidence that Wingless could play the role of the morphogen, at least within part of the segmental pattern. We looked at the cuticular structures that develop after different levels of uniform Wingless activity are added back to unsegmented embryos (wingless- engrailed-). Because it is rich in landmarks, the T1 segment is a good region to analyse. There, we find that the cuticle formed depends on the amount of added Wingless activity. For example, a high concentration of Wingless gives the cuticle elements normally found near the top of the presumed gradient. Unsegmented embryos are much shorter than wild type. If Wingless activity is added in stripes, the embryos are longer than if it is added uniformly. We suggest that the Wingless gradient landscape affects the size of the embryo, so that steep slopes would allow cells to survive and divide, while an even distribution of morphogen would promote cell death. Supporting the hypothesis that Wingless acts as a morphogen, we find that these stripes affect, at a distance, the type of cuticle formed and the planar polarity of the cells.

Combinatorial signalling by Xwnt-11 and Xnr3 in the organizer epithelium

The epithelium of the Spemann organizer plays an important role in embryonic axis formation and transplantation experiments have shown that epithelial organizer cells have potent axis-inducing potential. Known axis-inducing molecules like noggin and chordin are not expressed in the epithelium and cannot account for its inductive properties. Xwnt-11 is expressed in the epithelium but has only poor dorsalizing activity. In an expression screen for genes that are able to functionally cooperate with Xwnt-11 we have identified a cDNA encoding Xenopus nodal-related 3 (XNR3), a member of the TGF-beta family, coexpressed with Xwnt-11 in the organizer epithelium. Xwnt-11 and Xnr3 act highly cooperatively in inducing secondary embryonic axes and dorsalizing ventral mesoderm. Xwnt-11/Xnr3 interfere with BMP signalling without themselves inducing chordin or noggin. The results indicate that induction by the organizer epithelium may result from the combinatorial action of instructive Xnr3 and permissive Xwnt-11 signalling.

Direct and long-range action of a wingless morphogen gradient

Wingless (Wg), a founding member of the Wingless/Int-1 (Wnt) family of secreted proteins, acts as a short-range inducer and as a long-range organizer during Drosophila development. Here, we determine the consequences of ectopically expressing (i) a wild-type form of Wg, (ii) a membrane-tethered form of Wg, and (iii) a constitutively active form of the cytosolic protein Armadillo (Arm), which normally acts to transduce Wg, and we compare them with the effects of removing endogenous Wg or Arm activity. Our results indicate that wild-type Wg acts at long range, up-regulating the transcription of particular target genes as a function of concentration and distance from secreting cells. In contrast, tethered Wg and Arm have only short-range or autonomous effects, respectively, on the transcription of these genes. We interpret these findings as evidence that Wg can act directly and at long range as a gradient morphogen during normal development.

Ectopic expression of wingless in imaginal discs interferes with decapentaplegic expression and alters cell determination

We have expressed the segment polarity gene wingless (wg) ectopically in imaginal discs to examine its regulation of both ventral patterning and transdetermination. By experimentally manipulating the amount of Wg protein, we show that different thresholds of Wg activity elicit different outcomes, which are mediated by regulation of decapentaplegic (dpp) expression and result in alterations in the expression of homeotic genes. A high level of Wg activity leads to loss of all dorsal pattern elements and the formation of a complete complement of ventral pattern elements on the dorsal side of legs, and is correlated with repression of dpp expression. wg expression in dorsal cells of each disc also leads to dose-dependent transdetermination in those cells in homologous discs such as the labial, antennal and leg, but not in cells of dorsally located discs. When dpp expression is repressed by high levels of Wg, transdetermination does not occur, confirming that dpp participates with wg to induce transdetermination. These and other experiments suggest that dorsal expression of wg alters disc patterning and disc cell determination by modulating the expression of dpp. The dose-dependent effects of wg on dpp expression, ventralization of dorsal cells and transdetermination support a model in which wg functions as a morphogen in imaginal discs.

Organizing spatial pattern in limb development

Recent studies on the development of the legs and wings of Drosophila have led to the conclusion that insect limb development is controlled by localized pattern organizing centers, analogous to those identified in vertebrate embryos. Genetic analysis has defined the events that lead to the formation of these organizing centers and has led to the identification of gene products that mediate organizer function. The possibility of homology between vertebrate and insect limbs is considered in light of recently reported similarities in patterns of gene expression and function.

Visualization of gene expression in living adult Drosophila

To identify genes involved in the patterning of adult structures, Gal4-UAS (upstream activating site) technology was used to visualize patterns of gene expression directly in living flies. A large number of Gal4 insertion lines were generated and their expression patterns were studied. In addition to identifying several characterized developmental genes, the approach revealed previously unsuspected genetic subdivisions of the thorax, which may control the disposition of pattern elements. The boundary between two of these domains coincides with localized expression of the signaling molecule wingless.

Antagonistic interactions between wingless and decapentaplegic responsible for dorsal-ventral pattern in the Drosophila Leg

Subdivision of the limb primordia of Drosophila into anterior and posterior compartments triggers cell interactions that pattern the legs and wings. A comparable compartment-based mechanism is used to pattern the dorsal-ventral axis of the wing. Evidence is presented here for a mechanism based on cell interaction, rather than on compartment formation, that distinguishes dorsal from ventral in the leg. Mutual repression by Wingless and Decapentaplegic signaling systems generates a stable regulatory circuit by which each gene maintains its own expression in a spatially restricted domain. Compartment-independent patterning mechanisms may be used by other organisms during development.

wingless signaling in the Drosophila eye and embryonic epidermis

After the onset of pupation, sensory organ precursors, the progenitors of the interommatidial bristles, are selected in the developing Drosophila eye. We have found that wingless, when expressed ectopically in the eye via the sevenless promoter, blocks this process. Transgenic eyes have reduced expression of acheate, suggesting that wingless acts at the level of the proneural genes to block bristle development. This is in contrast to the wing, where wingless positively regulates acheate to promote bristle formation. The sevenless promoter is not active in the acheate-positive cells, indicating that the wingless is acting in a paracrine manner. Clonal analysis revealed a requirement for the genes porcupine, dishevelled and armadillo in mediating the wingless effect. Overexpression of zeste white-3 partially blocks the ability of wingless to inhibit bristle formation, consistent with the notion that wingless acts in opposition to zeste white-3. Thus the wingless signaling pathway in the eye appears to be very similar to that described in the embryo and wing. The Notch gene product has also been suggested to play a role in wingless signaling (J. P. Couso and A. M. Martinez Arias (1994) Cell 79, 259–72). Because Notch has many functions during eye development, including its role in inhibiting bristle formation through the neurogenic pathway, it is difficult to assess the relationship of Notch to wingless in the eye. However, we present evidence that wingless signaling still occurs normally in the complete absence of Notch protein in the embryonic epidermis. Thus, in the simplest model for wingless signalling, a direct role for Notch is unlikely.

Complementary and mutually exclusive activities of decapentaplegic and wingless organize axial patterning during Drosophila leg development

Growth and patterning of the Drosophila leg are organized by three secreted proteins: Hedgehog (Hh), Wingless (Wg), and Decapentaplegic (Dpp). Hh is secreted by posterior cells; it acts at short range to induce dorsal anterior cells to secrete Dpp and ventral anterior cells to secrete Wg. Here we show that the complementary patterns of dpp and wg expression are maintained by mutual repression: Dpp signaling blocks wg transcription, whereas Wg signaling attenuates dpp transcription. We also show that this mutual repression is essential for normal axial patterning because it ensures that the dorsalizing and ventralizing activities of Dpp and Wg are restricted to opposite sides of the leg primordium and meet only at the center of the primordium to distalize the appendage.

hiiragi, a gene essential for wing development in Drosophila melanogaster, affects the Notch cascade

A recessive mutation, hiiragiP1, on the second chromosome of Drosophila was obtained by P element insertion mutagenesis. Flies homozygous for hiiragiP1 have notched wing margins. Genetic interactions between hiiragi and the genes that encode components of Notch signaling, such as Notch, Hairless, Serrate and deltex, strongly support the involvement of hiiragi in the signal transduction cascade of Notch. It has been reported that Serrate and Delta, other components of Notch signaling, share EGF-like repeats and a second conserved cysteine-rich motif, and that these components interact physically with the same region of Notch. In hiiragiP1; SerrateD/+ double mutants, we observed synergistic enhancement of the notched phenotype of wing margins. In contrast, Delta FX3 had no phenotypic effect on hiiragiP1 in hiiragiP1; Delta FX3/+ double mutants. Taken together, these results indicate that hiiragi is involved in the Notch signaling cascade induced by Serrate rather than by Delta.

Decapentaplegic restricts the domain of wingless during Drosophila limb patterning

Signalling proteins in the BMP-decapentaplegic (dpp), WNT-wingless (wg) and Shh-hedgehog (hh) families have been implicated in limb and appendage development in both invertebrates and vertebrates. In Drosophila, dpp protein (Dpp) induces distal outgrowth and patterning of legs and wings, but the molecular responses to Dpp are not well characterized. Analysis of clones mutant for the Dpp receptors encoded by punt or thickveins (tkv) reveals that repression of wg expression is one critical function of Dpp signalling in leg and wing discs. Distal clones that lie on the anterior edge of the anterior-posterior compartment boundary ectopically express wg and cause pattern abnormalities, suggesting that Dpp represses Hh activation of wg in the distal primordia of the leg and wing. By repressing wg expression in the leg, Dpp signalling limits the region that responds to high levels of Wg and Dpp to the site of distal outgrowth. Such negative regulatory feedback loops between signalling molecules are likely to be critical for limb patterning in other species.

Two distinct mechanisms for long-range patterning by Decapentaplegic in the Drosophila wing

Secreted signalling molecules provide cells with positional information that organizes long-range pattern during the development of multicellular animals. Evidence is presented that localized expression of Decapentaplegic instructs cells about their position along the anterior-posterior axis of the Drosophila wing in two distinct ways. One mechanism is based on the local concentration of the secreted protein; the other is based on the ability of the cells to retain an instruction received at an earlier time when their progenitors were in close proximity to the signal. Both mechanisms are involved in axis formation.

decapentaplegic, a target gene of the wingless signalling pathway in the Drosophila midgut

dishevelled, shaggy/zeste-white 3 and armadillo are required for transmission of the wingless signal in the Drosophila epidermis. We show that these genes act in the same epistatic order in the embryonic midgut to transmit the wingless signal. In addition to mediating transcriptional stimulation of the homeotic genes Ultrabithorax and labial, they are also required for transcriptional repression of labial by high wingless levels. Efficient labial expression thus only occurs within a window of intermediate wingless pathway activity. Finally, the shaggy/zeste-white 3 mutants revealed that wingless signalling can stimulate decapentaplegic transcription in the absence of Ultrabithorax, identifying decapentaplegic as a target gene of wingless. As decapentaplegic itself is required for wingless expression in the midgut, this represents a positive feed-back loop between two cell groups signalling to each other to stimulate each other's signal production.

Serrate signals through Notch to establish a Wingless-dependent organizer at the dorsal/ventral compartment boundary of the Drosophila wing

Growth and patterning of the Drosophila wing is controlled by organizing centers located at the anterior-posterior and dorsal-ventral compartment boundaries. Interaction between cells in adjacent compartments establish the organizer. We report here that Serrate and Notch mediate the interaction between dorsal and ventral cells to direct localized expression of Wingless at the D/V boundary. Serrate serves as a spatially localized ligand which directs Wg expression through activation of Notch. Ligand independent activation of Notch is sufficient to direct Wg expression, which in turn mediates the organizing activity of the D/V boundary.

wingless inhibits morphogenetic furrow movement in the Drosophila eye disc

Differentiation of the Drosophila eye imaginal disc is an asynchronous, repetitive process which proceeds across the disc from posterior to anterior. Its propagation correlates with the expression of decapentaplegic at the front of differentiation, in the morphogenetic furrow. Both differentiation and decapentaplegic expression are maintained by Hedgehog protein secreted by the differentiated cells posterior to the furrow. However, their initiation at the posterior margin occurs prior to hedgehog expression by an unknown mechanism. We show here that the wingless gene contributes to the correct spatial localization of initiation. Initiation of the morphogenetic furrow is restricted to the posterior margin by the presence of wingless at the lateral margins; removal of wingless allows lateral initiation. Ectopic expression of wingless at the posterior margin can also inhibit normal initiation. In addition, the presence of wingless in the center of the disc can prevent furrow progression. These effects of wingless are achieved without altering the expression of decapentaplegic.

Invagination centers within the Drosophila stomatogastric nervous system anlage are positioned by Notch-mediated signaling which is spatially controlled through wingless

The gut-innervating stomatogastric nervous system of Drosophila, unlike the central and the peripheral nervous system, derives from a compact, single layered epithelial anlage. Here we report how this anlage is initially defined during embryogenesis by the expression of proneural genes of the achaete-scute complex in response to the maternal terminal pattern forming system. Within the stomatogastric nervous system anlage, the wingless-dependent intercellular communication system adjusts the cellular range of Notch-dependent lateral inhibition to single-out three achaete-expressing cells. Those cells define distinct invagination centers which orchestrate the behavior of neighboring cells to form epithelial infoldings, each headed by an achaete-expressing tip cell. Our results suggest that the wingless pathway acts not as an instructive signal, but as a permissive factor which coordinates the spatial activity of morphoregulatory signals within the stomatogastric nervous system anlage.

Axes, boundaries and coordinates: the ABCs of fly leg development

Recent studies of gene expression in the developing fruitfly leg support a model--Meinhardt's Boundary Model--which seems to contradict the prevailing paradigm for pattern formation in the imaginal discs of Drosophila--the Polar Coordinate Model. Reasoning from geometric first principles, this article examines the strengths and weaknesses of these hypotheses, plus some baffling phenomena that neither model can comfortably explain. The deeper question at issue is: how does the fly's genome encode the three-dimensional anatomy of the adult? Does it demarcate territories and boundaries (as in a geopolitical map) and then use those boundaries and their points of intersection as a scaffolding on which to erect the anatomy (the Boundary Model)? Or does it assign cellular fates within a relatively seamless coordinate system (the Polar Coordinate Model)? The existence of hybrid Cartesian-polar models shows that the alternatives may not be so clear-cut: a single organ might utilize different systems that are spatially superimposed or temporally sequential.

The dishevelled protein is modified by wingless signaling in Drosophila

Wingless (Wg) is an important signaling molecule in the development of Drosophila, but little is known about its signal transduction pathway. Genetic evidence indicates that another segment polarity gene, dishevelled (dsh) is required for Wg signaling. We have recently developed a cell culture system for Wg protein activity, and using this in vitro system as well as intact Drosophila embryos, we have analyzed biochemical changes in the Dsh protein as a consequence of Wg signaling. We find that Dsh is a phosphoprotein, normally present in the cytoplasm. Wg signaling generates a hyperphosphorylated form of Dsh, which is associated with a membrane fraction. Overexpressed Dsh becomes hyperphosphorylated in the absence of extracellular Wg and increases levels of the Armadillo protein, thereby mimicking the Wg signal. A deletional analysis of Dsh identifies several conserved domains essential for activity, among which is a so-called GLGF/DHR motif. We conclude that dsh, a highly conserved gene, is not merely a permissive factor in Wg signaling but encodes a novel signal transduction molecule, which may function between the Wg receptor and more downstream signaling molecules.

Wingless induces transdetermination in developing Drosophila imaginal discs

Drosophila imaginal discs, the precursors of the adult fly appendages, have been the subject of intensive developmental studies, particularly on cell determination. Cultured disc fragments are recognized not only for the ability to maintain their determined state through extra cell divisions but also for the ability to transdetermine, or switch to the determined state of a different disc. An understanding of transdetermination at a molecular level will provide further insight into the requirements for maintaining cell determination. We find that ectopic expression of the Drosophila gene wingless induces transdetermination of foreleg imaginal disc cells to wing cells. This transdetermination occurs in foreleg discs of developing larvae without disc fragmentation. The in situ-transdetermining cells localize to the dorsal region of the foreleg disc. This wingless-induced transdetermination event is remarkably similar to the leg-to-wing switch that occurs after leg disc culture. Thus we have identified a new approach to a molecular dissection of transdetermination.

Glycogen synthase kinase-3 and dorsoventral patterning in Xenopus embryos

Glycogen synthase kinase 3 (GSK-3) is homologous to the product of the Drosophila gene shaggy (zeste-white 3), which is required for signalling by wingless during Drosophila development. To test whether GSK-3 is also involved in vertebrate pattern formation, its role was investigated during early Xenopus development. It was found that dominant-negative GSK-3 mutants induced dorsal differentiation, whereas wild-type GSK-3 induced ventralization. These results indicate that GSK-3 is required for ventral differentiation, and suggest that dorsal differentiation may involve the suppression of GSK-3 activity by a wingless/wnt-related signal.

Compartments and appendage development in Drosophila

The appendages of Drosophila develop from the imaginal discs. During the extensive growth of these discs cell lineages are for the most part unfixed, suggesting a strong role for cell-cell interactions in controlling the final pattern of differentiation. However, during early and middle stages of development, discs are subdivided by strict lineage restrictions into a small number of spatially distinct compartments. These compartments appear to be maintained by stably inheriting states of gene expression; the compartment-specific expression of two such 'selector'-like genes, engrailed and apterous, are critical for anterior-posterior and dorso-ventral compartmentalization, respectively. Recent work suggests that one purpose of compartmentalization is to establish regions of specialized cells near compartment boundaries via intercompartmental induction, using molecules like the hedgehog protein. Thus, compartments can act as organizing centers for patterning within compartments. Evidence for non-compartmental patterning mechanisms will also be discussed.